Atlantis Cruise 3-31

Southern EPR 17°-19°S:
The STOWA Expedition


Text of NSF Proposals

Alivin Proposal

  • Introduction
  • The NAUDUR Program
  • Proposed Work
  • DSL 120 Surveys
  • Alvin Dives
  • Night Program
  • Cruise Plan
  • Shore-Based Progreams
  • Project Responsibilities
  • Relevancy to RIDGE and International Programs
  • References
  • ABE Proposal

  • Project Summary
  • Introduction
  • Scientific Objectives
  • Study Area
  • ABE Vehicle
  • Proposed Work
  • At Sea Operations
  • Post-Cruyise Analysis
  • Relevance to the RIDGE/NSF
  • References Cited
  • Figure Captions

  • [ATL331 home page | Atlantis Home Page]

    ALVIN proposal

    Title: Volcanological Investigations of a Superfast-Spreading Mid-Ocean Ridge

    P.I.s: John Sinton, Rodey Batiza, and Ken Rubin

    Introduction [back]
    One of the least understood phenomena associated with mid-ocean ridges concerns eruptive processes. For example, there are few places on the seafloor where single eruptive units have been clearly identified and mapped, a vital step in understanding the temporal and spatial evolution of a volcanic system. Existing evidence suggests that eruptions along intermediate to fast spreading ridges mainly occur as fissure eruptions, resulting in low-relief lava flows. In contrast, the isolated volcanoes and pillow mounds [Ballard and Van Andel, 1977; Bryan and Moore, 1977; Smith and Cann 1990;1992], characterizing the axial valley regions of slower spreading ridges might be taken to indicate that eruptions there mainly occur at point-source vents and lower effusion rates (but see also Head et al. [1996]). Because the thickness of crust is more or less constant over the spreading rate spectrum for oceanic crust generated at mid-ocean ridges away from hotspots and axial discontinuities, magma supply bears a systematic correlation with spreading rate. Although one might assume that eruptions are more frequent at high spreading rates and consequent magma supplies, this is only true if the volumes of individual eruptions do not also increase with spreading rate.

    If the volumes of individual eruptive units were known for various magma supplies, then recurrence intervals could be calculated. For example, the average thickness of normal oceanic crust is about 6 km, but the average thickness of the extrusive part of this crust is only about 0.5 km. Where reasonble geologic maps of the seafloor are available [e.g. Ballard et al., 1979; Ballard and Van Andel, 1977; Bryan and Moore, 1977; Smith and Cann 1990;1992; Chadwick and Embley, 1994; Haymon et al., 1993], estimated volumes for what might be interpreted to be individual eruptive units mapped on the seafloor are less than 0.05-0.2 km3. Gregg et al. [1996] estimate the volume of the 1991 eruption on the EPR near 9°50N to be closer to 0.005 km3. Macdonald et al. [1989] described a huge (~220 km2) lava field close to the axis of the EPR near 8°S, but Hall et al. [1993; 1996] showed that this field is the product of several different eruptions. Using values of .01-0.2 km3 for typical axial eruptive volumes gives recurrence intervals of about 50-1000 years for a 20 km-long segment of ridge spreading at 20 mm/yr full rate, decreasing to about 7-135 years for one spreading at 150 mm/yr, and a prediction for one eruption approximately every 1-20 years on the EPR between 17°25S and 18°40S, the area we are proposing to study.

    Constraining the volumes of individual eruptions reveals vital information about the subvolcanic magma storage and delivery system. For example, Sinton and Detrick [1992] suggested that no more than about 25% of the subaxial magma volume may be eruptible, due the dramatic increase in viscosity associated with crystallization. Although the size, shape and internal crystallinity of the subaxial magma system in our proposed study area are being investigated by seismic studies [e.g. Newell et al., 1993; Carbotte et al., 1994; Mutter et al., 1995; Hussenoeder et al., 1995], determining typical eruptive volumes at various spreading rates and magma supplies can provide another way to assess the size and internal dynamics of magma chambers beneath mid-ocean ridges. If we can constrain eruptive volumes in this area, we will be able to assess the relative amount of this subaxial magma involved in individual eruptions. This result will help us estimate the driving pressures of eruptions at mid-ocean ridges, which in turn should constrain the geometry of the magma delivery systsem and primary eruptive vents. Eruptive volumes and the nature of primary eruptive vents are critical parameters which allow for calculation of such things as magma rise rates in dikes and linear effusion rates [e.g. Wilson and Head, 1981; Delaney and Pollard, 1982; Gregg et al, 1996]. Knowledge of effusion rates and magma rise rates also help to define the processes occuring in the magmatic storage and delivery system.

    Although the study of hand samples can provide information about lava viscosity, chemical composition and eruption temperature, it tells us nothing about eruption rates. However, using laboratory simulations, in which polyethylene glycol is extruded at a constant rate into a tank filled with cold sucrose solution, Gregg and Fink [1995] produced a series of flow morphologies that are remarkably similar to those observed on the seafloor. By relating these laboratory results to submarine flow types, it is possible to estimate effusion rates, provided that eruption temperature and viscosity are known or can be estimated. It has generally been assumed that pillowed flows result from the lowest effusion rates and sheet flows from much higher effusion rates [e.g. Ballard et al., 1979; Luyendyk and Macdonald, 1985]. Gregg and Fink [1995] related submarine lava flow morphology to a dimensionless parameter, Y, which is the ratio of a time scale required for the surface of the flow to solidify (ts) to a time scale for the rate of heat advection within the flow (ta). In experiments performed on slopes
    <10¡, pillowed flows form when Y<3; rifted flows result from 3<Y<13, folded flows when 13<Y<25, and leveed flows require Y>25 [Fink and Griffiths, 1990]. As underlying slope increases these transitional values decrease, so that, for example, conditions which produce a lineated sheet flow on a 10¡ slope would generate a folded flow on a 50¡ slope. Because Y incorporates all key physical parameters of both an extrusion and its environment, each range of values appropriate to a specific submarine flow type can be associated with a range of effusion rates. Thus, effusion rates for submarine lava flows can be estimated if morphology, eruption temperature and eruption viscosity are known (Fig. 1). Once effusion rates for submarine lavas are constrained, this information can be coupled with information about flow volume and spreading rate to estimate the frequency and duration of eruptions along mid-ocean ridges, revealing vital information about the magmatic processes at active spreading centers.

    Figure 1. Effusion rate, lava viscosity and flow morphology for basalts emplaced on seafloor on 10° slope. Dashed lines indicate linear source (qo lava extruded per unit length); solid lines indicate point source (Qo). As slope increases, lines of constant move downward to right. Solid dots and solid bar indicate observed submarine lavas (from Gregg and Fink [1995]).

    It is notable that linear effusion rates can be estimated by two independent methods. One is that based on lava morphology, as described above, if eruption temperature and viscosity are known (both can be calculated from lava chemistry). Alternatively, effusion rates can be determined from knowledge of the flow volumes and nature of the eruptive conduit, which can be determined or estimated from detailed geological observations. Gregg et al. [1996] found convergence of these two methods at values of ~3 m3/sec for the 1991 EPR 9°50N eruption, a rate similar to that observed for pahoehoe eruptions in Hawaii.

    We propose to investigate eruptive processes at the superfast spreading East Pacific Rise, between 17° and 19°S. Specifically we propose to constrain eruptive ages and volumes, and lava flow morphology along three individual segments with contrasting axial morphology (axial depth and cross section, apparent extent of inflation, amount of rifting, extent of development of hydrothermal activity with implications for the amount of magma cooling, etc.). In addition we will attempt to determine the nature of eruptive vents, i.e. whether elongate fissures or point-source vents, the uniformity of chemical composition within individual eruptive units (with implications for the along-axis and vertical heterogeneity within subaxial magma reservoirs), and relate lava flow morphology to slope angle and effusion rates. Within the proposed study area is the full range of lava flow morphologies, with sheet flows being relatively more abundant that pillowed flows. Furthermore, we have already identified the products of individual eruptions in several places and are confident that further field work will allow us to better define the limits of these eruptions and identify others. Taken together, the volcanological characteristics of the area along with the range and juxtaposition of different submarine flow types will allow us to determine the range of possible effusion rates, and estimate volumes, durations and frequency of eruption along this superfast spreading mid-ocean ridge, as well as refine existing models for effusion rates and the internal dynamics of magma plumbing systems beneath mid-ocean ridges.

    The NAUDUR Program, EPR 17°-19°S [back]
    The East Pacific Rise (EPR) between 17° and 19°S has been mapped using Seabeam and SeaMARC II and HMR-1 side-scan sonar systems [Lonsdale, 1989; Bäcker et al., 1985; Cormier and Macdonald, 1994]. In 1993 we participated in the NAUDUR program involving 23 Nautile dives to the area [Auzende et al., 1994a,b,c; 1996]. This program was a reconnaissance program intended to provide first-order observations on the nature of tectonic, volcanic and hydrothermal activity at superfast spreading. Four individual segments were visited along with 2 dives on three near-axis seamounts and one dive to investigate an abandoned ridge near 18°45S. In the following we describe each of the four main segments along with observations relating to the status of each segment with respect to volcanic and hydrothermal activity. Though the sedimentation rate in this area is not well known, published estimates [Marchig et al., 1986; Lyle et al., 1987] suggest a value close to 2 cm/1000 yrs. Thus, lavas underlying 1 mm of sediment are estimated to be less than 50 years old. Two general characteristics with respect to the entire area is that hydrothermal activity is very widespread, and that there is compelling evidence for volcanic eruptions having occurred within the last few years to decades.

    17°10-17°27S. The axis in this region is an axial dome culminating at less than 2600 m. Bulbous and elongate pillows dominate off-axis but the near-axis and axial region consist of much more fluid, lobate, and both smooth and jumbled sheet flows. Between 17°10 and 17°12S, the axis is a zone less than 100 m-wide consisting of discontinuously collapsed lava lakes, 30-50 m deep. Locally lava was seen to have surrounded apparently extinct hydrothermal chimneys that were still colonized by vent animals; in other places a very fresh, unsedimented and uncolonized lava flow was seen to partly cover a field of vent animals (mainly bivalves) that had colonized an older lava flow. Between 17°25 and 17°27S the axis varies from completely untectonized lava fields to discontinuous, 10-30 m-wide fissures. This region was visited by Cyana divers in 1984 [Renard et al., 1985] (Fig. 2); at this time the axis consisted of an axial alignment of discontinuous lava lake collapses. Fossil hydrothermal deposits were found in the axial fissures but there were no high temperature (>250°C) smokers. A deep-towed vehicle lost during an earlier survey was seen by Cyana divers to be stuck into the walls of a fairly large collapsed area near 17°26S. In 1993 this collapsed area, along with the survey vehicle, apparently had disappeared; in places the axis was covered over with fresh lobate lava completely devoid of sediment, only very locally collapsed along narrow fissures. The floors of the collapsed areas typically were covered with crinkly sheet flows. The largest areas of collapse are no more than about 60 m across and mainly much less than 15 m deep. Glass compositions from lavas recovered in the youngest looking areas [Sinton et al., 1994; Auzende et al., 1996] have no counterparts in collections made by Sinton and Mahoney in 1987 [Sinton et al., 1991] or Cyana in 1984 [Renard et al., 1985]. There also are a number of biological indicators of very recent eruptive activity in this area, i.e. filamentous enteropneust (hemichordate) worms and bacteria [Auzende et al., 1994c]. An extensive field of very high temperature hydrothermal chimneys has developed near 17°25S. Taken together, the evidence suggests that significant changes have occurred in this area in the last decade; the chemical data suggest that two separate eruptions may have occurred within this time frame.

    18°10-18°22S. This segment (Fig. 3) is characterized by an axial graben bounded by walls culminating at 2650 m. The graben steps down along two main faults on each side; each set of walls had total relief of 30-40 m for a maximum graben depth of about 80 m. The axis

    Figure 2. Geologic sketch map of the EPR near 17°25S (left-from Auzende et al., 1994c) and lava flow map based on glass analyses from NAUDUR dives, Renard et al [1985] and Sinton et al. [1991] (right). Highly tentative lava ages are based on chemical comparison with previous collections.

    bottom is heavily fissured. Chaotic, jumbled sheet flows flanked the axial graben, thick pillow lavas lined the inner edge of the graben, the inner axis of which mainly consists of a collapsed, sedimented, lobate lava lake with pillars 10-15 m high. No recent lava was observed in this segment. Twenty hydrothermal sites were discovered along 20 km of the eastern wall, although only two black smokers (up to 310°C) were seen; the abundance of silicic chimneys is remarkable compared to other areas investigated [Auzende et al., 1994a]. Surprisingly, glass analyses indicate that samples from the collapsed lava lake have nearly identical compositions over a distance of at least 12 km, which can be extended to about 18 km when S&M dredges [Sinton et al., 1991] are considered (Figs. 3-5). These data suggest that the axis in this area had previously been the site of a huge lava lake, 12-18 km long. The lava lake outcrops occur over nearly the entire 800 m-wide graben, but the area appears to have been extended by subsequent seafloor spreading. Estimating the original width as 500 m and the thickness to be about 30 m, gives a total eruptive volume of 0.18 to 0.27 km3. There is a slight hump in MgO centered at about 18°16S, indicating slightly higher magma temperature in this area (Fig. 5). Perhaps this was the center of the feeder system to the eruption that produced this large lava lake.

    18°22S to 18°36S. This complex segment consists of an asymmetric, medial graben 200-500 m-wide; the eastern wall rises to less than 2600 m whereas the western wall rises to about 2700m. The central axial graben is about 100 m deep, consisting of pillowed horsts cut by deep fissures. Most of the axis shows considerable sediment cover. Seventeen new hydrothermal sites were found within a 2 km-long survey of the eastern wall of the graben. Only near 18°33S is the axial graben filled by a relatively unsedimented, lobate lava flow that is unaffected by recent tectonism (Fig. 6). In this region warm water shimmered directly off of the surface of the flow which was not yet colonized by vent animals other than crabs. Temperatures up to 160°C were measured in cracks in the flow.

    Figure 3. Map of the 18°10-18°20S segment. Dots on dive tracks denote sample localities. Heavy bars are dredges of Sinton et al. [1991]. Lava lake samples with uniform chemistry are enclosed by the line (see Figs. 4,5).

    Figure 4. Glass chemical variations in the segment shown in Fig. 3. Lava lake samples with nearly uniform composition are in the shaded field.

    Figure 5. Glass compositions in the segment shown in Fig. 3, plotted against latitude. When S&M dredges are included the lava lake appears to continue to at least 18°10N. Note the slight hump in MgO centered near 18°16S.

    18°37S. One dive near 18°37S traversed two segments separated by a small OSC that was 2 km wide (Fig. 6). The overlap basin was almost entirely composed of constructional lava showing flow directions down from the east. The traverse up to the southern segment axis encountered a series of jumbled and smooth sheet flows, locally with well-developed levees, with lesser lobate lava (Fig 7). The axis consists of a young sheet flow colonized by anemones, bivalves, crabs, gastropods and other vent animals, particularly concentrated along cracks in

    Figure 6. Map showing NAUDUR dives and S&M dredges. Very young lava occurs near 18°33 and 18°37S.

    pressure ridges in the expansive sheet flow. Uniform chemistry characterizes samples from the eastward traverse up this flank (Table 1), suggesting that the entire slope consists of a succession of thin flow lobes produced by a sustained eruption from the axial region.

     

    Table 1. Glass compositions from ND dive 12 samples shown in Figure 7.
    sample 12-5 12-6 12-7
    Lava
    Type jumbled sheet lobate smooth sheet
    SiO2 51.03 50.94 50.90
    TiO2 1.66 1.68 1.63
    Al2O3 14.49 14.48 14.47
    FeO* 10.83 10.99 10.70
    MnO 0.20 0.20 0.20
    MgO 7.15 7.14 7.30
    CaO 11.66 11.65 11.64
    Na2O 2.90 2.93 2.92
    K2O 0.08 0.08 0.09
    P2O5 0.13 0.14 0.15

    Figure 7. Profile along part of NAUDUR dive 12 showing the flow morphologies and the location of samples 5-7.

    Volcanological Summary
    Of the four segments studied, two are in a state of inflation with the axes dominated by discontinuous collapses of recently formed lava features. The other two segments have been rifted into graben. Recent volcanism dominates the two inflated segments but also was found in one place in the graben of one of the rifted segments. Geological and geochemical evidence supports the conclusion that there have been eruptions near 17°26S since 1984 and possibly since 1987 when the region was sampled by dredging. This is the place where Detrick et al. [1991] found an unusual spike in the multichannel seismic reflector to about 800 m below the seafloor. Mutter et al. [1995] and Sinton et al. [1994] argued that this spike was indicative of active eruption. Thus, the combined evidence supports the conclusion that eruptions have occurred in this area within the last few years. The age of the graben-filling flow near 18°33S is largely unknown but may be slightly older than those to the north. Relatively young sheet flows also characterize the southern segment axis near 18°37S but because this lava is heavily colonized by vent animals, it too probably is older than that in the northernmost area studied.

    The products of individual eruptive events were identified in several places. Although volume estimates are highly tentative, due to the reconnaissance nature of the surveys done so far, initial observations suggest that those near 17°26S and 18°33S may be less than 0.01 km3, whereas the lava lake between 18°10-18°20S might have had a consolidated volume >0.25 km3. Although we do not presently know the areal extent of the apparently co-eruptive sequence of lavas near 18°37S, the profile of Figure 7 suggests that it is about 30 m thick near the axis, thinning to only a few meters thick near its end in the overlap basin. Simple relations suggest that if this flow has an average width of 2 km, the eruptive volume would be ~0.05 km3.

    Lava flow morphologies include bulbous and elongate pillows, lobate lavas, and a variety of sheet flows. Some sheet flows are very smooth with longitudinal pressure ridges and/or levees, whereas others are a chaotic jumble of folded sheet fragments. Near 18°37S a range of flow types have uniform chemistry and probably are co-eruptive.

    PROPOSED WORK [back]
    We intend to attempt to determine the ages, volumes, source vents, and chemical heterogeneity of individual eruptive units within the study area, and investigate lava flow morphology in the context of topographic gradient and apparent discharge rates. These observations and collections will greatly further our understanding of eruptive processes at superfast spreading centers. In fact, this will constitute the first dedicated volcanological investigation at any mid-ocean ridge. The proposed study area is ideal because (1) previous work has fully characterized the regional petrological variations [Sinton et al., 1991; Mahoney et al., 1994; Niu et al., 1996; Auzende et al., 1996], (2) specific areas of volcanological interest have already been identified, (3) detailed work on the subvolcanic magma system is being studied seismically, (4) the indicated frequency of eruptions associated with superfast spreading suggests that we may be able to detect volcanologic changes (such as new eruptions) which have occurred since this area was last visited, and (5) the combination of apparently high eruption frequency and a high sedimentation rate provides a spatial and temporal resolution that has rarely been identified on the seafloor. We plan to survey two areas with the Woods Hole DSL 120 KHz side-scan deep-tow vehicle, and conduct dives in three areas as described below. In addition we are proposing a night program of rock dredging and wax coring in four areas, and a comprehensive shore-based analytical study.

    Despite the paramount importance of determining the volume of individual eruptive units to understanding volcanic processes, few previous seafloor studies have mapped complete flow boundaries or explicitly measured flow thicknesses. We recognize that exactly measuring the thickness of a lava flow is difficult, even on land, when the pre-flow topography is not precisely known. However, the thickness of submarine lava flows can be estimated using observations made from Alvin, where the flows have lapped or ponded against topographic highs, by measuring the depth of collapse pits within flows, or by using the heights of lava pillars. Furthermore, reconstructions from traverses (e.g. Figure 7) also can yield thickness information. Although these methods mostly yield minimum flow depths, it is important to recognize that existing volcanological models of effusion rates, eruption durations and magma supply rates require only order-of-magnitutde constraints on flow volumes; we will easily be able to obtain such estimates using Alvin and DSL 120 data. We identified several such boundaries during the NAUDUR program, and our experience is that chemical mapping is a powerful technique for discerning the limits of the products of single eruptive events. For example, the geologic boundaries shown on Figure 2 are the limits of lavas with uniform chemistry, and nearly uniform composition appears to characterize the large lava lake region between 18¡10-18¡22õS and the flow sequence shown in Figure 7. This approach works if the eruptive products are chemically uniform and different from those of previous eruptions. Although most subaerial and submarine basaltic eruptions are chemically uniform, the Cleft new pillow mounds appear to show internal chemical variation [Embley and Chadwick, 1994; Smith et al., 1994]. We will combine extensive sample collections and analysis with visual and DSL 120 observations to constrain the limits of coeruptive units and explicitly test for their internal chemical variation.

    DSL 120 surveys [back]
    The WHOI side-looking sonar vehicle [Bowen et al., 1993; Blondel et al., 1993] (Fig. 8) is towed ~300-500 m above the seafloor and will provide high resolution maps of the axial features we wish to study. We expect to be able to image features a few m in size and produce bathymetric maps at ~ 2m contours. Specific objectives of the side-scan surveys are described below. Results of these surveys will help guide the dive and night programs. In a separately funded program, R. Haymon and K. Macdonald will study the region between 17°18 and 17°42S using this vehicle along with ARGO photo-acoustic mapping [R. Haymon, pers. comm., 1995]. Hence, we are planning no additional surveys for this area in the present proposal.

    Figure 8. Examples of processed 120 Khz sonar output from the DSL 120. Left = backscatter; center = phase bathymetry; right = shaded bathymetry. Scale across bottom = 1 km. Figures are of the TAG region, MAR, courtesy of M. Kleinrock and S. Humphris. The TAG mound is the round feature at mid-right edge of records; relief of TAG mound is ~ 50 m.

    18°05-18°20S. We intend to map the entire segment between 18°05 and 18°20S using a single swath centered on the axis, in order to better constrain the limits of the large lava lake identified in the NAUDUR program. The maps generated by this system will provide continuous coverage of the axis and help us constrain the extent of fissuring and other tectonism, between dive tracks. These observations will be coupled with those from dives to reconstruct the original volume of the lake by correcting for the amount of extension and/or collapse that has occurred since the original lake completely filled the axis.

    18°33-18°40S. We would like to map the northern end of the southern Hump segment in order to determine the limits of young volcanism in this area, as well as try to map out individual eruptive units. NAUDUR dives indicate that at least two, relatively recent eruptions have occurred here, including one sequence showing a range of lava morphologies over its ~ 1.8 km length. We propose to do two swaths of this region, spaced ~ 750 m apart, which will provide a high resolution map ~ 1.5 km wide x 13 km long. The DSL 120 survey data will be used to provide the regional scale geologic context of the new dive observations (see below).

    Alvin Dives [back]
    We are proposing 21 Alvin dives that will investigate volcanological and petrological problems throughout the region. We intend to try to map out and sample individual eruptive units which will require on-bottom observations and real-time navigation by geologists in the submersible. We do not expect to be able to fully define the limits of many of these units with the DSL 120 data, especially where several recent eruptions have occurred, such as near 17°26S. Therefore we need to make on-bottom observations and sample accordingly. Furthermore, we will need to carefully document the samples that are collected, i.e. sheet flows in the bottoms of narrow collapse structures, with a resolution unobtainable with remote systems. Several of the measurements that we hope to make concerning flow morphologies, such as fold wavelength and amplitude on jumbled flows, lobe size and aspect ratio, and generally how flow morphology varies along its length, can only be made from the submersible.

    We have assembled a dive team of exceptional experience in order to make the required observations and collections. Although we have no specific hydrothermal or biological objectives in this proposal, the dive team includes appropriate experts in hydrothermal processes and vent biology and we will make collections as appropriate. Vent fluids will be collected using titanium bottles and will be studied by J-L. Charlou in France at no cost to this project. Sulfides will be studied by Y. Fouquet in France, also at no cost to this project. The dive team is as follows:
    John Sinton, U. Hawaii - geology/petrology
    Jean-Marie Auzende, IFREMER - geology
    Rodey Batiza, U. Hawaii - geology/petrology
    Yves Fouquet, IFREMER - geology/sulfide mineralogy
    Tracy Gregg, WHOI - volcanology
    Rachel Haymon, UCSB - geology/hydrothermal processes
    Yves Lagabrielle, U. Brest - structural geology
    Ken Rubin, U. Hawaii - geochronology
    Cindy Van Dover, U. Alaska - biology
    Specific dives and their objectives are outlined for each area below:

    17°24-17°30S. We propose to conduct 8 dives in this area. Preliminary results of the NAUDUR dives indicate that approximately six individual eruptive units, as indicated by glass chemical compositions, are present between 17°22 and 17°27S (Fig. 2). We intend to use about 3-4 dives to extend the coverage to the south of the dome near 17°30S, which was not covered by NAUDUR dives. An additional 4-5 dives will be used to check for possible changes since the NAUDUR program in December, 1993, and to study specific eruptive sequences identified from DSL 120 and ARGO data (Haymon and Macdonald program) and other dives. These dives will collect samples and make measurements of flow characteristics in order to determine flow volumes, lava effusion rates, and intraflow chemical variability.

    18°05S-18°20S. Six dives in this area will investigate the large lava lake discovered during the NAUDUR program. These dives will be used to make measurements of apparent lava lake thickness, throws on faults, determine the lateral limits of the lake and extend coverage to the north of 18°13S where we suspect the lava lake continues based on the composition of samples collected by dredging in 1987. We note that an active lava lake with these dimensions (12-18 km-long, perhaps several hundred meters wide) has never before been observed, either above or below sea level. It is critical that we investigate this feature further. The amount of heat that such an eruption might have imparted to the water column has important hydrothermal and oceanographic implications.

    18°35S-18°39S. We know from the NAUDUR program that this highly inflated region has experienced relatively recent eruptions but only 1/2 dive covered the area of interest. That dive found a sequence of lavas varying from lobate to a variety of sheet morphologies that all appear to have been part of a single eruptive event. We would like to use this area as a primary study area for our investigation of the effects of slope angle and effusion rate on flow morphologies. Furthermore we wish to further explore the area, both to the north and south of the single NAUDUR dive in order to better assess the extent of recent volcanism, determine the number of recent flows, their volumes, morphologies and internal chemical variation. A total of seven dives are planned for this area..

    Night Program [back]
    During the nights when we are not doing DSL 120 surveys or navigating transponders we intend to carry out a program involving rock dredging and wax coring. Specifically we plan to devote about 7 nights to sampling the axial region between 17°25S and 18°00S. Five to six nights will employ wax coring along the axis at intervals of about 0.5-1 km spacing, and another 1-2 nights for dredging in the region. The axis is poorly known south of 17°27S; we have only two dredges from our 1987 cruise here. Another 5-6 nights will be devoted to wax coring in the region of the large lava lake between 18°05 and 18°20S. In this region we plan to do profiles transverse to the axis, with wax coring stations spaced about 200-300 m apart. We expect to complete 5-6 profiles, one each night. These profiles will be used to tightly constrain the lateral limits of the lava lake chemical composition.

    Although the segment between 18°22 and 18°36S was extensively sampled by Preussag AG [Sinton et al., 1991], there is considerable uncertainty with respect to the precise location of many of these samples. For example it is quite unclear whether samples come from the axial graben walls or from horsts inside the axial graben. Although we are not proposing additional dives here in the present program, we propose to spend 3 nights coring the axis between 18°31S and 18°35S at a sample spacing of about 500 m. These samples will be used to study the fine detail of along-axis chemical variations in this segment, as well as contribute to the definition of the limits of young volcanism occurring near 18°33S. An additional 3-4 nights will be devoted to sampling the segment south of 18°37S not visited by Alvin.

    Cruise Plan [back]
    We propose a 35 day cruise, assuming Easter Island as one of the ports and Valparaiso, Chile as the other. The area lies about 7 days steaming from Tahiti, Valparaiso, Panama and Acapulco; the use of any of these ports will not affect the total cruise length significantly. We intend to use the same transponder deployments for the DSL 120 surveys and Alvin dives. The following cruise plan allows for the DSL 120 to be towed at 1.2 kts with 4 hrs per turn, and 4 hrs per transponder deployment and navigation. It also allows for trip time for the DSL vehicle.

    Transit from Easter Island 1.5 days
    Deploy 3 transponders:18°33-18°40S 0.5 days
    DSL 120 surveys, 18°33-18°40S 1.3 days
    Seven Dives, and night programs:18°33-18°40S 7.0 days
    Deploy 6 transponders, 18°05-18°20S 1.0 days
    DSL 120 survey - 18°05-18°20S 1.2 days
    Six Alvin dives and night programs, 18°05-18°20S 6.0 days
    Deploy 3 transponders, 17°25-17°30S 0.5 days
    Seven Alvin dives + night programs, 17°25-17°30S 7.0 days
    Contingency 0.7 days
    Transit to Valparaiso or Tahiti 7.0 days
    Total Cruise 35 days

    Shore-based Program [back]
    Geology and Volcanology

    We will work with the DSL-120 data, Alvin mesotech data, and video and still camera images (including video from the NAUDUR program) to compile information on flow types and general geology for all areas studied. Incorporating the observations with chemical data (see below) will allow us to make flow unit maps for these areas. In addition we will study individual flows, incorporating submersible observations with video images to measure fold wavelengths and amplitudes of folded flows, of individual chunks of crust blocks in jumbled flows, and of channels where appropriate. These will be combined with morphologic transitions, i.e. changes in regional slope, constriction in channels, etc., in order to identify the range of morphologies within individual flows. One primary goal of these endeavors is to make morphologic maps for individual areas and then to transcribe the observations into effusion rate maps. If we are successful in mapping out individual eruptive units and determining their eruptive volumes, then the above data will allow us to assess eruption durations and frequency. Compilation of fissure counts, width statistics, and fault throws will be used for reconstructing the original lava lake dimensions for the area between 18°05-18°20S.

    Petrography and Geochemistry
    Chemical analyses will be essential for characterizing all samples collected from Alvin and from the night program. For those areas visited in 1993, we will compare the new data with earlier analyses as a method for determining which, if any, lavas were erupted in the intervening period. Chemical data will be used for assessing eruption temperature and for calculation of magma viscosities, e.g. by the methods of Weaver and Langmuir [1990] and/or Ghiorso et al. [1994], for use in the volcanology program outlined above. Although major elements alone will provide important data for many of these objectives, we also need to do trace element analyses if we are to understand the way the subaxial magma chamber is tapped. For example, although individual flows or groups of related flows might be homogeneous with respect to major elements we need to check to see whether or not they are homogeneous for trace elements. If we find heterogeneity we certainly will need to acquire trace element data to help determine the underlying causes of the heterogeneity, e.g. crystal fractionation or some more complicated processes, with implications for the nature of compositional zoning in subvolcanic magma chambers, and the way these chambers are tapped during eruptions. Thin sections will be vital for assessing the role of crystal distributions in controlling chemical variations.

    We will analyze natural glasses by electron probe for major and minor elements, and whole rock samples by XRF for major, minor and trace elements. A subset of these samples will be analyzed by ICP-MS for trace elements (REE, Ba) beyond the capabilities of the XRF. The glass analyses are particularly useful for a first-order chemical characterization and a perspective of the continuity of single chemical types in space. The whole rock XRF and ICP-MS analyses will give us more precise chemical data and also allow us to assess the extent of crystal redistribution during flow, and other consideations as outlined above.

    Geochronology [back]
    Despite the great need by the RIDGE community for absolute eruption ages of submarine Holocene lava flows, determinations of eruption chronologies are rare in the geological and oceanographic literature. This is due, in general, to compositional and logistical restrictions imposed by the few available geochronological tracers for submarine basalts. In recent years, chronometers using 230Th-238U, 231Pa-235U and 226Ra-230Th radioactive disequilibrium have been used to determine semi-quantitative, relative ages in the ranges 20-350 ka, 10-175 ka and 0.5-8 ka, respectively [e.g. Rubin and Macdougall, 1990; Goldstein et al., 1991, 1993, Volpe and Goldstein, 1993]. Although useful, age relationships among volcanic features on the seafloor that can be determined with the above three chronometers do not provide the fine-scale resolution required to examine volcanological phenomena occurring during individual eruptive episodes. Rather, they provide clues as to the general distribution of lava ages within the oceanic crust over fairly long time scales. Recently, the need to determine ages of very young mid-ocean ridge lavas spurred the development of a new tracer using radioactive disequilibrium between 210Po and 210Pb [Rubin et al., 1994]. With this chronometer we were able to determine the absolute ages of lavas erupted at 9°50'N on the EPR to within 1-2 months resolution over a period of 2 to 3 years. The unprecedented temporal resolution provided by these data provided the first quantitative evidence regarding the lifetime of a mid-ocean ridge eruption episode and the time scale over which eruptive vents can migrate during an episode.

    To get the most complete picture of the development and life history of an active mid-ocean ridge volcano, it is necessary to date lavas over the entire interval between today and 10,000 years ago. At present, the published actinide-series techniques available allow us to bracket this age range, but do not provide much information on processes occurring over the decadal or century time scale. To also address these time scales, we propose to use additional tracers in the actinide decay chains that, until now, have not been used to constrain submarine lava ages. A particularly useful new tracer will likely involve 210Pb-226Ra disequilibrium (see Table below), for which disequilibrium exists in a variety of young basalts [Rubin and Macdougall, 1989; Rubin et al., 1989]; preliminary evidence indicates this is also the case in young MORB from 9°50N [Rubin and Macdougall, unpublished data]. In the actinide series decay chains, the mere presence or absence of radioactive disequilibria between various nuclides provides evidence of the relative youth of rocks and gives a qualitative upper or lower limit on their age. Two nuclides are said to be in a state of secular equilibrium when the activity of a parent and daughter are equal; chemical perturbations during petrogenesis and/or eruption introduce radioactive disequilibrium into the system, after which any initial inequality of the parent and daughter activities will be erased with time, i.e., ~99% of the original signature will have decayed away after 6 half lives of the daughter nuclide. To obtain quantitative radiometric ages from the radioactive disequilibrium requires that the initial and final abundances of the important nuclides be known accurately.

    We propose to use the nuclide pairs outlined in the table below to determine the relative ages of lava flows in the study region over the entire Holocene. As implied above, the mere presence or absence of radioactive disequilibrium between various nuclide pairs will give us much-needed semi-quantitative constraint on the ages of collected lavas. We will use external and/or internal isochrons to determine ages using 226Ra-230Th and 230Th-238U as appropriate. In addition, we plan, wherever possible, to apply the same types of approaches used in the application of the more-common, longer-lived, U-series tracers to constrain more recent ages using some combination of tracers 1-4 in the table below (of which only #1 is a published technique.


    Nuclide Pair Time Scale Measurement Technique
    210Po-210Pb 1 month - 3 yrs -counting, mass spectrometry
    228Th-228Ra 2-10 yrs -counting, mass spectrometry
    228Ra-232Th 3-30 yrs mass spectrometry
    210Pb-226Ra 10-120 yrs mass spectrometry
    226Ra-230Th 500-8000 yrs mass spectrometry
    230Th-238U >10,000 yrs mass spectrometry

    Our approach will be to analyze in each of the lavas the various isotopes of Ra, Pb, Po, Th and U described above. Once the approximate age bracket for each sample is defined, we will use normalization for estimating the initial ratio to extract the most quantitative age possible from that nuclide pair in the table above. We emphasize that, whether or not we obtain fully quantitative age data for all samples at all age ranges, we are sure to collect semi-quantitative data that will constrain age relationships over spatial and temporal scales far more comprehensively than has ever before been attained on the seafloor.

    Project Responsibilities [back]
    Sinton will oversee the project and serve as Chief Scientist on the cruise. A PhD student at the University of Hawaii will be involved in the geochemical and geochronological data acquisition, and work with Sinton, Batiza and Rubin on the interpretation of results. Batiza will be responsible for the geochemical acquisition by ICP-MS. Rubin will be responsible for the geochronology program; Gregg will be responsible for the lava flow morphology study and for quantifying lava effusion rates.

    Relevancy to RIDGE and International Programs [back]
    As this program will investigate variations in magma chamber and eruptive processes over several segments varying in apparent extent of recent inflation and/or tectonism, this program is highly relevant to the Crustal Accretionary and Segment Scale Processes (CASP) Program Element of RIDGE. The RIDGE-sponsored RISES Workshop identified the study of active processes in this area to be of prime importance, and specific studies of this kind were strongly endorsed at the recent RIDGE Workshop on Processes and Fluxes on a Superfast Spreading Ridge: the southern East Pacific Rise. Because we will be assessing the temporal variability in eruptive processes and lava compositions within the area, this project also is relevant to the Temporal Variability Program Element of RIDGE. The region near 17°S is the site of the Mantle Electromagnetic and Tomographic (MELT) experiment and hence our proposed work also is relevant to the Mantle Flow and Melt Generation program element of RIDGE. Although not formally part of the French-American FARA project in the Atlantic, several French scientists will participate in the proposed work at no cost to this project.

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    ABE proposal: [back]

    Title: Collaborative research: a piggy-back near-bottom geophysical survey along the ultrafast EPR at 17°24'-18°39'S using the ABE vehicle

    P.I.s: Marie-Helene Cormier, William B. F. Ryan, Albert M. Bradley, Barrie B. Walden, Dana R. Yoerger

    Project Summary [back]

    The characterization of discrete eruptive events along the superfast spreading East Pacific Rise at 17°24'-18°39'S will be the focus of a funded survey led by John Sinton, Rodey Batiza, and Ken Rubin, which will include Alvin submersible dives, towed DSL-120 side-looking sonar, wax-coring, and dredging. Based on the identification of individual flow units during previous submersible and sampling cruises, three areas have been targeted for detailed geological mapping. By adding acquisition of near-bottom high-resolution bathymetry, magnetic data, and stereo snapshot video to their night programs, the dimensions, along-axis continuity, and morphology of individual flows could be determined with a greatly improved accuracy.

    There are no substitute for direct observations of the seafloor from a submersible, and the capability for experienced observers to guide exploration in real-time. However, remotely operated or autonomous underwater vehicles can gather near-bottom images between Alvin tracks, as well as systematic high-resolution geophysical profiling over an entire study area. The Autonomous Benthic Explorer (ABE) vehicle recently developed at WHOI is uniquely suited to supplement Alvin surveys, and we propose to use it during most night programs of the upcoming cruise. During one deployment, ABE can cover more than three time the distance logged during a typical Alvin dive. It follows pre-programmed track lines on its own with great accuracy, and is equipped with a variety of sensors, including a 3-component magnetometer, an Imagenix pencil scanning sonar, a digital stereo camera system, a CTD sensor, and an optical backscatter sensor.

    Near-bottom magnetic data acquired by ABE during 21 night programs will be analyzed to reveal and outline recent, highly magnetized lava flows, and potentially, the associated non-magnetized feeder dikes. For additional constraints, the magnetization properties of the basalt samples collected with Alvin and dredging will be measured. Three of the ABE deployments will be dedicated to acquire near-bottom magnetic transects of the axial high at 17°27'S for successive elevations above the seafloor. These profiles will be used to tightly constrain the magnetization distribution within the upper crust at the EPR. Stereo images will bridge gaps between Alvin tracks, and allow to delineate flow morphologies and boundaries with improved continuity. Closely spaced bathymetry profiles with a resolution of ~10 cm will provide needed measurements on the fine-scale morphology of the neovolcanic zone, including fissures and lava lakes dimensions, and seafloor slopes.

    This program will represent the first intensive use of ABE, which has already proven its effectiveness in several pilot projects over the past two years. While it will greatly facilitate the stated objectives of the funded project, such as the quantification of eruptive volumes and flow rates, it will not require additional ship time and impact minimally the operation of the cruise.

    Introduction [back]
    Despite the importance of determining the volume of individual eruptive units to understanding volcanic processes, few previous seafloor studies have mapped complete flow boundaries or explicitly measured flow thicknesses. The major difficulty in mapping flow units on mid-ocean ridges arises from the spatial limitation of near-bottom observations. However, the thickness of submarine lava flows can be estimated where they have lapped or pounded against topographic highs, by measuring the depth of collapse pits within flows, or by using the heights of lava pillars [Gregg et al., 1996]. The thickness of recent flows can also be estimated with geophysical techniques such as near-bottom magnetic surveys [Tivey et al., 1997, and submitted]. The lateral extent of individual flows has been estimated from a combination of direct observations, chemical typing of collected samples, analysis of side-scan sonar images, and repeat swath bathymetry surveys.

    In 1998, John Sinton, Rodey Batiza, and Ken Rubin will investigate eruptive processes at the superfast spreading East Pacific Rise between 17° and 19°S (Figure 1). Their funded program is dedicated to volcanological investigation and combines the use of Alvin dives during daytime, and of DSL-120 sonar system, wax-coring or dredging at night. Eruptive ages and volumes and lava flow morphologies will be constrained along nearby segments with varying axial morphologies (Figures 2 and 3). The nature of eruptive vents, whether elongate fissures or point-source vents, will also be investigated, as well as the uniformity of chemical composition within individual eruptive units. Lava flow morphologies will be examined in relation to slope angle and effusion rates.

    We propose to use the ABE (Autonomous Benthic Explorer) vehicle as an add-on program to this cruise, and to supplement the volcanological investigations with a systematic near-bottom camera and geophysical survey. Because ABE navigates pre-programmed missions on its own, it will be deployed during the night programs while wax-coring or dredging is carried out nearby. In that way, the proposed program will not require additional ship time and will minimally affect the operation of the funded cruise. High resolution topography, magnetic field, snapshot stereo video, and water column properties (conductivity, temperature, and optical backscatter) will be routinely acquired by ABE along closely spaced profiles navigated several meters only from the seafloor. These data will be acquired on a tighter, more systematic grid that can be achieved with Alvin dives, and will provide invaluable information for fully characterizing individual eruptive units.

    Scientific objectives [back]
    The Alvin diving program will focus on three areas of the southern EPR located at 17°25'-30'S, 18°05'-20'S, and 18°33'-40'S, and belonging to three distinct ridge segments (Figures 1, 2 and 3). These areas have been selected for the following reasons: (1) previous work has fully characterized the regional petrological variations [Sinton et al., 1991; Mahoney et al., 1994; Niu et al., 1996; Auzende et al., 1996], (2) Individual flow units have been already identified in several places during a survey with the Nautile submersible [Auzende et al., 1996], (3) the subvolcanic magma system is being studied seismically [Detrick et al., 1993; Mutter et al., 1995; Carbotte et al., 1997; Hooft et al., in revision; Forsyth et al., 1996], (4) Superfast spreading rates (~144 mm/yr) suggests that volcanological changes -such as new eruptions- may have occurred since these areas were last visited. The proposed ABE program will cover the same three areas, aiming at supplementing the volcanological investigation with systematic, near-bottom camera and geophysical surveying.

    The funded program will combine extensive sample collections and analysis with visual and DSL 120 observations to constrain the ages, volumes, source vents, and chemical heterogeneity of individual eruptive units within the study area, and investigate lava flow morphology in the context of topographic gradient and apparent discharge rates. Ultimately, the scientific objectives are to determine the range of possible effusion rates, and estimate volumes, durations and frequency of eruption along this superfast spreading mid-ocean ridge, as well as refine existing models for effusion rates and the internal dynamics of magma plumbing systems beneath mid-ocean ridges.

    The use of the ABE vehicle during 21 night programs to acquire data along a dense, systematic pattern of tracks will further help achieving the above objectives:

    The acquisition of topography profiles with the down-looking sonar and the Imagenix scanning sonar will determine the seafloor slopes and the dimensions of the collapsed lava lakes, lava pillars, and fissures with an accuracy 10 times superior to that of the DSL-120 system (~10 cm versus 1-2 m).

    The outline of recent flows will be extrapolated between Alvin tracks based on their associated anomalously high magnetization and on digital stereo images. The thickness of these flows will also be estimated from forward modeling of the magnetic filed [Tivey et al., 1997; Tivey et al., submitted]. To further constrain the results, the magnetization properties of the rock samples collected by Alvin and by dredging will be measured in laboratory.
    Potentially, recent feeder dikes, which may go undetected to direct observations because of an extensive lava flow cover, may be revealed by their anomalous magnetic signature.
    In addition, the magnetization structure of the axial high will be directly constrained from a series of magnetic profiles acquired at different elevations above the same location. For added control on the crustal structure, these precisely navigated profiles will be acquired over the site of a 1991 seismic line [Mutter et al., 1995].

    The study area [back]
    Because the study area belongs to a long, "uncomplicated" section of the ultrafast spreading EPR (Figure 1), located far from any transform fault, large overlapping spreading centers, hot spot influence, or the magnetic equator, it has been extensively surveyed in recent years. Surveys have aimed at characterizing the along-axis variations of numerous parameters at the regional scale, including morphology [Macdonald et al., 1988a and 1988b; Lonsdale, 1989; Scheirer and Macdonald, 1993; Scheirer et al., 1996a], petrology and geochemistry [Sinton et al., 1991; Bach et al., 1994; Mahoney et al., 1994], crustal and mantle structures [Detrick et al., 1993; Mutter et al., 1995; Carbotte et al., 1997; Hussenoeder et al., 1996; Hooft et al., in review; Forsyth et al., 1996], density structures [Magde et al., 1995; Cormier et al., 1995], and hydrothermal activity [Urabe et al., 1995; Auzende et al., 1996; Baker et al., 1996]. In addition, several other surveys have focused on near-bottom observations and rock sampling [Renard et al., 1985; Morton and Ballard, 1986; Macdonald et al., 1988b; Marchig et al., 1988; Sinton et al., 1991; Fujioka et al., 1995; Auzende et al., 1996; Haymon and Macdonald, SOJNMV-2 1996 cruise report]

    The upcoming Alvin program and the proposed ABE deployments will focus on three 8-25 km-long areas, located at 17°25'-30'S, 18°05'-20'S, and 18°33'-40'S (Figure 3). All three targeted segments have a broad, inflated cross-sections (Figure 2), characteristic of a robust magmatic budget [Scheirer and Macdonald, 1993]. Accordingly, a clear seismic reflector is detected along these segments which indicates the presence of an axial magma chamber only 1-1.5 km below seafloor [Detrick et al., 1993]. However, the segments differ in their eruptive styles and in the detailed morphology of their crestal regions.

    At 17°25'-30'S and 18°33'-40'S, the ridge crest has a smooth profile in cross-section (Figure 2). Near-bottom surveys and direct observations reveal discontinuous collapses of recently formed lava features, 5-12 m deep and at most a few 100 m wide. In contrast, at 18°05'-20'S the ridge axis is notched by a large summit trough, 1.0-1.5 km wide and 50-100 m deep. These dimensions are an order of magnitude greater than those of the collapse lava lakes present along the other two targeted areas. The floor of that trough is quite sedimented and heavily fissured. A nested inner trough, 500-700 m wide, consists mainly of collapsed, sedimented, lobate lava lakes, with lava pillars 10-15 m high. That inner trough lays at a relatively constant depth along its entire length: 2715-2730 m according to multibeam surveys, and ~2700 m according to the depth sensor mounted on the Nautile submersible. The uniformity of basalt samples collected between 18°10'-22'S suggest that they all belong to a single lava lake, at least 18 km long [Auzende et al., 1996]. This and several other clues suggest that this large summit trough may result from foundering of the uppermost crust over a large but waning magma reservoir, rather than from extensional tectonics [Lagabrielle et al., 1996].

    Geological and geochemical evidence supports the conclusion that there have been eruptions near 17°26'S since 1984 and possibly since 1987 when the region was sampled by dredging [Auzende et al., 1996]. Relatively young sheet flows also characterize the southern segment axis near 18°37'S, but because this lava is heavily colonized by vent animals, it probably is older than that at 17°26'S.

    The ABE vehicle [back]
    The Autonomous Benthic Explorer (ABE) is being developed at WHOI (Figure 4). It determines its position using four long baseline acoustic transponders mounted in-hull with a backup at the surface., and uses this knowledge to follow preprogrammed track lines. It can either fly at pre-selected depths, or it can follow bottom keeping a pre-selected interval. ABE is extremely stable, and can hold its heading better than 1° and its depth better than 0.15 m. Its normal transit speed is 0.7 m/s (~1.4 knots). During one lowering, ABE can survey a pre-programmed course approximately 10 km long in about 4.5 hours. Data collected by ABE are stored on board and retrieved after the dive. Standard ABE sensors include a 3-component magnetometer (Develco 9200C-01), a CTD, an optical backscatter sensor (Seapoint Turbidity Meter), and a digital monochrome stereo camera system. The digital stereo camera can be turned on and off depending on proximity to bottom. At present, it can acquire images at intervals of 10 sec or more, and could provide up to 2500 image pairs on a single dive.

    A 675 kHz Imagenix pencil scanning sonar has been installed recently for acquiring cross-track bathymetry with a depth resolution of ~0.1 m. Although Imagenix is not yet operational on ABE, it is now being tested and is expected to be fully operational by next year. Imagenix scans through 120° perpendicular to the track every 24 seconds. Assuming a cruising altitude of 10 m, the width of seafloor scanned along track will average 40 m. At the normal ABE cruising speed of 0.7 m/s, successive sweeps are separated by an average of 17 m. The Imagenix system is very similar to, but an improved version of the Mesotech scanner which has been used on Alvin for several years. Figure 5a illustrates the seafloor coverage pattern of the Mesotech survey of the Juan de Fuca Ridge (generously provided by W.W. Chadwick and R.W. Embley). Figure 5b illustrates the fine resolution provided with Mesotech along ~60 m of a graben on the Coaxial FLOW site (Juan de Fuca Ridge). Systematic changes in the graben dimension (10-20 m wide, 5-15 m deep) and in the roughness of the seafloor are evident. Similarly, Imagenix data have a vertical resolution of ~10 cm, and can accurately characterize features a few meters across. In comparison, the towed DSL-120 sonar system (towed 300-500 m above seafloor) has a vertical resolution of ~1 m.

    Unfortunately, the addition of a gravimeter to ABE's array of sensors is not feasible with the current technology, as a gravimeter and its stabilization platform would be too bulky to fit in the pressure housing.Within a few hours from recovery of ABE, the data collected are provided to the science party on PC floppy disks in either ascii or Matlab format. Digital image data are too large for floppy disks, and are usually transferred over the shipboard network. Over the past two years, ABE has successfully operated 21 missions, and carried out several near-bottom 3-D magnetic surveys of the Juan de Fuca Ridge. Results are reported by Yoerger et al. [1996], Tivey et al. [1997], and are also posted on the Internet:
    http://www.marine.whoi.edu/ships/auvs/abe_description.htm

    The attractive and unique capability of ABE is that it operates on its own while other experiments are being carried out. It relies on the same transponder system as Alvin and DSL-120, making its use on the same cruise as these vehicles cost- and time-effective. This also facilitate the spatial co-registration of the different data set acquired during a combined survey. Because the capability does not exist yet to operate ABE simultaneously with Alvin and DSL-120, its deployments will be restricted to night programs when doing wax-coring or dredging. Altogether, with careful planning, its use during the funded survey will not require supplementary ship time and will not affect other operations.

    Proposed work: [back]

    Rationale
    Recent studies suggest that lava ages and cooling feeder dikes may be distinguished by their anomalous magnetization. Huge magnetic anomalies (> 10,000 nT) are associated with fresh lava flows along the Juan de Fuca Ridge [Tivey and Johnson, 1995; Johnson and Tivey, 1995; Tivey et al., 1997] (Figure 6). These authors propose a rapid (on the order of years) loss of magnetization for the youngest lavas. Similarly, magnetic data collected along the EPR at 13°N a few 100 m above the ridge crest (W.B.F. Ryan, unpublished data) reveal that high anomalies are systematically associated with the fresh lava flows identified from the high-resolution SeaMARC I side-scan sonar data [Ryan and Barone, 1986; Crane, 1987]. Tivey and Johnson [1995] also observe a notch-like negative anomaly across the center of a fresh lava flow along the Juan de Fuca Ridge, which they interpret as a non-magnetic dike, possibly indicating temperatures above the Curie isotherm (Figure 7).

    For submarine lava flows, the eruption temperature, effusion rate and volume, and preexisting topography are important controlling factors on their surficial expression [Gregg and Fink, 1994; Gregg et al., 1996; Gregg and Chadwick, 1996]. Accurate measurements of surficial slopes and a dense photographic coverage of the study area will aid in the quantitative assessment of these factors along the superfast EPR.

    Fine scale bathymetry will also allow the study of lava lake levels, and help constrain eruptive processes. For instance, if neighboring lava lakes have different elevations, then they either formed during distinct eruptive events, or else it is likely that the lava flowed downslope outward from the source region, from lava lake to lava lake. On the other hand, if adjacent lava lakes have the same level, then their magma source is probably connected at depth.

    Axial fissures could either represent eruptive fissures, form above subsurface dikes, form in response to tectonic extension, reflect a tumescence of the axial region, or be activated by collapse of the brittle carapace over a waning magma chamber. Existing fissures can also widen by mass wasting of their margins, or be reactivated by hydrothermal circulation. An accurate measurement of fissure dimensions is critical to understanding which tectonic, magmatic, or hydrothermal processes control their geometry.

    Finally, ABE will be used for investigating the magnetization structure of the axial high. The analysis of Deep-Tow magnetic profiles just south of the study area (19.5°S) indicates a very rapid magnetization reduction within only a few km from the ridge axis [Perram et al., 1990; Gee and Kent, 1994]. Superimposed onto this broader (2-3 km) central anomaly magnetization high, a 1-km wide magnetization low is also detected (significantly wider than the <100 m notch in the magnetic anomaly expected to be associated with recent feeder dikes). Perram et al. [1990] suggest that this broad low indicates an elevated Curie isotherm, only 200-300 m below the ridge crest. Alternatively, Gee and Kent [1994] argue that this magnetization pattern results from the competing effects of the rapid thickening of the extrusive layer off-axis, and of its rapid loss of magnetization. ABE offers the unique opportunity to exactly repeat the track of a magnetic profile at a variety of elevations above the seafloor. Such repeat profiles, oriented transverse to the axial high, will be used to tightly constrain the thickness and magnetization intensity of the source layer.

    At-sea operations [back]
    The program described below will be implemented as an add-on to the cruise of John Sinton, Rodey Batiza, and Ken Rubin, which is tentatively scheduled for the first half of 1998 on the R/V Atlantis. Their cruise plan is as follows:

    Transit from Valparaiso: 7.0 days
    Deploy 3 transponders, 18°33'-40'S: 0.5 days
    DSL 120 surveys, 18°33'-40'S: 1.3 days
    7 dives and night programs,18°33'-40'S: 7.0 days
    Deploy 6 transponders, 18°05'-20'S: 1.0 days
    DSL 120 survey, 18°05'-20'S: 1.2 days
    6 dives and night programs,18°05'-20'S: 6.0 days
    Deploy 3 transponders, 17°25'-30'S: 0.5 days
    8 dives and night programs,17°25'-30'S: 8.0 days

    Contingency: 1.0 days
    Transit to Easter Island: 1.5 days

    Total cruise:
    35 days

    The ABE program will be folded into this plan and only minimally affect its operation. ABE will be deployed during night programs not making use of DSL-120, when wax-coring or dredging.

    With Valparaiso as the departure port, ABE will be deployed about 8.5 days from leaving port, providing ample lead time for programming its first mission. ABE will then be deployed for 7 consecutive nights near 18°33'-40'S. For the next 2.5 days, no deployments would be possible while transponders are being redeployed and DSL-120 data are being acquired. ABE will then be used for another 6 consecutive nights at 18°05'-20'S, followed by about 1 day to redeploy the transponders at 17°25'-30'S, and 8 additional night deployments of ABE. Partly because of the intensive use of ABE, and partly to obtain a systematic acquisition pattern for near-bottom geophysical data, similar surveys will be programmed for most deployments.

    Track parameters will be adjusted based on the high resolution bathymetry and side-scan data acquired earlier with the DSL-120 system. DSL-120 data for the 17°25'-30'S area have been acquired last November during cruise SOJNMV-2 [Haymon and Macdonald, P.I.s], and maps will be made available for the upcoming cruise.

    ABE will be navigated along a zig-zag pattern whose overall trend follows that of the ridge axis (about 013°E). This zig-zag pattern will extend over a zone 500-700 m wide, which corresponds to the dimension of the inner trough at 18°05-20'S, but is much larger than the 8-50 meter-wide axial summit caldera present at 17°25'-30'S and 18°37'S (Auzende et al., 1996; Haymon and Macdonald, personal communication). The angle between adjacent tracks and the number of tracks achieved during each deployments will depend on the length of ridge that must be covered. To achieve a systematic, regular track pattern over each study area, and assuming ABE will cover 10 km of track each time, the angle between successive tracks should be ~22° at 17°25'-30'S (5 zig-zag deployments), ~12° at 18°35'-39'S (7 deployments), and ~55° at 18°05'-20'S (6 deployments only). Depending on the width of axial zone that will be surveyed, about 12 to 20 oblique profiles could be navigated during each deployment. Their spacing will average 260-370 m along the axial trough at 18°05'-20'S, but only 65-140 m in the other two study areas.

    Tracks oriented nearly normal to the strike of the ridge are ideal for detecting ridge-parallel magnetic anomalies, as may be associated with dike-fed lava flows. It is less optimally oriented for the Imagenix instrument, which scans normal to tracks, and thus nearly parallel to fissures. For all the zig-zag deployments, a constant survey altitude will be adopted. This approach (rather than tracking bottom) will minimize the battery consumption of ABE, and hence optimize the track coverage. Because the topography within each study area is not expected to vary by more than 30 m, this approach will still provide high quality Imagenix and magnetic data. The survey altitude will be selected to be only a few to several meters above the bottom at the shallowest point of its mission. Photographs will be taken at regular interval whenever ABE is within 10 meters from bottom.

    Three deployments will be devoted to acquiring long profiles at different elevations near 17°27'S, right over the site of seismic profile 1113. A series of seismic profiles were run perpendicular to the strike of the EPR in the study area and tightly constrain the thickness of layer 2A, thought to represent the extrusive layer [Hooft et al., 1996; Carbotte et al., 1997]. Our goal will be to investigate the characteristics of the magnetized upper layer, and compare these against the shape of seismic layer 2A. The profiles will be about 7-8 km long, which corresponds to the width of accumulation for the extrusive layer at this location [Carbotte et al., 1997]. Data will be analyzed in collaboration with Jeff Gee and other researchers at Scripps, who will acquire a series of long magnetic profiles in the same area as part of a research project recently funded by NSF. ABE will be run successively at 10, 50, and 150 m from the seafloor. To obtain precise positioning beyond the crestal region, and therefore outside of the Alvin transponder network, a new navigation software will be implemented with ABE. The method has been tested successfully in 1978 to navigate Alvin onto the tailings of a DSDP drill site. Its principle is to take advantage of the highly accurate positioning of the ship with P-code GPS while it zig-zags in the neighborhood of the area surveyed with ABE, acquiring dredges and wax-cores. An additional advantage of this positioning method is that it will allow to deploy ABE even if wax-coring and dredging operations were co-located with pingers within the transponder network.

    In addition, Alvin's magnetometer will be mounted and log data during all the dives. While this will not interfer with the volcanological missions of Alvin, it will supplement the ABE near-bottom magnetics data at no added cost.

    The program described above is primarily designed to characterize recent eruptive units in a systematic way. In that respect, if targets of particular interest were identified during some Alvin dives (such as the emplacement of a new eruptive unit), some ABE deployments would probably be dedicated to investigate these areas instead.

    Post-cruise analysis [back]
    It is anticipated that much of the preliminary processing for the Imagenix bathymetry and magnetic data will be completed at sea. Computer programs exist for processing these data, and both the topography and magnetic profiles will be examined after each mission as a routine quality check and for planning the subsequent missions.

    Digital stereo images will be processed on computers at Lamont, where the necessary algorithm exists already.

    Near-bottom magnetics require many processing steps to retrieve the seafloor magnetization information. The necessary software will be provided by Maurice Tivey. Using a similar approach as described in Tivey and Johnson [1995] and Tivey et al. [1997], both magnetic inversion and forward modeling will be used to constrain the crustal magnetization distribution. The analysis of a series of shipboard magnetic surveys over the study area indicate that peak diurnal variations probably average 40-60 nT [Cormier and Macdonald, 1994]. Because magnetic data will be acquired at night, the amplitude of the recorded diurnal variations should be much smaller (< 10 nT). Magnetic anomalies associated with fresh lava flows, if of the same order of magnitude (>10,000 nT; Figures 6 and 7) as those observed along the Juan de Fuca Ridge [Tivey and Johnson, 1995], will be obvious. If negative notches are marking the high amplitude magnetic anomaly over fresh flows, this might indicate the presence of a recent feeder dike. The spatial geometry of such negative notches would be mapped from the successive transverse profiles, and interpreted in relation to the visual observations (Alvin video and ABE digital stereo images) and the fine scale bathymetry (ABE and Alvin down-looking sonar, and Imagenix). The thickness of the recent flows will be evaluated with iterative forward modeling of the magnetic field anomaly. Results will be compared to thicknesses estimated from direct visual observations.

    The magnetization properties of selected basalt samples collected during the cruise with Alvin and dredging will be measured at the LDEO Paleomagnetics Laboratory, in collaboration with Dennis Kent. These measurements will be taken into account for the forward and inverse analysis of the magnetic field.

    The three parallel magnetic profiles acquired at 17°27'S will be processes for determining the magnetization distribution in the upper crust of the axial high. Two-dimensional forward models of the magnetic field will be iteratively computed for varying magnetization distribution until a model is found that satisfactorily accounts for the observed magnetic anomalies at the different elevations. Initial model will rely on results from the 2-D near-bottom magnetic survey for the uppermost crust, and on known seismic structure of the area for the deeper levels [Mutter et al., 1995; Harding et al., 1996; Carbotte et al., 1997]. In collaboration with Alberto Malinverno (now at Schlumberger-Doll Research), we will also experiment with using inverse theory on this exceptional data set.

    An Imagenix and magnetic survey of the axial zone at 17°16'-40'S has been carried out recently during cruise SOJNMV-2 with the ARGO system [R.M. Haymon and K.C. Macdonald are PIs, and D.S. Scheirer is associate investigator]. Comparison of that data set to the one acquired by ABE will reveal whether any changes have occurred between 17°25'-30'S during the ~1.5 year separating the two cruises. In case no differences can be detected between the two data sets, they will be merged to produce very finely gridded bathymetry and magnetic maps. This would be particularly appropriate since the SOJNMV-2 ARGO lines were all navigated along-strike, while the ABE lines will be navigated across-strike.

    Collected data will be analyzed in conjunction with the other investigators participating in that cruise: John Sinton (U. Hawaii), Rodey Batiza (U. Hawaii), Ken Rubin (U. Hawaii), Jean-Marie Auzende (Ifremer, France), Yves Fouquet (Ifremer), Tracy Gregg (WHOI), Rachel Haymon (UCSB), Yves Lagabrielle (U. Brest, France), and Cindy van Dover (U. Alaska). Data relevant to the two programs will be exchanged freely, and collaboration among investigators while at sea and during the second year will insure that the stated objectives are met.

    Water column measurements (CTD and optical backscatter) will be routinely collected by ABE. These measurements will not be analyzed by the PIs, and will be made available to the interested cruise participants. In particular, Rachel Haymon surveyed the hydrothermal activity in the proposed study area with the ARGO platform (cruise SOJNMV-2, November 1996), and Yves Fouquet has investigated hydrothermal discharge in this same area during the Naudur cruise (December 1993). The 1993 and 1996 data, together with the 1998 ABE data, will constitute a data base for a time series analysis of hydrothermal discharge.

    Relevance to the NSF-RIDGE initiative [back]
    This piggy-back project is an add-on to a RIDGE sponsored project, and will greatly contribute to its stated objectives. The funded program and this piggy-back project will investigate variations in eruptive processes over several segments with differing extent of inflation and/or tectonism, and as such is highly relevant to the "crustal accretionary and segment scale processes" program element of RIDGE. High resolution mapping of the ultrafast EPR like the one we propose were strongly endorsed at the 1996 workshop on "Processes and fluxes on a superfast spreading ridge: the southern East Pacific Rise" in Monterey. Because it will evaluate the variability in eruptive processes and contrast the results to previous expedition in the area, this program is also relevant to the "temporal variability program" element of RIDGE. Finally, this region is the site of the Mantle Electromagnetic and Tomographic (MELT) experiment and hence, the proposed work is also relevant to the "mantle flow and melt generation" program element of RIDGE.

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    Figure captions: [back]

    Figure 1. Plate boundaries of the southeast Pacific.
    Study area is shaded.

    Figure 3. Shaded relief images of the ridge axis for the three targeted areas; illumination is from the east. Bathymetry data have been acquired recently with SeaBeam 2000 (Cormier et al., 1997); swath width is about 10 km. (Left and right) Dash lines mark the precise ridge axis, and extend over the length of the proposed study areas. (Center) Facing arrowheads point to the bounding scarps of the ~1 km wide, 50 m deep summit trough, and delimit the length of the third study area.

    Figure 2. Typical depth cross-sections for each of the three targeted areas. Vertical exaggeration is 5. Each profile averages a few successive pings acquired along-axis with SeaBeam 2000 or Hydrosweep swath bathymetry systems. Within each frame, the bottom profile is shown at its correct depth, while successive ones are offset vertically by 50 m; 120-150 m along-track separate adjacent profiles.

    Figure 4. Photo showing the autonomous underwater vehicle ABE being launched. The upper pods contain flotation, acoustic navigation transponders, and the video cameras. The pressure housing on the bottom contains the main electronics, batteries, and magnetic sensor. (after Tivey et al., 1997)

    Figure 5a. (From W.W. Chadwick and R.W. Embley, Graben formation associated with recent dike intrusions and volcanic eruptions on the mid-ocean ridge, J. Geophys. Res., in revision)
    Map showing the zig-zag pattern of seafloor coverage acquired along NNE tracks by the Mesotech, a pencil scanner sonar similar to Imagenix. Data were collected from the Alvin submersible along CoAxial segment (Juan de Fuca Ridge).

    Figure 5b. (From W.W. Chadwick and R.W. Embley, Graben formation associated with recent dike intrusions and volcanic eruptions on the mid-ocean ridge, J. Geophys. Res., in revision)
    A series of successive cross-sections of a narrow graben acquired with the Mesotech pencil scanning sonar (no vertical exaggeration). Successive cross-sections are offset vertically by 15 m. The approximate total length of graben shown is 60 m. Small arrows are anchored at Alvin's position, and point in the direction of scanning. Because Alvin flew to the right of the graben center, the graben's right wall is not illuminated.

    Figure 6. From Tivey et al. [1997]. (Left) The bathymetry of the new lava area (10-m contours) on the Coaxial ridge segment of the Juan de Fuca Ridge. The 1993 lava flow extent determined from differential bathymetric mapping is shaded gray and overlain by ABE tracks and the magnetic field anomaly measured along track. Note the strong positive anomaly located over the new flow. Eastern gray areas represent a 10-year-old eruption. (Right) The calculated magnetic thickness of the 1993 lava flow in comparison with the extent of the lava flow, which was determined by depth differencing from surface ship surveys (bold line).

    Figure 7. From Tivey and Johnson [1995]. Magnetic profile collected with Alvin across the 1993 Coaxial lava flow. (Bottom) Bathymetry along the profile with the new flow shaded. (Top) Forward magnetic models. Dash line is the upward continued magnetic anomaly. Thin line is the magnetic field due a constant magnetization of 26 A/m produced by the pre-1993 bathymetry. Bold line is the magnetic field produced by the new flow, magnetized to 127 A/m, combined with the pre-1993 anomaly. The notch in the anomaly high is better modeled with a 10-m-wide nonmagnetic dike (Right) than without (Left)


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