Events During 1 to 30 November 1996
Current Geologic Activity and Research at Loihi
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10 Nov 1996 Ken Rubin offers this summary of radiogeochemical studies on recently collected Loihi rocks (it is based upon the work of the following people (Ken Rubin and Lester Sacks-radioactive isotopes; Khal Spencer-Pb isotopes; Kevin Johnson-trace elements by ICP-MS)
Analyses are under way to determine the age of 2 fresh RR cruise
lavas using the 210Po-210Pb technique, which was successfully applied to
verify and date recent eruptive activity at 9ø50'Non the East Pacific
Rise in 1991 and 1992 (Rubin et al., 1994). Essentially, an age is
determined from the rate of return of degassed 210Po to secular
equilibrium with grand-parental 210Pb in a lava following eruption. The
half-life for this process is 138.4 days, so the technique can provide
ages of lavas erupted within the past 2.5 years. Three to four
separate analyses of 210Po in a lava are required (each several months
apart) to establish the lava's age. Thus, we will not have final ages
on these rocks for about a year. Instead, at this early stage, we can
only bracket samples within fairly broad age ranges.
Nevertheless, initial results indicate that: (a) the two lavas were both erupted within the past year; (b) the two lavas probably are of different ages (being separated in time by at least 3 months); and (c) the minimum ages of these lavas predate the initiation of the recent seismicity at Loihi by anywhere from 1 to 10 weeks.
Additionally, 210Po, Pb isotopes and a host of volatile metals are being analyzed in particle-enrichment seawater (0.22 g/L) collected during the RR cruise in search of indicators in the water column expected from magmatic degassing of metals. Particulates show enrichments (10-40x) relative to Loihi summit rocks of Pb, Po, Mo, Sb, As and Tl) and a Pb isotopic composition that is indistinguishable from that of Loihi summit lavas (the latter being less than 0.5% higher than values in one of the young rocks noted above). This strongly implicates Loihi as the ultimate source of the trace elements adhering to the particulates, although the relative youth of the particulates awaits repeat 210Po analyses for the same reasons as the rocks do.
8 Nov 1996 Mike Garcia offers this summary of geochemistry and petrology from recently collected Loihi rocks (it is based upon the work of the following people (M. Garcia/Marc Norman/J.M. Rhodes/D.A. Clague):
Rocks and sediment were collected from Loihi using the Pisces V submersible
during three cruises following this summer's seismic event. This summary
includes basic descriptions of these materials and the highlights of our
petrologic work on RR rocks. The RR rocks are from a young breccia deposit
on the western flank of Loihi's summit (Fig. a). The rocks from the other
two cruises were collected along the southern margin of the summit and the
upper south rift zone. They include talus fragments [some with sulfides
(pyrite, marcasite, sphalerite) and amorphous silica (?) coatings], in situ
lavas and sediment (black sand with paper-thin bubble wall fragments of
glass, mixed with abundant planktonic foraminifera).
Three of the RR rocks are the freshest lavas that have been collected from Loihi. All of the other rocks and sediment have reddish-orange and/or whitish stains on some surfaces from hydrothermal alteration. The RR rocks are low SiO2 tholeiites (typical of recent Loihi lavas; Garcia et al., 1995) and contain 1 to 2 vol.% phenocrysts and microphenocrysts of clinopyroxene (rare in Hawaiian tholeiites) and two populations of olivine (~forsterite 81-82 and 86-87). The clinopyroxenes have inclusions of strongly resorbed olivine, indicating a reaction relation, and are reversely zoned near their rims, indicating magma mixing shortly before or during eruption. These rocks range in MgO content from 8.2 to 10.3 wt% and the olivines are in equilibrium with these endmembers (assuming 10% oxidized iron). Glassy margins of these rocks record a lower pressure, clinopyroxene-dominated crystallization trend, which is reflected in the microphenocryst assemblage. Laser ICPMS trace element glass analyses yield trace element ratios (e.g., La/Yb) which continue the temporal geochemical trend for young Loihi tholeiites (Garcia et al., 1995).
Modeling of equilibrium crystallization from 1 atmosphere to 6 kb for the most mafic RR rock composition using the MELTS program (Ghiorso and Sack, 1995) indicates that the olivine reaction could occur at moderate pressures (~3.5 kb) and that clinopyroxene is a late crystallizing phase at lower pressures (1-2 kb, typical of the summit magma chamber for Kilauea).
In summary, our preliminary results indicate that the RR rocks are young and were stored at moderate depths (~ 10 km) before being mixed with a more mafic magma, which may have trigger their eruption. The bubble glass fragments indicate that some Loihi eruptions are more violent than previously thought.
1 Nov 1996 Dave Clague (recently of HVO, now at MBARI) offered this description of geological observations made during the LONO expedition to Loihi (the last in a series of trips to the seamount following the activity this past July/August)
Hydrothermal sulfides were recovered from vent site 5 in the pit.
These deposits consist of pyrite, marcasite, amorphous silica, sphalerite,
and bornite(?). It occurs as thin coatings on sand- to fist-sized talus
fragments. The thin coatings of sulfide are inconsistent with the idea that
stockwork beneath the old Pele's Vents was sampled. Instead, the sulfides
appear to be recently formed deposits that postdate the formation of the
pit and precipitated from fluids venting through talus deposits.
Abundant sedimentary deposits of black sand was observed in the summit region and along specific sections of the south rift. Recovered samples include paper-thin bubble wall fragments of glass produced during submarine fire fountains or mild steam explosions. Examination of these glass fragments reveals that all are slightly altered, have spotty hydrothermal nontronite deposits on their surfaces, and are mixed with abundant planktonic foraminifera and dense glass fragments. These glass fragments did not form during this past summer, but indicate that some Loihi eruptions in the past were more violent than previously thought.
The collapse of the summit pit was accompanied by formation of ground cracks and fissures on the south rift between about 1400 and 1300 m depth. These fissures crosscut another, previously unknown, low-temperature hydrothermal vent field. Despite the open fissures created as part of the events of this past summer, we found no evidence of a recent eruption. The CTD casts along the south rift also failed to detect any temperature anomalies deeper than this new vent field. These observations, coupled with the small volume of the pit collapse (between 0.05 and 0.15 km3), suggest that the collapse was caused by magma intrusion into the south rift, but that no eruption ensued. Based on the apparent lack of new ground cracks between 1465 m and 1400 m depth, we infer that the intrusion was limited to the uppermost part of the south rift.
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Last page update on 10 November 1996