Active hydrothermal vents were first discovered at Loihi in the late
1980's (e.g., Karl et al., (1988) Nature, vol 335, 532-535). These
vents, while remarkably similar to those found at mid-ocean ridge
spreading centers, had some compositional and thermal differences,
leading researchers to believe that (possibly) mid-plate volcanoes
supported a unique style of hydrothermalism.
Two prominent vent fields at the summit were named Pele's vents and Kapo's vents (after the Hawaiian volcano goddess, said by legend to reside at nearby Kilauea volcano, and her sister). These vents were considered as "low temperature vents" by the scientific community as their waters were only 30°C or so. The vents were characterized by relatively high dissolved Fe and CO2 concentrations.
The eruption and seismic event of 1996 changed all that. In the location where Pele's vents once stood (near the highest point on Loihi, in fact), a new pit crater was formed.
|Inside this new "Pele Pit" (shown to the left), high temperature venting was initiated at a number of sites, including Lohiau vent field (left "x" on the northwest wall of the new pit) and the Forbidden Vents (right "x" at the bottom of the pit). Exit temperatures of 77 °C were measured in Oct., 1996 at Lohiau vents. Additionally, in Sept. 1996, high temperature vents were observed on the floor of Pele's Pit but temperature readings were not made and, unfortunately, it was deemed too unsafe for the October Expedition participants to visit go to Forbidden Vents at this time (so temperature readings were not made). In Sept. 1997, the Forbidden Vents were examined and submersible divers measured exit temperatures of over 200°C. This vent had, within the course of a year spawned new hydrothermal chimneys comprised mostly of Pyrite and Barite (see below). You can view additional images of these hydrothermal minerals at: www.mbari.org/~davisa/Loi.hydrothermal2.html. In addition, new lower temperature vents were discovered in 1996 on the shallow portions of the Loihi South Rift Zone. These new vents will likely be a valuable research resource for some time to come.|
Hydrothermal Mineral Images © Courtesy of Alice Davis and Dave Clague,
(Click on the images to view larger versions)
Details of Vent Locations
(Peles' and Kapo's vents data from Karl et al., Nature, 1987; Other data complied from 1996 and 1997 HURL cruise reports by Ken Rubin)
|Kapo's||1280m||upper S rift||1987||no longer venting in 1996|
|Lohiau (slow, retarded)||?||Pele's Pit||1996||77°C|
|Pahaku (rocky)||1196m||south rift||1996||17°C|
|Ula (red)||1099m||south summit||1996||diffuse venting|
|Maximilian||1249m||west flank off summit||1990s||diffuse venting|
|Naha ("cracked" or "broken")||1325m||south rift||1996||23°C|
|Naha vent field is approximately 20 x 30 m, and is heavly covered with nontronite deposits and tan bacterial mats. The field contains many small vents, as well as diffuse flow through fractured pillows and large fissures. water depth The extensive vent field includes many fresh fractures, including a 1-3 m wide fissure that vented large volumes of water|
Details of Recent Chemical Observations of Hydrothermal Vents and Plumes
Immediately after the 1996 eruption, the plumes issuing from te new
vents at Loihi were very acidic. They had pH as low as 5.6 (in
samples from Pele's Pit) and temperature anomalies of approximately
3.0°C over ambient sea water. They also had alkalinity
as high as 3.15 meq/l, implying a total CO2 concentration
of > 9 mM (roughly 4 times the concentration in sea water). Dissolved
Fe and Mn levels in the plumes were also very high (maximum Mn levels
of 0.4 uM in the plumes and 5.5 uM in Pele's Pit). (Loihi Science
Team, 1997, EOS (Transactions of the American Geophysical Union) vol. 78, 229-233).
Reasearch on these systems is on going.
Particles and sea water from this new pit had large anomalies of radioactive 210Po (about 5-10 times that of sea water) and the particles large compositional anomalies of volatile metails such as Mo, W, As, Pb, Pb and Sb (about 10-30 times that of sea water) (Rubin, 1997, Geochim. et Cosmochim. Acta, vol 61, 3525-3542).
Carbon dioxide is the dominant gas in Loihi vent waters. However, it's ratio to dissolved silica and vent temperature (heat) has changed dramatically since before the 1996 eruption. Between 1987 and 1992, dissolved CO2 had decreased by about 30% (Sedwick et al., 1994). This trend has continued since the eruption (samples from new vent fields fall on the same CO2-versus-temperature and dissolved silica trends. Overall, the CO2/heat ratio has decreased by about 95% since 1987 (G. McMurtry, G. Wheat and F. Goff, unpubl. data). These decreases had previously been ascribed to progressive degassing from an older magmatic intrusion (Sedwick et al., 1994, Geochim. Cosmochim. Acta, vol. 58, 1219-1227). However, with the continuation of this trend following a confirmed new eruption and lots of associated CO2 degassing, this is now not so certain.
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Last page update on 22 Jul 1998