Other
Scientific paper
Oct 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.3643s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 56, Issue 10, pp.3643-3667
Other
9
Scientific paper
Hydrothermal fluids were sampled from Pele's Vents on the summit of Loihi Seamount, an intraplate, hotspot volcano, on four occasions from February 1987 to September 1990. The warm ( 31°C) vent solutions are enriched in dissolved Si, CO 2 , H 2 S, alkalinity, K + , Li + , Rb + , Ca 2+ , Sr 2+ , Ba 2+ , Fe 2+ , Mn 2+ , NH 4 + , and possibly Ni 2+ , and depleted in SO 4 2- , O 2 , Mg 2+ , 87 Sr / 86 Sr , NO 3 - , and sometimes Cl - and Na + (calculated), relative to ambient seawater. Dissolved Si correlates linearly with sample temperature, suggesting that the solutions sampled from numerous vents in the ~ 20 m diameter field have a common source and that Si can be used as a conservative tracer for mixing of the vent fluids with ambient seawater. There are general similarities of the vent waters with ridge-axis warm springs on the Galapagos Rift and Axial Seamount, but also striking differences: very high total dissolved CO 2 (> 200 mmol/kg), high alkalinity (> 8 meq/kg) and dissolved Fe 2+ (almost 1 mmol/kg), and relatively low pH (~ 4.2-4.4 estimated, in situ, sws) and dissolved H 2 S (several mol/kg). The Mg 2+ and SO 4 2- data are inconsistent with the "Galapagos model" proposed for the warm springs at 86°W, Galapagos Rift, whereby the warm fluids result from sub-seafloor mixing of a high-temperature (~ 350°C) hydrothermal endmember with essentially unaltered seawater. The variable Cl - depletions in the vent fluids, however, suggest that the warm vent fluids do contain a high-temperature (> 200°C) component. The fluid history can be qualitatively described by a modified "Galapagos model" which includes the overprint of reactions resulting from the addition of juvenile CO 2 and SO 2 to the circulating fluids; the CO 2 attacks the basalt releasing metal cations, HCO 3 - and possibly Si into solution, and the SO 2 is hydrolyzed to SO 4 2- . These juvenile inputs likely reflect the shallow, hotspot setting of this hydrothermal system. A simple quantitative fluidhistory model is considered and shown to be consistent with mass-balance constraints and saturationstate calculations, which suggest that the Si concentration of the fluids may be controlled by amorphous silica saturation at ~ 31°C. Observed temporal variations in fluid composition between expeditions--specifically, in Cl - , A T , C T , Na + (calculated), Mg 2+ , Ca 2+ , Sr 2+ , 87 Sr / 86 Sr , Fe 2+ , Mn 2+ and perhaps NH 4 + , relative to Si--are, excepting Mg 2+ , 87 Sr / 86 Sr , and Mn 2+ , consistent with the effects of variable phase segregation at the proposed high-temperature endmember.
Macdougall Douglas J.
McMurtry Gary M.
Sedwick Peter N.
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