HR: 17:00h
AN: V34B-05    [Abstracts]
TI: The Role of Magmatic Volatile Input, Near-surface Seawater Entrainment and Sulfide Deposition in Regulating Metal Concentrations Within Manus Basin Hydrothermal Systems
AU: * Craddock, P R
EM: pcraddock@whoi.edu
AF: Woods Hole Oceanographic Institution, Dept. Marine Chemistry and Geochemistry, Woods Hole, MA 02543, United States
AU: Tivey, M K
EM: mktivey@whoi.edu
AF: Woods Hole Oceanographic Institution, Dept. Marine Chemistry and Geochemistry, Woods Hole, MA 02543, United States
AU: Seewald, J S
EM: jseewald@whoi.edu
AF: Woods Hole Oceanographic Institution, Dept. Marine Chemistry and Geochemistry, Woods Hole, MA 02543, United States
AU: Rouxel, O
EM: orouxel@whoi.edu
AF: Woods Hole Oceanographic Institution, Dept. Marine Chemistry and Geochemistry, Woods Hole, MA 02543, United States
AU: Bach, W
EM: wbach@uni-bremen.de
AF: University of Bremen, Dept. Geosciences, Bremen, 28834, Germany
AB: Analyses of Fe, Mn, Cu, Zn, Pb, Ag, Cd, Co and Sb in vent fluid samples from four hydrothermal systems in the Manus back-arc basin, Papua New Guinea, were carried out by ICP-MS. Vienna Woods is located on the well- defined, basalt-dominated Manus Spreading Center, while the other systems are hosted in felsic volcanics on the Pual Ridge (PACMANUS), within a caldera (DESMOS), and on volcanic cones (SuSu Knolls). Metal concentrations were coupled with other fluid data (pH, SO₄, Ca, H₂S) to discriminate effects of deep- seated water-rock reaction and magmatic volatile input from near surface seawater entrainment, mixing, and consequent mineral precipitation and metal remobilization. Both magmatic volatile input (e.g. SO₂, HCl, HF) and sulfide precipitation can increase fluid acidity and thus affect the aqueous mobility of metals. At Vienna Woods, 280°C end-member (Mg = 0) fluids have high pH (>4.2) and low metal contents (Fe <160 uM, Cu <10 uM, Zn <40 uM) relative to most mid-ocean ridge (MOR) vent fluids. The high pH and lack of evidence for magmatic volatile input are consistent with fluid compositions regulated by subsurface seawater- basalt/andesite reactions. Despite low aqueous Zn concentrations, Zn-rich (wurtzite-lined) chimneys are common at Vienna Woods active vents, reflecting deposition from fluids characterized by low Fe and Cu and high pH. At PACMANUS, black smoker fluids (T >300°C, pH ~ 2.7) are enriched in sulfide-forming metals by an order of magnitude relative to Vienna Woods fluids. Enrichments at PACMANUS reflect efficient leaching of metals at low pH, with the lower pH likely a result of input of magmatic volatiles. In addition, some vents fluids show clear evidence for seawater entrainment, subsurface precipitation of Cu-Fe-sulfides and preferential remobilization of Zn-sulfides (lower T, non-zero Mg, lower Fe, Cu, H₂S and pH (2.3–2.4), but higher Zn, Pb, Cd and Ag, compared to black smokers). The higher metal concentrations and lower pH of fluids from PACMANUS versus Vienna Woods are reflected in chimney deposit compositions with Zn-poor sulfide linings composed of Cu-Fe-sulfides and As-Sb-sulfosalts in high T and lower T vents, respectively. At DESMOS caldera, fluid data suggest extensive magmatic volatile input (e.g. pH <1.5, elevated F and SO₄) but lesser reaction with the basement felsic rocks (low Li, Rb, Mn). Sampled "acid-sulfate" fluids are low temperature (T ~180°C) with Mg >46 mM, and very high concentrations of some metals for these Mg concentrations (Fe >5 mM, Zn >50 – 400 uM). At SuSu Knolls, vent fluid compositions similar to those at both PACMANUS and DESMOS are observed. Smoker fluids have high but variable metal concentrations of similar magnitude to PACMANUS. Acid-sulfate fluids from North Su have low pH (<2), non-zero Mg (>40 mM), and high Fe and Zn concentrations, similar to DESMOS fluids. At SuSu Knolls, fluid compositions reflect either high temperature water-rock reaction (smoker fluids) or magmatic volatile input (acid-sulfate fluids). As at PACMANUS, chimney deposits that correspond to venting fluids are Cu-Fe-As-Sb-rich and Zn-poor, likely reflecting deposition from low pH, high Cu and Fe fluids.
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 3616 Hydrothermal systems (0450, 1034, 3017, 4832, 8135, 8424)
DE: 4832 Hydrothermal systems (0450, 1034, 3017, 3616, 8135, 8424)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting