HR: 1340h
AN: V13C-0563    [Abstracts]
TI: Helium and Carbon Isotope Systematics of Cold Seep Fluids at Monterey Bay (California, USA)
AU: * Fueri, E
EM: efuri@ucsd.edu
AF: Geoscience Research Division, Scripps Inst. Oceanography, 9500 Gilman Drive , La Jolla, CA 92093-0244 United States
AU: Hilton, D R
EM: drhilton@ucsd.edu
AF: Geoscience Research Division, Scripps Inst. Oceanography, 9500 Gilman Drive , La Jolla, CA 92093-0244 United States
AU: Brown, K M
EM: kmbrown@ucsd.edu
AF: Geoscience Research Division, Scripps Inst. Oceanography, 9500 Gilman Drive , La Jolla, CA 92093-0244 United States
AU: Tryon, M
EM: mtryon@ucsd.edu
AF: Geoscience Research Division, Scripps Inst. Oceanography, 9500 Gilman Drive , La Jolla, CA 92093-0244 United States
AB: We report ³He/⁴He ratios, δ¹³C values as well as He and CO₂ abundances for cold seep fluids collected in Monterey Bay, CA. Fluids were collected in 1/8" copper coils attached to Chemical and Aqueous Transport (CAT) meters (Tryon et al., Deep-Sea Research, 2001), deployed in March and August 2004 for periods of 2 and 3 weeks, respectively. Volatiles were extracted from adjacent sections of the coils, allowing the resolution of temporal variations in the volatile content. The aims of this study were to establish the feasibility of new instrumentation in capturing submarine fluids for their He and C characteristics and to test whether an input of mantle-derived volatiles is observed in the cold seep fluids. Results from four copper coils show that all fluid samples are supersaturated with respect to seawater in helium and CO₂, indicating addition of extraneous He and CO₂. CO₂/³He ratios are significantly greater than anticipated from mantle values (MORB ~ 2 x10⁹). The ³He/⁴He ratios range from 0.46 to 2.42 R_A (where R_A equals the ³He/⁴He in air = 1.4 x 10^-6), between the value expected for mantle-derived He (mid-ocean ridge He = 8 R_A) and radiogenic He produced in crust and/or sediments (0.05 R_A). Using a simple 2-component mixing model for He, this indicates that up to ~30% of the total He in the cold seep fluids is mantle-derived. This is consistent with prior studies of other localities in the vicinity of the San Andreas Fault Zone, also showing evidence for an input of mantle-derived He (e.g. Kennedy et al., 1997). The isotopic composition of CO₂ varies between -5 and -25‰. This is consistent with an input of both organic-derived carbon (δ¹³C = -25‰) and mantle-derived CO₂ (δ¹³C = -5‰). The correlation between higher δ¹³C values and greater CO₂ concentrations indicates subsurface mixing involving a high-CO₂, high-δ¹³C endmember (mantle) and a low-CO₂, low-δ¹³C endmember (sediment). The data demonstrate remarkable temporal variation in the volatile content of the cold seep fluids. However, higher δ¹³C values and greater CO₂ concentrations correlate with higher ³He/⁴He ratios, implying input of mantle-derived volatiles, at least periodically.
DE: 1030 Geochemical cycles (0330)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 3021 Marine hydrogeology
DE: 4806 Carbon cycling (0428)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005