Geochemistry of low-molecular weight hydrocarbons in hydrothermal fluids from Middle Valley, northern Juan de Fuca Ridge

Abstract

Hydrothermal vent fluids from Middle Valley, a sediment-covered mid-ocean ridge on the northern Juan de Fuca Ridge, were sampled in July, 2000. Eight different vents with exit temperatures of 186 to 281°C were sampled from two areas of venting: the Dead Dog and ODP Mound fields. Fluids from the Dead Dog field are characterized by higher concentrations of ΣNH 3 and organic compounds (C 1 -C 4 alkanes, ethene, propene, benzene and toluene) compared with fluids from the ODP Mound field. The ODP Mound fluids, however, are characterized by higher C 1 /(C 2 +C 3 ) and benzene:toluene ratios than those from the Dead Dog field. The aqueous organic compounds in these fluids have been derived from both bacterial processes (methanogenesis in low-temperature regions during recharge) as well as from thermogenic processes in higher-temperature portions of the subsurface reaction zone. As the sediments undergo hydrothermal alteration, carbon dioxide and hydrocarbons are released to solution as organic matter degrades via a stepwise oxidation process. Compositional and isotopic differences in the aqueous hydrocarbons indicate that maximum subsurface temperatures at the ODP Mound are greater than those at the Dead Dog field. Maximum subsurface temperatures were calculated assuming that thermodynamic equilibrium is attained between alkenes and alkanes, benzene and toluene, and carbon dioxide and methane. The calculated temperatures for alkene-alkane equilibrium are consistent with differences in the dissolved Cl concentrations in fluids from the two fields, and indicate that subsurface temperatures at the ODP Mound are hotter than those at the Dead Dog field. Temperatures calculated assuming benzene-toluene equilibrium and carbon dioxide-methane equilibrium are similar to observed exit temperatures, and do not record the hottest subsurface conditions. The difference in subsurface temperatures estimated using organic geochemical thermometers reflects subsurface cooling processes via mixing of a hot, low-salinity vapor with a cooler, seawater salinity fluid. Because of the disparate temperature dependence of alkene-alkane and benzene-toluene equilibria, the mixed fluid records both the high and low temperature equilibrium conditions. These calculations indicate that vapor-rich fluids are presently being formed in the crust beneath the ODP Mound, yet do not reach the surface due to mixing with the lower-temperature fluids.2

Gas geochemical characteristics of hydrothermal plumes at the HAKUREI and JADE vent sites, the Izena Cauldron, Okinawa Trough

“…Notably, the H 2 /CH 4 ratio of the HAKUREI fluid was comparable with those of thermogenic gases produced by hydrothermal experiment using the Guaymas Basin sediment (0.02~0.2: Seewald et al, 1994) as well as other sediment-related hydrothermal sites (Fig. 6) (Chiba et al, 1993;Lilley et al, 1993;Cruse and Seewald, 2006;McCollom, 2008). This thermogenic signature at the HAKUREI site is consistent with the thick layer of sediment present.…”

“…6). The low H 2 /CH 4 ratio in the JADE fluid would result from involvement of microbial methanogenesis at recharge zone (Lilley et al, 1993;Cruse and Seewald, 2006) that provides CH 4 with little H 2 .…”

“…Because of requirement of high temperature, thermal decomposition of sedimentary organic matter could occur at deep reaction zone and during fluid upwelling. Alternatively, microbial methanogenesis in low-temperature sediment environment during fluid recharge has also been discussed as the possible origin of isotopically light methane in vent fluids at sediment-related hydrothermal systems (Lilley et al, 1993;Cruse and Seewald, 2006). Thus "sedimentderived methane" potentially represents both thermogenic and biogenic methane.…”

Pathways for abiotic organic synthesis at submarine hydrothermal fields

“…Aliquots were collected in Optima HCl-cleaned high-density polyethylene bottles for shore-based analysis of Mg, Cl, and total formate species (ΣHCOOH = HCOOH + HCOO − ) in samples stored frozen. Shipboard measurement of dissolved H 2 and CH 4 was accomplished by molecular-sieve gas chromatography (GC) with thermal conductivity detection after a headspace extraction (3,34). Aliquots for shorebased total dissolved inorganic carbon (ΣCO 2 = CO 3 2− + HCO 3 − + H 2 CO 3 )…”

“…Dissolved He was extracted from fluid samples on board the ship using a portable vacuum line and transferred to evacuated aluminosilicate glass break-seal tubes for shore-based He isotope analysis. Fluid aliquots were transferred into sealed glass tubes fitted with Teflon and stainless steel valves for shore-based C 2 H 6 and C 3 H 8 analysis using a purge and trap device interfaced to molecular-sieve GC with flame ionization detection (34). Dissolved Cl and ΣHCOOH abundances were determined by ion chromatography (36,37).…”