georefid:2011-102183SEDIS Publication Catalogue

ana.macario@awi.de

http://sedis.iodp.org/pub-catalogue/informationpointOfContact2013-07-08T00:00:00Zhttp://sedis.iodp.org/pub-catalogue/index.php?id=10.1016/j.chemgeo.2011.05.0052011-01-01publicationgeorefid:2011-102183doi:10.1016/j.chemgeo.2011.05.005Alford, Susan E.University of Michigan, Department of Geological Sciences, Ann Arbor, MI, United StatesauthorAlt, Jeffrey C.U. S. Geological Survey, United StatesauthorShanks, Wayne C., IIIauthorElsevier, Amsterdam, NetherlandspublisherdocumentHardcopyChemical Geology286 (3-4)185-195Sulfide petrography plus whole rock contents and isotope ratios of sulfur were measured in a 1.5 km section of oceanic gabbros in order to understand the geochemistry of sulfur cycling during low-temperature seawater alteration of the lower oceanic crust, and to test whether microbial effects may be present. Most samples have low SO (sub 4) /Sigma S values (< or =0.15), have retained igneous globules of pyrrhotite + or - chalcopyrite + or - pentlandite, and host secondary aggregates of pyrrhotite and pyrite laths in smectite + or - iron-oxyhydroxide + or - magnetite + or - calcite pseudomorphs of olivine and clinopyroxene. Compared to fresh gabbro containing 100-1800 ppm sulfur our data indicate an overall addition of sulfide to the lower crust. Selection of samples altered only at temperatures < or =110 degrees C constrains microbial sulfate reduction as the only viable mechanism for the observed sulfide addition, which may have been enabled by the production of H (sub 2) from oxidation of associated olivine and pyroxene. The wide range in delta (super 34) S (sub sulfide) values (-1.5 to +16.3 ppm) and variable additions of sulfide are explained by variable epsilon (sub sulfate-sulfide) under open system pathways, with a possible progression into closed system pathways. Some samples underwent oxidation related to seawater penetration along permeable fault horizons and have lost sulfur, have high SO (sub 4) /Sigma S (> or =0.46) and variable delta (super 34) S (sub sulfide) (0.7 to 16.9 ppm). Negative delta (super 34) S (sub sulfate) -delta (super 34) S (sub sulfide) values for the majority of samples indicate kinetic isotope fractionation during oxidation of sulfide minerals. Depth trends in sulfide-sulfur contents and sulfide mineral assemblages indicate a late-stage downward penetration of seawater into the lower 1 km of Hole 735B. Our results show that under appropriate temperature conditions, a subsurface biosphere can persist in the lower oceanic crust and alter its geochemistry. Abstract Copyright (2011) Elsevier, B.V.completedIsotope geochemistryIgneous and metamorphic petrologyalterationaqueous alterationAtlantis II fracture zonebiochemistrybiogenic structurescrustelectron probe datagabbrosgeochemistryigneous rocksIndian Oceanisotope ratiosisotopesLeg 176mass spectramicroorganismsmid-ocean ridgesmineral assemblagesmineral compositionOcean Drilling Programocean floorsoceanic crustODP Site 735oxidationpetrographyplutonic rocksprecipitationreductionS-34/S-32sea watersecondary mineralssedimentary structuresSouthwest Indian Ridgespectrastable isotopessulfatessulfidessulfurtemperaturetexturesupliftsurn:org.iodp:exp:176urnlargerWorkCitationcampaignurn:org.iodp:exp:118:site:735urnlargerWorkCitationstudyurn:org.iodp:exp:118urnlargerWorkCitationcampaignurn:org.iodp:exp:176:site:735urnlargerWorkCitationstudyurn:org.iodp:exp:176urnlargerWorkCitationcampaignEnglishgeoscientificInformation57.155757.1618-32.4327-32.4318