Alford, Susan E. et al. (2011): Sulfur geochemistry and microbial sulfate reduction during low temperature alteration of uplifted lower oceanic crust; insights from ODP Hole 735B
Leg/Site/Hole:
Related Expeditions:
ODP 176
Identifier:
ID:
2011-102183
Type:
georefid
ID:
10.1016/j.chemgeo.2011.05.005
Type:
doi
Creator:
Name:
Alford, Susan E.
Affiliation:
University of Michigan, Department of Geological Sciences, Ann Arbor, MI, United States
Role:
author
Name:
Alt, Jeffrey C.
Affiliation:
U. S. Geological Survey, United States
Role:
author
Name:
Shanks, Wayne C., III
Affiliation:
Role:
author
Identification:
Title:
Sulfur geochemistry and microbial sulfate reduction during low temperature alteration of uplifted lower oceanic crust; insights from ODP Hole 735B
Year:
2011
Source:
Chemical Geology
Publisher:
Elsevier, Amsterdam, Netherlands
Volume:
286
Issue:
3-4
Pages:
185-195
Abstract:
Sulfide 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.
Language:
English
Genre:
Serial
Rights:
URL:
Coverage: Geographic coordinates: North:-32.4318 West:57.1557 East:
57.1618 South:-32.4327
Keywords: Isotope geochemistry; Igneous and metamorphic petrology; alteration; aqueous alteration; Atlantis II fracture zone; biochemistry; biogenic structures; crust; electron probe data; gabbros; geochemistry; igneous rocks; Indian Ocean; isotope ratios; isotopes; Leg 176; mass spectra; microorganisms; mid-ocean ridges; mineral assemblages; mineral composition; Ocean Drilling Program; ocean floors; oceanic crust; ODP Site 735; oxidation; petrography; plutonic rocks; precipitation; reduction; S-34/S-32; sea water; secondary minerals; sedimentary structures; Southwest Indian Ridge; spectra; stable isotopes; sulfates; sulfides; sulfur; temperature; textures; uplifts;
.