The mineralogy and stable (O and C) and Sr isotopic compositions of low-temperature alteration phases were determined in Hole 735B gabbroic rocks in order to understand the processes of low-temperature alteration in this uplifted block of lower oceanic crust. Phyllosilicates include smectite (saponite, Mg montmorillonite, and nontronite), chlorite/smectite, chlorite, talc, and serpentine. Other phases include prehnite, albite, K-feldspar, analcite, natrolite, thompsonite, pyrite, and titanite. The low-grade mineral assemblages mainly represent zeolite facies and lower-temperature "seafloor weathering" processes. Phyllosilicates formed over a range of temperatures but may also reflect variable reaction progress. Alteration temperatures were probably somewhat greater below 1300 meters below seafloor. Mineralogy and isotopic data indicate that conditions were mostly reducing and that seawater solutions were rock dominated. Carbonates formed late from cold and generally oxidizing seawater solution, however, as seawater penetrated downward as the result of fracturing and faulting in the uppermost portion of the uplifted crustal block.
1Alt, J.C., and Bach, W., 2001. Data report: Low-grade hydrothermal alteration of uplifted lower oceanic crust, Hole 735B: mineralogy and isotope geochemistry. In Natland, J.H., Dick, H.J.B., Miller, D.J., and Von Herzen, R.P. (Eds.), Proc. ODP, Sci. Results, 176 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/176_SR/chap_01/chap_01.htm>. [Cited YYYY-MM-DD]
2Department of Geological Sciences, 2534 C.C. Little Building, The University of Michigan, Ann Arbor MI 48109-1063, USA. jalt@umich.edu
3Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, 360 Woods Hole Road, MS #8, Woods Hole MA 02543, USA.
Initial
receipt: 17 April 2000
Acceptance: 11 July 2001
Web publication: 18 December 2001
Ms 176SR-013