![]() ![]() Variations in alteration are correlated with texture, with reduced, low SiO 2 assemblages in mesh cores recording very low water/rock ratios, juxtaposed with adjacent veins recording much higher ratios. fO 2 deep in Holes BA1A, BA1D, and BA4A is 3–4 log units above the H 2O-H 2 limit, controlled by equilibria involving serpentine and brucite. ![]() Oxygen fugacity deep in Holes BA3A, NSHQ14, and BA2A is fixed by the reaction 2H 2O = 2H 2 + O 2 combined with oxidation of ferrous iron in serpentine, brucite, and olivine. Sulfur enrichment in Hole BA1B may result from oxidative leaching of sulfur from the upper 30 m, coupled with sulfate reduction and sulfide precipitation at 30–150 m. Ongoing hydration is accompanied by SiO 2 addition. Serpentine veins are intergrown with, and cut, carbonate veins with measurable 14C. Gradients in water and core composition are correlated. In Hole BA1B, alteration extent decreases with depth. Ferric to total iron ratios vary from 50% to 90%. Harzburgites and dunites are 65%–100% hydrated. Dunites show evidence for “reactive fractionation,” in which cooling, crystallizing magmas reacted with older residues of melting. Host rocks are partially to completely serpentinized, residual mantle harzburgites, and replacive. This article reviews the geology of the MBO region, summarizes recent research, and provides new data constraining ongoing alteration. The Oman Drilling Project “Multi-Borehole Observatory” (MBO) samples an area of active weathering of tectonically exposed peridotite.
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