Ongoing alteration of mantle peridotite in the weathering horizon: Initial results from the Oman Drilling Project Multi-Borehole Observatory
Journal Article
Overview
abstract
; The Oman Drilling Project “Multi-Borehole Observatory” (MBO) samples; an area of active weathering of tectonically exposed peridotite. This; paper reviews the geology of the MBO region, summarizes recent research,; and provides new data constraining ongoing alteration. Host rocks are; partially to completely serpentinized, residual mantle harzburgites and; replacive. Dunites show evidence for “reactive fractionation”, in; which cooling, crystallizing magmas reacted with older residues of; melting. Harzburgites and dunites are 65-100% hydrated. Ferric to total; iron ratios vary from 50 to 90%. In Hole BA1B, alteration extent; decreases with depth. Gradients in water and core composition are; correlated. Serpentine veins are intergrown with, and cut, carbonate; veins with measurable; 14; C. Ongoing hydration is; accompanied by SiO; 2; addition. 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.; Oxygen fugacity deep in Holes BA3A, NSHQ14 and BA2A is fixed by the; reaction 2H; 2; O = 2H; 2; + O; 2; combined with oxidation of ferrous iron in serpentine, brucite and; olivine. fO; 2; deep in Holes BA1A, BA1D and BA4A is 3-4; log units above the H; 2; O-H; 2; limit,; controlled by equilibria involving serpentine and brucite. 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. The proportion of reduced mesh cores vs oxidized veins increases; with depth, and the difference in fO; 2; recorded in cores; and veins decreases with depth.;
