Ongoing alteration of mantle peridotite in the weathering horizon:
Initial results from the Oman Drilling Project Multi-Borehole
Observatory
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 14C. Ongoing hydration is
accompanied by SiO2 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 2H2O = 2H2 + O2
combined with oxidation of ferrous iron in serpentine, brucite and
olivine. fO2 deep in Holes BA1A, BA1D and BA4A is 3-4
log units above the H2O-H2 limit,
controlled by equilibria involving serpentine and brucite. Variations in
alteration are correlated with texture, with reduced, low
SiO2 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 fO2 recorded in cores
and veins decreases with depth.