The Oman Drilling Project “Multi-Borehole Observatory” (MBO) samples an area of active weathering of tectonically exposed peridotite. This article 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 versus oxidized veins increases with depth, and the difference in fO₂ recorded in cores and veins decreases with depth.

中文翻译：

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阿曼钻井项目多孔观测台的初步结果：风化层中地幔橄榄岩的成岩作用和持续变化

阿曼钻探项目“多钻孔天文台”(MBO) 在构造暴露的橄榄岩活跃风化区取样。本文回顾了 MBO 地区的地质，总结了最近的研究，并提供了限制正在进行的蚀变的新数据。主岩部分至完全蛇纹石化，残余地幔斜方辉石，和替换。Dunites 显示了“反应性分馏”的证据，其中冷却、结晶的岩浆与较旧的熔融残留物发生反应。Harzburgites 和 Dunite 的水合度为 65%–100%。三价铁与总铁的比率从 50% 到 90% 不等。在 BA1B 孔中，蚀变程度随着深度的增加而减小。水和岩心成分的梯度是相关的。蛇纹石脉与碳酸盐脉共生和切割，具有可测量的¹⁴C. 持续的水合作用伴随着 SiO _{2 的}添加。BA1B 孔中的硫富集可能是由于硫从上部 30 m 处氧化浸出，再加上 30-150 m 处的硫酸盐还原和硫化物沉淀。孔 BA3A、NSHQ14 和 BA2A 深处的氧逸度是通过反应 2H ₂ O = 2H ₂  + O ₂与蛇纹石、水镁石和橄榄石中亚铁的氧化反应确定的。孔 BA1A、BA1D 和 BA4A 中的fO ₂比 H ₂ O-H ₂极限高 3-4 个对数单位，由涉及蛇纹石和水镁石的平衡控制。蚀变的变化与质地相关，具有减少的、低的 SiO ₂网状岩心中的组合记录了非常低的水/岩石比，与相邻的脉并列记录了更高的比值。减少的网状核心与氧化的脉的比例随着深度增加，并且在核心和脉中记录的fO ₂的差异随着深度而减小。