Initial Results From the Oman Drilling Project Multi-Borehole Observatory: Petrogenesis and Ongoing Alteration of Mantle Peridotite in the Weathering Horizon
Initial Results From the Oman Drilling Project Multi-Borehole Observatory: Petrogenesis and Ongoing Alteration of Mantle Peridotite in the Weathering Horizon
复制标题
阿曼钻井项目多孔观测站的初步结果：风化层中地幔橄榄岩的岩石成因和持续蚀变
DOI：
10.1029/2021jb022729
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发表时间：
2021
期刊：
影响因子：
3.4
通讯作者：
Kelemen P
中科院分区：
文献类型：
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作者：
Kelemen P
阿曼钻探项目“多钻孔观测站”（MBO）对构造暴露的橄榄岩的活跃风化区域进行了采样。本文回顾了MBO地区的地质情况，总结了最近的研究，并提供了新的数据来约束正在进行的蚀变。寄主岩部分至完全蛇纹岩化，残余地幔黑斑岩，并有替代作用。邓恩岩显示出“反应分馏”的证据，即冷却、结晶的岩浆与较老的熔融残留物发生反应。哈尔茨伯尔岩和邓恩岩为65%-100%水合。铁与总铁的比率从50%到90%不等。BA1B井蚀变程度随深度减小。水的梯度与岩心成分具有相关性。蛇形脉与碳酸盐脉共生或切割，具有可测的14c。持续的水合作用伴随着sio2的加入。BA1B孔的硫富集可能是由30 m上部硫的氧化浸出，以及30 - 150 m处的硫酸盐还原和硫化物沉淀所致。BA3A、NSHQ14和BA2A孔深处的氧逸度由2H2O = 2H2+ o2反应与蛇纹石、水辉石和橄榄石中的亚铁氧化作用固定。在BA1A、BA1D和BA4A井中，受蛇纹岩和水镁石的平衡控制，fo2比H2O‐H2limit高3-4个对数单位。蚀变的变化与结构有关，网状岩心中减少的低sio2组合记录了非常低的水/岩比，而邻近的矿脉记录了高得多的水/岩比。还原岩心与氧化岩脉的比例随深度增加而增加，岩心与岩脉中所记录的fo2差异随深度减小。
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 measurable14C. Ongoing hydration is accompanied by SiO2addition. 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+ O2combined with oxidation of ferrous iron in serpentine, brucite, and olivine. fO2deep in Holes BA1A, BA1D, and BA4A is 3–4 log units above the H2O‐H2limit, controlled by equilibria involving serpentine and brucite. Variations in alteration are correlated with texture, with reduced, low SiO2assemblages 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 fO2recorded in cores and veins decreases with depth.