Geochemical Profiles Across the Listvenite-Metamorphic Transition in the Basal Megathrust of the Semail Ophiolite: Results From Drilling at OmanDP Hole BT1B
Geochemical Profiles Across the Listvenite-Metamorphic Transition in the Basal Megathrust of the Semail Ophiolite: Results From Drilling at OmanDP Hole BT1B
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DOI：
10.1029/2021jb022733
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发表时间：
2021-12-01
影响因子：
3.9
通讯作者：
Kelemen, P. B.
中科院分区：
文献类型：
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作者：
Godard, M.;Carter, E. J.;Kelemen, P. B.
在ICDP OmanDP孔BT1B钻探了从Smail蛇绿岩地幔到下伏变质底的过渡过程。我们分析了地幔成因的李氏岩系和变质岩的主体主量、挥发分和微量元素组成，目的是限制与橄榄岩碳酸盐作用有关的化学转移沿蛇绿岩底部逆冲。列文岩系列由不同的碳酸盐蛇纹岩和(含玄武岩)列文岩组成。它们有很高的二氧化碳(高达43wt.%)和可变的H2O(0-12wt.%)。然而，它们的成分与大多数主要和亲岩石的基性条带橄榄岩的成分接近，其中含玄武岩的列表岩组分与含角闪石的基性二辉橄榄岩在成分上重叠(例如，Al_2O_3=0.1-2.2 wt.%；Yb=0.05-1 x CI球粒陨石)。Listvenite系列的原岩在结构和成分上很可能与蛇纹状条带橄榄岩相似，后者直接覆盖在阿曼其他地方的变质鞋底之上。与Smail橄榄岩相比，Listvenite系列富含流体活动元素(FME)(最高可达10(3)-10(4)x原始地幔)，其浓度与俯冲变质玄武岩和/或变质岩中Cs、Sr和Ca的浓度相似，有时Pb、Li、As和Sb的浓度甚至更高(例如Li高达130mU g/g；As高达170mU g/g)。我们还观察到了锶钙富集物与其他FME之间的解耦，表明它与沿基底逆冲带输送的几批深部富CO2流体相互作用。这些结果表明，橄榄岩碳化作用可能是碳、水和FME沿汇聚边缘捕获和释放的主要机制之一。
The transition from the Semail ophiolite mantle to the underlying metamorphic sole was drilled at ICDP OmanDP Hole BT1B. We analyzed the bulk major, volatile and trace element compositions of the mantle-derived listvenite series and metamorphic rocks, with the aim to constrain chemical transfers associated with peridotite carbonation along the ophiolite basal thrust. The listvenite series comprise variously carbonated serpentinites and (fuchsite-bearing) listvenites. They have high CO2 (up to 43 wt.%) and variable H2O (0-12 wt.%). Yet, they have compositions close to that of the basal banded peridotites for most major and lithophile trace elements, with fuchsite-bearing listvenites overlapping in composition with amphibole-bearing basal lherzolites (e.g., Al2O3 = 0.1-2.2 wt.%; Yb = 0.05-1 x CI-chondrite). The protolith of the listvenite series was likely similar in structure and composition to serpentinized banded peridotites which immediately overlie the metamorphic sole elsewhere in Oman. The listvenite series are enriched in fluid mobile elements (FME) compared to Semail peridotites (up to similar to 10(3)-10(4) x Primitive Mantle), with concentrations similar to the underthrusted metabasalts and/or metasediments for Cs, Sr and Ca and sometimes even higher for Pb, Li, As, and Sb (e.g., Li up to 130 mu g/g; As up to 170 mu g/g). We also observe a decoupling between Sr-Ca enrichments and other FME, indicating interactions with several batches of deep CO2-rich fluids transported along the basal thrust. These results suggest that peridotite carbonation could represent one of the major trap-and-release mechanisms for carbon, water and FME along convergent margins.