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首页> 外文期刊>Contributions to Mineralogy and Petrology >Zircon record of fractionation, hydrous partial melting and thermal gradients at different depths in oceanic crust (ODP Site 735B, South-West Indian Ocean)
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Zircon record of fractionation, hydrous partial melting and thermal gradients at different depths in oceanic crust (ODP Site 735B, South-West Indian Ocean)

机译：在海洋地壳（ODP Site 735B，西南印度洋）的不同深度下分馏，湿偏熔和热梯度的分馏的锆石记录

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摘要

Felsic veins (plagiogranites) are distributed throughout the whole oceanic crust section and offer insight into late-magmatic/high temperature hydrothermal processes within the oceanic crust. Despite constituting only 0.5% of the oceanic crust section drilled in IODP Site 735B, they carry a significant budget of incompatible elements, which they redistribute within the crust. Such melts are saturated in accessory minerals, such as zircon, titanite and apatite, and often zircon is the only remaining phase that preserves magmatic composition and records processes of felsic melt formation and evolution. In this study, we analysed zircon from four depths in IODP Site 735B; they come from the oxide gabbro (depth approximately 250 m below sea floor) and plagiogranite (depths c. 500, 860, 940 m below sea floor). All zircons have similar epsilon Hf composition of c. 15 units indicating an isotopically homogenous source for the mafic magmas forming IODP Site 735B gabbro. Zircons from oxide gabbro are scarce and variable in composition consistent with their crystallization from melts formed by both fractionation of mafic magmas and hydrous remelting of gabbro cumulate. On the other hand, zircon from plagiogranite is abundant and each sample is characterized by compositional trends consistent with crystallization of zircon in an evolving melt. However, the trends are different between the plagiogranite at 500 m bsf and the deeper sections, which are interpreted as the record of plagiogranite formation by two processes: remelting of gabbro cumulate at 500 m bsf and fractionation at deeper sections. Zircon from both oxide gabbro and plagiogranite has delta O-18 from 3.5 to 6.0%. Values of delta O-18 are best explained by redistribution of delta O-18 in a thermal gradient and not by remelting of hydrothermally altered crust. Tentatively, it is suggested that fractionation could be an older episode contemporaneous with gabbro crystallization and remelting could be a younger one, triggered by deformation and uplift of the crustal pile.
机译：铰接静脉（Plagiogranites）在整个海底剖面分布，并在海外壳内提供对后岩岩/高温水热过程的洞察。尽管在IODP网站735B中仅构成了0.5％的海洋地壳部分，但它们载有重大预算的不相容元素，它们在地壳内重新分配。这种熔体在辅助矿物质中饱和，例如锆石，二钛矿和磷灰石，并且通常锆石是唯一保留岩浆组合物的剩余阶段，并记录肠熔体形成和进化的记录过程。在这项研究中，我们分析了IODP Site 735B中的四个深度的锆石;它们来自氧化物gabbro（深度约250米以下）和斑块（深度C. 500,860,940米）。所有锆石都有类似的εce组合物的c。 15个单位，表明MAFIC岩浆形成IODP网站735B Gabbro的同位素同质源。来自氧化物Gabbro的锆石是稀缺和变量的组合物，其符合其通过由MAFIC磁带的分馏和Gabbro累积的湿质重熔形成的熔体结晶。另一方面，来自沉淀物的锆石是丰富的，每个样品的特征在于与锆的结晶在不断发展的熔体中一致的成分趋势。然而，在500米的BSF和更深的部分之间的斑块之间的趋势是不同的，并且通过两种方法被解释为沉淀物形成的记录：Gabbro重熔在500μm的BSF和更深部分的分馏。来自氧化物Gabbro和Plagiogranite的锆石具有3.5至6.0％的δO-18。最佳ΔO-18的值是通过在热梯度的热梯度中重新分布而不是通过再冻结水热改变的地壳来解释ΔO-18的值。暂定地，建议分馏可以是较旧的剧集与加布布罗结晶和重熔可以是较小的剧集，由地壳桩的变形和隆起触发。

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