Zircon record of fractionation, hydrous partial melting and thermal gradients at different depths in oceanic crust (ODP Site 735B, South-West Indian Ocean)
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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DOI：
10.1007/s00410-016-1324-y
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发表时间：
2017-03
影响因子：
3.5
通讯作者：
A. Pietranik;C. Storey;J. Koepke;Stéphanie Lasalle;Stéphanie Lasalle
中科院分区：
文献类型：
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作者：
A. Pietranik;C. Storey;J. Koepke;Stéphanie Lasalle;Stéphanie Lasalle
长英质脉（斜长花岗岩）分布于整个洋壳剖面，为了解洋壳内的晚期岩浆/高温热液过程提供了线索。尽管仅占IODP 735 B站点钻探的洋壳部分的0.5%，但它们携带了大量不相容元素，并在地壳内重新分布。这种熔体中含有饱和的副矿物，如锆石、钛铁矿和磷灰石，锆石通常是唯一保留岩浆成分并记录长英质熔体形成和演化过程的相。在这项研究中，我们分析了来自IODP 735 B站点四个深度的锆石;它们来自氧化物辉长岩（海底以下约250米）和斜长花岗岩（深度c。500、860、940米）。所有锆石具有相似的εHf成分c。15个单元表明形成IODP站点735 B辉长岩的镁铁质岩浆的同位素均匀来源。氧化物辉长岩的锆石是稀缺的，可变的组成与他们的结晶从熔融形成的基性岩岩浆分馏和辉长岩堆晶的含水重熔。另一方面，锆石从斜长花岗岩是丰富的，每个样品的特点是组成趋势一致的结晶锆石在不断发展的熔体。然而，在500 m bsf的斜长花岗岩和更深的部分，这是不同的趋势被解释为斜长花岗岩形成的两个过程的记录：在500 m bsf的辉长岩堆晶重熔和分馏在更深的部分。氧化物辉长岩和斜长花岗岩中的锆石δ 18 O为3.5 ~ 6.0‰。δ 18 O值的最佳解释是δ 18 O在热梯度中的再分布，而不是热液蚀变地壳的重熔。因此，分馏作用可能是与辉长岩结晶作用同时发生的一个较老的事件，而重熔作用可能是一个较年轻的事件，是由地壳堆体的变形和抬升所触发的。
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 εHf 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 δ18O from 3.5 to 6.0‰. Values of δ18O are best explained by redistribution of δ18O 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.