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首页> 外文期刊>Contributions to Mineralogy and Petrology >Oxygen isotope record of oceanic and high-pressure metasomatism: a P-T-time-fluid path for the Monviso eclogites (Italy)
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Oxygen isotope record of oceanic and high-pressure metasomatism: a P-T-time-fluid path for the Monviso eclogites (Italy)

机译：海洋和高压交代作用的氧同位素记录：Monviso榴辉岩的P-T-时间流体路径（意大利）

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摘要

Fluids are considered a fundamental agent for chemical exchanges between different rock types in the subduction system. Constraints on the sources and pathways of subduction fluids thus provide crucial information to reconstruct subduction processes. The Monviso ophiolitic sequence is composed of mafic, ultramafic and minor sediments that have been subducted to similar to 80 km depth. In this sequence, both localized fluid flow and channelized fluids along major shear zones have been documented. We investigate the timing and source of the fluids that affected the dominant mafic rocks using microscale U-Pb dating of zircon and oxygen isotope analysis of mineral zones (garnet, zircon and antigorite) in high-pressure rocks with variable degree of metasomatic modification. In mafic eclogites, Jurassic zircon cores are the only mineralogical relicts of the protolith gabbros and retain delta O-18 values of 4.5-6 parts per thousand, typical of mantle melts. Garnet and metamorphic zircon that grew during prograde to peak metamorphism display low delta O-18 values between 0.2 and 3.8 parts per thousand, which are likely inherited from high-temperature alteration of the protolith on the sea floor. This is corroborated by delta O-18 values of 3.0 and 3.6 parts per thousand in antigorite from surrounding serpentinites. In metasomatized eclogites within the lower shear zone, garnet rim formed at the metamorphic peak shows a shift to higher delta O-18 up to 6 parts per thousand. The age of zircons in high-pressure veins and metasomatized eclogites constrains the timing of fluid flow at high pressure at around 45-46 Ma. Although the oxygen data do not contradict previous reports of interaction with serpentinite-derived fluids, the shift to isotopically heavier oxygen compositions requires contribution from sediment-derived fluids. The scarcity of metasediments in the Monviso sequence suggests that such fluids were concentrated and fluxed along the lower shear zone in a sufficient amount to modify the oxygen composition of the eclogitic minerals.
机译：流体被认为是俯冲系统中不同岩石类型之间进行化学交换的基本介质。因此，对俯冲流体的来源和途径的限制为重建俯冲过程提供了关键信息。蒙维索（Monviso）的滑石序列由镁铁质，超镁铁质和次要沉积物组成，这些沉积物已俯冲至80 km左右的深度。按此顺序，已记录了沿主要剪切带的局部流体流动和通道化流体。我们使用锆石的微量U-Pb测年和对高压岩体中的矿物区（石榴石，锆石和反蛇纹石）进行氧同位素分析，研究了变质变质程度不同的影响主镁铁质岩体的流体的时机和来源。在镁铁质榴辉岩中，侏罗纪锆石岩心是原生岩辉长岩的唯一矿物遗迹，其δO-18值为4.5-6千分之几，这是地幔熔体的典型特征。石榴石和变质锆石在逐渐发展到峰值变质过程中生长，其δO-18值较低，在千分之0.2至3.8之间，这很可能是由于海底原生岩的高温蚀变所遗传的。来自周围蛇纹石的反蛇纹石的δO-18值分别为千分之3.0和3.6，证实了这一点。在较低剪切带内的交代榴辉岩中，在变质峰上形成的石榴石边缘显示出向较高δO-18的移动，最高可达千分之六。高压静脉中锆石的年龄和交代的榴辉岩的年龄限制了在45-46 Ma左右高压下流体流动的时间。尽管氧气数据与先前与蛇纹石衍生的流体相互作用的报道并不矛盾，但向同位素更重的氧气组成的转变需要来自沉积物的流体的贡献。 Monviso序列中变质沉积物的稀少表明，这些流体被浓缩并沿下剪切带流动，流量足以改变浮生矿物的氧组成。

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