玄武岩浆对蛇纹化地幔的快速同化作用
Fast assimilation of serpentinized mantle by basaltic magma
摘要：
最丰富的陆地熔岩，洋中脊和洋岛玄武岩（MORB和OIB），通常被认为是来自亏损的MORB地幔成分和更具体的，富集地幔柱源。然而，从熔体中发出的大洋熔岩和镁铁质堆晶，富含氯，并具有放射性的Sr比的发现，可以归因于软流圈衍生的熔体和岩石圈橄榄岩之间的相互作用，由于热液水渗透到莫霍面以下。为了解释这种相互作用的机制和速率，我们报道了蛇纹岩和拉斑玄武岩熔体之间反应的实验结果。结果表明，该反应是通过多阶段机制进行的：（i）蛇纹岩转化为含孔隙流体的富铬尖晶石方辉橄榄岩，（ii）方辉橄榄岩组合部分熔融溶解形成填隙式含水熔体，（iii）富铬尖晶石方辉橄榄岩/纯橄榄岩最终同化形成高MgO和高Cr、Ni含量的混合玄武质熔体。蛇纹岩被玄武质熔体同化可能发生在熔体/岩石比升高的情况下，并可能导致铬铁矿的形成。我们的实验提供的证据表明，MORB和高镁铬斜方辉石丰富的累积亏损的不相容元素，可以从常见的洋中脊玄武岩熔体与水化岩石圈橄榄岩反应修改。我们建立了蛇纹橄榄岩同化的速率是由二氧化硅扩散反应含水玄武质熔体。
英文摘要：
The most abundant terrestrial lavas, mid-ocean ridge and ocean island basalt (MORB and OIB), are commonly considered to be derived from a depleted MORB-mantle component and more specific, variably enriched mantle plume sources. However, findings of oceanic lavas and mafic cumulates issued from melts, enriched in chlorine and having a radiogenic Sr ratio, can be attributed to an interaction between the asthenosphere-derived melts and lithospheric peridotite variably hydrated due to penetration of hydrothermal water down to and below Moho level. To constrain mechanisms and rates responsible for the interaction, we report results of experiments of reaction between serpentinite and tholeiitic basaltic melt. Results show that the reaction proceeds via a multi-stage mechanism: (i) transformation of serpentinite into Cr-rich spinel-bearing harzburgite containing pore fluid, (ii) partial melting and dissolution of the harzburgite assemblage with formation of interstitial hydrous melts, and (iii) final assimilation of the Cr-rich spinel-bearing harzburgite/dunite and formation of hybrid basaltic melts with high MgO and elevated Cr and Ni contents. Assimilation of serpentinite by basaltic melt may occur under elevated melt/rock ratios and may lead to chromitite formation. Our experiments provide evidence that MORB and high-Mg-Cr orthopyroxene-rich cumulates depleted in incompatible elements can be produced from common mid-ocean ridge basaltic melts modified by reaction with hydrated lithospheric peridotite. We established that the rate of assimilation of serpentinized peridotite is controlled by silica diffusion in the reacting hydrous basaltic melt.
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