A reactive porous flow control on mid-ocean ridge magmatic evolution
A reactive porous flow control on mid-ocean ridge magmatic evolution
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DOI：
10.1093/petrology/egw074
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发表时间：
2016-11
影响因子：
3.9
通讯作者：
C. Lissenberg;C. MacLeod
中科院分区：
文献类型：
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作者：
C. Lissenberg;C. MacLeod
洋中脊玄武岩（MORB）提供了地球上地幔成分和熔融过程的基本信息。为了利用MORB来进一步了解地幔，必须很好地了解它们的地壳演化，从而在估计原始熔体成分时可以解释。在这里，我们提出了发生反应性多孔流的证据，即迁移熔体与洋中脊岩浆房中的晶体糊反应。这一证据包括从下洋壳回收的岩石的结构和矿物主要和微量元素地球化学，并发生在一系列尺度上。反应结构包括矿物中的溶解前沿、不同相之间的不规则晶界和单斜辉石-棕色角闪石合片岩。然而，一个重要的发现是，反应，即使是普遍的，也同样不会留下任何纹理证据。从地球化学角度来看，反应性多孔流导致矿物模式（例如单斜辉石对橄榄石的净置换）和成分（例如单斜辉石Mg-Ti-Cr关系）偏离分离结晶预测的模式。此外，单斜辉石微量元素记录了一个渐进的核心边缘过度富集（相对于分离结晶）的多到少不相容的元素作为反应性多孔流的结果。事实上，这种过度富集发生在一个距离高达8毫米，单斜辉石显示这种签名保存分区的铁镁，排除了微量元素分布的扩散控制。相反，它可以解释为在晶体糊状物中的晶体熔化反应。这些数据表明，反应流不仅发生在颗粒尺度上，而且还发生在样品尺度上，它可以将一种岩石类型转变为另一种岩石类型[例如，橄榄岩到橄榄辉长岩，橄榄辉长岩到（氧化物）辉长岩]，并延伸到整个下洋壳的尺度。熔体经历这些反应过程中的组合物的变化，这可以解释MORB组合物的主要元素和微量元素阵列。特别是，反应性多孔流可以解释MORB MgO-CaO-Al 2 O3关系，以前被解释为高压（高达8 kbar）晶体分馏的结果，以及与分离结晶的影响相比，不相容元素的过度富集。大洋中脊岩浆房中反应性多孔流的重要作用的发现非常符合地球物理学证据，即这些岩浆房即使在最快的扩张速度下也由晶体糊主导，并与晶体糊行为的模型预测非常吻合。总之，这些观察表明，反应性多孔流是一个共同的，如果不是无处不在，糊状岩浆房固有的过程，它有一个显着的控制洋中脊岩浆演化。
Mid-ocean ridge basalts (MORB) provide fundamental information about the composition and melting processes in the Earth’s upper mantle. To use MORB to further our understanding of the mantle, is imperative that their crustal evolution is well understood and can thus be accounted for when estimating primary melt compositions. Here, we present the evidence for the occurrence of reactive porous flow, whereby migrating melts react with a crystal mush in mid-ocean ridge magma chambers. This evidence comprises both the textures and mineral major and trace element geochemistry of rocks recovered from the lower oceanic crust, and occurs on a range of scales. Reaction textures include dissolution fronts in minerals, ragged grain boundaries between different phases and clinopyroxene–brown amphibole symplectites. However, an important finding is that reaction, even when pervasive, can equally leave no textural evidence. Geochemically, reactive porous flow leads to shifts in mineral modes (e.g. the net replacement of olivine by clinopyroxene) and compositions (e.g. clinopyroxene Mg–Ti–Cr relationships) away from those predicted by fractional crystallization. Furthermore, clinopyroxene trace elements record a progressive core–rim over-enrichment (relative to fractional crystallization) of more-to-less incompatible elements as a result of reactive porous flow. The fact that this over-enrichment occurs over a distance of up to 8mm, and that clinopyroxenes showing this signature preserve zoning in Fe–Mg, rules out a diffusion control on trace element distributions. Instead, it can be explained by crystal–melt reactions in a crystal mush. The data indicate that reactive flow occurs not only on a grain scale, but also on a sample scale, where it can transform one rock type into another [e.g. troctolite to olivine gabbro, olivine gabbro to (oxide) gabbro], and extends to the scale of the entire lower oceanic crust. Melts undergoing these reactive processes change in composition, which can explain both the major element and trace element arrays of MORB compositions. In particular, reactive porous flow can account for the MORB MgO–CaO–Al2O3 relationships that have previously been interpreted as a result of high-pressure (up to �8 kbar) crystal fractionation, and for over-enrichment in incompatible elements when compared with the effects of fractional crystallization. The finding of a significant role for reactive porous flow in mid-ocean ridge magma chambers fits very well with the geophysical evidence that these magma chambers are dominated by crystal mush even at the fastest spreading rates, and with model predictions of the behaviour of crystal mushes. Together, these observations indicate that reactive porous flow is a common, if not ubiquitous, process inherent to mushy magma chambers, and that it has a significant control on mid-ocean ridge magmatic evolution.