Reactive melt migration in the lower oceanic crust and its implications for the evolution of mid-ocean ridge basalt

大洋中脊玄武岩(MORB)是地球上最丰富的岩浆。它是在大洋中脊之下通过上涌地幔减压熔融而产生的，在较低的地壳岩浆室中处理后，喷发到海底。四十多年来，火成岩岩石学家和地球化学家一直依赖MORB作为了解地幔的主要窗口，从喷发的熔岩中推导出其组成、熔融过程和熔体迁移机制。然而，这种方法假定地壳岩浆室中熔体的变质完全是通过分步结晶作用发生的，因此可以很容易地进行修正。在过去的十年里，有证据表明，熔体在穿过大洋中脊岩浆室时，可能会与现有的积云晶体发生广泛的反应。这一过程可以强烈地改变熔体的主元素和微量元素组成，导致与分步结晶趋势的显著偏差。在最近的一篇论文中，我第一个重建了下地壳反应性熔体迁移对岩浆成分的潜在影响，认为这一过程可能会向MORB传递强烈的化学信号。如果是这样的话，这就需要从根本上重新评估MORB的岩石成因及其作为地幔信使的用途。因此，为了了解MORB的岩石成因及其对地幔研究的意义，需要一个充分描述海洋岩浆室中反应流动的性质和程度的可靠模型。然而，到目前为止，还没有对反应熔体迁移进行系统的研究，数据仅限于来自两个地点的少量样品。因此，没有数据有力地评估反应熔体迁移的程度、性质和空间分布，这阻碍了对MORB岩石成因及其用于研究地幔的了解。这一建议旨在确定大洋中脊岩浆室中的反应性熔体迁移的性质和程度，并验证其在MORB岩石成因中发挥重要作用的假设。这将通过对以前被证明是古代大洋中脊岩浆室(735B孔，西南印度洋海脊)内的完整、离散侵入体进行系统、高分辨率的研究来实现。我将获得广泛的结构和矿物化学分析，获得关于下部洋壳反应熔体迁移的第一个系统数据集。根据岩石记录推断出的熔体-岩石反应历史将被建模，从而使其在MORB演化中的作用得以量化。综合起来，这些数据和模型将提供一种前所未有的视角，了解大洋中脊岩浆室中的反应性熔体迁移及其在MORB演化中的作用。最终，这将决定MORB作为地幔属性和过程记录器的保真度。

Mid-ocean ridge basalt (MORB) is the most abundant magma on Earth. It is generated beneath mid-ocean ridges by decompression melting of upwelling mantle, and, following processing in lower crustal magma chambers, erupted onto the seafloor. For more then four decades igneous petrologists and geochemists have relied upon MORB as their major window into the mantle, deriving its composition, melting processes and melt migration mechanisms from the erupted lavas. However, this approach assumes that modification of melts in crustal magma chambers occurs exclusively by fractional crystallisation, and can thus be easily corrected for. Within the last decade evidence has emerged that melt may react extensively with existing cumulus crystals as it migrates through mid-ocean ridge magma chambers. This process can strongly modify both the major and trace element compositions of the melt, leading to significant deviations from fractional crystallisation trends. In a recent paper, I was the first to reconstruct the potential effect of reactive melt migration in the lower crust on magma compositions, arguing that the process may impart strong chemical signals on MORB. If so, this requires a fundamental reassessment of MORB petrogenesis and its use as a messenger from the mantle. Thus, in order to understand MORB petrogenesis, and its implications for mantle studies, a robust model that fully characterises the nature and extent of reactive flow in oceanic magma chambers is required. However, no systematic study of reactive melt migration has been done to date, with data limited to a small number of samples from only two locations. As a result, no data exist to robustly assess the extent, nature and spatial distribution of reactive melt migration, hampering an understanding of MORB petrogenesis and its use for studying the mantle. This proposal seeks to determine the nature and extent of reactive melt migration in mid-ocean ridge magma chambers, and test the hypothesis that it plays an important role in MORB petrogenesis. This will be achieved by conducting a systematic, high-resolution study of what has previously been shown to represent a complete, discrete intrusion within an ancient mid-ocean ridge magma chamber (ODP Hole 735B, Southwest Indian Ridge). I will obtain extensive textural and mineral chemical analyses, acquiring the first systematic dataset on reactive melt migration in the lower oceanic crust. The melt-rock reaction history as deduced from the rock record will then be modelled, allowing its role in MORB evolution to be quantified. Combined, the data and models will provide an unprecedented view of reactive melt migration through mid-ocean ridge magma chambers and its role in the evolution of MORB. Ultimately, this will determine the fidelity of MORB as recorders of mantle properties and processes.