Near-fractional Melting of Pyroxenite: Experimental Investigations and Applications to Basalt Petrogenesis

构成地壳的岩石受到生命循环的影响——新地壳通过地幔（地壳下面的物质）的融化和岩浆向地表的运输在分散的板块边界（扩张中心）形成，而旧地壳则在收敛的板块边界沉入地幔。具体来说，海洋地壳向地幔的回归产生了化学和矿物学上的非均质性（富含橄榄石和辉石岩的岩性的混合物），当这些物质在扩张中心和地幔柱下再次上升时，会导致复杂的熔融行为。地壳旋回的丰度和时间尺度是板块构造旋回的重要方面，但很难仅根据地壳岩石的岩石学和地球化学来加以限制。定量建模需要了解不同岩石类型的熔化行为。辉石岩是地幔中最重要的岩石类型之一，本研究的目的是获得新的高质量的辉石岩熔融实验数据。实验将在地幔条件下进行，那里的温度比地球表面高出数千度，压力是地球表面的数万倍。所得结果将被整合到非均质地幔熔融正演模型中，模拟在世界扩张中心和地幔柱下运行的动力学过程。该项目的主要目的是了解辉石岩近分数熔融的影响，以更好地量化辉石岩在玄武岩成因中的作用。实验方法将包括进行增量熔化实验，在每个阶段，熔体分数的增量小于5 wt. %，对以前通过批量熔化实验研究的三种不同成分。所选择的成分跨越了地幔中辉石岩岩性的可能成分范围。每个温度增量下的熔体成分将通过微脉技术确定，并用于定义新的熔融参数化，这些参数化可用于模拟近分数熔融，并将其整合到非均质地幔熔融的正演模型中。用于确定典型玄武岩岩浆及其源区辉石岩贡献的NiO和MnO指标将根据实验结果重新校准。由于MORB和洋岛玄武岩形成过程中的熔体提取很可能是通过近分次熔融而不是批量熔融发生的，这些新的校准将使我们能够更好地量化地球上最丰富岩浆的地幔源中辉石岩的比例。

The rocks that make up Earth's crust are subject to a cycle of life - new crust is formed at divergent plate boundaries (spreading centers) via the melting of the mantle (material beneath the crust) and transport of magma to the surface, while old crust sinks back into the mantle at convergent plate boundaries. Specifically, the return of oceanic crust to the mantle creates chemical and mineralogical heterogeneities (mixtures of olivine-rich and pyroxene-rich lithologies) that can lead to complex melting behavior when this material rises again beneath spreading centers and in mantle plumes. The abundance and time scales of crustal recycling are important aspects of the plate tectonic cycle, but difficult to constrain based solely on the petrology and geochemistry of crustal rocks. Quantitative modeling requires knowledge of melting behavior of different rock types. The goal of this research is to obtain new high quality experimental data on the melting of pyroxenite, which represents one the most important rock types that are recycled into the mantle. Experiments will be performed at conditions found in the mantle where temperatures are thousands of degrees hotter and pressures tens of thousands of times greater than found at the Earth's surface. The results obtained will be integrated into forward models of melting of heterogeneous mantle that simulate the dynamic processes operating beneath the world's spreading centers and in mantle plumes. The primary objective of this project is to understand the effect of near-fractional melting of pyroxenites, to better quantify the role of pyroxenite in basalt genesis. The experimental approach will consist of performing incremental melting experiments, with increments of melt fraction less than 5 wt. % at each stage, on three different compositions previously investigated by batch melting experiments. The selected compositions span the likely compositional range of pyroxenite lithologies in the mantle. Melt composition at each temperature increment will be determined with the microdike technique and used to define new melting parameterizations that can be used to model near-fractional melting and integrate them into forward models of melting of heterogeneous mantle. NiO and MnO proxies used to determine the pyroxenite contribution in typical basaltic magmas and their source regions will be recalibrated based on the experimental results. Because melt extraction during generation of MORB and ocean island basalts is likely to occur via near fractional melting rather than batch melting, these new calibrations will allow us to better quantify the proportion of pyroxenite in the mantle source of the most abundant magmas on Earth.