Using wet pyroxenes to reconstruct magma degassing history

本文提出了一种新的方法，通过测量辉石中H+和Al的含量来量化弧中母基性岩浆的含水量。该方法利用了熔体-辉石对水的分配系数与实验确定的辉石Al含量之间的线性关系。在过去，这种方法主要应用于地幔捕虏体来推断地幔的含水量，除了一项针对中美洲火山岩的研究。在Soufriere Hills火山和许多其他处于弧形环境中的中硅酸火山，人们认为岩浆入侵触发并维持了喷发。在SHV，基性岩浆向富含晶体的安山岩提供硫和热量，引发对流自混合和喷发。气体排放和地面变形数据集都表明岩浆在6-12千米深的地方侵入，岩石学研究显示，在喷发的产物中出现了条纹状和带状安山岩，或圆形包裹体，这表明在两个端段岩浆之间发生了“混合”，而不是混合。如果侵入岩浆具有较高的挥发性含量，它将在岩浆界面淬火和脱气时为系统提供大量的气相。流体含量高的岩浆具有高度可压缩性，可以抑制地面变形信号。在氧化系统中，硫在深处强烈地分解成蒸汽，因此高流体含量更有可能从熔体中“剥离”出硫，使其分离，并使其迁移到表面或引起爆炸性喷发。本研究旨在比较基性岩浆（以包体形式出现）中的辉石岩与安山岩中的辉石岩，以评估两个端部岩浆喷发前的H2O含量，并进一步评估是否有可能通过辉石岩剖面和扩散损失来评估熔融水含量。研究结果将对一般的弧岩浆具有广泛的意义。

This proposal seeks to quantify the water content of parental mafic magmas in arcs, using a new method involving the measurement of H+ and Al in pyroxenes. The method utilises the linear relationship between the melt-pyroxene partition coefficient for water, and the Al content of the pyroxene, as determined experimentally. In the past, this method has been applied primarily to mantle xenoliths for inferring the water content of the mantle, with the exception of one study which looked at volcanic rocks from Central America. At Soufriere Hills Volcano and many other intermediate-silicic volcanoes in arc settings, mafic intrusion is thought to trigger and sustain eruptions. At SHV, the mafic magma supplies sulphur and heat to the crystal-rich resident andesite, provoking convective self-mixing and eruption. Gas emissions and ground deformation datasets all point to intrusion of magma at 6-12 km depth, and petrological studies show streaked and banded andesites, or rounded enclaves in the erupted products, suggesting that 'mingling', rather than mixing occurs between the two end member magmas. If the intruding magma has a high volatile content, it will supply a significant vapour phase to the system on quenching and degassing at the interface between the magmas. Magma with a high fluid content is highly compressible, damping ground deformation signals. Sulphur partitions strongly into vapour at depth in oxidised systems, so a high fluid content has a higher potential for 'stripping out' sulphur from the melt, segregating it and allowing it to migrate to the surface or to cause explosive eruption. This proposal seeks to compare pyroxenes from mafic magma (which occurs as enclaves) with those from the resident andesite, to assess the pre-eruptive H2O contents of both end member magmas and further, to evaluate whether it is possible to assess melt water contents through time using profiles across the pyroxenes and accounting for diffusive loss. The results will have broad implications for arc magmas in general.