Using wet pyroxenes to reconstruct magma degassing history

这项建议旨在量化的水含量的母岩浆弧，使用一种新的方法，涉及测量的H+和铝的辉石。该方法利用水的熔体-辉石分配系数和辉石的Al含量之间的线性关系，如实验所确定的。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，除了一项研究，该研究着眼于中美洲的火山岩。在苏弗里埃山火山和许多其他弧环境中的中温火山，基性岩侵入被认为是触发和维持喷发的原因。在SHV，镁铁质岩浆提供硫和热量的晶体丰富的居民安山岩，挑起对流自混合和喷发。气体排放和地面变形数据集都指向6-12公里深处的岩浆侵入，岩石学研究表明，条纹和带状安山岩，或圆形包体的喷发产物，这表明“混合”，而不是混合发生在两个端元岩浆之间。如果侵入岩浆具有高挥发分含量，它将在岩浆之间的界面处淬火和脱气时向系统提供重要的气相。流体含量高的岩浆具有高度可压缩性，从而衰减地面变形信号。硫在氧化系统的深处强烈地分裂成蒸汽，因此高流体含量具有更高的从熔体中“剥离”硫的可能性，使其分离并允许其迁移到表面或引起爆炸性喷发。本提案旨在比较辉石从镁铁质岩浆（发生的包体）与居民安山岩，以评估喷发前的H2O含量的两个端员岩浆，并进一步，以评估是否有可能评估通过时间使用剖面的辉石和占扩散损失的融水含量。这些结果将对一般的弧岩浆产生广泛的影响。

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.