Testing In-situ Crystallization of High-S Apatite in Silicic Magmas at Pinatubo Volcano: Key for Developing Apatite as Monitor for Magmatic Sulfur

磷灰石是一种普遍存在的副矿物，由多种岩浆类型结晶而成。当磷灰石结晶时，除了磷和钙之外，它还含有少量元素，磷和钙是这种矿物的主要成分。硫和稀土元素通过化学取代作用结合到磷灰石结构中，其浓度反映了这些元素在岩浆中的含量。这项研究将研究硫在磷灰石中的吸收，以及这种吸收在多大程度上不仅取决于熔体中的硫浓度，还取决于挥发出的汽泡中的硫浓度。该项目的总体目标是利用磷灰石中的硫浓度来推断岩浆储集层中硫浓度的变化。这项研究将集中在1991年顶峰喷发所记录的菲律宾皮纳图博火山的岩浆系统。1991年的皮纳图博喷发是值得注意的，非常适合作为该项目的案例研究，因为它向大气中释放了过量的硫磺气体，这些气体可能以单独的蒸汽相的形式储存在大气中。这项研究将继续调查人员之前的一项研究，该研究发现，与根据实验数据预测的硫的溶解度和在这种矿物中的分配系数相比，一些天然磷灰石似乎硫含量过高。目前尚不清楚是什么过程导致硅质岩浆中出现如此高的S磷灰石。了解这一点对于使用磷灰石作为岩浆硫的监测至关重要，并可能是解释磷灰石中硫含量经常观察到的颗粒间和颗粒内变化的缺失环节。位于德国弗莱堡大学美国地质调查局和瑞士苏黎世联邦理工学院的PI及其合作者将调查1991年皮纳图博火山喷发熔岩中高S磷灰石的赋存情况。他们试图测试这一命题，即存在于皮纳图博山硅质熔体中的高S磷灰石受到来自底板镁铁质岩浆发出的富硫流体的影响。为了验证这一假设，他们将从皮纳图博岩浆中获得自然形成的磷灰石的综合常量、次要和微量元素数据集，并将以皮纳图博英安岩为原料进行实验。最终目标是评估高硫磷灰石在何时何地结晶，以及这些磷灰石相对于其宿主熔体成分是否确实过度浓缩。

Apatite is a ubiquitous accessory mineral that crystallizes from many magma types. As apatite crystallizes, it incorporates minor elements in addition to phosphorus and calcium that are the main constituents of this mineral. Sulfur and rare earth elements are incorporated into the apatite structure by chemical substitutions, in concentrations that reflect the contents of these elements in the magmas. This study will investigate the uptake of sulfur into apatite and to what extent the uptake is not only dependent on the sulfur concentration in the melt but also on the concentration of sulfur in exsolved vapor bubbles. The overall aim of the project is to enable the use of sulfur concentration in the apatite to infer changing sulfur concentrations in the magmatic reservoirs. This study will be focused on the magmatic system of the Pinatubo volcano, Philippines as recorded by the climactic 1991 eruption. The 1991 Pinatubo eruption is remarkable and well suited as case-study for this project because it released excessive amounts of sulfur gases to the atmosphere that were probably stored as a separate vapor phase.This study will follow up on a prior study by the investigator that identified some natural apatites that appeared to be overly sulfur enriched compared to what is predicted from the experimental data for sulfur solubility and partition coefficients into this mineral. It is unknown what process could lead to such high-S apatite in silicic magma. Understanding this is of critical importance in the use of apatite as monitor of magmatic sulfur and may be the missing link in explaining the often observed inter- and intra-grain variability of sulfur content in apatite. The PI and collaborators located at the USGS, University of Freiburg, Germany, and the Swiss Federal Institute of Technology, (ETH, Zürich), Switzerland will investigate the occurrence of high-S apatite in lavas of the 1991 Pinatubo eruption. They seek to test the proposition that high-S apatites that are present in silicic melts at Mount Pinatubo were affected by input from sulfur-rich fluids emanating from underplated mafic magmas. In order to test this hypotheses, they will acquire a comprehensive major, minor and trace element data set on naturally occurring apatite from Pinatubo magmas and will perform experiments using Pinatubo dacite as starting material. The ultimate goal is to evaluate when and where the high sulfur apatite crystallized and whether these apatites are indeed overly enriched relative to their host melt composition.