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辅助矿物，从许多岩浆类型中结晶出来。当磷灰石结晶时，除了磷和钙之外，它还含有少量的元素，这是磷灰石的主要成分。硫和稀土元素通过化学取代进入磷灰石结构，其浓度反映了这些元素在岩浆中的含量。本研究将探讨硫在磷灰石中的吸收，以及吸收在多大程度上不仅取决于熔体中的硫浓度，还取决于溶解蒸气泡中的硫浓度。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层中硫浓度的变化。本研究将集中于1991年菲律宾皮纳图博火山喷发所记录的岩浆系统。1991年的皮纳图博火山喷发非常引人注目，非常适合作为这个项目的研究案例，因为它向大气中释放了大量的硫气体，这些气体可能以单独的蒸汽相储存起来。这项研究将在之前的研究的基础上进行，该研究确定了一些天然磷灰石，与从实验数据中预测的硫溶解度和分配系数相比，这些磷灰石似乎过度富集硫。目前尚不清楚是什么过程导致了硅质岩浆中如此高硫磷灰石的形成。了解这一点对于使用磷灰石作为岩浆硫的监测至关重要，并且可能是解释经常观察到的磷灰石中硫含量的粒间和粒内变化的缺失环节。PI和位于美国地质调查局、德国弗莱堡大学和瑞士联邦理工学院（ETH, z<e:1> rich）的合作者将调查1991年皮纳图博火山喷发的熔岩中是否存在高硫磷灰石。他们试图验证这样一种观点，即皮纳图博火山的硅熔体中存在的高硫磷灰石受到了来自地幔下基性岩浆的富硫流体的影响。为了验证这一假设，他们将获得Pinatubo岩浆中天然磷灰石的主要、次要和微量元素的综合数据集，并将以Pinatubo英安岩作为起始材料进行实验。最终目标是评估高硫磷灰石何时何地结晶，以及这些磷灰石是否确实相对于它们的宿主熔体成分过度富集。

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.