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.
