Calcium sulfate in crystalline form (anhydrite) and as melt blebs has been found in a few natural samples of volcanic rocks from circum-Pacific volcanoes in the Andes, Philippines, Cascades, and Aleutians. It has also been produced in a few laboratory experiments. Such sulfate-bearing magma potentially carried much of the sulfur from deep sources in the Earth's mantle to the shallow crust. Where arc volcanoes have erupted historically, sulfate-rich magmas have degassed abundant sulfur dioxide that enters the atmosphere and may produce up to 2 years of global climatic cooling with attendant human impact. Where sulfate-rich magmas crystallize to granite in the shallow crust they may release water, up to one gigaton of sulfur, and metals that are responsible for formation of large mineral deposits of copper, molybdenum, gold, silver and other economic metals critical to the USA economy. In order to understand better the contributions of calcium sulfate-bearing magmas to sulfur in the atmosphere and mineral deposits, the proposed research will undertake laboratory experiments on calcium sulfate in magmas, and document the occurrence of minute calcium sulfate inclusions preserved in natural magmatic samples.
The first approach includes petrologic experiments on the stability of sulfate in oxidized and water-rich arc magmas that are planned at pressures ranging from the base of the crust (~1GPa) to the upper crust (~50MPa). High pressure experiments on basaltic andesite will provide information on the transfer of sulfur in arc magmas through the crust, and the stability, nature and composition of calcium sulfate-rich melt phase. Initial experiments have produced such an immiscible Ca-SO4-Na-Mg-Cl melt. Analysis of both the silicate and sulfate melt phases will directly determine partition coefficient data for sulfur, chlorine and other species including ore metals such as gold. Low pressure experiments on dacite will be saturated in aqueous fluid, and the concentrations of sulfur in all three phases (silicate melt, anhydrite, aqueous fluid) can be measured and partition coefficients can be determined. Direct measurement of the sulfur content of the one or two aqueous fluid phases will make us of laser ablation-ICP-MS on fluid inclusions trapped in situ in fractured quartz. The second approach is to use modern imaging techniques (SEM, QEMSCAN, EMPA element mapping) to search arc volcanic and plutonic samples for minute inclusions of sulfate minerals in robust phenocryst minerals such as apatite, titanite, quartz, hornblende, pyroxene, and feldspar. A special focus will be plutonic samples from porphyry copper deposits to learn whether or not these magmas were initially calcium sulfate-bearing. The data from this work will allow improved and quantitative modeling magmatic cooling, crystallization and depressurization that causes the breakdown of magmatic sulfate and consequent transfer sulfur species (SO2, H2S) to a separate water-rich fluid phase that may either enter the atmosphere during volcanic eruptions or react in the crustal with metals in the aqueous fluid to form economic mineral deposits.
