Anhydrite in Arc Magmas and its Relationship to Sulfur Degassing and Ore Formation

在安第斯山脉、菲律宾、喀斯喀特山脉和阿留申群岛的环太平洋火山的一些天然火山岩样品中发现了结晶形式（硬石膏）和熔融气泡形式的硫酸钙。 它也在一些实验室实验中产生。 这种含硫酸盐的岩浆可能将大部分来自地幔深处的硫带到地壳浅层。 在历史上曾经爆发过弧火山的地方，富含硫酸盐的岩浆将大量的二氧化硫脱气，进入大气层，并可能造成长达两年的全球气候变冷，伴随着人类的影响。 富硫酸盐的岩浆在地壳浅层结晶成花岗岩，它们可能释放出水、高达10亿吨的硫和金属，这些金属形成了铜、钼、金、银和其他对美国经济至关重要的经济金属的大型矿床。 为了更好地了解含硫酸钙岩浆对大气和矿床中硫的贡献，拟议的研究将对岩浆中的硫酸钙进行实验室实验，并记录天然岩浆样品中保存的微小硫酸钙包裹体的存在。 第一种方法包括对氧化和富水弧岩浆中硫酸盐的稳定性进行岩石学实验，这些实验计划在地壳底部（~1GPa）到上地壳（~ 50 MPa）的压力范围内进行。 对玄武安山岩的高压实验将提供关于弧岩浆中硫通过地壳的转移以及富硫酸钙熔融相的稳定性、性质和组成的信息。 最初的实验已经产生了这种不混溶的Ca-SO 4-Na-Mg-Cl熔体。 对硅酸盐和硫酸盐熔体相的分析将直接确定硫、氯和其他物质（包括矿石金属如金）的分配系数数据。 英安岩的低压实验将在含水流体中饱和，可以测量所有三相（硅酸盐熔体、硬石膏、含水流体）中的硫浓度并确定分配系数。直接测量一个或两个水相流体中的硫含量，将使我们的激光烧蚀-电感耦合等离子体质谱（ICP-MS）的流体包裹体现场捕获在破碎的石英。 第二种方法是使用现代成像技术（SEM，QEMSCAN，EMPA元素映射），以搜索弧火山岩和深成岩样品中的硫酸盐矿物的微小包裹体的强大的斑晶矿物，如磷灰石，钛，石英，角闪石，辉石和长石。 一个特别的重点将是斑岩铜矿的深成样品，以了解这些岩浆最初是否含有硫酸钙。 这项工作的数据将允许改进和定量建模岩浆冷却，结晶和减压，导致岩浆硫酸盐的分解和随后的硫物质（SO2，H2S）转移到一个单独的富水流体相，可以在火山爆发期间进入大气或在地壳中与含水流体中的金属反应，形成经济的矿藏。

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.