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

在菲律宾安第斯山脉、喀斯喀特山脉和阿留申山脉的环太平洋火山的几个天然火山岩样品中，发现了结晶形式(硬石膏)和熔化气泡的硫酸钙。它也已经在几个实验室实验中被生产出来。这种含有硫酸盐的岩浆可能会将地球地幔深部来源的大部分硫带到地壳浅层。在历史上弧形火山喷发的地方，富含硫酸盐的岩浆将大量进入大气的二氧化硫脱气，并可能产生长达两年的全球气候降温，并伴随着人类的影响。在富含硫酸盐的岩浆结晶成浅地壳花岗岩的地方，它们可能释放出水，高达10亿吨的硫，以及导致铜、钼、金、银和其他对美国经济至关重要的经济金属大型矿藏形成的金属。为了更好地了解含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，并记录保存在天然岩浆样品中的微小硫酸钙包裹体的存在。第一种方法包括关于硫酸盐在氧化和富水的弧形岩浆中稳定性的岩石学实验，这些岩浆计划在从地壳底部(~1 Gpa)到地壳上部(~50 MPa)的压力范围内进行。对玄武岩安山岩的高压实验将提供有关弧状岩浆中硫在地壳中的转移以及富钙熔体相的稳定性、性质和组成的信息。初步实验已经产生了这样一种不相容的Ca-SO4-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、氯和包括金等矿石金属在内的其他物种的分配系数数据。对英安岩的低压实验将在水中饱和，可以测量所有三相(硅酸盐熔体、硬石膏和水流体)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，将使激光烧蚀-电感耦合等离子体质谱(ICPMS)用于原位捕获裂隙石英中的流体包裹体。第二种方法是利用现代成像技术(SEM、QEMSCAN、EMPA元素图)在弧形火山和深成岩样中寻找粗壮斑晶矿物(如磷灰石、钛铁矿、石英、角闪石、辉石和长石)中硫酸盐矿物的微量包裹体。一个特别的焦点将是来自斑岩铜矿床的深成岩样，以了解这些岩浆最初是否含有硫酸钙。这项工作的数据将有助于改进和定量模拟岩浆冷却、结晶和降压，导致岩浆硫酸盐的分解，并随后将硫物种(SO2、H2S)转移到单独的富水流体相，该流体相可能在火山喷发期间进入大气，或者在地壳与水溶液中的金属反应，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.