Sulfur Isotope Fractionation and Sulfur Partitioning between Apatite and Silicate Melts

太平洋边缘的火山与铜、金、铁矿的形成有着密切的关系。这些金属是我国民用和军事基础设施的重要组成部分，了解这些矿藏是如何形成的，对于确保我们不断增长的人口的可持续供应至关重要。对与自然火山有关的矿石系统的观察表明，硫元素几乎总是与铜、金和铁联系在一起，这表明硫可能在将这些金属浓缩到允许公司开采它们获利的水平方面发挥了作用。在自然界中，硫以几种不同的状态存在，这取决于氧气的含量，科学家们已经确定，硫的状态可以调节火山系统中铜、金和铁的流动性。在这个项目中，科学家们正在研究如何利用矿物磷灰石中的硫浓度来更好地限制金属沉积物的形成，并更广泛地了解是什么控制了火山系统中硫的运动。磷灰石是制造人类牙齿和骨骼的同一种矿物。科学团队包括大学教职员工、一名博士后研究员、一名家里第一个大学毕业的博士生，以及几名将与资深科学家密切合作的本科生。科学家们正在与德国汉诺威大学的教师和学生合作，他们将在那里学习新技术，然后在密歇根大学实施。这个项目的主要目标是提供限制地球硫循环的关键信息。科学家们已经确定，海洋地壳和上覆沉积物中的硫在俯冲带被回收到上地幔，部分通过弧火山作用返回地表。现有资料表明，海洋地壳的硫同位素特征与火山气体和弧岩浆有显著差异。弧岩浆通常硫含量低，但硫-34/硫-32同位素值范围广，从-0.5到+20.7 / mil不等，而海洋地壳通常更富硫，其特征是硫-34/硫-32的平均值接近0 / mil。解释这些差异的有效假设包括脱气、交代和氧化态变化。在这项研究中，我们正在测试磷灰石晶体和共存的硅酸盐熔体中的总硫含量和硫同位素丰度是否有助于阐明地球的硫循环。要解释磷灰石中的总硫和硫同位素特征，并将其应用于理解影响俯冲带岩浆挥发物的过程，一个先决条件是对磷灰石中各种氧化态硫形态的定量认识。在与俯冲带火山环境相关的条件下，我们对磷灰石和硅酸盐熔体之间的总硫和硫同位素分配进行了实验研究。该方法将平衡分馏和动态减压实验与高精度硫和硫同位素分析相结合。他们计划使用微x射线吸收近边光谱来确定单个磷灰石晶体和共存的熔融相中硫的氧化状态。实验将产生一个强大而全面的数据集，这将有助于解释自然系统中磷灰石和共存的硅酸盐熔体中的总硫和硫同位素特征。在进行实验计划的同时，我们正在测量来自活火山（马扎马火山、皮纳图博火山、默拉皮火山和奎兹卡普火山）样本的总硫和硫同位素丰度。假设磷灰石可以保存俯冲板块上方脱水熔融形成的岩浆的S（同位素和物种）信号，我们的实验数据可以根据对自然样品的分析，估计来自板块的相关流体的S（同位素）组成，确定岩浆在磷灰石形成时的氧化还原，阐明岩浆的脱气历史。

Volcanoes around the rim of the Pacific Ocean are intimately related to the formation of copper, gold, and iron ore deposits. These metals are vital components of our nation's civil and military infrastructure, and understanding how these ore deposits form is fundamental to ensuring a sustainable supply for our growing population. Observations of natural volcano-related ore systems indicate that the element sulfur is almost always associated with copper, gold and iron, which suggests that sulfur may play a role in concentrating these metals to levels that allow companies to mine them profitably. In nature, sulfur exists in several different states, depending on how much oxygen is present, and scientists have determined that the state of sulfur moderates the mobility of copper, gold and iron in volcanic systems. In this project, the scientists are investigating how to use sulfur concentrations in the mineral apatite, which is the same mineral that makes human teeth and bones, as a way to better constrain the formation of metal deposits and understand more broadly what controls the movement of sulfur in volcanic systems. The science team includes university faculty, a post-doctoral researcher, one doctoral student who is the first in his family to graduate from college, and several undergraduate students who will work closely with the senior scientists. The scientists are collaborating with faculty and students at the University of Hannover, Germany, where they will learn new techniques and then implement them at the University of Michigan.This project has as a main goal to provide critical information to constrain Earth's sulfur cycle. Scientists have determined that sulfur in the oceanic crust and overlying sediments is recycled to the upper mantle at subduction zones and partly returned to the surface via arc volcanism. Existing data demonstrate that the sulfur isotope signature of oceanic crust differs significantly from that of volcanic gases and arc magmas. Arc magmas typically have low sulfur contents, but cover a wide range of sulfur-34/sulfur-32 isotope values from -0.5 to +20.7 per mil, whereas oceanic crust is typically more sulfur-enriched and characterized by average sulfur-34/sulfur-32 values that are close to 0 per mil. Working hypotheses to explain these discrepancies include degassing, metasomatism, and variations in oxidation state. In this study, we are working to test whether or not the total sulfur content and the sulfur isotopes abundances in apatite crystals and coexisting silicate melt can help elucidate Earth's sulfur cycle. A prerequisite for the interpretation of total sulfur and sulfur isotope signatures in apatite and application of this to understanding processes affecting volatiles in subduction zone magmas is a quantitative knowledge of the sulfur speciation in apatite over a wide range of oxidation states. We are investigating experimentally total sulfur and sulfur isotope partitioning between apatite and silicate melt at conditions relevant to subduction zone volcanic environments. The proposed approach combines equilibrium fractionation and dynamic decompression experiments with high precisions sulfur and sulfur isotope analyses. They plan to use micro-Xray absorption near edge spectroscopy to determine the oxidation state of sulfur within single apatite crystals and the coexisting melt phase. The experiments will yield a robust and comprehensive dataset, which will facilitate the interpretation of total sulfur and sulfur isotope signatures in apatites and coexisting silicate melt in natural systems. Simultaneous with the experimental program, we are measuring total sulfur and sulfur isotope abundances among samples from active volcanoes (Mt. Mazama, Mt. Pinatubo, Mt. Merapi, and Quizapu). Assuming that apatites can preserve the S (isotope and speciation) signal of a magma formed due to dehydration melting above the subducted slab, our experimental data may enable the estimation of the S (isotopic) composition of the involved fluids derived from the slab, determine the redox of a magma at apatite formation and elucidate the degassing history of a magma, all based on the analyses of natural samples.