Sulfur Isotope Fractionation and Sulfur Partitioning between Apatite and Silicate Melts

太平洋边缘的火山与铜、金和铁矿床的形成密切相关。这些金属是我国民用和军事基础设施的重要组成部分，了解这些矿床的形成方式对于确保我们不断增长的人口的可持续供应至关重要。对与火山有关的天然矿石系统的观察表明，硫元素几乎总是与铜、金和铁有关，这表明硫可能在将这些金属浓缩到允许公司有利可图的水平方面发挥作用。在自然界中，硫以几种不同的状态存在，这取决于存在的氧气量，科学家已经确定硫的状态会缓和火山系统中铜，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，磷灰石是制造人类牙齿和骨骼的同一种矿物，作为一种更好地限制金属沉积物形成的方法，并更广泛地了解是什么控制了火山系统中硫的运动。科学团队包括大学教师，一名博士后研究员，一名博士生，他是家里第一个从大学毕业的人，还有几名本科生，他们将与资深科学家密切合作。科学家们正在与德国汉诺威大学的师生合作，在那里他们将学习新技术，然后在密歇根大学实施。该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再循环到上地幔，并通过弧火山作用部分返回到地表。 现有资料表明，洋壳的硫同位素特征与火山气体和弧岩浆的硫同位素特征有很大不同。弧岩浆的硫含量通常较低，但硫-34/硫-32同位素值的范围很广，从-0.5到+20.7每密耳，而洋壳通常更富硫，其特征是平均硫-34/硫-32值接近0每密耳。解释这些差异的工作假设包括脱气，交代作用和氧化态的变化。在这项研究中，我们正在努力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。磷灰石中的总硫和硫同位素特征的解释和应用，以了解俯冲带岩浆中的挥发物的影响过程的先决条件是在广泛的氧化态磷灰石中的硫形态的定量知识。我们正在实验研究总硫和硫同位素分配之间的磷灰石和硅酸盐熔体在有关俯冲带火山环境的条件。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射线吸收近边光谱法来确定单个磷灰石晶体和共存熔体相内硫的氧化态。这些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硅酸盐熔体中的总硫和硫同位素特征。与实验计划同时，我们正在测量活火山（Mt.马扎马山皮纳图博山Merapi和Quizapu）。假设磷灰石可以保存的S（同位素和物种形成）信号的岩浆形成的脱水熔融以上俯冲板，我们的实验数据可以使估计的S（同位素）组成的有关流体来自板，确定在磷灰石形成的岩浆的氧化还原和阐明岩浆的脱气历史，所有基于天然样品的分析。

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.