Sulfur Isotope Fractionation and Sulfur Partitioning between Apatite and Silicate Melts

环太平洋的火山与铜、金、铁矿的形成密切相关。这些金属是我国民用和军用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。对与火山有关的天然矿石系统的观察表明，元素硫几乎总是与铜、金和铁有关，这表明硫可能在将这些金属浓缩到使公司能够有利可图地开采它们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，这取决于存在多少氧气，科学家们已经确定，硫的状态调节了火山系统中铜、金和铁的活动性。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矿床的形成，并更广泛地了解是什么控制了火山系统中硫的运动。磷灰石是构成人类牙齿和骨骼的同一种矿物。科学团队包括大学教师、一名博士后研究员、一名他家第一个从大学毕业的博士生，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职员工和学生合作，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关键信息，以限制地球的硫循环。科学家们已经确定，洋壳和上覆沉积物中的硫在俯冲带回收到上地幔，并通过弧火山作用部分返回地表。已有的资料表明，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。弧状岩浆的硫含量通常很低，但硫-34/硫-32的同位素值范围很广，从-0.5到+20.7/mil，而洋壳通常硫更丰富，其特点是平均硫-34/硫-32值接近每mil 0。解释这些差异的工作假说包括脱气、交代和氧化状态的变化。在这项研究中，我们致力于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。解释磷灰石中总硫和硫同位素特征并将其应用于理解影响俯冲带岩浆中挥发分的过程的一个先决条件是对磷灰石中各种氧化状态下硫的形态进行定量了解。我们正在实验研究与俯冲带火山环境相关的条件下总硫和硫同位素在磷灰石和硅酸盐熔体之间的分配。该方法将平衡分馏和动态减压实验与高精度的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内硫的氧化状态和共存的熔体相。这些实验将产生一个强大和全面的数据集，这将有助于解释磷灰石和自然系统中共存的硅酸盐熔体中的总硫和硫同位素特征。在实验计划的同时，我们正在测量活火山(Mt.Mazama，Mt.皮纳图博，Mt.Merapi和Quizapu)。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(同位素和形态)信号，我们的实验数据可能有助于估计来自板块的相关流体的S(同位素)组成，确定磷灰石形成时岩浆的氧化还原，并阐明岩浆的脱气历史，所有这些都建立在对自然样品的分析基础上。

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.