| 英文摘要: | 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. |