Collaborative Research: Mass Independent Fractionation of Sulfur Isotopes, Re-Os Dating of Sedimentary Pyrite, and the Composition of the Late Archean Atmosphere

摘要2.45Ga前硫化物和硫酸盐中硫同位素质量独立分数（MIF）的发现为研究大气-海洋系统的化学演化打开了一扇新的窗口。目前，太古代沉积物中硫的MIF数据较少，且相互矛盾。我们建议进行一个深入的研究，硫的MIF在黄铁矿的三对高度碳质沉积岩单位沉积在2.7和2.5 Ga在西澳大利亚州，南非和加拿大。黄铁矿将被选择为这项研究的基础上的结构，化学和同位素证据表明同生或成岩起源。六个岩石单元中的黄铁矿将采用Re-Os法测定年代。初步资料表明，对2.3Ga沉积黄铁矿的年龄测定精度可达百万年。从黄铁矿等时线推断的初始187 Os/188 Os比值将提供海水中Os同位素组成长期变化的信息，这可能反映了氧化风化和大陆Os运输的变化。因此，太古代沉积黄铁矿的定年可以产生关于Os循环的重要信息。MIF测量将在华盛顿的地球物理实验室的MAT 253质谱仪上进行，使用激光扫描和连续流入口技术。Re-Os测年将在科罗拉多州立大学（CSU）的AIRIE实验室使用Carius管溶解和负热电离质谱法进行。样品制备、显微镜检查和微量元素分析将在哈佛大学完成。我们希望我们的数据将表明，在何种程度上，当地条件决定了这些岩石单元中硫同位素的MIF，在何种程度上，全球差异是重要的，以及全球差异在多大程度上反映了太古代最后200 Ma大气成分的变化。更广泛的影响标准该项目将通过各级学生（博士后、研究生、本科生和高中生）的参与促进培训和学习，至少年轻学生来自代表性不足的群体。所有人都将参与最先进技术的应用，并鼓励他们在专业会议上介绍他们的研究结果。通过这个项目，在哈佛大学和科罗拉多州立大学之间，在三大洲的研究人员之间，以及在地质子学科之间建立了新的合作。成果的传播将超越直接的研究社区，因为哈佛小组通过参与天体生物学小组向美国宇航局报告，而CSU的AIRIE计划以国际合作为基础，并将新技术和概念迅速转移到直接用于矿物勘探的应用中。

ABSTRACTThe discovery of mass independent fraction (MIF) of the sulfur isotopes in pre-2.45 Ga sulfides and sulfates opened a new window on the chemical evolution of the atmosphere-ocean system. At present, the MIF data for sulfur in Archean sediments are few and contradictory. We propose to undertake a thorough study of the MIF of sulfur in pyrite for three pairs of highly carbonaceous sedimentary rock units deposited between 2.7 and 2.5 Ga in Western Australia, South Africa, and Canada. Pyrite will be selected for this study on the basis of textural, chemical, and isotopic evidence indicative of a syngenetic or diagenetic origin. Pyrite in each of the six rock units will be dated by the Re-Os method. Preliminary data show that it is possible to determine the age of 2.3 Ga sedimentary pyrites with a precision of 1 m.y. Initial 187Os/ 188Os ratios inferred from the pyrite isochrons will provide information on secular variations in the Os isotopic composition of seawater, which probably reflects changes in oxidative weathering and transport of Os from the continents. The dating of Archean sedimentary pyrite can therefore yield important information regarding Os cycling.The MIF measurements will be made on the MAT 253 mass spectrometer at the Geophysical Laboratory in Washington, D.C, using laser fluorination and continuous flow inlet technology. The Re-Os dating will be done at the AIRIE laboratory at Colorado State University (CSU) using Carius tube dissolution and negative thermal ionization mass spectrometry. Sample preparation, microscopy and trace element analyses will be done at Harvard University.We expect that our data will demonstrate to what extent local conditions determined the MIF of the sulfur isotopes in these rock units, to what extent global differences were important, and to what extent global differences reflected changes in the composition of the atmosphere during the last 200 Ma of the Archean.Broader Impacts CriterionThe project will promote training and learning through involvement of students at all levels (postdoctoral, graduate, undergraduate, and high school) with at least the younger students drawn from underrepresented groups. All will be engaged in application of state-of-the-art techniques, and will be encouraged to present their findings at professional meetings. New collaborations are established through this project between Harvard University and CSU, among researchers on three continents, and across geologic sub-disciplines. Dissemination of results will reach beyond immediate research communities, as the Harvard group reports to NASA through participation on the Astrobiology Team, and the AIRIE Program at CSU is based on international collaboration, and transfers new techniques and concepts quickly to applications of direct use in mineral exploration.