Tourmaline geochemistry and cassiterite geochronology of highly evolved tin granites and their hydrothermal systems in eastern Australia

Three models have been proposed for cassiterite (SnO2) mineralisation in magmatic–hydrothermal environments: (1) magmatic crystallisation from a granitic melt, (2) late-stage magmatic partition of Sn into a fluid or vapour phase and subsequent cassiterite deposition, and (3) hydrothermal leaching of Sn from granite and/or country rocks and subsequent deposition. The complex chemistry of the ‘tin’ granites, and the large and pervasive hydrothermal systems which can overprint and destroy primary features make understanding the processes responsible for Sn enrichment difficult. Two new analytical methods were developed. Firstly, a method for the determination of Rb–Sr and Sm–Nd isotopic compositions of magmatic and hydrothermal tourmalines, which can record the compositional evolution of magmas and their hydrothermal fluids. Secondly, cassiterite U–Pb geochronology to constrain the absolute age and duration of magmatic–hydrothermal Sn systems. These data, together with major and trace element compositions of tourmaline, whole-rock geochemistry, quartz δ18O values and zircon U–Pb geochronology are applied to two Sn deposits associated with the Ardlethan and Mole granites of eastern Australia. The geochemical and isotope data of tourmaline show large compositional changes across the magmatic–hydrothermal transition. In the Ardlethan Granite, tourmaline 87Rb–86Sr isotope compositions, which provide robust estimates of 87Sr/86Sr(i) because of their low 87Rb/86Sr, are used to model the assimilation and fractional crystallisation processes that lead to a 30-times enrichment of Sn in residual melts relative to the source rocks. However, caution must be taken with interpreting 87Sr/86Sr(i) tourmaline data as high 87Rb/86Sr of parental melts and fluids can lead to significant in-situ decay of 87Rb prior to tourmaline precipitation. This phenomon is hypothesised for the parental melts of the Mole Granite which due toextreme fractional crystallisation have extreme 87Rb/86Sr of ~900. Subsequently 87Sr/86Sr(i) tourmaline compositions are more evolved the the whole rock composition. The Sn concentration of tourmaline increases from magmatic to hydrothermal settings within the Ardlethan and Mole granites, recording the exsolution of a fluid from a silicate melt. The enrichment of Sn during fluid fractionation, recorded by tourmaline, agrees with experimentally determined melt–fluid partitioning coefficients. Fluid fractionation is the dominant enrichment process for greisen deposits of the Ardlethan Granite, and all deposits of the Mole Granite. Fluid leaching of host rocks is evidenced by convergence of Fe/(Fe+Mg), Sr, 87Sr/86Sr(i) and εNd(i) in hydrothermal tourmaline from the original source rock composition to the host rock composition. At Ardlethan, the host rock of mineralised breccia pipes is enriched in Sn (~50 ppm) and fluid leaching results in an increase of Sn in the mineralising fluids. Although fluid leaching occurs around the Mole Granite, the low Sn concentrations in the host rocks limits Sn enrichment. Melt/fluid-mineral partitioning is a major uncertainty in the interpretation of tourmaline trace element geochemistry. Natural studies performed here provide some constraints, however, more targeted experimental work is required. A new method for U–Pb characterisation of cassiterite by ID-TIMS has provided a matrix-matched reference material for in-situ techniques. However, common-Pb corrections of in-situ techniques remain a large uncertainty in cassiterite geochronology. At Ardlethan, the common-Pb compositions are appropriately estimated by terrestrial Earth models and are more precise than isochron ages. Conversely, the common-Pb associated with the Mole Granite appears variable between a terrestrial Earth composition and a highly evolved composition. Cassiterite U–Pb geochronology of both the Ardlethan and Mole granite mineralisation systems indicate precipitation synchronous with emplacement. The method does not have sufficient precision (~4 % absolute) to distinguish the age of cassiterite precipitation from that of zircon, however, the magmatic–hydrothermal systems of the Ardlethan and Mole granites persisted for a maximum of 4.2 Ma

Coordinated Multi-Agent Imitation Learning

We study the problem of imitation learning from demonstrations of multiple coordinating agents. One key challenge in this setting is that learning a good model of coordination can be difficult, since coordination is often implicit in the demonstrations and must be inferred as a latent variable. We propose a joint approach that simultaneously learns a latent coordination model along with the individual policies. In particular, our method integrates unsupervised structure learning with conventional imitation learning. We illustrate the power of our approach on a difficult problem of learning multiple policies for fine-grained behavior modeling in team sports, where different players occupy different roles in the coordinated team strategy. We show that having a coordination model to infer the roles of players yields substantially improved imitation loss compared to conventional baselines.Comment: International Conference on Machine Learning 201

The Commitment to Securing Perpetual Journal Access: A Survey of Academic Research Libraries

Current and emerging trends raise questions about the extent to which academic research libraries should continue to seek perpetual access provisions for journal acquisitions. To describe the questions being raised, this paper begins by framing perpetual access commitments within the contexts of the past, present, and future. The paper then assesses current views and practices by describing and analyzing the results of a survey of librarians. The results show that, while the respondents' libraries generally espouse strong commitments to perpetual access, a combination of factors is leading many libraries to take actions that weaken perpetual access provisions

Should NASIG Develop a Code of Ethics

This editorial discusses whether NASIG should develop a code of ethics

Habitat Utilization and Seasonal Movements of Black Bears in the Great Smoky Mountains National Park

A radiotelemetry study to determine seasonal movements and habitat utilization of black bears (Ursus americanus) in the Great Smoky Mountains National Park was undertaken from June 1980 to May 1982. Annual home range size in a year of poor hard mast production was 119 km2 and 13 km2 for males and females, respectively, and 36 km2 and 6 km2 in a year of good hard mast production. Bear movements were governed by seasonal food availability. Bears exhibited an affinity to summer home ranges but traveled to widely dispersed fall ranges. Seasonal range shifts were more evident in years of poor hard mast than good hard mast. Eleven of 14 radiocollared bears traveled extensively in fall 1980, a poor mast year. Three of 6 females and every one of 8 males traveled to various parts of North Carolina; bears spent time in the Park, the Cherokee National Forest, the Nantahala National Forest, and private lands adjoining these federal lands. Three males were killed illegally, 1 was hunter-harvested, and the 7 other bears returned to the study area from fall 1980 ranges. Only 1 bear traveled widely in fall 1981, and no radiocollared bears were killed. Bears used different forest cover types non-randomly during different seasons. Oak forests are extremely important to bear survival in the Southern Appalachians. Abundant spring fruits, summer berries, and fall hard mast make the oak types critical habitat for bears. Bears regularly crossed roads and trails according to their spatial arrangement in their home ranges. Limiting road access into bear range is important to bear survival