1,262 research outputs found
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 magmaticâhydrothermal 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 âtinâ 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 RbâSr and SmâNd isotopic compositions of
magmatic and hydrothermal tourmalines, which can record the
compositional evolution of magmas and their hydrothermal fluids.
Secondly, cassiterite UâPb geochronology to constrain the
absolute age and duration of magmaticâhydrothermal Sn systems.
These data, together with major and trace element compositions of
tourmaline, whole-rock geochemistry, quartz ÎŽ18O values and
zircon UâPb 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 magmaticâhydrothermal
transition. In the Ardlethan Granite, tourmaline 87Rbâ86Sr
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 meltâfluid
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 ΔNd(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 UâPb 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 UâPb 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
magmaticâhydrothermal 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
Taking difference seriously : the role of historicity in Herbert Marcuse's early philosophical writings
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
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