ALBERT

Description: The Tropicana gold deposit (7.9 Moz) is hosted by the Neoarchean, amphibolite- to granulite-grade Tropicana Gneiss. These rocks form part of the Plumridge terrane, a tectonic unit that separates the in situ eastern margin of the Yilgarn craton from Proterozoic elements of the Albany-Fraser orogen, Western Australia. The gneissic host package to the orebody originally constituted a suite of medium- to high-K calc-alkaline and adakitic basaltic to rhyolitic lavas emplaced in a relatively thick crustal continental margin arc. Amphibolitized mafic intrusions have back-arc basin basalt-like compositions consistent with their having formed during extension and incipient rifting of the arc. Gold mineralization occurs mainly within a sheet-like, syenitic gneiss that ranges compositionally from basaltic andesite to rhyolite, and derived through metasomatism of compositionally equivalent lavas within an interpreted shear zone. Coarse perthite textures in microcline, rutile exsolution in brown biotite, and occasional pyrite inclusions away from fractures in microcline porphyroblasts indicate that this shear zone predated peak metamorphism. Relative to the unmineralized host gneisses, the mineralized syenitic gneisses show significant enrichments in S and the ore elements (Mo, Te, Tl, Ag, Au, W and, occasionally, Bi and Sb, but not As, Cu, or Zn), in the K-group elements, and also in P, light rare earth elements (LREE), Th, U, Zr, and Hf. Depletions in most major elements (Si, Ti, Fe, Mg, and Ca), as well as V, Cr, Ni, and Co, are considered to reflect dissolution of relatively low volume mafic phases (hornblende, augite, Fe-Ti oxides) and plagioclase in the protoliths. Some of the distinctive enrichments (P, LREE, Zr, Hf) are linked to volume loss (in excess of 50%) associated with dissolution of silica and concentration of insoluble apatite and zircon in the mineralized unit, but the ore elements and strongly enriched (~ x 10) Th and U were added from the hydrothermal fluids involved. Exhumation of the Tropicana Gneiss into the upper crust was accompanied by fluid ingress and retrogression. This produced mm- to cm-scale, wavy biotite-pyrite shears, solution seams, and disseminations throughout the syenitic gneiss which were further retrogressed to sericite-chlorite assemblages in places, both of which host most of the gold in the deposit. Thus, although the present deportment of gold is unambiguously associated with the postpeak metamorphic, greenschist-grade retrogression at ca. 2520 Ma, there is compelling evidence that the paucity of quartz and carbonate veining, atypical element enrichments, and sheet-like nature of the Tropicana deposit in part reflect metasomatism associated with a prepeak metamorphic, relatively deep shear zone. At present, we have no direct evidence for the timing of introduction of gold, and a protracted history of gold precipitation, redistribution, and enrichment is implied.

Description: Sedimentary charcoal is a widely used tool for reconstructing past fire histories. These fossil charcoals are normally quantified as the number or area of particles present in a sediment sample, but it is not known how well either measure is correlated with their volume. Areal measurements are often treated as the true measure of quantity, which may introduce bias to fire histories. The relationship of measured area to volume is a function of shape, and if this relationship can be understood, areal measurements may be adjusted to better reflect volume. We show that particle elongation may be particularly important when making such adjustments. We also establish a high degree of correlation between volume and area in a mid-Holocene mesocharcoal assemblage, which indicates that the bias may be small in similar cases.

Description: Recent geological studies demonstrate that the Isthmus of Panama emerged some 10 m.y. earlier than previously assumed. Although absent today in Panama, Central American savanna environments likely developed in connection with the onset of Northern Hemisphere glaciations. As is widely recognized, most of the mammals crossing the isthmus since 2.5 Ma lived in savannas. Could climate-induced vegetational changes across Panama explain the delayed migration of mammals, rather than terrestrial connectivity? We investigate the congruence between cross-continental mammal migration and climate change through analysis of fossil data and molecular phylogenies. Evidence from fossil findings shows that the vast majority of mammals crossed between South and North America after ca. 3 Ma. By contrast, dated mammal phylogenies suggest that migration events started somewhat earlier, ca. 4–3 Ma, but allowing for biases toward greater ages of molecular than geologic dating and uncertainties in the former, we consider this age range not to be significantly earlier than 3 Ma. We conclude that savanna-like environments developed in response to the vast Laurentide ice sheet at the first Quaternary glaciation triggered the initiation of the Great American Biotic Interchange in mammals.

Description: Electrical performance and characterization of deep levels in vertical GaN P-i-N diodes grown on low threading dislocation density (∼10 4   - 10 6  cm −2 ) bulk GaN substrates are investigated. The lightly doped n drift region of these devices is observed to be highly compensated by several prominent deep levels detected using deep level optical spectroscopy at E c -2.13, 2.92, and 3.2 eV. A combination of steady-state photocapacitance and lighted capacitance-voltage profiling indicates the concentrations of these deep levels to be N t  = 3 × 10 12 , 2 × 10 15 , and 5 × 10 14  cm −3 , respectively. The E c -2.92 eV level is observed to be the primary compensating defect in as-grown n -type metal-organic chemical vapor deposition GaN, indicating this level acts as a limiting factor for achieving controllably low doping. The device blocking voltage should increase if compensating defects reduce the free carrier concentration of the n drift region. Understanding the incorporation of as-grown and native defects in thick n -GaN is essential for enabling large V BD in the next-generation wide-bandgap power semiconductor devices. Thus, controlling the as-grown defects induced by epitaxial growth conditions is critical to achieve blocking voltage capability above 5 kV.