ALBERT

Description: Accessory mineral thermometry and thermodynamic modelling are fundamental tools for constraining petrogenetic models of granite magmatism. U–Pb geochronology on zircon and monazite from S-type granites emplaced within a semi-continuous, whole-crust section in the Georgetown Inlier (GTI), NE Australia, indicates synchronous crystallisation at 1550 Ma. Zircon saturation temperature (Tzr) and titanium-in-zircon thermometry (T(Ti–zr)) estimate magma temperatures of ~ 795 ± 41 °C (Tzr) and ~ 845 ± 46 °C (T(Ti-zr)) in the deep crust, ~ 735 ± 30 °C (Tzr) and ~ 785 ± 30 °C (T(Ti-zr)) in the middle crust, and ~ 796 ± 45 °C (Tzr) and ~ 850 ± 40 °C (T(Ti-zr)) in the upper crust. The differing averages reflect ambient temperature conditions (Tzr) within the magma chamber, whereas the higher T(Ti-zr) values represent peak conditions of hotter melt injections. Assuming thermal equilibrium through the crust and adiabatic ascent, shallower magmas contained 4 wt% H2O, whereas deeper melts contained 7 wt% H2O. Using these H2O contents, monazite saturation temperature (Tmz) estimates agree with Tzr values. Thermodynamic modelling indicates that plagioclase, garnet and biotite were restitic phases, and that compositional variation in the GTI suites resulted from entrainment of these minerals in silicic (74–76 wt% SiO2) melts. At inferred emplacement P–T conditions of 5 kbar and 730 °C, additional H2O is required to produce sufficient melt with compositions similar to the GTI granites. Drier and hotter magmas required additional heat to raise adiabatically to upper-crustal levels. S-type granites are low-T mushes of melt and residual phases that stall and equilibrate in the middle crust, suggesting that discussions on the unreliability of zircon-based thermometers should be modulated.

Description: Centre of Excellence for Core to Crust Fluid Systems, Australian Research Council http://dx.doi.org/10.13039/100012537

Description: Ruhr-Universität Bochum (1007)

Description: The late Proterozoic Ntaka Ultramafic Complex is a body of dominantly pyroxenitic cumulate rocks containing cyclic alternations of olivine–orthopyroxene cumulates. Chemical zoning in the pyroxenes has been imaged at 25–40 µm resolution using desktop microbeam X-ray fluorescence mapping followed up with laser ablation–inductively coupled plasma mass spectrometry analysis for minor and trace elements on selected samples. Poikilitic and granular harzburgites are finely intermingled, in some cases on a centimetre scale in the same thin section. Poikilitic varieties display spectacular textures, ranging from isolated equant orthopyroxene oikocrysts within olivine-rich heteradcumulate harzburgites to rocks composed entirely of interlocking centimetre-sized anhedral orthopyroxene oikocrysts containing sharply bounded idiomorphic Cr-enriched cores. The poikilitic harzburgites are interlayered with cumulate pyroxenites in which orthopyroxene grains show a variety of zoning patterns: Cr-rich cores similar to those in the oikocrysts; sharply bounded oscillatory zoned cores; and reverse zoning with Cr-poor cores and Cr-enriched rims. A further variation is the presence of a mingled harzburgite lithology in which dunite or poikilitic harzburgite is invaded on a centimetre scale by diffuse vein networks or patches of coarse orthopyroxenite. This range of textures and lithologies attests to a more complex set of processes than implied by the standard cumulus theory model in which oikocrysts are considered to have crystallized from intercumulus liquid within a permeable crystal mush. A range of hypotheses is proposed, including infiltration metasomatism of original olivine cumulates by migrating orthopyroxene-saturated pore fluid; however, the textural relationships, whole-rock chemistry and Cr zoning within the grains can best be explained by a model in which the orthopyroxene oikocrysts form in part or whole as mechanically accumulated cumulus grains. The complexity of zoning patterns is attributed to stirring of entrained olivine and orthopyroxene crystals within a heterogeneous flowing crystal mush, where the transporting magma has a wide range of silica contents owing to poorly stirred incorporation of siliceous country-rock material. The Cr-rich orthopyroxenite component grew from Si-enriched chromite-saturated magma. Mingled lithologies developed after accumulation as a result of percolation and infiltration metasomatism by Si-enriched liquid derived by melting of xenoliths within the crystal pile. The model may be more generally applicable: dunite–harzburgite cycles, common in many layered intrusions, may reflect variable degrees of contamination rather than cycles of fractional crystallization and replenishment.

Description: An Early Cretaceous siliceous large igneous province (SLIP) that developed on the eastern margin of Gondwana produced some of the most voluminous siliceous volcaniclastic deposits known globally. We report U-Pb ages and trace-element and Hf-isotopic signatures of detrital zircons from the Madura Shelf (onshore Bight Basin), Western Australia. These zircons include a geochemically distinct 106 Ma component with age and Hf characteristics that match SLIP volcanics some 2300 km distant in eastern Australia. This young subpopulation shows limited grain abrasion, which contrasts with older detrital components that are stratigraphically persistent. Regional detrital zircon provenance demonstrates that sediment routing systems were disconnected in the eastern and western Bight Basin, negating terrestrial transport mechanisms as a possible vector of the zircons from the SLIP to their recovered position. Palynology indicates that the 106 Ma zircons are syn-depositional, and we interpret them as being significantly transported in an eruption plume. Given the grain size and distance from source, such distal zircon emplacement suggests previously undocumented 106 Ma super-eruptions. The 106 Ma zircons likely reflect Southern Hemisphere winter eruptions when tropospheric polar easterly winds would have been favored across southeastern Australia.

Description: The Yao’an porphyry Au system, Machangqing porphyry Cu-Mo system, and Beiya porphyry-skarn Au system, are spatially and temporally associated with potassic felsic intrusions emplaced during the Eocene to Oligocene epochs at 37 to 33 Ma in a postcollisional intracontinental setting in western Yunnan, western Yangtze craton, China. The Yao’an monzonite and quartz monzonite porphyry intrusions are alkaline and potassic with high K 2 O/Na 2 O ratios (1.1–1.5). They have Sr-Nd-Pb isotopes similar to coeval lamprophyres and are characterized by uniform zircon Hf (–6.4 to –8.7) and 18 O values (6.6–7.0). They are interpreted as products of fractional crystallization of lamprophyre-like potassic mafic magma derived from ancient metasomatized lithospheric mantle, a scenario similar to the mid-Cretaceous postcollisonal Scheelite Dome gold system in Yukon, Canada. The Machangqing granitic intrusions are high K calc-alkaline and show high Sr, Sr/Y, and La/Yb, but low Y and Yb geochemical signatures. They have Sr-Nd-Pb isotope compositions similar to amphibolite xenoliths hosted by potassic felsic intrusions in western Yunnan. The zircon Hf values of the Machangqing granitic intrusions are positive (0.3–4.7), and the zircon-depleted Hf mantle model ages are 1.1 to 0.8 Ga. They also have mantle-like zircon 18 O values (5.5–6.4). The Machangqing granites were most likely derived from partial melting of Neoproterozoic lower crust. The Beiya granitic intrusions are alkaline, with high K 2 O/Na 2 O (1.9–2.7), Sr/Y and La/Yb ratios, high Sr contents, and low Y and Yb contents. They contain abundant zircon inheritance and have variable magmatic zircon Hf (–4 to +4) and the highest magmatic zircon 18 O values (6.6–7.8). The Beiya felsic intrusions are interpreted to be derived from partial melting of a K-rich mafic source mixed with a metasedimentary component. The Eocene-Oligocene intracontinental potassic intrusions and associated mineralization in western Yunnan are located proximal to the Mesozoic Jinsha suture, suggesting that this Mesozoic lithospheric boundary may have provided a first-order control on localization of Cenozoic mineral systems. These potassic felsic intrusions are coeval with regional potassic mafic magmatism in western Yunnan and were emplaced between 37 to 33 Ma, after the collision between India and Asia at ca. 60 to 55 Ma. It is therefore postulated that continental collision may have preferentially thickened the continental lithospheric mantle (CLM) adjacent to the Jinsha suture, in which overthickened lower continental lithospheric mantle was subsequently removed during 37 to 33 Ma, inducing melting of residual metasomatized lithospheric mantle as well as lower crust. The gold-rich Yao’an and Beiya intrusions are alkaline and potassic, characterized by high zircon 18 O values (〉6.5), which is consistent with supracrustal contributions. In contrast, the Cu-Mo-rich Machangqing intrusions are high K calc-alkaline with mantle-like zircon 18 O values (〈6.5) and juvenile Hf signatures, indicating negligible supracrustal recycling. Empirically, source compositions played an important role in determing the metal endowment among intrusions formed under the same tectonic setting with similar ages in western Yunnan. In western Yunnan, gold tends to be associated with alkaline and potassic melts with a supracrustal contribution, whereas Cu-Mo mineralization seems to be more related with juvenile crustal sources with little supracrustal influence.

Description: Recent UV-optical surveys have been successful in finding tidal disruption events (TDEs), in which a star is tidally disrupted by a supermassive black hole (BH). These TDEs release a huge amount of radiation energy E rad  ~ 10 51 –10 52 erg into the circum-nuclear medium. If the medium is dusty, most of the radiation energy will be absorbed by dust grains within ~1 pc from the BH and re-radiated in the infrared. We calculate the dust emission light curve from a 1D radiative transfer model, taking into account the time-dependent heating, cooling and sublimation of dust grains. We show that the dust emission peaks at 3-10 μm and has typical luminosities between 10 42 and 10 43 erg s –1 (with sky covering factor of dusty clouds ranging from 0.1 to 1). This is detectable by current generation of telescopes. In the near future, James Webb Space Telescope will be able to perform photometric and spectroscopic measurements, in which silicate or polycyclic aromatic hydrocarbon features may be found. Dust grains are non-spherical and may be aligned with the magnetic field, so the dust emission may be significantly polarized. Observations at rest-frame wavelength ≥ 2 μm have only been reported from two TDE candidates, SDSS J0952+2143 and Swift J1644+57. Although consistent with the dust emission from TDEs, the mid-infrared fluxes of the two events may be from other sources. Long-term monitoring is needed to draw a firm conclusion. We also point out two nearby TDE candidates (ASASSN-14ae and -14li) where the dust emission may be currently detectable. Detection of dust infrared emission from TDEs would provide information regarding the dust content and its distribution in the central pc of non-active galactic nuclei, which is hard to probe otherwise.

Description: Hibernation has been a key area of research for several decades, essentially in small mammals in the laboratory, yet we know very little about what triggers or ends it in the wild. Do climatic factors, an inte...

Description: Scheelite is well developed in hydrothermal deposits, providing a window into genetic processes and facilitating comparative studies, however, few studies have focused on characterizing scheelite in skarn-type W-Mo deposits. The primary ore mineral in the Jitoushan and Baizhangyan skarn-type W-Mo deposits (Anhui Province, Eastern China), scheelite was analyzed for major, trace, and rare earth element (REE) abundance and for Sr-Nd isotopes. The analysis revealed two unique geochemical characteristics that distinguish the scheelite from skarn-type W-Mo deposits to that from vein-type Au-W and porphyry-type W-Mo deposits: higher Mo content with a negative correlation between MoO 3 and WO 3 and a strong HREE depletion. Skarn-type scheelite mainly inherited REE signatures from ore-forming fluids, and the early precipitation of skarn minerals (e.g., garnet, diopside, and amphibole) has most likely resulted in the observed strong HREE depletion in scheelite and the decoupling of LREEs and HREEs. Of the numerous substitution mechanisms suggested by previous workers, 3Ca 2+ = 2REE 3+ + Ca (where Ca is a Ca-site vacancy) is preferred for the substitution of REE 3+ for Ca 2+ and in this study, particularly given the low salinity of ore fluids. As the scheelite Eu anomalies were inherited from ore-forming fluids with variable redox conditions and pH, the complex dEu/Mo correlation indicates that Mo increasingly entered the scheelite under oxidizing conditions and reached a maxim at dEu values of 0.8 to 1. In contrast, under reducing conditions, Mo contents in scheelite decrease gradually and Mo is precipitated as molybdenite as a result of the change in dominant valence state. Unlike the Sr-Nd isotope compositions of scheelite from vein-type Au-(W) and W-(Sb-Au) deposits, the scheelite from skarn-type W-Mo deposits has low ( 143 Nd/ 144 Nd)(t) (most 〈0.5125) and intermediate ( 87 Sr/ 86 Sr)(t) values (most between 0.708 and 0.715). The eNd(t) values of the scheelite varied from –16 to –12.3 in the Baizhangyan deposit and from –9.5 to –9.1 in the Jitoushan deposit, indicating that the ore-forming materials in the two W-Mo deposits were mainly derived from crustal sources.

Description: Recent studies argue that subduction-modified, Cu-fertilized lithosphere controls the formation of porphyry Cu deposits in orogenic belts. However, it is unclear if and how this fertilization process operates at cratonic edges, where numerous large non-arc Au-rich deposits form. Here we report data from lower crustal amphibolite and garnet amphibolite xenoliths hosted by Cenozoic stocks that are genetically related to the Beiya Au-rich porphyry deposits along the western margin of the Yangtze craton, China. These xenoliths are thought to represent cumulates or residuals of Neoproterozoic arc magmas ponding at the base of arc at the edge of the craton that subsequently underwent high-pressure metamorphism ca. 738 Ma. The amphibolite xenoliths are enriched in Cu (383–445 ppm) and Au (7–12 ppb), and a few garnet amphibolite xenoliths contain higher Au (6–16 ppb) with higher Au/Cu ratios (2 x 10 –4 to 8 x 10 –4 ) than normal continental crust. These data suggest that metal fertilization of the base of an old arc at the edge of the craton occurred in the Neoproterozoic via subduction modification, and has since been preserved. The whole-rock geochemical and zircon Hf isotopic data indicate that melting of the Neoproterozoic Cu-Au–fertilized low-crustal cumulates at 40–30 Ma provided the metal endowment for the Au-rich porphyry system at the cratonic edge. We therefore suggest that the reactivated cratonic edges, triggered by upwelling of asthenosphere, have the potential to host significant Au ore-forming systems, especially non-arc Au-rich porphyry deposits.