Carbon isotopic thermometry and geobarometry of sillimanite isograd in thermal aureoles: the depth of emplacement of upper crustal granitic bodies

Carbon isotope fractionation between coexisting calcite and graphite (C ) has been studied in metamorphosed limestones from three thermal aureoles around Cretaceous granitic bodies (i.e., Tanohata, Tono, and Senmaya aureoles) in the Kitakami Mountains, Northeast Japan. C in each aureole decreases toward the granitic bodies, and becomes virtually uniform near the sillimanite isograd for metapelites, although calcite has variable isotopic ratios reflecting the original sedimentary compositions. The relationships indicate that isotopic equilibrium has been attained in metamorphosed limestone of sillimanite grade. Estimated C at the sillimanite isograd is similar in the Tanohata and Tono aureoles, but different in the Senmaya aureole with smaller carbon isotopic fractionations. From the temperature dependence of C and the negative dP/dT of andalusite–sillimanite equilibrium, we conclude that the sillimanite isograd in the Senmaya aureole was under higher temperature and lower pressure than in the other two localities. Temperatures at the sillimanite isograd are estimated by using existing calibrations of carbon isotopic exchange between calcite and graphite, whereas pressures are estimated from carbon isotopic temperatures and the andalusite–sillimanite equilibrium (Holdaway and Mukhopadhyay 1993a). Consistency of the P–T estimates is examined in the light of phase equilibria in the pelitic system. The estimated pressures at the sillimanite isograd are at about 2.1–2.7(±0.2) kbar for the Tanohata and Tono aureoles and less than 1 kbar for the Senmaya aureole, respectively. Geobarometry of sillimanite isograd in thermal aureoles indicates a marked difference in the depth of solidification of upper crustal granitoids: the Senmaya pluton has intruded and solidified at a very shallow level of less than 4 km whereas the Tanohata and Tono plutons are more deep-seated (ca. 8–10 km). The method can also be an effective tool in studying low-pressure type metamorphism in which geothermobarometry using garnet is not always applicable.Editorial responsibility: J. Hoefs     

Role of Iron(III) and Aluminum Hydroxides in Concentration/reduction of Au(III) Complexes

Abstract: The adsorption of gold on iron(III) and aluminum hydroxides from solutions containing Au(III) complexes has been studied as a function of pH and chloride concentration at 30C. Iron(III) hydroxide was more effective in adsorbing gold from solution than aluminum hydroxide. However, both hydroxides controlled the behavior of Au(III) complex with very similar manner. The most effective gold adsorption occurred in aqueous solution with near neutral pH and low Cl concentration. In this solution condition, Au(III) complexes were mainly dissolved as AuCl₂(OH)₂^- and AuCl(OH)₃^-, and the surface charge for both hydroxides was positive. In addition, the adsorbed Au(III) complexes were spontaneously reduced to elemental gold in spite of the absence of a specific reducing agent.
The results of this study suggest that adsorption and spontaneous reduction of gold complexes on the surface of hydrous metal oxides with positive charge play an important role in gold precipitation in subsurface environment.     

Global mapping of the degree of space weathering on asteroid 25143 Itokawa by Hayabusa/AMICA observations

Abstract— We obtained color images of near‐Earth asteroid 25143 Itokawa by the Hayabusa multiband imaging camera to characterize the regional color properties. Images were obtained for the whole disk from the gate position (GP) and home position (HP) at a spatial resolution of 0.8–3.7 m/pixel. Whole‐disk spectra are adjusted to the telescopic data obtained by the University of Hawai'i's 88‐inch telescope using the Eight Color Asteroid Survey (ECAS) system. The disk‐resolved measurements show large variations in the three visible channels. We present a map of an index related to the degree of space weathering, which has been newly developed based on laboratory measurements. We find large variations in the degree of space weathering on Itokawa. Fresh materials are observed in regions of steep slopes and craters, whereas mature materials are ubiquitously distributed. This result suggests that pristine ordinary chondrite‐like materials have been exposed through weathered layers by excavation. By also examining close‐up images obtained during touchdown rehearsal, we find that most rocks in Itokawa's rough terrains are weathered. Instead of a regolith blanket, the surface of this small asteroid is covered with weathered rocks and gravels.     

Molecular clouds and star formation in the Magellanic Clouds and the Milky Way

Star formation is a fundamental process that dominates the life-cycle of various matters in galaxies: Stars are formed in molecular clouds, and the formed stars often affect the surrounding materials strongly via their UV photons, stellar winds, and supernova explosions. It is therefore revealing the distribution and properties of molecular gas in a galaxy is crucial to investigate the star formation history and galaxy evolution. Recent progress in developing millimeter and sub-millimeter wave receiver systems has enabled us to rapidly increase our knowledge on molecular clouds. In this proceedings, the recent results from the surveys of the molecular clouds in the Milky Way and the Magellanic Clouds as well as the Galactic center as the most active regions in the Milky Way are presented. The high sensitivity with unrivaled high resolution of ALMA will play a key role in detecting denser gas that is tightly connected to star formation.