¹Peng Ni, ¹Youxue Zhang, ¹Adam Simon, ²Joel Gagnon
American Mineralogist 102, 1287-1301 Link to Article [https://doi.org/10.2138/am-2017-5885]
¹Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109, U.S.A.
²Department of Earth and Environmental Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada
Copyright: The Mineralogical Society of America

Copper diffusion plays an important role in natural processes, such as metal transport during the formation of magmatic-hydrothermal porphyry-type ore deposits and Cu isotope fractionation during tektite formation. Copper diffusion data in natural silicate melts, however, are limited. In this study, chalcocite (Cu2S) “dissolution” experiments were carried out using chalcocite-rhyolite diffusion “couples” to study Cu (and S) diffusion in rhyolitic melts. Instead of chalcocite dissolution as initially expected, our experiments show that Cu is transferred from the chalcocite crystal to the rhyolitic melt, and Fe is transferred from the rhyolitic melt to chalcocite, whereas the S concentration profile in the rhyolitic melt is essentially flat. From the Cu and Fe exchange profiles in the rhyolitic melts, Cu diffusivities and Fe diffusivities are obtained and reported. Copper diffusivity in rhyolitic melts containing 0.10 to 5.95 wt% H2O at temperatures of 750 to 1391 °C and pressures of 0.5 to 1.0 GPa can be described as: Embedded Image

which allows the estimation of an activation energy for diffusion in dry rhyolitic melts to be 96.8 ± 4.1 kJ/mol. In the above equation, diffusivity (D) is in m2/s, T is the temperature in K, w is the H2O concentration in the rhyolitic melts in wt% and all errors reported are at 1σ level. Combining Cu diffusion data from this study and previous data in basaltic melt gives a general equation for Cu diffusivity in natural silicate melts: Embedded Image

where Si+Al-H is the cation mole fraction of Si plus Al minus H in the silicate melt on a wet basis. Iron diffusivities obtained in this study, in anhydrous to 6 wt% H2O rhyolite, are combined with previous data to get a general equation for Fe diffusion in rhyolitic melts: Embedded Image

Our data demonstrate that Cu diffusion is faster than H2O or Cl in rhyolitic melts containing 6 wt% water, which indicates that the scavenging and transport of Cu by a magmatic volatile phase during formation of porphyry-type ore deposits is not limited by diffusion of Cu. Based on our experimental data, Cu diffusivity is almost four orders of magnitude higher than Zn in anhydrous rhyolitic melts, which supports the explanation of more diffusive loss of Cu leading to more fractionated Cu isotopes than Zn isotopes in tektites.

¹D. J. Joswiak, ¹D. E. Brownlee, ²A. N. Nguyen, ²S. Messenger
Meteoritics&Planetary Sciences (in Press) Link to Article [DOI: 10.1111/maps.12877]
¹Department of Astronomy, University of Washington, Seattle, Washington, USA
²Robert M. Walker Laboratory for Space Science, ARES, NASA JSC, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

Transmission electron microscope examination of more than 250 fragments, >1 μm from comet Wild 2 and a giant cluster interplanetary dust particle (GCP) of probable cometary origin has revealed four new calcium-aluminum-rich inclusions (CAIs), an amoeboid olivine aggregate (AOA), and an additional AOA or Al-rich chondrule (ARC) object. All of the CAIs have concentric mineral structures and are composed of spinel + anorthite cores surrounded by Al,Ti clinopyroxenes and are similar to two previous CAIs discovered in Wild 2. All of the cometary refractory objects are of moderate refractory character. The mineral assemblages, textures, and bulk compositions of the comet CAIs are similar to nodules in fine-grained, spinel-rich inclusions (FGIs) found in primitive chondrites and like the nodules may be nebular condensates that were altered via solid–gas reactions in the solar nebula. Oxygen isotopes collected on one Wild 2 CAI also match FGIs. The lack of the most refractory inclusions in the comet samples may reflect the higher abundances of small moderately refractory CAI nodules that were produced in the nebula and the small sample sizes collected. In the comet samples, approximately 2–3% of all fragments larger than 1 μm, by number, are CAIs and nearly 50% of all bulbous Stardust tracks contain at least one CAI. We estimate that ~0.5 volume % of Wild 2 material and ~1 volume % of GCP is in the form of CAIs. ARCs and AOAs account for <1% of the Wild 2 and GCP grains by number.

^1,2Marco Pignatari, ³Peter Hoppe, ^4,2Reto Trappitsch, ^5,2Chris Fryer, ^6,7,2F.X. Timmes, ^8,9,2Falk Herwig, ^9,10,2Raphael Hirschi
Geochmica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.005]
¹E. A. Milne Centre for Astrophysics, University of Hull, Hull, HU6 7RX, UK
²The NuGrid Collaboration (http://www.nugridstars.org)
³Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
⁴Department of the Geophysical Sciences, The University of Chicago, and Chicago Center for Cosmochemistry, Chicago, IL 60637, USA
⁵Computational Physics and Methods (CCS-2), LANL, Los Alamos, NM, 87545, USA
⁶Arizona State University (ASU), PO Box 871404, Tempe, AZ, 85287-1404, USA
⁷The Joint Institute for Nuclear Astrophysics, Notre Dame, IN 46556, USA
⁸Department of Physics & Astronomy, University of Victoria, Victoria, BC, V8P5C2 Canada
⁹Keele University, Keele, Staffordshire ST5 5BG, United Kingdom
¹⁰Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8583, Japan
Copyright Elsevier

Carbon-rich presolar grains are found in primitive meteorites, with isotopic measurements to date suggesting a core-collapse supernovae origin site for some of them. This holds for about 1-2 % of presolar silicon carbide (SiC) grains, so-called Type X and C grains, and about 30 % of presolar graphite grains. Presolar SiC grains of Type X show anomalous isotopic signatures for several elements heavier than iron compared to the solar abundances: most notably for strontium, zirconium, molybdenum, ruthenium and barium. We study the nucleosynthesis of zirconium and molybdenum isotopes in the He-shell of three core-collapse supernovae models of 15, 20 and 25 M☉ with solar metallicity, and compare the results to measurements of presolar grains. We find the stellar models show a large scatter of isotopic abundances for zirconium and molybdenum, but the mass averaged abundances are qualitatively similar to the measurements. We find all models show an excess of 96Zr relative to the measurements, but the model abundances are affected by the fractionation between Sr and Zr since a large contribution to 90Zr is due to the radiogenic decay of 90Sr. Some supernova models show excesses of 95,97Mo and depletion of 96Mo relative to solar. The mass averaged distribution from these models shows an excess of 100Mo, but this may be alleviated by very recent neutron-capture cross section measurements. We encourage future explorations to assess the impact of the uncertainties in key neutron-capture reaction rates that lie along the n-process path.

^1,2,3Adam R. Sarafian, ⁴Erik H. Hauri, ⁵Francis M. McCubbin, ⁶Thomas J. Lapen, ⁷Eve L. Berger, ^2,3Sune G. Nielsen, ^2,8Horst R. Marschall, ²Glenn A. Gaetani, ⁵Kevin Righter, ^1,2Emily Sarafian
Philosophical Transactions of the Royal Society A 375, 2094 Link to Article [https://doi.org/10.1098/rsta.2016.0209]
¹Massachusetts Institute of Technology – Woods Hole Oceanographic Institution Joint Program in Oceanography/Applied Ocean Science and Engineering, Cambridge, MA 02139, USA
²Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
³NIRVANA Laboratories, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
⁴Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
⁵NASA JSC, Mailcode XI2, 2101 NASA Parkway, Houston, TX 77058, USA
⁶Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA
⁷GeoControl Systems Inc., Jacobs JETS Contract, NASA JSC, Houston, TX, USA
⁸Goethe Universität Frankfurt, Institut für Geowissenschaften, Altenhöferallee 1, 60438 Frankfurt am Main, Germany

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