¹Junko Isa, ²Chi Ma, ^1,3Alan E. Rubin
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California 90095, U.S.A.
²Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
³Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095, U.S.A.

Joegoldsteinite, a new sulfide mineral of end-member formula MnCr2S4, was discovered in the Social Circle IVA iron meteorite. It is a thiospinel, the Mn analog of daubréelite (Fe2+Cr2S4), and a new member of the linnaeite group. Tiny grains of joegoldsteinite were also identified in the Indarch EH4 enstatite chondrite. The chemical composition of the Social Circle sample determined by electron microprobe is (wt%) S 44.3, Cr 36.2, Mn 15.8, Fe 4.5, Ni 0.09, Cu 0.08, total 101.0, giving rise to an empirical formula of (Mn0.82Fe0.23)Cr1.99S3.95. The crystal structure, determined by electron backscattered diffraction, is a Fd3m spinel-type structure with a = 10.11 Å, V = 1033.4 Å3, and Z = 8.

Reference
Isa J, Ma C, Rubin AE (2016) Joegoldsteinite: A new sulfide mineral (MnCr2S4) from the Social Circle IVA iron meteorite. American Mineralogist 101,1217-1221
Link to Article [http://dx.doi.org/10.2138/am-2016-5594]
Copyright: The Mineralogical Society of America

¹Sota Arakawa, ¹Taishi Nakamoto
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan

We present a novel model showing that compound chondrules are formed by collisions of supercooled droplets. This model reproduces two prominent observed features of compound chondrules: the nonporphyritic texture and the size ratio between two components.

Reference
Arakawa S, Nakamoto T (2016) Compound chondrule formation via collision of supercooled droplets. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.04.041]
Copyright Elsevier

¹Justin I. Simon, ²Jennifer E.P. Matzel, ³Steven B. Simon, ⁴D. Kent Ross, ²Peter K. Weber, ^3,6Lawrence Grossman
¹Center for Isotope Cosmochemistry and Geochronology, Astromaterials Research and Exploration Science Division, Exploration, Integration, and Science Directorate, NASA Johnson Space Center, Houston, TX 77058
²Lawrence Livermore National Laboratory, Livermore, CA 94451
³Department of the Geophysical Sciences, 5734 S. Ellis Ave, The University of Chicago, Chicago, IL 60637
⁴University of Texas at El Paso / Jacobs Technology, Houston, TX 77058
⁵Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637

Oxygen isotopic variations across the outer margins and Wark-Lovering (WL) rims of a diverse suite of six coarse-grained Types A and B refractory inclusions from both oxidized and reduced CV3 chondrites suggest that CAIs originated from a 16O-rich protosolar gas reservoir and were later exposed to both relatively 17,18O-rich and 16O-rich reservoirs. The O-isotope profiles of CAIs can be explained by changes in the composition of gas near the protoSun or the migration of CAIs through a heterogeneous nebula. Variability within the inclusion interiors appears to have been set prior to WL rim growth. Modeling the isotopic zoning profiles as diffusion gradients between inclusion interiors and edges establishes a range of permissible time-temperature combinations for their exposure in the nebula. At mean temperatures of 1400 K, models that match the isotope gradients in the inclusions yield timescales ranging from 5×103 to 3×105 years. Assuming CAIs originated with a relatively 16O-rich (protosolar) isotopic composition, differences among the melilite interiors and the isotopic gradients in their margins imply the existence of a number of isotopically distinct reservoirs. Evidence at the edges of some CAIs for subsequent isotopic exchange may relate to the beginning of rim formation. In the WL rim layers surrounding the interiors, spinel is relatively 16O-rich but subtly distinct among different CAIs. Melilite is often relatively 16O-poor, but rare relatively 16O-rich grains also exist. Pyroxene generally exhibits intermediate O-isotope compositions and isotopic zoning. Olivine in both WL and accretionary rims, when present, is isotopically heterogeneous. The extreme isotopic heterogeneity among and within individual WL rim layers and in particular, the observed trends of outward 16O-enrichments, suggest that rims surrounding CAIs contained in CV3 chondrites, like the inclusions themselves, formed from a number of isotopically distinct gas reservoirs. Collectively, these results support numerical protoplanetary disk models in which CAIs were transported between several distinct nebular reservoirs multiple times prior to accretion onto a parent body.

Reference
Simon JI, Matzel JEP, Simon SB, Ross DK, Weber PK, Grossman L (2016) Oxygen isotopic variations in the outer margins and Wark-Lovering rims of refractory inclusions. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.04.025]
Copyright Elsevier

^1,2Andrew W. Needham, ¹Scott Messenger, ²Jangmi Han, ¹Lindsay P. Keller
¹Robert M. Walker Laboratory for Space Science, ARES, EISD Directorate, NASA JSC, 2101 NASA Parkway, Houston TX 77058, USA
¹Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston TX 77058, USA

Corundum-bearing Ca-Al-rich inclusions (CAIs) are a rare class of high-temperature condensates from the inner regions of the protoplanetary disk. Their mineralogy is intermediate between isolated corundum grains and CAIs where corundum has been replaced by lower-temperature phases. These inclusions sample a critical transitional period of the inner nebula where both the Sun and protoplanetary disk were rapidly evolving. We conducted O isotopic, Al-Mg chronological, petrographic, and crystallographic studies of four corundum-bearing inclusions in the Murchison CM2 and ALHA 77307 CO3.0 carbonaceous chondrites. Within each inclusion, corundum, hibonite, and spinel have indistinguishable 16O-rich compositions. The O isotopic compositions from all inclusions fall within a narrow range of Δ17O = -22.8 ± 3.6 ‰ that matches values of most previously studied micrometer-sized corundum grains and mineralogically pristine CAIs. These data indicate that, with few exceptions, the most refractory inclusions in carbonaceous chondrites formed from the same O isotopic reservoir. One CAI from ALHA 77307, ALH-61, exhibits a continuous corundum mantle overlying a hibonite core, opposite the equilibrium condensation sequence at typical nebular pressures and dust/gas ratios. Transmission electron microscopy examination of the hibonite-corundum interface suggests that the corundum condensed on the hibonite and was itself then partially overlain with spinel. Additionally, high dust/gas ratios are interpreted from the W- and Mo-depleted composition of a refractory metal nugget within a second corundum-bearing CAI, ALH-160. Together, these observations show that the primary formation conditions of some corundum-bearing CAIs involved non-equilibrium condensation in environments with elevated dust-gas ratios.

The corundum-bearing CAIs studied here have inferred initial 26Al/27Al ratios that fall within the roughly bimodal distribution of values observed in most CAIs. ALH-160 retains no resolvable excess 26Mg while ALH-61 has a well-resolved initial 26Al/27Al ratio of 4. 2 ± 0.4 x 10-5. The presence or absence of live 26Al at the time of CAI formation may record distinct chronology if 26Al was initially homogeneously distributed in the early Solar System. Alternatively, variations in 26Al/27Al ratios may reflect late injection and/or heterogeneous distribution of 26Al. Regardless of which model for 26Al distribution is correct, the data presented here indicate that formation of corundum-bearing CAIs was repeated during multiple heating and non-equilibrium condensation events throughout early Solar System history and within a single oxygen isotopic reservoir.

Reference
Needham AW, Messenger S, Han J, Keller LP (2016) Corundum-hibonite inclusions and the environments of high temperature processing in the early solar system. Geochmica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.04.022]
Copyright Elsevier

¹Chi Ma, ²Oliver Tschauner, ¹John R. Beckett, ³Yang Liu, ¹George R. Rossman, ⁴Stanislav V. Sinogeikin, ⁴Jesse S. Smith, ⁵Lawrence A. Taylor
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
²High Pressure Science and Engineering Center and Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154, USA
³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA
⁴HPCAT, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, Illinois 60439, USA
⁵Planetary Geosciences Institute, Department of Earth and Planetary Science, University of Tennessee, Knoxville, Tennessee 37996, USA

Ahrensite (IMA 2013-028), γ-Fe2SiO4, is the natural Fe-analog of the silicate-spinel ringwoodite (γ-Mg2SiO4). It occurs in the Tissint Martian meteorite, where it forms through the transformation of the fayalite-rich rims of olivine megacrysts or Fe-rich microphenocrysts in contact with shock melt pockets. We report the first comprehensive set of crystallographic, spectroscopic, and quantitative chemical analysis of type ahrensite, and show that concentrations of ferric iron and inversion in the type material of this newly approved mineral are negligible. We also report the occurrence of nanocrystalline ringwoodite in strained olivine and establish correlations between grain size and distance from melt pockets. The ahrensite and ringwoodite crystals show no preferred orientation, consistent with random nucleation and incoherent growth within a highly strained matrix of olivine. Grain sizes of ahrensite immediately adjacent to melt pockets are consistent with growth during a shock of moderate duration (1-10 ms).

Reference
Ma C, Tschauner O, Beckett JR, Liu Y, Rossman GR, Sinogeikin SV, Smith JS, Taylor LA (2016) Ahrensite, γ-Fe2SiO4, a new shock-metamorphic mineral from the Tissint meteorite: implications for the Tissint shock event on Mars. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.04.042]
Copyright Elsevier

The most popular papers on Cosmochemistry Papers in April were:

1-Wallner A et al. (2016) Recent near-Earth supernovae probed by global deposition of interstellar radioactive 60Fe. Nature 532, 69–72
Link to Article [doi:10.1038/nature17196]

2-Breitschwerdt D, Feige J, Schulreich MM, de Avillez MA, Dettbarn C, Fuchs B (2016)
The locations of recent supernovae near the Sun from modelling 60Fe transport
Nature 532, 73–76
Link to Article [doi:10.1038/nature17424]

3-Maio U, Tescari E (2015) Origin of cosmic chemical abundances. Monthly Notices of the Royal Astronomical Society 453, 3798-3820.
Link to Article [doi: 10.1093/mnras/stv1714]

4-Fimani F et al. (2016) Interstellar Fe60 on the Surface of the Moon
Physical Review Letters 116, 151104
Link to Article [DOI:http://dx.doi.org/10.1103/PhysRevLett.116.151104]

5-Han J, Brearley AJ (2016) Microstructural constraints on complex thermal histories of refractory cai-like objects in an amoeboid olivine aggregate from the ALHA77307 CO3.0 chondrite. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.04.011]