^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 Their mineralogy is intermediate between isolated corundum grains and CAIs where corundum has been replaced by lower-temperature phases.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