Paque, J. M. and Beckett, J. R. and Ishii, H. A. and Aléon-Toppani, A. and Burnett, D. S. and Teslich, N. and Dai, Z. R. and Bradley, J. P. (2009) The formation of boundary clinopyroxenes and associated glass veins in type B1 CAIs. Meteoritics and Planetary Science, 44 (5). pp. 665-687. ISSN 1086-9379 http://resolver.caltech.edu/CaltechAUTHORS:20090817-144815407
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We used focused ion beam thin section preparation and scanning transmission electron microscopy (FIB/STEM) to examine the interfacial region between spinel and host melilite for spinel grains in type B1 inclusions from the Allende and Leoville carbonaceous chondrites. Boundary clinopyroxenes decorating spinel surfaces have compositions similar to those of coarser clinopyroxenes from the same region of the inclusion, suggesting little movement after formation. Host melilite displays no anomalous compositions near the interface and late-stage minerals are not observed, suggesting that boundary pyroxenes did not form by crystallization of residual liquid. Allende spinels display either direct spinel-melilite contact or an intervening boundary clinopyroxene between the two phases. Spinel-melilite interfacial regions in a Leoville B1 are more complex, with boundary clinopyroxene, as observed in Allende, but also variable amounts of glass, secondary calcite, perovskite, and an Mg-, Al-, OH-rich and Ca-, Si-poor crystalline phase that may be a layered double hydrate. One possible scenario of formation for the glass veins is that open system alteration of melilite produced a porous, hydrated aggregate of Mg-carpholite or sudoite + aluminous diopside that was shock melted and quenched to a glass. The hydrated crystalline phase we observed may have been a shocked remnant of the precursor phase assemblage, but is more likely to have formed later by alteration of the glass. In the mantle, boundary clinopyroxenes may have been crystallized from Ti-rich liquids formed by the direct dissolution of perovskite and an associated Sc-Zr-rich phase or as a reaction product between dissolving perovskite and liquid. In the core, any perovskite and associated Ti-enriched liquids that may have originally been present disappeared before the growth of boundary clinopyroxene, and the observed boundary clinopyroxene may have nucleated and grown from the liquid, along with the larger core clinopyroxene.
|Additional Information:||© The Meteoritical Society, 2009. (Received 03 June 2008; revision accepted 13 February 2009) Reviews by S. Simon and A. N. Krot were useful and appreciated. We thank Steve Simon and Larry Grossman for the loan of the section of TS-34 and Glenn MacPherson (Smithsonian Institution) for USNM Leoville 3537-2. This work was funded in part by NASA grants NNG04GG14G, NNG05GH797, and NAG5-11640. Portions of this work were also performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory in part under Contract W-7405-Eng-48 and in part under Contract DE-AC52-07NA27344.|
|Subject Keywords:||Refractory inclusions; Calcium-aluminum-rich inclusions (CAIs); Alteration; Glass|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||George Porter|
|Deposited On:||08 Sep 2009 18:15|
|Last Modified:||26 Dec 2012 11:12|
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