Mathematics – Logic
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p12b..01s&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P12B-01
Mathematics
Logic
[3620] Mineralogy And Petrology / Mineral And Crystal Chemistry, [3662] Mineralogy And Petrology / Meteorite Mineralogy And Petrology, [6040] Planetary Sciences: Comets And Small Bodies / Origin And Evolution, [6099] Planetary Sciences: Comets And Small Bodies / General Or Miscellaneous
Scientific paper
We identified two types of melt inclusions in olivine crystals from the Brahin main-group pallasite, similar to those described in [1, 2]. Both types are abundant and coexist within a same olivine grain. The first type are isolated, 1-10 µm large, elongated to tubular, and oriented along the [001] axis of the olivine host (as determined by EBSD). These inclusions are phosphate-rich, typically made of stanfieldite, a bubble, often phosphoran olivine (21-32 wt% P2O5), plus occasional minute phases including metal-sulfide blebs and daubreelite. These isolated inclusions look primary, but microprobe mapping of P revealed the scars of former fractures joining these inclusions. The second type are 2-dimensional arrays of abundant chromite inclusions (<100 µm) and metal-sulfide assemblages (<50 µm). Typical metal-sulfide inclusions contain metal (kamacite and/or taenite ) + troilite + phosphoran olivine (6-24 wt% P2O5) + occasional minute grains of withlockite, shreibersite, daubreelite, and silica. These 2D arrays show the same morphological features that characterize secondary inclusions and healed fractures in olivine xenocrysts and other terrestrial minerals. Discussion: The phosphoran olivines are known in pallasites [3] but to our knowledge it is the first description of this mineral in inclusions. We identify this phase as olivine based on its steochiometry, (Mg,Fe,Ca)4-x(Si1-xPx)2O8, and its structure (as evidenced by EBSD). The value of 32 wt% P2O5 is the highest of all P-rich olivines reported so far [3, 4, 5, 6]. Such high P contents suggest an extended solid solution between olivine and chopinite [7]. Melt inclusions in Brahin seem to record two distinct HT events. According to phase diagrams, a single silicate melt enriched in P might be sufficient to form these phosphate-rich inclusions at T < 1200°C. On the contrary, at least two immiscible melts are required to form the secondary inclusions: a silicate-chromite melt and a metal-sulfide one. The abundance of chromite requires temperatures > ca. 1800°C in order to accommodate Cr in the silicate melt. It seems likely that these secondary inclusions result from a shock that took place while the stony-iron assembly of the Brahin pallasite was already formed. On the other hand, the lack of metal-sulfide associated with the phosphate-rich inclusions may imply that the iron part of the Brahin pallasite was not present when these inclusions were formed. References: [1] Buseck P.R. 1977. Geochimica and Cosmochimica Acta 41:711-740. [2] Koviazin S.V. and Podgornykh N.M. 2006. 37th Lunar and Planetary Science Conference pp. 1235-1236. [3] Buseck P.R. and Clark J. 1984. Mineralogical Magazine 48:229-235. [4] Agrell S.O. et al. 1998. Mineralogical Magazine 62:265-269. [5] Tropper P. et al. 2004. European Journal of Mineralogy 16:631-640. [6] Boesenberg J.S. et al. 2004. 35th Lunar and Planetary Science Conference pp. 1366-1367. [7] Grew E.S. et al. 2007. European Journal of Mineralogy 19:229-245.
Devidal J.
Devouard Bertrand
Provost Ariel
Sonzogni Y.
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