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Goldschmidt Conference Abstracts 2008

A1049

Modeling Fe-Ni metal and silicate melt compositions produced by thermal reduction of nebular condensates above the liquidus
O.I. Y
AKOVLEV
1

Structure of MgSiO3 glass at high pressure and temperature
A. Y
1

AMADA

1

*, C.E. LESHER1, S.J. GAUDIO1, T. I 3 AND K. FUNAKOSHI

NOUE

2

*, A.A. ARISKIN, G.S. BARMIN K.A. BYCHKOV

A AND
2

Vernadsky Institute, Kosygin str., 19, Moscow, Russia (*correspondence: yakovlev@geokhi.ru) The revised version of the METEOMOD model [1, 2] allows simulations of the compositional evolution of the Fe-Ni metal exsolved from protochondritic liquids during equilibrium heating above the silicate liquidus. Results of preliminary calculations have demonstrated a similarity between the calculated proportions and compositions, and those observed in LL to H chondrites [3]. This led us to a fundamental conclusion that differences in the nebular redox conditions may play a subordinate role in the formation of the main components of ordinary chondrites. Due to the highly endothermic nature of the FeOmelt Fe0+0.5O2 reaction, the presence of a reducing agent (carbon, hydrogen gas) is not required to reduce large amounts of metal from silicate melts during a flush heating event over 1600-1800oC. To constraint the hypothesis numerically, we performed low-pressure calculations simulating thermal reduction of primordial dustenriched solar nebular condensates [4] which composition was derived from the CWPI program [5]. The simulation has been carried out on mineral assemblages condensated at 1000-1200oC and then completely melted above the olivine liquidus in the range of 1550-2100oC at a constant oxygen fugacity (logfO2=-6.5). As a result, a sequence of expected equilibrium low FeO to almost iron-free melt compositions were generated displaying a complimentary increase in SiO2 and mg#. A non-trivial result of this modelling includes relations between the calculated amount of metal and its Ni/(Ni+Fe) ratios that were found to be similar to the average characteristics of the metal phase observed in the ordinary LL, L, and H chondrites [6]. This argues that a flush increase of temperature may be the main factor differentiating the nebular source into a primary metallic phase and molten silicate (chondrules?) residues that have been evolving to more reduced and magnesian compositions. [1] Ariskin et al. (1997) Met. Planet. Sci. 32, 123-133. [2] Bychkov et al. (2006) Abs. 69th Met. meeting. [3] Ariskin et al. (2006) Ibid. [4] Anders & Grevesse (1989) GCA 53, 197-214. [5] Petaev & Wood (2005) In: ASP Conf. Ser. 341, 373-406. [6] Schaefer & Fegley (2007) Icarus 186, 462­483.

3

Dept. of Geology, Univ. of California, Davis, CA 95616, USA (*correspondence: yamada@geology.ucdavis.edu) Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan Japan Synchrotron Radiation Institute, SPring-8, Sayo 6795198, Japan

X-ray diffraction studies of vitreous MgSiO3 (v-En) were performed at BL04B1 at SPring-8 and ARNE5C at KEK using 6/8 and cubic-type multianvil devices up to 21 GPa and 863K. Energy dispersive patterns were obtained with a polychromatic X-ray beam (30-180 keV) at diffraction angles (2) between 3є and 25є. Samples were compressed cold and diffraction data collected prior to heating and at successively higher temperatures until crystallization was encountered. The figure below presents the interatomic distances for Si-O (triangles) and Mg-O (diamonds) for v-En just below the crystallization temperature at 6.5, 9.2, 14 and 21 GPa. Open symbols represent interatomic distances for 1 atm. Si-O and Mg-O bond lengths show broadly antithetic variations with pressure. The average Si-O increases and Mg-O decreases from 0 to ~7 GPa, followed by a reversal up to ~11 GPa. At higher pressures the Mg-O bond shortens and Si-O bond initially lengthens and then may decrease. Raman spectroscopy shows permanent changes in structure for glasses recovered from above 6.5 GPa, consistent with the development of a more depolymerized silicate network.

We interpret these variations as reflecting changes in topology related to compression of the Si and Mg polyhedra and coordination changes. The increase and decrease in Si-O and Mg-O bond lengths, respectively, above ~10 GPa are consistent with an increase in Si coordination and accompanying collapse of Mg polyhedra. The production of high coordinated Si is supported by the recent NMR study of Gaudio et al. [GCA, 2008, 72] reporting [5]Si and [6]Si in v-En after heating at 10 GPa and rapid decompression. NMR studies underway on annealed glasses decompressed from >10 GPa will further aid in the interpretation of our diffraction data.