Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004m%26ps...39.2043x&link_type=abstract
Meteoritics & Planetary Science, Vol. 39, No. 12, p.2043-2054
Physics
12
Scientific paper
We report a previously undocumented set of high-pressure minerals in shock-induced melt veins of the Umbarger L6 chondrite. High-pressure minerals were identified with TEM using selected-area electron diffraction, and energy-dispersive X-ray spectroscopy. Ringwoodite (Fa30), akimotoite (En11Fs89), and augite (En42Wo33Fs25) were found in the silicate matrix of the melt vein, representing the crystallization from a silicate melt during the shock pulse. Ringwoodite (Fa27) and hollandite-structured plagioclase were also found as polycrystalline aggregates in the melt vein, representing solid-state transformation or melting with subsequent crystallization of entrained host-rock fragments in the vein. In addition, Fe2SiO4-spinel (Fa66-Fa99) and stishovite crystallized from a FeO-SiO2-rich zone in the melt vein, which formed by shock melting of FeO-SiO2-rich material that had been altered and metasomatized prior to shock. Based on the pressure stabilities of the high-pressure minerals, ringwoodite, akimotoite, and Ca-clinopyroxene, the melt vein crystallized at approximately 18 GPa. The Fe2SiO4-spinel + stishovite assemblage in the FeO-SiO2-rich melts is consistent with crystallization of the melt-vein matrix at the pressure up to 18 GPa. The crystallization pressure of ?18 GPa is much lower than the 45-90 GPa pressure one would conclude from the S6 shock effects in melt veins (St?ffler et al. 1991) and somewhat less than the 25-30 GPa inferred from S5 shock effects (Schmitt 2000) found in the bulk rock.
Sharp Thomas G.
Xie Zhidong
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