Details

Autor(en) / Beteiligte

• Miyahara, Masaaki
• El Goresy, Ahmed
• Ohtani, Eiji
• Nagase, Toshiro
• Nishijima, Masahiko
• Vashaei, Zahra
• Ferroir, Tristan
• Gillet, Philippe
• Dubrovinsky, Leonid
• Simionovici, Alexandre

Titel

Evidence for fractional crystallization of wadsleyite and ringwoodite from olivine melts in chondrules entrained in shock-melt veins

Ist Teil von

• Proceedings of the National Academy of Sciences - PNAS, 2008-06, Vol.105 (25), p.8542-8547

Ort / Verlag

United States: National Academy of Sciences

Erscheinungsjahr

2008

Quelle

EZB Electronic Journals Library

Beschreibungen/Notizen

• Peace River is one of the few shocked members of the L-chondrites clan that contains both high-pressure polymorphs of olivine, ringwoodite and wadsleyite, in diverse textures and settings in fragments entrained in shock-melt veins. Among these settings are complete olivine porphyritic chondrules. We encountered few squeezed and flattened olivine porphyritic chondrules entrained in shock-melt veins of this meteorite with novel textures and composition. The former chemically unzoned (Fa₂₄₋₂₆) olivine porphyritic crystals are heavily flattened and display a concentric intergrowth with Mg-rich wadsleyite of a very narrow compositional range (Fa₆-Fa₁₀) in the core. Wadsleyite core is surrounded by a Mg-poor and chemically stark zoned ringwoodite (Fa₂₈-Fa₃₈) belt. The wadsleyite-ringwoodite interface denotes a compositional gap of up to 32 mol % fayalite. A transmission electron microscopy study of focused ion beam slices in both regions indicates that the wadsleyite core and ringwoodite belt consist of granoblastic-like intergrowth of polygonal crystallites of both ringwoodite and wadsleyite, with wadsleyite crystallites dominating in the core and ringwoodite crystallites dominating in the belt. Texture and compositions of both high-pressure polymorphs are strongly suggestive of formation by a fractional crystallization of the olivine melt of a narrow composition (Fa₂₄₋₂₆), starting with Mg-rich wadsleyite followed by the Mg-poor ringwoodite from a shock-induced melt of olivine composition (Fa₂₄₋₂₆). Our findings could erase the possibility of the resulting unrealistic time scales of the high-pressure regime reported recently from other shocked L-6 chondrites.