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

doi:10.1073/pnas.0801518105

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

*Institute of Mineralogy, Petrology, and Economic Geology, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan;
^‡Bayerisches Geoinstitut, Universität Bayreuth, D-95440, Bayreuth, Germany;
^§Tohoku University Museum, Tohoku University, Sendai 980-8578, Japan;
^¶Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan;
^‖Laboratoire des Sciences de la Terre, Université de Lyon, Ecole Normale Supérieure de Lyon, Université Claude Bernard Lyon 1, Centre National de la Recherche Scientifique 46 Allée d'Italie, 69364 Lyon Cedex 07, France; and
**Observatoire des Sciences, Laboratoire Géophysique Interne et Tectonophysique, Université de Grenoble, 38041 Grenoble Cedex 9, France

Edited by David Walker, Lamont–Doherty Earth Observatory of Columbia University, Palisades, NY, and approved May 3, 2008 (received for review February 15, 2008)

Abstract

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 (Fa_24–26) 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 (Fa_24–26), starting with Mg-rich wadsleyite followed by the Mg-poor ringwoodite from a shock-induced melt of olivine composition (Fa_24–26). 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.

Footnotes

^†To whom correspondence should be addressed. E-mail: miyahara{at}ganko.tohoku.ac.jp
Author contributions: M.M., A.E.G., and E.O. designed research; M.M., A.E.G., T.N., M.N., Z.V., T.F., P.G., L.D., and A.S. performed research; M.M. analyzed data; and M.M. and A.E.G. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.