Host rock solid-state transformation in a shock-induced melt vein of Tenham L6 chondrite

Zhidong Xie, Thomas Sharp

Research output: Contribution to journalArticle

38 Scopus citations

Abstract

The host-rock fragments entrained in a 580-μm-wide melt vein of the Tenham L6 chondrite were investigated using field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) to better understand the solid-state transformation mechanisms and the shock conditions. The melt vein consists of a matrix of silicate plus metal-sulfide that crystallized from immiscible melts, and sub-rounded host-rock fragments that have been entrained in the melt and transformed to polycrystalline high-pressure silicates by solid-state transformation mechanisms. These high-pressure phases include ringwoodite, low-Ca majorite, clinoenstatite, hollandite-structured plagioclase and Ca-rich majorite. The Ca-rich majorite occurs as a symplectitic intergrowth with a Ca-poor amorphous silicate phase in a 200 μm-diameter chondrule in the vein. This intergrowth seems to be the result of a disproportionate breakdown of a Ca-rich clinopyroxene precursor into Ca-rich majorite and (FeMg)SiO3 perovskite, which subsequently vitrified upon pressure release. The TEM observations suggest that most solid-state transformations in the Tenham are reconstructive. The transformation of olivine to polycrystalline ringwoodite appears to involve incoherent intracrystalline nucleation and interface-controlled growth. Lamellae in partially transformed olivine are not continuous coherent lamellae, but rather lamellae of polycrystalline ringwoodite, which is inconsistent with a coherent lamellar transformation mechanism. Growth rate calculations based on published kinetic data suggest that the time required to grow 1 μm ringwoodite crystal is ∼ 100 ms at 1600 K, suggesting that the minimum shock pulse of approximately 100 ms.

Original languageEnglish (US)
Pages (from-to)433-445
Number of pages13
JournalEarth and Planetary Science Letters
Volume254
Issue number3-4
DOIs
StatePublished - Feb 28 2007

Keywords

  • Ca-rich majorite
  • Tenham
  • growth rate
  • ringwoodite lamellae
  • shock metamorphism
  • solid-state transformation

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

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