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

• Published in: Earth and planetary science letters Vol. 254; no. 3; pp. 433 - 445
• Main Authors: Xie, Zhidong, Sharp, Thomas G.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 28.02.2007
• Subjects: Ca-rich majorite; growth rate; ringwoodite lamellae; shock metamorphism; solid-state transformation; Tenham; Ca-rich majorite; growth rate; shock metamorphism; ringwoodite lamellae; solid-state transformation; Tenham
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2006.12.001

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)SiO 3 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.