TY - JOUR
T1 - Host rock solid-state transformation in a shock-induced melt vein of Tenham L6 chondrite
AU - Xie, Zhidong
AU - Sharp, Thomas
N1 - Funding Information:
We thank Carlton Moore and the Center for Meteorite Studies at Arizona State University for supplying the sample. We also thank John Wheatley, Karl Weiss, and Zhengquan Liu, and the Center for High Resolution Microscopy at ASU for assistance with the electron microscopy. We are grateful to Paul DeCarli and Adrian Brearley for reviews of the manuscript. We acknowledge NASA Cosmochemistry grants NAG5–7285, NAG5–9381 and NAG5–1977 for supporting this research.
PY - 2007/2/28
Y1 - 2007/2/28
N2 - 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.
AB - 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.
KW - Ca-rich majorite
KW - Tenham
KW - growth rate
KW - ringwoodite lamellae
KW - shock metamorphism
KW - solid-state transformation
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U2 - 10.1016/j.epsl.2006.12.001
DO - 10.1016/j.epsl.2006.12.001
M3 - Article
AN - SCOPUS:33846580081
SN - 0012-821X
VL - 254
SP - 433
EP - 445
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
IS - 3-4
ER -