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Origin of a unique impact‐melt rock—the L‐chondrite Ramsdorf
Author(s) -
YAMAGUCHI Akira,
SCOTT Edward R. D.,
KEIL Klaus
Publication year - 1999
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.1999.tb01731.x
Subject(s) - chondrule , pyroxene , olivine , porphyritic , chondrite , phenocryst , geology , pigeonite , geochemistry , shock metamorphism , mineralogy , electron microprobe , meteorite , augite , plagioclase , volcanic rock , astrobiology , physics , paleontology , quartz , volcano
— We have studied a unique impact‐melt rock, the Ramsdorf L chondrite, using optical and scanning microscopy and electron microprobe analysis. Ramsdorf contains not only clast‐poor impact melt (Begemann and Wlotzka, 1969) but also a chondritic portion (>60 g) with what appears at low magnification to be a normal, well‐defined chondritic texture. However, detailed studies at high magnification show that >90 vol% of the crystals in the chondritic portion were largely melted by the impact: the chondrules lack normal microtextures and are ghosts of the original features. The only relics from the precursor chondrules are olivine crystals, which have the highest melting temperature (∼1620 °C). Pyroxene‐rich chondrules were so extensively melted that no phenocrysts were preserved and the melt crystallized in situ before significant mixing with exterior olivine‐rich melts. Fine‐grained pyroxene chondrule ghosts have sharper boundaries with the matrix than porphyritic olivine and pyroxene chondrule ghosts, probably because pyroxene‐rich melts are significantly more viscous. Complex textures that formed by injection of melt along cracks and fractures in relic olivines suggest that the chondritic portion of Ramsdorf formed directly from petrologic type 3–4 material by strong shock. We infer that Ramsdorf was largely melted by shock pressures of ∼75–90 GPa and that chondrule ghosts and relic olivine phenocrysts were locally preserved by rapid cooling. Quenching was not due to the addition of cold clasts into the melt but to heterogeneous shock heating that only caused internal melting of large olivines and pyroxenes. Ramsdorf appears to be one of the most heavily shocked meteorites that has retained some trace of its original texture.