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Orthopyroxene as a geospeedometer: Thermal history of Kapoeta, Old Homestead 001, and Hughes 002 howardites
Author(s) -
Domeneghetti M. c.,
Molin G. m.,
Triscari M.,
Zema M.
Publication year - 2000
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.1945-5100.2000.tb01780.x
Subject(s) - augite , breccia , geology , mineralogy , microprobe , materials science , closure temperature , geochemistry , analytical chemistry (journal) , crystallography , chemistry , composite material , metamorphic rock , quartz , plagioclase , chromatography
— Single crystals of orthopyroxene from small fragments of the Kapoeta, Old Homestead 001, and Hughes 002 howardites were studied by x‐ray diffraction and microprobe analyses. The Fe‐Mg equilibrium distribution coefficients k D of the crystals were used to calculate the closure temperatures ( T c ) using the calibration by Stimpfl et al. (1999). The compositions, the presence of exsolved augite lamellae, and the T c values (from 365 to 385 °C) obtained for Kapoeta orthopyroxene s suggest that our fragment comes from a diogenitic cumulate clast. The more Fe‐rich composition, the absence of exsolved lamellae, and the higher T c values (from 583 to 605 °C) measured in the Old Homestead 001 orthopyroxenes suggest that this fragment comes from a cumulitic clast affected by fast cooling at high temperature. For the Hughes 002 orthopyroxenes, close in composition to Old Homestead 001, the different T c values (339, 358, and 607 °C) recorded by the various crystals and the presence of augite lamellae in the crystals with the lowest T c support the hypothesis that this howardite sample is an unheated breccia containing a mixture of cumulitic orthopyroxenes with different thermal histories.