Zaoyang Chondrite Cooling History from Fe2+-Mg Intracrystalline Ordering in Pyroxenes

Gian Mario Molin, Mario Tribaudino and Elisabetta Brizi
Dipartimento di Mineralogia e Petrologia, Università di Padova, Corso Garibaldi 37, I-35100 Padova, Italy
Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Via Valperga Caluso 37, I-10125 Torino, Italy
Dipartimento di Scienze della Terra, Università di Perugia, Piazza Università, I-06100 Perugia, Italy

Abstract: The crystal chemistry of clinopyroxene, orthopyroxene and olivine from a crushed fragment of the H5 Zaoyang chondrite has been investigated by X-ray structure refinement and detailed microprobe analysis. The meteoritic pyroxenes have cell and polyhedral volumes which compare well with such data from terrestrial pyroxenes that typically crystallize at low-pressure. Fe2+ and Mg are rather disordered in M1 and M2 sites of clino- and orthopyroxenes; the closure temperatures of the exchange reaction are 600 and 512°C respectively, which is consistent with a reasonably fast cooling rate, estimated to be of the order of 1°C/day.

The closure temperature for the intercrystalline Ca-Mg exchange reaction for clino- and orthopyroxenes is 900°C as calculated from clino- and orthopyroxene intergrowth.

The cooling rates obtained from Fe2+-Mg intracrystalline partitioning suggest a cooling of the order of degrees per day at temperatures of 600–500°C due to a strong loss of heat by irradiation.

Keywords: chondrite • pyroxene • crystal-chemistry • cooling rate

Mineralogical Magazine; March 1994 v. 58; no. 390; p. 143-150; DOI: 10.1180/minmag.1994.058.390.13
© 1994, The Mineralogical Society
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