Thermal history of Acapulco and ALHA81261 acapulcoites constrained by Fe2+Mg ordering in orthopyroxene

Petrogenesis of acapulcoites and lodranites: A shock-melting model

“…29, 2 as transitional between acapulcoites and lodranites. Zema et al (1996); these workers measured Fe 2+ -Mg ordering in orthopyroxene in Acapulco and ALH 81261 (paired with ALH A77081) and determined cooling rates of $0.17°C/day at 490°C and $0.05°C/day at 465°C, respectively. These large discrepancies in cooling rates point to a complicated thermal history for the acapulcoites and lodranites.…”

Fe-Mg order-disorder in orthopyroxenes

“…Orthopyroxene has two crystallographically distinct octahedral sites, M1 and M2 in the structure, with Fe 2ϩ (referred to as Fe hereafter) preferring the larger and more distorted M2 site and Mg 2ϩ (referred to as Mg hereafter) preferring M1 (Ghose, 1965). The Fe-Mg distribution between the two sites is of great interest because of its effect on the thermodynamic property of orthopyroxene and its potential application to the determination of cooling history of terrestrial and extraterrestrial rocks (Ghose and Hafner, 1967;Virgo and Hafner, 1970;Anovitz et al, 1988;Skogby, 1992;Ganguly et al, 1994;Zhang, 1994;Ganguly and Domeneghetti, 1996;Zema et al, 1996;Kroll et al, 1997;Domeneghetti et al, 2000;Heinemann et al, 2000). These applications all rely on a knowledge of equilibrium distribution and exchange kinetics.…”

Cooling rates of diogenites: A study of Fe2+‐Mg ordering in orthopyroxene by single‐crystal x‐ray diffraction

“…Achondrites from the A-L (acapulcoite-lodranite) parent body fall in a distinct region at higher closure temperatures. In particular, Acapulco and ALHA81261 acapulcoites (Zema et al, 1996b) (Zema et al, 1996~). Much lower closure temperatures are exhibited by Estherville and Bondoc mesosiderites and by a silicate inclusion of Landes IAB iron meteorite (Stimpfl et al, 1993).…”