Equilibrated ordinary chondrites: Constraints for thermal history from iron‐magnesium ordering in orthopyroxene

Abstract

Abstract-Iron-magnesium ordering was determined in orthopyroxenes from two suites of unshocked (shock stage S 1, S2), equilibrated L-and LL-chondrites (10 grains from 5 meteorites and 7 grains from 4 meteorites, respectively) by means of single crystal x-ray diffraction (SCXRD). This study, together with a previous investigation of H-chondrites (1 3 grains from 8 meteorites), produces an internally consistent data set about the thermal record in equilibrated ordinary chondrites (EOCs).The major feature outlined by cation ordering in EOC orthopyroxenes is that H-, L-and LL-chondrites share a common low-temperature record, that is, a common range of similar cooling rates in the 340-480 "C interval for the petrographic types 4 to 6. As a consequence, the thermal evolution of EOCs consists of at least two subsolidus stages; the first stage occurred at temperatures >480 "C where petrographic types were established in distinct environments; the second stage occurred when EOCs, irrespective of chemical class and type, cooled through 34M80 "C in environments characterized by close temperature-time conditions.Quantitative estimates of minimal cooling rates for EOCs range from a few "Cka to -102 "Cka in the 340480 "C interval. Possibly, final ordering was attained in environments where moderate radiative heat-loss was possible and, thus indicating shallow burial depths in the parent body.

Reclassification and thermal history of Trenzano chondrite

“…rates of equilibrated H chondrites near their closure temperature T c (450-550 °C) is in all cases very fast compared to Trieloff et al (2003), and is independent of the petrologic type, as observed by Folco et al (1997). The observed cooling rate at around 520 °C of Trenzano could indicate fast cooling after shock-reheating, or cooling in a shallow environment, possibly caused by denudation as a consequence of excavation of formerly overlying rocks or by fragmentation of the parent body into sizable blocks.…”

“…Shock heating and annealing may obscure pre-shock thermal history and, depending on the highest temperature reached during shock, may selectively disorder specific mineral closing temperatures. However, Trenzano is an S2 chondrite, and such weakly shocked meteorites are not expected to have undergone significant reheating due to shock (Folco et al 1997). According to Rubin (2004), all ordinary chondrites reached shock stage S3-S6 due to collisional events and then underwent post-shock annealing, which not only obliterated most of their primary shock features but also contributed significantly to the final thermometamorphism displayed.…”

“…Applications of orthopyroxene single crystal X-ray diffraction (XRD) studies to chondrites (Molin et al 1994;Folco et al 1996Folco et al , 1997 have shown a restricted orthopyroxene T c range for each chemical group (e.g., 384 ± 48 to 480 ± 28 °C for H chondrites; 346 ± 27 to 459 ± 26 °C for L chondrites; and 340 ± 28 to 463 ± 17 °C for LL chondrites) and no systematic correlation with petrologic type. Cooling rate estimates around the T c (103-105 °C/Myr), misquoted in Kessel et al (2007), are two to three orders of magnitude faster than those obtained from metallographic and 244 Pu fission track data (Folco et al 1997). Such fast cooling is in evident contrast with the observation that at temperatures below 500 °C, ordinary chondrite cooling rates are usually only a few °C/Myr (Kessel et al 2007).…”

Unraveling the evolution of chondrite parent asteroids by precise U-Pb dating and thermal modeling

“…Evaluation of cooling rates of chondrites based on precise U-Pb dating of pyroxene and phosphates has several differences from the methods of determination of cooling rates based on metallography, 244 Pu fission tracks, and Fe-Mg ordering in orthopyroxene (Wood, 1979;Lipschutz et al, 1989;Folco et al, 1997). First and most important, the cooling rate values are precisely linked to the absolute time scale.…”

Chondrites and Their Components

“…The cooling rates determined by the metallographic technique, which are mostly 1-10 3 C Ma -1 in the temperature range of 400-500 C, are compatible with cooling rates in the range 100-500 C determined by the fission track technique. A third technique based on Fe-Mg ordering in orthopyroxene gives cooling rates in the range 340-480 C that are systematically higher by several orders of magnitude (Folco et al, 1997), probably because metamorphism was insufficient to reequilibrate the ordering imposed during chondrule formation (Artioli and Davoli, 1994). Cooling rates can also be estimated from radiometric age data for mineral grains that close isotopically at different temperatures Bogard, 1995Bogard, , 2011Ganguly and Tirone, 2001).…”