Thermal history of lodranites Yamato 74357 and MAC88177 as inferred from the chemical zoning of pyroxene and olivine

Abstract

The chemical zoning profiles of pyroxene and olivine in the Yamato (Y) 74357 and Mac Alpine Hills (MAC) 88177 acapulcoite‐lodranite primitive achondrites were measured by using an electron probe microanalyzer in order to study their cooling histories. The reverse zoning of Fe and Mg, and MnO enrichment were observed within a few tens of micrometers from the rim of orthopyroxene in Y74357; this is probably related to reduction. The CaO of orthopyroxene in Y74357 and MAC88177 decreases gradually toward the rim, as do the Cr2O3, Al2O3, TiO2, and Na2O in orthopyroxene and augite. These results suggest that interstitial melt was present and was then extracted at a late stage in the formation of these meteorites. In addition to these observations, the absence of Fe‐Mg reverse zoning and MnO enrichment at the rim of MAC88177 orthopyroxene implies that the reduction process of Y74357 took place after partial melting. A calculated cooling rate of 2–3°C/yr gives the best fit for the observed Fe‐Mg and CaO profiles of orthopyroxene, and CaO of olivine in Y74357, suggesting that the zonings were formed during the same event that probably caused the reduction. Thermal model calculations show that the maximum attainable temperature in the parent body was about 1000°C, if the decay of 26Al is the heat source and if the 26Al/27Al ratio is 5×10−6, identical to that used for the ordinary chondrite parent body. Partial melt was probably produced at this temperature; this is an important part of the history of some primitive achondrites.