Two-pyroxene thermometry in chondrules from Nuevo Mercurio H5 ordinary chondrite, Mexico

Abstract

In this work we reconstructed the thermal history of some chondrules of Nuevo Mercurio H5 ordinary chondrite. We selected pyroxene-rich chondrules of different textural groups (RP, C and PP) showing diopsidic rims and exsolution textures and obtained 149 WDS microprobe chemical analyses. In order to calculate the temperature, we used the QUILF program for two-pyroxene thermometry, which yielded minimal values of crystallization because Ca-poor pyroxene formed prior to Ca-rich pyroxene. Presence of contraction cracks, clinoenstatite Raman spectra and enclosed olivine in enstatite phenocrysts suggest formation temperatures much greater than 1200 ºC. The highest pyroxene temperatures were obtained for augite blebs and grains (1065–1232 ºC), pigeonite lamellae (1037–1103 ºC) and enstatite cores (1277–1284 ºC). On the other hand, calculated temperatures for augite lamellae and diopsidic rims are 933–817 ºC, whilst enstatite reached temperatures from 915–988 ºC. We found in the five chondrules of Nuevo Mercurio that diopside rims and Ca-rich augite lamellae reached a minimum temperature range between 817 ºC and 933 ºC, and interpreted this temperature range as the peak of thermal metamorphism. We conclude that pyroxene temperatures from Nuevo Mercurio chondrules had two origins: the highest, albeit minimum temperatures (1283–915 ºC) preserved from chondrule formation, and the lowest temperatures (933–817 ºC) reflecting prograde thermal metamorphism well inside the chondritic parental body, as suggested by the low cooling rate reported by other authors of 15 ºC/Ma. We performed mineragraphic analysis of troilite + Fe-Ni alloys in order to verify that the metamorphic peak did not reach the eutectic temperature of this opaque minerals association at 988 ºC. X-ray diffraction data detected sodic plagioclase, a secondary recrystallization product confirming the thermal metamorphism grade 5.