Pyroxene equilibration temperatures in metamorphosed ordinary chondrites

Abstract

Ordinary chondrites are divided into petrographic types based on observed mineralogical and textural properties consistent with progressive thermal metamorphism from low grade (type 3) to high (type 7). Regardless of the exact cause of the metamorphism, higher-type chondrites should retain information concerning peak temperatures reached and for what duration. Using the two-pyroxene geothermometer of Lindsley, we have calculated the equilibration temperatures for 26H, L and LL type 5 and 6 ordinary chondrites, to investigate the relative peak temperatures and equilibration-states reached by these various meteorite classes. The Lindsley thermometer relies on a detailed accounting of non-quadrilateral components in pyroxenes, whose recalculated compositions are then plotted onto an empirically-derived polythermal diagram from which temperatures can be interpolated. The reported uncertainty of this method is plus or minus 50 C; in addition, close spacing of isotherms on the graph (particularly for orthopyroxene compositions) increase this uncertainty. We have parameterized the Lindsley polythermal quadrilateral for 1 atm pressure (less than 2 bar), and interpreted recalculated coordinates directly in terms of 25 C temperature intervals. Meteorites selected for this study include both relatively shocked and unshoked specimens; heavily weathered or visibly brecciated specimens were avoided. Temperatures were calculated from orthopyroxene (opx) and clinopyroxene (cpx) analyses within one relative percent of ideal sums and stoichiometry. Histograms summarizing the calculated temperatures for type 5 and 6 ordinary chondrites are shown.