The thermal history of equilibrated ordinary chondrites and the relationship between textural maturity and temperature

Abstract

The compositions and textures of phases in eleven equilibrated ordinary chondrites from the H, L, and LL groups spanning petrographic types 4–6 were studied and used to constrain the thermal histories of their parent bodies. Based on Fe–Mg exchange between olivine and spinel, average equilibration temperatures for type 4–6 chondrites encompass a small range, 586–777 °C, relative to what is commonly assumed for peak temperatures (600–950 °C). The maximum temperatures recorded by individual chondrites, which are minima relative to peak metamorphic temperatures, increase subtly but systematically with metamorphic type and are tightly clustered for H4-6 (733–754 °C) and LL4-6 (670–777 °C). For the Ls, Ausson (L5) records a higher maximum olivine–spinel temperature (761 °C) than does the L4 chondrite Saratov (673 °C) or the L6 chondrite Glatton (712 °C). Our data combined with olivine–spinel equilibration temperatures calculated for other equilibrated ordinary chondrites using mineral compositions from the literature demonstrate that, in general, type 4 chondrites within each chemical group record temperatures lower than or equal to those of types 5–6 chondrites. For H chondrites, the olivine–spinel closure temperature is a function of spinel grain size, such that larger grains, abundant in types 5–6 chondrites, record temperatures of ∼740 °C or more while smaller grains, rare in types 5–6 but abundant in type 4 chondrites, record lower temperatures. Olivine–spinel temperatures in the type 6 chondrites Guareña and Glatton are consistent with rapid (50–100 °C/Myr) cooling from high temperatures in the ordinary chondrite parent bodies. With one exception (∼500 °C/Myr), olivine–spinel data for St.-Séverin (LL6) are consistent with similar cooling rates. Cooling rates of order 100 °C/Myr at ∼750 °C for type 6 chondrites are considerably higher than previously determined cooling rates for lower temperatures (⩽550 °C) based on metallography, fission tracks, and geochronology. For H chondrites, current thermal models of an "onion shell" parent body are inconsistent with a small range of peak temperatures based on olivine–spinel and two pyroxene thermometry combined with a wide dispersion of cooling rates at low temperatures. Equilibrated chondrites may have sampled regions near a major transition in physical properties such as near the base of a regolith pile.

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