Other
Scientific paper
Apr 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007gecoa..71.1855k&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 71, Issue 7, p. 1855-1881.
Other
13
Scientific paper
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.
Beckett John R.
Kessel Ronit
Stolper Edward M.
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