Reclassification and thermal history of Trenzano chondrite

We present a new single-crystal X-ray diffraction (XRD) study performed on a suite of six orthopyroxene grains from the low-shocked H6 Trenzano meteorite. The quenched intracrystalline Fe2+-Mg ordering state in orthopyroxene preserves the memory of the cooling rate near closure temperature T-c, thus yielding useful constraints on the last thermal event undergone by the host rock. The orthopyroxene T-c of 522 +/- 13 degrees C, calculated using a new calibration equation obtained by Stimpfl (2005b), is higher than in previously published H chondrite data. The orthopyroxene cooling rate at this T-c is about 100 degrees C/kyr. This fast rate is inconsistent with the much slower cooling rate expected for H6 in the onion shell structural and thermal model of chondrite parent bodies. A petrographic study carried out at the same time indicated that the Trenzano meteorite is an H5 chondrite and not an H6 chondrite, as it is officially classified. Furthermore, the two-pyroxene equilibrium temperature of Trenzano (824 +/- 24 degrees C), calculated with QUILF95, is similar to the two-pyroxene temperature of 750-840 degrees C obtained for the Carcote (H5) chondrite (Kleinschrot and Okrusch 1999).