Thermal history of ALH 84001 meteorite by Fe2+-Mg ordering in orthopyroxene

A single orthopyroxene crystal from the Martian meteorite Allan Hills (ALH) 84001 was studied by X-ray diffraction (XRD) and electron microprobe analysis (EMPA) to retrieve information about its thermal history. Both sets of data were used to measure the Fe2+-Mg order degree between the M1 and M2 sites expressed by the distribution coefficient k(D). The 529 +/- 30 degrees C closure temperature (T-c) of the Fe2+-Mg ordering process of ALH 84001 orthopyroxene (FS28) was calculated using Stimpfl (2005a, 2005b) Ink(D) versus I/T equation obtained for intermediate iron sample. At this T-c, the orthopyroxene cooling rate, calculated by Ganguly's (1982) numerical method, was 0.1 degrees C/day. This study puts new constraints on the last high-temperature thermal episode recorded by orthopyroxene. With reference to the geological history (Treiman 1998), we ascribe this episode to the 13 event, and we interpret the T-c of 529 degrees C as a lower limit for this impact heating. Our data confirm that experimentally defined physical conditions for the formation of magnetite from decomposition of carbonates took place on the Martian surface during event 13.