Thermal history of nakhlites : a comparison between MIL 03346 and its terrestrial analogue Theo's flow

High resolution single-crystal X-ray diffraction (HR-SCXRD) and Mössbauer spectroscopy of the intracrystalline cation distribution have been performed on augitic core-crystals from a Miller Range nakhlite (sample MIL 03346,13) with approximate composition of En36Fs24Wo40. The Mössbauer data on the single-crystal yielded a very low Fe3+ content [Fe3+/Fetotal - 0.033(23)a.p.f.u.] that, together with the Electron microprobe analysis (EMPA) and the X-ray structural data allowed us to obtain the accurate cation site distribution and the Fe2+-Mg degree of order. This leads to a closure temperature (Tc) of 500 with a standard deviation of ±100°C that would correspond to a slow cooling rate, which is in disagreement with petrologic evidence that indicates that this sample originates from a fast cooled (~3-6°C/h) lava flow.In order to clarify this discrepancy we undertook (i) a SC-XRD study of an augite (~En49Fs9 Wo42) from a pyroxenite (TS7) of Theo's flow, a 120-m-thick lava flow regarded as a terrestrial analogue of MIL 03346; (ii) an annealing experiment at 600°C on a crystal from exactly the same fragment of MIL 03346. SC-XRD data from TS7 augite yields a Tc=600(20)°C, consistent with the cooling rate expected at 85m below the surface. This Tc is higher, although similar within error, to the Tc=500(100)°C obtained for MIL 03346; thus suggesting relatively slower cooling for MIL 03346 with respect to TS7. The annealing experiment on the MIL 03346 crystal clearly showed that the degree of order remained unchanged, further confirming that the actual Tc is close to 600°C.This result appears inconsistent with the shallow depth of origin (~<2m) assumed for MIL 03346, further supporting the discrepancy between MIL 03346 textural and petrologic evidence of fast cooling and the abovementioned Tc results obtained for augite. Therefore, a tentative scenario is that, soon after eruption and initial quench and while still at relatively high-T (~600°C), MIL 03346 was blanketed with subsequent lava flows that slowed down the cooling rate and allowed the augite Fe2+-Mg exchange reaction to proceed.