A new calibration to determine the closure temperatures of Fe‐Mg ordering in augite from nakhlites

Recently it has been shown that the relatively low closure temperature ( T c ) of 500 (100)°C calculated for augite from Miller Range nakhlite ( MIL 03346,13) using the available geothermometers would correspond to a slow cooling rate inconsistent with the petrologic evidence for an origin from a fast‐cooled lava flow. Moreover, previous annealing experiments combined with HR ‐ SC ‐ XRD on an augite crystal from MIL 03346 clearly showed that at 600 °C, the Fe 2+ ‐Mg degree of order remained unchanged, thus suggesting that the actual T c is close to this temperature. In order to clarify this discrepancy, we undertook an ex situ annealing experimental study at 700, 800, and 900 °C, until the equilibrium in the intracrystalline Fe 2+ ‐Mg exchange is reached, using an augite crystal from Miller Range nakhlite ( MIL 03346,13) with a composition of about En 36 Fs 24 Wo 40 . These data allowed us to calculate the following new geothermometer calibration for Martian nakhlites:ln k D = − 4421 ( ± 561 ) / T ( K ) + 1.46 ( ± 0.52 ) ( R 2 = 0.988 ) , whereK D = [ ( F e M 1 2 + ) ( M g M 2 ) / ( F e M 2 2 + ) ( M g M 1 ) ] . The application of this new equation to other Martian nakhlites ( NWA 988 and Nakhla) suggests that for augite with composition close to that of MIL 03346, the T c is up to 170 °C higher with respect to the one calculated using the previous available geothermometer equation, thus suggesting a significantly faster cooling in agreement with petrologic evidence.