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

• DOI: 10.1111/maps.12436
• Publication Date: 2015-03-17T00:23:29Z
• Authors: M. Alvaro; M. C. Domeneghetti; A. M. Fioretti; F. Cámara; L. Marinangeli
• Keywords: Mineralogy, Space and Planetary Science, geothermometer, Meteorites; augite; closure temperature; Martian nakhlite; single crystal X-ray diffraction; Thermal history; Geothermometer; 01 natural sciences; ORTHO-PYROXENE; MARTIAN METEORITE; CRYSTAL-CHEMISTRY; AEOLIAN ISLANDS; THERMAL HISTORY; COOLING RATES; THEOS FLOW; MIL 03346; DISORDER; KINETICS; Augite Geothermometer Meteorite Nakhlite Mars; 0105 earth and related environmental sciences

AbstractRecently it has been shown that the relatively low closure temperature (Tc) 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 Fe2+‐Mg degree of order remained unchanged, thus suggesting that the actual Tc 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 Fe2+‐Mg exchange is reached, using an augite crystal from Miller Range nakhlite (MIL 03346,13) with a composition of about En36Fs24Wo40. These data allowed us to calculate the following new geothermometer calibration for Martian nakhlites: urn:x-wiley:10869379:media:maps12436:maps12436-math-0001where 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 Tc 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.