Premium
A new calibration to determine the closure temperatures of Fe‐Mg ordering in augite from nakhlites
Author(s) -
Alvaro M.,
Domeneghetti M. C.,
Fioretti A. M.,
Cámara F.,
Marinangeli L.
Publication year - 2015
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.12436
Subject(s) - augite , annealing (glass) , closure temperature , martian , geology , calibration , materials science , atmospheric temperature range , mineralogy , analytical chemistry (journal) , thermodynamics , chemistry , physics , petrology , plagioclase , metallurgy , mars exploration program , astrobiology , quartz , chromatography , metamorphic rock , quantum mechanics
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.