The role of Fe content on the Fe-Mg exchange reaction in augite

Abstract

The study of the intracrystalline Fe-Mg exchange between M 1 and M 2 crystallographic sites in clinopyroxene on samples a variety of geological settings has provided a framework to understand the thermal history of pyroxene-bearing rocks. The Fe-Mg exchange reaction has successfully been exploited as a geothermometric tool in the study of orthopyroxene and pigeonite-bearing rocks, but relatively few data are available for clinopyroxene. A strong correlation between total iron content and the slope of the Fe 2+ -Mg equilibrium distribution coefficient ( k D ) as a function of temperature has been found for orthopyroxene and pigeonite, and we investigate this relationship in augite. We carried out new equilibrium annealing experiments at 800, 900, and 1000 °C followed by single-crystal X-ray diffraction and structural refinement to obtain a new geothermometric calibration for augite from a 120 m thick lava flow from Ontario, Canada [Theo’s Flow, En 49 Fs 9 Wo 42 hereafter also referred as Fs 9 where Fs = 100·ΣFe/(ΣFe+Mg+Ca) with ΣFe = Fe 2+ +Fe 3+ +Mn]. This new calibration enabled us to evaluate the compositional effects (mainly Fe content) by comparison with the data previously obtained on augite from MIL 03346 martian sample (En 36 Fs 24 Wo 40 hereafter referred to as Fs 24 ). The extremely good agreement observed between the data obtained on Theo’s Flow and Miller Range (MIL 03346) augite demonstrate that for the range of compositions between Fs 9 and Fs 24 , total iron content has a negligible or null influence on equilibrium behavior. Furthermore, linear regression of data from Theo’s Flow and MIL 03346 gave a single calibration equation: ln k D = − 4040 ( ± 180 ) / T ( K ) + 1 . 12 ( ± 0 . 17 ) ( R 2 = 0 . 988 ) . This new calibration describes the equilibrium behavior of augite and can be reliably used to determine the closure temperature ( T c ) of augite with composition ranging between Fs 9 and Fs 24 .