The broadest application of in-tracrystalline Fe2+-Mg partitioning between the M1 and M2 crystallographic sites in the pyroxene structure is the determination of the closure temperature (Tc) of the Fe2+-Mg exchange reaction that provides important con-straint on the cooling rate of the pyroxene-bearing host rocks (eg. [1]). Although this approach has been suc-cessfully developed and applied for orthopyroxene and pigeonite-bearing rocks, relatively few data are availa-ble for clinopyroxenes (cpxs). The most recent calibra-tion for cpxs has been obtained by [2]. Calculations per-formed for cpxs in some Earth and planetary contexts, provided (i) Tc consistent among different samples and coherent with their respective geological setting; (ii) cooling rates for different samples from the same con-text in significant disagreement one to another. [3] showed that the relative low Tc calculated for augite from Miller Range nakhlite (MIL 03346) 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. In order to account for these discrepancies [4] per-formed an ex situ equilibrium annealing study on augite crystals from Miller Range 03346 nakhlite (MIL 03346, Fs24). This calibration has been then applied also to the augite from Theo's Flow always regarded as the terres-trial analogue for MIL03346. With the new calibration it is clear that the nakhlites Tc is about 600°C thus lower than that of 720°C for TS7 sample, which was sup-posed to be cooled at a burial depth of 85m. However, despite the similar geological setting, the difference in Tc could be just ascribed to the differences in Fe content that may affect the Mg-Fe equilibrium behaviour.
Fe-Mg exchange reaction in clinopyroxene and its application to the thermal history of planetary bodies
AM Fioretti;
2016
Abstract
The broadest application of in-tracrystalline Fe2+-Mg partitioning between the M1 and M2 crystallographic sites in the pyroxene structure is the determination of the closure temperature (Tc) of the Fe2+-Mg exchange reaction that provides important con-straint on the cooling rate of the pyroxene-bearing host rocks (eg. [1]). Although this approach has been suc-cessfully developed and applied for orthopyroxene and pigeonite-bearing rocks, relatively few data are availa-ble for clinopyroxenes (cpxs). The most recent calibra-tion for cpxs has been obtained by [2]. Calculations per-formed for cpxs in some Earth and planetary contexts, provided (i) Tc consistent among different samples and coherent with their respective geological setting; (ii) cooling rates for different samples from the same con-text in significant disagreement one to another. [3] showed that the relative low Tc calculated for augite from Miller Range nakhlite (MIL 03346) 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. In order to account for these discrepancies [4] per-formed an ex situ equilibrium annealing study on augite crystals from Miller Range 03346 nakhlite (MIL 03346, Fs24). This calibration has been then applied also to the augite from Theo's Flow always regarded as the terres-trial analogue for MIL03346. With the new calibration it is clear that the nakhlites Tc is about 600°C thus lower than that of 720°C for TS7 sample, which was sup-posed to be cooled at a burial depth of 85m. However, despite the similar geological setting, the difference in Tc could be just ascribed to the differences in Fe content that may affect the Mg-Fe equilibrium behaviour.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
