Cation Ordering in Spinel from Calcium-Aluminum-Rich Inclusions in Carbonaceous Chondrites NWA 2364 and NWA 6991 to Quantify Temperature in the Early Solar System

Abstract

Abstract Minerals extracted from two calcium-aluminum-rich inclusions, one each from NWA 2364 and NWA 6991 CV3 chondrite meteorites, were examined using micro X-ray diffraction, 27Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) and Triple Quantum (3Q) MAS NMR. In situ examination by micro X-ray diffraction was used to confirm the presence of spinel (MgAl2O4) and to identify co-existing minerals. Aluminum-27 3Q MAS NMR was used to confirm the identity of co-existing minerals by their NMR signature in this two-dimensional experiment and to ensure that their NMR peaks did not overlap with those attributed to spinel. Aluminum-27 MAS NMR was used to quantitatively measure cation ordering between the tetrahedral ([4]Al) and octahedral ([6]Al) sites in the spinel. The measured cation distribution was used to calculate the inversion parameter, x, for each sample: x = 0.077 ± 0.007 for NWA 2364, x = 0.027 ± 0.001 for NWA 6991 (gehlenite-rich fraction), and x = 0.052 ± 0.003 for NWA 6991 (Al-bearing diopside-rich fraction). The measured cation-ordering data were input into six literature calibration curves to estimate the temperature of formation or the most recent equilibration temperature of the spinel. The NWA 2364 sample yielded temperatures between 420 and 707 K and the NWA 6991 sample yielded temperatures between 153 and 615 K, depending on the calibration curve used. These temperatures are lower than expected for nebular condensation temperatures, however, reordering may have occurred during cooling from high temperatures, so these values may be taken to represent temperature minima. The calculated spinel-related temperatures may thus represent equilibration temperatures related to subsequent nebular or CV chondrite parent body processes.