A test for oxidation during metamorphism of L and LL chrondrites

Subtle but progressive changes in the olivine and low-Ca pyroxene abundances and the olivine, low-Ca pyroxene,and diopside compositions are observed with increasing metamorphic grade in equilibrated (types 4-6)ordinary chondrites. These changes suggest that the meteorites experienced oxidation of metallic Fe during heating according to the reaction:

Fe + ½ O₂ + (Mg,Fe)Si0₃ => (Mg, Fe)₂Si0₄

metal    pyroxene    olivine

hypothesized by McSween and Labotka (1993), based on calculated normative mineralogy as well as incorporation of Fe²⁺ into ferromagnesian silicate phases. From this study, two observations support this hypothesis: (1) the modal olivine: low-Ca pyroxene ratio increases with petrologic type in L and LL chondrites and (2) Fe-enrichment in olivine and low-Ca pyroxene are observed from petrologic types 4-6 in the L chondrites. The behavior of the LL chondrites studied is inconsistent; however,a general Fe-enrichment in silicates is apparent.

Characteristic spectral absorption features in visible infrared spectra (VIS- NIR) are sensitive to the olivine/ low-Ca pyroxene abundance ratio and the Ca²⁺ and Fe²⁺ contents of pyroxenes. Therefore, metamorphic grade (petrologic type) in ordinary chondrites might be identified using characteristic spectra since each petrologic type has different olivine/ pyroxene abundances and pyroxene compositions. This work is applicable to the NEAR mission to 433 Eros and other ground-based spectroscopic studies of asteroids.