Pyroxene spectroscopy revisited: Spectral-compositional correlations and relationship to geothermometry.

Pyroxene reflectance and transmittance spectra have been examined in a search for systematic relationships between spectral features and compositional variations and to assess the applicability of reflectance spectroscopy to pyroxene geothermometry. Orthopyroxenes containing up to ~11% Wollastonite show a positive correlation between Fe2+ content and wavelength positions of the major absorption bands. Aluminum-rich orthopyroxenes display absorption bands at lower than expected wavelengths. Spectral-compositional relationships are more complex for clinopyroxenes, showing both positive and negative correlations between band positions and major cation abundances. These relationships are further complicated by the presence of significant amounts of other transition series elements such as Ti and Cr and by the presence of exsolved phases and compositional zonations. Even small-scale exsolutions ( < 1 μm wide) seem to be sufficient to significantly alter spectral properties. Contours of the wavelength positions of band minima projected onto the pyroxene tetralateral generally exhibit concave downward shapes. The orientations of the contours relative to pyroxene geotherms are such as to effectively preclude the use of spectroscopy to significantly constrain the temperatures of formation of pyroxenes.The authors wish to thank the various agencies which have provided the necessary support for this project including grants-in-aid of research from Sigma Xi, The Scientific Research Society (to E.A.C.), the Geological Society of America grant 3741-87 (to E.A.C.), and NASA Planetary Geology and Geophysics grant NAGW 642 (to M.J.G.).The authors wish to thank the various agencies which have provided the necessary support for this project including grants-in-aid of research from Sigma Xi, The Scientific Research Society (to E.A.C.), the Geological Society of America grant 3741-87 (to E.A.C.), and NASA Planetary Geology and Geophysics grant NAGW 642 (to M.J.G.).https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/91JE0251