Analysis of terrestrial and Martian volcanic compositions using thermal emission spectroscopy: 1. Determination of mineralogy, chemistry, and classification strategies

We have examined and applied existing classification schemes for volcanic r ocks and developed new schemes using thermal emission spectra Of terrestria l volcanic rocks. Laboratory thermal infrared spectra (5-25 mum, at 2 cm sp ectral sampling), deconvolved modal mineralogies, and derived mineral and b ulk rock chemistries were used to distinguish basalt, basaltic andesite, an desite, and dacite. Modal mineralogies derived from linear deconvolution of terrestrial volcanic rocks were compared to modes measured by an electron microprobe phase mapping technique to determine the accuracy of linear deco nvolution in modeling specific mineral abundances. One sigma standard devia tions of the absolute differences between modeled and measured mineral abun dances range from 2.4 to 12.2 vol %, with an average standard deviation of 4.8 vol % being in agreement with average uncertainties calculated in previ ous studies. Weighted average compositions of feldspars in the deconvolutio n generally overlap the measured ranges of plagioclase compositions and the presence of low-calcium and high-calcium pyroxenes was correctly identifie d. Bulk chemistries of volcanic rocks were derived with a relatively high d egree of accuracy (1 sigma standard deviations ranging from 0.4 to 2.6 vol %) by combining the compositions of spectrally modeled phases in proportion to their relative abundances in a particular sample. These data were colle ctively used to examine existing and develop new volcanic rock classificati on schemes. However, no single classification scheme was effective in accur ately classifying all samples. Multiple steps of classification were requir ed to distinguish volcanic rocks, reflecting the mineralogic diversity and continuum of compositions that exists in volcanic rock types. In a companio n paper [Hamilton et al., this issue] these schemes are applied to the clas sification of Martian surface compositions.