CRYOGENIC WEATHERING OF THE ANTARCTIC FERRAR DOLERITE: A TERRESTRIAL ANALOG FOR MARTIAN BASALTIC LITHOLOGIES AND SURFACE PROCESSES

The study of terrestrial analogs to Martian igneous rock compositions and surface features is important in order to gain insight into past and present dynamic processes experienced on Mars.  For this study, we investigate petrological and mineralogical properties of the Antarctic Ferrar dolerites as an analog to the Martian basaltic meteorites.  The Ferrar is exposed to some of the coldest and driest weathering environments on Earth.  As a result, these rocks are an excellent terrestrial analog to Martian igneous lithologies and their weathering on a variety of scales.  We present mineralogical analyses from a suite of Ferrar dolerites samples, ranging from relatively unexposed gray-green specimens to ones displaying distinctively advanced weathering features (e.g., cavernous weathering pits). 

Quantitative microprobe results confirm the presence of two pyroxenes (augite and pigeonite), two feldspars (labradorite and alkaline feldspars), quartz, and a variety of Fe- and Ti-oxides (e.g., ilmenite, titanomagnetite).  The Ferrar pyroxene assemblage is, therefore, a good match to the Shergottite pyroxenes in terms of both range and average composition.  The modal abundance of pyroxenes and feldspars in the Ferrar differs from that of the Shergottites; feldspar is more abundant than pyroxene in the former, while the reverse is generally true in the Shergottites. 

Augite in weathered Ferrar samples appears deficient in FeO compared to unweathered samples.  Augites from these samples cluster around ³⁰Wo⁵⁶En¹⁴Fs to ³⁰Wo²⁵En⁴⁵Fs.  The likely cause of this FeO deficiency is oxidation to Fe₂O3, resulting in the prevalent green-to-red color transition in more weathered samples.

Mšssbauer spectra from three Ferrar samples are currently being collected to determine the relative amount of ferric iron in the silicate phases.  We are also conducting X-ray diffraction with thermal analysis on specimens that have been kept at ambient (-15°C) during transport and storage to identify cryogenic weathering phases in situ.