Basalt versus Andesite in the Martian Crust : New Geochemical Perspectives

Introduction: Data from the MGS Thermal Emission Spectrometer (TES) has shown that surface type 1 (ST1) materials, interpreted as basalt, dominate the ancient crust in the southern highlands of Mars [1]. Large expanses of surface type 2 (ST2) materials in the northern lowlands have been variously interpreted as andesite [1, 2], oxidized crystalline basalt [3], or partly weathered basalt [4]. The ambiguity in interpreting the ST2 lithology arises because volcanic siliceous glass (a major component of andesite) is spectrally similar to some oxidation products and/or clay minerals in TES data. Distinguishing between these multiple working hypotheses is crucial to understanding the planet's crust composition and differentiation. This study offers new geochemical data that complement an earlier consideration of this problem [5]. One Size Fits All?: An important but often overlooked perspective is that a single interpretation of ST2 spectra may not be warranted everywhere on Mars (Ockam wasn't always right). THEMIS data from Mars Odyssey reveal adjacent volcanic flows of ST1 and ST2 materials emanating from the same caldera [6]. These flows have high thermal inertias and probably represent lava outcrops. For these units an igneous origin , involving successive eruptions of basaltic to ande-sitic lavas, offers a reasonable interpretation. For ST2 sediments, however, partly weathered basalt may be more likely. For example, [7] described deposits of ST1 sand dunes on the floors of large craters in Oxia Palus, adjacent to ST2 materials on the downwind side of the crater walls. In this case, ST2 is more plausibly explained as a weathered finer-grained fraction (containing some clays) winnowed by winds from coarser basaltic sediment on the crater floor. If oxidized basalt particles are fine grained, they too might be winnowed and concentrated on crater walls. Materials of the Northern Plains: In evaluating the bulk composition of the martian crust, we must focus on interpreting global-scale deposits of ST2 materials in the northern plains. These plains, mapped as the Vastitas Borealis Formation (VBF), have recently been interpreted as sedimentary deposits derived from outside the basin [8] or altered sediments formed through local reworking of earlier deposits by perma-frost processes [9]. The ST2 sediments are underlain by ancient ridged plains interpreted as volcanic flows. A few local SF1 deposits identified in northern plains