Andesites in the Primitive Martian Crust: Products of Hydrous

Introduction: Deconvolution of thermal emission spectra from Mars orbiting spacecraft, in-situ analysis of martian rocks by Mars Pathfinder, Spirit and Opportunity rovers, as well as investigation of martian meteorites, all point towards a wide magmatic diversity at the surface of the planet. Compositions varying from olivine-rich basalt to high-silica granite / rhyolite through basaltic andesites, andesites and dacites have all been identified [1]. Primitive olivine-rich basalts (45-46 wt% SiO2) have been analyzed by the Spirit rover inside Gusev Crater [2]. Similar olivine-rich rocks have also been identified by infrared spectrometry in different places on Mars [1,3]. These basaltic rocks have been interpreted as partial melts of an undepleted martian mantle [4], and might form part of the primitive martian crust. Infrared and thermal emission spectroscopy, however, show that most of the martian surface is not covered by primitive basalts [5], but by more silicarich basaltic andesites (52-55 wt% SiO2, surface type 1) or andesites (56-59 wt% SiO2, surface type 2). Andesitic rocks have also been analyzed by the Mars Pathfinder mission [6]. Andesites are common rocks on Earth that can be formed through two main processes: fractional crystallization of basaltic rocks and partial melting in the presence of water. In this abstract, we discuss the potential role of water in the generation of basaltic andesites and andesites, through an experimental study of hydrous peridotite melting and comparison with magmatic processes that occur on Earth. We argue that if magmatic rocks of basalticandesitic or andesitic composition are present in large amounts in the Martian crust, their genesis is highly likely to have involved the presence of water. Water in the Martian mantle: There is compelling evidence that Mars was quite wet, at least through part of its history. Well-developed valley networks suggest that there has been running water during early Martian history [7]. Extensive hydrous alteration of the Martian surface [8], evaporite deposits [9] and polar water ice deposits [10], confirm the presence of surface water throughout the planet's history. Although controversial, analytical and experimental evidence from Martian meteorites suggests magmatic water contents as high as 1.8 wt% [11]. Experimental study of hydrous melting: Watersaturated experiments have been performed in a piston-cylinder apparatus, at 1.0-1.5 GPa and temperatures between 975 °C and 1025 °C on a primitive (mantle + crust) Martian composition [12]. Glasses from three experiments, saturated with olivine, orthopyroxene, clinopyroxene, spinel, and, for one experiment, amphibole, have been analysed by electron microprobe. They have very similar andesitic compositions, with SiO2 = 57-60 wt%, Al2O3 = 18-21 wt%, FeO = 2-6 wt%, MgO = 1-3 wt% and CaO = 810 wt%. Melt fractions have been calculated by massbalance, and are between 9 and 10 wt%. Generation of the Martian crust: Olivine-rich basaltic rocks, like those observed at Gusev crater, are very similar to dry partial melts of the Martian mantle [4]. Dry fractional crystallization of such "tholeiitic" basalts is characterized by an important iron enrichment [13] that is not observed in surface types 1 or 2 (Fig. 1). Furthermore, dry crystallization leads to almost constant silica content until very high amounts of crystallization are reached. Any basaltic-andesitic or andesitic composition like surface types 1 or 2 is thus unlikely to be derived by dry crystallization of primitive Martian mantle melts.