Hot and Steamy: Alteration of the Primordial Martian Crust by Supercritical Fluids during Magma Ocean Cooling

Introduction: Mars’ basaltic crust has been altered by H2O to form hydrated silicates (mostly clays) that are distributed globally [1,2] and extend to great depth [3]. However, climate models for early Mars are challenged to generate warm and wet environments needed to form these minerals via weathering near or at the surface [4,5], and geomorphic evidence for surface hydrology seems to postdate most of this alteration: preexisting clays were simply moved around in the fluvial-lacustrine systems visible from orbit today [e.g., 6]. While some clays probably formed through low-grade metamorphism in an early, wet crust [7], the Pre-Noachian period has not been explored for possible clay formation. The Pre-Noachian was a time when Mars had to have been warm and wet because the planet was still cooling from a magma ocean [8,9]. What fraction of the secondary minerals observed today could have originally formed during this period? Here, we conduct a set of laboratory studies to determine if, and to what extent, a massive outgassed atmosphere could have altered the primordial martian crust during magma ocean cooling. Model: As a martian magma ocean evolved, 10s to 100s of bars of outgassed H2O/CO2 vapor likely sat emplaced on the silicate liquid surface [9] (Fig. 1a). As the magma ocean cooled further, this atmosphere may have entered the supercritical field [9], creating an interface between supercritical fluid and the primordial basaltic crust, enhanced by seepage into fractures caused by impact bombardment (Fig. 1b). Further cooling should lead to hot water condensing and percolating through the fractured crust as Mars evolved toward clement conditions [10]. All three phases of water (vapor, supercritical fluid and liquid) can likely alter basalt, but engineering literature demonstrates the unique properties of supercritical fluids: they effuse through solids like gases and act as powerful solvents [e.g., 11]. However, few studies have investigated the rate and extent of basalt alteration by these fluids because the conditions imagined in Fig. 1b fall well outside typical P-T metamorphic pathways (Fig. 2) considered for Earth or Mars [12]. Experimental: We synthesized rock samples similar to the estimated composition of Mars’ crust [13] (Fig. 3). Samples were created in a 1 atm furnace at QFM-1 and cooled from above the liquidus to 1000 °C at 5 °C/hr, then to room temperature at 250 °C/hr. The resulting material contains domains of both coarse (~200 μm) and fine (~30 μm) crystals with plagioclase (zoned) > clinopyroxene > olivine (zoned) > Ti-rich spinel > glass (with quench crystals). This material was then ground and sieved to a 500-1000 μm fraction, with clinging fines removed by washing under ethanol. We will mix the basalt with water in a 1:1 ratio by