Reactive Transport Modeling of Phosphate Mineral Dissolution in High-p

Introduction: Phosphate is among the nutrients considered critical for all known life [1-4]. The ion is a component in ATP, DNA, RNA, phospholipid cell membranes and required in numerous fundamental biochemical reactions [5]. Phosphorus, either as phosphate or a more reduced species such as phosphite, is also considered crucial in pre-biotic reactions that may have led to the origin of life on Earth [5-7]. A determining factor for the potential of Mars to develop and maintain life may therefore be the availability of phosphorus. Though Mars is rich in phosphate (5-10x Earth) [811], the presence of phosphate alone is not equivalent to phosphate availability. Unlike other bioessential nutrients, phosphate has no significant volatile phase and remains locked in phosphate-bearing minerals within rocks until rock/water interactions release it into the environment [12]. On Earth, the most common primary phosphate mineral is fluorapatite (Ca5(PO4)3F). The dissolution of fluorapatite in igneous rocks is a major source of inorganic phosphate available for biological reactions. In contrast, meteorite evidence suggests the most abundant primary phosphate minerals on Mars are Clrich apatite (Ca5(PO4)3Cl) and the extraterrestrial mineral merrillite (Ca9(Na,Fe,Mg)(PO4)7) [13]. The dissolution and release of phosphate from these primary phosphate minerals is likely the main source of phosphate in Martian environments, and therefore important in investigating the potential for the origin and persistence of life on Mars. In previous studies we have synthesized chlorapatite and merrillite (Figures 1 and 2) [14, 15] and measured the dissolution rates and solubilities of these more Mars-relevant primary minerals [11, 16]. In addition, we have also examined phosphate mobility in basalts of a Mars-analog environment, Craters of the Moon National Monument in Idaho [17]. Here we use results of these previous studies and reactive transport modeling to investigate the dissolution and release of phosphate from phosphate-rich martian rocks (e.g. Wishstone) during rock/water interactions, to gain insight into past and present martian phosphate availability, aqueous interactions, and the implications for life, past, present, or future on Mars. Methods: Modeling is being carried out using the reactive transport code CrunchFlow. CrunchFlow is a computer model written by Carl Steefel [18] for simulating multicomponent, multi-dimensional, reactive transport in porous media. The program reads required thermodynamic and kinetic data from an included user modifiable database and allows mineral dissolution and precipitation reactions to be modeled. The code has been used in previous studies including those applied to Mars [19-23]. In our current modeling a Wishstone class rock is conceptualized as a column of 100 cells of 50 μm depth representing the rock surface to the interior. Rock mineralogy is based on a CIPW normative of major minerals using MER Spirit APXS and Mössbauer data [24]. Because the exact primary phosphate mineral in Wishstone class rocks is unknown, we run separate models for a merrillite, and a chlorapatite bearing Wishstone rock. For comparison we also model mineral dissolution in a fluorapatite containing Wishstone rock. Thermodynamic and kinetic data for the Ca-phosphate minerals are derived from our previous work [11]. For the remaining minerals we use previously published values [25-27]. Parameters such as porosity, tortuosity, and mineral surface area are based on typical basalt values [28-30] and our observations from basalts of Craters of the Moon National Monument [17]. In our current modeling, rock/water interaction is modeled with a low ionic strength water of variable pH at 1°C and under martian atmospheric conditions. During modeling, mineral volumes, surface areas, porosity, and other parameters are updated within CrunchFlow after each time iteration. Models are run for up to 100,000 years of water-rock interaction.