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Phosphorus mobility on the martian surface is fundamental to habitability because the element is essential for biochemical reactions and biological structures. The Spirit, Opportunity, and Curiosity rovers have discovered a range of P concentrations on the martian surface suggesting pervasive mobility of the element [e.g., 1, 2, 3]. A key pathway for P mobilization is via dissolution of primary phosphates (apatite, merrillite) [4, 5] and reprecipitation of P-rich phases in soils, veins, alteration haloes, localized features and/or rock matrix. The secondary P-bearing phases are not fully characterized by rover observations; where primary apatite is not apparent, P is variably associated with Ca, Fe, Al, and Mn with no detectable affinity with any single cation [e.g., 1, 2, 3]. In Mars analogue samples from the summit of Maunakea, Hawai’i, we have identified an association of P with S in secondary aluminum-phosphate-sulfate (APS) minerals. Here, we describe the occurrence of APS minerals in analogue samples and propose a similar fate for P in S-rich martian systems. Samples and Methods: Two Maunakea summit samples representative of acid-sulfate alteration processes were investigated: (1) a tephra fragment with an unaltered interior and an ~1-cm-thick alteration rind (HWMK953) and (2) breccia containing highly altered lithic fragments cemented by sulfates, phyllosilicates, and amorphous silica-rich material (HWMK959). The average unaltered parent composition of the summit tephra is hawaiite. A field-emission SEM was used to acquire an EDS element map of the breccia and spot EDS analyses on polished thin sections of both samples. Bulk chemistry and mineralogy of the breccia sample was obtained with powder X-ray diffraction (XRD) and X-ray fluorescence (XRF). Results: Unaltered tephra: The least-altered zone of the tephra (HWMK953) is typical for unaltered hawaiitic/mugeartitic volcanics capping Maunakea, which have bulk P2O5 ~1 wt% [6, 7]. Phosphorus is contained within apatite, which occurs as 1-100 μm hexagonal and acicular grains (mostly skeletal) in the glass matrix and as inclusions in olivine. F, Cl, and OH were not analyzed with EDS and S was below detection limits. Altered lithic material: The altered rock fabric largely maintains its primary trachytic texture, albeit with silica pseudomorphs after feldspar, pyroxene, and olivine, and Ti-oxide pseudomorphs after Fe-Ti-oxide. No residual primary apatite grains were found. Figure 1: Element maps of P, S, and Si in acid-sulfate altered breccia (HWMK959). Si indicates altered lithic fragments (green) cemented together by Pand S-rich secondary materials (magenta and light violet).

The Fate of Mobile Phosphorus in Sulfur-Rich, Mars-Relevant Systems