Phosphorous Immobility During Formation of the Layered Sulfate Deposits of the Burns Formation at Meridiani Planum

The relatively high abundance of phosphorous (P) in rocks from the Martian crust makes it a potentially useful element for tracking geochemical processes. Owing to the high solubility of common P-bearing minerals such as apatite at low pH, P is widely thought to be mobile under acid-sulfate conditions. Accordingly, transport of P by acidic fluids has been invoked to explain enrichments or depletions of this element in some rocks and soils from Mars. The sediments that compose the layered-sulfate deposits of the Burns formation at Meridiani Planum were originally derived from a basaltic precursor but appear to have been exposed to acidic, sulfate-rich fluids at several intervals during their history. Yet, assessment of the chemical composition of the bedrocks indicates that they maintain pristine igneous P:Ti ratios and P abundances similar to Martian basalts, indicating that there has been little or no transport of P into or out of the deposits at any stage in their formation. Potential reservoirs for P in the deposits include basaltic glass, secondary Fe- or Al-phosphate minerals, substitution of PO₄ for SO₄ in alunite-jarosite group minerals, or surface adsorption. Although Fe-phosphates were not among the minerals detected by the Mössbauer spectrometer onboard the Opportunity rover at Meridiani, it might be difficult to detect such minerals even if they are present because they are below the detection limit or have signatures similar to other minerals. Overall, the results demonstrate that P can be immobile even in some acid-sulfate environments on Mars and Earth.