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.
Bibliographical noteFunding Information:
This work was supported by funds from NASA Mars Fundamental Research Program grant NNX12AI02G and Mars Data Analysis Program grant NNX16AJ40G. The authors thank Bethany Ehlmann for providing spectral analysis of minerals and Karyn Rogers and Sally Potter-McIntyre for assistance with fieldwork. The authors are grateful for the comments of the reviewers and Editor that helped to refine our interpretations and improve the manuscript. All data discussed in this report are included in the spreadsheets provided as supporting information. The Institute for Rock Magnetism is supported by the Instruments and Facilities Program of the NSF Division of Earth Science. This is IRM contribution number 1709. The authors declare no financial conflicts of interest.
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- Meridiani Planum