The Mobility of Phosphorus and Potassium in Altered Hawaiian Volcanics: Constraints on Fluid Alteration on the Martian Surface

Phosphorus and potassium are a fluid-mobile element pair that constrains fluid conditions on the martian surface. The geochemical conditions under which P and K were incorporated into primary phases, released via alteration into fluids, and precipitated into secondary phases are recorded in the sedimentary units of the Murray fm. in Gale crater. The two elements are detectable (>300 ppm) by the Alpha Particle X-ray Spectrometer (APXS) on the Mars rover Curiosity, and enrichments have been discovered in Gale crater. Volcanics from Maunakea and Kohala on Hawai'i are compositionally analogous to Gale crater, and the mobility of P and K during alteration of Hawaiian lava and tephra may indicate similar processes on Mars. Here, we report results for P₂O₅ and K₂O in Gale crater (APXS) and Hawaiian volcanics (XRF) and discuss possible analogue geochemical processes that may have acted on sediments in Gale crater.

The Murray fm. has elevated P­₂O₅ (0.9 - 1.3 wt%) relative to the average martian crust (0.9 wt%). Diagenetic features (n ~15) with P₂O₅ enrichments (1.5 - 7.5 wt%) are evidence that phosphate was mobile in-situ after the sediment was deposited. In Murray bedrock, K₂O is consistently elevated (0.76 ± 0.12 wt%) relative to soil (0.47 ± 0.07 wt%). In situ mobility is indicated by two diagenetic features with high K₂O (1.9 wt% - 2.3 wt%) that have K:Fe:S approximating jarosite.

The bulk chemical and mineralogical composition (XRF and XRD) of altered Maunakea and Kohala samples show that, after emplacement, P and K mobility is dependent on the alteration regime. In a circumneutral pedogenic regime, the primary apatite has low solubility and P is retained in weathered residue, whereas K is mobilized and leached out of the rock. In contrast, Maunakea tephra altered in an acid-sulfate regime retains both K and P in altered residue, where K occurs in jarosite and P likely occurs in amorphous Fe-, Al-phosphates.

As observed in the Hawaiian samples, the in situ mobility of both P and K in the Murray fm. is consistent with a fluid pH < 6. The detection of jarosite by the CheMin XRD indicates a lower pH < 4.5. Under these conditions, amorphous Fe-, Al-phosphates are probable. These observations suggest that acidic fluids interacted with Gale crater sediment after it was deposited and may have altered the materials.