Hypotheses on the Source of Potassium Enrichment in Some Gale Crater Rocks

Friday, 19 December 2014: 5:45 PM
Martin R Fisk1, Melinda Darby Dyar2, John Bridges3, Ryan B Anderson4, Mariek E Schmidt5, Olivier Gasnault6, Nicolas Mangold7, Robert L Tokar8, Roger C Wiens9, Ralf Gellert10, David Frederick Blake11, Susanne P Schwenzer12 and Peter Edwards3, (1)Oregon State Univ, Corvallis, OR, United States, (2)Mount Holyoke College, South Hadley, MA, United States, (3)University of Leicester, Leicester, LE1, United Kingdom, (4)USGS Astrogeology Science Center, Flagstaff, AZ, United States, (5)Brock University, St Catharines, ON, Canada, (6)Universite de Toulouse, Toulouse Cedex 4, France, (7)LPGN Laboratoire de Planétologie et Géodynamique de Nantes, Nantes Cedex 03, France, (8)Planetary Science Institute, Santa Fe, NM, United States, (9)Space Science and Applications, Los Alamos, NM, United States, (10)University of Guelph, Guelph, ON, Canada, (11)NASA Ames Research Center, Moffett Field, CA, United States, (12)Open University, Milton Keynes, MK7, United Kingdom
K2O contents of rocks and soils were measured or estimated with three instruments on Curiosity in Gale Crater, Mars: ChemCam (laser induced remote sensing), APXS (α-particle X-ray spectroscopy), and CheMin (X-ray diffraction analysis). In some sedimentary rocks the K2O contents are consistent with the rock’s mineralogy, but in others K2O is higher than expected for the mineralogy. We have considered five hypotheses for the high abundance of K2O in Gale Crater sediments.

Hypothesis I; K2O is enriched in plagioclase feldspar due to crystallization at high temperatures and pressures. Hypothesis II, some sedimentary rocks are enriched in K2O-rich feldspars. Hypothesis III; rocks may have been enriched in K2O by secondary mineral formation. Hypothesis IV; dust has been incorporated into the rocks. Hypothesis V, K2O is contained in an amorphous or a nanocrystalline phase. Hypotheses IV and V are easily invalidated because the measured and inferred compositions of dust and amorphous and nanocrystalline material are not sufficiently enriched in K2O to cause the observed K2O in sedimentary rocks.

Hypothesis I is possible because high-K2O plagioclase feldspars are sometimes observed in martian meteorites and sediments may have incorporated feldspars of these compositions. Hypothesis II is supported if potassium-rich feldspars, which have been identified by XRD of Gale Crater sediments and have been found in some martian meteorites, are concentrated in Gale sediments by fluvial processes. Hypothesis III is supported if correlations of K2O with other components of alteration minerals or secondary phases are found such as in a K2O-rich phyllosilicate, jarosite, alkali feldspar, or a potassium and fluorine-bearing phase.

Hypotheses I and II may explain the elevated K2O in Gale Crater sediments, however, potassium enrichments must be compatible with mineralogy. Hypotheses III is supported if a K2O-rich secondary phase or phases can be identified. The cause of K2O enrichment could reveal much about past igneous, geochemical, and physical processes in Gale Crater.