X-ray amorphous materials in phyllosilicate- and sulfate-bearing units in Gale crater, Mars

Session: Advancing Mineral Science and Exploring Planetary Surfaces: In Honor of MSA Dana Medalist, Elizabeth B. Rampe, Part I

Presenting Author:

Sarah Simpson

Authors:

Simpson, Sarah L¹, VanBommel, Scott J², Rampe, Elizabeth B³, Bristow, Tom⁴, Clark, Joanna⁵, Meusburger, Johannes⁶, Morris, Richard⁷, Morrison, Shaunna⁸, Thorpe, Michael⁹, Tu, Valerie¹⁰, Achilles, Cherie¹¹, Berger, Jeff¹², Blake, Dave¹³, Chipera, Steve¹⁴, Des Marais, Dave¹⁵

(1) Amentum JETS II, NASA Johnson Space Center, Houston, TX, USA; Texas State University, San Marcos, TX, USA, (2) Washington University in St. Louis, St. Louis, MO, USA, (3) NASA, NASA Johnson Space Center, Houston, TX, USA, (4) NASA, NASA Ames, Moffett Field, CA, USA, (5) Amentum JETS II at NASA Johnson Space Center, Houston, TX, USA; Texas State University, San Marcos, TX, USA, (6) NASA Ames - ORAU, Moffett Field, CA, USA, (7) NASA Johnson Space Center, Houston, TX, USA, (8) Rutgers University, New Brunswick, USA, (9) University of Maryland - NASA Goddard Space Flight Center, Greenbelt, USA, (10) Amentum - JETS II at NASA Johnson Space Center, Houston, TX, USA; Texas State University, San Marcos, USA, (11) NASA Goddard Space Flight Center, Greenbelt, USA, (12) Amentum JETS II at NASA Johnson Space Center, Houston, USA, (13) NASA Ames, Moffett Field, CA, USA, (14) PSI, Tucson, USA, (15) NASA Ames, Moffett Field, CA, USA, (16) University of Arizona, Tucson, USA, (17) University of Nevada Las Vegas, Las Vegas, USA, (18) Carnegie Institution for Science, Washington DC, USA, (19) NASA Johnson Space Center, Houston, USA, (20) LPI-USRA, Houston, USA, (21) Amentum JETS II at NASA Johnson Space Center, Houston, USA, (22) LPI-USRA, Houston, USA, (23) University of Calgary, Calgary, Canada, (24) PSI, Tucson, AZ, USA, (25) JPL, Pasadena, USA,

Abstract:

X-ray amorphous materials comprise ~20-70 wt% of every rock and soil sample analyzed by the CheMin X-ray diffractometer (XRD) on board the Curiosity rover in Gale crater. Here we use the term ‘X-ray amorphous materials’ to describe nanocrystalline phases that scatter X-rays (i.e., they may contain short-range atomic order). The origin(s) of these phases remains speculative; in Gale crater this material is likely a complex mixture of primary aluminosilicate glass(es), sulfate(s) and other secondary alteration or diagenetic products. As Curiosity traversed across the smectite-rich region, transitional units and upwards into the sulfate unit, several noteworthy sedimentological and mineralogical trends were observed that indicate a period of gradual drying including the loss of phyllosilicates, from older clay-rich sandstones and mudstones to the younger, phyllosilicate-barren, hydrated Mg-sulfate-rich (+/- Fe(II) carbonate) sandstones. Here we present major trends observed in the abundances and compositions of the amorphous component across these regions of major change, as determined using FULLPAT modeling combined with a mass balance calculation, and derive their context in combination with the mineralogy.

The proportion of amorphous material in each sample has gradually increased across the traverse (smectite-rich strata ~36 wt%; transitional units ~50 wt%; and sulfate unit ~58 wt%). This increase in amorphous abundance coincides with the loss of smectites and correlates with the inferred exhaustion of surface water. This may have been punctuated by brief wet-dry cycling, precluding the crystallization of widespread, abundant phyllosilicates while allowing the amorphous material to persist without forming long-range order. The amorphous material is primarily composed of SiO₂, FeO_T, MgO and SO₃, with varying amounts of other minor phases. There is a consistent anticorrelation between SiO₂ and SO₃ which suggests there are at least two amorphous endmembers: one that is silicate (+/- FeO_T) and one that is more sulfate (+/- MgO) -rich, reflective of mixture(s) of primary and secondary materials (e.g., basaltic glasses, sulfates with a similar composition to the crystalline phases detected by CheMin). The silicate endmember could also contain X-ray amorphous clay minerals, representative of destroyed clay mineral components, or nontronite-like precursors that did not crystallize further due to the lack of sufficient water in the upper strata. Curiosity will continue to explore these relationships to help elucidate the alteration history across this region in Gale crater.