P069-0007
The X-ray Amorphous Composition of Rocks Sampled from the Greenheugh Pediment and Underlying Murray Formation
Tuesday, 15 December 2020
Poster
Cherie Achilles1, Richard V Morris2, Thomas Bristow3, Travis S. J. Gabriel4, Elizabeth B Rampe2, Brad Sutter5, Albert Yen6, Michael Thorpe7, Steve Chipera8, Amy McAdam9, Douglas W Ming2, David Frederick Blake3, David Vaniman10, Shaunna M Morrison11, Robert T Downs12, Valerie Tu13, David Des Marais14, Robert Hazen11, Allan H Treiman15, Nicholas Castle8, Patricia Craig16, John P Grotzinger17, Gordon W Downs18 and Tanya Peretyazhko5, (1)NASA Goddard Space Flight Center, Greenbelt, MD, United States, (2)NASA Johnson Space Center, Houston, TX, United States, (3)NASA Ames Research Center, Moffett Field, CA, United States, (4)Arizona State University, Tempe, AZ, United States, (5)Jacobs Technology, NASA Johnson Space Center, Houston, TX, United States, (6)NASA Jet Propulsion Laboratory, Pasadena, CA, United States, (7)NASA JSC, Houston, TX, United States, (8)Planetary Science Institute, Tucson, United States, (9)NASA Goddard SFC, Greenbelt, MD, United States, (10)Planetary Science Institute, Tuscon, CA, United States, (11)Carnegie Institution for Science, Earth and Planets Laboratory, Washington, DC, United States, (12)University of Arizona, Department of Geosciences, Tucson, AZ, United States, (13)Jacobs at NASA Johnson Space Center, Houston, United States, (14)NASA Ames, Mountain View, CA, United States, (15)Lunar & Planetary Inst, Houston, TX, United States, (16)Planetary Science Institute Tucson, Tucson, United States, (17)Caltech, Pasadena, CA, United States, (18)University of Arizona, Tucson, United States
Abstract:
Data from the CheMin X-ray diffraction instrument on the Mars Science Laboratory (MSL) rover has provided the relative abundance of minerals, clay minerals, and X-ray amorphous materials in 28 samples from Gale crater. On average, ~40 wt% of CheMin-analyzed rocks are composed of X-ray amorphous materials. Like crystalline phases, amorphous materials are important for assessing a rock’s alteration history, however, distinct phases are difficult to identify with the MSL suite of instruments, therefore, this component is best characterized by its chemical composition. The composition of the amorphous fraction is estimated using the APXS bulk rock composition and the quantity and composition of the minerals. An assessment of the combined crystalline and amorphous components suggests that silicate alteration, clay mineral formation, and changing depositional environments are the primary factors affecting the majority amorphous of assemblages observed.
We examine the amorphous composition of rocks drilled at and below the Greenheugh Pediment (GP) to investigate, 1) uniformity among Stimson fm. sandstones, 2) the depositional and diagenetic settings compared to rocks sampled lower in the section, and 3) detrital materials. Preliminary data show that although the mineralogy of the GP sandstone site is different from previously drilled, unaltered Stimson fm. sandstones, the amorphous composition is similar in abundance and composition. Rocks below the GP contact are nontronite-rich, a shift from from the saponite- and montmorillonite-bearing mudstones below the Glen Torridon unit. Additionally, these rocks have some of the lowest proportions of amorphous SiO2 and FeOT among all clay-bearing samples in Gale crater. These data are consistent with nontronite formation from ultramafic detritus or precipitation in Fe-rich waters. An investigation into the expected amorphous chemistry if nontronite was detrital is underway. As observed in previous samples, diagenetic processes can strongly influence amorphous materials, especially in rocks near stratigraphic contacts. Investigations of two mudstones from the unit underlying the GP (one contact-adjacent and one not) will provide an opportunity to again assess the extent of alteration and impact of diagenetic events on the formation of amorphous materials.
