Mineralogy of a Mudstone at Yellowknife Bay, Gale Crater, Mars

Abstract

X-ray diffraction analysis of the Rocknest scoop sample is described in (23); similar analyses were performed for John Klein and Cumberland. John Klein and Cumberland were the first two drill samples collected by Curiosity. All scooped or drilled samples pass through the Collection and Handling for In situ Martian Rock Analysis (CHIMRA) sample collection and processing system (10). All powders for X-ray diffraction are processed through a 150-m sieve before delivering a portion to the CheMin inlet funnel.The sieved drill powders were placed into sample cells with 6 μm thick Mylar® windows. Mylar® contributes a minor, broad scattering signature in diffraction patterns that is generally "swamped" by diffraction from the loaded sample. In addition, an aluminized light shield also contributes "peaks" to the observed diffraction patterns. Only ~10 mm 3 of material is required to fill the active volume of the sample cell, which is a disc-shaped volume 8 mm in diameter and 175 m thick. A collimated ∼70 μm diameter X-ray beam illuminates the center of the sample cell. A piezoelectric vibration system on each cell pair shakes the material during analysis, causing grains in the cell to pass through the X-ray beam in random orientations.CheMin measures XRD and XRF data simultaneously using Co radiation in transmission geometry (11). The instrument operates in single-photon counting mode so that between each readout the majority of CCD pixels are struck by either a single X-ray photon or by no photons. In this way, the system can determine both the energy of the photons striking the CCD (XRF) and the two-dimensional (2-D) position of each photon (XRD). The energy and positional information of detected photons in each frame are summed over repeated 10-sec measurements into a "minor frame" of 30 min of data (180 frames). The 2-D distribution of Co K X-ray intensity represents the XRD pattern of the sample. Circumferential integration of these rings, corrected for arc length, produces a conventional 1-D XRD pattern. For conversion of the 2-D CCD pattern to a 1-D pattern we have used FilmScan © software from Materials Data, Inc.CheMin generally operates for only a few hours each night, when the CCD can be cooled to its lowest temperature, collecting as many minor frames as possible for the available analysis time, usually five to seven per night. XRD data were acquired over multiple nights for the John Klein and Cumberland drill samples to provide acceptable counting statistics. Total data collection times were 33.9 hr for John Klein and 20.2 hr for Cumberland. The data for individual minor frames and for each night's analysis were examined separately, and there was no evidence of any changes in instrumental parameters as a function of time over the duration of these analyses. Before sample delivery and analysis, the empty cell was analyzed to confirm that it was indeed empty before receiving the sample. The flight instrument was calibrated on the ground before flight using a quartz-beryl standard, and measurement of this st...

Low HesperianPCO2constrained from in situ mineralogical analysis at Gale Crater, Mars

“…S2). Formulating the model in this way is also consistent with detection of small amounts of hematite in YKB sediments (9). This state is represented by the following chemical reaction:…”

“…In drawing the stability boundaries for magnetite, redox state is set at the magnetite/hematite boundary, as detailed in the main text. All thermodynamic data come from the Lawrence Livermore National Laboratory dataset (15) matrix and the lack of detection of evaporative minerals in the mudstone (9,11,12). This is compatible with bulk geochemical evidence of limited chemical alteration of sediments during erosion and transport from the source area (11).…”

“…The threshold is higher when SiO 2(aq) is saturated with respect to amorphous silica (long dashed line) rather than quartz (short dashed line). In both cases Al 3+ is set to saturation with respect to albite-reasonable given that the Sheepbed contains ∼20 wt % plagioclase feldspar (9). In drawing the stability boundaries for magnetite, redox state is set at the magnetite/hematite boundary, as detailed in the main text.…”

“…Sharp), the MSL team documented a ∼70 m sedimentary section of mudstones, siltstones, sandstones, and conglomerates deposited by lakes and rivers on the floor of Gale crater between 3.8 and 3.1 Ga (8). Sedimentary rocks analyzed by the CheMin X-ray diffraction (XRD) instrument during the traverse share many of the same components including (i) crystalline silicates including pigeonite, intermediate calcic plagioclase, and olivine from a basaltic source or sources; (ii) poorly crystalline clay minerals; (iii) various Fe oxides including magnetite; (iv) Ca-sulfate minerals, which mainly occur in cross-cutting fractures and veins; and (v) an X-ray amorphous component (9,10). Thus far, carbonate minerals have not been definitively detected in Gale Crater sedimentary rocks.…”

“…It is proposed that isochemical alteration during the deposition of lacustrine mudstone at YKB involved dissolution of Fe 2+ -bearing olivine (Fo 62 ) providing a source of Fe 2+ and Mg 2+ in pore waters for subsequent coeval precipitation of magnetite and Fe 2+ -bearing, ferrian saponitetype clay minerals (9,(11)(12)(13)(14). The detection of these secondary phases indicates anoxic to poorly oxidizing conditions (13), and places lower limits on the pore water activity of Fe 2+ and Mg 2+ released from olivine.…”