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Mineralogical Constraints on Diagenesis of the Sheepbed Mudstone, Gale Crater, Mars, as Determined by the CheMin XRD Instrument and Examples of the Use of NASA Spinoff Technology in the Oil and Gas Industry

Abstract

Sediments of the Yellowknife Bay formation (Gale crater, Mars) include the Sheepbed member, a mudstone cut by light-toned veins. Two drill samples, John Klein and Cumberland, were collected and analyzed by the CheMin XRD/XRF instrument during Curiosity's Yellowknife Bay campaign. The CheMin XRD analysis shows that the mudstone contains basaltic minerals (Fe-forsterite, augite, pigeonite, plagioclase), as well as Fe-oxide/hydroxides, Fe-sulfides, ∼20% trioctahedral phyllosilicates and ∼20-30% amorphous materials. The light-toned veins that intersect the drill hole contain Ca-sulfates; anhydrite and bassanite are detected by XRD but gypsum is also indicated from Mastcam spectral mapping. These sulfates appear to be almost entirely restricted to late-diagenetic veins. The presence of phyllosilicates indicates that the activity of water was high during their formation and/or transport & deposition (should they have been detrital). Lack of chlorite places limits on the maximum temperature of alteration (likely <100 C). The presence of Ca-sulfates rather than Mg- or Fe-sulfates suggests that the pore water pH was near-neutral and of relatively low ionic strength. The presence of a surplus of magnetite (∼7-9%) and the near absence of olivine (∼0–3%) when compared to previous CheMin soil measurements and Mars normative basalts suggests that saponitization of olivine (a process analogous to serpentinization) could have occurred. All of these early diagenetic features appear to have been preserved in this more than 3 billion year old rock with minimal alteration since their formation, as indicated by the ease of drilling (weak lithification, lack of cementing phases), the presence of amorphous material, and the late-stage fracturing with emplacement of calcium sulfate veins. The CheMin XRD/XRF now operating robotically on Mars is the product of a 22-year NASA technology development effort. CheMin is 30×30×30 cm, weighs 10 kg, has only one moving part (the wheel which holds the transmission-geometry sample cells) and operates on 50 watts. Commercial spinoff instruments have been developed using CheMin technology that are now widely used in the oil & gas industries. One example is the Terra XRD/XRF, a battery powered field portable unit that can assist in real-time mudlogging to provide full mineralogical information useful in the geosteering of horizontal wells. Terra is currently being deployed at the well site by some of the major service companies.