X-Ray Amorphous Components in Sedimentary Rocks of Gale Crater, Mars: Evidence for Ancient Formation and Long-Lived Aqueous Activity

The CheMin instrument on the Mars Science Laboratory rover Curiosity detected ubiquitous high abundances (∼15–70 wt%) of X-ray amorphous components (AmCs) in ancient sedimentary rocks of Gale crater. Mechanisms and timing of formation for the AmCs are poorly constrained, and could include volcanic, impact, or aqueous processes. We explore trends in AmC composition and abundance, and look for systematic compositional variation between sites within Gale crater. AmC compositions were estimated indirectly based on bulk chemistry and the nature and abundance of the crystalline phases for 19 sedimentary rock samples. AmC abundances positively correlate with AmC SiO₂ contents, and a mixing relationship appears to exist between SiO₂-rich and FeO_T-rich AmC endmembers. Endmember compositions are inconsistent with volcanic or impact glass alone, and so we conclude that the SiO₂ and FeO_T contents formed largely through aqueous processes. Cross-cutting relationships and geologic context provide evidence that the most SiO₂-rich AmCs observed in Gale crater thus far may result from interactions with localized fluids during late diagenesis. AmCs with moderate to low SiO₂ contents likely formed earlier (before or soon after sediment deposition). Thus, the AmC SiO₂ and FeO_T contents in Gale crater rocks represent mixtures of sedimentary materials formed over most of the sedimentary history of Gale crater, starting before the first sediments were deposited in the crater (late Noachian), and ending well after the youngest sediments were lithified (at least mid-Hesperian). However, it remains unclear how these metastable minerals have persisted through billions of years of diagenesis in Gale crater sediments.