GEOCHEMICAL CONSTRAINTS ON THE ORIGIN OF SECONDARY MINERAL ASSEMBLAGES IN SEDIMENTARY ROCKS ON MARS

One proposed paradigm for the evolution of surficial environments on Mars suggests that they changed from benign and water-rich to more acidic and water-limited with a fairly rapid transition during the mid- to late Noachian. Foundations of this model rest on orbital observations indicating a dominance of clay minerals in early to mid-Noachian secondary mineral assemblages but dominance of sulfates in late Noachian – Hesperian secondary assemblages. In situ observations of the ferric sulfate mineral jarosite in Meridiani Planum sedimentary rocks appeared to confirm acidic conditions during the late Noachian – early Hesperian and numerous low pH aqueous alteration experiments produced secondary mineral assemblages broadly supportive of this model. On the other hand, occurrence of clay minerals in the early Hesperian or younger Yellowknife Bay Fm. in Gale Crater is less obviously consistent and there is growing evidence for localized redox processes influencing pH.

Sedimentary clay minerals mainly form by any of three basic (non-mutually exclusive) mechanisms: chemical weathering of sources to form clays that are transported into sedimentary basins, a variety of diagenetic processes (e.g., authigenesis, replacement, cementation) and less often in situ chemical weathering (e.g., pedogenesis). Distinguishing these mechanisms is not straightforward on Mars because textural and microanalytical data of sufficient resolution typically are lacking. In favorable cases, bulk chemistry, when combined with mineralogical detections, can help to distinguish among these processes because bulk sedimentary rock chemistry constrains time-integrated levels of element loss during aqueous alteration. We present two examples for illustration. Yellowknife Bay Fm. (Gale Crater) sedimentary compositions are inconsistent with open-system chemical transport, thus limiting the degree of chemical weathering, and accordingly providing support for low water-rock ratio diagenesis causing a diverse secondary mineral assemblage (including clays). In contrast, chemical and mineralogical compositions of Burns Fm. sandstones at Meridiani Planum, can be deconvolved in a manner that demonstrates an early history of circum-neutral chemical weathering of the provenance overprinted by complex diagenetic processes.