PLAGIOCLASE DISSOLUTION RATES AND MECHANISMS IN NEAR SATURATED BRINES: IMPLICATIONS FOR POST-NOACHIAN AQUEOUS ALTERATION ON MARS

Plagioclase feldspars with compositions of An_18-36 and ~An₅₇ have been reported in Gusev Crater and Gale Crater, Mars respectively. Sodic plagioclase was also detected by CRISM in association with clay minerals in Marwth Vallis. Sulfate and chloride evaporite deposits observed on Mars suggest brines likely were active in the past and may even be stable at low temperatures present on Mars’ surface today. In order to investigate the role of high ionic strength brines on feldspar weathering, albite (An₁₃) and labradorite (An₅₈) were reacted three aqueous solutions: ultrapure water (UPW; aH₂O=1.00), 1.67 molal MgSO₄ brine (aH₂O=0.91), and 4.5 molal CaCl₂ brine (aH₂O=0.35) for 60 day batch reactor dissolution experiments at 295K.

Feldspar dissolution was incongruent in all experimental trials. Surface area normalized initial dissolution rates (log mol m² s^-1) are slightly higher in UPW for labradorite (-10.02) than albite (-10.14), as well as MgSO₄ brine (-9.94 for labradorite;-10.32 for albite). PhreeqC modeling of labradorite dissolution in MgSO₄ brine indicates sulfate complexes increase Ca release by ~10x relative to UPW, thereby decreasing anorthite mineral lifetimes in MgSO₄ brines relative to UPW. Increased pyroxene dissolution rates in sulfate brines have been previously attributed to Ca-sulfate complexation, suggesting sulfate brines may significantly leach calcium from primary minerals, resulting in secondary calcium sulfate minerals Dissolution rates in CaCl₂ brines were significantly slower for both labradorite (-10.67) and albite (-10.12), which may result from decreasing water activity, and/or common ion effects in the case of labradorite.

PhreeqC modeling of albite and labradorite dissolution in UPW and MgSO₄ brine reactions predicts clays, including kaolinite, Al-hydroxides, including gibbsite, and SiO_2(am) will form. Increased feldspar weathering rates in sulfate brines due to Ca-complexation, may explain the associations of clay-rich and Si-rich rocks associated with sulfate vein networks in outcrops observed on Mt. Sharp, Gale Crater, Mars. However, preservation of ~An₅₇ in some places in Gale Crater argues that aqueous alteration may have been restricted in area and/or duration.