ASSESSING THE EFFECTS OF SOLUTION CHEMISTRY ON THE δD COMPOSITION WEATHER MINERALS: ISOTOPE ANALYSIS OF STRUCTURE WATERS IN SYNTHETIC PHYLLOSILICATES FORMED IN EXTREME PH AND SALINE FLUIDS

Phyllosilicate minerals are a useful proxy of paleoenvironment and paleoclimate. The distribution and accumulation of these minerals can be used to reconstruct temperature, pressure, and even some details of aqueous chemistry. Barring diagenetic forcings, common clay minerals such as kaolinite, illite, and smectite preserve a δD composition that can be used to interpret the isotopic composition of the waters in which they formed. The fractionation between δD in the precipitating mineral and the formation water is temperature dependent and has been determined empirically for circumneutral waters on Earth. These fractionation factors have been widely used to reconstruct paleowater δD on Earth, and this technique has also been applied to martian meteorites and in situ measurements made on smectites at Gale Crater with the SAM (Sample Analysis at Mars) instrument. This experiment poses a special problem however since formation waters recorded by materials throughout Gale Crater may not necessarily be dilute and circumneutral like terrestrial meteoric waters but rather acidic and saline. This work evaluates the effects of acidity, salinity and water activity on fractionation between formation waters and clay minerals in order to improve our estimates of the δD in the lakes of Gale Crater in the past.

Synthetic phyllosilicate minerals were formed from basalts and basaltic glasses in D-enriched fluids under extreme pH (both acid and alkaline) and highly saline conditions. The δD of the starting aqueous solution was analyzed, as well as the δD of the final post-reaction solution and the synthesized clay minerals. The identity and abundance of the minerals formed during reaction was determined by XRD. The clay-size fraction of the reaction product contained a variety of smectites and zeolites These experiments have found that the δD expression of a mineral can be greatly influenced by solutes and suggests that fractionation factors for clays formed in saline, extreme pH fluids may not be the same as those for circumneutral conditions. These results would recommend fractionation factors usef for reconstruction of meteoric water δD must be sensitive to the chemistry of the formation solution.