Sediment-water interactions in three Holocene saline, alkaline playas, south-central Oregon: Mineralogy and geochemistry

Mineralogical, chemical, and isotopic analyses of clay minerals from outcrops and playa sediments, as well as chemical and isotopic analyses of groundwater and interstitial playa pore waters, have been used to improve understanding of the nature of sediment/water reactions in playa environments. Outcrop samples and 5 to 7 m deep playa cores were studied from three closed basins in south-central Oregon. Al-rich dioctahedral smectite dominates outcrop samples from the Alkali and Abert basins, whereas Al- and Fe-rich dioctahedral smectite is most abundant in the Alvord basin. The composition of smectite from outcrop samples reflects the composition of parent rock in each basin. In Alkali and Abert playas, randomly interstratified Mg-rich trioctahedral kerolite/smectite (Ke/Sm) formed, whereas in the Alvord playa Mg-smectite did not form, due to a lower degree of evaporative concentration. Thus, mineral authigenesis in playa sediments is controlled by the composition of pore waters. Although the pore waters from all three playas plot in the stability field for trioctahedral Mg-smectite, progressive evaporation reaction-path calculations of dilute playa pore waters predict that Mg-silicate minerals should not precipitate from Alvord pore waters until the maximum extent of evaporative concentration observed at Alvord. In contrast, calculations predict precipitation of Mg-silicates at Alkali and Abert playas after little evaporative concentration. Additional reactions in playa sediments are formation of clinoptilolite {dollar}(pm{dollar} searlesite) at 2 to 4 m depth in the Alkali playa, probably reflecting an influx of volcanic ash and K{dollar}sp+{dollar}-fixation within collapsed smectite interlayers at the Alvord playa. Authigenic illite was not found.; Mixed-layer Ke/Sm forms by a neoformation mechanism based on the approach to oxygen isotopic equilibrium with evaporatively concentrated pore waters at the surface. Ke/Sm exhibits few chemical and mineralogical variations with depth in the playa suggesting that it forms relatively rapidly (in {dollar}<{dollar}16,000 yrs) near the surface due to a higher reactivity of shallow pore waters and the greater frequency of wetting and drying. The chemical and isotopic similarity of Al/Fe-smectite from outcrops and playa sediments at Alvord supports a solid-state mechanism for K{dollar}sp+{dollar}-fixation in Al/Fe-smectite.; Evaporative concentration controls the chemical and isotopic variations in playa pore waters. Playa pore waters are the most saline and isotopically heavy at the surface. The oxygen and hydrogen isotopic analyses of playa pore waters extrapolate to the meteoric water line at {dollar}-18perthous{dollar} for {dollar}sp{lcub}18{rcub}{dollar}O and {dollar}-134perthous{dollar} for D. During evaporative concentration, Ca{dollar}sp{lcub}2+{rcub},{dollar} Mg{dollar}sp{lcub}2+{rcub},{dollar} and SiO{dollar}sb2{dollar} are removed from solution whereas K{dollar}sp+{dollar} and Na{dollar}sp+{dollar} appear to be conserved.