Enrichment of heavy calcium isotopes in saprolite due to secondary mineral formation

Abstract

Calcium stable isotopes were analyzed in well-characterized saprolite samples developed on a Mesozoic metadiabase dike near Cayce, South Carolina, to determine the degree and sense of isotopic fractionation during chemical weathering. The δ44/40Ca values (relative to NIST SRM 915a) of the saprolites, measured using double-spike TIMS, vary significantly (0.91‰ to 1.26‰), and are generally higher than the δ44/40Ca value of the unweathered metadiabase (0.98‰), which overlaps, within analytical uncertainties, with the δ44/40Ca estimate of the bulk silicate Earth. The δ44/40Ca values of the saprolites negatively correlate with bulk density, and Ti-normalized calcium concentrations, and positively correlate with chemical index of alteration (CIA) values. These trends reflect loss of light Ca isotopes to the hydrosphere during metadiabase weathering. Using insoluble incompatible elements ratios (e.g., Th/Nb) we estimate that the influence of aeolian dust is minimal (3% - 11%) and therefore unable to explain the observed variation in δ44/40Ca values of the saprolites. Selective weathering of rock-forming minerals like plagioclase and clinopyroxene cannot explain the high δ44/40Ca values (> 1 ‰) of the saprolites based on mass balance constraints. Samples with higher kaolinite/smectite ratio have higher δ44/40Ca values and lower concentrations of Ca, which suggests the loss of Ca from smectite-rich clays during progressive weathering. In this case, the lighter isotopes are preferentially lost, leaving the residual clays enriched in heavier isotopes of Ca. The δ44/40Ca values of saprolites correlate positively with δ26Mg and negatively with δ7Li values of the same samples, which further suggests that Ca stable isotopic variability of the Cayce metadiabase saprolites is controlled by formation of clay minerals and progressive loss of Ca during saprolitization. The enrichment of heavy Ca isotopes in saprolites with progressive loss of Ca can be modeled using Rayleigh distillation, with apparent fractionation factors between saprolites and fluid (α) of 1.00005 to 1.00015.