R. I. Gabitov

Partitioning of strontium between calcite and fluid

The Sr/Ca ratio of biogenic carbonate is widely used as a proxy for paleotemperature. This application is supported by empirical calibrations of Sr/Ca as a function of temperature, but it is also known that Sr uptake in calcite gauged by KdSr = is affected by other variables, including bulk precipitation rate (KdSr increases with increasing precipitation rate). There are no data from controlled experiments specifically addressing the effect of radial growth rate of individual crystals on KdSr. For this reason, we conducted two series of experiments to explore Sr partitioning at varying growth rates: (1) growth from a CaCl2–NH4Cl–SrCl2 solution by diffusion of CO2 from an ammonium carbonate source (“drift” experiments) and (2) “drip” precipitation of calcite on a substrate, using a steady flow of CaCl2–SrCl2 and Na2CO3 solutions, mixed just before passage through a tube and dripped onto a glass slide precoated with calcite (“cave-type” experiments). The growth rates of individual crystals were determined by periodic monitoring of crystal size through time or, roughly, by comparison of the final size with the duration of the experiment. Electron microprobe analyses across sectioned crystals grown in the drift experiments show that the concentration of Sr is high in the center (where radial growth rates are highest) and decreases systematically toward the edge. The center-to-edge drop in Sr concentration is a consequence of the slowing radial growth rate as individual crystals become larger. In general, high crystal growth rate (V) enhances Sr uptake in calcite due to a type of kinetic disequilibrium we refer to as “growth entrapment.” The apparent KdSr ranges from 0.12 to 0.35 as V increases from 0.01 nm/s to 1 μm/s at 25°C.

In situ δ7Li, Li/Ca, and Mg/Ca analyses of synthetic aragonites

In situ secondary ion mass spectrometry (SIMS) analyses of δ7Li, Li/Ca, and Mg/Ca were performed on five synthetic aragonite samples precipitated from seawater at 25°C at different rates. The compositions of δ7Li in bulk aragonites and experimental fluids were measured by multicollector inductively coupled plasma–mass spectrometry (MC-ICP-MS). Both techniques yielded similar δ7Li in aragonite when SIMS analyses were corrected to calcium carbonate reference materials. Fractionation factors α7Li/6Li range from 0.9895 to 0.9923, which translates to a fractionation between aragonite and fluid from −10.5‰ to −7.7‰. The within-sample δ7Li range determined by SIMS is up to 27‰, exceeding the difference between bulk δ7Li analyses of different aragonite precipitates. Moreover, the centers of aragonite hemispherical bundles (spherulites) are enriched in Li/Ca and Mg/Ca relative to spherulite fibers by up to factors of 2 and 8, respectively. The Li/Ca and Mg/Ca ratios of spherulite fibers increase with aragonite precipitation rate. These results suggest that precipitation rate is a potentially important consideration when using Li isotopes and elemental ratios in natural carbonates as a proxy for seawater composition and temperature.

Chemical speciation of heterogeneously reduced Pu in synthetic brines

Summary X-ray absorption fine structure (XAFS) spectroscopy has been used to determine the speciation of Pu precipitates prepared by the heterogeneous reduction of Pu(VI) with Al and Fe in 5 M NaCl and an ERDA-6 brine, a simulant from the Waste Isolation Pilot Plant in Carlsbad, New Mexico. NaOCl was added to some of these solutions to determine its effect on Pu speciation. Analysis of the Pu LIII spectra showed that all solids consisted of PuO2+x-y(OH)2y·z H2O, compounds with characteristics identical to those prepared by hydrolysis and with Pu-O and Pu-Pu distances identical to those treated at elevated temperature. Additionally, reduction with Al gave compounds with different site distributions than reduction with Fe, and reduction with Al or the addition of NaOCl appeared to suppress the formation of oxo groups and their associated Pu(V) sites.