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

In situ secondary ion mass spectrometry (SIMS) analyses of delta Li-7, Li/Ca, and Mg/Ca were performed on five synthetic aragonite samples precipitated from seawater at 25 degrees C at different rates. The compositions of delta Li-7 in bulk aragonites and experimental fluids were measured by multicollector inductively coupled plasma-mass spectrometry (MC-ICP-MS). Both techniques yielded similar delta Li-7 in aragonite when SIMS analyses were corrected to calcium carbonate reference materials. Fractionation factors alpha Li-7/Li-6 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 delta Li-7 range determined by SIMS is up to 27%, exceeding the difference between bulk delta Li-7 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.