|Title:||In situ d7Li, Li/Ca, and Mg/Ca analyses of synthetic aragonites|
|Authors:||R. I. Gabitov, A. K. Schmitt, M. Rosner, K. D. McKeegan, G. A. Gaetani, A. L. Cohen, E. B. Watson, and T. M. Harrison|
|Publication:||Geochemistry Geophysics Geosystems, v. 12, p. .|
In situ secondary ion mass spectrometry (SIMS) analyses of δLi, Li/Ca, and Mg/Ca were performed on five synthetic aragonite samples precipitated from seawater at 25°C at different rates. The compositions of δLi in bulk aragonites and experimental fluids were measured by multicollector inductively coupled plasma–mass spectrometry (MC–ICP–MS). Both techniques yielded similar δLi in aragonite when SIMS analyses were corrected to calcium carbonate reference materials. Fractionation factors ?7Li/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 δLi range determined by SIMS is up to 27‰, exceeding the difference between bulk δLi 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.