Sr/Mg variation during rock-water interaction: implications for secular changes in the elemental chemistry of ancient seawater

Prior studies have recently demonstrated a positive linear covariance in the Sr and Mg contents of abiotic marine carbonates such that the slope of this relation is dependent upon the Sr/Mg ratio of seawater at the time of precipitation. Samples of ancient abiotic marine calcite cements that have undergone diagenesis through alteration by meteoric waters exhibit similar linear trends that range from primary marine ratios to low values that converge to zero. Thus, Sr/Mg ratios in diagenetic precipitates are potential proxies for the reconstruction of past variations in the elemental chemistry of ancient seawater. To expand upon limited existing Sr/Mg data, late Silurian and middle Triassic abiotic calcite cements were analyzed for stable isotope and minor element compositions. Silurian cements from the Pipe Creek Junior Quarry, Grant County, Indiana, show a linear covariance of Sr and Mg concentrations expressed by the following relationship: Sr/Ca = 0.0306 (±0.0091) Mg/Ca + 2.349 × 10−6 (±5.812 × 10−5). Triassic cements from the Cozzo di Lupo Formation, Sicily, Italy, exhibit a linear covariance given by: Sr/Ca = 0.0103 (±0.0009) Mg/Ca + 3.818 × 10−5 (±2.800 × 10−5). To evaluate the mechanisms by which Sr/Mg ratios are preserved in diagenetic calcite, a progressive rock-water interaction model was employed that traces the simultaneous evolution of δ13C, δ18O, Sr, and Mg contents of both fluid and solid phases. Results from modeling indicate that the Sr/Mg ratio of diagenetic calcite is primarily controlled by the ratio of distribution coefficients for Sr and Mg in calcite. When DSr/DMg is equal to unity, covariance of Sr/Ca and Mg/Ca in diagenetic calcite is linear, with a slope equivalent to the Sr/Mg value of the original dissolved solid phase. When DSr/DMg deviates from unity, the incorporation of Sr/Ca and Mg/Ca in diagenetic precipitates becomes progressively non-linear. A general agreement of theoretical and empirical results for Pliocene abiotic calcite cements of Frank and Lohmann (1996) indicates that model simulations adequately represent natural diagenetic settings. Additionally, at rock-water ratios where δ13C has converged to and δ18O has shifted toward original rock composition, elemental contents, and thus the Sr/Mg ratio, are effectively preserved. Thus, retention of original Sr/Mg chemistries is possible across a broad range of rock-water ratios, even where primary δ18O values have been significantly modified. Given the correlation between theoretical and empirical Sr/Mg results, we suggest that variation in the Sr/Mg composition of abiotic calcite cements reflect fluctuations in the Sr/Mg chemistry of ancient seawater.