Microanalytical characterization of the calcium isotopic composition of low temperature calcite veins in the ocean crust

海洋地壳低温蚀变（LTA）过程中形成的方解石脉可以记录周围海水的化学和同位素组成。基于这些方解石脉的锶、镁、钙元素比值重建显示了晚新生代以来的重要趋势。钙同位素可用于检验LTA方解石作为古海水成分记录仪的稳健性和可靠性。LTA方解石脉钙同位素记录显示，随着年龄和地层温度的增加，钙同位素向轻同位素转变的趋势明显。这表明，在温度略微升高的海洋地壳变化过程中，或者在数百万年的时间尺度上，海洋地壳中方解石的埋藏过程中，玄武岩中浸出的钙可能大量涌入。在这两种情况下，我们都可以预料到钙同位素组成在方解石脉中的系统性微米级变化。我们将使用一种创新的微量分析工具（SIMS，次级离子质谱仪）在微米尺度上研究LTA方解石脉的钙和氧同位素组成。钙同位素的小尺度变化应揭示玄武岩钙（轻同位素组成）和海水钙（重同位素组成）对方解石脉的影响。氧同位素可以用来重建方解石沉淀或再结晶时的温度。不同的变化趋势可以表明方解石生长过程中流体成分的变化或与周围玄武岩同位素交换后流体成分的变化。低温下保存下来的小块原始方解石可以用来重建方解石形成时的古海水成分。

Calcite veins that form during low-temperature alteration (LTA) of the ocean crust can record the chemical and isotopic composition of ambient seawater. Reconstructions of strontium, magnesium and calcium element ratios based on such calcite veins show significant trends since the late Cenozoic. Calcium isotopes can be used to test the robustnes and reliability of LTA calcites as recorders of paleo-seawater composition. A record of calcium isotopes from LTA calcite veins shows a strong trend to light isotopes with increasing age and formation temperature. This points to a possible influx of calcium leached from basalts during ocean crust alteration at slightly elevated temperatures or during burial of calcite in the ocean crust on million-year time-scales. In both cases one can expect systematic micrometer-scale variations in the calcium isotopic composition across calcite veins. We will investigate the calcium and oxygen isotopic composition of LTA calcite veins on a micrometer scale using an innovative microanalytical tool (SIMS, secondary ion mass spectrometer). Small scale variations of calcium isotopes should reveal the influence of basaltic calcium (light isotopic composition) and seawater calcium (heavy isotopic composition) on the calcite veins. Oxygen isotopes can be used to recontruct the temperature at the time of calcite precipitation or recrystallization. Different trends can indicate variable fluid compositions during the calcite growth or later diagenetic changes in the composition by isotopic exchange with surrounding basalt. Preserved small patches of original calcite fromed at low temperatures could be used to reconstruct the paleo-seawater composition at the time of formation of the calcites.