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

洋壳低温蚀变 (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。基于这种方解石脉重建的锶、镁和钙元素比例显示出自新生代晚期以来的显着趋势。钙同位素可用于测试 LTA 方解石作为古海水成分记录器的稳健性和可靠性。 LTA 方解石矿脉的钙同位素记录显示，随着年龄和地层温度的增加，轻同位素呈强烈趋势。这表明，在温度稍高的海洋地壳蚀变过程中，或者在数百万年的时间尺度上，方解石埋藏在海洋地壳中时，玄武岩中的钙可能会大量涌入。在这两种情况下，我们可以预期方解石脉中的钙同位素组成会出现系统性的微米级变化。我们将使用创新的微量分析工具（SIMS，二次离子质谱仪）在微米尺度上研究 LTA 方解石脉的钙和氧同位素组成。钙同位素的小规模变化应该揭示玄武岩钙（轻同位素成分）和海水钙（重同位素成分）对方解石脉的影响。氧同位素可用于重建方解石沉淀或重结晶时的温度。不同的趋势可以表明方解石生长过程中流体成分的变化，或者通过与周围玄武岩的同位素交换而导致成分的后期成岩变化。保存下来的小块低温形成的原始方解石可用于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.