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

在海洋地壳低温蚀变（LTA）过程中形成的方解石脉可以记录环境海水的化学和同位素组成。锶，镁，钙元素比值的重建基于这样的方解石脉显示出显着的趋势，自晚新生代。钙同位素可以用来检验LTA方解石记录古海水成分的可靠性。从LTA方解石脉的钙同位素记录表明，随着年龄和形成温度的增加，轻同位素的强烈趋势。这表明，在海洋地壳的变化过程中，在稍微升高的温度下，或在数百万年的时间尺度上，方解石埋藏在海洋地壳中时，可能会有从玄武岩中沥滤出来的钙流入。在这两种情况下，我们可以预期在整个方解石脉的钙同位素组成系统的微米尺度的变化。我们将调查的钙和氧同位素组成的LTA方解石脉在微米尺度上使用一种创新的微量分析工具（西姆斯，二次离子质谱仪）。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重结晶时的温度。不同的趋势可以表明在方解石的生长或后来的成岩变化的组成与周围的玄武岩同位素交换的流体成分的变化。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.