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

在洋壳低温改造(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。基于这种方解石矿脉的锶、镁和钙元素比值的重建显示出自晚新生代以来的显著趋势。钙同位素可以用来检验LTA方解石作为古海水成分记录器的健壮性和可靠性。LTA方解石矿脉的钙同位素记录表明，随着年龄和形成温度的增加，轻同位素有强烈的趋势。这表明，在洋壳微温变化期间或在百万年时间尺度上方解石在洋壳中埋藏期间，可能有从玄武岩中浸出的钙的流入。在这两种情况下，方解石矿脉的钙同位素组成都会出现微米级的系统性变化。我们将使用一种创新的显微分析工具(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)对方解石矿脉的影响。氧同位素可用来重建方解石沉淀或重结晶时的温度。通过与周围玄武岩的同位素交换，不同的趋势可以指示方解石生长过程中流体成分的变化或后来成岩成分的变化。保存下来的小块低温形成的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.