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

• Applicant: Professor Dr. Anton Eisenhauer
• Subject Area: Palaeontology
• Term: from 2014 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 260997739

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.

• DFG Programme: Infrastructure Priority Programmes
• Subproject of: SPP 527: Bereich Infrastruktur - "International Ocean Discovery Program" (IODP)
• Participating Persons: Professor Dr. Wolfgang Bach; Privatdozent Dr. Andreas Klügel; Ana Kolevica; Dr. Volker Liebetrau (†); Dr. Svenja Rausch; Professor Dr. Michael Wiedenbeck