High Temperature Calcium Isotopic Fractionation Andits Potential For Tracing Deep Carbon Recycling

In order to use Ca isotopes as a proxy for tracing deep carbon recycling, I investigated Ca isotopic fractionation under high temperature geological processes. This thesis includes:(1) constraining Ca isotopic behaviors during magmatic differentiation; and(2) evaluation of Ca isotopes as a proxy for tracing deep carbon recycling. The major results are the following.(1)Ca isotopic composition of geological samples were precisely measured with double spike technique at Harvard University. I also provide the MATLAB codes that can be used to easily determine Ca double spike data reduction algorithm. The long-term external reproducibility is 0.02‰(2σmean).(2)With improvement of Ca isotopic measurement, Ca isotopes are used as a proxy to study high temperature geological processes. However, important geochemical questions contain whether the terrestrial mantle is homogeneous in Ca isotopes and the magnitude of Ca isotopic fractionation induced by these processes. I chose Kilauea Iki lava lake(Hawaii, USA) basalts to study the possible Ca isotopic fractionation during basaltic magmatic differentiation. Kilauea Lava Lake, Hawaii, cooled and crystallized as a closed system, which is an ideal location for studying magmatic differentiation. First I use the programme MELTS to place constraints on the process of its fractionation crystallization. Then I presented high-precision Ca isotopic measurement on these samples. Although undergone fractionation crystallization and chemical differentiation, they show no resolvable variations in Ca isotope composition(δ44/40Ca= 0.91±0.03, 2σmean). This suggests Ca isotope fractionation is insignificant at whole-rock scale during basalt magmatic differentiation at current analytical precision. The Ca isotopic composition of fresh basalts should represent their source region. Hence, it is appropriate to use Ca isotopic composition of basalts to trace deep carbon recycling.(3)Deep carbon recycling is an essential part of the global carbon cycle. Carbonates at the bottom of the ocean are brought to the mantle through subduction zones. Subsequently, mantle carbon is released to the atmosphere in the form of CO2 throughvolcanism.Theproportionof subduction-related carbon and primary mantle-derived carbon in CO2 released by volcano is an important issue. Carbon isotopes can easily distinguish organic carbon from inorganic carbon. However, ~95% of subduction-related and primary mantle-derived carbon released by volcano is inorganic, which cannot be distinguished using carbon isotopes.Recently, Ca and Mg isotope geochemistry has provided important constraints on tracing recycling of crust-derived material. Here I introduce the principles of C, Mg, and Ca isotopes in tracing deep carbon recycling. I also summarize the research progress on the total storage and phases of carbon in deep Earth, CO2 fluxes, partial melting of carbon-bearing mantle, and carbon behaviour at subduction zone. Previous study indicates that recycling of sedimentary carbonates into the mantle may cause the heterogeneous isotopic composition of mantle.