METHOD DEVELOPMENT FOR LINKING PLATE TECTONICS AND MANTLE OXIDATION USING CE AND EU OXIDATION STATES ZIRCONS
The analysis of the Rare Earth Elements (REE) Ce and Eu contents of a suite of zircons from SE Greenland, representative of early Proterozoic-style subduction and a 3.0 Ga continental crust block, could provide a definitive link between the onset of plate tectonics and mantle oxidation. Ce and Eu are multivalent elements that can exist in oxidation states other than REE3+ under modern magmatic conditions; enrichments and/or depletions in these elements relative to the other REE can be used as proxies for mantle fO2. Under oxidizing conditions, Ce4+ is preferentially accommodated into zircon versus Ce3+.
A method for determining REE concentrations using a high resolution, single collector ICP-MS coupled with a high power excimer (ArF) laser system (193 nm wavelength) is demonstrated through the analysis of a compilation of standard reference glasses. Although REE analyses of zircons have been performed traditionally by implementing low resolution (m/Δm ≤ 500) mass discrimination (standard for quadrupole instruments), higher resolutions provide more confidence for the separation/isolation of isobaric interference.
EMPA analyses were performed on the zircon suite to obtain internal standard concentrations (Yb, Hf). GeoREM glass standard values were used for internal standard concentrations. For LA-ICP-MS analysis, multiple mass stations of elements La through Hf were selected taking into account atomic, oxide, argide, and hydride isobaric interferences. Glass standards were analyzed using 150µm, 65µm, and 25µm spot sizes. A recommended method for high-resolution REE analysis of zircons will be presented.