METHOD DEVELOPMENT FOR LINKING PLATE TECTONICS AND MANTLE OXIDATION USING CE AND EU OXIDATION STATES ZIRCONS

Oxygen fugacity (fO₂) is a major control on trace element partitioning and during planetary differentiation. During the earliest stages of planet formation the Earth’s magma ocean was reduced enough for liquid iron to precipitate and coalesce at the planet’s center to form the core. Today, the mantle is more oxidized, and iron partitions primarily between silicate phases during mantle melting. Plate tectonics processes are thought to have influenced the progression of redox conditions in Earth’s interior, however an unambiguous link has yet to be established.

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.