Zircon as a recorder of the oxygen fugacity of magmas

Authors: Antony Burnham (Imperial College London, U.K.)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: July 2012

Abstract: Zircon (ZrSiO4) is a durable mineral found in most igneous rocks; it is highly retentive of the trace element concentrations it acquires at crystallisation, and its high initial concentrations of U and Th relative to Pb make it the most important mineral for radiometric dating. Therefore, it is a valuable archive of magmatic processes, particularly with regard to those occurring on the early Earth. Based on crystal chemistry, anomalous Ce and Eu concentrations in zircon relative to other rare earth elements (REE) seem likely to reflect the oxidation state of the magma. Zircons were grown experimentally under controlled conditions of oxygen fugacity (fO2) and the crystals and coexisting glass were analysed by SIMS and LA-ICP-MS to examine the covariation of the partition coefficients of Ce and Eu, as well as those of other trace elements. This revealed that with increasing fO2, Ce becomes more compatible and Eu and U become less compatible. There is a narrow window of fO2s in which a Ce and a Eu anomaly coexist. Literature data allow the partitioning data obtained for the heavy REE in this study to be extrapolated to other temperatures. To allow extrapolation of the partitioning experiments, Ce- and Eu-doped glasses of various melt compositions were prepared at a range of fO2s and temperatures. X-ray absorption near edge structure (XANES) spectroscopy of these glasses was carried out at the LIII-edge of these elements to determine their oxidation state ratios. Because of beam damage effects for the Eu-bearing glasses, a limited number of XANES spectra were recorded in situ at 1400 °C, and some samples were analysed by electron paramagnetic resonance spectroscopy. The results obtained were compared to trace element concentrations in zircons from some natural samples, and suggestions for future work made.

Journal Keywords: Rare earth; Zircon; Europium; Cerium

Subject Areas: Earth Science, Chemistry

Instruments: B18-Core EXAFS , I18-Microfocus Spectroscopy

Other Facilities: ESRF

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