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.