Redox evolution of silicic magmas: Insights from XANES measurements of Ce valence in Bishop Tuff zircons

Abstract

<p>The potential for zircon to record continuous and evolving magmatic redox conditions is investigated by quantifying Ce valence in natural and synthetic crystals by X-ray Absorption Near Edge Structure (XANES). Valence was determined at high spatial resolution (2 × 4 μm) by analysis of the Ce L₃ edge for synthetic zircons and crystals from the Bishop Tuff Ig2E sequence; analyses included both core-to-rim and cross-sector measurements. Core-to-rim zonation among natural grains reveals a systematic increase in Ce^{4 +}/ΣCe, with core regions that range from ~ 0.4 to 0.6 Ce^{4 +}/ΣCe (i.e., ~ 40–60% Ce^{4 +}), while zircon rims range from ~ 0.7 to 1.0 Ce^{4 +}/ΣCe (i.e., ~ 70–100% Ce^{4 +}). Repeat analysis on an individual point suggests, on average, a Ce^{4 +}/ΣCe reproducibility at the 5% level or less. Changes in spectral features with grain orientation were also investigated by rotating and analyzing synthetic zircons every 45o. This resulted in changes to the calculated Ce valence of < 5%, which is much smaller than the range observed in natural samples. The core-to-rim increase in Ce^{4 +}/ΣCe of Bishop Tuff samples may indicate a continuous crystal-melt evolution to more oxidizing conditions prior to eruption, but this cannot be uniquely decoupled from other effects that may influence Ce valence in zircon, which potentially include temperature changes or kinetic processes related to the mineral growth surface. Cathodoluminescence imaging couples with XANES spectra for Bishop Tuff samples reveal that different sectors yield notably different Ce^{4 +}/ΣCe, implying anisotropic decoupling of Ce^{3 +} and Ce^{4 +} in the zircon near-surface during crystallization. Broadly correlative (albeit with lower spatial resolution) Ti-thermometry and light rare earth element LA-ICP-MS data are also reported for zircon grains; there is no correlation between measured Ce anomalies and Ce^{4 +}/ΣCe. Cerium valence measurements of zircon may be able to constrain magma redox evolution with time, without relying on the nearly ubiquitous low concentrations of La and Pr, which are classically used to calculate Ce anomalies.</p>