Abstract
As a proxy of magmatic oxidation state, the accurate characterization of the Ce anomaly of zircon is of great significance since it can give important information for provenance studies of rocks as well as for exploration of intrusion-related mineral deposits. The magnitude of the zircon Ce anomaly has been traditionally described by Ce/Ce*, where Ce* is the theoretical Ce value derived from a chondrite-normalized rare earth element (REE) pattern. More recently, the Ce4+/Ce3+ method based on the lattice strain model has been proposed, since the latter method does not need La and Pr contents for zircon, both of which are commonly below the limit of detection and susceptible to contamination from melt/mineral inclusions. In this contribution we show that the Ce4+/Ce3+ method is confronted with some problems in practice and should be further improved. In contrast, by re-examining chondrite-normalized REE patterns of zircon, we find that Ce* can be estimated according to a logarithmic function curve without involvement of La and Pr contents. Application of this new method to zircon data from 11 giant to supergiant porphyry Cu deposits suggests this revised method as a more valid measure in evaluating magmatic oxidation state. The revised Ce/Ce* method is of particular importance for analyses where the provenance of the analyzed zircon is unknown or in question, since the method does not require knowledge of the melt composition.
Highlights
As one of the most important physico-chemical parameters of magmas (Barberi et al 1971), there is a consensus that the oxidation state can act as a valuable archive of geochemical information
Zircon (ZrSiO4) is an accessory mineral enriched in rare earth element (REE) that is common in igneous rocks of intermediate to felsic composition
Regression calculation clearly shows that REE patterns of zircons from Online Resource 2 can be matched by the logarithmic function (y = alog(x) + b, where y is the logarithm of chondrite-normalized REEs) (Fig. 4), with more than 99% of 847 fitting curves characterized by the coefficient of determination (R2) of >0.95 (Online Resource 2)
Summary
As one of the most important physico-chemical parameters of magmas (Barberi et al 1971), there is a consensus that the oxidation state can act as a valuable archive of geochemical information. In Online Resource 1, trace element data of a total of 35 experimental and natural zircon samples were considered to examine the optimum radius (rn0þ ) for ions substituting the Zr site of zircon lattice They were complied from publications of Burnham and Berry (2012); Colombini et al (2011); Luo and Ayers (2009); Mahood and Hildreth (1983); Marshall et al (2009); Murali et al (1983); Sano et al (2002) and Thomas et al (2002). This database was chosen because zircon, and melt compositions (by analyzing co-existing groundmass in volcanic rocks or melt inclusions) were measured directly, giving exact estimation for partition coefficients of trace elements In this table, r30þ values were calculated by fitting the partition coefficients of trivalent REE (and Sc where possible) ions to a parabolic curve in the Onuma diagram. Zircon Ce/Ce* and Eu/Eu* values calculated by either method exhibit some overlaps between barren and fertile intrusions, those calculated by the revised method behave more regularly and can more clearly distinguish fertile
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