Aluminum in zircon as evidence for peraluminous and metaluminous melts from the Hadean to present

Abstract

<p>Zircon structurally accommodates a range of trace impurities into its lattice, a feature which is used extensively to investigate the evolution of silicate magmas. One key compositional boundary of magmas is defined by whether the molar ratio of Al₂O₃/(CaO+Na₂O+K₂O) is larger or smaller than unity. Here we report ~800 Al in zircon concentrations from 19 different rocks from the Lachlan Fold Belt(southeastern Australia), New England (USA), and Arunachal leucogranites (eastern Himalaya) with Al₂O₃/(CaO+Na₂O+K₂O) whole rock values that range from 0.88 to 1.6. Zircons from peraluminous rocks yield an average Al concentration of~10 ppm, which distinguishes them from crystals found in metaluminous rocks (~1.3 ppm). This difference is related to the materials involved in the melting, assimilation, and/or magma differentiation processes; for example, magmas that assimilate Al-rich material such as metapelites are expected to produce melts with elevated alumina activities, and thus zircons with high Al concentrations. These observations are applied to the Archean and Hadean Jack Hills detrital zircon record. Detrital Archean zircons, with ages from about 3.30 to 3.75 Ga, yield Al in zircon concentrations consistent with origins in peraluminous rocks in ~8% of the cases (n=236). A single zircon from the pre-3.9 Ga age group(n=39) contains elevated Al contents, which suggests that metaluminous crustal rocks were more common than peraluminous rocks in the Hadean. Weathered material assimilated into these Hadean source melts was not dominated by Al-rich source material.</p>