ESTIMATING HADEAN MAGMA COMPOSITIONS USING IMPROVED ZIRCON/MELT KDS: COOL PETROGENESIS, ARCHEAN CONNECTIONS? (Invited Presentation)

Zircon/melt partition coefficients based on rim and natural glass measurements display very strong negative correlations with temperature and T proxies like Ti-in-zircon. Kd-Ti_zrc correlations can thus be used to estimate Kds for individual trace element and Ti analyses of zircon (Claiborne et al. 2017). We use individually tailored Kds to estimate elemental concentrations in Hadean melts (emphasizing REE) and compare results to model melts determined using younger zircons from known tectonic settings: MORB (Grimes et al. 2007), Iceland: oceanic rift/hotspot (Carley et al. 2014), and Mount St. Helens: continental arc (Claiborne et al. 2010). An initial survey of model melts using zircon from these modern environments reveals REE patterns that are distinguishable and consistent with typical patterns for these settings. Ranges in model concentrations are large, probably because of sensitivity to measured Ti, but median concentrations and patterns appear robust.

We use two sets of zircon data for Early Earth melt modeling: (1) a compilation of Jack Hills Hadean zircon (n = 76) which includes ages, an extensive suite of trace elements, Ti, and careful vetting for alteration (Bell et al. 2016); and (2) previously published but unpaired Hadean REE (n = 176) and Ti (n = 600, median 5 ppm used in Kds) compiled in Carley et al. 2014. Both data sets yield very similar results: Hadean model melts are distinct from the modern examples, characterized by HREE depletion and concave-up M-HREE/chond patterns with an inflection around Tm; median Tm/chond for both are <5. These characteristics contrast strongly with Iceland and MORB (M-HREE straight, HREE/chond 60-100) and, less so, with Mount St. Helens (slightly concave-up, Tm/chond ~10). Furthermore, the same patterns (HREE depletion, concave with Tm/chond <5) persist for model melts for Archean zircons (3.4-4.0 Ga) in the Jack Hills data of Bell et al. (2016; n = 119), suggesting kinship in petrogenesis of early silicic magmas across the Hadean-Archean boundary. The shared Hadean-Archean model melt REE pattern is strikingly similar to those of Archean TTGs (e.g., Martin 1987, Moyen and Martin 2012), and suggests that that melts from which these Hadean zircons grew were influenced by fractionation of garnet and/or amphibole and thus reflect genesis under relatively cool, wet conditions.