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