Reconciling heterogeneity in crystal zoning data: An application of shared characteristic diagrams at Chaos Crags,Lassen Volcanic Center,California

Isotope, trace element, and textural crystal zoning patterns record heterogeneity in magmatic systems not resolved by whole rock analyses. These zoning data are used to infer crystal residence times, magma mixing, and other magmatic processes in many magmatic systems. We present the shared characteristic diagram (SCD) as an organizational framework for crystal zoning data that compares information from different phases and chemical tracers in a common framework. An example from Chaos Crags in the Cascade arc, produces three main results. (1) Anorthite zoning profiles in plagioclase have fewer shared characteristics in mafic inclusions than in the host rhyodacite. (2) Single-crystal ⁸⁷Sr/⁸⁶Sr data from previous studies (Tepley et al. 1999) are consistent with more shared history between crystals than in anorthite profiles. This difference reflects a more homogeneous distribution of ⁸⁷Sr/⁸⁶Sr than the intensive parameters controlling plagioclase composition. (3) The Chaos Crags system exhibits a layer of heterogeneity in crystal populations that is not represented in whole-rock analyses that indicate only simple binary mixing. The inconsistency between ⁸⁷Sr/⁸⁶Sr and anorthite zoning data highlights decoupling between compositionally controlled and temperature/water-controlled zoning in plagioclase from Chaos Crags.