242 Crustal Petrology, Part I

This session explores various igneous and metamorphic processes that have shaped Earth’s crust. We welcome contributions from the micro- to macroscale, from the deep to shallow crust, and from field- to laboratory- to modeling-based.

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 216AB

Not your average zircon – assessing geochemical variability across igneous compositions

Paper number: 242-14
Program: Technical Sessions
Time: 11:30 AM – 11:45 AM

Authors

Abstract

Crustal magmatism encompasses a suite of complex processes spanning time and spatial scales [1]. Zircon serves as a premier geochronometer to date magmatic processes and its trace element geochemistry can record the thermochemical evolution of the surrounding magma. This chemical and physical history can be studied using diverse proxies backed by experiments, case studies, and, more recently, “big data” compilations.

These proxies are especially useful for interpreting the evolution and complexities of single magma systems, but when applied to magmatic systems across tectonic settings, zircons show variable patterns of compositional heterogeneity and homogeneity that are difficult to interpret on face value. There is also an inherent zircon “core bias” in many of these measurements because most zircon analyses are spots on the polished interior of grains; an exemplar effect of this is seen in the significant compositional overlap between I- and S-type magmatism, despite zircon rims often preserving more disparate compositions. Depth profiling, analyzing a grain from the rim inwards (roughly orthogonal to growth zoning), presents one avenue to improve our resolution of magmatic evolution unique to tectonic setting and locality. Importantly, depth profiling allows access to outer, lower-temperature growth zones that may capture more complex late-stage magma dynamics, which should better reflect the complexity of magma bodies in plutonic and volcanic settings [2].

Our new depth profiling dataset from samples of different igneous classifications (I, S, A1, and A2) (~45,000 analyses) allow us to perform a robust statistical comparison of the variability of grain chemistry across multiple sampling scales. We expand significantly on prior work by examining how the average chemistry and corresponding variability affect zircon comparison at the grain, hand sample, locality, and granite class scales. Notably, individual grains record parts of the magmatic evolution, which translates to expanded variability in hand samples (coefficient of variation (CV) = 0.5-2.5), and comparable averages/variabilities inter-sample. These lead to statistical similarity intra-class and discernible differences inter-class. These results allow for a simple classification scheme based on average compositions and characterize the variability induced in different types of magmatism.  The averages and variability across different scales highlight a proposed minimum range of zircon compositions imparted by different magmatic settings and differentiation processes.

[1] Annen et al., 2025, Nature

[2] Ratschbacher et al., 2024, G-Cubed

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414103

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