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Trace element compositional trends in zircons separated from single hand samples have been used to infer dynamic processes in magma reservoirs. Here, we compile published zircon trace element chemistry to quantify any systematic difference between the range of compositions observed in zircon from individual volcanic and plutonic hand samples and compare these results with geochemical modeling to derive implications for magma reservoir dynamics. We find that both rock types span a wide range of hand‐sample scale variability (i.e., wide range of coefficients of variation), but there is no systematic difference in the average variability between plutonic and volcanic samples (i.e., no difference in the mean coefficient of variation). This indicates that dynamic processes related to eruption are not necessarily required as a fundamental process to create hand sample‐scale compositional heterogeneity beyond what is present due to dynamic processes in the reservoir recorded in plutonic samples. Modeling of felsic systems (>68.5 wt.% SiO2) indicates that the similar average variability in felsic volcanic and plutonic hand samples cannot be reproduced by closed‐system crystallization of compositionally distinct melts locally within a magma reservoir (i.e., isolated melt pockets in a crystal mush) but requires mixing of at least two felsic melt compositions at a small spatial scale. This study provides a framework for focused studies on individual volcanic‐plutonic systems exploring how plutonic and volcanic zircon compositional variability records the time and length scales of magma reservoir processes. Plain Language Summary Studies of volcanic rocks (erupted magmas) and plutonic rocks (unerupted magmas) provide insights into dynamic processes operating in magma reservoirs (e.g., mixing, crystal‐melt separation, etc.). However, contemporaneous volcanic and plutonic rocks of the same magmatic system are rarely exposed together, thus conceptual models of magma reservoir dynamics are seldom integrated directly between volcanic and plutonic studies. Zircon is a common mineral in crustal magmas (volcanic and plutonic) and is capable of recording melt evolution via its trace element chemistry. This study aims to gain insights into magma reservoir dynamics by systematically comparing trace element compositional variability of zircon separated from individual volcanic and plutonic hand samples. Our study shows that there is no systematic difference in the average compositional variability between plutonic and volcanic hand samples. This indicates that processes leading to eruptions do not necessarily introduce compositional heterogeneity beyond what is present due to dynamic processes in the reservoir recorded in plutonic samples. We further show using geochemical modeling that the observed similar average variability of zircon in felsic volcanic and plutonic hand samples cannot be reproduced by closed‐system crystallization (i.e., isolated melt pockets in a crystal mush) but requires mixing of at least two felsic melts (i.e., open‐system behavior). Key Points Zircon trace element chemistry from volcanic and plutonic hand samples does not show a difference in their average compositional variability Similar variability suggests that processes leading to eruption do not introduce systematically more heterogeneity than present in unerupted parts of the reservoir Crystallization modeling requires open‐system behavior at a scale of decimeters to reproduce the average variability in both volcanic and plutonic hand samples

Phenocryst assemblages of lavas from the long-lived Aucanquilcha Volcanic Cluster (AVC) have been probed to assess pressure and temperature conditions of pre-eruptive arc magmas. Andesite to dacite lavas of the AVC erupted throughout an 11-million-year, arc magmatic cycle in the central Andes in northern Chile. Phases targeted for thermobarometry include amphibole, plagioclase, pyroxenes, and Fe–Ti oxides. Overall, crystallization is documented over 1–7.5 kbar (~25 km) of pressure and ~680–1,110 °C of temperature. Pressure estimates range from ~1 to 5 kbar for amphiboles and from ~3 to 7.5 kbar for pyroxenes. Pyroxene temperatures are tightly clustered from ~1,000–1,100 °C, Fe–Ti oxide temperatures range from ~750–1,000 °C, and amphibole temperatures range from ~780–1,050 °C. Although slightly higher, these temperatures correspond well with previously published zircon temperatures ranging from ~670–900 °C. Two different Fe–Ti oxide thermometers (Andersen and Lindsley 1985 ; Ghiorso and Evans 2008 ) are compared and agree well. We also compare amphibole and amphibole–plagioclase thermobarometers (Ridolfi et al. 2010 ; Holland and Blundy 1994 ; Anderson and Smith 1995 ), the solutions from which do not agree well. In samples where we employ multiple thermometers, pyroxene temperature estimates are always highest, zircon temperature estimates are lowest, and Fe–Ti oxide and amphibole temperature estimates fall in between. Maximum Fe–Ti oxide and zircon temperatures are observed during the middle stage of AVC activity (~5–3 Ma), a time associated with increased eruption rates. Amphibole temperatures during this time are relatively restricted (~850–1,000 °C). The crystal record presented here offers a time-transgressive view of an evolving, multi-tiered subvolcanic reservoir. Some crystals in AVC lavas are likely to be true phenocrysts, but the diversity of crystallization temperatures and pressures recorded by phases in individual AVC lavas suggests erupting magma extensively reams and accumulates crystals from disparate levels of the middle to upper crust.