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Zirconium (Zr) stable isotope variations occur among co-existing Zr-rich accessory phases as well as at the bulk-rock scale, but the petrologic mechanism(s) responsible for Zr isotope fractionation during magmatic differentiation remain unclear. Juvenile magma generation and intra-crustal differentiation in convergent continental margins may play a crucial role in developing Zr isotope variations, and the Northern Volcanic Zone of the Andes is an ideal setting to test this hypothesis. To investigate the influence of these processes on Zr stable isotope compositions, we report δ94/90ZrNIST of whole rock samples from: 1) juvenile arc basalts from the Quaternary Granatifera Tuff, Colombia; 2) lower crust-derived garnet pyroxenites (i.e., arclogites), hornblendites, and gabbroic cumulates found in the same unit; and 3) felsic volcanic products from the Doña Juana Volcanic Complex, a dacitic composite volcano in close proximity to and partially covering the Granatifera Tuff. The basalts have δ94/90ZrNIST values ranging from −0.025 ± 0.018 ‰ to +0.003 ± 0.015 ‰ (n = 8), within the range of mid-ocean ridge basalts. The dacites have δ94/90ZrNIST values ranging from +0.008 ± 0.013 ‰ to +0.043 ± 0.015 ‰ (n = 14), slightly positive relative to the Granatifera and mid-ocean ridge basalts. In contrast, the (ultra)mafic cumulates have highly variable, predominantly positive δ94/90ZrNIST values, ranging from −0.134 ± 0.012 ‰ to +0.428 ± 0.012 ‰ (n = 15). Individual grains and mineral fractions of major rock-forming phases, including garnet (n = 21), amphibole (n = 9), and clinopyroxene (n = 18), were analyzed from 8 (ultra)mafic cumulates. The mineral fractions record highly variable Zr isotopic compositions, with inter-mineral fractionation (Δ94/90Zrgarnet-amphibole) up to 2.067 ‰. Recent ab initio calculations of Zr–O bond force constants in rock-forming phases predict limited inter-mineral Zr isotope fractionation in high-temperature environments, suggesting that the large fractionations we observe are not the product of vibrational equilibrium processes. Instead, we propose a scenario in which large Zr isotopic fractionations develop kinetically, induced by sub-solidus Zr diffusion between coexisting phases via changes in Zr distribution coefficients that arise from changes in temperature. Altogether, Zr isotope variability in this calc-alkaline continental arc setting exhibits no correlation with indices of magmatic differentiation (e.g., Mg#, SiO2), and is not a simple function of fractional crystallization. Furthermore, the garnet clinopyroxenite cumulates studied here represent density-unstable lower arc crust material; consequently, material with isotopically variable δ94/90Zr can be recycled into the mantle as a consequence of lower crustal foundering. 

Abstract The thermal and compositional structure of arcs influence magmatic differentiation and lower-crustal foundering, two key processes impacting the evolution of the continental crust. Although many studies have proposed time scales of lithospheric recycling based on convective downwelling calculations, these models depend on the composition, density (ρ), and thermal structure of the lower crust and mantle, which are difficult to quantify in active continental arcs. Here, we constrained these properties for the Andean Northern Volcanic Zone using direct petrologic observations from a unique suite of lower-crust and mantle xenoliths from Mercaderes, Colombia. Chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb dates for zircons within the host tuff indicate the xenoliths erupted no earlier than 238 (±19) ka and thus capture a recent snapshot of the arc and subarc mantle. Equilibrium pressure-temperature (P-T) estimates for 81 xenoliths define three distinct thermal domains, interpreted as (1) a steep conductive geothermal gradient in the lower arc crust; (2) a convecting mantle wedge; and (3) cooled mantle in proximity to the subducting slab. Our results indicate the presence of an ~10–14-km-thick, high-density lithospheric root that is ~0.1 g/cm3 denser than the underlying mantle. Unlike records from exhumed paleoarcs, Rayleigh-Taylor instability calculations using our P-T-ρ constraints are unrealistically short for the northern Andes. We suggest the presence of partial melts in this hot arc root as a potential source of buoyancy preventing or significantly slowing down foundering. 

Zirconium (Zr) plays a key role in the development of phases like zircon (ZrSiO 4 ) and baddeleyite (ZrO 2 ) in magmatic systems. These minerals are crucial for the study of geologic time and crustal evolution, and their high resistivity to weathering and erosion results in their preservation on timescales of billions of years. Although zircon and baddeleyite may also preserve a robust record of Zr isotope behavior in high-temperature terrestrial environments, little is known about the factors that control Zr isotope partitioning in magmatic systems, the petrogenetic significance of fractionated compositions, or how these variations are recorded in Zr-rich accessory phases. Here, we describe a new analytical protocol for accurately determining the Zr stable isotope composition of zircon by multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS), using the double-spike method to correct for procedural and instrumental mass bias. We apply this technique to test whether zircon crystallization in carbonatite magmatic systems is a driver of Zr isotope fractionation by interrogating the internal zonation of a zircon megacryst from the Mud Tank carbonatite (MTUR1). We find the MTUR1 megacryst to lack internal zoning within analytical uncertainties with a mean μ 94/90 Zr NIST = −55 ± 28 ppm (2 SD, n = 151), which suggests that zircon crystallization is not a driver of Zr isotope fractionation in carbonatite magmas. This observation is in stark contrast with those made in silicate magmatic systems, raising the possibility that the bonding environment of Zr 4+ ions may be fundamentally different in carbonatite vs. silicate melts. Because of its remarkable homogeneity, the MTUR1 megacryst is an ideal natural reference material for Zr isotopic analysis of zircon using both solution and spatially resolved methods. The reproducibility of a pure Zr solution and our chemically purified zircon fractions indicate that the external reproducibility of our method is on the order of ±28 ppm for μ 94/90 Zr, or ±7 ppm per amu, at 95% confidence.