25-7 Zinc Isotopes Trace Lithogenic and Biogenic Recycling in Cordilleran Continental Arcs

Session: Integrating metamorphism, mass transfer, and magmatism across the American Cordillera (Posters)

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Abstract:

Arc magmatism has played a fundamental role in shaping the composition of the oceans and atmosphere and, consequently, the evolution of life on Earth. However, the extent to which biological processes modulate arc magmatism remains poorly understood. A significant fraction of the CO2​ emitted by arc volcanoes is thought to originate from subducted microfossils, organic matter, and carbonate sediments; yet, the fate of carbon within terrestrial reservoirs remains difficult to trace because of its complex partitioning among minerals, fluids and gases. Zinc isotopes have emerged as promising tracers of subducted carbon due to the isotopic contrasts between silicic and carbonate lithologies. In the ocean, Zn behaves as a micronutrient: organic tissues preferentially incorporate light isotopes, while biogenic carbonate shells are enriched in heavy Zn isotopes. Consequently, biogenic carbonate sediments display heavier Zn isotopes (δ66Zn≈+0.94‰) than the continental crust (δ66Zn≈+0.27‰) or the upper mantle (δ66Zn≈+0.16‰), suggesting that Zn isotopes may distinguish subducted carbonate from lithogenic inputs in arc magmas. Despite this potential, previous studies of arc volcanic rocks have not identified a clear carbonate-derived Zn isotopic signal, whereas heavy Zn isotopes are instead observed in intraplate basalts where the influence of subduction is difficult to recognize.

To resolve this apparent discrepancy, we analyzed Zn isotopic compositions of well-characterized arc volcanoes from Mexico and Colombia: archetypal Cordilleran arcs developed on thick, ancient continental crust. In both regions, the dominant volcanism consists of high-Mg# andesites that are virtually identical in their major-element compositions. However, andesites from the two arcs differ markedly in radiogenic isotopes and trace-elements, reflecting contrasting subduction inputs. In Mexico, intense forearc subduction erosion introduces predominantly continental material into the mantle wedge. Conversely, the non-erosive character of the Colombian margin allows biogenic carbonates and organic-rich clays to be transported into the subduction zone. Consistent with these contrasting inputs, Mexican andesites display significantly lighter Zn isotopic compositions (δ66Zn≈0.23±0.02‰) than their Colombian counterparts (δ66Zn≈0.31±0.03‰). The Zn isotopic variations are independent of proxies for partial melting or crystal fractionation, but correlate with trace-element ratios and radiogenic isotopes sensitive to lithogenic and biogenic inputs (e.g., Ba/Nb,U/La,87Sr/86Sr). Together, these results demonstrate that Zn isotopes provide a robust link between surface biogeochemical processes and deep carbon cycling in Cordilleran convergent systems.

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