Sr isotopes and geochemistry of Cerro Overo maar provide a unique window into arc magma genesis in the Central Volcanic Zone of the Andes

Cerro Overo maar is the most mafic lava (54 wt% SiO2, 7.4 wt% MgO) found within the modern subduction arc of the Central Andes. Structural weaknesses in the crust allowed magma to rapidly ascend while avoiding extensive crustal contamination, interactions with the mid-crustal Altiplano-Puna Magma Body, or storage in magma chambers. Cerro Overo lava thus preserves chemical and mineralogical signals from deep-crustal or upper-mantle magmatism which are otherwise absent in the modern Andes Central Volcanic zone (CVZ). Radiogenic isotope ratios of CVZ rocks are characteristically enriched, and researchers have debated for decades whether this enrichment is derived from an enriched mantle source or the result of crustal processes. Phenocrysts of high-forsterite olivine (Fo86) from Cerro Overo contain three populations of silicate melt inclusions: 1. basaltic andesite inclusions chemically identical to the host lava, 2. rhyodacite inclusions representing assimilated upper crust, and 3. basaltic melt inclusions (SiO2 = 49.0 wt%) of minimally-altered magma, reflecting an early-stage melt which may be common to the active arc of the Altiplano-Puna region. Melt inclusion 87Sr/86Sr (0.70376 to 0.70720) also form three clusters, including the most primitive Sr isotopic ratios reported for the modern CVZ arc: direct evidence that the sub-arc mantle source retains non-radiogenic Sr signatures. The least-evolved Olivine-hosted melt inclusions from Cerro Overo match the composition of postulated arc endmembers and provide direct evidence that the overall radiogenic isotopic characteristic of the modern CVZ results from lithospheric processes and not inheritance from a mantle source with an initially radiogenic 87Sr/86Sr signature.