Abstract

Globally, low-delta O-18 (<5.7 parts per thousand) magmas are rare, especially in subduction zone settings. Here we report the variation in oxygen isotope composition of 0 to 12 Ma magmatic rocks across the Central Volcanic Zone (CVZ) of the Andes. The analysed samples range from andesite to dacite lavas (SiO2 62 to 72 wt%), ignimbrite sheet deposits, and lava domes. The delta O-18 value of quartz phenocrysts range between 7.1 and 10.7 parts per thousand (n = 83) and reflect those of the magma and give an average estimated magma delta O-18 value of 9.33 parts per thousand. Biotite and quartz phenocryst pairs have delta O-18 values that are consistent with oxygen isotope equilibrium at magmatic temperatures. In this study, we actively sought hydrothermally altered rocks for bulk rock analysis. Bulk rock delta O-18 values (n = 74) range from 5.0 to 22.3 parts per thousand, with the majority of visibly altered samples having high delta O-18 values. The 5D values of biotite phenocrysts are similar to those of bulk rock (-71 to -138 parts per thousand). We, therefore, find little evidence for either low delta O-18 magmas or low-delta O-18 rocks in the CVZ; even in zones of hydrothermally alteration. Previous explanations why low-delta O-18 rocks and magmas are not present in the CVZ highlight the low precipitation combined with high evaporation rates, which limited the influx of meteoric water to hydrothermal systems. Our data show that even when hydrothermal alteration occurred it did not generate low-delta O-18 rocks, without which, low-delta O-18 magmas could not be generated. The CVZ and other subduction-related volcanoes in the Caribbean and Indonesia share similar ranges of whole-rock delta O-18 and 5D values. This is consistent with a common alteration fluid dominated by subduction-related magmatic fluid that limits the influx of O-18-depleted surface water into sub-volcanic hydrothermal systems.