Magma–cumulate mixing identified by U–Th disequilibrium dating

Abstract

Magma mixing is believed to be important in magmagenesis¹, particularly in volcanic arcs², where its influence is revealed by heterogeneous phenocryst populations³ and quenched inclusions⁴. Nevertheless isotope studies, particularly of uranium-series dis-equilibria, have concentrated on the analysis of bulk rock samples^5–9; comprehensive U–Th mineral-separate data from arc volcanics are scarce^10–14. Here we present the first U–Th isotopic evidence for mixing between a crystal mush and a magma in an andesite from the island of Santorini, in the Aegean arc. The lava contains crystal populations from two sources of distinct thorium isotope composition: one from a basic cumulate; the other phenocrysts from a dacite magma. The crystal populations have statistically indistinguishable crystallization ages of 79⁺¹⁴₋₁₂kyr and 93⁺²⁹₋₂₂kyr, and are thought to have physically intermingled just before eruption. The crystals did not re-equilibrate and hence retain the isotopic compositions and ages of the parental materials.