Altered mineral uptake into fresh arc magmas: insights from U–Th isotopes of samples from Andean volcanoes under differential crustal stress regimes.

Abstract

Uranium–thorium (U–Th) isotope compositions of whole rocks, groundmasses and minerals from mafic to intermediate Andean arc magmas were determined to assess the influence of crustal stress on rates of pre-eruptive crystallization and the significance of crystal uptake. Volcanoes investigated include Lascar in the central Andes, situated in a compressional regime, and Callaqui and Lonquimay in the southern Andes, situated in a transtensional regime. In the southern Andes, Calbuco volcano, which experienced variations in the local stress field due to volcano unloading triggered by large debris avalanches, was also targeted. In U–Th equiline space, whole rock and groundmass coincide in each studied sample, and are close to secular equilibrium or in slight U-excess. No isochronal relationships are displayed by the mineral separates, although many are out of secular equilibrium. Further, (234U/238U) activity ratios of some mineral separates from the southern Andes are in disequilibrium and vary significantly between different phases of individual rock samples. The combined data indicate that many crystals are foreign to the melts they are carried in, and that some mineral phases have experienced incipient weathering before their incorporation into the melt. Using Calbuco as an example, we speculate that volcano loading may affect secondary alteration processes at depth.