U-series evidence for crustal involvement and magma residence times in the petrogenesis of Parinacota volcano, Chile

In this study, we present Th-U disequilibria as well as radiogenic and trace element data for recent volcanic rocks from the Nevados de Payachata volcano which erupted through similar to 70 km of continental crust in the Central Volcanic Zone of the Andes (18 degrees S, 69 degrees W). Both lavas and mineral separates were analyzed by mass spectrometry for U-238-Th-230 disequilibria. The Lavas are characterized either by Th-230 enrichment or depletion relative to its parent nuclide U-238. Mineral separates are used to derive U-Th isochron ages and these ages compare favorably with inferred stratigraphic ages or K-Ar ages, although in one case the U-Th age is significantly older than the stratigraphic age. Despite relatively constant Sr, Nd, and Pb isotope ratios, the lavas display inverse trends in Th-230/U-238 versus Ce/Yb or Ba/Hf diagrams. These trends cannot be interpreted by simple two-component mixing. Rather, there must be three (and perhaps four components) involved in the genesis of the Parinacota lavas. A mantle wedge, a slab fluid, and a lower crustal component can be identified. A sediment component is more difficult to detect as it is difficult to decipher its signature because of the strong crustal influence. The existence of binary arrays can be explained by variable amounts of crustal material. The process of crust-mantle interaction must have been short enough to preserve U-Th disequilibrium ( < 300 ka).