U-Th-Ba elemental fractionation during partial melting of crustal xenoliths and implications for U-series disequilibria in continental arc rocks

Understanding U-series isotopic disequilibria of partially melted crust is integral in determining the effect that crustal assimilation has on the U-series signature of magmas. Unfortunately, U-series isotopes are too low in abundance to determine by any micro-beam technique. In this work U, Th and Ba (as a proxy for Ra) abundances were measured in quenched glass in partially melted crustal xenoliths of granitic (71-73% SiO2) composition from the Mirador volcano in the Chilean Southern Volcanic Zone using micro-beam techniques (EPMA and LA-ICP-MS). Since the crustal xenoliths are old (Miocene) they can be assumed to be at secular equilibrium. The eruption of Mirador occurred in 1979, and partial melting/quenching of the xenoliths accompanied the eruption. Any recent fractionation of U from Th or Th from Ra by partial melting will result in isotopic disequilibrium. A comparison of the ratios of the elements (Ba/Th & U/Th) in the quenched glasses with those of the whole rock reveals how much fractionation has occurred during partial melting. An EPMA was used to locate (through BSE images) and analyze glass pockets in the samples for major elements establishing a baseline idea for the composition of the potential assimilates with SiO2 contents between 54% and 75%, Al2O3 (13% - 27%) and K2O (0.2% - 7%). Laser ablation ICP-MS was used to analyze U, Th and Ba elements in the quenched partial melts, and ICP-MS was used for the whole rocks. Measured (Ba/Th glass)/(Ba/Th whole rock) are between 0.2 to 51 and (Th/U glass)/(Th/U whole rock) range from 0.3 to 7. The vast majority of analyses show barium and uranium excesses relative to Th in the glasses. Different ratios of U, Th and Ba (Ra) compared to the whole rock substantiate fractionation of these elements via partial melting. The results suggest that assimilation of partial melts of crustal rocks can play a role in producing U-series isotopic disequilibria, which is commonly observed in continental arc magmas.