Extreme U-Th Disequilibrium in Rift-Related Basalts, Rhyolites and Granophyric Granite and the Timescale of Rhyolite Generation, Intrusion and Crystallization at Alid Volcanic Center, Eritrea

Rhyolite pumices and co-erupted granophyric (granite) xenolithsyield evidence for rapid magma generation and crystallizationprior to their eruption at 15·2 ± 2·9 kaat the Alid volcanic center in the Danikil Depression, Eritrea.Whole-rock U and Th isotopic analyses show ²³⁰Th excesses upto 50% in basalts <10 000 years old from the surroundingOss lava fields. The 15 ka rhyolites also have 30–40%²³⁰Th excesses. Similarity in U–Th disequilibrium, andin Sr, Nd, and Pb isotopic values, implies that the rhyolitesare mostly differentiated from the local basaltic magma. Giventhe (²³⁰Th/²³²Th) ratio of the young basalts, and presumablythe underlying mantle, the (²³⁰Th/²³²Th) ratio of the rhyolitesupon eruption could be generated by in situ decay in about 50000 years. Limited (5%) assimilation of old crust would hastenthe lowering of (²³⁰Th/²³²Th) and allow the process to takeplace in as little as 30 000 years. Final crystallization ofthe Alid granophyre occurred rapidly and at shallow depths at20–25 ka, as confirmed by analyses of mineral separatesand ion microprobe data on individual zircons. Evidently, 30000–50 000 years were required for extraction of basaltfrom its mantle source region, subsequent crystallization andmelt extraction to form silicic magmas, and final crystallizationof the shallow intrusion. The granophyre was then ejected duringeruption of the comagmatic rhyolites. KEY WORDS: U-series; zircon; ion microprobe; volcano; geochronology