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Time scale of magma differentiation in arcs from protactinium-radium isotopic data

¹ Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
² Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA
³ Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA

Absolute chronology of magma differentiation processes has been a long-desired goal, given its importance in understanding magma chamber dynamics and its connection to a fundamental understanding of the style and frequency of volcanic eruptions. Broad estimates of the duration of magma differentiation and overall crustal residence times have been made based on a variety of indirect approaches, such as physical models of magma chamber cooling, rates of crystal growth and settling, and long-lived radiogenic isotopes. In contrast, combined ²³¹Pa-²³⁵U data may provide a robust measure of the time scale of magma differentiation. Based on ²³¹Pa-²³⁵U, ²³⁰Th-²³⁸U and ²²⁶Ra-²³⁰Th data from Taal volcano, Luzon Arc, Philippine Archipelago, we show that ²³¹Pa-²³⁵U data may provide a robust direct measure of the time scale of magma differentiation. A closed-system magma fractionation model gives a ²³¹Pa-²³⁵U differentiation time scale in the range of 30 k.y., while the ²²⁶Ra-²³⁰Th time scale is considerably younger. The time scales are reconciled if we consider either fluid-mixing or magma-mixing models. The fluid-mixing model gives a time scale of differentiation similar to the ²³¹Pa-²³⁵U closed-system time scale and is supported by the ²³⁰Th-²³⁸U data. The magma-mixing model gives a considerably longer time, in the range of 55 k.y. The combined observations support the robustness of the ²³¹Pa-²³⁵U chronology, indicating a differentiation time scale in the range of 30 k.y., although this time scale for other volcanoes may vary depending on size and thermal state of the magma chamber. The ²²⁶Ra-²³⁰Th closed-system model ages, which yield much younger estimates for magma differentiation, are not likely to reflect time scales of magma differentiation.

Key Words: time scale • differentiation • ²³¹Pa • ²²⁶Ra • magma • uranium series

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