## Abstract

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 ^{231}Pa-^{235}U data may provide a robust measure of the time scale of magma differentiation. Based on ^{231}Pa-^{235}U, ^{230}Th-^{238}U and ^{226}Ra-^{230}Th data from Taal volcano, Luzon Arc, Philippine Archipelago, we show that ^{231}Pa-^{235}U data may provide a robust direct measure of the time scale of magma differentiation. A closed-system magma fractionation model gives a ^{231}Pa-^{235}U differentiation time scale in the range of 30 k.y., while the ^{226}Ra-^{230}Th 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 ^{231}Pa-^{235}U closed-system time scale and is supported by the ^{230}Th-^{238}U 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 ^{231}Pa-^{235}U 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 ^{226}Ra-^{230}Th closed-system model ages, which yield much younger estimates for magma differentiation, are not likely to reflect time scales of magma differentiation.

Original language | English (US) |
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Pages (from-to) | 633-636 |

Number of pages | 4 |

Journal | Geology |

Volume | 33 |

Issue number | 8 |

DOIs | |

State | Published - Aug 1 2005 |

## Keywords

- Differentiation
- Magma
- Pa
- Ra
- Time scale
- Uranium series