127-8 Ra/Th Dating of Alkali Feldspars in Fogo and Furnas Trachytes of the Azores

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session

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The young age of historic volcanic eruptions pose challenges for understanding magmatic processes occurring prior to eruption. The Fogo 1563 CE and B and Furnas 1630 CE and I trachytic eruptions of São Miguel, Azores are no exception. Here, ²²⁶Ra/²³⁰Th dating of individual alkali feldspar crystals is used to constrain crystallization ages and indicate crystal residence times using lattice strain modeling. Equilibrium between crystals and melt is essential for calculating the Ra partition coefficient used for constraining a crystallization age. 

In this study, three of the four eruptions have alkali feldspar crystals in disequilibrium with the melt. Two scenarios may be relevant: 1) Remobilization or assimilation of xenocrystic crystals from a pre-existing cumulate mush. 2) Contamination of the groundmass from country rock occurring during eruption or post emplacement contamination from sea spray. ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁶Pb analyse of both single crystals and groundmass also provides evidence for melt and crystal disequilibrium occurring in 3 of the 4 assessed eruptions. Additionally, ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁶Pb ratios aid in magma source identification. Furnas 1630 alkali feldspars yield young crystallization ages that occurred soon before or nearly at eruption. Fogo 1563, Fogo B, and Furnas I alkali feldspars are younger than 8 ka since Ra is present, but crystallization ages could not be calculated due to disequilibrium between the crystals and the melt. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5362

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