Late-stage volatile saturation as a potential trigger for explosive volcanic eruptions

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Magma reservoirs are thought to grow relatively slowly, assembling incrementally under volatile-saturated conditions. Eruptions may be triggered by injections of volatile-rich melt, or generation of over-pressure due to protracted crystallization. Here, we analyse fluorine, chlorine and water in apatite crystals trapped at different stages of magma evolution, and in melt inclusions from clinopyroxene and biotite crystals expelled during an explosive eruption of the Campi Flegrei caldera, Italy, about 4,000 years ago. We combine our geochemical analyses with thermodynamic modelling to reconstruct the evolution of magmatic volatile contents leading up to the explosive eruption. We find that the magma reservoir remained persistently water-undersaturated throughout most of its lifetime. Even crystals in contact with the melt shortly before eruption show that the magma was volatile-undersaturated. Our models suggest that the melt reached volatile saturation at low temperatures, just before eruption. We suggest that late-stage volatile saturation probably triggered the eruption, and conclude that ‘priming’ of the magma system for eruption may occur on timescales much shorter than the decadal to centennial timescales thought typical for magma reservoir assembly. Thus, surface deformation pulses that record magma assembly at depth beneath Campi Flegrei and other similar magmatic systems may not be immediately followed by an eruption; and explosive eruptions may begin with little warning.

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Figure 1: Textural relations of hydrous phases in the Astroni 1 magma.
Figure 2: Volatile compositions of Astroni 1 apatites.
Figure 3: The volatile contents of Astroni 1 hydrous glasses.
Figure 4: Rhyolite-MELTS thermodynamic model for Astroni 1.


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This research was funded by a NERC studentship NE/K500811/01 awarded to M.J.S. and a NERC Edinburgh Ion Microprobe Facility grant (IMF519/0514). M.C.S.H. was supported by a Royal Society University Research Fellowship. M.C.S.H. and V.C.S. acknowledge funding from NERC grant NE/K003852/1. This work has benefited from discussions with R. Brooker, J. Riker and P. Candela. The review of W. Bohrson significantly improved the manuscript. We are grateful to R. Hinton and N. Charnley for assistance with SIMS and SEM analysis, respectively. We also thank D. Harlov for providing synthetic apatite standards for SIMS calibration and R. van Elsas for technical support during mineral separation.

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M.J.S., M.C.S.H. and V.C.S. conceived the project and analytical strategy. V.C.S. and R.I. collected samples, M.J.S. and V.C.S. performed the EPMA and SIMS analyses and M.J.S. and M.C.S.H. performed the modelling. M.J.S. analysed the data and wrote the first draft of the manuscript, which was revised by all authors.

Correspondence to Michael J. Stock.

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Stock, M., Humphreys, M., Smith, V. et al. Late-stage volatile saturation as a potential trigger for explosive volcanic eruptions. Nature Geosci 9, 249–254 (2016) doi:10.1038/ngeo2639

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