Wim Degruyter

Abstract Storage pressures of magma chambers influence the style, frequency and magnitude of volcanic eruptions. Neutral buoyancy or rheological transitions are commonly assumed to control where magmas accumulate and form such chambers. However, the density of volatile-rich silicic magmas is typically lower than that of the surrounding crust, and the rheology of the crust alone does not define the depth of the brittle–ductile transition around a magma chamber.

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Analytical methods Juvenile clasts representative of 23 different eruptions were sampled at Campi Flegrei caldera and in more distal outcrops (Fig. 1 and Supplementary Materials). All samples were analyzed for bulk rock composition by x-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICPMS) at Eidgenössische Technische Hochschule (ETH) Zürich.