Electron-probe microanalysis (EPMA). These measurements were performed at the Tephrochronological Analytical Unit at the University of Edinburgh using a Cameca SX100 electron microprobe with five vertical wavelength dispersive spectrometers, providing oxide values (wt. %) for 10 major elements (see supplementary information).Beam size: 5Settings:Na, Al, Si, K†, Ca, Fe, Mg :15 keV 2nAMn, P†, Ti: 15 keV 80 nA