Crystals in volcanic rocks provide records of magma-reservoir processes and timescales prior to eruptions 7. A crystal growing from a magmatic melt incorporates trace elements in quantities governed by thermodynamic and kinetic laws 8,9 . If the crystal is subsequently mixed into another melt, trace elements that 2 diffuse sufficiently fast will begin to re-equilibrate with the new host melt, generating intra-crystalline diffusion gradients than can be used to obtain time information [10][11][12] (Supplementary Fig. S1). (Fig. 1g). In some crystals, the core also contains a euhedral to anhedral inner core of An 88-58 ('calcic inner core' ; Fig 1 a, c). A spectrum of type 1 crystals is observed from those with thick rims (up to 100 µm), to those with thin rims, to those in which the rim is absent (Fig. 1f). In rimmed type 1 crystals the plagioclase in contact with the host glass is ~ An 40, whereas in rimless ones it is ~An 50 (i.e., the core composition). Rimmed type-1 crystals occur in pumices from all four eruptive phases, but rimless ones have only been observed in pumices from phase 4. Type 2 crystals are very rare; they are reversely zoned, with cores of An 36-30 mantled by rims identical to those of type 1 (Fig. 1e). The broad range of plagioclase compositions in Minoan pumice shows that the rhyodacite was the product of open-system magmatic processes involving multiple, compositionally diverse magmas.Trace elements were analysed in four rimmed type-1 crystals, one rimless type-1 crystal, some interstitial glasses and some inclusions of glass contained within the crystals ( Fig. 2; Tables S1 and S2). Mg, Sr and Ti are particularly useful elements for characterizing coexisting melt compositions and mixing time scales because they partition differently between melt and plagioclase 8 and diffuse at different rates. Mg diffuses faster than Sr 7 ; Ti probably diffuses slowly due to its high charge. Published An-dependent partition coefficients 8 were used to invert melt trace element contents to those of equivalent plagioclase (Fig. 2), and vice versa (Fig. 3).Rimmed type-1 crystals have core-to-rim gradients in all the three elements (Fig. 2). Rim compositions record equilibrium with the interstitial glass, but concentrations of Mg, Sr, and Ti in the cores and calcic inner cores are significantly higher than those calculated to be in equilibrium with the glass (green lines, Fig. 2). The rimless type-1 crystal has concentrations of Mg and Sr throughout that are too high to have been in equilibrium with the glass. None of these crystals resided in the host melt long enough for any of these the elements to reach total equilibrium with the host melt.This observation is reinforced by a comparison of calculated Mg, Ti, and Sr concentrations of melts in equilibrium with the different plagioclase zones (referred to as Mg melt , Sr melt , and Ti melt ), with those of Santorini lavas, pumices, and glasses. The latter represent an approximate liquid line of descent of the magmatic system 3 (Fig. 3a, b). The calculated c...

The most voluminous silicic volcanic eruptions in the geological past were associated with caldera collapses above giant silicic magma reservoirs. The thermal evolution of these subcaldera magma reservoirs controls the volume of eruptible magma and eruptive style. Here we combine high-precision zircon U-Pb geochronology, trace element analyses of the same mineral grains, and mass balance modeling of zircon trace element compositions allowing us to track the thermal and chemical evolution of the Oligocene Fish Canyon Tuff magma reservoir (Colorado, United States) as a function of absolute time. Systematic compositional variations in U-Pb dated zircons record ~440 k.y. of magma evolution. An early phase of volumetric growth was followed by a period of cooling and crystallization, during which the Fish Canyon magma approached complete solidifi cation. Subsequent remelting, due to underplated andesitic recharge magmas, began 219 ± 45 k.y. prior to eruption, and led to the generation of ~5000 km3 of eruptible crystal-rich (~45 vol%) dacite. Age-equivalent, but compositionally different, zircons in an andesite enclave from late-erupted Fish Canyon Tuff tie the growth and thermal evolution of the upper-crustal reservoir to a lower-crustal magma processing zone. Our results demonstrate that the combination of high-precision dating and trace element analyses of accessory zircons can reveal invaluable information about the chemical and thermal histories of silicic magmatic systems and provides critical input parameters for fl uid dynamic modeling

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