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...

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[1] Nominally anhydrous minerals in 46 peridotite xenoliths hosted by Cenozoic basalts from five localities (Fangshan, Penglai, Qixia, Changle, and Hebi) of the eastern part of the North China Craton (NCC) have been investigated by Fourier transform infrared spectrometry (FTIR). The water contents (H 2 O wt %) of clinopyroxene (cpx), orthopyroxene (opx), and olivine (ol) range from 27 to 223 ppm, 8 to 94 ppm, and ∼0 ppm, respectively. On the basis of (1) the homogenous H 2 O content within single pyroxene grains and (2) the equilibrium partitioning of H 2 O between cpx and opx, it is suggested that the pyroxenes largely preserve the H 2 O content of their mantle source, although possible H loss during xenolith ascent cannot be excluded for ol. The recalculated whole-rock H 2 O contents, using mineral modes and assuming a partition coefficient of 10 for water between cpx and ol, range from 6 to 56 ppm (average of 23 ± 13 ppm). In combination with previously reported data, the recalculated whole-rock water contents of peridotite xenoliths (105 samples from 9 localities) hosted by Cenozoic basalts from the eastern part of the NCC range from 6 to 85 ppm (average of 25 ± 18 ppm). The Cenozoic lithospheric mantle of the eastern part of the NCC is therefore characterized by a low water content compared to continental lithospheric mantle worldwide represented by typical cratonic and off-cratonic peridotites (normally 40-180 ppm, with average values of 119 ± 54 ppm and 78 ± 45, respectively) and to oceanic mantle values (>50 ppm) inferred from MORB and OIB. Peridotite xenoliths have low-to-moderate spinel Fe 3+ /SFe (0.02-0.34) and whole rock DFMQ values (from −4.2 to 2.2, normally between −2.5 and 1.5), which are not correlated with pyroxene H 2 O contents. Therefore, the low water contents cannot have resulted from oxidation of the mantle xenoliths and may have been caused instead by heating from an upwelling asthenosphere flow that acted in concert with NCC lithospheric thinning during the late Mesozoic to early Cenozoic. If so, the present eastern NCC lithospheric mantle represents essentially relict ancient lithospheric mantle after the thinning event, rather than newly accreted and cooled asthenospheric mantle.