North Motzfeldt is an intrusive igneous centre within the Igaliku (formerly spelt Igaliko) complex, Gardar Province, South Greenland. A detailed field description of the centre is given, with preliminary geochemical and isotopic data. The North Motzfeldt centre is intruded into Ketilidian granitoid basement rocks, and current exposure retains parts of the original roof against earlier Gardar eruptives and clastic sediments of the Eriksfjord formation. The unconformity between the Ketilidian and the overlying Eriksfjord is believed to have been crucial in the siting of the centre. The centre is subdivided into two major units, NM1 and NM2. This was followed by significant peralkaline nepheline microsyenite in sheets, characterized by rare element-rich accessory minerals including rinkite-mł sandrite, pyrochlore and låvenite. A significant microsyenite body is called NM3. A preliminary Rb-Sr isochron of 1226±27 Ma indicates a far younger age for the centre than previously thought (~;1350 Ma). Cross-cutting relationships between North Motzfeldt and rocks of the adjacent Motzfeldt centre require a reappraisal of the chronology of Motzfeldt magmatism. We suggest that rocks previously believed to comprise the Motzfeldt centre represent the products of multiple phases of magmatism from Early- to Mid-Gardar times.

Weakly peralkaline syenite ejecta within the plinian fall deposits of Agua de Pau volcano (São Miguel, Azores Islands) are divisible into (1) silica-undersaturated and (2) silica-saturated/oversaturated groups. The first group occurs in the pre-Fogo-A pyroclastic falls (15.2 ka) whereas the second occurs in the Fogo-A pumices (5 ka). The petrography indicates that the syenites of both groups crystallized at shallow depths and whole-rock geochemistry shows that they have compositions reflecting those of the Agua de Pau trachyte magmas. The syenites are inferred to be derived from solidification zones around the margins of the parental magma chamber. Mineral parageneses in both syenite groups permit distinction of three magmatic stages of crystallization: (1) an early crystallization stage; (2) a main crystallization stage during which most of the feldspar growth occurred; and (3) a late interstitial stage. Crystallization of the trachytic magmas yielded peralkaline residuals, highly enriched in volatiles, alkalis, HFSE and REE. In the silica-undersaturated syenites, the complex Zr-Ti silicate minerals, eudialyte and a låvenite-like mineral, crystallized interstitially from the residual magmas. In contrast, the zircono-silicate dalyite, and REE-rich minerals (chevkinite-(Ce), britholite-(Ce) and pyrochlore), together with thorite, were late-stage interstitial products of the silica-saturated/oversaturated syenites.

Wedge-shaped layers of ultramafic and mafic cumulates in Intrusion II of the Caledonian Honningsvåg Intrusive Suite suggest crystallization on an inclined magma chamber floor from a compositionally-zoned and density-stratified magma.

Cyclic unit 8 (140–100 m thick) consists of a distally-thinning olivine gabbro (denoted paoC) macrolayer overlain by a distally-thickening gabbronorite, pahC. New mineral data in four traverses across cyclic unit 8 show systematic compositional changes; the Mg# of the mafic phases decreases upwards through the unit and distally, both along the base and along the paoC/pahC interface.

A crystallization model based on an effectively continuously-zoned magma chamber with numerous, relatively thin, double-diffusive magma layers is proposed. Differential migration of horizontal isopleths (e.g. Mg# and aSiO2) in response to fractional crystallization and assimilation of country rock can explain the variations in the Mg# of the cumulates.

In the 1944 Vesuvius eruption, the shallow magma chamber was disrupted during the highly energetic explosive phases. Abundant cognate xenoliths such as subvolcanic fergusites and cumulates, hornfels, skarns and rare marbles occur in tephra deposits.

Mineral chemistry, melt inclusions in minerals and glassy matrix compositions show that fergusites (highly crystalline rocks made of leucite, clinopyroxene, plagioclase, olivine, apatite, oxides and glass) do not correspond to melt compositions but result from combined sidewall accumulation of crystals, formed from K-tephriphonolitic magma resident in the chamber, and in situ crystallization of the intercumulus melt. Very low H2O contents in the intercumulus glass are revealed by FTIR and apatite composition. Whole rock compositions are essentially determined by the bulk mineral assemblages.

Glass–bearing fergusites constitute the outer shell of the magma chamber consisting of a highly viscous crystal mush with a melt content in the range 20–50 wt.%. The leucite/(clinopyroxene+olivine) modal ratio, varies with the extraction order of magmas from the chamber, decreasing upwards in the stratigraphic sequence. This reflects a vertical mineralogical zonation of the crystal mush. These data contribute to the interpretation of the subvolcanic low–pressure crystallization processes at the magma chamber sidewalls affecting alkaline potassic magmas.

The Island of Linosa is a small part of the large submarine volcanic complex which is locatedat the SW edge of the Linosa Graben, Sicily Channel. The island was formed between 1.06±0.10 and 0.53±0.07 Ma, through three main stages of activity: Paleo-Linosa, Arena Bianca and Monte Bandiera. Major and trace element data show that the compositional variability of the three activity stages is limited, with most of the rocks showing basaltic to hawaiitic composition. Evolved benmoreites and trachytes are foundas lithics in some pyroclastic units of Paleolinosa. The mafic rocks of the three stages show porphyritic texture, with phenocryst assemblages characterized by olivine, clinopyroxene and plagioclase. The volume ratio of olivine vs. clinopyroxene decreases from early to late stages of activity in mafic rocks with comparable major element composition. Clinopyroxene phenocrysts from mafic rocks of the three stages have poorly variable composition, clustering in the augite field. Phenocrysts from the first activity stage (Paleo-Linosa), show a slight increase in TiO2, Al2O3 and CaO, and a decrease of Fe2O3 (total) with the increasing SiO2 content of the host rocks. Crystals from the second and the third stage (Arena Bianca and Monte Bandiera) display a slightly more restricted range of FeOtot, frequently with very high MgO, Al2O3 and TiO2 contents. Crystal chemical investigation of clinopyroxenes from rocks of the three stages with comparable degrees of evolution, revealed significant variation of structural parameters, in particular VM1 and Vcell. These show a consistent decrease, passing from clinopyroxenes of the early stage to crystals extracted from the mafic lavas of stages 2 and 3. Given the similar compositional ranges of the host rocks, structural variations of clinopyroxenes are interpreted to reveal modifications of crystallization pressure, which increased, passing from Paleo-Linosa to the Arena Bianca and Monte Bandiera stages. Given this information, the observed crystal-chemical variations provide information on the depth of magma reservoirs and on the evolution of the plumbing system of Linosa volcano.

The Murotomisaki Gabbroic Intrusion is a sill-like layered gabbro emplaced in sedimentary strata of Tertiary age in southwest Japan. The zoning (including resorption structures) and the compositional variations of plagioclase from throughout the intrusion were studied, and it was found that the zoning pattern may be classified into four types, which may well correlated with the hosting rock types, the mode of occurrences and their stratigraphic positions in the intrusion. The plagioclase zoning was successfully decoded, and the sequence of events that took place during the magmatic differentiation was deduced and further interpreted in the context of a stratified basal boundary layer slowly ascending in a solidifying magma body. It was shown that various layered structures – modal layering, podiform gabbroic pegmatites and anorthositic layers – observed in the Murotomisaki Gabbro were formed within the moving basal boundary layer by flushing of H2O-rich fluid and fractionated silicate melts from below. By the fluxing of hydrous fluids, plagioclase crystals preferentially dissolved and then melt fraction increased in the basal boundary layer. Under these circumstances, plagioclase-rich fractionated melts diapirically segregated from the crystal pile. Calcic plagioclases, which are out of equilibrium in the central part of the intrusion, may have originated from the basal boundary layer in this manner.

Hekla is a Holocene volcanic ridge in southern Iceland, which is notable for the link between repose periods and the composition of the first-erupted magma. The two largest explosive silicic eruptions, H4 and H3, erupted about 4200 and 3000 years ago. Airfall deposits from these eruptions were sampled in detail and analysed for major and trace elements, along with microprobe analyses of minerals and glasses. Both deposits show compositional variation ranging from 72 % to 56 % SiO2, with mineralogical evidence of equilibrium crystallization in the early erupted rhyolitic component but disequilibrium in the later erupted basaltic andesite component. The eruptions started with production of rhyolitic magma followed by dacitic to basaltic andesite magma. Sparse crystallization of the intermediate magma and predominant reverse zoning of minerals, trending towards a common surface composition, indicate magma mixing between rhyolite and a basaltic andesite end-member. The suggested model involves partial melting of older tholeiitic crust to produce silicic magma, which segregated and accumulated in deep crustal reservoir. Silicic magma eruption is triggered by basaltic andesite dyke injection, with a proportion of the dyke magma contributing to the production and eruption of a mixed hybrid magma. Both the volume of the silicic partial melt, and the proportion of the hybrid magma depend on the pre-eruptive repose time.

Post-caldera eruptions of Santorini, Greece, over the past 3000 years resulted in the formation of the Kameni Islands, which comprise a series of compositionally similar dacitic lava flows. Each lava flow has a distinct population of partially-crystalline mafic enclaves, which we propose were derived from the break-up of a layer of replenishing magma responsible for triggering the eruption. Five of the recent flows (erupted in 1570, 1939, 1940, 1941 and 1950) include enclaves of essentially identical andesitic bulk compositions, which formed by crystallization of originally aphyric melts prior to eruption. Detailed examination of angles subtended at the junctions between pairs of plagioclase grains demonstrates that enclaves from each flow have a characteristic textural signature, with distinct differences in the extent of quench-related modification of the original population of dihedral angles formed by impingement of growing grains. These variations suggest that the temperature difference between the host dacite and the replenishing andesite at the time of layer overturn and eruption differed between flows. The uniformity of major element compositions of both the replenishing magma and the host dacitic lava flows demonstrates that the critical parameter in determining the timing of layer-overturn is the pre-eruptive H2O content of both host dacite and replenishing magma. We suggest that the replenishing magma responsible for two of the three eruptions in the period 1939–1941 was significantly wetter than that responsible for the later 1950 eruption. The enclaves with the least amount of quench-related modification occur in the 1570 flow. We suggest that in this case the intruding magma was relatively dry.

The thermal and compositional conditions of magma chambers constrain the type and complexity of their metasomatic aureoles. Fragments of these aureoles, disrupted during explosive eruptions of Vesuvius, provide quenched snapshots of interactions between potassic magmas and carbonate country rocks. Near the highly crystallized peripheral portions of the chambers, represented by magmatic xenoliths (fergusites, syenites and pyroxenites), skarn aureoles form. Open conduit reservoirs and the lower portions of plinian and subplinian eruption magma chambers develop aureoles controlled by diffusive metasomatism by high-T-modified silicate melts, generating magmatic skarn. In contrast, infiltrative metasomatism by magmatic fluids exsolved from crystallizing margins is responsible for endoskarn (magmatic protolith) and exoskarn (carbonate protolith) formation in the upper portions of plinian and subplinian eruption magma chambers.

Temporal and spatial variations in the Nd isotopic compositions of Tertiary caldera-forming rhyolite tuffs, and Cretaceous and Tertiary granites of the western U.S.A. are used as a basis for a model that accounts for the observed proportions of crustal versus mantle contributions to silicic magmas in terms of two parameters: the ambient crustal temperature and the rate of supply of basaltic magma from the mantle. The crustal contribution to silicic igneous rocks is measured in terms of the Neodymium Crustal Index (NCI). The relationships between crustal temperature, basalt supply and NCI are quantified using a model of a magma chamber undergoing continuous recharge, wall-rock assimilation and fractional crystallisation. From the model, a critical value of the ratio of basalt recharge-to-assimilation, (r/a)c, is deduced, which increases with decreasing crustal temperature. The r/a value must exceed (r/a)c to allow the volume of differentiated magma to increase, a prerequisite for developing large volumes of silicic magma. Strongly peraluminous (or S-type) magmas (NCI = 0·8–1), form under conditions of high crustal temperature and low basalt supply. Metaluminous or I-type granites form over a wide range of conditions (NCI = 0·1–1), generally where basalt supply is substantial. In individual long-lived volcanic centres, the large-volume high-silica ignimbrites are associated with the highest r/a and lowest NCI values, indicating that these magmas are typically differentiates of mantle-derived basaltic parents.