Trachytic Magma Research Articles

The magmatic feeding system of the Campi Flegrei caldera: Architecture and temporal evolution

To develop a model of both the structure and evolution of the Campi Flegrei caldera (CFc) magmatic feeding system, geochronological, geochemical and Sr, Nd, Pb and B isotopic data of representative volcanic products of the past 15 ka have been combined with geophysical and melt inclusion literature data, structural setting and dynamics of the resurgent caldera. According to previous petrological data, the CFc magmatic feeding system consists of a deep reservoir, in which mantle-derived K-basaltic parental magmas differentiate to shoshonite, latite and trachyte, through combined crustal contamination and fractional crystallization processes, and shallow reservoirs where the evolved magmas further differentiate and mingle/mix before eruptions. The Sr, Nd, Pb, and B isotope data allow recognition of three distinct magmatic components. One component is believed to be residual magma from the Neapolitan Yellow Tuff (NYT) caldera forming eruption. The NYT component ( 87Sr/ 86Sr of 0.70750–53, 143Nd/ 144Nd ratio of ca. 0.51246, 206Pb/ 204Pb of ca. 19.04 and δ 11B of ca. –7.9‰), has been the most prevalent component over the past 15 ka being mixed, in most cases, with the other two components. One of these other components is best recognized in the Minopoli 2 magma, first erupted 10 ka ago. Minopoli 2 magma is shoshonitic in composition and is the most enriched in radiogenic Sr ( 87Sr/ 86Sr of ca. 0.70860) and unradiogenic Nd and Pb ( 143Nd/ 144Nd ratio of ca. 0.51236, 206Pb/ 204Pb of ca. 18.90), and is characterised by δ 11B value of ca. –7.32‰. The third component is trachytic in composition and has higher 206Pb/ 204Pb (ca. 19.08), lower 87Sr/ 86Sr (ca. 0.70726) and δ 11B (−9.8‰) and higher 143Nd/ 144Nd (ca. 0.51250), with respect to the NYT component. This third component is best recognized in the Astroni 6 magma and did not appear until ca. 4 ka. The identified isotopically distinct magmatic components were erupted in different sectors of the CFc. During both I (< 14.9–9.5 ka) and II (8.6–8.2 ka) epochs of volcanic activity, magmas similar to the NYT component, and those resulting from mixing between Minopoli 2 and NYT components were erupted from vents located mostly on the marginal faults of the NYT caldera. During the III epoch (4.8–3.8 ka) magmas either similar to NYT, or resulting from mixing between Astroni 6 and NYT components were erupted from vents located along faults bordering the La Starza resurgent block and, subordinately, the NYT caldera. Moreover, magmas resulting from mixing between Minopoli 2 and NYT components were erupted from vents located along NE–SW regional faults activated during caldera resurgence. The inferred present structure of the feeding system is characterised by a deep reservoir, whose top is at about 8 km depth, that hosts shoshonitic–trachytic magmas. Remnants of the NYT magma reside at shallower depth in different sectors of the crust underlying CFc, and were sometimes intercepted by volatile-rich magmas of deep provenance during the three epochs of CFc volcanic activity.

40Ar-39Ar dating of volcanic rocks from the Fernando de Noronha Archipelago

The Fernando de Noronha Archipelago is situated345 km off the Brazilian coast. In terms of tectonics, thearchipelago is confined to a transform fracture zone. TheFernando de Noronha lineament extends a considerabledistance westward and is traced by the Atol das Rocas, aseamount chain, and occurrences of alkaline magmatismin the continent. According to tectonic conceptions, thisstructure is interpreted as a hotspot track.The universally accepted differentiation of the volcanic rocks of the Fernando de Noronha Archipelago wasproposed by Almeida [1]. According to his data, three formations can be distinguished: Remedios, Quixaba, andSao Jose. The igneous rocks of the Remedios Formationare dominated by phonolites and trachytes. The pyroclastic material was produced by explosive eruptions of phonolitic and trachytic magmas. The rocks of the QuixabaFormation are dominated by intercalating ankaratriteflows and pyroclastic layers of the same composition andlie on an erosion surface. Sedimentary rocks were notfound at the stratigraphic boundary between the Remedios and Quixaba formations. Therefore, the sequence offormation of these complexes cannot be established bygeological methods. The nepheline basanites of Sao JoseIsland were distinguished into a separate formation.Almeida argued that these basanites are younger than theankaratrites of the Quixaba Formation.The dating of igneous rocks from the Fernando deNoronha (FN) Islands is important for the understandingof the magmatic evolution of this hot spot. The K–Ar agesreported by Cordani [2] and Kogarko et al. [3] showedthat the alkali basanites of the Sao Jos e Formation and thetrachytes of the Remedios Formation are the oldest igneous rocks of the archipelago (11.7 ± 0.6 Ma and 11.8–11.7 Ma, respectively). According to the same data, thephonolites of the Remedios Formation are somewhatyounger (9.5–8.6 Ma), and the olivine melanephelinitesof the Quixaba Formation are the youngest rocks. In contrast to this model, Almeida [1] supposed previously thatthe Sao Jose basanites are younger than the Quixabaankaratrites (note that the contact between these rocks islocated below the sea level).The classical K–Ar method ignores the possible presence of excess Ar (i.e., Ar absorbed and accumulated during the existence of the mantle block producing the magmatism of the island, rather than radiogenic Ar formed

Unravelling carbonatite–silicate magma interaction dynamics: A case study from the Velay province (Massif Central, France)

Mechanisms of interaction between a carbonatite magma and a silicate magma are inferred on the basis of the textural and geochemical features of a carbonate-bearing tuff from the Massif Central, characterized by the presence of carbonatite globules within a silicate glass matrix. A two-fluid dynamical model is proposed in order to constrain the formation of the carbonatite globules that underwent no geochemical re-equilibration with the trachyte magma due to rapid thermal re-equilibration and crystallization of dolomite. It is suggested that a viscous fingering instability occurred simultaneously with the injection of a carbonatite magma into a chamber filled with a more viscous trachytic magma. Instantaneous fragmentation of the fingers, induced by the action of gradient stresses, resulted in the formation of a magmatic emulsion. The replenishment event triggered immediate evacuation of the magma chamber such that interaction between the two magmas occurred over a short time scale and large-scale mixing was prevented. The exsolution of volatiles, perhaps as a consequence of the recharge event shortly before the eruption, drove a metasomatic event that produced the subtle chemical variations observed at the interface between carbonatite globules and silicate matrix.

Megacrysts and salic xenoliths in Scottish alkali basalts: derivatives of deep crustal intrusions and small-melt fractions from the upper mantle

AbstractCa-poor and typically Na-rich feldspar megacrysts are common associates of spinel lherzolitic and pyroxenitic xenoliths in Scottish alkalic basalts. Associated megacrysts and composite megacrysts and salic xenoliths include apatite, magnetite, zircon, biotite, Fe-rich pyroxene(s) and corundum. The salic xenoliths and related megacrysts are referred to collectively as the ‘anorthoclasite suite': the majority of the samples are inferred to derive from the disaggregation of coarse-grained, typically Na-rich, syenitic protoliths at depth. Rare occurrences of euhedral anorthoclase megacrysts, together with zircon dating, imply that the suite crystallized at, or very shortly before, their entrainment by the basaltic host magmas. Some evidence suggests that the anorthoclasite suite protoliths lie within ultramafic (pyroxenitic) domains in the deep crust. The latter are inferred to be pegmatites, crystallized from carbonated trachytic magmas with widely variable Ca, Na, K, Ba and trace-element contents, and to have ranged from metaluminous to peraluminous. Crystal zonation and resorption textures within the salic xenoliths imply that the crystallization of the parent magmas was complex. Confirmation of this comes from cathodoluminescence studies of the feldspars showing that early ('primary’) anorthoclases and potassian albites exhibit partial replacement by a more potassic feldspar. A third generation of potassic feldspar (enriched in an assortment of trace elements and deduced to have crystallized from a carbonated high-K melt) forms transecting zoned veins in which carbonate fills the axial zone.Whereas most of the anorthoclasite suite materials are inferred to have grown from metaluminous magmas, the occurrence of magmatic corundum in salic xenoliths indicates crystallization from magmas that were peraluminous. The corundum-bearing samples also contain Nb-rich oxide minerals and their associated feldspars have the highest rare-earth element (REE)contents. Accordingly, the peraluminous trachyte magmas are deduced to have been specifically enriched in high field-strength trace elements. It is proposed that formation of the anorthoclasite suite protoliths is a phenomenon closely related to that of salic glass ‘pockets', well known from spinel lherzolite xenoliths around the world. Not only are there compositional affinities, but both sets of phenomena appear to have closely pre-empted the ascent of alkali basalt (host) magmas. We propose that the two sets of phenomena are linked and that the anorthoclasite suite derived from coarse-grained sheets, generated by the aggregation of salic melt fractions rising from the shallow mantle and heralding the onset of basaltic magmatism.

The pre-eruptive volatile contents of recent basaltic and pantelleritic magmas at Pantelleria (Italy)

Pantelleria Island, located in the Sicily Channel Rift Zone (Italy), is the type locality for the peralkaline rhyolitic rocks called pantellerites. In the last 50 ka, after the large Green Tuff caldera-forming eruption, volcanic activity at Pantelleria has consisted of effusive and explosive eruptions mostly vented inside and along the rim of the caldera and producing silicic lava flows, lava domes and poorly dispersed pantelleritic pumice fall deposits. Basaltic cinder cones and lava flows are only present outside the caldera in the NW sector of the island. The most recent basaltic (Cuddie Rosse, ∼ 20 ka) and pantelleritic (Cuddia Randazzo and Cuddia del Gallo, ∼ 6 ka) pyroclastic products were sampled to investigate magmatic volatile contents through the study of melt inclusions. The melt inclusions in pyroxene and olivine phenocrysts of Cuddie Rosse scoriae have an alkali basalt composition. The dissolved volatiles comprise 0.9–1.6 wt.% H 2O, several hundred ppm of CO 2, 1600–2000 ppm of sulphur and 500–900 ppm of chlorine. The water–carbon dioxide couple gives a confining pressure ∼ 2 kbar prior to the eruption. This result indicates that episodes of magma ponding and crystallization occurred in the upper crust prior to eruption. The melt inclusions in feldspar, fayalite and aenigmatite phenocrysts of Cuddia del Gallo and Cuddia Randazzo pumice have a pantelleritic composition (Agpaitic Indices 1.3–2.1), up to 4.4 wt.% H 2O, 8700 ppm Cl, 6000 ppm F, and CO 2 below the detection limit. Sulphur averaging 420 ppm has been measured in Cuddia Randazzo melt inclusions. These data indicate relatively high volatile contents for these low-energy Strombolian-type eruptions. Melt inclusions in Cuddia del Gallo pumice show the most evolved composition (Agpaitic Indices 2–2.1) and the highest volatile content, in agreement with fluid saturation conditions in the magma chamber prior to the eruption. This implies a confining pressure of ∼ 1 kbar for the top of the pantelleritic reservoir. The composition of melt inclusions and mineralogical assemblage of Cuddia Randazzo pumice indicate that it has a lower evolutionary degree (Agpaitic Indices 1.3–1.8) and lower pre-eruptive Cl and H 2O contents than Cuddia del Gallo pumice. An increase in pressure due to the exsolution of volatiles in the upper part of the pantelleritic reservoir may have triggered the Cuddia del Gallo explosive eruption. Evidence of widespread pre-eruptive mingling between trachytes and pantellerites suggests that the intrusion of trachytic magma into the pantelleritic reservoir likely played a major role in destabilizing the magma system just prior to the Cuddia Randazzo event.

Exhumation of an active magmatic–hydrothermal system in a resurgent caldera environment: the example of Ischia (Italy)

Abstract: Ischia is a rare case of a well-exposed caldera system that has experienced rapid recent resurgence, which can be used to dissect the anatomy of an otherwise inaccessible active hydrothermal system developed in a caldera-forming environment. Integrated analyses of melt and fluid inclusions, mineralogy and stable isotopic compositions of pumices, tuffs and syenitic xenoliths of the Ischia volcanic system provide strong physico-chemical constraints on the shallow magmatic reservoir and the hydrothermal system. The hydrothermal system has been exposed by the rapid uplift of the Mt. Epomeo resurgent block. The engine of the hydrothermal system of Ischia can be identified in the shallow magmatic system (at around 2 km depth) that hosts hot ( c . 1000 °C) trachytic magma. The hydrothermal system developed principally within thick intracaldera ignimbrite deposits, and extended to a depth of at least 1 km, defining a series of facies characterized by associations of alteration mineral assemblages typical of a seawater-dominated high-temperature geothermal system. Supplementary material: Sample descriptions and analytical methods are available at http://www.geolsoc.org.uk/SUP18371 .

A model for Ischia hydrothermal system: Evidences from the chemistry of thermal groundwaters

Ischia volcano, in Central Italy, has long been known for its copious surface hydrothermal manifestations, signs of a pervasive circulation of hot fluids in the subsurface. Because of the significant chemical heterogeneity of fumarolic gas discharges and hot spring discharges, evidences of a complex hydrothermal setting, a definite model of fluid circulation at depth is currently unavailable, in spite of the several previous efforts. Here, we report on the chemical and isotopic composition of 120 groundwater samples, collected during several sampling surveys from 2002 to 2007. The acquired data suggest that the composition of surface manifestations reflect contributions from meteoric water, sea water, and thermal fluids rising from two distinct hydrothermal reservoir, with equilibrium temperatures of respectively ~ 150 °C and ~ 270 °C, and depths of 150–300 m and > 300 m (but possibly > 1000 m). We also make use of an isotopic characterization of the dissolved gas phase in thermal waters to demonstrate that the Ischia hydrothermal system is currently supplied by a deep-rising gas component (DGC), characterized by CO 2 ~ 97.7 ± 1.2 vol.% (on a water-free basis), δ 13C CO2 = − 3.51 ± 0.9‰, and helium isotopic ratio of about 3.5 Ra ( 3He/ 4He ratio normalized to the air ratio, R a), likely magmatic in origin. An assessment of the thermal budget for Ischia hydrothermal system is also presented, in the attempt to derive a first estimate of the size and rate of degassing of the magmatic reservoir feeding the gas emissions. We calculate that a heat flow of about 153–222 MW presently drives hydrothermal circulation on the island, which we suggest is supplied in convective form (e.g., by the ascent of a high- T magmatic vapour phase) by complete degassing of 2.2–3.3 · 10 7 m 3 yr − 1 of trachytic magma (with ~ 2.1 wt.% dissolved H 2O content). If extrapolated to entire period of quiescence lasting since the Arso eruption in 1302 A.D., this volume corresponds to 1.6–2.3·10 10 m 3 of magma degassed in about 700 years of quiescent activity.

Estimation of pre-eruptive magmatic water fugacity in the Phlegrean Fields, Naples, Italy

We estimated the water fugacity (fH(2)O) in the trachytic magma of Phlegrean Fields using the biotite-magnetite-sanidine equilibrium. We confirm that the partly ionic model is the most appropriate to estimate annite activity (a(annite)) for the experimental biotite. Crystallization experiments were carried out on a representative sample of trachytic Breccia Museo eruption, Naples, Italy. Experiments were performed in the temperature and pressure range or 725-870 degrees C and 50-200 MPa, and redox conditions ranging from NNO (nickel- nickel oxide) + 1 (+/- 0.2) to FMQ (fayalite + magnetite + quartz). Most experiments were done under conditions of H2O saturation (P-water = P-total). Few experiments were done using a mixed H2O-CO2 fluid phase. The pre-eruptive fH(2)O of the Phlegrean Fields magma is estimated at 735 bars, which compares well with available melt inclusion constraints. Our results confirm that this geohygrometer can be used in any magmatic systems bearing the worked assemblage.

Fractionation of Peralkaline Silicic Magmas: the Greater Olkaria Volcanic Complex, Kenya Rift Valley

The Greater Olkaria Volcanic Complex is a young (20 ka) multi-centred system in the central Kenya Rift Valley, mainly represented at outcrop by peralkaline rhyolites. The rhyolites show significant compositional variation; peralkalinity [mol. (Na2O + K2O)/Al2O3] varies from 1·01 to 1·55, Zr contents from 442 to 3640 ppm and Rb contents from 262 to 1056 ppm. More peralkaline rhyolites were generated along multiple, but generally closely similar, liquid lines of descent by 75% fractional crystallization of alkali feldspar–quartz-dominated assemblages from mildly peralkaline parental magmas, themselves probably derived by fractionation of trachytic magmas. Crystal fractionation took place at more than one upper crustal level. The rhyolite magmas were erupted from 13 centres, each having an eruptive history and geochemical evolution broadly similar to, but distinct from, those of the other centres. For most of the life span of the Olkaria system, almost the whole spectrum of peralkaline rhyolite compositions was erupted from vents in the complex at any one time. Apparent partition coefficients are presented for 34 trace elements in sanidine, fayalite, ferrohedenbergite, amphibole, biotite, ilmenite and chevkinite-(Ce). With the exception of certain values for Ba, Rb and Sr in sanidine, most coefficients vary systematically with whole-rock composition.

Mingling of carbonate and silicate magmas under turbulent flow conditions: Evidence from rock textures and mineral chemistry in sub-volcanic carbonatite dykes, Chagatai, Uzbekistan

The Triassic Chagatai Complex, Uzbekistan, comprises explosive pipes and dykes, dominantly of silicocarbonatite composition, with cross-cutting relationships indicating multi-stage emplacement. Although the dykes have been reported as diamond-bearing, they have not previously undergone detailed investigation in terms of their mineral chemistry or rock texture. The xenolith-rich dykes contain irregularly-shaped microscopic magmatic enclaves of silicate composition within carbonatite magma and corroded microphenocrysts with crystal overgrowths that record synmagmatic geochemical disequilibrium. Quench crystals of apatite and aegirine, and anhedral baryte, which formed after corrosion of apatite and magnetite microphenocrysts but prior to formation of crystal overgrowths and mantles, indicate contemporaneous rapid undercooling. The anhedral baryte formed as a by-product of an oxidising hydrous reaction from Ba-rich biotite and pyrite to chlorite. The rock and microphenocryst textures suggest that mingling between two magmas occurred and a post-mingling mineral assemblage, including baryte, crystallised in a partially hybridised heterogeneous magma. An initial carbonatite mineral assemblage is identified as calcite + magnetite + apatite ± augite ± barium-rich biotite ± melilite ± pyrite. Changes in mineral chemistry of the carbonatite assemblage that are contemporaneous with the disequilibrium reaction textures suggest addition of a hydrous, Na–Si–Al-rich magma, and the mineral assemblage in the magmatic enclaves is similar to that of trachyte dykes in the Chagatai Complex. Using primarily rock textures and mineral chemistry, supported by mass balance calculations and isotope data, the silicate material is interpreted as a hydrous trachyte magma that had assimilated upper crustal material. The trachyte magma was entrained by carbonatite that was rapidly and turbulently ascending through the crust, shortly before emplacement as silicocarbonatite. The interpretation of magma mingling textures in the Chagatai Complex are unique amongst reported carbonatite occurrences: previously reported carbonate–silicate magma systems either formed globular textures or were interpreted as the products of assimilation of country rock only.

The origin of trachyte and pantellerite from Pantelleria, Italy: Insights from major element, trace element, and thermodynamic modelling

Trachyte and peralkaline rhyolite (pantellerite and comendite) frequently comprise the felsic end-member in bimodal suites in continental rift and oceanic island settings. In these settings, the relationship between the mafic (mildly alkaline, or transitional, basalt) and felsic lavas is ambiguous; major- and trace-element models and isotopic data are often consistent with an origin for felsic lavas from either fractional crystallization of transitional basalt or partial melting of alkali gabbro followed by fractional crystallization. In this paper, we present representative mineral analyses and whole-rock analyses from forty samples of a basalt–trachyte–pantellerite suite collected at Pantelleria, Italy, in the Strait of Sicily Rift Zone, and compare the results of major- and trace-element modelling with the results of thermodynamic (MELTS) modelling. From these results we conclude that metaluminous trachyte formed as a result of 70 to 75% low-pressure (0.1 GPa) fractional crystallization of an assemblage of plagioclase, clinopyroxene, olivine, magnetite, and apatite from a hydrous (1.0–1.5 wt.% H 2O) transitional basalt magma at relative oxygen fugacities approximately one log unit below the fayalite–magnetite–quartz buffer (FMQ−1). The “Daly gap”–a lack of intermediate (~ 49–62 wt.% SiO 2) volcanic rocks–at Pantelleria is concluded to be primarily the result of rapid differentiation through that interval. Relatively rapid crystallization at low pressure may have effected the partial degassing of water-saturated (~ 4 wt.% H 2O) metaluminous trachyte magma. Some metaluminous trachyte lavas have positive Eu anomalies, high K/Rb ratios, high concentrations of Ba, and low concentrations of incompatible trace elements; these are interpreted to be the result of up to 40% accumulation of alkali feldspar. Comenditic trachyte, pantelleritic trachyte, and pantellerite formed after an additional 20 to 80% fractional crystallization of an assemblage dominated by alkali feldspar from metaluminous trachyte magma. The most evolved pantellerite lavas and tuffs are the result of a total of ~ 95% fractional crystallization of transitional basalt, with phenocrysts that equilibrated at low temperatures (< 700 °C), low oxygen fugacities, and high (> 4 wt.%) water contents.

The Roles of Fractional Crystallization, Magma Mixing, Crystal Mush Remobilization and Volatile–Melt Interactions in the Genesis of a Young Basalt–Peralkaline Rhyolite Suite, the Greater Olkaria Volcanic Complex, Kenya Rift Valley

The Greater Olkaria Volcanic Complex is a young ( 20 ka) multi-centred lava and dome field dominated by the eruption of peralkaline rhyolites. Basaltic and trachytic magmas have been erupted peripherally to the complex and also form, with mugearites and benmoreites, an extensive suite of magmatic inclusions in the rhyolites. The eruptive rocks commonly represent mixed magmas and the magmatic inclusions are themselves two-, three- or four-component mixes. All rock types may carry xenocrysts of alkali feldspar, and less commonly plagioclase, derived from magma mixing and by remobilization of crystal mushes and/or plutonic rocks. Xenoliths in the range gabbro–syenite are common in the lavas and magmatic inclusions, the more salic varieties sometimes containing silicic glass representing partial melts and ranging in composition from anorthite ± corundum- to acmite-normative. The peralkaline varieties are broadly similar, in major element terms, to the eruptive peralkaline rhyolites. The basalt–trachyte suite formed by a combination of fractional crystallization, magma mixing and resorption of earlier-formed crystals. Matrix glass in metaluminous trachytes has a peralkaline rhyolitic composition, indicating that the eruptive rhyolites may have formed by fractional crystallization of trachyte. Anomalous trace element enrichments (e.g. 2000 ppm Y in a benmoreite) and negative Ce anomalies may have resulted from various Na- and K-enriched fluids evolving from melts of intermediate composition and either being lost from the system or enriched in other parts of the reservoirs. A small group of nepheline-normative, usually peralkaline, magmatic inclusions was formed by fluid transfer between peralkaline rhyolitic and benmoreitic magmas. The plumbing system of the complex consists of several independent reservoirs and conduits, repeatedly recharged by batches of mafic magma, with ubiquitous magma mixing.

Magma evolution in the Pliocene–Pleistocene succession of Kos, South Aegean arc (Greece)

This study investigates the petrogenesis of Pliocene−Quaternary andesites, dacites and rhyolites of the island of Kos. These volcanic rocks differ from other volcanic centres in the South Aegean arc in the narrow range of Pliocene volcanic products, the abundance of high-silica rhyolite, the lower ε Nd for a given Sr isotope composition, and greater depth to the subducting slab. Pliocene and early Pleistocene dacite stocks and rhyolite domes are succeeded by younger tuffs, notably the 0.16 Ma Kos Plateau Tuff derived from a super-eruption of an andesite stratocone now subsided beneath the sea south of Kos. Volcanic products in tuffs have been sampled from lithic clasts. Andesite, dacite and rhyolite all have ε Nd ∼+ 1.5 to −1.5 and 86Sr/ 87Sr ∼ 0.7042; this unusual composition is argued to be the result of subduction of sediments derived from the River Nile. All rock types show structures indicative of widespread magma mixing, including complexly zoned plagioclase, clinopyroxene and amphibole containing glass inclusions of trachyte and rhyolite compositions. The observed rocks result from fractionation and mixing of three principal magma types: (a) calc-alkaline high-Al basalt that fractionated to andesite at the base of crust; (b) partially melted metabasaltic amphibolite underplated at the base of crust, that fractionated to produce high-SiO 2 rhyolite; and (c) a minor component of trachytic magma from partial melting of enriched subcontinental lithospheric mantle. The complexly zoned phenocrysts with glass inclusions provide specific evidence for mixing of these three components. Specifically, it was the emplacement of the andesite into a voluminous rhyolite magma in a mid-crustal magma chamber that led to the explosive Kos Plateau Tuff super-eruption.

Heterogeneities in magma chambers: Insights from the behavior of major and minor elements during mixing experiments with natural alkaline melts

We present new electron microprobe and Sr-isotope analytical results from mixing experiments using natural volcanic samples. In order to constrain the dynamics of such mixing events, we applied a Taylor–Couette flow, simulating forced convection under very low Reynolds numbers, in a time series ranging from 1 h up to 1 week. The end-member melts derive from samples of the Campanian Ignimbrite (CI), in Italy. The CI is thought to represent a layered reservoir formed in 3 stages: 1) a resident phono-trachytic magma reservoir (end-member A); replenished by 2) a less evolved trachybasaltic–trachytic magma (end-member B of trachytic composition); 3) short-term pre-eruptive mixing in the shallow chamber between a new trachytic and the phono-trachytic resident magmas. Our experiments are motivated by this hypothesis. The two end-members are stirred together, under constant low flow velocity (0.5 rotations per minute). This initially generates single convection cells, which cause progressive homogenization of some major components. This is the case after 1, 4 and 9 h. After 16 h the 87Sr/ 86Sr-isotopic system is homogenized and the starting compositions are fully mixed. Then separate convection cells and compositional layering for major and minor elements emerged. Based on microprobe measurements of quenched melts (glass) from the 16-hour, 25-hour and 1-week long experiments, we confirm the separation of layers having different densities. This phenomenon is locally complicated by the production of micro-volumes of unmixed melts. Our results support the effectiveness of the interplay between convection and diffusion, enhanced by a double-diffusive–convection-driven differentiation for moderately high-silica magmas under high (near- liquidus) temperatures, attesting that differentiation initiates in the liquidus before the onset of fractional crystallization.

Isotopic evidence for open system processes within the Campanian Ignimbrite (Campi Flegrei–Italy) magma chamber

New Sr and Nd isotope data for whole rocks, glasses and minerals are combined to reconstruct the nature and origin of mixing end-members of the 200 km3 trachytic to phonolitic Campanian Ignimbrite (Campi Flegrei, Italy) magmatic system. The least-evolved magmatic end-member shows equilibrium between host glass and the majority of the phenocrysts and is less radiogenic in Sr and Nd than the most-evolved magma. On the contrary, only the Fe-rich pyroxene from the most-evolved erupted magma is in equilibrium with the matrix glass, while all other minerals are in isotopic disequilibrium. These magmas mixed prior to and during the Campanian Ignimbrite eruption and minerals were freely exchanged between the magma batches. Combining the results of the geochemical investigations on magma end-members with geophysical and geological data, we develop the following scenario. In stage 1, a parental, less differentiated magma rose into the middle crust, and evolved through combined crustal assimilation and crystal fractionation. In stage 2, the differentiated magma rose to shallower depth, fed the pre-Campanian Ignimbrite activity and evolved by further open-system processes into the most-evolved and most-radiogenic Campanian Ignimbrite end-member magma. In stage 3, new trachytic magma, isotopically distinct from the pre-Campanian Ignimbrite magmas, rose from ca. 6 km to shallower depth, recharged the most-evolved pre-Campanian Ignimbrite magma chamber, and formed the large and stratified Campanian Ignimbrite magmatic system. During the course of the Campanian Ignimbrite eruption, the two layers were tapped separately and/or simultaneously, and gave rise to the range of chemical and isotopic values displayed by the Campanian Ignimbrite pumices, glasses and minerals.

The Breccia Museo formation, Campi Flegrei, southern Italy: geochronology, chemostratigraphy and relationship with the Campanian Ignimbrite eruption

The Breccia Museo is one of the most debated volcanic formations of the Campi Flegrei volcanic district. The deposit, made up of six distinctive stratigraphic units, has been interpreted by some as the proximal facies of the major caldera-forming Campanian Ignimbrite eruption, and by others as the product of several, more recent, independent and localized events. New geochemical and chemostratigraphical data and Ar–Ar age determinations for several units of the Breccia Museo deposits (~39 ka), correlate well with the Campanian Ignimbrite-forming eruption. The chemical zoning of the Breccia Museo deposits is interpreted here to be a consequence of a three-stage event that tapped a vertically zoned trachytic magma chamber.

The Campanian Ignimbrite (southern Italy) geochemical zoning: insight on the generation of a super-eruption from catastrophic differentiation and fast withdrawal

More than ca 100 km3 of nearly homogeneous crystal-poor phonolite and ca 100 km3 of slightly zoned trachyte were erupted 39 ka during the Campanian Ignimbrite super eruption, the most powerful in the Neapolitan area. Partition coefficient calculations, equilibrium mineral assemblages, glass compositions and texture were used to reconstruct compositional, thermal and pressure gradients in the pre-eruptive reservoir as well as timing and mechanisms of evolution towards magma chamber overpressure and eruption. Our petrologic data indicate that a wide sill-like trachytic magma chamber was active under the Campanian Plain at 2.5 kbar before CI eruption. Thermal exchange between high liquidus (1199°C) trachytic sill and cool country rocks caused intense undercooling, driving a catastrophic and fast (102 years) in situ fractional crystallization and crustal assimilation that produced a water oversaturated phonolitic cap and an overpressure in the chamber that triggered the super eruption. This process culminated in an abrupt reservoir opening and in a fast single-step high decompression. Sanidine phenocrysts crystal size distributions reveal high differentiation rate, thus suggesting that such a sill-like magmatic system is capable of evolving in a very short time and erupting suddenly with only short-term warning.

Experimental constraints on the differentiation process and pre-eruptive conditions in the magmatic system of Phlegraean Fields (Naples, Italy)

Phase relations of two samples of the Breccia Museo Eruption (BME), BME is an explosive event that took place (about 20 ky ago) during the caldera-forming phase of the Ignimbrite Campana eruption, have been determined experimentally as a function of temperature (700 to 885 °C), pressure (50 to 200 MPa) and water content of the melt. The crystallization experiments were carried out at fO 2 = NNO + 1 log unit. Melt water content ranged from 3.4 to 8 wt.% (H 2O saturation). The synthetic products are compared to the natural phases to constrain the pre-eruptive conditions of trachytic magma in the presence of an H 2O-rich fluid. The major phases occurring in the BME have been reproduced. The stability of biotite is favoured at pressures higher than 135 MPa. Phase equilibria at 200 MPa reproduce the phase assemblage of the magma only at temperatures below 775 °C. Phase abundances and melt fractions indicate that the eruption tapped a magma body that was at a temperature of 780 °C and a pressure in the range 200–140 MPa. The observed major element variations are fully consistent with a fractional crystallization of a sanidine-dominated assemblage starting from the least differentiated trachytes. The compositions of the experimental products are compatible with the progressive tapping of a shallow magma chamber that was chemically zoned. These results suggest that after an early eruptive phase during which the upper, most differentiated level of the magma chamber was tapped, the sudden collapse of the roof of the reservoir triggered drainage of the less evolved remaining magma.

Peralkaline syenite autoliths from Kilombe volcano, Kenya Rift Valley: Evidence for subvolcanic interaction with carbonatitic fluids

Mineral chemistry, textures and geochemistry of syenite autoliths from Kilombe volcano indicate that they crystallized in the upper parts of a magma chamber from peralkaline trachytic magmas that compositionally straddle the alkali feldspar join in the “residuum system” (ne = 0–1.03; qz = 0–0.77). Mineral reaction and/or overgrowth processes were responsible for the replacement of (i) Mg–hedenbergite by aegirine–augite, Ca–aegirine and/or aegirine, (ii) fayalite by amphiboles, and (iii) magnetite by aenigmatite. Ti–magnetite in silica-saturated syenites generally shows ilmenite exsolution, partly promoted by circulating fluids. By contrast, the Fe–Ti oxides in the silica-undersaturated (sodalite-bearing) syenites show no signs of deuteric alteration. These syenites were ejected shortly after completion of crystallization. Ilmenite–magnetite equilibria indicate fO 2 between − 19.5 and − 23.1 log units ( T 679–578 °C), slightly below the FMQ buffer. The subsequent crystallization of aenigmatite and Na-rich pyroxenes suggests an increase in the oxidation state of the late-magmatic liquids and implies the influence of post-magmatic fluids. Irrespective of silica saturation, the syenites can be divided into (1) “normal” syenites, characterized by Ce/Ce ⁎ ratios between 0.83 and 0.99 and (2) Ce-anomalous syenites, showing distinct negative Ce-anomalies (Ce/Ce ⁎ 0.77–0.24). “Normal” silica-saturated syenites evolved towards pantelleritic trachyte. The Ce-anomalous syenites are relatively depleted in Zr, Hf, Th, Nb and Ta but, with the exception of Ce, are significantly enriched in REE. The silica-saturated syenites contain REE–fluorcarbonates (synchysite-bastnaesite series) with negative Ce-anomalies (Ce/Ce ⁎ 0.4–0.8, mean 0.6), corroded monazite group minerals with LREE-rich patches, and hydrated, Fe- and P-rich phyllosilicates. Each of these is inferred to be of non-magmatic origin. Fractures in feldspars and pyroxenes contain Pb-, REE- and Mn-rich cryptocrystalline or amorphous material. The monazite minerals are characterized by the most prominent negative Ce-anomalies (Ce/Ce mean ⁎ = 0.5), and in the most altered and Ca-rich areas (depleted in REE), Ce/Ce ⁎ is less than 0.2. It is inferred that carbonatitic fluids rich in F, Na and lanthanides but depleted in Ce by fractional crystallization of cerian pyrochlore, percolated into the subvolcanic system and interacted with the syenites at the thermal boundary layers of the magma chamber, during and shortly after their crystallization. Chevkinite–(Ce), pyrochlore, monazite and synchysite-bastnaesite, occurring as accessory minerals, have been found for the first time at Kilombe together with eudialyte, nacareniobsite–(Ce) and thorite. These latter represent new mineral occurrences in Kenya.

Temporal evolution of a long-lived syenitic centre: The Kangerlussuaq Alkaline Complex, East Greenland

We present new mineral and whole-rock compositions and Sr–Nd–Hf–Pb–O–H isotope data on samples from the Kangerlussuaq Alkaline Complex (∼ 1000 km 2) in central East Greenland, part of the North Atlantic Igneous Province. This complex mainly consists of the Kangerlussuaq Intrusion but includes at least 13 separate satellite intrusions emplaced in the uppermost crust close to the unconformity between Archaean gneisses and overlying Palaeogene flood basalts. The complex is divided into (i) older satellite intrusions (∼ 55–53 Ma) composed of multiple syenites and granites and minor gabbros and peridotites, (ii) the voluminous Kangerlussuaq Intrusion (∼ 50 Ma), which displays a gradual transition from quartz syenites (nordmarkites) at the margin to nepheline syenites (foyaites) in the centre, and (iii) younger satellite intrusions (∼ 47–45 Ma) of minor syenites, granites and diorites concentrated southeast of the Kangerlussuaq Intrusion. The complex displays a temporal evolution in which SiO 2 decreases (74–56 wt.%) and total alkalis (6–16 wt.%), amphibole Na + K content, 206Pb/ 204Pb meas, ε Ndi and ε Hfi (+ 3 to + 11) increase from the older intrusions through the nepheline syenites. This is followed by a reversal to higher silica (62–73 wt.%) and lower total alkalis (9–12 wt.%), amphibole Na + K content, 206Pb/ 204Pb meas, ε Ndi and ε Hfi (− 13 to + 2) in the younger satellite intrusions. Temporal changes in the location of magma plumbing systems and in magma production rates played a profound role in controlling silica content, alkalinity and degree of crustal contamination during development of the complex. Phonolitic magma was only generated after prolonged magmatism had shielded the conduits from interaction with country rock. The parental magmas were probably basanitic to alkali olivine basaltic in composition. The older satellite intrusions and the Kangerlussuaq Intrusion have low δ 18O magma values (− 1 to + 6‰) compared to the younger, more crustally contaminated, satellite intrusions (+ 4.5 to + 7‰). It appears that the magmas only had sufficient over-pressure to intrude the basalt cover when larger volumes of less contaminated trachyte magma were produced, resulting in the generation of low-δ 18O magmas due to dehydration of hydrothermally altered basalt xenoliths.