Compositional Layering and Syn-eruptive Mixing of a Periodically Refilled Shallow Magma Chamber: the AD 79 Plinian Eruption of Vesuvius

A detailed study of the pyroclastic deposits of the AD 79 ‘Pompei’Plinian eruption of Vesuvius has allowed: (1) reconstructionof the thermal, compositional and isotopic (⁸⁷Sr/⁸⁶Sr) pre-eruptivelayering of the shallow magma chamber; (2) quantitative definitionof the syn-eruptive mixing between the different magmas occupyingthe chamber, and its relationships with eruption dynamics; (3)recognition of the variability of mafic magma batches supplyingthe chamber. During the different phases of the eruption 25–30%of the magma was ejected as white K-phonolitic pumice, and 70–75%as grey K-tephri-phonolitic pumice. The white pumice resultsfrom the tapping of progressively deeper magma from a body (T= 850–900%C) consisting of two distinct layers mainlyformed by crystal fractionation. The grey pumice results fromsyn-eruptive mixing involving three main end-members: the phonolitic‘white’ magmas (salic end-member, SEM), mafic cumulates(cumulate end-member, CEM) and a crystal-poor ‘grey’phono-tephritic magma (mafic end-member, MEM), which was nevererupted without first being mixed with ‘white’ magma.Evidence is provided that mixing occurred within the chamberand was characterized by a transition with time from physicalmixing at a microscopic scale to chemical hybridization. TheMEM magma had a homogeneous composition and constant ⁸⁷Sr⁸⁶Srisotopic ratio, possibly as a result of sustained convection.No unambiguous liquidus phases were found, suggesting that theMEM magma was superheated (T = 1000–1100C); its verylow viscosity was a main cause in the establishment of a physicaldiscontinuity separating the white and the grey magmas. Thewhite-grey boundary layer possibly consisted of a multiply diffusiveinterface, periodically broken and recreated, supplying thephonolitic body through mixing of moderate amounts of fractionatedgrey melts with the overlying white magma. The presence of alarge overheated mass indicates the young, growing stage ofthe AD 79 chamber, whose main engine was the periodic arrivalof hot mafic magma batches. These were characterized by K-tephriticto K-basanitic compositions, high temperatures (>1150C),high volatile contents (20–25% H₂O +Cl+F+S), low viscosities[(1+2 10² poises)] and relatively low densities (2500–2600kg/m³). The birth of the Pompei chamber followed the repeatedarrival of these batches (on average characterized by ⁸⁷Sr/⁸⁶Sr070729)into a reservoir containing a tephriticphonolitic, crystal-enriched,magma, a residue from the preceding ‘Avellino’ Plinianeruption (3400 BP).In fact, about half of magma ejected duringthe AD 79 eruption could have been inherited from pre-Avellinotimes. KEY WORDS: Vesuvius; magma chamber; magma mixing; compositional layering phonolites; magma supply; potassic magmas ^*Correponding author     

Fountains in Magma Chambers

Cyclic layering is a common feature of the ultramafic zone oflayered intrusions and is usually attributed to the entry ofnew pulses of dense magma into the chamber. Since the crystallizationof olivine and bronzite lowers the density of the magma, a newpulse of the parent magma will be denser than the fractionatedmagma in the chamber. If the new pulse enters with excess momentumit will initially rise up into the host magma to form a fountain,then fall back around the feeder when negative buoyancy forcesovercome the initial momentum of the pulse. Laboratory experimentsusing aqueous solutions with both point and line sources havebeen conducted to obtain a quantitative understanding of thefluid-dynamical processes that are important in fountains. Itis observed that convection within the fountain is highly turbulent,resulting in appreciable entrainment of the host magma. A gravity-stratifiedhybrid layer develops at the floor and this breaks up into aseries of double-diffusive convecting layers if the new pulseis hotter than the host magma. The number of layers that formdepends on a number of factors, especially R, the ratio of thecontributions of composition and heat to the total density differencebetween the host magma and the new pulse. Raising the valueof R, results in the formation of more, thinner layers. The thickness of the hybrid layer at any time t is given byH = h₀+(V₀/A)t where V₀ is the volume flux through the feederand A is the horizontal area of the chamber. h₀ is related tothe initial steady-state height of the fountain and, for a linesource, is given by h₀=CU₀^4/3 d^–1(g/)^–2/3 whereU₀ is the volume flux per unit length, g is the accelerationdue to gravity, d is the width of the feeder, is the densityof the host magma, is the density difference between the magmasand C is a constant. Calculations based on these results and the consideration ofthe flow in the feeder dykes below the chamber indicate thata fountain will rise at least 350 m in a continental magma chamberif the feeder width is greater than 10 m. This will lead toextensive mixing between the new pulse and the fractionatedmagma in the chamber, producing a zoned hybrid layer at thefloor that is commonly over 1000 m thick. If the chamber receivesmany pulses of dense magma, the resulting zoning may persistthroughout much of the life of the chamber, especially if thefirst pulse to enter becomes contaminated by light magma releasedby melting at the margins. The highest Mg/Fe ratio for olivineand pyroxenes from cyclic units from the ultramafic zones oflayered intrusions is often well below the value expected forminerals crystallizing from a melt derived directly from themantle, supporting the hypothesis that new pulses of dense magmacan mix extensively with the fractionated magma in the chamber. The feeder dykes to some oceanic magma chambers, such as theBay of Islands Ophiolite, are believed to be narrower, so thatfountains do not rise more than a few metres above the floorof the chamber. This restricts mixing between the input magmaand the host magma and can result in the formation of a hybridzone that is only a few metres thick.     

Assimilation and Fractional Crystallization Controlled by Transport Process of Crustal Melt: Implications from an Alkali Basalt-Dacite Suite from Rishiri Volcano, Japan

Mechanisms of fractional crystallization with simultaneous crustalassimilation (AFC) are examined for the Kutsugata and Tanetomilavas, an alkali basalt–dacite suite erupted sequentiallyfrom Rishiri Volcano, northern Japan. The major element variationswithin the suite can be explained by boundary layer fractionation;that is, mixing of a magma in the main part of the magma bodywith a fractionated interstitial melt transported from the mushyboundary layer at the floor. Systematic variations in SiO₂ correlatewith variations in the Pb, Sr and Nd isotopic compositions ofthe lavas. The geochemical variations of the lavas are explainedby a constant and relatively low ratio of assimilated mass tocrystallized mass (‘r value’). In the magma chamberin which the Kutsugata and Tanetomi magmas evolved, a strongthermal gradient was present and it is suggested that the marginalpart of the reservoir was completely solidified. The assimilantwas transported by crack flow from the partially fused floorcrust to the partially crystallized floor mush zone throughfractures in the solidified margin, formed mainly by thermalstresses resulting from cooling of the solidified margin andheating of the crust. The crustal melt was then mixed with thefractionated interstitial melt in the mushy zone, and the mixedmelt was further transported by compositional convection tothe main magma, causing its geochemical evolution to be characteristicof AFC. The volume flux of the assimilant from the crust tothe magma chamber is suggested to have decreased progressivelywith time (proportional to t^–1/2), and was about 3 x 10^–2m/year at t = 10 years and 1 x 10^–2 m/year at t = 100years. It has been commonly considered that the heat balancebetween magmas and the surrounding crust controls the couplingof assimilation and fractional crystallization processes (i.e.absolute value of r). However, it is inferred from this studythat the ratio of assimilated mass to crystallized mass canbe controlled by the transport process of the assimilant fromthe crust to magma chambers. KEY WORDS: assimilation and fractional crystallization; mass balance model; magma chamber; melt transport; Pb isotope     

Petrogenesis of the Zoned Laacher See Tephra

The late Quaternary Laacher See phonolitic tephra deposit (EastEifel, W. Germany) is mineral-ogically and chemically zonedfrom highly evolved, volatile-rich and crystal-poor at its basetowards a mafic, crystal-rich phonolite at the top (Wörner& Schmincke, 1984). This zonation is interpreted as theresult of a continuous eruption from a zoned magma column. Majorand trace element evidence shows that the last erupted maficULST (Upper Laacher See Tephra) phonolite can be derived froma basanite parent magma via fractional crystallization of 30per cent clinopyroxene, 24 per cent amphibole, 4 per cent phlogopite,3.8 per cent magnetite, 2.5–3.0 per cent olivine and 1per cent apatite, leaving a derivative of 30 per cent evolvedmagma. Starting from the mafic (ULST) phonolite as a parent, the zonedsequence is postulated to have been formed by progressive fractionalcrystallization of the observed phenocryst phases. This modelwas tested by a series of 7 step-by-step mass balance fractionationcalculations. Abundance, modal composition and relative variationsof calculated fractionated phases agree well with the observedphenocryst abundances: sanidine followed by plagioclase andminor amounts of mafic phases are to be fractionated to givethe observed zoned sequence. The most evolved phonolite, however, cannot be generated bysubtraction of phenocrysts from the underlying phonolite. Processessuch as liquid-state differentiation may therefore have chemicallymodified the upper part (cupola) of the Laacher See magma columnsubsequent to crystal fractionation. The erupted phonolite magma (5.3 km³) was calculated to havestarted with a volume of 56 km³ of parental basanite magma whichfractionated to form 16.6 km³ of mafic phonolite. This magmafurther differentiated to give a 5.3 km³ zoned (erupted) phonolitecolumn. The non-erupted volume of 50 km³ is postulated to forma cooling cumulate body below the present day Laacher See volcano. The Laacher See magma system represents a complex end-membertype of a highly evolved small volume composition ally zonedmagma chamber with steep major and trace element gradients,the uppermost volatile rich magma layer resembling the stableroof part of rhyolitic chambers.     

Magmatic Evolution of the La Pacana Caldera System, Central Andes, Chile: Compositional Variation of Two Cogenetic, Large-Volume Felsic Ignimbrites

La Pacana is one of the largest known calderas on Earth, andis the source of at least two major ignimbrite eruptions witha combined volume of some 2700 km³. These ignimbrites have stronglycontrasting compositions, raising the question of whether theyare genetically related. The Toconao ignimbrite is crystal poor,and contains rhyolitic (76–77 wt % SiO₂) tube pumices.The overlying Atana ignimbrite is a homogeneous tuff whose pumiceis dacitic (66–70 wt % SiO₂), dense (40–60% vesicularity)and crystal rich (30–40 % crystals). Phase equilibriaindicate that the Atana magma equilibrated at temperatures of770–790°C with melt water contents of 3·1–4·4wt %. The pre-eruptive Toconao magma was cooler (730–750°C)and its melt more water rich (6·3–6·8 wt% H₂O). A pressure of 200 MPa is inferred from mineral barometryfor the Atana magma chamber. Isotope compositions are variablebut overlapping for both units (⁸⁷Sr/⁸⁶Sr_i 0·7094–0·7131;¹⁴³Nd/¹⁴⁴Nd 0·51222–0·51230) and are consistentwith a dominantly crustal origin. Glass analyses from Atanapumices are similar in composition to those in Toconao tubepumices, demonstrating that the Toconao magma could representa differentiated melt of the Atana magma. Fractional crystallizationmodelling suggests that the Toconao magma can be produced by30% crystallization of the observed Atana mineral phases. Toconaomelt characteristics and intensive parameters are consistentwith a volatile oversaturation-driven eruption. However, thelow H₂O content, high viscosity and high crystal content ofthe Atana magma imply an external eruption trigger. KEY WORDS: Central Andes; crystal-rich dacite; eruption trigger; high-silica rhyolite; zoned magma chamber     

Phase Equilibrium Constraints on Intensive Crystallization Parameters of the Ilimaussaq Complex, South Greenland

The 1·13 Ga Ilímaussaq intrusive complex, SouthGreenland, is composed of various types of alkali granite andsilica-undersaturated alkaline to agpaitic nepheline syenitesrelated to three subsequently intruded magma batches. Mineralchemistry indicates continuous fractionation trends within eachrock type, but with distinct differences among them. The last,peralkaline magma batch is the most fractionated in terms ofX_Fe^{mafic mineral}, feldspar composition and mineral assemblage.This indicates that an evolving magma chamber at depth discontinuouslyreleased more highly fractionated alkaline melts. Fluid inclusionsin some sodalites record a pressure drop from 3·5 to1 kbar indicating that crystallization started during magmaascent and continued in the high-level magma chamber. On thebasis of phase equilibria and preliminary fluid inclusion data,crystallization temperature drops from >1000°C (augitesyenite liquidus) to <500°C (lujavrite solidus) and silicaactivity decreases from     

Petrology of Rhyolitic and Mixed Magma Ejecta from the 1875 Eruption of Askja, Iceland

Volcanic activity in Askja central volcano and its fissure swarmin 1875 occurred in response to a crustal rifting episode inIceland, resulting in up to 70 km lateral flow of magma withinthe crust, caldera collapse and a plinian eruption of acid magma(0·2 km³ dense-rock equivalent). Petrologic studies ofthe predominantly rhyolitic and crystal-poor ejecta reveal thata complex array of other liquid compositions was also present,including icelandite (0.75 per cent) and basalt (1.9 per cent),as well as leucocratic xenoliths of trondhjemite type. Mineralgeothermometers indicate that the rhyolite evolved at 990 to1010 °C and 0·5 Kb P_H2O, the icelandite at 1005 to1020 °C and at f_O2 10^–10 atm. and the basalt at 1140to 1170 °C. A petrologic model of Askja in 1875 consists of a density-stratifiedmagma chamber with a rhyolitic upper part and a lower part offerrobasalt, with an intervening layer of icelandite. The modelcalculations show that the icelandite can be derived from ferrobasaltby 50 per cent fractional crystallization, but one-stage fractionalcrystallization models cannot account for generation of theacid magma. Simple partial or complete fusion of the field-associatedtrondhjemite xenoliths cannot produce the acid magma. Instead,a more complex fusion, hybridization and fractional crystallizationmodel is presented, which is consistent with the available petrologicevidence. This model involves large-scale fusion of pre-existingtrondhjemite intrusions or reactivation of previously consolidatedroof-rock in the magma chamber followed by hybridization ofthe acid magma with 7 to 14 per cent basaltic magma. Finally,10 to 11 per cent fractional crystallization of the dacite hybridis required to produce the observed compositional range withinthe rhyolite ejecta. The 1875 explosive eruption was causedby the ascent of tholeiitic basalt magma from depth during crustalrifting. Influx of new basalt magma in 1874–75 triggeredconvective mixing and hybridization in the compositionally zonedmagma chamber.     

Microchemical and Sr Isotopic Investigation of Zoned K-feldspar Megacrysts: Insights into the Petrogenesis of a Granitic System and Disequilibrium Crystal Growth

K-feldspar megacrysts (Kfm) are used to investigate the magmaticevolution of the 7 Ma Monte Capanne (MC) monzogranite (Elba,Italy). Dissolution and regrowth of Kfm during magma mixingor mingling events produce indented resorption surfaces associatedwith high Ba contents. Diffusion calculations demonstrate thatKfm chemical zoning is primary. Core-to-rim variations in Ba,Rb, Sr, Li and P support magma mixing (i.e. high Ba and P andlow Rb/Sr at rims), but more complex variations require othermechanisms. In particular, we show that disequilibrium growth(related to variations in diffusion rates in the melt) may haveoccurred as a result of thermal disturbance following influxof mafic magma in the magma chamber. Initial ⁸⁷Sr/⁸⁶Sr ratios(I_Sr) (obtained by microdrilling) decrease from core to rim.Inner core analyses define a mixing trend extending towardsa high I_Sr–Rb/Sr melt component, whereas the outer coresand rims display a more restricted range of I_Sr, but a largerrange of Rb/Sr. Lower I_Sr at the rim of one megacryst suggestsmixing with high-K calc-alkaline mantle-derived volcanics ofsimilar age on Capraia. Trace element and isotopic profilessuggest (1) early megacryst growth in magmas contaminated bycrust and refreshed by high I_Sr silicic melts (as seen in theinner cores) and (2) later recharge with mafic magmas (as seenin the outer cores) followed by (3) crystal fractionation, withpossible interaction with hydrothermal fluids (as seen in therim). The model is compatible with the field occurrence of maficenclaves and xenoliths. KEY WORDS: Elba; monzogranite; K-feldspar megacrysts; zoning; magma mixing; trace element; Sr isotopes; petrogenesis     

Iron-Titanium Oxide Minerals in the Bjerkrem-Sogndal Massif, South-western Norway

Ilmenite and magnetite are investigated from the point of viewof their distribution, microtexture, and chemical composition(major and minor elements) in the Bjerkrem-Sogndal massif (Egersundarea, South-Rogaland, SW. Norway). This massif is an igneouslayered synkinematic lopolith made up of cumulates of the anorthosite-mangeritesuite. The lower part of the massif presents a rhythmic structure. The microtextures of ilmenite result from simple exsolutionof ilmenite-hematite solid solutions. Magnetite contains intergrowthsof ilmenite formed by oxidation-exsolution of ulv?spinel-magnetitesolid-solutions. In the stratigraphic sequence, on a large scale, ilmenite appearsfirst alone, and is then accompanied by magnetite; its hematitecontent decreases towards the top of the massif, while the titaniumcontent of the magnetite increases. On the scale of the rhythms,similar trends but of lesser amplitude are also observed. Evidence of deuteric readjustment of the orthomagmatic compositionof the two oxides is provided (1) by the observation of microtexturesat the contact between grains (zoning of primary ilmenite andrim of secondary ilmenite) (2) by the existence of differencesin chemical composition between isolated grains and grains incontact, and (3) by the determination of the equilibrium temperatureby means of the Buddington and Lindsley geothermometer. Reconstitution of the T-fo₂ orthomagmatic conditions in twoparticular levels of the massif shows that the reducing characterof the magma increases during differentiation. The sudden changesin the oxide assemblage at the base of the rhythms reflect asudden increase in the fo₂ of the magma. These increases, asshown by variation in Cr, Ni, and Co, are due to recurrencesof the basic character of the magma. The variations of the minor elements Mn, V, Ga, and Zn are interpretedin terms of the influence of the deuteric readjustment. It followsthat the ratios Mn/Fe²⁺, Ga/Fe³⁺, and Zn/Fe²⁺ increase and thatthe ratio V/Fe³⁺ decreases in the magma in the course of differentiation.The distribution of Mn between ilmenite and magnetite is discussed. Intermittent supplies of undifferentiated magma are proposedas the geological mechanism controlling the chemical recurrencesassociated with the rhythmic structure.     

Open System Magmatic Evolution of the Taos Plateau Volcanic Field, Northern New Mexico: 3. Petrology and Geochemistry of Andesite and Dacite

Calc-alkaline olivine andesite and two-pyroxene dacite of theTaos Plateau volcanic field evolved in an open magmatic system.mg-numbers of spatially and temporally associated ServilletaBasalt (54–61) and ohvine andesite (49–59) are comparableand preclude fractional crystallization of ferromagnesian mineralsas the major differentiation process. If Servilleta olivinetholeiite is assumed to be the parental magma type, enrichmentsof highly incompatible trace elements (up to 17 ?) oVer concentrationsin the basalts require that andesitic and dacitic magmas containa substantial proportion of assimilated crust. Isotopic compositionsof andesite and dacite, which have slightly higher ⁸⁷Sr/⁸⁶Srratios than the basalts but lower ¹⁴³Nd/¹⁴⁴Nd, ²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios, are consistent with contaminationof parental basalt by old, low Rb/Sr, low U/Pb, and low Th/Pbcontinental crust. Concentrations of highly incompatible traceelements in andesite and dacite lavas are decoupled from majorelement compositions; the highest concentrat ions of these elementsoccur in andesitic, rather than dacitic compositions, and andesitelavas are more variable in trace element contents. Assimilationof heterogeneous crust concurrent with fractional crystallizationof varying mineral assemblages could cause this decoupled behavior.High mg-numbers in andesite and dacite, skeletal olivine phenocrysts,and reversely zoned pyroxene phenocrysts are manifestationsof mafic replenishment and magma mixing in the Taos Plateaumagmatic system. Taos Plateau volcanoes are monolithologic and are distributedin a semi-concentric zoned pattern that is a reflection of thecomplex subvolcanic magmatic system. A central focus of basaltshields developed above the main basaltic conduit system; thesemagmas contain 10–35% admixed andesitic and dacitic magma.Basalt shields are surrounded by a partial ring of olivine andesiteshield volcanoes, where replenishment of basaltic magma providedthe heat necessary for prolonged assimilation of crust, resultingin intermediate-composition lavas. Dacite shields are locatedaround the periphery of the more mafic volcanoes and reflecta decrease in mafic input on the fringes of the magmatic system.     

Magmatic Differentiation at an Island-arc Caldera: Okmok Volcano, Aleutian Islands, Alaska

Okmok volcano is situated on oceanic crust in the central Aleutianarc and experienced large (15 km³) caldera-forming eruptionsat 12 000 years BP and 2050 years BP. Each caldera-forming eruptionbegan with a small Plinian rhyodacite event followed by theemplacement of a dominantly andesitic ash-flow unit, whereaseffusive inter- and post-caldera lavas have been more basaltic.Phenocryst assemblages are composed of olivine + pyroxene +plagioclase ± Fe–Ti oxides and indicate crystallizationat 1000–1100°C at 0·1–0·2 GPain the presence of 0–4% H₂O. The erupted products followa tholeiitic evolutionary trend and calculated liquid compositionsrange from 52 to 68 wt % SiO₂ with 0·8–3·3wt % K₂O. Major and trace element models suggest that the moreevolved magmas were produced by 50–60% in situ fractionalcrystallization around the margins of the shallow magma chamber.Oxygen and strontium isotope data (¹⁸O 4·4–4·9,⁸⁷Sr/ ⁸⁶Sr 0·7032–0·7034) indicate interactionwith a hydrothermally altered crustal component, which led toelevated thorium isotope ratios in some caldera-forming magmas.This compromises the use of uranium–thorium disequilibria[(²³⁰Th/ ²³⁸U) = 0·849–0·964] to constrainthe time scales of magma differentiation but instead suggeststhat the age of the hydrothermal system is 100 ka. Modellingof the diffusion of strontium in plagioclase indicates thatmany evolved crystal rims formed less than 200 years prior toeruption. This addition of rim material probably reflects theremobilization of crystals from the chamber margins followingreplenishment. Basaltic recharge led to the expansion of themagma chamber, which was responsible for the most recent caldera-formingevent. KEY WORDS: Okmok; caldera; U-series isotopes; Sr-diffusion; time scales; Aleutian arc     

Raft-like Metabasaltic Inclusions in the Fongen-Hyllingen Layered Intrusive Complex, Norway, and their Implications for Magma Chamber Evolution

The Hyllingen Series comprises the southern part of the Caledoniansynorogenic Fongen- Hyllingen layered mafic intrusion, whichoccupies an area of 160 km², southeast of Trondheim, Norway.Large, raft-like inclusions form an important part of the HyllingenScries. Most of these are of fine-grained, equigranular rocksof basaltic composition with lithologies matching those of theadjacent country rocks. The rafts, which compose up to 22% ofthe lower part of the Hyllingen Series, are broadly concordantwith modal layering in the host gabbroic rocks. Individual bodiescan be up to 1500 m long and over 100 m thick. Some of the raftsare branching, and appear locally to form a threedimensionalnetwork. Impact structures are associated with small metabasicinclusions but not with the large rafts. The Hyllingen magma chamber is believed to have developed asa southerly expanding, thin wedge, forming the upper part ofthe Fongen chamber. The magma was compositionally zoned andcrystallized along the inclined floor of the wedge-shaped chamber.The wedge expanded as a result of the influx of dense, primitivemagma in the northern part of the chamber. The highly evolvedmagma at the top of the chamber penetrated along fractures inthe roof and spread laterally to form sill-like bodies. Theroof zone consisted of a network of veins and sills penetratingan interconnected framework of metabasic hornfels. Continuedcrystallization at the floor, while the magma chamber expanded,finally resulted in the interconnected rafts being engulfedby the crystallization front. Fragments detached from the roofsank to the floor to cause the observed impact structures. Thelarge, raft-like, fine-grained, granular, gabbroic bodies areconsidered to be in situ country rock inclusions. Reprints available from J. R. Wilson     

Emplacement of the Cleveland Dyke: Evidence from Geochemistry, Mineralogy, and Physical Modelling

The igneous rocks of the British Tertiary Volcanic Province(BTVP) comprise intrusive central complexes and associated lavafields in northwest Scotland and northern Ireland. These centresare associated with linear dyke swarms which are radial aroundthe major central complexes. The most extensive dyke swarm isrelated to the Mull intrusive complex and includes the Clevelanddyke, which appears to extend some 430 km from Mull throughthe Scottish Midland Valley (SMV) to the coast of northeastEngland. The dyke may have been emplaced by lateral magma migrationfrom Mull, by vertical magma migration, or by a combinationof these processes associated with the emplacement of the Mullcentre and the presence of a regional stress field in northernBritain. Petrographic, mineralogical, and geochemical data for samplescollected across and along the Cleveland dyke have been usedto evaluate its petrogenesis and emplacement mechanism. Thesegment of the dyke north of, and along, the Southern UplandsFault, the southern boundary of the SMV, is not comagmatic withthat to the south, which is now defined as the Cleveland dykesensu stricto. The Cleveland dyke is an olivine-free, plagioclase-and pyroxene-phyric basaltic andesite. Plagioclase mineralogyand bulk composition indicate that it experienced a complexmagmatic history involving polybaric fractional crystallizationand minor crustal contamination. Despite this complex evolution,the dyke magma is relatively homogeneous and shows chemicalcharacteristics closely similar to tholeiitic rocks from Mull.The data substantiate lateral emplacement from this BVTP centre,rather than by vertical emplacement through heterogeneous lithosphere. Numerical modelling of dyke dynamics is consistent with emplacementof the Cleveland dyke as a single pulse of magma from the Mullcentre, flowing in a manner transitional between laminar andturbulent conditions. According to this model, the dyke (volumec. 85 km³ was initiated in a large magma chamber below Mullsubject to a small excess magmatic pressure. Lateral migrationat relatively high velocity (1–5 ms^–1) caused emplacementof the dyke in 1–5 days. Following emplacement, minorvertical ascent of magma may have contributed to the local enechelon distribution of dyke segments.     

Crystal-Melt Separation and the Development of Isotopic Heterogeneities in Hybrid Magmas

If a magma is a hybrid of two (or more) isotopically distinctend-members, at least one of which is partially crystalline,separation of melt and crystals after hybridization will leadto the development of isotopic heterogeneities in the magmaas long as some of the pre-existing crystalline material (antecrysts)retains any of its original isotopic composition. This holdstrue whether the hybridization event is magma mixing as traditionallyconstrued, bulk assimilation, or melt assimilation. Once a magma-scaleisotopic heterogeneity is formed by crystal–melt separation,it is essentially permanent, persisting regardless of subsequentcrystallization, mixing, or equilibration events. The magnitudeof the isotopic variability resulting from crystal–meltseparation can be as large as that resulting from differentialcontamination, multiple isotopically distinct sources, or insitu isotopic evolution. In one model, a redistribution of one-thirdof the antecryst cargo yielded a crystal-enriched sample with⁸⁷Sr/⁸⁶Sr of 0·7058, whereas the complementary crystal-poorsample has ⁸⁷Sr/⁸⁶Sr of 0·7068. In other models, crystal-richsamples are enriched in radiogenic Sr. Isotopic heterogeneitiescan be either continuous (controlled by the modal distributionof crystals and melt) or discontinuous (when there is completeseparation of crystals and liquid). The first case may be exemplifiedby some isotopically zoned large-volume rhyolites, formed bythe eruptive inversion of a modally zoned magma chamber. Inthe latter case, the isotopic composition of any (for example)interstitial liquid will be distinct from the isotopic compositionof the bulk crystal fraction. The separation of such an interstitialliquid may explain the presence of isotopically distinct late-stageaplites in plutons. Crystal–melt separation provides anadditional option for the interpretation of isotopically zonedor heterogeneous magmas. This option is particularly attractivefor systems whose chemical variation is otherwise explicableby fractionation-dominated processes. Non-isotopic chemicalheterogeneities can also develop in this fashion. KEY WORDS: isotopic heterogeneity; zoning; hybrid magma; crystal separation; Sr isotopes; aplite; rhyolite     

The Peralkaline Volcanic Suite of Aden and Little Aden, South Arabia

The volcanic rocks of Aden, Little Aden, and Ras Imran, heredesignated as belonging to the Aden Volcanic Series, were eruptedthrough central-vent, strato-volcanoes about 5 m.y. ago. Inits major element chemistry the Aden Volcanic Series is intermediatebetween the alkaline and tholeiitic associations, and this isdemonstrated by comparing it with the alkaline suite of Hawaiiand the tholeiitic series of Thingmuli, Iceland. It is proposedthat the most acceptable ‘parental’ magma is a mildlyalkaline olivine basalt which, on fractionation, produced aseries ranging from trachybasalts through trachyandesites andtrachytes to rhyolites. These rhyolites are peralkaline as themolecular proportion of alumina is less than that of the combinedalkalis, and are comenditic as the series is poor in normativefemic constituents. Trace element data suggest that the peralkalinesilicic eruptives are chemically comparable with those of MayorIsland, New Zealand, where a mildly alkaline olivine basaltparent has also been postulated. Although the age of eruption of c. 5 m.y., given by K-Ar measurements,is entirely consistent with an age deduced from geomorphologicalcriteria, an ⁸⁷Sr/⁸⁶Sr versus ⁸⁷Rb/⁸⁶Sr isochron plot suggeststhat the series is related to a thermal event some 20-30 m.y.older than the age of eruption. As this earlier age correspondsdirectly to the age of the previous magmatic episode, the eruptionof the Yemen Trap Series, the upper part of which is petrologicallysimilar to the Aden Volcanic Series, and as the initial ⁸⁷Sr/⁸⁶Srratios suggest that the magma originated in the mantle, it isproposed that the most acceptable petrogenetic scheme, whichwould also explain the anomalously old Rb-Sr age, is: (a) Partialfusion in the upper mantle giving rise to the alkaline YemenTrap Series, (b) After the cessation of surface activity, alarge body of magma existed in the upper mantle and this magma,on crystallizing, fractionated to produce a layered sequence,(c) About 5 m.y. ago some event, either pressure relief or furtherthermal activity, resulted in the partial remelting of thisfractionated plutonic sequence and the liquids so formed reachedthe surface without significant mixing or chemical fractionation.     

Oxygen isotope study of the Long Valley magma system, California: isotope thermometry and convection in large silicic magma bodies

Products of voluminous pyroclastic eruptions with eruptive draw-down of several kilometers provide a snap-shot view of batholith-scale magma chambers, and quench pre-eruptive isotopic fractionations (i.e., temperatures) between minerals. We report analyses of oxygen isotope ratio in individual quartz phenocrysts and concentrates of magnetite, pyroxene, and zircon from individual pumice clasts of ignimbrite and fall units of caldera-forming 0.76 Ma Bishop Tuff (BT), pre-caldera Glass Mountain (2.1-0.78 Ma), and post-caldera rhyolites (0.65-0.04 Ma) to characterize the long-lived, batholith-scale magma chamber beneath Long Valley Caldera in California. Values of '¹⁸O show a subtle 1‰ decrease from the oldest Glass Mountain lavas to the youngest post-caldera rhyolites. Older Glass Mountain lavas exhibit larger (~1‰) variability of '¹⁸O(quartz). The youngest domes of Glass Mountain are similar to BT in '¹⁸O(quartz) values and reflect convective homogenization during formation of BT magma chamber surrounded by extremely heterogeneous country rocks (ranging from 2 to +29‰). Oxygen isotope thermometry of BT confirms a temperature gradient between "Late" (815 °C) and "Early" (715 °C) BT. The '¹⁸O(quartz) values of "Early" and "Late" BT are +8.33 and 8.21‰, consistent with a constant '¹⁸O(melt)=7.8ǂ.1‰ and 100 °C temperature difference. Zircon-melt saturation equilibria gives a similar temperature range. Values of '¹⁸O(quartz) for different stratigraphic units of BT, and in pumice clasts ranging in pre-eruptive depths from 6 to 11 km (based on melt inclusions), and document vertical and lateral homogeneity of '¹⁸O(melt). Worldwide, five other large-volume rhyolites, Lava Creek, Lower Bandelier, Fish Canyon, Cerro Galan, and Toba, exhibit equal '¹⁸O(melt) values of earlier and later erupted portions in each of the these climactic caldera-forming eruptions. We interpret the large-scale '¹⁸O homogeneity of BT and other large magma chambers as evidence of their longevity (>10⁵ years) and convection. However, remaining isotopic zoning in some quartz phenocrysts, trace element gradients in feldspars, and quartz and zircon crystal size distributions are more consistent with far shorter timescales (10²-10⁴ years). We propose a sidewall-crystallization model that promotes convective homogenization, roofward accumulation of more evolved and stagnant, volatile-rich liquid, and develops compositional and temperature gradients in pre-climactic magma chamber. Crystal + melt + gas bubbles mush near chamber walls of variable '¹⁸O gets periodically remobilized in response to chamber refill by new hotter magmas. One such episode of chamber refill by high-Ti, Sr, Ba, Zr, and volatile-richer magma happened 10³-10⁴ years prior to the 0.76-Ma caldera collapse that caused magma mixing at the base, mush thawing near the roof and walls, and downward settling of phenocrysts into this hybrid melt.     

The Generation of Granitic Magmas by Intrusion of Basalt into Continental Crust

When basalt magmas are emplaced into continental crust, meltingand generation of silicic magma can be expected. The fluid dynamicaland heat transfer processes at the roof of a basaltic sill inwhich the wall rock melts are investigated theoretically andalso experimentally using waxes and aqueous solutions. At theroof, the low density melt forms a stable melt layer with negligiblemixing with the underlying hot liquid. A quantitative theoryfor the roof melting case has been developed. When applied tobasalt sills in hot crust, the theory predicts that basalt sillsof thicknesses from 10 to 1500 m require only 1 to 270 y tosolidify and would form voluminous overlying layers of convectingsilicic magma. For example, for a 500 m sill with a crustalmelting temperature of 850 ?C, the thickness of the silicicmagma layer generated ranges from 300 to 1000 m for countryrock temperatures from 500 to 850?C. The temperatures of thecrustal melt layers at the time that the basalt solidifies arehigh (900–950?C) so that the process can produce magmasrepresenting large degrees of partial fusion of the crust. Meltingoccurs in the solid roof and the adjacent thermal boundary layer,while at the same time there is crystallization in the convectinginterior. Thus the magmas formed can be highly porphyritic.Our calculations also indicate that such magmas can containsignificant proportions of restite crystals. Much of the refractorycomponents of the crust are dissolved and then re-precipitatedto form genuine igneous phenocrysts. Normally zoned plagioclasefeldspar phenocrysts with discrete calcic cores are commonlyobserved in many granitoids and silicic volcanic rocks. Suchpatterns would be expected in crustal melting, where simultaneouscrystallization is an inevitable consequence of the fluid dynamics. The time-scales for melting and crystallization in basalt-inducedcrustal melting (10²–10³ y) are very short compared tothe lifetimes of large silicic magma systems (>10⁶ y) orto the time-scale for thermal relaxation of the continentalcrust (> l0⁷ y). Several of the features of silicic igneoussystems can be explained without requiring large, high-level,long-lived magma chambers. Cycles of mafic to increasingly largevolumes of silicic magma with time are commonly observed inmany systems. These can be interpreted as progressive heatingof the crust until the source region is partially molten andbasalt can no longer penetrate. Every input of basalt triggersrapid formation of silicic magma in the source region. Thismagma will freeze again in time-scales of order l0²–10³y unless it ascends to higher levels. Crystallization can occurin the source region during melting, and eruption of porphyriticmagmas does not require a shallow magma chamber, although suchchambers may develop as magma is intruded into high levels inthe crust. For typical compositions of upper crustal rocks,the model predicts that dacitic volcanic rocks and granodiorite/tonaliteplutons would be the dominant rock types and that these wouldascend-from the source region and form magmas ranging from thosewith high temperature and low crystal content to those withhigh crystal content and a significant proportion of restite.     

A Fluid-Dynamical Study of Crystal Settling in Convecting Magmas

Thermal convection in magma chambers is believed to be almostalways highly time-dependent, or ‘turbulent’, andpredicted convective velocities are commonly orders of magnitudelarger than settling velocities for typical crystals calculatedfrom Stokes' Law. To understand crystal settling in magma chamberswe have therefore undertaken a theoretical and experimentalstudy of particle settling in a turbulently convecting fluid. The regime of interest is where the ratio, S, of the Stokes'Law settling velocity, v_s, to the root mean square verticalcomponent of convective velocity, W, at mid-depth in the fluidis less than unity. Although v_s < W, settling is still possiblebecause convective velocities are height-dependent and mustdecrease to zero at the boundaries of the fluid. Particles immediatelyadjacent to the bottom boundary settle out with their full Stokes'settling velocities. At the same time, convection is vigorousenough to ensure that the distribution of particles in the fluidis uniform. It follows that the number of particles in suspensiondecays with time according to an exponential law, and the decayconstant is simply the ratio of v_s to h, the depth of the fluid.Experiments confirm this relationship, at least for low particleconcentrations, provided S < 0.5 and there is no re-entrainmentof particles from the floor of the tank. We apply this relationship to crystals in magma chambers andso calculate residence times for typical crystals. We find thatfor basaltic magmas the predicted residence times are smallcompared with the many thousands of years that a chamber takesto solidify if cooling is dominated by conduction through thecountry rock. We therefore conclude that crystal settling maybe an efficient differentiation mechanism. Significant magmaticevolution may, however, take place on time-scales that are competitivewith these residence times. If the settling of crystals is the rate-limiting step duringthe crystallization of a magma chamber it is expected that asteady state will be achieved at which the rate of supply ofcrystals into the convecting magma by crystallization balancesthe rate at which crystals settle out. We show how this ideacan explain both the lack of hydraulic equivalence in cumulaterocks and the commonly observed discrepancy between the relativeproportions of phenocrysts of various phases in fractionatedbasaltic lavas and the calculated relative proportions of thesemineral phases in the fractionating assemblage. Finally, anattempt is made to calculate the steady-state crystal contentof convecting magma chambers. Comparison of the predicted crystalcontents with the observed phenocryst contents of typical basalticlavas suggests that magma chambers may often cool more rapidlythan would be expected for conduction through the country rockalone.     

Evidence for Multi-stage Magmatic Evolution during the past 60 kyr at Campi Flegrei (Italy) Deduced from Sr, Nd and Pb Isotope Data

The Campi Flegrei caldera, an active volcanic field in the Campanianprovince, Italy, is a nested structure generated by the CampanianIgnimbrite (37 ka BP) and the Neapolitan Yellow Tuff (12 kaBP) eruptions. Since at least 60 ka BP Campi Flegrei has producedmagmas with variable chemical and Sr isotopic compositions.⁸⁷Sr/⁸⁶Sr ratios increase through time from 0·7068 to0·7086, with the highest ratios detected in the least-evolvedshoshonitic products. The origin of this progressive Sr isotopicvariability has been investigated using new Sr, Nd and Pb isotopicdata for volcanic rocks and entrained xenoliths. The data obtainedare combined and discussed with previous geochemical and Srisotope data and used to suggest a multi-stage evolution forthe magmatic system, mainly involving deeper and shallower crustalmagma storage reservoirs. The deeper reservoir is proposed tobe a magma chamber periodically refilled by primitive maficmagmas which subsequently undergo contamination by crustal material.The assimilated crustal material is represented by xenolithsrecovered in the shoshonitic pyroclastic products. Magma batchesoriginating from the deeper reservoir migrated towards the surfaceand fed a shallower complex magmatic system. The deeper chamberwas tapped during the eruption of least evolved magmas by regionalfault systems. In addition to crystal–liquid fractionation,open-system processes occurred in the shallower system. KEY WORDS: Campi Flegrei; crustal contamination; xenoliths     

Rates of Thermal and Chemical Evolution of Magmas in a Cooling Magma Chamber: a Chronological and Theoretical Study on Basaltic and Andesitic Lavas from Rishiri Volcano, Japan

Rates of magmatic processes in a cooling magma chamber wereinvestigated for alkali basalt and trachytic andesite lavaserupted sequentially from Rishiri Volcano, northern Japan, bydating of these lavas using ²³⁸U–²³⁰Th radioactive disequilibriumand ¹⁴C dating methods, in combination with theoretical analyses.We obtained the eruption age of the basaltic lavas to be 29·3± 0·6 ka by ¹⁴C dating of charcoals. The eruptionage of the andesitic lavas was estimated to be 20·2 ±3·1 ka, utilizing a whole-rock isochron formed by U–Thfractionation as a result of degassing after lava emplacement.Because these two lavas represent a series of magmas producedby assimilation and fractional crystallization in the same magmachamber, the difference of the ages (i.e. 9 kyr) is a timescaleof magmatic evolution. The thermal and chemical evolution ofthe Rishiri magma chamber was modeled using mass and energybalance constraints, as well as quantitative information obtainedfrom petrological and geochemical observations on the lavas.Using the timescale of 9 kyr, the thickness of the magma chamberis estimated to have been about 1·7 km. The model calculationsshow that, in the early stage of the evolution, the magma cooledat a relatively high rate (>0·1°C/year), and thecooling rate decreased with time. Convective heat flux fromthe main magma body exceeded 2 W/m² when the magma was basaltic,and the intensity diminished exponentially with magmatic evolution.Volume flux of crustal materials to the magma chamber and rateof convective melt exchange (compositional convection) betweenthe main magma and mush melt also decreased with time, from 0·1 m/year to 10^–3 m/year, and from 1 m/yearto 10^–2 m/year, respectively, as the magmas evolved frombasaltic to andesitic compositions. Although the mechanism ofthe cooling (i.e. thermal convection and/or compositional convection)of the main magma could not be constrained uniquely by the model,it is suggested that compositional convection was not effectivein cooling the main magma, and the magma chamber is consideredto have been cooled by thermal convection, in addition to heatconduction. KEY WORDS: convection; magma chamber; heat and mass transport; timescale; U-series disequilibria