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     

Andesites from northeastern Kanaga Island,Aleutians

Kanaga island is located in the central Aleutian island arc. Northeastern Kanaga is a currently active late Tertiary to Recent calc-alkaline volcanic complex. Basaltic andesite to andesite lavas record three episodes (series) of volcanic activity. Series I and Series II lavas are all andesite while Series III lavas are basaltic andesite to andesite. Four Series II andesites contain abundant quenched magmatic inclusions ranging in composition from high-MgO low-alumina basalt to low-MgO highalumina basalt. The spectrum of lava compositions is due primarily to fractional crystallization of a parental low-MgO high-alumina basalt but with variable degrees of crustal contamination and magma mixing. The earliest Series I lavas represent mixing between high-alumina basalt and silicic andesite with maximum SiO₂ contents of 65–67 wt %. Later Series I and all Series II lavas are due to mixing of andesite magmas of similar composition. The maximum SiO₂ content of the pre-mixed andesites magmas is estimated at 60–63 wt %. The youngest lavas (Series III) are all non-mixed and have maximum estimated SiO₂ contents of 59 wt %. The earliest Series I lavas contain a significant crustal component while all later lavas do not. It is concluded that the maximum SiO₂ contents of silicic magmas, the contribution of crustal material to silicic magma generation, and the role of magma mixing all decrease with time. Furthermore, silicic magmas generated by fractional crystallization at this volcanic center have a maximum SiO₂ content of 63 wt %. All of these features have also been documented at the central Aleutian Cold Bay Volcanic Center (Brophy 1987). Based on data from these two centers a model of Aleutian calc-alkaline magma chamber development is proposed. The main features are: (1) a single low pressure magma chamber is continuously supplied by primitive low-alumina basalt; (2) non-primary high-alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase; (3) sidewall crystallization accompanied by crustal melting produces buoyant silicic (>63 wt % SiO₂) liquids that pond at the top of the chamber, and; (4) continued sidewall crystallization, now isolated from the chamber wall, produces silicic liquids with 63 wt % SiO₂ that increase the thickness and lowers the overall SiO₂ content of the upper silicic zone. It is suggested that the maximum SiO₂ content of 63% imposed on fractionation-generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal-liquid mush along the chamber wall.     

Intrusion of basaltic magma into a crystallizing granitic magma chamber: The Cordillera del Paine pluton in southern Chile

The Cordillera del Paine pluton in the southernmost Andes of Chile represents a deeply dissected magma chamber where mafic magma intruded into crystallizing granitic magma. Throughout much of the 10x15 km pluton, there is a sharp and continuous boundary at a remarkably constant elevation of 1,100 m that separates granitic rocks (Cordillera del Paine or CP granite: 69–77% SiO₂) which make up the upper levels of the pluton from mafic and comingled rocks (Paine Mafic Complex or PMC: 45–60% SiO₂) which dominate the lower exposures of the pluton. Chilled, crenulate, disrupted contacts of mafic rock against granite demonstrate that partly crystallized granite was intruded by mafic magma which solidified prior to complete crystallization of the granitic magma. The boundary at 1,100 m was a large and stable density contrast between the denser, hotter mafic magma and cooler granitic magma. The granitic magma was more solidified near the margins of the chamber when mafic intrusion occurred, and the PMC is less disrupted by granites there. Near the pluton margins, the PMC grades upward irregularly from cumulate gabbros to monzodiorites. Mafic magma differentiated largely by fractional crystallization as indicated by the presence of cumulate rocks and by the low levels of compatible elements in most PMC rocks. The compositional gap between the PMC and CP granite indicates that mixing (blending) of granitic magma into the mafic magma was less important, although it is apparent from mineral assemblages in mafic rocks. Granitic magma may have incorporated small amounts of mafic liquid that had evolved to >60% SiO₂ by crystallization. Mixing was inhibited by the extent of crystallization of the granite, and by the thermal contrast and the stable density contrast between the magmas. PMC gabbros display disequilibrium mineral assemblages including early formed zoned olivine (with orthopyroxene coronas), clinopyroxene, calcic plagioclase and paragasite and later-formed amphibole, sodic plagioclase, mica and quartz. The early formed gabbroic minerals (and their coronas) are very similar to phenocrysts in late basaltic dikes that cut the upper levels of the CP granite. The inferred parental magmas of both dikes and gabbros were very similar to subalkaline basalts of the Patagonian Plateau that erupted at about the same time, 35 km to the east. Mafic and silicic magmas at Cordillera del Paine are consanguineous, as demonstrated by alkalinity and trace-element ratios. However, the contemporaneity of mafic and silicic magmas precludes a parent-daughter relationship. The granitic magma most likely was derived by differentiation of mafic magmas that were similar to those that later intruded it. Or, the granitic magma may have been contaminated by mafic magmas similar to the PMC magmas before its shallow emplacement. Mixing would be favored at deeper levels when the cooling rate was lower and the granitic magma was less solidified.     

Synplutonic mafic injections into crystallizing granite pluton from Gurgunta area,northern part of Eastern Dharwar Craton: Implications for magma chamber processes

In this paper we document widespread coeval felsic-mafic magma interaction and progressive hybridization near Gurgunta in the northern part of Eastern Dharwar Craton (EDC) where mafic magma pulses have injected into a 2.5 Ga granite pluton. The pluton contains voluminous pink porphyritic facies with minor equigranular grey facies. The mafic body shows compositional variation from diorite to meladiorite with hornblende as the chief mafic mineral with lesser clinopyroxene and biotite. The observed variation on binary diagrams suggests that granite was evolved by fractional crystallization. Chemical characteristics such as higher Al₂O₃ and moderate to high CaO, Mg#, Ni, Cr, Co and V are interpreted by slab-melting. Mafic bodies show lower SiO₂, Na₂O and K₂O; but higher CaO, Mg#, FeO, Cr, Ni and V; higher LREE with moderate to higher HREE which suggest their derivation from mantle. A major active shear zone has played an important role at the time of synplutonic mafic injection and hybridization process. Field evidences suggest that the synplutonic mafic body has injected into the crystallizing felsic magma chamber in successive stages. The first stage injection has resulted in extensive mixing and hybridization due to the liquidus state of resident felsic magma to which hot mafic magma was injected. However, progressive mixing produced heterogeneity as the xenocrysts started mechanically dispersed into hybrid magma. The second stage injection, after a time gap, encountered colder and viscous hybrid magma in the magma chamber, which inhibited free injection. As a consequence, the mafic magma spread into magma chamber as flows, producing massive mafic bodies. However, with the continued mafic pulses and the heat gradient, the viscosity contrasts of mafic magma and felsic magma were again lowered resulting in second stage mixing. This episode was followed by mingling when the granite was almost crystallized, but still viscous enough to accommodate lamellar and ribbon like mafic penetrations to produce mingling. The successive mixing and mingling processes account for the observed heterogeneity in the granite pluton.     

Field evidence,mineral chemical and geochemical constraints on mafic-felsic magma interactions in a vertically zoned magma chamber from the Chotanagpur Granite Gneiss Complex of Eastern India

The Nimchak granite pluton (NGP) of Chotanagpur Granite Gneiss Complex (CGGC), Eastern India, provides ample evidence of magma interaction in a plutonic regime for the first time in this part of the Indian shield. A number of outcrop level magmatic structures reported from many mafic-felsic mixing and mingling zones worldwide, such as synplutonic dykes, mafic magmatic enclaves and hybrid rocks extensively occur in our study domain. From field observations it appears that the Nimchak pluton was a vertically zoned magma chamber that was intruded by a number of mafic dykes during the whole crystallization history of the magma chamber leading to magma mixing and mingling scenario. The lower part of the pluton is occupied by coarse-grained granodiorite (64.84–66.61?wt.% SiO₂), while the upper part is occupied by fine-grained granite (69.80–70.57?wt.% SiO₂). Field relationships along with textural and geochemical signatures of the pluton suggest that it is a well-exposed felsic magma chamber that was zoned due to fractional crystallization. The intruding mafic magma interacted differently with the upper and lower granitoids. The lower granodiorite is characterized by mafic feeder dykes and larger mafic magmatic enclaves, whereas the enclaves occurring in the upper granite are comparatively smaller and the feeder dykes could not be traced here, except two late-stage mafic dykes. The mafic enclaves occurring in the upper granite show higher degrees of hybridization with respect to those occurring in the lower granite. Furthermore, enclaves are widely distributed in the upper granite, whereas enclaves in the lower granite occur adjacent to the main feeder dykes.Geochemical signatures confirm that the intermediate rocks occurring in the Nimchak pluton are mixing products formed due to the mixing of mafic and felsic magmas. A number of important physical properties of magmas like temperature, viscosity, glass transition temperature and fragility have been used in magma mixing models to evaluate the process of magma mixing. A geodynamic model of pluton construction and evolution is presented that shows episodic replenishments of mafic magma into the crystallizing felsic magma chamber from below. Data are consistent with a model whereby mafic magma ponded at the crust-mantle boundary and melted the overlying crust to form felsic (granitic) magma. The mafic magma episodically rose, injected and interacted with an overlying felsic magma chamber that was undergoing fractional crystallization forming hybrid intermediate rocks. The intrusion of mafic magma continued after complete solidification of the magma chamber as indicated by the presence of two late-stage mafic dykes.     

Interaction of coeval felsic and mafic magmas from the Kanker granite,Pithora region,Bastar Craton,Central India

Field and petrographic studies are carried out to characterize the interactions of mafic and felsic magmas from Pithora region of the northeastern part of the Bastar Craton. The MMEs, syn-plutonic mafic dykes, cuspate contacts, magmatic flow textures, mingling and hybridization suggest the coeval emplacement of end member magmas. Petrographic evidences such as disequilibrium assemblages, resorption textures, quartz ocelli, rapakivi and poikilitic textures suggest magma mingling and mixing phenomena. Such features of mingling and mixing of the felsic and mafic magma manifest the magma chamber processes. Introduction of mafic magmas into the felsic magmas before initiation of crystallization of the latter, results in hybrid magmas under the influence of thermal and chemical exchange. The mechanical exchange occurs between the coexisting magmas due to viscosity contrast, if the mafic magma enters slightly later into the magma chamber, then the felsic magma starts to crystallize. Blobs of mafic magma form as MMEs in the felsic magma and they scatter throughout the pluton due to convection. At a later stage, if mafic magma enters the system after partial crystallization of felsic phase, mechanical interaction between the magmas leads to the formation of fragmented dyke or syn-plutonic mafic dyke. All these features are well-documented in the study area. Field and petrographic evidences suggest that the textural variations from Pithora region of Bastar Craton are the outcome of magma mingling, mixing and hybridization processes.     

浙东晚白垩世酸性岩浆的自混合作用及其意义

岩浆混合作用是造成火成岩多样性的主要原因之一,也是诱发火山喷发的重要机制。以往的研究多集中于基性和酸性岩浆之间的混合作用,但近年来酸性岩浆之间的混合作用受到越来越多的关注和研究。本文报道了浙东小雄破火山一个次级火山口内粗面质和流纹质两种酸性岩浆之间的混合现象。野外调查及岩相学研究显示,粗面质岩浆多呈大小不一的条带状以及透镜体状分布于流纹质岩浆内,局部发生扩散,粗面岩中斑晶大多为粗大的正长石斑晶,强烈熔蚀且聚斑结构普遍;在副矿物聚晶(由钛磁铁矿+磷灰石+锆石组成)的周围常可见反应边结构。流纹岩的斑晶主要由正长石、透长石及石英组成,晶体粒径较小,且熔蚀现象不发育。全岩主、微量元素特征及其他地质证据均显示,两种酸性岩浆之间以机械混合为主,其地球化学成分变化趋势主要受结晶分异过程控制。粗面质及流纹质岩浆在矿物组成、结构等方面的差异表明两者来源于同一层状岩浆房内的不同部位,其中粗面质岩浆应代表岩浆房底部及边部富晶体、贫熔体的粥状层部分(正长石+磁钛铁矿+锆石+磷灰石);而分异程度较高的流纹质岩浆则聚集于岩浆房上部形成富熔体、贫晶体的部分。两种酸性岩浆的混合现象是它们在地壳浅部层状岩浆房内自混合的结果,这一过程可能受岩浆房底部基性岩浆的聚集作用所控制,当更热、更基性的岩浆聚集时,岩浆房下部晶粥区内的粗面质岩浆迅速升温、活化,从而向上运移并与上部富熔体贫晶体的流纹质岩浆发生自混合作用。这一发现为我们理解中国东南沿海地区晚中生代大规模酸性火山喷发及岩浆演化机制、岩浆房结构提供了重要的参考,同时也为认识地壳浅部岩浆房内岩浆之间的自混合作用提供了可靠的例证。     

Magma mixing in the acidic-basic complex of Ardnamurchan: Implications on the evolution of shallow magma chambers

The Ardnamurchan net-veined complex consists of three rock types: a porphyritic mafic rock, an aphyric intermediate rock and a silicic rock. Pillows of mafic and intermediate rock are included in the silicic rock and contain crenulated and some chilled margins. Liquid-liquid relationships are inferred for these three magmas. The trace element data, using ratio-ratio plots, are consistent with magma mixing being the dominant process and are inconsistent with any process that is dominated by crystal fractionation or melting. The major element data, using multiple linear regression techniques, are consistent with magma mixing of high-silica silicic magma and primitive mafic magma, along with about 35 percent crystal fractionation to form the intermediate rock type. All of the data taken together are consistent with a magma mixing model with some fractionation to produce the variation observed. The simplest model is that a fractionating basaltic magma comes into contact with a silicic magma and limited mixing produces the intermediate magma.This net-veined complex may be the only evidence available for interaction of mafic and silicic melts that occurred in the Ardnamurchan high-level magma chamber before the silicic magma was lost to eruptive and surface processes. In general the chemical and field relationships are consistent with Smith's model for the evolution of high-level, magma chambers.     

Development of viscous fingering between mafic and felsic magmas: evidence from the Terra Nova Intrusive Complex (Antarctica)

Summary A wide range of types of contact morphology among mafic and felsic magmas are observed in outcrops on Vegetation Island (Terra Nova Intrusive Complex, Antarctica). Image analysis and fractal geometry techniques were applied for in-depth study of the mafic/felsic interface, with the aim of studying the origin of the varied morphologies. In particular, the length (IP_N) and fractal dimension (D_box) of interfaces were measured. Results indicate that there is a close exponential dependence of IP_N on D_box.The observed morphologies are identical to those observed during viscous fingering processes induced by the displacement of a more viscous fluid by a less viscous one. To test if viscous fingering was responsible in this case too, IP_N and D_box values were measured on viscous fingering structures obtained experimentally using various viscosity ratios (V_R) from the literature. Results indicate that, as in the natural case, there is an exponential dependence of IP_N on D_box, leading to the conclusion that the varied interface morphologies between mafic and felsic magmas are the result of viscous fingering dynamics. In addition, experimental studies clearly show that there is an exponential relationship between the viscosity ratio of fluids and the interface fractal dimension (D_box), and the ratio between the two types of magma was estimated using this relationship. It is shown that viscosity contrasts between mafic and felsic magmas varied considerably, ratios ranging from ca. 6 to 49. These results, together with outcrop evidence, provide indications regarding the evolution of the magmatic system, which generated the actual mafic/felsic associations on Vegetation Island.     

Remobilization of Highly Crystalline Felsic Magma by Injection of Mafic Magma: Constraints from the Middle Sixth Century Eruption at Haruna Volcano, Honshu, Japan

The latest eruption of Haruna volcano at Futatsudake took placein the middle of the sixth century, starting with a Plinianfall, followed by pyroclastic flows, and ending with lava domeformation. Gray pumices found in the first Plinian phase (lowerfall) and the dome lavas are the products of mixing betweenfelsic (andesitic) magma having 50 vol. % phenocrysts and maficmagma. The mafic magma was aphyric in the initial phase, whereasit was relatively phyric during the final phase. The aphyricmagma is chemically equivalent to the melt part of the phyricmafic magma and probably resulted from the separation of phenocrystsat their storage depth of 15 km. The major part of the felsicmagma erupted as white pumice, without mixing and heating priorto the eruption, after the mixed magma (gray pumice) and heatedfelsic magma (white pumice) of the lower fall deposit. Althoughthe mafic magma was injected into the felsic magma reservoir(at 7 km depth), part of the product (lower fall ejecta) precedederuption of the felsic reservoir magma, as a consequence ofupward dragging by the convecting reservoir of felsic magma.The mafic magma injection made the nearly rigid felsic magmaerupt, letting low-viscosity mixed and heated magmas open theconduit and vent. Indeed the lower fall white pumices preservea record of syneruptive slow ascent of magma to 2 km depth,probably associated with conduit formation. KEY WORDS: high-crystallinity felsic magma; magma plumbing system; multistage magma mixing; upward dragging of injected magma; vent opening by low-viscosity magma     

Nature and timing of magma interactions before,during, and after the caldera-forming eruption of Volcán Ceboruco,Mexico

Volcán Ceboruco, Mexico, erupted ~1,000 years ago, producing the Jala pumice and forming a ~4-km-wide caldera. During that eruption, 2.8 to 3.5 km³ of rhyodacite (~70 wt% SiO₂) magma and 0.2 to 0.5 km³ of mixed dacite (~67 wt% SiO₂) magma were tapped and deposited as the Jala pumice. Subsequently, the caldera was partially filled by extrusion of the Dos Equis dome, a low-silica (~64 wt% SiO₂) dacite dome with a volume of ~1.3 km³. Petrographic evidence indicates that the Jala dacite and Dos Equis dacite originated largely through the mixing of three end-member magmas: (1) rhyodacite magma, (2) dacite magma, and (3) mafic magma. Linear least-squares modeling and detailed modal analysis indicate that the Jala dacite is predominantly a bimodal mixture of rhyodacite and dacite with a small additional mafic component, whereas the Dos Equis dacite is composed of mostly dacite mixed with subordinate amounts of rhyodacite and mafic magma. According to Fe–Ti oxide geothermometry, before the caldera-forming eruption the rhyodacite last equilibrated at ~865 °C, whereas the dacite was originally at ~890 °C but was heated to ~960 °C by intrusion of mafic magma as hot as ~1,030 °C. Zoning profiles in plagioclase and/or magnetite phenocrysts indicate that mixing between mafic and dacite magma occurred ~34–47 days prior to eruption, whereas subsequent mixing between rhyodacite and dacite magmas occurred only 1–4 days prior to eruption. Following the caldera-forming eruption, continued inputs of mafic magma led to effusion of the Dos Equis dome dacite. In this case, timing between mixing and eruption is estimated at ~93–185 days based on the thickness of plagioclase overgrowth rims.Editorial responsibility: T.L. Grove     

Textural and compositional evidence for magma mixing and its mechanism,Abu volcano group,southwestern Japan

Quaternary basalts, andesites and dacites from the Abu monogenetic volcano group, SW Japan, (composed of more than 40 monogenetic volcanoes) show two distinct chemical trends especially on the FeO^*/MgO vs SiO₂ diagram. One trend is characterized by FeO^*/MgO-enrichment with a slight increase in SiO₂ content (Fe-type trend), whereas the other shows a marked SiO₂-enrichment with relatively constant FeO^*/MgO ratios (Si-type trend). The Fe-type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma. Rocks of the Si-type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and/or fine-grained basaltic inclusions. They are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz, sodic plagioclase and hornblende phenocrysts. Petrographic variation as well as chemical variation from basalt to dacite of the Si-type trend is accounted for by various mixing ratios of basalt and dacite magmas. Pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing. Olivine and spinel, and quartz, sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas, respectively, before the mixing. Within a lava flow, the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow, and the number of resorbed phenocrysts varies inversely, suggesting zonation in the magma chamber.The mode of mixing changes depending on the mixing ratio. In the mafic mixture, basalt and dacite magmas can mix in the liquid state (liquid-liquid mixing). In the silicic mixture, on the other hand, the basalt magma was quenched and formed inclusions (liquid-solid mixing). During mixing, the disaggregated basalt magma and the host dacite magma soon reached thermal equilibrium. Compositional homogenization of the mixed magma can occur only when the equilibrium temperature is sufficiently above the solidus of the basalt magma. The Si-type trend is chemically and petrographically similar to the calc-alkalic trend. Therefore, a calc-alkalic trend which is distinguished from a fractional crystallization trend (e.g. Fe-type trend) may be a product of magma mixing.     

Isotopic responses to basaltic injections into silicic magma chambers: a whole-rock and microsampling study of macrorhythmic units in the Pleasant Bay layered gabbro-diorite complex, Maine, USA

The Pleasant Bay layered gabbro-diorite complex (420 Ma) formed via repeated injections of mafic magma into a felsic magma chamber. It is dominated by repeating sequences (macrorhythmic units) with chilled gabbroic bases which may grade upward into medium-grained gabbro, diorite and granite. Each unit represents an injection of mafic magma into the chamber followed by differentiation. Increases in Sr_i and decreases in )_Ndi with stratigraphic height indicate open-system isotopic behaviour and exchange between the mafic and felsic magmas. Isotopic variations of whole-rock samples in individual macrorhythmic units do not conform to bulk mixing or AFC models between potential parental magmas. Sr isotopic studies of single feldspar crystals from one macrorhythmic unit indicate that exchange of crystals between the resident felsic magma and mafic influxes was important, that some of the rocks contain feldspar xenocrysts, and that the rocks are isotopically heterogeneous on an intercrystal scale. Xenocryst abundance increases with stratigraphic height, suggesting that crystal exchange occurred in situ. The lack of disequilibrium textures in the xenocrystic feldspar indicates the evolved macrorhythmic magma and resident silicic magma were of a similar composition and likely in thermal equilibrium at the time of crystal transfer. Mafic chilled margins are enriched in alkalis and isotopically evolved compared with mafic dikes (representing the parental melts) and suggest rapid in-situ diffusional exchange following emplacement of individual mafic replenishments.     

Magmatic inclusions in rhyolites,contaminated basalts,and compositional zonation beneath the Coso volcanic field,California

Basaltic lava flows and high-silica rhyolite domes form the Pleistocene part of the Coso volcanic field in southeastern California. The distribution of vents maps the areal zonation inferred for the upper parts of the Coso magmatic system. Subalkalic basalts (<50% SiO₂) were erupted well away from the rhyolite field at any given time. Compositional variation among these basalts can be ascribed to crystal fractionation. Erupted volumes of these basalts decrease with increasing differentiation. Mafic lavas containing up to 58% SiO₂, erupted adjacent to the rhyolite field, formed by mixing of basaltic and silicic magma. Basaltic magma interacted with crustal rocks to form other SiO₂-rich mafic lavas erupted near the Sierra Nevada fault zone.Several rhyolite domes in the Coso volcanic field contain sparse andesitic inclusions (55–61% SiO₂). Pillow-like forms, intricate commingling and local diffusive mixing of andesite and rhyolite at contacts, concentric vesicle distribution, and crystal morphologies indicative of undercooling show that inclusions were incorporated in their rhyolitic hosts as blobs of magma. Inclusions were probably dispersed throughout small volumes of rhyolitic magma by convective (mechanical) mixing. Inclusion magma was formed by mixing (hybridization) at the interface between basaltic and rhyolitic magmas that coexisted in vertically zoned igneous systems. Relict phenocrysts and the bulk compositions of inclusions suggest that silicic endmembers were less differentiated than erupted high-silica rhyolite. Changes in inferred endmembers of magma mixtures with time suggest that the steepness of chemical gradients near the silicic/mafic interface in the zoned reservoir may have decreased as the system matured, although a high-silica rhyolitic cap persisted.The Coso example is an extreme case of large thermal and compositional contrast between inclusion and host magmas; lesser differences between intermediate composition magmas and inclusions lead to undercooling phenomena that suggest smaller T. Vertical compositional zonation in magma chambers has been documented through study of products of voluminous pyroclastic eruptions. Magmatic inclusions in volcanic rocks provide evidence for compositional zonation and mixing processes in igneous systems when only lava is erupted.     

Chemical gradient at diffusive interfaces in magma chambers

The results of high pressure experiments on diffusion and Soret separation in natural silicate melts show that the diffusive behaviour between natural silicic and mafic magmas can be approximately modelled as if the system were a binary mixture of SiO₂ and other components such as MgO+FeO+CaO. Steady state compositional profiles across a diffusive interface between silicic and mafic magma layers are calculated on the basis of phenomenological relationships for the fluxes of chemical species and heat in the binary mixtures, using the experimental data of diffusion and Soret coefficients in natural silicate melts. The compositional profiles show a curvature with a minimum SiO₂ value within the interface due to the Soret effect and temperature dependence of diffusion coefficient. The compositional gradient at the lower half of the diffusive interface is similar to that resulting from the Soret separation of a mafic melt regardless of the composition of the silicic magmas. These results suggest that picritic magma can be formed in the interfacial region between the mafic and silicic magma layers. The compositional gradient explains chemical variation of mafic to picritic inclusions in a mixed andesite of the Abu Volcano Group, Japan.     

Amphibole perspective to unravel pre-eruptive processes and conditions in volcanic plumbing systems beneath intermediate arc volcanoes: a case study from Ciomadul volcano (SE Carpathians)

Ciomadul is the youngest volcano in the Carpathian–Pannonian region produced crystal-rich high-K dacites that contain abundant amphibole phenocrysts. The amphiboles in the studied dacites are characterized by large variety of zoning patterns, textures, and a wide range of compositions (e.g., 6.4–15 wt% Al₂O₃, 79–821 ppm Sr) often in thin-section scale and even in single crystals. Two amphibole populations were observed in the dacite: low-Al hornblendes represent a cold (<800 °C) silicic crystal mush, whereas the high-Al pargasites crystallized in a hot (>900 °C) mafic magma. Amphibole thermobarometry suggests that the silicic crystal mush was stored in an upper crustal storage (~8–12 km). This was also the place where the erupted dacitic magma was formed during the remobilization of upper crustal silicic crystal mush body by hot mafic magma indicated by simple-zoned and composite amphiboles. This includes reheating (by ~200 °C) and partial remelting of different parts of the crystal mush followed by intensive crystallization of the second mineral population (including pargasites). Breakdown textures of amphiboles imply that they were formed by reheating in case of hornblendes, suggesting that pre-eruptive heating and mixing could take place within days or weeks before the eruption. The decompression rim of pargasites suggests around 12 days of magma ascent in the conduit. Several arc volcanoes produce mixed intermediate magmas with similar bimodal amphibole cargo as the Ciomadul, but in our dacite the two amphibole population can be found even in a single crystal (composite amphiboles). Our study indicates that high-Al pargasites form as a second generation in these magmas after the mafic replenishment into a silicic capture zone; thus, they cannot unambiguously indicate a deeper mafic storage zone beneath these volcanoes. The simple-zoned and composite amphiboles provide direct evidence that significant compositional variations of amphiboles do not necessarily mean variation in the pressure of crystallization even if the Al-tschermak substitution can be recognized, suggesting that amphibole barometers that consider only amphibole composition may often yield unrealistic pressure variation.     

Mixing of rhyolite,trachyte and basalt magma erupted from a vertically and laterally zoned reservoir,composite flow P1, Gran Canaria

The 14.1 Ma composite welded ignimbrite P1 (45 km³ DRE) on Gran Canaria is compositionally zoned from a felsic lower part to a basaltic top. It is composed of four component magmas mixed in vertically varying proportions: (1) Na-rhyolite (10 km³) zoned from crystal-poor to highly phyric; (2) a continuously zoned, evolved trachyte to sodic trachyandesite magma group (6 km³); (3) a minor fraction of Na-poor trachyandesite (<1 km³); and (4) nearly aphyric basalt (26 km³) zoned from 4.3 to 5.2 wt% MgO. We distinguish three sites and phases of mixing: (a) Mutual mineral inclusions show that mixing between trachytic and rhyolitic magmas occurred during early stages of their intratelluric crystallization, providing evidence for long-term residence in a common reservoir prior to eruption. This first phase of mixing was retarded by increasing viscosity of the rhyolite magma upon massive anorthoclase precipitation and accumulation. (b) All component magmas probably erupted through a ring-fissure from a common upper-crustal reservoir into which the basalt intruded during eruption. The second phase of mixing occurred during simultaneous withdrawal of magmas from the chamber and ascent through the conduit. The overall withdrawal and mixing pattern evolved in response to pre-eruptive chamber zonation and density and viscosity relationships among the magmas. Minor sectorial variations around the caldera reflect both varying configurations at the conduit entrance and unsteady discharge. (c) During each eruptive pulse, fragmentation and particulate transport in the vent and as pyroclastic flows caused additional mixing by reducing the length scale of heterogeneities. Based on considerations of magma density changes during crystallization, magma temperature constraints, and the pattern of withdrawal during eruption, we propose that eruption tapped the P1 magma chamber during a transient state of concentric zonation, which had resulted from destruction of a formerly layered zonation in order to maintain gravitational equilibrium. Our model of magma chamber zonation at the time of eruption envisages a basal high-density Na-poor trachyandesite layer that was overlain by a central mass of highly phyric rhyolite magma mantled by a sheath of vertically zoned trachyte-trachyandesite magma along the chamber walls. A conventional model of vertically stacked horizontal layers cannot account for the deduced density relationships nor for the withdrawal pattern.     

Origin of composite dikes in the Gouldsboro granite, coastal Maine

Composite dikes, consisting of aphyric basaltic margins and phenocryst-rich rhyolitic interiors, cut the Gouldsboro granite of coastal Maine at many localities. Limited hybridization (exchange of crystals, commingling, and mixing) occurs in most of the dikes and indicates that the two magmas were contemporaneous with emplacement of rhyolitic magma following closely in time the initial emplacement of the basaltic dike. Petrographic characteristics and geochemistry indicate that the source of the rhyolite was resident magma in the Gouldsboro granite magma chamber. The composite dikes formed when basaltic dikes ruptured the Gouldsboro magma chamber, permitting partly crystallized magma from the margin of the chamber to flow outward into the center of the basaltic dikes. Field relations of similar composite dikes in other areas (e.g., Iceland, Scotland) are consistent with this model. A second type of composite dike (silicic margins with chilled basaltic pillows) commonly cuts mafic intrusions along the Maine coast and probably formed when a granitic dike ruptured an established chamber of mafic magma, permitting resident mafic magma to collapse downward into the still Liquid granitic dike. Most composite dikes have probably formed when a magma chamber was disrupted by a dike of contrasting magma rather than by tapping a stratified magma chamber.     

Convection and mixing in magma chambers

This paper reviews advances made during the last seven years in the application of fluid dynamics to problems of igneous petrology, with emphasis on the laboratory work with which the authors have been particularly involved. Attention is focused on processes in magma chambers which produce diversity in igneous rocks, such as fractional crystallization, assimilation and magma mixing. Chamber geometry, and variations in the density and viscosity of the magma within it, are shown to play a major role in determining the dynamical behaviour and the composition of the erupted or solidified products.Various convective processes are first reviewed, and in particular the phenomenon of double-diffusive convection. Two types of double-diffusive interfaces between layers of different composition and temperature are likely to occur in magma chambers. A diffusive interface forms when a layer of hot dense magma is overlain by cooler less dense magma. Heat is transported between the layers faster than composition, driving convection in both layers and maintaining a sharp interface between them. If a layer of hot slightly less dense magma overlies a layer of cooler, denser but compositionally lighter magma, a finger interface forms between them, and compositional differences are transported downwards faster than heat (when each is expressed in terms of the corresponding density changes).Processes leading to the establishment of density, compositional and thermal gradients or steps during the filling of a magma chamber are considered next. The stratification produced, and the extent of mixing between the inflowing and resident magmas, are shown to depend on the flow rate and on the relation between the densities and viscosities of the two components. Slow dense inputs of magma may mix very little with resident magma of comparable viscosity as they spread across the floor of the chamber. A similar pulse injected with high upward momentum forms a turbulent “fountain”, which is a very efficient mechanism for magma mixing, as is a turbulent plume of less dense magma rising through the host magma to the top of the chamber.The form of convection in the filled magma chamber is controlled by the shape and size of the chamber, the viscosity of the magma (through the Rayleigh number which is usually high in the early stages of cooling), and by processes at the boundary which produce lighter or denser fluid than that in the interior of the chamber. Compositional convection due to fluid released by crystallization often dominates over thermal convection. If crystallization at the bottom of a funnel-shaped chamber releases a light magma, this convects away from the floor, causing turbulent convection which tends to homogenize the overlying melt. If the magma released is dense, it flows down the sloping floor and stratifies the magma at the base of the chamber. Convection driven by crystallization in an inverted funnel has the reverse effect, e.g. dense fluid released at the sloping roof now has a homogenizing influence. Assimilation of wall rocks can also lead to identical dynamical effects and thus to zoning in magma chambers. Melting of a light roof, for instance, can produce a layer of cool felsic magma overlying the hotter more basic magma in the lower part of the chamber, with a diffusive interface between them. Assimilation has also been discussed for other geometries: assimilation of the walls of dykes, sills and lava flows can occur when the flow is hot and turbulent, whereas if the flow is laminar the magma will chill against the adjacent rocks and protect them from assimilation.When the magma in a chamber is layered, crystallization can cause the composition and density to change in several ways which may lead to mixing. A crystallizing lower layer of hot dense magma can evolve till it has the density of the magma above it, causing sudden overturning and thorough mixing. On the other hand, with a much more viscous layer above, light fluid is released continuously during crystallization and rises to the top of the chamber with little mixing. Overturning of a gas-rich mafic lower layer into a cooler silicic layer can cause a sudden quenching, with the rapid release of gas which could trigger an explosive eruption. Mixing can also occur during eruption, as two layers are drawn up simultaneously from a stratified chamber when a critical flow velocity is exceeded, and they then mix in the outlet vent. Laboratory experiments suggest, however, that magma mixing is inhibited by large viscosity differences, both during the filling and emptying of a magma chamber. Scaling these results to magmas indicates that a basaltic magma can flow into the bottom of a chamber containing rhyolite with little or no mixing between them, and that these two magma types can also flow out through the same exit vent with limited mixing.Each of the phenomena discussed in this review has been studied, at least in a qualitative way, using laboratory experiments to identify and understand a significant physical process occurring in magma chambers. The field of geological fluid mechanics and its application to these problems is still very new, and further advances seem assured as new phenomena are identified and more detailed and quantitative analogue experiments are developed to study them.     

The Influence of Viscosity on Fountains in Magma Chambers

Geological observations suggest that basaltic magmas mix readilybut that rhyolites and basalts can erupt through the same volcanicvent without mixing. The implication is that viscosity may havean important influence on the mixing of magmas. This paper examines the influence of viscosity on magma mixingwhen a magma of low viscosity is injected as a turbulent fountaininto a chamber containing a magma of higher viscosity. Threeseries of experiments were carried out. In the first, the lowviscosity fluid was injected into the tank at a Reynolds number(Re₁) of about 1000 to ensure that flow within the fountainwas fully turbulent. Both fluids were maintained at the sametemperature and the viscosity of the host fluid was varied systematically.If the viscosities of the two fluids are similar, turbulencein the fountain leads to extensive mixing and to formation ofa density-stratified layer which collects at the bottom of thetank. However, if the viscosity ratio of the fluids used isgreater than 400, no detectable mixing occurs. Motion withinthe fountain is again fully turbulent but the high momentumof the input fluid is not transmitted to the more viscous hostfluid. Experiments carried out at intermediate viscosity ratiosresulted in intermediate amounts of mixing. Theoretical considerations suggest that the criterion for mixingin turbulent fountains, when a fluid of low viscosity v₁ isinjected into a fluid of much higher viscosity v₂ is wd>kv₂ where w is the flow velocity through an input pipe of diameterd and k is a constant. This relationship is believed to be ageneral one, applying equally to jets and plumes, although thevalues of k may vary with the nature of the flow. The secondseries of experiments was designed to test this relationshipand determine values of k for fountains. The results show that,if wd/v₂ > 70, the inflowing fluid mixes with the host fluidas there was no viscosity difference between them. However,if wd/v₂ < 7, little or no mixing occurs even if motion withinthe fountain is fully turbulent. In the third series of experiments the temperature of the inputfluid was 70?C above that of the host fluid. This led to heatingofa thin boundary layer of the host fluid, lowering the viscosityof the host fluid adjacent to the fountain and thus allowingadditional mixing between the fluids. This effect was most obviousin the experiment with the highest viscosity ratio (1229) wherea small amount of mixing was detected, compared with no mixingin the equivalent experiment in the first series. The criterion for mixing, wd> kv₂ can be applied to naturalmagmas. The results show that if a primitive basaltic magmais injected, as a turbulent fountain or plume, into a chambercontaining fractionated basaltic magma, the two magmas willmix readily. However, if basaltic magma is injected into a chambercontaining granitic melt, little or no mixing occurs.