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.     

Enrichment of basalt and mixing of dacite in the rootzone of a large rhyolite chamber: inclusions and pumices from the Rattlesnake Tuff, Oregon

 A variety of cognate basalt to basaltic andesite inclusions and dacite pumices occur in the 7-Ma Rattlesnake Tuff of eastern Oregon. The tuff represents ∼280 km³ of high-silica rhyolite magma zoned from highly differentiated rhyolite near the roof to less evolved rhyolite at deeper levels. The mafic inclusions provide a window into the processes acting beneath a large silicic chamber. Quenched basaltic andesite inclusions are substantially enriched in incompatible trace elements compared to regional primitive high-alumina olivine tholeiite (HAOT) lavas, but continuous chemical and mineralogical trends indicate a genetic relationship between them. Basaltic andesite evolved from primitive basalt mainly through protracted crystal fractionation and multiple cycles (≥10) of mafic recharge, which enriched incompatible elements while maintaining a mafic bulk composition. The crystal fractionation history is partially preserved in the mineralogy of crystal-rich inclusions (olivine, plagioclase ± clinopyroxene) and the recharge history is supported by the presence of mafic inclusions containing olivines of Fo₈₀. Small amounts of assimilation (∼2%) of high-silica rhyolite magma improves the calculated fit between observed and modeled enrichments in basaltic andesite and reduces the number of fractionation and recharge cycles needed. The composition of dacite pumices is consistent with mixing of equal proportions of basaltic andesite and least-evolved, high-silica rhyolite. In support of the mixing model, most dacite pumices have a bimodal mineral assemblage with crystals of rhyolitic and basaltic parentage. Equilibrium dacite phenocrysts are rare. Dacites are mainly the product of mingling of basaltic andesite and rhyolite before or during eruption and to a lesser extent of equilibration between the two. The Rattlesnake magma column illustrates the feedback between mafic and silicic magmas that drives differentiation in both. Low-density rhyolite traps basalts and induces extensive fractionation and recharge that causes incompatible element enrichment relative to the primitive input. The basaltic root zone, in turn, thermally maintains the rhyolitic magma chamber and promotes compositional zonation. Received: 1 June 1998 / Accepted: 5 February 1999     

Silicic recharge of multiple rhyolite magmas by basaltic intrusion during the 22.6 ka Okareka Eruption Episode, New Zealand

Deposits of the 22.6 ka Okareka Eruption Episode from Tarawera Volcanic Complex record the sequential and simultaneous eruption of three discrete rhyolite magmas following a silicic recharge event related to basaltic intrusion. The episode started with basaltic eruption ( 0.01 km³ magma), and rapidly changed to a plinian eruption involving a moderate temperature (750 °C), cummingtonite-bearing rhyolite magma (T1) with a volume of  0.3 km³. Hybrid basalt/rhyolite clasts demonstrate direct basaltic intrusion that helped trigger the eruption. Crystals, shards and lapilli of two other rhyolite magmas then joined the eruption sequence. They comprise a cooler (720 °C) crystal-rich biotite–hornblende rhyolite magma (T2) ( 0.3 km³), and a hotter (780 °C), crystal-poor, pyroxene–hornblende rhyolite magma (T3) ( 4.5 km³). All mid to late-stage ash units contain various mixtures of T1, T2 and T3 components with a general increase in abundance of T3 and rapid decline of T1 with time. About 4 km³ of T3 magma was extruded as lavas at the end of the episode. Contrasts in melt composition, crystal and volatile contents, and temperatures influenced viscosity and miscibility, and thus limited pre-eruption mixing of the rhyolite magmas. The eruption sequence and the restricted direct basaltic intrusion into only one magma (T1) is consistent with the rhyolites occupying separate melt pods within a large crystal-mush zone. Melt–crystal equilibria and volatile contents in melt inclusions indicate temporary magma storage depths of < 8 km. Each of the magmas display quartz crystals containing melt inclusions that are compositionally highly evolved relative to the accompanying matrix glass, and thus point to a stage of more complete crystallisation. The matrix glass, enriched in Sr and Ti, represents a re-melting event of underlying the crystal pile induced by basaltic intrusion, presumably part of the same event that erupted scoria at the start of the eruption. This recharge rhyolite melt percolated upward and hybridised with the resident melts in each of the three magma pods. The Okareka episode rhyolites contrast with other well-documented rhyolites that are either continuously or discontinuously zoned, or have been homogenised during re-activation to a uniform composition. Rapid basalt dike intrusion to shallow levels appears to have (prematurely?) triggered the Okareka rhyolites into eruption, so that their early ponding in separate melt pods has been recorded before it could be masked by mixing or stratification had amalgamation into a larger body occurred.     

A cascade of magmatic events during the assembly and eruption of a super-sized magma body

We use comprehensive geochemical and petrological records from whole-rock samples, crystals, matrix glasses and melt inclusions to derive an integrated picture of the generation, accumulation and evacuation of 530 km³ of crystal-poor rhyolite in the 25.4 ka Oruanui supereruption (New Zealand). New data from plagioclase, orthopyroxene, amphibole, quartz, Fe–Ti oxides, matrix glasses, and plagioclase- and quartz-hosted melt inclusions, in samples spanning different phases of the eruption, are integrated with existing data to build a history of the magma system prior to and during eruption. A thermally and compositionally zoned, parental crystal-rich (mush) body was developed during two periods of intensive crystallisation, 70 and 10–15 kyr before the eruption. The mush top was quartz-bearing and as shallow as ~3.5 km deep, and the roots quartz-free and extending to >10 km depth. Less than 600 year prior to the eruption, extraction of large volumes of ~840 °C low-silica rhyolite melt with some crystal cargo (between 1 and 10%), began from this mush to form a melt-dominant (eruptible) body that eventually extended from 3.5 to 6 km depth. Crystals from all levels of the mush were entrained into the eruptible magma, as seen in mineral zonation and amphibole model pressures. Rapid translation of crystals from the mush to the eruptible magma is reflected in textural and compositional diversity in crystal cores and melt inclusion compositions, versus uniformity in the outermost rims. Prior to eruption the assembled eruptible magma body was not thermally or compositionally zoned and at temperatures of ~790 °C, reflecting rapid cooling from the ~840 °C low-silica rhyolite feedstock magma. A subordinate but significant volume (3–5 km³) of contrasting tholeiitic and calc-alkaline mafic material was co-erupted with the dominant rhyolite. These mafic clasts host crystals with compositions which demonstrate that there was some limited pre-eruptive physical interaction of mafic magmas with the mush and melt-dominant body. However, the mafic magmas do not appear to have triggered the eruption or controlled magmatic temperatures in the erupted rhyolite. Integration of textural and compositional data from all available crystal types, across all dominant and subordinate magmatic components, allow the history of the Oruanui magma body to be reconstructed over a wide range of temporal scales using multiple techniques. This history spans the tens of millennia required to grow the parental magma system (U–Th disequilibrium dating in zircon), through the centuries and decades required to assemble the eruptible magma body (textural and diffusion modelling in orthopyroxene), to the months, days, hours and minutes over which individual phases of the eruption occurred, identified through field observations tied to diffusion modelling in magnetite, olivine, quartz and feldspar. Tectonic processes, rather than any inherent characteristics of the magmatic system, were a principal factor acting to drive the rapid accumulation of magma and control its release episodically during the eruption. This work highlights the richness of information that can be gained by integrating multiple lines of petrologic evidence into a holistic timeline of field-verifiable processes.     

Origin of metaluminous and alkaline volcanic rocks of the Latir volcanic field,northern Rio Grande rift,New Mexico

Volcanic rocks of the Latir volcanic field evolved in an open system by crystal fractionation, magma mixing, and crustal assimilation. Early high-SiO₂ rhyolites (28.5 Ma) fractionated from intermediate compositionmagmas that did not reach the surface. Most precaldera lavas have intermediate-compositions, from olivine basaltic-andesite (53% SiO₂) to quartz latite (67% SiO₂). The precaldera intermediate-composition lavas have anomalously high Ni and MgO contents and reversely zoned hornblende and augite phenocrysts, indicating mixing between primitive basalts and fractionated magmas. Isotopic data indicate that all of the intermediate-composition rocks studied contain large crustal components, although xenocrysts are found only in one unit. Inception of alkaline magmatism (alkalic dacite to high-SiO₂ peralkaline rhyolite) correlates with, initiation of regional extension approximately 26 Ma ago. The Questa caldera formed 26.5 Ma ago upon eruption of the >500 km³ high-SiO₂ peralkaline Amalia Tuff. Phenocryst compositions preserved in the cogenetic peralkaline granite suggest that the Amalia Tuff magma initially formed from a trace element-enriched, high-alkali metaluminous magma; isotopic data suggest that the parental magmas contain a large crustal component. Degassing of water- and halogen-rich alkali basalts may have provided sufficient volatile transport of alkalis and other elements into the overlying silicic magma chamber to drive the Amalia Tuff magma to peralkaline compositions. Trace element variations within the Amalia Tuff itself may be explained solely by 75% crystal fractionation of the observed phenocrysts. Crystal settling, however, is inconsistent with mineralogical variations in the tuff, and crystallization is thought to have occurred at a level below that tapped by the eruption. Spatially associated Miocene (15-11 Ma) lavas did not assimilate large amounts of crust or mix with primitive basaltic magmas. Both mixing and crustal assimilation processes appear to require development of relatively large magma chambers in the crust that are sustained by large basalt fluxes from the mantle. The lack of extensive crustal contamination and mixing in the Miocene lavas may be related to a decreased basalt flux or initiation of blockfaulting that prevented pooling of basaltic magma in the crust.     

Magmatic processes under Quizapu volcano,Chile, identified from geochemical and textural studies

Quizapu is part of a linear system of active volcanos in central Chile. The volcanic petrology and geology have been used to infer the plumbing system beneath the volcano. The 1846–1847 eruption (~5 km³) started with small flows of dacite, then changed to a range of andesite–dacite compositions and finally terminated with large flows of dacite. Andesitic enclaves (<10 %) occur in some of these flows. Activity restarted explosively in 1932 (~4 km³ DRE) with an initial andesite–dacite ash, followed by uniform dacite ash and then a terminal andesite ash. All samples, including the enclaves, have chemical compositions that lie on an almost perfect mixing line, with a few exceptions. The abundant plagioclase macrocrysts in the matrix were divided into five petrographic classes on the basis of colour in cold-cathode cathodoluminescence images and zonation in visible light. All populations of macrocrysts have CSDs characteristic of coarsening, although they differ in detail. Two classes can be ascribed to growth in andesite and dacite magmas, but the three other classes are associated with particular magma batches. A model is developed which started with ponding of andesite magma in the crust. This differentiated to produce a dacite magma, most of which probably solidified to make a granodiorite batholith. Activation of a N–S fault enabled volcanism: andesite magma traversed the dacite-filled chamber, heating and raising it up into storage areas hosted by the fault, where it mixed to form a homogeneous magma. A short time before the 1846–1847 eruption, more andesite magma was injected into the shallow part of the system where it mingled with existing mixed magmas. The first magma to be erupted from Quizapu was a dacite, but soon other storage areas along the fault started to feed the system—first mixed magmas, then back to dacites. The eruption then terminated until 1932 when renewed injection of andesite into the system created a conduit that tapped an undegassed dacite chamber and resulted in a strong explosive eruption. The whole story is one of continual andesite magmatism, modulated by storage, degassing and mixing.     

The 26{middle dot}5 ka Oruanui Eruption, Taupo Volcano, New Zealand: Development, Characteristics and Evacuation of a Large Rhyolitic Magma Body

The caldera-forming 26·5 ka Oruanui eruption (Taupo,New Zealand) erupted 530 km³ of magma, >99% rhyolitic, <1%mafic. The rhyolite varies from 71·8 to 76·7 wt% SiO₂ and 76 to 112 ppm Rb but is dominantly 74–76 wt% SiO₂. Average rhyolite compositions at each stratigraphiclevel do not change significantly through the eruption sequence.Oxide geothermometry, phase equilibria and volatile contentsimply magma storage at 830–760°C, and 100–200MPa. Most rhyolite compositional variations are explicable by28% crystal fractionation involving the phenocryst and accessoryphases (plagioclase, orthopyroxene, hornblende, quartz, magnetite,ilmenite, apatite and zircon). However, scatter in some elementconcentrations and ⁸⁷Sr/⁸⁶Sr ratios, and the presence of non-equilibriumcrystal compositions imply that mixing of liquids, phenocrystsand inherited crystals was also important in assembling thecompositional spectrum of rhyolite. Mafic compositions comprisea tholeiitic group (52·3–63·3 wt % SiO₂)formed by fractionation and crustal contamination of a contaminatedtholeiitic basalt, and a calc-alkaline group (56·7–60·5wt % SiO₂) formed by mixing of a primitive olivine–plagioclasebasalt with rhyolitic and tholeiitic mafic magmas. Both maficgroups are distinct from other Taupo Volcanic Zone eruptivesof comparable SiO₂ content. Development and destruction by eruptionof the Oruanui magma body occurred within 40 kyr and Oruanuicompositions have not been replicated in vigorous younger activity.The Oruanui rhyolite did not form in a single stage of evolutionfrom a more primitive forerunner but by rapid rejuvenation ofa longer-lived polygenetic, multi-age ‘stockpile’of silicic plutonic components in the Taupo magmatic system. KEY WORDS: Taupo Volcanic Zone; Taupo volcano; Oruanui eruption; rhyolite, zoned magma chamber; juvenile mafic compositions; eruption withdrawal systematics     

Lead and strontium isotopic evidence for crustal interaction and compositional zonation in the source regions of Pleistocene basaltic and rhyolitic magmas of the Coso volcanic field,California

The isotopic compositions of Pb and Sr in Pleistocene basalt, high-silica rhyolite, and andesitic inclusions in rhyolite of the Coso volcanic field indicate that these rocks were derived from different levels of compositionally zoned magmatic systems. The 2 earliest rhyolites probably were tapped from short-lived silicic reservoirs, in contrast to the other 36 rhyolite domes and lava flows which the isotopic data suggest may have been leaked from the top of a single, long-lived magmatic system. Most Coso basalts show isotopic, geochemical, and mineralogic evidence of interaction with crustal rocks, but one analyzed flow has isotopic ratios that may represent mantle values (⁸⁷Sr/⁸⁶Sr=0.7036,²⁰⁶Pb/²⁰⁴Pb=19.05,²⁰⁷Pb/²⁰⁴Pb=15.62,²⁰⁸Pb/²⁰⁴Pb= 38.63). The (initial) isotopic composition of typical rhyolite (⁸⁷Sr/⁸⁶Sr=0.7053,²⁰⁶Pb/²⁰⁴Pb=19.29,²⁰⁷Pb/²⁰⁴Pb= 15.68,²⁰⁸Pb/²⁰⁴Pb=39.00) is representative of the middle or upper crust. Andesitic inclusions in the rhyolites are evidently samples of hybrid magmas from the silicic/mafic interface in vertically zoned magma reservoirs. Silicic end-member compositions inferred for these mixed magmas, however, are not those of erupted rhyolite but reflect the zonation within the silicic part of the magma reservoir. The compositional contrast at the interface between mafic and silicic parts of these systems apparently was greater for the earlier, smaller reservoirs.     

Mixed Magmas, Mush Chambers and Eruption Triggers: Evidence from Zoned Clinopyroxene Phenocrysts in Andesitic Scoria from the 1995 Eruptions of Ruapehu Volcano, New Zealand

Juvenile ejecta from the September and October 1995 eruptionsof Ruapehu volcano, New Zealand, indicate that mixing occurredbetween relatively higher- and lower-temperature (high-T andlow-T) andesitic magmas. Compositional zonations in clinopyroxenephenocrysts provide direct evidence for a pre-eruption crystal–meltmush chamber containing low-T magma, and elucidate the processesof magma mixing and eruption, following the injection of high-Tmagma. Many phenocrysts with Fe-rich cores derived from low-Tmagma have extremely reverse zoned mantles around slightly resorbedcores. Mg-value [100Mg/(Mg + Fe)] increases from 65–70to     

Post-caldera dacites from the Santorini volcanic complex,Aegean Sea,Greece: an example of the eruption of lavas of near-constant composition over a 2,200 year period

The post-caldera Kameni islands of the Santorini volcanic complex, Aegean Sea, Greece are entirely volcanic and were formed by eleven eruptions between 197 B.C. and 1950. Petrographic, mineral chemical and whole-rock major and trace element data are presented for samples of lava collected from the products of seven eruptive cycles which span the entire period of activity. The main phenocryst phases are plagioclase, clinopyroxene, orthopyroxene and titaniferous magnetite, which are weakly zoned (e.g. plagioclase — An₅₅ to An₄₂). The lavas are typical calc-alkaline dacites and show a restricted range of composition (from 64.1 to 68.4 wt. % SiO₂). The phenocrysts were in equilibrium with the melts at temperatures of 960–1012 °C, pressures of 800–1500 bars and oxygen fugacities of 10^–9.6-10^–9.9 bars. The pre-eruptive water content of the magmas was 3–4 wt. % but since the lavas contain only 0.1–0.4 wt. % H₂O, a considerable amount (about 0.01–0.015 km³) of water was lost prior to or during eruption. This indicates that the magmas rose to the surface gradually allowing the (largely) non-explosive loss of volatiles. The lavas were probably extruded initially from more or less cylindrical conduits which developed into fissures as the eruptions proceeded. The post-caldera lavas evolved from more mafic parental magmas (basalt-andesite) via fractional crystallization. The small range of compositional variation shown by these lavas can be explained in terms of near-equilibrium crystallization. Analyses of samples of lavas belonging to single eruption cycles and to individual flows indicate that the underlying magma chamber is compositionally zoned. The average composition of erupted magma has remained approximately constant since 1570 A.D. but that fact that the 197 B.C. magma was sligthly richer in SiO₂ provides additional evidence that the magma chamber is compositionally zoned. Crystal settling has not affected the composition of the magma over a 2,200 year period of time which indicates that the melts do not behave as Newtonian fluids. Zonation was thus probably established prior to the 197 B.C. eruption though it is possible that it is developed and maintained by crystal-liquid differentiation processes other than crystal settling (e.g. boundary layer crystallization). The data indicate that there has been no significant cooling during 2,200 years; the maximum amount of cooling is <50 °C and is probably less than 30 °C. Two hypotheses are considered to explain the thermal and chemical buffering of the post-caldera magma chamber: (i) The magma chamber is large and heat losses due to conduction are largely compensated by latent heat supplied by thick, partially crystalline cumulate sequences. (ii) Periodic influx of hot mafic magma, which does not mix with the dacitic magma, inhibits cooling. The second alternative is favored because the post-caldera lavas differ geochemically from the pre-caldera lavas which signifies that a new batch of magma was formed and/or emplaced after the catastrophic eruption of 1390 B.C., and hence that mafic magmas may still be reaching upper crustal levels.     

长白山天池火山粗面玄武岩的喷发历史与演化

本文新提出的年代学和岩石化学结果，进一步从天池火山与区域火山活动的关系，论述了天池火山造盾、造锥历史和岩浆结晶分异转型的时间约束，早更新世早期（2Ma前）开始粗面玄武岩的造盾，早更新世晚期（约1Ma）粗面玄武岩向粗安岩、粗面岩演化，中更新世是粗面岩造锥的主阶段，到了晚更新世（约0．1Ma）粗面岩向碱流岩演化。在中-晚更新世来自地壳岩浆房的粗面岩、碱流岩造锥过程中，来自地幔的粗面玄武岩浆喷发活动始终没有间断过。由于来自地幔粗面玄武质岩浆持续向地壳岩浆房补给，所以天池火山是一座长寿命的火山。岩浆的结晶分异作用和混合作用是天池火山岩浆演化的两个最重要过程，前者形成天池火山双峰式火山岩分布特征，后者成为天池火山喷发的触发机制。天池火山在晚更新世-全新世碱流质岩浆主喷发期兼有少量玄武质粗安岩、粗安岩或粗面质岩浆的交替喷出，揭示了天池火山的地壳岩浆房熔体的分层结构特点，由于来自地幔粗面玄武质岩浆注入地壳岩浆房，导致不同层位岩浆的扰动和混合作用，触发天池火山的喷发。     

Mineral and chemical variations within an ash-flow sheet from Aso caldera,Southwestern Japan

Although products of individual volcanic eruptions, especially voluminous ash-flow eruptions, have been considered among the best available samples of natural magmas, detailed petrographic and chemical study indicates that bulk compositions of unaltered Pleistocene ash-flow tuffs from Aso caldera, Japan, deviate significantly from original magmatic compositions.The last major ash-flow sheet from Aso caldera is as much as 150 meters thick and shows a general vertical compositional change from phenocryst-poor rhyodacite upward into phenocryst-rich trachyandesite; this change apparently reflects in inverse order a compositionally zoned magma chamber in which more silicic magma overlay more mafic magma. Details of these magmatic variations were obscured, however, by: (1) mixing of compositionally distinct batches of magma during upwelling in the vent, as indicated by layering and other heterogeneities within single pumice lumps; (2) mixing of particulate fragments—pumice lumps, ash, and phenocrysts—of varied compositions during emplacement, with the result that separate pumice lenses from a single small outcrop may have a compositional range nearly as great as the bulk-rook variation of the entire sheet; (3) density sorting of phenocrysts and ash during eruption and emplacement, resulting in systematic modal variations with distance from the caldera; (4) addition of xenocrysts, resulting in significant contamination and modification of proportions of crystals in the tuffs; and (5) ground-water leaching of glassy fractions during hydration after cooling.Similar complexities characterize ash-flow tuffs under study in southwestern Nevada and in the San Juan Mountains, Colorado, and probably are widespread in other ash-flow fields as well. Caution and careful planning are required in study of the magmatic chemistry and phenocryst mineralogy of these rocks.Publication authorized by the Director, U. S. Geological Survey.     

Pre-eruptive magma mixing in ash-flow deposits of the Tertiary Rum Igneous Centre,Scotland

The Northern Marginal Zone of the Rum Igneous Centre is a remnant of an early caldera and its infill. It is composed of intra-caldera breccias and various small-volume pyroclastic deposits, overlain by prominent rhyodacite ash-flow sheets of up to 100 m thickness. The ash-flows were fed from a feeder system near the caldera ring-fault, and intrusive rhyodacite can locally be seen grading into extrusive deposits. A variety of features suggest that the ash-flows were erupted from a magma chamber that contemporaneously hosted felsic and mafic magmas: (i) chilled basaltic inclusions in rhyodacite; (ii) formerly glassy basaltic to andesitic enclaves with fluid-fluid relationships; (iii) feldspars with thick reaction rims enclosed in the basaltic to andesitic inclusions, yet with cores chemically resembling those of the rhyodacite: (iv) trace element compositions of the rhyodacite and the mafic enclaves form a mixing line between the end-member rhyodacite and basalt compositions. Additionally, textural and chemical features in the rhyodacite feldspar phenocrysts are consistent with magma mixing; (v) feldspars with resorption embayments cutting through internal zonation of the crystals; (vi) complexly zoned crystals with sieve-textured zones that are overgrown with euhedral zones; (vii) oscillatory zonation of feldspar phenocrysts in the rhyodacite, showing sharp increases in anorthite (An 10%) followed by gradual decrease in An-content (An 4%). This evidence points to eruption of ash-flows from a felsic magma chamber that was periodically replenished by injection of mafic magma. Diffusional mixing between the two magmas was permitted by temperature and compositional differences, but was slow due to the contrast in viscosities and densities. The Fe–Ti–P-enriched basic magma that replenished the chamber was degassing on entering the lower temperature environment and soon equilibrated thermally, followed by chemical exchange between the two end-member magmas. This process formed hybrid andesite enclaves enriched in trace elements beyond that caused by simple mixing, implying trace element diffusion in addition to bulk mixing. Eruption was caused by replenishment with, and degassing of, the basic magma and the chamber partially evacuated while the process of hybridisation was underway. The erupted products record magma mixing by chamber replenishment, blending of two magmas and elemental exchange in the magma chamber, and also physical mingling in the eruptive conduit.     

Relationships between Kuroko volcanogenic massive sulfide (VMS) deposits, felsic volcanism, and island arc development in the northeast Honshu arc, Japan

The northeast (NE) Honshu arc was formed by three major volcano-tectonic events resulting from Late Cenozoic orogenic movement: continental margin volcanism (before 21?Ma), seafloor basaltic lava flows and subsequent bimodal volcanism accompanied by back-arc rifting (21 to 14?Ma), and felsic volcanism related to island arc uplift (12 to 2?Ma). Eight petrotectonic domains, parallel to the NE Honshu arc, were formed as a result of the eastward migration of volcanic activity with time. Major Kuroko volcanogenic massive sulfide (VMS) deposits are located within the eastern marginal rift zone (Kuroko rift) that formed in the final period of back-arc rifting (16 to 14?Ma). Volcanic activity in the NE Honshu arc is divided into six volcanic stages. The eruption volumes of volcanic rocks have gradually decreased from 4,600?km³ (per 1?my for a 200-km-long section along the arc) of basaltic lava flows in the back-arc spreading stage to 1,000?C2,000?km³ of bimodal hyaloclastites in the back-arc rift stage, and about 200?km³ of felsic pumice eruptions in the island arc stage. The Kuroko VMS deposits were formed at the time of abrupt decrease in the eruption volume and change in the mode of occurrence of the volcanic rocks during the final period of back-arc rifting. In the area of the Kuroko rift, felsic volcanism changed from aphyric or weakly plagioclase phyric (before 14?Ma), to quartz and plagioclase phyric with minor clinopyroxene (12 to 8?Ma), to hornblende phyric (after 8?Ma), and hornblende and biotite phyric (after 4?Ma). The Kuroko VMS deposits are closely related to the aphyric rhyolitic activity before 14?Ma. The rhyolite was generated at a relatively high temperature from a highly differentiated part of felsic magma seated at a relatively great depth and contains higher Nb, Ce, and Y contents than the post-Kuroko felsic volcanism. The Kuroko VMS deposits were formed within a specific tectonic setting, at a specific period, and associated with a particular volcanism of the arc evolution process. Therefore, detailed study of the evolutional process from rift opening to island arc tectonics is very important for the exploration of Kuroko-type VMS deposits.     

Time Scales of Crystal Fractionation in Magma Chambers--Integrating Physical, Isotopic and Geochemical Perspectives

A simple heat balance model for an evolving magma chamber isused to make predictions of the time scales for magma differentiation,which are compared with geological and isotopic constraintson the rates of crystallization and differentiation. In a 10km³ magma chamber releasing thermal energy at a rate of 100MW, basalt and rhyolite magmas should reach 50% crystallizationafter     

Compositional evolution of the zoned calcalkaline magma chamber of Mount Mazama,Crater Lake,Oregon

The climactic eruption of Mount Mazama has long been recognized as a classic example of rapid eruption of a substantial fraction of a zoned magma body. Increased knowledge of eruptive history and new chemical analyses of 350 wholerock and glass samples of the climactic ejecta, preclimactic rhyodacite flows and their inclusions, postcaldera lavas, and lavas of nearby monogenetic vents are used here to infer processes of chemical evolution of this late Pleistocene — Holocene magmatic system. The 6845±50 BP climactic eruption vented 50 km³ of magma to form: (1) rhyodacite fall deposit; (2) welded rhyodacite ignimbrite; and (3) lithic breccia and zoned ignimbrite, these during collapse of Crater Lake caldera. Climactic ejecta were dominantly homogeneous rhyodacite (70.4±0.3% SiO₂), followed by subordinate andesite and cumulate scoriae (48–61% SiO₂). The gap in wholerock composition reflects mainly a step in crystal content because glass compositions are virtually continuous. Two types of scoriae are distinguished by different LREE, Rb, Th, and Zr, but principally by a twofold contrast in Sr content: High-Sr (HSr) and low-Sr (LSr) scoriae. HSr scoriae were erupted first. Trace element abundances indicate that HSr and LSr scoriae had different calcalkaline andesite parents; basalt was parental to some mafic cumulate scoriae. Parental magma compositions reconstructed from scoria wholerock and glass data are similar to those of inclusions in preclimactic rhyodacites and of aphyric lavas of nearby monogenetic vents.Preclimactic rhyodacite flows and their magmatic inclusions give insight into evolution of the climactic chamber. Evolved rhyodacite flows containing LSr andesite inclusions were emplaced between 30000 and 25000 BP. At 7015±45 BP, the Llao Rock vent produced a zoned rhyodacite pumice fall, then rhyodacite lava with HSr andesite inclusions. The Cleetwood rhyodacite flow, emplaced immediately before the climactic eruption and compositionally identical to climactic rhyodacite (volatile-free), contains different HSr inclusions from Llao Rock. The change from LSr to HSr inclusions indicates replenishment of the chamber with andesite magma, perhaps several times, in the latest Pleistocene to early Holocene.Modeling calculations and wholerock-glass relations suggest than: (1) magmas were derived mainly by crystallization differentiation of andesite liquid; (2) evolved preclimactic rhyodacite probably was derived from LSr andesite; (3) rhyodacites contain a minor component of partial melt from wall rocks, and (4) climactic and compositionally similar rhyodacites probably formed by mixing of evolved rhyodacite with HSr derivative liquid(s) after replenishment of the chamber with HSr andesite magma. Density considerations permit a model for growth and evolution of the chamber in which andesite recharge magma ponded repeatedly between cumulates and rhyodacite magma. Convective cooling of this andesite resulted in rapid crystallization and upward escape of buoyant derivative liquid which mixed with overlying, convecting rhyodacite. The evolved rhyodacites were erupted early in the chamber's history and(or) near its margins. Postcaldera andesite lavas may be hybrids composed of LSr cumulates mixed with remnant climactic rhyodacite. Younger postcaldera rhyodacite probably formed by fractionation of similar andesite and assimilation of partial melts of wallrocks.Uniformity of climactic rhyodacite suggests homogeneous silicic ejecta from other volcanoes resulted from similar replenishment-driven convective mixing. Calcalkaline pluton compositions and their internal zonation can be interpreted in terms of the Mazama system frozen at various times in its history.     

Processes and timescales in the evolution of a chemically zoned trachyte: Fogo A,Sao Miguel,Azores

U-series disequilibria analyses have been combined with chemical and petrographic analyses in order to assess both the timescales and processes involved in the formation of the chemically zoned Fogo A trachytes. Least squares major element modelling demonstrates that the mafic trachytes could have evolved from a parental alkali basalt via trachybasalt with 70% fractionation of augite (35–36%), plagioclase (23%), magnetite (16%), kaersutite (15%), olivine (8%) and apatite (2–3%). Derivation of the mafic trachytes from a basanite parent is inconsistent with calculated fractionation paths. Major and trace element variations in 25 pumice samples collected from throughout the stratigraphic extent of the Fogo A deposit show that the trachytes represent the inverted, extrusive equivalent of a strongly chemically zoned magma chamber. The zonation is attributed to 70–75% Rayleigh fractional crystallization of the observed phenocryst phases. Wallrock assimilation and magma mixing did not contribute significantly to the observed chemical trends. The maximum age of the Fogo A trachytic magma based on radioactive disequilibria between ²³⁰Th and ²³⁸U is 300000 years. However, a calculated model age suggests that the time of evolution of the Fogo A trachytes from a parent alkali basalt is only 90000 years. Constant element variations and Th-isotopic ratios in Fogo C, Fogo A and 1563 A.D. trachytes suggest that a single long-lived trachytic magma chamber has been the source of at least the past 15.2 Ka of trachytic volcanism from Agua de Pao. After each eruption an evolved cupola reformed and became zoned prior to the next eruption. The maximum time necessary to form the zonation is 4600 years, the time between the Fogo A and 1563 A.D. eruptions. Low (²²⁶Ra)/(²³⁰Th)_i ratios in the Fogo A and 1563 A.D. trachytes suggest that alkali feldspar fractionation continued up to the time of the respective eruptions.     

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     

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

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

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.