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.     

EXPERIMENTAL STUDIES OF MAGMA MIXING

The paper is devoted to experiments on mixing of natural melts of different compositions at 1300-1850° C and 1-12 kbars. Two series of experiments were carried out: one involving gravity-driven convective mixing and one involving diffusive mixing. The results demonstrate the effectiveness of mixing of contrasting magmas in the course of relative motion. Less viscous mafic melt transforms into andesitic much more easily than viscous silicic melt. The latter tends to “dissolve” into the mafic melt. Diffusive runs revealed selective behavior of alkalies and other components due to diffusion. Uphill diffusion of alkalies may cause double-diffusive convection in intercoupled melts. Diffusive interaction of two contrasting melts is explained as a multistage chemical reaction following the principle of acid-base interaction of components in silicate melts.     

An experimental study of anorthosite dissolution in lunar picritic magmas: Implications for crustal assimilation processes

Experiments characterizing the kinetics of anorthosite dissolution in lunar picritic magmas (very low-Ti, low-Ti, and high-Ti picritic glasses) were conducted at 0.6 GPa and 1250-1400 °C using the dissolution couple method. Reaction between the anorthosite and lunar picritic magmas at 1250-1300 °C produced a spinel + melt layer. Reaction between the anorthosite and an olivine-saturated low-Ti magma at 1250-1300 °C produced a crystal-free region between the spinel + melt layer and the olivine-saturated magma. The anorthosite dissolution experiments conducted at 1400 °C simply dissolved anorthosite and did not result in a crystal-bearing region. The rate of anorthosite dissolution strongly depends on temperature and composition of the reacting melt. Concentration profiles that develop during anorthosite dissolution are nonlinear and extend from the picritic glass compositions to anorthite. These profiles feature a large and continuous variation in melt density and viscosity from the anorthosite-melt interface to the initial picritic magmas. In both the low-Ti and high-Ti magmas the diffusive fluxes of TiO₂, Al₂O₃, and SiO₂ are strongly coupled to the concentration gradients of CaO and FeO. Anorthosite dissolution may play an important role in producing the chemical variability of the lunar picritic magmas, the origin of spinel in the lunar basalts and picritic glasses, and the petrogenesis of the high-Al basalts.     

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.     

Solution properties of silicate liquids from thermal diffusion experiments

Experimental thermal diffusion (Soret) fractionation of silicate liquids shows SiO₂ to be the principal ingredient of the component fractionating to the hot end of experimental charges. The chemical characteristics of disequilibrium Soret separations resemble those of equilibrium 2-liquid phase separation. This reflects the fact that liquid structural constitution does not depend upon the process leading to chemical separation. The activity-composition relations which lead to silicate liquid immiscibility strongly influence the Soret fractionation behavior of liquids near the solvus. This is especially well illustrated in the system Fa-Lc-Q. The temperature versus composition features of the steady state thermal diffusion profiles may be adequately described by an asymmetric regular solution model for silicate liquids, in contrast to any simpler solution model. This model can be used to quantify Margules parameters and the heat of transport for SiO₂ in these silicate liquids. Observation of the rise time and steady state values of Soret fractionation allows calculation of ordinary and thermal diffusion coefficients for SiO₂.     

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.     

Experimental investigation of K and Na partitioning between miscible liquids

Model silicate melts with variable Al₂O₃ and SiO₂ contents were experimentally saturated with alkalis at a total pressure of 1 atm and temperatures of 1300–1470°C, using the crucible supported loop technique. It was shown that Al₂O₃ content has little influence on the degree of silicate melt saturation with K and Na. In contrast, SiO₂ content strongly affects the solubility of alkalis in silicate melts. Model calculations were performed to evaluate the behavior of alkalis during the contamination/mixing of basic and silicic magmas.     

The dynamics of magma-mixing during flow in volcanic conduits

The production of mixed magmas (streaky pumice) during flow in a volcanic conduit has been modelled in the laboratory by studying the flow of two miscible fluids of differing viscosity passing concentrically through a vertical pipe. In the experiments reported in this paper, the outermost fluid is the more viscous, as would be the case when two magmas are simultaneously tapped from a zoned chamber in which silicic magma overlies mafic magma. At a Reynolds number (Re) which is much less than that required for turbulence in isoviscous pipe flow, the interface between two liquids of different viscosity can become unstable. Growth of the instability and mixing proceed when Re, based on the properties of the inner, less viscous fluid (Re _i), is greater than approximately 3 if between 10% and 90% of the flowing fluid is composed of the more viscous fluid. Outside this range of flow rate ratios, higher Re _i and viscosity ratios are required to ensure mixing. When the viscosity ratio U10 the unstable flow takes the form of an asymmetric, sinusoidal wave and at higher viscosity ratios axisymmetric, bead-like waves are the dominant instability. Entrainment across the boundaries of these wavy interfaces results in the production of streaky mixtures of the two liquids. The degree of mixing increases with Re ₁, U and distance downstream. Application of experimental results to magmatic situations shows that mixing will be possible in eruptions which tap layers of different viscosity from a stratified chamber. If a volcanic feeder is allowed to become lined by silicic magma before a mafic magma layer is drawn up from the chamber then a mixed pumice (or lava) sequence will ensue. Alternatively, if draw-up occurs when the feeder is still propagating away from the chamber, the slower flowing silicic magma may be overtaken by the faster flowing mafic magma. The advancing conduit will then have mafic or hybrid chilled margins enclosing a silicic interior, i.e. the usual arrangement in composite dykes and sills. Simultaneous tapping of silicic and underlying mafic magmas from a chamber can thus lead to magma mixing and to the emplacement of either mixed pumice sequences or composite intrusions, depending on the history of magma withdrawal and the dynamics of flow in the conduit.     

Petrogenesis of Early Cretaceous bimodal volcanic rocks in the Fanchang Basin,SE China: an energy-constrained assimilation–fractional crystallization model

Volcanic rocks in the Middle–Lower Yangtze River Valley (MLYRV) constitute a bimodal magmatic suite, with a significant compositional gap (between 50% and 63% SiO₂) between the mafic and felsic members. The suite is characterized by a relatively wide spectrum of rock types, including basalts, trachytes, and rhyolites. The basaltic rocks have low-to-moderate SiO₂ contents of 46.00–50.01%, whereas the trachytes and rhyolites possess SiO₂ contents in the range of 63.08–77.61%. Rocks of the bimodal suite show moderate enrichment of LILEs, negative Nb, Ta, and Ti anomalies, and are significantly enriched in LREEs. The basalts were most likely generated by parental mafic magmas derived from enriched lithospheric mantle with minor assimilation of crustal materials involving coeval crystal fractionation during magma evolution. The results of energy-constrained assimilation and fractional crystallization simulations demonstrate that the felsic magma was produced by the mixing of 5–20% lower crustal anatectic melts with an evolved mafic magma (～48% SiO₂) and accompanied by extensive clinopyroxene, plagioclase, biotite, and Fe–Ti oxide fractionation. Our model for the genesis of felsic rocks in bimodal suites is different from the traditional models of crustal melting and fractional crystallization or assimilation–fractional crystallization of basaltic liquids.     

Petrogenesis of mafic and associated silicic end-member magmas for calc-alkaline mixed rocks in the Shirataka volcano,NE Japan

Eruptive products of the Shirataka volcano (0.9–0.7 Ma) in NE Japan are calc-alkaline andesite–dacite, and are divisible into six petrologic groups (G1–G6). Shirataka rocks possess mafic inclusions—basalt–basaltic andesite, except for G3 and G4. All rocks show mixing and mingling of the mafic and silicic end-members, with trends defined by hosts and inclusions divided into high-Cr and low-Cr types; both types coexist in G1, G2, and G5. Estimated mafic end-members are high-Cr (1120–1170°C, 48–51% SiO₂, olv ± cpx ± plg) and low-Cr type magmas (49–52% SiO₂, cpx ± plg) except for the Sr isotopic composition. In contrast, the silicic end-members of both types have similar petrologic features (790–840°C, 64–70% SiO₂, hbl ± qtz ± px + plg). High-Cr type mafic and corresponding silicic end-members have lower ⁸⁷Sr/⁸⁶Sr ratios than the low-Cr ones in each group. The trace element model calculations suggest that the low-Cr type mafic end-member magma is produced through ca. 20% fractional crystallization (olv ± cpx ± plg) from the high-Cr type one with assimilation of granitoids (r = 0.02–0.05). The silicic magmas are producible through <30% partial remelting of previously emplaced basaltic magma with assimilation of crustal components. The compositional difference between the low-K and medium-K basalts in the Shirataka volcano is mainly attributed to the different degrees of the effect of subduction derived fluid by dehydration of phlogopite. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Quantitative field constraints on the dynamics of silicic magma chamber rejuvenation and overturn

The catastrophic eruption of large-volume, crystal-rich silicic magmas is often proposed to be a consequence of reheating, melting and overturn of partially molten, buoyant silicic material following repeated injection of dense, hot mafic magma. To test this “rejuvenation hypothesis”, we analyze at high spatial resolution 33 examples of deformed interfaces between intrusive mafic and silicic layers in two plutons of the Coastal Maine Magmatic Province, USA. These deformed interfaces are thought to record the buoyant overturn of silicic crystal mush layers, apparently in response to the injection and cooling of hot, dense mafic magmas. We use spectral analysis and scaling theory along with petrologic and textural data to identify, characterize, and understand periodic deformations from the scale of individual crystals (≈1 cm) to the thicknesses of mafic and silicic layers. Deformations at the largest scale lengths (>100 m) are at wavelengths comparable to, or greater than, silicic layer thicknesses and support a conjecture that mafic recharge can cause large-scale Rayleigh–Taylor-type overturning of silicic mushy layers. By contrast, the smallest scales of individual crystals probably record effects related to production and buoyancy-driven rise of melt from the tops of silicic mushes in contact with overlying hot basalt, whereas intermediate scales are explained by compaction. Our results constrain the evolution of a thermal rejuvenation event and potentially identify a condition for a large-scale overturn of the magma chamber that may lead to eruption. This work provides the first quantitative field-based constraints on some of the key physical processes inherent to the rejuvenation hypothesis.     

Liquid immiscibility in deep-seated magmas and the generation of carbonatite melts

This paper reviews the results of investigations of melt inclusions in minerals of carbonatites and spatially associated silicate rocks genetically related to various deep-seated undersaturated silicate magmas of alkaline ultrabasic, alkaline basic, lamproitic, and kimberlitic compositions. The analysis of this direct genetic information showed that all the deep magmas are inherently enriched in volatile components, the most abundant among which are carbon dioxide, alkalis, halides, sulfur, and phosphorus. The volatiles probably initially served as agents of mantle metasomatism and promoted melting in deep magma sources. The derived magmas became enriched in carbon dioxide, alkalis, and other volatile components owing to the crystallization and fractionation of early high-magnesium minerals and gradually acquired the characteristics of carbonated silicate liquids. When critical compositional parameters were reached, the accumulated volatiles catalyzed immiscibility, the magmas became heterogeneous, and two-phase carbonate-silicate liquid immiscibility occurred at temperatures of ≥1280–1250°C. The immiscibility was accompanied by the partitioning of elements: the major portion of fluid components partitioned together with Ca into the carbonate-salt fraction (parental carbonatite melt), and the silicate melt was correspondingly depleted in these components and became more silicic. After spatial separation, the silicate and carbonate-silicate melts evolved independently during slow cooling. Differentiation and fractionation were characteristic of silicate melts. The carbonatite melts became again heterogeneous within the temperature range from 1200 to 800–600°C and separated into immiscible carbonate-salt fractions of various compositions: alkali-sulfate, alkali-phosphate, alkali-fluoride, alkali-chloride, and Fe-Mg-Ca carbonate. In large scale systems, polyphase silicate-carbonate-salt liquid immiscibility is usually manifested during the slow cooling and prolonged evolution of deeply derived melts in the Earth’s crust. It may lead to the formation of various types of intrusive carbonatites: widespread calcite-dolomite and rare alkali-sulfate, alkali-phosphate, and alkali-halide rocks. The initial alkaline carbonatite melts can retain their compositions enriched in P, S, Cl, and F only at rapid eruption followed by instantaneous quenching.     

Mid-Tertiary magmatism of the Toquima caldera complex and vicinity,Nevada: development of explosive high-K,calc-alkaline magmas in the central Great Basin,USA

The Toquima caldera complex (TCC) lies near the middle of a west-northwest-trending belt of Oligocene to early Miocene volcanic rocks that stretches from southwestern Utah to west-central Nevada. Three overlapping to eccentrically nested calderas, called Moores Creek, Mt. Jefferson, and Trail Canyon, comprise the TCC. The calderas formed due to eruption of the tuffs of Moores Creek, Mt. Jefferson, and Trail Canyon at 27.2 Ma, 26.4 Ma, and 23.6 Ma, respectively. In total, 900+ km³ of magma was erupted from the complex. The high-silica rhyolite tuff of Moores Creek is the least strongly zoned in silica (78.0–76.8 wt% SiO₂), and the tuff of Mt. Jefferson is the most strongly zoned (77.5–65.3 wt% SiO₂); the tuff of Trail Canyon is moderately zoned (75.9–70.4 wt% SiO₂). All eruptive products contain plagioclase, sanidine, quartz, biotite, Fe–Ti oxides, and accessory zircon, allanite, and apatite. Amphibole and clinopyroxene join the assemblage where compositions of bulk tuff are 74 wt% SiO₂ and 70 wt% SiO₂ respectively. Proportions and compositions of phenocrysts vary systematically with composition of the host tuff. Compositional zoning trends of sanidine and biotite suggest the presence of a high Ba-bearing magmatic component at depth or its introduction into the Mt. Jefferson and Trail Canyon magma chambers at a late stage of magmatic evolution. Rocks of the complex constitute a high-K, calc-alkaline series.Empirical data from other systems and results of published phase-equilibria and thermo-chemical studies suggest that magma erupted from the TCC was oxidized ( 1.5 to 2.0 log units above NNO), thermally zoned ( 700–730° C for high-silica rhyolite to 800–840° C for dacite) and water-rich (5.0–5.5. wt% H₂O for highsilica rhyolite to 4.0 wt% H₂O for dacite). Geologic relations and amphibole compositional data are consistent with total pressures of 1.5 to 2 kbars.Onset of mid-Tertiary magmatism in vicinity of the TCC began with intrusion of a small granodioritic stock and a northeast-trending dike swarm at 37–34 Ma. The dikes are broadly bimodal assemblage of silicic andesite and rhyolite. Voluminous ash-flow-tuff magmatism commenced at 32.3 Ma and persisted for 9 m.y. without eruption of intermediate to mafic magmas (<62 wt% SiO₂). As such, the TCC is probably a remnant of a more extensive complex of calderas whose identities are obscured by recurrent volcanism and by late Tertiary basin-range block faulting. The change from small-volume, broadly bimodal volcanism to voluminous outpourings of silicic magma is similar to that which occurred in east-central Nevada, where magmatism and rapid crustal extension overlapped in space and time. Although supracrustal extension at the time of formation of the TCC apears limited, the comparable magmatic histories and compositional characteristics of rocks erupted from east-central Nevada and the TCC suggest that fundamentally similar magmatic processes acted at depth and that extension may have been more pronounced in the lower and middle crust below the TCC and vicinity. Because strain is partitioned heterogeneously in the upper crust, the magmatic record, rather than surface structural features, may reflect better the actual state of crustal stress during volcanism.Mid-Tertiary magmatism in the TCC and vicinity probably began with intrusion of mantle-derived basalt into the lower crust, which led to crustal heating, local partial melting of crustal rocks, and intrusion of rhyolitic melts and contaminated basaltic differentiates (alkalirich andesite) into the upper crust. With time, intrusion to extrusion ratios increased as silicic melts retarded the rise of mafic magmas and mixing between mafic magmas and crustal partial melts occurred. The oxidized, water-rich, and low-temperature nature of these magmas reflects protracted crustal residence and interaction prior to eruption. The resulting hybridized and differentiated magmas ultimately erupted to form extensive deposits of silicic ash-flow tuff. By contrast, silicic lavas are scarce possibly because of coherent roof rocks that limited volatile degassing between major pyroclastic eruptions.     

Lamprophyres of the Tomtor Massif: A result of mixing between potassic and sodic alkaline mafic magmas

The paper presents data on inclusions in minerals of the least modified potassic lamprophyres in a series of strongly carbonatized potassic alkaline ultramafic porphyritic rocks. The rocks consist of diopside, kaersutite, analcime, apatite, and rare phlogopite and titanite phenocrysts and a groundmass, which is made up, along with these minerals, of potassic feldspar and calcite. The diopside and kaersutite phenocrysts display unsystematic multiple zoning. Chemically and mineralogically, the rock is ultramafic foidite and most likely corresponds to monchiquite. Primary and secondary melt inclusions were found in diopside, kaersutite, apatite, and titanite phenocrysts and are classified into three types: sodic silicate inclusions with analcime, potassic silicate inclusions with potassic feldspar, and carbonate inclusions, which are dominated by calcite. Heating and homogenization of the inclusions show that the potassic lamprophyres crystallized from a heterogeneous magma, with consisted of mixing mafic sodic and potassic alkaline magmas enriched in a carbonatite component. The composition of the magmas was close to nepheline and leucite melanephelinite. The minerals crystallized at 1150–1090°C from the sodic melts and at 1200–1250°C from the potassic ones. The sodic mafic melts were richer in Fe than the potassic ones, were the richest in Al, Mn, SO₃, Cl, and H₂O and poorer in Ti and P. The potassic mafic melts were not lamproitic, as follows from the presence of albite in the crystallized primary potassic melt inclusions. The diopside, the first mineral to crystallize in the rock, started to crystallize in the magmatic chamber from sodic mafic melt and ended to crystallize from mixed sodic–potassic melts. The potassic mafic melts were multiply replenished in the chamber in relation to tectonic motions. The ascent of the melts to the surface and rapidly varying P–T parameters of the magma were favorable for multiple separations of carbonatite melts from the alkaline mafic ones and their mixing and mingling.     

The petrogenetic characterization of intermediate and silicic charnockites in high-grade terrains: a case study from southern India

Large charnockite massifs occur in some of the Precambrian high-grade terrains like the southern Indian granulite terrain. The Cardamom Hill charnockite massif from the Madurai Block, southern India, consists of an intermediate type and silicic type, with the intermediate type showing similarities to high-Ba−Sr granitoids with low K₂O/Na₂O ratios and the silicic type showing similarities to high-Ba–Sr granitoids with high K₂O/Na₂O ratios. Within the constraints imposed by near basaltic composition of the most mafic samples and their relatively high concentrations of both compatible and incompatible elements, comparison with recent experimental studies on various source compositions, and trace- and rare-earth-element modeling, the distinctive features of the intermediate charnockites can be best explained in terms of assimilation–fractional crystallization (AFC) models involving interaction between a mantle-derived basaltic magma and lower crustal materials. Silicic charnockites on the other hand are high temperature melts of moderately hydrous basaltic magmas. A two-stage model which involves an initial partial melting of hydrous basaltic magma and later fractionation explains the geochemical features of the silicic charnockites, with the fractionation stage most probably an open system AFC. It is suggested that for massifs showing spatial association of intermediate and silicic charnockites, a model taking into account their compositional difference in terms of the effect of variations in the conditions (e.g., temperature, water fugacity) that prevailed, can account for plausible petrogenetic scenarios.     

地壳硅质岩浆演化的动力学机制SCIEI北大核心CSCD

硅质岩浆的成因及演化机制研究是认识大陆地壳成分结构演化机制的关键途径。地壳硅质岩浆储库的基本性质及其演化的动力学过程是制约硅质岩浆演化及其多样性的根本原因,也是受到广泛关注的前沿和热点问题。本文总结了硅质岩浆储库的基本性质及其演化机制研究的相关进展,重点探讨了岩浆体系演化的动力学机制,即其物质成分及温度压力条件的动态演化过程,亦即岩浆储库在补给驱动下的存留、活化、分异、喷发过程。经由累积生长的方式形于地壳浅部的硅质岩浆储库以晶粥为主要赋存形式,具有较低的温度压力范围和高硅高结晶度高粘度的特征。岩浆储库的基本性质导致其自身不具有持续演化的能力,只有在基性岩浆补给的驱动下才能长时间存留分异或快速活化喷发。基性岩浆的补给及其携带的热及流体/挥发份深刻地改变了岩浆储库的物质成分及物理条件,是导致硅质岩浆分异演化或喷发的根本原因。本文还结合研究进展探讨了东南沿海地区中生代火山-侵入岩的成因演化机制及相关问题。     

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.     

Ferric/ferrous ratio in silicate melts: a new model for 1 atm data with special emphasis on the effects of melt composition

The effect of MgO and total FeO on ferric/ferrous ratio in model multicomponent silicate melts was investigated experimentally in the temperature range 1300–1500 °C at 1 atm total pressure in air. We demonstrate that the addition of these weak network modifier cations results in an increase of Fe³⁺/Fe²⁺ ratio in both mafic and silicic melts. Based on present and published experimental data, a new empirical equation is proposed to predict the ferric/ferrous ratio as a function of oxygen fugacity, temperature and melt composition. In contrast to previous equations, the compositional effect of melts on the Fe³⁺/Fe²⁺ ratio is not only modeled by the sum of the molar fraction of the individual oxide components. Additional interactions terms have also been incorporated. The main advantage of the proposed model is its applicability for a wide compositional range. However, its application to felsic melts (>?68 wt% SiO₂) is not recommended. Other advantages of this equation and differences when compared with previous models are discussed.     

The Cold Bay Volcanic Center,Aleutian Volcanic Arc

The Cold Bay Volcanic Center has experienced two major stages of eruptive activity. Early (M-Series) acitivity produced bimodal Hi-Alumina basalt and calc-alkaline andesite lavas while later (FPK-Series) activity produced only calc-alkaline andesite. The spectrum of basalt compositions is believed to be due to high pressure (8 kb) fractionation at or near the base of the crust. Abundant mineralogical and geochemical evidence support a lower pressure mixing origin for all andesites. Inspection of the mineralogical data has shown that the earliest (M-Series) andesites were produced by mixing of basalt (<53 wt% SiO₂) and silicic andesite (60.5 to 62.5 wt%) while later (FPK-Series) andesites resulted from the mixing of basaltic-andesite (53 to 56 wt%) and less silicic andesite (58.5 to 60.0 wt%). The major element and trace element geochemical data are consistent with a low pressure fractionation origin for the silicic endmember magmas and support the temporal variations in both mafic and silicic endmember compositions. The complete lack of crustal inclusions in all lavas is taken as evidence for a minimal crustal melting and/or assimilation role in the origin of the silicic endmembers. Many of the features of all andesites, including the important long term convergence of endmember magma compositions, are consistent with the process of liquid fractionation, accompanied by large scale magma mixing. A deduced upper limit of 62.5 wt% SiO₂ for the silicic endmember magmas suggests that liquid fractionation, in the absence of major crustal melting, cannot produce more silicic magmas. A possible explanation is the presence of a rheological barrier, based on the concept of critical crystallinity (Marsh 1981), which prohibits more silicic liquids from being extracted from a crystal-liquid suspension.     

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.