Magmatic evolution of the basaltic shield volcanoes of Reunion Island

Chemical data are presented for the basic lavas of the two volcanic shields, Piton des Neiges and Piton de la Fournaise, which comprise Reunion Island. In addition, data for cumulate xenoliths have been used to predict mineral/melt distribution coefficient values for the Reunion magmas.The younger volcanic shield, Piton de la Fournaise, comprises two lava sequences, the >0.5−0.2-m.y. B.P. Primary Shield lavas, and the <0.2-m.y. B.P. Caldera Series lavas. Fractional crystallization models for these lavas indicate that olivine is the major fractionating phase during the evolution from the parental basalt composition to the average basaltic liquid. Only during the evolution of the older, Primary Shield lavas has the common fractionation of an ol + cpx + plag + mt assemblage resulted in the eruption of hawaiitic, ankaramitic and feldspar-phyric lavas. The restriction of the Caldera Series liquids predominantly to olivine fractionation and the extensive cotectic fractionation during the evolution of the Primary Shield sequences is interpreted in terms of the maturity of the volcanic center. The younger stages of evolution involve high magma input into a well-developed feeder and reservoir system, thus maintaining the liquids above a cotectic surface. Whereas, during the evolution of the Primary Shield lavas, lower magma input rates into a less well-developed feeder system increased the probability of the fractionating liquid attaining a cotectic surface. Fractional crystallization accounts for all the chemical variation observed for the Piton de la Fournaise basaltic magmas. The analytical data are closely comparable to the rare earth element (REE) and trace element fractionation curves predicted by least-squares calculations, this supports the use of such models in quantitative evaluation of fractional crystallization.A preliminary survey of Sr isotope values indicates that the oldest (>2 m.y. B.P.) lava sequences of Piton des Neiges may be derived from a source which was isotopically distinct from that of the <2 m.y. B.P. lavas of both volcanic shields. These latter sequences are remarkably consistent in both isotopic and trace element abundance implying a homogeneous source material and an invariable partial melting process. Partial melting calculations indicate that the basaltic lavas have been derived by 5–10% melting of a garnet-poor peridotite (cpx/gt 9). Systematic differences in the light- and heavy-REE patterns between similar basaltic provinces are interpreted to be a result of variation in the nature of the phases buffering the entry of light- and/or heavy-REE into the melt during partial fusion.     

Magma dynamics, crystallization, and chemical differentiation of the 1959 Kilauea Iki lava lake, Hawaii, revisited

Using constraints from an extensive database of geological and geochemical observations along with results from fluid mechanical studies of convection in magma chambers, we identify the main physical processes at work during the solidification of the 1959 Kilauea Iki lava lakes. In turn, we investigate their quantitative influence on the crystallization and chemical differentiation of the magma, and on the development of the internal structure of the lava lake. In contrast to previous studies, vigorous stirring in the magma, driven predominately by the descent of dense crystal-laden thermal plumes from the roof solidification front and the ascent of buoyant compositional plumes due to the in situ growth of olivine crystals at the floor, is predicted to have been an inevitable consequence of very strong cooling at the roof and floor. The flow is expected to have caused extensive but imperfect mixing over most of the cooling history of the magma, producing minor compositional stratification at the roof and thermal stratification at the floor. The efficient stirring of the large roof cooling is expected to have resulted in significant internal nucleation of olivine crystals, which ultimately settled to the floor. Additional forcing due to either crystal sedimentation or the ascent of gas bubbles is not expected to have increased significantly the amount of mixing. In addition to convection in the magma, circulation driven by the convection of buoyant interstitial melt in highly permeable crystal-melt mushes forming the roof and the floor of the lava lake is envisaged to have produced a net upward flow of evolved magma from the floor during solidification. In the floor zone, mush convection may have caused the formation of axisymmetric chimneys through which evolved magma drained from deep within the floor into the overlying magma and potentially the roof. We hypothesize that the highly evolved, pipe-like ‘vertical olivine-rich bodies’ (VORBs) [Bull. Volcanol. 43 (1980) 675] observed in the floor zone, of the lake are fossil chimneys. In the roof zone, buoyant residual liquid both produced at the roof solidification front and gained from the floor as a result of incomplete convective mixing is envisaged to have percolated or ‘leaked‘ into the overlying highly-permeable cumulate, displacing less buoyant interstitial melt downward. The results from Rayleigh fractionation-type models formulated using boundary conditions based on a quantitative understanding of the convection in the magma indicate that most of the incompatible element variation over the height of the lake can be explained as a consequence of a combination of crystal settling and the extensive but imperfect convective mixing of buoyant residual liquid released from the floor solidification front. The remaining chemical variation is understood in terms of the additional influences of mush convection in the roof and floor on the vertical distribution of incompatible elements. Although cooling was concentrated at the roof of the lake, the floor zone is found to be thicker than the roof zone, implying that it grew more quickly. The large growth rate of the floor is explained as a consequence of a combination of the substantial sedimentation of olivine crystals and more rapid in situ crystallization due to both a higher liquidus temperature and enhanced cooling resulting from imperfect thermal and chemical mixing.     

The lesser antilles — A discussion of the Island arc magmatism

The active island arc of Lesser Antilles marks the junction between the Atlantic and Carribbean lithospheric plates. With the exception of the alkali basalts of Grenada, the volcanics of the arc can be regarded as belonging to the low-K, island arc, calc-alkaline suite. Although compositions ranging from basalt to rhyolite have been described, porphyritic andesite appears to be the dominant rock type on most volcanoes (intermediate centers). Variable amounts of basalt and basaltic andesite occur and rarely predominate over andesite (latter are basic centers), whereas the more silicic members are only occasionally found. The calc-alkaline suite is characterized by relatively high Al₂O₃ and CaO and low K₂O, Rb and Ni. Variations, especially in the alkali elements, occur both with space and time. A characteristic feature of many of the volcanoes is the occurrence in the basalt and basaltic andesite volcanics of plutonic blocks, often showing cumulate textures. The blocks which ware composed of plagioclase — amphibole — olivine — clinopyroxene — magnetite are thought to be the products of fractionation. The differences between basic and intermediate centers is probably due to the frequency that the magma ascended to the surface or remained in high level chambers where fractionation occurred.     

Petrology of gabbroic xenoliths in 1960 Kilauea basalt: crystalline remnants of prior (1955) magmatism

The 1960 Kapoho lavas of Kilauea’s east rift zone contain 1–10 cm xenoliths of olivine gabbro, olivine gabbro-norite, and gabbro norite. Textures are poikilitic (ol+sp+cpx in pl) and intergranular (cpx+pl±ol±opx). Poikilitic xenoliths, which we interpret as cumulates, have the most primitive mineral compositions, Fo_82.5, cpx Mg# 86.5, and An_80.5. Many granular xenoliths (ol and noritic gabbro) contain abundant vesicular glass that gives them intersertal, hyaloophitic, and overall ‘open’ textures to suggest that they represent ‘mush’ and ‘crust’ of a magma crystallization environment. Their phase compositions are more evolved (Fo_80–70, cpx Mg# 82–75, and An_73–63) than those of the poikilitic xenoliths. Associated glass is basaltic, but evolved (MgO 5 wt%; TiO₂ 3.7–5.8 wt%). The gabbroic xenolith mineral compositions fit existing fractional crystallization models that relate the origins of various Kilauea lavas to one another. FeO/MgO crystal–liquid partitioning is consistent with the poikilitic ol-gabbro assemblage forming as a crystallization product from Kilauea summit magma with ∼8 wt% MgO that was parental to evolved lavas on the east rift zone. For example, least squares calculations link summit magmas to early 1955 rift-zone lavas (∼5 wt% MgO) through ∼28–34% crystallization of the ol+sp+cpx+pl that comprise the poikilitic ol-gabbros. The other ol-gabbro assemblages and the olivine gabbro-norite assemblages crystallized from evolved liquids, such as represented by the early 1955 and late 1955 lavas (∼6.5 wt% MgO) of the east rift zone. The eruption of 1960 Kapoho magmas, then, scoured the rift-zone reservoir system to entrain portions of cumulate and solidification zones that had coated reservoir margins during crystallization of prior east rift-zone magmas. Received: January 7, 1993/Accepted: November 23, 1993     

Acid volcanoes of San Miguel,Azores

The trachytic volcanoes of San Miguel Island are emplaced on either side of a young basaltic area, which could correspond to some active branch of the mid-Atlantic rift. Geological and geomagnetic surveys suggest crustal drift of about 1 cm/year on either direction. From the seismic anomaly data, shallow magma chambers have been inferred for those acid volcanoes, where owing to the crustal drift the main vents appear to have been shifted relatively to the fractures feeding the chambers from the deep upper mantle supply. The primitive magma is certainly basaltic; when the surface vents are directly above the deep feeding fractures, only basaltic lavas of some primitive composition can be extruded; but, when the upper vents are laterally displaced, the magma is delayed in the chambers and will differentiate, eventually into trachytic material capable of producing huge pumice explosions.     

Magmatic evolution of Pacaya and Cerro Chiquito volcanological complex,Guatemala

Four major phases are distinguished during the building of the Pacaya volcanological complex (Guatemala): (1) the ancestral volcano, now much eroded, covered by younger deposits and battered by faulting and landslides; (2) the initial cone made up of large lava flows and dated at about 0.5 Ma; (3) andesito-dacitic domes (Cerro Chiquito dome and others) emplaced during an extrusive phase at about 0.16 Ma; and (4) the active Pacaya volcano. Lavas of phases 2 and 4 are basalts and basaltic andesites with almost the same major and trace element compositions. Classical enrichment in LILE and depletion in HFSE are observed. Phase 3 domes show magma-mingling features. The dacitic host rock includes basaltic andestic enclaves, 20 to 30% in volume. According to geochemical and mineralogic data (Mg/Fe ratios of basic minerals higher in dacite, groundmass glasses sodic in dacite and potassic in basaltic andesite), the basaltic andesites and dacites of phase 3 cannot be related by a simple fractional crystallization process. The existence of such differences suggests that magma mingling/mixing processes were involved by a connection between the two magma chambers prior to the extrusion of the andesito-dacitic domes. However, some trace element data clearly suggest that fractional crystallization played a significant role in the differentiation of these lavas. Remelting of amphibole-bearing cumulates from the dacite may also have played a role in the basaltic andesitic liquid genesis. Thermodynamical parameters of each liquid are contrasted. The basaltic andesitic magma, at a high temperature (1037°C) and in relatively small amounts, is embayed in the cooler (905° C) dacitic magma. The former liquid, denser (2.72) and less viscous (10^3.31 poises for free crystal liquid) may crystallize while the latter, lighter (2.60) and more viscous (10^4.46 poises), remains still liquid. Isotopic data (0.70383<⁸⁷Sr/⁸⁶Sr <0.70400; 0.512785<¹⁴³Nd/¹⁴⁴Nd<0.512908; 18.61<²⁰⁶Pb/²⁰⁴Pb<18.66; 15.56<²⁰⁷Pb/²⁰⁴Pb <15.58; 38.30<²⁰⁸Pb/²⁰⁴Pb<38.40) indicate that all the lavas (from Pacaya as well as from Cerro Chiquito) are cogenetic and derive from the same mantle source. Sr, Nd and Pb isotope ratios are similar to those of OIBs. (²³⁰Th/²³²Th) activity ratios on two historical lavas are respectively 1.2 and 1.3. The Th excess is similar to that of other calcalkaline volcanoes emplaced on a continental crust. These lavas evolved, possibly in separate magma chambers, through processes of fractional crystallization and magma mixing.     

Emplacement and arrest of sheets and dykes in central volcanoes

Sheet intrusions are of two main types: local inclined (cone) sheets and regional dykes. In Iceland, the inclined sheets form dense swarms of (mostly) basaltic, 0.5–1 m thick sheets, dipping either at 20–50° or at 75–90° towards the central volcano to which they belong. The regional dykes are (mostly) basaltic, 4–6 m thick, subvertical, subparallel and form swarms, less dense than those of the sheets but tens of kilometres long, in the parts of the volcanic systems that are outside the central volcanoes. In both types of swarms, the intrusion intensity decreases with altitude in the lava pile. Theoretical models generally indicate very high crack-tip stresses for propagating dykes and sheets. Nevertheless, most of these intrusions become arrested at various crustal depths and never reach the surface to supply magma to volcanic eruptions. Two principal mechanisms are proposed to explain arrest of dykes and sheets. One is the generation of stress barriers, that is, layers with local stresses unfavourable for the intrusion propagation. The other is mechanical anisotropy whereby sheet intrusions become arrested at discontinuities. Stress barriers may develop in several ways. First, analytical solutions for a homogeneous and isotropic crust show that the intensity of the tensile stress associated with a pressured magma chamber falls off rapidly with distance from the chamber. Thus, while dyke and sheet injection in the vicinity of a chamber may be favoured, dyke and sheet arrest is encouraged in layers (stress barriers) at a certain distance from the chamber. Second, boundary-element models for magma chambers in a mechanically layered crust indicate abrupt changes in tensile stresses between layers of contrasting Young’s moduli (stiffnesses). Thus, where soft pyroclastic layers alternate with stiff lava flows, as in many volcanoes, sheet and dyke arrest is encouraged. Abrupt changes in stiffness between layers are commonly associated with weak and partly open contacts and other discontinuities. It follows that stress barriers and discontinuities commonly operate together as mechanisms of dyke and sheet arrest in central volcanoes.     

Eruption and emplacement of a basaltic welded ignimbrite during caldera formation on Gran Canaria

The 14.1 Ma old composite ignimbrite cooling unit P1 (45 km³) on Gran Canaria comprises a lower mixed rhyolite-trachyte tuff, a central rhyolite-basalt mixed tuff, and a slightly rhyolite-contaminated basaltic tuff at the top. The basaltic tuff is compositionally zoned with (a) an upward change in basalt composition to higher MgO content (4.3–5.2 wt.%), (b) variably admixed rhyolite or trachyte (commonly <5 wt.%), and (c) an upward increasing abundance of basaltic and plutonic lithic fragments and cognate cumulate fragments. The basaltic tuff is divided into three structural units: (I) the welded basaltic ignimbrite, which forms the thickest part (c. 95 vol.%) and is the main subject of the present paper; (II) poorly consolidated massive, bomb- and block-rich beds interpreted as phreatomagmatic pyroclastic flow deposits; and (III) various facies of reworked basaltic tuff. Tuff unit I is a basaltic ignimbrite rather than a lava flow because of the absence of top and bottom breccias, radial sheet-like distribution around the central Tejeda caldera, thickening in valleys but also covering higher ground, and local erosion of the underlying P1 ash. A gradual transition from dense rock in the interior to ash at the top of the basaltic ignimbrite reflects a decrease in welding; the shape of the welding profile is typical for emplacement temperatures well above the minimum welding temperature. A similar transition occurs at the base where the ignimbrite was emplaced on cold ground in distal sections. In proximal sections the base is dense where it was emplaced on hot felsic P1 tuff. The intensity of welding, especially at the base, and the presence of spherical particles and of mantled and composite particles formed by accretion and coalescence in a viscous state imply that the flow was a suspension of hot magma droplets. The flow most likely had to be density stratified and highly turbulent to prevent massive coalescence and collapse. Model calculations suggest eruption through low pyroclastic fountains (<1000 m high) with limited cooling during eruption and turbulent flow from an initial temperature of 1160°C. The large volume of 26 km³ of erupted basalt compared with only 16 km³ of the evolved P1 magmas, and the extremely high discharge rates inferred from model calculations are unusual for a basaltic eruption. It is suggested that the basaltic magma was erupted and emplaced in a fashion commonly only attributed to felsic magmas because it utilized the felsic P1 magma chamber and its ring-fissure conduits. Evolution of the entire P1 eruption was controlled by withdrawal dynamics involving magmas differing in viscosity by more than four orders of magnitude. The basaltic eruption phase was initially driven by buoyancy of the basaltic magma at chamber depth and continued degassing of felsic magma, but most of the large volume of basalt magma was driven out of the reservoir by subsidence of a c. 10 km diameter roof block, which followed a decrease in magma chamber pressure during low viscosity basaltic outflow.     

Crustal Structure of the Flood Basalt Province of Ethiopia from Constrained 3-D Gravity Inversion

The Oligocene Afar mantle plume resulted in the eruption of a large volume of basaltic magma, including major sequences of rhyolitic ignimbrites, in a short span of time across Ethiopia. In order to assess the impact of these magmatic processes on the crust and to investigate the general crustal configuration beneath the Ethiopian plateau, northern part of the Main Ethiopian Rift and the Afar depression, analysis and modeling of the gravity field have been conducted. The Bouguer gravity map is dominated by long-wavelength anomalies that primarily arise from the isostatic compensation of the topography. Consequently, anomalies within the crust/upper mantle are masked and quantitative interpretation becomes difficult. The long-wavelength anomalies are approximated using admittance technique and subsequently removed from the Bouguer anomalies to obtain the residual isostatic anomalies. The residual map contains both short- and intermediate-wavelength anomalies related to geologic and tectonic features. The long-wavelength regional isostatic field is used to map the crust-mantle interface and the results are in good agreement with those determined by other geophysical methods. Seismic constrained gravity inversion was performed on the isostatic residual field and series of three-dimensional models have been constructed for the structures of the crust and upper mantle beneath the uplifted and rifted flood basalt province of northern Ethiopia. The inversion results have shown that the NW plateau has thick crust that rests on normal lithospheric mantle. Afar, On the other hand, is marked by thin stretched crust resting on a low-density upper mantle indicating a hotter thermal regime and partial melt. No lithospheric mantle is observed beneath Afar. The models further indicate the presence of an extensive sub-crustal thick (~12 km on average) and high-density (~3.06 gm/cc) mafic accreted igneous layer of fractionated cumulate (magmatic underplating) beneath the NW plateau. The study suggests that the underplate was fundamental to the accretion process and may have played a role in compensating most of the plateau uplift and in localizing stresses.     

Rayleigh fractionation with continuous removal of liquid

In its simple form the Rayleigh surface equilibrium fractionation equation is only valid for a closed system in which solid separates from liquid, and no liquid is lost. Many natural systems, particularly magma chambers, lose liquid continuously as they fractionate. Here it is shown that evolution in such a system can, if loss of liquid proceeds in a constant ratio to precipitation of solid, be modelled by a modified Rayleigh equation where the solid/liquid distribution coefficientD is replaced byD*=(q·D + 1)/(q + 1), whereq is the ratio of solid precipitated to liquid extracted. This can be visualised as treating the liquid removed as an additional phase extracted during fractionation for whichD = 1. The effect of this is particularly important for incompatible elements, where asD → 0,D* → 1/(q + 1), so that whenq = 1, the most incompatible element givesD* = 0.5.     

The Ploskie Sopki volcanic massif: Geology,petrochemistry, mineralogy,and petrogenesis (Klyuchevskoi Volcanic Cluster,Kamchatka)

This paper is concerned with the geological history and petrology of a major polygenic volcanic edifice dating back to Upper Pleistocene to Holocene time. This long-lived volcanic center is remarkable in that it combines basaltic and trachybasaltic magmas which are found in basaltic andesite and trachybasaltic–trachyandesite series. The inference is that the coexisting parent magmas are genetically independent and are generated at different sources at depth in an upper mantle volume. The associated volcanic rocks have diverse compositions, stemming from a multi-stage spatio–temporal crystallization differentiation of the magmas and mixing of these in intermediate chambers.     

Analytical Solutions of Three‐Dimensional Contaminant Transport Models with Exponential Source Decay

Despite the availability of numerical models, interest in analytical solutions of multidimensional advection‐dispersion systems remains high. Such models are commonly used for performing Tier I risk analysis and are embedded in many regulatory frameworks dealing with groundwater contamination. In this work, we develop a closed‐form solution of the three‐dimensional advection‐dispersion equation with exponential source decay, first‐order reaction, and retardation, and present an approach based on some ease of use diagrams to compare it with the integral open form solution and with earlier versions of the closed‐form solution. The comparison approach focuses on the relative differences associated with source decay and the effect of simulation time. The analysis of concentration contours, longitudinal sections, and transverse sections confirms that the closed‐form solutions studied can be used with acceptable approximation in the central area of a plume bound transversely within the source width, both behind and beyond the advective front and for concentration values up to two orders of magnitude less than the initial source concentration. As the proposed closed‐form model can be evaluated without nested numerical computations and with simple mathematical functions, it can be very useful in risk assessment procedures.     

Gravity evidence for an extinct magma chamber beneath Syrtis Major, Mars: a look at the magmatic plumbing system

Syrtis Major is an ancient basaltic shield volcano on Mars with a basal diameter of 1100 km. The free-air gravity anomaly is 126 mGal at spherical harmonic degree 50 and reaches its maximum amplitude over the 2 km deep topographic caldera. The observed gravity anomaly cannot be explained by flexurally supported surface topography and requires the presence of a buried, high-density load. The geologically most reasonable interpretation of this high-density load is that it represents the magma chamber of Syrtis Major, now solidified and filled at least in part by dense igneous cumulates. Pyroxene is likely to be the dominant cumulate mineral in this system, although olivine may also be present. Gravity models presented here define the structure of the buried load and in essence provide a look at the magmatic plumbing system of this volcano. The preferred model involves a buried load that is approximately 300×600 km across, roughly twice as large as the topographic caldera. Both the buried load and the caldera are elongated in the north-south direction. In the center of the buried load, the minimum thickness is 2.8 km for an olivine-dominated cumulate system or 3.9 km for a pyroxene-dominated system. The best terrestrial analog for this structure is the Bushveld Complex, an igneous cumulate body that is similar in size and thickness to the Syrtis Major structure. Assuming that the mean crustal density is 2600 kg m⁻³ due to impact brecciation, the elastic lithosphere at Syrtis Major was 10-15 km thick at the time when the topographic load was emplaced. This corresponds to a lithospheric thermal gradient of 28-52 K/km and a surface heat flux of 70-130 mW m⁻². Higher resolution gravity data, such as that which is planned for the 2005 Mars Reconnaissance Orbiter, will permit further refinement of the dimensions of this structure.     

Petrology of basaltic xenoliths in andesitic to dacitic host lavas from Martinique (lesser antilles): Evidence for magma mixing

Some recent calc-alkaline andesites and dacites from southern and central Martinique contain basic xenoliths belonging to two main petrographic types:

The most frequent one has a hyalodoleritic texture (« H type ») with hornblende + plagioclase + Fe-Ti oxides, set in an abundant glassy and vacuolar groundmass.

The other one exhibits a typical porphyritic basaltic texture (« B type ») and mineralogy (olivine + plagioclase + orthopyroxene + clinopyroxene + Fe-Ti oxides and scarce, or absent hornblende).

Gradual textural and mineralogical transitions occur between these two types (« I type ») with the progressive development of hornblende at the expense of olivine and pyroxenes. Mineralogical and chemical studies show no primary compositional correlations between the basaltic xenoliths and their host lavas, thus demonstrating that the former are not cognate inclusions; they are remnants of basaltic liquids intruded into andesitic to dacitic magma chambers. This interpretation is strengthened by the typical calc-alkaline basaltic composition of the xenoliths, whatever their petrographic type (« H », « I » or « B »). The intrusion of partly liquid, hot basaltic magma into colder water-saturated andesitic to dacitic bodies leads to drastic changes in physical conditions. The two components; the basaltic xenoliths are quenched and homogeneized with their host lavas with respect to T^o;fO₂ andpH₂O conditions. « H type » xenoliths represent original mostly liquid basalts in which such physical changes lead to the formation of hornblende and the development of a vacuolar and hyalodoleritic texture. The temperature increase of the acid magma depends on the amount of the intruding basalt and on the thermal contrast between the two components. The textural diversity which characterizes the xenoliths reflects the cooling rate of the basaltic fragments and/or their position relative to the basaltic bodies (chilled margins or inner, more crystallized, portions). In addition to physical equilibration (T, fO₂) between the magmas, mixing involves:

mechanical transfer of phenocrysts from one component to another, in both directions;

volatile transfer to the basaltic xenoliths, with chemical exchanges.

It is here demonstrated that a short period of time (some ten hours to a few days) separates the mixing event from the eruption, outlining the importance of magma mixing in the triggering of eruption. The common occurrence of basaltic xenoliths (generally of « H » type) in calc-alkaline lavas is emphasized, showing that this mechanism is of first importance in calc-alkaline magma petrogenesis.     

Magma plumbing system beneath Ontake Volcano, central Japan

Ontake Volcano in central Japan was last active from ~ 100–35 Ka. The eruptions contained rhyodacite pumice and lavas in the first stage (stage O1, > 33 km³), followed by eruptions of andesite lavas and pyroclastics (stages O2 and O3, > 16 km³). Modeling of major and incompatible elements with Sr isotope ratios suggests that the primary magma was a high-alumina basalt. One andesite magma type appears to have evolved from the basalt in a closed system magma chamber, in part by fractional crystallization, and its generation included crustal assimilation. The other andesite magma type is considered to have evolved in an open system magma chamber in which repeated input of primary magma occurred together with wall-rock assimilation and fractional crystallization. The rhyodacite is inferred to have evolved in a closed system magma chamber by fractional crystallization of the second type of andesite. These genetic relationships require that the magma chamber functioned alternately as an open and a closed system. Geobarometry indicates that there may have been multiple magma chambers, located in the upper crust for the rhyodacite, near the upper–lower crust interface for the andesite and in the mid-lower crust for the basalt. These chambers were stacked to form the magma plumbing system of Ontake. Incompatible element compositions of the basalt are considered to have changed during the eruptions, suggesting that two different plumbing systems for stage O1 magma and for stages O2, O3 magmas existed during the 65 Ka of activity. Evolutionary history of the systems implies that the primary magma was introduced into the magma plumbing system each for ~ 17 500 years and that the life span of a magma plumbing system was shorter than 40 Ka.     

Evidence of an early alteration process driven by magmatic fluid in Mururoa volcano

Evidence for a deuteric alteration process induced by a magmatic fluid has been found in the feeder zone of the Mururoa volcano (French Polynesia). Within the dikes, where basaltic glass does not show any evidence of pervasive alteration, vesicles are filled with dioctahedral smectites and calcite, while olivine phenocrysts are replaced by dioctahedral smectites, ankerite and calcite.The ¹³C signature of carbonates, the carbon and H₂O content of the whole rocks and their impoverishment in deuterium are compatible with the presence of magmatic CO₂ during the crystallization of intruding lavas and exclude contamination by seawater. Mass balance calculations on selected thin sections photographs of partly filled up vesicles and replaced olivine crystals, constrain, assuming a closed system interaction, the chemical composition of the initial fluid and the respective amounts of the initial solid phases involved in the alteration process. Thermodynamic modelings using the EQ3/6 software package correctly predict the mineralogic, chemical and isotopic exchanges accompanying alteration, thus validating the closed system assumption. The model which allows prediction of the influence of CO₂ on the alteration products, shows that, above a 0.25 CO₂ mole fraction in the initial fluid, the alteration is entirely controlled by the chemical composition of the initial solid phases. The presence of CO₂ implies the precipitation of dioctahedral smectites and carbonates instead of the magnesian smectites commonly observed in CO₂-free systems.The Mururoa feeder zone shows alteration features typical of a closed system interaction between the basaltic rock and a magmatic fluid in which seawater did not take part.     

Magma mixing: petrological process and volcanological tool

Magma mixing is a widespread, if not universal igneous phenomenon of variable importance. The evidence for magma mixing is found primarily in glassy tephra; the consolidation of lava obscures the evidence. Inclusions of glass in big crystals in tephra, because of their greater range in composition compared to the whole rock and the residual glass, indicate that the big crystals were derived from separate systems which mixed together prior to and during eruption. The observed or reconstructed concentration of K₂O in inclusions of glass in large crystals represent the composition of the contaminant and host systems. Selective enrichment in K₂O during entrapment of melt by growing crystals is shown to be negligible. The weight percents of K₂O in host, contaminant and residual glass and bulk rock determine the proportions of contaminant and host required to yield either the residual glass or bulk rock. In several cases the proportion of contaminant required is substantially larger than the proportion of crystals in the hybrid magma; therefore, by heat budget argument, the contaminant was partly liquid when contamination began. In some tephra individual phenocrysts contain glasses which are more silicic toward the center of the crystal indicating that the crystal grew from a melt whose composition changed in the opposite sense to that expected for progressive solidification of a closed system. Space time associations of compositionally distinct glassy tephra with contaminated magmas suggest coexistence of basaltic and silicic melts within magma systems. Evidence of contamination is present in most tephra studied so far. Magma mixing appears to be the prevalent process whereby contamination occurs. Magma mixing seems to be particularly evident in systems where there is independent evidence for a vapor-saturated magma reservoir. Probably vapor saturation promotes mixing in magma systems. Magma mixing probably is an important mechanism of compositional diversification (differentiation) of volcanic rocks from continental margin and possibly other environments.Textural evidence of the onset of magma mixing can be related to disturbance of a complex reservoir immediately before ascent and eruption. Thus, conditions before mixing can be ascribed to the reservoir. In this way it is possible to learn about the reservoir: its composition, its diversity, its depth, its walls. It is also possible to learn about the causes of eruption: whether by increase in gas pressure due to either progressive consolidation, or heating from below by an injection of hot magma, or by encounter with ground water; whether by buoyant rise. Evaluation of these problems requires also a thorough knowledge of the chronology of particular eruptions. Thus, magma mixing is a useful volcanological tool.     

Upscaling relative permeabilities in a structured porous medium

Upscaling of multi-phase flow problems for a heterogeneous porous medium requires modification of constitutive functions at the grid-block scale. A particular type of heterogeneity that has important environmental consequences involves thin, continuous streaks of high permeability through lower-permeability background rocks. These streaks, which may correspond to features like abandoned wells in mature sedimentary basins, can become preferential flow paths for an invading fluid. Quantification of flow through these types of heterogeneities in deep, geological formations is necessary for estimates of migration and possible leakage of injected fluids such as hazardous liquid wastes, municipal liquid wastes, and, possibly, carbon dioxide. One of the important constitutive functions for proper estimation of flow through these flow paths is the relative permeability function. In the simple case of a single high-permeability streak in a uniform rock matrix, with both materials having identical (local) relative permeability functions, the upscaled relative permeability must be changed significantly to capture the proper leakage. Standard petroleum reservoir pseudo-functions for relative permeability capture the general features of the upscaled function, but they still produce errors of several hundred percent in the leakage estimation. Detailed three-dimensional numerical simulations and associated upscaled calculations demonstrate the proper form for the upscaled relative permeability, and provide a modified derivation of pseudo-functions to capture the leakage behavior in upscaled models.     

Memory effect in M ≥ 7 earthquakes of Taiwan

The M?≥?7 earthquakes that occurred in the Taiwan region during 1906–2006 are taken to study the possibility of memory effect existing in the sequence of those large earthquakes. Those events are all mainshocks. The fluctuation analysis technique is applied to analyze two sequences in terms of earthquake magnitude and inter-event time represented in the natural time domain. For both magnitude and inter-event time, the calculations are made for three data sets, i.e., the original order data, the reverse-order data, and that of the mean values. Calculated results show that the exponents of scaling law of fluctuation versus window length are less than 0.5 for the sequences of both magnitude and inter-event time data. In addition, the phase portraits of two sequent magnitudes and two sequent inter-event times are also applied to explore if large (or small) earthquakes are followed by large (or small) events. Results lead to a negative answer. Together with all types of information in study, we make a conclusion that the earthquake sequence in study is short-term corrected and thus the short-term memory effect would be operative.     

Melt inclusion investigation of the volatile behaviour in historic alkali basaltic magmas of Etna

Crystallization paths of basaltic (1763 eruption) and hawaiitic (1865 and 1329 eruptions) scoria from Etna were deduced from mineralogy and melt inclusion chemistry. The volatile behaviour was investigated through the study of melt inclusions trapped in the phenocrysts and those of the whole rocks and the matrix glasses. The results from the 1763 eruption point to the early crystallization of olivine Fo 81.7 from a water-rich alkaline basalt, with high Cl (1750–2000 ppm) and S (2100–2400 ppm) concentrations. The hawaiitic melt inclusions trapped in olivine Fo 74, salite and plagioclase are characterized by a decrease in Cl/K₂O and S/K₂O ratios. In each investigated system there is good correlation between K₂O and P₂O₅. In the whole rocks, Cl ranges from 980 to 1680 ppm, from basaltic to hawaiitic lavas, whereas S (110–136 ppm) remains low. Cl and S behaviour in the 1763 magma suggests an early degassing stage of Cl and S, with CO₂ and a water-rich gaseous phase for a pressure close to 100 MPa, consistent with a permanent outgassing at the summit craters of Etna. During the eruption, the sulphur remaining in the hawaiitic liquid is lost, and the degassing of chlorine is limited. Such a degassing model can be extended to the 1865 and 1329a.d. eruptions.