the fluid dynamics of a basaltic magma chamber replenished by influx of hot dense ultrabasic magma this paper describes a fluid dynamical investigation of the influx of hot dense ultrabasic magma into a reservoir containing lighter fractionated basaltic magma this situation is compared with that which develops when hot salty water is introduced under cold fresh water theoretical and empirical models for salt water systems are adapted to develop a model for magmatic systems a feature of the model is that the ultrabasic melt does not immediately mix with the basalt but spreads out over the floor of the chamber forming an independent layer a non turbulent interface forms between this layer and the overlying magma layer across which heat and mass are transferred by the process of molecular diffusion both layers convect vigorously as heat is transferred to the upper layer at a rate which greatly exceeds the heat lost to the surrounding country rock the convection continues until the two layers have almost the same temperature the compositions of the layers remain distinct due to the low diffusivity of mass compared to heat the temperatures of the layers as functions of time and their cooling rate depend on their viscosities their thermal properties the density difference between the layers and their thicknesses for a layer of ultrabasic melt 18 mgo a few tens of metres thick at the base of a basaltic 10 mgo magma chamber a few kilometres thick the temperature of the layers will become nearly identical over a period of between a few months and a few years during this time the turbulent convective velocities in the ultrabasic layer are far larger than the settling velocity of olivines which crystallise within the layer during cooling olivines only settle after the two layers have nearly reached thermal equilibrium at this stage residual basaltic melt segregates as the olivines sediment in the lower layer depending on its density the released basalt can either mix convectively with the overlying basalt layer or can continue as a separate layer the model provides an explanation for large scale cyclic layering in basic and ultrabasic intrusions the model also suggests reasons for the restriction of erupted basaltic liquids to compositions with mgo 10 and the formation of some quench textures in layered igneous rocks

trends of sr and nd isotopes through time near the japan sea in northeastern japan nd and sr isotopic analyses were made on selected volcanic rocks of known stratigraphic age from a limited area in the green tuff region northeastern japan and in adjacent areas two ultramafic inclusions from ichinomegata were also analyzed the volcanic rocks range in age from 27 to 6 ma and vary in e nd from 0 9 to 5 7 and in e sr from 14 1 to 16 3 the isotopic data plot generally along the region of the mantle array when plotted against time the isotopic ratios are found to be a regular function of geologic age the results indicate that 1 the volcanic rocks in the area are part of a related sequence or evolutionary pattern over a time period of 27 ma 2 the evolutionary pattern goes from an initially enriched source to that of a very depleted source the shift in isotopic composition appears to reflect the ongoing lining of magma conduits or the decreasing extraction of lower crustal contributions with increasing infiltration of oceanic magma types in an environment where the crust is thinning based on the geotectonic development of this area due to the back arc spreading of the japan sea we identify two different stages the pre opening stage 15 ma and the post opening stage 15 ma the volcanic rocks of the pre opening stage are characterized by an enriched isotopic signature and those of the post opening era by a depleted signature the host rocks of the kuroko ore deposit lie within this evolutionary trend and contain a substantial amount of continentally derived materials processed through an oceanic environment in general we infer that the degree of continental crust involvement is high during the pre opening stages of a back arc basin when it is associated with a rifted continental mass

the dynamics of magma mixing in a rising magma batch the conditions under which two magmas can become mixed within a rising magma batch are investigated by scaling analyses and fluid dynamical experiments the results of scaling analyses show that the fluid behaviours in a squeezed conduit are determined mainly by the dimensionless number i mu 1 u g delta rho r 2 where 1 is the viscosity of the fluid u is the velocity g is the acceleration due to gravity is the density difference between the two fluids and r is the radius of the tube the parameter i represents a balance between the viscous effects in the uppermost magma which prevent it from being moved off the conduit walls and the buoyancy forces which tend to keep the interface horizontal the experiments are carried out using fluid pairs of various density and viscosity contrasts in a squeezed vinyl tube they show that overturning of the initial density stratification and mixing occur when i order 10 1 the two fluids remain stratified when i order 10 3 transitional states are observed when 10 3 i 10 1 these results are nearly independent of reynolds number re 1 rho 1 ur mu 1 and viscosity ratio mu 1 mu 2 in the range of 10 2 mu 1 mu 2 10 3 and re 1 300 applying these results to magmas shows that silicic to intermediate magmas overlying mafic magma will be prone to mixing in a rising magma batch this mechanism can explain some occurrences of small volume mixed lava flows

textural and compositional evidence for magma mixing and its mechanism abu volcano group southwestern japan quaternary basalts andesites and dacites from the abu monogenetic volcano group sw japan composed of more than 40 monogenetic volcanoes show two distinct chemical trends especially on the feo mgo vs sio2 diagram one trend is characterized by feo mgo enrichment with a slight increase in sio2 content fe type trend whereas the other shows a marked sio2 enrichment with relatively constant feo mgo ratios si type trend the fe type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma rocks of the si type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and or fine grained basaltic inclusions they are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz sodic plagioclase and hornblende phenocrysts petrographic variation as well as chemical variation from basalt to dacite of the si type trend is accounted for by various mixing ratios of basalt and dacite magmas pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing olivine and spinel and quartz sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas respectively before the mixing within a lava flow the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow and the number of resorbed phenocrysts varies inversely suggesting zonation in the magma chamber

aftershocks of the andreanof islands earthquake of june 10 1996 and local seismotectonics three questions have been addressed in connection with an mw 7 7 earthquake that occurred in the andreanof islands alaska on june 10 1996 first was there any variation in seismic activity rate during the previous five years that might have been diagnostic of the imminent occurrence of this event the answer based on the usgs pde catalog is no second though there were abundant pre event earthquakes and aftershocks on both sides of adak canyon why was there very little activity under the canyon itself a property that has been observed previously it is not possible to resolve the cause of the behavior with existing seismological data finally how does one explain the eastern group of aftershocks which fell within the zone of abundant aftershocks of the 1986 mw 8 andreanof islands earthquake when very little of the 1996 mainshock moment was released under or to the east of adak canyon one explanation of these aftershocks is that the recent aftershocks are late aftershocks of 1986 whose occurrence within a short time was induced by the stress pulse but not seismic slip from the 1996 earthquake though large earthquakes in the aleutians occur in distinct tectonic blocks the boundaries of these blocks may not always be defined reliably by aftershock zones

magma mixing processes in volcanic contexts a thermodynamic approach with the examples of st vincent soufriere volcano west lndies and cerro chiquito guatemala summary different modalities and thermodynamic conditions of magma mixing are compared heterogeneity and chemistry temperature and water pressure volatile water content and water solubility density and viscosity such data are somewhat scarce in the literature two case studies have been studied in detail to highlight this review 1 st vincent soufriere west indies magma mixing may have been a major process in triggering the 1979 explosive phase all products whole rock analyses are basaltic andesite 52 3 56 2 wt sio2 but dacitic 60 64 wt sio2 glass occurs as micropumice or inclusions in crystals 2 cerro chiquito guatemala a quaternary extinct dome consisting of dacite 62 5 67 1 wt sio2 enclosing 20 30 in volume of basaltic andesitic 54 9 55 3 wt sio2 enclaves the nearly thorough mixing process of case 1 strongly differs from the commingling process of case 2 the main phases may be mafic case 1 or acidic case 2 thermodynamic parameters are calculated using classical geothermometers and formulae the range of values and errors are estimated 60 80 c in case of temperature in addition other processes such as crystallization of mineralo gical phases and fluid vesiculation may modify thermodynamical conditions difference of magma temperature ranges from about 60 c case 1 up to about 130 c case 2 water concentrations of 2 5 wt in mafic magmas and 4 wt in acidic magmas are always lower than the upper limit of water solubility at depths where contrasting magmas were mixed the difference between the viscosities of the two magmas varies from one order of magnitude up to five various mixing scenarios may occur each of them being linked to different petrological processes including effusive or explosive volcanism as well as plutonism

evidence for two stage mixing in magmatic inclusions and rhyolitic lava domes on niijima island japan abstract fine grained inclusions are found in the achiyama and the mukaiyama rhyolitic lava domes on niijima island japan the existence of fine grained rims in the inclusions indicates that these inclusions are a quenched mafic magma incorporated into their rhyolite host the chemical composition and the phenocryst assemblage of the inclusions in the achiyama dome are identical with those of the effusive basaltic lava on niijima island wakago basalt the inclusions of the mukaiyama dome are dacitic and have a disequilibrium phenocryst assemblage olivine clinopyroxene orthopyroxene hornblende biotite plagioclase quartz and opaque minerals whole rock compositions lie on a mixing line between the host rhyolite and wakago basalt the dacitic magma is therefore a product of mixing of the basaltic and rhyolitic magmas the occurrence of two different inclusions basaltic and dacitic in two different domes can be explained by a two stage magma mixing model the dacite is formed in the first stage by mixing in a vertically stratified magma chamber caused by an interfacial instability between rhyolitic and basaltic layers a small amount of rhyolitic magma is entrained into the lower basaltic magma layer and two magmas can mix essentially in a liquid state the mixed dacitic magma is formed in the lower layer the second stage is mixing caused by forced convection during the ascent through a conduit just before eruption the stably stratified silicic and mafic magmas in a magma chamber can turn over and efficiently mix because of the formation of an unstable flow in a conduit under this condition the system reaches thermal equilibrium immediately after the mafic magma is disaggregated and dispersed in the silicic host the mafic magma solidifies when the silicic endmember is predominant in the mixture hence the equilibrium temperature is below the solidus of the mafic magma mafic inclusions are formed at this stage whether the inclusions are basaltic or dacitic depends on whether the final ascent through a conduit eruption occurred before or after the formation of the layer of a mixed dacitic magma in a magma chamber

petrology and geochemistry of late stage intrusions of the a type mid proterozoic pikes peak batholith central colorado usa implications for petrogenetic models abstract the 1 08 ga anorogenic a type pikes peak batholith front range central colorado is dominated by coarse grained biotite amphibole syenogranites and minor monzogranites collectively referred to as pikes peak granite ppg the batholith is also host to numerous small late stage plutons that have been subdivided into two groups e g wobus 1976 studies in colorado field geology colorado school of mines professional contributions colorado 1 a sodic series sio 2 44 78 wt k na 0 32 1 36 composed of gabbro diabase syenite quartz syenite and fayalite and sodic amphibole granite and 2 a potassic series sio 2 70 77 wt k na 0 95 2 05 composed of biotite granite and minor quartz monzonite differences in major and trace element and nd isotopic characteristics for the two series indicate different petrogenetic histories potassic granites of the late stage intrusions appear to represent crustal anatectic melts derived from tonalite sources based on comparison of their major element compositions with experimental melt products in addition nd isotopic characteristics of the potassic granites e nd 1 08 ga 0 2 to 2 7 overlap with those for tonalites granodiorites ca 1 7 ga boulder creek intrusions e nd 1 08 ga 2 4 to 3 6 exposed in the region some of the partial melts evolved by fractionation dominated by feldspar the late stage potassic granites share geochemical characteristics with most of the ppg which is also interpreted to have an anatectic origin involving tonalitic crust the origin of monzogranites associated with the ppg remains unclear but mixing between granitic and mafic or intermediate magmas is a possibility syenites and granites of the sodic series cannot be explained as crustal melts but are interpreted as fractionation products of mantle derived mafic magmas with minor crustal input high temperature and low oxygen fugacity estimates e g frost et al 1988 american mineralogist 73 727 740 support a basalt fractionation origin as do e nd values for sodic granitoids e nd 1 08 ga 2 2 to 0 7 which are higher than e nd values for colorado crust at 1 08 ga ca 1 0 to 4 0 enrichments in incompatible elements e g rare earth elements rb y and depletions in compatible elements e g cr sr ba in the sodic granitoids compared to coeval mafic rocks are also consistent with fractionation accessory mineral fractionation release of fluorine rich volatiles and or removal of pegmatitic fluids could have modified abundances of ce nb zr and y in some sodic granitoid magmas gabbros and mafic dikes associated with the sodic granitoids have e nd 1 08 ga of 3 0 to 3 5 which are lower than depleted mantle at 1 08 ga and their trace element characteristics suggest derivation from mantle sources that were previously affected by subduction related processes however it is difficult to characterize the mantle component in these magmas because assimilation of crust during magma ascent could also result in their observed geochemical features the pikes peak batholith is composed of at least two petrogenetically different granite types both of which exhibit geochemical characteristics typical of a type granites models proposed for the petrogenesis of the granitoids imply the existence of mafic rocks at depth and addition of juvenile material to the crust in central colorado at 1 1 ga

magma mixing in mantle xenolith bearing calc alkalic ejecta ichinomegata volcano northeastern japan abstract the latest pleistocene ejecta from the ichinomegata volcano ne japan composed of three maars show a wide compositional variation ranging from basalt 51 sio 2 to dacite 65 sio 2 these have the same phenocryst assemblage of olivine augite hornblende biotite quartz plagioclase and titanomagnetite upper mantle peridotite xenoliths are found not only in basalts but also in andesite whole rock major chemistry and modal compositions indicate that the chemical and modal variations have been caused by the mixing of three magmatic components primitive high alumina basalt sio 2 51 wt mgo 8 5 wt with olivine fo 88 and augite phenocrysts differentiated basaltic andesite sio 2 54 wt mgo 5 wt which has the same but slightly fe rich phenocryst phases as the primitive basalt and porphyritic dacite sio 2 65 wt mgo 2 0 wt with phenocrysts of hornblende biotite quartz plagioclase and titanomagnetite the upper mantle peridotite bearing andesite may not be derived directly from the upper mantle and its peridotite xenoliths may have been carried by the primitive basaltic magma the estimated density for three magmatic end components requires a high flow velocity of basaltic magma and a small volume 5 m 3 of the dacite magma body to facilitate breaking of the stratification in the dacite magma body this is consistent with the retention of peridotite xenoliths in the mixed magma and the preserved disequilibrium petrographic features

magma mixing versus xenocryst assimilation the genesis of trachyandesites in sancy volcano massif central france sr and nd isotopic compositions have been determined on basaltic and acid trachyandesites bta ata from the sancy volcano mont dore massif france these represent more than 80 of the lavas erupted during its activity between 0 9 and 0 2 ma these lavas have been recently interpreted as the result of two component magma mixing during and after repeated injections of basaltic magmas in trachytic reservoirs magmatic heterogeneities in the ata s large to small enclaves banded lavas megacrysts testify to the mingling event complete mixing is supposed to have been achieved in the hybrid bta s which contain sanidine plagioclase and clinopyroxene megacrysts in disequilibrium with their host the megacrysts are interpreted as relicts of the trachytic end member isotopic data on basic comagmatic enclaves and host ata matrix samples from three different cycles of mingling succession of heterogeneous pyroclastics heterogeneous ata lava flows or domes and occasionally homogeneous bta lava flows are not incompatible with two component mixing but could just reflect the heterogeneity of the analysed samples however the bta s have sr contents and sr isotopic ratios which are too high to be simple binary mixing products between the postulated end members three hypotheses are considered to explain this discrepancy 1 the analysed end members are not those involved in bta genesis 2 some crustal contamination occurred during and after the mixing event 3 sr rich sanidine xenocrysts with radiogenic 87sr86sr have been assimilated and digested in the bta s in this third hypothesis that i favour it is not necessary to resort to magma mixing to explain the genesis of the bta s assimilation of xenocrysts by basaltic hawaiitic or mugearitic magmas accounts for both mineralogical disequilibria and isotopic characteristics of these lavas

the origin of mount st helens andesites abstract mount st helens volcano has intermittently produced mainly dacitic products but occasionally erupted a more diverse suite of lavas including basalts and andesites petrogenetic relations between these magmas provide insight into the dynamics of the subjacent magma system mineralogical and geochemical features of representative lavas erupted during the past 2200 years can distinguish three basaltic and three andesitic variants the mafic lavas include a transitional olivine plagioclase basalts with low k 2 o and incompatible trace element abundances b calc alkaline olivine plagioclase clinopyroxene basalts enriched in k 2 o tio 2 and incompatible trace elements and c calc alkaline olivine plagioclase basaltic andesites with incompatible trace element contents transitional between the two basalt types intermediate lavas include a tholeiitic two pyroxene andesites b calc alkaline plagioclase two pyroxenes olivine amphibole mafic andesites 56 59 wt sio 2 and c calc alkaline plagioclase two pyroxenes amphibole high silica andesites 61 62 wt sio 2 eruption of these magmatic variants within the same eruptive phase implies the existence of different petrogenetic lineages and that the plumbing system is sufficiently complex to simultaneously isolate and preserve numerous magma batches it is unlikely that any of the andesites or dacites are derived by fractional crystallization of the recognized basaltic variants formation of the andesites simply by contamination or assimilation fractional crystallization of basaltic magma is also improbable more plausibly the andesites represent mixing between basaltic and dacitic end member magmas each of which may be somewhat heterogeneous or vary in composition with time in this model efficient mixing must occur in some parts of the magma plumbing system while some conduits or storage reservoirs must be effectively isolated

the aleutian earthquake of june 10 1996 mw 7 9 ruptured parts of both the andreanof and delarof segments the epicenter of the aleutian earthquake of june 10 1996 is located in the delarof segment where a large subduction earthquake is expected however the aftershock area of the earthquake is not only in the delarof segment but also in the andreanof segment where the 1986 andreanof earthquake occurred previously we estimate the fault parameters of the 1996 earthquake by numerically computing the tsunami and comparing the waveforms with that recorded at adak the fault width is 30 km or less and the average slip is about 4 m the observed far field tsunamis around the pacific are also explained by the synthetic tsunami computed using the above fault parameters the results indicate that the 1996 earthquake ruptured parts of both the delarof and andreanof segments however the 1996 earthquake did not rupture the region of the largest moment release by the 1986 earthquake

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

grain boundary enrichment of incompatible elements in some mantle peridotites electron microprobe analyses sensitive to 15 40 ppm at 3 confidence have been made for minerals in a spinel dunite from san carlos arizona u s a and a spinel lherzolite from ichinomegata akita ken japan whereas both rocks contain an appreciable amount of k2o p2o5 and tio2 the constituent minerals contain only trace amounts of these elements microprobe analyses across the grain boundaries on polished sections and on mineral surfaces before and after acid leaching show that most of the incompatible elements in the rocks are contained within a surface film al k emission spectroscopy suggests that the surface film is neither an aggregate of fine grained intergranular minerals nor a quenched glass it could be an adsorbate on the irregular sites of crystal surfaces and was probably formed by any of diffusion from interiors of minerals i e segregation mantle metasomatism and partial melting the contribution of incompatible elements from the grain boundaries into partial melts has been documented by heating experiments on peridotite chips under controlled fo2 the spinel dunite yields at 1300 c a 4 5 vol partial melt with about 1 k2o 1 6 na2o 0 14 p2o5 0 8 tio2 210 ppm sr 140 ppm ba and 40 ppm ce along the grain boundaries grain boundaries are an important storage of incompatible elements in peridotites and they in turn can be the dominant source of these elements in magmas during partial melting of the upper mantle

a comparison of the spatio temporal distribution of moment release for the 1986 andreanof islands earthquake with relocated seismicity we compare relocated teleseismically recorded seismicity before and after the may 7 1986 andreanof islands earthquake mw 8 to the spatial and temporal distribution of seismic moment release during the mainshock relocated seismicity from 1964 through april 1987 clusters strongly along the main thrust zone in similar locations for the pre and post mainshock periods we performed simultaneous iterative inversions of broadband teleseismic gdsn and nars p waveforms using the method of kikuchi and fukao 1987 fault plane parameters e g strike dip depth and extent of fault plane consistent with regional tectonics and aftershock locations were specified the results of the inversions are spatial and temporal maps of moment release on the fault plane most of the moment found by the inversion is localized in several regions of the fault plane resolution of the location and timing of the major subevents is good the rupture was bilateral with a highly variable rupture velocity a shallow updip region of the fault plane ruptured toward the end of the mainshock process the mainshock moment release tended to occur in regions of the fault plane in which no or few aftershocks or preshocks are located this tendency suggests that regions of high moment release in the 1986 andreanof islands earthquake resulted from rupture of mechanically strong regions i e asperities on the fault plane which exhibited little seismicity before or after the mainshock that ruptured them we infer that the asperity regions were locked and quiescent before the mainshock while surrounding seismically active areas did not store as much strain and slipped mostly aseismically

plate coupling variation and block translation in the andreanof segment of the aleutian arc determined by subduction zone modeling using gps data 1 we use gps measurements in the first geodetic study of plate coupling on the andreanof segment aleutian subduction zone convergence of 7 3 cm yr at the subduction zone results in northwestward displacements greater than 1 cm yr at stations located on the west end of the andreanof region but velocities at the east end are smaller in magnitude and oriented west southwest these velocity variations are caused by differences in plate coupling along the subduction zone which correlate with the rupture zones of the 1986 and 1996 mw8 earthquakes we construct a dislocation model to estimate the velocity of the arc in the andreanof region relative to north america and fault plane coupling coefficients our best model shows a southwestward velocity of 7 8 mm yr a high degree of coupling in the main thrust zone at the west end of the subduction zone and little to no coupling at the east end

evolution of magma mixing in an alkaline suite the grande cascade sequence monts dore french massif central geochemical modelling abstract trachyandesitic lavas from an alkaline suite grande cascade sequence monts dore french massif central have been investigated for evidence of mechanical magma mixing a wide variety of textures are present in the sequence as pillow like enclaves bandings and emulsified facies the grande cascade flows and pyroclasts range from basalt to trachyte 44 67 wt sio 2 pillow like enclaves intricate commingling and local diffusive mixing at the mafic felsic interface in emulsified types show that mafic inclusions were incorporated in their trachytic host as blobs of magma such inclusions were probably dispersed throughout the trachytic magma by mechanical mixing different stages of the evolution of a mechanical mixing process were recognized in relation with the stratigraphy and chronology of the products the most heterogeneous features enclaves bandings are restricted to the bottom of the sequence where basalt and trachyte coexist at the top texturally homogeneous trachyandesites occur relict phenocrysts are observed in both basaltic enclaves oligoclase fe rich clinopyroxene sanidine of the trachyte and trachytic hosts mg rich olivine mg rich clinopyroxene ca rich plagioclase of basalt the intermediate rocks basic trachyandesites contain xenocrysts of mg rich olivine clinopyroxenes oligoclase sanidine together with reversely zoned plagioclase and pyroxene phenocrysts this assemblage is presumed to result from the incorporation of xenocrysts related both to mafic and felsic endmembers all mineral assemblages are out of equilibrium reversed compositional zoning profiles in plagioclase and clinopyroxene crystals were produced by the mixing event we are able to identify the compositional characteristics of the components that were mixed a primary alkaline basalt mg olivine labrador mg clinopyroxene magnetite and a silicic trachyte oligoclase fe clinopyroxene sanidine are the endmembers of mixing observed chemical variations in major and trace elements can be modelled as pure binary mixtures between basaltic and trachytic endmembers using the best mixing line method provost and allegre 1979 the calculations emphasize that trachyandesites are the result of complete hybridization between basalt and trachyte the data on mesostasis show that melt mixing and not only crystal transfers has produced the intermediate lavas of the grande cascade sequence textural criteria petrography mineral chemistry and geochemical data major and trace elements support the magma mixing model

a model of reverse differentiation at dikii greben volcano kamchatka progressive basic magma vesiculation in a silicic magma chamber dikii greben volcano is the largest modern volcano with silicic rocks in the kurile kamchatka island arc it consists of many domes and lava flows of rhyodacite dacite and andesite which were erupted in a reverse differentiation sequence non equilibrium phenocryst assemblages quartz mg rich olivine an rich an poor plagioclase etc abundance of chilled mafic pillows in the dacites and andesites and linear variations of rock compositions in binary plots are considered as mineralogical textural and geochemical evidence for mixing mafic pillows in volcanics have a lower density because of high porosity and contain the same non equilibrium phenocryst assemblages as the host rocks their groundmass contains skeletal microlites of plagioclase and amphibole proving that the groundmass as well as the pillows themselves formed from a water rich basaltic magma at depth they are considered as supercooled vesiculated floating drops of a hot hybrid layer in the magma chamber which formed after refilling the lower density of the inclusions allows them to float in the host magma and to concentrate at the top of the chamber prior to eruption magma mingling was effected by mechanical disintegration of the inclusions in the host magma during eruption the rhyodacitic and basic end members of the mixing series cannot be linked by low p fractionation though high p amphibole rich fractionation is not excluded

petrology of the younger andesites and dacites of iztaccihuatl volcano mexico i disequilibrium phenocryst assemblages as indicators of magma chamber processes

mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab slab derived fluids play an important role in heat and material transfer in subduction zones dehydration and decarbonation reactions of minerals in the subducting slab have been investigated using phase equilibria and modeling of fluid flow nevertheless direct observations of the fluid chemistry and pressure temperature conditions of fluids are few this report describes co2 bearing saline fluid inclusions in spinel harzburgite xenoliths collected from the 1991 pinatubo pumice deposits the fluid inclusions are filled with saline solutions with 5 1 1 0 wt nacl equivalent magnesite crystals co2 bearing vapor bubbles and a talc and or chrysotile layer on the walls the xenoliths contain tremolite amphibole which is stable in temperatures lower than 830 c at the uppermost mantle the pinatubo volcano is located at the volcanic front of the luzon arc associated with subduction of warm oceanic plate the present observation suggests hydration of forearc mantle and the uppermost mantle by slab derived co2 bearing saline fluids dehydration and decarbonation take place and seawater like saline fluids migrate from the subducting slab to the mantle wedge the presence of saline fluids is important because they can dissolve more metals than pure h2o and affect the chemical evolution of the mantle wedge

chemical modification of enclave magma by post emplacement crystal fractionation diffusion and metasomatism hornblende bearing microgranitoid enclaves from the swifts creek pluton scp se australia display mineralogical and textural variations from their margins to their centers margins are fine grained and display quench textures and are enriched in amphibole biotite and in some cases magnetite relative to their coarser grained centers enclaves of this type and their adjacent granitoid host rocks have been sectioned into 0 3 to 1 cm thick slabs in order to determine the chemical variations associated with these mineralogical changes the fine grained margins are variably enriched in ti al mg fe mg k rb ba nb y sc v ni and the ree up to a factor of three relative to the enclave centers this enrichment is compensated by a depletion of si and zr elements like ca na and sr show less coherent variation from margin to center host rocks in immediate contact with these enclaves are depleted in mg na k rb and ba relative to host rocks that are not exposed to enclaves no one single process can account for all the variations instead we propose that the chemical variations are related to a combination of in situ crystal fractionation of isolated magma globules mass transfer by diffusion and metasomatic exchange

contamination of deep formation waters by drilling fluids correction of the chemical and isotopic composition and evaluation of errors abstract contamination of deep formation waters by drilling fluids is a problem that concerns most types of drilling operations petroleum wells geothermal boreholes and is crucial in the course of feasibility and safety studies of potential radioactive waste repositories residual contamination of formation water samples has an important impact on the accuracy of the characterisation of the natural hydrogeochemical background of the study sites based on a literature review and on experience acquired on the sites of the french agency for nuclear waste disposal andra this article proposes a general method for the correction of residual contamination including estimates of the associated errors the quantification of the contamination is based on tracing techniques and on a geochemical survey during the pumping test preceding sampling the correction and estimation of errors require repeated measurements of the tracer s and the chemical and isotopic species during pumping the method is applied to a pumping test in a research well in deep granite of the vienne district france where multi tracing of drilling fluids has been used

on the na2o content of convergent zone high alumina basalts abstract high alumina basalts in the convergent zone are classified into high medium and low na type the high na high alumina basalts occur in arcs without low alkali tholeiites where younger and warmer oceanic lithospheres are subducting the low na high alumina basalts occur in arcs where on the contrary older and cooler plates are descending and occur also in arcs with extensional tectonics even though younger lithospheres are underthrusting a good correlation is observed between the na2o content of high alumina basalts and the age of the subducting lithosphere this suggests that generation of high alumina basalt magma is controlled by the nature of the descending plate

absence of strain accumulation in the western shumagin segment of the alaska subduction zone gps measurements in the western shumagin segment of the alaska subduction zone show no significant strain the elastic dislocation model that best fits the data has the entire plate interface slipping freely the simplest explanation is that steady slip at the plate convergence rate is occurring on the entire plate interface although we cannot rule out the existence of a shallow locked patch close to the trench large enough to generate a m 7 earthquake the 99 confidence upper bound on the down dip width of the locked patch is 55 km assuming the trench is the the updip limit of the locked zone unless there is significant temporal variability in the locking of the plate interface or viscoelastic effects from a past earthquake the shumagin segment cannot by itself release enough moment to generate a great m 8 earthquake

revel a model for recent plate velocities from space geodesy 1 we present a new global model for recent plate velocities revel describing the relative velocities of 19 plates and continental blocks the model is derived from publicly available space geodetic primarily gps data for the period 1993 2000 we include an independent and rigorous estimate for gps velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates the velocity fields for north america eurasia and antarctica clearly show the effects of glacial isostatic adjustment and australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field two thirds of tested plate pairs agree with the nuvel 1a geologic 3 myr average velocities within uncertainties three plate pairs caribbean north america caribbean south america and north america pacific exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in nuvel 1a due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities most other differences probably reflect real velocity changes over the last few million years several plate pairs arabia eurasia arabia nubia eurasia india move more slowly than the 3 myr nuvel 1a average perhaps reflecting long term deceleration associated with continental collision several other plate pairs including nazca pacific nazca south america and nubia south america are experiencing slowing that began 25 ma the beginning of the current phase of andean crustal shortening

peridotite xenoliths in silica rich potassic latite from the transition zone of the colorado plateau in north central arizona potassic latite in the transition zone of the colorado plateau near chino valley arizona contains abundant eclogite and amphibolite xenoliths and minor websterite and pyroxenite xenoliths one unit contains peridotite xenoliths analyzed samples have mg ratios of 68 and 71 58 and 63 wt sio2 and are enriched in potassium and other large ion lithophile lil elements rare earth element ree patterns are light ree enriched with la greater than 100 times chondritic abundance the peridotite xenoliths are partly to totally altered but contain remnant olivine orthopyroxene and clinopyroxene one harzburgite nodule also contains spinel mineral compositions from the xenoliths are relatively refractory and similar to those in other spinel peridotite xenoliths from the colorado plateau geothermometry on olivine spinel and two pyroxene pairs indicates equilibration temperatures of less than 800 c for the peridotite nodules the relatively low temperatures calculated from mineral equilibria are consistent with temperature estimates for other mantle nodules from under the colorado plateau

a model for mixing basaltic and dacitic magmas as deduced from experimental data it has been demonstrated experimentally that basaltic and dacitic magmas can be easily mixed to form both banded dacite and homogeneous andesite in less than a few hours the presence of phenocrysts larger than 0 5 mm increased considerably the efficiency of mixing flow patterns in the experimental system were visualized using pt spheres which indicated that convection occurs in basalt melt but not in dacite melt the reynolds numbers of the basaltic and dacitic melts in the experimental system were calculated to be about 10 3 and 10 6 respectively mixing proceeds initially by mechanical mixing of the two magmas in a large scale but later by coupling interfacial convection and mutual diffusion thus depending on the depth where vesiculation and following disruption of the magma occurs banded pumice homogeneous pumice and homogeneous andesite lava are erupted the observed textures of mixed rocks of plinian type eruption and the limiting occurrence of banded pumice are satisfactorily accounted for on this model

seismicity of the bering strait region evidence for a bering block newly compiled russian and u s seismological data support an independent bering block in motion relative to the north american plate this motion is likely to be driven by the westward extrusion of southwestern alaska resulting from compression in southern alaska due to subduction of the pacific plate and terrane accretion seismicity extends from central alaska through the bering strait and into chukotka in eastern chukotka several southwest trends are evident some of which continue through the koryak highlands to kamchatka the seismicity outlines the bering block which includes most of the bering sea chukchi peninsula seward peninsula and parts of western alaska focal mechanisms young basaltic volcanism and normal faults in western alaska and chukotka indicate that the bering strait is under northeast southwest extension this in conjunction with thrust faulting in the koryak highlands indicates that the bering block is rotating clockwise relative to the north american plate

life time of a stratified magma chamber recorded in ultramafic xenoliths from ichinomegata volcano northeastern japan variations in major element chemistry and modal compositions of the mantle xenolith bearing calc alkalic ejecta from ichinomegata volcano are inferred to be due to mixing of three magmatic end members basalt i sio2 51 wt mgo 8 5 wt basalt ii sio2 54 wt mgo 5 wt and dacite sio2 65 wt mgo2 wt ultramafic xenoliths are found in mafic mixtures of dacite basalt i and dacite basalt ii the thermal histories of the xenoliths in both mixtures are compared with each other chemical compositions of olivine and orthopyroxene in xenoliths suggest that xenoliths in basalt i were equilibrated at about 800 c while those in basalt ii were also equilibrated originally at about 800 c but were subsequently annealed at about 1000 c for more than 102 3 years prior to the eruption