diapiric transfer of melt in kilauea iki lava lake hawaii a quick efficient process of igneous differentiation kilauea iki lava lake formed in 1959 is a large pond of picritic basalt average mgo content 15 34 by weight which has cooled and crystallized as a small self roofed magma chamber repeated drilling of the upper crust of the lake down to its molten core and more recent 1981 drilling through the thermal maximum and part way through the lower crust have made it possible to monitor the differentiation processes in the lake in detail differentiation processes recognized as active in the lake include rather inefficient settling of the larger 2 10 mm olivine phenocrysts formation of segregation veins and formation of diapir like vertical olivine rich bodies all processes which occur in one or more of the other kilauean lava lakes as well in addition most of the central part of kilauea iki has been affected by diapiric melt transfer in this process relatively low density liquid present at 1145 1160 c rose from within the loose crystal mush at the base of the lens of melt and intruded the equivalent thermal horizon at the top of the lens of melt passing through the hotter but denser melt in the core of the lake without mixing the source volume from which the low density liquid was extracted is depleted in tio 2 and other incompatible elements and enriched in feo and cao the upper part of the lake shows the opposite effects the crystalline assemblage present was olivine augite minor plagioclase the crystallinity of both the source and receiving layers was low enough that no obvious textural imprint was left by the transfer process the principal evidence for its occurrence is the chemical zonation of the lake seen in core from depths of 13 to 80 m diapiric melt transfer was active from 1960 to 1971 and has affected most of the central part of the lake from 13 m to at least 80 m the process ran simultaneously with the other three main differentia tion processes but started and stopped independently of the others calculations suggest that between 21 and 42 wt liquid has been extracted from the depleted zone at 56 78 m in the center of the lake making this a very efficient process of chemical differentiation

physical properties of partly melted tholeiitic basalt melting experiments on centimetre scale blocks of tholeiitic holyoke basalt from the mesozoic hartford basin of connecticut reveal that when this basalt is only 30 crystallized a relatively strong network of crystals exists cubes of the basalt melted to this degree retain their shape as the interstitial liquid drains easily from the crystal network the intrinsic permeability of a sample containing 34 crystals was determined to be 3 10 10 m2 which is similar to that of coarse gravel serial sections through partly melted samples reveal that the network owes its strength to monomineralic strings of plagioclase and pyroxene crystals although these strings are clearly evident once the rock has been partly melted they are difficult to see in thin sections of the initial basalt but they are present the compressive strength of the mush increases dramatically from 336 to 4333 pa as the percentage of crystals increases from 33 to 37 this rapid strengthening is believed to explain why sheets of segregation liquid in tholeiitic flood basalts have compositions that correspond to no more than 35 fractional crystallization of the host basalt these sheets are proposed to form through compaction of the crystal mush in the lower part of thick flows the compaction occurring only while the mush remains weak that is while it has 35 crystals

linking ocean and continental records of paleoclimate determining accord and discord in the records climate forcing at milankovitch and sub milankovitch frequencies are evaluated by linking climate proxy data from terrestrial and ocean records at two scales of temporal resolution 1 relatively coarse resolution over the last 500 ka and 2 higher resolution over the last 200 ka we determine that climate proxy data from hole 810c rep resent regional atmospheric and oceanic processes by comparison to core v21 146 both from the northwest pacific ocean we then compare records that represent regional deposition from china and the western pacific ocean to focus on determining milankovitch type periodicities grain size and magnetic susceptibility data from hole 810c and baoji china are contrasted using a time sensitive wavelet analysis the analyses indicate a general correspondence to milankovitch frequencies although there is no specific frequency that dominates throughout the records additionally the dominant 100 000 year frequency does not occur at the same time in the records these factors suggest that differ ences in the physical mechanisms of deposition may influence the preserved apparent periodicities in these environ ments lastly climate events over the last 200 000 years in marine and terrestrial records are correlated at a high resolution in the depth domain the last interglacial glacial boundary is consistently placed at the base of a cold period c20 in the terrestrial and marine records however the cold period in the ocean data 18 o is very subdued relative to terrestrial records additionally over the last 200 ka periods of minimum insolation intensity do not correspond lin early to periods of maximum grain size and ice volume variations revealed in the loess and ocean records respectively however during the warm interstadials of the last two glaciations peaks in insolation intensity compare favourably with the times of strongest pedogenic development in the continental records and minimum ice volume in the ocean records resume le forcage du climat a des frequences milankovitch et sous milankovitch est evalue en faisant un lien entre les donnees indirectes de sources terrestres et oceaniques a deux echelles de resolution dans le temps 1 une resolu tion relativement grossiere pour les dernieres 500 ka et 2 une plus haute resolution pour les dernieres 200 ka nous determinons que les donnees indirectes du trou 810c representent des procedes atmospheriques et oceaniques regionaux par rapport a la carotte v21 146 les deux provenant du nord ouest de l ocean pacifique nous avons ensuite compare les registres qui representent une deposition regionale en provenance de la chine et de l ouest de l ocean pacifique afin de se concentrer sur la determination de frequences de type milankovitch des donnees granulometriques et de suscep tibilite magnetique provenant du trou 810c et de baoji china sont mises en contraste au moyen d une analyse tempo relle des ondelettes les analyses indiquent une bonne correspondance generale aux frequences de milankovitch bien qu aucune frequence specifique ne domine a travers les registres de plus la frequence principale de 100 000 ans ne se presente pas au meme moment dans les registres ces facteurs suggerent que des differences dans les mecanismes phy siques de deposition peuvent influencer les periodicites apparentes preservees dans ces environnements finalement dans le domaine de la profondeur il y a correlation a haute resolution entre les registres marins et terrestres des evene ments climatiques au cours des derniers 200 000 ans la derniere limite glaciaire interglaciaire est constamment placee a la base d une periode froide c20 dans les registres marins et terrestres toutefois la periode froide dans les don nees oceaniques 18 o est tres attenuee par rapport aux registres terrestres de plus au cours des dernieres 200 ka des periodes d intensite minimale d insolation ne correspondent pas de facon lineaire aux periodes de granulometrie maximale et de variations des volumes de glace qui sont revelees respectivement dans les registres oceaniques et de loess toutefois au cours des rechauffements interstadiaires des deux dernieres glaciations des sommets dans l intensite d insolation se comparent favorablement avec les temps du plus fort developpement pedogenique dans les re gistres continentaux et avec le volume minimal de glace dans les registres oceaniques traduit par la redaction 848

convective exchange between pore fluid and an overlying reservoir of denser fluid a post cumulus process in layered intrusions abstract we describe experiments on buoyancy driven exchange between fluid in a porous bed of glass balls and an overlying reservoir of denser fluid fingers of the dense fluid penetrated downwards into the porous medium when the two fluids had equal viscosities the rate of convective exchange was proportional to permeability and the density difference between the fluids and was inversely proportional to viscosity when the fluids had different viscosities the rate further depended on the viscosity ratio raised to the power 0 4 if a layer of dense new melt is emplaced above a porous cumulus pile forming the floor of a magma chamber an analogous process may occur as the new melt can percolate down into the pore space and expel the less dense intercumulus liquid convective exchange may proceed at rates of metres per year to tens of metres per year in basic to ultrabasic magmatic systems this conclusion strongly depends on the porosity of the cumulus pile and is less sensitive to the magmatic viscosities chemical and textural effects will result from this process such as re equilibration and resorption of cumulus crystals when intercumulus melt exchange is incomplete lateral geochemical gradients may be preserved in cumulate rocks

crystal mush compaction and the origin of pegmatitic segregation sheets in a thick flood basalt flow in the mesozoic hartford basin connecticut

interactions between bubbles in magmas and lavas effects of bubble deformation the interactions between two deformable bubbles are studied in order to determine the effects of deformation on bubble dynamics and to determine the limits in which the effects of deformation can be ignored deformation due to hydrodynamic interactions with other bubbles leads to alignment of horizontally offset bubbles and thus an enhanced rate of coalescence bubble alignment may produce spatial inhomogeneities in bubble concentrations in magmas which implies temporal variations in the concentration and size of bubbles in erupting magmas and spatial variations in solidified lavas an approximate quantitative model for the rate of coalescence of deformable bubbles is developed based on a series of experiments the effects of deformation can be ignored if the largest bubbles have radii less than about 5 mm in silicate magmas

mixing of stratified liquids by the motion of gas bubbles application to magma mixing mafic magma underlying more silicic magma in a reservoir can exsolve volatiles as it crystallizes forming bubbles that segregate upwards to the interface with the overlying silicic magma we describe laboratory experiments designed to investigate how gas bubbles migrate into the more viscous upper layer in one regime bubbles move individually across the interface entraining a small quantity of lower layer fluid and the two layers become progressively stirred into a homogeneous mixture in a second regime bubbles form a thin foam layer at the interface which becomes gravitationally unstable giving rise to two phase plumes which rise and produce a coarse mixture of the liquids for a given viscosity ratio the transition between these regimes occurs when the gas flux is greater than a critical value which we found to decrease with increasing viscosity ratio we estimate the critical gas flux for magmatic conditions and how vesiculation may vary with pressure degree of crystallization and the composition of volatile components mafic inclusions which are often found in silicic to intermediate lavas can form in the plume regime analysis of the instability of the foam predicts wavelengths of between 1 cm and 1 m which is consistent with the observed sizes of inclusions volatiles from the mafic magma may be efficiently transported by this mechanism into the viscous silicic magma as bubble plumes despite low stokes velocities for individual vesicles at small viscosity ratios or low gas fluxes the bubbling regime can produce hybrid magmas

segregation processes in vesiculating crystallizing magmas vesiculation of crystallising magma can produce either a mobile vesicular magma or a rigid network of crystals containing vesicular liquid where partially crystallized rigid mush underlies less crystallized magma such as near the base of a lava flow or in the cumulus pile of a magma chamber evolved interstitial melt and or gas may escape into the main body of magma the consequences of this may include contamination of the overlying liquid with gas and interstitial melt or intrusion of diapirs of vesicular evolved liquids to form vertical vesicle cylinders and other segregation features found in many basaltic lava flows and sills analog experiments were used to investigate some of the phenomena that can arise during vesiculation within a crystal mush which was simulated by pumping air through a porous plate that formed the floor of a container filled with a viscous liquid floored with a layer of glass beads experiments used either a single liquid or two stably stratified liquids with a liquid interface either coincident with the top of the porous layer of beads or slightly above the porous layer for a range of liquid viscosities and air flow rates vesiculation rates individual bubbles emerged from the top of the porous layer of beads and carried a thin trail of interstitial liquid into the overlying liquid the number of bubble trains leaving the surface of the porous bed increased with decreasing liquid viscosity and flow rate and with increasing bead size and hence with increasing permeability analog vesicle cylinders composed of diapirs of bubbly interstitial liquid were produced only when a layer of buoyant bubbly liquid lay above the surface of the porous layer the relative size of the bubbles and constrictions within the porous layer are argued to control whether individual bubbles leading to bubble trains or vesicular liquid leading to vesicle cylinders leaves the porous layer and hence whether vesicle cylinders can form

gas driven filter pressing in magmas most silicic and some mafic magmas expand via second boiling if they crystallize at depths of about 10 km or less the buildup of gas pressure due to second boiling can be relieved by expulsion of melt out of the region of crystallization and this process of gas driven filter pressing assists the crystallization differentiation of magmas for gas driven filter pressing to be effective the region of crystallization must inflate slowly relative to buildup of pressure and expulsion of melt these conditions are satisfied in undercooled magmatic inclusions and in thin sheets of primitive magma underplating cooler magma reservoirs gas driven filter pressing thereby adds fractionated melt to magma bodies gas driven filter pressing is probably the dominant process by which highly evolved melts segregate from crystal mush to form aplitic dikes in granitic plutons this process could also account for the production of voluminous crystal poor rhyolites

an experimental investigation of volatile exsolution in evolving magma chambers abstract previous laboratory experiments investigating the fluid dynamics of replenished magma chambers have been extended to model effects resulting from the release of gas turbulent transfer of heat between a layer of dense hot and volatile rich mafic magma overlying cooler more evolved magma can lead to crystallization and exsolution of volatiles in the lower layer small gas bubbles can cause the bulk density to decrease to that of the upper layer and thus produce sudden overturning and initiate mixing followed by further exsolution of gas and explosive eruption these processes have been modelled in the laboratory using a chemical reaction between sodium or potassium carbonate and nitric acid to release small bubbles of co 2 we have investigated both the initial overturning produced by gas release in the lower layer and the subsequent evolution of gas due to intimate mixing of the two layers the latter experiments in which the reactants remained isolated in the two layers until overturning occurred demonstrated unambiguously that the fluxes of chemical components across the interfaces between convecting layers are very slow compared to the flux of heat this shows that the evolution of layers of magma of different origins and composition can take place nearly independently of each other the magmas can coexist in the same stratified chamber until their bulk densities become equal and they mix together the processes illustrated in these experiments could occur in h 2 o bearing magmas such as in the calcalkaline association and in co 2 bearing mafic magmas such as in silica undersaturated suites

control of early formed vesicle cylinders on upper crustal prismatic jointing in compound pāhoehoe lavas of elephanta island western deccan traps india upper crustal prismatic joints and vesicle cylinders common in p hoehoe lava flows form early and late respectively and are therefore independent features however small scale compound p hoehoe lava lobes on elephanta island western deccan traps india which resemble s type spongy p hoehoe in some aspects contain vesicle cylinders which apparently controlled the locations of upper crustal prismatic joints the lobes are decimeters thick did not experience inflation after emplacement and solidified rapidly they have meter scale areas that are exceptionally rich in vesicle cylinders up to 68 cylinders in 1 m2 with a mean spacing of 12 1 cm separated by cylinder free areas and pervasive upper crustal prismatic jointing with t curved t and quadruple joint intersections a majority 76 5 of the cylinders are located exactly on joints or at joint intersections and were not simply captured by downward growing joints as the cylinders show no deflection in vertical section we suggest that large numbers of cylinders originated in a layer of bubble rich residual liquid at the top of a basal diktytaxitic crystal mush zone which was formed very early probably within the first few minutes of the emplacement history the locations where the rising cylinders breached the crust provided weak points or mechanical flaws towards which any existing joints formed by thermal contraction propagated new joints may also have propagated outwards from the cylinders and linked up laterally some cylinders breached the crust between the joints and thus formed a little later than most others the elephanta island example reveals that whereas thermal contraction is undoubtedly valid as a standard mechanism for forming upper crustal prismatic joints abundant mechanical flaws such as large concentrations of early formed crust breaching vesicle cylinders can also control the joint formation process

compositional convection in a reactive crystalline mush and melt differentiation fractional crystallization and partial melting involve relative motion of liquid and solid phases and chemical and thermal interactions between them to elucidate some physical principles of thermosolutal convection in a reactive porous medium we describe experiments on the directional solidification of aqueous ammonium chloride solutions the addition of small amounts of a polymerizing agent permits variation of the solution viscosity independently of thermal conditions the phase diagram and permeability solutions were cooled from below and crystallization developed at the base of the tank generating at first a field of thin plumes of light residual fluid released from the boundary layer at the top of the mush the interstitial fluid within the mush also became unstable eventually the onset of convection occurring when the porous medium rayleigh number of the mush reached a critical value this threshold value was found to be 25 at low initial superheat and to decrease with increasing superheat local thermodynamic equilibrium between crystals and liquid within the mush coupled the evolution of temperature composition and porosity convective motions locally caused dissolution and precipitation and hence fluctuations of porosity developed dissolution occurred preferentially in the central parts of upwellings and the upflow gradually focused being ultimately channelized into narrow chimneys devoid of crystals all else being equal the areal density of chimneys was greater the lower the viscosity chimney diameter increased with increasing solution viscosity in the liquid above the growing mush the convective plumes were very similar to salt fingers depending on solution viscosity and temperature gradient they exhibited a phenomenon of collective instability such that vertical motion was disrupted by wave instabilities the base plate temperature chosen was above the eutectic and hence the total amount of crystals at the end of experiments with the same initial composition and the same final temperature was a constant fixed by the phase diagram the spatial distribution of crystals and the final porosity of the mush were however determined by the strength of compositional convection as measured by the porous medium rayleigh number when no convection occurred mush thickness finally became equal to the initial layer thickness and the system was homogeneous when compositional convection occurred the overlying reservoir underwent chemical evolution and mush growth slowed dramatically in experiments with progressively lower solution viscosity and hence more vigorous convection final mush thickness was progressively less and final porosity lower final mush thickness was found to scale with solution viscosity to the power 0 33 during fractional crystallization of magma the effects of compositional convection can be recorded in the chemical and mineralogical features of cumulate rocks we speculate that fossil chimney structures can be found in the lower zone of the bushveld ultramafic complex in the form of iron rich platinum bearing dunite pipes this could explain the adcumulate nature of the lower zone rocks and the chemical and mineralogical similarities between the pipes and the overlying merensky reef the merensky reef would be interpreted as a hiatus in mush growth the results may also have applications to the flow structure in regions of partial melting and in the earth s core

a slippery problem with explosive consequences water has remarkable effects on the properties of geochemical materials such as magma the result is that a small change in water concentration can make the difference between explosive volcanic eruptions and less violent ones in his perspective spera discusses results reported in the same issue by ochs and lange on new measurements of the amount of water in molten silicates the findings will add new insight to the atomic structure of these materials as well as the effects of water on them

the importance of pahoehoe abstract pahoehoe lava flows are common in every basaltic province and their submarine variants pillow lavas and sheet flows cover the bulk of the earth pahoehoe flows are emplaced by inflation the injection of molten lava underneath a solidified crust only in the past few years has an understanding of the inflation process and the ability to recognize ancient inflated lava flows been achieved all large terrestrial basaltic flow fields studied to date including flood basalts were emplaced as thermally efficient inflated compound pahoehoe sheet flows this leads us to propose that this is the standard way of emplacing large lavas the swell hypothesis the atmospheric impact of such flood basalt eruptions could have been protracted and severe providing a plausible link between flood basalt eruptions and mass extinctions

the yield strength of subliquidus basalts experimental results yield strength is an important property of particle fluid suspensions in basaltic lavas that crystallize during flow emplacement the onset of yield strength may result in threshold transitions in flow behavior and flow surface morphology however yield strength crystallinity relations are poorly known particularly in geologic suspensions where difficulties of experimental and field measurements have limited data acquisition in the subliquidus temperature range here we describe two complementary experimental approaches designed to examine the effect of particle shape on the low shear yield strength of subliquidus basalts the first involves melting cubes of holocrystalline basalt samples with different initial textures to determine the temperature crystallinity at which these samples lose their cubic form these experiments provide information on the minimum crystal volume fractions 0 20 0 35 required to maintain the structual integrity of the cube the second set of experiments uses suspensions of corn syrup and neutrally buoyant particles to isolate the effect of particle shape on yield strength development from these experiments we conclude that the shape is important in determining the volume fraction range over which suspensions exhibit a finite yield strength as anisotropic particles may orient during flow the effect of particle shape will be controlled by the orientation distribution of the constituent particles we find that the so called excluded volume can be used to relate results of experiments on anisotropic particles to those of suspensions of spherical particles recent measurements of yield strength onset in basaltic melts at crystal volume fractions near 0 25 are consistent with our observations that crystal frameworks develop at low to moderate crystal volume fractions when crystals are anisotropic e g plagioclase we further suggest that conditions leading to yield strength onset at low crystallinities include rapid cooling increased crystal anisotropy heterogeneous nucleation which promotes extensive crystal clustering and large cluster anisotropy and static conditions random crystal orientations

middle to late holocene glacial variations periglacial processes and alluvial sedimentation on the higher apennine massifs italy the major climatic variations that have affected the summit slopes of the higher apennine massifs in the last 6000 yr are shown in alternating layers of organic matter rich soils and alluvial glacial and periglacial sediments the burial of the soils triggered by environmental climatic variations took place in several phases for the last 3000 yr chronological correlations can be drawn between phases of glacial advance scree and alluvial sedimentation and development of periglacial features during some periods the slopes were covered by vegetation up to 2700 m and beyond while in other phases the same slopes were subject to glacial advances and periglacial processes and alluvial sediments were deposited on the high plateaus around 5740 5590 1560 1370 and 1300 970 cal yr b p organic matter rich soils formed on slopes currently subject to periglacial and glacial processes the mean annual temperature must therefore have been higher than at present furthermore on the basis of the variations in the elevation of the lower limit reached by gelifraction it can be concluded that the oscillations in the minimum winter temperatures could have ranged between 3 0 c lower ca 790 150 cal yr b p and 1 2 c higher ca 5740 5590 cal yr b p than present minimum winter temperatures during the last 3000 yr the cold phases recorded by the calderone glacier advance in the apennines essentially match basically the phases of glacial advance in the alps

a possible link between gabbros bearing high temperature iddingsite alteration and huge pegmatoid intrusions the society islands french polynesia this paper reports new field observations and new petrological textural and geochemical data on two gabbroic intrusions and one pegmatoid dyke from the french polynesian islands maupiti and bora bora respectively olivine crystals from the faataufi and barque de hiro gabbros maupiti include high temperature iddingsite hti exhibiting three distinct morphological facies chemical and crystallographical data performed by electron microprobe scanning microscope x ray diffractometry and near infrared spectroscopy show that hti results from modifications of olivine structure through intense hydroxylation and fe oxidation the hti free 120 m wide cloche de hiro dyke bora bora exhibits a textural zonation from heterogranular intergranular gabbro peripheral part to typical pegmatoid inner area we propose a textural tetrahedral classificatory diagram which can be used to clearly distinguish the textural characteristics of the pegmatoids from those of the other coarse grained samples through comparison with a lava flow containing vesicle rich segregation sheets we suggest that the cloche de hiro dyke may correspond to a huge segregation structure derived from a maupiti type hti bearing gabbro through a vapor differentiation process both intrusions might be considered as representative of two superposed levels in a vertically arranged magmatic complex in this view the maupiti gabbroic bodies would represent the lower part of the postulated system under high oxygen fugacity olivine crystals would have undergone important modifications of their structure leading to hti the huge bora bora pegmatoid dyke would be the upper part of the complex after expulsion from the hti bearing gabbroic solidification zone by build up of gas pressure vesicle rich residual melt would have gathered into a shallow level and crystallized there as a pegmatoid body

a physical model for metal extraction and transport in shallow magmatic systems 1 the highest concentrations of metals e g cu au ag mo in the earth s crust are found in porphyry type deposits the metals are ultimately sourced from magmas and appear to be concentrated hundred to thousand fold from typical magmatic contents ppm ppb in the exsolved volatile phase to better quantify the purging and transport of metals we develop a physical model of volatile evolution in an incrementally built upper crustal magma reservoirs that considers 1 partitioning of metals from the melt to an exsolved volatile phase and 2 advection of the buoyant volatile phase using a single dimensionless parameter the peclet number pe ratio of advection rate over diffusion rate we propose that metal extraction and segregation from magmas can occur in 3 stages with different pe 1 during exsolution of the magmatic volatile phase in shallow crystal poor magma bodies slow volatile advection pe 1 2 during the growth of volatile channels that develop in the reservoir as crystallinity increases pe 1 and 3 during advection in connected channels rapid volatile advection high pe 1 for each stage a metal enrichment factor can be calculated allowing insight into the optimal conditions to maximize metal mass flux into the overlying hydrothermal system the model predicts that the most efficient purging of metals occurs for magmas with intermediate volatile contents and is enhanced during late stage magmatic activity as the reservoirs reach high crystallinity and are not disturbed by volcanic venting in agreement with natural observations suggesting that ore formation post dates volcanic activity

segregation vesicles gas filter pressing and igneous differentiation some vesicles in certain subaerial flows of basalt and basaltic andesite are partially filled with dark partly glassy segregation material the segregation material has about three times the concentration of k o tio and p o as the associated host rock therefore the segregation materials are inferred to be frozen residual liquids that migrated into the vesicles several possible processes for filling the vesicles are examined within the context of the cooling time implied by heat conduction the preferred process is one in which the residual liquid migrates through a porous and permeable but rigid network of interlocking crystals in response to a pressure gradient generated by vapor saturated crystallization as crystallization proceeds h o is concentrated in the residual liquid and gas this produces a higher pressure in the gas poor matrix than in the vesicles and forces liquid to migrate into the vesicles textural features suggestive of or consistent with the process include 1 vesiculation

rejuvenation of the fish canyon magma body a window into the evolution of large volume silicic magma systems voluminous unzoned phenocryst rich pyroclastic deposits considered as erupted batholiths provide a unique opportunity to investigate magmatic processes in silicic magmas the fish canyon tuff a well documented example of these monotonous ignimbrites displays evidence for simultaneous dissolution of feldspars 1 quartz and crystallization of hydrous phases during gradual near isobaric reheating from 720 to 760 8c these observations along with a high crystallinity 45 and near solidus mineral assemblage suggest that the fish canyon magma cooled to a rigid crystal mush before being partly remelted prior to eruption rejuvenation was triggered by intrusion of water rich mafic magmas at the base of the fish canyon mush but the mechanisms of heat transfer remain poorly understood the growth of amphibole during reheating requires addition of mafic components but the absence of any measurable gradients and the paucity of mafic enclaves in the fish canyon magma rule out a reheating event dominated by convective mixing with a mafic magma closed system processes such as heat conduction and convective self mixing could not account for the transport of externally derived mafic components we performed numerical simulations of upward percolation of a hot low density h2o co2 fluid phase gas sparging through a crystalline framework saturated with rhyolitic melt to assess the efficiency of such a process in rejuvenating silicic mushes in open systems sparging by 20 40 km 3 of gas extracted from 3000 km 3 of mafic magma is capable of reheating 7500 km 3 of silicic crystal mush by 40 8c in 150 200 k y moreover the vertical thermal gradient after 150 k y in most of the mush is small 25 8c in the upper 65 gas sparging also produces an increase in the internal pressure of silicic crystal mushes and may lead to the formation of crystal poor rhyolites by expelling interstitial melt however our simulations predict that filter pressing driven by sparging of externally derived gas could not solely account for the generation of the most voluminous rhyolites

evidence for volatile influenced differentiation in a layered alkali basalt flow penghu islands taiwan an approximately 20 m thick alkali basalt flow on the penghu islands contains 20 cm thick horizontally continuous 50 m vesicular layers separated by 1 5 m of massive basalt in its upper 8 5 m the three layers contain ocelli like vesicles filled with nepheline and igneous carbonate they are coarse grained and enriched in incompatible elements relative to the massive basalt with which they form sharp contacts these vesicular layers segregation veins formed when residual liquid in the underlying crystal mush was forced gas filter pressing or siphoned into three thermally induced horizontal cracks that opened successively in the advancing crystal mush of the flow s upper crust most vesicular layer trace elements can be modelled by residual melt extraction after 25 40 fractional crystallization of massive basalt underlying each layer sulphur cl as zn pb k na rb and sr show large concentration changes between the top middle and bottom layers with each vesicular and underlying massive basalt forming a chemically distinct pair the large changes between layers are difficult to account for by crystal fractionation alone because other incompatible elements e g la sm yb zr nb and the major elements change little the association of these elements s cl etc with fluids in various geologic environments suggests that volatiles influenced differentiation perhaps by moving alkali alkaline earth and chalcophile elements as magma dissolved volatile complexes volatiles may have also led to large grain sizes in the segregation veins by lowering melt viscosities and raising diffusion rates the chemical variability between layers indicates that a convection and concentration mechanism acted within the flow the specific process cannot be determined but different rates of vesicle plume rise through the flow and or accumulation in the upper crust s crystal mush might account for the chemical pairing and extreme variations in cl s as and c this study emphasizes the importance of sampling vesicular rocks in flows it also suggests that volatiles play important physical and chemical roles in rapidly differentiating mafic magmas in processes decoupled from crystal fractionation

on bimodal differentiation by solidification front instability in basaltic magmas part 1 basic mechanics extensive coarse grained silicic lenses are common in the upper portions of large diabase sills lava lakes and gabbroic intrusions these lenses which often contain up to 10 wt more silica than their host rocks record a clear process of bimodal differentiation that may play a fundamental role in generating significant quantities of silicic magma in predominantly basaltic systems the spatial compositional textural and contact relations suggest that these silicic segregations form by infilling of tears formed during gravitational instability of the upper solidification front this process is investigated in some detail by exploring the thermal and mechanical conditions leading to internal failure of the front and segregation formation as the upper solidification front grows inward and thickens the local strength of the developing crystalline network must increase upward through the front sufficiently fast to offset the increasing weight of the front itself this necessary local variation in strength dictates how crystallinity must vary to prevent failure but the actual variation in crystallinity is determined by phase equilibria which in general varies in a fashion that is noncovariant with that required for mechanical stability it is this mismatch that apparently leads to failure and formation of silicic segregations the rate of filling of the tears by porous flow of interstitial melt from below regulates the rate of tear opening and the overall size of the tears is regulated by the local rate of advance of the solidification front segregations therefore thicken downward as the growth of the front slows and then die out altogether with approach of the lower solidification front this process irreversibly introduces siliceous noise into basaltic systems the silicic signal can be enhanced through subsequent reprocessing of the host rock to form volumetrically important masses of silicic magma this process may be fundamental to achieving the strong compositional diversity of the igneous rocks to yielding continental masses and to the differentiation of earth itself