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

magmatic fractionation by compositional convection in a sheet like magma body constraints from the nosappumisaki intrusion northern japan the mechanism of fractionation in a sheet like magma body was investigated based on observations from the nosappumisaki intrusion northern japan this is one of a number of late cretaceous shoshonite sills in the nemuro peninsula and has a compound structure comprising a bottom cumulate zone containing 50 vol cumulus crystals and a overlying middle zone free of such cumulus crystals these zones are sandwiched between porphyritic rocks of upper and lower chilled and marginal zones it is inferred that massive crystal settling or flotation from a magma initially laden with 20 vol of crystals primocrysts in a sheet like magma body formed a cumulus pile and an overlying crystal free melt layer after the formation of the compound structure with partial freezing of the upper and lower marginal zones the internal part of the intrusion slowly solidified during this period the melt in the central part of the intrusive sheet underwent fractional crystallization this is shown by patterns of vertical variation in the composition of the interstitial melt the overall patterns are inverted mirrorimage s shaped and s shaped for incompatible and compatible elements respectively the incompatible element abundances show maxima near the bottom of the middle zone which corresponds to the final solidification horizon they show minima in the cumulate zone which suggests discharge of an evolved melt from the crystal pile fractionation is inferred to have taken place via compositional convection without settling of crystals grown after the intrusion event or collapse of the upper boundary layer the melt in the central melt layer descended into the permeable crystal pile counterbalancing discharge of buoyant evolved melt formed by crystallization of the pore melt in the crystal pile this conclusion is based on the following observations 1 there is no accumulation of crystals grown after the intrusion of the magma above the cumulate zone 2 leucocratic pipe like structures which represent relict pathways for the evolved melt are developed in the cumulate zone these structures increase in diameter decrease in number density and increase in abundance of incompatible elements with height 3 cumulus augite in the cumulate zone has ca rich and al poor dissolved rims the extent of dissolution increases with stratigraphic height suggesting downwelling of an h2o rich melt from the overlying melt layer 4 there is no evidence for extensive compaction accompanying pore melt crystallization in the main part of the cumulate zone 5 the initial melt composition estimated from the groundmass of the chilled margin rocks and the average composition of the material interstitial to the primocrysts for the whole sill are statistically identical a mass balance model for compositional convection was constructed to quantify the melt transportation processes model parameters were optimized by fitting the observed vertical variation of the interstitial melt compositions the optimized results suggest that the evolved melts discharged from the crystal pile and rose through the central melt layer as plumes with minor mingling with the surrounding melt the initially formed compound structure is a highly preferential environment for compositional convection

fractionation of the platinum group elments and re during crystallization of basalt in kilauea iki lava lake hawaii article i nfo kilauea iki lava lake formed during the 1959 summit eruption of kilauea volcano then crystallized and differentiated over a period of 35 years it offers an opportunity to evaluate the fractionation behavior of trace elements in a uniquely well documented basaltic system a suite of 14 core samples recovered from 1967 to 1981 has been analyzed for 5 platinum group elements pge ir os ru pt pd plus re these samples have mgo ranging from 2 4 to 26 9 wt with temperatures prior to quench ranging from 1140 c to ambient 110 c five eruption samples were also analyzed osmium and ru concentrations vary by nearly four orders of magnitude 0 0006 1 40 ppb for os and 0 0006 2 01 ppb for ru and are positively correlated with mgo content these elements behaved compatibly during crystallization mostlylikelybeingconcentratedintracephases alloyorsulfide presentinolivinephenocrysts or included chromite iridium also correlates positively with mgo although less strongly than os and ru the somewhat poorer correlation for ir compared with os and ru may reflect variable loss of ir as volatile irf6 in some of the most magnesian samples rheniumisnegativelycorrelated withmgo behavingas anincompatibletraceelement itsbehavior inthe lava lake is complicated by apparent volatile loss of re as suggested by a decrease in re concentration with time of quenching forlakesamplesvs eruptionsamples platinum andpdconcentrationsarenegatively albeit weakly correlatedwithmgo sotheseelementsweremodestlyincompatible duringcrystallizationof themajorsilicate phases palladium contents peaked before precipitation of immiscible sulfide liquid however and decline sharply in the most differentiated samples in contrast pt appears to have been unaffected by sulfide precipitation microprobe data confirm that pd entered the sulfide liquid before re and that pt is not strongly chalcophile in this system occasional high pt values in both eruption and lava lake samples suggest the presence of unevenly distributed unidentified pt rich trace phases in some kilauea iki materials estimated mineral olivine chromite melt d values for os ir ru and pt for equilibrium crystallization for samples from 7 to 27 wt mgo are 26 8 2 19 and 0 55 respectively these os ir and ru estimates are somewhat higher than previous estimates for similar systems if fractional crystallization is instead assumed d values are much more similar results confirm many prior observations in other mafic systems that olivine together with included phases has a major effect on absolute and relative abundances of re and the pge the relatively linear correlations between these elements and mgo potentially permit accurate estimation of the concentrations of these elements in the primary melts of comparable systems especially in instances where the mgo content of the primary melt is well constrained

from alkali basalt to phonolite in hand size samples vapor differentiation effects in the bouzentes lava flow cantal france abstract the bouzentes lava flow is a 20 m thick alkali basalt flow emplaced during the last stage of formation of the cantal stratovolcano at 4 2 ma its upper part has 1 to 20 cm thick vesicle rich segregation sheets which recur every 0 1 2 m these horizontal veins are hawaiitic in composition they are characterized by hypertrophic development of their minerals pegmatoids and by glassy phonolitic segregation vesicles internal differentiation within the bouzentes lava flow was triggered by an unusually high water content as suggested by pre emptive iddingsite alteration of olivine phenocrysts the proposed model of formation of the segregation sheets includes the upward motion of diapirs of residual melt plus addition of vapor from the bottom of the central liquid lens to the base of the upper solidified crust of the cooling lava flow olivine settling appears to have been inhibited or at least retarded by upward migration of melt plus vesicles most of the features observed in bouzentes recall the internal differentiation processes usually described within thick hawaiian lava lakes the segregation vesicles are believed to result from an increase of gas solubility in residual melt during the crystallization process

boundary layer fractionation constrained by differential information from the kutsugata lava flow rishiri volcano japan detailed investigation of an erupted magma with limited compositional diversity provides instantaneous information on incremental magma differentiation processes in a magma chamber kutsugata lava a suitable target of such study is a quaternary alkali basalt 51 3 53 2 wt in sio2 from rishiri volcano northern japan despite the narrow range in the whole rock compositional variation chemical and modal compositions of mineral phases crystallized in the magma chamber vary systematically with the whole rock composition in the north lava 51 3 51 9 wt in sio2 the less differentiated portion of the kutsugata lava most crystals which include low ni olivine and plagioclase were derived from the mushy boundary layer the main part of the magma body was principally aphyric 30 vol by such mechanisms as compaction and compositional convection this fractionated melt was mixed with the overlying main magma causing differentiation of the kutsugata magma the average temperature of the extracted melt is 1010 c significantly lower than 1100 c estimated for the main magma a quantitative model of magmatic differentiation which includes thermal and compositional evolution of a mushy boundary layer can successfully reproduce the observed compositional trends of the north lava

joint inversion for three dimensional s velocity mantle structure along the tethyan margin abstract for purposes of studying the lateral heterogeneity as well as for ultimately predicting seismograms for this region we construct a new 3 d s velocity model by jointly inverting a variety of different seismic data we jointly invert regional waveforms surface wave group velocity measurements teleseismic s arrival times and crustal thickness estimates from receiver functions refraction lines and gravity surveys these data types have complementary resolving power for crust and mantle structures vertical and lateral variations shallow and deep mantle features local and global structure therefore a joint inversion of these data sets might help unravel the complexity of this tectonically diverse area these measurements are made from a combination of mantle investigation of the deep suture between europe and africa midsea program for array seismic studies of the continental lithosphere passcal geoscope geofon global seismographic network gsn international deployment of accelerometeres ida mednet national networks and local deployments throughout the study region which extends from the western mediterranean region to the hindu kush and encompasses northeastern africa the arabian peninsula the middle east and part of the atlantic ocean for reference we have fitted the waveforms of regional s and rayleigh waves from over 3800 seismograms using partitioned waveform inversion we include over 3000 crustal thickness estimates from receiver functions gravity measurements and refraction profiles we include rayleigh wave group velocities for hundred thousands of paths transecting the region we have over 3000 teleseismic s arrival times measured through cross correlation and over 170000 from picks originally reported to the international seismological centre isc

cooling history and differentiation of a thick north mountain basalt flow nova scotia canada a thick 175 m north mountain basalt flow at mckay head nova scotia canada shows 25 cm thick differentiated layers separated by 130 cm of basalt in its upper 34m upper layers 5 m below the lava top are highly vesicular whereas lower ones are pegmatitic and contain a thin 2 cm rhyolite band the layering of the flow closely resemble that of some hawaiian lava lakes the eesicular basalts and mafic pegmatites are inferred to be liquid rich segregations which drained into horizontal cracks that formed within a crystalline mush the cracks resulted from a thermal contraction associated with cooling and shrinkage of the mush rhyolites were formed by in situ differentiation gas overpressures fractured the pegmatites and gas effervescence filter pressing forced silicarich residual liquid from pegmatite interstices into the fractures creating bands chemical differences between the pegmatitic layers and early formed highly differentiated upper vesicular layers may reflect a role for volatiles in the differentiation process along with crystal fractionation

the kulanaokuaiki 3 tephra 900 ce products of a remarkably energetic pyroclastic eruption at kīlauea volcano hawaiʻi usa

mechanisms of differentiation in shallow mafic alkaline intrusions as illustrated in the big bend area western texas abstract syenitic bodies are a common feature in alkaline sills and laccoliths that range in composition from syenogabbroic to syenodioritic the syenitic bodies are generally accepted to be the result of in situ differentiation such bodies are usually called segregations but relatively little discussion is given to the actual ways they may form subhorizontal sheets or layers are most common although rounded ocelli and vertically elongated cylindroidal forms are also common these are all systematically arranged in the intrusions such features are found in at least ten sills in the big bend area of texas and are reported as well in montana utah australia sakhalin scotland and new zealand detailed chemical and mineralogical analyses demonstrate crystal fractionation very convincingly for the generation of syenitic liquids that form the bodies the analyses also allow the calculation of viscosities cooling time and mineral settling rates in the sill studied most thoroughly these factors indicate or are compatible with proposed mechanisms for the formation of syenite bodies in many intrusions there is a bimodal distribution of syenite compositions one type is relatively plagioclase rich and the other type is plagioclase poor mass balance calculations on several intrusions show that a residual liquid of plagioclase rich composition will form after about 30 of the parent magma has crystallized whereas a residual liquid of plagioclase poor composition will form only after about 50 of the original magma has crystallized the several mechanisms by which plagioclase rich residual liquids are aggregated into the different forms of syenite bodies include localized crystal settling and sagging of a crystaline framework on a scale of tens of centimeters to a meter within an upper solidification front formation of cylindroidal columns of syenitic differentiate in a crystal mush within a lower solidification front and diapiric rise of such masses after the amount of magma crystallized reaches 50 or more rupture of a rigid crystalline framework occurs through the processes of contraction upon cooling shearing caused by deformation and expansion of bubbles in the interstitial residual liquid while plagioclase poor differentiate flows into spaces created in these processes important for the effectiveness of these processes is the buildup of volatiles in the residual liquids which results from crystallization of anhydrous minerals that causes second boiling and lowering of the viscosity of the liquids

the skaergaard layered series part iii non dynamic layering layering in the skaergaard intrusion has been divided into two introduction general types one produced by magmatic flow and another by recent studies of the structural and textural features of processes resulting from variations of rates of nucleation and crysthe skaergaard intrusion mcbirney nicolas 1997 tallization and in the case of the layered series by compactionhave distinguished two broad types of layering one related processes modal variations caused by shifts of cotectic produced by the dynamic effects of magmatic flow and proportions produce thick layers which in the layered and upper another by processes that operate in situ such as varied border series are diffuse and normally lack strong foliation and nucleation and growth of crystals recrystallization or by lineation in the marginal border series the layers are thinner and compaction related mechanisms we refer to layering sharper possibly because the rate of accumulation was slower formed by these latter processes as non dynamic to oscillatory nucleation may have played a role in producing fineemphasize that it is not the result of fluid dynamic scale cyclic layers but it was less important than solution and processes although most layering combines elements of reprecipitation during slow cooling and ostwald ripening evidence more than one process the contribution of each mechfor compaction is found in deformed plagioclase laths and a relative anism can usually be recognized from its distinctive form deficiency of incompatible elements in rocks formed on the floor and setting layering related to compaction becomes sharper with increasing the distinguishing features of dynamic magmatic layheight in the layered series until it suddenly disappears above the ering have been described in part ii of this series mcbirtrough horizon near the base of upper zone b mechanical sorting ney nicolas 1997 they are best seen near the steep during compaction may have produced crude layering but if it did margins of the layered series in what wager brown the evidence has long since been destroyed by the superimposed effects 1968 referred to as the cross bedded zone where the of solution and reprecipitation when interstitial liquid rose through layering is disrupted by slumping and channeling and the overlying crystals and re equilibrated with them numerical the rocks have a marked foliation and lineation simulations illustrate how small differences of surface energy caused layering not associated with magmatic flow takes a by variations of grain size textural dependence of solubility and variety of forms but we can distinguish two general pressure solution can cause segregation of minerals into layers during end members each of which has a distinct origin and solution and reprecipitation characteristics the first results from varied rates of

magmatic processes under mid ocean ridges a detailed mineralogic study of lavas from east pacific rise 9 30 n 10 30 n and 11 20 n 1 detailed petrologic study has been made for lavas from the northern east pacific rise epr 9 30 n 11 20 n and 10 30 n 9 30 n and 11 20 n have robust magma supply and a shallow melt lens while 10 30 n has nonrobust magma supply and no shallow melt lens lavas from all three localities are sparsely phyric and glassy containing plagioclase olivine pyroxene typically the lavas contain several to many up to seven distinct chemical groups of plagioclase that are not always distinct texturally the lavas may also contain up to three chemically distinct groups of olivine and two groups of pyroxene the lavas contain both individual crystals and groups comprising reticulate and dendritic clots that we interpret to represent bits of crystal networks forming in mushy zones of an axial magma chamber 21 of the 23 samples studied in detail have diverse crystal compositions and require mixing which most likely occurs when mostly liquid magma passes through mushy zones we find no significant systematic differences between robust and nonrobust segments in terms of their crystal content proportion of texturally distinct xenocrysts crystal size aspect ratio roundness modal abundance magma residence time number of diverse mineral chemical groups and characteristics of mixing like the total range of composition of disequilibrium minerals present and the magnitude of the chemical gaps between disequilibrium and equilibrium compositions this unexpected and remarkable similarity suggests that the presence or absence of a seismically imaged shallow melt lens has essentially no effect on the mineralogy of erupted lavas we conclude that the fundamentally important magmatic process under mid ocean ridges from slow to fast is the formation of crystal networks and their subsequent compaction the seismically detected shallow melt lenses likely contain highly evolved magma formed by expelled interstitial melt during crystal network compaction and play very little role in crustal accretion our conclusion implies that the thick 3 km low velocity zone near the base of the crust produces the thin 30 m shallow melt lenses not the other way around

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

a rare occurrence of a crater filling clastogenic extrusive coherent kimberlite victor northwest ontario canada kimberlite pipes can contain significant proportions of dark and dense kimberlite that have mostly been interpreted as intrusive coherent hypabyssal in origin this study reports a well documented occurrence of a fresh intra crater clastogenic extrusive coherent kimberlite that is concluded to have formed as a result of lava fountaining this paper focuses on a dark dense competent generally crystal rich massive kimberlite unit within the victor northwest kimberlite pipe ontario canada using a comprehensive volcanological and petrographic analysis of all available drill cores it is shown that this unit has a fresh well crystallised coherent groundmass and is extrusive and pyroclastic in origin the proposed clastogenic coherent extrusive origin is based on deposit morphology gradational contacts to enveloping pyroclastic units as well as the presence of remnant pyroclast outlines and angular broken olivines this paper and an increasing number of other studies suggest that fragmental extrusive coherent kimberlite in intra crater settings may be more common than previously thought the emplacement history and volcanology of these pipes need to be reconsidered based on the emerging importance of this particular kimberlite facies

partial melting and recrystallization of granite and their application to deep disposal of radioactive waste part 2 recrystallization abstract a previously published scheme for the high temperature disposal of nuclear waste in deep boreholes requires that granite host rock partially melted by the heat from the waste can be completely recrystallized on the time scale of the cooling of the waste experimental studies of dynamic recrystallization of a typical s type crustal granite were therefore undertaken under the pressure 0 15 gpa and other conditions likely to arise during implementation of this scheme linear cooling experiments at 1 c h 0 1 c h and 1 c day were carried out over the temperature range 800 560 c these demonstrate that the partially melted granite can be fully recrystallized when cooled at 0 1 c h or slower crystal growth rates for the main phases formed during cooling are linearly proportional to total h 2 o content and decrease from alkali feldspar through biotite to quartz recrystallizing the partially melted granite at 0 1 c h compared with 1 c h increases grain size by a factor of 2 5 on the basis of partial melting experiments reported previously by us lithos 2003 this issue for the same granite and the results presented here it is concluded that an s type granite similar to the one studied would be a suitable host rock for the proposed high temperature very deep disposal scheme for radioactive waste

thermal and compositional evolution of a cooling magma chamber by boundary layer fractionation model and its application for primary magma estimation a boundary layer fractionation model for the thermal and compositional evolution of a basaltic magma chamber is presented the model utilizes a multicomponent thermodynamic approach for relating the compositional structure with the thermal structure of a cooling magma body the magma composition in the main magma body evolves by transportation of a fractionated interstitial melt from the mushy boundary layer the consideration of both the thermal and compositional evolution of the magma body enables a realistic simulation of magmatic differentiation as a function of time and space the model is used to evaluate a primary magma estimation from volcanic rock series it is shown that the common procedure of olivine addition gives an estimation which is significantly different from the true primary composition if the magma actually differentiated by boundary layer fractionation this can cause significant errors in estimating the conditions at which the primary magma was generated

quantitative modeling of granitic melt generation and segregation in the continental crust 1 we present a new quantitative model of granitic in a broad sense melt generation and segregation within the continental crust we assume that melt generation is caused by the intrusion of hot mantle derived basalt and that segregation occurs by buoyancy driven flow along grain edges coupled with compaction of the partially molten source rock we solve numerically the coupled equations governing heating melting and melt migration in the source rock and cooling and crystallization in the underlying heat source our results demonstrate that the spatial distribution and composition of the melt depends upon the relative upward transport rates of heat and melt if melt transport occurs more quickly than heat transport then melt accumulates near the top of the source region until the rock matrix disaggregates and a mobile magma forms as the melt migrates upward its composition changes to resemble a smaller degree of melting of the source rock because it thermodynamically equilibrates with rock at progressively lower temperatures we demonstrate that this process of buoyancy driven compaction coupled with local thermodynamic equilibration can yield large volumes of mobile granitic magma from basaltic and meta basaltic amphibolitic protoliths over timescales ranging from 4000 years to 10 myr the thickness of basaltic magma required as a heat source ranges from 40 m to 3 km which requires that the magma is emplaced over time as a series of sills concurrent with melt segregation these findings differ from those of previous studies which have suggested that compaction operates too slowly to yield large volumes of segregated granitic melt

226ra 230th excess generated in the lower crust implications for magma transport and storage time scales comment and reply comment dufek and cooper 2005 1 hereafter dc present an interesting and provocative paper on the origins of 226ra excess in arc lavas they develop an incongruent continuous melting model to argue that 226ra excess can be created by partial melting in the lower crust however despite the novelty of

sediment ingestion by worms and the production of bio clays a study of macrobiologically enhanced weathering and early diagenetic processes the experiments reported here show that a range of early diagenetic clay minerals can develop within ingested sand in the guts of creatures at a rate that is at least 2 or 3 orders of magnitude greater than in an abiotic environment early diagenesis in marine environments may be strongly influenced by the occurrence of sediment ingestion and excretion by animals using a set of artificial marine experiments and the common lugworm arenicola marina i t is shown that sediment ingestion by macrobiota represents a new way in which to precipitate clay minerals or bio clays significant quantities of clay minerals can be the product of biological interaction within sediment tanks were constructed with artificially layered sediment and natural seawater the experiments were conducted at room temperature for a total duration of 9 months the silicate material fed to the worms was an unweathered and thus clay mineral free basalt from iceland less than 2 lm fractions from original control and faecal samples were periodically analysed using x ray diffraction xrd and fourier transform infrared spectroscopy ftir faecal casts underwent mineral alteration as denoted by loss of primary minerals especially plagioclase new minerals in faecal casts from the lugworms included kaolinite illite quartz and possibly chlorite inorganic weathering of similar parent basalt would probably produce an identical mineral assemblage but many more times slowly than with the macrobiotic mechanism demonstrated here

the graveyard point intrusion an example of extreme differentiation of snake river plain basalt in a shallow crustal pluton the graveyard point intrusion is the only known example of a wellexposed differentiated mafic pluton associated with the late miocene pleistocene magmatism of the western snake river plain srp it is exposed in a 6 km by 4 km area adjacent to the oregon idaho border and exposures range in thickness from 20 to 160 m the thicker parts of the intrusion are strongly differentiated and contain a 25 60 m thick section of well laminated cumulus textured gabbros that grade upward into pegmatoidal ferrogabbro evolved liquids formed sheets of fe rich siliceous granophyre at least two injections of magma are indicated by abrupt discontinuities in the rock and mineral compositions and by the lack of mass balance between the bulk intrusion and its chilled borders the laminated gabbros are interpreted to have formed from a tongue of augite and plagioclase crystals that were carried in with the second pulse of magma following the final emplacement of the intrusion in situ differentiation proceeded through a two stage process the ferrogabbros are explained as interstitial liquids forced out of the crystal mush by compaction and the siliceous granophyres are interpreted to be residual liquids that migrated out of the partly crystallized ferrogabbros in response to the exsolution of volatiles because the geochemical trend inferred for the mafic to intermediate composition liquids in the graveyard point intrusion is similar to the trend for many western snake river plain lavas the pluton may be a good model for shallow sub volcanic magma chambers elsewhere in the srp however some western srp lavas contain anomalously high concentrations of p2o5 which are best explained by mixing within the active crustal mush column or with partial melts of previously formed differentiated mafic intrusions

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

internal differentiation of kutsugata lava flow from rishiri volcano japan processes and timescales of segregation structures formation article i nfo internal differentiation processes in a solidifying lava flow were investigated for the kutsugata lava flow from rishiri volcano in northern japan in a representative 6 m thick lava flow that was investigated in detail in this study segregation products darker than the host lavas manifested mainly in the form of pipes vesicle cylinders and layers vesicle sheets occurring around 0 5 2 3 m and 2 0 4 0 m above the base respectively both the cylinders and sheets are significantly richer in incompatible elements such as tio2 and k2o than the host lavas which suggest that these products essentially represent residual melt produced during solidification of the lava flow field observation and the geochemical features of the lavas suggest that the vesicle cylinders grew upward from near the base of the flow by continuous feeding of residual melt from the neighboring host lavas to the heads of the cylinders on the other hand the vesicle sheets were produced in situ in the solidifying lava flow as fracture veins caused by horizontal compression the vesicle cylinders have a remarkably higher mgo content up to 8 wt than the host lava b6 wt whereas the vesicle sheets display mgo depletion as low as 3 5 wt the relatively high mgo content of the vesicle cylinders cannot be explained solely by the mechanical mixing of olivine phenocrysts with the residual melt it is suggested that the vesicle cylinders were produced by the extraction of olivine bearing interstitial melt from an augite plagioclase network in the host lava whereas the vesicle sheets were formed by the migration of the residual melt from a crystal network consisting of plagioclase augite and olivine in the host lava into platy fractures we infer that this selective crystal fractionation for forming the vesicle cylinders resulted from processes in which abundant vesicles rejected from the upward migrating floor solidification front prevented olivine crystals from being incorporated into the crystal network in the host lava the vesicle cylinders are considered to have formed in 1 day after the lava flow came to rest while relatively large vesicle sheets n1 cm thick appeared much later after 9 days the formation of these segregation products was essentially complete within 20 days after the lava emplacement