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

limnologic analysis of gusev crater paleolake mars abstract the survey of the hydrogeologic system formed by gusev crater and ma adim vallis aeolis subquadrangle of mars points out evidence for the existence of an ice covered lake in gusev crater a first lake was formed by the drainage of the aquifer in the region surrounding gusev before the entry of ma adim vallis in the crater the existence of a former lake in gusev is deduced from the morphology of the ma adim delta its comparison with terrestrial antarctic analogs argues for the presence of an ice covered lake in gusev at the time that the southern part of the crater s rampart was breached by ma adim first release and for a subice lacustrine construction of the valley s delta our survey shows that ma adim vallis may have entered gusev crater as late as late hesperian early amazonian as part of a second lake episode the relationship between the variation of the gusev lake water level the volume of the lake and the surface of the lake bed is established by our bathymetric model the elevation of the former lake is deduced from the elevation of the mesa like structures in the delta of ma adim vallis furthermore the correlation of the crater frequency of gusev rampart with mars stratigraphic age shows that lakes may have occupied gusev crater over a period of time covering 2 gyrs from the formation of the crater to the last episode of water release from ma adim vallis though it is most likely that the lake was episodical the recurrence of abundant water in gusev crater makes this site a high priority for missions either for martian resource exploration or for the search of life

postcumulus processes in layered intrusions during the postcumulus stage of solidification in layered intrusions fluid dynamic phenomena play an important role in developing the textural and chemical characteristics of the cumulate rocks one mechanism of adcumulus growth involves crystallization at the top of the cumulate pile where crystals are in direct contact with the magma reservoir convection in the chamber can enable adcumulus growth to occur to form a completely solid contact between cumulate and magma another important process may involve compositional convection in which light differentiated melt released by intercumulus crystallization is continually replaced by denser melt from the overlying magma reservoir this process favours adcumulus growth and can allow adcumulus growth within the pore space of the cumulate pile calculations indicate that this process could reduce residual porosities to a few percent in large layered intrusions but could not form pure monomineralic rocks intercumulus melt may also be replaced by more primitive melt during episodes of magma chamber replenishment dense magma emplaced over a cumulate pile containing lower density differentiated melt may sink several metres into the underlying pile in the form of fingers reactions between melt and matrix may lead to changes in mineral compositions mineral textures and whole rock isotope compositions another important mechanism for forming adcumulate rocks is compaction in which the imbalance of the hydrostatic and lithostatic pressures in the cumulate pile causes the crystalline matrix to deform and intercumulus melt to be expelled for cumulate layers from 10 to 1000 metres in thickness compaction can reduce porosities to very low values 1 and form monomineralic rocks the characteristic time scale for such compaction is theoretically short compared to the time required to solidify a large layered intrusion during compaction changes of mineral compositions and texture may occur as moving melts interact with the surrounding matrix both compaction and compositional convection can be interrupted by solidification in the pore spaces compositional convection will only occur if the rayleigh number is larger than 40 if the residual melt becomes lower in density and the convective velocity exceeds the solidification velocity measured by the rate of crystal accumulation in the chamber orthocumulates are thus more likely to form in rapidly cooled intrusions where residual melt is frozen into the pore spaces before it can be expelled by compaction or replaced by convection

progradational gravel beach sequences in a moderate to high energy microtidal marine environment the anatomy of progradational gravel beaches was analysed in two different tectonic settings the strongly subsiding messinian foreland basin of the eastern southern alps and the rising pleistocene marine terraces of the outer margin of the apenninic chain progradation took place near the mouths of fan delta or braid delta distributaries debouching in microtidal landlocked basins palaeo adriatic and palaeo ionian seas a wind wave climate comparable to that of the present day was characterized by periodic intense storm activity most of the beachface progradation is thought to have taken place during the post storm recovery and fair weather stages whereas the impact of storms is mostly recorded by the cutting of erosional surfaces on the beachface and by emplacement of poorly sorted and coarse grained gravels on the shoreface by storm intensified seaward trending flows and behind the highest berms by washover processes analysis of the sequences led to the identification of a number of typical divisions which are differently developed in the two settings compared here the characteristics of the apenninic sequences suggest a strongly variable wind wave climate with periodic changes from a relatively dissipative barred morphology during storms accompanied by strong longshore currents and rip development to a more reflective state typical of recovery and fair weather stages the southern alpine sequences on the other hand are characterized by poor evidence of barred morphology and of longshore currents and widespread effects of shear and gravity sorting in the lower beachface gravels these features indicate a comparatively more reflective average state due to location of beaches along the deeply embayed head of the palaeo adriatic and the very narrow directional width of the incoming wave spectrum which was almost shore normal most of the time in addition to well developed shape zonation of gravels the common presence of wave generated gravel megaripples in the southern alpine sequences indicates the greater influence of long period waves due to the greater available fetch distances in both settings the cyclicity is thought to be genetically related to minor changes of sea level

sequential crystal overproduction triggering mg cr ti v p mree enrichment in a single pulse tholeiitic mafic sill in the central iberian zone spain abstract we study the causes of mg cr ti v p mree enrichment in a 134 m thick mafic sill emplaced in a single magma pulse whole rock chemistry indicates that tio2 up to 5 51 wt and p2o5 up to 1 60 wt enrichment occurred in a closed system immiscibility processes during magmatic crystallization and interaction with late fluids seem to have played little or no role for the ti p distribution liquid lines of descent for anhydrous and hydrous conditions reveal that the melt did not follow a fenner trend anhydrous which may lead to strong ti or p enrichment but a bowen trend hydrous during crystallization at low fo2 fmq 2 and low water contents 0 5 h2o the mg cr ti v p mree rich samples are cumulates and their composition is offset from the liquid line of descent due to postcumulus compaction and compositional convection theoretical formulation of compaction and compositional convection demonstrates that classical postcumulus scenarios of strong undercooling sill cr ti v or p mree rich cumulates the efficiency of postcumulus processes declines once the latent heat does not compensate heat loss to the country rock leading to cumulates with high intergranular liquid fractions and without cr ti and p enrichment high water contents in the upper level of the sill facilitate postcumulus processes and migration of residual liquids and fluids upwards leading to the formation of abundant pegmatoid pockets and the development of a sandwich horizon that does not follow the liquid line of descent

evolution of tholeiitic diabase sheet systems in the eastern united states examples from the culpeper basin virginia maryland and the gettysburg basin pennsylvania abstract high tio 2 quartz normative htq tholeiite sheets of early jurassic age have intruded mainly late triassic sedimentary rocks in several early mesozoic basins in the eastern united states field observations petrographic study geochemical analyses and stable isotope data from three htq sheet systems in the culpeper basin of virginia and maryland and the gettysburg basin of pennsylvania were used to develop a general model of magmatic differentiation and magmatic hydrothermal interaction for htq sheets the three sheet systems have remarkably similar major oxide and trace element compositions cumulus and evolved diabase in comagmatic sheets separated by tens of kilometers are related by igneous differentiation differentiated diabase in all three sheets have petrographic and geochemical signatures and fluid inclusions indicating hydrothermal alteration beginning near magmatic temperatures and continuing to relatively low temperatures sulfur and oxygen isotope data are consistent with a magmatic origin for the hydrothermal fluid the three sheet systems examined apparently all had a similar style of crystal liquid fractionation that requires significant lateral migration of residual magmatic liquid the proposed magmatic model for htq sheets suggests that bronzite laden magma was intruded in an upper crustal magma chamber with bronzite phenocrysts collecting in the lower part of the magma chamber near the feeder dike early crystallization of augite and ca poor pyroxene before significant plagioclase crystallization resulted in density driven migration of lighter residual magmatic liquids along lateral and vertical pressure gradients towards the upper part of the sheet the influence of water on the physical properties of the residual liquid including density viscosity and liquidus temperature may have facilitated the lateral movement more than 15 km up dip in the sheets exsolution of a cl and s rich metal bearing aqueous fluid from residual magma resulted in concentration and redistribution of incompatible and aqueoussoluble elements in late stage differentiated rocks this proposed hydrothermal mechanism has important economic implications as it exerts a strong control on the final distribution of noble metals in these types of diabase sheets

terraces and gilbert type deltas in crater lakes in ismenius lacus and memnonia mars paleolacustrine environments have been widely recognized from landform associations on the surface of mars in contrast to paleolakes in more open areas putative crater lakes commonly display well developed sedimentary landforms especially wave formed terraces and gilbert type deltas this study focuses on terraces and deltas of western memnonia and ismenius lacus as on earth these landforms provide important indications of water dynamics and sedimentary processes the putative martian paleolakes must have been active for thousands of years to have permitted the water movement and wave formation necessary to form one or more terraces fan shaped landforms at the mouths of channels entering crater lake areas may indicate various channel water interaction of these gilbert type deltas are particularly informative their presence indicates multiple flooding events whereby the inlet channels conveyed coarse grained sediment to the crater lake both terraces and deltaic progradation indicate that the crater lakes of memnonia and ismenius lacus were active for long time periods at least for thousands of years during the post heavy bombardment period of the later martian history the unequivocal presence of standing bodies of water of relatively long durations indicates that climatic settings very different from those prevailing today must have occurred at least locally and temporarily during parts of the martian planetary history

compaction of igneous cumulates when initially deposited mafic igneous cumulates consist of a solid matrix of mineral phases plus 50 60 interstitial liquid due to its greater density the matrix compacts squeezing the interstitial liquid upward a numerical fluid mechanical model of this process indicates that if the cumulate deposition rate is constant compaction occurs in one of two distinct models for rapid deposition the system is permeability controlled with compaction occurring only at the base of the cumulate pile for slowly deposited cumulates an additional compaction zone that is partially viscosity controlled develops at the top of the pile if the extent of compaction and thickness of this zone are known for a real system the model can be used to estimate the permeability and effective viscosity of the compacting cumulates such calculations yield a permeability of 1 5 times 10 3 a 2 f 3 1 f 2 for olivine cumulates in the muskox intrusion reliable viscosity estimates await an empirical law relating ma

concentration of platinum group elements by magmatic fluids in layered intrusions evidence leading to a model which supports the formation of pt pd reefs in a layered intrusion by the action of magmatic fluids is presented as well as some of the constraints imposed on such a model the stillwater complex is used as an example of this model within the stillwater complex no laterally continuous sulfide enriched cumulates occur above the lowermost parts of the ultramafic series until one reaches the platiniferous j m reef this sulfide poor stratigraphic interval beneath the j m reef is characterized by unusually cl rich apatite although reported sulfide modes and whole rock sulfur contents in the cumulates below the j m reef are approximately an order of magnitude lower than that expected for a sulfide saturated magma sulfide saturation prior to the formation of the j m reef is suggested by relatively high pd and pt concentrations reported in the cumulates below the j m reef as well as by the presence of sulfide as inclusions in silicate and oxide cumulus minerals hence it is suggested that the footwall cumulates to the j m reef were originally sulfide saturated but later lost sulfur to a fluid phase exsolving from inter cumulus liquid sporadic sulfide bearing pegmatoids in some of these rocks are additional evidence for late loss of sulfur these observations as well as the association of the j m reef at the first significant stratigraphic break in the cl cl f ratio of apatite and the relatively high solubility of pt and pd as aqueous cl complexes are consistent with the formation of the j m reef by exsolution of cl and volatile rich fluids from footwall intercumulus liquids which leached and transported ore components both the pge and s upward to the level of the reef using published whole rock platinum group element pge concentrations for the ultramafic and lower banded series the cumulates below the j m reef must have been enriched by a factor of at most 1 24 for pt and 1 75 for pd if these rocks were to be the source for the metals in the j m reef model raleigh fractionation calculations suggest that the cumulates beneath the j m reef could have had the required metal concentrations if during the crystallization of a parent magma containing no more than 15 ppb of either metal the average bulk solid liquid distribution coefficients for pt and pd were no more than 5 these values are consistent with assumption of sulfide saturation prior to the formation of the j m reef during solidification of intercumulus liquid the separation and loss of a cl rich fluid will deplete the residual assemblage of s and pge if 0 1 to 0 5 wt percent fluid containing 5 percent s evolved during solidification of the footwall cumulates the amount of fluid expected to be exsolved by the crystallization of approximately 2 to 10 percent fluid saturated intercumulus liquid the fluid liquid distribution coefficients for pt and pd would need to be almost as large as the sulfide liquid silicate liquid distribution coefficients to remove significant amounts of pt and pd much smaller distribution coefficients are needed if sulfide is completely lost to fluid during degassing the latter possibility is consistent with the low s contents of the footwall cumulates beneath the j m reef numerical models of the degassing of intercumulus liquids suggest that thick cumulus sequences can act as chromatographic columns to separate the pge and s during the degassing of intercumulus silicate liquids the pge being enriched at a sulfide dissolution front as upward migrating sulfide undersaturated fluids resorb cumulus sulfides zones of pge and s enrichment can occur where degassing fronts encounter stratigraphic physical chemical discontinuities such as changes in the fluid saturation temperature of the intercumulus silicate liquids

timing and duration of the central atlantic magmatic province in the newark and culpeper basins eastern u s a abstract new major and trace element data and 40ar 39ar plateau ages constrain the timing duration and time related geochemical evolution of the central atlantic magmatic province in the u s a newark and culpeper basins and refine correlations with basaltic lava flows from other late triassic early jurassic circum atlantic basins the precise statistically robust 40ar 39ar plateau ages were obtained on biotite and on fresh plagioclase and calculated using the latest 40k decay constants these ages are supported by a general consistency of the ca k calculated from 37ar 39ar of the plateau steps and the ca k obtained by detailed electron microprobe analyses on plagioclase phenocrysts the ages of five analyzed basalt lava flows from all three lava flow units in the newark basins and the ages of two sill samples are indistinguishable indicating a brief magmatic peak phase at 201 8 0 7 ma recalibrated 40ar 39ar plateau ages from the entire province indicate a near synchronous onset and peak volcanic activity at the triassic jurassic boundary within the circum atlantic basins from the u s a canada and morocco the early erupted magmas moroccan lower to upper basalts the fundy basin north mountain basalt and orange mountain and equivalent u s a flows yield an enriched geochemical signature e g with relatively high la yb whereas late magmas in the u s a hook mountain and hampden basalts and morocco recurrent basalt yield relatively depleted geochemical compositions low la yb a slight but significant age difference for eruption of hook mountain and hampden basalts 200 3 0 9 ma and recurrent basalts 198 2 1 1 ma is interpreted as evidence of a diachronous northward rift drift transition during break up of pangea our data indicate also a prolonged intrusive sequence that continued until about 195 ma at the palisades sill and is consistent with sporadic late camp magmatism for dykes from the south eastern u s a and for intrusions from guinea

convection and crystal settling in sills it has been advocated that convective and crystal settling processes play significant and perhaps crucial roles in magmatic differentiation the fluid dynamics of magma chambers have been extensively studied in recent years both theoretically and experimentally but there is disagreement over the nature and scale of the convection over its bearing on fractionation and possibly over whether it occurs at all the differential distribution of modal olivine with height in differentiated alkaline basic sills provides critical evidence to resolve this controversy at least for small to medium large magma chambers our own and others published data for such sills show that irrespective of overall olivine content modal olivine contents tend to increase in a roughly symmetrical manner inwards from the upper and lower margins of the sill i e the distribution patterns are more often approximately d shaped rather than the classic s shape generally ascribed to gravity settling we concur with the majority of other authors that this is an original feature of the filling process which has survived more or less unchanged since emplacement we therefore conclude that the magmas have not undergone turbulent convection and that gravity settling has usually played only a minor modifying role since the intrusion of these sills we offer a possible explanation for the apparent contradiction between fluid dynamical theory and the petrological evidence by suggesting that such sills rarely fill by the rapid injection of a single pulse of magma rather they form from a series of pulses or a continuous pulsed influx over a protracted interval during which marginal cooling severely limits the potential for thermal convection

evolution of late stage differentiates in the palisades sill new york and new jersey abstract the palisades sill at upper nyack ny contains evolved rocks that crystallized as ferrodiabase and ferrogranophyre and occupy 50 to 60 of the local thickness 143 nd 144 nd isotope values for rocks representing palisades diversity range between 0 512320 and 0 512331 and indicate a homogeneous source for the palisades and little or no contamination from shallow crustal sediments petrographic analysis of ferrodiabase suggests that strong iron enrichment was the result of prolonged quiescence in cycles of magmatic input ferrogranophyres in the updip northern palisades at upper nyack are members of a suite of cogenetic rocks with similar composition to sandwich horizon rocks of the southern palisades at fort lee nj but display distinct mineralogical and textural features differences in textural and mineralogical features are attributed to a updip lateral migration of residual liquid as the sill propagated closer to the surface b deformation caused by tectonic shifts and c crystallization in the presence of deuteric hydrothermal fluids resulting in varying degrees of alteration a model connecting multiple magmatic pulses compaction and mobilization of residual liquid by compositional convection closed system differentiation synchronous with tapping of the sill for extrusion of coeval basaltic subaerial flows is presented the persistence of a low temperature mushy layer represented by ferrogranophyres supports the possibility of a long lived conduit subject to reopening after periods of quiescence in magmatic input leading to the extrusion of the multiple flows of the orange mountain basalt and perhaps even subsequent preakness basalt flows depending on solidification conditions a sub newark basin network of sills subjected to similar protracted input of pulses as hypothesized for the palisades was likely responsible for 600 ka of magmatic activity required to emplace a third set of watchung flood basalts the hook mountain basalt

oxygen isotope exchange kinetics of mineral pairs in closed and open systems applications to problems of hydrothermal alteration of igneous rocks and precambrian iron formations the systematics of stable isotope exchange between minerals and fluids are examined in the context of modal mineralogical variations and mass balance considerations both in closed and in open systems on mineral pair 18 o plots samples from terranes that have exchanged with large amounts of fluid typically map out steep positively sloped non equilibrium arrays analytical models are derived to explain these effects these models allow for different exchange rates between the various minerals and the external fluids as well as different fluid fluxes the steep arrays are adequately modelled by calculated isochron lines that involve the whole family of possible exchange trajectories these isochrons have initially steep near vertical positive slopes that rotate toward a 45 equilibrium slope as the exchange process proceeds to completion the actual data point array is thus analogous to the hand of an isotopic clock that measures the duration of the hydrothermal episode the dimensionless ratio of the volumetric fluid flux to the kinetic rate parameter u k determines the shape of each individual exchange trajectory in a fluid buffered system u k 1 the solutions to the equations 1 are independent of the mole fractions of the solid phases 2 correspond to taylor s open system water rock equation and 3 yield straight line isochrons that have slopes that approach 1 f where f is the fraction reacted of the more sluggishly exchanging mineral the isochrons for this simple exchange model are closely congruent with the isochrons calculated for all of the more complex models thereby simplifying the application of theory to actual hydrothermal systems in nature in all of the models an order of magnitude of time in units of kt separates steep non equilibrium arrays e g slope 10 from arrays approaching an equilibrium slope of unity on a diagram because we know the approximate lifetimes of many hydrothermal systems from geologic and heat balance constraints we can utilize the 18 o 16 o data on natural mineral assemblages to calculate the kinetic rate constants k s and the effective diffusion constants d s for mineral h 2o exchange these calculated values k qtz 10 14 k feld 10 13 10 12 agree with experimental determinations of such constants in nature once the driving force or energy source for the external infiltrating fluid phase is removed the disequilibrium mineral pair arrays will either 1 remain frozen in their existing state if the temperatures are low enough or 2 re equilibrate along specific closed system exchange vectors determined solely by the temperature path and the mineral modal proportions thus modal mineralogical information is a particularly important parameter in both the open and closed system scenarios and should in general always be reported in stable isotopic studies of mineral assemblages these concepts are applied to an analysis of 18 o 16 o systematics of gabbros plagioclase clinopyroxene and plagioclase amphibole pairs granitic plutons quartz feldspar pairs and precambrian siliceous iron formations quartz magnetite pairs in all these examples striking regularities are observed on and plots but we point out that plots have many advantages over their equivalent diagrams as the latter are more susceptible to misinterpretation using the equations developed in this study these regularities can be interpreted to give semiquantitative information on the exchange histories of these rocks subsequent to their formation in particular we present a new interpretation indicating that precambrian cherty iron formations have in general undergone a complex fluid exchange history in which the iron oxide magnetite precursor has exchanged much faster with low temperature 400 c fluids than has the relatively inert quartz

geochemical evolution of a 10 m thick intrusive body the south brenterc h diabase dyke western armorican massif france the south brenterc h 9 5 m thick diabase dyke belongs to the intermediate ti quartz normative tholeiitic magmatism which has intruded the western end of britanny france near the trias lias boundary it has been previously identified as a simple dyke resulting from a single magmatic injection the present study is based on ten samples collected along a half transverse section of this dyke geochemical analyses and some textural parameters have been used to point out two flow related differentiation processes partially masked by alteration these late magmatic mechanisms have induced subtle geochemical variations among samples which sometime counteract each other the processes pointed out here are i expulsion of phenocrysts from the borders during flow differentiation likely by the way of an hydrodynamic grain dispersive pressure and ii emplacement of a more evolved magma in the central part of the intrusion probably due to a viscosity segregation mechanism during the flow

ta and sn concentration by muscovite fractionation and degassing in a lens like granite body the case study of the penouta rare metal albite granite nw spain abstract the penouta peraluminous low phosphorous granite is the most important low grade high tonnage sn ta nb bearing albite granite from the iberian massif a sheet or laccolith shape instead of a stock is inferred for the penouta granite maybe in relation with the low viscosity and high mobility of a fluorine bearing melt subhorizontal lateral extension of the magma is also inferred via vertical and horizontal geochemical variations the absence of compositional gaps in variation diagrams coupled with continuous evolutionary trends of compatible and incompatible elements with height discard a multi pulse intrusion and point to a single magma pulse mineral chemistry trace element and least squares mass balance modelling support a differentiation process from bottom to top in the emplacement place the absence of switch from incompatible to compatible behaviour bell shaped trends in sn nb and ta variation diagrams coupled to experimental constraints on tantalite and cassiterite saturation suggest that nb ta oxides and probably cassiterite were not fractionated mineral phases their crystallisation being related to concentration gradients within a trapped intercumulus melt major and trace element modelling support that the concentration upwards of ta and the ta nb ratio could be a consequence of mineral fractionation with a key role of muscovite mainly primary for the ta nb ratio as this mineral has a higher partition coefficient for nb than ta our results suggest that fluorine and peraluminosity had a limited effect in the ta nb ratio variations hence ta enrichment is mainly controlled by fractional crystallisation processes in most cases sn enrichment was also concomitant with ta indicating that crystal melt fractionation processes also played an important role in sn concentration nevertheless the strongest sn enrichment in the granite e g central part of the granite body does not correspond to a significant ta enrichment the high affinity of sn for fluids and the high partitioning of ta for melt could explain this decoupling nevertheless the magmatic signature of cassiterites in these strongly sn enriched zones central part of the granite body rules out a hydrothermal subsolidus origin for this fluid by analogy with models carried out in sill like bodies it seems likely that the sn enrichment in the central part of the granite body is related to fluid saturation degassing occurred in the lower margin as a consequence of cooling and crystallisation of mostly anhydrous minerals i e second boiling the vapour exsolved migrated into the hotter melt up to the central part where it probably was reabsorbed yielding cassiterite with a magmatic signature moreover we suggest that heat loss in the upper margin of the granite body might also contribute to the formation of a second fluid saturated zone as a result pegmo aplites and greisen were developed

crystallization and compositional convection in a porous medium with application to layered igneous intrusions growing crystals can provide a local source of buoyancy by depleting melt of certain chemical components and vigorous convective motions in the fluid can result we report a quantitative experimental study in which saturated aqueous solutions of cuso4 and na2so4 in porous media of either glass balls or cuso4 5h2o crystals were crystallized by cooling from below the crystals growing in the pore space released light chemically depleted solution which had the potential to convect out of the porous medium and be replaced by additional saturated solution from above we explore the physical and chemical effects of compositional convection during the complete solidification of the interstitial solution by varying the permeability of the porous medium the cooling rate and the initial composition of the solution this type of convection can form adcumulate rocks by enabling continuous circulation of melt from the main magma reservoir through the pores of a cumulus pile adcumulus growth can occur by this mechanism as long as the convective velocity in the porous medium is greater than the velocity of the solidification front we scale the experimental results to magmatic conditions and infer that this process should occur commonly in layered igneous intrusions when our porous media had a small permeability compositional convection enabled the adcumulate growth of a completely solid contact between melt and cumulate trapping of dynamically unstable melt beneath this impermeable layer of crystals also occurred in these experiments and may be important geologically small shallowly emplaced intrusions and the cumulate zones at the base of komatiites are probably cooled too rapidly for significant intercumulus convection to occur

magmatisme fissural triasico liasique dans l ouest du massif armoricain france petrologie geochimie âge et modalites de la mise en place the dolerite dykes at the western end of britanny france which is located on the north atlantic margin were emplaced in a nw se field of faults at the trias lias time limit between 210 and 195 ma these rocks are quartz tholeiites enriched in tio2 with a trace element composition close to that of the deccan basalts the initial concentrations of ba k and rb though slightly modified by the alteration and of nb seem to have been affected by crustal contamination during the magma transfer these dykes are comparable in age and composition to the tholeiitic dykes exposed in the iberian peninsula in the moroccan atlas and in the northern appalachians they however have a different orientation furthermore these various domains differ in their postintrusion history the rifting leading to the opening of the central atlantic occurred soon after the intrusions of the dykes whereas the opening of the north atlantic began several tens of millions of years later following the direction of the armoric

lateglacial retreat chronology of the scandinavian ice sheet in finnmark northern norway reconstructed from surface exposure dating of major end moraines abstract we report results from a comprehensive surface exposure dating campaign in eastern finnmark located in the northernmost part of norway and close to the norwegian russian border this is a palaeo glaciologically important region as it sits near the proposed border zone between the former scandinavian and barents sea ice sheets however until now the deglaciation history has few direct dates onshore and the chronology of ice front retreat is instead found by correlating ice marginal deposits with isostatically raised shorelines and marine sediment cores we measured the content of 10 be n 22 and 36 cl n 17 from boulders located at the crest of major moraine ridges at four localities kjaes kongsfjorden vardo and kirkenes these are key localities of existing regional reconstructions of ice recession in this area despite some spread in age results from each locality due to methodological challenges associated with surface exposure dating the large numbers of samples from each site except kjaes still allow for obtaining clusters of similar ages which are used for arriving at a likely chronology of ice front retreat our results show that the kongsfjorden and vardo moraines were deposited 14 3 1 7 ka and 13 6 1 4 ka respectively and thus point to a older dryas age of the proposed outer porsanger deglaciation sub stage moraine ridges belonging to the main sub stage near kirkenes were dated to 11 9 1 2 ka corresponding well with the ice retreat chronology farther west in northern norway and suggesting that the maximum younger dryas ice sheet extent was attained in the late younger dryas along a more than 500 km long stretch in northernmost scandinavia

physical processes in the evolution of magmas the most striking feature of the igneous rock suite is its compositional diversity and the explanation of this fact is the main goal of igneous petrology the production from a relatively homogeneous mantle source of a large range of compositions which go to make up the crust renders igneous processes of major interest to the earth sciences furthermore magma migration and eruption involve a fascinating range of phenomena making them a rewarding object of study in this chapter we discuss several aspects of natural convection in fluids and the application of these ideas to the dynamics of magma chambers

density changes during the fractional crystallization of basaltic magmas fluid dynamic implications the dynamical behaviour of basaltic magma chambers is fundamentally controlled by the changes that occur in the density of magma as it crystallizes in this paper the term fractionation density is introduced and defined as the ratio of the gram formula weight to molar volume of the chemical components in the liquid phase that are being removed by fractional crystallization removal of olivine and pyroxene whose values of fractionation density are larger than the density of the magma causes the density of residual liquid to decrease removal of plagioclase with fractionation density less than the magma density can cause the density of residual liquid to increase during the progressive differentiation of basaltic magma density decreases during fractionation of olivine olivine pyroxene and pyroxene assemblages when plagioclase joins these mafic phases magma density can sometimes increase leading to a density minimum calculations of melt density changes during fractionation show that compositional effects on density are usually greater than associated thermal effects in the closed system evolution of basaltic magma several stages of distinctive fluid dynamical behaviour can be recognised that depend on the density changes which accompany crystallization as well as on the geometry of the chamber in an early stage of the evolution where olivine and or pyroxenes are the fractionating phases compositional stratification can occur due to side wall crystallization and replenishment by new magma with the most differentiated magma tending to accumulate at the roof of the chamber when plagioclase becomes a fractionating phase a zone of well mixed magma with a composition close to the density minimum of the system can form in the chamber the growth of a zone of constant composition destroys the stratification in the chamber a chamber of well mixed magma is maintained while further differentiation occurs unless the walls of the chamber slope inwards in which case dense boundary layer flows can lead to stable stratification of cool differentiated magma at the floor of the chamber in a basaltic magma chamber replenished by primitive magma the new magma ponds at the base and evolves until it reaches the same density and composition as overlying magma successive cycles of replenishment of primitive magma can also form compositional zonation if successive cycles occur before internal thermal equilibrium is reached in a chamber in a chamber containing well mixed plagioclase saturated magma the primitive magma can be either denser or lighter than the resident magma in the first case the new magma ponds at the base and fractionates until it reaches the same density as the evolved magma mixing then occurs between magmas of different temperatures and compositions in the second case a turbulent plume is generated that causes the new magma to mix immediately with the resident magma