fluid absent melting of a layered crustal protolith implications for the generation of anatectic granites we report the result of h2o undersaturated melting experiments on charges consisting of a layer of powdered sillimanite bearing metapelite hq36 and a layer of powdered tonalitic gneiss agc150 experiments were conducted at 10 kbar at 900 925 and 950 c when run alone the pelite yielded 40 vol strongly peraluminous granitic melt at 900 c while the tonalite produced only 5 vol weakly peraluminous granitic melt at 950 c the pelite and the tonalite yielded 50 vol and 7 vol granitic melt respectively when run side by side the abundance of melt in the tonalite was 10 times higher at all temperatures than when it was run alone in the pelite the melt abundance increased by 25 vol when run alone biotite dehydration melting in the tonalite yielded orthopyroxene and garnet in addition to granitic melt when run side by side only garnet was produced in addition to granitic melt experiments of relatively short duration however also contained al rich orthopyroxene we suggest that the large increase in melt fraction in the tonalite is mainly a result of increased activity of al2o3 in the melt which lowers the temperature of the biotite dehydration melting reaction in the pelite the increase in the abundance of melt is caused by transport of plagioclase component in the melt from the tonalite layer to the pelite layer this has the effect of changing the bulk composition of this layer in the direction of minimum temperature granitic liquids our results show that rocks which are poor melt producers on their own can become very fertile if they occur in contact with rocks that contain components that destabilize the hydrous phase s and facilitate dehydration melting because of this effect the continental crust may have an even greater potential for granitoid melt production than previously thought our results also suggest that many anatectic granites most likely contain contributions from two or more different source rocks which will be reflected in their isotopic and geochemical compositions

velocity distribution of turbulent open channel flow with bed suction this study investigates theoretically and experimentally the velocity distributions of turbulent open channel flow with bed suction a velocity profile with a slip velocity at the bed surface and an origin displacement under the bed surface is proposed and discussed based on this assumption a modified logarithmic law is derived the measured experimental velocity distribution verifies the accuracy of the theoretically derived profile the data show a significant increase in the near bed velocity and a velocity reduction near the water surface resulting in the formation of a more uniform velocity distribution the values of the origin displacement slip velocity and shear velocity are found to increase with increasing relative suction the measured data show the occurrence of two flow regions in the suction zone a transitional region in which the velocity readjusts rapidly and an equilibrium region

melting of plagioclase quartz assemblages at 2 kbar water pressure the melting of plagioclase and quartz has been investigated at ph2o 2kb a single crystal of plagioclase was surrounded by quartz powder and water a reaction rim consisting of glass and of an rich plagioclase developed around unchanged starting plagioclase microprobe determinations of melt and coexisting plagioclase compositions reveal a strong fractionation of plagioclase components between melt and new plagioclase for example at 850 c the approximate xan of melt is 0 3 and that of plagioclase is 0 8 the temperature interval between beginning of melting and complete melting of cotectic compositions is 100 c or more for quartz plagioclase mixtures with plagioclases between an 40 and an 75 in comparison to the system ab an h2o the plagioclase melting loop is somewhat wider in the investigated system qz ab an h2o but the temperature interval is much smaller 100 c than in the system ab an h2o 200 c the solidus data indicated by the new plagioclase compositions are practically identical with those observed in beginning of melting experiments the results show that fractionation of plagioclase components between partial melts and restite plagioclase can be more pronounced in multi component rocks than in the pure plagioclase system this finding is important for the development of albite rich rocks from more basic compositions

the trace element compositions of s type granites evidence for disequilibrium melting and accessory phase entrainment in the source within individual plutons the trace element concentrations in s type granites generally increase with maficity total iron and magnesium content and expressed as atomic fe mg in this study the degree of variability in trace element concentration also expands markedly with the same parameter the strongly peraluminous high level s type granites of the peninsular pluton cape granite suite south africa are the product of biotite incongruent melting of a metasedimentary source near the base of the crust leucogranites within the suite represent close to pure melts from the anatectic source and more mafic varieties represent mixtures of melt and peritectic garnet and ilmenite trace elements such as rb ba sr and eu that are concentrated in reactant minerals in the melting process show considerable scatter within the granites this is interpreted to reflect compositional variation in the source in contrast elements such as lree zr and hf which are concentrated within refractory accessory phases zircon and monazite show well defined negative correlations with increasing sio2 and increase linearly with increasing maficity this is interpreted to reflect coupled co entrainment of accessory minerals and peritectic phases to the melt leucocratic rocks cannot have evolved from the more mafic compositions in the suite by a process of fractional crystallisation because in this case they would have inherited the zircon saturated character of this hypothetical earlier magma trace element behaviour of granites from the peninsular pluton has been modelled via both equilibrium and disequilibrium trace element melting in the disequilibrium case melts are modelled as leaving the source with variable proportions of entrained peritectic phases and accessory minerals but before the melt has dis solved any accessory minerals thus the trace element signature of the melt is largely inherited from the reactants in the melting reaction with no contribution from zircon and monazite dissolution in the equilibrium case melt leaves the source with entrained crystals after reaching zircon and monazite saturation a significant proportion of the rocks of the peninsular pluton have trace element concentrations below those predicted by zircon and monazite saturation in the case of the most leucocratic rocks all compositions are zircon undersaturated whilst the majority of the most mafic compositions are zircon oversaturated however in both cases zircon is commonly xenocrystic thus the leucocratic rocks represent close to pure melts which escaped their sources rapidly enough that some very closely match the trace element disequilibrium melting model applied in this study zircon dissolution rates allow the residency time for the melt in the source to be conservatively estimated at less than 500 years

chemical characteristics of migmatites accessory phase distribution and evidence for fast melt segregation rates equilibration between melt and solid is inhibited by rapid melt extraction and by restricted equilibration armouring slow dissolution when segregation occurs by channelised migration along high porosity pathways melt migration is more rapid than trace element diffusion rates in silicates and faster than accessory phase dissolution rates evidence for channelised flow and deformation enhanced melt segregation into boudin necks fractures and micro shears at low melt fractions is present in the moine kirtomy migmatitie suite kms in sutherland scotland melt migration distances are on a metre to tens of metres scale concordant leucosomes in stromatic migmatities in the kms have low zr contents low lree light rare earth element and h heavy ree contents and positive eu anomalies ree patterns of this type can be produced by removal of leucosome before complete equilibration with source due to the inhibited dissolution of lree and hree bearing accessory phases in water undersaturated melts melting in the kms however occurred at or near the wet granite solidus leaving biotite as a residual phase detailed back scattered electron imaging shows that ree bearing accessory phases remained as residual phases and were concentrated in the melanosome and at the melanosome leucosome boundary irregularly shaped patches of diatexite contain a small proportion of excess zr consistent with entrainment of melanosome schlieren enriched in zircon these data indicate that deformation enhanced melt extraction led to the rapid migration of small melt fractions from the melting site on a time scale less than that required to saturate the melt in zr leucosomes were thus prevented from equilibrating with accessory phases before extraction

granitic melt viscosity and silicic magma dynamics in contrasting tectonic settings models for granitic melt viscosities reveal the influences of h 2 o temperature and tectonics on melt segregation ascent and emplacement low t magmas are wetter than those formed at high t so log 10 melt viscosity pa s is 3 2 6 3 for leucogranite to tonalite only 10 1000 times more viscous than basalts there should be no correlation between critical dyke width to prevent freezing and tectonic setting granitic magma ascent rates in dykes vary by

selective entrainment of peritectic garnet into s type granitic magmas evidence from archaean mid crustal anatectites entrainment of restite is commonly invoked to explain both the origin of relatively mafic granites and granodiorites as well as the chemical connection between granite magmas and their sources this concept has become linked to models for magma migration out of the source as restite entrainment is considered to take place when diatexitic sources mobilise en masse this is at odds with the common occurrence of relatively mafic granites as high level intrusions in the crust or their eruptive equivalents that must have formed from markedly water undersaturated magmas that ascended through narrow conduits we investigate pelitic migmatites from the mkhondo valley metamorphic suite mvms in swaziland where a mid crustal heating event produced metatexitic migmatites with minimal post anatectic recrystallisation in these rocks all the garnet is peritectic having arisen through biotite fluid absent melting which produced garnet poikiloblasts characterised by inclusions of melt quartz and biotite leucosomes that represent sites of melt transfer carry similar smaller typically 1 mm entrained garnet poikiloblasts which were capable of amalgamating to form larger composite grains in anatectic structures where melt was present for longer entrained garnet was extensively recrystallised via a dissolution precipitation process to adopt a more magmatic character the peritectic garnet in the pelitic source appears to have grown out of equilibrium with feldspar and hree rich accessory phases while the recrystallised garnet in the larger melt filled structures became progressively better equilibrated with these minerals thus peritectic garnet in the source grew sufficiently rapidly to prevent trace element equilibrium with the bulk rock composition and concurrent rapid magma segregation prevented the development of diatexitic source conditions the segregated magma consisted of melt the peritectic assemblage principally garnet and the accessory minerals monazite and zircon these rocks illustrate that mafic granites may arise purely as mixtures of melt and the peritectic assemblage produced by the incongruent melting reaction importantly under the circumstances which produced the mvms anatectites peritectic garnet is entrained as 1 mm poikiloblasts demonstrating how mafic granitic magmas can migrate out of the source without the source becoming diatexitic

back reaction between restite and melt implications for geothermobarometry and pressure temperature paths partial melting is an important process in high grade metamorphism and is responsible for the production of large volumes of melt in the earth9s crust unless melt extraction is complete in situ crystallizing melt back reacts with the restite upon cooling in the northampton block western australia spinel formed by high temperature 850 c partial melting of metapelites is separated from leucosome by garnet sillimanite or cordierite sillimanite coronas formation of the coronas by back reaction with a silica undersaturated melt is inferred because 1 textural evidence indicates that quartz was exhausted as a result of progressive dehydration melting before formation of spinel melt 2 the coronitic phases are never in direct contact with quartz of the leucosome 3 barometry on cordierite and garnet bearing equilibria with spinel gives conflicting pressures 8 0 0 5 and 3 3 0 2 kbar respectively at 850 c if reactions with quartz are assumed but pressure estimates converge on 6 5 0 5 kbar at lower silica activity a sio 2 0 85 0 02 these results indicate that the use of corona textures for the derivation of pressure temperature p t conditions and pressure temperature vectors in migmatites should be reconsidered such coronas may result from a heating cooling cycle without substantial change in pressure back reaction with residual melt is likely to be an important process in migmatites generally affecting both the solid residuum and the leucosome composition

origin of k poor leucosomes in a metasedimentary migmatite complex by ultrametamorphism syn metamorphic magmatism and subsolidus processes two types of stromatic leucosomes are identified in metasedimentary rocks from the skagit migmatite complex north cascades washington state u s a both types are trondhjemitic and appear similar in outcrop but although both contain low abundances of ree one type consists of leucosomes that are relatively ree enriched compared to the other and contains 1 small 700 c water rich fluid present inferences about the origin of the above listed mineralogical and fluid inclusion features and modeling of leucosome trace element abundances the second type of leucosome is interpreted to have formed entirely by subsolidus processes e g metamorphic differentiation because these leucosomes lack features consistent with an origin by partial melting k poor tonalitic trondhjemitic leucosomes associated with metasedimentary biotite bearing source rocks may form by water saturated partial melting or by subsolidus processes both general leucosome forming mechanisms may operate at different times during upper amphibolite facies regional metamorphism partial melting may be initiated by syn metamorphic magmatic activity if crystallizing plutons serve as external sources of the water rich fluid necessary for ultrametamorphism in the middle crust during orogenesis large scale migmatite complexes such as the skagit migmatites may form at least in part in response to contact effects of plutonism associated with high grade metamorphism so although migmatite complexes are a volumetrically substantial part of many orogenic belts they may not themselves represent a significant original source of magma for larger scale igneous bodies

effect of seepage flow on incipient motion of sand particles in a bed subjected to surface flow abstract understanding the interaction of the groundwater seepage flow and sediment transport along permeable boundaries is essential for managing soil beds and bank erosion a significant bank failure may occur due to sediment transport under the boiling condition in the present study an experiment is conducted to investigate the effects of injection or upward seepage on the critical shear stress of a cohesionless soil bed subjected to surface flow the upward seepage force acting on the soil bed particles may reduce the virtual weight of the particles in turn it may also reduce the critical shear stress acting on the particles a theoretical analysis examines how the upward seepage force acts to modify the critical shear stress for the sediment entrainment due to the increase in the hydraulic gradient of the seepage flow a force analysis reveals that the ratio of the critical shear stress with seepage to that without seepage is dependent on the ratio of the hydraulic gradient of seepage to its value at the quicksand condition a test apparatus was built and the effects of injection on the critical shear stress of cohesionless soil bed particles were measured an acrylic channel with a rectangular cross section was prepared and a section of a cohesionless soil bed subjected to an upward seepage flow was installed at the bottom of the channel the experiment demonstrates that the induced seepage affects the dynamics of the channel flow even though the magnitude of the seepage flow velocity is significantly smaller than that of the free surface water flow the test results show that the dimensionless critical shear stress of the cohesionless soil bed particles decreases slightly as the hydraulic gradient of the seepage flow is increased when critical shear stress is determined at the upstream edge of the seepage zone furthermore the dimensionless critical shear stress decreases sharply with the increment of the hydraulic gradient of the seepage flow when critical shear stress is determined in the seepage zone however it does not decrease in either case as was theoretically assumed even when the hydraulic gradient approaches to the critical hydraulic gradient or the quicksand condition

h2o diffusion models in rhyolitic melt with new high pressure data abstract water diffusion in silicate melts is important for understanding bubble growth in magma magma degassing and eruption dynamics of volcanos previous studies have made significant progress on water diffusion in silicate melts especially rhyolitic melt however the pressure dependence of h2o diffusion is not constrained satisfactorily we investigated h2o diffusion in rhyolitic melt at 0 95 1 9 gpa and 407 1629 c and 0 2 5 2 wt total water h2ot content with the diffusion couple method in a piston cylinder apparatus compared to previous data at 0 1 500 mpa h2o diffusivity is smaller at higher pressures indicating a negative pressure effect this pressure effect is more pronounced at low temperatures assuming h2o diffusion in rhyolitic melt is controlled by the mobility of molecular h2o h2om the diffusivity of h2om dh2om at h2ot 7 7 wt 403 1629 c and 1 9 gpa is given by d h 2 o m d 0 e x p a x with d 0 e x p 13 375 1 8875 p 12939 3625 6 p t and a 37 256 75884 t where d0 is in m2 s x is mole fraction of h2ot on a single oxygen basis t is temperature in k and p is pressure in gpa h2ot diffusivities dh2ot in m2 s can be calculated from h2om diffusivity or directly from the following expression l n d h 2 o t x 13 47 49 996 x 7 0827 x 1 8875 p 9532 3 91933 x 13403 x 3625 6 p t at low h2ot content up to 2 wt if an error of a factor of 2 is allowed h2ot diffusivity is approximately proportional to h2ot content d h 2 o t c c 0 e x p 9 5279 1 8875 p 9698 5 3625 6 p t where c is h2ot content in wt and c0 is 1 wt the new expressions for h2o diffusion not only reproduce our own data but also match data in literature from different laboratories and using different methods indicating good inter laboratory and multi method consistency the new expressions cover a wide range of geological conditions and can be applied to h2o diffusion in rhyolitic melts in various volcanic and magmatic processes

the ti saturation surface for low to medium pressure metapelitic biotites implications for geothermometry and ti substitution mechanisms the ti content of biotite can serve as a geothermometer for graphitic peraluminous metapelites that contain ilmenite or rutile and have equilibrated at roughly 4 6 kbar the relationship between ti content temperature and mg mg fe value was calibrated empirically using an extensive natural biotite data set 529 samples from western maine and south central massachusetts in combination with the petrogenetic grid of spear et al 1999 the calculated ti saturation surface is curved such that for a given mg mg fe value ti concentration increases as a function of temperature in a nonlinear fashion and for a given temperature ti concentrations decrease with an increase in mg mg fe the fit to the ti saturation surface can be reformulated as the geothermometric expression t ln ti a c x mg 3 b 0 333 in which t is temperature in degrees celsius ti is the number of atoms per formula unit apfu normalized on the basis of 22 o atoms x mg is mg mg fe a 2 3594 b 4 6482 10 9 and c 1 7283 the calibration range for this expression is x mg 0 275 1 000 ti 0 04 0 60 apfu and t 480 800 c precision of the ti in biotite geothermometer is estimated to be 24 c at the lower temperature range and improves to 12 c at higher temperatures application of the ti in biotite geothermometer to ilmenite or rutile bearing graphitic peraluminous metapelites equilibrated at 3 6 kbar is generally consistent with independent temperature determinations but with some deviations that represent local reequilibration consequently the ti systematics in biotite can also serve as the basis of a very sensitive indicator of chemical equilibrium or lack thereof application of the geothermometer to metapelites not containing the requisite mineral assemblages can lead to minor to significant errors in estimated temperatures biotite ti substitution mechanisms are controlled by several factors based on the biotite calibration data set magnesian biotites x mg 0 65 incorporate ti in accordance with the exchange vector tial2r 1si 2 where r is the sum of the divalent cations mg fe mn this substitution mechanism is primarily a response to misfit of the octahedral and tetrahedral layers in magnesian biotites intermediate biotites x mg 0 65 particularly at higher temperatures exhibit enhanced ti concentrations most consistent with the ti deprotonation tio2r 1 oh 2 exchange vector dominance of ti deprotonation substitution is largely a function of reduction of h2o activity at higher metamorphic grades supplementary biotite data from metaluminous amphibolites and mafic granulites metamorphosed isothermally with variable h2o activities reveal that low al biotite incorporates significantly higher concentrations of ti relative to peraluminous biotite as a result of a combination of the exchange vectors tio2r 1 oh 2 and rsial 2 substituting in roughly an 8 1 ratio

insights into the complexity of crustal differentiation k2o poor leucosomes within metasedimentary migmatites from the southern marginal zone of the limpopo belt south africa differentiation of the continental crust is the result of complex interactions between a large number of processes which govern partial melting of the deep crust magma formation and segregation and magma ascent to significantly higher crustal levels the anatectic metasedimentary rocks exposed in the southern marginal zone of the limpopo belt represent an unusually well exposed natural laboratory where the portion of these processes that operate in the deep crust can be directly investigated in the field the formation of these migmatites occurred via absent incongruent melting reactions involving biotite which produced cm to m scale k2o poor garnet bearing stromatic leucosomes with high ca na ratios relative to their source rocks field investigation combined with geochemical analyses and phase equilibrium modelling designed to investigate some aspects of disequilibrium partial melting show that the outcrop features and compositions of the leucosomes suggest several steps in their evolution 1 melting of a portion of the source with restricted plagioclase availability due to kinetic controls to produce a magma melt entrained peritectic minerals in variable proportions relative to melt 2 segregation of the magma at near peak metamorphic conditions into melt accumulation sites mas a k a future leucosome 3a reequilibration of the magma with a portion of the bounding mafic residuum via chemical diffusion h2o k2o which triggers the co precipitation of quartz and plagioclase in the mas 3b extraction of melt dominated magma to higher crustal levels leaving peritectic minerals entrained from the site of the melting reaction and the minerals precipitated in the mass to form the leucosome in the source the key mechanism controlling this behaviour is the kinetically induced restriction of the amount of plagioclase available to the melting reaction this results in elevated melt h2o and k2o and chemical potential gradient for these components across the leucosome mafic residuum contact the combination of all of these processes accurately explains the composition of the k2o poor leucosomes these findings have important implications for our understanding of melt segregation in the lower crust and minimum melt residency time which according to the chemical modelling is 5 years we demonstrate that in some migmatitic granulites the leucosomes constitute a type of felsic refractory residuum rather than evidence of failed magma extraction this provides a new insight into the ways that source heterogeneity may control anatexis this article is protected by copyright all rights reserved

geochronology of the hout river shear zone and the metamorphism in the southern marginal zone of the limpopo belt southern africa abstract in this paper monazite u pb and zircon evaporation dates stepleaching pb pb results on garnet staurolite and kyanite and hornblende ar ar data are presented which constrain the timing of granulite facies metamorphism in the southern marginal zone of the limpopo belt and its thrusting onto the kaapvaal craton the southern marginal zone of the limpopo belt is considered to be a lower crustal equivalent of the northern kaapvaal craton granulite exhumation is associated with southward thrusting along the hout river shear zone which is a set of thrust and strike slip shear zones zircon ages for the matok intrusive complex which was emplaced within the zone during this thrusting charno enderbites 2671 4 ma granodiorites between 2667 and 2664 ma have previously been interpreted as evidence for rapid exhumation of the southern marginal zone within only 7 ma we have obtained a u pb date of 2691 7 ma for monazite from the bandelierkop quarry in the southern marginal zone interpreted as the age of high grade metamorphism a single zircon evaporation pb pb date of 2643 1 ma from a leucosome band at the same locality may indicate longer lasting metamorphism or decompression melting during exhumation anatexis of metapelitic xenoliths within the matok intrusive complex was dated at 2663 4 ma by u pb on monazite indistinguishable from existing zircon ages for this complex pb pb step leaching dates obtained on synkinematically grown garnet 2691 20 ma staurolite 2712 37 ma and kyanite 2672 51 ma from the khavagari hills in the giyani greenstone belt in the immediate footwall of the hout river shear zone indicate that early thrusting was contemporaneous with peak metamorphism in the southern marginal zone ar ar dating and geochemistry on syntectonic hornblende separates from the same shear zone system yielded disturbed spectra and indicated multiple populations probably reflecting repeated or continuous tectonic activity of the hout river shear zone up to about 2600 ma

microstructures of melt inclusions in anatectic metasedimentary rocks the occurrence of crystallized and glassy melt inclusions mi in high grade partially melted metapelites and metagraywackes has opened up new possibilities to investigate anatectic processes the present study focuses on three case studies khondalites from the kerala khondalite belt india the ronda migmatites spain and the barun gneiss nepal himalaya the results of a detailed microstructural investigation are reported along with some new microchemical data on the bulk composition of mi these inclusions were trapped within peritectic garnet and ilmenite during crystal growth and are therefore primary inclusions they are generally isometric and very small in size mostly 15 m and only rarely reaching 30 m they occur in clusters in most cases inclusions are crystallized nanogranites and contain a granitic phase assemblage with quartz feldspar and one or two mica depending on the particular case study commonly with accessory phases mainly zircon apatite rutile in many cases the polycrystalline aggregates that make up the nanogranites show igneous microstructures e g granophyric intergrowths micrographic quartz in k feldspar and cuneiform rods of quartz in plagioclase further evidence for the former presence of melt within the investigated inclusions consists of melt pseudomorphs similar to those recognized at larger scale in the host migmatites moreover partially crystallized inclusions are locally abundant and together with very small 8 m glassy inclusions may occur in the same clusters both crystallized and partially crystallized inclusions often display a diffuse nanoporosity which may contain fluids depending on the case study after entrapment inclusions underwent limited microstructural modifications such as shape maturation local necking down processes and decrepitation mainly in the barun gneiss which did not influence their bulk composition re homogenized nanogranites and glassy inclusions show a leucogranitic and peraluminous composition consistent with the results of partial melting experiments on metapelites and metagraywackes anatectic mi should therefore be considered as a new and important opportunity to understand the partial melting processes

cfd investigations of scour beneath a submarine pipeline with the effect of upward seepage abstract a numerical model of scour beneath subsea structures considering the effect of upward seepage in the seabed is proposed a small seepage can cause significant changes to the hydrodynamic force on the bed surface and stability of bed particles which can further affect the sediment transport processes and scour patterns around subsea structures the present model is developed based on a fully coupled hydrodynamic and morphologic sediment transport model the unsteady reynolds averaged navier stokes urans equations are solved together with the k turbulence closure in the presence of seepage the bed friction velocity the bed shear stress and the bed load transport rate will be changed as compared to conditions without seepage the sediment particle stability is also affected and the critical shields parameter is changed in the presence of upward seepage forces the repose angle of the sediment is also reduced the present model is validated against existing experiments in terms of streamwise flow velocity distribution subjected to upward seepage the fully coupled hydrodynamic and morphologic model is validated against existing experiments of scour beneath a pipeline in the live bed regime and clear water regime respectively the validated model is then applied to investigate the scour development beneath a submarine pipeline subjected to different upward hydraulic gradients it is found that the equilibrium scour width is increased with a large upward hydraulic gradient the equilibrium scour depth stays in the range of 0 6 0 8 of the pipeline diameter for the live bed cases for the clear water case with a large upward hydraulic gradient the equilibrium scour depth slightly decreases

disequilibrium melting during crustal anatexis and implications for modeling open magmatic systems contamination of ascending mantle derived magmas by the continental crust was investigated and modeled for a suite of volcanic rocks and entrained crustal xenoliths from the central andes using bulk geochemical compositions for mantle derived and crustal end members as dictated by traditional approaches the assumption that the crustal contaminant in these open magmatic systems is a single composition was assessed through in situ analysis of quenched anatectic melt trapped within its crustal xenolith our results show for the first time significant chemical and sr isotopic disequilibrium between melt and source over submillimeter length scales in a natural system sampled glass is rhyolitic in nature enriched in large ion lithophile elements lile and depleted in heavy rare earth elements hree analysis of the melt for its 87sr 86sr composition revealed isotopic heterogeneity ranging from 0 7164 to 0 7276 the isotopic disequilibrium between melt and source is understood to reflect the melting of minerals with different rb sr and therefore 87sr 86sr more quickly than the isotopic composition can diffusively equilibrate between melt and minerals our results suggest that the mechanism of crustal anatexis produces contaminating melts which are geochemically heterogeneous both spatially and temporally furthermore time scales of sr diffusion and anatectic melt segregation promote the preservation of isotopic disequilibrium at the micro submillimeter and macro crustal scale this highlights the need for detailed microscopic investigations coupled with petrogenetic modeling in order to develop more robust characterization and quantification of contamination in open magmatic systems

migmatites granites and orogeny flow modes of partially molten rocks and magmas associated with melt solid segregation in orogenic belts abstract this paper presents a model for the genesis of migmatites and granites during orogenic evolution based on the analysis of several phanerozoic crustal segments and a review of the physical properties of partially molten rocks and magmas this model inventories the modes of bulk flow of partially molten rocks and magmas and the mechanisms of melt solid segregation for each mode partial melting of rocks is associated with a strength decrease of two to three orders of magnitude leading to strain partitioning expressed by channel flow driven either by forces related to plate tectonics vertical channel flow or by the gravity force associated with lateral variations of crustal thicknesses horizontal channel flow although the structural characteristics of migmatites indicate that deformation plays a role in melt migration the emplacement of laccoliths of leucogranites above migmatites attests to the efficiency of the buoyancy force another decrease in apparent strength of about ten orders of magnitude is associated with the loss of continuity of the solid framework marking the transition from partially molten rocks to magmas the breakdown of the solid framework also allows for settling of the solid in suspension increasing the buoyancy of the remaining magma accordingly domes cored by diatexites former heterogeneous magmas and mantled by metatexites former partially molten rocks are interpreted as gravitational instabilities driven by the relative buoyancy of the magma and permitted by the weakness of the partially molten rocks this model provides a first order framework to elucidate the development of a crustal scale horizontal layering during the evolution of orogenic belts that are affected by partial melting in this case the middle crust is dominated by migmatites with domes cored by diatexites and mantled by metatexites that correspond to a partially molten and magmatic zone respectively the granitic dikes and sills and the associated laccoliths of leucogranites correspond to an intrusive zone overlying the partially molten zone the refractory lower crust is potentially in part formed by accumulation of solids segregated from the overlying heterogeneous magmas

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

regional geological setting of the limpopo belt abstract the 2 7 ga limpopo belt granulite terrane is situated between two lower grade granite greenstone cratons the kaapvaal craton in the south and the zimbabwe craton in the north this granulite facies terrane is subdivided into three zones each with a distinctive geological signature and separated from each other and from the adjacent cratons by well defined shear zones 1 the southern marginal zone composed of granite greenstone material very similar to that of the kaapvaal craton has retrogressed granulites on its southern margin in the hanging wall of the northward dipping terrane boundary hout river shear zone separating the granulite terrane from the adjacent kaapvaal craton 2 the northern marginal zone a granite greenstone terrane at granulite grade which is separated from the zimbabwe craton by a southward dipping terrane boundary and 3 the central zone a unique crustal element with distinctive carbonate platform lithologies and a metagabbroic layered complex which intruded before or at 3250 ma this zone cannot be related to the kaapvaal craton or to the zimbabwe craton it is separated from the two marginal zones by mylonitic shear zones with horizontal stretching lineations the entire limpopo belt is characterized by a distinctive deformational and metamorphic style which contrasts markedly from that of the adjacent granite greenstone terranes of the kaapvaal and zimbabwe cratons this distinctive pattern developed during the limpopo orogeny probably as a consequence of continental collision during the late archaean

source of biotite in the wuluma pluton replacement of ferromagnesian phases and disaggregation of enclaves and schlieren abstract experimental glasses and naturally quenched melts from pelitic and psammitic protoliths both contain markedly less feo mgo than typical s type granites the entrainment of residual peritectic phases and contamination by country rock material are two mechanisms commonly proposed to account for their higher abundance in granites this study examines the origin of the ferromagnesian minerals specifically biotite contained in the wuluma pluton a heterogeneous granite body in central australia that lies within its granulite facies pelitic and psammitic source rocks heterogeneity at the margins of the wuluma pluton arises from abundant enclaves and innumerable dykes of granite separated by screens of country rock towards its center heterogeneity arises from abundant biotite rich enclaves schlieren and aggregates based on field observations the microstructural relationships between minerals and the composition of minerals obtained by electron microprobe and la icp ms analysis the origin of three of the most common types of schlieren were determined enclave related schlieren are the remains of isolated enclaves that interacted with the host granite long schlieren developed from screens of country rocks trapped between sheets of granite and wispy schlieren are derived from porphyroblasts of garnet cordierite and orthoproxene in the granite once incorporated in the granite all the enclaves schlieren porphyroblasts of garnet and cordierite are progressively replaced by biotite most of the newly formed biotite and the magmatic biotite subsequently re equilibrate with the host granite and acquires the same composition very little of the biotite retains the compositional traits of the minerals it has replaced disaggregation of the enclaves and schlieren that have been replaced by biotite adds biotite to the granite and this constitutes the major source of biotite in the wuluma pluton that can reach 15 modal

the behaviour of nd and pb isotopes during 2 0 ga migmatization in paragneisses of the central zone of the limpopo belt south africa and botswana we have carried out major and trace element analyses and nd pb isotope measurements on different components of two migmatite outcrops in the limpopo belt lose quarry botswana and palala south africa in order to assess the effect of migmatization during a high grade metamorphic event on sm nd systematics and nd model ages the outcrops studied are located in the mahalapye tshipise straightening zone at the south western part of the limpopo central zone close to its contact with the kaapvaal craton leucosomes of peraluminous granitic composition were formed by incongruent biotite melting in an fe rich metagreywacke lose quarry and in a metapelite palala respectively monazite u pb and garnet sm nd ages yield close to 2 0 ga for the high grade event confirming other recent work in the central zone the chemical compositions of the main mineral phases feldspar biotite and garnet indicate minimum melt formation at t 750 c and low pressure isocon plots and pb isotope data indicate that the partial melts are locally derived however lil trace element distribution between the leucosomes and paleosomes indicate the incomplete equilibration of the nascent melts with the residual feldspars and biotite in the paleosomes sm nd results on leucosomes and paleosomes of the metagreywacke migmatite show considerable scatter when calculated back to 2 0 ga the paleosomes fall in a narrow range of nd 4 and give tdm model ages between 2 45 and 2 66 ga the nd values of leucosomes are on average four units lower causing them to have apparent tdm ages of 2 7 2 92 ga on average 250 ma older than those of the paleosomes the ree th p2o5 and zr budgets of paleosomes and leucosomes indicate dominance of the lree by monazite and the hree by apatite incorporation of proportionally more monazite than apatite into the melt is inferred as the cause for the nd isotope discrepancy and hence the uncertainty of the average protolith mantle derivation age in contrast pb pb results define an apparent age of 2025 29 ma reflecting nearly full pb isotope exchange in the case of the palala metapelitic migmatite the nd values at 2 0 ga of all but two migmatitic constituents cluster between 7 and 9 5 pointing out extensive nd isotope equilibration during migmatization whereas the pb isotopes yield an errorchron date of 2311 65 ma indicating incomplete pb isotope exchange here garnet dominates the hree distribution whereas the lree are controlled by feldspar and monazite considering that nd isotope equilibration was nearly reached during migmatization nd model ages on the whole rocks represent a good approximation of those of the original protolith 3 0 3 2 ga in the regional context the pb isotope data from the lose quarry locality combined with the nd model age uncertain but within the range of 2 5 2 9 are reminiscent of the northern marginal zone of the limpopo belt and the southern zimbabwe craton whereas the nd and pb isotope data from the palala locality are typical for the limpopo southern marginal zone and the northern kaapvaal craton

large scale melt depletion in granulite terranes an example from the archean ashuanipi subprovince of quebec this study uses field petrographic and geochemical methods to estimate how much granitic melt was formed and extracted from a granulite facies terrane and to determine what the grain and outcrop scale melt flow paths were during the melt segregation process the ashuanipi subprovince located in the north eastern superior province of quebec is a large 90 000 km2 metasedimentary terrane in which 85 of the metasediments are of metagreywacke composition that was metamorphosed at mid crustal conditions 820 900 c and 6 7 kbar in a late archean dextral transpressive orogen decrease in modal biotite and quartz as orthopyroxene and plagioclase contents increase together with preserved former melt textures indicate that anatexis was by the biotite dehydration reaction biotite quartz plagioclase melt orthopyroxene oxides using melt orthopyroxene ratios for this reaction derived from experimental studies the modal orthopyroxene contents indicate that the metagreywacke rocks underwent an average of 31 vol partial melting the metagreywackes are enriched in mgo cao and feot and depleted in sio2 k2o rb cs and u have lower rb sr higher rb cs and th u ratios and positive eu anomalies compared to their likely protolith these compositions are modelled by the extraction of between 20 and 40 wt granitic melt from typical archean low grade metagreywackes a simple mass balance indicates that about 640 000 km3 of granitic melt was extracted from the depleted granulites the distribution of relict melt at thin section and outcrop scales indicates that in layers without leucosomes melt extraction occurred by a pervasive grain boundary porous flow from the site of melting across the layers and into bedding planes between adjacent layers in other rocks pervasive grain boundary flow of melt occurred along the layers for a few to tens of centimetres followed by channelled flow of melt in a network of short interconnected and structurally controlled conduits visible as the net like array of leucosomes in some outcrops the leucosomes contain very little residual material 5 biotite orthopyroxene indicating that the melt fraction was well separated from the residuum left in situ as melt depleted granulite only 1 3 vol percentage melt remained in the melt depleted granulites hence the extraction of melt generated by biotite dehydration melting in these granulites was virtually complete under conditions of natural melting and strain rates in a contractional orogen

melt segregation in the continental crust distribution and movement of melt in anatectic rocks the grain and outcrop scale distribution of melt has been mapped in anatectic rocks from regional and contact metamorphic environments and used to infer melt movement paths at the grain scale anatectic melt is pervasively distributed in the grain boundaries and in small pools consequently most melt is located parallel to the principal fabric in the rock typically a foliation short branched arrays of linked melt bearing grain boundaries connect melt depleted parts of the matrix to diffuse zones of melt accumulation protoleucosomes where magmatic flow and alignment of euhedral crystals grown from the melt developed the distribution of melt leucosome and residual rocks normally melanocratic in outcrop provides different but complementary information the residual rocks show where the melt came from and the leucosomes preserve some of the channels through which the melt moved or sites where it pooled different stages of the melt segregation process are recorded in the leucosome melanosome arrays regions where melting and segregation had just begun when crystallization occurred are characterized by short arrays of thin branching leucosomes with little melanosome a more advanced stage of melting and segregation is marked by the development of residual rocks around extensive branched leucosome arrays generally oriented along the foliation or melting layer places where melting had stopped or slowed down before crystallization began are marked by a high ratio of melanosome to leucosome because most of the melt has drained away very few leucosomes remain to mark the melt escape path this is common in melt depleted granulite terranes many migmatites contain abundant leucosomes oriented parallel to the foliation mostly these represent places where foliation planes dilated and melt drained from the matrix via the branched grain boundary and larger branched melt channel leucosome arrays collected melt collected in the foliation planes was partially or fully expelled later when discordant leucosomes formed leucosomes or veins oriented at high angles to the foliation layering formed last and commonly lack melanocratic borders hence they were not involved in draining the matrix of the melting layer discordant leucosomes represent the channels through which melt flowed out of the melting layer

a linear hf isotope age array despite different granitoid sources and complex archean geodynamics example from the pietersburg block south africa abstract combined u pb and lu hf isotope data from zircon populations are widely used to constrain hadean archean crustal evolution linear hf isotope age arrays are interpreted to reflect the protracted internal reworking of crust derived from the depleted mantle during a short lived magmatic event and related 176lu 177hf ratios are used to constrain the composition of the reworked crustal reservoir results of this study however indicate that hf isotope age arrays can also result from complex geodynamic processes and crust mantle interactions as shown by u pb and lu hf isotope analyses of zircons from well characterized granitoids of the pietersburg block pb northern kaapvaal craton south africa apart from scarce remnants of paleoarchean crust most granitoids of the pb with ages between 2 94 and 2 05 ga n 32 define a straight hf isotope age array with low 176lu 177hf of 0 0022 although they show a wide compositional range were derived from various sources and emplaced successively in different geodynamic settings the crustal evolution occurred in five stages i predominately mafic crust formation in an intra oceanic environment 3 4 3 0 ga ii voluminous ttg crust formation in an early accretionary orogen 3 0 2 92 ga iii internal ttg crust reworking and subduction of ttg derived sediments in an andean type setting 2 89 2 75 ga iv post collisional high k magmatism from both mantle and crustal sources 2 71 2 67 ga and v alkaline magmatism in an intra cratonic environment 2 05 2 03 ga the inferred array results from voluminous ttg crust formation during stage ii and involvement of this crust during all subsequent stages by two different processes i internal crust reworking through both partial melting and assimilation at 2 89 2 75 ga leading to the formation of biotite granites coeval with minor ttgs and ii subduction of ttg derived sediments underneath the pb causing enrichment of the mantle that subsequently became source for high k granitoids and mafic rocks at 2 68 and 2 05 ga some scatter along the array might have resulted either from significant assimilation of ancient crust intracrustal lu hf fractionation or melting of heterogeneous mantle sources those results show that without any information about the nature and composition of zircon hosting granitoids hf isotope age data are of limited use to constrain hadean archean magmatogenesis and geodynamics

suction effects on sediment transport in closed conduit flows abstractlaboratory experiments were conducted to investigate the influence of suction on sediment transport in closed conduit flows in this study the results show that the bed load transport rate essentially remains unchanged for small values of suction rates before abruptly increasing beyond a certain threshold theoretical analyses of the forces acting on a spherical particle also are conducted and a conceptual model set up to analyze suction effects on particle mobility by considering the near bed velocities the model hypothesizes that 1 the bed particle experiences an additional downward vertical drag force induced by suction and 2 increasing suction will lead to larger horizontal and vertical near bed velocities which enhances both the driving force and effective weight of the particle to qualitatively examine how suction affects the near bed flow behavior physical modeling capability software was used to simulate the physical system and the results confirm that the near bed velocities incr

a geochemical and experimental study of the role of k feldspar during water undersaturated melting of metapelites abstract the production of s type granites has been related to the extraction of melts during water undersaturated melting of metasediments in the lower continental crust if this mechanism is of widespread applicability a chemically distinctive granulitic restite must be left behind with a positive eu anomaly and depleted incompatible element concentrations however little geochemical evidence has been found in support of this hypothesis the rare earth element geochemistry of lower crustal granulites and associated melts will depend on the behaviour of the phases in which ree and trace elements are concentrated i e zircon monazite and k rich feldspar new experimental data on synthetic metapelites show that k feldspar can be a product or a reactant during water undersaturated melting of biotite depending on the h2o k2o of the melt relative to biotite past confusion over the role of k feldspar during melting is due to the reaction being close to degeneracy and changing its stoichiometry in response to only small changes in bulk composition coherent interpretation of the role of k feldspar in different experimental studies requires a better knowledge of phase water contents than has commonly been available the leucosomes formed in some antarctic granulite facies migmatites are likely to have had low h2o k2o ratios which suggests that k feldspar would have been a reactant during melting this could cause the positive eu anomalies observed in the leucosomes through the disequilibrium melting of k feldspar during water undersaturated melting the undersaturation of trace elements and ree in the leucosomes strongly suggests that disequilibrium melting took place caused by the incomplete dissolution of accessory phases monazite and zircon these phenomena give the melts a distinctive geochemical character which has also been seen in the other granulite facies leucosomes analysed to date this geochemical character contrasts with that of s type granites and therefore the production of granulites in the lower crust may not necessarily be associated with this type of magmatism if lower crustal granulite facies migmatites are the source region for peraluminous s type granites then the melts generated must undergo considerable modification during ascent

fluids p t paths and the fates of anatectic melts in the earth s crust this paper presents a model for the longevity and physical behaviour of anatectic melts in the earth s crust it is assumed that a thermal anomaly has caused partial fusion of biotite plagioclase quartz rocks to produce a granitic melt and restitic solids the focus is on the potential chemical and physical outcomes as a function of the various p t fluid conditions rocks undergoing fluid present melting will sustain an increase in bulk density and will become more gravitationally stable diapiric up welling of such material should not occur in fluid absent melting the bulk density will decrease and foster gravitational instability stresses in the surrounding rocks could however lead to fracture and rapid evacuation of the melt with excess fluid and without melt segregation isobaric cooling will cause immediate freezing if t rises above the biotite quartz melting reaction freezing will still occur at the original reaction t diapiric up welling should not occur biotite bearing diatexites might be formed by large degrees of fusion but magma ascent is unlikely in isothermal decompression immediate freezing will ensue and the migmatite should contain orthopyroxene rather than biotite with excess fluid and melt segregation cooling will induce freezing at the fluid saturated granite solidus magma ascent could however occur even for xflh2o 1 in isothermal decompression melt will persist to the granite solidus for the imposed xflh2o limited magma ascent could occur all granites should contain primary igneous biotite for fluid deficient rocks without melt segregation freezing will occur at the t of the original melting if the reaction is overstepped solidification will occur at the original xflh2o magma formation is unlikely migmatites should contain either pyroxene retrogressed to biotite or biotite alone on decompression immediate freezing is likely if t remains at the biotite quartz reaction for higher t melt will survive decompression and freeze at the fluid saturated solidus for some increased xflh2o magma ascent is possible and the melt will freeze very close to the wet granite solidus in fluid deficient rocks with melt segregation cooling and crystallization will cause fluid saturation in situ segregated melt will freeze at the original xflh2o melt ascent will however cause exsolution of a co2 rich fluid with fluid escape melt will freeze close to the wet granite solidus an increase in melt proportion may accompany magma ascent or source up welling any low p granite intrusion should be charnockitic for fluid absent rocks without melt segregation freezing will occur on cooling to the t of the original reaction only transitory migmatites could be formed but melt will survive decompression almost to the h2o saturated granite solidus the rocks will reveal former melt presence only in the existence of veins of coarse retrograde biotite with isothermal decompression further melting may occur forming an orthopyroxene bearing granulite facies migmatite in fluid absent situations with melt segregation magma could be efficiently transported to higher crustal levels if the melt proportion were insufficient for magma formation granulite facies migmatites would result ascending melt would progressively dissolve any entrained restite the granites produced should be charnockitic to aid in interpretation of the modelling results we suggest that it may be slightly more common for the highest grade metamorphic terranes to exhibit isobaric cooling paths that fluid present anatexis is normally limited to the shallower parts of the crust and that fluid absent reactions are more likely at greater depths we conclude that the best models for natural fluid present anatexis are those dealing with essentially pure h2o fluid and that fluid deficient conditions will dominate over fluid excess we also believe that it is common for melts to be physically segregated on scales larger than the diffusion paths of many elements

water saturated and undersaturated melting of metaluminous and peraluminous crustal compositions at 10 kb evidence for the origin of silicic magmas in the taupo volcanic zone new zealand and other occurrences

accessory phase controls on the geochemistry of crustal melts and restites produced during water undersaturated partial melting the profound geochemical conseqences of accessory phase behaviour during partial melting of highgrade metapelites are demonstrated with reference to two geochemically distinct crustal melts produced by biotite dehydration melting reactions under granulite facies kbar 860 cc conditions these two leucogneiss suites from the brattstrand bluffs coastline eastern antarctica have similar field relations transport distances 10 100 s of metres and major element chemistry type 1 leucogneisses have low zr th and lree positive eu anomalies and zr zr and lreet lreet values less than 1 0 i e less than required to saturate the melt mass balance constraints suggest that these melts have been extracted before equilibration with host melanosomes the dry peraluminous nature of vapour undersaturated melts inhibits monazite and zircon solubility and results in concentration of these phases in the residue melts are consequently depleted in lree and hree melanosomes show complementary enrichment in lree while hree patterns are dominated by residual garnet type 2 leucogneisses in contrast have strongly enriched zr th and lree abundances negative eu anomalies and zr zr and lreet lreet 1 resulting from accessory phase entrainment vapour absent partial melting under moderate 6 8 kbar pressure granulite facies conditions of a pelitic source containing monazite is likely to give disequilibrium melts depleted in lree and hree as monazite and garnet are concentrated in the residue if temperatures are high enough 850 870 c to permit relatively large degrees of partial melting then the feldspar component of the source will be removed almost completely giving melts with large positive eu anomalies melts formed under vapour present conditions are unlikely to show such extreme lree and hree depletion or positive eu anomalies even at high degrees of partial melting disequilibrium melting coupled with source entrainment could fortuitously produce ree and trace element signatures similar to those typical of s type granites and usually ascribed to equilibrium melting conditions

constraints on melting and magma production in the crust major intrusions of granitic rocks are found in several tectonic settings and in all cases crustal melts may contribute to the volumes of granitic magma high grade metamorphism and partial melting of the crust take place predominantly under fluid absent conditions we present a model for calculating the amounts of melt that may be formed by fluid absent breakdown of micas and amphiboles in common crustal rock types pelitic quartzofeldspathic intermediate and mafic melt proportions depend mainly on the kind of source rock the pressure at which melting takes place the temperature and the hydrous mineral h2o content of the source as a consequence of the pressure dependence of water solubility in silicate melts any given source rock will produce more melt by a given fluid absent reaction at lower pressure at a given pressure higher temperature reactions can produce more melt from a given source rock based on a survey of the compositions of common rock types we show that the amounts of melt can vary from 50vol thus crustal rocks vary widely in their fertility as magma sources depending on the types and amounts of hydrous phases they contain in general muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation biotite breakdown in pelites occurs at higher temperature and because most high grade pelites below granulite grade are biotite rich can yield up to about 50 vol of granitoid melt rocks of intermediate composition and hornblende rich mafic rocks are potentially highly fertile magma sources also provided that the high temperatures necessary for biotite and hornblende breakdown are realized pyroxene rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with 30and probably 20vol melt the pressure temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria

regional scale grenvillian age uht metamorphism in the mollendo camana block basement of the peruvian andes the mollendo camana block mcb is a 50 150 km precambrian inlier of the andean belt that outcrops along the pacific coast of southern peru it consists of stromatic migmatites of paleoproter ozoic heritage intensely metamorphosed during the grenville event c 1 ga u pb and u th pb ages on zircon and monazite in the migmatites aluminous mesosomes fmas and quartzofeldspathic leucosomes kfmash contain various amounts of k feldspar kfs orthopyroxene xmg opx 0 86 plagioclase pl sillimanite sil exceptionally kyanite ky ilmenite ilm magnetite mag quartz qtz and minor amounts of garnet xmg grt 0 60 sapphirine xmg spr 0 87 cordierite xmg crd 0 92 and biotite xmg bt 0 83 the ubiquitous peak mineral assemblage is opx sil kfs qtz grt in most of the mcb which together with the high al content of orthopyroxene 10 al2o3 and the local coexistence of sapphirine quartz attest to regional uht metamorphism 900 c at pressures in excess of 1 0 gpa fluid absent melting of biotite is responsible for the massive production of orthopyroxene that proceeded until exhaustion of biotite and most of the garnet in the southern part of the mcb mollendo cocachacra areas in this area a first stage of decompression from 1 1 1 2 to 0 8 0 9 gpa at temperatures in excess of 950 c is marked by the breakdown of sil opx to spr opx crd assemblages according to several bivariant fmas reactions high t decompression is also shown by mg rich garnet being replaced by crd spr and crd opx bearing symplectites and reacting with quartz to produce low al opx sil symplectites in quartz rich migmatites neither osumilite nor spinel quartz assemblages being formed isobaric cooling at about 0 9 gpa probably followed the initial decompression and proceeded with massive precipitation of melts towards the os invariant point as demonstrated by bt qtz pl symplectites in quartz rich migmatites melt opx sil bt grt kfs qtz finally opx rims around secondary biotite attest to late fluid absent melting compatible with a second stage of decompression below 900 c the two stages of decompression are interpreted as due to rapid tectonic denudation whereas the regional extent of uht metamorphism in the area probably results from large scale penetration of hot asthenospheric mantle at the base of an over thickened crust

fluid absent melting and the roles of fluids in the lithosphere a slanted summary abstract the presence or absence nature and compositions of lithospheric fluids are crucial factors in the evolution of igneous and metamorphic rocks despite this confusion still surrounds perceptions of the degrees of fluid involvement in many geological settings however experimental and field investigations constrain these roles and we are able to make some valid generalizations studies of prograde metamorphism have highlighted the spatial heterogeneity of fluid concentrations in many crustal environments this commonly indicates internal lithological control of volatile activities under fluid present conditions crustal fluids do not appear to mix readily fluid flow is generally structurally or lithologically controlled the transition from amphibolite to granulite facies is characterized by a decrease in ah2o as well as a temperature increase in many terranes these phenomena correlate with the appearance of migmatites some granulites are not migmatitic but have chemical and mineralogical characteristics of melt depleted restites this indicates efficient extraction of melts incongruent melting reactions involving the breakdown of crystalline hydrates in crustal rocks produce granitoid melts plus denser restites that are nearly or actually anhydrous experimental and theoretical treatments of fluid absent melting show that micas and amphiboles decompose at 700 to 900 c for mid to lower crustal pressures the melts resemble the compositions of s and i type granitoids pervasive aqueous fluids could not generally have been present during this melting since exposed granulite terranes are not characterized by high melt proportions and the magmas themselves are water undersaturated likewise the general presence of very h2o poor pervasive fluids is unlikely since this would allow only subsolidus dehydration at normally attainable crustal temperatures in this case the amphibolite granulite transition is possible at relatively low temperatures and the resultant rocks would not contain evidence of in situ melting the granulite granite connexion and effective crustal differentiation seem viable only during high temperature fluid absent metamorphism data on continental geotherms show that the lower crust is not entirely granulitic granulites are produced in regions of anomalously high heat flow the mantle connexion thus inferences on the nature of the lower crust should not be entirely based on studies of exposed granulite terranes mineral assemblages in eclogites also indicate widespread internal buffering of volatile activities and probable fluid absent conditions mantle derived basaltic magmas too are volatile bearing and it is plausible that volatile fixing phases also occur in the mantle volatiles may largely be introduced into the mantle through subduction processes and only released during the melting attending the breakdown of hydrous minerals as in the crust

h2o in rhyolitic glasses and melts measurement speciation solubility and diffusion dissolved h2o in silicate melts and glasses plays a crucial role in volcanic eruptions on terrestrial planets and affects glass properties and magma evolution in this paper major progress on several aspects of the h2o melt or glass system is reviewed consistency among a variety of data is investigated discrepancies are evaluated and confusion is clarified on the infrared measurement of total h2o and species concentrations calibration for a variety of glasses has been carried out at room temperature the measurements for h2o in rhyolitic glasses have undergone the most scrutiny resulting in the realization that absorptivities for the near infrared bands depend on total h2o content although the variation of the absorptivities does not seem to significantly affect the determination of total h2o it does affect the determination of molecular h2o and oh species concentrations calibration of the infrared technique for h2o in rhyolitic glasses still needs much improvement especially at high total h2o furthermore it is now almost certain that the molar absorptivities also depend on the measurement temperature in in situ studies hence it will be necessary to carry out calibrations in situ at high temperatures on h2o speciation results from two experimental approaches the quench technique and the in situ technique are very different leading to controversy in our understanding of true speciation a solution is presented to reconcile this controversy it is almost certain that the quench technique does not suffer from a quench problem but interpretation of in situ results suffered from ignoring the dependence of the molar absorptivities on measurement temperature accurate calibration at high temperatures is necessary for the quantitative application of the in situ technique to h2o speciation in silicate melts and glasses on h2o solubility in silicate melts recent experimental work has significantly expanded the t p range of solubility measurements and recent solubility models fill a gap for predicting solubility for a wide range of melt compositions i present a solubility model for rhyolitic and quasi rhyolitic melts over a wide range of t and p 500 1350 c 0 8 kbar by incorporating the role of speciation the solubility model is able to recover the experimental solubility data and has extrapolative value although the partial molar volume of h2o derived from the solubility model differs from that derived from density measurements on h2o diffusion recent studies on h2o diffusion in a quasi rhyolitic melt at 800 1200 c 0 5 5 kbar and up to 7 total h2o not only provide important new diffusion data but are also challenging earlier understanding of h2o diffusion based on data in rhyolitic glasses at 400 550 c 1 bar and 0 2 1 8 total h2o a comparison between the earlier model and recent data is made the recent high temperature diffusivities at total h2o 2 can be predicted by the earlier model however at higher total h2o the earlier model fails new work is under way to understand the diffusion mechanisms at high h2o contents

partial melting and the origin of metapelitic granulites in the southern marginal zone of the limpopo belt south africa abstract two potentially realistic models exist to explain the origin of the granulite facies rocks of the southern marginal zone i co2 fluxing inducing the necessary high temperature dehydration reactions and ii melting of hydrous minerals to yield an h2o undersaturated granitic melt in equilibrium with a dehydrated granulitic assemblage petrographic and field evidence from the bandelierkop quarry in the southern marginal zone indicate a sequence of melting reactions involving muscovite and biotite relict textural evidence indicates that the reaction responsible for the generation of most of the melt bi sil qz plag melt ga ksp was fluid absent the melting reactions can be shown to have occurred during the prograde metamorphic path with the highest temperature reaction bi qz plag hy cord melt occurring at approximately peak metamorphic conditions 850 c and less than 9 5 kbar this suggests a fluid absent mode of granulite formation a subsequent retrograde reaction cord ksp h2o ky qz related to a rise in h2o during in situ crystallisation of the melt did not destroy the granulite assemblages and hypersthene remains stable in areas of the southern marginal zone that record pressures of greater than 6 5 kbar during retrogression

the link between migmatites and s type granites in the turku area southern finland abstract metamorphism and compositions of metasedimentary migmatites and related small scale granites are investigated in the turku area sw finland phase assemblages and mineral compositions suggest peak p t conditions of 800 c and 6 kbar the portion of melt produced in the migmatites by dehydration melting is estimated to be 20 wt the partial melt is k 2 o rich and water undersaturated 4 wt h 2 o the leucosomes and small scale granites have almost the same proportions of felsic minerals but the mafic minerals grt crd bt are enriched in the granites along with the contents of tio 2 feo tot mgo and hfse the leucosomes have low total ree values and strongly positive eu anomalies the ree patterns of the granites are characterized by relatively high lree low hree and distinct negative eu anomalies the distribution of major elements and trace element signatures suggests that leucosomes and granites are related as cumulates and evolved residual melts respectively the decimeter to meter scale leucosomes are interpreted to be the product of crystal fractionation which occurred during near isobaric cooling and the granites are the crystallized residual melts which could move further on and form larger bodies within and outside of the migmatite complex

seepage effects on dune dimensions this note presents an experimental study of seepage effects on the dimensions of a dune its height length and lee side slope the critical slope of a cohesionless material was found to be related to the ratio of the hydraulic gradient of seepage to its critical value under the quick condition the theoretical relationship is successfully used to predict seepage effects on the lee side slope of dunes measured laboratory data support the assumption that the effect of seepage on the lee side slope of dunes is the same as that on the critical slope of the sediment the data reveal that the slope is reduced and increased with injection and suction respectively additionally the study explores seepage effects on the development of dune height and length experimental results show that suction and injection increase and decrease the equilibrium height of dunes by 25 and 40 respectively on the other hand the dune length is found to increase by approximately 30 and decrease by 20 under the influence of suction and injection respectively

spatial distribution of melt bearing structures in anatectic rocks from southern brittany france implications for melt transfer at grain to orogen scale abstract in this study we present quantitative spatial information on the one and two dimensional distribution of inferred melt bearing structures in anatectic supracrustal rocks of the southern brittany migmatite belt south of the transcurrent south armorican shear zone sasz based on these data we infer the mechanism of melt extraction from partially molten crust former melt bearing structures include foliation parallel leucosomes and cross cutting granitic leucosomes that infill inter boudin partitions and extensional shear surfaces as well as discordant dykes of granite petrographic i e mineralogical and microstructural continuity of granite from structure to structure suggests that they once formed a continuous melt bearing network measurements along one dimensional line traverses perpendicular to layering of stromatic migmatite exposed in clean sub horizontal outcrop surfaces provide information about thickness and spacing distributions of foliation parallel leucosomes most leucosome thicknesses fall in the range of 1 10 mm with upper limits around 20 30 mm the number of thicker layers decreases abruptly with increasing thickness which is inconsistent with scale invariance this suggests that leucosome formation was controlled by short range melt movement along grain boundaries to form melt rich layers constrained by pre existing compositional layering spacing distributions also are not scale invariant however the large percentage of leucosomes 40 60 in these line traverses suggests that spacing distributions may be controlled in part by impingement of leucosomes making it difficult to derive genetic information from these data qualitative observation of inferred melt bearing structures in mutually perpendicular two dimensional exposures from the same outcrop reveals anisotropy of the leucosome network related to a well developed sub horizontal quartz feldspar lineation reflecting stretching associated with transcurrent movement along the sasz analysis of these two dimensional distributions using the box counting method corroborates the observed anisotropy but indicates that leucosome morphology and perhaps distribution is not scale invariant the applicability of the box counting method or of fractal analysis to understanding melt movement in migmatites is discussed in light of these results based on the anisotropy of melt bearing structures we infer that melt movement in structures now represented by layer parallel leucosomes was primarily sub horizontal these layers fed steeply dipping structures now represented by cross cutting leucosomes in particular those developed at inter boudin partitions and granite dykes the formation and orientation of these steeply dipping structures was in part controlled by far field stresses related to dextral displacement along the sasz melt extraction is inferred to have occurred along these steeply dipping structures extracted melt accumulated in plutons at higher crustal levels such as the quiberon sarzeau and guerande granites

melt production during granulite facies anatexis experimental data from primitive metasedimentary protoliths we conducted fluid absent partial melting experiments at 0 5 and 1 0 gpa in the temperature range 750 to 1000 c to investigate the influence of bulk rock mg 100mg mg fe and the effects of additional tio2 on the granulite grade anatectic evolution of relatively magnesian metapelites and metagreywackes in these experiments melting began between 780 and 830 c by the incongruent breakdown of biotite to produce quartz saturated granulite facies residual mineral assemblages in equilibrium with h2o undersaturated granitic melt the glass quenched melt compositions produced in this study vary little generally the glasses have compositions similar to those of many natural strongly peraluminous leucogranites the solidus temperatures in both rock types increase with increasing mg but are unaffected by the presence or absence of a tio2 component at 0 5 gpa the metapelites melted at temperatures up to 50 c lower than the equivalent metagreywackes but at 1 gpa there was no discernible difference this study suggests that the fluid absent solidus has a steep positive dp dt slope in metapelites and steep negative dp dt slope in metagreywackes the pattern of melt production with increasing temperature is strongly controlled by the upper limit of biotite stability in tio2 free compositions this was found to increase by 15 to 20 c in the metapelites and by 30 to 40 c in the metagreywackes as a function of increasing mg from 49 to 81 the presence of a tio2 component increases the upper limit of biotite stability by 50 c in the metapelites and by 80 c in the metagreywackes over that observed in the equivalent tio2 free compositions in consequence in the tio2 free samples large pulses of melt up to 35 wt are produced over narrow temperature ranges as little as 15 c in these experiments between 830 and 875 c in the tio2 bearing samples the major pulse of melt production occurs more gradually between 830 and 900 c

microstructures and composition of melt inclusions in a crustal anatectic environment represented by metapelitic enclaves within el hoyazo dacites se spain abstract this article reports microstructures and compositions of melt inclusions in anatectic metapelites found as enclaves within el hoyazo dacites in the neogene volcanic province of southeastern spain the enclaves represent fragments of continental crust partially melted at 800 850 c and 5 7 kbar and brought to surface rapidly within the host volcanics while they still were in a molten state rapid cooling produced the solidification of silicate melt to glass in the rock matrix and in inclusions within minerals melt inclusions mi are present within nearly all mineral phases in the enclaves and their associated microstructures indicate a primary or pseudo secondary origin the entrapment mechanisms of mi within plagioclase were associated with 1 the presence of micron sized solids mostly graphite on the surface of growing crystals 2 partial resorption of crystals generation of embayments and later growth in the presence of melt and 3 entrapment of melt during crystal growth within crystallographic controlled planar structures e g on crystal faces melt inclusions in garnet are commonly associated with regularly oriented planar discontinuities filled with glass interpreted either as spaces left between adjacent mineral growth spirals or as crystallographic controlled cracks generated in between stages of mineral growth melt was trapped or percolated along these microstructures and later on necking down phenomena individualized inclusions with negative crystal shape melt inclusions in biotite are abundant show negative crystal shape and parallel cleavage planes melt inclusions in cordierite alkali feldspar and ilmenite are isolated sparse and sometimes rather large the composition of glass from all mi is leucogranitic and peraluminous with small differences among glasses in each mineral host there is some variation in glass composition within each textural location as well compositional heterogeneity can be partially explained by a combination of processes such as generation of boundary layers during mineral growth crystallization of daughter phases and hydrogen loss via diffusion through the host mean glass compositions in the several textural locations however can be tentatively interpreted as reflecting the evolution of melt chemistry during the prograde low to medium pressure anatexis of quartz poor metapelites

maximum and minimum water contents of granitic melts generated in the crust a reevaluation and implications abstract recent results of studies aimed to determine the water solubilities and the liquidus temperatures in synthetic and natural granitic systems are used to constrain the minimum water contents of granitic melts generated in the crust as a function of pressure temperature melt composition and water activity it is concluded that melts generated in the crust at high temperature and low water activities such as a type granites contain higher water contents than previously assumed up to 20 at 800 900 c and 3 7 kbar as a consequence the melt fraction produced at a given amount of available water can be up to 20 lower the minimum water content of melts generated by partial melting of crustal protoliths especially quartzofeldspathic rocks can be predicted adequately from the liquidus curves in granitic systems this water content can be higher if the melt composition departs from a minimum or eutectic quartzofeldspathic composition melts generated from quartzofeldspathic rocks at low temperature 750 c and below are necessarily water rich 5 wt h 2 o and have approximately the same viscosity or even slightly lower viscosities than melts generated at higher temperatures viscosities in the range 10 4 3 10 5 4 pa s recent experimental data also show that the value of the positive d p d t slope of the liquidus curves for fixed water contents above 800 c is lower than predicted by previous models thus adiabatic decompression produces a higher amount of melt than previously assumed at high temperature the formation of melt by adiabatic decompression can be up to 50 higher than assumed

partial melting and the formation of granulite facies assemblages in namaqualand south africa abstract dehydration melting reactions in which water from a hydrous phase enters the melt leaving an anhydrous solid assemblage are the dominant mechanism of partial melting of high grade rocks in the absence of externally derived vapour equilibria involving melt and solid phases are effective buffers of ah2 o the element partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which for most cases ah2o is driven to lower values the mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at t 850 c is relatively small 10 20 and probably insufficient to mobilize a partially melted rock body granulite facies metapelite biotite gneiss and metabasic gneiss in namaqualand contain coarse grained discordant unfoliated anhydrous segregations surrounded by a finer grained foliated matrix that commonly includes hydrous minerals the segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration melting reactions bt sil qtz pi grt crd kfs l metapelite bt qtz pl opx kfs l biotite gneiss and hbl qtz opx cpx pl l metabasic gneiss the size shape distribution and modes of segregations suggest only limited migration and extraction of melt growth of anhydrous poikiloblasts in matrix regions development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity also they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles the preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that h2o in the melt was released as vapour on crystallization this model combined with the natural observations suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt

leucosome forming small scale geochemical processes in the metapelitic migmatites of the turku area finland abstract selected outcrops of the metapelitic migmatites of the turku area southwestern finland have been investigated for their petrographic and chemical properties three different leucosome types could be distinguished leucosome sills in situ leucosomes and composite leucosomes almost all leucosomes are rich in or dominated alkali feldspar only a few composite leucosomes show plagioclase rich layers in addition all leucosomes are of very similar composition except the rare plagioclase rich ones and assumed to be generated by dehydration melting the melt proportion formed by this reaction at peak metamorphic conditions 800 c 5 6 kbar is estimated from experimental data to be approximately 20 wt the different leucosome types are considered to be the result of various degrees of melt segregation and transportation the in situ leucosomes were formed almost in place the composite leucosomes are the outcome of segregation and transportation of partial melts over relatively short distances

fluid absent melting behavior of a two mica metapelite experimental constraints on the origin of black hills granite we have studied experimentally the vapor absent melting behavior introduction of a proterozoic metapelite hp 60 1 from the black hills south peraluminous leucogranites such as the miocene granites dakota to assess whether the high do tourmaline bearing granites of the himalaya and tourmaline granites of the harney of the harney peak suite resulted from muscovite dominated depeak suite in south dakota are widely considered to be hydration melting at 10 kbar samples were sealed with no added the product of muscovite dehydration melting yet few h2o in gold capsules then run in a 0 5 inch 1 27 cm pistonexperimental studies of muscovite dehydration melting cylinder apparatus over the temperature interval 812 975 c reactions have been performed because the low temexperiments were conducted at 10 kbar to allow comparison with peratures of the reactions hinder the approach to equiother metapelite melting studies mass balance calculations imply librium in addition melt fractions generated are typically the melting reaction muscovite biotite plagioclase quartz low resulting in small melt pockets which are very difficult melt alkali feldspar aluminosilicate garnet until to analyze with the electron microprobe in this study muscovite is exhausted 812 c then the reaction biotite we assess the possible genetic relationship between the plagioclase quartz alkali feldspar aluminosilicate tourmaline bearing granites of the harney peak suite melt garnet the inferred muscovite reaction is completed below and the aluminous metasediments these granites intrude 812 c and produces 1 2 wt melt melt production of the although the attempt to investigate the muscovite decontinuous biotite reaction increases steadily with rising temperature hydration melting reaction was hindered by the surto 32 wt in our highest temperature run the low melt prisingly low melt productivity we were able to infer a productivity of the muscovite reaction at 10 kbar suggests that balanced reaction illuminating the dominantly peritectic melting at lower pressures may be necessary to generate mobile melt nature of fluid absent muscovite breakdown under the fractions by muscovite dominated reactions in our starting material conditions investigated in addition melts of hp 60 1 are considerably more mafic than isotopic and trace element studies indicate a genetic the tourmaline bearing granites with average combined tio2 relationship between s type granites and aluminous metamgo feo contents of 1 85 wt and 0 84 wt respectively sediments in the black hills of south dakota where the in particular melts of hp 60 1 are enriched in ti and would harney peak granite hpg intruded proterozoic and probably stabilize biotite rather than tourmaline upon cooling archean schists and metagreywackes undergoing regional arguing against hp 60 1 as a potential source rock for the metamorphism the timing and mode of emplacement of tourmaline granites the hpg and the deformation and regional metamorphic

scales of disequilibrium and rates of equilibration during metamorphism mounting evidence suggests that partial disequilibrium meaning disequilibrium for some elements but not for others may be a common but rarely detected phenomenon during metamorphic mineral growth even in ordinary prograde reactions that progress to completion detailed examination of compositional variations in garnet crystals over a range of metamorphic grade suggests distinctly different scales of equilibration for common elements with strong temperature dependence under lower greenschist facies conditions fe and mg may equilibrate at hand sample scale whereas mn and ca may not equilibrate even at millimeter scale although mn may achieve hand sample scale equilibration under upper greenschist facies conditions ca and many trivalent cations e g rees may not do so until temperatures exceed those of the middle amphibolite facies even in the lower granulite facies some elements e g y yb show indications of disequilibrium at sub centimeter scales during garnet growth analysis and numerical modeling of undisputed disequilibrium textures demonstrate that the most common impediment to equilibration during metamorphism is the sluggishness of intergranular diffusion the same mechanism known to govern porphyroblast crystallization in many metamorphic environments despite this importance to petrology few quantitative determinations exist of inter granular diffusion rates under metamorphic conditions using numerical simulations of coupled intergranular and intracrystalline diffusion processes in coronal textures around partially resorbed garnet crystals from the llano uplift u s a a very precise and relatively accurate estimate is obtained for the rate of intergranular diffusion of al in fluid undersaturated systems this result and an earlier estimate for fluid saturated systems provide bracketing values for al diffusivity during metamorphism

from migmatites to plutons power law relationships in the evolution of magmatic bodies magma is generated by partial melting from micrometre scale droplets at the source and may accumulate to form 100 km scale plutons magma accumulation thus spans well over ten orders of magnitude in scale here we provide measurements of migmatitic leucosomes and granitic veins in drill cores from the estonian proterozoic basement and outcrops at masku in sw finland and montemor o novo central portugal despite the differences in size and number of measured leucosomes and magmatic veins differences in host rock types and metamorphic grades the cumulative width distribution of the studied magmatic leucosomes veins follows a power law with exponents usually between 0 7 and 1 8 published maps of the se australian lachlan fold belt were used to investigate the distribution of granitoid pluton sizes the granites occupy ca 22 of the 2 6 105 km2 area the cumulative pluton area distributions show good power law distributions with exponents between 0 6 and 0 8 depending on pluton area group using the self affine nature of pluton shapes it is possible to estimate the total volume of magma that was expelled from the source in the 2 6 105 km2 map area giving an estimated 0 8 km3 of magma per km2 it has been suggested in the literature that magma batches in the source merge to form ever bigger batches in a self organized way this leads to a power law for the cumulative distribution of magma volumes with an exponent m v between 1 for inefficient melt extraction and 2 3 for maximum accumulation efficiency as most of the volume resides in the largest batches that can escape from the source if m v 1 the mass of the magma is dominated by small batches in case m 2 3 about 50 of all magma in the system is placed in a single largest batch our observations support the model that the crust develops a self organized critical state during magma generation in this state magma batches accumulate in a non continuous step wise manner to form ever larger accumulations there is no characteristic length or time scale in the partial melting process or its products smallest melt segregations and km scale plotuns form the end members of a continuous chain of mergers of magma batches