radiative thermal conductivity in obsidian and estimates of heat transfer in magma bodies the optical transmission spectra of four ryholitic obsidian samples were measured in order to determine the importance of radiative heat transfer in granite magmas the spectra obtained in the temperature range 20 800 sup 0 c show that the radiative spectral window in these samples is limited by a charge transfer band in the uv 400 nm and si o stretching overtone in the ir 4500 nm within this window the main obstacles to radiative transfer in order of decreasing importance are background scattering a water band centered at 2800 nm and an fe sup 2 crystal field band at 1100 nm unlike crystalline silicates the absorption bands in obsidian do not broaden significantly as temperature increases as a result the temperature dependence of the calculated radiative thermal conductivity k sub r is dominated by the t sup beta term actual values of k sub r increase from 9 x 10 sup 5 to 1 x 1 sup 3 cal cm sup 1 s sup 1 deg sup 1 between 300 sup 0 and 800 sup 0 c the high temperature value being comparable to the lattice thermal conductivity in obsidian and a lower limit for k sub r in granitic melts as the scattering coefficient in melts is probably significantly lower than in obsidian the radiativemore conductivity in active plutons is likely to be much higher as an example if scattering and the water band are removed from the observed spectra of the obsidian samples calculated values of k sub r could increase by a factor of 5 to about 5 x 10 sup 3 cal cm sup 1 s sup 1 deg sup 1 at 1000 sup 0 c less

diffusive loss of argon in response to melt vein formation in polygenetic impact melt breccias many planetary surfaces in the solar system have experienced prolonged bombardment with each impact new rocks can be assembled that incorporate freshly generated impact melts with fragments of older rocks some breccias can become polygenetic containing multiple generations of impact melt products and can potentially provide important insights into the extensive bombardment history of a region however the amount of chronological information that can be extracted from such samples depends on how well the mineral isotopic systems of geochronometers can preserve the ages of individual melt generations without being disturbed by younger events we model the thermal evolution of impact melt veins and the resulting loss of ar from k bearing phases common in impact melt breccias to assess the potential for preserving the 40ar 39ar ages of individual melt generations our model results demonstrate that millimeter scale clast free melt veins cause significant heating of adjacent host rock minerals and can cause detectable ar loss in contact zones that are generally thinner than and at most about the same thickness as the vein width the incorporation of cold clasts in melt veins reduces the magnitudes of heating and ar loss in the host rocks and ar loss can be virtually undetectable for sufficiently clast rich veins quantitative evidence of the timing of impacts as measured with the 40ar 39ar method can be preserved in polygenetic impact melt breccias particularly for those containing mm scale bodies of clast bearing melt products

strain heating as a mechanism for partial melting and ultrahigh temperature metamorphism in convergent orogens implications of temperature dependent thermal diffusivity and rheology 1 we explore the importance of strain heating in ductile shear zones for production of leucogranites and high temperature metamorphism in collisional orogens with temperature dependent thermal diffusivity d and rock rheology new measurements on metamorphic rocks from the bohemian massif show that d is 1 3 mm2 s at 25 c and decreases exponentially to as low as 0 4 mm2 s at 600 c this temperature dependence causes model lithospheric geotherms to be straighter compared to geotherms calculated with a constant d using power law parameters for rheology we show that deformation of quartzite at strain rate of 3 10 13 s 1 produces 100 w m3 at 550 c and 7 w m3 at 800 c deformation of stronger clinopyroxenite at this strain rate produces 8 w m3 even at 1050 c when strain heating is introduced by a 3 km thick ductile shear zone with quartz rheology at the depth of 35 km in a lithosphere that has 70 km thick crust and 60 km thick lithospheric mantle the schist solidus is reached by 8 myr in the vicinity of the shear zone deforming at the strain rate of 3 10 13 s 1 for clinopyroxenite rheology ultrahigh temperature uht metamorphic conditions are reached in 40 myr after initiation of strain heating two dimensional models that replicate the exhumation of the greater himalaya crystalline rocks above the main central thrust produce extensive partial melting and an inverted metamorphic field gradient occurrences of leucogranites within crustal shear zone systems may be evidence of the coupling between deformation and heat production in the crust

mesoproterozoic arc magmatism in se india petrology zircon u pb geochronology and hf isotopes of the bopudi felsic suite from eastern ghats belt abstract the southern segment of the eastern ghats mobile belt egmb in india was an active convergent margin during mesoproterozoic prior to the final collision in neoproterozoic during the assembly of the rodinia supercontinent here we present mineralogical whole rock geochemical zircon u pb and hf isotopic data from a granitoid suite in the bopudi region in the eggb the granitoid complex comprises quartz monzodiorite with small stocks of rapakivi granites the monzodiorite locally porphyritic contains k feldspar megacrysts plagioclase quartz biotite and ortho amphibole the presence of mantled ovoid megacrysts of alkali feldspar embaying early formed quartz and the presence of two generations of the phenocrystic phases in the rapakivi granites indicate features typical of rapakivi granites the k feldspar phenocrysts in the rapakivi granite are mantled by medium grained aggregates of microcline ab7 or93 which is compositionally equivalent to the rim of kfs phenocryst and pl an23 24 ab75 the geochemistry of both the granitoids shows arc like features for ree and trace elements la icp ms zircon analyses reveal 207pb 206pb ages of 1582 mswd 1 4 for the rapakivi granite 1605 3 ma mswd 3 9 for the monzodiorite the zircons from all the granitoid samples show high ree contents prominent hree enrichment and a conspicuous negative eu anomaly suggesting a common melt source the zircons from the monzodiorite have a limited variation in initial 176hf 177hf ratios of 0 28171 0 28188 with ehf t values of 2 2 to 2 8 correspondingly their two stage hf isotope model ages tdm2 ranging from 2 15 to 2 47 ga probably suggest a mixed source for the magma involving melting of the paleoproterozoic basement and injection of subduction related juvenile magmas the prominent mesoproterozoic ages of these granitoids suggest subduction related arc magmatism in a convergent margin setting associated with the amalgamation of the columbia derived fragments within the neoproterozoic rodinia assembly

compositional variation and the origin of deep crustal reflections abstract identifying the sources of crustal reflections is essential for deriving more geological information from deep crustal reflection profiles theoretical and model studies help place constraints on the role of compositional variation in producing deep crustal reflections analysis of laboratory derived velocities and densities of rock types typical for the continental crust indicates that reflection coefficients are generally small but significantly 17 of the possible reflection coefficients have magnitudes between 0 1 and 0 2 comparison between reflection coefficients derived from well logs and reflections observed in associated seismic profiles indicates that constructive interference associated with geological layering is at least as important as the magnitude of the reflection coefficients for producing detectable reflections constructive interference can increase reflection amplitude by two to three times but is limited to a relatively narrow range of layer thickness for a typical 10 40 hz seismic wavelet and typical crustal velocities of about 6 km s constructive interference occurs for layer thickness ranging between about 35 and 80 m layers thinner than 35 m interfere destructively if reflections result from compositional variation seismic models of hypothetical and observed geologic relations provide analogs for interpreting complex reflection patterns observed in deep crustal reflection profiles such models show reflection patterns similar to those observed in the reflection profiles the models indicate that the reflections could originate in the complexly deformed and intruded terranes that are common in the crystalline crust and it may not be necessary to appeal to unobserved phenomena such as special lamellae or fluid filled fractures to explain the reflections

plutonic rock types series discrimination of various granitoid series and related rocks abstract the streckeisen qap diagram discriminates four characteristic type series among the large variety of granitoids associated in intrusions three of them integrate varied compositions from basic to acid terms 1 the calcalkaline series with its low k or trondhiemite medium k or granodioritic high k or monzonitic variants 2 the alkaline series with its characteristic syenitic members 3 the tholeiitic series deprived of k feldspars the fourth 4 corresponds to mobilizates associated with migmatites of varied composition but always quartz rich the generalization in the qapf system allowing the plotting of undersaturated alkaline series provides an excellent visual display of the major associations and trends for the plutonic rocks

the chimalpahad anorthosite complex and associated basaltic amphibolites nellore schist belt india magma chamber and roof of a proterozoic island arc abstract new major and trace element data on the proterozoic chimalpahad layered anorthositic complex and associated basaltic amphibolites of the nellore schist belt of south india provide new constraints on their petrogenesis and geodynamic setting the complex consists of layered anorthosites leucogabbros gabbros ultramafic rocks and is spatially associated with basaltic amphibolites despite deformation and metamorphism primary cumulate textures and igneous layering are locally well preserved throughout the complex whereas the amphibolites display diverse ree systematics the chimalpahad anorthositic gabbroic rocks are characterized by moderately depleted to strongly enriched lree patterns and by flat to depleted hree patterns the field relations major and trace element compositions of the basaltic amphibolites suggest that they are petrogenetically related to the anorthositic gabbroic rocks by fractional crystallization the anorthositic rocks and the basaltic amphibolites share the depletion of nb relative to th and la on primitive mantle normalized diagrams they exhibit signatures of arc magmatic rocks such as high lile and lree relative to the hfse and hree as well as high ba nb ba zr sr y la yb ratios that mimic chondrite normalized ree and primitive mantle normalized trace element patterns of arc magmas similarly on log transformed tectonic discrimination diagrams the chimalpahad rocks plot within the field of phanerozoic magmatic arcs consistent with a subduction zone origin on the basis of field relations and geochemical characteristics the chimalpahad complex is interpreted as a fragment of a magma chamber of an island arc which is tectonically juxtaposed against its original volcanic cover a new preliminary sm nd date of anorthosite from the chimalpahad complex indicates a model age of 1170 ma

the fold illusion the origins and implications of ogives on silicic lavas abstract folds on the surfaces of mafic lavas are among the most readily recognized geological structures and are used as first order criteria for identifying ancient lavas on earth and other planetary bodies however the presence of surface folds on the surface of silicic lavas is contested in this study and we challenge the widely accepted interpretation that silicic lava surfaces contain folds using examples from the western united states and sardinia italy we interpret the ridges and troughs on their upper surfaces typically referred to as ogives or pressure ridges as fracture bound structures rather than folds we report on the absence of large scale buckle style folds and note instead the ubiquitous presence of multiple generations and scales of tensile fractures comparable to crevasses in glaciers and formed in ways similar to already recognized crease structures we report viscosity data and results of stress analyses that preclude folding ductile deformation in compression of the upper surface of silicic lavas at timescales of emplacement weeks to months therefore analysis of fold geometry wavelength amplitude etc is erroneous and instead the signal produced reflects the strength and thickness of the brittle upper surface stretching over a ductile interior the presence of ogives on the surfaces of lavas on other planetary bodies may help to elucidate their rheological properties and crustal thicknesses but relating to their tensile strength not viscosity

thermal diffusivity of fe rich pyroxene glasses and their melts abstract we provide new measurements of thermal diffusivity d of 8 synthetic glasses and melts along the enstatite ferrosilite compositional binary 4 remelted crystals which are close to the binary plus one synthetic ternary composition fs15wo40en45 thermal diffusivity of these glasses is between 0 50 and 0 62 mm2 s 1 at room temperature decreases to 0 45 to 0 50 mm2 s 1 near 1000 k then drops upon melting at 1100 k to 0 34 mm2 s 1 a roughly linear trend in d exists across the en fs binary at 298 k with fe rich compositions having lower d whereas at high temperatures all melts have similar d the trend is not smooth due to variable amounts of crystallites and residual strain in glasses both of which affect d values above about 700 k d t is visibly positive for moderate fe contents i e fe fe mg 0 1 to 0 3 which we attribute to diffusive radiative transfer being enhanced by electronic vibronic coupling all data are well represented by equations of the form ft g ht where f g and h are fitting parameters the fitting reveals a weak increase in d at high t for all of our glasses consistent with weak radiative transfer in the infrared although data could only be collected on melts over a narrow temperature range they also appear to have a positive d t which is consistent with previous results on less mafic iron bearing glasses and melts our results suggest that fe rich melts in the lower mantle could have thermal diffusivity higher than their crystalline counterparts due to the response of d to temperature

shrimp u pb ages of zircons from mafic granulites of the eastern ghats belt se india implications for the evolution of the palaeoproterozoic arc crust abstract we determined shrimp u pb ages of zircons from three mafic granulite samples of the polymetamorphosed multiply deformed kondapalli ongole domain of the eastern ghats belt egb se india our goals are 1 to evaluate the origin of zircons in the high grade mafic granulites and 2 to clarify the petrotectonic evolution of magmatic and metamorphic rocks along the se margin of india the zircons record three distinct paleoproterozoic u pb age groups igneous protoliths of the mafic granulites apparently crystallized at 1715 ma and underwent granulite facies high temperature metamorphism at 1676 ma with a subsequent fabric defining garnet granulite grade metamorphic overprint at 1610 ma based on textures cathodoluminescence images and geochemistry we identified three zircon types among the 1610 ma zircons type i and ii zircon grains recrystallized or newly grown in equilibrium with feldspar and garnet respectively type iii zircons crystallized from aqueous anatectic melt in quartzofeldspathic veins some zircons of all the three types show alteration effects the 1715 ma event likely represents formation of the kondapalli ongole magmatic arc crust whereas the 1610 ma high grade metamorphic event reflects suturing of the arc to proto india and cratonization of the egb the interval between the formation of u poor low th yb zircons equilibrated with garnet and u rich high th yb zircons equilibrated with fluid rich melt is geologically insignificant suggesting that tectonic accretion of the egb its final suturing and orogenic collapse took place rapidly

analysis of multiscale scattering and poroelastic attenuation in a real sedimentary rock sequence compressional waves in heterogeneous permeable media experience attenuation from both scattering and induced pore scale flow of the viscous saturating fluid for a real finely sampled sedimentary sequence consisting of 255 layers and covering 30 meters of depth elastic and poroelastic computer models are applied to investigate the relative importance of scattering and fluid flow attenuation the computer models incorporate the known porosity permeability and elastic properties of the sand shale sequence in a binary medium plane layered structure the modeled elastic scattering attenuation is well described by stochastic medium theory if two length scale statistics are applied to reflect the relative thickness of the shale layers when compared to the sand layers under the poroelastic biot squirt flow model fluid flow attenuation from the moderate permeability 10 14 m2 sands may be separated in the frequency domain from the attenuation due to the low permeability 5 10 17 m2 shale layers based on

radiative heat transfer in molten and glassy obsidian we have measured optical transmittance spectra in rhyolitic obsidian samples in the wavelength range 380 5500 nm and at temperatures t from 19 1145 c above and below the softening point from the transmittance we calculated the absorption coefficient t and the radiative thermal conductivity kr t kr ranged from 3 10 3 cal cm 1 s 1 k 1 1 2 w m 1 k 1 at 700 c to 12 10 3cal cm 1 s 1 k 1 5 w m 1k 1 at 1145 c the 700 c value is comparable with lattice thermal conductivity klof about 4 10 3 cal cm 1 s 1k 1 1 7 w m 1k 1 removing scattering effects due to bubbles from the transmittance spectra by lowering the absorption baseline increased kr to 20 10 3 cal cm 1 s 1 k 1 8 4 w m 1 k 1 at 1145 c because scattering by bubbles is likely to be small in confined magmas these numbers are probably minimum values for kr and indicate that in active plutons radiative heat transport could be greater than lattice conductivity by more than a factor of 2 at 1000 c thus melting markedly strengthens kr and radiative heat transport is probably the dominant component of the total conductivity k kl kr in silicic magmas these relatively large values of k can be applied to models of the thermal evolution of magma bodies and to cooling of intrusives

thermal diffusivity of rhyolitic glasses and melts effects of temperature crystals and dissolved water thermal diffusivity d was measured using laser flash analysis on pristine and remelted obsidian sam ples from mono craters california these high silica rhyo lites contain between 0 013 and 1 10 wt h2o and 0 to 2 vol crystallites at room temperature dglass varies from 0 63 to 0 68 mm 2 s 1 with more crystalline samples having higher d a st increases dglass decreases approaching a constant value of 0 55 mm 2 s 1 near 700 k the glass data are fit with a simple model as an exponential function of temperature and a linear function of crystallinity dissolved water contents up to 1 1 wt have no statistically significant effect on the thermal diffusivity of the glass upon crossing the glass transition d decreases rapidly near 1 000 k for the hydrous melts and 1 200 k for anhydrous melts rhy olitic melts have a dmelt of 0 51 mm 2 s 1 thermal conduc tivity k0d cp of rhyolitic glass and melt increases slightly with t because heat capacity cp increases with t more strongly than density and d decrease the thermal conductivity of rhyolitic melts is 1 5 wm 1 k 1 a nd should vary little over the likely range of magmatic temperatures and water contents these values of d and k are similar to those of major crustal rock types and granitic protoliths at magmatic temperatures suggesting that changes in thermal properties accompanying partial melting of the crust should be relatively minor numerical models of shallow rhyolite intrusions indicate that the key difference in thermal history between bodies that quench to obsidian and those that crystallize results from the release of latent heat of crystallization latent heat release enables bodies that crystallize to remain at high temperatures for much longer times and cool more slowly than glassy bodies the time to solidification is similar in both cases however because solidification requires cooling through the glass transition in the first case and cooling only to the solidus in the second

thermal properties of vesicular rhyolite abstract thermal diffusivity of rhyolite melt and rhyolite foam 70 80 porosity has been measured using the radial heat transfer method cylindrical samples length 50 55 mm diameter 22 mm of rhyolite melt and foam have been derived by heating samples of little glass mountain obsidian using available data on heat capacity and density of rhyolite melt the thermal conductivity of samples has been determined the difference in thermal conductivity between rhyolite melt and foam at igneous temperatures 1000 c is about one order of magnitude the effect of thermal insulation of magmas due to vesiculation and foaming of the top layer is discussed in terms of the data obtained using a simple illustrative model of magma chamber convection

dismembered ophiolites in paleoproterozoic nappe complexes of kandra and gurramkonda south india abstract the constitution and structural architecture of ophiolites is important in the understanding of cratonic growth through subduction collision accretion history in the precambrian terrains the nellore schist belt nsb sandwiched between the eastern ghats belt egb and the eastern dharwar craton south india is known to have deformed remnants of ocean plate stratigraphy ops as at kanigiri and kandra suggesting multiple cycle of ocean opening and closure in the paleoproterozoic mesoproterozic interval the outcrop details and structural architecture of the paleoproterozoic kandra complex with geochemical character of subduction related ophiolites is described and analyzed to better constrain the accretion collision history along the southeastern margin of the indian craton imbricate thrust slices between kandra and arimanupadu host slivers of hornblende metagabbro thick massive to pillowed metabasalt sheets sheeted meta dolerite dykes and thin metapsammite and metapelite interstratified with metabasalt comparable to layer 2a of oceanic crust contractional deformation in interlayered metabasalt and quartzite around kandra is accommodated by south dipping imbricate thrust systems and n ne vergent folds the wnw trending thrust system with dominant tectonic transport toward northeast is oblique to overall n s trend of the nsb or the egb further east intercalated metabasalts deep water turbidites and thin metacherts around gurramkonda are deformed into sw vergent imbricate thrust system here occurrence of metabasalt sheets along individual thrusts suggest syntectonic emplacement of some basalts as the subducted and scraped upper oceanic crust was incorporated in the acrcetionary prism the rock sequences around kandra and gurramkonda comprising incomplete ops are tectonically stacked over granites of the eastern dharwar craton margin bivergent thrust systems around kandra possibly root into a detachment separating layers 1 2 from more rigid deeper layers cf layer 3 of oceanic crust as indicated by absence of significant volume of ultramafic rocks at the current outcrop level

the origin of felsic microgranitoid enclaves insights from plagioclase crystal size distributions and thermodynamic models abstract magma mixing is widely recognized in contemporary petrology as one of the primary igneous processes microgranitoid enclaves mes are considered to be remnants of such mixing processes and the term has a well established genetic implication however microgranitoid enclaves span a wide range of compositions and felsic varieties are also frequently reported nd sr isotope and textural data from felsic microgranitoid enclaves fmes mafic microgranitoid enclaves mmes and host granites from the salto pluton itu granitic province show that the cm sized mmes are dioritic have medium grained igneous textures and xenocrysts of alkali feldspar and quartz the fmes are cm to meter sized have spheric shapes show corrugated contacts with the host granites and have resorbed feldspars and deformed quartz crystals interpreted as xenocrysts set in a fine grained groundmass compared to the host granites both mme and fme samples have increased feo mgo tio2 p2o5 and zr contents but their sr and nd isotope signatures are identical fme 87sr 86sri 0 7088 0 7063 endi 10 0 to 10 2 mme 87sr 86sri 0 7070 endi 10 5 host granite 87sr 86sri 0 7056 0 7060 endi 10 2 to 10 3 these indicate that the enclaves derive from a similar source although the melts from which they formed were probably hotter and chemically more primitive than their host granites crystal size distributions csds of plagioclase in samples drilled from rinds and cores of three fmes show that the rind samples are systematically finer grained than the samples from the cores which indicates that the fmes cooled inwards and contradict interpretations that the fmes are autoliths thermal modeling suggests that a slightly more primitive hotter magma would be thermally equilibrated with an evolved resident melt within weeks after mixing mingling upon thermal equilibrium the fmes would have an increased crystal cargo and the resulting touching framework would impart a solid like behavior to the fme forming magma which would lead to a contrast in rheology fragmentation dragging and preservation of felsic replenishment batches as distinct enclaves

u pb zircon chronology of the pangidi kondapalle layered intrusion eastern ghats belt india constraints on mesoproterozoic arc magmatism in a convergent margin setting abstract the pangidi kondapalle layered intrusion is emplaced within high grade supracrustal rocks in the southern sector of the eastern ghats granulite belt proximal to the south eastern periphery of the indian plate this layered complex consists mainly of anorthosites and gabbronorites with pyroxenite cumulates containing orthopyroxenites and chromitites in a regressive intrusive sequence here we report laser ablation icp ms data and ree geochemistry of zircons from three co spatial anorthositic bodies in this complex our results show mean 207 pb 206 pb ages of 1634 18 ma mswd 0 55 for the anorthosite from pangidi two anorthosite samples from kondapalle yield mean 207 pb 206 pb ages of 1628 36 ma mswd 0 54 and 1693 110 ma mswd 5 5 the zircons from all the anorthosite samples show high ree contents prominent hree enrichment and a conspicuous positive eu anomaly suggesting a common melt source the prominent mesoproterozoic ages revealed from our u pb dating of zircons from these layered complexes suggest subduction related magmatic underplating in a convergent margin setting associated with the amalgamation of the columbia derived fragments within the neoproterozoic supercontinent

reflectivity and seismic properties of the deep continental crust the recovery of drill core from a section of upper amphibolite facies terrane located in the inner piedmont of south carolina provides a rare opportunity to study in detail the seismic character of rocks which were once part of the deep continental crust seismic reflection profiles over the inner piedmont in the vicinity of the drill hole reveal subhorizontal high amplitude reflections the origin of these reflections has been investigated by measuring compressional wave velocities as a function of confining pressure and densities in 150 samples obtained from 305 m of drill core synthetic seismograms generated from the velocity and density measurements demonstrate that reflections originate from subhorizontal metamotphic layering probably originating from lower crustal ductile flow fine tuning of the metamotphic layering which is temperature and pressure related can produce bright spots in the deep crust anisotropy resulting from preferred mineral orientation is another important propeny of this sectim velocities are low for vertical wave propagatim whereas horizontal velocities are relatively high horizontal velocities show mly a slight variatim with azimuth which is unlikely to be detected using standard refractim techniques reflectivity decreases with increasing anisotropy critical to this study has been the continuous coring of a major section of high grade metamotphic terrane which has provided the necessary stratigraphic and lithologic cmtrol for the generation of accurate synthetic seismograms

water transfer during magma mixing events insights into crystal mush rejuvenation and melt extraction processes many plutons preserve evidence of magma mixing between hydrous mafic magmas and resident felsic crystal rich mushes to investigate water transfer processes in such systems following thermal equilibration we conducted 24 h experiments to establish the petrological evolution of a water undersaturated 4 wt h 2 o in the interstitial melt quartz bearing dacite crystal mush 0 5 0 8 in crystal fraction intruded by a water saturated 6 wt h 2 o initially crystal free andesite magma at 950 c and 4 kbar 12 km depth our results show isothermal undercooling resulting from a change in liquidus temperatures of the interacting magmas due to their changing water content specifically mafic samples dramatically crystallize during water escape into the felsic end members and consequent increase in liquidus temperature conversely the addition of water to the felsic mush reduces the liquidus temperature leading to an increase in melt fraction the experiments provide insights into how volatiles contribute to crystal mush rejuvenation i e increase of melt fraction however h 2 o diffusion alone is not sufficient to promote melt extraction from short and long lived mushes in the earth s crust

thermal constraints on the emplacement rate of a large intrusive complex the manaslu leucogranite nepal himalaya the emplacement of the manaslu leucogranite body nepal himalaya has been modelled as the accretion of successive sills the leucogranite is characterized by isotopic heterogeneities suggesting limited magma convection and by a thin 100 m upper thermal aureole these characteristics were used to constrain the maximum magma emplacement rate models were tested with sills injected regularly over the whole duration of emplacement and with two emplacement sequences separated by a repose period additionally the hypothesis of a tectonic top contact with unroofing limiting heat transfer during magma emplacement was evaluated in this latter case the upper limit for the emplacement rate was estimated at 3 4 mm year or 1 5 myr for 5 km of granite geological and thermobarometric data however argue against a major role of fault activity in magma cooling during the leucogranite emplacement the best model in agreement with available geochronological data suggests an emplacement rate of 1 mm year for a relatively shallow level of emplacement granite top at 10 km uninterrupted by a long repose period the thermal aureole temperature and thickness and the isotopic heterogeneities within the leucogranite can be explained by the accretion of 20 60 m thick sills intruded every 20 000 60 000 years over a period of 5 myr under such conditions the thermal effects of granite intrusion on the underlying rocks appear limited and cannot be invoked as a cause for the formation of migmatites

dynamics and time evolution of a shallow plumbing system the 1739 and 1888 90 eruptions vulcano island italy abstract in this work we analyze the morphology of latitic enclaves occurring in the rhyolitic lava flow of pietre cotte island of vulcano italy we show that enclave morphology is a feature inherited from the shallow plumbing system of the volcano during the invasion of the latitic magma into the rhyolitic magma the complexity of enclave morphology is quantified by fractal analyses using the empirical relationship given by perugini et al 2005 relating the fractal dimension and viscosity ratio the range of viscosity ratios that developed during the injection of the latitic magma into the rhyolitic one is estimated thermodynamical and rheological modeling indicates that the most plausible scenario to explain the variability of observed viscosity ratios is represented by a plumbing system where a large volume of latitic magma intruded a smaller volume of rhyolitic magma the comparison of volume proportions of magmas on the outcrop with those estimated by the modeling allows us to infer that most of the latitic magma remained in the plumbing system after the ending of the 1739 eruptive cycle the strong similarity of compositions from the pietre cotte lava flow and products erupted during the last volcanic activity of vulcano 1888 1890 is interpreted as the reactivation of the latitic magma that possibly evolved through fractional crystallization to produce trachy rhyolitic compositions during the 1888 1890 eruptive cycle we suggest that the methodological approach presented here can represent a further tool for hazard mitigation in volcanic areas in particular it allows obtaining information about the dynamics of plumbing systems of active volcanoes and their time evolution

p wave velocity across a noncoaxial ductile shear zone and its associated strain gradient consequences for upper crustal reflectivity in order to simulate a normal incidence reflection profile across a noncoaxial ductile shear zone we determined p wave velocities of samples cut parallel and normal to mylonite foliation along a closely spaced profile 27 cm long through a transition zone and its associated strain gradient the ductile shear zone developed within an aplitic leucogranite was sampled from a kilometer wide ductile transcurrent fault in the northern french massif central strain analysis indicates that the sample experienced heterogeneous and progressive simple shear deformation shear strain systematically increases from zero in the undeformed protolith to 30 in the mylonite the transition zone thickness t is about 30 cm and the mylonite thickness m is about 10 cm the amount of quartz and mica increases relative to feldspar toward the mylonite indicating that a mineralogical composition change accompanied mylonitization mica and quartz developed a strong crystallographic preferred orientation cpo in the least strained domain seismic anisotropy is low and mean vp is 6 km s at 600 mpa anisotropy increases up to 10 and vp decreases up to 5 35 km s for propagation normal to the mylonite foliation through the transition zone this systematic velocity change correlates with the increasing through the transition zone and can be directly related to the cpo of mica and the increase in volume percent mica within the mylonite zone these results indicate that velocity and anisotropy gradients may in some cases be associated with ductile shear zones and that mylonite boundaries may not represent first order discontinuities the reflectivity of a ductile shear zone depends on the thickness of the transition zone relative to the seismic wavelength and on the t m ratio synthetic seismograms show that for a given seismic wavelength the reflectivity decreases when the transition zone thickness increases and when the ratio t m increases we show that layers with second order boundaries velocity gradients in transition zones are only seismically detectable within a narrow thickness range extrapolation to thicker shear zones is based on the assumption that the strain gradient thickness relative to shear zone thickness is to a first approximation scale independent in granitic domains ductile shear zones with similar geometrical and petrophysical features to the example studied here will be detected on deep seismic profiles only if their width is between 20 and 400 m development of ductile shear zones with strain gradients of the appropriate thickness to enhance reflectivity is favored under low temperature conditions in the granitic upper crust indeed low temperature strain gradient may explain the high seismic reflectivity of the upper crust in the scandinavian caledonides whereas high temperature strain gradient may explain in part the relative transparency of the european variscan upper crust

the magmatic metamorphic and tectonic evolution of the northern marginal zone of the limpopo belt in zimbabwe the northern marginal zone of the limpopo belt in southern africa comprises a plutonic assemblage of granitoids including a distinctive suite of porphyroclastic granites and a much less abundant supracrustal assemblage of metabasites and iron formations these rocks are at granulite facies above a normal thickness of continental crust most of the plutonic assemblage are intrusive rocks that crystallized from dry melts from 2800 to 2600 ma with a relatively simple thermal history they may have been derived from partial melting of a mafic source some supracrustal rocks have experienced two thermal events at granulite facies a reverse sense shear zone forms the boundary of the northern marginal zone with the zimbabwe craton the southern boundary is the triangle shear zone which is proven as a continuous structure along a much greater strike length than previously documented a widespread sub vertical foliation in the northern marginal zone and the reverse shear zone formed during progressive nnw sse shortening crustal thickening occurred both magmatically and tectonically in the late archaean and was accompanied by synchronous uplift protracted magmatism provides a mechanism to incorporate supracrustal rocks into the lower crust and can explain the occurrence of more than a single thermal event

the influences of incremental pluton growth on magma crystallinity and aureole rheology numerical modeling of growth of the papoose flat pluton california the papoose flat pluton in the white inyo range california is one of the best examples of forcefully emplaced plutons within an arc crust having internal fabrics and a contact aureole that deformed in a ductile manner a 2 d numerical model for the thermo rheological evolution of the pluton wall rock system is proposed we explore how the frequency of magma input from instantaneous episodic to continuous affects magma chamber crystallinity and rheology of both the pluton and its contact aureole we model pluton growth in the depth range of 10 13 km which is at the brittle ductile transition of the crust and in the 7 4 km depth range where the host rocks are initially brittle for incremental growth episodic and continuous the pluton begins as a sill with subsequent injections to the bottom the pluton grows into a laccolith results of mid crustal models show that the ductile region above the papoose flat pluton is related to thermal weakening the ductile region during incremental growth is 100 150 m thick matching the observed thickness it is ten times thinner than in the instantaneous growth model in episodic and continuous models the upper part of the pluton is fully or quasi crystalline throughout growth during continuous growth it is likely to remain ductile with potential for the development of solid state fabrics during episodic growth strain rates within the pluton during each injection may become sufficiently high to cause embrittlement of magma in no case a ductile aureole develops above the pluton at the upper crustal level but may develop below the pluton which serves as thermal insulator thus the pluton s floor may sag during incremental growth most of the pluton is below the solidus and brittle the results suggest that large volcanic eruptions are unlikely to occur by slow pressurization of magma chambers instead they require rapid injections of large melt volumes

the theoretical seismic reflection response of sedimentary sequences a sedimentary sequence can quite often be considered very fruitfully as a discrete markov chain and thus can be described by a stochastic matrix knowing this matrix and the thickness probability distributions of the layers of the various participating lithologies while assuming that their densities and velocities are normally distributed an expression is derived for the autocorrelation or power spectrum of the seismic reflection response this expression can bring out essential characteristics of the power spectra i e the low frequency droop and its slope this then leads to the possible estimation of some physical and sedimentary parameters of the sequence such as the scatter of the reflection coefficients and the sand shale ratio or sedimentary cycle thickness the case of a binary sequence as the predominant sequence of the sediments of the gulf coast area and nigeria is analyzed in detail the theoretical results conform well with field data and lead to the interesting conclusion that indee

the fracture behaviour of volcanic glass and relevance to quench fragmentation during formation of hyaloclastite and phreatomagmatism quench fragmentation is a non explosive process that occurs when molten magma is super cooled to glass upon contact with ambient water this occurs when coherent lavas are erupted subaqueously when they flow into water when magma intrudes into water saturated sediments and when magma and water interact explosively during phreatomagmatism quench fragmentation also occurs alongside explosive phreatomagmatic activity although products of quench fragmentation hyaloclastite sensu stricto have been discussed qualitatively in the volcanological literature compared to explosive fragmentation processes very little is known about the exact dynamics of quench fragmentation of magma and how this relates to the rheology and physical properties of volcanic glass based on literature from materials engineering we present a detailed review of the processes by which glass forms the properties of glass and the fracture mechanics that cause it to fragment non explosively we also consider how this can be applied to understanding the dynamics behind the volcanological processes of in situ glass fragmentation during quenching in wet environments and phreatomagmatism important parameters for the occurrence of quench fragmentation are the temperature difference between the magma and the ambient water and how much the ambient water is superheated above its leidenfrost temperature the geometry of the lava or magma intrusion the thermal conductivity and the thermal expansion are also of great importance the resistance of the magma against fragmentation can be increased with the presence of crystals provided the thermal expansion of the crystals does not greatly exceed that of the glass vesicles have the opposite effect unless the magma is highly vesicular this overview then provides a solid basis for further quantitative study of quench fragmentation and hyaloclastite formation

random scattering in magnetotellurics typical well logs show substantial variations of formation electrical resistivity over small spatial scales down to the resolution of the logging tool using a plane stratified earth model we examine the effects of this fine scale microstructure on scattering of the naturally occurring electromagnetic em waves used in magnetotellurics we show how 1 d magnetotelluric mt data may be viewed as arising statistically from a smoothed effective medium version of the resistivity depth profile the difference between the data produced by the true medium and the effective medium is attributable to random scattering noise this noise is fundamental to magnetotellurics and other diffusive wave em exploration methods since it arises from the very small spatial scales that are usually ignored the noise has unique statistical properties which we characterize we show that if scattering is the dominant noise source a thin layer of increased resistivity at depth can be reliably detected only if the noise statist

petrological and geochemical characterization of proterozoic ophiolitic melange nellore khammam schist belt se india abstract the ophiolitic melange occurs close to a major fault within the metavolcanics of kanigiri area in the western margin of the khammam nellore schist belt andhra pradesh india the unique feature of kanigiri melange is the occurrence of ophiolitic bodies of various sizes most of which display petrological and geochemical characteristics typical of supra subduction zone oceanic crust the ophiolitic melange is composed mainly of blocks of variably deformed and metamorphosed pillow basalts serpentinized ultramafic rocks meta gabbros cherts and pelagic sediments in a pervasive scaly argillite matrix the melange has a tectonic contact with a granite intrusion and numerous thrust faults cut across the melange exposures primary magmatic features such as drainage cavities in pillow basalts magmatic layering in gabbros and clinopyroxene cumulates in ultramafic rocks are well preserved in low strain domains mafic components enclosed in the melange are characterized by low pressure secondary mineral assemblages retrograded from amphibolite down to greenschist facies as commonly observed in subducted and exhumed orogens all amphiboles in the studied fragments are calcic and cluster in the range from winchite actinolite to hornblende amphibole compositions are characterized by al 2 o 3 2 4 13 8 feo 5 13 k 2 o 0 04 0 7 and x mg 0 67 0 77 the metabasalt and wherlite samples exhibit light rare earth element lree enriched patterns and negative high field strength element hfse zr and ti consistent with a suprasubduction zone geodynamic setting a notable feature of both metabasalt and wherlite samples is their pronounced negative ce anomaly and a positive eu anomaly as observed in a number of modern island arc suites as the petrological features and the geochemical characteristics of the remnant fragments we analyzed in this study are reminiscent of their phanerozoic ophiolite counterparts we suggest that these rocks were derived from a proterozoic oceanic plate belonging to a suprasubduction ophiolite assemblage subduction zone geochemical signatures and ophiolite like crustal lithologies in the study region are consistent with the accretion of juvenile oceanic fragments to the pre existing continental blocks through a forearc continent collision

thermodynamic and rheological properties of rhyolite and andesite melts the heat capacities of a rhyolite and an andesite glass and liquid have been investigated from relative enthalpy measurements made between 400 and 1800 k for the glass phases the experimental data agree with empirical models of calculation of the heat capacity for the liquid phases the agreement is less good owing to strong interactions between alkali metals and aluminum which are not currently accounted for by empirical heat capacity models the viscosity of both liquids has been measured from the glass transition to 1800 k the temperature dependence of the viscosity is quantitatively related to the configurational heat capacity determined calorimetrically through the configurational entropy theory of relaxation processes for both rhyolite and andesite melts the heat capacity and viscosity do not differ markedly from those obtained by additive modeling from components with mineral compositions

transport properties of low sanidine single crystals glasses and melts at high temperature thermal diffusivity d was measured using laser flash analysis from oriented single crystal low sani dine k0 92na0 08al0 99fe 3 si2 95o8 and three glasses near kalsi3o8 viscosity measurements of the three super cooled liquids in the range 10 6 8 to 10 12 3 pa s confirm near arrhenian behavior varying subtly with composition for crystal and glass d decreases with t approaching a constant near 1 000 k dsat 0 65 0 3 mm 2 s 1 for bulk crystal and 0 53 0 03 mm 2 s 1 for the glass a rapid decrease near 1 400 k is consistent with crossing the glass transition melt behavior is approximated by d 0 475 0 01 mm 2 s 1 thermal conductivity klat of glass calculated using previous heat capacity cp and new density data increases with t because cp strongly increases with t for melt klat reaches a plateau near 1 45 w m 1 k 1 and is always below klat of the crystal melting of potassium feldspars impedes heat transport providing positive thermal feedback that may promote further melting in continental crust

transport properties of glassy and molten lavas as a function of temperature and composition abstract we provide measurements of thermal diffusivity d heat capacity cp and viscosity for 12 remelted natural lavas and 4 synthetic glasses and melts ranging in composition from leucogranite to low silica basalt and calculate their thermal conductivity both viscosity and the glass transition temperature decrease with decreasing melt polymerization for basaltic glasses d is low 0 5 mm2 s 1 at room temperature decreases slightly with increasing temperature and then drops upon melting to 0 25 to 0 35 mm2 s 1 other samples behave similarly despite scatter clear correlations exist between d of glass or melt with si content density nbo t and most strongly with fragility m glass thermal diffusivity is represented by d ft g ht where f g and h are fitting parameters for melts d t was resolved only for dacite andesite and morb a positive slope is consistent with other iron bearing samples glass and liquid cp depend on density and other physical properties but not exactly in the same manner as d we calculate thermal conductivity k from these data and demonstrate that k for glasses is described by a maier kelly formula large scatter exists for k at 298 k but silicic to intermediate melts have k between 1 8 and 1 3 wm 1 k 1 whereas basaltic melts are constrained to 1 4 0 1 wm 1 k 1 low values for thermal diffusivity and viscosity for basaltic melts suggests that basalts transfer heat much more efficiently by advection than by conduction alone and that partially molten zones in the mantle quickly become more thermally insulating than non molten zones potentially contributing to melt localization during decompression melting

an investigation of the spectral properties of primary reflection coefficients this paper investigates the form of the nonwhiteness found in the reflection coefficients from a wide variety of rock sequences around the world in all but one case densities are taken as constant due to the paucity of suitable density data the reflection sequences are pseudo white only above a corner frequency below which their power spectrum falls away according to a power law f where is between 0 5 and 1 5 this spectrum can be adequately modelled in practice very simply with an arma 1 1 process which acts on a white innovation sequence the corollary of this is that before wavelet estimation methods are applied all of which except those based on synthetic seismograms presuppose white reflection sequences or deconvolution filters estimated seismic traces should be filtered with the inverse of this process interestingly the estimated arma processes group themselves into two clearly differentiated categories having very different indices of predictability or strictly indices of linear determinism the two categories apparently correlate precisely with two kinds of sedimentation one which consists largely of sequences of rocks with repeating properties called repetitive in this paper but perhaps loosely describable as cyclic and the other which is randomly bedded with no apparent pattern of components the former has indices of predictability which are two to four times as great as those of the latter another probably related property is that for the repetitive sequences tends to be greater than that for non repetitive rock columns the observed power spectra are shown to be consistent with a simple model for the logarithm of acoustic impedance consisting of a mixture of processes where the distribution of time scale parameters is reciprocal detailed effects of block averaging and sampling the logs are shown to depend on the type of sequence under examination

geologically constraining india in columbia the age isotopic provenance and geochemistry of the protoliths of the ongole domain southern eastern ghats india abstract the ongole domain in the southern eastern ghats belt of india formed during the final stages of columbia amalgamation at ca 1600 ma yet very little is known about the protolith ages tectonic evolution or geographic affinity of the region we present new detrital and igneous u pb hf zircon data and in situ monazite data to further understand the tectonic evolution of this columbia forming orogen detrital zircon patterns from the metasedimentary rocks are dominated by major populations of palaeoproterozoic grains ca 2460 2320 2260 2200 2100 2080 2010 1980 1920 1850 and 1750 ma and minor archaean grains ca 2850 2740 2600 and 2550 ma combined u pb ages and lu hf zircon isotopic data suggest that the sedimentary protoliths were not sourced from the adjacent dharwar craton instead they were likely derived from east antarctica possibly the same source as parts of proterozoic australia magmatism occurred episodically between 1 64 and 1 57 ga in the ongole domain forming felsic orthopyroxene bearing granitoids isotopically the granitoids are evolved producing ehf values between 2 and 12 the magmatism is interpreted to have been derived from the reworking of archaean crust with only a minor juvenile input metamorphism between 1 68 and 1 60 ga resulted in the partial to complete resetting of detrital zircon grains as well as the growth of new metamorphic zircon at 1 67 and 1 63 ga in situ monazite geochronology indicates metamorphism occurred between 1 68 and 1 59 ga the ongole domain is interpreted to represent part of an exotic terrane which was transferred to proto india in the late palaeoproterozoic as part of a linear accretionary orogenic belt that may also have included south west baltica and south eastern laurentia given the isotopic geological and geochemical similarities the proposed exotic terrane is interpreted to be an extension of the napier complex antarctica and may also have been connected to proterozoic australia north australian craton and gawler craton

dynamic elasticity of microbedded and fractured rocks microbedded rocks have an anisotropic frequency dependent sound speed which depends on the intrinsic sound speeds of the individual microbeds and on the o doherty anstey effect fractured rocks have an anisotropic frequency dependent sound speed which depends on the intrinsic sound speed of the unfractured rock the frequency dependent phase shift that occurs during reflection or transmission across a fracture and the interfracture o doherty anstey effect these effects are neglected by the quasistatic methods presently used to generate elastic constants here i introduce a new method for generating elastic constants that contain all the above effects first a statistical description of the rock is used to generate a sample of the rock then an exact two way method is used to propagate just a few plane waves of frequency a distance of several wavelengths from the source if an equivalent homogeneous medium exists at frequency then the computed motions must also satisfy a one way elastic wave equation for that equivalent medium this one way wave equation is used to invert for the elastic coefficients when no equivalent medium exists perhaps because is too large this is indicated by the inversion possible applications of the method are prediction of seismic sound speeds from measurements of bed thicknesses in cores analysis of laboratory data for fracture constitutive relations and inversion of multioffset vertical seismic profiling data for elastic coefficients comparable with those predicted from cores

thermal diffusivity of olivine group minerals at high temperature thermal diffusivity d data from 12 oriented single crystals and seven polycrystalline samples of olivine group minerals were acquired with the laser flash method at temperatures t of up to 1500 c samples included forsterite fe mg binary olivines sinhalite and chrysoberyl specimens were characterized using infrared spectroscopy and electron microprobe analysis crystal orientation and chemistry both affect d for our single crystals d 100 d 001 d 010 at all temperatures thermal diffusivity decreases with increasing t and becomes virtually constant at high temperatures at room temperature d 001 of pure forsterite has the highest observed values but substitution of a small amount of co in forsterite 0 3 wt coo lowers d by 20 substitution of 10 fe for mg in forsterite as in typical mantle olivine lowers d by 50 at room temperature mantle olivine has d 3 25 1 66 and 2 59 mm2 s for the 100 010 and 001 orientations respectively the values decrease to 0 93 0 87 mm2 s at 790 985 c for 100 0 54 51 mm2 s at 590 740 c for 010 and 0 83 0 79 mm2 s at 740 890 c for 001 two dunite samples have d of 0 55 0 56 mm2 s at 890 1080 c showing the effect of preferred orientation of grains dominated by 010 thermal diffusivity of polycrystalline samples is controlled by the large amounts of olivine present minor phases offset the curves for d t from the value of the olivine mineral our laser flash measurements isolate the phonon component of heat transfer from radiative transfer and show that the phonon contribution becomes nearly constant for the high temperatures expected in the mantle the other microscopic component diffusive radiative transfer depends strongly on temperature and this temperature dependence likely exerts greater control on mantle convection

transport properties of high albite crystals near endmember feldspar and pyroxene glasses and their melts to high temperature thermal diffusivity d was measured using laser flash analysis lfa from oriented single crystal albite and glasses near lialsi3o8 naalsi3o8 caal2si2o8 lialsi2o6 and camgsi2o6 compositions viscosity measurements of the supercooled liquids over 2 6 108 to 8 9 1012 pa s confirm strongly non arrhenian behavior for caal2si2o8 and camgsi2o6 and near arrhenian behavior for the others as t increases d glass decreases approaching a constant near 1 000 k upon crossing the glass transition d decreases rapidly for feldspars d for the melt is 15 below d of the bulk crystal whereas for pyroxenes this difference is 40 thermal conductivity k lat c p d of crystals decreases with increasing t but k lat of glasses increases with t because heat capacity c p increases with t more strongly than density and d decrease for feldspars k lat for the melt is 10 below that of the bulk crystal or glass whereas this decrease for pyroxene is 50 therefore melting substantially impedes heat transport providing positive thermal feedback that could promote further melting

simultaneous measurements of thermal conductivity and thermal diffusivity for garnet and olivine under high pressure abstract thermal conductivity and thermal diffusivity have been measured simultaneously for garnet al73py25gr1sp1 and olivine fo93fa7 single crystals up to 8 3 gpa and 1100 k by using a pulse heating method anisotropy of thermal conduction is investigated along the three crystallographic axes 1 0 0 0 1 0 and 0 0 1 in olivine the pressure dependence of or dln dp is determined to be 0 03 0 04 gpa 1 for the three crystallographic direction in olivine and 0 03 gpa 1 for garnet while dln dp is determined to be 0 04 gpa 1 for olivine and 0 03 gpa 1 for garnet the anisotropy in thermal diffusivity or thermal conductivity of olivine is clearly observed in the present experimental range of pressure and temperature it is likely that the anisotropy in thermal conduction would be maintained throughout the olivine stability field in the mantle down to 410 km depth heat capacities of olivine and garnet are calculated from the present and both the present heat capacities of olivine and garnet are consistent with the previously reported values within 5 those obtained for the three crystallographic direction in olivine are also consistent with each other within 6 further the effect of basalt eclogite transition on heat transportation is discussed based on the present results for garnet it implies that the eclogite layer of the slab transmits thermal energy more effectively than the upper basaltic layer

thermal transport properties of major archean rock types to high temperature and implications for cratonic geotherms abstract the thermal structure of archean terranes depends in part on the thermal transport properties of their distinctive rock types we determined thermal diffusivity d of a suite of 14 greenstone mafic granulite and tonalite trondhjemite granodiorite ttg rocks at temperatures up to 1000 c at atmospheric pressure using laser flash analysis which lacks systematic errors associated with conventional contact techniques at room temperature d ranges from 3 8 mm 2 s 1 for banded iron formation to 0 8 mm 2 s 1 for a quartz poor trondjhemite for all samples thermal diffusivity decreases with increasing temperature such that d for the suite converges around 0 7 0 1 mm 2 s 1 at 700 c combining these results with measured density and heat capacity calculated from modal mineralogy provides thermal conductivity as a function of temperature for each rock suite whereas near surface thermal conductivity varies from 2 7 to 4 2 w m 1 k 1 depending on rock type thermal conductivity of the lower crust is 2 w m 1 k 1 and only weakly dependent on lithology which is more thermally resistive than the uppermost mantle 3 5 w m 1 k 1 use of temperature dependent thermal transport properties in numerical models reveals that surface geotherms can be relatively insensitive to crustal heat production because high heat producing ttg rocks typically have high thermal diffusivity and conductivity at low temperatures and consequently are more efficient conductors of the heat they produce this finding implies that calculations of crustal heat production from surface borehole measurements may entail significant uncertainties

heat transfer during partial melting an experimental study of a simple binary silicate system abstract heat transfer during partial melting of a composition in a simple binary silicate system relevant to geological processes was investigated experimentally a 600 g cylindrical charge of nepheline 85 8 wt sodium disilicate 14 2 wt contained in a platinum crucible was brought to its solidus temperature 768 c the crystalline mass was then suddenly heated from below to an interface temperature of 1175 c the heat flux just below the bottom of the crucible measured with a heat flux gauge consisting of 750 differential thermocouples was time dependent at both the top and bottom of the charge and can be described by functions of the type f 1 e bt where b is a constant the composition and temperature range were selected so that the degree of melting under equilibrium conditions would be 40 at the base of the charge and 20 at the top the temperature profiles were recorded sequentially every 47 seconds by a vertical planar array of 54 thermocouples in the charge a quasi steady state thermal profile resulted after 2 5 hours the thermocouple array within the charge was effectively supported during the 5 day experiment because crystal bridging observed in independent experiments was within the einstein limit the observed thermal profiles were similar to those obtained independently on a refractory aluminosilicate 75 al 2 o 3 and 25 sio 2 in which no melting took place the enthalpy of melting of the sodium bearing system was therefore treated as an anomalous heat capacity the calculated stefan number c p t h is about 5 6 independent time studies revealed that more than several hours were required to achieve phase equilibrium hence the melting process is slower than that of heat transfer the melting process appears to be decoupled from the heating process so that melting takes place simultaneously throughout the silicate charge at various rates depending on the degree of superheat above the solidus temperature no adiabatic regions were observed in the thermal profiles hence it is concluded that convection did not occur through the porous media convection therefore may not be a principal factor in the generation of homogeneous magma at the site of origin but may be of importance after segregation in the absence of abrupt changes in the thermal profiles expected from the enthalpy of partial melting there was no evidence of a stefan boundary marking the progressive advance of a melt front the relatively high viscosity of the melt in the temperature range investigated 10 3 10 8 poises and the sluggish melting kinetics even for small m crystals may be the principal rate controlling factors in the partial melting process in silicate systems relevant to magma generation the results indicate that magma is generated throughout a mass at temperatures above its solidus where the rate of increase of the thermal gradient greatly exceeds the rate of melting permeability and porosity are established simultaneously in those parts of the mass at temperatures above the beginning of melting the advancement of melting in a granitic mass would be expected to be much slower than that in a basaltic mass with the same heat flux migmatization appears to be an incomplete rate determined process tending toward anatexis if the bulk compositions are at or near a minimum or eutectic like invariant point in applications to contact metamorphism the experiments suggest that the heating process is decoupled from the reaction process and therefore contact metamorphism will not be progressive each zone recording only that reaction taking place at the highest temperature sustained by the imposed thermal profile

rheological controls on the emplacement of extremely high grade ignimbrites extremely high grade lava like welded ignimbrites are produced by many large explosive eruptions with volumes typically 101 103 km3 however understanding of the physical properties of these unusual deposits and their transport and depositional mechanisms is incomplete the lava like and rheomorphic grey s landing ignimbrite idaho western united states provides abundant field evidence supporting the upward migration of a transient 2 m thick sub horizontal ductile shear zone at the interface between the pyroclastic density current and the deposit through which all of the aggrading pyroclastic material passed here we use a combination of rheological experiments and thermo mechanical modeling to test the syndepositional shear zone model we show that syndepositional welding and ductile flow are achievable within a very restricted field of likely temperature strain rate space where rapid deformation is favored by higher emplacement temperatures 850 c the field of ductile deformation is broadened significantly by accounting for strain heating which permits a sustained temperature increase of up to 250 c within the shear zone and helps to explain the enormous extents of lava like lithofacies and the intense rheomorphism recorded in extremely high grade ignimbrites recognition of strain heating within rheomorphic ignimbrites suggests that large pyroclastic density currents may travel over a hot substrate potentially hotter than the density current itself

extended cooling and viscous flow of large hot rhyolite lavas implications of numerical modeling results abstract a common misconception about rhyolite lava flows is that they cannot advance far from their vents and are constrained to be small due to their high viscosities although a high viscosity lava will indeed flow quite slowly it may advance a great distance if the lava efficiently retains heat and thus mobility and if the available magma supply is large to evaluate this possibility a finite difference numerical model was used to determine the rate of heat loss from units interpreted to be large rhyolite lavas cooling of such 100 300 m thick units is very slow and is further retarded by a vesicular carapace and by evolution of latent heat thermal history alone suggests that large thick lava flows could remain active for several decades to constrain the duration of flow advance of these units their inferred apparent viscosities are compared with those of active lava flows mean flow velocity of some large rhyolites in southwestern idaho usa was estimated based on the observation that the units must still have been advancing by viscous flow at the start of spherulite crystallization indicating perhaps a 10 to 18 year flow duration based on studies of obsidian dome california mean flow velocities would have been roughly 0 59 to 2 5 km yr and calculated average apparent viscosities of the flows ranged from 1 8 to 3 6 orders of magnitude greater than the viscosity of the lava itself this relationship also holds for active andesite and basaltic andesite block lava flows which have apparent viscosities 2 7 to 4 orders of magnitude greater than lava viscosities this implies that large rhyolites may indeed have been emplaced in the same manner as less silicic block lavas and that long distance viscous flow is not unrealistic for rhyolite lavas a good example of a large volume lava is the 15 km 3 rhyolite of the badlands sw idaho where vent relations show that lava effused from a fissure and advanced up to 9 km more extensive rhyolites could also be emplaced solely by effusion extended flow of these lavas may lead to the remelting and removal of basal crumble breccias and to the formation of welded shard textures in their margins both processes tending to obscure the lava flow origin of the units the great dimensions of lava like rhyolitic units with volumes of 10 to at least 200 km 3 are not a sufficient reason to assume they erupted as ignimbrites

evolution of the continental crust in the proterozoic eastern ghats belt india and new constraints for rodinia reconstruction implications from sm nd rb sr and pb pb isotopes abstract for this study nd sr and pb isotope compositions were analyzed for ortho and paragneisses from the eastern ghats belt of india in order to determine its crust formation and crustal evolution this belt represents a proterozoic orogen that extends along the east coast of peninsular india and forms part of the mobile belts in east gondwana and rodinia the eastern ghats belt was affected by mesoproterozoic granulite facies metamorphism in the western segment western charnockite zone and a grenvillian regional scale high grade event in the central and eastern segments western khondalite zone charnockite migmatite zone and eastern khondalite zone as well as a local pan african overprint the results of the isotope studies are used for the large scale reconstruction of the indo antarctic part of the rodinia supercontinent based on nd model ages and pb isotope ratios from leached feldspars four crustal domains can be distinguished in the eastern ghats belt these domains can in part be correlated with the lithological division of the belt 1 the western charnockite zone south of the godavari graben is characterized by nd model ages between 2 3 and 2 5 ga for orthogneisses and 2 6 and 2 8 ga for metasediments domain 1 the pb isotopes are primitive indicating reworking of dominantly archean and mixing with minor proterozoic material 2 north of godavari graben nd model ages for orthogneisses are significantly higher with values ranging from 3 2 to 3 9 ga the pb isotopes are strongly retarded 3 the north eastern parts of the charnockite migmatite zone and western khondalite zone form a distinct and almost homogeneous crustal domain domain 3 with nd model ages between 1 8 and 2 2 ga 4 between the isotopically homogeneous terranes stretches a broad transition zone domain 2 enclosing parts of the western khondalite zone charnockite migmatite zone and eastern khondalite zone the nd model ages for metasediments 2 1 2 5 ga are younger than paragneiss ages of the adjoining western charnockite zone the nd model ages for orthogneisses 1 8 3 2 ga display a large spread which is consistent with the pb isotope signatures that indicate mixing of archean with proterozoic material the border zone between domain 3 and the archean eastern indian craton forms a second transition zone domain 4 characterized by metasediments with nd model ages between 2 2 and 2 8 ga and orthogneisses with model ages around 3 2 ga reworking of archean crustal material is most intense along the border zones of the belt and juvenile material is more dominant away from the orogenic front this scenario is indicative of an active continental margin setting for the two proterozoic episodes of orogenesis in the eastern ghats belt a correlation of domain 3 with the rayner complex and the prydz bay region antarctica for the early crustal evolution is supported by the similarity of the isotope signatures the napier complex is very different to the eastern ghats belt and an early joint evolution of these terranes is ruled out on the basis of the pb pb and sm nd systematics

the north aegean region a tectonic paradox in the past two decades several publications have been presented concerning the recent and active fault geometry kinematics and geodynamics of the aegean region and particularly of the northern sector data and results are often contradictory and because of the complexity of the area most hypotheses and models should be considered carefully the right lateral movement of the north anatolia fault continues into some branches of the north aegean fault system there strike slip motion along ne sw trending faults coexists with dip slip e w trending faults in the frame of an extensional regime related to n s crustal stretching if we take into account the geodynamic environment of the region several mechanical problems arise to the east the aegean is compressed by the westward convergence of anatolia while to the south and west along the hellenic arc a hemiradial compression occurs due to subduction although the north anatolia north aegean trough fault system resembles a restraining bend the whole area is in fact affected by pure extension and local transtension along ne sw trending structures accordingly the major paradox of the area and especially in the western sector fault termination is the occurrence of extension where compression should regionally or at least locally predominate

the building blocks of continental crust evidence for a major change in the tectonic setting of continental growth at the end of the archean abstract oceanic arcs are commonly cited as primary building blocks of continents yet modern oceanic arcs are mostly subducted also lithosphere buoyancy considerations show that oceanic arcs even those with a felsic component should readily subduct with the exception of the arabian nubian orogen terranes in post archean accretionary orogens comprise 50 produced in continental arcs felsic igneous rocks in oceanic arcs are depleted in incompatible elements compared to average continental crust and to felsic igneous rocks from continental arcs they have lower th yb nb yb sr y and la yb ratios reflecting shallow mantle sources in which garnet did not exist in the restite during melting the bottom line of these geochemical differences is that post archean continental crust does not begin life in oceanic arcs on the other hand the remarkable similarity of incompatible element distributions in granitoids and felsic volcanics from continental arcs is consistent with continental crust being produced in continental arcs during the archean however oceanic arcs may have been thicker due to higher degrees of melting in the mantle and oceanic lithosphere would be more buoyant these arcs may have accreted to each other and to oceanic plateaus a process that eventually led to the production of archean continental crust after the archean oceanic crust was thinner due to cooling of the mantle and less melt production at ocean ridges hence oceanic lithosphere is more subductable widespread propagation of plate tectonics in the late archean may have led not only to rapid production of continental crust but to a change in the primary site of production of continental crust from accreted oceanic arcs and oceanic plateaus in the archean to primarily continental arcs thereafter

metallogeny of the north china craton link with secular changes in the evolving earth abstract the north china craton ncc has experienced a complex geological evolution since the early precambrian and carries important records of secular changes in tectonics and metallogeny here we synthesize the salient geological and tectonic features of the evolution and destruction of the ncc vis a vis major metallogenic events and the formation of potential ore deposits we identify a close relationship between the major geological events in the ncc and those reported elsewhere on the globe we trace the records of a regular change in the pattern of metallogeny mineral deposit character spatial distribution and genetic mechanisms which closely match the timing and styles of the major geological and tectonic events in this craton the ncc went through five major tectonic cycles 1 neoarchean crustal growth and stabilization 2 paleoproterozoic rifting subduction accretion collision with imprints of the great oxidation event goe 3 late paleoproterozoic neoproterozoic multi stage rifting 4 paleozoic orogenesis at the margins of the craton and 5 mesozoic extensional tectonics associated with lithospheric thinning and decratonization coinciding with these major geological events are five major metallogenic systems identified as follows 1 an archean bif system 2 paleoproterozoic cu pb zn and mg b systems 3 a mesoproterozoic ree fe pb zn system 4 a paleozoic orogenic cu mo system and 5 mesozoic intracontinental au and ag pb zn and mo systems the ore deposit types in each of these metallogenic systems show distinct characteristics and tectonic affinities from early precambrian through late precambrian to paleozoic and mesozoic the ncc records a transition from primitive to modern style plate tectonics evidence for imbricated oceanic plate stratigraphy in a subduction accretion setting and collisional orogenesis along at least three major zones of ocean closure are documented major transitions in tectonic style and surface environmental changes recorded in other parts of the world are also reflected in the geological history and metallogenic events in the ncc large scale gold deposits formed through intraplate tectonics during the mesozoic provide important insights into mantle dynamics and crust mantle interaction associated with lithospheric thinning and craton destruction the ncc provides one of the best examples for documenting secular changes in the geological history and metallogenic epochs of an evolving earth

collision tectonics in the late precambrian eastern ghats mobile belt mesoscopic to satellite scale structural observations regional mapping of a section across the eastern ghats mobile belt egmb north of the godavari graben in eastern peninsular india by using landsat thematic mapper data enables recognition of a number of shear zones lineaments and structural domes and basins a conspicuous megashear occurs at the western boundary of the granulite facies rocks of the egmb adjacent to the archean granite greenstone craton the confinement of a suite of alkaline igneous rocks to this shear zone is a notable feature the strike extensions of this shear belt extend through to the elchuru alkaline complex prakasam district andhra pradesh and the syenite plutons of koraput district orissa the contrasting lithologies metamorphism and structural history on either side of the shear zone suggests that it might be a precambrian suture zone the mesoscopic structural features in the egmb include prominent foliation with moderate to steep dips folds faults shears s c fabrics pinch and swell structures and other linear fabric elements these observations favour the consideration of drastic crustal shortening and thickening and a complex deformational sequence the major rock units in this part of egmb comprise garnetiferous sillimanite gneisses quartzites and calc granulites forming the khondalitic suite of rocks and a wide variety of charnockitic rocks the contact of the two rock units is generally sheared and often migmatised the structural fabric suggests two major tectonic events an essentially horizontal tectonic regime resulting in thrust systems and associated structures subsequently followed by strike slip tectonics characterized by high shear strains features such as westward verging thrusts large scale recumbent folds major shear zones structural domes and basins indications of tectonic crustal shortening extensive calc alkali magmatism and widespread migmatization in the region are attributed to collisional processes during proterozoic times the spatial disposition of the egmb and its linkage with the distribution of similar rock units during the late precambrian time in a global tectonic scenario are discussed

groundmass crystallisation and cooling rates of lava like ignimbrites the grey s landing ignimbrite southern idaho usa constraining magmatic and eruptive processes is key to understanding how volcanoes operate however reconstructing eruptive and pre eruptive processes requires the ability to see through any post eruptive modification of the deposit the well preserved grey s landing ignimbrite from the central snake river plain provides an opportunity to systematically investigate the post eruptive processes occurring through a single deposit sheet despite overall compositional homogeneity in both bulk and glass compositions the grey s landing ignimbrite does preserve differences in the abundance of li in plagioclase crystals which are strongly associated with the host lithology li abundances in plagioclase from the quickly cooled upper and basal vitrophyres are typically low average 5 ppm n 262 while plagioclase from the microcrystalline interior of the deposit has higher li contents average 33 ppm n 773 given that no other trace elemental parameter in plagioclase varies we interpret the variability in li to reflect a post depositional process groundmass crystallisation of a rhyolite like grey s landing requires 50 crystallisation of sanidine and variable amounts of a silica rich phase quartz tridymite cristobalite and plagioclase to satisfy mass balance we suggest the low affinity of li for sanidine causes migration of groundmass li into plagioclase during crystallisation even within the microcrystalline interior of the deposit the morphology of the groundmass varies the more marginal finer grained regions are dominated by cristobalite as the sio2 rich phase while tridymite and quartz are additionally found in the more slowly cooled coarser grained portions of thick sections of the ignimbrite numerical models of cooling and crystallisation tested against field observations indicate that the groundmass crystallisation occurred relatively rapidly following emplacement a maximum of a few years where the ignimbrite is thickest these numerical models also illustrate that the time at which the potential for rheomorphism ceases either the ignimbrite is a solid glass or 50 crystalline is diachronous across the deposit

glass forming ability and crystallisation behaviour of sub alkaline silicate melts abstract the intrinsic solidification behaviour and glass forming ability gfa of natural sub alkaline silicate melts have been experimentally quantified via cooling induced solidification approach gfa is measured by the critical cooling rate rc the rate at which a melt solidifies 2 area of crystals cooling rates of 9000 1800 180 60 7 and 1 c h have been run between 1300 c super liquidus region and 800 c quenching temperature at air f o 2 and ambient p for six silicate melts with compositions ranging from basalt b to rhyolite r i e b 100 b 80 r 20 b 60 r 40 b 40 r 60 b 20 r 80 and r 100 and water contents comprised between 53 b 100 and 384 b 20 r 80 ppm the ranges of cooling rates and chemical compositions used in this study are the broadest ever investigated in the earth sciences the phase proportions area were determined by image analysis on about 500 back scattered electron images collected over different magnifications phases are glass clinopyroxene cpx spinel sp and plagioclase plg sp is ubiquitous with abundance of few area and nucleates earlier than silicate crystals cpx solidifies in all runs except in r 100 and its abundance follows asymmetric broad gaussian like trends with tails towards low rates as a function of cooling rate moving from b 100 to b 40 r 60 these trends conserve their shape but shift progressively to lower cooling rates and mineral abundances plg crystallises only at low cooling rates and in sio 2 poor compositions run products with low amounts of crystals 5 area clearly show that cpx preferentially nucleates on surfaces of sp whereas a significant crystallisation of cpx 5 area is observed with decreasing cooling rate and with changing composition from b 100 to b 20 r 80 the crystallisation of silicate crystals is related to the chemical diffusivity of components in the melt also the initial crystallisation of plg occurs preferentially on cpx in general the amount of crystals decreases as the cooling rate increases however in some cases the amount of crystals remains constant or even decreases for b 80 r 20 with decreasing cooling rate rc values change over 5 orders of magnitude being 100 b 20 r 80 b 40 r 60 b 60 r 40 and b 80 r 20 and b 100 respectively the variation of rc can be modelled through nbo t nonbridging oxygen per tetrahedron parameter by the following equation rc a 1 e nbo t b c where a b and c are fitting parameters equal to 9214 0 297 and 0 040 respectively similarly to other glass forming liquids network metallic and molecular systems rc for natural sub alkaline silicate melts is inversely related to the reduced glass transition parameter trg trg tg tm and can be quantified with the equation rc a trg b where a and b are 1 19 10 4 and 28 7 respectively these results may be used to retrieve the solidification conditions of aphyric degassed and oxidised lavas in addition our data provide general constrains on the crystallisation kinetics of natural crystal bearing silicate melts erupted on earth e g lavas with phenocrysts the relationship between crystal content and cooling rate suggests that the solidification path induced by degassing can be also complex and nonlinear the growth of crystals with size up to 1 mm from a nearly anhydrous superheated silicate melt indicates that variable cooling conditions of lavas have to be accounted to discriminate among minerals formed before during and after eruptions moreover our results can be used to design glass ceramics from naturally available easy to find low cost starting materials

thermal diffusivity of igneous rocks at elevated pressure and temperature thermal diffusivity measurements of seven igneous rocks were made to temperatures of 400 sup 0 c and pressures of 200 mpa the measuring method was based on the concept of cylindrical symmetry and periodic heat pulses the seven rocks measured were westerly rhode island granite climax stock nevada quartz monzonite pomona washington basalt atikokan ontario canada granite creighton ontario canada gabbro east bull lake ontario canada gabbro and stripa sweden granite the diffusivity of all the rocks showed a positive linear dependence on inverse temperature and excluding the east bull lake gabbro showed a linear dependence on quartz content quartz content varied from 0 to 31 by volume diffusivity in all cases rose or remained steady with increasing confining pressure the pressure effect was strongest at lowest pressures and vanished by levels between 10 and 100 mpa depending on rock type the pressure effect measured as a percentage change in diffusivity is stronger in the four rocks of granite composition than in the three of basaltic composition our results agree well with existing thermal diffusivity measurements at atmospheric pressure

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method