prolonged residence times for the youngest rhyolites associated with long valley caldera 230th 238u ion microprobe dating of young zircons abstract we describe a new ion microprobe method for dating magmatic zircon growth that is based on in situ measurement of the magnitude of238u 230th disequilibrium our results support independent inferences that zircon can remain suspended for long periods 100 ka in the convecting potions of the magma from which it crystallizes because the crystallization ages date when the magma cooled to its zircon saturation temperature even when the zircons have long magmatic residence times 238u 230th zircon dating can be used to constrain the thermochemical evolution of silicic magmas 238u 230th ages have been determined for individual zircons from rhyolites associated with the long valley magmatic system of eastern california the samples are from deer mountain an 115 3 ka low silica moat rhyolite and from the coarsely porphyritic low silica rhyolite of south deadman dome one of the 0 6 ka inyo domes previous investigations have suggested that the two lavas were derived from the same magma reservoir a few of the zircon model ages calculated with respect to the isotopic characteristics of the whole rocks are within error of that for eruption of the deer mountain rhyolite however the majority of zircons from both lavas cluster around an age of 230 ka this common interval of zircon nucleation and growth for petrologically similar lavas suggests that the younger inyo dome lava may have tapped the same magma body from which the deer mountain rhyolite erupted more than 100 ka before on the other hand most of the zircon model ages are younger than previous episodes of silicic volcanism in the long valley caldera suggesting that the rhyolites may have been generated during development of a silicic upper crustal magma chamber in the western portion of long valley caldera zircon saturation temperatures for the rhyolites studied 795 810 c are the same as those obtained from coexisting fe ti oxides 809 4 c showing that the magma cooled to

coupled mass transport and kinetically limited isotope exchange applications and exchange mechanisms recent studies provide theoretical models for kinetically limited isotope exchange during fluid flow however few have attempted to apply these models to natural systems in this paper we interpret stable isotopic data from fossil hydrothermal systems in two different tectonic settings by using combined equations of mass transport and kinetically limited isotope exchange our models provide alternative interpretations for the geometry of flow responsible for the development of 18 o depletion annuli surrounding epizonal intrusions and for the geometry of flow in shallow level normal faults our models also provide information about relative isotope exchange rates and by extension mechanisms of isotope exchange our results indicate that dynamic recrystallization in fault zones promotes exchange by surface reaction whereas exchange in chemically nonreactive and undeformed minerals may be limited by solid state diffusion

partial transformation of gabbro to coesite bearing eclogite from yangkou the sulu terrane eastern china an ultra high pressure uhp metamorphic slab at yangkou beach near qingdao in the sulu region of china consists of blocks of eclogite facies metagabbro metagranitoid ultramafic rock and mylonitic orthogneisses enclosed in granitic gneiss a gradational sequence from incipiently metamorphosed gabbro to completely recrystallized coesite eclogite formed at ultra high pressures was identified in a single 30 m block metagabbro is preserved in the core whereas coesite eclogite occurs along the block margins the metagabbro contains an igneous assemblage of pl aug opx qtz bt ilm ti mag it shows relict magmatic textures and reaction coronas fine grained garnet developed along boundaries between plagioclase and other phases primary plagioclase broke down to ab ky ms zo grt amp augite is rimmed by sodic augite or omphacite whereas orthopyroxene is rimmed by a corona of cum act and omp qtz layers or only omp qtz in transitional rocks augite and orthopyroxene are totally replaced by omphacite and the lower pressure assemblage ab ky phn zo grt coexists with domains of omp jd70 73 ky phn in pseudomorphs after plagioclase both massive and weakly deformed coesite bearing eclogites contain omp ky grt phn coe qtz rt and preserve a faint gabbroic texture coesite inclusions in garnet and omphacite exhibit limited conversion to palisade quartz some intergranular coesite and quartz pseudomorphs after coesite also occur assemblages of the coronal stage transitional and uhp peak occurred at about 540 50 c at c 13 kbar 600 800 c at 15 25 kbar and 800 850 c at 30 kbar respectively garnet from the coronal through the transitional to the eclogite stage rocks show a decrease in almandine and an increase in grossular pyrope components garnet in low grade rocks contains higher mno and lower pyrope components the growth textures of garnet within pseudomorphs after plagioclase or along grain boundaries between plagioclase and other phases are complex the application of garnet zoning to estimate p t should be carried out with caution some garnet enclosing quartz aggregates as inclusions shows radial growth boundaries these quartz aggregates as well as other primary and low p phases persisted metastably at uhp conditions due to sluggish reactions resulting from the lack of fluid during prograde and retrograde p t evolution

ion microprobe evidence for the mechanisms of stable isotope retrogression in high grade metamorphic rocks retrograde interdiffusion is widely proposed as the dominant factor in producing the stable isotopic fractionation among minerals in slowly cooled igneous and metamorphic rocks mineral zonation consistent with interdiffusion of stable isotopes has never been directly observed however leaving doubt as to the mechanism responsible for the bulk mineral isotopic compositions commonly measured ion microprobe analyses of oxygen isotope ratios in magnetite were combined with conventional bulk mineral analyses and diffusion modeling to document the relationship between mineral zonation and the mechanism of retrogression inferred from bulk mineral data two samples of magnetitebearing quartzo feldspathic lyon mountain gneiss from the adirondack mountains n y were studied in detail conventional stable isotope analysis of both samples indicates that isotope thermometers are discordant and were reset by as much as 200 c from the estimated peak temperature of 750 c the relative order of apparent temperatures recorded by various thermometers differs between the two samples however with t qtz fsp t mt qtz and t mt fsp in one sample and t qtz fsp t mt qtz and t mt fsp in the other diffusion modeling using the fast grain boundary model shows that the former pattern of apparent temperatures is consistent with closed system interdiffusion during cooling whereas the latter is not the modeling predicts that 0 5 mm diameter magnetite grains common to this rock type will contain isotopic zonation of 1 rims lower in 18 o than cores and that the cores of smaller 0 1 mm grains will be similarly lower than to the cores of large 0 5 mm grains ion microprobe analysis reveals that the zoning patterns of magnetite grains from the first sample contain clear core to rim zonation in multiple grains core rim 1 1 0 4 and predicted grain size vs core composition variations consistent with diffusion controlled resetting of bulk mineral fractionations in contrast the second sample shows irregular inter and intra granular variations over an 8 range consistent with open system alteration these results provide direct documentation of the importance of interdiffusion in affecting stable isotope distributions in slowly cooled rocks the correlations of bulk mineral resetting with zonation show that bulk mineral data when interpreted with detailed modeling can be used to determinate what processes controlling retrogression

constraints on magma ascent emplacement and eruption geochemical and mineralogical data from drill core samples at obsidian dome inyo chain california systematic chemical and mineralogical variability occurs in samples from drill holes through obsidian dome the conduit to the dome and a nearby associated feeder dike the drill hole samples from the margins of the conduit and most of the lower part of the dome are high ba low silica rhyolites they contain two populations of phenocrysts and represent commingled magmas whereas samples from the dike and upper parts of the dome are low ba higher silica rhyolites that do not reflect commingled magmas samples from the center of the conduit are low ba higher silica rhyolites that are only slightly mixed a major part of the variability within the drill core samples of the dome and conduit reflects the juxtaposition and commingling of two distinct magmas during their passage through the conduit

constancy of silicate mineral weathering rate ratios between natural and experimental weathering implications for hydrologic control of differences in absolute rates abstract inverse models which apportion watershed effluxes geochemical mass balance over estimated mineral surface area give weathering rates for individual silicate minerals e g feldspar 1 3 orders of magnitude slower than laboratory rates comparisons of rates calculated from several recent watershed geochemical mass balance studies with experimentally determined rates reveal that the ratio of mineral weathering rates for a given silicate mineral pair determined in an internally consistent field study equals the ratio of rates for the same minerals in any internally consistent set of laboratory experimental data thus the coefficient which corrects the weathering rate for any mineral from a laboratory data set to a field setting is identical to the corresponding coefficient for any other mineral in the same data sets the correction factor linking experimental and natural rates must be independent of composition physical e g hydrologic controls rather than compositional or chemical controls cause the difference between weathering rates in nature and the laboratory because flow in natural weathering profiles is spatially heterogeneous not all of the potentially available surface in natural systems actually participates in reactions with pore fluids

high precision microsampling and preparation of zoned garnet porphyroblasts for sm nd geochronology abstract zoned garnet porphyroblasts contain one of the longest continuous records of tectonic processes with growth histories spanning many millions of years well known challenges exist in the physical preparation purification and geochronologic analysis of zoned garnets which have limited the progress and application of this potentially powerful geochronologic tool here we present details of an integrated methodology specifically for the acquisition of high resolution sm nd geochronologic records from zoned garnets that overcome these challenges we develop and demonstrate our methods on large test garnets from a shear zone in the tauern window in western austria chemically contoured microdrilling permits physical sampling of many discrete age annuli each as narrow as 100 m within a single garnet refined hno 3 hf hclo 4 partial dissolution methods cleanse coarse crushed 75 150 m size garnet annuli of detrimental inclusions yielding high 147 sm 144 nd ratios 0 9 to 4 6 and consequently accurate and precise ages experiments on powdered garnet 147 sm 144 nd 143 nd 144 nd with 10 20 ppm 2 rsd precision good enough for 147 sm 144 nd 1 0 in total this integrated sampling and analytical protocol permits the acquisition of 3 11 discrete high precision

the metamorphic aureole of the nisa alburquerque batholith sw iberia implications for deep structure and emplacement mode the nisa alburquerque granitic batholith southern variscan belt iberian peninsula has been studied by petrological structural and geophysical approaches obtaining contrasting models for its deep structure and emplacement sequence in order to test these models and gain knowledge on the thermal increase induced by the intrusion we have studied its contact aureole which was developed in similar country rock lithologies mica schists alternating with metasandstones and feldespatic schists all along the northern external contact of the batholith our results indicate no change in metamorphic grade and some variations in aureole width which narrows toward the western sectors of the batholith cordierite is the only contact metamorphic mineral developed together with a high temperature biotite probably related to the granite thermal input by considering these new data together with zircon saturation temperatures within the granite and previous petrological and geophysical studies we propose a model in which the feeder zones of the granitic magmas were an eastern main one and a western secondary one we have also made comparisons of the metamorphic grade in the country rocks and the xenoliths within the granite most of the xenoliths have the same metamorphic facies as the country rocks crd zone though some of them contain slightly different assemblages and crd which could be explained in different ways 1 differences in the primary schist compositions 2 increased time span of xenoliths in contact with the melt and 3 xenolith incorporation at slightly higher depths during final granite ascent

degassing of rhyolitic magma during ascent and emplacement the degassing history of a rhyolitic igneous system was documented from analyses of drill core samples through the extrusive and intrusive portions of obsidian dome and of surface samples of associated tephra the initial volatile composition of the inyo magma was estimated to be 4 0 wt h2o 500 ppm f 800 ppm cl and 80 ppm s retained volatile contents of glassy and crystalline samples reflect the effects of decompression and second boiling decompression is rapid and involves loss of water rich fluid until a close approach to lithostatic equilibrium is achieved second boiling is a slower process and produces a chlorine rich fluid some of which can be trapped during development of extremely fine crystallization textures nearly complete dewatering during decompression of surface extruded magma strongly undercools the system t 175 c suppressing crystallization and yielding glassy rhyolitic lava partial degassing of shallowly intruded magma permits pervasive crystallization even at high cooling rates the subvolcanic intrusive regime is the zone of maximum volatile release because second boiling is incomplete in extrusives and volatile bearing crystalline phases are stable in magma crystallized at greater depth

textural development of mixed finely porphyritic silicic volcanic rocks inyo domes eastern california magma mixing in silicic volcanic rocks of the inyo chain is manifest by bimodal mineral chemistry and distinctive banding defined by either variations in 1 glass color and chemistry or 2 microlite abundance within colorless glass bands composed of brown or microlite rich colorless glass are characterized by more mg rich mafic silicates and more calcic plagioclase than microlite poor colorless glass domains a rhyolitic and a dacitic mixing end member can be defined on the basis of this mineral distribution thermobarometry using the mineral assemblages unique to each end member indicates that prior to mixing the dacite was more oxidized and water rich than the rhyolitic magma determined mineral crystallization conditions compare favorably with experimentally derived phase equilibria for melts of broadly similar composition the spatial association of contrasting mineral assemblages with the flow banding in these rocks implies that the banding is a direct product of magma mixing it formed as either 1 volumes of the comingled magmas were immediately vented to the surface and quenched preserving domains of two distinct glass compositions or 2 bands of dacitic magma precipitated microlites when comingled with the rhyolitic magma in an attempt for the two magmas to approach equilibrium

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

morphology of intergranular pores and wetting angles in pelitic schists studied by transmission electron microscopy in pelitic schists composed mainly of quartz and albite grains the morphology of intergranular pores which were filled with water was studied by transmission electron microscopy tem although some pores are defined by crystallographic planes f face most of their form has an ideal shape determined by interface tensions between grains and fluid high resolution tem observations demonstrate that pore free regions at grain boundaries are tight even at the nanometer scale showing that the wetting angle is larger than 0 in this rock the pore distribution in two grain junctions can be compared to a necklace microstructure developed by instability of a fluid film along the boundary induced by microcracking wetting angles for pores located at grain edges of quartz and albite decrease in the order albite albite quartz quartz and quartz albite the quartz quartz wetting angle in a calcite free sample is smaller than that in a calcite containing sample this angle also changes due to grain misorientation our results confirm that solid solid and solid fluid interfacial energies control the geometry of intergranular fluid in natural rocks

a new monthly pressure dataset poleward of 60 s since 1957 abstractthis study presents a new monthly pressure dataset poleward of 60 s from 1957 to 2016 based on a kriging interpolation from observed pressure anomalies across the antarctic continent ove

fluid flux and reaction rate from advective diffusive carbonation of mafic sill margins in the dalradian southwest scottish highlands abstract greenschist facies mafic sills in the dalradian of the southwest highlands scotland have been carbonated by infiltration of a co 2 bearing hydrous fluid from adjacent calc phyllites the primary amphibole epidote bearing assemblage is preserved in the cores of many sills in which the margins were altered to calcite chlorite and quartz the asymmetric widths of the carbonated margins allow determination of flow direction and magnitude and indicate that sill margins were buffered to nearly constant fluid composition by copious flow in the more permeable phyllites partially carbonated amphibole epi dote assemblages within the reaction fronts preserve evidence for sluggish reaction kinetics downstream margins developed by diffusion against the flow direction and this allows calculation of reaction front broadening due to both diffusion and reaction kinetics an advective diffusive transport model with linear reaction kinetics has been fitted to the reaction progress profile for a sill at port cill maluaig knapdale this implies a cross layer time integrated fluid flux of 62 1 1 3 m 3 m 2 a damkohler number of 22 4 4 2 and a peclet number of 66 2 20 3 1 errors the peclet number and time integrated fluid flux imply that the flow event lasted between 0 02 and 20 ma for plausible porosities in the range 10 3 to 10 6 the inferred rate constant for the reaction kinetics is two or more orders of magnitude slower than that calculated by extrapolation of experimentally determined surface reaction rates either the kinetic dispersion arose from factors additional to reaction kinetics or fluid solid reaction was controlled by a slower mechamism such as diffusion away from flow channels the latter conclusion implicates deformation as an important control on the rates of fluid infiltration and fluid rock reaction

a comprehensive study of pumice formation and dispersal the cretaio tephra of ischia italy the cretaio tephra is a small volume 0 02 km3 pumice deposit on the island of ischia in italy it was erupted during historical time and is associated with a tuff ring dome complex extruded along a n s trending normal fault system bordering the east side of the mt epomeo resurgent block 14c ages of the deposit average 1860 a b p which is in agreement with the potsherds found in paleosols underlying and overlying the deposit the eruption sequence consists of 7 members most of these members are pumice fall deposits one is an ash and pumice fallout and another is a surge bed the tephra have a trachytic phonolitic composition all chemical characteristics including major and trace elements and sr isotope composition are homogeneous throughout the sequence the vesicularity of pumice clasts does not show any systematic change the erupted magma had the same amount of volatiles the same flux and vesicle growth behavior throughout the eruption sequence grain size analyses of 90 tephra samples representative of all the members at variable distance from the supposed vent have been processed using the sequential fragmentation transport theory in order to define the effects of eruptive fragmentation and subsequent transport on the particle sizes distribution four significant subpopulations have been identified and related to transport mechanisms

white mica 40ar 39ar age spectra and the timing of multiple episodes of high p metamorphic mineral growth in the cycladic eclogite blueschist belt syros aegean sea greece research support providedby arc discovery projects dp0877274 anddp1201035 54 forster ackno wledges the support ofan australian research council arc australianresearch fellowship

kinetics of diffusion controlled mineral growth in the christmas mountains texas contact aureole chert nodules in limestone in the aureole of the christmas mountains gabbro are rimmed by wollastonite in the interval 102 25 m from the intrusive contact and by tilleyite or spurrite and wollastonite within 25 m of gabbro wollastonite rims thicken from 4 5 mm at 102 m to 7 mm at 80 m to 30 mm at 49 m and diminish in thickness from 79 to 53 mm over the interval 13 2 m from the gabbro wollastonite has two textural components polygonal matrix wollastonite grains coarsen from 0 011 mm at 102 m to 0 053 mm at 46 m to 0 2 0 3 mm within 20 m of the contact whereas scattered wollastonite porphyroblasts appear at 30 m and coarsen to impingement with a diameter of 1 07 mm at 12 m kinetic models for normal grain growth and for layer thickening set a function of the square of the grain diameter or layer thickness equal to the product of a material constant and a temperature time integral that includes the arrhenius function for the diffusion coefficient the time index for non isothermal diffusion controlled mineral growth is the temperature time integral numerically evaluated along t t curves for the thermal history of the contact aureole the thermal history of the christmas mountains aureole is obtained using a numerical model for a cylindrical intrusion radius 823 m that matches maximum temperatures in the contact aureole of 600 940 1000 and 1030 c at 115 23 13 and 0 m by convection for 700 yr followed by crystallization and cooling the log of the t t integral varies linearly with distance from the gabbro with a slope proportional to activation energy and an intercept proportional to the material constant a plot of the log of the square of the grain diameter of matrix wollastonite against distance from the gabbro is linear and the normal grain growth model yields the following arrhenius function for the diffusion of oxygen in wollastonite grain boundaries d gb o 2 9 333 x 10 4 m s exp 185 rt kj mole where is grain boundary width the fixed ratio of tilleyite to wollastonite rim thickness on nodules from the inner aureole yields a molar ratio of tilleyite to wollastonite in c t w q structures of 1t 24w and requires consumption of calcite and quartz in the ratio 29c 26q truncation of fully coarsened impingement wollastonite by columnar tilleyite implies that the tilleyite layer grew at expense of wollastonite and that wollastonite coarsened at temperatures greater than 940 c solution of a system of mass balance conservation and flux ratio equations describing diffusion controlled growth of a c t w q nodule matches the net mass transfer and observed textural features for onsager diffusion coefficient ratios of l cao l sio 2 42 and l cao l co 2 grain diameter of impingement wollastonite varies linearly with the radius of spherical wollastonite nodules that ceased growth on consumption of the quartz core implying that coarsening coincided in time with wollastonite rim growth grain boundary cross section of wollastonite decreased by two orders of magnitude during layer growth coupling of the kinetics of coarsening of impingement wollastonite with diffusion controlled growth of the wollastonite layer yields the following arrhenius function for the onsager diffusion coefficient for diffusion of cao in wollastonite grain boundaries l gb cao 1 731 x 10 4 rt mole 2 j s exp 220 rt kj mole the pre exponential term for grain boundary diffusion of sio 2 in wollastonite is obtained from the relation l cao l sio 2 42 values of the diffusion coefficients derived from analysis of grain coarsening and layer growth in the christmas mountains contact aureole satisfy the diffusion compensation relations for grain boundary diffusion of oxygen and cations in oxides based on laboratory experiment the value of the coefficient for diffusion of cao in tilleyite grain boundaries at 1000 c based on a time integrated aver age temperature is essentially identical to that obtained for wollastonite using the full non isothermal analysis suggesting that the values of onsager coefficients for grain boundary diffusion of a given cation are not strongly dependent on identity of solid phase wollastonite rims grow to 82 to 95 of their ultimate thickness during the period of heating that coincides with magmatic convection because growth of the wollastonite and tilleyite layers does not begin until temperature rises to 600 and 940 c respectively rim growth is not synchronous across the aureole

monitoring of helium and carbon isotopes in the western eger rift area czech republic relationships with the 2014 seismic activity and indications for recent 2000 2016 magmatic unrest abstract we report new data of the regional distribution pattern of total gas compositions as well as he and co2 isotopic compositions from 25 gas exhalations in the western eger rift and its surroundings additionally the first time series data from gas exhalations in a clay pit within the cheb basin cb are given at 21 degassing locations the first data were obtained 20 years ago from 7 locations within the degassing center cb and from 3 degassing sites belonging to the marianske lazn ml degassing center neon and argon isotope compositions were determined also co2 is the major component at all degassing sites the 13c values display a small range 1 7 to 5 1 and the 3he 4he ratios vary from 1 9 to 5 9 ra the highest 3he 4he ratios are found at locations along the pocatky plesna fault zone followed by the degassing site in the clay pit on the nova ves fault and the locations on the ml fault at the edge of the cb although gas flow and co2 concentrations in all degassing centers are very high the fractions of mantle derived helium are different with presently up to 94 in relation to the sclm 3he 4he of 6 32 ra in the cb up to 73 in the ml and up to 35 in the karlovy vary degassing center at the locations in the eastern part of the cb a clear progressive increase of the 3he 4he ratio has been observed since the first sampling campaigns there in 1993 and 1994 whereas at the other degassing sites the helium isotope ratio remained essentially the same the progressive increase of the 3he 4he ratio in the eastern part of the cb together with further short time increases up to 6 3 ra at one location bublak before both the 2000 and 2008 earthquake swarms indicate an ongoing magmatic process beneath this area which seems to be associated with the occurrence of seismicity the cb is located close to the nov kostel focal zone where since the beginning of our investigations four strong periods of seismicity with magnitudes 3 occurred the latest gas data confirm our earlier findings time series studies showed that in relation with seismic events decreased 3he 4he ratios were repeatedly observed due to admixed seismically released crustal helium presently the eastern part of the cb is the most active non volcanic region in the european cenozoic rift system with gas signatures similar to those found in free mantle derived gases from the east african rift system

reactive dispersion of stable isotopes by mineral reaction during metamorphism abstract the process of reactive dispersion is introduced whereby reaction in one chemical system imparts hydrodynamic dispersion to another apparently unrelated reactive transport system reactive dispersion is illustrated with a one dimensional finite difference model for coupled heat flow fluid flow mineral reaction and oxygen isotopic exchange during contact metamorphism of siliceous dolomite infiltration driven decarbonation reactions impart isotopic heterogeneity to an initially homogeneous system the effect on fluid composition is explored using a breakthrough curve that tracks fluid composition through time the corresponding effects on rock composition are illustrated with plots of isotopic alteration vs distance reactive dispersion is especially efficient during infiltration of h 2 o into carbonate rocks because 18 o is strongly fractionated between co 2 and h 2 o and because mineral reactions may buffer the co 2 h 2 o content of fluid the spatially heterogeneous isotopic alteration predicted to develop during decarbonation reactions in a homogeneous protolith is similar in magnitude and distribution to that observed downstream of the isotopic depletion front in contact metamorphic aureoles

oxygen isotope equilibrium between eclogite minerals and its constraints on mineral sm nd chronometer abstract sm nd and oxygen isotope analyses were carried out for mineral separates of ultrahigh pressure eclogites from the sulu terrane in eastern china the results show a direct correspondence in equilibrium or disequilibrium state between the oxygen and sm nd isotope systems of eclogite minerals the omphacite garnet pairs of oxygen isotope equilibrium at eclogite facies conditions yield meaningful triassic sm nd isochron ages whereas those of oxygen isotope disequilibrium give non triassic ages of geological meaninglessness this can be reasonably interpreted by the fact that the rates of oxygen diffusion in garnet and pyroxene are lower than or close to those of nd diffusion and thus attainment of isotopic equilibrium in the omphacite garnet o system suggests achievement of nd isotope equilibrium in the same mineral pairs the presence or absence of fluid in the eclogite protoliths is a major rate controlling factor for isotopic equilibration during high grade metamorphism it appears that the state of oxygen isotope equilibrium between cogenetic minerals can provide a critical test for the validity of the sm nd mineral chronometer in addition the exact timing of the ultrahigh pressure metamorphism in the dabie sulu terranes is constrained at early triassic rather than late triassic

timescales and mechanisms of fluid infiltration in a marble an ion microprobe study using a recently developed ion microprobe technique a detailed oxygen isotope map of calcite grains in a coarse grained marble has been constructed supported by trace element mn sr fe analysis and cathodoluminescence cl imaging in order to constrain scales of oxygen isotope equilibrium timescales and mechanisms of metamorphic fluid infiltration and fluid sources and pathways results are compared with a previous study of this sample wada 1988 carried out using a cryo microtome technique and conventional oxygen isotope analysis the marble from the high temperature low pressure hida metamorphic belt in north central japan underwent granulite facies followed by amphibolite facies metamorphic events the latter associated with regional granite intrusion the cl imaging indicates two types of calcite a yellow luminescing ylc and a purple luminescing plc variety the ylc which occupies grain boundaries fractures replacement patches and most of the abundant deformation twin lamellae post dates the dominant plc calcite and maps out fluid pathways systematic relationships were established between oxygen isotope and trace element composition calcite type and texture based on 74 18o 16o and 17 trace element analyses with 20 30 m spatial resolution the ylc is enriched in mn and fe and depleted in 18o and sr compared to plc and is much more 18o depleted than is indicated from conventional analyses results are interpreted to indicate infiltration of 18o depleted metamorphic or magmatic fluid initial 18o 9 10 5 along grain boundaries fractures and deformation twin lamellae depleting calcite grains in sr and enriching them in mn and fe the sample is characterised by gross isotopic and elemental disequilibrium with important implications for the application of chromatographic theory to constrain fluid fluxes in metacarbonate rocks

modeling metamorphic fluid flow with reaction compaction permeability feedbacks existing models of metasomatic flow do not allow for the effect that reaction has on the flow patterns instead it is assumed that the volatiles produced are negligible in volume compared to those infiltrated and that reaction does not modify permeability this is clearly unlikely to be true for infiltration driven decarbonation reactions the rates of porosity creation by reaction and porosity loss by creep have been calculated for a representative volume of calcite quartz wollastonite marble and it is found that even for a weak calcite matrix the rate of porosity generation by reaction is likely to outstrip the collapse of porosity as long as the system is out of equilibrium we have applied a self consistent 1d finite difference model to the reaction of calcite quartz to wollastonite in a 10m thick marble in response to influx of h2o rich fluid with fixed boundary conditions the model allows us to evaluate the effect of reaction on the porosity structure and fluid pressure variation across the layer from which local darcy fluxes can be evaluated the progress of reaction that we model is constrained by hydrological considerations with the requisite parameters recalculated as reaction progresses assuming creep compaction of rock under the stress difference between lithostatic and fluid pressures we fnd that the volume of fluid realised by decarbonation driven by influx of h20 is sufficient to create a back flow so that further advancement of the reaction front is only possible as a result of diffusion of water against the darcy flux the effect of creep driven by differences between fluid pressure and lithostatic pressure is to reduce the permeability of the layer and especially reduce the secondary porosity developed in the zone at and behind the advancing reaction front we predict that in a 3d situation the porous zone of reacted marble becomes a conduit for layer parallel flow and the secondary porosity is infilled by calc silicate minerals due to silica metasomatism

geochronology of the larderello geothermal field new data and the closure temperature issue the larderello geothermal field is generally accepted to have been produced by a granite intrusion at 4 9 km depth hydrothermal parageneses and fluid inclusions always formed at temperatures greater than or equal to the current ones which implies that the field has always undergone a roughly monotonic cooling history fluctuations 40 k since intrusion of the granite at 4 ma the heat required to maintain the thermal anomaly over such a long period is supplied by a seismically anomalous body of 32000 km3 rooted in the mantle borehole minerals from larderello are thus a unique well calibrated natural example of thermally induced ar and sr loss under geological conditions and time spans the observations biotites retain ar above 450 c agree well with other albeit less precise geological determinations but contrast with laboratory determinations of diffusivity from the literature we therefore performed a hydrothermal experiment on two larderello biotites and derived a diffusivity dlab 370 c 5 3 10 18 cm2s 1 in agreement with published estimates of diffusivity in annite from dlab and the rejuvenation of the k ar ages we calculate maximum survival times at the present in hole temperatures they trend smoothly over almost two orders of magnitude from 352 ka to 5 3 ka anticorrelating with depth laboratory diffusivities are inconsistent not only with geological facts but also among themselves from the geologically constrained lifetime of the thermal anomaly we derive a diffusivity dg 370 c 3 81 1021 cm2s 1 3 1 orders of magnitude lower than dlab the cause of these discrepancies must be sought among various laboratory artefacts overstepping of a critical temperature t enhanced diffusivities in wet experiments presence of fast pathway dislocation and pipe diffusion and of dissolution reprecipitation reactions which we imaged by scanning electron microscopy these phenomena are minor in geological settings in the absence of mineral transformation reactions complete or near complete resetting is achieved only by volume diffusion therefore laboratory determinations will necessarily result in apparent diffusivities that are too high compared to those actually effecting the resetting of natural geochronometers

field measurement of high temperature bulk reaction rates i theory and technique knowledge of metamorphic reaction rates is crucial to accurately interpret rock and mineral chemistry the local equilibrium assumption used in geochronology geothermobarometry and material flux estimates requires that local reaction rates among system phases are fast relative to local rates of p t x change natural metamorphic reaction rates are essentially unknown and difficulties and disagreements exist regarding extrapolations of existing laboratory data to natural conditions growing recognition of natural effects that could be attributed to slower reaction rates justifies the need for a field based quantification of reaction rates to assess the accuracy of lab based predictions we describe in detail the theory and methodology of a technique for extracting bulk reaction rates directly from isotopic data derived from natural samples baxter and depaolo 2000 reaction rates measured using this technique may be judiciously applied to isotopic exchange or net reaction kinetics in other natural systems the technique requires collection of whole rock and garnet 87sr 86sr data along a sampling traverse normal to a lithologic contact where there was prior to metamorphism a sharp isotopic discontinuity garnet data provide information on syn metamorphic conditions and the time interval for the exchange process forward modeling of the reactive transport process using numerical methods and the equations for diffusive reactive transport allow determination of the reaction rate and bulk sr diffusivity that provides the best fit to the data measurement of reaction rates is best constrained if an isotopic step is preserved at the contact the size of which is directly proportional to the bulk reaction rate this contribution is intended to serve as a template for future use and development of this technique to acquire natural reaction rate data from metamorphic systems

field measurement of high temperature bulk reaction rates ii interpretation of results from a field site near simplon pass switzerland bulk metamorphic reaction rates have been measured using sr isotopes as tracers of reactive transport processes at a field site near simplon pass switzerland employing the technique described in part i of this study baxter and depaolo 2002a the reaction rate r inferred similar to the value first reported in baxter and depaolo 2000 is 1 4 1 1 0 4 x 10 7 yr 1 or r 7 0 5 5 2 0 x 10 9 g cm 2 yr normalized to the geometric surface area of plagioclase which is several orders of magnitude slower than extrapolations of laboratory data this rate suggests that attainment of local equilibrium may require 10 s of millions of years and consequently equilibrium may not be closely approached in dynamic metamorphic systems if significant changes in p t x occur over shorter timescales these data suggest that there is a problem in current extrapolations or applicability of laboratory kinetic data to natural high temperature systems possible reasons for this discrepancy include differences in reactive surface area reactant supply limited by slow diffusion of major elements or an armoring effect by which access to grain interiors is limited on the basis of microchemical patterns in plagioclase we identify grain boundary migration gbm as the rate limiting process for accessing grain interiors and thus for bulk reaction calculation of reaction quotients for six balanced mineral reactions suggests local disequilibrium for garnet with respect to its ca content the characteristic equilibrium lengthscales le for sr and ca are both about 1 meter equilibrium based geochemical tools such as isochron geochronology or geothermobarometry are compromised within a distance le of the lithologic contact bulk sr diffusivity measured at the field site changes from 10 9 to 10 7 m yr suggesting that the characteristics of the intergranular transporting medium itm changed significantly during the early retrograde history of the field site

timing of prograde metamorphism in the zanskar himalaya the timing of the prograde metamorphism of continental crust buried during the collision of india and eurasia is of importance to our understanding of the mechanics of mountain building and crustal metamorphism our knowledge of the himalaya indicates that continental collision at 55 50 ma culminated in crustal melting and leucogranitic magmatism at 24 18 ma however the timing of events in the intervening 30 m y is poorly understood here we present sm nd isotope analyses of garnets from the zanskar himalaya that also preserve information on crustal and orogenic evolution during that period the prograde p t t paths indicate that heating and burial from 6 to 10 kbar continued until at least 25 30 ma and imply peak temperatures 700 c close to those necessary to cause the crustal melting in the himalaya at ca 20 ma without a requirement for additional heat sources such as shear heating on major crustal shear zones the persistence of high temperatures as late as 20 ma in the himalaya may be related to postcollisional thrusting south of the suture zone and within the indian plate at 40 35 ma

constraints on the upper crustal structure of the long valley mono craters volcanic complex eastern california from seismic refraction measurements during the summers of 1982 and 1983 field work was completed on 10 new seismic refraction profiles in the long valley mono craters region located within the technically active zone between the eastern sierra nevada and the basin and range province the four profiles presented here sample the upper 7 10 km of the crust within the western half of long valley cladera the mono craters ring fracture system and the normal crust north and northeast of long valley the entire area shares a common crystalline basement consisting primarily of mesozoic granitic rocks at a depth of 2 km beneath the topographic surface the p wave velocity is uniformly 5 6 km s beneath which the velocity increases with a gradient of about 0 1 s 1 clear secondary arrivals observed on a profile crossing the west part of the caldera support evidence from 1972 seismic refraction profiles for a reflecting boundary at a depth of 7 8 km beneath the west margin of the resurgent dome which may represent the top of a magma chamber structural details resolved in the upper 2 km of the crust lead to the following conclusions 1 p wave velocities in the crystalline basement at depths shallower than 2 km vary from 3 6 to 5 0 km s this variation reflects differences in the degree of fracturing of the upper portion of the basement 2 an apparently homogeneous basement in the upper 7 10 km beneath the mono craters ring fracture system indicates that any extensive magma chamber must be at least 10 km deep there 3 the downdropped crystalline basement beneath the western half of long valley caldera dips genty 5 10 to the northeast apparent offsets along steeply dipping caldera bounding faults are 1 0 1 7 km along the northern western and southern sides of the caldera with largest offsets in the north 4 rocks forming the caldera fill fall into essentially three p wave velocity groups from the surface downward these velocity groups are 100 400 of 1 2 1 8 km s material corresponding to unconsolidated fluvial or lacustrine deposits or highly fractured rhyolite flows 200 400 m of 2 8 3 1 km s material corresponding to sparsely jointed postcaldera rhyolite rhyodacite and basalt flows and approximately 1000 m of 3 9 4 4 km s material corresponding to deposits of bishop tuff

structure and stratigraphy beneath a young phreatic vent south inyo crater long valley caldera california an 861 m long hole has been cored on a slanted trajectory that passed directly beneath south inyo crater in the west moat of long valley caldera california the purpose of the hole was to investigate the magmatic behavior that led to surface deformation and phreatic activity during the 600 year old eruption of the inyo vent chain the hole was sited 216 m southwest of the crater passed beneath its center at a depth of 566 m and terminated 79 m northeast of the crater center at a depth of 810 m metamorphic basement was encountered at a depth of 779 m the volcanic and sedimentary sequence consists solely of post bishop tuff caldera fill including 319 m of moat basalt and 342 m of early rhyolite and is nearly 900 m thinner than in a unocal corporation well 900 m to the southeast apparently a major fault lies between the two holes and forms part of the western structural boundary of the caldera 3 4 km inboard of its topographic boundary breccia zones that intrude the caldera fill were intersected at 12 0 9 3 m and 1 2 0 8 m sw and 8 5 25 1 m ne of the crater center the largest breccia unit is symmetrically zoned from margins rich in vesicular rhyolite and locally derived rhyolite wall rock with most clasts 0 1 m to a center of up to 50 vol basalt with individual basalt clasts to 1 m in intersected length the basalt appears to be an early feeder for the moat basalt sequence the vesicular rhyolite is chemically distinct from both the rhyolite wall rock and previously recognized 600 year old inyo eruptives but matches fresh appearing pyroclasts in the crater ejecta this component of the breccia is therefore interpreted as representing juvenile magma that vesiculated and fragmented as it rose along the margins of an older basalt intrusion during this process wall rock fragments were ejected from depths extending to at least 800 m progression to a magmatic eruption was probably prevented by rapid influx of groundwater into the feeder the chemical data do not support the hypothesis that a single simple dike exists under the entire segment of the inyo chain that was active 600 years ago if such a dike exists it apparently tapped different magma chambers along its 10 km length the excess of surface extension over that needed to accommodate the intruded juvenile material may reflect the presence of a larger intrusion at depth below the level of fragmentation or the contribution of tectonic activity to the surface deformation

a sr isotopic comparison between thermal waters rocks and hydrothermal calcites long valley caldera california abstract the 87 sr 86 sr values of thermal waters and hydrothermal calcites of the long valley caldera geothermal system are more radiogenic than those of young intracaldera volcanic rocks five thermal waters display 87 sr 86 sr of 0 7081 0 7078 but show systematically lighter values from west to east in the direction of lateral flow we believe the decrease in ratio from west to east signifies increased interaction of deeply circulating thermal water with relatively fresh volcanic rocks filling the caldera depression all types of pre syn and post caldera volcanic rocks in the west and central caldera have 87 sr 86 sr m between about 0 7060 and 0 7072 and values for sierra nevada granodiorites adjacent to the caldera are similar sierran pre intrusive metavolcanic and metasedimentary rocks can have considerably higher sr isotope ratios 0 7061 0 7246 and 0 7090 0 7250 respectively hydrothermally altered volcanic rocks inside the caldera have 87 sr 86 sr m slightly heavier than their fresh volcanic equivalents and hydrothermal calcites 0 7068 0 7105 occupy a midrange of values between the volcanic plutonic rocks and the sierran metamorphic rocks these data indicate that the long valley geothermal reservoir is first equilibrated in a basement complex that contains at least some metasedimentary rocks reequilibration of sr isotope ratios to lower values occurs in thermal waters as convecting geothermal fluids flow through the isotopically lighter volcanic rocks of the caldera fill

some aspects of fluid motion during metamorphism most of the mineralogical chemical and textural features that can be observed in metamorphic rocks are produced during devolatilization reactions fyfe et al 1978 thompson 1983 in many cases only the last such event is recorded between devolatilization pulses fluid rock interaction will be minimal and microstructural evidence of even the last event is often modified by recrystallization ridley thompson 1986 the mechanisms by which the fluids escape varies depending upon whether a connected porosity or fracture network is available and the magnitudes of temperature and fluid pressure gradients e g norton knight 1977 etheridge et al 1984 yardley 1986 these factors will change during the metamorphic process fluids released deeper in the metamorphic pile do not necessarily flow pervasively through the overlying rocks but rather appear to be focused into high permeability channelways through a variety of processes fyfe et al 1978 wood walther 1986 the patchiness of retrogressive alteration points also to the focused flow of fluid after a metamorphic peak and perhaps under different rheological conditions metamorphism in the absence of fluid results in incomplete mineral reactions and other disequilibrium features in metamorphic rocks thompson 1983 ridley dixon 1984 many papers have appeared in recent years dealing with aspects of fluid motion during metamorphism they have variously demonstrated the transient nature of fluid flow indicated some controls on different modes of fluid behaviour and suggested that mechanical responses will be different depending upon whether a rock is undergoing devolatilization or not attention is focused here on

petrology and emplacement dynamics of intrusive and extrusive rhyolites of obsidian dome inyo craters volcanic chain eastern california drilling at obsidian dome has provided continuous core samples of the distal and proximal portions of obsidian dome its conduit and an associated feeder dike both the dome and conduit are chemically and mineralogically zoned and consist of a finely porphyritic high ba low silica rhyolite occurring in the basal portion of the dome and margins of the conduit and a finely porphyritic low ba higher silica rhyolite occurring in the upper portion of the dome and center of the conduit the high ba rhyolite contains two distinct phenocrysts assemblages with two distinct compositions and represents mingled magmas the low ba rhyolite in the dome and conduit contains significantly fewer disequilibrium phenocrysts and is only slightly mingled the dike sampled at 600 m depth as well as a related tephra fall from obsidian dome vent are entirely low ba rhyolite that contain no evidence of magma mingling end members of the mingled magma calculated using two different methods are a 63 silica end member and a silicic end member identical in composition to the dike and tephra fall from obsidian dome vent this silicic end member was the first magma emplaced in the dike and comprised much or all of the first magma vented to the surface during formation of the obsidian dome vent when eruption rates were high magma mingling of mafic and rhyolite magmas occurred during formation of the conduit a comparison of the magma types at obsidian dome with the other 600 year old magma types in the inyo chain demonstrates that the finely porphyritic rhyolites at obsidian dome are similar to the finely porphyritic rhyolites at the other domes all of these originated by mingling of two end members the coarsely porphyritic rhyolites that occur at glass creek and deadman domes are distinctly different from the finely porphyritic rhyolites the two rhyolites can not be related by any fractionation scheme and could not have been in thermal or chemical equilibrium with each other these observations require considerable heterogeneity in the inyo domes magmatic system two possible interpretations of the chemical variation and emplacement sequence are explored in the first interpretation the chemical patterns observed in the surface and subsurface samples at obsidian dome are explained by differential draw up of magma from a stratified dikelike reservoir this differential draw up is the result of changing flow rates during the course of the eruption when the magma initially vented to the surface during the main part of the explosive phase flow rates were very high and the deepest layer of the reservoir was drawn into the conduit where magma mingling occurred during the subsequent effusive phase flow rates were low draw up depths were reduced and only magma from the uppermost layer in the reservoir erupted only slightly contaminated by earlier hybrid magma drawn up similar processes most likely occurred at the other inyo domes however the low ba rhyolite was less abundant and highly viscous coarsely prophyritic rhyolite was near the surface and drawn up as a late pluglike structure an alternate interpretation is that the dike initially consisted of heterogeneous magma domains and that the zonation observed in the conduit of obsidian dome and on the surface of the three inyo domes is the result of the tendency of less viscous magma to encapsulate more viscous magma during sustained flow in a conduit

simultaneous calculation of metamorphic fluid fluxes reaction rates and fluid rock interaction timescales using a novel inverse modeling framework abstract to study metamorphic carbonation at greenschist facies conditions in the sw scottish highlands a novel inverse modeling framework which combines solutions of the transport equation with a global optimization method of differential evolution was developed using this framework we calculated simultaneously time integrated and time averaged metamorphic fluid fluxes of 83 4 35 4 m 3 m 2 and 10 10 1 0 5 m 3 m 2 s 1 respectively a time averaged reaction rate constant of 10 12 7 10 2 m s 1 and comparable timescales for fluid flow and fluid driven reaction of 10 4 3 0 5 yr and 10 2 7 5 2 yr respectively these parameters were calculated using an empirical estimate of the coefficient of molecular diffusion and a calculated value for metamorphic porosity our estimates are 1 consistent with single pass flow of fluid released by metamorphic devolatilization 2 within the range where heat is transported by conduction and matter is transported by advection 3 in agreement with an emerging consensus that metamorphic events are relatively short lived and 4 supportive of applying laboratory based estimates of kinetic parameters to metamorphic systems based on a sensitivity analysis we show that 1 selecting the diffusion coefficient rather than fluid velocity reaction rate or flow duration as an input parameter yields more robust estimates of metamorphic fluid flow parameters and 2 ignoring reaction dependent porosity and reaction rates can result in an order of magnitude uncertainty in best fit flow parameters evaluated from concentration profiles finally similarity between our calculated time averaged metamorphic fluid fluxes which were obtained numerically and those which were obtained analytically confirms the validity of using the quasi stationary state assumption to quantify metamorphic fluid flow parameters

fluid flow and diffusion in the waterville limestone south central maine constraints from strontium oxygen and carbon isotope profiles oxygen carbon and strontium isotopic profiles across the margin of the waterville limestone member are used to investigate advective and diffusive transport during metamorphism of the waterville formation in south central maine usa rb sr isotopic systematics were homogenized on the 10 cm hand specimen scale at ages that are within error of the 366 6 ma rb sr whole rock age of the syn metamorphic hallowell pluton this is consistent with a plutonic heat source for this low pressure andalusite and sillimanite grade acadian metamorphic terrane advective displacements of all three isotope profiles at the garnet grade blue rock quarry indicate fluid flow to the east into the limestone and the oxygen isotope profile implies a time integrated fluid flux of 3 2 1 5 m 3 m 2 2 error this cross layer flux is insufficient to cause the observed reaction progress of the muscovite ankerite quartz to biotite anorthite in plagioclase calcite reaction in the 100 m thick waterville limestone member and much of the fluid flow responsible may have been layer parallel the isotope profiles indicate advective diffusive homogenization over distances of 1 5 m 13 c to 6 m 18 o and such homogenization distances are difficult to reconcile with observations of order of magnitude variations in reaction progression the centimetre scale or less it is possible that infiltration occurred during events short lived compared with diffusion that the reactions started at different temperatures dependent on bulk composition or that diffusion of water from layers with less reactants to layers with more reactants was important in driving the biotite producing reaction however variations of fluid composition inferred from the mineral assemblages are apparently inconsistent with diffusion driving reactions progress and models of precursor assemblages do not indicate significant compositional control of the temperature of the first appearance of biotite in the rocks irrespective of the details of flow and diffusive exchange on the centimetre scale the average reaction progress in the waterville limestone member requires significant layer parallel fluid fluxes

diffusion controlled growth of wollastonite rims between quartz and calcite comparison between nature and experiment growth rates of wollastonite reaction rims between quartz and calcite were experimentally determined at 0 1 and 1 gpa and temperatures from 850 to 1200 c rim growth follows a parabolic rate law indicating that this reaction is diffusion controlled from the rate constants the d values of the rate limiting species were derived i e the product of grain boundary diffusion coefficient d and the effective grain boundary width in dry runs at 0 1 gpa wollastonite grew exclusively on quartz surfaces from volume considerations it is inferred that d cao d sio2 1 33 and that sio2 diffusion controls rim growth d sio2 increases from about 10 25 to 10 23 m3 s 1 as temperature increases from 850 to 1000 c yielding an apparent activation energy of 330 36 kj mol 1 in runs at 1 gpa performed in a piston cylinder apparatus there were always small amounts of water present here wollastonite rims always overgrew calcite rims around calcite grains in quartz matrix are porous and their growth rates are controlled by a complex diffusion advection mechanism rim growth on matrix calcite around quartz grains is controlled by grain boundary diffusion but it is not clear whether cao or sio2 diffusion is rate limiting d increases from about 10 21 to 10 20 m3 s 1 as temperature increases from 1100 to 1200 c d sio2 or d cao in rims on calcite is c 10 times larger than d sio2 in dry rims at the same temperature growth structures of the experimentally produced rims are very similar to contact metamorphic wollastonite rims between metachert bands and limestone in the bufa del diente aureole mexico whereby noninfiltrated metacherts correspond to dry and brine infiltrated metacherts to water bearing experiments however the observed diffusivities were 4 to 5 orders of magnitude larger during contact metamorphism as compared to our experimental results

transport mechanisms by fluid flow in metamorphic rocks oxygen and strontium decoupling in the trois seigneurs massif a consequence of kinetic dispersion

disequilibrium for ca during growth of pelitic garnet compositional zoning in hundreds of almandine rich garnets in amphibolite by facies micaceous quartzites from the picuris range north central new mexico usa indicates that although mn mg and fe achieve chemical equilibrium at hand sample scale during garnet growth ca does not instead ca concentrations at the surface of growing garnets appear to depend strongly on kinetic factors that govern the local chemical environment yielding disequilibrium for ca at scales larger than the region immediately surrounding an individual porphyroblast detailed zoning profiles were obtained for 371 garnet crystals in a small volume of a single sample of garnetiferous quartzite and core analyses were made of 97 additional crystals each analysis was made on a section that passed precisely through the morphological centre of the crystal located by means of 3 d imagery from computed x ray tomography the data reveal strong correlations between crystal size and concentrations of mn mg and fe but not ca in garnet cores a relationship between crystal size and isolation rigorous cross correlations among concentrations of mn mg and fe but not ca and systematic variations in ca concentrations as a function of crystal size and core composition that are anomalous in comparison to the behaviour of the other divalent cations we interpret these observations as the result of thermally accelerated diffusion controlled garnet growth in circumstances that promoted rapid intergranular diffusion and thus rock wide equilibration of mn mg and fe but that prevented equilibration at similar scale for ca because of its more sluggish intergranular diffusion the anomalous behaviour of ca is made evident in these garnets by the presence of sharp spikes in ca concentration which are demonstrably not a consequence of any simultaneous rock wide event such as a change in pressure temperature or some other intensive parameter instead ca concentrations probably reflect the local extent of reaction in the immediate vicinity of each porphyroblast to the degree that such kinetic factors introduce departures from chemical equilibrium for ca thermobarometric estimates that involve grossular contents of pelitic garnet will be in error

natural constraints on metamorphic reaction rates abstract quantitative constraints on the rates at which metamorphic reactions proceed in nature are now available from several sources most common are predictions made on the basis of laboratory kinetic data however the applicability of such laboratory based predictions has long been questioned and many observations in the field now suggest much slower rates here published quantitative field based constraints on high temperature 400 c reaction rates are assembled from a variety of sources reaction rates attending regional metamorphism are four to seven orders of magnitude slower than most laboratory based predictions a general rate law for regional metamorphism has been derived which best describes these field based data log 10 r net 0 0029 t 9 6 1 where r net is the net reaction rate g cm 2 a and t is temperature c at the same time natural reaction rates attending contact metamorphism differ from laboratory based predictions by less than two orders of magnitude and are in close agreement at higher temperatures thus while existing laboratory based kinetic data may be judiciously applied to some contact metamorphic systems laboratory based kinetic predictions clearly misrepresent regional metamorphism to explain this kinetic discrepancy regional metamorphic reaction rates may be limited by slow intergranular transport due to comparatively limited or transient availability of aqueous fluid in the intergranular medium the general field based rate law may be applied to regional metamorphic and other environments i e ultrahigh pressure or ultrahigh temperature metamorphism if similar system characteristics mainly low aqueous fluid content can be inferred

the unique radar properties of silicic lava domes 1 silicic lava domes exhibit distinct morphologic characteristics at scales of centimeters to kilometers multiparameter radar observations capture the unique geometric signatures of silicic domes in a set of radar scattering properties that are unlike any other natural geologic surfaces backscatter cross section values are among the highest observed on terrestrial lava flows and show only a weak decrease with incidence angle cross polarization backscatter hv shows a unique behavior increasing with increasing wavelength circular polarization ratios are relatively high in the 0 3 0 95 range and increase with increasing wavelength field measurements of boulder size frequency distributions and microtopography indicate that silicic dome surfaces are among the roughest ever measured rms heights at a 1 m lateral scale range from 13 cm to 50 cm rms slopes at 1 m spacing range from 12 to 43 degrees modeling of the scattering behavior suggests it results from a combination of rough surface facet scattering and scattering from block edges that act as a random collection of dipoles the unusual wavelength dependence of the radar parameters appears to result from a higher component of edge scattering at large wavelengths producing for example higher cross polarized backscatter at p band 68 cm steep sided volcanic domes on venus superficially resemble terrestrial silicic domes in plan view gross morphology but few similarities remain when the radar scattering and three dimensional shapes of the features are compared the unique radar scattering properties suggest that such volcanic surfaces can be identified with multiparameter radar observations in future planetary radar missions and in active terrestrial volcanoes where dome development can represent serious hazards

three dimensional gravity modeling of the geologic structure of long valley caldera a 48 mgal gravity low coincides with long valley caldera and is mainly attributed to low density caldera fill gravity measurements by unocal geothermal have been integrated with u s geological survey data vastly improving gravity station coverage throughout the caldera a strong regional gravity trend is mainly attributed to isostasy a best fitting based on regional control of basement densities airy heiskanen isostatic model was used for the regional correction a three dimensional multiple unit gravity modeling program with iterative capabilities was developed to model the residual gravity the density structure of long valley caldera and vicinity was modeled with 22 discrete density units most of which were based on geologic units information from drill hole lithologies surface geology and structural geology interpretations constrain the model some important points revealed by the three dimensional gravity modeling are that 1 the volume of ejected magma associated with the bishop tuff eruption is greater than previously thought 2 the caldera structure is strongly influenced by precaldera topography and the extensions of major active faults 3 the main west ring fracture is coincident with the inyo domes mono craters fracture system 4 a relatively low density region probably underlies the caldera and 5 a silicic magma chamber may underlie devils postpile

diffusion controlled corona reaction and overstepping of equilibrium in a garnet granulite yenisey ridge siberia diffusion modelling is applied to layered garnet pyroxene quartz coronas formed by a pressure induced reaction between plagioclase and primary pyroxene in a metabasic granulite the reconstructed reaction involves some change in composition of reactant minerals the distribution of minerals between layers is satisfactorily explained by diffusion controlled reaction with local equilibrium in which the diffusion coefficient for al was smaller than those for fe mg and ca by a factor of approximately four diffusion of mg towards plagioclase implies a chemical potential gradient for mgo component in a direction opposite to the changing mg content of garnet this is explained by the influence of al2o3 on the chemical potential of the pyrope end member grain boundary diffusion is suggested to have operated possibly with composition gradients different from those in the bulk minerals chemical potential differences across the corona are estimated from the variation in garnet composition enabling affinity the free energy change driving the reaction to be estimated as 6 9 1 8 kj per 24 oxygen mole of garnet produced this implies that the pressure for equilibrium among the minerals was overstepped by 1 4 0 4 kbar the probable p t conditions of reaction were in the range 650 790 c 8 10 kbar assuming a timescale of reaction between 106 and 108 years estimated diffusion coefficients for fe mg and ca are in the range 9 10 23 to 5 10 20 m2 s 1 these are consistent with experimental values in the literature for solid state diffusion including grain boundary diffusion

raman spectroscopic carbonaceous material thermometry of low grade metamorphic rocks calibration and application to tectonic exhumation in crete greece we present new raman spectra data of carbonaceous material cm to extend the range of the raman spectra of cm thermometer rscm to temperatures as low as 100 8c previous work has demonstrated that raman spectroscopy is an excellent tool to describe the degree of graphitization of cm a process that is independent of pressure but strongly dependent on metamorphic temperature a linear relationship between temperature and the raman parameter r2 derived from the area of the defect band relative to the ordered graphite band forms the basis of a previous thermometer because r2 shows little variability in low temperature samples 330 8c serves as a lower limit on the existing thermometer herein we present raman spectra from a suite of low temperature 100 to 300 8c samples from the olympics mountains and describe other aspects of the raman spectra of cm that vary over this range in particular the raman parameter r1 the ratio of heights of the disordered peak to ordered peak varies regularly between 100 and 350 8c these data together with published results from higher temperature rocks are used to calibrate a modified rscm thermometer applicable from 100 to 700 8c application to low grade metasediments in the otago region in the south island of new zealand gives temperatures consistent with previous estimates demonstrating the reliability of the modified rscm thermometer we apply the modified rscm thermometer to 53 samples from crete to evaluate the role of the cretan detachment fault in exhuming miocene high pressure low temperature metamorphic rocks exposed there the metamorphic rocks below the detachment the plattenkalk and phyllite quartzite units give metamorphic temperatures that range from 250 to 400 8c consistent with previous petrologic estimates we also demonstrate that the tripolitza unit which lies directly above the detachment gives an average metamorphic temperature of about 260 8c the modest break in metamorphic temperature in central crete indicates that the cretan detachment accounts for only 5 to 7 km of exhumation of the underlying hp lt metamorphic rocks which were initially accreted at 35 km we argue that the bulk of the exhumation 28 km out of 35 km total occurred by pervasive brittle stretching and erosion of structural units above the detachment d 2005 elsevier b v all rights reserved

metasomatism and graphite formation at a lithological interface in malaspina alpine corsica france multiple pieces of geologic evidence suggest that interfaces between contrasted lithologies exert a strong control on the fate of volatiles in subduction zones here we present results from a contact between serpentinites and sediments located in corsica and metamorphosed in the blueschist facies during the alpine orogeny it was shown previously that carbonates in the sediments have been reduced to graphitic carbonaceous material within a 5 10 cm thick reaction zone at the contact with serpentinites in an effort to investigate the mechanisms governing this unusual process bulk rock geochemical analyses incorporating a statistical analysis of compositional data are presented observations show that the fate of c was decoupled from that of other elements such as o h and large ion lithophile elements e g k sr ba as that were extensively leached from the reaction zone notably na is strongly enriched in the reaction zone and structurally linked to pectolite reducing conditions manifested by the depletion of o in the reaction zone compared to the bulk metasediment were likely maintained by the presence of fe ii in the serpentinite moreover thermodynamic calculations show that the low solubility of carbon in coh fluids at high pressure and low temperature conditions was the main driver for graphite precipitation synchronously with carbonate destabilization this may have been kinetically favored by the presence of already existing graphitized carbonaceous material and phengite in the metasediment limited lateral flow might have contributed as well to the geochemical and petrological patterns observed in these rocks

contact metamorphic devolatilization of shales in the karoo basin south africa and the effects of multiple sill intrusions abstract quantification of fluid generation during contact metamorphism of shale is important for the understanding of metamorphic processes fluid flow in sedimentary basins and perturbations of the global carbon cycle in this study we provide geochemical and numerical analyses from the organic rich ecca group in the karoo basin south africa which was affected by contact metamorphism from multiple sill intrusions in the early jurassic organic matter was efficiently converted to hydrocarbons during contact metamorphism and complete loss of organic carbon in the innermost aureole is common mineral dehydration reactions are evident from the occurrence of metamorphic minerals like biotite and loss of the clay fraction towards the intrusive contact we have developed a numerical model in order to quantify fluid production from both inorganic and organic reactions during contact metamorphism the modelling results are constrained by data from two case studies in the karoo basin in order to obtain reliable estimates of the carbon loss from metamorphism of shale we show that single thin 15 m thick sills have a gas production potential of several gigatons of methane ch4 if emplaced over a 1000 km2 area furthermore the vertical spacing between simultaneously emplaced sills has an important influence on the gas generation potential when two sills are emplaced with a vertical spacing of 7 times the intrusion thickness the total ch4 generation is up to 35 more than for two separate sills data and modelling from five sills emplaced within the ecca group show hydrocarbon generation throughout the organic rich section with total carbon loss next to the sills this has implications for the fluid production and metamorphism in volcanic basins where multiple sills are common

simple models of co2 release from metacarbonates with implications for interpretation of directions and magnitudes of fluid flow in the deep crust simple one dimensional models of coupled advection hydrodynamic surroundings can account for basic t fluid composition reaction dispersion reaction are used to investigate processes of co2 release progress relationships observed in much of the acadian orogen of from metacarbonate beds during deep crustal 8 kbar acadian new england the results provide an alternative to up t flow prograde metamorphism in new england usa two broad models scenarios that account for these relationships by large pervasive in which reaction progress is controlled by gradients in h2o co2 horizontal fluid fluxes up regional t gradients fluid composition between diverent rock types are presented in the first divusional exchange of volatiles across lithologic contacts is significant co2 generated during prograde temperature t rise is transported away from metacarbonate layers to surrounding 1 metapelitic layers which generate h2o by dehydration and or 2 key words metamorphism fluid flow flow direction carbon dioxide

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

magnetotelluric transect of long valley caldera resistivity cross section structural implications and the limits of a 2 d analysis twenty four magnetotelluric mt soundings have been collected in an east west profile across the center of long valley caldera the average station spacing is approximately 1 km and appears adequate to sample the important features of the upper crustal and deeper resistivity structures additional control on the shallowest resistivity is provided by a continuous profile of time domain electromagnetic soundings coincident with the western portion of the mt line our mt data set reveals numerous resistivity structures which illuminate the evolution and present state of the long valley system many of these have been quantified through two dimensional 2 d finite element modeling emphasizing the transverse magnetic tm mode important structural components include low resistivity layers 0 5 1 5 km deep under the eastern half of the caldera beneath the axial graben of the resurgent dome and under the west caldera moat most of this layering appears to lie in post caldera early rhyolite tuffs and the upp

physical rock properties in and around a conduit zone by well logging in the unzen scientific drilling project japan abstract the objective of the unzen scientific drilling project usdp is not only to reveal the structure and eruption history of the unzen volcano but also to clarify the ascent and degassing mechanisms of the magma conduit conduit drilling usdp 4 was conducted in 2004 which targeted the magma conduit for the 1990 95 eruption the total drilled length of usdp 4 was 1995 75 m geophysical well logging including resistivity gamma ray spontaneous potential sonic wave velocity density neutron porosity and fullbore formation microimager fmi was conducted at each drilling stage variations in the physical properties of the rocks were revealed by the well log data which correlated with not only large scale formation boundaries but also small scale changes in lithology such variations were evident in the lava dike pyroclastic rocks and breccias over depth intervals ranging from 1 to 40 m these data support previous models for structure of the lava conduit in that they indicate the existence of alternating layers of high resistivity and high p wave velocity rocks corresponding to the lava dikes in proximity to narrower zones exhibiting high porosity low resistivity and low p wave velocity these narrow low porosity zones are presumably higher in permeability than the adjacent rocks and may form preferential conduits for degassing during magma ascent

development and emplacement of the inyo domes magmatic suite california evidence from geological textural csd and geochemical observations of ash and lava the inyo domes magmatic system erupted 650 years ago in eastern california it is a series of three lava domes and associated pyroclastic rocks made up of phenocryst poor fp and phenocryst rich compositions cp we examined the chemical composition quantitative petrographic texture and geology of the ensemble the fp unit has considerable compositional variation that was produced by fractional crystallisation of feldspars and pyroxenes in a plexus of conduits and magma storage areas under or close to the inyo domes magmatic system there is no magmatic link with the synchronous north mono silicic eruption 20 km to the north the cp unit is compositionally more uniform and originated from a mushy feldspar cumulate formed in an earlier rhyolitic magma chamber the eruption started with the reactivation of a major fault which ruptured the surface initially the fault intersected a single hydrous magma source in the plexus and produced a pyroclastic eruption in the south other regions of the plexus to the north were rapidly co opted making further pyroclastic eruptions as the magmas became degassed lava domes started to form from fissures finally the cp magma source became reactivated by the injection of new magma felsic or mafic into the chamber the cp magma was forced out into a conduit of the fp unit where it formed a mixed unit im the quantity of the cp component increased until it dominated at this point the fp units were solid and the cp unit piled up to make the central knobs of the southern domes

excess 40ar transport scale and mechanism in high pressure phengites a case study from an eclogitized metabasite of the dora maira nappe western alps application of the bulk separate furnace heating laser step heating and in situ laser agemapping 40ar 39ar techniques reveals that high pressure phengites from an alpine eclogite of the dora maira basement nappe western alps were pervasively contaminated with excess 40ar to yield ages 130 ma spot fusion dating of individual grains with the 40ar 39ar laserprobe reveals concentric radial patterns with core ages as high as 320 ma showing that this excess component was trapped at an early stage and subsequently remobilized by volume diffusion during a late greenschist thermal pulse the single grains and population from this hand sample retain major differences in apparent ages indicating that the extent of the excess 40ar contamination varies from grain to grain attempt to model these differences in terms of grain size dependent fractional loss falls short and demonstrates that strong cm scale argon isotopic gradients locally prevailed during syn kinematic high pressure metamorphism on petrostructural grounds a model of internal isotopic buffering is outlined whereby the excess 40ar is argued to have been internally derived from within the eclogite protolith itself low intergranular ar diffusion rates due to reduced water activity buffered below unity by the phengite assemblage resulted in the local segregation and partitioning of the excess 40ar along grain boundaries during ongoing ductile deformation supporting evidence for this model comes from the recent literature on fluid and stable isotope behaviour in alpine eclogites and very high pressure metasediments which shows that volatile recycling during high pressure metamorphism may occur on a very limited scale

grain scale stable isotope disequilibrium during fluid rock interaction 2 an example from the penninic austroalpine tectonic contact in eastern switzerland at the tectonic contact between the penninic platta nappe and the lower austroalpine err unit in eastern switzerland carbon and oxygen isotope fronts are preserved at the interface of serpentinites and low grade regional metamorphic metacarbonates the carbon and oxygen isotope compositions of calcite are delta 13 c 2 3 per mil pdb and delta 18 o 21 0 per mil smow in the unaltered metacarbonates at a distance of about 20 m from the lithologic contact and they are delta 13 c 0 0 per mil and delta 18 o 14 0 per mil at the metacarbonate serpentinite interface the concomitant shift in the oxygen isotope composition of quartz is from 24 0 to 17 0 per mil a peculiarity of the isotope pattern is the systematic variation of the quartz calcite oxygen isotope fractionations across the isotopic front quartz calcite fractionations are relatively small nearly equal 3 0 per mil in the unaltered metacarbonate and at the metacarbonate serpentinite interface and they increase in the transitional zone in between with a maximum of 7 1 per mil at a distance of about 2 m from the contact this pattern is interpreted as a result of advective dispersive material transport across the lithologic contact and coupled first order kinetic mineral fluid exchange the oxygen isotope systematics indicate that calcite fluid isotope exchange was fast and local calcite fluid equilibrium prevailed during fluid rock interaction in contrast quartz fluid exchange was relatively sluggish and the quartz calcite oxygen isotope fractionations deviated significantly from their equilibrium values front geometries suggest that dispersive processes contributed substantially to material transport across the metacarbonate serpentinite contact cross layer advective flow was minor with a maximum time integrated volumetric flux of 21 m 3 m 2 from the influence of quartz fluid exchange kinetics on the intermineral fractionations quantitative relations among transport velocities the rates of mineral fluid exchange and the duration of fluid rock interaction are derived these relations are then combined with geologic and experimental data to identify feasible scenarios of fluid rock interaction

from nanometer to megameter isotopes atomic scale processes and continent scale tectonic models abstract earth sciences deal with physical entities spanning 15 orders of magnitude from nanometer crystal lattice unit cells to megameter faults and orogens the understanding of megameter scale processes is greatly improved by the study of the nanometer scale processes the growth of crystals their deformation under shear and the transport of isotopes used for dating from a physical point of view the transport of atoms across a crystal with high potential barriers and no interatomic voids is not accurately described by fick s equations which were developed for dilute solutions with unhindered particle mobility a variety of more general descriptions have been recently proposed all of which share features such as complex temperature dependence as opposed to linear arrhenian behaviour from a petrological point of view the ubiquitous recognition of microstructures in chemical and isotopic disequilibrium underscores the fact that diffusive reequilibration is a much slower process than creating the structures themselves diffusion in an inert matrix fickian diffusion is observed to be negligible with respect to the growth of discrete mineral generations as the latter frequently involves aqueous fluids isotope geoscientists should follow petrologists and stress the geohygrometric properties of minerals dissolution reprecipitation the fact that mineral reactions and microstructure development do not solely depend on temperature may explain why some thermochronological models from the literature have made predictions whose field tests have failed the demise of one class of models might initially seem unfortunate actually it is a great benefit because it shifts our attention to the reality of petrogenesis and to the sequence of events that any mineral assemblage records

kinetically limited isotope exchange in a shallow level normal fault mineral mountains utah we have conducted a stable isotope and geochemical study of the corral canyon fault zone a shallow level low angle normal fault on the west flank of the mineral mountains in southwest utah our stable isotope and geochemical data document the infiltration of local meteoric water during brittle deformation and hydrothermal alteration of the shear zone during tertiary basin and range extensional faulting within the fault zone 18o depletion correlates with progressive hydrothermal alteration and cataclasis demonstrating that the processes of fluid infiltration hydrothermal alteration fracturing comminution of grains and 18o exchange took place concurrently interpretation of our data using a model of combined fluid flow and kinetically limited isotope exchange leads us to conclude that the exposed fault zone was infiltrated by isotopically unevolved fluids from the wall rock adjacent to the fault zone and that an important component of fluid flow in this fault was between the wall rock and the fault zone rather than strictly within the fault plane we propose that the rate of isotopic alteration in shallow level fault zones is enhanced by comminution and recrystallization of grains during deformation so that fluids in the wall rock because of the relatively coarser grain size remain relatively unevolved until they infiltrate the fault zone where they rapidly become isotopically evolved due to exchange with cataclasized grains we present a general model for calculating the duration of isotopic alteration in flow systems characterized by kinetically limited isotope exchange calculated exchange durations for the corral canyon fault zone are short on the order of thousands to a few hundreds of thousands of years compared to the likely duration of deformation of millions of years the geologically short duration of isotope exchange leads us to suggest that the fluid flow responsible for the isotopic alteration was episodic of short duration and possibly related to discrete seismic events this result corroborates diverse lines of evidence that support a model of alternating fluid flow and permeability sealing by hydrothermal precipitation within fault zones

helium isotope and gas discharge variations associated with crustal unrest in long valley caldera california 1989 1992 the onset of anomalous seismic activity in 1989 beneath mammoth mountain on the southwestern rim of the long valley caldera california was followed within approximately 4 months by a large increase in sup 3 he sup 4 he in vapor discharged from a fumarole on the north side of the mountain the helium isotopic ratio at this vent rose to a maximum of 6 7 r sub a in july 1990 and subsequently declined to values near 5 r sub a potential sources of the sup 3 he rich vapors include degassing of fresh magma degassing from fresh surfaces generated in newly fractured igneous rocks and volatile release from a sup 3 he rich gas chamber situated above previously emplaced intrusives the magnitude of the increase in helium isotopic composition from 3 8 to 6 7 r sub a the persistence of relatively high values 5 r sub a over a period of 3 years the increase in the flux of total he relative to gases in air saturated water and the increases in the rates of discharge of steam and gas from this fumarole indicate that magmatic intrusion did in fact begin in 1989 beneath mammoth mountain seismic activity and limited measurements of extensional deformation at the surface suggest that the depth of intrusion maymore be as shallow as 2 km consistent with the prompt appearance of increased sup 3 he sup 4 he ratios in the fumarolic gas and that the intrusive process may have persisted for approximately 1 year in contrast a similar combination of magmatic intrusion and anomalous seismic activity beneath the resurgent dome south moat region during the 1989 1991 period resulted in at most relatively small changes in sup 3 he sup 4 he in fumarolic discharge at the southern edge of the resurgent dome the more subdued response may result from a combination of greater intrusive depths and greater dilution of sup 3 he rich inputs to thermal fluid reservoirs in the shallow hydrothermal system in this area compared with mammoth mountain 60 refs 11 figs 4 tabs less

can metamorphic reactions proceed faster than bulk strain available constraints on metamorphic reaction rates derived from the study of natural systems are similar to or slightly lower than the bulk strain rates measured in the same rocks here we explore whether this apparent relationship is merely coincidence or due to a more fundamental mechanistic link between reaction and strain grain boundary migration accommodated dislocation creep gbmdc or grain boundary diffusion creep gbdc i e pressure solution both of which involve dissolution precipitation as we define it will occur simultaneously with mineral reactions involving dissolution precipitation in the presence of a non zero deviatoric stress the exact relationships between reaction and strain are different depending on whether gbmdc or gbdc is controlling strain but the mechanistic link exists in both cases we present theoretical arguments which show that bulk strain by gbmdc or gbdc which may additionally be accommodated by processes not involving dissolution precipitation such as dislocation glide and climb or grain boundary sliding should in most cases be somewhat faster than the bulk reaction rates as observed with few exceptions for natural metamorphic systems undergoing plastic deformation strain rates provide an upper limit for bulk reaction rates occurring simultaneously in the same rocks the data suggest that mineral reaction rates may typically be within one order of magnitude of the strain rate

diffusion control of garnet growth harpswell neck maine usa a detailed analysis of chemical zoning in two garnet crystals from harpswell neck maine forms the basis of an interpretation of garnet nucleation and growth mechanisms garnet apparently nucleates initially on crenulations of mica and chlorite and quickly overgrows the entire crenulation giving rise to complex two dimensional zoning patterns depending on the orientation of the thin section cut contours of ca zoning cross those of mn fe and mg indicating a lack of equilibrium among these major garnet constituents zoning of fe mg and mn is interpreted to reflect equilibrium with the rock matrix whereas ca zoning is interpreted to be controlled by diffusive transport between the matrix and the growing crystal image analysis reveals that the growth of garnet is more rapid along triple grain intersections than along double grain boundaries moreover different minerals are replaced by garnet at different rates the relative rate of replacement by garnet along double grain boundaries is ordered as muscovite chlorite plagioclase quartz flux calculations reveal that replacement is limited by diffusion of si along double grain boundaries to or from the local reaction site it is concluded that multiple diffusive pathways control the bulk replacement of the rock matrix by garnet with si and al transport being rate limiting in these samples

surface and subsurface facies architecture of a small hydroexplosive rhyolitic centre in the mesoproterozoic gawler range volcanics south australia abstract at menninnie dam south australia a drilling program has revealed a complete section through the subsurface feeder system and erupted products of a small hydroexplosive rhyolitic centre within the mesoproterozoic gawler range volcanics porphyritic rhyolite intruded near vertical faults in the palaeoproterozoic basement and at less than a few hundred metres depth interacted with fault hosted hot groundwater hydrofracturing of the wall rock occurred in advance of and at the margins of the rhyolitic intrusions the rhyolitic intrusions have peperitic margins and grade into discordant lithic rich pb facies the advancing fragmentation front intersected the palaeosurface triggering phreatic eruptions that deposited a poorly sorted lithic rich explosion breccia rhyolite then rose to the surface through the intrusive breccias and shallow seated magma water interaction occurred in the conduit within 800 m from the inferred source the products include muddy sandstone and pumice breccia at the end of the eruption rhyolitic lava was extruded in the form of a small dome the presence of contemporaneous pb zn ag mineralisation in the wall rocks suggests that an active hydrothermal system may have been involved in the formation of the menninnie dam hydroexplosive volcanic centre

rhyolite intrusions in the intracaldera bishop tuff long valley caldera california abstract drilling of the long valley exploratory well on the resurgent dome in the long valley caldera revealed 300 m cumulative thickness of granophyric intrusions within the 1180 m thick 760 ka intracaldera bishop tuff the intrusions are aphyric to sparsely plagioclase phyric high silica high barium and low strontium rhyolites they resemble the lavas of the early rhyolite the first phase of post caldera volcanism a mean 40 ar 39 ar age of 590 17 ka from a part of a shallow intrusion is coeval with early rhyolite volcanism a second mean age of 454 17 ka from the same intrusion may reflect either younger early rhyolite activity with no external equivalent or hydrothermal resetting of the argon system hydrothermal alteration of the intrusions is characterized by introduction of quartz calcite and pyrite and formation of illite smectite high co 2 content of fluids apparently inhibited zeolite formation alteration varies locally within intrusions and intrusive groups and does not vary systematically with depth oxygen shows consistent depletion of the 18 o isotope from an initial magmatic composition of 6 0 to 8 5 to values ranging from 1 4 to 0 4 the constant oxygen isotope depletion most likely reflects alteration of intrusions due to local emplacement induced hydrothermal circulation rather than a caldera scale hydrothermal system in contrast 18 o depletion of the host bishop tuff increases regularly with depth except at an intrusive contact a pre early rhyolite geothermal gradient of approximately 70 c km was inferred this is substantially higher than the current gradient but substantially lower than expected for the case of a conductive regime over a shallow residual magma chamber either the intrusions were fed from a deep chamber or a cool hydrologic recharge regime was established early in caldera history the age thickness and suspected lateral extent of these shallow intrusions are such that emplacement of the intrusions rather than inflation of a shallow chamber is responsible for resurgence of the central long valley caldera similar intrusions occur in another well on the resurgent dome lv13 21 but not in wells located off the dome

fluid flow and chemical reaction kinetics in metamorphic systems the treatment and effects of chemical reaction kinetics during metamorphism are developed along with the incorporation of fluid flow diffusion and thermal evolution the interplay of fluid flow and surface reaction rates the distinction between steady state and equilibrium and the possible overstepping of metamorphic reactions are discussed using a simple analytic model this model serves as an introduction to the second part of the paper which develops a reaction model that solves the coupled temperature fluid flow chemical composition differential equations relevant to metamorphic processes consideration of stable isotopic evidence requires that such a kinetic model be considered for the chemical evolution of a metamorphic aureole a general numerical scheme is discussed to handle the solution of the model the results of this kinetic model allow us to reach several important conclusions regarding the factors controlling the chemical evolution of mineral assemblages during a metamorphic event 41 refs 19 figs 5 tabs

the effect of feldspar on quartz h 2 o co 2 dihedral angles at 4 kbar with consequences for the behaviour of aqueous fluids in migmatites an investigation was made of the effect of trace amounts of feldspar na and or k on dihedral angles in the quartz h2o co2 system at 4 kbar and 450 1050 c quartz quartz h2o dihedral angles in feldspar bearing quartz aggregates are observed to be the same as those in pure quartz aggregates at temperatures below 500 c above this temperature they decrease with increasing temperature until the solidus the final angle at the inception of melting is about 65 for microcline quartz h2o and microcline albite quartz h2o and much less than 60 the critical value for formation of grain edge fluid channels in an isotropic system for the albite quartz h2o system co2 was observed to produce a constant quartz quartz fluid dihedral angle of 97 in feldspar bearing quartz aggregates at all temperatures studied also examined were the dihedral angles for the two co existing supersolidus fluids in quartz aggregates in all systems the quartz volatile fluid angle is greater than 60 whereas the quartz melt angle is lower than 60 both supersolidus angles decrease with increasing temperature the transition from nonconnected to connected porosity with increasing temperature observed in the quartz albite h2o system some tens of degrees below the solidus termed a permeability transition if a common feature of rocks near their melting points will play an important role in controlling the permeability of high grade rocks to aqueous fluids

compositional changes of minerals associated with dynamic recrystallizatin the rate of compositional and isotopic exchange between minerals may be enhanced significantly if the rock is deformed simultaneously the enhanced exchange rate may result from a reduction in grain size shorter distance for volume diffusion dissolution and growth of grains by diffusion creep pressure solution or the movement of high angle grain boundaries through strained grains during recrystallization in the dislocation creep regime the migration of high angle grain boundaries provides high diffusivity paths for the rapid exchange of components during recrystallization the operation of the latter process has been demonstrated by deforming aggregates consisting of two plagioclases an1 and an79 at 900 c 1 gpa confining pressure and a strain rate of 2x10 6s 1 the polygonal recrystallized grains were analyzed using an analytical transmission electron microscope and have a variable but often intermediate composition at the conditions of these experiments the volume interdiffusion rate of nasi caal is too slow to produce any observable chemical change and microstructural chemical relations indicate that the contribution from diffusion creep was insignificant except for initially fine grained 2 10 m aggregates these results indicate that strain induced recrystallization can be an effective mechanism for enhancing the kinetics of metamorphic reactions and for resetting the isotope systematics of minerals such as feldspars pyroxenes and amphiboles

structures and lithofacies of inferred silicic conduits in the parana etendeka lip southernmost brazil abstract extensive silicic units in the parana etendeka lip have been long interpreted as pyroclastic density currents rheomorphic ignimbrites derived from the messum complex in namibia in recent literature however they have been characterized as effusive lava flows and domes in this paper we describe structures and lithofacies related to postulated silicic lava feeder conduits at mato perso sao marcos and jaquirana cambara do sul areas in southern brazil inferred conduits are at least 15 25 m in width and the lithofacies include variably vesicular monomictic welded and non welded breccias in the margins to poorly vesicular banded spherulitic and microfractured vitrophyres in the central parts flat lying coherent vitrophyres and massive obsidian are considered to be the subaerial equivalents of the conduits large scale regional tectonic structures in southern brazil include the ne sw aligned porto alegre suture leao and acotea faults besides the antas lineament a curved tectonic feature accompanying the bed of antas river south of the antas lineament smaller scale nw se lineaments limit the exposure areas of the inferred conduits ne sw and subordinate nw se structures within this smaller scale lineaments are represented by the main postulated conduit outcrops and are parallel to the dominant sub vertical banding in the widespread banded vitrophyre lithofacies upper lava flows display flat lying foliation pipe like and spherical vesicles and have better developed microlites petrographic characteristics of the silicic vitrophyres indicate that crystal poor magmas underwent distinct cooling paths for each inferred conduit area the vitrophyre chemical composition is defined by the evolution of trachydacitic dacitic vitrophyres with 62 65 wt sio 2 to rhyodacite and rhyolite with 66 68 wt sio 2 the more evolved rocks are assigned to the latest intrusive grey vitrophyre outcropping in the center of the conduits degassing pathways formed during fragmentation and fracturing episodes within the conduits may have helped to inhibit the explosivity of the eruptions based on the documented lithofacies architecture we attribute the source of the silicic lava flows in the studied localities to tectonic controlled local conduits rather than pyroclastic density currents from distant vent areas

experimental kinetic study of the reaction calcite quartz wollastonite carbon dioxide from 1 to 3 kilobars and 500 degrees to 850 degrees c

garnet zoning and the identification of equilibrium mineral compositions in high pressure temperature granulites from the moldanubian zone austria a detailed investigation of the compositional variation in garnet has been undertaken in a garnet pyroxene bearing granulite from the high grade gfohl unit moldanubian zone lower austria textural observations together with the interpretation of the preserved garnet chemistry enables the recognition of both prograde core and peak metamorphic garnet mantle growth stages an extremely rare feature in high p t granulite facies rocks initial thermobarometric calculations undertaken across whole garnet zoning profiles show how correct interpretation of a zoning profile is essential if the maximum peak metamorphic p t conditions are to be recovered the effect of retrograde decompression and cooling driven reactions on inclusion and host garnet compositions has also been assessed the results indicate that caution should be exercised when utilizing inclusion and adjacent garnet compositions for the thermobarometric evaluation of peak metamorphic equilibration conditions peak p t conditions were determined by the tweequ thermobarometric method utilizing the core compositions of matrix phases combined with the interpreted high p t garnet mantle composition to give 15 6 kbar and 1090 c consistent with previously determined results for moldanubian granulites similar high p t estimates are also provided by a re evaluation of previously published results for a granulite sample from the same lithological unit using a modified interpretation of garnet and plagioclase compositional data the new estimates presented confirm the previously disputed idea that the gfohl unit underwent a high pressure granulite facies stage and is therefore distinctly different from the underlying tectonostratigraphic units it is emphasized that any interpretation of the peak metamorphic conditions in high grade rocks must be based on detailed petrographic observations combined with a thorough understanding of the co existing equilibrium mineral compositions

syndeformational recrystallization dynamic or compositionally induced dynamic recrystallization in the strict sense of the term is the reconstitution of crystalline material without a change in chemical composition driven by strain energy in the form of dislocations driving potentials additional to internal strain energy may contribute to the recrystallization of naturally deformed minerals which form solid solutions such as feldspar amphiboles and pyroxenes if they change their composition during recrystallization to estimate the relative importance of these driving potentials the chemical composition of porphyroclasts and recrystallized grains of plagioclase clinopyroxene and hornblende have been investigated in samples from a high grade shear zone of the ivrea zone italy the plagioclases show two different recrystallization microstructures bulging recrystallization at grain boundaries and discrete zones of recrystallized grains across porphyroclasts probably involving fracturing deformation took place under amphibolite facies conditions on a retrograde p t path porphyroclast and recrystallized compositions from bulging recrystallization microstructures differ only in their or content and yield a g between mean host grain and mean recrystallized grain composition at fixed p t conditions of approximately 5 joules 10 4 m3 extreme compositional variations yield approximately 60 j 10 4 m3 the increase of free energy due to dislocations calculated for common glide systems in plagioclase are on the order of 100 joules 10 4 m3 for high values of dislocation densities of 1014 m 2 thus the effect of chemically induced driving energies on grain boundary velocity appears small for mean compositions but may be as great as that of deformational energies for larger chemical differences in the other type of microstructure porphyroclasts and recrystallized grains in discrete zones differ in their anorthite content the maximum g induced by the compositional disequilibrium is on the order of 100 j 10 4 m3 this maximum value is of the same magnitude as the g derived from high dislocation densities of 1014 m 2 the resulting combined g is approximately twice as high as for deformational g alone and heterogeneous nucleation may become a feasible recrystallization mechanism which is evident from the microstructures the recrystallization mechanism depends on the nature of the driving potential grain boundary migration gbm and heterogeneous nucleation can release gibbs free energy induced by compositional disequilibrium whereas this is not likely for subgrain rotation therefore only gbm and heterogeneous nucleation may link metamorphism and deformation so that syndeformational recrystallization may represent a transitional process ranging from dynamic recrystallization to metamorphic reaction

hydrothermal alteration and past and present thermal regimes in the western moat of long valley caldera abstract hydrothermal alteration in caldera fill early rhyolite and moat rhyolite was investigated in six western moat drillcores in long valley caldera with an emphasis on clay mineralogy in two of the cores examined a complete sequence was observed in argillic alteration of tuffaceous rocks progressing from smectite often with kaolinite to mixed layer illite smectite i s to illite with increasing depth portions of this sequence were observed in the other four drillcores lower permeability rhyolitic lavas typically showed much sparser clay alteration than tuffs in cores where alteration progressed through smectite to i s or illite the distribution of the clay minerals and of compositional and structural variation in the i s phases allowed estimation of pre existing alteration temperatures and thermal gradients i s and illite occurred at the shallowest depths and highest elevations in the shady rest sr core and comparable alteration occurred at progressively deeper levels in plv 1 and inyo 4 presently observed temperatures are well below preexisting temperatures at comparable depths in these three holes with the exception of the deeper portion of sr paleotemperatures in the zone of i s alteration in the sr hole 200 250 m depth were 65 c higher than at present with a thermal gradient at least twice the steep gradient existing there now in plv 1 temperatures near the base of the hole have declined even further than in sr despite the steep gradient currently observed in inyo 4 hydrothermal circulation somewhat cooler than that in sr and plv 1 occurred at one time in the deeper portion of the hole and the steep thermal gradient currently observed at shallower levels in inyo 4 may be the truncated remnant of a gradient that once continued to greater depth in mlgrap 1 hydrothermal circulation occurred previously near the base of the hole but at temperatures probably not far above present ones no evidence of high temperature alteration was observed in cores from mlgrap 2 or plv 2

the origin of olivine plagioclase coronas in metagabbros from the adirondack mountains new york olivine plagioclase coronas in metagabbros from the adirondack mountains new york usa are spatially well organized reaction textures consisting most commonly of sequential layers of orthopyroxene clinopyroxene plagioclase and garnet the textures are characteristic of diffusion controlled reaction kinetics although similar coronas have been interpreted by previous workers in terms of an isochemical steady state diffusion model petrographical relations and material balance calculations establish that coronas in the adirondack metagabbros cannot be treated as isochemical and do not form in a single stage steady state process instead they evolve through time in a complex open system reaction in this study the isochemical diffusion model is modified to account for elemental fluxes across the outer boundaries of the coronal reaction band thereby approximating the open system behaviour of the coronas the sequence and relative proportions of product minerals calculated by the open system steady state model correspond closely to those observed in coronas of the adirondacks over a wide range of values for the relative diffusivities of chemical components involved in the reaction regardless of the particular method used to determine material balance in the reaction texture despite this correspondence petrographical evidence for successive replacement of coronal product layers reveals that the adirondack coronas evolved through one or more transient states rather than forming in a single stage steady state process there is no evidence that the successive replacement of coronal product layers resulted from changes in pressure or temperature but there is petrographical evidence that these changes resulted from modification of the composition of reactant plagioclase as the corona forming reaction proceeded this is confirmed by the fact that the evolution of the coronas over time can be replicated with the open system diffusion model by simulating the effect of the gradual exhaustion of plagioclase as a source of the ca and si components required for reaction these simulations suggest that successive stages in the evolution of the coronas are characterized by these product sequences i orthopyroxene clinopyroxene plagioclase garnet ii orthopyroxene clinopyroxene garnet and iii orthopyroxene garnet all of these stages and the transitions between them are observed petrographically coronas in adirondack metagabbros appear therefore to have originated in a complex open system diffusion controlled reaction in which the product assemblages changed as the reaction progressed

evidence for 600 year old basalt and magma mixing at inyo craters volcanic chain long valley caldera california andesite inclusions are found within the vent areas of two 600 year old rhyolite domes in the inyo craters volcanic chain eastern california both domes lie within the present northwestern sector of 0 73 ma long valley caldera inclusion morphology indicates that they were partially molten and ductile during incorporation within coarsely porphyritic rhyolite the xenocryst assemblage within the inclusions plagioclase hornblende biotite quartz sanidine is identical to the phenocryst assemblage within the host rhyolite microprobe analyses of xenocrysts within the andesite inclusions show that they have compositions similar to phenocrysts in the host rhyolite these data suggest that the andesite inclusions are the result of mixing of the coarsely porphyritic rhyolite with a more mafic end member whole rock geochemistry of the inclusions generally supports a simple mixing model between the coarsely porphyritic rhyolite of the inyo domes and typical postcaldera mafic lavas found throughout the western caldera moat the inclusions provide the first evidence for involvement of basaltic magma in the 600 year old rhyolite eruptions of the inyo craters volcanic chain and suggest the possibility that the rhyolitic eruptions were initiated by injection of basalt into the lower part of the silicic magma system periodic thermal replenishment of the magmatic system through basaltic injection may explain the relatively high temperatures encountered in drill holes within the western caldera moat the inclusions further suggest that although rhyolite eruptions are statistically more likely from the inyo craters volcanic chain future basaltic eruptions cannot be ruled out

local origin of high pressure vein material in eclogite facies rocks of the zermatt saas zone switzerland prograde dehydration reactions within metabasalts at the blueschist eclogite facies transition produce kyanite veins usually with quartz sometimes with omphacite or chloritoid of up to several meters length and 20 cm width bulk rock analyses show clear depletion haloes for al2o3 and sio2 near the veins mass balance calculations support the suggestion that the vein material must have been derived from the immediate surroundings these veins are interpreted as local segregation phenom ena and not as fluid dehydration channels of the eo alpine subduction zone calcula tions have been performed for a stagnant vein formation model that considers the rate of the dehydration reactions k responsible for providing the h2o for al and si transport and the rate of diffusion d of these from wall rock to growing vein the model is satisfied for quite a wide range of values for k d and upper limits on the duration of diffusional vein formation of about 1 ma are indicated

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

surface deformation and shallow dike intrusion processes at inyo craters long valley california the inyo craters are the two largest of four phreatic craters that lie within a 2 5 km long 500 to 700 m wide n s trend of faults and fissures at the south end of the inyo volcanic chain in eastern california the alignment of these features with dike fed silicic volcanic centers of the same age a few kilometers to the north suggests that they were produced during intrusion of a dike at about 650 550 yr b p e w extension south of south inyo crater is ten to several tens of meters suggesting that the dike is at least that thick to understand how the faults and fissures developed we mapped and studied the fault and fissure pattern used a theoretical boundary element model to determine the surface strain profile above a shallow dike in a purely elastic medium and conducted physical model experiments of fault and fissure growth results of the field studies and experiments indicate that deformation develops according to the following sequence 1 extension fractures and perhaps other inelastic deformation develop immediately above the dike top and on the limbs of a shallow syncline which forms above the dike 2 fissures form along two parallel trends at the surface on opposite sides of the dike plane leaving a relatively unfractured region in between 3 dip slip movement on subsurface fractures and linkage of these fractures with surface fissures produces inward facing normal fault scarps that bound a nested graben above the dike experimental graben widths are up to several times narrower than predicted from the locations of theoretical strain maxima above a dilating crack in a linearly elastic medium this fact is apparently due to the growth of a zone of inelastic deformation above the experimental dike top in the subsurface if the ratio between the depth to the dike top and the distance between the outermost surface fissures at inyo craters is within the range of ratios measured experimentally then the depth to the dike top is between 250 m and several times that a slant hole drilled in the summer of 1987 by the department of energy office of basic energy sciences intersected three breccia bodies of possibly intrusive origin directly below the center of south inyo crater between 550 m and 610 m below the crater rim no intact juvenile igneous rock was intersected suggesting that the hole passed above the magma filled dike top perhaps passing through the zone of inelastic deformation which is inferred to exist from our model results

the mount perkins block northwestern arizona an exposed cross section of an evolving preextensional to synextensional magmatic system the steeply tilted mount perkins block northwestern arizona exposes a cross section of a magmatic system that evolved through the onset of regional extension new 4oar 39ar ages of variably tilted 0 90 o volcanic strata bracket extension between 15 7 and 11 3 ma preextensional intrusive activity included emplacement of a composite miocene laccolith and stock trachydacite dome complex and east striking rhyolite dikes related volcanic activity produced an 18 16 ma stratovolcano cored by trachydacite domes and flanked by trachydacite trachyandesite flows and 16 ma rhyolite flows similar compositions indicate a genetic link between the stratovolcano and granodioritic phase of the laccolith magmatic activity synchronous with early regional extension 15 7 14 5 ma generated a thick felsic volcanic sequence a swarm of northerly striking subvertical rhyolite dikes and rhyolite domes field relations and compositions indicate that the dike swarm and felsic volcanic sequence are cogenetic modes of magma emplacement changed during the onset of extension from subhorizontal sheets east striking dikes and stocks to northerly striking subvertical dike swarms as the regional stress field shifted from nearly isotropic to decidedly anisotropic with an east west trending horizontal least principal stress preextensional trachydacitic and preextensional to synextensional rhyolitic magmas were part of an evolving system which involved the ponding of mantle derived basaltic magmas and ensuing crustal melting and assimilation at progressively shallower levels major extension halted this system by generating abundant pathways to the surface fractures which flushed out preexisting crustal melts and hybrid magmas remaining silicic melts were quenched by rapid upper crustal cooling induced by tectonic denudation these processes facilitated eruption of mafic magmas accordingly silicic magmatism at mount perkins ended abruptly during peak extension 14 5 ma and gave way to mafic magmatism which continued until extension ceased

basement structure and implications for hydrothermal circulation patterns in the western moat of long valley caldera california detailed surface mapping subsurface drill hole data and geophysical modeling are the basis of a structural and hydrothermal model for the western part of long valley caldera six fault zones are recognized in the western caldera with dominant orientations of north northwest and northeast sub parallel to regional fault trends in the surrounding sierran basement the internal structural geometry of the cores of 12 exogenous domes inside and outside the caldera suggests that the domes erupted at the intersections of these principal fault trends rather than along the axis of a single dike gravity modeling and subsurface data from deep geothermal wells indicate that the floor of the caldera is segmented into a number of discrete fault blocks with varying offsets one of the northeast trending fault zones designated discovery fault in this paper appears to be part of the original sierran embayment that existed before caldera collapse recent hydrothermal alteration occurs along discovery fault strands and composite vertical offset of intracaldera volcanic units across the entire fault zone may be as much as 400 m field relationships geophysical interpretations and interpretaions of drill hole data suggest that this fault is a fundamental flaw in the western caldera the preexisting tectonic framework of the basement rocks controlled the configuration of the western caldera floor and through it the location of postcollapse eruptive centers these deep basement structures may also provide the high fracture density which controls circulation in the present geothermal system of long valley