diffusion obstacles and opportunities in petrochronology many of the approaches in petrochronology are rooted in the assumption of equilibrium diffusion is an expression of disequilibrium the movement of mass in response to chemical potential gradients and isotopes in response to isotopic gradients it is extremely important that we be aware of how the effects of diffusion can place obstacles across our path towards petrochronologic enlightenment conversely the effects of diffusion also provide opportunities for understanding rates processes and conditions experienced by rocks the enormity of the field does not permit us to provide a comprehensive review of either the mathematics of diffusion or quantitative data that have been obtained relevant to the interpretation of diffusive processes in rocks and minerals many resources cover these topics including rimg volume 72 diffusion in minerals and melts zhang and cherniak 2010 and several textbooks crank 1975 glicksman 2000 particularly relevant to the discussion of petrochronology are summaries of the theory and controls on diffusion brady and cherniak 2010 zhang 2010 as well as diffusion rates in feldspar cherniak 2010a accessory minerals cherniak 2010b garnet ganguly 2010 mica pyroxene and amphibole cherniak and dimanov 2010 and melts zhang and ni 2010 zhang et al 2010 rather than duplicate that material our goal is to explore the obstacles and opportunities presented by the effects of diffusion as they inform the rates of petrologic processes to achieve this goal we emphasize key principles and illustrative examples quantitative interpretation of the effects of diffusion assumes predictability of numerous factors that may affect chemical or isotopic transport including temperature initial and boundary conditions water and oxygen fugacities activities of other components multiple mechanisms of diffusion and crystal chemistry coupling of the substitution of elements into different crystallographic sites additionally the extraction of meaningful ages durations of events and temperatures requires

analysis of experimental data on divalent cation diffusion kinetics in aluminosilicate garnets with application to timescales of peak barrovian metamorphism scotland we present a new statistical framework to analyze the diffusion data for divalent cations fe mg mn and ca in aluminosilicate garnet using published experimental data and an arrhenius relationship that accounts for dependence on temperature pressure garnet unit cell dimension and oxygen fugacity the regression is based on bayesian statistics and is implemented by the markov chain monte carlo approach all reported experimental uncertainties are incorporated and the data are weighted by the precision of the experimental conditions we also include a new term the inter experiment bias to compensate for possible inconsistencies among experiments and to represent any experimental variability not explicitly presented in the arrhenius relationship e g water content defect density at high temperatures where most experiments were conducted the diffusion coefficients calculated with the new parameters agree well with previous diffusion models e g chakraborty and ganguly in contrib mineral petrol 111 74 86 1992 carlson in am mineral 91 1 11 2006 however the down temperature extrapolation leads to notable differences at lower temperatures for common petrological applications for example at 600 c the diffusion coefficients of fe and mn are one half to one order of magnitude faster and the diffusion coefficient of ca is about one order of magnitude slower than calculated with parameters in carlson 2006 our statistical analysis also provides well defined uncertainty bounds for timescale estimates based on garnet diffusion profiles application of the newly derived coefficients indicates that the timescale of the thermal peak of barrovian metamorphism dalradian belt of scotland is about four to seven times longer than that estimated using previously published diffusion coefficients the peak is still geologically brief however of the order of 106 years 0 75 myr 0 70 0 36 myr 1 this brevity requires pulsed advective heat input as provided by syn orogenic mafic magmatism in these rocks

discontinuous precipitation of rutilated quartz grain boundary migration induced by changes to the equilibrium solubility of ti in quartz we experimentally investigated changes to the titanium concentrations of quartz rich rocks caused by grain boundary migration synthesis experiments were performed to make high ti quartz starting materials for usage in recrystallization experiments we isothermally recrystallized quartz starting material in separate experiments at 10 25 kbar to reduce the solubility of ti in quartz microstructural and geochemical results indicate that the high ti quartz starting material recrystallized to form low ti quartz with acicular rutile inclusions during recrystallization high ti quartz was consumed to form low ti quartz and acicular rutile crystals precipitated behind moving grain boundaries via the discontinuous precipitation unmixing process experimentally imposed changes to the equilibrium solubility concentration of ti in quartz by amounts as low as 0 02 weight 200 g g or ppm by weight created a free energy driving force of 8 7 kj mol that was sufficient to drive large scale recrystallization of the quartzites microstructures of relict grains of the high ti quartz starting materials are characterized by cuspate and irregular grain boundaries whereas new quartz grains exhibit subhedral to euhedral grain morphologies and abundant three grain junctions discontinuous precipitation can significantly change mineral compositions and in the absence of a free fluid phase can operate much faster than other types of mass transfer processes commonly associated with solid state reactions

p t d histories from quartz a case study of the application of the titaniq thermobarometer to progressive fabric development in metapelites 1 this study investigates the ability of quartz to record segments of the pressure temperature deformation p t d path in poly deformed metamorphic terranes and the associated application of the ti in quartz thermobarometer titaniq metapelites from the strafford dome eastern vermont were selected for this study because they record progressive fabric development during prograde metamorphism and have well constrained p t d histories from previous work results of this investigation are in agreement with previous studies and demonstrate that quartz in these samples records additional distinct intervals of the p t d path six preserved quartz equilibration events include 1 early prograde growth during burial 2 kyanite in quartz producing reactions 3 prenappe emplacement equilibration during isobaric heating 4 precipitation of quartz in quartz feldspar domains during crenulation cleavage development due to solution transfer 5 quartz producing chlorite out reactions during heating postnappe emplacement and 6 retrograde quartz overgrowths from an influx of siliceous fluids modeling quartz volume allows for the identification of quartz producing reactions in p t space the same may be done for ti phases to better constrain activities during reequilibration events an integration of cathodoluminescence imaging microstructural and petrographic investigation isochemical forward stability modeling and thermobarometry allows identification and interpretation of zoning patterns in quartz to unravel histories that would otherwise not be obtainable

geochronologic and thermobarometric constraints on the evolution of the main central thrust central nepal himalaya the main central thrust mct juxtaposes the high grade greater himalayan crystallines over the lower grade lesser himalaya formation an apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust garnet bearing assemblages sampled along the marysandi river and darondi khola in the annapurna region of central nepal show striking differences in garnet zoning of mn ca mg and fe above and below the mct thermobarometry of mct footwall rocks yields apparent inverted temperature and pressure gradients of 18 c km 1 and 0 06 km mpa 1 respectively pressure temperature p t paths calculated for upper lesser himalaya samples that preserve prograde compositions show evidence of decompression during heating whereas garnets from the structurally lower sequences grew during an increase in both pressure and temperature in situ i e analyzed in thin section ion microprobe ages of monazites from rocks immediately beneath the greater himalayan crystallines yield ages from 18 to 22 ma whereas late miocene and pliocene monazite ages characterize rocks within the apparent inverted metamorphic sequence a lesser himalayan sample collected near the garnet isograd along the marysandi river transect contains 3 3 0 1 ma monazite ages p 0 72 gpa t 535 c this remarkably young age suggests that this portion of the mct shear zone accommodated a minimum of 30 km of slip over the last 3 ma i e a slip rate of 10 mm yr 1 and thus could account for nearly half of the convergence across the himalaya in this period the distribution of ages and p t histories reported here are consistent with a thermokinematic model in which the inverted metamorphic sequences underlying the mct formed by the transposition of right way up metamorphic sequences during late miocene pliocene shearing

metamorphic rates in collisional orogeny from in situ allanite and monazite dating the prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates in individual samples from the northern lepontine central alps allanite is partly replaced by monazite at 560 580 c relic allanite retains its characteristic growth zoning acquired at greenschist facies conditions 430 450 c coexisting monazite and allanite were dated in situ to delimit in time successive stages of the barrovian metamorphism in situ sensitive high resolution ion microprobe shrimp u th pb dating of allanite 31 5 1 3 and 29 2 1 0 ma and monazite 18 0 0 3 and 19 1 0 3 ma constrains the time elapsed between 430 450 c and 560 580 c which implies an average heating rate of 8 15 c m y combined with new fission track ages zircon 10 9 ma apatite 7 5 6 5 ma metamorphic rates of the entire orogenic cycle from prograde to final cooling can be reconstructed

ti diffusion in quartz abstract we have measured ti diffusion in quartz under dry 1 atm conditions experiments were performed using synthetic and natural quartz and a tio2 powder source with ti profiles obtained by rutherford backscattering spectrometry rbs over the temperature range 700 1150 c the following arrhenius relation was obtained for diffusion parallel to 001 d ti 7 10 8 exp 273 12 kj mol 1 r t m 2 sec 1 similar diffusivities were obtained for both synthetic and natural quartz and for a range of ti source materials including natural titanite powder and rutile quartz single crystal diffusion couples although ti diffusion appears slightly slower in the direction normal to c this difference is not great using these diffusion parameters calculations indicate that distances over which diffusional alteration of ti concentrations in quartz could occur in a million years would be on order of 500 m at 800 c and 15 m at 600 c the arrhenius relation above should be broadly applicable and may find application in constraining metamorphic histories under conditions where the ti in quartz geothermometer can be reasonably applied this relation can also be used to constrain peak temperatures and cooling histories from observations of exsolved rutile needles in quartz and the width of the ti depleted zone that develops around the growing needles finally these findings may find utility in evaluating crystal residence times in magmatic systems when coupled with observations of the sharpness of zoning patterns in quartz

ti diffusion in quartz inclusions implications for metamorphic time scales quartz inclusions in garnet from samples collected from the staurolite zone in central new england are zoned in cathodoluminescence cl the cl intensity is interpreted to be a proxy for ti concentration and the zoning attributed to ti diffusion into the quartz grains driven by ti exchange between quartz and enclosing garnet as a function of changing temperature the cl zoning has been interpreted using a numerical diffusion model to constrain the time scales over which the diffusion has occurred temperature time histories are sensitive to the presumed peak temperature but not to other model parameters the total time of the metamorphic heating and cooling cycle from around 450 c to the peak temperature 550 600 c back to 450 c is surprisingly short and encompasses only 0 2 2 million years for peak temperatures of 600 550 c the metamorphism was accompanied by large scale nappe and dome formation and it is suggested that this occurred as a consequence of in sequence thrusting resulting in a mid crustal ductile duplex structure

cathodoluminescence imaging and titanium thermometry in metamorphic quartz cathodoluminescence cl of quartz from metamorphic rocks representing a range of conditions from the garnet grade to the migmatite grade reveals a variety of textures that is a function of metamorphic grade and deformation history ti concentrations determined by electron microprobe and ion microprobe generally correlate with cl intensity blue wavelengths and application of the ti in quartz thermometer titaniq reflects the temperature of quartz growth or recrystallization and in some settings modification by diffusion quartz from garnet grade samples is not visibly zoned records temperatures of 425 475 c and is interpreted to have recrystallized during fabric formation quartz grains from staurolite grade samples are zoned in cl with markedly darker cores and brighter rims some of which are interpreted to have been produced by the dominant staurolite producing reaction whereas others are interpreted as having formed by diffusion of ti into quartz rims quartz from the matrix of kyanite and sillimanite grade samples are generally unzoned although locally displays slightly brighter rims higher ti quartz inclusions within garnet and staurolite have distinctly brighter rims which are interpreted as having been produced by diffusive exchange with the host mineral quartz from migmatite grade samples displays highly variable cl intensity which is dependent on the location of the grain matrix grains in melanosomes are largely unzoned or rarely zoned with darker cores leucosome quartz is strongly zoned with bright cores and dark rims and is interpreted as having formed during crystallization of the melt locally within the leucosome is observed oscillatory zoned quartz which is interpreted as a subsolidus recrystallization to achieve strain relaxation quartz inclusions within garnet or plagioclase crystals often show bright domains separated by zones of dark cl these enigmatic textures possibly reflect local melting fluxed by fluid inclusions temperatures calculated from the ti in quartz thermometer are a function of the metamorphic grade of the sample the textural setting of the quartz the reaction history and the deformation history of the rock the titaniq temperatures can be used to constrain the conditions at which various metamorphic processes have occurred

time scales of porphyry cu deposit formation insights from titanium diffusion in quartz porphyry dikes and hydrothermal veins from the porphyry cu mo deposit at butte montana contain multiple generations of quartz that are distinct in scanning electron microscope cathodoluminescence sem cl images and in ti concentrations a comparison of microprobe trace element profiles and maps to sem cl images shows that the concentration of ti in quartz correlates positively with cl brightness but al k and fe do not after calibrating cl brightness in relation to ti concentration we use the brightness gradient between different quartz generations as a proxy for ti gradients that we model to determine time scales of quartz formation and cooling model results indicate that time scales of porphyry magma residence are 1 000s of years and time scales from porphyry quartz phenocryst rim formation to porphyry dike injection and cooling are 10s of years time scales for the formation and cooling of various generations of hydrothermal vein quartz range from 10s to 10 000s of years these time scales are considerably shorter than the 0 6 m y overall time frame for each porphyry style mineralization pulse determined from isotopic studies at butte montana simple heat conduction models provide a temporal reference point to compare chemical diffusion time scales and we find that they support short dike and vein formation time scales we interpret these relatively short time scales to indicate that the butte porphyry deposit formed by short lived episodes of hydrofracturing dike injection and vein formation each with discrete thermal pulses which repeated over the 3 m y generation of the deposit

a comment on titaniq under pressure the effect of pressure and temperature on the solubility of ti in quartz by jay b thomas e bruce watson frank s spear philip t shemella saroj k nayak and antonio lanzirotti trace concentrations of ti in quartz are used to indicate the pressure and temperature conditions of crystallization in the titaniq geothermobarometer of thomas et al contrib miner petrol 160 743 759 2010 it utilises the partitioning of ti into quartz as an indicator of the pressures and or temperatures of crystal growth for a given value of tio2 activity in the system if temperatures are inferred to 20 c pressure is constrained to 1 kbar and vice versa there are significant contrasts however between the conclusions from titaniq and those for natural quartz as well as other mineral phases in volcanic rocks application of the titaniq model to quartz from the 27 ka oruanui and 760 ka bishop high silica rhyolites where the values of t p and tio2 activity are constrained by other means fe ti oxide equilibria melt inclusion entrapment pressures in gas saturated melts melt and amphibole compositions yields inconsistent results if realistic values are given to any two of these three parameters then the value of the third is wholly unrealistic the model yields growth temperatures at or below the granite solidus pressures in the lower crust or upper mantle or tio2 activities inconsistent with the mineralogical and chemical compositions of the magmas cl imagery and measurements of ti and other elements in quartz are of great value in showing the growth histories and changes in conditions experienced by crystals but direct linkages to p t conditions during crystal growth cannot be achieved

constraints on the duration of high pressure metamorphism in the tauern window from diffusion modelling of discontinuous growth zones in eclogite garnet an eclogite sample from the grossgockner region of the hohe tauern austria contains garnet with a pronounced compositional discontinuity between a mn rich core and an fe rich rim this jump in composition was caused by a garnet consuming reaction followed by growth of the garnet rim omphacite and marks the prograde transition from epidote amphibolite to eclogite facies metamorphism garnet growth ended at peak metamorphic conditions of 570 c 17 kbar but intracrystalline diffusion continued until about 450 c 4 kbar on the retrograde path this garnet overgrowth texture represents a natural diffusion couple and a time span of 1 myr was calculated from the diffusion profile developing out of the original sharp compositional step for typical crustal densities this time corresponds to a minimum average velocity in the range 4 6 7 4 cm yr 1 for vertical movement which is one of the fastest exhumation rates reported the diffusion of all divalent cations of four profiles was modelled both analytically and numerically both approaches gave comparable results but the times computed for each element were always discrepant up to a factor of 2 variations of diffusion coefficients within 2 in analytical calculations remedied this and gave consistent upper time limits numerical modelling does not require the simplifications introduced in the analytical approach on the other hand error propagation was computationally unfeasible with this method

a late miocene pliocene origin for the central himalayan inverted metamorphism abstract perhaps the best known occurrence of an inverted metamorphic sequence is that found immediately beneath the himalayan main central thrust mct generally thought to have been active during the early miocene however in situ 208 pb 232 th dating of monazite inclusions in garnet indicates that peak metamorphic recrystallization of the mct footwall occurred in this portion of the central himalaya at only ca 6 ma the apparent inverted metamorphism appears to have resulted from activation of a broad shear zone beneath the mct which tectonically telescoped the young metamorphic sequence this explanation may resolve some outstanding problems in himalayan tectonics such why the mct and not the more recently initiated thrusts marks the break in slope of the present day mountain range it also renders unnecessary the need for exceptional physical conditions e g high shear stress to explain the generation of the himalayan leucogranites

petrology of the regional sillimanite zone west central new hampshire u s a with implications for the development of inverted isograds abstracr sillimanite from the regional sillimanite zone in west central new hampshire is fibrolitic and overprints f2 folds nappe stage of earlier mica foliation regional sillimanite zone samples show no evidence for earlier staurolite parageneses despite the fact that staurolite is abundant at lower grade because sillimanite was produced directly from garnet chlorite by the prograde heating reaction garnet chlorite muscovite quartz sillimanite biotite h20 the pressure at which this reaction occurs is sensitive to the mno and cao contents of garnet and phase equilibrium arguments reveal that at the regional pressures of west central new hampshire 2 4 kbar staurolite parageneses are only possible in rocks with low mno cao the inferred p t path is counterclockwise with nearly isobaric initial heating at 2 kbar followed by loading t heating to a peak metamorphic temperature of 600 t 25 c at 4 kbar followed by nearly isobaric cooling garnets were compositionally homogenized near the metamorphic peak but subsequent cooling was rapid 100 c m y these data argue against the folded isotherm model of chamberlain 1986 instead the present distribution of metamorphic grades is interpreted to be the result of regional stacking of high grade thrust sheets on lower grade rocks followed by depression of high grade rocks to lower structural levels

tio2 activity in metamorphosed pelitic and basic rocks principles and applications to metamorphism in southeastern canadian cordillera the activity of tio2 can be precisely defined as a function of pressure temperature and activities of other components for common mineral assemblages in metapelites ilmenite quartz garnet plagioclase al2sio5 and in metabasites plagioclase sphene ilmenite quartzgarnet these mineral assemblages can be modelled by the equilibria 1 3ilmenite al2sio5 2quartz almandine 3tio2 2 anorthite 2sphene grossular 2tio2 quartz 3 3anorthite 3quartz 6ilmenite grossular 6tio2 2almandine these mineral assemblages can be used at a text tio text 2 1 rutile saturation and a given t to get maximum pressure limits of some metapelites and metabasites when electron microprobe analyses of mineral grains adjacent to ti bearing phases are made these data give maximum pressure estimates in reasonable agreement with other geobarometers the activity of tio2 in many metapelites is very near rutile saturation but for metabasites the activity of tio2 in some sillimanite zone rocks is as low as 0 6 the solubility of tio2 in biotite hornblende and garnet is a complex function of t p the activities of components in coexisting minerals and crystal chemical constraints in these minerals at a given p and t the solubility of tio2 in biotite and hornblende does not appear to be strongly dependent upon a text tio text 2 1 for sphene and ilmenite versus rutile bearing assemblages

metamorphic p t paths from calcic pelitic schists from the strafford dome vermont usa metamorphism of the gile mountain formation and waits river formation in the strafford dome and townshend brownington syncline in east central vermont records two nappe style events d1 and d2 followed by doming d1 formed a muscovite biotite ilmenite schistosity subparallel to compositional layering so and was followed by heating to garnet grade the temperature and pressure at the end of d1 are estimated to be c 450d c and 6 8 kbar d2 variably crenulated and folded s1 during a nearly isothermal pressure increase of 1 2 kbar calculated from compositions of garnet which have inclusions trails with progressive crenulation and rotation of the s1 fabric similar p t paths are computed for most of the area suggesting that the later schistosity developed during emplacement of a regional nappe 3 6 km thick there is a general lack of d3 dome stage microstructures near the strafford willoughby arch staurolite and kyanite overgrew s2 in pelites and plagioclase with increasing xan overgrew s2 in calcic pelites reflecting post d2 heating to a maximum of 550 600d c metamorphic pressures at the end of d2 are fairly constant on the west side of the dome indicating minor dome stage uplift in contrast pressures at the thermal peak of metamorphism decrease by more than 4 kbar east of the dome the observed pattern of isotherms and isobars is mainly the result of post metamorphic differential uplift and unroofing finally a minor retrograde metamorphism produced the assemblage albite epidote k feldspar muscovite chlorite with grade increasing east toward the connecticut river

the duration of near peak metamorphism from diffusion modelling of garnet zoning garnet in a staurolite kyanite zone sample from central vermont displays a bell shaped mn growth zoning with diffusional modification over the outer 100 m the diffusion is driven by the prograde net transfer reaction garnet chlorite kyanite biotite as is evidenced by a well defined resorption zone on the rim analysis of the reaction history and resorbed garnet composition suggests that the peak temperature attained was 620 660 c diffusional modelling of the rim diffusion provides an estimate of the duration of the metamorphic episode over which significant garnet diffusion occurs the duration is a function of the assumed peak temperature and garnet diffusivities and range from a few hundred thousand years to a few million years such short durations require rapid tectonic burial and exhumation of relatively thin tectonic slices

geothermobarometric history of subduction recorded by quartz inclusions in garnet 1 burial histories of subduction zone rocks are often difficult to accurately constrain owing to a lack of robust mineral geobarometers applicable to high pressure mineral assemblages knowledge of the depth histories of subduction is however required for our understanding of global geochemical cycles subduction related seismicity and the evolution of destructive tectonic boundaries the high spatial resolution of quartz inclusion geobarometry can be used to determine pressure evolution during metamorphic growth of individual garnet crystals quartz inclusions in garnet from sifnos greece preserve such a record of the pressure of garnet growth allowing detailed reconstruction of the metamorphic evolution of these rocks pressure dependent raman spectra of quartz inclusions were combined with elastic modeling to infer the conditions at which they were trapped during garnet growth all measured inclusions suggest that garnet growth occurred between 19 and 20 5 kbars with little evidence for significant pressure variation during the garnet growth interval which is interpreted to record 100 c of heating coupled with thermometry and geochronology these results show that early cold burial was followed by a phase of rapid heating which immediately preceded exhumation garnet growth occurred primarily during this heating phase

petrochronology based on ree minerals monazite allanite xenotime apatite ree minerals monazite xenotime and allanite are ree1 minerals sensu stricto because lanthanides la lu and yttrium are critical constituents in them apatite does not require ree but because it contains substantial ree in many rocks it is included in this review all four minerals also host unusually high radionuclide concentrations notably th and u forming the basis of their utility as geochronometers this quartet of accessory minerals is playing an increasingly important role in petrochronology because they provide ways to link robust spot ages to petrogenetic p t conditions so can lend petrogenetic context to chronology based on other minerals part i of this review assembles the basic requisites prior to integrative petrochronologic analysis individual characteristics of the four ree minerals are addressed first i e their crystal chemistry and stability relations thermobarometers and trace element geochemistry used for tracing petrogenesis are discussed next and finally their chronology is summarized part ii presents case studies to highlight the specific strengths of ree minerals used to resolve the dynamics of a broad range of processes from diagenetic to magmatic conditions finally a brief section at the end outlines a few of the current challenges and promising perspectives for future work to introduce the four ree minerals in style let us recall the origins of their names the three phosphates have well respected greek grandparents and allanite has solid scottish roots yet of all four of them show idiosyncracies in etymology or type material apatite had long puzzled naturalists as it shows great chemical and physical variability and can resemble other minerals once properly identified abraham gottlieb werner named it apatite his reasoning referred to the greek root and giving the precise latin translation decipio taken literally both mean i deceive or i mislead which sounds like an apt confession from this mineral for having fooled

a holocene coral algal reef at mavra litharia gulf of corinth greece structure history and applications in relative sea level change a holocene coral algal reef at mavra litharia south central coast of gulf of corinth greece is exposed from ca 2 to 9 3 m above sea level on an uplifting footwall associated with the eliki fault the reef lacks sea level critical species but its coralline algal assemblage indicates a ca 10 m water depth reef frame components date from 9280 8730 years bp to 6343 5993 years bp so the reef frame grew between ca 10 000 and 6000 years bp the youngest dated shells 1860 305 years bp from the site are accessory organisms collected from the lowest 2 m of outcrop one of which dendropoma grew at sea level stiros s c and pirazzoli p 1998 late quaternary coastal changes in the gulf of corinth greece tectonics earthquake archaeology guidebook for the gulf of corinth field trip patras university greece patras september 14 16 1998 reef history has four phases a growth and lithification of reef b development of smooth walled dissolution pipes and caves in the reef c colonisation of dissolution surfaces by mn fe crusts that may be bacterially formed barnacles serpulid worms and rock boring bivalves and d uplift to present position where much of the reef is eroded sea level history after 11 500 years bp when rising post glacial sea level overtopped the rio sill and returned the gulf of corinth to a marine environment is reconstructed calculations of interplay between sea level rise and tectonic uplift suggest that between 11 500 and 10 000 years bp sea level rose very quickly associated with deglaciation at the close of the younger dryas mwp 1b at a maximum of 25 6 mm year broadly consistent with other studies then slowed to ca 4 4 mm year until 6000 years bp when sea level was ca 3 m below modern after which sea level rose at ca 0 5 mm year to modern day tectonic uplift rate of maximum 3 mm year slower than sea level rise means that the reef could not catch up to sea level until recent times explaining its fossil assemblage the pipe cave system is interpreted as having formed when the reef top was emerged but much dissolution occurred beneath sea level because of insufficient time to allow uplift and resubmergence intense rainfall during the mid holocene pluvial phase ca 6000 years bp for the eastern mediterranean which affected large areas of the mediterranean is suggested as the mechanism and timing of submarine dissolution

evolution of nd and pb isotopes in central pacific seawater from ferromanganese crusts abstract hydrogenetic ferromanganese crusts incorporate elements from ambient seawater during their growth on seamounts by analysing nd pb and be isotope profiles within crusts it is possible to reconstruct seawater tracer histories depth profiles of 10be 9be ratios in three pacific ferromanganese crusts have been used to obtain growth rates which are between 1 4 and 3 8 mm ma nd and pb isotopes provide intact records of isotopic variations in pacific seawater over the last 20 ma or more there were only small changes in pb isotope composition in the last 20 ma this indicates a constant pb composition for the erosional sources and suggests further that erosional nd inputs may have been uniform too end values vary considerably with time and most probably reflect changes in ocean circulation the end values of the crusts not only vary as a function of age but also as a function of water depth from 25 to 0 ma crust va13 2 from 4 8 km water depth has a similar pattern of end variation to the two shallower crusts from 1 8 and 2 3 km but about 1 0 to 1 5 units more negative this suggests that end stratification in pacific seawater as demonstrated for the present day has been maintained for at least 20 ma each crust shows a decrease in end from 3 5 ma to the present which is interpreted in terms of an increase in the nadw component present in the pacific from 10 to 3 5 ma ago the crusts show an increase in end this suggests a decreasing role for a deep water source with end less than circum pacific sources in this regard the panamanian gateway restriction from 10 ma with final closure at 3 5 ma may have played an important role in reducing access of atlantic derived nd to the pacific

the nature and origin of the barrovian metamorphism scotland diffusion length scales in garnet and inferred thermal time scales abstract in this paper we examine the length scales of major element diffusion in garnet during the barrovian metamorphism the role of diffusion in the flattening of mn zoning profiles in garnet with increasing metamorphic grade across the barrovian metamorphic series is ambiguous however the loss of distinct mn defined secondary compositional zoning in barrovian garnets with increasing metamorphic grade and preservation of geochemical textures around mn free inclusions within high grade barrovian garnets provide robust evidence for c 1000 m mn diffusion in sillimanite zone garnets during the barrovian regional metamorphism sillimanite grade garnets from the barrovian metamorphic series also preserve c 100 m mn diffusion textures between sillimanite grade rim domains and lower grade cores bimodality in diffusion length scales requires bimodality in thermal time scales length scales of chemical diffusion are considered within the context of recent duration estimates for the barrovian metamorphism of a few million years we conclude that heat associated with the barrovian regional metamorphism accumulated following numerous short time scale heating events responsible for the smaller scale diffusion textures this self similar thermal regime was likely accommodated by the operation of shear zones in the highest grade regions of the barrovian metamorphic series which concentrated small scale episodic heat sources such as sheeted magmas fluids and or mechanical work supplementary material a table of quantitative electron microprobe analyses an example of the calibration curve used to produce quantitative electron microprobe maps from count maps and y maps are available at http www geolsoc org uk sup18415

using microstructures and titaniq thermobarometry of quartz sheared around garnet porphyroclasts to evaluate microstructural evolution and constrain an alpine fault zone geotherm abstract interpretations of deformation processes within ductile shear zones are often based on the characterisation of microstructures preserved in exhumed rocks however exhumed microstructures provide only a snapshot of the closing stages of deformation and we need ways of understanding how microstructures change through time and at what rate this occurs to address this problem we study optical microstructures and electron backscatter diffraction ebsd data from samples of quartz layers deflected around garnet porphyroclasts which generate local stress and strain rate perturbations during mylonitic deformation in the alpine fault zone of new zealand during shearing around rigid garnet porphyroclasts quartz undergoes grain size reduction in response to locally increased stresses while c axes reveal increasing components of rhomb and prism slip reflecting a local increase in shear strain and strain rate titaniq thermobarometry and quartz microstructures suggest a rather narrow range of recorded quartz deformation temperatures around 450 500 c which we propose reflects the cessation of grain boundary migration driven deformation given that temperatures well above the brittle ductile transition for quartz 350 c are preserved we anticipate that rapid cooling and exhumation must have occurred from the 500 c isotherm ultimately we propose a modified geotherm for the central alpine fault zone hanging wall which raises the 500 c isotherm to 11 km depth near the brittle ductile transition our updated alpine fault zone geotherm implies a hotter and weaker middle to lower crust than previously proposed

ore forming fluids and metallogenesis of mengku iron deposit in altay xinjiang the mengku large size iron deposit occurs in metavolcanic sedimentary sequence of the upper silurian lower devonian kangbutiebao formation with the host rocks being garnet skarn granulitite leucogranulitite and marble ore bodies are largely concordant with the bedding of the strata and the distribution of the ore bodies is spatially related to skarn inclusions in garnet of the skarn period are dominantly glassy melt inclusions fluid melt inclusions and fluid inclusions fluid inclusions in minerals of the middle skarn stage are liquid inclusions two phase aqueous type inclusions fluid inclusions in minerals of the regional metamorphic period mostly contain liquid inclusions and daughter mineral bearing inclusions homogenization temperatures of melt inclusions in the skarn period is 1 100 and homogenization temperatures of fluid inclusions in the early skarn stage vary from 193 to 499 with the peaks around 450 350 and 230 homogenization temperatures of fluid inclusions in the late skarn stage vary from 263 to 550 with the peak around 350 fluid inclusions in the regional metamorphic period show a wide range of homogenization temperatures from 132 to 513 c with three peaks around 350 230 and 190 respectively salinity ranges from 1 23 to 60 31 wt nacl equiv ore fluid densities range from 0 64 to 1 16 g cm3 the 18osmow values of garnet quartz and calcite range from 0 2 to 8 4 with the corresponding 18oh2o values between 5 1 and 5 33 and d values of fluid inclusions between 127 and 81 the combined isotopic data imply that the ore forming fluids of the skarn period in the mengku iron deposit were mainly derived from magmatic fluids with a minor part from the meteoric water and the ore forming fluids of the regional metamorphic period was mainly derived from the meteoric water with a minor part from the metamorphic water the 13cpdb values of calcite from the mengku iron deposit define a narrow range of 6 1 to 2 3 indicating that the carbon in the fluids was mainly derived from the mantle or a deep seated place the mengku iron mineralization took place in the early period of early devonian slightly later than 404 400 ma with its metallogenesis related to the retrogressive metamorphism of the skarn

single shot laser ablation split stream ss lass petrochronology deciphers multiple short duration metamorphic events single shot laser ablation split stream ss lass petrochronology offers spatial resolution of 1 m per surface analysis this enables the technique to interrogate metamorphic zircon overgrowths that are very thin or that preserve a composite age structure to demonstrate the advantages of ss lass the technique was applied to metamorphic zircon overgrowths in five rocks from the cordillera de la costa venezuela in these rocks ss lass was able to decipher discrete short duration 106 yr zircon growth events at c 33 0 c 28 3 c 23 0 and c 18 2 ma comparison with existing geo thermochronology suggests that the ss lass dates represent hydro thermal events that mark distinct episodes of tectonism affecting the northern margin of south america with an external error of 5 incorporated into isotope ratios as required to achieve satisfactory analytical precision ss lass dates for reference materials were accurate to within 1 5 of published values the ss lass technique is thus in favorable cases capable of providing 106 yr resolution for cenozoic rocks methods in geospeedometry which employ forward models to reproduce observed diffusion length scales have obtained anomalously short time scales of 104 106 yr for metamorphism the high spatial and temporal resolution of ss lass offers tremendous promise for investigating the veracity of these claims

trace element zoning of sulfides and quartz at sheba and fairview gold mines clues to mesoarchean mineralisation in the barberton greenstone belt south africa abstract the fairview and sheba mines are two of the major gold mines in the paleoarchean barberton greenstone belt of southern africa at these mines gold is associated with quartz carbonate rutile veins and occurs both as invisible gold finely dispersed in sulfides primarily pyrite and arsenopyrite and as visible electrum grains hosted in pyrite up to approximately 1000 ppm au are contained in pyrite and up to approximately 1700 ppm in arsenopyrite mapping of trace element distribution in sulfide minerals using electron microprobe and proton probe techniques revealed multiple events of ore formation and au mineralisation at fairview mine three stages of pyrite formation were identified the last of which is associated with arsenopyrite electrum and other sulfide minerals sphalerite chalcopyrite galena gersdorffite and sb sulfides at sheba mine pyrite was deposited in two stages and electrum is associated with the second stage at both mines the last stage of sulfide formation is the main stage of au deposition and is associated with mobilisation of au as sb cu zn and ni the host rock composition seems to have affected the composition of pyrite since higher ni and co concentrations up to 1 4 and 1 6 wt respectively have been measured in meta ultra mafic host rocks in comparison with chert and metagreywacke arsenopyrite is chemically zoned and has sb and s rich cores and as and ni rich rims this zoning indicates variations in fluid compositions decreasing sb and increasing ni and crystallisation conditions increasing as content for increasing temperature geothermometric estimates based on the as content of arsenopyrite as 32 at indicate temperatures up to 420 c for the crystal rims petrographic and cathodoluminescence observations of quartz associated with gold mineralisation show only local brittle deformation and no plastic deformation this supports the notion that the ore transporting veins were emplaced late in the deformation history variations of cathodoluminescence of quartz are correlated with changing al contents al 0 16 wt and can be related to fluctuations in the ph of the mineralising fluids

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

high temperature reduced granulite facies nature of garnetites in the khabarny mafic ultramafic massif southern urals evidence from fluid and mineral analyses