ion microprobe analysis of oxygen isotopes in garnets of complex chemistry abstract accurate ion microprobe analysis of oxygen isotope ratios in garnet is possible if appropriate standards are employed to correct for instrumental bias a component of which depends on the cation chemistry of the analyzed mineral in this study 26 garnet standards including 14 new standards that span the compositional range of pyrope almandine grossular spessartine and andradite were analyzed repeatedly by ion microprobe to develop a new method of correcting for instrumental bias in garnets all analyses were normalized to a single master garnet standard uwg 2 before bias from cation composition was considered bias due to cation composition in garnet was found to correlate with grossular content in pyralspite garnets and with andradite in ugrandite garnets bias is correlated with molar volume in garnets of all compositions in this study although this correlation is suitable as a correction scheme for bias a more accurate correction scheme based on the grossular and andradite compositions of garnet is proposed this method reproduces the bias of all but one standard to within a range of 0 4 an accuracy that is on the same order as the reproducibility 0 3 2 s d of the master garnet standard uwg 2 but that remains an independent source of error the new correction scheme is used to successfully reproduce laser fluorination analyses along a traverse of a polymetamorphic zoned skarn garnet from the adirondack mountains while previous analyses were at the mm scale the new data resolve a gradient of 18 o of 2 1 over 16 m if experimentally derived diffusion coefficients are correct these new results show that granulite facies metamorphism was significantly faster than previously assumed and the thermal peak was less than 5 myr

coesite eclogites from the roberts victor kimberlite south africa abstract coesite or pseudomorphic quartz is more common than previously recognized in the eclogite xenolith suite from the roberts victor mine south africa all coesite eclogites in this study 18 samples are classified as group i and have mineral compositions that span virtually the entire compositional range of group i eclogites described from this locality most of the roberts victor group i eclogite suite may be in equilibrium with free silica thus weakening the case for these rocks representing residues of partial melting oxygen isotope compositions of garnets from 15 samples are in the range 5 32 6 95 similar to other roberts victor group i eclogites these observations are consistent with a model in which protoliths of these rocks were oceanic basalts and intrusive rocks that underwent exchange with seawater at relatively low temperatures to variable extents prior to subduction and metamorphism to eclogite facies without suffering significant partial melting during or following subduction

high temperature deformation during continental margin subduction exhumation the ultrahigh pressure western gneiss region of norway abstract a new dataset for the high pressure to ultrahigh pressure western gneiss region allows the definition of distinct structural and petrological domains much of the study area is an e dipping homocline with e plunging lineations that exposes progressively deeper more strongly deformed more eclogite rich structural levels westward although eclogites crop out across the wgr scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area the scandian reworking increases westward culminating in strong scandian fabrics with only isolated pockets of older structures the dominant scandian deformation was coaxial e w stretching the sinistrally sheared more trondelag fault complex and nordfjord mylonitic shear zone bound these rocks to the north and south there was moderate top e amphibolite facies deformation associated with translation of the allochthons over the basement along its eastern edge and the nordfjord sogn detachment zone underwent strong lower amphibolite facies to greenschist facies top w shearing a northwestward increase in exhumation related melting is indicated by leucosomes with hornblende plagioclase and scandian sphene in the western 2 3 of the study area exhumation related amphibolite facies symplectite formation in quartzofeldspathic gneiss postdated most scandian deformation further deformation was restricted to slip along biotite rich foliation planes and minor local folding that the western gneiss region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation implies that continental crust in general need not undergo pervasive deformation during subduction

kinematics of eastern salt range and south potwar basin pakistan a new scenario the salt range of northern pakistan represents an emergent thrust front along which the potwar plateau has been translated southward based on the analysis of subsurface data we have constructed a balanced cross section exhibiting a frontal fault bend fold and a set of detachment folds below the potwar plateau from the restored section we are able to propose a forward kinematic modelling using the thrustpack software the main characteristics of the model are the alternation of periods during which deformation was concentrated on the frontal thrust between 10 and 5 ma and since 1 9 ma and an intervening episode during which deformation was distributed along the whole section between 5 and 1 9 ma the latter phase is marked by development of out of sequence structures the episodes of frontal thrust activation are characterised by high denudation rates on the contrary out of sequence folding occurred during periods of important sedimentary accumulation the consequences for petroleum exploration are also discussed however timing of the petroleum system is strongly dependent on hypotheses made on basal heat flow

eclogites and eclogites in the western gneiss region norwegian caledonides the western gneiss region wgr marks the outcrop of a composite terrane consisting of variably re worked proterozoic basement and parautochthonous or autochthonous cover units the wgr exhibits a gross structural petrographic and thermobarometric zonation from southeast to northwest reflecting an increasing intensity of scandian late palaeozoic

isotope evidence for the involvement of recycled sediments in diamond formation diamonds as chemically robust containers of material from the mantle have provided important information regarding the evolution of the earth s interior1 yet despite much effort2 9 the origins of diamonds themselves have remained controversial10 13 sulphur isotope ratios measured in most mantle or meteoritic samples exhibit minimal deviation from the meteoritic standard whereas the ratios in crustal materials deviate by as much as 70 ref 14 for this reason isotopic studies of sulphide mineral inclusions in diamonds and mantle material are an attractive means of investigating the possible role of crust mantle interaction in the formation of diamonds15 recent ion microprobe studies have found departures from mantle values in the sulphur isotope compositions of individual diamond inclusions16 and some mantle rocks17 which would be consistent with subduction of altered ocean crust into the region of diamond growth here we present new sulphur and lead isotope data from diamond sulphide inclusions which suggest that sedimentary material might also be subducted into the mantle and that the crustal recycling process has been operative for at least 1 000 million years

trace element geochemistry of coesite bearing eclogites from the roberts victor kimberlite kaapvaal craton trace element characteristics of seven coesite bearing eclogitic xenoliths from the roberts victor kimberlite demonstrate that this suite of eclogites originated as gabbroic cumulates in oceanic crust that was subsequently subducted all but one of the garnets show positive eu anomalies accompanied by a flat heavy rare earth pattern which is atypical of garnet but characteristic of plagioclase arguing for a considerable amount of plagioclase in the protoliths forward modelling of the accumulation of liquidus minerals from primitive komatiitic picritic and basaltic liquids suggests that at least some of the eclogite protoliths were not derived from basaltic parental liquids whereas derivation from either komatiitic or picritic liquids is possible the reconstructed eclogite bulk rocks compare favourably with oceanic gabbros from odp hole 735b sw indian ridge even to the extent that oxygen isotopic systematics show signs of low temperature seawater alteration however the oxygen isotope trends are the reverse of what is expected for cumulates in the lower section of the oceanic crust these new findings show that 18o values in eclogitic xenoliths despite being sound indicators for their interaction with hydrothermal fluids at low pressure do not necessarily bear a simple relationship with the inferred oceanic crustal stratigraphy of the protoliths

equilibrium oxygen hydrogen and carbon isotope fractionation factors applicable to geologic systems as demonstrated by the chapters in this short course stable isotope techniques are an important tool in almost every branch of the earth sciences central to many of these applications is a quantitative understanding of equilibrium isotope partitioning between substances indeed it was harold urey s 1947 thermodynamically based estimate of the temperature dependence of 18o 16o fractionation between calcium carbonate and water and a recognition of how this information might be used to determine the temperatures of ancient oceans that launched the science of stable isotope geochemistry the approach pioneered by urey has since been used to estimate temperatures for a wide range of geological processes e g emiliani 1955 anderson et al 1971 clayton 1986 valley this volume in addition to their geothermometric applications equilibrium fractionation data are also important in the study of fluid rock interactions including those associated with diagenetic hydrothermal and metamorphic processes baumgartner and valley this volume shanks this volume finally a knowledge of equilibrium fractionation is a necessary first step in evaluating isotopic disequilibrium a widespread phenomenon that is increasingly being used to study temporal relationships in geological systems cole and chakraborty this volume in the fifty four years since the publication of urey s paper equilibrium fractionation data have been reported for many minerals and fluids of geological interest these data were derived from 1 theoretical calculations following the methods developed by urey 1947 and bigeleisen and mayer 1947 2 direct laboratory experiments 3 semi empirical bond strength models and 4 measurement of fractionations in natural samples each of these methods has its advantages and disadvantages however the availability of a variety of methods for calibrating fractionation factors has led to a plethora of calibrations not all of which are in agreement in this chapter we evaluate the major methods for determining fractionation factors

optically continuous silcrete quartz cements of the st peter sandstone high precision oxygen isotope analysis by ion microprobe a detailed oxygen isotope study of detrital quartz and authigenic quartz overgrowths from shallowly buried 1 km quartz arenites of the st peter sandstone in sw wisconsin constrains temperature and fluid sources during diagenesis quartz overgrowths are syntaxial optically continuous and show complex luminescent zonation by cathodoluminescence detrital quartz grains were separated from 53 rocks and analyzed for oxygen isotope ratio by laser fluorination resulting

carbon and oxygen isotope variations in diamond and graphite eclogites from orapa botswana and the nitrogen content of their diamonds abstract in eclogite xenoliths from orapa botswana 13 c of graphite varies between 4 6 and 7 8 vs pdb while that of diamonds ranges from 4 0 to 22 3 in graphite eclogites 18 o values of garnets fall between 5 2 and 7 vs smow and those of clinopyroxenes between 4 6 and 6 5 for diamond eclogites the respective ranges are wider i e 3 9 to 9 2 and 5 2 to 8 3 the 18 o enrichment above normal mantle oxygen isotopic compositions 5 to 6 5 and the wide 13 c range appear to occur in parts of the mantle in which diamond is stable eclogites with 13 c values in the range 4 to 8 and 13 o values in the normal mantle range tend to have positive garnet clinopyroxene 18 o fractionations and lower fe ca and fe mg ratios than eclogites which have simultaneously 13 c values lower than 8 and 18 o values higher than 6 5 in these eclogites the garnet clinopyroxene fractionations tend to be negative positive garnet clinopyroxene 18 o fractionations increase slightly in the sequence diamond eclogites 0 36 0 15 n 6 graphite diamond eclogites 0 46 0 36 n 3 and graphite eclogites 0 53 0 15 n 3 neither igneous fractionation processes subduction nor metasomatic alteration can satisfactorily explain all observed chemical and isotopic trends

unusually low δ18o ultra high pressure metamorphic rocks from the sulu terrain eastern china eclogite and quartzite interlayers containing ultra high pressure uhp metamorphic mineral assemblages occur at qinglongshan in the sulu terrain eastern china these rocks exhibit some of the most negative oxygen isotope compositions that have ever been reported for metamorphic rocks the 18 value of minerals in eclogite ranges from 10 4 to 9 0 o and in quartzite ranges from 10 2 to 7 3 near isotopic equilibrium between minerals and rocks suggests that these rocks must have acquired such low oxygen isotope compositions before the uhp metamorphism through near surface meteoric water rock interactions the rocks were then subducted to a great depth during continent continent collision forming uhp minerals with concomitant local oxygen isotope redistribution it is significant that the rocks have retained such negative oxygen isotope compositions despite subduction into the upper mantle

spatial variations of the effective elastic thickness te using multitaper spectral estimation and wavelet methods examples from synthetic data and application to south america 1 improved recovery of the spatial variability in the effective elastic thickness te of the lithosphere is important to interpret earth s structure and geodynamics here we use synthetic topography and gravity data to systematically compare the recovery of te using the multitaper windowing scheme of perez gussinye et al 2004 2007 with the wavelet approach of kirby and swain 2008 we find that spuriously high te estimates previously present in synthetic tests with the multitaper are artifacts arising from fourier transform edge effects introduced by splitting the data into land and ocean areas prior to load deconvolution hence we adopt the mixed land ocean loading scheme of kirby and swain 2008 using this approach we find that underestimation of te when the transitional wavelength approaches the window size is largely related to the limited size of the analysis window and not to random correlations of initial loading surfaces as occurs in the wavelet method to attenuate this bias we apply a correction factor to our resulting multitapered te and find that this technique more accurately recovers steep te gradients and small scale te anomalies than the wavelet method finally we recalculate te of south america using the multitaper and the mixed land ocean approach the new te distribution is very similar to that of perez gussinye et al 2007 2008 suggesting that fourier transform edge effects are less severe in the real earth than in synthetic data this difference arises because coastlines are much less rough in the real earth than in our synthetic data on the basis of the synthetic results we interpret that most of the small scale features present in the multitaper but not in wavelet te estimates of south america presented by tassara et al 2007 are real these include high te over the andean flat subduction zones and low te along the dyke swarms of the parana flood basalts the tacutu graben and amazonian basin

oxygen isotope constraints on metamorphic fluid flow townshend dam vermont u s a fluid rock interaction during amphibolite facies metamorphism has been investigated for rocks exposed in a single 400 m long lithologically heterogeneous outcrop near townshend vermont usa oxygen isotopic compositions have been measured in profiles across single garnet crystals from thirteen samples and from hornblende and garnet separates from thirty three fine grained samples this outcrop was previously studied who measured large oxygen isotope gradients 3 per thousand in garnets from one sample and inferred large amounts of pervasive fluid flow all of the garnets that we have analyzed show less isotopic zonation le 1 per thousand twelve of them have zonation 0 5 per thousand and the mineral separate data imply both a strong correlation of isotopic composition with rock type and large gradients in peak metamorphic fluid isotopic compositions although devolatilization reactions in these rocks must have produced metamorphic fluids the data preclude cross foliation time integrated fluid fluxes greater than 300 600 cm sup 3 cm sup 2 during prograde amphibolite facies metamorphism the isotopic trends can all be interpreted in terms of closed system behavior channeled fluid flow or diffusive exchange of oxygen in an interconnected grain boundary fluid and within uncertainty cross strike advective fluxes could have been zero any significant flow at this locality was dominantly either layer parallel or channeled out of the system in veins more if all fluid flow occurred in veins then their spacing must have been less than 300 600 m the data are inconsistent with massive and pervasive metamorphic fluid flow across strike but do not address layer parallel flow less

low δ18o eclogites from the kokchetav massif northern kazakhstan oxygen isotopic compositions of silicates in eclogites and whiteschists from the kokchetav massif were analyzed by whole grain co2 laser fluorination methods systematic analyses yield extremely low 18o for eclogites as low as 3 9 for garnet these values are comparable with those reported for the dabie sulu uhp eclogites oxygen isotopic compositions are heterogeneous in samples of eclogite even on an outcrop scale schists have rather uniform oxygen isotope values compared to eclogites and low 18o is not observed isotope thermometry indicates that both eclogites and schists achieved high temperature isotopic equilibration at 500 800 c this implies that retrograde metamorphic recrystallization barely modified the peak metamorphic oxygen isotopic signatures a possible geological environment to account for the low 18o basaltic protolith is a continental rift most likely subjected to the conditions of a cold climate after the basalt interacted with low 18o meteoric water it was tectonically inserted into the surrounding sedimentary units prior to or during subduction and uhp metamorphism

eclogite facies fluid infiltration constraints from δ 18 o zoning in garnet in situ analysis reveals that eclogite facies garnets are zoned in 18o with lower values in the core and rims that are 1 5 to 2 5 higher this pattern is present in 9 out of 12 garnets analyzed by sims from four orogenic eclogite terranes and correlates with an increase in the mole fraction of pyrope and mg fe ratio from core to rim indicating prograde garnet growth at the maximum temperatures and the time scales experienced by these garnets calculated intragranular diffusion distances for oxygen are small 5 from two kimberlites are homogeneous in 18o possibly due to diffusive exchange which is possible for prolonged periods at higher mantle temperatures homogeneity of 18o in garnets outside the normal mantle range 5 6 may be common in kimberlitic samples

extreme crustal oxygen isotope signatures preserved in coesite in diamond the anomalously high and low oxygen isotope values observed in eclogite xenoliths from the upper mantle beneath cratons have been interpreted as indicating that the parent rock of the eclogites experienced alteration on the ancient sea floor1 recognition of this genetic lineage has provided the foundation for a model of the evolution of the continents whereby imbricated slabs of oceanic lithosphere underpin and promote stabilization of early cratons2 early crustal growth is thought to have been enhanced by the addition of slab derived magmas leaving an eclogite residuum in the upper mantle beneath the cratons3 but the oxygen isotope anomalies observed in eclogite xenoliths are small relative to those in altered ocean floor basalt and intermediate stage subduction zone eclogites and this has hindered acceptance of the hypothesis that the eclogite xenoliths represent subducted and metamorphosed ocean floor basalts we present here the oxygen isotope composition of eclogitic mineral inclusions analysed in situ in diamonds using an ion microprobe secondary ion mass spectrometer the oxygen isotope values of coesite a polymorph of sio2 inclusions are substantially higher than previously reported for xenoliths from the subcratonic mantle but are typical of subduction zone meta basalts and accordingly provide strong support for the link between altered ocean floor basalts and mantle eclogite xenoliths

neoproterozoic anomalous kimberlites of guaniamo venezuela mica kimberlites of isotopic transitional type in venezuela kimberlites have so far only been found in the guaniamo region where they occur as high diamond grade sheets in massive to steeply foliated paleoproterozoic granitoid rocks the emplacement age of the guaniamo kimberlites is 712f6 ma i e neoproterozoic the guaniamo kimberlites contain a high abundance of mantle minerals with greater than 30 olivine macrocrysts the principal kimberlite indicator minerals found are pyrope garnet and chromian spinel with the overwhelming majority of the garnets being of the peridotite association chrome diopside is rare and picroilmenite is uncommon chemically the guaniamo kimberlites are characterized by high mgo contents with low al2o3 and tio2 contents and higher than average feo and k2o contents these rocks have above average ni cr co th nb ta sr and lree concentrations and very low p yand particularly zr and hf contents the nb zr ratio is very distinctive and is similar to that of the aries australia kimberlite the guaniamo kimberlites are similar in petrography mineralogy and mantle mineral content to ilmenite free group 2 mica kimberlites of south africa the nd sr isotopic characteristics of guaniamo kimberlites are distinct from both kimberlite group 1 and group 2 being more similar to transitional type kimberlites and in particular to diamondiferous kimberlites of the arkhangelsk diamond province russia the guaniamo kimberlites form part of a compositional spectrum between other standard kimberlite reference groups they formed from metasomatised subcontinental lithospheric mantle and it is likely that subduction of oceanic crust was the source of this metasomatised material and also of the eclogitic component which is dominant in guaniamo diamonds d 2004 published by elsevier b v

protolith ages and exhumation histories of ultra high pressure rocks across the western gneiss region norway the timing of protolith formation ultrahigh pressure uhp subduction and subsequent exhumation for the ultrahigh pressure to high pressure units across the eastern part of the western gneiss region norway were assessed using u pb zircon th pb monazite and 40 ar 39 ar white mica ages u pb zircon ages from eclogites demonstrate that oceanic and continental allochthons were emplaced onto the baltica basement before the entire mass was subducted to ultra high pressure eclogites within the allochthons across the entire western gneiss region are caledonian and show a degree of zircon re crystallization that increases with peak pressure permitting the interpretation that the entire region underwent synchronous subduction 40 ar 39 ar white mica ages of 399 ma indicate that the eastern part of the western gneiss region had been exhumed to shallow crustal levels while uhp metamorphism was ongoing farther west indicating a westward dip to the slab the 40 ar 39 ar white mica ages also show a clear east to west gradient across the entire western gneiss region indicating that the western gneiss region rose diachronously to crustal levels from east to west between 399 and 390 ma

oxygen isotopes in coexisting garnets clinopyroxenes and phlogopites of roberts victor eclogites implications for petrogenesis and mantle metasomatism abstract 18 o values of coexisting garnet clinopyroxene and phlogopite for twelve compositionally and texturally diverse roberts victor eclogite xenoliths range from 3 8 to 7 1 4 0 to 7 4 and 5 9 to 7 4 respectively differences between the 18 o values of coexisting garnets and clinopyroxenes are normally zero however there is some variation in the 18 o values of different fractions of the same mineral in four samples which suggests the presence of isotopic zonation and inhomogeneity possibly resulting from the introduction of a secondary fluid which metasomatized the eclogites and resulted in the formation of phlogopite amphibole and celsian the 18 o value of the metasomatic fluid is generally buffered by the isotopic composition of the primary garnet and clinopyroxene as indicated by a correlation between the isotopic composition of phlogopite and the primary pyroxene and garnet the large range in 18 o values of the eclogites and the similarity in the isotopic composition of coexisting pyroxene and garnet support the interpretation that the roberts victor eclogites represent metamorphosed altered basalts the eclogites were subjected to infiltration metasomatism in the mantle prior to their incorporation in the kimberlite and the source of this fluid was probably unrelated to the eclogite

a new look at stable isotope thermometry interdiffusion between coexisting minerals affects all rocks and causes resetting and discordance of stable isotope geothermometers that is commonly observed in slowly cooled igneous and metamorphic rocks the fast grain boundary fgb model describes the stable isotope fractionations and intracrystalline zonation which result from closed system interdiffusion eiler et al 1991 1992 this model assumes that grain boundary diffusion is much faster than volume diffusion and it accounts for exchange among all minerals in a rock previous models of closure temperature violate mass balance restrictions and will be inaccurate in most rocks modeling results are described for amphibolites and hornblende granites and gneisses biotite granites schists and gneisses pelitic and semi pelitic rocks garnet peridotites anorthosites gabbros pyroxenites and related rocks and calc silicate rocks examples of mineral pairs and specific rock types that allow accurate stable isotope thermometry include plagioclase pyroxene in pyroxene bearing anorthosites and garnet quartz in garnetiferous quartzites in contrast the same mineral pairs in related rocks such as pyroxenites and pelitic schists will exhibit reset apparent temperatures closed system processes are capable of producing a variety of patterns of stable isotope resetting discordance mineral zonation and fractionation reversals examples include large reversals of quartz feldspar fractionations in micaceous rocks and oscillatory zonation in feldspar from some quartz rich rocks these results permit reinterpretation of many studies of stable isotope thermometry speedometry and retrograde alteration history fgb modeling of mineral zonation provides an important new guide to applying recently developed micro analytical tools to slowly cooled rocks application of the fgb model to quartzo feldspathic gneisses from the adirondack mountains new york demonstrates the usefulness of diffusion modeling in discriminating closed system diffusion controlled retrogression from open system retrogression and illustrates the possible importance of incorporating the effect of water activity on mineral diffusivity

geochronology and geochemistry of eclogites from the marianske lazně complex czech republic implications for variscan orogenesis the marianske lazn complex mlc is located in the bohemian massif along the north western margin of the tepla barrandian microplate and consists of metagabbro amphibolite and eclogite with subordinate amounts of serpentinite felsic gneiss and calcsilicate rocks the mlc is interpreted as a metaophiolite complex that marks the suture zone between the saxothuringian rocks to the north west and the tepla barrandian microplate to the south east sm nd geochronology of garnet omphacite pairs from two eclogite samples yields ages of 377 7 and 367 4 ma samples of eclogite and amphibolite do not define a whole rock sm nd isochron even though there is a large range in sm nd ratio implying that the suite of samples may not be cogenetic eclogites do not have correlated nd values and initial 87sr 86sr ratios five of the eight eclogite samples have high nd values 10 2 to 7 1 consistent with derivation from a morb like source but variable 87sr 86sr ratios 0 7033 to 0 7059 which probably reflect hydrothermal seawater alteration three other eclogite samples have lower nd values 5 4 to 0 8 and widely variable 87sr 86sr ratios 0 7033 to 0 7096 such low nd values are inconsistent with derivation from a morb source and may reflect a subduction or oceanic island basalt component in their source the mlc is an important petrotectonic element in the bohemian massif providing evidence for cambro ordovician formation of oceanic crust and interaction with seawater late devonian frasnian famennian high and medium pressure metamorphism related to closure of a saxothuringian ocean basin early carboniferous visean thrusting of the tepla terrane over saxothuringian rocks and late visean extension

uniformly mantle like δ18o in zircons from oceanic plagiogranites and gabbros lower ocean crust is primarily gabbroic although 1 2 felsic igneous rocks that are referred to collectively as plagiogranites occur locally recent experimental evidence suggests that plagiogranite magmas can form by hydrous partial melting of gabbro triggered by seawater derived fluids and thus they may indicate early high temperature hydrothermal fluid circulation to explore seawater rock interaction prior to and during the genesis of plagiogranite and other late stage magmas oxygen isotope ratios preserved in igneous zircon have been measured by ion microprobe a total of 197 zircons from 43 plagiogranite evolved gabbro and hydrothermally altered fault rock samples have been analyzed samples originate primarily from drill core acquired during ocean drilling program and integrated ocean drilling program operations near the mid atlantic and southwest indian ridges with the exception of rare distinctively luminescent rims all zircons from ocean crust record remarkably uniform 18o with an average value of 5 2 0 5 2sd the average 18o zrc would be in magmatic equilibrium with unaltered morb 18o wr 5 6 5 7 and is consistent with the previously determined value for equilibrium with the mantle the narrow range of measured 18o values is predicted for zircon crystallization from variable parent melt compositions and temperatures in a closed system and provides no indication of any interactions between altered rocks or seawater and the evolved parent melts if plagiogranite forms by hydrous partial melting the uniform mantle like 18o zrc requires melting and zircon crystallization prior to significant amounts of water rock interactions that alter the protolith 18o zircons from ocean crust have been proposed as a tectonic analog for 3 9 ga detrital zircons from the earliest hadean earth by multiple workers however zircons from ocean crust are readily distinguished geochemically from zircons formed in continental crustal environments many of the 3 9 ga zircons have mildly elevated 18o 6 0 7 5 but such values have not been identified in any zircons from the large sample suite examined here the difference in 18o in combination with newly acquired lithium concentrations and published trace element data clearly shows that the 3 9 ga detrital zircons did not originate by processes analogous to those in modern mid ocean ridge settings

fractionation of oxygen and iron isotopes by partial melting processes implications for the interpretation of stable isotope signatures in mafic rocks abstract recycling of oceanic crust into the deep mantle via subduction is a widely accepted mechanism for creating compositional heterogeneity in the upper mantle and for explaining the distinct geochemistry of mantle plumes the oxygen isotope ratios 18o of some ocean island basalts oib span values both above and below that of unmetasomatised upper mantle 5 5 0 4 and provide support for this hypothesis as it is widely assumed that most variations in 18o are produced by near surface low temperature processes here we show a significant linear relationship between 18o and stable iron isotope ratios 57fe in a suite of pristine eclogite xenoliths the 18o values of both bulk samples and garnets range from values within error of normal mantle to significantly lighter values the observed range and correlation between 18o and 57fe is unlikely to be inherited from oceanic crust as 57fe values determined for samples of hydrothermally altered oceanic crust do not differ significantly from the mantle value and show no correlation with 18o it is proposed that the correlated 57fe and 18o variations in this particular eclogite suite are predominantly related to isotopic fractionation by disequilibrium partial melting although modification by melt percolation processes cannot be ruled out fractionation of fe and o isotopes by removal of partial melt enriched in isotopically heavy fe and o is supported by negative correlations between bulk sample 57fe and cr content and bulk sample and garnet 18o and sc contents as cr and sc are elements that become enriched in garnet and pyroxene bearing melt residues melt extraction could take place either during subduction where the eclogites represent the residues of melted oceanic lithosphere or could take place during long term residence within the lithospheric mantle in which case the protoliths of the eclogites could be of either crustal or mantle origin this modification of both 57fe and 18o by melting processes and specifically the production of low 18o signatures in mafic rocks implies that some of the isotopically light 18o values observed in oib and eclogite xenoliths may not necessarily reflect near surface processes or components

stable isotope thermometry at high temperatures determination of accurate temperatures for geological events has been the grail of stable isotope geochemistry since the seminal 1947 paper by urey in theory this should be simple calibrated mineral pairs are common analysis is rapid and there is no significant pressure correction however in spite of widespread application the promise of reliable thermometry has been elusive stable isotope temperatures in metamorphic and igneous rocks are often controversial is the fractionation between two phases a thermometer speedometer rate dependant hygrometer p h2o dependant or chimera a number of factors have contributed to this uncertainty including incomplete and sometimes conflicting calibration of isotope fractionation factors limited understanding of the kinetics of mineral exchange and the lack of microanalytical techniques this situation has improved dramatically over the past decade permitting reliable and detailed thermal histories to be inferred in a landmark paper bottinga and javoy 1975 empirically calibrated the oxygen isotope fractionation factors among ten rock forming minerals at t 500 c their compilation shows that 84 of analyzed samples contain at least three minerals meeting a concordance test for equilibrium they concluded the great majority of igneous and metamorphic rocks has conserved a state of oxygen isotope exchange equilibrium if correct this bodes well for thermometry however this conclusion was contested by deines 1977 who critically examined the existing data and concluded that less than half of the rocks analyzed to date would yield concordant 18o derived temperatures while seeming irreconcilable both conclusions are based on fact all rocks contain some degree of disequilibrium but many also preserve a thermometric record it is the purpose of this review to discuss the new strategies for reliable isotope thermometry and to explore disequilibrium processes so they may be recognized and properly interpreted the organization of this paper will be to discuss the assumptions