transport of pb and sr in leaky aquifers of the bufa del diente contact metamorphic aureole north east mexico the sr and pb isotopes from the 31 6 0 3 ma 2 old diente del bufa alkali syenite northeastern mexico and marbles of its contact aureole were used to trace the sources and the mobility of these metals during hydrothermal activity chert layers form aquifers within the marbles the marbles represent aquitards during fluid wallrock reaction the chert layers developed wollastonite rims early wollastonite rims have sr and pb isotopic compositions similar to those of their immediate host marbles which indicates that the isotopic composition of sr and pb is initially buffered by the marble later wollastonite and other replacement minerals rimming the aquifer have sr and pb isotopic compositions that carry with time increasingly larger contributions from the high salinity magmatic brine the sr and pb contributions from the alkali syenite can be traced isotopically for more than 90 m away from the contact of the intrusion in contrast sr and pb originating from the alkali syenite are traceable within the marbles only for 3 to 5 cm from the aquifer marble boundary this distance is comparable to the spatial distribution of isotopic alterations of c and o implying that sr and pb were transported into the marbles through a fluid phase the isotopic variation of sr pb c and o across the aquifer marble profiles reflects infiltration as a transport mechanism rather than diffusion because sr and pb are minor components in both the infiltrating fluid and the rock and because their concentrations are strongly affected by the distribution coefficients among the solid phases present there is little correlation between the isotopic compositions of the trace elements sr and pb and those of c and o which are major components in fluid and rock very thin meta argillite rinds at the outer margin of the aquifer represent residual material after the dissolution of calcite they are distinctly enriched in rb sr and u the rb and sr are to some extent residual from the original limestone mineralogy whereas u is dominantly derived from the magmatic fluid and leaked from the aquifer with the escaping immiscible co2 rich h2o co2 fluid that was produced by decarbonation the 238u 204pb values ranging from 100 to 250 and distinctly lowered th u in the meta argillite rims 1 demonstrate that u was transported with the magmatic fluid along the aquifer and 2 imply that during unmixing of the highly saline magmatic fluid u fractionated into the co2 rich h2o co2 fluid from which it precipitated selectively in the meta argillite band across the aquifer radioautographs demonstrate that the upper meta argillite rim has 20 to 40 times more u than the lower rim which implies that 20 to 40 times more co2 rich h2o co2 fluid has left through the upper aquifer contact

ultrasonic velocity and anisotropy of hydrocarbon source rocks an experimental study of the physical properties of black kerogen rich shales also including maturation analysis scanning electron microscope sem observations and physical modeling revealed fairly peculiar petrophysical parameters specifically these rocks have very low porosity and density but most importantly both p and s ultrasonic velocities normal to bedding are extremely low whereas they are much higher parallel to bedding giving rise to a strong anisotropy even at high confining pressures we found that these parameters primarily reflect kerogen content microstructure and maturation level of these rocks we found also that microcracks inferred from ultrasonic velocity measurements occur only in mature shales these microcracks are parallel to the bedding plane and further enhance strong intrinsic anisotropy notably at low effective pressure our results show that on a small scale kerogen rich shales are transversely isotropic rocks and can be effectively modeled using the thin laye

composition and source of fluids in high temperature graphite bearing granitoids associated with granulites examples from the southern marginal zone limpopo complex south africa abstract p t conditions fluid regime and carbon isotope composition of graphite and fluid inclusions from garnet sillimanite bearing leucocratic tonalites trondhjemites and granites associated with orthopyroxene bearing granulite metapelites in the southern marginal zone smz of the limpopo complex south africa are presented in the paper re integrated compositions of perthitic alkali feldspars and antiperthitic plagioclase as well as p t and t xco2 phase equilibria modeling using perple x software indicate that the granitoids began to crystallize at temperatures of 900 940 c and pressures of 7 9 kbar and were equilibrated with a fluid phase with xco2 0 5 0 6 as is recorded in dense fluid inclusions in quartz a small fraction of a saline fluid accumulated during cooling only average 13cpdb values for graphite 6 52 to 8 65 and fluid inclusions 2 50 to 5 58 from the granitoids differ substantially from the values which have been obtained in previous studies for graphite from the surrounding smz metapelites isotope data thus indicate that fluids associated with the granitoid magmas of the smz originated from a source unrelated to the host metapelites in terms of the heavy isotope signatures of carbon the granitoids might carry fluids which have been produced during devolatilization of mafic rocks amphibolites interlayered with hydrothermal carbonate veins in the adjacent granite greenstone successions of the kaapvaal craton that have been buried underneath the smz granulites during their exhumation the cratonic rock could serve as a source for the trondhjemite magmas intruding the smz granulites in this scenario the studied granitoids crystallized from these magmas variously contaminated by metapelitic material graphite precipitated via reduction of co2 rich fluid as the result of dissolution of pyrrhotite rich metapelites in the magmas

contrasting fluid flow patterns at the bufa del diente contact metamorphic aureole north east mexico evidence from stable isotopes impure limestones with interstratified metachert layers were contact metamorphosed and metasomatized by the bufa del diente alkali syenite massive marbles exhibit mineralogical and stable isotope evidence for limited fluid infiltration confined to a 17 m wide zone at the contact influx of magmatic brines along most metacherts produced up to 4 cm thick wollastonite rims according to calcite cc quartz qz wollastonite wo co2 and were observed at distances of up to 400 m from the contact the produced co2 exsolved as an immiscible low density co2 rich fluid chert protolith isotope compositions were 18o qz 27 30 and 18o cc 24 27 many wollastonites in infiltrated metacherts have low 18o ranging from 11 17 and confirm that decarbonation occurred in presence of a magmatic signatured fluid large gradients in 18o wo across the rims may reach 6 the 18o of remaining quartz is often lowered to 15 20 whereas caleites largely retained their original compositions the isotopic reversals of up to 10 between quartz and calcite along with reaction textures demonstrate non equilibrium between infiltrating fluid in the aquifer and the assemblage calcite quartz wollastonite this is compatible with the assumption of a down temperature flow of magmatic fluids that occurred exclusively in the remaining quarzite layer the 13c cc and 18o cc of marble calcites measured perpendicular to two metachert bands reveal significant isotopic alterations along distances of 4 5 cm and 7 5 cm from the wollastonite marble boundary only into the hanging wall marble suggesting an advection process caused by a fluid phase which movel upwards covariation trends of 13c cc and 18o cc across the alteration front indicate that this fluid was co2 rich mass balance calculations show that all co2 rich fluid produced by the decarbonation reaction was lost into overlying marble the metachert aquifers did not leak with respect to water rich fluids

thermal overprint on blueschists of the seward peninsula the lepontine in alaska on the seward peninsula alaska blueschists and greenschists of the nome group and amphibolites and granulites of the kigluaik group all formed during a single episode of continent arc collision high pressure low temperature mineral assemblages that initially developed in a structurally coherent package of miogeoclinal rocks were then variably overprinted during later uplift by lower p higher t barrovian facies series assemblages derived pressure temperature time p t t paths illustrate the early passage of both the nome and kigluaik groups through high pressure 12 kbar conditions subsequent nearly isothermal decompression of nome group blue schists and divergence during decompression of nested p t paths to amphibolite and granulite conditions in the underlying kigluaik group similar field occurrences are described from the lepontine alps and the greek island of naxos standard geobarometric techniques applied to medium grade kigluaik group rocks do not record the earliest burial phase and therefore underestimate peak pressures by 8 kbar

the transformation of amphibolite facies gneiss to charnockite in southern karnataka and northern tamil nadu india amphibolite facies metamorphic grade gives way southward to the granulite grade in southern karnataka as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes petrologic investigations in the transitional zone south of mysore have established the following points 1 the transition is prograde amphibole bearing gneisses intimately associated with charnockite at kabbal and several similar localities are not retrogressive after charnockite as proved by patchy obliteration of their foliation by transgressive very coarse grained charnockite high fluorine content of biotite and amphibole in gneisses and high large ion lithophile element contents in gneisses and charnockites these features are in contrast to very low fluorine in retrogressive amphiboles and biotites very low large ion lithophile element contents and zonal bleaching of charnockite in clearly retrogressive areas as at bhavani sagar tamil nadu 2 metamorphic temperatures in the transitional areas were 700 800 c pressures were 5 7 kbar and h2o pressures were 0 1 0 3 times total pressures based on thermodynamic calculations using mineral analyses dense co2 rich fluid inclusions in the kabbal rocks confirm the low h2o pressures at the first appearance of orthopyroxene farther to the south in the nilgiri hills and adjacent granulite massif areas peak metamorphic temperatures were 800 900 c pressures were 7 9 kbar and water pressures were very low so that primary biotites and amphiboles those with high f contents are rare 3 the incipient granulite grade metamorphism of the transitional areas was introduced by a wave of anatexis and k metasomatism this process was arrested by drying out under heavy co2 influx charnockites so formed are hybrids of anatectic granite and metabasite of metabasite and immediately adjacent gneiss or are virtually isochemical with pre existing gneiss despite gross recrystallization to granulite mineralogy these features show that partial melting and metasomatism are attendant rather than causative in charnockite development copious co2 from a deep crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis granulite metamorphism of both neosome and paleosome followed the process is very similar to that deduced for the madras granulites by weaver 1980 the massif charnockites for the most part extremely depleted in lithophile minor elements show many evidences of having gone through the same process

inversion for parameters of tensile earthquakes the tensile source model generalizes the shear source model by assuming that the slip vector can be arbitrarily oriented with respect to the fault and is not constrained to lie within the fault plane the proposed inversion for the parameters of tensile sources is based on the evaluation of the isotropic iso compensated linear vector dipole clvd and double couple dc components in seismic moment tensors the most significant parameters inverted are the ratio at the fault denoted as the parameter and the inclination of the slip vector from the fault the parameter is significant for discriminating noisy moment tensors of shear earthquakes from those of tensile earthquakes the inclination can be accurately determined from the dc component in the moment tensor because the dc component rapidly decreases with increasing for example the inclination of 20 causes dc being 50 60 only the inversion is applied to earthquakes which occurred in january 1997 in west bohemia czech republic it is shown that some of these earthquakes display tensile faulting the parameter is 0 1 the inclination of the slip from the fault attains values of up to 20 this inclination is a result of tensile traction and reduced shear traction along the fault and high fluid pressure in the region

co 2 melts and granulite metamorphism the discovery that granulites contain co2 rich fluid inclusions while amphibolites in the same terrane contain h2o rich or mixed co2 h2o inclusions1 has led to a debate over the processes that produce granulite metamorphism one theory2 4 maintains that granulites form as the result of a massive influx of co2 from lower crustal or mantle sources such an influx could be responsible for the marked depletion many granulite terranes show in large ion lithophile elements2 5 others however maintain that granulite facies metamorphism is a necessary complement of crustal melt ing6 9 recently lamb and valley10 11 have argued that granulite metamorphism may be a product of any one of three processes co2 streaming partial melting or recrystallization of originally dry rocks here we suggest that these processes need not be independent and that they all may be related to the passage of melts through the crust

fluid flow mineral reactions and metasomatism a general model that relates fluid flow along a temperature gradient to chemical reaction in rocks can be used to quantitatively interpret petrologic and geochemical data on metasomatism from ancient flow systems in terms of flow direction and time integrated fluid flux the model is applied to regional metamorphism quartz veins and a metasomatized ductile fault zone

on the feasibility of inversion of single well microseismic data for full moment tensor the existing paradigm widely accepted in the microseismic community asserts the impossibility of unambiguous estimation of full seismic moment tensor from three component data recorded in a single straight borehole the statement of inversion nonuniqueness related to the lack of illumination of the off plane moment component in the zero solid angle ray apertures tacitly assumes the slowness vectors normal to the relevant wave fronts to be confined to the plane containing the ray trajectories although such arrangements of the rays and slownesses do take place for vertical wells drilled in horizontally layered isotropic and vti transversely isotropic with a vertical symmetry axis formations azimuthal anisotropy commonly expected for hydraulically fractured hydrocarbon bearing shales makes the slowness vectors deviate from a plane arrange themselves into nonzero solid angle apertures and yield theoretically unique inverse problems for seismic moment tensors this paper examines those problems for the anisotropy parameters characterizing fractured shales and siltstones in the bakken their strong azimuthal anisotropy entails not only formally unique but importantly for practical applications well posed inverse problems resulting in robust estimates of the complete moment tensors from single borehole data

fluid heterogeneity during granulite facies metamorphism in the adirondacks stable isotope evidence the preservation of premetamorphic whole rock oxygen isotope ratios in adirondack metasediments shows that neither these rocks nor adjacent anorthosites and gneisses have been penetrated by large amounts of externally derived hot co2 h2o fluids during granulite facies metamorphism this conclusion is supported by calculations of the effect of fluid volatilization and exchange and is also independently supported by petrologic and phase equilibria considerations the data suggest that these rocks were not an open system during metamorphism that fluid rock ratios were in many instances between 0 0 and 0 1 that externally derived fluids as well as fluids derived by metamorphic volatilization rose along localized channels and were not pervasive and thus that no single generalization can be applied to metamorphic fluid conditions in the adirondacks

structure petrology and metamorphic history of the nome group blueschist terrane salmon lake area seward peninsula alaska an 80 mi 2 area adjacent to salmon lake mapped at a scale of 1 36 680 includes a section of the nome group schist that is at least 2 000 m thick it is relatively intact with relict primary lithologic layering and not melange like or highly disrupted by faults the nome group metasediments include pelitic schist graphitic quartzite pure and impure marble calcareous schist chlorite albite schist chloritic quartz albite and mafic schists metaigneous rocks include fe ti rich metabasite and granitic to tonalitic orthogneiss the protoliths are precambrian to paleozoic in age and radiometric ages indicate that the high pressure metamorphism of the nome group is late jurassic blueschist facies assemblages occur in nome group metabasites and in most metasedimentary schists the blueschists are synkinematic they formed during an episode of widely distributed penetrative deformation and mesoscopic intrafolial isoclinal folding of consistent orientation the nome group of the salmon lake area underwent a prograde monocyclic polyfacial high p t metamorphism initial metamorphism was in the lower t regime of the lawsonite glaucophane field 300 c 7 kb and prograded with increasing t and p into the epidote almandine glaucophane field through an apparent p t maximum as indicated by the assemblage glaucophane omphacite garnet 420 c 9 10 kb the later stages of the metamorphism were characterized by increased t p likely during initial stages of decompression and the return of the normal geothermal gradient to the transition between the blueschist facies and the high p albite epiclote barroisite field 450 to 525 c 7 8 kb this high p t cycle was followed by an incipient static retrogressive metamorphism at low greenschist facies conditions during the later uplift of the nome group in the late cretaceous and early tertiary

steep oxygen isotope gradients at marble metagranite contacts in the northwest adirondack mountains new york usa products of fluid hosted diffusion steep gradients in oxygen isotope ratios are recorded from two contacts between grenville marbles calcite 18o 21 24 and metamorphosed granitic intrusives whole rock 18o 11 12 in the northwest adirondacks oxygen and carbon isotope fractionations between coexisting minerals in the marbles and the metagranites are consistent with isotopic equilibration at peak metamorphic temperatures the 18o profiles are sigmoidal with inflection points at the contacts and are developed over 5 6 m quantitative fluid infiltration diffusion modeling suggests that the isotopic gradients are best explained as the result of fluid hosted diffusion for porosities of 10 3 to 10 6 the profiles could have formed in 103 107 years via an interconnected grain boundary fluid the granites were probably emplaced at shallow crustal levels prior to the regional grenville metamorphism and isotopic exchange is most likely to have occurred at that time contact metamorphism at these localities involved devolatilisation of the marbles and the time estimates may reflect the periods over which fluid producing reactions took place and an enhanced porosity was maintained in the marbles there is a progressive increase in the percentage of diopside and k feldspar at the expense of phlogopite and tremolite towards the metagranites these mineralogical changes probably occurred due to the temperature increase associated with granite intrusion and diffusion of h2o from the granite the two localities lie on either side of the regional metamorphic amphibolite to granulite facies transition the preservation of isotopic profiles formed during contact metamorphism suggests that there was little pervasive fluid flow at this crustal level during the regional metamorphism or at any time thereafter steep isotopic gradients are present at orthogneiss marble contacts elsewhere in the adirondacks suggesting that this conclusion may be generally applicable to this terrain

petrology of biotite cordierite garnet gneiss of the mccullough range nevada i evidence for proterozoic low pressure fluid absent granulite grade metamorphism in the southern cordillera proterozoic migmatitic paragneisses exposed in the mccullough range southern nevada consist of cordierite almanditic garnet biotite sillimanite plagioclase k feldspar quartz ilmenite hercynite this assemblage is indicative of a low pressure facies series at hornblende gr anulite grade textures record a single metamorphic event involving crystallization of cordierite at the expense of biotite and sillimanite thermobarometry utilizing cation exchange between garnet biotite cordierite hercynite and plagioclase yields a preferred temperature range of 590 750 c and a pressure range of 3 4 kb equilibrium among biotite sillimanite quartz garnet and k feldspar records ahlo between 0 03 and 0 26 the low ah o together with low fol qfm and optical properties of cordierite indicate metamorphism under fluid absent conditions preserved mineral compositions are not consistent with equilibrium with a melt phase earlier limited partial melting was apparently extensive enough to cause desiccation of the pelitic assemblage the relatively low pressures attending high grade metamorphism of the mccullough range paragneisses allies this terrane with biotite cordierite garnet granulites in other orogenic belts closure pressures and temperatures require a transient apparent thermal gradient of at least 50 c km during part of this proterozoic event in the southern cordillera

limited fluid rock interaction at marble gneiss contacts during cretaceous granulite facies metamorphism seward peninsula alaska stable isotope profiles show that flat lying marble units acted as impermeable barriers to upward fluid flow in transitional amphibolite granulite grade rocks of the kigluaik mountains seward peninsula alaska the degree of permeability is related to the composition of the marble the margin of a thick pure dolomite marble chemically reacted with underlying metasyenite ah2o 0 2 to form a 2 cm boundary layer of calcite forsterite by introduction of sio2 no fluid penetrated past this reaction front although the high temperature of metamorphism 800 c allowed transport of carbon and oxygen isotopes for an additional 2 cm by diffusion through the solid dolomite a second marble with a higher silica content underwent more decarbonation which enhanced porosity and lead to a greater extent of isotope transport 2 3 m in contact with quartzo feld spathic gneiss below an estimate of total fluid flux across the bottom of this marble layer based on the shape of the isotope profile is 1 cm3 cm2 directed down out of the marble at two other marble gneiss contacts steep isotopic gradients coincide with lithologic contacts indicating very little cross lithology fluid flow the extent of diffusional transport of isotopes in the marbles is limited and interpreted as indicating the transient presence of a pore fluid generated by thermally driven devolatilization reactions no wholesale pervasive advection of c o h fluid occurred across the thick continuous marble units near the exposed base of the kigluaik group section during the entire regional metamorphic cycle activities of pore fluid species were controlled by internal processes movement of volatiles and stable isotopes between contrasting rock types was dominantly diffusive channelized fluid pathways through the marble units developed during uplift and cooling but were not present during peak metamorphism heating of the section occurred by conduction probably from an underlying magma source and not by advection of a c o h fluid

elemental dispersion and stable isotope fractionation during reactive fluid flow and fluid immiscibility in the bufa del diente aureole ne mexico evidence from radiographies and li b sr nd and pb isotope systematics the 31 6 0 3 ma old bufa del diente alkali syenite ne mexico intruded a sequence of cretaceous limestones with intercalated sub horizontal chert layers the cherts acted as aquifers that facilitated transport of brines and pegmatitic melts within the shallow level 1 kbar contact metamorphic aureole fluid driven reactions between chert and marble wallrock and the influx of late melts and various fluids gave rise to distinct chemical and isotopic signatures within the aquifer and across the zones of infiltration and fluid driven reaction aqueous brines of magmatic origin produced thick wollastonite mantles around the chert layers wollastonite formation occurred at the expense of limestone and chert and generated co2 this co2 induced fluid unmixing into an aqueous brine and a low density co2 rich fluid which was lost to the overlying marble where it oxidized organic matter and caused 13c and 18o shifts in a zone some 5 10 cm wide after wollastonite formation the chert aquifers were locally intruded by pegmatite veins carrying alkali feldspar quartz aegirine augite eudialyte zircon and apatite aqueous fluids that exsolved during crystallization of the pegmatite veins escaped along late cross fractures and migrated along the inner and outer borders of the wollastonite margins chemical dispersion patterns of u al na k p s fe and ree across the chert to marble boundary and its metasomatic rims are shown by autoradiography and neutron induced radiography scavenging of cations at mineralogical contacts and cation transport into the marbles occurred only on the mm to cm scale isotopic data for pb and sr across a simple metachert marble boundary and for pb sr nd b and li across a metachert pegmatite marble sequence demonstrate the following 1 the pb and sr isotopic signature of early fluids was buffered by the carbonate wallrock only late fluids shielded from wallrock interaction by a wollastonite mantle variably preserved a memory of their initial magmatic signature 2 since the nd isotope signature of marble and chert is bound to calcite and clay minerals systematic shifts to unradiogenic nd in marble reflect loss of carbonate bound nd as the wollastonite margin is approached nd in the wollastonite margin is dominated by nd originally bound to clay minerals the later emplacement of the pegmatite which carried the nd isotope signature of its alkali syenite source had little effect on the nd isotopic composition of the wollastonite rim 3 although the li and b isotopic compositions reflect the alkali syenite source they are also affected by isotopic fractionation and partitioning between melt fluid and solids

incipient charnockite characterization at the type localities abstract fifty years after the discovery of incipient charnockite orthopyroxene bearing granitic rocks partially replacing amphibole biotite gneiss in southern india this phenomenon has been found in transitional amphibolite facies to granulite facies terrains on every continent and may be a characteristic feature of progressive deep crustal metamorphism the alteration is essentially a metasomatic transformation mass balance calculations for localities in southern india and sri lanka for which analytic data are most comprehensive show that the main reaction producing orthopyroxene in orthogneiss is breakdown of hornblende by reaction with biotite with loss of feo mgo tio 2 and cao as well as h 2 o from the host rock there is concomitant gain of sio 2 no significant change of alkalis can be demonstrated as essential to the transformation though advanced charnockitic alteration at the type locality of kabbal karnataka may also show large increase of k 2 o with removal of mafic constituents very similar orthopyroxene bearing veins in garnet biotite gneiss at the ponmudi type locality in kerala show definite but more subtle metasomatism with loss of feo and mgo and increase of sio 2 and alkalis both the kabbal and ponmudi alterations occurred at mid crustal levels 5 6 kbar and 700 750 c it is postulated that both types of charnockitic alteration may have been produced as open system processes by much the same kind of metasomatizing fluids nearly coeval co 2 rich and ultrasaline fluids based on fluid inclusion evidence the paragneiss alteration is mainly biotite breakdown with some involvement of garnet and leads to a distinct metamorphic facies type the presence of graphite in some of the garnet biotite gneisses at ponmudi would have resulted in lower oxygen fugacity and probably lower h 2 o activity than in the similar kabbal type metamorphism the fluid rock ratios for both types were small about 10 mass percent of the rocks and the fluids whatever their origin were largely rock governed the source of the metasomatizing fluids has not been definitely identified carbon isotopes of fluid inclusions seem to show an igneous or primitive mantle source at both type localities but ultrasaline fluids may be more plausibly explained by metasedimentary sources and calcareous metasediments have been identified as a source of co 2 in some occurrences much remains to be learned about incipient charnockite and its significance in processes of crustal evolution

matrix analysis of metamorphic mineral assemblages and reactions assemblage diagrams are widely used in interpreting metamorphic mineral assemblages in simple systems they can help to identify assemblages which may represent equilibrium states to determine whether differences between assemblages reflect changes in metamorphic grade or variations in bulk composition and to characterize isograd reactions in multicomponent assemblages these questions can be approached by investigating the rank composition space range and reaction space null space of a matrix representing the compositions of the phases involved singular value decomposition svd provides an elegant way of 1 finding the rank of a matrix and detemining orthonormal bases for both the composition space and the reaction space needed to represent an assemblage or pair of assemblages and 2 finding a model matrix of specified rank which is closest in a least squares sense to an observed assemblage although closely related to least squares techniques the svd approach has the advantages that it tolerates errors in all observations and is computationally simpler and more stable than non linear least squares algorithms models of this sort can be used to interpret multicomponent mineral assemblages by straightforward generalizations of the methods used to interpret assemblage diagrams in simpler systems svd analysis of mineral assemblages described by lang and rice 1985 demonstrates the utility of the approach

polyphase metamorphic fluid flow in the lower calcsilicate unit reynolds range central australia abstract lower calcsilicate unit metasediments and napperby gneiss metagranites at conical hill in the reynolds range central australia underwent at least two periods of fluid flow the ingress of surface derived fluids 18 o 2 h 18 o values in both the lower calcsilicate unit and the napperby gneiss fluid flow during cooling from the peak of 1600 ma regional metamorphism formed quartz garnet veins and caused epidotisation of the lower calcsilicate unit the fluids in this later event were water rich x co 2

relations among net transfer reaction progress 18 o 13 c depletion and fluid infiltration in a clinohumite bearing marble a siliceous dolomitic marble xenolith within a mid crustal jurassic diorite exhibits mineralogical and stable isotopic evidence for infiltration of water rich fluid adjacent to endoskarn which bounds the xenolith forsterite has been replaced by clinohumite as a result of a devolatilization reaction driven by addition of aqueous fluid and extraction of heat isotopic compositions of calcite also record infiltration of aqueous fluid concentrated near contacts with endoskarn marble calcite 18osmow values range from 19 5 at the center of the xenolith to 12 6 adjacent to endoskarn calcite 13cpdb decreases sympathetically from 3 0 to 1 4 the calculated equilibrium composition of coexisting c o h f fluid and a quantitative characterization of the whole rock reaction which produced clinohumite and consumed graphite are used to place inequality contraints on the composition of the infiltrated fluid continnum mechanical transport models based on the resulting fluid compositions suggest that a small time integrated darey flux of 44 cm3 cm2 coupled with diffusive transport in the fluid was sufficient to produce both the isotopic shifts and the net transfer reactions evidenced in the xenolith the calculations demonstrate the importance of graphite as an indicator of time integrated flux the maximum possible ch4 content of the infiltrated fluid is sufficiently high to impart a 2 0 uncertainty in the 13c of the fluid the isotopic composition of the fluid is consistent with a magmatic origin when this uncertainty is taken into account

stable isotopic evidence for large scale seawater infiltration in a regional metamorphic terrane the trois seigneurs massif pyrenees france oxygen isotopic analyses of 95 metamorphic and igneous rocks and minerals from a hercynian metamorphic sequence in the trois seigneurs massif pyrenees france indicate that all lithologies at higher metamorphic grades than the andalusite in isograd have relatively homogeneous 18o values the extent of homogenization is shown by the similarity of 18o values in metacarbonates metapelites and granitic rocks 11 to 13 and by the narrow range of oxygen isotopic composition shown by quartz from these lithologies these values contrast with the 18o values of metapelites of lower metamorphic grade 18o about 15 homogenization was caused by a pervasive influx of hydrous fluid mass balance calculations imply that the fluid influx was so large that its source was probably high level groundwaters or connate formation water hydrogen isotopic analyses of muscovite from various lithologies are uniform and exceptionally heavy at d 25 to 30 suggesting a seawater origin many lines of petrological evidence from the area independently suggest that metamorphism and anatexis of pelitic metasediment occurred at depths of 6 12 km in the presence of this water rich fluid the composition of which was externally buffered deep penetration of surface waters in such environments has been hitherto unrecognized and may be a key factor in promoting major anatexis of the continental crust at shallow depth three types of granitoid are exposed in the area the leucogranites and the biotite granite quartz diorite are both mainly derived from fusion of local paleozoic pelitic metasediment because all these rocks have similar whole rock 18o values 11 to 13 the post metamorphic biotite granodiorite has a distinctly different 18o 9 5 to 10 0 and was probably derived from a deeper level in the crust rare mafic xenoliths within the deeper parts of the biotite granite quartz diorite also have different 18o 8 0 to 8 5 and possibly represent input of mantle derived magma which may have provided a heat source for the metamorphism

structurally focused fluid flow during orogenesis the islay anticline sw highlands scotland abstract displacement of isotopic compositions at boundary layers across strata of contrasting composition is commonly used to investigate hydrothermal fluid flow during orogeny this study investigates whether hydrothermal fluid flow was focused along the islay anticline islay sw highlands of scotland as shown in the axial zone of the neighbouring ardrishaig anticline four localities from the limb to the axial plane of the islay anticline were investigated for isotopic homogenization of metacarbonate units to silicate values at mull of oa on the limb of the anticline metacarbonate samples show limited isotopic resetting and the fluid flux is estimated to be 3 m 2 within the axial zone of the islay anticline metacarbonate units from port a9 chotain and bagh an da dhoruis show complete isotopic homogenization to silicate values indicating higher fluid fluxes fluid flow was enhanced along localized parasitic folds such as at port an t sruthain where metacarbonates have been isotopically reset and there are abundant quartz carbonate veins that precipitated during d 1 d 2 deformation metamorphic fluid flow was higher in the axial zone of the islay anticline and in localized antiformal structures fluid fluxes are estimated to be considerably lower than at the neighbouring ardrishaig anticline

inversion of microseismic data for triclinic velocity models modern downhole microseismic surveys often employ geometries in which ray trajectories generated by a collection of locatable events provide full polar and azimuthal coverage making it possible to estimate the in situ seismic anisotropy we show that traveltimes and particle motions of the direct p and shear waves acquired in such geometries can constrain stiffness tensors of triclinic media while obtaining all 21 stiffness coefficients of a homogeneous triclinic space simultaneously with locating pertinent microseismic events from data recorded in a single vertical well is relatively straightforward the same methodology does not necessarily succeed in layered formations because the combination of their vertical heterogeneity and azimuthal anisotropy might invalidate the commonly adopted approximation of the event azimuths by those of the p wave polarization vectors when the event azimuths cannot be derived from the particle motions traveltimes observed in two or more wells are required to locate the events and build layered triclinic or higher symmetry azimuthally anisotropic velocity models as our numerical tests indicate the multi well event location methods are expected to perform better than their single well counterparts because they rely solely on triangulation and eliminate the usually pronounced azimuthal uncertainties in the event locations that stem from noises adversely affecting hodogram analysis

fluid rock interaction in the granulites of madagascar and lithospheric scale transfer of fluids abstract the nature of synmetamorphic fluids and their flow is examined in the granulitic lower crust of madagascar part of a precambrian crustal scale network of vertical ductile shear zones based on three independent data sets field and satellite mapping c o and h isotope geochemistry and gravimetry this crust is divided into three zones outside of shear zones minor shear zones 350 km long and 20 35 km wide the major shear zones are rooted in and are controlled by the mantle they tapped mantle derived co2 with carbon fluxes of the same order of magnitude as oceanic ridge degassing one major shear zone shows abundant phlogopite diopside apatite calcite mineralizations a well known paragenesis in mantle metasomatism due to mantle fluid infiltration and their interaction with the crust carbonatitic magmas possibly collected in the major shear zones at the base of the crust and may be the source for co2 upwellings as well as other metasomatic agents small scale minor shear zones were controlled by crustal deformation processes and focused crustally derived h2o rich fluids pervasive fluid circulation was restricted to the vicinity

equilibrium dihedral angles in the system h2o co2 nacl calcite and implications for fluid flow during metamorphism fluid calcite calcite dihedral angles have been measured for fluids in the system h2o co2 nacl between 1 and 2 kbar and 550 750 c it is found that the calcite calcite h2o dihedral angle decreases steadily with addition of nacl from a value of about 80 pure water to 44 60 wt nacl the co2 h2o system displays a well defined minimum at x co 2 0 5 with a dihedral angle of 50 in contrast to those of pure co2 and h2o which are 90 and 80 respectively experiments containing fluids which are immiscible at run conditions showed a bimodal distribution of dihedral angles in the co2 h2o nacl system which can be approximately correlated with the compositions of the two fluid phases such bimodality was only observed for immiscible fluids in the h2o nacl system if the quench rate exceeded about 200 c per min this is probably due to the extremely rapid establishment of the single phase dihedral angle on quenching the fluid phase topology in devolatilising marbles will only be a connected network for very saline brines and fluids with x co 2 close to 0 5 fluids trapped in fluid inclusions in calcite grains in marbles may be predominantly h2o rich or co2 rich and of low salinity all other fluid compositions in the h2o co2 nacl calcite system will occupy isolated pores the largest of which will grow at the expense of the smallest escape of fluid produced during devolatilisation reactions under such conditions will occur by fluid overpressuring and hydrofracture in contrast previous experimental studies of quartz fluid dihedral angles between 950 and 1100 c watson and brenan 1987 predict that quartz dominated lithologies will permit pervasive flow of h2o nacl fluids but not of h2o co2 fluids documented geological examples of differences in permeability and fluid flow mechanism between metamorphic argillites psammites and limestones which support the results of the experimental studies are discussed

regional progressive high pressure metamorphism seward peninsula alaska blueschist facies rocks on the seward peninsula constitute a structurally coherent terrane measuring at least 100 150 km radiometric age data indicate that high pressure metamorphism probably occurred in jurassic rather than in palaeozoic or precambrian time as previously suggested protolith sediments nome group are of intracontinental basin or continental margin type and of lower palaeozoic and possibly late precambrian age thus predating the high pressure metamorphism by more than 200 m y blueschist facies mineral assemblages were developed in almost all lithologies of the nome group and are best preserved in feti rich metabasites glaucophane almandine epidote and pelites glaucophane chloritoid phengite a lawsonite crossite subfacies was developed in possible nome group rocks on the east flank of the darby mountains albite epidote amphibolite facies assemblages characterize nome group rocks in the southwestern part of the peninsula metamorphism in the central zone of the terrane passed from early lawsonitic to subsequent epidote almandine glaucophane schist subfacies with the local development east of the nome river of eclogitic assemblages the high pressure metamorphic minerals were synkinematic with the development of mesoscopic scale intrafolial isoclinal folds and a flattening foliation of consistent orientation initiation of uplift probably corresponded to the growth of barroisite rims on earlier sodic and actinolitic amphiboles and partial post kinematic greenschist facies replacements record later stages of decompression ophiolites and melange are not associated with the seward peninsula blueschists the high pressure metamorphism was caused by tectonic loading of a continental plate by an allochthon of indeterminate origin the pt conditions of high pressure metamorphism were approximately 9 11 kbar 400 450 c thus falling between the pt paths of the shuksan and franciscan terranes

fluid inclusions in rocks from the amphibolite facies gneiss to charnockite progression in southern karnataka india direct evidence concerning the fluids of granulite metamorphism fluid inclusion studies of rocks from the late archaean amphibolite facies to granulite facies transition zone of southern india provide support for the hypothesis that co2 rich h2o poor fluids were a major factor in the origin of the high grade terrain charnockites closely associated leucogranites and quartzo feldspathic veins contain vast numbers of large co2 rich inclusions in planar arrays in quartz and feldspar whereas amphibole bearing gray gneisses of essentially the same compositions as adjacent charnockites in mixed facies quarries contain no large fluid inclusions inclusions in the northernmost incipient charnockites as at kabbal karnataka occasionally contain about 25 mol of immiscible h2o lining cavity walls whereas inclusions from the charnockite massif terrane farther south do not have visibile h2o microthermometry of co2 inclusions shows that miscible ch4 and n2 must be small probably less than 10mol combined densities of co2 increase steadily from north to south across the transitional terrane entrapment pressures calculated from the co2 equation of state range from 5 kbar in the north to 7 5 kbar in the south at the mineralogically inferred average metamorphic temperature of 750 c in quantitative agreement with mineralogic geobarometry this agreement leads to the inference that the fluid inclusions were trapped at or near peak metamorphic conditions calculations on the stability of the charnockite assemblage biotite orthopyroxene k feldspar quartz show that an associated fluid phase must have less than 0 35 h2o activity at the inferred p and t conditions which agrees with the petrographic observations high tio2 content of biotite stabilizes it to lower h2o activities and the steady increase of biotite tio2 southward in the area suggests progressive decrease of ah2o with increasing grade oxygen fugacities calculated from orthopyroxene magnetite quartz are considerably higher than the graphite co2 o2 buffer which explains the absence of graphite in the charnockites the present study quantifies the nature of the vapours in the southern india granulite metamorphism it remains to be determined whether co2 flushing of the crust can by itself create large terranes of largeion lithophile depleted granulites or whether removal of h2o bearing anatectic melts is essential

the relationship between metamorphic charnockite and marble near speculator central adirondack mountains new york granites and charaockites with similar bulk compositions are exposed in a series of roadcuts along rt 8 3 east of the central adirondack town of speculator the mineralogy of these rocks is correlated with their associated rock types with the orthopyroxene bearing varieties occurring in the marble rich portion of the sequence mineral assemblages in a charnockite boudin enclosed in marble appear to be consistent with equilibration with a co 2 rich fluid under high grade t 700 c p 7 kb conditions while high density co 2 rich fluid inclusions occur in a quartz vein enclosed within the boudin it is suggested that small amounts of charnockite may have been produced in this area by the influx of co 2 produced by devolatilization reactions in the marble however carbonic metamorphism in the adirondacks appears to have been at most a minor local process and did not operate on the regional scale that has been suggested for some other granulite terranes thus the presence of abund

genesis and mobility of the h2o co2 fluid phase during regional greenschist and epidote amphibolite facies metamorphism a petrological and stable isotope study in the scottish dalradian during high pressure low temperature greenschist and epidote amphibolite facies metamorphism in dalradian rocks of the sw scottish highlands mineral assemblages in metabasites and calcareous metasediments were dominantly controlled by infiltration of hydrous fluids consequently mineral assemblages capable of buffering the fluid phase composition were rare equilibrium prograde h 2 o co 2 fluids usually contained less or much less than about 1 2 mol co 2 three fluid infiltration events are recognized during prograde greenschist facies metamorphism metabasic sills were infiltrated by large volumes of co 2 bearing hydrous fluid carbon isotope studies indicate that the co 2 was locally derived by widespread oxidation of graphite or other organic carbon in adjacent metasediments this may have occurred under approximately lower greenschist facies conditions as a result of mixing of fluids of varying f o 2 initiated by thermal expansion of water during heating decompression and consequent hydraulic fracturing in the epidote amphibole facies garnet zone dehydration reactions in metabasites generated large quantities of water which removed carbonate from metabasites on a regional scale and infiltrated calcareous metasediments to produce assemblages containing grossular diopside k feldspar amphibole clinozoisite and sphene a late retrograde infiltration of co 2 bearing hydrous fluid under lower greenschist facies conditions generated assemblages containing k feldspar chlorite rutile dolomite in calcareous rocks and albite prophyroblast schists in zones of intense secondary deformation large scale infiltration of fluid into greenschist facies metadolerite sills was intimately related to and possibly controlled by penetrative deformation and in the absence of a penetrative deformation grain boundary diffusion by itself was an ineffectual mechanism of fluid transport

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

stable isotope evidence for fluid present and fluid absent metamorphism in metapelites from the damara orogen namibia a large difference of about 5 in the o isotopic composition of quartz characterizes the transition from very low grade or even anchimetamorphic slates to pelites metamorphosed at greenschist facies conditions a further increase in temperature does not result in systematic isotopic shifts mean around 15 5 however immediately at the isograds marking the transition from greenschist to amphibolite facies and amphibolite facies to higher amphibolite facies the quartz 18o values are significantly higher by 1 this increase is restricted to the area immediately at the isograds

oxygen isotope fractionation in quartz albite anorthite and calcite laboratory measurements of equilibrium oxygen isotope fractionation in quartz albite anorthite and calcite have been carried out by anhydrous exchange between silicates and calcite at temperatures of 600 c and above exchange in these systems is as rapid as exchange between silicates and water fractionation factors can be summarized in the following equations qc 0 38 106t 2 abc 0 57 106t 2 anc 1 59 106t 2 from which the silicate pair fractionations are readily obtained these results are compared with published theoretical estimates as well as data derived from hydrothermal experiments some significant differences are found in particular it is difficult to reconcile all of the hydrothermal data either with theoretical calculations or with the present experimental data the new experiments provide an internally consistent set of fractionation factors suitable for isotopic thermometry and for test of disequilibrium in natural systems

on the retrieval of moment tensors from borehole data the complete moment tensors of seismic sources in homogeneous or vertically inhomogeneous isotropic structures cannot be retrieved using receivers deployed in one vertical borehole the complete moment tensors can be retrieved from amplitudes of p waves provided that receivers are deployed in at least three boreholes using amplitudes of p and s waves two boreholes are in principle sufficient similar rules also apply to transversely isotropic media with a vertical axis of symmetry in the case of limited observations the inversion can be stabilized by imposing the zero trace constraint on the moment tensors however this constraint is valid only if applied to observations of shear faulting on planar faults in isotropic media which produces double couple mechanisms for shear faulting on non planar faults for tensile faulting and for shear faulting in anisotropic media the zero trace constraint is no longer valid and can distort the retrieved moment tensor and bias the fault plane solution numerical modelling simulating the inversion of the double couple mechanism from real data reveals that the errors in the double couple and non double couple percentages of the moment tensors rapidly decrease with increase in the number of boreholes used for noisy p and s wave amplitudes with noise of 15 of the top amplitude at each channel and for a velocity model biased by 10 the errors in the double couple percentage attain 25 13 and 6 when inverting for the double couple mechanism from one two and three boreholes

determining the direction of contact metamorphic fluid flow an assessment of mineralogical and stable isotope criteria one dimensional fluid advection dispersion models predict differences in the patterns of mineralogical and oxygen isotope resetting during up and down temperature metamorphic fluid flow that may in theory be used to determine the fluid flow direction with respect to the palaeotemperature gradient under equilibrium conditions down temperature fluid flow is predicted to produce sharp reaction fronts that separate rocks with isobarically divariant mineral assemblages in contrast up temperature fluid flow may produce extensive zones of isobarically univariant mineral assemblages without sharp reaction fronts however during contact metamorphism mineral reaction rates are probably relatively slow compared with fluid velocities and distended reaction fronts may also form during down temperature fluid flow in addition uncertainties in the timing of fluid flow with respect to the thermal peak of metamorphism and the increase in the variance of mineral assemblages due to solid solutions introduce uncertainties in determining fluid flow directions equilibrium down temperature flow of magmatic fluids in contact aureoles is also predicted to produce sharp 18o fronts whereas up temperature flow of fluids derived by metamorphic devolatilization may produce gradational 18o vs distance profiles however if fluids are channelled significant kinematic dispersion occurs or isotopic equilibrium is not maintained the patterns of isotopic resetting may be difficult to interpret the one dimensional models provide a framework in which to study fluid rock interaction however when some of the complexities inherent in fluid flow systems are taken into account they may not uniquely distinguish between up and down temperature fluid flow it is probably not possible to determine the fluid flow direction using any single criterion and a range of data is required

petroleum reservoir characterization using downhole microseismic monitoring imaging of microseismic data is the process by which we use information about the source locations timing and mechanisms of the induced seismic events to make inferences about the structure of a petroleum reservoir or the changes that accompany injections into or production from the reservoir a few key projects were instrumental in the development of downhole microseismic imaging most recent microseismic projects involve imaging hydraulic fracture stimulations which has grown into a widespread fracture diagnostic technology this growth in the application of the technology is attributed to the success of imaging the fracture complexity of the barnett shale in the fort worth basin texas and the commercial value of the information obtained to improvecompletions and ultimately production in the field the use of commercial imaging in the barnett is traced back to earlier investigations to prove the technology with the cotton valley imaging project and earlier experiments at the m site in the piceance

persistence of pre metamorphic c and o isotopic signatures in marbles subject to pan african granulite facies metamorphism and u th mineralization tranomaro southeast madagascar abstract the status of fluid regimes during the pan african granulite facies metamorphism in southern madagascar was examined by means of a mineralogical and stable isotope study of marble lithologies in the granulitic tranomaro area which is made up of metasedimentary rocks metapelitic leptynitic felsic and calc silicate metasediments and intruded by syn granulitic granitic bodies syn granulitic mineralization th u and metasomatic transformations marbles pyroxenites demonstrate that fluid circulation has occurred the tranomaro marbles can be subdivided into two compositional groups either mg poor or mg rich this division reflects pre metamorphic compositional differences rather than different p t x co 2 histories on a regional scale the marbles display a large variation in 18 o from 19 to 6 5 relative to smow but have a more restricted 13 c range from 1 4 to 2 5 relative to pdb the lowest 18 o values are observed in magnesian marbles whereas calcic ones do not have 18 o values lower than 13 5 both initially low and high 18 o marbles may have locally been infiltrated by the syn metamorphic fluid flow which gave rise to pyroxenites and th u ores thorianite 18 o values of carbonate in mg rich marbles evolve from 7 up to 8 5 towards the contact with a metasomatic pyroxenite in a meter scale profile across an infiltration zone the mineralogy as well as the large and small scale isotopic results argue for i a pre metamorphic origin for the isotopic variations probably related to pre granulitic dolomitization ii a lack of pervasive fluid infiltration capable of erasing pre metamorphic isotopic compositional differences iii an absence of mantle derived c bearing fluids in the infiltrated zones for the area as a whole the present data are consistent with the decarbonation of impure siliceous limestones and dolostones coupled with fluid release from syn metamorphic granitic intrusions

oxygen isotope geochemistry of the scourian complex northwest scotland the scourian complex of northwest scotland underwent granulite facies metamorphism at 2 7 ga granulite facies orthogneisses have whole rock 18o values similar to those of igneous rocks of comparable compositions ultrabasic gneisses 5 5 5 9 basic gneisses 6 4 6 8 tonalitic gneisses 9 5 9 7 and steep 18o gradients of up to 3 in 10cm are recorded at two lithological contacts these data suggest that large volumes of pervasive metamorphic fluids have not infiltrated the rocks but do not preclude localized fluid rock interaction scourian paragneisses have lower 18o values 8 5 9 9 than typical metasediments and may have undergone fluid hosted isotopic exchange with the surrounding orthogneisses felsic sheets within tonalitic and basic gneisses have similar 18o values to their hosts these data combined with petrological and geochemical information suggest that the sheets were formed by local anatexis during granulite facies metamorphism fluid infiltration during the 2 5 ga amphibolite facies inverian retrogression caused homogenization of 18o values of the scourian gneisses unaltered tholeiitic scourie dykes have low 18o values of 2 0 it is unlikely that heated surface derived fluids infiltrated the scourian complex and these dykes were probably emplaced as low 18o magmas

strain collapsed metamorphic isograds in a sillimanite gneiss dome seward peninsula alaska unusually closely spaced barrovian series isograds have been described along the flanks of the kigluaik mountains seward peninsula alaska where they separate a high grade gneiss complex intruded by granites of cretaceous age from surrounding regionally developed blueschist to greenschist facies rocks structural mapping of the transition zone between the two metamorphic types indicates that their juxtaposition was aided by significant syn to late metamorphic solid state flow that served to attenuate the overlying rock column and thus collapse the field metamorphic gradient on the basis of field relations structural data petrography and geochronologic data strain appears to have accompanied the rapid adiabatic rise of high temperature rocks from several tens of kilometers to less than 10 km depth during the cretaceous in an event younger than the unrelated to high p metamorphism granite cored gneiss domes on the seward peninsula may have formed during extension of previously thickened continental crust resulting in the approximately 35 km thick crust and near sea level elevations of the region today

palaeomagnetic evidence for a displaced terrain in western antartica an outstanding problem in antarctic tectonics is the relationship between the east antarctic craton and the late palaeozoic cenozoic orogenic belts that comprise western antarctica1 antarctic peninsula ellsworth land eights coast and marie byrd land see fig 1a related problems are the place of western antarctica in reconstructions of gondwanaland and the evolution of the pacific margin of gondwanaland we present here palaeomagnetic data derived from folded cambrian argillites that crop out in the ellsworth mountains which were collected during the 1979 80 austral field season although the early palaeozoic palaeomagnetic field of gondwanaland is characterized by rapid apparent polar wander at the present level of coverage the data are consistent with rotation of the ellsworth mountain block with respect to the east antarctic craton as postulated by schopf2 this result supports a microplate nature of western antarctica

timing and mechanisms of carbon isotope exchange in granulite facies calc silicate boudins rauer group east antarctica mineralogically zoned decimeter diameter calc silicate boudins enclosed within paragneisses from little italy island rauer group east antarctica underwent granulite facies metamorphism p 5 700 900 mpa t 5 840 6 40 8c followed by near isothermal decompression to 200 400 mpa during decompression several mineral reactions occurred in the presence of a pore fluid 0 4 the calc silicate boudins show a general xco2 increase in calcite d 13 c values from core as low as 217 to rim 2 9t o 2 10 by contrast d 18 o values show little variation across the boudins the d 13 c profiles are similar to those predicted to result from diffusion within a sphere that is surrounded by a homogeneous reservoir diffusion of carbon isotopes probably occurred synchronous with the post peak metamorphic mineral reactions at that time centimeter scale diffusion would have been facilitated by the presence of the fluid while isotopic exchange between the minerals and the fluid would have been promoted by recrystallization for metamorphic porosities of 10 25 2 10 23 the d 13 c profiles could have formed in a few thousand to hundreds of thousands of years small millimeter to centimeter scale variations in d 13 c values that may have been initially present within the boudins would have been homogenized on much shorter timescales than those required to form the profiles the calculated timescales may reflect the time over which metamorphic recrystallization occurred and a reactionenhanced porosity was present graphite formed locally in the paragneisses only at margins of the boudins probably reflects the local escape of co 2 bearing fluids from the boudins into relatively low rocks fo2