crystallization environment of kazakhstan microdiamond evidence from nanometric inclusions and mineral associations nanometric solid inclusions in diamond incorporated in garnet and zircon from felsic gneiss of the kokchetav massif kazakhstan have been examined utilizing electron microscopy and focused ion beam techniques host garnet and zircon contain numerous pockets of multiple inclusions which consist of 1 3 diamond crystals intergrown with quartz phengite phlogopite albite k feldspar rutile apatite titanite biotite chlorite and graphite in various combinations recalculation of the average chemical composition of the entrapped fluid represented by multiple inclusion pockets indicates that such fluid contained a low wt of sio2 suggesting a relatively low temperature fluid rather than a melt transmission electron microscopy revealed that the diamond contains abundant nanocrystalline inclusions of oxides rare carbonates and silicates within the 15 diamond crystals studied abundant inclusions were found of sio2 tio2 f e xoy c r 2o3 zrsio4 and single grains of thxoy baso4 mgco3 fecr2o4 and a stoichiometric fe rich pyroxene the diversity of trace elements within inclusions of essentially the same stoichiometry suggests that the kokchetav diamond crystallized from a fluid containing variable amounts of si fe ti cr zr ba mg and th and other minor components such as k na p s pb zn nb al ca cl most of the components in crystals included in diamond appear to have their origin in the subducted metasediments but some of them probably originate from the mantle it is concluded that kokchetav diamond most likely crystallized from a coh rich multicomponent supercritical fluid at a relatively low temperature hence the apparently low content of rock forming elements and that the diversity of major and minor components suggests interactions between subducted metasediments and mantle components

water in h2o saturated magma fluid systems solubility behavior in k2o al2o3 sio2 h2o to 2 0 gpa and 1300 c abstract the distribution of h 2 o alkali silicate and alkali aluminosilicate components between coexisting h 2 o saturated melt and silicate and aluminosilicate saturated aqueous fluids has been determined in the pressure and temperature range 0 8 2 0 gpa and 700 1300 c respectively the solubility of silicate and aluminosilicate materials in aqueous fluid is between 1 and 12 mol this solubility is a positive function of both pressure and temperature the solubility increases with increasing alkali content and diminishes as the system becomes more aluminous the h 2 o solubility in coexisting h 2 o saturated peralkaline silicate and aluminosilicate melt ranges between 10 and 50 mol the h 2 o solubility is a positive and near linear function increasing pressure and of decreasing temperature the h 2 o saturation values are marginally sensitive to aluminum content the si k and al k ratios in the aqueous fluid are less than those of the coexisting aluminosilicate melt the partial molar volume of h 2 o in the melts v h 2 o melt is 14 cm 3 mol and decreases slightly with increasing temperature at the same temperature v h 2 o melt decreases as the melts become more aluminous the partial molar volume of h 2 o in the silicate and aluminosilicate saturated aqueous fluids v h 2 o fluid 16 7 22 cm 3 mol generally is less than that of pure h 2 o at the same pressure and temperature the v h 2 o fluid is insensitive to pressure in the 0 8 to 2 0 pressure range the v h 2 o fluid p t is much smaller than that of pure h 2 o in the 0 8 2 0 gpa pressure range the partial molar volumes of h 2 o in silicate melts and silicate saturated aqueous solutions were combined with published data to estimate the energy release during exsolution of h 2 o from h 2 o saturated magma in shallow magma chambers such as those feeding explosive dacitic eruptions 0 2 gpa and 800 1000 c about 1 9 10 9 ergs g h 2 o were obtained at 800 c and 0 2 gpa this value decreases by about 10 between 0 2 gpa and ambient pressure thus the exact pressure during such eruptions is not needed to evaluate the effect of exsolved h 2 o on the energy budget the energy contribution from exsolved h 2 o appears to be 2 to 4 of the total energy budget of such volcanic eruptions

crust derived potassic fluid in metamorphic microdiamond abstract a highly potassic coh fluid with dissolved phosphate chloride and sulfate sulfide was identified as nanometer size inclusions in metamorphic microdiamonds of kokchetav dolomite marble and garnet quartz pyroxene rock using an analytical electron microscope aem coupled with an energy dispersive x ray edx spectrometer this provides a direct piece of evidence for the formation of metamorphic microdiamonds from such a fluid although this crust derived potassic fluid and the associated solid precipitates are in some respects similar to the fluid inclusions in fibrous cloudy mantle diamonds distinct chemical differences exist this potassic coh fluid in addition has the potential to trigger high to ultrahigh k magmatic and k metasomatic processes in mantle

migration of fluid phases and genesis of basalt magmas in subduction zones dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of h2o in the production of magmas at convergent plate boundaries the subduction of oceanic lithosphere occurring with increasing pressures and rising temperatures causes liberation of h2o from the slab amphibole which can be stable to the highest pt conditions among hydrous phases in the slab decomposes at around 90 km depth it follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab derived h2o is not a direct trigger for the production of arc magmas instead the h2o released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals serpentine talc amphibole chlorite and phlogopite this hydrated peridotite is dragged downward on the slab toward higher pt regions and releases h2o to shallower potential magma source regions in the mantle wedge combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge it is concluded that those minerals decompose beneath the forearc region on the other hand high pt experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front phlogopite in the peridotite decomposes to release h2o at a deeper level about 200 km h2o liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites along with the migration of h2o through the above processes subduction components especially large ion lithophile elements can be overprinted on the magma source region which governs the geochemical characteristics of arc magmas

mantle fluid evolution a tale of one diamond abstract microinclusions analyzed in a coated diamond from the diavik mine in canada comprise peridotitic minerals and fluids the fluids span a wide compositional range between a carbonatitic melt and brine the diamond is concentrically zoned the brine microinclusions reside in an inner growth zone and their endmember composition is k19na25ca5mg8fe3ba2si4cl32 mol the carbonatitic melt is found in an outer layer and its endmember composition is k11na21ca11mg26fe7ba2si10al3p2cl5 the transition in inclusion chemistry is accompanied by a change in the carbon isotopic composition of the diamond from 8 5 in the inner zone to 12 1 in the outer zone we suggest that this transition reflects mixing between already evolved brine and a freshly introduced carbonatitic melt of different isotopic composition the compositional range found in diamond on dvk 294 is the widest ever recorded in a single diamond it closes the gap between brine found in cloudy octahedral diamonds from south africa and carbonatitic melt analyzed in cubic diamonds from zaire and botswana thus all microinclusions analyzed to date fall along two arrays connecting the carbonatitic melt composition to either a hydrous silicic endmember or to a brine endmember this connection suggests that many diamonds are formed from fluids derived form a mantle source not significantly influenced by local heterogeneities

nanometer size p k rich silica glass former melt inclusions in microdiamond from the gneisses of kokchetav and erzgebirge massifs diversified characteristics of the formation media of metamorphic microdiamond in uhp rocks due to host rock buffering abstract nanometer size p k rich silica glass former melt inclusions were identified within metamorphic microdiamonds from garnets of ultrahigh pressure uhp gneisses of the kokchetav and the erzgebirge massifs by analytical electron microscopy aem the chemical characteristics of these inclusions within microdiamonds are surprisingly similar among various gneissic rocks from both kokchetav and erzgebirge but are significantly different from the si poor ultrapotassic fluid inclusions within microdiamonds from garnets of the kokchetav uhp marble these contrasting findings not only provide constraints on the characteristics compositions of the formation media of metamorphic microdiamonds but also imply that the formation media must have been buffered by the hosting rocks resulting in the observed diversities as reported here in addition depending on the rock types and thus on the nature of the formation media from which metamorphic microdiamonds were formed the respective characteristic morphologies of the microdiamonds differ the p k rich silica melt tends to form octahedral or cubo octahedral microdiamonds within garnet in gneissic rocks whereas the si poor ultrapotassic fluid tends to form spheroids cuboid microdiamonds with rugged surfaces within garnet in marble consequently the buffered media in hosting rocks played a decisive role in determining the different morphologies and growth rates mechanisms of metamorphic microdiamonds in general

microdiamond daughter crystals precipitated from supercritical coh silicate fluids included in garnet erzgebirge germany microdiamonds associated with phlogopite quartz paragonite phengite apatite and rutile are found as regular constituents of minute polyphase inclusions in garnet of ultrahigh pressure p metamorphic gneiss lenses within migmatites of the gneiss eclogite unit erzgebirge germany these aggregates are interpreted to represent original inclusions of a supercritical dense coh fluid rich in k na and sio 2 from this fluid diamond was precipitated as a daughter crystal due to cooling at ultrahigh p conditions brittle failure of the garnet host due to overpressure during release of confining pressure is demonstrated by healed radial cracks during further cooling the silicate phase assemblage of the inclusions crystallized at reduced internal pressures outside the stability field of diamond as indicated by the presence of quartz paragonite and plagioclase it is proposed that the mica dominated mineral assemblage of the inclusions formed by reaction between the fluid and the garnet host the previously formed diamond daughter crystals being preserved metastably these diamond bearing inclusions provide an unequivocal record of dense supercritical coh fluids rich in alkalies and silica within subducted continental crust during ultrahigh p metamorphism

very high pressure orogenic garnet peridotites mantle derived garnet peridotites are a minor component in many very high pressure metamorphic terranes that formed during continental subduction and collision some of these mantle rocks contain trace amounts of zircon and micrometer sized inclusions the constituent minerals exhibit pre and postsubduction microstructures including polymorphic transformation and mineral exsolution experimental mineralogical petrochemical and geochronological characterizations using novel techniques with high spatial temporal and energy resolutions are resulting in unexpected discoveries of new phases providing better constraints on deep mantle processes

precipitates of α cristobalite and silicate glass in uhp clinopyroxene from a bohemian massif eclogite

diamond graphite relationships in ultrahigh pressure metamorphic rocks from the kokchetav massif northern kazakhstan graphite inclusions in diamond k bearing clinopyroxene garnet and kyanite and graphite coatings around microdiamonds from the ultrahigh pressure kokchetav massif northern kazakhstan were investigated by means of scanning electron microscopy laser raman spectroscopy and x ray diffraction important observations include the increasing size of the graphite crystals from the diamond graphite interface to the external part of the graphite coatings the high volume fraction of graphite in the graphite coatings the presence of composite crystals of successively formed diamond core graphite mantle zone diamond rim external graphite coating the spherulitic texture of the graphite coatings the formation of both ordered and disordered graphite and the absence of any fluid associated with the graphite inclusions in diamond and other ultrahigh pressure metamorphic uhpm minerals these features combined with the presence of oriented graphite flakes in k bearing clinopyroxene and in diamond bearing garnet and kyanite and the presence of intergranular diamonds without graphite coatings in the quartzo feldspathic matrix require reconsideration of the generally accepted diamond graphitization model for interpreting the formation of graphite in uhpm rocks metastable growth of graphite within the diamond stability field is suggested and the following crystallization scheme for the c polymorph is proposed graphite was the first c polymorph formed in the diamond stability field followed by diamond the graphite coatings around diamond formed during the final stage of uhpm conditions

pinwarian to grenvillian magmatic evolution in the central grenville province new constraints from id tims u pb ages and coupled lu hf s mc icp ms data understanding the magmatic evolution of the rocks once comprising the hinterland of the grenville orogen through their mesoproterozoic formation is a key to understanding the grenvillian orogeny as a whole in this contribution we present high precision isotope dilution thermal ionization mass spectrometry id tims u pb and coupled solution multicollector inductively coupled plasma mass spectrometry s mc icp ms lu hf zircon data from magmatic rocks occurring in the allochthonous belt of the grenville orogen in the central part of the grenville province we document the presence of a large tract of pinwarian crust represented by a 1497 5 ma granitic gneiss as well as large late geon 14 to early geon 13 1434 7 11 1413 12 1393 8 1383 1 ma magmatic complexes one grenvillian plutonic suite of 1015 2 ma that cross cuts the host rock metamorphic fabric has also been dated this age provides a minimum age of ottawan metamorphism in the region the hf isotopic data show that the magmatic roc

discovery of metamorphic diamonds in central china an indication of a 4000 km long zone of deep subduction resulting from multiple continental collisions the central orogenic belt cob of china is a major continental collision zone that contains extensive outcrops of deeply subducted and exhumed rocks at both the eastern and the western end of the belt here we report discovery of microdiamonds from both eclogites and felsic gneisses in the north qinling zone in the central portion of the cob this discovery demonstrates that the country rocks of continental affinity shared in the ultra high pressure metamorphic uhpm event and provides a bridge connecting the two previously recognized uhpm terranes thereby establishing the existence of a uhpm belt extending more than 4000 km geochronological dating yields early palaeozoic ages in the west and early mesozoic ages in the east recording two separate continental collisions overprinted within the cob occurrence of uhp metamorphism during recurrent continental collision here and in the alps suggests that deep subduction of continental material during such collisions is probably common rather than exceptional with significant implications for processes of plate tectonic reorganization and mantle mixing over time

geochemistry and age of ultrahigh pressure metamorphic rocks from the kokchetav massif northern kazakhstan isotopic and geochemical data of the zerenda series metamorphic rocks from the kokchetav massif are reported some of these rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism p 40 kbar t 900 1000 c the diamond bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks the ree patterns range from lree depleted to slightly lree enriched chondrite normalized la yb n 0 1 5 4 with a negative eu anomaly they are depleted in incompatible elements e g sr ba u th with respect to the upper crust in contrast non diamondiferous rocks of the zerenda series exhibit normal crustal geochemistry all rocks of the zerenda series have very radiogenic lead isotopes the measured values 238u 204pb compared with those calculated for the interval between crust formation and ultrahigh pressure uhp metamorphism suggest a decrease by factors of up to 200 during the uhp metamorphism the sm nd mineral isochrons from the diamond bearing rocks and other rock types of the zerenda series give a middle cambrian 524 535 ma age of metamorphism the nd model ages show that crust formation occurred about 2 3 ga ago significant fractionation of sm and nd and loss of incompatible elements may be due to partial melting of the protoliths the ar ar age determinations of secondary biotite and muscovite from the diamond bearing rocks yield an age of 517 5 ma this cooling age requires a short time interval between uhp metamorphism and uplift to a crustal level ultrahigh pressure metamorphism might be a significant source of pb for the mantle we propose that the radiogenic pb of the oceanic array is the contamination traces of numerous uhp events beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using nd isotopes we compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the sm nd system during very high grade metamorphic events

geometry of the 1954 fairview peak dixie valley earthquake sequence from a joint inversion of leveling and triangulation data in 1954 four earthquakes greater than ms 6 0 occurred within a 30 km radius and in a period of 6 months elevation and angle changes calculated from repeated leveling and triangulation surveys which span the coseismic period provide constraints on the fault geometries and coseismic slip of the faults which were activated the quality of the coseismic geodetic data is assessed corrections are applied to the leveling data for subsidence due to groundwater withdrawal in the fallon area and a rod miscalibration error of 150 30 ppm is isolated in leveling surveys made in 1967 the leveling and triangulation observations are then simultaneously inverted using the single value decomposition svd inversion method to determine fault geometries and coseismic slip using svd it is possible to determine on which faults slip is resolvable given the data distribution the faults are found to dip between 50 and 80 and extend to depths of 5 to 14 km the geodetically derived slip values are generally equal to or greater than the maximum observed displacement along the surface scarps where slip is resolvable the geodetic data indicates the 1954 sequence contained a significant component of right lateral slip this is consistent with the n15 w trending shear zone which geodetic surveys have detected in western nevada

focused ion beam technique and transmission electron microscope studies of microdiamonds from the saxonian erzgebirge germany a focused ion beam of ga ions is a relatively new technique that has been developed for microelectronic industries now researchers of the earth sciences find it to be a promising tool for studying various geological materials using the fib technique and an fei strata db 235 dual beam system we have successfully prepared several electrontransparent foils which crossed wm sized diamonds included in host minerals such as zircon and garnet from quartzofeldspathic rocks of the saxonian erzgebirge germany scanning and transmission electron microscopy applied to these foils revealed that the diamonds contain crystalline nanometric inclusions these inclusions consist of minerals of known stoichiometries such as sio2 and al2sio5 whereas others are characterized by different combinations of si k p ti and fe in the presence of oxygen stoichiometries are not clear at this stage of research one suite of inclusions is assumed to be represented by archerite kh2po4 which is known to be stable at pressures of 4 22 gpa and one nanocrystal containing pb oxygen and carbon is interpreted to be pbxoy or pbco3 along with solid crystalline inclusions the diamonds contain cavities filled by liquid gas that escaped during sample preparation these are associated with dislocations of diamond growth our data are consistent with the concept of diamond crystallization from a coh rich multicomponent supercritical fluid and suggest that the composition of such a fluid is more consistent with a local crustal source rather than that of a mantle origin 2003 elsevier science b v all rights reserved

complete miscibility between silicate melts and hydrous fluids in the upper mantle experimental evidence and geochemical implications the phase relationships between silicate melts and hydrous fluids were studied by direct visual observation in an externally heated diamond anvil cell complete miscibility of silicate melt and water was observed for a wide range of melt compositions including nepheline jadeite dacite haplogranite and ca bearing granite some evidence for complete miscibility was also observed in the system basalt h2o the critical temperatures in all systems decrease rapidly with pressure at 15 kbar the critical temperature for nepheline is around 550 c for jadeite around 800 c and for granitic compositions it is close to 900 c in general the critical temperatures appear to increase with silica content in the system our results suggest that there is complete miscibility between silicate melts and water in most of the upper mantle except at very shallow depths this means that a water saturated solidus cannot be defined any more in the deeper parts of the upper mantle very silica rich melt inclusions found in spinel lherzolites associated with fluid inclusion are probably the result of the unmixing of a supercritical fluid containing comparable amounts of water and silicate components the decomposition of amphibole in a subducted slab occurs at conditions where the miscibility gap between fluid and silicate melt is not yet closed while the decomposition of phengite and lawsonite occurs far beyond the critical curve accordingly the fluids released by the breakdown of these minerals should have very different properties highly mobile hydrous fluids containing little dissolved silicate should be produced by amphibole breakdown while the decomposition of lawsonite and phengite will lead to much more silicate rich and less mobile fluid phases

a comparison of the evolution of diamondiferous quartz rich rocks from the saxonian erzgebirge and the kokchetav massif are so called diamondiferous gneisses magmatic rocks abstract the petrography and chemical composition of minerals of quartz rich diamondiferous rocks from the kokchetav massif especially the zonation of garnet were studied and compared with diamondiferous quartzofeldspathic rocks from the saxonian erzgebirge many compositional and textural features were found to be similar for instance microdiamonds are enclosed systematically in a specific intermediate growth zone of garnet in these rocks on the basis of experimental data a magmatic scenario was constructed to check if the quartz rich diamondiferous rocks are of magmatic origin by this the p t paths derived here for the kokchetav rocks and the textural observations it is concluded that the minerals of the diamondiferous rocks have crystallized from silicate melts these melts originated by anatexis of deeply submerged metasediments erzgebirge at t as high as 1200 c kokchetav massif at 50 100 c lower t and ascended from at least 200 km depth relics of the pre anatectic evolution are still present for instance as garnet cores after ascent and emplacement of the magma in deep portions of thickened continental crust kokchetav massif 45 50 km close to 800 c erzgebirge 55 60 km at 30 50 c lower t considerable quantities of white and or dark micas formed by peritectic reactions from melt for instance garnets could be resorbed at this stage and biotite grew instead after the magmatic stage retrogression took place much stronger in the kokchetav massif this was accompanied by deformation transforming broadly the magmatic texture of quartz rich diamondiferous rocks from the kokchetav massif to a gneissic texture

metamorphic diamonds mechanism of growth and inclusion of oxides we report a detailed series of electron microscope observations of metamorphic microdiamonds included in and separated from garnets and zircons from a single specimen of garnet biotite feldspar gneiss from the kokchetav massif kazakhstan the morphology of the diamonds ranges from skeletal forms composed of thin 111 plates through cuboid and octahedral forms included within the diamonds is a diverse suite of nanometric oxides suggesting that the c o h fluid from which the diamonds grew may have carried chemical components derived from both the sediments and the mantle the spectrum of morphologies and their abundant tiny inclusions can all be explained by a simple model based on the ratio of the rate at which 111 plates grow and the rate of random nucleation of new plates at their edges

evidence of fluid inclusions in metamorphic microdiamonds from the kokchetav massif northern kazakhstan abstract microdiamonds from garnet clinopyroxenites of the kokchetav massif northern kazakhstan and associated alluvial diamonds have been investigated using infrared spectroscopy all diamonds have high nitrogen contents from 747 up to 4488 20 at ppm and are of type ib iaa nitrogen data are consistent with a diamond formation over a narrow temperature range microdiamonds from garnet clinopyroxenites are characterized by water and carbonate inclusions similar to those ocurring in fibrous coated diamonds from kimberlite which suggest that diamonds grew from a c h o fluid in contrast alluvial microdiamonds do not contain any carbonate inclusions and h2o is absent or present in only minor amounts alluvial diamonds are considerably more n rich but have a similar range of nitrogen aggregation states it is suggested that alluvial diamonds belong to a separate diamond population compared with the microdiamonds from garnet clinopyroxenites the kokchetav microdiamonds are distinct from kimberlitic fibrous diamonds in that they contain abundant ib centres have high nitrogen contents and show different populations depending on host rock type these characteristics support a metamorphic origin for the microdiamonds from the uhpm rocks of the kokchetav massif

genesis of microdiamonds from melt and associated multiphase inclusions in garnet of ultrahigh pressure gneiss from erzgebirge germany abstract the common association of microdiamonds with the phase assemblage phlogopite apatite paragonite and quartz containing amorphous na al silicate inclusions as inclusions in garnets of quartzofeldspathic rocks from the saxonian erzgebirge germany was studied by analytical electron microscopy the assemblage implies a precedent melt which coexisted with the microdiamonds before and after entrapment in the garnet host and subsequently crystallized the formation of microdiamonds in this metamorphic rock could be explained by cotectic induced partial melting of a subducted continental slab at about 4 6 gpa and 1000 c as constrained by the occurrence of tio 2 with an pbo 2 type structure at the peak metamorphic conditions and by the catalytic effect of siderophile and chalcophile elements

synthesis of skeletal diamonds implications for microdiamond formation in orogenic belts diamond was synthesized at high temperature and pressure in a walker style multianvil apparatus from various supercritical c o h rich fluid compositional systems depending on the type and amount of impurities added to the system diamonds of vary ing morphology and bearing solid oxide inclusions were found within the run product these characteristics bear a striking re semblance to metamorphic microdiamonds recovered from the kokchetav massif kazakhstan these natural diamonds share mor phological traits with our synthesized diamonds and both the nat ural and synthesized diamonds contain similar solid oxide inclu sions our data favor formation of kokchetav diamonds via crystallization from a supercritical c o h fluid rather than from carbonate or carbonatite melt as others have suggested

k2o prograde zoning pattern in clinopyroxene from the kokchetav diamond grade metamorphic rocks missing part of metamorphic history and location of second critical end point for calc silicate system abstract despite extensive studies of calc silicate rocks of the kokchetav massif there is no satisfactory explanation of the origin of potassium bearing clinopyroxene in alkali poor metamorphic rocks in this paper we report the finding of potassium bearing clinopyroxene with prograde zonation k 2 o increases from core to rim from diamond grade but diamond free uhp calc silicate rocks of the kokchetav massif we believe that the crystallization of potassium bearing clinopyroxene started on the prograde stage and slightly prior to the peak of uhp metamorphism thus prograde metamorphic history is only traceable in diamond free uhp calc silicate rocks while in diamond bearing uhpm rocks it is completely reset fluid and polyphase solid inclusions originally representing melt inclusions occur in the core of potassium bearing clinopyroxene and imply that melt and fluid may coexist in calc silicate rocks even at 1000 1100 c and 6 7 gpa

oxygen carbon and strontium isotope geochemistry of diamond bearing carbonate rocks from kumdy kol kokchetav massif kazakhstan abstract diamond bearing carbonate rocks from kumdy kol kokchetav massif kazakhstan were strongly altered by fluids flowing through fractures and infiltrating along grain boundaries during exhumation alteration includes retrogradation of high grade silicate assemblages by hydrous minerals replacement of diamond by graphite and of dolomite by calcite diamond bearing carbonate rocks are among the most intensely altered isotopically with 18 o vsmow values as low as 9 13 c vpdb 9 and 87 sr 86 sr as high as 0 8050 evidence of isotopic equilibration between coexisting dolomite and high mg calcite during ultrahigh pressure metamorphism uhpm is preserved only rarely in samples isolated from infiltrating fluids by distance from fractures isotopic heterogeneity and isotopic disequilibrium are widespread on a hand specimen scale because of this lack of homogeneity bulk analyses cannot provide definitive measurements of 13 c 12 c fractionation between coexisting diamond and carbonate our study adequately documents alteration on a scale commensurate with observed vein structures but testing the hypothesis of metamorphic origin of microdiamonds has not fully succeeded because our analytical spatial resolution limited to 0 5 mm is not small enough to measure individual dolomite inclusions or individual diamond crystals

aeromagnetic signature of an exhumed double dome system in the sw grenville province canada analysis of aeromagnetic data in the grenville province reveals the presence of two regional scale unmapped structural domes the morin and mekinac taureau domes with an oval shaped magnetic pattern bounded by regional scale shear zones and a geometry that is similar to that produced in crustal flow models under extension which predict two upright domes of foliation double dome separated by a steep shear zone the mekinac taureau dome a metamorphic core complex and the morin dome may have been exhumed by channel flow exhumation occurred by a combination of thrust normal sense and wrench shear zones the preservation of paragneisses in the morin dome suggests that it underwent a lesser degree of exhumation than did the mekinac taureau dome this study shows how the integration of local field information with magnetic data in a regional tectonic setting can reveal and delineate concealed gneiss domes and highlights a role for strike slip tectonics in the creation of regional structures involving the exhumation of deep crust this article is protected by copyright all rights reserved

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c