retrograde phases of former bridgmanite inclusions in superdeep diamonds abstract mg fe sio3 bridgmanite is the dominant phase in the lower mantle however no naturally occurring samples had ever been found in terrestrial samples as it undergoes retrograde transformation to a pyroxene type structure to identify retrograde phases of former bridgmanite single phase and composite inclusions of mg fe sio3 in a series of superdeep diamonds have been examined with electron microscopy electron microprobe raman spectroscopy and x ray diffraction techniques our study revealed that mg fe sio3 inclusions are represented by orthopyroxene orthopyroxenes in single phase and composite inclusions inherit initial chemical composition of bridgmanites including a high al and low ni contents in composite inclusions they coexist with jeffbenite ex tapp and olivine the bulk compositions of these composite inclusions are rich in al ti and fe which are similar but not fully resembling al rich bridgmanite produced in experiments on the morb composition the retrograde origin of composite inclusions due to decomposition of al rich bridgmanite may be doubtful because each of observed minerals may represent coexisting hp phases i e bridgmanite or ringwoodite

formation of diamond with mineral inclusions of mixed eclogite and peridotite paragenesis two unusual diamonds were studied from kimberlites from china which contain both ultramafic and eclogitic mineral inclusions in the same diamond hosts diamond l32 contains seven fe rich garnets four omphacites and one olivine inclusion four olivine one sanidine and one coesite were recovered from diamond s32 both garnet and omphacite inclusions have similar compositions as those from other localities of the world and show basaltic bulk composition all the garnet and omphacite inclusions in diamond l32 have positive eu anomalies eu eu 1 64 1 79 these observations support the proposal that mantle eclogite is the metamorphic product of subducted ancient oceanic crust the mg mg fe ratio of the olivine inclusions from the two diamonds 91 92 are evidently lower than the normal olivine inclusions in diamonds from the same kimberlite pipe 92 95 the following model is proposed for the formation of diamonds with mixed mineral inclusions ascending diamond bearing eclogite recycled oceanic crust entrained in mantle plumes may experience extensive partial melting whereas the ambient peridotite matrix remains subsolidus in the diamond stable field this provides a mechanism for the transport of diamond from its original eclogitic host to an ultramafic one subsequent re growth of diamond in the new environment makes it possible to capture mineral inclusions of different lithological suites partial melts of basaltic sources may interact with the surrounding peridotite resulting in the relatively lower mg mg fe ratios of the coexisting olivine inclusions from the studied diamonds diamonds with mixed mineral inclusions demonstrate that plume activity also occurred in the archean cratons

deep electrical structure of northern alberta canada implications for diamond exploration geophysical studies of upper mantle structure can provide constraints on diamond formation teleseismic and magnetotelluric data can be used in diamond exploration by mapping the depth of the lithosphere asthenosphere boundary studies in the central slave craton and at fort a la corne have detected conductors in the lithospheric mantle close to or beneath diamondiferous kimberlites graphite can potentially explain the enhanced conductivity and may imply the presence of diamonds at greater depth petrologic arguments suggest that the shallow lithospheric mantle may be too oxidized to contain graphite other diamond bearing regions show no upper mantle conductor suggesting that the correlation with diamondiferous kimberlites is not universal the buffalo head hills in alberta host diamondiferous kimberlites in a proterozoic terrane and may have formed in a subduction zone setting long period magnetotelluric data were used to investigate the upper mantle resistivity structure of this region magnetotell

taltson basement gneissic rocks upb and nd isotopic constraints on the basement to the paleoproterozoic taltson magmatic zone northeastern alberta the taltson magmatic zone is a 300 km long northerly trending belt of archean to paleoproterozoic granitic basement gneiss amphibolite supracrustal gneissic rocks and voluminous paleoproterozoi

introduction to special issue of canadian journal of earth sciences the alberta basement transect of lithoprobe from outcrops in northeastern alberta to exposures within structural culminations in the canadian cordillera the crystalline basement that lies buried beneath the phanerozoic alberta basin of western canada represents a significant part of the canadian shield ca 750 000 km2 the advent of modern high resolution aeromagnetic data and u pb geochronology have opened the doors for the next era of investigation of the crystalline basement of the alberta basin aeromagnetic data and geochronology provided important physical and temporal links to the exposed canadian shield which allowed its structural framework to be extrapolated into the subsurface the most recent chapter in this evolving story has been the alberta basement transects of lithoprobe a 12 year program which has used multi disciplinary geoscience studies to further elucidate the nature history and geometric characteristics of basement domains beneath the alberta basin this special issue is the second in a three issue set of publications

retrograde isochemical phase transformations of majoritic garnets included in diamonds a case study of subcalcic cr rich majoritic pyrope from a snap lake diamond canada abstract homogeneity of a peridotitic garnet inclusion in diamond demonstrating excess in si concentration i e presence of majorite component was investigated by tem using fib prepared foils the host diamond is a low nitrogen brown stone which can be related to type iia with features of strong plastic deformation the studied sample is represented by ca poor cr pyrope of harzburgitic h paragenesis from snap lake dyke canada the garnet had been previously reported to contain si 3 16 apfu the revised examination of the sample resulted in detection of extremely fine grained symplectite consisting of low ca orthopyroxene clinopyroxene cr spinel and coesite completely located and isolated in the inner part of the garnet crystal which forms a sharp interface with the surrounding homogeneous garnet xrd study confirmed the presence of the minerals constituting the symplectite epma showed an identical bulk chemistry of the nanometer sized symplectite and garnet further polishing of the garnet inclusion on the same surface with diamond removed the symplectite which possibly was present as a thin lens within garnet the remaining garnet is completely homogeneous as checked by two profiles and contains unusually high ni 118 2 ppm and depleted ree patterns estimated pt formation conditions of this garnet are 10 8 gpa and 1450 c within asthenosphere symplectite testifies partial retrograde isochemical phase transformation of the examined garnet which is suggested to be caused by decompression along with plastic deformation of diamond within the coesite stability field at t 1000 c and depth no less than 100 km because previously published studies of rare majoritic garnets composition were performed by epma only it is possible that the traces of partial phase transformation symplectite formation could have been overlooked without additional xrd and or tem aem studies

li as a barometer for bimineralic eclogites experiments in natural systems abstract experiments performed in eclogitic systems show that the partitioning of li between garnet and clinopyroxene is pressure dependent in natural eclogitic bulk compositions this supports experimentally the hypothesis based on the analysis of li in eclogitic xenoliths and inclusions in diamond that the partitioning of this particular element between clinopyroxene and garnet is very sensitive to changes in pressure and could be calibrated as a barometer for bimineralic eclogites in order to calibrate this pressure dependence into a barometer 16 multianvil experiments were performed from 4 to 13 gpa and 1100 1500 c using starting materials sourced from a well preserved mg rich eclogitic xenolith from the roberts victor kimberlite pipe south africa our findings reinforce the general trend we observed in the cmas system that d li cpx grt for li decreases with increasing p and that at p 12 gpa garnet is able to incorporate more li than clinopyroxene multiple linear regression was applied to our experimental results to create the barometer p 0 000963 t ln d li cpx grt 1 581 0 252 where p is pressure in gpa t is temperature in c and d li cpx grt is defined as the partitioning coefficient of li obtained by dividing the concentration of li in cpx by the concentration of li in garnet this barometer reproduces the experimental conditions to 0 2 gpa it is applicable to mg rich eclogitic xenoliths and to peridotitic and eclogitic inclusions in diamond application of the barometer to diamond bearing xenoliths results in pressures in the diamond stability field clinopyroxene is easily corrupted in xenoliths and also preferentially takes in li during short lived metasomatic processes care must be taken therefore to analyse primary unaltered clinopyroxene our preliminary application to natural samples shows that the barometer can be applied beyond the experimental range to pressures down to 3 5 gpa this is higher than the pressures encountered in crustal eclogites and thus the barometer is not applicable to these eclogites

chromium bearing phases in the earth s mantle evidence from experiments in the mg2sio4 mgcr2o4 system at 10 24 gpa and 1600 c

exhumation of serpentinized peridotite in the northern manila subduction zone inferred from forward gravity modeling the taiwan integrated geodynamic research program taiger collected two wide angle and reflection seismic transects across the northern manila subduction zone that provide constraints on the seismic velocity structure of the crust two dimensional gravity modeling along these two transects shows a significant relatively high density 3 12 and 3 02 g cm3 in the fore arc region at the interface between the subducting eurasian plate and the accretionary prism in front of the luzon arc on the overriding philippine sea plate the anomalous density in this zone is higher than that in the fore arc crust and the accretionary prism but lower than that in mantle numerous geophysical and geological data together with numerical models have indicated that serpentinization of the fore arc mantle is both expected and observed serpentinization of mantle rocks can dramatically reduce their seismic velocity and therefore their seismic velocity in a density to velocity conversion therefore the source of the high density material could be serpentinized fore arc mantle with serpentinization caused by the dehydration of the subducting eurasian plate we interpret that positive buoyancy combined with weak plate coupling forces in the northern manila subduction zone is resulting in this serpentinized fore arc mantle peridotite being exhumed

potassium argon dating of fine grained basalts with massive ar loss application of the 40ar39ar technique to plagioclase and glass from the kirkpatrick basalt antarctica 40ar39ar incremental heating measurements are reported for separated plagioclase and matrix fractions and a glass sample of the kirkpatrick basalt from victoria land antarctica these results are used to address the limitations of whole rock analyses of glassy or very fine grained basalts the use of plagioclase for determining ages of such rocks the ar behavior of matrix glass and feldspar components the use of k cl ratios in interpretation of step heating results and the age of the kirkpatrick basalt the precise age of the kirkpatrick basalt has been a long standing problem the wide range of reported k ar dates has been attributed to loss of ar despite massive ar loss from the matrix plagioclase is shown to yield good 40ar39ar plateaus plateaus are observed for increments above 700 c and are defined not only by consistent apparent ages but also appropriately low k ca and high k cl ratios cl relations help identify ar released from small amounts of matrix which can seriously contaminate a plagioclase separate an age of 176 8 1 8 ma defines the time of formation of the youngest lava at mt frustum in north victoria land a glass sample from south victoria land gives the same age indicating that terrestrial basaltic glasses as old as jurassic do not inherently lose ar the step heating profile of glass with very fine crystals shows discordance which is an artifact of ar recoil during irradiation

early paleozoic structural and metamorphic evolution of western sierra de san luis argentina in relation to cuyania accretion abstract structural metamorphic and isotopic data obtained from the nogoli metamorphic complex of western sierra de san luis indicate that the early paleozoic famatinian orogeny overprinted an already structured and metamorphosed older basement the older geological features are relict nw trending fabric associated with high grade amphibolite facies regional metamorphism preserved within thin strips of schists and paragneisses and in the core of mafic to ultramafic lenses arc magmatism medium p barrovian type high t amphibolite to granulite facies regional metamorphism and penetrative nne to ne trending foliation are related to the building of the famatinian orogenic belt the p t conditions of the famatinian prograde metamorphism reached a pressure peak of ca 8 kb with a thermal peak from 750 c up to 820 c u pb conventional and chemical dating and ar ar plateau ages constrain the peak of the main orogenic phase related to the famatinian belt to 470 457 ma early to mid ordovician greenschist facies retrograde metamorphism closely associated with shear zones and secondary ar ar plateau and sm nd ages suggest that a late to post orogenic phase of the famatinian belt was active at least since 445 ma this phase continued during the silurian to late devonian times through multiple reactivation of early shear zones the famatinian orogeny reset a previous thermal history and therefore the timing of the relict fabric could not be constrained conclusively with radiometric dates despite this difficulty a range of 520 to 490 ma suggests some inheritance from pampean events registered by the older nw se fabric the early to mid ordovician regional metamorphism and ductile deformation of the western sierra de san luis is interpreted as the orogenic effects of the collision of the allochthonous cuyania terrane with the autochthonous proto pacific margin of gondwana during the famatinian orogeny

mineral inclusions and geochemical characteristics of microdiamonds from the do27 a154 a21 a418 do18 dd17 and ranch lake kimberlites at lac de gras slave craton canada a mineral inclusion carbon isotope nitrogen content nitrogen aggregation state and morphological study of 576 microdiamonds from the do27 a154 a21 a418 do18 dd17 and ranch lake kimberlites at lac de gras slave craton was conducted mineral inclusion data show the diamonds are largely eclogitic 64 followed by peridotitic 25 and ultradeep 11 the paragenetic abundances are similar to macrodiamonds from the do27 kimberlite davies r m griffin w l o reilly s y 1999 diamonds from the deep pipe do27 slave craton canada in gurney j j gurney j l pascoe m d richardson s h eds the j b dawson vol proc 7th internat kimberlite conf red roof designs cape town pp 148 155 but differ to diamonds from nearby kimberlites at ekati e g lithos 2004 tappert r stachel t harris j w brey g p 2004 mineral inclusions in diamonds from the panda kimberlite s p canada 8th international kimberlite conference extended abstracts and snap lake to the south dokl earth sci 380 7 2001 806 that are dominated by peridotitic stones eclogitic diamonds with variable inclusion compositions and temperatures of formation 1040 1300 c crystallised at variable lithospheric depths sometimes in changing chemical environments a large range to very 13c depleted c isotope compositions 13c 35 8 to 2 2 and an nmorb bulk composition calculated from trace elements in garnet and clinopyroxene inclusions are consistent with an origin from subducted oceanic crust and sediments carbon isotopes in the peridotitic diamonds have mantle compositions 13c mode 4 0 mineral inclusion compositions are largely harzburgitic variable temperatures of formation garnet tni 800 1300 c suggest the peridotitic diamonds originate from the shallow ultra depleted and deeper less depleted layers of the central slave lithosphere carbon isotopes 13c av 5 1 and mineral inclusions in the ultradeep diamonds suggest they formed in peridotitic mantle 670 km the diamonds may have been entrained in a plume and subcreted to the base of the central slave lithosphere poorly aggregated nitrogen iaa without platelets in a large number of eclogitic 67 and peridotitic 32 diamonds with similar nitrogen contents indicates the diamonds were stored in the mantle at low temperatures 1060 1100 c following crystallisation in the archean type iaa diamonds have largely cubo octahedral growth forms and type ii and type iaab diamonds with higher nitrogen aggregation states mostly have octahedral morphologies however no correlation between these groups and their mineral inclusion compositions c isotopes and n contents rules out the possibility of unique source origins and suggests eclogitic and peridotitic diamonds experienced variable mantle thermal states variation in mineral inclusion chemistries in single diamonds possible overgrowths of 13c depleted eclogitic diamond on diamonds with peridotitic and ultradeep inclusions and type i ultradeep diamond with low n aggregation is consistent with diamond growth over time in changing chemical environments

phase relations and formation of sodium rich majoritic garnet in the system mg 3 al 2 si 3 o 12 na 2 mgsi 5 o 12 at 7 0 and 8 5 gpa the pseudo binary system mg3al2si3o12 na2mgsi5o12 modelling the sodium bearing garnet solid solutions has been studied at 7 and 8 5 gpa and 1 500 1 950 c the na bearing garnet is a liquidus phase of the system up to 60 mol na2mgsi5o12 nagrt at higher content of nagrt in the system enstatite up to 80 mol and then coesite are observed as liquidus phases our experiments provided evidence for a stable sodium incorporation in garnet 0 3 0 6 wt na2o and its control by temperature and pressure the highest sodium contents were obtained in experiments at p 8 5 gpa near the liquidus t 1 840 c the equilibrium concentration of na2o in garnet is 0 7 0 8 wt 6 mol na2mgsi5o12 with the temperature decrease na concentration in grt increases and the maximal na2mgsi5o12 content of 12 mol 1 52 wt na2o is gained at the solidus of the system t 1 760 the data obtained show that most of natural diamonds with inclusions of na bearing garnets usually containing 1 wt na2o may crystallize at a pressure range of 7 0 8 5 gpa however the upper pressure limit for the formation of naturally occurring na bearing garnets is restricted by the eclogite garnetite bulk composition

phase relations on the diopside jadeite hedenbergite join up to 24 gpa and stability of na bearing majoritic garnet abstract phase relations on the diopside di hedenbergite hd jadeite jd system modeling mineral associations of natural eclogites were studied for the compositions mol di70jd30 di50jd50 di30jd70 di20hd80 and di40hd10jd50 using a toroidal anvil with hole 7 gpa and a kawai type 6 8 multianvil apparatus 12 24 gpa we established that di hd and jd form complete series of solid solutions at 7 gpa and melting temperatures of pure di 1980 c and jd 1870 c for that pressure were estimated experimentally the melting temperature for the di50jd50 composition at 15 5 gpa is 2270 c the appearance of garnet is clearly dependent on initial clinopyroxene composition at 1600 c the first garnet crystals are observed at 13 5 gpa in the jadeite rich part of the system di30jd70 whereas diopside rich starting material di70jd30 produces garnet only above 17 gpa the proportion of garnet increases rapidly above 18 gpa as pyroxene dissolves in the garnet structure and pyroxene free garnetites are produced from diopside rich starting materials in all experiments garnet coexists with stishovite st at a pressure above 18 gpa pyroxene is completely replaced by an assemblage of majorite maj st casio3 perovskite ca pv in ca rich systems whereas maj is associated with almost pure jd up to a pressure of 21 5 gpa above 22 gpa maj and st are associated with naalsio4 with calcium ferrite structure cf we established that an hd component also spreads the range of pyroxene stability up to 20 gpa in the di70jd30 system at 24 gpa an assemblage of maj ca pv mgsio3 with ilmenite structure mg il was obtained the experimentally established correlation between na si and al contents in maj and pressure in grt maj pyroxene assemblages may be the basis for a majorite geobarometer the results of our experiments are applicable to the upper mantle and the transition zone of the earth 400 670 km and demonstrate a wide range of transformations from eclogite to perovskite bearing garnetite in addition the mineral associations obtained from the experiments allowed us to simulate parageneses of inclusions in diamonds formed under the conditions of the transition zone and the lower mantle

lithoprobe s impact on the canadian diamond exploration industrythis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent earth sciences sector contribution 20070261 lithoprobe contribution 1479 lithoprobe aided and will continue to guide diamond exploration in canada via a number of diverse ways and at scales from cratons to individual ore bodies specific transects outlined the domainal nature geometry and age relations of crust subjacent mantle and diamond bearing roots in unusually rich detail for cratonic settings such as those of the superior and slave cratons as well as pericratonic shield settings such as those of the sask craton and buffalo hills terrane the superior and slave examples of craton construction by multiple underthrust layers as inferred from structural geometries observed in lithoprobe seismic and magnetotelluric surveys remain the clearest resolved to date the first definition of the sask craton occurred 10 15 years ago and helped fuel the diamond industry s ongoing revision of clifford s rule for exploration so as to include targets in proterozoic shield settings main lithoprobe transect results and supporting geoscience studies have helped to demonstrate the

na bearing majoritic garnet in the na2mgsi5o12 mg3al2si3o12 join at 11 20 gpa phase relations structural peculiarities and solid solutions abstract na majorite na 2 mgsi 5 o 12 namaj an end member of sodium rich majoritic garnet was synthesized and garnet pyroxene transition was studied in kawai type multi anvil experiments at 11 20 gpa and 1500 2300 c na majorite was obtained at 16 gpa and 1500 c its stability spreads to the high temperature region with pressure 1900 c at 17 gpa and 2100 c at 19 5 gpa experimental study of the mg 3 al 2 si 3 o 12 prp na 2 mgsi 5 o 12 join at 11 20 gpa demonstrated the following phase assemblages changing with pressure garnet two pyroxenes enstatite rich and na pyroxene garnet pyroxene enstatite jadeite na pyroxene stishovite garnet stishovite garnet naalsio 4 with calcium ferrite type structure stishovite for a given bulk composition prp 50 namaj 50 a regular increase of both na and si concentrations and consequently namaj mole portion in garnets was observed from 8 5 to 20 gpa the concentration of na 2 o increased from 1 52 to 5 71 wt which correspond to a namaj content of 40 mol the highest na 2 o content 12 24 wt was registered at 18 5 gpa and 1600 c for the prp 20 namaj 80 starting composition experiments at 18 5 gpa and 1600 c on the pyrope na majorite join allowed us to study mixing peculiarities for sodium rich majoritic garnet the transition from cubic ia 3 d to tetragonal i 4 1 acd symmetry was observed for the composition with 80 mol na 2 mgsi 5 o 12 which is consistent with the similar change of the structure in the pyrope majorite mg 4 si 4 o 12 system significant na majorite solubility in pyrope as well as findings of natural garnets with high na concentrations 1 wt na 2 o allow us to consider na bearing majoritic garnet as an important concentrator of sodium in the deep upper mantle and transition zone

archean diamonds from wawa canada samples from deep cratonic roots predating cratonization of the superior province with an age of ca 2 7 ga greenschist facies volcaniclastic rocks and lamprophyre dikes in the wawa area superior craton host the only diamonds emplaced in the archean available for study today nitrogen aggregation in wawa diamonds ranges from type iaa to iab suggesting mantle residence times of tens to hundreds of millions of years the carbon isotopic composition 13c of cube diamonds is similar to the accepted mantle value 5 0 octahedral diamonds show a slight shift by 1 5 to isotopically less negative values suggesting a subduction derived isotopically heavy component in the diamond forming fluids syngenetic inclusions in wawa diamonds are exclusively peridotitic and similar to many diamond occurrences worldwide are dominated by the harzburgitic paragenesis compositionally they provide a perfect match to inclusions from diamonds with isotopically dated paleo to mesoarchean crystallization ages several high cr harzburgitic garnet inclusions contain a small majorite component suggesting crystallization at depth of up to 300 km combining diamond and inclusion data indicates that wawa diamonds formed and resided in a very thick package of chemically depleted lithospheric mantle that predates stabilization of the superior craton if late granite blooms are interpreted as final stages of cratonization then a similar disconnect between paleo to mesoarchean diamondiferous mantle lithosphere and neoarchean cratonization is also apparent in other areas e g the lac de gras area of the slave craton and may suggest that early continental nuclei formed and retained their own diamondiferous roots

the application of c isotope measurements to the identification of the sources of c in diamonds a review abstract in the past decade the isotopic compositions of c in 600 inclusion bearing diamonds have been determined such analyses have revealed the following isotopic characteristics 1 peridotitic diamonds which typically contain garnet chromite olivine and or orthopyroxene inclusions with refractory compositions high mg cr have 13 c values predominantly between 10 and 1 with a sharp peak in the distribution near 5 2 eclogitic diamonds which commonly contain inclusions of omphacitic clinopyroxene cr poor pyrope and or eclogitic accessory minerals such as rutile kyanite coesite or sanidine have 13 c values between 34 and 3 with a smaller peak near 5 3 the isotopic compositions obtained for suites of diamonds from individual occurrences are in general unique and do not resemble the range and distribution obtained by amalgamating the diamond isotope data from a number of localities 4 isotopic zoning patterns and heterogeneities are found in some diamonds cores of coated diamonds tend to be depleted in 13 c relative to the rims and within single octahedral diamonds 13 c variations of nearly 6 have been reported because expected c isotope fractionations at mantle temperatures are small attempts to model the full range of diamond isotope values through fractionating a homogeneous mantle c source have been unsuccessful nevertheless fractionation is probably responsible for some of the observed variation in 13 c values two other models have also been proposed to account for the diamond characteristics outlined above the primordial model suggests that the range and distribution of c isotope compositions are inherited from primordial c in the mantle which has an inhomogeneous isotopic composition such as that found in meteorites the subduction model suggests that subducted crustal c is the source of c in diamonds as organic and inorganic c compounds in the crust exhibit a range of 13 c values similar to that observed in diamonds this paper reviews the c isotope characteristics of diamonds and compares the models which have been proposed to explain the origins of these characteristics

early paleozoic orogenic belt of the andes in southwestern south america result of laurentia gondwana collision the late precambrian to early paleozoic age rock units of the pampean ranges the puna and the north patagonian massif of southwestern south america constitute the famatinian orogenic belt they are interpreted as an ordovician collisional belt between the occidentalia terrane and the gondwana craton they include mafic and ultramafic belts of neoproterozoic to early paleozoic age an intense tectonothermal event resulted from the collision syntectonic granitoids represent crustal melting in that collision syntectonic to late tectonic foreland basins developed the recently proposed juxtaposition of laurentia and east antarctica australia in the neoproterozoic raises the possibility that laurentia and western south america were close together in the early paleozoic and therefore that the famatinian belt resulted from laurentia gondwana collision occidentalia which is bordered by a cambrian carbonate platform similar to that of eastern north america may be a sliver detached from laurentia during late ordovician time

inclusions in diamonds from the k14 and k10 kimberlites buffalo hills alberta canada diamond growth in a plume analyses of mineral inclusions carbon isotopes nitrogen contents and nitrogen aggregation states in 29 diamonds from two buffalo hills kimberlites in northern alberta canada were conducted from 25 inclusion bearing diamonds the following paragenetic abundances were found peridotitic 48 eclogitic 32 eclogitic websteritic 8 websteritic 4 ultradeep 4 and unknown 4 diamonds containing mineral inclusions of ferropericlase and mixed eclogitic asthenosphericwebsteritic and eclogitic websteritic mineral associations suggests the possibility of diamond growth over a range of depths and in a variety of mantle environments lithosphere asthenosphere and possibly lower mantle eclogitic diamonds have a broad range of c isotopic composition d 13 c y21 x to y5x peridotitic websteritic and ultradeep diamonds have typical mantle c isotope values d 13 c y4 9 x av except for two 13 c depleted peridotitic d 13 c y11 8 x y14 6 x and one 13 c depleted websteritic diamond d 13 c y11 9 x infrared spectra from 29 diamonds identified two diamond groups 75 are nitrogen free type ii or have fully aggregated nitrogen defects type iab with platelet degradation and low to moderate nitrogen contents av 330 ppm n 25 have lower nitrogen aggregation states and higher nitrogen contents f30 iab 1600 ppm n the combined evidence suggests two generations of diamond growth type ii and type iab diamonds with ultradeep peridotitic eclogitic and websteritic inclusions crystallised from eclogitic and peridotitic rocks while moving in a dynamic environment from the asthenosphere and possibly the lower mantle to the base of the lithosphere mechanisms for diamond movement through the mantle could be by mantle convection or an ascending plume the interaction of partial melts with eclogitic and peridotitic lithologies may have produced the intermediate websteritic inclusion compositions and can explain diamondsofmixedparageneses andtheoverlapinc isotopevaluesbetweenparageneses strongdeformationandextremelyhigh nitrogen aggregation states in some diamonds may indicate high mantle storage temperatures and strain in the diamond growth environment a second diamond group with type iaa iab nitrogen aggregation and peridotitic inclusions crystallised at the base of the cratonic lithosphere all diamonds were subsequently sampled by kimberlites and transported to the earth s surface crown copyright d 2004 published by elsevier b v all rights reserved

stacked uppermost mantle layers within the slave craton of nw canada as defined by anisotropic seismic discontinuities 1 a 20 station seismic array in nw canada recorded 336 teleseismic events with distribution in back azimuth and epicentral distance sufficient to characterize uppermost mantle discontinuities between depths typical of the mohorovicic and lehman discontinuities following wavefield decomposition groups of seismograms were source normalized through simultaneous deconvolution to estimate the near receiver impulse response and thus detect major discontinuities beneath each seismic station stations within the lac de gras kimberlite field display an unusually strong negative impulse on the radial component within the nw quadrant and two moderate impulses on the transverse component forward modeling of these impulses suggests a mantle layer dipping at 22 to the southeast with a mildly anisotropic 2 upper discontinuity at 120 135 km depth and another mildly anisotropic 2 discontinuity at about 170 km depth superimposed on these layers is another stronger anisotropic 4 layer between 110 and 180 km depths that dips to the west stations outside of the lac de gras field but within the southeastern slave craton display more numerous but weaker impulses the most prominent of these occurs at about 150 km depth on the transverse component and has opposite polarity to that observed farther north the prominent negative impulse observed on the radial component is interpreted to arise from structural preferred orientation in the form of a stockwork of wehrlite dykes beneath the lac de gras field interpretation of the other layers in the context of known surface geology as well as xenolith petrology and garnet geochemistry of diamondiferous kimberlites favors previous suggestions that they represent 4000 2900 ma depleted harzburgite and eclogite layers underthrust from the northwest at 2600 or 1880 ma the layer beneath the se slave craton has a similar but distinct tectonic history of nw verging underthrusting associated with the 2635 2615 ma defeat suite of plutonism taken together these interpretations indicate that the slave craton was assembled from at least four lithospheric blocks prior to its cratonization about 2580 ma each block is 90 120 km thick and all four blocks abut across a steep suture boundary beneath mackay lake in the central slave craton significant amounts of carbon and eclogites could have become incorporated into the central slave mantle within the diamond stability field during the proposed underthrusting of lithosphere to explain their relatively common occurrence in kimberlite eruptions within the slave craton