remobilization in the cratonic lithosphere recorded in polycrystalline diamond polycrystalline diamonds framesites from the venetia kimberlite in south africa contain silicate minerals whose isotopic and trace element characteristics document remobilization of older carbon and silicate components to form the framesites shortly before kimberlite eruption chemical variations within the garnets correlate with carbon isotopes in the diamonds indicating contemporaneous formation trace element radiogenic and stable isotope variations can be explained by the interaction of eclogites with a carbonatitic melt derived by remobilization of material that had been stored for a considerable time in the lithosphere these results indicate more recent formation of diamonds from older materials within the cratonic lithosphere

peraluminous xenoliths in kimberlite metamorphosed restites produced by partial melting of pelites abstract three new varieties of aluminous assemblages from the jagersfontein kimberlite are recognized as xenoliths consisting of garnet spinel alkremite corundum garnet corganite and corundum garnet spinel corgaspinite accessory minerals include phlogopite k pargasite fassaitic pyroxene and secondary spinel bulk rock major elements are depleted in sio2 and enriched in al2o3 and mgo relative to basalt the compatible trace elements ni cr zn ga and v reflect the modal mineralogy and the incompatible trace elements k rb and ba vary as a function of metasomatism chondrite normalized ree patterns are characterized by fanning light and heavy ree variations calculations show that the patterns are consistent with partial melting of pelitic rocks from which a granitic low temperature portion has been removed by anatexis to produce a restite containing the observed geochemical variations sr isotopic ratios of garnet separates are high 0 780 0 804 and also indicate a sedimentary origin for the assemblages a two stage process involving partial melting of archean sediments followed by prograde metamorphism of the residual restite is tentatively proposed initiated by exotic terrain accretion or subduction at the edge of the kaapvaal craton

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

oxygen and strontium isotopic investigations of subduction zone volcanism the case of the volcano arc and the marianas island arc abstract recent fresh volcanic rocks of the intra oceanic mariana and volcano arcs were analyzed for o and sr isotopic compositions in order to determine the source of these magmas fresh non arc volcanic rocks from the regions surrounding the mariana volcano arcs and some dsdp sediments were also analyzed for comparison the oxygen isotopic ratios of the arc lavas 5 5 6 8 exhibited a small inter island variation that cannot be entirely explained by fractional crystallization the sr isotopic composition of the arc lavas is remarkably uniform 0 70332 0 70394 for the marianas three models are considered in order to explain the observed isotopic characteristics 1 bulk mixing and melting of morb type mantle with a subducted sediments and b subducted oceanic crust excluding sediments 2 melting of a mixture of sediment derived fluids and morb type mantle and 3 melting of a mixture of sediment derived fluids and oceanic island or hot spot type mantle the last model fits the data best the conclusion that very small and variable amounts of sediment derived fluid 1 are required to explain the observed inter island o isotopic variation is consistent with that of other workers who used different isotopic and trace element methods the generation of magmas in the mariana volcano arcs involves very little sediment and the source region of mariana lavas is isotopically indistinguishable from that of hot spot basalts

40ar 39ar ages in mantle xenolith phlogopites determining the ages of multiple lithospheric mantle events and diatreme ascent rates in southern africa and malaita solomon islands kimberlites are extraordinary natural phenomena ascending through the earth s lithosphere entraining xenoliths to erupt at the surface within hours to days of their inception deep within the lithospheric mantle with the realization that some ar ar phlogopite grain core ages may be indicative of geological events we have undertaken high spatial resolution ar ar dating of phlogopites in xenoliths and megacrysts from kimberley monastery and letseng in southern africa and malaita in the solomon islands to est whether other mantle phlogopite cores may yield meaningful ages modelling of ar diffusive loss profiles from phlogopite grain boundaries to cores provides information on both the eruption age and the duration of outgassing within the kimberlite magma and hence yields estimates on diatreme ascent rates the ascent durations are very similar for all of the southern african pipes studied yielding durations of 0 9 6 9 days assuming an average kimberlite magma temperature of 1000 c these can be compared to estimates from phlogopite xenoliths from siberian diamond bearing kimberlites yielding ascent durations of 2 15 hours assuming the same magma temperature

eclogites with oceanic crustal and mantle signatures from the bellsbank kimberlite south africa part 2 sr nd and o isotope geochemistry the sr nd and o isotopic compositions of garnet and clinopyroxene mineral separates from nine eclogite xenoliths from the bellsbank kimberlite erupted at 120 ma define three groups group a eclogites considered to be mantle cumulates are characterized by 6180 and 87sr srsr values typical of mantle derived materials 5 1 to 5 6 o and 0 7042 0 7046 respectively and very low sm nd ratios apparent rb sr ratios and nallz01 values 0 057 0 078 0 00005 0 00136 and 14 to 16 respectively the ree and isotopic data for these eclogites can be modelled in terms of crystallization from a group ii kimberlite magma at 1 1 5 ga group b and c eclogites believed to be the metamorphosed products of ancient subducted oceanic crust are characterized by low 3180 2 9 to 4 7 extremely high nd a20 40 to 219 and radiogenic 87sr s6sr ratios 0 708 0 710 the sm nd ratios of the group b eclogites are very high up to 1 6 the data for group b and c eclogites define a linear correlation on sm nd and 1 nd vs nal 201 diagrams these relationships are consistent with mixing of the bellsbank kimberlite cno 1201 10 sm nd 0 10 with a depleted eclogite end member end 120 219 sm nd 1 6 during a cryptic metasomatic event the sr isotopic variations in group b and c eclogites cannot be generated by simple two component mixing the st nd and o isotope data for group b and c eclogites probably reflect a complex sequence of depletion and enrichment events in both crust and mantle settings enrichments which possibly affected the group b and c eclogites include seawater alteration of a morb like protolith which lowered the 6180 and raised the svsr 86sr ratio but left the nd isotopic compositions unchanged and cryptic metasomatism by the magmatism that produced the bellsbank kimberlite the high sm nd ratio of the depleted eclogite end member cannot be generated by extraction of a melt from a modern morb composition rather it is argued that such high sm nd ratios are produced as a result of partitioning during the recrystallization of a morb like component to eclogite during subduction nd model ages suggest that this process occurred 2 3 2 4 ga

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

the oxygen isotope composition of mantle eclogites as a proxy of their origin and evolution a review abstract in the past decade the oxygen isotope composition in rock forming minerals of mantle eclogites 18o 2 12 has been actively discussed as a most reliable proxy of their origin and evolution in the present study the possibility of using the oxygen isotope composition as a proxy for the origin of mantle eclogites is carefully tested and conclusions regarding its application limits are drawn contributions of all processes leading to variations in the isotope composition from protolith oceanic crust formation to ascent of eclogites in kimberlite to the surface are discussed including hydrothermal alteration by sea water fractionation of oxygen isotopes upon metamorphism dehydration and the removal of volatile components in subduction zone metasomatism in subduction zone partial melting mantle metasomatism and variation in the oxygen isotope composition upon the exhumation of mantle xenoliths by kimberlitic magma the total contribution of all processes excluding mantle metasomatism typically does not exceed a deviation of 3 5 from the initial 18o value established in a protolith variations of 18o in mantle eclogites 2 12 and 8 which subducts into the mantle up to 5 and the preferential erosion and partial melting of the uppermost layers of the oceanic crust the statistical data show that eclogite garnet mainly preserves more ancient oxygen isotope signatures including the initial 18o of protolith than clinopyroxene garnets with 18o of 1 5 a total of 451 samples oxygen isotope composition in a considerable portion of mantle eclogites that originated from the oceanic gabbro based on their chemical composition 18o 6

3 d x ray tomography of diamondiferous mantle eclogite xenoliths siberia a review abstract diamonds form over billions of years hundreds of kilometers beneath the earth s surface and in combination with inclusions trapped within provide important constraints on the evolution of the mantle over geological time diamonds are generally studied as individual crystals sourced from highly explosive kimberlite pipes which entrain and subsequently disaggregate mantle fragments xenoliths en route to the surface this has resulted in a general absence of robust textural descriptions of diamonds relative to their hosting mantle protolith the textural associations of diamonds within their mantle host rocks are reviewed here on the basis of a compilation of x ray tomographic data for 17 diamondiferous eclogite xenoliths from siberian kimberlites this review represents a comprehensive description of diamonds relative to their host silicates the lack of such descriptions in previous studies is largely due to the rarity of these xenoliths the difficulty in preparing petrographic thin sections containing diamonds and their high monetary value high resolution computed x ray tomography hrcxt produces up to 1200 sequential 2 d slices through individual xenoliths each of which represents a pseudo thin section with a resolution on the order of 5 20 m the improved resolution of x ray imaging in recent studies allows for the identification of not only primary minerals but metasomatic minerals assemblages including spongy textured clinopyroxene phlogopite k richterite and hercynitic spinel allowing for the delineation of distinct metasomatic pathways through the xenoliths and their relationship to diamonds diamonds are observed in three distinct textural settings potentially representing several temporally distinct diamond growth events these setting includes 1 diamonds completely enclosed in garnet 2 diamonds associated with highly embayed silicate grain boundaries and 3 diamonds contained within distinct metasomatic plumbing systems diamonds observed completely enclosed in garnets suggest an early diamond forming event prior to major re crystallization and eclogite formation during subduction the occurrence of diamond in association with embayed garnets suggests that diamond grew at the expense of the hosting silicate protolith in addition the spatial relationships of diamonds with metasomatic pathways which are generally interpreted to result from late stage proto kimberlitic fluid percolation indicate a period of diamond growth occurring close to but prior to the time of kimberlite emplacement furthermore the paragenesis of sulfides within eclogite xenoliths are described using 3 d models for entire xenoliths volumes providing important constraints of the timing of sulfide mobilization within the mantle three d animations created using x ray tomography data for ten of the xenoliths can be viewed at the following link http eps utk edu faculty taylor tomography php

latter day origin of diamonds of eclogitic paragenesis syngenetic mineral inclusions in diamonds provide the best means of determining their origin among such inclusions the peridotitic paragenesis of olivine orthopyroxene and chrome pyrope garnet is on average far more abundant than the eclogitic paragenesis of pyrope almandine garnet and omphacitic clinopyroxene1 3 diamonds of peridotitic paragenesis from the 100 myr old kimberley and finsch kimberlites in southern africa were previously shown to have originated in 150 200 km thick lithosphere beneath the kaapvaal craton 3 300 myr ago4 5 at a few localities such as the premier kimberlite in southern africa and the argyle lamproite in northwestern australia diamonds of eclogitic paragenesis predominate6 8 allowing recovery of sufficient material for complementary analysis the results of which are reported here eclogitic garnet and clinopyroxene inclusions in premier and argyle diamonds yield sm nd isochron ages of 1 150 and 1 580 myr respectively compared with host diatreme emplacement ages9 10 of 1 100 1 200 myr eclogitic inclusion ages and precursor isotopic signatures indicate a second genetically distinct origin of diamonds apparently related in time and space to kimberlite or lamproite magmatism

strontium alteration in the troodos ophiolite implications for fluid fluxes and geochemical transport in mid ocean ridge hydrothermal systems new and published strontium isotope analyses from the troodos ophiolite constrain fluid solid exchange processes and the magnitude and circulation paths of the hydrothermal fluids the87sr 86sr profile reflects alteration in the recharge zone of an evolving hydrothermal system fluid rock strontium isotope exchange in the upper 1 5 km of extrusive lavas was kinetically limited and seawater derived fluids emitted from the base of this zone were buffered to87sr 86sr ratios between 0 7047 and 0 7059 in contrast over the next 1 km depth interval of sheeted dykes and the uppermost plutonics 87sr 86sr values cluster about0 7054 7 2 and fluid flow is inferred to have been pervasive with near equilibrium fluid rock exchange quartz chlorite and epidosite zones the probable pathways of the concentrated high temperature upwelling fluids have identical87sr 86sr ratios to adjacent diabase dykes on troodos a time integrated fluid flux in excess of2 9 107 kg m 2 is required to transport the strontium isotope composition of 0 7054 set in the kinetically controlled exchange zone through the 1 km of sheeted dykes and into the zones of concentrated upwelling the uniformity of the87sr 86sr ratios in the diabase sheeted dykes and high temperature epidosite and quartz chlorite rocks indicate that the strontium isotopic alteration took place during the high temperature phase of hydrothermal circulation the inferred minimum time integrated fluid flux of2 9 107 kg m 2 substantially exceeds that of 5 106 kg m 2 inferred from thermal models of high temperature circulation but is comparable with estimates of the hydrothermal flux from oceanic budgets of3he mg and87sr the high flux estimate for troodos is consistent with the ophiolite venting fluids with87sr 86sr elevated significantly above rock values which contrasts with the near morb87sr 86sr ratios of fluids from active high temperature vents at mid ocean ridges and the87sr 86sr profile in dsdp hole 504b this requires that either the modern systems are immature and there is a protracted phase of lower temperature circulation or that circulation in the troodos ophiolite differed from that in normal mid ocean ridges and oceanic strontium budgets are balanced by another source the lack of very elevated87sr 86sr values in the vent fluids precludes significant channelling in the recharge zones of the active systems

early proterozoic oceanic crust and the evolution of subcontinental mantle eclogites and related rocks from southern africa seven eclogitic nodules from kimberlites in southern africa have been studied in detail for whole rock major and trace element geochemistry petrography and mineral chemistry by electron microprobe high purity mineral separates from six of these samples have been analyzed for trace elements and for the isotopic composition of sr nd and oxygen three eclogite groups are recognized group a eclogites have very high mg fe low na in pyroxene moderate 18o 4 7 to 5 3 and low 87sr 86sr and 143 144nd olivine and enstatite may also be present as accessory phases in this group group b eclogites have moderate to high mg fe ratios high na in pyroxene low 18o 3 0 to 3 4 high 87sr 86sr and 143nd 144nd ratios and extremely lree depleted hree enriched garnets with no eu anomalies group c eclogites have low mg fe ratios high na in pyroxene and variable sr nd and o isotope compositions accessory feldspar is present in one sample that may be of crustal origin mineral separates of garnet and pyroxene have positive eu anomalies the group a eclogites are too refractory to represent magma compositions and must have formed as cumulate dike rocks in the upper mantle that contain a minor trapped liquid component this is supported by the presence of accessory olivine and enstatite and by their ksm and knd which are similar to empirical pyroxene garnet partition coefficients the group b eclogites are extremely depleted in incompatible elements and have ndvalues 10x to 20x morb the high 87sr 86sr of these eclogites is not consistent with their strongly depleted lree and high nd and cannot be primary it must have been imposed on the protolith the low 18o of these eclogites cannot form by mantle fractionation processes and must also be inherited from the protolith both the sr and oxygen isotope data are consistent with high temperature hydrothermal alteration of a basaltic protolith followed by partial melting to form the refractory compositions observed now the hydrothermal fluid may have been sea water but secondary enrichment of the protolith in rb is required to generate the observed high sr ratios the high na content of the pyroxenes supports a spilitic alteration event the major and trace element characteristics of the group c eclogites are consistent with recrystallization of a cumulate gabbro protolith one group c rock is probably a garnet granulite derived from the lower crust the other may represent the plutonic portion of oceanic crust a reconstructed whole rock isochron for the three bellsbank eclogites yields an age of 2 1 0 1 b y this implies that plate tectonic processes involving the generation and subduction of oceanic crust have been active since the early proterozoic the early proterozoic ocean crust consisted of two components a plutonic section of gabbro cumulates group c eclogites and a volcanic section of basalt group b eclogites the volcanic component has undergone substantial hydro thermal alteration and subsequent partial melting to form a refractory residue the plutonic component is less modified

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

diamond growth history from in situ measurement of pb and s isotopic compositions of sulfide inclusions in a continuing effort to understand crust mantle dynamics we have determined the s and pb isotopic compositions of mantle sulfides encapsulated within diamonds from under the siberian craton and compared these results to those of previously investigated african counterparts because diamond inclusions are isolated from exchange with surrounding mantle they may preserve the history of diamond growth and act as direct tracers of the origins of mantle materials study of these inclusions may thus offer the best chance of recognizing global scale interaction between earth9s crust and mantle although 34 s values of the siberian sulfides do not deviate significantly from the mantle value of 0 3 pb isotopic compositions are highly variable pb isotopic compositions of sulfides from peridotitic suite diamonds generally plot near the terrestrial pb growth curve with model ages ranging between 0 and 2 ga whereas sulfides from eclogitic suite diamonds have radiogenic compositions plotting beyond the growth curve these results which are similar to those for sulfides in african diamonds suggest that the sulfides from eclogitic suite diamonds were derived from a source with an unusually high u pb ratio and may indicate a common process such as subduction of crystal materials into the mantle operating beneath africa and siberia the absence of extremely radiogenic pb in sulfides from eclogite xenoliths suggests that the radiogenic material from which eclogitic suite diamonds grew was a transient feature of the mantle associated with diamond growth the ultimate origin of this high u pb signature however remains enigmatic large variations in pb isotopic composition of sulfides from different zones in a single peridotitic suite diamond document 1 crystallization of the diamond9s core near 2 0 ga 2 growth of its outer zone in an environment with a high u pb ratio similar to the growth environment of eclogitic suite diamonds and 3 growth of the diamond rim near the time of kimberlite emplacement

diamond growth from c h n o recycled fluids in the lithosphere evidence from ch 4 micro inclusions and δ 13 c δ 15 n n content in marange mixed habit diamonds abstract mixed habit octahedral cuboid diamonds from the marange alluvial deposits in the eastern zimbabwe craton have high nitrogen and hydrogen contents that provide an opportunity to evaluate diamond growth mechanisms and c n h o bearing fluids in the lithospheric keel light grey cuboid sectors with hydrogen containing defects trap abundant dispersed ch 4 inclusions raman peaks at 2917 cm 1 associated with graphite raman peaks at 1580 cm 1 clear octahedral sectors are richer in nitrogen and free of any such inclusions core to rim co variations of 13 c 15 n and n content can be explained by a mixing trend between earlier fluids that are ch 4 rich and later fluids that are more co 3 or co 2 rich marange diamonds have limited overall 13 c variation but do show fractionation during growth towards higher 13 c values this trend can be explained by diamond precipitation from mixed ch 4 and co 2 fluids where isotopic fractionation occurs as the amount of fluid wanes calculated 15 n values for diamond source fluids evolving in this manner are between 2 3 and 6 4 these n isotopic compositions require ch 4 rich and co 3 co 2 rich end member fluids to have a recycled metasedimentary component perhaps introduced with subduction of eclogite

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

potentially harmful elements and lead isotopes distribution in a heavily anthropized suburban area the casoria case study italy this study presents the results of 12 trace elements that have been classified by italian environmental law as potentially dangerous to human health and new pb isotope data for topsoils and soil profiles collected in the casoria municipal area napoli elemental concentrations were determined in 126 topsoil samples and were produced interpolated distribution midw and baseline maps using geodas software results show casoria soils to be significantly enriched in several elements e g cd cu pb zn two geochemical sources were determined associating elemental distribution with the background values of neapolitan soils one geogenic and another one anthropogenic high as co se and tl concentrations are coherent with bedrock lithology and elemental concentrations show the same values typical of neapolitan volcanic soils higher cd hg pb and zn concentrations can be linked with anthropic activities coherent with previous studies in the neapolitan area cr cu sb and v geochemical concentration and distribution shows both geogenic and anthropogenic influence pb isotope analyses allow the determination of the source of the pb and the level of anthropogenic geogenic influence on their concentration pb sources in the casoria soils overlap the isotopic compositions typical of industrial soils aerosols this anthropic influence on pb concentration is quantified by anthropogenic fraction af casoria topsoil shows very high amount of anthropogenic pb af is 41 58 in profile soil samples leached l af is 31 43 topsoils and 38 56 bottom soils in profile soil samples residues r af is shifted toward the geologic signature 18 43 topsoils and 25 50 bottom soils

nano inclusion suite and high resolution micro computed tomography of polycrystalline diamond framesite from orapa botswana abstract a single polycrystalline diamond aggregate from the orapa kimberlite botswana contains a syngenetic micro and nano inclusion suite of magnetite pyrrhotite omphacite garnet rutile and c o h fluid in order of abundance this suite of inclusions is distinctly different from those in fibrous diamonds although the presence of sub micrometer fluid inclusions provides evidence for a similarly important role of fluids in the genesis of polycrystalline diamond it is the first study of polycrystalline diamond by high resolution ct computed tomography reaching a resolution of 1 3 m using polychromatic x rays combined with focused ion beam assisted transmission electron microscopy the method reveals epigenetic replacement coatings of hematite and late stage sheet silicates around the magnetites showing that magnetites are often but not always interstitial to the diamond and thus were open to late stage overprinting it is proposed that the polycrystalline diamond formed by a redox reaction between a small scale carbonatitic melt and a sulfide bearing eclogite the water released from the melt during diamond precipitation fluxed local melting of the surrounding eclogite and oxidation of sulfide phases to magnetite which mingled with the carbonatitic melt and re precipitated locally

a gap in the north american central plains conductivity anomaly abstract the palaeotectonic models of the north american continent have been deeply influenced by the concept of a north american central plains conductivity anomaly nacpca which was postulated as a result of large area wide spaced array studies using the magnetic deep sounding mds method a recent closely spaced magnetotelluric mt survey across this proposed anomaly near the canadian u s boundary by jones and savage 1986 failed to detect the nacpca at the location suggested by the mds however this survey did verify a conductivity anomaly discovered by rankin and kao 1979 and a new anomaly 75 km east of nacpca our present study verifies this latter anomaly and quantitative results are presented the striking isotropy in apparent resistivity does not prevent the interpretation of anomalous resistivity distributions this is discussed in the text the failure of the mds array studies to detect our anomaly is readily explainable by the wide station spacing however the failure of the mt survey to detect the nacpca leads us to the conclusion that it may not be a continuous feature

stable and radiogenic isotope study of eclogite xenoliths from the orapa kimberlite botswana eclogite xenoliths from orapa can be accurately classified as group i or group ii on the basis of na2o in garnet and k2o in clinopyroxene group i xenoliths are commonly diamondiferous while group ii xenoliths are diamond free both xenolith varieties may contain graphite isotopic character is to some degree correlated with major and trace element chemistry group ii samples with ca poor garnet have clinopyroxenes with radiogenic 87sr86sr 0 705 0 709 and the least radiogenic 143nd144nd 0 5122 0 5125 group i eclogites with higher ca and fe in garnets have less radiogenic 87sr86sr 0 702 0 7066 and bulk earth or higher 143nd144nd ratios group i eclogites have more radiogenic 206pb204pb 18 6 19 than group ii xenoliths 16 5 18 6 in contrast group ii xenoliths have more variable and in some cases more radiogenic 206pb204pb 36 6 39 3 than group i xenoliths 38 3 38 4 the sr sm nd and pb concentrations of minerals in orapa group i eclogite xenoliths are much lower than in group ii samples all the group ii xenoliths are inferred to be enriched in light rare earth elements while group i xenoliths are probably characterised in many cases by light rare earth element depletion constituent garnet and clinopyroxene in both group i and ii eclogite xenoliths are essentially in isotopic equilibrium at the time of pipe emplacement mineral as well as calculated whole rock 143nd144nd compositions of most of the group i eclogites are too close to bulk earth and depleted mantle estimates in order to obtain useful model age information depleted mantle model ages derived from the much lower 143nd144nd compositions of the group ii eclogite xenoliths range from 661 to 1248 my with an average clinopyroxene model age of 908 my and an average whole rock model age of 1016 my on the basis of an observed covariation of o and sr isotopic compositions the entire orapa group i eclogite xenolith suite can be modelled as mixtures of oceanic basalt with or without a few percent of ocean floor sediment the group ii xenoliths might have crystallised from a melt which derives from a protolith with time averaged lree depletion their radiogenic sr isotope character could be due to interaction of the melt with metasomatised lithosphere or might be a superimposed metasomatic signature