stable isotope composition of magmatic and deuteric carbonate phases in hypabyssal kimberlite lac de gras field northwest territories canada abstract the oxygen and carbon isotope compositions of carbonates from thirteen late cretaceous to paleogene hypabyssal intrusions from the lac de gras kimberlite field indicate that the carbonates are well preserved and have a complex paragenetic formation history kimberlites at ten localities studied are calcite bearing based on the presence of phenocrysts microphenocrysts and or millimeter scale calcite only segregations of sr rich calcite the calcite bearing kimberlites also contain large cm scale sr poor calcite serpentine segregations that are texturally distinct from the smaller sr rich calcite only segregations kimberlites at three other localities studied are dolomite bearing based on the presence of calcite dolomite segregations some of which preserve complex oscillatory and banded textures of calcite dolomite and magnesian calcite 18 o values for whole rock calcite in calcite bearing kimberlite and for micro samples of calcite from individual segregations in calcite and dolomite bearing kimberlite generally vary from 6 to 9 consistent with their formation at magmatic temperatures e g 750 c from a kimberlite melt in contrast higher 18 o values of 9 to 14 characterize micro samples of calcite from calcite serpentine segregations in calcite bearing kimberlite as well as whole rock dolomite and micro samples of dolomite from segregations in dolomite bearing kimberlite these data are consistent with formation from deuteric fluids late stage magmatic at sub solidus temperatures e g 500 to 100 c these higher 18 o values for whole rock calcite also correlate with increased intensity of forsterite alteration increased abundance of calcite serpentine segregations textural overprint of primary groundmass minerals by anhedral calcite and serpentine and development of atoll spinels all interpreted as a result of deuteric fluids in contrast whole rock calcite in dolomite bearing kimberlite has notably lower 18 o values of 1 5 to 5 5 interpreted to have formed from or been overprinted by locally derived high co 2 h 2 o deuteric fluids at 500 to 100 c 13 c values of the various types of calcite and dolomite 8 to 3 have no systematic variation with 18 o values and therefore there is no isotopic record of volatile degassing from these hypabyssal bodies

parental carbonatitic melt of the koala kimberlite canada constraints from melt inclusions in olivine and cr spinel and groundmass carbonate abstract this study of the hypabyssal kimberlite with particularly fresh olivine from the palaeogene koala pipe ekati cluster slave craton canada is focused on reconstruction of the kimberlite parental melt the koala kimberlite sample is a typical ultramafic rock archetypal or group i kimberlite containing large abundance of forsteritic olivine set in a carbonate rich matrix the euhedral olivine ii shows significant zoning in terms of forsterite content and trace elements only the rims of olivine ii are considered to be magmatic they are uniform in terms of fo 91 5 0 2 mol and host primary crystal apatite cr spinel and melt inclusions the magmatic spinel phenocrysts are strongly zoned from titanian magnesian chromite to pleonaste with the cores having the same composition as spinel inclusions in olivine the primary and pseudo secondary melt inclusions in both magmatic cr spinel and olivine comprise a complex assemblage of daughter phases dominated by carbonate minerals dolomite eitelite na 2 mg co 3 2 shortite na 2 ca 2 co 3 3 nyerereite na 2 ca co 3 2 zemkorite na k 2 ca co 3 2 and fairchildite k 2 ca co 3 2 in addition to the dominant carbonates the melt inclusions contain phosphates chlorides and sulphates bradleyite na 3 mg po 4 co 3 halite sylvite aphthitalite na 0 25 k 0 75 2 so 4 but the silicate component is represented by tetraferriphlogopite only the melt inclusions in olivine studied in heating experiments show immiscibility of two carbonate liquids on heating and cooling and homogenise at low temperature 2 o sro and bao in wt 0 37 0 65 1 17 2 40 2 37 4 18 respectively such compositions are compared to secondary calcite replacing nyerereite and shortite in the natrocarbonatite lavas of the modern oldoinyo lengai volcano the results on mineralogy composition and homogenisation temperature of melt inclusions and the groundmass of the koala kimberlite imply that the parental kimberlite liquid was a si poor ca mg carbonate enriched in alkali elements lithophile trace elements phosphorus chlorine and sulphur

discussion of kimberlites and aillikites as probes of the continental lithospheric mantle by d francis and m patterson lithos v 109 p 72 80 article the origin of kimberlites and volatile rich alkaline ultramafic magmas such as aillikites has been a matter of considerable scientific interest and the recent paper by francis and patterson francis and patterson 2009 kimberlites and aillikites as probes of the continental lithospheric mantle lithos 109 72 80 attempts to summarize advances in this research field they introduce new major element geochemical discrimination diagrams and present a petrogenetic model that allegedly has implications for diamond exploration contrary to the statements of francis and patterson we argue that kimberlite bulk chemical compositions are not determined by contamination with lithospheric mantle material alone and cannot be used to predict reliably the diamond grade of new discoveries we dispute their petrogenetic model specifically the proposal that calls for carbonate rich aillikite melts being parental to kimberlite magmas rather we promote the idea of carbonated ultramafic lamprophyre magmas such as aillikites being parental to many carbonatite intrusions in areas of rifted cratonic lithosphere from which archetypal kimberlites appear to be absent furthermore we believe that the geochemical discrimination diagrams si versus k and fe respectively introduced by francis and patterson do not effectively distinguish between kimberlites and aillikites because there is significant data overlap and bias of particular concern is the omission of relevant compositional data for the ultramafic lamprophyre group in this discussion we re iterate that bulk chemical analyses of kimberlites and alkaline ultramafic rocks need to be accompanied by petrographic descriptions and mineralogical data only by the inclusion of such data can the petrogenesis of the rocks considered be discussed

carbonate xenoliths hosted by the mesoproterozoic siddanpalli kimberlite cluster eastern dharwar craton implications for the geodynamic evolution of southern india and its diamond and uranium metallogenesis a number of limestone and metasomatised carbonate xenoliths occur in the 1 090 ma siddanpalli kimberlite cluster raichur kimberlite field eastern dharwar craton southern india these xenoliths are inferred to have been derived from the carbonate horizons of the kurnool palnad and bhima proterozoic basins and provide evidence for a connection between these basins in the geological past a revised mesoproterozoic age is proposed for the bhima and kurnool palnad basins based on this kimberlite association and is in agreement with similar proposals made recently for the chattisgarh and upper vindhyan sediments in central india the observed bhima kurnool interbasinal uplift may have been caused by 1 extension or plume related mafic alkaline magmatism that included the emplacement of the southern indian kimberlites at 1 1 ga 2 mantle plume related doming of the peninsular india during the cretaceous or 3 quaternary differential uplift in this region it is not possible with the currently available geological information to constrain the exact timing of this uplift the deep erosion of primary diamond sources in the raichur kimberlite field in the upper reaches of the krishna river caused by this uplift could be the elusive source of the alluvial diamonds of the krishna valley mesoproterozoic sedimentary basins can host world class unconformity type uranium deposits in light of its inferred mesoproterozoic age a more detailed stratigraphic and metallogenic analysis of the kurnool basin is suggested for uranium exploration

mineralogy of primary carbonate bearing hypabyssal kimberlite lac de gras slave province northwest territories canada abstract a suite of fresh late cretaceous to eocene hypabyssal kimberlites from the lac de gras field were studied in order to understand better carbonate silicate and oxide paragenesis the samples have excellent preservation of textures and primary mineralogy and are archetypal or group 1 kimberlite five kimberlite localities are identified as calcite bearing based on the presence of high sr ba calcite as phenocrysts microphenocrysts and in segregations three kimberlite localities are identified as dolomite bearing based on the presence of mixed calcite dolomite segregations containing oscillatory and banded textures of calcite dolomite solid solution and dolomite magnesite sr ba calcite are characterized by high xca 0 95 and are enriched in sr 4900 11 100 ppm and ba 3200 14 200 ppm the calcite dolomite and dolomite magnesite solid solution compositions span the xca range from 0 42 to 0 95 and typically have sr and ba contents in the range of 1000 4000 ppm the carbonate silicate and oxide mineral compositions suggest that the origin of the calcite bearing versus dolomite bearing kimberlites studied is related to subtle differences in parent magma composition in particular the co2 h2o ratio formation of the carbonates reflects the latter part of a protracted magmatic crystallization sequence in which sr ba calcite precipitates from an evolved kimberlite melt subsequently calcite dolomite solid solution and dolomite is precipitated from localized mg rich carbonate fluids at relatively high temperatures higher than serpentine stability

oxygen and hydrogen isotopic composition of phlogopites and amphiboles in diamond bearing kimberlite hosted marid xenoliths constraints on fluid rock interaction and recycled crustal material in the deep continental lithospheric mantle abstract marid mica amphibole rutile ilmenite diopside xenoliths are transported from the deep cratonic lithosphere to the earth s surface by cretaceous kimberlites emplaced in the kimberley area of the kaapvaal craton marid xenoliths have high modal abundances 70 80 vol of mica and amphibole indicating their origin from a hydrous source the 18o values 4 7 18o 6 9 of phlogopite micas from 14 marid samples indicate that these minerals are both 18o enriched and 18o depleted with respect to the average upper mantle 18o value of 5 8 0 6 the range of 2h values of phlogopites 83 2h 53 n 14 of marid xenoliths are slightly larger than the average mantle 2h value 70 10 the oxygen 18ophlogopites amphibole 0 4 and 0 4 and hydrogen 2hphlogopite amphibole 14 and 36 isotopic disequilibrium recorded from two marid xenoliths suggests the duration of the last isotopic exchange possibly just before the kimberlite emplacement between these minerals and metasomatic fluids was too short to reach isotopic equilibrium our model calculation indicates that the phlogopites of marid xenoliths underwent isotopic exchange with fluids of 18o 5 5 to 10 2h 62 to 90 the range of 18o value of the calculated metasomatic fluids resembles the oxygen isotopic composition of the primary mantle carbonate 6 9 suggesting interaction between carbonatic melt and marid xenoliths in the continental lithospheric mantle furthermore 18o values of phlogopites together with previously published nitrogen isotope data 11 15n 9 banerjee et al 2015 indicates incorporation of inhomogeneously distributed recycled crustal material from subducted crust within their source magma therefore o h n isotope data for marid xenoliths indicates their crystallization from geochemically heterogeneous magma in the upper continental mantle and subsequent metasomatism with mantle fluids

some aspects of the organic geochemistry of petroleum fluids the most important influences on the compositions of petroleum fluids are in many cases the composition of a petroleum sample may allow the depositional environment of its source rock and its temperature history before and after expulsion to be determined this information assists the search for more petroleum in a region we suggest that this approach should be used more frequently to validate models of petroleum generation and migration in the second part of the paper we discuss the origin and nature of the differences in petroleum composition which are frequently found within individual petroleum reservoirs petroleum accumulations in permeable reservoirs are normally found to be inmechanical equilibrium and verticalchemical equilibrium this is manifested by constant subsurface fluid density at any given depth across the accumulation and increases in subsurface density with depth known as gravitational segregation however even in permeable reservoirs lateral differences in composition inherited from the reservoir filling process are not totally eliminated by mixing processes such as diffusion and convection thus lateral chemical equilibrium is often not achieved and lateral compositional variations are observed in present day oil and gas reservoirs these observations are justified in terms of order of magnitude estimates for the rates at which reservoirwide mixing occurs a case history is presented

ultrafresh salty kimberlite of the udachnaya east pipe yakutia russia a petrological oddity or fortuitous discovery an ultrabasic ultramafic composition of kimberlite magmas is difficult to reconcile with existing models of the kimberlite mantle source and melting conditions inferred magma temperatures and rheological properties and the style of magma ascent and emplacement the inconsistencies in current thinking indicate serious flaws in understanding kimberlite magma compositions much of the uncertainty over true kimberlite compositions may stem from almost ubiquitous hydration and leaching of kimberlite rocks this study presents petrographic and geochemical data for kimberlite samples largely unaffected by postmagmatic modification from the devonian udachnaya east pipe in siberia these samples are unusually enriched in chlorine and sodium yet they are essentially anhydrous these features are consistent with the phase composition of the groundmass which is dominated by minerals such as na ca carbonates na k chlorides and sulphates which appear to be in our samples co magmatic with common silicates and oxides but are unknown in other kimberlites or rarely found within magmatic assemblages we suggest that a kimberlite parent melt of essentially non silicate composition with high concentrations of alkalis co2 and cl may be a viable alternative to the currently favoured water rich high mg model primary melt entrainment of mantle silicates into such a melt en route to the surface followed by gravitational accumulation of mantle olivine and liquidus oxides perovskite cr spinel at the bottom of vertically extensive magma bodies after emplacement would explain the observed properties of kimberlite magma rock notably enrichment in olivine and trace elements in the hypabyssal kimberlite facies a carbonate melt composition would retain attributes of the standard model such as trace element enrichment via low degrees of partial melting it would explain low temperatures of crystallisation and the exceptional rheological properties that enable kimberlite primary melts to segregate from the lithospheric source and buoyantly ascend at high speed while mixing and reacting with country rocks

alteration styles in the orion central volcanic complex fort a la corne kimberlite field saskatchewan and their effects on primary volcaniclastic textures implications for facies mapping and diamond exploration diamond grade distributions within individual kimberlite bodies are controlled by physical emplacement processes which are generally determined from volcaniclastic facies characteristics however the effects and distribution of secondary alteration styles which usually mask primary kimberlite facies have been given little attention at the fort a la corne kimberlite field saskatchewan complex overprinting alteration textures and mineral phases are recognized this study documents the characteristics overprinting relationships and spatial distribution of alteration styles within a crater filling volcaniclastic deposit body 219 of the orion central volcanic complex data were acquired from macroscopic drill core observations microscopic petrography sem imaging eds element mapping and xrd analyses olivine crystals display a mesh fracture pattern and have been replaced by multiple generations of serpentine and magnetite late stage serpentine has migrated into the interstitial medium bulk rock alteration textures include 1 fine grained crystal rich domains which are alteration pseudoclasts and or relict juvenile pyroclasts 2 a heterogeneous interstitial medium varying from early stage disseminated calcite rich serpentine to amorphous serpentine and crystalline calcite and 3 overprinting serpentine carbonate and magnetite veins and serpentine nodules alteration styles defined by unique alteration characteristics are spatially zoned through the body 219 volcaniclastic deposit and this is a result of multiple reaction fronts associated with the influx of fluids of varying composition over time and permeating different parts of the deposit recognition of alteration processes their paragenesis and spatial controls can assist in defining and constraining the distribution of primary kimberlite facies and by consequence the distribution of diamond grades

the volatile content of hypabyssal kimberlite magmas some constraints from experiments on natural rock compositions kimberlites are volatile rich magmas that ascend from deep in the mantle at high velocities then as they reach a root zone at 1 3 km in depth they either discharge explosively through to the surface or stall to form dykes and sills understanding this eruptive behaviour is difficult due to a lack of data on volatile solubility particularly at conditions where the magmas enter the root zone 30 80 mpa in this study we perform experiments on some putative primary kimberlite magma compositions to assess the amount of co2 and h2o retained if these compositions represent magma as it enters the root zone at the conditions investigated 100 200 mpa and 1 275 1 100 c the results suggest that none of these particular kimberlite compositions reproduce a magma that can retain the observed high volatile content when intruded at these pressures 4 8 km in our experiments the low volatile retention is due to a combination of factors including a high proportion of solid phases none of which are volatile bearing and inadequate volatile solubilities in the subordinate amounts of melt present modelled solubilities also suggest that the dissolved volatile contents remain too low even at super liquidus temperatures i e 100 melt for water the higher values observed in natural rocks can be explained by the addition of h2o associated with ubiquitous post emplacement serpentinization the high co2 contents in hypabyssal rocks are unlikely to be related to alteration we suggest that most kimberlites originally had lower sio2 contents and as such may have been transitional between silicate and carbonate melts this results in both higher co2 solubilities and lower liquidus temperatures for such compositions it is possible that both co2 and water solubility may first decrease and then increase as magmas decompress and crystallize such unusual behaviour can help explain why kimberlite magmas can be very explosive or form shallow hypabyssal intrusions

arrested diatreme development standing rocks east hopi buttes navajo nation usa abstract maar diatreme volcanoes defined by their relatively large pyroclastic debris filled subsurface structures and craters that cut into the pre eruptive land surface are typically found in small volume mafic to ultramafic monogenetic volcanic fields diatremes are associated with strong explosions throughout most of their development focused along feeder dikes and generally attributed to magma water interaction or high magmatic volatiles detailed mapping of the magnificently exposed standing rocks east sre diatreme shows evidence of additional eruptive complexity and offers new insights into how the plumbing and vent structures of small volume volcanoes evolve during an eruption sre is part of a larger basanitic volcanic complex that includes several diatremes formed along a series of irregular offset nw se trending dikes exposed 300 m below the pre eruptive land surface its similarly oriented elliptical shaped diatreme structure comprises predominantly country rock lithic rich breccia of coarse inhomogeneously mixed wall rock blocks sourced from above and below the current surface plus sparse juvenile material domains of pyroclastic deposits crosscut the country rock breccia deposits and the best exposed is the nw massif rising 35 m above the current erosional surface it represents a cross section of an evolving crater floor and comprises matrix rich lapilli tuff and spatter deposits cut by irregularly distributed dikes some with very complex textures the most significant deposit in terms of volume is an unbedded lapilli tuff that is poorly sorted and has a well mixed population of wall rock and juvenile clast varieties thus resembling deposits typical of diatremes it is overlain by and locally intercalated with spatter deposits and this irregular contact demarcates the base of what was during eruption an uneven evolving crater floor the generally massive variably welded spatter deposits constitute mostly lapilli sized juvenile clasts with fluidal folded over shapes and ropy surfaces subordinate thermally altered wall rock and variegated domains of lapilli tuff sre shows a progressive transition from fissure to diatreme and overall evolution from more explosive to weakly explosive eruption styles recorded at the conduit crater transition diatreme development was initiated by deep quarrying explosive eruptions along a fissure to form the country rock rich breccia only parts of the fissure remained active as magma feeding the highly explosive eruptions along the fissure localized into discrete point sources forming the matrix rich lapilli tuff deposits these superimposed deposits record the passage of multiple debris jets and subvertical fallback from shallow cratering arising from explosions triggered by magma water interaction at numerous discrete sites however instead of continuing to build a well formed diatreme the system switched to weak spattering with intermittent explosive activity and near surface dike emplacement into the unconsolidated anisotropic pyroclastic debris of the crater floor dominant spatter from strombolian style bursts accumulated on the topographically varied evolving unstable syn eruptive crater floor and led to local failure and remobilization this study demonstrates how the combination of fissure behavior and sensitivity of the shallow plumbing system to local conditions during an eruption can lead to a decrease in eruptive footprint within the diatreme structure and an overall decrease in explosivity resulting in the arrested development of an immature diatreme

the origin of high hydrogen content in kimberlitic olivine evidence from hydroxyl zonation in olivine from kimberlites and mantle xenoliths abstract olivine macrocrysts in kimberlites are the most h rich natural olivines known their xenocrystic provenance makes unclear whether h enrichment occurred in the mantle prior to entrainment in kimberlite magma or during ascent we present a fourier transform infrared ftir spectroscopy based study of h zonation in kimberlite derived olivine macrocrysts and or olivine in xenoliths from the kimberlites jericho nunavut canada beartooth northwest territories canada and pipe 200 and matsoku lesotho the objective of the study was to determine whether changes in the concentration or speciation of h defects in proximity to the host kimberlite could yield insights into the origin of h enrichment within the group 1 region of the ftir spectrum 3420 3700 cm 1 we find that olivines within xenoliths and macrocrysts show weak zonation or significant h depletions in rims peaks in the group 2 region 3260 3420 cm 1 in beartooth olivines show marked decrease in the crystal margins in comparison to group 1 peaks we find no preserved evidence for kimberlite related h enrichment in our dataset we ascribe h depletion to diffusive h loss to infiltrating kimberlitic media diffusion models using coefficients previously applied to dehydrogenation of olivine and assuming a dry environment produce extremely short ascent durations incompatible with the results of other geospeedometers this mismatch suggests that the poorly developed zonation in jericho and beartooth olivines indicates water enrichment and or faster ascent of these melts in comparison to pipe 200 and matsoku resulting in greater retention of mantle derived h a further significant factor in the mismatch is the differing speciation of h in most natural mantle derived olivines in comparison to those used in diffusion experiments we argue that the presence of presumably mantle derived group 2 h in olivine macrocrysts indicates rapid ascent of magma exsolving water rich fluid permitting preservation of these faster diffusing defects only in particularly rapidly ascending kimberlite magmas this interpretation explains empirical correlations between the presence of group 2 bands water derived diamond resorption features and volcaniclastic kimberlite facies

carbon elemental and isotopic composition in mantle xenoliths from spain insights on sources and petrogenetic processes abstract the carbon elemental concentration c wt and isotopic 13c composition of mantle xenoliths from the tallante and calatrava volcanic occurrences in south east and central spain respectively have been investigated to identify carbon sources and processes occurring in distinct geodynamic settings of the iberian peninsula the peridotitic mantle xenoliths from calatrava show elemental c ranging from 0 11 to 2 87 wt which is coupled with a continuous isotopic variation from very negative values 13c 26 1 to typical mantle values 13c 5 9 on the other hand the tallante mantle xenolith suite displays lower c contents 0 06 0 15 wt showing a tighter variation with 13c depleted values ranging between 20 1 and 23 7 higher elemental c up to 0 41 wt displaying distinctly less negative isotopic values 13c between 13 8 and 11 9 have been recorded in veins crosscutting tallante peridotites plausibly representing the product of metasomatic reactions the data from the two investigated xenolith suites invariably display a good correlation between elemental and isotopic composition suggesting a mantle origin for carbon and rayleigh type fractionation as the process responsible for the observed c 13c variation however the correlation between the carbon isotopic data with other isotopic tracers e g 87sr 86sr 3he 4he used to identify distinct mantle components and metasomatic reactions indicates systematic differences between the two xenolith suites suggesting that beneath the betic cordillera where tallante is located the deep c cycle involves recycling via subduction preceding accompanying continental collision of crustal components back in the mantle coherently geochemical trends observed in the tallante xenoliths seem to be influenced by metasomatic agents generated by melting of crustal lithologies that according to the analysis of a metasedimentary xenolith can contain c up to 1 2 wt having 13c of ca 18 0

characteristics and alteration origins of matrix minerals in volcaniclastic kimberlite of the muskox pipe nunavut canada abstract the matrix of volcaniclastic kimberlite vk from the muskox pipe northern slave province nunavut canada is interpreted to represent an overprint of an original clastic matrix muskox vk is subdivided into three different matrix mineral assemblages that reflect differences in the proportions of original primary matrix constituents temperature of formation and nature of the altering fluids using whole rock x ray fluorescence xrf whole rock x ray diffraction xrd microprobe analyses back scatter electron bse imaging petrography and core logging we find that most matrix minerals serpentine phlogopite chlorite saponite monticellite fe ti oxides and calcite lack either primary igneous or primary clastic textures the mineralogy and textures are most consistent with formation through alteration overprinting of an original clastic matrix that form by retrograde reactions as the deposit cools or in the case of calcite by precipitation from ca bearing fluids into a secondary porosity the first mineral assemblage consists largely of serpentine phlogopite calcite fe ti oxides and monticellite and occurs in vk with relatively fresh framework clasts alteration reactions driven by deuteric fluids derived from the juvenile constituents promote the crystallisation of minerals that indicate relatively high temperatures of formation 400 c lower temperature minerals are not present because permeability was occluded before the deposit cooled to low temperatures thus shielding the facies from further interaction with fluids the other two matrix mineral assemblages consist largely of serpentine phlogopite calcite diopside and chlorite they form in vk that contains more country rock which may have caused the deposit to be cooler upon emplacement most framework components are completely altered suggesting that larger volumes of fluids drove the alteration reactions these fluids were likely of meteoric provenance and became heated by the volcaniclastic debris when they percolated into the vk infill most alteration reactions ceased at temperatures 200 c as indicated by the absence or paucity of lower temperature phases in most samples such as saponite recognition that muskox vk contains an original clastic matrix is a necessary first step for evaluating the textural configuration which is important for reconstructing the physical processes responsible for the formation of the deposit

the geology of kimberlite pipes of the ekati property northwest territories canada abstract this paper reviews key characteristics of kimberlites on the ekati property nwt canada to date 150 kimberlites have been discovered on the property five of which are mined for diamonds the kimberlites intrude archean basement of the central slave craton numerous proterozoic diabase dykes intrude the area the precambrian rocks are overlain by quaternary glacial sediments no phanerozoic rocks are present however mudstone xenoliths and disaggregated sediment within the kimberlites indicate that late cretaceous and tertiary cover likely

oxygen and carbon isotope and cation geochemistry of metasomatic carbonates and fluids bergell aureole northern italy abstract oxygen and carbon isotope geochemistry and ca mg geothermometry are used to investigate temperatures fluid compositions and mechanisms of vein formation in dolomitic marble roof pendants of the bergell intrusive complex northern italy ca mg and oxygen isotope geothermometry suggests that the mineralogically zoned clinohumite chondrodite calcite forsterite calcite diopside calcite and tremolite calcite veins formed over a range of temperature of approximately 520 to 380 c most veins apparently formed near 400 c from h2o rich c o h fluids xco2 0 1 0 25 abrupt 5 14 isotopic discontinuities over several millimeters across the vein front are coincident with the mineralogically indicated vein fronts suggesting that mass transport was controlled by an infiltration mechanism decarbonation did not play a significant role in determining the isotopic composition of the vein calcite the isotopic data suggest a metasomatic fluid of rather constant composition and fluid rock ratios as high as 100 such ratios are minimum estimates but not inconsistent with fluid rock ratios calculated for complete reaction of the dolomitic wall rock with a fluid which provides the required silica

salts in southern yakutian kimberlites and the problem of primary alkali kimberlite melts abstract alkali carbonates sodalite gypsum anhydrite halite and sylvite are present in the groundmass and matrix of many kimberlites in the southern part of the yakutian kimberlite province the kimberlites were emplaced through 2 km thick evaporite bearing carbonate sediments saturated with brines in the global context southern yakutian kimberlites are unprecedented in the amount of the crustal carbonate and evaporite material included in the pipes as evidenced by the bulk major element chemistry and isotopic compositions of sr c o cl and s we present geological and hydrogeological data on country rocks and kimberlites of the udachnaya mir and international naya pipes the secondary crustal origin of na k cl and s rich minerals is supported by the following 1 a regional correlation between the geology and hydrogeology of the local country rocks and the kimberlite mineralogy in particular the difference between southern and northern yakutian kimberlites 2 a restriction of halite or gypsum mineralization in the mir and international naya pipes to depths where pipes intersect country rock strata with similar mineralogy 3 the localization of the highest abundances of na k cl s bearing minerals in the udachnaya east kimberlite at a depth interval that correlates across three magmatic phases of kimberlites and coincides with the roof of the halite bearing country rock and an aquifer carrying anomalously na rich brines 4 the presence of evaporite xenoliths and veins of halite gypsum and carbonate cutting through the kimberlite and xenoliths 5 a secondary origin of halite and alkali carbonates as observed in their textural relationships to serpentine and other groundmass minerals 6 the geochemical and isotopic evidence for crustal contamination addition of crustal salts to kimberlite melt began prior to the volcanic fragmentation as a result of preferential melting and assimilation of evaporite xenoliths and may have continued in situ after the pipe emplacement via reactions with external saline fluids the hybrid alkali s and cl rich compositions of residual melts and fluids were trapped in secondary inclusions in olivine the crustal origin of salts in the udachnaya east kimberlite demonstrated here is incompatible with a model of alkali rich primary kimberlite melt it is significantly more ca na b and s rich than the deep seated mantle k and ba rich fluid inclusions in fibrous diamonds our analysis suggests that the udachnaya east kimberlite is not exceptional and uniquely fresh but rather typical of the other crust contaminated southern yakutian kimberlites

petrographic and melt inclusion constraints on the petrogenesis of a magmaclast from the venetia kimberlite cluster south africa abstract kimberlitic magmaclasts are discrete ovoid magmatic fragments that formed prior to emplacement from disrupted kimberlite magma to provide new constraints on the origin and evolution of the kimberlite melts we document the mineralogy and petrography of a magmaclast recovered from one of the ca 520 ma venetia kimberlites south africa the sample bi9883 has a sub spherical shape and consists of a 10 mm diameter central olivine macrocryst surrounded by porphyritic kimberlite the kimberlitic material consists of concentrically aligned altered olivine phenocrysts set in a crystalline groundmass of calcite chromite perovskite phlogopite apatite ilmenite titanite sulphides rutile and magnetite along with abundant alteration phases i e serpentine talc and secondary calcite these features are typical of archetypal hypabyssal kimberlites we examined primary fluid melt inclusions in chromite perovskite and apatite containing a diversity of daughter phases chromite and perovskite host polycrystalline inclusions containing abundant alkali carbonates i e enriched in k na ba sr phosphates na k chlorides sulphides and equal to lesser quantities of olivine phlogopite and pleonaste in contrast apatite hosts polycrystalline assemblages with abundant alkali carbonates and na k chlorides and lesser amounts of olivine monticellite and phlogopite numerous solid inclusions of shortite na 2 ca 2 co 3 3 na sr carbonates and apatite occur in groundmass calcite along with fluid inclusions containing daughter crystals of na carbonates and na chlorides the primary inclusions in chromite perovskite and apatite are considered to represent remnants of fluid s melt s trapped during crystallisation of the host minerals whereas the fluid inclusions in calcite are probably secondary in origin the component proportions of these primary fluid melt inclusions were estimated in an effort to constrain the composition of the evolving kimberlite melt these estimates suggest melt evolution from a silicate carbonate kimberlite melt that became increasingly enriched in carbonates phosphates alkalis and chlorides in response to the fractional crystallisation of constituent minerals i e olivine to apatite the concentric alignment of crystals around the olivine kernel and ovoid shape of the magmaclast can be ascribed to the low viscosity of the kimberlite melt and rapid rotation whilst in a liquid or partial crystalline state or to progressive layer by layer growth of the magmaclast although the mineralogy of our sample is similar to hypabyssal kimberlites worldwide it differs from hypabyssal kimberlite units in the main venetia pipes which contain monticellite phlogopite rich assemblages and segregationary matrix textures therefore magmaclast bi9883 probably originated from a batch of magma distinct from those that produced known hypabyssal units within the venetia kimberlite cluster

apatite compositions and groundmass mineralogy record divergent melt fluid evolution trajectories in coherent kimberlites caused by differing emplacement mechanisms kimberlites are pipe like igneous bodies consisting of a pyroclastic crater and diatreme commonly underlain by coherent root zone rocks and with associated dyke sill complexes the processes that control the different modes of coherent kimberlite emplacement remain uncertain in addition late evolution of kimberlite melts during emplacement into the upper crust remains poorly constrained therefore it is unclear whether there is a link between melt composition evolution and the emplacement mechanism of coherent kimberlites i e planar dykes sills vs irregular bodies in the root zone an absence of comparative studies on late stage magmatic phases across the different emplacement modes of coherent kimberlite from the same locality hamper resolution of these issues therefore we report petrographic and mineral chemical data for groundmass apatite in samples of dyke sill and root zone kimberlites from the kimberley cluster south africa early crystallised phases olivine spinel mg ilmenite in dyke sill and root zone kimberlites have indistinguishable compositions and hence crystallised from similar primitive melts conversely apatite compositions are generally distinct in dyke sill low sr high and variable si and root zone kimberlites high and variable sr low si the si enrichment of apatite in dykes sills is attributed to the coupled incorporation of co32 and sio44 for po43 reflecting higher co2 contents in their parental melts and potentially higher si contents due to the preferential crystallisation of carbonates over mica monticellite the low sr contents of apatite in dyke sill kimberlites reflect equilibrium with a kimberlite melt i e dsr is close to unity for carbonate and silicate melts whereas the higher sr contents of apatite in root zone kimberlites require crystallisation from or overprinting by a h2o co2 fluid significantly higher dsr the relative enrichment of co2 in kimberlite dykes sills is evident from the abundance of carbonates the presence of mesostasis dolomite and calcite phenocrysts in some samples and concomitant reduced proportions of other groundmass phases e g serpentine mica monticellite during late alteration of kimberlite dykes sills monticellite is typically replaced by carbonates whereas olivine and pleonaste are relatively stable indicating the melts which form dykes sills evolve to higher co2 h2o ratios it is unlikely that these two distinct evolutionary paths were caused by crustal contamination before or during near surface magma emplacement because crustal assimilation is not recorded in the o and sr isotopic composition of late crystallising olivine rinds or carbonates respectively we suggest that higher concentrations of co2 are retained in kimberlite dykes sills due to higher confining pressures i e lack of breakthrough to the surface in contrast exsolution of co2 from root zone kimberlites increased melt h2o co2 ratios and promoted the crystallisation of mica and monticellite at the expense of dolomite and calcite apatite compositions have the potential to aid in the discrimination of kimberlites from lamproites higher lree sr f and s lower si contents and carbonatites higher lree f cl and s lower fe contents however the compositions of kimberlitic apatite overlap those from aillikites probably due to similar late stage melt compositions

maar diatreme volcanoes a review maar diatreme volcanoes are produced by explosive eruptions that cut deeply into the country rock a maar is the crater cut into the ground and surrounded by an ejecta ring while the diatreme structure continues downward and encloses diatreme and root zone deposits here we attempt an evenhanded review of maar diatreme volcanology that extends from mafic to kimberlitic varieties and from historical maar eruptions to deeply eroded or mined diatreme structures we conclude that maar diatreme eruptions are episodic ejecta rings provide invaluable insight into eruption processes and sequence but are incomplete records of diatreme formation deposits within the diatreme structure include in varying proportions lower unbedded deposits sometimes typified by subvertical contacts among domains of debris emplaced sequentially and upper bedded deposits formed by sedimentation on surfaces open to the atmosphere a basal root zone comprises the transition from coherent magmatic feeder dike to clastic deposits formed by fragmentation of magma and enclosing country rock root zones are irregular in form and the clastic deposits are typically intruded by contorted dikes irregular root zone like chaotic breccias cut by contorted dikes are also present within diatreme deposits where they represent intra diatreme fragmentation zones and record changes in the location of the explosion locus during eruption

carbon and oxygen isotopic covariations in hydrothermal calcites isotopic covariations of carbon and oxygen in hydrothermal calcites are quantitatively modeled in terms of the following three mixing processes 1 mixing between two different fluids which leads to the precipitation of calcite 2 mixing between fluid and rock a calcite precipitation due to fluid rock interaction b secondary alteration of primary calcite by interaction with a subsequent fluid the models are derived from mass balance equations a distinction among the three mixing processes can be made on a 13c vs 18o diagram which places important constraints on the genesis of hydrothermal mineralization the variables which control the ultimate isotopic composition of hydrothermal calcites include the composition of the initial fluid and the wallrock temperature and dissolved carbon species owing to significant temperature dependent fractionation effects during equilibrium precipitation of calcite from a hydrothermal fluid the mixing processes may be distinguished by telltale patterns of isotopic data in 13c vs 18o space in particular caution must be exercised in postulating the fluid mixing as the cause for mineral deposition this is demonstrated for hydrothermal pb zn deposits in the western harz mountains germany a positive correlation between 13c and 18o values is observed for calcites from the bad grund deposit in the upper harz two sample profiles through calcite veins show similar correlations with the lowest values at the center of the veins and the highest values at the vein margins because the correlation array has a greater slope than for calcite precipitation at equilibrium in a closed system and because fluid mixing may not proceed perpendicular to the vein strike it is assumed that a fluid rock interaction is responsible for the observed correlation and thus for the precipitation of calcite a deep seated fluid is inferred with a 13c value of 7 and a 18o value of 10 as well as h2co3 as the dominant dissolved carbon species precipitation temperatures of the calcites are estimated to be about 280 170 c quite different isotopic distributions are observed for calcites from the st andreasberg deposit in the middle harz an alteration model is suggested based mainly on the isotopic distribution through a calcite vein in addition to a primary fluid which has the same isotopic composition as that in the bad grund deposit and thus seems to be responsible for the precipitation of calcite associated with sulfides an evolved hco 3 dominant subsurface fluid with 13c about 20 15 and 18o 0 is deduced to alter the primary calcite at low temperatures of 70 40 c