inclusions of nanocrystalline hydrous aluminium silicate phase egg in superdeep diamonds from juina mato grosso state brazil abstract inclusions in alluvial diamond from juina mato grosso brazil have been investigated by tem methods electron diffraction hrtem aem haadf eels and raman spectroscopy the inclusion paragenesis of juina diamonds is dominated by ultrahigh pressure superdeep phases one of these diamonds sample 1 1 4 contains several micrometer sized approximately 200 m by 50 70 m inclusions which have been studied tem foils prepared applying focused ion beam fib technique revealed that these inclusions consist of a porous nanocrystalline groundmass which is composed of nanometre sized crystals of a hydrous aluminium silicate phase with al si approximately 1 1 and chemical composition of phase egg alsio 3 oh a minor volume fraction of nanocrystalline stishovite and pore space which was originally filled with a fluid or gas the nanocrystalline hydrous aluminium silicate phase is idiomorphic randomly oriented approximately 20 30 nm in size predominantly with tetragonal crystal structure a 0 0 743 nm c 0 0 706 nm the monoclinic structure of synthetic phase egg determined at ambient conditions m w schmidt l w finger r j ross r e dinnebier synthesis crystal structure and phase relations of alsio 3 oh a high pressure hydrous phase american mineralogist 83 1998 881 888 is only occasionally observed the fluid filling in the porosity has been released into the vacuum of the fib during tem specimen preparation quench products of the fluid containing minor concentrations of f p s cl k ca and ba were detected at the walls of the pores in addition phase egg is identified by raman spectroscopy within a second sample rs 43a from the same location the presence of phase egg in the inclusions in diamond may suggest that crustal material has been subducted to a depth of the lower transition zone although metastable growth of nanocrystalline high pressure phases or extension of their respective stability fields to lower pressure can not ruled out completely

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

nanometric inclusions of carbonates in kokchetav diamonds from kazakhstan a new constraint for the depth of metamorphic diamond crystallization abstract previous studies have revealed that microdiamonds from the kokchetav ultra high pressure metamorphic terrane of kazakhstan contain nanometric scale inclusions of si fe ti and cr oxides because the structure of sio 2 inclusions was not confirmed to be coesite or stishovite due to their very small size such diamonds formerly served only as an indicator of a minimum pressure ca 4 gpa geothermobarometry applied to kokchetav diamond bearing rocks yielded a wide range of conditions t 700 c 1250 c and p 4 9 gpa our paper presents transmission electron microscopy studies with focused ion beam assistance that indicate that diamonds from marbles contain inclusions of aragonite caco 3 and magnesite mgco 3 and that aragonite and nitrogen bearing nanometric particles are associated with dislocations reflecting diamond growth at relatively high temperature conditions we determined the boundary of dolomite stability using the reaction camg co 3 2 dolomite caco 3 aragonite mgco 3 this allowed us to utilize available experimental data to evaluate the pressure at which diamond was crystallized taking into consideration uncertainties existing between experimental data produced in different laboratories we propose the pressure for kokchetav diamond crystallization to be 6 to 9 gpa this evaluation is based on the assumption that temperature was determined correctly as 980 c minimum and 1250 c maximum for diamond grade dolomitic marbles our data provide strong evidence that the metasedimentary rocks of the kokchetav massif containing diamonds were subducted to the depth of 190 280 km

serpentinization of oceanic peridotites 2 kinetics and processes of san carlos olivine hydrothermal alteration 1 the kinetics of the reaction mg fe olivine h 2 o serpentine magnetite brucite h 2 were investigated at 500 bars in the 250 350 c range using natural olivine san carlos fo 91 with grain sizes between 1 and 150 mm and for run durations up to 514 d the amount of magnetite produced which directly relates to reaction progress was accurately monitored using up to 24 time resolved magnetic measurements per experiment eighty percent of serpentinization was achieved after 60 d for olivine grain sizes of 5 15 mm and after 500 d for grain sizes of 50 63 mm serpentinization kinetics were found to be inversely proportional to the geometrical surface area of the starting olivine grains they were one or two orders of magnitude slower than serpentinization kinetics commonly used for modeling serpentinization related processes the nature of the serpentine mineral product depended on the initial olivine grain size igs for igs in the 5 150 mm range lizardite formed and olivine dissolution was the rate limiting process at igs below 5 mm chrysotile crystallized instead of lizardite and the relationship between olivine surface area and reaction kinetics no longer held we infer that for such small olivine grain sizes dissolution is no longer the rate limiting process serpentinization in our experiments was associated with the creation of new reactive surface area according to two cooperative processes etch pits formation associated with dissolution and grain fracturing for igs above 20 mm interestingly fractures and etch pits with similar geometry and sizes were also observed for residual olivine with a typical grain size of 50 mm in serpentinized peridotite samples from the southwest indian ridge this suggests that the processes governing olivine serpentinization kinetics in our experiments are similar to those prevailing in natural systems we therefore suggest that the new kinetic data set that we present here which encompasses a range of olivine grain sizes and reaction temperatures is relevant to the serpentinization of olivine in the oceanic crust insofar as water is available

focused ion beam fib combined with sem and tem advanced analytical tools for studies of chemical composition microstructure and crystal structure in geomaterials on a nanometre scale abstract in order to make meaningful interpretations of trace element and isotopic information from geomaterials prior characterization of the material of interest with respect to structural and chemical homogeneity radiation damage and microstructure is often required transmission electron microscopy tem is a powerful analytical tool that may be used to fully characterise a wide range of synthetic and natural materials the focused ion beam technique fib is an ideal tool for tem sample preparation that allows for the fabrication of electron transparent foils with typical dimensions of 15 10 0 150 m from any region of interest i e site specific and in virtually any material fib sample preparation consumes only a small volume of material approximately 2300 m 3 for a standard foil with the dimensions 15 10 0 150 m leaving most of the material essentially unaffected by the sampling process modern tem facilitates the measurement of chemical composition with unsurpassed spatial resolution e g nanometer scale high resolution imaging and electron diffraction provide important complimentary structural information it is the combination of crystal structure information and chemical composition that allows for the complete characterization of the volume of interest high resolution elemental mapping or line scans may be used to acquire complete energy dispersive x ray analysis edx spectra of a material pixel by pixel subsequent evaluation of the data allows for the correlation of the edx spectrum with the respective elemental map or line scan with an image thus illustrating spatially resolved chemical composition variations at the 0 5 1 0 wt level frequently minerals enclose sub micrometer or even nanometre sized inclusions of minerals fluids or silicate melt which are of great interest to geoscientists nanoinclusions e g in diamond or olivine are of particular interest because they may have preserved their original crystal structure thus providing a geobarometer recording the pressure conditions at the time of formation of the inclusion fib tem are appropriate techniques to sample and subsequently define the chemical composition and the structural state of mineral inclusion on the nanometre scale the combination of fib with an sem allows for 3d information to be obtained from samples including 3d imaging e g phase distribution and volume of the individual phases in symplectites 3d distribution of elements in a specific volume 3d elemental maps and three dimensional texture analysis using electron backscatter diffraction ebsd

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

incipient hydration of mantle lithosphere at ridges a reactive percolation experiment a reactive percolation experiment was conducted by injecting seawater into a permeable olivine aggregate at 190 c and 19 mpa to explore the relationships between hydration reactions and hydrodynamic properties during the onset of serpentinization in the ultramafic lithosphere exposed at mid ocean ridges the experiment was stopped after 23 days when the sample became impermeable the initial flow rate was 0 2 ml h and then was decreased down to 0 06 ml h after 8 days permeability decreased continuously throughout the experiment the analyses of fluid chemistry time series and of the mineralogy and structure of the reacted sample showed olivine dissolution and precipitation of proto serpentine brucite and fe oxides these reactions are controlled by coupled hydrodynamic and chemical processes interacting at different time and spatial scales their first characteristic is the production of silica rich outlet fluids in disequilibrium with the observed reacted mineral assemblages these compositions are interpreted as resulting from a suite of coupled dissolution precipitation reactions controlled at the pore scale by surface kinetic processes that rapidly reaches steady state constant fluid composition independent of fluid flux hence the effective rate of serpentinization was controlled to the first order by the transport of reactants during the experiment mass balance calculations show that the rate of olivine conversion was fast 0 2 0 5 wt day yet only 8 wt of the olivine sample reacted because the permeability drop limited fluid circulation porosity varied little during the experiment compared to permeability changes the decrease of permeability was explained by the structure of the newly formed serpentine favouring the clogging of fluid paths the rate at which permeability decreased was the fastest at low flux conditions this suggests that permeability changes were not controlled simply by the kinetics of the serpentinization reactions but mainly reflected the development of low peclet pore scale zones forming micro environments that control the serpentinization reaction paths these results outline the need to take into account the coupling of hydrodynamic and chemical processes when modelling the effects of reactive transport within the hydrating oceanic lithosphere

mineral inclusions in diamonds from the sloan kimberlites colorado mineral inclusions have been recovered from 40 small 1 5 mm diamonds from the sloan 1 and 2 kimberlite pipes in northern colorado primary inclusions i e proto or syngenetic are olivine clinopy roxene sanidine rutile and possibly native fe and some phlogopite phases interpreted as epigenetic are acmite richterite most phlogopite perovskite mn ilmenite cr spinel magnetite calcite serpentine and possibly mg ilmenite acmite richterite and perovskite have not previously been reported in diamond the epigenetic minerals commonly occur as inclusion assemblages such as phlogopite richterite phlogopite perovskite ilmenite magnetite serpentine and phlogopite clinopyroxene spinel magnetite calcite the majority of the primary inclusions have an eclogitic suite affinity which is uncommon but similar to diamonds in the premier orapa and mbuji mayi kimberlites of southern and central africa and from a few sites in russia and western australia secondary inclusions reflect

diamonds through time diamonds form in the upper mantle during episodic events and have been transported to the earth s surface from at least the archean to the phanerozoic small diamonds occur as inclusions in robust minerals in tectonically activated ultrahigh pressure metamorphosed crustal rock establishing an association with subduction processes and recycled carbon but providing no economic deposits diamonds in economic deposits are estimated to be mainly 99 derived from subcontinental lithospheric mantle and rarely approx 1 from the asthenosphere harzburgite and eclogite are of roughly equal importance as source rocks followed by lherzolite and websterite diamonds which provide evidence of extensive residence time in the mantle are with minimal exceptions smooth surfaced crystalline diamonds scd with potential commercial value the oldest prolific scd formation event documented on the world s major diamond producing cratons occurs in archean lithospheric mantle harzburgite metasomatized by likely subduction related potassic carbonatitic fluids disaggregation of the diamondiferous carbonated peridotite on decompression during volcanic transit gives rise to the association between diamonds g10 garnets and diamond inclusion type chromites well used in diamond exploration within the mantle domains of diamond stability there have been repeated episodes of further diamond crystallization and or growth these are associated with old often proterozoic subduction related melt generation metasomatic fluid migration and reaction with preexisting mantle eclogite websterite and peridotite using improved methods of isotope analysis diamond formation ages can be correlated with specific major processes such as craton accretion craton edge subduction and magmatic mantle refertilization fibrous cuboid diamond and fibrous coats on scd are rough surfaced diamonds with abundant fluid inclusions they have low mantle residence time forming rapidly from late stage metasomatic fluids in diamond stable domains that may already contain scd the symbiotic relationship between formation of fibrous diamond and magmatic sampling and transport of diamonds into the crust suggest that the associated fluids contribute diamond friendly volatile loading of the deep lithospheric mantle shortly before the triggering of a volcanic eruption continuing a process of volatile enrichment in the lithospheric mantle already identified in the archean harzburgite diamond event mantle derived scd commonly shows evidence of resorption illustrating that diamond unfriendly processes including lamproite and kimberlite generation are also active and may have a substantial negative effect in extreme cases on scd crystal size exposure of scd to a long period of changing conditions during mantle residence contributes to the difficulty of assigning specific parageneses and ages to individual inclusion free diamonds with our current state of knowledge

hydrothermal alteration of olivine in a flow through autoclave nucleation and growth of serpentine phases a single pass flow through experiment was conducted to induce serpentinization and to relate serpentine mineralogy to reaction progress during open system alteration of san carlos olivine by air saturated water the fluid flowed at 300 c and 300 bars for 1368 hours through crushed olivine contained in a tubular titanium reaction cell chemical analysis of the fluids indicated a steady state composition with respect to mg and ph after 550 hours which was interpreted to reflect saturation with brucite and one or more serpentine group minerals the alteration products were characterized by x ray diffraction xrd scanning electron microscopy sem and high resolution transmission electron microscopy hrtem nucleation and growth characteristics of serpentine phases on the fracture surfaces of olivine were studied by tem using platinum carbon replica techniques after reaction olivine fragments displayed clear signs of dissolution and growth of neoformed phases brucite and magnetite were distributed uniformly through the reaction cell although magnetite was more abundant there is however a distinct zonal distribution of the serpentine phases from the inlet to the outlet of the reaction cell lizardite is the dominant serpentine phase at the inlet whereas fibrous serpentines are predominant near the outlet lizardite crystals are more numerous but smaller in the middle of the cell and are topotactically aligned along a single preferential crystallographic orientation of the olivine cylindrical chrysotile formed predominantly on the surfaces of lizardite conical or polygonal serpentine the morphology and sorting of the serpentine phases through the reaction cell suggest that crystallization of lizardite was controlled by heterogeneous nucleation on olivine under supersaturated conditions and crystallization of chrysotile by nucleation on lizardite at higher levels of supersaturation

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 early earth geochemical and isotope record of serpentinizing environments from archean stichtite mg6cr2 oh 16 co3 4h2o abstract a record of the conditions and duration of archean serpentinization is preserved as chemical isotopic and textural properties of the magnesium chromium hydroxycarbonate mineral stichtite this provides a window into the conditions associated with a potentially habitable environment on early earth and mars stichtite is associated with archean chromite rich serpentinite rocks of south africa western australia and southern india stichtite compositions range within the cr fe al hydrotalcite group with trends unique to each discrete serpentinite host body elemental and textural analysis of stichtite associated chromite indicates that stichtite forms in fore arc layered intrusive boninite setting rocks through interaction of chromite and methane rich serpentinizing fluids the degree to which chromite is replaced by stichtite is inferred to correlate with the length of time that the host rocks spent within the stichtite window carbon stable isotope values of archean stichtite suggest carbon sourcing from marine kerogen with minor marine dissolved inorganic carbonate in most samples as opposed to the strongly kerogen dominated phanerozoic stichtite the carbon and hydrogen stable isotope profile of archean stichtite ranges from the methane fields of active serpentinizing zones to geothermal with trends suggestive of methane oxidation rather than the substrate depletion noted in phanerozoic stichtite thus isotope values of stichtite appear to record oceanic oxygenation levels the association of the stichtite 2h polytype nee barbertonite with aragonite antigorite suggests this is a higher pressure temperature polytype of stichtite reaction textures isotopic values and qualitative mineral thermobarometric indicators indicate that stichtite forms during serpentinization in a methane h2 rich environment within fluid conduits ranging from near the surface to depths where pressure is up to 2 8 gpa and temperature is up to 300 c

abundance and distribution of diamonds in eclogite revealed by volume visualization of ct x ray scans a diamond bearing eclogite has been studied using industrial high energy computed tomography and volume visualization techniques using this non destructive technique in which distinction between phases is based on density differences seven diamonds five completely enclosed in the rock have been isolated and viewed in three dimensions six diamonds appear to be single crystals of well formed to malformed octahedra one is a spinel twin macle partial to complete rimming by secondary barite provides sufficient density contrast with surrounding silicates to allow the diamonds to be easily recognized by their morphology they appear to be confined to a 1 cm wide layer diamonds range in size from 0 9 to 2 4 mm in maximum dimension and are estimated to constitute 0 19 wt of the sample equivalent to a grade of 9500 carats per metric ton several models for diamond genesis can be ruled out for this sample e g pseudomorphs of graphite by diamond precipitation as glomeroporphyritic aggregates from

influence of pyroxene and spinel on the kinetics of peridotite serpentinization hydrothermal experiments were performed at 311 degrees c and 3 0 kbar on natural olivine and peridotite to investigate the kinetics of serpentinization the results show that the rates of reaction strongly depend on grain sizes of solid reactants with smaller grain sizes resulting in faster kinetics after 27 days of reaction the reaction extent was 99 for peridotite with grain sizes of 30 mu m and the reaction extent was 28 for grain sizes of 100 177 mu m compared to peridotite olivine is serpentinized at much slower rates e g 5 3 of reaction extent was achieved for olivine with grain sizes of 100 177 mu m after 27 days approximately five times lower than that reached during peridotite serpentinization such contrasting results are due to the presence of pyroxene and spinel an interpretation which is supported by a marked increase in reaction extents for experiments with the addition of pyroxene and spinel the reaction extent achieved in experiments with 3 wt spinel greatly increased to 98 after 27 days much higher than that achieved during olivine serpentinization these results appear to be related to pyroxene and spinel releasing al and cr during serpentinization as indicated by compositions of serpentine orthopyroxene lost similar to 60 of al at a reaction extent of 59 influence of al and cr is suggested by a dramatic increase in reaction extents with the addition of al2o3 and cr2o3 powders olivine in natural geological settings is commonly associated with pyroxene and spinel consequently serpentinization kinetics may be much faster than previously thought

systematic regional variations in diamond carbon isotopic composition and inclusion chemistry beneath the orapa kimberlite cluster in botswana abstract the carbon isotopic composition nitrogen content and aggregation state of 95 inclusion bearing diamonds collected from the production of damtshaa kimberlites were investigated the damtshaa kimberlites belong to the group of southern african kimberlites showing 12c enrichment in eclogitic compared to peridotitic diamonds the results were compared with similar studies of the nearby orapa and letlhakane kimberlites providing the opportunity to examine variations in mantle composition on the kilometer scale while orapa letlhakane and damtshaa diamonds may share some overall similarities in their 13c distribution there are significant differences among them these differences support the concept that diamonds from different kimberlite pipes have distinctive features in their 13c distribution and that this is true for kimberlite occurrences which are spatially separated by only a few tens of km compared to orapa peridotitic garnet inclusions from damtshaa and letlhakane are derived in part from highly depleted dunitic harzburgitic reservoirs extending to greater depth in all three kimberlites diamonds with eclogitic garnet inclusions displaying comparatively low al2o3 cr2o3 ratios characteristic for mantle compositions show a wide range in carbon isotopic composition from about 4 to 18 vs pdb the observed relationship between 13c and al2o3 cr2o3 ratios indicates the presence of 13c depleted mantle sources that cannot be explained by subduction estimates of possible pressure temperature conditions based on the fe mg distribution coefficients of coexisting eclogitic garnets and clinopyroxenes and the ca mole fraction in garnets lead to the conclusion that beneath the orapa letlhakane and damtshaa kimberlites there is a mantle zone depleted in 13c approximately 20 30 km thick and extending over an area of at least 200 km2

high pressure fluids in the system mgo sio 2 h 2 o under upper mantle conditions fluids and melts have been trapped and analysed in high pressure experiments in the model mantle system mgo sio2 h2o at 6 to 10 5 gpa and 900 to 1 200 c the fluid melt traps consisted of a diamond layer that was added to the experimental charge and was separate from the silicate phases the recovered diamond traps were analysed by laser ablation icp ms starting materials were synthetic mixtures of brucite talc and silica with variable mg si containing 11 31 wt h2o experiments on a serpentine starting composition mg3si2o5 oh 4 result in mgo sio2 weight ratios in the subsolidus fluids close to 1 at 6 gpa and close to 2 at 9 gpa melt compositions at 6 and 9 gpa have mgo sio2 ratios close to that of forsterite at a single pressure the amount of dissolved silicate in the fluid increases steadily with increasing temperature up to 1 150 c where a sudden increase of both sio2 and mgo is observed this discrete step marks the solidus which is more clearly developed at 6 than at 9 gpa thus hydrous melts within the model mantle subsystem mg2sio4 mg2si2o6 h2o are chemically distinct from aqueous fluids up to at least 9 gpa corresponding to 300 km depth extrapolation of the current data set implies that total convergence between fluid and melt along the solidus probably occurs at 12 13 gpa 400 km i e close to the earth s mantle transition zone beneath cratons interactions of hydrous fluids with upper mantle lithologies cause relative silica depletion olivine enrichment at depths greater than 200 km and silica orthopyroxene enrichment at shallower depths

deformation microstructures reveal a complex mantle history for polycrystalline diamond 1 detailed quantitative electron backscatter diffraction ebsd analysis of polycrystalline diamond rocks diamondites provides insights into their formation and significance the fine grain size of diamondites is usually attributed to rapid crystallization however ebsd reveals significant intragrain bending distinct low angle boundaries and straight to highly irregular high angle grain boundaries highly deformed grains may be in contact with others showing little or no deformation these features are typical for crystal plastic deformation in which differential stress generates dislocations according to the dominant slip systems in diamond dislocations accumulate to form subgrain boundaries with grain size reduction by rotation of subgrains nucleation and growth of new grains and migration of grain boundaries such features are seen in other polygranular materials such as deformed metals and quartzite deformed at high temperatures during this process interaction with fluids produced interstitial garnets with compositions different from those of the primary inclusions oscillatory cl zoning in diamonds developed through diffusion along subgrain boundaries rather than being a primary growth feature diamondites are thus not simply products of primary crystallization but may be strongly deformed recrystallized and modified integrated ebsd cl and in situ chemical and isotopic analysis provide the spatial control and new insights into mantle processes

mineralogical evidence for h2 degassing during serpentinization at 300 c 300 bar abstract hydrogen is produced in large amounts during hydrothermal alteration of peridotite in low spreading rate mid ocean ridges this production is directly linked to reducing conditions in hydrothermal fluids induced by the oxidation of fe2 in primary minerals olivine and pyroxene to fe3 in secondary minerals magnetite and serpentine a better knowledge of iron speciation in serpentine is therefore crucial to the quantification of hydrogen production during the serpentinization process for the first time we have determined the amount of ferric iron in altered peridotite as a function of alteration time we investigated experimentally the alteration of powdered lherzolite in pure water at 300 c 300 bar for each experimental run 0 7 18 34 and 70 days h2 degassing was measured using in situ gas chromatography and the experimental products were analyzed using xrd raman and x ray absorption spectroscopy at the iron k edge in parallel aqueous solutions were analyzed by icp aes our results show quasi complete serpentinization at 70 days with replacement of primary olivine and pyroxene by secondary lizardite and magnetite the fe3 fetotal ratio is linearly dependent on the hydrogen production and ranges from 0 to 0 66 at the end of the experiment our results reveal strong variations in fe3 in serpentine for different alteration times from 0 to 100 of ferric iron including up to 12 of tetrahedral iron hydrogen was produced in three main stages 1 a first stage during which the h2 production rate reaches a maximum at 18 days and is controlled by the crystallization of magnetite 2 an intermediate stage during which serpentine incorporates ferric iron and thus plays a major role up to 50 in the hydrogen formation and 3 a final stage during which magnetite amount increases from 2 to 5 of the mineral assemblage the last alteration stage is accompanied by a slight increase of the fe3 fetotal ratio while the rate of hydrogen production decreases at the end of the experiment consequently variations of the ferric iron contents in natural oceanic peridotites may constitute a good indicator of the hydrogen potential of various ultramafic hydrothermal fields

petrologic and stable isotopic studies of a fossil hydrothermal system in ultramafic environment chenaillet ophicalcites western alps france processes of carbonate cementation abstract the late jurassic chenaillet ophiolitic complex western alps represents parts of an oceanic core complex of the liguria piemonte domain a model for the origin and evolution of the chenaillet ophicalcites based on textural and isotopic characterization is presented the chenaillet ophicalcites correspond to brecciated serpentinized peridotites that record seafloor shallow serpentinization at a minimum temperatures of 150 c followed by authigenic carbonation carbonation starts with a network of micrometric to millimetric pre or syn clast formation calcite veins accompanied by a pervasive carbonation of residual olivine and serpentine inside the serpentinite mesh core a matrix of small calcite 13 c vpdb values that range between 5 and 0 4 the lower values were obtained for calcite within the serpentinite clasts the 18 o vsmow values have a range between 11 and 16 in carbonated clasts the 18 o values in the matrix are fairly homogeneous with an average at 12 and the late calcite veins have values between 12 5 and 15 5 these values suggest a relatively high temperature of formation for all the carbonates carbonates within clast are mainly characterized by a formation temperature in the range of 110 c to 180 c assuming a 18 o value of seawater of 0 the matrix forms at a temperature of ca 165 c late veins are characterized by a formation temperature ranging between 120and 155 c we propose a model where serpentinization is followed by discrete carbonation then brecciation and cementation as a consequence of continuous hydrothermal fluid circulation in the serpentinite basement this is comparable to observations made in the stockwork of present day long lived oceanic hydrothermal systems

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

late ordovician early silurian cryptospore occurrences on anticosti island ile d anticosti quebec canada residues from hirnantian upper ordovician and aeronian lower silurian strata of anticosti island quebec canada have yielded the oldest occurrence of terrestrial based palynomorphs cryptospores from eastern north america a low abundance low diversity cryptospore assemblage containing specimens of pseudodyadospora velatitetras and alete monads has been recovered from the velleda member of the ellis bay formation upper ordovician hirnantian the gun river formation lower silurian aeronian contains a low abundance low diversity assemblage composed of specimens of velatitetras laevolcancis and alete monads the occurrence of these assemblages provides evidence for early land plants in this part of north america during the late ordovician and early silurian and documents the existence of a terrestrial based flora during and after the saharan late ordovician glaciation

nanoinclusions in microdiamonds from neogenic sands of the ukraine samotkan placer a tem study abstract mineral and fluid inclusions in seven microdiamonds ca 0 2 mm in diameter from the neogene samotkan placer of the ukrainian shield were investigated by tem various types of submicron or nanometre sized inclusions such as olivine and orthopyroxene fe sn oxide and assemblages of mineral multi phases were observed in an octahedral microdiamond crystal numerous olivine and enstatite micro and nanocrystals were observed their composition is close to that of other olivine and enstatite inclusions found in kimberlitic diamonds worldwide an unusual feature of these minerals in the samples studied however is a relatively high nio content 0 50 0 80 wt in olivine fo 91 3 92 1 and 0 40 0 70 wt in enstatite fo 94 1 95 4 these peridotitic mineral associations represent the host possibly lherzolitic mantle in which the diamonds grew the mean mg values of olivine and enstatite inclusions in samotkan s microdiamond are respectively 91 43 and 94 83 which are closer to mg values of these minerals in lherzolite rather than in harzburgite assemblages included in diamond in addition enstatite clinoenstatite k richterite graphite and fe sn oxide were also identified as nanoinclusions in samotkan diamond the fe sn oxides and magnetite were found in two microdiamonds of the transitional 111 110 form the k richterite and graphite have been attributed to epigenetic inclusions the origin of fe sn oxide is not clear assemblages of multi phase minerals were observed in four microdiamonds of variable morphology octahedron transitional 111 110 form cube and cube coated diamond these phases belong to fluid bearing inclusions and are enclosed in cavities which as a rule are not filled completely by the solid phases the remainder of the volume is occupied by a fluid that for the most part has been released during sample preparation in the majority of cases the multi phase assemblages consist of carbonate mica rutile ilmenite apatite and sylvite carbonates are the most abundant phases in the fluid inclusions they are calcium rich carbonates frequently with admixtures of fe mg and sr the mica inclusions are enriched in silicon with si varying from ca 6 7 to 7 0 a p f u the multi phase mineral assemblages present in samotkan s microdiamond are similar to fluid bearing microinclusions found previously in other diamond types e g fibrous diamonds the fibrous coat of coated diamonds or the internal clouds of octahedral diamonds from kimberlites of different provinces worldwide they represent the carbonate alkali and chlorine rich mantle fluid composition present during crystallisation of the samotkan s microdiamond judging from the composition of the multi phase assemblages it may be assumed that the samotkan s microdiamond grew from a carbonatitic to a slightly silicic melt rich in alkali and volatile components

new insight into polycrystalline diamond genesis from modern nanoanalytical techniques abstract technical developments in analytical methods that reach nanometer spatial resolution have enabled the interrogation of smaller submicron sized inclusions in diamond that had previously been elusive this has inspired and enabled studies of non classical diamond species from different geological settings resulting in a strongly faceted and dynamic picture of diamond formation this article reviews the leap of knowledge achieved by employing state of the art analytical methods with high spatial resolution to polycrystalline diamonds from different settings i e from kimberlite from crustal ultra high pressure metamorphic terranes and alluvial carbonados while crustal metamorphic diamonds are generally formed under oxidizing conditions polycrystalline diamond from the earth s mantle and carbonado have inclusion suites reflecting variable and sometimes extreme redox conditions diamond fluid compositions however fall in the same compositional field for worldwide diamond fluids regardless of their geodynamic environment on the basis of thermodynamic equilibrium data for c h o fluids in the mantle we argue that submicron inclusions in diamonds are products of local remobilization connected to fluid fluxed partial melting and redox freezing thus evidence from these inclusions complements information from classical work on larger inclusions and allows a unique direct insight into the medium in which diamond formed

control of the products of serpentinization by the fe2 mg 1 exchange potential of olivine and orthopyroxene it is argued that the high mg content mg number1 4 95 3 of the serpentine minerals in serpentinized peridotite is a consequence of the environmental femg 1 exchange potential imposed on the system by the abundance of olivine and orthopyroxene mass balance in the serpentinization reaction then requires the precipitation of an ironrich mineral that in most cases is magnetite this causes hydrogen to be evolved in an oxygen conserved reaction the low variance mineral assemblage ol srp brc mag sets the chemical potentials of h2o sio2 and o2 internally at an early stage in the process but the paragenetic assessment of serpentinites is rendered difficult by the variable and usually unknown fe3 content of the serpentine minerals particularly lizardite whole rock analyses of highly to completely serpentinized peridotites reveal fe sfe ratios40 4 with an average value 0 69 similar to that of magnetite 0 67 this feature may be attributed to the presence of high fe lizardite as has been found in mo ssbauer spectroscopy studies electron microprobe and scanning electron microcope analyses in the literature exhibit element trends e g decreasing si vs sfe a p f u for olivinepseudomorph lizardite and with some exceptions for bastite lizardite that show a substitution of the cronstedtite component fe charge balanced ontand m sites cronstedtite substitution will be favoured at low temperature and or low hydrogen fugacity and in these circumstances less magnetite will be evolved during serpentinization in some cases none at all some bastite lizardites from sea floor settings show evidence of m site vacancy substitution of fe for fe in the course of progressive serpentinization micrometer to millimeter scale variations in sio2 potential may well be present but their influence on fe in lizardite seems to be limited to a few cases of lizardite associated with orthopyroxene chrysotile is on average more mg rich and less variable in fe mg ratio than lizardite facts that may be attributed to the greater fe content of lizardite chrysotile veins provide the best record available to us of the environmental femg 1 exchange potential in the pore fluid attending serpentinization this potential serves as a robust control on serpentine and brucite compositions although it may fail after olivine and orthopyroxene have been armoured or eliminated and in more open system environments high water rock ratio such as on the sea floor or at serpentinite host rock contacts the default assumption in microprobe analyses that measured iron is all fe can lead to inappropriate petrological conclusions in the case of serpentinites

dynamic control on serpentine crystallization in veins constraints on hydration processes in oceanic peridotites deformation and hydration processes are intimately linked in the oceanic lithosphere but the feedbacks between them are still poorly understood especially in ultramafic rocks where serpentinization results in a decrease of rock density that implies a volume increase and or mass transfer serpentinization is accompanied by abundant veining marked by different generations of vein filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation we use these veins to constrain the role of deformation and mass transfer processes during hydration of oceanic peridotites at slow spreading ridges we have selected a representative set of veins from ocean floor serpentinites of the mid atlantic ridge near kane transform fault 23 n and characterized these in detail for their microstructures and chemistry by coupling optical and electron microscopy sem tem with electron microprobe analyses four main veining episodes v1 to v4 accompany the serpentinization the first episode identified as vein generation v1 is interpreted as the tectonically controlled penetration of early seawater dominated fluid within peridotites enhancing thermal cracking and mesh texture initiation at 3 4 km up to 8 km depth and at t 300 350 c the two following vein stages v2 and v3 formed in a closed diffusive system and accommodate volume expansion required to reach almost 50 serpentinization of the protolith the cracks exploited by these veins were caused by the progressive unroofing at depths of 4 to 2 km along a detachment fault degree and rate of serpentinization seem to be controlled by the capacity of the system to create space and to drive the mass transfer needed for ongoing serpentinization and this capacity is in turn linked to the exhumation rate and local tectonics during this period water consumed by hydration may prevent the establishment of convective hydrothermal cells the onset of an open hydrothermal system in the shallow lithosphere 2 km where brittle fracturing and advective transfer dominate and enable the completion of serpentinization is marked by the last vein generation v4 these results show a complete history of alteration with the crystallization of different types of serpentine recording different tectonic events chemical conditions and modes of hydrothermal alteration of the lithosphere

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

sulfide mineralogy and redox conditions in some shergottites magmatic sulfide mineralogy has been studied in 2 olivine phyric shergottites dag 476 and sau 005 and 4 basaltic shergottites zagami shergotty los angeles and nwa 480 modal abundances of magmatic sulfides as estimated by image analysis on thin section are high 0 16 to 0 53 area percent and correlate positively with abundances of fe ti oxides sulfides are mesostasis minerals being mostly interstitial grains or locally enclosed in post cumulus melt inclusions e g sau 005 in olivine sulfides in shergottites are composed of major pyrrhotite containing pentlandite exsolutions associated with minor amounts of cu sulfides chalcopyrite and or cubanite hot desert finds e g dag 476 show abundant fracture filling iron oxy hydroxides of probable terrestrial origin unaltered sulfides show metal rich hexagonal pyrrhotite compositions with metal sulfur m s ratio ranging between 0 936 0 005 and 0 962 0 01 this compositional range corresponds to the two phase structural domain 2c nc of the fe s system however the high temperature disordered hexagonal 1c pyrrhotite structure would be in better agreement with magnetic properties of shergottites ni contents in pyrrhotite increase from los angeles 0 05 at to shergotty zagami and nwa 480 0 2 0 5 at and dag 476 and sau 005 up to 3 at the higher ni values of pyrrhotite in olivine phyric shergottites correlate with the abundance of pentlandite exsolutions both reflecting more primitive ni rich sulfide liquids where abundant olivine crystallized this result and the strong correlation between sulfide abundances and fe ti oxides argue for a primary magmatic origin of these sulfides although they reproduce the trend of magmatic oxygen fugacity conditions determined from fe ti oxide pairs observed pyrrhotite compositions are systematically more metaldeficient compared to those calculated from the fe s o system this suggests post magmatic oxidation during cooling on mars followed by terrestrial weathering for hot desert finds

lizardite versus antigorite serpentinite magnetite hydrogen and life the serpentinization of peridotite operates according to one or the other or a combination of two end member mechanisms in low temperature environments 50 300 c where lizardite is the predominant serpentine mineral olivine is consumed by reaction with h2o but its composition mg remains unchanged mg rich lizardite magnetite and dihydrogen gas brucite are products of the reaction at higher temperatures 400 600 c rates of mgfe diffusion in olivine are orders of magnitude faster with the result that the growth of mg rich antigorite can be accommodated by a compositional adjustment of olivine eliminating the need to precipitate magnetite and evolve hydrogen this latter end member mechanism probably best reflects the situation in the forearc mantle wedge

unusual upper mantle beneath guaniamo guyana shield venezuela evidence from diamond inclusions the geochemistry of mineral inclusions in diamond is an important source of information about the composition of the continental lithospheric mantle at depths exceeding 120 150 km at these depths two main types of geochemical environment support diamond formation they are ultramafic or peridotitic u type and eclogitic e type environments as shown by minerals that occur as inclusions in diamonds in primary diamond bearing kimberlite or lamproite rock the ratio of diamonds from these two geochemical environments varies widely between localities the u type environment dominates for the majority of diamond occurrences worldwide whereas the e type environment dominates for a very limited number of localities in south africa north america and australia the present study shows that an uncommonly high percentage 99 4 of e type diamonds with extremely variable inclusion assemblages is found in kimberlites in the northwestern part of the guyana shield at guaniamo venezuela these variations range from previously unknown silica undersaturated corundum eclogite to abundant silica rich coesite eclogite assemblages representing 22 of all e type diamonds some of which contain syngenetic ilmenite and magnetite the compositional variations of garnet and omphacite inclusions are extremely broad the wide variability of the eclogitic source in which diamonds formed beneath the guyana shield indicates a broadly basaltic chemistry in this environment which may represent ancient subducted oceanic crust such specific features have not been reported for any other diamond occurrence and reflect an unusual composition of the deep lithospheric mantle in this area

in situ oxygen isotope analyses in serpentine minerals constraints on serpentinization during tectonic exhumation at slow and ultraslow spreading ridges abstract peridotites exhumed along detachment faults at slow and ultraslow spreading ridges undergo variable and complex serpentinization here we present in situ oxygen isotope analyses in serpentine minerals from a textural sequence identified in samples drilled at the atlantis massif mid atlantic ridge mar 30 n and dredged along the easternmost southwest indian ridge swir 62 65 e the textural sequence is similar at both locations and involves mesh texture and bastite formation after olivine and orthopyroxene mesh texture recrystallization into chrysotile and antigorite dominated textures and banded and fibrous veins the 18o in serpentine decreases with the textural sequence which we interpret to record an increase in the time integrated water rock ratio under a nearly constant temperature while mesh texture development starts at stoichiometric water rock ratios and creates an isotopic variability at a scale of 100 m recrystallized textures indicate serpentinization temperatures on the order of 260 290 c seawater dominated fluid hypothesis or 320 360 c hydrothermally altered seawater hypothesis the subsequent banded veins record the evolution towards colder and seawater dominated fluids potentially present in shallower levels of the detachment footwall serpentinization leading to the mesh textures appears faster and more heterogeneous in atlantis massif samples than in swir samples we propose that such features are influenced by the amount of gabbros intruded in the peridotite in addition to influencing fluid pathways gabbros also provide a source for observed si enrichments in some atlantis massif samples and control the ph which in turn influences the mobility of al

unusual micro and nano inclusions in diamonds from the juina area brazil abstract a series of uncommon micro and nano inclusions has been identified in diamonds from the rio soriso placer deposit in mato grosso state brazil the micro inclusions are variable in size from 1 to 300 m usually they are polymineralic being formed predominantly by intergrowths of carbonates silicates and other minerals carbonates are represented mostly by dolomite and occasionally by calcite silicates found are coesite wollastonite ii cuspidine and monticellite sulphides and ilmenite form micro inclusions as well nano inclusions are different from micro inclusions not only in size not exceeding 200 nm they are usually included in micro inclusions among nano inclusions halides nacl kcl cacl2 and pbcl2 anhydrite spinel phlogopite pbo2 tio2 with an pbo2 structure native fe and other phases are identified all these minerals are of the eclogitic association they are either associated with coesite or are included in a diamond with a light organic carbon isotopic composition with 13c from 14 to 25 pdb this confirms our earlier conclusion that diamonds from the juina area may have formed as a result of subduction of the crustal material to depths of at least the lower transition zone or even the lower mantle the pressure estimates for the investigated diamonds vary in a range from 3 to 10 gpa an interesting assemblage of calcite cuspidine wollastonite monticellite fluid identified in one of the studied diamonds is considered as a product of a reaction of wollastonite fluid forming cuspidine monticellite the presence of numerous pores cavities and bubbles in mineral inclusions and identification of an association of volatile containing mineral inclusions such as halides nacl kcl cacl2 and pbcl2 fluorine containing silicate cuspidine and phlogopite emphasize the important role of volatiles particularly chlorine and fluorine in the formation of the diamonds

micrometer scale cavities in fibrous and cloudy diamonds a glance into diamond dissolution events abstract micrometer sized internal cavities in diamonds preserve evidence of diamond dissolution events combining the methods of focused ion beam fib sample preparation and transmission electron microscopy tem enables these features to be studied in detail micrometer scale cavities are found in the inner parts of fibrous and cloudy kimberlitic diamonds their filling consists of amorphous matrix secondary nano crystals volatiles and in some cases larger resorbed crystals trapped minerals include corundum kappa alumina quartz olivine moissanite 6h and ca mg carbonates this is the first observation of kappa alumina in nature secondary nano minerals are observed within the amorphous matrix and include carbonates al oxide fluorite ilmenite and secondary diamond crystals the amorphous matrix is spongy and its composition is dominated by amorphous carbon nitrogen chlorine and also contains water when no crystalline phases are observed the matrix is also enriched in alumina silica and in some cases calcium we propose that micrometer scale cavities in diamonds form during dissolution events induced by the introduction of oxidizing hydrous fluids into the diamond growth area hydrous fluids are the main dissolving agents for most kimberlitic diamonds fedortchouk y canil d semenets e 2007 mechanisms of diamond oxidation and their bearing on the fluid composition in kimberlite magmas am mineral 92 1200 1212 at diamond forming conditions silica and alumina are enriched in hydrous fluids that are in equilibrium with eclogites kessel r ulmer p pettke t schmidt m w thompson a b 2005 the water basalt system at 4 to 6 gpa phase relations and second critical endpoint in a k free eclogite at 700 to 1400 c earth planet sci lett 237 873 892 this is consistent with the increased solubility of alumina with increased pressure and temperature in the na cl bearing fluids manning c e 2006 mobilizing aluminum in crustal and mantle fluids j geochem explor 89 251 253 manning c e 2007 solubility of corundum kyanite in h2o at 700c and 10 kbar evidence for al si complexing at high pressure and temperature geofluids 7 258 269 additionally hydrous fluids may leach grain boundaries that are enriched in alumina in peridotitic environments hiraga t anderson i m kohlstedt d l 2004 grain boundaries as reservoirs of incompatible elements in the earth s mantle nature 427 699 703 wirth r 1996 thin amorphous films 1 2 nm at olivine grain boundaries in mantle xenoliths from san carlos arizona contrib mineral petrol 124 44 54 diamond dissolution will form oxidized carbon species and may decrease the solubility of silica and alumina in the dissolving agent leading to their precipitation diamond forming fluids that are trapped in sub micrometer inclusions in the same fibrous diamonds are the dominant fluid component in the diamond growth area corrosive hydrous fluids are less common and appear as short discrete events followed by the return of the common diamond forming fluids and continued diamond precipitation

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

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

conditions of stichtite mg 6 cr 2 oh 16 co 3 4h 2 o formation and its geochemical and isotope record of early phanerozoic serpentinizing environments abstract stichtite is a magnesium chromium hydroxycarbonate mineral found in association with early phanerozoic chromite rich serpentinite rocks of tasmania australia and tehuitzingo mexico elemental analysis of stichtite shows a range of compositions within the cr fe al hydrotalcite group with compositional trends associated with each discrete serpentinite host body elemental and textural analysis of stichtite associated chromite indicates that stichtite forms in fore arc setting rocks through interaction of chromite and methane rich serpentinizing fluids the degree to which chromite is replaced by stichtite is inferred to correlate with the length of time that the host rocks spent within the stichtite window carbon stable isotope analyses of stichtite suggest carbon sourcing from marine kerogen with a minor marine carbonate component in some samples the carbon and hydrogen stable isotope profile of stichtite ranges from the field of methane from active serpentinizing zones to organic thermogenic methane the association of the stichtite 2h polytype nee barbertonite with aragonite antigorite suggests this is a higher pressure temperature polytype of stichtite reaction completion textures isotopic values and qualitative mineral thermobarometric indicators indicate that stichtite forms during serpentinization of fore arc setting rocks in a methane h2 rich environment within fluid conduits ranging from low temperatures and pressures near the surface to depths where pressure is up to 0 8 1 2 gpa and temperature is up to 300 c these unique chemical isotopic and textural properties of stichtite from distinct serpentinite bodies likely record the duration of serpentinization at specific thermobarometric conditions and provide a window into the conditions associated with a potentially habitable environment on early earth and other bodies of the solar system

trace elements in sulfide inclusions from yakutian diamonds sulfide inclusions in diamonds may provide the only pristine samples of mantle sulfides and they carry important information on the distribution and abundances of chalcophile elements in the deep lithosphere trace element abundances were measured by proton microprobe in 50 sulfide inclusions sdi from yakutian diamonds about half of these were measured in situ in polished plates of diamonds providing information on the spatial distribution of compositional variations many of the diamonds were identified as peridotitic or eclogitic from the nature of coexisting silicate or oxide inclusions known peridotitic diamonds contain sdis with ni contents of 22 36 consistent with equilibration between olivine monosulfide solid solution mss and sulfide melt whereas sdis in eclogitic diamonds contain 0 12 ni a group of diamonds without silicate or oxide inclusions has sdis with 11 18 ni and may be derived from pyroxenitic parageneses eclogitic sdis have lower ni cu and te than peridotitic sdis the ranges of the two parageneses overlap for se as and mo the mo and se contents range up to 700 and 300 ppm respectively the highest levels are found in peridotitic diamonds among the in situ sdis significant zn and pb levels are found in those connected by cracks to diamond surfaces and these elements reflect interaction with kimberlitic melt significant levels of ru 30 1300 ppm and rh 10 170 ppm are found in many peridotitic sdis sdis in one diamond with wustite and olivine inclusions and complex internal structures have high levels of other platinum group elements pges as well and high chondrite normalized ir pd comparison with experimental data on element partitioning between crystals of monosulfide solid solution mss and sulfide melts suggests that most of the inclusions in both parageneses were trapped as mss while some high cu sdis with high pd rh may represent fractionated sulfide melts spatial variations of sdi composition within single diamonds are consistent with growth histories shown by cathodoluminescence images in which several stages of growth and resorption have occurred within magmatic environments that evolved during diamond formation

first find of polycrystalline diamond in ultrahigh pressure metamorphic terrane of erzgebirge germany this article describes polycrystalline diamond from the erzgebirge ultrahigh pressure metamorphic uhpm terrane of the bohemian massif related to the variscan 340 ma continent continent collision the foils polycrystalline diamond included in zircon were prepared with focused ion beam fib and studied with transmission electron microscopy tem each domain of the polycrystalline diamond consisted of a series of diamond monocrystals from 1 to 4 up to 15 containing numerous nanometric fluid and crystalline inclusions of sio2 caco3 kcl and kalsi3o8 the integrated chemistry of the nanoinclusions suggests that the polycrystalline diamond was crystallized from a mixture of two end members which includes a carbon rich hydrous silicic fluid containing si al k and a hydrous saline fluid rich in cl k na traces of carbonate sulphate and phosphate coupled with the presence of metallic cations such as fe mg ti cr co zn zr pb as and hg found in the diamond are coherent with the local geochemical diversity of the host metasedimentary rocks the oxidation state of the diamond forming fluid was close to c co buffer for example fo2 6 log units which is at the upper limit of the diamond stability region the presence of dislocation arrays indicating low angle grain boundaries together with zig zag features or kinks at diamond grain boundaries reflects rapid crystallization of the polycrystalline diamond triangular fluid pockets with steps at the surface related to 110 and 111 planes of diamond could promote the formation of c h and c oh bonds that probably could facilitate the process of diamond graphitization the microstructural features of polycrystalline diamond from the erzgebirge uhpm terrane as well as their undisputed formation from crustal organic carbon can be used for better understanding of the origin of any polycrystalline diamond formation from both kimberlitic and alluvial sources including enigmatic polycrystalline black diamond commonly called carbonado

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