phyllosilicate orientation demonstrates early timing of compactional stabilization in calcite cemented concretions in the barnett shale late mississippian fort worth basin texas u s a abstract calcite cemented zones in the prolific gas producing barnett shale ft worth basin texas preserve very early compactional fabrics that contrast markedly with fabrics in the enclosing host rocks the compactional state of these zones based upon alignments of phyllosilicates as determined by high resolution x ray goniometry hrxtg complemented by x ray powder diffraction xrpd back scattered electron imaging and elemental x ray mapping points to a prelithification near syndepositional timing of initial authigenic calcite emplacement thus these carbonate cemented zones are of particular interest for the information they provide on the character of grain assemblages and sediment types at the time of deposition and also on the early post depositional environmental chemistry of the barnett shale strata

tem and xrd determination of crystallite size and lattice strain as a function of illite crystallinity in pelitic rocks average crystallite size and mean square strain of illite in rock specimens and clay separates were measured independently in tem images and by single line fourier voigt method profile analysis of the c 1 nm peak of xrd patterns for a prograde sequence of pelitic rocks illite crystallinity indices 0 17 0 58 d2h from the gaspepeninsula quebec the tem determined crystallite sizes in clay separates approxi mate those determined by fourier profile analyses and those calculated from illite crystallinity indices by the scherrer equation with the exception of the diagenetic sample the crystallite sizes and mean square strains of illite in rock samples exhibit a trend similar to that determined by profile analyses but the average crystallite sizes are up to five times larger than those measured for clay separates tem images show that all rock samples have a wide range of crystallite sizes and the proportions of larger crystallites increase with metamorphic grade the diagenetic illite is defect rich fine grained mean thickness by volume c 70 nm 1m d material anchizonal illite tends to occur as separate aggregates of small 1m d and larger 2m 1 crystals c 200 nm comprising arrays of subparallel coalescing packets the epizone sample has thick c 400 nm defect free crystals of muscovite occurring in stacks of parallel layers or subhedral crystals intergrown with large angle boundaries cleaved crystals that are free of intracrystalline layer terminations are dominant in clay separates of all samples having ranges of smaller sizes with volume average thicknesses of c 43 43 and 81 nm c 14 28 67 nm by the voigt method respectively for the three zones the results suggest that illite crystallinity indices do not provide a direct measure of a single microstruc tural state of illite in rocks although they yield consistent limits for average crystallite sizes for the anchizone 23 48 nm here therefore they serve as a general parameter of the degree of recrystallization on a relative basis in part because the contributions of all peak broadening variables mixed layering size and strain decrease regularly with prograde regional metamorphism of pelites the microstructural changes caused by rock disaggregation are probably a function of those variables as well the data collectively demonstrate a trend from metastable defect rich small crystals towards a stable assemblage of larger defect free crystals through dissolution of strained crystals and neocrystallization consistent with the ostwald step rule

structural evolution within the luning fencemaker fold thrust belt nevada progression from back arc basin closure to intra arc shortening abstract the luning fencemaker fold thrust belt lftb of central nevada reflects major mesozoic shortening in the western us cordillera and involved contractional deformation in triassic and lower jurassic back arc basinal strata structural analyses in the santa rosa range in the northern lftb provide new insight into the evolution of this belt four phases of deformation are recognized in the santa rosa range d 1 involved tight to isoclinal folding cleavage development under low grade metamorphic conditions and reverse faulting this deformation phase reflects nw se shortening of 55 70 in the early and or middle jurassic d 2 structures are similar in orientation to d 1 but involved much less overall strain and are well developed only to the southeast d 2 appears to be related to thrusting along the eastern margin of the lftb in the middle and or late jurassic d 3 deformation reflects very minor shortening 4 deformation demonstrably occurred synchronously with emplacement of cretaceous granitoids dated at 102 ma u pb zircon based on syntectonic relations between d 4 structures and thermal metamorphism associated with intrusion and an upgrade in d 4 strain in the thermally softened metamorphic aureoles of the intrusions this last phase of deformation reflects minor regional ne sw shortening coupled with localized strain associated with pluton emplacement formation of the lftb structural province was accomplished during the d 1 and d 2 phases of deformation and most shortening occurred during the d 1 event this jurassic deformation led to structural closure of the back arc basin by top to the se tectonic transport and development of a largely ductile fold thrust belt subsequent deformation d 3 and d 4 is 50 m y younger and unrelated to development of the lftb the younger deformation reflects a combination of minor regional shortening interpreted to be related to the cretaceous sevier orogeny plus localized shortening related to emplacement of cretaceous intrusions

fabric and anisotropy of slates from classical studies to new results abstract slates are low grade metamorphic argillaceous rocks which have developed an intense tectonic fabric referred to as slaty cleavage due to their unique properties they have long been used as building materials especially tiles for roofs and floors the fabric characterized by strong alignment of platy phyllosilicate crystals produces extreme anisotropy that has been of great interest in structural geology what different fabric types are observed how do they form here we review crystal preferred orientation cpo and add new information to establish a comprehensive base for future studies slates are composed of fine grained phyllosilicates dominantly white mica and chlorite sometimes chloritoid quartz and accessories texture analyses of these polyphase rocks with high energy synchrotron x ray diffraction and high resolution ebsd reveal strong alignment of mica and chlorite in some cases exceeding any previously reported texture strength including metals interestingly the cpo of quartz with highly flattened grains shape preferred orientation or spo is practically random recrystallization microstructures observed by scanning electron microscopy suggest that the phyllosilicate preferred orientation is largely caused by crystal growth under stress and that quartz was subject to pressure solution and reprecipitation producing an anisotropic shape while phyllosilicates preferred orientation in slates is much higher than reports for shale schists and gneiss elastic seismic anisotropy for slates is similar to shales where a significant component is due to aligned porosity

fabric anisotropy induced by primary depositional variations in the silt clay ratio in two fine grained slope fan complexes texas gulf coast and northern north sea article i nfo high resolution x ray texture goniometry hrxtg has been used to quantify the alignment anisotropy of phyllosilicate minerals in a series of diagenetically altered mudstone samples from the deep subsurface of the texas gulf coast and the northern north sea the sampled formations have undergone a simple burial history with no overprint of tectonic deformation that may have influenced the phyllosilicate orientation observations have been made in two constrained temperature windows 148 150 c north sea data and 169 210 c texas gulf coast wherein less than 20 smectite remains in the mixed layer phase illite smectite quantitative x ray powder diffraction xrpd shows quartz which is dominantly detrital and phyllosilicate content having an inverse relationship quantified illite smectite fabric intensity phyllosi licate alignments ranges from low alignment values of 2 42 m r d multiples of a random distribution to high alignment values 6 75 m r d and chlorite kaolinite ranges from 1 80 to 6 46 m r d we propose that the broad range of phyllosilicate fabric intensity observed reflects the range of quartz phyllosilicate ratios that are the imprint of depositional setting bedform truncation and initial grain size detrital quartz acts to disrupt fabric intensity by being non platy producing areas sheltered from effective stress pressure shadows and acting as a matrix support there is no evidence that bioturbation is a significant control on phyllosilicate alignment in these quartz phyllosilicate mixtures these findings have important implications for the role of grain scale alignment on seismic anisotropy in mudstone systems fabric intensity in post diagenetic samples can be predicted from 1 quartz content and 2 total clay content where

open system constant volume development of slaty cleavage and strain induced replacement reactions in the martinsburg formation lehigh gap pennsylvania no important changes in the volume of mudrocks occur across the mudstone to slate transition in the martinsburg formation at lehigh gap pennsylvania mass balance calculations based on chemical analyses and specific gravity measurements of 48 mudstones and 26 graywackes from the martinsburg formation across this 130 m strain gradient show constant ratios of al2o3 tio2 feot mgo mno y v and zr this reflects the relatively low differential mobility of these components during diagenesis and slaty cleavage development as well as the uniformity of compositions of the mudstones and graywackes at deposition using minimum strain samples and al2o3 as references the calculations show no loss in sio2 but large losses of cao na2o ba and sr from the outcrop that are directly proportional to strain and reflect an open system on the scale of kilometers analyses of graywacke mudstone metagraywacke slate pairs show that losses in sio2 na2o and volume and gains in k2o and ba in the graywacke metagraywacke are balanced by opposite changes in the adjacent mudstone slate this documents a local mobility of sio2 on the scale of centimeters the inertness of most major components over the 100 m outcrop especially sio2 however requires 1 that the composition of the fluid passing through the rocks was closely buffered by the quartz albite muscovite chlorite assemblage 2 that the volume of the fluid was small and 3 that the p t gradient down which it traveled was gentle the changes in chemical composition identified can be explained by the inferred replacement reaction plag 1m musc k 2m musc qz ca 2 na sr 2 ba 2 the strong correlation of minor element chemistry and strain indicates that this reaction was strain induced and that changes in bulk chemical composition are defined by the progress of this reaction

a 10 000 year record of dune activity dust storms and severe drought in the central great plains dune fi elds and loess deposits of the great plains of north america contain stratigraphic records of eolian activity that can be used to extend the short observational record of drought we present a 10 000 yr reconstruction of dune activity and dust production in the central great plains region based on 95 optically stimulated luminescence ages the integration of data from both eolian sand and loess is an important new aspect of this record clusters of ages defi ne episodes of extensive eolian activity which we interpret as a response to frequent severe drought at 1 0 0 7 ka and 2 3 4 5 ka with peaks centered on 2 5 and 3 8 ka sustained eolian activity occurred from 9 6 to 6 5 ka parts of this record may be consistent with hypotheses linking holocene drought to sea surface temperature anomalies in the pacifi c or atlantic oceans or to the el nino southern oscillation phenomenon but the record as a whole is diffi cult to reconcile with any of these hypotheses

microfracturing and microporosity in shales shales are ubiquitous rocks in sedimentary basins where their low permeability makes them efficient seals for conventional oil and gas reservoirs and underground waste storage repositories waste waters co2 nuclear fuels moreover when they contain organic matter they form source rocks for hydrocarbons that may escape towards a more porous reservoir during burial a process referred to as primary migration and when the hydrocarbons cannot escape these rocks can be exploited as oil or shale gas reservoirs while the presence of fractures at the outcrop scale has been described the existence of fractures at smaller scales their link with microporosity the mechanisms that created them their persistence over geological times and their effect on the petrophysical properties of shales represent scientific challenges for which drillings in various sedimentary basins over the past decades may hold timely key data here we review and synthetize the current knowledge on how microfractures and micropores in shales can be imaged and characterized and how they control their anisotropic mechanical properties and permeability one question is whether such microfractures when observed in outcrops or in drilled core samples extracted from boreholes are related to decompaction and do not exist at depth another question is whether veins observed in shales represent microfractures that were open long enough to have acted as flow paths across the formation the mechanisms of microfracture development are described some have an internal origin fracturing by maturation of organic matter dehydration of clays while others are caused by external factors tectonic loading importantly the amount of microfracturing in shales is shown to depend strongly on the content in 1 organic matter and 2 strong minerals the nucleation of microfractures depends on the existence of mechanical heterogeneities down to the nanometer scale their propagation and linkage to create a percolating network will depend on the presence of heterogeneities at the meso to macro scales such percolating microfracture networks could control both the long term sealing capabilities of cap rocks and the further propagation of hydraulic fracturing cracks finally possible areas of research for describing the mechanism of microfracture formation in greater detail and how this impacts the transport and mechanical properties of shales are also discussed

evidence for syntectonic crystallization for the mudstone to slate transition at lehigh gap pennsylvania u s a data primarily for phyllosilicates have been obtained for the continuous transitional sequence from mudstone to slate with well developed slaty cleavage at lehigh gap pennsylvania and for slates from quarries in the same area samples were studied by optical microscopy powder x ray diffraction and electron microprobe analysis with emphasis on transmission and analytical electron microscopy mineral grains are virtually free of deformation induced strain concomitant with the gradual development of cleavage normal to bedding the following changes are observed or confirmed 1 the orientation of phyllosilicate grains changes discontinuously from being preferentially parallel to bedding to being parallel to cleavage 2 crystal imperfections as expressed in layer terminations low angle grain boundary like features and other defects decrease in density 3 complex mixed layering is replaced by homogeneous packets of layers of single phases and 4 illite transforms to muscovite with increase in k a1 and change from a lmd to 2m polytype slaty cleavage apparently develops due in part to pressure solution of phyllosilicates oriented parallel to bedding mass transport of components and crystallization to form new grains parallel to cleavage it reflects transitions from imperfect metastable phases toward ordered stable phases in a low temperature 225 c metamorphic environment

preferred orientation of phyllosilicates effects of composition and stress on resedimented mudstone microfabrics abstract development of a preferred orientation of clay minerals is investigated in response to changes in vertical effective stress and composition in resedimented material boston blue clay boston blue clay bbc an illitic glacio marine clay composed of 57 clay sized particles

sedimentology and possible paleoceanographic significance of mudstone turbidites and associated deposits of the pen argyl member martinsburg formation upper ordovician eastern pennsylvania abstract the pen argyl member the stratigraphically highest unit of the upper ordovician martinsburg formation of eastern pennsylvania consists of the three mudstone facies laminated mudstone uniform mudstone and black shale these deposits conformably overlie a coarsening upward sequence of fine grained and classic sandstone turbidites of the underlying bushkill and ramseyburg members respectively petrographic and x radiographic analysis suggests that laminated and uniform mudstone facies beds accumulated from muddy turbidity currents interbedded black shale facies beds however are interpreted to reflect slow gravitational settling of sediment in a density stratified anoxic basin the marked transition from sandstone and siltstone turbidite deposits of the ramseyburg member to mudstone turbidite and black shale deposits of the pen argyl member may reflect the combined effects of a caradocian rise in sea level increased organic carbon production lack of sand size sediment and a tropical to subtropical location of the basin density stratification such a model however does not explain adequately the very thick laminated and uniform mudstone facies beds common only to the pen argyl member an alternative hypothesis involves amplification of global eustatic and climatic related oceanographic effects by local or basinal influences the very thick beds of the laminated and uniform mudstone facies by comparison with lithologically similar deposits of the eastern mediterranean are interpreted to have been ponded in a topographically restricted basin accordingly deposition of the pen argyl member may record a significant change in basin morphology from a wide extensive basin to a topographically restricted or enclosed anoxic basin

slaty cleavage development and magnetic anisotropy fabrics the shale to slate transition preserved in the ordovician martinsburg formation at the lehigh water gap pennsylvania provides an opportunity to study the relationship between magnetic anisotropy fabrics and the development of slaty cleavage our previous work has indicated that anisotropy of magnetic susceptibility ams does not record changes in finite strain associated with cleavage development in these rocks but instead measures the degree of dissolution and new growth of ehlorite additional ams data presented in this paper lend further support to this conclusion conversely anhysteretie remanent magnetization anisotropy arma which is not affected by paramagnetie ehlorite accurately reflects the strain induced rock fabrics associated with cleavage formation arma results show that magnetite dimensional orientations vary from bedding parallel in shale samples to cleavage parallel in samples with well developed slaty cleavage samples with weak and pencil cleavage display scattered arma orientations which lie in between bedding and cleavage these intermediate orientations may be due to either passive rotation of magnetite from bedding parallel to cleavage parallel or re crystallization of magnetite if rotation occurred grain rotation was highly heterogeneous in the samples with incipient cleavage the intermediate arma orientations may also reflect the varying contribution of two magnetite preferred orientations a depositional orientation parallel to bedding and a new growth orientation parallel to cleavage

high resolution x ray texture goniometry high resolution x ray texture goniometry hrxtg significantly improves peak background ratios and allows for the determination of crystallographic preferred fabrics of small regions 1 mm2 in heterogeneously deformed rocks the basic instrument is a commercially available automated single crystal x ray diffractometer equipped with a mo source a stage for transmission mode was constructed that is driven by the original motors so the unit is fully computer controlled absorption corrections are calculated which removes the need for generally difficult to obtain empirical corrections this combined with equipment optimization enables us to use small irradiated volumes of rocks with variable compositions the advantage of hrxtg in heterogeneously deformed rocks is demonstrated in a cleaved mudstone analysis of 1 mm2 areas in a 2 cm bed showing a gradation from micaceous sandstone to slate reveals that fabric intensity of both mica and chlorite increases significantly dm from 0 64 to 1 03 and from 0 45 to 0 78 respectively without appreciable change in the oblate shape km of the fabric thus hrxtg can record a strain state at the mm scale of heterogeneously deformed rocks

the relationship of phyllosilicate orientation x ray diffraction intensity ratios and c b fissility ratios in metasedimentary rocks of the helvetic zone of the swiss alps and the caledonides of jamtland central western sweden phyllosilicate preferred orientations x ray diaraction intensity ratios and c b fissility ratios were determined from 23 metasedimentary rocks from the helvetic zone of the swiss alps and from the caledonides of jamtland sweden the relationships between these parameters of phyllosilicate orientation depend strongly on the lithology and the cleavage morphology diaerences in orientation direction and in degree of preferred orientation reflect both deformation intensities and lithologies whereas polyphase deformed fine grained samples generally have phyllosilicates that are oriented parallel to cleavage showing high orientation intensities phyllosilicates in less deformed and or coarse grained rocks are oriented parallel to bedding with low orientation intensities weak orientation intensities in samples with phyllosilicates that are oriented parallel to bedding reflect compaction strain the higher strain in samples with micas and chlorites that are parallel to cleavage is a combination of compaction and tectonic strain due to the development of crenulation cleavage 2000 elsevier science ltd all rights reserved

static recrystallization and preferred orientation of phyllosilicates michigamme formation northern michigan usa abstract the michigamme formation of the marquette district in michigan s upper peninsula comprises a sequence of cleaved rocks of increasing metamorphic grade because metamorphism in the area occurred after cleavage formation the rocks provide an opportunity to study preferred orientation development of phyllosilicates under conditions of static recrystallization x ray texture goniometry on samples from the greenschist facies zone that were collected at varying distances from the bounding biotite in and garnet in isograds shows that 1 the preferred orientation of phyllosilicates is always parallel to the mesoscopic cleavage and 2 the degree of preferred orientation of phyllosilicates improves as a function of increasing metamorphic grade from 9 m r d scanning electron microscopy on these samples shows that 1 the length width ratio increases with increasing grade and 2 grain shapes are better defined with increasing grade previous work on slates showed mechanical processes dominate at very low grade metamorphism whereas chemical processes are favored at higher grades the michigamme samples show that improvement of preferred orientation occurrred by grain dissolution and crystallization noncleavage parallel phyllosilicate grains were preferentially dissolved probably facilitated by internal strain energy from mineral defects aided by chemical energy whereas cleavage parallel phyllosilicates were hosts for new growth along their basal planes these results show that significant fabric strengthening can be achieved by grain dissolution and crystallization in the absence of tectonic stress

nitrogen aggregation in metamorphic diamonds from kazakhstan microdiamonds that occur in high grade metamorphic rocks of the kokchetav massif in kazakhstan have been interpreted as part of the metamorphic mineral assemblage implying previously unrecorded pressures for the metamorphism of crustal rocks analysis of the infrared and visible absorption spectra from kokchetav microdiamonds extracted from recent stream sediments draining the metamorphic rocks shows roughly 48 63 aggregation of single n substitution to a centres but no measurable further aggregation calculated residence times assuming peak metamorphic temperatures of 800 900 c are from 0 5 ma to 40 ma and support the petrographic evidence that these diamonds formed during the high pressure metamorphism rather than being inherited from premetamorphic sediments

chlorite mica aggregates morphology orientation development and bearing on cleavage formation in very low grade rocks abstract chlorite mica aggregates in slates from northern spain have been investigated in very thin thin sections specimens from various parts of a fold show different morphologies that are dependent on the operating deformation mechanism from the limb to the hinge rigid body rotation becomes less important and folding and intragranular kinking are more common this is associated with an increasing aspect ratio width length for the aggregates chlorite mica aggregates were in part present before cleavage development and are statistically parallel to the bedding enveloping surface variation in degree of distortion of chlorite suggests continuous solution and growth during deformation muscovite on the other hand is generally more highly deformed suggesting that it was all early splitting of micas along 001 planes occurs creating extension sites where chlorite growth can take place thus an early diagenetic metamorphic formation with mimetic growth of chlorite on detrital micas is favoured for the origin of the aggregates the behaviour of the aggregates during deformation provides information on a mechanism for cleavage development as a result of intragranular kinking high angle boundaries are created with parts of the deformed grains at a low angle to the cleavage plane mechanical rotation appears to be the important mechanism for cleavage development in these rocks modification by recrystallization solution and growth processes can account for the observed microstructure

zircon response to diamond pressure metamorphism in the kokchetav massif ussr ultrahigh pressure metamorphism of the diamond bearing gneisses in the kokchetav massif ussr has been dated at 530 7 ma 2 sgr by shrimp ion microprobe analysis of metamorphic zircons zircon xenocrysts as old as 2000 ma have retained original isotopic compositions through the extreme metamorphic conditions and indicate that the protolith that was buried to diamond forming pressures included early proterozoic components highly variable reset ting of u pb isotopic systems in zircon xenocrysts at the time of metamorphism suggests that diffusive lead loss had commenced but was incomplete a disequilibrium observation consistent with relatively transient exposure of the rocks to extreme crustal depths

reorientation mechanisms of phyllosilicates in the mudstone to slate transition at lehigh gap pennsylvania abstract the mudstone to slate transition of the martinsburg formation at lehigh gap pennsylvania was re examined using a new transmission mode x ray texture goniometer supplemented by sem xrd and optical studies three mesoscopic zones are recognized in the outcrop i mudstone ii transition and iii slate zone in the mudstone zone the mica basal planes are parallel to bedding whereas the preferred orientation of the chlorite basal planes is up to 30 shallower than bedding the angular difference between chlorite and mica decreases towards the transition zone becoming subparallel at ca 50 m from the contact with the overlying shawangunk formation in the transition zone the preferred orientations of both mica and chlorite are intermediate to bedding and cleavage orientations which is consistent with mechanical reorientation of phyllosilicates this is supported by a decrease in march strain with a minimum at ca 95 m from the contact for both mica and chlorite sem observations similarly show the importance of grain rotation in large detrital grains in the slate zone both chlorite and mica orientations are parallel to cleavage chlorite and mica in the cleavage orientation of the slate zone have high fe contents whereas low fe mica and mg fe chlorite dominate in the mudstone and transition zone which indicates that dissolution neocrystallization is the dominant mechanism in the slate zone thus mechanical rotation of large detrital grains is important in the early stages of cleavage development with dissolution neocrystallization occurring at all stages and becoming dominant in the more evolved stages

the anisotropy of magnetic susceptibility ams in low grade cleaved pelitic rocks influence of cleavage bedding angle and type and relative orientation of magnetic carriers

criteria for incipient slaty and crenulation cleavage development in tertiary flysch of the helvetic zone of the swiss alps abstract two parameters of incipient cleavage development are compared in a series of tertiary flysch from the helvetic zone of the swiss alps 1 the overall relative phyllosilicate orientation as obtained from the x ray diffraction intensity ratio of their 00l reflections in cleavage and bedding parallel slabs i 10a 0 5 c b and 2 the dimensional aspect ratios of fissility fragments the cleavage bedding fissility ratio c b these two parameters show a roughly linear power relationship with a wide scatter appreciably better fits are obtained if populations showing different cleavage morphology closely vs widely spaced cleavage crenulation vs non crenulation cleavage and lithology silt and fine sand size quartz content carbonate content are plotted separately in contrast there is no clear link between the two criteria of cleavage intensity and the degree of incipient metamorphism as expressed by illite crystallinity samples with crenulation cleavage show both lower i 10a 0 5 c b and c b cleavage bedding fissility ratios than most of those with non crenulation domainal cleavages with closely spaced cleavage domains these lower ratios reflect the presence of an earlier largely bedding parallel cleavage fabric this difference in the ratios between these groups of cleavage morphologies proves that the cleavage bedding fissility ratio is not only dependent on the overall phyllosilicate orientation but also on the cleavage microstructure in particular the morphology and spacing of the cleavage domains and the persistence of a bedding parallel cleavage fabric in the microlithons of the crenulated samples the relationship between the c b cleavage bedding fissility ratio and the i 10a 0 5 c b ratio can provide a useful indication of the type of incipient cleavage fabric developed and help in detecting the presence of an earlier slaty cleavage additional measurement of the intersection bedding fissility ratio i b on the fissility fragments is likely to help in assessing the effect of lithology particularly the effect of the initial bedding anisotropy the differences in the illite chlorite ratios in the cleavage and bedding planes as determined from the i 10a i 7a c b ratio provides information regarding the relative formation of illite and chlorite in the cleavage domains and the chlorite mica stacks during development of the cleavage fabric

first investigation of quartz and calcite shape fabrics in strained shales by means of x ray tomography abstract we document the evolution of the 3d fabric of shale along a km long strain gradient in the jaca basin southern pyrenees spain with respect to the distance from a thrust samples were collected in the cleavage free domain at the onset of the pencil cleavage domain within the pencil cleavage domain and within the slaty cleavage domain by combining high resolution x ray computed tomography xct and energy dispersive x ray spectroscopy eds the morphology and the shape preferred orientation spo of thousands of quartz grains calcite grains and pores was studied in the least deformed samples quartz and calcite display mean foliation parallel to bedding with comparable dispersion in the pencil cleavage domain quartz foliation still follows the bedding while calcite foliation is mostly governed by cleavage in the slaty cleavage domain calcite shows a much better organization with foliation parallel to cleavage mimicking closely the pore fabric by contrast the quartz shape fabric is much less defined scattered between bedding and cleavage planes this suggests that quartz grain act as rigid marker in the ductile matrix while calcite grain orientation is governed by dissolution precipitation processes

the relationship of strain and preferred orientation of phyllosilicate grains in rocks a review abstract the calculation of strain from the preferred orientation of platy phyllosilicate grains taking strain to be homogeneous is surprisingly often successful from a pragmatic point of view although numerous theoretical objections to this procedure can be and have been raised for the calculation to succeed the original distribution of mica or clay mineral grains must be known or assumed the usual assumption that the starting distribution was random and therefore nearly uniform implies that the strain history of pelitic sediments is taken to start when the watery sediment was last stirred by waves or bioturbation or if it was laid down in still anoxic waters from the moment of first deposition it therefore includes compaction as part of the strain history which is usually neglected by structural geologists originally random distribution of clay flake orientations has been observed in recently deposited sediments in many instances and has been theoretically explained in various ways exceptions probably exist but their frequency has not been estimated biologically disturbed sediments may be brought nearer to a state of random orientation of clay flakes than they were at deposition with the exception of whole beds or concretions in which pores are early filled with cement mudrocks are almost always compacted by the weight of later deposits the strain so induced is axially symmetric about the vertical and thus about the pole of the bedding planes compaction acts on soft unconsolidated rocks but may continue with increasing overburden on rocks strengthened by cementation and by increasing interlocking of grains tectonic deformation may start early before the rock is fully compacted or it may begin only much later and the mechanisms of deformation increase in complexity with the increasing temperature and hydrostatic stress of deeper burial up to a certain level of mild metamorphism however phyllosilicate grains seem to be rather permanent if not as mineral species then at least by preservation of their constituent sheets of silica tetrahedra given this permanence the mechanisms of rock deformation whether intergranular slip cataclasis or dissolution and reprecipitation and various intragranular processes in the non phyllosilicate matrix hardly matter as far as the recording of strain is concerned even a certain amount of stress solution at especially heavily loaded points of the phyllosilicate grains themselves does not destroy their value as strain markers as long as the dissolved material is disposed of mainly by crystallographically continuous overgrowth on existing grains the same holds for local dissolution of phyllosilicates in portions of old grains that have become thermodynamically unstable by bending widespread irregular or random nucleation and growth of newly nucleated phyllosilicate grains at intermediate or high grades of metamorphism can reduce the reliability of the strain record and so can domainal structure of the deformed rock if both the volume fraction of phyllosilicate grains and the intensity of total strain differ sharply from one domain to another if distinct domains are not equant and have themselves a strong preferred orientation of shape nevertheless rocks containing an anastomosing network of zones rich in phyllosilicate grains as slates and other deformed mudrocks commonly do have in many cases been found to have average preferred orientations of mica or chlorite which if used to estimate the strain give results similar to those obtained from independent strain markers in the same rocks thus the most significant verification of the usefulness of strain estimates by measurement of preferred orientation is the mutual confirmation with independently determined strain the limit to such verification is the rarity of strain markers so that the coexistence of two independent ones is exceptionally rare for this reason however it is important to make full use of the orientation of phyllosilicate grains where they are available the alternative to have no strain measurements at all causes structural geologists to miss potential information

geology and structure of diamond bearing rocks of the kokchetav massif kazakhstan abstract two crustal settings for microdiamond formation have been described from eclogite bearing metamorphic areas 1 the economic concentration of microdiamonds in metasedimentary gneisses and calc silicate rocks northern kazakhstan and 2 microdiamonds recently found in eclogite garnet pyroxenite and jadeitite from dabie shan mountain eastern china the latter occurrence is interpreted to be the product of ultra high pressure metamorphism in a mesozoic collision zone there are however a number of discrepancies between the geological structural and geochemical data for kazakhstan microdiamond deposits and an interpretation in terms of a deep subduction zone model the geodynamic setting of the kokchetav massif can be defined as a continental rise prism environment related to a passive continental margin where rifting predominated during early palaeozoic orogeny the kumdikol microdiamond province is closely associated with a tectonic melange zone involved in ductile to semi ductile shearing abundant in graphite microdiamonds of the kumdikol area have a dual setting in the rocks they appear to be included not only in refractory garnet and zircon but also in almost all rock forming minerals as biotite phlogopite diopside quartz and secondary sericite chlorite and sericite chlorite calcite aggregates after garnet pyroxene and plagioclase and in spite of softness of the host phases microdiamonds are well preserved on the other hand the ore body extends along the shear zone and high concentrations of the microdiamonds within it are distributed without any lithological control along the local s c surfaces of the main kumdikol strike slip shear zone this duality of microdiamond settings in absence of practically all ultra high pressure minerals except diamond itself weakens the interpretation of this occurrence in terms of very deep subduction and very fast uplift and exhumation during 15 10 ma according to recent geochronological data these microdiamonds could have been formed metastably under anisotropic stress conditions in fact the available data from the kumdikol province provides certain support for either metastable and stable hypothesis

strain history of the los prietos syncline santa maria basin california a case of post tectonic compaction abstract strain measurements of the monterey shale along a fold profile of the los prietos syncline in the santa maria basin california provide an opportunity to study the strain history of the syncline by using transmitted x ray goniometry we first measured the degree of preferred orientation of chlorites in 19 samples along the profile from the preferred orientations total strains were calculated according to march s theory based on the variation of the total strains along the asymmetrical syncline a plausible history could have been as follows compaction first reduced the porosity from 86 to 79 before beds were fully consolidated they were buckled and then homogeneously strained on the average by strains of 1 00 0 27 0 32 with a nominal standard deviation of 0 01 the rheology during deformation of the unconsolidated rocks made the folding style resemble one of soft sediment deformation finally the porosity was further reduced by a post tectonic vertical compaction to 40 of the original pore volume the deformation history of the syncline appears to be related to the development of the pull apart santa maria basin the relationship between the geometry of the syncline and the orientation of the principal axes of the homogeneous tectonic strain are consistent with a tectonic strain history in three increments basin opening folding and westward tilting

implanted radiogenic and other noble gases in crustal diamonds from northern kazakhstan noble gases were extracted in steps from grain size fractions of microdiamonds 15 m the concentration also decreases with the combustion step both results are clear evidence that 4he has been implanted into the diamonds from decaying elements in the surrounding matrix the saturation concentration of 4he 5 6 10 4 cm3 stp g is among the very highest observed in any terrestrial diamonds fission xenon from the spontaneous fission of 238u accompanies the radiogenic 4he the 136xef 4he ratio of 2 5 0 3 10 9 agrees well with the production ratio of 2 3 10 9 expected in a reservoir where th u 3 3 radiogenic 40ar is predominantly 90 set free upon combustion it also resides in the diamonds and appears to have been incorporated into the diamonds upon their formation 3he on the other hand is mainly released during pyrolysis and hence is apparently carried by contaminants the concentration in the diamonds proper is of the order of 4 10 12 cm3 stp g with a 3he 4he ratio of 1 10 8 excess 21ne similarly appears to be present in contaminants as well as in diamonds proper these two nuclides in the contaminants must have a nucleogenic origin but it is difficult to explain their high concentrations

magnetite dissolution and neocrystallization during cleavage formation paleomagnetic study of the martinsburg formation lehigh gap pennsylvania paleomagnetic study of the shale to slate transition in the ordovician martinsburg formation at lehigh gap pennsylvania reveals that the remanence of the relatively undeformed shales predates cleavage formation and that the slates were remagnetized during cleavage development the shales display two components of remanence component 1 c1 was the only magnetization direction in the majority of the shales and the first removed component in 10 shale specimens component 2 c2 is observed as a single component remanence with a low 300 400 c unblocking temperature range in samples from one shale station and as a second removed component in 10 shale specimens the mean directions are declination inclination 347 04 346 47 in situ tilt corrected for c1 and 80 63 135 40 for c2 the pencil slates and well cleaved slates both display single component remanences the mean directions are 198 12 201 26 for the pencil slates and 199 24 225 54 for the well cleaved slates using the relationship between cleavage development and remagnetization in the slates the timing of cleavage development and folding and the regional geology we can constrain the relative ages of the remanences as follows 1 the shale c1 direction is most likely prefolding and represents a primary upper ordovician direction 2 the slate directions are younger in age than the shale directions and are probably of permo carboniferous alleghenian age these results show that remagnetization occurred during development of slaty cleavage in these rocks we interpret the mechanism for this remagnetization to have been strain induced dissolution and neocrystallization of magnetite the additional difference in the characteristic directions of the pencil slates and well cleaved slates may be attributed to strain reorientation of the pencil slate remanence after new growth of magnetite the unknown temporal relationship between dissolution and neocrystallization and strain precludes any quantitative attempt to correct remanence for strain in these samples our study shows that paleomagnetic studies of deformed sediments must consider the possibility of dissolution and neocrystallization of remanence carriers in addition to strain induced rotation

disentangling magnetic subfabrics and their link to deformation processes in cleaved sedimentary rocks from the internal sierras west central pyrenees spain abstract here we present a detailed study of the magnetic fabrics and subfabrics of remagnetized upper cretaceous limolites that crop out in the internal sierras west central pyrenees affected by a penetrative pressure solution cleavage the bulk magnetic fabrics of these rocks rt ams show variable orientations that do not conform to what is typically reported for cleaved sedimentary rocks in contrast the paramagnetic subfabrics lt ams show remarkably constant directional properties so that their k max and k min axes cluster parallel to the intersection lineation and to the poles to bedding respectively these lt ams subfabrics indicate a preferred orientation of phyllosillicates that is consistent with a syn sedimentary late cretaceous period of nne oriented layer parallel shortening noticeably these phyllosillicate subfabrics are not further altered by the subsequent formation of cleavage in the late eocene early oligocene the ferrimagnetic subfabrics aarm also show remarkably constant orientation so that their k max axes are strikingly parallel to the shortening direction in the area we interpret this preferred orientation of ferrimagnetic grains as being caused by subhorizontal shear associated to cleavage formation which is consistent with the age and mechanisms authigenic growth and rotation of pre existing magnetite grains proposed for the pervasive remagnetization that affects the studied rocks

the clay fabric of shales is a strain gauge abstract shales with their large clay fraction are prone to deform in an almost ductile way under tectonic loading here we propose to monitor and quantify the evolution of strained shales clay fabric in the field of illite diagenesis by using magnetic fabric in the footwall of a south pyrenean thrust 281 shales samples have been collected along a 2 km long exceptional exposure the cleavage evolves from pencil structures at 1 km of the thrust to pencil cleavage at 850 m and slaty cleavage at 170 m the average magnetic foliation of illite remains nearly parallel to bedding over a distance of 600 m despite development of pencil cleavage it is at 600 m of the thrust that illite foliation shifts gradually from bedding to cleavage by contrast the magnetic fabric scalar data reflect a more continuous evolution of the deformation the application of a rigid rotation model of the clays allows to quantify the strain magnitude using eigen values of tensor this strain magnitude is consistent with distance to the thrust and conforms favorably with a trishear modelling of this thrust damage we demonstrate therefore that the magnetic fabric of illite rich shales is a strain gauge

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

early proterozoic ultrahigh pressure metamorphism evidence from microdiamonds microdiamonds from the akluil k minette dykes nunavut canada are similar to diamonds formed in subducted metamorphic rocks high concentrations of unaggregated nitrogen and positive 15 n suggest that the microdiamonds formed within rocks subducted to ultrahigh pressures before being sampled by the minette magma 1 8 billion years ago this ultrahigh pressure metamorphism in north america probably related to the trans hudson orogen about 2 billion years ago extends the occurrence of ultrahigh pressure metamorphism from 0 6 billion years to before 1 8 billion years ago and suggests that phanerozoic type subductions were active by the early proterozoic

gradational development of domainal slaty cleavage its origin and relation to chlorite porphyroblasts in the martinsburg formation eastern pennsylvania abstract slaty cleavage frequently consists of narrow folia or domains of phyllosilicates alternating with microlithons richer in quartz within cleavage domains the 001 planes of phyllosilicates are usually parallel or subparallel to the domain boundaries but they may be at a small but persistent angle in microlithons the phyllosilicates are commonly not parallel to the cleavage domains domainal cleavage may grade into pervasive slaty cleavage with few or no microlithons and a parallel orientation of all the phyllosilicates or may grade within distances as short as a few millimeters into pelite with no cleavage and with the phyllosilicates either with no preferred orientation or a weak to strong orientation parallel to bedding study of cleavage specimens mainly from the martinsburg formation of pennsylvania and new jersey indicates that cleavage development is related to strain variation and to volume loss at least as measured by deformed quartz calcite veins in the xz plane of a fold mass transfer of the more soluble constituents under conditions of low grade metamorphism chlorite zone and under the influence of deviatoric stress leads to the development of solution ways initiated at heterogeneities in the fabric which result in solution cleavage that is extended by terminal and lateral propagation stress gradients result in gradational development of solution cleavage with minimal development in strain shadows the solution ways develop into cleavage domains by accumulation of inert materials particularly carbonaceous grains iron ores such as framboidal pyrite and phyllosilicates however the preferred orientation of phyllosilicates is mainly the result of crystallization of new grains in the domains because of the breakdown of detrital or diagenetic phyllosilicates and of other minerals such as feldspar or epidote under low grade metamorphic conditions this interpretation is supported by lack of bent or deformed phyllosilicates in and along the boundaries of cleavage domains larger detrital phyllosilicates with 001 at a high angle to cleavage commonly show deformation further support is provided by the occurrence of chlorite grains with their 001 planes approximately parallel to the cleavage in strain shadows on competent grains such as framboidal pyrite and by the shape change of diagenetic chlorite porphyroblasts as traced from zones with no domainal cleavage into zones with well developed domainal cleavage mechanical rotation appears to be of minimal importance in the development of cleavage domains that are unrelated to a crenulation structure there will be grain boundary sliding and other adjustments as a consequence of removal of material along the solution ways deformation of a rock possessing a strong preferred orientation of phyllosilicates parallel to bedding or to a previous cleavage may result in the development of a crenulation structure and thenec a differentiated crenulation cleavage mechanical re orientation will in this case be a more important element in formation of cleavage although mass transfer and crystallization are still necessary to produce a domainal cleavage

xrd based 40ar 39ar age correction for fine grained illite with application to folded carbonates in the monterrey salient northern mexico abstract due to their minute size 40ar 39ar analysis of illite faces significant analytical challenges including mineral characterization and especially effects of grain size and crystallography on 39ar recoil quantifying the effects of 39ar recoil requires the use of sample vacuum encapsulation during irradiation which permits the measurement of the fraction of recoiled 39ar as well as the 39ar and 40ar retained within illite crystals that are released during step heating total gas ages tga are calculated by using both recoiled and retained argon which is functionally equivalent to k ar ages while retention ages ra only involve retained ar in the crystal natural applications have shown that tga fits stratigraphic constraints of geological processes when the average illite crystallite thickness ict is smaller than 10 nm and that ra matches these constraints for icts larger than 50 nm we propose a new age correction method that takes into account the average ict and corresponding recoiled 39ar for a sample with x ray corrected ages xca lying between total gas and retention ages depending on ict this correction is particularly useful in samples containing authigenic illite formed in the anchizone with typical ict values between 10 and 50 nm in three samples containing authigenic illite from cretaceous carbonates in the monterrey salient in northern mexico there is a range in tgas among the different size fractions of 46 49 36 43 and 40 52 ma while ras range from 54 64 47 52 and 53 54 ma respectively xca calculations produce tighter age ranges for these samples of 52 5 56 45 5 48 5 and 49 52 5 ma respectively in an apparent age vs ict or 2m 1illite plot authigenic illite grains show a slope that is in general slightly positive for tga slightly negative for ra but close to zero for xca with thinner crystallites showing more dispersion than thicker ones in order to test if dispersion is due to a different formation history or the result of retention capability degassing spectra were modeled for site xca averages and overall xca average modeling shows that local site age average best match the measured spectra instead of a global average age indicating that illite growth reflects local deformation and is not the result of regional metamorphism modeling also shows that ar degassing spectra are very sensitive to grain size such that age interpretation based on ar plateaus is meaningless for most fine grained clays

contrasting roles of detrital and authigenic phyllosilicates during slaty cleavage development eight samples from a prograde slate sequence from central wales ranging from the diagenetic zone to the epizone were studied by x ray texture goniometry sem and tem x ray pole figure data imply that in the four lower grade samples the crystallographic preferred orientations of both mica and chlorite change from parallel to bedding to intermediate to bedding and cleavage to parallel to cleavage orientation with increasing grade there is no difference in the degree of preferred orientation between mica and chlorite in individual samples the sample with an intermediate preferred orientation has the lowest degree of preferred orientation in the four higher grade samples the preferred orientations of mica are primarily in the cleavage orientation whereas those of chlorite are largely parallel to bedding observations by sem and tem show that the samples have two populations of phyllosilicates 1 large tens of pm chlorite mica stacks of detrital origin dominated by chlorite in a matrix of 2 authigenic fine grained tens of nm mica and chlorite with dominant illite muscovite preferred orientation measurements of chlorite primarily reflect the chlorite in the stacks however preferred orientation measurements of mica are dominated by the mica in the stacks for four lower grade samples but by mica in the fine grained matrix with 001 parallel to cleavage for the four higher grade samples these observations collectively show that 1 mechanical rotation of detrital grains was the dominant reorientation mechanism of cleavage formation in the lower grade samples 2 in the higher grade samples the preferred orientation of matrix mica and chlorite transformed from bedding parallel to cleavage parallel primarily by dissolution and neo crystallization copyright 1996 elsevier science ltd

determination of march strain from phyllosilicate preferred orientation a semi numerical method abstract the tensor for the strain in a fine grained phyllosilicate rich rock can be estimated according to the march theory an extended march equation with seven coefficients relates march strain in a rock to the angular density distribution function adf of the basal planes of its phyllosilicates the adf is mapped from 420 intensity measurements by an automatic x ray pole figure goniometer in the transmission mode five of the coefficients are the independent components of the constant volume strain tensor the two additional coefficients pertain to x ray goniometry the strain tensor and the adf share principal directions by theory by visually determining these directions from the fabric diagram and expressing the extended march equation in reference coordinates coinciding with the principal directions the seven coefficients in the equation are reduced to four applying the levenberg marquardt routine we calculate the remaining four coefficients by a best fit of the adf to the march equation in principal coordinates as an example the strain in a slate from north wales was measured by this method as 0 77 0 05 0 30 0 04 0 57 0 00 which is compatible with the results from the direct measurement of ellipsoidal strain indicators 0 80 0 31 0 57 this is an improvement over earlier estimates on the same sample by means of a graphic approximation 0 81 0 38 0 60 the stability and uniqueness of the solution was tested by convergences starting from a wide range of guess values and its sensitivity to bad estimates of principal direction by test calculations starting from two misalignments of different severity

multiple origins of alluvial diamonds from new south wales australia in eastern australia diamonds occur in alluvial deposits overlying phanerozoic basement the origin of the diamonds is not known no feeder pipes of kimberlite or lamproite are known and none of the usual diamond indicator minerals occurs in association with the diamonds alluvial diamonds from wellington bingara copeton and airly mountain in new south wales form two distinct groups here termed a and b group a diamonds are similar to those found in kimberlites and lamproites globally and are thought to have formed in precambrian lithospheric mantle group b diamonds have unusual characteristics that indicate they formed in a subduction environment surface features of diamonds of both groups indicate that all were brought to the surface by magmas group a diamonds are similar in primary crystal form internal structure mineral inclusion composition mainly peridotitic and carbon isotopes to diamonds found in kimberlitic and lamproitic hosts in archean and proterozoic cratons worldwide re os ages 3 4 and 2 1 ga pearson et al 1998 of sulfide inclusions in two group a diamonds constrain the origin of these diamonds to ancient mantle sources this along with the nature of the surface abrasion structures and radiation damage suggests that the group a diamonds represent an older group of diamonds that have been in secondary collectors for a significant time if this is so it is feasible that the group a diamonds may have been derived from a number of primary sources including possible sources in antarctica group b diamonds are unlike any other diamond suites worldwide in their combination of shape surface features strained and irregular internal structures 13c enrichment and ca rich eclogitic mineral inclusions the features are best explained as a product of diamond growth under varying p t conditions in a high p t dynamic environment such as a subducting slab the diamonds may well have formed during arc continent collision at the time of the development of the new england fold belt this would explain why the major known concentration of group b diamonds is within the new england fold belt at copeton and bingara and is consistent with the phanerozoic ages for diamond emplacement determined from mineral inclusions

tem and aem constraints on the origin and significance of chlorite mica stacks in slates an example from central wales u k chlorite mica stacks grains of intergrown chlorite and white mica in a matrix of fine grained white mica and chlorite have been studied using xrd sem empa tem and aem methods the stacks occur in a weakly cleaved llandoverian mudstone central wales which has a white mica illite crystallinity index of 0 35 a20 corresponding to the lower anchizone white mica occurs as packets 100 to 8 m thick interleaved with dominant chlorite packets in stacks with both apparent coherent interfaces or cross cutting small angle boundaries with chlorite layers it is well crystallized 2m polytype with phengitic composition and low paragonite component na na k 0 0 07 chlorite in stacks is fe rich and relatively homogeneous tio2 crystals surround stacks and occur within chlorite and white mica is ti rich compared to matrix white mica fine grained white mica and chlorite in the matrix have two different orientations one parallel to bedding and one parallel to cleavage which is approximately 30 50 to bedding matrix white mica is predominantly a 2m1 polytype but some cleavage parallel white mica is 3 t and some bedding parallel white mica is 1md it is na rich na na k 0 14 0 42 and relatively heterogeneous some discrete paragonite and phengitic muscovite are observed to coexist in the cleavage orientation matrix white mica and chlorite each contain less fe than corresponding white mica and chlorite in stacks both matrix and stack chlorite are one layer polytypes the data imply that chlorite in the stacks is largely derived from the replacement of volcanogenic biotite and other ferromagnesian minerals probably via intermediate expandable trioctahedral phyllosilicates most intergrowths of chlorite and mica in stacks formed by mica replacement of chlorite and altered biotite along cleavage fissures subsequent deformation caused further modification of pre existing chlorite mica stacks whereas partial dissolution of stacks and bedding parallel matrix phyllosilicates resulted in the formation of cleavage parallel phyllosilicates

diamonds from myanmar and thailand characteristics and possible origins diamonds occur in modern alluvial deposits at several localities in myanmar thailand and sumatra the deposits do not contain typical diamond indicator minerals and no obvious primary sources have been found in the surrounding terrane which is characterized by phanerozoic tectonic and magmatic activity detailed studies of diamonds from theindaw and momeik in myanmar and phuket in thailand have been undertaken to clarify their origin syngenetic mineral inclusions in the diamonds are largely of the peridotitic paragenesis with a smaller eclogitic component carbon and nitrogen isotope compositions are typical of kimberlitic and lamproitic diamond suites worldwide nitrogen aggregation states indicate a long residence and or significant deformation at mantle temperatures many stones show plastic deformation features the rounded and polished surfaces of most of the diamonds reflect resorption in a corrosive magma these features do not support an exotic or unusual origin for the diamonds for example by subduction exhumation extensive abrasion and abundant brown radiation damage spots suggest long surface transport their distribution within the sibumasu terrane and their close association with carbo permian glacial marine sediments suggest that these diamonds were derived from primary sources in northwestern australia or within the terrane itself prior to the early permian separation of the sibumasu terrane from the gondwanaland margin the isotopic data and the dominantly peridotitic nature of syngenetic inclusions rule out the argyle lamproite as a possible source and also distinguish the myanmar thailand diamonds from morphologically similar stones from eastern australia

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

preferred orientation and elastic anisotropy in shales anisotropy in shales is becoming an important issue in exploration and reservoir geophysics in this study the crystallographic preferred orientation of clay platelets that contributes to elastic anisotropy was determined quantitatively by hard monochromatic x ray synchrotron diffraction in two different shales from drillholes off the coast of nigeria to analyze complicated diffraction images with five different phases illite smectite kaolinite quartz siderite feldspar and many overlapping peaks we applied a methodology based on the crystallographic rietveld method the goal was to describe the intrinsic physical properties of the sample phase composition crystallographic preferred orientation crystal structure and microstructure and compute macroscopic elastic properties by averaging single crystal properties over the orientation distribution for each phase our results show that elastic anisotropy resulting from crystallographic preferred orientation of the clay particles can be determined quantitatively this provides a possible way to compare measured seismic anisotropy and texture derived anisotropy and to estimate the contribution of the low aspect ratio pores aligned with bedding