submillimeter scale zonation of δ18o in quartz and feldspar isle of skye scotland abstract laser probe analysis of oxygen isotope ratios in submillimeter domains of quartz and feldspar in hydrothermally altered granite reveals complex intragranular zonation that can be contoured within individual grains two samples collected within 10 m of each other in the maol na gainmhich epigranite mge isle of skye scotland are similar petrographically yet display very different 18o zonation patterns sk1 has isotopically zoned quartz 18 o 1 8 8 2 with intragranular gradients of 18o as great as 5 mm and individually homogeneous grains of k feldspar 0 2 which also display a large range of values 18 o 1 7 to 7 3 oxygen isotope gradients of more than 11 mm were measured in this sample in contrast sk4 has homogeneous quartz 18o 5 7 0 3 n 14 and a zoned k feldspar phenocryst 18 o 3 3 to 6 9 isotopic heterogeneity in quartz correlates with the degree of chlorite alteration in adjacent hornblende grains the isotopic heterogeneity revealed by the laser probe in single grains of quartz and feldspar is greater than the entire range of values displayed by bulk mineral analysis of samples collected throughout the mge pluton although the average values of 18o obtained by laser probe are identical to the average bulk values values in k feldspar suggest exchange at 400 c with infiltrating meteoric water 18 o 10 the marked differences in zonation profiles between samples on the outcrop scale suggest that permeability and isotope exchange during hydrothermal alteration was highly heterogeneous these microanalytical data for quartz and feldspar provide the first oxygen isotope evidence from pervasively altered granites that paleopermeability is caused at least in part by intracrystalline microfractures

porosity and permeability in volcanic rocks a case study on the serie tobifera south patagonia argentina the middle to late jurassic serie tobifera belongs to the chon aike province and extends all over patagonia and the antarctic peninsula it consists largely of ignimbrites epiclastics and rhyolitic lavas and was considered only a minor reservoir rock for oil with fracture permeability petrographic and petrophysical data in selected core samples from the austral basin were collected to determine the processes controlling the porosity and permeability of these volcanic rocks the sequence of processes occurring during cooling and in the post cooling stages can modify sometimes substantially their original petrophysic characteristics the results show that the highest porosity and permeability occur in rocks with quench fractured glasses and in non welded ignimbrites with gas pipe structures followed by autobrecciated rhyolites welded ignimbrites massive glasses and fresh rhyolites have the lowest permeabilities the new data indicate that tectonic fracturing is not as significant as was considered before and application of these concepts are relevant in the assesment of volcanic reservoir quality

unsolved problems in the lowermost mantle 1 many characteristics of d layer may be attributed to the recently discovered mgsio3 post perovskite phase without chemical heterogeneities they include a sharp discontinuity at the top of d regional variation in seismic anisotropy and a steep clapeyron slope however some features remain unexplained the seismically inferred velocity jump is too large in comparison to first principles calculations and the sharpness of the discontinuity may require a chemical boundary chemical heterogeneity may play an important role in addition to the phase transformation from perovskite to post perovskite phase transformation and chemical heterogeneity and the attendant changes in physical properties such as rheology and thermal conductivity are likely to play competing roles in defining the dynamical stability of the d layer revealing the relative roles between phase transition and chemical anomalies is an outstanding challenge in the study of the role of d in thermal chemical evolution of the earth

molecular dynamics simulations of seismic discontinuities and phase transitions of mgsio3 from 4 to 6 coordinated silicate via a novel 5 coordinated phase this paper reports detailed molecular dynamics simulations over a large pressure temperature range 0 140 gpa and 300 2000 k in the earth s mantle starting from the upper mantle phase enstatite mgsio 3 the simulated seismic velocities show several discontinuities corresponding to the phase transitions of enstatite with increasing pressure enstatite mgsio 3 transforms first to a new novel five coordinated silicon phase and then to the lower mantle perovskite phase involving six coordinated silicon atoms the new intermediate phase is crystalline but orientationally disordered the calculated seismic velocities and densities across the phase transitions for a pure mgsio 3 mantle are consistent with previous estimates these studies suggest that the major discontinuities between the upper mantle transition zone and the lower mantle could arise partially due to the changes in the silicon coordination

linking collision slab break off and subduction polarity reversal in the evolution of the central indian tectonic zonetectonic evolution of the central indian tectonic zonet deshmukh n prabhakar the central indian tectonic zone demarcates the zone of amalgamation between the north indian craton and the south indian craton presently the major controversies in the existing tectonic models of the central indian tectonic zone revolve around the direction of subduction and the precise timing of accretion between the north indian craton and the south indian craton a new model for the tectonic evolution of the central indian tectonic zone is postulated in this contribution based on recent geological and geophysical evidence combined with previously documented tectonic configurations the present study employs the slab break off hypothesis and subsequent polarity reversal to explain the tectonic processes involved in the evolution of the central indian tectonic zone we propose that the subduction initiated c 2 5 ga in a s directed system producing island arc sequences on the south indian craton the southward subduction regime culminated with slab break off underneath the south indian craton between c 1 65 ga and 1 55 ga which subsequently induced subduction polarity reversal and set the course for n directed subduction

u pb monazite ages in amphibolite to granulite facies orthogneiss reflect hydrous mineral breakdown reactions sveconorwegian province of sw norway in the rogaland vest agder terrain of the sveconorwegian province of sw norway two main sveconorwegian metamorphic phases are reported a phase of regional metamorphism linked to orogenic thickening m1 and a phase of low pressure thermal metamorphism associated with the intrusion of the 931 2 ma anorthosite charnockite rogaland igneous complex m2 phase m1 reached granulite facies to the west of the terrane and m2 culminated locally at 800 850 c with the formation of dry osumilite bearing mineral associations monazite and titanite u pb geochronology was conducted on 17 amphibolite to granulite facies orthogneiss samples mainly from a suite of 1050 2 8 ma calc alkaline augen gneisses the feda suite in these rocks prograde negatively discordant monazite crystallized during breakdown of allanite and titanite in upper amphibolite facies at 1012 1006 ma in the feda suite and other charnockitic gneisses concordant to slightly discordant monazite at 1024 997 ma probably reflects breakdown of biotite during granulite facies m1 metamorphism a spread of monazite ages down to 970 ma in biotite hornblende samples possibly corresponds to the waning stage of this first event in the feda suite a well defined monazite growth episode at 930 925 ma in the amphibolite facies domain corresponds to major clinopyroxene formation at the expense of hornblende during m2 growth or resetting of monazite was extremely limited during this phase in the granulite facies domain up to the direct vicinity of the anorthosite complex the m2 event was shortly followed by cooling through ca 610 c as indicated by tightly grouped u pb ages of accessory titanite and titanite relict inclusions at 918 2 ma over the entire region a last generation of u poor monazite formed during regional cooling below 610 c in hornblende rich samples at 912 904 ma this study suggests 1 that monazite formed during the prograde path of high grade metamorphism may be preserved 2 that monazite ages reflect primary or secondary growth of monazite linked to metamorphic reactions involving redistribution of rees and th and or fluid mobilisation 3 that the u pb system in monazite is not affected by thermal events up to 800 850 c provided that conditions were dry during metamorphism

feldspar diagenesis in the frio formation brazoria county texas gulf coast tremendous quantities of detrital feldspar have been dissolved or albitized below about 14 000 ft 4 267 m in the frio formation oligocene chocolate bayou field brazoria county texas some sandstones no longer contain any unmodified detrital feldspar grains material transfer involved in these reactions is immense affecting at least 15 of the rock volume thus albitization has important implications for several other diagenetic processes that involve feldspars or their components these processes include formation of secondary porosity precipitation of quartz and carbonate cements and the evolution of na ca cl formation water

on the connected porosity of mineral aggregates in crystalline rocks the 14 c pmma impregnation of rock samples and mineral staining methods provide the connected porosity map and the spatial distribution of mineral aggregates respectively combined image analysis of mineral and porosity maps allows quantification of the connected porosity distribution in rock forming mineral aggregates after the two maps have been superimposed numerically the 14 c pmma method provides an indication of the extent of pore connectivity for each pixel in the analyzed area which can be used to obtain the porosity distribution as a function of modal mineralogy when applied to undeformed and deformed kivetty granodiorite samples from finland the method allows for a detailed analysis of the rock porosity porosity distributions related to the undeformed rock are unimodal and approximately identical to each other on the other hand porosity distributions of the deformed granodiorite are multimodal and vary significantly constituent porosity sets of the deformed samples were albite crystals free of alkali feldspar patches and alkali feldspar phenocrysts average porosity 0 21 albite containing alkali feldspar patches 0 59 rapakivi albitic mantles 1 15 quartz 0 39 and mafic minerals 5 8 moreover the analysis indicates that the numerous micropores observed under sem within alkali feldspar phenocrysts and albite crystals free of alkali feldspar patches are unconnected in 3d

experimental study of zircon coarsening in quartzite h2o at 1 0 gpa and 1000 c with implications for geochronological studies of high grade metamorphism the rate and mechanism of zircon coarsening in quartzite h 2 o at 1 0 gpa and 1000 c were characterized by performing time series experiments in a piston cylinder apparatus anhydrous experiments show no significant growth of zircon or textural modifications after 24 h in the presence of 1 2 wt h 2 o zircon 2 wt in coarsely powdered quartz recrystallized in the first 8 hours so that mean crystal size decreased relative to the starting material after 8 h zircon grew by ostwald ripening and by coalescence of crystals and maintained its position on quartz grain boundaries even while quartz crystals coarsened by recrystallizing in the fluid fitting the experimental data produced the growth rate equation log n log 2 2 log t to log k n log t to 5 9 where is mean diameter in micrometers the subscript 0 denotes initial value at time zero t is experiment duration in seconds and k n is the growth rate constant with units of m 2 s five experiments r 2 0 988 a value of n 2 suggests that growth is interface controlled with the rate limited by dissolution and precipitation on the crystal surface an equivalent growth rate equation is k 1 n t k 1 n n t 1 n 1 12 x 10 3 m s 1 2 t 1 2 a parabolic rate law equation that can be used to demonstrate that zircon in the presence of h 2 o at 1000 c grows fast enough by ostwald ripening to produce micrometer thick overgrowths in 10 y rapid coarsening of zircon by ostwald ripening will occur during thermal metamorphism when fluid is present addition of fluid and possibly melt greatly increases the rate of coarsening suggesting that most metamorphic rims form nearly instantaneously during fluid influx or anatexis and that u pb rim ages date such events

recent developments in dating ancient crustal fluid flow several isotopic techniques have recently been developed or substantially improved allowing the accurate determination of the timing of fluid related processes these include k ar rb sr and sm nd dating of diagenetic minerals in clastic rocks u pb dating of carbonates sm nd dating of fluorite and uraninite re os and rb sr dating of sulfides 40ar 39ar and rb sr dating of fluid inclusions and u pb dating of ilmenite magnetite sphene and rutile while many of these techniques require further evaluation some have already resulted in new constraints on a variety of crustal fluid flow models

the archaean proterozoic palaeoboundary in the lulea area northern sweden field and isotope geochemical evidence for a sharp terrane boundary abstract extensive sm nd isotope work has been carried out in northern sweden with the intention of studying crustal reactivation and assimilation processes relative to juvenile arc accretion at an archaean cratonic margin previous regional work has shown a gradual change from negative nd t values in northernmost sweden to positive values towards the skellefte sulphide ore district farther south isotopic variation and geophysical data show a break in the character of the crust along a wnw trending zone it is suggested that this zone which passes close to the towns of lulea and jokkmokk represents the transition from the archaean craton in the northeast to proterozoic juvenile crust in the southwest a profile across the eastern part of this transition zone has been studied in detail in the lulea area where archaean rocks recently have been discovered the distribution and character of the archaean rocks have been studied these occur as fragments ranging in size from several km 2 to dm in scale and consist of ca 2 7 ga orthogneisses of granodioritic to dioritic composition and porphyritic granitoids of quartz monzodioritic to granodioritic composition plutonic rocks of ca 1 9 ga age and archaean rocks were analysed for major and trace elements and for sm nd isotopes the calc alkaline ca 1 9 ga granitoids are dominated by granodiorites tonalites quartz diorites and quartz monzodiorites and there is a distinct boundary rather than a transition zone between intrusions with postive nd t values in the southwest and intrusions with negative nd t values in the northeast the locations of the outcropping archaean fragments coincides with this distinct boundary which is interpreted as a terrane boundary separating a ca 1 9 ga juvenile arc terrane in the southwest from the archaean continent in the northeast a possible explanation for this is that the proterozoic juvenile terrane was obducted onto the archaean craton during the final stages of the svecofennian orogeny

observation of primitive clay precursors during microcline weathering high resolution transmission electron microscopy of clay formation on microcline has revealed a poorly ordered hydrated silicate phase with an ultrathin 200 250 a circular form which occurs on the feldspar surface and appears to represent the first phase before the development of normal crystalline clays probe analysis at ca 100 a intervals reveals that during the formation of this material the feldspar loses silicon potassium and aluminium while the intermediate incorporates iron this primitive structure may result from rearrangement of the feldspar framework following the potassium hydrogen ion exchange process involved in weathering

the relationship between shear and compressional velocities at high pressures reconciliation of seismic tomography and mineral physics the value of the parameter from low pressure laboratory measurements disagrees with the value from seismic observations the parameter relates the isobaric change in shear velocity vs to the change in compressional velocity vp seismic evidence indicates exceeds 2 0 in the lower mantle whereas data on a variety of minerals at high temperature t and ambient pressure p result in lower values we reconcile these differences our ab initio model calculations on mgo show that increases with p and is 2 0 2 5 at lower mantle pressures there is no need to assume partial melting to explain the seismic data these calculations also provide insight into the p and t dependence of the dimensionless parameter 1 g g t p where g is the isotropic shear modulus and is the volume thermal expansivity using measured values of thermoelastic parameters coupled with thermodynamic identities we seek constraints on s 1 ks ks t p where ks is the adiabatic bulk modulus and confirm that p causes s to decrease we find poisson s ratio increases with p and t altogether these results show that for mgo increases from 1 3 at ambient conditions to over 2 at lower mantle conditions we expect other mantle minerals to behave similarly therefore we find that reconciliation of the mineral physics approach with that of seismic tomography concerning does not require special assumptions about the state of the lower mantle

nucleation and growth of myrmekite during ductile shear deformation in metagranites myrmekite is extensively developed along strain gradients of continuous lower amphibolite facies shear zones in metagranites of the gran paradiso unit western alps to evaluate the role of stress strain energy and fluid phase in the formation of myrmekite we studied a sample suite consisting of weakly deformed porphyric granites wdgs foliated granites fgs representative of intermediate strains and mylonitic granites mgs in the protolith most k feldspar is microcline with different sets of perthite lamellae and fractures in the wdgs abundant quartz oligoclase myrmekite developed inside k feldspar only along preexisting perthite lamellae and fractures oriented at a high angle to the incremental shortening direction in the wdgs stress played a direct role in the nucleation of myrmekites along interfaces already characterized by high stored elastic strain because of lattice mismatch between k feldspar and albite in the fgs and mgs k feldspar was progressively dismembered along the growing network of microshear zones exploiting the fine grained recrystallized myrmekite and perthite aggregates this was accompanied by a more pervasive fluid influx into the reaction surfaces and myrmekite occurs more or less pervasively along all the differently oriented internal perthites and fractures independently of the kinematic framework of the shear zone in the mgs myrmekite forms complete rims along the outer boundary of the small k feldspar porphyroclasts which are almost completely free of internal reaction interfaces therefore we infer that the role of fluid in the nucleation of myrmekite became increasingly important as deformation progressed and outweighed that of stress mass balance calculations indicate that in al si conservative conditions myrmekite growth was associated with a volume loss of 8 5 this resulted in microporosity within myrmekite that enhanced the diffusion of chemical components to the reaction sites and hence the further development of myrmekite

ostwald ripening as a possible mechanism for zircon overgrowth formation during anatexis theoretical constraints a numerical model and its application to pelitic migmatites of the tickalara metamorphics northwestern australia a fundamental dichotomy exists between the low solubility of zircon in peraluminous melt predicted by experimental and geochemical studies and the large volume proportions of zircon overgrowths formed during high temperature metamorphism and anatexis that are revealed by cathodoluminescence imaging we investigate the potential of ostwald ripening as a possible mechanism for overgrowth formation by presenting a numerical solution to the continuity equation governing open system diffusion rate limited ostwald ripening in a zircon saturated melt application of the model to a typical log normal initial zircon crystal size distribution csd suggests that despite uncertainties associated with the interfacial free energy of zircon significant grain coarsening is possible via this mechanism under geological conditions and time scales relevant to high grade metamorphism primary influences on the rate at which ostwald ripening proceeds are i the temperature of the system ii the duration of the time interval for which the system is above its solidus and iii the nature of the initial premelting zircon csd to test the viability of the model we examine zircon csds from three high grade pelitic migmatites of the tickalara metamorphics northwestern australia assuming that zircon crystals hosted by melanosome biotite were permanently occluded from the melt and therefore approximate the premelting csd the model predicts that within 1 to 2 ma these biotite hosted zircon csds will evolve into the observed leucosome hosted zircon csds via melt present ostwald ripening under geological conditions applicable to peak metamorphism although we have not conclusively demonstrated that ostwald ripening contributed to changes in zircon csds during anatexis of the tickalara metapelites our results suggest that ostwald ripening is a viable mechanism for zircon volume transfer in a zircon saturated melt and capable of playing a significant role in overgrowth formation in rocks where the total volume of zircon overgrowths substantially exceeds the concentration of zircon dissolvable in the coexisting melt

mineralization and deformation of the malanjkhand terrane 2 490 2 440 ma along the southern margin of the central indian tectonic zone the malanjkhand cu mo au deposit located near the northwest margin of the malanjkhand batholith terrane is a strategic and significant porphyry style deposit that experienced a protracted 50 m y deformational history shortly after its formation at 2 490 8 ma stein et al 2004 in a recent study panigrahi et al 2004 averaged u pb shrimp zircon data from a pooled set of samples from the malanjkhand batholith to advocate a meaningless intermediate age of 2 476 ma for the malanjkhand granitoid and its cu mo au deposit in the northwest part of the malanjkhand batholith re os dating of occurrence specific molybdenite captures not only the age of porphyry style mineralization and associated magmatism but also elucidates a complex deformational history that extends to 2 450 ma in the central part of the malanjkhand batholith re os dating of delicate spindles of accessory molybdenite occurring with pristine muscovite in miarolitic cavities within the undeformed microgranitoid at the devgaon mo prospect unequivocally shows that deformation ceased at this location no later than 2 470 2 465 ma the deformational history recorded at the malanjkhand deposit in the northwest most likely reflects prolonged transpressive convergence and docking of the malanjkhand terrane with units in the poorly understood proto central indian tectonic zone citz along its southern margin the central indian shear zone the timing for this convergence is late archean early paleoproterozoic

first principles theory of mantle and core phases the earth s interior is an extreme environment where the pressure and temperature up to 3 6 mbar or 360 gpa and approximately 6000 k respectively at earth s center are sufficiently high to affect the structure physics and chemistry of minerals in often surprising ways that may confound our intuition based on studies at near ambient conditions though the earth s interior deeper than 12 km has not been subject to in situ observation observational evidence from seismology combined with experimental and theoretical studies of earth materials under extreme conditions has allowed us to construct a picture of this remote region of the planet fig 1 although the mineralogy of the uppermost mantle is familiar from studies of xenoliths the deepest portion of the silicate mantle 2890 km depth or 136 gpa is more than twice as dense as average continental crust dziewonski and anderson 1981 and is thought to be composed primarily of a phase not yet seen at the earth s surface an mg rich meta silicate with the perovskite structure the earth s core is thought to be composed primarily of iron and is subdivided into a liquid outer part and a solid inner part the inner core is 65 denser than iron at ambient conditions partly due to the effects of compression and partly to the stabilization at high pressure of a close packed paramagnetic phase of iron figure 1 properties of the earth s interior the density bold line as determined seismologically and the pressure light line calculated from the density distribution dziewonski and anderson 1981 the density profile reflects the divisions of the earth s interior into mantle and core and the further subdivision into upper mantle transition zone and lower mantle liquid outer core and solid inner core the temperature distribution cannot be directly observed below the uppermost crust and must be inferred the bold

seismic velocities of major silicate and oxide phases of the lower mantle we have recently investigated the elastic properties of five compounds generally considered as potential consituents of the earth s lower mantle full elastic constant tensors of mgsio3 and casio3 perovskites mgo cao and sio2 have been determined as a function of pressure up to 140 gpa and at zero temperature using the first principles plane wave pseudopotential method within the local density approximation we report comprehensive comparisons of the calculated high pressure elastic wave velocities of these phases with seismically derived lower mantle properties the effects of iron and temperature are estimated in the case of mg silicate perovskite based on available experimental data comparison of the p and s wave velocity profiles of perovskite along a series of high temperature isotherms to the lower mantle supports the view that this region is primarily composed of mg rich silicate perovskite our study also suggests that among the candidate phases usually considered none are invisible minor phases which may be present including magnesiowustite casio3 perovskite and the high pressure polymorphs of sio2 and cao have elastic wave velocities that differ significantly from those of mg rich silicate perovskite and may be detectable seismically

intracratonic stability a comparison of paleomagnetic data from the north and the south of dharwar craton india abstract the prominent bend of dharwar craton india was recently suggested to post date the stabilization phase of the craton via a 30 anticlockwise vertical axis intracratonic rotation the prima facie evidence for this rotation was the realignment of dyke trends following correction of the inferred deformation the proposal also appeared to be consistent with the observed structural grain in the host archean basement we argue that using the trend of dyke swarms is not the ideal method to assess intracratonic rotation and instead we use variations in paleomagnetically determined declination data from these dykes we find that multiple early paleoproterozoic dyke swarms the 2367 2216 and 2082 ma generations preserve a systematic offset with a large uncertainty the mean paleomagnetically permissible anticlockwise rotation is 19 17 dykes emplaced during the 1888 ma lip do not show the same offset which suggests that rotation occurred prior to this time therefore the 2082 1888 ma interval brackets the window for any deformation and tectonism required to bend the dharwar craton by about 20 no large scale deformational events are documented in the dharwar craton during this critical interval apart from the earliest stages of cuddapah basin formation we argue that the observed rotation results from biases associated with how trends of dykes and their magnetic directions are compiled when age data are limited the curvature of geological features in dharwar craton is likely the tectonic inheritance of the neoarchean collision between the western dharwar and eastern dharwar cratons

origins of the surface roughness of unweathered alkali feldspar grains abstract the origins of surface roughness of powdered feldspar grains the ratio of the total surface area quantified by gas adsorption to the surface area estimated by assuming an average grain geometry has been investigated by a detailed study of alkali feldspars from a range of igneous rocks and from pegmatites the of 125 63 m powders of ten different feldspar samples ranges from 5 16 22 00 and correlates well with mineral microtexture powders of feldspars which are homogeneous or have straight lamellar exsolution microtextures yield mean values of 5 59 whereas powders of vein macroperthites give a mean of 6 64 and powders of braid macroperthites yield mean values of 12 15 microtexture influences by partly controlling the manner in which feldspar grains break during grinding and this affects the frequency of steps on grain surfaces feldspars which are homogeneous or have straight lamellar exsolution microtextures break to produce a relatively high proportion of grains with smooth step poor external surfaces whereas vein macroperthites and braid microperthites break to produce grains with much rougher step rich surfaces microtexture is able to influence style of breakage because the presence size and crystallographic orientation of exsolution lamellae has a significant influence on the trajectories of fractures propagating through grains as they fragment during grinding we found little correlation between internal surface area due to micropores expressed as either microporosity or mean perimeter area of micropores and indicating that microporosity is a significantly less important contributor to than external surface area due to steps this conclusion is supported by an analysis of the of a number of different size fractions of two of the feldspar powders the origin of is only one part of the more important question of how closely measured surface areas are related to the proportion of grain surfaces which are reactive during experimental dissolution and natural weathering

dislocation formation and albitization in alkali feldspars from the shap granite orthoclase rich alkali feldspars in the lower devonian shap granite northwest england contain two generations of albite rich feldspar these have partially replaced earlier exsolution microtextures consisting of albite lamellae coarse semicoherent albite films and fine coherent albite platelets in tweed orthoclase the earlier generation of replacive albiterich feldspar ab90an9ori occurs together with orthoclase rich feldspar ab10or90 i n veins that crosscut exsolution microtextures throughout grain interiors this episode of recrystallization was mediated by magmatic fluids at 410 8c estimated from two feldspar geothermometry and was driven by stored elastic strain energy which was relatively homogeneously distributed throughout the microtextures the later generation of replacive albite rich feldspar which is restricted to grain margins and is compositionally pure ab 99 was produced by magmatic hydrothermal fluids at 370 8c this generation of albite rich feldspar does not crosscut exsolution microtextures and has selectively replaced volumes of highly elastically strained feldspar surrounding edge dislocations along semicoherent albite films marked differences in controls of the localization of the two generations of replacive albite rich feldspar by pre existing exsolution microtextures indicate that significant numbers of edge dislocations developed along albite films after the first phase of fluid feldspar interaction and associated albitization but before the second phase this relation indicates that edge dislocations formed between 410 and 370 8c these observations have important implications for understanding the factors that control the interaction of alkali feldspars with fluids both in cooling igneous rocks and in clastic sedimentary rocks during diagenesis

a sem study of dissolution textures of detrital feldspars in proterozoic sandstones sweden a sem study of dissolution textures of detrital feldspars in proterozoic sandstones from sweden

application of 40ar39ar laser probe and step heating techniques to the dating of diagenetic k feldspar overgrowths abstract the feasibility of determining the age of diagenetic k feldspar overgrowths utilizing 40 ar 39 ar step heating and laser probe dating techniques was tested on samples from the lower cretaceous arkoses of the angola offshore basin a previous study of these samples combining cathodoluminescence and k ar dating indicated that the k feldspar diagenetic overgrowths were 98 16 ma and the detrital cores were 516 26 ma conventional 40 ar 39 ar step heating of overgrown grains from the bulk k feldspar fraction failed to unambiguously distinguish detrital from diagenetic ages similarly step heating analysis of overgrowth concentrates 84 diagenetic did not yield any distinct plateau age attributable to the diagenetic k feldspar detrital and diagenetic k feldspars from the arkoses studied appear to release their ar over similar ranges of temperature partial laser fusion of individual overgrown grains using lowenergy short duration laser pulses carefully focussed on the overgrowth yielded mixed ages with ar contributed from both the overgrowth and the core heat conduction calculations indicate that conductive heating through the core overgrowth boundary is not responsible for partial degassing of the core rather beam scattering along intragrain structural defects and incipient melting of the core must occur a more successful approach involved total laser fusion of individual grains from the overgrowth concentrates which were physically separated from their detrital core one limitation of this approach was that because of the young age and the small size of the diagenetic feldspar simultaneous fusion of several grains was required for accurate age determination our results suggest that future investigations will greatly benefit from the use of on line cathodoluminescence capability to reduce the possibility of contamination by detrital material and the use of a laser beam whose wavelength is strongly absorbed by diagenetic k feldspar

a 2 5 ga porphyry cu mo au deposit at malanjkhand central india implications for late archean continental assembly abstract eight re os ages from six molybdenite samples representative of the cu mo au mineralization at the giant malanjkhand deposit in madhya pradesh were obtained using id ntims these data provide a clear late archean early paleoproterozoic age for this deposit and by implication for its calc alkaline granitoid host among other diverse models the origin of malanjkhand has been debated as orogenic lode style associated with development of a quartz reef in a shear zone or as porphyry style associated with a pink granitoid intrusion that is conspicuously present in the vicinity of the brittle ductile deformed quartz reef previous models included middle proterozoic ages for the cu mo au mineralization based on poorly constrained rb sr and k ar data direct dating of molybdenite shows that stringer mineralization in the quartz reef and disseminated and vein mineralization in the granite were contemporaneous at 2490 8 ma 2489 5 1 4 ma based on regression without uncertainty in the 187re decay constant mswd 0 5 n 5 additional molybdenite from the quartz reef dated at 2475 and 2450 ma represents at least two pulses of reworking that we suggest configured the elongate and arcuate quartz reef into its present position we suggest that the quartz reef is a high silica stockwork cap embedded in its primary potassic alteration halo and subsequently tilted so that it lies convex to the east with deeper levels exposed to the north petrographic evidence coupled with high re concentrations for molybdenites 400 650 ppm support this scenario and suggest that malanjkhand is a subduction related stockwork porphyry style cu mo au deposit involving mantle malanjkhand geology shares little affinity with three surrounding and tectonically juxtaposed terranes we suggest that the malanjkhand cu mo au deposit and the surrounding batholith containing multiple cu mo au prospects may be the vestiges of a microplate captured along the north margin of the nearly amalgamated southern indian craton at 2 5 ga as the southern craton began to converge with the northern indian craton or other protocontinents now embedded or consumed within the central indian tectonic zone citz this scenario could imply an early paleoproterozoic age for the earliest development of the citz at least in the region of the central indian shear suture cis whose marked ene trend imprints the northwestern part of the malanjkhand batholith and the cu mo au deposit based on a compilation of 2 5 ga events in both the southern and northern indian cratons we suggest that some units in the citz linked an extensive and continuous late archean early paleoproterozoic orogenic belt that may extend to the moyar bavali bhavani attur palghat and cauvery shear zones in southern india and include inboard units of the eastern ghats and the aravalli delhi orogenic belts robust geochronology that can be tied to the early history within these orogenic belts is scarce and extensive reworking during grenvillian and pan african time creates a pervasive overprint at some localities this late archean early paleoproterozoic belt preserves sutures within a coalesced late archean continent that included east antarctica napier and vestfold hills complexes at 2 5 ga

a speculation on the structure of the d layer the growth of anti crust at the core mantle boundary through the subduction history of the earth abstract the growth curve of the continental crust shows that large amounts of continental crust formed in the early part of the earth history are missing in order to test a hypothesis that the former crust was subducted to the deep mantle we performed phase assemblage analysis in the systems of mid oceanic ridge basalt morb anorthosite and tonalite trondhjemite granite ttg down to the core mantle boundary cmb conditions results show that all these materials can be subducted to the cmb leading to the development of a compositional layering in the d layer we speculate that there could be five layers of feo enriched melt from partial melting of morb morb crust anorthosite ttg and slab or mantle peridotite in ascending order although the polymorphic transformation of perovskite to post perovskite in mg fe sio3 may explain the seismic discontinuity at the top of the d layer d discontinuity the effects of solid solution on the sharpness of the transformation suggest that the compositional layering is more plausible for the origin of the d discontinuity the d layer can be an anti crust made up mostly of ttg anorthosite derived from the former continental crust tectonic style of the anti crust at the cmb is similar to that at the surface at both places chemically distinct layers are density stratified and are also characterized by the processes of accretion magmatism and metasomatism

processes controlling porosity and permeability in volcanic reservoirs from the austral and neuquen basins argentina volcanic rocks develop primary and secondary porosity and permeability depending on both their lithology and the sequence of processes involved in their formation primary processes welding deuteric crystal dissolution gas release flow fragmentation and crystal shattering may lead to high porosity and permeability the best example of which is a nonwelded ignimbrite with well developed gas pipe zones secondary processes different types of alteration however tend to decrease primary porosity however certain secondary processes such as dissolution and hydraulic fracturing may contribute to enhance total porosity and permeability these conclusions were developed through a systematic review of reservoir quality in volcanic rocks integrating lithology and process interpretation with petrophysical data the said information was taken from selected cores of volcanic rocks from the serie tobfera unit in the austral basin and the precuyano unit in the neuqun basin argentina a clear understanding of both primary and secondary processes may serve to predict the quality of volcanic reservoirs and could be used as a guide for oil and gas exploration and development in many parts of the world

argon loss by alkali feldspars abstract ar loss is generally thought to preclude the use of alkali feldspars for dating old rocks by the k ar method and has been thought to occur via structural features such as the boundaries of perthite lamellae or twins we have done a 40 ar 39 ar step heating study of a suite of alkali feldspars from the klokken syenite intrusion u pb age 1166 4 ma which have been fully characterized by transmission electron microscopy all are low albite low microcline intergrowths but some are non turbid strain controlled cryptoperthites while others are turbid coarse microperthites geological and textural evidence suggests that the turbidity developed by fluid rock interactions during the initial cooling of the pluton in the stability field of low microcline 5 a after crystallization the finest cryptoperthites have numerous fully coherent perthite lamellar boundaries lamellar periodicity 40 nm and twin composition planes the have retained radiogenic 40 ar and yield total degassing ages as high as 1125 16 ma and maximum ages on the age spectra of 1162 16 ma the coarse turbid microperthites with patchy intergrowths and twins on the mm scale gave total degassing ages as low as 650 ma maximum age 750 ma argon leakage occurs least in feldspars with the most lamellar interfaces but in proportion to the degree of turbidity which is thought to be caused by large numbers of micropores strain controlled intergrowths in perthites allow the recognition of volumes of alkali feldspar crystals which have not interacted with deuteric fluids these retain argon and can give reliable k ar ages in old rocks saddle shaped age spectra appear to result from the preferential degassing at 1000 c of part of the feldspar structure which has lost more of its argon in nature the laboratory degassing does not occur in a manner analogous to that which has occurred naturally

40ar39ar analysis of perthite microtextures and fluid inclusions in alkali feldspars from the klokken syenite south greenland laser probe andin vacuo crushing have been used to apply the40ar39ar technique to microtexturally well characterised pristine and turbid alkali feldspar from the 1166 ma klokken syenite individual crystals are several millimetres in size but sem and tem studies reveal complex microstructures which define a range of sub grain sizes over three orders of magnitude from 100 m in pristine areas with fine scale cryptoperthite down to 200 nm in turbid areas with coarsened perthites and numerous micropores laser probe40ar39ar measurements of turbid feldspar gave low apparent ages indicating significant40ar loss 40 average which can be accounted for by sustained heating of the small sub grains 1 m for 1166 ma at low temperature 150 c pristine feldspar gives high apparent ages indicating the presence of excess40ar this component was readily lost from the feldspar by laboratory heating at low temperature 600 c and was also released by crushing which indicates that it was released from fluid inclusions during crushing release of excess40ar is correlated with cl release and the40ar cl ratio of the fluid is similar to that of mantle fluids turbid feldspar also released a fluid during crushing but the40ar cl ratio was lower probably as a result of evolution of the original fluid during boiling anomalously high apparent ages are often a feature of low temperature ar release in stepped heating age spectra of feldspars the presence of excess40ar in fluid inclusions may substantially affect the lower temperature release in feldspar stepped heating spectra the character of the microtextures in the host feldspar provides information on the temperature of fluid trapping because inclusions formed above the coherent alkali feldspar solvus have little or no effect on the exsolution textures whereas fluids trapped at low temperatures are associated with major structural rearrangements leading to development of patch perthite

poisson s ratio in the lower mantle beneath alaska evidence for compositional heterogeneity 1 using waveform cross correlation we have measured 17 000 p and s wave differential travel times from earthquakes and receivers near a great circle path from japan across alaska to north america we jointly invert the data for variations in shear and bulk sound speed and derived poisson s ratio variations in the mantle beneath 1000 km depth we explore the model space using different levels of regularization and our conclusions are based on a range of models that give acceptable data fits the correlation between vs and vp variations is good to 1500 km depth but gradually degrades in the bottom 1000 km whereas the ratio r lnvs lnvp is between 1 0 and 1 8 at most depths our data suggest that there is no significant correlation between bulk and shear wave speed but they appear anticorrelated in several regions we estimate the effect on the poisson s ratio of changes in temperature iron content and the magnesiowustite mw to perovskite pv ratio at all depths the effect of temperature is largest and that of the pv to mw ratio smallest our data cannot resolve the trade off between thermal and compositional effects but explaining the poisson s ratio variability by temperature alone would require unrealistic perturbations and it would not explain the inferred anticorrelation between shear and bulk sound speed our results suggest that despite r being significantly smaller than 2 5 we need a combination of thermal and compositional effects in order to explain the elastic parameter variations in the deep mantle beneath alaska with realistic values of t of 300 800 k variation in xfe of 4 and pv enrichment or depletion of up to 10 just sufficient for explaining the data

development of microporosity diffusion channels and deuteric coarsening in perthitic alkali feldspars turbidity is an almost universal feature of alkali feldspars in plutonic rocks and has been investigated by us in alkali feldspars from the klokken syenite using sem and tem it is caused by the presence of myriads of tubular micro inclusions either fluid filled micropores or sites of previous fluid inclusions and is associated with coarsening of microperthite and development of sub grains micropores are abundant in coarsened areas in which porosities may reach 4 5 but are almost absent from uncoarsened pristine braind microperthite areas the coarsening is patchy and involves a scale increase of up to 103 without change in the composition of the phases low albite and low microcline or in the bulk composition of the crystal it occurs abruptly along an irregular front within individual crystals which retain their original shapes the coherent braid microperthite gives way across the front to an irregular semi coherent film perthite over a few m and then to a highly coarsened irregular patch perthite containing numerous small sub grains on scales of a few hundred nm in both phases the coarsening and micropore formation occured at a t 400 450 c and it is inferred to have been driven by the release of coherent strain energy low angle grain boundary migration being favoured by a fluid the patchy nature of the coarsening and the absence of a relationship with initial grain boundaries suggest that the fluid was of local origin possibly arising in part through exsolution of water from the feldspar the sub grain texture and microporosity modify profoundly the permeability of the rock and greatly enhance the subsequent reactivity of the feldspars

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

petrography and composition of authigenic feldspars oligocene frio formation south texas abstract detrital feldspar assemblages 1 to 4 km burial depth in oligocene sandstones from south texas are extensively altered by dissolution in some samples cementation in secondary pores principally by albite but also by authigenic k feldspar and other minerals occurs subsequent to the dissolution and can lead to wholesale replacement textures solid state diffusion of cations has not been important in the replacement process information from conventional light microscopy sem examination and back scattered electron imaging suggests that relative rates of dissolution versus replacement varied widely among different detrital grains through time even on the scale of a thin section dissolution of detrital feldspars and precipitation of relatively pure albite and k feldspar take place ver a large depth interval and thus can be markedly separated in time vacuolized albite occurring as irregular veins within detrital grains is associated with little visible secondary porosity and forms when precipitation is rapid relative to dissolution of the detrital grain more rapid dissolution results in formation of larger scale intragranular secondary pores commonly filled by subsequent precipitation of clear albite within the former grain volume electron microprobe analyses show that the composition of vacuolized albite and authigenic k feldspar deviate somewhat from ideal compositions presumed typical for authigenic feldspars regardless of their cause these subtle variations in composition have important ramifications for basin scale mass balance calculations requiring accurate estimates of the volume of authigenic feldspar

re os molybdenite pyrite and chalcopyrite geochronology lupa goldfield southwestern tanzania tracing metallogenic time scales at midcrustal shear zones hosting orogenic au deposits a detailed re os molybdenite pyrite and chalcopyrite geochronology at five shear hosted au occurrences kenge mbenge porcupine konokono and dubwana in the lupa goldfield southwestern tanzania is reported in this paper au occurrences within the lupa goldfield share many geologic similarities with the orogenic au deposit type and are situated within a paleoproterozoic magmatic arc that intruded the archean tanzanian cratonic margin pyrite chalcopyrite molybdenite bearing fault fill veins and mylonitic shear zones crosscut granitic host rocks and are associated with the highest au grades re os sulfide ages are deemed a suitable proxy to constrain the timing of au based on the occurrence of au bearing minerals as inclusions within pyrite and chalcopyrite whereas au bearing minerals filling pyrite fractures may represent a younger and undated metallogenic event molybdenite at kenge occurs as ultrafine disseminations within fault fill veins 1953 6 ma n 3 that possess nominally older weighted average re os ages than molybdenite hosted by stylolite like veins 1937 8 ma n 7 both sample sets are ca 70 m y older than a weighted average re os pyrite age from the mylonitic shear zones at kenge and mbenge 1876 10 ma n 13 which contain fault fill veins and record the timing of mylonitization molybdenite at porcupine occurs as ultrafine disseminations within quartz veins and mylonitized granite samples 1886 6 ma n 4 that are broadly equivalent in age to weighted average re os ages of molybdenite occurring as stylolite like veins 1873 5 ma n 6 and pyrite within oblique extension veins 1894 45 ma n 2 weighted average re os pyrite model ages at konokono 1880 14 ma n 9 and dubwana 1905 25 ma n 2 are also consistent with the ca 1 88 ga event observed at kenge mbenge and porcupine gold occurrences in the lupa goldfield therefore record a protracted hydrothermal history 1 95 1 87 ga comprising at least three temporally distinct hydrothermal events ca 1 95 1 94 and 1 88 ga which are each represented in detail by a complex vein history that occurred at a time scale less than the resolution of the re os method the sampling of broadly contemporaneous sulfides from five shear zones suggest that mylonitic shear zones represented an interconnected network of midcrustal permeable fluid conduits at ca 1 88 ga that permitted the transportation and deposition of gold comparison between re os sulfide and high precision u pb zircon ages for the granitic host rocks provides unequivocal evidence for sulfidation concomitant with magmatism however the range of re os ages argues against an intrusion related deposit model whereby metallogenic fluids are solely derived from an individual intrusion the regional ca 1 88 ga metallogenic event identified as part of this study occurred concurrently with eclogite facies metamorphism during the ubendian orogenic cycle and provides one of earth s earliest temporal links between subduction zone processes and orogenic au deposit formation during the paleoproterozoic

subsolidus deuteric hydrothermal alteration of eudialyte in lujavrite from the pilansberg alkaline complex south africa abstract the most evolved rocks of the pilansberg alkaline complex are aegirine lujavrites in which three varieties of eudialyte are recognized on the basis of textural relationships and composition manganoan eudialyte i is a relict orthomagmatic phase occurring as poikilitic plates or as relict grains in pseudomorphed euhedral phenocrysts late eudialyte ii ranges in composition from manganoan eudialyte through kentbrooksite to taseqite like varieties and is considered to be formed by cation exchange with eudialyte i and alkaline fluids eudialyte iii is a hydrothermal phase replacing eudialyte ii and has either taseqite like 5 7 3 wt sro

low rhenium molybdenite by metamorphism in northern sweden recognition genesis and global implications abstract re os dating of molybdenite is an accurate means to date intrusions and intrusion related ore deposits using the model age or isochron approach but molybdenite has a new niche in the greenschist to granulite facies metamorphic environment re os ages for metamorphic molybdenite may be used to construct regional metamorphic histories age significance and accuracy are established by analyzing multiple molybdenite separates extracted from single petrographically characterized molybdenite occurrences in this study twelve geologically distinct molybdenite bearing samples from two small mo districts in northern sweden trace a 150 m y paleoproterozoic svecofennian metamorphic history from 1900 to 1750 ma these data reveal a little known widespread and protracted late svecofennian anatexis in northern sweden the kataberget mo cu f deposit is located in the moskosel granite batholith north of the economically renown skellefte district four different molybdenite samples from outcrop at kataberget indicate an intrusion age of 1895 6 ma with the formation of later pegmatite aplite at 1875 6 ma the allebuoda bjorntjarn and munka mo w deposits in the rappen district are represented by three outcrop and five drill core samples of molybdenite bearing aplite pegmatite granite these two deposits were previously described as intrusion related climax type mo mineralization re os ages for molybdenites from these deposits range from 1865 to 1750 ma and significantly re concentrations are markedly low extending to the sub ppm level age agreement within the deposits is conspicuously lacking whereas with one exception age agreement within any single sample geologic occurrence as established by analysis of additional molybdenite separates is very good these data together with fundamental geologic observations discussed in this paper suggest that mo w mineralization in northern sweden is not intrusion related but the local product of episodic melting of archean paleoproterozoic supracrustal gneisses related to the svecofennian orogeny petrographic traverses across the boundary between widespread foliation parallel units of aplitic to pegmatitic pink granite and hosting biotite gneiss directly capture the process of ore formation dehydration breakdown of zircon rich biotite aligned with the foliation in the gneiss is accompanied by formation of new pristine post deformational biotite plus sulfides oxides hydrothermal zircon and fluorite all associated with microcline dominant leucosomes this process has profound implication for the traditional leucogranite intrusion related genesis attributed to the broad classification of mo w sn base and precious metal mineralization e g south mountain batholith nova scotia okiep namaqualand south africa mactung yukon pogo liese tintina alaska carajas and goias rio tocantins brazil new england batholith nsw australia bergslagen sweden nevoria western australia alpeinerscharte austria erzgebirge germany sardinia corsica batholith in addition to biotite metallogenic contributions e g mo w sn u bi cu pb zn fe ni co au ag te as sb ree in various combinations may also be controlled by breakdown of amphibole in effect the trace element composition of dehydrating or recrystallizing components in a gneissic rock essentially defines the local and district metallogenic suite in the absence of focusing structures e g shear zones sheeted vein development this process will generally form small and disconnected subeconomic deposits with erratic and unpredictable grades low re content in associated molybdenite is a key indicator for a subeconomic origin by local melting of biotite gneiss mo w or muscovite schist sn w

ion microprobe study of intragrain micropermeability in alkali feldspars alkali feldspar cleavage fragments from the klokken layered syenite south greenland were heated to 700 c at 0 1 gpa in 99 h218o for 75 h these samples were then polished and imaged by ion microprobe for 18o the feldspars were known to contain areas of pristine braid micro perthite which were not turbid and areas of deuteric patch perthite which were turbid turbidity is related to the presence of micropores in the feldspars on imaging the grain it was found that the 18o had penetrated into the parts of the grain which were microporous and not into the pristine areas micropores are therefore responsible for rendering the feldspars permeable as well as porous the implications of micropermeable feldspars in several areas of geology are discussed

microtextural controls of weathering of perthitic alkali feldspars the relationship between the microtexture and dissolution behaviour of fresh hf acid etched and naturally weathered alkali feldspar phenocrysts from the lower devonian shap granite has been investigated by sem and tem a novel resin impregnation technique has revealed the three dimensional shape and interconnectivity of etch pits beneath the weathered crystal surface further electron microscope work suggests that shap phenocrysts are representative of the alkali feldspar in the protolith of many soils fresh and unweathered shap feldspars have a complex microtexture comprising areas of pristine cryptoperthite and lamellar microperthite cross cut by volumes of microporous altered feldspar or patch perthites cryptoperthites are made up of 75 nm wide albite films the platelets are coherent but albite films have numerous edge dislocations along their interface with orthoclase 001 and 010 cleavage surfaces intersect 2 3 edge dislocations m2 in three dimensions these edge dislocations form an orthogonal net in the murchison plane of easy fracture close to 601 patch perthites are irregular semicoherent to incoherent intergrowths of albite and irregular microcline subgrains with 0 65 0 70 sub m to m sized pores m2 microporous patch perthites form by dissolution reprecipitation reactions with magmatic or hydrothermal fluids and pores are present before the alkali feldspars enter the weathering regime dissolution of shap feldspars during natural weathering and laboratory acid etching is controlled by their microtexture especially by dislocations and exsolution lamellae the core and strain energy associated with dislocation outcrops on 001 and 010 cleavage surfaces promotes rapid dissolution at those sites and formation of nn sized etch pits after 30 s of laboratory etching with hf acid vapour with progressive hf etching crystallographically controlled differences in the reactivity of etch pit walls cause them to expand more rapidly into orthoclase than albite naturally weathered feldspars were collected from the glacial erratic boulders fine gravels surrounding exposed granite surfaces and from peat soil overlying the granite during natural weathering etch pits on microperthites enlarge almost exclusively by dissolution of albite and resin casts demonstrate that they can penetrate 15 m below the cleavage surface forming an interconnecting ladder like grid of submicrometer wide channels in the murchison plane coherent albite platelets and volumes of albite between dislocations in films dissolve uniformly but faster than orthoclase this is probably because the albite lamellae have significant elastic coherency strain but this is much less per unit volume of albite than the core and strain energy associated with edge dislocations patch perthites etch in hf vapour and weather rapidly in nature to produce a honeycomb like texture of interconnecting nanometer to micrometer sized pits which nucleate at preexisting micropores or incoherent subgrain boundaries the size and density of etch pits on microperthite surfaces which is determined by rates of growth and coalescence may be a useful progress variable for natural and experimental dissolution all alkali feldspars are highly heterogeneous materials whose chemical composition and microtexture can vary on a submicrometer scale these microtextures are critical variables with regard to the origin of surface roughness of fresh and weathered grains the controls on absolute dissolution rates and why they commonly change over time the nonlinear variation of dissolution rate with grain size the ratio of alkali ions released into solution and disparities between laboratory dissolution rates and those observed in the field

artificial weathering of spanish granites subjected to salt crystallization tests surface roughness quantification for hundreds of years two types of granite zarzalejo and alpedrete from the madrid region spain have been extensively used as building stones fresh specimens of both stone types have been sampled from their respective quarries and subjected to sodium sulphate salt crystallization test sct the resulting physical and chemical weathering patterns have been characterized by polarized light optical and environmental scanning electron microscopy water absorption under vacuum conditions and mercury intrusion porosimetry techniques were used to determine the pre and post sct porosity and pore size distribution the following non destructive techniques were performed to assess stone durability and decay ultrasound velocity us and surface roughness determination sr of intra and inter granular quartz feldspar and biotite minerals at the centre as well as at the corners and edges of specimen surfaces before the sct us values were lower and sr values higher in zarzalejo zar than alpedrete alp granite after sct the us values declined while sr rose in both types of granites with greater average differences in zar than alp for both parameters feldspar and biotite and their inter granular contacts were found to be the weakest and therefore the most decay prone areas of the stone the initial sr parameters were generally higher and rose more steeply after sct at the corners and around the edges of the specimens while behaviour was found to be similar in the two types of granite variations were greater in zar the less durable and more decay prone of the two surface roughness measurement of mineral grains in granite stones is a very useful in situ non destructive technique for quantifying salt crystallization mediated physical and chemical weathering the resulting quantification of decay and of related durability provides insight into the future behaviour of this type of stone commonly used in historic buildings

filling characteristics reservoir features and exploration significance of a volcanostratigraphic sequence in a half graben basin a case analysis of the wangfu rift depression in songliao basin ne china abstract it has long been considered that thicker volcanostratigraphic sequences should in theory contain more favourable reservoir conditions than in thinner sequences particularly within large scale stratovolcanoes found in sedimentary basins however drilling results from the wangfu rift depression wrd in the songliao basin china show that the thick intermediate volcanostratigraphic sequence contains only one or two layers of favourable reservoir characteristics over approximately 1000 m of volcanic successions the volcanostratigraphic sequence that is common in the graben basin has a particular distribution pattern as a result of reservoir and origin processes the eruptive interval unconformity boundary eiub is introduced and the burial history is investigated in order to determine the reservoir characteristics and origin we also identify what controls the gas pool accumulation in the thick intermediate volcanostratigraphy according to the reworked volcanic rocks and weathered crust patterns the volcanostratigraphy of the basin has 13 wells containing one eiub and 2 wells with two eiubs the exposed and burial histories of the volcanostratigraphic sequence have produced a variety of pore space including primary pores secondary pores and secondary fractures the secondary pores are the most dominant followed by primary pores and finally fractures these pore spaces form a reservoir with moderate to low porosity and permeability and the most productive reservoirs are located 40 80 m below the eiub the secondary pores are primarily controlled by weathering and leaching as well as deep burial alteration due to the migration of fluids from the overlying strata into the volcanic rocks of the huoshiling formation below the top most layer dissolves first which significantly enhances the secondary porosity of this layer found 40 80 m below the eiub thus the number of eiubs should control the number of favourable reservoir layers based on analysis of the reservoir source and cap rocks in the study area we conclude that the best exploration targets in these thick volcanostratigraphic sequences specifically in half graben basins are the volcanic inherited uplift slope areas followed by the inherited uplift of the sag area and the uplifted zones the findings presented in this work provide an insight of the evolution of porosity in other volcanic systems elsewhere and contribute to the likelihood of locating the favourable exploration targets for thick volcanostratigraphic sequences in half graben basins

age of granitic activity associated with copper molybdenum mineralization at malanjkhand central india the malanjkhand copper molybdenum deposit in the bhandara craton central india is hosted by a granite complex which consists of regionally dominant grey granitoid and pink granitoid confined to the mineralized zone new shrimp rg data on zircons from both granite types are inferred to have crystallized during the same magmatic pulse at ca 2 48 ga the discrepancy between zircon age and earlier obtained rb sr whole rock age is attributed to modification of the rb sr system by hydrothermal overprint similarity in petrographic features and chemical affinity in combination with identical age strongly indicate that the pink granite is the hydrothermally altered variety microclinization and silicification of the grey granite the spatially associated main cu mo mineralization event at malankhand appears to be broadly contemporaneous with and genetically related to the emplacement of the host granitoids at about 2 48 ga

40 ar 39 ar thermochronology using alkali feldspars real thermal history or mathematical mirage of microtexture the multiple diffusion domain mdd theory of ar loss based on the 40ar 39ar method has been used to obtain thermal histories over tens of millions of years in the range 500 c to near surface temperatures of alkali feldspars from slowly cooled geological environments and recovered in situ from sedimentary basins and geothermal systems it assumes that ar diffuses in nature over geological time from the same domains and by the same processes that control its diffusion during laboratory step heating experiments mathematically the model has many stringent requirements the domains must form at t above those being modelled and must remain unchanged during geological time and during step heating must not interact must occur in discrete size ranges sharing one simple geometrical shape contain no compositional gradients in 40k and release ar into an infinite reservoir we describe the nature of possible diffusion domains and the fast pathways for diffusion between them known to exist in alkali feldspars from direct electron microscopic observations and explore their time temperature evolution based on established phase behaviour both over geological time and during step heating strain controlled microtextures whose morphology is controlled by the minimization of coherency strain energy do not provide fast pathways for ar loss and behave as crystallographically complex but single domains for ar subregular dislocations may form over a range of t in some strain controlled intergrowths but in nature the majority form closed loops and do not provide pathways to the crystal surface deuteric microtextures cut across strain controlled microtextures and are composed of microporous subgrain and grain mosaics with many pathways and a wide spectrum of possible domain sizes and shapes the deuterically altered zones provide pathways for ar diffusion out of the crystal although some ar is retained in fluid inclusions all microtextures likely to define diffusion domains form below 500 c over a range of t and in slowly cooled rocks of times the most important involve fluid feldspar reactions which can reset ar and continue to surface t feldspars in geothermal systems and young sedimentary basins are subject to ongoing chemical and microtextural changes because the microtextures are metastable during step heating experiments in the t range over which most ar is released domains and pathways for rapid diffusion of ar between domains will be significantly modified during step heating particularly above 750 c when departures from a simple arrhenius diffusion law are most marked pre melting is likely to be an important cause of complexity in ar loss behaviour few if any of the requirements of the mdd model are met and we conclude that thermal histories cannot be obtained from alkali feldspars using this method

a survey of hydrous species and concentrations in igneous feldspars the hydrous components in 85 feldspars from various igneous environments spanning the range of naturally occurring compositions were examined with infrared spectroscopy the feldspars contain structural oh 0 512 ppm h 2o h 2o 0 1350 ppm h 2o and nh 4 0 1500 ppm nh 4 groups as well as fluid inclusions and alteration products although composition and structure do influence the type of hydrous species that can be incorporated into a particular feldspar mineral the concentration of these species is not controlled by major element composition coarse perthitic microclines have a heterogeneous distribution of hydrous species and contain h 2o or nh 4 in k rich lamellae and fluid inclusions in na rich areas the structural oh in plagioclase feldspars is not associated with twin boundaries or exsolution lamellae all of the possible structural hydrous species are found in pegmatite feldspars whereas volcanic feldspars contain only structural oh the variation in oh concentration within a given feldspar composition suggests that fluids in the geologic environment play a role in determining the hydrogen concentration of each sample the vast majority of plutonic feldspars have undergone partial or total equilibration with meteoric fluids during low temperature 400 150 c hydrothermal exchange obliterating any structural hydrogen and creating substantial concentrations of fluid inclusions up to 4000 ppm h 2o in the exchanged regions the amount of water stored as fluid inclusions within feldspars in the upper crust 1 10 19 kg is small compared to the 1 35 10 21 kg of water in the oceans but is roughly equivalent to the reservoir of water stored in hydrous minerals in the upper crust

molybdenite re os dating constrains gravitational collapse of the sveconorwegian orogen sw scandinavia abstract re os dating of molybdenite from small deposits is used to define crustal domains exhibiting ductile versus brittle behaviour during gravitational collapse of the sveconorwegian orogen in sw scandinavia a 1019 3 ma planar quartz vein defines a minimum age for brittle behaviour in central telemark in rogaland vest agder molybdenite associated with deformed quartz and pegmatite veins formed between 982 3 and 947 3 ma in the amphibolite facies domain three deposits and between 953 3 and 931 3 ma west of the clinopyroxene in isograd two deposits in the vicinity of the 0 93 0 92 ga rogaland anorthosite complex the data constrain the last increment of ductile deformation to be younger than 0 95 and 0 93 ga in these two metamorphic zones respectively molybdenite is the product of an equilibrium between biotite oxide and sulfide minerals and a fluid or hydrated melt phase after the peak of 1 03 0 97 ga regional metamorphism molybdenite precipitation is locally episodic a model for gravitational collapse of the sveconorwegian orogen controlled by lithospheric extension after 0 97 ga is proposed in the west of the orogen the rogaland vest agder sector is interpreted as a large shallow gneiss dome formed slowly in two stages in a warm and structurally weak crust the first stage at 0 96 0 93 ga was associated with intrusion of the post collisional hornblende biotite granite suite the second stage at 0 93 0 92 ga restricted to the southwesternmost area was associated with intrusion of the anorthosite mangerite charnockite suite most of the central part of the orogen was already situated in the brittle upper crust well before 0 97 ga and did not undergo significant exhumation during collapse in the east of the orogen situated against the colder cratonic foreland exhumation of high grade rocks of the eastern segment occurred between 0 97 and 0 95 ga and included preservation of high pressure rocks but no plutonism

high pressure elastic properties of major materials of earth s mantle from first principles the elasticity of materials is important for our understanding of processes ranging from brittle failure to flexure to the propagation of elastic waves seismologically revealed structure of the earth s mantle including the radial one dimensional profile lateral heterogeneity and anisotropy are determined largely by the elasticity of the materials that make up this region despite its importance to geophysics our knowledge of the elasticity of potentially relevant mineral phases at conditions typical of the earth s mantle is still limited measuring the elastic constants at elevated pressure temperature conditions in the laboratory remains a major challenge over the past several years another approach has been developed based on first principles quantum mechanical theory first principles calculations provide the ideal complement to the laboratory approach because they require no input from experiment that is there are no free parameters in the theory such calculations have true predictive power and can supply critical information including that which is difficult to measure experimentally a review of high pressure theoretical studies of major mantle phases shows a wide diversity of elastic behavior among important tetrahedrally and octahedrally coordinated mg and ca silicates and mg ca al and si oxides this is particularly apparent in the acoustic anisotropy which is essential for understanding the relationship between seismically observed anisotropy and mantle flow the acoustic anisotropy of the phases studied varies from zero to more than 50 and is found to depend on pressure strongly and in some cases nonmonotonically for example the anisotropy in mgo decreases with pressure up to 15 gpa before increasing upon further compression reaching 50 at a pressure of 130 gpa compression also has a strong effect on the elasticity through pressure induced phase transitions in several systems for example the transition from stishovite to cacl2 structure in silica is accompanied by a discontinuous change in the shear s wave velocity that is so large 60 that it may be observable seismologically unifying patterns emerge as well eulerian finite strain theory is found to provide a good description of the pressure dependence of the elastic constants for most phases this is in contrast to an evaluation of birch s law which shows that this systematic accounts only roughly for the effect of pressure composition and structure on the longitudinal p wave velocity the growing body of theoretical work now allows a detailed comparison with seismological observations the athermal elastic wave velocities of most important mantle phases are found to be higher than the seismic wave velocities of the mantle by amounts that are consistent with the anticipated effects of temperature and iron content on the p and s wave velocities of the phases studied an examination of future directions focuses on strategies for extending first principles studies to more challenging but geophysically relevant situations such as solid solutions high temperature conditions and mineral composites

transient porosity resulting from fluid mineral interaction and its consequences the term porosity is very widely used in geosciences and normally refers to the spaces between the mineral grains or organic material in a rock measured as a fraction of the total volume these spaces may be filled with gas or fluids and so the most common context for a discussion of porosity is in hydrogeology and petroleum geology of sedimentary rocks while porosity is a measure of the ability of a rock to include a fluid phase permeability is a measure of the ability for fluids to flow through the rock and so depends on the extent to which the pore spaces are interconnected the distribution of pores and pore neck size as well as on the pressure driving the flow this chapter will be primarily concerned with how reactive fluids can move through tight rocks which have a very low intrinsic permeability and how secondary porosity is generated by fluid mineral reactions a few words about the meaning of the title will help to explain the scope of the chapter 1 fluid mineral interaction when a mineral is out of equilibrium with a fluid it will tend to dissolve until the fluid is saturated with respect to the solid mineral we will consider fluids to be aqueous solutions although many of the principles described here also apply to melts the generation of porosity by simply dissolving some minerals in a rock is one obvious way to enhance fluid flow dissolution of carbonates by low ph solutions to produce vugs and even caves would be one example however when considering the role of fluid mineral reaction during metamorphism the fluid provides mechanisms that enable re equilibration of the rock i e by replacing one assemblage of minerals by a more stable assemblage this not only involves the dissolution of the parent mineral phases but the

zircons from syros cyclades greece recrystallization and mobilization of zircon during high pressure metamorphism zircons were studied from high pressure low temperature metamorphosed meta igneous lithologies from syros these rocks carry several zircon generations related to each other by dissolution reprecipitation processes one generation is pristine zircon that shows growth zoning relatively elevated contents of trivalent cations and high th u ratios the other end member is a skeletal zircon generation with negligible trivalent cation contents and low th u ratios 0 1 texturally between these two there is a range of zircon crystals with complex inclusion populations of y hree th phases and fluid inclusions showing variable progress of replacement recrystallization both end members yield distinct sensitive high resolution ion microprobe shrimp u pb ages the pristine generation has an age of 80 2 1 6ma from a metagabbro and 76 4 2 1ma from a meta plagiogranite dyke the skeletal low th u zircon generation yields an age of 52 4 0 8ma the older late cretaceous zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust the younger eocene age however dates a zircon recrystallization event which possibly coincides with high solubility and mobility of high field strength elements in a high pressure aqueous fluid phase intergrowth relations between zircon and peak metamorphic garnet and excellent agreement of the u pb ages with white mica ar ar ages for the same samples support the conclusion that eocene is the true age of high pressure metamorphism on syros

thermodynamically consistent decompression implications for lower mantle composition estimates of the composition of the lower mantle are commonly obtained by a process of adiabatic decompression in which zero pressure mantle parameters are determined with a least squares fit of an equation of state to mantle densities we show that this procedure may lead to biased estimates of the zero pressure density and compressibility of the mantle the bias can be eliminated or minimized by subjecting the equation of state fit to a thermodynamic constraint at zero pressure such inversions provide equations of state that are compatible with the lower mantle as well as the thermal properties of selected mineral models we used this method to evaluate assemblages containing mg fe sio3 perovskite and mg fe o with fe fe mg ratios as high as 0 2 thermodynamic systematics were used to place bounds on the anderson gruneisen parameter s for perovskite we find a fairly large range of acceptable models for the composition of the lower mantle they range from uppermantle like compositions to assemblages that are enriched in both si and fe relative to the upper mantle however a pure perovskite lower mantle whether enriched in fe or not seems improbable small fe enrichments of the order of 1 given density contrasts across the 670 km discontinuity that are probably insufficient to prevent convective mixing of the upper and lower mantles we therefore suggest that unless the mantle is homogeneous in its major oxide components the lower mantle is likely to be enriched in fe by at least several percent

first principles computation of mantle materials in crystalline and amorphous phases abstract first principles methods based on density functional theory are used extensively in the investigation of the behavior and properties of mantle materials over broad ranges of pressure temperature and composition that are relevant a review of computational results reported during the last couple of decades shows that essentially all properties including structure phase transition equation of state thermodynamics elasticity alloying conductivity defects interfaces diffusivity viscosity and melting have been calculated from first principles using mgo the second most abundant oxide of earth s mantle as a primary example and considering many other mantle materials in their crystalline and amorphous phases we have found that most properties are strongly pressure dependent sometimes varying non monotonically and anomalously with the effects of temperature being systematically suppressed with compression the overall agreement with the available experimental data is excellent it is remarkable that the early calculated results such as shear wave velocities of two key phases mgo and mgsio 3 perovskite were subsequently reproduced by experimentation covering almost the entire mantle pressure regime as covered in some detail the defect formation and migration enthalpies of key mantle materials increase with pressure the predicted trend is that partial mgo schottky defects are energetically most favorable in mg silicates but their formation enthalpies are high so the diffusion in the mantle is likely to be in the extrinsic regime preliminary results on mgo and forsterite hint that the grain boundaries can accommodate point defects including impurities and enhance diffusion rates at all pressures the structures are highly distorted in the close vicinity of the defects and at the interface with excess space recent simulations of mgo sio 2 binary and other silicate melts have found that the melt self diffusion and viscosity vary by several orders of magnitudes with pressure temperature and composition the predicted high compressibility and complex dynamical behavior can be associated with structural changes involving non bridging oxygen oxygen tri clusters si o pentahedra etc occurring on compression we envision future prospect for massively parallel distributed computing of unprecedented magnitude and scope in the study of relevant materials for earth super earth and other planets a renewed computational theme perhaps should be the first principles simulations of large systems with long runs that are necessary to explore realistic natural compositions polycrystalline phases multi component melts crystal melt interfaces trace element partitioning etc

the malanjkhand copper molybdenum deposit india mineralization from a low temperature ore fluid of granitoid affiliation copper and subordinate molybdenum mineralization at malanjkhand occurs within a fracture controlled quartz reef enclosed in a pink granitoid body surrounded by grey granitoids constituting the regional matrix sulfide bearing stringers pegmatites with only quartz microcline and sulfide disseminations all within the pink granitoid represent other minor modes of occurrences despite this diversity in mode of occurrence the mineralogy of ores is quite consistent and conform to a common paragenetic sequence comprising an early ferrous stage of precipitation of magnetite i and pyrite i and the main stage chalcopyrite mineralization with minor sphalerite pyrite ii magnetite ii molybdenite and hematite both stages witnessed continuous precipitation of quartz microcline chlorite biotite and epidote the enclosing pink granitoid and the regional grey granitoids display alteration features such as saussuritization of plagioclase breakdown of hornblende and chloritization of biotite on a regional scale indicating interaction with a pervasive fluid quartz and microcline precipitation mostly restricted within the pink granitoid postdates this alteration four types of primary inclusions were encountered in quartz from ore samples 1 type i aqueous biphase l v inclusions the commonest variety in all ore types 2 type ii aqueous carbonic laq lcarb vcarb 3 type iii pure carbonic lcarb vcarb type ii and iii being restricted to stringer and pegmatitic ores and 4 rare polyphase laq vaq calcite gypsum inclusions quartz in granitoids contain primary type i inclusions only type i inclusions from ore samples furnish a temperature range after a rough pressure correction to the th maxima of 140 180 c of 150 275 c and a moderately low salinity of 4 12 wt nacl equivalent this is inferred to represent the signature of the major component f2 of the ore fluid a few type i inclusions of higher tm up to 380 c and low salinity and density represent the other f1 identifiable component of the ore fluid present in low proportion the th maxima and the total range in salinity of type i inclusions in quartz from granitoids are strikingly similar to those from the ore samples composition of syn ore chlorites furnished a temperature range of 185 327 c which conforms to the fluid inclusion microthermometric data pressure estimates using standard fluid inclusion geobarometric methods vary from 550 to 1790 bar in the stringer ores observed temperature salinity density relationships are best explained by a two stage evolution model of the ore fluid the first stage witnessed mixing of the two components f1 and f2 in unequal proportion bringing about mineralization the second stage of evolution was marked by the separation of a carbonic component on continued sulfide precipitation and attendant increase in salinity of the fluid the f1 component emerged as a distinct heated and co2 s charged entity due to steam heating and contamination of the early ingressed f2 fluid at the fracture zone the pervasive fluid phase in the surrounding granitoids contributed the f2 component

argon diffusion domains in k feldspar i microstructures in mh 10 samples of mh 10 a k feldspar thought to contain discrete diffusion domains for argon have been examined by light and tem microsopy to search for domain boudaries we examined both unheated k feldspar separates and samples heated in vacuum between 750 and 1150 c in order to characterize the initial microstructure as well as any changes resulting from laboratory heating the mh 10 rock shows almost no evidence of plastic deformation and the k feldspar does not possess networks of dislocations which might define a subgrain structure derived from external stresses three classes of substructure are present in the k feldspar but only the third appears to have been affected by laboratory heating 1 cross hatched extinction in light microscope is common but variably developed tem reveals almost no albite pericline twinning but tweed microstructure is ubiquitous 2 turbid zones occur with about 5 volume abundance micropores 1 to 2 m in diameter characterize these regions which often also contain bleds of albite up to 40 m indiameter tem shows these turbid zones to be very complex with intricate twin and tweed structures at the sub micron scale and numerous dislocation and strain features 3 albitic exsolution lamellae 1 m thick 20 m long and separated by 1 m are rarely seen in the light microscope but tem shows that 0 01 m thick by 0 2 1 m long lamellae separated by 0 1 to 0 5 m occur in about 20 of the sample these lamellae are disk shaped have a semi coherent or coherent relationship to the host k feldspar and seem to disappear upon heating to 750 c extensive diffusion studies have been conducted on hm 10 and the results analyzed in terms of distribution of sizes of diffusion domains the preferred result of the analyses was a distribution consisting of three distinct sizes the largest domain size identified in this way is approximately 50 to 100 m and this corresponds to blocks of k feldspar defined by the network of fractured turbid zones the smallest domain size inferred from diffusion analysis is 0 1 m which corresponds to the spacing between albite exsolution lamellae that may represent fast diffusion pathways alternatively the modulated argon distribution accompanying the lamellar variation in feldspar composition may result in an overestimation of the initial diffusion coefficients even though a promising agreement exists between domain analyses based upon diffusional and microstructural analyses at least two questions remain unanswered firstly no clear candidate has been identified for a domain of intermediate size secondly the diffusion analysis has been formulated with independent gas release from all domains whereas the observed structures are likely to be nested i e small domains exist inside bigger domains with the network of semi coherent lamellar boundaries having connectivity and permeability which can provide only a modest increase in argon diffusivity over bulk values

field evidence of magma mixing from microgranular enclaves hosted in palaeoproterozoic malanjkhand granitoids central india abstract the malanjkhand granitoids mg pluton about 1500 sq km occurs in the balaghat district of madhya pradesh the mg 2400 ma represent an episode of palaeoproterozoic felsic magmatism in central india and hosts potential cu mo au deposits the enclaves hosted in mg can be broadly classified into two categories microgranular enclaves dark coloured fine grained magmatic and xenoliths of country rocks the microgranular enclaves me may be rounded ellipsoidal discoid elongated lenticular or tabular and their size commonly reaches up to 2 metres across the me have sharp and in places diffuse contacts with their host granitoids the shape and size of me indicate contemporaneous flow and mingling of partly crystalline felsic mafic magmas some me exhibit dark crenulated margins giving them a pillow like form that has been attributed to undercooling of a me magma as globules intruded into a granitoid magma the presence of corroded felsic and mafic minerals xenocrysts in me is interpreted as the result of mechanical transfer during the mafic felsic magma interaction and mixing event mafic minerals biotite rim the quartz xenocrysts giving rise to ocellar texture which exhibit signatures of resorption under hybrid enclave magma conditions all these features suggest an origin for the calc alkaline intermediate granitoid magma in malanjkhand involving a magma mixing process

the problem of dating high pressure metamorphism a u pb isotope and geochemical study on eclogites and related rocks of the marianske lazně complex czech republic this study presents new geochemical major and trace element nd sr isotope and u pb zircon monazite titanite and rutile data for various rock types eclogite high pressure granulite amphibolite orthogneiss leucosome of the high grade metamorphic marianske lazn complex in the western bohemian massif concordant u pb zircon analyses from the mafic to intermediate samples disclose the marianske lazn complex as a mixture of c 540 ma oceanic rocks juxtaposed at depth with lower crustal rocks of the structurally overlying tepla barrandian unit this interpretation refutes earlier models in which the marianske lazn complex was interpreted as a dismembered cambro ordovician ophiolite complex affected by variscan subduction metamorphic zircon in mafic rocks of the marianske lazn complex as well as monazite from orthogneiss and titanite in leucosome yield ages around 380 ma on the basis of detailed petrography and cathodoluminescence imaging we conclude that the c 380 ma age reflects the timing of variscan exhumation and associated decompression melting under upper amphibolite facies to granulite facies conditions thereby the data derived from metamorphic zircon of eclogites and high pressure granulite unexpectedly do not date the timing of eclogitization which could have happened just before variscan exhumation or even shortly after late cadomian protolith formation

comparative petrogenesis and tectonics of paleoproterozoic malanjkhand and dongargarh granitoids central india abstract the malanjkhand granitoid mg and two units of dongargarh granitoids dgs represent contemporaneous episodes of paleoproterozoic felsic magmatism in the bastar craton central india a comparative geochemical study is carried out between the mineralized malanjkhand granitoid mg and barren dongargarh granitoids dgs that occur adjacent to each other separated by the dongargarh group of rocks major element oxides geochemistry reveals that mg is granite granodiorite whereas dg is strictly granite in composition and both plutons show calc alkaline affinity geochemical discrimination indicates that mg is an i type whereas an a type affinity can only be suspected in case of dg and both can be labeled as peraluminous trace element data from these paleoproterozoic granitoids indicate that they evolved in a tectonic environment similar to continental rift setting hfse ratios suggest variable degree of fractionation of highly differentiated granitic magma was a very complex crystallization process results in the formation of these two granitoids by common mechanism possibly fluid separation might be involved highly fractionated ree and depleted mree patterns with eu anomaly absent in mg and negative in dg inferred that the parental magma derived by low degree partial melting of heterogeneous mafic source probably amphibolitic in nature followed by progressive differentiation zircon saturation temperature and zr content reveals that mg crystallized from wet melt whereas dg crystallized from relatively dry melt but both are classify as hot granites thermobarometric calculations indicates that mg has emplaced at upper level whereas dg at relatively deeper or middle upper level in the continental crust the origin of palaeoproterozoic granitoids in central india has attributed to the magma mixing process between crustal derived felsic and mafic magma contributed by basaltic underplating in a continental rift environment

distribution of melt along the east pacific rise from 9 30 to 10 n from an amplitude variation with angle of incidence ava technique author posting the author s 2015 this article is posted here by permission of the royal astronomical society for personal use not for redistribution the definitive version was published in geophysical journal international 203 2015 1 21 doi 10 1093 gji ggv251

microstructural evolution during experimental albitization of k rich alkali feldspar crystals of k feldspar c2 m in contact with highly concentrated aqueous nacl solutions at 500 c and 200 mpa are pseudomorphically replaced by high albite c bar 1 as a result of an interface coupled dissolution reprecipitation process the reaction occurs at an extremely sharp reaction front 10 nm and involves the complete breakdown of the initial framework structure this results in the release of tetrahedrally incorporated elements such as fe3 and ti4 and a significant increase in si al disorder across the reaction interface the evolving microstructure is controlled by crystallographic relations between the phases this leads to highly anisotropic sawtooth shaped intergrowths of albite and initial k feldspar resulting in the least structural misfit between the two framework structures as a result the newly formed interfaces appear to be semicoherent and cracks across the reaction fronts even indicate elastic strain the reaction produces 2 distinctive albite types albite 1 and albite 2 both are polycrystalline with albite 2 showing significantly larger subgrain sizes this indicates a secondary coarsening step driven by the reduction in interfacial energy within the polycrystalline replacement product the reaction also produces a highly porous rim however the porosity is not evenly distributed resulting in a porous albite 1 and a non porous albite 2 that mostly surrounds large euhedral pores despite the substantial volume fraction of porosity in albite 1 no significant 3d interconnectivity could be detected making the presence of a pervasive porosity unlikely however the result of coarsening is the continuous modification of the 3d porosity distribution this could potentially provide a mechanism for fluid transport through the replacement rim until textural and chemical equilibration is achieved

dissecting through the metallogenic potentials of precambrian granitoids case studies from the bastar and eastern dharwar cratons india

lattice preferred orientation in deformed polycrystalline mg fe o and implications for seismic anisotropy in d abstract magnesiowustite mg fe o is an important constituent of the lower mantle probably occupying about 20 25 of its volume laboratory and theoretical studies have shown this mineral to be highly elastically anisotropic at lower mantle pressures and temperatures thus strain induced formation of lattice preferred orientation lpo in magnesiowustite is a candidate mechanism for the origin of anisotropic structure in d although observations of seismic anisotropy within d are robust both the occurrence and the style of that anisotropy are spatially variable two hypotheses have been offered to explain the observations of d anisotropy lpo of intrinsically anisotropic minerals or shape preferred orientation spo perhaps in the form of horizontal layering or oriented inclusions to investigate the first hypothesis we performed confined simple shear deformation experiments in the dislocation creep regime using a gas medium deformation apparatus over a range of compositions the mgo and feo endmembers and three intermediate compositions samples were deformed at 1273 1473 k at a confining pressure of 300 mpa to large shear strains 3 5 4 5 using deformation pistons cut at 45 after deformation the lpo was measured by electron backscatter diffraction ebsd the lpo produced varied for differing compositions indicating the activity on individual slip systems and or the nature of grain boundary migration in mg fe o are affected by composition and or homologous temperature we predicted seismic anisotropy from the measured lpos and theoretically determined single crystal elastic constants anisotropic behavior predicted from lpo agrees well with observations of d anisotropy so the lpo hypothesis appears to satisfy the seismological constraints our calculated anisotropy patterns suggest that if d anisotropy is due to lpo of mg fe o then azimuthal variations in anisotropy in the horizontal plane should be present such azimuthal variations are not generally predicted for spo type hypotheses and this may provide a means for distinguishing the cause of d anisotropy

diagenetic albitization of detrital k feldspar in jurassic lower cretaceous and tertiary clastic reservoir rocks from offshore norway i textures and origin abstract the origin of albitized grains in sandstones has been the topic of considerable discussion in recent years some workers believe that the albitization of feldspar can occur in situ during burial diagenesis but others argue that albitization is a feature inherited from source clastic grains our petrological and chemical studies of subsurface samples of the jurassic lower cretaceous and tertiary reservoir sandstones from offshore norway show that detrital grains of potassium feldspar have been albitized during burial diagenesis criteria suggesting that the albitization has occurred during burial diagenesis include 1 euhedral habit of albite crystals with sharp edges and comers and markedly smooth crystal faces 2 generally untwinned albitized grains mostly riddled with minute br wnish inclusions 3 lack of cathodoluminescence in albite 4 homogeneous and pure albite composition 99 ab 5 absence of albitized grains in carbonate cemented zones and 6 increase in the percentage and degree partial to complete of albitized k feldspar with depth under sem albitized grains display two types of fabrics type i albitized grains are composed of numerous tiny albite crystals 1 30 m growing along cleavage planes in parent grains and they have abundant dissolution porosity while type ii albitized grains show blocky albite crystals 5 80 m forming pseudomorphs of detrital k feldspar and they lack any dissolution porosity type i albitized grains are found commonly at shallower depths between 2 2 and 3 km and temperatures varying from 65 to 90 c whereas type ii albitized grains dominate at greater depths 3 5 km and at temperatures 90 c albitization proceeds by a dissolution precipitation mechanism at shallower depth dissolution of the parent k feldspar is more rapid than the precipitation of albite and rates of albite precipitation increase with depth favoring pseudomorphic replacement of k feldspars other factors that influence albitization processes are ana ak permeability reduction and surface dissolution kinetics of the parent grain

kaolinisation of granite in an urban environment under natural acid and wet conditions one of the main weathering processes affecting granitic rocks is the kaolinisation of na ca and k feldspar minerals by h2o co2 attack we here report the occurrence of authigenic kaolinite on the surface of an eighteenth century granitic monument covered with sulphate rich weathering patinas we suggest that in humid mesothermic climates anthropogenically derived gaseous so2 from air pollution is responsible for accelerated kaolinisation of feldspars in an urban environment so2 from air pollution thus plays a dual role in the weathering of silicate rocks being responsible for the well known sulphation of ca bearing materials leading to the formation of sulphate salts such as gypsum as well as the weathering of feldspar minerals to kaolinite the kaolinisation reaction weakens the granite microfabric leading to enhanced decay of the building stone

stable isotope thermometry at high temperatures determination of accurate temperatures for geological events has been the grail of stable isotope geochemistry since the seminal 1947 paper by urey in theory this should be simple calibrated mineral pairs are common analysis is rapid and there is no significant pressure correction however in spite of widespread application the promise of reliable thermometry has been elusive stable isotope temperatures in metamorphic and igneous rocks are often controversial is the fractionation between two phases a thermometer speedometer rate dependant hygrometer p h2o dependant or chimera a number of factors have contributed to this uncertainty including incomplete and sometimes conflicting calibration of isotope fractionation factors limited understanding of the kinetics of mineral exchange and the lack of microanalytical techniques this situation has improved dramatically over the past decade permitting reliable and detailed thermal histories to be inferred in a landmark paper bottinga and javoy 1975 empirically calibrated the oxygen isotope fractionation factors among ten rock forming minerals at t 500 c their compilation shows that 84 of analyzed samples contain at least three minerals meeting a concordance test for equilibrium they concluded the great majority of igneous and metamorphic rocks has conserved a state of oxygen isotope exchange equilibrium if correct this bodes well for thermometry however this conclusion was contested by deines 1977 who critically examined the existing data and concluded that less than half of the rocks analyzed to date would yield concordant 18o derived temperatures while seeming irreconcilable both conclusions are based on fact all rocks contain some degree of disequilibrium but many also preserve a thermometric record it is the purpose of this review to discuss the new strategies for reliable isotope thermometry and to explore disequilibrium processes so they may be recognized and properly interpreted the organization of this paper will be to discuss the assumptions

provenance studies from 40ar 39ar dating of mineral inclusions in diamonds methodological tests on the orapa kimberlite botswana abstract previous studies indicate that clinopyroxene inclusions extracted from their host diamonds give 40ar 39ar ages approaching the time of source kimberlite eruption this behaviour is attributed to diffusion of argon to the diamond inclusion interface region during mantle residence with subsequent loss of this component on cleaving of the diamond to release the inclusion s in this study we investigate the potential of the 40ar 39ar inclusion dating method as a niche provenance tool as diamondiferous kimberlites are uncommon igneous rocks largely restricted to archaean cratons and have limited age ranges provenance studies of detrital diamond occurrences should provide unique information on craton erosional histories palaeo drainage evolution and related basin development as a methodological test 40ar 39ar step heating and in vacuo crushing experiments were conducted on 50 eclogitic clinopyroxene inclusions extracted from diamonds from the 93 ma orapa kimberlite in botswana low temperature steps and crushing experiments produced anomalously old ages up to 2 98 0 10 ga consistent with partial retention of pre eruption inherited argon in sub micron defect sites high temperature fusion steps yielded significantly younger ages although only 35 are within error of the time of orapa kimberlite eruption however 77 of results are within 50 ma and 92 within 100 ma of the orapa eruption age these results mandate that individual fusion ages be treated as maximum estimates of source kimberlite eruption ages with analyses of several 6 inclusions required to date single age detrital populations for multi source diamond deposits only the youngest detrital population or populations separated by 100 ma will be resolvable despite these limitations we conclude that the 40ar 39ar inclusion dating method is still capable of resolving key provenance questions such as the source of detrital diamond deposits in southern africa where controversy exists as to whether the diamonds originated from proximal permo carboniferous glacial deposits or distal cretaceous jurassic kimberlites finally a model is presented to account for the variable retention of pre eruption argon in clinopyroxene inclusions extracted from their host diamonds the proportion of this component retained by inclusions is considered dependent on interface porosity volume partitioning of argon into defect structures and the timing of stress fracture development around inclusions during entrainment to surface

implications of new upb zircon chronology to early proterozoic crustal accretion in northern sweden abstract u pb ages ranging from 1 89 to 1 78 ga are reported for zircon populations formed during major magmatic events in the proterozoic development of northern sweden the studied area includes parts of the continental and the marine svecokarelian domains of the baltic shield the ages record crustal accretion and reactivation off the margin of an exposed archaean craton and show that magmatism was broadly coeval throughout the entire north swedish portion of the svecokarelian province irrespective of regions involved a detailed chronostratigraphy is presented for the skellefte district of northern sweden and comparisons are made with the evolution in other parts of the svecokarelian province two main magmatic phases commencing at 1 89 and 1 79 ga ago are recognized the first represents large scale volcanism and plutonism which in the skellefte district was characterized by the introduction of mainly mantle derived magmas or by magmas formed from rocks with short crustal residence time in the vicinity of the archaean craton some distance to the north the crust generating processes included the mobilization of archaean sources and appear to have continued until c 1 85 ga ago a relatively long period of consolidation and denudation separates this magmatic phase from the one at c 1 79 ga ago the generation of these later migmatite associated granites in the svecokarelian province coincided in time and was possibly associated with those processes which gave rise to the initial plutonism in the neighbouring transscandinavian granite porphyry belt