fracture characterization using frequency dependent shear wave anisotropy analysis of microseismic data summary the presence of fractures in hydrocarbon reservoirs can enhance porosity and permeability and consequently increase production the use of seismic anisotropy to characterize fracture systems has gained much interest in the last two decades however estimating fracture sizes from observations of seismic anisotropy has not been possible recent work has shown that frequency dependent anisotropy fda is very sensitive to the length scale of the causative mechanism for the anisotropy in this study we observe fda in a microseismic data set acquired from a carbonate gas field in oman the frequency dependent shear wave anisotropy observations are modelled using a poroelastic model which considers fluid communication between grain size pore spaces and larger scale fractures a grid search is performed over fracture parameters radius density and strike to find the model that best fits the real data the results show that fracture size varies from the microscale within the shale cap rocks to the metre scale within the gas reservoir to the centimetre scale within the non producing part of the carbonate formation the lateral variation in fracture density agrees with previous conclusions from ordinary shear wave splitting sws analysis cumulatively the results show the potential for characterizing fracture systems using observations of fda

ca eskola component in clinopyroxene experimental studies at high pressures and high temperatures in multianvil apparatus abstract in order to investigate the pressure implications of silica exsolution from clinopyroxene in ultrahigh pressure metamorphic rocks we have measured systematically the values of the calcium eskola ca esk component of clinopyroxenes over a considerable range of pressure and temperature a series of anhydrous experiments using powdered glass of composition wt sio2 59 29 al2o3 23 12 mgo 5 44 cao 10 37 and na2o 1 78 were conducted in a walker style multianvil apparatus in the p t range of 6 12 gpa and 900 1200 c we find that all clinopyroxenes synthesized in equilibrium with garnet kyanite and sio2 coesite or stishovite are cation deficient the highest value of the ca esk component 32 38 mol is reached at 6 gpa followed by slight decrease beginning at 8 gpa as the pyroxene progressively dissolves into garnet and more rapid decrease at 10 12 gpa when stishovite becomes stable in the assemblage comparison of our data with those available in the literature which all together covers a wider range of p and t shows that the ca esk activity depends strongly on bulk chemistry especially the silica content in general integrated analyses of published experiments on similar compositions show that the ca esk component rises rapidly to its highest values from 1 to 4 6 gpa then slightly decreases to 9 gpa followed by remarkable decrease to 15 gpa culminating in replacement of pyroxene by garnet in equilibrium with stishovite we conclude that the sio2 exsolution lamellae observed in clinopyroxenes from many ultrahigh pressure terranes can be explained by destabilization of the ca esk component during decompression however because silica solubility in clinopyroxene reaches its maximum at 6 8 gpa the practical upper pressure limit that can be inferred from this system is 200 250 km

effective medium theories for fluid saturated materials with aligned cracks there is general agreement between different theories giving expressions for the overall properties of materials with dry aligned cracks if the number density of cracks is small there is also very fair agreement for fluid filled isolated cracks however there are considerable differences between two separate theories for fluid filled cracks with equant porosity comparison with recently published experimental data on synthetic sandstones gives a good fit with theory for dry samples however although the crack number density in the laboratory sample is such that first order theory is unlikely to apply expressions correct to second order in the number density provide a worse fit it also appears that the ratio of wavelength to crack size is not sufficiently great for any detailed comparison with effective medium theories which are valid only when this ratio is large the data show dispersion effects for dry cracks and scattering neither of which will occur at sufficiently long wavelengths data from the water saturated samples indicate that the effect of equant porosity is significant although the two theories differ strongly as to just how significant once again and in spite of the reservations mentioned above a reasonable fit between theory and observation can be shown

fluid rock interactions during uhp metamorphism a review of the dabie sulu orogen east central china comprehensive review on the characteristics of petrology oxygen isotope fluid inclusion and nominally anhydrous minerals nams for many dabie sulu ultrahigh pressure uhp metamorphic rocks including drill hole core samples reveals that fluid has played important and multiple roles during complicated fluid rock interactions attending the subduction and exhumation of supracrustal rocks we have identified several distinct stages of fluid rock interactions as follows 1 the neoproterozoic supercrustal protoliths of uhp rocks experienced variable degrees of hydration through interactions with cold meteoric water with extremely low oxygen isotope compositions during neoproterozoic snow ball earth time 2 a series of dehydration reactions took place during triassic subduction of the yangtze plate beneath the sino korean plate the released fluid entered mainly into volatile bearing high pressure hp and uhp minerals such as phengite zoisite epidote talc lawsonite and magnesite as well as into uhp nams such as garnet omphacite and rutile 3 silicate rich supercritical fluid hydrous melt existed during the uhp metamorphism at mantle depths 100 km which mobilized many normally fluid immobile elements and caused unusual element fractionation 4 the fluid exsolved from the nams during the early exhumation of the dabie sulu terrane was the main source for hp hydrate retrogression and generation of hp veins 5 local amphibolite facies retrogression at crustal depths took place by infiltration of aqueous fluid of various salinities possibly derived from an external source 6 the greenschist facies overprinting and low pressure lp quartz veins were generated by fluid flow along ductile shear zones and brittle faults during late stage uplift of the uhp terrane

magnetite exsolution in ilmenite from the fe ti oxide gabbro in the xinjie intrusion sw china and sources of unusually strong remnant magnetization this study investigates magnetite exsolution in ilmenite from fe ti oxide gabbro in the xinjie intrusion sw china exsolved magnetite lamellae in ilmenite contain nearly pure fe3o4 with 1 wt tio2 ebsd based analyses indicate that the magnetite lamellae have close packed oxygen planes and directions parallel to those in the host ilmenite with 111 mag 0001 ilm and formula 1 the fe2 in the magnetite lamellae is probably derived from adjacent titanomagnetite by sub solidus inter oxide cation repartitioning of fe2 ti4 2fe3 during cooling it is thus suggested that only fe3 cations in the magnetite lamellae should be included into the composition of the ilm hemss precursor for the fe ti oxide oxy thermometer the existence of magnetite exsolution in ilmenite also provides an alternative explanation for unusually strong natural remnant magnetization in natural ilmenite 1 embed mml math 1 gif

first in situ x ray identification of coesite and retrograde quartz on a glass thin section of an ultrahigh pressure metamorphic rock and their crystal structure details to ensure the presence of coesite and its transformed polymorph quartz in ultrahigh pressure uhp rocks and to examine the relic of the phase transformation crystal structures were analyzed by single crystal x ray diffraction xrd directly using the rock thin section mounted on a slide glass the rock sample used is a coesite bearing eclogite from the sulu uhp terrain eastern china the crystal structures were determined successfully by this new method and the presence of coesite and quartz in uhp rocks was identified for the first time by xrd the r factor r f converged to 0 046 for coesite and 0 087 for quartz the displacement ellipsoids for coesite and quartz are larger than those previously reported for these two phases which is consistent with expected effects of trapped strain due to the phase transformation from coesite to quartz during exhumation from the earth s mantle this paper is the first report of single crystal xrd of a rock thin section on a glass slide and establishes the technique and provides proof of concept of the method although the mineral species included in a thin section can often be identified by other methods such as raman spectroscopy an advantage of the reported method is that it can be applied to any mineral in a thin section and not just to the uhp minerals moreover it is applicable to an unknown or new mineral in a thin section discarding the spots of known minerals and constructing a lattice from the residual spots to find the structure of the unknown phase

eclogite facies relics and inferred ultrahigh pressure metamorphism in the north dabie complex central eastern china mineral assemblages and microstructures of a newly identified retrogressed eclogite sensu lato in the north dabie complex ndc central eastern china indicate early very high pressure metamorphic relics preserved in a dominant amphibolite facies host where no eclogite sensu stricto has been positively identified before the investigated eclogitic rock shows distinct multistage recrystallization with granulite and amphibolite facies assemblages overprinting eclogite facies relics the minimum temperature for the eclogite facies metamorphism is estimated to be 800 820 c a spectacular microstructure of oriented quartz needles 2 20 m wide 5 200 m long in matrix ca na clinopyroxene implies the prior existence of a non stoichiometric supersilicic omphacite stabilized at ultrahigh pressure uhp 25 kbar conditions although no coesite or coesite pseudomorphs have been found in the samples the absence of coesite may be due to the lack of free silica at uhp conditions or the consumption of silica during retrograde reactions the inferred uhp conditions metamorphism is further supported by sm nd ages that are equivalent to triassic metamorphic ages from uhp eclogites in the southeastern dabie mountains this finding expands the uhp terrane northward about 50 km spatial distribution of subduction collision related uhp rocks includes parts of the ndc

oriented precipitates of quartz and amphibole in clinopyroxene of metabasites from the greek rhodope a product of open system precipitation during eclogite granulite amphibolite transition in metabasites from the high grade metamorphic kimi complex of the greek rhodope lamellae of quartz and amphibole were observed together with either rutile or titanite precipitates in the cores of clinopyroxene that underwent a prolonged exhumation history from eclogite via granulite and amphibolite facies conditions lamellae are crystallographically oriented along clinopyroxene cleavage planes 010 and interpreted as the result of precipitation during a two step intracrystalline compositional adjustment from an omphacite to a ca tschermak rich clinopyroxene and finally to diopside even though retrograde diffusion profiles develop in clinopyroxene the diffusion rates of highly charged ions like si4 al3 and ti4 are too slow so precipitates rich in si4 quartz al3 amphibole and ti4 rutile or titanite form as viable kinetic alternatives these precipitation reactions are not possible isochemically and additional exchange of ions with the matrix is required for precipitation to occur this exchange of mainly rapidly diffusing low charge ions is postulated to proceed by bulk diffusion self diffusion and does not require special pathways or fluid even though these may play an additional role during metamorphism this hitherto undescribed process of precipitation made possible by diffusional exchange with the matrix through an intact host crystal lattice is termed here as open system precipitation osp no prior ca eskola rich ultrahigh pressure uhp composition of omphacite is required to explain the observed precipitates it is proposed that many precipitation phenomena that were hard to explain in a closed system scenario can now be explained more readily by osp calculation of an original solid solution composition as in an uhp phase from precipitate and host mineral analyses becomes imprecise to the degree that bulk diffusion and hence osp have been important

ultrahigh pressure minerals and metamorphic terranes the view from china abstract ultrahigh pressure uhp metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths associated mafic ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere but which subsequently have experienced p t conditions within or above the coesite stability field 2 7 gpa 700 c typical products include eclogite garnet peridotite and uhp varieties of metapelite quartzite marble paragneiss and orthogneiss uhp metamorphic assemblages require relatively cold lithospheric subduction to mantle depths some recrystallization even occurs under forbidden p t conditions characterized by a geotherm of the current explosion of research on continental uhp terranes reflects their significance for mantle dynamics and the tectonics of continental subduction collision exhumation mantle slab interactions and geochemical recycling a further characterization of uhp phases and positive identification of uhp minerals requires new experimental studies coupled with state of the art analyses for example the very rare occurrence of microdiamond inclusions in zircons from dabie sulu uhp rocks may reflect high f o 2 attending recrystallization inasmuch as epidote is rather common rutile needles within garnets from sulu uhp eclogitic rocks may not be the result of exsolution so in such cases the apparent uhp pressure may have been over estimated hadean igneous microdiamond inclusions in jack hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from new south wales might have been derived from unexposed uhp metamorphic terranes micro mineral intergrowth and nano size minerals may hold important key to deciphering the actual p t paths of subduction and mantle return flow although most exhumed terranes have returned surfaceward relatively rapidly after short time of uhp condition the long duration of storage at great depth and slow exhumation for the largest uhp terranes remain as major problems

zircon u pb ages ree concentrations and hf isotope compositions of granitic leucosome and pegmatite from the north sulu uhp terrane in china constraints on the timing and nature of partial melting granitic leucosome and pegmatite are widely distributed within biotite bearing orthogneiss in the northern part of the sulu ultrahigh pressure uhp metamorphic terrane eastern china a combined study of mineral inclusions cathodoluminescence cl images u pb shrimp dates and in situ trace element and lu hf isotope analyses of zircons provided insight into the nature and timing of partial melting in these rocks zircon grains separated from biotite bearing orthogneiss typically have three distinct domains 1 pre metamorphic magmatic cores with qtz kfs pl ap inclusions which record a neoproterozoic protolith age of 790 ma 2 mantles with coe phe ap inclusions that record triassic uhp age at 227 3 ma and 3 narrow rims with quartz inclusions that record hp granulite facies retrograde metamorphism at 210 3 ma in contrast zircons separated from granitic leucosome have only two distinct domains 1 the central uhp areas with coe phe ap inclusions record triassic uhp age of 227 3 ma and 2 outer magmatic areas with qtz kfs ab ap inclusions that record partial melting time of 212 2 ma zircons separated from pegmatite contain mineral inclusions of qtz kfs ap and show regular magmatic zoning from centre to edge the centres record partial melting time of 212 2 ma in line with the outer domains of granitic leucosome whereas the edges give a younger age of 201 2 ma related to pb loss and partial recrystallization during late triassic regional amphibolite facies retrogression these data indicate that partial melting in the north sulu uhp gneissic rocks took place during post uhp retrograde hp granulite facies metamorphism pre metamorphic magmatic zircon cores from biotite bearing orthogneiss give uniform 176hf 177hf of 0 28187 0 00003 2 sd standard deviation corresponding to ehf 790 and hf model ages tdm2 of about 16 3 and 2 41 ga respectively this is consistent with the generation of its protolith by reworking of paleoproterozoic to late archean crust in contrast uhp zircon domains from biotite bearing orthogneiss and granitic leucosome are characterized by distinct trace element composition with low lu hf 0 006 low th u 0 1 and considerably higher 176hf 177hf 0 28233 0 00002 2 sd than the pre metamorphic cores the uniform but significantly different hf isotope composition between the uhp ehf 227 14 6 0 8 2 sd and pre metamorphic ehf 227 27 7 domains indicates equilibration of the lu hf isotope system only within the uhp metamorphic mineral assemblage the disequilibrium between whole rock and uhp zircon suggests that about two thirds of the whole rock hf retained in the pre metamorphic zircon domains zircon domains crystallized during partial melting at 212 ma in granitic leucosome and pegmatites have a hf isotope composition indistinguishable from that of the uhp zircon domains this suggests that only hf and zr equilibrated during uhp metamorphism was remobilized during partial melting while pre metamorphic zircon remained stable or was not accessible in contrast the magmatic zircon edges from pegmatite have somewhat lower 176hf 177hf 0 28216 and ehf t 17 6 1 2 2 sd indicating some release of less radiogenic hf for instance by dissolution of pre metamorphic zircon during late regional amphibolite facies retrogression

supersilicic clinopyroxene and silica exsolution in uhpm eclogite and pelitic gneiss from the kokchetav massif kazakhstan abundant exsolved quartz rods occur in matrix clinopyroxene of eclogite from the kokchetav massif kazakhstan these rocks are diamond grade ultrahigh pressure uhp metamorphic rocks that recrystallized at p 6 gpa and t 1000 c zircon is an excellent container which effectively protects peak uhp metamorphic phases from retrogression therefore to ascertain the pre exsolution composition of the clinopyroxene we analyzed clinopyroxene inclusions in zircon of the eclogite and a diamond bearing biotite gneiss clinopyroxene in zircon has an excess of fe 3 vi al over iv al na k and calculated cation totals significantly less than 4 0 per six o atoms the stoichiometry of these pyroxenes can be reconciled if the ca eskola end member ca 0 5 0 5 alsi 2 o 6 is considered the zircon hosted clinopyroxene in the eclogite contains up to 9 6 mol of the ca eskola component and in the biotite gneiss up to 18 2 mol whereas the matrix clinopyroxene contains much less 1 3 mol on average recalculation of the composition of the matrix clinopyroxene prior to exsolution of quartz rods yields 6 8 mol ca eskola component which is consistent with the composition of the clinopyroxene inclusions in zircon we conclude that the ca eskola component in the peak metamorphic clinopyroxenes broke down by a retrograde reaction 2 ca 0 5 square 0 5 alsi 2 o 6 rightarrow caal 2 sio 6 3 sio 2 resulting in exsolution of the quartz rods in the matrix clinopyroxene subducted crustal and supracrustal rocks form the ca eskola clinopyroxene at high pressures and temperatures the vacancy containing clinopyroxene may have an important bearing on the physico chemical properties of the subducted slab at upper mantle depth

use of multicomponent seismic data for oil water discrimination in fractured reservoirs geophysicists have devoted great efforts to the problem of determining fluid saturation from seismic measurements with some notable successes it is commonly believed that fluid information is to be found in the p wave data with shear waves being insensitive to fluid and indeed almost all successful fluid detection methodologies have been based on analysis of the p wave

prograde metamorphism decompressional partial melting and subsequent melt fractional crystallization in the weihai migmatitic gneisses sulu uhp terrane eastern china abstract in order to decipher fluid melt activity during subduction and subsequent exhumation of felsic continental crust we carried out a combined study of zircon internal structure zircon u pb age zircon trace element composition and whole rock geochemistry on migmatitic gneisses from the weihai region in the sulu ultra high pressure uhp metamorphic terrane eastern china the weihai migmatitic gneisses are composed of intercalated compositional layers of melanosome and plagioclase pl rich leucosome and k feldspar kfs rich pegmatite veins whole rock geochemistry suggests that mgo fe2o3t cao tio2 p2o5 rare earth elements ree and large ion lithophile elements hfse are nearly completely retained in the melanosome whereas significant amounts of lile e g rb ba k sr pb and u are partitioned into the pl rich leucosome and the kfs rich pegmatite veins the kfs rich pegmatite vein has much higher k2o u pb and rb but lower cao and na2o contents than those of the pl rich leucosome layer protolith magmatic zircon domains with an upper intercept age of about 780 ma are common in the cores of zircon grains from the melanosome and the pl rich leucosome layers indicating that the protolith of the migmatitic gneiss is mid neoproterozoic magmatic rock metamorphic zircons with concordant ages ranging from 243 to 256 ma occur as overgrowth mantles on the protolith magmatic zircon cores they are characterized by remarkably flat heavy rare earth element hree patterns without obviously negative eu anomalies and have low y and th contents low th u and high hf y ratios indicating a prograde metamorphism in the hp eclogite facies during subduction recrystallized rims of zircons 228 2 ma in the pl rich leucosome layer are similar to magmatic zircons in terms of their hree enriched patterns with negative eu anomalies high y and low th contents and low hf y and th u ratios this indicates that the gneiss has experienced decompressional partial melting and the pl rich leucosome layers were formed by fractional crystallization of the primary melt during the early stage hot exhumation zircon grains 219 2 ma in the kfs rich veins are characterized by aqueous fluid melt related signatures with strongly hree enriched patterns high u pb and low th contents and low th u and high lu hf ratios indicating that the pegmatite vein was formed by crystallization of residual aqueous melt after the crystallization of pl rich leucosome layers during later stage cooling during exhumation our results suggest that the migmatitic gneiss successively recorded information concerning the protolith prograde hp metamorphism during subduction fractional crystallization of primary melt and crystallization of the residual melt during exhumation

ultrahigh pressure metamorphism in western tianshan china part i evidence from inclusions of coesite pseudomorphs in garnet and from quartz exsolution lamellae in omphacite in eclogites inclusions of coesite pseudomorphs in garnets have been found in type i eclogites interlayered with mafic blueschists and carbonate eclogites and quartz exsolution lamellae in omphacites have been recognized in type ii eclogites with pillow structure in western tianshan china based on mineralogy and petrology the metamorphic evolution of western tianshan eclogites can be divided into three stages 1 pre peak ultrahigh pressure uhp eclogite facies stage 356 443 c 8 10 kb 2 uhp eclogite facies stage 496 598 c 25 7 26 7 1 kbar and 3 retrograde epidote blueschist facies stage 500 530 c 10 12 kbar consequently the eclogites from the western tianshan region of china have undergone unambiguous uhp metamorphism

evidence of former stishovite in uhp eclogite from the south altyn tagh western china abstract we report here for the first time long prism needle shaped polycrystalline quartz aggregates and square parallelogram shaped single phase quartz inclusions in omphacite and garnet of ultrahigh pressure uhp eclogite from the yinggelisayi area in the south altyn uhp belt based on their morphology and mineral composition these quartz inclusions in omphacite and garnet are quartz paramorphs after stishovite the minimum peak pressure of the eclogite is estimated to be 8 9 gpa at 800 1000 c based on the stability field of stishovite this new evidence together with previous discovery of kyanite spineal exsolutions in former stishovite of the pelitic gneiss from the same region suggests a ultra deep subduction and exhumation of the south altyn continental rocks to from mantle depths in the stability field of stishovite evidence of ultra deep subduction of continental materials might be more common and diverse than previous thought

evaluation of anisotropy by shear wave splitting the polarizations of three component shear wavetrains carry unique information about the internal structure of the rock through which they pass specifically commonly observed shear wave splitting may contain information about the orientation of crack distributions this information cannot usually be recovered from shear waves recorded at the free surface because of interference with the interaction of the shear wave with the surface even for nearly vertical incidence however shear wave splitting in synthetic three component vertical seismic profiles in some cases may be interpreted directly in terms of the direction of strike of vertical cracks because the human eye is not skilled at identifying the phase relationships between three component seismograms played out conventionally as parallel time series the polarizations are displayed in orthogonal sections of the particle displacements to facilitate recognition and evaluation of the shear wave splitting estimating the orientations of cracks an

quartz exsolution topotaxy in clinopyroxene from the uhp eclogite of weihai china abundant oriented silica precipitates of quartz 4 0 1 0 vol in part coexisting with calcic amphiboles 0 2 vol have been identified in clinopyroxenes of eclogites from the weihai area sulu ultrahigh pressure terrane eastern china electron backscatter diffraction ebsd analyses demonstrate that the majority 97 of quartz precipitates have topotactic relationships with their host clinopyroxenes three types of crystallographic topotactic relationship have been identified between quartz and host clinopyroxene 1 52 quartz precipitates share the same orientation for the c axes with 0001 qz 001 cpx 2 34 quartz precipitates share the same orientation for the a axes with 112 0 qz 001 cpx and 3 11 quartz precipitates share the same orientation for the s planes with 112 1 qz 100 cpx other quartz axes and planes disperse in large or small girdles around the shared axes or planes many quartz rods needles are elongated parallel to the 001 cpx with the long axes of quartz being either 0001 qz or 112 0 qz amphibole precipitates have also a strong crystallographic relationship with host clinopyroxene i e 100 amp 100 cpx 010 amp 010 cpx and 001 amp 001 cpx these results provide quantitative microstructural evidence supporting an exsolution origin for oriented quartz needles rods in clinopyroxene and demonstrate that the exsolution of quartz from clinopyroxene occurred within the stability field of quartz rather than coesite the oriented precipitates of quartz in part coexisting with calcic amphiboles in host clinopyroxene are probably promoted by supercritical fluid or partial melting during the early exhumation of eclogites our results suggest that oriented quartz precipitates in clinopyroxene cannot be used as an indisputable uhp indicator

silica precipitates in omphacite from eclogite at alpe arami switzerland evidence of deep subduction observations of oriented sio2 precipitates in omphacite from eclogite with tholeiitic basalt protolith bordering the alpe arami garnet peridotite massif ticino switzerland and petrological studies of the eclogitic mineral assemblages suggest that this rock was subjected to higher pressure metamorphism than previously realized we employed various calibrations of the fe2 mg exchange thermometer and calculations of equilibria with thermodynamic data considering the calcium tschermak s component caal2sio6 of garnet pyroxene pairs from these calculations it is concluded that the eclogitic lenses have recorded at least four stages of mineral growth corresponding to the following stage i prograde c 2 4 gpa 700 c stage iia maximum recorded grade c 7 0 gpa 1100 c stage iib retrograde c 3 7 gpa 900 c stage iii retrograde c 2 1 gpa 750 c because of the preservation of stage i a relatively rapid subduction and exhumation of alpe arami eclogite is suggested the exhumation path of the eclogitic rock is in good agreement with most exhumation paths inferred for the alpe arami garnet lherzolite proposed previously by several authors based upon a variety of different observations although the eclogite and peridotite exhumation paths may diverge at depths greater than 120 km

an analytical electron microscopic study of a pyroxene amphibole intergrowth samples of a garnet granulite from the mafic border units of the lake chatuge georgia alpine peridotite body were found to contain lamellar intergrowths of a pargastic amphibole in augite having the typical appearance of an exsolution feature single crystal x ray diffraction optical electron microprobe and conventional and analytical electron microscopic studies have provided data limiting the compositions and structures of the coexisting phases individual lamellae of both materials are from 0 5 to 2 0 m in width with the lamellar interface parallel to 0 1 0 the formulae of the minerals as determined by a combination of electron microprobe and analytical electron microscopy are na0 1ca1 0mg0 6fe3 0 3 si1 8al0 2 o6 for the pyroxene and na0 7ca1 9 mg2 1fe2 1 4fe3 0 5ti0 1cr0 1al0 8 si5 9al2 1 o22 oh 2 for the amphibole several other studies have described intergrowths similar to those observed in this work in general favoring exsolution as the formation mechanism for the intergrowths in the lake chatuge samples however replacement of pyroxene by amphibole is in part indicated by continuous gradation of amphibole lamellae into amphiboles rimming the clinopyroxenes

crystallographic orientation relationships in host inclusion systems new insights from large ebsd data sets crystallographic orientation relationships cors between mineral inclusions and their hosts could potentially deliver information about inclusion formation processes and conditions most previous studies are based on small numbers of analyses this paper uses ebsd to study host inclusion cors in an inclusion rich permian metapegmatite garnet koralpe region eastern alps austria demonstrating the importance of large data sets and of ebsd in particular for the analysis of cors the distribution of measured orientations reflects host garnet point group symmetry for 89 of inclusions analyzed total n 530 each inclusion phase rutile corundum and ilmenite shows at least three different cors to host garnet statistical cors are introduced to describe distributions of inclusion orientations that have one or two degrees of freedom with respect to the host but still reflect host crystal symmetry two end member characteristics of statistical cors are distinguished rotation and dispersion most statistical cors observed show a mixture of both each inclusion phase shows at least one statistical cor multiple coexisting cors and statistical cors are not restricted to rutile re examination of previous garnet rutile cor studies in light of the new results indicates that cor information may have been overlooked when using small data sets variation in cor parameters correlates with broad differences in assumed metamorphic conditions for new and literature samples suggesting that petrogenetic information may be available if cor formation can be understood the favorability of the detected cors cannot be explained by a simple model involving minimization of misfit between lattice planes implying that other interface properties or the inclusion formation mechanism are important controls on cor development

oriented quartz calcic amphibole inclusions in omphacite from the saualpe and pohorje mountain eclogites eastern alps an assessment of possible formation mechanisms based on ir and mineral chemical data and water storage in eastern alpine eclogites abstract the composition of mineral phases and their modal proportions have been determined for three representative eoalpine eclogites from the saualpe type locality eastern austria sample skp31 and the pohorje massif slovenia cm31 03 and cm15 01 using electron microprobe laser icp ms ir spectroscopy and modal analysis to evaluate possible mechanisms for the formation of composite oriented calcic amphibole quartz inclusions cois in omphacite and to assess the relative importance of hydrous and nominally anhydrous phases as h 2 o carriers in these eclogites for omphacites in cm31 03 with a zonal distribution of cois a comparison of water and trace element concentrations of areas containing cois and those free of cois and a comparison with the trace element concentration of calcic amphibole indicate that cois have formed through an open system alteration of clinopyroxene and not through a closed system exsolution process in sample skp31 both textural and mineral chemical evidence suggests that cois did not form by exsolution involving a ca eskola component in clinopyroxene but formed by progressive growth under eclogite facies p t conditions and prior to the onset of retrogressive symplectite formation analogous to the formation of poikiloblastic quartz calcic amphibole grains in the matrix bulk h 2 o contents of the eclogites are between ca 750 and 2150 ppm with 6 25 of the total water contributed by nominally anhydrous minerals nams because of high modal amounts of 37 65 omphacite is the major nominally anhydrous water carrier containing 145 580 ppm h 2 o with significant concentration variations on a thin section scale due to their very low h 2 o concentrations of 2 o content calcic amphibole forming part of cois may be a major carrier of h 2 o as evidenced by cm31 03 containing both cois and texturally primary calcic amphibole in this sample calcic amphibole of the cois contributes 63 whereas primary calcic amphibole only accounts for 13 of the bulk water the relative order of h 2 o concentrations in nams is h 2 o rutile h 2 o omphacite h 2 o garnet for cm31 03 and cm15 01 and h 2 o rutile h 2 o omphacite h 2 o garnet in skp31

oriented inclusions of pyroxene amphibole and rutile in garnet from the luliangshan garnet peridotite massif north qaidam uhpm belt nw china an electron backscatter diffraction study

biot dispersion for p and s wave velocities in partially and fully saturated sandstones ultrasonic compressional and shear wave velocities have been measured on 34 samples of sandstones from hydrocarbon reservoirs the sandstones are all of low clay content high porosity and cover a wide range of permeabilities they were measured dry and brine saturated under hydrostatic effective stresses of 10 20 and 40 mpa for eight of the sandstones ultrasonic velocity measurements were made at different partial water saturations in the range from dry to fully saturated the gassmann biot theory is found to account for most of the changes in velocities at high effective stress levels when the dry sandstones are fully saturated with brine provided the lower velocities resulting when the dry sandstone initially adsorbs small amounts of moisture are used to determine the elastic properties of the dry sandstone at lower effective stress levels local flow phenomena due to the presence of open microcracks are assumed to be responsible for measured velocities higher than those predicted by the theory the partial saturation results are modelled fairly closely by the gassmann biot theory assuming heterogeneous saturation for p waves

the relationship between ductile shear zone and mineralization in the jiufeng sn deposit northern guangxi south china evidence from structural analysis and cassiterite u pb dating abstract the jiufeng sn deposit is located in the outer contact zone southwest of the neoproterozoic motianling granitic pluton in northern guangxi south china the deposit is characterized by a nne striking 30 200 m wide ductile shear zone named the jiufeng ductile shear zone majority of the sn orebodies including four large ones are hosted by the shear zone field and microstructural observations electron backscatter diffraction measurements and magnetic fabric analyses show that the shear zone is characterized by sinistral thrust shearing the shear zone has also slightly modified the sn orebodies quartz in the shear zone exhibits core mantle structure and undulose extinction quartz has experienced significant sub grain rotation and grain boundary migration recrystallization lattice preferred orientation analysis reveals primarily basal rhomb and prism slips cassiterite grains in the shear zone however demonstrate brittle deformation therefore the deformation temperature during the ductile shearing is estimated lower than 650 c and below u pb closure temperature of cassiterite as such the u pb isotopic composition of the cassiterite was not disturbed by the shearing and the u pb age of the cassiterite could be used for the age of the sn mineralization la icp ms u pb dating of cassiterite grains selected from a cassiterite bearing tourmalinite sample performed in this study yielded a tera wasserburg u pb lower intercept age of 832 3 5 3 ma this age is consistent within error with that of the neighboring neoproterozoic motianling granitic pluton which suggests a close genetic relationship between the sn mineralization and the granitic magmatism studies show that the jiufeng ductile shear zone was produced by a widespread caledonian or kwangsian tectonic event in south china indicating that the shearing was much later than the sn mineralization and has no generic relationship to the sn deposit

refining the timing of eclogite metamorphism a geochemical petrological sm nd and u pb case study from the pohorje mountains slovenia eastern alps high pressure metamorphism in the pohorje mountains of slovenia austroalpine unit eastern alps affected n morb type metabasic and metapelitic lithologies thermodynamic calculations and equilibrium phase diagrams of kyanite phengite bearing eclogites reveal pt conditions of 2 1 gpa at t 750 c but within the stability field of quartz metapelitic eclogite country rocks contain the assemblage garnet phengite kyanite quartz for which calculated peak pressure conditions are in good agreement with results obtained from eclogite samples the eclogites contain a single population of spherical zircon with a low th u ratio combined constraints on the age of metamorphism come from u pb zircon as well as garnet whole rock and mineral mineral sm nd analyses from eclogites a coherent cluster of single zircon analyses yields a 206pb 238u age of 90 7 1 0 ma that is in good agreement with results from sm nd garnet whole rock regression of 90 7 3 9 and 90 1 2 0 ma end 8 for two eclogite samples the agreement between u pb and sm nd age data strongly suggests an age of approximately 90 ma for the pressure peak of the eclogites in the pohorje mountains the presence of garnet omphacite and quartz inclusions in unfractured zircon indicates high pressure rather than ultrahigh pressure conditions the analysed metapelite sample yields a sm nd garnet whole rock scatterchron age of 97 15 ma these data probably support a single p t loop for mafic and pelitic lithologies of the pohorje area and a late cretaceous high pressure event that affected the entire easternmost austroalpine basement including the koralpe and saualpe eclogite type locality in the course of the complex collision of the apulian microplate and europe

salinity changes along the upper limb of the atlantic thermohaline circulation lagrangian analyses of a global ocean circulation model quantify the salinity changes experienced by the warm limb of the thermohaline circulation during the northward flow to the atlantic deep convection regions 6 sv out of the estimated 10 sv transfer from 45 s to 47 n flow through regions of prevailing surface evaporation the southern and northern formation regions of salinity maximum water and the gulf of cadiz mediterranean sea domain the remaining transport gains salinity through mixing with adjacent waters as much as 6 sv flow through the low salinity surface mixed layer at the latitudes of the itcz whose effect annihilates that of the southern region of salinity maximum water most of the salinity increase corresponds to the transformation of south to north atlantic central water with strong diapycnal transfers for the water that intersects the high and low salinity regions and nearly isopycnal modifications for the water that avoids these regions

role of partial melting in the evolution of the sulu eastern china ultrahigh pressure terrane strongly deformed potassium feldspar rich dikes are widely distributed in the northern part of the sulu ultrahigh pressure uhp metamorphic terrane eastern china the fact that the crystallization ages of these dikes overlap with the age of peak uhp metamorphic conditions implies the presence of melt during metamorphism sr isotopic ratios of the dikes are compatible with their origin as partial melts of the dominant felsic sulu gneiss partial melting may be the key to solving several unusual features of the sulu and other uhp terranes such as the almost complete lack of mineralogical evidence for uhp conditions and the limited growth of zircon during uhp conditions in the dominant felsic gneiss in addition because partial melting will cause a drastic reduction in the strength of the uhp gneisses the most likely exhumation mechanism is diapiric rise of a low viscosity partially molten mass containing entrained blocks of eclogite and not a thin sheet as usually proposed

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

unusual exsolution phenomena in omphacite and partial replacement of phengite by phlogopite kyanite in an eclogite from the erzgebirge in the erzgebirge crystalline complex eclogites occur in three different high pressure hp units 1 2 and 3 recording contrasting pressure p temperature t conditions eclogites from hp unit 1 experienced peak metamorphic conditions in the coesite stability field at about 33 kbar 850 c commonly these eclogites from hp unit 1 are all very similar with an eclogitic peak assemblage of omphacite garnet coesite k feldspar rarely accompanied by kyanite and omphacites systematically deviating from a stoichiometric composition in contrast an eclogite recently found near blumenau is mineralogically and geochemically different from the typical eclogites of hp unit 1 this unusual eclogite reveals the eclogitic equilibrium assemblage omphacite garnet coesite phengite phlogopite kyanite and yields metamorphic peak conditions of 870 c and 29 kbar there is clear textural evidence of the formation of phlogopite and kyanite under partial consumption of phengite and garnet moreover the omphacite is stoichiometric and contains abundant exsolution lamellae the thickest of which were identified as quartz by the electron microprobe the finer lamellae were studied by transmission electron microscopy tem oligoclase was identified as an exsolution phase other lamellae proved to consist of k white mica also interpreted as exsolution prior to exsolution the omphacite composition must have been cation deficient as that of the other common hp unit 1 eclogites these non stoichiometric compositions are ascribed to partial substitution by the ca eskola pyroxene component which calculates to an average of 8 mol for omphacite in hp unit 1 eclogites according to experiments this substitution becomes significant at p 30 kbar exsolution of k white mica may indicate hydroxyl defects in the original omphacite also favoured by high pressure oligoclase and k white mica exsolution from ca eskola rich clinopyroxene has not previously been reported the omphacite has a disordered c2 c structure and in just one case very small a few tens of nanometres antiphase domains resulting from the c2 c to p2 n transformation are present these features may indicate a brief thermal history and rapid tectonic processes

pyroxene exsolution topotaxy in majoritic garnet from 250 to 300 km depth we report here the detailed microstructure and chemistry of pyroxene exsolution from a polycrystalline garnet porphyroblast of the western gneiss region wgr garnet peridotite otroy norway for both clinopyroxene cpx and orthopyroxene opx the same basic crystallographic relationship is found with the host garnet 100 py 112 grt 010 px 110 grt and 001 px 111 grt for the majority 90 of its intracrystalline pyroxene rods in addition this pattern is also exhibited by some interstitial opx and a subpopulation of both pyroxenes shows a different pattern or no discernible pattern the results provide quantitative microstructural evidence demonstrating an exsolution precipitation origin of both the intracrystalline cpx and opx and the small interstitial opx crystals the reconstructed precursor majoritic garnet taking into account both the intracrystalline pyroxenes and interstitial opx was characterized by si 3 07 cation per formula unit that corresponds to a minimum pressure of 7 7 gpa 250 km depth we also deduce from the observation of opx being the majority of intracrystalline precipitates and 100 of the interstitial ones that the precursor majoritic garnet probably originated from a pressure less than 10 gpa 300 km depth a multistage decomposition hypothesis is proposed for this wgr majoritic garnet during exhumation of the peridotite from 250 to 300 km depth to explain the topotaxy and chemistry of the exsolved pyroxenes

quartz exsolution in clinopyroxene is not proof of ultrahigh pressures evidence from eclogites from the eastern blue ridge southern appalachians u s a oriented quartz needles in clinopyroxene have become one of the diagnostic indicators of ultrahigh pressure uhp metamorphism the presence of apparently exsolved quartz is taken as evidence of decompression of a non stochiometric ca eskola component ca0 5 0 5alsi2o6 caes that is presumed to be stable only at uhp conditions eclogite from the eastern blue ridge north carolina contains clinopyroxene jd20cats5ac5caesdi65hd5 with oriented needles of quartz and calcic amphibole that appear to have exsolved together the quartz amphibole intergrowths are surrounded by 1 5 m haloes of neoformed pyroxene jd10cats10ac5caesdi70hd5 the modes of quartz amphibole and clinopyroxene haloes were determined using bse images and reintegrated with the host clinopyroxene viewing the quartz and amphibole needles down the c axis of the pyroxene host provides a better estimate of their proportions than in prismatic sections reintegrated pyroxene compositions were nearly identical to the analyzed host pyroxene with no caes component clinopyroxene with caes solid solution has been repeatedly synthesized at uhp conditions however examination of the phase equilibria usually cited as evidence for caes stability at conditions of 25 kbar shows that clinopyroxene with 10 mol caes is stable well within the quartz field and provides a pressure minimum similar to the albite jadeite quartz barometer exsolution of quartz and associated amphibole is commonplace in clinopyroxene from the blue ridge eclogite that lacks coesite or other evidence for uhp metamorphism the presence of a diluted 5 10 caes component in clinopyroxene does not require uhp conditions

morphology and microstructure of magnetite and ilmenite inclusions in plagioclase from adirondack anorthositic gneiss it is well known that oriented iron and titanium oxide inclusions occur in pyroxenes and plagioclase of anorthosites and granulites and they are attributed to exsolution at subsolidus conditions the oxides occur as needles or platelets in this study we determine the morphology of oxide needles as well as their orientation in plagioclase an 30 35 in anorthosite gneiss from the adirondack mountains new york the investigation was done with electron backscatter diffraction ebsd in a scanning electron microscope as well as laue diffraction with a microfocus synchrotron x ray beam at the advanced light source in berkeley it was observed that the needle direction is 110 in magnetite and 1010 in ilmenite the needle direction is consistently parallel to 001 of plagioclase furthermore 111 of magnetite and 0001 of ilmenite are sub parallel to 120 and 120 of plagioclase we note that for directions 110 in the magnetite structure and 1010 in ilmenite o atoms are close packed and 111 and 0001 are close packed planes correspondingly in plagioclase 001 is a direction with open channels as well as approximate alignment of si tetrahedral edges thus providing nucleation sites with a coincidence lattice relationship 120 and 120 in this triclinic mineral are planes with approximate tetrahedral sides so that the relationship is structurally plausible from laue diffraction we can determine that the magnetite needle axis is subject to an extensional stress most likely attained during cooling of the inclusions within the plagioclase host

melting induced fluid flow during exhumation of gneisses of the sulu ultrahigh pressure terrane abstract hydrothermally altered rocks are products of fluid rock interactions and typically preserve numerous quartz veins that formed as chemical precipitates from fluids that fill up cracks thus quartz veins are the record of the fluid system that involved fracture flow in the direction of changing temperature or pressure in order to decipher the fluid activity in the sulu ultrahigh pressure uhp terrane in eastern china quartz veins together with an adjacent eclogite lens and the host gneiss were studied in one location a deformed quartz vein is located at the boundary between the host gneiss and the eclogite lens the amphibolite facies overprinting of the eclogite lens decreases from the rim to the core of the lens with fresh eclogite preserved in the core the foliated biotite gneiss contains felsic veins and residual phengites zircon rims from the gneiss are characterized by melt related signatures with steep hree patterns high hf contents and negative eu anomalies and a pool of weighted average 206 pb 238 u analyses reveal an age of 219 3 ma 2 which is younger than the uhp metamorphic age 236 2 ma 2 recorded by zircons from the eclogite lens this suggests that the gneiss in the sulu uhp terrane could have suffered from partial melting due to phengite dehydration during the hot exhumation stage the formation age of the quartz vein 219 2 ma 2 defined by zircon rims agrees well with the partial melting time 219 3 ma 2 of the host gneiss the initial 176 hf 177 hf ratios of zircon rims from the quartz vein are obviously lower than zircons from the eclogite lens but overlap with the coeval zircon domains from the nearby granite dikes produced by partial melting of orthogneiss these observations suggest that the quartz vein and corresponding fluid flow could be associated with partial melting of the host gneiss on the other hand amphibole bearing and hree rich zircon rims from the amphibolite pool an amphibolite facies metamorphic age of 217 5 ma 2 overlap with the formation age of the quartz vein this implies that retrogression of the eclogite lens could have been caused by melting induced fluid flow based on the above observations we speculate that partial melting of the gneiss in the continental subduction related uhp belt could have induced a significant fluid flow during the exhumation stage and thus contributed significantly to the extensive retrogression of eclogites in the sulu uhp terrane

ultrahigh pressure metamorphism and multistage exhumation of eclogite of the luotian dome north dabie complex zone central china evidence from mineral inclusions and decompression textures abstract the studied eclogite is located in the luotian dome in the southwestern part of the north dabie complex zone ndz central china and is a portion of deeply subducted mafic lower continental crust of the south china block petrologic analysis suggests that the eclogite underwent ultrahigh pressure uhp and high pressure eclogite facies metamorphism and subsequent hp granulite facies overprinting and amphibolite facies retrogression during continental subduction and exhumation as a result multiple decompression textures were produced suggesting a multistage exhumation history from eclogite to granulite to amphibolite facies conditions a striking feature of this eclogite is the widespread exsolution of different phases oriented needles of rutile clinopyroxene amphibole apatite in garnet quartz amphibole hypersthene sodic plagioclase in clinopyroxene and pyrrhotite in apatite most importantly we provide for the first time conclusive evidence of quartz pseudomorphs after coesite in garnet and relic coesite confirmed by in situ raman spectroscopy in zircon in addition there are two groups of apatite one is fluor f apatite that occurs as inclusions in garnet or in matrix which is rich in f 3 wt lakes exsolution features and formed at uhp metamorphic conditions the second is associated with the oriented pyrrhotite exsolution and formed during decompression microtextural and petrologic analysis suggests that the eclogites from the southwestern segment of the ndz are similar to eclogites from the northern segment of the ndz e g huangweihe and baizhangyan and underwent uhp metamorphism with a peak pressure 5 7 gpa these results combined with published geochronological data imply that the ndz experienced triassic uhp metamorphism as a coherent unit

frequency dependent anisotropy in wellington new zealand shear wave splitting measurements are made using teleseismic s scs and sks waveforms recorded at the gdsn broadband station snzo situated in south karori new zealand the average s and sks delay times are around 2 to 3 s among the highest in the world the average scs delay time is significantly smaller around 1 s this discrepancy appears to be due to differences in dominant frequency the most likely cause of frequency dependent anisotropy is oriented heterogeneities with a scale length much smaller than the wavelength the fast polarizations range between 21 and 79 with an average ne sw direction which is sub parallel to the trend of the local geologic structure and the strike of the hikurangi subduction zone azimuthal variations in delay time which cannot be explained by differences in period may be due to a dipping axis of symmetry or laterally varying anisotropy or a more complicated symmetry system

fabrics produced mimetically during static metamorphism in retrogressed eclogites from the zermatt saas zone western italian alps abstract lattice preferred orientations lpos are commonly interpreted to form by dislocation creep consequently they are used to infer deformation at the metamorphic grade at which the minerals were stable especially if those minerals show a shape fabric here we show that lpos can occur through mimicry of a pre existing lpo so they formed statically not during deformation omphacite and glaucophane lpos occur in eclogite facies rocks from the zermatt saas unit of the northwest italian alps barroisite grew during greenschist facies retrogression and has an lpo controlled significantly by the eclogite facies omphacite and glaucophane lpos rather than directly by deformation using spatially resolved lattice orientation data from the three key minerals collected using electron backscatter diffraction we deploy a new technique of interphase misorientation distribution analysis to prove this barroisite lpo develops by mimicry of omphacite via a particular lattice orientation relationship and by direct topotactic and epitactic replacement of glaucophane lpo in turn influenced anisotropic grain growth resulting in a barroisite grain shape fabric thus regional retrogression during exhumation of the zermatt saas high pressure rocks was in large part static rather than dynamic as previously interpreted in general the possibility of mimetic fabrics forming during metamorphic reactions must be borne in mind when interpreting direct structural observations and seismic anisotropy data in terms of deformation in both crust and mantle

natural occurrence of keatite precipitates in uhp clinopyroxene from the kokchetav massif a tem investigation we report the first natural occurrence of keatite also known as silica k discovered as a precipitate in the core of ultrahigh pressure uhp clinopyroxene cpx within garnet pyroxenite from the kokchetav massif kazakhstan high resolution transmission electron microscopy and electron diffraction demonstrate that sub micrometer and nano scale keatite precipitates have a definite crystallographic relationship with the host pyroxene diopside di 90 cpx 100 keatite 100 and cpx 010 keatite 001 clinopyroxene provides a template for keatite nucleation due to the close structural relationship and excellent lattice match between the diopside and keatite we propose that keatite micro precipitates are formed in localized low pressure micro environments produced as a result of exsolution of extra silica and vacancies held within uhp host diopside and stabilized by the pyroxene lattice low density metastable keatite and its relationship to the host pyroxene likely reflects the important influence of pyroxene precipitate interfacial energy on the micro and nano scales in controlling the nature of exsolved phases in exhumed uhp minerals

the 3d shear experiment over the natih field in oman the effect of fracture filling fluids on shear propagation this is the final paper in a series on the 3d multicomponent seismic experiment in oman in this experiment a 3d data set was acquired using three component geophones and with three source orientations the data set will subsequently be referred to as the natih 9c3d data set we present for the first time evidence demonstrating that shear waves are sensitive to fluid type in fractured media two observations are examined from the natih 9c3d data where regions of gas are characterized by slow shear wave velocities one is that the shear wave splitting map of the natih reservoir exhibits much larger splitting values over the gas cap on the reservoir this increase in splitting results from a decrease in the slow shear wave velocity which senses both the fractures and the fracture filling fluid using a new effective medium model it was possible to generate a splitting map for the reservoir that is corrected for this fluid effect secondly an anomaly was encountered on the shear wave data directly above the reservoir the thick fiqa shale overburden exhibits a low shear wave velocity anomaly that is accompanied by higher shear reflectivity and lower frequency content no such effects are evident in the conventional p wave data this feature is interpreted as a gas chimney above the reservoir a conclusion supported by both effective medium modelling and the geology with this new effective medium model we show that introduction of gas into vertically fractured rock appears to decrease the velocity of shear waves s2 polarized perpendicular to the fracture orientation whilst leaving the vertical compressional wave velocity largely unaffected this conclusion has direct implications for seismic methods in exploration appraisal and development of fractured reservoirs and suggests that here we should be utilizing s wave data as well as the conventional p wave data as a direct hydrocarbon indicator

the squirt flow mechanism macroscopic description we introduce a poroelasticity model that incorporates the two most important mechanisms of solid fluid interaction in rocks the biot mechanism and the squirt flow mechanism this combined biot squirt bisq model relates compressional velocity and attenuation to the elastic constants of the drained skeleton and of the solid phase porosity permeability saturation fluid viscosity and compressibility and the characteristic squirt flow length squirt flow length is a fundamental rock property that does not depend on frequency fluid viscosity or compressibility and is determined experimentally we find that the viscoelastic response of many sandstones is dominated by the squirt flow component of the bisq mechanism and that the viscoelastic properties of these rocks can be expressed through a single dimensionless parameter omega r 2 kappa where omega is angular frequency r is the characteristic squirt flow length and kappa is hydraulic diffusivity the biot mechanism alone does not give an adequate explanation of the observed velocity dispersion and attenuation and the viscoelastic behavior of many sandstones

experimental observation of water saturation effects on shear wave splitting in synthetic rock with fractures aligned at oblique angles fractured rocks are known to exhibit seismic anisotropy and shear wave splitting sws sws is commonly used for fractured rock characterization and has been shown to be sensitive to fluid type the presence of partial liquid gas saturation is also known to affect the elastic properties of rocks the combined effect of both fractures and partial liquid gas saturation is still unknown using synthetic silica cemented sandstones with aligned penny shaped voids we conducted laboratory ultrasonic experiments to investigate the effect fractures aligned at an oblique angle to wave propagation would have on sws under partial liquid gas saturation conditions the result for the fractured rock shows a saturation dependence which can be explained by combining a fractured rock model and a partial saturation model at high to full water saturation values sws decreases as a result of the fluid bulk modulus effect on the quasi shear wave this bulk modulus effect is frequency dependent as a result of wave induced fluid flow mechanisms which would in turn lead to frequency dependent sws this result suggests the possible use of sws for discriminating between full liquid saturation and partial liquid gas saturation

tio2 exsolution from garnet by open system precipitation evidence from crystallographic and shape preferred orientation of rutile inclusions we investigated rutile needles with a clear shape preferred orientation in garnet from ultra high pressure metapelites from the kimi complex of the greek rhodope by electron microprobe electron backscatter diffraction and tem techniques a definite though complex crystallographic orientation relationship between the garnet host and rutile was identified in that rt 001 is either parallel to grt or describes cones with opening angle 27 6 around grt each rt 001 small circle representing a cone on the pole figure displays six maxima in the density plots this evidence together with microchemical observations in tem when compared to various possible mechanisms of formation corroborates a precipitate origin a review of exchange vectors for ti substitution in garnet indicates that rutile formation from garnet cannot occur in a closed system it requires that components are exchanged between the garnet interior and the rock matrix by solid state diffusion a process we refer to as open system precipitation osp the kinetically most feasible reaction of this type will dominate the overall process the perhaps most efficient reaction involves internal oxidation of fe2 to fe3 and transfer from the dodecahedral to the octahedral site just vacated by text ti 4 6 text m 2 3 text tial left text alsi 2 right text o 1 2 6 text m 2 2 5 text tialsi 3 text o 1 2 10 text m 2 3 0 text al 1 8 text fe 0 2 text si 3 text o 1 2 text m 2 2 text e 1 2 text tio 2 osp is likely to occur at conditions where the transition of natural systems to open system behaviour becomes apparent as in the granulite and high temperature eclogite facies

relict coesite exsolution in omphacite from western tianshan eclogites china exsolution rods of relict coesite together with quartz were identified in omphacite in eclogites from western tianshan china they are oriented along the c axis of the host clinopyroxenes and have grain size up to 30 m long and 2 3 m wide raman spectra of exsolved lamellae yield consistent but weak bands at 521 270 181 151 and 118 cm 1 typical for coesite in addition to those of quartz and the host omphacite such occurrences together with textured observations suggest a two stage evolution of sio 2 exsolution rods in omphacite lamellae of coesite were apparently exsolved from supersilicic omphacite at p 5 0 gpa and the transformation from coesite to quartz occurred during retrograde metamorphism

p and s wave anisotropy of a synthetic sandstone with controlled crack geometryl1 a method is reported for the production of synthetic porous sandstones containing cracks of known dimensions and geometry with respect to the matrix a synthetic sandstone was manufactured from sand cemented with an epoxy glue the cracks of known geometry were introduced into the material in the manufacturing stage by emplacing thin metallic discs in the sand epoxy matrix these discs were chemically leached out of the consolidated porous sandstone acoustic anisotropy and shear wave splitting were observed in the synthetic sandstones for the dry sample the observed angular dependence of the p and s wave velocities at 100 khz compares well qualitatively with the theoretical models of hudson and of thomsen quantitatively however the experimental data fits hudson s model better for the case of a saturated sample the experimental results are in excellent agreement with thomsen s model hudson s model on the other hand predicts a different angular dependence for p waves this demonstrates that the concept of fluid transfer between cracks and the ambient porosity can be a significant process the results reported here are from the first successful experiment in which the theoretical models were tested on a porous material containing a known crack geometry

jadeite with the ca eskola molecule from an ultra high pressure metagranodiorite dora maira massif western alps a metagranodiorite from the ultra high pressure uhp brossasco isasca unit biu of the southern dora maira massif western alps contains relicts of jadeite developed as a pseudomorph after igneous plagioclase during the alpine uhp metamorphism the jadeite is characterised by a slight but significant shortage of total cations on average for 6 oxygens the sum of m2 na ca cations is 0 93 careful algebraic calculations on the jadeite end members indicate that ca and al combine to form the calcium eskola caes molecule 0 08 0 17 and minor calcium tschermack cats molecule 0 03 this is the first report of jadeite with significant amounts of caes component from a natural uhp metagranitoid rock the finding of caes molecule in the metagranodiorite is further evidence that the rock experienced uhp metamorphism during the alpine metamorphism

excess silica in omphacite and the formation of free silica in eclogite silica lamellae in eclogitic clinopyroxene are widely interpreted as evidence of exsolution during decompression of eclogite however mechanisms other than exsolution might produce free silica and the possible mechanisms depend in part on the nature and definition of excess silica excess silica may occur in both stoichiometric and non stoichiometric pyroxene although the issue has been debated we show that all common definitions of excess silica in non stoichiometric clinopyroxene are internally consistent interchangeable and therefore equivalent the excess silica content of pyroxene is easily illustrated in a three component condensed composition space and may be plotted directly from a structural formula unit or recalculated end members in order to evaluate possible mechanisms for the formation of free silica in eclogite we examined the net transfer reactions in model eclogites using a thompson reaction space we show that there are at least three broad classes of reactions that release free silica in eclogite i vacancy consumption in non stoichiometric pyroxene ii dissolution of ti phases in pyroxene or garnet iii reactions between accessory phases and either pyroxene or garnet we suggest that reliable interpretation of the significance of silica lamellae in natural clinopyroxene will require the evaluation not only of silica solubility but also of titanium solubility and the possible roles of accessory phases and inclusions on the balance of free silica

early paleozoic tectonic reactivation of the shaoxing jiangshan fault zone structural and geochronological constraints from the chencai domain south china abstract the shaoxing jiangshan fault zone sjfz as a fundamental neoproterozoic block boundary that separates the yangtze block from the cathaysia block is the key to understanding the evolution of south china from neoproterozoic block amalgamation to early paleozoic crustal reworking new structural observations coupled with geochronological ages from the chencai domain indicate that intense ductile deformation and metamorphism along the sjfz occurred at 460 420 ma in response to the early paleozoic orogeny in south china to the east of the sjfz the deformation involves widespread generations of ne striking foliation intrafolial folds and local development of sinistral oblique shear zones the shearing deformation occurred under amphibolite facies conditions at temperatures of 550 c locally even 650 c to the west of the sjfz the deformation corresponds to sinistral oblique shearing along ne striking steep dipping zones under greenschist facies conditions at temperatures of 400 500 c these deformation styles as typical mid crustal expressions of continental reworking reflect tectonic reactivation of the pre existing deeply rooted neoproterozoic block boundary in the early paleozoic we infer that the tectonic reactivation possibly induced by oblique underthrusting of north cathaysia facilitated ductile shearing and burial metamorphic reactions giving rise to the high strain zones and high grade metamorphic rocks with respect to pre existing mechanical weakness our work highlights the role of tectonic reactivation of early structures in localizing later deformation before it propagates into yet undeformed domains