26al in chondrules from unequilibrated l chondrites onset and duration of chondrule formation in the early solar system abstract al mg isotope records in sixteen ferromagnesian chondrules from five unequilibrated ordinary chondrites uocs four l chondrites with petrologic grades varying from 3 0 to 3 2 and one l ll chondrite with petrologic grade 3 1 were studied using an ion microprobe to look for presence of the now extinct nuclide 26 al at the time of their formation uocs of low petrologic grades were selected to avoid possible thermal perturbation of al mg isotope records in the analyzed chondrules the presence of resolved 26 mg excess due to decay of 26 al in al rich phases such as glassy mesostasis and plagioclase was found in all the chondrules barring a sole exception the inferred initial 26 al 27 al ratios in the chondrules range from 0 6 1 9 10 5 and most of them have values lying between 0 6 1 1 10 5 our data supports a late formation of chondrules relative to cais suggested in previous studies and indicates a time lag of at least 1 5 ma between cai formation and onset of an intense episode of chondrule formation in the early solar system that lasted less than a million years the paucity of chondrules 26 al 27 al 10 5 rules out active chondrule forming events at earlier epochs the presence of rare chondrules with initial 26 al 27 al 5 in uocs of low petrologic grades 3 0 to 3 2 analyzed by us and also reported previously in uocs of petrologic grade ranging up to 3 4 most probably represent parent body processes leading to disturbance of al mg isotope systematics in these cases an extended duration of chondrule formation lasting more than a couple of million years suggested in some earlier studies needs reassessment a long duration of chondrule formation is also difficult to reconcile with the problem of storage isolation mixing and assimilation of chondrules into their meteorite parent bodies

evolution of protoplanetary disk inferred from 26 al chronology of individual chondrules abstract we review the 26al ages of chondrules in various type 3 0 chondrites the 26al ages of chondrules are 1 3 myr after calcium aluminum rich inclusion cai for ll3 0 co3 0 and acfer 094 ungrouped c 3 0 available data for chondrules in cr chondrites indicate that many chondrules are relatively younger 3 myr although data from chondrules in cr3 0 are not yet available to confirm their younger ages the total ranges for the 26al ages of chondrules in a single chondrite group are more than 0 5 1 myr however most chondrules show relatively narrow range of ages in a single chondrite group 0 2 0 4 myr 1 sd which might be short enough to preserve the group specific chemical and isotope signatures against radial diffusion of solid in the disk distinct oxygen isotope reservoirs might exist in the protoplanetary disk simultaneously which could be spatially separated

fabric transitions in the saxony granulite terrain abstract quartz fabric transitions reported from the saxony granulite terrain and from the nearby ore mountains are interesting because of the changes in the pattern types and the variation in the opening angles of the fabric skeletons a short summary is presented of salient features in research documented in the east german literature between 1961 and 1978 the changes in fabric skeleton with decreasing metamorphic grade are proposed as mainly consequent to the deactivation of prism c glide systems as temperature decreases fabrics indicative of the appearance of the c burgers vector might be diagnostic for deformation in regimes of comparatively high temperatures or low strainrates or for deformation under conditions such that there is a relatively high kinetically effective hydroxyl content in the quartz structure

an experimental study of soil memory and preconsolidation adjacent to an active tectonic structure the meers fault oklahoma usa abstract special consolidation tests were run on undisturbed samples to study the ability of quaternary soils adjacent to the meers fault in southwestern oklahoma to record and remember the maximum effective preconsolidation stresses they experienced during the faulting process the results show that the soils record 60 of the applied total stresses as preconsolidation stresses in 2 s of loading time indicating that these stresses could have been recorded during an earthquake faulting event to record all of the applied total stresses as preconsolidation stresses 100 recording or memory the loading needs to last at least 4 5 min

26al in plagioclase rich chondrules in carbonaceous chondrites evidence for an extended duration of chondrule formation abstract the 26al 26mg isotope systematics in 33 petrographically and mineralogically characterized plagioclase rich chondrules prcs from 13 carbonaceous chondrites ccs one ungrouped acfer 094 six cr five cv and one co reveal large variations in the initial 26al 27al ratio 26al 27al 0 well resolved 26mg excesses 26mg from the in situ decay of the short lived nuclide 26al t1 2 0 72 ma were found in nine chondrules two from acfer 094 five from the cv chondrites allende and efremovka and one each from the paired cr chondrites eet 92147 and eet 92042 with 26al 27al 0 values ranging from 3 10 6 to 1 5 10 5 data for seven additional chondrules from three cv and two cr chondrites show evidence suggestive of the presence of 26al but do not yield well defined values for 26al 27al 0 while the remaining chondrules do not contain excess radiogenic 26mg and yield corresponding upper limits of 11 2 10 6 for 26al 27al 0 the observed range of 26al 27al 0 in prcs from ccs is similar to the range seen in chondrules from unequilibrated ordinary chondrites uocs of low metamorphic grade 3 0 3 4 however unlike the uoc chondrules there is no clear trend between the 26al 27al 0 values in prcs from ccs and the degree of thermal metamorphism experienced by the host meteorites high and low values of 26al 27al 0 are found equally in prcs from both ccs lacking evidence for thermal metamorphism e g crs and ccs where such evidence is abundant e g cvs the lower 26al 27al 0 values in prcs from ccs relative to most cais are consistent with a model in which 26al was distributed uniformly in the nebula when chondrule formation began approximately a million years after the formation of the majority of cais the observed range of 26al 27al 0 values in prcs from ccs is most plausibly explained in terms of an extended duration of 2 3 ma for the formation of cc chondrules this interval is in sharp contrast to most cais from ccs whose formation appears to be restricted to a narrow time interval of less than 105 years the active solar nebula appears to have persisted for a period approaching 4 ma encompassing the formation of both cais and chondrules present in ccs and raising important issues related to the storage assimilation and mixing of chondrules and cais in the early solar system

an fe isotope study of ordinary chondrites the fe isotope composition of ordinary chondrites and their constituent chondrules metal and sulphide grains have been systematically investigated bulk chondrites fall within a restricted isotopic range of 1 0 84 to 0 21 relative to the irmm 14 fe standard fe isotope compositions do not vary systematically with the very large differences in total fe concentration or oxidation state of the h l and ll chondrite classes similarly the fe isotope compositions of chondrules do not appear to be determined by the h l or ll classification of their host chondrite this may support an origin of the three ordinary chondrite groups from variable accretion of identical fe bearing precursors a close relationship between isotopic composition and redistribution of fe during metamorphism on ordinary chondrite parent bodies was identified the largest variations in chondrule compositions were found in chondrites of the lowest petrologic types the clear link between element redistribution and isotopic composition has implications for many other non traditional isotope systems e g mg si ca cr isotopic compositions of chondrules may also be determined by their melting history porphyritic chondrules exhibit a wide range in isotope compositions whereas barred olivine and radial pyroxene chondrules are generally isotopically heavier than the ordinary chondrite mean very large chondrules preserve the greatest heterogeneity of fe isotopes the mean fe isotope composition of bulk ordinary chondrites was found to be 0 06 0 12 2 sd this is isotopically lighter than the terrestrial mean composition and all other published non chondritic meteorite suites e g lunar and martian samples eucrites pallasites and irons ordinary chondrites though the most common meteorites found on earth today were not the sole building blocks of the terrestrial planets

a short duration of chondrule formation in the solar nebula evidence from 26al in semarkona ferromagnesian chondrules abstract the 26 al 26 mg systems of five ferromagnesian chondrules from the least metamorphosed ordinary chondrite semarkona ll3 0 were studied using a secondary ion mass spectrometer their glass or plagioclase portions contain excesses of 26 mg and in two chondrules the 26 mg excesses are well correlated with 27 al 24 mg which demonstrates the in situ decay of 26 al the initial 26 al 27 al ratios in these chondrules obtained from the slope of isochrons show a narrow range of between 6 10 6 and 9 10 6 indicating their short formation duration of less than 1 my if the solar nebula was initially homogeneous in al isotopes the chondrule formation ages are 2 my younger than those of cais our results based on the study of the least metamorphosed uoc are consistent with the previous studies on al rich chondrules that the chondrule formation started at least 2 my after cais alternatively the older records before 2 my were erased by chondrule recycling process it further suggests that the young apparent ages 3 to 5 my after cais for chondrules in type 3 4 uocs are due to the disturbance of the 26 al 26 mg system possibly during parent body metamorphism the result is not consistent with the extended nebular time scale of 5 my and the chondrule formation by planetary processes the ni isotopic analysis of the feo rich olivines in a type ii chondrule in semarkona did not show any detectable excess 60 ni in spite of their high fe ni ratios the upper limit of the initial 60 fe 56 fe ratio of the solar system was estimated to be 3 4 10 7 which is consistent with the previous estimate 0 2 1 9 10 7 from eucrites this result confirms that the 60 ni excess previously observed from cais was not due to the decay of the short lived nuclide 60 fe but a ni isotopic anomaly of nucleosynthetic origin

how did the meers fault scarp form paleoearthquake or aseismic creep a soil mechanical perspective abstract studies confirm that the meers fault in southwestern oklahoma has been active in recent times the most recent movement occurred about 1100 y ago in late holocene there is as much as 5 m vertical and possibly appreciably more left lateral strike slip displacements on the fault during faulting the quaternary soils 1 along the fault were folded as well as ruptured in some places almost all of the deformation is accommodated by ductile folding of the soils having this kind of deformation with no record of an earthquake associated with the meers fault during historical times raises the question whether the present scarp was formed seismically by earthquake event s or aseismically by slow deformation aseismic fault creep to determine how the scarp was formed i have developed a multidisciplinary study which involved geological soil mechanical and soil micromorphological techniques geological mapping delineated the deformation stratigraphy and any features that might be associated with the faulting the mapping was also needed to reconstruct the sequence of events that formed the scarp at the study site consolidation tests using the casagrande 1936 method for finding maximum effective stresses were used to determine the states of stresses imposed on the soil deposits when they were first faulted these states of stresses were then compared to the states of stresses needed to slowly deform or shear the soils in triaxial and direct drained shear tests the direction of maximum principal stress was determined by mohr s circle and soil micromorphological analyses the results of soil mechanical analysis show that the faulting of the quaternary soils along the fault had to be sudden or fast which indicates that the scarp was probably created seismically during faulting the soils were anisotropically consolidated or compacted thereby recording in their structure the states of stresses that caused faulting to occur the direction of the maximum principal horizontal stress is between n62 e and n72 e indicating rotation of principal stress axes the technique developed for this study can be used in similar areas where active faults offset quaternary soils with relatively unchanging moisture contents below some depth also for shallow depths this technique might give more reliable young tectonic stress measurements both magnitude and orientation than other techniques such as overcoring hydraulic fracturing and wellbore breakouts because it is used on geologically recent units which unlike lithified rocks have not yet existed through millions of years of deformation

fault valve behaviour in optimally oriented shear zones an example at the revenge gold mine kambalda western australia abstract quartz vein systems developed in and adjacent to shear zones host major gold deposits in the kambalda region of the norseman wiluna greenstone belt at the revenge mine two groups of mineralised reverse shear zones formed as conjugate near optimally oriented sets during ese subhorizontal shortening adjacent to a major transpressional shear system the shear zones developed at temperatures of about 400 c in a transitional brittle ductile regime deformation was associated with high fluid fluxes and involved fault valve behaviour at transiently near lithostatic fluid pressures during progressive evolution of the shear system early brittle and ductile deformation was overprinted by predominantly brittle deformation brittle shear failure was associated with fault dilation and the formation of fault fill veins particularly at fault bends and jogs a transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses the formation of subhorizontal stylolites locally subvertical extension veins and minor normal faults in association with thrust faulting indicates episodic or transient reorientation of the near field maximum principal stress from a subhorizontal to a near vertical attitude during some fault valve cycles local stress re orientation is interpreted as resulting from near total shear stress release and overshoot during some rupture events previously described fault valve systems have formed predominantly in severely misoriented faults the shear systems at revenge mine indicate that fault valve action and associated fluctuations in shear stress and fluid pressure can influence the mechanical behaviour of optimally oriented faults

26al records in chondrules from unequilibrated ordinary chondrites ii duration of chondrule formation and parent body thermal metamorphism abstract fossil records of the now extinct short lived nuclide 26al were analyzed in twelve chondrules from four unequilibrated ordinary chondrites uocs semarkona 3 0 bishunpur and y 791324 both 3 1 and alha76004 3 3 belonging to the ll group to infer their time of formation the present data along with those reported by us previously firmly establish a short duration of less than a million years for the major episode of uoc chondrule formation nebular processes responsible for chondrule formation started 1 ma after formation of cais and effectively stopped within another one and a half million years in the inner region of the asteroidal belt modeling of thermal evolution of chondritic parent bodies suggest that the longer duration of uoc chondrule formation inferred in some of the earlier studies could be attributed to thermal metamorphism experienced by some chondrules in uoc parent bodies the duration of formation of uoc chondrules is significantly shorter than that for carbonaceous chondrite chondrules

fractionation and fragmentation of glass cosmic spherules during atmospheric entry glass cosmic spherules are a sub type of s type cosmic spherules that are almost at the end phase of fractionation and volatilization during atmospheric entry a focused investigation of 36 glass spherules out of several hundred spherules reveals fractionation and fragmentation phenomena during atmospheric entry some of the glass spherules largely preserve relict pyroxene normative crystals within the spherules appear to have experienced temperatures of 1700 o c that resulted in vaporization of fe and other volatile elements such as s na and k from the silicate spherule resulting in the enrichment of sio2 mgo and cao in weight percent calculations with a chemical ablation model suggest an entry velocity of 11 16 km s to achieve the temperature required for glass spherule formation under these conditions the micrometeoroid undergoes a mass of loss of 20 any increase in entry velocity will increase the mass loss drastically and change its chemical composition most glass spherules beyond a certain threshold diameter of 300 m appear to undergo fragmentation resulting in two different spherules which could lead to an apparent enhancement of the percentage of glass spherules in unbiased collections siderophile and volatile element loss and subsequent development of a platinum group element pge nugget are observed in one of the spherules this appears to be the first reported observation of a platinum group nugget pgn in a glass spherule the nugget has a diameter of 3 m and contains all the six pge although some of the more volatile pge are depleted relative to an average chondritic composition interestingly the pge data of the nugget seem to have a trend which is similar to that of a pge nugget from a calcium aluminium rich inclusion cai the trace element data of these glass spherules suggest that their parent bodies were carbonaceous chondrites

stratigraphic evidence of holocene faulting in the mid continent the meers fault southwestern oklahoma stratigraphic relations and ten 14c ages show that movement occurred on the meers fault in late holocene time movement on the fault postdates the browns creek alluvium which began to be deposited between 14 000 and 13 000 yr b p and predates the east cache alluvium which was deposited between 800 and 100 yr b p surface warping along the fault led to local stream incision on the upthrown side of the fault and deposition of slopewash and fan alluvium on the down thrown side three 14c ages of charcoal and soil humus buried by fan alluvium indicate that faulting probably occurred between 1400 and 1100 yr b p the soil that formed in the fan alluvium is only slightly more developed than that in the east cache alluvium and the weak development of both soils indicates a geologically recent age that is consistent with the radiocarbon ages obtained for these deposits

river dynamics and slope processes along a sector of the villalvernia varzi line northern italy abstract the river dynamics and the slope processes along a sector of a regional tectonic line the villalvernia varzi line in the northwestern apennines northern italy were investigated this line has been referred to as a dextral transpressive system and has played an important but not yet well defined role in the structural context of the apennines the staffora river basin is characterised by a complex geological and structural setting of the outcropping soils consisting in sedimentary formations with a dominant clay component the planimetric and altimetrical characteristics and long term evolution of the riverbed were studied analysis of the thickness of the alluvial sediments allows division of the staffora river into two blocks the northern block with the minimum alluvial sediment thickness and the southern block with the highest values of thickness different factors could be responsible for this peculiarity including contribution of the tributaries presence of landslides and alluvial fans human intervention and tectonics the occurrence and distribution of the landslides with various influencing factors were analysed in order to verify the tectonics as factor of the slope movements and whose landslide characteristics could be related to it

bleached chondrules evidence for widespread aqueous processes on the parent asteroids of ordinary chondrites we present the first detailed study of a population of texturally distinct chondrules previously described by kurat 1969 christophe michel levy 1976 and skinner et al 1989 that are sharply depleted in alkalis and al in their outer portions these bleached chondrules which are exclusively radial pyroxene and cryptocrystalline in texture have porous outer zones where mesostasis has been lost bleached chondrules are present in all type 3 ordinary chondrites and are present in lower abundances in types 4 6 they are most abundant in the l and ll groups apparently less common in h chondrites and absent in enstatite chondrites we used x ray mapping and traditional electron microprobe techniques to characterize bleached chondrules in a cross section of ordinary chondrites we studied bleached chondrules from semarkona by ion microprobe for trace elements and h isotopes and by transmission electron microscopy chondrule bleaching was the result of low temperature alteration by aqueous fluids flowing through finegrained chondrite matrix prior to thermal metamorphism during aqueous alteration interstitial glass dissolved and was partially replaced by phyllosilicates troilite was altered to pentlandite but pyroxene was completely unaffected calcium rich zones formed at the inner margins of the bleached zones either as the result of the early stages of metamorphism or because of fluid chondrule reaction the mineralogy of bleached chondrules is extremely sensitive to thermal metamorphism in type 3 ordinary chondrites and bleached zones provide a favorable location for the growth of metamorphic minerals in higher petrologic types the ubiquitous presence of bleached chondrules in ordinary chondrites implies that they all experienced aqueous alteration early in their asteroidal histories but there is no relationship between the degree of alteration and metamorphic grade a correlation between the oxidation state of chondrite groups and their degree of aqueous alteration is consistent with the source of water being either accreted ices or water released during oxidation of organic matter ordinary chondrites were probably open systems after accretion and aqueous fluids may have carried volatile elements with them during dehydration individual radial pyroxene and cryptocrystalline chondrules were certainly open systems in all chondrites that experienced aqueous alteration leading to bleaching

gold quartz veins in metamorphic terranes and their bearing on the role of fluids in faulting gold quartz vein fields in metamorphic terranes such as greenstone belts provide evidence for the involvement of large volumes of fluids during faulting and may be products of seismic processes near the base of the seismogenic regime in the val d or district of the abitibi greenstone belt canada quartz tourmaline carbonate veins form a vein field 30 15 km in the hanging wall of a crustal scale fault zone which was the main channelway for upward migration of the deeply generated fluids the veins occur in small high angle reverse faults and in adjacent horizontal extensional fractures extending up to 75 m in intact rocks they have formed incrementally during active reverse faulting in response to crustal shortening at depths corresponding to those at the base of the seismogenic zone in actively deforming crust detailed structural and fluid inclusion studies provide evidence for generally lithostatic but fluctuating fluid pressures p of the order of 200 mpa and for cyclic stress reversals during vein formation and provide good support for the fault valve model a comparison of vein characteristics with standard earthquake rupture parameters suggests that each slip increment along veins in reverse faults was accompanied by a small earthquake 4 m 3 or less the large vein field thus represents both the extent of fluid dispersion in the hanging wall of a crustal scale channelway and the distribution of small earthquakes integrated over the lifetime of the hydrothermal system it is proposed that such small earthquakes along veins in reverse faults are related to large earthquakes m 6 nucleating near the base of the seismogenic regime along the nearby crustal scale fault either as aftershocks or as a precursory smarm

development of fluid conduits in the auriferous shear zones of the hutti gold mine india evidence for spatially and temporally heterogeneous fluid flow abstract the gold mineralization of the hutti mine is hosted by nine parallel n s trending steeply dipping 2 10 m wide shear zones that transect archaean amphibolites the shear zones were formed after peak metamorphism during retrograde ductile d 2 shearing in the lower amphibolite facies they were reactivated in the lower to mid greenschist facies by brittle ductile d 3 shearing and intense quartz veining the development of a s 2 s 3 crenulation cleavage facilitates the discrimination between the two deformation events and contemporaneous alteration and gold mineralization ductile d 2 shearing is associated with a pervasively developed distal chlorite sericite alteration assemblage in the outer parts of the shear zones and the proximal biotite plagioclase alteration in the center of the shear zones d 3 is characterized by development of the inner chlorite k feldspar alteration which forms a centimeter scale alteration halo surrounding the laminated quartz veins and replaces earlier biotite along s 3 the average size of the laminated vein systems is 30 50 m along strike as well as down dip and 2 6 m in width mass balance calculations suggest strong metasomatic changes for the proximal biotite plagioclase alteration yielding mass and volume increase of ca 16 and 12 respectively the calculated mass and volume changes of the distal chlorite sericite alteration ca 11 ca 8 are lower the decrease in 18 o values of the whole rock from around 7 5 for the host rocks to 6 7 for the distal chlorite sericite and the proximal biotite plagioclase alteration and around 5 for the inner chlorite k feldspar alteration suggests hydrothermal alteration during two stage deformation and fluid flow the ductile d 2 deformation in the lower amphibolite facies has provided grain scale porosities by microfracturing the pervasive steady state fluid flow resulted in a disseminated style of gold sulfide mineralization and a penetrative alteration of the host rocks alternating ductile and brittle d 3 deformation during lower to mid greenschist facies conditions followed the fault valve process ductile creep in the shear zones resulted in a low permeability environment leading to fluid pressure build up strongly episodic fluid advection and mass transfer was controlled by repeated seismic fracturing during the formation of laminated quartz gold veins the limitation of quartz veins to the extent of earlier shear zones indicate the importance of pre existing anisotropies for fault valve action and economic gold mineralization

short time interval for condensation of high temperature silicates in the solar accretion disk chondritic meteorites are made of primitive components that record the first steps of formation of solids in our solar system chondrules are the major component of chondrites yet little is known about their formation mechanisms and history within the solar protoplanetary disk spd we use the reconstructed concentrations of short lived 26al in chondrules to constrain the timing of formation of their precursors in the spd high precision bulk magnesium isotopic measurements of 14 chondrules from the allende chondrite define a 26al isochron with 26al 27al 1 2 0 2 10 5 for this subset of allende chondrules this can be considered to be the minimum bulk chondrule 26al isochron because all chondrules analyzed so far with high precision 50 chondrules from cv and ordinary chondrites have an inferred minimum bulk initial 26al 27al 1 2 10 5 in addition mineral 26al isochrons determined on the same chondrules show that their formation i e fusion of their precursors by energetic events took place from 0 myr to 2 myr after the formation of their precursors thus showing in some cases a clear decoupling in time between the two events the finding of a minimum bulk chondrule 26al isochron is used to constrain the astrophysical settings for chondrule formation either the temperature of the condensation zone dropped below the condensation temperature of chondrule precursors at 1 5 my after the start of the solar system or the transport of precursors from the condensation zone to potential storage sites stopped after 1 5 my possibly due to a drop in the disk accretion rate

26 al 26 mg systematics in chondrules from kaba and yamato 980145 cv3 carbonaceous chondrites we report the mineralogy petrography and in situ measured 26al 26mg systematics in chondrules from the least metamorphosed cv3 vigarano type chondrites kaba and yamato y 980145 two y 980145 chondrules measured show no resolvable excesses in 26mg 26mg a decay product of a short lived t1 2 0 7 ma radionuclide 26al plagioclase in one of the chondrules is replaced by nepheline indicative of thermal metamorphism the lack of 26mg in the y 980145 chondrules is most likely due to disturbance of their 26al 26mg systematics during the metamorphism although kaba experienced extensive metasomatic alteration 300 c it largely avoided subsequent thermal metamorphism and the 26al 26mg systematics of its chondrules appear to be undisturbed all eight kaba chondrules measured show 26mg corresponding to the initial 26al 27al ratios 26al 27al 0 ranging from 2 9 1 7 10 6 to 6 3 2 7 10 6 if cv parent asteroid accreted rapidly after chondrule formation the inferred 26al 27al 0 ratios in kaba chondrules provide an upper limit on 26al available in this asteroid at the time of its accretion the estimated initial abundance of 26al in the cv asteroid is too low to melt it and contradicts the existence of a molten core in this body suggested from the paleomagnetic records of allende carporzen et al 2011 magnetic evidence for a partially differentiated carbonaceous chondrite parent body proc natl acad sci usa 108 6386 6389 and kaba gattacceca et al 2013 more evidence for a partially differentiated cv parent body from the meteorite kaba lunar planet sci 44 abstract 1721

anorthite rich chondrules in cr and ch carbonaceous chondrites genetic link between ca al rich inclusions and ferromagnesian chondrules anorthite rich chondrules in cr and ch carbonaceous chondrites consist of magnesian low ca pyroxene and forsterite phenocrysts feni metal nodules interstitial anorthite al ti cr rich low ca and high ca pyroxenes and crystalline mesostasis composed of silica anorthite and high ca pyroxene three anorthite rich chondrules contain relic ca al rich inclusions composed of anorthite spinel al diopside and forsterite a few chondrules contain regions which are texturally and mineralogically similar to magnesian type i chondrules and consist of forsterite low ca pyroxene and abundant feni metal nodules anorthite rich chondrules in cr and ch chondrites are mineralogically similar to those in cv and co carbonaceous chondrites but contain no secondary nepheline sodalite or ferrosilite relatively high abundances of moderately volatile elements such as cr mn and si in the anorthite rich chondrules suggest that these chondrules could not have been produced by volatilization of the ferromagnesian chondrule precursors or by melting of the refractory materials only we infer instead that anorthite rich chondrules in carbonaceous chondrites formed by melting of the reduced chondrule precursors olivine pyroxenes feni metal mixed with the refractory materials including relic cais composed of anorthite spinel high ca pyroxene and forsterite the observed mineralogical and textural similarities of the anorthite rich chondrules in several carbonaceous chondrite groups cv co ch cr may indicate that these chondrules formed in the region s intermediate between the regions where cais and ferromagnesian chondrules originated this may explain the relative enrichment of anorthite rich chondrules in 16o compared to typical ferromagnesian chondrules russell et al 2000

origin of plagioclase olivine inclusions in carbonaceous chondrites plagioclase olivine inclusions pois are an abundant group of chondrule like objects with igneous textures found in carbonaceous chondrites pois consist of plagioclase olivine pyroxene and spinel and cover a wide range of compositions between type c ca al rich inclusions cais and ferromagnesian chondrules pois are distinguished from cais by the absence of melilite lack of refractory siderophile rich opaque assemblages more sodic plagioclase and abundance of olivine and aluminousenstatite rare accessory minerals including armalcolite zirconolite rutile and sapphirine are found in several pois the petrographie and chemical properties of pois indicate that they are not condensates or evaporative residues but formed by melting or partial melting of pre existing solids seven of fourteen pois contain isotopically fractionated mg and despite their textures these pois are not isotopically homogeneous spinel is the major carrier of fractionated mg in six pois the magnitude of mg isotopic fractionation fmg of spinel ranges from 8 to 11 amu among the pois but f mg is always either positive or negative within an individual pol within a single inclusion f mg of coexisting spinel is not constant but varies by up to 7 amu isotopic fractionation of mg in silicates is less common and always positive f mg 0 pyroxenes in two pois and plagioclase in a third contain isotopically heavy mg radiogenic 26 mg is rare in pois as only two inclusions show evidence of excess 26 mg with 26 mg 27 al 4 10 6 the preservation of isotopic heterogeneity in objects whose igneous textures suggest crystallization from a homogeneous melt implies that melting was incomplete allowing survival of relict phases a comparison of the essential characteristics of pois and cais suggests that the major processes leading to formation of pois including condensation dust gas fractionation aggregation of chemically and isotopically disparate materials and partial melting are common to most cais and chondrules we present a scenario for the formation of these objects and conclude that the homogeneity of the final assemblage cai poi or chondrule is primarily a reflection of the thermal history rather than the nature of precursor materials

dynamic recrystallization of quartz under greenschist conditions abstract the sem electron channelling technique has been used to investigate the dynamic recrystallization of three quartz grains representative of optical textures observed in quartzite deformed under greenschist facies conditions the degree of recrystallization has been determined at 10 20 50 and 100 respectively we demonstrate that dynamic recrystallization is a sequential process involving subgrain grain rotation and grainboundary migration it initiates via subgrain polygonization on a single crystal slip system and continues via subgrain and neoblast rotation on several slip systems in the early stages of recrystallization

alteration geochemistry and fluid inclusion characteristics of the greenstone hosted gold deposit of hutti eastern dharwar craton india gold mineralization of the hutti mine southern india is situated in closely spaced laminated quartz veins and associated alteration haloes along steeply dipping shear zones within a sequence of rather uniform amphibolites intense shearing has resulted in large scale mylonitization of the wall rocks anastomosing shear zones with intervening lensoid bodies of unsheared amphibolites are characteristic features of the deposit the general pattern of symmetrical alteration comprises a distal zone of chlorite rich rock with a proximal biotite rich zone adjacent to laminated quartz veins arsenopyrite thermometry yielded a temperature range of 350 477 c for the biotite alteration zone which preceded the formation of the laminated quartz veins mass balance calculations on the alteration zones indicate a gradual mass and volume loss during alteration the alteration is accompanied by intense potash metasomatism and addition of sulfur which resulted in the formation of arsenopyrite pyrrhotite and pyrite results of fluid inclusion studies suggest that low salinity 3 9 13 5 wt nacl equivalent h2o co2 rich fluids were responsible for gold rich laminated quartz vein formation in the hutti deposit these fluids constituted a later counterpart of the protracted fluid activity that first formed the biotite alteration zone the estimated p t values range from 1 0 to 1 7 kbar at 280 320 c these data along with the alteration assemblages and the characteristic gold sulfide association both in the altered wall rock and laminated quartz veins suggest that gold transported as reduced bisulfide complexes was deposited in response to sulfidation reactions in the wall rocks comparison of p t conditions of formation of gold quartz veins at hutti with two other large gold deposits in the eastern dharwar craton namely kolar 1 8 kbar 280 c and western ramagiri 1 45 1 7 kbar 240 270 c indicates broadly similar lode gold forming conditions in the dharwar craton

dominant coaxial deformation of veins during the interseismic stage of the fault valve cycle microfabrics of laminated quartz veins of the hutti gold mine india abstract auriferous shear veins formed during greenschist facies reactivation of amphibolite facies shear zones of the hutti mine the laminated vein fabric hydraulic breccias and fault gouge material formed by failure during the fault valve cycle microstructural observations in the laminated quartz veins indicate that porphyroclasts of quartz 1 scheelite 1 and pyrrhotite have an oblate shape quartz 2 grains with a width of 0 03 0 05 mm result from predominant subgrain rotation while predominant bulging recrystallization formed quartz 2 grains with a width of 0 01 0 03 mm the pole figures of the quartz texture are symmetrical with respect to a shortening direction perpendicular to the vein walls strain partitioning of shear stress into the rheologically weaker altered host rocks preserved these fabrics in the stronger quartz veins a non coaxial overprint of the veins is only weakly recorded by the microfabrics and quartz textures the study of microfabrics and quartz textures helps to understand processes of seismic activity in shear zones during fault valve behavior in the case of hutti coaxial deformation normal to the shear zone is indicated during the initial stages of shear stress and fluid pressure build up following the crack seal stages

polyphase deformation of mylonites from the renco gold mine zimbabwe identified by crystallographic preferred orientation of quartz abstract gold mineralization at renco mine in the northern marginal zone of the limpopo belt zimbabwe is hosted by a set of intersecting steep and shallow dipping shear zones locally termed reefs both steep and shallow reefs are closely associated with quartz mylonites the crystallographic preferred orientation of quartz from samples of these quartz mylonites indicate that deformation in steep reefs occurred under amphibolite facies conditions pole figures and microstructures of samples from shallow reefs in contrast show evidence for an origin of the auriferous mylonites at amphibolite facies conditions that were subsequently overprinted by deformation under lower greenschist facies conditions since mineralization is hosted by both reef geometries gold mineralization is inferred to have occurred during deformation and associated fluid infiltration along mylonites at amphibolite facies grades coeval with late archaean thrusting of rocks of the northern marginal zone onto rocks of the zimbabwe craton the crystallographic preferred orientation patterns of quartz point to a normal sense of movement during reactivation of the shallow reefs probably related to the meso proterozoic tectonometamorphic event that has affected the central parts of the limpopo belt at ca 2 0 ga normal faulting along the boundary between the northern marginal zone and zimbabwe craton has to date not yet been documented and may provide some important constraints for the understanding of the kinematics of the meso proterozoic tectonics in the limpopo belt

initial 26al 27al in carbonaceous chondrite chondrules too little 26al to melt asteroids we report 26mg excesses correlated with al mg ratios in five chondrules from the primitive co3 0 chondrite yamato 81020 that yield a mean initial 26al 27al ratio of only 3 8 0 7 10 6 about half that of ordinary chondrite oc chondrules even if asteroids formed immediately after chondrule formation this ratio and the mean al content of co chondrites is only capable of raising the temperature of a well insulated co asteroid to 940 k which is more than 560 k too low to produce differentiation the same ratio combined with the higher al content of cv chondrites results in a cv asteroid temperature of 1100 k we calculate that the mean initial 26al 27al ratio of about 7 4 10 6 found in ll chondrules is only able to produce small amounts of melting too little to produce differentiation these results cast serious doubt on the viability of 26al as the heat source responsible for asteroid differentiation inclusion of 60fe raises temperatures about 160 k but this increment is not enough to cause differentiation even of an ll chondrite asteroid

microstructures in feldspars from a major crustal thrust zone the grenville front ontario canada abstract samples collected across the nw grenville front in ontario canada have been used to document the change in feldspar microstructures produced during shearing under metamorphic conditions that range from upper amphibolite to lower greenschist grade the differential displacement across this 5 km thick section of the grenville front tectonic zone is at least 10 km at upper to mid amphibolite conditions both plagioclase and potassium feldspars behave plastically grainsize reduction occurs through grain boundary migration and or subgrain rotation and produces a core and mantle structure at middle to lower amphibolite grade grain size reduction is accomplished through grain boundary migration at lower amphibolite to upper greenschist grade grain size reduction occurs by the nucleation and growth of new grains with a different composition the abundance of intracrystalline deformation at this grade is inversely proportional to the amount of fine grained matrix in ultramylonite with a high matrix to porphyroclast ratio few intracrystalline strain features occur other than ubiquitous undulatory extinction myrmekite is well developed on the high stress boundaries in k feldspar at upper to mid greenshist conditions feldspars deform mainly by slip on synthetic shear fractures deformation twins undulatory extinction deformation bands and kink bands occur in plagioclase and perthite flames and myrmekite occur principally on high stress boundaries in k feldspar grain size reduction in feldspar is accomplished through cataclasis at middle to lower greenschist facies deformation in feldspars is dominated by antithetic fractures the transition from the dominance of synthetic to antithetic fractures is gradational below lower greenschist grade rocks deform by pervasive cataclasis of all minerals

soil particle and pore orientations during drained and undrained shear of a cohesive sandy silt clay soil the orientations of particles pores and other constituents of an artificially made cohesive sandy silt clay soil were studied to investigate how they change during drained and undrained shear th

dynamically recrystallised quartz c axis fabrics in greenschist facies quartzites singhbhum shear zone and its footwall eastern india influence of high fluid activity abstract oriented specimens of quartz tectonites from the singhbhum shear zone ssz an important tectonic element in the precambrian eastern indian shield and its footwall have been analysed for c axis fabrics of dynamically recrystallised quartz grains in individual specimens the fabric pattern is either an asymmetric type i cross girdle or an asymmetric kinked single girdle irrespective of whether the fabric is from the ssz or its footwall the asymmetry of the fabric confirms a southward thrust movement of the northern block on a northerly dipping shear zone the measured fabrics are characterised by a concentration near the y axis of the finite strain ellipsoid with an equally or subordinately developed concentration near the z axis although some earlier works suggest the y concentration of the quartz c axis fabric as due to the influence of higher temperature obtained in amphibolite facies the occurrence of a similar concentration in the quartz c axis fabric from quartz tectonites of the ssz and its footwall interpreted to have formed under greenschist facies condition is explained by formation of the fabric under high fluid activity during deformation

the role of fluids in syntectonic mass transport and the localization of metamorphic vein type ore deposists abstract in low to medium grade metamorphic environments the mobilization of species is closely associated with the operation of solution precipitation creep processes during foliation development such deformation processes involve dissolution of material from source sites which are usually discrete dissolution surfaces having orientations controlled by the applied stress and displacement fields mass transfer occurs via solute diffusion in an essentially static fluid network or by transport of dissolved species in a migrating fluid precipitation of material removed from dissolution sites occurs in a variety of sink sites including pore surfaces grain scale microcracks and veins sink sites may be close to or distantly removed from source sites the transport of material during deformation involving solution precipitation creep is largely controlled by the mobility of metamorphic fluids in regions of high fluid mobility significant mass transfer may occur over large distances fluid flow mass transfer paths and the development of sink sites in such cases are constrained by the structurally controlled development of high fluid pressure domains and the consequent mechanical enhancement of permeability by hydraulic fracture processes and the development of grain scale dilatancy these processes may result in focussed fluid flow and precipitation of mobilized species in restricted regions a proportion of syntectonically mobilized material is precipitated to form vein type deposits in hydraulic fracture arrays which develop in high fluid pressure environments the microstructures developed in minerals deposited in these sites reflect the interaction of a range of nucleation and growth processes which usually involve multiple generations of fracture growth and mineral crystallization the formation of syntectonic gold quartz vein deposits in central victoria australia illustrates the importance of structural controls on both the large scale migration of metamorphic fluids and the development of sites suitable for the precipitation of metamorphically mobilized species

relative timing of deformation and two stage gold mineralization at the hutti mine dharwar craton india gold mineralization at hutti is confined to a series of nine parallel n s to nnw sse trending steeply dipping shear zones the host rocks are amphibolites and meta rhyolites metamorphosed at peak conditions of 660 40 c and 4 1 kbar they are weakly foliated s1 and contain barren quartz extension veins the auriferous shear zones reefs are typically characterized by four alteration assemblages and laminated quartz veins which in places occupy the entire reef width of 2 10 m and contain the bulk of gold mineralization a 1 5 m wide distal chlorite sericite biotite calcite plagioclase alteration zone can be distinguished from a 3 5 m wide proximal biotite plagioclase quartz muscovite calcite alteration zone gold is both spatially and temporally associated with disseminated arsenopyrite and pyrite mineralization an inner chlorite k feldspar quartz calcite scheelite tourmaline sphene epidote sericite alteration halo which rims the laminated quartz veins is characterized by a pyrrhotite chalcopyrite sphalerite ilmenite rutile and gold paragenesis the distal chlorite sericite and proximal biotite plagioclase alteration assemblages are developed in microlithons of the s2 s3 crenulation cleavage and are replaced along s3 by the inner chlorite k feldspar alteration indicating a two stage evolution for gold mineralization ductile d2 shearing alteration and gold mineralization formed the reefs during retrograde evolution and fluid infiltration under upper greenschist to lower amphibolite facies conditions 560 60 c 2 1 kbar the reefs were reactivated in the d3 dextral strike slip to oblique slip environment by fault valve behavior at lower greenschist facies conditions ca 300 350 c which formed the auriferous laminated quartz veins later d4 crosscutting veins and d5 faults overprint the gold mineralization the alteration mineralogy and the structural control of the deposit clearly points to an orogenic style of gold mineralization which took place either during isobaric cooling or at different levels of the archean crust from overlaps in the tectono metamorphic history it is concluded that gold mineralization occurred during two tectonic events affecting the eastern dharwar craton in south india between ca 2550 2530 ma 1 the assemblage of various terranes of the eastern block and 2 a tectono magmatic event which caused late to posttectonic plutonism and a thermal perturbation it differs however from the pre peak metamorphic gold mineralization at kolar and the single stage mineralization at ramagiri notably greenschist facies gold mineralization occurred at hutti 35 90 million years later than in the western dharwar craton

contemporaneous formation of chondrules in distinct oxygen isotope reservoirs we investigated the 26al 26mg systematics of type i feo poor chondrules from one of the most primitive carbonaceous chondrites acfer 094 the inferred initial 26al 27al ratios of chondrules from acfer 094 ranged from 4 2 2 0 10 6 to 9 0 1 5 10 6 these chondrules have distinct oxygen isotope ratios 17o 5 2 and 0 though no correlation between inferred initial 26al 27al ratios and oxygen isotope ratios were observed these results indicate the regional oxygen isotope heterogeneity in the solar nebula over the duration of chondrule formation a few low ca pyroxene grains in chondrules have small but resolvable 26mg excesses at the level of 0 1 which may be the result of a partial melting of chondrules by reheating events if this is the case the true inferred initial 26al 27al ratios of these particular chondrule could be slightly lower 10 than estimates using mg isotope ratios of coexisting plagioclase and olivine stable isotope ratios of mg 25mg 24mg of many olivine grains in chondrules are positively fractionated by 1 relative to those in other phases within the same chondrules this suggests that olivine preserved a positively fractionated mg isotope ratios of the chondrule forming melt prior to significant condensation during the cooling stage of chondrule formation

near surface folding along an active fault seismic or aseismic abstract the meers fault in southwestern oklahoma has been active in recent times the most recent movement occurred about 1100 years ago in the late holocene during the faulting the quaternary age alluvial deposits along the fault were folded as well as ruptured in some places almost all of the deformation is accommodated by ductile folding of these deposits having this type of deformation with no record of an earthquake associated with the meers fault during historical times raises the question whether the present scarp was formed seismically by earthquake event s or aseismically by slow deformation aseismic fault creep triaxial shear tests with various shear rates were run to determine the conditions for brittle ductile and brittle ductile transition type failures relationships between ductility moisture content strain rate shear rate or failure time and confining stress were determined the results show that under the possible field conditions that existed during the faulting the ductile folding of these deposits is possible indicating the meers fault scarp could have been created contemporaneously with earthquake event s

the influence of grain boundary fluids on the microstructure of quartz feldspar mylonites quartz rich domains in pre alpine water deficient amphibolite facies 510 580 c 250 450 mpa pegmatite mylonites from mont mary mm western alps preserve a fine dynamically recrystallized grain size without significant annealing despite the high synkinematic temperatures and subsequent static greenschist facies alpine overprint the microstructure is dramatically different from more typical water rich amphibolite facies mylonites such as from the simplon fault zone in the central alps where the recrystallized grain size is on the millimetre scale the difference reflects the dominant strain induced recrystallization mechanism 1 progressive subgrain rotation and grain boundary bulging for the dry mm examples and 2 fast grain boundary migration for the wet simplon examples the grain boundary microstructure imaged with sem is also very different with most grain boundaries in the dry mm samples lacking porosity whereas grain boundaries in the wet samples are decorated by a multitude of pores quartz grain boundaries from both wet and dry samples are locally coated by thin 100 s of nanometres possibly amorphous silica films despite the differences in microstructure the crystallographic preferred orientations cpos of quartz rich domains from both areas are very similar water deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism s but apparently have little influence on the intracrystalline slip systems as reflected in the cpos strong c axis y maxima in mm mylonites both k feldspar and plagioclase an33 38 dynamically recrystallize consistent with the inferred metamorphic conditions under water deficient conditions mid to lower crustal rocks can deform heterogeneously under transitional ductile brittle conditions at high differential stress for mm ca 300 500 mpa as estimated for dry mohr coulomb failure and preserve this high stress microstructure because of the low mobility of dry grain boundaries

aluminum 26 in calcium aluminum rich inclusions and chondrules from unequilibrated ordinary chondrites in order to investigate the distribution of 26 a1 in chondrites we measured aluminum magnesium systematics in four calcium aluminum rich inclusions cais and eleven aluminum rich chondrules from unequilibrated ordinary chondrites uocs all four cais were found to contain radiogenic 26 mg 26 mg from the decay of 26 a1 the inferred initial 26 al 27 al ratios for these objects 26 al 27 al 0 5 10 5 are indistinguishable from the 26 al 27 al 0 ratios found in most cais from carbonaceous chondrites these observations together with the similarities in mineralogy and oxygen isotopic compositions of the two sets of cais imply that cais in uocs and carbonaceous chondrites formed by similar processes from similar or the same isotopic reservoirs or perhaps in a single location in the solar system we also found 26 mg in two of eleven aluminum rich chondrules the 26 al 27 al 0 ratio inferred for both of these chondrules is 1 10 5 clearly distinct from most cais but consistent with the values found in chondrules from type 3 0 3 1 uocs and for aluminum rich chondrules from lightly metamorphosed carbonaceous chondrites 0 5 10 5 to 2 10 5 the consistency of the 26 al 27 al 0 ratios for cais and chondrules in primitive chondrites independent of meteorite class implies broad scale nebular homogeneity with respect to 26 al and indicates that the differences in initial ratios can be interpreted in terms of formation time a timeline based on 26 al indicates that chondrules began to form 1 to 2 ma after most cais formed that accretion of meteorite parent bodies was essentially complete by 4 ma after cais and that metamorphism was essentially over in type 4 chondrite parent bodies by 5 to 6 ma after cais formed type 6 chondrites apparently did not cool until more than 7 ma after cais formed this timeline is consistent with 26 al as a principal heat source for melting and metamorphism

chondrules as products of dust collisions with totally molten droplets within a dust rich nebular environment an experimental investigation abstract chondrules are usually interpreted as the products of incomplete melting of mineral aggregates in the solar nebula and have textures controlled by the number of residual nuclei present after melting if barred olivine bo chondrules formed by nearly total melting at their respective liquidi i e over a wide temperature range it is surprising that there are so few totally glassy chondrules the occurrence of fine grained accretionary rims on chondrules suggests mineral dust in the chondrule forming environment which might have collided with totally molten droplets to produce chondrule textures we have therefore conducted experiments in which mineral dusts encounter molten droplets during their cooling at 500 c h and cause nucleation our experiments reproduce the majority of common chondrule textures the production of textures in our experiments is a function of the temperature at which dust is encountered the number of dust grains encountered and to a lesser extent the mineralogy of the encountered mineral grains crystal growth rates increase with lower dust encounter temperatures giving a textural range with increasingly skeletal or elongated crystals for lower temperature encounters we have produced some less commonly discussed types of chondrule textures e g barred olivine porphyritic olivine pyroxene bo pop by dust seeding that have not been reproduced by the incomplete melting of starting materials the reproduction of bo pop and excentroradial textures by dust collisions and the scarcity of natural totally glassy chondrules suggest that formation by seeding of total melts was common for chondrules with the lowest liquidus temperatures e g feo rich chondrules from these seeding experiments we suggest that the maximum melting temperature chondrules experienced is limited only to the temperature at which evaporation of chondrule liquid occurs however we have reproduced the very fine grained microporphyritic textures characteristic of type ia chondrules only by incomplete melting such chondrules could not have reached average internal temperatures above their liquidus temperatures the conclusion that at least some chondrules formed from total melts that collided with mineral dust during cooling indicates that chondrule formation occurred within a region of the nebula rich in mineral dust

a petrographic chemical and isotopic study of calcium aluminum rich inclusions and aluminum rich chondrules from the axtell cv3 chondrite petrographic compositional and isotopic characteristics were studied for three calcium aluminum rich inclusions cais and four plagioclase bearing chondrules three of them al rich from the axtell cv3 chondrite all seven objects have analogues in allende cv3 and other primitive chondrites yet axtell like most other chondrites contains a distinctive suite of cais and chondrules in common with allende cais cais in axtell exhibit initial 26 al 27 al ratios 26 al 27 al 0 ranging from 5 10 5 to 2 ma after the first cais as in other cv3 chondrites some objects in axtell show evidence of isotopic disturbance axtell has experienced only mild thermal metamorphism 600 c probably not enough to disturb the al mg systematics its cais and chondrules have suffered extensive metasomatism probably prior to final accretion these data indicate that cais and chondrules in axtell and other meteorites had an extended history of several million years before their incorporation into the axtell parent body these long time periods appear to require a mechanism in the early solar system to prevent cais and chondrules from falling into the sun via gas drag for several million years before final accretion we also examined the compositional relationships among the four plagioclase bearing chondrules two with large anorthite laths and two barred olivine chondrules and between the chondrules and cais three processes were examined 1 igneous differentiation 2 assimilation of a cai by average nebular material and 3 evaporation of volatile elements from average nebular material we find no evidence that igneous differentiation played a role in producing the chondrule compositions although the barred olivine compositions can be related by addition or subtraction of olivine methods 2 and 3 could have produced the composition of one chondrule axch 1471 but neither process explains the other compositions our study indicates that plagioclase bearing objects originated through a variety of processes

the microstructure of semarkona and bishunpur abstract the ordinary chondrites semarkona and bishunpur appear to have experienced in situ hydrous alteration here we report the results of the first detailed tem examination of them which supports this conclusion in semarkona all but the most magnesian mafic minerals have been destroyed and primary sulphides most likely troilite have been partially altered to phyllosilicates and ni rich pyrrhotite in bishunpur alteration is confined to the production of smectite from an amorphous feldspathic material in the chondrule rims and interchondrule matrix during hydrous alteration of both meteorites the temperature probably did not exceed 260 c the equilibrium composition of the gas present during the alteration of semarkona was not solar but was dominated by h2o the formation of smectite does not require the presence of liquid water however evidence for the redistribution of elements such as ca zn and si over tens of microns or more implies that there was at least some transient grain boundary fluid present the alteration in semarkona and bishunpur is far from complete thermodynamic modelling suggests that more altered ordinary chondrites would be mineralogically similar to the cm and ci carbonaceous chondrites superficially the mineralogy mineral chemistry and bulk chemistry of semarkona matrix rims resemble those of some chondritic interplanetary dust particles cidps the ordinary chondrite parent bodies may therefore be a source of some cidps

comment on known and suggested quaternary faulting in the midcontinent united states by russell l wheeler and anthony crone abstract studying previously published and other publicly available geologic evidence of quaternary faulting in the midcontinent united states wheeler and crone eng geol 62 2001 51 judged that the 30 km northwest extension of the meers fault in southwestern oklahoma suggested by cetin cetin h 1992 the northwest extension of the meers fault in southwestern oklahoma proc 28th symp eng geology and geotechnical eng university of idaho boise id pp 43 58 is not of quaternary age and claimed that holocene surface rupture has been limited to the previously reported 26 37 km section of the fault to the southeast surface rupture length is one of the most commonly used parameters to infer magnitudes of paleoearthquakes and thus potential future earthquakes in seismic hazard studies as a result underestimating fault dimensions may lead to serious errors in earthquake hazard evaluation evidence for the northwestward extension include displaced terrace deposits of pleistocene age displaced buried and or overthickened soil horizons fault related colluvium deposits colluvial wedges found near and only on the downthrown side of the fault active oil seepage near the fault deflection of stream alignments and the land use pattern along the fault the assessment of the northwestward extension of the meers fault by wheeler and crone thus appears to be mistaken

style and timing of holocene surface faulting on the meers fault southwestern oklahoma abstract stratigraphic relations and radiocarbon ages of deposits exposed in several trenches and excavations help to establish the timing sense of slip and style of the deformations that resulted from late holocene surface faulting on the meers fault in southwestern oklahoma the eastern half of the scarp is formed on relatively ductile permian hennessey shale and quaternary alluvium whereas the western half is formed on well lithified relatively brittle permian post oak conglomerate in the slick hills

the formation and alteration of the renazzo like carbonaceous chondrites iii toward understanding the genesis of ferromagnesian chondrules to better understand the formation conditions of ferromagnesian chondrules from the renazzo like carbonaceous cr chondrites a systematic study of 210 chondrules from 15 cr chondrites was conducted the texture and composition of silicate and opaque minerals from each observed feo rich type ii chondrule and a representative number of feo poor type i chondrules were studied to build a substantial and self consistent data set the average abundances and standard deviations of cr2o3 in feo rich olivine phenocrysts are consistent with previous work that the cr chondrites are among the least thermally altered samples from the early solar system type ii chondrules from the cr chondrites formed under highly variable conditions e g precursor composition redox conditions cooling rate with each chondrule recording a distinct igneous history the opaque minerals within type ii chondrules are consistent with formation during chondrule melting and cooling starting as s and ni rich liquids at 988 1350 c then cooling to form monosulfide solid solution mss that crystallized around olivine pyroxene phenocrysts during cooling fe ni metal crystallized from the s and ni rich liquid and upon further cooling mss decomposed into pentlandite and pyrrhotite with pentlandite exsolving from mss at 400 600 c the composition texture and inferred formation temperature of pentlandite within chondrules studied here is inconsistent with formation via aqueous alteration however some opaque minerals fe ni metal versus magnetite and panethite present in type ii chondrules are a proxy for the degree of whole rock aqueous alteration the texture and composition of sulfide bearing opaque minerals in graves nunataks 06100 and grosvenor mountains 03116 suggest that they are the most thermally altered cr chondrites

physical properties of ordinary chondrites physical properties intrinsic and bulk densities porosity compressional and shear wave velocities thermal diffusivity and conductivity are measured on 11 ordinary chondrites one carbonaceous chondrite and two achondritic clasts of a mesosiderite with the previously reported results from antarctic ordinary chondrites the data are useful in clarifying the difference between h and l chondrites for example the intrinsic density of h chondrites 3800 kg m3 is generally higher than that of l chondrites 3600 kg m3 the sample porosity less than 20 strongly controls its elastic wave velocities thermal diffusivity and conductivity the variations of elastic wave velocities and thermal properties with porosity show that as with lunar rocks chondrites contain many cracks these cracks are thought to be created on their parent bodies by the cumulative effects of many mutual collisions and impacts thermal properties are linear functions of elastic wave velocities for both h and l chondrites the difference between h and l chondrites can be attributed to the content of metallic fe ni correlations between petrologic types which roughly represent metamorphic temperature and porosity are less obvious for l chondrites than for h chondrites since porosity is not changed significantly by impact events it appears that the sintering process for l chondrites is independent of the metamorphic events represented by petrologic type

mg diffusion in anorthite implications for the formation of early solar system planetesimals we have measured the self diffusion coefficients of mg ca and sr in anorthitic plagioclase in order to assess the potential of mg isotopic heterogeneities in early solar system planetesimals to survive thermal metamorphism diffusion couples were constructed from polished single crystals of natural anorthite and synthetic isotopically enriched anorthite glass couples were annealed at atmospheric pressure and 1200 1400 c and isotopic concentration profiles were measured with an ion microprobe the results show that mg diffusion in anorthite is surprisingly fast with d mg being over 2 orders of magnitude greater than d sr this indicates that the diffusion coefficient of mg in anorthite cannot be approximated with that for sr mg diffusion in the c direction is also slightly faster than in the b direction while ca and sr diffusion appear to be isotropic the results provide important constraints on the thermochronological history of anorthite bearing mineral assemblages that preserve radiogenic 26 mg excesses in a planetesimal heated by the decay of 26 al the temperature at any point depends on the planetesimal size time of formation thermal conductivity and depth within the planetesimal given sufficient heating 26 mg heterogeneities produced by the in situ decay of 26 al in ca al rich inclusions cais and chondrules will be erased by diffusive equilibration using the self diffusion coefficient for mg in anorthite measured in this study we show that the common occurrence of 26 mg excesses in these inclusions requires that they must be stored in small 15 km bodies or the outermost rims of larger bodies for the first 1 2 million years of the solar system s history for early formed bodies larger than 15 km most of the mass will have been heated sufficiently for any radiogenic 26 mg to have been diffusively homogenized in the mg rich planetary environment

the onset of metamorphism in ordinary and carbonaceous chondrites ordinary and carbonaceous chondrites of the lowest petrologic types were surveyed by xray mapping techniques a variety of metamorphic effects were noted and subjected to detailed analysis using electron microprobe scanning electron microscopy sem transmission electron microscopy tem and cathodoluminescence cl methods the distribution of cr in feo rich olivine systematically changes as metamorphism increases between type 3 0 and type 3 2 igneous zoning patterns are replaced by complex ones and cr rich coatings develop on all grains cr distributions in olivine are controlled by the exsolution of a cr rich phase probably chromite cr in olivine may have been partly present as tetrahedrally coordinated cr3 separation of chromite is nearly complete by petrologic type 3 2 the abundance of chondrules showing an inhomogeneous distribution of alkalis in mesostasis also increases with petrologic type tem shows this to be the result of crystallization of albite residual glass compositions systematically change during metamorphism becoming increasingly rich in k glass in type i chondrules also gains alkalis during metamorphism both types of chondrules were open to an exchange of alkalis with opaque matrix and other chondrules the matrix in the least metamorphosed chondrites is rich in s and na the s is lost from the matrix at the earliest stages of metamorphism due to coalescence of minute grains progressive heating also results in the loss of sulfides from chondrule rims and increases sulfide abundances in coarse matrix assemblages as well as inside chondrules alkalis initially leave the matrix and enter chondrules during early metamorphism feldspar subsequently nucleates in the matrix and na re enters from chondrules these metamorphic trends can be used to refine classification schemes for chondrites cr distributions in olivine are a highly effective tool for assigning petrologic types to the most primitive meteorites and can be used to subdivide types 3 0 and 3 1 into types 3 00 through 3 15 on this basis the most primitive ordinary chondrite known is semarkona although even this meteorite has experienced a small amount of metamorphism allan hills alh a77307 is the least metamorphosed co chondrite and shares many properties with the ungrouped carbonaceous chondrite acfer 094 analytical problems are significant for glasses in type ii chondrules as na is easily lost during microprobe analysis as a result existing schemes for chondrule classification that are based on the alkali content of glasses need to be revised

oxygen isotope systematics of chondrules in the allende cv3 chondrite high precision ion microprobe studies the oxygen three isotope systematics of 36 chondrules from the allende cv3 chondrite are reported using high precision secondary ion mass spectrometer cameca ims 1280 twenty six chondrules have shown internally homogenous 17 o values among olivine pyroxene and spinel within a single chondrule the average 17 o values of 19 feo poor chondrules 13 porphyritic chondrules 2 barred olivine chondrules and 4 chondrule fragments show a peak at 5 3 0 6 2sd another 5 porphyritic chondrules including both feo poor and feo rich ones show average 17 o values between 3 and 2 and 2 other feo poor barred olivine chondrules show average 17 o values of 3 6 and 0 these results are similar to those for acfer 094 chondrules showing bimodal 17 o values at 5 and 2 nine porphyritic chondrules contain olivine grains with heterogeneous 17 o values as low as 18 indicating that they are relict olivine grains and some of them were derived from precursors related to refractory inclusions however most relict olivine grains show oxygen isotope ratios that overlap with those in homogeneous chondrules the 17 o values of four barred olivine chondrules range from 5 to 0 indicating that not all bo chondrules plot near the terrestrial fractionation line as suggested by previous bulk chondrule analyses based on these data we suggest the presence of multiple oxygen isotope reservoirs in local dust rich protoplanetary disk from which the cv3 parent asteroid formed a compilation of 225 olivine and low ca pyroxene isotopic data from 36 chondrules analyzed in the present study lie between ccam carbonaceous chondrite anhydrous mineral and young and russell lines these data define a correlation line of 17 o 0 982 0 019 18 o 2 91 0 10 which is similar to those defined by chondrules in cv3 chondrites and acfer 094 in previous studies plagioclase analyses in two chondrules plot slightly below the ccam line with 17 o values of 2 6 which might be the result of oxygen isotope exchange between chondrule mesostasis and aqueous fluid in the cv parent body

structural style deformation mechanisms and paleodifferential stress along an exposed crustal section constraints on the rheology of quartzofeldspathic rocks at supra and infrastructural levels bohemian massif abstract investigations of variscan shortening of quartzofeldspathic rocks have been carried out along an exposed crustal section located in the western part of the tepla barrandian unit bohemian massif the study focuses on determining differential stress deformation mechanisms and geometry of cadomian metagreywackes and cambrian granitoids which show different grade of variscan barrovian type metamorphism ranging from the greenschist subgreenschist facies boundary semibrittle deformation regime to the high grade amphibolite facies the semibrittle deformation regime t ca 300 c is characterized by large scale thrusting within a folding environment and by similar intensities of cataclasis diffusive mass transfer and crystal plasticity of quartz the latter including low temperature migration recrystallization within cadomian quartz veins of the greenschist facies part dislocation creep of quartz operated at differential stresses ranging from ca 70 mpa in the lowermost greenschist facies to ca 20 mpa in the uppermost greenschist facies these values have been derived by paleopiezometry using the grain size of recrystallized quartz they are interpreted as upper bounds for the bulk stress because of intense diffusive mass transfer within the adjacent metagreywackes a marking increase in recrystallized grain size of quartz occurs within the lower greenschist facies where low temperature grain boundary migration is replaced by subgrain rotation thus causion is needed when applying the recrystallized grain size paleopiezometer to this crustal level pressure solution of quartz and development of discrete crenulation cleavage was common in fine grained metagreywackes of the lower greenschist facies whereas the upper greenschist facies t ca 500 570 c is characterized by increasing intensity of metamorphic reactions grain coarsening and development of a schistosity the entire greenschist facies level forms a suprastructure where viscosity contrasts between mica and quartz rich layers were sufficiently high for buckle folding strain protected intercalated metagranitoids on the other hand do not show pervasive deformation fabrics the amphibolite facies part forms an infrastructure characterized by pervasive mylonitic shearing of both cambrian granitoids and metagreywackes it is suggested that the combination of the following processes has contributed to weaken the amphibolite facies level making nonlocalized flow possible 1 increased recrystallization of feldspar 2 crystal plastic softening of quartz due to high temperature grain boundary migration and activity of prism slip and 3 enhanced diffusive mass transfer in the form of metamorphic reactions the structural breaks at the semibrittle crustal level and at the greenschist amphibolite facies boundary supports the view that rheological boundaries are prone to form major detachment horizons in the continental crust

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

palaeoseismic events recorded in archaean gold quartz vein networks val d or abitibi quebec canada archaean gold quartz vein deposits are commonly hosted in high angle reverse shear zones and are interpreted to have formed in a regime of horizontal compression and high fluid pressure environment this paper presents the results of a combined structural and fluid inclusion study on three gold quartz vein deposits of the val d or area abitibi quebec consisting of subhorizontal extensional veins and e w steeply dipping shear veins crack seal structures tourmaline fibres stretched quartz crystals and open space filling textures indicate that the subhorizontal veins formed by hydraulic fracturing under supralithostatic fluid pressure co2 rich and h2o nacl fluid inclusions interpreted as two coexisting immiscible fluids occur typically in microcracks of different orientations interpreted to have formed in the 1 2 plane horizontal co2 rich fluid inclusion planes are contemporaneous with the opening of these veins 3 vertical vertical h2o nacl fluid inclusion planes as well as some microstructures such as deformed minerals indicate that the same extensional veins have experienced episodic vertical shortening 3 horizontal alternating with the opening events deformation and slip opening also occurred in shear veins in which preferred orientation of fluid inclusion planes is not clear except that the h2o nacl fluid inclusion planes tend to be oriented at high angles to the slip direction the successive opening and collapse events in subhorizontal extensional veins are correlated with deformation and slip opening events in shear veins respectively and are attributed to cyclic fluid pressure fluctuations in the system they are thus consistent with the fault valve model sudden drop in fluid pressure from supralithostatic to lower values induces fluid unmixing and occurs immediately post failure following seismic rupturing along the shear zone sealing of the shear veins allows the fluid pressure to build up again and the subhorizontal extensional veins to form the intermittent change in orientation of 3 in horizontal extensional veins is ascribed to a total shear stress release along the neighbouring shear veins after rupture the complex internal structures of the gold quartz veins provide an excellent record of archaean seismic activity along high angle reverse faults

preconsolidation stress in the vega baja and media areas of the river segura se spain causes and relationship with piezometric level changes financiado por el ministerio de ciencia y tecnologia y feder proyecto tec 2005 06863 por la generalitat valenciana proyecto gv06 179 y grupos03 085 y por la universidad de alicante proyecto vigrob 157

the influence of three dimensional grain size distributions on the rheology of polyphase rocks abstract in this study the relation between the deformational behaviour of two phase rocks and the grain size distributions of the constituent phases is investigated samples of hot isostatically pressed 50 50 volume percent mixtures of calcite and anhydrite were experimentally deformed in a triaxial apparatus at a temperature of 500 c a confining pressure of 300 mpa and a strain rate of 2 10 5 s 1 for calcite and anhydrite these are the deformational conditions for dislocation and diffusion creep respectively and for a grain size of 8 m these are the conditions of the equiviscous point it was found that samples from different batches of the synthetic material while having an identical average two dimensional grain size displayed significantly different rheological behaviour the ensuing full analysis revealed that the volumetric three dimensional grain size distributions of the different phases and the different batches varied significantly since the anhydrite is in the transition between the dislocation and the grain size sensitive diffusion creep regime the deformational behaviour of the bulk depends critically on the grain size distribution of the anhydrite phase the usefulness of the concept of an average grain size is discussed

reply to comment on evaluation of meers fault oklahoma in known and suggested quaternary faulting in the midcontinent united states by russell l wheeler and anthony j crone

the himalayan main central thrust pile and its quartz rich tectonites in central nepal as a consequence of the india asia collision a deeply rooted crustal slice has been moved southwards over the northern boundary of the indian plate at the present level of observation the main central thrust pile a rock pile of several kilometers thickness is affected by shear deformation which imprinted a flat foliation and a constant n s trending lineation these tectonites were deformed under a wide range of metamorphic conditions grading from lower greenschists up to the sillimanite field the m c t shear zone is studied from the view point of the quartz rich rocks over 100 samples helped to define a microstructural zonation roughly parallel to the thermal one 35 samples particularly rich in quartz have been used for the preferred orientation study of their c axes and a few of them for their a axes strong lattice orientations when undisturbed by mainly secondary recrystallization attest to large plastic strains in quartz the c axes are mostly distributed along unequally populated crossed girdles which may evolve into an inclined single girdle the a axes concentrate perpendicular to the more populated girdle concerning quartz the following is suggested 1 1 the deformation has a dominant shear component responsible for the dominant stretching lineation it could be simple shear in case of inclined single girdles 2 2 one set of a axes line up parallel to the shear direction slip on various planes in the a direction may be an important deformation mechanism 3 3 asymmetry in the lattice preferred orientations could be used to predict the sense of shear for the himalayan thrusting in more than 80 of cases

influence of micaceous impurity on dynamically recrystallized quartz c axis fabric in l s tectonites from the singhbhum shear zone and its footwall eastern india when temperature and strain rate remain constant the quartz c axis fabric in deformed pure quartz aggregates is largely dependent on deformation kinematics asymmetry of the fabric e g in type i asymmetric crossed girdle pattern in natural quartz tectonites simulated fabric or experimentally deformed quartz aggregates is generally related to sense of vorticity for a non coaxial flow natural quartz tectonites however often contain micaceous impurities measurement on a sample of 59 quartz tectonites with mesoscopic l s fabric and representing low ttm deformation under non coaxial flow from the singhbhum shear zone and dhanjori quartzites eastern india provides the basic data to quantitatively assess the influence of mica on i asymmetry of quartx c axis fabric and ii degree of crystallographic preferred orientation i e fabric intensity taking c axes of dynamically recrystallized quartz grains as a fabric element a fabric intensity parameter is defined as the ratio of the greatest eigenvalue to the least eigenvalue of the orientation tensor matrix corresponding to c axis orientations in each measured specimen the modal percent of mica in the sample varies from 2 to 35 that of recrystallized quartz grains as opposed to relict clasts from 45 to 98 the asymmetry of the fabrics in the above sample measured either as the am statistic or as the angle between the central segment of the fabric skeleton and direction of mineral elongation lineation is independent of mica content correlation regression analysis of the variables and demonstrate a negative correlation between micaceous impurity and the fabric intensity parameter the regression equation is of the form 0 13 1 11 0 617

the relative formation ages of ferromagnesian chondrules inferred from their initial aluminum 26 aluminum 27 ratios we performed a systematic high precision sims 26al 26mg isotopic study for 11 ferromagnesian chondrules from the highly unequilibrated ordinary chondrite bishunpur ll3 1 the chondrules are porphyritic and contain various amounts of olivine and pyroxene and interstitial plagioclase and or glass the chemical compositions of the chondrules vary from feo poor to feo rich eight chondrules show resolvable 26mg excesses with a maximum 26mg of 1 in two chondrules the initial 26al 27al ratios inferred for these chondrules range between 2 28 0 73 x 10 5 to 0 45 0 21 x 10 5 assuming a homogeneous distribution of al isotopes in the early solar system this range corresponds to ages relative to cais between 0 7 0 2 my and 2 4 0 4 0 7 my the inferred total span of the chondrule formation ages is at least 1 my which is too long to form chondrules by the x wind the initial 26al 27al ratios of the chondrules are found to correlate with the proportion of olivine to pyroxene suggesting that olivine rich chondrules formed earlier than pyroxene rich chondrules though we do not have a completely satisfactory explanation of this correlation we tentatively interpret it as a result of evaporative loss of si from earlier generations of chondrules followed by addition of si to the precursors of later generation chondrules

experimental intracrystalline plastic flow in hot pressed synthetic quartzite prepared from brazilian quartz crystals samples of synthetic ultrafine grained quartzite were prepared by hot pressing aggregates of crushed clear brazilian quartz of mean grain size 0 4 m at 300 mpa and 1373 and 1473 k the samples displayed rapid grain growth to ca 12 20 m with 2 porosity at 1473 k facilitated by the 0 6 wt of water adsorbed onto the grain boundaries during sample preparation this water could be driven off by preheating thereby preventing grain growth sufficient water was incorporated during growth into the coarsened samples to render them weak and ductile these were deformed experimentally in the quartz field using an argon gas medium apparatus at 300 mpa confining pressure at temperatures mainly between t 1273 and 1473 k ductile flow was described by the flow law 10 4 9 3 2 9 7 f h 2 o exp 242000 rt with stress a in mpa and grain size d in microns and strain rate in s 1 r is the gas constant and f h 2 o is water fugacity based on observation of grain flattening optical strain features shapes and densities of dislocation arrays and flow law parameters samples deformed dominantly by dislocation creep with some contribution from grain boundary sliding bearing in mind possible changes in the flow law parameters over the extrapolation interval and possible effects of the phase transition extrapolation to natural strain rates and temperatures predicts plastic flow at higher flow stresses at the same water fugacity at greenschist facies temperatures than previously published flow laws

deformation related recrystallization processes abstract recrystallization is a common microstructural transformation that occurs during deformation metamorphism and diagenesis of rocks studies on minerals and rock analogues have demonstrated that a wide range of recrystallization mechanisms can occur the range of mechanisms is related to the various ways in which two basic processes grain boundary migration and new grain boundary formation combine to transform the microstructure two recent papers drury et al 1985 urai et al 1986 have proposed different schemes for the description of recrystallization mechanisms the purpose of this paper is to provide a unified framework for the description of mechanisms recrystallization mechanisms are divided into three main types rotation mechanisms which principally involve the formation of new grain boundaries migration mechanisms which principally involve grain boundary migration and general mechanisms which involve both basic processes a further distinction is made on the basis of the continuity of the microstructural transformation with respect to time each of the three main types of mechanism can be divided into a number of sub types depending on whether the processes of grain boundary migration new grain boundary formation and new grain formation occur in a discontinuous or continuous manner with respect to time as the terms continuous and discontinuous have been used in the metallurgical literature to signify the spatial continuity of the microstructural transformation the terms discontinuai and continual are used to refer to the temporal continuity of the transformation it is recommended that the following aspects should be specified if possible in a general description of recrystallization mechanisms 1 1 how do the basic processes combine to transform the microstructure 2 2 if new grain development occurs what is the development mechanism and does new grain formation occur in a continual or discontinuai manner 3 3 if grain boundary migration is involved in the transformation what is the migration mechanism i e fast solute escape migration slow solute loaded migration fluid assisted migration etc and is migration a continual or discontinuai process the application of the unified scheme is illustrated by reviewing studies that have provided detailed information on the recrystallization mechanisms involved the complicating effects of solid solution impurities dispersed second phase particles and grain boundary fluid films are also considered and it is demonstrated that variations in content of these types of impurity can significantly effect the types of recrystallization that occur in a given material

fabric asymmetry and shear sense in movement zones asymmetry between lattice preferred orientation and shape fabrics is commonly observed in microstructural studies of many deformed mineral aggregates e g ice carbonate rocks peridotites and quartzites theoretical predictions are limited by the nature of the models available and more can be gained by discussion of the mechanisms which give rise to such asymmetry in specific natural and experimental situations