cambro ordovician anatexis and magmatic recycling at the thinned gondwana margin new constraints from the kouřim unit bohemian massif the kou im unit represents one of the largest pre variscan meta igneous complexes in the bohemian massif and a geochronological whole rock geochemical and sr nd isotopic study was conducted to better understand the magmatic evolution of the early paleozoic gondwana margin five orthogneisses yield u pb zircon ages ranging from 492 4 to 484 2 ma two leucogranites give u pb zircon ages of 500 4 and 485 2 ma interpreted to be inherited from their orthogneiss host rock two samples from the metasedimentary host rock are dominated by neoproterozoic cambrian detrital zircons the abundance of zircon inheritance in the orthogneisses and whole rock sr nd isotopic composition imply an origin from relatively mature continental crustal material the subalkaline subaluminous slightly peraluminous and high k calc alkaline arc like geochemical signature of the orthogneisses is interpreted as inherited from the recycled cadomian metasedimentary source and both the magmatic and metasedimentary rocks are correlated with similar occurrences in the adjacent moldanubicum and tepla barrandian unit the late cambrian early ordovician magmatic activity is linked to crustal anatexis which was probably initiated by thermal and gravitational relaxation of the thickened cadomian arc type crust followed by lithospheric thinning assisted by far field forces the extensional event led to the formation of a passive margin associated with the opening of the rheic ocean supplementary material analytical methods petrographic descriptions with photographs of studied samples collections of cl images of representative zircon grains results of la icp ms u pb analyses of zircons and an additional geochemical diagram are available at https doi org 10 6084 m9 figshare c 4649255

geometry and regional significance of neoproterozoic cadomian structures of the narcea antiform nw spain the narcea antiform is located at the hinterland to foreland transition of the variscan belt in the nw of the iberian massif it consists of an antiformal stack of fault propagation folds that developed in the hanging wall of allochthonous thrust sheets during the first variscan deformation event in the palaeozoic rocks the neoproterozoic narcea slates formation cropping out in the core of the narcea antiform records a cadomian deformation event that formed large asymmetric nw vergent folds with related axial planar cleavage developed in the lower greenschist facies these folds are truncated by the lower cambrian angular unconformity lower cambrian conglomerates contain pebbles with predepositional microstructures that are widespread in the underlying neoproterozoic rocks the narcea antiform provides valuable information concerning major tectonic cadomian events the large scale near recumbent cadomian folds developed between 560 and 540 ma top to the nw kinematics can be deduced for the cadomian plate convergence transverse to the variscan convergence in present coordinates it was followed shortly afterwards by large displacements along the northern gondwana margin that resulted in its accretion alongside the essentially undeformed neoproterozoic rocks of the central iberian zone before early cambrian times

review of geochemical variation in lower palaeozoic metabasites from the ne bohemian massif intracratonic rifting and plume ridge interaction

rupture characteristics of the deep bolivian earthquake of 9 june 1994 and the mechanism of deep focus earthquakes the m w 8 3 deep 636 kilometers bolivian earthquake of 9 june 1994 was the largest deep focus earthquake ever recorded seismic data from permanent stations plus portable instruments in south america show that rupture occurred on a horizontal plane and extended at least 30 by 50 kilometers rupture proceeded at 1 to 3 kilometers per second along the down dip azimuth of the slab and penetrated through more than a third of the slab thickness this extent is more than three times that expected for a metastable wedge of olivine at the core of the slab and thus appears to be incompatible with an origin by transformational faulting these large events may instead represent slip on preserved zones of weakness established in oceanic lithosphere at the earth s surface

early palaeozoic rift related magmatism in variscan europe fragmentation of the armorican terrane assemblage early palaeozoic bimodal rift related magmatism is widespread throughout much of the variscides of europe it is traceable from the polish sudetes to nw iberia granitic plutonism generally predates cambro ordovician bimodal magmatism in the n bohemian massif this early palaeozoic granitic plutonism was generated by partial melting of cadomian basement whereas contemporaneous alkali granites with a mantle component are typical of the nw iberian massif silurian devonian mafic magmatism in the n bohemian massif massif central and nw iberian massif is partly preserved as obducted ophiolites compositional diversity displayed by cambro ordovician mafic magmatism can be accounted for by interaction between a spreading centre and an upwelling mantle plume this indicates that combined tensional forces and mantle plume convection assisted the early palaeozoic dispersal of terranes from the n gondwana margin continued fragmentation resulted in development of an archipelago of related terranes separated by a network of seaways and formation of oceanic crust

coherent and subsequent incoherent ringwoodite growth in olivine of shocked l6 chondrites abstract pervasive replacement of original olivine grains by ringwoodite adjacent to a shock melt vein of yamato 791384 l6 chondrite was observed with a feg sem in these olivines we observed three distinct olivine ringwoodite textures arranged in a spatially successive arrangement from the wall of the shock melt vein into the olivine grains 1 polycrystalline ringwoodite 2 oriented several sets of ringwoodite lamellae and 3 oriented single lamellae tem images show that the polycrystalline and oriented several sets of lamellae parts consist of polycrystalline ringwoodite indicating that phase transformation from olivine to ringwoodite is controlled by incoherent growth mechanism on the other hand the set of oriented single lamellae consists of thin ringwoodite platelets

grain scale pressure variations and chemical equilibrium in high grade metamorphic rocks in the classical view of metamorphic microstructures fast viscous relaxation and so constant pres sure is assumed with diffusion being the limiting factor in equilibration this contribution is focused on the only other possible scenario fast diffusion and slow viscous relaxation and brings an alter native interpretation of microstructures typical of high grade metamorphic rocks in contrast to the pressure vessel mechanical model applied to pressure variation associated with coesite inclusions in various host minerals a multi anvil mechanical model is proposed in which strong single crystals and weak grain boundaries can maintain pressure variation at geological time scales in a polycrystalline material in such a mechanical context exsolution lamellae in feldspar are used to show that feldspar can sustain large differential stresses 10 kbar at geological time scales furthermore it is argued that the existence of grain scale pressure gradients combined with diffusional equilibrium may explain chemical zoning preserved in reaction rims assuming zero net flux across the microstructure an equilibrium thermodynamic method is introduced for inferring pressure variation corresponding to the chemical zoning this new barometric method is applied to plagioclase rims around kyanite in fel sic granulite bohemian massif czech republic yielding a grain scale pressure variation of 8 kbar in this approach kinetic factors are not invoked to account for mineral composition zoning preserved in rocks metamorphosed at high grade

ringwoodite growth rates from olivine with 75ppmw h2o metastable olivine must be nearly anhydrous to exist in the mantle transition zone it has been previously demonstrated that as little as 300 ppmw h 2o increases wadsleyite and ringwoodite growth rates to magnitudes that are inconsistent with the metastable olivine hypothesis to further test this hypothesis we present new ringwoodite growth rate measurements from olivine with 75 ppmw h 2o at 18 gpa and 700 900 and 1100 c these growth rates are nearly identical to those from olivine with 300 ppmw h 2o and significantly higher than those from nominally anhydrous olivine we infer that transformation of olivine with 75 300 ppmw h 2o is primarily enhanced by hydrolytic weakening of reaction rims which reduces the elastic strain energy barrier to growth we present a new method for fitting non linear nominally anhydrous data to demonstrate that reduction of growth rates by elastic strain energy is an additional requirement for metastable olivine based on previous thermokinetic modeling these enhanced growth rates are inconsistent with the persistence of metastable olivine wedges into the mantle transition zone metastable persistence of olivine into the mantle transition zone would therefore require 75 ppmw h 2o

high pressure metamorphism in the early variscan subduction complex of the sw iberian massif abstract several units exposed in the boundary area of the ossa morena zone omz and the south portuguese zone spz preserve petrographic and thermobarometric evidence for an early metamorphic episode m1 developed under a high p low to intermediate t gradient related to early variscan subduction in the sw iberian massif in the omz these are the cubito moura unit p min 9 2 kbar and t 395 45 c blueschists from its basal melange p min 12 4 kbar and t 310 11 c and the underlying fuenteheridos group p 10 9 0 4 kbar and t 449 31 c the equivalent units in the spz are the la minilla formation p 8 7 0 4 kbar and t 388 16 c and the lawsonite pseudomorphs bearing pulo do lobo formation all these units formed part of an approximately ne verging orogenic wedge present coordinates developed by the accretion of subducted slabs of the outermost margin of gondwana and other elements of the rheic ocean realm from at least the middle devonian to the lowermost tournaisian high pressure rocks were subsequently emplaced on more internal zones of the omz that only experienced a younger high t low to intermediate p metamorphism m2 this high t event was coeval with magmatic activity from the uppermost devonian to the middle mississippian probably as a consequence of transtensional lithospheric thinning and or delamination of the lower crust and mantle lithosphere of the gondwana margin pre late devonian synorogenic sedimentation in forearc and back arc basins of the subduction complex evolved to a late devonian middle mississippian foreland basin system in early collisional stages finally a new middle late mississippian fold and thrust belt with opposite sw vergence and new foreland basins developed during late collisional stages

the effect of small amounts of h2o on olivine to ringwoodite transformation growth rates and implications for subduction of metastable olivine abstract it has been suggested that in relatively cold subduction zones the interior of down going lithospheric plates may be cool enough to inhibit the transformation of olivine to its high pressure polymorphs wadsleyite and ringwoodite resulting in the persistence of metastable olivine in the mantle transition zone the rapid transformation of this metastable olivine has been identified as a possible cause of deep focus earthquakes however it is known that olivine can incorporate hydrogen at high pressure and this hydrogen may enhance transformation rates we conducted a series of high pressure experiments to hydrate and then partially transform olivine to ringwoodite we measured transformed rim thickness as a function of run duration and fit a linear function to our results to determine transformation growth rates the addition of h2o to olivine appears to promote olivine to ringwoodite transformation by increasing the growth rate decreasing the activation enthalpy for growth and weakening the ringwoodite and or olivine to allow for dissipation of elastic strain sufficient to maintain constant or near constant growth rates throughout transformation the addition of approximately 300 ppmw 2h2o to olivine increases the growth rate of olivine to ringwoodite transformation at 1100 c and 18 gpa by almost a factor of 40 from 4 0 0 8 10 9 ms 1 to 1 5 0 3 10 7 ms 1 there is a corresponding decrease in the activation enthalpy of growth from 392 kj mol to 237 kj mol assuming activation volume of 0 cm3 mole analyses of hydrogen distributions in partially transformed samples suggest that the hydrogen concentration in the bulk of the olivine crystal may be less important than hydrogen at the olivine and ringwoodite interface

ahrensite γ fe 2 sio 4 a new shock metamorphic mineral from the tissint meteorite implications for the tissint shock event on mars ahrensite ima 2013 028 fe 2sio 4 is the natural fe analog of the silicate spinel ringwoodite mg 2sio 4 it occurs in the tissint martian meteorite where it forms through the transformation of the fayalite rich rims of olivine megacrysts or fe rich microphenocrysts in contact with shock melt pockets the typical sequence of phase assemblages traversing across a tissint melt pocket into olivine is quenched melt or fayalite pigeonite intergrowth bridgmanite wustite ahrensite and or ringwoodite highly deformed olivine nanocrystalline ringwoodite deformed olivine we report the first comprehensive set of crystallographic spectroscopic and quantitative chemical analysis of type ahrensite and show that concentrations of ferric iron and inversion in the type material of this newly approved mineral are negligible we also report the occurrence of nanocrystalline ringwoodite in strained olivine and establish correlations between grain size and distance from melt pockets the ahrensite and ringwoodite crystals show no preferred orientation consistent with random nucleation and incoherent growth within a highly strained matrix of olivine grain sizes of ahrensite immediately adjacent to melt pockets are consistent with growth during a shock of moderate duration 1 10 ms

constraining the kinetics of mantle phase changes with seismic data summary in a system where two phases coexist a seismic wave can disrupt the pre existing equilibrium and induce a re equilibration process because the kinetics of the phase change is not instantaneous the transformation induces an attenuation of the wave that can be quantified using an appropriate physical theory kinetics of earth s phase transitions are not well known in this paper we show that they can be constrained by seismic attenuation data we quantify the influence of a phase transition upon seismic mode attenuation and body wave reflexion coefficient we perform a numerical application for the olivine to wadsleyite transition at 410 km depth assuming a phase loop thickness of 10 km we show that the relaxation time that controls the frequency band of attenuation and the velocity at which the interface evolves when submitted to a pressure disequilibrium is likely larger than 7000 s for this kinetics slower than typical seismic waves periods the transformation loop does not affect s waves attenuation but potentially that of p waves and normal modes

a process for low temperature olivine spinel transition under quasi hydrostatic stress 1 the behavior of fayalite the fe2sio4 olivine end member fa100 has been monitored during high pressure and low temperature experiments using in situ x ray synchrotron radiation in order to investigate the effect of stress on the olivine phase spinel phase or ringwoodite transition the run products were investigated by optical and transmission electron microscopy tem a low temperature mechanism for the transformation is documented this shear induced mechanism is operative in hydrostatically compressed samples at temperature as low as 450 c and is driven by the shear stress generated by the volume change of the transformation itself projection to mantle compositions suggests that metastable subduction of olivine will not occur in the deep transition zone

cambrian ensialic rift related magmatism in the ossa morena zone evora aracena metamorphic belt sw iberian massif sm nd isotopes and shrimp zircon u th pb geochronology abstract the late ediacaran c 560 550 ma serie negra sediments of the evora aracena metamorphic belt ossa morena zone sw iberian massif preserve a record of the erosion of an avalonian cadomian magmatic arc and subsequent related turbiditic sedimentation detrital zircon from the serie negra is characterized by predominantly ediacaran and cryogenian ages with few paleoproterozoic and archean cores and a marked lack of grenvillian ages these features when combined with the metasediments enrichment in lree la yb 14 negative eu anomalies low 147 sm 144 nd values 0 121 and negative e nd 550 5 5 indicate that the protolith serie negra sediments were derived from a continental magmatic arc a period of late cadomian ca 560 540 ma tectonism was followed by an extended episode of widespread bimodal magmatism related to cambrian ca 540 500 ma rifting this tectonic inversion is expressed in the geological record by a regional early cambrian unconformity shrimp zircon u th pb ages from four felsic orthogneisses from the evora massif record cambrian 527 10 ma 522 5 ma 517 6 ma and 505 5 ma crystallization ages for their igneous protoliths this confirms the existence of widespread lower paleozoic igneous activity in the ossa morena zone i a lower cambrian ca 535 515 ma igneous felsic dominated sedimentary complex with calc alkaline signature and associated carbonate and siliciclastic deposition and ii a middle cambrian ordovician ca 515 490 ma igneous bimodal sedimentary complex with calc alkaline and tholeiitic signatures and associated dominant siliciclastic deposition but also carbonate sediments the cambrian felsic magmatism was characterized by negative eu anomalies la lu n 0 8 11 147 sm 144 nd 0 1289 0 1447 and e nd 500 ranging from 1 5 to 0 8 a tendency towards peraluminous compositions suggests late fractionation low degrees of partial melting or the mixing of crustal and mantle derived material in the magma source region some felsic rocks possibly represent the last residual melts of high temperature zircon undersaturated mafic magmas later affected by crustal contamination while others indicate partial melting of crustal metasediments variably contaminated by basaltic liquids the transition from early felsic dominated to later more mafic magmatism suggests the gradual opening of the system to tholeiitic n e morb products th n ta n 147 sm 144 nd 0 1478 0 1797 and e nd 500 values ranging from 6 4 to 7 3 while the n morb amphibolites have 147 sm 144 nd 0 1818 0 1979 and e nd 500 values of 5 8 and 7 0 reaching a maximum of 9 1 in contrast other amphibolites have a negative ta anomaly 1 35 n ta n 147 sm 144 nd values ranging from 0 1639 to 0 1946 and e nd 500 values of 3 5 to 5 2 suggesting derivation by crustal assimilation processes the sub alkaline igneous precursors of the amphibolites were most likely generated in a rift setting by asthenospheric upwelling these results strengthen the proposed geodynamic scenarios for the sw iberian massif by which cadomian accretion gave rise to an ensialic rift that developed into a proto oceanic basin and incipient spreading opening of the rheic ocean a similar transition from a convergent to a divergent plate boundary during the ediacaran to cambrian ordovician has also been reported in other segments of the northern gondwana margin

strain and strain rate in a synkinematic trondhjemitic dike evidence for melt induced strain softening during shearing bohemian massif czech republic abstract emplacement related deformation of a cambrian trondhjemitic dike located at the western border of the tepla barrandian unit central european variscides is studied in detail the trondhjemitic melt was emplaced at t 750 c into an active low temperature ca 350 c east northeast trending dextral transcurrent shear zone thermal modelling indicates that the rheologically critical melt fraction and the solidus of the dike were achieved after 1 and 3 days of cooling respectively the major part of the shearing occurred within 8 days after the melt emplaced the zonation of the dike in strain magnitude mineralogy and geochemistry can be explained by the laterally varying rheology from margin to centre during cooling and shearing the deformation was volume constant and the strain data plot in the apparent constrictional field the unusually high value of the calculated longitudinal strain rate 2 8 10 6 s 1 equivalent to a displacement rate of 2 cm h 1 is probably related to the intruding melt that lubricated the shear zone and thus enhanced its displacement velocity consequently melts that intrude as dikes into active transcurrent shear zones may significantly weaken the strength of the middle and upper continental crust

kinetics of the olivine ringwoodite transformation and seismic attenuation in the earth s mantle transition zone abstract in regions of the mantle where multi phases coexist like at the olivine wadsleyite ringwoodite transitions the stress induced by the seismic waves may drive a mineralogical reaction between the low to high pressure phases a possible source of dissipation in such a situation the amount of attenuation critically depends on the timescale for the phase transformations to reach equilibrium relative to the period of the seismic wave here we report synchrotron based measurements of the kinetics of the olivine to ringwoodite transformation at pressure temperature conditions of the co stability loop for iron rich olivine compositions both microstructural and kinetic data suggest that the transformation rates are controlled by growth processes after the early saturation of nucleation sites along olivine grain boundaries transformation time data show an increase of reaction rates with temperature and iron content and have been fitted to a rate equation for interface controlled transformation g k 0 t exp n x fa exp h a p v r t 1 exp g r r t where x fa is the fayalite fraction the exponential factor n 9 7 ln k 0 9 1 m s 1 x fa 1 and h a 199 kj mol assuming v 0 cm 3 mol including these new kinetic results in a micro mechanical model of a two phase loop ricard et al 2009 we predict q k 1 and q 1 significantly higher than the prem values for both body waves and normal modes this attests that the olivine wadsleyite transition can significantly contribute to the attenuation of the earth s mantle transition zone

high pressure polymorphs in yamato 790729 l6 chondrite and their significance for collisional conditions shock pressure recorded in yamato y 790729 classified as l6 type ordinary chondrite was evaluated based on high pressure polymorph assemblages and cathodoluminescence cl spectra of maskelynite the host rock of y 790729 consists mainly of olivine low ca pyroxene plagioclase metallic fe ni and iron sulfide with minor amounts of phosphate and chromite a shock melt vein was observed in the hostrock ringwoodite majorite akimotoite lingunite tuite and xieite occurred in and around the shock melt vein the shock pressure in the shock melt vein is about 14 23 gpa based on the phase equilibrium diagrams of high pressure polymorphs some plagioclase portions in the host rock occurred as maskelynite sixteen different cl spectra of maskelynite portions were deconvolved using three assigned emission components centered at 2 95 3 26 and 3 88 ev the intensity of emission component at 2 95 ev was selected as a calibrated barometer to estimate shock pressure and the results indicate pressures of about 11 19 gpa the difference in pressure between the shock melt vein and host rock might suggest heterogeneous shock conditions assuming an average shock pressure of 18 gpa the impact velocity of the parent body of y 790729 is calculated to be 1 90 km s 1 the parent body would be at least 10 km in size based on the incoherent formation mechanism of ringwoodite in y 790729

shock induced p t conditions and formation mechanism of akimotoite pyroxene glass assemblages in the grove mountains grv 052082 l6 meteorite akimotoite mg fe sio 3 ilmenite was encountered in shock induced melt veins of grove mountains grv 052082 a highly equilibrated low iron ordinary chondritic meteorite l6 coexistence of ringwoodite majorite and majorite pyrope solid solution indicates the shock pressure at 18 23 gpa and temperature of 2000 2300 c during the natural dynamic event most low ca pyroxene clasts entrained in the melt veins have been partially or entirely transformed into akimotoite pyroxene glass assemblages which contain micrometer sized areas with various brightness in the backscattered electron images different from the chemically homogeneous grains in the host rock fs 20 5 21 3 the transmission electron microscopy study of a focused ion beam fib slice from the heterogeneous areas shows that the assemblages are composed of feo depleted and heterogeneous akimotoite fs 6 19 crystals 100 nm up to 400 nm in size scattered in feo enriched and relatively homogeneous pyroxene glass fs 31 39 all analyses of the akimotoite pyroxene glass assemblages plot on a fractionation line in feo mgo diagram with the host rock pyroxene at the middle between the compositions of feo depleted akimotoite and the feo enriched pyroxene glass these observations are different from previous reports of almost identical compositions of akimotoite bridgmanite mg fe sio 3 perovskite or pyroxene glass to the host rock pyroxene chen et al 2004 ferroir et al 2008 ohtani et al 2004 tomioka and fujino 1997 which is consistent with solid state transformation from pyroxene to akimotoite and preexisting bridgmanite that could be vitrified the observed fractionation trend and the granular shapes of akimotoite suggest crystallization from liquid produced by shock melting of the host rock pyroxene and the pyroxene glass matrix was probably quenched from the residual melt however this interpretation is inconsistent with the static experiments that expect crystallization of majorite mg fe sio 3 garnet instead of akimotoite from pyroxene liquid sawamoto 1987 our discovery raises the issue on formation mechanisms of the high pressure polymorphs of pyroxene and places additional constraints on the post shock high pressure and high temperature conditions of asteroids

gondwana break up and the northern margin of the saxothuringian belt variscides of central europe at the northwestern edge of the hercynian bohemian massif saxothuringian belt new u pb zircon age data from rift related magmatic rocks indicate that the initiation of gondwana break up in this area started during the middle to upper cambrian magmatic rocks from a bimodal morb to within plate volcanic sequence in the vesser area are dated between ca 517 and 501 ma the volcaniclastic sequences analysed exhibit basal layers of conglomerates and mature sandstones which can be correlated with a widespread gondwana derived onlap horizon of an uppermost cambrian tremadocian age that links the vesser area with the saxothuringian continental basin the association of the vesser rocks with the saxothuringian terrane as part of the armorican terrane assemblage is further demonstrated by a coeval magmatic development and by identical detrital components which are derived from a common cadomian basement white mica with a ca 539 ma k ar minimum age and inherited zircon signatures the vesser unit situated between the nw margin of the saxothuringian zone and the mid german crystalline zone probably represents a n facing remnant of an ocean continent transition of the or within the armorican terrane assemblage and involves sections of the early break up process at the peri gondwanan shelf south of the rheic ocean

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

a microstructural investigation of natural lamellar ringwoodite in olivine of the shocked sixiangkou chondrite abstract olivine adjacent to the shock produced veins of sixiangkou meteorite contains several sets of ringwoodite lamellae nucleated and grown along planar fractures in olivine these lamellae consist of two generations of crystallite aggregates of ringwoodite and show features of incoherent nucleation and growth of ringwoodite along fractures of olivine the lamellae are of zonal fe mg distribution characterized as fe rich rim and mg rich interior it indicates that the intracrystalline transformation of olivine to ringwoodite proceeds through fe mg interdiffusion during growth of ringwoodite the formation of the lamellae favors a fracture involved phase transformation mechanism firstly a thin layer of crushed or distorted olivine along a gliding plane of a fracture was produced and then transformed into the first generation of crystallite layer of ringwoodite subsequently the second generation of relatively coarse grained ringwoodite grew on both interfaces of the firstly formed thin layer of ringwoodite this study brings forward a new observation of intracrystalline olivine ringwoodite transformation in addition to other mechanisms since mechanical deformations resulting in planar fractures are common in olivine of terrestrial mantle rocks this proposed mechanism could play an important role in the subducting lithospheric slabs

mantle layering from scs reverberations 2 the transition zone this is the second paper in a four part sequence that examines the nature of mantle layering using the scsn phases and internal reflections observed within the reverberative interval of sh polarized seismograms mantle reflectivity profiles for 18 seismic corridors sampling portions of indonesia australia and the western and central pacific are constructed from low frequency 25 mhz scs reverberations i e scsn type phases reflected once from internal mantle discontinuities modeling the reflectivity profiles using synthetic seismograms yields precise path specific estimates of the travel times to the 410 km and 660 km discontinuities and their reflection coefficients the data are consistent with an olivine dominated model of the transition zone marked by two major phase transitions having average apparent depths of 414 2 km and 660 2 km and impedance contrasts of 0 046 0 010 and 0 072 0 010 respectively the interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km peak to peak and is negatively correlated consistent with an endothermic transition at 660 km whose clapeyron slope magnitude is comparable to that of the 410 km exothermic transition in the case of the 660 km discontinuity we have been able to supplement these long wavelength 1500 5000 km observations with an estimate of intrapath topography inferred from the frequency dependence of the 660 km reflection coefficient which gives peak to peak variations less than 40 km on scale lengths of 500 1500 km for the 660 km discontinuity the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction however we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660 km discontinuity which may involve small compositional variations plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition loss of reflected energy due to local curvature or roughness of the reflector and or extreme topography occurring on a small compositional component to the discontinuity 40 of the total impedance contrast by stripping the 410 km and 660 km peaks from the composite path reflectivity profile we identify three minor reflectors at mean depths of 520 710 and 900 km the shallowest may mark the phase spinel phase transition the 710 km discontinuity can be attributed to the ilmenite gamet perovskite phase transition although changes in perovskite symmetry can potentially provide explanations for both the 710 km and 900 km features

revising the shock classification of meteorites the current shock classification scheme of meteorites assigns shock levels of s1 unshocked to s6 very strongly shocked using shock effects in rock forming minerals such as olivine and plagioclase the s6 stage 55 90 gpa 850 1750 c relies solely on localized effects in or near melt zones the recrystallization of olivine or the presence of mafic high pressure phases such as ringwoodite however high whole rock temperatures and the presence of high pressure phases that are unstable at those temperatures and pressures of zero gpa e g ringwoodite are two criteria that exclude each other each type of high pressure phase provides a minimum shock pressure during elevated pressure conditions to allow the formation of this phase and a maximum temperature of the whole rock after decompression to allow the preservation of this phase rocks classified as s6 are characterized not by the presence but by the absence of those thermally unstable high pressure phases high pressure phases in or attached to shock melt zones form mainly during shock pressure decline this is because shocked rocks 60 gpa experience a shock wave with a broad isobaric pressure plateau only during low velocity 4 5 km s 1 impacts which rarely occur on small planetary bodies e g the moon and asteroids the mineralogy of shock melt zones provides information on the shape and temporal duration of the shock wave but no information on the general maximum shock pressure in the whole rock

flow behavior and microstructures of hydrous olivine aggregates at upper mantle pressures and temperatures deformation experiments on olivine aggregates were performed under hydrous conditions using a deformation dia apparatus combined with synchrotron in situ x ray observations at pressures of 1 5 9 8 gpa temperatures of 1223 1800 k and strain rates ranging from 0 8 10 5 to 7 5 10 5 s 1 the pressure and strain rate dependencies of the plasticity of hydrous olivine may be described by an activation volume of 17 6 cm3 mol 1 and a stress exponent of 3 2 0 6 at temperatures of 1323 1423 k a comparison between previous data sets and our results at a normalized temperature and a strain rate showed that the creep strength of hydrous olivine deformed at 1323 1423 k is much weaker than that for the dislocation creep of water saturated olivine and is similar to that for diffusional creep and dislocation accommodated grain boundary sliding while dislocation microstructures showing the 001 slip or the 001 100 slip system were developed at temperatures of 1633 1800 k a much stronger pressure effect on creep strength was observed for olivine with an activation volume of 27 7 cm3 mol 1 assuming a stress exponent of 3 5 water fugacity exponent of 1 2 and activation energy of 520 kj mol 1 i e power law dislocation creep of hydrous olivine because of the weak pressure dependence of the rheology of hydrous olivine at lower temperatures water weakening of olivine could be effective in the deeper and colder part of earth s upper mantle

using shear wave amplitude patterns to detect metastable olivine in subducted slabs 1 olivine within the lowest temperature regions of subducting oceanic lithosphere may persist metastably to depths well below the equilibrium phase boundary for transformation to phase this may produce a tapering low seismic velocity wedge within the slab in the transition zone transformational faulting of such metastable olivine has been proposed as a possible explanation of deep earthquakes detecting seismic velocity structure associated with metastable olivine within a slab using seismic wave arrival time information alone has proven problematic motivated by previous two dimensional numerical calculations for upgoing and downgoing seismic waves a three dimensional gaussian beam method is used to assess whether the azimuthally varying teleseismic amplitude signature produced by a slab with an internal wedge of metastable olivine is distinguishable from that for a slab with equilibrium transitions five slab velocity structures are considered a thermal model with horizontal phase transitions at the same depths as in ambient mantle two models incorporating equilibrium transitions within the slab and two models with tongues of metastable olivine a depth and temperature dependent thermal conductivity term and the effect of the latent heat of transformation of olivine to phase are included in the thermal modeling three dimensional dynamic ray tracing and gaussian beam summation are performed for events located at different depths and lateral positions for each of the slab models for sources near 400 km depth the presence of low velocity metastable olivine produces relatively complex azimuthal shear wave amplitude patterns compared to thermal and equilibrium models comparison with observed amplitudes for four events in the kurile slab indicates that allowing for internal slab velocity complexity in the transition zone provides better agreement with observations than simple thermal and equilibrium slab models this indicates that careful modeling of teleseismic shear wave amplitude patterns may have the resolution to detect and characterize the presence of metastable olivine in deep slabs

fracture related intracrystalline transformation of olivine to ringwoodite in the shocked sixiangkou meteorite magnesium iron olivine in the sixiangkou l6 chondrite contains abundant fractures induced by plastic deformation during shock metamorphism this study reports the discovery of lamellar ringwoodite that incoherently nucleated and grew along planar and irregular fractures in olivine magnesium iron interdiffusion took place between olivine matrix and crystallizing ringwoodite at high pressures and high temperatures which resulted in higher feo content in ringwoodite lamellae than in olivine this suggests that a quasi hydrostatic high pressure lasting for several minutes should have been produced in the shock veins of the meteorite the intracrystalline transformation of olivine to ringwoodite also has implications for phase transitions in subducting lithospheric slabs because planar and irregular fractures are commonly produced in olivine that suffered plastic deformation

kinetics of the olivine spinel transformation in subducting lithosphere experimental constraints and implications for deep slab processes abstract the persistence of metastable olivine to depths greater than 400 km in subducting slabs has implications for the generation of deep focus earthquakes the magnitude of buoyancy forces driving plate motion and the state of stress in the slab the depth to which metastable olivine can survive in a subduction zone and the depth interval over which transformation to or mg fe 2sio4 occurs have been evaluated from experimental kinetic data for the mg2geo4 and ni2sio4 transformations and the mg2sio4 transformation the data were extrapolated using a kinetic model for grain boundary nucleation and interface controlled growth under non isothermal and non isobaric conditions the results predict that metastable mg1 8fe0 2sio4 olivine survives to depths greater than 500 km in the cold interior of rapidly subducting slabs of old lithosphere the onset of transformation to mg fe 2sio4 spinel depends only on growth kinetics and coincides with the 550 50 c isotherm including the effects of latent heat production causes the transformation to occur by a runaway process over a very narrow depth interval at the onset of transformation high nucleation rates and low growth rates are consistent with the formation of very fine grained reaction products which are required for the transformational faulting mechanism of deep focus earthquakes when olivine crosses the equilibrium boundaries at higher temperatures e g higher than 700 c at 400 km depth transformation to or occurs much closer to equilibrium at a depth controlled by nucleation kinetics in this case the effect of latent heat production on the transformation kinetics is small and microstructural evolution is unlikely to result in transformational faulting below the depth of olivine breakdown cold slabs are likely to have a complex rheological structure owing to temperature dependent microstructural evolution during the phase transformation

crustal structure of the transpressional variscan orogen of sw iberia sw iberia deep seismic reflection profile iberseis 1 iberseis a 303 km long 20 s deep seismic reflection profile was acquired across the variscan belt in sw iberian peninsula the acquisition parameters were designed to obtain a high resolution crustal scale image of this orogen the seismic profile samples three major tectonic terranes the south portuguese zone the ossa morena zone and the central iberian zone which were accreted in late paleozoic times these terranes show a distinctive seismic signature as do the sutures separating them late strike slip movements through crustal wedges are apparent in the seismic image and have strongly modified the geometry of sutures the upper crust appears to be decoupled from the lower crust all along the seismic line but some deformation has been accommodated at deeper levels a sill like structure is imaged in the middle crust as a 1 2 s thick and 175 km long high amplitude conspicuous reflective band it is interpreted as a great intrusion of mafic magma in a midcrustal decollement taking into account surface geological data and the revealed crustal architecture a tectonic evolution is proposed for sw iberia which includes transpressional collision interacting during early carboniferous with a mantle plume the moho can be identified along the entire transect as subhorizontal and located at 10 to 11 s indicating a 30 35 km average crustal thickness its seismic signature changes laterally being very reflective beneath the south portuguese zone and the central iberian zone but discontinuous and diffuse below the ossa morena zone

the effect of strain energy on growth rates during the olivine spinel transformation and implications for olivine metastability in subducting slabs we have studied growth kinetics during the transformation of mg1 8fe0 2sio4 san carlos olivine to its high pressure polymorphs wadsleyite phase and ringwoodite phase at 800 1200 c and nominal pressures of 16 20 gpa in experiments in which a large 500 600 m olivine single crystal contained in a matrix of either fine grained olivine or nacl was transformed reaction rims of wadsleyite ringwoodite form on the margins of the single crystal by incoherent grain boundary nucleation and interface controlled growth contrary to theoretical expectations the growth rate of these reaction rims decreases sharply as a function of time for instance at 1100 c and 18 gpa growth ceases on an experimental timescale after the rim width reaches 20 25 m in order to explain this observation we develop an elastic model based on the theory of a misfitting inclusion comparing the results of this model with the experimental data suggests that elastic strain energy which develops because of the large volume decrease associated with the transformation is responsible for the decreasing growth rates on the other hand experimental and theoretical results suggest that elastic strain energy is relatively unimportant when grains of the product phase are randomly dispersed and distinct reaction rims do not form this is the case when the nucleation rate is low the growth rate is fast and the reactant olivine is fine grained on a geological timescale in subducting lithosphere where the grain size of the olivine is large the growth rates of grain boundary nucleated reaction rims are likely to be controlled by viscoelastic relaxation therefore current kinetic models of olivine metastability in subducting slabs which are based on simple extrapolations of experimental data and on the assumption that the growth rate is constant at fixed temperature and pressure need to be reevaluated

seismic evidence for very deep roots of continents abstract a major problem in geodynamics with seismic data is discussed how deeply do the continents penetrate into the mantle differential travel times of underside reflections from mantle discontinuities that appear as precursors to ss in large parts of the globe show a clear correlation with oceans and continents they are significantly larger beneath the asian and north american continents than underneath the neighbouring pacific from this observation we conclude that the asian and north american continents affect the mantle well below 410 km changes in the thickness of the transition zone can explain our observations which are in agreement with the hypothesis of petrological phase changes causing the 410 km and 660 km seismic discontinuities average thickness of the transition zone underneath continents is about 14 kim thicker than beneath oceans moreover our findings imply temperature variations about 100 200 k in the mantle transition zone weak reflections from 520 km depth corresponding to an impedance contrast of about 2 can be observed only in some areas of the earth while observations from other locations definitively show no signal from this depth therefore we propose that the 520 km reflector is only a regional feature

matrix rheology effects on reaction rim growth i evidence from orthopyroxene rim growth experiments the rim forming reaction quartz olivine orthopyroxene is used to investigate the effect of matrix rheology on rim growth rates orthopyroxene rim growth around olivine grains in quartz matrix is compared to rim growth around quartz grains in an olivine matrix at constant p t within one single capsule orthopyroxene rims grow faster around quartz clasts in olivine matrix than around olivine clasts in quartz matrix fourier transform infra red spectra indicate that the entire samples are water saturated because of water adsorption on the reactant grain surfaces the increased orthopyroxene growth rates in olivine matrix as opposed to quartz matrix are interpreted in terms of matrix rheology where in the two different matrix inclusion arrangements the olivine matrix behaves softer and the quartz matrix more rigid the strain energy associated with accommodation of the negative reaction volume is higher for the quartz than the olivine matrix and reduces the free energy that drives orthopyroxene rim growth growth textures in both kinds of orthopyroxene rims indicate that the diffusivity of mgo slightly exceeds the diffusivity of sio2 the relative mobility of mgo and sio2 at given p t fh2o seems to be controlled by energy minimization during orthopyroxene growth at the compressive ol opx interface our experiments provide evidence for two previously overlooked effects relevant to rim growth reactions in metamorphic rocks i diffusivity along chemical potential gradients to reaction sites is a function of rheology and ii the relative diffusivity of components during reaction rim or corona growth is a function of local volume changes at the rim s interfaces

transition zone velocity gradients and the 520 km discontinuity stacks of long period global digital seismograph network gdsn seismograms at 110 to 180 epicentral distance reveal precursors to ss that result from underside reflections off upper mantle seismic discontinuities the 410 and 660 km discontinuities are obvious in these stacks but identification and modeling of other transition zone discontinuities are complicated by sidelobes from the 410 and 660 km reflections these sidelobes result from the limited bandwidth of the gdsn instrument responses and the effect of crustal reverberations on the ss reference phase the crustal effects can be minimized by restricting the records to oceanic bounce points where the 6 km thick crust has little effect on the long period waveforms over 2000 long period transverse component seismograms with oceanic ss bounce points recorded by the gdsn from 1976 to 1991 are manually edited aligned on ss and then stacked using a new procedure that weights the records by data quality the resulting image shows a clear reflection from a 520 km discontinuity that cannot be explained as a sidelobe artifact confirming earlier results of shearer 1990 1991 and revenaugh and jordan 1991 by stacking along the expected travel time curves for discontinuity phases the time versus range image of the precursor wave field is reduced to a single trace that measures upper mantle reflectivity versus time the features in this reflectivity profile are sensitive to the brightness and depth of the transition zone discontinuities and to the steepness of the velocity gradients between the interfaces using geometrical ray theory and assuming a constant velocity versus density scaling relationship i fit this reflectivity profile with velocity models of the upper mantle using both forward modeling and direct inversion the inverse problem is addressed by performing a deconvolution of the profile with the ss reference phase after a correction for attenuation followed by a direct mapping of reflectivity versus time into velocity versus depth velocity depth profiles resulting from these procedures are roughly in agreement with standard upper mantle velocity models except that the ss precursor data require a minor discontinuity near 520 km and a steeper gradient just below the 660 km discontinuity estimated discontinuity shear impedance changes are 6 7 1 1 at 420 km 2 9 0 7 at 519 km and 9 9 1 5 at 663 km the impedance change near 520 km is consistent with current mineral physics results for the olivine to phase change and places constraints on the fraction of olivine in the transition zone

experimental constraints on the depth of olivine metastability in subducting lithosphere the hypothesis that metastable olivine persists in some subducting slabs into the transition zone has wide implications for mantle dynamics and rheology in order to evaluate this possibility we derive new thermo kinetic subduction zone models to predict the extent of olivine metastability within the stability fields of its high pressure polymorphs wadsleyite and ringwoodite our updated models improve on previous work by incorporating experimental kinetic data on realistic mantle compositions mg fe 2sio4 rather than analogue systems furthermore latent heat due to the transformation is fed back into both the kinetics and the thermal model we also consider the effects of transformation stress on growth kinetics and the possibility of an intracrystalline transformation mechanism previously thought to be important only at high shear stresses our models predict significantly smaller wedges of metastable olivine than previous work in the case of tonga for example where high values of lithospheric age 100 140 million years and convergence rate 14 cm per year are most favorable for metastability models considering only grain boundary nucleation and interface controlled growth predict olivine metastability to 600 km depth in contrast to 660 km predicted previously by kirby et al rev geophys 34 1996 261 when intracrystalline transformation is considered the depth of metastability is further reduced by as much as 100 km due to the large increase in the density of nucleation sites inhibition of growth by transformation stress can increase the depth interval over which the transformation takes place but is unlikely to be a dominant factor especially if the intracrystalline mechanism operates these results indicate that the existence of metastable olivine at depths corresponding to those of the deepest earthquakes 680 km requires subduction of old lithosphere 100 million years and a high vertical subduction velocity 15 cm per year such conditions currently may only be achieved in the very northern part of the tonga subduction zone yet earthquakes occur at depths down to 680 km in other subduction zones in which lithospheric age and or subduction velocity are relatively low e g indonesia and the marianas therefore mechanisms other than transformational faulting in metastable olivine must operate to cause the deepest earthquakes

tectonic evolution of the boundary between the central iberian and ossa morena zones variscan belt southwest spain three tectonometamorphic units can be differentiated in the boundary between the central iberian and ossa morena zones of the iberian massif variscan belt the three units have been named the northern unit central unit and southern unit the northern unit corresponds to the border of the central iberian zone it evolved at low temperature and intermediate or low pressure metamorphic conditions and was affected by top to the se ductile shearing the central unit placed under the northern unit preserves high pressure silurian eclogitic assemblages retrograded by high to medium temperature and finally low temperature ductile shearing with top to the nw sense of movement oblique left lateral the central unit is superposed on the southern unit the latter corresponds to the border of the ossa morena zone and underwent low pressure intermediate temperature metamorphism synchronous with right lateral top to the se ductile shearing the envisaged tectonic evolution is as follows after a stage of lower paleozoic rifting subduction of the central unit under the northern unit took place in silurian times as a result of the crustal thickening a gravitational instability developed giving way to left lateral extensional shearing that affected the entire central unit the combined action of oblique thrusting at the front of the wedge and oblique extensional shearing at the rear caused the exhumation of the high pressure rocks of the central unit this tectonic evolution reveals that the boundary between the central iberian and ossa morena zones is a suture of the variscan belt

dynamic phase boundaries of olivine wadsleyite in subduction zones in the western pacific 1 a modified coupling scheme is proposed to calculate the thermal structures and the kinetics of olivine to wadsleyite phase transformation in subduction zones which assures a high computational precision the phase and thermal structures of some subduction zones in the western pacific region are calculated computational results show that most subduction zones don t have large wedges of metastable olivine which lets the mechanisms of transformational faulting have difficulty explaining deep seismicity

metamorphic and deformational imprint of cambrian lower ordovician rifting in the ossa morena zone iberian massif spain abstract the high temperature metamorphism recorded in the valuengo and monesterio areas constitutes a rare occurrence in the ossa morena zone of southwest iberia where low grade metamorphism dominates the metamorphism of the valuengo area has been previously considered either cadomian or variscan in age whereas that of monesterio has been interpreted as a cadomian imprint however these areas share important metamorphic and structural features that point towards a common tectonometamorphic evolution the metamorphism of the valuengo and monesterio areas affects late proterozoic and early cambrian rocks and is syn kinematic with a top to the north mylonitic foliation which was subsequently deformed by early variscan folds and thrusts the u pb zircon age 480 7 ma we have obtained for an undeformed granite of the valuengo area is consistent with our geological observations constraining the age of the metamorphism we propose that this high temperature metamorphic imprint along a nw se ductile extensional shear zone is related to the crustal extension that occurred in the ossa morena zone during the cambro ordovician rifting in the same way the tectonothermal effect of the preorogenic rifting stage may have been wrongly attributed to orogenic processes in other regions as well as in this one

crystallographic preferred orientations and microstructure of a variscan marble mylonite in the ossa morena zone sw iberia the mylonitic marbles of the cherneca shear zone a variscan sinistral strike slip structure developed in the ossa morena zone sw iberia related to the emplacement of the santa olalla igneous complex were studied in detail different mylonites from protomylonites to ultramylonites were analyzed by electron back scattering diffraction ebsd in order to determine the crystallographic preferred orientation cpo of calcite the cpos show very good agreement with experimental torsion work giving suitable interpretations of the operative slip systems and strain quantifications thus the obtained natural cpos validate the experimental approaches a wide range of observed textures and microstructures are mainly controlled by the amount of shear strain the presence of secondary phases and the development of antithetic subsidiary shears the effect of secondary phases and the nucleation of antithetic shear bands on texture development are discussed

strength of mg fe 2sio4 wadsleyite determined by relaxation of transformation stress abstract the growth of a reaction rim around a crystal undergoing a polymorphic phase transformation generates elastic strain energy as a consequence of the reaction volume change the strain energy inhibits the transformation by counteracting the chemical free energy driving force for growth stress relaxation enables growth to continue at a rate controlled primarily by the mechanical properties of the rim we studied this process in high pressure experiments in order to constrain the rheology of wadsleyite one of the high pressure polymorphs of olivine single crystal spheres of san carlos olivine surrounded by a quasi hydrostatic pressure medium of nacl or au were partially transformed to wadsleyite at 1100 c and 16 17 gpa to determine the rate of growth as a function of time microstructural observations indicate that the relict olivine remains largely undeformed while the wadsleyite rims deform by dislocation creep application of an elastoplastic model to fit the observed decrease in growth rates with time constrains the yield strength of wadsleyite at these conditions to between 4 and 6 gpa a decrease in growth rates was also observed in another study in which varying amounts of water were added to the samples kubo t ohtani e kato t shinmei t fujino k 1998a experimental investigation of the transformation of san carlos olivine single crystal phys chem miner 26 1 6 kubo t ohtani e shinmei t fujino k 1998b effects of water on the transformation kinetics in san carlos olivine science 281 85 87 modeling these results gives values for the yield strength of wadsleyite between 2 and 5 gpa under various conditions a strength difference of 1 gpa is indicated by experiments at 1030 c conducted under nominally dry 200 ppm h2o and water added 500 ppm h2o conditions supporting the hypothesis that hydrolytic weakening is an important process in wadsleyite the calculated yield strengths for wadsleyite indicate that it is three to six times stronger than olivine at the same conditions based on an extrapolation of the low temperature plasticity flow law for olivine these results imply a strong increase in the strength of subducting slabs in the mantle transition zone provided that the deformation mechanism is the same as in the experiments furthermore inhibition of growth by transformation stress may be an important factor in extending the depth range over which metastable olivine transforms to its high pressure polymorphs in subducting slabs

age constraints to the relationships between magmatism metamorphism and tectonism in the aracena metamorphic belt southern spain the aracena metamorphic belt amb southwest iberian peninsula is characterized by the following geological elements a a high temperature low pressure ht lp metamorphic belt a few kilometres wide and more than 200 km long b a linear belt of oceanic amphibolites with a low pressure inverted metamorphic gradient c crustal scale ductile shear zones and d mafic noritic intrusions of high mg andesite boninite composition the relationships between these elements led to the proposal of a model of ridge subduction for this sector of the hercynian belt of europe this interpretation is supported by the age relationships displayed between the main rock units considered representative of the main tectonic and petrological processes responsible for the geological elements mentioned previously the results of a geochronological study ar ar rb sr and sm nd clearly support a late paleozoic tectonic evolution at an active continental margin the time evolution of the metamorphism in the oceanic domain ranging from 342 6 0 6 ma in the west to 328 4 1 2 ma in the east over a distance of 70 km along the metamorphic belt support a tectonic model of triple junction migration responsible for the creation at depth of a slab free window with decisive consequences for the thermal evolution of the region the origin of the linear metamorphic belt of ht lp regime may be explained by the migration along a continental margin of a punctual thermal anomaly induced by the creation of a triple junction at the continental margin

supergene enrichment of precious metals by natural amalgamation in the las cruces weathering profile iberian pyrite belt sw spain abstract natural au ag hg alloys occur in the las cruces ore deposit in the eastern part of the iberian pyrite belt they are mainly concentrated in the lower part of the gossan profile including a sheared black shale level where the gossan makes contact with a barren pyrite zone within the supergene cu rich mineralization drill core analyses show a heterogeneous distribution of au ag and hg within the weathering profile with mean values of 5 1 ppm 155 ppm and 52 ppm respectively in general the absolute tenures increase towards the bottom of the weathered profile mineralogical studies conducted on samples from the active mine workings indicate that hg and precious metals occur mainly as au ag hg alloys these associations constitute the best potential resource for precious metals at the las cruces deposit this paper describes how this unusual precious metal enrichment is produced along the weathering profile by supergene processes combining paragenetic information mineral chemistry and the data pertaining to the solubilities of au ag and hg in a weathering profile we suggest a two stage genetic model for the formation of the las cruces au ag hg mineralization 1 release of au ag and hg from the massive sulfide deposit by weathering processes during the gossan formation at ph 0 9 v conditions au ag and hg are mobilized downward through the weathering profile as chloride complexes and fixed as elemental au halides oxides and sulfates and 2 remobilization of hg ag and au in the gossan after the deposit was buried beneath the neogene carbonate rich sedimentary cover the buffering capacity of the percolating fluids due to their interaction with the carbonate rich sedimentary pile leads to significant mineralogical and geochemical changes at near neutral conditions ph 6 7 eh 0 v hg ag and au are newly remobilized as thiosulfate sulfate and hydroxide complexes and newly fixed by sorption during ferric hydroxide formation and as sulfates several cycles of dissolution precipitation of au ag and hg near the redox front occur by oscillations in the water table and changes in the ph eh conditions the interaction of downward migrating fluids with high reductant lithologies black shales and massive sulfides seems to be responsible for the reduction of different complexes and for the precipitation of cinnabar ag sulfides and sulfosalts as well as the precipitation of au ag hg amalgams

seismic imaging of upper mantle structure with new evidence for a 520 km discontinuity stacked images of long period seismic data reveal many phases some not previously observed which are caused by reflections and conversions at discontinuities in the earth s upper mantle these images support the existence of discontinuities at 410 520 and 660km depth but no evidence is found for a significant interface at 220 km

implications of slab mineralogy for subduction dynamics phase relations among mantle minerals are perturbed by the thermal environment of subducting slabs both under equilibrium and disequilibrium metastable conditions such perturbations yield anomalies not only in seismic velocities but also in density the buoyancy forces arising from these density anomalies may exert several important effects they contribute to the stress field within the slab in a fashion consistent with observed patterns of seismicity they may affect subduction rates both by inducing time dependent velocity changes under equilibrium conditions and by imposing velocity limits through a thermal feedback loop under disequilibrium conditions they may affect slab morphology possibly inhibiting penetration of slabs into the lower mantle and allowing temporary stagnation of deflected or detached slabs latent heat release from phase transitions under disequilibrium conditions in slabs can yield isobaric superheating which may generate adiabatic shear instabilities capable of triggering deep seismicity 2001 elsevier science b v all rights reserved

u pb age constraints on variscan magmatism and ni cu pge metallogeny in the ossa morena zone sw iberia new u pb zircon ages from the santa olalla igneous complex have been obtained which improve the knowledge of the precise timing of variscan magmatism in the ossa morena zone sw iberia this complex has a special relevance as it hosts the most important ni cu platinum group element pge mineralization in europe the aguablanca deposit u pb zircon ages have been obtained for seven samples belonging to the santa olalla igneous complex and spatially related granites with the exception of the cala granite 352 4 ma which represents an older intrusion the bulk of samples yield ages that cluster around 340 3 ma the santa olalla tonalite 341 5 3 ma the sultana hornblende tonalite 341 3 ma a mingling area at the contact between the aguablanca and santa olalla stocks 341 1 5 ma the garrote granite 339 3 ma the teuler granite 338 2 ma and dioritic dykes from the aguablanca stock 338 6 0 8 ma the bodonal cala porphyry which has also been dated 530 3 ma comprises a group of sub volcanic rhyolitic intrusions belonging to the bodonal cala volcano sedimentary complex which hosts the igneous rocks the knowledge that emplacement of the aguablanca deposit was related to episodic transtensional tectonic stages during the variscan orogeny will be fundamental in future mineral exploration in the ossa morena zone

tectonics of the beja acebuches ophiolite a major suture in the iberian variscan foldbelt the beja acebuches ophiolitic complex baoc south portugal spain corresponds to a high grade metamorphic belt along the boundary between ossa morena and the south portuguese zones and comprises a lithostratigraphic sequence including from top to bottom metabasalts metamorphosed multiple dyke intrusions in gabbro flasergabbros and metaserpentinites it is affected by three deformation phases d1 affects the ophiolite lower stratigraphic units and is represented by a mylonitic cleavage with a stretching lineation where shear criteria indicates the sense of shear to be towards the north north east this deformation event can be related to the ophiolite emplacement above the crystalline footwall of the serpa antiform affecting precambrian basement and cambrian cover rocks the obduction polarity ist north eastwards similar to the subduction polarity that generates the beja gabbroic complex bgc implying a flake geometry the second deformation phase d2 is represented by an intense wnw ese sinistral shear event which is responsible for the shattered appearance of the suture d2 is reactivated later by a more brittle d3 event involving thrusting to the south west again with a sinistral component 40ar 39ar isotopic ages were obtained for metamorphosed multiple dyke intrusions in the baoc s gabbro 342 6 1 4 ma for metagabbroic cumulates 340 7 1 9 ma and for the undeformed unmetamorphosed bgc 341 1 1 3 ma occurring to the north of the ophiolitic suture these ages reflect a last regional cooling event in the area which post dates the ophiolite emplacement and the intrusion of the bgc through this oceanic sequence

variscan intracontinental deformation the coimbra cordoba shear zone sw iberian peninsula the coimbra cordoba shear zone is a major lineament roughly parallel to the nw se variscan structures of the southern iberian peninsula inside the shear zone evidence for a non coaxial strain path exists from structural microstructural and lattice preferred orientation data on quartz plane strain is suggested by the study of microstructures in the xy and yz sections in all formations occurring in the shear zone with increasing strain a mylonitic series develops the deformation contemporaneous with epizonal to mesozonal metamorphism is responsible for 1 1 the regional subvertical foliation oblique to the shear zone 2 2 the well defined subhorizontal lineation striking nw se 3 3 the discrete planes of shear roughly parallel to the shear zone the data are interpreted as resulting from a left lateral slip motion with a displacement of at least 72 km in the light of plate tectonics it is inferred that the coimbra cordoba lineament could have been a suture zone which evolved into an intracontinental left lateral shear zone during the variscan orogeny of the ibero armorican arc

the strength of mantle silicates at high pressures and room temperature implications for the viscosity of the mantle the rheological properties of the upper mantle important in convective and tectonic processes have long been studied through low pressure creep experiments on olivine1 5 in comparison our understanding of deformation in the deep earth is incomplete because there are few experimental constraints on the rheology of the transition zone 400 670 km depth and the lower mantle estimates of the viscosity at these depths have had to rely on geophysical models of for example post glacial uplift the geoid and changes in the earth s rate of rotation6 13 recently we developed techniques for measuring solid state creep at mantle pressures14 and here we use these methods to make direct laboratory measurements of the strength of key mantle minerals we study mg fe 2sio4 olivine representative of about 60 of the upper mantle mg fe 2sio4 spinel which constitutes a large fraction of the transition zone and mg fe sio3 perovskite and perovskite mg fe o magnesiowiistite assemblages which comprise most of the lower mantle all at room temperature and pressures of up to 60 gpa our results suggest that the transition zone may form a layer of relatively high viscosity or strength between the upper and lower mantle a simple theological model shows that this behaviour may be compatible with current geophysical observations

the influence of olivine metastability on the dynamics of subduction abstract both seismological observations and mineral physics experiments suggest that old and cold subducting slabs might contain a wedge of metastable olivine mo dynamically consistent models of subducting slabs in which the kinetics of the olivine to spinel wadsleyite or ringwoodite transition is treated in a simplified way are carried out to evaluate the buoyancy effect of mo on the subduction velocity assuming slab thicknesses according to the cooling half space model we find that mo forms in slabs with ages greater than approximately 70 myr and that a significant slowing down of subduction velocity occurs for ages greater than 100 myr because a decrease in the subduction velocity for old lithospheric ages is not observed our models suggest that the amount of mo in slabs is less than approximately 5000 km2 in cross section and that for old lithosphere the cooling half space model is not applicable

mechanism and kinetics of the α β transition in san carlos olivine mg1 8fe0 2sio4 the mechanism and kinetics of the a b transformation in san carlos olivine a mg1 8fe0 2sio4 containing ca 526 wt ppm h2o were studied in situ by time resolved synchrotron x ray diffraction xrd in the range 12 5 14 5 gpa and 870 1150 k time series of xrd spectra reveal the early formation of an intermediate phase that further transforms to wadsleyite b phase this intermediate phase may be either a metastable ringwoodite g phase or the disordered spinelloid phase proposed in the pseudomartensitic reaction model both microstructural and kinetic data suggest that the transformation rates are controlled by growth processes after the early saturation of nucleation sites along olivine grain boundaries growth rates in the range 1 7 10 12 2 1 10 9 m s 1 are estimated by fitting the transformation time data to the cahn rate equation and define an activation enthalpy for growth ha 204 55 kj mol compared to the previous studies performed on pure forsterite mg2sio4 these new kinetic results point out an increase of transformation rates with the iron content hence the persistence of a metastable olivine wedge to depths 660 km might be restricted to even colder and or dryer subduction zones than previously estimated

evolutionary model of the himalaya tibet system geopoem based on new modelling geological and geophysical data a two dimensional thermo mechanical laboratory modelling of continental subduction was performed the subducting continental lithosphere includes a strong brittle upper crust a weak ductile lower crust and a strong upper mantle the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is denser than the asthenosphere the lithospheric layers are composed of material whose strength is sensitive to and inversely proportional to temperature throughout the experiment the model surface was maintained under relatively low temperature and the model base at higher temperature the subduction rate satisfied the peclet criterion modelling confirms that the continental crust can be deeply subducted and shows that slab break off delamination and tectonic underplating are fundamental events with drastic consequences on the subsequent evolution of the convergent system combining these results with previous purely mechanical modelling we elaborate a new evolutionary model for the himalaya tibet convergent system the principal successive stages are 1 subduction of the indian continental lithosphere to 200 250 km depth following subduction of the tethys oceanic lithosphere 2 failure and rapid buoyancy driven uplift of the subducted continental crust from ca 100 km depth to some depth that varies along the mountain belt 20 30 km on average 3 break off of the indian subducted lithospheric mantle with the attached oceanic lithosphere 4 subduction underplating of the indian lithosphere under asia over a few to several hundred kilometers 5 delamination roll back and break off of the indian lithospheric mantle 6 failure of the indian crust in front of the mountain belt formation of the main central thrust and underthrusting of the next portion of indian lithosphere beneath tibet for a few hundred kilometers at the beginning of stage 6 the crustal slice corresponding to the crystalline himalayas undergoes erosion activated uplift and exhumation fl 2000 elsevier science b v all rights reserved

a new estimate of the conditions for transition from basal a to prism c slip in naturally deformed quartz abstract quartz c axis fabrics of naturally deformed quartz in metacherts from the ryoke metamorphic belt in the yanai district southwestern japan suggest that the prism c prism a and basal a slip systems were activated during the plastic deformation under high temperature metamorphic conditions with increasing temperature quartz c axis fabric patterns change from girdle type through x maximum with a faint crossed girdle to x maximum the dominant orientation of subgrain boundaries in the quartz also changes from prism to basal with increasing temperature closely correlated with the change in quartz c axis fabric the fabric transition is also accompanied by an abrupt increase of recrystallized quartz grain size moreover observations with transmission electron microscopy tem clearly identify the dominant activation of c slip through the presence of basal subgrain boundaries and free dislocations with the 0001 burgers vector in the quartz from the metachert samples exhibiting x maximum c axis fabric based on these observations it is concluded that the mechanism switch from basal a to prism c slip systems occurred with increasing temperature in the metacherts the transition temperature is petrologically estimated to be ca 550 600 c at the natural strain rate

an experimental study of grain scale microstructure evolution during the olivine wadsleyite phase transition under nominally dry conditions abstract the grain scale microstructure evolution during the phase transformation of olivine to wadsleyite was investigated using a multi anvil high pressure apparatus we identified three stages in the microstructure evolution during olivine to wadsleyite phase transformation 1 nucleation of wadsleyite along potential nucleation sites e g grain boundaries and grain corners inside of grains dislocations stacking faults 2 development of an interconnected network of fine grained wadsleyite along grain boundaries 3 growth of wadsleyite into olivine our observations suggest that at low over pressures 3 gpa intra crystalline nucleation makes a significant contribution to the volume fraction transformed the rheological properties of a sample undergoing the phase transformation are controlled by the grain size and the connectivity of newly formed grains based on the model of grain size evolution involving both nucleation and growth we show that the initial grain size of wadsleyite is controlled by the competition between nucleation rate and growth rate when phase transformation takes place near the equilibrium boundary at relatively higher temperatures 1200 k but mostly by critical size for nucleation when phase transformation takes place away from the equilibrium boundary at relatively low temperatures

stability of clinopyroxene at pressure temperature conditions of the transition region abstract the stability of diopside at conditions of 16 19 gpa and 1250 1900 c was determined using a 6 8 type multi anvil apparatus at pressures of 17 18 gpa diopside undergoes a decomposition reaction to an assemblage of casio3 perovskite ca mg sio3 garnet or ilmenite depending on temperature the clapeyron slope for the breakdown reaction download download full size image is 2 mpa k 1 when coupled with a re examination of clinopyroxene stability in other more complex peridotite systems the experimental data are consistent with a recent proposal that the 520 km discontinuity in the transition region is caused by the exsolution of casio3 rich perovskite from garnet rich in majorite component

melt enhanced deformation a major tectonic process convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks anatectic and or mantle derived melts combine to form a melt weakened zone in the thickened lower crust the accumulation of melt eventually leads to crustal failure along melt lubricated shear zones rapid 1 mm yr vertical component movements of large crustal blocks result we refer to these crustal displacements as tectonic surges the melt lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons rocks that formed at different crustal levels are juxtaposed across these shear zones one result of surges with large lateral component of movement is metamorphic inversion with high pressure and high temperature metamorphic rocks structurally over lower pressure and temperature assemblages large and rapid vertical surges may displace crust containing abundant melt and may result in high t p including granulite facies metamorphism and preservation of metamorphic textures caused by rapid decompression

the effect of water on mantle phase transitions water has played an important role in the earth s evolution the incorporation of water as hydroxyl into solid mineral phases or as coexisting hydrous fluids and melts affects the chemical and physical properties of crust and mantle constituents i e it weakens rocks and minerals reduces viscosity and strength of the materials and depresses dramatically the melting temperature of silicate minerals e g karato 1990 inoue 1994 hirth and kohlstedt 1996 chen et al 1998 kubo et al 1998 mei and kohlstedt 2000 many recent studies have suggested the possible existence of water in the earth s mantle especially in the transition zone e g smyth and frost 2002 ohtani et al 2004 litasov et al 2005a hae et al 2006 where wadsleyite and ringwoodite can accommodate up to 3 wt of h2o in their structures e g kohlstedt et al 1996 low velocity zones observed seismologically at the top of the 410 km discontinuity may indicate the existence of trapped high density melt e g revenaugh and sipkin 1994 song et al 2004 matsukage et al 2005 sakamaki et al 2006 which is likely to be hydrous as it is not possible to melt the base of the upper mantle without water at these conditions electrical conductivity anomalies in the upper mantle and transition zone that are related to subduction zones have also been interpreted as an effect of water in the mantle e g fukao et al 2004 tarits at el 2004 hae et al 2006 koyama et al 2006 studies of the kinetics of the hydrous olivine wadsleyite transformation ohtani et al 2004 hosoya et al 2005 seem to be consistent with seismological observations e g koper et al 1998 that indicate the absence of a metastable olivine wedge in subducting

ediacaran palaeozoic tectonic evolution of the ossa morena and central iberian zones sw iberia as revealed by sm nd isotope systematics abstract sm nd isotopic analyses of palaeozoic sedimentary and igneous rocks in the southwest iberian massif western end of the european variscan belt are presented in order to unravel its complex poly orogenic evolution during the closure of the rheic ocean and the amalgamation of pangea the gondwanan margin in southwest iberia sw iberia is subdivided into the ossa morena and central iberian zones separated by the badajoz cordoba shear zone which represents a cryptic suture zone between these terranes the relationships between these terranes and between units preserved within the suture zone e g the sierra albarrana group during the palaeozoic and neoproterozoic are controversial sm nd isotopic studies of representative sedimentary sequences covering the entire pre variscan record of the ossa morena and central iberian zones show very similar characteristics from the uppermost ediacaran onwards these data indicate that their accretion to one another must have been completed by the late neoproterozoic ediacarin that time an event assigned to cadomian orogeny and that they never separated substantially from each other since that time the sm nd isotopic composition of the sierra albarrana group metasedimentary rocks is similar to that of the pre cadomian sequences of the ossa morena zone serie negra suggesting derivation from a common source the common provenance of the palaeozoic sequences in the two zones is identical to that of the pre cadomian serie negra of the ossa morena zone which in accordance with the data presented herein and published u pb zircon data indicates a west african affinity

tectonic setting of the north gondwana margin during the early ordovician a comparison of the ollo de sapo and famatina magmatic events abstract this paper presents a comparison of compiled geochronological and geochemical data from the ollo de sapo and famatina magmatic events the ollo de sapo magmatic sequence is located in northwest iberia and was emplaced during the early ordovician from 495 to 474 ma the famatina complex is a magmatic sequence located in northern argentina that was emplaced during the early to mid ordovician from 483 to 463 ma these magmatic events are currently interpreted to have been emplaced in different tectonic settings despite both having occurred along the north gondwana margin geochronological data indicates that these magmatic events occurred contemporaneously over at least 9 m y and therefore can provide a snapshot of the northern gondwana margin during the mid ordovician major element data indicates that both magmatic suites are calc alkaline to alkali calcic and trace element and ree data show magmatic signatures that are indistinguishable this study highlights the similarity between the ollo de sapo and the famatina magmatic suites and discusses alternative models for their emplacement based on paleomagnetic and paleobiogeographical data these data indicate that the ollo de sapo was likely emplaced in a subduction zone setting while the famatina magmatic suite may be of parautochthonous origin to gondwana implying that the pampeanan margin may not have been active during the early to mid ordovician

early paleozoic rifting and bimodal volcanism in the ossa morena zone of south west spain lower paleozoic volcanic members have been investigated by geological petrographical and geochemical means in a traverse across the ossa morena zone omz in south west spain

thermal modelling in low pressure high temperature metamorphic belts abstract low pressure high temperature metamorphic belts occur throughout the world they display a characteristic metamorphic style which has remained unchanged with time they are found in disparate tectonic settings including magmatic arcs regions of extension regions of thickened crust in continent continent collision zones and accretionary wedges they are commonly characterized by an abundance of granitoid plutons pressures of metamorphism are primarily less than or equal to that of the aluminosilicate triple point approximately 4 kbar while temperatures are in the range 500 750 c with rare exceptions inferred ptt paths are nearly isobaric and recrystallization was rapid and prograde metamorphic thermal gradients are commonly in the range 60 150 c km these conditions necessitate the presence of a large component of advective heat transfer and or anomalously high basal heat flow during metamorphism the large magmatic fluxes associated with the formation of magmatic arcs provide an adequate heat source for low pressure metamorphism lpm lithospheric extension is likely to produce lpm only if the degree of thinning is large 3 and there is additional heat from magmas the generation of lpm by crustal thickening requires either a large component of advective heating by magmas anomalously high basal heat flow or pre thickening lithospheric extension lpm in this environment must necessarily be accompanied by plutonic activity advection of heat by aqueous fluids may provide a significant contribution to the thermal budget but acting along is unlikely to produce lpm regional lpm induced by advective sources rarely occurs along a geotherm which increases monotonically with depth or synchronously across the region at a given depth it occurs over time as a series of overlapping temporally separate and spatially localized thermal events

magma emplacement in transpression the santa olalla igneous complex ossa morena zone sw iberia the santa olalla igneous complex a late variscan group of intrusions located in the ossa morena zone sw iberia has been the focus of a gravity and structural study the structure outlined by the foliation map is complex showing two different structural domains one characterized by vertical and the other by horizontal magmatic foliations the vertical fabrics are restricted to the ne half of the complex which is in direct contact with a variscan sinistral strike slip fault cherneca fault whereas the horizontal fabrics are developed in the sw half of the complex which is characterized by a horizontal tabular geometry gravity modeling indicates that the deeper floor of the plutons is closely related to the ne margin and the cherneca fault an emplacement and structural evolution model for this igneous complex is proposed following these structural and gravity results 1 magma ascent was favored by releasing bends in the trace of the cherneca fault 2 when magma reached the present level it intruded to the sw with a horizontal sheet geometry generating the subhorizontal foliation domain 3 after emplacement the ne half of the complex suffered the external tectonic stress field provoked by sinistral motion along the cherneca fault subsequently generating the subvertical magmatic foliation domain

space and time in the tectonic evolution of the northwestern iberian massif implications for the variscan belt recent advances in geochemical studies of igneous rocks isotopic age data for magmatism and metamorphism quantitative pressure temperature p t estimates of metamorphic evolution and structural geology in the northwestern iberian massif are integrated into a synthesis of the tectonic evolution that places the autochthonous

ordovician palaeogeography with new palaeomagnetic data from the montagne noire southern france a joint palaeomagnetic and ar 40 ar 39 study has been performed on two olistolithic blocks from the cabrieres wildflysch in the montagne noire region of the massif central in france there andesitic volcanic and volcaniclastic rocks of llanvirn early caradoc age ca 470 458 ma occur despite extensive secondary alteration destruction of the dominant magnetic mineral phase and ar 40 ar 39 whole rock experiments that demonstrate that the volcanic rocks suffered significant argon loss a positive fold test and the presence of dual polarities suggest that a primary ordovician magnetisation has mostly survived this is one of the few documented cases where the argon system was substantially reset whilst a subordinate set of small relatively unaltered magnetite grains probably hosted in silicates still carry the original in this case ordovician remanence the new data show that the montagne noire region was located at high southerly latitudes 68degrees 19 15 during the mid ordovician this latitude represents the location for nw gondwana of which the massif central was a part palaeomagnetic data from all the central european massifs and terranes demonstrate a close link to the gondwana margin during the lower and middle ordovician

coesite in subducted continental crust p t history deduced from an elastic model abstract coesite has been found in a metasedimentary rock in a continental unit of the western alps dora maira massif the rock preserves the high pressure low temperature assemblage of nearly pure pyrope talc phengite kyanite in a quartz matrix coesite exclusively occurs as inclusions in pyrope and is partly inverted to quartz textural observations and mechanical considerations suggest that the rock crystallised in the coesite stability field a simple elastic model for a coesite inclusion in pyrope explains why coesite has been exclusively preserved in the inclusions and not in the matrix combined with petrological data the model constrains the crystallisation conditions of the pyrope coesite rock p 28kbar 650 this finding has an important bearing on the significance of p t conditions estimated from minerals occurring as inclusions and on the actual behaviour of continental crust in collision zones

shock conditions recorded in nwa 8159 martian augite basalt with implications for the impact cratering history on mars nwa 8159 is an augite rich martian basalt formed by cooling of a relatively evolved ca rich ti poor and lree depleted lava under relatively oxidizing conditions during the early amazonian in addition to its distinct igneous petrogenesis and high fo 2 nwa 8159 is also set apart from most martian shergottites with respect to the low degree of shock metamorphism required to preserve crystalline igneous plagioclase an 50 65 in this study mineral transformations within and adjacent to shock veins in nwa 8159 were investigated using scanning electron microscopy raman spectroscopy and transmission electron microscopy to better constrain the unusal shock history of this meteorite the transformation of olivine to ahrensite fe ringwoodite along shock vein margins and tissintite and coesiteformed from igneous mineral labradorite and silica grains entrained as clasts within shock veins has been documented in this study we report on a previously unidentified mineral assemblage of ca na majoritic garnet sodic clinopyroxene and stishovite crystallized from shock melt this mineral assemblage indicates a crystallization pressure of approximately 16 gpa which is within the range of previous shock pressure estimates for this meteorite 15 23 gpa the presence of a majoritic garnet bearing assemblage throughout veins up to 0 6 mm wide indicates that the sample remained at high pressure throughout the melt vein quench based on thermal models the sample must have remained at high pressure for 100 ms this shock duration is an order of magnitude longer than those experienced by more highly shocked shergottites such as tissint or zagami 30 gpa 10 20 ms and would seem to imply a relatively large impact event recent numerical models demonstrate that a range of shock pressures and durations are realized by rocks within the ejected spall zone of a hypervelocity impact the shock conditions experienced by nwa 8159 therefore do not require an impact event distinct from other shergottites rather our findings suggest that this meteorite originated from near the martian surface at the edge of the impact site the shock history of nwa 8159 provides a picture of mars consistent with that derived from remote observation that of a random cratering process that samples a geologically long lived and complex planet

cadomian vs variscan evolution of the ossa morena zone sw iberia field and 40ar 39ar mineral age constraints abstract six hornblende and ten muscovite concentrates have been dated from three contrasting tectonic units exposed within the ossa morena zone of the iberian massif these include 1 the obejo valsequillo domain north of the badajoz cordoba shear zone 2 the sierra albarrana structural unit immediately south of the badajoz cordoba shear zone and 3 the thermal some exposed within the olivenza monesterio antiform zafra monesterio domain hornblende from foliated amphibolite in the obejo valsequillo domain siera negra unit displays internally discordant 40ar 39ar age spectra as a result of both intracrystalline contamination with extraneous argon and slight rejuvenation 36ar 40ar vs 39ar 40ar isotope correlations are generally well defined and yield ages of c 550 560 ma these are interpreted to date cooling following late precambrian early paleozoic cadomian tectonothermal activity this was followed by slight variscan rejuvenation at c 375 400 ma muscovite from this unit displays internally discordant age spectra which reflect more extensive variscan rejuvenation of intracrystalline argon systems which had initially cooled below appropriate blocking temperatures at c 560 ma hornblende and muscovite from the sierra albarrana domain sierra albarrana group record 40ar 39ar plateau and isotope correlation ages which range between c 392 ma and 351 ma these results indicate that complete variscan rejuvenation was followed by relatively rapid post metamorphic cooling hornblende separated from various lithologie elements exposed in the olivenza monesterio antiform yield variable results hornblende within a mafic xenolith in the monesterio granodiorite records an isotope correlation age of c 553 ma which is interpreted to date the last cooling through appropriate argon retention temperatures hornblende within amphibolite of the montemolin series lower serie negra records internally discordant age spectra which reflect extensive variscan c 390 400 ma rejuvenation of intracrystalline argon systems which had initially recorded ages c 500 ma muscovite was concentrated from several lithologie elements in this area including 1 mylonitic monesterio granodiorite 2 metasedimentary rocks of the montemolin succession displaying high grade sillimanite potassium feldspar bearing assemblages and 3 migmatitic schist of the siere negra group these muscovite concentrates display similarly discordant 40ar 39ar age spectra which suggest significant but incomplete variscan rejuvenation occurred at c 400 ma this appears to have affected intracrystalline argon systems which had initially cooled through appropriate closure temperatures sometime prior to c 450 ma following an initial high grade cadomian metamorphism the 40ar 39ar results clearly reflect a variable complex and polymetamorphic evolution for the ossa morena zone internal consistency of data within each regional tectonic unit compared with marked contrast between the units provides evidence of the extremely heterogeneous variscan tectonothermal overprint the results demonstrate that the present tectonic architecture of the ossa morena zone was not solely the result of late variscan wrench tectonics but instead a consequence of processes active since at least the lower middle devonian juxtaposition of regional structural units with contrasting tectonothermal histories and recording variable rates and extents of uplift are consistent with maintainence of an overall transpressional variscan tectonic regime

formation of a metastable olivine wedge in a descending slab we present a thermal model of a descending slab in which the transformation of olivine to spinel is controlled by pressure and temperature dependent reaction kinetics two different formulations of the kinetics are considered with the main discriminant being the temperature range over which olivine converts to spinel at pressures of about 15 gpa about 500 515 c and 560 650 c we use a finite element method to solve the coupled heat conduction perpendicular and parallel to the dip of the slab and kinetics equations and we include the latent heat of the phase transformation latent heat release together with heat conduction parallel to the dip of the slab reduces significantly the length of the metastable olivine wedge and results in a very thin 5 km two phase region we employ the thermal parameter sin is the velocity of the descending slab is the age of the slab and is the dip of the slab to the horizontal to interpret the results for the length of the metastable wedge for values of the thermal parameter smaller than about 4000 and 7000 km depending on the model of the kinetics no metastable olivine wedge exists the critical value of 7000 km for the thermal parameter corresponds to the kinetics model with the lowest transition temperature range the length of the metastable olivine wedge is also found to be very sensitive to the model of the kinetics and to the effects of adiabatic heating if the occurrence of deep earthquakes is related to the transformation of metastable olivine to spinel then data on earthquake depth versus thermal parameter require that the onset of the reaction takes place at temperatures of about 550 575 c in this case the slab thermal parameter must be larger than 10 000 km for the metastable olivine wedge to extend down to 660 km depth but deep earthquakes occur near 660 km depth in slabs with thermal parameter as small as about 5000 km south america for example either some deep earthquakes are unrelated to olivine metastability or our knowledge of olivine spinel reaction kinetics is incomplete

the structure of a major suture zone in the sw iberian massif the ossa morena central iberian contact abstract we have investigated the stratigraphy structure and metamorphism of the boundary between the ossa morena zone omz and the central iberian zone ciz two significant continental portions of the variscan iberian massif the omz ciz contact is marked by a strongly deformed and metamorphosed nw se trending narrow band namely the central unit in which partially retrogressed eclogites are included during the middle late devonian the ciz overthrust the omz and in the footwall km scale recumbent folds and thrusts developed with decoupling and underplating of the lower crust at the same time in the hanging wall there took place intense though localized back folding and back shearing in the early carboniferous a transtensional tectonic regime sank the overthrust block resulting in the exhumation of eclogites these eclogites probably came from the underthrust omz lower crust and they are at present included in the suture zone central unit of this continental collision the extension is responsible for the origin of a basin and bimodal magmatism on the southern border of the ciz a late episode of folding and fracturing significantly contributed to the final complex picture of this suture boundary

u pb zircon ages and structural development of metagranitoids of the tepla crystalline complex evidence for pervasive cambrian plutonism within the bohemian massif czech republic u pb zircon dating of three metagranitoids situated within a tilted crustal section at the northwestern border of the tepla barrandian unit tepla crystalline complex tcc yields similar cambrian ages the u pb data of zircons of the tepla orthogneiss define an upper intercept age of 513 7 6 ma the 207pb 206pb ages of 516 10 and 511 10 ma of nearly concordant zircons of the hanov orthogneiss and the lestkov granite are interpreted to be close to the formation age of the granitoid protolith similar to the cambrian granitoids of the southwestern part of the tepla barrandian unit doma lice crystalline complex dcc the middle cambrian emplacement of the tcc granitoids postdates cadomian deformation and metamorphism of the upper proterozoic country rocks but predates variscan tectonometamorphic imprints structural data as well as sedimentological criteria suggest a dextral transtensional setting during the cambrian plutonism related to the early paleozoic break up of northern gondwana due to strong variscan crustal tilting the degree of variscan tectonometamorphic overprint is strikingly different in the dated granitoids it is lowest in the weakly or undeformed lestkov granite located in the greenschist facies domain the tepla orthogneiss in the north underwent pervasive top to nw mylonitic shearing under amphibolite facies conditions there is no indication for a resetting of the u pb isotopic system of the tepla orthogneiss zircons that could be attributed to this imprint radiation damages accumulated until recent have probably caused lead loss

metamorphic degassing of carbonates in the contact aureole of the aguablanca cu ni pge deposit spain analysis of magmatic and sedimentary rocks of several large igneous provinces has demonstrated that the release of gas during plutonic metamorphic processes may be linked to global climate change and mass extinctions aguablanca one of the largest cu ni pge deposits in europe formed during the variscan orogeny when a mafic magma intruded limestones and shales creating a contact aureole composed of marble skarn and hornfels our petrological and geochemical investigation of the aureole provides evidence that a combination of the two processes led to the formation of the ore deposit the assimilation of terrigenous sediments supplied s to the magma while the assimilation of carbonates changed the oxygen fugacity and decreased the solubility of sulfur in the magma the metamorphic assemblages in the contact aureole are directly related to heterogeneity of the protolith and particularly to the original proportions of calcite and clay we modeled carbon dioxide degassing during contact metamorphism and showed that pure limestone is relatively unproductive because of its high reaction temperature the presence of clay however leads to the formation of calc silicates and significantly enhances co2 degassing our estimations suggest that degassing of the aguablanca contact aureole released about 74 8 mt of co2 a relatively low volume that we attribute to the composition of the host rock mainly a pure limestone a far larger volume of carbon dioxide was emitted by the contact metamorphism of dolostones in the contact aureole of panzhihua part of emeishan large igneous province sw china we propose that the level of emission of carbon dioxide depends strongly on the nature of the protolith and has to be considered when predicting environmental impact during the emplacement of large igneous provinces

akimotoite in the tenham meteorite crystal chemistry and high pressure transformation mechanisms the transformation of pyroxene to its high pressure polymorph akimotoite mgsio3 ilmenite type structure is documented in the shock induced melt veins of the l6 tenham chondrite four textural relationships between pyroxene and akimotoite are observed in former pyroxene grains entrained in the shear melt vein and in pyroxene grains attached to the wall of the melt vein in one of the entrained enstatite grains the transformation to akimotoite is partial one third of the grain is transformed to a polycrystalline aggregate of akimotoite with a scalloped interface with enstatite akimotoite fe0 4mg1 24ca0 07na0 12al0 14 si1 963al0 037 o6 is slightly enriched in ca cao 1 74 al and na with respect to enstatite cao 0 71 narrow bands of polycrystalline akimotoite with three crystallographic orientations and a chemical composition similar to the surrounding enstatite intersect the other part of the grain in a second grain entrained in the shear melt vein tablets of akimotoite are interwoven with a pyroxene glass with a lower na and fe content and a higher mg content compared to the adjacent akimotoite this pyroxene glass is probably the product of mg fe sio3 perovskite amorphization polycrystalline akimotoite is also formed at the expense of enstatite at the vein wall of the shear melt vein in that case akimotoite is also enriched in ca cao 2 85 na nao 1 72 and al al2o3 4 14 all these observations suggest that akimotoite is mainly formed by solid state transformation of former pyroxenes with subsequent diffusion of calcium aluminum and sodium from the chondritic melt of the shear melt vein finally the first complete x ray diffraction pattern of natural akimotoite is presented

3d gravity modelling of the aguablanca stock tectonic control and emplacement of a variscan gabbronorite bearing a ni cu pge ore sw iberia the aguablanca stock is a variscan mafic pluton located in the ossa morena zone southern iberian massif hosting an unusual ni cu pge mineralization associated with magmatic breccia pipes which intruded its northern part the emplacement of the aguablanca stock and the mineralized breccia pipes are related to the activity of the cherneca ductile shear zone a variscan sinistral shear zone that favoured magma ascent through the upper crust a detailed gravity study has been carried out in order to investigate the 3d geometry of the aguablanca intrusion and to get insights about the emplacement mechanism and tectonic controls of the mineralization the three dimensional gravity modelling shows that the stock has an inverted drop geometry with a feeder zone in contact with the cherneca ductile shear zone the inferred orientation of the feeder zone suggests that the emplacement probably took place along an open tensional crack formed within the strain field of the adjacent cherneca ductile shear zone the modelling of the breccia pipes hosting the ni cu pge ore shows that they are included inside the feeder zone thus their emplacement is probably controlled by successive opening events of this tensional crack

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

bulk attenuation in the earth s mantle due to phase transitions abstract the calculated bulk attenuation due to phase transition from mineral physics data is reported here with relaxation time less than 1 s the calculated value for a pyrolite mantle is consistent with the inverted bulk attenuation of the upper mantle from seismic observations the two important mechanisms of phase transitions diffusion controlled and kinetics controlled have different relaxation time as indicated by the models here the diffusion controlled is more likely to contribute to the observed seismic bulk attenuation than the kinetic controlled process based on the available diffusivity and kinetics data the correlation between the bulk attenuation and relaxation time emphasizes the importance of a number of parameters in the mineral physics database such as fe mg diffusivity and kinetics in olivine wadsleyite ringwoodite perovskite mg ca al si diffusivity and kinetics in pyroxene garnet ca perovskite some of which are still unknown

peridotite xenoliths in silica rich potassic latite from the transition zone of the colorado plateau in north central arizona potassic latite in the transition zone of the colorado plateau near chino valley arizona contains abundant eclogite and amphibolite xenoliths and minor websterite and pyroxenite xenoliths one unit contains peridotite xenoliths analyzed samples have mg ratios of 68 and 71 58 and 63 wt sio2 and are enriched in potassium and other large ion lithophile lil elements rare earth element ree patterns are light ree enriched with la greater than 100 times chondritic abundance the peridotite xenoliths are partly to totally altered but contain remnant olivine orthopyroxene and clinopyroxene one harzburgite nodule also contains spinel mineral compositions from the xenoliths are relatively refractory and similar to those in other spinel peridotite xenoliths from the colorado plateau geothermometry on olivine spinel and two pyroxene pairs indicates equilibration temperatures of less than 800 c for the peridotite nodules the relatively low temperatures calculated from mineral equilibria are consistent with temperature estimates for other mantle nodules from under the colorado plateau

analytical electron microscopy of a synthetic peridotite experimentally deformed in the β olivine stability field a synthetic harzburgite prepared by isostatic hot pressing of finely ground natural olivine admixed with 3 enstatite was deformed at high temperature and high pressure such that the olivine phase boundary was traversed both residual and new phases their grain sizes their deformation induced lattice defects and their compositions were then characterized by analytical electron microscopy owing to large heterogeneities in temperature pressure and deformation conditions contrasting microstructures were observed some areas consist essentially of untransformed olivine while the polymorph is the main constituent of other areas crystals of the polymorph are generally free of dislocations except at the grain boundaries of the larger grains where slip bands were nucleated probably induced by stress concentrations the activated glide systems are 021 100 the orthopyroxene grains have been transformed to very fine grained clinopyroxene and have reacted with chromite to form garnet with a majoritic component compared to olivine crystals of the phase contain significantly higher concentrations of fe and trivalent cations all crystals are free of inclusions confirming the conclusion from previous infrared spectroscopic analysis that the high oh concentration in this specimen is dissolved in the phase coupled with these previously measured oh concentrations our new measurements suggest that incorporation of hydrogen into the phase and substitution of trivalent cations into the octahedral sites may be coupled such increased complexity of mineral chemistry points to a need for detailed investigation of the spectrum of compositions stable in the structure and coexisting phases in order to better estimate the modal mineralogy of the transition zone

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

static compression and olivine flotation in ultrabasic silicate liquid piston cylinder techniques have been used to observe static compressions of ultrabasic silicate liquids to 6 gpa and 2000 c compressions are measured by bracketing sinking and floating of olivine crystals of known density at run conditions komatiite peridotite and their liquid mixtures with fe2sio4 all show constant density changes with pressure of 0 07 g cm3 gpa along their liquidi these results confirm shock wave studies and ultrasonic studies which show weak compositional effects on compressibility of basic to ultrabasic silicate liquid the techniques converge in supporting the conjecture of stolper et al 1981 that ultrabasic silicate liquids may be denser than their equilibrium coexisting olivines and pyroxenes at high compressions the pressure of this density inversion is 7 8 gpa for the compositions studied olivine flotation provides an important constraint upon earth evolution models that include extensive melting during early differentiation it also provides a mechanism for concentrating high mg si olivine in the upper mantle without unduly perturbing other geochemical parameters a full evaluation of the physical and geochemical consequences is required at an early date