an isotopic mixing model for the origin of granitic rocks in southeastern australia a simple mixing process was a significant element in the genesis of many southeastern australian granitic rocks major and trace element abundances are near linearly correlated nd and sr initial isotopic compositions define a simple hyperbolic trend and many initial 87sr 86sr ratios are a regular function of rb sr ratios geological arguments and regression of geochemical variation diagrams place limits on the nature of the end members the high si end member is meta sedimentary derived magma comparable in composition to the cooma granodiorite the one pluton in the region surrounded by and clearly derived from regional metamorphic rocks the low si end member is basaltic material with similarities to rare gabbros also found in the area isotope dilution calculations based upon initial sr and nd isotopic compositions deduced for the end members yield realistic hypothetical sr and nd concentrations for the end members in a demonstration of the internal consistency of the model all of the granitic rocks from hornblende tonalites to cordierite granodiorites are a single broad family products of variable mixing between distinct batches of basaltic material and a uniform partial melt of the regional basement

geochemistry and petrogenesis of late miocene granitoids cyclades southern aegean nature of source components abstract neogene igneous activity in the aegean region comprised both extrusive volcanism and intrusive granitoid plutonism granitoid plutonism in the cyclades was established in the mid miocene period and lasted until late miocene 18 to 9 ma the timing of granitoid intrusion was associated with the initiation of extensional tectonics in the aegean these intrusions form a broad belt about 200 km long running from the west lavrium and serifos to the eastern aegean both s type and i type granitoids are present the former generally being emplaced earlier than the i types 15 to 8 3 ma major and trace element variations reveal that three end member components are involved in the granitoids but the proportions of these vary in the different plutons initial isotopic compositions of all the granitoids are typical of crust derived magmas from heterogeneous metasedimentary sources i type 87 sr 86 sr 0 7091 0 712 e nd 6 4 to 10 4 s type 0 710 0715 e nd 7 5 to 10 1 three end member sources have been identified 1 one end member appears to be a metasedimentary biotite gneiss greywacke type such as that forming the metamorphic core complexes naxos and paros this is a dominant but not the only component in the s types 2a major trace element and sr nd isotopic composition correlations of the s type granitoids with basement gneiss require an extra source of sr and ca having lower initial 87 sr 86 sr indicating a more depleted metasedimentary source at depth due to possible interaction of metamorphic fluids with marbles and amphibolites and infiltration through the gneissic units at mid crustal levels 2b a possible second end member could be the marble component as indicated by the buffered values of the initial sr isotopic ratios major element variation of the mafic microgranular quartz diorites and tonalites enclaves are compatible with dehydration melting of a mafic source similar to the amphibolites island arc tholeiites at medium pressure 8 kb conditions 3 amphibolite is another end member which may contribute mostly to the source of the younger intrusions along the western flank of the arc

anelasticity and thermal structure of the oceanic upper mantle temperature calibration with heat flow data the thermal structure of the oceanic upper mantle is examined on the basis of seismic attenuation surface heat flow and mid ocean ridge basalt morb composition the comparison of seismic q with laboratory q determined at sufficiently high pressure and temperature allows us to constrain temperature in the earth since the inferred absolute temperature relies on knowledge of the frequency dependence of q if any we calibrate an unknown frequency effect on q by comparing the derived temperatures with geotherms calculated from surface heat flow and morb composition this comparison yields a correction factor found to be small for the geotherm derived from laboratory ultrasonic q examples of thermal structure are presented for the iceland plateau and the pacific ocean under the iceland plateau we expect partial melt for the asthenosphere only in the vicinity of the spreading ridge in asthenosphere older than 5 m y of the iceland plateau and older than 40 m y of the pacific ocean temperatures are below the dry peridotite solidus and no partial melt is expected these results are consistent with the thermal structure derived from a comparison of seismic and laboratory velocities it is shown that there is a good agreement between temperatures determined from seismic attenuation heat flow and morb chemistry since the laboratory measurements were made on a dry peridotite and we employ a dry peridotite solidus for determining the geotherm the consistency in temperatures derived from three independent data strongly suggests that the upper mantle remote from subduction zones is essentially dry it appears that the spreading process in mid ocean ridge causes asthenosphere material to ascend from greater depths where it was at higher temperature this gives rise to pressure release melting

thermal erosion by komatiites at kambalda western australia and the genesis of nickel ores huppert et al 1 calculated that komatiites flow turbulently transfer heat efficiently to their substrate and thus potentially produce thermal erosion channels in sequences onto which they were erupted their suggestion that komatiite hosted kambalda type2 nickel ores reside in such channels has led to resurgence of interest in volcanic models for ore emplacement3 5 and caused controversy6 7 such models have important implications not only to ore genetic studies but also to interpretation of the geochemistry of komatiites8 and their sm nd systematics9 10 here we demonstrate that interspinifex fe ni cu sulphide ores which underlie some hanging wall massive sulphide ores at kambalda the site of one of the world s major nickel deposits formed through thermal erosion of underlying komatiite flows and possibly also sediments providing proof of thermal erosion however we caution against the general application of thermal erosion models to the eruption of komatiites it seems that significant thermal erosion occurred rarely and only where basal concentrations of highly thermally conductive sulphide liquid were present at the base of localized lava channels isotope data11 12 and theoretical considerations13 suggest that the komatiites were contaminated by crustal rocks before eruption although there is limited evidence that localized contamination including sulphide assimilation4 may also have occurred below lava channels localized in pre existing embayments or troughs

anthropogenic pb component in hydrothermal ochres from marsili seamount tyrrhenian sea abstract recent hydrothermal fe oxyhydroxide precipitates ochres were sampled during two submersible expeditions to the ne part of the marsili seamount summit se tyrrhenian sea the marsili ochres belong to type 1 primary sulfide poor fe oxyhydroxides according to the classification of hekinian et al hekinian r hoffert m larque p cheminee j l stoffers p bideau d 1993 hydrothermal fe and si oxyhydroxide deposits from south pacific intraplate volcanoes and east pacific rise axial and off axial regions econ geol 88 2099 2121 they are x ray amorphous highly metalliferous al al mn fe 0 4 low in si transition metals co ni cu zn pb and rare earth elements ree and enriched in b v and mo their chemical composition and ree distribution patterns indicate precipitation from low temperature hydrothermal fluid mixed with seawater measured pb isotope compositions in the ochres indicate that the pb is dominantly anthropogenic in origin by contrast the pb isotope ratios in the nearby higher temperature hydrothermal deposits from the panarea arc volcano show that the pb is derived mainly from local volcanic rocks the fact that the pb isotopic signature of the marsili ochres is controlled by recent anthropogenic input suggests that the fe oxyhydroxides precipitated from fluids depleted in transition metals the low 206pb 207pb ratios in the ochres reflect the contribution of pb from australian ores indicating precipitation of the ochres somewhere between present day and the beginning of australian pb import to europe the young age of the ochres and the presence of other hydrothermal deposits along the marsili crest suggest that the magmatic chamber beneath the seamount may still be alive this study represents the first observation of anthropogenic influence on submarine hydrothermal deposits

petrology and isotope geochemistry of san vincenzo rhyolites tuscany italy two groups of rhyolites have been recognized at san vincenzo tuscany italy group a rhyolites are characterized by plagioclase quartz biotite sanidine and cordierite mineral assemblages they show constant mgo and variable cao and na2o contents initial87sr 86sr ratios in group a samples range between 0 71950 and 0 72535 whereas the nd isotopic compositions are relatively constant 0 51215 0 51222 group b rhyolites are characterized by orthopyroxene and clinopyroxene as additional minerals and show textural mineralogical and chemical evidence of interaction with more mafic magmas the sr and nd isotopic ratios range between 0 71283 0 71542 and 0 51224 0 51227 respectively magmatic inclusions of variable size 1 mm to 10 cm were found in groups b rhyolites these inclusions consist mainly of diopsidic clinopyroxene and minor olivine and biotite they are latitic in composition and represent blobs of hybrid intermediate magmas entrained in the rhyolitic melts these magmatic inclusions have relatively high sr contents 996 1529 ppm and sr and nd isotope ratios of 0 70807 0 70830 and 0 51245 0 51252 respectively 87sr 87sr data on minerals separated from both group a and b rhyolites and magmatic inclusions reveal strong isotopic disequilibria due to the presence of both restitic and newly crystallized phases in group a rhyolites and due to interaction of rhyolites with a mantle de rived magma in group b rhyolites isotopic data on whole rocks and minerals allow us to interpret the group a rhyolites as representative of different degrees of melting of an isotopically fairly homogeneous pelitic source conversely group b rhyolites underwent interactions with a mantle derived magma the crustal source as inferred from isotopic systematics would be characterized by87sr 86sr and143nd 144nd ratios close to 0 7194 and 0 51216 respectively the sub crustal magma would have sr isotopic composition close to 0 7077 and a143nd 144nd ratio greater than or equal to 0 51252 these isotopic features are different from those reported for the parental magmas postulated for vulsini and alban hills in the nearby roman magmatic province and are similar to those of the vesuvius and ischia magmas

petrogenesis of calc alkaline shoshonitic and associated ultrapotassic oligocene volcanic rocks from the northwestern alps italy along the western alps there is geological evidence of late alpine oligocene magmatic activity which clearly postdates the lepontine eocene early oligocene metamorphism and related deformation of the alpine nappe pile this magmatic activity was notably delayed in relation to the most important convergent processes and may be related to buoyancy of lithosphere tensional tectonics and thermal updoming subsequent to the collision between the eurasian and african plates the geochemical features of the rocks and the geophysical characteristics of the alpine chain suggest that a shoshonitic and calcalkaline melts may have been generated by partial melting of metasomatized peridotitic material and subsequent fractional crystallization and crustal contamination silicic andesites and latites however could have been also derived from metasomatized eclogite or deep continental crust material b the ultrapotassic lamprophyres with high k p lree th zr u and high 87sr 86sr ratios were generated by partial melting of strongly metasomatized mantle the varied sr isotopic ratios may partially also reflect additional radiogenic component from the continental crust following magma segregation from the source

is there a link between faulting and magmatism in the south central aegean sea a distinct spatial relationship between surface faulting magmatic intrusions and volcanic activity exists in the aegean continental crust in this paper we provide detailed structural observations from key onshore areas as well as compilations of lineament maps and earthquake locations with focal plane solutions from offshore areas to support such a relationship although pluton emplacement was associated with low angle extensional detachments the nne to ne trending strike slip faults also played an important role in localizing the middle miocene plutonism providing ready pathways to deeper magma batches and controlling the late stage emplacement and deformation of granites in the upper crust additionally the linear arrangements of volcanic centres from the quaternary volcanoes along the active south aegean volcanic arc are controlled primarily by ne trending faults and secondarily by nw trending faults these volcanic features are located at several extensional settings which are associated with the main ne trending faults such as i in the extensional steps or relay zones between strike slip and oblique normal fault segments ii at the overlap zones between oblique normal faults associated with an extensional strike slip duplex and iii at the tip zone of a ne trending divergent dextral strike slip zone the ne trend of volcano tectonic features such as volcanic cone alignments concentration of eruptive centres hydrothermal activity and fractures indicates the significant role of tectonics in controlling fluid and magma pathways in the aegean upper crust furthermore microseismicity and focal mechanisms of earthquakes in the area confirm the activity and present kinematics of these ne trending faults

deglacial dust provenance changes in the eastern equatorial pacific and implications for itcz movement abstract the provenance of eolian dust supplied to deep sea sediments has the potential to offer insights into changes in past atmospheric circulation specifically measuring temporal changes in dust provenance can shed light on changes in the mean position of the intertropical convergence zone itcz a region acting as a barrier separating wind blown material derived from northern versus southern hemisphere sources here we have analyzed nd sr and pb isotope ratios in the operationally defined detrital component extracted from deep sea sediments in the eastern equatorial pacific eep along a meridional transect at 110 w from 3 s to 7 n odp leg 138 sites 848 853 sr isotope results show that barite sr has a significant influence on 87sr 86sr isotope ratios of samples in the upwelling zone of the eep however sites located 3 or more away from the equator sites 852 and 853 are believed to not be affected by barite sr and provide useful detrital sr signals 208pb 206pb and 207pb 206pb ratios in all cores fall into the pb isotope space of five potential dust sources asia north and central south america sahara and australia with no distinct isotopic fingerprinting of the dominant source s end values were most valuable for discerning detrital source provenance and their values at all sites ranging from 5 46 to 3 25 were more unradiogenic for sediments deposited during the last glacial than for those deposited during the holocene there are distinct latitudinal trends in the end values with more radiogenic values further south and less radiogenic values further north excluding site 848 this distinction holds true for both holocene and last glacial periods for the most southerly site 848 we invoke for the first time a distinct southern hemisphere australian source as being responsible for the unradiogenic nd isotope ratios both average last glacial and holocene end values show similar sharp gradients along the transect between 5 29 n and 2 77 n suggesting little movement of the glacial itcz in the eep however during the deglacial this gradient is stronger and shifted further north between 5 29 n and 7 21 n suggesting a more northerly possibly stronger deglacial itcz

the nature of the fluids associated with the monte rosa gold district nw alps italy recent o isotope and fluid inclusion studies have provided evidence on the nature of the fluids associated with the late alpine quartz gold deposits of the monte rosa district the most abundant inclusions in quartz from these deposits contain a low salinity aqueous fluid about 2 to 10 wt nacl eq and a co2 phase usually less than 20 mol in places with minor methane co2 densities and total homogenization temperatures vary widely throughout the district reflecting diverse conditions of trapping p 1 to 3 kb t 300 to 450 c at miniera dei cani unmixing between co2 rich and h2o rich fluids possibly occurred a second type of inclusion contains an aqueous brine without recognizable co2 and is especially abundant at val toppa o isotope studies suggest that fluids were largely equilibrated in a metamorphic environment it is concluded that the gold related fluids in the district were mainly of a metamorphic nature at val toppa both isotopic and fluid inclusion data point to contributions of unexchanged meteoric waters mechanisms of gold transport and precipitation are less contrained a possible model involves transport of gold as bisulfide complexes and precipitation due to one or more of the following processes decrease of sulfur activity due to precipitation of sulfides wall rock reaction cooling dilution and or fluid unmixing

petrology and geochemistry of ultrapotassic rocks from the montefiascone volcanic complex central italy magmatic evolution and petrogenesis abstract the montefiascone volcanic complex belongs to the roman magmatic province of central italy the volcanic activity took place in an extensional post collisional setting during late pleistocene giving rise to lava flows and pyroclastic deposits the extrusive products consist of moderately to strongly undersaturated k rich lavas ranging in composition from trachybasalts through leucite basanites and leucititic tephrites to tephritic leucitites they show the typical geochemical and isotopic characters of the roman potassic magmas i e low tio2 low k2o al2o3 strong enrichment in lile high lile hfse ratios highly radiogenic 87 sr 86 sr ratios 0 71005 0 71112 and unradiogenic 143 nd 144 nd 0 51209 0 51229 corresponding to end 10 7 to 6 8 large chemical variations have been recognized within the montefiascone volcanics resulting both from the occurrence of different primary magmas and shallow level fractionation processes the differentiation mainly took place by means of closed system fractional crystallisation with local influence of crustal assimilation the leucite basanites represent primary mantle magmas which did not yield derivative products whereas the leucititic tephrites tephritic leucitites and trachybasalts comprise highly differentiated rocks strongly depleted in compatible elements and enriched in lile fractional crystallisation dominated respectively by clinopyroxene leucite and clinopyroxene plagioclase yielded the most evolved tephritic leucitites and trachybasalts in contrast assimilation of metamorphic basement rocks characterized by highly radiogenic 87 sr 86 sr is needed to explain the moderate increase of the 87 sr 86 sr ratio within the leucititic tephrites the geochemical and isotopic signatures shown by the montefiascone primary magmas require a clinopyroxene and phlogopite rich mantle source in particular partial melting of a veined lithospheric mantle can account for the occurrence of different primary magmas characterized by relatively constant sr and nd isotopic compositions depleted mantle nd model ages 1 1 1 5 ga suggest that the mantle enrichment may be a very old event unrelated to the subduction that preceded the roman magmatism this hypothesis is further supported by the strong similarity in age and isotopic composition between the montefiascone volcanics and the kamafugitic and carbonatitic rocks of the adjoining ultra alkaline umbria latium district

igneous and metamorphic enclaves in the s type deddick granodiorite lachlan fold belt se australia petrographic geochemical and nd sr isotopic evidence for crustal melting and magma mixing the deddick granodiorite a mafic s type pluton in the lachlan introduction fold belt contains abundant enclaves most are derived from enclaves in granites and volcanic rocks can provide high grade metasediments of pelitic psammitic composition and petrogenetic information not readily available from their migmatites are common among these discrete fragments of host rocks didier 1973 didier barbarin 1991 for melanosomes rich in cordierite and garnet are restites from partial example metasedimentary enclaves can place constraints melting however compositional and nd sr isotopic data indicate on the age composition and deformational history of they are not restite in equilibrium with the melt component of the magma source regions within continental crust fleming host magma but may have formed part of a spectrum of diverse 1991 steele et al 1991 enclaves formed by acmagma source lithologies alternatively they may be accidental cumulation of early phenocrysts autoliths document xenoliths regardless of their origin data for these enclaves suggest the early evolution of the host magma whereas others their precursors could represent deeply buried age equivalents of the may represent commingled magmas and illustrate the ubiquitous ordovician turbidites of the lachlan fold belt three possible role of hybridization didier 1987 types of microgranular enclaves with igneous textures are disenclave suites in the relatively mafic s type granitoids tinguished the most common type are small and rounded mafic of se australia are often diverse including abundant enclaves of tonalitic composition they carry xenocrysts derived from enclaves of metasedimentary origin and microgranular the host magma and some contain high mg pyroxene derived from enclaves phillips et al 1981 price 1983 chen et al a mafic magma isotopic data form an array with end from 6 to 1989 wyborn et al 1991 the metasedimentary en 12 and sr sr from 0 7130 to 0 7167 host rock 10 and claves are lithologically diverse including gneissic 0 715 respectively these enclaves formed as globules of hybrid migmatitic types and biotite rich surmicaceous types mafic magma commingled with and contaminated by the more apart from their potential as indicators of crustal structure felsic host magma other enclave types were derived from disrupted and composition they are of interest as possible fragments syn plutonic dykes having a distinct isotopic composition and from of source material and or refractory residue from which a cogenetic marginal facies of the host peraluminous melts were generated and extracted by

slab breakoff a model for syncollisional magmatism and tectonics in the alps slab breakoff is the buoyancy driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision in a recent paper davies and von blanckenburg 1994 have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere melting of its enriched layers and thus to bimodal magmatism breakoff will also lead to thermal weakening of the subducted crustal lithosphere thereby allowing buoyant rise of released crustal slices from mantle depths in this paper we present a test of this model in the tertiary evolution of the european alps in the alps both basaltic and granitoid magmatism occur between 42 and 25 ma following the closure of oceanic basins by subduction and continental collision the granitoids are now well established to result from mixing of basalt with assimilated continental crust to identify the tectonically crucial origin of the partial mantle melts we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole alpine arc their trace element and isotopic composition suggests that they have been formed by low degree melting of the mechanically stable lithospheric mantle we see no evidence for melting of asthenospheric mantle it was thus not decompressed to depths shallower than 50 km once initiated rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust this explains why all alpine magmatic rocks intruded almost synchronously along a strike slip fault the periadriatic lineament a compilation of ages from penninic high pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55 40 ma followed by uplift at 40 35 ma from the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff the slab breakoff model fulfills its predictions in the case of the alps and therefore supports the assumptions made in the theoretical model on a geological basis we believe that the characteristic association of magmatic activity with the return of high pressure rocks to the surface allows the identification of this process in the earth s mountain belts

ultrasonic measurements of vp and qp relaxation spectrum of complex modulus on basalt melts abstract ultrasonic compressional wave velocity v p and quality factor q p have been measured in alkali basalt olivine basalt and basic andesite melts in the frequency range of 3 4 22 mhz and in the temperature range of 1100 1400 c velocity and attenuation of the melts depend on frequency and temperature showing that there are relaxation mechanisms in the melts complex moduli are calculated from the ultrasonic data the results fit well a complex modulus of arrhenius temperature dependence with log normal gaussian distribution in relaxation times of attenuation the analysis yields average relaxation time its activation energy relaxed modulus unrelaxed modulus and width of gaussian distribution in relaxation times relaxed modulus is smaller 17 5 gpa for basic andesite melt of high silica and high alumina contents than for the other two basalt melts 18 1 18 4 gpa the most probable relaxation times decrease from 3 10 10 s for basic andesite to 10 11 s for alkali basalt at 1400 c activation energies of attenuation ranging from 270 to 340 kj mol 1 in the three melts are highest in basic andesite longitudinal viscosity values and their temperature dependences are also calculated from v p and q p data the volume viscosity values are estimated from the data using the shear viscosity values longitudinal volume and shear viscosities and their activation energies are highest in the basic andesite melt of the most polymerized structure

sm nd studies of archaean metasediments and metavolcanics from west greenland and their implications for the earth s early history metasedimentary and metavolcanic rocks from the archaean of west greenland have been examined for evidence of crustal components greater than 3 8 ga in age and for their compatibility with the presently adopted bulk earth sm nd parameters sm nd isotopic data have been obtained for the garbenschiefer metagabbro unit metasediments from the isua supracrustal belt gneisses interior to the isua belt and metasediments from the malene supracrustal belt using estimates of emplacement age t of between 3 77 and 3 67 ga for the parental volcanics to the garbenschiefer unit initial143nd 144nd ratios yield positiveendt values between 1 0 and 3 1 relative to the chur parameters for seven out of eight samples model sm nd ages for the isua gneisses and metasediments are only compatible with their estimated stratigraphic ages if their sources were ca 2end relative to chur at those times similarly model sm nd ages for the malene samples are only compatible with stratigraphic age constraints when based on a source evolution with positiveendt implications of these results for the early development of the earth s mantle are discussed

isotope geochemistry of recent magmatism in the aegean arc sr nd hf and o isotopic ratios in the lavas of milos and santorini geodynamic implications in this comparative study of variations in the isotopic compositions sr nd o and hf of the calc alkaline magmas of the largest two volcanoes milos and santorini of the aegean arc eastern mediterranean we demonstrate the complexity of the processes governing the evolution of the magmas on the scale both of the arc and of each volcano on santorini the crustal contamination processes have been limited effecting the magma gradually during its differentiation the most differentiated lavas rhyodacite and pumice are also the most contaminated on milos by contrast these processes are very extensive they are expressed in the143nd 144nd vs 87sr 86sr diagram as a continuous mixing curve between a mantle and a crustal end member pole defined by schists and metavolcanic rocks outcropping on these volcanoes in contrast with santorini the least differentiated lavas on milos are the most contaminated these isotopic singularities can be correlated with the geodynamic evolution of the aegean subduction zone consisting of alternating tectonic phases of distension and compression the genesis of rhyolitic magmas can be linked to the two phases of distension and the contamination of the calc alkaline mantle derived magmas with the intermediate compressive phase the isotopic characteristics of uncontaminated calc alkaline primitive magmas of milos and santorini are directly comparable to those of magmas generated in subduction zones for which a contribution of subducted sediments to partial melts from the mantle is suggested such as in the aleutian sunda and lesser antilles island arcs however in spite of the importance of the sediment pile in the eastern mediterranen oceanic crust 6 10 km the contribution of the subducted terrigenous materials remains of limited amplitude

i type plutonism in a continental back arc setting miocene granitoids and monzonites from the central aegean sea greece miocene extension in the aegean sea was accompanied by the intrusion of granitoid and monzonitic magmas at upper crustal levels with increasing distance from the trench the metaluminous to slightly peraluminous i type intrusives show a systematic regional variation in their compositions medium to high k granodiorites in the wsw high k low silica granites in the central region and high k high silica granites monzonites monzogranites and composite lamprophyric monzonitic dikes in the ene the granodioritic and low silica granitic plutons comprise similar ranges of sio2 63 5 71 6 wt and initial nd sr and o isotopic values end i 7 0 to 9 8 87sr 86sr i 0 7092 0 7115 18o 9 7 11 5 but display significant differences in their chemical compositions compared with the granodiorites the low silica granites have higher abundances of large ion lithophile light rare earth and some high field strength elements initial isotopic signatures of the high silica granites in the ene end i 9 0 to 9 6 87sr 86sr i 0 7128 0 7144 18o 10 8 12 0 are slightly more crust like than those of the low silica granites in chemical variation diagrams samples from the high silica granites continue the trends defined by those from the low silica granites it is suggested that all granitoid magmas originated by dehydration melting of metaluminous older crustal sources most probably metagreywackes u pb systematics of zircon populations in conjunction with zr sio2 systematics and zircon saturation temperatures suggest a high proportion of restitic or assimilated crustal material in these magmas monzonitic to monzogranitic rocks show chemical and isotopic characteristics end i 0 8 to 3 6 87sr 86sr i 0 7059 0 7071 18o 9 4 10 6 that are significantly different from those of the granitoids a small portion of an older crustal component in these magmas is indicated by u pb systematics of zircon populations lamprophyric dikes display chemical and isotopic signatures end i 0 5 87sr 86sr i 0 7044 18o 8 9 that are compatible with an origin from either a mantle source previously enriched by slab derived hydrous fluids melts as suggested by negative nb ta anomalies or a new k rich basaltic underplate abundant composite dikes indicate the coexistence of and mingling mixing between lamprophyric melts and monzonitic magmas

origin of basal dolomitic claystone in the marsili basin tyrrhenian sea abstract red brown dolomitic claystones overlay the marsili basin basaltic basement at odp site 650 sequential leaching experiments reveal that most of the elements considered to have a hydrothermal or hydrogenous origin in a marine environment such as fe cu zn pb co ni are present mainly in the aluminosilicate fraction of the dolomitic claystones their vertical distribution content and partitioning chemistry of trace elements and ree patterns suggest enhanced terrigenous input during dolomite formation but no significant hydrothermal influence from the underlying basaltic basement positive correlations in the c and o isotopes in the dolomites reflect complex conditions during the dolomitization the stable isotopes can be controlled in part by temperature variations during the dolomitization majority of the samples however form a trend that is steeper than expected for only temperature control on the c and o isotopes the latter indicates possible isotopic heterogeneity in the proto carbonate that can be related to arid climatic conditions during the formation of the basal dolomitic claystones in addition the dolostones stable isotopic characteristics can be influenced by diagenetic release of heavier 18o from clay dehydration and or alteration of siliciclastic material strontium and pb isotopic data reveal that the non carbonate fraction the dye of the dolomitic claystones is controlled by saharan dust 75 80 and by material with isotopic characteristics similar to the aeolian arc volcanoes 20 25 the non carbonate fraction of the calcareous ooze overlying the dolomitic claystones has a sr and pb isotopic composition identical to that of the dolomitic claystones indicating that no change in the input sources to the sedimentary basin occurred during and after the dolomitization event combination of climato tectonic factors most probably resulted in suitable conditions for dolomitization in the marsili and the nearby vavilov basins the basal dolomitic claystone sequence was formed at the initiation of the opening of the marsili basin 2 ma which coincided with the consecutive glacial stage the glaciation caused arid climate and enhanced evaporation that possibly contributed to the stable isotope variations in the proto carbonate the conductive cooling of the young lithosphere produced high heat flow in the region causing low temperature passive convection of pore waters in the basal calcareous sediment we suggest that this pumping process was the major dolomitization mechanism since it is capable of driving large volumes of seawater the source of mg2 through the sediment the red brown hue of the dolomitic claystones is terrigenous contribution of the glacially induced high eolian influx and was not hydrothermally derived from the underlying basaltic basement the detailed geochemical investigation of the basal dolomitic sequence indicates that the dolomitization was most probably related to complex tectono climatic conditions set by the initial opening stages of the marsili basin and glaciation

sm nd pb pb and rb sr systematics of the basement in the cigar lake area saskatchewan canada the sm nd pb pb and rb sr isotope geochemistry of graphitic metapelitic gneisses and their altered equivalents from the cigar lake area saskatchewan canada has been investigated some granitic gneisses were also analyzed for pb pb and rb sr sm nd data show that the metapelitic gneisses are composed of detritus from heterogeneous mainly mantle derived archean rocks 2 5 2 6 ga and that the sm nd system has not been significantly perturbed during subsequent alteration and metamorphic events the pb pb age for samples of the less altered graphitic metapelitic gneisses is 1 77 0 03 ga the crustal common pb is located on the pb pb isochron but there are different zones with high and variable u pb ratios 15 280 the pb pb age for the granitic gneisses is 1 79 0 11 ma the pb isotope data show that there has been no major uranium redistribution in the basement after the hudsonian orogeny however there has been a strong perturbation of the u pb system in the regolithic zone beneath the athab

shear modulus and q of forsterite and dunite near partial melting from forced oscillation experiments an apparatus designed to determine the complex shear modulus of rock samples by forced torsion oscillations at high temperature and in the seismic frequency band 0 003 30 hz is briefly described measurements were performed on natural dunite from aheim norway up to 1400 c and on polycrystalline forsterite up to 1500 c at 1 atm pressure the two materials were chosen to study by comparison the effect of melt on the elasticity and anelasticity of mantle rocks between 1000 and 1200 c the absolute values of the shear modulus g are almost equal for both materials above 1200 c g for natural dunite decreases progressively with temperature and at 1400 c and 1 hz reaches 13 of its value at 1100 c in contrast g of pure forsterite depends little on temperature for petrological reasons supported by simultaneous measurements of the electric resistivity there is strong evidence that the decrease of g in dunite above 1200 c is due to melt from the lower melting components of the dunite based on different models estimates of the melt fraction are made at high temperature in both materials q 1 is characterized by a monotonic decrease with frequency according to with 0 25 an apparent activation energy of 38 5 kcal mol 1 for forsterite and 48 8 kcal mol 1 for dunite was found with no significant change in the regime of partial melting from this it is concluded that q 1 even at partial melting is dominated by solid state high temperature background absorption there is no indication from these experiments for a constant q band at low seismic frequencies or an increase of q proportional to frequency as suggested by some seismologists the present results are in good qualitative agreement with those for young s modulus obtained previously by strain retardation experiments

geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy abstract serri g innocenti f and manetti p 1993 geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 117 147 the neogene quaternary magmatism of the northern apenninic arc took place in four phases separated in space and time which become progressively younger from west to east phase i 14 ma phase ii 7 3 6 0 ma phase iii 5 1 2 2 ma phase iv 1 3 0 1 ma this magmatism is the result of the activation of three physically separate sources 1 the adriatic continental crust extracted from the mantle in the late proterozoic 2 a strongly refractory recently k enriched harzburgitic mantle located in the mechanical boundary layer mbl of the lithosphere and 3 a recently metasomatized cpx rich mantle compositionally variable from iherzolite to wehrlite clinopyroxenite interpreted as an ephemerally k enriched asthenosphere the adriatic continental crust is the dominant source of the acid plutonic and volcanic rocks of the tuscan region the acid magmatism is mostly found inside an ellipsoidal area about 150 300 km centred on giglio island here defined as the tuscan crustal dome within this area mantle derived magmas unaffected by important crustal contamination processes and mixing with crustal anatectic melts have so far not been found pure crustal magmas are rare but are represented for example by some of the san vincenzo and roccastrada rhyolites virtually all the tuscan acid centres show evidence of mixing with potassic mantle derived magmas major and trace elements as well as 87 sr 86 sr and 143 nd 144 nd data on primitive rocks mg 65 reveal two groups of mantle derived magmas these define two distinct mantle enrichment trends both essentially due to the additions of k rich components which metasomatized separate compositionally diverse upper mantle sectors in both cases the most remarkable mineralogical effect of these enrichment processes is the production of variable amount of phlogopite through reaction between fluids and or melts with the mantle the rocks of group i ol hy and q normative lamproites ultrapotassic high mg latites ultrapotassic shoshonites and shoshonites saturated trend are considered to be derived by partial melting at low pressure 87 sr 86 sr 0 717 ce sr 0 3 and k 2 o na 2 o 6 7 and low ratios of 143 nd 144 nd 0 5121 0 5120 and ba la 87 sr 86 sr 0 712 and k 2 o na 2 o 8 9 values and low 143 nd 144 nd ba la 0 10 ratios these constraints do not allow to exclude a complete identity between the k rich components which metasomatized the mantle sources of the saturated and undersaturated trend magmas the geochemical and isotopic features of the components that metasomatized the mantle sources of the northern apenninic arc magmatism can be explained by a geodynamic process which causes a large amount of crustal materials to be incorporated within the upper mantle we propose that the delamination and subduction of the adriatic continental lithosphere related to the still ongoing northern apennine continental collision provide a viable mechanism to explain the genesis and eastward discontinuous migration of the magmatism in central italy the subduction of delaminated lithospheric mantle with lower crustal slivers would have exposed uppermost mantle adriatic mbl and crustal units previously imbricated in the apennine chain to the heating advected by the upwelling of a recently and ephemerally k enriched asthenospheric mantle wedge and by the underplating of magmas derived from it we consider that the diapiric uprising of a hot crustally contaminated asthenosphere occurs in the wake left above the sinking of the adriatic delaminated subducting continental lithosphere the delamination subduction process of the adriatic lithosphere has probably started in the early middle miocene but earlier than 15 14 ma ago as indicated by the age and petrologic characteristics of the first magmatic episode sisco lamproite of the northern apennine orogenesis

ultrasonic velocity and attenuation measurements on basalt melts to 1500 c role of composition and structure in the viscoelastic properties viscoelastic properties of basalt melts of a wide range of composition are determined from ultrasonic compressional wave velocity vp and quality factor qp measurements in the frequency range of 3 4 22 mhz and the temperature range of 1150 1500 c the basalt samples are grouped into two types low magnesia group a containing 5 9 wt mgo and high magnesia group b containing 18 23 wt mgo the vp values of the melts determined at 1400 c are in the range 2 53 2 64 km s for group a basalts and 2 79 2 86 km s for group b basalts the relaxed low frequency velocity vr is a principally linear function of the melt density least squares fits of the data give velocity density relations vr km s 0 691 0 712 g cm3 for group a basalts and vr 0 328 0 907 for group b basalts at 1400 c the mean molecular weights m are 65 0 0 3 and 58 2 0 5 g mol for groups a and b basalts respectively suggesting that m is also a principal factor determining the melt velocity the relaxed modulus mr of the basalt melts range from 16 5 to 22 8 gpa and is also a linear function of the melt density the values of longitudinal l and volume v viscosities are calculated from the measured vp and qp and the available shear viscosity s data the viscosities and related activation energies decrease almost linearly as nbo t nonbridging oxygens per tetrahedrally coordinated cation increases

the pb sr nd isotope geochemistry of some recent circum mediterranean granites pb sr and nd isotopic compositions have been analyzed in recent granites from northern africa northern italy and greece lead isotope compositions of k feldspars are rather homogeneous and cluster close to the modern lead of stacey and kramers 1975 but with slightly higher207pb 204pb and208pb 204pb ratios the cyclades samples however have higher206pb 204pb ratios addition of mantle derived lead was probably very limited which supports a quasi closed system evolution of this element in the continental crust the sr nd data fall in the enriched part of the143nd 144nd vs 87sr 86sr diagram and define a smooth hyperbolic mixing curve over a wide area straddling different orogens most granites may be accounted for by a binary mixture between a recycled crustal component and a depleted mantle like component no correlation is observed between either pb and sr or nd isotopic ratios or any isotopic ratio and major element contents quantitative modelling suggests that two cases fit the sr and nd characteristics of these granites they both require anatexis of the crust on a scale large enough to average the isotopic properties of heterogeneous terranes in the first case the mantle derived component may be represented by differentiated island arc type magmas and the granites result from mixing these magmas with anatectic melts in the second case mantle derived igneous rocks such as obducted ophiolites are part of the crustal source and their variable involvement in the anatectic process causes isotopic variations

lead isotopic composition of hercynian granitic k feldspars constrains continental genesis lead isotope compositions have been determined in many k feldspars from granitic rocks emplaced during the variscan hercynian orogeny sensu lato around 300 myr in various parts of europe and the eastern us the ratios show a regular geographical pattern with contours arched around southern france and parallel to the atlantic coast of the us this distribution parallels predriftpalaeogeographical zones during the palaeozoic the lead isotope pattern of variscan granitic rocks is similar to that of some orogenic volcanic suites but is better explained by crustal melting than by an hercynian mantle crust material mixing the continental crust from these areas must be older than 3 000 myr large scale fluid circulation in the crust during orogenic processes apparently explains the observed geographical patterns of lead isotopes

nd sr o isotopic and chemical evidence for a two stage contamination history of mantle magma in the central alpine bergell intrusion two different contamination processes have been identified as having been operative in the genesis of a plutonic suite initial contamination of a mantle source and subsequent crustal contamination of uprising partial melts from the mantle these processes are indicated by a detailed analyses of nd sr and oxygen isotopes together with major and trace elements of the 32 30 ma calc alkaline bergell intrusion this intrusion is located at the suture of the alpine continental collision zone and contains rock types capable of discriminating between mantle and intracrustal processes a range from basaltic andesitic dykes in the surrounding country rocks cumulitic hornblendites gabbros tonalite granodiorite and lamprophyres to pegmatites and aplites is exposed in this single intrusion the results of ree modelling and isotopic compositions of the basic members suggest that the cumulates were fractionated from a picrobasaltic liquid originating by partial melting of enriched subcontinental mantle end 4 increases in 87sr 86sr 0 7055 and 18o 6 7 in these samples relative to the mantle array and compositions of other periadriatic intrusions are most likely the result of an initial contamination of the mantle source by dehydration or partial melting of altered subducted oceanic crust slight differentiation of such a picrobasaltic liquid produced the basaltic andesitic dykes simultaneous fractional crystallization and contamination of the uprising magma by continental crust produced crustal isotopic signatures which increase with acidity to values of end 7 6 87sr 86sr 0 716 and 18o 10 the crustal imprint and lree enrichment in the dominating tonalite increase with decreasing crystallization depth which indicates that the tonalites were emplaced in several distinct batches with different degrees of contamination shoshonitic lamprophyres which intruded into the partly solidified granodiorite were generated in a deep strongly contaminated mantle source the posttectonic 26 ma novate leucogranite is not cogenetic with the main bergell body but rather formed from a predominantly crustal source if the described features are indeed due to mantle source contamination processes which are well known for volcanic arcs it must be concluded that these may also play a significant role in the genesis of calcalkaline plutonic suites

lead isotopic compositions of neogene volcanic rocks from the aegean extensional area abstract lead isotope composition has been determined for a diverse suite of cenozoic igneous rocks in the central and north aegean sea these include voluminous miocene shoshonitic and calc alkali volcanic rocks and correlative plutonic phases middle miocene adakitic high mg andesite and associated rhyolite and middle miocene to quaternary ne normative basalt trachyte and k rich andesite the trace element contents of most of the studied rocks suggests that pb isotopic composition is not a result of upper crustal contamination but rather is a feature of the mantle source for k rich rocks this mantle source is represented by primitive mafic to intermediate volcanic rocks with 87 sr 86 sr 0 7095 high 207 pb 204 pb and 208 pb 204 pb 1000 ppm sr and ba and 35 ppm pb the high 207 pb 204 pb requires a metasomatic origin that lowered u pb and increased th pb followed by a second stage residence time of 1 ga this k rich magma was generated as a result of late tertiary extension of the aegean in the south aegean arc there is more evidence for crustal contamination either from present crust or from subduction of nile derived sediment

zircon crystallization and recycling in the magma chamber of the rhyolitic kos plateau tuff aegean arc in contrast to most large volume silicic magmas in continental arcs which are thought to evolve as open systems with significant assimilation of preexisting crust the kos plateau tuff magma formed dominantly by crystal fractionation of mafic parents deposits from this 60 km3 pyroclastic eruption the largest known in the aegean arc lack xenocrystic zircons secondary ion mass spectrometry sims u pb ages on zircon cores never older than 500 ka and display sr nd whole rock isotopic ratios within the range of european mantle in an area with exposed paleozoic and tertiary continental crust this evidence implies a nearly closed system chemical differentiation consequently the age range provided by zircon sims u th pb dating is a reliable indicator of the duration of assembly and longevity of the silicic magma body above its solidus the age distribution from 160 ka age of eruption by sanidine 40ar 39ar dating smith et al 1996 1 to ca 500 ka combined with textural characteristics high crystal content corrosion of most anhydrous phenocrysts but stability of hydrous phases suggest 1 a protracted residence in the crust as a crystal mush and 2 rejuvenation reduced crystallization and even partial resorption of minerals prior to eruption probably induced by new influx of heat and volatiles this extended evolution chemically isolated from the surrounding crust is a likely consequence of the regional geodynamics because the thinned aegean microplate acts as a refractory container for magmas in the dying aegean subduction zone continent continent subduction 1 ref 32

pb isotopic zoning of k feldspar megacrysts determined by laser ablation multi collector icp ms insights into granite petrogenesis this study investigates pb isotopic zoning in magmatic k feldspar megacrysts from the monte capanne pluton elba italy using laser ablation multi collector icpms the studied crystals provide an ideal opportunity to use in situ techniques to assess the extent of open system processes and better characterize the components involved in the genesis of complex magma systems earlier investigations of the pluton identified the importance of magma mixing between mantle and crustal derived magmas the investigated k feldspar megacrysts exhibit strong zoning in 207pb 206pb and 208pb 206pb correlated with lead elemental variations we interpret these variations as reflecting growth zoning as opposed to secondary diffusive exchange despite a great variety of zoning patterns we were able to correlate different events of megacryst growth reflecting crystallization in a dynamic magma system our two step model includes 1 growth of a granitic magma chamber by addition of low 208pb 206pb magma to a high 208pb 206pb magma contaminated with crustal material i e the megacryst cores and 2 recharge by mantle derived magma i e the megacryst rims we interpret the thorogenic nature of the megacryst rims to reflect the mantle derived component involved in the mixing process taking account of other data from the tuscan magmatic province the mantle source is inferred to have been metasomatized by continental material during subduction tims sr isotopic data from microdrilled cores in one megacryst provides general support for the model but show that the two isotopic systems are decoupled

convergence of the indian and eurasian plates under eastern tibet revealed by seismic tomography 1 a three dimensional p wave velocity model of the crust and upper mantle down to 400 km depth beneath eastern tibet is obtained using many temporary seismic stations of the ascent project and the namche barwa broadband seismic network we collected 16 508 arrival times of p pn and pg phases from 573 local and regional earthquakes and 7 450 p wave arrivals from seismograms of 435 teleseismic events our high quality data set enables us to reconstruct the 3 d velocity structure under eastern tibet in more detail than the previous studies in the shallow depth our results show that the low velocity zones are not interconnected well no wide spread low velocity zones which may reflect the complex pattern of material flowing in the study region below the moho we find that the indian lithospheric mantle underthrusts sub horizontally under eastern tibet and the extent of the northward advancing indian lithosphere decreases from west to east in the north the asian lithospheric mantle is detected under the vicinity of the qaidam basin between the indian and asian lithospheric mantles there is an obvious low velocity anomaly which may reflect an upwelling mantle diapir

regional similarities in lead isotopes and trace elements in galena of the cyclades mineral district greece with implications for the underlying basement abstract polymetallic ore deposits of the aegean sea greece are temporally and spatially associated with the emplacement of miocene granitoids and pliocene to quaternary volcanic units along the active volcanic arc and occur adjacent to the crustal scale detachment systems a variety of mineral deposits e g carbonate replacement skarn porphyry vein type and low to high sulfidation epithermal deposits formed during different stages of back arc evolution and can be found in the footwall and hanging wall of the major detachment systems and all tectono stratigraphic units new galena pb isotope data collected by laser ablation inductively coupled plasma mass spectrometry la icp ms analysis exhibit a range of isotopic ratios 206pb 204pb 18 68 18 91 207pb 204pb 15 67 15 75 and 208pb 204pb 38 83 39 18 with a resolvable geographic pattern together with compiled pb isotope data of cycladic galena large scale regional similarities in the pb isotope signatures are recognized suggesting distinct sources of lead in the underlying basement base metal rich deposits in the west cyclades have a common lead source with a more radiogenic signature whereas lead in the deposits in the north central cyclades was derived from a less radiogenic source similarities between the pb isotopic signature of galena and regional host rocks suggest that the pre alpine cycladic basement was the main source of lead for the deposits in the north and central and the metamorphosed volcano sedimentary lower cycladic blueschist nappe was the source of lead for the deposits in the west moreover there is a close spatial relationship of the 206pb 204pb 18 84 line and trace of the proposed trans cycladic thrust regional trends were also observed in the new la icp ms trace element analysis in galena galena in carbonate replacement and skarn deposits in the west is enriched in base metals and silver whereas galena in vein type deposits in the north and low to high sulfidation epithermal deposits along the active volcanic arc is enriched in gold tellurides and selenium combined trace element and isotopic data of galena indicate distinct metal sources in the cyclades heterogenous rock assemblages of mixed provenance and age account for the differences in lead isotopes whereas the base metals and silver are ultimately sourced from the basement the precious metals might be derived from the metasomatized lithospheric mantle

seismic tomography of northern tibet and kunlun evidence for crustal blocks and mantle velocity contrasts although the crust and mantle of the tibet plateau reveal vital information for understanding the interplay of dynamic processes that has governed its recent uplift and growth their deep physical and thermal structure remains poorly understood in order to throw light on the structure and hence to constrain the processes and models tied to them we performed a teleseismic experiment on a 600 km long profile across the northern part of the plateau the kunlun range and the qaidam basin the 400 km deep tomographic image we obtain has a resolution 50 km over one order of magnitude better than achieved in previous broader scale studies at relatively shallow depth 100 km the tomographic cross section clearly reveals that the mid lower crust and upper lithosphere of northern tibet is an assemblage of blocks with different velocities and thicknesses hence different natures histories and ages the crust of the qiantang block appears to be thickest 70 km with the lowest velocity along the north edge of that block the jinsha suture although not remarkable in the local geology and topography stands as a particularly sharp and prominent crustal boundary the crust and lithosphere of the region between that suture and the qaidam bayan har songpan south kunlun appear to be thinnest 50 km and with the highest velocity at greater depth 150 km our experiment confirms the existence of a bulky low velocity zone in the mantle beneath the northernmost reaches of the plateau the tomogram helps assess with unprecedented accuracy the location and shape of this low velocity anomaly which forms a 250 300 km wide dome rising to about 150 200 km south of the kunlun range roughly coincident with the region where the strongest shear wave splitting hence horizontal anisotropy has recently been found that dome lies between two relatively high velocity zones with opposite dips one towards the north under the qaidam basin the other towards the south beneath the quangtang platform the implications that this new detailed image of the deep structure of northern tibet has upon the uplift and thermo mechanical evolution of the plateau are discussed

shear modulus and internal friction of calcite rocks at seismic frequencies pressure frequency and grain size dependence abstract the shear modulus g and associated internal friction q 1 have been measured at seismic frequencies for a suite of four essentially monomineralic calcite rocks with grain sizes ranging from 4 to 1000 m the measurements are based on a previously described technique for the observation of forced torsional oscillations of a series combination of a rock cylinder and a steel standard under high confining pressure at room temperature the behaviour of each rock has been studied as a function of strain amplitude 10 8 10 6 maximum at surface pressure 0 300 mpa and oscillation period 1 300 s no evidence is found of any departure from linearity in the range of strain amplitudes studied corrections which are significant only at the shortest periods 1 3 s have been applied to the raw data to account for the influence of torques exerted on the specimen assembly by the gas pressure medium up to about 100 mpa increase in pressure results in a marked increase in g and decrease in q 1 attributable to the closure of crack porosity except in the fine grained solnhofen limestone in which both g and q 1 are insensitive to change in pressure indicative of the virtual absence of crack porosity at higher pressures g is much less pressure sensitive and q 1 is essentially pressure independent with values less than 0 001 in all four rocks since the rock specimens were jacketted under conditions of normal humidity these relatively low values of q 1 indicate a diminished role for adsorbed water in the anelasticity of rocks at high effective pressures within the pressure independent regime q 1 is essentially independent of frequency for periods of 3 100 s and increases by only 30 as the grain size decreases from 1000 to 4 m thus within this regime it is concluded that the observed value of q 1 results to a large degree from an intracrystalline mechanism with a broad range of relaxation times the possibility of significant dislocation damping will be discussed

the use of laboratory velocity data for estimating temperature and partial melt fraction in the low velocity zone comparison with heat flow and electrical conductivity studies peridotite is widely considered to be the dominant component of the upper most mantle seismic velocities of a dry peridotite determined in the laboratory at high pressure and hypersolidus temperature show a rapid decrease with increasing temperature the strong temperature dependence of velocities can be used to estimate temperature and melt fraction in the low velocity zone of the earth the laboratory results show that the pressure dependence of both velocity and melt fraction appears to be well accounted for by the pressure dependence of the solidus temperature of the peridotite i e homologous temperature dependence this observation allows us to extrapolate laboratory results to higher pressures greater depths with some confidence requiring only a knowledge of the solidus as a function of pressure using the ratios of lithospheric to asthenospheric velocities temperature and melt fraction in the asthenosphere the low velocity zone can be determined from laboratory velocity data examples of thermal structures of the upper mantle beneath oceanic plates are presented we choose three locations of geophysical interest iceland plateau pacific ocean and philippine sea where reliable seismic velocities have been determined in the upper mantle from surface wave studies a large amount of partial melt 5 vol and a higher temperature than the dry peridotite solidus are inferred in 0 5 m y asthenosphere under the iceland plateau and the active marginal basin of the east philippine sea the partial melt zone appears to extend deeper 100 km and to greater ages 20 m y in the pacific ocean region than under the slowly spreading iceland plateau partial melting is not expected in the asthenosphere older than 5 m y under the iceland plateau even though the volume fraction of partial melt is not so large 3 vol beneath the pacific plate the melting zone may extend to a distance of above 2000 km from the ridge many seamounts observed in the pacific ocean may result from this vast region of melting under the plate we suggest a linear relation between the width of melting zone and the plate velocity the laboratory results are also applied to other low velocity zones it is suggested that a velocity drop up to about 6 in the asthenosphere does not necessarily require the existence of partial melt but can be explained by high subsolidus temperature this implies that partial melt may not exist generally but only in limited areas in the low velocity zone comparison of seismic results with theoretical models may overestimate the temperature and melt fraction if the velocity drop at subsolidus temperature is neglected temperature and melt fraction obtained in this study are discussed together with results from heat flow and electrical studies almost the same temperatures as inferred from seismic velocity and a dry peridotite solidus are calculated from heat flow data indicating that the upper mantle under mid ocean ridges may be fairly dry even a water or co2 undersaturated solidus 0 1 wt does not give a temperature consistent with the lithospheric geotherm and laboratory velocity results it is noted that electrical conductivity studies generally yield lower temperatures and larger melt fractions than this study unknown pressure frequency and impurity effects on laboratory conductivities of partially molten mantle rocks are suggested as possible reasons for this discrepancy

the langmuir volcanic peridotite associated nickel deposits canadian equivalents of the western australian occurrences the 2 7 b y old abitibi greenstone belt is a crudely s shaped zone of metamorphosed volcanic subvolcanic and sedimentary rocks and associated intrusions through it are scattered a dozen small nickel deposits in volcanic peridotites their hosts are volcanic and subvolcanic ultramafic rocks typically containing 40 to 45 percent anhydrous mgo with spinifex tops and overlain by thinner peridotitic to pyroxenitic flows at least one deposit dumont is hosted by a dunitic intrusion others e g dundonald langmuir 2 are partially hosted in sulfidic iron formations mining of the langmuir 2 deposit permitted extensive underground mapping core logging and sample collecting the other deposits are known mainly from poor surface exposures and drill core the langmuir 1 and 2 deposits provided sufficient data to evaluate models for their origin and to compare them to nickel sulfide deposits associated with volcanic peridotites in western australia a magmatic origin is supported by the localization of the main ore zone in a paleotrough by the presence of a possible volcanic feeder beneath the ore and by the regular sequence through the basal ultramafic flow of massive ore overlain by net textured ore then by barren peridotite and lastly by a spinilex textured cap to the flow an unusual spinilex textured and sulfide bearing rock and skeletal chromite also support a magmatic origin as do the base and precious metal contents of the ores the high s se ratios averge 23 000 and slightly lower sulfur isotopic ratios average 0 5 per mil delta 34 s compared to most magmatic nickel sulfide deposits are similar to the ratios for windarra western australia and may be explained by assimilation of sulfide rich cherty iron formation other features incompatible with a purely maginatic origin include the presence of sheared and breccia ore and the high sulfur content of some ores these features result from deformation and greenschist facies metamorphism but surviving features of an earlier magmatic history argue against the ores being formed metamorphically evidence of contemporaneous fumarolic activity was found at the south end of the mine but its importance as an ore forming process was not supported by detailed petrological and geochemical studies limited data from the other deposits in the abitibi belt suggest a common origin for the langmuir deposits with minor variations attributable to the volume of ultramafic magma emplaced the volume of contained nickel sulfides the presence of sulfidic iron formations and the severity of alteration and metamorphism the deposits in western australia probably were formed by similar processes in particular the langmuir deposits display strikingly similar sulfide chemistries to the windarra deposits which are probably dae to assimilation of sulfide rich cherty iron formation

geochemistry of the serifos calc alkaline granodiorite pluton greece constraining the crust and mantle contributions to i type granitoids the late miocene 11 6 9 5 ma granitoid intrusion on the island of serifos western cyclades aegean sea is composed of syn to post tectonic granodiorite with quartz monzodiorite enclaves cut by dacitic and aplitic dikes the granitoid a typical i type metaluminous calcic amphibole bearing calc alkaline pluton intruded the cycladic blueschists during thinning of the aegean plate combining field textural geochemical and new sr nd o isotope data presented in this paper we postulate that the serifos intrusion is a single zoned pluton the central facies has initial 87 sr 86 sr 0 70906 to 0 7106 e nd t 5 9 to 7 5 and 18 qtz 10 to 10 6 whereas the marginal zone or border facies has higher initial 87 sr 86 sr 0 711 to 0 7112 lower e nd t 7 3 to 8 3 and higher 18 qtz 10 6 to 11 9 the small range in initial sr and nd isotopic values throughout the pluton is paired with a remarkable uniformity in trace element patterns despite a large range in silica contents 58 8 to 72 wt sio 2 assimilation of a crustally derived partial melt into the mafic parental magma would progressively add incompatible trace elements and sio2 to the evolving mafic starting liquid but the opposite trend of trace element depletion during magma evolution is observed in the serifos granodiorites thermodynamic modeling of whole rock compositions during simple fractional crystallization fc or assimilation fractional crystallization afc processes of major rock forming minerals at a variety of pressure oxidation state and water activity conditions fails to reproduce simultaneously the major element and trace element variations among the serifos granitoids implying a critical role for minor phases in controlling trace element fractionation both saturation of accessory phases such as allanite and titanite at sio 2 71 wt to satisfy trace element constraints and assimilation of partial melts from a metasedimentary component to match isotopic data must have accompanied fractional crystallization of the major phases

the age of the kambalda greenstones resolved by ion microprobe implications for archaean dating methods abstract a chert horizon within the archaean basalt pile at kambalda western australia contains zircons from air fall tuffs erupted as the sediment was laid down ion microprobe analyses of these magmatic zircons constrain the long sought age of the kambalda lavas to 2692 4ma 2 and show that regional metamorphism and deformation of the greenstone pile occurred within 30 ma of deposition rare zircon xenocrysts are evidence of pre greenstone crust as old as 3450 ma exposed remnants of which are yet to be identified subsets of both the magmatic and xenocryst zircons show evidence of hitherto unsuspected low temperature u and or pb mobility in the proterozoic long after stabilisation of the craton the ion microprobe data unravel complexities of isotopic inheritance and alteration illustrating the hazards of using bulk or whole rock samples in archaean isotopic investigations identification of interflow cherts as a precisely dateable rock type opens the way to direct ion probe dating of mafic ultramafic piles and close numerical age control of the complex but ill exposed stratigraphic and structural relations in this richly mineralised greenstone belt

melting of garnet peridotite to 13 gpa and the early history of the upper mantle by analogy with the evolution of the moon it has been suggested that the earth may have had a magma ocean or liquid outer layer before 3 800 myr ago 1 its presence would have had profound implications for the primary stratification of the mantle into upper mantle transition zone and lower mantle2 3 an essential constraint on this hypothesis is a knowledge of the high pressure melting characteristics of mantle material generally assumed to be peridotite in the upper mantle we recently described4 the first melting experiments on mantle peridotite to 14 gpa using spinel iherzolite klb 1 from kilbourne hole new mexico we showed that partial melts close to the solidus at 5 7 gpa are komatiitic in composition and that the solidus and liquidus converge at high pressures here we report further melting experiments on garnet iherzolite phn1611 from thaba putsoa lesotho we confirm the convergence of the solidus and liquidus and we show a negative dt dp slope for the liquidus above 7 gpa these observations are briefly discussed in terms of their importance to the petrological evolution of the upper mantle and the presence of a magma ocean in the early archaean

tectonics of the punta bianca promontory insights for the evolution of the northern apennines northern tyrrhenian sea basin the structural setting of the eastern headland of the la spezia gulf punta bianca promontory provides some constraints on the contractional and extensional framework of the northern apennines northern tyrrhenian sea basin geodynamic system and allows a detailed reconstruction of a complex evolutionary path out of sequence thrusting is supposed in the inner side of the chain two different extensional events are identified the older early middle miocene may be related to the imbrication of the deeper metamorphic units of the apenninic thrust wedge younger extension fits very well into an eastward asymmetrical faulting framework related to the opening of the northern tyrrhenian sea basin from the late miocene to the pliocene pleistocene well exposed low angle normal faults developed during this event extensional structures were then overprinted and reactivated by a mainly strike slip regime in the quaternary

magmatic and hydrothermal ammonium in granites of the tuscan magmatic province italy abstract the granites of the tuscan magmatic province have total ammonium contents that vary from 0 to 636 ppm in the three largest intrusions of elba giglio and montecristo the granites have not been affected by hydrothermal alteration and have low ammonium contents ranging up to 50 ppm there are detectable differences between the separate intrusions and also within the biggest intrusion granitic samples from the smaller outcrops and boreholes on the tuscan mainland are nearly all hydrothermally altered and these rocks show much higher ammonium contents ranging up to 636 ppm in the hydrothermally altered rocks the ammonium contents vary considerably over short distances and where they have been most closely sampled in the s2 borehole of castel di pietra there is a clear correlation between the ammonium content and the intensity of sulphide mineralization the major conclusion of the study is that the amount of ammonium present in the granites is determined much more by post magmatic hydrothermal alteration than by magmatic variation the mean ammonium contents of the fresh and hydrothermally altered granites are 17 and 95 ppm respectively

the effect of accessory minerals on the redistribution of lead isotopes during crustal anatexis a model abstract the initial pb isotopic composition of anatectic granites is modelled as a mixture of pb derived from major minerals that have characteristically low 238 u 204 pb and 232 th 204 pb ratios i e feldspars biopyroboles etc with pb contributed from dissolved accessory minerals with characteristically high 238 u 204 pb and 232 th 204 pb ratios i e zircon monazite etc the initial pb isotopic composition of anatectic granites is a function of a the age type modal distribution and heterogeneity in the initial u and th content of the accessory minerals b variation in melt composition and temperature during anatexis c the fraction of the source melted and d the extent to which the melt was homogenized prior to crystallization the differential behavior of accessory minerals during anatexis can explain the common yet enigmatic decoupling of the pb isotopes from other chemical and isotopic systems in granite rocks granitic plutons derived by anatexis of the same source material are not required to have the same initial pb isotopic composition low temperature peraluminous granites are more likely to exhibit homogeneous initial pb isotopic compositions dominated by the pb contained within the major mineral assemblage of the source material in contrast the initial pb isotopic composition of higher temperature metaluminous or peralkaline anatectic granites is more likely to be heterogeneous unless the melts are homogenized during emplacement and will record a larger fraction of the radiogenic pb component contained in the accessory mineral assemblage of the source material heterogeneity in the initial pb isotopic composition of individual granites or granitic suites can preserve the weighted average age of the radiogenic pb present in the accessory mineral assemblage of the source region even if these minerals are entirely dissolved during anatexis the use of isotopic tracer studies in terrane analysis will be optimized by the isotopic characterization of granitic suites

tertiary granitoids of rhodope northern greece magmatism related to extensional collapse of the hellenic orogen abstract many granites that occur in orogenic belts such as the himalaya the hercynides or caledonides are thought to result from anatexis of the lower parts of over thickened thrust stacked crustal units just preceding termination of the orogenic cycle numerous tertiary granitoids occur in the rhodope zone of northern greece and adjacent regions of bulgaria a belt regarded as an eastern extension of the alpine system these granites could also be regarded as syn or late tectonic crustal melts in that some have been emplaced as sheet like bodies along major alpine thrust planes and show a strong marginal foliation parallel to the pervasive alpine compressional fabrics in detail however the geochemistry of the granitoids their timing and relationships with country rocks suggest they have been emplaced in an extensional regime that is related to the collapse of the hellenic orogen and more particularly to the crustal thinning that the aegean region has suffered since the mid tertiary rather than being products of crustal thickening it is suggested that their petrogenesis is related to the elevation of thermal gradients during the extensional phase which initiated thermal breakdown of hydrous minerals in the lithosphere and lower crust that had existed metastably during compression and the subduction of cool ocean lithosphere beneath the region in the late mesozoic early tertiary

emplacement and cooling of komatiite lavas we propose that komatiite lavas were emplaced as turbulent flows accompanied by vigorous forced convection with cooling rates often in excess of hundreds of c h 1 they melted and assimilated up to 10 of the ground over which they flowed forming deep channels nickel sulphide mineralization may have resulted from incorporation of sulphur rich sediment after emplacement high cooling rates persisted resulting in spinifex textures due to compositional convection

separation of intrinsic and scattering q for volcanic areas a comparison between etna and campi flegrei abstract we studied the seismic attenuation in the short period range 1 24 hz at the campi flegrei volcanic area using the method of frankel 1991 to separate intrinsic and scattering attenuation parameters the results compared to those obtained at etna volcano show that scattering phenomena play a main role in the attenuation mechanism of the seismic waves for these two volcanic areas the comparison of the present results with those obtained in non volcanic areas allow us to re consider the importance of the molten materials magma as a main cause of the seismic attenuation in the active volcanic areas

rock anelasticity measurements at high pressure low strain amplitude and seismic frequency a new apparatus has been constructed which will ultimately facilitate the study of rock anelasticity under conditions which closely approach those of teleseismic wave propagation simultaneous high pressure and temperature low frequency and strain amplitude and controlled pore pressure of volatiles its performance has been tested in a series of preliminary experiments conducted at room temperature measurements on a steel standard demonstrate the sensitivity of the apparatus to very small departures from ideal elasticity q 1000 experimental data for a fine grained granitic rock show that both the shear modulus g and quality factor q increase sharply with increasing pressure below 100 mpa beyond which pressure both parameters become markedly less pressure sensitive these observations are in accord with those of previous studies at higher frequencies and larger strains and are consistent with the view that the anelasticity of rocks at ambient pressure is dominated by mechanisms operative at open cracks and grain boundaries

unscrambling the lead model ages abstract a linear relation is derived for the secular evolution of 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb ratios that permits tests to be made for open system evolution on each system independently application of the method to conformable ore bodies of various geological age indicates that the available data do not demand an open system evolution for the last 3 8 b y 238 u 204 pb and 232 th 204 pb of 9 66 0 15 and 37 65 1 14 respectively fit best the data for this time interval a single stage evolution since 4 5 b y is unlikely however and the major events of continent formation postdate the earth accretion by at least 400 m y the larger scatter of 207 pb 204 pb data about the evolution line relative to the other isotopic ratios is also interpreted as resulting from a series of continental differentiation events taking place at 3 85 0 15 b y

geochemical comparison between minettes and kersantites from the western european hercynian orogen trace element and pbsrnd isotope constraints on their origin trace element and isotopic characteristics of late carboniferous to early permian minettes and kersantites have been determined these lamprophyres have been sampled throughout the western european hercynian orogen from brittany to the west to schwarzwald to the east in spite of sharp petrological differences reflected by mineralogy and major element geochemistry minettes and kersantites exhibit close identity with respect to trace element and isotopic features these features comprise enrichment in incompatible elements highcs rb and lowce pb ratios ta and ti relative depletion high abundance in transition elements and highni mg ratios pb isotope ratios are undistinguishable from those measured on hercynian continental crust initial143nd 144nd ratios are between0 5120 ei 5 and0 5122 ei 1 for minettes and kersantites whereas initial87sr 86sr ratios vary between 0 7055 0 710 for minettes and 0 707 0 708 for kersantites no simple mixing relations are visible on rbsr and smnd isochron diagrams the exceptional homogeneity of these geochemical characteristics along a 1000 km traverse does not allow for an hypothesis of enrichment through upper level assimilation and thus leads to propose that these rocks originated through melting of a mantle enriched by recycling of crustal material

radiogenic isotopes for deciphering terrigenous input provenance in the western mediterranean abstract radiogenic isotopic signatures in marine sediments can be used to trace terrigenous source areas and transport mechanisms which are in turn related to climate variability to date most of the published studies using this approach have been focused on eastern mediterranean sediments in contrast we study here the terrigenous input provenance in the westernmost mediterranean alboran sea basin by using radiogenic isotope proxies and nd model ages in a marine record spanning the last 20 ka nd sr and pb isotopes obtained from carbonate free samples from the

qp melting temperature relation in peridotite at high pressure and temperature attenuation mechanism and implications for the mechanical properties of the upper mantle the anelastic properties of compressional waves in a peridotite have been determined in the laboratory at sufficiently high temperatures to 1280 c and pressures to 0 73 gpa to warrant comparison with seismic measurements of the earth a substantial decrease of qp is observed at temperatures well below the onset of partial melting qp systematically increases with increasing pressure over the entire temperature range of major significance is the finding that qp is dependent on the ratio of the temperatures to the melting solidus temperature i e qp depends on the homologous temperature the pressure dependence of qp appears through the pressure dependence of the solidus of the peridotite within the uncertainties of measurement of both qp and the phase diagram it appears that melting and high temperature anelastic properties have a common origin in peridotite the homologous temperature dependence of qp suggests that we may estimate the temperature and pressure dependence of qp for peridotites of different compositions and possibly even for hydrous peridotites if solidus temperatures are known as a function of pressure a far easier measurement than elastic and anelastic properties the activation volume of qp is greatly reduced at high pressure since the slope of solidus versus pressure rapidly decreases with increasing pressure pressure dependence of seismic velocity and melt fraction in peridotite also appears to be related to the homologous temperature the qp homologous temperature relation suggests a connection between qp and the properties of the grain boundaries that is the major loss of seismic energy occurs at the grain boundaries grain boundary relaxation or high temperature background attenuation is a possible mechanism for the grain boundary damping no frequency dependence of qp is resolved 0 0 2 in qp f over the pressure temperature and frequency ranges of the measurement the present results and the model of grain boundary relaxation suggest that an appropriate choice of grain size may give an ultrasonic q that is applicable to the earth experimentally determined anelastic properties of a peridotite are critical for modeling mechanical properties of the upper mantle implications of the results are as follows 1 seismic data commonly interpreted as indicating a partially molten asthenosphere may instead reflect a hot solid asthenosphere at 90 100 of the solidus temperature 2 partial melting may not produce any abrupt change of seismic velocity and q rather elastic and anelastic properties of the upper mantle will change gradually at the boundary where the geotherm crosses the solidus 3 there may be no sharp mechanical boundary between the lithosphere and the asthenosphere

dependence of creep in olivine on homologous temperature and its implications for flow in the mantle creep data for olivine when normalized by the melting temperature can be extrapolated directly to the pressure temperature and chemical environment of the upper mantle by using the appropriate solidus and geotherms the data presented here collected over a broad pressure range predict effective viscosities that are consistent with geophysical and tectonic constraints for a dry upper mantle these systematics suggest that the effective viscosity of the mantle has risen over the age of the earth as outgassing has progressively raised the solidus

absorption band q model for the earth body wave surface wave and normal mode data are used to place constraints on the frequency dependence of q in the mantle with a simple absorption band model it is possible to satisfy the shear sensitive data over a broad frequency range the quality factor qs is proportional to in the band and to and 1 at higher and lower frequencies respectively as appropriate for a relaxation mechanism with a spectrum of relaxation times the parameters of the band are q min 80 0 15 and width 5 decades the center of the band varies from 101 seconds in the upper mantle to 1 6 103 seconds in the lower mantle the shift of the band with depth is consistent with the expected effects of temperature pressure and stress high qs regions of the mantle are attributed to a shift of the absorption band to longer periods to satisfy the gravest fundamental spheroidal modes and the scs data the absorption band must shift back into the short period seismic band at the base of the mantle this may be due to a high temperature gradient or high shear stresses a preliminary attempt is also made to specify bulk dissipation in the mantle and core specific features of the absorption band model are low q in the body wave band at both the top and the base of the mantle low q for long period body waves in the outer core an inner core qs that increases with period and low qp qs at short periods in the middle mantle the short period qs increases rapidly at 400 km and is relatively constant from this depth to 2400 km the deformational q of the earth at a period of 14 months is predicted to be 463

u th pb systematics of morphologically characterized zircon and allanite a high resolution isotopic study of the alpine rensen pluton northern italy abstract the rensen pluton is a typical member of the tertiary calc alkaline plutonic suite structurally connected to the periadriatic lineament it intrudes the austroalpine polymetamorphic basement of the eastern alps southwest of the tauern window northern italy zircons and allanites from tonalitic quartz dioritic and monzogranitic lithologies were studied systematically on the basis of refined grain by grain microanalytical techniques high resolution isotope analyses show that morphology and u pb systematics of zircon are highly correlated with magma evolution zircons from the main tonalitic part of the intrusion display a gradual increase of u content from early stage s type morphology to late stage g1 type morphology most of the zircon crystals include premagmatic seeds derived from old crustal sources unevolved s type zircons contain inherited components of late proterozoic early paleozoic apparent age whereas late stage g1 type zircons show inheritance of early proterozoic apparent age both types of inherited components are present in zircons from a fine grained tonalitic domain located at the center of the pluton which shows restitic structures the crustal affinity of this rock is corroborated by its relatively low e nd value 6 0 as compared to values obtained for the main tonalite 3 9 and the quartz diorite 3 5 the accuracy of cainozoic zircon ages is affected by initial radioactive disequilibrium in the u pb decay chains particularly uncertain is the fractionation behavior of protactinium which could be enriched in zircon due to the high stability of the pa 5 oxidation state because the th pb system is devoid of long lived intermediate radioactive nuclides the best approximation to the emplacement age of the rensen pluton is given by allanite th pb ages of 31 70 0 25ma 95 c l for the main tonalite and by a slightly younger age of 31 09 0 32ma for quartz diorite

temperature distribution in the crust and mantle in an attempt to understand the temperature distribution in the earth experimental constraints on the geotherm in the crust and mantle are considered the basic form of the geotherm is interpreted on the basis of two dominant mechanisms by which heat is transported in the earth 1 conduction through the rock and 2 advection by thermal flow data reveal that 1 the temperature distributions through continental lithosphere and through oceanic lithosphere more than 60 million years old are practically indistinguishable 2 crustal uplift is instrumental in modifying continental geotherms and 3 the average temperature through the archean crust and mantle was similar to that at present it is noted that current limitations in understanding the constitution of the lower mantle can lead to significant uncertainties in the thermal response time of the planetary interior

the genesis of felsic mafic plutonic associations a sr and nd isotopic study of the hercynian braga granitoid massif northern portugal abstract the late tectonic braga plutonic complex in the centro iberian zone north portugal was emplaced during the hercynian orogeny within a silurian sedimentary sequence and displays an acid basic association which consists of three well defined intrusions the biotite dominant braga granite minor bodies of gabbro to granodiorite composition and the mildly peraluminous gonca leucogranite these three plutonic suites present field relationships indicating a synchronous magmatic emplacement for which a rb sr age of 310 10 ma is obtained the distribution of the three plutonic units along regular curves in major and trace element diagrams suggests that the different units can be genetically related however the rb sr and sm nd isotopic results do not indicate a unique homogeneous source and a simple fractional crystallization process the gabbros have chemical characters high k ba sr and light rare earth elements and low nb ti and zr contents and isotopic compositions sri 0 70497 and nd 2 5 which suggest an alkaline magmatic affinity of shoshonitic type they are probably derived from an enriched mantle source in contrast the peraluminous gonca leucogranite sri 0 70933 and nd 6 8 is more likely the result of crustal anatexis the braga granite and the evolved members of the basic series have an intermediate isotopic composition sri 0 70532 to 0 70733 and nd 3 0 to 6 2 which can be interpreted in terms of an hybridization process between the two previous end members the chemical and sr nd isotopic compositions of the braga plutonic series can be explained by an assimilation fractional crystallization afc model between a mantle derived magma equivalent to the gabbros and a crust derived magma the gonca granite

a salt diapir related mississippi valley type deposit the bou jaber pb zn ba f deposit tunisia fluid inclusion and isotope study the bou jaber ba f pb zn deposit is located at the edge of the bou jaber triassic salt diapir in the tunisia salt diapir province the ores are unconformity and fault controlled and occur as subvertical column shaped bodies developed in dissolution collapse breccias and in cavities within the late aptian platform carbonate rocks which are covered unconformably by impermeable shales and marls of the fahdene formation late albian cenomanian age the host rock is hydrothermally altered to ankerite proximal to and within the ore bodies quartz as fine grained bipyramidal crystals formed during hydrothermal alteration of the host rocks the ore mineral assemblage is composed of barite fluorite sphalerite and galena in decreasing abundance the ore zones outline distinct depositional events sphalerite galena barite ankerite and fluorite fluid inclusions commonly oil rich have distinct fluid salinities and homogenization temperatures for each of these events sphalerite galena 17 to 24 wt nacl eq and th from 112 to 136 c ankerite barite 11 to 17 wt nacl eq and th from 100 to 130 c fluorite 19 to 21 wt nacl eq th from 140 to 165 c the mean temperature of the ore fluids decreased from sphalerite 125 c to barite 115 c and increased during fluorite deposition 152 c then decreased to 110 c during late calcite precipitation laser ablation inductively coupled plasma mass spectrometry la icp ms analyses of fluid inclusions in fluorite are metal rich hundreds to thousands ppm pb zn cu fe but the inclusions in barite are deficient in pb zn cu fe inclusions in fluorite have cl br and na br ratios of several thousand consistent with dissolution of halite while the inclusions analysed in barite have values lower than seawater which are indicative of a br enriched brine derived from evaporation plus a component of halite dissolution the salinity of the barite hosted fluid inclusions is less than obtained simply by the evaporation of seawater to halite saturation and requires a dilution of more than two times by meteoric water the higher k na values in fluid inclusions from barite suggest that the brines interacted with k rich rocks in the basement or siliciclastic sediments in the basin carbonate gangue minerals ankerite and calcite have 13c and 18o values that are close to the carbonate host rock and indicate fluid equilibrium between carbonate host rocks and hydrothermal brines the 34s values for sphalerite and galena fall within a narrow range 1 to 10 with a bulk value of 7 5 indicating a homogeneous source of sulfur the 34s values of barite are also relatively homogeneous 22 with 6 higher than the 34s of local and regional triassic evaporites 15 the latter are believed to be the source of sulfate temperature of deposition together with sulfur isotope data indicate that the reduced sulfur in sulfides was derived through thermochemical sulfate reduction of triassic sulfate via hydrocarbons produced probably from late cretaceous source rocks the 87sr 86sr ratio in the bou jaber barite 0 709821 to 0 711408 together with the lead isotope values of bou jaber galena 206pb 204pb 18 699 to 18 737 207pb 204pb 15 635 to 15 708 and 208pb 204pb 38 321 to 38 947 show that metals were extracted from homogeneous crustal source s the tectonic setting of the bou jaber ore deposit the carbonate nature of the host rocks the epigenetic style of the mineralization and the mineral associations together with sulfur and oxygen isotope data and fluid inclusion data show that the bou jaber lead zinc mineralization has the major characteristics of a salt diapir related mississippi valley type mvt deposit with superimposed events of fluorite and of barite deposition field relations are consistent with mineral deposition during the eocene miocene alpine orogeny from multiple hydrothermal events 1 zn pb sulfides formed by mixing of two fluids one fluid metal rich but reduced sulfur poor and a second fluid reduced sulfur rich 2 barite precipitation involved the influx of a meteoric water component that mixed with a barium rich fluid and 3 fluorite precipitated from a highly saline fluid with higher temperatures

climate forcing of the pb isotope record of terrigenous input into the equatorial atlantic abstract pb isotopic compositions of bulk sediment cores from the tropical atlantic are used here to infer variations in the provenance of terrigenous input to the tropical atlantic during pleistocene climate cycles the 200 ka high precision 2 100 ppm pb isotope records of the ceara rise western atlantic and sierra leone rise eastern atlantic cores show a clear glacial interglacial cyclicity reflected by alternating unradiogenic pb and radiogenic pb in both cores the glacial interglacial pb isotopic contrast is also observed in pb pb space and can be explained in terms of binary mixing variations along the mixing lines reflecting changes in the relative proportions of the glacial unradiogenic and interglacial radiogenic pb source s the pleistocene pb isotopic variability of the terrigenous input to the ceara rise can be linked to changes in the weathering styles in the amazon basin and as a result in the amazon river discharge these changes are reflected by a greater contribution from the highlands andes pb source during glacial times and strengthening of the lowlands shield pb source during interglacials on the other hand pb isotopic variations in the sierra leone rise core indicate increased pb inputs from the saharan dust plume during glacial times in agreement with the wind patterns over africa furthermore this result bears some implications on the cause of the enhanced glacial terrigenous fluxes to the tropical atlantic which we infer to be due to increase in winter wind transport rather than glacial hyperaridity the cyclicity of the ceara rise and sierra leone rise pb isotopic records together with changes in the proportions of mixing sources throughout the last 200 ka monitor changes in the hydrological cycle over south america and the wind systems over africa respectively both of which are linked to the seasonal latitudinal migration of the intertropical convergence zone the correlation found between pb isotope cycles and other paleoclimate proxies suggests that pb isotopes may be responding to variations in earth s orbital parameters

a sr nd and pb isotopic investigation of the transition between two megacyclic units of the bjerkreim sokndal layered intrusion south norway abstract the bjerkreim sokndal layered intrusion forms part of the proterozoic south rogaland igneous complex of southwest norway the intrusion has been divided into two parts an anorthositic to gabbro noritic layered series and a generally more massive mangeritic quartz mangeritic to charnockitic upper part on the basis of repeated lithological sequences the layered series has been divided into a number of megacyclic units which are interpreted as reflecting magma chamber replenishment initial 940 ma sr nd and pb isotope ratios for plagioclases separated from samples crossing the transition between megacyclic unit iii and megacyclic unit iv range from 87 sr 86 sr i 0 7048 143 nd 144 nd i 0 51156 end 2 6 and 206 pb 204 pb i 17 50 to 87 sr 86 sr i 0 7061 143 nd 144 nd i 0 51143 end 0 1 and 206 pb 204 pb i 17 73 the isotopic compositions vary systematically with stratigraphic height and the interplay between the three isotopic systems is used to divide the stratigraphic profile into two parts replenishment and post replenishment across the replenishment interval isotopic variations are consistent with mixing between a resident and a replenishing magma the replenishing magma was characterized by 87 sr 86 sr i 0 7048 143 nd 144 nd i 0 51156 end 2 6 and 206 pb 204 pb i 17 50 the positive end of this magma indicates that its source region was either in the mantle or was itself recently derived from the mantle in contrast the resident magma had 87 sr 86 sr i 0 7061 143 nd 144 nd i 0 51143 end 0 1 and 206 pb 204 pb i 17 73 indicating that it had assimilated crustal material in addition the isotopic correlations across the replenishment interval indicate that prior to mixing the concentrations of nd and pb in the resident magma had been strongly reduced probably by fractionation of apatite above the replenishment interval nd and pb isotopic variations are apparently decoupled from sr isotopic variations two mechanisms are proposed for achieving this the first is heterogeneous mixing of up to 20 of a third magmatic component into an already isotopically stratified magma chamber the second is the presence of 10 20 of post cumulus plagioclase that crystallized from a migrating intercumulus melt

linking danube river activity to alpine ice sheet fluctuations during the last glacial ca 33 17 ka bp insights into the continental signature of heinrich stadials abstract offshore archives retrieved from marine lacustrine environments receiving sediment from large river systems are valuable quaternary continental records in the present study we reconstruct the danube river activity at the end of the last glacial period based on sedimentological mineralogical and geochemical analyses performed on long piston cores from the north west black sea margin our data suggest that the danube river produced hyperpycnal floods throughout the ca 33 17 ka period four main periods of enhanced danube flood frequency each of 1 5 3 kyr duration are recorded at ca 32 5 30 5 ka equivalent to the first part of heinrich stadial hs 3 at ca 29 27 5 ka equivalent to greenland stadial 4 at ca 25 3 23 8 ka equivalent to hs 2 and at ca 22 3 19 ka based on mineralogical and geochemical data we relate these events to enhanced surface melting of the alpine ice sheet ais that covered 50 000 km2 of the danube watershed at the last glacial maximum lgm our results suggest that i the ais growth from the inner alps to its lgm position in the northern alpine foreland started from ca 30 5 ka ended no later than ca 25 3 ka and was interrupted by a melting episode ca 29 27 5 ka ii the ais volume drastically decreased from ca 22 3 to 19 ka as soon as summer insolation energy at the ais latitude increased and iii hss strongly impacted the ais mass balance through enhanced summer surface melt this together with evidence of severely cool winters and the rapid expansion of sea ice in the north atlantic implies strong seasonality in continental europe during stadials

high temperature experiments on the elastic and anelastic behaviour of magmatic rocks measurements of the complex shear modulus have been performed on various ultrabasic and basic rocks dunite peridotite gabbro and basalts and a synthetic forsterite at temperatures between 600 and 1500 c depending on the sample material and in the frequency range from 0 003 to 30 hz under forced torsional oscillations the maximum shear stress was 0 3 mpa and the strains were of the order 5 10 6 to 5 10 5 measurements of the electrical resistivity as a function of temperature have been carried out together with the mechanical experiments to indicate the beginning of partial melting the absolute values of the shear modulus below tm are comparable with the results of other authors the onset of melting shows up in a rapid decrease of both shear modulus and electrical resistivity at high temperatures t 0 8 tm the absorption factor q 1 follows a power law q 1 exp a rt with 0 15 0 30 and a between 100 and 200 kj mol 1 towards lower temperatures this high temperature background is superimposed by a flat absorption peak which shifts from high to low frequencies with falling temperature this peak can often be described by a lognormal distribution of simple relaxation processes with a standard deviation of about three and a mean relaxation time exp a rt with a 400 600 kj mol 1 above tm a third contribution to q 1 is observed that can be regarded as independent of frequency between 1 and 30 hz this last component which contributes 50 to the total absorption is attributed to the melt phase in the samples an explanation for the high temperature background and the peak is proposed which is based on the relaxation of pinned dislocation lines by some coupling mechanism via the reorganisation of the stress field with increasing deformation this process may be responsible for both features at very high temperatures and low frequencies a small influence of steady state flow on q 1 is observed and below t 0 6 tm thermal cracking or thermoelastic losses may contribute to q 1 the influence of lithostatic pressure on q 1 is estimated and the extrapolation of the experimental data to the p t conditions of the uppermost mantle are in good agreement with published earth models an attempt is made to establish a general q t relationship from the experimental data

o sr isotopic variations in miocene granitoids from the aegean evidence for an origin by combined assimilation and fractional crystallization sr o and d h isotopic compositions have been analyzed in miocene metaluminous to slightly peraluminous i type granitoids of the central aegean individual plutonic complexes display significant variations in their 18o and initial87sr 86sr compositions d and 18o compositions of minerals and whole rocks are mostly in the magmatic range some samples from naxos and mykonos delos show low d and 18o values characteristic of meteoric water hydrothermal interaction but as a whole the changes in 18o and sr isotopic compositions as a result of hydrothermal alteration were slight even in instances where marked alteration is petrographically observable consequently the bulk rock variations of 18o from 8 1 to 12 0 and of87sr 86sr from 0 70438 to 0 71450 may be regarded as primary and indicative of the conditions of their evolution heterogeneous isotopic compositions observed in the individual plutons of serifos ikaria samos and kos may be caused by the multiple intrusion of chemically and isotopically distinct magma pulses with high viscosities and relatively rapid consolidation in most cases preventing complete homogenization the granitoids of serifos ikaria and kos display weak correlations between the initial87sr 86sr and 18o and 1 sr the granitoid province shows a positive correlation between87sr 86sr and 18o and a non linear relationship between87sr 86sr and 1 sr whereby 1 sr increases more rapidly than the isotopic ratio as the degree of fractionation of the rocks increases it is argued that assimilation of older continental material by mantle derived arc magmas with combined fractionation afc is the most plausible model to explain the chemical and isotopic characteristics of the granitoids and the geological situation in which rock types trend from granodiorites in the south west near the inferred oligocene miocene suture to granites in the center and monzonites in the north east of the province

early precambrian crustal development changing styles of mafic magmatism the work of sutton watson 1951 was one of the cornerstones in the interpretation of the evolution of early precambrian archaean and early proterozoic gneiss complexes and in the documentation of mafic dyke swarms that intruded recently accreted archaean cratons since then the concept of geological terranes and terrane accretion has been extended back to the archaean and different events can be distinguished by increasingly precise isotopic dating techniques there is growing evidence that with certain clear provisos the principle of uniformitarianism can be applied as equally to the early precambrian as it can through the rest of the geological record in terms of mafic magmatism there are similarities between some ancient and modern suites both intrusive and extrusive but there are also some major differences such as the presence of komatiites in the archaean and the formation of large scale layered intrusions and extensive dyke swarms at the end of the late archaean and in the early proterozoic the mantle has evolved continuously with the progressive extraction of mafic magmas but allowing for this continental tholeiites appear to have changed little with time ancient mid ocean ridge basalts are elusive because of their poor preservation record the geochemistry of mafic volcanics occurring in many greenstone belts commonly suggests a continental contamination component and that these volcanic rocks are most closely akin to those from present day arc systems the relative abundance of early precambrian intrusive mafic suites parallels the crustal growth curve reflecting the global crustal accretion differentiation superevent towards the close of the archaean bimodal tholeiitic noritic mafic magmatism associated with many of these intrusive suites stems from this rapid turnover in the mantle mafic crust continental crust system this in turn gives a possible link between the respective evolutions of archaean to recent komatiitic noritic and boninitic suites

2 hydrothermal ore deposits related to post orogenic extensional magmatism and core complex formation the rhodope massif of bulgaria and greece abstract the rhodope massif in southern bulgaria and northern greece hosts a range of pb zn ag cu mo and au ag deposits in high grade metamorphic continental sedimentary and igneous rocks following a protracted thrusting history as part of the alpine himalayan collision major late orogenic extension led to the formation of metamorphic core complexes block faulting sedimentary basin formation acid to basic magmatism and hydrothermal activity within a relatively short period of time during the early tertiary large vein and carbonate replacement pb zn deposits hosted by high grade metamorphic rocks in the central rhodopean dome e g the madan ore field are spatially associated with low angle detachment faults as well as local silicic dyke swarms and or ignimbrites ore formation is essentially synchronous with post extensional dome uplift and magmatism which has a dominant crustal magma component according to pb and sr isotope data intermediate and high sulphidation pb zn ag au deposits and minor porphyry cu mo mineralization in the eastern rhodopes are predominantly hosted by veins in shoshonitic to high k calc alkaline volcanic rocks of closely similar age base metal poor high grade gold deposits of low sulphidation character occurring in continental sedimentary rocks of synextensional basins e g ada tepe show a close spatial and temporal relation to detachment faulting prior and during metamorphic core complex formation their formation predates local magmatism but may involve fluids from deep mantle magmas the change in geochemical signatures of palaeogene magmatic rocks from predominantly silicic types in the central rhodopes to strongly fractionated shoshonitic bulgaria to calc alkaline and high k calc alkaline greece magmas in the eastern rhodopes coincides with the enrichment in cu and au relative to pb and zn of the associated ore deposits this trend also correlates with a decrease in the radiogenic pb and sr isotope components of the magmatic rocks from west to east reflecting a reduced crustal contamination of mantle magmas which in turn correlates with a decreasing crustal thickness that can be observed today hydrogen and oxygen isotopic compositions of the related hydrothermal systems show a concomitant increase of magmatic relative to meteoric fluids from the pb zn ag deposits of the central rhodopes to the magmatic rock hosted polymetallic gold deposits of the eastern rhodopes

meta igneous origin of hercynian peraluminous granites in n w french massif central implications for crustal history reconstructions seventy samples of hercynian peraluminous granites gueret millevaches and saint sylvestre massifs and metamorphic units of the limousin area were analysed for rb sr and sm nd the source rocks of the peraluminous granites can be found in the metamorphic rocks of limousin among them meta igneous rocks were largely predominent over meta sedimentary rocks in the source of the three granites millevaches and gueret granites were generated by the partial melting of rocks comprising meta volcanics and meta sediments whereas the saint sylvestre granite was produced exclusively by the melting of late precambrian granites this leads to confusing t dm nd values the confusion being amplified by the segregation of monazite during the petrogenetic evolution of the peraluminous granites which leads to dramatic fractionation in sm nd ratios the data of the present study tend to demonstrate that peraluminous granites do not give a good representation of isotopic mean crustal estimates late precambrian time seems however to have been a period of extensive crustal generation in western europe

assimilation of continental crust by komatiites in the precambrian basement of the carswell structure saskatchewan canada the isotope geochemistry sm nd pb pb and rb sr of mafic gneisses from the basement of the carswell structure saskatchewan canada rich both in mg and incompatible elements k rb ree has been investigated a good sm nd alignment gives a slope corresponding to an age of 3 7 ga however comparison with major elements data strongly suggests that this alignment is a mixing line between mg rich high cao al2o3 magmas and the local felsic crust older than 2 9 ga the mafic magmas were probably of komatiitic affinity mgo 20 percent but nevertheless were extracted from a source with nearly chondritic to slightly enriched light ree distribution the age of the komatiite emplacement 1 9 2 9 ga is only loosely constrained by the oldest crustal residence age in the series and the subsequent metamorphic events the granulite facies climax is dated at ca 1 9 ga by concordant whole rock pb pb and sm nd garnet whole rock isochrons the rb sr systematics have been disturbed by later event s younger than 1 5 1 7 ga but do not permit a more precise assessment of the perturbation age

geophysical constraints on partial melt in the upper mantle this paper addresses the conditions under which partial melt can exist in the mantle in order to be observed as a geophysical anomaly typical observed anomalies are high electrical conductivity of the order of 0 1 s m or greater velocity decreases of 7 10 seismic q values less than 100 and a frequency band for seismic effects in the region near 1 hz existing theories of electrical conduction in partial melts and of frequency dependent seismic properties together with recent measurements of melt electrical conductivity viscosity and partial melt texture can be used to establish requirements for melt to be observed by geophysical methods from electrical anomalies mainly sensitive to melt volume and its interconnection one can require a minimum melt fraction of several percent at temperatures close to the solidus 1150 1300 c however seismic models demand only a small volume in very flattened shapes aspect ratio 0 001 melt fraction 0 1 further if melt configuration permits seismic dissipation in bulk that is there exist flattened voids intersecting more or less equant voids then it is possible to infer melt fractions for elastic anomalies that are consistent with the several percent required for electrical anomalies observed equilibrium textures of partly melted peridotite together with inferred melt solid surface energies suggest that melt on a grain size scale in a gravitational field segregates into a strongly anisotropic pattern thus if partial melt causes mantle geophysical anomalies it should exist in a variety of void shapes and probably of sizes while the association of electrical and elastic anomalies with indications of reduced density volcanism and high heat flow makes the hypothesis of partial melting an attractive explanation the minimum physical requirement is for existence of relatively high temperature

a nd isotope investigation of sediments related to crustal development in the british isles the times of original fractionation of the sm and nd component of clastic sediments from a mantle source crustal residence age have been estimated from sm nd model ages calculated relative to a depleted mantle evolution in this way the provenance and evolution of selected precambrian and phanerozoic sediments and metasediments from the british isles have been examined whereas some archaean and early proterozoic sediments have sm nd model ages that are close to their stratigraphic age the phanerozoic sediments analysed have model ages as much as 2 0 ga in excess of their stratigraphic age a more detailed study of lower palaeozoic sediments deposited on the northern margin of the iapetus ocean provides evidence for a marked change of provenance in the ordovician after the deposition of the dalradian supergroup a component with comparatively high143nd 144nd and sm nd ratio presumably basaltic is present in the sediments throughout the accretionary prism crustal residence age estimates average about 1 5 ga for both these lower palaeozoic sediments and modern pelagic clays and collectively fail to provide any evidence for significant continental growth during the phanerozoic

nd sr isotopic and geochemical characteristics of the southern adamello northern italy intrusives implications for crustal versus mantle origin among the periadriatic tertiary calc alkaline intrusions in northern italy the adamello batholith displays the most convincing evidence for a mafic parentage this is particularly the case for the southern adamello where minor ultramafic to gabbroic bodies form the margins of several distinct and dominantly tonalitic plutons isotopic analyses are presented for the val fredda and blumone ultramafic gabbroic to tonalitic leucoquartzdioritic suites for the more felsic rock types of the spatially predominant re di castello group as well as for a lamprophyre this dike of picrobasaltic composition is thought to represent the parental magma field relations and a large body of petrographic geochemical and mineral chemical data point to a cogenetic origin for all of these rocks and to magmatic differentiation with fractional crystallization and accumulation as the main evolutionary mechanism this is compatible with the isotopic results for the blumone series and the mafic rocks of val fredda with end 3 8 to 1 5 and esr 9 1 to 4 6 a common parental magma similar to the lamprophyre end 3 1 esr 4 4 also seems plausible the isotopically homogeneous felsic lithologies of val fredda end 0 1 to 0 3 esr 2 9 to 0 7 structurally intimately associated with their mafic counterpart indicate crustal contamination incurred prior to their differentiation for the re di castello group progressing crustal affinity of isotopic compositions end of 1 9 to 4 9 and esr of 16 5 to 40 9 with chemical differentiation implies large scale interaction with a granitoid crustal component attempts at quantifying this process reveal individual characteristics and evolution for each pluton the data indicate extraction of this component from a source in the lower crust with time integrated low rb sr esr approximating 50 and en close to 6 corresponding to a 1000 ma crustal residence age if a two stage evolution is assumed formation of a gabbroic reservoir from the above parental melt would require uptake of some 4 12 wt of granitoid component by combined assimilation fractional crystallization whereas subsequent evolution to tonalitic compositions would necessitate admixing of 40 70 wt of granitoid partial melt assimilation thus mainly occurs during evolution from gabbroic to tonalitic compositions when the mafic magma is ponded near the mantle crust boundary and where homogenization prior to resuming buoyant ascent is assured precise locations and more detailed petrography of the samples is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document 91b 002 2 50 payment must accompany order