high al gabbros in the laramie anorthosite complex wyoming implications for the composition of melts parental to proterozoic anorthosite high al gabbro represents one of the latest phases of magmatism in the 1 43 ga laramie anorthosite complex lac in southeastern wyoming this lithology which is mineralogically and geochemically the most primitive in the lac forms dikes and small intrusions that cross cut monzonitic and anorthositic rocks high al gabbro is characterized by high al2o3 15 19 wt ree patterns with positive europium anomalies eu eu 1 2 3 8 and the lowest initial 87sr 86sr as low as 0 7033 and highest initial nd up to 2 in the lac their sr and nd isotopic characteristics indicate a mantle origin followed by crustal assimilation during ascent intermediate plagioclase an50 60 and mafic silicate fo54 63 compositions suggest that they are not primary mantle melts and that they differentiated prior to final emplacement high al gabbros of the lac are similar compositionally to gabbros from several other proterozoic anorthosite complexes including rocks from the harp lake complex and the hettasch intrusion in labrador and the adirondack mountains of new york these gabbros are considered to be parental to their associated anorthositic rocks a theory that is supported by recent experimental work we interpret lac high al gabbros to represent mantle derived melts produced by the differentiation of a basaltic magma in an upper mantle chamber continued evolution of this magma eventually resulted in the formation of plagioclase rich diapirs which ascended to mid crustal levels and formed the anorthositic rocks of the lac because these gabbros intrude the anorthositic rocks they do not represent directly the magma from which anorthosite crystallized and instead are younger samples of magma formed by identical processes

petrogenesis of coarse grained intrusives from tahiti nui and raiatea society islands french polynesia this study is based on a set of coarse grained igneous rocks collected from two zoned plutons located in the central part of tahiti nui and raiatea the ahititera pluton central depression of tahiti nui comprises a great diversity of rocks ranging from ultrabasic to felsic in composition it shows a concentric zonation with nepheline free rocks in its periphery and nepheline bearing rocks in its central part the faaroa pluton central depression of raiatea is entirely mafic and includes only gabbros and theralites the two plutons have variable nd sr isotopic signatures especially the ahititera rocks which are subdivided into three groups based on their mineralogy geochemistry and isotope composition the isotopic variability probably reflects local heterogeneities in the society mantle plume petrographic and isotopic data have been used to define two magmatic suites in ahititera identifiable from their degree of si undersaturation the evolution of the mildly si undersaturated suite is controlled by simple fractional crystallization whereas the strongly si undersaturated suite requires additional h2o influx the third isotopic group includes only theralites the rare earth element ree compositions of the mafic rocks from both plutons do not correlate with their isotopic signature the ree patterns of the most si undersaturated rocks are systematically characterized by steeper slopes such features are also observed in lavas from seamounts located within the present day hotspot area it appears that ree concentrations in society lavas and intrusives are probably mainly governed by variable degrees of partial melting of a garnet free mantle source and are independent of their isotopic signature

the generation of oceanic rhyolites by crystal fractionation the basalt rhyolite association at volcan alcedo galapagos archipelago

mantle discontinuities and temperature under the north american continental keel a ubiquitous feature of upper mantle seismic velocity models has been the presence of high velocity keels beneath stable continental interiors1 2 3 4 5 uncertainty remains however regarding the maximum depth to which continental keels extend the degree to which they have cooled the mantle that surrounds them and their role in mantle flow here we investigate thermal anomalies across the eastern margin of the north american continental keel by imaging the seismic discontinuities at depths of 410 and 660 km with compressional to shear converted waves recorded by a 1 500 km long seismometer deployment in the eastern united states the thickness of the transition zone the region nominally between depths of 410 and 660 km and the depth to the 410 km discontinuity indicate that cold keel material and sub keel downwellings must be largely confined to the upper mantle and may impinge on the transition zone only in localized regions and with thermal anomalies of less than 150 k a 20 km depression of the 660 km discontinuity to the south of the westernmost stations coincides with a region of fast velocity in the deep transition zone2 and may be associated with the remnants of the subducted farallon plate1 2 4

shear wave velocity structure beneath europe the northeastern atlantic and western asia from waveform inversions including surface wave mode coupling summary waveforms of 449 seismograms have been inverted for s wave velocity structures beneath europe the northeastern atlantic and western asia down to a depth of 670 km recorded waveforms were used in the time window starting at the s wave arrival and ending after the fundamental mode rayleigh wave arrival the inversion method is based on the partitioned waveform inversion nolet 1990 with the difference that synthetic seismograms are calculated taking surface wave mode coupling into account in order to model body waves in laterally heterogeneous media more accurately the partitioning of the inversion procedure makes non linear optimization feasible even for inversions including surface wave mode coupling the non linear inversion of the waveforms resulted in linear constraints on the 3 d velocity structure in a second step these constraints were used in a linear inversion for the 3 d shear wave velocity model beneath europe the northeastern atlantic and western asia the eur sc 95 model is presented which is characterized by a wide range of length scales of the velocity structures in central europe where the ray density is highest small scale structures are recovered such as the presence of high velocities associated with the hellenic subduction zone on the edges of the inversion model where the ray density and therefore also the resolution is poorer we find larger scale features such as the relatively laterally homogeneous high velocity structure beneath the russian platform to a depth of 300 km in this paper we discuss the inversion method data processing parametrization difficulties due to the introduction of surface wave mode coupling spatial resolution of the model and structures in the eur sc 95 model

magma chamber processes in the tellnes ilmenite deposit rogaland anorthosite province sw norway and the formation of fe ti ores in massif type anorthosites the origin of igneous fe ti oxide ores associated with massif type anorthosites is investigated through a detailed study of the world class tellnes ilmenite deposit part of the late proterozoic 930 920 ma amc series of the rogaland anorthosite province sw norway more than 100 samples from drill cores reveal significant petrographical and compositional variations within the ore body four zones are defined based on variations in modal proportions and cumulus mineral assemblages the lower and upper central zones and the lower and upper marginal zones plagioclase and whole rock compositions discriminate the zones and display patterns interpreted as a result of mixing of either plagioclase ilmenite or plagioclase ilmenite orthopyroxene olivine cumulates with a melt of ferrodioritic jotunitic composition with a content decreasing from 80 to 20 from the margins to the central part of the ore body phase diagrams for a jotunitic parental magma reproduce the crystallization sequence at 5 kb the orthopyroxene olivine liquidus boundary is a peritectic in the bjerkreim sokndal layered intrusion and a cotectic in tellnes and this explains the differences in the sequence of crystallization of the two intrusions the high concentration of ilmenite well above cotectic proportions resulted from gravity sorting in the tellnes ore body which represents the lower part of a larger magma chamber uniform sr isotope ratios do not support magma mixing the cryptic layering of the ore body precludes injection as a crystal mush but favours in situ crystallization from an evolving magma in a sill like magma chamber the present trough shape and mineral orientations result from deformation during gravity induced subsidence and by up doming of the anorthosite fractional crystallization of a tio2 rich magma with ilmenite as an early liquidus mineral and plagioclase buoyancy are the principal mechanisms responsible for the formation of fe ti deposits in proterozoic massif type anorthosites

exploring fractionation models for martian magmas 1 martian primary compositions i e magmas that did not experience fractionation and or contamination after extraction from the mantle occur as a subset of martian meteorites and a few lavas analyzed on the planet s surface by rovers eruptions of primary magmas are rare on earth and presumably on mars previous studies of fractional crystallization of martian primary magmas have been conducted under isobaric conditions simulating idealized crystallization in magma chambers polybaric fractionation which occurs during magma ascent has not been investigated in detail for martian magmas using the melts algorithm and the pmelts revision we present comprehensive isobaric and polybaric thermodynamic calculations of the fractional crystallization of four primary or parental martian magmas humphrey fastball y 980459 shergottite and nakhlite parental melts using various pressure temperature paths oxygen fugacities and water contents to constrain how these magmas might evolve we then examine whether known martian alkaline rock compositions could have formed through fractional crystallization of these magmas under the simulated conditions we find that isobaric and polybaric crystallization paths produce similar residual melt compositions but given sufficient details we may be able to distinguish between them we calculate that backstay gusev crater likely formed by fractionation of a primary magma under polybaric conditions while jake m gale crater may have formed through melting of a metasomatized mantle crustal assimilation or fractional crystallization of an unknown primary magma the best fits for the backstay composition indicate that consideration of polybaric crystallization paths can help improve the quality of fit when simulating liquid lines of descent

shear wave velocity variations in the upper mantle beneath central europe summary a new model eur s91 of shear wave velocity variations in the upper mantle beneath central europe and surrounding regions down to a depth of 670km is presented the model is derived from the inversion of the waveforms of 217 seismograms using the partitioned waveform inversion method for the seismogram in the time window from the s wave arrival to the fundamental mode of the rayleigh wave the seismograms were mostly assembled from digitally recording long period and broad band stations in europe the resulting 3 d model accurately predicts most of the observed waveforms in a wide band of frequencies body waves were fit for frequencies up to 60mhz the fundamental rayleigh mode which at high frequencies is more prone to scattering and multipathing was generally low passed at 25 mhz the resolving power of the data set depends strongly on the density of available wave paths and varies as a function of geographical position small scale heterogeneities like the subducted lithosphere in the hellenic collision zone were imaged in the region with the highest density of wave paths which indicates an optimum resolution of better than 200 km the main new results of this study pertain to the transition between east and central europe we present new information about the structure below the tornquist teisseyre zone ttz the ttz is generally regarded as the boundary between the precambrian crust of the baltic shield russian platform and the younger crust of central europe our 3 d s velocity model reveals that below this line a sharp lateral boundary extends down to at least 140 km depth with high velocities beneath the baltic shield and russian platform contrasting to the low velocities beneath the younger regions of europe the observed velocity contrast across the ttz is largest between the pannonian basin and the russian platform where it is equal to 12 per cent at 80 km depth at depths of 300 400 km the ttz is underlain by a zone of low s velocity which indicates a local thinning of the deeply rooted high velocity structure or tectosphere that underlies most of eastern europe in addition to these new results on the transition region between east and central europe a number of features that are present in other tomographic results have been confirmed the precambrian provinces of europe are characterized by high s velocities low velocities are found beneath the pannonian basin the western mediterranean northern aegean sea turkey and a small region south west of the massif central the lithosphere beneath the paris basin has a positive s velocity anomaly the resolution of all these anomalies has been established with sensitivity tests

the geochemical evolution of anorthosite residual magmas in the laramie anorthosite complex wyoming olivine and pyroxene bearing fit enriched dioritic rocks in the 1434 ma laramie anorthosite complex are interpreted to represent variably fractionated and contaminated magmas residual after the crystallization of anorthosite geochemical characteristics of this suite include the following high contents ofti02 fe20s and p2o5 high incompatible trace element contents rare earth element patterns with a large range of eu anomalies and isotopic compositions that reflect the geographic location of individual samples with i r increasing and ejy decreasing from south to north after extraction from anorthosite fractionation offerrodioritic residual magmas resulted in secondary residual monzpdioritic melts and complementary oxide rich ferrodiorite cumulates geographic trends in isotopic composition reflect an increasing archean crustal component from south to north dioritic dikes and cumulates with isotopic compositions similar to associated anorthosites were derived locally large isotopic discrepancies between some diorites and their hosting anorthosites reflect preferential contamination of residual magma during ascent and emplacement of mantle derived plagioclase rich diapirs followed by subsequent extraction and isolation of fe enriched interstitial melt strong isotopic contrasts between anorthosite and associated fe enriched rocks in anorthosite complexes do not preclude a direct relationship between them and reflect the diversity and complexity of processes during their petrogenesis

the chemical and isotopic record of rock water interaction in the sherman granite wyoming and colorado chemical isotopic radiographic and rock leaching data are combined to describe the effects of rock water interactions in core samples of petrographically fresh 1 43 b y old sherman granite the data serve to identify sensitive indicators of incipient alteration and to estimate the degree pathways and timing of element mobilization unfractured core samples of sherman granite are remarkably fresh by most chemical or isotopic criteria but incipient alteration is indicated by the abundance and distribution of uranium and the degree of radioactive equilibration of uranium with its decay products uranium abundances which are out of equilibrium with lead decay products indicate remobilization of a portion 3 to 60 percent of original uranium in late phanerozoic time association of uranium with minor but pervasive secondary alteration products also indicates some remobilization the amount of apparent uranium mobility in unfractured sherman granite 3 to 60 percent is small compared to the results of similar studies of archean granites from nearby localities chemical and isotopic data evaluated as a function of core sample depth suggest a uranium migrational pathway involving near surface leaching and reconcentration at depth movement of solutions through the upper 200 ft 60 m of sherman granite is fracture controlled and brecciated granite shows more obvious petrographic chemical and isotopic evidence of alteration and multi element redistribution laboratory experiments using freshly crushed sherman granite confirm that uranium is leached in preference to elements such as si mg ca and k and that leachable uranium is situated close to the solid liquid interface perhaps as uranium along grain boundaries in crystal defects or on cleavage traces of minerals that exclude uranium from their structure

origin of differentiated volcanic and plutonic rocks from ascension island south atlantic ocean altered pre existing volcanic basement may have been melted or the first phase of felsic magmatism on ascension island in the assimilated during differentiation of some of the felsic magmas form of trachyte and rhyolite domes coulees lava flows and pyroclastic deposits created the central and eastern parts of the island between about 1 0 and 0 56 my ago the geochemical characteristics of the felsic rocks are largely consistent with an origin by fractional crystallization of high zr nb mafic magmas as

upper paleozoic mafic and intermediate volcanic rocks of the mount pleasant caldera associated with the sn w deposit in southwestern new brunswick canada petrogenesis and metallogenic implications abstract upper paleozoic 365 ma mafic and intermediate volcanic rocks of the piskahegan group constitute a subordinate part of the mount pleasant caldera which is associated with a significant polymetallic deposit tungsten molybdenum bismuth zones 33 mt ore with 0 21 w 0 1 mo and 0 08 bi and tin indium zones 4 8 mt with 0 82 sn and 129 g t in in southwestern new brunswick canada the epicontinental caldera complex formed during the opening of the late paleozoic maritimes basin in the northern appalachians the mafic and intermediate rocks make up two compositionally distinct associations the first association includes evolved rift related continental tholeiitic basalts and the second association comprises calc alkaline andesites although both associations were emplaced penecontemporaneously the basalts have low mg 0 34 0 40 smooth chondrite normalized ree patterns with la yb n 5 6 primitive mantle normalized trace element patterns without noticeable negative nb ta anomalies and their nd t ranges from 2 5 to 2 2 the basalts were generated by partial melting of a transition zone between spinel and garnet mantle peridotite at a depth of 70 90 km the calc alkaline andesites of the second association have chondrite normalized ree patterns that are more fractionated with la yb n 7 8 5 but without significant negative eu anomalies compared to the basaltic rocks they have lower nd t values ranging from 0 5 to 1 9 and their mantle normalized trace element plots show negative nb ta anomalies the nd t values display negative correlations with indicators of crustal contamination such as th la th nb and sio2 the andesitic rocks are interpreted to have formed by assimilation fractional crystallization processes which resulted in the contamination of a precursor basaltic magma with crustal material the parent basaltic magma for both suites underwent a different evolution the tholeiitic basalts experienced shallow seated fractional crystallization and evolved along a tholeiitic trend of early iron enrichment non oxic conditions the contaminated magma of the second association followed a calc alkaline fractionation trend of no iron enrichment oxidizing conditions characterized by a high ph2o and po2 environment at the mid crust levels the piskahegan group which is associated with an important polymetallic mineral deposit differs from the numerous non mineralized rift related volcanic suites of the regional upper devonian to lower carboniferous successions in the maritimes basin by the presence of a significant amount of coeval calc alkaline andesite which may be an indicator of potential mineralization

the nars deep project the nars deep project involves the deployment of broadband seismological stations in russia belarus and the ukraine six stations were installed in 1995 two in 1997 and more stations were to be installed from 1998 onwards the nars deep project was initiated as a temporary deployment but funding from the european community enabled the stations to become permanent in this paper we present first results of the nars deep project a surface waveform inversion for the upper mantle structure along a profile from egypt to spitsbergen shows evidence for strong variations in the shear velocity structure for the different tectonic units the most striking features of the model comprise strong lithospheric anisotropy beneath the eastern mediterranean and a lithospheric thickness of approximately 200 km beneath the shield areas the crustal structure beneath the seismic station in st petersburg was investigated using the receiver function method the results of a monte carlo inversion for the receiver functions of this station show evidence for a sedimentary layer with a thickness of less than 1 km overlying an upper crustal layer extending to a depth of approximately 16 km the moho depth cannot be resolved from the receiver functions of this station

40ar 39ar geochronology of the active phonolitic cadamosto seamount cape verde abstract cadamosto seamount is located in the sw of the cape verde archipelago in the central atlantic ocean off the west coast of africa many radiometric dates exist for the islands in the archipelago however no geochronological information has been obtained from the numerous seamounts the timescales for igneous processes in the submarine realm are thus poorly understood in this study we investigated five lavas that were sampled by dredging and rov remotely operated vehicle from the flanks and summit areas of the largely phonolitic cadamosto seamount during two different research cruises chemical zonation patterns of minerals were determined by electron microprobe and radiometric ages were obtained from single crystal total fusion and single multi grain step heating 40ar 39ar analyses of sanidine nepheline and sodalite group minerals our 40ar 39ar results reveal young sanidine eruption ages all the older 40ar 39ar ages of many sanidine and nepheline antecrysts also give us clues regarding older magmatic events at cadamosto seamount despite these grains having undergone resorption and phenocrystic rim overgrowths resulting in some radiogenic 40ar loss during entrainment in the subsequent magmas the antecrysts minimum ages extend back to 1 5215 0 0083 ma which supports the age progression of magmatism observed in the southern islands chain of the cape verde archipelago the youngest volcanic eruption period 21 14 0 62 ka occurred during the last glacial maximum a period of global sea level lowstands we suggest that the comparatively rapid crust unloading leading up to the lowstand may have reduced pressure conditions within the cadamosto seamount magma plumbing system and thus led to enhanced submarine eruption activity

the anisotropic seismic structure of the east european platform 1 we present models for the p and s velocity structure of the upper mantle beneath the east european platform including measurements of radial anisotropy the models were obtained by full waveform inversion of 3 component broadband seismograms from strike slip earthquakes located near the edge of the platform and recorded in russia and europe we used direct and multiply reflected body wave arrivals s ss p pp combined with fundamental mode love and rayleigh waves at source receiver distances between 15 and 50 to resolve mantle structure down to the 410 km discontinuity the platform is underlain by a radially anisotropic mantle lid extending to a depth of 200 km with a largely isotropic mantle below the model has a positive velocity gradient from 41 km to 100 km depth and a relatively uniform velocity structure from 100 km to 200 km depth with high sh and ph velocities 4 77 km s 8 45 km s shear anisotropy is uniform at 5 h v from 41 to 200 km depth drops to 2 from 200 to 250 km and is isotropic below that the average shear velocity from 100 to 250 km is also uniform at 4 65 km s and the drop in anisotropy is matched by a drop in h to 4 70 km s combined with an increase in v to 4 60 km s below 250 km there is a positive velocity gradient in both p and s velocity down to 410 km p anisotropy is not well resolved but p structure mimics the sh velocity structure suggesting that p is also anisotropic within the lid

deep structure of the east european platform according to seismic data abstract the paper presents a review and analysis of new seismic data related to the structure of the mantle beneath the east european platform analysis of observations of long range profiles revealed pronounced differences in the structure of the lower lithosphere beneath the russian plate and the north caspian coastal depression the highest p velocities found at depths around 100 km are in the range 8 4 8 5 km s 1 deep structure of the baltic shield is different from the structures of both these regions no evidence of azimuthal anisotropy in the upper mantle was found a distribution of p velocity in the upper mantle and in the transition zone consistent with accurate travel time data was determined the model involves several zones of small and large positive velocity gradients in the upper mantle rapid increases of velocity near 400 and 640 km depths and an almost constant positive velocity gradient between the 400 and 640 km discontinuities the depth of the 640 km discontinuity was determined from observations of waves converted from p to sv in the mantle

p and s wave anisotropy and shear wave splitting at pressure and temperature in possible mantle rocks and their relation to the rock fabric the seismic properties and their directional dependence have been measured in three potentially mantle derived rocks peridotite ivrea zone italy lherzolitic xenolith massif central france serpentinite the experimental data include the measurement of elastic wave velocities and densities at confining pressures up to 600 mpa and temperatures up to 600 c 600 mpa and the determination of velocity anisotropy and shear wave splitting the petrophysical data can be related to mineral composition and to microstructural characteristics of the rocks macroscopic anisotropy of p and s wave velocities is strongly controlled in the peridodites by the olivine and in the serpentinite by the serpentine antigorite lattice preferred orientation lpo respectively shear wave splitting was found to be an important characteristic of the anisotropic rocks the study shows that for the interpretation of shear wave splitting the direction of wave propagation in relation to the lpos of the major minerals and to the symmetry of their lattice fabric is of great importance in the ivrea peridotite exhibiting rhombic symmetry there is practically no shear wave splitting to be observed for s waves propagating parallel to the olivine 100 axes and the maximum shear wave splitting is not coupled with the direction of highest p wave velocity in the transversely isotropic massif central xenolith shear waves do not split when propagating parallel to the olivine 010 maximum coinciding with the direction of the radial symmetry axis of the olivine fabric the experimental data are principally in accordance with velocity calculations based on the measured olivine lpo and the olivine single crystal stiffness coefficients implications for the interpretation of seismic data are discussed

petrogenesis of basalt comendite and basalt pantellerite suites terceira azores and some implications for the origin of ocean island rhyolites petrogenetic modeling of the recent lava succession of santa barbara and pico alto volcanoes and associated basaltic lavas indicates that there are two discrete lava series present one erupted from the axial rift linking the two central volcanoes and one associated with monogenetic cones scattered around the flanks of santa barbara the felsic lavas of both volcanoes are peralkaline and appear to be derived from associated basalts by fractional crystallization of an assemblage including essential amphibole trace element abundances in the felsic lavas particularly those of sr and ree cannot be reconciled with an origin through partial melting of basaltic material at the base of the volcanic pile the difference between the comenditic and pantelleritic differentiation trends of santa barbara and pico alto is attributed primarily to fo2 control of the crystallizing assemblage probably related to thermal dissociation of magmatic water in the santa barbara magma chamber this effect may be augmented by minor differences in parent basaltic compositions the pico alto pantellerites being derived from the rift basalts whereas the santa barbara comendites are derived from the off rift basalts a compositional gap between 54 and 64 silica content in the lavas is not present if the suite is extended to include co magmatic hypabyssal xenoliths leading to the inference that the gap in this and other bimodal suites results solely from a relative inability of magma of intermediate composition to erupt

crystal fractionation and the evolution of intra plate hy normative igneous suites insights from their feldspars feldspars and normative feldspar constituents of bulk magma show complexes and anorogenic granites is associated with hotspot and continental rift environments a major chartrends supportive of fractional crystallization in the three main types of hy normative intraplate suites that contain qz oversaturated acteristic of this magmatism is the production of suites of compositionally diverse rocks that are temporally and rocks ocean island and continental alkalic suites ocean island tholeiitic suites and continental tholeiitic suites these suites are spatially associated much is still unknown about the processes that have led to the diversity of magmas within characterized by the presence of a single feldspar in each suite member a shift of this feldspar from plagioclase to alkali feldspar such suites or to the global diversity of intra plate suite types and k enrichment of alkali feldspar with decreasing temperature in the trachytic members the modal feldspars provide evidence for a many ocean island and continental hotspot and continental rift related suites are characterized by rocks with reaction relationship between feldspars and indicate a build up of magmatic volatile content towards saturation with progressive high alkali silica ratios that fall into the alkalic field of irvine baragar 1971 fig 1 regardless of their state fractionation of a parental magma having low initial volatile content the feldspar and normative feldspar evolutionary paths are unique of silica saturation such alkalic suites range from those that are strongly silica undersaturated containing rocks for each of the three suite types but similar for different suites within the same type this characteristic extends to the felsic members such as nephelinites or leucitites in the more potassic making it easy to distinguish between rhyolitic or granitic rocks suites to mildly silica undersaturated suites the kenfrom the different suite types the feldspars in natural volcanic nedy series of miyashiro 1978 containing alkali basalts suites commonly show evidence for a polybaric history particularly silica undersaturated trachytes and phonolites and in the least evolved suite members late stage feldspars of the finally to hy normative suites the coombs trend of intermediate members and feldspars of the most evolved members miyashiro 1978 that include olivine basalts trachytes show paths indicative of significantly lower temperature and pressure and rhyolites hy normative alkalic suites that contain regimes silica oversaturated rocks on ocean islands are widespread and include those from ascension island e g harris 1983 the samoan islands macdonald 1968 terceira azores e g white et al 1979 and clarion island mexico

tertiary and quaternary volcanism in the massif central france summary continental alkaline volcanics of the french massif central range in age from 65 ma to 3450 years bp and can be divided by geography and age into 20 separate areas there is no correlation between the age of an area and the geographic locality two magma series are present a silica undersaturated basanite tephrite phonolite series and an alkali basalt trachyandesite trachyte rhyolite series the evolved members of which are silica saturated rare nephelinites are also found primitive magmas of both main series may be derived from 2 20 partial melting of a mantle enriched with light rare earth elements with variable amounts of garnet in the residue nephelinites were formed from a source which was isotopically and chemically distinct fractional crystallization of observed phases accounts for the diversity of intermediate and evolved products amphibole fractionation in basalts at depth causes the trend towards silica saturation while alkali feldspar fractionation dominates the final stages of crystallization removal of small quantities of sphene and zircon explains trace element depletions in trachytes phonolites and rhyolites up to 30 crustal contamination has occurred in the evolved magmas but contamination is generally absent from their basaltic parents sr nd and pb isotope data indicate that the most probable contaminant is undepleted meta sedimentary granulite facies lower crust variations in sr and nd isotope ratios also constrain petrogenetic hypotheses such as amphibole fractionation and magma mixing mineralogical disequilibrium compositional banding and emulsification textures indicate extensive magma mixing some flows are homogeneous hybrid lavas in which only the unusual composition and rare mineral disequilibria indicate the mode of formation

alkaline volcanic rocks from the columbia hills gusev crater mars 1 irvine backstay and wishstone are the type specimens for three classes of fine grained or fragmental relatively unaltered rocks with distinctive thermal emission spectra found as float on the flanks of the columbia hills chemical analyses indicate that these rocks are mildly alkaline basalt trachybasalt and tephrite respectively their mineralogy consists of na and k rich feldspar s low and high ca pyroxenes ferroan olivine fe ti and possibly cr oxides phosphate and possibly glass the texture of wishstone is consistent with a pyroclastic origin whereas irvine and backstay are lavas or possibly dike rocks chemical compositions of these rocks plot on or near liquid lines of descent for most elements calculated for adirondack class rocks olivine rich basalts from the gusev plains at various pressures from 0 1 to 1 0 gpa we infer that wishstone backstay and irvine class magmas may have formed by fractionation of primitive oxidized basaltic magma similar to adirondack class rocks the compositions of all these rocks reveal that the gusev magmatic province is alkaline distinct from the subalkaline volcanic rocks thought to dominate most of the planet s surface the fact that differentiated volcanic rocks were not encountered on the plains prior to ascending husband hill may suggest a local magma source for volcanism beneath gusev crater

p wave mantle velocity structure beneath northern eurasia from long range recordings along the profile quartz high resolution seismic sounding of the mantle using two peaceful nuclear explosions pnes along the 3950 km long quartz profile in northern eurasia reveal major unexpected vertical and lateral mantle inhomogeneities in contrast to standard seismological earth models such as prem and iasp91 three rather than two mantle discontinuities are found between 400 and 700 km depth they are located at about 420 550 and 670 km depth lateral variations in depths amount to 10 20 km and associated p wave velocities vary by 0 1 km s 1 in the upper mantle the uppermost layer through which pn propagates extends down to about 100 km depth the mantle below this depth but above the 410 km discontinuity is characterized by alternating high and low p wave velocities in the depth range between 100 and 200 km the velocity fluctuates between 8 3 and 8 7 km s 1 and shows variations from shotpoint to shotpoint average velocities are clearly higher than standard seismological reference models this part of the mantle overlies a low velocity zone between about 200 and 300 km depth this channel also displays substantial lateral variation in depth which could be interpreted as a laterally variable asthenosphere beneath the west siberian basin the asthenosphere seems to be well established whereas it is only poorly defined under the uralian belt and the tectonic units to the northwest with the addition of more data from this and other profiles future aims include two dimensional and three dimensional tomographic modelling as well as modelling of petrological composition of the mantle down to 850 km depth beneath northern eurasia

energetics of glass fragmentation experiments on synthetic and natural glasses 1 natural silicate glasses are an essential component of many volcanic rock types including coherent and pyroclastic rocks they span a wide range of compositions occur in diverse environments and form under a variety of pressure temperature conditions in subsurface volcanic environments e g conduits and feeders melts intersect the thermodynamically defined glass transition temperature to form glasses at elevated confining pressures and under differential stresses we present a series of room temperature experiments designed to explore the fundamental mechanical and fragmentation behavior of natural obsidian and synthetic glasses pyrex under confining pressures of 0 1 100 mpa in each experiment glass cores are driven to brittle failure under compressive triaxial stress analysis of the load displacement response curves is used to quantify the storage of energy in samples prior to failure the brittle release of elastic energy at failure and the residual energy stored in the post failure material we then establish a relationship between the energy density within the sample at failure and the grain size distributions d values of the experimental products the relationship between d values and energy density for compressive fragmentation is significantly different from relationships established by previous workers for decompressive fragmentation compressive fragmentation is found to have lower fragmentation efficiency than fragmentation through decompression i e a smaller change in d value with increasing energy density we further show that the stress storage capacity of natural glasses can be enhanced approaching synthetic glasses through heat treatment

upper mantle stratigraphy of the slave craton canada insights into a new kimberlite province mantle xenoliths carried by kimberlite magma provide the only hard evidence of the rock types that constitute cratonic roots and the conditions under which they formed here we present the first such data for the slave craton northern canada based on samples of peridotite eclogite and pyroxenite collected from a middle jurassic kimberlite jericho pipe the petrology and thermobarometric analysis of these mantle xenoliths provide estimates of the thermal state and stratigraphy of the underlying mantle and constrain the evolution of this ancient craton the upper mantle beneath the north central slave is relatively cold as is typical of cratonic environments but it is also associated with relatively high values of surface heat flow 47 50 mw m 2 because of this craton9s highly radiogenic crust below 160 190 km the jericho p t array shows a significant high temperature disturbance caused by transient magmatic events the apparent source region for both porphyroclastic peridotite and other texturally unequilibrated magmatic rocks coincides with this thermal excursion and is taken to mark the base of the petrological lithosphere the depth of this petrological transition is also consistent with important geophysical discontinuities mapped below the slave craton

fractional crystallization and zoning in igneous feldspars ideal water buffered liquid fractionation lines and feldspar zoning paths diagrams giving plagioclase and sanidine fractionation paths and liquid fractionation lines under conditions of ideal water buffered fractional crystallization in the ternary feldspar system were constructed graphically using topological reasoning and experimental data and calculated phase relationships from the literature the liquidus lines and solidus or solvus paths are unique at constant p and ah2o the composition of a liquid evolves with time and moves along a fractionation line by removal of successive crystal fractions whereas the compositions of each of the crystal fractions lie on and define a solidus or solvus path most but not all such water buffered lines and paths differ only slightly from those in which water is free to build up during crystallization and ah2o to increase as in many rocks liquid compositions lying along liquidus fractionation lines are not normally preserved unless erupted as aphyric lavas the solidus or solvus paths may be preserved either as overgrowth zones in crystals zoning paths or as a series of crystal fractions in layered intrusions the topologies of the lines and paths depend mainly on the nature of the two feldspar boundary line separating the plagioclase and sanidine fields which is a function of ph2o or ah2o at constant p increases in either progressively lower the liquidi and solidi and cause larger intersections of the solidi with the solvus one feldspar solidus paths at high p and ah2o are simple whereas they are complex and may bend back on themselves at low ph2o or low ah2o at high p two feldspar paths may be simultaneous cotectic or sequential peritectic the former are simple and do not meet at high p and ah2o the critical solution line lying in the gap they are complex and may bend back or overlap at low ph2o or low ah2o at high p the position of the critical solution line being hard to determine liquids which have simultaneously fractionated two feldspars may fractionate only one towards the end crystallization changing from subsolvus to hypersolvus sequential paths may involve overgrowth of an early feldspar by a later one usually sanidine overgrowths on plagioclase but plagioclase overgrowths on sanidine occur these complexities explain in part the difficulties of unravelling the textural and compositional relationships of ternary feldspars in water poor felsic igneous rocks even in the absence of alteration or complex magma dynamics and of trying to deduce phase relationships from natural occurrences of feldspars

average lithospheric structure of southern africa summary we present a new 1 d earth model for the crust and upper mantle beneath the archaean core of southern africa the crustal structure is constrained by published seismic refraction reflection data and by the modelling of teleseismic receiver function data the mantle structure is constrained by traveltime measurements for p and s waves by waveform inversion of multimode surface waves and by forward modelling of p waveforms for regional earthquakes the 42 km thick crust of southern africa consists of four layers a 2 km thick 5 05 km s surface layer a 5 km thick 6 08 km s upper crust a 20 km thick 6 30 km s l mid crust and a 15 km thick 6 73 km s lower crust below the crust there is an 80 km thick upper mantle lid v and v are 8 09 and 4 62 km s l respectively at the top of the lid and the compressional and shearvelocity gradients through the lid are 0 0008 s l and 0 0013 s respectively below the lid there is a substantial shear wave lvz with the shear wave velocity dropping from 4 72 km s l in the lower part of the lid to 4 32 km s at 250 km depth the shear wave lvz is required to fit both the v measurements and the regional surface waveforms forward modelling of p suggests that no upper mantle p wave lvz exists there is an increase in the p wave velocity gradient at 125 km depth to 0 0015 s and a second increase at 250 km depth to 0 0035 s velocities and densities in the seismic model of the lid agree closely with velocity and density estimates from the geochemical analysis of garnet periodotite nodules from kimberlites of the kaapvaal craton implying that the estimates from kirnberlites are representative of a wider region of the archaean core of southern africa not just of the restricted region beneath which the kimberlites are found a comparison of the southern african seismic model with seismic models for other shield regions where nodule data are not available shows that the nodule results may also be representative of the upper mantle in those regions

the petrogenesis of a type magmas from the amram massif southern israel the 550 530ma alkaline magmatic suite of the amram massif southern israel was emplaced during the transition from an orogenic to an intra plate tectonic setting in the northeastern arabian nubian shield ans the suite ranges from 45 6t o 788 wt sio2 and consists of rhyolites alkali quartz syenites quartz syenites monzonites and co magmatic mafic to felsic alkaline dikes these rocks define a continuous chemical evolutionary trend and reveal a correlation between decreasing stratigraphic age and increasing silica content the felsic members of the suite display a type characteristics and are genetically linked through fractionation to the more mafic ones moderately positive initial end values a2 05 low initial 87 sr 86 sr values 07036 2 high mgo and fe2o3 concentrations 410 895 and 100 125 wt respectively and relatively flat rare earth element patterns la yb na 64 09 in the amram mafic dikes 456 495 wt sio2 suggest their derivation from the sub continental lithospheric mantle above the garnet stability zone the melts program was used to quantitatively model the chemical evolution of the suite extensive anhydrous fractionation 490 of plagioclase alkali feldspar clinopyroxene olivine and minor timagnetite and apatite from parental mafic magmas represented by the amram mafic dikes produced the rhyolitic compositions as well as the intermediate members of the suite this suggests the presence of a large unexposed body of cumulate rocks at depth as well as fusion of a large source region equivalent to an 5km layer in the lithospheric mantle regarded as a representative example for similar a type outcrops in this region this petrogenetic model further suggests that neoproterozoic early cambrian a type magmatism in the northeastern ans represents a significant post orogenic addition of mantlederived material to the juvenile crust this magmatic episode was of a similar magnitude to that of the cenozoic extensionrelated alkaline volcanism of the arabian plate

anisotropic models of the upper mantle long period rayleigh wave and love wave dispersion data particularly for oceanic areas have not been simultaneously satisfied by an isotropic structure in this paper available phase and group velocity data are inverted by a procedure which includes the effects of transverse anisotropy anelastic dispersion sphericity and gravity we assume that the surface wave data represents an azimuthal average of actual velocities thus we can treat the mantle as transversely isotropic the resulting models for average earth average ocean and oceanic regions divided according to the age of the ocean floor are quite different from previous results which ignore the above effects the models show a low velocity zone with age dependent anisotropy and velocities higher than derived in previous surface wave studies the correspondence between the anisotropy variation with age and a physical model based on flow aligned olivine is suggestive for most of the earth sh sv in the vicinity of the low velocity zone neat the east pacific rise however sv sh at depth consistent with ascending flow anisotropy is as important as temperature in causing radial and lateral variations in velocity the models have a high velocity nearly isotropic layer at the top of the mantle that thickens with age this layer defines the lid or seismic lithosphere in the pacific the lid thickens with age to a maximum thickness of 50 km this thickness is comparable to the thickness of the elastic lithosphere the lid thickness is thinner than derived using isotropic or pseudo isotropic procedures a new model for average earth is obtained which includes a thin lid this model extends the fit of a prem type model to shorter period surface waves

upper mantle shear and compressional velocity structure of the central us craton shear wave low velocity zone and anisotropy one dimensional upper mantle velocity structure is estimated by modeling p sv and sh body waveforms sampling the central united states the resulting model features a thick mantle lid with a positive velocity gradient a significant shear wave low velocity zone 6 velocity reduction is required below the lid however the compressional velocities are not reduced the paths studied lie along the fast axes of teleseismic shear wave splitting measurements and absolute plate motion we observe splitting of upper mantle refracted shear waves with the transverse component slow relative to the radial component in order to fit the shear wave arrivals and satisfy teleseismic shear wave splitting observations we infer azimuthal anisotropy maximum 1 6 distributed throughout the mantle lid and low velocity zone with increasing anisotropy with depth

anisotropic upper mantle stratigraphy and architecture of the slave craton the earth s physical properties show a dominantly radial structure which is the result of compositional differentiation isochemical phase changes1 and rheological layering2 rheological layering is perhaps the most difficult to investigate using conventional seismological techniques because the seismic manifestation of this property elastic anisotropy may closely mimic the effects of isotropic heterogeneity3 nonetheless an improved characterization of earth s rheological structure promises important insights into such processes as plate dynamics and continental evolution here i present a methodology for effectively characterizing sharp transitions in anisotropic upper mantle structure using the coda of teleseismic p waves application to seismograms from the slave craton reveals a well developed stratigraphy at least in part anisotropic with major boundaries occurring at nominal depths of 75 135 and 195 km the geometry and sharpness of these discontinuities suggest a structural origin perhaps involving shallow subduction

high velocities in the uppermost mantle of the siberian craton abstract anomalous high seismic velocities up to 8 8 km s have been reported suvorov et al 1985 in the mantle beneath the kimberlite areas of siberia high mantle velocities were also observed on some geon center profiles carried out using nuclear explosions egorkin et al 1987 to obtain clear general signatures of the mantle structure in siberia and the role of the high velocity blocks the geon data were analyzed and compared with other geophysical information this analysis showed that the largest blocks of 300 400 km width with velocities up to 8 6 km s are observed beneath the tunguss depression and in the southern part of the craton near its boundary with the baikal rift zone the blocks penetrate to depths of 100 110 km where they are underlain by low velocity zones or a horizontally homogeneous layer there is no evidence of these blocks in the gravity field velocity anisotropy is proposed as a main cause of the anomalous high velocities although the crossing profiles show that such velocities are observed in different directions the yakutia kimberlite fields are located in the region with average sub moho velocity of 8 3 8 4 km s but with higher velocity at depths of 70 80 km

the p wave upper mantle structure beneath an active spreading centre the gulf of california summary detailed analysis of short period travel time dt d and wave form data reveals the upper mantle structure beneath an oceanic ridge to depths of 900 km more than 1400 digital seismograms from earthquakes in mexico and central america recorded at scarlet yield 1753 travel times and 58 direct measurements of dt d as well as high quality stable waveforms the 29 events combine to form a continuous record section from 9 to 40 with an average station spacing of less than 5 km first the travel times are inverted using the tau method of bessonova et al the resultant model is adjusted to agree with the experimental p values further constraints arise from the observed relative amplitudes of mantle phases which are modelled by trial and error using wkbj synthetic seismograms chapman wiggins model gca which is appropriate for western mexico north of 20 latitude is similar to existing upper mantle models for shield tectonic continental and arc trench regimes below 400 km but differs significantly in the upper 350 km gca velocities are very low in this depth range the model catches up with the others with a very large velocity gradient from 225 to 390 km this well resolved feature is consistent with the shear wave model tna for western north america of grand helmberger the abundant data from 20 to 30 control the detailed shape of the 660 km discontinuity very large velocity gradients lie both above 620 660 km and below 661 680km a 2 8 per cent velocity change

upper mantle heterogeneity in the northern part of eurasia abstract a two dimensional velocity model of the upper mantle has been compiled from a long range seismic profile crossing the west siberian young plate and the old siberian platform it revealed considerable horizontal and vertical heterogeneity of the mantle a sharp seismic boundary at a depth of 400 km outlines the high velocity gradient transition zone its base lying at a depth of 650 km several layers with different velocities velocity gradients and wave attenuation are distinguished in the upper mantle they likewise differ in their inner structure for instance the uppermost 50 70 km of the mantle are divided into blocks with velocities from 7 9 8 1 to 8 4 8 6 km s 1 comparison of the travel time curves for the siberian long range profile with those compiled from seismological data for europe distinguished large scale upper mantle inhomogeneities of the eurasian continent and allowed for the correlation of tectonic features and geophysical fields the velocity heterogeneity of the uppermost 50 100 km of the mantle correlates with the platform age and heat flow i e the young plates of western europe and western siberia have slightly lower velocities and higher heat flows than the ancient east european and siberian platforms at greater depths 150 250 km the upper mantle velocities increase from the ocean to the inner parts of the continent the structure of the transition zone differs significantly beneath western europe and the other parts of eurasia the sharp boundary at a depth of 400 km traced throughout the whole continent as the boundary reflecting intensive waves transforms beneath western europe into a gradient zone this transition zone feature correlates with positions of the north atlantic west europe geoid and heat flow anomalies

lateral heterogeneity of europe s upper mantle as inferred from modelling of broad band body waves summary the upper mantle velocity structure beneath europe is investigated by body wave modelling of broad band seismograms from earthquakes in the eastern mediterranean recorded by stations in north west europe the gross features of the p and s wave velocity distribution as inferred from the data show a high velocity lid a low velocity zone and two discontinuities at depths of approximately 420 and 650 km the seismograms also present evidence for lateral heterogeneity of at least a few per cent 2 5 per cent for p 4 per cent for s to a depth of approximately 400 km many of the inferred velocity variations can be related to the tectonic setting of the region between source and receiver broad band seismograms are essential for such detailed study of upper mantle structure using body wave modelling

observation of high frequency teleseismic p n on the long range quartz profile across northern eurasia short period three component recordings of the seismic wave field of peaceful nuclear explosions pnes on long range profiles are used to determine the fine structure of the mantle lithosphere by analyzing the frequency content of the recorded phases and applying different band pass filters to the data it is possible to divide the wave field into two distinctly different constituents low frequency body waves traveling along fermat paths first arrivals and the high frequency teleseismic or long range pn phase traveling with a group velocity of 8 1 km s and accompanied by a long incoherent coda this high frequency teleseismic pn phase is observable from about 750 km where it separates from the faster first arrival to the maximum recording distance of 3145 km it is recorded from shots at different locations and appears to be almost unaffected by the major tectonic feature along the profile the urals the frequency spectrum of this pn phase contains more high frequency energy up to 12 hz than first arrivals that penetrate deeper into the upper mantle the teleseismic high frequency pn arrival has a remarkable coda which is incoherent between closely spaced stations the coda duration is dependent on the component of motion being shortest on the vertical and longest on the transverse component we propose a velocity model that is characterized by a zone extending from the crust mantle boundary to a depth of about 100 km this zone has randomly distributed spatially anisotropic velocity fluctuations we propose that these velocity heterogeneities are stretched in the horizontal direction this zone forms a scattering waveguide that confines the high frequency teleseismic pn there are indications that below this pn waveguide either the scale of the velocity fluctuations or the q factor changes this is expressed in a separation of the teleseismic pn phase from the phase diving deeper into the mantle lithosphere

on the lehmann discontinuity the lehmann discontinuity 200km in the upper mantle is proposed to result from a change in the preferred orientation of olivine as a result of a change in deformation mechanism in the shallow upper mantle deformation is due to dislocation creep and results in an anisotropic structure whereas in the deeper upper mantle diffusion creep becomes dominant due to a smaller activation volume resulting in an isotropic structure for anisotropy due to dislocation glide corresponding to a horizontal flow the resultant anisotropy will yield vsh vsv and vph vpv thus for waves propagating nearly vertically e g scs waves there will be discontinuous jumps in the s and p wave velocities at this discontinuity in this model the depth of the lehmann discontinuity depends on the deformation conditions such as stress level geotherm water fugacity and grain size giving a natural explanation for its observed variation with tectonic setting

residual liquid origin for a monzonitic intrusion in a mid proterozoic anorthosite complex the sybille intrusion laramie anorthosite complex wyoming the sybille intrusion 100 km 2 is one of three large monzonitic intrusions in the 1 43 ga laramie anorthosite complex of southeastern wyoming the petrographic geochemical isotopic and geophysical characteristics of sybille monzonitic rocks are consistent with an origin by extensive crystallization of liquids residual to nearby anorthositic cumulates ferrodiorites and contamination by archean wall rocks the exposed part of the intrusion is composed mainly of coarse grained monzosyenites with abundant alkali feldspar phenocrysts the monzosyenites preserve mineralogical evidence for high crystallization temperatures 1000 c mid crustal emplacement pressures 3 kbar relatively reduced crystallization conditions 2 log units below the fayalite magnetite quartz fmq oxygen buffer and they crystallized in the presence of a co2 rich fluid phase fuhrman et al 1988 frost and touret 1989 the eastern monzosyenites those adjacent to contemporaneous anorthosite are distinguished by an anhydrous mineral assemblage fo16 fo8 olivine high ca pyroxene lacking modal quartz silica contents of 60 wt and smaller eu anomalies eu eu 1 2 to 1 3 abundant xenoliths of archean wall rocks and anorthosite from the adjacent intrusions in all monzosyenites attest to a stoping emplacement mechanism near the roof of the chamber we propose that the monzosyenites represent a relatively thin 0 5 1 0 km thick roof to a magma chamber dominated by dense ferrodioritic cumulates at depth extensive open system fractionation of a ferrodioritic parent magma residual after crystallization of anorthosite produced fe enriched monzodioritic and or monzonitic magma in the upper part of the chamber and complementary fe and ti rich cumulates in the lower levels we have corroborated the production of monzonitic liquids from crystallization of ferrodiorite through a series of reconnaissance equilibrium crystallization experiments the presence of dense ferrodioritic cumulates at depth is consistent with the prominent positive gravity anomaly associated with the sybille intrusion hodge et al 1973 in the upper parts of the chamber the fractionated monzodioritic and or monzonitic magmas eventually became saturated in alkali feldspar owing to density contrasts the alkali feldspar phenocrysts floated to the roof of the chamber thus producing the exposed porphyritic monzosyenites in addition the roof of the chamber was the site of significant melting of archean gneiss and locally metapelite the sr and nd isotopic compositions of the monzosyenites with sr isotopic ratios becoming increasingly radiogenic from east i sr 0 7059 and initial nd 2 5 to west i sr 0 7092 and initial nd 2 6 are consistent with a 5 to 15 addition of archean orthogneiss to a ferrodioritic parent magma that had isotopic characteristics similar to adjacent anorthositic rocks the stratigraphic and compositional similarity of the sybille monzosyenites to mangerites in the bjerkreim sokndal intrusion of the rogaland anorthosite complex southern norway indicates that similar open system magmatic processes are capable of having produced high temperature k rich monzonitic rocks in other proterozoic anorthosite complexes

numerical simulations of depth dependent anisotropy and frequency dependent wave propagation effects a numerical investigation of the effects of shear wave splitting for vertical propagation in a smoothly varying anisotropic medium is presented through forward modeling we predict the olivine lattice preferred orientation lpo developed in the oceanic upper mantle in response to the absolute plate motion apm we consider the effect of a change in apm similar to the one that presumably caused the kink in the emperor hawaii seamount island chain in the north pacific this results in an oblique orientation between lithospheric and asthenospheric anisotropy numerical simulations of shear wave propagation are used to estimate the characteristics of shear wave splitting ray theory does not account for coupling between shear waves in the depth dependent anisotropic medium due to the implicit assumption of high frequency a forward propagator technique for calculating waveforms and splitting parameters is used to assess frequency dependent effects the results show that ray theory is valid for estimating the splitting only for frequencies above 1 hz at frequencies more realistic for sks propagation apparent splitting parameters exhibit a 2 dependence on the incoming shear wave polarization back azimuth for certain back azimuth ranges shear wave splitting is very frequency dependent with apparent delay times ranging from 1 to 4 s and apparent fast polarization directions changing rapidly by up to 80 thus stacking of shear wave splitting measurements for largely different initial polarizations and frequencies should be avoided depth dependent anisotropy implies that shear wave splitting analyses will be sensitive to filtering anisotropic depth variations cannot be resolved unambiguously from splitting observations at relatively long periods 5 s it is not possible for instance to discriminate between smooth and abrupt transitions separating the anisotropic regions shorter period waveforms provide further information on the fine structure of anisotropic depth variations a comparison between splitting calculations and observations from hawaii suggests a divergent past apm direction or may indicate an alternative mechanism responsible for the lithospheric anisotropy

feldspathic rocks on mars compositional constraints from infrared spectroscopy and possible formation mechanisms rare feldspar dominated surfaces on mars were previously reported based on near infrared nir spectral data and were interpreted to consist of anorthosite or felsic rocks using thermal infrared tir data over the feldspar detections with the largest areal extent in nili patera and noachis terra we rule out felsic interpretations basaltic or anorthositic compositions are consistent with tir measurements but the geologic contexts for these regions do not support a plutonic origin laboratory nir spectral measurements demonstrate that large plagioclase crystals 840 m can be detected in mixtures with as much as 50 vol mafics which is higher than the previously stated requirement of no more than 15 mafics thus anorthositic or felsic interpretations need not be invoked for all nir based feldspar detections plagioclase enriched basaltic eruptive products can be formed from martian basalts through partial crystallization at the base of a thick crust followed by low pressure crystallization of the residual liquids

magma mixing and crustal recycling recorded in ternary feldspar from compositionally zoned peralkaline ignimbrite a gran canaria canary islands miocene ignimbrite a on gran canaria contains three compositional endmember fiamme types two rhyolites and one trachyte each of which crystallized distinct feldspar various textural and compositional criteria are interpreted as reflecting a complex scenario within the magma chamber in which the crystals formed about 25 30 of the feldspar phenocrysts contain evidence for magma mixing in the form of 1 partial to severe dissolution resorption rims 2 distinct zones of drastically different compositions and 3 overgrowth textures on formerly resorbed crystals four major types of zoning in the oligoclase to anorthoclase feldspars of ignimbrite a include a normal and a reversely zoned type and two complexly zoned types the feldspars with normal and reverse zonation show only minor compositional amplitudes between individual zones ab or 4 whereas the complexly zoned types show compositional differences between zones of up to 18 mol ab and 20 mol or and are commonly associated with an internal dissolution surface complex zoning with large compositional amplitudes and dissolution textures is taken as evidence of crystal movements within the magma and across compositional boundaries between magma batches a multiple step cycle model involving growth and transport of a crystal into another magma batch and its return to the original host magma is suggested by the data moreover feldspars from one rhyolite compositional group are found to be substantially elevated in 18o suggesting an input of a high 18o component to this rhyolite the other endmember rhyolite appears to be related to the endmember trachyte by mainly crystal fractionation of anorthoclase feldspar this observation is consistent with trace element and rare earth element concentrations for the magma endmembers and their feldspars where contamination led to a depletion in incompatible trace elements and light rare earth elements in the contaminated rhyolite and its feldspar phenocrysts we suggest that the combination of textural and compositional variation in ternary feldspar of peralkaline rhyolitic systems is well suited to reconstruct dynamic processes such as magma mixing and contamination in evolving rhyolitic magma chambers

reduced rapakivi type granites the tholeiite connection reduced rapakivi type granites are the most iron enriched and reduced i e least oxidized of the anorogenic granite association the low oxygen fugacity and chemical composition of these granites severely limit their sources in this paper we argue that reduced rapakivi type granites and their eruptive equivalents high potassium fayalite rhyolites are derived from mafic sources because tholeiitic magmas and their derivatives have the required low oxygen fugacity reduced rapakivi type granites are produced either by extreme differentiation of basaltic melts or by partial melting of underplated basalts and their differentiated equivalents they form in extensional environments where the asthenosphere is present at shallow depths we envision three stages in the origin of these rocks 1 tholeiitic melts are emplaced at the base of the crust 2 continued introduction of heat partially remelts these tholeiitic rocks and 3 the hot dry melts so produced migrate into the middle crust to produce rapakivi batholiths or erupt as rhyolites partial melting of felsic continental crust may accompany the intrusion of rapakivi type magmas thereby producing the other metaluminous and peraluminous granite compositions of the anorogenic suite

seismological structure of the upper mantle a regional comparison of seismic layering abstract we investigate seismic layering i e discontinuities regions of anomalous velocity gradients and anisotropy and its lateral variability in the upper mantle by comparing seismic models from three tectonic regions old 100 ma pacific plate younger 40 ma philippine sea plate and precambrian western australia these models were constructed by combining two data sets scs reflectivity profiles which provide travel times and impedance contrasts across mantle discontinuities and observations of frequency dependent travel times of three component turning s ss ss sss sss sa and surface r1 g1 waves which constrain the anisotropic velocity structure between discontinuities the models provide a better fit to observed seismograms from these regions than the current generation of global tomographic models the australian model is characterized by high shear velocities throughout the upper 350 km of the mantle with no low velocity zone lvz in the isotropically averaged shear velocities in contrast the oceanic models are characterized by a thin high velocity seismic lid underlain by a distinct lvz with a sharp boundary the g discontinuity separating them the g is significantly deeper beneath the western philippine sea plate than beneath the older pacific 89 and 68 km respectively implying that thermal cooling alone does not control the thickness of the lid we interpret this discontinuity as a compositional boundary marking the fossilized base of the melt separation zone msz active during sea floor spreading no discontinuity is detected at the base of the lvz in the oceanic models the s velocity gradient between 200 and 410 km depth is much steeper in the oceans than beneath australia this high oceanic gradient is probably controlled by a decrease in the homologous temperature over this depth interval the relative depths of the transition zone tz discontinuities are consistent with clapeyron slopes expected for an olivine dominated mineralogy the 660 km discontinuity displays variability in its amplitude that appears to correlate with its depth shallow and bright beneath the pacific deep and dim beneath australia and the philippine sea such behavior is possibly caused by the juxtaposition of the olivine and garnet components of the phase transition radial anisotropy extends through the upper 250 km of the mantle in the australia model and through the upper 160 km of the two oceanic models the magnitude of anisotropy is consistent with that expected for models of horizontally oriented olivine and the localization of anisotropy in the shallowest upper mantle implies that it reflects strain associated with past or present tectonic events

upper mantle velocity structure beneath southern africa from modeling regional seismic data the upper mantle seismic velocity structure beneath southern africa is investigated using travel time and waveform data which come from a large mine tremor in south africa mb 5 6 recorded by the tanzania broadband seismic experiment and by several stations in southern africa the waveform data show upper mantle triplications for both the 410 and 670 km discontinuities between distances of 2100 and 3000 km auxiliary travel time data along similar profiles obtained from other moderate events are also used p wave travel times are inverted for velocity structure down to 800 km depth using the wiechert herglotz technique and the resulting model is evaluated by perturbing it at three depth intervals and then testing the perturbed model against the travel time and waveform data the results indicate a typical upper mantle p wave velocity structure for a shield p wave velocities from the top of the mantle down to 300 km depth are as much as 3 higher than the global average and are slightly slower than the global average between 300 and 420 km depth little evidence is found for a pronounced low velocity zone in the upper mantle a high velocity gradient zone is required above the 410 km discontinuity but both sharp and smooth 410 km discontinuities are permitted by the data the 670 km discontinuity is characterized by high velocity gradients over a depth range of 80 km around 660 km depth limited s wave travel time data suggest fast s wave velocities above 150 km depth these results suggest that the bouyant support for the african superswell does not reside at shallow depths in the upper mantle

upper mantle compressional velocity structure beneath the northwest atlantic ocean a compressional velocity model for the northwest atlantic ocean is derived by modeling waveform and travel time data from long period and short period world wide standard seismograph network and canadian network station recordings a 90 km thick lid with a velocity of 8 1 km s at the top gradually increasing to 8 3 km s at the bottom is obtained by fitting the travel time data of first arrivals and waveform data of almost purely oceanic paths at distances of 8 20 triplication p waveform data constrain the structure below the lid a distinct but smooth low velocity zone is located at a depth of 170 km combining these results with the shear wave structure derived by grand and helmberger 1984a for the same region we suggest an olivine rich mineralogy in the upper 100 km and partial melting possibly extending as deep as 300 km based on low seismic velocities and high vp vs

petrogenesis of the 1 43 ga sherman batholith se wyoming usa a reduced rapakivi type anorogenic granite the 1 43 ga sherman batholith southeastern wyoming usa origin that are found throughout the proterozoic anorogenic granite provinces of the southwestern usa adirshows extreme a type petrochemical characteristics compared with other mid proterozoic granite batholiths of north america it ondacks eastern canada southern greenland and the fennoscandian shield anderson 1983 phanerozoic consists of 1 the sherman granite a coarse grained biotite hornblende granite that locally contains fayalite and pyroxenes 2 examples include paleozoic granites of southeastern australia collins et al 1982 and of corsica poitrasson et the lincoln granite a medium grained biotite granite 3 a porphyritic biotite hornblende granite that probably formed by interaction al 1995 and the mesozoic nigerian younger granites jacobsen et al 1958 and the white mountain magma of granitic and mafic magmas and 4 iron enriched mafic dikes and pods the ilmenite series metaluminous sherman granite series foland allen 1991 anorogenic granite refers to plutons that are not associated with compressional exhibits extreme values of feo feo mgo and is rich in structures and that were emplaced long after any known k ree nb and y it crystallized at temperatures exceeding orogenic event anorogenic or a type granites loiselle 900 c and a pressure of 2 5 kbar with water activity of 0 7 wones 1979 are characterized by low h2o and o2 and dlog fo2 of 0 1 to 0 5 the lincoln granite which is fugacities along with high feo feo mgo k2o peraluminous and has less extreme a type geochemical characteristics na2o and k2o contents a type granites typically are rich crystallized at temperatures as low as 750 c and dlog fo2 of in incompatible elements including rare earth elements around 0 5 units above fmq fayalite magnetite quartz the ree zr nb and ta but poor in co sc cr ni ba rocks of the sherman batholith are chemically equivalent to lavas sr and eu minimum magma liquidus temperatures are from the yellowstone hotspot like the yellowstone magmas the 900 1000 c clemens et al 1986 creaser white sherman batholith probably originated by partial melting of under1991 some workers suggest these rocks are derived plated mantle derived mafic rocks from melting of tonalitic or more felsic crust anderson cullers 1978 collins et al 1982 clemens et al 1986 creaser et al 1991 whereas others relate the granites

can crystallization of olivine tholeiite give rise to potassic rhyolites an experimental investigation experiments were conducted to determine whether the rhyolites and basalts of the intraplate silica saturated potassic suites could be genetically related through crystallization extreme crystallization 96 97 of a high mgo 10 62 wt olivine tholeiite from the snake river plain with an initial bulk water content of 0 4 wt at a mid crustal pressure of 4 3 kbar generated potassic rhyolitic liquids similar in major element chemistry to those found in the quaternary rhyolite domes of the snake river plain and their plutonic equivalents in the proterozoic laramie anorthosite complex residual liquids comparable in composition to the bulk rock compositions of intermediate rocks found at the craters of the moon and cedar butte eruptive centers in the snake river plain are also generated along this crystallization path