the lost inca plateau cause of flat subduction beneath peru abstract since flat subduction of the nazca plate beneath peru was first recognized in the 1970s and 1980s a satisfactory explanation has eluded researchers we present evidence that a lost oceanic plateau inca plateau has subducted beneath northern peru and propose that the combined buoyancy of inca plateau and nazca ridge in southern peru supports a 1500 km long segment of the downgoing slab and shuts off arc volcanism this conclusion is based on an analysis of the seismicity of the subducting nazca plate the structure and geochemistry of the marquesas plateau as well as tectonic reconstructions of the pacific farallon spreading center 34 to 43 ma these restore three sub parallel pacific oceanic plateaus the austral tuamotu and marquesas to two farallon plate counterparts the iquique and nazca ridges inca plateau is apparently the sixth and missing piece in an ensemble of v shaped hotspot tracks formed at on axis positions we argue the mirror image of the inca plateau the marquesas plateau is an ancient edifice overprinted by recent volcanism in disagreement with the widely accepted young

signatures of rift environment in the production of garnet amphibolites and eclogites from tso morari region ladakh india a geochemical study abstract the geochemistry of eclogites and garnet amphibolites from tso morari region ladakh india has been investigated to characterize their protoliths on the basis of immobile elements especially trace elements including ree the eclogites and garnet amphibolites have coherent compositions except for the uhp metamorphic minerals being preserved in eclogites compositionally the metabasites range from depleted to enriched and span from within plate basalts wpb to morb fields and match with various enriched or transitional morb types e g on ti zr y and nb zr y ternary plots isotopically they have sr i ratio 0 706 which is similar to some of the ocean island basalt oib the rocks under study suggest that the enriched components are probably derived by melting of a mantle source with an enriched oib type component rather than due to the crustal contamination we propose a rift environment for their protoliths and relate to advanced intra continental rift situation furthermore our geochemical studies envisage an initial phase of plume activity cambrian or earlier resulting in basaltic magma in the eclogitic layers at sub lithospheric levels wherein they were subjected to crystallization under ultra high pressure conditions at a later stage the reactivation of faults probably during permo triassic times acted as channels for the emplacement of the high pressure rocks in the continental crust subsequently the ultra high pressure rocks got re equilibrated as amphibolites with some remaining as relict eclogites which later got exposed to the surface during various phases of the himalayan uplift

upper mantle flow beneath french polynesia from shear wave splitting summary upper mantle flow beneath the south pacific is investigated by analysing shear wave splitting parameters at eight permanent long period and broad band seismic stations and 10 broad band stations deployed in french polynesia from 2001 to 2005 in the framework of the polynesian lithosphere and upper mantle experiment plume despite the small number of events and the rather poor backazimuthal coverage due to the geographical distribution of the natural seismicity upper mantle seismic anisotropy has been detected at all stations except at tahiti where two permanent stations with 15 yr of data show an apparent isotropy the median value of fast polarization azimuths n67 5 w is parallel to the present pacific absolute plate motion direction in french polynesia apm n67 w this suggests that the observed sks fast polarization directions result mainly from olivine crystal preferred orientations produced by deformation in the sublithospheric mantle due to viscous entrainment by the moving pacific plate and preserved in the lithosphere as the plate cools however analysis of individual measurements highlights variations of splitting parameters with event backazimuth that imply an actual upper mantle structure more complex than a single anisotropic layer with horizontal fast axis a forward approach shows that a two layer structure of anisotropy beneath french polynesia better explains the splitting observations than a single anisotropic layer secondorder variations in the measurements may also indicate the presence of small scale lateral heterogeneities the influence of plumes or fracture zones within the studied area does not appear to dominate the large scale anisotropy pattern but may explain these second order splitting variations across the network

atypical hotspot chains evidence for a secondary melting zone below the marquesas french polynesia the atypical age distance to the hotspot relationships observed for the marquesas linear chain which present a considerable scatter are best explained considering 1 a pacific plate motion of 10 5 cm yr 1 in the n115 e direction and 2 rejuvenation of volcanism in ua huka island new k ar ages show that the main hotspot activity in ua huka emplaced successively shield olivine tholeiites and post shield alkali basalts between 3 11 0 04 and 2 43 0 04 ma then after a quiescence period of 1 28 0 06 myr two small basanitic volcanoes were emplaced between 1 15 0 02 and 0 763 0 013 ma with respect to the main hotspot volcanics their lavas originated from weaker partial melting rates of a deeper source field petrologic and geochemical arguments are consistent with the existence of a secondary melting zone located c 140 km downstream the marquesas hotspot which produced the rejuvenated volcanics

basalts from mururoa fangataufa and gambier islands french polynesia geochemical dependence on the age of the lithosphere abstract basaltic volcanics from mururoa fangataufa and gambier islands which belong to the pitcairn island chain have been studied for trace elements and sr nd and pb isotopic composition while the trace element ratios remain constant the 87 sr 86 sr ratios range from 0 70292 to 0 70367 143 nd 144 nd ranges from 0 51303 to 0 51288 and the 206 pb 204 pb isotopic composition ranges from 18 99 to 19 62 these values which are clearly distinct from those of pitcairn basalts vary according to distance from the present hot spot and are correlated with the age of the underlying lithosphere it is suggested that the basalts studied were produced by variable degrees of partial melting from an oceanic lithospheric plate metasomatized by prior passage over the easter hot spot

evidence of an early alteration process driven by magmatic fluid in mururoa volcano evidence for a deuteric alteration process induced by a magmatic fluid has been found in the feeder zone of the mururoa volcano french polynesia within the dikes where basaltic glass does not show any evidence of pervasive alteration vesicles are filled with dioctahedral smectites and calcite while olivine phenocrysts are replaced by dioctahedral smectites ankerite and calcite the 13c signature of carbonates the carbon and h2o content of the whole rocks and their impoverishment in deuterium are compatible with the presence of magmatic co2 during the crystallization of intruding lavas and exclude contamination by seawater mass balance calculations on selected thin sections photographs of partly filled up vesicles and replaced olivine crystals constrain assuming a closed system interaction the chemical composition of the initial fluid and the respective amounts of the initial solid phases involved in the alteration process thermodynamic modelings using the eq3 6 software package correctly predict the mineralogic chemical and isotopic exchanges accompanying alteration thus validating the closed system assumption the model which allows prediction of the influence of co2 on the alteration products shows that above a 0 25 co2 mole fraction in the initial fluid the alteration is entirely controlled by the chemical composition of the initial solid phases the presence of co2 implies the precipitation of dioctahedral smectites and carbonates instead of the magnesian smectites commonly observed in co2 free systems the mururoa feeder zone shows alteration features typical of a closed system interaction between the basaltic rock and a magmatic fluid in which seawater did not take part

assimilation of ocean crust by hawaiitic and mugearitic magmas an example from eiao marquesas abstract three holes were drilled in the northern dominique central sophie and southern naore parts of the volcanic shield of eiao island in the marquesas archipelago french polynesia these were continuously cored with an excellent recovery and reached depths of 500 m sophie and 800 m dominique and naore k ar dating indicates that the volcanic sequence penetrated was emplaced between 5 52 0 05 ma and 4 95 0 04 ma petrographic types include tholeiites alkali basalts picrobasalts hawaiites mugearites and trachytes intermediate lavas hawaiites and mugearites are fairly common they occur within stratigraphic units denoted d1 d2 d3 s1 s2 s3 n1 and n2 respectively d dominique s sophie n naore the d3 s1 and n1 hawaiites and mugearites appear to be derived from fractional crystallization of parental alkali basalt magmas the other groups are characterized by variable enrichments in sr d1 d2 and s3 p2o5 d2 th s2 and n2 and light rare earth elements d2 or by depletion in heavy rare earth elements s2 and n2 we suggest that these trace element characteristics reflect open system differentiation processes which occurred during the building of eiao volcanic shield two stages of assimilation coupled with fractional crystallization afc are proposed the calculated compositions of the two contaminants suggest that they were leucocratic materials located at different levels of the oceanic crust finally a water induced assimilation process is invoked to explain the origin of the s3 hawaiites these lavas are enriched in al2o3 na2o and sr inferred to reflect plagioclase breakdown within the eiao plumbing system

evolution of hawaiian basalts a hotspot melting model melt generation and extraction along the hawaiian volcanic chain should be largely controlled by the thermal structure of the hawaiian swell and the heat source underneath it we simulate numerically the time and space dependent evolution of hawaiian volcanism in the framework of thermal evolution of the hawaiian swell constrained by residual topography geoid anomalies and anomalous heat flow along the hawaiian volcanic chain the transient heat transfer problem with melting relationships and variable boundary conditions is solved in cylindrical coordinates using a finite difference method the model requires the lithosphere to be thinned mechanically by mantle plume flow melting starts quickly near the base of the plate when the hotspot is encountered thermal perturbation and partial melting are largely concentrated in the region where the original lithosphere is thinned and replaced by the mantle flow the pre shield loihi alkalic and tholeiitic basalts are from similar sources which are a mixture of at least three mantle components the mantle plume asthenosphere and the lower lithosphere the degree of partial melting averages 10 20 with a peak value of 30 near the plume center as a result of continuous compaction melts are extracted from an active partial melting zone of about 10 20 km thickness which moves upwards and laterally as the heating and compaction proceed the rate of melt extraction from the swell increases rapidly to a maximum value of 1 105 km3 m y over the center of the heat source corresponding to eruption of large amounts of tholeiitic lavas during the shield building stage this volume rate is adequate to account for the observed thickness of the hawaiian volcanic ridge melts from direct partial melting of the mantle plume at depth may be important or even dominant at this stage although the amount is uncertain at the waning stage mixing of melts from the mantle flow pattern with those from low degree partial melting of the lithosphere may produce postshield alkalic basalts after the plate moves off the heat source continuous conductive heating can cause very low degree partial melting less than 1 of the lithosphere at shallow depths for about one million years this process may be responsible for producing post erosional alkalic basalts the extraction time for removing such small amount of melts is about 0 4 2 m y similar to the time gap between the eruption of post erosional alkalic lavas and the shield building stage our results show that multi stage hawaiian volcanism and the general geochemical characteristics of hawaiian basalts can be explained by a model of plume plate interaction

quaternary atoll development new insights from the two dimensional stratigraphic forward modelling of mururoa island central pacific ocean knowledge about the quaternary evolution of mid ocean atolls comes mainly from drilling and field observations carried out on a number of pacific carbonate islands however little is known about the early to mid pleistocene atoll development history especially at margin and foreslope settings using previous field and subsurface data from mururoa atoll and a process based modelling software dionisos a two dimensional forward stratigraphic model of atoll development is proposed for the past 1 8 million years myr observational data from vertical to inclined coring seismic and bathymetric surveys indicate that from approximately 0 45 to 0 40 million years before present ma carbonate deposition at mururoa atoll resulted in a series of mostly prograding reef units the model is first constrained at the base by the shape and topography of the pre quaternary basement a number of sensitivity tests were performed to define the respective influence of variant parameters the best fit development scenario that accounts for the overall geometry and stratigraphic architecture of the quaternary sediment packages is obtained by using the sea level curve by miller et al 2005 uniform subsidence rate of 105 m myr 1 and carbonate production rates gradually increasing from 0 50 to 8 mm yr 1 between 1 80 ma and the present additional controlling parameters include subaerial erosion at a constant rate of 0 25 m kyr wave energy and sediment transport processes the stratigraphic forward model predicts a succession of three distinct types of carbonate systems that have developed since the earliest pleistocene toe of slope systems from 1 80 ma to about 0 80 ma open platform systems from 0 80 ma to 0 50 ma and framework reef systems from about 0 50 ma to the present the development of these different systems is most likely to be controlled by climate and changes in sea level cycles during the low amplitude 41 kyr cycle periods of the earliest pleistocene ambient conditions were not conducive to framework reef growth shallow water carbonate sedimentation was dominantly gravity driven operating along the platform foreslopes only during the mid pleistocene climate transition narrow open platform units have developed at the upper parts of the pre quaternary basement flanks with the onset of the high amplitude 100 kyr sea level modes and climate restoration reef frameworks started to be generated these models from mururoa agree with a number of previous studies suggesting that most of the true framework reefs were not initiated prior to 0 50 ma mururoa atoll is demonstrated to be a robust analogue for providing more realistic interpretations of the development history of pacific atolls further modelling with three dimensional dionisos could generate better predictions by taking into account hydrodynamic and transport parameters more accurately

short wavelength geoid bathymetry and the convective pattern beneath the pacific ocean summary the geoid and bathymetry data in the pacific ocean are used to shed new light on the structure of upper mantle convection at short wavelengths 200 km the geoid pattern is characterized by parallel lineations lineations however appear less clearly at larger wavelengths 400 or 600 km where the signal due to convective thermal anomalies might be expected the most prominent features among the short wavelength lineations are associated with the well known chains of seamounts and islands other lineations observed in the geoid coincide with computed tracks of other volcanic centres although these tracks are not continuous this suggests that long lived plumes are at the origin of the apparently short chains of seamounts apart from the lineations corresponding to the tracks of classical hotspots there are numerous similar lineations visible on the filtered geoid maps which have the same directions as hotspot tracks these preferential directions and the gravity to bathymetry ratios seem to indicate that the lineations are due to magmas implaced just above or within the lithosphere convective plumes fixed with respect to each other and much more numerous than the classical hotspots could be the source of these magmas a geometrical test of this hypothesis is performed by comparing the lineation pattern with the computed tracks for a set of tentative new hotspots in this way numerous plumes long lived and fixed with respect to each other are found to provide an explanation of the observed small wavelength geoid features

a reinterpretation of the timing position and significance of part of the sacramento mountains detachment fault southeastern california a contact previously considered to be part of the sacramento mountains detachment fault sdf exposed in the sacramento mountains metamorphic core complex is reinterpreted as an unconformity between tertiary rhyolite of eagle peak and cataclastically deformed crystalline lower plate rocks this reinterpretation is based on outcrop scale topographic relief and the absence of deformation along the base of the rhyolite even where underlying rocks are severely deformed and altered the rhyolite is dated at about 14 3 ma lower plate rocks of the sdf comprise gneiss intruded by granodiorite tonalite and leucogranite mylonitic fabrics are variably developed in lower plate rocks and are cut by brittle shear zones that contain ultracataclasite and by high and low angle faults upper plate rocks include gneiss nonmylonitic granodiorite tonalite and granite and middle miocene sedimentary and volcanic rocks on the south side of eagle peak a thick ultracataclasite zone dips about 30 west contains subhorizontal hematite stained slickenlines and is interpreted to be part of the sdf system this ultracataclasite zone is unconformably overlain by more gently dipping undeformed 14 3 ma rhyolite of eagle peak the unconformity records unroofing of a domed sdf after a period of 8 to 9 m y of progressive unloading of the lower plate in an evolving extensional shear zone extension on part of the sdf was accommodated by a system of high angle oblique slip faults

the elastic thickness of the lithosphere in the pacific ocean abstract in this study we present determinations of the effective elastic thickness t e of the oceanic lithosphere along pacific chains or archipelagoes t e is determined by computing the deflection of a continuous elastic plate under the load of volcanoes and constrained by geoid heights provided by seasat in the south central pacific estimates of 14 km for the marquesas and 6 km or less for the pitcairn mururoa gloucester chain are in good agreement with a previous work in this region cook austral and society chains around the line islands chain seasat data reveal that the bathymetry is poorly known preventing fine analysis meanwhile t e looks globally very low 6 km except for three volcanoes but these results may be unreliable the easter chain features low t e values 6 km with no noticeable variation along the chain higher values are found for a samoan island manuae 24 km and along the hawaiian emperor seamounts chain from 32 km at the eastern end of the chain to 21 5 km for the hawaiian volcanoes and from 25 5 to 15 km for the emperor seamounts the large number of t e estimates obtained in this study points out a noticeable difference between north and south pacific results those from the north pacific agree with the general trend increase with the square root of age plate at loading time while those from the south central pacific are much lower according to their plate age these low t e results from the south pacific are only partly explained by taking account of thermal perturbations using the rejuvenation model therefore these results then point out a regional difference in oceanic lithosphere

hydrothermal alteration at mururoa atoll french polynesia abstract petrological study of altered aerial basaltic flows submarine pillow lavas and oceanitic basalts of a rift zone sampled in two drill cores in the mururoa atoll viviane 1000 m depth fuchsia 550 m depth shows that during hydrothermal alteration processes the secondary clay minerals evolve the following three different trends controlled by permeability of the rocks and by rock fluid interaction mg saponite al saponite and al montmorillonite in the basaltic pillow lavas first celadonite saponite assemblage crystallizes in fresh glass rich rocks in altered and brecciated rocks the latter mineral assemblage is followed by fe saponite and mg saponite in breccias the glass is replaced by fe saponite the residual voids are sealed by zeolites mainly phillipsite apophyllite or gypsum indicating an upper temperature limit of 65 c in the aerial basaltic flows massive rocks are affected by celadonite and random mixed layer chlorite saponite crystallizations in the strongly altered rocks veined or brecciated the primary minerals and glass are partly replaced by al saponites and locally by chabazite the same al saponite fills up the veins and constitutes the major part of the cement as rock fluid interaction increases the si al and ca content in the solution increases inducing the late precipitation of aluminous smectite and chabazite in the oceanitic basalts sampled in the rift zone massive rocks are affected by crystallization of secondary clay minerals which is governed by local equilibrium femg clay minerals talc mg saponite celadonite partly replace the olivine phenocrysts and al saponite replaces the glass in veined and brecciated rocks the paragenesis is dioctahedral al smectite silica amorphous for a temperature range 2 content in the solution

formation of water escape structure during shock induced fluidization the role of permeability contrast abstract water escape structure is considered to be a product of fluidization the exact mechanism of formation of this structure has however not been adequately addressed in the present study laboratory experiments were carried out to observe the effect of cyclic loading on water saturated sediment in a close system with an objective to decipher the specific mechanism of formation of water escape structure present study reveals that an optimum permeability contrast between the successive sedimentary layers of a fining upward succession is the prerequisite for formation of water escape structures the onset of fluidization comes into existence when at a critical minor principal stress condition during shearing the underlying coarse grained layer attains drained contractive state and the overlying finer grained layer retains the undrained dilative state in the contractive layer the sediments tend to become more compact through rearrangements of the particles and displacement of the interstitial fluid the pore fluid is squeezed out and the sand structure suffers volume contraction as the upwelling fluid is obstructed by the dilative layer water filled cavities are formed at the interface on progressive shaking these water filled cavities keep on enlarging and finally the expelled water ruptures through the overlying sand layer resulting water escape structure this study also depicts that no influx of external fluid is essential in fluidization of sediments as was believed earlier

the transition from large magnitude extension to distributed dextral faulting in the buckskin rawhide metamorphic core complex west central arizona brittle fault data from the buckskin rawhide metamorphic core complex west central arizona document the miocene transition from large magnitude ne directed extension to distributed e w extension and dextral faulting the buckskin rawhide detachment fault locally records a clockwise rotation of the slip direction from dominant top ne directed slip to ene and e directed slip during the last stages of activity postmylonitic brittle deformation is dominated by normal and dextral slip along moderately to steeply ne dipping faults relative timing relationships indicate that most postmylonitic normal faulting was coeval with middle miocene detachment fault slip whereas dextral and oblique dextral faulting postdates detachment fault slip post 11 ma based on the persistent sw dip of lower plate fabrics and ne dip of abundant normal faults the amount of ne sw brittle extension across the mylonitic lower plate is estimated to be 20 30 in several areas the end of detachment faulting in the buckskin rawhide metamorphic core complex was marked by a transition to dominantly e w extension and associated dextral normal faulting at least 10 ne dipping postdetachment faults record 0 1 1 km dextral or oblique dextral displacement the cumulative amount of dextral shear across the core complex is probably 7 9 km which is the amount needed to restore the topographic trend of lower plate corrugations into alignment with the dominant extension direction postdetachment dextral transtensional faulting across the core complex reflects the increasing influence of the diffuse pacific north america transform plate boundary toward the end of the middle miocene

late stage evolution of the chemehuevi and sacramento detachment faults from apatite u th he thermochronometry evidence for mid miocene accelerated slip the unique low temperature sensitivity of the apatite u th he ahe technique provides valuable new information relating to the late stage thermal evolution of metamorphic core complexes in the southern basin and range province in all the ranges studied the data show greater complexity than previously seen from other thermochronometers in the chemehuevi and northern sacramento mountains a trend of decreasing ages toward the northeast applies only for the structurally shallowest parts of the footwall this is followed by essentially age invariant segments of 15 1 ma reflecting an increase in the rate of detachment fault movement the timing of the change suggested by the data agrees with recently published ahe results from the nearby harcuvar mountains located 125 km to the southeast suggesting that an underlying regional mechanism triggered this change plate reconstructions indicate that the pioneer mendocino fracture zone migrated northward through the area at this time leaving a slab window in its wake we suggest that the additional heat applied to the base of the lithosphere from the slab window resulted in mechanical weakening of the area and that this combined with the extensional strain regime present resulted in an increased rate of slip within the actively extending metamorphic core complexes this agrees well with timing constraints for extension onset in the central basin and range province of southern nevada and suggests that weakening of the southern basin and range province at 15 1 ma was partially responsible for the onset of extension in the central basin and range these findings demonstrate that improved constraints on changes in extension rate within the basin and range province should be possible through the application of the ahe technique to other metamorphic core complex footwalls

ontong java plateau and late neogene changes in pacific plate motion aided by a new geometrical technique called hot spotting we recently proposed a new model for the absolute motion of the pacific plate this model called wk97 assumed the hawaiian hotspot is under loihi and that there was a change in plate motion at 3 6 ma the model located the louisville hotspot close to the hollister ridge a shallow volcanic ridge associated with a geoid high seismicity and recent volcanism most of the tectonic implications of wk97 however were not addressed furthermore although several recent studies suggest that the geochemistry of hollister lavas may indicate a mixture between louisville melt material and pacific antarctic ridge mid ocean ridge basalt these studies strongly disagree with wk97 and consider it seriously flawed here we discuss numerous tectonic predictions of wk97 and present a large amount of observational evidence in support of it we find that a late neogene collision between the ontong java plateau and the northern margin of the australia plate appears to have altered the motion of the pacific plate as inferred from hotspot volcanism by forcing it to rotate counter clockwise this rotation about a low latitude pole appears to have induced right lateral shear stress along the entire pacific plate divergent boundary which resulted in the formation of extensional transform faults microplates and propagating ridges this change in motion also appears to have triggered concomitant circum pacific tectonism including trench migration and back arc rifting the wk97 suggests that long lived plume fed volcanism on the pacific plate appears very limited perhaps only to five to six hotspots hawaii louisville cobb caroline and perhaps marquesas and bowie the geometry of these chains are consistent with our model despite some evidence for volcanic age progressions most other pacific neogene hotspots are most likely crack spots i e sites of extensional volcanism at preexisting zones of weakness reactivated by plate stresses indeed several volcanic ridges such as hollister ridge near the louisville hotspot appear to have formed in response to stresses following the change in absolute plate motion wk97 also obviates the need for simultaneous different directions of mantle flow in the south central pacific transpression at the san andreas and alpine strike slip faults and aleutian arc explosivity since 6 ma also appear to be consequences of the changes in pacific plate motion furthermore both the wk97 model and observed relative plate motions suggest that adjoining plates have changed their absolute motions as well and that large changes in absolute plate motion do not necessarily imply large changes in relative plate motion our findings support slab pull as the dominant plate tectonic driving force highlight the rheological difference between strong oceanic lithosphere and weak continental crust and suggest that wk97 presently provides a unifying albeit preliminary framework for relating both intraplate and circum pacific tectonism and volcanism in the late neogene

morphostructural study and type of volcanism of submarine volcanoes over the pitcairn hot spot in the south pacific undersea volcanoes found at about 80 km southeast of the island of pitcairn are believed to be the manifestation of a hot spot activity located near 129 30 w 25 10 s along the strike of the mururoa gambier pitcairn volcanic alignment hydrothermal activities and recent volcanic flows were observed on the two largest 20 km in basal diameter and shallowest 60 m and 450 m depth volcanoes more than twenty other smaller volcanic edifices 0 25 made up of ancient morb type tholeiitic rocks are inferred to be inherited from the epr axial regions the shallow volcanic activity which occurred on the two largest edifices are classified as 1 reactive eruptions with hydromagmatic activities giving rise to volcanic ejecta bombs xenoliths and ash and 2 quiet eruptions which formed pillows and or lobated lavas and large massive flows intrusives dyke and sill were observed cutting through the volcanic ejecta near the summit 500 m depth of the seamounts the general structural orientations of the rift zones recognized from the bathymetry of individual volcanoes n160 180 n80 n30 and n120 are comparable to those observed in the society and austral hot spot regions these orientations were inferred as corresponding to the structural discontinuities of the ancient oceanic crust and to the regional stress field of the pacific plate rock samples from the pitcairn region consist of alkali basalts basanites trachyandesites and trachytes which are closer in chemical composition to some of the volcanics from the society rather than to those from the austral hot spot regions

petrology of the volcanic bedrock of mururoa atoll tuamotu archipelago french polynesia abstract the south pacific atoll of mururoa is located in the southeastern part of the tuamotu archipelago it is about 28 km long and represents the emergent part of a 60 km long submarine plateau this atoll is composed of submarine volcanic rocks resting on oceanic crust then above subaerial volcanic rocks and finally a coral coating about 300 m thick the volcanic rocks are markedly alkaline and this alkaline magmatic trend evolved by fractional crystallization from nepheline normative 10 wt basalts to quartz normative 5 wt trachyte fractionation of phlogopite played a significant role in this evolution the basic lavas contain olivine spinel magnetite ilmenite clinopyroxene and plagioclase phenocrysts rare feldspathoids occur in the groundmass of the hawaiites two trends can be distinguished the first one characterized by the presence of mica plagioclase and ilmenite has evolved under high p h 2 o the second characterized by amphibole richterite and arfvedsonite aegirine alkali feldspar and magnetite has evolved under lower p h 2 o and silica activity but higher f o 2 thermodynamic conditions of crystallization at depth deduced from olivine liquid plagioclase liquid and magnetite ilmenite equilibria are high temperature 1100 to 1200 c medium water pressure 1 to 1 5 kbar and medium to low oxygen fugacity 10 10 bar the mururoa magmas may have issued from fissures along the australes fault it may not result from the activity of the hot spot situated east of pitcairn island considered as responsible for the volcanism of others islands of the tuamotu archipelago fangataufa gambier pitcairn

anomalous elastic thickness of the oceanic lithosphere in the south central pacific to first order the elastic thickness te of the oceanic lithosphere under mid plate volcanoes increases linearly with the square root of plate age at the time of loading1 4 a large scatter around this simple relationship is observed however suggesting that factors other than age influence the elastic thickness and produce departure from the first order trend5 to look at second order effects we have conducted a worldwide analysis over the pacific atlantic and indian oceans and determined te under sixty volcanoes applying elastic flexure theory and using as observational constraints geoid height data from the seasat satellite we find that for the three main oceans the depth of the elastic layer under volcanoes increases with age of plate at loading time t according to the simple relationship te 2 70 0 15 t1 2 te in km t in myr except under all of the volcanic chains of the south central pacific where this relation does not apply the elastic layer is found to be much thinner there and indicates a broad regional anomaly presumably of thermal origin well correlated with several other geophysical anomalies reported in this region6 10

louisville seamount chain petrogenetic processes and geochemical evolution of the mantle source the louisville seamount chain is a 4300 km long chain of submarine volcanoes in the southwestern pacific that spans an age range comparable to that of the hawaiian emperor chain and is commonly thought to represent a hot spot track dredging in 2006 recovered igneous rocks from 33 stations on 22 seamounts covering some 49 myr of the chain s history all samples are alkalic similar to previous dredge and drill samples providing no evidence for a hawaiian type tholeiitic shield volcano stage major and trace element variations appear to be predominantly controlled by small but variable extents of fractional crystallization and by partial melting isotopic values define only a narrow range in agreement with a surprising long term source homogeneity relative to the length scale of melting and overlap with proposed fields for the c and fozo mantle end members trace element and isotope geochemistry is uncorrelated with either seamount age or lithospheric thickness at the time of volcanism except for a small number of lavas from the westernmost louisville seamounts built on young 20 ma old oceanic crust the louisville hot spot has been postulated to be the source of the 120 ma ontong java plateau but the louisville isotopic signature cannot have evolved from a source with isotopic ratios like those measured for ontong java plateau basalts on the other hand this signature can be correlated with that of samples dredged from the danger islands troughs of the manihiki plateau which has been interpreted as a rifted fragment of the greater ontong java plateau

geochemistry of kauai volcanics and a mixing model for the origin of hawaiian alkali basalts a comprehensive model is developed to explain the major trace element and strontium and neodymium isotopic characteristics of alkali basalts from hawaii the model is similar to that of chen and frey 1983 in that it requires mixing of a small melt fraction of morb source material with another component to generate the alkalic suite of a particular hawaiian volcano it differs from the chen and frey model in that the other end member must be different from primitive mantle if it is to be consistent with both trace element and isotopic data alkali basalts and tholeiites from kauai analyzed in this study show a nearly complete transition in sr and nd isotopes there is a relatively well constrained array on a nd sr isotope correlation plot that can be explained by two component mixing of kauai tholeiite magma and a small amount of melt of east pacific rise source rock after corrections are made for fractional crystallization involving primarily clinopyroxene and olivine the sr and ba concentrations of kauai lavas plot along mixing curves defined by the above sources providing positive tests of the mixing hypothesis implications of this model are 1 the main source of hawaiian shield building tholeiites is a mixture of subducted crust primitive mantle and depleted asthenosphere that has been homogenized prior to melting 2 early alkalic volcanism as at loihi seamount will be characterized by greater isotopic heterogeneity than will late stage alkali basalt production and 3 there are two fundamentally distinct types of alkalic lavas erupted towards the end of magmatism at a given hawaiian volcano one represents smaller degrees of melting of the same source that generated shield building tholeiites kohala type the other derives from the mixed source discussed in this paper haleakala kauai type