the differing effects of compositional and thermal buoyancies on the evolution of mantle diapirs diapirs rising through the earth s mantle and lithosphere are hotter and generally also of a different chemical composition than their surroundings experiments demonstrate that diapirs driven by a temperature difference alone entrain material from their surroundings and that their velocity of rise decreases as they enlarge these diapirs are referred to as thermals however when the driving buoyancy is due only to a difference in chemical composition there is no entrainment and diapirs rise at a constant velocity assuming the surroundings have a constant viscosity the different flow is a result of radically different diffusion coefficients for heat and composition between these two extremes is a range of behaviours characterised by the density ratio r i t where i is the contribution to the total density difference resulting from the composition or intrinsic difference and t is the contribution from the temperature difference for r of order one buoyancy given to surrounding material by heat loss from the diapir must be taken into account and entrainment persists during a diapir s rise through a scale height for cooling beyond this height entrainment ceases if the two buoyancy forces reinforce r 0 or gravitational instability causes breakup of the diapir if the buoyancy forces are opposed and thermal effects are initially dominant r 0 furthermore only diapirs driven by an intrinsic density difference but having temperatures high enough to produce a low viscosity lubricating boundary layer are able to exceed the stokes terminal velocity

crystal chemistry of hydration in aluminous orthopyroxene hydrogen incorporation in aluminous orthopyroxene may control the generation of melt and dominate the seismic properties at the base of the earth s lithosphere to clarify the substitution mechanism of h we have synthesized characterized and refined the crystal structure of this potentially significant variant of orthopyroxene the experimentally produced crystals are small needles up to approximately 20 20 100 m in size electron microprobe chemical analysis indicates about 11 7 wt al 2 o 3 ftir spectra indicate 7500 ppmw h 2 o with absorbance features qualitatively similar to natural mantle orthopyroxenes tem imagery indicates that the phase is pure orthopyroxene with low concentrations of defects and inclusions cell parameter refinement from single crystal x ray diffraction gives a 18 1876 7 a b 8 7352 7 a c 5 1789 5 a v 822 79 11 a 3 which is 1 2 smaller than pure mg anhydrous orthoenstatite the crystal structure has been refined from single crystal x ray intensity data measured using a rotating anode x ray generator micro focused x ray beam and ccd detector system the refined structure indicates about 5 vacancy in m2 and significant al occupancy in both m1 and t2 consistent with its composition mg 0 95 0 05 m2 mg 0 79 al 0 21 m1 al 0 25 si 0 75 t2 si t1 o 6 the existence of hydrous orthopyroxene in the mantle could absorb water released from olivine on decompression to delay the onset of melting in the spinel stability region in mantle peridotite compositions

easter microplate evolution geochemical evidence confirms that there is current active spreading on both the east and west rift zones of the easter microplate and northwards propagation of the east rift with the active tip at 25 s the east rift is influenced by lateral flow of plume derived material from a hotspot located beneath either sala y gomez or easter island spreading rate variation does not seem to be influential in controlling the composition of erupted basalts

microplate tectonics along a superfast seafloor spreading system near easter island there is a growing easter microplate between the pacific and nazca plates the eastern and western boundaries of the microplate are actively spreading and propagating north and south respectively the northern and southern boundaries are broad transform zones most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres

electrical conductivity of mantle clinopyroxene as a function of water content and its implication on electrical structure of uppermost mantle abstract the electrical conductivity of san carlos clinopyroxene aggregates with various water contents were measured under ni nio buffer at 1 5 gpa and 600 1200 k in a dia type apparatus the conductivity increases with increasing water content in clinopyroxene hidden conduction mechanism was detected because of the much smaller iron content in clinopyroxene which was usually covered by small polaron conduction in other nominally anhydrous minerals the identified activation enthalpies ranged from 0 70 0 75 ev to 1 23 1 37 ev our result reveals that the dominant charge carrying species in electrical conductivity could change with temperature and water content at high temperatures relevant to asthenospheric condition activation enthalpy for the conductivity agrees well with that for the hydrogen self diffusion the dominant charge carrier therefore might be m site vacancy however contrary to previous view that all hydrogens contribute to increasing conductivity equally our result shows that only a limited amount 20 40 of hydrogen acts as effective charge carrier in clinopyroxene on the other hand the activation enthalpy for the conductivity at low temperatures is significantly lower than that for the hydrogen self diffusion similar to what has been observed in olivine and orthopyroxene this type of conduction is probably caused by fast diffusion of specific hydrogen or fast hydrogen grain boundary diffusion at low temperatures the proton conduction of clinopyroxene is nearly one order and two orders of magnitude lower than those of olivine and orthopyroxene respectively and tends to converge at high temperatures using the present data combined with conductivity of olivine and orthopyroxene a laboratory based conductivity depth profile in the uppermost mantle shows that hydrous clinopyroxene cannot account for the high conductive regions observed beneath the ocean floor near eastern pacific rise the presence of partial melt would be unavoidable

a model for the plate tectonic evolution of the east central pacific based on seasat investigations abstract a complete seasat dataset is used to investigate the small scale bathymetry of a 15 by 15 area of the east central pacific northeast of pitcairn island which was involved in the miocene ridge jump and reorientation two fracture zones fz1 and fz2 are mapped along 450 km at an azimuth of n95 e while the signature of the austral fracture zone disappears east of 135 w several seamounts are discovered or confirmed along the oeno henderson ducie lineament and the degree of isostatic compensation of a number of islands at the eastern end of the tuamotu group is studied islands of the northern branch have practically no signatures in the geoid while the southern ones must have formed on lithosphere at least 10 ma old on the basis of these constraints we present a model of the evolution of spreading since 40 ma b p in which the deactivation of the austral fracture zone and the transfer of its offset to fz1 in the late oligocene is interpreted as due to rift propagation triggered by a northern tuamotu hotspot we propose that the oeno ducie chain is the surface expression of a southern tuamotu hotspot deviated 15 in azimuth by the presence of a young and fresh fracture zone thus our model predicts that hotspots and ridge systems can have a moderate influence on each other s kinematics and surface expressions which does not however extend to permanent trapping

hydroxide in pyroxene variations in the natural environment a suite of 51 pyroxenes from a wide range of geological environments was examined with infrared spectroscopy small amounts of oh occur in nearly all the samples suggesting that oh groups are a minor component of most pyroxenes the oh concentrations vary 0 002 0 12 wt oh as a function of geological setting with the greatest amounts occurring in mantle derived samples the intensities of certain oh absorption bands are somewhat correlated with pyroxene composition in particular with the presence of trivalent cations this suggests that crystal chemical or compositional factors also control the oh incorporation the oh incorporated in the pyroxene structure is easily distinguishable from oh owing to amphibole lamellae which usually occur in diopsides and often in orthopyroxenes oh in pyroxene is probably a function of the activity of hydrous components during crystallization but may also depend on postcrystallization changes in the geological environment

pito and orongo fracture zones the northern and southern boundaries of the easter microplate southeast pacific abstract the easter rapanui microplate is a case example of a large dual spreading center system in a region where the fastest seafloor spreading on earth is occurring today recent theoretical models of the tectonic evolution of dual spreading center systems have explored the effects of shear and rigid rotation on the boundaries and internal structure of microplates but the models must be critically constrained by improved relative motion and structural fabric data sets during the january 1987 rapanui expedition on the n o jean charcot we conducted a sea beam magnetics gravity survey of a portion of the microplate boundaries the method that was used was to fully map selected portions of the boundaries in order to establish precise structural relationships the northern terminus of the east rift or eastern boundary of the microplate is expressed as a series of parallel nw se trending valleys including what appears to be with 5890 m depth the deepest active rift axis mapped in the pacific today pito rift the northern end of the pito rift merges with an e w to 083 narrow band of linear faults interpreted to be a transform fault between the nazca and easter rapanui plates the northern triple junction between the easter rapanui nazca and pacific plates is a rff type with the two transform faults colinear along an approximately e w direction the southwestern boundary of the easter rapanui microplate is marked by a series of en echelon offsets outlined by depressions which merge into an approximately e w zone where shear must be predominant the southern triple junction is a rrf junction with an overlapping ridge system the structural data acquired during the survey provide strong constraints for kinematic models of the microplate the structural data need to be combined with crustal age determinations in order to derive a model for the evolution of the microplate

two way exchange between the easter mantle plume and the easter microplate spreading axis the conventional model whereby plume volcanism forms linear age progressive volcanic chains with the youngest activity occurring nearest a spreading axis at a hotspot has been challenged for the easter seamount chain1 4 whereas early work suggested the existence of a linear melting anomaly a hotline 1 2 more recent studies3 4 have proposed a hotspot near salas y gomez island connected with the easter microplate spreading system by an 800 km long volcanically active plume channel here we use geochemical geological and geochronological data to argue that the hotspot lies close to easter island moreover new isotopic data for lavas from the seamount chain provide evidence for bidirectional flow between the spreading axis and the plume thus supporting geophysical and fluid dynamical models of mantle flow in a plume spreading axis system5 7 material balance and flux considerations show the easter plume to be weak and cool compared with those beneath larger features such as iceland hawaii and the galapagos islands

deep tow and sea beam studies of dueling propagating ridges on the east pacific rise near 20 40 s nontransform ridge axis discontinuities such as propagating rifts overlapping spreading centers oscs and saddle points occur frequently along the mid ocean ridge system we used sea beam and deep tow to study an example of a relatively large nontransform offset near 20 40 s on the east pacific rise epr the two spreading center tips are 15 km apart and separated by a diffuse shear zone a broad v shaped wake of disturbed topography and magnetics occurs on the flanks of the epr near the offset on a fine scale the offset is tectonically much more complex than either the typical osc or propagating rift structures for two reasons first it is a dueling propagator in which structures produced by successive short term episodes of northward and southward propagation have overprinted each other with migration toward the south prevailing over the long term second its offset distance is such that its tectonic behavior may be transitional from that of a propagating rift to that of an osc system the offset s rate of migration to the south has been 20 mm yr over the last 2 m y far less than total opening rate of 162 mm yr distinct propagation events may have been much more rapid however surging north and south at rates of 160 230 mm yr in the process several fossil spreading ridges 30 50 km long have been abandoned and rafted to the side we propose a fine scale tectonic evolution in which a propagating spreading center either curves inside or runs outside of a soon to be abandoned propagating rift tip we call this process self decapitation dueling propagation at 20 40 s may be caused by competing magmatic sources at shallow points along the epr one to the north near 20 s and one to the south near 21 30 s the 20 40 s dueling propagator occurs at a long wavelength low in the axial depth profile between these competing sources in general the division of spreading centers into segments separated by ridge axis discontinuities occurs at low points along the axis and may be caused by the pattern of upper mantle convection and the factors governing flow of magma along the strike of the ridge

multi stage metasomatism of diamondiferous eclogite xenoliths from the udachnaya kimberlite pipe yakutia siberia the primary garnet pyrope almandine omphacite cpx 1 6 5 7 wt na2o sulfide fe ni co mss assemblage of the two diamondiferous eclogite xenoliths studied u33 1 and ux 1 experienced two mantle metasomatic events the metasomatic event i is recorded by the formation of platy phlogopite 10 wt k2o prior to incorporation of the xenoliths in the kimberlite the bulk of the metasomatic alteration consisting of spongy textured clinopyroxene cpx 2a 1 3 wt na2o coarser grained clinopyroxene cpx 2b 2 5 wt na2o pargasitic amphibole 0 8 wt k2o 3 3 5 wt na2o kelyphite cpx 3 mostly 1 wt na2o and zoned mg fe al spinel sodalite calcite k feldspar djerfisherite k5 95na0 02fe18 72ni2 36co0 01cu4 08s26cl and a small amount of k ca fe mg glass is ascribed to the metasomatic event ii that occurred also in the upper mantle but after the xenoliths were incorporated in the kimberlite a pervasive chloritic alteration mainly clinochlore magnetite that overprints earlier assemblages probably took place in the upper crustal environment the diamonds are invariably associated with secondary clinopyroxene and chlorite but the diamonds formed before the entrainment of the xenoliths in the udachnaya kimberlite

precipitates of α cristobalite and silicate glass in uhp clinopyroxene from a bohemian massif eclogite

extreme isotopic homogeneity among basalts from the southern east pacific rise mantle or mixing effect basaltic lavas which have erupted along the great oceanic ridge system are characterized by their overall chemical and isotopic homogeneity nevertheless they show evidence for variability in the chemical and isotopic composition of the ocean ridge basalt orb source whose scale and distribution remain uncertain isotopic compositions of sr nd pb and hf and ratios of some highly incompatible elements have been used as tracers of orb source compositions usually permitting source characteristics to be distinguished from petrogenetic effects the mid atlantic ridge mar in particular shows correlations in the isotopic compositions of several elements between isotopic compositions and topography and between these parameters and incompatible element concentrations and ratios1 3 we report here isotopic measurements for basalts dredged between approximately 30 and 34 s on the east pacific rise and adjacent maddingly rise which show highly uniform sr and nd isotopic compositions possibly resulting in part from mixing associated with a high rate of magma generation and rapid spreading 170 mm yr 1 however a ridge segment of comparable length between 18 and 22 s and with only a slightly lower spreading rate shows much greater isotopic variability hence we are apparently sampling small but detectable regional differences in mantle composition the isotopic homogeneity among the basalts from 30 to 34 s implies that any heterogeneity in the underlying upper mantle is uniformly distributed on a scale much smaller than the volume sampled in individual magma batches and is thus averaged out in the resulting lavas

mantle dynamics and generation of a geochemical mantle boundary along the east pacific rise pacific antarctic ridge a large scale mantle compositional discontinuity was identified along the east pacific rise epr and the pacific antarctic ridge par with an inferred transition located at the epr 23 degrees s 32 degrees s because of the epr easter hotspot interactions in this area the nature of this geochemical discontinuity remains unclear iodp sites u1367 and 1j1368 drilled into the ocean crust that was accreted at similar to 33 5 ma and similar to 13 5 ma respectively between 28 degrees s and 30 degrees s on the epr we use lavas from sites u1367 and u1368 to track this mantle discontinuity away from the epr the mantle sources for basalts at sites u1367 and u1368 represent respectively northern and southern pacific mantle sub domains in terms of sr nd pb hf isotopes the significant isotopic differences between the two iodp sites are consistent with addition of ancient subduction processed ocean crust to the south pacific mantle sub domain our modeling result shows that a trace element pattern similar to that of u1368 e morb can be formed by melting a subduction processed typical n morb the trace element and isotope compositions for site u1368 morbs can be formed by mixing a himu mantle end member with site u1367 morbs comparison of our data with those from the epr par shows a geochemical mantle boundary near the easter microplate that separates the pacific upper mantle into northern and southern sub domains on the basis of reconstruction of initial locations of the ocean crust at the two sites we find that the mantle boundary has moved northward to the easter microplate since before 33 5 ma a model in which along axis asthenospheric flow to where asthenosphere consumption is strongest explains the movement of the apparent mantle boundary crown copyright c 2013 published by elsevier b v all rights reserved

the dynamics of off axis plume ridge interaction in the uppermost mantle two decades of geochemical and geophysical observations have led to the plume channel model whereby buoyant off axis mantle plumes feed and interact with diverging plates at mid ocean ridge axes which act as sinks of upper mantle material here we present results from two dimensional 2 d numerical experiments which incorporate the essential physics and fluid dynamic aspects of the plume ridge upper mantle system in order to test the feasibility of this plume ridge interaction model specifically experiments test the relative importance of physical effects such as strong viscosity variations and thermal and chemical buoyancy forcing in plume ridge dynamics results indicate that both transient and steady state connections may be established between off axis plumes and ridges for a range of realistic mantle conditions the presence of a strongly sloping rheological boundary layer rbl is a necessary condition for long term communication between an off axis buoyant upwelling and a spreading ridge the flux of buoyant material to the ridge is also shown to increase with increasing plume to mantle density contrast decreasing plume viscosity and smaller plume ridge separation distances plume ridge interaction regimes are defined based on the competing effects of plate driven and buoyancy driven flow thermal erosion of the viscous lithosphere strongly inhibits long term plume ridge interaction by enhancing the plates ability to deflect the plume both head and conduit away from the ridge axis

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

comprehensive sonar imaging of the easter microplate recent gloria and seamarc ii sidescan sonar surveys have imaged virtually the whole of the easter microplate and revealed unprecedented detail and complexity in its tectonic fabric and history there is clear evidence of rapid microplate rotation of rapidly evolving plate boundaries and of complex deformation near the northern and southern microplate boundaries

87sr86sr and ree variations along the easter microplate boundaries south pacific application of multivariate statistical analyses to ridge segmentation abstract we report 87 sr 86 sr ree sc v cr and co analyses on 48 basaltic glasses dredged at 47 different localities along the spreading boundaries of the easter microplate emp the latitudinal 87 sr 86 sr variation along the east and west rifts closely parallels that of the pb isotope variation previously published and confirms the strong abnormality of the mantle underlying this part of the east pacific rise epr the data also further support the binary mantle mixing model previously proposed we evaluate the potential of multivariate statistical analyses in studying this and previously published geochemical data obtained on these very same 48 basaltic glasses the data base includes major and trace elements ranging from compatible e g cr to highly incompatible e g la and the pb and sr isotope ratios multivariate classification of these geochemical variables in q mode sample space allows to clearly distinguish major element grouping from those of moderate to highly incompatible elements and the isotope ratios reflecting mantle source heterogeneities present in the region the power of this multivariate geochemical classification is best represented in two dimensional space by combining principal components analysis pca with cluster analysis ca illustrated in the form of a dendrogram the method is further illustrated by classifying elements of the lanthanide series at different statistical level of affinities using the variance covariance matrix and correlation matrix in r mode we further demonstrate a close relationship between segmentation revealed by the multiple variation of the four isotope ratios and the actual tectonic segmentation both discriminant analysis and pca further indicate a close relationship between the geochemical anomaly located on the east rift of the emp and sala y gomez and easter island to a lesser extent the tuamotu chain whereas there is no close affinity between the emp and the more distant juan fernandez san felix or society islands in the isotopic space considered this provides further support that the geochemical anomaly located on the east rift of the emp may be related to the influence of a plume located either beneath the east rift or in the vicinity of sala y gomez or easter island in the last two models the plume would be laterally deflected at shallow depth towards the epr emp spreading complex an analysis of statistical generalized distances against geographical distances rules out that such influence is caused by radial dispersion of this plume in the upper mantle the flow between the plume and the east rift of the emp must be more confined or channelled the possibility that the plume would be located directly beneath the east rift cannot be ruled out but appears less likely

intraplate versus ridge volcanism on the pacific antarctic ridge near 37 s 111 w exploration of the foundation volcanic chain 33 s 131 w 37 s 111 w revealed the existence of different magmatic provinces with relation to their geological settings 1 the pacific antarctic ridge par is made up of several en echelon segments where both glassy midocean ridge basalts morbs with low incompatible elements k2o 150 ppm and ce 48 ppm at about 306 1300 km from the par axis 4 the old pacific seamounts built on a crust older than 23 m y located west of longitude 124 w 1300 km from the par axis consist of t and emorb on the par axis extensive crystal fractionation 65 produced the silicic lavas on the basis of pacific plate reconstruction using a half spreading rate of about 50 mm yr and integrating the observed compositional changes with respect to the structural settings it is inferred that the last volcanic events giving rise to the fs took place at about 110 km from the par axis about 5 m y ago the oblique ridges built between 5 m y and 23 m y with morb volcanics comparable to those of the the oblique ridge par provinces could also have been formed by an interaction between the foundation seamount dredge site 28 hotspot magmatism and that of an ancient accreting ridge magmatism precursor of the par

overlapping rift zones at the 5 5 s offset of the east pacific rise a seabeam and magnetometer survey of the pacific nazca plate boundary around 5 5 s mapped a nontransform offset whose geology and kinematics seem typical of a whole class of structures formed where fast spreading rises are laterally offset for distances less than the width of a subaxial magma chamber though the spreading center is abruptly displaced by 15 km right laterally there is no trace of strike slip faulting instead the ends of two 100 km long axial volcanic ridges veer 15 toward each other and overlap for more than 20 km in this overlapping region between 5 22 s and 5 32 s accretion of the upper oceanic crust by dike injection and eruption is partitioned between two adjacent and parallel rift zones with oblique and one sided spreading a continuous but dog legged magma chamber is inferred to underlie both rift zones and an intervening 9 km wide volcano studded basin a strong magnetic signature from the crust which accreted around this oblique chamber identifies the trace of the offset on the adjacent rise flank in the past 0 5 106 years the southern axial ridge has propagated north at 40 mm yr 55 of the spreading half rate

redistribution of high pressure fluids during retrograde metamorphism of eclogite facies rocks voltri massif italian western alps abstract high pressure clinopyroxenes in metasomatic mafic rocks rodingites and metagabbros containing titanoclinohumite and eclogites from the voltri massif ligurian western alps retain primary two phase aqueous inclusions h 2 o nacl kcl in the metasomatized rocks the inclusions align with the c axes and the cleavage planes of diopside the eclogites display mylonitic textures primary fluid inclusions are hosted in early porphyroclasts of omphacite omphacite i and are absent in later fine grained omphacite omphacite ii that forms the mylonitic foliation omphacite i has been frequently replaced by retrograde symplectites clinopyroxene plagioclase amphibole in such cases the fluid inclusions are either absent or occur in much lower number the textural observations suggest that formation of the symplectites has been controlled by availability of the fluid inclusions inside omphacite i the symplectites have been in turn overgrown by glaucophanic and winchitic amphiboles indicating that breakdown of the omphacite i to symplectites still occurred at relatively high pressure post entrapment changes have caused a large range of densities in the analyzed inclusions although the textural evidence constrains the entrapment of fluid inclusions to high pressure conditions the highest density isochores in the eclogites indicate pressure temperature conditions 8 kbar and 550 c lower than those attained during the high pressure stage such p t estimates approach those of symplectite formation after omphacite i thus the fluid inclusions preserved in omphacite were formed by low pressure re equilibration of a fluid presumably trapped at eclogite facies conditions the inclusions kinetically triggered the retrograde breakdown of the eclogitic clinopyroxenes the compositional overlap between the analyzed inclusions and those related to the latest greenschist evolutionary stage suggests further redistribution of the high pressure fluids and close system behaviour of these rocks during their overall exhumation path

effective elastic thickness of the lithosphere along the easter seamount chain bathymetry and gravity data collected during legs 5 6 and 7 of the 1993 gloria expedition and the recently released 2 min altimetry derived global gravity grid are used to determine the effective elastic thickness of the lithosphere along the easter seamount chain esc forward modeling admittance and coherence methods yield consistent restfits with the exception of the eastern and western ends of the esc the effective elastic thickness along the chain is 1 4 km the thin elastic thickness for the majority of the esc seamounts is compatible with a young seafloor age at the time of loading derived from new radiometric ages of the seamounts along the chain and a magnetic isochron age interpretation of the nazca plate seafloor age the elastic thickness southeast of the nazca fracture zone is 6 km apparently because of the seafloor age discontinuity across the fracture zone the elastic thickness near the san felix island at the eastern end of the esc is even greater 11 km which is compatible with the estimated seafloor age at the time of loading a slight increase in the effective elastic thickness of the far western part of the esc suggests dynamic compensation or less thermal weakening of lithosphere above a plume channel versus directly above the plume center these findings combined with published geochemistry support a hotspot origin for the esc complicated by large scale plate boundary reorganizations and channeling of plume material to the east pacific rise

the fixed hotspot hypothesis and origin of the easter sala y gomez nazca trace reconstructions of the pacific and nazca plates suggest that both the nazca and tuamotu ridges originated from a melting anomaly which existed beneath the pacific farallon ridge during the time interval between anomalies 19 and 11 as would be predicted from the fixed hotspot hypothesis the hotspot hypothesis interpreted according to morphology of the easter sala y gomez nazca trace would further suggest that the hotspot was located entirely beneath the nazca plate during the time interval between anomalies 11 and 2 at the end of this interval the eastern boundary of the easter plate formed centered above the hotspot according to this scheme the present location of the hotspot is about 200 km west of easter island however the easter model is incompatible with the hotspot model for the hawiian emperior chain if the hotspots are fixed relative to one another as the age of the point on the tuamotu ridge which corresponds with the hawaiian emperor bend 53 to 55 m y is significantly greater than the age of the bend 43 m y the fixed hostpot hypothesis can be satisfied by the two traces only if one or a combination of the following conditions is met the kinematic age of the hawaiian emperor bend is substantially greater than the isotopically dated age the magnetic anomaly time scale is in significant error and or magnetic anomaly identifications in the vicinity of the tuamotu ridge are erroneous it might appear that other mechanisms for the origin of hotspot traces are preferable to the hotspot hypothesis the most promising of these involves propagating fractures resulting from intraplate stress this hypothesis has the advantage of accommodating anomalies in some hotspot traces such as persistence of igneous activity long after passage of the trace over the hotspot

helium isotope ratios in easter microplate basalts abstract 3 he 4 he ratios in easter microplate basalt glasses show clear evidence of the effects of a mantle plume the east rift of the microplate between 26 and 28 s identified by la sm sr and pb isotopes and ridge crest elevation as the region of maximum plume influence has 3 he 4 he ratios spanning the entire range from 7 5 to 11 7 r a the easter microplate is the only section of the entire east pacific rise that is associated with a known hotspot track mantle plume and has elevated 3 he 4 he ratios although most of the west rift basalts contain morb helium 8 0 8 7 r a the basalt closest to the east rift has an elevated 3 he 4 he ratio 11 3 r a consistent with a significant plume component the diversity in isotopic signatures also indicates that homogenization of isotopic anomalies does not occur even in this region of super fast spreading the overall 3 he 4 he 206 pb 204 pb and 3 he 4 he 87 sr 86 sr trends have positive correlations although the high 3 he 4 he 87 sr 86 sr and 206 pb 204 pb component is not a single value but rather a broad band the apparent decoupling between the he and sr isotope distribution is modeled in the context of a plume source migrating ridge sink during channeling of the plume toward the ridge helium is preferentially lost from the center of the channeled plume resulting in lower he pb and he sr concentration ratios in the high 3 he 4 he component mixing trajectories in he sr isotopic space between a lile depleted asthenosphere and a variably degassed plume component provide a reasonably good fit to the data and may explain the isotope systematics of plume ridge interactions in the context of modern theories of plume dynamics

water contrast between precambrian and phanerozoic continental lower crust in eastern china 1 the presence of water even in small amounts in the continental lower crust may play a critical role in its physical and chemical properties and behavior however the environment and evolution of water in the deep crust remain poorly constrained investigation of water dissolved as h related point defects in minerals of lower crustal granulites may provide clues to clarify this issue the analyzed and compiled water data of nominally anhydrous clinopyroxene cpx orthopyroxene opx and plagioclase plag in lower crustal granulites from hannuoba nushan and daoxian in eastern china reveal significant contrast in water contents ppm h2o by weight between precambrian and phanerozoic samples e g 200 2330 versus 275 720 ppm for cpx 140 1875 versus 60 185 ppm for opx 145 900 versus 65 345 ppm for plag and 155 1120 versus 165 360 ppm for the bulk concentrations our data show consistently higher water contents in the precambrian granulites implying a more hydrous lower crust in the precambrian than in the phanerozoic such a difference may reflect variable water contents in the original melts indicating higher water contents in the precambrian upper mantle or a plume source for that part of the precambrian lower crust

laboratory investigation of the interaction of off axis mantle plumes and spreading centres mantle plumes and mid ocean ridge spreading centres are the dominant phenomena through which mass and heat are transported from the mantle to the earth s surface it now seems that the dispersion of near ridge plumes beneath the lithosphere is modulated strongly by mid ocean ridges in particular geochemical and geophysical observations have suggested that rising plumes are diverted towards and feed nearby ridges 1 7 here we confirm the feasibility of this model with laboratory experiments that incorporate the essential physical and fluid dynamic aspects of a plume ridge upper mantle system our results indicate that an off axis plume may communicate thermally and chemically with a spreading ridge through a narrow sub horizontal conduit instead of a broader radially spreading plume head a necessary condition for this communication is the presence of a lithospheric or rheological boundary layer that thickens away from the ridge axis owing to conductive cooling interestingly we find that for high plume temperatures increasing the plume thermal buoyancy may inhibit rather than enhance plume ridge interaction as a result of increased erosion of the overlying lithosphere

hydrous species in eclogitic omphacite implication for metamorphic dehydration during exhumation abstract water contents were analyzed by infrared spectroscopy for structural hydroxyl in randomly oriented cuboids and doubly polished wafers of single crystal omphacite from an ultrahigh pressure eclogite in the dabie orogen the results are quantitatively compared with bulk water contents measured by the tc ea ms continuous flow technique this enables recognition of various hydrous species in omphacite and their bearings on metamorphic dehydration during continental subduction the two cuboids yield constant maximal structural water contents of 378 ppm h2o wt using the omphacite specific infrared absorption coefficient which are much lower than the bulk water contents of 1496 ppm the excessive water is contributed by the presence of other hydrous species in the form of hydrous mineral inclusions hydrous symplectite and submicron fluid inclusions since the hydrous mineral and or fluid inclusions may be captured at different metamorphic stages during subduction and exhumation of continental crust and the hydrous symplectite is the secondary rather than the primary it is suggested that the bulk water contents represent a mixing of various hydrous species with distinct origins in this regard it is critical to examine whether water in these secondary hydrous species is of internal origin if not it has nothing to do with the transport capacity of water in deeply subducted crustal rocks upon stepwise heating up to 700 c omphacite exhibits a gradual decrease in bulk water contents but an increase in d values indicating the preferential loss of d depleted hydrous species as a whole due to the decompressional dehydration of uhp metamorphic rocks during exhumation therefore the d depleted hydrous species may be dominated in the hydrous symplectite and inclusion water

upper mantle heterogeneities and dynamics the width of geochemical and residual elevation anomalies along the mid ocean ridge system decrease as both the distance between migrating ridge segments and related hotspots and the local geochemical dispersion in the anomalies increase these observations provide further evidence that flows of mantle plumes in the upper mantle are deflected by migrating ridge sinks once they override hotspots

thsr isotopic relationships in morb abstract although morb are well known to exhibit a limited range in isotopic ratios such as 87sr 86sr a compilation of existing 230th 232th data which reflect present th u in the source shows a large dispersion this is in contrast to the admittedly limited data available for oceanic islands for which there is a broad correlation between 87sr 86sr and 230th 232th the distribution of data for morb is approximately bimodal with relatively depleted samples plotting from within to well above the band defined by the oceanic island correlation 87sr 86sr 0 70230 0 70270 and 230th 232th 1 1 2 3 and relatively enriched morb plotting from within to below this band 87sr 86sr 0 70270 70320 and 230th 232th 0 8 1 4 the data are consistent with the generally accepted idea that e morb are generated by the interaction of depleted mantle with more enriched material having higher th u and 87sr 86sr however the large range in source th u implied by the thorium isotopic compositions of both enriched and depleted morb requires complex melting and or other recent source modification processes much of this variability may be caused by multiple small degree melt extraction at the spreading ridge a process which would affect all incompatible trace element concentrations and ratios

tectonics of the easter plate abstract we present a new model for the easter plate in which rift propagation has resulted in the formation of a rigid plate between the propagating and dying ridges we use the distribution of earthquakes eleven new focal mechanisms and existing bathymetric and magnetic data to describe the tectonics of this area both the easter nazca and easter pacific euler poles are sufficiently close to the easter plate to cause rapid changes in rates and directions of motion along the boundaries the east and west boundaries are propagating and dying ridges the southwest boundary is a slow spreading ridge and the northern boundary is a complex zone of convergent and transform motion the easter plate may reflect the tectonics of rift propagation on a large scale where rigid plate tectonics requires boundary reorientation we use simple schematic models to illustrate the general features and processes which occur at plates resulting from large scale rift propagation

tectonic evolution of the easter microplate the plate tectonic history of the easter microplate has been reconstructed by closing the microplate in a series of steps using the pacific nazca magnetic anomalies north and south of the microplate and the nuvel 1 global plate motion model after each step the easter microplate was rotated rigidly to match the nazca and pacific anomalies gaps and overlaps formed by this kinematic treatment indicate compressional and tensional deformation respectively and show that rigid plate motions are insufficient to explain the complete tectonic evolution analysis of the magnetic anomaly data was guided by contoured seamarc ii sea beam and 3 5 khz bathymetry data and a lineament map derived from seamarc ii side scan and sea beam bathymetry data the patterns of lineaments and bathymetric structures suggest that rotational deformation of the nazca plate is the general mechanism that accommodates the space problems arising from transfer of the nazca plate to the microplate and rapid rotation of the microplate against the nazca plate similar but smaller amounts of deformation are predicted along the southern boundary of the microplate prior to the origin of the microplate the east pacific rise epr was offset in at least two places according to the older magnetic anomalies yet there is no evidence of linear fracture zones within the sparse data set except for occasional small consistent changes in regional depth across these age offsets the magnetic bathymetry and satellite altimetry data indicate that the microplate initially formed at or perhaps southeast of easter island near a left lateral offset of the epr sometime between anomaly 3 and 3 the east rift started propagating north from the present location of easter island at 4 5 ma which is 1 5 m y earlier than previously proposed however the magnetic data that support this interpretation are sparse and complicated by recent volcanic flows and associated rough bathymetry west of easter island the geometry of the microplate changes very rapidly during its evolution at the initial stages of development the microplate resembles a large propagating rift system suggesting that deformation may have been occurring throughout most of its interior up to about 2 47 ma at this time the length to width of overlap ratio of the two rifts reaches a value of 3 the northward propagation slows down the curved opening of the southwest rift becomes well established and rigid rotation of the previously deformed transferred lithosphere probably starts to predominate at this time the offset distance between the two overlapped rifts starts to increase some time after 2 47 ma and before 1 ma the east rift starts propagating northwestward probably in response to the microplate rotation and continues up until present also during this time period the east rift breaks into a series of northward propagating rifts each propagating into the microplate interior thereby transferring lithosphere from the microplate to the nazca plate and reducing the total growth rate of the microplate

morphology and distribution of seamounts surrounding easter island we investigate the morphology and distribution of a seamount population on a section of seafloor influenced by both superfast seafloor spreading and hotspot volcanism the population under investigation is part of a broad chain of seamounts extending eastward from the east pacific rise near easter island in order to define the morphological variability of the seamounts basal shape cross sectional area volume flatness and flank slope are plotted against height for 383 seamounts with heights greater than 200 m based on bathymetry data collected by glori b and seabeam 2000 during three cruises onboard the r v melville in the spring of 1993 nearly complete swath mapping coverage of the seamounts is available for the analysis of size and shape distribution we quantitatively describe the seamount population of this active region in which seamounts cover 27 of the seafloor and account for 4 2 of the total crustal volume over 50 of the total volume 61 000 km3 of seamounts used in this study is made up by the 14 largest seamounts and the remaining volume is made up by the 369 smaller seamounts 200 m in height our analysis indicates there are at least two seamount populations in the easter island salas y gomez island 25 29 s 113 104 w study area one population of seamounts is composed of short seamounts 1200 m shield like pointy cones flatness 1200 m originate exclusively from a hotspot source but only a portion of the smaller volcanoes 1200 m are formed from a hotspot source the remainder would be presumably formed by a normal mantle or mixed source

distribution of hydrous components in jadeite of the dabie mountains two kinds of jadeite occur in the jadeite quartzite from the dabie mountains eastern china associated either with weak or strong deformation the former shows a uniform composition while the latter shows both uniform and zoning composition these jadeites were examined with infrared ir spectroscopy all jadeite displays hydroxyl oh stretching bands implying that hydrous components commonly exist in jadeite the concentration of the hydrous components in the jadeite with weak deformation is homogenous whereas the concentration of the hydrous components in strongly deformed samples is variable the correlation between the intensity of oh band absorbance and compositional zoning of jadeite with strong deformation indicates that na rich jadeite can accommodate more oh than na poor jadeite its interpretation is that there is more na associated with greater oh availability in the initial crystallization environment our data of the comparatively high oh concentrations recorded in the core of the jadeite which contains relative high vacancies in m2 site imply that both oh content and the vacancies in m2 site could decrease during exhumation of the jadeite quartzite the decrease of oh solubility in jadeite would result in the formation of h2o fluid during the early exhumation of uhp metamorphic rocks the h2o fluid transformed from oh during exhumation could trigger and enhance the early retrograde metamorphism of the host rocks and facilitate plastic deformation of jadeite grain by dislocation creep and diffusion creep

two episodes of paleoproterozoic mafic intrusions from liaoning province north china craton petrogenesis and tectonic implications abstract mafic dyke swarms constitute a common occurrence within the cratonic provinces in the world since the archean proterozoic boundary and their geneses harbor critical information for facilitating regional tectonic and supercontinental reconstruction this zircon u pb dating and geochemical study documents two episodes of paleoproterozoic mafic intrusions from liaoning province north china craton ncc the haicheng suite hcs with an emplacement age of ca 2125 ma from the liaodong peninsula and the guohua suite ghs with an age of ca 1816 ma from the western liaoning terrane the hcs meta gabbros are tholeiitic in composition with an sio 2 range from 46 to 52 modest enrichment in light rare earth elements lree and depletion in high field strength elements hfse and possess radiogenic whole rock nd nd t 1 91 to 4 21 and zircon hf hf t 4 02 to 8 48 isotopic compositions these features endow the hcs with a juvenile subduction fluid metasomatized lithospheric mantle source by contrast the ghs exhibits a meta gabbroic to dioritic composition and a transitional character from tholeiitic to calc alkali with variably enriched lrees and more evolved isotopic signatures nd t 0 20 to 5 82 zircon hf t 0 09 to 4 91 than the hcs while the relatively intact high fe gabbroic samples are supposed to represent partial melts from a fertile cratonic sub continental lithospheric mantle source in the garnet spinel stability field the low fe dioritic ones tend to bear the imprints of subsequent assimilation and fractional crystallization process apart from making an important contribution to the present archive of the paleoproterozoic mafic dyke swarms in the ncc the present two mafic magmatic suites witnessed a shift in tectonic setting from a subduction related continental arc environment to a post orogenic extensional regime synthesizing two suites with coeval felsic igneous episodes as well as concomitant episodic metamorphic events in the jiao liao ji belt leads to the characterization of a possible divergent double subduction system in the eastern ncc during the time span of ca 2 2 1 80 ga synchronized beating of episodic magmatic and metamorphic pulses in the constituent mobile belts of the ncc carries further implication of attesting to the ubiquity and importance of divergent double subduction system in the cratonic amalgamation and its incorporation within the supercontinent columbia

geochemical constraints on magma sources and mixing processes in easter microplate morb se pacific a case study of plume ridge interaction the easter microplate is bounded in the east and west by two large overlapping spreading centres called the east and west rift respectively the east rift is located close to the easter hotspot the site of young intraplate volcanism above a mantle plume easter hotspot material creates a geochemical anomaly on the east rift spreading axis of the microplate and mainly northward directed material flow occurs beneath the propagating east rift the incompatible element depleted mantle of the easter microplate consists of two different sources one of which mixes with enriched easter hotspot material forming the east rift mid ocean ridge basalt morb the other depleted mantle source is present beneath the west rift and mixes with an enriched endmember apparently distinct from the easter hotspot source while the easter hotspot material leads to a bathymetric anomaly on the east rift no increased degree of melting is observed at the point of maximum inflow of enriched plume material implying low excess temperatures 50 c of the mantle the mixing processes between the easter hotspot and morb material probably take place in the partially molten asthenosphere during the horizontal flow of the plume material melting above the plume and at the spreading axes in the easter microplate region occurs over a range of pressures but there is no sign of melts from enriched garnet pyroxenite

fast rift propagation along the east pacific rise near easter island during a 1983 seabeam cruise we surveyed the easter nazca spreading center which forms the eastern boundary of the easter microplate we have analyzed all available bathymetry and magnetic anomaly data to determine the present fine scale tectonic configuration and recent tectonic evolution of this spreading center the narrowing of the axial brunhes anomaly is best explained by a young propagating rift growing northward at an average rate of 150 mm yr assuming an average spreading rate of 120 mm yr there are indications of a short episode of faster propagation 350 mm yr prior to 0 4 ma these are the fastest rates of rift propagation yet documented the tip of the propagating rift is at about 25 s 112 25 w 180 km south of the previously hypothesized position the recent spreading direction is n85 w a large well defined sheared zone to the west of the western pseudofault bounding the crust formed by the propagating rift suggests that the easter pacific spreading center 360 km to the west is being replaced by the fast propagator