oceanic plateau formation by seafloor spreading implied by tamu massif magnetic anomalies tamu massif is an immense mesozoic submarine volcano the main edifice of the shatsky rise oceanic plateau it is located at a spreading ridge triple junction but considered to be a shield volcano formed by effusive volcanism from an emerging mantle plume however it is unclear how tamu massif eruptions interacted with the spreading ridges which are enormous linear volcanoes themselves here we create a magnetic anomaly map for tamu massif which can provide clues about crustal formation for tamu massif we find dominantly linear magnetic field anomalies caused by crustal blocks of opposite magnetic polarity this pattern suggests that tamu massif is not a shield volcano but was emplaced by voluminous focused ridge volcanism if the magma source at the shatsky rise was a plume it was closely connected to and controlled by seafloor spreading by implication even the largest oceanic plateau edifices can be formed by seafloor spreading we suggest that the widely accepted analogy between continental flood basalts and oceanic plateaus requires reconsideration the shatsky rise oceanic plateau formed by spreading ridge volcanism according to analyses of linear magnetic anomalies over the tamu massif submarine volcano

unusual bulk rock compositions in eclogite facies rocks from syros and tinos cyclades greece implications for u pb zircon geochronology low temperature eclogite facies rocks from syros and tinos cyclades greece include meta ophiolitic blocks of unclear origin meta olistostrome or tectonic melange these blocks occur in a matrix of altered serpentinite and or semipelitic to tuffitic schists some samples are characterized by very high trace element concentrations for example eclogites from tinos contain up to 4950 ppm zr and up to 480 ppm y understanding of the geochemical controls on the petrogenesis of these rocks has important consequences for interpretation of geochronological results the emphasis of this study is on the behaviour of zr under eclogite facies p t conditions due to remarkably high modal amounts of zircon some samples from the meta ophiolite suite are suitable for u pb zircon chronology the geological significance of zircon ages can only be correctly interpreted by assessing whether zircon crystallized from a melt or formed during subsequent hydrothermal or metamorphic processes the results of this study lead us to conclude that infiltration of trace element enriched fluids during eclogite facies metamorphism caused local scale compositional changes textural observations document the presence of a fracture network favouring fluid infiltration under high pressure conditions such a system of microfractures might have provided infiltration paths for trace element enriched fluids during earlier stages of high pressure metamorphism in most cases subsequent recrystallization erased any evidence of the early infiltration channels but inclusion of high pressure phases in zircon provide evidence for this model suggesting metasomatic alteration in a subduction zone environment the original fluid source and the conditions e g high f environment highly saline brines under which commonly immobile elements were mobilized remain unclear it is speculated that eclogitization of closely associated metagabbros may have released zr and other trace elements the results of this contribution suggest that new zircon formed during high pressure metamorphism cretaceous u pb zircon ages are interpreted to date subduction metamorphism and not magmatic crystallization

an immense shield volcano within the shatsky rise oceanic plateau northwest pacific ocean most oceanic plateaux are massive basaltic volcanoes however the structure of these volcanoes and how they erupt and evolve is unclear because they are remote and submerged beneath the oceans here we use multichannel seismic profiles and rock samples taken from integrated ocean drilling program core sites to analyse the structure of the tamu massif the oldest and largest edifice of the shatsky rise oceanic plateau in the north western pacific ocean we show that the tamu massif is a single immense volcano constructed from massive lava flows that emanated from the volcano centre to form a broad shield like shape the volcano has anomalously low slopes probably due to the high effusion rates of the erupting lavas we suggest that the tamu massif could be the largest single volcano on earth and that it is comparable in size to the largest volcano in the solar system olympus mons on mars our data document a class of oceanic volcanoes that is distinguished by its size and morphology from the thousands of seamounts found throughout the oceans

discrete plumbing systems and heterogeneous magma sources of a 24 km3 off axis lava field on the western flank of east pacific rise 14 s the largest known mid ocean ridge off axis lava flow field occurs slightly off axis near 14 s along the east pacific rise epr it comprises at least four volcanologically discrete units we collected lava samples from 24 sites within the off axis lava field and 7 sites on the adjacent ridge crest using the shinkai 6500 submersible the lava field comprises at least three distinct morb compositions all of which are different from the lavas collected from the nearby ridge axis the east and west cones and the northern lobe of the lava field consist of normal morb n morb lavas with a low concentration of incompatible elements and low lile hfse and lree hree ratios by contrast the samples from the west plain of the field have a higher concentration of incompatible elements and higher lile hfse and lree hree ratios indicating t morb character the lava samples collected from the summit of the east cone show the highest concentration of lile elements and lrees among the lava field emorb lava the n morb of the off axis lava field are more depleted in incompatible elements than the adjacent epr axis lavas possibly reflecting the re melting of the residual mantle in the off axis region the e morb lava was probably derived from fertile mantle that did not undergo melting beneath the spreading center t morb which occupies the main part of the 14 s lava field is the product of magma mixing between n morb and e morb magmas 2007 elsevier b v all rights reserved

jurassic cretaceous boundary age and mid ocean ridge type mantle source for shatsky rise basalt sills cored in lowermost berriasian sediments on shatsky rise in the northwest pacific yield a mean 4 0 ar 3 9 ar incremental heating age of 144 6 0 8 ma providing a minimum estimate on the age of the jurassic cretaceous boundary the largest part of the rise was emplaced at rates similar to those of several continental flood basalts the rise formed at an oceanic triple junction and the sills as well as lavas from two dredge sites exhibit an ocean ridge type nd pb sr isotope signature this signature is provisionally consistent with either a perisphere or meteorite impact model but is not predicted by the plume head model of plateau formation however no model can yet be rejected given the very sparse sampling of shatsky rise

sea level fluctuations driven by changes in global ocean basin volume following supercontinent break up abstract long term variations in eustatic sea level in an ice free world which existed through most of the mesozoic and early cenozoic eras are partly driven by changes in the volume of ocean basins previous studies have determined ocean basin volume changes from plate tectonic reconstructions since the mesozoic however these studies have not considered a number of important elements that contribute to ocean basin volume such as regional differences in sedimentation or uncertainties within the plate tectonic model itself such as spreading asymmetries and the incomplete representation of back arc basins in the mesozoic additionally studies on long term changes in sea level related to the extension and rifting of passive margins have not been performed on a global scale and likely significantly underestimated the influence of this process in order to improve reconstructions of sea level on geologic time scales and assess the uncertainty in deriving the volume of ocean basins based on a global plate kinematic model we investigate the influence of back arc basins spreading asymmetry large igneous provinces lips sediment thickness and passive margins on ocean basin volume since 200 ma we find that less constrained plate tectonic elements such as the presence of back arc basins or spreading asymmetry may contribute up to 120 m or 150 m to sea level respectively changes in the sea level related to sedimentation and lips are respectively 75 165 m and 45 m changes in sea level associated with passive margin formation are almost negligible at present day though were much larger in the cretaceous and the assumed sedimentation style strongly influences the rate and magnitude of sea level change we incorporate predictions for these components during times where ocean basins are predominantly synthetic reconstructions and find that sea level driven by fluctuating ocean basin volume has changed by 200 m since the jurassic which is comparable to previous estimates our revised estimates will need to be combined with other processes driving long term sea level change including mantle convection driven dynamic topography and glacio eustasy for constructing a complete eustatic sea level curve understanding and quantifying the uncertainties in the volume of ocean basins has implications for modelling subduction flux the oceanic carbon cycle and heatflow and is important for exploring earth s evolutionary cycles especially during times in the geologic past where much of the ocean basin history has been lost

the mesozoic tectonic history of the magellan microplate in the western central pacific magnetic lineation mapping in the western central pacific has revealed a pair of opposite sensed fanned lineation patterns that define the accretionary boundaries of the fossil magellan microplate this tectonic synthesis results from extensive magnetic mapping of two new lineation patterns over a large area and extended mapping of previously identified lineations the entire evolutionary history of the magellan microplate is well constrained to a 9 m y period in the early cretaceous by synchronous spreading patterns and associated geologic data during this period the microplate grew and evolved as a generally rectangular structure to a final size of 700 km 600 km with spreading centers on two opposing sides and transform faults on the other two sides the lifetime and size of the magellan microplate are somewhat longer and larger respectively than presently active microplates on the east pacific rise however these modern structures are still evolving and growing and the tectonic behavior of the modern and cretaceous systems appears to be similar study of both active and fossilized microplates should provide additional insights on their common tectonic histories in particular we show that the magellan trough spreading center behaved as an asymmetric accretionary plate boundary that can be described with two separate poles of motion very close to this spreading center during much of its history the magellan trough spreading center then failed as a result of a larger ridge reorganization at the triple junction of the pacific farallon and phoenix plates at ml0n time microplate activity ceased when the microplate became welded to the pacific plate at m9 time

seaward dipping reflectors and the continent ocean boundary at passive continental margins abstract the definition of the continent ocean boundary at passive continental margins has proved to be an elusive task even the relatively direct method of seismic refraction experiments has yielded results that cannot always be interpreted unequivocally multichannel seismic reflection profiles on many passive margins have revealed the presence of remarkable suites of arcuate reflectors dipping seaward to form a wedge shaped structure their general characteristics and velocity structure suggest that they may be largely volcanic in nature but there is no agreed upon model for their origin nevertheless it is generally thought that they lie at or close to the boundary between continent and ocean and as such they are extremely important structural markers that may yield important evidence concerning the structure and evolution of passive margins

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

implications of updated magnetic anomalies for the late cretaceous tectonic evolution of walvis ridge in the south atlantic a reorganization of the mid atlantic ridge began before anomaly c34n 83 6 ma and ended before anomaly c30n 66 4 ma complicating tectonics of rio grande rise and older walvis ridge wr which formed together at the mid atlantic ridge this reorganization is poorly understood because magnetic anomalies c30n c34n are poorly defined near wr we interpreted these anomalies along westernwrto improve knowledge of rio grande rise wrtectonic development anomaly trends indicate that valdivia bank has an e w age progression perpendicular to that predicted by hot spot models anomaly spacing and width is irregular and anomalous near wr implying a series of ridge jumps and possibly a microplate between anomalies c34n and c32n eastward ridge jumps transferred microplate lithosphere to the south american plate this study shows that late cretaceous tectonic evolution of the rio grande rise wrlarge igneous provinces was more complex than previously understood

magnetic anomaly map of ori massif and its implications for oceanic plateau formation abstract many oceanic plateaus have been emplaced at or adjacent to mid ocean ridges to explain plateau volume and thickened crust compared to normal oceanic crust hotspot ridge interaction is commonly assumed but the manner of interaction remains unclear the shatsky rise oceanic plateau is a large volcanic mountain that formed at a triple junction during late jurassic and early cretaceous time recent drilling and seismic investigations suggest that the intermediate edifice in the rise ori massif is a central volcano paradoxically magnetic lineations were traced across parts of ori massif implying formation at a spreading ridge in this study we examined magnetic anomalies over and around ori massif to obtain insights about the formation of this volcanic edifice magnetic data from 21 cruises were corrected combined and gridded to construct a magnetic anomaly map forward and inverse magnetic modeling was done to investigate the magnetic structure of ori massif the results imply that this large volcanic edifice is predominantly characterized by linear magnetic anomalies resulting from alternating normal and reversed polarity magnetization blocks analogous to magnetic anomalies recorded by spreading ridges this magnetic structure is not expected for a central volcano that was built by long runout lava flows implying that ori massif eruptions must have been constrained near the ridge axis magnetic bights on the north and south boundaries of ori massif imply that it was bracketed by triple junctions indicating complex ridge tectonics during the formation of shatsky rise the surprising finding that ori massif is traversed by coherent linear magnetic anomalies indicates that oceanic plateaus can record seafloor spreading magnetic anomalies despite large crustal thickness other oceanic plateaus also record linear magnetic anomalies implying a link between divergent plate boundaries and oceanic plateau volcanism

rapid formation of the shatsky rise oceanic plateau inferred from its magnetic anomaly shatsky rise in the northwest pacific ocean is probably the oldest extant oceanic plateau and as with most such features its origin is uncertain both oceanic plateaus and continental flood basalts are thought to be formed by rapid voluminous eruptions that occur when the head of a newly born mantle plume ascends to the base of the lithosphere1 3 high eruption rates have been estimated for flood basalts for example 1 5km3 yr 1 for the deccan traps2 from dating of lava flows but the inaccessibility of oceanic plateaus makes it necessary to extrapolate dating information from a small number of samples and sites4 here we estimate the eruption rate of shatsky rise by a method that is indirect but has the virtue of sampling the entire volume of the plateau above the surrounding sea floor the main southern part of the plateau has a positive magnetic anomaly corresponding to a reversed geomagnetic polarity at the time of eruption using age constraints to identify the longest period of reversed polarity during which the plateau could have formed we estimate that 2 106 km3 of material erupted at a minimum rate of 1 7km3 yr 1 this issomewhat less than the rate of 8 22 km3 yr 1 estimated for the ontong java plateau4 but still represents a massive eruption consistent with the plume head hypothesis

plate kinematics of the south atlantic chron c34 to present a high resolution seafloor spreading history of the south atlantic since chron c34 is constrained by a combination of seasat altimeter data and underway marine geophysical data a set of 45 finite rotation poles defines the relative position of africa and south america at roughly 2 m y intervals a set of 12 stage poles constrain the relative motion of these two plates at 5 to 10 m y intervals the position of the stage poles continuously migrates reflecting the continuously changing azimuths of the fracture zones major changes in spreading direction are observed in the late cretaceous and early cenozoic as the fracture zones sweep out broad s shaped curves similar to the pattern seen on the kane fracture zone in the central atlantic small offset fracture zones were found to be the most accurate recorders of changes in plate motion large offset fracture zones such as the agulhas falkland fracture zone were the least reliable recorders at 30 s spreading rates decrease throughout the late cretaceous from a high of 75 mm yr at the end of chron c34 to a low of 30 mm yr around chron c27 a period of slow spreading between chron c30 and chron c20 corresponds to a zone of fracture zone proliferation an increase in the amplitude of geoid anomalies over fracture zones greater relief on topographic profiles and locally evidence of intraplate crustal deformation spreading rates increase at chron c20 to about 50 mm yr and then gradually decrease during the late paleogene and neogene a comparison of synthetic fracture zones based on the south atlantic stage poles to the observed trends of fracture zones in the equatorial atlantic indicates that the vema and marathon fracture zones were generated by south atlantic spreading as opposed to central atlantic spreading at least as far back as 35 m y ago and possibly 50 m y ago

intraplate deformation of oceanic crust near the rio grande rise in the south atlantic abstract globally oceanic crust has relatively uniform thickness and structure compared to continental crust with typical ocean crust being 5 8 km thick however a substantial proportion of oceanic crust is thickened outside the normal range in the form of oceanic plateaus and crust affected by hotspots the rio grande rise rgr is the most prominent bathymetric feature in the western south atlantic ocean and was thickened by activity of the tristan hotspot we use multichannel seismic data collected along the eastern margin of the rgr to investigate the relationship history between the thickened crust of the rgr and the adjacent normal crust we find numerous normal faults spatially coincident with the region where oceanic crust begins to thicken from normal to rgr crust using crosscutting relationships and superposition our interpretation supports protracted extensional deformation at the eastern margin of the rgr including recent activity that appears to deform the modern seafloor we attribute the faulting to differential subsidence linked to crustal thickness variations between the thickened rgr and normal oceanic crust given the fundamental processes we attribute to the cause of faulting and the proportion of thickened oceanic crust globally it is possible that regions of many oceanic plates experience intraplate deformation after formation

archaean hydrothermal zircon in the abitibi greenstone belt constraints on the timing of gold mineralisation abstract hydrothermal zircons have been found in archaean mesothermal au veins and altered wallrock selvages at val d or in the abitibi greenstone belt of canada the zircons are paragenetically associated with vein quartz tourmaline mica carbonate scheelite pyrite and gold zirconium mobility and the consequent occurrence of hydrothermal zircon may be associated with the intense tourmalinisation characteristic of archaean gold deposits in this district the shrimp ion microprobe has been used to analyse hydrothermal zircons from four separate mines spatially associated with the bourlamaque batholith and has yielded ages constraining formation of the au bearing vein systems to within 20 ma of emplacement of the pluton the ion microprobe data reveal multiple stages of hydrothermal zircon growth in the vein systems contemporaneous with the regional metamorphic peak and late kinematic activity along regional structures younger including proterozoic ages previously obtained for the veins using other minerals and isotopic schemes must reflect either alteration or renewed mineral growth during much later reactivation of fluids along the same structures up to 400 ma after initial formation of the veins

single grain argon laser probe dating of phengites from the blueschist to greenschist transition on sifnos cyclades greece measurements of 40ar 39ar age spectra on single white mica crystals of tertiary age is a new means of obtaining detailed geochronological information of alpine blueschists internal consistency of the data set as well as excellent agreement with previous conventional k ar results demonstrates that this new technique can be used with confidence to obtain information on the thermal evolution of young metamorphic belts

palaeomagnetism of cretaceous pacific seamounts summary the palaeomagnetism of cretaceous pacific seamounts as deduced from their magnetic anomalies is reviewed results from 35 seamounts are reported here for the first time of these 35 results 15 are of high enough quality to be useful this has resulted in a considerable increase in the available information concerning the palaeomagnetism of the pacific plate using only the most reliable results we calculate a palaeomagnetic pole for the upper cretaceous at 58 north 350 east with a 95 per cent cone of confidence of only 5 this pole is considerably more accurate than the one previously reported and is farther to the west the previously reported difference between the mean pole position from seamounts close to japan and the mean pole position from seamounts close to hawaii is not real although one group of seamounts close to japan gives an anomalous pole position several mechanisms have been suggested whereby errors might be introduced in the determinations of pole positions from seamounts no error is introduced in the pole position because of the restricted extent of surveys and the consequent inaccuracy in the determination of the regional field the demagnetization effect which can produce differences between the observed direction of magnetization and the direction of the magnetizing field in an elongated body is unimportant for seamounts the new pole position is sufficiently accurate for use in many types of calculations in which it is desired to locate the pacific plate with respect to the earth s spin axis

hydrous species in zircon a survey of hydrous species in zircon from a number of localities with a range of occurrences and structural states was carried out using infrared spectroscopy and electron microprobe analysis stepwise heating experiments on a crystalline and a nearly metamict zircon were also performed the region of ir spectra of oh bearing crystalline zircon from 4000 to 2500 cm 1 exhibits anisotropic oh bands resulting from at least three oh sites one at vacant tetrahedra and two that may be at si occupied tetrahedra one site associated with si occupied tetrahedra is the most important producing ir absorption bands at 3420cm 1 e c and 3385cm 1 e c oh maybe introduced during crystal growth by a coupled substitution of the form m 3 h 4 zr 4 the sites at vacant tetrahedra account for considerably less oh producing relatively weak ir absorption bands at 3510 cm 1 e c e c oh may be introduced during crystal growth by the hydrogrossular substitution of the form 4 oh sio 4 4 an additional oh band at 3270 cm 1 appears to be associated with an occupied tetrahedron heating experiments produced two more oh environments and indicate that h can migrate among the sites at high temperatures some crystalline samples are entirely free of oh ir spectra of metamict zircon exhibit broad asymmetric isotropic oh bands similar to h 2o bands but the absence of the h 2o bending mode band indicates that oh is the dominant hydrous species present ir spectra of partially metamict zircon samples retain some anisotropy the breadth of their oh bands is a function of degree of metamictizatron the maximum oh observed in any metamict or partially metamict zircon sample was approximately 0 1wt expressed as h 2o sucho h is probably secondary the maximum primary oh observed is 0 05 wt partially metamict zircon from a hydrothermal vug crystalline crustal zircon contains more than an order of magnitude less oh the maximum oh content observed in crystalline zircon 0 01 wt i s in a mantle zircon sample from a kimberlite suggesting high activity of h 2o in the mantle additional oh is present in some metamict and partially metamict zircon samples in the form of inclusions of hydrous silicates such as kaolinite fluid inclusions containing h 2o occur in some samples this may be a partial explanation of the high h 2o contents reported for some radiation damaged zircon based on loss on heating measurements

pb u and th diffusion in natural zircon zircon zrsio4 is one of the most widely used minerals for determining the age origin and thermal history of rocks by u th pb geochronology but the parameters describing the solid state volume diffusion rates of these elements in natural zircon which are themselves important for establishing the limits on the applicability of zircon for geochronological studies have remained poorly quantified1 this is because of the measurement difficulties associated with the low p p m concentrations and low diffusion rates of these elements in natural zircon and the chemical and physical heterogeneity present in most crystals here we present direct measurements of the uranium thorium and lead loss from a thermally treated gem quality natural zircon and show that lead diffuses much faster than uranium or thorium we find that the u th pb isotopic system in natural zircon will typically have a closure temperature greater than 900 c which explains why zircon is apparently such a robust geochrometer and is capable of remaining isotopically closed through extended periods of high grade metamorphism and partial melting of the host rock

the crystal chemical basis for ar retention in micas inferences from interlayer partitioning and implications for geochronology cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length strength in these minerals these data therefore provide a useful crystal chemical perspective on relative mass transfer kinetics of radiogenic isotopes and account for the observation that biotite is generally less retentive of 40ar and 87sr than coexisting muscovite partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite first the preferences of large rb cs in biotite and of small la3 and na in muscovite indicate a relatively spacious interlayer volume in biotite suggesting a longer mean k o bond second the preference of interlayer vacancies in biotite with some all possibly h2o h3o filled suggests that its adjacent 2 1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite third the relative exclusion of large ba2 from biotite despite its large interlayer sites is attributed to o h bonds pointing into the interlayer cavity sub normal to 001 k ba2 h repulsion thereby induced by the bare proton both destabilizes ba2 and weakens k o bonds in contrast muscovite offers a more favorable electrostatic environment for ba2 substitution since its o h bonds are directed into the vacant m1 octahedral site sub parallel to 001 this hypothesis is supported by the observation that progressive f oh 1 exchange enhances ba2 partitioning into biotite phlogopite relative to coexisting muscovite these crystal chemical differences between biotite and muscovite are mirrored in calculated values of ionic porosity z i defined here as the percentage of their interlayer unit cell volume not occupied by ions a monitor of ionic packing density and geometry z i is inversely correlated with k o bond strength which appears to be the rate determining kinetic common denominator for a variety of processes affecting micas including those responsible for loss of radiogenic isotopes in biotite and muscovite accordingly the relatively longer weaker k o bonds of biotite are envisioned as being more easily stretched during volume diffusion or broken during recrystallization or retrograde alteration this in turn accounts for common observations of enhanced radiogenic ar sr loss and younger 40ar 39ar and rb sr ages in natural biotite high z i relative to coexisting muscovite lower z i significantly this pattern may arise irrespective of isotopic loss mechanism diffusion or recrystallization etc and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field petrographic and petrologic constraints extension of this partitioning porosity based synthesis leads to prediction of corollary age retentivity composition effects among chemically diverse trioctahedral and dioctahedral micas which are best field tested in terranes that cooled slowly under dry static conditions pressure effects on argon retention are also inferred from the porosity model

rare earth elements in synthetic zircon part 1 synthesis and rare earth element and phosphorus doping zircon crystals were grown from a li mo flux 7 5 mol li2moo4 86 5 mol moo3 to which equal molar proportions zro2 and li2sio3 were added 3 mol each the crystals were initially grown undoped but later dy was added to the flux without any other charge compensating species with dy and p added in equal molar proportions the zircon crystals incorporated 1 37 mol 6 99 wt dy and 1 36 mol 1 33 wt p a factor of 5 3 increase in dy over the crystals grown without p intentionally added to the flux the other p ree doped zircon crystals displayed an approximately 1000 fold increase in ree and p from la through lu a result of decreasing ionic radii the incorporation of p5 allowed in a general sense the xenotime type coupled substitution of ree3 y3 p5 zr4 si4 from la to nd however p exceeds ree from sm to gd the ree are generally equal to p and from tb to lu the ree exceed p the y and p doped zircon behaved more like middle ree mree doped zircon than heavy ree hree doped zircon crystals in their ability to incorporate y and p and to maintain charge neutrality to investigate the incorporation of dy with no p added to the flux the p to lree excess and the hree to p excess in the doped zircon secondary ion mass spectrometry sims analyses were completed on selected crystals in the dy doped crystals the sims analyses revealed minor amounts of p li and mo in the crystals these elements contributed to charge balance required by the excess dy in ree and p doped zircon the sims analyses detected li and mo and the li and mo may also provide charge balance for excess ree in the hree p doped crystals

interaction between the mid atlantic ridge and the azores hot spot during the last 85 myr emplacement and rifting of the hot spot derived plateaus 1 multiple and single beam bathymetric data are compiled over the azores plateau to produce a 1 km 1 km grid between latitudes 32 n and 49 n and longitudes 22 w and 43 w mantle bouguer anomalies are then calculated from this grid and the satellite derived gravity these grids provide new insights on the temporal and spatial variations of melt supply to the ridge axis the elevated seafloor of the azores plateau is interpreted as resulting from the interaction of a mantle plume with the mid atlantic ridge mar the presence of a large region of elevated seafloor associated with a thick crust between the great meteor seamounts and the azores platform on the africa plate and less developed conjugate structures on the north america plate favors genetic relations between these hot spot derived structures this suggests that a ridge hot spot interaction has occurred in this region since 85 ma this interaction migrated northward along the ridge axis as a result of the sse absolute motion of the africa plate following a direction grossly parallel to the orientation of the mar kinematic reconstructions from chron 13 35 ma to the present allow a proposal that the formation of the azores plateau began around 20 ma and ended around 7 ma a sharp bathymetric step is associated with the beginning of important melt supply around 20 ma the excess of melt production is controlled by the interaction of the ridge and hot spot melting zones the geometry and distribution of the smaller scale features on the plateau record episodic variations of the hot spot melt production the periodicity of these variations is about 3 5 myr following the rapid decrease of widespread volcanism the plateau was subsequently rifted from north to south by the mid atlantic ridge since 7 ma this rifting begins when the mar melting zone is progressively shifted away from the 200 km plume thermal anomaly these results bear important consequences on the motion of the africa plate relative to the azores hot spot they also provide an explanation to the asymmetric geochemical signature of the azores hot spot along the mid atlantic ridge

new seismic images of oceanic crustal structure remarkable new images of the internal structure of oceanic crust have been recorded from a two ship multichannel seismic survey in the western north atlantic results include strong planar dipping reflectors in the lower crust evidence of whole crustal failure generating reflectors from the top of the basement to the moho and banded layering in the lower crust and in some places in the upper mantle anomalous crust in the blake spur fracture zone is documented from both normal incidence and wide angle seismic profiles the internal structure of the crust records the magmatic and tectonic history of the crust as it was generated at and then moved away from the spreading center

plateaus from seafloor spreading ocean floor plateaus are not voluminous lava flows from central volcanoes as thought but anomalously thick oceanic crust suggest magnetic anomaly patterns from the shatsky rise in the northwestern pacific ocean

the timing of partial melting barrovian metamorphism and granite intrusion in the naxos metamorphic core complex cyclades aegean sea greece abstract the relationship between partial melting and granite intrusion in a classic barrovian metamorphic terrane has been assessed thirteen samples were dated by shrimp u pb zircon geochronology from the island of naxos greece one of the aegean metamorphic core complexes the effect of partial melting during peak barrovian metamorphism on naxos is recorded by fine

bulk density of surface crusts depth functions and relationships to texture abstract a range of soils from pleistocene deposits with sandy to sandy loam textures and a group of loess derived soils with predominantly silty textures were subjected to 60 mm of simulated rainfall to form structural seals after drying samples of the surface crusts were collected to determine their bulk densities at a high resolution of depth 0 15 mm using an immersion method the bulk density data obtained for each soil sample were plotted as a function of depth beneath the soil surface two models were fitted to these plots the first was an exponential decay type function as proposed by mualem et al 1990 and the second was a sigmoidal type of function assuming that maximum compaction had already progressed to some depth below the soil crust surface all of the results indicated a gradual decrease in the bulk density with depth below the surface until convergence with the initial bulk density of the undisturbed soil was attained the maximum bulk densities recorded for crust segments representing the uppermost 2 mm of the crusts ranged from 1 713 to 1 91 g cm 3 for soils with silty sand loamy sand or sandy loam textures crusts of loess derived soils showed lower values ranging from 1 44 to 1 65 g cm 3 the maximum surface bulk density was shown to be highly significantly correlated with the log of geometric mean diameter of the primary grain size distribution in most cases both models showed good to very good fits to the measured data the exponential decay function appeared to better represent the initial stages of surface compaction and the sigmoidal function the later stages of structural crust formation

the iceland plate boundary zone propagating rifts migrating transforms and rift parallel strike slip faults unlike most of the mid atlantic ridge the north america eurasia plate boundary in iceland lies above sea level where magmatic and tectonic processes can be directly investigated in subaerial exposures accordingly geologic processes in iceland have long been recognized as possible analogs for seafloor spreading in the submerged parts of the mid ocean ridge system combining existing and new data from across iceland provides an integrated view of this active mostly subaerial plate boundary the broad iceland plate boundary zone includes segmented rift zones linked by transform fault zones rift propagation and transform fault migration away from the iceland hotspot rearrange the plate boundary configuration resulting in widespread deformation of older crust and reactivation of spreading related structures rift propagation results in block rotations that are accommodated by widespread rift parallel strike slip faulting the geometry and kinematics of faulting in iceland may have implications for spreading processes elsewhere on the mid ocean ridge system where rift propagation and transform migration occur

tectonic evolution of the northern pacific plate and pacific farallon izanagi triple junction in the late jurassic and early cretaceous m21 m10 abstract magnetic lineations in the northwestern pacific near the shatsky plateau were interpreted to trace the tectonic history of the northern pacific plate from the time of anomalies m21 to m10 at m22 time the pacific farallon izanagi p f i triple junction was located southwest of the plateau and migrating north northwest with respect to the pacific plate shortly thereafter the stress field acting on the three plates changed significantly and the pacific lzanagi ridge reacted by pivoting 24 clockwise from m21 to m19 time several ridge propagation events occurred while the ridge adjusted to the new spreading regime to the east of the m22 location of the junction it appears that this reorganization triggered a series of at least three ridge jumps along the pacific farallon ridge the geometry of magnetic isochron m21 suggests that several triple junctions existed simultaneously during the reorganization bordering one or more microplates differences in the spreading rate of the japanese magnetic lineations north of the plateau imply that one microplate which we call the shatsky microplate existed until about m15 m13 time it was probably captured by either the farallon or the izanagi plate and subsequently destroyed by subduction after m15 the p f i triple junction was reformed on seafloor that is now beneath the shatsky plateau the junction drifted generally northeast either in a ridge ridge ridge or in a ridge fault fault configuration perhaps alternating between the two from m11 m10 time the stress field on the three plates changed again and the pacific farallon ridge southeast of the plateau reoriented 15 clockwise at this time the p f i junction assumed a ridge ridge ridge geometry at the northeast end of the plateau

zircons from syros cyclades greece recrystallization and mobilization of zircon during high pressure metamorphism zircons were studied from high pressure low temperature metamorphosed meta igneous lithologies from syros these rocks carry several zircon generations related to each other by dissolution reprecipitation processes one generation is pristine zircon that shows growth zoning relatively elevated contents of trivalent cations and high th u ratios the other end member is a skeletal zircon generation with negligible trivalent cation contents and low th u ratios 0 1 texturally between these two there is a range of zircon crystals with complex inclusion populations of y hree th phases and fluid inclusions showing variable progress of replacement recrystallization both end members yield distinct sensitive high resolution ion microprobe shrimp u pb ages the pristine generation has an age of 80 2 1 6ma from a metagabbro and 76 4 2 1ma from a meta plagiogranite dyke the skeletal low th u zircon generation yields an age of 52 4 0 8ma the older late cretaceous zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust the younger eocene age however dates a zircon recrystallization event which possibly coincides with high solubility and mobility of high field strength elements in a high pressure aqueous fluid phase intergrowth relations between zircon and peak metamorphic garnet and excellent agreement of the u pb ages with white mica ar ar ages for the same samples support the conclusion that eocene is the true age of high pressure metamorphism on syros

the age of earth s largest volcano tamu massif on shatsky rise northwest pacific ocean this study presents laser step heating 40ar 39ar age determinations of basaltic lava samples from tamu massif the oldest and largest edifice of the submarine shatsky rise in the northwest pacific and earth s proposed largest volcano the rocks were recovered during integrated ocean drilling program expedition 324 which cored 160 m into the igneous basement near the summit of tamu massif the analyzed lavas cover all three major stratigraphic groups penetrated at this site and confirm a late jurassic early cretaceous age for the onset of shatsky rise volcanism lavas analyzed from the lower and middle section of the hole yield plateau ages between 144 4 1 0 and 143 1 3 3 ma with overlapping analytical errors 2 whereas a sample from the uppermost lava group produced a significantly younger age of 133 9 2 3 ma suggesting a late or rejuvenated phase of volcanism the new geochronological data infer minimum average melt production rates of 0 63 0 84 km3 a over a time interval of 3 4 million years consistent with the presence of a mantle plume

magnetic lineations within shatsky rise northwest pacific ocean implications for hot spot triple junction interaction and oceanic plateau formation oceanic plateaus are major ocean features yet their origins and development are poorly understood many are huge piles of basalt and it is widely accepted that mantle plumes are their source perhaps from eruptions of the voluminous head of an emerging plume shatsky rise is a basaltic plateau that formed during a period of geomagnetic reversals unlike many mid cretaceous plateaus so magnetic data can help us understand its tectonic history in this study we analyzed magnetic anomaly data from 131 cruises over and around shatsky rise and constructed a magnetic lineation chart for tectonic interpretation a significant finding is that magnetic lineations are traceable through low parts of the rise between volcanic massifs indicating nearly normal lithosphere and between large volcanic edifices many lineations form bights near the rise axis and show former locations of the pacific izanagi farallon triple junction they indicate that the junction was in a ridge ridge ridge rrr configuration and closely followed the rise axis from chron m20 146 ma to chron m4 127 ma lineation and bight geometries indicate that the triple junction jumped northeastward at least nine times often apparently in response to volcanic activity from the mantle plume some jumps however appear related to ridge reorientations numerous features within the rise are parallel to lineations or lineation offsets implying that the plateau formed near the triple junction and was modified by ridge faulting most of our observations support the hypothesis that shatsky rise was formed by a mantle plume that captured a triple junction

a 220 km2 recently erupted lava field on the east pacific rise near lat 8 s seamarc ii records show a 220 km2 area of unusually high reflectivity on the east pacific rise epr near lat 8 s which we interpret to be a recently erupted lava field this is the only segment of the epr exhibiting high reflectivity over such a large area in our continuous seamarc ii coverage from lat 8 n to 18 n and 3 s to 23 s the lavas appear to issue from the axial neovolcanic zone between lat 8 15 and 8 17 s the lava field extends up to 18km off axis and inundates scarps up to 100 m high on the basis of measurements of scarp heights just outside areas where they are buried we estimate the average thickness of the field to be 70 20 m thus the volume of the lava field is 15 4 km3 in comparison the average annual volcanic budget for the earth is approximately 4 5 km3 and the largest historic basaltic eruption from the icelandic laki fissure in 1783 was approximately 12 3 km3 when did the eruption occur comparative studies of acoustic reflectivity and photographed bottom characteristics elsewhere along the epr suggest that the lava field has little or no sediment cover and is very young this is also one of the only areas on the epr where earthquakes occur that do not appear to be caused by transform faulting if the earthquakes are related to the lava field then major eruptions may have occurred in 1964 1965 and 1969 a large helium plume near lat 13 s may be caused in part by eruptions of the lat 8 s lava field this is consistent with estimates of currents at 2 3 km depth which are such that a plume generated at lat 8 s would eventually appear off axis at lat 13 s our observations suggest that this spreading segment which is bounded by small ridge axis discontinuities at lat 8 05 s and 8 47 s may have been the most volcanically active segment along the epr during the past 25 yr

extending comprehensive models of the earth s magnetic field with orsted and champ data summary a new model of the quiet time near earth magnetic field has been derived using a comprehensive approach which includes not only pogo and magsat satellite data but also data from the orsted and champ satellites the resulting model shows great improvement over its predecessors in terms of completeness of sources time span and noise reduction in parameters with its well separated fields and extended time domain of 1960 to mid 2002 the model is able to detect the known sequence of geomagnetic jerks within this frame and gives evidence for an event of interest around 1997 because all sources are coestimated in a comprehensive approach intriguing north south features typically filtered out with other methods are being discovered in the lithospheric representation of the model such as the s atlantic spreading ridge and andean subduction zone lineations in addition this lithospheric field exhibits significantly less noise than previous models as a result of improved data selection the f region currents through which the satellites pass are now treated as lying within meridional planes as opposed to being purely radial results are consistent with those found previously for magsat but an analysis at orsted altitude shows exciting evidence that the meridional currents associated with the equatorial electrojet likely close beneath the satellite besides the model a new analysis technique has been developed to infer the portion of a model parameter state resolved by a particular data subset this has proven very useful in diagnosing the cause of peculiar artefacts in the magsat vector data which seem to suggest the presence of a small misalignment bias in the vector magnetometer

international geomagnetic reference field the eleventh generation the eleventh generation of the international geomagnetic reference field igrf was adopted in december 2009 by the international association of geomagnetism and aeronomy working group v mod it updates the previous igrf generation with a definitive main field model for epoch 2005 0 a main field model for epoch 2010 0 and a linear predictive secular variation model for 2010 0 2015 0 in this note the equations defining the igrf model are provided along with the spherical harmonic coefficients for the eleventh generation maps of the magnetic declination inclination and total intensity for epoch 2010 0 and their predicted rates of change for 2010 0 2015 0 are presented the recent evolution of the south atlantic anomaly and magnetic pole positions are also examined