mantle wedge anisotropy in the hikurangi subduction zone central north island new zealand 1 the anisotropic characteristics of the hikurangi subduction zone in central north island new zealand are studied using shear wave splitting measurements from 55 local earthquakes recorded on a 200 km profile of 19 stations the eastern fore arc shows trench parallel fast directions and a low average delay time of 0 2 s the source of anisotropy is confined to the upper 60 km and is likely to be deformation induced within the overriding australian plate central western north island shows n s trending fast anisotropy which may signify the presence of a viscous blanket of mantle material being entrained by the absolute motion of the australian plate between these regions a dramatic switch to extension parallel fast directions and delay times of 0 35 s are seen across the back arc in the taupo volcanic zone tvz these results suggest asthenospheric flow beneath the overriding plate with the olivine a axes oriented in the extension direction down to a maximum depth of 100 km

african laterite dynamics using in situ produced 10be abstract we have investigated the development of iron crust lateritic systems and rain forest soils in tropical environments using in situ produced 10be in quartz veins and cobbles the variability of its concentration as function of subsurface depth in soil profile provides criteria for distinguishing between allochthonous elsewhere formation and autochthonous in situ formation processes and hence for determining whether colluvial transport or in situ chemical weathering dominates soil development in these environments in the stable west african craton in southwest burkina faso at larafella the exponential decrease in 10be concentration observed along a quartz vein from the lowland lateritic system corresponds to the decrease in its production rate associated with attenuation of cosmic rays and thus indicates an autochthonous development models of the 10be depth profile in this unit suggest that this surface has been subject to erosion at a mean rate of 2 m my 1 and that 1 5 of the surface 10be production rate is associated with muon induced reactions in a contrasting environment tropical rain forest malemba congo 10be concentrations have been measured in quartz vein and in round and angular quartz cobbles incorporated in a downslope stone line comparison with data from larafella suggests that the angular cobbles have been transported a few meters from the quartz veins by downslope lateral creeping whereas the round cobbles have an allochthonous origin models of 10be distributions as this site yield an erosion rate of 12 m my 1 and a rate of lateral creeping on the order of 60 m my 1

a petrogenetic grid for pelites in the ballachulish and other scottish thermal aureoles the ballachulish granite is a composite devonian intrusive complex surrounded by a distinctive thermal aureole developed in regionally deformed and metamorphosed dalradian sediments on the w coast of scotland the most abundant rocks in the aureole are pelites which show a progression of assemblages from quartz muscovite chlorite up to a variety of high grade assemblages involving combinations of cordierite corundum spinel and rarely hypersthene and garnet metamorphic zones have been mapped around the granite which are defined by the following reactions going upgrade mu chl q cd bi v mu bi q cd kf v or mu cd q bi as v mu q as kf v q bi as cd kf v or mu cd bi as kf v mu cor kf v bi as cor kf cd v the restricted occurrences of assemblages involving spinel hypersthene and garnet do not allow higher grade zones to be mapped variations in the reaction sequence as a consequence of bulk compositional factors in particular the development of quartz bearing versus quartz absent assemblages are described details of the mineral assemblages from ballachulish are combined with high grade assemblage data from the belhelvie lochnagar and comrie aureoles to construct a comprehensive schematic petrogenetic grid the grid involves the minerals quartz chlorite muscovite biotite cordierite alumino silicate k feldspar corundum spinel hypersthene and garnet whose assemblage relationships are modelled in the system k 2 o feo mgo al 2 o 3 sio 2 h 2 o kfmash

igneous microstructures in migmatites clear differences between the microstructures of leucosomes and mesosomes occur in some nebulitic migmatites in the proterozoic arunta block central australia the leucosomes show crystal faces of k feldspar microcline microperthite cordierite andalusite and plagio clase against quartz indicating crystallization of a melt in contrast the mesosomes show predominantly polygonal grain shapes modified by 001 crystal faces of biotite and rare crystal faces in some porphyroblasts of garnet cordierite and orthopyroxene indicating re crystallization in the solid state the occurrence of abundant crystal faces in minerals such as feldspar cordierite and andalusite which typically grow crystal faces in magmas but generally not in metamorphic rocks is evidence of former melt therefore igneous microstructures can be preserved in leucosomes that have not undergone penetrative subsolidus deformation and recrystallization the cores of many of the crystals of cordierite and k feldspar have small locally oriented inclusions similar to those in the same minerals in the mesosomes these indicate that the cores remained solid during crystallization of the leucosome which produced inclusion free euhedral rims these crystals provide criteria for recognizing possible restite in granitoid magmas

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

seismological imaging of ridge arc interaction beneath the eastern lau spreading center from obs ambient noise tomography abstract the lau basin displays large along strike variations in ridge characters with the changing proximity of the adjacent subduction zone the mechanism governing these changes is not well understood but one hypotheses relates them to interaction between the arc and back arc magmatic systems we present a 3d seismic velocity model of the shallow mantle beneath the eastern lau back arc spreading center elsc and the adjacent tofua volcanic arc obtained from ambient noise tomography of ocean bottom seismograph data our seismic images reveal an asymmetric upper mantle low velocity zone lvz beneath the elsc two major trends are present as the ridge to arc distance increases 1 the lvz becomes increasingly offset from the ridge to the north where crust is thinner and the ridge less magmatically active 2 the lvz becomes increasingly connected to a sub arc low velocity zone to the south the separation of the ridge and arc low velocity zones is spatially coincident with the abrupt transition in crustal composition and ridge morphology our results present the first mantle imaging confirmation of a direct connection between crustal properties and uppermost mantle processes at elsc and support the prediction that as elsc migrates away from the arc a changing mantle wedge flow pattern leads to the separation of the arc and ridge melting regions slab derived water is cutoff from the ridge resulting in abrupt changes in crustal lava composition and crustal porosity the larger offset between mantle melt supply and the ridge along the northern elsc may reduce melt extraction efficiency along the ridge further decreasing the melt budget and leading to the observed flat and faulted ridge morphology thinner crust and the lack of an axial melt lens

cosmogenic 3 he in hematite and goethite from brazilian canga duricrust demonstrates the extreme stability of these surfaces helium isotopes were measured in hematite and goethite samples from several lateritiric duricrusts canga developed on banded iron formations these samples uniformly have high 3 he concentrations which must arise from long periods of cosmic ray exposure from coexisting phases from the quadrilatero ferrifero in east central brazil we determined the ratio of cosmogenic 3 he in hematite to that of 21 ne in quartz to be 3 96 0 19 combined with best current estimates of the 21 ne production rate in quartz this ratio implies a sea level high latitude slhl 3 he production rate in hematite of 68 1 8 1 atoms g yr from the chemical composition we estimate the 3 he production rate in goethite to be 5 higher we use these production rate estimates to interpret 3 he concentrations measured in goethite and hematite from a 10 m depth profile collected from a surface canga in carajas in the amazon basin of brazil we find that the carajas canga has experienced a very low rate of surface erosion 0 16 0 54 m myr over at least the last few millions of years this iron rich canga surface is remarkably resistant to erosion despite its location in a wet tropical environment details of the depth profile suggest that despite its stability the canga has also been internally dynamic translocation of material solution and reprecipitation over million year timescales

cosmogenic 3he concentrations in ancient flood deposits from the coombs hills northern dry valleys east antarctica interpreting exposure ages and erosion rates abstract in situ produced cosmogenic 3 he analyses provide independent support for the model of a stable hyper arid polar climate persisting in east antarctica since the mid miocene and provide quantitative constraints on long term rates of erosion within the dry valleys in the coombs hills area a series of cobble size boulders form mega ripples with wavelengths of approximately 50 m their topographic position and association with features characteristic of scabland such as stripped corrugated bedrock surfaces indicate the boulders were deposited by subglacial floodwaters such outburst flooding could only have occurred during overriding of the northern dry valleys by a greatly expanded east antarctic ice sheet timing of the overriding episode has been previously assigned to 14 8 to 13 6 ma by correlation with volcanic ash deposits dated by 40 ar 39 ar in the asgard range of the dry valleys cosmogenic 3 he concentrations in clinopyroxene from ferrar dolerite boulders are consistent with 8 6 to 10 4 ma exposure calculated using scaling factors appropriate for antarctica and assuming zero erosion these are among the oldest surface exposure dates yet measured on earth but are not however consistent with the 40 ar 39 ar chronology used to define the age of the landscape due to unconstrained levels of erosion erosion rates of 0 03 0 06 m ma 1 are necessary to have produced the measured boulder exposure age if they were deposited at 14 8 ma these are less than half the steady state erosion rate derived from cosmogenic 3 he in the nearby bedrock surfaces 0 17 m ma 1 and testify to the extreme stability of the landscape

infiltration driven dehydration and anatexis in granulite facies metagabbro grenville province ontario canada dark hornblende garnet rich quartz absent metagabbro boudins from the seguin subdomain ontario grenville province are transected by anastomosing light coloured veins rich in orthopyroxene clinopyroxene plagioclase and sometimes quartz the veins vary in texture from fine grained diffuse veins and patches that overprint the metagabbro to coarse tonalitic leucosomes with sharp borders the diffuse veins and patches are suggestive of channellized subsolidus dehydration of the metagabbro while the tonalitic leucosomes are suggestive of local internally derived anatexis all vein types grade smoothly into each other with the tonalitic leucosomes being the latest relative to the host metagabbro the veins have higher si na ba sr lower fe mg ca ti and similar al the coarser veins are enriched in k plagioclase becomes steadily enriched in na in the transition from host metagabbro an47 to the veins an35 and in the coarsest veins it is antiperthitic differences in composition of the other minerals between host metagabbro and vein are minor pressure temperature estimates are scattered but indicate a minimum temperature during vein formation of 700 c at about 8 kbar mass balance constraints indicate that the veins formed from the metagabbro in an open system the transecting veins are interpreted to represent pathways of si na ba sr k al enriched low ah2o fluids that metasomatized the host metagabbro to form the anhydrous veins an initial period of localized solid state dehydration of the metagabbro represented by the diffuse veins was followed by a transition to localized anatexis represented by the tonalitic leucosomes the change to anatexis may have been due to the addition of k to the infiltrating fluid the source and delivery mechanism of the fluids is unknown

from rifting to active spreading in the lau basin havre trough backarc system sw pacific locking unlocking induced by seamount chain subduction associated with pacific australia plate convergence the lau basin havre trough is an active back arc basin that has been opened since 5 5 ma by rifting and southward propagating oceanic spreading current back arc opening rates decrease from 159 mm yr 1 in the northern lau basin to 15 mm y 1 in the southern havre trough major tectonic changes occur at the transition between havre trough rifting and full oceanic spreading of the eastern lau spreading center elsc where the oblique to trench westward subducting louisville seamount chain lsc sweeps southwards along the tonga trench new swath bathymetry seismic reflection data and limited rock sampling in this area constrain a tectonic and kinematic back arc model that incorporates the effects of lsc subduction the elsc which extends southward to 24 55 s forms a deep rift valley propagating southward through older rifted arc basement present day seismicity and fresh and fractured pillow lavas at 23 42 s are consistent with rift valley neovolcanism conversely the northern havre trough has low seismicity and rifted volcanic basement ridges trending 25 45 oblique to the basin axis consistent with low levels of extensional tectonism and volcanism this latter structural fabric is interpreted as an early stage of rifting that is now locked due to compression on the arc exerted by lsc subduction while in the lau basin such effects have passed as the lsc swept along the tonga trench it is proposed that the lau havre back arc opening is controlled by tectonic constraints exerted at the limits of the system by the lsc subduction which determines the southward migration of the tonga arc pole of rotation and associated lau basin opening a discrete three stage back arc opening evolution is proposed comprising 1 an initial phase of back arc rifting along the whole length of the plate boundary beginning at 6 5 ma 2 a subsequent phase mostly present in the southern part of the back arc domain and still active in the havre trough of transpression and transtension starting at 4 ma in the north as the lsc starts to subduct and sweeps southward along the tonga trench and 3 a renewed opening phase in the northern segment of the back arc domain with rifting and spreading starting at 3 5 ma as subduction of the lsc along the northern tonga trench is progressively completed

shear wave polarization anisotropy and subduction induced flow in the mantle wedge of northeastern japan abstract shear wave polarization anisotropy in the central part of northeastern ne japan was investigated we analyzed s phases of 293 intermediate depth earthquakes recorded at 77 stations and obtained 1286 splitting parameters the obtained results show the systematic variation in splitting parameters across the arc the leading shear wave polarization directions fast direction obtained at stations in the western side of the study area are oriented nearly e w which is sub parallel to the direction of relative plate motion between the pacific plate and the north american okhotsk plate in contrast fast directions obtained at stations in the eastern side of the study area are oriented approximately n s stations in the western side have a larger average delay time compared to those in the eastern side these variations suggest that the nature of anisotropy is quite different between the eastern and the western sides both experimental studies on the deformation of olivine and numerical simulations of plate driven flow predict that the lattice preferred orientation of olivine causes the fast direction sub parallel to the flow direction for the conditions in the mantle wedge of ne japan we thus infer that the lattice preferred orientation of olivine generated by flow induced strain is the most likely candidate for anisotropy that produces splitting with e w fast direction three possible causes of anisotropy in the eastern side are considered deformation of water rich olivine in the mantle wedge trench parallel flow in the mantle wedge due to along strike dip variations in the slab and anisotropy in the crust and the slab we could not however determine which causes are dominant further systematic study is required to reveal the cause of anisotropy in the eastern side

cordierite as a sensor of fluid conditions in high grade metamorphism and crustal anatexis cordierite h2o and co2 volatile saturation surfaces derived from recent experimental studies are presented for p t conditions relevant to high grade metamorphism and used to evaluate fluid conditions attending partial melting and granulite formation the volatile saturation surfaces and saturation isopleths for both h2o and co2 in cordierite are strongly pressure dependent in contrast the uptake of h2o by cordierite in equilibrium with melts formed through biotite dehydration melting controlled by the distribution of h2o between granitic melt and cordierite dw dw wt h2o melt wt h2o crd is mainly temperature dependent dw 2 5 6 0 for the h2o contents 0 4 1 6 wt percentage typical of cordierite formed through biotite dehydration melting at 3 7 kbar and 725 900 c this range in dw causes a 15 30 relative decrease in the total wt of melt produced from pelites cordierite in s type granites are h2o rich 1 3 1 9 wt and close to or saturated in total volatiles signifying equilibration with crystallizing melts that achieved saturation in h2o in contrast the lower h2o contents 0 6 1 2 wt percentage preserved in cordierite from many granulite and contact migmatite terranes are consistent with fluid absent conditions during anatexis in several cases including the cooma migmatites and broken hill granulites the cordierite volatile compositions yield ah2o values 0 15 0 4 and melt h2o contents 2 2 4 4 wt compatible with model dehydration melting reactions in contrast h2o leakage is indicated for cordierite from prydz bay antarctica that preserve h2o contents 0 5 0 3 wt which are significantly less than those required 1 0 0 8 wt for equilibrium with melt at conditions of 6 kbar and 860 c the co2 contents of cordierite in migmatites range from negligible 0 1 wt to high 0 5 1 0 wt and bear no simple relationship to preserved cordierite h2o contents and ah2o in most cases the cordierite volatile contents yield total calculated fluid activities ah2o aco2 that are significantly less than those required for fluid saturation at the p t conditions of their formation whether this reflects fluid absence dilution of h2o and co2 by other components or leakage of h2o from cordierite is an issue that must be evaluated on a case by case basis

a record of impacts preserved in the lunar regolith abstract the absolute chronology of meteoroid impacts on the moon is largely quantified by only a few 40 ar 39 ar plateau ages of rocks thought to be associated with specific impact events stoffler et al 2006 we demonstrate a more broadly applicable approach by using high resolution 40 ar 39 ar thermochronometry to investigate the physical conditions responsible for partially reset k ar systems in lunar rocks seven rocks from apollo 16 regolith sample 63503 have plateau ages of either 3 9 billion yr ga or 4 2 ga and all experienced varying degrees of partial resetting concordance between diffusion kinetics and the degree of resetting among all samples shows that these observations are best explained by a heating event 3 3 ga ago that lasted between 10 3 s at 600 c and 20 yr at 300 c we conclude that partial resetting of the k ar systems in these samples record an impact event 3 3 ga ago that mixed several preexisting ejecta units in the cayley plains if partially reset 40 ar 39 ar ages of other lunar highland samples also constrain the timing of late stage reheating associated with impact events they constitute an additional record of impacts preserved in the lunar regolith a review of existing datasets from this perspective reveals that episodic pulses in the impactor flux in the inner system are common and most likely related to dynamical events in the asteroid belt or outer solar system

new insights into the carrier phase s of extraterrestrial 3he in geologically old sediments to better understand the composition characteristics of helium diffusion and size distribution of interplanetary dust particles idps responsible for the long term retention of extraterrestrial 3he we carried out leaching stepped heating and sieving experiments on pelagic clays that varied in age from 0 5 ma to 90 myr the leaching experiments suggest that the host phase s of 3he in geologically old sediments are neither organic matter nor refractory phases such as diamond graphite al2o3 and sic but are consistent with extraterrestrial silicates fe ni sulfides and possibly magnetite stepped heating experiments demonstrate that the 3he release profiles from the magnetic and non magnetic components of the pelagic clays are remarkably similar because helium diffusion is likely to be controlled by mineral chemistry and structure the stepped heating results suggest a single carrier that may be magnetite or more probably a phase associated with magnetite furthermore the stepped outgassing experiments indicate that about 20 of the 3he will be lost through diffusion at seafloor temperatures after 50 myrs while sedimentary rocks exposed on the earth s surface for the same amount of time would lose up to 60 the absolute magnitude of the 3he loss is however likely to depend upon the 3he concentration profile within the idps which is not well known contrary to previous suggestions that micrometeorites in the size range of 50 100 m in diameter are responsible for the extraterrestrial 3he in geologically old sediments stuart f m harrop p j knott s turner g 1999 laser extraction of helium isotopes from antarctic micrometeorites source of he and implications for the flux of extraterrestrial 3he flux to earth geochim cosmochim acta63 2653 2665 our sieving experiment demonstrates that at most 20 of the 3he is carried by particles greater than 50 m in diameter the size distribution of the 3he bearing particles implies that extraterrestrial 3he in sediments record the idp flux rather than the micrometeorite flux

using geochemistry to map mantle flow beneath the lau basin geochemical tracers can be used to map mantle flow beneath the lau basin tonga region over the past 6 m y and constrain the extent and means by which different components contribute to the magmas erupted there helium isotope data track the samoan plume to show that it infiltrated into the northwestern part of the lau basin at 40 mm yr 1 through an opening tear in the pacific plate in contrast to other recent studies our analysis indicates that this plume does not contribute to the arc lavas a louisville plume component is present in the northern tonga arc lavas but this was added from volcaniclastic sediments transported into the arc on the subducting slab and does not require the presence of plume material within the mantle wedge the louisville tracer has been used to infer that the mantle wedge beneath the island arc is downwelling at the rate of 20 40 mm yr 1 owing to viscous drag against the downgoing pacific plate a third geochemical tracer provided by the distinction between indian and pacific mantle indicates that indian mantle has migrated eastward at rates of 45 65 mm yr 1 the inferred rates of migration indicate that the translation of geochemical signatures occurs by mantle flow rather than by the order of magnitude faster movement of partial melts except for the flow in the mantle wedge the directions of mantle flow in this region are unrelated to the overlying plate motions

seismic anisotropy predicted for 2 d plate driven flow in the lau back arc basin abstract the lau back arc basin and spreading centers exhibit significant along strike variations in plate motion rates and distance between the spreading centers and the tonga trench the mantle processes are expected to be complex in this region and seismic anisotropy offers a key constraint linked geodynamic and mineral alignment models of the mantle wedge predict the scale of anisotropic variation and underlying assumptions can be tested by comparison to observed anisotropy three cross basin transects allow us to isolate the effects of trench spreading center distance as well as rates of convergence and spreading lattice preferred orientations lpo corresponding to each transect s 2 d flow field are calculated for peridotite polycrystals predicted p wave anisotropy of several percent and shear wave split times up to 3 5 s are determined with measurable differences predicted between transects variation in the 2 d estimates of flow driven by each transect s plate kinematics will induce along strike flow in the mantle wedge we find that the rate of such flow would likely not alter the subduction spreading induced lpo sufficiently to explain the reported trench arc parallel fast seismic directions in the lau basin additional geodynamic factors must play a role

neogene weathering and supergene manganese enrichment in subtropical south china an 40ar 39ar approach and paleoclimatic significance deep weathering profiles and associated supergene mn oxides deposits are widely distributed in south china however data on their ages are not available until most recently ar 40 ar 39 laser incremental heating analysis of potassium bearing supergene mn oxides collected from four weathering profiles in the qinzhou fangcheng mn belt guangxi province south china provides for the first time numerical constraints on timing of weathering and supergene mn enrichment thirty eight of 42 cryptomelane grains yield well defined plateau 36 grains or pseudo plateau 2 grains ages ranging from 17 6 0 5 ma to 4 71 0 08 ma 2 sigma the results when combined with data from mn oxides deposits from the adjacent guangdong and hunan provinces document a protracted history of weathering and supergene mn enrichment spanning from the earliest miocene to the latest pliocene in south china this indicates that warm and humid climatic conditions conducive to intense weathering and secondary mineralization prevailed across south china during the whole neogene the climatic conditions inferred from weathering geochronology are consistent with sedimentary records and fossil floral and fauna associations from cenozoic inland and marginal sea basins of south china the weathering geochronology data also provide temporal constraints on the initiation and intensification of the east asian monsoon that has been the cause for the south china s humid climate c 2007 elsevier b v all rights reserved

effects of plate boundary geometry and kinematics on mantle melting beneath the back arc spreading centers along the lau basin abstract the back arc spreading centers that extend along the lau basin exhibit trends in axial morphology crustal thickness and geochemistry which are opposite those typically observed at mid ocean spreading centers we develop 2d numerical models of mantle flow thermal structure and melting of the lau back arc tonga subduction system for each of three lau spreading centers valu fa ridge the eastern lau and the central lau our goal is to determine whether along strike variability in the circulation pattern or hydration within the mantle wedge could explain the trends observed we use present day plate and subducted slab geometries and velocities to explore a range of mantle potential temperature and water content scenarios and test whether predictions match observations of crustal thickness and water content of the magmas erupted at the spreading ridges within the range of mantle parameters tested we find that a potential temperature of 1300 c and source water contents greater than 0 22 wt are required to match observed crustal thickness and lava water contents at valu fa substantially less water in the sub arc mantle source is required to match the eastern lau and central lau observations at the same or higher mantle potential temperatures a small background hydration of 0 01 wt water in the mantle wedge is required to match the observations of water in the central lau magmas we predict that the arc and back arc melting regions are interconnected for all potential temperatures and sub arc water contents at the eastern lau spreading center and at the valu fa ridge while at the central lau they are only connected for cases when mantle potential temperature is 1400 c or greater we hypothesize that the longer lived eastern lau and central lau rifting and axial volcanism may have dehydrated the mantle wedge and slowed melt production beneath these spreading centers the valu fa ridge which is actively propagating into a more hydrated wedge associated with the tofua arc experiences enhanced melting relative to the other two spreading centers for the valu fa case we show fast subduction in combination with the proximity of the trench and spreading center results in enhanced upwelling and therefore increased crustal production slower subduction with a convergence rate of 45 mm yr a dry mantle and a 1350 c mantle potential temperature reduces the difference in crustal thickness between the central lau and valu fa to within 1 2 km in contrast for present day kinematics with a dry mantle and 1350 c mantle potential temperature our models predict the crustal thickness at valu fa to be 3 1 km thicker than central lau much closer to the observed values

maximum and minimum water contents of granitic melts implications for chemical and physical properties of ascending magmas new results obtained by the investigation of liquidus and solidus phase relations in the system qz ab or and by the determination of h2o solubility in haplogranitic melts are used to discuss the evolution of granitic quartzo feldspathic magmas during their ascent in the crust particular attention is given to the role of h2o and its effect on physical properties of ascending magmas the evolution of melt fraction h2o content of the melt and viscosity of melt and magma during ascent are discussed for several examples adiabatic ascent ascent with cooling ascent with crystal fractionation it is shown that decompression melting processes may be very important significant increase of the melt proportion especially at low pressure in magmas ascending along adiabats or with slow cooling rates the comparison of the discussed examples shows that melts formed at low t are significantly less viscous than at high t although the degree of melting may remain very low in the case of dehydration melting at low temperature the low viscosity of the generated melts may enhance the extraction of these melts from the source area to produce leucogranites

unusually large shear wave anisotropy for chlorite in subduction zone settings using first principle simulations we calculated the elasticity of chlorite at a density 2 60 g cm 3 the elastic constant tensor reveals significant elastic anisotropy vp 27 vs1 56 and vs2 43 the shear anisotropy is exceptionally large for chlorite and enhances upon compression upon compression the shear elastic constant component c44 and c55 decreases whereas c66 shear component stiffens the softening in c44 and c55 is reflected in shear modulus g and the shear wave velocity vs our results on elastic anisotropy at conditions relevant to the mantle wedge indicates that a 10 20 km layer of hydrated peridotite with serpentine and chlorite could account for the observed shear polarization anisotropy and associated large delay times of 1 2 s observed in some subduction zone settings in addition chlorite could also explain the low vp vs ratios that have been observed in recent high resolution seismological studies

hydrous cordierite in granulites and crustal magma production huge volumes of s type granitic and volcanic rocks as well as many migmatites are the products of fluid absent partial melting in metasedimentary protoliths at granulite facies conditions thus it is essential to know the controls on the fertility of metasedimentary rocks as magma sources in the present experiments on fluid absent partial melting of biotite in metasedimentary rocks cordierite proportion is inversely correlated with melt fraction at constant pressure p and temperature t this variation in melt proportion is a function of bulk rock mg number 100 mg mg fe and activity of alumina alpha al subscript 2o subscript 3 however melt compositions are essentially unaffected at 0 5 gpa the calculated h2o contents of cordierite decrease as t increases from 850 to 1000 c in aluminous metasedimentary rocks the cordierite stability field considerably overlaps with the conditions of fluid absent melting in the middle to lower crust severely restricting melt productivity consequently metapelites may be much less fertile sources of granitic magma than metagraywackes

application of in situ produced cosmogenic 10be and 26al to the study of lateritic soil development in tropical forest theory and examples from cameroon and gabon abstract depth profiles of in situ produced cosmogenic nuclides including 10 be t 1 2 1 5 10 6 years and 26 al t 1 2 0 73 10 6 years in the upper few meters of the earth s crust may be used to study surficial processes quantifying denudation and burial rates and elucidating mechanisms involved in landform evolution and soil formations in this paper we discuss the fundamentals of the method and apply it to two lateritic sequences located in african tropical forests

fluids and melting under upper amphibolite facies conditions internal buffering paths on temperature water activity diagrams for the pelitic model system naaio 2 kaio 2 caai 2 o 4 al 2 sio 5 sio 2 h 2 o give rise to sequences of assemblages which correspond to those observed in upper amphibolite facies terranes in particular the persistence of the assemblage quartz alkali feldspar plagioclase muscovite sillimanite across a zone and the rapidly changing composition of plagioclase across this zone can be accounted for in a natural way

meteoric 10be in soil profiles a global meta analysis abstract in order to assess current understanding of meteoric 10 be dynamics and distribution in terrestrial soils we assembled a database of all published meteoric 10 be soil depth profiles including 104 profiles from 27 studies in globally diverse locations collectively containing 679 individual measurements this allows for the systematic comparison of meteoric 10 be concentration to other soil characteristics and the comparison of profile depth distributions between geologic settings percent clay 9 be and dithionite citrate extracted al positively correlate to meteoric 10 be in more than half of the soils where they were measured but the lack of significant correlation in other soils suggests that no one soil factor controls meteoric 10 be distribution with depth dithionite citrate extracted fe and cation exchange capacity are only weakly correlated to meteoric 10 be percent organic carbon and ph are not significantly related to meteoric 10 be concentration when all data are complied the compilation shows that meteoric 10 be concentration is seldom uniform with depth in a soil profile in young or rapidly eroding soils maximum meteoric 10 be concentrations are typically found in the uppermost 20 cm in older more slowly eroding soils the highest meteoric 10 be concentrations are found at depth usually between 50 and 200 cm we find that the highest measured meteoric 10 be concentration in a soil profile is an important metric as both the value and the depth of the maximum meteoric 10 be concentration correlate with the total measured meteoric 10 be inventory of the soil profile in order to refine the use of meteoric 10 be as an estimator of soil erosion rate we compare near surface meteoric 10 be concentrations to total meteoric 10 be soil inventories these trends are used to calibrate models of meteoric 10 be loss by soil erosion erosion rates calculated using this method vary based on the assumed depth and timing of erosional events and on the reference data selected

high 3 he 4 he ratio in ocean sediments two decades ago merrihue1 reported 3he 4he ratios of 10 4 in ferromagnetic separates from a pacific deep ocean red clay and concluded that the high ratio is due to extraterrestrial debris amounting to 1 of the sediment a decade later krylov et al 2 compiled 3he 4he isotopic data on ocean sediments measured in the soviet union and observed that the 3he 4he ratio is generally higher in pelagic sediments where the sedimentation rate is lower they suggested that the high 3he 4he ratio was attributable to extraterrestrial materials which were concentrated in slowly accumulating ocean floor however these important discoveries were almost completely neglected until we re examined the problem we have measured 39 sediments from 12 different sites 10 sites from the western to central pacific and two sites from the atlantic ocean we find 3he 4he ratios 5 10 5 for six sites well above the values generally observed in common terrestrial materials the very high 3he 4he ratio in the sediments is probably due to input of extraterrestrial materials input of stratospheric dust of 1 p p m which corresponds to a fallout rate of 2 000 tons per year can explain the observation

seismic anisotropy of the upper mantle 1 factors that affect mineral texture and effective elastic properties 1 flow induced mineral alignment in the earth s mantle affects the nature of seismic wave propagation since measurements of seismic travel time and shear wave splitting are a key means by which the structure of the upper mantle can be imaged it is important to understand the factors that contribute to variability in elastic properties seismic anisotropy associated with lattice preferred orientations of minerals in peridotite is the focus of this study one way to better understand anisotropy in a convecting upper mantle is to simulate texture evolution based on certain assumptions simulations of the development of olivine and orthopyroxene alignment along streamlines of a mantle flow field illustrate how continuously varying strain conditions affect the resulting orientation distribution there are various uncertainties in rock texture simulations and the aim here is to investigate how much model assumptions may influence the results a model of upper mantle flow in the vicinity of an oceanic spreading center is used to illustrate these points first we assess how sensitive results are on assumptions of different polycrystal plasticity models specifically lower bounds and viscoplastic self consistent approaches we also investigate how recrystallization during deformation might affect the texture that develops along a streamline the effects of grain growth and nucleation produce as expected textures that are different from the deformation only case however the basic p wave structure of the predicted anisotropy is similar between models for streamlines in a slow spreading passive flow model as is commonly used for simulating flow near this type of plate boundary shear wave splitting patterns are more complex and differ somewhat more between the models in the off axis region we also compare predictions for texture development of olivine only aggregates to that of mixed composition with 70 olivine and 30 orthopyroxene although we consider only one aspect of such polyphase deformation i e plastic deformation on specified crystal slip systems our results are consistent with field observations of the orientation distributions in ophiolitic peridotites finally we determine how simplifying assumptions about the symmetry of the elastic anisotropy can bias interpretations of seismic travel time and shear wave splitting whereas a hexagonal approximation can lead to underestimates of the degree of anisotropy an orthorhombic approximation is found to closely match the results predicted for a general elastic tensor corresponding to an orientation distribution whose symmetry is as low as monoclinic or triclinic

interpretation of sks waves using samples from the subcontinental lithosphere abstract the seismic properties of five south african kimberlite nodules have been calculated from petrofabric measurements these garnet lherzolite and harzburgite composition nodules 56 80 olivine are known from previous studies to have originated at depths of 120 170 km in the subcontinental lithosphere their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature 900 1050 c and pressure 3 0 3 5 gpa conditions the average of the five samples has a maximum s wave anisotropy v s of 3 7 which is sufficiently high to explain previously reported teleseismic sks delay times t in continental shield areas of up to 1 7 s the biggest delay time is observed when the foliation is vertical the lineation horizontal and the fastest s wave polarized parallel to the lineation i e an orientation for transcurrent motion delay times of over 1 s can only be generated by this orientation a strong mantle crust mechanical coupling is suggested for such situations the potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine the most abundant phase 70 in the upper mantle the v p and v s seismic anisotropy increases with fabric strength and finite strain recrystallization tends to reduce fabric strength and seismic anisotropy resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50 60 strain or 8 9 v s for pure olivine aggregates a survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum s wave anisotropy for the olivine component of 9 or about 8 for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account the ophiolite samples are twice as anisotropic as the kimberlite nodules if an average anisotropy value is representative of the lithosphere then the sks delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal the magnitude of s wave anisotropy is less sensitive than v p to variations in olivine volume fraction in range 50 100 and to deformation or fabric intensity further suggesting that s wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness for smaller delay times there is some trade off between structural orientation and thickness

production rate of cosmogenic 21ne in quartz estimated from 10be 26al and 21ne concentrations in slowly eroding antarctic bedrock surfaces abstract we estimated the production rate of 21ne in quartz using a set of samples from slowly eroding sandstone surfaces in the antarctic dry valleys geologic evidence as well as cosmogenic 10be and 26al concentrations indicate that i these sites have experienced millions of years of surface exposure at low erosion rates and ii steady erosion has been sustained long enough that surface 10be and 26al concentrations have reached equilibrium with the erosion rate under these conditions surface cosmogenic 21ne concentrations should be a function only of the erosion rate and the 21ne production rate as the erosion rate can be determined from 10be and 26al concentrations this allows an estimate of the 21ne production rate estimating the reference 21ne production rate on this basis with the assumption that all 21ne production is by neutron spallation yields a poor fit to measured 21ne concentrations and a systematic residual that is correlated with the erosion rate of the sample site the same steady erosion assumption with a production model that includes production by deeply penetrating muons yields a good fit both to our measurements and to similar independent measurements from an antarctic bedrock core both data sets together yield a total reference 21ne production rate of 18 3 0 4 atoms g 1 a 1 of which 0 66 0 10 atoms g 1 a 1 is due to muon interactions

a complex pattern of mantle flow in the lau backarc shear wave splitting analysis of local events recorded on land and on the ocean floor in the tonga arc and lau backarc indicate a complex pattern of azimuthal anisotropy that cannot be explained by mantle flow coupled to the downgoing plate these observations suggest that the direction of mantle flow rotates from convergence parallel in the fiji plateau to north south beneath the lau basin and arc parallel beneath the tonga arc these results correlate with helium isotopes that map mantle flow of the samoan plume into the lau basin through an opening tear in the pacific plate

cenozoic variations in the flux of interplanetary dust recorded by 3he in a deep sea sediment helium 3 concentrations and 3he 4he ratios in modern pelagic sediments are known to be far in excess of terrestrial values as a result of micrometeorite fallout here i report that extraterrestrial helium is easily detected in a pelagic clay core dating back more than 70 myr the remarkable preservation of the extraterrestrial signature arises from high retention of 3he within interplanetary dust particles coupled with loss of radiogenic 4he from terrestrial mineral grains the core provides a continuous record of the fallout of extraterrestrial helium for the cenozoic era this record suggests that there have been significant variations in the influx of interplanetary dust through time probably related to asteroidal breakup events or the passage of comets through the inner solar system the results also show 3he to be a far more sensitive tracer of the interplanetary dust flux than is iridium

determining the orientations of ocean bottom seismometers using ambient noise correlation 1 the cross correlation of multicomponent ambient seismic noise can reveal both the velocity and polarization of surface waves propagating between pairs of stations we explore this property to develop a novel method for determining the horizontal orientation of ocean bottom seismometers obs by analyzing the polarization of rayleigh waves retrieved from ambient noise cross correlation we demonstrate that the sensor orientations can be estimated through maximizing the correlation between the radial vertical component and the phase shifted vertical vertical component of the empirical green s tensor we apply this new method to the elsc eastern lau spreading center obs experiment data set and illustrate its robustness by comparing the obtained orientations with results from a conventional method utilizing teleseismic p and rayleigh wave polarizations when applied to a large obs array the ambient noise method provides a larger number of orientation estimates and better azimuthal coverage than typically is possible with traditional methods

u pb geochronology of archean magmatism and basement reactivation in the carajas area amazon shield brazil abstract the archean carajas subprovince is a metallogenic district bif bauxite cu ni mn au located in the easternmost amazonian craton in brazil the main conclusions from a u pb study of the itacaiunas supergroup and a locality in the xingu complex are the following the xingu basement complex the oldest unit underwent migmatization at 2859 2 ma and also grew new zircon at 2851 4 ma the luanga mafic intrusion is 2763 6 ma old and coeval with the 2759 2 ma felsic volcanics of the grao para group amphibolites from the salobo group contain 2851 ma zircons were metamorphosed 2761 3 ma and intruded by granitic veins ca 2758 ma at the time of grao para magmatism and also grew new zircon at 2732 2742 ma the data indicate that the salobo group is part of the xingu complex and not of the itacaiunas supergroup as heretofore believed the pojuca group possibly coeval with the salobo group was metamorphosed at 2732 3 ma a late archean reactivation event of regional extent is marked by titanite growth at 2581 5 ma in the mirim area intrusion of the old salobo granite at 2573 2 ma new zircon growth in salobo group amphibolites at 2555 4 3 ma and monazite growth in a metamorphosed iron formation at 2551 2 ma in addition k metasomatism associated with shear zones formed titanite in salobo group amphibolites at 2497 5 ma and new titanite at 2519 5 ma in the xingu complex events in the 2581 2497 ma interval are probably associated with the latest activity of a wnw ese sinistral shear system which could also be responsible for the establishment of a transtensional basin filled by the volcano sedimentary sequence of the grao para bahia and rio fresco groups finally undeformed proterozoic granitoid intrusions were dated at 1883 2 ma cigano 1883 5 2 ma musa 1880 2 ma carajas and 1874 2 ma pojuca

ftir microspectroscopy and sims study of water poor cordierite from el hoyazo spain application to mineral and melt devolatilization abstract this paper reports the microchemical and microspectroscopic ftir study of cordierite from a partially melted graphite bearing granulitic enclave within the dacitic lava dome of el hoyazo se spain optically transparent single crystals hand picked from the rock were oriented using x ray diffraction and studied by fourier transform infrared ftir single crystal ftir spectroscopy shows that the examined cordierite is co2 rich and almost h2o free two weak and sharp peaks are observed at 3708 and 3595 cm 1 respectively which are strongly polarised for e a these peaks are assigned to combination modes of co2 very weak bands due to h2o molecules oriented with the h h vector c type i water are occasionally observed in certain zones of the grains associated with absorptions due to hydrated inclusions of alteration products the very intense bands observed in the 2600 2000 cm 1 region are assigned to co2 molecules oriented a the spectra also show the presence of 13c and 18o and weak amounts of co in the sample microspectrometric mapping shows that the distribution of c is relatively homogeneous whereas that of h2o is complicated by a very broad absorption extending from 3700 to 3100 cm 1 high resolution ftir imaging done using a focal plane array of detectors shows that this broad absorption is associated with microfractures sims analyses give an average concentration of h2o 0 033 0 007 wt and co2 0 21 0 07 wt on the basis of these data molar absorption coefficients can be calibrated for co2 eico2 integrated 11 000 4000 l mol cm 2 and elco2 linear 800 250 l mol cm 1 due to the extremely low amount of h2o and its inhomogeneous distribution calibration of absorption eh2o coefficients is unreliable the very low h2o contents in the el hoyazo cordierite indicate continued mineral melt volatile exchange during decompression from 5 kbar to significantly shallower levels

on the relation between seismic anisotropy and finite strain because seismic anisotropy in the upper mantle is due primarily to lattice preferred orientation lpo of olivine crystals induced by deformation observations of anisotropy can provide constraints on mantle deformation and flow in this paper we use a previously published theory for deformation induced lpo ribe and yu 1991 to quantify the relation between seismic anisotropy and deformation for two model compositions a pure olivine dunite aggregate and a harzburgite comprising 70 olivine and 30 enstatite analytical and numerical solutions of the governing equations show that the predicted lpo and hence also the seismic anisotropy depends only on the finite strain and not on the deformation path by which that strain is produced the magnitude of the anisotropy depends only on the ratios c1 c2 and c2 c3 where c1 c2 c3 are the lengths of the principle axes of the finite strain ellipsoid and the directions of maximum and minimum compressional wave velocity vp coincide with the axes c1 and c3 respectively we calculate the percent anisotropies for compresional and shear waves propagating along the three principle axes of the finite strain ellipsoid as functions of the axial ratios c1 c2 and c2 c3 for our two model compositions however inferences of the magnitude of finite strain obtained using these results are likely to be minimum estimates because our theory neglects the effects of dynamic recrystallization finally we present as a byproduct of this study a new direct method for calculating the finite strain produced by an arbitiary deformation history

water fluxed melting of the continental crust a review abstract water fluxed melting also known as fluid or water present melting is a fundamental process in the differentiation of continents but its importance has been underestimated in the past 20 years during which research efforts focused mostly on dehydration melting reactions involving hydrate phases in the absence of a separate aqueous phase the presence of a free aqueous phase in anatectic terranes influences all major physical and chemical aspects of the melting process from melt volumes viscosity and ability to segregate from rock pores to melt chemical and isotopic composition a review of the literature shows that melting due to the fluxing of aqueous fluids is a widespread process that can take place in diverse tectonic environments active tectono magmatic processes create conditions for the release of aqueous fluids and deformation driven transient high permeability channels capable of fluxing high temperature regions of the crust where they trigger voluminous melting water fluxed melting can be either congruent in regions at the water saturated solidus or incongruent at suprasolidus p t conditions incongruent melting reactions can give rise to peritectic hornblende or to nominally anhydrous minerals such as garnet sillimanite or orthopyroxene in this case the presence of an aqueous phase is indicated by a mismatch between the large melt fraction generated and the much smaller fractions predicted in its absence the relatively small volumes of aqueous fluids compared to that of rocks imply that melting reactions are generally rock buffered fluids tend to move upwards and down temperature however there are cases in which pressure gradients drive fluids up temperature potentially fluxing suprasolidus terranes crustal regions at conditions equivalent to the water saturated solidus represent a natural impediment to the up temperature migration of aqueous fluids because they are consumed in melting reactions in this case continued migration into supra solidus terranes take place through the migration of water rich melts thus melts become the transport agent of water into supra solidus terranes and responsible for water fluxed melting other processes such as the relatively rapid fluid migration through fractures also allow regional aqueous fluids to by pass the water saturated solidus fluid trap and trigger melting above solidus conditions when aqueous fluids or hydrous melts flux rocks at supra solidus conditions they equilibrate with the surroundings through further melting decreasing water activity and giving rise to undersaturated melts it is in these conditions that hornblende or anhydrous peritectic phases are stabilized unlike dehydration melting the melt fraction generated in this case is not limited by the water contained in hydrous minerals but by the volume of water added to the system unlike melting at the water saturated solidus these melts are capable of rising without freezing and do give rise to upper crustal granitic bodies

in situ cosmogenic 10be and 21ne in sanidine and in situ cosmogenic 3he in fe ti oxide minerals abstract we report concentrations of in situ cosmogenic 10 be and 21 ne from coexisting quartz and sanidine separates and of cosmogenic 3 he in coexisting fe ti oxide minerals from ignimbritic successions of northern chile oxaya and lauca ignimbrites new mineral isotope pairs such as sanidine and fe ti oxide minerals are helpful in quantitative geomorphology for geological settings where the lithology lacks the commonly used minerals quartz pyroxene or olivine production rates in sanidine and fe ti oxide minerals were determined by normalizing nuclide concentrations to established production rates in quartz the experimentally determined production rates are compared to model production rates calculated with new cross sections for 3 he 21 ne and 10 be production from the individual target elements the mean experimental 21 ne production rate for five sanidine samples is 30 4 3 7 atoms g 1 yr 1 30 4 5 4 atoms g 1 yr 1 including the uncertainty of the 21 ne production rate in quartz p 21 ne qtz this is in excellent agreement with the modelled value of 28 3 atoms g 1 yr 1 which has an estimated uncertainty of 20 the 21 ne production rate in sanidine is thus about 50 higher than that in quartz the cosmogenic neon in sanidine is entirely released in low temperature steps 400 600 c and no signs of an interfering nucleogenic neon component were observed this is in stark contrast to quartz and makes sanidine an attractive mineral for terrestrial cosmogenic neon studies 3 he diffuses out of the sanidine structure preliminary results also indicate that sanidine is well suited for 10 be studies the mean experimental 10 be production rate from two sanidine samples is 4 45 0 38 atoms g 1 yr 1 4 45 0 42 atoms g 1 yr 1 including uncertainty of p 10 be qtz very close to the modelled value of 4 55 atoms g 1 yr 1 which has an estimated uncertainty of 20 we emphasize that 10 be and 21 ne production in sanidine is composition dependent therefore major element analyses should be carried out and production rates calculated on a sample by sample basis fe ti oxide minerals retain 3 he quantitatively experimentally derived production rates are in excellent agreement with new values derived from physical modelling the mean experimental 3 he production rate for five fe ti oxide minerals samples is 120 11 atoms g 1 yr 1 120 12 atoms g 1 yr 1 including the uncertainties of p 21 ne qtz compared to a mean modelled value of 124 atoms g 1 yr 1 which has an estimated uncertainty of 20 the variable chemical and structural composition of the solid solution lines of fe ti oxide minerals has little effect on the total 3 he production rate cosmogenic 21 ne is not produced in significant quantities in fe ti oxide minerals due to the absence of suitable target elements

cosmogenic noble gas studies in the oldest landscape on earth surface exposure ages of the dry valleys antarctica extraordinarily high surface exposure ages have been determined for sirius group tillites of mt fleming and mt feather as well as at localities in the inner dry valleys using cosmogenic helium and neon ages of 10 ma at mt fleming 5 3 ma at mt feather and 6 5 ma at insel mountain are among the highest nominal exposure ages published so far these values are minimal ages as they are based on the assumption of zero erosion and uplift the mt feather sample independently confirms the pre pliocene age of the sirius group sediments in the dry valleys as previously determined at mt fleming the insel mountain samples provide evidence for a landscape formation of the inner dry valleys not later than late miocene time assuming conservatively low values of 2 5 cm ma 1 for erosion rate and 50 m ma 1 for uplift rate we infer that the sirius group tillites at mt fleming were deposited earlier than 20 ma ago this indicates that the overriding of the dry valleys block of the transantarctic mountains by the east antarctic ice sheet occurred not later than the early miocene maximum long term erosion rates in the inner dry valleys must be 15 cm ma 1 down to altitudes 1000 m since such low erosion rates require permanently cold and hyperarid conditions the response of antarctica to the pliocene warm climatic episode must have been small cosmogenic nuclide data from both the inner dry valleys and the sirius group sediment localities support the hypothesis of a stable east antarctic ice sheet since at least late miocene time implying that the climate of antarctica was decoupled from that of lower southern latitudes we present also new elemental 21ne production rates of p21 mg 196 atoms g 1 yr 1 and p21 al 55 atoms g 1 yr 1 at sea level and high geomagnetic latitude these figures are consistent with a 3he production rate of p3 110 atoms g 1 yr 1 similar to previously published values this consistency provides evidence that pyroxene is retentive for both helium and neon over at least 10 ma cosmogenic ne in quartz and pyroxene has a 22ne 21ne cos ratio of 1 266 0 040 and 1 159 0 040 respectively

brazilian laterite dynamics using in situ produced 10be abstract concentrations of in situ produced cosmogenic 10 be t 1 2 1 5 myr were measured using accelerator mass spectrometry ams in quartz samples from three brazilian lateritic systems in order to characterize and quantify processes involved in their development results from a bedrock embedded quartz vein outcropping at gentio do ouro lead to a calculated mean erosion rate of 2 5 m myr 1 for a muon participation to the total 10 be production set at 1 5 at itaberaba the 10 be content of the surficial sample yields a mean erosion rate of 9 0 0 5 m myr 1 in addition the systematic increase in 10 be concentrations of stone line quartz with distance from the quartz vein strongly suggests an autochthonous development the data constrain a model of stone line emplacement and imply a rate of lateral displacement on the order of 70 m myr 1 by contrast the constant 10 be concentration measured within the rounded quartz cobbles all along the cuiaba stone line suggests that it most likely results from almost contemporaneous rapid events that occurred at least 500 kyr ago the proposed scenario involves sheet washing at the surface of the lateritic paleolandscape leading to surficial dispersion of allochthonous quartz cobbles immediately followed by colluvial deposition of weathered lateritic material

weathering geochronology by u th he dating of goethite nine samples of supergene goethite feooh from brazil and australia were selected to test the suitability of this mineral for u th he dating measured he ages ranged from 61 to 8 ma and were reproducible to better than a few percent despite very large variations in u and th in all samples with internal stratigraphy or independent age constraints the he ages corroborated the expected relationships these data demonstrate that internally consistent he ages can be obtained on goethite but do not prove quantitative 4he retention to assess possible diffusive he loss stepped heating experiments were performed on two goethite samples that were subjected to proton irradiation to produce a homogeneous distribution of spallogenic 3he the 3he release pattern indicates the presence of at least two diffusion domains one with high helium retentivity and the other with very low retentivity at earth surface conditions the low retentivity domain which accounts for 5 of 3he contains no natural 4he and may represent poorly crystalline or intergranular material which has lost all radiogenic 4he by diffusion in nature diffusive loss of 3he from the high retentivity domain is independent of the macroscopic dimensions of the analyzed polycrystalline aggregate so probably represents diffusion from individual micrometer size goethite crystals the 4he 3he evolution during the incremental heating experiments shows that the high retentivity domain has retained 90 95 of its radiogenic helium this degree of retentivity is in excellent agreement with that independently predicted from the helium diffusion coefficients extrapolated to earth surface temperature and held for the appropriate duration considering both the high and low retentivity domains these data indicate that one of the samples retained 90 of its radiogenic 4he over 47 5 ma and the other retained 86 over 12 3 ma thus while diffusive loss corrections to supergene goethite he ages are required these initial results indicate that the corrections are not extremely large and can be rigorously quantified using the proton irradiation 4he 3he method

age and duration of weathering by 40k 40ar and 40ar 39ar analysis of potassium manganese oxides supergene cryptomelane k1 2 mn3 mn4 8o16 h2o samples from deeply weathered pegmatites in southeastern brazil subjected to 40k 40ar and 40ar 39ar analysis yielded 40k 40ar dates ranging from 10 1 0 5 to 5 6 0 2 ma million years ago laser probe 40ar 39ar step heating of the two most disparate samples yielded plateau dates of 9 94 0 05 and 5 59 0 10 ma corresponding within 2 to the 40k 40ar dates the results imply that deep weathering profiles along the eastern brazilian margin do not reflect present climatic conditions but are the result of a long term process that was already advanced by the late miocene weathering ages predate pulses of continental sedimentation along the eastern brazilian margin and suggest that there was a time lag between weathering and erosion processes and sedimentation processes

dating constituent formation in soils to determine rates of soil processes a review abstract rates of soil processes are poorly known one approach to determine them could be through the radiogenic isotopic dating of soil constituents such methods have been recently used to date minerals in soils and weathering profiles thanks to the development of high resolution techniques such as 40ar 39ar u series of micro drilled samples u series by laser ablation and u th he and to the development of suitable methodology expanding application of the techniques to a variety of supergene minerals e g fe and mn oxihydroxides carbonates etc in this paper we identify i the main soil constituents that can be dated through these isotopic methods that are mainly fe oxides mn oxides pedogenic carbonates and organic matter ii the main pedogenic processes that leads to their formation in soils and iii the different soil settings that allow transforming soil constituent ages into rates of soil processes namely growth bands transformation fronts and chronosequences

controls on back arc crustal accretion insights from the lau manus and mariana basins abstract together the lau manus and mariana basins encompass a broad range of conditions of back arc basin development marine surveys have determined the tectonic setting and reconnaissance scale geophysical and geochemical properties of the extension axes in these basins we review these data to examine crustal accretion characteristics in the back arc setting in each basin magmatism is enhanced in spreading centres near the arc volcanic front but decreases becoming deficient in axes further from the arc in the lau and manus basins the axes extend far behind the arc and develop typical mid ocean ridge mor like characteristics we propose that these variations are controlled by the subducting slab which shapes composition and flow within the mantle wedge water released from the slab lowers the mantle solidus and increases in concentration toward the volcanic arc spreading centres near the arc advect the hydrated mantle material enhancing melt production slab induced flow carries melt depleted mantle material back beneath the basin where it mixes with ambient mantle spreading centres further from the arc advect this mantle mixture and produce thinner than normal crust far from the arc spreading centres advect essentially mid ocean ridge basalt morb source mantle and their characteristics are mor like thus in addition to spreading rate effects on mantle melting that predominate at mors in back arc basins the subducting slab introduces additional systematic effects

u series isotope data on lau basin glasses the role of subduction related fluids during melt generation in back arc basins new thermal ionization mass spectrometry u series disequilibrium data are presented for 24 basaltic to dacitic glasses from active spreading centres in the back arc lau basin sw pacific together with additional inductively coupled plasma mass spectrometry trace element analyses and sr nd pb isotope data valu fa ridge samples adjacent to the arc front have high u th and 230th 238u 1 implying a recent 350 ka addition of a u rich slab derived fluid the valu fa data can be combined with existing 230th 238u data for the central tonga arc to infer a fluid addition event at 50 ka the similar sources and time scales for fluid transfer beneath the valu fa ridge and beneath the arc itself suggest that the valu fa ridge is propagating into the arc front region central lau basin samples further behind the arc have lower u th and 230th 238u 1 similar to typical mid ocean ridge basalts morb within the central lau basin a water rich subduction component is seen only in samples closest to the arc and this fluid does not have the high u th composition of the fluid at valu fa melt generation in the central lau basin appears to be dominated by normal ridge type processes but the relatively low 230th 238u for these shallow ridges compared with global morb could be a consequence of increased melt productivity as a result of the elevated water contents the transition from 238u to 230th excesses within the back arc basin is not a smooth function of decreasing addition of a u rich fluid moving away from the arc front but also reflects the complex dynamics between two major mantle domains within the mantle wedge pacific beneath valu fa ridge indian beneath the central lau basin

fluids p t paths and the fates of anatectic melts in the earth s crust this paper presents a model for the longevity and physical behaviour of anatectic melts in the earth s crust it is assumed that a thermal anomaly has caused partial fusion of biotite plagioclase quartz rocks to produce a granitic melt and restitic solids the focus is on the potential chemical and physical outcomes as a function of the various p t fluid conditions rocks undergoing fluid present melting will sustain an increase in bulk density and will become more gravitationally stable diapiric up welling of such material should not occur in fluid absent melting the bulk density will decrease and foster gravitational instability stresses in the surrounding rocks could however lead to fracture and rapid evacuation of the melt with excess fluid and without melt segregation isobaric cooling will cause immediate freezing if t rises above the biotite quartz melting reaction freezing will still occur at the original reaction t diapiric up welling should not occur biotite bearing diatexites might be formed by large degrees of fusion but magma ascent is unlikely in isothermal decompression immediate freezing will ensue and the migmatite should contain orthopyroxene rather than biotite with excess fluid and melt segregation cooling will induce freezing at the fluid saturated granite solidus magma ascent could however occur even for xflh2o 1 in isothermal decompression melt will persist to the granite solidus for the imposed xflh2o limited magma ascent could occur all granites should contain primary igneous biotite for fluid deficient rocks without melt segregation freezing will occur at the t of the original melting if the reaction is overstepped solidification will occur at the original xflh2o magma formation is unlikely migmatites should contain either pyroxene retrogressed to biotite or biotite alone on decompression immediate freezing is likely if t remains at the biotite quartz reaction for higher t melt will survive decompression and freeze at the fluid saturated solidus for some increased xflh2o magma ascent is possible and the melt will freeze very close to the wet granite solidus in fluid deficient rocks with melt segregation cooling and crystallization will cause fluid saturation in situ segregated melt will freeze at the original xflh2o melt ascent will however cause exsolution of a co2 rich fluid with fluid escape melt will freeze close to the wet granite solidus an increase in melt proportion may accompany magma ascent or source up welling any low p granite intrusion should be charnockitic for fluid absent rocks without melt segregation freezing will occur on cooling to the t of the original reaction only transitory migmatites could be formed but melt will survive decompression almost to the h2o saturated granite solidus the rocks will reveal former melt presence only in the existence of veins of coarse retrograde biotite with isothermal decompression further melting may occur forming an orthopyroxene bearing granulite facies migmatite in fluid absent situations with melt segregation magma could be efficiently transported to higher crustal levels if the melt proportion were insufficient for magma formation granulite facies migmatites would result ascending melt would progressively dissolve any entrained restite the granites produced should be charnockitic to aid in interpretation of the modelling results we suggest that it may be slightly more common for the highest grade metamorphic terranes to exhibit isobaric cooling paths that fluid present anatexis is normally limited to the shallower parts of the crust and that fluid absent reactions are more likely at greater depths we conclude that the best models for natural fluid present anatexis are those dealing with essentially pure h2o fluid and that fluid deficient conditions will dominate over fluid excess we also believe that it is common for melts to be physically segregated on scales larger than the diffusion paths of many elements

helium isotope variations in seafloor basalts from the northwest lau backarc basin mapping the influence of the samoan hotspot 1 new helium isotope results for seafloor basalts from the northern lau basin indicate that the signature of the samoan hotpot has penetrated beneath the rochambeau rifts rr and the northwest lau spreading center nwlsc all the way to the peggy ridge pr helium isotope ratios in 29 samples from the nwlsc rr range from 10 9 to 28 1 ra all clearly higher than typical depleted mantle or mor type values comparison with other helium results from the northern lau basin show the samoan hotspot influence is confined to the nwlsc rr extensional zone and is absent in the northeast lau basin and along the pr the absence of high 3he 4he ratios in the northeast lau basin may delimit the eastward extent of the pacific plate tear which allowed southward intrusion of the samoan plume or indicate deep mantle flow carrying the samoan plume signal westward from the samoan islands

effects of crystal preferred orientation on upper mantle flow near plate boundaries rheologic feedbacks and seismic anisotropy insight into upper mantle processes can be gained by linking flow induced mineral alignment to regional deformation and seismic anisotropy patterns through a series of linked micro macro scale numerical experiments we explore the rheologic effects of crystal preferred orientation cpo and evaluate the magnitude of possible impacts on the pattern of flow and associated seismic signals for mantle that includes a cooling thickening young oceanic lithosphere the cpo and associated anisotropic rheology computed by a micromechanical polycrystal model are coupled with a large scale flow model eulerian finite element method via a local viscosity tensor field which quantifies the stress strain rate response of a textured polycrystal cpo is computed along streamlines throughout the model space and the corresponding viscosity tensor field at each element defines the local properties for the next iteration of the flow field stable flow and cpo distributions were obtained after several iterations for the two dislocation glide cases tested linear and nonlinear stress strain rate polycrystal behaviour the textured olivine polycrystals are found to have anisotropic viscosity tensors in a significant portion of the model space this directional dependence in strength impacts the pattern of upper mantle flow for background asthenosphere viscosity of 1020 pa s and a rigid lithosphere the modification of the corner flow pattern is not drastic but the change could have geologic implications feedback in the development of cpo occurs particularly in the region immediately below the base of the lithosphere stronger fabric is predicted below the flanks of a spreading centre for fully coupled power law polycrystals than was determined using prior linear intermediate coupling polycrystal models the predicted sks splitting is modestly different 0 5 s between the intermediate and fully coupled cases for oceanic plates less than 20 myr old the magnitude of azimuthal anisotropy for surface waves on the other hand is predicted to be twice as large for fully coupled power law flow polycrystals than for linear intermediate coupled flow polycrystal models

40ar39ar analysis of supergene jarosite and alunite implications to the paleoweathering history of the western usa and west africa supergene alunite kal sub 3 so sub 4 sub 2 oh sub 6 and jarosite kfe sub 3 so sub 4 sub 2 oh sub 6 are often precipitated during the oxidation of sulfide bearing rocks by meteoric solutions dating of these phases by the sup 40 ar sup 39 ar method allows timing of the progression of the oxidation front during chemical weathering fine scale laser heating sup 40 ar sup 39 ar dating of hypogene alunite and supergene jarosites allows to precisely and accurately time hydrothermal alteration and subsequent supergene oxidation in goldfield nevada the results indicate that pervasive weathering occurred in the western usa during the late miocene approximately 10 ma ago similar application of this technique to the study of weathering and laterite formation in west africa indicates that the last pervasive oxidation event recorded in the weathering profile in this area also occurred in the miocene approximately 13 ma ago the occurrence of a pervasive mid to late miocene oxidation event recorded in these weathering profiles in the western usa and africa and also previously measured in brazil and chile indicates that climatic conditions at that time were conducive to worldwide development of deep weathering sequences subsequent weathering processes have not been as pervasive as the late miocene event indicating a general climatic transition to cooler more drier climates in most of the areas studied the results indicate that deep weathering profiles reflect past climatic conditions and may not be directly linked to the present climates less

lateral variation in crustal structure along the lesser antilles arc from petrology of crustal xenoliths and seismic receiver functions abstract we reconstruct crustal structure along the lesser antilles island arc using an inversion approach combining constraints from petrology of magmatic crustal xenoliths and seismic receiver functions xenoliths show considerable island to island variation in xenolith petrology from plagioclase free ultramafic lithologies to gabbros and gabbronorites with variable proportions of amphibole indicative of changing magma differentiation depths xenoliths represent predominantly cumulate compositions with equilibration depths in the range 5 to 40 km we use xenolith mineral modes and compositions to calculate seismic velocities v p v s and density at the estimated equilibration depths we create a five layer model of crustal structure for testing against receiver functions rf from island seismic stations along the arc lowermost layer 5 comprises peridotite with physical characteristics of mantle xenoliths from grenada uppermost layer 1 consists of 5 km of volcaniclastics and sediments whose physical properties are determined via a grid inversion routine the three middle layers 2 to 4 comprise igneous arc crust with compositions corresponding to the xenoliths sampled at each island by inversion we obtain a petrological best fit for the rf on each island to establish the nature and thicknesses of layers 2 to 4 along the arc we see variations in the depth and strength of both moho and mid crustal discontinuity mcd on length scales of tens of km moho depths vary from 25 to 37 km mcd from 11 and 32 km the moho is the dominant discontinuity beneath some islands st kitts guadeloupe martinique grenada whereas the mcd dominates beneath others saba st eustatius along arc variability in mcd depth and strength is consistent with variation in estimated magmatic h2o contents and differentiations depths that in turn influence xenolith lithologies a striking feature is steep along arc gradients in v p similar to those observed at other oceanic arcs these gradients reflect abrupt changes in rates and processes of magma generation in the underlying crust and mantle we find no evidence for large interconnected bodies of partial melt beneath the lesser antilles instead the crustal velocity structure is consistent with magma differentiation in vertically extensive crystal mush dominated reservoirs along arc variation in crustal structure may reflect heterogeneous upwelling within the mantle wedge itself driven by variation in slab derived h2o fluxes

a three plate kinematic model for lau basin opening 1 we present revised compilations of bathymetry magnetization acoustic imagery and seismicity data for the lau basin and surrounds we interpret these data to more precisely locate the plate boundaries in the region and to derive a three plate kinematic model for the opening of the lau basin during the brunhes chron our tectonic model includes the niuafo ou microplate which is separated from the australian plate along the peggy ridge lau extensional transform zone central lau spreading center and from the tongan plate along the newly discovered fonualei rift and spreading center our model shows that australia tonga plate motion between 15 5 s and 19 s is partitioned across the microplate and it resolves the former apparent conflict between geodetic versus seafloor spreading velocities regionally this implies that the angular rate of opening of the lau basin has been faster than that of the havre trough to the south and that the rapid current rates of pacific subduction at the tonga trench 240 mm yr at 16 s have been sustained for at least 0 78 m y

δ37cl systematics of a backarc spreading system the lau basin determinations of 37 cl in glasses and melt inclusions from backarc basin basalts of the lau basin southern pacific ocean constrain the effects of mantle source subduction and shallow assimilation in sections of the lau basin not heavily affected by subduction input central and eastern lau spreading centers cl derived from unfractionated seawater via assimilation in shallow crustal environments is the predominant source of excess cl and moderates 37 cl to near zero values in contrast low cl glasses from the mangatolu triple junction have distinctly negative 37 cl compatible with recent estimates for 37 cl of the depleted mantle source along the valu fa ridge subduction input manifests as 37 cl that is variable but dispersed toward more negative values in both lavas and melt inclusions these values are compatible with a second source of lighter cl within a slab derived flux derived from altered ocean crust sediments and or serpentinized mantle wedge

evolution of structurally contrasting anatectic migmatites in the 3 kbar ballachulish aureole scotland anatectic migmatites of contrasting structural style are found adjacent to the contacts of the ballachulish igneous complex argyllshire scotland on the east flank evidence for migmatization is largely restricted to the local development of millimetre centimetre scale kfs qtz rich leucocratic segregations which accompany fragmentation of brittle hornfels layers and ductile deformation of mm cm scale semipelitic layers large volumes of semipelitic rock rich in feldspar and quartz on the east flank show no migmatitic features and bedding is usually preserved undisturbed right up to the contact on the west flank in contrast similar semipelitic rocks show widespread migmatitic features and disruption of layering is substantial and widespread over a 400 m wide zone within the west flank migmatites 1 100 cm scale rigid bedding fragments schollen may be suspended and disoriented in a semipelitic matrix that underwent ductile deformation the p t conditions on both flanks are in the same range 3 kbar and 650 700 c the contrast in gross structural style is believed to result from differences in the volumes of melt produced and differences in the proportion of rock in which the critical melt fraction of the rocks was exceeded on the east flank only on a mm cm scale was enough melt locally accumulated to cause disruption of some layers and segregation of melt on the west flank melting proceeded substantially in a broad tract of semipelitic rocks resulting in larger scale contrasts in rheology that led to the present chaotic structures in this zone because migmatization occurred at a pressure too low for muscovite dehydration melting and at temperatures too low for substantial biotite dehydration melting the different amounts of melting on the east and west flanks most probably resulted from the introduction of differing amounts of externally derived water on the east flank and throughout most of the aureole the absence of melting even in quartzofeldspathic protoliths indicates that there was no substantial movement of fluid towards or away from the igneous complex during migmatization the contrasting situation on the west flank may have resulted from devolatilization of underlying quartz diorite magma 690 710 c which released heat and fluids into the overlying quartz and feldspar rich semipelites solidus temperature 650 680 c

cordierite as a monitor of fluid and melt h2o contents in the lower crust an experimental calibration cordierite can contain variable amounts of h 2 o and co 2 and the total amount and relative proportions of these volatiles reflect the environment in which the cordierite was formed in this study cordierite has been experimentally equilibrated with a peraluminous granite melt and the h 2 o contents of the phases have been determined by secondary ion mass spectrometry sims the results show that cordierite can have a range of h 2 o contents at a single pressure and temperature in this case 900 c 5 0 kbar the maximum h 2 o contents occur at saturation when h 2 o vapor is also present at which point cordierite contains 1 69 0 05 wt and the melt contains 10 0 0 42 wt however as the system becomes progressively more water undersaturated the phases contain less h 2 o and the partitioning coefficient d w h 2 o melt h 2 o cordierite in wt decreases from 5 9 0 3 to 4 4 0 6 the experiments show that equilibration with a volatile undersaturated melt provides an alternative to leakage as a mechanism for producing volatile poor cordierites application of this experimental calibration to cordierites from granulite migmatites predicts that the melts with which they last equilibrated contained only 2 7 3 8 wt h 2 o

quantitative geochemical approach to pedogenesis importance of parent material reduction volumetric expansion and eolian influx in lateritization abstract using mass balance techniques we test the prevalent view that laterite genesis is dominated by in situ residual enrichment during chemical weathering of bedrock through calculation of net mass fluxes through the laterite soils in mali west africa we show that residual enrichment by removal of mobile elements with a corresponding increase in bulk porosity and decrease in bulk density contributes only a very minor fraction of the enrichment of al fe si and au instead we demonstrate that the abundance of these elements is due to the influx and accumulation by selective retention of chemically mature detritus of local and foreign origin clearly evident in micromorphological infilling features at the same sample depths that accumulation reaches extreme values we show that volumetric expansion in excess of 200 has occurred locally we infer that these spatially coincidental zones of mass influx of rock forming metals fe al si and also au with dilational hyperstrains result from a mutually reinforcing mechanical interaction between material influx and the effects of subsurface deformational processes such as shrink swell cycles and root growth and decay we propose that with progressive infilling of available connected voids by illuvial microsedimentary deposits of insoluble resistate and neoformed minerals the capacity for the combined skeleton and plasma to remain isovulumetric is exceeded we speculate that the resultant space problem is relieved by upwards expansion towards the overlying free surface continued translocation and void infilling occur and are limited to the depth where the size of translocational particles is smaller than that of the connected voids consequently progressive hyperstrains accumulate above this critical depth by the long term influence of a proposed translocational wedge of chemically resistant minerals against which numerous generations of plant roots have exerted stresses eclectic surficial contaminants involved are continuously derived from above leaving no indication of a relict source region within the present soil profile from which they might have been extracted instead the source region is largely the existing regolith column itself which releases local material and in addition is supplemented by deposition of colluvial detritus shed nearby by escarpment retreat

direct dating of weathering phenomena by 40ar39ar and k ar analysis of supergene k mn oxides abstract potassium bearing manganese oxides cryptomelane k1 2 mn3 mn4 8 o16 xh2oand hollandite k ba 1 2 mn3 mn4 8 o16 xh2o are often authigenically precipitated in weathering profiles the presence of structural potassium in manganese oxides makes these minerals datable by the k ar and 40 ar 39 ar methods dating of these phases allows us to time the progression of oxidation fronts during weathering and pedogenic processes detailed age resolution of the weathering history allows us to use deep weathering profiles as land based stratigraphic records of past climatic and geomorphic events complementing evidence from the marine sedimentary and paleontological record potential problems in manganese oxide dating such as ar and or k losses excess argon 39ar loss by recoil during neutron irradiation contamination by primary minerals resistant to weathering the presence of multiple generations of manganese oxides and proportionately low radiogenic ar yield due to adsorbed atmospheric gases are addressed the k ar and 40 ar 39 ar analytical results indicate that ar and or k losses excess 40ar and 39ar recoil seem not to pose problems in manganese oxide dating this investigation suggests that fine scale laser probe 40 ar 39 ar technique is most appropriate for dating of weathering phenomena because this technique permits us to identify contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles k ar and 40 ar 39 ar dating of supergene k bearing manganese oxides formed during lateritization of archean and proterozoic bedrocks in the caraj s region amazonia brazil indicates that weathering started before 72 6 ma petrographic electron microscope and electron microprobe investigation reveal multiple generations of manganese oxide precipitation k ar and 40 ar 39 ar laser probe step heating results independently confirm the presence of these various generations of manganese oxides age clusters at 65 69 51 56 40 43 33 35 20 24 12 17 ma and zero age 0 2 0 2 ma suggest episodic precipitation of k mn oxides resulting from changing weathering conditions in the amazon throughout the cenozoic k ar and 40 ar 39 ar dating of supergene cryptomelane from weathering profiles in eastern minas gerais southeastern brazil suggests continuous weathering from 10 to 5 6 ma ago possibly reflecting local climatic conditions due to the proximity with the atlantic ocean

seismic anisotropy in the upper mantle 2 predictions for current plate boundary flow models 1 the anisotropic seismic structure due to flow induced mineral alignment is investigated for a series of models designed to simulate deformation in the upper mantle within a few hundred kilometers of a plate boundary the orientation distributions of olivine enstatite aggregates evolve along streamlines of each flow model based on each grains plastic response to the local stress strain field the effective elastic tensor for these textured aggregates provides predictions of p wave anisotropy and shear wave splitting throughout the model space p and s travel time delay patterns and fast shear wave polarization angles are found to vary significantly with incidence angle for a given model comparison of predicted fast p direction for our method versus a finite strain based estimate shows that agreement is acceptable for much of the model space but notable differences occur in regions up to several tens of kilometers in size two dimensional models of spreading center flow are presented for slow and fast rates and for several cases in which the ridge migrates over the deeper mantle the effect of flow in the third dimension is addressed in a few calculations for one comparison of flow in the mantle wedge at a subduction zone the introduction of trench parallel flow causes significant changes in the predicted patterns of p wave anisotropy magnitude more than orientation and sks splitting

dating paleochannel iron ore by u th he analysis of supergene goethite hamersley province australia u th he dating of late stage authigenic goethite combined with corrections for diffusive loss of he 4 by the he 4 he 3 methodology reveals strong correlation between a sample s age and its depth in ferruginized channel sediments from the yandicoogina deposit western australia corrected ages ranging from ca 18 ma near the surface to ca 5 ma at the bottom of the profile indicate that ferruginization of the aggraded channels becomes progressively younger with depth this trend is consistent with goethite precipitation at the groundwater atmosphere interface during water table drawdown driven by the aridification of western australia during the neogene the results demonstrate that the u th he system is ideal for dating goethite if diffusive loss corrections are applied the approach is suitable for dating weathering reactions on earth and should also be suitable for dating fe oxyhydroxides in the martian regolith

upper mantle structure of the tonga lau fiji region from rayleigh wave tomography we investigate the upper mantle seismic structure in the tonga lau fiji region by jointly fitting the phase velocities of rayleigh waves from ambient noise and two plane wave tomography the results suggest a wide low velocity zone beneath the lau basin with a minimum sv velocity of about 3 7 0 1 km s indicating upwelling hot asthenosphere with extensive partial melting the variations of velocity anomalies along the central and eastern lau spreading centers suggest varying mantle porosity filled with melt in the north where the spreading centers are distant from the tonga slab the inferred melting commences at about 70 km depth and forms an inclined zone in the mantle dipping to the west away from the arc this pattern suggests a passive decompression melting process supplied by the australian plate mantle from the west in the south as the supply from the australian mantle is impeded by the lau ridge lithosphere flux melting controlled by water from the nearby slab dominates in the back arc this source change results in the rapid transition in geochemistry and axial morphology along the spreading centers the remnant lau ridge and the fiji plateau are characterized by 60 80 km thick lithosphere underlain by a low velocity asthenosphere our results suggest the removal of the lithosphere of the northeastern fiji plateau lau ridge beneath the active taveuni volcano azimuthal anisotropy shows that the mantle flow direction rotates from trench perpendicular beneath fiji to spreading perpendicular beneath the lau basin which provides evidence for the southward flow of the mantle wedge and the samoan plume this article is protected by copyright all rights reserved

chemical mobilizations in laterites evidence from trace elements and 238u 234u 230th disequilibria geochemical and mineralogical investigations including measurements of major and trace elements sr isotope ratios and 238u 234u 230th activity ratios were made on an old african laterite to reconstruct its formation steps and assess recent chemical mobilization the present data support a scenario of discontinuous formation for the laterite with different bedrock weathering conditions during the formation of each unit rather than a scenario of continuous formation absolute accumulation of fe u and lanthanides in the uppermost ferruginous unit suggests an autochthonous origin of this iron cap by leaching of an older overlying profile present chemical distributions of lanthanides as well as of rb k ba and sr within the profile cannot be linked to the mineralogical distribution of both relictual primary and authigenic secondary phases complementary lanthanide patterns indicate that these elements were primarily accumulated in the uppermost ferruginous unit before further remobilization and accumulation in the underlying horizons these redistribution processes may be related to the chemical instability of the ferruginous cap the 238u 234u 230th disequilibria indicate that recent u mobilization occurs in the whole profile and that as for lanthanides there is a vertical redistribution of u from the uppermost ferruginous unit to the underlying horizons moreover these data show that both u losses and gains exist at each level of the profile a simple modeling of this double u mobilization process is proposed to interpret the 238u 234u 230th data differences in the mobilization and fractionation intensities of the u input and removal processes can account for the two evolution trends which distinguish the ferruginous unit from the underlying ones furthermore on the basis of this modeling the profile appears to be in a transient state because of recent changes in the u mobilization conditions which could correspond to major pleistocene climatic variations

seismic anisotropy indicates ridge parallel asthenospheric flow beneath the eastern lau spreading center seismic anisotropy beneath the eastern lau spreading center elsc is investigated using both rayleigh waves and shear waves using data from the 2009 2010 elsc ocean bottom seismograph experiment phase velocities of rayleigh waves determined by ambient noise cross correlation are inverted for azimuthally anisotropic phase velocity maps splitting of s waves from five intermediate and deep focus earthquakes was determined by waveform analysis taken together rayleigh wave and s wave data indicate that significant 2 anisotropy extends to at least 300 km depth both data sets indicate a fast direction aligned within a few degrees of the n10 e striking elsc and somewhat oblique to the n25 e strike of the neighboring volcanic arc we therefore describe the fast direction as spreading perpendicular not convergence perpendicular and interpret it as due to ridge parallel flow of the asthenosphere however the region arcward east of the elsc has the stronger anisotropy suggesting that the strongest flow gradients may occur in the region between the spreading center and the arc in contrast to being centered beneath the elsc fluids released from the underlying plate may produce anisotropic hydrous materials but more importantly lower the viscosity thus enhancing along strike flow both could contribute to an along strike fast direction signature seafloor spreading diminishes south of the seismic array ceasing altogether south of latitude 25 s 500 km south of the array center a region dominated by much slower tectonic extension suggesting that asthenosphere is inflowing from the north to accommodate the increase in asthenospheric volume associated with the seafloor spreading

seismic anisotropy and mantle flow seismic anisotropy in the mantle is primarily due to deformation induced lattice preferred orientation lpo of olivine crystals the aim of this paper is to better understand how such lpo is produced by flow and to learn whether observations of seismic anisotropy can be used to infer the local direction of flow within the mantle the basis of the work is a two dimensional theory which describes how lpo evolves during an arbitrary time dependent deformation ribe 1989 i investigate the evolution of lpo beneath ocean ridges above subducted slabs and in thermal convective flows the ocean ridge model predicts that the seismic anisotropy produced at a fast spreading ridge should be greater in magnitude than that produced at a slow spreading ridge and that the fast axis should be more nearly horizontal the subduction zone model predicts that strong seismic anisotropy should be produced above subducted slabs with the fast axis aligned with the slab regardless of the subduction angle the evolution of lpo in typical convective flows is extremely complex and depends strongly on such factors as the mode of heating and the surface boundary conditions in general there is no simple relation between the lpo and the local flow direction in any of the models considered the attempt to use seismic anisotropy to map flow patterns on a global scale is therefore probably unwarranted however the lpo does become aligned with the flow direction when the flow is a progressive simple shear such a flow occurs in limited portions of the mantle beneath the lithosphere at a fast spreading ridge above subducted slabs and in boundary layers beneath rigid surfaces such as continents observations of seismic anisotropy may therefore indicate the sense of shear at shallow depths in the mantle

the development of iron crust lateritic systems in burkina faso west africa examined with in situ produced cosmogenic nuclides we have investigated the development of iron crust laterites on the stable west african craton in northern burkina faso using cosmogenic radionuclides produced in situ in quartz veins and pebbles lateritic soils develop in tectonically stable slowly eroding tropical environments and are a major component of the earth s surface to examine processes affecting laterite formation we determined 10be and 26al in samples of quartz from three sites representing two sequential and connected iron crust laterite systems results from outcropping quartz veins suggest that the mean erosion rate in this region is about 3 8 m myr 1 in addition data from quartz cobbles and pebbles incorporated in iron crusts demonstrate that depth dependent distributions of these nuclides may be used to distinguish surfaces undergoing burial from those undergoing erosive loss results from sections of the lowland lateritic system are consistent with mean accumulation rates of a few metres per million years quartz cobbles presently at depths of a few metres in a paleochannel filled with rapidly deposited fluvial colluvial material have10be distributions that suggest that the lowland lateritic surface may have formed during an erosive episode presumably associated with a wetter climate roughly 300 kyr bp these results illustrate the practicality and the potential of the use of in situ produced cosmogenic nuclides for understanding the history of formation of laterites and for differentiating between systems formed through in situ chemical weathering and mechanical transport

shear wave anisotropy beneath the andes from the banjo seda and pisco experiments we present the results of a detailed shear wave splitting analysis of data collected by three temporary broadband deployments located in central western south america the broadband andean joint experiment banjo a 1000 km long east west line at 20 s and the projecto de investigacion sismologica de la cordillera occidental pisco and seismic exploration of the deep altiplano seda deployed several hunderd kilometers north and south of this line we determined the splitting parameters fast polarization direction and t splitting delay time for waves that sample the above and below slab regions teleseismic ks and s scs waves from local deep focus events as well as s waves from intermediate focus events that sample only the above slab region all but one of the ks stacks for the banjo stations show e w fast directions with t varying between 0 4 and 1 5 s however for ks recorded at most of the seda and pisco stations and for local deep focus s events north and south of banjo there is a rotation of to a more nearly trench parallel direction the splitting parameters for above slab paths determined from events around 200 km deep to western stations yield small delay times 0 3 s and n s fast polarization directions assuming the anisotropy is limited to the top 400 km of the mantle olivine stability field these data suggest the following spatial distribution of anisotropy for the above slab component as one goes from east where ks reflects the above slab component to west changes from e w to n s and delay times are substantially reduced this change may mark the transition from the brazilian craton to actively deforming e w shortening andean mantle we see no evidence for the strain field expected for either corner flow or shear in the mantle wedge associated with relative plate motion the small delay times for above slab paths in the west require the existence of significant spatially varying below slab anisotropy to explain the ks results the implied anisotropic pattern below the slab is not easily explained by a simple model of slab entrained shear flow beneath the plate instead flow induced by the retrograde motion of the slab in combination with local structural variations may provide a better explanation

the distribution of h2o co2 between cordierite and granitic melt under fluid saturated conditions at 5 kbar and 900 c experiments to determine the distribution of h2o and co2 between synthetic peraluminous granitic melt and natural fe mg cordierite have been conducted at 5 kbar and 900 c under fluid saturated conditions up to seven charges saturated with a range of h2o co2 fluid compositions were run simultaneously the h2o and co2 contents of the cordierites and the h2o contents of the melts were determined using secondary ion mass spectrometry as the co2 content of the fluid increases the h2o content of both the cordierite and the melt decrease in a similar manner to fluid undersaturated experiments in the h2o only system the cordierite h2o contents in the experiments range from 0 5 to 1 7 wt h2o the distribution coefficients dw wt h2o melt wt h2o cordierite are in the range 3 5 5 9 and agree with the model of harley and carrington for 5 kbar and 900 c the co2 content of the cordierite increases from 0 to 1 wt co2 in the most co2 rich experiment as the cordierite and melt h2o contents decrease our h2o co2 cordierite data allow us to model and predict the maximum co2 content in cordierite to be 0 16 0 01 molecules per formula unit i e 1 16 wt co2 at 5 kbar and 900 c this value and the channel co2 contents of cordierite in equilibrium with h2o co2 fluids at high xco2 are significantly lower than predicted using previous models on the basis of these experiments cordierites that are saturated in h2o co2 and have channel xco2 0 25 require aco2 of over 0 75

rapid mantle flow beneath the tonga volcanic arc abstract flow in the earth s asthenospheric mantle is typically assumed to be driven by motions of the overlying and adjacent plates suggesting flow rates comparable to plate velocities however arc parallel shear wave splitting and geochemical observations imply along strike flow in the lau basin presenting a challenge to geodynamic models of plate driven subduction systems as plate driven flow models predict arc perpendicular fast direction with simple a type mineral texturing although b type mineral texturing would result in the fast directions developing 90 relative to a type and likely occurs in the forearc mantle such texturing is unlikely in the asthenosphere wedge where higher temperatures and weak rheology likely prevail by tracking mineral texturing development within geodynamic flow models patterned after the tongan system in the lau basin that include along strike flow we find that the observations are best explained by models including rapid along arc flow within the subarc mantle the effect of melt on the anisotropy does not appear to be a primary control but needs to be investigated more fully the best fit to the observations is a model with along strike flow within a low viscosity channel beneath the arc at a rate of nearly 50 cm yr several times the plate motion velocities

a laboratory model of subduction zone anisotropy abstract measurements of split seismic shear waves near pacific subduction zones show that fast polarization directions are primarily trench parallel in the seaward side mantle beneath a subducting slab and variable in the mantle wedge beneath back arc basins it is generally accepted that the azimuth of the fast polarization direction defines the average orientation of the crystallographic a axis in olivine here we use laboratory experiments to determine the orientation of the olivine a axis in the upper mantle in the vicinity of a subducting slab a laboratory analog for olivine a axis motion during creep deformation is developed using small cylinders whiskers suspended in a viscous fluid subduction beneath a stationary overriding plate variable dip angles and realistic rollback and down dip plate kinematics are examined plate rollback produces strong plate parallel whisker alignments for all dip angles examined alignment in the wedge occurs in three layers a layer of dip parallel alignment above the downgoing plate a layer of subhorizontal alignment beneath the overriding plate and a core of intermediate orientations the amount of alignment beneath the overriding plate and the wedge core decreases with increasing dip angle our results indicate that olivine a axis orientation in the seaward side mantle is controlled by the amount of slab rollback and that orientations in the mantle wedge depend upon slab dip angle

partial melting and the formation of granulite facies assemblages in namaqualand south africa abstract dehydration melting reactions in which water from a hydrous phase enters the melt leaving an anhydrous solid assemblage are the dominant mechanism of partial melting of high grade rocks in the absence of externally derived vapour equilibria involving melt and solid phases are effective buffers of ah2 o the element partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which for most cases ah2o is driven to lower values the mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at t 850 c is relatively small 10 20 and probably insufficient to mobilize a partially melted rock body granulite facies metapelite biotite gneiss and metabasic gneiss in namaqualand contain coarse grained discordant unfoliated anhydrous segregations surrounded by a finer grained foliated matrix that commonly includes hydrous minerals the segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration melting reactions bt sil qtz pi grt crd kfs l metapelite bt qtz pl opx kfs l biotite gneiss and hbl qtz opx cpx pl l metabasic gneiss the size shape distribution and modes of segregations suggest only limited migration and extraction of melt growth of anhydrous poikiloblasts in matrix regions development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity also they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles the preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that h2o in the melt was released as vapour on crystallization this model combined with the natural observations suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt

axial crustal structure of the lau back arc basin from velocity modeling of multichannel seismic data located west of the tonga trench the lau back arc basin is a prime environment for studying the interplay between oceanic spreading systems and an active subduction zone within the basin lie two complementary intermediate rate spreading systems the central lau spreading center clsc and the eastern lau spreading center valu fa ridge elsc vfr that are positioned 170 to 40 km away from the active volcanic arc respectively multichannel seismic mcs images of both systems reveal systematic variations in axial crustal structure primarily related to the proximity of the volcanic arc but also related to spreading rate morphology and petrology upper crustal refraction data selected from the along axis seismic lines collected during the 1999 mcs survey were modeled in both the time versus range t x and intercept time versus slowness p domains to provide validation and detail to the seismic reflection observations the results show that as both the proximity to the arc and the spreading rate decrease southward 1 seismic layer 2a thickens by 0 6 km between the clsc and vfr from 0 4 km to 1 0 km 2 the average depth of the axial magma chamber amc increases from 1 5 km at the clsc to 2 8 km at the southern vfr excluding the northern section of the elsc that shows no continuous amc reflector but does show an isolated melt sill 3 the upper crustal basement velocity decreases from 2 1 km s at the clsc to 1 8 km s at the vfr and 4 the velocities of both layer 2a and 2b decrease between the clsc and northern vfr from 3 2 to 2 5 km s and 5 0 to 3 9 km s respectively along with a broad axial high morphology these features of the clsc thinner layer 2a shallower amc and faster crustal velocities correlate best with the fast spreading east pacific rise and intermediate spreading juan de fuca ridge conversely the structural characteristics of the central elsc vfr have no known counterpart in the global mid ocean ridge system we attribute this primarily to the volatile concentration in the magmas coming from the tonga subduction zone

seismic mapping of a magma chamber beneath the valu fa ridge lau basin we analyze a dense grid of seismic normal incidence and wide angle data collected over an active back arc spreading center in the lau basin of the southwest pacific our survey area covers the whole of a 35 km long morphological segment and a small overlapping spreading center a bright reflector which is coincident with a velocity inversion at a depth of 3 2 0 2 km below the seafloor is observed on every one of the 40 across axis profiles we interpret this reflector as being the roof of a crustal magma chamber the widest magma chamber reflector occurs beneath the overlapping spreading center where it extends up to 4 km being imaged beneath both ridges and in places beneath the overlap basin elsewhere the width of the reflector varies between 0 6 and 2 3 0 4 km the narrowest reflectors are observed beneath deviations devais of the ridge axis in addition to being observed on each of the kilometer spaced across axis profiles the reflector is also seen as a continuous event for at least 10 km on a profile along the ridge axis the reflection coefficient of the event is between 0 34 and 0 65 this requires the melt at the top of the chamber to have a p wave velocity less than 2 7 km s if the interface is planar or less than 3 8 km s if the interface is layered

variations in fluid activity across the etive thermal aureole scotland evidence from cordierite volatile contents the h2o and co2 content of cordierite was analysed in 34 samples from successive contact metamorphic zones of the etive thermal aureole scotland using fourier transform infrared spectroscopy ftir the measured volatile contents were used to calculate peak metamorphic h2o and co2 activities total volatile contents are compared with recently modelled cordierite volatile saturation surfaces in order to assess the extent of fluid present v fluid absent conditions across the thermal aureole in the middle aureole prior to the onset of partial melting calculated ah2o values are high close to unity and measured volatile contents intersect modelled h2o co2 saturation curves at the temperature of interest suggesting that fluid present conditions prevailed total volatile contents and ah2o steadily decrease beyond the onset of partial melting consistent with the notion of ah2o being buffered to lower values as melting progresses once free hydrous fluid is exhausted all sillimanite zone samples record total volatile contents that are significantly lower than modelled h2o co2 saturation surfaces implying that fluid absent conditions prevailed the lowest recorded ah2o values lie entirely within part of the section where fluid absent melting reactions are thought to have dominated samples within 30 m of the igneous contact appear to be re saturated possibly via a magmatically derived fluid in fluid absent parts of the aureole cordierite h2o contents yield melt h2o contents that are compatible with independently determined melt h2o contents the internally consistent cordierite volatile data and melt h2o data support the conclusion that the independent p t estimates applied to the etive rocks were valid and that measured cordierite volatile contents are representative of peak metamorphic values the etive thermal aureole provides the most compelling evidence suggesting that the cordierite fluid monitor can be used to accurately assess the fluid conditions during metamorphism and partial melting in a thermal aureole

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

mass spectrometric identification of cosmic ray produced neon in terrestrial rocks with multiple neon components studies of cosmic ray produced nuclides in terrestrial rocks are expected to provide important geomorphological and glaciological information such as surface exposure ages erosion rates and extent of glacial cover in the case of the stable nuclide21ne the cosmic ray produced component may represent but one of several components and a component resolution based on three isotope correlations is essential we developed analytical techniques which permit accurate corrections for all interfering ions we show for our modified nier source that corrections for doubly charged species based on constant charge state ratios are not satisfactory the improved techniques permit isotopic studies of 106 atoms of21ne the spallation ratio 22ne 21ne c from si was calibrated using a quartz separate from an allan hills sandstone and yields a value of1 243 0 022 this ratio is expected to be essentially constant for terrestrial quartz samples and should be useful in component resolutions the stepwise release of ne at increasing temperatures shows that cosmic ray produced ne is released from quartz at rather low temperatures this permits an estimate of the activation energy for ne diffusion in quartz 90 10 kj mol

protolith carbon isotope ratios in cordierite from metamorphic and igneous rocks cordierite commonly contains h 2 o and co 2 in the channels formed by its ring structure in many studies cordierite has been shown to have volatile contents and carbon isotope ratios consistent with high temperature equilibrium suggesting preservation of protolith carbon isotope ratios and motivating this survey of carbon isotopes in cordierite co 2 cordierite co 2 from pelitic country rocks in the etive aureole have 13 c values of 20 70 1 27 n 10 that are unaffected by a ca 150 c thermal gradient and fluid saturated and undersaturated regimes these 13 c values are consistent with expected carbon isotope ratios of organic carbon in protolith sediments similar lithologies from the cooma and huntly aureoles show more variable behavior in a more limited data set with some rocks preserving organic carbon 13 c values and others that may have been affected by externally derived fluids in cordierite gedrite gneisses carbon isotopes of cordierite crd are distinct from those of cordierite in pelites when excluding one outlier new data plus those from the literature average 13 c crd 12 51 2 45 n 17 these isotope ratios are higher than those of cordierite in typical metasedimentary protoliths and are similar to published carbon isotope ratios of trace carbonate in altered submarine volcanic rocks which are likely analogs for protoliths of many cordierite gedrite rocks igneous cordierite from granitic plutons have 13 c 23 61 2 08 n 13 which is interpreted as reflecting a magmatic carbon budget dominated by organic carbon from sedimentary source rocks in contrast small pegmatites reported in the literature have 13 c crd 10 20 3 06 n 6 which may indicate derivation from orthogneiss source materials these new data show that carbon isotopes in cordierite can be used to help understand the protolith of even carbon poor metamorphic rocks and can also shed light on carbon in the sources of magmatic rocks this latter approach has the potential for helping constrain the source rocks of peraluminous granitoids which is controversial determining the extent to which organic carbon 13 c is preserved in granitoids is important for understanding the deep carbon cycle and could serve as an important constraint in the search for low 13 c graphite inclusions in hadean detrital zircons which have been reported as a potential biosignature for the early earth

mantle wedge control on back arc crustal accretion at mid ocean ridges plate separation leads to upward advection and pressure release partial melting of fertile mantle material the melt is then extracted to the spreading centre and the residual depleted mantle flows horizontally away in back arc basins the subducting slab is an important control on the pattern of mantle advection and melt extraction as well as on compositional and fluid gradients modelling studies predict significant mantle wedge effects on back arc spreading processes here we show that various spreading centres in the lau back arc basin exhibit enhanced diminished or normal magma supply which correlates with distance from the arc volcanic front but not with spreading rate to explain this correlation we propose that depleted upper mantle material generated by melt extraction in the mantle wedge is overturned and re introduced beneath the back arc basin by subduction induced corner flow the spreading centres experience enhanced melt delivery near the volcanic front diminished melting within the overturned depleted mantle farther from the corner and normal melting conditions in undepleted mantle farther away our model explains fundamental differences in crustal accretion variables between back arc and mid ocean settings