the cleft revealed geologic magnetic and morphologic evidence for construction of upper oceanic crust along the southern juan de fuca ridge 1 the geology and structure of the cleft segment of the southern juan de fuca ridge jdfr have been examined using high resolution mapping systems observations by remotely operated vehicle rov rov mounted magnetometer and the geochemical analysis of recovered lavas bathymetric mapping using multibeam em300 coupled with in situ observations that focused on near axis and flank regions provides a detailed picture of 0 to 400 ka upper crust created at the southern terminus of the jdfr a total of 53 rock cores and 276 precisely located rock or glass samples were collected during three cruises that included sixteen rov dives our observations of the seafloor during these dives suggest that many of the unfaulted and extensive lava flows that comprise and or cap the prominent ridges that flank the axial valley emanate from ridge parallel faults and fissures that formed in the highly tectonized zone that forms the walls of the axial valley the geochemically evolved and heterogeneous nature of these near axis and flank eruptions is consistent with an origin within the cooler distal edges of a crustal magma chamber or mush zone in contrast the most recent axial eruptions are more primitive higher mgo chemically homogeneous lobate sheet and massive flows that generate a distinct magnetic high over the axial valley we suggest that the syntectonic capping volcanics observed off axis were erupted from near axis and flank fissures and created a thickened extrusive layer as suggested by the magnetic and seismic data this model suggests that many of the lavas that comprise the elevated ridges that bound the axial valley of the cleft segment were erupted during the collapse of a magmatic cycle not during the robust phase that established a new magmatic cycle

the sensitivity of seafloor compliance to two dimensional low velocity anomalies summary seafloor compliance the deformation of the seafloor under pressure forcing by long wavelength ocean surface waves is a passive source geophysical measurement that is particularly sensitive to subsurface s wave velocity existing inversions of seafloor compliance data for s wave structure assume a 1 d velocity model whereas in reality there may be significant lateral velocity variation the effect of 2 d structure on compliance measurements has previously been investigated using finite differences but computational constraints limit the number and type of models that can be tested we present a quasi analytical calculation of seafloor compliance over a cylinder that allows us to efficiently investigate the effect of an important subset of subsurface models including pure fluid inclusions the compliance displays the same sensitivity to regions of low shear modulus as is seen for 1 d situations generating a strong peak over the low velocity zone the spatial width of this peak which is a measure of the horizontal resolution of seafloor compliance is approximately proportional to the depth of the cylinder while its amplitude is controlled by the radius and depth of the cylinder and the s wave speed of the cylinder and the background minimum structure 1 d inversions of the compliance measured above a fluid cylinder recover distinct low velocity zones although the recovered s wave speed is much greater than zero and the low velocity zone is smeared out over a greater depth range than that occupied by the cylinder

thermal models of oceanic crustal accretion linking geophysical geological and petrological observations 1 models of oceanic crustal accretion should be able to reproduce both petrological and geological observations from ophiolites and geophysical observations from active spreading ridges we have developed thermal models of crustal accretion at fast spreading ridges that incorporate petrological variation and allow for a variable vertical distribution of crystallization including petrological variation has a major effect on the distribution of melt within the crust and the consequent interpretation of model results in terms of geophysical observables in particular the narrow melt lens imaged by seismic reflection surveys can be produced by the presence of large scale compositional stratification within the crust such as that observed in the oman ophiolite controlling the vertical distribution of crystallization allows a range of crustal accretion mechanisms to be tested including the many sills and gabbro glacier models both gabbro glacier and many sills models can broadly match the geophysical observations but the preferred model is a hybrid where the uppermost 25 or more of the lower crust forms in a gabbro glacier and the lowermost 25 or more of the lower crust forms by crystallization in situ the success of all models is dependent on the strength and distribution of hydrothermal cooling

the accretion of oceanic crust by episodic sill intrusion seismic refraction data from mature oceanic regions show remarkable consistency in crustal thickness between sites yet the characteristics of different spreading centers show great variation this leads to the paradox that the similarity in mature oceanic crust suggests that the igneous processes in crustal accretion are the same across a range of spreading rates but that the differences in the spreading centers themselves imply the opposite we have developed a simple two dimensional model for crustal accretion at oceanic spreading centers by the episodic addition of sills at a high level within the crust as implied by recent geophysical studies of spreading centers using our model we have calculated velocity and temperature fields in the crust including the effects of hydrothermal cooling we show that oscillations in the temperature field due to episodicities of 200 20000 years have an effect localized to the region within 50 500 m of the intrusion by assuming that all latent heat and excess specific heat introduced by the injection of sills is convected away by hydrothermal circulation we estimate that the maximum heat transfer associated with hydrothermal cooling in the upper crust is of the order of 10 times greater than the conductive flux we have calculated seismic velocities associated with the model temperature field in the crust and we show that these are consistent with results from both seismic refraction and reflection experiments the model may be applied to both fast and slow spreading ridges and it accounts for the apparent increases in both the mean crustal temperature and the frequency of presence of melt with increasing spreading rate hence resolving the paradox we also show that the implications of this model are consistent with geological observations of layering in ophiolites

constraining fault friction in oceanic lithosphere using the dip angles of newly formed faults at outer rises article history we investigate the mechanical properties of the oceanic lithosphere using earthquake focal mechanisms from subduction zone outer rises we study regions where faulting oblique to the pre existing mid ocean ridge fabric implies the formation of new faults the nodal plane dips of dip slip earthquakes on these faults are dominantly in the range 30 60 with a strong peak concentrated around 45 this distribution is inconsistent with the pattern that would result from high coefficients of friction e g 0 6 equivalent to normal faults forming at 60 and thrust faults at 30 we instead suggest that the observed distribution of dips implies that faults in the oceanic lithosphere have a low coefficient of friction due to either low friction clay minerals formed by hydrothermal alteration at the ridge or due to an intrinsic level of friction that is lower than that suggested by laboratory studies

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

some ir features of sio 4 and oh in coesite and its amorphization and dehydration at ambient pressure abstract coesite coe with grain size in the range of 30 80 m has been synthesized at 5 gpa and 1600 c for 12 h by using a cubic press its unpolarized single crystal absorption infrared ir spectra show 14 ir bands in the range of 1200 650 cm 1 five of which have high intensity at 1161 1109 1063 1028 and 994 cm 1 and are preliminarily assigned to the sio4 asymmetric stretching 3 in addition three sharp but relatively weak bands at 838 814 and 796 cm 1 are tentatively attributed to the sio4 asymmetric bending 4 the ir data also show six oh peaks in the range of 3700 3300 cm 1 with an estimated h2o content of 30 4 wt ppm following previous studies we have assigned the peaks at 3464 7 3421 8 3406 9 and 3377 cm 1 10 to the type ii hydrogarnet substitution and the peaks at 3500 6a and 3534 cm 1 6b to the b based defects with the latter aroused by possible b contamination in the synthesizing experiments annealing experiments conducted consecutively at 200 400 600 800 1000 and 1200 c with every heating step lasting for 24 h demonstrate that water diffuses quickly out of coe at t as low as 600 c the material annealed at 1200 c is completely dehydrated and amorphous a quick response of the water content in coe to the changes of p t and composition is thus possible which may be critical to the preservation of natural coe in relevant geological processes it further implies that water in coe and possibly in other nominally anhydrous minerals nams may behave distinctively different from the water located in the hydrous phases such as amphibole and mica and potentially makes significant contribution to the subduction zone related fluids

coesite sio2 as an extreme case of superheated crystal an x ray diffraction study up to 1776 k the dense sio2 polymorph coesite has been investigated by x ray powder diffraction from room temperature up to 1776 k no phase transition has been detected in this temperature interval thermal expansion shows little anisotropy and remains very small since the volume increases by only 1 6 between 300 and 1776 k as indicated by theoretical simulations the small dilation results from limited rotations of sio4 tetrahedra and also from slight expansion of si o bonds in addition the results demonstrate that coesite can be kept 900 above its metastable congruent temperature of fusion which is about 875 k comparisons with measurements made under the same conditions for quartz and cristobalite indicate that for sio2 polymorphs with 4 fold coordinated si crystal structure exerts a smaller influence than melt viscosity on the extent of superheating

factors in the preservation of coesite the importance of fluid infiltration the survival of coesite in ultrahigh pressure uhp rocks is most commonly attributed to rapid exhumation continuous cooling during uplift and inclusion in strong phases that can sustain a high internal over pressure during decompression exceptions to all of these criteria exist perhaps less attention has been paid to the role of fluid infiltration in the preservation of coesite we used infrared spectroscopy to measure water contents of coesite and coesite pseudomorphs in a variety of uhp rocks in all cases oh concentrations in coesite are below the detection limit of 100 ppm h2o the silica phases surrounding coesite however show varying amounts of h2o this is most spectacularly observed in pyrope quartzites from the dora maira massif that contain at least three phases of silica replacing coesite also distinguished by varying color of cathodoluminescence cl palisade textured quartz 100 ppm h2o red violet cl mosaic quartz which is actually chalcedony up to 0 4 wt h2o yellow brown cl and a rare highly hydrated silica phase interpreted to be opal 7 wt h2o dark blue cl very similar signatures are observed in a grospydite xenolith from the roberts victor kimberlite the quartz replacing coesite in other uhp samples studied contains on the order of 500 ppm h2o or less and most measurements are under the detection limit of our technique we infer that palisade quartz forms under dry or nearly dry conditions and at high temperatures during dilation of the host phase the formation of hydrous silica phases such as chalcedony and opal however must take place at much lower temperatures after cracking of the host phase which allows external fluids to infiltrate delay of fluid infiltration to low temperatures where kinetics are slow even in the presence of water is the most critical factor in the preservation of coesite

hydrothermal cooling of midocean ridge axes do measured and modeled heat fluxes agree abstract accurate estimates of near axis hydrothermal cooling are keys to understanding many geological chemical and biological processes at midocean ridges at present however field observations h heat flux vent field are few and imprecise and model results h m heat flux km of axis vary widely depending on assumptions about crustal formation crustal composition and the cooling efficiency and spatial and temporal extent of hydrothermal circulation in this paper i compare all published vent field scale measurements of h to a range of published h m for slow 40 mm yr and fast 40 mm yr ridges to assess the present state of knowledge about the magnitude of near axis hydrothermal cooling h has been evaluated 28 times either by point measurements using inventories of individual sources 362 552 mw n 12 measurements or by integrated measurements using water column observations 876 1011 mw n 16 comparable estimates of mean vent field expected h at any spreading rate can be derived by merging h m from various models with the predicted global inventory of active vent sites these estimates range from 1 1 5 gw field and are approximately constant for all spreading rates this range agrees with the best constrained results of h from slow ridge vent fields 1669 1354 mw but is considerably higher than those from fast ridges 245 170 mw inferences about the spatial and temporal variability of h can be gained by defining the parameter l h h m equivalent to the axis length needed to continuously power a hydrothermal field the resultant large l values 13 333 km for slow ridges 1 32 km for fast demand punctuated cooling where for short periods h h m the percentage of time that a given vent field might be active can be speculatively estimated by dividing l by the length of axis the field cools examples of this percentage range from a low of 50 for some fast ridge fields

accretion and oxidation of a superfast spread axial melt lens tims and sims zircon analyses of the iodp hole 1256d gabbros abstract iodp hole 1256d penetrates a complete section of upper oceanic crust spread from the east pacific rise at a full rate of up to 220 mm yr here we present u pb zircon data from hole 1256d gabbroic rocks using isotope dilution thermal ionization mass spectrometry id tims and secondary ion mass spectrometry sims three gabbro samples yielded th corrected weighted mean 206pb 238u sims dates of 15 03 0 14 ma 15 13 0 35 ma and 15 23 0 12 ma an id tims th corrected weighted mean 206pb 238u date of 15 191 0 040 ma from the first of the three samples provides verification and refinement of the timing of magmatism the u pb dates agree with the predicted age of the crust based on marine magnetic anomalies suggesting the gabbros most likely formed by intrusion and crystallization at or near the ridge axis zircons from two of the studied samples have anomalously high th u ratios likely reflecting oxidation of u prior to or during zircon crystallization the elevated o2 of late stage melts in the axial melt lens may be related to progressive crystallization assimilation of hydrothermally altered sheeted dikes and gabbros and or contamination of the melt lens by seawater derived saline brines

a mid ocean ridge thermal model constraints on the volume of axial hydrothermal heat flux the depth to the axial magma chamber at three oceanic spreading centers as indicated by seismic reflection surveys is greater than that predicted by conductive cooling thermal models of ridge crests the additional cooling at the ridge axis is owed to the circulation of seawater through the shallow crust a theoretical model for the temperature distribution at spreading centers which includes distributed heat sources and sinks is presented by representing the hydrothermal heat loss as a series of heat sinks the depths to the axial magma chamber at the east pacific rise at latitude 9 n the southern juan de fuca ridge and the lau basin spreading center are modeled heat sinks are added to the input model to increase the solidus depth until the computed magma chamber depth matches the magma chamber depth determined by seismic reflection surveys the thermal modeling indicates that the axial hydrothermal heat flux is between 7 1 106 and 13 8 106 mj m2 or 10 to 20 of the total missing heat at the ridge axis requiring extensive low temperature circulation off axis high temperature vents that are spaced 1 km apart and are active between 4 and 10 of the time would account for the axial hydrothermal heat loss

crustal growth by magmatic overplating in the galapagos the isotopic compositions of xenoliths hosted in lavas from floreana island indicate that they formed from magmas unlike those at present day floreana instead the xenoliths are geochemically more similar to magmas now erupting from sierra negra and cerro azul volcanoes at the leading edge of the galapagos hotspot this is the first evidence for compositional evolution at a galapagos volcano and indicates increasing contributions from an iso topically enriched source with time as the volcano is carried away from the focus of the hot spot clinopyroxenes in many of the xenoliths exhibit positive anomalies of sr and eu which are attributed to the breakdown of plagioclase the growth of clinopyroxene at the expense of plagio clase results from compression as the crust cools compression is caused by growth mostly from above as shallow intrusions and lavas load the middle and upper oceanic crust

near axis subsidence rates hydrothermal circulation and thermal structure of mid ocean ridge crests we systematically investigated near axis subsidence on the ridge flanks of intermediate and fast spreading mid ocean ridges using bathymetric data from well surveyed portions of the southeast indian ridge spreading at 72 76 mm yr the northern east pacific rise 91 96 mm yr and the southern east pacific rise 144 mm yr in all three regions the mean subsidence rate of young 1 1 4 ma seafloor is less than 220 m m y 1 2 the distribution of individual estimates shows a distinct peak at 180 220 m m y 1 2 with few profiles having subsidence rates greater than 275 m m y 1 2 the observed subsidence rates are significantly lower than both the worldwide average 350 m m y 1 2 and subsidence rates observed for older lithosphere at the same ridge segments intense hydrothermal circulation at the ridge axis can result in low subsidence rates on the adjacent ridge flanks provided the vigorous flow is confined to the immediate vicinity 5 km of the axis according to our model the extremely vigorous hydrothermal circulation ceases off axis and conductive heat flow becomes a primary mechanism of vertical heat transport on the ridge flanks the very low geothermal gradient within the cooled portion of the uppermost lithosphere retards conductive cooling and the cooled area needs to be heated from below before a geothermal gradient can be established which permits significant heat to be conducted out of the lithosphere as a result subsidence in very young 0 1 1 ma lithosphere is suppressed a simple one dimensional thermal model with a nusselt number parameterization was used to estimate the effect of hydrothermal circulation an upper layer with a high nusselt number and the half space that it overlies are initially at a temperature of 1300 c and the surface is maintained at 0 c after 0 1 m y of cooling about 3 5 to 7 km from the spreading axis the nusselt number of the top layer is set to 1 so that normal conduction is simulated in the cooled layer we used an explicit finite difference method to solve for the temporal changes in temperature with depth this model produces subsidence rates in the range that we observe for nu in the range of 15 30 isotherms resulting from the modeling imply rapid lithospheric thickening very near the axis which is incompatible with most current models for the formation of the bathymetric axial high observed at fast spreading mid ocean ridges

formation of fast spreading lower oceanic crust as revealed by a new mg ree coupled geospeedometer abstract a new geospeedometer is developed based on the differential closures of mg and rare earth element ree bulk diffusion between coexisting plagioclase and clinopyroxene by coupling the two elements with distinct bulk closure temperatures this speedometer can numerically solve the initial temperatures and cooling rates for individual rock samples as the existing mg exchange thermometer was calibrated for a narrow temperature range and strongly relies on model dependent silica activities a new thermometer is developed using literature experimental data when the bulk closure temperatures of mg and ree are determined respectively using this new mg exchange thermometer and the existing ree exchange thermometer this speedometer can be implemented for a wide range of compositions mineral modes and grain sizes applications of this new geospeedometer to oceanic gabbros from the fast spreading east pacific rise at hess deep reveal that the lower oceanic crust crystallized at temperatures of 998 1353 c with cooling rates of 0 003 10 2 c yr stratigraphic variations of the cooling rates and crystallization temperatures support deep hydrothermal circulations and in situ solidification of various replenished magma bodies together with existing petrological geochemical and geophysical evidence results from this new speedometry suggest that the lower crust formation at fast spreading mid ocean ridges involves emplacement of primary mantle melts in the deep section of the crystal mush zone coupled with efficient heat removal by crustal scale hydrothermal circulations the replenished melts become chemically and thermally evolved accumulate as small magma bodies at various depths feed the shallow axial magma chamber and may also escape from the mush zone to generate off axial magma lenses

a thermomechanical numerical model for crustal accretion of medium to fast spreading mid ocean ridges we propose a new thermomechanical numerical model of mid ocean ridge accretion aimed at investigating asymmetric spreading rates diverse configurations of lens and sill magma injections crystallization and depth and on and off axis patterns of hydrothermal cooling the numerical algorithm iteratively resolves temperature and motion equations until it reaches a stationary solution the motion equation was written in a vorticity stream function formalism with boundary and internal conditions applied to the stream function to impose the style of magma injection unlike in previous models our model does not assume an a priori shape for the temperature field which is initiated by an initial half space cooling according to the left and right spreading rates complex patterns of hydrothermal cooling are simulated by enhanced thermal diffusivity the model succeeds in describing the dynamic and thermal effects of spreading rates the style of magma intrusion and the hydrothermal cooling accurate descriptions of these are essential to study the cooling histories of crustal rocks and geophysical observables

water movement through a thick unsaturated zone underlying an intermittent stream in the western mojave desert southern california usa previous studies indicated that small amounts of recharge occur as infiltration of intermittent streamflow in washes in the upper mojave river basin in the western mojave desert near victorville california these washes flow only a few days each year after large storms to reach the water table water must pass through an unsaturated zone that is more than 130 m thick results of this study done in 1994 1998 show that infiltration to depths below the root zone did not occur at control sites away from the wash at these sites volumetric water contents were as low as 0 01 and water potentials measured as the combination of solute and matric potentials using a water activity meter were as negative as 14 000 kpa water vapor movement was controlled by highly negative solute potentials associated with the accumulation of soluble salts in the unsaturated zone highly negative matric potentials above and below the zone of maximum solute accumulation result from movement of water vapor toward the highly negative solute potentials at that depth the 18o and d delta oxygen 18 and delta deuterium isotopic composition of water in coarse grained deposits plots along a rayleigh distillation line consistent with removal of water in coarse grained layers by vapor transport beneath oro grande wash water moved to depths below the root zone and presumably to the water table about 130 m below land surface underneath oro grande wash volumetric water contents were as high as 0 27 and water potentials measured as matric potential using tensiometers were between 1 8 and 50 kpa on the basis of tritium data water requires at least 180 260 years to infiltrate to the water table clay layers impede the downward movement of water seasonal changes in water vapor composition underneath the wash are consistent with the rapid infiltration of a small quantity of water to great depths and subsequent equilibration of vapor with water in the surrounding material it may be possible to supplement natural recharge from the wash with imported water recharge to the wash may be advantageous because the unsaturated zone is not as dry as most areas in the desert and concentrations of soluble salts are generally lower underneath the wash

apatite in the dike gabbro transition zone of mid ocean ridge evidence for brine assimilation by axial melt lens we present textures and halogen and trace element compositions of apatites in intrusive rocks from the dike gabbro transition zone of a fast spreading mid ocean ridge which was formed at the east pacific rise and recently drilled by iodp hole 1256d these data are used to discuss the properties of parental magmas and seawater derived hydrothermal fluids at the roof of the axial melt lens during the formation of oceanic crust in general zoning of apatites from three different lithologies tonalites diorites and gabbros is common and shows a consistent evolution trend with depletion in cl and rees from core to rim the cores are usually homogenous in composition and interpreted as magmatic origin whereas zones with lower cl and rees are disseminated with heterogeneous concentrations indicating exchanges with hydrothermal fluids the apatite cores in tonalites are rich in both f and cl with x ap f proportion of fluorapatite end member up to 0 5 and x ap cl proportion of chlorapatite end member up to 0 4 in contrast the apatite cores in gabbros have high x ap cl up to 0 85 and very low x ap f s in some parts of the apatite crystals mainly rim can be explained by removal of these components via hydrothermal fluids based on available f cl oh exchange coefficients for apatite melt the very high cl oh and cl f ratios and high cl contents calculated for tonalitic melts cannot be reconciled with a formation of these felsic melts by partial melting of amphibole bearing metabasalts but indicate that an assimilation of high cl brines must have occurred similarly the low f chlorapatites in gabbros also imply an assimilation of high cl brines the source of high cl fluids in the axial magmatic system may result from seawater derived fluids which may form immiscible vapor and brine at high temperatures as a result of hydrothermal boiling

drilling to gabbro in intact ocean crust sampling an intact sequence of oceanic crust through lavas dikes and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid ocean ridges but it has been an elusive goal of scientific ocean drilling for decades recent drilling in the eastern pacific ocean in hole 1256d reached gabbro within seismic layer 2 1157 meters into crust formed at a superfast spreading rate the gabbros are the crystallized melt lenses that formed beneath a mid ocean ridge the depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid ocean ridges melt lenses occur at shallower depths at faster spreading rates the gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by hole 1256d

distribution of tritium in precipitation and surface water in california summary the tritium concentration in the surface hydrosphere throughout california was characterized to examine the reasons for spatial variability and to enhance the applicability of tritium in hydrological investigations eighteen precipitation samples were analyzed and 148 samples were collected from surface waters across california in the summer and fall of 2013 with repeat samples from some locations collected in winter and spring of 2014 to examine seasonal variation the concentration of tritium in present day precipitation varied from 4 0 pci l near the california coast to 17 8 pci l in the sierra nevada mountains concentrations in precipitation increase in spring due to the spring leak phenomenon the average coastal concentration 6 3 1 2 pci l in precipitation matches estimated pre nuclear levels surface water samples show a trend of increasing tritium with inland distance superimposed on that trend elevated tritium concentrations are found in the san francisco bay area compared to other coastal areas resulting from municipal water imported from inland mountain sources and local anthropogenic sources tritium concentrations in most surface waters decreased between summer fall 2013 and winter spring 2014 likely due to an increased groundwater signal as a result of drought conditions in 2014 a relationship between tritium and electrical conductivity in surface water was found to be indicative of water provenance and anthropogenic influences such as agricultural runoff despite low initial concentrations in precipitation tritium continues to be a valuable tracer in a post nuclear bomb pulse world

density contrasts in the upper mantle and lower crust across the continent ocean transition constraints from 3 d gravity modelling at the norwegian margin summary 3 d gravity models that consider the highly complex density structure of the norwegian passive volcanic continental margin require a significantly lower density in the mantle below the ocean than below the continent this is especially pronounced on the crust free residual gravity anomalies obtained by 3 d gravity stripping this density contrast in the mantle indicates different physical properties either related to thermal or compositional conditions beneath the two domains we assess the gravity response of thermal differences and find that a hotter oceanic mantle compared to a colder continental mantle could cause the density contrast accordingly 3 d lithospheric scale gravity models considering temperature dependent mantle densities indicate that the transition between continental and oceanic lithosphere is rather sharp with a steep gradient in thickness beneath the continent ocean transition zone furthermore a mantle wedge of reduced density extends from the oceanic jan mayen fracture zone to the continental jan mayen lineament and points to increased mantle temperatures and or compositional changes beneath the lineament the models further indicate the presence of high density bodies within the lower crust at the continent ocean transition that are continuous with the lowermost oceanic layer 3b in terms of geometry and density this supports the concept of underplated lower crust beneath the western norwegian margin finally ne sw trending linear zones of increased density are required in the lower crust beneath the eastern voring and more basins to fit the short wavelength gravity signal

constraining groundwater recharge and the rate of geochemical processes using tritium and major ion geochemistry ovens catchment southeast australia summary constraining recharge is essential for understanding and managing groundwater systems this study uses high precision tritium 3 h concentrations together with major ion and environmental isotope sr c o h geochemistry to constrain rates of recharge and geochemical processes in the ovens catchment of the southeast murray basin australia 3 h concentrations of groundwater in the ovens catchment decline with depth but not with position in the catchment implying that recharge occurs over broad areas recharge rates calculated using an exponential piston flow model are 120 180 mm yr in the coarse grained alluvial gravels of the upper and middle ovens and 30 45 mm yr in the finer grained sediments of lower ovens these recharge rates are similar to those calculated from the response of shallow bore hydrographs to rainfall the geochemistry of groundwater with low 60 160 mg l total dissolved solids tds from the upper and middle ovens indicates that dissolution of both silicate minerals and calcite has occurred major ion geochemistry 87 sr 86 sr ratios and 13 c values in the upper and middle ovens groundwater are correlated with groundwater residence times and constrain the progress of mineral reactions dissolution of silicates with high 87 sr 86 sr ratios biotite and k feldspar occurs prior to dissolution of silicates with low 87 sr 86 sr ratios plagioclase and calcite mineral dissolution mainly occurs in the upper 20 m of the aquifer system with relatively little change in geochemistry deeper in the flow system groundwater from the lower ovens has higher tds contents 600 9600 mg l due to higher degrees of evapotranspiration although a correlation between tds and groundwater residence times exists it is more likely to reflect differential recharge rates and not the rate of evapotranspiration the conclusion that recharge occurs across the ovens catchment indicates that protection of groundwater from surface contamination is dependent on the protection and management of large areas

interaction of highly saline fluid and olivine gabbro experimental simulation of deep hydrothermal processes involving amphibole at the base of the oceanic crust abstract natural occurrences of cl bearing amphibole indicate crystallization in the presence of a highly saline fluid amphibole with varying cl contents 0 1 to 5 wt cl found in lower oceanic crustal rocks demonstrates the activity of saline hydrothermal fluids at depth here we present an experimental study done in cold seal pressure vessels cspv and internally heated pressure vessels ihpv to illustrate the process by which gabbro hosted amphibole rich parageneses evolve in the presence of a hydrothermal saline fluid the starting material used was a natural gabbroic rock containing high ti magnesio hastingsite with the addition of a moderately to highly saline fluid nacl h2o with xnacl 0 02 0 07 and 0 24 6 20 and 50 wt nacl we evaluated a range of conditions from hydrothermal 500 750 c to magmatic 900 c at pressures of 200 mpa and fo2 close to nno fluid rock mass ratios used were 0 2 and 1 in subsolidus hydrothermal experiments and 0 07 in partial melting magmatic experiments new amphibole was formed on rims of the starting high ti magnesio hastingsite with product compositions corresponding to magnesio hastingsite high si magnesio hornblende tschermakite edenite hastingsite and ferro pargasite with varying cl contents up to 0 47 wt cl our results from subsolidus experiments demonstrate a decrease in olivine plagioclase and clinopyroxene in the starting rock and formation of new amphibole with decreased ival and ti with respect to starting amphibole product amphibole does not display any correlation between cl and fe2 ival and k in contrast to natural highly cl rich amphibole suggesting that these correlations hold true only at cl contents higher than those attained in our experiments in addition we found that increasing nacl in the fluid correlates with increased maximum cl contents in amphibole compositional variations found in product amphibole highlight the heterogeneities in fluid infiltration and cl activity that account for the complexity of hydrothermal fluid rock interactions in deep oceanic geological systems

the influence of oh in coesite on the kinetics of the coesite quartz phase transition metastable coesite is an important pressure indicator for ultrahigh pressure rocks however in many cases coesite does not survive exhumation but reacts back to quartz although it was shown experimentally that incorporation of h in coesite increases with increasing pressure most coesite relics found in nature are essentially dry i e oh concentrations are below detection limit 1 wt ppm h2o thus does the incorporation of h promote the back reaction of coesite to quartz during exhumation the influence of intrinsic oh on the kinetics of the coesite quartz phase transition was determined using synthetic dry coesite with approximate to10 wt ppm h2o and wet coesite with approximate to105 wt ppm h2o tem analysis of the quenched samples proved the presence and absence of water in the wet and dry samples respectively the kinetics of the coesite quartz transition was investigated in situ using the multi anvil apparatus max 80 at the hamburger synchrotron radiation laboratory hasylab the transition rates were measured by observing changes in selected diffraction line intensities as a function of time the transformation and growth rates were derived using cahn s model of nucleation and growth at grain boundaries under the same experimental conditions the transformation rate of the wet coesite is more than ten times higher than that of the dry coesite this difference may explain why oh bearing natural coesite is rare this study reveals the importance of structurally bound oh for the kinetics of phase transitions of nominally anhydrous minerals

hydroxyl contents of accessory minerals in mantle eclogites and related rocks infrared spectra in the region 3000 to 3800 cm 1 have been measured on coesite kyanite sanidine and rutile occurring in inclusions of eclogite and on garnet and spinel in an inclusion of alkremite from south african kimberites coesite and sanidine contain virtually no h 2o or oh although one sanidine crystal was observed to contain abundant h2o apparently as fluid inclusions of molecular h2o kyanite contains significant oh consistent with previous results for similar samples corundum gives no indication of the presence of h 2o or oh rutile containing significant amounts of fe cr al and nb also contains significant amounts of structural oh garnet in alkremite contains small amounts of molecular h 2o whereas the spinel coexisting with this garnet gives no indication of h 2o or oh indicating that the spinel structure is not able to accommodate significant amounts of oh ammonium contents of all samples were less than the detection limit about 10 ppm

near conductive cooling rates in the upper plutonic section of crust formed at the east pacific rise abstract a new geospeedometer based on diffusion modeling of mg in plagioclase is used to determine cooling rates of the upper section of the lower oceanic crust formed at fast spreading mid ocean ridges the investigated natural sample suites include gabbroic rocks formed at three different locations along the fast spreading east pacific rise these samples cover a depth interval of 0 840 m below the sheeted dike gabbro boundary and therefore allow the variation of cooling rate as a function of depth within the upper plutonic sequence to be determined we demonstrate that the cooling rates we obtained are robust reproducible and consistent across different vertical sections at fast spreading ridges and decrease significantly with increasing sample depth covering almost 4 orders of magnitude ranging from 1 c y 1 for the shallowest samples to 0 0003 c y 1 for the deepest samples both the absolute cooling rates and the rate of decrease of cooling rate with depth are consistent with conductive thermal models in contrast the absolute cooling rates determined from the deeper samples 300 m below dgb and the large decrease in cooling rate with depth are inconsistent with thermal models that include substantial cooling by off axis hydrothermal circulation within the upper plutonic section of the crust

crustal accretion and dynamic feedback on mantle melting of a ridge centred plume the iceland case abstract a major consequence of the interaction of a plume with an oceanic ridge is the enhanced melt production and associated crust generation in the case of iceland crustal thickness as large as 20 to 40 km has been reported crustal seismic velocities are high and have to be explained by thermal or chemical effects in the first part of the paper we address the question whether extraction of melt out of the plume beneath a slowly spreading ridge and deposition of extracted basalt volumes at the surface produces a dynamic feedback mechanism on mantle melting to study this question we solve the convection equations for a ridge centred plume with non newtonian rheology including melting melt extraction associated with deposition of cold crust at the surface of the model and using a simplified approach for compaction the assumption of cold crust is justified if the thickness of each deposited basaltic layer is less than roughly 1 km depending on the buoyancy flux of the plume crustal thicknesses between 10 and 40 km are modelled showing characteristic dipping structures resembling the rift ward dipping basaltic layers of east and western iceland comparing the resulting crustal thickness and magma generation rate with models in which the dynamic effect of crust deposition has been suppressed indicates that melt generation beneath a slowly spreading ridge is considerably damped by the dynamic feedback mechanism if the plume buoyancy flux exceeds 400 to 600 kg s based on the observed crustal thickness of iceland our models predict a plume buoyancy flux of 1140 kg s in the second part we study the accretion of the icelandic crust by a thermo mechanical model in more detail based on the navier stokes the heat transport and the mass conservation equations including volumetric sources hot 1200 c molten crustal material is injected into the newly forming crust with a constant rate at different crustal source regions a deep widespread emplacement of dykes and sills including crustal underplating b magma chambers at shallow to mid crustal level and c surface extrusions and intrusions in fissure swarms at shallow depth connected to volcanic centres we identify the material from the different source regions by a marker approach varying the relative dominance of these source regions characteristic crustal structures evolve showing shallow dipping upper crustal layers with dip angles between 10 and 15 the thermal structure of the crust varies between cold crust shallow source region dominating and hot crust deep source region dominating we use observations of maximum depth of seismicity to constrain the depth of the 650 c isotherm and seismological inferences on the lower crust to constrain temperatures in that region the best agreement with our models is achieved for crust formation dominated by deep dykes and underplating with a considerable influence of magma chamber accretion

magmatic accretion and thermal convection at the sheeted dike complex gabbro boundary in superfast spreading crust odp hole 1256d abstract the magmatic processes responsible for accretion of new oceanic plutonic crust at fast spreading centers remain unclear expedition 312 at ocean drilling program hole 1256d on the eastern flank of the east pacific rise barely drilled through the uppermost gabbros but offers unparalleled opportunities to constrain deep magmatic processes and the mode of construction of the gabbroic layer gabbro 1 and gabbro 2 respectively 52 and 24 m thick were intruded in an altered and baked sheeted dike complex both gabbros lack a macroscopic fabric and were until now considered isotropic here we use digital 3 d fabric analysis of plagioclase phenocrysts in 33 non azimuthally oriented samples in contrast with previous observations the two gabbros display a distinct and consistent shape preferred orientation both gabbros exhibit magmatic equilibrated microstructures with minor submagmatic microstructures the calculated rayleigh numbers range between 10 15 and 10 18 proving that the gabbroic magma was initially undergoing turbulent magmatic flow magmatic foliations dip moderately with a mean of 48 in gabbro 1 and 61 in gabbro 2 magmatic lineations plunge in gabbro 1 between 1 and 44 mean 28 while in gabbro 2 plunges range from 6 to 69 mean 41 these inclined magmatic structures combined with the inclined magmatic contacts between the gabbros and their host rock show that these two gabbro bodies instead of being horizontal sills more likely are inclined intrusions that cut obliquely through the sheeted dike complex seismic reflection data around hole 1256d shows inclined reflectors similar to those produced by gabbroic saucer shaped intrusions in passive margins on the basis of the inclined fabrics inclined contacts and the regional seismic data we propose that the uppermost gabbroic intrusions at fast spreading centers form with a saucer shape this new shape of intrusion never reported before in the oceanic environment may constitute a significant component of oceanic accretion

elasticity and phase transformation at high pressure in coesite from experiments and first principles calculations the crystal structure and equation of state of coesite space group c 2 c and its high pressure polymorph coesite ii space group p 2 1 n under pressure have been studied using x ray powder diffraction in a diamond anvil cell dac up to 31 gpa at room temperature and first principles calculations at 0 k up to 45 gpa new diffraction peaks appear above 20 gpa indicating the formation of coesite ii structure the calculated enthalpies provide theoretical support for the pressure induced phase transformation from coesite to coesite ii at 21 4 gpa compared with coesite the coesite ii structure is characterized by a doubled b axis and the breakdown of the linear si1 o1 si1 angle in coesite into two distinct angles one is 176 close to linear whereas the other decreases by 22 to 158 coesite is very anisotropic with the a axis the shortest and twice more compressible than the b and c axis by comparison coesite ii is not so anisotropic with similar compressibilities in its a b and c axis as analyzed by a third order eulerian finite strain equation of state the bulk modulus of coesite at 21 4 gpa is 182 3 gpa and that of coesite ii is 140 8 gpa indicating that coesite ii is much more compressible than coesite the existence of coesite ii in the coldest subduction zone will change the elasticity and anisotropic properties of the subducting materials dramatically

chemical and thermal constraints on focussed fluid flow in the lower oceanic crust the mechanism of heat extraction from the lower oceanic crust near the ridge axis is poorly constrained despite its importance for understanding both the process of accretion of the plutonic complex and the mass fluxes associated with ridge hydrothermal systems we have investigated the role of zones of focussed fluid flow in the plutonic complex of the oman ophiolite in the near axis cooling of the oceanic crust lineaments identified on aerial photographs that occur at 1 km spacing show evidence for extensive hydrothermal fluid flow through regions 10 to 50 m wide fluid flow is initiated in these regions at 800 c and continues at least into the lower greenschist facies strontium isotope analyses indicate that the fluid flux through these zones is sufficient to transport a metasomatic front from the base of the sheeted dike complex to close to the moho computed minimum fluid fluxes to transport a metasomatic front through the focussed fluid flow zones are 1x108 kgm2 modeling of diffusive exchange of calcium from olivine to clinopyroxene indicates enhanced cooling rates adjacent to the focussed fluid flow zones heat fluxes estimated from the enhanced cooling rates are broadly consistent with the fluid fluxes determined from modeling the sr isotopic composition of samples from the focussed fluid flow zones the combination of independent estimates of the fluid and heat fluxes such as these can provide more rigorous constraints on the thermal history than either approach used in isolation our results show that focussed fluid flow could play a major role in the cooling in the lower oceanic crust significant focussed fluid flow in the lower oceanic crust has important implications for predicting the total mass flux associated with hydrothermal circulation at mid ocean ridges this is because fluids flowing through channels become chemically rock buffered at smaller fluid fluxes than those flowing pervasively through a rock mass thus if focussed fluid flow is an important mechanism of heat loss from the lower oceanic crust the chemical fluxes from ridge hydrothermal systems into the oceans may be smaller than currently thought

interplay between faults and lava flows in construction of the upper oceanic crust the east pacific rise crest 9 25 9 58 n the distribution of faults and fault characteristics along the east pacific rise epr crest between 9 25 n and 9 58 n were studied using high resolution side scan sonar data and near bottom bathymetric profiles the resulting analysis shows important variations in the density of deformational features and tectonic strain estimates at young seafloor relative to older sediment covered seafloor of the same spreading age we estimate that the expression of tectonic deformation and associated strain on old seafloor is 5 times greater than that on young seafloor owing to the frequent fault burial by recent lava flows thus the unseen volcanically overprinted tectonic deformation may contribute from 30 to 100 of the 300 m of subsidence required to fully build up the extrusive pile layer 2a many longer lava flows greater than 1 km dam against inward facing fault scarps this limits their length at distances of 1 2 km which are coincident with where the extrusive layer acquires its full thickness more than 2 of plate separation at the epr is accommodated by brittle deformation which consists mainly of inward facing faults 70 faulting at the epr crest occurs within the narrow 4 km wide upper crust that behaves as a brittle lid overlying the axial magma chamber deformation at greater distances off axis up to 40 km is accommodated by flexure of the lithosphere due to thermal subsidence resulting in 50 inward facing faults accommodating 50 of the strain on the basis of observed burial of faults by lava flows and damming of flows by fault scarps we find that the development of layer 2a is strongly controlled by low relief growth faults that form at the ridge crest and its upper flanks in turn those faults have a profound impact on how lava flows are distributed along and across the ridge crest

melt to mush variations in crustal magma properties along the ridge crest at the southern east pacific rise the determination of along axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle eruption history along the ridge crest and the process of crustal accretion seismic reflection images1 2 3 4 have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths 250 4 500 m but variations in its seismic properties and thickness have remained elusive despite several attempts to constrain these properties5 6 7 here we report that the melt sill along the southern east pacific rise which is about 50 m thick undergoes abrupt changes in its internal properties ranging from pure melt to mush the 60 km long ridge crest segment near 14 00 s is characterized by three 2 4 km sections containing pure melt embedded within a magma chamber rich in mush these small pure melt pockets may represent a fresh supply of magma from the mantle capable of erupting and forming the upper crust conversely the 80 90 of the magma chamber which is mushy is unlikely to erupt and may influence the lower crustal accretion

strontium alteration in the troodos ophiolite implications for fluid fluxes and geochemical transport in mid ocean ridge hydrothermal systems new and published strontium isotope analyses from the troodos ophiolite constrain fluid solid exchange processes and the magnitude and circulation paths of the hydrothermal fluids the87sr 86sr profile reflects alteration in the recharge zone of an evolving hydrothermal system fluid rock strontium isotope exchange in the upper 1 5 km of extrusive lavas was kinetically limited and seawater derived fluids emitted from the base of this zone were buffered to87sr 86sr ratios between 0 7047 and 0 7059 in contrast over the next 1 km depth interval of sheeted dykes and the uppermost plutonics 87sr 86sr values cluster about0 7054 7 2 and fluid flow is inferred to have been pervasive with near equilibrium fluid rock exchange quartz chlorite and epidosite zones the probable pathways of the concentrated high temperature upwelling fluids have identical87sr 86sr ratios to adjacent diabase dykes on troodos a time integrated fluid flux in excess of2 9 107 kg m 2 is required to transport the strontium isotope composition of 0 7054 set in the kinetically controlled exchange zone through the 1 km of sheeted dykes and into the zones of concentrated upwelling the uniformity of the87sr 86sr ratios in the diabase sheeted dykes and high temperature epidosite and quartz chlorite rocks indicate that the strontium isotopic alteration took place during the high temperature phase of hydrothermal circulation the inferred minimum time integrated fluid flux of2 9 107 kg m 2 substantially exceeds that of 5 106 kg m 2 inferred from thermal models of high temperature circulation but is comparable with estimates of the hydrothermal flux from oceanic budgets of3he mg and87sr the high flux estimate for troodos is consistent with the ophiolite venting fluids with87sr 86sr elevated significantly above rock values which contrasts with the near morb87sr 86sr ratios of fluids from active high temperature vents at mid ocean ridges and the87sr 86sr profile in dsdp hole 504b this requires that either the modern systems are immature and there is a protracted phase of lower temperature circulation or that circulation in the troodos ophiolite differed from that in normal mid ocean ridges and oceanic strontium budgets are balanced by another source the lack of very elevated87sr 86sr values in the vent fluids precludes significant channelling in the recharge zones of the active systems

magmatic and tectonic segmentation of the intermediate spreading costa rica rift a fine balance between magma supply rate faulting and hydrothermal circulation 3 d tomographic modelling of wide angle seismic data recorded at the intermediate spreading costa rica rift has revealed a p wave seismic velocity anomaly low located beneath a small overlapping spreading centre that forms a non transform discontinuity at the ridge axis this low velocity zone displays a maximum velocity anomaly relative to the background ridge axis crustal structure of 0 5 km s 1 has lateral dimensions of 10 5 km and extends to depths 2 5 km below the seabed placing it within layer 2 of the oceanic crust we interpret these observations as representing increased fracturing under enhanced tectonic stress associated with the opening of the overlapping spreading centre that results in higher upper crustal bulk porosity and permeability evidence for ongoing magmatic accretion at the costa rica rift ridge axis takes the form of an axial magma lens beneath the western ridge segment and observations of hydrothermal plume activity and microearthquakes support the presence of an active fluid circulation system we propose that fracture pathways associated with the low velocity zone may provide the system through which hydrothermal fluids circulate these fluids cause rapid cooling of the adjacent ridge axis and any magma accumulations which may be present the costa rica rift exists at a tipping point between episodic phases of magmatic and tectonically enhanced spreading the characteristics inherited from each spreading mode have been preserved in the crustal morphology off axis for the past 7 myr using potential field data we contextualize our seismic observations of the axial ridge structure at the whole segment scale and find that the proposed balance between magmatic and tectonically dominated spreading processes observed off axis may also be apparent along axis and that the current larger scale magma supply system at the costa rica rift may be relatively weak based on all available geophysical observations we suggest a model for the inter relationships between magmatism faulting and fluid circulation at the costa rica rift across a range of scales which may also be influenced by large lithosphere scale structural and or thermal heterogeneity

the cooling history and the depth of detachment faulting at the atlantis massif oceanic core complex 1 oceanic core complexes occs are domal exposures of oceanic crust and mantle interpreted to be denuded to the seafloor by large slip oceanic detachment faults we combine previously reported u pb zircon crystallization ages with u th he zircon thermochronometry and multicomponent magnetic remanence data to determine the cooling history of the footwall to the atlantis massif occ 30 n mar and help establish cooling rates as well as depths of detachment faulting and gabbro emplacement we present nine new u th he zircon ages for samples from iodp hole u1309d ranging from 40 to 1415 m below seafloor these data paired with u pb zircon ages and magnetic remanence data constrain cooling rates of gabbroic rocks from the upper 800 m of the central dome at atlantis massif as 2895 1276 1162 c myr 1 from 780 c to 250 c the lower 600 m of the borehole cooled more slowly at mean rates of 500 125 102 c myr 1 from 780 c to present day temperatures rocks from the uppermost part of the hole also reveal a brief period of slow cooling at rates of 300 c myr 1 possibly due to hydrothermal circulation to 4 km depth through the detachment fault zone assuming a fault slip rate of 20 mm yr from u pb zircon ages of surface samples and a rolling hinge model for the sub surface fault geometry we predict that the 780 c isotherm lies at 7 km below the axial valley floor likely corresponding both to the depth at which the semi brittle detachment fault roots and the probable upper limit of significant gabbro emplacement

dating the growth of oceanic crust at a slow spreading ridge nineteen uranium lead zircon ages of lower crustal gabbros from atlantis bank southwest indian ridge constrain the growth and construction of oceanic crust at this slow spreading midocean ridge approximately 75 of the gabbros accreted within error of the predicted seafloor magnetic age whereas 25 are significantly older these anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or more likely crystallization of older gabbros at depths of 5 to 18 kilometers below the base of crust in the cold axial lithosphere which were uplifted and intruded by shallow level magmas during the creation of atlantis bank

relationships between hydrothermal activity and axial magma chamber distribution depth and melt content 1 hydrothermal activity especially high temperature discharge is commonly thought to require the presence of an axial magma chamber amc but this association has not been examined systematically the availability of six lengthy 170 560 km ridge sections with continuous surveys of both amcs and hydrothermal plumes now makes it possible to quantitatively compare the distribution and intensity of hydrothermal activity to amc extent depth and at a few locations melt content these six ridge sections span spreading rates from 55 to 145 mm a and total 20 second order segments at the section multisegment scale the linear incidence of hydrothermal plumes increases with amc incidence amci r2 0 64 excluding the hot spot affected galapagos spreading center for all six sections plume incidence increases as amc depth below the seafloor decreases amcz r2 0 66 at the second order segment scale plume incidence is poorly correlated with both amci r2 0 12 and amcz r2 0 25 finally at the subsegment or local scale 0 75 km long bins plume intensity increases as amcz shallows r2 0 85 of bins where plumes are most intense and thus closest to their seafloor sources 68 13 lie directly over an amc as do at least 37 of the 40 known high temperature vent fields the data also allow tests of other hypotheses linking amc properties and hydrothermal activity existing data though still sparse do not support the hypothesis that lenses of melt rich magma preferentially support vigorous long lasting venting also the suggestion that increased hydrothermal cooling within a segment locally depresses amcz finds no support within any ridge section evidence for magma bodies is much scarcer on slow spreading ridges but the data are nevertheless consistent with those from faster ridges observations from all spreading rates thus demonstrate that high temperature vent fields are almost universally associated with the presence or inference of magma hot rock or other nonmagmatic heat sources are insufficient

constraining the cooling rate of the lower oceanic crust a new approach applied to the oman ophiolite abstract knowledge of the cooling rate of newly formed lower oceanic crust is critical in constraining models of crustal accretion and hydrothermal circulation at mid ocean ridges in this study we calculate the cooling rate of the lower oceanic crust based on the closure temperature of down temperature diffusive exchange of ca from olivine to clinopyroxene in gabbros from the oman ophiolite modelling of the closure temperatures shows that the cooling rates for the lower oceanic crust decrease with depth changing rapidly in the upper third of the gabbros but more slowly in the lower two thirds there is several orders of magnitude variation in cooling rate between the top and bottom of the lower crust the cooling rate versus depth profile closely matches that of a conductive cooling model or a model in which conductive cooling is enhanced by a constant factor due to hydrothermal circulation

ductile deformation and the origin of layered gabbro in ophiolites layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3 6 km deep although large steady state magma chambers provide a possible explanation for the limited diversity of mid ocean ridge basalts their existence is not supported by geophysical observations sinton and detrick 1992 recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small 1 4 km wide magma chambers perched on large 8 km wide zones of low velocity material which are interpreted to consist of partially molten crystal cumulates in this paper we compare the geology of the gabbroic rocks of the samail ophiolite which is thought to have formed by fast spreading with results of a finite element analysis of the sinton detrick model for fast spreading centers our results demonstrate that flow similar to that postulated by sleep 1975 within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the samail gabbros as a consequence of this deformation 1 layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center 2 the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime let alone the magma chamber 3 feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering but those that invade farther from the ridge will be less deformed and retain crosscutting relationships and 4 total deviatoric strain should increase down section toward the moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state

reconciling temperatures of metamorphism fluid fluxes and heat transport in the upper crust at intermediate to fast spreading mid ocean ridges 1 current models of the mineralogical and chemical modification of the crust within mid ocean ridge hydrothermal systems at intermediate to fast spreading ridges are difficult to reconcile with the temperature distribution predicted by the required fluid fluxes in particular the sharp increase in metamorphic grade passing down from the lavas into the sheeted dike complex places important constraints on the flow of fluid through the crust this distribution of metamorphic temperatures has typically been discussed in terms of a step in permeability in the downwelling limb of a single pass convection cell simple modeling of this system shows that downwelling fluid cools the crust too efficiently for high temperature metamorphism of the upper dikes to occur in the downflow zone in fact temperatures throughout most of the sheeted dike complex are too high three alternative models are considered fluid rock reaction during the cooling of each dike from magmatic temperatures is difficult to reconcile with cooling rate estimates for dikes reaction rates required to grow metamorphic minerals and the relatively constant composition of many hydrothermal vents metamorphism during reheating of the crust after fluid flow slows or stops is difficult to reconcile with the required fluid fluxes during metamorphism based on the sr isotopic composition of the sheeted dike complex and cannot produce high enough temperatures in the upper dikes and low temperatures in the lower lavas in contrast fluid rock reaction in the sheeted dike complex as hot fluids migrate upward after being heated by the underlying magma source appears to be consistent with the data in this model downwelling seawater cools the recharge zone and the fluid only becomes hot enough to significantly alter the crust once it approaches the conductive boundary layer overlying the magma chamber metamorphism of the sheeted dike complex then largely occurs in the discharge zone

what do variable magnetic fabrics in gabbros of the oman ophiolite reveal about lower oceanic crustal magmatism at fast spreading ridges the magmatic processes responsible for accretion of the lower oceanic crust remain one of the least constrained components of the global seafloor spreading system samples of gabbroic rocks recovered by scientific ocean drilling are too limited to allow effective assessment of spatial variations in magmatic flow within in situ lower crust extensive exposures of gabbros in ophiolites on the other hand provide opportunities to study accretion processes in three dimensions across wide areas and at a resolution that allows variations in magmatic fabrics through the crust to be quantified here we show that magnetic anisotropy provides a reliable proxy for lower crustal magmatic fabrics in the world s largest ophiolite in oman important differences in magnetic fabrics are detected that reflect variations in magmatic processes on a range of scales fabrics in layered gabbros are aligned with modal layering and display a consistency in the orientation of maximum principal axes of anisotropy between localities at a regional scale these fabrics are compatible with subhorizontal preferred alignment of crystals orthogonal to the inferred orientation of the oman spreading axis resulting from magmatic flow or deformation of melt rich crystal mushes during spreading in contrast magnetic anisotropy in foliated gabbros at the top of the lower crust reveals for the first time distinctly different linear and anastomosing fabric styles between localities sampled at the same pseudostratigraphic level these differences reflect spatial variations in the style and trajectory of flow in the crystal mush beneath the axial melt lens during upward melt migration at the spreading axis

cooling of the lower oceanic crust thermal models of mid ocean ridges that balance the influx of heat from magmatic sources with the removal of heat by conduction and hydrothermal circulation allow quantification of cooling of young oceanic crust these models reproduce key observations relating to crustal accretion and hydrothermal cooling at fast spreading ridges the rate of cooling is constrained both by the bathymetry of ridge axes and by olivine compositions from ophiolite gabbros successful models involve extensive hydrothermal cooling of the lower crust within 20 km of the ridge with 50 70 kw of hydrothermal cooling for every 1 m of ridge axis at crustal ages of 0 1 0 4 ma these timates can be used to refine global models of geochemical and thermal fluxes close to spreading ridges

kinetics of the coesite to quartz transformation abstract the survival of coesite in ultrahigh pressure uhp rocks has important implications for the exhumation of subducted crustal rocks we have conducted experiments to study the mechanism and rate of the coesite quartz transformation using polycrystalline coesite aggregates fabricated by devitrifying silica glass cylinders containing 2850 h 10 6 si at 1000 c and 3 6 gpa for 24 h conditions were adjusted following synthesis to transform the samples at 700 1000 c at pressures 190 410 mpa below the quartz coesite equilibrium boundary reaction proceeds via grain boundary nucleation and interface controlled growth with characteristic reaction textures remarkably similar to those seen in natural uhp rocks we infer that the experimental reaction mechanism is identical to that in nature a prerequisite for reliable extrapolation of the rate data growth rates obtained by direct measurement differ by up to two orders of magnitude from those estimated by fitting a rate equation to the transformation time data fitting the rates to turnbull s equation for growth therefore yields two distinct sets of parameters with similar activation energies 242 or 269 kj mol but significantly different pre exponential constants extrapolation based on either set of growth rates suggests that coesite should not be preserved on geologic time scales if it reaches the quartz stability field at temperatures above 375 400 c the survival of coesite has previously been linked to its inclusion in strong phases such as garnet that can sustain a high internal pressure during decompression other factors that may play a crucial role in preservation are low fluid availability possibly even less than that of our nominally dry experiments and the development of transformation stress which inhibits nucleation and growth these issues are discussed in the context of our experiments as well as recent observations from natural rocks

detailed analysis of near tectonic features along the east pacific rise at 16 n near the mathematician hot spot spreading processes at the axes of fast spreading ridges are mainly controlled by magmatic activity whereas tectonic activity dominates further away from the axis high resolution near bottom bathymetry data photographs videos and human observations from submersible surveys are used to develop a detailed tectonic analysis of the 16 n segment of the east pacific rise epr these data are used to evaluate how a highly magmatic segment close to a hot spot affects the nucleation and evolution of faulting patterns and impacts the evaluation of tectonic strain within 2 km of the spreading axis our study shows that 1 the growth of tectonic features differs in response to dike intrusion and tectonic extension 2 the initiation of brittle extension is strongly controlled by the location of the axial magma lens and the development of layer 2a and 3 the high magmatic budget and the off axis magma lens in the west part of the plateau do not significantly impact the initiation of brittle extension along the central portion of the 16 n segment within the axial summit region more than 2 of plate separation at 16 n on the epr is accommodated by brittle extension as is observed at other epr segments the interaction of the mathematician hot spot with this epr segment has no significant influence on the initiation of the tectonic deformation but it does reduce the development of the brittle deformation

a fossil melt lens in the oman ophiolite implications for magma chamber processes at fast spreading ridges from seismic evidence we know that a small body of magma is present at mid crustal levels beneath most fast spreading ridges and that this small sill or melt lens overlies a much broader area of hot but largely solid material a crystal mush that constitutes the lower ocean crust beyond this however we have little direct knowledge of the physical and chemical processes that operate in and below the melt lens nor the role the melt lens plays in the storage and aggregation of mid ocean ridge basalts morb we here offer constraints on the processes of melt transport modification and residence beneath fast spreading ridges from a coupled structural and geochemical study through the crustal section of the oman ophiolite concentrating on the uppermost plutonic rocks and transition into sheeted dykes we find that almost the entire oman plutonic suite the layered gabbro and most of the high level gabbro is dominated by cumulate rocks containing very low proportions of trapped interstitial melt the exception to this is the very uppermost part of the section within 150 metres of the base of the sheeted dyke complex at which level the gabbros instead display a marked heterogeneity of structure texture and composition and plutonic rocks of generally basaltic composition occur for the first time associated pegmatitic ferrogabbros formed by in situ fractionation of these liquids this horizon passes up into a more homogeneous microgabbroic facies also of basaltic composition within which isolated asymmetric doleritic chilled margins are found and thence into doleritic sheeted dykes we argue that the thin heterogeneous gabbro horizon is a fossilised melt lens containing pooled liquids of net basaltic composition mg of 65 expelled from the crystal mush beneath existing models have suggested that steep fabrics observed in homogeneous gabbros beneath this horizon formed by subsidence of originally horizontal cumulus layers formed at the base of the melt lens but this is not supported by the field relationships instead we propose that the fabrics record the buoyant ascent of magma through the mush pile probably in part by porous flow and reflect crystallisation essentially in situ without significant vertical transport of the mush low residual melt porosities in the sub melt lens region imply efficient extraction of interstitial liquid and at least episodically high permeabilities in the upper part of the lower crust

cooling rates and the depth of detachment faulting at oceanic core complexes evidence from zircon pb u and u th he ages oceanic detachment faulting represents a distinct mode of seafloor spreading at slow spreading mid ocean ridges but many questions persist about the thermal evolution and depth of faulting we present new pb u and u th he zircon ages and combine them with magnetic anomaly ages to define the cooling histories of gabbroic crust exposed by oceanic detachment faults at three sites along the mid atlantic ridge ocean drilling program odp holes 1270d and 1275d near the 15 20 n transform and atlantis massif at 30 n closure temperatures for the pb u 800 c 850 c and u th he 210 c isotopic systems in zircon bracket acquisition of magnetic remanence collectively providing a temperature time history during faulting results indicate cooling to 200 c in 0 3 0 5 myr after zircon crystallization recording time averaged cooling rates of 1000 c 2000 c myr assuming the footwalls were denuded along single continuous faults differences in pb u and u th he zircon ages together with independently determined slip rates allow the distance between the 850 c and 200 c isotherms along the fault plane to be estimated calculated distances are 8 4 4 2 km and 5 0 2 1 km from holes 1275d and 1270d and 8 4 1 4 km at atlantis massif estimating an initial subsurface fault dip of 50 and a depth of 1 5 km to the 200 c isotherm leads to the prediction that the 850 c isotherm lies 5 7 km below seafloor at the time of faulting these depth estimates for active fault systems are consistent with depths of microseismicity observed beneath the hypothesized detachment fault at the tag hydrothermal field and high temperature fault rocks recovered from many oceanic detachment faults

protracted construction of gabbroic crust at a slow spreading ridge constraints from 206pb 238u zircon ages from atlantis massif and iodp hole u1309d 30 n mar sensitive high resolution ion microprobe shrimp u pb zircon ages of 24 samples from oceanic crust recovered in integrated ocean drilling program iodp hole u1309d and from the surface of atlantis massif mid atlantic ridge mar 30 n document a protracted history of accretion in the footwall to an oceanic detachment fault ages for 18 samples of evolved fe ti oxide gabbro and felsic dikes collected 40 1415 m below seafloor in u1309d yield a weighted mean of 1 20 0 03 ma mean square of weighted deviates 7 1 however the ages range from 1 08 0 07 ma and 1 28 0 05 ma indicating crustal construction occurred over a minimum of 100 200 ka the zircon ages along with petrologic observations indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka the oldest ages are observed below 600 mbsf an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault the data are most consistent with a multiple sill model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1 4 km zircon ages from broadly spaced samples collected along the southern ridge of atlantis massif yield a detachment fault slip rate of 28 7 6 7 mm a and imply significant asymmetric plate spreading up to 100 on the north american plate for at least 200 ka during core complex formation

slow cooling of the lowermost oceanic crust at the fast spreading east pacific rise the cooling history of the lowermost ocean crust formed at the east pacific rise and exposed at the hess deep rift is constrained using fresh primitive olivine gabbro and troctolite cores recently recovered by integrated ocean drilling program expedition 345 the mg in plagioclase and ca in olivine geospeedometers constrain the initial subsolidus cooling rates down to closure temperatures in the range of 900 750 c these independent geospeedometers yield similar slow cooling rates 0 005 0 0001 c yr 1 with a mean value of 0 0011 c yr 1 making these results robust despite the uncertainties inherent in the methods the slow cooling rates derived using natural samples provide a crucial test of predictions of the thermal structure of the lower crust based on remote imaging techniques and thermal modeling the cooling rates are most consistent with thermal models that are dominated by conductive cooling within the plutonic complex this implies that lower crust formed at fast spreading rates remains hot in the near axis environment indicating limited hydrothermal fluid ingress and reaction

causes for axial high topography at mid ocean ridges and the role of crustal thermal structure mid ocean ridge topography is modeled as the flexural response to loads using a thin plate approximation and setting thermal structure of the lithosphere to allow but not require a region of rapid cooling near the axis loads on the lithosphere arise from the presence of low density melt densification due to cooling with distance from the ridge axis and thermal contraction stresses we find two end member classes of temperature and melt structure that can produce axial high topography and gravity observed at the east pacific rise epr one class is very similar to previous models requiring a narrow column of melt extending to at least 30 km depth within the mantle and lithosphere which cools and thickens very gradually with distance from the ridge axis the other is a new class predicting lithosphere which cools rapidly within a few kilometers of the axis and then slowly farther from the axis with melt which is contained primarily within the crust the latter solution is consistent with tomography and compliance studies at the epr which predict rapid crustal cooling within a few kilometers of the axis that is attributed to hydrothermal circulation this solution also allows the melt region to be coupled to crustal thermal structure and requires no melt anomaly within the mantle model fits predict 0 30 melt in the lower crust depending on how temperatures are distributed within the lithosphere and the degree to which thermal contraction stresses are assumed to contribute to topography the model generally predicts a wider axial high for lithosphere which is thin over a wider region near the axis this is consistent with previous correlations between large cross sectional area of the high and indicators of higher melt presence or a warmer crustal thermal regime for a slightly slower rate of lithospheric cooling at distances more than 5 km from the axis the model predicts a trough at the base of the axial high such troughs have been previously observed at the base of the high on the western flank of the southern epr where subsidence rates are anomalously low finally thick axial lithosphere reduces the amplitude of the high making it sometimes difficult to distinguish from long wavelength subsidence this morphology is comparable to that of some intermediate spreading ridges where topography is relatively flat suggesting a transition from fast to intermediate style morphology

a numerical model of hydrothermal cooling and crustal accretion at a fast spreading mid ocean ridge 1 there are two competing models for lower crustal accretion at fast spreading ridges in the gabbro glacier model all the lower crust solidifies in a magma lens at the dike gabbro boundary and then flows to its final level in the sheeted sill model the lower crust is formed by sills emplaced at a variety of depths we present a steady state numerical representation of the sill model that explicitly includes hydrothermal circulation the crust is accreted uniformly at the ridge axis at all depths and hydrothermal circulation occurs in crustal rocks that have cooled below a threshold temperature the results show that when the permeability exceeds a threshold value 4 10 14 m2 for a cracking temperature of 800 c and a uniform permeability hydrothermal circulation can cool the entire crust near the ridge axis producing vertical isotherms in the lower crust at lower permeabilities the properties of seawater lead to bimodal cooling with the lowermost crust cooling more slowly than shallower depths these results are reasonably consistent with studies of the seismic structure of the east pacific rise and of plagioclase crystal size distributions in gabbros from oman we infer that the sheeted sill model cannot be discarded on thermal grounds

volatiles co2 s f cl br in the dike gabbro transition zone at iodp hole 1256d magmatic imprint versus hydrothermal influence at fast spreading mid ocean ridge abstract the dike gabbro transition zone of mid ocean ridges mors is a critical site for the accretion of oceanic crusts as it is the main zone of interactions between the hydrothermal and magmatic systems in this study volatiles contents of co 2 s f cl and br for a variety of lithologies from the dike gabbro transition zone of an intact oceanic crust were investigated in order to examine the magmatic imprint versus hydrothermal influence the studied samples include plutonic rocks gabbros diorites and tonalites hornfelses i e granoblastic dikes an amphibole rich vein and albitites these rocks were recently sampled by the iodp integrated ocean drilling program at site 1256 representing parts of an oceanic crust that formed at the fast spreading ridge of the east pacific rise the bulk co 2 contents 500 10 000 ppm show no lithological dependence and are similar to those of fresh lavas highly variable bulk s concentrations 3 900 ppm show significant depletions compared to undegassed morb melts which might be caused by exsolution of magmatic fluids during crystallization the plutonic samples have cl concentrations of 500 1000 ppm remarkably higher than the hornfelses 200 600 ppm cl and erupted lavas average 200 ppm cl except for the albitites that contain very low f 20 30 ppm and the hydrothermal amphibole rich vein that contains extremely high f 1000 ppm the other lithologies have similar f concentrations within 100 350 ppm the br concentrations of different lithologies are largely overlapping within 0 5 3 5 ppm petrological and mineralogical evidence indicates that bulk f concentrations are dominated by the presence of amphibole and apatite whereas cl and br are mainly hosted in fluid inclusions rather than in hydrous minerals the variation of f cl and br cl ratios may trace the mixing between morb magmas and seawater derived fluids crystallization of apatite and amphibole and or extraction of magmatic fluids our data support the current s budget of oceanic crust but indicate that the budgets of c and cl may have been underestimated

kinetic and mineralogic controls on the evolution of groundwater chemistry and 87sr 86sr in a sandy silicate aquifer northern wisconsin usa abstract substantial flowpath related variability of 87sr 86sr is observed in groundwaters collected from the trout lake watershed of northern wisconsin in the extensive shallow aquifer composed of sandy glacial outwash groundwater is recharged either by seepage from lakes or by precipitation that infiltrates the inter lake uplands 87sr 86sr of groundwater derived mainly as seepage from a precipitation dominated lake near the head of the watershed decreases with progressive water chemical evolution along its flowpath due primarily to enhanced dissolution of relatively unradiogenic plagioclase in contrast 87sr 86sr of groundwater derived mainly from precipitation that infiltrates upland areas is substantially greater than that of precipitation collected from the watershed due to suppression of plagioclase dissolution together with preferential leaching of sr from radiogenic phases such as k feldspar and biotite the results of a column experiment that simulated the effects of changing residence time of water in the aquifer sand indicate that mobile waters obtain relatively unradiogenic sr whereas stagnant waters obtain relatively radiogenic sr nearly the entire range of strontium isotope composition observed in groundwaters from the watershed was measured in the experimental product waters the constant mobility of water along groundwater recharge flowpaths emanating from the lakes promotes the dissolution of relatively unradiogenic plagioclase perhaps due to effective dispersal of clay mineral nuclei resulting from dissolution reactions in contrast episodic stagnation in the unsaturated zone along the upland recharge flowpaths suppresses plagioclase dissolution perhaps due to accumulation of clay mineral nuclei on its reactive surfaces differences in redox conditions along these contrasting flowpaths probably enhance the observed differences in strontium isotope behavior this study demonstrates that factors other than the calculated state of mineral saturation must be considered when attempting to simulate chemical evolution along flowpaths and that reaction models must be able to incorporate changing contributions from reacting minerals in the calculations

estimation of oceanic hydrothermal heat flux from heat flow and depths of midocean ridge seismicity and magma chambers the difference between the heat flow predicted by thermal models of the lithosphere and that measured at the seafloor can be used to estimate the cooling by hydrothermal circulation however this approach may yield an overestimate because measurements in thinly sedimented young crust are typically made at topographic lows where hydrothermal circulation may systematically lower heat flow to circumvent this bias we estimate the cooling using the depths of midocean ridge earthquakes and magma chambers in addition to heat flow data the results indicate deeper hydrothermal circulation at slow spreading ridges and higher near axial hydrothermal heat loss at fast spreading ridges the predicted global hydrothermal heat flux is 80 of that for models constrained only by heat flow

hydrothermal cooling along the eastern lau spreading center no evidence for discharge beyond the neovolcanic zone heat transported from the mantle beneath spreading centers creates an astonishingly narrow ribbon of convective heat discharge at plate boundaries as apparently demonstrated by exhaustive exploration for hydrothermal discharge sites over the last three decades recent observations and models are now challenging this assumption of exclusively axis centric high temperature venting one example is the proposal that intense cooling along the vertical boundaries of a broad low velocity volume lvv of hot crust could generate high temperature fluids several kilometers off axis to test the hypothesis that substantial hydrothermal discharge might occur beyond the lvv we conducted a dense survey grid of the ridge and surrounding seafloor up to 5 km along 175 km of the eastern lau spreading center and valu fa ridge 1800 km of track line our sampling array extended from 50 to 400 m above bottom and included light scattering oxidation reduction potential and hydrographic sensors attached to the tow line and beneath the imi120 sonar mapping system the surveys successfully mapped plumes from several vent fields in the neovolcanic zone 1 5 km about the axis but did not detect evidence of significant discharge anywhere farther off axis at a few locations on the valu fa ridge however we did record oxidation reduction potential anomalies with hydrographic density signatures that imply low temperature hydrothermal sources on the axial flank although these sites are hundreds of meters deeper than the adjacent crest they are above not beyond the previously mapped lvv our results thus do not support a simple picture of high temperature fluids ascending undiluted through the crust to the seafloor several kilometers off axis however we cannot exclude the possibilities that the largely unmapped lvv is narrower here than seen on other ridges that hydrothermal fluids formed beyond the lvv are channeled to the axis or that discharge beyond the neovolcanic zone occurs only as dispersed very low temperature fluids our observations do demonstrate that high temperature discharge predominantly exits the seafloor within a narrow 1 5 km axial ribbon regardless of the presence or absence of an axial magma chamber

crustal accretion of thick mafic crust in iceland implications for volcanic rifted margins rifting near hotspots results in mantle melting to create thick mafic igneous crust at volcanic rifted margins vrms this mafic crust is transitional between rifted continental crust with mafic intrusions landward and oceanic crust into which it grades seaward seismic velocities crustal drilling and exhumed margins show that the upper crust in these areas is composed of basaltic lava erupted in subaerial to submarine conditions intruded by downward increasing proportions of dikes and sparse gabbroic intrusions the lower crust of these regions is not exposed but is inferred from seismic velocities vp 6 5 km sec and petrological constraints to be gabbroic to ultramafic in composition limited access to crustal sections generated along vrms have raised questions regarding the composition and structure of this transitional crust and how it evolves during the early stages of rifting and subsequent seafloor spreading active processes in iceland provide a glimpse of subaerial spreading with the creati

cooling rates in the lower crust of the oman ophiolite ca in olivine revisited debate over the mechanism of accretion of the lower oceanic crust has centered around the gabbro glacier and the sheeted sills models the thermal profile of the crust specifically the roles of hydrothermal circulation and cooling rate is a key component in distinguishing between these two models results of this study show no systematic variation of cooling rate with depth in the lower crust of the khafifah section in the wadi tayin massif of the oman ophiolite additionally very high rates recorded near the base of the crust suggest that hydrothermal circulation plays an important role in the removal of heat throughout the crust while the conclusions presented here do not rule out the possibility of accretion by gabbro glacier they do contradict the initial tenet of the gabbro glacier model convective heat loss by hydrothermal circulation can penetrate to moho depths and crystallization is not by necessity constrained to the upper crust on the basis of these results it can be concluded that the sheeted sills model is a thermally viable mechanism for accretion of oceanic lower crust

dating cave drip water by tritium summary speleothems are increasingly used as an archive of past climate but some of the proxy signals encoded in these deposits reflect hydrological characteristics of the karst aquifer and not necessarily climate variability a central aspect in karst hydrology is the time required for the rainwater to reach the point of discharge in a cave e g the tip of the stalactite one promising approach in determining this residence time is drip water dating by tritium 3 h in contrast to traditional tritium dating we do not refer directly to tritium concentrations in precipitation as input function but to an infiltration weighted annual mean of the rainwater values using concentration differences between this infiltration weighted mean and the drip water an age is calculated from the radioactive decay law assuming piston flow the approach was tested in three adjacent caves in northwestern germany which were monitored for about two years all of the studied drip sites yielded drip water ages between 2 and 4 years with uncertainties on the order of 1 year these results were confirmed at several drip sites by oxygen isotope data which show rather constant values with insignificant intra annual variability attempts to apply the 3 h 3 he method resulted in comparable ages despite several complicating factors

cooling history of atlantis bank oceanic core complex evidence for hydrothermal activity 2 6 ma off axis 1 we report 26 u th he zircon ages from atlantis bank southwest indian ridge which constrain time scales and rates of lower crustal cooling in ultraslow spreading oceanic crust in this setting samples from the detachment fault surface indicate that denuded oceanic crust cooled rapidly 1200 c ma consistent with existing models for the cooling of oceanic crust u th he zircon ages from samples collected along n s and e w trending faults scarps record young ages inconsistent with standard cooling models for lower oceanic crust these samples have a mean u th he zircon age 2 6 ma younger than their corresponding igneous crystallization ages and record cooling through 200 c well outside the rift valley similar anomalously young ages are recorded by zircon sphene and apatite fission track data from odp hole 735b we interpret these young ages as recording an off axis thermal heating event associated with localized high temperature 300 c hydrothermal fluid flow resulting from underplated mafic magmas

pressure induced phase transitions in coesite high pressure behavior of coesite was studied on single crystals using diamond anvil cells with neon as the pressure transmitting medium by means of in situ raman spectroscopy up to pressures of 51 gpa the experimental observations were complemented with theoretical computations of the raman spectra under similar pressure conditions we find that coesite undergoes two phase transitions and does not become amorphous at least up to 51 gpa the first phase transition coesite i to coesite ii is reversible and occurs around 23 gpa the second transition coesite ii to coesite iii at about 35 gpa is also reversible but involves a large hysteresis samples recovered from the highest pressure achieved 51 gpa show raman spectra of the initial coesite the ab initio calculations gave insight into the initiation mechanism of the first phase transition implying from the analysis of unstable phonon modes that it is probably a displacive phase transition due to shearing of the four membered rings of sio4 tetrahedra upon compression the transition to the lowest symmetry phase coesite iii is possibly a first order phase transition that leads to a very distinct structure none of the metastable high pressure phases of coesite has been previously studied and it was widely accepted that coesite undergoes pressure induced amorphization at significantly lower pressures 30 gpa the study of the high pressure behavior of coesite is important to better constrain the metastable phase diagram of silica further crystallographic investigations are necessary for characterizing the structures of these metastable coesite forms crystalline or amorphous metastable phases derived from coesite under high pressure conditions are of particular interest because they can be used as potential tracers of peak transient pressures stress reached in processes such as impacts or faulting

evidence and implications of crustal magmatism on the flanks of the east pacific rise article i nfo article history formation of oceanic crust along the east pacific rise is thought to be complete within a few kilometers of the rise axis here however we present evidence for magmatism 20 km from the spreading center in 300 ka old crust seismic data reveal an intrusive complex 2 km beneath the seafloor which is limited in lateral extent b5 km and comprises a melt lens underlain by low velocity high attenuation crust the crustal magma body overlies a region of off axis delivery of mantle melt that results from a long lived skew between the axes of mantle upwelling and plate spreading magmatism on the flanks of the epr may be commonplace thus affecting the architecture of oceanic crust and providing the necessary conditions to drive off axis hydrothermal activity

effects of the legacy of axial cooling on partitioning of hydrothermal heat extraction from oceanic lithosphere the predicted and observed heat flux across the seafloor these methods have assumed a dynamic steady state thermal system we show that this assumption is not warranted and leads to an incorrect partitioning of hydrothermal circulation between ridge axes and flanks to more accurately estimate hydrothermal heat loss on ridge axes and flanks we consider the spatial and temporal extent and vigor of axial hydrothermal circulation in calculating hydrothermal heat extraction axial fluid circulation perturbs the thermal state of oceanic lithosphere for 5 ma after that circulation ceases reducing the hydrothermal heat extraction on ridge flanks we find 30 of hydrothermal heat extracted on axis 10 extractednearaxis fromendofaxialhydrothermalcirculationto1ma and 60 extracted from lithosphere 1 ma

tritium in australian precipitation a 50 year record summary tritium in precipitation has been measured in australia over the past 50 years as an essential research tool in hydro climate studies and to contribute to the global network for isotopes in precipitation gnip tritium a component of the water molecule hto is the only true age tracer for waters the elevated level of tritium in the environment as a result of last century s atmospheric thermonuclear testing delivers the benefit of tracing groundwater systems over a 100 year timeframe the concentration of tritium in australian precipitation reached a maximum of 160 tritium units tu in 1963 during one of the most intense periods of nuclear weapons testing from 1963 to present we observe a rapid drop in the concentration of tritium more than expected from natural decay and this can be attributed to the wash out of tritium into the oceans and groundwater spikes in the tritium level are superimposed over this general trend the first around 1969 with levels ranging from 39 4 to 84 4 tu was due to french atmospheric weapon testing and again in 1990 levels peaked between 6 6 and 12 9 tu which is attributed to tritium leaking from underground tests in the french pacific since 1990 the levels of tritium have declined globally and regionally currently the levels of tritium in australia are stabilising to around 2 3 tu increasing with latitude across the continent suggesting that today the tritium in precipitation is predominantly natural the spatial distribution of tritium is presented and found to be dominated by the annual stratosphere troposphere exchange in combination with latitude and continental effects a precipitation amount effect is also observed for inland sites