seismic evidence for anomalous crustal structure beneath mesozoic fracture zones in the gambia basin eastern equatorial atlantic abstract five sonobuoy refraction profiles shot in the eastern equatorial atlantic reveal the presence of thin and heterogenous crust in a zone at least 200 km wide near the southern margin of the gambia basin oceanic layer 2 v p 4 1 4 4 km s is detected on all profiles on some sections it is underlain by crustal material with apparent seismic velocities between 5 7 km s and 7 0 km s on others layer 2 directly overlies a zone with apparent velocities characteristic of normal upper mantle velocities 8 km s the formation of anomalous crust is attributed to a modification of accretion processes resulting from a close spacing of fracture zones during a period of slow spreading in the early cretaceous

crystallization environment of kazakhstan microdiamond evidence from nanometric inclusions and mineral associations nanometric solid inclusions in diamond incorporated in garnet and zircon from felsic gneiss of the kokchetav massif kazakhstan have been examined utilizing electron microscopy and focused ion beam techniques host garnet and zircon contain numerous pockets of multiple inclusions which consist of 1 3 diamond crystals intergrown with quartz phengite phlogopite albite k feldspar rutile apatite titanite biotite chlorite and graphite in various combinations recalculation of the average chemical composition of the entrapped fluid represented by multiple inclusion pockets indicates that such fluid contained a low wt of sio2 suggesting a relatively low temperature fluid rather than a melt transmission electron microscopy revealed that the diamond contains abundant nanocrystalline inclusions of oxides rare carbonates and silicates within the 15 diamond crystals studied abundant inclusions were found of sio2 tio2 f e xoy c r 2o3 zrsio4 and single grains of thxoy baso4 mgco3 fecr2o4 and a stoichiometric fe rich pyroxene the diversity of trace elements within inclusions of essentially the same stoichiometry suggests that the kokchetav diamond crystallized from a fluid containing variable amounts of si fe ti cr zr ba mg and th and other minor components such as k na p s pb zn nb al ca cl most of the components in crystals included in diamond appear to have their origin in the subducted metasediments but some of them probably originate from the mantle it is concluded that kokchetav diamond most likely crystallized from a coh rich multicomponent supercritical fluid at a relatively low temperature hence the apparently low content of rock forming elements and that the diversity of major and minor components suggests interactions between subducted metasediments and mantle components

an ion microprobe study on trace element composition of clinopyroxenes from blueschist and eclogitized feti gabbros ligurian alps northwestern italy some petrologic considerations abstract detailed microtextural investigations coupled with ion microprobe trace element determinations have been performed on clinopyroxenes from blueschist and eclogitized fe ti gabbros from ligurian alps northwestern italy ion microprobe analyses have also been carried out on garnets from eclogitized fe ti gabbros the aim is to understand the relations between metamorphic reactions and trace element compositions of clinopyroxenes in high pressure low temperature terranes trace and major element analyses on relicts of igneous diopsides indicate similar protoliths for both eclogites and blueschists metamorphic clinopyroxenes have variable trace element compositions which reflect the compositions of original mineral phases in particular the clinopyroxenes replacing the igneous diopside commonly retain geochemical signatures of the precursor mineral such as relatively high sc y and hree contents the trace element variations among high pressure clinopyroxenes in different textural domains are greater for blueschists than for eclogites this reflects the more limited major element equilibration of blueschists relative to eclogites in deformed eclogites the clinopyroxenes lack the geochemical signatures of the original igneous minerals most probably due to the enlargement of reaction domains caused by synmetamorphic deformation trace element partitioning between omphacite and garnet suggests that in both undeformed and deformed eclogites chemical equilibrium is locally approached and therefore the trace element composition of clinopyroxenes is controlled by the coexisting minerals for instance clinopyroxenes in eclogites are strongly hree and lree depleted i e with bell shaped pattern due to the equilibration with garnet and allanite respectively the partition coefficients for rees between omphacite and garnet are markedly lower than those reported in the literature on the basis of crystal chemical considerations we suggest that this is probably due to lower temperature conditions during formation

ultrahigh pressure macro diamonds from copeton new south wales australia based on raman spectroscopy of inclusions abstract mining of cenozoic alluvial deposits at copeton and bingara eastern australia has produced two million macrodiamonds 0 25 ct median size raman spectroscopy is used to identify included minerals within uncut copeton diamonds with sealed chamber remnant pressures of 31 7 to 35 6 kbar for coesite 13 6 and 22 7 kbar for clinopyroxene and 7 6 kbar for grossular garnet assuming elastic behaviour these values generate inclusion entrapment pt loci which intersect restricting diamond formation conditions from 250 c 43 kbar to 800 c 52 kbar larger than error 100 c and 4 kbar this range shows a systematic variation in inclusion composition with diamond zoning and n properties published research shows 1 copeton and bingara diamonds are unique and 2 modern alluvium in the bingara district carries mantle formed garnet captured by post tectonic alkali basalt from an extensive diamondiferous ultrahigh pressure uhp terrane that stalled at depth because it is dominated by mafic eclogite the combined raman and geological results indicate two sets of subduction uhp diamond formation conditions protolith are required firstly cooler oceanic slab and secondly including higher temperature continental crust the copeton and bingara stones are uhp macrodiamonds and carboniferous 40 ar 39 ar age dates on clinopyroxene inclusions should be interpreted as ages of crystallisation representing the termination of subduction the characteristic features of ruptured inclusions and etched percussion marks on copeton and bingara diamond indicate volcanic delivery to the earth s surface alluvial deposits elsewhere in eastern australia may carry similar diamond along with diamond of different origin

changes in streamflow components following logging and regeneration in the southern forest of western australia two small experimental catchments were established in the south west of western australia to study the effects of logging and subsequent regeneration on the mechanism of streamflow generation following a six year pre treatment calibration period 1976 1981 one catchment march road was logged and reforested in 1982 and the other april road south remained as a control logging resulted in an increase in groundwater levels and subsequently groundwater discharge area the deep permanent groundwater levels in the valley and upslope areas rose until 1986 and then began to decline the maximum rise was 5 m in the upslope areas the duration of shallow intermittent groundwater system perched on underlying clay was extended from 2 3 months in winter before logging to 5 6 months after logging the shallow groundwater level rose in the valley and began to discharge at the ground surface in 1986 logging resulted in an increase in streamflow the maximum increase 18 of annual rainfall was in 1983 one year after logging the increase in streamflow was due to a substantial decrease in interception and evapotranspiration increased recharge to the shallow groundwater system decreased soil moisture deficit and consequently an increase in throughflow the increase in base flow was about twice that of quick flow the changes in streamflow and its components in the subsequent years were closely related to the groundwater discharge area most of the quick flow was generated as saturation excess overland flow from the groundwater discharge area in the valley the expansion of the groundwater discharge area increased soil moisture content higher groundwater level and the presence of the shallow groundwater system for the extended periods were responsible for the process of streamflow generation

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

origin of titanian pargasite in gabbroic rocks from the northern apennine ophiolites italy insights into the late magmatic evolution of a mor type intrusive sequence abstract gabbroic rocks from northern apennine ophiolites contain accessory titanian pargasite in interstices between plagioclase and clinopyroxene and as rims around interstitial fe ti oxide phases the origin of titanian pargasite has been evaluated by combining major volatile and trace element microanalyses titanian pargasites show variable amounts of f and low cl 0 03 0 23 and 0 03 wt respectively and mg value ranging from 0 78 to 0 70 and from 0 61 to 0 53 in mg and fe rich rocks respectively geothermometric calculations based on amphibole plagioclase equilibrium yield temperatures of 900 50 c and 840 50 c for mg and fe rich rocks respectively titanian pargasites are characterized by lree depletion nearly flat hree and variable negative eu anomalies total ree contents in titanian pargasites are higher and lower than in associated clinopyroxenes and apatites respectively in the chondrite normalized patterns of titanian pargasites ba k and sr are markedly depleted relative to lree whereas nb zr and ti are slightly enriched to slightly depleted relative to neighboring ree a separate of titanian pargasite from a fe rich gabbroic rock was analyzed for sr isotopic composition its initial 87sr 86sr falls within the range of modern n morb and is consistent with those of fresh mg rich gabbroic rocks titanian pargasite most likely formed by an igneous liquid with relatively high h2o content 4 4 6 1 wt and a slight lree enrichment the origin of this liquid has been ascribed to the percolation in the gabbroic crystal mush of a h2o rich agent of igneous origin which could be a trondhjemite liquid or an exsolved fluid probably such interaction triggered a post cumulus crystallization process that finally yielded the precipitation of titanian pargasite

raman spectroscopic study of garnet inclusions in diamonds from the mantle transition zone raman spectra of syngenetic majoritic garnets were obtained from the brazilian sno luiz diamond suite these garnets have a low majoritic content 3 have a characteristic signature in the frequency region of the sio4 sio6 stretching vibrations this vibration gives a broad peak between 800 and 900 cm 1 just before in intense band 900 930 cm 1 classically assigned to si o stretching vibrations of the sio4 tetrahedra in all known garnets this broad band is also present in non majoritic garnets si 3 that result front the transformation within the diamonds of majoritic garnets into low pressure garnet pyroxene intergrowths we show that the presence of significant tio2 content in these normal garnets leads to raman spectra mimicking those of majoritic garnets the occurrence of a broad shoulder between 900 and 900 cm 1 as well as broad bands near 960 and 1030 cm 1 in the raman spectra are no longer diagnostic features indicative of a majoritic garnet when the tio2 content or the garnets is high 1 wt as is the case for the sao luiz diamonds the full width at half maximum fwhm of the strong raman mode near 910 cm 1 is the only reliable signature that discriminates si 3 from si 3 garnets the frequency of the diamond raman mode used as an in situ piezometer shows that the actual pressure of the garnet inclusions is between 0 5 and 0 8 gpa

design and application of a constant head well permeameter for shallow high saturated hydraulic conductivity soils a new constant head well permeameter has been designed to measure saturated hydraulic conductivities ks ranging from 10 6 to 10 3 ms 1 application to shallow soils of western australia in this ks range highlighted problems with different aspects of the borehole discharge response firstly steady state borehole discharge was only reached in 13 out of 50 tests conducted this was a consequence of the constraint placed on the volume of water storage 40 l of the permeameter for some non steady state cases techniques of estimating steady state discharge from the available data are examined secondly borehole discharge was observed to increase with time in 34 per cent of cases a satisfactory physical explanation of this phenomenon is not presently available other limitations in the application of the constant head well permeameter are noted examination of a number of recent theoretical developments in the determination of ks led to a preference of saturated unsaturated flow solutions by philip 1985 and reynolds et al 1985 these solutions involve the sorptive number to which the solutions were sensitive but not critically sensitive the theory was used to establish design criteria for the permeameter and to identify an optimum application strategy in terms of well dimensions and heads error estimation indicated that the designed permeameter was a practically useful field instrument

anomalous seismic crustal structure of oceanic fracture zones summary the seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the north atlantic the crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3 anomalous crust is present in both large and small offset fracture zones since they are among the most common tectonic features in the ocean basins and are particularly closely spaced on slow spreading ridges fracture zones represent a major source of seismic crustal heterogeneity we interpret the anomalous crust as a thin intensely fractured faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that in places are extensively serpentinized the unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness culminating in the very thin crust of the fracture zones at either end of the ridge segment

ultramafic and mafic rocks from the garret transform fault near 13 30 s on the east pacific rise igneous petrology serpentinized peridotites metagabbros and fresh basaltic rocks were dredged from the garret transform fault near 13 s on the east pacific rise two dredge hauls taken on the northern wall from about 3100 3600 m consisted of aphyric and picritic basalts while peridotites gabbros and moderately phyric basalts were recovered in a single dredge located near the deepest part 4616 4820 m of the transform valley well preserved igneous textures and mineralogical assemblages enable us to differentiate between tectonites including harzburgites and clinopyroxene bearing harzburgites and cumulates consisting of plagioclase dunite troctolites olivine gabbros and ilmenite gabbros the harzburgites are likely to represent deep seated peridotites left after extraction of basaltic melt during upper mantle partial melting of a lherzolite the ultramafic cumulates underwent some deformation and show textural and mineralogical evidences which suggest formation at a minimum depth corresponding to the very lower crust it is also inferred that the composition of the coexisting liquid along with early cumulus crystals has a ratio mg mg fe2 of 0 7 high cao na2o ratios and a low ni content about 150 ppm when compared to similar rock types from atlantic fracture zones subsequent uplift during rifting of the oceanic lithosphere enhanced plastic deformation subsolidus recrystallization and retrogressive metamorphism of the gabbros and the ultramafics

diffusion between olivine and spinel application to geothermometry abstract heating experiments were carried out at 1100 c and 1200 c under controlled oxygen fugacity on olivine spinel assemblages from new caledonia this mineral pair constitutes a perfect experimental material for such studies the result was a marked change in chemical composition of the minor phase olivine it becomes enriched in iron and chromium and the magnesium and nickel contents decrease careful studies of the influence of oxygen fugacity show that though it has a strong effect on the rate of re equilibration it does not affect the equilibrium composition of the phases the stability limits of the mineralogical assemblage used are also examined our results show the possibility of an important subsolidus re equilibration in natural assemblages containing olivine and spinel a new equation for the geothermometer based upon fe 2 mg 2 partitioning between olivine and spinel is proposed and some applications discussed

the kerguelen isles southern indian ocean new results on deep structure from refraction profiles abstract additional seismic refraction profiles shot across the kerguelen archipelago terres australes et antarctiques francaises known as taaf located in the southern indian ocean substantiate the oceanic type crust of the archipelago and the kerguelen plateau as well the crust mantle boundary is better modelled by a transition than by a discrete interface seismic velocities of 6 60 6 80 km s are in the range of those within oceanic layer 3 velocities of 7 2 7 5 km s observed at long ranges may be interpreted according to at least two distinct hypotheses 1 1 notwithstanding their rather low values they represent upper mantle material and might be assigned to peridotites serpentinised by hydrothermal circulations as reported for various oceanic and continental features 2 2 these velocities are assigned to the lower crust intruded by melt produced by adiabatic decompression of upper mantle material during rifting underplating a small increase in mantle temperature caused by the presence of heat sources i e the southeast indian ridge and the kerguelen mantle plume nearby may have produced huge quantities of melt facilited by the slow movement of the antarctic plate then building up the aseismic kerguelen plateau

comparison of three hydraulic property measurement methods hydraulic functions of soils may differ depending on the different measuring methods used the performance of three different methods for measuring soil hydraulic properties of a heterogeneous field were evaluated the experiments were conducted using three different sizes of undisturbed soil cores collected systematically along a 31 m long transect of a well drained sandy loam soil having three soil horizons ap 0 0 25 m c1 0 25 0 55 m c2 0 55 1 00 m the laboratory studies involved 1 detailed unsteady drainage flux experiments performed on fifteen columns of 1 m length and 0 3 m diameter 2 combined crust test and hot air methods applied to thirty columns of 0 2 m length and 0 2 m diameter and to a subset of sixty cylinders of 0 1 m length and 0 045 m diameter respectively taken from the ap horizon and 3 desorption experiments carried out on a total of one hundred eighty cores of 0 051 m length and 0 05 m diameter collected evenly from the three horizons mean soil hydraulic properties were inferred from experimental data characterizing either selected depths or the soil profile as a whole the results revealed considerable differences among estimated mean soil properties as obtained with different measuring techniques although the application of scaling theory substantially reduced variation in the measured pressure heads h and conductivities k the results revealed that scaling parameters determined from soil pressure head were not identical to scaling factors determined from hydraulic conductivity the results also show that k scaling factors in general were much more variable than h scaling factors and that the observed variability in scaling factors also depend upon the measurement technique used c 1997 elsevier science b v

ca rich majorite derived from high temperature melt and thermally stressed hornblende in shock veins of crustal rocks from the ries impact crater germany shock veins up to 1 1 mm thick were found within non porous lithic clasts from suevite breccia of the nordlinger ries impact structure these veins were studied by optical microscopy in transmitted and reflected light and by scanning electron microscopy in shocked amphibolites two types of ca rich majorite occur within and adjacent to the veins the first type crystallized from shock induced melts within the veins si contents of these majorites suggest dynamic pressure of 15 17 gpa implying minimum temperatures in the range of 2 150 2 230 c the second type of majorite was formed adjacent to the shock veins within pargasitic hornblende this majorite contains significant amounts of h2o 0 7 0 9 wt based on the textural setting the shrinkage cracks and the chemical compositions of both phases a solid state mechanism is deduced for the hornblende to majorite phase transition both genetic types of ca rich majorite are described for the first time from a terrestrial impact crater along with stishovite majorite constitutes the second silicate mineral displaying sixfold coordination of si at ries using micro raman spectroscopy jadeite coesite and jadeite grossular were identified within local melt glasses of alkali feldspar and plagioclase composition respectively stishovite aggregates produced by solid state reaction along with shock induced high pressure melt glasses of almandine composition were also detected in shock veins of a garnet cordierite sillimanite restite the quenched homogeneous almandine glasses point to melting temperatures of more than 2 500 c for the veins our findings demonstrate that terrestrial shock veins can give valuable information on shock induced mineral transformations and transient high pressures of host rocks during a natural impact event

a comparison of three measuring methods for estimating the saturated hydraulic conductivity in the shallow subsurface layer of mountain podzols abstract saturated hydraulic conductivity ks is a key input parameter for modeling water flow in soils well established methods exist but there is no methodological standard or benchmark this paper presents a comparison of three well established measurement methods which we used in the specific conditions of extremely heterogeneous mountain podzols the following devices were applied to undisturbed soil core samples diameter of 5 6 cm ks was estimated for a shallow subsurface ash gray layer the geometric mean values for ks measured using these three methods were gp 4 41 10 6 ms 1 sr 3 20 10 6 ms 1 and lp 7 09 10 6 ms 1 the mean values obtained using field methods were not statistically different the laboratory permeameter yielded slightly higher values than the field methods from the engineering perspective all tested methods provided similar mean values

water infiltration underneath single ring permeameters and hydraulic conductivity determination summary a single ring permeameter is suitable for measurement of soil hydraulic conductivity at a large field scale because of its simple structure easy operation portability small amount of recharging water and particularly because of its variable radius the authors established a simple single ring permeameter which consists of a hollow cylinder that is simply inserted into the top soil based on the theories of the philip and nestingen methods for single ring permeameter tests this paper presents new equations for describing 1 d infiltration within the inserted depth of the single ring infiltrometer and 3 d flow beyond the depth under both gravity and soil suction forces the soil hydraulic parameters of hydraulic conductivity and soil suction were optimized based on the declines of the ponded water depth within the single ring cylinder the new method was validated using numerical simulations of the declines of the water column within the cylinder through the variably saturated flow modflow vsf modflow the new method was then successfully applied in laboratory troughs and on a hillslope in huanjiang county northwest guangxi in southwest china the results demonstrate that the hydraulic conductivity determined by the new method is more accurate than those from the 1 d green ampt method and the 3 d nestingen method

a hillslope scale experiment to measure lateral saturated hydraulic conductivity 1 one of the most challenging parameters in hillslope and watershed scale distributed hydrologic models is the lateral saturated hydraulic conductivity ks in this paper we present a methodology to determine the hillslope scale lateral ks above a moderately deep sloping restrictive layer in an 18 35 m hillslope plot using perched water level measurements and drain tile outflow data the hillslope scale lateral ks was compared to small scale ks measured with small soil cores and the guelph permeameter our results show that small scale ks measurements underestimate the actual hillslope scale ks the hillslope scale ks measurements were 13 7 4 1 and 3 2 larger than small soil core measurements in the a b and e horizons respectively we argue that the gap between small scale and hillslope scale ks within the same porous medium is foremost a measurement problem data analysis provided the ks distribution with depth showing a sharp decrease in ks within the first 0 1 m of the soil and an exponential decline in ks below 0 1 m the distribution of ks with depth was best described by a double exponential relationship overall results indicate the importance of macroporosity perhaps of biological origin in determining ks at a hillslope scale

falling head ponded infiltration with evaporation comment reference ecol article 1994 002doi 10 1016 0022 1694 94 90013 2 url http www elsevier com wps find journaldescription cws home 503343 description description record created on 2005 12 09 modified on 2016 11 26

crustal structure of an extinct rift axis in the labrador sea preliminary results from a seismic refraction survey abstract a preliminary crustal structure for the extinct rift axis in the labrador sea is presented based on a one dimensional analysis of ocean bottom seismometer obs refraction profiles within the extinct rift low p wave velocities are observed for crustal 3 6 6 3 km s and upper mantle 7 3 8 1 km s refractors the velocity depth structures for obss sampling crust within the extinct rift suggest a marked thinning and variability in crustal thickness from 4 7 km to 2 6 km in comparison to typical thicknesses of 6 4 km observed in the adjacent flanks the crustal structures at the extinct rift resemble those observed at the active and inactive transforms of oceanic fracture zones following arguments for crustal thinning at fracture zones we speculate that the first order thinning of the lower crust is due to a reduction in the supply of partial melt to the accretionary axis at the extinct rift the reduced supply is attributed to changes in the thermal field and spreading rate as rifting ceased this is consistent with thermal models which predict that crustal thickness decreases appreciably for spreading rates less than 20 mm yr when plate spreading predominates over buoyancy in driving mantle upwelling

trace element redistribution in high temperature deformed gabbros from east ligurian ophiolites northern apennines italy constraints on the origin of syndeformation fluids c n r centro di studio per la cristallochimica e la cristallografia via abbiategrasso 209 1 271 00 pavia abstract mg gabbros from east ligurian ophiolites northern apennines italy display a high temperature low pressure recrystallization localized along ductile shear zones in deformed gabbros the igneous diopside is recrystallized into granoblastic aggregates of neoblastic diopside and minor red brown amphibole the latter displays a pargasitic composition with high amounts of ai na a and ti 1 8 0 7 and 0 4 atoms per formula unit respectively major element composition of neoblastic minerals highlight equilibration temperature conditions in the range 800 950 c red brown ti pargasite also occurs as a minor interstitial constituent presumably growing from a residual trapped liquid in the differentiated lithologies fe ti diorites of the plutonic ophiolitic complex by means of ion microprobe sims technique rare earth la ce nd sm eu gd dy er yb and selected trace elements sr y cr v sc zr ti have been analysed in igneous and neoblastic diopside as well as in ti pargasites ti pargasites have also been analysed for f and c1 and compared with the halogen composition of the amphiboles mainly hornblendes to actinolites which are related to the subsequent low temperature brittle evolution neoblastic ti pargasite from deformed mg gabbros bears close compositional similarities with igneous ti pargasite from undefonned fe ti diorites whereas it is geochemically distinct from the amphiboles post dating the ductile event in particular ti pargasites have relatively high contents of f ree y zr and sr which are not consistent with crystallization in the presence of seawater derived hydrothermal fluids high grade recrystallization probably developed in the presence of volatile rich igneous fluids either trapped between the cumulus minerals or injected into the ductile shear zones from outside the local system an alternative hypothesis comprises the absence of fluid phase and the development of ductile shear zones in ti pargasite rich layers the petrological features of deformed gabbros recovered from present day slow spreading ridges and fracture zones bear close similarities with the investigated ophiolitic metagabbros in east ligurian ophiolites high grade ductile shear zones have been related to the initial stages of the uplift of the gabbro peridotite complex to the sea floor

the oceanographer fracture zone i crustal structure from seismic refraction studies summary this paper presents the results of four lookm long seismic refraction lines carried out in and around the active transform of the oceanographer fracture zone which offsets the mid atlantic ridge rightlaterally by 130 km near 35 n the seismic data are interpreted using standard travel time inversions the time term method and two dimensional ray tracing techniques the results show that beneath the oceanographer fracture zone the crust is typically only 4 5 km thick it consists of a thin upper crustal layer in which the p wave velocity increases rapidly with depth and a main crustal layer with velocity increasing from 6 4 km s l at the top to 6 8 km s at the base of the crust some degree of crustal thinning is found to extend for distances of 30 40 km on either side of the fracture zone axis in addition to this a small area 10 15 km long of crust less than 2 5 km thick is found beneath the centre of the transform valley crust less than 4 km thick is also observed on a smaller offset fracture zone 85 km to the south of the oceanographer but no evidence is found for significantly thin crust beneath another small offset fracture zone 65 km to the north of it we conclude that the moderate thnning of the crust at the fracture zone is due to a progressive reduction in the crustal production rates of the oceanic spreading centres as they approach the transform fault however we attribute the occurrence locally of extremely thin crust to a different process in which a thin lid of mafic and ultramafic rocks overlying unaltered upper mantle material is accreted at the junction of a spreading centre and a transform fault this may happen if the accretion rate at the end of the spreading centre becomes so slow that the usual processes which give rise to normal oceanic crust can no longer operate it therefore seems that there are two distinct types of crust either of which may be found at different points along a fracture zone this might account for the extreme variability in crustal thickness found in different fracture zones by previous studies

retrograde hydration sequence in disordered mg amphiboles a tem investigation acicular mg amphibole in thermal metamorphic serpentinites from elba island italy is affected by polytypic disorder with the possible coexistence of pnmn pnma and c 2 m structures chain width defects and retrograde hydration reactions based on hr tem nanostructures we documented three main hydration stages 1 formation of chain multiplicity faults cmfs 2 formation of fibrous topotactic talc elongated parallel to 001 tlc and resulting from progressive polymerization of cmfs and 3 pervasive serpentinization incipient serpentinization produces isolated inclusions within mg amphibole rhombic in shape and parallel to 210 mg am or elongated parallel to 100 mg am the inclusions are filled by not topotactic curved 001 srp layers or chrysotile partial fibers as the mg amphibole to serpentine replacement proceeds the isolated serpentine inclusions merge together producing a sponge like texture with interconnected fluid paths from which serpentine can rapidly grow at the expense of enclosing amphibole we suggest that whereas the first hydration reactions to cmfs and fibrous talc take place at relatively high t and low fluid rock ratio fluid circulation confined to microfractures cleavage planes and subgrain boundaries subsequent serpentinization requires significant changes in physical conditions with definitely lower t and pervasive fluid circulation

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

the indus yarlung zangbo ophiolites from nanga parbat to namche barwa syntaxes southern tibet first synthesis of petrology geochemistry and geochronology with incidences on geodynamic reconstructions of neo tethys the purpose of this first synthesis is to summarize findings on the yarlung zangbo suture zone yzsz ophiolites and discuss still remaining problems the yzsz studied for almost 30 years and is the youngest of the sutures recognized on tibet plateau it is now acknowledged that the yzsz is a complex assemblage of sedimentary metamorphic and igneous rocks created during and shortly after the collision between india and eurasia the ages of the various lithologies span a time interval from the jurassic to the middle miocene with some permian and devonian exotic blocks from melange zone the yzsz is characterized by ophiolitic complexes and ophiolitic melange the ophiolites are of two types non dismembered and dismembered sections the non disturbed sections although tectonically reworked are observed along the segment from dazhuqu to jiding in xigaze area and spontang ophiolite the dismembered sequences are found in various locations such as nidar kiogar jungbwa saga sangsang xigugabu and luobusa the incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins the ophiolites are distributed into two groups of ages the luobusa zedang and kiogar sequences being jurassic lower cretaceous whereas all other sequences are of lower cretaceous age compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 my from their initial genesis to obduction which occurred around 70 90 my ago ophiolites differ in terms of petrological and geochemical aspects however they were all generated in suprasubduction zone and more specifically in arc few fore arc and back arc settings synthesis of more than 700 geochemical analyses show variable mixing of components from n morb type to iat cab and to oib end members the jurassic ophiolites show the maximum of arc component while the lower cretaceous ones show little to strong mixing in addition most ophiolites were created in short lived 30 my basins and generated close to the eurasiatic continental margin we propose that ladakh tibet ophiolites were generated in a suprasubduction context similar to mariana arc interarc and back arc or tonga lau system the variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system

regression models for hydraulic conductivity and field test of the borehole permeameter the saturated hydraulic conductivity ks of sediments in the vadose zone is an important parameter in predicting the seepage rates of water and contaminants the borehole permeameter is an in situ technique to test a relatively large sample size at any depth solutions are presented which account for the effects of unsaturated flow these solutions are derived from a regression analysis of results of numerical simulations in which unsaturated hydraulic conductivity is represented by one or two parameters the results of a borehole permeameter test in a uniform sand are compared with field ponding tests and air entry permeameter tests the regression based solutions for the borehole permeameter which account for capillarity provide very good agreement with other permeameter results depending upon the approach used to solve the borehole problem ks values determined by methods which neglected and included capillary effects varied by a factor of only about two for the soil tested

the ocean continent boundary off the western continental margin of iberia ii crustal structure in the tagus abyssal plain summary an 80 km long reversed seismic refraction line line 5 was shot over the tagus abyssal plain off portugal the main p wave reflected and refracted phases were modelled both for traveltime and amplitude the resulting p wave velocity depth model has the following features a an extremely thin crust of about 2 km b the absence of oceanic layer 3 and c very low upper mantle velocities between 7 6 and 7 9 km s this very unusual seismic velocity crustal structure is quite unlike thinned continental crust but is remarkably similar to the seismic crustal structures found at atlantic fracture zones and in particular to the structures found in profiles shot along the transform valley and near ridge transform intersections a magnetic anomaly chart seems to allow the possibility of several fracture zones one of which could intersect the centre of line 5 as an alternative to the fracture zone hypothesis we show that if the oceancontinent transition in the tagus abyssal plain is located at about 11 30 w in a symmetric position with respect to the ocean continent transition in the conjugate south newfoundland basin then magnetic anomalies can be modelled simply by assuming sea floor spreading west of 11 45 w at 10 mm yr beginning at m11 time 133 myr bp and blocks of rifted continental crust to the east the location of the proposed ocean continent transition in the tagus abyssal plain is marked by a well defined n s linear magnetic anomaly which is adjacent to the oldest sea floor spreading block east of the proposed ocean continent transition there is an increase in the depth to basement similar to that found east of the ocean continent transition in the iberia abyssal plain and elsewhere this model also allows us to explain why purdy s 1975 seismic refraction line a ar in the tagus abyssal plain cannot be interpreted as a conventional reversed pair because most of line a was shot over the ocean continent transition zone and most of line ar over thinned continental crust remarkably similar velocity depth structures to that under line 5 are found close to the ocean continent transition zone off the whole of western iberia in areas which show no clear evidence of fracture zones therefore it appears more likely that the seismic structure of line 5 is due to its proximity to the ocean continent transition than to a local association with a fracture zone and further that its structure is typical of this transition off the western margin of iberia we also suspect that the low upper mantle velocities associated with the ocean continent transition indicate the widespread occurrence of serpentinized peridotite

comment on variable head ponded infiltration under constant or variable rainfall by j r philip keywords time equation reference ecol article 1996 002doi 10 1029 95wr03645 url http www agu org journals wr record created on 2005 12 09 modified on 2016 08 08

comparison of methodological uncertainties within permeability measurements permeability measurements are critical to the calculation of water flow within hillslopes despite this errors in permeability measurements are often ignored and can be very large particularly in disturbance sensitive gley soils this work compares the uncertainties associated with six field methods of permeametry applied to a gleyed soil in upland britain slug tests constant head borehole permeametry and falling head borehole permeametry were undertaken on established piezometers additionally ring permeametry and two types of trench tests were evaluated method related uncertainty due to proximity of impeding layers of high sorptivity soils produces under and over estimates of permeability by a factor of up to 0 2 and 5 respectively this uncertainty band is smaller than the observed effects of anisotropy and temporal variability had smearing and soil ring leakage errors not been minimized the methodological uncertainties would have been so large that they would have distorted the true spatial field of permeability and its estimated impact on the balance of vertical and lateral flow

geology and seismic structure of the northern section of the oman ophiolite in the north oman mountains a continuous ophiolite succession is exposed from tectonized harzburgites and dunites at the base through layered gabbros and peridotites high level gabbros and plagiogranite to a dike swarm and pillowed volcanics overlain by pelagic shales the upper part of this sequence possesses a static metamorphic overprint which passes downward from greenschist facies in the lowermost volcanics and most of the dike swarm to amphibolite facies in the lowermost dike swarm and in the high level intrusives these pervasively altered rocks are underlain by layered gabbros and peridotites where hydration is restricted to fractures compressional and shear wave velocities have been measured to confining pressures of 6 kbar for 139 water saturated cores from this sequence the samples were field oriented and collected from known stratigraphic levels which allows the construction of velocity depth profiles and an examination of anisotropy within the ophiolite compressional wave velocities measured for the basalt section are quite variable ranging from 4 5 to 6 0 km s at in situ pressures within the sheeted dike section compressional and shear wave velocities vary from 5 4 to 6 3 km s and from 2 9 to 3 6 km s respectively from the basalt sediment contact through the dike section velocities increase rapidly with depth this gradient is related to increasing metamorphic grade and a decrease in grain boundary porosity the top of seismic layer 3 is located near the lower boundary of the sheeted dike swarm and marks the transition from greenschist facies to amphibolite facies metamorphics the dikes decrease in abundance downward into the high level intrusives coarse grained gabbroic and granitoid rocks which often contain abundant hornblende with isotropic textures and lacking compositional layering mean compressional and shear wave velocities increase uniformly from 6 7 and 3 6 km s respectively in the highlevel intrusives to 7 5 and 3 9 km s respectively at the base of the layered sequence measured velocities of the ultramafic tectonites are highly variable because of the anisotropy and serpentinization petrofabrics of relic olivine in the wadi ragmi region show strong preferred fabrics for the tectonites and calculated compressional wave velocities for the serpentine free rocks range from 7 8 to 8 5 km s with the fast direction parallel to the direction of spreading inferred from dike orientations in excellent agreement with observed upper oceanic mantle seismic anisotropy the crust mantle boundary which is defined by the abrupt increase in seismic velocities coincides with the petrological contact between layered gabbros and peridotites and tectonized harzburgite and dunites

late paleozoic gabbroic rocks of the bridge river accretionary complex southwestern british columbia geology and geochemistry gabbroic bodies in the bralorne gold bridge area of southwestern british columbia are associated with the oceanic bridge river complex of the western canadian cordillera one of the suspect terranes accreted to north america in the jurassic the gabbros are locally cut by tonalites and are structurally interleaved with ultramafic rocks phyllites graphitic cherts and carbonate lenses that comprise the lower part of the bridge river complex their late carboniferous crystallization age overlaps the depositional age of affiliated supracrustal rocks mississippian jurassic some of which have been metamorphosed to blueschist facies compositionally the gabbros resemble mafic plutonic rocks of ophiolitic complexes and gabbroic rocks of the nearby shulaps range they display some affinity to oceanic island arc tholeiitic suites the bralorne and shulaps gabbros include cumulates and appear to have been derived from a single light ree depleted peridotitic source by melting and subsequent fractional crystallization accumulation of various combinations of plagioclase pyroxenes and olivine the tonalites are compositionally distinct from typical ophiolitic plagiogranites but might be related to the associated gabbros the gabbroic bodies occur within tectonic slivers derived from the oceanic crust that floored a deep ocean basin that existed during the late paleozoic and early mesozoic the bridge river complex comprises fragments of oceanic crust that were tectonically incorporated into an east verging accretionary prism during a middle late triassic to jurassic collisional event

petrology and mineralogy of dredged gabbro from gettysburg bank eastern atlantic gabbros have been dredged from gettysburg bank 110 km west of portugal on the azores gibraltar fracture zone primary minerals in olivine pyroxene and brown hornblende gabbros are partially replaced by metamorphic minerals igneous textures are inhomogeneously overprinted by a granular polyhedral deformation and a cataclastic deformation amphiboles show characteristics which indicate a transition from crystallisation in a magma chamber to formation of amphibole in solid gabbro under metasomatic conditions of the amphiboles analysed chlorine was present in the green amphiboles but below 0 05 in the brown suggesting the penetration of sea water after the formation of the brown amphibole but during the formation of the green

falling head ponded infiltration it is remarkable that the green ampt or delta function solution for falling head ponded infiltration takes precisely the same functional form as that for constant head infiltration only the values of the constants change falling head infiltration experiments are thus as amenable to green ampt analysis as more complicated constant head experiments

infiltration characteristics of a complex lateritic soil profile large infiltration ponds 10 15 m2 were used in conjunction with a ring infiltrometer and a well permeameter to determine the infiltration characteristics of a complex lateritic soil profile in the jarrah eucalyptus marginata forest of western australia simultaneous measurements of soil water content and soil water potential allowed a description of the infiltration and redistribution in the soil profile the infiltration ponds effectively measured the conductivity of a subsurface lateritic duricrust which was found to have a relatively high saturated hydraulic conductivity ks of 2 7 m d 1 despite its apparently massive and extensive nature removal of the topsoil identified large 1 m2 infilled holes penetrating the duricrust over about 6 by area measurements indicated that these large holes had a high ks value 10 m d 1 whereas the remaining duricrust had a lower ks value 2m d 1 these results have implications for probable maximum flood design calculations and assessing the hydrological impact of extensive open cut bauxite mining

a drive point application of the guelph permeameter method for coarse textured soils abstract the guelph permeameter gp is a widely used well auger hole method for in situ determination of field saturated hydraulic conductivity kfs and sorptive number in the vadose or unsaturated zone its application can be difficult however in gravelly materials which are problematic to auger and in un cohesive sandy materials which collapse during the augering process or upon wetting of the unlined well we circumvented these issues by replacing the gp well with a conically tipped well screen or drive point which is pushed driven rotated or vibrated to the desired depth we also developed a steady flow analysis to account for the fact that water discharge through the drive point screen is solely radial rather than both radial and vertical as in the original gp method the hydrus 2d numerical simulation model was used to determine appropriate shape values cdp for the drive point analysis and an empirical regression equation was developed to convert the discrete cdp values into continuous shape functions for porous materials with negligible moderate strong and very strong capillarity in a well sorted medium coarse sand the drive point method produced kfs and values which were plausible and consistent with other data from the field site the original gp method on the other hand under estimated the drive point results by factors of 2 1 3 1 which may have been caused by progressive collapse of the unlined gp well and or gradual sinking of the gp outflow tip into the unprotected well base it was concluded that the drive point gp method can provide convenient and accurate estimates of kfs and in materials where hydraulic properties are not substantially altered by the disturbance of drive point installation

on the occurrence and boron isotopic composition of tourmaline in ultra high pressure metamorphic rocks abstract the extensive p t stability and the high chemical variability of tourmaline tur together with its common occurrence in metasediments proves its high potential for petrological and isotope geochemical studies on fluid rock interaction in subduction and collision related rocks this paper reviews the occurrence major element chemistry and boron isotopic composition of tur in high and ultrahigh pressure metamorphic uhpm rocks in addition it presents a new discovery of coesite bearing tur schorl from the erzgebirge germany as well as tur dravite related to the retrograde history of coesite and diamond bearing rocks from the erzgebirge and the kokchetav massif kazakhstan the scarce data on worldwide occurrences of u hpm tur reveal a high occupation of the crystallographic x site dominated by na and the possible presence of excess b with little further distinctiveness in its major element composition when compared with tur from medium grade rocks high k 2 o contents in tur are probably not related to uhp growth or equilibration the b isotopic composition of u hpm tur ranges in 11 b from 16 to 1 with many samples in or below the range of continental crust in contrast tur formed during retrograde fluid influx typically shows high 11 b values up to 28 suggesting heavy b fluids infiltrating the exhuming u hpm units coesite inclusions in tur characterized by raman spectroscopy are regarded as the best indicator for its uhp stability supplementary material analytical methods tourmaline compositions and boron isotope values are available at http www geolsoc org uk sup18354

genesis of mud volcano fluids in the gulf of cadiz using a novel basin scale model approach abstract mud volcanism and fluid seepage are widespread phenomena in the gulf of cadiz sw iberian margin in this seismically active region located at the boundary between the african and eurasian plates fluid flow is typically focused on deeply rooted active strike slip faults the geochemical signature of emanating fluids from various mud volcanoes mvs has been interpreted as being largely affected by clay mineral dehydration and recrystallization of upper jurassic carbonates here we present the results of a novel fully coupled 1d basin scale reactive transport model capable of simulating major fluid forming processes and related geochemical signatures by considering the growth of the sediment column over time compaction of sediments diffusion and advection of fluids as well as convective and conductive heat flow the outcome of the model is a realistic approximation to the development of the sediment pore water system over geological time scales in the gulf of cadiz combined with a geochemical reaction transport model for clay mineral dehydration and calcium carbonate recrystallization we were able to reproduce measured concentrations of cl strontium and 87sr 86sr of emanating mud volcano fluids these results support previously made qualitative interpretations and add further constraints on fluid forming processes reaction rates and source depths the geochemical signature at porto mv posed a specific problem because of insufficient constraints on non radiogenic 87sr 86sr sources at this location we favour a scenario of basement derived fluid injection into basal upper jurassic carbonate deposits hensen et al 2015 although the mechanism behind such basement derived flow e g along permeable faults remains speculative at this stage it provides an additional source of low 87sr 86sr fluids and offers an idea on how formation water from the deepest sedimentary strata above the basement can be mobilized and eventually initiate the advection of fluids feeding mvs at the seafloor the dynamic reactive transport model presented in this study provides a new tool addressing the combined simulation of complex physical geochemical processes in sedimentary systems the model can easily be extended and applied to similar geological settings and thus help us to provide a fundamental understanding of fluid dynamics and element recycling in sedimentary basins

the seismic structure of 140 myr old crust in the western central atlantic ocean summary a detailed seismic refraction experiment using explosive sources and ocean bottom hydrophone obh receivers was carried out over mesozoic magnetic anomaly m17 about 300 km south west of bermuda amplitude and travel time interpretations show this 140myr old crust to be uniform on the seismic scale over a lateral distance of almost 1ookm the best estimate of the velocity structure consists of an average 700 m of sediment overlying a total thickness of igneous crust of 7 2 km the principle components beginning with a 5 km s l velocity at the top of layer 2 are a 0 4 km thickness with gradient 1 1 s a 1 9km thickness with gradient 0 64s 1 7 km thickness with gradient 0 1 s a 3 2 km thickness with zero gradient and constant velocity of 7 km s below which lies a 0 5 km thick moho transition zone the uppermost few kilometres of the upper mantle apparently has little or no velocity gradient the normal incidence two way reflection time through this structure agrees to better than 0 1 s with the location of the moho reflection seen on the ipod usgs multichannel reflection profile that passes within 1 km of this experiment the travel times of the converted shear wave arrivals constrain poisson s ratio in the igneous crust to 0 28 k 0 01 a delay time study of a 20km radius circle of shots fired around a small 6 km five element array of ocean bottom hydrophone receivers confirms the uniformity of structure of this old crust on the scale of a few kilometres the observed differences in shallow crustal delay time between the five obh instruments can be wholly attributed to differences in sediment thickness beneath the receivers as determined by a deep towed hydrophone seismic reflection profile these experiments were located clear of fracture zones or other structural anomalies in a region of well defined linear constant amplitude magnetic anomalies we propose that this was the primary cause of our result of lateral uniformity of structure on the scale of a few kilometres we suggest that the reason that such a simple uniform structural model for oceanic crust is not supported by the historical seismic refraction dataset is that a large proportion of the old experiments were poorly located relative to fracture zones and other structural anomalies unknown at the time and the

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

mineral chemistry of ultramafic cumulates from the north arm mountain massif of the bay of islands ophiolite evidence for high pressure crystal fractionation of oceanic basalts the mineral chemistry of ultramafic cumulates forming the basal portions of the plutonic section of the north arm mountain massif bay of islands ophiolite complex is not consistent with crystal liquid fractionation of a primitive mid ocean ridge basalt at low pressures the appearance of clinopyroxene and orthopyroxene having very high mg numbers 100 mg mg fe 85 to 92 and the absence of plagioclase as early fractionating phases coprecipitating with forsteritic olivine fo86 92 suggest that moderate to high pressure phase relationships are applicable we propose on the basis of mineral chemistry crystallization sequences and structural relationships that the basal ultramafic cumulates on the north arm massif represent the products of high pressure 10 kbar polybaric crystal fractionation of primary basaltic melts generated within the mantle beneath a mid ocean spreading center these basal ultramafic cumulates were probably precipitated due to in situ crystallization processes along the margin of a vertical conduit extending downward from a crustal level magma chamber to mantle depths that exceed 30 km after their formation the cumulates were transported to the near surface along the axis of upwelling and divergence as the lithosphere was separating beneath a mid ocean ridge

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

comment on falling head ponded infiltration by j r philip reference ecol article 1995 001doi 10 1029 94wr02565 url http www agu org record created on 2005 12 09 modified on 2016 08 08

coesite in subducted continental crust p t history deduced from an elastic model abstract coesite has been found in a metasedimentary rock in a continental unit of the western alps dora maira massif the rock preserves the high pressure low temperature assemblage of nearly pure pyrope talc phengite kyanite in a quartz matrix coesite exclusively occurs as inclusions in pyrope and is partly inverted to quartz textural observations and mechanical considerations suggest that the rock crystallised in the coesite stability field a simple elastic model for a coesite inclusion in pyrope explains why coesite has been exclusively preserved in the inclusions and not in the matrix combined with petrological data the model constrains the crystallisation conditions of the pyrope coesite rock p 28kbar 650 this finding has an important bearing on the significance of p t conditions estimated from minerals occurring as inclusions and on the actual behaviour of continental crust in collision zones

anomalously thin crust in oceanic fracture zones new seismic constraints from the kane fracture zone three detailed and carefully positioned seismic refraction experiments have recently been carried out along the kane fracture zone near 24 n 44 w in order to place better geophysical constraints on the extent and origin of the anomalously thin crust reported from parts of this and other large atlantic fracture zones these three experiments when combined with earlier studies result in reversed structural control along about a 300 km long segment of the kane fracture zone extending from the ridge transform intersection out onto crust about 25 m y old an analysis of refraction lines obtained along the fracture zone trough by using both travel time modeling and delay time function techniques reveals large variations in crustal thickness and or velocity however along most of the eastern fracture zone trough upper mantle type velocities are found at depths of only 2 3 km below the seafloor less than half the typical depth to moho in the ocean basins this anomalous fracture zone crust is generally characterized by unusually low compressional wave velocities relatively high velocity gradients throughout most of the thinned crustal section about 1 2 s 1 and a distinct crust mantle boundary the very thin crust appears to be confined to the deepest parts of the kane fracture zone trough 10 km wide although a more gradual crustal thinning may extend up to several tens of kilometers from the fracture zone a specially designed delay time experiment at the intersection of the kane fracture zone and the mid atlantic ridge also revealed the presence of a thinned crustal section of a relatively uniform thickness about 3 km except beneath the nodal basin where the crust may be less than 1 km thick the anomalous crust in the intersection area appears to extend at least part way down the median valley we interpret this anomalous fracture zone crust as a primary accretionary feature that forms as a result of a restricted magma supply near ridge transform intersections although tectonic processes may also play a part at least locally in thinning the crust our results support a ribbon model of the gross seismic structure of the ocean basins in which bands of relatively homogeneous so called normal crust 50 100 km wide extend away from spreading centers each formed at a discrete ridge crest segment and each separated by the thin crust underlying the fracture zone trough

falling head ponded infiltration in the nonlinear limit the green and ampt infiltration solution represents only an extreme example of behavior within a larger class of very nonlinear delta function diffusivity soils the mathematical analysis of these soils is greatly simplified by the existence of a sharp wetting front below the soil surface solutions for more realistic delta function soil models have recently been presented for infiltration under surface saturation without ponding after general formulation of the problem solutions for a full suite of delta function soils are derived for ponded surface water depleted by infiltration exact expressions for the cumulative infiltration as a function of time or the drainage time as a function of the initial ponded depth may take implicit or parametric forms and are supplemented by simple asymptotic expressions valid for small times and small and large initial ponded depths as with surface saturation without ponding the green ampt model overestimates the effect of the soil hydraulic conductivity at the opposing extreme a low conductivity model is identified that also takes a very simple mathematical form and appears to be more accurate than the green ampt model for larger ponded depths between these two the nonlinear limit of gardner s soil is recommended as a physically valid first approximation relative discrepancies between different soil models are observed to reach a maximum for intermediate values of the dimensionless initial ponded depth and in general are smaller than for surface saturation without ponding

hydrothermally reset magmatic spinels in retrograde serpentinites formation of ferritchromit rims and chlorite aureoles serpentinized harzburgites from southern tuscany italy host different kinds of spinels a relic magmatic al spinels b hydrothermally altered spinels occurring as ferritchromit rims c syn and post serpentinization magnetites the composition of relic al spinels suggests 5 15 partial melting of a fertile spinel lherzolite after the main serpentinization al spinels are progressively replaced by ferritchromit rims by a dissolution recrystallization process transmission electron microscopic investigation shows that ferritchromit actually consists of a complex nanometric association of cr magnetite mg0 03 fe0 972 al0 11 cr0 89 fe3 1 00 o4 chlorite and lizardite with 001 chl liz always parallel to 111 cr mag mg and al released during the al spinel ferritchromit replacement interact with mesh textured serpentine giving rise to chloritic aureoles i e randomly intergrown chlorite lizardite and septechlorite that overgrow and postdate mesh textures

seismic constraints on the crustal structure within the vema fracture zone results from recent studies of the in well surveyed areas such as the famous area in kane and oceanographer fracture zones suggest that the central north atlantic ramberg et al 1977 significant variations in crustal thickness are transform faults are found every 50 to 100 km associated with parts of these fracture zones making them one of the most common tectonic with total crustal thicknesses in places less than features in the ocean basins morphologically half the thickness of normal oceanic crust the fracture zones on slowly spreading ridges display extent of this anomalous crust and its origin are many similar characteristics deep closed con still very poorly constrained by available geo tour depressions are usually present at ridge physical data here we report results from a 75 axis transform intersections sleep and biehler km long refraction line shot in late 1977 over 1970 the axis of the transform is typically three hawaii institute of geophysics ocean bottom defined by a deep linear trough bounded by escarpments rising eimom uuyuu withe ve valley near its western intersection with the mid transform valley floor a diverse suite of atlantic ridge 10o53 n 43o39 w we show that the apparent crustal thickness beneath this por tion of the vema transform valley is about 5 km not substantially different from crustal thick nesses typically associated with oceanic crust however the vema fracture zone crust is clearly anomalous and similar in many respects to crust reported from the kane and oceanographer fracture zones it is characterized by low compressional wave velocities a relatively uniform velocity gradient throughout most of the crust and the absence of a typical layer 3 refractor this anomalous crust is interpreted as intensely fractured hydrothermally altered and serpenti nized crustal and upper mantle rocks overlain by locally thick accumulations of basaltic rubble from the adjacent fracture zone walls the large variation in moho depths found in these large offset slowly slipping transforms is attributed to differences in the depth and extent of serpen tnnization of ultramafics lying beneath a thin and highly fractured basaltic and gabbroic layer one important consequence of the relatively low seismic velocities and densities inferred for the crust in these fracture zones is that transform valleys are not in local isostatic equilibrium with the surrounding seafloor

seismic evidence for hydrothermally altered mantle beneath old crust in the tydeman fracture zone abstract to study the structure of old atlantic fracture zones seismic refraction profiles were obtained using ocean bottom seismometers and two ship multichannel expanding spread experiments on two inactive sections of the tydeman fracture zone the results indicate that a crustal seismic velocities are low compared with normal oceanic crust b there is a marked absence of an identifiable layer 3 c the depth to the moho is shallow though not exceptionally so and d anomalously low seismic velocities of 7 2 7 5 km s 1 are observed over ranges at which upper mantle velocities are normally detected s wave arrivals corresponding to both crustal and mantle refractions are also observed which enable the first estimates of poisson s ratio within a fracture zone to be made using these results and those from other surveys we show that the seismic velocity in the upper mantle beneath fracture zones decreases systematically with increasing age these findings suggest that hydrothermal circulation which can lower the upper mantle velocity by serpentinization persists under fracture zones for at least 70 ma and occurs at greater depths than for normal oceanic crust the serpentinization process is not sufficiently rapid to obscure the location of the original crust mantle boundary

survival of majoritic garnet in diamond by direct kimberlite ascent from deep mantle 1 majoritic garnets discovered as diamond inclusion in several localities originated from the deep upper mantle or the mantle transition zone some of them show evidence of partial transformation to low pressure phases of pyropic garnet and clinopyroxene during transportation to the earth s surface the degree of partial transformation can be used potentially as a unique speedometer on the average ascent rate of diamond through the entire upper mantle here we present experimental results on the back transformation kinetics of majoritic garnet into pyropic garnet and clinopyroxene by time resolved in situ synchrotron x ray diffraction measurements we observed that back transformation occurs by grain boundary nucleation and diffusion controlled growth at 6 5 7 5 gpa and 1020 1300 c the transformation mechanisms are similar to those observed in natural diamond the temperature dependence of diffusion controlled growth rates were determined from the kinetic data obtained which was used to constrain the survival time of majoritic garnet in diamond we found that the ascending velocity in the normal mantle convection is far too slow to satisfy the time limitation for the survival of majoritic garnet indicating that such diamonds have been transported directly from the deep mantle by the rapid kimberlite magma within a few days

soil characterization in and around a proposed waste disposal facility at nuclear fuel complex hyderabad india generation of site specific hydrogeological parameters such as porosity bulk density hydraulic conductivity pore water velocity etc are the pre requisites to carry out studies on the low probability event of migration of radionuclides along with groundwater due to long term leaching from a waste disposal site in the present work the hydrogeological parameters are estimated for soil samples collected from various locations in and around a proposed solid waste disposal site at the nuclear fuel complex hyderabad india the groundwater velocity is estimated using darcy s law the range of values for hydraulic conductivity porosity and bulk density are determined to be 16 24 76 90 m day 1 average 48 12 m day 1 0 42 0 53 average 0 47 and 1 01 1 43 g cm 3 average 1 24 g cm 3 respectively using the estimated values of hydraulic conductivity porosity and the average value of hydraulic gradient between various wells at the site the average groundwater velocity has been estimated to be around 1 0 m day 1 which matches fairly well with the groundwater velocity estimated using radioactive tracer techniques these site specific results will be useful in the radiological impact assessment of a proposed near surface waste disposal facility at nuclear fuel complex hyderabad india

ultradeep greater than 300 kilometers ultramafic upper mantle xenoliths geophysical discontinuities in earth s upper mantle and experimental data predict the structural transformation of pyroxene to garnet and the solid state dissolution of pyroxene into garnet with increasing depth these predictions are indirectly verified by omphacitic pyroxene exsolution in pyropic garnet bearing xenoliths from a diamondiferous kimberlite conditions for silicon in octahedral sites in the original garnets are met at pressures greater than 130 kilobars placing the origin of these xenoliths at depths of 300 to 400 kilometers these ultradeep xenoliths support the theory that the 400 km seismic discontinuity is marked by a transition from peridotite to eclogite

seismic crustal structure and morphotectonic features associated with the chain fracture zone and their role in the evolution of the equatorial atlantic region oceanic transform faults and fracture zones fzs represent major bathymetric features that keep the records of past and present strike slip motion along conservative plate boundaries although they play an important role in ridge segmentation and evolution of the lithosphere their structural characteristics and their variation in space and time are poorly understood to address some of the unknowns we conducted interdisciplinary geophysical studies in the equatorial atlantic ocean the region where some of the most prominent transform discontinuities have been developing here we present the results of the data analysis in the vicinity of the chain fz on the south american plate the crustal structure across the chain fz at the contact between 10 and 24 ma oceanic lithosphere is sampled along seismic reflection and refraction profiles we observe that the crustal thickness within and across the chain fz ranges from 4 6 5 9 km which compares with the observations reported for slow slipping transform discontinuities globally we attribute this presence of close to normal oceanic crustal thickness within fzs to the mechanism of lateral dike propagation previously considered to be valid only in fast slipping environments furthermore the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower order mid atlantic ridge segmentation around the equator

microdiamonds frontier of ultrahigh pressure metamorphism a review abstract this is a comprehensive review paper devoted to microdiamonds from ultrahigh pressure metamorphic uhpm terranes incorporated in orogenic belts formed at convergent plate boundaries in paleozoic mesozoic alpine time when in 1980 the first small diamonds were discovered within amphibolite granulate facies metamorphic rocks it came as a great surprise that buoyant continental crust could be subducted to depths of hundreds of kilometers and then subsequently exhumed since then much progress has been made in understanding the mechanism of these diamonds formation and the number of new diamond bearing uhpm terranes was significantly increased especially within european orogenes moreover new variations in tectonic settings in which uhp rocks can be formed and exhumed came to the attention of geologists simply due to the finding of diamonds in places previously forbidden for their formation e g oceanic islands ophiolites and forearc environments over the past decade the rapidly moving technological advancement has made it possible to examine microdiamonds in detail and to learn that part of them has a polycrystalline nature that they contain nanometric multiphase inclusions of crystalline and fluid phases and that they keep a crustal signature of carbon isotopes scanning and transmission electron microscopy focused ion beam techniques synchrotron infrared spectroscopy micro x ray diffraction and nano secondary ion mass spectrometry studies of these diamonds provide evidence that they keep traces of fluid originated from both crustal and mantle reservoirs and that they probably interacted with deep mantle plumes hypotheses proposed for diamond formation in subduction zones founded on both analytical and experimental studies are discussed the paper also emphasizes that the discovery of these microdiamonds as well as coesite triggered a major revision in the understanding of deep subduction processes leading to a clear realization of how continental materials can be recycled into the earth s mantle and geochemically rejuvenate it

the correction for the travel time effects of seafloor topography in the interpretation of marine seismic data problems associated with the application of topographic corrections to deep ocean marine seismic data are described synthetic travel time data are used to show that corrections for the delay time introduced by the water column are inferior to corrections for the true water path water path corrections place the shots at the ray entry points on the seafloor require no assumptions about the structure under investigation and maintain the correct distribution of data with range the water path correction can be most accurately applied during the stacking operation that transposes the observed data into intercept time slowness space there is then no requirement to make determinations of slowness and for experiments involving seafloor receiver and surface shots the data may be transposed to the seafloor with complete objectivity water path corrected stacks from different locations may then be compared in the sure knowledge that any observed differences are due to real structural changes and not to topographic correction errors or to invalid assumptions of lateral homogeneity

the tydeman morphology and seismic structure of old fracture zone crust the tydeman fracture zone was identified 160 km se of the azores in 1977 from a detailed bathymetry and magnetic survey between 22 309 and 23 409w twigt et al 1979 the fracture zone forms a linear trough here with a width of 20 km water depth of 4 7 km and orientation of 085 this study extends the path of this fracture zone between 18 and 30 w though not yet to the ridge crest using a larger dataset of bathymetric seismic reflection and magnetic profiles a discontinuity in the fracture zone valley indicates an interruption in its formation possibly during the formation of the azores platform two separate wide angle seismic experiments carried out over old crust in the active section of the tydeman fracture zone show low crustal velocities compared with normal oceanic crust of comparable age an absence of an identifiable layer 3 and a shallower than normal moho overlying a region of anomalously low upper mantle velocities estimates of poisson9s ratio suggest that hydrothermal circulation may continue for as long as 70 ma after crustal formation

shear wave velocity structure of the oceanic lithosphere from ocean bottom seismometer studies summary a comprehensive interpretation of two shear wave refraction profiles recorded on ocean bottom seismometers provides velocity depth models for the oceanic crust and upper mantle the data were obtained as part of an investigation of lithospheric structure carried out in the nootka fault zone area off the west coast of canada one dimensional s wave velocity depth models are derived from the data by trial and error comparisons with wkbj synthetic seismograms the results of this study confirm crustal characteristics interpreted previously from the compressional wave data layers 3a and 2b in the upper crust are characterized by rapid increases in seismic velocities shear wave velocity increases from 2 2 to 3 6 km s l over an interval of 3 5 km although the exact nature of the shearwave velocity gradient is uncertain in contrast the shear velocity in layer 3a remains fairly constant with depth at values near 3 6km s no velocity discontinuity is observed at the moho instead the transition from crust to upper mantle is marked by the presence of a velocity gradient zone layer 3b the shear wave velocities in the upper mantle are 4 5 and 4 6 km s l in directions approximately parallel to the spreading ridge and parallel to the direction of spreading respectively this low degree of velocity anisotropy contrasts sharply with the 10 per cent anisotropic effect observed for the pn velocities combining the p wave interpretation with this s wave study enables evaluation of poisson s ratio as a function of depth except for low values of poisson s ratio found in the upper crust the seismic properties of the oceanic crust and upper mantle interpreted in this study agree remarkably well with those of laboratory measurements on ophiolite samples

the association of continental crust rocks with ophiolites in the northern apennines italy implications for the continent ocean transition in the western tethys abstract the late cretaceous sedimentary melanges from the external liguride units of the northern apennines include large slide blocks of ophiolites and lower and upper continental crust rocks representative of a continent ocean transition between the internal liguride oceanic domain and the thinned continental margin of the adria plate the slide blocks preserve a record of the long lived history of rifting which led to opening of the jurassic western tethys basin the external liguride ophiolites consist of 1 undepleted spinel peridoties partly re equilibrated under plagioclase facies conditions which were interpreted as unroofed subcontinental mantle 2 rare gabbroic rocks mainly troctolite to olivine bearing gabbro probably crystallised from n morb magmas and 3 basalts with n to t morb affinity covered by late callovian early oxfordian radiolarian cherts both gabbroic rocks and basalts locally intrude the mantle peridotites and postdate their re equilibration to plagioclase facies conditions the slide blocks of lower continental crust are composed of gabbro derived mafic granulites and felsic granulites the latter include quartzo feldspathic granulites and rare quartz poor to quartz free charnockitic rocks in both mafic and felsic granulites granulite facies re equilibration was followed by a retrograde metamorphic evolution to amphibolite greenschist and subgreenschist facies conditions retrogression is commonly accompanied by deformations progressively changing from plastic to brittle the upper crustal rocks occurring as slide blocks consist of hercynian granitoids with orogenic affinity mainly biotite bearing granodiorites and peraluminous two mica leucogranites locally the granitoids are intruded by basaltic dykes or capped by basaltic flows and radiolarian cherts the granitoids underwent polyphase brittle deformations under subgreenschist facies conditions which predated the basalt emplacement the tectono metamorphic evolution recorded by the slide blocks of the external liguride units started in the late carboniferous early permian about 290 ma with the emplacement at deep crustal levels of the gabbroic protoliths for the mafic granulites the associated felsic granulites likely represent the remnants of the lower continental crust intruded by the gabbro derived granulites mafic and felsic granulites subsequently underwent tectonic exhumation in permo triassic times as testified by the development of granulite to amphibolite facies ductile shear zones the granulites were finally exhumed to shallow levels probably in association with the subcontinental mantle in late triassic middle jurassic times the latter period was most likely characterized by extensive brittle faulting at shallow crustal levels thus giving rise to extensional allochthons formed by stretched slices of granitoids the western tethys opening is finally testified by the basalt intrusion and effusion in the late jurassic followed by deep sea pelagic sedimentation the external liguride crustal stratigraphy can be regarded as a fossil example of the transitional realm at the continent ocean boundary this reconstruction fits well with the available data on the present day continental margins derived from passive lithosphere stretching

the casanova complex of the northern apennines a melange formed on a distal passive continental margin abstract the cretaceous palaeocene casanova complex occurs in two thrust sheets of the eugeosynclinal ligurids of the northern apennines it is a sedimentary melange with ophiolitic and quartzose turbidites or limestone shale olistostrome submarine debris flows as matrix exotic blocks of ophiolite and granite serpentinite breccias and lenticular ophiolitic breccias and olistostromes contribute to the melange character of the complex deformational structures include soft sediment slump folds indicating a sw dipping palaeoslope and boudins a gradational slumped top to the melange small scale faults in chert blocks and deformation associated with the emplacement of the exotic slide blocks the blocks were shed as rotational slides from submarine fault scarps and are surrounded by haloes of debris created by submarine weathering the stacking pattern of the blocks with the originally stratigraphically highest ophiolite lithologies lowest in the pile of blocks is explained by a diverticulation model with progressively deeper erosion mechanical analysis shows that the blocks were stable when partly exposed resting on a soft sediment substratum criteria which distinguish the casanova complex from a tectonic melange and which may be of value in other melanges are discussed previous interpretations of the complex as a precursor olistostrome to northeastward nappe emplacement the bracco ridge model are rejected the melange is believed to have formed on ocean crust as a result of turbidite and debris flow sedimentation soft sediment deformation block faulting gravity sliding and submarine erosion at the distal edge of a uniformly sw dipping continental margin

vema fracture zone central atlantic tectonic and magmatic evolution of the median ridge and the eastern ridge transform intersection domain the eastern vema fracture zone intersection with the mid atlantic ridge mar axis was surveyed with the french submersible nautile during the vemanaute cruise in 1989 at this ridge transform intersection rti an elongated e w ridge more than 50 km long is present in the transform valley this median ridge rises up to 1000 m above the surrounding seafloor the crest of the median ridge is lower and presents an arcuate shape at the tip of the mar axis dive observations on both the southern and northern flanks of the median ridge as well as sample studies suggest that this morphological feature is not a serpentinized mantle protrusion or a recent volcanic constructional ridge but represents a sliver of uplifted oceanic lithosphere covered by a sedimentary breccia formation this detrital cover consists of polymictic sedimentary breccias sandstones and siltstones composed of basaltic doleritic and gabbroic clasts with less frequent serpentinite and spinel fragments which originated from the disaggregation of shallow to deep levels of tectonically uplifted oceanic crust and upper mantle most of these clasts have undergone greenschist facies metamorphism prior to their incorporation in the detrital formation disaggregation mass wasting and rapid emplacement of detrital formations on the valley floor by gravity flows are likely to be related to a major tectonic episode that affected one or both the fracture valley walls this event could be related to the uplift of the southern wall of the fracture zone the transverse ridge which took place probably between 10 and 3 ma ago since this uplift episode the transverse ridge which is now undergoing subsidence and the detritus covered transform valley floor separated by the transform fault zone have migrated westward and eastward respectively vertical tectonics of the median ridge at the approach of the rti can not be explained solely by the hypothesis of a diapiric intrusion of serpentinite as proposed by earlier authors a possible interpretation follows the suggestions that the anomalous crust of the fracture valley near the western rti is more than 1 km out of isostatic equilibrium recent tectonic and magmatic events including subsidence and lava emplacement which occurred at the tip of the mar axis have been recorded on the southern flank of the median ridge several stages in the very recent tectonic volcanic history of the eastern rti that is roughly during the last 300 000 years can thus be defined the lower elevation and narrow arcuate shape of the median ridge east of 40 57 w are inferred to have resulted from tectonic extension during the creation of the nodal basin

experimental evidence for the origin of two kinds of inclusions in diamonds from the deep mantle abstract the conditions of origin for the type iii inclusions in diamonds from brazil hutchison thesis 1997 and the napx en inclusion from china wang and sueno miner j 18 1996 9 16 were experimentally determined using a split sphere anvil apparatus ussa 2000 the type iii inclusions formed at a minimum pressure and temperature of 25 gpa and 2000 c while the origin of the napx en inclusion was close to 23 gpa and 1900 c both determinations suggest that the temperature at the corresponding depths is about 300 c higher than predicted by most geotherms for a convecting mantle without a thermal boundary layer at 660 km both kinds of inclusions required rapid single stage transport by carbonate melt to the earth s surface which is consistent with the depths of origin for this melt greater than 660 km the unusual composition of the napx en inclusion is the result of metasomatism by carbonate melt enriched in na k and mg and depleted in si and al since this melt is not kimberlitic in composition exchange of material between the melt and the mantle was necessary for the melt to become kimberlitic by the time it reached the surface the resulting metasomatism taking place over a long period of time could cause major changes in the mineral and chemical composition and the structure of the earth s mantle and thus play an important role in its evolution

mineral chemistry of ultramafic tectonites and ultramafic to gabbroic cumulates from the major oceanic basins and northern apennine ophiolites italy a comparison abstract the paper presents the mineralogy of upper mantle tectonites and associated lower crustal ultramafic and mafic cumulates from the jurassic northern apennine ophiolites nao in a comparison assay with equivalent rocks from the major oceanic basins mob east pacific rise and atlantic fracture zones legs 37 and 45 among the mantle rocks both tectonite lherzolites and harzburgites have been so far widely found in mob whereas lherzolites largely prevail in nao the mg mg mg mg fe 2 range in mafic minerals in nao tectonites is restricted to high values ol 88 90 opx 90 92 cpx 91 93 cr spinels show large variation of the cr cr cr al ratio found in spinels from lherzolites and very rare cpx bearing harzburgites cr 10 and 15 respectively the varied compositions largely reflect the relative greater degree of partial melting that affected mob tectonites compared to nao tectonites from a restored stratigraphic log of the nao cumulates the order of appearance of the cumulus phases both in nao and mob cumulates is ol chr pl cpx opx fe 1bti oxides ap the sequence gave rise to the following main lithotypes dunites troctolites minor wehrlites ol gabbros gabbronorites ferrogabbros and ferrodiorites the olivine from early ultramafic cumulates has high fo content fo90 for nao fo91 for mob with a nio content ranging from 0 21 to 0 31 wt cr diopside generally shows higher tio2 than found in tectonite peridotites the coexisting spinel is typically a ti chromite with high cr 43 60 the most mg rich olivine coexisting with an rich plagioclase in the most primitive cumulates are fo88 and an72 and were in equilibrium with a basaltic melt these facts coupled with the lack of intense plastic deformation in the early cumulates allow us to exclude a high pressure fractionation system formation of very little volume percent of amphibole indicates a very low h2o activity in the magma starting from the early fractionating cumulus phases a recalculated primary magma composition is proposed from these calculations it is concluded that nao cumulates derived from a tholeiitic basalt liquid similar to primitive mob liquid except for a higher na2o content in particular present day analogues are found among high na2o mid ocean ridge basalts from the mid cayman rise and southwest indian ridge the composition of calculated primary magmas indicated for nao 1 a much lower degree of melting than average mob also sustained by evident differences in residual peridotites 2 a crustal thickness of 2 3 km also supported by the reconstructed maximum thickness of crustal sequences of 2 km 3 a spreading axis with an average water depth of 5 km by analogy with the mid cayman rise and southwest indian ridge half spreading rates of