vertical magnetic structure of ocean crust determined from near bottom magnetic field measurements consensus on the source of marine magnetic anomaly stripes has remained one of the most elusive aspects of the vine and matthews seafloor spreading model which has been of such fundamental importance to global plate tectonics the first attempt to measure the vertical magnetic structure of oceanic crust exposed at a scarp face using near bottom magnetic measurements is reported along with the theoretical basis for the reduction and analysis of these measurements magnetometers mounted to deep tow and submersible platforms were used to determine the horizontal and vertical variation of crustal magnetization of the upper 2 km of young oceanic crust exposed by a steep submarine escarpment at the blanco fracture zone in the northeast pacific ocean results show a large amplitude magnetic anomaly is consistently found at the extrusive to intrusive transition indicating that the extrusives are strongly magnetized compared to the weakly magnetized intrusive crust profiles also show the systematic presence of more than one polarity with depth indicating the possibility of dipping polarity boundaries within the extrusives forward models find that a dipping polarity boundary model correctly predicts the amplitude and shape of the overlying sea surface anomaly while a block model with vertical polarity boundaries overestimates the amplitude these in situ results are used to directly assess the contribution made by the extrusive section to the overlying magnetic anomaly signal in essence to define the magnetic source layer it is concluded that the extrusive lavas constitute the main magnetic source for magnetic anomalies in oceanic crust less than 2 m y in age

on the origin of el chichon volcano and subduction of tehuantepec ridge a geodynamical perspective the origin of el chichon volcano is poorly understood and we attempt in this study to demonstrate that the tehuantepec ridge tr a major tectonic discontinuity on the cocos plate plays a key role in determining the location of the volcano by enhancing the slab dehydration budget beneath it using marine magnetic anomalies we show that the upper mantle beneath tr undergoes strong serpentinization carrying significant amounts of water into subduction another key aspect of the magnetic anomaly over southern mexico is a long wavelength 150 km high amplitude 500 nt magnetic anomaly located between the trench and the coast using a 2d joint magnetic gravity forward model constrained by the subduction p t structure slab geometry and seismicity we find a highly magnetic and low density source located at 40 80 km depth that we interpret as a partially serpentinized mantle wedge formed by fluids expelled from the subducting cocos plate using phase diagrams for sediments basalt and peridotite and the thermal structure of the subduction zone beneath el chichon we find that 40 of sediments and basalt dehydrate at depths corresponding with the location of the serpentinized mantle wedge whereas the serpentinized root beneath tr strongly dehydrates 90 at depths of 180 200 km comparable with the slab depths beneath el chichon 200 220 km we conclude that this strong deserpentinization pulse of mantle lithosphere beneath tr at great depths is responsible for the unusual location singularity and probably the geochemically distinct signature adakitic like of el chichon volcano

physical connectivity between pulley ridge and dry tortugas coral reefs under the influence of the loop current florida current system abstract the pulley ridge and dry tortugas coral reefs are among the most pristine but also fragile marine ecosystems of the continental united states understanding connectivity processes between them and with surrounding shelf and deep areas is fundamental for their management this study focuses on the physical processes related to the connectivity of these reefs unprecedented in situ time series were used at these specific reef locations together with satellite observations and numerical simulations to investigate the dynamics controlling local circulation on the southwestern florida shelf swfs under oceanic influence the approach of the loop current and florida current lc fc system to the swfs slope can induce 0 5 to 1 m s offshore flows impacting the pulley ridge and dry tortugas reefs on the other hand when the lc fc system retreats from the slope onshore flows can carry open sea waters over the coral reefs local formation of cyclonic eddies is possible near the dry tortugas reefs in the lc approach case and passage of upstream lc frontal eddies is possible in the lc retreat case offshore currents 1 m s over the swfs slope were also found during periods of anticyclonic lc eddy separation a novel finding is the shedding and northward propagation of mesoscale anticyclonic eddies from the core of the lc along the west florida shelf eddy shedding may have a broader effect on the dynamics of the shelf around the study reef areas long periods of lc fc domination over these coral reefs reaching several weeks to months are characterized by strong 1 m s along shelf currents and continuous upwelling processes which may weaken the slope stratification and bring colder deeper waters over the shelf break and toward the shallower shelf region

constructional features of the troodos ophiolite and implications for the distribution of orebodies and the generation of oceanic crust the map pattern of the troodos ophiolite in cyprus is an expression of a simple elongate anticlinal structure the cross sectional form of the anticline can be described by a sinusoidal dip model on the northern flank of the anticline the thickness of various units number of gossans etc were measured along profiles at 2 5 km intervals normal to strike using existing maps supplemented by new observations profiles are approximately perpendicular to the spreading direction during crustal construction and uncertainties in unit thickness are smaller than the range of unit thicknesses data on the profiles were analyzed quantitatively by harmonic analysis using the fourier transform method the harmonic analysis discloses cyclic variation in the thickness of major units number of gossans etc that provides a link with the regular spacing of five groups of cu fe zn orebodies groups of orebodies occur preferentially where sheeted dikes are at a shallow crustal depth sheet and massive flows and dike

magnetic studies of magma supply and sea floor metamorphism troodos ophiolite dikes abstract dikes of the eastern troodos ophiolite of cyprus intruded at slow ocean spreading axes with dips ranging up to 15 from vertical and with bimodal strikes now ne sw and n s due to post 88 ma sinistral microplate rotation varied dike orientations may represent local stress fields during dike crack propagation but do not influence the spatial distributions or orientation distributions of dikes magnetic fabrics nor of their palaeomagnetic signals anisotropy of magnetic susceptibility ams integrates mineral orientation distributions from each of 1289 specimens sampled from dikes at 356 sites over 400 km2 in the eastern troodos ophiolite of cyprus in 90 of dikes ams fabrics define a foliation kmax kint parallel to dike walls and a lineation kmax that varies regionally and systematically magma flow alignment of accessory magnetite controls the ams with a subordinate contribution from the mafic silicate matrix that is reduced in anisotropy by sea floor metamorphism titanomagnetite has less influence on anisotropy occasionally intermediate and minimum susceptibility axes are switched so as to be incompatible with the kinematically reasonable flow plane but maximum susceptibility kmax still defines the magmatic flow axis such blended subfabrics of kinematically compatible mafic silicate and misaligned multidomain magnetite subfabrics are rare areas of steep magma flow kmax plunge 70 and of shallow magma flow alternate in a systematic and gradual spatial pattern foci of steep flow were spaced 4 km parallel to the spreading axes and 6 km perpendicular to the spreading axes ridge parallel separation of steep flow suggest the spacing of magma feeders to the dikes whereas ridge perpendicular spacing of 6 km at a spreading rate of 50 mm a implies the magma sources may have been active for 240 ka the magma feeders feeding dikes may have been 2 km in diameter stable paleomagnetic vectors in some cases verified by reversal tests are retained by magnetite and titanomagnetite in all specimens the stable components were isolated by three cycles of low temperature demagnetization ltd followed by 10 steps of incremental thermal demagnetization td 47 of primary a components 338 2 57 2 n 207 95 3 9 mean tub 397 8 c are overprinted by a b component 341 4 63 5 n 96 95 8 7 mean tub 182 11 c a and b components are ubiquitous and shared equally by the n s and ne sw striking dikes a component unblocking temperatures tub are zoned subparallel to the fossil spreading axis their spatial pattern is consistent with chemical remagnetization at some certain off axis distance determined by sea floor spreading a components indicate less microplate rotation and more northerly palaeolatitudes that are consistent with metamorphic remagnetization after some spreading from the ridge axis thus their magnetizations are younger than those of the overlying volcanic sequence for which chrms are commonly reported as 274 33 88 ma

vertical distribution and alteration of dikes in a profile through the troodos ophiolite in situ oceanic crust at intermediate depths of from one to five km is presently largely inaccessible to continuous examination and sampling except in the possibly special circumstances of major transform faults the study of ophiolites over this interval where thin sub vertical dikes are usually important may provide guidance to the nature of in situ oceanic crust at these depths the likelihood that similarities exist between in situ and ophiolitic oceanic crust has recently been strengthened by the possible identification of a dike complex below 0 8 km depth in dsdp hole 504b1 sub vertical basic dikes occur over a 4 6 km depth interval from 0 2 to 4 8 km crustal depth in a profile through the troodos cyprus ophiolite exponential growth and arctangent decay in dike density occur over 2 3 km and 1 0 km intervals respectively secondary magnetite is the dominant dike magnetic material of the dikes and associated screens these results have implications for the construction alteration history and magnetization of in situ oceanic crust at intermediate depths

the late cretaceous palaeolatitude of the neotethyan spreading axis in the eastern mediterranean region abstract a compilation of available palaeomagnetic data from the troodos cyprus and baer bassit syria ophiolitic terranes of the eastern mediterranean tethyan orogenic belt is presented the ophiolites represent fragments of oceanic lithosphere generated at a neotethyan spreading axis in the late cretaceous although debate continues over the tectonic setting of this spreading axis and its position within the eastern mediterranean palaeogeography two types of model reconstructions have been proposed type 1 the ophiolites formed in a southerly neotethyan basin by spreading above an oceanic subduction zone the baer bassit ophiolite was then emplaced a relatively short distance tens of kilometers southwards on to the arabian continental margin leaving the troodos ophiolite isolated in an intra oceanic setting to the west and type 2 the ophiolites formed in a northerly neotethyan basin by spreading at a normal oceanic ridge with subsequent large scale thrusting hundreds of kilometers to the south of emplaced ophiolites over microcontinental fragments to reach their present positions palaeomagnetic determination of the palaeolatitude of the neotethyan spreading axis is therefore of considerable interest previous palaeomagnetic analyses have demonstrated the presence of significant and in some cases extreme relative tectonic rotations of a variety of origins in both ophiolites to allow palaeomagnetic data from these rotated units to be combined an inclination only formulation of the palaeomagnetic tilt test is employed this provides unequivocal evidence that both ophiolites retain pre deformational remanent magnetizations which are interpreted as original ocean floor magnetizations acquired close to the time of crustal formation in the late cretaceous the mean inclinations of 37 0 2 6 for the troodos terrane and 41 1 3 4 for the baer bassit terrane indicate respective palaeolatitudes for the spreading axes of 20 6 n 1 8 and 23 6 n 2 5 consistent with a late cretaceous position between the arabian and eurasian margins these data together with a well defined palaeolatitude of 25 5 n 4 5 for the eastern pontides previously reported in the literature provide constraints which must be incorporated in any successful tectonic reconstruction of the eastern mediterranean tethys the implications of these constraints for type 1 and 2 models are discussed using a series of plate tectonic cross sections constructed along a line extending northwards from the arabian continental margin in the absence of palaeomagnetic data from late cretaceous rocks of the eastern taurides however it is presently impossible to use these palaeolatitudinal constraints to resolve the root zone debate on a purely palaeomagnetic basis solutions which satisfy the constraints may be found for both types of model reconstruction additional published field based geological considerations however strongly support models in which the troodos and baer bassit and other southerly ophiolites were generated in a southern neotethyan basin rather than those involving generation in a northerly basin and subsequent large scale thrust displacement to the south

reduced crustal magnetization beneath the active sulfide mound tag hydrothermal field mid atlantic ridge at 26 n abstract a detailed near bottom magnetic field survey was carried out by the submersiblealvin over the actively venting mound located within the tag trans atlantic geotraverse hydrothermal field on the mid atlantic ridge at 26 08 n 44 49 w three dimensional analysis of these data clearly shows a distinct zone of reduced magnetization directly beneath the active mound this magnetization low is consistent with the highly altered upflow zone of a hydrothermal vent system that feeds the actively venting mound structure in contrast the sea surface magnetic anomaly is associated with a broad 2 8 km magnetization low elongated along the axis that includes both the active and inactive mounds the short wavelength 250 m near bottom magnetic anomaly over the active mound is far too small to produce the long wavelength 8 km sea surface magnetic anomaly at tag however and even a collection of mounds with similar magnetic structure cannot produce the magnetic moment needed to generate the sea surface anomaly other hypotheses including reversely magnetized crust and structurally thinned crust could account for the sea surface anomaly but are considered unlikely the existence of vigorous hydrothermal activity at tag and the lack of microseismic activity in the tag area suggests that thermal demagnetization is the prime contributor to the sea surface anomaly the thermal halo associated with a largely solid but still hot intrusion would provide sufficient demagnetization on a kilometer scale to produce the long wavelength sea surface anomaly pervasive alteration at depth would also be an important factor in the destruction of crustal magnetization and is the only way that such a long wavelength magnetic signal could be preserved in the crust the overall model of crustal magnetization at a hydrothermal field with discrete zones of demagnetization in the upper crust and a broader zone of demagnetization at depth is consistent with studies of hydrothermal systems in ophiolite suites these studies show narrow alteration pipes in the upper crust feeding the exhalative seafloor deposits and pervasive alteration at depth which commonly have associated late stage intrusive bodies 1 while detailed magnetic surveys may provide some clues to the location of oceanic hydrothermal upflow zones only drilling will ultimately test these hypotheses

equivalent radiolarian ages from ophiolitic terranes of cyprus and oman radiolarian biostratigraphy shows that umberiferous strata overlying the troodos ophiolite in cyprus are turonian in age and are thus essentially contemporaneous with similar strata that overlie the samail ophiolite in oman however this radiolarian age is markedly older than campanian isotopic ages measured on the underlying rocks of the troodos ophiolite the revised age for the umbers indicates that the troodos lavas were formed no later than turonian time the presence of overlying autochthonous maastrichtian chalks restricts the emplacement of the troodos ophiolite to the late cretaceous santonian to early maastrichtian

re computing palaeopoles for the effects of tectonic finite strain abstract the pre messinian limestone cover 58 8 ma to the troodos ophiolite 88 ma of southern cyprus is penetratively strained as shown by ubiquitous magnetic fabrics and in many sites stylolitic cleavage these define a gently n dipping foliation and an n plunging extension south vergent folding and thrusting is well known in very localized whereas the bulk of the strained limestone cover dips gently south disturbed by faulting the magnetic fabrics and stylolitic cleavage define the axes of finite strain in all sites studied and the calcite matrix was suitably ductile to permit the original palaeomagnetic directions to be de strained assuming continuum behaviour the optimum de straining 30 40 shortening in a flattening strain is compatible with the stylolitic cleavage development restores bedding to the near horizontal and restores the characteristic remanent magnetization vectors chrms to concentrated symmetrical fisherian distributions the strain corrected chrms yield more reasonable palaeopole locations for the lefkara and pakhna limestone and more uniform micro plate rotation rates corrected palaeopoles reveal a relatively uniform anticlockwise rotation of the troodos plate since the creation of the late cretaceous 88 ma ocean lithosphere it did not accelerate during the deposition of the limestone cover as required by palaeopoles calculated from data not corrected for finite strain but turned at 1 5 ma 1 since 58 ma

dsdp hole 504b the first reference section over 1 km through layer 2 of the oceanic crust hole 504b provides a reference section for correlation of in situ petrological and geophysical studies with regional crustal models based on dredged samples and remote surveys and at least for the medium spreading rate costa rica rift hole 504b confirms that the main features of the upper part of an idealized ophiolite sequence occurs in the oceanic crust

age dependent three dimensional magnetic modeling of the north pacific and north atlantic oceanic crust at intermediate wavelengths three dimensional magnetic modeling of the north atlantic and northeast pacific is performed at intermediate wavelengths using three models for the acquisition of a natural remanent magnetization it is shown that a remanent magnetization which is dependent on the crustal age is the dominant source for the intermediate wavelength pattern in both basins however a pure thermoremanent magnetization of layer 2 alone is insufficient to model the intensity and shape of the observed magnetic anomalies at satellite altitude we conclude that the best fitting magnetization model for both basins is a combination of a chemical remanent magnetization in the altered upper crust and a thermoviscous remanent magnetization of the slowly cooling lower crust and upper mantle the north pacific requires a bulk magnetization which is 50 higher than that of the north atlantic in order to fit the magsat field geological processes associated with a faster spreading rate such as a faster hydrothermal alteration and the growth of a thicker gabbro layer at the expense of a weakly magnetized sheeted dike layer are plausible explanations for the higher north pacific magnetization the lineated positive magnetic anomaly observed over the north atlantic spreading center is not well reproduced by our models this anomaly is likely due to a highly magnetized body along or in the vicinity of the spreading center this highly magnetic body could be an unstable serpentinized lens of crustal material younger than 20 m y which develops within the zone cooled by hydrothermal circulation

chemical remanent magnetization in oceanic sheeted dikes marine lineated magnetic anomalies often exhibit anomalous skewness that indicates non vertical boundaries between normally and reversely magnetized crust the acquisition of secondary magnetization components well after formation of the crust has been suspected as one possible cause for anomalous skewness a petrographic and rock magnetic study on sheeted dikes recovered from the lower part of the deepest drill hole in the oceanic crust odp ocean drilling program hole 504b demonstrates that these rocks carry a secondary chemical remanent magnetization crm and not a thermoremanent magnetization trm primary titanomagnetites have mostly been altered to non magnetic phases while the remanent magnetization resides in secondary magnetite that formed below its curie temperature 600 c at ambient pressure mainly by the alteration of olivine the natural remanent magnetization nrm has been compared with laboratory anhysteretic remanent magnetization arm and trm the intensities relate nrm arm trm each by a factor of two because previous studies have shown that crms are much lower in intensity than trms and because of the common presence of secondary magnetite we conclude that the nrms of the lower hole 504b dikes are crms the dikes exhibit the same reversed magnetization as the overlying extrusive basalts and thus the timing of crm acquisition is most likely confined to the time span between the crustal age of hole 504b and the next younger field reversal i e 0 25 m y after emplacement

the magnetization of oceanic crust contribution to knowledge from the troodos cyprus ophiolite an extensive study of a segment of the troodos cyprus ophiolite using both outcrop and drill core samples and extending from the sediment extrusive interface through sheeted dikes to cumulate ultramafics has allowed a number of key questions regarding the magnetization of oceanic crust to be addressed these include the number of strongly magnetized intervals with depth their lateral variability and controls on their occurrence comparison has also been made with the section in ocean drilling program odp hole 504b and a reinterpretation of its constructional setting is offered two strongly magnetized intervals occur in the area studied the upper is in the extrusive sequence extends on average from 0 2 to 0 6 km depth and has a thickness of 0 4 km here magnetization is dominated by remanence the lower interval extends from the lowest level at which flows occur with dikes average depth 0 9 km into the sheeted complex average depth 1 2 km and has a thickness of 0 3 km here magnetization is dominantly induced no other strongly magnetized intervals occur in the section the extent of dike intrusion is closely related to the position of the lower limit of the highremanence layer and to the occurrence of the high induced magnetization layer in both cases the replacement of primary magnetite which can carry a strong remanence by magnetically soft secondary magnetite appears to be the controlling process comparison of the troodos and hole 504b magnetization profiles shows close similarity in the upper remanence dominated magnetic interval the absence of the deeper interval of high induced magnetization in the hole 504b profile is interpreted as meaning that sheeted dikes have not been penetrated by the drill hole

magnetization of the oceanic crust beneath the labrador sea the aeromagnetic anomalies of the oceanic crust northeast of newfoundland canada are strongly skewed with an anomalous skewness of 33 52 with respect to the anomalies calculated using the standard sea floor spreading model this paper examines the possibility that the anomalous skewness is on account of an oceanic crust with a thermo viscous remanent magnetization where unlike the standard model the lower crust is magnetic the thermo viscous remanent magnetization of the lower crust beneath the labrador sea has dominant contributions to broad scale marine magnetic anomalies and to the negative magsat anomaly of the sea

the iceland research drilling project crustal section variation of magnetic properties with depth in icelandic type oceanic crust an attempt has been made to identify the processes that give rise to a number of depth trends in the magnetization of a 3 1 km vertical section of icelandic type oceanic crust and to assess the possibility that similar processes act and depth trends occur in typical oceanic crust the depth trends in the icelandic section consist of a general increase in saturation and induced magnetization to 2 km crustal depth below which flow magnetization decreases while dike magnetization remains constant and of large changes in flow magnetization that occur on a scale of a few hundred metres below 3 km crustal depth increase in saturation and induced magnetization with depth in the upper 2 km is thought to be the result of two processes a decrease in low temperature oxidation from the original lava surface to 700 800 m crustal depth thence an increase in hydrothermal alteration with depth this interpretation is based on oxide petrography and curie temperatures which show a weakly defined minimum in the 700 8

a new mixing diagnostic and gulf oil spill movement chaotic advection has served as the paradigm for mixing in fluid flows with simple time dependence its skeletal structure is based on analysis of invariant attracting and repelling manifolds in fluid flows here we develop a finite time theory for two dimensional incompressible fluid flows with arbitrary time dependence and introduce a new mixing diagnostic based on it besides stretching events around attracting and repelling manifolds this allows us to detect hyperbolic mixing zones we used the new diagnostic to forecast the spatial location and timing of oil washing ashore in plaquemines parish and grand isle louisiana and pensacola florida in may 2010 and the flow of oil toward panama city beach florida in june 2010

thermoviscous remanent magnetization of oceanic lithosphere inferred from its thermal evolution remanent magnetization patterns of the oceanic lithosphere determined on the basis of the thermal evolution of the lithosphere while taking into account both thermal and viscous magnetizations give rise to magnetic anomalies similar to those observed by magsat over the cretaceous quiet zones of the atlantic ocean and by marine magnetic surveys over anomaly 33 viscous magnetization acquired during a given core field polarity period is not removed completely by that acquired in the next period of opposite polarity because of cooling of the lithosphere and the strong temperature dependence of viscous magnetization the magnetization pattern of layer 2a resembles that of the standard seafloor spreading magnetic model consisting of normally and reversely magnetized zones separated by vertical boundaries and produces symmetric magnetic anomalies the magnetization pattern of the middle crust is similar to that of layer 2a for high magnetic blocking temperature models but it is slightly skewed with respect to layer 2a for low magnetic blocking temperature models the magnetization pattern of the lower crust is strongly skewed relative to that of layer 2a and has diffuse boundaries with pronounced slopes between normally and reversely magnetized zones it produces magnetic anomalies with appreciable skewness suggesting that observed skewness of marine magnetic anomaly 33 arises from the magnetization of this layer the magnetization pattern of the upper mantle is appreciably skewed and almost devoid of small scale variations it produces strongly skewed magnetic anomalies at satellite altitudes which may explain the observed skewness of magsat anomalies over the cretaceous quiet zones of the atlantic ocean

cyclonic eddies in the eastern gulf of mexico abstract cold domed cyclonic eddies juxtaposed to the cyclonic shear side of the gulf loop current are observed in simultaneously obtained hydrographic current meter mooring and satellite infrared data the cyclones are initially observed in the satellite data as cold perturbations on the northern extreme of the current and grow either into a cold tongue or a quasi stable meander off the dry tortugas florida areal shipboard surveys show closed isopleths of temperature and salinity and surface geostrophic current speeds relative to 1000 db are in excess of 100 cm s 1 the diameter of the cold domes varied from 80 to 120 km separation of large anticyclonic rings is always observed to be preceded by cyclonic eddies in the transition zone between campeche bank and the west florida shelf but only on the eastern side not every cyclonic eddy off dry tortups is associated with the separation of an anticyclonic ring some are eroded away by the florida current but they have never been observed in 10 years

temporal and spatial characteristics of summer upwelling along florida s atlantic shelf abstract temperature data from shelf waters along florida s atlantic coast are used to characterize upwelling during the summer of 1980 bottom temperatures recorded at seven stations across the shelf and during 14 weekly cruises between early july and mid september show a continual cross shelf movement of the isotherms but only one primary upwelling event during the first three weeks of august upwelling ends with an exceptionally rapid warming over a one week period the largest net temperature increase of about 10 c occurs at mid and near bottom depths over the middle shelf temperature recorder data from four stations spaced along the 10 m isobath suggest that upwelling events arrive on the inner shelf along relatively restricted sections of coastline and persist between one and three weeks surface drogue data reveal an increase in current speed coinciding with lowest bottom temperatures this lends support to the likelihood that the florida current is the primary case of upwelling in florida at

remanent magnetization of the oceanic upper mantle skewnesses of the marine and magsat magnetic anomalies of the atlantic ocean are explained in terms of the remanent magnetization gradually acquired by the oceanic lithosphere during its cooling magnetizations of the lower crust and the upper mantle are about 20 50 and 10 30 percent of the magnetization of the oceanic basaltic layer 2a respectively the oceanic upper mantle controls the skewness of the magsat anomalies whereas the skewness of the marine anomalies arise primarily from the oceanic lower crust

kankan diamonds guinea i from the lithosphere down to the transition zone peridotitic inclusions in alluvial diamonds from the kankan region of guinea in west africa are mainly of lherzolitic paragenesis nevertheless extreme cr2o3 contents max 17 wt in some of the exclusively lherzolitic garnets document that the diamond source experienced a previous stage of melt extraction in the spinel stability field this initial depletion was followed by at least two metasomatic stages 1 enrichment of lree and sr and 2 introduction mainly of mree hree and other hfse ti y zr hf the ti and hfse poor character of stage 1 points towards a cho rich fluid or carbonatitic melt the high hfse in stage 2 favour silicate melts as enriching agent eclogitic inclusions are derived from a large depth interval ranging from the lithosphere through the asthenosphere into the transition zone the occurrence of negative eu anomalies in garnet and clinopyroxene from both lithosphere and transition zone suggests a possible relationship to subducted oceanic crust lithospheric eclogitic inclusions are derived from heterogeneous sources that may broadly be divided into a low ca group with lree depleted trace element patterns and a high ca group representing a source with negative lree hree slope that is moderately enriched in incompatible elements relative to primitive mantle high ca inclusions of majoritic paragenesis are significantly more enriched in incompatible elements such as in sr and lree calculated whole rock compositions require metasomatic enrichment even if a derivation from morb is assumed

thermoremanent magnetization of the oceanic lithosphere inferred from a thermal evolution model implications for the source of marine magnetic anomalies abstract thermoremanent magnetization models of the oceanic crust and the uppermost part of the oceanic mantle are derived on the basis of a typical thermal evolution model of the oceanic lithosphere using different magnetic blocking temperature ranges the magnetization of the basaltic layer 2a has a similar pattern but with a lower intensity to that of the standard sea floor spreading magnetic model however the magnetization patterns of the lower crust and upper mantle are skewed so that in a given polarity period the total magnetization of a lithospheric column which is created early in the period is greater than that of a column which is created later in the period magnetic anomalies of the magnetization models are calculated at sea level and at magsat elevation for slow moderate and fast sea floor spreading velocities although the magnetization models are derived based on purely physical reasoning without using any a priori information about the observed marine and magsat magnetic anomalies they do yield magnetic anomalies which are similar to the observed anomalies the models explain several features of the marine and magsat anomalies the anomalous skewness of marine magnetic anomalies is explained in terms of the magnetization of the oceanic lower crust the magnetization of the oceanic upper mantle may account for the anomalous skewness observed over the cretaceous quiet zones in the atlantic ocean the skewness of the magnetic anomalies calculated at sea level decreases with the increase in the sea floor spreading velocity

strange bedfellows a deep water hermatypic coral reef superimposed on a drowned barrier island southern pulley ridge sw florida platform margin abstract the southeastern component of a subtle ridge feature extending over 200 km along the western ramped margin of the south florida platform known as pulley ridge is composed largely of a non reefal coastal marine deposit modern biostromal reef growth caps southern pulley ridge spr making it the deepest hermatypic reef known in american waters subsurface ridge strata are layered lithified and display a barrier island geomorphology the deep water reef community is dominated by platy scleractinian corals leafy green algae and coralline algae up to 60 live coral cover is observed in 60 75 m of water although only 1 2 of surface light is available to the reef community vertical reef accumulation is thin and did not accompany initial ridge submergence during the most recent sea level rise the delayed onset of reef growth likely resulted from several factors influencing gulf waters during early stages of the last deglaciation 14 kyr b p including cold low salinity waters derived from discrete meltwater pulses high frequency sea level fluctuations and the absence of modern oceanic circulation patterns currently reef growth is supported by the loop current the prevailing western boundary current that impinges upon the southwest florida platform providing warm clear low nutrient waters to spr the rare discovery of a preserved non reefal lowstand shoreline capped by rich hermatypic deep reef growth on a tectonically stable continental shelf is significant for both accurate identification of late quaternary sea level position and in better constraining controls on the depth limits of hermatypic reefs and their capacity for adaptation to extremely low light levels