the ocean continent boundary off the western continental margin of iberia i crustal structure at 40 30 n summary the western continental margin of the iberian peninsula has the characteristics of a rifted non volcanic margin with half graben and tilted fault blocks seen in several places the ocean continent boundary ocb is therefore expected to be where thinned continental crust and oceanic crust are juxtaposed as at many similar margins worldwide it is particularly useful to locate the ocb off western iberia in order to constrain the pre rift fit of north america to iberia and by implication the shape of the proto bay of biscay the fit is only marginally constrained by sea floor spreading magnetic anomalies because anomaly 34 is believed to be far to the west of the ocb and it is even possible that all older oceanic crust was created during the cretaceous constant polarity interval the best way to distinguish oceanic crust from thinned continental crust appears to be the crustal seismic velocity structure therefore in 1986 a series of seismic refraction lines was shot parallel to and normal to the continental margin these lines enabled us to bracket the location of the ocb a further constraint on the location was obtained by modelling an east west magnetic profile which included the enigmatic j anomaly this anomaly can be explained as either just pre anomaly mo or as part of the cretaceous constant polarity interval depending on whether spreading began about 127 or after 118 myr ago respectively the evidence favours the former explanation lastly the depth to acoustic basement was contoured from a compilation of seismic reflection profiles this indicated a new fracture zone at 41 15 n which offsets the ocb a few key reflection profiles also suggest that the ocb can be identified by an abrupt landward step down in acoustic basement we conclude that the ocb in the eastern iberia abyssal plain lies between 12 10 and 12 30 w and has a trend just east of north this westerly location is consistent with recent estimates of the location of the ocb off the grand banks but brings into question the proposed location at about 11 w of the ocb in the tagus abyssal plain

discriminating hydrothermal and terrigenous clays in the okinawa trough east china sea evidences from mineralogy and geochemistry abstract the okinawa trough ot in the east asian continental margin is characterized by thick terrigenous sediment and ubiquitous volcanic hydrothermal activities in this study the clays collected during iodp expedition 331 to the middle ot iheya north knoll were analyzed for mineralogical and geochemical compositions by comparing with the clays from the east china sea shelf and surrounding rivers we examine different clay origins the hydrothermal field in the mid ot is dominated by mg rich chlorite while the recharge zone has clay mineral assemblages similar to the shelf and rivers showing high content of illite subordinate chlorite and kaolinite and scarce smectite compared to the terrigenous clays the hydrothermal clays in the ot have high concentrations of mg mn and zr but low fe na k ca ba sr p sc and ti while the hydrothermal clays in the mid ocean ridge are relatively enriched in fe and v and depleted in al mg zr sc and ti different fractionation patterns of rare earth elements also register in the terrigenous and hydrothermal clays diagnostic of variable clay origins we infer that the ot hydrothermal clay was primarily formed by the chemical alteration of detrital sediments subject to the hydrothermal fluids the remarkably different compositions of hydrothermal clays between the sediment rich back arc basin like ot and the sediment starved ocean ridge suggest different physical and chemical processes of hydrothermal fluids and fluid rock sediment reactions under various geologic settings

deep reaching liquefaction potential of marine slope sediments as a prerequisite for gravity mass flows results from the dsdp abstract plots of water content w and atterberg liquid limits wl versus depth coupled with measurements from young submarine mass flows support the concept that many fine grained but low cohesive slope sediments are susceptible to liquefaction and can as a result of dynamic loading earthquakes and storm waves be transformed into mud flows without the uptake of additional water the in situ water content w can be higher or approximately equal to wl boundary between the plastic and liquid state of a soil usually in the uppermost few meters but frequently also tens or even some hundreds of meters below the sea floor such a substantial liquefaction potential is particularly common in sediments rich in silt size biogenic particles and in regions of high sedimentation rates causing underconsolidation it is not clear whether changes in the salt concentration and chemistry of interstitial water versus depth in core exert some influence on wl and the liquefaction potential the average field liquid limit of the total mass flow can be even lower than wl of small only fine grained homogenized samples if part of the dislocated material is carried unchanged as lumps of mud by a liquefied matrix w and wl often decrease in a similar way versus depth in sediments rich in microfossils due to the onset of slight chemical diagenesis with increasing burial depth thus in spite of increasing compaction a high liquefaction potential may be maintained until finally substantial lithification reduces w to values lower than the plastic limit wp and completely alters all the mass physical properties

evidence for transform margin evolution from the ivory coast ghana continental margin the relative motions between lithospheric plates can be reduced to three major types convergence divergence and transform motion convergence leads to an active margin and eventual collision divergence results in the construction of a rifted passive margin transform motion generates a transform or sheared margin although passive margins have been extensively studied1 2 and many models have been proposed for their origin and subsequent evolution3 5 little is known about transform margins with the exception of a few studies of their morphologies6 7 shallow structures or crustal sections8 9 here we present the main conclusions derived from a recent study of the northern gulf of guinea margins particularly off the eastern ivory coast and ghana where the continental margin is one of the best preserved examples of an extinct transform margin10 12 our observations support a four stage model for transform margin evolution technically active transform contacts first between normal continental crusts and then between thinned margins induce characteristic structures such as pull apart grabens and shear folds the next stage in which thermal exchange between oceanic and continental lithos pheres controls a complex subsidence is followed by the transition to a true intra oceanic fracture zone

consolidation characteristics of sediments from the peru chile continental margin and implications for past sediment instability abstract one dimensional consolidation tests were performed on eight near surface samples 0 5 to 3 5 m from the peru chile continental margin these tests show that the consolidation behavior of these deposits is strongly influenced by the presence of high concentrations of organic matter resulting from the occurrence of coastal upwelling along this margin of south america organic carbon concentrations as high as 12 are found in samples from an upper slope mud lens between 10 5 to 13 6 s that lies beneath an area of intense upwelling with increasing organic content both the compressibility and rate of secondary compression are found to increase the increase in void ratio with increasing organic content and the ease by which organic matter is deformed appear responsible for these trends outside the mud lens the compressibility also increases as a result of increasing concentrations of clay size material and the abundance of smectite and illite all of the sediments along the margin behave as if they were consolidated under a pressure greater than the present effective overburden pressure evidence of erosion as a cause of the overconsolidation is present only on the upper slope mud lens at 13 6 s where 14 c dating verifies the erosion of 3 5 to 7 m of sediment numerous unconformities and intervals of deformed bedding in a core from 13 6 s attest to the past instability of sediments at this site at the other locations along the margin the increase in the overconsolidation ratio with increasing organic carbon content suggests that the bonding of sediment particles by organic matter may be responsible for the apparent overconsolidation low sedimentation rates on the slope off northern chile may also be a contributing factor to the apparent overconsolidation of sediments in this area

the physical properties of peru chile continental margin sediments the influence of coastal upwelling on sediment properties physical properties sediment texture water content wet bulk density grain specific gravity porosity shear strength and atterberg limits were determined for near surface sediments 0 to 4 m from the peru chile continental slope and eastern nazca plate these properties are strongly affected by organic matter which is concentrated in the slope deposits as a result of the high productivity in coastal upwelling areas off peru sediments with unique properties occur in a lens of organic rich mud on a portion of the upper slope 10 5 degrees to 13 6 degrees s which coincides with an area of intense upwelling the mud lens sediments are anomalously fine grained and have the highest water content and plasticity and lowest wet bulk density and grain specific gravity found along the margin variation of these properties on the mud lens and on the remainder of the slope and nazca plate correlates well with changes in organic content the undrained shear strength of the mud lens sediments is higher than might be expected for their high water content possibly resulting from the development of clay organic aggregates a distinctive feature of the upper slope mud lens is a surface layer 0 to 15 cm characterized by organic carbon concentrations of up to 20 percent dry weight extremely high water content maximum 853 dry weight and very low wet bulk density minimum 1 09 mg m 3 outside the mud lens organic matter is less abundant and the amount of clay size material in the sediment exerts a greater influence on the physical properties progressive fining of the sediments from south to north along the margin and with increasing distance from shore is accompanied by an increase in water content and plasticity and a decrease in wet bulk density and shear strength these sediments typically show less change in physical properties with depth of burial than those of the upper slope mud lens differences in sediment mineralogy and sedimentation rate also affect the physical properties but to a lesser extent than the variation of organic content or amount of clay

87sr 86sr ratios in hydrothermal waters and deposits from the east pacific rise at 21 n abstract 87 sr 86 sr ratios of three hydrothermal waters collected on the east pacific rise at 21 n define a mixing line between seawater and a hydrothermal end member at 0 7030 which is derived by seawater basalt interaction at ca 350 c and water rock ratio of about 1 5 sr concentrations are not affected in the process while mg uptake from seawater is almost complete up to2 3 of this hydrothermal component is involved in anhydrite precipitation while the sr isotopic ratio in sulfides chalcopyrite sphalerite cannot be distinguished from that of sulfate it is estimated that ca 1 10 10 moles of strontium are yearly cycled in the hydrothermal systems of mid oceanic ridges thereby affecting the 87 sr 86 sr budget of seawater mass balance between river runoff limestone precipitation and ridge basalt alteration suggests that the 87 sr 86 sr ratios of the river runoff are in the range 0 7097 0 7113 and are largely dominated by limestone alteration

the ocean continent boundary off the western continental margin of iberia crustal structure west of galicia bank a seismic refraction transect across the galicia bank continental margin shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin parallel peridotite ridge pr immediately west of the pr oceanic crust is only 2 5 3 5 km thick but farther west oceanward it thickens to 7 km the pr caps a similar to 60 km wide lens shaped serpentinized peridotite body underlying both thinned continental and thin oceanic crust when superimposed on a reflection time version of the velocity model the s reflector is clearly intracrustal at its east end westward s cuts down to lower crustal levels eventually coinciding with the top of the serpentinized peridotite lens original crust mantle boundary these observations render almost impossible the seafloor exposure of the pr by s acting as a top to the west detachment fault numerical models of melting and borehole subsidence information constrain our rifting model the easternmost continental crust experienced a total stretching factor of 4 3 most likely in two stages it probably occurred over similar to 25 m y with the highest fate of stretching at the beginning of the main earlier rift phase valanginian 141 135 ma the 3 4 7 km thick continental crust depending on whether serpentinized peridotite is assigned to crust or mantle which may include melt products requires stretching factors 6 more than 11 7 and a rift duration of more than 25 13 m y the thin oceanic crust immediately west of the pr is explained by conductive cooling of the mantle during the long prebreakup stretching phase which temporarily caused reduced melting immediately after breakup

the origin of hydrothermal chlorite and anhydrite rich sediments in the middle okinawa trough east china sea abstract during the integrated ocean drilling program iodp expedition 331 five sites were drilled into the iheya north knoll hydrothermal system in the okinawa trough ot a back arc basin characterized by thick terrigenous sediment following up on the previous study by shao et al 2015 we present new mineralogical geochemical and sr nd isotope data to investigate the origin of the hydrothermal sediments and characterize the hydrothermal system the substrate at the iheya north knoll is dominated by pumiceous sediment and other volcanoclastic materials interbedded with hemipelagic terrigenous and biogenous sediments impermeable layers separate the hydrothermal sediments into distinct units with depth that are characterized by various assemblages of alteration materials including polymetallic sulfides sulfates chlorite and kaolinite rich sediments the rare earth elements rees and nd isotope data suggest that the chlorite rich and kaolinite rich layers primarily resulted from the alteration of pumiceous materials in different chemical and physical conditions kaolinite rich sediment likely reflects low ph and low mg concentration fluids while chlorite rich sediment formed from fluids with high ph and increased mg contents likely at higher temperatures the sr isotopic compositions of subsurface anhydrite reflect high seawater hydrothermal fluid ratios in the mid ot hydrothermal area compared with chlorite rich sediments from other sediment covered or felsic hosted hydrothermal systems the chlorite rich sediments in the mid ot are characterized by lower concentrations of al and fe but much higher y zr hf th and rees indicative of the distinct nature of the precursor rocks in this region

arctic ocean gravity field derived from icesat and ers 2 altimetry tectonic implications a new detailed marine gravity field for the persistently ice covered arctic ocean derived entirely from satellite data reveals important new tectonic features in both the amerasian and eurasian basins reprocessed geoscience laser altimeter system glas data collected by nasa s ice cloud and land elevation satellite icesat between 2003 and 2005 have been combined with 8 years worth of retracked radar altimeter data from esa s ers 2 satellite to produce the highest available resolution gravity mapping of the entire arctic ocean complete to 86 degrees n this arctic satellite only arcs marine gravity field uniformly and confidently resolves marine gravity to wavelengths as short as 35 km arcs relies on a gravity recovery and climate experiment grace only satellite gravity model at long 580 km wavelengths and plainly shows tectonic fabric and numerous details imprinted in the arctic seafloor in particular in the enigmatic amerasian basin ab for example in the makarov basin portion of the ab two north south trending lineations are likely clues to the highly uncertain seafloor spreading history which formed the ab

reinterpretation of morphology and crustal structure in the central arctic ocean basin ice station bathymetric profiles and submarine echograms reveal a steep escarpment at least 200 km long with up to 1700 m of relief subparallel to the lomonosov ridge this feature formerly called the marvin spur is here interpreted as the makarov facing flank of the alpha ridge east of the 160 w meridian the intervening makarov basin is a narrow grabenlike trough between 25 and 60 km wide filled with horizontally stratified sediments that were deposited largely after the marvin seamounts were constructed along the basin axis the association of basement relief with geomagnetic field variations within the trough plus evidence of normal faulting on the lomonosov ridge suggest that this segment of the makarov basin is an extensional feature produced by other than typical seafloor spreading processes gravity models indicate that the crust below the makarov flank of the alpha ridge ranges in thickness between 16 and 20 km considerably less than the 26 km known from seismic refraction work to exist below the lomonosov ridge it is argued on the basis of gravity data that adjacent to the lomonosov ridge the fram basin crust which is probably truly oceanic is 3 4 km thinner than that of the makarov basin

back arc extension in the okinawa trough the okinawa trough lying to the east of china is a back arc basin formed by extension within continental lithosphere behind the ryukyu trench arc system middle to late miocene uplift associated with normal faulting of the initially adjacent ryukyu nonvolcanic arc and the taiwan sinzi folded belt corresponds to the first rifting phase the timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern okinawa trough the rifting occurred after a major early miocene change in the motion of the philippine plate with respect to eurasia and ceased during the pliocene a second rifting phase started about 2 m y ago at the plio pleistocene boundary and has continued until the present time it has proceeded to a more advanced stage in the middle and southern okinawa trough than it has farther north detailed bathymetric sea beam seismic reflection and magnetics data collected during the pop 1 cruise of the r v jean charcot reveal the principal features of the extensional processes the back arc spreading phase started very recently in the southern and middle okinawa trough as exemplified by several en echelon and in some cases overlapping active central graben oriented n70 e n80 e some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies transform faults between these en echelon active rifts are not obvious we suggest that the major part of the southern okinawa trough is underlain by a thinned continental crust and that except for the system of en echelon rifts of the southern okinawa trough the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough the system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of okinawa island whereas active volcanoes in the ryukyu arc occur only north of okinawa island we refer to this transition between active arc and back arc volcanism as the volcanic arc rift migration phenomenon vamp globally back arc volcanism propagated from the southern okinawa trough to the okinawa vamp area rifting continues to occur in the northern okinawa trough but is not yet accompanied by associated volcanism the okinawa vamp area is characterized by a series of parallel basaltic ridges oriented n75 e with associated linear magnetic anomalies characteristic of dyke intrusions we suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending n75 e and that this zone presently ends at the southern extremity of the active volcanic chain the initial phase of formation of back arc basin oceanic crust is non steady state and is characterized by the lack of a developed fracture zone pattern the termination of the vamp area in the direction of the volcanic zone of the arc is consistent with the suggestion of molnar and atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc spreading occurs apparently back arc extension initially occurred within the continental lithosphere located westward of the ryukyu arc along its whole length but the subsequent back arc volcanism was initiated in the southernmost portion of the region and then moved northward this migration was accompanied by the shutting down of volcanic activity along the abandoned portions of the arc it is this transfer of volcanism that we call the vamp process thus arc and back arc basin volcanism seem to be associated in such a manner that spreading tends to migrate simultaneously with a cessation in volcanic activity along the arc this interplay of arc and back arc activity is probably linked to changes in the parameters of plate convergence since the plate motion in the phillippine sea is oblique to the trench at least in the southern part of the okinawa trough we suggest that the oblique resisting force applied to the edge of the overriding plate engenders the development of en echelon extensional features within and behind the arc the motion of the ryukyu platelet with respect to eurasia is consequently an extensional strike slip motion trending roughly parallel to the okinawa trough

sr isotope evolution of seawater the role of tectonics we use a high resolution seawater sr isotopic evolution curve for the last 100 m y in conjunction with modern riverine sr flux measurements and also geologic tectonic and geochronological data to make the case for a close relationship between seawater sr isotopic composition and the india asia continental collision using a simple seawater sr budget model we begin by showing that the sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater sr isotopic curve of the last 100 m y the flux of dissolved sr carried by rivers originating in the himalaya tibet region on the other hand is presently a significant fraction of the global sr budget we calculate how this riverine flux would have had to change with time in order to match the observed seawater sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of india with asia but then increases very significantly after collision we note that the period of most rapid change in seawater sr isotopic ratio from 20 ma to 15 ma is also a period of exceptionally high erosion in parts of the himalayas and the tibetan plateau as further evidence that sr derived from the collision of india with asia plays a major role in the sr isotopic evolution of seawater we show that the total amount erosion of the himalaya tibetan plateau since collision which we calculate separately represents a total amount of sr that is very nearly the same as the cumulative amount required by the sr isotopic change of seawater since collision the relationship between erosion and riverine sr flux allows us to use the sr isotopic evolution of seawater to reconstruct a history of erosion since collision and we find that the erosion rate accelerates with time since collision with the present having the largest rate when we apply the sr budget model to the entire phanerozoic using a new compilation of deformed continental area versus time we find that we can account for the large scale structure of the seawater sr isotopic curve but fail to reproduce several local maxima and minima especially in the period 100 300 ma the present high s7sr s6sr of seawater and similar highs in the devonian and cambrian do correlate with extensive deformation on the continents

early diagenesis impact of organic matter on mass physical properties and processes california continental margin abstract high resolution scales of 2 5 mm to submicrometer interdisciplinary studies of california continental slope sediments 0 80 cm reveal the importance of marine organic material and mineralogy on mass physical properties in fine grained sediments during very early diagenesis the combination of organic material 2 wt with smectite 5 is associated with high depositional porosities low effective stresses low porosity reduction and dewatering and the impedance of interstitial fluid flow living microbiota and nonliving mucopolysaccharides organic matter affecting the physical properties was revealed by transmission electron microscopy and carbon isotope studies calculations using the redfield ratio show that sediments containing high organic carbon 5 wt have hydrated organic material in the pore spaces that can occupy a volume that is up to 20 of the total sediment porosity pore water chemistry and x ray diffraction show evidence of early diagenetic processes that may significantly influence sediment fabric an important geochemical process near the sediment water interface that affects sediment fabric is the rapid dissolution of biogenic opal

late cretaceous paleogene paleogeography and paleocirculation evidence of north polar upwelling abstract quantitative determination of biogenic silica in late cretaceous and paleogene deep sea sediment cores from the central arctic ocean provides evidence of open ocean polar upwelling the timing of polar upwelling coincides with periods of a weakened meridional thermal gradient suggesting that heat transport to the poles by oceanic circulation may have been important the timing of biogenic silica deposition in the arctic precedes its deposition in both the norwegian greenland sea of the north atlantic and he bering sea of the north pacific tectonic events may be resposnible for the timingg and sitting of sites of deposition of biogenic silica in high northern latitudes particularly the tectonic evolution of sites of deep water exchange between the arctic and the world ocean we outline three phases in the post mid cretaceous history of silica deposition in high northern latitudes during phase i the arctic is a silica sink with deep water formation but with no deep water outflow the transition to phase ii is brought about by opening of the svalabard greenland strait to deep water outflow from the arctic to the norwegian greenland sea the transition to phase iii is initiated by submergence of the faroe iceland ridge and deep water outflow from the arctic to the north atlantic climatic conditions in the arctic during late cretaceous and paleogene time are predicted to have favored open ocean upwelling due to a circulation pattern dominated by cyclonic conditions resulting from the establisment of a semi permanent atmospheric low over the alpha ridge bathymetry of the alpha ridge may have intensified paleo upwelling

in situ water contents liquid limits and submarine mass flows due to a high liquefaction potential of slope sediment results from dsdp and subaerial counterparts water content depth curves and atterberg liquid limits support the conception that many finegrained but lowcohesive thick slope sediments are susceptible to liquefaction and therefore can as a result of dynamic loading earthquakes storm waves be transformed into mud flows without the uptake of additional water usually in the uppermost meters but frequently also tens or even hundreds of meters below the sea floor the in situ water content can be higher or approximately equal to the liquid limit boundary between the plastic and liquid state of a soil under these conditions the sediments have a high liquefaction potential this situation is particularly common in regions of high sedimentation rates causing underconsolidation as well as in sediments rich in silt size biogenic particles whether depthrelated changes in the salt concentration of interstitial waters exert some influence is not clear if part of the material is carried unchanged as mud lumps by a liquified matrix the average field liquid limit of the total mass flow can be even lower than the atterberg liquid limit of small only finegrained homogenized samples

morphology and structure of the lomonosov ridge arctic ocean the lomonosov ridge is a band of continental crust that stretches across the arctic ocean and separates the mesozoic amerasian basin from the cenozoic eurasian basin from about 87 n north of greenland across the pole to about 86 n the lomonosov ridge is a single highstanding blocky ridge with minimum depths of 950 1400 m south of 86 n on the siberian side the ridge breaks up into a series of ridges spread over a width of about 200 km in this region a highstanding blocky ridge with minimum depths of 650 1400 m bounds the eurasian basin and continues to the siberian continental margin this ridge is continuous with the single ridge making up the lomonosov ridge toward north america and is the former outermost continental shelf of eurasia bounding the amerasian basin the eurasian basin margin of the lomonosov ridge consists of a series of rotated fault blocks stepping down to the basin that result from nearly orthogonal rifting to form the eurasian basin no rotated fault blocks are observed on the amerasian basin margin of the lomonosov ridge on the amerasian basin side marvin spur a linear ridge separated from lomonosov ridge by a deep basin parallels lomonosov ridge on the north american side of the pole at the bend in the lomonosov ridge near the north pole marvin spur continues along strike across the makarov basin south of 86 n toward siberia a continuous outer ridge makes up the amerasian basin edge of the lomonosov complex with a series of basins and ridges between it and the former eurasian shelf the outer ridge marks an abrupt boundary between the lomonosov ridge complex and the apparently oceanic crust of the makarov basin the outer ridge and marvin spur very closely follow small circles about a pole located on the mackenzie delta the observed structure on the amerasian basin side of the lomonosov ridge is analogous to that observed at well studied shear margins and supports rotational models for the development of the amerasian basin

petrology and major element geochemistry of peru margin phosphorites and associated diagenetic minerals authigenesis in modern organic rich sediments abstract new petrographic and major element geochemical data from modern peru margin upper slope outer shelf phosphorites are presented which provide insight into their origin and paragenetic relationship with other authigenic minerals glauconite pyrite and dolomite occurring in organic rich sediments glauconites are precipitated relatively early following the partial reduction of ferric iron and following this process phosphate pyrite and then dolomite precipitation take place at progressively deeper levels in the sediment in association with microbial reduction of sulfate as in many ancient economic phosphorite deposits the phosphatic facies here consist of nodules crusts coatings and strata composed of phosphatic pelletal grains ooids structureless grains intraclasts clumps and biogenic grains in association with organic rich biosiliceous sediments all are considered to have formed within a few centimeters or within a few tens of centimeters below the sediment water interface important factors that influence which morphology will tend to develop include the amount of available pore space the presence of suitable nucleation sites the amount and size of siliciclastic detritus incorporated as inclusions and the relative solution chemistries of the precipitating solutions bacterial mediation may play an important but as yet unspecified role in the precipitation process textural data and factor analysis of chemical data suggest that structureless pellets are relatively inclusion free na f mg co3 substituted pore water precipitates whereas ooids are inclusion rich pore water precipitates poor in lattice substituted components variations in nodular cement birefringence and crystallinity are suggested to have been produced by similar lattice substitutions that directly reflect pore water carbonate ion concentrations and thus relative degrees of organic matter degradation phosphate and dolomite are intimately mixed yet mineralogically distinct phases in phosphatized dolomicrites depth stratified threshold carbonate ion concentrations may control the lower limit at which phosphatic minerals may precipitate below depths of a few centimeters excessive carbonate ion concentrations and diminished reactive iron and sulfate concentrations favor the development of dolomite while precluding further development of phosphatic minerals and pyrite periodic sediment reorganization bioturbation current winnowing and erosion mass wasting etc plays an important role in both concentrating pelletal grains and maintaining nodules and crusts at critical depth levels in the sediment as well as mixing ordered mineral parageneses into complicated sequences

the okinawa trough genesis of a back arc basin developing along a continental margin abstract the okinawa trough is the only example of a marginal back arc basin type at a young stage of evolution that developed along a continental margin the first phase of crustal stretching and subsidence occurred during late miocene early pliocene times a second major phase occurred after the latest pliocene or early pleistocene with en echelon rift axes along the axes of the okinawa trough oblique to the general trend of the arc and trench crustal separation and active spreading has occurred in the central and southern okinawa trough but the north okinawa trough is only in a rifting stage the development of the basin has several correlations with the general tectonism of the arc system 1 1 there was a synchronism between opening and subsidence of the okinawa trough and tilting and subsidence of the fore arc terrace 2 2 back arc rifting was initiated in the volcanic arc and propagated along it during the neogene remnants of a volcanic arc are found on the continental side of the basin the beginning of the back arc spreading observed in the central and southern okinawa trough correspond to a relative minimum in the activity of the volcanic arc 3 3 the timing and kinematic evolution of the taiwan collision and the south okinawa trough opening suggest a connection between these two events such a collision lateral back arc opening model could explain the initiation of the opening of back arc basins but additional phenomena such as thermal convection retreating trench model or anchored slab model are indispensable to maintain extension in the back arc basin

mg rich clay mineral formation associated with marine shallow water hydrothermal activity in an arc volcanic caldera setting abstract shallow water hydrothermal activity represented by venting of hydrothermal fluid around 200 c occurs in the wakamiko submarine crater at 200 m water depth in kagoshima bay southwest japan the crater is the center of large eruptions that formed a volcanic caldera which is semi submerged at present the crater is covered with thick volcanic sediments of felsic composition we studied the distribution and chemical composition of hydrothermal clay minerals that are abundant in the sediment collected by piston coring we also conducted chemical analysis of pore fluids squeezed from the sediment to understand hydrothermal interactions that resulted in formation of these clay minerals the pc 2 core 340 cm in length collected in the vicinity of a high temperature fluid venting site was characterized by abundant mg saponite that is limited to a layer between 270 and 300 centimeters below the seafloor cmbsf and montmorillonite throughout the core below 55 cmbsf vertical profiles of pore fluid chemistry suggest that saponite formation is related to the interface between the seawater and the hydrothermal component in the sediment layer formation temperatures of the montmorillonite were estimated to be 118 163 c based on oxygen isotope thermometry formation of the montmorillonite is attributed to hydrothermal interaction between seawater dominant pore fluid and volcanic glass the formation temperature of the saponite was estimated to be 164 c based on oxygen isotope thermometry formation of the saponite is attributed to hydrothermal interaction between seawater dominant pore fluid and the montmorillonite which had been formed at a prior stage the pc 1 core 240 cm in length collected from a relatively low temperature fluid shimmering site was characterized by the occurrence of kerolite in the lower section 210 240 cmbsf vertical profiles of pore fluid chemistry suggest that the kerolite formation occurred at the interface between seawater and the hydrothermal component of the sediment layer formation temperature of the kerolite was estimated to be about 211 c based on oxygen isotope thermometry formation of the kerolite is attributed to precipitation from a fluid that was a mixture of a hydrothermal component and seawater this study revealed the occurrence of mg rich clay minerals saponite and kerolite beneath a submarine hydrothermal field that developed within sediment of felsic composition during hydrothermal interactions that formed these clay minerals seawater penetrated into the sediment and was an important mg source formation of mg rich clay minerals saponite and kerolite are controlled by pore fluid chemistry which varies from a seawater dominant to hydrothermal dominant component exclusive formation of mg rich clay minerals at different sites could be explained by different water rock ratios of the hydrothermal interaction saponite formation at low water rock ratio and kerolite precipitation at high water rock ratio occurrence of mg rich clay minerals provides clues to the hydrological structure in sediment covered hydrothermal systems in an arc volcanic caldera setting

mesozoic stratigraphy of canadian arctic archipelago and implications for opening of amerasian basin abstract mesozoic strata in the canadian arctic archipelago occur mainly in the sverdrup basin where the succession is up to 9 km thick the strata consist of alternating sandstone dominant units and argillaceous intervals and record 25 regional transgressions and regressions these resulted from the interplay of sedimentation tectonics and eustatic sea level changes subaerial unconformities occur on the basin margins and disappear basinward most of these unconformities are interpreted to be the product of eustatic sea level fall on the basis of correlation with global sea level charts however unconformities of latest aalenian late callovian hauterivian and coniacian ages are interpreted to be related to tectonic activity and the formation of the amerasian basin normal faulting and basic dike and sill intrusion occurred in the sverdrup basin from middle jurassic to early late cretaceous the latest aalenian and late callovian unconformities probably reflect early rifting events hauterivian uplift was widespread and coincides with the final phase of rifting and initiation of sea floor spreading in the amerasian basin marine onlap across the breakup unconformity began in barremian time and by albian much of the arctic archipelago had been transgressed basalt flows are intercalated with barremian to turonian strata in the northeastern sverdrup basin and represent the cratonward extension of the alpha mendeleyev volcanic ridge which was active during the opening of the amerasian basin coniacian uplift coincided with the cessation of volcanism in the sverdrup basin and of sea floor spreading in the ocean basin to the north end of article last page 253

the crustal structure across the transform continental margin off ghana eastern equatorial atlantic forward modeling of a suite of wide angle seismic lines across the transform continental margin at the eastern end of the romanche fracture zone off ghana has shown the transition from continental to oceanic crust to be confined to a narrow 6 to 11 km wide zone located at the foot of the steep continental slope the structure of the adjacent oceanic and continental crusts has also been resolved these results are confirmed and enhanced by gravity and magnetic models the crust of the ocean continent transition zone is characterized by high velocities 5 8 7 3 km s 1 a high density 3 10 mg m 3 and high magnetizations 1 1 1 25 a m 1 this rules out a purely continental origin for the zone and suggests that it may be formed of basic igneous rocks intruded when the hot oceanic spreading center migrated along the margin the oceanic crust within 70 km of the margin is abnormally thin compared to normal atlantic oceanic crust and shows an average thickness of just 4 4 km we suggest that the region of abnormally thin oceanic crust is the result of a reduced magma supply due to a combination of closely spaced fracture zones and the conductive loss of heat from the upwelling oceanic mantle in small basins surrounded on at least three sides by cold continental lithosphere the preferred seismic model also shows a new fracture zone 30 km southeast of the ocean continent transition which is characterized by low velocities in the upper crust a lens shaped layer with velocities of 7 2 7 4 km s 1 at the base of the crust and a crustal thickness of just 3 4 km the continental crust appears largely unaffected by the proximity of the adjacent oceanic crust there is no evidence for underplating of the continental crust adjacent to the ocean continent transition zone

submarine hydrothermal mineralization in the okinawa trough sw of japan an overview abstract the okinawa trough is a heavily sedimented rifted back arc basin formed in an intracontinental rift zone submarine hydrothermal activity is located within the six back arc rifts located in the middle and southern okinawa trough and its distribution is controlled principally by tectonic factors subduction of the daito and gagua ridges beneath the ryukyu arc has resulted in fracturing of the brittle lithosphere beneath the okinawa trough hydrothermal activity is strongest in the volcanic arc rift migration phenomenon vamp area plus the jade site and southernmost part of the okinawa trough spot area which form the prolongation of these two ridges these areas are characterized by extremely high heat flow locally submarine hydrothermal fluids from the okinawa trough tend to be strongly influenced by interaction of the hydrothermal fluids with organic matter in the sediment resulting in high alkalinity and nh 4 concentrations of the fluids the fluids also contain high concentrations of co 2 of magmatic origin submarine hydrothermal mineralization in the trough is diverse the clam site consists principally of carbonate chimneys interaction of the hydrothermal fluid with organic matter in the sediment is particularly strong at this site this is most probably a sediment hosted deposit in which sulphide minerals have deposited within the sediment column leaving spent ore fluids to emerge at the seafloor the jade site consists of active and inactive sulphide sulphate chimneys and mounds the zn pb rich sulphides at this site contain the highest concentrations of pb ag and au so far recorded in submarine hydrothermal sulphide deposits at minami ensei knoll and hatoma knoll active and inactive chimneys consist principally of anhydrite and barite as a result of phase separation of the hydrothermal fluids beneath the seafloor an intense black smoker has recently been discovered at yonaguni knoll in the spot area if it is confirmed that sulphide mineralization is dominant at this site this could be a highly prospective area the most prospective areas for economic grade minerals in the okinawa trough appear to be the jade site and the spot area

comments about the age of the canada basin abstract the canada basin appears to have formed over a 60 ma interval during the cretaceous period the opening process can be divided into two parts continental breakup widespread in hauterivian through aptian time about 113 131 ma ago along what was about to become the polar margin of north america and seafloor formation active during the extended cretaceous interval of normal geomagnetic polarity to marine magnetic anomaly 33 time about 118 79 ma ago presently available age controls indicate that all parts of the north american polar margin could have formed at about the same time two styles of margin formation are evident from brock to axel heiberg island there appears to be an extended 70 ma stratigraphic hiatus and northeastward from ellef ringnes island extensive tholeiitic igneous activity associated with continental breakup from banks island to northeastern alaska the breakup interval is abbreviated 20 30 ma and few igneous rocks are observed a progressive difference may be indicated in the amount and or rate of crustal stretching during margin formation with relatively less slower stretching having occurred along the segment southwest of brock island and relatively more faster stretching having taken place along the segment to the northeast

insight into the nature of the ocean continent transition off west iberia from a deep multichannel seismic reflection profile a high quality multichannel seismic reflection profile across the west iberia margin confirms the existence of a 130 km wide transition zone between tilted continental fault blocks and unequivocal oceanic crust the zone is characterized by a 1 0 2 5 km thick seismically unreflective upper basement layer lying over reflective basement that contains major landward dipping reflectors that mostly truncate against the base of the upper layer and penetrate to depths of 10 19 km below the basement surface and minor seaward dipping reflectors moho reflections are absent in this zone we suggest that the unreflective layer represents tectonically exposed upper mantle that has undergone extensive serpentinization enhanced by vigorous seawater circulation about 40 km farther north the transition zone narrows to 35 km and the unreflective layer is less continuous and widespread

seafloor hydrothermal clay alteration at jade in the back arc okinawa trough mineralogy geochemistry and isotope characteristics seafloor hydrothermal activity at jade has resulted in extensive alteration of the host epiclastic sediments and pumiceous tuffs forming mica kaolins kaolinite and halloysite mg rich chlorite talc montmorillonite and a mixed layer mineral of dioctahedral chlorite and montmorillonite chl mont clay mineral assemblages show a vertical variation which reflects variable amounts of cold seawater incorporated into hot hydrothermal fluids in subsurface sediments and tuff however mixing alone cannot explain the occurrence of abundant kaolin minerals at jade the formation of kaolin minerals requires much more acidic fluid than expected from simple mixing of hydrothermal fluids and cold seawater low ph values are likely attained by oxidation of h2s either dissolved in the hydrothermal fluid or released from the fluid during decompression the fluid reaching the seafloor is discharged into cold seawater which caused precipitation of sulfides close to vents and native sulfur and barite at the margins of the vent areas halloysite barite and anhydrite show sr isotope compositions similar to marine sr indicating the derivation of marine sr directly from seawater or by the dissolution of calcareous nannoplanktons the isotopic compositions of kaolinite 18o 7 4 d 23 chl mont 18o 7 0 d 32 and mica 18o 5 4 to 9 9 d 30 to 26 suggest fluids of a heated seawater origin the o isotopic data yielded formation temperatures of 170 c for kaolinite 61 to 110 c for halloysite and 145 to 238 c for mica barite 34s values 21 0 to 22 5 are very similar to the marine sulfate value confirming that the barite formation took place due to mixing of ba bearing hydrothermal fluids and sulfate rich seawater native sulfur shows a large variation in 34s in one hand specimen probably because of rapid disequilibrium precipitation of s during fluid exhalation on the seafloor sulfur in hydrothermal fluids is usually consumed to form metal sulfides therefore abundant native sulfur at jade suggests high h2s metals ratios of the hydrothermal fluids the alteration assemblages and isotopic data of hydrothermal minerals from jade are very similar to those of kuroko type barite deposits of middle miocene age which formed from fluids of high s metals ratios at less than 200 c at jade there is only one black smoker actively discharging high temperature 320 c fluid but there are many fossil sulfide chimneys and mounds in the area the mineralogy and high au and cu in these precipitates suggest highly metalliferous hydrothermal activity in the past these activities likely resulted in discharge of hydrothermal plumes and fall outs of sulfides and sulfates on the seafloor these fall outs were incorporated in sediments far from the vent areas they are now recorded as high metal contents in sediments with no petrographic and mineralogical evidence of in situ hydrothermal activity some are high as 8 100 ppm for cu 12 500 ppm for zn 1 000 ppm for as 100 ppm for ag and 21 000 ppm for pb detrital grains of montmorillonite in such sediments are coated with fe oxyhydroxides during the suspension in seawater before settling on the seafloor the depths of such metal anomalies in sediments suggest high levels of metalliferous hydrothermal activities from 1 800 to 300 ybp

strontium and oxygen isotopic constraints on fluid mixing alteration and mineralization in the tag hydrothermal deposit abstract strontium and oxygen isotopic measurements of samples recovered from the trans atlantic geotraverse tag hydrothermal mound during leg 158 of the ocean drilling program provide important constraints on the nature of fluid rock interactions during basalt alteration and mineralization within an active hydrothermal deposit fresh mid ocean ridge basalt morb with a 87 sr 86 sr of 0 7026 from the basement beneath the tag mound was altered at both low and high temperatures by seawater and altered at high temperature by near end member black smoker fluids pillow breccias occurring beneath the margins of the mound are locally recrystallized to chlorite by interaction with large volumes of conductively heated seawater 200 c the development of a silicified sulfide mineralized stockwork within the basaltic basement follows a simple paragenetic sequence of chloritization followed by mineralization and the development of a quartz pyrite paragonite stockwork cut by quartz pyrite veins initial alteration involved the development of chloritic alteration halos around basalt clasts by reaction with a mg bearing mixture of upwelling high temperature 300 c black smoker type fluid with a minor 300 c interaction between the wallrock and these mg bearing fluids results in the complete recrystallization of the wallrock to chlorite quartz pyrite the quartz pyrite paragonite assemblage replaces the chloritized basalts and developed by reaction at 250 360 c with end member hydrothermal fluids having 87 sr 86 sr 0 7038 similar to present day vent fluids the uniformity of the 87 sr 86 sr ratios of hydrothermal assemblages throughout the mound and stockwork requires that the 87 sr 86 sr ratio of end member hydrothermal fluids has remained relatively constant for a time period longer than that required to change the interior thermal structure and plumbing network of the mound and underlying stockwork precipitation of anhydrite in breccias and as late stage veins throughout most of the mound and stockwork down to at least 125 mbsf records extensive entrainment of seawater into the hydrothermal deposit 87 sr 86 sr ratios indicate that most of the anhydrite formed from 2 1 mixture of seawater and black smoker fluids 65 15 seawater oxygen isotopic compositions imply that anhydrite precipitated at temperatures between 147 c and 270 c and require that seawater was conductively heated to between 100 c and 180 c before mixing and precipitation occurred anhydrite from the tag mound has a sr ca partition coefficient kd 0 60 0 28 2 this value is in agreement with the range of experimentally determined partition coefficients kd 0 27 0 73 and is similar to those calculated for anhydrite from active black smoker chimneys from 21 n on the east pacific rise the 18 o so 4 of tag anhydrite brackets the value of seawater sulfate oxygen 9 5 dissolution of anhydrite back into the oceans during episodes of hydrothermal quiescence provides a mechanism of buffering seawater sulfate oxygen to an isotopically light composition in addition to the precipitation and dissolution of anhydrite within the oceanic basement during hydrothermal recharge at the mid ocean ridges

tracing the chemical evolution of fluids during hydrothermal recharge constraints from anhydrite recovered in odp hole 504b a unique record of the chemical evolution of seawater during hydrothermal recharge into oceanic crust is preserved by anhydrite from the volcanic sequences and sheeted dike complex in odp hole 504b chemical and isotopic analyses 87sr 86sr 18o 34s of anhydrite constrain the changing composition of fluids due to reaction with basalt there is a general trend of decreasing 87sr 86sr of anhydrite corresponding to the minor incorporation of basaltic strontium with depth in the volcanic rocks 87sr 86sr ratios decrease rapidly with depth in the dikes to values identical to host basalt 0 7029 sr ca ratios 0 1 mmol mol suggest that recharge fluids have very low sr concentrations and fluids evolve by first precipitating sr bearing phases before extensive exchange of sr with the host basalt there is a background trend of decreasing sulfate 18o with depth from 12 13 in the lower volcanics to 7 in the lower sheeted dikes recording an increase in recharge fluid temperature from 150 to 250 c and confirming the presence of sulfate in hydrothermal fluids at elevated temperatures from the amount of anhydrite recovered from hole 504b and the amount of seawater sulfur that has been reduced to sulfide a minimum seawater recharge flux can be calculated this value is 4 25 times lower than estimates of high temperature fluid fluxes based on either thermal constraints or global chemical budgets and suggests that there is significant deficit of seawater derived sulfur in the oceanic crust only a minor proportion of the seawater that percolates into the crust near the axis is heated to high temperatures and exits as black smoker type fluids a significant proportion of the axial heat loss must be advected at 200 250 c by sulfate bearing hydrothermal solutions that egress diffusely from the crust these fluids penetrate into the dikes and exchange both heat and chemical tracers without the extensive clogging of porosity by anhydrite precipitation which would halt hydrothermal circulation for any reasonable fluid flux the heating of the major proportion of hydrothermal fluids to only moderate temperatures 250 c reconciles estimates of hydrothermal fluxes derived from thermal models and global geochemical budgets the flux of hydrothermal sulfate would be of a magnitude similar to the riverine input and oxygen isotopic exchange at 200 250 c between dissolved sulfate and recharge fluids during hydrothermal circulation provides a mechanism to continuously buffer seawater sulfate oxygen to the light isotopic composition observed

seabed erosion on the lomonosov ridge central arctic ocean a tale of deep draft icebergs in the eurasia basin and the influence of atlantic water inflow on iceberg motion 1 the submarine lomonosov ridge which bisects the arctic ocean is covered by 450 m of hemipelagic sediments high resolution chirp sonar and side scan data have documented erosion of a section of the ridge here we present multichannel seismic and chirp sonar data which show removal of unconsolidated sediments along the eurasia basin side of the lomonosov ridge where the central ridge is shallower than 1000 m the incomplete erosion of the hemipelagic drape and probable topographic control of ice motion suggest that the deepest draft glacier ice which reached the central arctic ocean from the eurasia basin was in the form of armadas of large icebergs embedded in sea ice rather than a single continuous floating ice shelf massive rapid discharge of glacier ice was probably required to produce the deepest draft 800 m icebergs eastward and northward drift trajectories from a likely iceberg source area on the kara barents sea margin would reflect the relative strength of atlantic inflow through the fram strait however only icebergs with drafts exceeding bathymetric thresholds on the lomonosov ridge would leave an erosive imprint on the seabed advection of atlantic water may have been relatively strong at times of discharge from the saalian ice sheet icebergs exiting the arctic ocean caused massive erosion and redeposition of the hemipelagic sediments on top of yermak plateau

shear margins continent ocean transform and fracture zone boundaries as exploration extends into deeper water it has become more important to understand the nature of the earth s crust beneath offshore sediments that is heat flow and related source rock potential usually require that sediments be deposited over continental crust hence delineating the boundary between oceanic and continental crust becomes important the material in this article was originally presented at the workshop the crust and its structure at seg s recent annual meeting in calgary these results part of a larger dissertation research project also have been presented to several oil and gas exploration companies in houston

shallow structures and evolution of the ivory coast and ghana transform margin abstract seismic profiles across the transform continental margin off the ivory coast and ghana western africa illustrate the structural style resulting from the early cretaceous phase of shear stress which leads to the final separation between the african and brazilian continental margins in this area most of the characteristic tectonic features observed along this portion of margin asymmetric grabens on the ghanean platform folds of the deep ivory coast basin the ivory coast ghana marginal ridge are believed to result from progressive transform contacts between the african and brazilian continents as their margins were created during early cretaceous time a major tectonic unconformity inferred to be of upper albian lower cenomanian age may be a direct consequence of the final separation of the continental margins the later evolution of the transform margin is chiefly explained by thermal subsidence

structure and tectonics of the agulhas falkland fracture zone abstract the 1200 km long agulhas falkland transform developed during the early cretaceous break up of west gondwanaland on the african plate the continent bordering the agulhas fracture zone extends between the agulhas bank and the south tugela reentrant it is divided into four distinct parts mallory trough segment i diaz ridge segment ii east london segment iii and durban segment iv from southwest to northeast each segment differs from the ohters in its physiography and in the nature of the continent ocean crustal boundary in segment i a wedge like eastward narrowing deep water basin separates the steep continental slope from a marginal fracture ridge of probable volcanic origin along the transform fault trace along segment ii the south african continental slope is deeply embayed and a great thickness of sedimentary strata has been ponded behind a buried ridge along the northern side of the fracture zone trace above a probable fragment of oceanic crust representing a northerly extension of the jurassic falkland plateau basin in segment iii the continental margin is steep especially along the middle slope it lies directly along the transform trace but conspicuously rugged along the continental slope due to the effects of submarine canyoning and sediment slumping in segment iv the continent ocean boundary also lies directly along the transform fault trace and the margin shows evidence of seaward tilting above landward dipping faults along the middle or lower slope the segmentation is inherited from jurassic structural fabrics formed prior to early cretaceous transform motion when tectonic rotation of microplates caused rifting and partial oceanization along the future agulhas falkland megafault trajectory

rare earth element abundances in hydrothermal fluids from the manus basin papua new guinea indicators of sub seafloor hydrothermal processes in back arc basins abstract rare earth element ree concentrations are reported for a large suite of seafloor vent fluids from four hydrothermal systems in the manus back arc basin vienna woods pacmanus desmos and susu knolls vent areas sampled vent fluids show a wide range of absolute ree concentrations and chondrite normalized reen distribution patterns lan smn 0 6 11 lan ybn 0 6 71 eu n eu n 1 55 reen distribution patterns in different vent fluids range from light ree enriched to mid and heavy ree enriched to flat and have a range of positive eu anomalies this heterogeneity contrasts markedly with relatively uniform reen distribution patterns of mid ocean ridge hydrothermal fluids in manus basin fluids aqueous ree compositions do not inherit directly or show a clear relationship with the ree compositions of primary crustal rocks with which hydrothermal fluids interact these results suggest that the rees are less sensitive indicators of primary crustal rock composition despite crustal rocks being the dominant source of rees in submarine hydrothermal fluids in contrast differences in aqueous ree compositions are consistently correlated with differences in fluid ph and ligand chloride fluoride and sulfate concentrations our results suggest that the rees can be used as an indicator of the type of magmatic acid volatile i e presence of hf so2 degassing in submarine hydrothermal systems additional fluid data suggest that near seafloor mixing between high temperature hydrothermal fluid and locally entrained seawater at many vent areas in the manus basin causes anhydrite precipitation anhydrite effectively incorporates ree and likely affects measured fluid ree concentrations but does not affect their relative distributions

mineralogy and geochemical behavior of trace elements of hydrothermal alteration types in the volcanogenic massive sulfide deposits ne turkey abstract volcanogenic massive sulfide vms deposits of the eastern pontides turkey are hosted by the maastrichtian eocene dacite and rhyodacite series accompanied by lesser andesite and basalts as well as their pyroclastic equivalents with tholeiitic to calc alkaline affinity the ore mineral assemblages are chalcopyrite sphalerite galena chalcocite covellite bornite and tetrahedrite potassic phyllitic sericitic argillic kaolinitic and smectitic silicic propylitic and hematitic alteration is commonly associated with these deposits hfse lile trte and ree contents show strong variability in different alteration types resulting from interaction with acid or alkaline fluids sample groups showed chondrite normalized enrichment of lree relative to hree and sub parallel trends except for the hematitic and phyllitic alteration types mree are strongly depleted in the zones of most intense silicification and kaolinization most sample groups have strongly to slightly negative eu anomalies ranging from 0 35 to 0 88 mean hematitic 1 45 and propylitic altered rocks 1 11 have slightly to moderately positive anomalies the negative eu anomalies indicate the low temperatures of fluids 200 c no ce anomaly was observed except for phyllitic alteration where a slight positive anomaly was noted the chondrite normalized trace and ree patterns of the altered rocks are similar to each other suggesting that they were derived from a common felsic source the alteration groups formed from acid intermediate and alkaline hydrothermal solutions some transition base and precious metals and volatile elements were clearly enriched especially in the hematitic silicic kaolinitic and phyllitic altered samples the other elements exhibit different behaviors in different sample groups ree behavior is relatively immobile in the silicic hematitic kaolinitic and partially in moderately and propylitic altered rocks based on mass balance calculations lile and hfse appear mobile in the altered sample groups except in the propylitic altered rocks trte behave as relatively immobile in most of samples except in some of the silicic and phyllitic altered rocks and especially in the hematitic altered samples hfse most of the transition w mo cu and sb and some other trace elements pb as hg bi se and tl are enriched in the hematitic altered samples and in the some silicic altered samples the highest as bi mo se and hg concentrations in the hematite altered samples can be used to distinguish other alteration types and may be a useful indicator in a prospect scale base metal exploration

deep water flow over the lomonosov ridge in the arctic ocean the arctic ocean likely impacts global climate through its effect on the rate of deep water formation and the subsequent influence on global thermohaline circulation here the renewal of the deep waters in the isolated canadian basin is quanitified using hydraulic theory and hydrographic observations the authors calculate the magnitude of this renewal where circumstances have thus far prevented direct measurements a volume flow rate of q 0 25 0 15 sv sv 10 6 m 3 s 1 from the eurasian basin to the canadian basin via a deep gap in the dividing lomonosov ridge is estimated deep water renewal time estimates based on this flow are consistent with 14 c isolation ages the flow is sufficiently large that it has a greater impac to n the canadian basin deep water than either the geothermal heat flux or diffusive fluxes at the deep water boundaries

geochemical constraints on the origin of bimodal magmatism at the okinawa trough an incipient back arc basin abstract the magmatism at the axial zone of the middle okinawa trough a young continental back arc basin comprises a bimodal basaltic rhyolitic suite accompanied by minor intermediate rocks we report major and trace element and sr nd isotopie data for the intermediate to silicic suites to provide constraints on their petrogenesis the rhyolites recovered as lava and pumice fall into three geochemical groups type 1 2 and 3 rhyolites type 1 rhyolites have 87sr 86sr 0 7040 0 7042 and 143nd 144nd 0 5128 0 5129 identical to those of associated basalts and are characterized by highly fractionated ree patterns petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt in contrast type 2 rhyolites and andesite have slightly higher 87sr 86sr 0 7044 0 7047 but similar 143nd 144nd 0 5128 compared to those of the basalts the compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization afc processes of the basalt magma quantitative analysis suggests assimilation fractional crystallization ma mc ratios of 0 05 hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present we emphasize that mechanical extension in this part of the okinawa trough involves gabbro ic lower crust that resulted from fractionation of mantle derived basaltic magmas type 3 rhyolite occurs only as pumice which makes its derivation questionable this rhyolite has major and trace element compositions and sr nd isotopie ratios which suggests that it may be derived from volcanic activity on the southern ryukyu volcanic front and arrived in the okinawa trough by drifting on the kuroshio current

on the sr isotope and ree compositions of anhydrites from the tag seafloor hydrothermal system abstract sr isotope analyses have been conducted on anhydrite samples from the tag trans atlantic geotraverse active hydrothermal mound 26 08 n mid atlantic ridge that have previously been shown to exhibit two distinct patterns of ree behavior when normalized to tag end member hydrothermal fluid despite differences in ree patterns the sr isotope data indicate that all the anhydrites precipitated from fluids with a similar range of hydrothermal fluid and seawater components and all but one were seawater dominated 52 75 speciation calculations using the eq3 6 software package for geochemical modeling of aqueous systems suggest that the ree complexation behavior in different fluid mixing scenarios can explain the variations in the ree patterns anhydrites that exhibit relatively flat ree patterns labs ybbs 0 8 2 0 subscript bs indicates normalization to end member black smoker hydrothermal fluid and a small or no eu anomaly eubs eu bs 0 8 2 0 are inferred to have precipitated from mixes of end member hydrothermal fluid and cold seawater ree complexes with hard ligands e g fluoride and chloride are less stable at low temperatures and trivalent eu has an ionic radius similar to that of ca2 and the other ree and so they behave coherently in contrast anhydrites that exhibit slight lree depletion labs ybbs 0 4 1 4 and a distinct negative anomaly eubs eu bs 0 2 0 8 are inferred to have precipitated from mixes of end member hydrothermal fluid and conductively heated seawater the lree depletion results from the presence of very stable lree chloro complexes that effectively limit the availability of the lree for partitioning into anhydrite above 250 c eu is present only in divalent form as chloride complexes and discrimination against eu2 is likely due to both the mismatch in ionic radii between eu2 and ca2 and the strong chloro complexation of divalent eu which promotes stability in the fluid and inhibits partitioning of eu2 into precipitating anhydrite these variations in ree behavior attest to rapid fluctuations in thermal regime fluid flow and mixing in the subsurface of the tag mound that give rise to heterogeneity in the formation conditions of individual anhydrite crystals

lithospheric strength variations as a control on new plate boundaries examples from the northern red sea region abstract the complex plate boundary between arabia and africa at the northern end of the red sea includes the gulf of suez rift and the gulf of aqaba dead sea transform geologic evidence indicates that during the earliest phase of rifting the red sea propagated nnw towards the mediterranean sea creating the gulf of suez subsequently the majority of the relative movement between the plates shifted eastward to the dead sea transform we propose that an increase in the strength of the lithosphere across the mediterranean continental margin acted as a barrier to the propagation of the rift a new plate boundary the dead sea transform formed along a zone of minimum strength we present an analysis of lithospheric strength variations across the mediterranean continental margin the main factors controlling these variations are the geotherm crustal thickness and composition and sediment thickness the analysis predicts a characteristic strength profile at continental margins which consists of a marked increase in strength seaward of the hinge zone and a strength minimum landward of the hinge zone this strength profile also favors the creation of thin continental slivers such as the levant west of the dead sea transform and the continental promontory containing socotra island at the mouth of the gulf of aden calculations of strength variations based on changes of crustal thickness geotherm and sediment thickness can be extended to other geologic settings as well they can explain the location of rerifting events at intracratonic basins of backarc basins and of major continental strike slip zones

pore water fluoride in peru continental margin sediments uptake from seawater abstract fluoride analyses display downward decreasing pore water gradients in peru shelf phosphatic muds that require diffusion from the overlying seawater into the sediment column and removal by reaction within the upper few tens of centimeters presumably by incorporation into carbonate fluorapatite the profiles can be modeled as first order f removal with rate constants of 3 yr 1 and asymptotic f concentrations deep in the cores of 35 45 m almost one half the seawater value the integrated flux of fluoride from seawater into organic rich shelf sediments in coastal upwelling zones phosphatic muds yields a contemporaneous global f burial of 0 54 10 10 mol f yr 1 about one fifth the burial in other sinks mostly carbonates and opal the associated burial flux of phosphorus in shelf phosphorites is about 1 6 10 10 mol p yr 1 comparable to p burial in the deep sea with organic matter 1 4 10 10 mol yr 1 and biogenic carbonates 1 4 10 10 mol yr 1 thus phosphorite formation on the peru shelf is a significant contemporaneous process

rare earth element geochemistry of hydrothermal deposits from the active tag mound 26 n mid atlantic ridge the rare earth element ree geochemistry of various phases from the active tag hydrothermal mound has been examined and related to their mineralogy and fluid chemistry the mound deposits range from black and white smoker chimneys massive anhydrite sulphide mixtures oxides and ochres all phases except black smoker chimney anhydrite demonstrate a positive eu anomaly when normalised to chondrite ree values ree substitution into sulphide and sulphate phases appears to be strongly influenced by crystallographic control for all ree other than eu precipitation of anhydrite within the tag mound is the major mechanism for removal of ree during mound circulation and 0 15 0 35 g anhydrite is inferred to precipitate from every kg of fluid venting from the white smoker chimneys oxides from the mound fall into three different categories with distinct ree patterns oxide rims on sulphides atacamite bearing oxides and silica rich fe oxides and ochres the oxide rim phases contain sulphide and seawater derived rees whereas the atacamite bearing oxides and the ochreous material exhibit no seawater signature which suggests precipitation from or alteration by a modified hydrothermal fluid

the shallow structures of the guinea and ivory coast ghana transform margins their bearing on the equatorial atlantic mesozoic evolution abstract recent geological and geophysical data from a marine survey conducted off guinea and the ivory coast ghana region equatorial atlantic are discussed we interpret the southern guinean margin firstly as an area linked to the central jurassic atlantic and secondly as an area affected by the progressive transform opening of the equatorial atlantic in the early cretaceous off the ivory coast and ghana we have demonstrated distinct structural domains interpreted as consequences of the progressive transform contact between the west african and brazilian cratons also during the early cretaceous on both margins we observe a clear tectonic unconformity of probable late albian early cenomanian age we propose that this unconformity results from the final disruption between the two continental west african and brazilian crusts the geological structures generated along transform margins can be considered as good geological tools to constrain early phases of plate motions particularly around the equatorial atlantic

ree geochemistry of fine grained sediments from major rivers around the yellow sea abstract to investigate the distribution pattern and controlling factors of rare earth elements rees in riverine sediments river mouth sediments were collected at five geographically different rivers around the yellow sea two 1 m hcl leached and residual fractions and five step sequential extraction schemes the sedex method were applied to size separated sediments

experimental study of the hydrothermal formation of kaolinite kaolinite was hydrothermally precipitated starting from amorphous aluminosilicates with si al ratio from 1 8 to 0 76 at temperatures of 150 175 200 225 and 250 c at time periods varying from 6 h to 60 days the solutions were analysed for ph si al and k and their saturation state was calculated by using the computer program solmineq 88 the solids were studied by surface area xrd and dta tg analyses kaolinite was the only crystalline phase found in the products of the runs and its formation and crystallinity depended on time temperature and si al ratio of the starting material the kaolinite yield increased as starting material was aged for longer times products synthesized at higher temperature contain more kaolinite which was more crystalline than in the experiments carried out at lower temperature the gel with si al 0 99 produced the most disordered kaolinite and better crystallinity was obtained from products richest in al or si the process of kaolinite formation was the result of two stages during the first one aggregates of domains having a kaolinite like structure were formed the second stage corresponded to the formation of hexagonally outlined platy crystals the formation rates indicated that the first stage was approximately one order of magnitude faster than the second stage with average activation energies of 82 5 kj mol 1 and 71 5 kj mol 1 respectively si al ratio of the starting material exerted only a slight influence on the activation energies the precipitation rates obtained for the second stage were consistent with literature data and may be considered precipitation rates of kaolinite in hydrothermal environments

seismic facies of the permian section of the barents shelf analysis and interpretation abstract the palaeozoic succession of the barents shelf has been investigated with data from only a few wells however a considerable amount of high quality seismic data complements the sporadic well control permitting an extensive seismic study of the three major intervals distinguished in the permian section the lowermost interval i consists of an evaporitic sequence both vertical and horizontal gradations of the seismic facies are recognized and interpreted in terms of basin evolution in the centre of the basin a lower unit characterized by high amplitude parallel reflections grades upwards to a mounded reflection free upper unit the mounds are interpreted as salt pillows overlying a lower unit with lithology dominated by anhydrite a rather thick evaporite interval in basinal setting passes laterally to a mixed carbonaceous anhydritic section on the margins and basinal highs the thickness distribution and the areal extent of the seismic facies provide information on the configuration of the evaporite basin on syn sedimentary tectonism and on evaporite diagenesis as a result of renewed transgression the overlying interval ii was deposited in open marine conditions on a shallow shelf belts and patches of seismic mounds interpreted as carbonate build ups are widely spread on the basinal highs and on the basin margins depositional environments are inferred from the distribution of build ups during the progressive drowning of the shelf a back stepping belt of seismic mounds were deposited on the southern basin margin this transgressive episode which constitutes interval iii culminated in the generalized deposition of deep water facies

late paleozoic to cenozoic reconstruction of the arctic abstract the plate tectonic evolution of the arctic is reassessed in the context of the known histories of the north atlantic and north pacific oceans and of the tectono stratigraphic development of the lands around the arctic ocean computer map drawing facilities were used to provide geometrical constraints on the reconstructions which are presented in the form of eight palinspastic maps stratigraphic similarities among presently dispersed continental areas identify fragments of a former barents plate collision of this plate with the euramerican plate was the cause of the late devonian ellesmerian orogeny in later paleozoic time the siberian continent also joined pangea by collision with the combined barents and euramerican plates along the urals taymyr suture the mesozoic cenozoic history of the arctic is concerned with the fragmentation and dispersal of the former barents plate as well as the accretion of new continental fragments from the pacific of the major basins of the present day arctic ocean the eurasia basin formed contemporaneously with the north atlantic ocean and is still spreading while the earlier opening of the canada basin was largely connected with events in the pacific the canada basin formed by the separation of northern alaska from the area now occupied by the alpha ridge initial rifting in the late jurassic was contemporaneous with the earliest major accretionary events in eastern siberia and the northwestern cordillera of north america an early to mid cretaceous age for the main phase of spreading is confirmed by the age depth relationship for the floor of the canada basin after this time the canada basin formed part of the north atlantic plate and subsequent movements related to the opening of the orth atlantic and eurasia basins were taken up within siberia and the bering sea area the history of the latter is not yet clear for times earlier than the late eocene the earliest time for which it is possible to make a geometrically realistic reconstruction of that area earliest stages of spreading in the northern north atlantic caused the initial separation of greenland from north america the eurekan orogeny in the eastern canadian arctic is a local result of this spreading palinspastic restoration of the eastern canadian arctic islands is required to fill the gap otherwise left in reconstruction of that area the eurasia basin opened contemporaneously with the norwegian sea and thus entirely postdates the canada basin geometrical constraints suggest that the makarov basin between the alpha and lomonosov ridges formed during the eocene end of article last page 679

the ivory coast ghana transform margin a marginal ridge structure deduced from seismic data abstract the ivory coast ghana icg marginal ridge is a prominent feature of the icg transform margin and includes a fossil ridge partially buried by a thick undeformed sedimentary cover the fossil icg ridge is 130 km long and 25 km wide and towers over the adjacent rifted basin deep ivorian basin dib and the oceanic crust by 1 3 km and more than 4 km respectively it formed in three successive stages 1 1 during the rifting of the dib both vertical and horizontal motions between the dib and the south american plate varied along the plate boundary this relative motion occurred in an accommodation zone that tilted the northern slope of the icg ridge along en echelon mainly strike slip faults 2 2 after the rifting of the dib the relative motion remained constant along the transform plate boundary at this time strike slip deformation was localized into a narrow and highly deformed belt that truncated the accommodation zone 3 3 finally the transform motion occurred between the dib and an occanic plate thermal exchanges between the two adjacent plates induced thermal uplift of the icg ridge that amplified previous tectonic tilting

mechanisms of extension at nonvolcanic margins evidence from the galicia interior basin west of iberia 1 we have studied a nonvolcanic margin the west iberia margin to understand how the mechanisms of thinning evolve with increasing extension we present a coincident prestack depth migrated seismic section and a wide angle profile across a mesozoic abandoned rift the galicia interior basin gib the data show that the basin is asymmetric with major faults dipping to the east the velocity structure at both basin flanks is different suggesting that the basin formed along a paleozoic terrain boundary the ratios of upper to lower crustal thickness and tectonic structure are used to infer the mechanisms of extension at the rift flanks stretching factor 2 the ratio is fairly constant indicating that stretching of upper and lower crust was uniform toward the center of the basin 3 5 5 5 fault block size decreases as the crust thins and faults reach progressively deeper crustal levels indicating a switch from ductile to brittle behavior of the lower crust at 3 5 faults exhume lower crustal rocks to shallow levels creating an excess of lower crust within their footwalls we infer that initially extension occurred by large scale uniform pure shear but as extension increased it switched to simple shear along deep penetrating faults as most of the crust was brittle the predominant brittle deformation might have driven small scale flow 40 km of the deepest crust to accommodate fault offsets resulting in a smooth moho topography the gib might provide a type example of nonvolcanic rifting of cold and thin crust

occurrence of kaolinite and mixed layer kaolinite smectite in hydrothermal sediments of grimsey graben tjornes fracture zone north of iceland the sediments at a site situated among high temperature vents in the grimsey graben tjornes fracture zone north of iceland exhibit features of strong hydrothermal alteration 1 almost total dissolution of the volcaniclastic material composing the background sediment 2 sulfate and sulfide precipitation 3 kaolinitisation smectite precipitated in the shallowest sediment is gradually replaced downward by mixed layer kaolinite smectite and pure well crystallised kaolinite the kaolinite smectite is interstratified with up to 10 swelling smectitic layers according to the oxygen isotope composition kaolinite smectite mixed layer mineral most likely formed at temperatures near 160 degrees c the vertical sequence kaolinite kaolinite smectite smectite as well as the distinct zonation across the kaolinitic veins almost pure kaolinite in the central zone and kaolinite smectite along the rim suggest hydrothermal transformation of initially formed smectite kaolinite smectite kaolinite most probably this conversion occurred in an evolving from alkaline to slightly acidic hydrothermal environment c 2004 elsevier b v all rights reserved

cote d ivoire ghana margin seismic imaging of passive rifted crust adjacent to a transform continental margin i i summary during may 1990 and january february 1991 an extensive geophysical data set was collected over the cote d ivoire ghana continental margin located along the equatorial coast of west africa the ghana margin is a transform continental margin running subparallel to the romanche fracture zone and its associated marginal ridge the cote d ivoire ghana ridge from this data set an explosive refraction line running 150 km ene wsw between 3 55 n 3 21 w and 4 23 n 2 4 w has been modelled together with wide angle airgun profiles and seismic reflection and gravity data this study is centred on the cote d ivoire basin located just to the north of the cote d ivoire ghana ridge where bathymetric data suggest that a component of normal rifting occurred rafher than the transform motion observed along the majority of the equatorial west african margin traveltime and amplitude modelling of the ocean bottom seismometer data shows that the continental moho beneath the margin rises in an oceanward direction from 24 km below sea level to 17 km in the centre of the line where the crust thins most rapidly there exists a region of anomalously high velocity at the base of the crust reaching some 8km in thickness this higher velocity region is thought to represent an area of localized underplating related to rifting modelling of marine gravity data collected coincident meth the seismic line has been used to test the bestfitting seismic model this modelling has shown that the observed free air anomaly is dominated by the effects of crustal thickness and that a region of higher density is required at the base of the crust to fit the observed data this higher density region is consistent in size and location with the high velocities required to fit the seismic data

controls of fluid chemistry and complexation on rare earth element contents of anhydrite from the pacmanus subseafloor hydrothermal system manus basin papua new guinea ocean drilling program odp leg 193 successfully drilled four deep holes 126 to 386 m into basement underlying the active dacite hosted pacmanus hydrothermal field in the eastern manus basin anhydrite is abundant in the drill core material filling veins and vesicles cementing breccias and occasionally replacing igneous material we report rare earth element ree contents of anhydrite from a site of diffuse venting site 1188 which show extreme variability in terms of both absolute concentrations e g 0 08 28 3 ppm nd and pattern shape lan smn 0 08 3 78 smn ybn 0 48 23 1 eu eu 0 59 6 1 the range of ree patterns in anhydrite includes enrichments in the middle and heavy rees and variable eu anomalies the patterns differ markedly from those of anhydrite recovered during odp leg 158 from the tag hydrothermal system at the mid atlantic ridge which display uniform lree enriched patterns with positive eu anomalies very similar to tag vent fluid patterns as the system is active the host rock composition is uniform and the anhydrite veins appear to relate to the same hydrothermal stage we can rule out predominant host rock and transport control instead we propose that the variation in ree content reflects waxing and waning input of magmatic volatiles hf so2 and variable complexation of rees in the fluids ree speciation calculations suggest that increased fluoride and possibly sulfate concentrations at pacmanus may affect ree complexation in fluids whereas at tag only chloride and hydroxide complexes play a significant role the majority of the anhydrites do not show positive eu anomalies suggesting that the fluids were more oxidizing than in typical mid ocean ridge hydrothermal systems we use other hydrothermal fluids from the manus basin vienna woods and desmos which bracket the pacmanus fluids in terms of acidity and ligand concentrations to examine the dependence of ree complexation on fluid composition geochemical modeling reveals that under the prevailing conditions at pacmanus ph 3 5 t 250 300 c eu oxidation state and the relative importance of fluoride versus chloride complexing are very sensitive to small variations in oxygen fugacity temperature and ph patterns with extreme mid ree enrichment may reflect speciation effects free ion abundance coupled with crystal chemical control we conclude that the great variability in ree concentrations and pattern shape is likely due to variable fluid composition and ree complexation in the fluids

the subseafloor thermal gradient at iheya north knoll okinawa trough based on oxygen and hydrogen isotope ratios of clay minerals abstract we measured the oxygen and hydrogen isotope ratios of smectite chlorite and illite in hydrothermally active sediments at the iheya north knoll okinawa trough obtained by seafloor drilling of active hydrothermal fields during the integrated ocean drilling program iodp expedition 331 formation temperatures calculated from oxygen isotope data were compared to the downhole temperatures that were measured during the same expedition the 18o values ranged from 9 9 to 13 3 for smectite 1 3 to 3 0 for chlorite or chlorite smectite mixed layer mineral and 2 1 to 5 1 for illite or illite smectite mixed layer mineral with minor chlorite for samples of illite that contained minor chlorite 18o values for illite or illite smectite mixed layer mineral of 4 4 to 6 1 were calculated based on the proportion of the two clay minerals calculated from chemical composition of the individual mineral and the mixture determined by tem eds and epma we estimated the formation temperatures of the minerals using oxygen isotope fractionation data and assumed values of 0 to 1 2 for the fluid that was equilibrated with the minerals based on the reported values of the seawater and vent fluid measured in previous studies the temperature range calculated was 111 175 c for a shallow layer less than about 25 m below seafloor mbsf that contained smectite and a minor kaolinite component 177 297 c for a deeper layer between about 25 and 40 mbsf that was dominated by chlorite or chlorite smectite mixed layer mineral and 220 314 c for the deepest layer between about 40 and 120 mbsf that contained illite or illite smectite mixed layer mineral with less chlorite the temperatures of mineral formation calculated from the oxygen stable isotope data were slightly higher at some depths but both calculated and measured temperatures demonstrated a steep step wise thermal gradient below the active hydrothermal field where temperatures of 200 c were determined at as little as 50 mbsf hydrogen isotope values mostly ranged between 40 and 55 and did not vary significantly or systematically between the layers except in the clay fractions containing smectite above 25 mbsf that had relatively low values between 65 and 72 hydrogen isotope results of the clay minerals were problematic to interpret and not consistent with interpretation of the oxygen data calculated temperatures from illite and smectite were mostly negative whereas temperatures calculated from chlorite were more positive average 145 c but lower than those from the oxygen isotope data additional work is still required to determine the hydrogen isotope fractionation between clay minerals and water with respect to temperature

hydrothermal alumina rich clays and boehmite on the gorda ridge abstract hydrothermal alteration of basalt on the northern gorda ridge produced an alteration crust up to 1mm thick that can be divided into two layers based on mineralogy an inner layer next to the basalt surface is an al rich interlayered smectite chlorite similar to high temperature al rich clays recovered from the east pacific rise 21 n with minor quartz zeolite phillipsite and relict plagioclase an exterior layer of boehmite alo oh and minor anatase results from severe leaching of the underlying clays the alteration crust is progressively enriched in al ti v and cr from interior to exterior by residual weathering the presence of boehmite and absence of a silica rich phase in the exterior of the crust indicates the hydrothermal fluids were silica undersaturated or silica was maintained in solution by extremely acidic fluids this constitutes the first reporting of boehmite as a major alteration mineral in a mid ocean ridge hydrothermal environment

europium redox equilibria in aqueous solution thermochemical data and recent advances in theoretical aqueous solution chemistry enable prediction of the relative stabilities of aqueous eu2 and eu3 over a wide range of temperatures and pressures at low temperatures near earth surface conditions the aqueous geochemistry of europium should be dominated by the trivalent state except possibly in the most reducing alkaline pore waters of anoxic marine sediments however at temperatures greater than about 250 c and elevated pressures divalent europium should predominate even with significant amounts of complexing trivalent europium is not stable at elevated temperatures and pressures consequently under most hydrothermal and metamorphic conditions europium in aqueous solution should be divalent at intermediate temperatures around 100 c significant activities of both eu2 and eu3 and related complexes can occur in aqueous solutions depending on the oxidation state and the ph of the solutions and the activities of potential ligands such as sulfate carbonate and chloride the predicted stability of divalent europium in aqueous solution at elevated temperatures is consistent with the large positive europium anomalies in rare earth element patterns of high temperature barites of hydrothermal and metamorphic origin reported by guichard et al 5 and with the observed depletion of europium as a result of high temperature sericitization of feldspar bearing assemblages discovered by alderton et al 1 it is suggested that significant fractionation of europium relative to the other rare earth elements may take place during high temperature hydrothermal alteration processes such as in the mid oceanic ridge systems because of the expected stability of divalent europium during these processes

nd and sr isotopic characteristics of chinese deserts implications for the provenances of asian dust silicate nd sr isotopes of the fine grained fractions of the 10 major deserts and sandy lands in north china and the loess in chinese loess plateau were systematically investigated wide ranges in nd sr isotopic compositions have been observed the results of the 7 0 b the deserts on the northern margin of tibetan plateau with end 0 ranging from 11 9 to 7 4 and c the deserts on the ordos plateau with the lowest end 0 11 5 the distribution of the threes isotopic regions is controlled by the tectonic setting in north china which implies that the materials of the deserts are derived from the locally eroded rocks from the surrounding mountains and the nd sr isotopic signatures of these deserts could be quit stable over the past million years on the sub tectonic time scales if there is any desert at those times the nd sr isotopic compositions of the loess are mostly close to those of the deserts in isotopic region b suggesting that the main source regions of the last glacial loess in the chinese loess plateau are badain jaran desert tengger desert and qaidam desert also the comparison between the nd sr isotopes of the 5 lm silicate fractions of the deserts and the ancient dust falls in the north pacific and greenland show that the asian end members of these dust falls are derived most from the deserts in the isotopic region b and less from those in the isotopic region c

crustal structure of the lomonosov ridge and the fram and makarov basins near the north pole reversed refraction surveys were conducted along and across the lomonosov ridge as part of the 1979 lomonosov ridge experiment interpretation of the strike profiles indicates a 5 km thick upper crustal layer with a velocity of 4 7 km s overlying a 15 to 20 km thick layer of 6 6 km s material an upper mantle velocity of 8 3 km s is indicated by a few pn arrivals high amplitude reflection events recorded from this boundary can be successfully modeled by a transition zone of rapidly changing velocity over a depth interval of 5 km ray trace modeling of the dip profiles suggests a root structure extending to about 28 km depth flanked by crust thinning to a depth near 13 km beneath the makarov basin and a more gradual thinning to near 16 km beneath the fram basin the similarity between the crust of the lomonosov ridge and that beneath the barents and kara seas supports the suggestion that the ridge is a slice rifted from the baltic shelf

a palaeomagnetic study of 143 ma kimberlite dikes in central new york state summary a palaeomagnetic study of 143 ma serpentinized kimberlite dikes near ithaca new york yields a dual polarity high unblocking temperature and high coercivity magnetization which passes the reversal test and two baked contact tests the mean pole position 58 n 203 e a95 3 8 n 7 differs from published late jurassicearly cretaceous north american poles currently used to define the apparent polar wander path the angular dispersion in mean directions 3 5 is low but the presence of reversals argues that the ithaca kimberlites magnetization should represent sufficient time for averaging of palaeosecular variation similar findings apparently typify palaeomagnetic studies of other serpentinized kimberlite supporting the suggestion that thermo chemical remanent magnetization in this lithology prolongs the duration of magnetization acquisition sufficiently to average secular variation per dike a consistent but weak foliation in anisotropy of arm parallels the n s and vertical orientation of the ithaca dikes but is apparently unrelated to the northwest down or southeast up remanence the ithaca kimberlites pole may therefore record a previously undocumented sharp bend or cusp at 143ma and the initiation of a cretaceous and cenozoic interval of apparent polar wander that generally follows a great circle along the 200 e meridian to geographic north a coeval 145 ma kimberlite pole from southern africa transferred to north america agrees with the ithaca kimberlites pole position whereas reported poles from the berriasian stratotype southern france and 144ma svalbard dolerites provide less diagnostic tests of the ithaca kimberlites pole due in part to uncertainties in europe north america reconstructions

experimental nucleation and growth of smectite and chlorite coatings on clean feldspar and quartz grain surfaces abstract laboratory experiments were performed to investigate high temperature 100 150 c authigenic formation of clay coatings smectites chlorites on clean feldspar and quartz surfaces artificial formation waters were used with mg concentrations in the range of north sea brines experiments were run for 21 50 days in brines composed of magnesium chloride and sodium carbonate adjusted to circum neutral ph the experiments suggest that the silica activity is the main factor determining if grain coating smectites or chlorite form the clay minerals form easily on both clean quartz and feldspar surfaces but chlorite coatings were formed only on feldspar surfaces while smectite coatings were formed on both feldspar and quartz the chlorite morphology varies between honeycomb edge to face and rosette patterns while all smectite formed with honeycomb like textures the clay coatings produced in this study are morphologically similar to naturally occurring diagenetic clay minerals in natural sediments and sedimentary rocks the formation of clay coatings is promoted by pre existing clay drapings on mineral grains the growth substrates used in this study were not coated with such a natural precursor materials this suggests that the nucleation of clay coatings in nature may be possible on clean quartz and feldspar surfaces but pre existing clay minerals may impact the allowed levels of supersaturations and thereby the formation of the grain coating phases

flexural unloading and uplift along the cote d ivoire ghana transform margin equatorial atlantic recent ocean drilling program sampling of the cote d ivoire ghana margin of west africa provides for the first time the opportunity to study the development of a marginal ridge that formed along a sheared passive margin adjacent to the continent ocean transition after the end of intracontinental wrenching we model its evolution using a two dimensional flexural backstripping technique the model is constrained by existing seismic refraction and reflection data on the crustal structure and stratigraphy and paleobathymetric evidence from the cores following rifting at 120 ma aptian intracontinental wrenching continued until 105 ma mid albian when south america and africa separated along this transform while the possible presence of thicker crust under the marginal ridge compared to the deep ivorian basin may explain some of the ridge s topography the entire uplift can be readily modeled as a flexural response to unloading along a shallow dipping 25 fault at the time of continental separation forward subsidence modeling of the marginal ridge suggests that an effective elastic thickness of 2 5 km at 105 ma is most appropriate to match the observed structure conduction of heat from the oceanic plate across the continent ocean transition drove temporary uplift of at least 1200 m peaking just before 89 ma when sedimentary data show deposition within the photic zone 0 50 m this study shows that significant transient thermal uplift can be found in sheared continental passive margin settings even where magmatism is insignificant

provenance weathering and erosion records in southern okinawa trough sediments since 28ka geochemical and sr nd pb isotopic evidences abstract geochemical and sr nd pb isotopic compositions of the detrital sediments from odp hole 1202b and taiwan rivers were measured in this study aiming to reveal changes in sediment provenance in the southern okinawa trough sot since 28 ka and to examine the weathering and sediment transport processes in response to monsoon climate variability large variations in sr nd pb isotopic ratios at 11 9 ka suggest changes in detrital sediment provenance in the sot from a dominance of the paleo changjiang yangtze river and or continental shelf sediment during the late deglaciation and to west taiwan rivers since 9 5 ka volcanic rocks and eastern taiwan sediments have not significantly contributed to the sot the large shift in sediment provenance during the early holocene marks a major change in oceanic circulation mainly caused by the intrusion of the kuroshio current into the trough clay mineral and geochemical proxies suggest that the taiwan derived sediments accumulated during the early mid holocene climate optimum ca 9 5 4 ka might be tightly related to the reworking of older altered sediments from terraces and floodplains rather than having experienced more intense silicate weathering than in the late holocene 4 0 ka overall silicate weathering in taiwan was greatly inhibited by accelerating sediment production and transfer from land to ocean caused by monsoon intensification in holocene our study illustrates that the radiogenic isotopic and geochemical compositions of fine grained detrital sediments are sensitive tools for fingerprinting sediment sources and for reconstructing changes in oceanic currents and monsoon climate in river dominated east asian continental margin

nearshore sediment characteristics and formation of mudbanks along the kerala coast southwest india in order to gain insight into the formation dynamics of mudbanks off the kerala coast of india extensive surveying of the nearshore bathymetry along with sediment characterization was undertaken the textural and geotechnical properties of the surficial sediments of a mudbank were determined during pre monsoon monsoon and post monsoon periods the mudbank sediments were clayey silts with high water and organic carbon contents high atterberg limits and low bulk density and therefore very susceptible to entrainment during the monsoon the mudbank regime was characterised by enhanced turbidity and a benthic fluff layer triggered by the increasing swell of the early monsoon period re suspension exposed a more consolidated previously sub bottom layer which exhibited lower water content and greater shear strength than the pre monsoon seabed texturally the monsoon seabed was similar to the pre monsoon seabed with the same modal grain size but the proportions of sand and coarse silt increased nearshore while the proportions of fine and very fine silt increased offshore there was a seaward fining textural gradient at all times but this became pronounced during the monsoon period paradoxically the monsoonal seabed displayed greatly reduced wet bulk density it is hypothesized that this was due to the presence of gas probably methane in the sediments while the pre monsoon sediments were fully saturated the monsoon sediments were only 83 saturated we speculate that the gas was forced into the surficial sediments either by wave pumping at the onset of the monsoon or by seaward flowing subbottom freshwater derived from monsoonal rains with the waning of the monsoon the benthic fluid mud layer rapidly disappeared and the seabed returned to its pre monsoon state as suspended sediments were redeposited the mudbank regime is therefore essentially an in situ phenomenon it is suggested that the mudbanks are palimpsest marshy lagoonal deposits rich in organic matter and derived gas that were submerged after a marine transgression the surficial sediment is annually entrained during the monsoon but erosion is limited by the formation of the benthic fluid mud layer which attenuates wave generated turbulence although some fine sediment disperses alongshore and offshore most is returned to the seabed as the monsoon declines

deposition of talc kerolite smectite smectite at seafloor hydrothermal vent fields evidence from mineralogical geochemical and oxygen isotope studies abstract talc kerolite smectite smectite chlorite smectite and chlorite samples from sediments chimneys and massive sulfides from six seafloor hydrothermal areas have been analyzed for mineralogy chemistry and oxygen isotopes samples are from both peridotite and basalt hosted hydrothermal systems and basaltic systems include sediment free and sediment covered sites mg phyllosilicates at seafloor hydrothermal sites have previously been described as talc stevensite or saponite in contrast new data show tri octahedral mg phyllosilicates ranging from pure talc and fe rich talc through kerolite rich kerolite smectite to smectite rich kerolite smectite and tri octahedral smectite the most common occurrence is mixed layer kerolite smectite which shows an almost complete interstratification series with 5 to 85 smectitic layers the smectite interstratified with kerolite is mostly tri octahedral the degree of crystal perfection of the clay sequence decreases generally from talc to kerolite smectite with lower crystalline perfection as the proportion of smectite layers in kerolite smectite increases our studies do not support any dependence of the precipitated minerals on the type subtype of hydrothermal system oxygen isotope geothermometry demonstrates that talc and kerolite smectite precipitated in chimneys massive sulfide mounds at the sediment surface and in open cracks in the sediment near seafloor are high temperature 250 c phases that are most probably the result of focused fluid discharge the other end member of this tri octahedral mg phyllosilicate sequence smectite is a moderate temperature 200 250 c phase forming deep within the sediment 0 8 m chlorite and chlorite smectite which constitute the alteration sediment matrix around the hydrothermal mounds are lower temperature 150 200 c phases produced by diffuse fluid discharge through the sediment around the hydrothermal conduits in addition to temperature other two controls on the precipitation of this sequence are the silica activity and mg al ratio i e the degree of mixing of seawater with hydrothermal fluid higher silica activity favors the formation of talc relative to tri octahedral smectite vent structures and sedimentary cover preclude complete mixing of hydrothermal fluid and ambient seawater resulting in lower mg al ratios in the interior parts of the chimneys and deeper in the sediment which leads to the precipitation of phyllosilicates with lower mg contents talc and kerolite smectite have very low trace and rare earth element contents some exhibit a negative or flat eu anomaly which suggests eu depletion in the original hydrothermal fluid such eu depletion could be caused by precipitation of anhydrite or barite sinks for eu2 deeper in the system ree abundances and distribution patterns indicate that chlorite and chlorite smectite are hydrothermal alteration products of the background turbiditic sediment

mineralogical geochemical and isotopic characteristics of hydrothermal alteration processes in the active submarine felsic hosted pacmanus field manus basin papua new guinea during odp leg 193 4 sites were drilled in the active pacmanus hydrothermal field on the crest of the felsic pual ridge to examine the vertical and lateral variations in mineralization and alteration patterns we present new data on clay mineral assemblages clay and whole rock chemistry and clay mineral strontium and oxygen isotopic compositions of altered rocks from a site of diffuse low temperature venting snowcap site 1188 and a site of high temperature venting roman ruins site 1189 in order to investigate the water rock reactions and associated elemental exchanges the volcanic succession at snowcap has been hydrothermally altered producing five alteration zones 1 chlorite illite cristobalite plagioclase alteration apparently overprinted locally by pyrophyllite bleaching at temperatures of 260 310degreesc 2 chlorite mixed layer clay alteration at temperatures of 230degreesc 3 chlorite and illite alteration 4 illite and chlorite illite mixed layer alteration at temperatures of 250 260degreesc and 5 illite chlorite alteration at 290 300degreesc felsic rocks recovered from two holes 1189a and 1189b at roman ruins although very close together show differing alteration features hole 1189a is characterized by a uniform chlorite illite alteration formed at similar to250degreesc overprinted by quartz veining at 350degreesc in contrast four alteration zones occur in hole 1189b 1 illite chlorite alteration formed at similar to300degreesc 2 chlorite illite alteration at 235degreesc 3 chlorite illite and mixed layer clay alteration and 4 chlorite illite alteration at 220degreesc mass balance calculations indicate that the chloritization illitization and bleaching silica pyrophyllite assemblages alteration stages are accompanied by different chemical changes relative to a calculated pristine precursor lava the element cr appears to have a general enrichment in the altered samples from pacmanus the clay concentrate data show that cr and cu are predominantly present in the pyrophyllites illite shows a significant enrichment for cs and cu relative to the bulk altered samples considerations of mineral stability allow us to place some constraints on fluid chemistry hydrothermal fluid ph for the chloritization and illitization was neutral to slightly acidic and relatively acidic for the pyrophyllite alteration in general the fluids especially from roman ruins and at intermediate depths below snowcap show only a small proportion of seawater mixing 10 fluids in shallow and deep parts of the snowcap holes in contrast show stronger seawater influence copyright c 2004 elsevier ltd

the s reflector west of galicia spain evidence from prestack depth migration for detachment faulting during continental breakup the mechanism of continental extension and breakup and the role of detachment faults in these processes are currently the subject of intense debate one possible detachment fault is the s reflector imaged as an undulating in time locally discontinuous reflection on existing seismic profiles west of iberia here we present new images in depth of the four margin normal profiles across the west galicia rifted margin where the s reflector was originally defined and is best imaged it is shown that faults bounding wedge shaped units of late synrift sediments which hence were active during rifting immediately prior to breakup appear to detach at shallow levels onto the s reflector s itself appears as a continuous locally domal feature and does not generally appear offset the waveform of s is compatible with a reflection from a single interface such as a sharp tectonic boundary the depth sections show that s was active at 1 3 km below the seafloor during final rifting s is interpreted as a brittle detachment fault which controlled the final breakup of the continent west of galicia furthermore the data provide constraints on the sense of shear of s analogy with detachment terranes the present synrift and structural dips of s and the identification of a breakaway to s imply that s accommodated top to the west shear toward the east of the profiles s becomes more complex possibly because of different phases of detachment faulting and the development of both incisement and excisement structures from the geometry of wedge shaped sedimentary units deposited during faulting above s it also appears that s was active at an angle of 20 or less and hence may be considered a genuine low angle normal fault

evaluation of the bgm 3 sea gravity meter system onboard r v conrad the first bell aerospace bgm 3 marine gravity meter system available for academic use was installed on r v robert d conrad in february 1984 the bgm 3 system consists of a forced feedback accelerometer mounted on a gyrostabilized platform its sensor requiring no cross coupling correction is a significant improvement over existing beam and spring type sea gravimeters such as the gss 2 a gravity survey over the wallops island test range together with the results of subsequent cruises allow evaluation of the precision accuracy and capabilities of the new system over the test range the bgm 3 data were compared directly to data obtained by a gss 2 meter onboard r v conrad the rms discrepancy between free air gravity anomaly values at intersecting ship tracks of r v conrad was 0 38 mgal for bgm 3 compared to 1 60 mgal for the gss 2 moreover bgm 3 s platform recovered from abrupt changes in ship s heading more rapidly than did the platform of gss 2 the principal factor limiting the accuracy of se

correlations between the lomonosov ridge marvin spur and adjacent basins of the arctic ocean based on seismic data abstract seismic profiles across the lomonosov ridge marvin spur and adjacent basins acquired near the north pole by the drifting ice station np 28 provide a reflection image of the upper parts of the ridge that is readily correlatable with those acquired by the alfred wegner institute closer to the siberian margin a prominent flat lying composite reflection package is seen in most parts of the ridge at a few hundred meters below the sea bottom underlying reflections are variable in intensity and also in dip the base of this reflection package is often accompanied by a sharp increase in p velocity and defines a major angular discontinuity referred to here as the lomonosov unconformity the arctic coring expedition acex cored the first c 430 m section on the lomonosov ridge near the north pole in 2004 defining the deeper water character of the neogene and the shallower water paleogene sediments these boreholes penetrated the composite reflection package towards the base of the hole and identified sediments our unit iii of late paleocene and early eocene age campanian beds at the very base of the hole were thought to be representative of the units below the lomonosov unconformity but the p velocity data suggest that this is unlikely correlation of the lithologies along the top of the lomonosov ridge and to the marvin spur indicates that the marvin spur is a sliver of continental crust closely related to and rifted off the ridge this narrow 50 km wide linear basement high can be followed into beneath and across the makarov basin supporting the interpretation that this basin is partly resting on thinned continental crust in the makarov basin the paleogene succession is much thicker than on the ridge thus the condensed shallow water succession with hiati was deposited on the ridge during rapid eocene to miocene subsidence of the basin in the amundsen basin adjacent to the lomonosov ridge the sedimentary successions thicken towards the canadian margin and the reflections on the ridge are not readily identifiable the approximate ages of the sedimentary units are inferred from their relationships to the linear magnetic anomalies in the basin lomonosov acoustic basement dips gently into the basin over a distance of about 100 km and the linear negative anomaly previously thought to be chron 25 is probably related to a rift related mafic intrusive complex

organic matter in anoxic sediments off central peru relations of porosity microbial decomposition and deformation properties abstract microstructure of central peru continental margin sediments suggests that in environments of rapid hemipelagic sedimentation organic matter decomposition has a significant effect on geotechnical properties in highly porous surface sediments particles resembling the remains of fecal pellets appear to be coated by bacterial sheaths but after burial sheaths disappear as an increasingly uniform and closed structure of organo mineral aggregates develops interstitial water nutrient and solid phase c n and p profiles support the interpretation that this progression corresponds to decomposition and metabolite regeneration from labile organic matter by microbial sulfate reduction it is proposed that early aggregation has a strengthening effect and that this combined with the low density of near surface sediments results in an effective overburden pressure lower than experimentally determined preconsolidation pressures more basic knowledge of the lifetime of organo mineral aggregates and the influence of gross and local microstructure on the rates and products of diagenetic reactions is needed in order to verify the relationship between organic matter accumulation and sediment stability in continental margin environments

tectonic subsidence of the lomonosov ridge the cenozoic sedimentary record revealed by the integrated ocean drilling program s arctic coring expedition acex to the lomonosov ridge microcontinent in 2004 is characterized by an unconformity attributed to the period 44 18 ma according to conventional thermal kinematic models the microcontinent should have subsided to 1 km depth owing to rifting and subsequent separation from the barents kara sea margin at 56 ma we propose an alternative model incorporating a simple pressure temperature p t relation for mantle density using this model we can explain the missing stratigraphic section by post breakup uplift and erosion the pattern of linear magnetic anomalies and the spreading geometry imply that the generation of oceanic crust in the central eurasia basin could have been restricted and confined by non volcanic thinning of the mantle lithosphere at an early stage ca 56 40 ma in response to a rise in temperature the mantle mineral composition may have changed through breakdown of spinel peridotite and formation of less dense plagioclase peridotite the consequence of lithosphere heating and related mineral phase transitions would be post breakup uplift followed by rapid subsidence to the deep water environment observed on the lomonosov ridge today

chemistry of ore forming fluids and mineral formation rates in an active hydrothermal sulfide deposit on the mid atlantic ridge the chemical composition of diffuse effluent from the tag hydrothermal mound 26 n mid atlantic ridge directly demonstrates that mixing of entrained seawater and high temperature black smoker fluids leads to the precipitation of sulfides silica and anhydrite and to the zone refining of metals including cu zn u and the rare earth elements in an actively forming sulfide ore body rates of mineral formation deduced from these data are extremely rapid and are up to an order of magnitude greater for anhydrite 5 15 107 kg yr than for sulfides 106 107 kg yr comparison with mineral inventories for tag suggests that virtually all of the anhydrite in the tag mound is contemporary and confirms that the sulfide precipitated episodically during the history of mound growth and that much of it has been lost by oxidation and reworking

geochemical composition and provenance discrimination of coastal sediments around cheju island in the southeastern yellow sea sediments from the northern coastal area of cheju island south sea of korea southeastern yellow sea were analyzed for grain size composition elemental compositions and clay mineralogy in order to investigate their provenance rare earth element ree compositions and geochemical discrimination diagrams were revealed to be useful indices for identifying the origin of sediments in the study area these indices together with clay mineral compositions suggest that both the sandy and muddy sediments originated from weathering of the volcanic rocks of cheju island whereas sediments originating from the changjiang and huanghe rivers cannot be traced in the study area similarly river borne matter from the korea peninsula has little influence on the sediment deposited therefore the suggestion that fine grained changjiang sediments can reach the nearshore area of cheju island the cheju strait suggested by oceanic circulation patterns in the yellow and east china seas cannot be supported by this study based on geochemical and mineralogical analyses

mobilization of beryllium in the sedimentary column at convergent margins studies of be distributions in subduction zone sediments will help to understand questions regarding the enrichments of cosmogenic be 10 in arc volcanic rocks analyses of be 10 and be 9 in sediments of ocean drilling program site 808 nankai trough and be 9 in porewaters of site 808 and sites 671 and 672 barbados ridge complex show significant decreases in solid phase be 10 and large increases of porewater be 9 at the location of the decollement zone and below or at potential flow conduits these data imply the potential mobilization of be during pore fluid expulsion upon sediment burial experiments involving reaction between a decollement sediment and a synthetic nacl cacl2 solution at elevated pressure and temperatures were conducted in an attempt to mimic early subduction zone processes the results demonstrate that be is mobilized under elevated pressure and temperature with a strong ph dependence the be mobilization provides an explanation of be 10 enrichment in arc volcanic rocks and supports the argument of the importance of the fluid processes in subduction zones at convergent margins