ocean crust formation processes at very slow spreading centers a model for the mohns ridge near 72 n based on magnetic gravity and seismic data the mohns ridge in the norwegian greenland sea is one of the slowest spreading centers of the mid ocean ridge system 8 mm yr half rate sea beam data acquired with r v jean charcot near 72 n show that its rift valley floor is characterized by en echelon volcanic ridges oriented obliquely relatively to the average strike of the ridge axis these ridges are regularly spaced along the axis about every 40 km and are separated by nontransform discontinuities sharp positive magnetic anomalies centered over the topographic highs suggest that they are eruptive centers considered as the surficial expression of active spreading cells over the rift valley bouguer anomalies obtained by subtracting the predicted effects due to seafloor topography from the measured free air gravity field are consistent with a low density body within the lower crust having its upper surface lying at about 2 km below the sea surface this body if it exists probably corresponds to the zone of low viscosity that can be inferred from the model of chen and morgan 1990b which predicts the existence of a decoupling region between the upper crust and the asthenophere below its width varies rapidly along strike from less than about 5 km to more than 15 km in plan view it has a pinch and swell form which defines a series of spreading cells the center of one cell being where the bouguer anomaly is widest short wavelength less than 10 to 20 km along strike variations such as bouguer anomaly lows centered on the topographic highs reflect local effects associated with the presence of the eruptive centers seismic tomography data from a 20 10 km active oblique volcanic ridge near 72 22 n tend to indicate that the links between the main low velocity body at depth and the magma injections centers which lie within the rift valley inner floor are probably complex

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

upper crustal seismic structure of the slow spreading mid atlantic ridge 35 n constraints on volcanic emplacement processes 1 the upper crustal seismic structure of the slow spreading mid atlantic ridge is studied using a genetic algorithm based waveform inversion of multichannel streamer data four single ship multichannel profiles from 35 n are analyzed one in the rift valley and three in the rift mountains along 0 7 1 6 and 1 9 ma crust a layer 2a horizon is continuously imaged along three profiles and is associated with a sharp velocity increase from extrusives to dikes its depth and regularity in the rift valley indicate that most of the extrusive section is built on the inner valley floor through a pattern of deposition and fault bounded uplift into the rift mountains its variability along one line however shows that this process is disrupted during tectonically dominated periods a thickening of layer 2a toward the oceanographer fracture zone may be the result of along axis magma transport the interval of rapid velocity increase at the base of layer 2a thins with age a possible response to enhanced hydrothermal mineralization within the zone of mixed dikes and extrusives transition zone 200 m and off axis layer 2a thicknesses 350 600 m are similar to those at other spreading centers this indicates that equivalent extrusive volumes are produced at all spreading rates along a relatively narrow zone of dike emplacement however differences in on axis layer 2a thickness between this area and fast spreading ridges suggest that the exact pattern of thickening varies between spreading regimes relative to fast spreading ridges the moderate velocity increase with age recorded in the upper crust from 2 3 to 2 7 km s 1 within 2 myr may be due to a less active hydrothermal system and hence slower porosity reduction

microvertebrate assemblages from the upper silurian of cornwallis island arctic canada microvertebrate assemblages from four upper silurian ludlow pridoli localities on cornwallis island arctic canada comprise mainly scales plus dentition cones and jaw fragments from ischnacanthid acanthodians with rare scales assigned to heterostracan lepidaspis sp chondrichthyan arauzia sp and placodermi gen et sp indet most of the scales in sample c 11460 are assigned to the poracanthodid acanthodian poracanthodes canadensis sp nov which shows closest affinity to poracanthodes punctatus brotzen variants from the baltic pridoli the flank scales of the new species resemble those of p punctatus s s silurian variant the zone fossil for the late pridoli in the standard silurian microvertebrate scheme with their superposed crown growth zones rows of small pores aligned with the underlying zones number of radial canals and arcade canals connecting these radial canals they differ in having numerous anterior crown riblets zig zag rather than straight crown pore rows and v shaped arcade canals resume les assemblages de microvertebres de quatre localites du silurien superieur ludlow pridoli sur l ile de cornwallis dans l arctique au canada comprennent principalement des ecailles plus des cones de dentine et des fragments de m choire d acanthodiens ischnacanthides et aussi de rares ecailles assignees a heterostrace lepidaspis sp chondrichthyen arauzia sp et placoderme gen et sp indetermines la majorite des ecailles dans l echantillon c11460 sont assignees a l acanthodien poracanthodes canadensis sp nov qui possede une affinite nettement dominante avec les variants de poracanthodes punctatus brotzen du pridoli de la baltique les ecailles des flancs de la nouvelle espece ressemblent a celles de p punctatus s s variant silurien zone fossile du pridoli tardif dans le schema des microvertebres du silurien standard leurs zones de croissance de la couronne sont superposees leurs rangees de petits pores sont alignees avec les zones sous jacentes les canaux radiaux nombreux sont connectes par les canaux en arcade elles se distinguent par leurs nombreuses petites rides sur la couronne anterieure les pores disposes en zig zag sur la couronne plutot qu en rangees droites et les canaux en arcade en forme de v traduit par la redaction burrow et al 361

gas pressures in geothermal systems abstract the partial pressures of co2 h2s and h2 in geothermal reservoir waters are fixed by temperature dependent mineral equilibria the dissolved gases have a significant effect at which depth the rising hot waters start to boil if their temperature exceeds 250 c and particularly when it exceeds 300 c the following functions describe the temperature dependence of pco2 ph2s and ph2 respectively log p co 2 2 81 5012 7t 1 0 00919t 6 464 log t log p h 2 s s 18 75 24738 9t 1 0 10133t 43 170 log t log p h 2 s d 2 76 5758 2t 1 0 00850t 6 359 log t log p h 2 s 10 14 17763 1t 1 0 04084t 23 368 log t log p h 2 d 9 85 7290 3t 1 0 07202t 22 651 log t where p is partial pressure in bars abs and t is temperature in kelvins two functions are given for each of h2s and h2 one set indicated by subscript d is valid for all waters 300 c and waters in the range 200 300 c if containing 500 ppm cl it is considered that the temperature functions are valid for waters in basaltic to acidic volcanic rocks and also for some waters at least in sedimentary rocks the following function describes crudely total gas pressure plus ph2o in geothermal systems log p total 0 00543 621 4t 1

reconstruction of the central indian ocean abstract sixteen plate tectonic reconstructions are presented illustrating the development of the central indian ocean from the latest cretaceous anomaly 29 to the present the history of seafloor spreading in the indian ocean can be divided into five distinct tectonic epochs 1 1 latest cretaceous early paleocene a29 a27 2 2 middle paleocene early eocene a27 a22 3 3 middle eocene late eocene a22 a18 4 4 late eocene early miocene a18 a8 and 5 5 early miocene recent a8 a5 during each of these tectonic epochs subtle changes took place in the directions and rates of spreading along the central southeast and southwest indian ridges the most dramatic changes in plate motion occurred between a21 and a18 time due to the collision of india and eurasia

crustal structure of the southwest indian ridge at the atlantis ii fracture zone the southwest indian ridge is a slow spreading end member of the mid ocean ridge system the deepest borehole penetrating the lower oceanic crust ocean drilling program hole 735b lies on the eastern transverse ridge of the atlantis ii fracture zone at 57 e a wide angle seismic survey in the vicinity of the borehole reveals a crustal structure that is highly heterogeneous to the east of atlantis bank on which hole 735b is located the crust consists of a 2 2 5 km thick high velocity gradient oceanic layer 2 and a 1 2 km thick low velocity gradient layer 3 the transform valley has a 2 5 3 km thick crust with anomalously low velocities interpreted to consist largely of highly serpentinized mantle rocks the seismically defined crust is thickest beneath the borehole where layer 2 is thinner and the lower crust is inferred to contain 2 3 km of partially serpentinized mantle the seismic velocity models are consistent with gravity data which show weak residual mantle bouguer anomalies because the regions of thinner crust have lower crustal densities stress variations deduced from mass balances between the transform valley floor and the adjacent transverse ridges are much larger than the likely threshold for lithospheric failure and therefore indicate that the relief is supported dynamically the variation of crustal thickness with spreading rate defined by data from the southwest indian ridge and elsewhere is consistent with models of melt generation in which the upwelling mantle is cooled by conductive heat loss at very slow spreading rates resulting in reduced melt generation under the spreading axis large segment scale variations in crustal thickness suggest subcrustal along axis migration of melt toward segment centers

do lower crustal rocks record reversals of the earth s magnetic field magnetic petrology of oceanic gabbros from ocean drilling program hole 735b ocean drilling program odp leg 118 at the southwest indian ridge recovered the first vertically oriented continuously cored gabbros from in situ ocean crust magnetic properties and oxide petrography of drillcore samples were analyzed in order to determine the processes which control the remanent magnetization of oceanic layer 3 and the ability of gabbros to record reversals of the earth s magnetic field primary igneous magnetite is common only in evolved fe ti oxide rich gabbros and averages by volume 2 of the rock although substantially higher contents are common secondary magnetite produced by high temperature exsolution and hydrous alteration of olivine and pyroxene is observed in all gabbros studied and is the most important magnetic phase in the majority of recovered gabbros cumulate olivine gabbros because of the high temperatures associated with this alteration it is likely that the primary magnetization of this crustal section was a thermal remanent magnetization or a very high temperature chemical remanent magnetization acquired very near the ridge crest hysteresis loop parameters indicate that all of the studied samples are capable of carrying significant stable remanent magnetization finally previously published drilling results and field studies from odp site 735b indicate that although this crustal section has been tectonically uplifted it formed at a mid ocean ridge spreading center similar to much of the oceanic crust therefore the results of this magnetic study can be extrapolated to include the oceanic gabbro section in general and argue strongly that layer 3 crustal rocks are capable of recording reversals of the earth s magnetic field

constructing the upper crust of the mid atlantic ridge a reinterpretation based on the puna ridge kilauea volcano the volcanic morphology of a number of segments of the slow spreading mid atlantic ridge mar have been reinterpreted based on our understanding of dike emplacement dike propagation and eruption at the east rift zone of kilauea volcano hawaii and its submarine extension the puna ridge the styles of volcanic eruption at the submarine puna ridge are remarkably similar to those of the axial volcanic ridges avrs constructed on the median valley floor of the mar we use this observation to relate volcanic processes occurring at kilauea volcano to the mar we now consider that volcanic features e g seamounts and lava terraces built on the flanks of the avrs are secondary features that are fed from lava tubes or channels not primary features fed directly from an underlying dike we examine simple models of pipe flow and conclude that lava tubes can transport lava down the flanks of submarine rifts to build all of the volcanic features observed there in addition deep water lava tubes are strong enough to withstand the pressures of a few megapascals that the building of a volcanic structure 150 m high at the end of the tube would generate the volumes of individual volcanic terraces and seamounts on the puna ridge and at the mar are large 0 1 1 km3 and similar to the volumes of lava flows that are broadly distributed at the subaerial east rift zone of kilauea this striking difference in the volcanic morphology on a scale of 1 2 km producing terraces and seamounts underwater and low relief flows on land must be related to the enhanced cooling and to the greater mechanical stability of tubes in the submarine environment we suggest that at the mar a crustal magma reservoir most likely located beneath shallow flat sections of the segment provides magma to the rift axis through dikes that propagate laterally tens of kilometers the zone of dike intrusion at least in the neighborhood of the magma body is likely narrower than the width resurfaced by flows yielding a crustal structure that has a rapid vertical transition from lavas to sheeted dikes at segment ends the zone of dike intrusion is likely to be wider giving a resulting structure with a more gradual transition from lavas to dikes

tectonic evolution of the southwest indian ocean since the mid cretaceous plate motions and stability of the pole of antarctica africa for at least 80 myr summary the southwest indian ridge is the boundary where seafloor is being created between the african and antarctic plates east of the newly recognized du toit fracture zone at 27 e it trends north easterly and is cut by a series of six deep near north south clefts marking transform faults this series starts with the dual prince edward fracture zone near 35 e and includes discovery i1 also dual at 42 e indomed at 46 e and gallieni at 52 e terminating with the spectacular atlantis i1 and melville fracture zones at 57 30 e and 60 30 e respectively the trend of these fracture zones is compatible with an instantaneous pole of relative motion for antarctica africa at 8 4 n 42 4 w between 35 e and 55 e the development of the southwest indian ridge has separated the elevated madagascar ridge from the discontinuous crozet plateau that was contiguous to it prior to eocene time in this entire sector the ocean floor overall is elevated with respect to ocean floor of equivalent age elsewhere between these features anomalies 0 22 0 53 ma have been identified north and 13 26 35 60 ma south of the ridge axis these anomalies and 0 34 0 80 ma located just to the west in the mozambique basin lie at the same distance from the ridge axis as a similar sequence east of the bouvet triple junction the similarity in distance from the ridge axis of these anomaly sequences is evidence that africa and east antarctica have been separated by a single plate boundary from 80ma to the present the pole representing the motion of these two plates at the present has not moved in totality far from the position it occupied in the late cretaceous 80 ma furthermore calculation of intermediate finite rotation poles for four intervals within this span reveals only minor 15 excursions from the present pole the spatial reconstruction of the southwest indian ridge at 80ma anomaly 34 places the ridge midway between africa and eastern antarctica and lying at right angles to the north trending mozambique ridge the separation of antarctica from africa following initial opening in the jurassic may have been as straightforward as it appears to have been from 80 ma to the present

magnetic properties and opaque mineralogy of drilled submarine intrusive rocks summary measurements of natural remanent magnetization nrm curie temperature ferrimagnetic and paramagnetic susceptibility saturation induced and remanent magnetizations coercive forces alternating field properties and viscous magnetization are reported for 50 submarine intrusive rocks drilled during legs 30 37 and 45 of dsdp the collection includes doleritic sill rocks fresh and serpentinized cumulate gabbros and serpentinized cumulate peridotites and serpentinized lherzolites believed to have originated in layers 2b 3b and 4 respectively magnetite with a curie temperature between 520 and 580 c is the principal magnetic mineral in all samples there is no indication of maghemitization or of metamorphism to greenschist facies or above the magnetite in the doleritic and cumulate gabbros is a product of deuteric alteration of titanomagnetite and pyroxene the stable nrm is a primary trm the magnetite in the serpentinized rocks is a secondary product of serpentinization the stable nrm is a crm in most rock types the magnetite is of single domain or pseudo single domain size and soft components of nrm are small the magnetite grain size in some of the doleritic gabbros is much coarser these rocks acquire large viscous magnetizations which however are readily removed by alternating field cleaning the cleaned nrms of legs 30 and 45 rocks have approximately dipole inclinations but the nrms of most of the mutually intruded leg 37 units have been dispersed by tectonic rotations the doleritic gabbros and the serpentinized peridotites have stable directionally coherent nrms 10 3emu cm 3 in intensity their counterparts in layers 2b and 3b 4 are likely contributors to oceanic magnetic anomalies layer 3b cumulate gabbros and tectonic serpentinites in the middle and upper crust are less likely anomaly sources

a systematic analysis of the mid atlantic ridge morphology and gravity between 15 n and 40 n constraints of the thermal structure multibeam bathymetry data obtained along a 2400 km long section of the mid atlantic ridge mar from 15 n to the azores platform 40 n and satellite derived gravity data were used to calculate the mantle bouguer anomaly mba along this portion of the mar both data sets were used to determine the relations between gravity anomalies and topographic variations and discuss these in terms of thermal difference a long wavelength influence of the azores hot spot is characterized by a gentle continuous slope of the average ridge axial depth and a general gradient in the along axis mba profile this thermal influence of the azores hot spot controls a systematic southward propagation of the spreading segments at least to 26 30 n south of 26 30 n the direction of the segment propagation is controlled by the local difference in thermal state between adjacent segments except on the azores platform the systematic along axis 11 90 km long wavelength segmentation is independent of the long wavelength influence of the azores at the segment center the axial morphology is linked to the thermal state of the segments between 1 hotter segments characterized by a smooth axial morphology a well defined shallow inner valley high mba and a long length 2 colder segments which present a rough axial morphology with a deep wide and well defined rift valley a low mba and a small length for hotter segments the formation of the abyssal hills is mainly due to a magmato tectonic cycle over periods of 0 3 to 1 myr whereas on colder segments the axial morphology is mainly controlled by a tectonic rift valley formation we propose that these different segment types correspond to a temporal evolution of the rift valley morphology over periods of several million years

east pacific rise 18 19 s asymmetric spreading and ridge reorientation by ultrafast migration of axial discontinuities a detailed bathymetric side scan and magnetic survey of the east pacific rise out to a seafloor age of 1 ma has been carried out between 18 and 19 s it reveals that some left stepping axial discontinuities have been migrating southward at rates an order of magnitude faster than the spreading rates 1000 mm a or higher these rapid migration events have left on the nazca plate discordant features striking nearly parallel to the ridge axis a discontinuity with an offset of several kilometers has migrated in two stages at around 0 45 and 0 3 ma and has left two large discordant zones consisting of a series of unfaulted hummocky basins bounded to the east by short ridges oriented about n s oblique to the ambient 013 fabric the morphology and reflectivity characteristics of these discordant zones are akin to the overlap basins and abandoned ridge tips which make up the migration trails of large slowly migrating overlapping spreading centers between 18 35 and 19 03 s the ridge axis is flanked a few kilometers to the east by a prominent sedimented ridge previously recognized as a recently abandoned ridge axis the present ridge segment steadily deepens and narrows southward which suggests the abandoned ridge has been rafted onto the nazca plate during the ultrafast southward propagation of the ridge segment rather than by one discrete ridge jump by transferring pacific lithosphere to the nazca plate these migration events account for most of the asymmetric accretion observed faster to the east this process is consistent with the features common to asymmetric spreading namely the sudden onset or demise of asymmetric spreading and the ridge segment to ridge segment variability because the discordant zones left by these rapid migration events are near parallel to the ambient seafloor fabric they are unlikely to be detected by conventional bathymetry or magnetic surveys and so called ridge jumps may actually often represent ultrafast propagation of a ridge segment variations in fault azimuth with age show there has not been any significant change in spreading direction over the past 0 8 m y instead the counterclockwise trend of the east pacific rise relative to the brunhes matuyama reversal 0 78 ma mostly reflects that ultrafast propagation of ridge segments has transferred a larger amount of the pacific lithosphere to the nazca plate at 18 s than at 19 s in keeping with the regional features of the magnetic anomalies we propose that an 8 to 10 km left stepping discontinuity which was located between 17 and 17 30 s at 0 78 ma has been recently redistributed into the present staircase of small left stepping discontinuities between 16 and 19 s this smoothing of the ridge geometry probably occurred through repeated small lateral steps of the ridge segments inside of the discontinuities during ultra fast propagation episodes and may be the consequence of a significant replenishment of the magma reservoir between 17 and 17 30 s during the past 1 m y

gas changes in the krafla geothermal system iceland abstract a massive influx of gas was encountered in the krafla geothermal system iceland following the start of magmatic activity in the area in 1975 the gas was of magmatic origin but its composition had been modified to predominantly carbon dioxide and a small amount of hydrogen sulphide the gas concentrations of krafla borehole fluids have been monitored since and the pattern has been similar for wells in the affected leirbotnar field i e a maximum in 1977 1978 a secondary maximum in 1980 and a steady decline since the more southerly su urhli ar and hvitholar fields were not affected fumaroles were sampled extensively in 1978 1979 and 1984 1985 a similar decline was observed in the affected area but a geographical trend that had been noticed for the borehole fluids was much clearer in this case the decline is smallest in the northeastern part of the field but more pronounced to the south and west it is suggested that magmatic gas has not entered the geothermal system after 1980 and that the excess gas observed now comes from a supply located in the northeastern part of the field near the explosive crater viti

marine magnetic anomalies the origin of the stripes the results of recent observational and theoretical investigations of lineated magnetic anomalies on the ocean floor are summarized in tables graphs and diagrams and analyzed topics addressed include early lineation models inversions of magnetic anomalies to obtain source functions deep tow studies of magnetic anomalies evidence from the long wavelength component of the magnetic field including magsat observations and direct measurements of the magnetic properties of oceanic rocks it is concluded that the source of the lineated anomalies must reside in most of the oceanic crust not just in the pillow lavas of layer 2a

vertebrate microremains from the upper silurian winnica formation of the holy cross mountains poland vertebrate microremains from the upper silurian winnica formation of the holy cross mountains poland

magmatic gases in well fluids aid the mapping of the flow pattern in a geothermal system abstract gas composition and silica concentrations of well fluids are used in conjunction with pressure temperature and enthalpy data to obtain a model of the drilled part of the krafla geothermal field northeast iceland a magma chamber is located at 3 8 km depth under the field magmatic gases emanate from the chamber and travel via a channel reaching the surface at the hveragil eruptive fissure the composition of the gases is apparently modified on the way in that sulphur which presumably is in the form of sulphur dioxide to begin with is removed on the way and what remains is in the form of hydrogen sulphide at the end it is suggested that the major removal mechanism is the deposition of pyrite and pyrrhotite during the passage of the gases through the hydrothermal system the hydrothermal system is divided into a lower part whose temperature exceeds 300 c and whose fluids are to a varying extent affected by the magmatic gases and an upper part whose temperature is ca 200 c this upper part is a run off from the lower part but the magmatic effects have mostly disappeared there

evidence for motion between nubia and somalia along the southwest indian ridge the east african rift marks the northern boundary of the nubian west african and somalian east african plates and has formed by horizontal stretching due to the separation of these plates1 south of 20 s any expression of deformation or seismicity due to the relative motion of these two distinct plates vanishes although the boundary must continue until it intersects another plate boundary the nearest such boundary is that of the antarctic plate marked by the southwest indian ridge but previous analyses of plate motion data have indicated no significant difference between nubia antarctica and somalia antarctica motion2 3 here we show using a large compilation of plate motion data that nubia antarctica motion does differ from somalia antarctica motion and we determine a relative angular velocity of the two plates that has compact confidence limits our analysis places the pole of rotation near to the southern limit of african seismicity implying that the southern part of the nubian somalian plate boundary is a diffuse zone of convergence up to 2 mm yr 1 whereas up to 6 mm yr 1 of separation is accommodated across the east african rift about half the separation rate of the slowest mid ocean ridge

asymmetric sea floor spreading caused by ridge plume interactions crustal accretion at mid ocean ridges is generally modelled as a symmetric process regional analyses however often show either small scale asymmetries which vary rapidly between individual spreading corridors or large scale asymmetries represented by consistent excess accretion on one of the two separating plates over geological time spans1 2 3 4 5 6 in neither case is the origin of the asymmetry well understood here we present a comprehensive analysis of the asymmetry of crustal accretion over the past 83 myr based on a set of self consistent digital isochrons7 and models of absolute plate motion8 9 we find that deficits in crustal accretion occur mainly on ridge flanks overlying one or several hotspots we therefore propose that asymmetric accretion is caused by ridge propagation towards mantle plumes or minor ridge jumps sustained by asthenospheric flow10 11 between ridges and plumes quantifying the asymmetry of crustal accretion provides a complementary approach to that based on geochemical12 and other geophysical data13 14 in helping to unravel how mantle plumes and mid ocean ridges are linked through mantle convection processes

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

hydrothermal interaction of columbia plateau basalt from the umtanum flow washington u s a with its coexisting groundwater abstract a series of experiments have been performed in which tholeiitic basalt from the umtanum flow of the columbia river plateau was reacted with the synthetic equivalent of groundwater with which it is in contact at a pressure of 300 bar temperatures of 100 200 and 300 c and water rock mass ratios of between 5 and 50 reaction products formed included silica illite scapolite wairakite heulandite k feldspar and smectite the alteration mineral assemblage formed during the experiments are similar to those predicted for equilibrium with geothermal waters in a tholeiitic system solution ph decreased relative to that of the initial solution the final calculated high temperature ph values were near 7 6 at 300 c 7 5 at 200 c and 7 2 at 100 c the ph decrease is much less than that of basalt seawater experiments the ph decrease may be due to incorporation of na into secondary products solution composition was modified silica al k and sulfide increased whereas na decreased the oxidation state of the solution became reduced during the experiment approaching the hematite magnetite pyrite invariant point at 300 c the solution composition from the experiments compare favorably with the compositions of some icelandic geothermal fluids

variations in oceanic crustal magnetization systematic changes in the last 160 million years the amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age globally the amplitudes decrease with age over the first 20 to 30 million years a phenomenon that has been attributed to the low temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks in oceanic crust older than 40 million years however the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years in order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust we compiled all of the existing rock magnetic data from relevant deep sea drilling project and ocean drilling program sites that sampled extrusive rocks from normal ocean crust this compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes both in the decrease due to oxidation in the first 20 to 30 million years and in the increase in magnetization in older 80 million years crust this positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks further the curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years and does not increase significantly beyond that point finally the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust specifically we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic feti oxides in the older crustal rocks this large increase in abundance of the feti oxides may be related to a systematic increase in bulk feti content in older tholeiitic basalts or more likely is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide phases

chemical geothermometry and geobarometry in hydrothermal aqueous solutions a theoretical investigation based on a mineral solution equilibrium model abstract the theoretical compositions of an aqueous solution in equilibrium with a mineral assemblage made up of low albite k feldspar either a ca al silicate or calcite clinochlore muscovite quartz anhydrite and fluorite under varying t pco2 mcl conditions of geothermal interest indicate that 1 1 the total so4 content as well as the na k k 2 mg and so 4 f 2 ratios are potential geothermometers 2 2 the total hco3 content as well as the k 2 ca ca mg hco 3 f and hco 3 2 so 4 ratios are potential pco2 indicators 3 3 the na k ca and mg total contents as well as the na 2 mg and na 2 ca ratios are mainly controlled by the total ionic salinity and are therefore hardly suitable as geoindicators a preliminary test of the equations involving total hco3 content as well as k 2 ca and hco 3 f ratios as pco2 indicators have provided satisfactory results

magnetic polarity structure of the lower oceanic crust a newly developed rock drill was used to take the first fully oriented samples from in situ gabbros exposed on atlantis bank which formed 11 m y ago on the southwest indian ridge swir these paleomagnetic data locate the seabed outcrop of a normal polarity magnetic lineation within an outcrop of lower crust that correlates with overlying deep towed and sea surface magnetic field measurements a systematic lateral offset between the seabed boundary and measured anomalies implies that the reversal boundary dips 25 to the south away from the ridge axis paleomagnetic results suggest that following magnetization acquisition the platform tilted south by 20 range 33 south to 7 north implying an original reversal boundary dip of only 5

ridge segmentation and the magnetic structure of the southern mid atlantic ridge 26 s and 31 35 s implications for magmatic processes at slow spreading centers along axis profiles of three dimensional magnetic inversions for the mid atlantic ridge mar 31 35 s show low magnetization near the middle of ridge segments and high magnetization at the segment tips for three adjacent spreading segments thus there is an inverse relation between axial magnetization and axial topography the ridge segment at 26 s on the mar has the same inverse relationship between magnetization and topography the common occurrence of this relationship suggests that it reflects a fundamental process of crustal accretion at the mar we analyze the rock magnetic properties from 42 locations within the four ridge segments in the south atlantic to constrain the inherent trade off between source intensity and source thickness in the magnetization model the natural remanent magnetization nrm intensities from the four ridge segments averaged together correlate with the magnetic inversion profiles this finding implies that changes in the magnetization of the extrusives may account for much of the observed magnetic anomaly amplitude variation a direct correlation of feo content and magnetization suggests that magnetic anomaly amplitudes may be an indicator of feti rich basalts at the slow spreading mar even though the iron content of the basalts from high magnetization areas is not as high as observed at pacific spreading centers despite the different magma plumbing systematics of the pacific spreading centers and the mar it appears that the segment scale magma system of the mar also results in segment scale crustal magnetization variations further evidence that the axial magnetic variations result from source intensity variations is that older isochrons have higher intensities near the ridge discontinuities similar to the behavior on axis between 0 and 5 ma the decay in magnetization is 50 independent of location within a spreading segment

reassessment of ischnacanthus scheii spjeldnaes acanthodii ischnacanthiformes from the latest silurian or earliest devonian of ellesmere island arctic canada articulated specimens of jawed fishes and assemblages of disarticulated elements that can be assigned to a single biological species are extremely rare from pre devonian deposits the acanthodian species ischnacanthus scheii spjeldnaes is based on a monospecific assemblage comprising fin spines dentigerous jaw bone fragments and scales from the siluro devonian boundary beds of the devon island formation in central west ellesmere island canadian arctic archipelago nunavut a new examination of the type material in particular by scanning electron microscopy and thin sectioning of scales shows that the species is a porosiform poracanthodid that is now assigned to radioporacanthodes scheii comb nov scales of the same species are also recognized from the upper pridoli of cornwallis island and the pridoli or lochkovian of north greenland

the mid atlantic ridge between 29 n and 31 30 n in the last 10 ma abstract the segmentation of the mid atlantic ridge between 29 n and 31 30 n during the last 10 ma was studied within our survey area the spreading center is segmented at a scale of 25 100 km by non transform discontinuities and by the 70 km offset atlantis transform the morphology of the spreading center differs north and south of the atlantis transform the spreading axis between 30 30 n and 31 30 n consists of enechelon volcanic ridges located within a rift valley with a regional trend of 040 south of the transform the spreading center is associated with a well defined rift valley trending 015 magnetic anomalies and the bathymetric traces left by non transform discontinuities on the flanks of the mid atlantic ridge provide a record of the evolution of this slow spreading center over the last 10 ma migration of non transform offsets was predominantly to the south except perhaps in the last 2 ma the discontinuity traces and the pattern of crustal thickness variations calculated from gravity data suggest that focused mantle upwelling has been maintained for at least 10 ma south of 30 30 n in contrast north of 30 30 n the present segmentation configuration and the mantle upwelling centers inferred from gravity data appear to have been established more recently the orientation of the bathymetric traces suggests that the migration of non transform offsets is not controlled by the motion of the ridge axis with respect to the mantle the evolution of the spreading center and the pattern of segmentation is influenced by relative plate motion changes and by local processes perhaps related to the amount of melt delivered to spreading segments relative plate motion changes over the last 10 ma in our survey area have included a decrease in spreading rate from 32 mm a 1 to 24 mm a 1 as well as a clockwise change in spreading direction of 13 between anomalies 5 and 4 followed by a counterclockwise change of 4 between anomaly 4 and the present interpretation of magnetic anomalies indicates that there are significant variations in spreading asymmetry and rate within and between segments for a given anomaly time these differences as well as variations in crustal thickness inferred from gravity data on the flanks of spreading segments indicate that magmatic and tectonic activity are in general not coordinated between adjacent spreading segments

variations in layer 2a thickness and the origin of the central anomaly magnetic high the seismically determined off axis thickening of the extrusive layer is apparently at odds with the magnetic anomaly high t ypic ally associated with the ridge crest the positive magnenzauon contrast at the ridge crest is most likely caused by rapid alteration of the extrusive source layer which occurs over spatial scales 2 3 km comparable to that of the proposed layer 2a thickening we present magnetic rema nence data from basalts dredged on and near the east pacific rise axis at 12on which are compatible with a rapid magnetization reduction 20 k y to decay to l e together with near bottom magnetic profiles from the ultra fast spreading east pacific rise at 19 5os these data suggest that previous estimates of the time constant of alteration inferred from slow spreading ridges 0 5 m y may be more than an order of magnitude too high seismic studies of the fast spreading east pacific rise epr indicate a rapid thickening of seismic layer 2a within a few kilometers of the ridge axis vera et al 1990 toomey et ai 1990 detrick et al 1993 these seismic data have been interpreted to reflect an approximate doubling of the extrusive layer thickness from 100 200 m to 300 500 m through addition of flows whi ch may extend 1 2 km off axis christeson et al 992 vera and diebold in press such thickening of the extrusive source layer had been argued on the basis of the widths of magnetic reversal transitions e g macdonald et al 1983 however this geometry for a uniformly magnetized source layer should produce a marked magnetic anomaly minimum over the ridge axis and is therefore difficult to reconcile with the pronounced anomaly high typically associated with the ridge crest klitgord 1976 sea surface magnetic profiles from the ultra fast spreading 77 mrn yr half rate epr near 19 5os figure 1 illustrate the short wavelength axial anomaly high within the 120 km wide central anomaly which has been termed the central anomaly magnetic high camh klitgord 1976 the camh typically has a wavelength of 8 10 km in sea surface profiles at in termediate spreading ridges 15 45 mm yr the camh occupies a larger proportion of the central anomaly and at slow spreading ridges it is not readily distinguishable in sea surface profiles from the central anomaly because the wavelengths become similar near bottom magnetic profiles figure 1 typically have higher amplitude and shorter wavelength 2 4 kin axial anomalies as expected from the closer proximity to the source klitg6rd et al 1975 miller 1977 perram et al 1990 and even allow distinction of the camh from the central anomaly at some slow spreading ridges macdonald 1977 the higher temporal resolution of faster spreading ridges therefore provides the best opportunity to evaluate the processes responsible for the camh thickness variations in the extraslye layer at fast spreading fi dges have a profound effect on the axial magnetic anomaly since the presence of an axial magma chamber and the low re rnanent intensity of intrusive rocks e g smith and banerice 1986 pariso et al 1991 minimize deeper source contributions in the near ridge environment based on seismic

segmentation and crustal structure of the western mid atlantic ridge flank 25 25 27 10 n and 0 29 m y we conducted a detailed geological geophysical survey of the west flank of the mid atlantic ridge between 25 25 n and 27 10 n and from the ridge axis out to 29 ma crust acquiring hydrosweep multibeam bathymetry hawaii mr1 sidescan sonar imagery gravity magnetics and single channel seismic reflection profiles the survey covered all or part of nine spreading segments bounded by mostly nontransform right stepping discontinuities which are subparallel to flow lines but which migrated independently of one another some discontinuities alternated between small right and left stepping offsets or exhibited zero offset for up to 3 4 m y despite these changes the spreading segments have been long lived and extend 20 m y or more across isochrons a large shift 9 in relative plate motion about 24 22 ma caused significant changes in segmentation pattern the nature of this plate boundary response together with the persistence of segments through periods of zero offset at their bounding discontinuities suggest that the position and longevity of segments are controlled primarily by the subaxial position of buoyant mantle diapirs or focused zones of rising melt within segments there are distinct differences in seafloor depth morphology residual mantle bouguer gravity anomaly and apparent crustal thickness between inside corner and outside corner crust this demands fundamentally asymmetric crustal accretion and extension across the ridge axis which we attribute to low angle detachment faulting near segment ends cyclic variations in residual gravity over the crossisochron run of segments also suggest crustal thickness changes of at least 1 2 km every 2 3 m y these are interpreted to be caused by episodes of magmatic versus relatively amagmatic extension controlled by retention and quasiperiodic release of melt from the upwelling mantle detachment faulting appears to be especially effective in exhuming lower crust to upper mantle at inside corners during relatively amagmatic episodes creating crustal domes analogous to turtleback metamorphic core complexes that are formed by low angle detachment faulting in subaerial extensional environments

dynamical consequences of compositional and thermal density stratification beneath spreading centers extraction of partial melt to form oceanic crust reduces the density of upwelling mantle beneath spreading centers this compositional buoyancy can significantly affect both the rate of upwelling and the pattern of mantle flow the relative importance of buoyant upwelling and upwelling due to plate spreading depends on the mantle viscosity for a low enough mantle viscosity buoyant upwelling can significantly enhance the thickness of the crust relative to that which would be produced by plate spreading alone for a mantle viscosity of 1022 pa s upwelling due to plate spreading is dominant and crustal thickness is predicted to be a strong function of spreading rate however for a mantle viscosity of 1019 pa s buoyancy dominates mantle upwelling and the calculated crustal thickness like the oceanic crust is nearly independent of spreading rate thermal and compositional density variations result in opposing buoyancy forces that can cause time dependent upwelling for a mantle viscosity of 1020 pa s and a low 1 cm yr spreading rate this behavior takes the form of undamped crustal thickness oscillations with a single well defined period for even slightly lower mantle viscosity the frequency increases and crustal thickness oscillations become more complex with two or more frequencies present while numerical experiments have thus far been carried out only for viscosities significantly greater than that likely to characterize the mantle beneath spreading centers we speculate that these crustal thickness oscillations may explain important aspects of seafloor structure

low temperature oxidation in morb of titanomagnetite to titanomaghemite a gradual process with implications for marine magnetic anomaly amplitudes mid ocean ridge basalt morb samples varying in age from recent to jurassic were selected for electron microscopic and rock magnetic studies our observations indicate that the degree of oxidation of titanomagnetite in morb increases only gradually with sample age the titanomagnetite in recent morb 100 ka both electron microscopic observations and rock magnetic data support the notion that low temperature oxidation of titanomagnetite to titanomaghemite in morb is a gradual process moreover the rate of maghemitization is controlled by many factors on both macroscopic and microscopic scales including regional oceanic crustal structures lithological features grain size and surrounding matrices pillow lavas and the tops of massive flows tend to have higher degrees of oxidation than the interiors of massive flows owing to higher porosity and permeability of the former in contrast interstitial glass can protect fine titanomagnetite grains from alteration the natural remanent magnetization nrm intensity of morb varies as a function of age magnetic granulometry concentration of magnetic materials and the degree of alteration fine grained morb samples typically have higher nrm intensity the nrm intensity appears to decrease substantially with increasing degree of maghemitization and hence with age the envelope of the smoothed magnetic anomaly amplitudes resembles the change in nrm intensity for morb samples of the last 30 million years but the underlying assumption that the highest degree of maghemitization observed in the samples of a given age is entirely responsible for these intensity changes is not supported maghemitization of the micrometer sized and larger grains is probably only partly responsible whereas a significant contribution to nrm intensities is inferred from those submicrometer sized titanomagnetite grains that remained protected from oxidation by the surrounding matrix of interstitial glass

axial magnetic anomaly amplitude along the mid atlantic ridge between 20 n and 40 n after reduction to correct for the topography spreading rate latitude and lineation azimuth effects axial magnetic anomalies amplitudes are compared with bathymetric gravity seismological and geochemical data along a 2600 km long section of the mid atlantic ridge axis between 20 n and 40 n significant results are obtained at two different scales a regional one 100 km related to sublithospheric thermal perturbations such as those associated to hotspots and a local one 100 km related to ridge segmentation at a regional scale two long wavelength highs of the axial magnetic anomaly amplitudes are observed between 37 5 n and 40 n and between 27 n and 30 n the first high associated with marked bathymetric gravity geoid seismic velocity and geochemical anomalies is related to the azores hotspot and is interpreted as reflecting the existence of a thick and or fe ti enriched magnetic source layer a second long wavelength high centered at 28 5 n corresponds to a weaker bathymetric and gravity signature it is associated with a large low seismic velocity anomaly and a marked intermediate wavelength positive geoid high which suggest higher sublithospheric temperatures deeper and more extensive partial melting and a thicker and or fe ti enriched magnetic source layer at a local scale all but one of the 25 ridge segments investigated show that axial magnetic anomaly amplitude is higher at segment ends by a factor of 2 compared to segment centers a clear correlation is observed between the range of axial magnetic anomaly amplitude and the range of the mantle bouguer anomaly mba within each segment more complex relationships exist between median amplitude amplitude at segment centers and at segments ends and mba separation of segments in two groups hotter and colder based on mba shows different types of variations among segment with grouped amplitudes at segment centers and scattered amplitudes at segment ends for hotter segments and the opposite for colder segments these observations support shallow fractionation and the presence of serpentinized bodies in the vicinity of discontinuities as the major processes which control the axial magnetic anomaly amplitude variations

morphology and tectonics of the mid atlantic ridge 25 27 30 s from sea beam and magnetic data high resolution sea beam bathymetry and sea surface magnetic data have been collected between 25 s and 27 30 s on the mid atlantic ridge analysis of the morphology and structure of the ridge axis and flanking terrain as well as the results of a three dimensional inversion of sea surface magnetic data are presented this 227 km long portion of the mid atlantic ridge is offset 42 km by the right lateral rio grande transform at 25 40 s and 9 5 km by the left lateral nontransform moore discontinuity at 26 30 s a third nontransform discontinuity the midway discontinuity exists near 26 12 s and is defined by a small displacement of the axial magnetization distribution and the disruption in the along strike continuity of ridge parallel terrain elements although the detailed survey presented in this paper extends out to only 5 to 7 m y old crust a regional compilation of magnetic data from this area by cande et al 1988 indicates that the relative positions and dimensions of the spreading cells have characterized this part of the mid atlantic ridge for at least the past 55 m y the offsets dimensions and geometries however have changed markedly because of prolonged differential asymmetric spreading between segments between and within the accretionary units that are spreading at uniform rates 35 mm yr total opening rate the morphology of the ridge axis varies dramatically from a well defined rift valley 20 25 km wide 1 1 5 km deep to an axial swell that rises 1500 m above the adjacent valley floor the axial magnetic structure is also highly variable along strike the inversion solution shows a series of generally linear short wavelength 10 km magnetization highs within the central brunhes anomaly over each of the four segments in the survey area these anomalies are interpreted to be the central anomaly magnetization high that is observed over the neovolcanic zones of many of the world s mid ocean spreading centers the distinct axial magnetization highs can extend out to 2 m y old crust at ridge discontinuity intersections suggesting that in some instances they may also be the product temporal variations in the geochemistry of the extrusive rocks e g highly fractionated and strongly magnetized basalts enriched in iron and titanium the magnetic inversion shows that the central anomaly is locally attenuated at the minimum depth points of three of the four spreading segments in the survey area both inversion and forward modeling of magnetic data show that topography alone cannot produce the anomalously low values observed in the case of an axial swell centered at 26 s attenuation of the central anomaly is attributed to the effective thinning of the source layer by demagnetization effects of a crustal thermal anomaly and or a zone of substantial alteration taken together the bathymetric data and magnetic modeling document strong along axis gradients in crustal and upper mantle properties

magnetization of 0 29 ma ocean crust on the mid atlantic ridge 25 30 to 27 10 n a sea surface magnetic survey over the west flank of the mid atlantic ridge from 0 to 29 ma crust encompasses several spreading segments and documents the evolution of crustal magnetization in slowly accreted crust we find that magnetization decays rapidly within the first few million years although the filtering effect of water depth on the sea surface data and the slow spreading rate 13 km m y preclude us from resolving this decay rate a distinctly asymmetric along axis pattern of crustal magnetization is rapidly attenuated off axis suggesting that magnetization dominated by extrusive lavas on axis is reduced off axis to a background value off axis we find a statistically significant correlation between crustal magnetization and apparent crustal thickness with thin crust tending to be more positively magnetized than thicker crust indicative of induced magnetization in thin inside corner ic crust in general we find that off axis segment ends show an induced magnetization component regardless of polarity and that ic segment ends tend to have slightly more induced component compared with outside corner oc segment ends possibly due to serpentinized uppermost mantle at ic ends we find that remanent magnetization is also reduced at segment ends but there is no correlation with inside or outside corner crust even though they have very different crustal thicknesses this indicates that remanent magnetization off axis is independent of crustal thickness bulk composition and the presence or absence of extrusives remanence reduction at segment ends is thought to be primarily due to alteration of lower crust in oc crust and a combination of crustal thinning and alteration in ic crust from all these observations we infer that the remanent magnetization of extrusive crust is strongly attenuated off axis and that magnetization of the lower crust may be the dominant source for off axis magnetic anomalies

thin crust beneath ocean drilling program borehole 735b at the southwest indian ridge abstract a wide angle seismic experiment at the atlantis ii fracture zone southwest indian ridge together with geochemical analyses of dredged basalt glass samples from a site conjugate to ocean drilling program hole 735b has allowed determination of the thickness and the most likely lithological composition of the crust beneath hole 735b the measured na8 composition of 3 3 0 1 corresponds to a melt thickness of 3 1 km a result consistent with rare earth element inversions which indicate a melt thickness of between 1 5 and 4 5 km the seismic crustal thickness to the north and south of the atlantis platform on which hole 735b is located is 4 1 km and probably consists largely of magmatic material since the seismic and inferred melt thicknesses agree within experimental uncertainty beneath hole 735b itself the moho is at a depth of 5 1 km beneath the seafloor the seismic model suggests that on average about 1 km of upper crust has been unroofed on the atlantis platform however allowing for the inferred local unroofing of 2 km of upper crust at 735b the base of the magmatic crust beneath this location is probably about 2 km beneath the seafloor and is underlain by a 2 3 km thick layer of serpentinised mantle peridotite the p wave velocity of 6 9 km s for the serpentinised peridotite layer corresponds to a 35 10 vol serpentine content the moho beneath hole 735b probably represents a serpentinisation front

do layer 3 rocks make a significant contribution to marine magnetic anomalies in situ magnetization of gabbros at ocean drilling program hole 735b we estimate the average magnetization of a vertical section of oceanic gabbros using paleomagnetic and downhole magnetic logging techniques and evaluate the ability of these crustal rocks to contribute to marine magnetic anomalies results from ocean drilling program hole 735b show that the drilled crustal section has a mean effective remanent magnetization of 2 5 a m olivine gabbros which make up 60 of the section have an average effective magnetization between 1 and 2 a m we used two dimensional forward modeling and the average effective magnetization of olivine gabbros to predict the magnitude of a typical layer 3 anomaly observable at sea surface these results indicate that if polarity boundaries within oceanic layer 3 are near vertical crustal sections similar to 735b would contribute from 25 to 75 of the overlying marine magnetic anomalies at the sea surface the shape of the polarity boundaries in lower crust will be controlled by its thermal evolution and potentially could be constrained by systematic analyses of the amplitude and skewness of marine magnetic anomalies at individual spreading centers

no evidence from multichannel reflection data for a crustal magma chamber in the mark area on the mid atlantic ridge the role of crustal magma chambers in the construction of oceanic crust along the slowly spreading mid atlantic ridge mar is still highly disputed1 some investigators have argued for small short lived crustal magma bodies2 whereas others have postulated the presence of permanent reservoirs centred between ridge offsets which expand and contract in response to a fluctuating magma supply3 thermal modelling4 5 detailed seismic refraction experiments at several different locations along the mar6 9 and micro earthquake10 11 and teleseismic source mechanism studies12 preclude the existence of large steady state magma chambers under the entire mar rift valley however smaller localized crustal magma bodies could be present at least intermittently here we use multichannel seismic reflection profiles to look for evidence of a crustal magma chamber along the mar south of the kane fracture zone in one of the most magmatically robust and hydro thermally active areas presently known along the mar although some subbasement reflectors are imaged we do not see a shallow large amplitude intracrustal reflector that might be associated with a crustal magma body if a crustal magma chamber was associated with the most recent volcanic episode along this part of the mar our results suggest that it was extremely short lived

variations in magnetization intensity and low temperature titanomagnetite oxidation of ocean floor basalts the remanent magnetization of the oceanic crust exhibits a systematic long term variation which correlates with the amplitudes of marine magnetic anomalies after a sharp initial decrease of natural remanent magnetization intensity a minimum is reached at 20myr followed by a gradual increase up to ages of 120 myr the progressive sea floor alteration of the magnetic minerals carrying the crustal magnetism is proposed as a cause for this behaviour

formation of the axial relief at the very slow spreading southwest indian ridge 49 to 69 e the comparison of segment lengths relief and gravity signature along the very slow spreading southwest indian ridge swir between 49oe and 69oe suggests that the marked change in segmentation style that occurs across the melville transform 60o45 e reflects a change in the modes of formation of the axial topography we propose that the axial relief east of melville is largely due to volcanic constructions that load the axial lithosphere from above by contrast the axial relief in segments west of the melville fracture zone appears to be primarily due as proposed for segments of the faster spreading mid atlantic ridge to along axis changes in the depth of the axial valley and to partial compensation of negative loads thicker lower crust and or lighter upper mantle acting within the plate or at the bottom of the plate in terms of geology this means that the contribution of the uppermost effusive part of the crust to along axis crustal thickness variations may be greater east of melville than in other regions of the study area regional axial depths suggest that the ridge east of melville is also characterized by a low melt supply and is underlain by cold mantle a simple model of mantle melting and regional isostatic compensation suggests that differences in mantle temperature and in melt thickness between this deep eastern ridge region and the shallower region west of the gallieni transform 52o20 e are of the order of 80oc and 4 km respectively

environment of the early paleozoic vertebrates abstract a comprehensive review of the known pre devonian vertebrate localities suggests that the non marine environment was not invaded until about wenlock early mid silurian time known well dated late cambrian ordovician and early silurian vertebrate localities are all in the marine environment considerations of paleogeography for the pre wenlock indicate that most of the vertebrate localities are too distant from shorelines to provide any basis for transport from the non marine environment to a marine depositional site as a favored explanation the articulated nature of the vertebrates from some of these pre wenlock occurrences also supports this interpretation reconsideration of both the physiological and embryological evidence relative to living vertebrates and some related chordates is also consistent with the view that the vertebrates originated in the marine environment

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

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

ridge jumps and propagations in the south atlantic ocean the normal pattern of mid ocean ridge divergent plate boundaries comprises straight spreading segments with narrow zones of crustal accretion bounded by transform faults where these segments are offset normally the configuration of spreading segments and transform faults is stable for long periods but in places on fast and medium rate spreading ridges the transform offsets have propagated along the ridge axis as some spreading segments have grown at the expense of adjacent ones1 7 although such behaviour has hitherto not been well documented on slow spreading ridges we have mapped multiple ridge jumps and propagating rifts in two areas of the slow spreading 18 mm yr 1 half rate south atlantic ridge crest we attribute this anomalous behaviour in the south atlantic to the presence of hotspots near our study areas causing elevated asthenospheric mantle temperatures and lateral mantle flow with consequent excess volcanism and disruption of the normally well focused melt delivery processes at the ridge crest

geothermal mineral equilibria the dominant reaction determining the chemistry of fluids in a geothermal system of the new zealand type consists of the conversion of primary plagioclase by co2 to calcite and clays with log pco2 15 26 7850 t 273 2 temperature t in c subsequent reactions involving secondary minerals control relative co2 h2s contents the distribution of mineral phases throughout a geothermal system reflects the stepwise conversion of thermodynamically unstable primary phases through a series of intermediate metastable phases to a thermodynamically stable secondary assemblage the relative stabilities of these phases was evaluated on the basis of their solubilities the least soluble aluminiumsilicate representing the thermodynamically most stable phase under a given set of conditions observed assemblages of secondary minerals in geothermal systems represent indicators allowing mineral fluidinteraction conditions to be evaluated on the basis of multi component mineral stability diagrams

hydrothermal alteration characteristics of the chating cu au deposit in xuancheng city anhui province china significance of sericite alteration for cu au exploration abstract the chating cu au deposit in xuancheng city anhui province is characterized by porphyry style mineralization with intensive hydrothermal alteration short wave infrared reflectance spectroscopy was used to systematically analyze the alteration of drill cores from two cross sections eleven alteration minerals sericite smectite kaolinite dickite chlorite epidote gypsum carbonate iron oxide saponite and prehnite were identified and the relative abundances of individual minerals at various depths were calculated by depth of absorption peak in these minerals iron oxides and gypsum are probably related to supergene process and other alteration minerals are hydrothermal alteration minerals the mineral assemblages and their spatial distributions indicate that the alteration patterns in this cu au deposit were controlled mainly by water rock interaction of the magmatic hydrothermal process resulting in a typical porphyry style hydrothermal mineralization system a saponite smectite k feldspar anhydrite gypsum alteration assemblage is located mainly in the host quartz diorite porphyry and is minor in the wall rocks of limestone and a post mineralization diorite porphyry a sericite chlorite alteration assemblage was widely developed in the quartz diorite porphyry and the intensity of alteration decreases gradually outward from the center to the margins of the deposit a kaolinite calcite alteration assemblage is mainly located on the top of the quartz diorite porphyry and in the surrounding limestone the physiochemical conditions of the hydrothermal fluid such as temperature and ph control the compositional variations of specific alteration minerals the most remarkable feature is a subtle change in octahedral al cation content of sericite as identified by the change in wavelength of sericite al oh absorption peak position muscovite is the main component of sericite with a trace amount of phengite the cu au grades are negatively correlated with the wavelength of sericite al oh absorption peak position because high ore grades are associated with relatively shorter al oh wavelengths corresponding to a high al si ratio in sericite i e towards muscovite this correlation suggests that the ore metal tends to be enriched and precipitated in an acidic environment of the alteration system in addition a lateral variation across the deposit in al oh wavelength indicates that the acidity of the hydrothermal fluids decreases gradually from the center of the deposit outward to the wall rocks whereas the variation of sericite al oh wavelength with depth suggests an increase in fluid ph with decreasing depth the alteration patterns imply that during mineralization the ore forming fluids probably migrated from the southwest in the deep part of the porphyry system towards the northeast near the surface based on the spectral alteration mapping it is concluded that there were at least three stages of hydrothermal activity in the chating mining district an early potassic skarn hydrothermal alteration stage an intermediate propylitic sericite medium low temperature alteration stage and a late kaolinite dickite epithermal stage the sericite alteration stage is temporally related to cu au mineralization leading to the formation of the main orebody at chating as the current exploration is still focused in the sericite alteration zone it is inferred that deep exploration has great potential in the chating district

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

gravity anomalies and crustal thickness variations along the mid atlantic ridge between 33 n and 40 n multibeam bathymetry and gravity data have been obtained along an 800 km long section of the mid atlantic ridge from just south of the hayes fracture zone at 33 n to the northern edge of the azores platform near 40 n a three dimensional analysis of these gravity and topography data combined with results from earlier seismic refraction studies in this area reveal two different scales of crustal heterogeneity a systematic along axis segment scale 8 9 km near the middle of spreading segments and the thinnest crust 3 4 km near segment offsets the magnitude of this along axis variation in crustal thickness is proportional to the length of the spreading segment and the size of the adjacent ridge offset there also is a distinct asymmetry in crustal structure across the rift valley near large segment offsets with gravity highs inferred to be thinner crust beneath the inside corner highs adjacent to these offsets this segment scale crustal heterogeneity is similar to that reported from the kane to atlantis section of the mid atlantic ridge and from parts of the intermediate spreading southern mid atlantic ridge it is superimposed on a second longer wavelength variation in gravity and crustal thickness associated with the azores hot spot the most pronounced effect of the azores hot spot on the mid atlantic ridge occurs between 38 n and 40 n where the ridge axis rapidly shoals by more than 1000 m the crust thickens by over 2 km and the rift valley largely disappears the absence of a deep axial rift valley on the azores platform and near the midpoints of some ridge segments along the mid atlantic ridge to the south agrees well with the predictions of recent ridge crest thermal and rheological models that suggest a dependence of axial morphology on both crustal thickness and spreading rate the transition from a rift valley to an axial high morphology at these spreading rates 11 mm yr half rate occurs at a crustal thickness of about 9 1km

crustal structure of a super slow spreading centre a seismic refraction study of mohns ridge 72 n summary a series of eight high resolution seismic refraction profiles from the ultra slow spreading 16 mm yr 1 full spreading rate mohns ridge in the norwegian greenland sea has been treated with modern inversion methods the profiles were shot parallel to the ridge at an off axis distance of 0 135 km corresponding to crustal ages of 0 22 ma the resulting models are constrained by synthetic seismograms and gravity modelling the crustal thickness in all profiles is well below the global average for typical oceanic crust and shows a high variability with a mean thickness of 4 0 0 5 km this is mainly due to a very thin and variable lower crustal layer layer 3 generally the crust is thicker beneath basement highs and thinner beneath basins implying local isostatic compensation the top of the basement layer 2a consists of a zone with low p wave velocities 2 5 3 0 km s 1 the mean thickness of this layer decreases with distance from the ridge beneath it lies a layer with slightly higher velocities layer 2b its thickness shows less variability along a given profile and an overall increase with age the combined average thickness of the upper two layers remains nearly constant indicating that the boundary between layer 2a and 2b may represent an alteration front upper mantle velocities are generally slow around 7 5 km s 1 for the profile directly within the rift valley a model without a third layer incorporating a constant gradient up to upper mantle velocities and a model with a moho depth inferred from neighbouring profiles and upper mantle velocity as slow as 7 2 km s 1 fit the seismic and gravity data equally well the crustal structure is not mature below the ridge these observations support previous models suggesting the presence of low densities and velocities at about 2 km below the rift axis poisson s ratios determined from converted s wave modelling are incompatible with a layer 3 consisting of purely serpentinized peridotite however a volume fraction of 10 40 per cent serpentinite cannot be ruled out

geothermal mineral equilibria reply to a comment by m a grant abstract the dominant process limiting co 2 contents of fluids in high temperature 240 liquid dominated systems consists of the conversion of primary plagioclase by co 2 to calcite and clay according to log p co 2 15 26 7850 t 273 2 temperature t in c with a likely error in log p co 2 due to variations in the activities of the anorthite and kaolinite components of the mineral phases involved of around 1 log unit secondary processes such as adiabatic expansion conductive cooling and mixing with cooler groundwaters are largely responsible for subsequent variations in p co 2 at lower temperatures 240

mineralogical and fluid inclusion features of rock alterations in the seigoshi gold silver mining district western part of the izu peninsula japan abstract propylitic and advanced argillic alterations occur in the seigoshi gold silver mining district in the western part of the izu peninsula japan the propylitic alteration has the following zonal arrangement from the deeper portion to the shallower portion an epidote prehnite k feldspar chlorite zone a wairakite laumontite zone and a stilbite heulandite montmorillonite zone lateral and vertical zoning is conspicuous in the advanced argillic alteration the inner zone is silica rich and this grades laterally and vertically through alunite to clay rich zones fluid inclusions from the epidote prehnite k feldspar chlorite zone are liquid dominated and filling temperatures of this zone are in the range of 225 285 c filling temperatures for the zeolite zone are variable being in the range of 240 380 c this wide range suggests that boiling of the fluids was responsible for the zeolite zone filling temperatures of the advanced argillic alteration range widely from 210 to 430 c this wide range and the coexistence of liquid and vapor dominated fluid inclusions in a given sample suggest that liquid vapor separation simultaneously occurred during the advanced argillic alteration process it is deduced that the ranges of gaseous fugacity are quite different for each type of alteration for instance f s 2 and f o 2 of the advanced argillic alteration are estimated to be higher than those of the propylitic alteration based on the alteration mineral assemblage chemical composition and mode of occurrence of alteration minerals fluid inclusions estimated ranges of gaseous fugacities and comparison of these features with those of active geothermal systems it is concluded that the propylitic alteration minerals were probably precipitated due to the loss of gases such as co 2 and that the formation of advanced argillic alteration was caused by subsurface mixing of volcanic gas and or condensed hot water with groundwater these coexisting propylitic and advanced argillic alterations are commonly found in the other au ag mining districts in japan

long lived mid ocean ridge segmentation of the pacific antarctic ridge and the southeast indian ridge the pacific antarctic and the southeast indian ridges are examined for evidence of long lived tens of millions of years tectonic segmentation of the mid ocean ridge by examining a variety of geophysical parameters similar to those previously documented in the south atlantic kane and hayes 1992 these parameters include subsidence rate of the oceanic crust individual flanks and asymmetry zero age depth and geoid height decrease with age geoid rate the variability in these parameters along strike of the mid ocean ridge is systematic and serves to define a large scale ridge segmentation of the order of hundreds of kilometers the southeast indian ridge exhibits long lived segmentation in all parameters examined and is subdivided into five flow line corridors with boundaries between corridors occurring at or near major fracture zones while the pacific antarctic ridge exhibits conspicuous asymmetric crustal subsidence evidence of segmentation exists primarily in the along strike geoid rate the observations in these two areas are found to be internally inconsistent with simple thermal conduction models for oceanic crust strongly suggesting that factors other than cooling and isostasy are influencing the creation and subsequent modification of the oceanic crust and lithosphere

the magnetic layer of the ocean crust how thick is it abstract special features of linear marine magnetic anomalies and the magnetic measurements made on samples of the ocean crust require that the thickness of the marine magnetic source layer be thicker than the 0 5 km thick pillow lava layer seismic layer 2a alone it is proposed here that the magnetic properties of the samples studied to date indicate a two layered source to be the most likely an upper 0 5 km thick pillow lava layer with a natural remanent magnetization of 5 a m and a lower 3 5 km thick dike and upper gabbro layer with a magnetization of 0 5 a m

low degree melting under the southwest indian ridge the roles of mantle temperature conductive cooling and wet melting abstract both low mantle temperatures and conductive cooling have been suggested as the cause of the atypically thin oceanic crust and the incompatible element enrichment characteristic of very slow spreading ridges here we present a model of melting under the southwest indian ridge swir which takes into account mantle temperature conductive cooling source composition and wet melting the model parameters are constrained by oceanic crustal thickness lava chemistry and isotopic composition and water content the results suggest that conductive cooling to a depth of around 20 km expected in areas with a full spreading rate of 15 mm yr is necessary to generate the swir lava chemistry but not that from faster spreading rate ridges at 23 n on the mid atlantic ridge or 45 n on the juan de fuca ridge the mantle potential temperatures of 1280 c estimated for the swir lavas are close to the global average of the upper mantle mantle water contents of 150 300 ppm can explain the observed melt water contents and allow sufficient melting at depth to explain the observed heavy rare earth element depletions in the melts

observations of ridge hotspot interactions in the southern ocean the evolution of ridge hotspot systems is not well understood in this investigation satellite derived marine gravity data are used in conjunction with underway bathymetric and magnetic anomaly profiles to investigate the nature of ridge hotspot interaction at four sparsely explored systems in the southern ocean these systems illustrate three different stages of ridge hotspot interaction in which a migrating spreading center approaches a hotspot pacific antarctic louisville passes over or is captured by the hotspot mid atlantic shona discovery and ultimately migrates away from the hotspot southeast indian kerguelen all of these systems show some evidence of discrete ridge jumps in the direction of the hotspot as the spreading center attempts to relocate toward the hotspot by asymmetric spreading interestingly these ridge jumps show no evidence of propagating offsets as have been seen on many other ridge hotspot systems a simple model predicts that typical plume excess temperatures can weaken the lithosphere sufficiently to promote asymmetric spreading and possibly allow a discrete ridge jump the presence of previously uncharted obliquely oriented aseismic ridges and gravity lineations between the ridge and the hotspot supports the notion of asthenospheric flux from the plume to the spreading center both before and after the time when the hotspot is ridge centered the azimuths of the aseismic ridges cannot be explained by plate kinematics alone they consistently extend from the ends toward the centers of the adjacent spreading segments suggesting some interaction between plume derived asthenospheric flux and local lithospheric structure the features discussed here also indicate that the transfer of asthenospheric material from the plume to the spreading center is influenced by the local plate boundary configuration and interaction with transform offsets

crustal magnetization and accretion at the southwest indian ridge near the atlantis ii fracture zone 0 25 ma 1 we analyze geophysical data that extend from 0 to 25 myr old seafloor on both flanks of the southwest indian ridge swir lineated marine magnetic anomalies are consistent and identifiable within the study area even over seafloor lacking a basaltic upper crust the full spreading rate of 14 km myr has remained nearly constant since at least 20 ma but crustal accretion has been highly asymmetric with half rates of 8 5 and 5 5 km myr on the antarctic and african flanks respectively this asymmetry may be unique to a 400 km wide corridor between large offset fracture zones of the swir in contrast to the mid atlantic ridge crustal magnetization amplitudes correlate directly with seafloor topography along the present day rift valleys this pattern appears to be primarily a function of along axis variations in crustal thickness rather than magnetic mineralogy off axis magnetization amplitudes at paleo segment ends are more positive than at paleo segment midpoints suggesting the presence of an induced component of magnetization within the lower crust or serpentinized upper mantle alteration of the magnetic source layer at paleo segment midpoints reduces magnetization amplitudes by 70 80 within 20 myr of accretion magnetic and ocean drilling program odp hole 735b data suggest that the lower crust cooled quickly enough to lock in a primary thermoremanent magnetization that is in phase with that of the overlying upper crust thus magnetic polarity boundaries within the intrusive lower crust may be steeper than envisioned in prior models of ocean crustal magnetization as the crust ages the lower crust becomes increasingly important in preserving marine magnetic stripes

magnetic telechemistry and magmatic segmentation on the southern east pacific rise results from axial dredges and a profile inversion of magnetic anomaly data along the axis of the east pacific rise epr at 13 23os provide an estimate of the average degree of fractionation for the extrusive layer at this ultrafast spreading 145 mm yr full rate ridge we find a high correlation rd 0 81 between dredge mean feo total iron as feo and natural remanence for 34 axial dredges with multiple samples having coincident geochemical and magnetic data we attribute this good correlation to detailed sampling spanning the full range of cooling related magnetization changes within a flow and to the young age 0 6 ka of these axial samples which effectively minimizes time dependent magnetization changes due to geomagnetic intensity or alteration a composite axial magnetic anomaly profile shows large amplitude up to 400 nt fluctuations with wavelengths of 50 200 km which theoretical considerations suggest can reliably be related to the magnetization directly beneath the ship for much of the southern epr seismic data provide independent limits on the axial thickness 259 55 m and the pattern of off axis thickening of the extrusive magnetic source layer these data also provide evidence for an axial magma lens that effectively eliminates anomaly contributions from deeper magnetic sources inversion of the axial magnetic anomaly data utilizing these geophysical constraints yields a magnetization solution which through use of the regression relating feo and natural remanence may be related to the average degree of differentiation of the extrusive source layer the magnetic data reveal a pattern of magmatic segmentation that closely parallels the tectonic segmentation of the ridge suggesting that magma supply may be an important control on the average degree of differentiation of the extrusive layer 1998 elsevier science b v all rights reserved

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data

direct measurement of magnetic reversal polarity boundaries in a cross section of oceanic crust magnetic field measurements made by submersible define the cross sectional geometry of a magnetic polarity rever sal boundary and the vertical variation of crustal magnetization in upper oceanic crust measured polarity boundaries show a systematic pattern of shallow dip towards the spreading axis within the upper extrusive lavas and steeper dip in the lower extrusive lavas this geometry is a consequence of the em placement of extrusive lava at a midocean ridge reversal boundary geometry and magnetization estimates are used to calculate the magnetic contribution of the extrusive lava se quence to the overlying marine magnetic anomaly signal from the forward modeling the highly magnetized extrusive lavas contribute the majority 50 75 of the observed sea surface magnetic anomaly consistent with the extrusive crust forming the primary source layer for young marine magnetic anomalies