plastic deformation and magmatic impregnation in serpentinized ultramafic rocks from the garrett transform fault east pacific rise the microstructural study of a set of serpentinized ultramafic samples dredged in the garrett transform fault shows that these rocks have been deformed plastically at temperatures probably in excess of 1000 c and under moderate deviatoric stresses below 1 kbar such temperature and stress conditions probably prevail in the asthenosphere or in the deepest and hottest part of the lithosphere below the ridge the deformed paragenesis of the studied samples belong to two groups harzburgites with petrological characteristics typical of residual mantle and plagioclase dunites a melt circulated in these two groups of deformed ultramafic rocks at the end of their deep seated plastic deformation this melt crystallized plagioclase clinopyroxene and also possibly orthopyroxene in one sample in dykelets and in interstices between the minerals of the deformed host rock the deformation and magmatic impregnation history of the studied ultramafic samples recall the results of on land studies of the crust mantle transition zone of ophiolitic massifs this suggests that our set of samples may represent fragments of the east pacific rise crust mantle transition zone the deformed ultramafic paragenesis of these samples was chemically modified by interactions with the impregnating melt the deformed spinel was enriched in tio2 and cr2o3 the deformed clinopyroxene and orthopyroxene of the harzburgitic samples were also enriched in tio2 and the orthopyroxene was significantly depleted in al2o3 the high mobility of titanium during the impregnation process suggests that the ti content of a melt often used as an indicator of its degree of evolution or of the degree of partial melting undergone by its source rock may also be modified by melt rock interactions during the circulation of the magma through the crust and upper mantle

reconstruction of the geometry of accretion during formation of the bay of islands ophiolite complex two types of highly allochthonous ophiolitic terrains crop out adjacent to one another in the bay of islands region of southwestern newfoundland canada the first called the coastal complex cc is exposed as a long linear belt along the west coast it consists of a highly deformed ophiolitic assemblage interpreted as oceanic crust and upper mantle with a previous transform fault tectonic history the second called the bay of islands complex boic crops out to the east and consists of a series of four relatively undeformed ophiolitic massifs interpreted as oceanic crust and upper mantle with a more normal seafloor spreading tectonic history an autochthonous relationship can be demonstrated between the two types of ophiolitic terrains in the southernmost of these massifs indicating that each evolved along a single ridge transform system the internal structure of both the boic and the cc is consistent with the above interpretations the ne sw structural trends in the cc transform are oriented approximately parallel to the sea floor spreading direction inferred from the average orientation of the sheeted diabase dikes within the boic one way chilling statistics from the sheeted diabase dike unit growth directions of dendritic crystals along inclined comb layers in the plutonic section and the sense of shear along the cc transform all indicate that the boic was accreted on the ne side of a nw se trending spreading center present geographic reference frame juxtaposition of the boic with the ne sw trending cc occurred at a ridge transform intersection the transform exhibited a left lateral offset magma chamber profiles have been identified in the layered gabbroic sections of the boic by using the patterns outlined by the attitudes of fine scale igneous layering these patterns depict arcuate structures which appear to define roughly a cross sectional view of a systematically oriented half cylinder the long axis of which is oriented parallel with the plane of the sheeted dikes and major lithologic unit contacts thus we believe that a large cylindrical shaped crustal level subaxial magma chamber extended in a direction parallel with the ridge axial plane at the time of formation of the bolc the cc marks the abrupt truncation of this long linear magma chamber petrologic constraints demonstrate that the ultramafic and transition zone cumulates which form the base of the plutonic section crystallized at a range of high to intermediate pressures possibly along a long narrow conduit extending to 30 km depth beneath the crustal level magma chamber after formation these cumulates were deformed at high temperatures in the zone of mantle upwelling and divergence then tectonically transported to crustal or near crustal levels to form the basement upon which the remainder of the ophiolite stratigraphy was accreted a geometric and tectonic model is presented for the accretion process responsible for formation of the bay of islands ophiolite

variations in os isotope ratios of pyrrhotite as a result of water rock and magma rock interaction constraints from virginia formation duluth complex contact zones abstract os isotope ratios in pyrrhotite bearing pelitic rocks of the 1 85 ga virginia formation are variable with perturbations linked to the emplacement of the 1 1 ga duluth complex pyrrhotite in footwall rocks of the contact aureole show evidence for a mixing event at 1 1 ga involving a low 187 os 188 os fluid however because rocks with perturbed pyrrhotite os isotope ratios occur 1 km or more from the duluth complex the fluid is unlikely to have been of magmatic origin fluid inclusions in layer parallel quartz veins provide evidence of the involvement of a boiling fluid at temperatures between 300 and 400 c analyses of fluid inclusions via la icp ms show that the fluids contain up to 1 7 wt na 1 1 wt k 4330 ppm fe 2275 ppm zn and 415 ppm mg the veins also contain pyrite or pyrrhotite plus minor amounts of chalcopyrite bornite pentlandite and sphalerite the re os isotopic ratios of pyrite from the veins indicate that they crystallized from low 187 os 188 os fluids 18 o values of vein quartz range from 7 7 to 9 5 consistent with an origin involving fluid with a relatively low 18 o value between 2 and 5 meteoric water with such a low 18 o value could have interacted with the igneous rocks of the complex and would have acquired os with a low 187 os 188 os ratio strongly serpentinized olivine rich rocks of the complex are commonly characterized by such low 18 o values and we propose that the fluid involved in serpentinization was also responsible for the perturbation of the os isotopic system recorded by pyrrhotite in the virginia formation two important observations are that only pyrrhotite bearing assemblages in the contact aureole show isotopic perturbation and that intervals showing os exchange are spatially restricted and not uniformly distributed os exchange and mixing has occurred only where temperatures were sufficient to convert pyrite to pyrrhotite and where time integrated water rock ratios in the aureole were high enough to provide a supply of os troctolitic and gabbroic rocks of the partridge river intrusion duluth complex are characterized by os isotope ratios that are indicative of variable degrees of crustal contamination os values of 0 543 xenoliths of carbonaceous and sulfidic pelitic rocks of the virginia formation found in the igneous rocks provide evidence that os was released by organic matter and pyrite in the sedimentary rocks and assimilated by mantle derived magma however residual pyrrhotite produced as a result of pyrite breakdown in the xenoliths is characterized by 187 os 188 os ratios that are much lower than anticipated and similar to those of pyrrhotite in the contact aureole the os exchange and addition shown by pyrrhotite in the xenoliths highlight an unusual cycle of re os liberation during devolatilization kerogen maturation and pyrite to pyrrhotite conversion processes that contribute to magma contamination followed by os uptake by pyrrhotite during back reaction involving magma and or fluid characterized by a relatively low 187 os 188 os ratio the extreme os uptake recorded by pyrrhotite in the xenoliths as well as the lesser degree of uptake recorded by pyrrhotite in the contact aureole is in line with the high os diffusivity in pyrrhotite experimentally determined by brenan et al 2000 our data confirm that os isotope ratios in pyrrhotite bearing rocks may be readily perturbed for this reason caution should be exercised in the interpretation of os isotope ratios in rocks where pyrrhotite may be the primary host of os

mineral chemistry of ultramafic and mafic cumulates as an indicator of the arc related origin of the mersin ophiolite southern turkey the mersin ophiolite represented by approximately 6 km thick oceanic lithospheric section on the southern flank of the taurus calcareous axis formed in the mesozoic neo tethyan ocean some time during late cretaceous in southern turkey the ultramafic and mafic cumulates having over 3 km thickness consist of dunite chromite wehrlite clinopyroxenite at the bottom and pass into gabbroic cumulates in which leucogabbro olivine gabbro and anorthosite are seen crystallization order is olivine fo91 80 chromian spinel cr 60 80 clinopyroxene mg 95 77 plagioclase an95 6 91 6 and orthopyroxene mg 68 77 mineral chemistry of ultramafic and mafic cumulates suggest that highly magnesian olivines clinopyroxenes and absence of plagioclase in the basal ultramafic cumulates are in good agreement with products of high pressure crystal fractionation of primary basaltic melts beneath an island arc environment major trace element geochemistry of the cumulative rocks also indicate that mersin ophiolite was formed in an arc environment coexisting ca rich plagioclase and forich olivine in the gabbroic cumulates show arc cumulate gabbro characteristics field relations as well as the geochemical data support that mersin ophiolite formed in a supra subduction zone tectonic setting in the southern branch of the neo tethys in southern turkey

mantle crust transition zone and origin of wehrlitic magmas evidence from the oman ophiolite abstract abundant exposure and well preserved outcrops in the oman ophiolite allow detailed observation of the transition from mantle to crust in an oceanic environment the mantle crust transition zone shows large lateral variations in thickness and composition but consists essentially of residual dunites and a magmatic component present as clinopyroxene and plagioclase impregnations and gabbro lenses compositionally the transition zone marks a gradational passage from mantle to crust the moho may be placed at the base of the continuous gabbroic crustal section structures both within the transition zone and near the base of the crust are nearly parallel to the moho however the dominant flow mechanisms differ from the upper mantle to the lower crust plastic flow dominates in the upper mantle while viscous flow resulting in strong magmatic fabrics dominates at crustal levels wehrlitic bodies within the crustal gabbro section are intruded from the transition zone into which they are rooted the parent magma of the wehrlitic instrusions is in fact a crystal melt mixture composed of olivine and chromite probably including a component of mantle xenocrysts and xenoliths and of reequilibrated basaltic melt

comments on composition and depth of origin of primary mid ocean ridge basalts by d c presnall and j d hoover presnall and hoover 1984 in their evaluation of the nature of primary mid ocean ridge basalts morbs concluded that earlier data suggesting many of the most primitive morbs are derived from high mgo picritic magmas are not valid in reaching this conclusion presnall and hoover 1984 ignore several important considerations regarding 1 analytical uncertainties 2 tio2abundances in morbs 3 least squares calculations 4 complex models of magmatic evolution and 5 enstatite saturation i briefly discuss these points below

crustal processes at spreading centers recent discoveries and research on mid ocean ridges mor s in the last four years have greatly increased our knowledge of this fundamental type of plate boundary the last iugg review of mor research was in 1975 aumento et al 1975 and its omission as a topic in 1978 may have reflected a feeling that spreading centers were reasonably well understood since that time however there has been a surfeit of important new observations and exciting research and there is now a turmoil of innovative ideas about the processes which create and leave their imprint on two thirds of the earth s crust new seismic measurements have been made and interpreted as evidence for the existence of shallow axial magma chambers amc beneath intermediate total opening rate 5 9 cm yr and fast 9 18 cm yr spreading centers fig 1 yet there is controversy as to whether they are steady state whether they persist beneath slow ridges as well in the steady state what the dimensions and shape of the chamber are its relation to oceanic crustal stratigraphy and its relation to observations in ophiolites black smoker hydrothermal vents have been discovered in the pacific spewing forth 350 c fluids blackened by polymetallic sulfide precipitates this discovery is embroiled in debate as to how long the vents last whether they occur at slow spreading rates as well whether the enormous heat flux of the vents freeze the magma chamber even at fast spreading rates their influence on geochemical balances in the ocean and the value of the sulfide deposits commercially it has been found that our initial view of mor s as two dimensional linear structures has served us well as an initial working model but must be significantly modified to account for variations in tectonic volcanic and hydrothermal activity along strike as well as for episodicity of these processes deep sea drilling project dsdp samples have raised fundamental questions about formation of the magnetic source layer hydrothermal circulation and fault mechanisms along the mor spreading centers which penetrate existing lithosphere along strike as propagating rifts have modified our views of how plate boundaries reorient themselves and how v shaped magnetic and topographic trends are formed we have also found that transform faults may modify spreading center processes for some distance away from their intersections with mor s and that rigid plate transform faults may fail to develop at fast spreading centers under certain conditions resulting in en echelon overlapping of spreading centers this review will focus on these developments and several others only example references to these topics will be given in the text and many important topics will not be discussed directly e g petrology and geochemistry mantle convection research on lithosphere older than 1 m y however i have tried to compile and have included an extensive bibliography for u s research on a broader range of mor topics for mid 1978 to mid 1982 which is the main purpose of this report pre 1978 references are occasionally included where appropriate abstracts are not included

the importance of symbiont bearing benthic foraminifera for west pacific carbonate beach environments abstract in the tropical central and west pacific as well as in the east indian ocean symbiont bearing benthic foraminifera often are the main producers of calcium carbonate grains deposited at beaches the proportion of foraminiferal tests in medium sized to coarse sand grains may range from 20 to 95 this depends on two factors productivity and transport because foraminifers do not live on or in the sandy bottom near the beaches but prefer firm substrate in high energy environments close to the front of coral reefs foraminiferal tests are produced in extreme numbers on the reef crest of sesoko island okinawa japan 0 6 kg m 2 year 1 which is covered by macroalgae or in the transition zone between the crest and the reef moat 1 3 kg m 2 year 1 empty tests released by reproduction or death are entrained at the reef crest by waves and transported by currents this transport depends on the direction and intensity of currents connecting the habitat reef crest and transition to moat with the deposition area central moat lagoon beach habitat destruction and or the interruption or diversion of water flow hinders the accumulation of foraminiferal tests at the beach where deepening the moat intensifies beach parallel currents supply to and deposition at the beach are reduced while the remaining sand is eroded by tropical cyclones understanding the ecology especially population dynamics of larger foraminifera in combination with factors influencing the transport of empty tests is essential to preserve the balance between deposition and removal of sand at carbonate beaches in the west pacific and east indian ocean

hurricane induced sediment transport in open shelf tropical systems an example from st croix u s virgin islands hurricane hugo passed directly over st croix on 17 september 1989 sustained winds in excess of 110 knots gusts to 165 knots and waves 6 7 m in height accompanied the storm along the north coast wave height was lower ca 3 4 m due to the leeward position of the shelf in the deeper reefs at cane bay and salt river damage was confined primarily to the soft bodied benthic community e g sponges gorgonians coral damage was much less severe largely because of the buffering effects of the water column the greatest change observed after the storm was wholesale flushing of sand from shelf edge areas in salt river submarine canyon a minimum of 2 million kg of sediment were flushed into deeper water the transport rate associated with the storm was eleven orders of magnitude above that measured during fair weather and the volume of sediment that was removed from the canyon equalled roughly a century of normal sediment accumulation at cane bay 336 000 kg of sediment were flushed from a single channel with similar amounts removed from adjacent breaks in the shelf edge reef a current meter in salt river submarine canyon provided information on the timing and intensity of the oceanographic processes related to hurricane hugo as the storm approached waves piled water against the shoreface and in salt river bay as the storm passed over st croix the change in wind direction followed by a decrease in wave height triggered a release of water trapped in the bay and along the adjacent shoreline by waves earlier in the storm for a period of 4 6 hours net down canyon currents reaching 2 m s and oscillatory flows up to 4 m s occurred along the base of the western canyon wall removing up to 2 m of sand similar events were likely responsible for the wholesale removal of sand in eastern cane bay the paradoxical concurrence of wholesale sediment transport and low level reef damage is related to the protection from waves but not wind afforded by the north coast of st croix facing away from the direction of storm approach these observations and measurements provide our first opportunity to relate sediment export in such a high energy event to the physical processes that were responsible calculations based on post hugo measurements are in agreement with an earlier sediment budget for salt river canyon sediment export in cane bay exceeded the volume similarly predicted because such events are probably common on all exposed carbonate shelf margins storms like hurricane hugo are among the most important factors in the cycling of sediment through exposed open marine environments both now and in the geologic past the patterns of reef damage and sediment transport are much more complicated than previously envisioned and more thoughtful consideration of their variability and the processes responsible is essential to an understanding of the signature that will be left by major storms

coral reef records of quaternary changes in climate and sea level abstract although coral reefs have long been regarded as documenting climate change largely through its effects on sea level the temporal continuity and sensitivity to disturbance of their records appears to fall short of those provided by ice cores deep sea sediments and similar proxies the reasons for this anomaly relate principally to shifts in the loci of deposition the temporal windows in which accretion has been able to occur in a given place reflecting sea level at a particular height relative to the present datum are severely constrained where it has been possible coral growth and reef accretion have been and are sensitive to the signals provided by the environment in the form of changes in water depth temperature salinity nutrient supply turbidity and ph acidity concomitant with changes in sea level itself changes in these environmental parameters are directly or indirectly controlled by climate however because the responses of corals to stimuli are commonly site specific and the nature of the changes evoked so disparate it is likely that only major sea level cycles will provide evidence of a consistent global response in what has been a segmented record

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

history of the cretaceous osbourn spreading center the osbourn trough is a fossil spreading center that rifted apart the manihiki and hikurangi plateaus during cretaceous time previous models of the osbourn spreading center are based on data collected near the trough axis and therefore only constrain the history of the osbourn spreading center during the last few ma of spreading our data set includes multibeam data collected northward to the manihiki plateau allowing us to examine seafloor morphology created during the entire active period of the osbourn spreading center as well as several additional multibeam data sets that provide the opportunity to examine the relationship between the osbourn paleospreading center and the cretaceous pacific phoenix ridge the axial gravity of the trough is similar to the gravity found at other extinct slow intermediate spreading rate ridges magnetic field measurements indicate that spreading at the trough ceased during chron c34 abyssal hill trends indicate that spreading during the early history of the osbourn spreading center occurred at 15 20 the east west component of this spreading explains the modern east west offset of the manihiki and hikurangi plateaus spreading rotated to 2 5 prior to extinction abyssal hill rms amplitudes show that a decrease in spreading rate from 7 cm yr to 2 6 cm yr full spreading rate occurred 2 6 ma prior to ridge extinction our data analysis is unable to determine the exact spreading rate of the osbourn spreading center prior to the slowing event the temporal constraints provided by our data show that the osbourn spreading center ceased spreading prior to 87 ma or 93 ma depending on whether the manihiki and hikurangi plateaus rifted at 115 ma or 121 ma our model resolves the conflict between regional models of osbourn spreading with models based on trough characteristics by showing that spreading at the osbourn spreading center was decoupled from pacific phoenix spreading

the influence of water on the petrogenesis of subductionrelated igneous rocks the presence of dissolved water can significantly change the sequence in which different minerals crystallize from a silicate magma1 3 this will alter the compositional path followed by residual liquids the liquid line of descent and modify the types and compositions of crystals that accumulate at the site of cooling the sharp decrease in the solubility of water in silicate melts with decreasing pressure makes direct measurements of pre eruptive concentrations in magmas difficult because much of the dissolved water boils off before eruption here we report experimental results on the crystallization of basalts that contain substantial dissolved water up to 6 wt and show how the results can be used to infer the amount of dissolved h2o involved in the petrogenesis of lavas that preserve a record of their liquid lines of descent or the accumulated minerals left behind after solidification the presence of abundant magmatic water is a signature of subduction zone volcanism4 and can be used to associate a suite of magmas or igneous cumulates with a convergent margin back arc or interarc tectonic setting

metamorphic evolution of oceanic gabbros recrystallization from subsolidus to hydrothermal conditions in the mark area odp leg 153 abstract the 117 38 m of gabbroic core drilled during the ocean drilling program odp leg 153 at sites 921 to 924 in the mid atlantic ridge mar between 23 n and the kane fracture zone exhibits a remarkable primary compositional heterogeneity such as magmatic layering intrusive contacts and late magmatic veining which express a succession of magmatic events textural indicators suggest that the cooling of the crystal mush occurred in a dynamic environment with infiltration of progressively evolved liquids magmatic features include random shape fabric and magmatic lamination the subsequent deformational overprint occurred in subsolidus conditions the ductile deformation generally concentrated in discrete domains of the gabbro is associated with continuous re equilibration of the metamorphic assemblages of 1 olivine clinopyroxene orthopyroxene plagioclase ilmenite ti magnetite 2 olivine clinopyroxene plagioclase ilmenite ti magnetite red hornblende at lower temperatures brittle deformation prevails and subsequent fractures control the development of metamorphic assemblages 3 clinopyroxene plagioclase red brown hornblende ti magnetite magnetite ilmenite 4 plagioclase brown hornblende ti magnetite magnetite hematite titanite ti oxide 5 plagioclase green hornblende magnetite titanite 6 plagioclase actinolite chlorite titanite magnetite 7 albite actinolite chlorite prehnite epidote titanite and 8 albite prehnite chlorite smectite assemblages 1 to 8 express increasing water rock ratios and decreasing degrees of recrystallization during the ductile phase red hornblende is stable and its abundance increases with deformation intensity possibly as an effect of the introduction of hydrous fluids during the brittle phase water diffusion controls the development of the fracture filling mineral assemblages and re equilibration of the adjacent rock temperatures decrease further as demonstrated by mineral zoning and incompletely re equilibrated assemblages the lowest temperatures correspond to the development of hydrothermal assemblages compared with oceanic gabbros from fast spreading transform environments high temperature ductile phases granulite and amphibolite are well developed whereas brittle phases are widespread as microcracks prevalent on fracturing associated with discrete veins

a possible explanation of gravity anomalies over mid ocean ridges gravity fields over the crests of slow spreading mid ocean ridges are characterized by small amplitude free air gravity anomaly lows 30 70 mgal with wavelengths of 40 60 km flanked by smaller gravity highs in general the amplitude and wavelength of these gravity lows decrease with increasing spreading rate until at fast spreading ridges 5 cm yr free air gravity highs 10 20 mgal are observed previous explanations of these anomalies which involve uncompensated ridges thermally expanded ridges and elastic plate compensation models are generally not consistent with the geological evidence regarding the nature of the crust and upper mantle beneath the ridge crest in particular the postulated presence of wide zones of partial melt in the uppermost mantle is inconsistent with petrologic data from mid ocean ridge basalts which indicates that primary melts are in equilibrium with and extracted from residual mantle at depths of 30 km or more in addition similar gravity lows observed over extinct spreading ridges suggest that the anomalies are not mainly due to any dynamic aspect of the accretion process taking place at active spreading centers geometrical models of mid ocean ridges derived from detailed structural and petrologic studies of the bay of islands ophiolite complex have been used to generate gravity anomalies over ridges with various spreading rates modeling results show that four main sources serve to define the amplitude and wavelength of the gravity anomaly the ridge topography the crustal level magma chamber the configuration of the isotherms near the ridge crest and a low density partially gabbroic cumulate root extending vertically into the mantle beneath the ridge systematic variations in the contributions from these sources with increasing spreading rate control the changes in character of the gravity anomaly from slow to fast spreading centers major contributions are found to come from the topography and from the lowdensity root which broadens with slower spreading rates resulting in both increasing amplitude and increasing wavelength with decreasing spreading rate at extinct spreading centers contributions from the magma chamber isotherms and topography gradually diminish with time once the ridge ceases to spread whereas the low density root remains a permanent part of the oceanic lithosphere and continues its contribution to the gravity field seismic evidence from slow spreading centers appears to support the existence of the lowdensity gabbroic root zone inferred from the study of the bay of islands ophiolite and the gravity analysis presented here

cyclone pumping sediment partitioning and the development of the great barrier reef shelf system a review abstract the modern great barrier reef gbr is part of the world s largest and best known mixed terrigenous carbonate continental margin the gbr shelf contains three shore parallel sedimentary belts an inner shelf zone of terrigenous sedimentation at depths of 0 22 m a middle shelf zone of sediment starvation at depths of 22 40 m and an outer shelf reef tract with its inner edge at ca 40 m depth these zones are controlled by the dynamics of northward fair weather along shelf drift driven by southeasterly trade winds and by the regular passage of tropical cyclones cyclones cause wind driven north directed middle shelf flows in excess of 130 cm s which erode the seabed concentrate the sparse mobile sediment into sand ribbons and advect suspended load onto the outer part of the nearshore terrigenous sediment prism and into inter reef depocentres within the outer shelf reef tract cyclones largely control the input of new sediment into the gbr system via river flooding seabed erosion or reef breakage they also help to control the partitioning and dispersion of the three main shore parallel belts of sediment and hence stratigraphic accumulation acting as a sediment pump especially during interglacial highstands cyclones have exerted great control on the development of the modern gbr province and its sediments by maintaining a broad shelf parallel zone of episodically mobilised sediment and scoured seabed upon which coral reefs have been unable to form cyclones may also have partly controlled the timing of initiation of the first gbr at 0 6 mybp contrary to current models gbr storm beds are most likely to be preserved intact close to the shoreline and become coarser grained away from the shoreline for the central gbr highstand shedding only applies to carbonate sediment at the scale of local reefs system wide oceanographic controls cause high rates of carbonate sedimentation on the slope during both sea level rise and highstand concomitantly terrigenous sediment accumulates fastest on the slope during sea level rise and slowest during sea level lowstand and highstand

magma chamber profiles from the bay of islands ophiolite complex models of the oceanic lithosphere derived from constraints imposed by the geology of ophiolite complexes require a magma chamber to produce the cumulate and other plutonic portions of the oceanic crust and upper mantle evidence from the bay of islands ophiolite complex indicates that layered plutonic rocks have not formed solely due to differential crystal settling to the horizontal floor of a magma chamber but have formed largely by heterogeneous nucleation and in situ crystallization along gently inclined as well as steeply inclined bounding surfaces of a large magma reservoir coherent well defined and systematically oriented large scale arcuate patterns described by mesoscopic igneous layering have been identified in the plutonic sections of two ophiolite massifs in the bay of islands complex these layering patterns seem to outline the shape of a large continuously evolving steady state magma chamber that extended along a ridge axis and ended at a transform fault intersection

examination of the deep levels of an island arc evidence from the tonsina ultramafic mafic assemblage tonsina alaska the tonsina ultramafic and mafic assemblage located in the northern chugach mountains along the trans alaska crustal transect tact is interpreted to be the remains of a magma chamber that crystallized at the base of a mature interoceanic island arc it is one of a number of isolated masses of peridotite and gabbro that comprise the basal part of the peninsular terrane along the northern fringe of the chugach mountains in southern alaska these masses the roots of a mid jurassic island arc have been uplifted along the north dipping border ranges fault system and crop out sporadically for 700 km from kodiak island to eastern alaska the tonsina ultramafic mafic assemblage is a layered sequence whose basal rocks are dunite and harzburgite 1 km thick overlain by a narrow zone of websterite and clinopyroxenite above these ultramafic rocks is a 5 to 6 km thick layered gabbro that is garnet bearing at its base textural relationships and chemical trends throughout the series of dunite pyroxenite garnet bearing gabbro and spinel gabbronorite point to an origin by igneous accumulation from a hydrous tholeiitic parent magma at deep levels i e at the base of a mature island arc before being emplaced along the border ranges fault the rocks were crystallized from a melt at pressures between 9 5 and 11 kbar 28 to 33 km depth and equilibrated at temperatures of 800 to 90o c based on phase relationships in the cumulus assemblages pyroxene thermobarometry and olivine spinel thermometry transitional sequences from ultramafic to mafic rocks are characterized by increasingly al rich pyroxenes and spinels or garnet and the absence of olivine plagioclase assemblages deep level fractionation of al poor phases such as olivine and pyroxene at moderately high pressures and temperatures resulted in concentration of al in the evolving magma as crystallization continued the phases recorded this alumina concentration and the resultant gabbros are al rich this observed trend supports the theory of high alumina basalt generation by fractional crystallization of olivine and pyroxene comparison of the tonsina rocks with aleutian lower crustal xenoliths as well as other exposures of arc related lower crustal magma chambers in japan pakistan and western north america indicates that this process is a common mechanism for island arc magma evolution at depth

cryptic mineral chemistry variations in a detailed traverse through the cumulate ultramafic rocks of the north arm mountain massif of the bay of islands ophiolite newfoundland

the supra subduction zone pozanti karsanti ophiolite southern turkey evidence for high pressure crystal fractionation of ultramafic cumulates the pozanti karsanti ophiolite pko forming one of the late cretaceous discontinuous oceanic lithosphere remnants in the eastern tauride belt in southern turkey is characterized by mantle tectonites ultramafic and mafic cumulates isotropic gabbros sheeted dikes and volcanics well preserved crustal cumulate rocks are mainly composed of dunitefchromite wehrlite olivine clinopyroxenite clinopyroxenite olivine websterite and low ti gabbro the crystallization order in the plutonic section is olivine fo93 83 fchromian spinel cr cr al 79 clinopyroxene mg mg fe 93 87 orthopyroxene mg mg fe 90 83 the mineral chemistry of the ultramafic cumulates forming the basal portion of the plutonic section of the pozanti karsanti ophiolite pko is not consistent with crystal liquid fractionation of primitive midocean ridge basalts at low pressures the presence of highly magnesian clinopyroxene and orthopyroxene together with the absence of plagioclase as early fractionating phases indicate medium to high pressure up to 10 kbar crystal fractionation of primary basaltic melts mineralogical and geochemical data suggest that the ultramafic cumulates are distinct from rocks in midocean ridge and back arc basin ophiolites the ultramafic rocks instead represent part of the plutonic core of an intraoceanic island arc suprasubduction zone tectonic setting it is believed that the arc crust was thick for the pko during the late cretaceous with the existence of an active magma chamber situated at the base of the crust around pressure of 8 10 kbar for the high pressure products and a related active shallower level magma chamber for the low pressure products d 2002 elsevier science b v all rights reserved

reconstructing 7000 years of north atlantic hurricane variability using deep sea sediment cores from the western great bahama bank q1 enso show abrupt changes around 6000 yrs b p but most coastal sedimentary records do not span this time period establishing longer records is essential for understanding mid holocene patterns of storminess and their climatic drivers which will lead to better forecasting of how climate change over the next century may affect tropical cyclone frequency and intensity storms are thought to be an important mechanism for transporting coarse sediment from shallow carbonate platforms to the deep sea and bank edge sediments may offer an unexplored archive of long term hurricane activity here we develop this new approach reconstructing more than 7000 years of north atlantic hurricane variability using coarse grained deposits in sediment cores from the leeward margin of the great bahama bank high energy event layers within the resulting archive are 1 broadly correlated throughout an offbank transect of multi cores 2 closely matched with historic hurricane events and 3 synchronous with previous intervals of heightened north atlantic hurricane activity in overwash reconstructions from puerto rico and elsewhere in the bahamas lower storm frequency prior to 4400 yrs b p in our records suggests that precession and increased nh summer insolation may have greatly limited hurricane potential intensity outweighing weakened enso and a stronger west african monsoon factors thought to be favorable for hurricane development

petrologic evolution of large failed rifts in the eastern pacific petrology of volcanic and plutonic rocks from the mathematician ridge area and the guadalupe trough

when did the subduction first initiate in the southern paleo asian ocean new constraints from a cambrian intra oceanic arc system in west junggar nw china abstract we report here a cambrian southward subducting intra oceanic arc system in the southern west junggar nw china where an immature arc occurred initially on ssz type ophiolites and finally evolved into a more mature one the immature arc is dominantly represented by early middle cambrian 510 ma low k tholeiitic felsic rocks whereas the mature arc is characterized by late cambrian 495 ma medium and high k calc alkaline felsic and mafic rocks the ssz type ophiolites show remarkable depletion of nb and ta and contain high cr spinel cr 0 6 resembling these formed in the forearc altogether they record the initiation of subduction and transformation of crust during early subduction of the paleo asian ocean in the southern part of the central asian orogenic belt caob the subduction initiation might occur in the early cambrian 515 ma as constrained by both the ssz type ophiolites 516 ma and the oldest arc plutons 515 509 ma that crosscut the ophiolites the immature felsic plutons have high sio 2 72 wt contents and variable mgo 0 42 1 49 wt and mg values 22 62 crustal anatexis may be responsible for the genesis of those plutons and thus the transformation from oceanic to continental crust these results combined with regional data convincingly indicate that it is one of the oldest intra oceanic arc systems in the southern caob which may mark the initial subduction of the paleo asian ocean in its southern part much later than those reported in the north an archipelago type model is proposed for the evolution of the southern west junggar and has implications for the development of the southern part of the caob

structure and petrology of peridotites clues to their geodynamic environment the first part of this paper is a review on the main mineralogical petrological and structural criteria which can be used to unravel the history of the various types of mantle peridotites met at the earth s surface these criteria are applied in the second part to show that most peridotites share a common history which is that of an asthenospheric uprise followed by specific fates corresponding to their lithospheric history the unifying concept is that of adiabatic ascent rate related to rifting and spreading rates rates lower than 0 5 cm yr in continental graben correspond to departure from adiabatic conditions at depths greater than 30 km this inhibits further melt extraction with as consequences a limited melt extraction and comparatively fertile spinel lherzolites as residue higher rates probably not exceeding 1 cm yr correspond to oceanic rifts with the main melt extraction completed around 15 km generating a thin oceanic crust and plagioclase lherzolites as residue finally rates greater than 1 2 cm yr correspond to oceanic ridges with melt being extracted at moho depth thus generating a 6 km thick crust and leaving depleted harzburgites as residue thus examination of the peridotite type and associated crustal formations give some clue to trace back the environment of origin this conclusion must however be tempered by the fact that the spreading rate envisioned here as the principal controlling parameter can change with space and time in a given environment mantle diapirism rifting seafloor spreading crust generation

are reefs and mud mounds really so different although both ecologic reefs and mud mounds are demonstrably rigid framework reefs they are still considered to be distinct in terms of their dominant processes of formation and preferred environmental settings this distinction has rested largely upon the assumption that ecologic reefs are dominated by skeletal metazoans growing in shallow waters in contrast to the complex autochthonous micrite supported cavity systems that characterise deep water mud mounds now considered to represent either organomineralic deposits where carbonate precipitation has taken place in association with nonliving organic substrates to form automicrite or various types of microbialite where carbonate forms as a direct result of the physiological activity or decay of benthic microorganisms yet such autochthonous micrite is increasingly recognised as an important component of many ancient shallow ecologic reefs as well as some modern coral reefs and indeed may contribute locally up to 80 of the reef rock these observations raise doubts as to the validity of current fabric based definitions used to distinguish between mud mounds and ecologic reefs whether the autochthonous micrite in mud mounds proves to be dominated by either automicrite or microbialite both require particular environmental conditions for their formation automicrites form where surplus organic matter from metazoans has degraded to release quantities of acidic amino acids with a significant ability to bind ca 2 and microbialite formation also often requires either unusual marine chemistries or ecological conditions such conditions might include changes in terrigenous influx ground water seepage local anoxia and increases in the ph of interstitial reef waters or in nutrient concentration d 2001 elsevier science b v all rights reserved

the border ranges ultramafic and mafic complex south central alaska cumulate fractionates of island arc volcanics a discontinuous elongate zone of mafic and ultramafic plutonic rock crops out in south central alaska for a distance of more than 1000 km intermediate and detailed scale geologic mapping petrographic study and compositional data suggest that the plutonic rocks are compositionally petrologically and mineralogically distinct from rocks in mid ocean ridge and back arc basin ophiolites the mafic and ultramafic rocks instead represent part of the plutonic core of an intraoceanic island arc the mafic ultramafic zone referred to as the border ranges ultramafic and mafic complex brumc is composed of ultramafic cumulates gabbronorite cumulates and massive gabbronorites a very minor amount of tectonized ultramafic rock of mantle origin is present in the southern part of the brumc a thick sequence of andesitic volcanic rocks the talkeetna formation of early jurassic age lies to the north of and structurally above the mafic ultramafic zone voluminous calcalkaline plutons composed of quartz diorite

depth transport functions and erosion deposition diagrams as indicators of slope inclination and time averaged traction forces applications in tropical reef environments abstract the trigonometric relationship between slope inclination the horizontallyacting time averaged traction force and the vertical depth of transport allowsthe estimation of one factor when both others are known depth transportfunctions can be deduced by comparing the depth distributions of livingorganisms and their skeletal remains and this paper simpli es this comparisonusing foraminifera in which a single test represents an individual differencesin distribution parameters between living individuals and empty tests allowdepth transport functions to be determined these functions differ betweenspecies at a single transect according to the varying buoyancies of the tests within a single species differences in depth transport functions betweenlocations are based on either slope inclination or traction intensities afterestablishing a mean depth transport function by averaging species characteristic functions the time averaged traction force acting on thestudied transect can be calculated transport intensities are also estimatedusing an erosion deposition diagram that combines the relative frequencydistributions of living individuals and empty tests the proportion of eroded parautochthonous and allochthonous tests mirrors the in uence of bothslope inclination and traction force for the deposition of empty tests to test themodel six species of symbiont bearing benthic foraminifers were investigatedat two transects in front of a nw paci c coral reef one transect is distinguishedby a strong slope attening below the steep reef slope 30 m whereas furthersteepening characterizes the equivalent part in the other transect thesedifferences are mirrored in the depth transport functions as well as in theerosion deposition diagrams of all species the time averaged traction forcesdiffer in intensities between transects because of the position of the reef frontwith respect to the predominant wind direction however the form of thefunctions is identical and distinguished by an increase from the surface to 35 mdepth followed by a decrease down to 105 m this can be explained bysuccessive onshore and offshore forces acting on the shallow slope such as thetropical cyclones that cross the region every summer keywords carbonate sands coral reef slopes foraminifera sediment trans port traction force tropical cyclones introductionsediment transport on the slopes and forereefareas greatly in uences the topography of coralreefs as carbonate produced by reef buildingorganisms is rapidly reduced to sediment especially at the reef front e g hubbard et al 1990 the overwhelming of reef growth bysedimentology 2001 48 1025 1046

mineral compositions of plutonic rocks from the lewis hills massif bay of islands ophiolite the lewis hills massif is interpreted as having been formed at an oceanic fracture zone fz prior to its obduction the compositions of minerals in the residual mantle and cumulate rocks of the massif are similar to those in other massifs of the bay of islands ophiolite and those in comparable rocks from the oceanic basins the residual mantle rocks consist of olivine fo89 9 91 7 ni0 0 37 0 46 orthopyroxene 100 mg mg fe ranges from 90 6 to 91 8 spinel 100 cr cr al 43 7 57 4 and 100 mg mg fe2 51 2 67 7 and rare clinopyroxene these minerals have compositions similar to those of the most depleted abyssal peridotites the cumulate units are mixtures of olivine fo88 8 78 0 ni0 0 32 0 08 plagioclase an88 4 68 8 clinopyroxene 100 mg mg fe 93 0 75 6 and cr al spinel 100 cr cr al 36 0 8 8 and 100 mg mg fe2 73 7 50 0 the ultramafic cumulates appear to have crystallized at moderate to high pressures and the gabbroic rocks at low pressures differences between the magmatic processes recorded in the lewis hills massif during its formation adjacent to a fz and those inferred for the north arm mountain massif while it formed more distantly from the fz are 1 the more depleted nature of the mantle near the fz 2 the higher ti02 contents of diabase dikes near the fz 3 the greater abundance of feldspathic ultramafic rocks near the fz and 4 the absence of orthopyroxene as a cumulate phase near the fz

major element variations in basalts and diabases from the north arm mountain massif bay of islands ophiolite implications for magma chamber processes at mid ocean ridges abstract the major element compositions of aphyric diabases from the north arm mountain massif range from primitive basalts to highly evolved ferrobasalts from modeling of these compositional variations it is clear that the diabases have undergone highly variable 10 to 90 increments of crystallization at low pressures the dominant minerals inferred to have crystallized from the liquids that formed these diabases are olivine plagioclase and clinopyroxene the same minerals that dominate the underlying cumulate gabbroic and ultramafic rocks the projection of these compositions onto pseudo liquidus phase diagrams suggests that the primary magma s from which this north arm suite was produced were generated by melting at 10 25 kbar or greater pressure there is no indication in the major element variations of these diabases of the operation of high pressure crystal factionation processes which is not surprising because low pressure fractionation processes will effectively mask evidence for earlier high pressure processes it is suggested that the tapping of intercumulus liquid from crystal mush areas and the tapping of discrete fractionated magmas within the roof zone of magma chambers is a method of producing a wide variety of magma compositions from a single magma chamber in which the majority of the chamber may be actively convecting and therefore remain relatively uniform in composition

mineral relationships in gabbroic rocks from fracture zones of indian ocean ridges evidence for extensive fractionation parental diversity and boundary layer recrystallization

arabian carbonate reservoirs a depositional model of the arab d reservoir in khurais field saudi arabia the upper jurassic arab formation in the arabian peninsula the most prolific oil bearing interval of the world is a succession of interbedded thick carbonates and evaporites that are defined stratigraphically upsection as the arab d arab c arab b and arab a the arab d reservoir is the main reservoir in khurais field one of the largest onshore oil fields of the kingdom of saudi arabia in khurais field the arab d reservoir is composed of the overlying evaporitic arab d member of the arab formation and the underlying upper part of the jubaila formation it contains 11 lithofacies listed from deepest to shallowest 1 hardground capped skeletal wackestone and lime mudstone 2 intraclast floatstone and rudstone 3 pelletal wackestone and packstone 4 stromatoporoid wackestone packstone and floatstone 5 cladocoropsis wackestone packstone and floatstone 6 clypeina and thaumatoporella wackestone and packstone 7 peloidal packstone and grainstone 8 ooid grainstone 9 crypt microbial laminites 10 evaporites and 11 stratigraphically reoccurring dolomite the arab d reservoir lithofacies succession represents shallowing upward deposition which from deepest to shallowest reflects the following depositional environments offshore submarine turbidity fans lithofacies 1 and 2 lower shoreface settings lithofacies 3 stromatoporoid reef lithofacies 4 lagoon lithofacies 5 and 6 shallow subtidal settings lithofacies 7 and 8 peritidal settings lithofacies 9 and sabkhas and salinas lithofacies 10 the depositional succession of the reservoir represents a prograding shallow marine reef rimmed carbonate shelf that was subjected to common storm abrasion which triggered turbidites

a budget of carbonate framework and sediment production kailua bay oahu hawaii abstract sediments of the bay and coastal plain of kailua oahu hawaii are 90 biogenic carbonate produced by destruction of primary reef framework coral and encrusting coralline algae and by direct sedimentation through the biological activity of calcifying organisms the green alga halimeda the branching coralline alga porolithion gardineri molluscs and benthic foraminifera field measurements of benthic community structure gross carbonate production bioerosion and direct sedimentation in 17 physiographic zones in kailua bay are used to calculate modern calcareous sediment production rates in the 12 km2 fringing reef system total gross carbonate productivity by corals and encrusting coralline algae based on mapped percent cover and known growth rates occurs at an average rate of 1 22 0 36 kg m 2 y 1 over hard substrates of the reef platform corresponding to 0 8 0 2 mm y 1 using a bulk density of 1 48 g cm 3 the average of coral and coralline algae coralline algae contributes 42 of the total gross productivity bioerosion of coral and coralline algae facies at and near the reef surface estimated from slabbed reef samples occurs at average rates of 0 10 1 15 kg m 2 y 1 and releases 1 911 436 m3 of unconsolidated carbonate sediment annually mechanical erosion coral breakage likely contributes an additional 315 m3 y 1 carbonate sediment is also produced directly by the green alga halimeda branching coralline algae molluscs and benthic foraminifera at a combined rate of 1 822 200 m3 y 1 using densities specific to sediment origin the total rate of production of unconsolidated carbonate sediment in kailua bay is the sum of these sources amounting to 4 048 635 m3 y 1 normalizing gross sediment production in kg y 1 by reef habitat area in m2 generates average rates of productivity in kg m 2 y 1 that are directly comparable to each other and to the literature such rates can also be employed and tested in other reef settings in kailua bay the total production of calcareous sediment corresponds to an average normalized rate of 0 53 0 19 kg m 2 y 1 with 0 33 0 13 kg m 2 y 1 contributed through erosion of the coralgal framework and 0 20 0 06 kg m 2 y 1 contributed by direct sediment production on the reef surface applying these modern sediment production rates over the 5 000 years that kailua bay has been completely inundated by postglacial sea level rise an estimated 20 2 3 2 106 m3 of unconsolidated carbonate sediment has been produced in the system the volume of sediment stored in the various reef channels and holes in kailua bay is 3 7 0 3 106 m3 or 19 of that produced since 5 000 y bp the volume of sand in the modern beach is 1 0 0 1 106 m3 or 5 of holocene sediment production the volume of carbonate sediment stored in the coastal plain is estimated using core log data and associated radiocarbon ages to be 10 0 1 8 106 m3 or 51 of holocene sediment production the remaining 25 likely represents sediment loss due to the natural processes of dissolution attrition and transport offshore these export terms are not well understood and emphasize the need for sediment dynamics to be incorporated into reef and sediment budgets although sediment production in this reef system is prodigious the rate of new sediment supplied to the beachface is less than 2 of what moves on and off the beach seasonally

origins and development of holocene coral reefs a revisited model based on reef boreholes in the seychelles indian ocean until recently concepts of coral reef growth and accumulation have been predominantly based on a darwinian model in this the upwards and outwards growth of a reef core a coral framework takes place over a foreslope consisting of reef talus with the simultaneous filling of the back reef lagoon by reef derived debris the principal adaptations of this pattern relate to the influence of relative changes in sea level and commonly ignore oceanographic factors such as storm frequency and severity boreholes through the outer edge of a fringing reef in the seychelles western indian ocean reveal a record of holocene sediment accumulation first established approximately 8 ka ago faunal and floral associations show that growth of this body began in relatively deep water but that this shallowed to 5 m within 1 ka subsequent accumulation was of keep up style but as the rate of sea level rise slowed shoaling became more frequent and aggradation was limited by reducing accommodation space constructional facies are characterised either by massive corals including leptastrea porites and faviids or by branching corals typically acropora of the danai robusta group coral surfaces may be encrusted by red algae foraminifera and vermetids and are commonly bored by filamentous algae clionids and molluscs however detrital facies are volumetrically dominant and the paucity of a constructional framework requires a re evaluation of models of reef accretion new models relate the geometry of accretion to the interplay between extreme storm events and fairweather hydrodynamic conditions these suggest that a contiguous framework forms in areas of moderate fairweather energy without extreme storm events severe storms destroy the continuity of reef structures and generate increasing volumes of coarse detritus low storm severity coupled with low fairweather hydrodynamic energy may promote the accumulation of fine grained reef derived sediments that inhibit framework growth while ecology reflects year by year sea conditions lithology and structure are controlled by exceptional storms with the effects of changing sea level superimposed

mineralogic variation in a layered ultramafic cumulate sequence at the north arm mountain massif bay of islands ophiolite newfoundland in order to investigate magmatic processes operative within magma chambers underlying oceanic spreading centers a suite of 59 samples was collected over an 8 m stratigraphic interval at the base of the cumulate ultramafic unit exposed on the north arm mountain massif in the bay of islands ophiolite complex newfoundland interlayered dunites and wehrlites are present in the 8 m section low abundances of incompatible elements ti y zr and other evidence suggest that these rocks originally formed as adcumulates olivine compositions are in the range fo84 5 89 9 and clinopyroxene mg mg mg fe2 100 range from 89 7 to 93 3 spinel compositions are highly variable and mg and al2o3 contents of spinels have been modified by subsolidus reequlibration with olivine and clinopyroxene cryptic variation patterns defined by the mg of olivine and clinopyroxene are characterized by successive cryptic regressions that have a repeat interval averaging 1 m the cryptic regressions are believed to record influxes of fresh relatively unfractionated magma into a magma chamber or conduit the gradual decrease in the mg of olivine and clinopyroxene and the decrease in olivine ni contents between successive cryptic regressions indicate that 10 crystallization of each batch of magma occurred before a new batch of magma entered the crystallization site of these cumulate rocks in some cases the cryptic regressions in mineral chemistry correlate with abrupt lithologic changes from wehrlite to dunite suggesting rapid displacement of the fractionated magma by more primitive incoming magma or rapid mixing of fresh incoming magma and resident magma in other cases inverse fractionation trends and fine scale interlayering of the cumulate rocks appear to represent zones of less rapid mixing between fractionated and fresh magmas comparison of cryptic variation patterns at the base and the top of the cumulate ultramafic section on north arm mountain indicates that in some cases the liquids that crystallized to form the upper part of the unit were more primitive higher mgo feo than the liquids that crystallized to form the base of the unit this feature is an indication of the complexity of processes that may operate in magma chambers underlying present day oceanic spreading centers

from platform top to adjacent deep sea new source to sink insights into carbonate sediment production and transfer in the sw indian ocean glorieuses archipelago abstract over the past ten years a huge amount of source to sink studies have aimed to unravel the tectonic climatic and other processes that shape the landscape from mountains to the deep ocean interestingly these studies have been mainly dedicated to siliciclastic or mixed systems for which the connection between drainage basins continental shelves slope and basin environments are often well constrained here we present a study focusing on a source to sink study dedicated to a pure carbonate system located in the sw indian ocean glorieuses archipelago extensive field sampling and geophysical acquisition across the carbonate platform have allowed us to estimate the composition the lateral variability and volumes of neritic sands deposited on the platform top additional seismic and bathymetric surveys across the platform interior illustrate the presence of plurimetric sandy bodies deposited along the leeward platform edge corresponding to the export of carbonate sediments from the platform top toward the platform edge under the influence of dominant currents and wind driven processes high resolution seismic bathymetric data and sediment cores acquired along the leeward slope and basin adjacent to the carbonate platform highlight the presence of channel levee complexes and turbiditic lobes which have accumulated on the seafloor on top of a 250 m thick sedimentary basin at 2000 3400 m water depth our study points out that carbonate sands and aragonitic mud produced on the platform top during the holocene have been shed to the adjacent basin we also demonstrate that this routing system was active at least throughout the last three glacial interglacial cycles this study has important consequences for our understanding of carbonate sedimentation processes occurring in the vicinity of isolated carbonate platforms 1 it highlights the role of oceanographic conditions in the distribution of sediment on a platform top and its export along the platform edge and the adjacent basin 2 it contributes to quantify the productivity of a carbonate platform as well as its sediment storage capacity 3 a first estimate of a carbonate source to sink system is proposed demonstrating that 0 57 km3 of sediments have been produced during the holocene 0 3 km3 being presently stored on the platform and the remaining having been exported to the deep basin

cumulate gabbros from the southwest indian ridge 54 s 7 16 e implications for magmatic processes at a slow spreading ridge a diverse volcanic and plutonic rock suite was recovered from the center of the 80 km long ridge segment of the southwest indian ridge 54 s 7 16 e between the islas orcadas and shaka fracture zones the cumulus nature of the gabbroic rocks in the suite is indicated by phase modal and cryptic layering igneous lamination and low incompatible element abundances we present a mass balance model for calculating the proportions and compositions of cumulus phases and crystallized intercumulus liquid from bulk rock major element compositions the model is based on the ability to define a compositional array of basaltic liquids and on the assumption that cumulus minerals are initially in equilibrium with trapped liquid calculated proportions of trapped liquid range from 3 15 values that are characteristic of adcumulates to mesocumulates models of postcumulus crystallization indicate significant enrichments of incompatible elements and buffering of compatible elements in residual trapped liquids thus explaining the high tio2 contents observed in magnesian clinopyroxenes cumulus phase assemblages and compositions suggest solidification in shallow level magma chambers but disequilibrium plagioclase compositions suggest some crystallization at greater depth furthermore basalt compositions projected onto the olivine clinopyroxenequartz pseudoternary suggest magma generation over a range of pressures from less than 10 to greater than 20 kb as well as polybaric fractional crystallization we suggest that the southwest indian ridge is characterized by low magma supply with small batches of melt that either ascend directly to the surface having undergone limited polybaric crystallization or are trapped in shallow crustal magma chambers where they evolve and solidify to form cumulate gabbros the adcumulus nature of the gabbros investigated here suggests slow cooling rates typical of large intrusions implying relatively large but ephemeral magma chambers below segments of the southwest indian ridge

gabbroic xenoliths from the northern gorda ridge implications for magma chamber processes under slow spreading centers abundant gabbroic xenoliths in porphyritic pillow basalt were dredged form the northern gorda ridge the host lava is a moderately fractionated normal mid ocean ridge basalt with a heterogeneous glass rind mg numbers 56 60 other lavas in the vicinity range from near primary mg number 69 to fractionated mg number 56 on the basis of textures and mineral compositions the xenoliths are divided into five types type a xenoliths consist of large clinopyroxene oikocrysts enclosing euhedral to subhedral plagioclase olivine and glass are present only in trace amounts type b xenoliths consist of olivine and plagioclase spinel with abundant intersertal glass partially resorbed olivine with deeply embayed margins in places subophitically encloses small plagioclase crystals olivine and large plagioclase crystals often contain glass inclusions and or spinel type c xenoliths are a composite of types a and b with areas of large clinopyroxene oikocrysts adjoining areas of loosely joined clusters of olivine and plagioclase in glass type d is represented by a single large xenolith that except for containing a larger proportion of crystals appears identical to the host lava type e is represented by a single small xenolith that is mineralogically similar to type b but with a distinct fine grained tightly interlocking texture formed by small olivine oikocrysts enclosing euhedral plagioclase in a small amount of intersertal glass many single crystals and glomerocrysts in the host lava appear to be xenocrysts some forsteritic olivine and anorthitic plagioclase contain glass inclusions more primitive than the host lava while rare glass inclusions in skeletal olivine suggest entrapment of melt more evolved than the host lava the overabundance of plagioclase crystals and their textural features suggest accumulation the xenoliths are not cognate to the host lava but they are genetically related chemistry of mineral phases in conjunction with textural features suggests that the xenoliths formed in different parts of a convecting magma chamber that underwent a period of closed system fractionation the chamber was filled with a large proportion of crystalline mush when new more primitive and less dense magma was injected and mixed incompletely with the contents in the chamber forming the hybrid host lava plagioclase rich leucogabbro layers in ophiolites attest to similar processes in magma chambers of past spreading centers however xenoliths in ferrobasalt from the southern juan de fuca ridge differ significantly from the gorda ridge xenoliths and suggest that the magma chamber at the southern juan de fuca ridge is located at shallower depth and in cooler crust than the one at the northern gorda ridge

gravity anomalies over extinct spreading centers a test of gravity models of active centers gravity fields over active mid ocean ridges display long wavelength free air gravity highs with superimposed short wavelength anomalies over the spreading axis these axial anomalies are negative over slow spreading active ridges with amplitudes of typically 30 70 mgal and wavelengths of 40 60 km but decrease in amplitude and wavelength with increasing spreading rate until at fast spreading centers the anomalies are positive 10 20 mgal with wavelengths of about 20 30 km similar free air anomalies are observed over extinct spreading ridges and typically show the same paleo spreading rate dependency suggesting that a large component of the anomaly is not dynamically maintained calculation and removal of gravitational effects from basement relief results in significant residual gravity lows at the extinct ridges analyzed here even though the thermal effects associated with active spreading have diminished and no longer contribute to such short wavelength gravity lows the amplitudes of the residual lows at slow paleo spreading rates are 25 45 mgal with half widths of about 20 45 km and decrease with increasing spreading rate to about 5 mgal amplitude and 10 15 km half width over fast paleo spreading ridges the amplitudes and wavelengths of these residual anomalies are nearly identical to those of the residual anomalies observed over active spreading centers most previously proposed models of mid ocean ridge structure are unable to explain these anomalies and many are inconsistent with a least some aspect of the large body of knowledge currently available for mid ocean ridges this research provides a further constraint on geometric models of accretion as these models predict vastly different gravity fields results indicate that a frozen in low density accretionary structure is probably confined to within about 10 km of either side of the extinct axis this conclusion is consistent with the presence of a low density gabbroic root situated at shallow mantle depths beneath the ridge axis this root tends to narrow with increasing spreading rate producing the required residual gravity field variation

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

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

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

geologic and seismic velocity structure of the crust mantle transition in the bay of islands ophiolite complex geological investigations of the bay of islands ophiolite complex show that while a typical layered ophiolite suite is present the thickness of major lithologic units is extremely variable from place to place the composition and internal structure of map scale kilometers across lithologic units as well as the contacts that bound them are laterally variable inferred velocity depth functions of the crust mantle moho transition reconstructed for this terrane as oceanic lithosphere suggest an extremely complex internal seismic structure the crust mantle transition from mafic to ultramafic lithologies varies across the ophiolite from a sharp geologic and seismic velocity discontinuity to a complexly interlayered transition zone as much as 3 km thick the mafic ultramafic transition is characterized by laterally discontinuous lithologic units on the order of hundreds of meters to a few kilometers thick and up to several kilometers long these units megalenses may be composed of lithologies with higher or lower seismic velocities than those of the surrounding units solid state deformation has produced seismically anisotropic materials in the upper mantle and lower crustal units substantial relief and slopes of up to at least 12 over lateral distances of about 10 km occur on the top of the crust mantle transition zone as well as other geologic unit contacts although these types of geological features are striking to the field geologist working in ophiolites and are significant to the understanding of crustal accretion processes many of them occur on such a small scale that they might go undetected in different types of seismic experiments in contemporary oceanic lithosphere

a detrital sediment budget of a maldivian reef platform abstract sediment dynamics are an important control on the morphology and development of reef systems by actively removing and redistributing excess detrital sediment this study presents quantitative data from direct point measurements of sediment transport on the platform surface and fore reef slope of vabbinfaru reef north male atoll maldives a suite of sediment traps were used to construct actual rates of platform sediment fluxes and off reef export over different spatial and temporal seasonal scales to establish key sediment transport pathways findings showed that high sediment fluxes occur on vabbinfaru platform in the absence of major storm activity up to 1905 g m 1 d 1 with 95 of annual transport occurring during the southwest monsoon as a result of increased wave energy climate driven changes in the platform process regime caused a reversal of net sediment transport pathways between each monsoon season off reef export rates were high reaching a maximum of 12 58 kg m 1 y 1 for gravel and 407 g m 1 d 1 for sand sized sediment an estimated 127 120 kg is exported from the platform annually equating to a significant loss from the reef sediment budget and contributing to the long term geomorphic development of the fore reef slope and atoll basin detrital sediment reservoirs on vabbinfaru are not purely depositional carbonate sinks but rather temporary stores that are important in the transfer of sediment between reef zones

petrology of a leucogabbroic interval within basal layered gabbros at north arm mountain bay of islands ophiolite data from detailed sample traverses in the layered gabbro unit of the north arm mauntain massif bay of islands ophiolite allow meter scale resolution of magmatic processes in spreading ridge magma chambers one suite of 46 samples from a 195 m interval near the base of the layered gabbro unit contains cumulus plagioclase an73 7 87 5 average modal abundance 75 clinopyroxene mg 80 3 86 0 18 and olivine fo76 6 82 1 6 with intergranular orthopyroxene mg 78 0 83 3 1 and accessory cr al spinel cr 32 3 41 4 ilmenite rims spinel in one sample whole rock zr contents range from 6 to 15 ppm plots of stratigraphic height in the traverse versus petrogenetic indicators e g mg s of mafic phases and an in plagioclase reveal both normal and reverse cryptic variation patterns the patterns for all indices are generally correlated the normal portions of the patterns formed during fractional crystallization of basalt batches ranges of mineral compositions in the normal trends suggest that 29 38 crystallization of each batch of basalt occurred before magmatic replenishment the reverse cryptic trends formed by crystallization of hybrid magmas produced during periods of magma mixing other evidence for magma mixing is the systematic association of spinel and reversely zoned plagioclase with the reverse trends experiments and observations of natural assemblages indicate that 55 modal plagioclase crystallizes from basalts at the olivine plagioclase clinopyroxene liquid piercing point the average plagioclase content of this suite of leucogabbros from north arm mountain is too high to have formed from simple crystallization at the piercing point petrologic modeling indicates the leucogabbros may have formed from basalts into which a small amount 10 of plagioclase was resorbed during mixing the initial compositions of these hybrid basalts lie in the plagioclase primary phase volume other suites of layered gabbros from north arm mountain are not so plagioclase rich as the leucogabbros described above crystallization of basalts in the plagioclase primary phase volume and the consequent formation of plagioclaserich gabbros may occur in restricted portions of zoned magma chambers underlying oceanic spreading centers or may occur episodically in the overall lifetimes of the magma chambers

anatomy of a modern coral reef flat a recorder of storms and uplift in the late holocene abstract cores from kabira reef ishigaki island southwest japan reveal the internal structure and temporal changes in sedimentary process of a complete coral reef flat the reef crest caught up with sea level at about 4000 yr bp since then it expanded oceanward and the reef pavement has grown landward the backreef structure is composed of bioclasts derived from the reef framework of all the bioclasts corals and coralline algae are most abundant coral fragments coarser than 3 0 o have dominated the backreef sediments since about 2000 yr bp benthic foraminiferal tests first occurred at about 4000 yr bp and their abundance increased significantly starting around 2000 yr bp shallowing of the reef crest is also indicated by the presence of the shallow water benthic foraminifera baculogypsina sphaerulata tests after 2000 yr bp the date 2000 yr bp is coincident with tectonic uplift at kabira reef kawana 1989 we consider this uplift to have caused a relative sea level fall that aided the deposition of coral fragments transported from the reef pavement by storms the relative sea level fall also caused subaerial exposure of the reef crest during low tides and the transition of reef building organisms from corals to shallow water species of benthic foraminifera resulting in a change noticeable in the constituents of backreef sediments the results of this study suggest that the coral reef flat and especially the backreef can be a faithful recorder of relative sea level changes

greenstones in the mino paleozoic mesozoic terrane of the east takayama area central japan evidence for magmatism evolution from normal ridge to plume volcanism abstract greenstones of early permian age occur extensively in the mino terrane of the east takayama area greenstones are mainly tholeiitic basalt and subordinately alkalic basalt tholeiitic basalt comprises a lava sequence about 1000 m thick and tectonically contacts alkalic basalt fusuline fossils in associated limestones show that alkalic basalt is slightly younger than tholeiitic basalt chrome spinel occurs locally in tholeiitic and alkalic basalts it is occasionally aluminous and similar to spinel in tholeiitic and alkalic basalts of oceanic ridge regions basalts in the lower portion of the tholeiite sequence are depleted in tio2 zr and nb and are characterized by extremely low nb y ratios the lower basalts appear to be normal mid oceanic ridge basalt n morb the middle upper basalts are enriched in tio2 zr and nb and show nb y ratios that are high relative to zr y they are similar to icelandic tholeiite basalt chemically transitional from n morb to icelandic basalt occurs in the l

a paleoproterozoic ultramafic mafic assemblage and associated volcanic rocks of the boromo greenstone belt fractionates originating from island arc volcanic activity in the west african craton abstract the loraboue birimian ultramafic mafic assemblage located in the boromo greenstone belt burkina faso is interpreted as the remains of a magma chamber that crystallized at the base of an island arc the ultramafic rocks exhibit an heteradcumulate texture being generally made up of wehrlites and more rarely dunites the crystallization sequence inferred from the cumulates is olivine chromite followed by clinopyroxene amphibole orthopyroxene biotite the gabbroic rocks are mainly layered and grade into more differentiated facies with sub pegmatitic texture containing up to 70 modal plagioclase including zircon and more commonly apatite crystals textural relationships and mineral phase chemistry are indicative of crystallization at elevated pressures 8 kbar the parental magma being generated by a moderate to high degree of partial melting of a mantle source affected by previous metasomatic events above a subducted oceanic slab the loraboue volcanic formations exhibit a range of geochemical features they consist dominantly of calc alkaline basalts pyroclastites and rhyolite and more rarely of basalt dolerite and gabbro of tholeiitic affinity these different types of basalt as well as the dolerite and the isolated massive gabbro show the classic features of arc magmatic suites namely lile and pb enrichment depleted hfse patterns and high ce nb and th nb ratios thus the calc alkaline plutonic and volcanic assemblages of loraboue could represent the roots of an island arc and the associated coeval volcanic rocks the paleoproterozoic crust of the west african craton was heterogeneous and was not the consequence of a single process of genesis as some modern igneous province the birimian crust was generated by both volcanic arc accretion and oceanic plateau accretion

the petrogenesis of plagioclase phyric basalts at mid ocean ridges 1 plagioclase ultraphyric basalts pubs have been sampled along most mid ocean ridges with ultraslow to intermediate spreading rates over the past 40 years the prevalent models for their origin assume positive buoyancy of plagioclase in basaltic liquids resulting in selective concentration of plagioclase phenocrysts by floatation however when the global population of pub lavas is examined this hypothesis becomes less compelling pub host lavas demonstrate a large range of compositions and densities similar to aphyric glasses from the same ridge segments most importantly the majority of pub host liquids are less dense than their phenocryst cargo meaning that plagioclase floatation within a magma chamber cannot be the driving force for phenocryst enrichment furthermore pub lavas have never been sampled on axis at fast spreading centers or from locations with noted contemporaneous axial magma chambers where pubs should be abundant if plagioclase is buoyant in mid ocean ridge basalt morb instead we argue that the high modal abundance of plagioclase results from interaction between magma and preexisting zones of crystal cumulates within the lower crust possibly followed by loss of olivine during magma ascent pubs erupt when the magma maintains an ascent velocity greater than the settling rate of the plagioclase phenocrysts which precludes long crustal residence times for these magmas in addition to being a proxy for lower spreading rates our findings also suggest that pub eruption can also be used as a proxy for the absence of a magma chamber or transport through a conduit system that bypassed an axial chamber

a new isostatic model for the east pacific rise crest we have developed a new isostatic model for the east pacific rise epr that explains the origin of both the axial topographic high and the rise crest gravity anomaly our model involves a thin elastic plate with a constant thickness crust that is broken at the ridge crest we assume that a buoyant force beneath the plate bends the free edge of the plate upward at the rise axis forming the topographic high and the rise crest gravity anomaly we have used this model and two conventional isostatic models to examine the isostasy of three segments of the epr at 9 l4 n 6 11 s and 16 21 s rather than modeling individual profiles we have stacked summed the gravity and bathymetry data in each area and computed a single 200 km long mean mirrored profile for each different ridge segment these stacked profiles are a remarkable improvement over even the best individual profiles and clearly show both the axial topographic high and its associated free air gravity anomaly in all three areas the best fitting model parameters are similar and give average compensation depths of about 6 7 km below the seafloor near the base of the oceanic crust and an upper limit on the flexural rigidity of the plate that increases from about 1018 n m at the ridge axis to about 3 1019 n m 5 km from the axis and 1020 n m 25 km or more from the rise crest these results indicate that the epr crest is compensated at much shallower depths than suggested by previous investigators our results are consistent with the presence of a small low density magma chamber in the lower crust although at least part of the compensating root for the axial high must extend into the upper mantle we interpret the low upper mantle densities below the rise crest as a zone of partial melt which accumulates at the base of the oceanic crust between episodic rifting events when the overlying crustal magma chamber is replenished

petrologic and geologic variations along the southern explorer ridge northeast pacific ocean mid ocean ridge basalts morb from the well defined southern explorer ridge segment ser in the northeast pacific ocean are moderately to strongly enriched in incompatible elements enriched morb were erupted at the highest and widest part of the segment culmination where the magma supply may be the greatest and also at the northern and southern ends of the ser variations in basalts incompatible element enrichment occur over short distances and suggest that the underlying mantle is heterogeneous on a small scale the variations also preclude the existence of a long lived well mixed magma chamber beneath the robust culmination instead magma chambers are probably temporally and spatially isolated as evidenced by the presence of highly differentiated lavas less enriched transitional morb were erupted along the central part of the ser 11 27 km south of the culmination and were supplied to the ridge separately from a less enriched pan of the mantle a more continuous magma chamber cannot be ruled out for this section on geochemical grounds all of the morb from the ser have undergone significant amounts of fractional crystallization unlike certain segments of the east pacific rise the most evolved morb were erupted near the culmination of the ridge segment aphyric lavas have been recovered only from the vicinity of the culmination while phyric lavas have erupted along the entire length of the ser there is no correlation between composition and total crystal abundances despite the requirement of multiple parental magmas most of the lavas from the culmination and the distal ends of the ridge fall on a common liquid line of descent indicating that they formed at similar depths and extents of partial melting the transitional morb from the central part of the ridge have probably formed by lesser extents of partial melting and separated from their mantle source at greater depths based on abundances of na and moderately incompatible elements such as sm and zr enriched high melt fraction lavas at the magma starved southern end of the ser closely resemble lavas from the culmination either the southernmost lavas have flowed laterally within the crust from near the culmination or there is another melting anomaly that supplies the southern end of the ser if the latter case is true correspondence between ridge morphology and mantle thermal state is poor

the significance of unusual zoning in olivines from famous area basalt 527 1 1 major element ni mn and ca electron microprobe analyses of olivine phenocrysts in one of the most primitive basalts from the famous area of the mid atlantic ridge 527 1 1 reveal two different olivine populations distinguished by their zoning characteristics the often skeletal group i olivines have zoning profiles with high forsterite high ni low mn and low ca cores these profiles can be explained by low pressure crystallization from the 527 1 1 magma on cooling the equant often megacrystic group ii olivines also have high forsterite low mn cores but the cores have low ni compared to rims thus they are normally zoned with respect to mg fe and mn but reversely zoned with respect to ni for example ni ranges from 1 700 ppm at fo90 5 in the core to 2 100 ppm at fo89 5 at the rim in view of the published whole rock and mineral data of le roex et al 1981 the most likely explanation for these data is that the group ii olivines are xenocrysts assimilated from solidified plagioclase pyroxene basalts through which the 527 1 1 basalt ascended the diffusion rate of ni and size of the xenocrystic olivines are used to calculate the residence time of the assimilated olivines in the magma an alternative hypothesis would be a high pressure origin for the group ii olivines this would be possible if future experiments show that with increasing pressure the partition coefficient for ni decreases relative to the partition coefficient for mg for a given bulk composition current evidence suggests this is unlikely the data from 527 1 1 and other samples from the famous area require magmas with distinct ni mgo feo characteristics in general morb from different ridge segments fall on distinct trends on plots of mgovs ni as well as mgovs feo calculation of the mgo and ni contents of primary magmas suggests a mechanism by which such distinct trends could come about

holocene reef development along the northeastern st croix shelf buck island u s virgin islands abstract eight cores were recovered from buck island underwater national monument u s virgin islands facies were defined based on recovered coral species fabrics observed in core slabs and thin sections and detailed notes on drilling character thirty six radiometric dates constrained the timing of reef accretion together these data provide a detailed history of reef development under varying regimes of sea level rise and physical oceanography holocene reefs around buck island initiated atop a broad antecedent bench at 13 16 meters below present sea level shelf flooding near buck island occurred as early as 9 500 years ago calbp but preserved reefs lagged by as much as 1 800 years earliest reef development was dominated by branching acropora palmata near the shelf edge and massive corals closer to buck island by 7 200 calbp a palmata apparently declined near the platform margin and was absent until ca 5 200 calbp throughout the study area over time the reefs closer to buck island built upward ca 16 m and seaward ca 50 m as the rate of sea level rise slowed and carbonate production increasingly exceeded the accommodation space that was being created reef topography and zonation became progressively more distinct with a palmata dominating the shallow reef crest branching coral apparently disappeared again between ca 3 030 and 2 005 calbp for reasons that are not clear this and the previous decline of a palmata mimic patterns seen around st croix and throughout the caribbean by 1 000 calbp the reefs close to buck island had largely assumed their present character and continued to track slowly rising sea level until the present around 1 200 calbp vertical accretion along buck island bar apparently ceased paradoxically the surface of this outer reef has historically been dominated by large stands of a palmata since the area was first described but rapid coral growth has not resulted in preservation of this species over the last millennium modern community structure mimics facies patterns seen in cores over the past 7 700 years the southern reef crest appears to have remained slightly shallower than its northern counterpart a condition that persists today observations after hurricane hugo in 1989 suggest that this difference in elevation is related to the piling up of debris on the broader southern reef crest by high waves from storms passing south of st croix also facies along the southern reef are more variable in species composition than their northern counterparts a condition that is exhibited by the modern reef community coral abundance and diversity in the cores total coral 20 30 dominated by a palmata are comparable to the community structure present in the late 1970s bythell et al 1993 hubbard et al 1993 in contrast fossil coral abundance and diversity are consistently higher than what was measured in the 1980s and early 1990s total coral 7 14 a palmata 2 after the onset of white band disease a putative pathogen which has recently decimated branching acroporids throughout the region the dominance of branching a palmata in the cores would seemingly reflect an absence of disease or other factors that would discourage its continued abundance in apparent contrast two lengthy gaps in the a palmata record reflect previous disappearances that roughly correspond to similar lapses elsewhere in the caribbean thus the spatial persistence of a species in the fossil record cannot necessarily be equated with its temporal continuity comparisons between changes in modern reefs on a time scale of decades and their fossil forebears must be made with great care understanding the role of short term changes and how they are reflected in the preserved record is thus critical to relating the late holocene a palmata gaps to the recent decline of the species this has important implications for our understanding of how preserved community structure relates to what actually existed in the past and could limit our ability to use the recent geologic record as a proxy to short term future changes in coral reefs

geochemical evidence for formation of the bay of islands ophiolite above a subduction zone the bay of islands boi complex in western newfoundland is one of the world s best known ophiolites and has generally been interpreted as a fragment of ocean floor emplaced on the eastern margin of north america1 2 the similarities of the boi to litho sphere formed at mid ocean ridges has led many investigators to use the boi as an onland laboratory in which to study the geochemical tectonic and geophysical properties of the oceanic lithosphere1 9 there have been nonetheless a few suggestions that the boi ophiolite formed above a subduction zone10 12 as have most other ophiolites13 15 here i report the existence of a substantial tantalum depletion in boi magmas jenner et al 16 demonstrate that boi magmas have a niobium depletion as well these depletions are characteristic of magmas erupted above subduction zones but not at mid ocean ridges indicating that the boi is not after all a good analogue for lithosphere formed at mid ocean ridges

coral indicators of past sea level change a global repository of u series dated benchmarks fossil corals provide valuable data for reconstructing past sea levels as they are often well preserved in the fossil record and can be dated with u series methods here we present a global and internally consistent database of usingle bondth dated fossil coral sea level indicators including full consideration of all known associated uncertainties both vertical and chronological we include carefully determined taxon specific depth distributions rather than blanket depth uncertainty terms as used in most previous work this is based on a synthesis of extensive modern ecological information on depth ranges these ranges are found to be spatially variable between ocean basins between regions and on sub regional scales because depth itself is not limiting instead depth distributions arise from complex physical chemical and biological interactions with coral reef growth distribution and composition one of the main causes for recognition of the greater depth variability of coral taxa has been the routine inclusion of deep diving and rov surveys in coral ecological studies over the past few decades which has broken through the shallow water bias of early surveys by adding frequent observations on deeper occurrences although more are needed it is also clear from our assessment that coral habitat depth distributions must be determined on the species level to reduce uncertainties in reconstructions of past sea levels and that application to sea level studies then requires these studies also to identify fossil corals to the species level samples identified only to the genus level give rise to wide uncertainties in habitat depth and hence sea level our database contains extensive metadata to assist evaluations of dating quality as well as geomorphic and stratigraphic metadata we demonstrate with examples how such metadata can help to evaluate sea level reconstructions for example by identifying outlier points one example discusses the last interglacial lig where we use the available data with their uncertainties to assess probabilistically the time at which local sea levels exceed that of the present which yields a mean age of 124 6 ka with 95 probability bounds at 118 5 and 129 5 ka we conclude with identification of key outstanding issues relating to i current incomplete understanding of tectonic setting including the current lack of independent verification of uplift subsidence rates and reliance of somewhat unsatisfactory and circular use of the elevation of last interglacial deposits ii the depth distributions of coral taxa and iii the complete documentation of stratigraphic geomorphological and other contextual information with suggestions for strategies to address these issues

geochemical and geological evidence bearing on the origin of the bay of islands and coastal complex ophiolites of western newfoundland abstract suggestions that ophiolitic rocks of the bay of islands complex boic and coastal complex cc of southwestern newfoundland were formed at a seafloor spreading center are supported by 1 1 whole rock trace and major element data from diabases and basalts 2 2 spinel mineral chemistry from ultramafic rocks 3 3 the internal geology of the ophiolite terranes and 4 4 regional geologic and tectonic constraints in the western newfoundland appalachians diabase and basalt samples from the boic and the northern cc massifs are tholeiitic and geochemically indistinguishable from present day mid ocean ridge basalts morbs many diabases from the lewis hills massif the southernmost part of the cc are also identical to morbs but there is also an unusual suite of diabases from the lewis hills that is unlike any other suite yet reported from a variety of tectonic settings including island arcs ocean islands or spreading centers this suite is interpreted to have been produced by unusual magma generation processes in the deep structural levels of an oceanic ridge ridge transform fault approximately 90 of spinels from ultramafic rocks in the boic and cc have cr cr al ratios that are less that 0 60 comparable to those values reported from ultramafic rocks of the present day ocean basins these geochemical constraints in combination with previously described geological relationships within the ophiolite terranes and regional tectonic relationships in the western newfoundland appalachians suggest that the boic and cc ophiolites formed within a main ocean basin i e the early paleozoic iapetus ocean along a seafloor spreading center and ridge ridge transform fault respectively there is little evidence to suggest formation in either an island arc or back arc basin tectonic setting as such the boic and cc ophiolites are probably well suited for detailed petrologic structural and geophysical studies to determine the processes involved in the generation and evolution of contemporary oceanic crust and upper mantle

the tsunamite problem the genetic term tsunamite is used for a potpourri of deposits formed from a wide range of processes overwash surges backwash flows oscillatory flows combined flows soft sediment deformation slides slumps debris flows and turbidity currents related to tsunamis in lacustrine coastal shallow marine and deep marine environments tsunamites exhibit enormous variability of features e g normally graded sand floating mudstone clasts hummocky cross stratification etc these sedimentary features may also be interpreted as deposits of turbidity currents turbidites debris flows debrites or storms tempestites however sedimentary features play a passive role when these same deposits are reinterpreted as tsunamites on the basis of historical evidence for tsunamis and their triggering mechanisms e g earthquakes volcanic explosions landslides and meteorite impacts this bipartite sedimentological vs historical approach which allows here classification of the same deposit as both turbidite and tsunamite has blurred the distinction between shallow marine and deep marine facies a solution to this problem is to classify deposits solely by a descriptive sedimentological approach the notion that tsunami waves can directly deposit sediment in the deep sea is unrealistic because tsunami waves represent transfer of energy and they are sediment starved during tsunamis and major storms submarine canyons serve as the physical link between shallow water and deep water environments for sediment transport tsunami related deposition involves four progressive steps 1 triggering stage offshore 2 tsunami stage incoming waves 3 transformation stage near the coast and 4 depositional stage outgoing sediment flows in this progression deep water deposition can commence only after the demise of incoming tsunami waves due to their transformation into outgoing sediment flows deposits of these sediment flows already have established names e g debrite and turbidite therefore the term tsunamite for these deposits is obsolete

reef talus a popular misconception reef fronts have traditionally been regarded as comprising debris derived by contemporaneous erosion of the reef however evidence from wave transport indicates that on present day reefs the bulk of the debris generated in this way accumulates in the back reef area with only finer grained sediment carried off reef by retreating flows or by overwash nevertheless in contrast to this observation fore reef debris slopes are commonly considered characteristic of phanerozoic reefs this apparent error reflects the conflation of processes defining contemporary growth and accretion of the reef and the corresponding long term accretion of the carbonate platform on which it rests present day reefs are commonly although not exclusively additions to long lived carbonate platforms growth of the latter is intermittent and has been moderated by changes in sea level that for recent reefs have been on time scales of less than 100 ka during low sea level stands growth ceases or is translated downslope and earlier deposits are subject to lithification and subaerial erosion similar changes are applied on a larger scale to the aggrading growth of carbonate platforms but the bulk accretion of these includes quite different processes and reflects far longer timescales during low sea level stands the margins of platforms commonly become unstable with instability reflected in slope failure and in the shedding of blocks ranging from metres to kilometres in diameter associated with the generation of debris flows and turbidites it is argued that these are the materials that are commonly described as reef talus in ancient structures although their formation is largely independent of any contemporary reef growth difficulties arise where the reef and the platform are treated as a single functional entity it is important to recognize the conceptual distinction between them reef talus is a misleading description of the debris predominantly generated by platform erosion and slope failure

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

carbonate platform scale correlation of stacked high frequency sequences in the arab d reservoir saudi arabia abstract the late jurassic arab formation contains a number of hydrocarbon bearing carbonates the most important of which is the lowermost arab d reservoir the reservoir lithofacies in khurais field are couplets of 1 lime mud and 2 intraclastic lithofacies representing basinal turbidites 3 pelletal lithofacies representing lower shoreface sands and silts 4 stromatoporoid lithofacies representing a reef 5 cladocoropsis and 6 dasyclad lithofacies representing a lagoon 7 peloidal and 8 oolitic lithofacies representing shore attached sand sheets 9 cryptomicrobial lithofacies representing supratidal flats 10 anhydrites representing sabkha followed by salina deposits and 11 stratigraphically reoccurring dolomite these are arranged in two partially preserved third order sequences the upper of which represents the arab d member and the lower of which represents the upper jubaila formation within these sequences lie six fourth order high frequency sequences composed of fifth order parasequences and parasequence scale cycles the preserved upward shallowing trend of the arab d reservoir is manifested laterally by a regional eastward thickening interpreted to be the result of an eastward progradation across the shallow late jurassic epeiric shelf and into the relatively deep arabian intrashelf basin this study presents a correlation model that explains the drastic thickening and downward climb of the reservoir lithofacies that is observed between the outcrops south of riyadh and the subsurface in ghawar field this model is different from the one currently used and predicts an eastward porosity improvement in the upper part of the reservoir accompanied by a porosity reduction in the lower part assuming a null diagenetic modification effect