geothermobarometry of basaltic glasses from the tamu massif shatsky rise oceanic plateau 1 we present the results of a petrological study of core samples from tamu massif site u1347 recovered during the shatsky rise integrated ocean drilling program iodp expedition 324 the basaltic glasses from site u1347 are evolved tholeiitic basalts containing 5 2 6 8 wt mgo and are principally located within the compositional field of mid ocean ridge basalts morbs but they have systematically higher feo lower al2o3 sio2 and na2o concentrations and the cao al2o3 ratios are among the highest known for morbs in this sense glasses from site u1347 more closely resemble basaltic magmas from the ontong java plateau ojp although they still have lower sio2 concentrations in contrast to morb and similar to ojp our fractionation corrected values of na2o and cao al2o3 indicate more than 20 of partial melting of the mantle during the generation of the parental magmas of tamu massif the water contents in the glasses determined by midinfrared fourier transform infrared ftir spectroscopy are morb like and vary between 0 18 and 0 6 wt h2o the calculated pressure p temperature t conditions at which the natural glasses represent cotectic olivine plagioclase clinopyroxene compositions range from 0 1 to 240 mpa and 1100 to 1150 c reflecting magma storage at shallow depth the variation of the glass compositions and the modeled p t conditions in correlation with the relative ages indicate that there were at least two different magmatic cycles characterized by variations in eruptive styles massive flows or pillow lavas chemical compositions volatile contents and preeruptive p t conditions each magmatic cycle represents the progressive differentiation in course of polybaric crystallization after the injection of a more primitive magma batch magma crystallization and eruption episodes are followed by magmatic inactivity reflected in the core sequence by a sedimentary layer our data for tamu massif demonstrate that similar to ontong java ocean plateau the crystallization beneath shatsky rise occurs at different crustal levels

implications for magmatic processes at ontong java plateau from volatile and major element contents of cretaceous basalt glasses 1 major elements cl co2 and h2o were determined in unaltered tholeiitic basalt glasses from drilling program sites 803d and 807c on ontong java plateau ojp and sites 462a in nauru basin nb and 802a in east mariana basin emb glasses are moderately evolved mg 53 60 with low na2o indicating they formed by large extents of melting extent of melting is less for glasses from ojp than from emb requiring faster or more focused mantle upwelling or a longer period of activity for ojp glasses within each hole define 1 3 different compositional types each compositional type is made up of many glass bounded flow units whose chemistry is identical within analytical uncertainty suggesting that each compositional type formed during a single eruptive episode with multiple magma surges the lack of diversity within each type also suggests that the magmatic system was large and well buffered major element chemistry also requires that significant shallow crystallization occurred during plateau formation and limits the amount of deep crystallization that might be responsible for the thick crust of the plateau chlorine contents are constant within each compositional type and are 580 ppm in type a and 850 ppm in types c g from hole 807c and 360 ppm in glasses from hole 803d glasses from 802a and 462a have 240 ppm and 1975 ppm cl respectively cl k ratios range from 0 2 to 2 33 well above mid ocean ridge basalt morb mantle values 0 07 suggesting that most liquids assimilated hydrothermally altered material as they ascended and crystallized high cl k ratios also suggest that abundant hydrothermal activity may have accompanied plateau formation low dissolved co2 i e co32 contents in one ojp compositional type 54 ppm in 807c type a are similar to those on shallow sections of the mid ocean ridge and are consistent with a maximum depth of eruption of 1320 250 m in contrast to the greater eruption depths inferred from fossils in the overlying sediments our proposed eruption depth falls in the morb field on a plot of depth versus na8 0 supporting a ridge or near ridge origin the shallow depth estimate allows for more postformation subsidence and diminishes the importance but does noteliminate the need for additional processes such as prolonged underplating to account for ojp s shallow depth higher co2 in other glasses including types c g from hole 807c suggests that some liquids were oversaturated with co2 when emplaced and therefore did not travel far from their point of eruption h2o contents are similar to morb and range from 0 13 wt for nb to 0 25 wt 807c types c g to 0 49 wt 807c type a ratios of h2o to rare earth elements ree are slightly higher than most morb and are similar to those from northern mid atlantic ridge mar which has distinctively high h2o h2o ce constant in depleted through enriched morb ranges from 200 to 400 compared to 220 380 for northern mar glasses and 150 250 for morb elsewhere and 500 for arc related glasses the slightly higher h2o contents in the ojp mantle source could have contributed very little to the extensive melting that built the plateau so a substantial temperature anomaly is still required

jurassic cretaceous boundary age and mid ocean ridge type mantle source for shatsky rise basalt sills cored in lowermost berriasian sediments on shatsky rise in the northwest pacific yield a mean 4 0 ar 3 9 ar incremental heating age of 144 6 0 8 ma providing a minimum estimate on the age of the jurassic cretaceous boundary the largest part of the rise was emplaced at rates similar to those of several continental flood basalts the rise formed at an oceanic triple junction and the sills as well as lavas from two dredge sites exhibit an ocean ridge type nd pb sr isotope signature this signature is provisionally consistent with either a perisphere or meteorite impact model but is not predicted by the plume head model of plateau formation however no model can yet be rejected given the very sparse sampling of shatsky rise

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

depths of partial crystallization of h2o bearing morb phase equilibria simulations of basalts at the mar near ascension island 7 11 s phase equilibria simulations were performed on naturally quenched basaltic glasses to determine crystallization conditions prior to eruption of magmas at the mid atlantic ridge mar east of ascension island 7 11 s the results indicate that mid ocean ridge basalt morb magmas beneath different segments of the mar have crystallized over a wide range of pressures 100 900 mpa however each segment seems to have a specific crystallization history nearly isobaric crystallization conditions 100 300 mpa were obtained for the geochemically enriched morb magmas of the central segments whereas normal n morb magmas of the bounding segments are characterized by polybaric crystallization conditions 200 900 mpa in addition our results demonstrate close to anhydrous crystallization conditions of n morbs whereas geochemically enriched morbs were successfully modeled in the presence of 0 4 1 wt h2o in the parental melts these estimates are in agreement with direct fourier transform ir measurements of h2o abundances in basaltic glasses and melt inclusions for selected samples water contents determined in the parental melts are in the range 0 04 0 09 and 0 30 0 55 wt h2o for depleted and enriched morbs respectively our results are in general agreement within 200 mpa with previous approaches used to evaluate pressure estimates in morb however the determination of pre eruptive conditions of morbs including temperature and water content in addition to pressure requires the improvement of magma crystallization models to simulate liquid lines of descent in the presence of small amounts of water

bathymetry of shatsky rise northwest pacific ocean implications for ocean plateau development at a triple junction oceanic plateaus are large igneous edifices thought to have been created by nascent mantle plumes but owing to sparse data their origins remain uncertain understanding plateau evolution is important because they are significant ocean features and may provide clues about mantle plume dynamics we constructed a bathymetry map of shatsky rise a large pacific plateau combining multibeam and wide beam echosounder data from 87 cruises and u s navy multibeam contours the rise consists of three large isolated volcanic edifices massifs surrounded by nearly normal lithosphere a linear volcanic ridge and a group of about 80 scattered seamounts massif flank slopes are typically gentle 1 5 and often parallel magnetic lineations or fracture zones the slope angles imply effusive volcanism similar to flood basalts whereas the rise shape suggests formation near the pacific izanagi farallon triple junction with modification of volcano flanks by spreading ridge tectonics edifice sizes and inferred ages imply a trend of decreasing volume and age from southwest to northeast furthermore gaps between massifs suggest episodic volcanism existing data are broadly consistent with the plume head hypothesis wherein the largest edifice formed by massive plume head eruptions the ridge formed from the plume tail and the two massifs in between represent a transition seamounts east of the rise are morphologically distinct which suggests that they may have been formed by a different source or mechanism than shatsky rise proper

molybdenum and other heavy metals of the mecca quarry and logan quarry shales pennsylvanian age mecca quarry shale from illinois and indiana contains enriched values of mo and other heavy metals average values range up to 1 160 ppm mo 8 970 ppm zn 160 ppm u 320 ppm pb 250 ppm se 420 ppm cu 450 ppm ni and 5 970 ppm v for full thicknesses pennsylvanian logan quarry shale of indiana carries similar enrichments of heavy metals for both shales trace element contents correlate markedly with specific lithologies most samples were analyzed by quantitative x ray fluorescence near shore deposition in brackish waters for the indiana locations deeper more characteristically marine environment for illinois despite high trace element values in both shales e g near ore grades for mo their thinness generally less than 1 m and the fine grained dispersed nature of their host phases relegate these shales to the status of long term resources unless improved beneficiation techniques can be devised or appreciably higher grades can be found the association of these shales with subjacent minable coals however improves their economic outlook modified journal abstract

mineralogy of the carbonate sediments western continental shelf of india abstract an x ray diffraction study of forty six sediment samples and three oolitic limestone samples from the western continental shelf of india shows that aragonite is the dominant carbonate mineral 99 maximum followed by low magnesium calcite 77 maximum and high magnesium calcite 39 maximum mgco 3 in the high magnesium calcite ranges from 5 to 21 maximum in the limestone with the most prominent mode at 13 followed by lesser modes at 16 9 and 6 5 these modes are attributed to the presence of various organic constituents in the sediments and rocks the sediments and the limestones are similar in mineralogy thus we conclude that no diagenetic change has taken place in the limestones after their formation

unconsolidated reefal sediments in the jubal strait region red sea egypt abstract the strait of jubal lies in a geologically active zone forming the junction between the old gulf of suez and the comparatively younger red sea the western side of the strait is bordered by a series of high islands and extensive coral reef formations the coast of the mainland is characterized by a developing fringing reef and intense sedimentation of terrigenous clastics this area is under the influence of an arid climate and strong tidal currents the beach and bottom sediments have different origins the beach sediments are siliciclastics of terrigenous origin whereas the bottom sediments comprise carbonate reef products both sediment types are poorly sorted reflecting the instability of the hydrodynamic conditions and the high rate of sedimentation it appears that lime mud originates in the northern agitated part of the area and drifts southeastwards to become trapped in the deep water areas the mineral assemblage of the reefal sands comprises high mg calcite aragonite and low mg calcite in decreasing order of abundance while the lime mud is composed of low mg calcite calcite and aragonite the terrigenous mineral association of the beach sands includes quartz and feldspars magnetite and ilmenite are the most common heavy minerals

the role of polybaric crystallization in genesis of andesitic magmas phase equilibria simulations of the bezymianny volcanic subseries abstract using the updated comagmat model the crystallization sequences of a bezymianny volcano basaltic andesite kamchatka russia are simulated in a wide range of thermodynamic conditions p t f o 2 as a function of h 2 o concentration comparison of the modeled liquid lines of descent with petrochemical trends of the volcanic suite indicates the parental melts contain 1 5 2 wt h 2 o stored under 490 520 mpa pressure in the magma plumbing system beneath bezymianny volcano the initial magma originates as a result of the polybaric evolution of mantle derived high mg basaltic magmas of the adjacent kliuchevskoi volcano the subsequent evolution of derivative hydrous and alumina rich basaltic andesite magmas may proceed under polybaric conditions with an average decompression of 12 mpa per 1 of crystallization in the course of polybaric crystallization compositions of pyroxene bearing andesites can be numerically reproduced and the modeled liquid compositions follow the natural liquid line of descent however hornblende bearing magmas cannot be produced as a result of continued crystallization from parental basaltic andesite through the stage of pyroxene bearing andesite formation they require high water contents and high pressures of crystallization in this case liquid composition should deviate from the chemical trend defined by the whole rock compositions

isotopic evidence for the origin of the manihiki and ontong java oceanic plateaus abstract pb nd and sr isotopic results for lavas of the cretaceous ontong java and manihiki oceanic plateaus fall well within the modern day oceanic island or hotspot field the data provide no evidence of old continental basements but indicate a major involvement of kerguelen type or em i like mantle in the sources of both plateaus which appear to have probably been formed at least in part by hotspots however the presently active hotspots that pacific plate reconstructions suggest might have been possible plateau sources lack kerguelen type isotopic compositions either these hotspots did not participate in the formation of the two plateaus or if they did kerguelen type material must have been volumetrically much more important early in their existence two hypotheses for the origins of these plateaus which involve hotspot sources are consistent with the sparse available geochemical geochronological and geophysical data the first holds that the plateaus formed cataclysmically in association with surfacing plume heads the second posits a relatively steady but robust hotspot at or near a ridge crest and requires a much longer period of formation a near ridge origin appears to be indicated by evidence that most of the pacific plateaus were built largely on relatively young ocean crust however we suggest that a near ridge origin is also compatible with the plume head concept in that plume heads appear very likely to become associated with spreading axes through their influence on rift propagation which should be substantially greater than for ordinary hotspots in either case the lack of hotspot tracks seamount chains attached to the two plateaus would be a consequence of ridge migration or rift propagation in a near ridge setting

developing carbonate platforms southern gulf of suez northern red sea abstract the ashrafi reef complex represents two small 8 10 2 km carbonate platforms and associated shoals located along the western side of the jubal strait at the gulf of suez s southern end these features are developing in an active marine environment that is operating within the geologic constraints of a tectonic trough characterized by mountainous fault controlled margins origin of the platforms is uncertain but the present morphology appears to be a product of the dynamic marine setting in which they are developing tidal flow between the red sea and the gulf results in strong rectilinear currents commonly 50 cm s 1 of the current velocities 5 10 cm s 1 can be attributed to surface currents driven by a regional unidirectional wind that blows from north to south down the axis of the gulf throughout the year side scan sonar data coupled with echo sounder profiles direct observations via scuba and bottom sampling indicate that the ashrafi platforms are actively building northward by windward reef accretion and extending to the south by sediment transport and accumulation the drumstick shape is a product of wave induced reef development along the northern margin and sediment distribution forced by strong tidal currents which coupled with unidirectional wind drift favors net transport to the south small reefs and hard grounds along the downdrift platform flanks assume a linearity consistent with the long axis of the platform the intensely mounded sediment of the platform flanks suggests that bioejection of particles in the mound building process together with a strong flow field is a downdrift sediment transport agent although the ambient salinity of surrounding gulf water is 41 platform top water masses were measured at 47 a lack of significant coral growth on the shallow platform top and the almost total exclusion of sediment producing calcareous green algae may be related to these hypersaline conditions sediments for the platform flanks are largely provided by the breakdown of platform margin reef communities and in situ organisms largely foraminifera

the age of earth s largest volcano tamu massif on shatsky rise northwest pacific ocean this study presents laser step heating 40ar 39ar age determinations of basaltic lava samples from tamu massif the oldest and largest edifice of the submarine shatsky rise in the northwest pacific and earth s proposed largest volcano the rocks were recovered during integrated ocean drilling program expedition 324 which cored 160 m into the igneous basement near the summit of tamu massif the analyzed lavas cover all three major stratigraphic groups penetrated at this site and confirm a late jurassic early cretaceous age for the onset of shatsky rise volcanism lavas analyzed from the lower and middle section of the hole yield plateau ages between 144 4 1 0 and 143 1 3 3 ma with overlapping analytical errors 2 whereas a sample from the uppermost lava group produced a significantly younger age of 133 9 2 3 ma suggesting a late or rejuvenated phase of volcanism the new geochronological data infer minimum average melt production rates of 0 63 0 84 km3 a over a time interval of 3 4 million years consistent with the presence of a mantle plume

reefal sediments of al ghardaqa northern red sea egypt abstract reefal sediments are abundant along the red sea samples of such sediments were studied in the al ghardaqa area along the red sea coast of egypt twenty samples were collected from the beach intertidal zone and lagoon in the study area geochemical analyses were carried out by atomic absorption spectrophotometer and flamephotometer while mineralogical compositions of the sediments were studied by x ray diffraction the average concentrations of the different chemical constituents were as follows sio 2 21 4 al 2 o 3 8 5 tio 2 0 12 fe 2 o 3 0 38 mno 0 021 cao 21 7 mgo 5 01 sro 0 82 na 2 o 0 23 k 2 o 0 11 zn 31 ppm and cu 21 ppm the chemical constituents are categorized into four groups regarding their possible sources aragonite and high mg calcite are the major carbonate minerals characterizing the reefal sediments on the other hand quartz and feldspar are the common minerals of terrigenous origin

magnetic lineations within shatsky rise northwest pacific ocean implications for hot spot triple junction interaction and oceanic plateau formation oceanic plateaus are major ocean features yet their origins and development are poorly understood many are huge piles of basalt and it is widely accepted that mantle plumes are their source perhaps from eruptions of the voluminous head of an emerging plume shatsky rise is a basaltic plateau that formed during a period of geomagnetic reversals unlike many mid cretaceous plateaus so magnetic data can help us understand its tectonic history in this study we analyzed magnetic anomaly data from 131 cruises over and around shatsky rise and constructed a magnetic lineation chart for tectonic interpretation a significant finding is that magnetic lineations are traceable through low parts of the rise between volcanic massifs indicating nearly normal lithosphere and between large volcanic edifices many lineations form bights near the rise axis and show former locations of the pacific izanagi farallon triple junction they indicate that the junction was in a ridge ridge ridge rrr configuration and closely followed the rise axis from chron m20 146 ma to chron m4 127 ma lineation and bight geometries indicate that the triple junction jumped northeastward at least nine times often apparently in response to volcanic activity from the mantle plume some jumps however appear related to ridge reorientations numerous features within the rise are parallel to lineations or lineation offsets implying that the plateau formed near the triple junction and was modified by ridge faulting most of our observations support the hypothesis that shatsky rise was formed by a mantle plume that captured a triple junction

geochemical evidence for a mid cretaceous superplume basalt lavas with a high nb y ratio for a given nb zr ratio occur in the polynesian superswell region of the south pacific which probably formed by upwelling of a deep mantle superplume the distinctive geochemical characteristics of the polynesian basalts may be attributed to melting of a mantle source that is more enriched in a basaltic ancient mid oceanic ridge basalt component basalts displaying such chemical signatures have been found on shatsky rise the ontong java plateau and greenstones from subduction zone complexes of sakhalin island the occurrence of polynesian type basalts together with an estimate of their ages suggests that the south pacific superplume was active as long ago as 90 150 ma the superplume activity preceded the onset of the superchron supporting an idea that the superplume acted as a trigger for such a global event

partition coefficients for rare earth elements in mafic minerals of high silica rhyolites the importance of accessory mineral inclusions abstract ree concentrations of mafic mineral separates from high silica rhyolites measured by inaa are high and variable compared to electron microprobe analyses of the minerals themselves the mafic phases commonly contain inclusions or have adhering grains of accessory rare earth element ree rich minerals optical and electron microscopic observation revealed discrete grains of chevkinite rare earth titano silicate included within clinopyroxenes from the sierra la primavera mexico rhyolite and monazite grains adhering to ortho and clinopyroxenes from the bishop tuff california during hand picking of mineral separates inclusions are only partly removed as a result the magnitude and variability of true mineral melt partition coefficients for light ree have been overestimated the true ree partition coefficients of la primavera and bishop tuff pyroxenes obtained by microprobe are only slightly higher than they are in lower silica rhyolites and are not as variable as previously thought the partitioning slope is positive as in less silicic systems the relative partitioning behavior of ree in high silica rhyolites is dominated by crystal chemical controls and not by liquid structural effects when a partition coefficient is used for crystallization calculations adhering phases and inclusions should be retained or the minor phases must be accounted for separately alternatively a bulk distribution coefficient can be calculated using whole rock and glass compositions and the glass mode

the seismic moho structure of shatsky rise oceanic plateau northwest pacific ocean abstract oceanic plateaus are large igneous provinces formed by extraordinary eruptions that create thick oceanic crust whose structure is poorly known owing to the lack of deep penetration seismic data multichannel seismic mcs reflection and wide angle refraction data allow us to show moho structure beneath a large part of the shatsky rise oceanic plateau in the northwest pacific ocean moho reflectors in the two data sets can be connected to trace the interface from the adjacent abyssal plain across much of the interior the reflectors display varied character in continuity shape and amplitude similar to characteristics reported in other locations beneath normal crust the moho is observed at 13 km depth 7 km below the seafloor in mcs data and disappears at 20 km depth 17 km below the seafloor beneath the high plateau moho at the distal flanks dips downward towards the center with slopes of 0 5 1 increasing to 3 5 at the middle flanks seismic moho topography is consistent with airy isostasy confirming this widely applied assumption data from this study show that crustal thickness between the massifs in the interior of the plateau is nearly twice normal crustal thickness despite the fact that this crust records apparently normal seafloor spreading magnetic lineations the moho model allows improved estimates of plateau area 5 33 10 5 km 2 and volume 6 90 10 6 km 3 the latter assuming that the entire crust was formed by shatsky rise volcanism because the massifs formed at spreading ridges this study is unique in showing moho depth and structure over an extraordinarily large area beneath an oceanic plateau giving insight to plateau structure and formation

a review of the origins of metal rich pennsylvanian black shales central u s a with an inferred role for basinal brines abstract compared to most shales even most black shales numerous thin pennsylvanian marine black shales of the u s midwest are very enriched in organic c 5 40wt and heavy elements for example instrumental neutron activation analysis of 74 samples of mostly black and dark gray pennsylvanian shales of the u s midwest average 1300 ppm zn 85 ppm u 655 ppm mo 130 ppm se and 55 ppm cd amounts sufficient to raise concerns about heavy element pollution direct precipitation of sulfides and fixation by abundant organic matter during sedimentation and early diagenesis may account for heavy metals contained by shales in most cases metal supply calculations however indicate that a special source would be needed to supply metals for a strictly syngenetic origin possible sources for such metals include submarine hot springs and pennsylvanian seas that may have been more enriched in metals than modern ocean water at least at some localities additional metal values may have been added later from basinal brines e g zn or from modern groundwaters e g mo se u the actions of modern supergene processes may fix metals mitigating detrimental effects from black shales

magma evolution revealed by anorthite rich plagioclase cumulate xenoliths from the ontong java plateau insights into lip magma dynamics and melt evolution abstract petrologic studies of igneous rocks often rely on whole rock analyses alone to deduce source and melting histories whole rock compositions represent a mixture of the processes responsible for the final texture and composition of the rock but do not emphasize the evolutionary processes that the rock may have experienced we examine the major minor and trace element compositions of plagioclase phenocrysts and plagioclase rich cumulate xenoliths from a sequence of kwaimbaita pillow basalt and massive flows recovered from ocean drilling program odp site 1183 on the expansive 122 ma ontong java plateau ojp and from subaerial outcrops on the island of malaita solomon islands these data are used to reconstruct equilibrium liquid compositions to understand magma dynamics magma evolution and the distribution basalt types across the ojp results reveal that crystals frequently are out of equilibrium with their host basalt two distinct compositional zones are present in plagioclase phenocrysts and xenolith crystals an80 86 and an65 79 equilibrium magma compositions derived from each of these zones are distinct in relative terms the following distinctions are made an80 86 sr y and eu richer ba and lree depleted higher sr ti lower ba sr and la y an65 79 sr y and eu poorer ba and lree enriched lower sr ti higher ba sr and la y we observed several an65 79 parent magmas with higher la y lower sr and higher lree than the majority of an65 79 zones which suggests that they grew from highly fractionated magmas the an80 86 zones of both xenolith crystals and phenocrysts appear to have crystallized relatively early and are attributed to growth in magmas more primitive than parent magmas of an65 79 zones or the host magma using melts modeling we demonstrate a significant role for h2o is not necessary to form an rich crystals from known ojp magma types these types of zones can instead form when relatively primitive ojp magmas ascend and partially crystallize at low pressures crystal mush layers are rheological elements of multiply saturated solidification fronts and conditions for extreme fractionation are met deep with thermally and mechanically insulated mush solidification fronts conditions in a homogenous magma body alone cannot generate the range of magma compositions and textures recorded in ojp plagioclase crystals we suggest that the ojp magma chamber system consisted of crystal mush dominated regions and liquid dominated regions in a laterally and vertically extensive system of interconnected dikes and sills solidification front disruption was common during magma recharge and transport which freed debris to mix with ojp magmas the volume of crystalline debris from disrupted solidification fronts was small relative to the overall magma volume which leads to little change of bulk magma chemistry this can explain the dominance of the kwaimbaita basalt type across the ojp evidence of solidification front disruption is best preserved as allochthonous crystals both more and less primitive than bulk magma chemistry both of which were observed in ojp basalts coincident with a zone of neutral buoyancy the shallow ojp magma chamber system existed at a depth of 0 7 km the ojp magma chamber system was thinner near the plateau margins which provided less density filtration of magmas this allowed more diverse magma types to reach the surface along the margins it is envisaged that as the ojp formed the zone of neutral buoyancy slowly migrated upward leading to slow yet pervasive assimilation of overlying seawater altered basalt at the base of the lava pile

variety and origin of magmas on shatsky rise northwest pacific ocean 1 shatsky rise consists of thick 30 km maximum basaltic crust with various geochemical compositions geochemistry data indicate that four magma types exist on the plateau namely normal low ti high nb and u1349 types the normal type is the most abundant in volume and appears on all three large edifices of the plateau tamu ori and shirshov massifs composition of the normal type is similar to normal mid ocean ridge basalt n morb composition but with slight relative enrichment of the more incompatible elements the low ti type is distinguished from the normal type basalt by slightly lower ti content at a given mgo composition of the high nb type is characterized by distinctively high contents of incompatible trace elements u1349 type basalts are composed of more primitive and depleted compositions compared with the others the normal type basalts constitute 94 of the lava units of the oldest tamu massif and non normal types i e the other three types basalts comprise 57 on the younger ori massif implying that geochemical compositions may have become heterogeneous with time petrological examination demonstrates that compositions of the normal low ti and high nb type basalts evolved through fractional crystallization of olivine plagioclase and augite in shallow magma chambers 200 mpa model calculations of immobile trace elements estimate that the normal type basalt can be formed by 15 melting of a depleted mantle source in the presence of residual garnet this degree of melting is similar to n morb but the larger effect of residual garnet during petrogenesis implies that a greater depth of melting

variations in organic geochemistry of anoxic oxic black shale carbonate sequences in the pennsylvanian of the midcontinent u s a abstract two cores containing thin organic rich phosphatic black shale members the excello and little osage shale and transitional lithologies from pennsylvanian cyclothems of southeast kansas and northeast oklahoma were characterized by geochemical and organic petrographic methods by very detailed sampling geochemical analyses included organic carbon determination rock eval pyrolysis bitumen extraction and chromatographic separation into saturated aromatic and nonhydrocarbon fractions gas chromatography of saturated hydrocarbons carbon isotope analysis of coexisting saturated and aromatic fractions and whole rock v and ni analyses organic petrography included maceral identification on polished whole rock fragments and vitrinite reflection and thermal alteration index tai determinations of kerogen concetrates the analytical results demonstrate large and systematic variations in geochemical properties within the black shale units in both cases an abrupt increase in toc 20 25 level and hi 400 level at the initiation of phosphatic black shale deposition is followed by a gradual decline to values of less than 1 toc and 100 hi organic constituents include both liptinitic algal derived and humic terrestrial components variations in their relative amounts control the nature and quality of the organic profile of the little osage shale level of anoxia varied within this sequence and influenced several geochemical parameters pr ph ratios decrease nonhydrocarbons increase relative to hydrocarbons saturated hydrocarbons decline relative to aromatics carbon isotopic composition of coexisting saturated and aromatic hydrocarbons converge and v and ni and the v ni v ratio increase with increased anoxia eustatic rise of sea level and rapid marine transgression over the continental craton with consequent flooding of adjacent extensive brackish marine peat swamps and the resultant influx of a large supply of nutrients and humic detritus appear to be the key geologic control of black shale deposition

relationship between inferred redox potential of the depositional environment and geochemistry of the upper pennsylvanian missourian stark shale member of the dennis limestone wabaunsee county kansas u s a analyses of 21 samples collected from a core of the 52 8 cm thick stark shale member of the dennis limestone in wabaunsee county kansas demonstrate four cycles with two orders of magnitude variations in contents of cd mo p v and zn and order of magnitude variations in contents of organic carbon cr ni se and u the observed variability in amounts and or ratios of many metals and amounts and compositions of the organic matter appear related to the cause and degree of water column stratification and the resulting absence presence of dissolved o2 or h2s high cd mo u v zn and s contents a high degree of pyritization dop 0 75 0 88 and high high v v ni 0 84 0 89 indicate the presence of h2s in a strongly stratified water column intermediate contents of metals and s intermediate dop 0 67 0 75 and intermediate v v ni 054 0 82 indicate a less strongly stratified anoxic water column whereas low metal contents and low v v ni 0 46 0 60 indicate a weakly stratified dysoxic water column high p contents at the top of the organic matter rich intervals within the stark shale member indicate that phosphate precipitation was enhanced near the boundary between anoxic and dysoxic water compositions relatively abundant terrestrial organic matter in intervals deposited from the more strongly stratified h2s bearing water column indicates a combined halocline thermocline with the fresher near surface water the transport mode for the terrestrial organic matter the predominance of algal organic matter in intervals deposited from a less strongly stratified water column indicates the absence of the halocline and the presence of the more generally established thermocline relatively low amounts of degraded hydrogen poor organic matter characterize intervals deposited in a weakly stratified dysoxic water column the inferred variability in chemistry of the depositional environments may be related to climate variations and or minor changes in sea level during the general phase of deeper water deposition responsible for this widespread shale member