multiple sources of quaternary tephra layers in the mariana trough abstract tephra layers in quaternary sediments stolper and newman 1994 model of binary mixing of source components with one endmember being the nmorb source of stolper and newman 1994 and the other one being a compositionally variable slab derived h2o rich component similar to the h2o component of stolper and newman 1994 this interpretation implies that the variation in k2o content in the arc derived glasses about a factor of 3 at 60 wt sio2 reflects a magmatic diversity that is inherent to the central mariana arc and is not necessarily reflective of temporal changes of the composition of arc magma sources

contrasting compositions of mariana trough fallout tephra and mariana island arc volcanics a fractional crystallization link discrete quaternary 400 ka tephra fallout layers mostly 1 cm thick within the siliceous oozes of the central mariana trough at 18 n are characterized by medium k to high k subalkalic volcanic glasses k2o 0 8 3 2 wt with high large ion lithophile elements lile high field strength elements hfse ratios and nb depletion ba la 35 ba zr 3 5 la nb 4 typical for convergent margin volcanic rocks compositional zoning within layers ranges from basaltic to dacitic sio2 48 71 wt mgo 0 7 6 5 wt all layers contain basaltic andesites the tephra layers are interpreted as single explosive eruptive events tapping chemically zoned reservoirs the sources being the mariana arc volcanoes mav due to their proximity 100 400 km and similar element ratios mav ba la 36 7 ba zr 3 5 0 9 the glasses investigated however contrast with the contemporaneous basaltic to dacitic lavas of the mav by being more enriched in tio2 1 2 wt mav 0 8 wt feo 10 wt mav 8 9 wt k2o 1 1 wt mav 0 8 wt and p2o5 0 4 wt mav 0 2 wt semi incompatible trace elements including rare earth elements ree of the basaltic andesitic and dacitic glasses match those of the dacitic mav lavas which became enriched by fractional crystallization moreover the glasses follow a tholeiitic trend of fractionation in contrast to mav transitional trends and have a characteristic p2o5 trend that reaches a maximum of 0 6 wt p2o5 at 57 wt sio2 whereas mav lavas increase linearly in p2o5 from 0 1 to 0 3 wt with increasing silica both explosive and effusive series are interpreted to have evolved in common magma reservoirs by convective fractionation similar parental magmas are suggested to have separated into coexisting si andesitic to dacitic and basaltic melts by in situ crystallization the differentiated melt is interstitial in an apatite saturated crystalline mush of plag px ox ol at the cooler chamber margins in contrast to the less differentiated basaltic to basaltic andesitic magmas which are not yet saturated in apatite and occupy the chamber interior reinjection of interstitial melt into the chamber interior and mixing with larger melt fractions of the interior liquid mixing ratios about 1 8 9 can explain the paradoxical behavior of apatite controlled p and mree variation in the basaltic andesite glasses and their mav dacite like fractionation patterns the process may also account for the exclusively tholeiitic trend of fractionation of the glass shard series but in situ crystallization alone cannot cause their absolute enrichment in semi incompatible elements the newly mixed melt is suggested to form the basaltic end member of the glass shard series however it must have become physically separated from the main mav magma body possibly by density driven convective fractionation in order to allow for further evolution of the contrasting geochemical paths as well as differentiation

tectonic hydrothermal brecciation associated with calcite precipitation and permeability destruction in mississippian carbonate reservoirs montana and wyoming the mississippian madison formation contains abundant fracture zones and breccias that are hydrothermal in origin based on their morphology distribution and geochemical signature the hydrothermal activity is related to crustal shortening during the laramide orogeny brecciation is accompanied by dedolomitization late stage calcite precipitation and porosity occlusion especially in outcrop dolomites the tectonic hydrothermal late stage calcite reduces permeability in outcrops and potentially high quality subsurface reservoir rocks of the subsurface madison formation bighorn basin the reduction of permeability and porosity is increased along the margins of the bighorn basin but not predictable at outcrop scale the destruction of porosity and permeability by hydrothermal activity in the madison formation is unique in comparison to studies that document enhanced porosity and permeability and invoke hydrothermal dolomitization models hydrothermal breccias from the owl creek thrust sheet are classified into four categories based on fracture density calcite volume and clast orientation shattered breccias dominate the leading edge of the tip of the owl creek thrust sheet in the eastern owl creek mountains where tectonic deformation is greatest whereas fracture mosaic and chaotic breccias occur throughout the bighorn basin the breccias are healed by calcite cements with 18o values ranging between 26 5 and 15 1 peedee belemnite pdb indicating that the cements were derived from isotopically depleted fluids with elevated temperatures in the chaotic and mosaic breccia types large rotated and angular clasts of the host rock float in the matrix of coarse and nonzoned late stage calcite this appearance combined with similar 18o values across even large calcite veins indicates that the calcite precipitated rapidly after brecciation values for 13c 5 12 pdb from the frontal part of the owl creek thrust sheet indicate equilibrium between methane and co2 bearing fluids at about 180c fluid inclusions from the eastern basin margin show that these cements are in equilibrium with fluids having minimum temperatures between 120 and 140c and formed from relatively low salinity fluids less than 5 wt nacl strontium isotope ratios of these hydrothermal fluids are more radiogenic than proposed values for mississippian seawater suggesting that the fluids mixed with felsic rich basement before migrating vertically into the madison formation we envisage that the tectonic hydrothermal late stage calcite cemented breccias and fractures originated from undersaturated meteoric groundwaters that migrated into the burial environment while dissolving and incorporating ca2 and and radiogenic sr from the dissolution of the surrounding carbonates and the felsic basement respectively in the burial environment these fluids were heated and mixed with hypersaline brines from deeply buried parts of the basement expulsion of these fluids along basement rooted thrust faults into the overlying strata including the madison formation occurred most likely during shortening episodes of the laramide orogeny by earthquake induced rupturing of the host rock the fluids were injected forcefully and in an explosive manner into the madison formation causing brecciation and fracturing of the host rock whereas the subsequent and sudden decrease in the partial pressure of co2 caused the rapid precipitation of calcite cements the explosive nature of hydrothermal fluid migration ultimately produces heterogeneities in reservoir quality carbonates in general flow units in the madison formation are related to sequence boundaries which create vertical subdivisions in the porous dolomite the late stage calcite cement surrounds hydrothermal breccia clasts and invades the dolomite reducing porosity and permeability of the reservoir quality rock as a consequence horizontal flow barriers and compartments are established that are locally unpredictable in their location and extent and regionally predictable along the margins of the bighorn basin

coupled manganese and carbon isotopic events in marine carbonates at the cenomanian turonian boundary abstract the boundary interval between the cenomanian and turonian stages of the upper cretaceous shows a positive excursion in the carbon isotopic composition of organic matter and carbonate in many epicontinental and oceanic sections of the atlantic tethyan realm some of the studied epicontinental sections contain an excursion in the manganese content of carbonates coincident with the shift in carbon isotopic values in a key reference section at pueblo colorado the stratigraphic position and magnitude of the manganese excursion are described with respect to the carbon isotopic excursion coincident excursions in manganese contents and carbon isotopic values are verified for a section in normandy france the depositional setting for these two sections is persistently to intermittently we l oxygenated with the accumulation of interbedded chalky and marly sediments no shift in manganese values was detected through the isotopic excursion in a section from northern nebraska where fossils are preserved only as bedding plane impressions due to extensive early diagenetic dissolution of carbonate the carbon isotopic and manganese excursions begin at the same stratigraphic position but manganese contents return to background values within the lower half of the carbon isotopic excursion the relative timing and duration of these signals suggest the existence of a large manganese reservoir that was rapidly mobilized transfer of manganese from deep sea oxides mn4 to epicontinental carbonates mn2 is inferred to have occurred during expansion of oceanic oxygen minimum zones with associated changes in redox gradients and boundaries simultaneous excursions in carbon isotopic values and manganese contents can occur at sites of carbonate accumulation when the carbon and manganese cycles are linked through the reducing potential of widespread oxygen deplete water masses

forty million years of magmatic evolution in the mariana arc the tephra glass record tephra glasses retrieved from 10 deep sea drilling project dsdp cores around the mariana arc system make up a remarkable record of explosive volcanism in the marianas over the past 40 m y major element compositions for approximately 1800 tephra glasses from basalt through rhyolite are reported and used to examine the nature and history of this activity three maxima of volcanic explosivity presumably related to especially vigorous volcanism in the mariana arc are identified with the biggest maximum at around 18 11 ma and two other maxima at 35 24 and 6 0 ma the two younger maxima are contemporaneous with peaks in explosive volcanism observed for other western pacific arcs explosive arc volcanism has been predominantly tholeiitic since shortly after arc inception no boninitic glasses were found the tephra glasses belong to the low to medium k suite except during an enigmatic phase of medium to high k explosive volcanism during the late miocene 11 7 ma even though mariana tephra glasses are largely similar in composition to mariana arc lavas the tephra show a much larger compositional range than that found for subaerial mariana arc lavas which totally lack dacitic to rhyolitic volcanic products the tephra glasses define a bimodal population in terms of silica content with a pronounced minimum or daly gap around 65 66 sio2 mariana tephra glasses are fractionated with the least evolved glass being fe rich and having a magnesium number mg 100mg mg fe 2 of 55 basaltic tephra glasses contain 16 a1203 average 14 3 a1203 and are not high alumina basalts this contrasts with the observation that modern mariana arc lavas contain 15 21 average 17 4 a1203 the high alumina basalt lavas of the mariana arc probably reflect plagioclase accumulation and not liquid compositions all tephra glasses plot near low pressure cotectics and reaction curves on the subprojection olivine clinopyroxene quartz mafic and felsic samples define distinct trends the mafic trend reflects fractional crystallization of mantle derived basaltic magma whereas the felsic trend may be due either to anatexis of mariana arc crust or to fractionation of mafic melts the tephra glass data reinforce the model that the magmatic evolution of the mariana arc has been dominated by low pressure fractionation perhaps accompanied by anatexis episodic changes in melting regime to generate miocene potassic tephra may be related to changing mantle sources and processes related to episodes of back arc basin spreading these episodic changes are superimposed on a long term increase in potassium that reflects progressive metasomatism of the mantle source long term increases in k20 contents for mariana arc magmas inferred from the tephra glass record are 0 004 wt m y mafic 0 011 wt m y intermediate and 0 023 wt m y felsic

delle phosphatic member an anomalous phosphatic interval in the mississippian osagean meramecian shelf sequence of central utah phosphatic rocks are a distinctive part of the lower and lower upper mississippian delle phosphatic member of the woodman formation at the type locality of the member in the lakeside mountains northwestern utah there as elsewhere in central and western utah the delle is composed of a relatively thin anomalous depositional interval characterized by phosphorite siliceous microfossils secondary chert distinctive dense lime mudstone and argillaceous rocks stratigraphically sandwiched within a thick sequence of subtidal and peritidal mississippian shelf carbonate and craton derived siliciclastic rocks strongly reducing in sediment conditions evidently prevailed at the onset of delle deposition yet sedimentologic features in the delle such as endolithic algal borings in pisolitic phosphorite and features in the rocks above and below the delle such as tidally laminated fecal pellet limestones and exclusively tractional sedimentary structures indicate shallow shelf depositional environments these rocks do not support the concept of a deep sediment starved depositional basin the deseret deep starved basin of previous authors rather the nature of the delle suggest incursion far onto the broad early mississippian shelf of nutrient rich organically productive waters possibly produced by upwelling in the coeval antler foreland trough in central nevada this interpretation places the early mississippian shelf margin to the west ofmore the lakeside mountains probably west of utah rather than in central utah as required by the deseret deep starved basin model less

application of the ce anomaly as a paleoredox indicator the ground rules much attention has been paid in recent years to the potential application of the ce anomaly measured in various marine phases as a paleoceanographic indicator of widespread marine anoxia in this paper we present and discuss results from recent studies of present day rare earth element ree distributions and hence ce anomaly distributions in the marine environment which are particularly pertinent to paleoceanography subsequently we review and discuss the validity of the recent literature in which ce anomalies have been employed as paleoredox indicators

mississippian wood grained chert and its significance in the western interior united states abstract wood grained chert is nodular chert which has internal light and dark colored concentric three dimensional compositional banding resembling grain in wood the light colored bands are dolomitic and the dark colored bands are quartzose with carbonaceous matter wood grained chert is recognized in the mississippian lodgepole deseret and great blue limestones and some correlatives of these formations in montana utah and idaho the deseret starved basin model sandberg and gutschick 1980 gutschick and sandberg 1983 provides a paleogeograpic framework that restricts the wood grained cherts in this basin to the foreslope between the oxygenated carbonate platform margin 300 m basin wood grained chert may mark th approximate position of the lower limit of the pycnocline in the deseret starved basin upwelling of nutrient rich water along the foreslope generated high biological productivity which in turn supplied abundant biogenic silica and organic matter to the sediments of the foreslope and basin siliceous microfossils dissolved and provided a source of silica for the formation of porcellanite opal ct nodules within the foreslope sediments during burial and compaction calcium carbonate diffused into the nodules from the surrounding mixed calcareous siliceous muds and early diagenetic conversion of opal ct to quartz released magnesium reduction of sulfate a dolomite inhibitor in the associated anoxic organic rich sediments below the pycnocline permitted dolomite formation dolomite precipitated in bands as a result of a periodic supersaturation precipitation nucleation d pletion process ostwald prager theory of liesegang band formation possibly augmented by competitive particle growth petrographic evidence and the apparent inverse relationship between band spacing and time support our theory

rare earth major and trace element composition of monterey and dsdp chert and associated host sediment assessing the influence of chemical fractionation during diagenesis abstract chert and associated host sediments from monterey formation and deep sea drilling project dsdp sequences were analyzed in order to assess chemical behavior during diagenesis of biogenic sediments the primary compositional contrast between chert and host sediment is a greater absolute sio2 concentration in chert often with final sio2 98 wt this contrast in sio2 and si al potentially reflects precursor sediment heterogeneity diagenetic chemical fractionation or both sio2 concentrations and si al ratios in chert are far greater than in modern siliceous oozes however and often exceed values in acid cleaned diatom tests compositional contrasts between chert and host sediment are also orders of magnitude greater than between multiple samples of the host sediment calculations based on the initial composition of adjacent host observed porosity reductions from host to chert and a postulated influx of pure sio2 construct a chert composition which is essentially identical to observed sio2 values in chert thus precursor heterogeneity does not seem to be the dominant factor influencing the current chert composition for the key elements of interest in order to assess the extent of chemical fractionation during diagenesis we approximate the precursor composition by analyzing host sediments adjacent to the chert the sio2 concentration contrast seems caused by biogenic sio2 dissolution and transport from the local adjacent host sediment and subsequent sio2 reprecipitation in the chert along with sio2 other elements are often added with respect to al to monterey and dsdp chert during silicification although absolute concentrations decrease the two monterey quartz chert nodules investigated in contrast to the opal ct and quartz chert lenses formed primarily by extreme removal of carbonate and phosphate thereby increasing relative sio2 concentrations dsdp chert formed by both carbonate phosphate dissolution and sio2 addition from the host manganese is fractionated during chert formation resulting in mno al 2 o 3 ratios that no longer record the depositional signal of the precursor sediment ree data indicate only subtle diagenetic fractionation across the rare earth series ce ce values do not change significantly during diagenesis of either monterey or dsdp chert eu eu decreases slightly during formation of dsdp chert la n yb n is affected only minimally as well during formation of one monterey opal ct chert lens ree al ratios show subtle distribution changes at gd and to a lesser extent near nd and ho ree compositional contrasts between diagenetic states of siliceous sediment and chert are of a vastly smaller scale than has been noted between different depositional environments of marine sediment indicating that the paleoenvironmental ree signature is not obscured by diagenetic overprinting

evolution of intra oceanic arc trench systems abstract intra oceanic island arc trench systems represent a locus of crustal thickening through the addition of mantle derived magmas which form the volcanoes and subjacent stratiform plutons the crust of immature island arcs and adjacent back arc basins is oceanic in chemistry in that it is dominated by mafic e g tholeiitic magmas a magma sequence is recognized in many arc systems which progress from boninite to arc tholeiite to calc alkaline and in some arcs to shoshonites the adjacent back arc basins are formed of morb like tholeiite arc systems may be dominated by extension rather than compression if the subducted and overriding plates are decoupled extension of the arc system may be associated with seaward retreat or roll back of the trench axis alternate cycles of extension and compression of the arc system can lead to both tectonic erosion of the fore arc and accretion of fragments from the incoming sea floor the izu bonin mariana tonga yap and palau trench arc systems are examples of different stages in a dynamic process which starts when subduction begins along old fracture zone traces crustal extension causes both fore arc subsidence and opening of back arc basins island arc volcanic chains and back arc basins are new crust formed largely of fractional melts of the mantle the evolution of an arc system may stop when blocking of the subduction zone or changes in relative plate motion causes overthrusting of back arc lithosphere on to former volcanic arcs many ophiolites preserve remnants of arc system crust and upper mantle

island arc carbonates characterization and recognition in the ancient geologic record abstract carbonates of island arc origin that are preserved in paleozoic mesozoic terranes of the north american cordillera exhibit a distinctive suite of paleontologic and lithologic features and share a fundamental similarity with limestones forming in modern volcanic arcs this study provides the first detailed synthesis of carbonate depositional systems in island arcs and documents primary sedimentary constituents based on facies relationships and faunal communities models are developed that show patterns in the long term evolution of shallow marine organisms and the construction evolution and demise of carbonate platforms in island arcs a suite of criteria is identified that may be used to differentiate island arc carbonates from limestones that accumulated in other platform settings biogeographic isolation prolonged subsidence steep submarine slopes and tectonic instability of volcanic edifices contribute to the development of relatively high levels of species endemism impoverished normal marine faunas complex provincial affinities and relict biotas in limestones that are characterized by exceptionally thick platform and periplatform sequences fringing and barrier reefs at the shelf margin extensive lagoonal deposits and rapid lateral and vertical facies changes although destructive tectonic and geologic processes in island arcs may hinder determining the original size and extent of the carbonate platform and particular facies types may not be represented e g fringing and barrier reefs may be replaced by sand shoals at the platform margin many characteristics have potential value for identifying carbonates of island arc origin in the ancient rock record apart from being associated with calc alkaline volcanic and volcaniclastic assemblages the most valuable suite of features for recognizing island arc carbonates is marine biotas that exhibit elevated levels of endemism and mixed paleobiogeographic affinities extraordinary thicknesses of platform shallow marine and periplatform carbonates and rapid facies changes between volcanic and carbonate rocks and between shallow and deep water limestones recognizing that an ensemble of features is distinctive within island arc carbonates considerably enhances identification of volcanic arcs in the ancient geologic record and thus improves the likelihood of successfully unraveling the complex geologic history of ocean basins

three dimensional crustal structure of the mariana island arc from seismic tomography 1 a three dimensional 3 d seismic refraction survey was acquired over the mariana volcanic arc at 14 5 18 5n and 145 147e first arrival traveltimes from this survey and from a separate 2 d survey acquired approximately perpendicular to the arc have been simultaneously inverted for a 3 d p wave velocity model using seismic tomography subject to smoothness constraints the active arc which initiated only 3 4 ma ago has an average crustal thickness of 18 km approximately 40 km to the east the inactive remnant of the rifted eocene arc has an average crustal thickness of 21 km due primarily to a thicker lower crustal layer with velocities of 6 5 7 0 km s 1 crustal production clearly varies both temporally and spatially with some crustal layers including the igneous forearc crust varying in thickness by a factor of up to 2 along strike average p wave velocities within the upper crust of the modern arc are 240 360 m s 1 lower than in the eocene arc but are 280 m s 1 higher within the lower crust middle crust with velocities of 6 0 6 5 km s 1 is best developed beneath the eocene arc these results suggest an evolution of arc structure with increasing age we infer closure of fractures and porosity in the upper crust through hydrothermal circulation and a reduction in the mafic character of the middle to lower crust as a result of intracrustal differentiation although tonalitic rocks may predominate in the transition from upper to middle crust the bulk of the crust is essentially basaltic

geochemical evidence for sundering of the west mariana arc in miocene ash from the parece vela basin abstract glass and mineral fragments from discrete volcanic ash layers were sampled from dsdp ipod site 450 in the parece vela basin philippine sea and analyzed by electron microprobe the ashes are interpreted as eruptive products of the adjacent west mariana arc system between 25 and 14 ma b p and have compositions between basaltic andesite and rhyolite and rarely boninite continuous chemical trends appear to reflect mixing of mafic and silicic magmas discontinuous trends between these end members are relatively few and are consistent with liquid lines produced by fractional crystallization andesitic tephra become progressively richer in mgo and cao through the middle miocene while boninite appears towards the end of the sequence between 14 and 15 ma b p coeval rhyolitic glasses become richer in k 2 o and na 2 o with maximum concentrations at about 15 ma b p chronologic changes in fractionation type and composition of parent magmas are interpreted to reflect the subaerial volcanic evolution of the west mariana arc the appearance of boninite is believed to signal early stages of arc sundering and corresponds temporally with regional uplift of the sea floor above the carbonate compensation depth precursor to a new pulse of back arc spreading

volcanism accompanying back arc basin development in the southwest pacific abstract the age and composition of magmatism in fiji and to a lesser extent vanuatu and tonga are evaluated in the light of the fragmentation which has sundered the island arc system they once collectively constituted any of six magmatic phenomena during the past 10 m y may be consequences of adjacent plate reconfigurations trondhjemitic plutonism about 10 m y change from tholeiitic to more calc alkaline volcanism between 10 and 6 m y eruption near the rifted arc margin of voluminous crustally derived high si low k rhyolites 7 m y ago shoshonite dominated basaltic volcanism 5 5 to 3 0 m y change to ocean island like basaltic volcanism at 3 0 0 2 m y and geochemically peculiar as well as tectonically anomalous acid andesites 0 7 3 0 m y ago in kandavu southwestern fiji crustal derivation of the low k rhyolites may reflect maximal heating during initial rifting shoshonites may accompany extreme regional extension above a still arc like source eruption of ocean island like basalts begins once subducted lithosphere no longer contributes to erupted melts or separates fiji from ocean island sources coeval eruption of hybrid andesites may accompany oblique subduction of south fiji basin crust beneath kandavu along the northeast hunter fracture zone

challenges of anticipating the 2011 tohoku earthquake and tsunami using coastal geology 1 can the magnitude of a giant earthquake be estimated from paleoseismological data alone attempts to estimate the size of the jogan earthquake of ad 869 whose tsunami affected much of the same coast as the 2011 tohoku tsunami offers an excellent opportunity to address this question which is fundamental to assessing earthquake and tsunami hazards at subduction zones between 2004 and 2010 examining stratigraphy at 399 locations beneath paddy fields along 180 km of coast mainly south of sendai we learned that a tsunami deposit associated with the ad 869 jogan earthquake had run inland at least 1 5 km across multiple coastal lowlands and that one of the lowlands had subsided during the jogan earthquake and an earlier earthquake as well radiocarbon ages just below above sand deposits left by the pre jogan tsunamis suggested recurrence intervals in the range of 500 to 800 years modeling inundation and subsidence we estimated size of the jogan earthquake as moment magnitude 8 4 or larger and a fault rupture area 200 km long we did not consider a longer rupture like the one in 2011 because coastal landform and absence of a volcanic ash layer make any jogan layer difficult to identify along the sanriku coast still sendai tsunami geology might have reduced casualties by improving evacuation maps and informing public awareness campaigns

fore arc basalts and subduction initiation in the izu bonin mariana system recent diving with the jamstec shinkai 6500 manned submersible in the mariana fore arc southeast of guam has discovered that morb like tholeiitic basalts crop out over large areas these fore arc basalts fab underlie boninites and overlie diabasic and gabbroic rocks potential origins include eruption at a spreading center before subduction began or eruption during near trench spreading after subduction began fab trace element patterns are similar to those of morb and most izu bonin mariana ibm back arc lavas however ti v and yb v ratios are lower in fab reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid ocean ridges and back arc basins some fab also have higher concentrations of fluid soluble elements than do spreading center lavas thus the most likely origin of fab is that they were the first lavas to erupt when the pacific plate began sinking beneath the philippine plate at about 51 ma the magmas were generated by mantle decompression during near trench spreading with little or no mass transfer from the subducting plate boninites were generated later when the residual highly depleted mantle melted at shallow levels after fluxing by a water rich fluid derived from the sinking pacific plate this magmatic stratigraphy of fab overlain by transitional lavas and boninites is similar to that found in many ophiolites suggesting that ophiolitic assemblages might commonly originate from near trench volcanism caused by subduction initiation indeed the widely dispersed jurassic and cretaceous tethyan ophiolites could represent two such significant subduction initiation events

magmatism of the westernmost komandorsky segment of the aleutian island arc abstract four magmatic associations are distinguished in the westernmost komandorsky segment of the aleutian island arc basalt plagiorhyolite 45 28 ma trachybasalt teschenite 27 16 ma diorite granitoid 12 5 8 5 ma and dacite rhyolite 0 7 0 ma the latter association is represented by the lavas of a recently discovered active submarine volcano piip volcano about 40 km north of the komandorsky islands and 700 km west of the quaternary center on buldir is detailed petrogeochemical data including ree and 143 nd 144 nd and 87 sr 86 sr isotopic ratios show that the parental melts of the komandorsky is were of mantle origin and their sources remained isotopically homogeneous during the whole geological history of the aleutian arc 10be concentrations in lavas of the piip volcano 2 7 106 at g indicate subduction related incorporation of young pelagic sediments in their magmatic sources or contamination of ascending melts by non solidified sedimentary material directly beneath the volcano the komandorsky segment of the aleutian arc evolved from a depleted arc tholeiitic series through a markedly alkaline series to later calc alkaline andesites and dacites and finally to late stage calc alkaline dacites and rhyodacites erupted inboard of the older volcanic centers

petrogenesis of volcanic rocks from saipan and rota mariana islands and implications for the evolution of nascent island arcs an 40 ar 39 ar age of 45 1 ma determined for lavas from northern saipan confirms that these high silica rhyolites erupted during the proto arc stage of volcanism in the izu bonin mariana system which is characterized elsewhere by eruption of boninitic lavas incompatible trace element concentrations and sr hf nd and pb isotope ratios for these rhyolites are transitional between those of c 48 ma boninitic lavas and post 38 ma first arc andesites and dacites from saipan and rota that have typical subduction related compositions these transitional compositions are modeled by crystal fractionation of parental tholeiitic basalt combined with assimilation of young boninitic crust a second stage of rayleigh fractionation in the upper crust is required by sio 2 concentrations that exceed 77 wt and near zero compatible element concentrations first arc magma compositions are consistent with fractionation of basalt and assimilation of crust similar in composition to the first arc magmas themselves the mantle sources of the proto arc and first arc lavas from saipan and rota are similar to those of philippine back arc basin basalts based on nd and hf isotopic compositions the pb isotope compositions of these lavas are between those of pacific sea floor basalts and jurassic and younger cherty and clay rich sediments this contrasts with the boninitic proto arc volcanic rocks from guam and deep sea drilling project sites 458 and 459 that have pb isotope compositions similar to pacific basin basalts and volcaniclastic sediments the preferred explanation for the difference in the nature of proto arc volcanism between saipan and other fore arc locations is that the crust ceased extending 3 4 myr earlier beneath saipan this was caused by a change from mantle upwelling fore arc extension and shallow melting to an environment dominated by more normal mantle wedge convection stable crust and deeper melting

geochemistry of the mississippian delle phosphatic event eastern great basin u s a abstract the middle mississippian delle phosphatic event marks a significant change in depositional conditions in the devonian mississippian antler foreland basin of north america stratigraphic and petrographic studies show that deposition of delle event rocks ended a long period of open marine carbonate sedimentation when facies shifted to phosphorites cherts fine grained siliciclastics and lime mudstones from highly productive waters that may have been episodically anoxic this drastic change in lithology is reflected in the chemistry of the two suites of rocks 18o values of the delle event lime mudstones and phosphatized wackestones packstones affected by the delle event are 3 5o oo lighter than underlying and overlying open marine wackestones packstones sr concentrations of delle event lime mudstones and open marine wackestone packstone facies are low suggesting that both of these lithologies have undergone similar postdiagenetic alteration the uniform sr concentrations and variable 18o data suggest that either the younger delle event rocks were exposed to meteoric water during diagenesis or that the relatively light 18o values represent a primary brackish water depositional signal either interpretation is consistent with sedimentological evidence of the delle event occurring in a predominantly shallow water setting 13c values of delle event lime mudstones are also anomalously light indicating a high input of organic carbon during their formation mn concentrations of the delle event lime mudstones and stratigraphic equivalents are highly variable suggesting fluctuating oxygen concentrations during their deposition and diagenesis rare earth element data from phosphorites and cherts of the delle event indicate oxic to suboxic depositional conditions

submarine arc volcanism in the southern mariana arc as an ophiolite analogue supra subduction zone ssz ophiolites are being recognized more and more frequently in the geologic record submarine volcanism in intra oceanic arcs is one tectonic setting in which such ophiolites might form the southern seamount province of the mariana arc represents the best documented example of an active intra oceanic submarine arc which can serve as a modern analogue for some ssz ophiolites the southern seamount province comprises nine submarine volcanoes at least seven of which are active or dormant these edifices have erupted plagioclase clinopyroxene olivine phyric basalts andesites dacites and dacitic pumices these lavas are similar to those of the subaerial edifices in the central marianas sio2 49 70 tio2 0 4 1 3 mgo 1 2 5 9 ba 140 450 ppm 87sr 86sr 0 70346 0 70354 but are somewhat more enriched in the light rare earth elements the geochemical characteristics of subaerial mariana arc lavas are preserved in the submarine lavas and the ssp lavas are slightly enriched in incompatible elements relative to those of the subaerial edifices we are ignorant of the composition of any lavas erupted between the principal edifices which may be expected to dominate the volcanic section of ssz ophiolites nevertheless the characteristic high ratio lil hfsc low ti ni and cr and diagnostic mineral compositions calcic plagioclase relatively fe rich olivine are likely to be preserved in unaltered submarine arc lavas the mariana active arc is built on back arc or thinned frontal arc crust and any ophiolite formed from such an arc is likely to exhibit complex intrusive and eruptive relationships of plutonic hypabyssal and volcanic rocks of back arc frontal arc and active arc origin such complicated intrusive relationships if preserved in structurally disrupted ophiolites will make it difficult to reconstruct the geologic history of an ssz ophiolite

phosphogenesis carbon isotope stratigraphy and carbonate platform evolution along the lower cretaceous northern tethyan margin we distinguish three different stages in the evolution of the tithonian late jurassic to aptian early cretaceous northern tethyan carbonate platform 1 carbonate production in the coral oolite mode late tithonian to early valanginian early barremian to early aptian 2 carbonate production in the crinoid bryozoan mode early valanginian haute rivian late aptian and 3 platform retrogradation and destruction condensation and phosphogenesis that is platform drowning early valanginian to early hauterivian middle hauterivian late hauterivian to early barremian early to early late aptian and latest aptian to earliest albian transitions from the coral oolite mode to the crinoid bryozoan mode and consequently to platform drowning may have been driven by increases in nutrient levels on the shelf phases of relative sea level rise in times of carbonate production in the coral oolite mode are named constructive because of the observed platform regeneration following phases of widespread emersion during late sea level highstands in contrast phases of relative sea level rise in times of platform drowning are termed destructive the 13 c stratigraphies obtained from valanginian hauterivian and aptian albian hemipelagic successions beyond the platform correlate well with the early cretaceous pelagic 13 c record positive excursions in the pelagic 13 c record correspond in time to episodes of platform drowning this suggests the existence of a coupling mechanism between changes in the global carbon cycle and platform drowning in our view the early cretaceous crises in carbonate platform growth were the consequence of reinforced greenhouse conditions which may have been triggered by episodes of extensive flood basalt volcanism strong greenhouse conditions may have induced the following chain of feedback mechanisms enabling the biosphere to return to normal conditions climate warming sea level rise accelerated water cycle intensified weathering nutrient mobilization platform destruction increased productivity increased phosphogenesis and carbon burial weakened greenhouse conditions

geologic consequences of globe encircling equatorial currents many black shales phosphorites and cherts that formed at low paleolatitudes on the north american continent during the late paleozoic were a direct consequence of equatorial upwelling and an equatorial undercurrent in an ocean that spanned much of the globe in equatorial parts of modern oceans wind stress divergence leads to high surface productivity sinking organic matter is remineralized in the strong eastward flowing equatorial undercurrent the undercurrent thus acts as a nutrient trap that becomes progressively oxygen poor and nutrient rich as it moves eastward the late paleozoic global ocean was 60 to 80 wider than the modern pacific ocean 24 000 km total width the nutrient trapping equatorial current system of this globe encircling ocean was probably anoxic and may have been sulfate reducing nutrient rich anoxic water from the undercurrent would have had direct consequences for the genesis of black shale facies in devonian and pennsylvanian epicontinental seaways as well as possibly providing the source water for coastal upwelling in settings such as the phosphoria sea

intra oceanic arcs of the paleo asian ocean abstract the paper reviews and integrates geological geochronological geochemical and isotope data from 21 intra oceanic arcs ioa of the paleo asian ocean pao which have been identified in the central asian orogenic belt the world largest accretionary orogeny the data we discuss structural position of intra oceanic arc volcanic rocks in association with back arc terranes and accretionary complexes major periods of intra oceanic arc magmatism and related juvenile crustal growth lithologies of island arc terranes geochemical features and typical ranges of nd isotope values of volcanic rocks four groups of ioas have been recognized neoproterozoic early cambrian early paleozoic middle paleozoic and late paleozoic the neoproterozoic early cambrian or siberian group includes eleven intra oceanic arcs of eastern and western tuva sayan southern siberia russia northern and southwestern mongolia and russian altai the early paleozoic or kazakhstan group includes selety urumbai bozshakol chingiz and baydaulet aqastau arc terranes of the kazakh orocline the middle paleozoic or southern group includes six arc terranes in the tienshan orogen chinese altai east kazakhstan west junggar and southern mongoia only one late paleozoic intra oceanic arc has been reliably identified in the caob bogda in the chinese tienshan probably due to pao shrinking and termination the lithologies of the modern and fossil arcs are similar although the fossil arcs contain more calc alkaline varieties suggesting either their more evolved character or different conditions of magma generation of special importance is identification of back arc basins in old accretionary orogens because boninites may be absent in both modern and fossil ioas the three typical scenarios of back arc formation active margin rifting intra oceanic arc rifting and fore arc rifting were reconstructed in fossil intra oceanic arcs some arcs might be tectonically eroded and or directly subducted into the deep mantle therefore the structural and compositional records of fossil intra oceanic arcs in intracontinental orogens allow us to make only minimal estimations of their geometric length life span and crust thickness

a model for tsunami flow inversion from deposits tsuflind abstract modern tsunami deposits are employed to estimate the overland flow characteristics of tsunamis with the help of the overland flow characteristics the characteristics of the causative tsunami wave can be estimated the understanding of tsunami deposits has tremendously improved over the last decades there are three prominent inversion models a moore s advection model moore et al 2007 b soulsby s model soulsby et al 2007 and c tsusedmod jaffe gelfenbuam 2007 tsuflind incorporates all three models and adds new modules to better simulate tsunami deposit formation and calculate flow condition tsuflind takes grain size distribution thickness water depth and topography information as inputs tsuflind computes sediment concentration grain size distribution of sediment source and initial flow condition to match the sediment thickness and grain size distribution from field observation furthermore tsuflind estimates the flow speed froude number and representative wave amplitude the model is tested by using field data collected at ranganathapuram india after the 2004 india ocean tsunami tsuflind reproduces the field measurement grain size distribution with less than 5 error tsunami speed in this test case is about 4 7 ms 1 at 150 m inland and decreases to 3 3 ms 1 350 m inland from the shoreline the estimated wave amplitude of the largest wave for this test case is about 5 to 7 m

paleomagnetic data from the philippine sea plate and their tectonic significance new paleomagnetic data from guam and saipan have been obtained and are used in conjunction with previously published results from around the philippine sea plate to interpret the origin and history of the philippine sea plate a lower miocene direction i 3 5 d 54 5 a95 17 4 k 20 3 and a middle oligocene direction i 5 3 d 68 4 a95 12 8 k 52 4 have been obtained from the palau islands only one reliable result a middle oligocene direction i 15 1 d 66 1 a95 11 1 k 20 1 has been obtained from guam many characteristic remanent magnetic directions have been obtained from the lower eocene facpi formation however the results are too scattered k 4 5 to be considered reliable two reliable directions have been obtained from saipan a middle miocene result i 30 7 d 28 1 a95 8 4 k 64 6 and a lower eocene result i 12 0 d 43 0 a95 12 5 k 16 0 two lower eocene results from chichijima i 9 5 d 105 3 a95 12 6 k 6 1 and anijima i 0 8 d 91 7 a95 16 9 k 4 9 are the reliable results from the bonin islands there are two end member interpretations for these data 1 small scale local rotation of blocks along the plate margin or 2 rotation of the philippine sea plate as a whole to distinguish between these interpretations we reconstructed the east philippine sea province back to its prerifting configuration the rotation caused by deformation can be estimated by comparing the paleomagnetism of guam saipan and the bonin islands to palau prior to reconstruction the older declination values are scattered closing the marginal basins reduces the declination scatter and all of the islands show increasing clockwise deflection with time this suggests that the philippine sea plate has rotated up to 80 clockwise and moved northward around 20 since the eocene and deformational rotation is a second order feature the finite pole of rotation of the philippine sea plate relative to eurasia must be somewhere near the southeast comer of the plate because this is the only location that is compatible with both the change in orientation and paleolatitude for the plate these data cannot distinguish between a backarc origin or trapped crust origin for the west philippine sea province the existing models require major modification to be consistent with the paleomagnetic data

geochemistry of a tertiary sedimentary phosphate deposit baja california sur mexico abstract the san gregorio formation in baja california sur a phosphate enriched sedimentary unit of late oligocene to early miocene age has been analyzed in two areas la purisima and san hilario for its chemical composition major oxides cu cd cr co v and rare earth elements ree and isotopic composition 18 o and 13 c a detrital and a marine component were determined from major oxides the detrital component consists of an unaltered volcanic ash fraction and a terrigenous clay silt fraction the marine component which accumulated initially as biogenic and hydrogenous material is now present as opal a opal ct caco 3 organic matter and an authigenic phosphate fraction mostly pelletal and composed of the carbonate fluorapatite mineral francolite the minor elements have been partitioned into these components by assuming a constant composition for the two detrital fractions the composition of the marine component of minor elements can then be interpreted by assuming that the stoichiometry of the original accumulating organic matter was equal to that of modern plankton the cu and cd contents in the marine component of all rocks require that the seawater derived fractions of these two metals were supplied to the seafloor solely by organic matter enrichments of cr and v at both sites required an additional marine input on the basis of their geochemistry in the modern ocean cr and v could have precipitated or been adsorbed onto settling particles from an o 2 minimum zone in which the o 2 content was low enough to promote denitrification rather than oxygen respiration an enrichment of the ree now within the apatite fraction resulted from their adsorption onto particulates also in the o 2 minimum zone and to the dissolution and alteration of biogenic phases predominantly silica within the sediment co and fe 2 o 3 show no enrichment above a detrital contribution the 18 o values of apatites from the la purisima site are heavier than those of apatites from the san hilario site whereas the 13 c values show the opposite trend one possible interpretation of these variations is that 18 o reflects seawater values and 13 c sediment pore water values this interpretation suggests that upwelling rates and primary productivity within the water column were greater at la purisima an interpretation that is corroborated by a greater abundance of apatite measured in outcrop at la purisima the ce anomalies of the phosphate enriched samples also differ between the two sites indicating that they also recorded water masses similar to the 18 o values

oxygen and carbon isotopic composition of marine carbonate concretions an overview abstract compilation of stable isotope data 18o and 13c for marine calcite siderite and dolomite concretions from a large number of studies reveals consistent and unanticipated isotopic trends although many of the dolomites have 18o values that are compatible with precipitation from unaltered seawater at low temperatures most of the calcites and siderites are far too depleted in 12o for such an origin this is surprising as most of the concretions appear to have formed very early in the diagenetic history of the sediments often within several meters of the sediment water interface the most likely cause of the anomalously depleted oxygen values is a decrease in pore water 18o values during early diagenesis a result perhaps of a significant influx of meteoric water into the marine shelf environment or water mineral interaction e g precipitation of 18o enriched minerals the carbon isotopic field for dolomite is quite different from that for calcite and siderite highly positive 13c values are fairly common only for dolomites this indicates that while dolomite concretions often form in sediments where methanogenesis is rapid precipitation of calcite and siderite concretions is restricted to sediments where either n methanogenesis takes place or where methanogenesis is not extensive enough to generate highly positive pore fluid 13c values this variation in carbon isotopic composition among the different minerals suggests that the rates of organic carbon oxidation and sedimentation may control the mineralogy of carbonate concretions high organic carbon oxidation rates favor dolomite and lower oxidation rates favor calcite or siderite

widespread effects of middle mississippian deformation in the great basin of western north america stratigraphic analyses in central and eastern nevada reveal the importance of a deformation event in middle mississippian time that caused widespread deformation uplift and erosion it occurred between middle osagean and late meramecian time and resulted in deposition of both synorogenic and postorogenic sediments the deformation resulted in east west shortening expressed as east vergent folding and east directed thrusting it involved sedimentary rocks of the antler foredeep as well as strata associated with the roberts mountains allochthon a latest meramecian to early chesterian unconformity with correlative conformable lithofacies changes postdates this deformation and occurs throughout nevada a tectonic highland created in the middle mississippian and lasting into the pennsylvanian and centered in the area west and southwest of carlin nevada shed sediments eastward across the antler foreland burying the unconformity post oro genic strata are late meramecian to early chesterian at the base and are widespread throughout the great basin the tectonism therefore occurred 20 to 30 m y after inception of the late devonian antler orogeny significantly extending the time span of this orogeny or representing a generally unrecognized orogenic event in the paleozoic evolution of western north america we propose a revised stratigraphic nomenclature for mississippian strata in nevada based on detailed age control and the recognition of unconformities this approach resolves the ambiguity of some stratigraphic names and emphasizes genetic relationships within the upper paleozoic section we take advantage of better stratigraphic understanding to propose two new stratigraphic units for southern and eastern nevada the middle mississippian gap wash and late mississippian captain jack formations

paleoredox variations in ancient oceans recorded by rare earth elements in fossil apatite abstract rare earth element concentrations in biogenic apatite of conodonts fish debris and inarticulate brachiopods were determined in over 200 samples from cambrian to modern sediments tests for experimental bias caused by the chemicals used to separate fossils from the rock matrix and for interlaboratory interexperiment or interspecies related variations clearly show that no resolvable fractionation of ree occurs incorporation of ree in biogenic apatite of recent fish debris occurs near the sediment water interface soon after deposition and reflects characteristics of seawater the original ree signature apparently survives subsequent burial and diagenesis cerium variations in fish debris from modern environments are controlled by redox potential ce is fractionated by co precipitation with metallic oxides under oxidizing conditions this fractionation produces a negative ce anomaly ce anom in seawater that is reflected in the sedimented fish debris conversely no fractionation of ce occurs under reducing conditions resulting in ce concentrations that are normal to enriched in anoxic seawater and fish debris deposited under local or basinwide anoxic conditions extrapolation of observations of the redox control of ce anom in modern oceans to fossil apatite indicates that anoxic conditions were prevalent in lower paleozoic and lower triassic oceans whereas in the upper paleozoic the world ocean was generally oxidizing

the origin of low k rhyolites from the mariana frontal arc low k rhyolites and overlying 2 pyroxene andesites both of late eocene age comprise the oldest volcanic units exposed on saipan the mineralogy and geologic setting of these rocks indicate they were erupted in a volcanic arc setting the presence of andesite and lack of basalts of similar age suggest that the rhyolites are not part of a typical bimodal basalt rhyolite suite

degassing at anatahan volcano during the may 2003 eruption implications from petrology ash leachates and so2 emissions abstract on 10 may 2003 anatahan volcano located at 16 21 n 145 40 e on the mariana arc entered its first historical eruptive episode sending ash to 12 km into the atmosphere abundant accretionary lapilli quenched pumice textures and hydrothermal minerals in the earliest eruptive deposits indicate hydromagmatic interaction and active mining of the pre eruptive hydrothermal system whole rock compositions of the products erupted within the first week are chemically homogenous with sio 2 61 mgo 2 1 k 2 o 1 4 na 2 o 4 1 and fe 2 o 3 9 1 the products are classified as medium k andesites with tholeiitic affinity slightly more silicic matrix glass compositions up to 63 sio 2 in microlite rich matrices overlap with whole rock suggesting limited crystal fractionation with microlite crystallization responsible for the more evolved residual melt decreasing corrected loi values 2 3 1 4 wt upsection are consistent with waning hydrothermal mineral contributions as the eruption progressed oxygen fugacity calculations based on the ferric to ferrous iron ratio of bulk samples indicate an oxidized magma with nno 1 two pyroxene equilibrium thermometry suggests magmatic temperatures of 1050 1100 c matrix glass volatile contents show a degassed residual melt with 2 o 1000 2000 ppm cl 480 780 ppm f 50 150 ppm s and 2 a magmatic so 2 flux of 3 4 5 kt day was measured by cospec on 21 may ash leachate data indicate a decreasing s cl ratio 3 3 0 7 in the eruptive plume between 10 and 21 may with a relatively constant cl concentration assuming a constant cl flux an so 2 flux of 14 22 kt day is calculated for 10 may the average s concentration from ash leachates 1230 mg kg suggests that at least 25 of the so 2 60 kt erupted from anatahan between 10 and 21 may was removed from the plume by the precipitation of sulphate salts in the eruption column adsorbtion onto ash particles and subsequent deposition molar ratios in ash leachates elucidate caso 4 and nacl as the most likely soluble salts formed in the plume total element abundances molar s ca 1 and ca mg na and k ratios in the leachates suggest a hydrothermal fluid contribution to elements present as water soluble salts adsorbed onto ash sulfur budget calculations based on estimates of pre eruptive magmatic and residual melt s contents mass of erupted magma and total so 2 output fluxes require an additional source of s other than the erupted magma multiple lines of evidence including high so 2 emissions early in the eruption the presence of accretionary lapilli and hydrothermal minerals in the early eruptive deposits quenched pumice textures and cation and anion ratios and abundances in ash leachates suggest that a s rich free volatile phase exsolved from a large magma body magmatic volatiles were stored as components of the hydrothermal system pressurized gases hydrothermal fluids and or hydrothermal minerals to be remobilized early in the eruption to contribute to the total so 2 output

emplacement growth and gravitational deformation of serpentinite seamounts on the mariana forearc summary serpentinite seamounts representing some of the first material outputs of the recycling process that takes place in subduction zones are found on the outer mariana forearc multichannel seismic mcs and bathymetric data collected in 2002 image the large scale structures of five seamounts as well as the pre seamount basement geometry and sediment stratigraphy we present data from three edifices that provide insights into seamount growth and internal deformation processes and allow us to support the interpretation that serpentinite mud volcanoes are formed by the episodic eruption of mud flows from a central region the presence of thrust faulting at the base of turquoise and big blue seamounts along with the low surface slopes 5 18 of all the seamounts studied lead us to infer that these edifices spread laterally and are subject to gravitational deformation as they grow numerical simulations using the discrete element method dem were used to model their growth and the origins of features that we see in mcs sections such as basal thrusts inward dipping reflections and mid flank benches the dem simulations successfully reproduced many of the observed features simulations employing very low basal and internal friction coefficients 0 1 and 0 4 respectively provide the best match to the overall morphology and structures of the serpentinite seamounts however the simulations do not capture all of the processes involved in seamount growth such as withdrawal of material from a central conduit leading to summit deflation compaction dewatering and degassing of mud flows mass wasting in the form of sector collapse and growth upon a dipping substrate a strong reflection beneath the summit of big blue the largest serpentinite seamount on the mariana forearc represents the floor of a summit depression that has been partially in filled by younger muds supporting the idea that serpentinite seamounts grow by episodic mud volcanism boundaries of mud flow units are visible in bathymetric data and as normal polarity subhorizontal reflections on seismic profiles big blue seamount displays complex nesting relationships as it merges with other seamounts to form a large composite edifice flank flows of serpentinite muds on big blue and celestial seamounts downlap pre existing forearc substrate the interface between serpentinite seamounts and the underlying forearc sediments is represented by a reverse polarity reflection beneath big blue and celestial seamounts suggesting that the substrate is undercompacted overpressured and may be a zone of fluid migration dem simulations suggest that this boundary represents a distinct decollement along which the seamounts slide laterally in contrast turquoise seamount grows laterally not by stable sliding along the top of forearc sediments but by incorporating them into large basal thrusts

petrogenesis of a tholeiite boninite sequence from ayios mamas troodos ophiolite evidence for splitting of a volcanic arc lavas and intrusives from ayios mamas at the western end of the limassol forest complex south of the arakapas fault belt troodos comprise three groups a a basal sequence of pillow lavas intruded by dikes and sills with the phenocryst assemblage plag cpx mt ol b a middle sequence of pillows and massive flows cpx plag ol and c an upper sequence mostly of pillows and breccia ol opx cpx sp the lower group is tholeiitic and the upper two groups boninitic group c whole rock and glass mg numbers exceed 72 and range up to 78 with corresponding olivine compositions of fo92 they are strongly depleted in high field strength elements and on the basis of cr y variation and major element mass balances are interpreted to reflect partial melting of a basalt depleted cpx harzburgite source group a tholeiites in contrast are interpreted to derive from a more fertile lherzolite source magmaphile and compatible trace element variation is best modelled after an open magma system involving periodic replenishment tapping and fractionation although neither tholeiites nor boninites represent steady state compositions the boninites belong to a cao rich sio2 poor variant resembling those from guam but contrasting with those from bonin and cape vogel morb normalized enrichment of low field strength elements k sr ba and rb in all groups indicates subduction derived source contamination it is proposed that the ayios mamas section reflects splitting of a volcanic arc prior to back arc spreading in an environment analogous to that of the present day andaman sea

petrology and geochemistry of boninite series volcanic rocks from the mariana trench boninite series volcanic rocks have been recovered from three dredge hauls on the inner slope of the mariana trench these hauls included olivine boninites boninites boninitic andesites and boninitic dacites as well as island arc tholeiitic basalts and andesites the boninite series volcanics range from 52 to 68 sio2 and are characterized by very low abundances of high field strength cations and heavy rare earth elements boninites and olivine boninites have phenocrysts of olivine and orthopyroxene the andesites phenocrysts of orthopyroxene and clinopyroxene and the dacites orthopyroxene clinopyroxene and plagioclase most of the major and trace element variation in the series from boninite to boninitic dacite can be modelled by fractionation of olivine orthopyroxene clinopyroxene and plagioclase in the proportions 2 5 4 1 2 leaving 47 residual liquid the fractionation must be in part open system reverse zoned phenocrysts resorbed olivine and plagioclase xenocrysts and bulk rock compositions which cannot be fit by simple closed system crystallization indicate some magma mixing and phenocryst accumulation two boninitic magma stems can be identified with similar high field strength element abundances but different amounts of ca na al and light rare earth elements there is also evidence for a magma stem transitional in chemistry from the boninites to arc tholeiites the compositions of these boninites are consistent with hypotheses for boninite formation by partial melting of a depleted mantle mixed with an incompatible element enriched fluid the mariana forearc boninite series lacks a strong iron enrichment but produces andesites with lower ti al and y zr and higher mg ni and cr than typical calcalkaline arc andesites and dacites boninites in the mariana system were erupted only in the earliest phases of subduction zone activity

sedimentary processes in modern and ancient oceanic arc settings evidence from the jurassic talkeetna formation of alaska and the mariana and tonga arcs western pacific abstract sediment deposited around oceanic volcanic ares potentially provides the most complete record of the tectonic and geochemical evolution of active margins the use of such tectonic and geochemical records requires an accurate understanding of sedimentary dynamics in an arc setting processes of deposition and reworking that affect the degree to which sediments represent the contemporaneous volcanism at the time of their deposition we review evidence from the modern mariana and tonga arcs and the ancient arc crustal section in the lower jurassic talkeetna formation of south central alaska and introduce new data from the mariana arc to produce a conceptual model of volcaniclastic sedimentation processes in oceanic arc settings all three arcs are interpreted to have formed in tectonically erosive margin settings resulting in long term extension and subsidence debris aprons composed of turbidites and debris flow deposits occur in the immediate vicinity of arc volcanoes forming relatively continuous mass wasted volcaniclastic records in abundant accommodation space there is little erosion or reworking of old volcanic materials near the arc volcanic front tectonically generated topography in the forearc effectively blocks sediment flow from the volcanic front to the trench although some canyons deliver sediment to the trench slope most volcaniclastic sedimentation is limited to the area immediately around volcanic centers arc sedimentary sections in erosive plate margins can provide comprehensive records of volcanism and tectonism spanning 10 my the chemical evolution of a limited section of an oceanic arc may be best reconstructed from sediments of the debris aprons for intervals up to 20 my but no longer because subduction erosion causes migration of the forearc basin crust and its sedimentary cover toward the trench where there is little volcaniclastic sedimentation and where older sediments are dissected and reworked along the trench slope

geodynamic setting of izu bonin mariana boninites the izu bonin mariana ibm forearc is characterized by the occurrence of boninite like lavas the study of the cenozoic setting of the genesis of these boninitic lavas in light of modern geodynamic contexts in the tonga and fiji regions lead us to define three tectonic settings that favour the formation of boninites in back arc basins in addition to previous settings that involve the presence of a mantle plume 1 propagation at low angle between a spreading centre and the associated volcanic arc 2 intersection at a high angle of an active spreading centre and a transform fault at the termination of an active volcanic arc and 3 intersection at a right angle between an active spreading centre and a newly created subduction zone a geodynamic model of the philippine sea plate shows that boninites in the bonin islands are related to the second mechanism mentioned above whereas mariana forearc boninites are relevant to the third mechanism in the early eocene the transform plate boundary bounding the eastern margin of the philippine sea plate at the location of the present day mariana arc evolved into a subduction zone that trends perpendicular to the active spreading centre of the west philippine basin some where around 43 47 ma the presence of a mantle plume in the vicinity of the subduction zone bounding the northern ibm arc explains boninites that erupted in its northern part but only in early eocene time

the phosphorus cycle phosphogenesis and marine phosphate rich deposits abstract phosphorus in the form of phosphate is an essential nutrient and energy carrier on many different levels of life and a key element in mediating between living and lifeless parts of the biosphere one of the most important aspects of the phosphorus cycle is its vital role in governing productivity thereby interacting with the exogenic part of the carbon cycle which in turn is important in regulating earth s climate phosphorus is a prime element to be traced in earth s history because it allows for the reconstruction of long term feedback mechanisms between climate environment and ecology and of global change as such marine sedimentary phosphate deposits are particularly suited to study aspects of the phosphorus cycle because in the case of ubiquity their origin may result from a general acceleration of the global phosphorus cycle sources of sedimentary phosphate are microbial breakdown of buried organic matter and redox driven phosphate desorption from iron and manganese oxyhydroxides dissolved sea water phosphate represents an additional source which may become important in the formation of phosphatic hardgrounds the main locus of phosphogenesis is near the sediment water interface but phosphogenesis also occurs at greater sediment depths current induced winnowing and transport processes along the sea floor concentrate phosphate precipitates into deposits which exhibit internal stratification patterns typical for the prevailing hydraulic energy regime in a sequence stratigraphic context phosphate deposits preferentially occur along marine or maximum flooding surfaces consequent sedimentary reworking may result in the transfer of phosphates to highstand or lowstand deposits bio chemical weathering on continents represents the most significant source of bioavailable phosphorus this implies that long term changes in marine phosphorus levels and with these changes in marine ecology productivity rates and ratios of exported carbonate carbon and organic carbon are a response to changes in continental weathering rates a compilation of marine sedimentary phosphorus burial rates for the last 160 myr suggests that natural variations have occurred that span one order of magnitude for the late jurassic cretaceous and most of the paleogene the phosphorus cycle appears to have been accelerated in times of climate warming which was most likely due to the spreading of zones of humid climate and more intense continental weathering in the neogene the phosphorus cycle appears to have responded to changes in glacially induced weathering this suggests that uniform interpretations with respect to the emplacement of major phosphorite deposits should be treated with caution integrated analyses of the sedimentary and biogeochemical context of phosphorite occurrences may help to identify paleoenvironmental conditions as well as to improve our understanding of periods of enhanced phosphate accumulation periods which were usually characterized by steep gradients in the development of climate and environment with regard to the complexity of feedback mechanisms between the phosphorus cycle and the biosphere the present day input rates of phosphate into the world s oceans should be of great concern they are more than doubled by anthropogenic means and affect ecological systems on a rapidly increasing scale

boninitic and low ti subduction related lavas from intraoceanic arc backarc systems and low ti ophiolites a reappraisal of their petrogenesis and original tectonic setting a comprehensive review of boninitic and other low ti subduction related lavas from modern intraoceanic arc backarc systems and of low ti ophiolites shows a complete compositional range from boninite to island arc basalts transitional low ti types distinguishable from the main magma types are well represented and defined here as 1 transitional boninites mainly from low ti ophiolites and 2 low ti island arc basalts major compatible and incompatible elements are clearly decoupled in all these low ti subduction related magmas in general their petrogenesis reflects a high degree of hydrous but not necessarily water saturated melting at pressures lower than 15 20 kbar of severely depleted mantle sources enriched in lile and h2o which are residual after the earlier extraction of basalt s as the residues after the extraction of low ti subduction related magmas very likely have a dunitic clinopyroxenefree harzburgitic composition it is proposed that the cao al2o3 ratios of the primary melts reflect those of their mantle sources particularly for the boninites and transitional boninites in this model the source of boninites sensu stricto would show a variable degree of depletion from clinopyroxene bearing to clinopyroxene free harzburgite whereas those of transitional boninites low ti island arc basalts would have clinopyroxene bearing harzburgite clinopyroxene poor lherzolite compositions the metasomatizing components involved in the genesis of low ti island arc basalts have a very close affinity with those involved with the sources of normal island arc tholeiites the common eruptive setting indicates that these low ti lavas represent extreme magma types of an otherwise normal island arc tectonomagmatic regime the petrogenesis of tertiary boninites and transitional boninites and the comparable magma types of the low ti ophiolites require at least these components 1 a variably depleted mantle source probably a subarc mantle residual after island arc basalt extraction 2 a hydrous fluid probably derived from dehydration of the subducted altered oceanic crust which carries most of sr ba k and rb in the subarc source and 3 a lree zr and nb enriched component with a low 143nd 144nd ratio probably derived from an ocean island type source it is proposed that this third component also carried significant amounts of rb and k which totally obliterated the ba spike typical of island arc tholeiites iats and produced the distinctive rb spike typical of the boninite transitional boninite magmas this component is hypothetically identified in liquids produced by partial melting of small scale large magnitude k richerite phlogopite bearing heterogeneities within the hot rising asthenospheric diapirs required in the model of crawford et al 1981 for the genesis boninite transitional boninite magmas geochemical thermal and geological constraints are best explained in a model which restricts the genesis of boninite transitional boninite magmas to the early stages of intraoceanic interarc spreading and to important rifting phases of the arc and or early interarc basin structures connected with hot rising asthenospheric mantle diapirs

titanium mobility in metamorphic systems a review abstract titanium is a dispersed lithophile element commonly regarded as chemically immobile at sub magmatic temperatures however a growing number of field studies demonstrate that ti is somewhat mobile in certain environments this review examines the current state of knowledge concerning the mobility of ti during alteration and metamorphism of crustal rocks also considered are relevant ti oxide solubility data from experiments and determinations of ti concentrations in natural waters where ti mobility is well documented its length scale is 10 m however ti may move greater distances in deep crustal shear zones the solubility of ti oxides in geologic fluids is strongly temperature dependent under low to medium grade metamorphic conditions ti mobility is limited by the low solubility of ti oxides in the associated fluids under eclogite conditions ti mobility is probably limited not by ti oxide solubility which reaches 1 ti but by restricted fluid flow in low permeability rocks

brachiopods as indicators of original isotopic compositions in some paleozoic limestones the original stable isotopic composition of paleozoic marine carbonate rocks is important to establish a baseline for evaluating diagenetic alteration and providing a reliable data base for modeling exogenic geochemical cycles of the elements we have compared the 18 o and 13 c values in the nonluminescent portions of brachiopods with similar published data of whole brachiopods other whole fossils and estimates of original isotopic composition of marine cements the oxygen and carbon isotopic records for each of these paleozoic components show generally good internal consistency significant discrepancies however were noted between results on the different components isotopic and trace element data presented in this study suggest that permo carboniferous whole brachiopods are little altered chemically nonetheless a more reliable but not fool proof approach for obtaining pristine isotopic values is microsampling of least altered regions based on cathodoluminescence in conjunction with staining techniques and elemental analyses the carbon and oxygen isotopic compositions of all components decrease in 18 o and 13 c in geologically older material neither temporal trend is monotonic however oxygen isotopic results suggest an important positive shift in 18 o of about 2 during the early carboniferous whereas the carbon isotope trend shows a major positive shift 2 3 during the mid carboniferous

geochemistry of brachiopods oxygen and carbon isotopic records of paleozoic oceans abstract combined trace element and isotope studies of 319 brachiopods covering the ordovician to permian time span show that 13c and 18o in well preserved specimens varied during the paleozoic the overall 13c secular trend is in accord with the previously published observations but its details are obscured by vital isotopic fractionation effects at generic level nonetheless the results suggest that the negative correlation between marine 13ccarbonate and 34ssulphate deteriorates at time scales of 106 years due to the long residence time and thus slow response of so42 in the ocean for oxygen isotopes all devonian and older specimens have 18o of 4 while the well preserved permian samples have near present day 18o of about 1 pdb this isotopic dichotomy is probably not due to post depositional phenomena salinity or biogenic fractionation effects this leaves open the perennial arguments for a change in 18 o 16 o of sea water versus warmer ancient oceans the present data are difficult to explain solely by the temperature alternative the coincidence of the proposed shift in 18o with the large late paleozoic changes in marine 87 sr 86 sr 13 c 12 c 34 s 32 s and sea level stands argues for a tectonic cause and for a change in 18 o 16 o of sea water although such explanation is difficult to reconcile with global balance considerations and with isotopic patterns observed in alteration products of ancient basalts and ophiolites whatever the precise cause or combination of causes the implications for tectonism and or paleoclimatology are of first order significance

ancient depleted mantle as a source of boninites in the izu bonin mariana arc evidence from os isotopes in cr spinel and magnetite abstract boninite is a volcanic rock derived from shallow melting of highly depleted hydrous mantle fluxed with water from subducted slabs the eruption of boninite early in the history of the izu bonin mariana ibm arc 48 45 ma suggests generation by melting of upper mantle material that was relatively unmodified by subducted components thus the boninite composition should largely reflect that of the sub arc mantle for better understanding of the mantle sources of nascent arc settings and the contributions of different components to arc melts we analyzed os isotope ratios 187os 188os of bulk rocks and mineral separates euhedral cr spinel from boninites and euhedral cr spinel magnetite mixtures from tholeiites younger than 45 ma that erupted after boninites from the bonin islands and guam the age corrected initial os isotope ratios of the whole rock samples 0 1179 0 2050 were more radiogenic and variable than those of the mineral separates possibly because of contamination with crustal materials during magma ascent or alteration after emplacement the age corrected os isotope ratios of euhedral cr spinel in boninite from the bonin islands 0 1187 0 1254 and from guam 0 1220 0 1269 are unradiogenic relative to primitive mantle and those of the cr spinel magnetite mixtures from the tholeiites from the bonin islands are similar to or slightly more radiogenic 0 1224 0 1382 the most depleted os isotope ratio of the cr spinel from boninite yielded a model re depletion trd age of 1 4 ga suggesting that the mantle source of the boninite experienced melt extraction prior to 1 4 ga the source of the boninites is interpreted to be mostly highly depleted mantle with a small contribution of slab flux arising from altered oceanic crust that has radiogenic os components with or without contributions from components with relatively unradiogenic os such as volcaniclastics of oceanic island basalt affiliation or very young mid ocean ridge basalt

the δ18o record of phanerozoic abiotic marine calcite cements monomineralic abiotic marine cements formed in low latitude phanerozoic reefs provide the direction and amplitude of secular variation of delta sup 13 c and delta sup 18 o in marine calcite and defines two end member compositions 580 to 360 my minus 7 to minus 5 per thousand delta sup 18 o sub pdb and 360 to present minus 3 to 0 per thousand delta sup 18 o sub pdb sampling of the devono carboniferous transition 375 320 my at several global sites reveals a rapid change in carbonate isotopic compositions bracketed within fammenian to early visean aged strata a 7 to 15 my time interval this shift corresponds to a 2 offset in mean delta sup 13 c and 3 4 offset in delta sup 18 o the abruptness of such change and its overall correlation with variations in sup 87 sr sup 86 sr delta sup 34 s delta sup 13 c and li al ratios in marine sediments suggests a primary offset in marine water composition

petrology geochemistry and tectonic implications of volcanics dredged from the intersection of the yap and mariana trenches abstract rocks dredged from the forearc very close to the intersection of the yap and mariana trenches include a suite of highly depleted arc tholeiites and several samples of transitional to slightly alkaline basalt the tholeiites range from magnesian quartz tholeiites with 0 46 0 6 tio2 to andesites with up to 62 sio2 and 8 2 feo all show pronounced lree depletion and have very low contents of ba and sr they are postulated to have been produced by partial melting of upper mantle peridotite residual after morb extraction following influx of hydrous fluids from the subducted slab while these fluids were responsible for small enrichments in ba k rb and sr in melts generated lree were not involved in the metasomatism and the strong lree depletion probably reflects the unmodified depleted source peridotite the second lava suite includes slightly ne normative ti augite bearing basalts with convex upward ree patterns showing slight lree depletion la sm n 0 76 the chemical features of these basalts support affinities with basalts erupted during the earliest stages of backarc basin opening a k ar age on one sample 7 8 1 3m y is in good agreement with the initial opening of the mariana trough the tectonic significance of the dredged arc tholeiite suite is less obvious a k ar age of10 8 0 4 my on one andesite and the occurrence of similar lavas in dredges from at least 300 km along the length of the yap arc suggest that subduction was occurring beneath the yap arc in the late miocene after overthrusting of the yap greenschist allochthon and while calc alkaline arc magmatism was occurring further north on the west mariana ridge we suggest that the depleted arc tholeiites in dredge 1438 were generated by abnormally shallow melting of upper mantle beneath the yap forearc following subduction beneath this area of young hot sorol trough crust these arc tholeiites represent a magma type transitional between more typical arc tholeiites e g tongan and high mg andesites and boninites

comment and reply on delle phosphatic member an anomalous phosphatic interval in the mississippian osagean meramecian shelf sequence of central utah

climatic and oceanographic isotopic signals from the carbonate rock record and their preservation stable isotopic data from marine limestones and their constituent fossils and marine cements can provide quantitative evidence for changes in global climate and ocean circulation oxygen isotopic data can indicate changes in temperature and ocean composition whereas stratigraphic variation in carbon isotope ratios may reflect changes in the carbon cycle that can be linked to changes in oceanic productivity and atmospheric greenhouse gases terrestrial carbonates meteoric cements calcretes and speleothems similarly offer significant potential for understanding the evolution of terrestrial climates by providing evidence for the composition of rainwater and the nature of vegetative cover primary environmental isotopic signals may be obscured by the effects of post depositional diagenetic alteration cementation and replacement reactions can take place in a wide range of diagenetic environments the diagenetic history of an individual limestone is determined by a combination of its mineralogical diagenetic potential and depositional setting together with subsequent changes in relative sea level and burial history carbon isotopic values are less prone to alteration during diagenesis than oxygen values but shifts can be significant where organogenic carbon is incorporated linear covariation of carbon and oxygen values is not a reliable indicator of diagenetic alteration water rock interaction and fluid mixing may produce non linear distributions attempts to determine long term changes in climatic and oceanographie conditions through isotope stratigraphy of shallow water limestones must include an assessment of the diagenetic history of the materials analysed petrographic examination using conventional microscopy backed up where appropriate by cathodoluminescence and scanning electron microscopy together with elemental and strontium isotopic analysis can help to identify the effects of diagenetic alteration where material with a range of different degrees of alteration is preserved in the same sediment it may be possible to compare patterns of isotopic and elemental variation and to attempt to unravel the effects of diagenesis in order to determine primary environmental isotopic signals recent research has shown that these techniques can be successfully employed in both phanerozoic and precambrian sediments

petrological and chemical evolution of the northeastern troodos extrusive series cyprus four magma series are distinguished in the northeastern troodos extrusive series a a lower low ti series lo lts of basaltic andesites b a high ti series hts of basaltic andesites to rhyodacites c a low ti series lts and d a depleted low ti series dlts the last two being of basaltic andesite trace element characteristics vary systematically from series a to d and are interpreted in terms of a variable contribution of three major source components scs lile enriched water rich fluids sci derived from dehydration of a subducted lithosphere slab were continuously added to the overlying mantle wedge increasing lile hfse and lile ree ratios and decreasing absolute hfse and ree concentrations from series a to d indicate progressive depletion of the actual mantle source scii a negative ta anomaly in the lavas decreases from series a to d and is interpreted to have resulted from partial melting of the lower crust scii where ta nb ti may be fractionated by ti rich accessory phases the contribution of sciii decreases when the eruptive sites successively move away from the central axial zone and the temperature of the lower crust decreases preventing partial melting of the lower crust chemical compositions of fresh glass separates and phenocrysts indicate a change of major petrogenetic processes from series a to d lo lts and hts lavas are intrepreted to be directly related by open system fractional crystallization in crustal magma chambers removal of observed phenocryst phases clinopyroxene orthopyroxene plagioclase and magnetite and repeated subsequent mixing of dacitic to rhyodacitic magmas with batches of replenishing basaltic andesites are the major processes possibly induced by vesicle formation in the mafic layer after a period of some crystallization lts and dlts magmas were directly fed to the surface without stagnating at crustal levels with feeder dykes positioned marginal to the central rift zone and thus by passing the central magma chambers these magmas apparently experienced only limited fractional crystallization of 10 15 wt olivine clinopyroxene chromite probably at the mantle crust boundary

isotope signatures associated with early meteoric diagenesis the environments in which carbonate diagenesis proceeds have been documented in previous studies of holocene and late pleistocene sediments and limestones on barbados west indies variations in the carbon and oxygen isotopic composition of limestones produced during early freshwater diagenesis have been observed in this study to occur in specific patterns six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework 1 the subaerial exposure surface is characterized by strongly 12c enriched limestones 13c compositions of underlying limestones grow progressively heavier with increasing depth 2 the subaerial exposure surface may also be marked by slight 18o enrichment 3 an abrupt shift in 18o values may differentiate sediments above the exposure surface from those below 4 sediments altered in the marine meteoric mixing zone may be characterized by positive covariance between their 18o and 13c compositions 5 the vadose phreatic boundary may be marked by a sharp increase in 13c values in the seaward portions of a fresh groundwater system and 6 samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of 18o and a wide range of 13c compositions analysis of samples from five palaeozoic and mesozoic formations which contained petrographic evidence of early freshwater diagenesis showed that isotope patterns similar to those observed in barbados limestones have been preserved in rocks as old as mississippian these isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks they might be used to identify limestones diagenetically altered in meteoric environments to identify mixing zone cements and dolomites and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations

evolution of the mariana convergent plate margin system the mariana convergent plate margin system of the western pacific provides opportunities for studying the tectonic and geochemical processes of intraoceanic plate subduction without the added complexities of continental geology the system s relative geologic simplicity and the well exposed sections of lithosphere in each of its tectonic provinces permit in situ examination of processes critical to understanding subduction tectonics its general history provides analogs to ancient convergent margin terranes exposed on land and helps to explain the chemical mass balance in convergent plate margins the mariana convergent margin s long history of sequential formation of volcanic arcs and extensional back arc basins has created a series of volcanic arcs at the eastern edge of the philippine sea plate the trenchward edge of the overriding plate has a relatively sparse sediment cover rocks outcropping on the trench s inner slope are typical of the early formed suprasubduction zone s lithosphere and have been subjected to various processes related to its tectonic history pervasive forearc faulting has exposed crust and upper mantle lithosphere many large serpentinized peridotite seamounts are within 100 km of the trench axis from these we can learn the history of regional metamorphism and observe and sample active venting of slab fluids ocean drilling recovered suprasubduction zone lava sequences erupted since the eocene that suggest that the forearc region remains volcanologically dynamic seismic studies and seafloor mapping show evidence of deformation throughout forearc evolution large portions of uplifted southern forearc are exposed at the larger islands active volcanoes at the base of the eastern boundary fault of the mariana trough vary in size and composition along strike and record regional differences in source composition their locations along strike of the arc are controlled in part by cross arc structures that also facilitate formation of submarine volcano chains extending from the base of the fault westward into the back arc basin the western boundary is the west mariana ridge the western portion of the volcanic arc active prior to formation of the mariana trough the trough evolved in a two stage extension process of rifting and subsequent seafloor spreading the back arc basin varies along strike from rifted arc lithosphere with scattered volcanoes but no real spreading center in the north to a complex mid ocean type spreading center south of 20 n the change from initial rifting to true seafloor spreading is also evident across the mariana trough from rifted topography near the west mariana ridge to spreading ridges in the central to eastern basin south of 20 n this morphologic change indicates an early stage of extension with basin and range type topography predominant and volcanism restricted to fissure eruptions along fault block boundaries the spreading ridges and abyssal hill morphology evolved later as new lithosphere was generated at elongate volcanic ridges located in the center of rift valleys the center of extension intersects the active volcanic front differently at either end of the mariana trough in the north extension is by rifting of arc lithosphere where it intersects the arc in the south a major strike slip fault extends from the trench axis across the forearc through the volcanic arc and into the back arc basin arc magmas apparently leak along this fault zone into the forearc and the back arc spreading center the complexity of interrelated tectonism and magmatism in this convergent margin is daunting but studies of arc systems such as this provide the best hope of interpreting many of the exposed terranes accreted to continents comparison of subaerial terranes with recent studies of intraoceanic convergent margins will add to our understanding of plate interactions and of the evolution of the volcanic arcs and extensional back arc basins generated within such environments

simultaneous boninitic and arc tholeiitic volcanisms in the izu forearc region during early arc volcanism based on odp leg 125 site 786 early arc volcanism during eocene to oligocene in the izu forearc region was investigated during odp legs 125 and 126 in 1989 and effusive and intrusive volcanics were recovered from leg 125 site 786 these rocks were all classified into boninites and associated rocks by leg 125 shipboard scientific party and they concluded that boninitic volcanism had occurred before 40 ma and arc tholeiitic volcanism began after 40 ma in this study lava flows and breccias that classified into boninite series are divided into two groups tholeiite and boninite based on petrographical and petrological properties both series are also distinguished by bulk rock composition it is considered that the sources of both rock types have similar depleted compositions because of their similar very low bulk hfse concentrations we suggest that boninitic and tholeiitic volcanism occurred closely in time and space and reflected different temperature and water condition

paleomagnetic dating of burial diagenesis in mississippian carbonates utah 1 the objective of this study is to test models for the origin of widespread secondary magnetizations in the mississippian deseret limestone the delle phosphatic member of the deseret limestone is a source rock for hydrocarbons and modeling studies indicate that it entered the oil window in the early cretaceous during the sevier orogeny paleomagnetic and rock magnetic results from the deseret limestone and the stratigraphically equivalent chainman shale in central and western utah indicate that the units contain two ancient magnetizations residing in magnetite burial temperatures are too low for the magnetizations to be thermoviscous in origin and they are interpreted to be chemical remanent magnetizations crms fold tests from western utah indicate the presence of a prefolding triassic to jurassic crm geochemical 87sr 86sr 13c and 18o and petrographic analyses suggest that externally derived fluids did not alter these rocks this crm was acquired at the beginning of the oil window and is interpreted to be the result of burial diagenesis of organic matter a second younger crm in western central utah is apparently postfolding and is probably late cretaceous to early tertiary in age on the basis of the thermal modeling the timing overlaps with the oil window these results are consistent with a connection between organic matter maturation and remagnetization modeling of the smectite to illite transformation in the deseret limestone suggests a mean age prior to acquisition of both crms although the range for illitization overlaps with the triassic to jurassic crm the results of this study support the hypothesis that pervasive crms can be related to burial diagenetic processes in addition paleomagnetism can be used to determine the timing of such processes which can benefit hydrocarbon exploration efforts

rare earth major and trace elements in chert from the franciscan complex and monterey group california assessing ree sources to fine grained marine sediments abstract rare earth element ree major and trace element analyses of 77 samples from the mesozoic franciscan complex and miocene monterey group of california usa indicate that magnitudes of the ce anomaly ce ce and total ree abundances sree in these rocks are controlled dominantly by a the amount of included metalliferous material b the amount of direct terrigenous input and c the overall burial rate the relative importance of these individual processes varies across an ocean basin in such a way that ree relative fractionations and sree correspond with its depositional environment our conclusions are supported by an extensive review of published ree data from various river coastal open ocean and ridge influenced waters as well as terrigenous pelagic and metalliferous sediment the chert sequences record deposition in spreading ridge open ocean basin and continental margin environments the relative importance of metalliferous and terrigenous influences and the role of scavenging from seawater may be estimated from a sample s inferred depositional distance km from the spreading ridge the major element chemistry and the stratigraphie context the ce ce values are low 0 29 in the vicinity of spreading ridges and trend to successively higher 1 values with decreasing metalliferous and increasing terrigenous influences other ree indicators ree lu n la n eu eu also respond to metalliferous and terrigenous influences near the spreading ridge chert ree appears determined by burial rate while shale ree responds to metalliferous abundance in open ocean and continental margin environments ree is controlled by sediment exposure time and therefore is related inversely to burial rate ree data are consistent with the major elements fe2o3 mno and al2o3 in reflecting the respective metalliferous and terrigenous sources p2o5 and cao do not vary with ce ce in any environment but cao shows a weak negative correlation with ree trace element trends e g v cr cu ge si rb sr y th u in general can similarly be accounted for in the context of depositional position relative to metalliferous and terrigenous inputs rees in chert appear less affected by post depositional processes than either major or trace elements indicating that rees are better tools for paleoceanographic or tectonic reconstructions that are based on chert chemistry this work complements recent efforts to elucidate the factors determining large scale ree distributions in marine sediments and has direct bearing on previous attempts to use the ce anomaly in older rocks and sediments as a recorder of secular variations and anoxic events in the overlying water column

shoshonitic volcanism in the northern mariana arc 1 mineralogic and major and trace element characteristics petrographic and geochemical characteristics of samples dredged from 23 submarine volcanic edifices in the northern mariana and southern volcano arcs define two distinct rock series the central and northern mariana arc to 23 n and the northern volcano arc north of iwo jima are characterized by plagioclase clinopyroxene orthopyroxene titantomagnetite bearing subalkaline rocks including both low k and medium k series the northern mariana arc and southern volcano arc from 23 n to iwo jima are erupting rocks of a shoshonitic series with phenocrysts of plagioclase clinopyroxene olivine and biotite these rocks are less saturated than those of the subalkaline provinces and are substantially enriched in ba 400 900 ppm sr 600 1000 ppm k2o 1 4 5 and k2o na2o 0 4 1 2 relative to the subalkaline lavas ba y and ba zr increase by a factor of 3 to 4 in the shoshonitic rocks but k rb ba sr and k ba are relatively constant throughout the arc there is no relationship between degree of enrichment and volcano volume or degree of fractionation seamounts and islands within each province have the same range of compositions much of the intra edifice variation in lava composition can be modeled by 10 70 crystallization of plagioclase and clinopyroxene with smaller amounts of olivine orthopyroxene and titanomagnetite or by accumulation of 10 40 phenocrysts dominantly plagioclase the differences between parental lavas in the alkalic and subalkalic provinces require melting of two distinct mantle sources the occurrence of this enriched mantle in the northern marianas may be a consequence of the propagation of the mariana trough spreading center into the volcano arc

crustal accretion and tectonic setting of the troodos ophiolite cyprus abstract the troodos ophiolite is composed of two distinct magmatic suites an early relatively high tio 2 basaltic suite constitutes the upper gabbros the sheeted dike complex and the andesitic to rhyodacitic lower pillow lavas a late low tio 2 basaltic andesite suite constitutes the lower ultramafic to gabbroic cumulates and the upper pillow lavas parental magmas to the early suite originated as partial melts from hydrous relatively undepleted peridotite diapirs rising in a mantle wedge in response to dehydration of a subducting crust the late basaltic andesite suite is highly depleted in incompatible elements and show similarities with second stage or refractory basalts from oceanic spreading centers the troodos basaltic andesites and associated cumulates may thus have formed in response to continued adiabatic upwelling as second stage melts from the hydrous water undersaturated peridotite diapirs from which the earlier spreading related magmas were extracted the basaltic andesites formed in a relatively slow spreading or non spreading tectonic environment and dominantly crystallized in magma chambers underplating the troodos ophiolite the extrusion of the basaltic andesites of the upper pillow lavas occurred simultaneously with still active evolving volcanic centers belonging to the earlier magmatic suite an alternative possibility would be that the basaltic andesites separated from renewed diapirs rising as a consequence of re initiation of subduction and liberation of fluids from the subducting slab into the overlying depleted peridotite the existence of lree and lil enriched lavas among the upper pillow lavas support this possibility the apparent conflict between a spreading related geology and the arc petrology and geochemistry of the troodos ophiolite may be an artifact of lack of detailed studies of the geology and chemistry of oceanic and back arc rifting environments however it is proposed that the available data for the troodos ophiolite are consistent with its formation in a short lived spreading basin formed by splitting of an immature submarine arc

mid ocean ridge and supra subduction affinities in the pindos ophiolites greece implications for magma genesis in a forearc setting abstract the pindos ophiolitic massif is considered an important key area within the albanide hellenide ophiolitic belt and is represented by two tectonically distinct ophiolitic units 1 a lower unit including an intrusive and a volcanic section and 2 an upper ophiolitic unit mainly including mantle harzburgites both units share similar metamorphic soles and tectono sedimentary melanges at their bases the intrusive section of the lower unit is composed by an alternation of troctolites with various ultramafic rock types including dunites lherzolites olivine websterites olivine gabbros anorthositic gabbros gabbros and rare gabbronorites the volcanic and subvolcanic sequence of the lower unit can geochemically be subdivided into three groups of rocks 1 basalts and basaltic andesites of the lower pillow section showing a clear high ti affinity 2 basaltic andesites of the upper pillow section with high ti affinity but showing many geochemical differences with respect to the first group 3 very low ti boninitic basaltic and basaltic andesitic lava flows separating the lower and upper pillow sections and dykes widespread throughout the pindos ophiolites these different magmatic groups originated from fractional crystallization from different primary magmas which were generated in turn from partial melting of mantle sources progressively depleted by previous melt extractions group 1 volcanics may have derived from partial melting ca 20 of an undepleted lherzolitic source while group 2 basaltic rocks may have derived from partial melting ca 10 of a mantle that had previously experienced mid ocean ridge basalt morb extraction finally the group 3 boninites may have derived from partial melting ca 12 17 of a mantle peridotite previously depleted by primary melt extraction of groups 1 and 2 primary melts in order to explain the coexistence of these geochemically different magma groups two petrogenetic models formerly proposed for the albanian ophiolites are discussed

tectonically controlled origin of three unusual rock suites in the woodlark basin we propose alternative mechanisms for the origin of three unusual rock suites high mg andesites nati basalts and arclike rocks that have been dredged from the woodlark basin southwest pacific ocean we show that the high mg andesites and nati basalts are associated with an unusually cool ridge environment the cooling is due to increased hydrothermal circulation stimulated by an unusually high crustal permeability the high permeability is mainly due to cracking of the woodlark basin lithosphere as it passes over the flexural bulge in front of the subduction zone although local processes such as faulting in fracture zones may also make some contribution this increased convective cooling affects magma dynamics and chemistry at the ridge crest the high mg andesites occur where the hydrothermal circulation lowers the temperatures in the upper oceanic crust causing the magma chamber to sit in the mantle rather than in the crust and promoting the interaction of basalt magma with harzburgite the nati basalts are also the indirect result of increased crustal cooling which causes anomalously low degrees of partial melting of their depleted mantle source the arclike rocks are caused by interaction with volatiles from lithosphere that was emplaced beneath the edge of the basin less than 6 m y ago by a now inactive subduction zone to the north of the currently active one simple thermal models indicate that this formerly subducting plate is still cold enough to retain volatiles and to remain seismically active as the old plate loses volatiles they rise into the mantle convection cell which feeds the woodlark basin ridge crest because ridge subduction increases the likelihood of ophiolite obduction these observations and explanations of woodlark basin tectonics are potentially important for ophiolites

geology and geochemistry of early arc volcanic rocks from guam the island of guam is located at the southern end of the mariana fore arc and was the site of early arc volcanism between 43 and 32 m y ago two volcanic units were erupted during the early evolution of guam the late middle eocene facpi formation and the late eocene to early oligocene alutom formation the facpi formation is composed largely of interbedded boninite series pillow lavas pillow breccias and dikes although arc tholeiite series rocks cap the formation in some areas the alutom formation is composed of interbedded volcanic breccias tuffaceous sandstones lava flows and sills calc alkaline arc tholeiite and boninite series compositions are all found in the alutom formation primitive boninite series rocks are relatively high in sio 2 mgo and ni and low in a1 2 o 3 and tio 2 contents compared to basaltic rocks from the arc tholeiite series additionally they have low ti zr ratios and high k zr rb zr and ba zr ratios although no calc alkaline basalts are found in the early arc formations of guam basalts with chemical characteristics transitional between boninite and tholeiite series rocks may be genetically linked to the calc alkaline series among the analyzed samples here are a few with anomalously high y and rare earth element ree contents chemical variation in the boninite series can be explained by early fractionation of olivine and clinopyroxene followed by plagioclase clinopyroxene and orthopyroxene the arc tholeiite series chemical trends can be explained by early olivine fractionation followed by plagioclase clinopyroxene orthopyroxene and magnetite the composition of the least silicic calc alkaline series andesite can be modeled by fractional crystallization of olivine clinopyroxene plagioclase and magnetite from a transitional basalt derivation of more silicic calc alkaline series rocks requires magma mixing or crustal assimilation in addition to crystal fractionation involving hornblende all basalts from the early volcanic series of guam were erupted in an arc setting the boninite series magmas were produced by hydrous partial melting of depleted mantle at relatively shallow levels whereas the arc tholeiite and calc alkaline series magmas were generated by partial melting of less depleted mantle at deeper levels we suggest that because the calc alkaline series parent basalts most likely were richer in incompatible high field strength hfs elements and sio 2 than were the tholeiitic parents they were generated by lower degrees of melting