origin of the adakite high nb basalt association and its implications for postsubduction magmatism in baja california mexico constraining the origin of the adakite high nb basalt hnb association in baja california mexico is critical to a better understanding of global arc magmatism currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the subducted farallon cocos plate thus providing support for the slab melting origin of adakites elsewhere moreover a tectono magmatic model involving the production of both adakite and hnb from slab melts offers a comprehensive explanation for the origin of the atypical arc related postsubduction magmatism in baja california this paper proposes alternative models for the origin of hnb and postsubduction magmatism in baja california wherein the unusual geologic setting of western mexico and westward movement of north america permitted the influx of pacific asthenosphere beneath the adjacent gulf of california after the cessation of subduction unlike the previous tectono magmatic model the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in baja california as tholeiites and hnb respectively fractional crystallization of some of the hnb magmas plus assimilation of tholeiitic materials produced nb enriched basalts neb the influx of pacific asthenosphere after the cessation of subduction also provided thermal energy to melt the mafic lower baja california crust producing adakite rocks and the preexist ing metasomatized mantle wedge producing bajaites and calc alkaline magmas

thermomechanical erosion of the lithosphere by mantle plumes a simplified semianalytic theory is presented that describes the process of lithosphere thinning through thermomechanical erosion by a hot mantle upwelling or plume the theory compares well with published numerical models but predicts that the rate of thinning is very dependent on the lateral scale of the upwelling and on the viscosity through the lithosphere asthenosphere transition a plume tail is likely to cause rapid local thinning reducing the lithosphere thickness substantially within a few million years plume heads or broadly spreading plume tail material are unlikely to cause significant thinning by this mechanism alone these results are consistent with geophysical evidence for localized thinning under the hawaiian volcanic chain but not under the broader hawaiian swell they are also qualitatively consistent with the distribution of basaltic volcanism in oceanic and continental hotspot tracks the theory predicts that thin continental lithosphere may be further thinned sufficiently by a plume tail to cause crustal melting within about 20 m y thus this mechanism may account for some delayed silicic magmatism on some slow continental plates higher thinning rates would have occurred in the archean

remobilization of authigenic uranium in marine sediments by bioturbation uranium behaves as a nearly conservative element in oxygenated seawater but it is precipitated under chemically reducing conditions that occur in sediments underlying low oxygen bottom water or in sediments receiving high fluxes of particulate organic carbon sites characterized by a range of bottom water oxygen bwo and organic carbon flux ocf were studied to better understand the conditions that determine formation and preservation of authigenic u in marine sediments our study areas are located in the mid latitudes of the northeast pacific and the northwest atlantic oceans and all sites receive moderate 0 5 g cm2 kyr to high 2 8 g cm2 kyr ocf to the sediments bwo concentrations vary substantially among the sites ranging from 3 to 270 m a mass balance approach was used to evaluate authigenic u remobilization at each site within each region studied the supply of particulate nonlithogenic u associated with sinking particles was evaluated by means of sediment traps the diffusive flux of u into sediments was calculated from pore water u concentration profiles these combined sources were compared with the burial rate of authigenic u to assess the efficiency of its preservation a large fraction one third to two thirds of the authigenic u precipitated in these sediments via diffusion supply is later regenerated even under very low bwo concentrations 15 m bioturbating organisms periodically mix authigenic u containing sediment upward toward the sediment water interface where more oxidizing conditions lead to the remobilization of authigenic u and its loss to bottom waters

discovery of hydrothermal vent fields on alarcon rise and in southern pescadero basin gulf of california author s paduan jb zierenberg ra clague da spelz rm caress dw troni g thomas h glessner j lilley md lorenson t lupton j neumann f santa rosa del rio ma wheat cg abstract 2018 the authors hydrothermal vent fields located in the gap between known sites in guaymas basin and 21 n on the east pacific rise were discovered on the alarcon rise and in southern pescadero basin the alarcon rise spreading segment was mapped at 1 m resolution by an autonomous underwater vehicle individual chimneys were identified using the bathymetric data vent fields were interpreted as active from temperature anomalies in water column data and observed and sampled during remotely operated vehicle dives the ja sit pericu and meyibo active fields are near the eruptive fissure of an extensive young lava flow vent fluids up to 360 c from meyibo have compositions similar to northern east pacific rise vents the tzab ek field is 850 m west of the volcanic axis and active chimneys rise up to 33 m above a broad sulfide mound the inactive field is 10 km north northeast along the rift axis and most sulfide chimneys are enriched in zn and associated elements that are transported at lower temperature compared to the more cu rich active fields in southern pescadero basin the auka field is on the margin of a sediment filled graben at 3 670 m depth discharging fluids are clear contain hydrocarbons and have neutral ph elevated salinity and temperatures up to 291 c they have deposited massive mounds of calcite with minor sulfide the fluids are compositionally similar to those in guaymas basin produced by high temperature basalt seawater interaction followed by reaction with sediment the paucity of sulfide minerals suggests subsurface deposition of metals

geologic and geochemical reconnaissance of isla san esteban post subduction orogenic volcanism in the gulf of california abstract isla san esteban is a calc alkaline volcanic island in the center of the gulf of california a zone of active continental rifting its rocks range in composition from andesitic to rhyolitic with dacitic flows and sills making up more than 80 of its volume most other islands of the north central gulf of california are fragments of continental crust and include extensive metamorphic and sedimentary as well as igneous units k ar age determinations have resulted in ages of 2 5 to 2 9 m y for samples from isla san esteban these ages indicate that a volcano was erupting siliceous lava flows and pyroclastic units in the center of the gulf of california 9 5 m y after subduction beneath the continental margin ceased and a minimum of 4 5 m y after extension began to open the gulf a site of young orogenic volcanism at one other location in the gulf of california region is further evidence that a lag time of more than 9 m y may elapse between the end of subduction of a lithospheric plate and the last eruption of orogenic rocks a fragment of subducted oceanic crust may be the source of the calc alkaline magma and the complex tectonic setting of the region may provide the environment necessary for the unusual volcanism

early cambrian ocean anoxia in south china the cause of the most marked changes in the evolution of life which define the first order stratigraphic boundary between the precambrian and the phanerozoic eon remains enigmatic and a highly topical subject of debate a global ocean anoxic event triggered by large scale hydrogen sulphide h 2 s release to surface waters has been suggested by wille et al on the basis of two data sets from south china and oman to explain the fundamental biological changes across the precambrian cambrian pc c boundary here we report a new precise shrimp u pb zircon age of 532 3 0 7 million years myr ago fig 1 for a volcanic ash bed in the critical unit that reflects the ocean anoxic event the lowermost black shale sequence of the niutitang formation in the guizhou province south china this age is significantly younger than the precise pc c boundary age of 542 0 0 3 myr ago approximately 10 myr younger than the extinction of the ediacaran fauna and thus challenging the view of a major ocean anoxic event having been responsible for the major changes in the direction of evolution at the pc c boundary

palaeoceanographic redox environments for the lower cambrian hetang formation in south china evidence from pyrite framboids redox sensitive trace elements and sponge biota occurrence abstract the lower cambrian hetang formation a black shale sequence contains a stone coal a flammable organic rich mudstone unit abundant pyrite framboids occur in this unit with the average mean sizes falling in a narrow range from 4 6 to 5 4 m and the maximum diameters being around 10 m the size distribution pattern of the pyrite framboids indicates a euxinic depositional environment the redox sensitive trace metals of the stone coal samples reveal a large enrichment of mo 10 180 times v 4 40 times u 10 60 times and ni 2 20 times compared to the average upper continental crust value consistent with an anoxic environment for their deposition the redox sensitive element ratios th u v v ni v cr indicate that the depositional environment for the lower part of the stone coal unit was the most anoxic and euxinic in contrast a sponge biota including eleven species of demosponges and hexactinellids and two undetermined forms indicate an oxic or dysoxic environment to reconcile these two facts we propose that although an anoxic euxinic environment predominated during the deposition of the lower cambrian hetang formation black shales occasional currents may also have brought free oxygen to the bottom water column to allow the growth of the sponges on the sea floor

late cenozoic basin and range structure in western mexico adjacent to the gulf of california the area that surrounds the gulf of california was intensely faulted during the late cenozoic prior to opening of the gulf in a representative 10 000 km 2 area of southern sinaloa the faults consist of a predominant north northwest striking set having normal displacement and a subsidiary east northeast set having largely strike slip displacement two domains are recognized one in which both fault trends are abundant and one in which east northeast faults are minor in the former north northwest faults form a series of mostly southwest tilted half grabens filled with upper tertiary sediments displacement on individual faults is commonly as much as several kilometers east northeast faults probably represent accommodation zones between areas of differential extension in the second domain north northwest faults form an extensive graben system major faults dipping 40 to 70 into the graben and having several kilometers of cumulative displacement are spaced every 5 to 10 km bedding attitudes indicate that fault blocks are rotated as much as 65 dependent upon assumptions about subsurface geometry of the faults total extension may range from 20 to 50 fault geometry and stress orientations calculated from fault slip data indicate that the least principal stress was east northeast k ar ages of three north northwest striking dikes indicate that east northeast extension began as early as 32 ma ages of tilted volcanic rocks however indicate that measurable tilting and therefore most faulting began after about 17 ma faulting having similar timing style and orientation occurred throughout the area surrounding the gulf of california as far south as nayarit on the mexican mainland and as far north as areas in sonora that are unequivocally within the basin and range province this continuity and similarity in characteristics of faulting around the gulf to those of early basin and range faulting in the united states indicate that the area around the gulf is part of the basin and range province this basin and range faulting probably created the widely recognized but debated proto gulf of california the present gulf of california opened by ocean floor spreading and transform faulting in a zone already weakened by basin and range extension the lack of correspondence between areas of extension and likely areas of crustal thickening related either to laramide contraction or magmatism argues against a model of basin and range extension related solely to spreading of overthickened crust

petrologic consequences of rift propagation on oceanic spreading ridges abstract the production of anomalously differentiated lava compositions at several mid ocean spreading centers can be attributed to magmatic processes associated with propagating rifts the degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers low supply rates and moderate cooling rates allow advanced degrees of closed system fractionation to occur high supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma propagating rift tips are a special class of ridge transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge this balance is affected by the spreading rate the propagation rate of the rift the length of the bounding transform and proximity to hotspots maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion geochemical anomalies may persist though changing in degree and extent for several million years on ridge segments that stop propagating the concept of magnetic telechemistry is generally supported by our study but in the vicinity of hotspots magnetic anomaly amplitude may be controlled more by bathymetric and or thickened magnetic layer effects than by geochemistry

geochemical changes in the early cambrian interval of the yangtze platform south china implications for hydrothermal influences and paleocean redox conditions abstract the possible triggering mechanisms of the ediacaran cambrian transition in south china have been debated for many years in this study 80 samples from liuchapo niutitang and mingxinsi formations were collected from well yk 1 located in southeastern chongqing a high resolution chemostratigraphic study including evaluation of rare earth elements rees and trace elements was used to elucidate the paleoenvironments of the upper yangtze platform margin the mingxinsi gray shale and the liuchapo formation exhibit flat post archean australian normalized ree patterns with weakly negative ce anomalies which is typical of detritus obvious positive eu anomalies in chert samples of the basal niutitang formation imply a short term influence of hydrothermal fluid the ree patterns of the basal niutitang phosphatic rocks show significant enrichment of heavy rees significantly negative ce anomalies and positive y anomalies similar to the ree distribution pattern in seawater demonstrating that these sediments were produced under oxic seawater conditions moderately positive eu anomalies and positive y anomalies of the lower niutitang shales may indicate mixing of hydrothermal fluids and seawater the flat ree patterns with weak eu anomalies of the upper niutitang shale may have been generated by hydrothermal inputs diluted by detrital materials dramatic increases of ce anomalies in the basal niutitang formation reveal a significant decrease of the oxygen level in seawater trace elements such as ba u v u cu cr zn ni and sr are commonly enriched in these formations compared to the upper continental crust authigenic abundances of al normalized u and al normalized v as well as ratios of th u and v sc are used as indicators of redox conditions these distributions demonstrate that the redox conditions of the bottom water during the ediacaran cambrian transition abruptly changed from oxic in the nemakit daldynian age to anoxic in the early tommotian age and gradually became less reducing in the late tommotian age and the atdabanian age a strong correlation between total organic carbon toc concentrations and ratios of u al and v al in the entire section except in the phosphatic rocks implies non sulfidic conditions most trace elements cannot be used as proxies for redox conditions associated with the phosphatic rocks in the basal niutitang formation because abundant trace elements were especially absorbed in apatite the toc concentrations correlated well with the redox conditions of bottom water we infer that the section deposited in the platform margin was influenced by episodic hydrothermal activities that began at the beginning of the tommotian age and these activities may have terminated at the beginning of the atdabanian age enrichments of organic matter that were mainly derived from the photic zone were controlled by the redox conditions of the bottom water

the composition and emplacement of basaltic magmas produced during the development of continental margin basins the gulf of california mexico the gulf of california is a region of active ensialic basin development and transform faulting legs 64 and 65 of the deep sea drilling project have demonstrated the diversity of basement stratigraphy drilling at sites 474 482 483 and 485 at the mouth of the gulf recovered massive basaltic flows and occasional pillow lavas and sills whereas drilling in the central part of the gulf in the guaymas basin recovered a sequence of sediments and massive doleritic sills very high sedimentation rates over 1000 m ma are characteristic of the guaymas basin and prevent extrusion of the basaltic magma onto the seafloor thus resulting in at least partial intercalation of oceanic layers 1 and 2 such a process of ocean crust formation could be locally important during the early stages of ocean and marginal basin development when sedimentation rates may be very high over 200 samples of basalts and dolerites from the gulf of california have been analysed by x ray fluorescence and neutron activation techniques basalts from the mouth of the gulf are tholeiitic and contain very low abundances of the more hygromagmatophile more hyg elements k rb ba th ta nb and light ree relative to the hyg elements y zr ti and heavy ree thus resembling depleted n type morb from elsewhere along the e pacific rise epr dolerites from the guaymas basin also have a tholeiitic chemistry and some epr basalt characteristics e g la n sm n

structural development of the lower paleozoic belt of south china genesis of an intracontinental orogen abstract an extensive structural study of wuyishan and surrounding areas south china brings data on the structures formed prior to the devonian unconformity building the lower paleozoic belt an ordovician tectonic event was responsible for i south directed structures in the wuyishan proper and its southern border related to both thin and thick skinned tectonics accompanied by metamorphism and crustal thickening ii north directed structures to the west of ganjiang fault and north of jiangshan shaoxing fault where only thin skinned tectonics is visible the southward deformation accommodating the shortening includes emplacement of thrust sheets involving deep crustal material and some mantle peridotite and likely repetition of continental material both responsible for crustal thickening the orogeny was due to the underthrusting of the southern part of the south china block beneath the northern part of this block closing the pre existing nanhua rift created at around 850 800 ma and involving the rift sedimentary fill in the southward thrusting the syn metamorphic tectonic piling was followed by anatexis and granite emplacement at around 440 390 ma our new u th pb epma monazite dating gives ages of 433 9 ma and 437 5 ma early silurian for the main anatectic event and a younger age of 412 5 ma late silurian early devonian for a late re heating process the lower paleozoic belt is an intracontinental orogen without oceanic suture record it shows some similarities with the cenozoic european pyrenees chain with a strong asymmetry

origin of nb enriched basalts and adakites in baja california mexico revisited reply maury et al 2009 present new and detailed field observations and petrologic analyses to argue that the new model that i propose as an alternative explanation for postsubduction magmatism in baja california mexico castillo 2008 1 is incorrect specifically they claim that their new

biogenic origin of intergrown mo sulphide and carbonaceous matter in lower cambrian black shales zunyi formation southern china abstract the lower cambrian zunyi formation in southern china hosts complex intergrown phases of mo sulphides and anthracite like carbonaceous matter and graphite mosc pyrite and ni sulphides these sulphides occur in a few centimeters thick polymetallic stratiform layer in black shales above phosphorites cherts dolomites and barite the ore horizon contains 7 wt mo 2 wt ni 19 wt s and 13 wt c org and c tot co 2 58 wt 14 wt cao 1 2 wt as 2000 ppm se 5400 ppm zn 450 ppm u and v and up to 1 ppm platinum group elements three types of mosc phases always including framboidal pyrite are distinguished 1 rounded structures few hundred microns large 2 relict inclusions in diagenetic pyrite 3 grains interstitial to sulphurised and phosphatised organic matter at the nanoscale mosc is represented by nanometric porous rounded structures of mos 2 finely intergrown with carbonaceous matter of typical nano onion structures mosc also contains nanocrysts of quartz clays ca sulphate carbonate and ni sulphides micro raman spectroscopy shows that the carbon phase has a maturation grade of anthracite indicating temperatures below 300 c mosc is mainly composed of 25 wt mo 23 wt s 5 6 wt fe 2 5 wt ni 33 wt c 1 5 wt o 0 6 wt n and several hundreds of ppm of cu as se sb pd zn co and sn electron energy loss spectra show that n is related to nanosized carbonaceous matter the mean c n ratio 57 of mosc is similar to those observed in marine organic matter and those reported from zooplankton at hydrothermal vents the mineralized shales are crosscut by hydrothermal veins composed of carbonate or barite always associated with quartz apatite and fe and zn sulphides a paragenesis also observed as nanocrysts in mosc although mo originated from seawater its extreme concentration is due to an enhanced growth of bacterial consortia in a diffuse hydrothermal vent system these consortia may be dominated by nitrogen fixing and mo dependant hyperthermophilic or mesophilic species containing nitrogenase composed also of mo s fe organic matter decay led thus to the formation of mosc and the included framboidal pyrite

extreme enrichment of polymetallic ni mo pge au in lower cambrian black shales of south china an os isotope and pge geochemical investigation extreme metal enrichments occur in the lower cambrian niutitang formation a thick black shale sequence deposited on the yangtze platform of south china and form a regionally distributed conformable polymetallic ni mo pge au sulfide horizon the origin for the extreme enrichment of metals in these black shales is discussed based on a comprehensive geochemical investigation of re os isotopes and platinum group elements the black shales of the lower cambrian niutitang formation in guizhou province yielded an re os isochron age of 535 11 ma with an initial 187 os 188 os ratio of 0 80 0 04 which is similar to the initial 187 os 188 os ratio 0 78 0 19 of the ni mo sulfide ores but slightly lower than the initial 187 os 188 os ratio 1 18 0 02 of lower cambrian black shales from the lesser himalaya india a comparison of os isotope data among the chinese ni mo sulfide ores the seafloor massive sulfide deposits and contemporaneous seawater suggests a possible similar hydrothermal forming mechanism for these deposits the pge concentrations pt anomaly pt pt pt pd ir pd au pd ratios of the chinese ni mo sulfide ores are similar to those metal rich black shales of submarine volcanogenic hydrothermal origin from czech republic namibia and canada but are different from those data of the ocean floor fe mn crusts that have scavenged their pge and other metals directly from seawater it is therefore suggested that the ni mo sulfide ores from the niutitang black shale sequence in south china had a submarine hydrothermal origin the anoxic environments and abundant organic matter in the ocean basin also played a key role for the extreme metal enrichments the discharge of hydrothermal fluids into the cambrian ocean may have had a great effect on life during the cambrian explosion 2007 elsevier b v all rights reserved

alteration and remelting of nascent oceanic crust during continental rupture evidence from zircon geochemistry of rhyolites and xenoliths from the salton trough california abstract rhyolite lavas and xenoliths from the salton sea geothermal field southern california provide insights into crustal compositions and processes during continental rupture and incipient formation of oceanic crust salton buttes rhyolite lavas contain xenoliths that include granophyres fine grained altered rhyolites felsite and amphibole bearing basalts zircon is present in lavas and xenoliths surprisingly even in the basaltic xenoliths where it occurs in plagioclase rich regions interpreted as pockets of crystallized partial melt zircons in the xenoliths are exclusively late pleistocene holocene in age and lack evidence for inheritance u th isochron ages are 20 5 1 2 1 2 ka granophyres 18 3 3 5 3 6 ka felsite 30 1 12 4 14 1 ka and 9 2 6 6 7 0 ka basalts all errors 1 the dominant zircon population in the rhyolite lavas yielded u th ages between 18 and 10 ka with few pre quaternary xenocrysts present 18ozircon values are lower than typical crustal basement values thus ruling out rhyolite genesis by melting of continental crust moreover 18ozircon values are 0 5 1 0 lower than compositions achievable by zircon crystallization from residual melt in equilibrium with unaltered mid ocean ridge basalt suggesting that basaltic crust and silicic plutons in the subsurface of the salton sea geothermal field isotopically exchanged with meteoric waters this is evidence for deep reaching hydrothermal circulation and indicates rhyolite genesis by episodic remelting of altered basalts instead of fractional crystallization of unaltered basaltic magma

thick deltaic sedimentation and detachment faulting delay the onset of continental rupture in the northern gulf of california analysis of seismic reflection profiles 1 the transition from distributed continental extension to the rupture of continental lithosphere is imaged in the northern gulf of california across the obliquely conjugate tiburon upper delfin basin segment structural mapping on a 5 20 km grid of seismic reflection lines of petroleos mexicanos demonstrates that 1000 extension is accommodated on a series of nne striking listric normal faults that merge at depth into a detachment fault the detachment juxtaposes a late neogene marine sequence over thinned continental crust and contains an intrabasinal divide due to footwall uplift two northwest striking dextral oblique faults bound both ends of the detachment and shear the continental crust parallel to the tectonic transport a regional unconformity in the upper 0 5 s two way travel time and crest erosion of rollover anticlines above the detachment indicates inversion and footwall uplift during the lithospheric rupture in the upper delfin and lower delfin basins the maximum length of new crust in both delfin basins is less than 40 km based on the lack of an acoustic basement and the absence of a lower sedimentary sequence beneath a wedge shaped upper sequence that reaches 5 km in thickness a fundamental difference exists between the tiburon delfin segment and the guaymas segment to the south in terms of presence of low angle normal faults and amount of new oceanic lithosphere which we attribute to thermal insulation diffuse upper plate extension and slip on low angle normal faults engendered by a thick sedimentary lid

mo total organic carbon covariation in modern anoxic marine environments implications for analysis of paleoredox and paleohydrographic conditions 1 sedimentary molybdenum mo s has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic rich marine facies deposited under oxygen depleted conditions a detailed analysis of mo s total organic carbon toc covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that 1 mo s while useful in distinguishing oxic from anoxic facies is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and 2 patterns of mo s toc covariation can provide information about paleohydrographic conditions especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal these inferences are based on data from four anoxic silled basins the black sea framvaren fjord cariaco basin and saanich inlet and one upwelling zone the namibian shelf representing a spectrum of aqueous chemical conditions related to water mass restriction in the silled basin environments increasing restriction is correlated with a systematic decrease in mo s toc ratios from 45 5 for saanich inlet to 4 5 1 for the black sea this variation reflects control of mo s by mo aq which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal the basin reservoir effect the temporal dynamics of this process are revealed by high resolution chemostratigraphic data from framvaren fjord and cariaco basin sediment cores which exhibit long term trends toward lower mo s toc ratios following development of water column stratification and deepwater anoxia mo burial fluxes peak in weakly sulfidic environments such as saanich inlet owing to a combination of greater mo aq availability and enhanced mo transport to the sediment water interface via fe mn redox cycling and are lower in strongly sulfidic environments such as the black sea and framvaren fjord these observations demonstrate that at timescales associated with deepwater renewal in anoxic silled basins decreased sedimentary mo concentrations and burial fluxes are associated with lower benthic redox potentials i e more sulfidic conditions these conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction e g silled basins and are not valid for low oxygen facies in open marine settings such as continent margin upwelling systems

paleo marine environments of the early cambrian yangtze platform abstract previous studies have generated conflicting interpretations of the nature of oceanographic conditions on the early cambrian yangtze platform we investigated paleo environmental conditions there through an analysis of major elements and redox sensitive trace elements in the lower cambrian niutitang formation from two new drillcores ry1 and ry2 in guizhou province high toc and excess ba concentrations record elevated productivity in the study area redox variation was documented based on enrichment factors efs for redox sensitive trace elements mo u v cr ni and co and c org p ratios these proxies indicate that the lower unit of the niutitang formation was deposited under fluctuating but mostly anoxic conditions whereas the upper unit was deposited under predominantly suboxic conditions mo toc ratios suggest persistently moderate to strong basinal restriction although mo u covariation patterns show no evidence for operation of a particulate shuttle despite the semi restricted anoxic nature of the deep watermass episodic deep water oxygenation events occurred within the yangtze platform sea and longer term secular variation may record eustatic influences additionally patterns of trace metal enrichment record evidence of submarine hydrothermal inputs we propose an integrated paleoenvironmental model to account for the observed productivity redox and hydrographic patterns in the early cambrian yangtze platform on the basis of paleo marine environmental analysis we ascribe the scarcity of trace fossils in the bottom to oxygen deficiency and deny considering the songlin biota as the onset of the intense biogenic evolution after global atmospheric and oceanic oxygenation

the ellsworth terrane coastal maine geochronology geochemistry and nd pb isotopic composition implications for the rifting of ganderia the ellsworth terrane is one of a number of fault bounded blocks that occur along the eastern margin of ganderia the westernmost of the peri gondwanan domains in the northern appalachians that were accreted to laurentia in the paleozoic geologic relations detrital zircon ages and basalt geochemistry suggest that the ellsworth terrane is part of ganderia and not an exotic terrane in the penobscot bay area of coastal maine the ellsworth terrane is dominantly composed of bimodal basalt rhyolite volcanic sequences of the ellsworth schist and unconformably overlying castine volcanics we use new u pb zircon geochronology geochemistry and nd and pb isotopes for these volcanic sequences to constrain the petrogenetic history and paleotectonic setting of the ellsworth terrane and its relationship with ganderia u pb zircon geochronology for rhyolites indicates that both the ellsworth schist 508 6 0 8 ma and overlying castine volcanics 503 5 2 5 ma are middle cambrian in age two tholeiitic basalt types are recognized type tb 1 basalt present as pillowed and massive lava fl ows and as sills in both units has depleted la and ce la nd n 0 53 0 87 values fl at heavy rare earth element ree values and no positive th or negative ta anomalies on primitive mantle normalized diagrams in contrast type tb 2 basalt present only in the castine volcanics has slightly enriched lree la yb n 1 42 2 92 values and no th or ta anomalies both basalt types have strongly positive e nd 500 values tb 1 7 9 8 6 tb 2 5 6 7 0 and relatively enriched pb isotopic compositions 206

hydrogen sulphide release to surface waters at the precambrian cambrian boundary changes in environmental conditions at the precambrian cambrian transition around 542 million years ago have been suggested as a possible explanation for the apparent rapid increase in abundance of multicellular organisms known as the cambrian explosion the nature of the environmental changes is still a matter of debate however wille et al now report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of about 542 million years with the help of a box model of the oceanic molybdenum cycle they find that intense upwelling of hydrogen sulphide rich deep ocean water best explains the observed early cambrian molybdenum isotope signal this suggests that the early cambrian animal radiation may have been triggered by a major change in ocean circulation following a long period during which the ocean was stratified with sulphidic deep water this paper reports molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 million years the findings suggest that the early cambrian animal radiation may have been triggered by a major change in ocean circulation terminating a long period during which the proterozoic ocean was stratified with sulphidic deep water animal like multicellular fossils appeared towards the end of the precambrian followed by a rapid increase in the abundance and diversity of fossils during the early cambrian period an event also known as the cambrian explosion 1 2 3 changes in the environmental conditions at the precambrian cambrian transition about 542 myr ago have been suggested as a possible explanation for this event but are still a matter of debate1 2 3 here we report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 myr we find a transient molybdenum isotope signal immediately after the precambrian cambrian transition using a box model of the oceanic molybdenum cycle we find that intense upwelling of hydrogen sulphide rich deep ocean water best explains the observed early cambrian molybdenum isotope signal our findings suggest that the early cambrian animal radiation may have been triggered by a major change in ocean circulation terminating a long period during which the proterozoic ocean was stratified with sulphidic deep water

diversity and spatial zonation of volcanic rocks from the east pacific rise near 21 n a substantial range of petrologic rock types has erupted on the accreting plate boundary near 21 n on the east pacific rise epr young olivine basalts have fo89 86 phenocrysts low bulk tio2 1 1 1 3 ba 7 10 ppm and high ni contents 100 ppm older plagioclase olivine pyroxene pop basalts have fo86 81 phenocrysts high tio2 1 4 1 7 ba 9 40 ppm and low ni 100 ppm the youngest olivine basalts erupt immediately around a segmented axial fissure system progressively older more fractionated pop basalts have spread farther from the same fissure system producing a stratigraphically controlled zonal pattern of basalt type distribution around the eruptive fissures a topographic and morphologic en echelon displacement of the ridge axis fissure of 1 7 km to the nw near 20 54 n offsets this zonal distribution pattern low pressure crystal fractionation of olivine plagioclase and clinopyroxene 2 5 1 from an olivine basalt parent would yield pop basalts of the observed zr ti y p and major element chemistry however very incompatible elements ba and k are too variably enriched in pop basalts for this model to be viable small variable degrees of mantle partial melting is not a viable model either because of the substantial depletion of ni which correlates with incompatible element enrichment and because of the precise low pressure cotectic character of pop basalts the in situ fractionation model of langmuir 1987 can explain these features the relative abundance of fractionated lavas their small scale areal chemical zonation the petrochemical correlation between types and geophysical evidence point to the existence of shallow fractionating magma reservoirs beneath the epr at 21 n

geochemical and structural studies of the lamont seamounts seamounts as indicators of mantle processes abstract a chain of seamounts located just west of the east pacific rise epr axis at 09 55 n herein called the lamont seamounts have been investigated using sea marc i and sea beam sonar surveys alvin submersible dives deep sea camera tows and dredging the most recently active volcanic vents on the summits of these volcanoes are located at the periphery of each caldera or crater rim or at the margins of caldera floors the distribution of recent volcanic vents and the morphology of the summit plateaus suggest the possible existence of ring fractures on these seamounts ring fractures could be important in focusing post caldera collapse eruptions around summit craters and building summit plateaus structural and morphological data and the petrology of recovered lavas suggest that calderas in the lamont seamounts probably formed by collapse associated with magma withdrawal into large primary vertical magma conduit systems rather than by repeated inflation deflation cycles associated with steady state shallow level magma chambers calderas in the lamont seamounts have evolved through coalescence of adjoining craters and the plan view shape and morphology of each caldera suggest that primary magma conduits have migrated beneath the summit of each seamount through time compared to lavas from the adjacent epr the majority of lamont seamount and cone lavas are more primitive depleted in incompatible elements and compositionally heterogeneous variations in light rare earth elements relative to major elements e g tio 2 na 2 o na 2 o cao and radiogenic isotopes cannot be ascribed to differences in the amount of partial melting of a single n type morb source rather it appears that lamont seamount lavas were derived from heterogeneous mantle sources variably depleted in highly incompatible elements comparison with lavas from the adjacent ridge crest suggests that magma plumbing and ascent mechanisms beneath the lamont seamounts are separate from those along the epr in particular seamount magmas appear to reflect near primary melting events in the mantle whereas epr lavas are the products of shallow level crystallization and mixing in crustal magma chambers absence of long lived magma chambers within the lamont seamounts may account for the low level of hydrothermal activity observed throughout the chain furthermore because of the tight control on sample location and the recovery of lavas having distinct parents from individual volcanoes as well as the lamont chain as a whole we propose that the scale of mantle chemical heterogeneities is less than 5 km

contrasting styles of seafloor spreading in the woodlark basin indications of rift induced secondary mantle convection the woodlark basin in the southwest pacific is a young ocean basin which began forming by 6 ma following the rifting of continental and arc lithosphere the n s striking moresby transform divides the oceanic basin into eastern and western parts which have contrasting characteristics seafloor spreading west of moresby transform began after 2 ma and although spreading rates decrease to the west the western basin has faster spreading characteristics than the eastern basin these include 1 500 m shallower seafloor 2 bouguer gravity anomalies that are 30 mgals lower 3 magnetic anomaly and modeled seafloor magnetization amplitudes that are higher 4 a spreading center with an axial high in contrast to the axial valleys of the eastern basin 5 smoother seafloor fabric and 6 exclusively nontransform spreading center offsets in contrast to the eastern basin which has transform faults and fracture zones that extend across most of the basin overall depth contrasts and bouguer anomalies can be matched by end member models of thicker crust 2 km or thinner lithosphere 1 3 in the western basin correlated with these contrasts the surrounding rifted margins abruptly thicken westward of the longitude of moresby transform we examine alternative explanations for these contrasts and propose that rift induced secondary mantle convection driven by thicker western margin lithosphere is most consistent with the observations although rift induced convection has been cited as a cause for the voluminous excess magmatism at some rifted margins the observations in the woodlark basin suggest that this mechanism may significantly affect the morphology structure and geophysical characteristics of young ocean basins in alternate ways which resemble increased spreading rate

diagenetic uptake of rare earth elements by bioapatite with an example from lower triassic conodonts of south china abstract the distribution patterns of rare earth elements rees are frequently used as proxies for ancient seawater chemistry or paleomarine environmental conditions however recent work has shown that diagenesis can lead to remobilization and inter elemental fractionation of rees and that these effects often occur in conjunction with redox reactions in sediment porewaters here we review existing literature on the diagenetic fluxes of rees in marine sediments and porewaters in order to systematize existing knowledge on this subject rees undergo significant redistribution among sediment phases during both early and late diagenesis as a consequence of adsorption and desorption processes remobilization of rees commonly leads to inter elemental fractionation variously leading to enrichment or depletion of the light middle or heavy ree fractions further ree remobilization can be facilitated by redox changes e g through reductive dissolution of host phases in suboxic and anoxic porewaters characteristic ree distribution patterns develop through these processes 1 a flat distribution signifying predominantly terrigenous siliciclastic influence 2 a middle ree bulge probably due to adsorption of light and heavy rees to mn and fe oxyhydroxides respectively and 3 heavy ree enrichment indicative of hydrogenous seawater influence note all patterns in this paper are normalized to the ree composition of average upper continental crust or ucc in the second part of this study we undertake an analysis of the ree distributions in conodonts and whole rock samples from west pingdingshan a permian triassic boundary section in south china using ree th and y ho ratios we show that almost all of the conodont samples have a strong diagenetic overprint and that the hydrogenous ree fraction is small and not isolatable furthermore the conodonts contain two diagenetic ree components one characterized by low ree 100 300 ppm high ree th ratios 1000 strong middle ree enrichment and eu eu ratios of 1 5 2 0 and the second by high ree 300 2000 ppm low ree th ratios 20 30 little or no middle ree enrichment and eu eu ratios of 1 0 the first component exhibits a pronounced middle ree bulge that represents an early diagenetic signature associated with suboxic conditions possibly related to adsorption of rees onto fe mn oxyhydroxides in the shallow subsurface environment the second component shows a flat ree distribution that is similar to both our whole rock samples and average ucc indicating derivation from rees released from detrital siliciclastics e g clay minerals probably at a range of burial depths from shallow to deep failure of the conodont samples to yield an isolatable hydrogenous component demonstrates that bioapatite does not always preserve a primary marine ree signature given that bioapatite rees have been widely used for this purpose often on the assumption of minimal or no diagenetic influence our findings are likely to necessitate a re evaluation of the results of many earlier studies in general we counsel caution in inferring a hydrogenous origin for rees in bioapatite owing to frequent diagenetic alteration of ree distributions

geology of the alarcon rise southern gulf of california author s clague da caress dw dreyer bm lundsten l paduan jb portner ra spelz madero r bowles ja castillo pr guardado france r le saout m martin jf santa rosa del rio ma zierenberg ra abstract 2018 the authors meter scale auv bathymetric mapping and rov sampling of the entire 47 km long alarcon rise between the pescadero and tamayo transforms show that the shallowest inflated portion of the segment hosts all four active hydrothermal vent fields and the youngest hottest and highest effusion rate lava flows this shallowest inflated part is located 1 3 of the way between the tamayo and pescadero transforms and is paved by a 16 km2 channelized flow that erupted from 9 km of en echelon fissures and is larger than historic flows on the east pacific rise or on the gorda and juan de fuca ridges starting 5 km south of the pescadero transform 6 5 km of the alarcon rise is characterized by faulted ridges and domes of fractionated lavas ranging from basaltic andesite to rhyolite with up to 77 3 wt sio2 these are the first known rhyolites from the submarine global mid ocean ridge system silicic lavas range from g11 7 ka to as young as 1 1 ka a basalt to basaltic andesite sequence and an andesite to dacite to rhyolite sequence are consistent with crystal fractionation but some intermediate basaltic andesite and andesite formed by mixing basalt with dacite or rhyolite magmatism occurred along the bounding tamayo and pescadero transforms as extensive channelized flows the flows erupted from ring faults surrounding uplifted sediment hills inferred to overlie sills the transforms are transtensional to accommodate magma migration from the adjacent alarcon rise

further shrimp geochronology on the early cambrian of south china zircon 206pb 238u ages are reported for bentonites from two cambrian sections in south china the meishucun and the fandian the analyzed zircons vary widely in age because of the presence of detrital and altered grains spot ages were corrected for calibration drift during analysis and the age of reference zircon sl13 was calibrated as 580 ma against other standards using sims use of mixture modeling was essential to resolve the different age components and to identify the volcanic ages two age groups occur within the meishucun bed 5 bentonite of which the larger at 518 7 2 5 ma is interpreted as a post depositional zircon growth event and the other at 539 4 2 9 ma as the volcanic age bentonites from the meishucun bed 9 and its fandian equivalent have two age groups 526 5 1 1 ma and 540 2 1 0 ma we interpret the younger of these as the tuff magmatic age and the older as detrital small shelly fossils indicate meischucunian bed 5 is an equivalent of the late nemakit daldynian and its 539 4 2 9 ma age both closely times the base of the cambrian and is a maximum for the basal tommotian the 526 5 1 1 ma for bed 9 and its equivalents approximates the close of the tommotian

accretionary orogenesis of the chinese altai insights from paleozoic granitoids zircon u pb dating and whole rock major oxide trace element and nd sr isotope compositions have been determined for four representative granitic intrusions in the sw chinese altai in order to understand the orogenesis and history of crustal growth in the central asian orogenic belt caob the ashile and halatas bodiesare relatively small intrusions formed in the late carboniferous 318 6ma andpermian 267 4ma respectively thelargertarlang tb andkeketuohaibatholiths kb havemagmaticagesof 359 to 412 ma the granitoids consist chiefly of tonalite granodiorite and granite and are metaluminous to weakly peraluminous in composition asi 0 8 1 1 the ashile pluton is characterized by relatively high mgo 2 73 3 54 wt and sr 395 456 ppm contents with low k2o na2o ratios 0 31 0 42 and mantle like nd sr isotopic compositions endt 2 85 to 3 26 initial 87 sr 86 sr 0 7047 0 7051 it probably formed by fractional crystallization of a mantle derived magma with limited assimilation of oldcrustalmaterial theotherintrusionshavenear zeroornegative endtvalues 0 67to 4 41 andhighersrisotopiccompositions initial 87 sr 86 sr 0 7067 0 7092 indicating a mixture of juvenile and old components in their genesis these intrusions are characterized by relatively low k rb cs contents and low rb sr ratios mostly b0 6 and generally exhibit moderate to high heavy rare earth element hree concentrations ybn 8 26 and weak hree fractionation dy ybnb1 5 with or without negative europium anomalies eu eu 0 5 1 0 these characteristics are consistent with an origin by dehydration melting of a hornblendebearing mid crustal source above the garnet stability field dominated by mafic to intermediate rocks this suggests that the deep crust of the chinese altai may contain a considerable proportion of juvenile material mantle derived magmatism probably in an active extensional continental margin setting played an important role in the formation of the chinese altai acting not only as a heat sourceforcrustalmeltingbutasasourceofjuvenilecomponents therelativelysmallvolumesanddistinctgeochemicalcompositions of the ashile and halatas plutons may imply a substantial change in the geodynamic setting in the late paleozoic 2007 elsevier b v all rights reserved

petrology of alarcon rise lavas gulf of california nascent intracontinental ocean crust 1 the alarcon basin in the southern gulf of california originated by intracontinental rifting of the southwestern margin of north america and has evolved into an intercontinental ocean basin by 3 7 m y of seafloor spreading lava samples collected on the axial ridge along alarcon rise two of the many near axis seamounts on the northwest flank of the rise and an abyssal hill on 0 6 ma crust on the southeast flank were examined petrographically and analyzed for major and trace element and he sr nd pb isotope composition most samples are typical mid ocean ridge basalt morb but rocks at the axial ridge site closest to continent are more differentiated rocks from the axial ridge and abyssal hill are genetically related through separate episodes of fractional crystallization seamount samples are relatively more primitive than seafloor samples and may have been formed by smaller degree of melting of the same mantle source as the seafloor lavas tamayo transform fault has no apparent effect on the geochemistry of axial ridge lavas erupted close to it this is contrary to the postulated transform fault effect alarcon basin differs from the salton trough and guaymas basin in that its youngest seafloor shows no evidence for contamination with continental crust or a subduction component we propose that pacific morb source mantle underlies the gulf of california because the western north american margin has overridden the pacific suboceanic mantle miocene rifting and pliocene spreading in alarcon basin have created a window allowing the most recent morb melts to rise uncontaminated to the surface

age refinement of the messinian salinity crisis onset in the mediterranean we propose a revised age calibration of the messinian salinity crisis onset in the mediterranean at 5 971 ma based on the recognition of an extra gypsum cycle in the transitional interval of the perales section sorbas basin spain and the revision of the magnetostratigraphy of the monticino section vena del gesso basin italy this age re calibration allows to state more accurately that i the interval encompassing the msc onset is continuous thus ruling out any erosional feature or stratigraphic hiatus related to a major sea level fall affecting the mediterranean ii the first gypsum was deposited during the summer insolation peak at 5 969 ma associated with an eccentricity minimum and roughly coincident with glacial stage tg32 iii the msc onset was preconditioned by the tectonically driven reduction of the hydrological exchanges with the atlantic ocean and finally triggered by glacial conditions in the northern hemisphere and by arid conditions in northern africa

strontium neodymium and lead isotope constraints on near ridge seamount production beneath the south atlantic studies1 7 of seamounts near the east pacific rise epr have shown that although most seamount lavas are petrographically and chemically identical to mid ocean ridge basalt they are chemically and isotopically more diverse than those erupted on the rise axis they are also generally more primitive higher mgo content and in some cases more depleted in incompatible elements than the axial basalts this indicates that although near ridge sea mounts and the epr share a common mantle source there must be significant differences in their magma supply processes sea mounts are probably built by small batches of melt that rise rapidly to the surface preserving evidence of heterogeneity in the mantle source region by contrast the processes of melting and melt segregation storage and extrusion along the fast spreading 9 13 cm yr 1 epr are more efficient in mixing and homogenizing basalt compositions1 7 here we show that the relationship of strontium neodymium and lead isotope data between seamounts in the south atlantic and the nearby axis of the slow spreading mid atlantic ridge mar is similar to that seen in the pacific this indicates that the processes leading to formation of near ridge seamounts are similar at a wide range of spreading rates differences in the specific isotope signatures of lavas from near ridge seamounts and axes of the epr and mar reflect regional differences in the upper mantle source of mid ocean ridge basalts

authigenic molybdenum formation in marine sediments a link to pore water sulfide in the santa barbara basin pore water and sediment mo concentrations were measured in a suite of multicores collected at four sites along the northeastern flank of the santa barbara basin to examine the connection between authigenic mo formation and pore water sulfide concentration only at the deepest site 580 m where pore water sulfide concentrations rise to 0 1 mm right below the sediment water interface was there active authigenic mo formation at shallower sites 550 430 and 340 m where pore water sulfide concentrations were consistently 0 05 mm mo precipitation was not occurring at the time of sampling a sulfide concentration of 0 1 mm appears to be a threshold for the onset of mo fe s co precipitation a second threshold sulfide concentration of 100 mm is required for mo precipitation without fe possibly as mo s or as particle bound mo mass budgets for mo were constructed by combining pore water and sediment results for mo with analyses of sediment trap material from santa barbara basin as well as sediment accumulation rates derived from 210 pb the calculations show that most of the authigenic mo in the sediment at the deepest site is supplied by diffusion from overlying bottom waters there is however a non lithogenic particulate mo associated with sinking particles that contributes 15 to the total authigenic mo accumulation analysis of sediment trap samples and supernant brine solutions indicates the presence of non lithogenic particulate mo a large fraction of which is easily remobilized and perhaps associated with mn oxides our observations show that even with the very high flux of organic carbon reaching the sediment of santa barbara basin active formation of sedimentary authigenic mo requires a bottom water oxygen concentration below 3 mm however small but measurable rates of authigenic mo accumulation were observed at sites where bottom water oxygen ranged between 5 and 23 mm indicating that the formation of authigenic mo occurred in the recent past but not at the time of sampling copyright 2000 elsevier science ltd

seismic evidence of a ridge parallel strike slip fault off the transform system in the gulf of california 1 a left lateral strike slip fault perpendicular to the major transform fault that occurred at the extended continental crust in the middle of the gulf of california produced a remarkably active main shock and aftershock seismic sequence that began on 1 september 2007 in contrast to observations from oceanic transform fault systems this event induced unusually active seismicity by means of an inversion procedure we obtained the moment tensor solutions of all the events greater than mw 4 4 and we observed multiple types of faulting some very similar to the main event and others that included a tensional cracking mechanism with a significant nondouble couple component static stress change for optimally oriented left lateral faults predicts the seismic activity this event has implications for the californian peninsular geology especially for the late miocene fault systems that were optimally reoriented to promote active faulting

the mechanisms of earthquakes and faulting in the southern gulf of california abstract accurate earthquake locations and their focal mechanisms can illuminate the distribution and mode of deformation at rifted continental margins the pacific north america pa na plate boundary within the gulf of california goc provides an excellent opportunity to explore the evolution and kinematics of rifting as continental extension in the north transitions to seafloor spreading in the south from october 2005 to october 2006 an array of eight four component ocean bottom seismographs deployed in the goc recorded seismicity as part of the sea of cortez ocean bottom array scooba experiment by combining the data with those from the onshore network of autonomously recording seismographs nars baja array we detect and locate 700 earthquakes m w 2 2 6 6 mainly along the northwest southeast striking transform faults that delineate the plate boundary for 36 events m w 3 5 6 6 with high signal to noise ratio in a long period 10 20 s band we determine deviatoric moment tensors and associated double couple focal mechanisms by regional waveform inversion many focal mechanisms are consistent with right lateral strike slip faulting along the pa na transform fault system which suggests that the transform faults primarily accommodate seismic deformation within the gulf in addition we capture a swarm of events on baja california along the right lateral northwest southeast striking las viboras el azufre fault which may be related to ongoing geothermal activity and volcanic deformation within the peninsula the combination of high resolution earthquake locations and focal mechanisms improves our understanding of the distribution of seismic deformation within the greater extensional zone in the southern goc online material earthquake catalog of the 695 events detected and located during our combined on offshore deployment of seismometers

enhanced trapping of molybdenum by sulfurized marine organic matter of marine origin in mesozoic limestones and shales there have been many studies devoted to trace metals and their value in assessing the paleoredox conditions of ancient marine deposition among them molybdenum mo is frequently cited as an effective proxy for sediments and sedimentary rocks recently helz et al helz g r miller c v charnock j m mosselmans j l w pattrick r a d garner c d vaughan d j 1996 mechanisms of molybdenum removal from the sea and its concentration in black shales exafs evidences geochim cosmochim acta 60 3631 3642 and adelson et al adelson j m helz g r miller c v 2001 reconstructing the rise of recent coastal anoxia molybdenum in chesapeake bay sediments geochim cosmochim acta 65 237 252 suggested that mo does not behave conservatively in the water column when h2s reaches a threshold concentration above this concentration a switch operates and mo is scavenged by forming bonds with metal rich notably iron particles sulfur rich organic molecules and pyrite in this paper mo trapping by sulfur rich organic matter om in ancient marine deposits is emphasized the following mesozoic geological formations were selected for study because of their relatively high concentration of sulfurized om the akkuyu formation turkey the calcaires d orbagnoux france and kimmeridge clay uk and its timeequivalent in boulonnais france the kashpir oil shales russia and the la luna formation venezuela the sulfur rich om is identified by either measured organic s abundance or kerogen microscope observation our results show that mo is systematically more enriched relative to the other redox sensitive sulfide forming elements studied u v ni cu zn cr and mo enrichment is positively correlated with the amount of sulfurized om but not with pyrite abundance these results illuminate the role played by sulfurized om in geologic scale mo capture and retention but they also underline the role played by reactive iron significant om sulfurization is only possible when reactive iron is limited nevertheless pyrite formation though limited could act as an initial mo trap prior to mo uptake by om that is sulfurized after the pyritization step in future paleoenvironmental reconstructions attention must be paid to this enhanced mo enrichment in the presence of sulfurized organic matter in such cases the use of mo could lead to overestimation of the reducing conditions of the depositional environment

buried rhyolites within the active high temperature salton sea geothermal system abstract previously unrecognized pulses of rhyolite volcanism occurred in the salton trough between 420 8 ka and 479 38 ka 2 based on high spatial resolution u pb zircon geochronology presently these rhyolite lavas tuffs and shallow subvolcanic sills are buried to depths between 1 6 and 2 7 km at ambient temperatures between 200 and 300 c and are overprinted by propylitic to potassic hydrothermal alteration mineral assemblages consisting of finely intergrown quartz k feldspar chlorite epidote and minor pyrite alteration resistant geochemical indicators whole rock nd isotopes zircon oxygen isotopes reveal that these rhyolites are derived from remelting of morb type crust that was chilled and hydrothermally altered by deep circulating hydrothermal waters u pb zircon dating confirms the presence of bishop tuff in well state 2 14 at 1 7 km depth approximately 5 km ne of the geothermal wells that penetrated the buried rhyolites these results indicate accelerated subsidence towards the center of the salton trough increasing from 2 2 mm a to 3 8 mm a based on these results the present day salton sea geothermal field is identified as a focus zone of episodic rhyolitic volcanism intense heat flow and metamorphism that predates present day geothermal activity and holocene volcanism by at least 400 ka

cenozoic volcanism and two stage extension in the salton trough southern california and northern baja california pronounced differences in age and geochemistry between cenozoic volcanic rocks within the salton trough southern california and northern baja california imply their formation within two distinct tectonic settings rather than the previously proposed single phase of continental rifting the emplacement of oligocene miocene alkaline lavas overlapped with crustal extension and basin formation in the region during this time post 4 ma subalkaline magmatism related to the gulf of california east pacific rise divergent plate margin exploited crust already ruptured during miocene extension new rare earth element and sr nd isotopic data and k ar 40ar 39ar and u pb ages for basalt andesite and granophyric granite xenoliths contained within the salton buttes rhyolite domes part of the post 4 ma oceanic spreading regime indicate that they are the components of a differentiated magmatic system these data also refute the presence of crystalline basement beneath this part of the salton trough we suggest that the rupture of crystalline basement beneath the axis of the salton trough was more effective during oligocene miocene time

petrology and sr nd and pb isotope geochemistry of mid ocean ridge basalt glasses from the 11 45 n to 15 00 n segment of the east pacific rise 1 basaltic glasses from the geophysically well studied section of the east pacific rise epr between 11 45 n to 15 00 n range from normal mid ocean ridge basalts morb to transitional morb and their major element variations correlate with isotopic and trace element indices of enrichment to first order basalts enriched in na8 0 incompatible elements 87sr 86sr and 206pb 204pb but low in fe8 0 and 143nd 144nd are more prevalent along the shallow portions of the ridge axis in detail the samples can be divided into two chemical and geographical groups the southern bathymetric dome extending from the 11 45 n overlapping spreading center to 14 10 n and the northern group extending from 14 10 n to the orozco transform the boundary between these two groups is apparent in a change in isotopic composition results indicate that there are three mantle source components that produce the compositional variability observed among samples from the 11 45 n to 15 00 n segment of the epr a depleted mantle component a seamount type enriched mantle component and an indian morb like mantle component south of 14 10 n the geochemical variability is dominated by binary mixing between a depleted mantle component and an enriched component similar to near ridge seamounts north of 14 10 n the low 206pb 204pb high 207pb 204pb indian morb like component exerts a major influence on the geochemical variability of the axial lavas regional averages of major element composition e g na8 0 and fe8 0 show relatively limited variability consistent with the restricted range in depth for this region and plot within the pacific field of the previously defined global trends major element variations among individual samples however parallel the global array and their correlation with indices of mantle enrichment supports the idea that the pacific type local trend results from small scale heterogeneities in the mantle beneath the epr our results also indicate that tectonic segmentation and magmatic boundaries are probably not causally related in this study area and that the sizes of the present magma chambers are not a dominant factor in determining the compositional variability of erupted lavas

trace and rare earth element geochemistry of phosphate nodules from the lower cambrian black shale sequence in the mufu mountain of nanjing jiangsu province china abstract phosphate nodules are abundant in the lowermost section of the lower cambrian black shale sequence along a ne trend belt in the yangtze platform south china in this study we examined the neoproterozoic cambrian mufu mountain section near nanjing of jiangsu province and conducted a detailed trace and rare earth element ree study on the phosphate nodules we employed a step leaching acid dissolution protocol of 1 m hac and 1 m hcl respectively to separate carbonate from phosphate mineral contributions in the nodules both the hac leached and hcl leached fractions of the phosphate nodules show similar symmetrical variations in trace and ree concentrations and ratios the total rees and a number of redox sensitive trace metals such as v cr mo ni abundances show a systematic increase from the core to rim whereas the u and mn abundances decrease from the core to rim in most of the nodules similar trends are also observed for the rb sr u th co ni and v ni ratios but these trends in the hac leached fractions are less pronounced than the hcl leached fractions all of the hcl leached fractions for the nodule cores show seawater like shale normalized ree patterns but the rims of the nodules display slightly mree enriched and hree depleted patterns we suggest that the phosphate nodules may have formed in a basinal setting beneath a stratified water column during early cambrian and the compositional variations of rees and redox sensitive trace elements from the core to rim may record a changing redox condition and fluid mixing during different stages of nodule growth the cores of the nodules preserve more pronounced negative ce anomalies and progressively hree enriched patterns which may retain primary early cambrian seawater chemistry whereas the nodule rims record chemistry consistent with changed redox conditions and or influences from pore fluids such as generated from degradation of organic matter

a model for uranium rhenium and molybdenum diagenesis in marine sediments based on results from coastal locations abstract the purpose of this research is to characterize the mobilization and immobilization processes that control the authigenic accumulation of uranium u rhenium re and molybdenum mo in marine sediments we analyzed these redox sensitive metals rsm in benthic chamber pore water and solid phase samples at a site in buzzards bay massachusetts usa that has high bottom water oxygen concentrations 230 300 mol l and high organic matter oxidation rates 390 mol c cm2 y the oxygen penetration depth varies from 2 to 9 mm below the sediment water interface but pore water sulfide is below detection release of rsm to pore waters during the remineralization of solid phases occurs near the sediment water interface at depths above the zone of authigenic rsm formation this release occurs consistently for mo at both sites but only in the winter for re in buzzards bay and intermittently for u at the buzzards bay site re removal to the solid phase extends to the bottom of the profile while the zone of removal is restricted to 2 9 cm for u and mo authigenic re formation is independent of the anoxic remineralization rate which is consistent with an abiotic removal mechanism the rate of authigenic u formation and its modeled removal rate constant increase with increasing anoxic remineralization rates and is consistent with u reduction being microbially mediated authigenic mo formation is related to the formation of sulfidic microenvironments the depth and extent of mo removal from pore water is closely associated with the balance between iron and sulfate reduction and the consumption of pore water sulfide via iron sulfide formation pore water rsm reach constant asymptotic concentrations in sulfidic sediments but only pore water re is constant at depth in buzzards bay the increases in pore water u at the buzzards bay site are consistent with addition via irrigation and subsequent upward diffusion to the removal zone deep pore water mo concentrations exceed its bottom water concentration due to irrigation induced oxidation and remobilization from the solid phase in sulfidic sediments there is no evidence for higher pore water u or mo concentrations at depth due to the absence of irrigation and or the presence of more stable authigenic rsm phases there are good correlations between benthic fluxes and authigenic accumulation rates for u and mo in sulfidic sediments however results from buzzards bay suggest irrigation ultimately results in the partial loss of u and mo from the solid phase with accumulation rates that are 20 30 of the modeled flux irrigation can augment re possibly u or compromise u mo authigenic accumulation in sediments and is important when determining burial rates in continental margin sediments

spatial variation in sediment fluxes redox conditions and productivity in the permian triassic panthalassic ocean abstract two permian triassic boundary sections in central japan provide a rare window into environmental conditions within the panthalassic ocean which encompassed more than half the earth s surface at 252 ma integration of petrographic geochemical and time series data provides new insights regarding the fluxes of major and trace components to the sediment as well as environmental conditions in both the deep and intermediate water masses at each study site the ubara section was located in a high productivity peri equatorial location whereas the gujo hachiman section was located in a moderate productivity location at some distance from the paleoequator an upward transition from gray organic poor cherts to black siliceous mudstones at both sites occurred in conjunction with increased primary productivity intensified euxinia within the oxygen minimum zone omz and decimation of the radiolarian zooplankton community euxinia in the omz of the equatorial panthalassic ocean developed episodically for a 200 250 kyr interval during the late permian followed by an abrupt intensification and lateral expansion of the omz around the permian triassic boundary throughout the study interval bottom waters at both sites remained mostly suboxic a finding that counters hypotheses of development of a superanoxic permo triassic deep ocean as a consequence of stagnation of oceanic overturning circulation

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california

oceanic magmatism in sedimentary basins of the northern gulf of california rift rift related magmatism in the northernmost gulf of california and the adjacent subaerial salton trough and cerro prieto basins comprises intermediate to rhyolitic surfi cial and buried lava fl ows and domes including their xenolith cargo in addition geothermal drill wells frequently penetrate subsurface gabbroic to granitic sills and dikes which intruded into colorado river delta fl uviatile and lacustrine sediments combined singlecrystal u th pb and u th he zircon ages reveal late pleistocene to holocene eruption ages for three volcanic centers in adjacent rift basins from n to s salton buttes eruption age 2 48 0 47 ka 95 confi dence cerro prieto maximum eruption age 73 7 ka and roca consag eruption age 43 6 ka u th zircon and allanite crystallization ages are close to the eruption ages with the exception of roca consag lava the zircon population of which is dominated by zircon with ca 1 ma crystallization ages a population interpreted to be recycled from an unknown crustal source underlying the wagner basin nd isotopic ratios for subsurface micro gabbros from cerro prieto end 8 9 overlap with values for midoceanic ridge basalts morb from the east pacifi c rise adjacent to the southern gulf of california cerro prieto microgranites and salton sea basaltic xenoliths have similarly elevated end values the lowest end value for late pleistocene holo cene igneous rocks from the northern gulf of california is for cerro prieto dacitic lava end 0 6 this value implies minor 20 assimilation of continental crustal rocks which however is an upper limit because of crystal scale evidence for magma contamination by unconsolidated sediment at the time of eruption zircon crystals in felsic rocks rhyolite lavas intrusive microgranites and granophyre xenoliths have trace element and submantle 18 o compositions that are robust indicators for a mafi c source that has exchanged oxygen by interacting with meteoric hydrothermal fl uids collectively these data imply that oceanic rifting has initiated in the salton trough and cerro prieto basins there morb type magmas formed mafi c intrusions within thick sedimentary basin fi ll where they became exposed to deep reaching hydrothermal fl uids diverse intermediate to high silica rhyolitic magmas that are prevalent at the surface are produced by fractional crystallization of mafi c parental magmas with minor assimilation of sediments or pre rift basement rocks and by partial melting of hydrothermally altered mafi c intrusions

geochemistry of basalt from escanaba trough evidence for sediment contamination mid ocean ridge basalts morbs erupted in escanaba trough introduction the southernmost segment of the gorda ridge have greater isotopic crustal contamination of basaltic magmas is generally and geochemical variability than those from the remainder of the recognized for continental volcanoes e g taylor 1980 ridge samples from the sediment free northern sites in the trough leeman hawkesworth 1986 glazner et al 1991 the are depleted normal morb similar to those from the northern composition thickness and low melting temperatures of gorda ridge however samples from the sediment covered portion continental crustal rocks facilitate assimilation that can of the trough are significantly enriched in incompatible elements be easily identified however the extent to which basaltic and sr nd and pb isotopic ratios are shifted toward the composition magmas are contaminated by oceanic crust is a topic of of local sediments pb isotopic ratios in particular lie on a mixing intense debate e g bohrson reid 1995 clague et al line with local sediments assimilation fractional crystallization 1995 eiler et al 1996 bohrson et al 1996 and references afc calculations suggest that 25 40 fractional crystallization therein thirlwall et al 1997 the debate has focused coupled with assimilation of 1 2 of local sediments can produce on distinguishing the chemical characteristics magmas the enrichment observed for some escanaba basalts variations in acquire during ascent crustal storage and emplacement the amount of enrichment of elements with similar incompatibility from those that have been used to identify heterogeneous for comparably differentiated basalts and uncorrelated isotopic mantle sources for most ocean island volcanoes the ratios suggest variable amounts of assimilation of compositionally crustal components available to serve as contaminants heterogeneous assimilants by parental magmas assimilation of are poorly characterized but include the underlying sediment by morb magmas probably resulted when laccolithic altered oceanic crust pelagic to hemipelagic sediment intrusions ponded at the sediment basement interface beneath the and the hydrothermally altered volcanic edifice thick sediment cover in the southern part of escanaba trough mid ocean ridge basalts morbs are relatively unigeochemical and isotopic evidence for contamination can be detected form geochemically with depleted incompatible element in these lavas because of the strong compositional contrast between and radiogenic isotope compositions compared with lavas the parental magma and assimilant the depleted morb magma from ocean island or continental volcanoes however and highly enriched sediment assimilant are end member comexcess chlorine content and cl k ratios in fresh morb positions that provide a sensitive indicator for the smallest amount glasses support the idea that morb magmas may also of contamination of oceanic magma by crustal processes be affected by assimilation of hydrothermally altered oceanic crust e g michael schilling 1988 michael cornell 1996 chlorine like other incompatible elements shows the greatest enrichment in strongly frac