ammonium in granites and its petrogenetic significance abstract ammonium is present as a trace constituent in all granites with an average concentration of 45 ppm nh4 equivalent to 35 ppm of elemental n it shows wide variations related to petrography and region but the only significant correlation between ammonium and other geochemical parameters is that it is most abundant in peraluminous granites and least abundant in peralkaline granites these variations can be related to a the amount of sedimentary material in the magmatic source region and b redox conditions in the source region the ammonium content of granitic magmas can also be modified by fractionation or contamination hydrothermal alteration has a major effect on the ammonium content of granitic rocks and variation due to this cause may exceed the magmatic variation most hydrothermally altered granites are enriched in ammonium as a result of the transfer of ammonium from sedimentary country rocks by the hydrothermal fluids

buried topography of utopia mars persistence of a giant impact depression knobs partially buried craters ring fractures and some mesas permit a qualitative determination of the topography buried beneath younger northern plains materials these features are widely distributed in the utopia area but are absent in a large roughly circular region centered at about 48 degree n 240 degree w this implies the existence of a circular depression about 3 300 km in diameter buried beneath utopia planitia that is here interpreted to represent the central part of a very large impact basin the presence of buried curved massifs around part of this depression and a roughly coincident mascon lend further support present topography areal geology and paleotopography of buried surfaces all point to the persistence of this major depression for almost the entire history of mars

toward a boot strap hypothesis of plate tectonics feedbacks between plates the asthenosphere and the wavelength of mantle convection abstract the solid earth system is characterized by plate tectonics a low viscosity zone beneath plates the asthenosphere and long wavelength flow in the convecting mantle we use suites of numerical experiments to show 1 how long wavelength flow and the operation of plate tectonics can generate and maintain an asthenosphere and 2 how an asthenosphere can maintain long wavelength flow and plate tectonics plate subduction generates a sub adiabatic temperature gradient in the mantle which together with temperature dependent viscosity leads to a viscosity increase from the upper to the lower mantle this allows mantle flow to channelize in a low viscosity region beneath plates an asthenosphere forms dynamically flow channelization in turn stabilizes long wavelength convection the degree of dynamic viscosity variations from the upper to the lower mantle increases with the wavelength of convection and drops toward zero if the system transitions from plate tectonics to a single plate planet the plate margin strength needed to initiate that transition increases for long wavelength cells long wavelength flow allows plate tectonics to exist over a wider range of plate margin strength the coupled feedbacks allow for a linked causality between plates the asthenosphere and the wavelength of mantle flow with none being more fundamental than the others and the existence of each depending on the others under this hypothesis the asthenosphere is defined by an active process plate tectonics which maintains it and is maintained by it and plate tectonics is part of an emergent self sustaining flow system that bootstraps itself into existence

geology the start of the world as we know it plate tectonics has created oceans and pushed up mountain ranges but when did the process that shapes the planet get going alexandra witze joins the geologists debating the issue

the diversity of tectonic modes and thoughts about transitions between them plate tectonics is a particular mode of tectonic activity that characterizes the present day earth it is directly linked to not only tectonic deformation but also magmatic volcanic activity and all aspects of the rock cycle other terrestrial planets in our solar system do not operate in a plate tectonic mode but do have volcanic constructs and signs of tectonic deformation this indicates the existence of tectonic modes different from plate tectonics this article discusses the defining features of plate tectonics and reviews the range of tectonic modes that have been proposed for terrestrial planets to date a categorization of tectonic modes relates to the issue of when plate tectonics initiated on earth as it provides insights into possible pre plate tectonic behaviour the final focus of this contribution relates to transitions between tectonic modes different transition scenarios are discussed one follows classic ideas of regime transitions in which boundaries between tectonic modes are determined by the physical and chemical properties of a planet the other considers the potential that variations in temporal evolution can introduce contingencies that have a significant effect on tectonic transitions the latter scenario allows for the existence of multiple stable tectonic modes under the same physical chemical conditions the different transition potentials imply different interpretations regarding the type of variable that the tectonic mode of a planet represents under the classic regime transition view the tectonic mode of a planet is a state variable akin to temperature under the multiple stable modes view the tectonic mode of a planet is a process variable that is something that flows through the system akin to heat the different implications that follow are discussed as they relate to the questions of when did plate tectonics initiate on earth and why does earth have plate tectonics this article is part of a discussion meeting issue earth dynamics and the development of plate tectonics

a spectrum of potentially diamondiferous lamproites and minettes from the jharia coalfield eastern india abstract abundant cretaceous 110 ma lamprophyric sills in the gondwana coalfields show a comagmatic continuum from olivine lamproites to lamproites and minettes four sills in the jharia coalfield have exotic mineralogy dominated by olivine fo 85 92 and phlogopite 5 10 tio 2 5 13 al 2 o 3 with subordinate potassium titanian richterite and or arfvedsonite and spectacularly zoned sr rich apatite up to 20 sro accessory to minor phases include clinopyroxene titanian diopside to aegirine augite k feldspar or 96 variable spinel up to 47 cr 2 o 3 perovskite niobian ilmenite niobian rutile sphene and ba poor priderite complex primary carbonate assemblages involve breunnerite ferroan dolomite witherite alstonite and strontianite the whole rocks show low al 2 o 3 5 7 combined with high k 2 o 4 5 mgo up to 30 and cr up to 1500 ppm plus extreme tio 2 up to 8 5 sr zr up to 6000 ppm each ba up to 8000 ppm and lree ce up to 3000 ppm in many rocks k al molecular 0 7 na k al 1 and k na k 0 6 radiogenic isotopes sr 7 to 21 and nd 1 5 to 3 1 imply derivation from a metasomatized lithosphere mantle source with time integrated rb sr and sm nd slightly higher than bulk earth intensive carbonation up to 14 co 2 in some rocks may reflect fluids derived from the magmas themselves from carbonatite like fluids and perhaps from coking of the invaded coal seams minor crystal fractionation within sills as little as 2 5 m thick has yielded complementary olivine lamproites and feldspathic lamproites the latter grade ultimately into minettes via increase of al 2 o 3 and decrease of mgo positive indicators of diamond potential in these rocks include their bulk compositional similarity to diamondiferous olivine lamproites elsewhere e g argyle their cr rich spinels and perhaps their tectonic position negative factors include their quiet emplacement the noted differentiation and the associated coking of the coal seams

nitrogen and carbon isotopic composition in the diamonds of mbuji mayi zaire the concentration and isotopic composition of nitrogen measured in large diamonds gram size from the mbuji mayi kimberlite district zaire show a large range of variation 100 n 2100 ppm 11 2 15n 6 0 the15n depleted nitrogen is associated with the higher nitrogen concentrations the large diamonds are individually rather homogeneous in13c range of 13c 0 9 while variations occur within small octahedral diamonds from the same district range up to 5 8 the total range of 13c variation is about the same for the large diamonds 10 5 13c 5 5 the small octahedral diamonds 10 13c 4 6 and the carbonates from local kimberlites 11 8 13c 5 5 the diamond carbon isotopic data could indicate a complex story of crystallization within a deep seated system fractionating its carbon isotopes the nitrogen results indicate that nitrogen in diamonds is on the average markedly depleted in15n weighted average 5 15 relative to atmosphere sediments and upper mantle

roman comagmatic province central italy evidence for subduction related magma genesis geochemical data on mafic volcanics show that important affinities exist between the roman and the calc alkaline rocks from the aeolian arc south tyrrhenian sea these affinities together with the close association of calc alkaline and k rich volcanics in the aeolian arc and in the naples area the continuity in the variation of abundances of incompatible elements from calc alkaline to potassic suites and the similarity in terms of major element geochemistry support a genetic relationship of the roman magmatism and the subduction processes that affected the apennines in tertiary time and are still active under the aeolian arc in the genetic model presented here both calc alkaline and k rich magmas were generated within a mantle heterogenously enriched in lil elements composition of the mantle was modified by addition of material probably sediments dragged down by the undergoing slab the geochemical and petrological differences displayed by the calc alkaline and k rich volcanics are accounted for by the different conditions of melting as well as by chemical and isotopic heterogeneities of the source 26 references 3 figures 1 table

a vestige of earth s oldest ophiolite a sheeted dike complex within the 3 8 billion year old isua supracrustal belt isb in southwest greenland provides the oldest evidence of oceanic crustal accretion by spreading the geochemistry of the dikes and associated pillow lavas demonstrates an intraoceanic island arc and mid ocean ridge like setting and their oxygen isotopes suggest a hydrothermal ocean floor type metamorphism the pillows and dikes are associated with gabbroic and ultramafic rocks that together make up an ophiolitic association the paleoarchean isua ophiolite complex these sheeted dikes offer evidence for remnants of oceanic crust formed by sea floor spreading of the earliest intact rocks on earth

geological archive of the onset of plate tectonics plate tectonics involving a globally linked system of lateral motion of rigid surface plates is a characteristic feature of our planet but estimates of how long it has been the modus operandi of

possible long term decline in impact rates 1 martian geological data abstract quantin et al quantin c allemand p mangold n delacourt c 2004a icarus 172 555 572 tabulated crater count data for 56 landslides along the walls of valles marineris under the assumption of a constant cratering rate after about 3 gyr ago as used in the 1999 2005 iterations of the crater chronology isochron system of hartmann and in the hartmann and neukum system these data indicate a regularly increasing rate of landslides which would be difficult to explain we suggest that these data may support a decline in inner solar system cratering rates by about a factor of 3 since 3 gyr ago not unlike predictions based on asteroid belt collision models such a decline is also supported by our review of data on lunar impact melts and glass spherules in a companion paper hartmann w k quantin c mangold n 2007 icarus 186 11 23 such models produce not only a more uniform rate of landslides over the last 3 gyr but also a more uniform rate of resurfacing processes which also had an apparent increase under the assumption of a constant cratering rate

initiation of subduction zones along transform and accreting plate boundaries triple junction evolution and forearc spreading centres implications for ophiolitic geology and obduction

influence of the conditions of deposition on the chemistry and the reflectance variations of the brent coals specular reflectance infrared i r microspectroscopy was performed in order to account for reflectance anomalies in the brent coal series bituminous coals of the upper jurassic of the north sea main bands of i r spectra recorded on a nicolet system 800 coupled with a nic plan microscope were integrated these data allowed us to relate two distinct reflectance trends top and basal brent coals to depositional conditions top brent coals may originate from reducing environments where organic matter was well preserved for this series defined as a reference series good correlations were obtained between the i r data based on oh aromatic and aliphatic c h as well as c o band intensity and maturity parameters such as vitrinite reflectance vr o and relative depth basal brent coals associated with coastal and upper delta plain environments may originate from more oxidizing depositional settings where organic matter was altered spectra exhibited strong oxygen bearing bands oh and c o and weak aliphatic c h bands because of such variations in the chemistry of vitrinites the reflectance measurements were strongly biased aromatic c h 3000 3100 cm 1 i r bands were also accurately studied the results suggest that aromatic c h concentration is rank controlled and independent of depositional setting large intra brent vr c variations observed in a single well 0 2 in 4 m depth were possibly related to a diagenetic oxidation process providing that all the samples were extracted from oxidizing environments those variations may also strongly correlate with an increasing degree of oxidation of the upper and coastal delta plain depositional settings these data suggest that vitrinite reflectance may be strongly influenced by oxidation and must be carefully handled when extrapolating r o data to the reconstruction of the thermal history of sedimentary basins infrared microspectroscopy is demonstrated to be particularly adapted to establish the oxidation of vitrinite that may lead to overestimation of r o values

detection of natural weathering of upper mcalester coal and woodford shale oklahoma u s a abstract natural weathering refers to the natural oxidation of organic and inorganic matter in rocks at the surface and in the shallow subsurface the objective of this study is to apply routine geochemical analyses including organic petrography to coal and shale samples extending from the weathered to unaltered zones to illustrate the effects of weathering on organic matter in coal and shale some significant observations from the upper mcalester coal and the woodford shale are 1 a sulfate index si using sulfur form analysis data that reveals the transformation of pyrite to sulfate minerals is a very sensitive weathering indicator it increased in weathered samples 2 rock eval pyrolysis data such as hydrogen index hi and oxygen index oi are very sensitive to weathering oi hi increased in weathered samples 3 vitrinite reflectance decreased in weathered samples and 4 petrographic signs of weathering include microfractures pitted surface dark reaction rims and high relief sulfate index and petrographic signs of weathering should be used on surface and shallow subsurface rock samples as an indication of the extent of weathering the validity of other analyses can then be evaluated accordingly

on the initiation of subduction estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation three environments proposed as preferential sites of incipient subduction are investigated passive continental margins transform faults fracture zones and extinct ridges none of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins the attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates this is the only tectonic force sufficient to trigger the nucleation of a new subduction zone the ubiquitous distribution of transform faults and fracture zones combined with the common proximity of these features to mature subduction complexes suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere

the evolution of hadean eoarchaean geodynamics abstract geodynamic modelling of hadean eoarchaean tectonics typically requires higher rates of internal heat production and higher mantle temperatures in models that possess temperature dependent viscosity and a yield criterion in such models under hadean conditions for a wide range of geodynamic configurations and modelling approaches subduction has a propensity to fail this has led to the suggestion that the predominant tectonic regime in the hadean was stagnant lid convection with intermittent recycling events various lines of evidence support this suggestion from i the long mixing time of mantle isotopic anomalies or compositional heterogeneities such as 142nd 182w and platinum group elements to ii the long residence time of the hadean protolith to the jack hills zircons and iii thermal evolution models which typically require lower heat flux in the past to avoid the archaean thermal catastrophe the framework provided by stagnant lid or episodic overturn convection iv provides an explanation for the formation of early archaean ttgs and greenstones and v explains the interleaving arc plume sequence observed in many archaean terranes suggesting subduction initiation events may have been common increasing their preservation potential implications include a low magnetic field strength in the hadean which is consistent with emerging paleointensity data from these times

giant quartz vein systems in accretionary orogenic belts the evidence for a metamorphic fluid origin from δ 15 n and δ 13 c studies abstract nitrogen isotope compositions of hydrothermal micas in giant structurally hosted quartz vein systems place constraints on the origin of the hydrothermal fluids the vein systems are from neoarchean terranes in the superior province of canada and the norseman terrane western australia the four quartz vein systems studied in different terranes formed at metamorphic grades ranging from lower greenschist to lower amphibolite facies coevally with metamorphism of the host terrane nitrogen resides predominantly as nh 4 in hydrothermal mica for lower to upper greenschist facies quartz veins n contents and 15 n values of micas are between 40 and 200 ppm and 15 and 21 respectively whereas in quartz veins formed at the greenschist to amphibolite transition and lower amphibolite facies micas have n contents of 20 70 ppm and 15 n of 11 24 in contrast micas and k feldspars from granitoids in the neoarchean abitibi and red lake greenstone belts are characterized by systematically lower 15 n of 5 to 5 and generally lower n contents of 20 50 ppm comparable to other granitoids carbon and oxygen isotope compositions of hydrothermal ferroan dolomite and calcite show systematic depletions with increasing metamorphic grade the mean values range from 2 2 to 3 7 for 13 c and 13 8 to 12 3 for 18 o in veins formed at lower to upper greenschist facies but from 5 8 to 7 1 for 13 c and 11 4 to 9 8 for 18 o at higher metamorphic grades nitrogen isotope compositions rule out mantle 15 n mean 5 magmatic 15 n 5 to 15 n mean 4 4 2 0 for the quartz veins accordingly the results are consistent with fluids derived from metamorphic dehydration of oceanic crust and sediments within accretionary orogenic belts

deep fluids and volatile recycling crust to mantle abstract there is now clear evidence that geofluids are involved in many stress release processes at many depths modern studies of crustal regions show that surface fluids may penetrate to depths of many kilometres at subduction zones volatiles may be recycled to mantle depths there is increasing evidence for the complexity of subduction structures with flaking off of major fragments which may devolatilize and revolatilize lower crust and upper mantle the same processes may be associated with massive magma underplating of continental crust the new techniques of modern surface chemistry open new possibilities for the study of fluid tracks thin films on mineral grains which may influence many properties like deep electrical conductivity

kinetic nitrogen isotope fractionation associated with thermal decomposition of nh3 experimental results and potential applications to trace the origin of n2 in natural gas and hydrothermal systems abstract ammonia nh3 is the major intermediate phase in the pathway of nitrogen n transfer from the fixed n phases e g in crustal material to free n2 e g in natural gas reservoirs and volcanic gases yet the n isotopic behavior during these n cycling processes remains poorly known in an attempt to contribute to the understanding of n cycling using n isotopes we carried out laboratory experiments to investigate the n isotopic effect associated with thermal decomposition of ammonia 2nh3 n2 3h2 pure nh3 with initial 15 n nh 3 of 2 relative to air standard was sealed into quartz tubes and thermally decomposed at 600 700 or 800 c from 2 hours to 500 days with the progress of the reaction the 15n of the remaining nh3 and the accumulated n2 increased from 2 to 35 and from 20 to 2 respectively the differences of the n isotope fractionations at the three temperatures are not significant modeling using the rayleigh distillation model yielded similar kinetic n isotope fractionation factors n 2 nh 3 of 0 983 0 002 for 600 700 and 800 c applied to geological settings this significant isotope discrimination 17 associated with partial decomposition of nh3 nh4 from crustal sources 15naverage 6 3 can produce mantle like i e 5 or even lower 15n values of n2 this may explain the large variation of 15n 20 to 30 of n2 in natural gas reservoirs it can also possibly explain the extreme 15n depletion of n2 in some volcanic gases this possibility has to be carefully considered when using n isotopes to trace geological n cycling across subduction zones by analysis of volcanic n2

the geochemistry of potassic lavas from vulsini central italy and implications for mantle enrichment processes beneath the roman region major and trace element and 143nd 144nd 0 51209 0 51216 and 87sr 86sr 0 70879 0 71105 isotope analyses are presented on a representative group of lavas from the vulsini district of the roman magmatic province three distinct series are identified the high k and low k series are similar to those described from other italian volcanoes while the third is represented by a group of relative ly undifferentiated leucite basanites which are thought to be near primary mantle melts major and trace element variations within the high k series are consistent with fractional crystallisation from a parental magma similar to the most magnesian leucitites crustal contamination resulted in an increase in 87sr 86sr with increasing fractionation but it was superimposed on magmas which had already inherited a range of incompatible element and isotope ratios from enrichment processes in the sub continental mantle these are reviewed using the available results from vulsini roccamonfina and ernici transition element abundances and ta yb ratios indicate that the pre enrichment mantle was similar to that of e type morb and that these elements were not mobilised by the enrichment process mixing calculations suggest that three components were involved in the enrichment process mantle comparable with the source of morb and two other components rich in trace elements one the low k component had high sr nd th ta and ba nb and no europium anomaly while the second had lower sr nd a negative europium anomaly and very high th ta it was also characterised by low nb ba and high rb ba ratios similar to those reported from phlogopite rich peridotite xenoliths the trace element enrichment processes are therefore thought to have occurred in the mantle wedge above a subduction zone with the trace element characteristics of the high k end member reflecting the subduction of sediments and the stabilisation of mantle phlogopite

on the initiation of subduction zones analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere numerical model studies show that if after a short evolution of a passive margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin

the martian hemispheric dichotomy may be due to a giant impact mars is divided into two fundamentally different geological provinces of approximately hemispherical extent1 3 the more southerly province is heavily cratered contains relatively old geological units and superficially resembles the lunar and mercurian highlands the northern province is relatively lightly cratered and contains younger geological units including extensive plains volcanic edifices and volcanic calderas except for the tharsis and elysium regions and other large volcanoes most of the younger northern province consists of lowlands which lie an average of 3 km below the highlands lowlands occupy about one third of mars they are separated from the highlands by a distinct scarp or by a sloping transitional zone as much as 700km wide in which highland materials have been disrupted and partly replaced by lowland deposits fig 1 the transition is characterized by a variety of landforms unknown on other planets the highlands and lowlands are in isostatic equilibrium across the transitional zone4 no generally accepted explanation for the cause of the highland lowland dichotomy has been proposed although thinning of the lithosph re in the northern hemisphere by internal processes has been suggested5 large impacts which were a feature of early solar system history have also been suggested as an important cause of such early planetary surface heterogeneities3 6 7 subcircular lowlands such as argyre chryse hellas and isidis planitia and the plains south of tharsis occupy previously recognized impact basins8 10 we propose here that the largest expanse of lowlands also results from formation of a large impact basin early in mars history that has profoundly shaped the planet s surface

nitrogen isotope geochemistry of organic matter and minerals during diagenesis and hydrocarbon migration the magnitude of isotopic variations between organic and inorganic nitrogen was examined in samples from three stacked hydrocarbon reservoirs in the fordoche field louisiana gulf coast basin usa measurements were made of 15n in kerogen bitumen oil formation water and fixed nh4 extracted from mudstones nonproductive sandstones and productive sandstones nitrogen isotope fractionation occurs because 14n is released preferentially to 15n from organic molecules during thermal maturation released 14n goes into solution or may be adsorbed by minerals leaving crude oil enriched in 15n diagenetic clay minerals e g illite commonly form in the temperature range of hydrocarbon generation and nh4 may be fixed in clay interlayers with an isotopic ratio similar to that of the migrating fluids results indicate that the influence of organic matter on mineral 15n depends on the timing of authigenic mineral formation relative to fluid migration the average 15n of kerogen 3 2 0 3 and fixed nh4 from mudstones 3 0 1 4 is similar while bitumen increases from 3 5 to 5 1 with depth in deep reservoir sandstones 100 c the 15n of crude oil averages 5 2 0 4 similar to the 15n of bitumen in the proposed source rocks formation waters are 14n enriched with an average 15n of 2 2 2 6 fixed nh4 15n values lie between that of the oil and water the average 15n of fixed nh4 is 3 0 1 2 in productive sandstones and 0 2 2 4 innonproductive sandstones in the shallower reservoir sandstones 90 c fixed nh4 is apparently not influenced by the presently associated fluids productive and nonproductive sandstones have distinctly low average 15n values 1 2 0 8 yet crude oil 11 1 0 3 and water 3 8 0 1 have been 15n enriched by 6 relative to the deeper reservoirs this suggests that the present fluids migrated into the reservoir after authigenic illite had formed fluids become enriched in 15n during migration and the amount of enrichment may be a function of the amount of interaction with argillaceous sediments

impact driven subduction on the hadean earth impacts could have driven transient subduction events on the hadean earth according to numerical simulations the scenario reconciles evidence for tectonic activity with that for an otherwise tectonically stagnant early earth

constraints on plate tectonics initiation from scaling laws for single cell convection abstract the earth is the only planet known to have plate tectonics while other planets are covered with a stagnant lid on the earth the initiation of subduction which is thought to be the fundamental process for plate tectonics initiation is caused not only by the negative buoyancy of the lithosphere but also by the forces from plate motions however for planets which do not have plate tectonics the very first episode of lithospheric failure has to be caused by forces other than plate motions sublithospheric convection has been proposed as a possible mechanism that provides lithospheric instability through inducing stresses in the lithosphere and lithospheric failure can occur when the yield stress is below a critical value we test the applicability of scaling laws for the critical yield stress obtained in single cell convection simulations to strongly time dependent multi cell systems we show that with an appropriate choice of characteristic aspect ratio for the convective system the scaling laws from single cell simulations can be used to evaluate the conditions on the terrestrial planets in the inner solar system for plate tectonics to exist in agreement with previous studies the estimated values for critical yield stress and coefficient of friction are much lower than the expected values for the earth s lithosphere

geochemistry and mineralogy of the minette lamproite association from the indian gondwanas abstract in northeast india early cretaceous lamprophyres intrude both the gondwana permian lower cretaceous sedimentary sequence and older cratonic blocks spessartite is the typical lamprophyre of the proterozoic meghalaya craton whereas a minette lamproite suite intrudes the gondwana coal bearing sequence cores from exploration boreholes and rocks from deep coal mines have provided excellent fresh samples the petrography mineral chemistry geochemistry and isotope geochemistry of such samples have been examined essential characteristics of the minette lamproite suite are a depletion in ca al and na b enrichment in k ti and other incompatible elements and c presence of phlogopite megacrysts of possible mantle origin preliminary sr pb and nd isotopic data demonstrate significant differences between the minette lamproite suite and the spessartites it is concluded that the minette lamproite suite evolved from a primary magma generated by the partial melting of a carbonate bearing harzburgite that had been extensively metasomatised

nitrogen isotope fractionations during progressive metamorphism a case study from the paleozoic cooma metasedimentary complex southeastern australia abstract the well studied paleozoic cooma metamorphic complex in southeastern australia is characterized by a uniform siliciclastic protolith of uniform age with a continuous range of metamorphic grade from subgreenschist to upper amphibolite facies and migmatite grade in an annular pattern around the cooma granodiorite those conditions are optimal for investigating variations of n concentrations and 15 n values during progressive metamorphism nitrogen concentrations decrease and 15 n increases with increasing metamorphic grade sub chlorite zone 120 ppm n 15 n 2 3 chlorite zone 110 ppm n 15 n 3 0 biotite and andalusite zone 85 ppm n 15 n 3 8 sillimanite and migmatite zones 40 ppm n 15 n 10 7 covariation of k and n contents is consistent with n substituting for k as nh 4 in micas observed trends of increasing 15 n values with decreasing nitrogen concentrations can be explained by a continuous release of nitrogen depleted in 15 n with progressive metamorphism which causes an enrichment of 15 n in the residual nitrogen of the rock equilibrium models for rayleigh distillation and batch volatilisation for data of the greenschist and amphibolite facies metasedimentary rocks can be explained by n 2 nh 4 exchange at temperatures of 300 600 c whereas observed large fractionations for the upper amphibolite facies and melt products in the migmatite grade samples may be interpreted as nh 3 nh 4 exchanges at temperature of 650 730 c lower values in the highest grade zones may also stem in part from input of 15 n depleted fluids from the granodiorite the magnitude of isotope fractionation of nitrogen is about 1 2 during progressive metamorphism of metasedimentary rocks from sub chlorite zone to biotite andalusite zone which is consistent with previous studies consequently the large spread of 15 n values in archean greenschist facies metasedimentary rocks of 6 to 30 can be accounted for by variable mixtures of mantle plume dominated volatiles with a 15 n of 5 and a 15 n enriched marine sedimentary kerogen component inherited from a ci chondrite veneer having 15 n of 30 to 42

semianthracite anthracite and meta anthracite in central canadian cordillera their geology characteristics and coalification history abstract in the jurassic and early cretaceous bowser basin and late cretaceous sustut basin in the north central canadian cordillera significant resources of coal ranging in rank from medium volatile bituminous coals to meta anthracite occur coal seams are up to 5 m thick have calorific values daf ranging up to 39 400 kj kg and fixed carbon daf up to 99 the coal seams are vitrinite rich have moderate to high sulphur contents and are variably argillaceous in the groundhog coal field of the bowser basin the coals are mainly anthracites and meta anthracites which are commonly cut by carbonate and quartz veins and are variably oxidized the orientation of the optical indicating surface together with regional maturation patterns suggest that the coalification in the groundhog coal field was pre tectonic and thus pre mid cretaceous extrapolation of the measured coalification gradients suggests that the coal measures were buried to depths of the order of 5000 m stratigraphic variation in the coalification gradients exist and are a product of varying thermal conductivity of the strata higher coalification gradients and levels of coalification in the southern part of the coal field reflect a combination of deeper depth of burial and higher paleoheat flow numerical modeling of the coalification gradients and burial histories suggest that paleoheat flows on the order of 80 mw m2 existed such values are consistent with deposition in a back arc basin in the sustut basin the coal ranges in rank from medium volatile bituminous coal to semianthracite and is up to about 2 m thick preliminary analyses indicate a good correlation between levels of coalification and stratigraphy which suggests that the coalification is pre tectonic pre mid tertiary the prominent variation in coal rank in the sustut basin is probably a result of the marked variation in depth of burial of the strata in isolated fault bouuded basins

the measurement of δ15n in crustal rocks by static vacuum mass spectrometry application to the origin of the ammonium in the cornubian batholith southwest england the release of ammoniacal nitrogen from muscovite biotite orthoclase microcline and a whole rock granite by vacuum pyrolysis has been investigated under the heating regimes employed it was found that micas lose most of their nitrogen below the temperature at which the powders begin to sinter ca 1100 c the peak release of n occurring in either the 900 c or 1000 c heating step it is possible that the loss of ammonium is linked to the loss of h2o and perhaps f for the feldspars there was insignificant release of n below the melting point of the minerals about 1150 c suggesting that diffusion of ammonium or ammonia is the principal control for both micas and feldspars there was significant kinetic fractionation of the nitrogen isotopes during stepped heating from these results a technique has been devised for the routine determination of 15n and ammonium concentration in whole rock granites by static vacuum mass spectrometry the ammonium concentrations and 15n values have been determined in whole rock samples from the cornubian batholith southwest england the ammonium concentrations were in the range 8 to 187 ppm and the 15n values were in the range 5 1 to 10 2 most of the granite samples had 15n values between 8 4 and 10 2 and are within the range that have been obtained from amphibolite facies metasediments these granites are also typified by high 87sr86srinit 0 7130 to 0 7166 and high normative corundum 2 31 to 2 55 taken together the data suggest that the ammonium present within these granites is of biological origin and that it was incorporated via the anatexis of metasediments which originally contained organic material the samples were collected over a distance of 120 km and the limited range in 15n suggests either 1 1 the 15n values within the metasedimentary protolith were rather homogeneous or 2 2 the nitrogen isotopes were homogenised during the incorporation of ammonium into the granite magmas two of the granites had lower 15n values of 5 1 and 7 0 these granites also have lower ammonium concentrations 8 and 15 ppm lower 87sr86srinit 0 7094 0 7095 and lower normative corundum 1 44 and 1 63 the preferred explanation is that they were derived from or have had a contribution from a second crustal source containing sedimentary or metasedimentary material with a lower proportion of pelitic to other constituents

a comparative study on structural differences of xylite and matrix lignite lithotypes by means of ft ir xrd sem and tga analyses an example from the neogene greek lignite deposits abstract the ft ir spectra for both neogene xylite and matrix lignite samples from six different greek lignite deposits nw greece show significant differences in particular in the aliphatic stretching region 3000 2800 cm 1 the intensities of the vibrations are more prominent in the xylite lithotype as in the matrix lignite lithotype the intense bands in the region 3402 3416 cm 1 are attributed to oh stretching of h2o and phenol groups for both xylite and matrix lignite lithotypes the bands at 3697 cm 1 and 3623 cm 1 as well as at 538 cm 1 and 470 cm 1 which are more evident in the ft ir spectra of the matrix lignite are attributed to a higher clay content in the samples of this lithotype data resulting from x ray diffraction analysis xrd scanning electron microscopy sem and thermoanalytical methods tg dtg and dta indicate main differences of the xylite and matrix lignite lithotype as well typical peaks in x ray diagrams confirm the high content of clay minerals in matrix lithotype and their minor contribution in the xylite lithotype scanning electron microscopy sem combined with the ft ir and xrd results reveals prevalence of gypsum pyrite marcasite quartz and clays in the matrix lignite lithotype as compared with the xylite lithotype and also textural differences such as the heterogeneity in the mass of matrix lignite and the homogeneity of the xylite compositional differences of the examined lignite materials as identified by ft ir were also confirmed by tg dtg dta while cellulose decomposition was observed by an endothermic peak of dta curves at around 360 c in the xylite samples lignin degradation became a prominent process observed in the temperature range from 200 c to 400 c in the matrix lignite samples this was also observed by an exothermic peak on dta curves each of the heat effects was accompanied with a partial mass loss registered on tg curves as a whole the data resulting from the combined research by means of ft ir xrd sem and tg dtg dta of both xylite and matrix lignite lithotypes confirm the significant differences between these two lignite lithotypes reflecting their different structure texture and maturity stage

distribution of ammonium in minerals of metamorphic and granitic rocks abstract sixty nine analyses are given for nh4 in minerals of metamorphic and granitic rocks mostly from the ryoke belt japan the distribution of nh4 in coexisting minerals is quite systematic suggesting that nh4 is one of the stable geochemical components in high temperature processes biotite has the highest content of nh4 followed by muscovite k feldspar and plagioclase pure quartz is almost free from nh4 calcic plagioclase contains less nh4 than does sodic plagioclase the partition coefficients d pl bi d kf bi and d kf bi are on the average 0 11 0 38 and 0 43 respectively the fractionation of nh4 in these minerals is quite similar to that of rb but much smaller than that of cs distribution of nh4 as well as those of rb and cs appears to be explained by its ionic radius and the shortest cation o distances in alkali positions of minerals

stable isotope o h s c and n systematics of quartz vein systems in the turbidite hosted central and north deborah gold deposits of the bendigo gold field central victoria australia constraints on the origin of ore forming fluids the central and north deborah gold deposits are located in one of 15 mining centers of the bendigo gold field central victoria australia a major world class gold province developed in a turbidite terrane gold mineralization occurs primarily within domes and reverse fault systems in multiply deformed lower ordovician metaturbidites deformation and thrusting occurred between 450 and 420 ma coevally with regional metamorphism at zeolite to greenschist facies in the central and north deborah deposits six stages of quartz veining each with characteristic geometry and mineral paragenesis developed sequentially stages 1 2 and 4 are gold bearing whereas stages 3 5 and 6 are barren gold related quartz veins were deposited at temperatures of 350 25 c based on oxygen isotope geothermometry of silicate phases quartz muscovite and sulfur isotope compositions of coexisting sulfide mineral pairs sphalerite galena this result is consistent with temperatures inferred from fluid inclusions the 18o values of gold bearing vein quartz and coexisting hydrothermal muscovite are uniform within and between the six vein stages ranging from 15 9 to 17 2 per mil and 11 6 to 12 8 per mil respectively the d values of hydrothermal muscovites are also uniform at 68 to 55 per mil ore fluids have calculated 18o values of 8 to 11 per mil and d values of 37 to 17 per mil pyrite of synsedimentary diagenetic origin in slate and sandstone displays sulfur isotope compositions of 23 5 to 8 0 per mil and 0 1 to 11 9 per mil respectively sulfides from gold bearing quartz veins have a range of 7 4 to 8 1 per mil median ca 2 5 the relatively uniform vein pyrites indicate a large uniform sulfur source most likely from desulfidation reactions of sulfide minerals interaction of the ore forming fluid with depleted sulfides at the site of deposition may have shifted the sulfur isotope signatures of source sulfides to more negative values carbon isotope compositions of carbonates range from 5 1 to 9 0 per mil median ca 6 5 nitrogen concentrations and nitrogen isotope compositions of hydrothermal muscovite from quartz veins are 652 to 895 ppm and 2 84 to 4 49 per mil respectively collectively the data define a band in n content vegh rsus 15n coordinate space from higher n and lower 15n in the phanerozoic metasedimenary hosted central and north deborah gold deposits to lower n and higher 15n in predominantly meta igneous hosted neoarchean gold provinces collectively the h o s c and n isotope compositions of ore related hydrothermal minerals indicate that the dilute aqueous carbonic and n bearing ore forming fluids were generated from metamorphic dehydration reactions at deeper crustal levels and were variably influenced by exchange with wall rocks during transport to the site of deposition this genetic model for the central and north deborah gold deposits is consistent with other turbidite hosted lode gold deposits and counterparts in volcanic plutonic terranes in contrast these data are not consistent with either mantle derived fluids or magmatic fluids nor do they support a deeply circulated meteoric surface water model for vein formation

high resolution tephra and u pb chronology of the 3 33 3 26 ga mendon formation barberton greenstone belt south africa abstract the mendon formation in the barberton greenstone belt of south africa marks the boundary between the onverwacht and fig tree groups these groups are characterized by mafic to ultramafic volcanism and felsic volcanism with related epiclastic sedimentation respectively this transition marks the end of komatiitic volcanism in the barberton greenstone belt and is accompanied by numerous impact related spherule layers this study characterizes the upper mendon formation texturally and geochemically over a wide areal extent and across structure and facies changes in an attempt to better understand the evolution of tectonic processes at this boundary a suite of whole rock and handheld x ray fluorescence analyses are presented in conjunction with textural information stratigraphic relationships and u pb ages to create a temporal and chemostratigraphic framework for the mendon formation local and regional stratigraphic variations including absence of distinctive layers and variation in layer thickness seen across the mendon preclude ascription of a single stratigraphy that accurately describes the 1 2 km of section present in this formation these variations indicate diachronous deposition of the mendon formation over a wide areal extent and into multiple basins or sub basins by more than one magmatic source 204 pb corrected 206 pb 238 u and 207 pb 235 u concordia model ages of 3279 9 1 ma and 3287 3 2 9 ma for two samples from upper portions of the mendon formation provide temporal context for deposition two samples from the basal 10 m of the fig tree group above the s2 spherule bed that marks the boundary between the onverwacht and fig tree groups give model ages of 3267 8 6 9 ma and 3261 18 ma these ages provide added constraints for the onverwacht fig tree boundary and confirm that the weltevreden formation is roughly age correlative with the uppermost mendon formation while the mendon and weltevreden formations are in part age correlative and have similar lithologies they do not appear to be genetically related the dominance of ultramafic volcanic rocks and the paucity of felsic volcanic and terrigenous sedimentary rocks within the mendon and weltevreden formations indicate that the primary mode of crustal formation was likely plume related magmatic accretion and not subduction the relatively sharp transition within the barberton greenstone belt from ultramafic volcanic sequences to more felsic volcanic and epiclastic sedimentary sequences is everywhere marked by impact related spherule layers which suggest that major impacts may have played a role in the evolution of early tectonics to more modern subduction related styles

isotopic evidence for the cretaceous tertiary impactor and its type high precision mass spectrometric analysis of chromium in sediment samples from the cretaceous tertiary k t boundary coincident with the extinction of numerous organisms on earth confirms the cosmic origin of the k t phenomenon the isotopic composition of chromium in k t boundary samples from stevns klint denmark and caravaca spain is different from that of earth and indicates its extraterrestrial source the chromium isotopic signature is consistent with a carbonaceous chondrite type impactor the observed differences in the chromium isotopic composition among various meteorite classes can serve as a diagnostic tool for deciphering the nature of impactors that have collided with earth during its history

origin of the martian global dichotomy by crustal thinning in the late noachian or early hesperian the marked dichotomy in topography surface age and crustal thickness between the northern lowland and southern upland of mars has been explained as due to an initially inhomogeneous crust a single mega impact event several overlapping large basin impacts and first order convective overturn of the martian mantle all of the published hypotheses propose that the dichotomy was formed early in martian history before the end of the primordial heavy bombardment a primordial origin is inherent in the initial crustal inhomogeneity hypothesis and required for both impact hypotheses endogenic hypotheses are not so constrained geological data indicate episodes of fracturing and faulting in the late noachian and the early hesperian this fracturing and faulting occurred primarily within the northern lowland and along the boundary between lowland and highland igneous activity also peaked in the late noachian and early hesperian these data suggest a tectonic event near the noachian hesperian boundary characterized by enhanced heat loss and extensive fracturing including formation of the faults that define much of the highland lowland boundary we argue that the major result of this tectonic event was formation of the dichotomy by thinning of the crust above a large convection cell or plume

molecular structure of reactive coals during oxidation carbonization and hydrogenation an infrared photoacoustic spectroscopic and optical microscopic study abstract coals from western canada were examined for structural changes occurring during oxidation carbonization and hydrogenation by means of ft ir spectroscopy optical microscopy and chemical analysis the coals are of cretaceous to eocene age ranging in rank from subbituminous c to medium volatile bituminous 0 42 1 47 r o results indicate that the reactivity of the coals during the above mentioned processes depends on rank and petrological composition infrared spectroscopy is shown to be sensitive to the molecular structural changes occurring during these processes and provides a chemical basis for interpreting data from chemical analysis and optical microscopy

icp ms a powerful tool for high precision trace element analysis in earth sciences evidence from analysis of selected u s g s reference samples abstract inductively coupled plasma mass spectrometry icp ms is a powerful analytical technique with considerable application to geochemistry in this paper we demonstrate the power of icp ms as a tool in petrogenetic studies data for 28 elements in seven u s g s basalt and andesite reference samples agv 1 bcr 1 bhvo 1 bir 1 dnc 1 w 1 and w 2 are presented accuracy and precision standard deviation for 26 of these elements is either excellent a detailed description of the icp ms technique we have developed is given a simple hf hno 3 dissolution in screwtop teflon bombs was used for sample preparation icp ms instrumental sensitivity was measured by external solutions with surrogate calibration for 4 elements matrix correction was made by use of standard addition naturally occurring internal standards rb y ce and pb were used to correct for instrumental drift between sample and spiked sample measurements we report data for the elements li cs rb sr ba zr hf nb ta y sc u th pb and all 14 of the ree in addition the procedure acquired data for be mo w tl and bi many elements in this array are crucial for the evaluation of petrogenetic models and source tracing herein lies the real power of icp ms one technique one dissolution small sample size good limits of detection good to excellent accuracy and good precision for many geochemically important elements

identifying impact events within the lunar cataclysm from 40ar 39ar ages and compositions of apollo 16 impact melt rocks abstract lunar impact melt breccias provide a unique record of the timing and frequency of collisional events during the early history of the inner solar system prior to the development of a significant rock record on earth the predominance of ages clustering between 3 8 and 4 0 ga was a major unexpected discovery obtained from geochronological studies of lunar impact melts and is the basis of the concept that a cataclysmic bombardment of large planetesimals struck the earth and moon and possibly the entire inner solar system about 3 85 0 10 billion years ago as a test of the cataclysm hypothesis we measured high resolution 20 50 steps 40 ar 39 ar age spectra on 25 samples of apollo 16 impact melt breccias using a continuous laser heating system on sub milligram fragments twenty one of these 25 breccias produced multi step plateaus that we interpret as crystallization ages with 20 of these ages falling in the range 3 75 3 96 ga we propose that at least four different melt producing impact events can be distinguished based on the ages bulk compositions and petrographic characteristics of apollo 16 melt breccias the recognition of multiple impact events within the apollo 16 melt breccia suite shows that numerous impact events occurred on the lunar surface within a relatively narrow time interval providing additional evidence of a heavy bombardment of the moon during the early archean

state of stress and age offsets at oceanic fracture zones and implications for the initiation of subduction abstract the recycling of oceanic lithosphere back into the earth s interior through subduction is a central component of plate tectonics the process by which new subduction zones initiate however remains poorly understood several different mechanisms for subduction initiation have been proposed including passive margin collapse aided by sediment loading and or rheological weakening due to the presence of volatiles and forced convergence across a zone of preexisting lithospheric weakness in this paper we focus on the latter type of model which identifies three conditions necessary for subduction initiation a zone of weakness such as a fracture zone an age and therefore density offset along the fracture zone and significant normal compressive stress which leads to shortening we identify regions on the present day earth which meet these conditions and which may correspond to regions of relatively likely subduction initiation in the near future using a digital seafloor age model we have created a database of oceanic fractures and quantified the associated age offsets we have evaluated the state of stress on these lithospheric weak zones using two different global stress models we find that the conditions needed to initiate subduction via the forced convergence model are relatively rare on the present day earth and that there is little indication of incipient subduction at regions identified as relatively likely for subduction initiation using the same technique we have evaluated the state of stress and seafloor age offset at regions of inferred present day incipient subduction and find that most of these regions are not associated with both high far field compressive stresses and large age and thus density offsets subduction has likely initiated via forced convergence across preexisting zones of lithospheric weakness in the past but our results indicate that the conditions needed for this type of subduction initiation are rare on the present day earth

global tectonic evolution of venus from exogenic to endogenic over time and implications for early earth processes venus provides a rich arena in which to stretch one s tectonic imagination with respect to non plate tectonic processes of heat transfer on an earth like planet venus is similar to earth in density size inferred composition and heat budget however venus lack of plate tectonics and terrestrial surficial processes results in the preservation of a unique surface geologic record of non plate tectonomagmatic processes in this paper i explore three global tectonic domains that represent changes in global conditions and tectonic regimes through time divided respectively into temporal eras impactors played a prominent role in the ancient era characterized by thin global lithosphere the artemis superstructure era highlights sublithospheric flow processes related to a uniquely large super plume the fracture zone complex era marked by broad zones of tectonomagmatic activity witnessed coupled spreading and underthrusting since arrested these three tectonic regimes provide possible analogue models for terrestrial archaean craton formation continent formation without plate tectonics and mechanisms underlying the emergence of plate tectonics a bolide impact model for craton formation addresses the apparent paradox of both undepleted mantle and growth of archaean crust and recycling of significant archaean crust to the mantle this article is part of a discussion meeting issue earth dynamics and the development of plate tectonics

cooling of lava flows on venus the coupling of radiative and convective heat transfer 1 a lava flow extruded on the surface of venus cools by radiating heat and by advecting heat upward by thermal convection unlike on other planetary bodies these processes are intimately coupled on venus because the dense co2 rich atmosphere has strong absorption bands in the infrared relative to a transparent atmosphere absorption and emission of radiation within short distances above a lava reduces the radiant heat loss and substantially diminishes the vigor of convection by dampening the thermal gradients within the boundary layer for these reasons lavas on venus cool significantly more slowly 30 40 than they would if the atmosphere were transparent in the infrared as assumed in previous calculations considering these effects indicates that lavas on venus cool more slowly than their equivalents on earth rather than more rapidly as previously estimated the reduction in cooling rate due to the suppression of convection by radiation extends our understanding of how some lavas on venus reach extraordinary lengths on the order of a thousand kilometers before solidifying

coal petrographic genetic units in deltaic plain deposits of the campanian mesa verde group new mexico usa the campanian rocks of the mesa verde group in the san juan basin new mexico usa form a littoral wedge composed of offshore shoreface and onshore facies this siliciclastic wedge was interpreted as a landward to seaward stepping of grouped genetic units i e intermediate term cycles the continental facies consist of coastal plain deposits coals shales and sandstones represented by three main domains progressively distant from the palaeoshoreline the deltaic plain the intermediate plain and the alluvial plain domains in the deltaic plain grouped aggradational retrogradational genetic units consist of preserved coal shale doublets alternating with thick shales although previous works have identified genetic units in the shoreface facies the existence of the latter has not been proven in the coastal plain deposits as coal shale doublets may represent such genetic units an organic petrography study was performed at high resolution on an 80 cm thick 1 5 km long coal seam to demonstrate this hypothesis in this work the depositional environments of the initial peatlands were rebuilt utilizing two complementary methods maceral and microlithotype analyses the results reveal the existence of internal sequences in the coal seam corresponding to recurrent patterns of a palaeogeographical model proposed for a littoral domain the authors interpret this coal sequence as the record of successive progradational and aggradational events furthermore a previous study of the organic content of the shales located above the coal demonstrated that the preservation of these facies is related to higher accommodation peat production and accommodation sedimentary supply ratios coal shale doublets are then interpreted as elemental genetic units of the deltaic plain formation this study demonstrates that contrary to original concepts littoral coal seams are not just markers of stable shoreline conditions at a high time scale but also record landscape migration

subduction origin on early earth a hypothesis i propose a hypothesis for the origin of subduction on early earth that is directly coupled with mantle upwelling and the formation of mafi c crust the hypothesis invokes spatial and temporal overlap of both endogenic and exogenic processes broad quasi cylindrical mantle upwelling and large bolide impact respectively leading to subduction and spreading two signature processes of modern plate tectonics the spatial and temporal intersection of these processes could have occurred variably across early earth and thus subduction could have begun at different locations and times globally the hypothesis postulates that the ability of a terrestrial planet to evolve plate tectonics results from a balance between the strength of its lithosphere and the size bolide it can attract and survive both factors to a fi rst order are a function of planet size

on the emergence of plate tectonics at present young oceanic lithosphere is positively buoyant and it does not become negatively buoyant until it is older than about 20 m y if in the past the mantle was hotter the oceanic crust would have been thicker and the lithosphere9s age of neutral buoyancy greater on the other hand the hotter mantle would have had a lower viscosity and convected faster so the average age of oceanic plates at subduction would have been less than the present 100 m y at some time in the past average oceanic lithosphere would have only just become negatively buoyant as it reached a subduction zone it is estimated here that this condition occurred when the mantle was only about 50 c hotter than at present and as recently as 0 9 1 4 b y ago transformation of basaltic crust to eclogite might have enhanced the vigor of the plates but possibly only modestly and intermittently in earlier times plate tectonics could have operated but more slowly so that it could not by itself have accomplished the necessary rate of heat removal from the earth a different tectonic mode must also have operated perhaps subcrustal delamination or dripping plate tectonics would have gradually taken over from the earlier mode plumes are a omplementary mode of mantle convection driven by a lower thermal boundary layer and so they could have operated in conjunction with both regimes of the upper boundary layer

isotopic variations of the fixed nitrogen in metamorphic rocks the content and isotopic composition of fixed nitrogen nh 4 n in metasedimentary rocks phyllites mica schists and gneisses from prograde metamorphic zones in the erzgebirge saxon ore mountains g d r were measured and show that the nitrogen content decreases and 15n increases with increasing metamorphic grade the geochemical and isotopic behaviour of nitrogen during contact metamorphism was investigated in a mica schist profile through the contact aureole of a granitic pluton the nitrogen concentrations of the wall rocks decrease towards the contact whereas the 15n values become higher various model experiments were carried out in the laboratory to explain the changes observed in the rock samples the nitrogen isotope fractionation factor was determined for the isotopic exchange in the system ammonium sulphate ammonia at 250 c 1 0143 0 0012 and 350 c 1 0128 0 0006 in accordance with the model experiments the conclusion seems warranted that the change in nitrogen content and its isotopic composition in rocks during metamorphic processes depend mainly on the metamorphic temperature

nitrogen 15 enriched precambrian kerogen and hydrothermal systems 1 nitrogen isotope data are reported for kerogen from precambrian black shales at prehnite pumpellyite to greenschist facies and for micas from postmetamorphic hydrothermal quartz vein systems nitrogen in micas was acquired from n bearing aqueous metamorphic hydrothermal fluids generated from breakdown of sedimentary kerogen and dehydration of k silicates at deeper crustal levels the 2 7 ga kerogens and hydrothermal k micas yield 15n values of 15 24 compared to existing data of 2 6 in phanerozoic counterparts paleoproterozoic equivalents have intermediate 15n of 7 12 implying a secular decrease in 15n of shale kerogen the 15n enriched nitrogen in archean shales and hydrothermal vein systems cannot be caused either by n isotopic shifts accompanying metamorphism or rayleigh fractionation because premetamorphic and postmetamorphic samples from the same terrane are both enriched and lack covariation of 15n with n c n ratios or metamorphic grade the magnitude of shifts during progressive metamorphism of sedimentary rocks in previous studies is constrained at 1 to greenschist and about 3 to amphibolite facies furthermore 15n enriched values cannot stem from long term preferential diffusional loss of 14n as samples were selected from terranes where 40ar 39ar ages are within a few million years of concordant u pb ages nitrogen is structurally bound in micas whereas ar is not it is possible that the 15n enriched values stem from a different n cycle in the archean with large biologically mediated fractionations yet the magnitude of the fractionations observed exceeds any presently known and chemoautotrophic communities tend to depleted values earlier results on archean cherts show a range from 6 to 30 given the temporal association of chert banded iron formation with mantle plumes the range is consistent with mixing between mantle n2 of 5 and the enriched marine reservoir identified in this study we attribute the secular trend from archean kerogen of 15 24 to phanerozoic kerogen averaging 3 4 to a secondary atmosphere derived from ci chondrite like material and comets with 15n of 30 42 shifts of 15n to its present values of 0 can be accounted for by a combination of sequestration of atmospheric n2 into sedimentary rocks and mantle degassing of 5 mantle n2

lithospheric scale gravitational flow the impact of body forces on orogenic processes from archaean to phanerozoic in the archaean the combination of warmer continental geotherm with a lighter sub continental lithospheric mantle suggests that gravitational forces played a more signifi cant role in continental lithospheric deformation to test this hypothesis we compare the evolution of the deformation and the regional state of stress in archaean like and phanerozoic like lithospheres submitted to the same boundary conditions in a triaxial stress field with imposed convergence in one direction for plausible physical parameters thickening of normal to cold phanerozoic lithospheres produces relatively weak buoyancy forces either extensional or compressional in contrast for archaean continental litho spheres or for anomalously warm phanerozoic lithospheres lateral gravitationally driven flow prevents significant thickening this conclusion is broadly consistent with 1 the rela tive homogeneity of the erosional level now exposed at the surface of archaean cratons 2 the sub aerial conditions that prevailed during the emplacement of up to 20 km of green stone cover 3 the relatively rare occurrence in the archaean record of voluminous detrital sediments 4 the near absence of significant tectonic metamorphic and magmatic age gra dients across archaean cratons 5 the relative homogeneity of strain across large areas and 6 the ubiquitous presence of crustal scale strike slip faults in many late archaean cratons

nitrogen geochemistry as a tracer of fluid flow in a hydrothermal vent complex in the karoo basin south africa abstract we have investigated the n geochemistry of minerals and rocks from contact metamorphic aureoles and hydrothermal vent complexes hvc in the karoo basin in south africa the hvc formed during phreatic eruptions associated with rapid devolatilization and pressure build up in contact aureoles around early jurassic sill intrusions by combining outcrop data from a hvc and core data from contact aureoles we investigate the relationship between light element release during metamorphism and vertical fluid migration sandstone and breccia from the hvc contain early diagenetic ammonium nh 4 bearing feldspar buddingtonite and illite ammonium occupies up to 95 of the a site in feldspar corresponding to concentrations up to 5 2 wt n bulk rock n isotope data for rocks from inside and outside the hydrothermal vent complex fall into two distinct groups background samples have 15nair between 1 5 and 4 9 whereas minerals from the vent complex have 15n in the range 7 5 to 10 6 the n geochemistry of contact metamorphic shale from the lower stratigraphic units of the karoo basin shows that the vitrinite reflectance and 15n values are positively correlated shale with reflectivity values 4 ro are enriched in 15n with 15n values between 6 and 14 implying the release of isotopically light n into metamorphic fluids probably as n2 we suggest that the relatively high 15n values of the early diagenetic buddingtonite in the hvc reflect exchange of buddingtonite with n bearing fluids ascending from greater depth after their release during contact metamorphism and dehydration we present a qualitative model whereby hydrothermal vent complexes represent fluid flow structures after their formation focusing n bearing metamorphic fluids sourced in deeper levels of the basin the release of organic n from sediments at depth in volcanic basins could play a role in the geochemical cycle of n becoming particularly important during periods of intense volcanic activity

15 n enriched gondwana lamproites eastern india crustal n in the mantle source abstract a comagmatic suite of lamproites ultramafic lamprophyres and minettes was intruded at 100 ma into thick siliciclastic sequences deposited in five intracratonic basins which developed during gondwana rifting in eastern india lamproites are dominated by microphenocrysts of ti rich phlogopite and olivine pseudomorphed by serpentine and talc with variable modal abundances of aegirine amphibole apatite carbonates cr spinel fluorite perovskite priderite and titanomagnetite compositionally lamproites are characterized by extremely high contents of tio2 4 2 10 wt k2o and p2o5 over a range of mg from 80 to 75 in keeping with lamproites worldwide they have high abundances of ba th u and ree 1019 5400 ppm with pronounced fractionation of ree where la smcn 3 5 5 6 and gd ybcn 7 3 18 consistent with residual garnet in the mantle source nitrogen isotope compositions range from 1 6 to 8 7 with an average of 3 8 and n contents average 107 ppm mantle 15n averages 5 with a n content of 2 ppm whereas continental crust varies from 2 to 6 with 30 90 ppm n atmospheric n2 is sequestered by n fixing microorganisms and stored as kerogen with an average 15n of 3 in sedimentary rocks some of the organic n is converted to nh4 which substitutes for k in crustal k silicates the 15n enriched lamproites are interpreted to have formed from partial melting of enriched harzburgitic mantle lithosphere by decompressional melting accompanying extension enrichment resulted from subduction erosion of continental crust or subduction of sediments low degree partial melting hybridization of the melts with mantle lithosphere and incubation prior to decompressional melting consequently some crustal n is recycled to the mantle

martian plate tectonics the northern lowlands of mars may have been produced by plate tectonics preexisting old thick highland crust was subducted while seafloor spreading produced thin lowland crust during late noachian and early hesperian time in the preferred reconstruction a breakup margin extended north of cimmeria terra between daedalia planum and isidis planitia where the highland lowland transition is relatively simple south dipping subduction occurred beneath arabia terra and east dipping subduction beneath thatsis montes and tempe terra lineations associated with gordii dotsum are attributed to ridge parallel structures while phelegra montes and scandia colles are interpreted as transform parallel structures or ridge fault fault triple junction tracks other than for these few features there is little topographic roughness in the lowlands seafloor spreading if it occurred must have been relatively rapid quantitative estimates of spreading rate are obtained by considering the physics of seafloor spreading in the lower c 0 4 g gravity of mars the absence of vertical scarps from age differences across fracture zones and the smooth axial topography to the first order the height of vertical scarps across fracture zones does not involve gravity crustal thickness at a given potential temperature in the mantle source region scales inversely with gravity thus the velocity of the rough smooth transition for axial topography also scales inversely with gravity plate reorganizations where young crust becomes difficult to subduct are another constraint on spreading age possible plate reorganizations for example the end of spreading through alba patera occur when the ridge axis is far from the trench that is rapid plate motions are inferred to have placed young oceanic crust far from the ridge axis the preferred full spreading rate 900 from the plate pole is 80 mm yr 1 plate tectonics if it occurred dominated the thermal and stress history of the planet a geochemical implication is that the lower gravity of mars allows deeper hydrothermal circulation through cracks and hence more hydration of oceanic crust so that more water is easily subducted than on the earth age and structural relationships from photogeology as well as median wavelength gravity anomalies across the now dead breakup and subduction margins are the data most likely to test and modify hypotheses about mars plate tectonics

high ammonium contents in the 3800 ma isua supracrustal rocks central west greenland abstract ammonium contents of whole rock and biotite samples from the metasedimentary sequence of the 3800 ma isua supracrustal belt central greenland have been determined and compared with the ammonium contents of volcanic sedimentary metamorphic and granitic rocks from the late archaean early proterozoic a sample of the amitsoq gneiss has an nh4 content whole rock 9 ppm biotite 29 ppm that is similar to many younger igneous rocks ammonium contents of the supracrustal rocks vary with chemistry and mineralogy of the rock and most likely were inherited from the original sediments a series of carbonate bearing and basic plagioclase bearing biotite rich schists from sequence a have varying nh4 contents whole rock 6 24 ppm that increase with an increasing chemical clastic component and decreasing volcanogenic component biotites in these rocks together with biotites in mica schists of the same sequence have high nh4 contents which vary over a small range of 54 95 ppm suggesting that the nh4 in these rocks was contained mostly in authigenic clays of the original sediments all these data imply that clay minerals were the major sink of nh4 or other nitrogen compounds on the earth s surface at 3800 ma two samples of graphite bearing garnet staurolite mica schist from sequence b have remarkably high nh4 contents whole rock 68 and 86 ppm biotite 234 and 253 ppm which are as high as or even higher than those of common late archaean early proterozoic argillites it appears almost certain that the original thick clay rich bottom sediments contained a large amount of nh4 or other nitrogen compounds the most likely major source of nitrogen in these nh4 enriched graphite bearing mica schists might be organic nitrogen

the behavior of nitrogen and nitrogen isotopes during metamorphism and mineralization evidence from the otago and alpine schists new zealand abstract metamorphism is a major mechanism for the re distribution of fluids and mass in the earth s crust with these processes most prominently highlighted by the occurrence of major gold resources within these terranes however although orogenic gold deposits have contributed over 20 of the global gold production their origins remain controversial the nitrogen concentration and isotopic composition of rocks and minerals are potentially powerful tracers of crustal metamorphism and mineralization but there have been few detailed applications of this approach to date although nitrogen isotopes have recently been used to elucidate the source of fluids in some neoarchean orogenic gold deposits and proterozoic to paleozoic mountain belts due to their age and geological complexity of these terranes major uncertainties as to the behavior of nitrogen remain the otago and alpine schists in the south island of new zealand comprise a large comparatively young the range of 15n values for mica and whole rock samples from the schists spans 0 2 to 7 0 and the nitrogen concentration from 23 to 3483 ppm sample provenance and rock type have minimal influence on the nitrogen concentration and isotopic value which appears to have been inherited from the original sedimentary kerogen there is no systematic variation between metamorphic temperature and 15n or n concentration in micas suggesting that there has been little discernible loss of 15n depleted fluids from silicates with increasing metamorphic temperature comparison of fluid mobile alkali element k rb and cs carbon and nitrogen whole rock concentrations indicates that for the otago and alpine schists in rocks up to upper greenschist facies significant nitrogen remains hosted in phases other than micas most likely poorly matured carbonaceous material samples from macraes flat a major gold producer have a similar range of 15n values to the host terrane but show distinctly higher nitrogen concentrations relative to unmineralized samples due to the incorporation of nitrogen from the mineralizing hydrothermal fluid this suggests that there is only a subtle metamorphic re distribution of nitrogen during mineralization albeit with minimal isotopic fractionation in the case of nitrogen at least the isotopic signatures of mineralized rocks support a metamorphic fluid source and are inconsistent with mantle or meteoric fluid reservoirs however due to the high relative abundance of nitrogen in sedimentary rocks compared to other potential reservoirs unless fluid fluxes are very large and well channeled nitrogen signatures are not sensitive recorders of fluid inputs from mantle magmatic or meteoric reservoirs conversely the absence of a sedimentary source nitrogen isotope signature similar to the host rock in an orogenic deposit would be a very strong indicator of an external exotic source for the mineralizing fluids

archean oceanic flake tectonics assuming that the archean mantle was hotter and the oceanic crust thicker than at present we estimate that the archean oceanic lithosphere had a sandwich rheology with the lower crust behaving as a soft ductile layer accordingly the archean oceanic lithosphere had a rheological structure similar to present day continental lithosphere but unlike present day oceanic lithosphere the thickness of the ductile layer is a decreasing function of age while its strength is an increasing function of age the buoyancy problem in archean subduction could be overcome by delamination of oceanic upper crust but mass balance calculation indicates that most archean oceanic crust must have been recycled where the right dynamic conditions existed for delamination oceanic flake tectonics may have been an important process leading to the preservation of oceanic upper crust in greenstone belts that are characteristic of archean terrains

a preliminary study of 15n14n in octahedral growth form diamonds abstract the 13 c 15 n and nitrogen concentrations have been determined in 43 octahedral diamonds from southern africa australia and north america the sample set included locations such as finsch south africa a mine which produces diamonds having 13c values close to 5 and argyle australia a mine which produces 13c depleted diamonds the nitrogen within high 13c diamonds 13 c 6 4 to 2 9 was generally depleted in 15n relative to atmospheric nitrogen 15n 12 3 to 5 9 mode 4 similar to the results which have been obtained previously from fibrous diamond it is believed that the volatiles from which these diamonds grew are primitive being derived from a source located beneath the continental lithosphere which may have changed little in terms of 13c and 15n since the mid archean the low 13c diamonds 13c 19 4 to 9 5 contained nitrogen generally enriched in 15n relative to air 15n 3 0 to 16 4 mode 5 it is suggested that the major carbon isotope heterogeneity within the mantle as represented by diamonds such as these is located in deep mechanically unstable regions of the continental lithosphere the isotopic compositions of both c and n are consistent with this heterogeneity resulting from the subduction of crustal material however isotope fractionation related directly to diamond growth cannot be ruled out

the geochemistry and petrogenesis of the lavas of the vulsinian district roman province central italy the vulsinian lavas are dominated by a suite of undersaturated leucite bearing basic to intermediate compositions the remaining lavas are mainly oversaturated and have shoshonitic affinities one hundred and thirty five samples have been analysed for major elements and most for 20 trace elements twenty seven lavas have been analysed for ree they are all perpotassic for the undersaturated lavas k2o na2o 2 8 and have very high lil element concentrations e g rb 400 800 ppm th 25 150 ppm ree reecho c 200 lree hree cho c 20 even in the most basic rocks

multiple growth events during diamond genesis an integrated study of carbon and nitrogen isotopes and nitrogen aggregation state in coated stones the internal structure and growth history of diamonds have been investigated by the study of carbon isotopes and the abundance isotope composition and aggregation state of nitrogen polished plates of zoned diamonds coated stones were laser sectioned for infrared mapping and isotope profiling nine small cubic and three small octahedral diamonds from zaire and two coated stones and five small cubic diamonds specifically from the mbuji mayi area of that country were also investigated the isotope composition and infrared characteristics of the diamonds of cubic habit including coats were found to be indistinguishable with 13c in the range 5 9 to 7 5 and all the nitrogen being present as the paired structure iaa with a 15n of between 3 1 and 8 0 the diamonds of octahedral habit including the cores of the coated stones were more variable 13c varied from 4 9 to 9 8 infrared spectra of the octahedral diamond exhibited either strong iaa characteristics or a combination of iaa and iab platelets were present in most cases 15n values from octahedral diamonds varied from 5 8 to 13 4 the data suggest that the octahedral diamonds represent a variety of growth events separated in time and or space and that the diamonds resided in the lithosphere for a period sufficient for thermal maturation in the case of the coated stones the octahedral diamonds were later involved in an event which added a component from a single relatively homogeneous reservoir cubic habit diamonds were formed at the same time this latter event is believed to be related to the kimberlite eruption comparison of the data with carbon isotope analyses of cubic diamond from other sources suggests that this reservoir could be widespread

petrogenesis and tectonic setting of the roman volcanic province italy abstract the volcanism in the roman province of italy can be modelled by the partial melting of heterogeneously enriched mantle sources the heterogeneity was created by materials derived from a subducted slab which can still be traced geophysically beneath the central apennines new petrographical and chemical data are presented for the high k calc alkaline and the shoshonitic volcanics of the campania region primary magmas are present only locally the existence of spatial zonation in the volcanism of campania is documented for the first time the shoshonitic leucite basanitic and leucititic volcanics of the phlegraean fields procida ischia and the somma vesuvius areas are at similar degrees of evolution about two times richer in nb and ba than those of northwestern campania and the latium part of the roman province accordingly distinct north western and south eastern subprovinces can be defined the evaluation of enrichment factors that is the abundance ratio between the average contents of each element in the relatively primitive lavas of the low k and the high k suites shows that the mantle sources prior to k metasomatism were different in the two sub provinces of the roman province in the north western one they resembled the sources of ocean island tholeiites and moderately enriched mor basalts in the south east they were similar to those of ocean island alkaline lavas and enriched morb s modelling based on k p ce sr rb ba th sm eu gd y nb and 87 sr 86 sr was carried out it indicates that the range of mantle sources of the volcanics in northwestern campania and latium can be modelled by the addition of 3 to 20 of materials derived by partial melting of carbonaceous pelites to a sr enriched mantle wedge comparable to the honolulu mantle source least enriched in nb the production of sr enriched mantle wedge requires either the action of fluids produced by dehydration of subducted oceanic crust or a small amount of metasomatism caused by the presence of carbonatite melts the near absence of ti ta nb yb and the highly fractionated ree in the metasomatizing component requires the presence of residual garnet and accessory ti rich minerals during the partial melting of the subducted sediments the writers propose that the mantle wedge overlying the subducted slab was hybridized by melts produced by partial fusion of subducted material derived from the continental crust probably sediments this process played a dominant role in the generation of the mantle sources from which the high k calc alkaline shoshonitic leucite basanitic and leucititic magmas of the roman province were derived

large impact basins and the mega impact origin for the crustal dichotomy on mars the origin of the fundamental crustal dichotomy on mars remains the outstanding unsolved problem in martian geologic evolution the hypothesis that this dichotomy is due to a single giant mega impact early in martian history is tested by comparing the observed number of large impact basins on mars with the number expected from a d 2 distribution this test is based on the assumption that the mega impact borealis basin is the largest member of such a distribution the difference between observed and expected basins is the number of missing basins that might have been obscured by subsequent geologic events if the borealis basin were the largest member of a d 2 impact population there would be a very large number of missing basins on mars 1 larger than 5000 km 10 larger than 2000 km and 40 50 larger than about 1000 km diameter the area occupied by these missing basins allowing for overlap is too large to be hidden by the younger surface units a large population of presently unrecognized large impact basins would have to be located in the well preserved cratered highlands by contrast if chryse is the largest member of a d 2 impact population the more modest number of missing basins could in principle be confined to areas of mars that have been resurfaced or reworked by subsequent geologic processes a possible alternative to the mega impact hypothesis is that the crustal dichotomy is due to the cumulative effects of multiple overlapping large but not giant impacts

nitrogen isotope compositions of metasedimentary rocks in the catalina schist california implications for metamorphic devolatilization history abstract in the catalina schist subduction zone metamorphic complex california metasedimentary rocks show a decrease in n concentration and an increase in 15nair with increasing metamorphic grade lowest grade lawsonite albite rocks contain 632 185 ppm n with 15 n 1 9 0 6 whereas highgrade amphibolite equivalents contain 138 76 ppm n with 15 n 4 3 0 8 loss of n accompanied devolatilization reactions that evolved h2o rich c o h s n fluids through consumption of chlorite and phengitic white mica and production of mineral assemblages containing muscovite biotite garnet and kyanite whole rock n concentrations of up to 200 ppm in veins and pegmatites produced during high p t metamorphism reflect the redistribution of n during devolatilization and partial melting of the metasedimentary rocks bulk fluid rock n isotope fractionations 15n 15nfluid 15nrock of 1 5 1 were calculated with the rayleigh distillation equation taking into account variability in rock composition by comparison of samples with similar k2o concentrations these fractionations are similar to but slightly lower than published calculated fractionations for n2 nh4 exchange at the temperature range of 350 600 c over which most of the devolatilization occurred in the catalina schist approximately 3 4 to 2 25 the n systematics appear to be explained by n2 nh4 exchange and a devolatilization process intermediate in behavior to batch volatilization and rayleigh distillation the observed shifts in n concentration and 15n cannot be explained by nh3 nh4 exchange at these temperatures using the equilibrium models the distillation devolatilization process implicated in this study may govern the behavior of other trace elements partitioned into hydrous fluids during devolatilization e g b cs u similarity of the calculated fluid 15n 1 5 to 5 5 with compositions of natural gases inferred to be derived from metasedimentary sources indicates the possibility of using n as a tracer of large scale volatile transport

mechanisms for lithospheric heat transport on venus implications for tectonic style and volcanism the tectonic and volcanic characteristics of the surface of venus are poorly known but these characteristics must be closely related to the mechanism by which venus rids itself of internal heat on the other solid planets and satellites of the solar system lithospheric heat transport is dominated by one of three mechanisms 1 plate recycling 2 lithospheric conduction and 3 hot spot volcanism we evaluate each mechanism as a candidate for the dominant mode of lithospheric heat transfer on venus and we explore the implications of each mechanism for the interpretation of venus surface features despite claims made to the contrary in the literature plate recycling on venus cannot be excluded on the basis of either theoretical arguments or present observations on topography and radar backscatter landforms resulting from plate convergence and divergence on venus would differ substantially from those on the earth because of the high surface temperature and the absence of oceans on venus the lack of free or hydrated water in subducted material the possibility that subduction would more commonly be accompanied by lithospheric delamination and the rapid spreading rates that would be required if plate recycling removes a significant fraction of the internal heat if plate recycling occurs on venus the rolling plains and lowlands provinces would be approximate analogs to terrestrial ocean basins in terms of age igneous rock type and formative process highlands on venus would be roughly analogous to terrestrial continents the hypothesis that lithospheric conduction dominates shallow heat transfer on venus leads to the prediction that the lithosphere is thin if venus has a global heat loss per mass equal to that for earth then temperatures marking the base of the thermal lithosphere on earth would be reached on venus at an average depth of about 40 km unless the mantle convective planform can maintain lithospheric regions of persistently low heat flow or unless the present atmospheric greenhouse on venus is geologically recent then such a lithospheric thickness leads to the conclusion that the topographic features contributing to the 13 km of relief on the planet must be geologically young the hypothesis that hot spot volcanism dominates lithospheric heat transfer on venus leads to the prediction that the surface must be covered with numerous active volcanic sources in particular if a typical venus hot spot has a volcanic flux equal to the average flux for the hawaiian hot spot for the last 40 m y then 104 such hot spots are necessary to remove the venus internal heat by volcanism such a number would produce enough volcanic material to resurface the entire planet to a depth of 1 km every 2 m y few areas of the planet would escape resurfacing for geologically long periods of time we find that none of the mechanisms for lithospheric heat transfer on venus can be excluded as unimportant at present it is likely that as on earth a combination of mechanisms operates on venus the strongest conclusion to emerge from this evaluation is that most of the major topographic features and probably many of the surface geological units on venus are young by comparison with the surfaces of the smaller terrestrial planets

boron lithium and nitrogen isotope geochemistry of nh4 illite clays in the fossil hydrothermal system of harghita bai east carpathians romania abstract coarse 2 0 0 2 m and fine boron in nh4 i s clays ranges from 513 to 1457 ppm and reached 1000 ppm in the k i nh4 k i the 11b measured in nh4 i s ranges from 12 6 to 22 4 0 3 and in k i nh4 k i is consistently 5 5 to 5 1 0 3 boron isotopes systematically become lighter in the nh4 i s series as temperature increased from 90 to 270 c low li content 1 to 8 ppm was found in illitic clay fractions the 7li shows negative values ranging from 8 6 to 12 3 0 8 for the coarser 2 0 0 2 m nh4 i s clays and from 4 3 to 14 1 1 for the finer the 11b of k i nh4 k i clays reflect a magmatic source whereas the nh4 i s series is consistent with the influx of isotopically light b waters derived from hydrothermal leaching of continental evaporites and or organic rich sediments the 7li signature measured on k i nh4 k i clays also support a magmatic fluid enriched in heavy li followed by precipitation of coarser nh4 i s from more recent sedimentary contributions of isotopically light li this interpretation is also supported by the 15n which reflect an influx of waters from an organic sediment origin the 15n of 14 6 0 6 measured on nh4 i s could be attributed to the presence of meteoric waters mixed with hydrothermal fluids the isotopic data obtained trace the mobility of magmatic and organic sedimentary components in the upper continental crust

nitrogen rich fluid in the upper mantle fluid inclusions in spinel dunite from lanzarote canary islands fluid inclusions ranging from pure n2 to pure co2 occur in olivine porphyroclasts in spinel dunite xenoliths chrome diopside suite from two localities within the quaternary to historic alkaline lavas of lanzarote canary islands this is the first report of fluid inclusions containing major amounts of n2 in mantle xenoliths the nitrogen rich fluid inclusions predate at least one generation of nitrogen free carbon dioxide inclusions textural evidence indicates that the inclusions were trapped within the upper mantle some of the nitrogen rich fluid inclusions are intimately associated with solid inclusions of spinel the nitrogen rich fluid was most likely produced in situ by oxidation dehydration reactions destabilizing ammonium bearing silicate minerals phlogopite amphibole increasing oxygen fugacity or possibly increasing temperature of the mantle this process could be related to an event of co2 and silicate melt injection at 6 8 kbar neumann et al in press or to some earlier event in the evolution of the mantle beneath lanzarote the existence of a n2 rich fluid phase in at least some mantle lithology ies at certain conditions is demonstrated by these data this discovery has consequences for the understanding of the evolution of the mantle below the canary islands as well as for the global nitrogen budget