creation of a continent recorded in zircon zoning we have discovered a robust microcrystalline record of the early genesis of north american lithosphere preserved in the u pb age and oxygen isotope zoning of zircons from a lower crustal paragneiss in the neoarchean superior province detrital igneous zircon cores with 18 o values of 5 1 7 1 record creation of primitive to increasingly evolved crust from 2 85 0 02 ga to 2 67 0 02 ga sharp chemical unconformity between cores and higher 18 o 8 4 10 4 metamorphic overgrowths as old as 2 66 0 01 ga dictates a rapid sequence of arc unroofing burial of detrital zircons in hydrosphere altered sediment and transport to lower crust late in upper plate assembly the period to 2 58 0 01 ga included 80 m y of high temperature 700 650 c nearly continuous overgrowth events reflecting stages in maturation of the subjacent mantle root huronian continental rifting is recorded by the youngest zircon tip growth at 2512 8 ma 600 c signaling magma intraplating and the onset of rigid plate behavior this 150 m y microscopic isotope record in single crystals demonstrates the sluggish volume diffusion of u pb and o in zircon throughout protracted regional metamorphism and the consequent advances now possible in reconstructing planetary dynamics with zircon zoning

high precision u pb ages for granulite metamorphism and deformation in the archean kapuskasing structural zone ontario implications for structure and development of the lower crust abstract precise u pb isotopic ages for two subhorizontal granulite gneisses in the kapuskasing structural zone indicate that formation and emplacement occurred more than 50 m y after cessation of volcanism in the steeply folded nearby greenstone belts by successive stages of ductile underplating across a zone of crust mantle delamination volcanism and batholith formation had ceased by 2700 and 2680 ma respectively while granulite formation occurred between 2660 and 2640 ma and ductile deformation continued until 2585 ma emplacement of the oldest granulite caused the development of shear planes and melting in the older overlying tonalite while emplacement of the younger granulite caused rupturing and rehydration of the overlying older granulite widespread tectonic underplating is implied since all superior province granulites have similar young ages anomalous ar ages that post date accretion by about 150 m y and scrambled pb pb and rb sr whole rock systems also suggest that late low temperature alteration accompanied this event temporal relationships and the presence of post volcanic 2670 ma conglomerate cobbles at granulite grade support a model of ductile underplating of a post accretion thermally softened back arc fore arc complex with components welded onto an overlying volcanic plutonic complex at successively deeper levels multistage deformation in active fault zones and hydrothermal emplacement of major gold deposits in greenstone belts are coeval with ductile underplating with ages between 2670 and 2585 ma anomalously abundant gold production from archean rocks may indicate that shallow underplating granulitization and devolatization was a widespread phenomenon at this time post accretion ductile underplating must be considered as a primary cause of seismic reflectors in archean terrains

holocene deceleration of the greenland ice sheet recent peripheral thinning of the greenland ice sheet is partly offset by interior thickening and is overprinted on its poorly constrained holocene evolution on the basis of the ice sheet s radiostratigraphy ice flow in its interior is slower now than the average speed over the past nine millennia generally higher holocene accumulation rates relative to modern estimates can only partially explain this millennial scale deceleration the ice sheet s dynamic response to the decreasing proportion of softer ice from the last glacial period and the deglacial collapse of the ice bridge across nares strait also contributed to this pattern thus recent interior thickening of the greenland ice sheet is partly an ongoing dynamic response to the last deglaciation that is large enough to affect interpretation of its mass balance from altimetry

greenland ice sheet mass balance reconstruction part i net snow accumulation 1600 2009 ice core data are combined with regional atmospheric climate model version 2 racmo2 output 1958 2010 to develop a reconstruction of greenland ice sheet net snow accumulation rate at g spanning the years 1600 2009 regression parameters from regional climate model rcm output regressed on 86 ice cores are used with available cores in a given year resulting in the reconstructed values each core site s residual variance is used to inversely weight the cores respective contributions the interannual amplitude of the reconstructed accumulation rate is damped by the regressions and is thus calibrated to match that of the rcm data uncertainty and significance of changes is measured using statistical models a 12 or 86 gt yr 21 increase in ice sheet accumulation rate is found from the end of the little ice age in 1840 to the last decade of the reconstruction this 1840 1996 trend is 30 higher than that of 1600 2009 suggesting an accelerating accumulation rate the correlation of at g with the average surface air temperature in the northern hemisphere satnht remains positive through time while the correlation of at g with local near surface air temperatures or north atlantic sea surface temperatures is inconsistent suggestinga hemispheric scale climateconnection an annualsensitivityof at g to satnhtof 6 8 k 21 or 51 gt k 21 is found the reconstuction at g correlates consistently highly with the north atlantic oscillation index however at the 11 yr time scale the sign of this correlation flips four times in the 1870 2005 period

identification of ammonium bearing minerals by shortwave infrared reflectance spectroscopy at the esquel gold deposit argentina the esquel low sulfidation epithermal vein deposit in chubut province argentina is hosted by a late jurassic bimodal arc assemblage shortwave infrared swir reflectance spectroscopy of vein samples using a hand held field spectrometer identified ammonium bearing illite jarosite and adularia buddingtonite in banded quartz displaying colloform and crustiform textures the swir technique proved to be a rapid and accurate means of detecting ammonium bearing species particularly buddingtonite which provides evidence for paleoboiling zones and thus of precious metal mineralization

intracrustal extension of an archean orogen revealed using single grain u pb zircon geochronology the metaplutonic wawa gneiss domain in the southern superior province is part of an uplifted block that was exhumed during the proterozoic exposing the lower crustal 30 km levels of the late archean 2720 to 2670 ma abitibi wawa orogen previous structural and geophysical studies of the midcrustal wawa gneiss domain reveal subhorizontal shear zones that formed during orogen parallel extension and correspond to gently dipping seismic reflectors present at depth elsewhere in the orogen in order to bracket the age of this extensional deformation the ages of metre scale igneous bodies that crystallized before and after extension have been determined in four outcrops using single grain u pb zircon dating techniques the results constrain the time of extension to the interval between 2660 and 2645 ma and show that deformation was coeval with pegmatite emplacement partial melting of tonalite gneisses as old as 2925 35 ma and regional circa 2660 ma granulite facies metamorphism of adjacent more deeply eroded crust this extension and associated high grade metamorphism in the wawa gneiss domain occurred 10 to 25 m y after the cessation of regional compression major transcurrent fault movement and sediment deposition in the upper structural levels of the orogen the middle and lower crust was therefore mechanically decoupled from the upper crust during this period of orogen parallel extension this intracrustal style of deformation is interpreted to be the consequence of gravitational forces working on a thermally softened middle crust the detachment of lithosphere from the orogen base is the favored mechanism for generating these forces and for introducing the heat that caused synextension metamorphism cyclic construction and delamination of orogen lithosphere could account for even younger metamorphic strain events evident in nearby exposures of lower crust

the lithoprobe abitibi grenville transect two billion years of crust formation and recycling in the precambrian shield of canada we summarize the results of lithoprobe studies in the neoarchean southeastern superior province and the mesoproterozoic grenville province in the southeastern precambrian shield of canada through two composite cross sections based on seismic reflection data which define dramatically different styles of crust formation and tectonic accretion in the neoarchean and mesoproterozoic in the neoarchean the structures at the surface are steep with discontinuous and flatter structures at depth much of the crust appears to be juvenile and the predominant process of crustal growth is inferred to have been subduction accretion of primitive crust in a prograding arc system in the mesoproterozoic surface structures are shallow and the seismic character of the crust is continuous over the entire cross section archean parautochthonous rocks and reworked archean crust comprise a very significant proportion of the preserved crust in the mesoproterozoic and provided the backstop to the grenvillian orogeny result

eemian interglacial reconstructed from a greenland folded ice core efforts to extract a greenland ice core with a complete record of the eemian interglacial 130 000 to 115 000 years ago have until now been unsuccessful the response of the greenland ice sheet to the warmer than present climate of the eemian has thus remained unclear here we present the new north greenland eemian ice drilling neem ice core and show only a modest ice sheet response to the strong warming in the early eemian we reconstructed the eemian record from folded ice using globally homogeneous parameters known from dated greenland and antarctic ice core records on the basis of water stable isotopes neem surface temperatures after the onset of the eemian 126 000 years ago peaked at 8 4 degrees celsius above the mean of the past millennium followed by a gradual cooling that was probably driven by the decreasing summer insolation between 128 000 and 122 000 years ago the thickness of the northwest greenland ice sheet decreased by 400 250 metres reaching surface elevations 122 000 years ago of 130 300 metres lower than the present extensive surface melt occurred at the neem site during the eemian a phenomenon witnessed when melt layers formed again at neem during the exceptional heat of july 2012 with additional warming surface melt might become more common in the future

laterally extensive modified placer gold deposits in the paleoproterozoic mississagi formation clement and pardo townships ontario local gold concentrations are found in laterally extensive pyrite bearing framework supported cobble and boulder conglomerates in the basal 30 m of the mississagi formation in the south eastern part of the huronian outcrop belt these were deposited as part of a valley fill succession in shallow gravel bed braided rivers with local hyperconcentrated flows the basal contact with underlying archean psammites is typically highly irregular and shows no obvious sign of weathering suggesting that deposition took place soon after retreat of the glaciers responsible for deposition of the ramsey lake formation highest gold concentrations are associated with moderately well sorted medium to large pebble conglomerate that show some signs of reworking during low flow events depositional elements are typically lenticular and of limited lateral extent unlike the older pyritic quartz pebble conglomerates at the base of the matinenda formation in the elliot lake and blind river areas these conglomerates contain no uraninite and are polymict with material derived from a highly restricted catchment area with marked local and regional topographic relief porous detrital allogenic pyrite and euhedral post depositional pyrite have overlapping generally positive delta 34 s values indicating a closed system during diagenesis and metamorphism the presence of biotite enriched rims on many of the metavolcanic and metasedimentary clasts in the conglomerates suggests that gold was partly leached from the allogenic pyrite grains at the peak of the penokean orogeny at 1 85 1 5 ga the potential source of the au bearing detrital allogenic pyrite appears to be an as yet undiscovered archean volcanic hosted massive sulfide vhms deposit located 1 to 8 km north of the deposit

in situ u pb granulite hosted zircon dates kapuskasing structural zone ontario a late archean large igneous province lip as a substrate for juvenile crust abstract a geochronological study of the kapuskasing structural zone ksz a section of late archean lower crust within the southern superior province canadian shield was undertaken to test a plate tectonic model for the late archean abitibi opatica terrane as a modified large igneous province lip new geochronological data allow us to interpret 1 construction of the lip in an environment where inherited zircons were derived from an older sialic crust and preserved in the granulite grade lower crust and 2 high grade metamorphism in the lower crust related to crustal refinement of the lip u pb and pb pb dates were obtained by la icp ms with beams of 8 30 m the study yielded 21 interpretable zircon dates spanning 500 m yr from one mafic granulite and two garnetiferous metatonalites nine of the dates are significantly older than the oldest known magmatic event recorded in the upper crust of the abitibi opatica terrane and indicate 2760 ma proto crustal material in the ksz lower crust the rema

new age ca 2970 ma mantle source composition and geodynamic constraints on the archean fiskenaesset anorthosite complex sw greenland the archean fiskenaesset complex sw greenland consists of an association of ca 550 meter thick layered anorthosite leucogabbro gabbro and ultramafic rocks peridotite pyroxenite dunite hornblendite the complex was intruded by tonalite trondhjemite and granodiorite ttg sheets now orthogneisses during thrusting that was followed by several phases of isoclinal folding the trace element systematics of the fiskenaesset complex and associated volcanic rocks are consistent with a supra subduction zone geodynamic setting the fiskenaesset anorthosites leucogabbros gabbros and ultramafic rocks collectively yield an sm nd errorchron age of 2973 28 ma mswd 33 with an average initial epsilon nd 3 3 0 7 consistent with a long term depleted mantle source regression of pb isotope data define an age of 2945 36 ma mswd 44 and the regression line intersects the average growth curve at 3036 ma slightly lower pb pb errorchron age is interpreted as reflecting partial disturbance of the u pb system in gabbros leucogabbros and ultramafic rocks during intrusion of ttgs complex internal structures in zircons from orthogneisses reveal several episodes of zircon growth and recrystallization taking place between ca 3200 and 2650 ma zircon ages peak at about 3200 3100 3000 2950 2820 and 2750 ma the 3200 3000 ma zircon cores are interpreted as inherited xenocrysts from older reworked crustal rocks 2950 ma is considered as an approximate intrusion age of sampled ttgs the 29402650 ma ages are attributed to metamorphic overgrowth and recrystallization in response to multiple tectonothermal events that affected the fiskenaesset region on the basis of recently published trace element data and new nd and pb isotope and u pb zircon age data a three stage geodynamic model is proposed to explain the evolution of the fiskenaesset complex stage 1 represents the formation of depleted shallow mantle source 3000 ma epsilon nd 3 3 0 7 for the complex stage 2 corresponds to the development of an intra oceanic island arc between 3000 2950 ma stage 3 is characterized by the collision of the island arc with either a passive continental margin or with an older arc between 2950 2940 ma c 2010 elsevier b v all rights reserved less

holocene thinning of the greenland ice sheet the nature of the response of the greenland ice sheet gis to climate change is a fundamental question in climate science but the details remain unclear during the holocene climate optimum 6 000 to 9 000 years ago for example many locations in the northern hemisphere showed unusual warmth yet changes in greenland appeared inconsistent vinther et al now standardize the previously disparate seeming ice core records of climate history on the gis during the holocene and find consistent evidence for a stronger climate optimum than was previously recognized together with elevation reduction and marginal ice sheet thinning the response of the greenland ice sheet gis to changes in climate remains uncertain during the holocene climatic optimum an unusually warm period from about 9 000 to 6 000 years ago changes in greenland appear inconsistent with the rest of the northern hemisphere here the greenland holocene temperature history and the evolution of gis surface elevation are extracted from four gis locations the results may resolve this disparity on entering an era of global warming the stability of the greenland ice sheet gis is an important concern1 especially in the light of new evidence of rapidly changing flow and melt conditions at the gis margins2 studying the response of the gis to past climatic change may help to advance our understanding of gis dynamics the previous interpretation of evidence from stable isotopes 18o in water from gis ice cores was that holocene climate variability on the gis differed spatially3 and that a consistent holocene climate optimum the unusually warm period from about 9 000 to 6 000 years ago found in many northern latitude palaeoclimate records4 did not exist here we extract both the greenland holocene temperature history and the evolution of gis surface elevation at four gis locations we achieve this by comparing 18o from gis ice cores3 5 with 18o from ice cores from small marginal icecaps contrary to the earlier interpretation of 18o evidence from ice cores3 6 our new temperature history reveals a pronounced holocene climatic optimum in greenland coinciding with maximum thinning near the gis margins our 18o based results are corroborated by the air content of ice cores a proxy for surface elevation7 state of the art ice sheet models are generally found to be underestimating the extent and changes in gis elevation and area our findings may help to improve the ability of models to reproduce the gis response to holocene climate

development of sedimentary basins in the archean abitibi belt canada an overview sedimentation in the archean abitibi greenstone belt occurred during four depositional episodes i sedimentary cycle 1 2730 2720 ma ii sedimentary cycle 2 2715 2705 ma iii sedimentary cycle 3 2700 2687 ma and iv sedimentary cycle 4 2685 2675 ma records of the first two sedimentary cycles are preserved in basins within the northern volcanic zone whereas basins formed during the latter two sedimentary cycles are located within the southern volcanic zone of the abitibi belt sedimentary cycles 1 and 3 represent deep water facies as indicated by turbidites resedimented conglomerates pelagic sediments and ubiquitous iron formations subaerial deposits have not been identified in contrast sedimentary cycles 2 and 4 show a prevalence of fluvial to shallow water marine and or lacustrine deposits tectono magmatic influence on sedimentation during cycles 2 and 4 is documented by i the presence of numerous unconformities underlain by plutonic and volcanic rocks ii locally voluminous sh

clay mineral authigenesis in coal and shale from the anthracite region pennsylvania textural chemical and mineralogical analyses of authigenic clays in anthracite rank coal and associated shale from eastern pennsylvania have allowed a better understanding of the parameters controlling diagenesis and perhaps coalification in this region minerals in anthracite occur in distinct assemblages associated with the following microenvironments coal matrix two orthogonal joint sets termed systematic and nonsystematic cleat and a third joint set mineralogical differences among microenvironments allow inferences about clay mineral origins and the parameters that controlled mineral authigenesis kaolinite occurs in the shale and all microenvironments within the coal seam authigenic minerals that replaced preexisting kaolinite during the latest stage ofcoalification anthracitization at t 200 c include nhu rich illite pyrophyllite and the following minerals which are primarily restricted to the systematic cleat set tosudite rl ordered dioctahedral mixed layer chlorite smectite sudoite di trioctahedral chlorite and rectorite rlordered mixedjayer paragonite smectite alteration of smectite to illite may be responsible for formation of authigenic illite and na bearing illite which are present only in the shale and coal matrix and fe al rich chlorite and quartz in the third joint set the chemical components for authigenesis appear to have multiple sources ai and si from preexisting kaolinite and quartz n from local organic matter mg and na for tosudite sudoite and rectorite largely from metasomatic hydrothermal fluids fe and mg for fe al rich chlorite from smectite illitization minerals in the shale the coal matrix and the nonsystematic cleat set are interpreted to represent authigenesis in a low permeability environment closed system alteration however the assemblage sudoite tosudite rectorite in the systematic cleat set is interpreted to be the result ofone or two stages ofhydrothermal alteration open system alteration we suggest hat differences in minerals between the two nearly perpendicular cleat sets are the result of permeability differences which were maintained by an anisotropic lateral stress field created by plate convergence during the alleghanian orogeny hydrothermal alteration may be related to large scale basinal flow induced by alleghanianage uplift such migrating fluids could also have transported heat from depth and thereby significantly increased the rate and rank ofcoalification in this region

experimental determination of the ammonium partitioning among muscovite k feldspar and aqueous chloride solutions abstract the cation exchange equilibria muscovite nh 4 cl tobelite kcl kal 2 alsi 3 o 10 oh 2 nh 4 cl aq nh 4 al 2 alsi 3 o 10 oh 2 kcl aq k feldspar nh 4 cl buddingtonite kcl kalsi 3 o 8 nh 4 cl aq nh 4 alsi 3 o 8 kcl aq are determined by hydrothermal experiments at 400 500 600 c at 400 mpa and piston cylinder experiments at 500 600 c at 1500 mpa along the entire k nh4 compositional range the composition of solid phases are determined by xrd emp and ftir methods nh4 k ratios in coexisting 2 3 molal chloride solutions by ion chromatography muscovite tobelite and k feldspar buddingtonite form complete solid solutions series at all conditions consistent phase relations and mass balances indicate equilibrium between solid solutions and fluids in both the k nh4 muscovite and k nh4 feldspar fluid systems nh4 strongly fractionates into the fluid in the range from 400 to 600 c 400 to 1500 mpa fractionations are slightly temperature and pressure dependent the distribution coefficient k d solid fluid x nh 4 solid x k solid x k fluid x nh 4 fluid varies within the experimental range from 0 10 at xkbulk 0 81 to 0 46 at xkbulk 0 20 for muscovite fluid and from 0 05 at xkbulk 0 91 to 0 70 at xkbulk 0 15 for feldspar fluid nh4 is preferentially incorporated into feldspar relative to mica with kdfsp ms ranging between 1 36 and 2 0 at 400 mpa and between 1 13 and 1 5 at 1500 mpa as xkbulk varies from 0 15 to 0 89 results are evaluated thermodynamically assuming equal mixing of nh4cl and kcl in the fluid mixing energies of solids were estimated using multidimensional regression and a regular solution model for the muscovite tobelite equilibrium values of s 11 7 j k 1 mol 1 v 2 1 j mpa 1 mol 1 and w 4 6 kj mol 1 and for the k feldspar buddingtonite equilibrium s 8 8 j k 1 mol 1 v 1 1 j mpa 1 mol 1 and w 5 4 kj mol 1 were derived results are used to calculate partition coefficients d among phases at very low nh4 bulk concentrations between 400 and 600 c dnh4fluid ms ranges from 7 to 8 at 400 mpa and is 5 at 1500 mpa dnh4fluid ms is 6 to 7 at 400 mpa and about 5 at 1500 mpa dnh4fsp ms is between 1 and 1 2 at all conditions partition coefficients are valid for nh4 contents of up to several hundred ppm in mica and feldspar well within the concentration range observed in many rocks combining the data with that of k nh4 partitioning between phlogopite and fluid results in dnh4phl ms 3 5 and dnh4phl ms 3 at 550 c 200 to 400 mpa nh4 concentrations in coexisting muscovite biotite and k feldspar from a variety of rocks show near equilibrium distributions nh4 k partitioning between major k bearing minerals and saline fluids allows for assessment of the dehydration history of metamorphic rocks during prograde metamorphism water is progressively produced by dehydration reactions and expelled along rock s p t path nitrogen is subsequently removed due to preferred fractionation of nh4 into fluids the remaining nh4 is continuously redistributed among muscovite and biotite and at higher grades k feldspar with biotite as the main solid phase carrier of ammonium the large fractionation effect of nh4 among most phases highlights its potential as tracer of devolatilization processes and fluid rock interactions

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

plumes and hotspots as sources of greenstone belts abstract greenstone belts that are older than 0 18 ga formed under circumstances that generate a buoyant garnet poor root because they do not generate enough depletion in the lithospheric mantle juvenile arcs involving average temperature early precambrian mantle were not a significant source of early precambrian greenstone belts instead early precambrian greenstone belts originated from mantle plumes and hotspots we focus on the origin of highly deformed morb like greenstone belts that are underlain by tonalitic to trondhjemitic plutons and surrounded by metamorphosed accretionary wedge like sediments these greenstone belts could form as the consequence of subducting a ridge segment within 500 km of a hotspot located beneath the continent as has happened on the taitao peninsula in chile the tonalitic to trondhjemitic plutons result from partial melting of oceanic crust thickened by extra melt derived from the hotspot the morb like rocks result from decompressive partial melting within the slab window produced by the subducting ridge the stratigraphy and tectonic setting of the recently emplaced taitao ophiolite are a possible phanerozoic analog of some morb like archean and early proterozoic greenstone belts

u pb age constraints for the magmatic and tectonic evolution of the pontiac subprovince quebec new u pb zircon titanite and monazite ages help constrain the history of magmatism and tectonism within the pontiac subprovince of western quebec the pontiac subprovince resembles other metasedimentary belts of the superior province however the stratigraphic relationships between the dominantly sedimentary rocks of the pontiac and the adjacent volcanic dominated abitibi belt to the north and west remain controversial volcanic rocks of the belleterre volcanic zone in the southern part of the pontiac subprovince have been interpreted by other workers as klippen of abitibi strata that were thrust southward onto the pontiac subprovince however volcanic rocks in the belleterre zone give crystallization ages of 2689 2682 ma which are younger than any extrusive rocks dated thus far from the abitibi belt single detrital zircon grains from pontiac sedimentary rocks give ages as young as 2683 ma indicating that the sediments are similar in age or younger than the volcanic units the volcanic rocks pro

ice sheet in peril earth s large ice sheets in greenland and antarctica are major contributors to sea level change at present the greenland ice sheet see the photo is losing mass in response to climate warming in greenland 1 but the present changes also include a long term response to past climate transitions on page 590 of this issue macgregor et al 2 estimate the mean rates of snow accumulation and ice flow of the greenland ice sheet over the past 9000 years based on an ice sheet wide dated radar stratigraphy 3 they show that the present changes of the greenland ice sheet are partly an ongoing response to the last deglaciation the results help to clarify how sensitive the ice sheet is to climate changes

geochemical oxygen and neodymium isotope compositions of metasediments from the abitibi greenstone belt and pontiac subprovince canada evidence for ancient crust and archean terrane juxtaposition abstract the abitibi greenstone belt agb and pontiac subprovince ps in the southwestern superior province are adjacent greenstone plutonic and metasedimentary dominated terranes respectively separated by a major fault zone metasediments from these two contrasting terranes are compared in terms of major and trace element and o and nd isotope compositions and detrital zircon ages the following two compositional populations of metasediments are present in the low grade abitibi southern volcanic zone 1 1 a mafic element enriched population mep characterized by flat depleted ree patterns enhanced mg cr co ni and sc low incompatible element contents and minor or absent normalized negative troughs at nb ta and ti and 2 2 a low mafic element population lmep featuring lree enriched patterns enhanced rb cs ba th and u contents and pronounced normalized negative troughs at nb ta and ti these geochemical features are interpreted to indicate that the mep sediments were derived from an ultramafic and mafic dominated oceanic provenance whereas the lmep sediments represent mixtures of mafic and felsic arc source rocks the ps metasediments are essentially indistinguishable from abitibi lmep on the basis of majorelement and transition metal abundances suggesting comparable types of source rocks and degrees of maturity but are distinct in terms of some trace elements and o isotope compositions the pontiac metasediments are depleted in 18o and enriched in cs ba pb th u nb ta hf zr and total ree and also have higher ratios of rb k cs rb ba rb ta nb th la and ba la relative to the abitibi lmep two subtypes of ree patterns have been identified in ps metasediments both have moderately fractionated distributions with la yb n 3 9 17 and 7 8 13 6 respectively but the first has minor eu anomalies eu eu 0 89 1 16 whereas the second features pronounced negative anomalies eu eu 0 52 0 81 coupled with high concentrations of incompatible elements such as k rb th u nb and ta the first subtype is interpreted to be derived from provenances of mixed mafic and felsic volcanic rocks whereas the eu depleted type has geochemical features that are typical of post archean sediments or archean k rich granites and volcanic equivalents abitibi metasediments have nd 2 7 ga values of 0 71 to 1 83 corresponding to model tchur 2 82 2 55 ga with the majority around 2 65 ga close to the depositional age of 2 7 ga in contrast ps metasediments are characterized by nd 2 7 ga 0 95 to 3 51 corresponding to t dhur 2 7 to 3 1 ga consistent with a detrital zircon age spectrum of 2 69 ga to 3 1 ga pontiac subprovince and abitibi metasediments have contrasting provenances suggesting their separate initial geodynamic settings and later tectonic juxtaposition major and trace elements o and nd isotopic compositions and detrital zircon ages collectively suggest that some of the ps metasediments were derived mainly from juvenile rocks whereas others require various degrees of contribution from older 3 0 3 5 ga evolved crust mixed with juvenile 2 7 ga rocks older components were derived either directly from erosion of an ancient continental arc or from contemporaneous volcanic rocks which may contain recycled older components if the ps represents in part an accretionary prism to the abitibi southern volcanic zone then oblique subduction may have been involved which allowed the pontiac subprovince to juxtapose or sample exotic source rocks

u pb geochronology of the opatica tonalite gneiss belt and its relationship to the abitibi greenstone belt superior province quebec u pb ages for zircon monazite and titanite from samples collected from two transects across the opatica tonalite gneiss belt in the superior province of quebec indicate that the belt contains rocks that are significantly older than those in the adjacent low grade northern abitibi belt tonalite and tonalite gneiss which make up most of the belt formed over an interval of 100 ma from pre 2800 to 2702 ma five samples have ages of ca 2807 13 2773 23 and 2702 3 ma zircon growth at 2718 2721 ma in the two oldest samples may record an early period of high grade metamorphism in the belt hornblende diorite monzodiorite and tonalite plutons were intruded at 2693 2696 ma particularly along the southern boundary with the abitibi belt these include the canet pluton at ma the lac ouescapis pluton at 2693 2 ma and the barlow pluton at 2696 3 ma pink granite plutons and dykes are the youngest intrusive rocks in the belt three samples have yielded zircon ages of 2690 2 and 2686 4 ma

contrasting sources for upper and lower continental crust the greenstone connection abstract greenstones may be useful in tracking the role of oceanic plateaus in the growth of continents lithologic proportions th ta ratios and ni concentrations in greenstone basalts of all ages show that arc related greenstones greatly exceed oceanic plateau and morb related greenstones in abundance this distribution may be accounted for by the preferential obduction of arcs formed on top of oceanic plateaus during collision of plateaus with continents because thick oceanic plateaus resist subduction a significant volume may be accreted to continental margins during collisions and over time these plateaus may evolve into lower continental crust this idea has important implications for continental development in that the lower continental crust may come chiefly from accreted oceanic plateaus while upper continental crust forms by subduction related processes consistent with this model are high seismic wave velocity layers in the lower crust of proterozoic cratons similar to high velocity layer

orogen parallel and transverse shearing in the opatica belt quebec implications for the structure of the abitibi subprovince the late archean opatica granitoid gneiss belt is situated within the northern abitibi subprovince along the northern margin of the abitibi greenstone belt approximately 200 km of structural section was mapped along three traverses within the previously unstudied opatica belt the earliest preserved structures are penetrative foliations and stretching and mineral lineations recording regional ductile shearing d1 late d1 deformation was concentrated into kilometre scale ductile fault zones typically with l s tectonite fabrics two families of lineations are associated with d1 indicating shearing both parallel and transverse to the east northeast trend of the belt lineations trending east northeast or northwest southeast tend to be dominant within domains separated by major fault zones in light of the abundant evidence for early north south compression documented throughout southern superior province including the abitibi greenstone belt d1 is interpreted in terms of mid crustal thrusting prob

pb isotopic geochemistry of granitoids and gneisses from the late archean pontiac and abitibi subprovinces of canada abstract initial pb isotopic compositions were determined on k feldspars for thirty granitoid plutons and gneiss complexes in the abitibi and the pontiac subprovinces of canada the gneisses cover a wide spectrum of initial pb isotopic compositions and undoubtedly demonstrate that very radiogenic reservoirs were present in the pontiac subprovince and along its southern boundary with the grenville province in the late archean the gra nitoid rocks also define a large spectrum covering 0 3 unit of the 207 pb 204 pb ratio ranging from compositions typical of mafic mantle melts to those of sedimentary banded iron formations the older intrusives emplaced between 2 72 and 2 68 ga were principally derived from juvenile mantle sources they may have been formed in a volcanic arc environment but they still frequently record the signature of crustal precursors which may have been represented by subducted sediments or by older assimilated crustal components a sharp change in the provenance of the granitoid magmas occurred close to 2 68 ga an age that also corresponds to an episode of alkaline activity and fault related sedimentation for the following 40 ma magmatism chiefly resulted from melting of crustal precursors in a thickened crustal lithosphere the pb isotopic compositions of these crustal melts demonstrate that the basement of the pontiac subprovince contains a major component of rocks older than 3 0 ga in comparison the abitibi subprovince may not contain basement components older than 2 9 ga or else they represent a negligible fraction of the crust this supports the concept that the abitibi and pontiac subprovinces formed as discrete crustal segments tectonically juxtaposed finally the grenville front tectonic zone was metamorphosed and partially melted in the late archean and is probably the deep crustal equivalent of the pontiac subprovince

hf isotope evidence for effective impact melt homogenisation at the sudbury impact crater ontario canada abstract we report on the first zircon hafnium oxygen isotope and trace element study of a transect through one of the largest terrestrial impact melt sheets the differentiated melt sheet at the 1 85 ga originally ca 200 km in diameter sudbury impact crater ontario canada yields a tight range of uniform zircon hf isotope compositions e hf 1850 of ca 9 to 12 this is consistent with its well established crustal origin and indicates differentiation from a single melt that was initially efficiently homogenised we propose that the heterogeneity in other isotopic systems such as pb in early emplaced impact melt at sudbury is associated with volatility related depletion during the impact cratering process this depletion leaves the isotopic systems of more volatile elements more susceptible to contamination during post impact assimilation of country rock whereas the systems of more refractory elements preserve initial homogeneities zircon oxygen isotope compositions in the melt sheet are also restricted in range relative to those in the impacted target rocks however they display a marked offset approximately one third up the melt sheet stratigraphy that is interpreted to be a result of post impact assimilation of 18 o enirched rocks into the base of the cooling impact melt given that impact cratering was a more dominant process in the early history of the inner solar system than it is today and the possibility that impact melt sheets were sources of ex situ hadean zircon grains these findings may have significance for the interpretation of the early zircon hf record we speculate that apparent e hf time arrays observed in the oldest terrestrial and lunar zircon datasets may be related to impact melting homogenising previously more diverse crust we also show that spatially restricted partial melting of rocks buried beneath the superheated impact melt at sudbury provided a zircon crystallising environment distinct to the impact melt sheet itself

thermal transport properties of major archean rock types to high temperature and implications for cratonic geotherms abstract the thermal structure of archean terranes depends in part on the thermal transport properties of their distinctive rock types we determined thermal diffusivity d of a suite of 14 greenstone mafic granulite and tonalite trondhjemite granodiorite ttg rocks at temperatures up to 1000 c at atmospheric pressure using laser flash analysis which lacks systematic errors associated with conventional contact techniques at room temperature d ranges from 3 8 mm 2 s 1 for banded iron formation to 0 8 mm 2 s 1 for a quartz poor trondjhemite for all samples thermal diffusivity decreases with increasing temperature such that d for the suite converges around 0 7 0 1 mm 2 s 1 at 700 c combining these results with measured density and heat capacity calculated from modal mineralogy provides thermal conductivity as a function of temperature for each rock suite whereas near surface thermal conductivity varies from 2 7 to 4 2 w m 1 k 1 depending on rock type thermal conductivity of the lower crust is 2 w m 1 k 1 and only weakly dependent on lithology which is more thermally resistive than the uppermost mantle 3 5 w m 1 k 1 use of temperature dependent thermal transport properties in numerical models reveals that surface geotherms can be relatively insensitive to crustal heat production because high heat producing ttg rocks typically have high thermal diffusivity and conductivity at low temperatures and consequently are more efficient conductors of the heat they produce this finding implies that calculations of crustal heat production from surface borehole measurements may entail significant uncertainties

uranium lead zircon systematics in the sudbury impact crater fill implications for target lithologies and crater evolution the sudbury impact structure is one of only a few terrestrial impact craters capable of providing insight into large impact processes however despite more than a century of study no consensus exists regarding its depths of excavation and melting this study presents 3920 u pb zircon dates for target lithologies and the crater fill as well as new pb isotope data for target lithologies in an attempt to further constrain these depths and understand the behaviour of zircon in large impacts only 1 5 of the crater fill zircons have dates within uncertainty of the 1 85 ga impact event with most seeming to preserve their pre impact u pb systematics the preponderance of undisturbed zircon in the crater fill suggests that this mineral is an effective means of tracing target lithologies in impact basins a significant fraction of crater fill zircon was dated between 2 50 and 2 61 ga which is negligible amongst known target lithologies and therefore identifies previously unrecognized target rocks the pb isotope systematics are compatible with the proposal that the 2 6 ga rock represents typical mid to lower crustal basement and could have been an important contributor to the melt sheet the combined data argue against a shallow melting scenario with regard to hadean terrestrial zircon the lesson from sudbury is that zircon can be recycled through multiple large impact events and still preserve the age of the original crust supplementary materials sample information trace element and pb isotope data for granitoid and gneiss feldspar pb modelling parameters u pb zircon data concordia diagrams and airborne radiometric images are available at http www geolsoc org uk sup18881

radiostratigraphy and age structure of the greenland ice sheet 2015 the authors several decades of ice penetrating radar surveys of the greenland and antarctic ice sheets have observed numerous widespread internal reflections analysis of this radiostratigraphy has produced valuable insights into ice sheet dynamics and motivates additional mapping of these reflections here we present a comprehensive deep radiostratigraphy of the greenland ice sheet from airborne deep ice penetrating radar data collected over greenland by the university of kansas between 1993 and 2013 to map this radiostratigraphy efficiently we developed new techniques for predicting reflection slope from the phase recorded by coherent radars when integrated along track these slope fields predict the radiostratigraphy and simplify semiautomatic reflection tracing core intersecting reflections were dated using synchronized depth age relationships for six deep ice cores additional reflections were dated by matching reflections between transects and by extending reflection inferred depth age relationships using the local effective vertical strain rate the oldest reflections dating to the eemian period are found mostly in the northern part of the ice sheet within the onset regions of several fast flowing outlet glaciers and ice streams reflections typically do not conform to the bed topography disrupted radiostratigraphy is also observed in a region north of the northeast greenland ice stream that is not presently flowing rapidly dated reflections are used to generate a gridded age volume for most of the ice sheet and also to determine the depths of key climate transitions that were not observed directly this radiostratigraphy provides a new constraint on the dynamics and history of the greenland ice sheet

water isotope diffusion rates from the northgrip ice core for the last 16 000 years glaciological and paleoclimatic implications abstract a high resolution 0 05 m water isotopic record o 18 is available from the northgrip ice core in this study we look into the water isotope diffusion history as estimated by the spectral characteristics of the o 18 time series covering the last 16 000 years the diffusion of water vapor in the porous medium of the firn pack attenuates the initial isotopic signal predominantly having an impact on the high frequency components of the power spectrum higher temperatures induce higher rates of smoothing and thus the signal can be used as a firn paleothermometer we use a water isotope diffusion model coupled to a steady state densification model in order to infer the temperature signal from the site assuming the accumulation and strain rate history as estimated using the gicc05 layer counted chronology and a dansgaard johnsen ice flow model the temperature reconstruction accurately captures the timing and magnitude of the bolling allerod and younger dryas transitions a holocene climatic optimum is seen between 7 and 9 ky b2k with an apparent cooling trend thereafter our temperature estimate for the holocene climatic optimum points to a necessary adjustment of the ice thinning function indicating that the ice flow model overestimates past accumulation rates by about 10 at 8 ky b2k this result is supported by recent gas isotopic fractionation studies proposing a similar reduction for glacial conditions finally the record presents a climatic variability over the holocene spanning millennial and centennial scales with a profound cooling occurring at approximately 4000 years b2k the new reconstruction technique is able to provide past temperature estimates by overcoming the issues apparent in the use of the classical o 18 slope method it can at the same time resolve temperature signals at low and high frequencies

magmatic and hydrothermal ammonium in granites of the tuscan magmatic province italy abstract the granites of the tuscan magmatic province have total ammonium contents that vary from 0 to 636 ppm in the three largest intrusions of elba giglio and montecristo the granites have not been affected by hydrothermal alteration and have low ammonium contents ranging up to 50 ppm there are detectable differences between the separate intrusions and also within the biggest intrusion granitic samples from the smaller outcrops and boreholes on the tuscan mainland are nearly all hydrothermally altered and these rocks show much higher ammonium contents ranging up to 636 ppm in the hydrothermally altered rocks the ammonium contents vary considerably over short distances and where they have been most closely sampled in the s2 borehole of castel di pietra there is a clear correlation between the ammonium content and the intensity of sulphide mineralization the major conclusion of the study is that the amount of ammonium present in the granites is determined much more by post magmatic hydrothermal alteration than by magmatic variation the mean ammonium contents of the fresh and hydrothermally altered granites are 17 and 95 ppm respectively

was the greenland ice sheet thinner in the late wisconsinan than now ice of wisconsinan origin which constitutes the basal layers of the ice caps in arctic canada and greenland flows three to four times more readily than the holocene ice above a model based on simple ice sheet profile theory is set up for the thickness response of the interior ice sheet regions to the progressive thinning of this soft layer the model is applied to calculate the thickness response of the greenland ice sheet at the locations dye 3 and crete and of the devon island ice cap in arctic canada it is concluded that the mechanism contributes significantly to the thickness change of the greenland ice sheet presently at a rate of about 1 cm yr 1 and that this rate of change will persist potentially over thousands of years to come as regards the devon island ice cap most of the estimated 15 thickness increase has already been accomplished a further consequence is that in the late wisconsinan ice thicknesses of the interior regions of the greenland ice sheet were likely to have been no greater and possibly even less than at present in spite of the larger geographical extent of the ice sheet it is argued that the glacial interglacial cycles of accumulation rate and ice temperature are likely to enhance this ice thickness variation

large scale reconstruction of accumulation rates in northern greenland from radar data internal layers in polar ice sheets have been mapped in many areas using information from radio echo sounding surveys the layers contain imprints of past accumulation patterns and ice flow and can potentially provide estimates of past accumulation rates in this study average accumulation rates for the past 14 7 ka in north and central greenland are recovered from the mapped depth of one pronounced isochrone using a simple ice flow model accounting for both vertical and horizontal flow combined with formal inversion techniques using this method upstream variations in the accumulation and flow are taken into account when inferring the accumulation rates from the deep layer this resulted in larger spatial variations in the inferred accumulation rates compared to approaches using a one dimensional ice flow model the largest differences were found in areas of significant horizontal flow the inferred accumulation rates agree with holocene estimates from ice cores in the interior of the ice sheet across the central ice divide the inferred accumulation pattern shows a pronounced gradient in accumulation rates similar to the present day pattern suggesting that the large scale pattern in the interior of central northern greenland has been consistent throughout the past 14 7 ka

precambrian ammonium biogeochemistry a study of the moine metasediments scotland abstract the content and isotopic composition of ammoniacal nitrogen have been determined in a suite of amphibolite facies metasediments of precambrian age whole rock samples and mica separates from the moine succession proterozoic of scotland were analysed by a combination of capacitance manometry and static vacuum mass spectrometry prior to the analyses the presence of the ammonium ion was confirmed by obtaining infrared ir absorption spectra from the micas principally biotites whole rock ammonium contents are high for amphibolite facies metasediments 140 422 ppm the ammonium ion was not detected in the ir absorption spectra of biotites from one schist however this sample contains neither garnet nor white mica for most of the samples the 15n values are very positive between 12 4 and 16 6 with one sample having a lower value of 8 4 the major variations between samples can be interpreted in terms of original sedimentary features for the ammonium bearing samples muscovites have concentrations between 365 and 633 ppm biotites between 845 and 1739 ppm for individual samples nh4 mu nh4 bi varies between 0 31 and 0 43 a much narrower range than has been reported previously there are no significant or consistent differences in 15n between co existing biotite and white mica nor between the 15n values of the mineral separates and their respective whole rock values the latter suggests minimal selective partitioning of isotopes between micas and plagioclase feldspar the only other potential host for ammonium in the rocks assuming that the ammonium has been derived from organic matter originally present at the time of deposition and by comparison with previous studies it can be inferred that the original 15n value of the moine sediments was somewhere between 5 and 10 similar to recent sediments this suggests that the nitrogen cycle in the proterozoic shelf sea from which the moine sediments were deposited was broadly similar to the present day overall the study underlines how under developed the n system is compared to other stable isotope systems notably c and s and we use the opportunity to highlight some areas that could be developed in the future however one aspect is clear future ammonium studies of metamorphic rocks will need to be performed at high resolution visible scale of heterogeneity with strong mineralogical petrological and field controls

u pb zircon and monazite ages from the kapuskasing uplift age constraints on deformation within the ivanhoe lake fault zone a decade of u pb dating of zircon and monazite from high grade metamorphic rocks in the kapuskasing uplift has identified a series of magmatic and metamorphic events between 2700 and 2585 ma and indicates that the onset of regional granulite metamorphism took place at mid crustal levels of the southern superior craton ca 2660 ma new u pb ages for zircon and monazite have been used to constrain the age of ductile deformation fabrics at two sites in the ivanhoe lake fault zone the structure along which the granulite facies kapuskasing structural zone was uplifted these results suggest that the fault zone was probably active in the late archean as young as 2630 ma and again at approximately 2500 ma

ammonium concentration and nitrogen isotope composition in metasedimentary rocks from different tectonometamorphic units of the european variscan belt the content and isotopic composition of ammonium nh4 were measured by using kjeldahl distillation for 63 samples of predominantly metasedimentary rocks from two different geological settings of the european variscides i e the erzgebirge and the zone of erbendorf vohenstrauss in germany the studied erzgebirge schists represent an early paleozoic passive margin sequence that reappears in different deep subducted metamorphic units this allows an efficient examination of nitrogen loss and isotope fractionation during prograde metamorphism ammonium is found to be progressively depleted accompanied by a shift in 15n from the low grade units 2 kbar 300 c with 638 124 ppm and 15n 2 2 0 6 to the garnet phyllite unit 9 kbar 470 c with 621 190 ppm and 15n 3 5 0 9 the mica schist eclogite unit 12 kbar 550 c with 394 113 ppm and 15n 3 9 0 8 and to the gneiss eclogite unit 12 kbar 730 c with 99 32 ppm and 15n 7 7 2 0 using equilibrium models for rayleigh distillation and batch volatilization suggest that the nitrogen depletion took place by ammonia release only for the mica schists this isotope fractionation can be explained by the loss of molecular nitrogen in comparison with the early paleozoic schists the proterozoic ortho and paragneisses of the erzgebirge gneiss unit 6 8 kbar 650 c contain significantly lower amounts of ammonium 70 ppm and reach relatively low 15n values 2 5 to 3 6 the paragneisses from the zone of erbendorf vohenstrauss zev sampled from the ktb pilot hole represents a separate tectonometamorphic unit that has undergone polyphase metamorphic conditions with the major imprint being amphibolite facies 7 kbar 650 700 c the metagraywackes have nh4 contents of 80 ppm and 15n of approximately 6 gneisses in the vicinity of a late cataclastic shear zone at 2000 m depth are characterized by a shift in nh4 250 ppm and 15n up to 12 which suggest late fluid rock interactions with a nitrogen and 15n enriched fluid

u pb zircon geochronology of the kapuskasing structural zone and vicinity in the chapleau foleyet area ontario geological development of the archean abitibi and wawa greenstone granite subprovinces of the superior province occurred in three main stages 1 a volcanic sedimentary depositional stage 2750 2696 ma ago 2 a metamorphic tectonic event 2700 2685 ma ago and 3 a late plutonic event at 2685 2668 ma the area between wawa in the west and ivanhoe lake in the east represents an oblique section through some 20 km of the crust where geochronological relationships can be examined at different structural levels much of the eastern wawa subprovince is characterized by tonalite granodiorite gneiss containing abundant mafic xenoliths and represents a crustal level intermediate between high level metavolcanic rocks near lake superior and the deep seated gneisses of the kapuskasing structural zone a tonalitic phase of the complex gneiss of the wawa subprovince has a minimum u pb zircon date of 2707 ma but the fine zircons are partly reset possibly in response to lit par lit injections of gneissic to foliat

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

nitrogen isotope record of fluid rock interactions in the skiddaw aureole and granite english lake district the skiddaw granite and its contact metamorphic aureole in the english lake district provide an excellent opportunity to test the capability of the n isotope system to trace devolatilization and large scale transfer of crustal fluids in the aureole skiddaw group metasedimentary rocks with relatively uniform lithology and major element compositions show a dramatic decrease in n content toward the granite contact from or 800 ppm at distances 2 5 km from the contact to 1 5 km these rocks have extremely uniform delta 15 n air near 3 7 per mil whereas closer to the contact or 1 km delta 15 n is shifted to higher values up to 8 7 per mil the coupled decreases in n content and increases in delta 15 n are compatible with the removal of n having low delta 15 n in fluids during continuous prograde devolatilization reactions involving the break down of white mica and the stabilization of biotite cordierite and andalusite bearing assemblages in the same metasedimentary rocks the lack of obvious trends in major element concentrations including sio 2 tio 2 sio 2 al 2 o 3 and the ratios of other major oxides to tio 2 and al 2 o 3 with distance from the granitic contact is consistent with minimal change in major element composition during the contact metamorphism ratios of whole rock n b rb and ba concentrations to whole rock k 2 o content are believed to reflect the differing fluid mica partitioning and involving varying relative proportions of white mica and biotite of these trace elements during devolatilization reactions greisenized skiddaw granite from a borehole is enriched in n range of 17 225 ppm for whole rocks and white mica separates relative to the unaltered granite whole rock 30 ppm and has delta 15 n of 1 0 to 4 8 per mil the n concentrations and delta 15 n of the wall rocks and greisenized granites combined with c isotopic data carbonate and carbonaceous matter for the same rocks are consistent with the mobilization of fluids having low delta 15 n and delta 13 c pdb values from the devolatilized aureole into the cooling intrusive body such transport is consistent with the predictions of recent theoretical models of late stage hydrothermal evolution in cooling intrusive systems

diagenesis of ammonium during hydrocarbon maturation and migration wilcox group louisiana u s a abstract as organic matter matures n is released in the reduced form as nh4 and substitutes for k in clay minerals as oil forms an abundance of n is generated coincident with the formation of authigenic illite this study suggests that nh4 released from organic matter during maturation migrates through sandstones together with the pore fluids and volatile phases generated from the migrating crude oil this causes anomalously high nh4 substitution in authigenic illites fixed nh4 concentrations in illite are a product of both hydrocarbon maturation and migration concentrations of fixed nh4 from organic rich mudstones in two wilcox group oil fields show a general increase with depth coincident with increasing organic maturation reservoir sandstones containing crude oils have fixed nh4 concentrations as much as two times those of the mudstones when values are normalized to the quantity of clay minerals fixed nh4 concentrations in the gaseous portion of the reservoir are three times as high two out of five sandstones from a dry hole adjacent to a producing oil field contain anomalously high 2 above mean concentrations of fixed nh4 with respect to total organic centent this together with a high production index and hydrogen index in those units suggests that migrating hydrocarbons may have influenced the fixed nh4 concentrations

relations of ammonium minerals at several hydrothermal systems in the western u s abstract ammonium bound to silicate and sulfate minerals has recently been located at several major hydrothermal systems in the western u s utilizing newly discovered near infrared spectral properties knowledge of the origin and mineralogic relations of ammonium minerals at known hydrothermal systems is critical for the proper interpretation of remote sensing data and for testing of possible links to mineralization submicroscopic analysis of ammonium minerals from two mercury and gold bearing hot springs deposits at ivanhoe nevada and mclaughlin california shows that the ammonium feldspar buddingtonite occurs as fine grained euhedral crystals coating larger sulfide and quartz crystals ammonium feldspar seems to precipitate relatively late in the crystallization sequence and shows evidence for replacement of nh4 by k or other monovalent cations some buddingtonite is observed in close association with mercury but not with gold ammonioalunite is found in a variety of isolated crystal forms at both deposits nitrogen isotopic values for ammonium bearing minerals show a 14 range in composition precluding assignment of a specific provenance to the nitrogen the correlations of nitrogen isotopic values with depth and ammonium content suggest some loss of nitrogen in the oxidizing supergene environment possibly as a metastable mineral the high ammonium content in these hydrothermal systems the close association to mercury and the small crystal size of the ammonium bearing minerals all suggest that ammonium may be transported in a late stage vapor phase or as an organic volatile such a process could lead to the formation of a non carbonaceous organic aureole above a buried geothermal source the discovery of a 10 km outcrop of ammonium minerals confirms that significant substitution of ammonium in minerals is possible over an extensive area and that remote sensing is a feasible means to detect such aureoles