sulfur isotopes in diamonds reveal differences in continent construction neoproterozoic west african diamonds contain sulfide inclusions with mass independently fractionated mif sulfur isotopes that trace archean surficial signatures into the mantle two episodes of subduction are recorded in these west african sulfide inclusions thickening of the continental lithosphere through horizontal processes around 3 billion years ago and reworking and diamond growth around 650 million years ago we find that the sulfur isotope record in worldwide diamond inclusions is consistent with changes in tectonic processes that formed the continental lithosphere in the archean slave craton diamonds that formed 3 5 billion years ago do not contain any mif sulfur younger diamonds from the kaapvaal zimbabwe and west african cratons do contain mif sulfur which suggests craton construction by advective thickening of mantle lithosphere through conventional subduction style horizontal tectonics

forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

middle archean continent formation by crustal delamination the processes that created the first large cratonic areas such as the pilbara and the kaapvaal remain poorly understood models based on the uniformitarian extrapolation of present day arc volcanic processes to a hotter early earth have not adequately explained the observations in these terranes here we propose an alternative mechanism for the formation of the earliest continental crust the formation of continental crust may be achieved by delamination of the lower eclogitic part of an oceanic plateau like protocrust such delamination results in uplift extension and the production of tonalite trondhjemite and granodiorite ttg suites as recorded in middle archean cratons the available geologic and geophysical observations in combination with model calculations permit this scenario as an alternative to subduction based hypotheses

petrology and geochemistry of mafic rocks in the acasta gneiss complex implications for the oldest mafic rocks and their origin abstract the acasta gneiss complex located in the western part of the slave province canada is widely recognized as the oldest eoarchean terrane in addition to felsic gneisses with the ages of 3 6 4 0 ga minor mafic rocks occur as rounded to elliptical enclaves and inclusions within the felsic gneisses despite serving as potential sources of geochemical information on the hadean mantle the mafic rocks have received less attention in previous studies thus we conducted a comprehensive geological petrological and geochemical investigation on the acasta mafic rocks to constrain their petrogenesis and geodynamic setting the mafic rocks comprise massive to weakly foliated amphibolite garnet amphibolite and hornblendite with variable abundances of hornblende plagioclase chlorite and quartz and subordinate clinopyroxene garnet and cummingtonite they commonly underwent high grade metamorphic recrystallization under amphibolite to upper amphibolite facies conditions the observed variations in mineral assemblages abundances and compositions reflect large differences in whole rock compositions likely caused by crustal anatexis during the eoarchean thermal events responsible for the generation of the surrounding felsic gneisses infiltration or extraction of felsic melts formed due to partial melting of precursor rocks can account for an overall negative correlation between al 2 o 3 and mgo contents and variable enrichments in the incompatible elements despite the widespread influence of anatexis on the geochemistry of acasta mafic rocks we identified the precursor compositions of the least modified amphibolites as basaltic magmas they are characterized by intermediate al 2 o 3 and mgo contents on the observed array and by near chondritic patterns for incompatible trace elements except for slightly negative nb and ta anomalies we considered two scenarios to explain the origin of eoarchean basaltic rocks with nb ta anomalies 1 generation of nb ta deficient basaltic magma in a suprasubduction setting analogous to modern arcs derived magmas and 2 generation of nb ta deficient basaltic magma from the melting of a nb ta deficient primitive mantle possible if the core contains significant proportions of the earth s nb and ta budget although the operation of plate tectonics and the presence of subduction zones at the end of hadean may be an attractive explanation for the observed nb ta depletions the chondritic relative proportions of other immobile trace elements for acasta mafic rocks leave open the possibility of their formation from an nb ta deficient primitive mantle

tracking partial melting and protolith nature by zircon u pb and hf o isotope compositions of migmatites in the north dabie terrane with emphasis on paleozoic low δ18o magmatism

parentage of archean basement within a paleoproterozoic orogen and implications for on craton diamond preservation slave craton and wopmay orogen northwest canada the wopmay orogen is a paleoproterozoic accretionary belt preserved to the west of the archean slave craton northwest canada reworked archean crystalline basement occurs in the orogen and new bedrock mapping u pb geochronology and sm nd isotopic data further substantiate a slave craton parentage for this basement detrital zircon results from unconformably overlying paleoproterozoic supracrustal rocks also support a slave craton provenance rifting of the slave margin began at ca 2 02 ga with a second rift phase constrained between ca 1 92 and 1 89 ga resulting in thermal weakening of the archean basement and allowing subsequent penetrative deformation during the calderian orogeny ca 1 88 1 85 ga the boundary between the western slave craton and the reworked archean basement in the southern wopmay orogen is interpreted as the rifted cratonic margin which later acted as a rigid backstop during compressional deformation age isotopic characteristics of plutonic phases track the extent and evolu

erosion of tephra from the 1980 eruption of mount st helens erosion of tephra on hillslopes north of mount st helens was measured at intervals of several months between 1980 and 1983 using large arrays of stakes rill erosion was initially greater than sheetwash but its importance decreased comparatively quickly both rill erosion and sheetwash decreased by one to two orders of magnitude over the three year period because a stable rill network developed and more permeable and less erodible substrates were created or exposed the decrease occurred prior to recovery of plants the causes for the rapid decline in erosion at mount st helens explain patterns of erosion following several other recent tephra eruptions basin wide annual rates of erosion were computed by mapping the surface cover the hillslope gradient and the texture and thickness of tephra and by using statistical relations developed between them and erosion rates annual erosion decreased from 26 mm between may 1980 and may 1981 to 1 8 mm between may 1982 and may 1983 projecting this rate of decline into the future indicated that only about one sixth of the tephra on hillslopes will be removed by water erosion before soil creep and other forms of mass wasting again dominate hillslope evolution

genesis of adakitic granitoids by partial melting of thickened lower crust and its implications for early crustal growth a case study from the huichizi pluton qinling orogen central china abstract adakitic rocks are often considered as a key to deciphering the genesis of archean ttgs and the early crustal growth granites from the huichizi pluton in the north qinling nq unit have high sr y and la yb n ratios similar to adakites their relatively high sio 2 k 2 o and na 2 o and very low mgo cr and ni contents are in the range of high sio 2 adakites and early archean ttgs and are compositionally similar to experimental melts derived from metabasalt sources new sims zircon u pb dating constrains the emplacement age of the huichizi pluton at 422 5 ma rock samples from the huichizi pluton have e nd t and zircon e hf t values similar to the neoproterozoic metabasalts in the nq unit in combination with their normal mantle like 18 o zir values these adakites are best explained by partial melting of the neoproterozoic mafic crustal root due to subduction of the shangdan ocean regional geological data suggest that the crust was probably thickened by a ca 490 ma arc collision process prior to the emplacement of the huichizi pluton our results confirm that underplating of mafic magma and its subsequent fusion triggered by slab subduction under high pressure conditions could be an important mechanism for the formation of early continental crust

the link between rossby wave breakings and weather regime transitions abstractthe link between rossby wave breaking rwb and the four wintertime weather regimes over the north atlantic domain is studied in this paper using the 40 yr ecmwf re analysis era 40 data frequencies of occurrence of anticyclonic and cyclonic wave breaking awb and cwb respectively events are computed each weather regime has its own characteristic pattern of rwb frequencies cwb events are found to be most frequent for the greenland anticyclone weather regime whereas awb events occur more for the atlantic ridge and the zonal regimes time lagged composites show that the rwb events characterizing each weather regime occur more often during the formation of the regime rather than during its decay this suggests a reinforcement of the weather regime by rwb an exception is the blocking weather regime which is destroyed by an increase of cwb events south of greenland weather regime transitions are then studied using the low frequency streamfunction tendency budget two types of precursors for th

generation of eoarchean continental crust from altered mafic rocks derived from a chondritic mantle the 3 72 ga aktash gneisses tarim craton nw china abstract our understanding of crust mantle evolution and geodynamics of the early earth has been hampered by the scarcity of hadean to eoarchean rocks and by the almost ubiquitous isotopic disturbance of the oldest rocks here we present in situ zircon u pb ages o oh and lu hf isotopes as well as whole rock sr nd hf pb isotopes for the recently identified eoarchean aktash gneisses in the tarim craton nw china the new age data confirm that the aktash gneisses were formed at least by 3 72 ga and were affected by two metamorphic events at 3 56 and 2 0 ga respectively whole rock isotopic data show that the sr nd hf pb isotopic systematics have all been variously disturbed by 2 0 ga regional granulite facies metamorphism resulting in decoupling of nd hf isotopic systematics that is in some samples similar to that observed in sw greenland the least disturbed sample 16alt67 however yielded chondritic to slightly sub chondritic whole rock end 0 3 and ehf 0 9 this is consistent with the in situ magmatic zircon ehf weighted mean 0 2 to 1 8 for most samples of the aktash gneisses suggesting a mafic source derived from chondritic mantle with a short crustal residence time our data thus provide no evidence for the presence of an eoarchean depleted mantle or a hadean enriched crust implying that early earth crust mantle differentiation was either limited or balanced by large scale crustal recycling in situ zircon o oh data show that metamorphism and later alteration tend to decrease the zircon 18 o of the aktash gneisses and the weighed mean 18 o of pristine magmatic zircons 6 2 6 3 from the least disturbed samples indicate that the mafic source was mildly altered at low temperature by the early hydrosphere facilitating partial melting and formation of some of the earliest preserved continental crust known on earth

unraveling the complexity of zircons from the 4 0 2 9 ga acasta gneiss complex abstract the acasta gneiss complex agc of the northwest territories canada contains some of the oldest evolved terrestrial crust and has a sustained record of more than a billion years of magmatism 4 0 2 9 ga these rocks provide an opportunity to investigate the nature of continental crust formation on the early earth because complexities in zircons and bulk rocks are characteristic of the early earth record including the agc strategies are needed to extract accurate and meaningful age and isotopic information in order to evaluate the accuracy of the early earth hf isotope record we examined agc zircons from a range of lithologies using paired chemical abrasion isotope dilution u pb age and solution lu hf isotope analysis and compared these with previous results obtained using laser ablation split stream lass analysis we describe an approach whereby lass is used to identify rocks with the least complex zircons and when appropriate solution methods are then used to refine the age and hf isotopic composition to the highest precision this two pronged analytical approach results in a more robust determination of the age and hf isotopic record of complex rocks and zircons and allows identification of complexity in the hf isotopic record that would not be apparent by solution analysis alone thereby refining the record of magmatic evolution on the early earth despite the better precision solution techniques are unsuitable for rocks with complexly zoned zircons in particular zircons from the two oldest agc rocks in this study have positive initial ehf values when determined by solution analysis 2 8 and 4 9 at 4 0 and 3 9 ga respectively but have negative ehf values when determined by lass 3 4 and 4 7 respectively this is attributable to the presence of later radiogenic overgrowths on the zircon grains which are incorporated in solution analyses but can be avoided using lass this provides important clarity to the agc hf isotope record in total all but one of the agc rocks we analyzed including the oldest samples have negative ehf values and indicate derivation from an enriched reservoir none of these samples in contrast to whole rock nd isotope compositions have sufficiently positive ehf values to indicate their derivation from a depleted mantle reservoir despite the presence of ancient agc crust there is no record of corresponding mantle depletion this implies that extraction of hadean crust in this region did not happen in sufficient volume to result in widespread mantle depletion in the agc source by the eoarchean our results underscore the importance of identifying different components in ancient zircons and the rocks that contain them and accurately determining the age and isotopic composition of those components this is critically important for clarifying the record of the formation of enriched crust and development of the depleted mantle in earth s early history

formation of paleoarchean continental crust through infracrustal melting of enriched basalt abstract paleoarchean rocks of the tonalite trondhjemite granodiorite ttg series require a basaltic source region more enriched in k lile th and lree than the low k tholeiites typical of archean supracrustal sequences most ttg of the pilbara craton in northwestern australia formed between 3 5 and 3 42 ga through infracrustal melting of a source older than 3 5 ga basaltic to andesitic rocks of the 3 51 ga coucal formation at the base of the pilbara supergroup are amongst the only well preserved remnants of pre 3 5 ga supracrustal material on earth and may have formed a large proportion of pre 3 5 ga pilbara crust these rocks are significantly enriched in k lile th and lree compared to post 3 5 ga paleoarchean basalts and andesites and form a compositionally suitable source for ttg enrichment in these basalts was not the result of crustal assimilation but was inherited from a mantle source that was less depleted than modern morb source and was enriched in recycled crustal components we suggest that the formation of paleoarchean ttg and of their voluminous mafic source regions reflects both a primitive stage in the thermal and compositional evolution of the mantle and a significant prehistory of crustal recycling

occurrence and geochronology of the eoarchean 3 9 ga iqaluk gneiss in the saglek block northern labrador canada evidence for the oldest supracrustal rocks in the world abstract understanding earth s early evolution requires decoding the cryptic geological record that is preserved in a few extremely rare terrains on earth the saglek hebron block in the north atlantic craton is one of the oldest terrains in the world to better understand the eoarchean history of the saglek hebron block we undertook a comprehensive geological and geochronological investigation of this terrain the saglek hebron locality contains orthogneisses and supracrustal rocks which are classified into two groups of the eoarchean and mesoarchean suites based on field associations defined by intrusion of the mesoarchean mafic saglek dykes the eoarchean suites contain the nulliak supracrustal rocks and uivak gneisses in this paper we report cathodoluminescence cl observations and u pb ages of zircons from the eoarchean orthogneisses to constrain the evolution of the saglek hebron block we made detailed sketch maps 1 20 of critical outcrops to establish relative chronological relationships between lithologies at least seven generations of mafic and felsic units were recognized in an outcrop of the st john s harbour south area the first and second generations are mafic supracrustal rocks whereas the third to seventh generations are tonalitic gneisses which are intruded into the supracrustal units we study cl observation geochemistry and u pb dating of zircons from the oldest and youngest tonalitic gneisses the cl observation of internal structures of the zircons showed they typically comprise three domains of core mantle and rim the cores have clear oscillatory zoning whereas the mantles lack oscillatory zoning and exhibit dark cl emission and structureless cl images the obvious correlation between the ages and chemical compositions of u contents and th u ratios indicates that the cores with low u contents and high th u ratios preserve the protolith age the cores with clear oscillatory zoning low u contents and high th u ratios of zircons from the oldest suite plot on a concordia line with ages ranging from 3953 to 3797 ma the oldest and average ages of the six oldest spots on the concordia line are 3953 54 and 3920 49 ma respectively on the other hand the average age of the four old spots of zircons from a younger generation of orthogneiss is 3869 63 ma the geological and geochronological relationships between the third and seventh generation orthogneisses are consistent with each other confirming an eoarchean age of ca 3 9 ga for the oldest suite of the uivak gneisses we name this oldest suite the iqaluk gneiss as the protolith of the iqaluk gneiss was clearly intruded into the nulliak supracrustal rocks the nulliak supracrustals must have a minimum age of ca 3 9 ga indicating that they are the oldest supracrustal rocks on earth but further studies should be necessary to obtain more precise age of the iqaluk gneiss and nulliak supracrustal rocks

silicic magma petrogenesis in iceland by remelting of hydrothermally altered crust based on oxygen isotope diversity and disequilibria between zircon and magma with implications for morb terra nova 24 227 232 2012 abstract petrogenesis of silicic magmas in iceland has fundamental significance for understanding the relative importance of fractional crystallization of mantle derived basalt and partial melting of hydrothermally altered basaltic crust in formation of the earliest continental crust first results of in situ oxygen isotope investigation of zircons in large volume silicic eruptive products of three volcanoes in iceland askja torfajokull and hekla demonstrate isotope diversity and disequilibria and long u th zircon pre eruptive residence of 103 104 year this suggests that zircons did not grow from their host melts but instead were inherited from older magma batches and leftover cumulates with generally low and variable 18o values this study demonstrates that segregation of cubic kilometres of silicic magma is faster than mineral diffusive or recrystallization time scales estimated at 103 years and it suggests that partial melting of hydrothermally altered and oxidized oceanic crust is the mechanism that best explains silicic rocks in iceland and early earth environments

low δ18o rhyolites from yellowstone magmatic evolution based on analyses of zircons and individual phenocrysts the yellowstone plateau volcanic field is one of the largest centers exchange time to form zoned zircons is between a few hundred and a few thousand years which reflects the residence time of lowo of rhyolitic magmatism on earth major caldera forming eruptions are followed by unusual lowo rhyolites new oxygen isotope magmas after formation and before eruption petrologic and geochemical data from rhyolites belonging to the 2 0 my eruptive history of yellowstone are presented with emphasis on the genesis of lowo magmas erupted after the huckleberry ridge tuff 2 0 ma 2500 km and lava creek tuff 0 6 ma

low 18o silicic magmas why are they so rare low 180 silicic magmas are reported from only a small number of localities e g yellowstone and iceland yet petrologic evidence points to upper crustal assimilation coupled with fractional crystallization afc during magma genesis for nearly all silicic magmas the rarity of 10w l o magmas in intracontinental caldera settings is remarkable given the evidence of intense 10w l o meteoric hydrothermal alteration in the subvolcanic remnants of larger caldera systems in the platoro caldera complex regional ignimbrites 150 1000 km3 have plagioclase 6180 values of 6 8 0 1 whereas the middle tuff a small volume est 50 100 km3 post caldera collapse pyroclastic sequence has plagioclase 8 80 values between 5 5 and 6 8 o on average the plagioclase phenocrysts from the middle tuff are depleted by only 0 3 0 relative to those in the regional tuffs at yellowstone small volume post caldera collapse intracaldera rhyolites are up to 5 5 o depleted relative to the regional ignimbrites two important differences between the middle tuff and the yellowstone 10w 180 rhyolites elucidate the problem middle tuff magmas reached water saturation and erupted explosively whereas most of the 10w l 80 yellowstone rhyolites erupted effusively as domes or flows and are nearly devoid of hydrous phenocrysts comparing the two eruptive types indicates that assimilation of 10w 180 material combined with fractional crystallization drives silicic melts to water oversaturation water saturated magmas either erupt explosively or quench as subsurface porphyrins bejiire the magmatic 180 can be dramatically lowered partial melting of low 180 subvolcanic rocks by near anhydrous magmas at yellowstone produced small volume 10w 180 magmas directly thereby circumventing the water saturation barrier encountered through normal afc processes

simple coupled midlatitude climate models a set of simple analytical models is presented and evaluated for interannual to decadal coupled ocean atmosphere modes at midlatitudes the atmosphere and ocean are each in sverdrup balance at these long timescales the atmosphere s temperature response to heating determines the spatial phase relation between sst and sea level pressure slp anomalies vertical advection balancing heating produces high low slp lying east of warm cold sst anomalies as observed in the antarctic circumpolar wave acw the decadal north pacific mode and the interannual north atlantic mode zonal advection in an atmosphere with a rigid lid produces low slp east of warm sst however if an ad hoc equivalent barotropic atmospheric response is assumed high slp lies east of warm sst relaxation to heating produces behavior like the observed north atlantic decadal pattern with low slp over warm sst meridional advection in the atmosphere cannot produce the observed sst slp patterns the dominant balance in the ocean s temperature equation determines the phase speed of the modes the coupled mode is nondispersive in all models examined here indicating the need for additional processes for modes with an sst slp offset as observed in the acw and north pacific ekman convergence acting as a heat source causes eastward propagation relative to the mean ocean flow sverdrup response to ekman convergence acting on the mean meridional temperature gradient causes westward propagation relative to the mean ocean flow when the ocean temperature adjusts through surface heat flux alone the mode is advected by the mean ocean flow and is damped relaxation to heating in the atmosphere when operating with sverdrup response in the ocean produces the only complete solution presented here that exhibits growth with an e folding timescale of order 100 days this solution appears appropriate for the north atlantic decadal mode in northern hemisphere basins with meridional boundaries the same sets of dynamics create the observed sst slp phase relation an additional factor is the creation of sst anomalies through variations in the western boundary current strengths which are related to the zonally integrated wind stress curl over the whole basin if barotropic and hence fast adjustment is assumed the resulting positive feedback can maintain or strengthen the coupled anomalies in the north pacific and interannual north atlantic modes

cyclic formation and stabilization of archean lithosphere by accretionary orogenesis constraints from ttg and potassic granitoids north china craton this study is financially supported by the national natural science foundation of china grant nos 41502179 41530207 41472165 and central university basic scientific research business expenses of china university of geosciences beijing grant no 2652015038 pac acknowledges support from australian research council grant fl160100168

source of gold in neoarchean orogenic type deposits in the north atlantic craton greenland insights for a proto source of gold in sub seafloor hydrothermal arsenopyrite in the mesoarchean abstract given that gold au mostly remained in the incipient earth mantle until ca 3 9 3 8 ga a proto source of gold may have been present in the dominantly mafic crust precursor born through first stage melting of the early earth mantle in south westernmost greenland a fragment of the north atlantic craton is characterised by greenstone belts comprising mafic volcanic and magmatic rocks and harzburgite cumulates that were emplaced at ca 3 0 ga prograde amphibolite facies metamorphic assemblages and caused local dissolution of arsenopyrite during this retrograde tectono metamorphic stage in gold rich shear zones the re os geochronometer in arsenopyrite was reset to a neoarchean age while invisible gold was liberated and deposited as free gold with 2 66 ga pyrite re os isochron ages the initial os isotope ratios of neoarchean arsenopyrite 187os 188osi 0 13 0 02 and gold bearing pyrite 0 12 0 02 overlap with the estimated 187os 188os ratio of the mesoarchean mantle 0 11 0 01 and preclude contribution of radiogenic crustal os from evolved lithologies in the accretionary arc complex but instead favour a local contribution in os from basaltic rocks and serpentinised harzburgite protoliths by metamorphic fluids thus the ca 2 66 ga lode gold mineralisation identified in the north atlantic craton may illustrate a gold endowment in shear zones in earth s stabilizing continental crust at the time of the 2 75 2 55 ga global gold event through metamorphic upgrading of bulk gold which had originally been extracted from the mesoarchean mantle and concentrated in hydrothermal arsenopyrite deposits in oceanic crust beneath the overall reduced mesoarchean ocean

in situ evidence for continental crust on early mars understanding of the geologic evolution of mars has been greatly improved by recent orbital 1 3 in situ 4 5 and meteorite 6 8 data but insights into the earliest period of martian magmatism 4 1 to 3 7 billion years ago remain scarce 9 the landing site of nasa s curiosity rover gale crater which formed 3 61 billion years ago 10 within older terrain 11 provides a window into this earliest igneous history along its traverse curiosity has discovered light toned rocks that contrast with basaltic samples found in younger regions 12 here we present geochemical data and images of 22 specimens analysed by curiosity that demonstrate that these light toned materials are feldspar rich magmatic rocks the rocks belong to two distinct geochemical types alkaline compositions containing up to 67 wt sio2 and 14 wt total alkalis na2o k2o with fine grained to porphyritic textures on the one hand and coarser grained textures consistent with quartz diorite and granodiorite on the other hand our analysis reveals unexpected magmatic diversity and the widespread presence of silica and feldspar rich materials in the vicinity of the landing site at gale crater combined with the identification of feldspar rich rocks elsewhere 9 13 14 and the low average density of the crust in the martian southern hemisphere 15 we conclude that silica rich magmatic rocks may constitute a significant fraction of ancient martian crust and may be analogous to the earliest continental crust on earth

could iceland be a modern analogue for the earth s early continental crust for the last two decades iceland and other oceanic plateaux have been considered as potential analogues for the formation of the early earth s continental crust this study examines the compositions of silicic rocks from modern oceanic plateaux revealing their differences to archaean continental rock types trondhjemite tonalite granodiorite or ttg and thereby emphasising the contrasted mechanisms and or sources for their respective origins in most oceanic plateaux felsic magmas are thought to be formed by fractional crystallization of basalts in iceland the interaction between mantle plume and the mid atlantic ridge results in an abnormally high geothermal gradient and melting of the hydrated metabasaltic crust however despite the current archaean like high geothermal gradients melting takes place at a shallow depth and is unable to reproduce the ttg trace element signature consequently oceanic plateaux are not suitable environments for the genesis of the archaean continental crust however their subduction could account for the episodic crustal growth which has occurred throughout the earth s history

southern hemisphere blocking onsets associated with upper tropospheric divergence anomalies abstract upper tropospheric divergence anomalies and divergence tendencies prior to and during the onset of blocking have been investigated for selected cases over the southern hemisphere in search of links between the upper tropospheric response to tropical convective activity and the onset of blocking in midlatitudes climatologies of blocking defined by an objective index and divergence are established for the southern hemisphere and the southern pacific respectively relative blocking frequency versus longitude reveals a region of maximum blocking activity between 160 e and 75 w blocking frequencies for each enso phase indicate a shift toward the late austral fall and early winter during the warm phase whereas during the cold and neutral phases the highest frequencies are in june and july respectively composites of area averaged divergence anomalies for the selected blocking cases reveal more anomalous divergence than during nonblocking periods over the blocking regions and the immediate upstre

a yilgarn seed to the pilbara craton australia evidence from inherited zircons knowledge of the age and compositional architecture of archean cratonic lithosphere is critical for models of geodynamics and continental growth on early earth but can be difficult to unravel from

assessment of the interrill and rill contributions to total soil loss from an upland field plot abstract govers g and poesen j 1988 assessment of the interrill and rill contributions to total soil loss from an upland field plot geomorphology 1 343 354 erosion measurements were carried out on an upland field plot in the loam region of belgium the sediment being detached by splash on interrill areas is transported to the channel system mainly by interrill wash over the entire 7500 m field plot rill and gully erosion is more important than interrill erosion however the relative importance of interrill erosion varies in time and in space due to changes of the interrill surface characteristics and the activation of sidewall and gullying processes in the channel network these factors should be taken into account in a realistic rill interrill erosion model introduction at present few available field data permit the separation of rill and or gully and interrill contributions to sediment removal from arable land the scarcity of this type of field data is a major factor inhibiting the validation and fur ther development of recently developed physi cally based models describing upland erosion by water e g foster 1982 rose et al 1983 knowledge of the contribution of rill and inter rill processes to total soil loss has also some practical interest if the ratio between rill and interrill erosion were known even approxi mately it would be possible to obtain a quick estimate of total erosion by making a volumet ric recording of rill patterns plot measure

magmatic complexity on early mars as seen through a combination of orbital in situ and meteorite data abstract until recently mars was considered a basalt covered world but this vision is evolving thanks to new orbital in situ and meteorite observations in particular of rocks of the ancient noachian period in this contribution we summarise newly recognised compositional and mineralogical differences between older and more recent rocks and explore the geodynamic implications of these new findings for example the msl rover has discovered abundant felsic rocks close to the landing site coming from the wall of gale crater ranging from alkali basalt to trachyte in addition the recently discovered martian regolith breccia nwa 7034 and paired samples contain many coarse grained noritic monzonitic clasts demonstrably noachian in age and even some clasts that plot in the mugearite field olivine is also conspicuously lacking in these ancient samples in contrast to later hesperian rocks the alkali suite requires low degree melting of the martian mantle at low pressure whereas the later hesperian magmatism would appear to be produced by higher mantle temperatures various scenarios are proposed to explain these observations including different styles of magmatic activity i e passive upwelling vs hotspots a second petrological suite of increasing interest involves quartzo feldspathic materials that were first inferred from orbit in local patches in the southern highlands and in the lower units of valles marineris however identification of felsic rocks from orbit is limited by the low detectability of feldspar in the near infrared on the other hand the msl rover has described the texture mineralogy and composition of felsic rocks in gale crater that are granodiorite like samples akin to terrestrial ttg tonalite trondhjemite granodiorite suites these observations and the low average density of the highlands crust suggest the early formation of continental crust on mars although the details of the geodynamic scenario and the importance of volatiles in their generation are aspects that require further work

geology of the eoarchean 3 95 ga nulliak supracrustal rocks in the saglek block northern labrador canada the oldest geological evidence for plate tectonics the earth is a unique planet which has been highly evolved diversified and complicated through geologic time and underwent many key events including giant impact magma ocean core formation large scale mantle differentiation and late heavy bombardment especially in its dawn but our knowledge of early earth is limited due to the lack of the hadean supracrustal rocks the supracrustal rocks with the eoarchean ages provide key evidence for the earth s early evolution but few supracrustal rocks have been comprehensively investigated therefore we mapped in seven areas of the saglek block northern labrador where ancient supracrustal sequences are interleaved with a diverse assemblage of orthogneisses early studies suggested that some of them have the mesoarchean ages because of the lack of the mesoarchean saglek dyke but we found the saglek dykes in the areas to recognize the eoarchean nulliak supracrustal rocks and uivak gneiss in all the areas recent reassessment of u pb dating and cathodoluminescence observation of zircons from the oldest suites of the uivak gneiss showed that the uivak gneiss has the eoarchean age 3 95 ga and forms the iqaluk uivak gneiss series because our geological survey clearly showed that the iqaluk uivak gneisses were intruded into the nulliak supracrustal belts the nulliak supracrustal rocks are the oldest supracrustal rock in the world the supracrustal belts consist of piles of fault bounded blocks which are composed of the ultramafic rocks mafic rocks and sedimentary rocks in ascending order similar to modern ocean plate stratigraphy ops in addition small scale duplex structures are found over the areas the presence of duplex structure and ops indicates that the 3 95 ga nulliak supracrustal belts originate from an accretionary complex the presence of the accretionary complex ophiolite and granitic continental crust provides the oldest evidence for the plate tectonics on the early earth

earth s earliest evolved crust generated in an iceland like setting the setting in which earth s first continental crust formed is unclear because few rocks survive from that time geochemical analysis of 4 03 billion year old rocks discovered in the acasta gneiss complex canada reveals rocks that are strikingly similar to those formed in iceland today implying earth s first continental crust was generated in an iceland like setting

interpreting low frequency modes of southern hemisphere atmospheric variability as the rotational response to divergent forcing abstract the principal modes of southern hemisphere low frequency variability have recently been calculated using a 39 yr record of 300 hpa streamfunction fields from the ncep ncar reanalysis dataset the authors attempt to interpret these modes as the rotational response to some divergent forcing for a range of mean states the linearized barotropic vorticity equation bve is used to solve for the divergent wind that would generate or at least be consistent with the observed vorticity modes several of these low frequency modes can be generated by forcing the bve with fairly simple divergent wind fields that could easily be interpreted as resulting from anomalous tropical convection in particular this is found to be true for streamfunction anomalies with el nino southern oscillation enso high latitude mode south pacific wave and madden julian oscillation structure the authors speculate that it may be possible to relate these calculated divergent wind fields to recently observed olr fields and h

the composite archaean grey gneisses petrological significance and evidence for a non unique tectonic setting for archaean crustal growth abstract the geodynamic context of formation of the archaean continental crust is a matter of debate the crust is largely made of grey gneiss complexes a composite rock assemblage dominated by granitoids that are generally regarded as belonging to the ttg tonalite trondhjemite granodiorite series using a large database of published ttg and grey gneiss compositions it is possible to show that the granitoids forming grey gneiss complexes actually belong to at least four main geochemical groups i a potassic component made of granodiorites and formed by melting of existing crustal lithologies and ii three sodic groups ttg proper that comprise low medium and high pressure groups the geochemistry of the low pressure group is consistent with derivation from a plagioclase and garnet amphibolite the medium pressure group was formed in equilibrium with a garnet rich plagioclase poor amphibolite whereas the high pressure group derived from a rutile bearing eclogite as the temperature of melting of metamafic rocks is largely independent from pressure this corresponds to melting along a range of contrasting geothermal gradients in turn reflecting a range of tectonic sites for the formation of the archaean continental crust

evolution of the archean continental crust some 20 years of plate tectonic theory combined with new insights into the fine structure of the lithosphere the application of multielement geo chemical and isotopic studies paleomagnetism and geophysical modeling of mantle processes have profoundly influenced present thinking on the origin and evolution of the earth s early continental crust previously our knowledge of the continental crust was based almost exclusively on field geological observations although there is now general agreement on how the earth worked for the last 200 m y because of observable evidence in the oceans and continents e g bird 1980 condie 1982 it has proved difficult to extend this history into more ancient times in view of the lost oceanic record and the ambiguity and complexity of the pre mesozoic rock relationships in the continents dewey 1982 however preserved characteristic rock as semblages uniquely identifying modern type wilson cycle processes i e opening and closure of oceans underlain by oceanic crust have now been recognized in continental terranes as old as 900 m y and these assemblages provide strong evidence for the conclusion that the present global tectonic regime has governed the evolution of the lithosphere at least since the late precambrian kroner 1977 1981a b goodwin 1981 profound disagreement on the older crustal history however prevails to the present day since typical features characterizing phanerozoic accretionary terranes such as obducted ophiolites blueschists and franciscan type melanges have not been found in more ancient regions thus two types of evolutionary models have been developed one type postulates uniformitarian development back to the earliest archean burke

disturbances in the sm nd isotope system of the acasta gneiss complex implications for the nd isotope record of the early earth abstract the whole rock 143nd isotope record of the acasta gneiss complex agc has been used to argue for the development of both depleted mantle and enriched crustal reservoirs during the hadean the 147sm 143nd isotope compositions of rocks from agc however fall on an array with an errorchron date of 3 3 ga despite the majority of samples yielding substantially older zircon u pb ages this has led to the suggestion of a major sm nd resetting event at this time in the agc to better understand the sm nd bulk rock systematics of the agc we investigate the u pb age and sm nd isotope compositions of apatite and titanite from the agc using the laser ablation split stream lass method apatite from all samples yield broadly similar u pb dates of 1 89 to 1 83 ga in agreement with resetting during the 1 9 ga wopmay orogen the sm nd isotope compositions of the apatite however lie off the wr errorchron and show a high degree of initial nd isotope heterogeneity conversely titanite sm nd systematics are consistent with the 3 3 ga whole rock errorchron further supporting sm nd modification at 3 3 ga taken together these show that the oldest rocks in the agc underwent at least two post crystallization events at 3 3 and 1 9 ga that modified the sm nd isotope system these results also demonstrate the relative mobility of the sm nd isotope system during orogenic events which may compromise the ability to determine robust bulk rock 143nd isotope signatures

the role of interactions between multiscale circulations on the observed zonally averaged zonal wind variability associated with the madden julian oscillation abstractthe mechanisms driving the upper tropospheric zonal mean intraseasonal zonal wind associated with the madden julian oscillation are examined through budget analysis during boreal winter to diagnose the role of nonlinear and cross scale interactions the wind fields are decomposed into three temporal bands including the intraseasonal time scale 30 100 days and periods shorter and longer than the intraseasonal time scales the intraseasonal zonal mean circulation and its driving mechanisms are first examined based on the leading eofs of the intraseasonal zonal wind consistent with previous studies of intraseasonal atmospheric angular momentum the upper troposphere zonal mean intraseasonal zonal wind anomaly begins in the tropics and propagates poleward results show that interaction between the background state and intraseasonal time scale zonally symmetric and asymmetric circulation helps drive changes in the tropical intraseasonal zonal wind and its poleward propagation the intraseasonal a

tropical connections to climatic change in the extratropical southern hemisphere the role of atlantic sst trends abstractthe austral spring relationships between sea surface temperature sst trends and the southern hemisphere sh extratropical atmospheric circulation are investigated using an atmospheric general circulation model agcm a suite of simulations are analyzed wherein the agcm is forced by underlying sst conditions in which recent trends are constrained to individual ocean basins pacific indian and atlantic allowing the impact of each region to be assessed in isolation when forced with observed global sst the model broadly replicates the spatial pattern of extratropical sh geopotential height trends seen in reanalyses however when forcing by each ocean basin separately similar structures arise only when atlantic sst trends are included it is further shown that teleconnections from the atlantic are associated with perturbations to the zonal walker circulation and the corresponding intensification of the local hadley cell the impact of which results in the development of atmospheric rossby w

horizontal structure and seasonality of large scale circulations associated with submonthly tropical convection the relationship between deep tropical convection and large scale atmospheric circulation in the 6 30 day period range is examined regression relationships between filtered outgoing longwave radiation at various locations in the tropics and 200 and 850 mb circulation are mapped for the standard seasons and the spatial structure and seasonal dependence of the results are interpreted in view of the basic state circulation in regions where the convection is embedded in upper level easterlies anomalous equatorial easterly flow is typically present at 200 mb within and to the west of the convective signal along with patterns of meridional outflow into subtropical anticyclonic perturbations lagged relationships suggest that the convection is forcing the circulation in many of these cases the outflow and subtropical circulations are strongest into the winter hemisphere during the solstitial seasons with more symmetric signals about the equator seen in the equinoctial seasons the longitudinal positioning of the subtropical features with respect to the convection varies but is generally located due poleward or just to the east of the convection there tends to be a first baroclinic mode vertical structure to these circulations such that equatorial westerlies are present at 850 mb within the convection with closed circulations on either side of the equator resembling equatorial rossby modes especially common over the atlantic and pacific sectors as a contrast in regions located within upper level westerlies or along the margin of influence of upper westerly disturbances convection appears to be forced by upper level wave energy propagating into the deep tropics with the heating located in the upward motion region ahead of upper level troughs this occurs over the atlantic and eastern pacific sectors during northern winter and spring and over australia the south pacific and south america during southern summer when upper westerlies are at relatively low latitudes where they can interact with deep tropical convection the results confirm theoretical and modeling ideas that suggest that rossby wave energy is able to propagate into the deep tropics in regions where upper level westerlies exist in the tropics

late neoarchean subduction related crustal growth in the northern liaoning region of the north china craton evidence from 2 55 to 2 50 ga granitoid gneisses abstract the north china craton ncc dominated by 2 6 2 5 ga tectonothermal events provides a natural laboratory to study neoarchean crustal growth and geodynamic evolution late neoarchean granitoid gneisses are well exposed in the northern liaoning province located north of the ancient anshan benxi terrane along the northeastern margin of the eastern block eb of the ncc la icpms zircon u pb isotopic data reveal that granitoid gneisses in the qingyuan area can be grouped into two major episodes i e 2559 2534 ma strongly gneissic quartz dioritic and tonalitic to trondhjemitic gneisses and 2529 2495 ma weakly gneissic to massive quartz monzodioritic and monzogranitic gneisses with subordinate tonalitic to trondhjemitic gneisses the late magmatic episode was accompanied by regionally high grade metamorphism 2510 2495 ma most granitoid gneisses display highly depleted zircon ehf t2 values 4 2 to 8 1 whereas one monzogranitic gneiss shows negative values of 4 7 to 1 0 indicating late neoarchean crustal growth with minor involvement of ancient continental materials probably sourced from the anshan benxi terrane geochemical and petrogenetic studies reveal that the quartz dioritic magmas were derived from partial melting of plagioclase poor garnet amphibolites or eclogites metamorphosed from oceanic slab materials with slab melts contaminated by mantle wedge peridotites during ascent the tonalitic to trondhjemitic magmas stemmed from partial melting of mainly juvenile metabasaltic rocks with minor metagreywackes of lower arc crust in comparison the quartz monzodioritic and monzogranitic magmas were derived respectively from partial melting of depleted mantle sources metasomatized by slab derived fluids and metagreywackes with different crustal resident ages at middle to lower crustal levels combined with previous studies of supracrustal metavolcanic rocks the northern liaoning province records late neoarchean crustal growth evolving from mid ocean ridge through initiation and maturation of an intra oceanic arc to arc continent collision arc continent accretion and possibly slab rollback processes may have triggered reworking of both juvenile arc crust and minor ancient continental margin materials generating the magmatic precursors for the monzogranitic gneisses overall the intense late neoarchean crustal growth of the eb was controlled mainly by arc continent accretion possibly linked to global assembly of cratonic fragments

coupled zircon lu hf and u pb isotopic analyses of the oldest terrestrial crust the 4 03 ga acasta gneiss complex abstract the acasta gneiss complex of the northwest territories canada contains some of the earliest terrestrial continental crust and thus provides a critical sample set for characterization of crust forming processes on the early earth here we report the results of a combined lu hf and u pb isotopic study of zircons from predominantly felsic orthogneisses from the acasta gneiss complex that crystallized between 4 0 and 2 9 ga many of which contain complex zoning and therefore require an analytical treatment suited to distinguish amongst compositionally distinct age and hf isotopic domains to ensure the reliability of the analyses and of subsequent geologic interpretations we employed the laser ablation split stream lass technique to concurrently measure the lu hf and u pb isotopic systems in zircon our results confirm prior findings of precursor hadean crust 4 0 ga in the source of these rocks and the continued involvement of this reservoir until 3 6 ga we present evidence for the input of relatively more juvenile material at 3 6 ga which we suggest corresponds to a fundamental change in the source of the magmas this study extends the lower bound of the published acasta hf isotopic record from 3 6 ga to 2 9 ga and demonstrates that the 3 6 ga 2 9 ga interval is largely represented by reworking of relatively juvenile 3 6 ga crust and the diminution of the 4 0 ga crustal signal significantly there is no evidence that rocks within the acasta gneiss complex were derived from a strongly depleted mantle

petrogenesis and tectonics of the acasta gneiss complex derived from integrated petrology and 142nd and 182w extinct nuclide geochemistry abstract the timing and mechanisms of continental crust formation represent major outstanding questions in the earth sciences extinct nuclide radioactive systems offer the potential to evaluate the temporal relations of a variety of differentiation processes on the early earth including crust formation here we investigate the whole rock 182w 184w and 142nd 144nd ratios and zircon 17 o values of a suite of well studied and lithologically homogeneous meta igneous rocks from the acasta gneiss complex northwest territories canada including the oldest known zircon bearing rocks on earth in the context of previously published geochemical data and petrogenetic models the new 142nd 144nd data indicate that formation of the hadean eoarchean acasta crust was ultimately derived from variable sources both in age and composition although 4 02 ga crust was extracted from a nearly bulk earth source heterogeneous 142nd signatures indicate that eoarchean rocks of the acasta gneiss complex were formed by partial melting of hydrated hadean age mafic crust at depths shallower than the garnet stability field by 3 6 ga granodioritic granitic rocks were formed by partial melting of archean hydrated mafic crust that was melted at greater depth well into the garnet stability field our 182w results indicate that the sources to the acasta gneiss complex had homogeneous high 182w on the order of 10 ppm a signature ubiquitous in other eoarchean terranes no significant deviation from the terrestrial mass fractionation line was found in the triple oxygen isotope 16o 17o 18o compositions of acasta zircons confirming homogeneous oxygen isotope compositions in earth s mantle by 4 02 ga

sediment routing in a small drainage basin in the blast zone at mount st helens washington u s a abstract a quantitative description of the routing of 400 10 3 t km 2 of sediment delivered to a 2 4 km 2 basin by the may 18 1980 eruption of mount st helens has been developed for the period may 1980 june 1981 on hillslopes this sediment resided in primary and secondary remobilized primary storage sites hillslope erosion processes redistributed sediment on hillslopes and delivered sediment to the channel system these processes included shallow sliding from slopes steeper than 0 70 sheet erosion and rill erosion the most important discrete sites of storage on hillslopes were colluvial wedges and sites upslope of logs or other organic debris most tephra delivered to the basin by the eruptions is likely to be preserved in the stratigraphic record twelve percent of the sediment delivered to the study area was transported to the channel system directly by the eruption and by hillslope erosion processes of this approximately half remained in storage in the channel system and the remainder was exported from the drainage basin important sites of alluvial sediment storage included alluvial fans sites related to logs channel beds and alluvial terraces

a new look at the midlatitude mjo teleconnection in the northern hemisphere winter the midlatitude to madden julian mjo teleconnection in the northern hemisphere wintertime was investigated using twenty years of outgoing long wave radiation and national center for atmospheric research national centers for environmental prediction re analysis data it is revealed through empirical orthogonal functions and regression analyses that the tropical upper level divergence convergence associated with enhanced reduced tropical convection has a subtropical counterpart of upper level convergence divergence as the convective region of mjo moves eastward from the indian ocean to the western pacific the divergent circulation connecting the tropics and subtropics moves eastward from an analysis of vertical motion diagnosed by the generalized omega equation it was revealed that the position of the subtropical rossby gyres of the mjo with respect to the asian pacific jet was the most important factor affecting the existence of the localized divergence convergence and associated lower level vertical motion in the subtropics as the subtropical rossby gyres pass by the longitudinal position of the asian pacific jet vertical motion occurs to meet the balance required by quasi geostrophic theory since this anomalous vertical motion is large enough to cause convection at that region the diabatic heating process reinforces the vertical motion furthermore other dynamical processes neglected in quasi geostrophic theory are activated and contribute to the vertical motion balancing the diabatic heating as a result of the vertical motion near the jet entrance region in the subtropics the vorticity advection by vertical wind and the tilting of vorticity become important since these two terms are similar in shape and magnitude they reinforce each other during the development phase of the midlatitude mjo teleconnection combining these two terms we define the baroclinic jet source bjs in order to evaluate the bjs as a source for the midlatitude mjo teleconnection a numerical experiment is carried out the linear barotropic model is forced by only two terms yet the model captured clearly the main feature of the midlatitude mjo teleconnection copyright 2006 royal meteorological society

tectonics changing of the plates the composition of earth s crust depends on the style of plate tectonics and of the melting regimes in the mantle analyses of the oldest identified rocks suggest that these styles and the resulting crust have changed over earth s history

tracking deep crust by zircon xenocrysts within igneous rocks from the northern alxa china constraints on the southern boundary of the central asian orogenic belt abstract the southern boundary of the central segment of the central asian orogenic belt caob with the precambrian alxa block is not well constrained due to poor recognition of deep crust statistical analysis of zircon xenocrysts within igneous rocks from the northern alxa and its adjacent regions was applied to resolve this problem we compiled new and previously published geochronological zircon age data obtained by shrimp sims tims and la icp ms for 316 igneous rock samples of which 61 samples contain zircon xenocrysts new and previously published hf isotopic compositions of these zircon xenocrysts are combined with zircon ages in this study the zircon xenocrysts are mainly contained within permian rocks from the yabulai nurgong honggueryulin ynh zone and igneous rocks from the northwestern margin of the north china craton wncc a few xenocrysts were also found in permian igneous rocks from the zongnaishan shalazhashan zs zone neo proterozoic and paleo proterozoic intrusive rocks in the ynh zone as well as devonian to carboniferous granitoids and volcanic rocks from southern mongolia altogether we analyzed more than 270 zircon xenocrysts and considered only ages that are less than 10 discordant xenocryst ages within the permian igneous rocks from the zs zone are mainly around ca 350 ma ca 600 ma and ca 1400 ma the oldest age of zircon xenocrysts in this zone is similar to those of zircon xenocrysts from the caob 1 1 ga by contrast abundant zircon xenocrysts within permian igneous rocks from the ynh zone show highly variable age populations at ca 2 6 2 1 ga 1 8 1 6 ga 930 750 ma and 460 300 ma zircon xenocrysts from the zs zone have positive e hf t values of 6 3 to 13 9 whereas those from the ynh zone display highly variable e hf t values from 16 1 to 11 6 from the southern caob and the zs zone to the ynh zone the zircon xenocrysts show a significant shift from juvenile to crustal hf isotopic compositions suggesting that the area between the zs and the ynh zones constitutes the southern boundary of the caob in the deep crust within this region our study indicates that statistical analysis or isotopic mapping of zircon xenocrysts is an effective method to trace the nature of the deep continental crust and to separate between newly formed orogenic domain and ancient cratons

links between tropical convection and variations of the extratropical circulation during toga coare abstract data from the intensive observation period of the tropical ocean global atmosphere coupled ocean atmosphere response experiment november 1992 february 1993 have been used to investigate the links between intraseasonal variations in tropical convection and those in forcing of upper tropospheric rossby waves in the extratropics the primary databases are geostationary meteorological satellite imagery and tropical wind analyses from the bureau of meteorology australia a number of 5 day periods showing convection in different locations were chosen for each period mean fields of divergence cloud top temperature and upper tropospheric rossby wave source are presented vorticity budgets are used to demonstrate the processes responsible for the rossby wave source patterns the approach follows earlier studies of links between interannual variations of tropical convection associated with the southern oscillation and variations of the extratropical circulation it is shown that the regions of tropi

the nature of mesoarchaean seawater and continental weathering in 2 85 ga banded iron formation slave craton nw canada abstract banded iron formations bif have been extensively used as proxies to infer the chemical composition of ancient bulk seawater however their proximity to ancient crust suggests that they might also be used to reveal the composition of emergent continental landmass at the time of their deposition here we use the combination of geochemistry and sm nd isotopes on a layer by layer basis to interpret the relative contributions of hydrothermal hydrogenous and terrestrial input to one of the oldest documented superior type bif in the world the 2 85 ga central slave cover group bif is deposited within a rift basin related to a continental margin and is found associated with basement gneisses as well as shoreline and shallow shelf type facies such as fuchsitic quartzite and pebble to cobble conglomerate that confirm a near shore depositional setting for the bif the bif ranges from a pure chemical oxide magnetite silicate grunerite actinolite sediment with low al 2 o 3 2 o 3 10 wt and the occurrence of ferro hornblende biotite and garnet the silica bands have low trace metal content e g ni low ree average of 6 ppm and a shale normalized rare earth and yttrium rey pattern that is hree to lree enriched with pr yb sn values reaching sn of 0 5 during active rifting of the basement excess of eu 2 impacted the basin yielding seawater with eu anomalies eu eu sn as high as 3 85 average 2 75 larger than similarly aged bif high resolution geochemistry shows that there is more silica 19 4 wt sio 2 in the iron bands than iron 8 7 wt fe 2 o 3 in the silica bands implying that dissolved fe 2 came to the bif site in pulses and that silica likely represents background deposition consistently radiogenic e nd t values for the iron bands average 1 7 show that the dissolved rey in the source water during ferric iron precipitation was provided by submarine hydrothermal fluids with relatively uniform 143 nd 144 nd the silica bands by contrast reveal high variation in seawater 143 nd 144 nd as evident from the bimodal e nd t distribution with one segment exhibiting negative e nd t values averaging 1 1 and another with positive e nd t values averaging 2 5 this suggests input of dissolved rey into the upper seawater from weathering of isotopically different crustal components in the source region collectively we speculate that the low rey in the upper seawater and the overall low ni content implies a highly weathered crustal surface that was unable to contribute a significant dissolved load to the shelf environment

a pole to pole west pacific atmospheric teleconnection during august 1 an observational analysis is presented that reveals an august teleconnection involving both the northern hemisphere nh and the southern hemisphere sh the teleconnection includes three primary anomalies in the monthly average surface pressure field 1 a high southern latitude component including wilkes land antarctica the nearby southern ocean and the ross sea 2 a sh midlatitude component near australia and new zealand and 3 a nh subtropical component over the extreme western pacific ocean surface pressure for the sh high latitude anomaly is negatively correlated to the other two primary anomalies the sh midlatitude and nh subtropical component are positively correlated furthermore the teleconnection is correlated to surface pressure near the aleutian islands the teleconnection anomalies result from tropical convection on intraseasonal timescales monsoon rainfall over east asia during august is correlated to the teleconnection furthermore august precipitation at several other locations in the nh and sh is modulated by the teleconnection two different sets of numerical experiments with the national center for atmospheric research ncar community climate model version 2 ccm2 show a similar appearing teleconnection during late boreal summer the modeled teleconnection however is forced from high southern latitudes

long term preservation of early formed mantle heterogeneity by mobile lid convection importance of grainsize evolution abstract the style of tectonics on the hadean and archean earth particularly whether plate tectonics was in operation or not is debated one important albeit indirect constraint on early earth tectonics comes from observations of early formed geochemical heterogeneities 142nd and 182w anomalies recorded in hadean to phanerozoic rocks from different localities indicate that chemically heterogeneous reservoirs formed during the first 500 myrs of earth s history survived their remixing into the mantle for over 1 gyrs such a long mixing time is difficult to explain because hotter mantle temperatures expected for the early earth act to lower mantle viscosity and increase convective vigor previous studies found that mobile lid convection typically erases heterogeneity within 100 myrs under such conditions leading to the hypothesis that stagnant lid convection on the early earth was responsible for the observed long mixing times however using two dimensional cartesian convection models that include grainsize evolution we find that mobile lid convection can preserve heterogeneity at high mantle temperature conditions for much longer than previously thought because higher mantle temperatures lead to larger grainsizes in the lithosphere these larger grainsizes result in stronger plate boundaries that act to slow down surface and interior convective motions in competition with the direct effect temperature has on mantle viscosity our models indicate that mobile lid convection can preserve heterogeneity for 0 4 1 gyrs at early earth mantle temperatures when the initial heterogeneity has the same viscosity as the background mantle and 1 4 gyrs when the heterogeneity is ten times more viscous than the background mantle thus stagnant lid convection is not required to explain long term survival of early formed geochemical heterogeneities though these heterogeneities having an elevated viscosity compared to the surrounding mantle may be essential for their preservation

the effect of boreal late autumn snow cover over western and central china on the northern hemisphere wintertime blocking frequency abstractthe impact of snow cover in western and central china during late autumn on wintertime blocking occurrence is investigated using reanalysis data the study results show that wintertime atmospheric circulations affected by late autumn snow cover anomalies form favorable conditions for increased blocking frequency bf especially in the north pacific and north atlantic evidence is also presented that the stratosphere troposphere interactions are the key mechanism of the lag response of wintertime north pacific and north atlantic bfs to the late autumn snow cover that is positive anomalous snow cover can induce a dipole anomaly in the geopotential height field over the lower stratosphere due to the decrease of the 300 1000 hpa thickness and the concurrent variation between the east asian plateau jet and the polar front jet the associated positive geopotential height anomalies are located over northwestern eurasia meanwhile western and central china features remarkably negative geopotential he

eoarchean to paleoproterozoic crustal evolution in the north china craton evidence from u pb and hf o isotopes of zircons from deep crustal xenoliths abstract granulite facies xenoliths entrained within igneous rocks shed a light on poorly understood yet critical questions about age origin and history of the lower continental crust here we present the first coupled in situ u pb lu hf and o isotope data for the precambrian zircons from fourteen deep crustal xenoliths from the north china craton the geochronological data demonstrates that the oldest lower crustal remnants formed at 3 82 ga and related magmatism continued until 3 55 ga the eo paleoarchean zircons with exception of one analysis have subchondritic initial hf isotopes with negative ehf t and are characterized by normal mantle to elevated 18o values 5 37 6 90 indicating a development of a low lu hf reservoir and hydrosphere crust interactions in the early earth magmatic zircons from lower crustal xenoliths of north china define a strongly episodic distribution of ages at 3 82 3 55 ga 2 7 ga 2 5 ga and 1 95 1 85 ga and a non linear hf isotope age array for nearly 2 gyr indicating episodic crustal generation and reworking through time the lower crustal xenoliths record a change in zircon o isotope compositions from a restricted range at 3 8 2 6 ga to more variable after 2 5 ga both heavier 18o up to 10 25 and lighter 18o low to 2 16 o isotope compositions than the normal mantle are observed in 2 5 ga magmatic zircons the secular change in zircon o isotopes documents an increase in recycling rate of surface derived materials including high 18o sediments weathered and altered rocks and low 18o altered oceanic crust into magma sources at the end of archean which in turn is possibly linked to modern style subduction processes and maturation of the crust at that time

modelling u pb discordance in the acasta gneiss implications for fluid rock interaction in earth s oldest dated crust abstract the u pb isotopic system in zircon is the tool of choice to interrogate high temperature geological processes yet this system has potential to investigate lower temperature fluid rock interaction as well in some cases removal of radiogenic pb is incomplete potentially allowing regression of discordant u pb data on a concordia diagram to determine both the age of crystallization and the timing of fluid driven isotopic disturbance however in rocks preserving more complex histories simple regression is not effective at resolving multiple pb loss events here we use a concordant discordant comparison cdc test to establish the times of u pb disturbance in the acasta gneiss complex agc canada agc c 4 03 to c 3 40 ga orthogneisses experienced a long and complex post crystallization history for which u pb zircon data reflects not only the heterogeneous nature of the rock but also the varying degrees and duration of crustal reworking that inevitably involved open system processes the cdc test calculates the similarity between the concordant age structure and a modelled age structure the latter inferred from discordant analyses over a wide range of potential disturbance times our analysis reveals concordant zircon components implying new growth and or recrystallization at 3992 5 3501 6 3442 5 and 3126 6 ma in addition we establish episodes of radiogenic pb loss driven by fluid rock interaction at 3150 50 ma and probably at 2875 50 ma and c 2590 ma these pb loss episodes correlate with previously recognised events recording growth of zircon rims during metamorphism granite emplacement and unroofing pb loss within the agc shows an antithetic relationship in different samples that are in close geographic proximity we suggest that zircon alteration and associated new growth effectively rendered those grains that underwent pb loss at a particular time less susceptible to alteration during the next episode of fluid interaction

blocking over the south pacific and rossby wave propagation abstract atmospheric blocking events over the south pacific are investigated using a 39 yr record of 500 hpa height fields from the ncep ncar reanalysis dataset the analysis extends earlier work using a 16 yr record and confirms that the occurrence of blocking over the southeast pacific is strongly modulated by the enso cycle during austral spring and summer comparison of results at 500 hpa with the 300 hpa meridional wind component showed that blocking events are associated with large scale wave trains lying across the south pacific from the region of australia to southern south america similar wave trains are evident in both hemispheres in singular value decomposition analyses between 300 hpa meridional wind components and tropical pacific outgoing longwave radiation olr anomalies the hypothesis that the divergence associated with tropical olr anomalies forces an extratropical wave response that results in enhanced blocking over the southeast pacific was tested using a linearized barotropic vorti

dependency of the north pacific winter storm tracks on the zonal distribution of mjo convection 1 we investigate the effects of the tropical madden julian oscillation mjo convection on the zonal location and intensity of storm activity during the boreal cool season nov 1 to feb 28 120 days over the pacific ocean as tropical convection shifts eastward from the eastern indian ocean to the western north pacific mjo induced local hadley circulation leads to an eastward displacement of extratropical rossby wave source rws consequently this influence leads to an eastward shift of the peak area of the storm track the storm track is strongest with the mjo convection over the eastern indian ocean phase 3 and it experiences the suppression when mjo convection is located over the maritime continents phase 4 and it increases again when the mjo propagates to the western pacific phase 5 after which it weakens again phase 6 the intensity of the storm track particularly over the western north pacific 120 e 180 is determined by the integrated effects of three factors the midlatitude convective forcing mcf the jet stream and rws these factors are associated with the locations of mjo convections it is estimated that mjo induced suppression accounts for only 2 5 of the total observed midwinter suppression mws however the pattern generated by the mjo is remarkably similar to the observed meridional displacement of north pacific storm track associated with sub seasonal variation we conclude that the spatial distribution of mjo affects the variation of the pacific storm track but is not a cause of the mws

early earth a new recipe for old crust the processes responsible for the growth of earth s first continents are enigmatic the geochemical signature of 4 billion year old rocks discovered in canada points to a key role for shallow magmatic processes above upwelling mantle rocks

detachment and transportation of loose sediments by raindrop splash summary laboratory experiments on sediments ejected by splash consisted of two parts quantities ejected and mean projected distances were measured since the splash tray was 100 cm long and 20 cm wide and since both measurements were made on a receiving screen of one square metre it was obvious that only part of the translocated sediment was caught it was subject to serious doubt that the measured distances were representative for all sediment translocated if it is stated that the ejected sediment is distributed according to an exponential function around any point of impact located on the tray and if by definition tray and rainfall are perfectly homogeneous the ratio of the true to the measured distance and the ratio of the caught to the translocated sediment are only a function of the exponent of the real function the apparent distance being also a function of this exponent is thus the only factor needed to solve the two questions hence all empirical data were transformed into true data