constraints on mantle evolution from 187os 188os isotopic compositions of archean ultramafic rocks from southern west greenland 3 8 ga and western australia 3 46 ga initial 187os 188os isotopic compositions for geochronologically and geologically well constrained 3 8 ga spinel peridotites from the itsaq gneiss complex of southern west greenland and chromite separates from 3 46 ga komatiites from the pilbara region of western australia have been determined to investigate the osmium isotopic evolution of the early terrestrial mantle the measured compositions of 187os 188os 0 0 10262 2 from an olivine separate and 0 10329 3 for a spinel separate from 3 8 ga peridotite g93 42 are the lowest yet reported from any terrestrial sample the corrections for in situ decay over 3 8 ga for these low re os phases are minimal and change the isotopic compositions by only 0 5 and 2 2 for the spinel and the olivine respectively resulting in 187os 188os 3 8 ga 0 1021 0 0002 and 0 1009 0 0002 respectively these data extend direct measurement of os isotopic compositions to much earlier periods of earth history than previously documented and provide the best constraints on the os isotopic composition of the early archean terrestrial mantle analyses of pilbara chromites yield 3 46 ga mantle compositions of 0 1042 0 0002 and 0 1051 0 0002 these new data combined with published initial os isotopic compositions from late archean and early proterozoic samples are compatible with the mantle or at least portions of it evolving from a solar system initially defined by meteorites to a modern composition of 187os 188os 0 0 1296 0 0008 as previously suggested from peridotite xenolith data meisel et al 2001 the associated 187re 188os 0 0 435 0 005 thus chondritic 187os 188os compositions were a feature of the upper mantle for at least 3 8 billion years requiring chondritic re os ratios to have been a characteristic of the very early terrestrial mantle in contrast nonchondritic initial compositions of some archean komatiites demonstrate that os isotopic heterogeneity is an ancient feature of plume materials reflecting the development of variable re os mantle sources early in earth history the lower average 187os 188os 0 1247 for abyssal peridotites snow and reisberg 1995 indicate that not all regions of the modern mantle have evolved with the same re os ratio the relative sizes of the various reservoirs are unknown although mass balance considerations can provide some general constraints for example if the unradiogenic 187os 188os modern abyssal peridotite compositions reflect the prevalent upper mantle composition then the complementary high re os basaltic reservoir must represent 20 to 40 by mass of the upper mantle taken here as 50 of the entire mantle depending on the mean storage age the difficulties associated with efficient long term storage of such large volumes of subducted basalt suggest that the majority of the upper mantle is not significantly re depleted rather abyssal peridotites sample anomalous mantle regions the existence of 3 8 ga mantle peridotites with chondritic 187os 188os compositions and with os concentrations similar to the mean abundances measured in modern peridotites places an upper limit on the timing of a late accretionary veneer these observations require that any highly siderophile element rich component must have been added to the earth and transported into and grossly homogenized within the mantle by 3 8 ga either large scale mixing of impact materials occurred on very short 0 100 myr timescales or the interpretation preferred here the late veneer of highly siderophile elements is unrelated to the lunar terminal cataclysm estimated to have occurred at 3 8 to 3 9 ga

40ar 39ar ages for the alkaline volcanism and the basement of gorringe bank north atlantic ocean abstract gorringe bank is situated on the europe africa plate boundary at the eastern end of the azores gibraltar fracture zone it has two summits gettysburg bank to the southwest and ormonde bank to the northeast we applied the 40 ar 39 ar stepwise heating method to date six samples of the alkaline volcanic rocks two gabbros from the ormonde bank and a dolerite from the gettysburg bank the results that the alkaline volcanism lasted probably for less than 6 ma 66 60 ma although the nature of this volcanism precludes any subduction feature during its setting the alkaline volcanism of ormonde is probably linked to upper cretaceous eocene compressive tectonic events the basement rocks of gorringe bank reveal distrubed 40 ar 39 ar age spectra one plagioclase and one biotite from a gabbro give evidence for a thermic event whose age is tentatively estimated at about 75 ma and related to a variation in the direction of the relative movement between europe and africa the more probable age given by a plagioclase of another gabbro and by a dolerite 110 ma corresponds to tilting northeastward of the gorringe massif

2 7 ga metamorphic peridotites from southeast greenland record the oxygen isotope composition of archean seawater abstract the 18 o value of seawater is presently buffered by high temperature hydrothermal alteration and low temperature weathering of lithospheric rocks it is much debated whether the 18 o of seawater has been buffered to this steady state value throughout earth s history or alternatively whether it gradually increased towards the present value since the archean a third possibility is that the 18 o of seawater was buffered at a higher value before continents emerged and has been buffered at its current value 18 o 1 since the emergence of continents in this contribution we reconstruct the 18 o of archean seawater from triple oxygen isotope 17 o 18 o variations in 2 7 ga ultramafic rocks that reacted with fluids that in turn had been derived from seawater in the archean the samples that were studied are peridotites from the ivnartivaq complex an ultramafic lens in the archean kuummiut terrane rae craton southeast greenland that have unusually low 18 o values for peridotites in olivine 1 7 18 o 4 6 relative to vsmow bulk rock trace element concentrations mineral compositions and u pb dating of zircon grains in the peridotites indicate that these samples are metamorphic peridotites that formed by the dehydration of serpentinites 2 7 ga ago the serpentinite protoliths in turn had formed by alteration of ultramafic cumulate rocks at high temperatures 250 450 c by fluids that were derived from seawater triple oxygen isotope variations in olivine from the peridotites indicate that the fluids could not have been derived from seawater with a significantly higher or lower 18 o value than that of seawater in the phanerozoic but could have been derived from seawater with a value of 18 o 1 i e the modern ice free seawater composition we conclude that the 18 o of seawater had reached its current steady state value by 2 7 ga ago

progressive mixing of meteoritic veneer into the early earth s deep mantle komatiites are ancient volcanic rocks mostly over 2 7 billion years old from the archaean era that formed through high degrees of partial melting of the mantle and therefore provide reliable information on bulk mantle compositions1 in particular the platinum group element pge contents of komatiites provide a unique source of information on core formation mantle differentiation and possibly core mantle interaction2 3 4 5 6 7 8 most of the available pge data on komatiites are from late archaean 2 7 2 9 gyr old or early proterozoic 2 0 2 5 gyr old samples here we show that most early archaean 3 5 3 2 gyr old komatiites from the barberton greenstone belt of south africa and the pilbara craton of western australia are depleted in pge relative to late archaean and younger komatiites early archaean komatiites record a signal of pge depletion in the lower mantle resulting from core formation this signal diminishes with time owing to progressive mixing in to the deep mantle of pge enriched cosmic material that the earth accreted as the late veneer during the early archaean 4 5 3 8 gyr ago meteorite bombardment

significance of high field strength and rare earth element distributions in deciphering the evolution of the inner solar system abstract major processes affecting high field strength hfse and rare earth ree element ratios in planetary basalts are degree of melting separation of metal sulfide melt fractions addition and loss of silicate melt ilmenite fractionation and subduction fractional crystallization of planetary magma oceans has left a surviving imprint on only three bodies for which we have data the moon vesta and the angrite parent body thorium mobilization in aqueous fluids may account for decoupling of th and nb in planetary systems and this is especially notable on earth but also possible on mars the moon and some asteroids on earth hfse and ree ratios in young basalts characterize hydrated hm enriched em and depleted dm mantle sources associated with respectively subduction mantle plumes and ocean ridges terrestrial hydrated and depleted mantle were in existence by at least 4 ga and possibly they may have been produced in a stagnant lid tectonic regime before 3 ga also removal of nb in metal sulfide melts can force the composition of silicate restitic material into the hydrated mantle field on hfse ree graphs thus not requiring hydration such an origin is probable for hydrated mantle in primitive achondrites and plutonic angrites the record of all three types of mantle in basalts from other bodies in the solar system indicates the three mantle reservoirs are not diagnostic of plate tectonics but can be produced in stagnant lid settings enriched mantle is thus far recognized only in earth and possibly mars there are at least two enriched mantle reservoirs in earth a primordial 4 ga reservoir perhaps hidden in the d layer above the core and rarely sampled by basalts and a recycled plate reservoir although there are several processes by which nb can be fractionated from ta in planetary bodies the low nb ta

40ar 39ar dating and geochemistry of tholeiitic magmatism related to the early opening of the central atlantic rift tholeiitic effusive and intrusive magmatism from iberia morocco algeria and mali related to the early opening of the central atlantic rift was dated by the 40ar 39ar step heating method four plateau ages ranging from 203 7 2 7 to 197 1 1 8 ma were obtained on plagioclase from dykes from the taoudenni area mali and two lava flows from morocco the messejana dyke iberia which previously yielded discrepant conventional kar dates did not furnish any 40ar 39ar plateau dates however there is a clear inverse relationship between apparent age and k ca ratio for gas fractions from a plagioclase separate proportional to the alteration degree which combined with dates obtained on amphibole biotite and pyroxene allows us to determine an age of around 200 ma for this body these data and those obtained on the foum zguid morocco and the ksi ksou algeria dykes give evidence of a brief magmatic event between 206 and 195 ma ago which affected a large area ca 2500 km long trace element modelling shows that most of these formations originated from a homogeneous enriched source material such a brief magmatic episode related to the opening of a continental rift is in agreement with findings in other magmatic provinces e g the deccan traps and the red sea rift precisely dated by the 40ar 39ar method as well

tungsten isotope composition of the acasta gneiss complex high precision tungsten 182w 184w isotope measurements on well characterised mafic and felsic samples of the ca 3960 ma acasta gneiss complex agc northwest territories canada show radiogenic e182w values between 0 06 to 0 15 two ca 3600 ma felsic samples from this terrane have e182w 0 and are the oldest samples so far documented to have a w isotopic composition indistinguishable from that of the modern mantle the e182w data are correlated with e142nd roth et al 2014 and we attribute this variability to incomplete metamorphic homogenisation of the 3960 ma protolith with more recent material in a 3370 ma tectono thermal event notably the value of the positive e182w anomalies seen in the 3960 ma agc samples that are least affected by metamorphic homogenisation is comparable to that observed in other early archean rocks isua supracrustal belt greenland nuvvuagittuq supracrustal belt canada and the late archean kostomuksha komatiites karelia this demonstrates a globally constant signature we infer that the presence of a pre late veneer mantle represents the most straightforward interpretation of a uniform distribution of ew182 0 15 value in archean rocks of different ages we show that such a notion is compatible with independent constraints from highly siderophile element abundances in mafic and ultra mafic archean mantle derived rocks the absence of anomalous e182w and e142nd so far measured in samples younger than ca 2800 ma suggests progressive convective homogenisation of silicate reservoirs the downward mixing of an upper mantle rich in late delivered meteoritic material might account for these combined observations

hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago earth s lithosphere probably experienced an evolution towards the modern plate tectonic regime owing to secular changes in mantle temperature radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time with some estimates suggesting that over 70 of the present continental crustal reservoir was extracted by the end of the archaean eon patterns of crustal growth and reworking in rocks younger than three billion years gyr are thought to reflect the assembly and break up of supercontinents by wilson cycle processes and mark an important change in lithosphere dynamics in southern west greenland numerous studies have however argued for subduction settings and crust growth by arc accretion back to 3 8 gyr ago suggesting that modern day tectonic regimes operated during the formation of the earliest crustal rock record here we report in situ uranium lead hafnium and oxygen isotope data from zircons of basement rocks in southern west greenland across the critical time period during which modern like tectonic regimes could have initiated our data show pronounced differences in the hafnium isotope time patterns across this interval requiring changes in the characteristics of the magmatic protolith the observations suggest that 3 9 3 5 gyr old rocks differentiated from a 3 9 gyr old source reservoir with a chondritic to slightly depleted hafnium isotope composition in contrast rocks formed after 3 2 gyr ago register the first additions of juvenile depleted material that is new mantle derived crust since 3 9 gyr ago and are characterized by striking shifts in hafnium isotope ratios similar to those shown by phanerozoic subduction related orogens these data suggest a transitional period 3 5 3 2 gyr ago from an ancient 3 9 3 5 gyr old crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3 2 gyr ago that involved juvenile crust generation by plate tectonic processes

the eoarchean legacy of isua greenland worth preserving for future generations abstract the eoarchean 3600 ma or millions of years ago folded and metamorphosed isua supracrustal belt and the adjacent orthogneiss exposures of greenland contain rare low deformation lenses that display some uniquely preserved components of earth s oldest rock record these include world s oldest but contested stromatolites in dolomitic carbonates conglomerates pillow basalts demonstrating submarine eruption slivers of upper mantle rocks formation of earliest continental crust by multistage tonalite diorite emplacement followed by intracrustal granite production all these diverse occurrences are keys to establish early earth s processes at the start of the geological record although some of these features are preserved at several localities other critical ones are exposed on only a few m2 of rock at single localities or are of historical significance none of these sites are currently protected and there is a reliance on responsible sampling to keep them intact for future generations given the high interest in the archean eon combined with the increased ease of fieldwork in remote localities many significant deep time localities in not only greenland but worldwide are in danger of eradication here five key isua area geological sites are presented with an explanation of their significance and worthiness for initially reliance on already collected samples but hopefully ultimately government protection this highlights an increasing problem of destruction of in situ evidence of earth s unique early geological heritage and the need for collaboration in protecting and archiving of these key scientific resources

lack of late accreted material as the origin of 182w excesses in the archean mantle evidence from the pilbara craton western australia abstract we report 182w and 142nd isotopic compositions 187re 187os systematics and abundances of highly siderophile elements hse re os ir ru rh pt pd and au for a suite of komatiites and basalts from the 3 3 ga ruth well formation and the 3 45 ga warrawoona group of the pilbara craton western australia the 182w compositions from all samples are indistinguishable from each other and more radiogenic than modern bulk silicate earth with a mean 182w value of 9 1 4 2 2sd by contrast the 142nd values for all samples are indistinguishable from each other and terrestrial standards with a mean 142nd value of 1 6 3 2 2sd the 146sm 142nd and 187re 187os systematics are consistent with chondritic sm nd and re os ratios in the mantle source during the lifetime of 182hf and the observed 182w excesses therefore cannot be accounted for by early hf w fractionation by magma ocean processes neither by silicate liquid crystal fractionation nor by high p t metal silicate equilibration the estimated abundances of hse in the mantle source however are significantly lower than modern bulk silicate earth with only 51 9 1sd of modern bulk silicate earth abundances these results are consistent with a partial lack of late accreted material within the pilbara source at 3 3 ga to account for the 182w excesses further widespread 182w excesses of similar magnitude in other archean mantle derived rocks worldwide strongly suggests that a common process most likely incomplete addition of late accreted material was responsible the apparent mismatch between late accreted 182w hse systematics for some other localities likely reflects either the inherent difficulties associated with estimating source hse abundances and or dissociation of w and hse by mantle processes finally the combined average 182w hse systematics of archean samples indicate that the pre late accretion bse likely had a 182w value similar to that of the lunar mantle which strongly suggests post giant impact earth moon equilibration and indicates that the moon formed after 182hf extinction

a comparison of fission track ages of apatite and zircon to the k ar ages of illite smectite i s from ordovician k bentonites southern appalachian basin fission track ages of apatite and zircon from the middle ordovician deicke k bentonites collected over a wide area of the southern appalachian basin bracketed the previously determined k ar ages 270 300 ma of illite smectite i s separated from these k bentonties the zircon fission track ages ranged from 306 pm 32 to 410 pm 50 ma while the apatite fission track ages ranged from 165 pm 14 to 223 pm19 ma basin wide the zircon fission track ages were partially reset while the apatite fission track ages were completely reset the partially reset zircon fission track ages at the distal margin suggest that the k bentonites were affected by a pulse of heated solutions 175 200 c for a duration of at least 1 m y as opposed to being heated by deep burial the fission track data are consistent with previous data suggesting that i s formed by reaction with heated saline solutions expelled during alleghanian orogeny these fission track ages are also consistent with apatite fission track ages

u pb geochronology of grossular andradite garnet abstract this study presents a new laser ablation inductively coupled plasma mass spectrometry la icpms based u pb geochronometric method for dating grossular andradite grandite garnet grandite is a primary skarn mineral therefore dating its growth directly dates hydrothermal activity as zircon u pb geochronometry provides a high resolution record of magmatic processes grandite u pb dating has the potential to provide a complementary record for hydrothermal systems this study characterizes four garnets of variable grossular andradite content and age as potential reference materials for la icpms u pb geochronology willsboro andradite 1020 ma adirondacks usa red and yellow mali grandite 200 ma southern mali and lake jaco grossular 35 ma coahuila mexico isotope dilution thermal ionization mass spectrometry id tims u pb analyses of willsboro andradite yield a mean 206pb 238u date of 1022 16 ma we use willsboro andradite as a primary reference material for la icpms u pb characterization of the two other garnets the non radiogenic pb pbc concentration in grandite is variable between specimens willsboro andradite and mali grandites contain almost purely radiogenic pb pb however lake jaco grossular has a higher pbc concentration comparatively and yields discordant u pb ratios for specimens with highly variable pb pbc linear regression is employed to derive a lower intercept age la icpms u pb dates for yellow mali grandite 202 2 ma 206pb 238u weighted mean and lake jaco grossular 35 2 ma lower intercept agree with independent id tims u pb 202 0 1 2 ma and u th he dates 35 5 ma derived for each specimen the precision of la icpms u pb dates varies between 1 10 2 for the analyzed garnets considering the short estimated lifespans of pluton related hydrothermal systems tens of thousands to a few million years neogene skarns present the best opportunity to test for and resolve separate episodes of garnet growth at these precision levels as a performance test for the method we present new u pb andradite data from a late miocene skarn system on serifos island greece this garnet yields a lower intercept age of 9 15 0 36 ma in agreement with biotite rb sr and zircon u pb dates from the causative pluton

dating multistage paleofluid percolations a k ar and 18o 16o study of fracture illites from altered hercynian plutonites at the basement cover interface poitou high france isotopic age determination of fluid events in basement fractures is of prime importance for the reconstruction of paleo hydraulic regimes however material from fracture or wall rock is not generally easily dated in the case of fractures filled by minerals such as phyllosilicates it can be expected that subsequent fluid circulation may cause partial alteration or recrystallization of the previous phyllosilicates disturbing the isotope signature using a specific methodology including the extraction process without contamination of the finest 0 2 m authigenic fractions identification of clay fractions by x ray diffraction xrd scanning electron microscopy and electron microprobe analysis of stable isotope composition d h 18o and k ar dating fluid event ages can be estimated this methodology was applied to the northwestern margin of the french massif central where basinal brines have interacted with the sedimentary cover infra toarcian formations and the fractured hercynian crystalline basement during periods of unknown age precipitates from brines sealed most basement fractures as carbonates and a recrystallization of early hercynian phengite chlorite into illite and illite smectite minerals occurred in the earlier fractures xrd patterns show that the clay fraction is dominated by illite and ordered mixed layer minerals having a relatively high amount of illite i s r3 in the shallowest samples beneath the paleo surface a late assemblage is identified as a mixture of i s minerals a part of them being characterized by significant amounts of swelling layers i s r0 the d values of the clay fractions are fairly constant around 50 10 but 18o values of the same clay fractions display a wide range from 8 to 18 smow the study of the size fractions between 2 and 0 2 m reveals a correlation between k ar ages and 18o values which cannot be related to a change in the relative amounts of clay minerals deep fractures at depths of 570 and 923 m are characterized by 18o values around 10 3 0 4 smow and old ages ranging from 253 to 272 ma the finest fractions 0 2 m of clays located at shallow level 300 m beneath the paleo surface yield the highest 18o values 15 7 18 2 and the youngest k ar dates 188 to 198 ma these dates are likely to record the extensional episode and thermal anomaly related to the rifting of central atlantic ocean c 190 ma the other important thermal extensional episode which affected the aquitaine basin namely the opening of the gascogne gulf at c 120 ma is not detected by the k ar method on the clay fractions we believe that resetting of clay type k ar ages linked to recrystallization was favoured in the upper part of the basement where the ancient hercynian clay minerals were destabilized by subaerial low temperature alteration during permo triassic times from a methodologic point of view the analytical characterization of several size fractions of clays from reactivated fractures using both stable isotope geochemistry and k ar isotope geochemistry can be considered as a powerful tool for dating fluid events when no suitable material for conventional methods is available

oxygen isotopes of an early archaean layered ultramafic body southern west greenland implications for magma source and post intrusion history a large number of metamorphosed ultramafic bodies are found in the region sw of the isua greenstone belt in southern west greenland enclosed in archaean banded tonalitic gneisses these enclaves of dunite and harzburgite range in size from a few to several hundred meters the ujarassuit nunat layered body occurs as a large enclave 800 m 100 m entrained within tonalitic gneisses and preserves primary igneous layering and textures this enclave has been subjected to amphibolite grade metamorphism but for the most part the magmatic mineralogy remains intact separated minerals have been analysed for 18o by co2 laser fluorination 18o values range from 4 49 to 4 89 for olivine and from 5 70 to 5 86 for orthopyroxene all chromite values are 2 5 closure temperatures recorded for olivine orthopyroxene pairs vary from 700 to 900 c much lower than expected for an ultramafic magma widespread late magmatic re equilibration has resulted in significant decreases in 18o for chromites and small changes of 0 3 in olivine but orthopyroxene values show little variation from typical magmatic signatures the lowering of 18o in chromite is associated with significant fe enrichment thermometry suggests that the re equilibration took place at 680 750 c with equilibration between plagioclase and amphibole in residual gabbro anorthosite melt around 700 c hydration possibly associated with this late melt stage resulted in further changes to chromite chemistry formation of secondary amphibole phlogopite and chlorite and may have further enhanced 18o depletion in disseminated chromites all these events are related to late magmatic processes only the later metamorphic mineral ruby corundum present in amphibole harzburgite adjacent to an intrusive gneiss vein clearly indicates an external oxygen input to this system olivines from an unlayered dunite body 3 5 km away from ujarassuit nunat have higher 18o values around 5 25 typical of more recent high temperature mantle peridotite nodules and suggesting that mantle oxygen composition at 3 8 ga was already similar to the present composition

paleomagnetism and potassium argon age of the messejana dike portugal and spain angular limitation to the rotation of the iberian peninsula since the middle jurassic abstract a paleomagnetic and potassium argon dating investigation has been carried out on a 530 km long dike system which transects the western iberian peninsula in a northeasterly direction the k ar age determinations were made on mineral separates exclusively they range between 160 and 200 ma and the authors suppose that this reflects the actual time interval of the intrusion in accord with previous results the paleomagnetic pole derived from 12 sites regularly distributed along the dike 71 n 236 e coincides well with other mesozoic paleomagnetic poles from the western africa a contemporaneous pole from stable europe is tentatively deduced from african and north american late triassic early jurassic poles using different reconstruction models around the north atlantic ocean the divergence between this pole and the iberian pole corresponds to the result obtained for permian poles

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

a review of structural patterns and melting processes in the archean craton of west greenland evidence for crustal growth at convergent plate margins as opposed to non uniformitarian models abstract the archean craton of west greenland consists of many fault bounded eoarchean to neoarchean tectonic terranes crustal blocks these tectonic terranes are composed mainly of tonalite trondhjemite granodiorite ttg gneisses granitic gneisses metavolcanic dominated supracrustal belts layered anorthositic complexes and late to post tectonic granites rock assemblages and geochemical signatures in these terranes suggest that they represent fragments of dismembered oceanic island arcs consisting mainly of ttg plutons tholeiitic to calc alkaline basalts boninites picrites and cumulate layers of ultramafic rocks gabbros leucogabbros and anorthosites with minor sedimentary rocks the structural characteristics of the terrane boundaries are consistent with the assembly of these island arcs through modern style of horizontal tectonics suggesting that the archean craton of west greenland grew at convergent plate margins several supracrustal belts that occur at or near the terrane boundaries are interpreted as relict accretionary prisms the terranes display fold and thrust structures and contain numerous 10 cm to 20 m wide bifurcating ductile shear zones that are characterized by a variety of structures including transposed and redistributed isoclinal folds geometrically these structures are similar to those occurring on regional scales suggesting that the archean craton of west greenland can be interpreted as a continental scale accretionary complex such as the paleozoic altaids melting of metavolcanic rocks during tectonic thickening in the arcs played an important role in the generation of ttgs non uniformitarian models proposed for the origin of archean terranes have no analogs in the geologic record and are inconsistent with structural lithological petrological and geochemical data collected from archean terranes over the last four decades the style of deformation and generation of felsic rocks on outcrop scales in the archean craton of west greenland and the mesozoic sulu orogenic belt of eastern china are similar consistent with the formation of archean continental crust by subduction zone processes

highly siderophile element abundances in eoarchean komatiite and basalt protoliths plume derived mg rich volcanic rocks komatiites high mg basalts and their metamorphic equivalents can record secular changes in the highly siderophile element hse abundances of mantle sources an apparent secular time dependent enrichment trend in hse abundances from paleoarchean to paleoproterozoic mantle derived rocks could represent the protracted homogenization of a late veneer chondritic contaminant into the pre late veneer komatiite source to search for a possible time dependence of a late accretion signature in the eoarchean mantle we report new data from rare 3700 myr old mafic and ultramafic schists locked in supracrustal belts from the inukjuak domain quebec canada and the akilia association west greenland our analysis shows that some of these experienced hse mobility and or include a cumulate component touboul et al in chem geol 383 63 75 2014 whereas several of the oldest samples show some of the most depleted hse abundances measured for rocks of this composition we consider these new data for the oldest documented rocks of komatiite protolith in light of the late veneer hypothesis

highly siderophile element and 182w evidence for a partial late veneer in the source of 3 8 ga rocks from isua greenland abstract the higher than expected concentrations of highly siderophile elements hse in earth s mantle most likely indicate that earth received a small amount of late accreted mass after core formation had ceased known as the late veneer small 182w excesses in the moon and in some archaean rocks such as the source of 3 8 billion year old isua magmatics also appear consistent with the late veneer hypothesis with a lower proportion received however 182w anomalies can also relate to other processes including early mantle differentiation to better assess the origin of these w isotope anomalies and specifically whether they relate to the late veneer we have determined the hse abundances and 182w compositions of a suite of mafic to ultramafic rocks from isua from which we estimate hse abundances in the source mantle and ultimately constrain the 182w composition of the pre late veneer mantle our data suggest that the isua source mantle had hse abundances at around 50 65 of the present day mantle consistent with partial but not complete isolation from the late veneer these data also indicate that at least part of the late veneer had been added and mixed into the mantle at the time the isua source formed prior to 3 8 ga for the same isua samples we obtained a 13 4 ppm 182w excess compared to the modern terrestrial mantle in excellent agreement with previous data using combined 182w and hse data we show that the moon isua and the present day bulk silicate earth bse produce a well defined co variation between 182w composition and the mass fraction of late accreted mass as inferred from hse abundances this co variation is consistent with the calculated effects of various late accretion compositions on the hse and 182w signatures of earth s mantle the empirical relationship therefore implies that the moon isua source and bse received increasing proportions of late accreted mass supporting the idea of disproportional late accretion to the earth and moon and consistent with the interpretation that the lunar 182w value of 27 4 ppm represents the composition of earth s mantle before the late veneer was added in this case the isua source can represent ambient mantle after the giant moon forming impact into which only a part of earth s full late veneer was mixed rather than an isotopically distinct mantle domain produced by early differentiation which would probably require survival through the giant moon forming impact

geochemistry of early mesozoic tholeiites from morocco abstract mesozoic dolerites from two areas of morocco the high atlas fold belt between marrakech and demnat and the anti atlas belt in the area of foum zquid are most high ti quartz normative tholeiites whichi in many respects resemble mesozoic dolerite dikes from eastern north america the dolerites display a wide range of major and trace element compositions some of which are due to fractional crystallization the doleritic sequences from high atlas also show vertical stratigraphic zonation which is characterized by a progressive depletion of lithophile elements toward the top this trend together with regularities of trace element ratio variations are indicative of a dynamic melting of an initially homogeneous source it is suggested that the continental upper mantle source for dolerites of morocco was enriched in several incompatible elements in comparison with the upper mantle source for ocean floor tholeiites