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

emergence of modern continental crust about 3 billion years ago the continental crust provides a record of earth s evolution analysis of the geochemical signature of continental crust formed since the hadean points to the initiation of plate tectonics about 3 billion years ago

transition of subduction related magmatism from slab melting to dehydration at 2 5 ga abstract the archean plate tectonics and subduction related magmatism have been concerned for several decades here we investigate the archean plate subduction from a new perspective using the secular records of global scale non ttg tonalite trondhjemite granodiorite felsic rocks with arc like geochemical affinities the statistical results show that the la yb and sm yb of non ttg arc felsic rocks evolved at high values in the archean and decreased sharply at 2 5 ga this abrupt transition indicates a significant change of the petrogenesis during the archean the oceanic crust subducted into the mantle along a high geothermal gradient and reached the conditions of partial melting with garnet stable in the residue resulting in high la yb and sm yb in the arc felsic rocks as a consequence of the secular cooling of the mantle the geothermal gradient of a post archean subducting cold oceanic crust intersected with the stability limits of hydrous minerals before it reached the wet solidus of basalts so that the dehydration occurred preferentially and resulted in a drier crust with a much higher temperature of solidus as such the post archean arc felsic rocks were mostly originated from melting of the hydrated mantle wedge overlying the subducted oceanic slab the magma source of the arc felsic rocks thereafter became much shallower corresponding to the low la yb and sm yb of those rocks as a consequence of the less stable garnet in the residue we thus conclude that the subduction related magmatism changed from slab melting to dehydration model at 2 5 ga

neoarchaean volcanic rocks central hearne supracrustal belt western churchill province canada geochemical and isotopic evidence supporting intra oceanic supra subduction zone extension abstract the kaminak segment of the central hearne supracrustal belt chsb western churchill province canada comprises a diverse sequence of neoarchaean volcanic and less abundant metasedimentary rocks that were emplaced as two assemblages between 2695 2711 ma and 2681 2686 ma respectively these are intruded by uncommon pre tectonic diorites and tonalites ca 2691 ma voluminous syn tectonic tonalites and granodiorites ca 2679 2686 ma and rare post tectonic potassic monzogranite and syenite ca 2659 2666 ma metasedimentary rocks include turbidites epiclastic tuffs minor iron formation oxide and rare volcanic conglomerates volcanic rocks of assemblage i include abundant pillowed and massive basalts to andesites with less common silicic lavas tuffs and volcaniclastic debris flow deposits assemblage ii contains voluminous silicic tuff and volcaniclastic debris flow deposits but fewer basaltic to andesitic flows the critical diagnostic feature of the chsb is the stratigraphic intercalation of compositionally diverse basaltic andesitic and felsic volcanic rocks throughout both assemblages mapping u pb geochronology and lithogeochemistry suggest that an initial morb like basaltic plain containing widespread intercalations of dacite to rhyolite was replaced at ca 2688 ma by a relatively short lived dacite to rhyolite dominated magmatic environment characterized by localized felsic volcanic centres and a bloom of 2686 2679 ma tonalitic to granodioritic plutons basaltic to andesitic rocks are dominated by iron rich tholeiites although the proportion of calc alkaline rocks increases with silica content felsic volcanic rocks all exhibit calc alkaline affinities the wide range in chemistry of the basaltic to andesitic rocks of both volcanic assemblages implies diverse mantle sources capable of generating voluminous morb with less common arc neb oib and rare babb like rocks similarly the variable composition of the felsic volcanic rocks indicates both anatexis of eclogitic to garnetiferous mafic crust and also extensive fractionation of mafic precursors in crustal magma chambers two geochemically distinct arc like mafic suites were generated through contamination of primary mantle derived magmas by juvenile ca 2700 ma silicic crust either in their mantle source or through assimilation upon ascent ndt isotopic data are comparable to contemporaneous depleted mantle with only local evidence for incorporation of older 2700 ma crust the chsb may have formed via tectono magmatic processes comparable to those of the eocene infant arcs of the sw pacific whereby the formation of a thick sequence of coeval intercalated compositionally diverse mantle and crustal derived rocks are generated in an extensional supra subduction setting the cessation of supra subduction zone extension at ca 2688 ma was followed by the short lived development of felsic volcanic edifices incipient arc the extrusion of mafic to felsic magmas and the concomitant intrusion of voluminous syn kinematic tonalitic plutons this accompanied a major change in the tectono magmatic setting accompanying and presumably following the termination of extensional supra subduction zone processes

observation of coherent echoes with narrow spectra near 150 km altitude during daytime away from the dip equator 1 we present the rare observation of backscatter echoes perpendicular to the geomagnetic field from the 150 km altitude vicinity over a region situated away from the dip equator namely at gadanki 13 5 n 79 2 e 6 4 dip latitude a layer of weak echoes having signal to noise ratio snr of the order of 12 to 3 db was seen drifting downward from 157 km at 0930 lt to about 148 km at 1028 lt on july 12 1997 the doppler velocity as well as doppler width was in the range 10 20 ms 1 while there was no quasi periodicity as such in terms of echo occurrence the echo properties were in other respects very similar to 150 km echoes seen in equatorial regions this included the descending phase with time during the morning hours these constitute the first observations of daytime 150 km echoes away from the dip equator and only add to the mystery regarding the origin of these structures

detection of temperature and sea ice extent changes in the antarctic and southern ocean 1949 96 abstract some global climate models indicate that future global warning from increased atmospheric concentrations of greenhouse gases may be greatest in the polar regions over areas where the sea ice cover is reduced the reduction of sea ice area in the models also gives rise to a strong positive feedback to the warming from the increase of atmospheric greenhouse gas concentration to date and the results of transient climate models an estimate of the expected change in the antarctic temperatures and sea ice extent can be made the existing data for observed changes in temperatures of the antarctic and southern ocean extending back to approx 1956 and approx 1945 respectively are analyzed along with the data of sea ice cover commencing in 1973 to examine the extent to which the anticipated warming trends and sea ice decrease are being realized in spite of high temporal and spatial variability the data does support small significant trends of temperature increase and sea ice cover decrease compatible in magnitude to those expected as a consequence of atmospheric greenhouse gas increase the seasonal cycle shows a delayed period of autumn winter sea ice growth with a longer period of open water this supports a mechanism for positive feedback between decreasing sea ice cover and increasing temperature

adakites from continental collision zones melting of thickened lower crust beneath southern tibet adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones here we report the first example of such magmas from southern tibet in an active continental collision environment the tibetan adakites were emplaced from ca 26 to 10 ma and their overall geochemical characteristics suggest an origin by melting of eclogites and or garnet amphibolites in the lower part 50 km of thickened tibetan crust this lower crustal melting required a significantly elevated geotherm which we attribute to removal of the tectonically thickened lithospheric mantle in late oligocene time the identification of collision type adakites from southern tibet lends new constraints to not only the himalayan tibetan orogenesis how and when the indian lithosphere started underthrusting asia can be depicted but also the growth of the early continental crust on earth that consists dominantly of the tonalite trondhjemite granodiorite suites marked by adakitic geochemical affinities

emergence of silicic continents as the lower crust peels off on a hot plate tectonic earth the processes for crustal recycling during the archaean are unclear numerical simulations suggest that dense lower crust would have peeled off into the mantle leading to a rapid concentration of buoyant silicic rocks that formed the continents

physical volcanology and geochemistry of a late archaean volcanic arc kurnalpi and gindalbie terranes eastern goldfields superterrane western australia abstract late archaean 2715 2704 ma calc alkaline volcanic complexes in the kurnalpi terrane are identical in facies and geochemistry to modern intra arc volcanic complexes preserved volcanic rocks and associated feldspathic sedimentary rocks represent proximal to medial submarine facies of submarine to locally emergent intra oceanic arc related volcanoes the least altered volcanic rocks are dominantly andesite but range from basalt and basaltic andesite to rare rhyolite and are the product of magmas similar in composition to those of modern arcs intermediate magmas were most likely derived from large ion lithophile element lile enriched mafic magmas by fractional crystallisation with or without assimilation processes the kurnalpi terrane thus provides an example of a late archaean volcanic arc in which hydrated mantle wedge melting was the main mechanism for magma genesis ttd suite volcanism derived from slab melting was either absent or played a subordinate role the adjacent gindalbie terrane contains younger 2692 2680 ma bimodal basalt rhyolite volcanic complexes with associated quartz rich sedimentary rocks and calc alkaline intermediate silicic volcanic complexes volcanosedimentary basement to the gindalbie volcanic sequences is deeply eroded equivalents of the calc alkaline volcanic complexes their associated sedimentary rocks and younger basaltic sequences found in the kurnalpi terrane gindalbie basalt rhyolite successions locally contain volcanic hosted massive cu zn mineral deposits rhyolites derived by low pressure fractional crystallisation or melting of andesites are more abundant than in the kurnalpi volcanic complexes facies associations and the local occurrence of distinctive incompatible element enriched mildly peralkaline a type rhyolites are consistent with rifting of a mature arc system the two terranes most likely represent volcanic evolution of the same late archaean volcanic arc but are different tectonic fragments that were welded together by 2660 ma

thickness distribution of antarctic sea ice 1 ship based observations are used to describe regional and seasonal changes in the thickness distribution and characteristics of sea ice and snow cover thickness around antarctica the data set comprises 23 373 observations collected over more than 2 decades of activity and has been compiled as part of the scientific committee on antarctic research scar antarctic sea ice processes and climate aspect program the results show the seasonal progression of the ice thickness distribution for six regions around the continent together with statistics on the mean thickness surface ridging snow cover and local variability for each region and season a simple ridge model is used to calculate the total ice thickness from the observations of level ice and surface topography to provide a best estimate of the total ice mass including the ridged component the long term mean and standard deviation of total sea ice thickness including ridges is reported as 0 87 0 91 m which is 40 greater than the mean level ice thickness of 0 62 m analysis of the structure function along north south and east west transects revealed lag distances over which sea ice thickness decorrelates to be of the order of 100 300 km which we use as a basis for presenting near continuous maps of sea ice and snow cover thickness plotted on a 2 5 5 0 grid

solar flare effects on 150 km echoes observed over jicamarca waccm x simulations submitted manuscript accepted for publication in geophysical research letters https doi org 10 1029 2019gl084790 copyright 2019 american geophysical union further reproduction or electronic distribution is not permitted

studies of the antarctic sea ice edge and ice extent from satellite and ship observations abstract passive microwave derived ice edge locations in the antarctic are assessed against in situ observations from ships between 1989 and 2000 during the growth season march october the ship data agrees with satellite data very well with r2 values of 0 99 and 0 97 for the bootstrap and team algorithms respectively during the melt season november february the agreement is not so good with the passive microwave ice edge typically 1 2 of latitude south of the observations this is due to the low concentration and saturated nature of the ice and the r2 values for this period are 0 92 and 0 80 for the bootstrap and team algorithms respectively sensitivity studies show that such an offset in the summer ice edge location can cause significant errors in trend studies of the extent of sea ice cover in the southern ocean the passive microwave ice concentration at the ice edge observed by ship varies greatly averaging 14 for the team algorithm and 19 for the bootstrap comparisons between passive microwave data and sar landsat and ols data during the ice growth season show that while small scale details in ice edge location are lost the passive microwave data generally provide good and consistent representation of the higher resolution imagery

variations of surface air temperature and sea ice extent in the western antarctic peninsula region the western antarctic peninsula wap region has experienced a statistically significant warming trend during the past half century in addition a statistically significant anticorrelation between air temperatures and sea ice extent as determined from satellite passive microwave data during the past two decades has been observed for this region consistent with this strong coupling sea ice extent in the wap area has trended down during this period of satellite observations further much of the variability in both air temperature and sea ice in the wap region has been shown to be influenced by contrasting maritime warm moist and continental cold dry climate regimes as part of the palmer long term ecological research program the ecological influence of these trends and variability is being studied and effects have already been demonstrated at all trophic levels here we extend earlier observations to include the past decade and focus on the annual cycles of air temperature and sea ice extent for the past few years with the aim of placing these recent observations within the context of changes seen in the longer term records the more recent years have seen an increasing maritime influence in the wap region with corresponding effects on the marine ecosystem

archean upper crust transition from mafic to felsic marks the onset of plate tectonics the archean eon witnessed the production of early continental crust the emergence of life and fundamental changes to the atmosphere the nature of the first continental crust which was the interface between the surface and deep earth has been obscured by the weathering erosion and tectonism that followed its formation we used ni co and cr zn ratios in archean terrigenous sedimentary rocks and archean igneous metaigneous rocks to track the bulk mgo composition of the archean upper continental crust this crust evolved from a highly mafic bulk composition before 3 0 billion years ago to a felsic bulk composition by 2 5 billion years ago this compositional change was attended by a fivefold increase in the mass of the upper continental crust due to addition of granitic rocks suggesting the onset of global plate tectonics at 3 0 billion years ago

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

high resolution observations of 150 km echoes at jicamarca high resolution soundings of equatorial upper e region echoes at jicamarca exhibit gravity wave modulated signal intensity distributions and doppler velocities corresponding to vertical plasma drifts a gravity wave wind driven interchange instability is suggested as a possible generation mechanism of the field aligned plasma irregularities responsible for the echoes the echoes may be utilized in gravity wave studies and in zonal electric field measurements by radars with modest sensitivities

the effect of snow on antarctic sea ice simulations in a coupled atmosphere sea ice model the effect of a snow cover on sea ice accretion and ablation is estimated based on the zero layer version sea ice model of semtner and is examined using a coupled atmosphere sea ice model including feedbacks and ice dynamics effects when snow is disregarded in the coupled model the averaged antarctic sea ice becomes thicker when only half of the snowfall predicted by the atmospheric model is allowed to land on the ice surface sea ice gets thicker in most of the weddell and ross seas but thinner in east antarctic in winter with the average slightly thicker when twice as much snowfall as predicted by the atmospheric model is assumed to land on the ice surface sea ice also gets much thicker due to the large increase of snow ice formation these results indicate the importance of the correct simulation of the snow cover over sea ice and snow ice formation in the antarctic our results also illustrate the complex feedback effects of the snow cover in global climate models in this study we have also tested the use of a mean value of 0 16 wm 1 k 1 instead of 0 31 for the thermal conductivity of snow in the coupled model based on the most recent observations in the eastern antarctic and bellingshausen and amundsen seas and have found that the sea ice distribution changes greatly with the ice becoming much thinner by about 0 2 m in the antarctic and about 0 4 m in the arctic on average this implies that the magnitude of the thermal conductivity of snow is of considerable importance for the simulation of the sea ice distribution an appropriate value of the thermal conductivity of snow is as crucial as the depth of the snow layer and the snowfall rate in a sea ice model the coupled climate models require accurate values of the effective thermal conductivity of snow from observations for validating the simulated sea ice distribution under the present climate conditions

partial convective overturn of archaean crust in the east pilbara craton western australia driving mechanisms and tectonic implications strain varies systematically from weakly developed outward dipping s tectonites in the 3320 3310 10 ma mount edgar batholith to intensely deformed subvertical l tectonites in greenstones of the warrawoona syncline a consistent greenstone down batholith up sense of shear is recorded in batholithic domal margins and adjacent high grade supracrustal rims lineations converge to a central zone of subvertical extension zone of sinking along the synclinal axis at domal margins early kinematic granitoid sheets and intrusive diatexites are subconcordant to a well developed dome parallel schistosity but late to post kinematic intrusives are discordant high level plutons all granitoids are the same age within analytical error these syn doming features conform with structural tests for diapirs and differ from those expected during metamorphic core complex formation and cross folding diapirism is part of a larger process involving partial convective overturn of the crust based on strain patterns and kinematic criteria we argue that deformation was initiated by sinking of greenstones which rapidly subsided the domes then rose passively as accommodation structures ongoing doming d4 partitioned strain to the southern rim of the batholith and uplifted a wedge of the warrawoona syncline keel we consider that crustal overturn occurred in response to mantle plume activity eruption of a 5 10 km thick 3325 ma mafic ultramafic greenstone pile onto an older granite greenstone terrain created a negatively buoyant crust but convective overturn occurred some 5 20 ma later triggered by widespread anatexis thermal softening convective overturn may have been a common archaean process occurring in response to a hotter mantle and represents an end member deformation mechanism that includes thrust accretion of lithotectonic assemblages in other archaean terranes such as the superior province of canada

simulations of southern hemisphere warming and antarctic sea ice changes using global climate models a combination of modelling techniques is used in conjunction with the limited available observational data to examine antarctic sea ice changes with global warming over the past century firstly a coupled global climate model is forced by prescribing the effect of increasing greenhouse gases from last century to the present secondly the gisst u k meteorological office global sea ice and sea surface temperature observational dataset is used to force an atmosphere sea ice model to compute changes in the antarctic sea ice from last century to the present thirdly the global sea surface temperature sst anomalies derived from the coupled model are used to force the atmosphere sea ice model over the same period the change in the southern hemisphere annual mean surface temperature simulated by the coupled model with greenhouse gas forcing is about 0 6 c which is similar to the observed change over the antarctic poleward of 60 s the corresponding simulated change is about 0 7 c which also appears compatible with observations the reduction in summer sea ice extent simulated by the csiro commonwealth scientific and industrial research organisation coupled model is 0 44 lat which is in general less than the observed change for the two sst forcing cases the changes are in general larger than indicated by the observations it is concluded that future changes of reduced sea ice extent from increasing greenhouse gases as simulated by the csiro coupled model are not expected to be overestimates

secular changes in tonalite trondhjemite granodiorite composition as markers of the progressive cooling of earth archean tonalite trondhjemite granodiorite associations ttg are classically thought to generate through partial melting of hydrous metabasalts however the chemical composition of the least differentiated ttg parental magmas evolved from 4 0 to 2 5 ga during this interval the mg as well as the ni and cr contents increased which is interpreted as reflecting increased interactions between felsic melts generated by metabasalt melting and mantle peridotite similarly cao na2o and sr also increased over time thus reflecting an increase in the abundance of plagioclase in the melt residue the presence or absence of residual plagioclase is interpreted in terms of melting depth the demonstrated interaction between ttg parental magmas and the mantle rules out their genesis by fusion of previously underplated metabasalt and favors the melting of subducted slab material at 4 0 ga earth s geothermal gradient was sufficiently high to allow slab melting at shallow depths where plagioclase was stable consequently due to the small thickness of the overlying mantle wedge felsic magmas interacted little with the mantle at 2 5 ga however owing to lower geothermal gradients the melting depth was greater and plagioclase became no longer stable in the thick mantle wedge overlying the subducted slab as a result felsic magmas reacted strongly with the mantle peridotite the changes of ttg composition during archean time can be thus interpreted as reflecting the progressive cooling of earth

covariation of sea ice and methanesulphonic acid in wilhelm ii land east antarctica sea ice plays an important role in ocean atmosphere heat exchange global albedo and the marine ecosystem knowledge of variation in sea ice extent is essential in order to understand past climates and to model possible future climate scenarios this paper presents results from a short firn core spanning 15 years collected from near mount brown wilhelm ii land east antarctica variations of methanesulphonic acid msa at mount brown were positively correlated with sea ice extent from the coastal region surrounding mount brown 60 1208 e and from around the entire antarctic coast 0 3608 e previous results from law dome identified this msa sea ice relationship and proposed it as an antarctic sea ice proxy curran and others 2003 with the strongest results found for the local law dome region our data provide supporting evidence for the law dome proxy at another site in east antarctica but a deeper mount brown ice core is required to confirm the sea ice decline suggested by curran and others 2003 results also indicate that this deeper record may also provide a more circum antarctic sea ice proxy

observations of change in the southern ocean the southern ocean has been in a state of disequilibrium with its atmosphere and cryosphere during recent decades ocean station and drifting float observations have revealed rising temperatures in

anomalous peak in antarctic sea ice area winter 1998 coincident with enso the results of an updated satellite analysis of hemispheric and regional antarctic sea ice cover are presented based on october 1987 september i999 special sensor microwave imager ssm i data these show an ongoing slight but significant hemispheric increase of 3 7 0 3 in extent and 6 6 1 5 in area in the two principal sectors weddell sea ice extent area decreased by 3 4 1 0 3 9 4 6 and ross sea ice extent area increased by 10 9 1 0 18 3 4 6 hemispheric ross sea and western pacific ocean ice peaks in september 1998 were anomalously high and may have been related to atmospheric and oceanic anomalies in the pacific ocean and beyond associated with that year s exceptionally strong enso preliminary comparison of antarctic sea ice concentration data with european centre for medium range weather forecasts ecmwf operational analyses fields suggests that the unusually extensive sea ice in winter 1998 was concomitant with an equatorward shift of the circumpolar westerly surface winds over the southern pacific ocean

the little ice age as recorded in the stratigraphy of the tropical quelccaya ice cap the analyses of two ice cores from a southern tropical ice cap provide a record of climatic conditions over 1000 years for a region where other proxy records are nearly absent annual variations in visible dust layers oxygen isotopes microparticle concentrations conductivity and identification of the historical a d 1600 huaynaputina ash permit accurate dating and time scale verification the fact that the little ice age about a d 1500 to 1900 stands out as a significant climatic event in the oxygen isotope and electrical conductivity records confirms the worldwide character of this event

filling in the juvenile magmatic gap evidence for uninterrupted paleoproterozoic plate tectonics abstract despite several decades of research on growth of the continental crust it remains unclear whether the production of juvenile continental crust has been continuous or episodic throughout the precambrian models for episodic crustal growth have gained traction recently through compilations of global u pb zircon age frequency distributions interpreted to delineate peaks and lulls in crustal growth through geologic time one such apparent trough in zircon age frequency distributions between 2 45 and 2 22 ga is thought to represent a pause in crustal addition resulting from a global shutdown of magmatic and tectonic processes the 2 45 2 22 ga magmatic shutdown model envisions a causal relationship between the cessation of plate tectonics and accumulation of atmospheric oxygen over the same period here we present new coupled u pb hf and o isotope data for detrital and magmatic zircon from the western churchill province and trans hudson orogen of canada covering an area of approximately 1 3 million km2 that demonstrate significant juvenile crustal production during the 2 45 2 22 ga time interval and thereby argue against the magmatic shutdown hypothesis our data is corroborated by literature data showing an extensive 2 22 2 45 ga record in both detrital and magmatic rocks on every continent and suggests that the operation of plate tectonics continued throughout the early paleoproterozoic while atmospheric oxygen rose over the same time interval we argue that uninterrupted plate tectonics between 2 45 and 2 22 ga would have contributed to efficient burial of organic matter and sedimentary pyrite and the consequent rise in atmospheric oxygen documented for this time interval

late pleistocene and holocene landscape evolution and environmental change in the lake sulivan area falkland islands south atlantic abstract late pleistocene and holocene landscape evolution around lake sulivan falkland islands south atlantic is assessed using morphological stratigraphical palaeoecological and 14c dating evidence data indicate a complex landscape history involving frost related activity ventifact development growth and degradation of peat terrain expansion and contraction of lakes and the development stabilisation and subsequent reworking of alluvial and aeolian sand bodies late pleistocene cold conditions favoured production of sandy diamicts on hillslopes and ventifacts on outcrops and blockfield clasts lateglacial climatic amelioration was marked by early growth of peat ca 16 500 cal yr bp since then peat accumulation changes in lake areas and depths and alluvial and or aeolian sand deposition have been recurring and dominant features of landscape change which throughout the late pleistocene and early holocene may have been a response to near synchronous climatic fluctuations along the south america antarctica axis marked landscape changes around lake sulivan coincided with intervals of glacier readvance in south america and south georgia at ca 3200 1900 and 700 100 cal yr bp with lower lake levels and aeolian sand transport from exposed lakebeds and shorelines implying greater aridity in the falklands than at present

ice core evidence for antarctic sea ice decline since the 1950s the instrumental record of antarctic sea ice in recent decades does not reveal a clear signature of warming despite observational evidence from coastal antarctica here we report a significant correlation p

investigation of the dynamic sea ice component of a coupled atmosphere sea ice general circulation model this paper discusses the development of a coupled atmospheric ice model with a comprehensive ice dynamics cavitating fluid rheology and an open water parameterization the results center on the ice thickness and concentration distribution in the arctic basin where the inclusion of ice rheology has maximum impact the influence of the wind stresses calculated by an atmospheric model with stability dependent surface drag coefficients is discussed along with the role of the ocean currents and ocean ice drag coefficient under winter wind stress conditions the model had a tendency to buildup ice on the siberian coast the introduction of a partial slip condition at land ice boundaries in the model designed to prevent this buildup is discussed and the best choice for the amplitude of the slip at the land boundaries in this model is found to be 0 4 the ice model results are compared with the available observational data sets on ice thickness concentration and ice velocity fields to test the validity of the model simulations

continental shelf drift deposit indicates non steady state antarctic bottom water production in the holocene a late quaternary current lain sediment drift deposit over 30 m in thickness has been discovered on the continental shelf of east antarctica in an 850 m deep glacial trough off george vth land radiocarbon dating indicates that a period of rapid deposition on the drift averaging 290 cm kyr occurred in the mid holocene between about 3000 and 5000 yr before present slower rates of around 10 cm kyr during the past 0 3000 yr and from 5000 to about 13000 yr bp coincides with deposition of bioturbated ice rafted debris ird rich sandy mud under an energetic bottom current regime in contrast the mid holocene 3000 5000 yr bp sediments are fine grained laminated to cross laminated with minimal ird content and are contemporaneous with a period of warmer marine conditions with less sea ice production this pattern suggests that bottom currents were weaker than present day in the mid holocene and that the rate of dense bottom water production was reduced at that time this scenario is consistent with the hypothesis of non steady state rates of antarctic bottom water production through the holocene as recently proposed by broecker and his colleagues

short term episodicity of archaean plate tectonics by combining geochemical data and geodynamical models evidence is provided to address the existence and style of archaean plate tectonics a topic of vigorous debate for decades using careful analyses of lithostratigraphic archaean assemblages and numerical model results we illustrate that a short term episodic style of subduction was a viable style of tectonics in the early earth modeling results show how due to the low strength of slabs in a hotter earth frequent slab break off events prevented a modern style long lived subduction system and resulted in frequent cessation and re initiation of the subduction process on a typical time scale of a few million years results fit with geochemical observations that suggest frequent alternation of arc style and non arc style volcanism on a similarly short time scale such tectonics could provide the link between early pre plate tectonic style of tectonics or stagnant lid convection and modern style plate tectonics in which short term episodes of proto subduction evolved over time into a longer term more successful style of plate tectonics as mantle temperature decayed

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

primary results of glaciological studies along an 1100 km transect from zhongshan station to dome a east antarctic ice sheet the chinese national antarctic research expedition ghinare carried out three traverses from zhongshan station to dome a princess elizabeth land and inaccessible area east antarctic ice sheet during the 1996 97 to 1998 99 antarctic field seasons the expeditions are part of the chinese international trans antarctic scientific expedition program in this project glaciological investigations of mass balance ice temperature ice flow stratigraphy in snow pits and snow firn ice cores as well as the glaciochemical study of surface snow and shallow ice cores have been carried out in the 1998 99 field season chinare extended the traverse route to 1128 km inland from zhongshan station the density profiles show that firnification over princess elizabeth land and inaccessible area 290 1100 km along the route is fairly slow and the accumulation rate recovered from snow pits along the initial 460 km of the route is 4 6 21 cm 46 210 kg m 2a 1 water equivalent the initial 460 km of the route can be divided into four sections based on the differences of accumulation rate this pattern approximately coincides with the study on the lambert glacier basin lgb by australian scientists during the past 50 years the trends of both air temperature and accumulation rate show a slight increase in this area in contrast to the west side of the lgb data on surface accumulation rates and their spatial and temporal variability over ice drainage areas such as the lgb are essential for precise mass balance calculation of the whole ice sheet and are important for driving ice sheet models and testing atmospheric models

plate tectonics on the early earth limitations imposed by strength and buoyancy of subducted lithosphere the tectonic style and viability of modern plate tectonics in the early earth is still debated field observations and theoretical arguments both in favor and against the uniformitarian view of plate tectonics back until the archean continue to accumulate here we present the first numerical modeling results that address for a hotter earth the viability of subduction one of the main requirements for plate tectonics a hotter mantle has mainly two effects 1 viscosity is lower and 2 more melt is produced which in a plate tectonic setting will lead to a thicker oceanic crust and harzburgite layer although compositional buoyancy resulting from these thick crust and harzburgite might be a serious limitation for subduction initiation our modeling results show that eclogitization significantly relaxes this limitation for a developed ongoing subduction process furthermore the lower viscosity leads to more frequent slab breakoff and sometimes to crustal separation from the mantle lithosphere unlike earlier propositions not compositional buoyancy considerations but this lithospheric weakness could be the principle limitation to the viability of plate tectonics in a hotter earth these results suggest a new explanation for the absence of ultrahigh pressure metamorphism uhpm and blueschists in most of the precambrian early slabs were not too buoyant but too weak to provide a mechanism for uhpm and exhumation

modeling of antarctic sea ice in a general circulation model a dynamic thermodynamic sea ice model is developed and coupled with the melbourne university general circulation model to simulate the seasonal cycle of the antarctic sea ice distribution the model is efficient rapid to compute and useful for a range of climate studies the thermodynamic part of the sea ice model is similar to that developed by parkinson and washington the dynamics contain a simplified ice rheology that resists compression the thermodynamics is based on energy conservation at the top surface of the ice snow the ice water interface and the open water area to determine the ice formation accretion and ablation a lead parameterization is introduced with an effective partitioning scheme for freezing between and under the ice floes the dynamic calculation determines the motion of ice which is forced with the atmospheric wind taking account of ice resistance and rafting the simulated sea ice distribution compares reasonably well with observations the seasonal cycle of ice extent is well simulated in phase as well as in magnitude simulated sea ice thickness and concentration are also in good agreement with observations over most regions and serve to indicate the importance of advection and ocean drift in the determination of the sea ice distribution

state of the antarctic and southern ocean climate system this paper reviews developments in our understanding of the state of the antarctic and southern ocean climate and its relation to the global climate system over the last few millennia climate over this and earlier periods has not been stable as evidenced by the occurrence of abrupt changes in atmospheric circulation and temperature recorded in antarctic ice core proxies for past climate two of the most prominent abrupt climate change events are characterized by intensification of the circumpolar westerlies also known as the southern annular mode between 6000 and 5000 years ago and since 1200 1000 years ago following the last of these is a period of major trans antarctic reorganization of atmospheric circulation and temperature between ad1700 and 1850 the two earlier antarctic abrupt climate change events appear linked to but predate by several centuries even more abrupt climate change in the north atlantic and the end of the more recent event is coincident with reorganization of atmospheric circulation in the north pacific improved understanding of such events and of the associations between abrupt climate change events recorded in both hemispheres is critical to predicting the impact and timing of future abrupt climate change events potentially forced by anthropogenic changes in greenhouse gases and aerosols special attention is given to the climate of the past 200 years which was recorded by a network of recently available shallow firn cores and to that of the past 50 years which was monitored by the continuous instrumental record significant regional climate changes have taken place in the antarctic during the past 50 years atmospheric temperatures have increased markedly over the antarctic peninsula linked to nearby ocean warming and intensification of the circumpolar westerlies glaciers are retreating on the peninsula in patagonia on the sub antarctic islands and in west antarctica adjacent to the peninsula the penetration of marine air masses has become more pronounced over parts of west antarctica above the surface the antarctic troposphere has warmed during winter while the stratosphere has cooled year round the upper kilometer of the circumpolar southern ocean has warmed antarctic bottom water across a wide sector off east antarctica has freshened and the densest bottom water in the weddell sea has warmed in contrast to these regional climate changes over most of antarctica near surface temperature and snowfall have not increased significantly during at least the past 50 years and proxy data suggest that the atmospheric circulation over the interior has remained in a similar state for at least the past 200 years furthermore the total sea ice cover around antarctica has exhibited no significant overall change since reliable satellite monitoring began in the late 1970s despite large but compensating regional changes the inhomogeneity of antarctic climate in space and time implies that recent antarctic climate changes are due on the one hand to a combination of strong multi decadal variability and anthropogenic effects and as demonstrated by the paleoclimate record on the other hand to multi decadal to millennial scale and longer natural variability forced through changes in orbital insolation greenhouse gases solar variability ice dynamics and aerosols model projections suggest that over the 21st century the antarctic interior will warm by 3 4 1oc and sea ice extent will decrease by 30 ice sheet models are not yet adequate enough to answer pressing questions about the effect of projected warming on mass balance and sea level considering the potentially major impacts of a warming climate on antarctica vigorous efforts are needed to better understand all aspects of the highly coupled antarctic climate system as well as its influence on the earth s climate and oceans

deep water properties and surface buoyancy flux as simulated by a z coordinate model including eddy induced advection abstract a parameterization for the adiabatic transport effect of eddies is introduced into a world ocean model based on the bryan cox code the model topography is only lightly smoothed and retains realistic sill depths and representation of continental shelves commensurate with the model s 1 6 latitude by 2 8 longitude resolution the parameterization allows the model to be run without horizontal diffusivity though only under certain conditions as discussed a first control run features a typical horizontal diffusivity and no eddy induced transport a second run features the eddy induced transport scheme and zero horizontal diffusivity substantial changes occur between the runs in subsurface temperature salinity and density throughout the model ocean the most profound changes occur in the deep ocean and feature a marked increase in density in the second run associated with a substantial decline in temperature especially in the atlantic and an increase in salinity especially in the southern

equatorial 150 km irregularities observed at pohnpei first observations of meter scale equatorial 150 km plasma irregularities outside the american longitude sector are reported the observations conducted at pohnpei western pacific with a wind profiling st radar demonstrate the feasibility of making ionospheric electric field measurements with low cost instruments and also reveal a potentially important anisotropy in the angular distribution of 150 km vhf radar echoes

characteristics of 150 km echoes linked with solar eclipse and their implications to the echoing phenomenon 1 this paper presents a study based on gadanki radar observations of 150 km echoes made during the solar eclipse of 15 january 2010 that occurred during 1122 1515 ist radar echoes were observed only during 1150 1215 ist and 1344 1356 ist linked with the entry and recovery phases of the eclipse respectively the most striking observation found is the unusual ascending and descending features of the echoing regions observed during these two time periods although these echoes occurred at higher altitudes than those of control days snr and spectral width of these echoes are similar to those of the lower echoing region observed on the control days further doppler velocities suggest the presence of westward electric field unlike those of the control days concurrent ionosonde observations showed ascent and descent of the f1 layer very similar to those observed in the 150 km echoes these observations and related analysis suggest that the observed echoes were due to the combined action of the electron density gradients and the reduced recombination rate linked with the solar eclipse effect these observations are first of its kind and elucidate the role of density gradient and recombination rate in the 150 km echoing process

recent southern indian ocean climate variability inferred from a law dome ice core new insights for the interpretation of coastal antarctic isotopic records stable isotopes in water have been measured along a very high accumulation ice core from law dome on the east antarctic coast these enable a detailed comparison of the isotopic records over sixty years 1934 1992 with local antarctic station data and remote meteorological observations atmospheric reanalyses and sea surface temperature estimates on a seasonal to inter annual time scale using both observations and isotopic atmospheric general circulation model gcm results we quantify the relationships between stable isotopes 18o d and deuterium excess d d 8 18o with site and source temperature at seasonal and decadal time scales showing the large imprint of source conditions on law dome isotopes these calibrations provide new insights for the quantitative interpretation of temporal isotopic fluctuations from coastal antarctic ice cores an abrupt change in the local meridional atmospheric circulation is clearly identified from law dome deuterium excess during the 1970s and analysed using gcm simulations

origin and early growth rate of continents abstract data suggest that early continents formed over mantle sinks and grew rapidly between 2 7 and 3 0 ga ago nd isotopic results indicate that archean tonalites which are largely responsible for early continental growth are derived chiefly from ancient depleted mantle sources trace element and oxygen isotope data however indicate enriched sources for such tonalites these constraints can be explained if the mantle is fractionated into depleted dmr and enriched emr reservoirs during early degassing of the earth 4 4 ga blocks of emr are recycled back into the mantle and the emr dmr mixtures serve as sources for archean basalts these basalts are enriched in incompatible elements by deep sea alteration metasomatism in sinks or by other mantle processes and then undergo wet partial melting to produce tonalite magmas enrichment must occur not more than 150 ma prior to tonalite production basalt plateaus accumulate at sinks and as cooling continues geotherms at sinks pass into the eclogite stability field dense eclogite roots form on the basalt plateaus which pulls them into the mantle between 2 7 and 3 0 ga ago resulting in rapid tonalite production and thus in rapid continental growth

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

the southern hemisphere westerlies in the australasian sector over the last glacial cycle a synthesis abstract the southern hemisphere westerlies in the southwest pacific are known to have waxed and waned numerous times during the last two glacial cycles though even semi continuous histories of the westerlies extend back no more than about 20 000 years we have good evidence for at least three scales of events a westerly maximum occurs at the last glacial maximum there is less conclusive evidence for another westerly maximum in the late holocene and for a minimum at ca 11 ka it is too early to ascribe even a cycle to these data but there are grounds to suggest that milankovitch precessional forcing may underlie the observed pattern there is also a quasi 2600 year cycle present in antarctic ice cores that appears to correlate to variation in westerly flow there is strong centennial scale variability in historical times the little ice age lia ca 1400 1850 was associated with a poleward shift in the circumpolar trough in the southern ocean strengthened westerly circulation over tasmania and a strengthening of southwesterly circulation and neoglaciation in southern new zealand while the preceding period 800 1400 ad was less certainly marked by reduced westerly flow from modern records we know that decadal and inter annual variability is important e g pacific decadal oscillation high latitude mode also known as the antarctic oscillation el nino southern oscillation only a minority of the proxies examined can identify changes on these temporal scales but data from tree rings ice cores and laminated lake sediments do indicate systematic changes in these phenomena through time rossby wave patterns are shown to play a critical role in long duration events as well as at the synoptic scale we conclude that westerly circulation is as strong now as at any time in the last glacial cycle in addition changes in latitudinal boundaries in the westerlies may be nearly as large in inter annual zonal shifts ca 2 maximum as in glaciation interglaciation movements ca 3 4 there is however reasonable evidence of strengthening weakening across much of the westerly belt at westerly maxima minima such as the lia early holocene

on the sheet like nature of 150 km f1 radar echoes 1 the so called 150 km or f1 echoes are shown to arise from field aligned irregularities fai that are embedded in a sheet like region using a 49 92 mhz radar on christmas island which has an antenna beam directed toward geographic east at a zenith angle of 29 5 high resolution doppler velocity variations within the radar scattering volume are used to infer that the sheet like region could have both tilt and curvature this finding is used to infer that plasma density n must be enhanced within the sheet like region and that fai could be generated by an interchange instability driven by a vertical electric field or a zonal neutral wind that acts on the horizontal component of the gradient in n how tilted sheet like structures could form in the f1 region is discussed

daytime 150 km echoes observed with the equatorial atmosphere radar in indonesia first results 1 results of the daytime 150 km echoes from 10 36 s magnetic latitude observed using the equatorial atmosphere radar ear which are first of its kind from magnetic southern hemisphere are presented the echoing region of 145 165 km shows forenoon descent and afternoon ascent with signal intensity modulation with time resembling a necklace and the doppler spectra are narrow which are quite similar to those reported earlier from other locations westward and upward southward irregularity drifts are found to be consistent with daytime background electric fields providing a tool to investigate low latitude daytime electrodynamics detectability of these echoes suggests the absence of strong latitudinal dependence in contrast to the earlier belief suggesting a possibility of their detection at mid latitudes also the echoes show zonal anisotropy that is opposite to that observed over pohnpei the implications of these observations are discussed in the light of present understanding of 150 km echoing phenomenon

a change in the geodynamics of continental growth 3 billion years ago models for the growth of continental crust rely on knowing the balance between the generation of new crust and the reworking of old crust throughout earth s history the oxygen isotopic composition of zircons for which uranium lead and hafnium isotopic data provide age constraints is a key archive of crustal reworking we identified systematic variations in hafnium and oxygen isotopes in zircons of different ages that reveal the relative proportions of reworked crust and of new crust through time growth of continental crust appears to have been a continuous process albeit at variable rates a marked decrease in the rate of crustal growth at 3 billion years ago may be linked to the onset of subduction driven plate tectonics

eight centuries of volcanic signal and climate change at talos dome east antarctica 1 during the 1996 programma nazionale di ricerche in antartide international trans antarctic scientific expedition traverse two firn cores were retrieved from the talos dome area east antarctica at elevations of 2316 m td 89 m long and 2246 m st556 19 m long cores were dated by using seasonal variations in non sea salt nss so42 concentrations coupled with the recognition of tritium marker level 1965 1966 and nss so42 spikes due to the most important volcanic events in the past pinatubo 1991 agung 1963 krakatoa 1883 tambora 1815 kuwae 1452 unknown 1259 the number of annual layers recognized in the td and st556 cores was 779 and 97 respectively the d record obtained from the td core has been compared with other east antarctic isotope ice core records dome c epica south pole taylor dome these records suggest cooler climate conditions between the middle of 16th and the beginning of 19th centuries which might be related to the little ice age lia cold period because of the high degree of geographical variability the strongest lia cooling was not temporally synchronous over east antarctica and the analyzed records do not provide a coherent picture for east antarctica the accumulation rate record presented for the td core shows a decrease during part of the lia followed by an increment of about 11 in accumulation during the 20th century at the st556 site the accumulation rate observed during the 20th century was quite stable

little ice age neoglacial paleoenvironmental conditions at siple station antarctica the 550 year records of 18 o and dust concentrations from siple station antarctica suggest warmer and less dusty atmospheric conditions from 1600 to 1830 a d which encompasses much of the northern hemisphere little ice age lia dust and 18 o data from south pole station indicate that the opposite conditions e g cooler and more dusty were prevalent there during the lia meteorological data from 1945 85 show that the lia temperature opposition between amundsen scott and siple inferred from 18 o is consistent with the present spatial distribution of surface temperature there is some observational evidence suggesting that under present conditions stronger zonal westerlies produce a temperature pattern similar to that of the lia these regional differences demonstrate that a suite of spatially distributed high resolution ice core records will be necessary to characterize the lia in antarctica

first results from the ionospheric extension of waccm x during the deep solar minimum year of 2008 new ionosphere and electrodynamics modules have been incorporated in the thermosphere and ionosphere extension of the whole atmosphere community climate model waccm x in order to self consistently simulate the coupled atmosphere ionosphere system the first specified dynamics waccm x v 2 0 results are compared with several data sets and with the thermosphere ionosphere electrodynamics general circulation model tie gcm during the deep solar minimum year comparisons with thermosphere ionosphere mesosphere energetics and dynamics satellite of temperature and zonal wind in the lower thermosphere show that waccm x reproduces the seasonal variability of tides remarkably well including the migrating diurnal and semidiurnal components and the nonmigrating diurnal eastward propagating zonal wavenumber 3 component there is overall agreement between waccm x tie gcm and vertical drifts observed by the communication navigation outage forecast system c nofs satellite over the magnetic equator but apparent discrepancies also exist both model results are dominated by diurnal variations while c nofs observed vertical plasma drifts exhibit strong temporal variations the climatological features of ionospheric peak densities and heights nmf2 and hmf2 from waccm x are in general agreement with the results derived from constellation observing system for meteorology ionosphere and climate cosmic data although the waccm x predicted nmf2 values are smaller and the equatorial ionization anomaly crests are closer to the magnetic equator compared to cosmic and ionosonde observations this may result from the excessive mixing in the lower thermosphere due to the gravity wave parameterization these data model comparisons demonstrate that waccm x can capture the dynamic behavior of the coupled atmosphere and ionosphere in a climatological sense

geochemistry and geochronology of the bimodal volcanic rocks of the suguti area in the southern part of the musoma mara greenstone belt northern tanzania abstract the suguti volcanic rocks of the southern musoma mara greenstone belt in northern tanzania comprise mainly of a bimodal suite of tholeiitic basalts basaltic andesites and calc alkaline rhyolites with a subordinate amount of intermediate rocks zircon u pb and whole rock sm nd geochronology suggests that the two suites are cogenetic and were emplaced at 2755 1 ma with a common initial nd value of 2 1 the tholeiitic basalts are characterised by relatively flat chondrite normalised ree patterns with la ybcn ratios of 0 8 1 6 mean 1 0 the basalts also exhibit negative ti and nb anomalies in primitive mantle normalised multi element diagrams the flat ree patterns the presence of prominent negative nb anomalies and the positive initial nd value of 2 1 suggest that the basalts were formed by low pressure melting of a mantle wedge in an active continental margin setting compared to the tholeiitic basalts the calc alkaline rhyolites are characterised by low abundances of the transition elements cr

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry

partial melting of metabasic rocks and the generation of tonalitic trondhjemitic granodioritic ttg crust in the archaean constraints from phase equilibrium modelling abstract rocks of tonalitic trondhjemitic granodioritic ttg composition preserved in archaean terranes represent fragments of the earth s earliest formed continental crust and are thought to have formed via partial melting of hydrated metabasalt the geodynamic environments in which such high grade metamorphism and anatexis may have occurred in the early earth is strongly debated constraining the pressure p and temperature t conditions at which melts of appropriate composition can be derived from protoliths containing plausible mineral assemblages is central to addressing this question phase equilibrium modelling has been undertaken for an enriched archaean tholeiite bulk composition a suggested protolith for early earth ttg magmas using newly parameterised thermodynamic models that were specifically developed to evaluate the anatectic behaviour of metabasalt assuming minimal h 2 o saturation at the wet solidus the potential fertility of the studied metabasalt is greatest if the solidus is crossed at a pressure of 11 kbar where the solidus temperature reaches a minimum of 610 c major element compositions and proportions of calculated partial melts show the best correlation with those of natural archaean ttgs when in the p t range 800 950 c and 10 18 kbar which we suggest are optimal conditions for their petrogenesis normative geochemistry suggests that these melts would crystallise to broadly trondhjemitic or tonalitic lithologies calculated modal proportions of garnet plagioclase amphibole and rutile in the residuum which control diagnostic trace element signatures in the melt also show the best agreement with natural and experimental data at these conditions importantly although partial melts calculated outside of this p t range still produce ttg like major element compositions and normative mineralogies they would poorly match the diagnostic trace element signatures of natural archaean examples owing to the absences of key minerals in the residuum this optimal p t range of 800 950 c and 10 18 kbar defined herein is most likely to characterise metamorphism of hydrated basalt at the base of a 40 km thick archaean oceanic plateau overthickened crust or else tectonic underplating via shallow subduction which we therefore suggest are the most likely tectonic settings for the formation of the first voluminous continental crust on the early earth