compositional recycling structure of an archean super plume nb th u lree systematics of archean komatiites and basalts revisited the volcanic stage of the 2 7 ga abitibi greenstone belt canada is dominated by bimodal arc magma series and komatiite basalt sequences the latter represents an aerially extensive oceanic plateau erupted from an anomalously hot super plume komatiites define a linear array of nb th vs nb u extending from nb th 8 20 and nb u 26 58 whereas basalts plot on a separate but overlapping field extending to higher th u but lower nb th values inter element ratios of th u nb and lree of komatiites and basalts plot with phanerozoic and modern ocean plateau basalts th u nb and lree are fractionated in subduction zones into low nb th nb u and nb lree arc crust and complementary high nb th nb u and nb lree residual slab accordingly the archean komatiite basalt association may be explained by a plume that likely originated from the core mantle boundary with komatiites erupted from a hot axis containing recycled oceanic crust and basalts erupted from the plume annulus that entrained upper mantle containing recycled oceanic and continental crust high nb th and nb u of plume related volcanic sequences documented in abitibi yilgarn and baltic archean greenstone belts suggest that the extraction and recycling of continental crust may have occurred early in the archean

efficient removal of recalcitrant deep ocean dissolved organic matter during hydrothermal circulation oceanic dissolved organic carbon doc is an important carbon pool similar in magnitude to atmospheric co2 but the fate of its oldest forms is not well understood hot hydrothermal circulation may facilitate the degradation of otherwise un reactive dissolved organic matter playing an important role in the long term global carbon cycle the oldest most recalcitrant forms of doc which make up most of oceanic doc can be recovered by solid phase extraction here we present measurements of solid phase extractable doc from samples collected between 2009 and 2013 at seven vent sites in the atlantic pacific and southern oceans along with magnesium concentrations a conservative tracer of water circulation through hydrothermal systems we find that magnesium and solid phase extractable doc concentrations are correlated suggesting that solid phase extractable doc is almost entirely lost from solution through mineralization or deposition during circulation through hydrothermal vents with fluid temperatures of 212 401 c in laboratory experiments where we heated samples to 380 c for four days we found a similar removal efficiency we conclude that thermal degradation alone can account for the loss of solid phase extractable doc in natural hydrothermal systems and that its maximum lifetime is constrained by the timescale of hydrothermal cycling at about 40 million years

lead isotopic compositions of the western dharwar craton southern india evidence for distinct middle archean terranes in a late archean craton abstract lead and sr isotopic compositions of whole rocks and feldspar separates from several fielddefined suites of rocks from the western dharwar craton of southern india elucidate a complex history of the area the evolution of the craton earlier than 3 0 ga is difficult to determine because of thermal and metasomatic overprinting at 3 0 ga many whole rocks were completely reset then but feldspar separates preserve some of the earlier history apparently a series of terranes evolved from the mantle over the period 3 4 to 3 0 ga to form the western dharwar craton in each case metamorphism and anatexis of older terranes accompanied extraction of juvenile material from the mantle the oldest gneisses 3 4 ga had radiogenic pb and sr isotopic compositions when they formed and were derived from older crustal protoliths that were plausibly accreted to the crust at or even before 3 8 ga the old gneisses were greatly depleted in rb probably at 3 4 ga when the protoliths of younger magmatic rocks may have separated from the mantle younger gneisses apparently formed from these mafic intermediate protoliths in the age range of 3 2 to 3 0ga the 3 0 ga event in the central part of the craton was accompanied by intrusion of small diapiric trondhjemites formed by melting of older crust and penecontemporaneous metasomatism of country rocks by hydrous fluids carrying u and other lithophilic elements the resultant deep crust was greatly depleted in heat producing elements and the craton has been inactive since 3 0 ga except for production of minor granites from depleted source regions at 2 5 2 6 ga the lack of activity at 2 5 ga contrasts strongly with extensive magmatism and metamorphism in surrounding areas that had not undergone depletion at 3 0 ga lower crustal depletion also has caused present day reduced heat flow to be very low

mobilization and redistribution of rees and thorium in a syenitic lateritic profile a mass balance study abstract ree th geochemistry and mineralogy have been studied in a lateritic profile derived from a syenite at akongo in sw cameroon ree and th mass balance calculations for the host rock minerals show that at least 70 of the lree s and 50 of the hrees are contained in allanite apatite titanite and epidote and at least 50 of the th is controlled by the same accessory minerals which represent about 2 wt of the unaltered syenite these accessory phases are destroyed during the first stages of weathering causing most of the rees and th to be rapidly released into the soil comparison of the variation in the zr ti and th content as a function of the apparent density of the different zones of the saprolite shows that th is the least mobile element the presence of secondary thorianite tho2 the etched surface on zircon grains and the presence of ti in secondary cerianite support this geochemical interpretation the concentration of thorium was therefore chosen as invariant relative to the concentration of the other elements especially the rees in mass balance calculations most of the rees are leached in the iron rich upper horizons loose nodular horizon iron crust and top of mottled clay horizon where the groundwater table moves saprolite and bottom of the mottled clay horizon the rees are fractionated and redistributed there is a juxtaposition of leached and accumulation zones with precipitation of lree aluminous hydrated phosphates this study supports the existence of two different cycles for the redistribution of elements in the soil 1 1 as dissolved ions in the saprolite horizon and 2 2 as individual particles in the upper part of the profile

contrasting high and intermediate pressures of metamorphism in the archaean sargur schists of southern india the archaean karnataka craton of southern india contains eastern and western crustal blocks separated by a major thurst in which sargur schists occur as lenses within tonalitic peninsular gneisses the schist complex comprises pelites quartzitic psammites carbonates and calc silicates iron formations and basic rocks and thus provides many mineral assemblages ideal for the calculation of pt conditions with their gneisses the sargur rocks are unconformably overlain by the dharwar greenstone belts and are generally thought to be older than 3 000 my

the influence of alteration on the trace element and nd isotopic compositions of komatiites abstract to investigate the effects of hydrothermal alteration and metamorphism on the chemical and isotopic compositions of komatiites we studied samples from the alexo and texmont regions in the 2 7 ga abitibi belt of canada and from the weltevreden mendon and komati formations of the 3 2 3 5 ga barberton belt of south africa particular emphasis was placed on multiple samples from individual layered spinifex olivine cumulate flows the argument being that if these flows showed variations in the ratios of elements incompatible with olivine then these variations were most likely due to chemical mobility during alteration data for rare earth ree and high field strength elements hfse reveal anomalies non chondritic hfse ree ratios particularly in the most altered samples after taking into account the limits of analytical precision these anomalies are attributed to element mobility not to fractionation of high pressure minerals as suggested by other authors for abitibi komatiites in barberton komatiites three separate processes produced hfse anomalies element mobility during alteration crust assimilation in some samples and majorite fractionation the effects of the latter process were recognized from systematic relationships between hfse ree and al ti that coincided with calculated majorite fractionation trends initial nd isotopic compositions of the komatiites were determined by analysis of magmatic pyroxenes the initial nd of pyroxene from the 2 7 ga alexo komatiite is 3 8 slightly higher than in whole rocks 0 6 to 3 5 average 2 5 in the carbonatized texmont samples the range in initial nd is far greater 6 2 to 8 8 the variable initial nd values in the rocks are attributed to isotopic exchange of nd with surrounding rocks during early alteration and fractionation of sm nd during later events pyroxene from 3 3 ga barberton komatiitic basalt had initial nd of 2 3

progressive metamorphism of mafic rocks from greenschist to granulite facies in the dharwar craton of south india in the archean dharwar craton of southern india n s trending belts of metabasic rocks are exposed which underwent regional metamorphism at about 2 5 ga ago the progressive changes in the assemblages and mineral chemistry of metabasites was studied in a n s traverse covering the chitradurga and nagaman gala belts the sargur area and the nilgiri hills towards the south with increasing metamorphic grade greenschists chl act ab ep pm carb qtz give way to amphibolites hbl plag pm qtz gar cumm and mafic granulites pyx plag gar pm hbl qtz the amphibole composition changes from actinolite in the greenschist zone to tschermakitic hornblende in the amphibolite zone and pargasitic hastingsitic hornblende in the granulite zone the ti content of the amphiboles systematically increases in this direction the plagioclase composition changes from albite in the greenschist zone to oligoclase andesine in the amphibolite zone and andesine labradorite in the granulite zone almandin

pb and nd isotope constraints on the origin of high mg and tholeiitic amphibolites kolar schist belt south india the late archean north south trending kolar schist belt in south india 4 km wide by 80 km long is thought to be a suture between two gneiss terranes krogstad et al 1989 within this volcanics dominated belt are recognized both tholeiitic and high mg komatiitic and picritic amphibolites which make up some 70 and 5 respectively of the exposed outcrops a massive tholeiitic amphibolite separates the belt into western and eastern parts a pb pb whole rock age of 2732 155 ma on samples from a single outcrop of massive tholeiite is a minimum age for this rock samples of this rock have nd values at 2700 ma that range between 3 8 and 6 8 1 initial 238u 204pb of 7 5 and k1 initial 232th 238u of about 4 two different types of high mg amphibolites are recognized from the western part of the belt a picritic or p type and a komatiitic or k type the p type have highly variable ce al ratios all greater than chondritic nd yb ratios greater than chondritic nd at 2700 ma of 1 5 to 8 and pb isotope compositions variable in 207pb 204pb with 1 of about 8 0 and k1 of about 4 the trace element data suggest that the light ree enrichment is a character of the mantle source and is not due to residual garnet the k type amphibolites have near chondritic ce al and nd yb ratios nd at 2700 ma of 1 5 to 8 and 1 of about 8 and k1 of about 4 although the p type is light ree enriched compared to the k type both types have similar ce nd ratios as well as initial pb and nd isotopes if the 2696 136 ma age for the sm nd isochron which includes both types of high mg amphibolite has any significance it dates the time of light ree enrichment of the mantle source for the p type komatiitic amphibolites the high mg amphibolites in the eastern part of the belt are light ree enriched have pb isotopic compositions that are variable in 207pb 204pb with a 1 about 8 5 and nd at 2700 ma of 1 8 to 4 5 hydrothermal fluids associated with metamorphism and shearing prior to about 2400 ma ago were responsible for the introduction of gold quartz carbonate veins into the kolar schist belt the pb isotope composition of galena in these veins suggests that these fluids may have also introduced extraneous pb from adjacent older granitoid gneisses into the amphibolites which could be responsible for the variability in the 207pb 204pb ratios of the samples this extraneous pb probably is not responsible for the distinct pb isotope character of each type of amphibolite

a smnd and pb isotope study of archaean greenstone belts in the southern kaapvaal craton south africa abstract a sm nd and pb study on a wide variety of lithologies in archaean greenstone belt fragments in the southern kaapvaal craton reveals a complex petrogenetic history the fragments are important because they represent a 350 km transect through the craton south of barberton to its southern margin the commondale greenstone belt yields a precise sm nd age of 3334 18ma on an exceptionally well preserved peridotite suite of komatiitic affinity the wide range of sm nd from 0 6 to 1 0 is attributed to the unusual occurrence of orthopyroxene in the spinifex bearing rocks a considerably younger age of about 3 2 ga is suggested for the nondweni greenstone belt close to the southern margin of the craton on the basis of separate sm nd isochrons on individual lithologies ranging from komatiite through komatiitic basalt and basalt to felsic volcanic rocks the most precise sm nd age for the greenstone belt of 3208 87ma is based on a combined mineral whole rock isochron for komatiitic basalts but the possibility cannot be excluded that this represents a later metamorphic age on the basis of the present study the greenstone belts appear to have been emplaced at progressively younger ages towards the southern margin of the craton of greater significance is that irrespective of age the individual lithologies of the nondweni sequence have isotopically distinct initial ratios combining isotopic data from these lithologies invariably yields apparent older ages the isotopic characteristics are consistent with a model of progressive contamination by older sialic crust thought to be the basement to the nondweni sequence

smnd ages of archaean metavolcanics of the dharwar craton south india abstract well constrained sm nd arrays of whole rocks are reported for the first time from three important stratigraphic units of the dharwar volcano sedimentary sequence of south india if interpreted as isochrons these arrays will correspond to the following ages and initial nd ratios relative to chur 2 911 0 049 ga and 0 20 0 40 for the kalasapura mafic basal metavolcanics of the bababudan greenstone belt 2 848 0 070 ga and 0 88 0 82 for the santaveri formation comprising mafic to felsic metavolcanics overlying the kalasapura rocks and 2 747 0 015 ga and 1 72 0 10 for the ingaldhal mafic felsic metavolcanic suite of the chitradurga greenstone belt the possibility that these sm nd arrays are mixing lines between unrelated components depleted and enriched respectively in light rare earth elements has been discussed leading to the conclusion that the ages closely correspond to their time of eruption the older suites from the bababudan belt from a mantle source of chur like composition the younger suite from the chitradurga belt from one with a prior history of lree depletion

late archaean granites a typology based on the dharwar craton india abstract extensive field work in the eastern dharwar craton associated with petrographic and geochemical major and trace elements investigations allows four main types of late archaean granitoids to be distinguished 1 na rich granitoids of trondhjemitic tonalitic and granodioritic composition ttg that are characterised by strongly fractionated ree patterns and low hree contents and generally interpreted as slab melts generated by partial melting of metamorphosed hydrated basalt most likely in a subduction environment 2 sanukitoids which are k and mg rich monzonites and granodiorites with ttg like ree patterns associated with marked lile enrichment and considered to result from the reaction of slab melts generated in a subduction environment with and assimilation of mantle wedge peridotite 3 uncommon high hfse mg and k granites with strongly ree and lile enrichment that probably formed by partial melting of an enriched mantle source unlike in the genesis of sanukitoids in this case the slab melt is considered to be wholly consumed by reaction with mantle minerals resulting in mantle enrichment subsequent melting of this enriched mantle probably in a post subduction setting gives rise to high hfse mg and k magmas as demonstrated for the closepet granite dharwar craton the hot mantle derived magma can induce melting of continental crust and then mix with the anatectic products 4 k rich mg poor anatectic biotite granites with ree patterns that are less fractionated and show negative eu anomalies these granites result from the remelting of old basement or recently accreted plutons both with ttg compositions such anatexis can occur either in a subduction or in a post subduction environment this typology based on the well exposed dharwar craton can be extended to late archaean granitoids from all over the world about such 500 analysis were compiled and a number of discriminant diagrams based on both major and trace elements are proposed the most likely tectonic setting for the observed rock types is an accretionary orogen with accretion of continental blocks above a subduction system followed by thermal reworking of the newly accreted continental material however it has been found that archaean subduction related granitoids are significantly different from their modern counterparts implying progressive changes in the modes of magma generation at convergent margins from the archaean to the present

loss of isotopic nd o and chemical ree memory during metamorphism of komatiites new evidence from eastern finland komatiites are often considered to depict the chemical and isotopic composition of their source rocks in the archean mantle however a weakness of these rocks in tracking the initial compositional heterogeneity of the earth s mantle is the ubiquitous presence of metamorphic recrystallization which casts some doubt about the preservation of primary chemical and isotopic characteristics two spinifex textured komatiite flows from the 2 75 ga old kuhmo greenstone belt siivikkovaara area of eastern finland document this weakness both flows have experienced low to medium grade metamorphism t 450 50 c and now consist entirely of secondary metamorphic assemblages of amphibole chlorite plagioclase with minor proportions of magnetite and ilmenite mgo contents range from 25 to 8 which suggests that low pressure differentiation was likely controlled by olivine and clinopyroxene fractional crystallization however neither major nor trace elements fall on olivine and or clinopyroxene control lines this is particularly well illustrated by the ree as there is an overall 60 variation of ce sm n ratios 0 38 to 0 91 which far exceeds that expected from olivine and clinopyroxene fractionation alone in fact careful evaluation of petrographic including mineral composition data and chemical characteristics shows that most elements of geological interest including the reputedly immobile ree were mobile on a whole rock scale during metamorphic recrystallization of these two flows this view is fully supported by sm nd isotopic data since both whole rock and mineral amphibole and plagioclase samples lie on a single isochron relationship at t 1800 ma an age which corresponds to the time of regional metamorphism thus the meta komatiite flows from siivikkovaara document a case of komatiite flow units in which metamorphism has induced whole rock scale resetting of primary ree patterns and sm nd isotope systematics as regards the nature of the mechanism responsible for this resetting it is suggested here that the secondary mineralogy played an important role as there are correlations between whole rock 147sm 144nd ratios major and trace element chemistry oxygen isotopic ratios and modal proportions of secondary minerals consideration of oxygen isotopic data as well as previous results from a komatiite flow from the nearby tipasjarvi belt further enable us to propose that the ree carrier was likely a co2 rich fluid the process of secondary ree redistribution prevents estimation of the true initial nd isotopic compositions of the two flows taken as a whole the results presented in this paper show that great care should be be exercised in the use of meta komatiites as probes of archean mantle composition

boninites from the neoarchaean gadwal greenstone belt eastern dharwar craton india implications for archaean subduction processes abstract boninites are rare in archaean arcs all over the world whilst there are some reports from the phanerozoic convergent margins in this paper boninites are reported for the first time from an archaean greenstone belt of india which have not been reported from any other indian greenstone belt as yet the greenstone belts of western dharwar craton are characterized by high mg basalts and komatiites whereas in the greenstone belts of eastern dharwar craton boninites represent the high mg lavas these boninites are characterized by 1 high mgo 12 24 wt 2 enriched in transition elements such as ni 423 1184 ppm cr 943 2765 ppm 3 low tio2 0 22 0 36 wt 4 high al2o3 tio2 ratio 29 50 5 gd ybn less than one 6 slight enrichment in lree depletion in mree compared to lree and hree resulting in u shaped ree patterns 7 negative nb ta zr hf ti and v anomalies despite the depletion in mree these characteristics differentiate them from deep mantle plume related magmas these boninites are compositionally similar to those found in modern and fossil island arcs from intraoceanic subduction settings and provide evidence to propose that two stage melting of the archaean mantle wedge generated this type of high mg magma the other high mg type magma komatiitic suite found in the greenstone belts of western dharwar craton has an enriched deep mantle plume source and its dry melting whereas boninites are generated by wet melting of the refractory depleted mantle wedge under high temperature and fluid pressure generated by dehydration of the slab the geological setting hfse and ree data show that possibly the 2 7 ga peak of geological activity had transitional characteristics between those of paleoarchaean and phanerozoic high thermal and fluid flux is inferred to generate these mg enriched and mree hfse lfse depleted suite this discovery of boninites from gadwal greenstone belt indicate that even in archaean subduction of small young and hot oceanic plates produced magmatic assemblage resembling to those of some of the cenozoic subduction complexes

sm nd isotopic data from archean metavolcanic rocks at holenarsipur south india results of a sm nd isotopic analysis of archean metavolcanics in the holenarsipur greenstone belt karnataka south india give a whole rock isochron age of 2 62 ga for lightly deformed metabasaltic amphibolites in the northern part of the belt this is within error of the age of high grade metamorphism and crustal thickening which affected areas further to the south during the late archean together with the geochemical affinities of these and other metavolcanics in karnataka and results of regional structural analysis this unexpected age supports a model relating volcanism and crustal thickening to northward subduction and crustal accretion during the late archean data from basic and ultrabasic metavolcanics from the more strongly deformed and higher grade southern arm of the holenarsipur belt do not permit an age greater than 3 0 ga previously these rocks were regarded as part of an older supracrustal sequence that predated the local 3 0 to 3 3 ga gneissic complex the new dates therefore consider

archean metavolcanics from the rouyn noranda district abitibi greenstone belt quebec 2 mobility of trace elements and petrogenetic constraints qualitative evaluation of altered mafic pillows from the rouyn noranda district of the abitibi metavolcanic belt reveals that for low grade metamorphism the high field strength elements hfs zr y ti and the rare earth elements ree are immobile however where alteration is extreme the ree elements are leached and redeposited by carbonate rich and k rich metasomatic fluids whilst zr y and zr ti remain unchanged rubidium ba and to a lesser extent sr are mobile at all levels of alteration however provided certain filters are applied the original geochemical values of the lavas for the alkaline elements the hfs elements and the ree can be defined the geochemical variations thus identified for the volcanic series of the rouyn noranda district are ascribed to a combination of contamination and fractional crystallization of a primitive tholeiitic magma

age of younger tonalitic magmatism and granulitic metamorphism in the south indian transition zone krishnagiri area comparison with older peninsular gneisses from the gorur hassan area abstract a major episode of continental crust formation associated with granulite facies metamorphism occurred at 2 55 2 51 ga and was related to accretional processes of juvenile crust dating of tonalitic trondhjemitic granitic gneisses and charnockites from the krishnagiri area of south india indicates that magmatic protoliths are 2550 2530 5 ma as shown by both u pb and 207pb 206pb single zircon methods monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510 10 ma these data provide precise time constraints and sr nd isotopes confirm the existence of late tonalitic granodioritic juvenile gneisses at 2550 ma pb single zircon ages from the older peninsular gneisses gorur hassan area are in agreement with some previous sr ages and range between 3200 20 and 3328 10 ma these gneisses were derived from a 3 3 3 5 ga mantle source as indicated from nd isotopes they did not participate significantly in the genesis of the 2 55 ga juvenile magmas all these data together with previous work suggest that the 2 51 ga granulite facies metamorphism occurred near the contact of the ancient peninsular gneisses and the 2 55 2 52 ga juvenile tonalitic trondhjemitic terranes during synaccretional processes subduction mantle plume rb sr biotite ages between 2060 and 2340 ma indicate late cooling probably related to the dextral major east west shearing which displaced the 2 5 ga juvenile terranes toward the west

petrography chemistry and isotopic ages of peninsular gneiss dharwar acid volcanic rocks and the chitradurga granite with special reference to the late archean evolution of the karnataka craton southern india abstract new petrographic major and trace element and isotopic age data are presented for four suites of rocks from the low to medium grade part of the south indian archaean craton in central karnataka two suites are from the peninsular gneiss basement to the late archaean dharwar supergroup the third is a suite of dharwar potassic rhyolites and the fourth is from the chitradurga granite which has intrusive contacts with the dharwar supergroup the chikmagalur granite granodiorite which with its host gneisses forms part of the basement peninsular gneiss to the dharwar supergroup in the bababudan basin yields rb sr and pb pb whole rock isochron dates of 3080 110 ma and 3175 45 ma respectively an initial 87 sr 86 sr ratio of 0 7013 0 0009 and a 1 value 238 u 204 pb of 7 98 host tonalitic gneisses to the chikmagalur granite give an rb sr whole rock isochron date of 3060 160 ma with an initial 87 sr 86 sr ratio of 0 7015 0 0004 and a pb pb whole rock isochron date of 3190 100 ma with a model 1 value of 8 00 a suite of granites and granodioritic tonalitic gneisses chitradurga granitic rocks and gneisses forming part of the peninsular gneiss basement to the dharwar supergroup west of the chitradurga belt yields rb sr and pb pb whole rock isochron dates of 2970 100 ma and 3028 28 ma respectively with an initial 87 sr 86 sr ratio of 0 7035 0 0013 and a model 1 value of 7 62 the suite of dharwar acid volcanic rocks from north of honnali defines a poorly fitted 207 pb 206 pb linear array mswd 15 8 which gives an apparent age of 2565 28 ma and a model 1 value of 7 46 the poor fit of this isochron shows that a closed system was not maintained a conclusion supported by high k abundances which are attributed to a syn or post magmatic ion exchange process the chitradurga granite which intrudes the older rocks of the dharwar supergroup in the chitradurga belt yields a well fitted pb pb whole rock isochron date of 2605 18 ma with a model 1 value of 7 68 the u pb relations in these suites from karnataka are in marked contrast with the commonly severe u depletion and consequently unradiogenic pb isotopic compositions observed in deeply eroded high grade archaean gneiss terranes such as those bordering the north atlantic this contrast indicates that the cratonic rocks in central karnataka represent a relatively high level in the original archaean continental crust

plate tectonics 2 5 billion years ago evidence at kolar south india the archean kolar schist belt south india is a suture zone where two gneiss terranes and at least two amphibolite terranes with distinct histories were accrted amphibolites from the eastern and western sides of the schist belt have distinct incompatible element and isotopic characteristics sugesting that their volcanic protoliths were derived from dint mantle sources the amphibolite and gneiss terranes were juxtaposed by horizontal compression and shearing between 2530 and 2420 million years ago ma along a zone marked by the kolar schist belt this history of accretion of discrete crustal terranes resembles those of phanerozoic convergent margins and thus suggests that plate tectonics operated on earth by 2500 ma

3 35ga komatiite volcanism in the western dharwar craton southern india constraints from nd isotopes and whole rock geochemistry abstract we present field petrographic sm nd whole rock isochron and whole rock geochemical data for komatiites from sargur group greenstone belts of the western dharwar craton field evidence such as pillow structure indicates their eruption in a marine environment petrographic data reveal that the igneous mineralogy has been altered during post magmatic hydrothermal alteration processes corresponding to greenschist to lower amphibolite facies conditions with rarely preserved primary olivine and orthopyroxene a 16 point sm nd whole rock isochron gives an age of 3352 110 ma for the timing of eruption of komatiite lavas about 60 of the studied komatiite samples show al depletion whilst the remaining are al undepleted the al depleted komatiites are characterised by high cao al2o3 ratios 1 01 1 34 and low al2o3 tio2 5 16 whereas al undepleted komatiites show lower cao al2o3 ratios 0 59 0 99 and higher al2o3 tio2 17 26 trace element distribution patterns of komatiites suggest that most of the primary geochemical and nd isotopic compositions are preserved with only minor influence of post magmatic alteration processes and negligible crustal contamination the chemical characteristics of al depleted komatiites such as high gd yb n together with lower hree y zr and hf imply their derivation from deeper upper mantle with garnet majorite involvement whereas lower gd yb n slightly higher hree y zr and hf suggest derivation from shallower upper mantle without garnet involvement the observed chemical characteristics cao al2o3 al2o3 tio2 mgo ni cr nb zr y hf ree indicate derivation of the komatiite magmas from different depths in a plume setting whereas sub contemporaneous felsic volcanism and ttg accretion can be attributed to an arc setting in order to explain the spatial association of komatiite volcanism with contemporaneous mafic felsic volcanism and ttg accretion we propose a combined plume arc setting nd isotope data of the studied komatiites indicate depleted mantle reservoirs which may have evolved by early 4 53 ga global differentiation of the silicate earth as suggested by boyet and carlson boyet m carlson r w 2005 142nd evidence for early 4 53 ga global differentiation of silicate earth science 309 577 581 or extraction of continental crust during the early archaean

sediment transport processes and morphodynamics on a reflective beach under storm and non storm conditions abstract new field experiments of sediment transport on the shoreface of a reflective beach r 0 4 0 6 during pre storm storm and post storm conditions are reported data were collected outside the breakpoint and include water surface elevations cross shore and long shore flow velocity suspended sediment concentrations bedform dimensions and the morphological response instruments included two pulse coherent adps an adv multiple obs and a pencil beam sonar the depth averaged mean velocity was always offshore and increased by a factor five during the storm and this increase is attributed to undertow being enhanced by strong backwash plumes from cusp bays estimates of cross shore suspended sediment transport are landward during the pre and post storm phases consistent with the morphological response during the storm the shoreface quickly appeared to reach equilibrium despite energy levels being maintained with sediment transport rates reducing rapidly after the initial phase of the storm streaming in the wave boundary layer was observed and strongest during the pre storm phase consistent with the observed shoreward transport during the storm the near bed streaming was too small to reverse the increased offshore current consistent with the observed net offshore transport the generally small orbital velocity skewness observed on this steep reflective beach suggests that wave reflection may reduce the overall skewness of the velocity in turn contributing to the stability of reflective beach types

the petrogenesis of gorgona komatiites picrites and basalts new field petrographic and geochemical constraints gorgona island colombia is remarkable not only because it contains the only phanerozoic komatiites but also because it has mafic to ultramafic lavas with a wide range of compositions from moderately enriched to extremely depleted relative to bulk earth the komatiite flows are in many respects similar to archaean komatiites they formed from mgo rich 18 liquids and have upper spinifex zones and lower cumulate zones the cumulate zones of archaean komatiites contain many solid grains in contrast more than 90 of the olivine in the gorgona cumulates is highly skeletal this combined with the fact that the gorgona cumulate zones are thinner than those in archaean komatiites suggests that the komatiite magma became strongly superheated en route to the surface the komatiites have trace element contents intermediate between those of the basalts and the ultramafic tuffs some basalts have isotope compositions indicative of long term enrichment in incompatible elements whereas other basalts and ultramafic volcanics have isotopic signatures that imply corresponding depletion it is apparent that the plume source region of the gorgona magmas was markedly heterogeneous with at least two source components contributing to the observed variation in composition this heterogeneity may have resulted from the incorporation of different components into the plume source or it may be the result of complex melting and melt extraction processes during the ascent of a heterogeneous plume despite earlier suggestions that there may have been a significant age gap between depleted komatiite and basalt flows and the enriched basal new 4oar 39ar dating of basalts and gabbros are more consistent with all being generated at 87 ma during formation of the caribbean colombian plateau possibly at the galapagos hotspot