iron zinc magnesium and uranium isotopic fractionation during continental crust differentiation the tale from migmatites granitoids and pegmatites abstract the causes of some stable isotopic variations in felsic rocks are not well understood in particular the origin of the heavy fe isotopic compositions i e high 56 fe values deviation in of the 56 fe 54 fe ratio relative to irmm 014 of granites with sio 2 70 wt compared with less silicic rocks is still debated it has been interpreted to reflect isotopic fractionation during late stage aqueous fluid exsolution magma differentiation partial melting or soret thermal diffusion the present study addresses this issue by comparing the fe isotopic compositions of a large range of differentiated crustal rocks whole rocks of migmatites granitoids and pegmatites mineral separates with the isotopic compositions of zn mg and u the samples include granites migmatites and pegmatites from the black hills south dakota usa as well as i s and a type granitoids from lachlan fold belt australia the nature of the protolith i e i or s type does not influence the fe isotopic composition of granitoids leucosomes partial melts in migmatites tend to have higher 56 fe values than melanosomes melt residues indicating that partial melting of continental crust material can possibly fractionate fe isotopes no clear positive correlation is found between the isotopic compositions of mg u and fe which rules out the process of soret diffusion in the systems studied here zinc isotopes were measured to trace fluid exsolution because zn can easily be mobilized by aqueous fluids as chloride complexes pegmatites and some granitic rocks with high 56 fe values also have high 66 zn values in addition high sio 2 granites show a large dispersion in the zn fe ratio that cannot easily be explained by magma differentiation alone these results suggest that fluid exsolution is responsible for some of the fe isotopic fractionation documented in felsic rocks and in particular in pegmatites however some granites with high 56 fe values have unfractionated 66 zn values and were presumably poor in fluids e g a type for these samples iron isotopic fractionation during magma differentiation is a viable interpretation equilibrium fe isotopic fractionation factors between silicic melts and minerals remain to be characterized to quantitatively assess the role of fractional crystallization on iron isotopes in granitoids

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

inter mineral fe isotope variations in mantle derived rocks and implications for the fe geochemical cycle iron isotope and major and minor element compositions of coexisting olivine clinopyroxene and orthopyroxene from eight spinel peridotite mantle xenoliths olivine magnetite amphibole and biotite from four andesitic volcanic rocks and garnet and clinopyroxene from seven garnet peridotite and eclogites have been measured to evaluate if inter mineral fe isotope fractionation occurs in high temperature igneous and metamorphic minerals and if isotopic fractionation is related to equilibrium fe isotope partitioning or a result of open system behavior there is no measurable fractionation between silicate minerals and magnetite in andesitic volcanic rocks nor between olivine and orthopyroxene in spinel peridotite mantle xenoliths there are some inter mineral differences up to 0 2 in 56fe 54fe in the fe isotope composition of coexisting olivine and clinopyroxene in spinel peridotites the fe isotope fractionation observed between clinopyroxene and olivine appears to be a result of open system behavior based on a positive correlation between the 56feclinopyroxene olivine fractionation and the 56fe value of clinopyroxene and olivine there is also a significant difference in the isotopic compositions of garnet and clinopyroxene in garnet peridotites and eclogites where the average 56feclinopyroxene garnet fractionation is 0 32 0 07 for six of the seven samples the one sample that has a lower 56feclinopyroxene garnet fractionation of 0 08 has a low ca content in garnet which may reflect some crystal chemical control on fe isotope fractionation the fe isotope variability in mantle derived minerals is interpreted to reflect subduction of isotopically variable oceanic crust followed by transport through metasomatic fluids isotopic variability in the mantle might also occur during crystal fractionation of basaltic magmas within the mantle if garnet is a liquidus phase the isotopic variations in the mantle are apparently homogenized during melting processes producing homogenous fe isotope compositions during crust formation

fe isotopes and the contrasting petrogenesis of a i and s type granite abstract we present new fe isotope data of 42 s i and a type ferroan granites from the cambrian delamerian orogen in south australia the palaeozoic lachlan fold belt and western usa interpretation of these data together with modelling suggests that magmatic processes do result in quite complex fe isotopic differentiation trends and can lead to granites with isotopically heavy iron with 57 fe 0 35 by comparison mid ocean ridge basalts morbs have 57 fe 0 15 teng et al 2013 these variations are similar to those previously reported poitrasson and freydier 2005 heimann et al 2008 telus et al 2012 but contrary to some interpretations beard and johnson 2006 heimann et al 2008 heavy values are not necessarily the product of late stage hydrothermal fluid loss though this process is undoubtedly also an important factor in some circumstances a type ferroan granites reach very heavy 57 fe values 0 4 0 5 whereas i types are systematically lighter 57 fe 0 2 s type granites show a range of intermediate values but also tend to be isotopically heavy 57 fe 0 2 0 4 our results show that the iron isotopic values and trends are signatures that reflect granite generation processes a modelling using the rhyolite melts software suggests that contrasting trajectories and end points in fe isotope evolution towards granite depend on oxidation state of the evolving magma and whether or not the system is oxygen buffered iron isotopic evolution supports an origin of ferroan a type granite from protracted closed magma chamber fractionation of moderately reduced mafic magmas in these systems magnetite saturation is delayed and the ferric iron budget is finite i type systems originate with the supply of relatively oxidised hydrous subduction related magmas from the mantle wedge to the upper plate crust these then experience oxygen buffered open system afc processes in lower crustal hot zones s type magmas are crustal melts that crystallise under reduced conditions initially imposed at source by sulphidic or graphitic sedimentary protoliths the composition of the resulting melts reflects the domination of partial melting where conditions are hence buffered open system followed by subsequent late stage closed system fractionation of these extracted reduced magmas

the geotectonic environments of late proterozoic mineralisation in the southern arabian shield abstract using the results of recent research the geotectonic setting of selected mineral occurrences in the saudi arabian late proterozoic shield south of latitude 21 n is described and an initial metallogenic cycle is presented the occurrences selected are types common in the region or are economically significant they include volcanic hosted copper zinc sulfides stratabound copper in sedimentary and minor volcanic rocks zinc in sedimentary rocks and tungsten hosted by mafic plutonic rocks the copper zinc occurrences are typical of volcanogenic mineralisation they are hosted by intraoceanic volcanic arc complexes one 100 200 ma younger and geochemically more evolved than the other the stratabound copper which has some features comparable with sedimentary hosted copper deposits elsewhere developed in the lower part of a transgressive sequence in a basin marginal to the younger volcanic arc complex the sedimentary hosted zinc mineralisation belongs to a small unit of rocks accreted to the younger arc complex from a terrane east of the shield the mafic plutonic rocks hosting tungsten are part of the post tectonic pan african phase of magmatism in the region the discussion of these occurrences assumes that the region developed through the processes of ocean plate subduction plate collision and cratonisation of the resulting deformed and metamorphosed accreted rocks by the intrusion of large volumes of post tectonic pan african granitic plutons a metallogenic cycle similar to those developed for other regions of accreted arc terranes results

early cretaceous adakitic granites in the northern dabie complex central china implications for partial melting and delamination of thickened lower crust abstract to date few adakitic rocks have been reported in direct association with contemporary intra continental extensional structures which has cast doubt on genetic models involving partial melting of the lower crust this study presents early cretaceous 143 129 ma new sensitive high resolution ion microprobe shrimp zircon u pb ages adakitic granites which are directly associated with a contemporary metamorphic core complex i e the northern dabie complex in the dabie area these granites exhibit relatively high sr contents negligible to positive eu and sr anomalies high la yb and sr y ratios but very low yb and y contents similar to subducted oceanic crust derived adakites they are also characterized however by very low mgo or mg and ni values and nd sr isotope compositions end t 14 6 to 19 4 and 87sr 86sr i 0 7067 0 7087 similar to triassic continent derived eclogites subducted in the dabie sulu orogen additionally late granitic dikes in the adakitic intrusions exhibit low sr contents clearly negative eu and sr anomalies low la yb and sr y ratios but relatively high yb and y contents similar to 118 105 ma granites in the northern dabie complex based on composition and geochronology data of neoproterozoic amphibolites and orthogneisses triassic high to ultra high pressure metamorphic rocks and early cretaceous mafic ultramafic intrusive rocks and the constraints provided by experimental melt data for tonalites metabasaltic rocks and eclogites we suggest that the adakitic granites were most probably generated by partial melting of thickened amphibole or rutile bearing eclogitic lower crust as a consequence of triassic middle jurassic subduction and thrusting the late dikes probably originated from plagioclase bearing intermediate granulites moreover we suggest that late mesozoic delamination or foundering of thickened eclogitic lower crust is also a more plausible mechanism for the petrogenesis of early cretaceous mafic ultramafic intrusive rocks in the dabie area and probably involved partial melting of a mixed source comprising eclogitic lower crust that had delaminated or foundered into upper lithospheric or asthenospheric mantle peridotite asthenospheric upwelling in response to post collisional delamination of lithospheric mantle was likely to have provided the heat source for the cretaceous magmatism

coupled lithium and iron isotope fractionation during magmatic differentiation abstract in this study we investigated fe and li isotope fractionation between mineral separates of olivine pheno and xenocrysts including one clinopyroxyene phenocryst and their basaltic hosts samples were collected from the canary islands teneriffa la palma and some german volcanic regions vogelsberg westerwald and hegau all investigated bulk samples fall in a tight range of li and fe isotope compositions 56 fe wr 0 06 0 17 and 7 li ma 2 5 5 2 assuming 7 li of the olivine free matrix is virtually identical to that of the bulk sample for mass balance reasons in contrast olivine phenocrysts display highly variable but generally light fe and mostly light li isotope compositions compared to their respective olivine free basaltic matrix which was considered to represent the melt with 56 fe ol 0 24 to 0 14 and 7 li ol 10 5 to 6 5 respectively single olivine crystals from one sample display even a larger range of 56 fe ol between 0 7 and 0 1 one single clinopyroxene phenocryst displays the lightest li isotope composition 7 li cpx 17 7 but no fe isotope fractionation relative to melt the olivine phenocrysts show variable mg and ni correlated in most cases that range between 0 89 and 0 74 and between 300 and 3000 g g respectively these olivines likely grew by fractional crystallization in an evolving magma one sample from the vogelsberg volcano contained olivine xenocrysts mg 0 89 and ni 3000 g g in addition to olivine phenocrysts this sample displays the highest li and the second highest fe isotope fractionation between olivine and melt 7 li ol melt 13 56 fe ol melt 0 29 our data i e the variable olivine at constant whole rock and matrix isotope compositions strongly indicate disequilibrium i e kinetic fe and li isotope fractionation between olivine and melt for li also between cpx and melt during fractional crystallization 7 li ol melt is correlated with the li partitioning between olivine and melt i e with li ol li melt indicating li isotope fractionation due to preferential faster diffusion of 6 li into olivine during fractional crystallization olivine with low 7 li ol melt also have low 56 fe ol melt indicating that fe isotope fractionation is also driven by diffusion of isotopically light fe into olivine potentially as fe mg inter diffusion the lowest 56 fe ol melt 0 40 was observed in a sample from westerwald germany with abundant magnetite indicating relatively oxidizing conditions during magma differentiation this may have enhanced equilibrium fe isotope fractionation between olivine and melt or fine dispersed magnetite in the basalt matrix may have shifted its fe isotope composition towards higher 56 fe the decoupling of li and fe isotope fractionation in cpx is likely due to faster diffusion of li relative to fe in cpx implying that the large investigated cpx phenocryst resided in the magma for only a short period of time which was sufficient for li but not for fe diffusion the absence of any equilibrium fe isotope fractionation between the investigated cpx phenocryst and its basaltic host may be related to the similar fe 3 fe 2 of cpx and melt in contrast to cpx the generally light fe isotope composition of all investigated olivine separates implies the existence of equilibrium in addition to diffusion driven isotope fractionation between olivine and melt on the order of 0 1

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

late cretaceous neo tethyan slab roll back evidence from zircon u pb o and whole rock geochemical and sr nd fe isotopic data of adakitic plutons in the himalaya tibetan plateau

iron isotope fractionation during crustal anatexis constraints from migmatites from the dabie orogen central china abstract iron fe isotope fractionation could occur during crustal melting which may contribute to the isotopic variations of granites our current knowledge on fe isotope fractionation during crustal melting remains rare thirteen leucosomes 9 melanosomes and 1 amphibolitic schollen in 11 migmatites from the dabie orogen central china were measured to investigate fe isotope fractionation during crustal anatexis the melanosomes and amphibolitic schollen yield 56fe values from 0 018 0 031 to 0 152 0 027 with an average of 0 106 the leucosomes have 56fe values from 0 107 0 035 to 0 512 0 028 variably higher than their coexisting melanosomes by 0 00 0 36 the 56fe of leucosomes and melanosomes as well as apparent isotope fractionation 56fel m between them do not correlate with the abundance of crystalline carbonate loss on ignition and th u indicating the insignificant effect of fluid components high 56fe values of three leucosomes 0 34 giving large 56fel m from 0 22 to 0 36 were likely produced by feldspar accumulation evidenced by their high eu low feot al2o3 and high plagioclase abundance up to 50 vol capture of peritectic amphiboles in another leucosome may explain its low 56fel m 0 after screening out these samples 7 leucosome melanosome pairs yield identical 56fel m averaging 0 093 0 056 2sd n 7 within analytical uncertainties no correlation between 56fel m and mg feot al2o3 or tio2 feot rules out the possibility that the fractionation observed here is produced by fractional crystallization and or sub solidus isotope re equilibrium between leucosomes and melanosomes therefore we suggest these consistent 56fel m should record equilibrium fe isotope fractionation during crustal anatexis producing these migmatites the leucosomes and melanosomes have comparable fe3 fe 0 40 and 56fel m approximates 0 07 at fe 3 fe leucosome fe 3 fe menalosome 1 fe isotope fractionation during crustal anatexis revealed here thus is not dominantly controlled by distribution of fe3 and fe2 but by the difference in coordination number of iron between granitic melts and residual mafic minerals the observation here argues that fe isotope fractionation during crustal partial melting should also contribute to the previously revealed 56fe variation in high silica granitic rocks

routine isotopic analysis of iron by hr mc icpms how precise and how accurate abstract high temperature isotopic variations documented in natural materials span a narrow range and call for precise and accurate isotopic analyses precisions better than 0 03 95 confidence interval for 56 fe have been obtained by high resolution multi collector inductively coupled plasma mass spectrometry hr mc icpms whether these uncertainties encompass all sources of error has not been carefully evaluated the current study examines all parameters that can affect accuracy and precision of fe isotopic measurements by hr mc icpms it is demonstrated that accurate 56 fe measurements can be routinely achieved within the quoted uncertainties

pyrite rb sr sm nd and fe isotopic constraints on the age and genesis of the qingchengzi pb zn deposits northeastern china abstract sr nd fe isotopes are reported for pyrites and whole rocks of ores and associated rocks for the qingchengzi pb zn deposits in the northeasern china to investigate the age sources and processes of the pb zn mineralization rb sr leaching isotopic dating of pyrites that crystallized coeval with galena and sphalerite from five subsamples of ores yields isochron ages of 143 12 ma to 159 12 ma with a weighted mean age of 151 8 5 2 ma this age can be interpreted to be the timing of pb zn mineralization coeval with the emplacement of late jurassic granites in the region the pyrites have negative end 152 ma values of 17 6 to 22 0 among the neoarchean ttg rocks paleoproterozoic meta sedimetary rocks i e wall rocks and granites and mesozoic mafic dykes and granites however the initial 87sr 86sr ratio of 0 7188 0 0004 is similar to those of late jurassic granites and paleoproterozoic rocks 0 7082 0 7183 distinct from those of mafic dykes 0 7069 0 7110 indicating the ore forming materials i e pb and zn were mainly derived from wall rocks and late jurassic granites combined with the previously published h o isotopes the new age and sr nd isotopic data indicate that the mineralizaing fluids were mainly derived from dehydration of granitic magmas interacted with meteoric water that extracted pb and zn from the wall rocks the 56fe values of pyrites in the pb zn ores and paleoproterozoic meta sedimentary rocks range from 0 23 to 1 09 and 0 49 to 0 17 respectively distinct from those of paleoproterozoic basement rocks and mesozoic rocks 0 03 to 0 28 combined with nd isotopes the large range of 56fe values for pyrites would be the result of rapid precipitation of pyrites in the wall rocks and contemperal deposition of pyrites in the residual fluids during interaction between wall rocks and ore forming fluids collectively our new sr nd fe isotopes of pyrites not only constrain the age and sources of pb and zn mineralization but also reveal the detailed mineralization processes providing a new method to constrain the genesis of hydrothermal pb zn deposits

mg isotope fractionation during partial melting of garnet bearing sources an adakite perspective abstract garnet that has a distinctly light mg isotopic composition compared to other mafic minerals is commonly present in deep crustal and mantle sources whether partial melting of these garnet bearing sources can lead to significant mg isotope fractionation or not remains poorly understood in order to investigate mg isotope fractionation during partial melting of garnet bearing sources we report mg isotopic data of 17 well characterized low mg adakitic rocks lma from the dabie orogen central china these lma are known for derivation from mafic sources leaving eclogitic residua with variable abundance of garnet our data together with previously published results show that fresh dabie lma have homogeneous mg isotopic compositions with an average 26mg of 0 22 0 08 2sd n 18 except that two samples display higher 26mg 0 05 to 0 01 due to alteration during sericitization and chloritization of biotite the lack of correlation between 26mg and dy yb n of the dabie lma indicates an unresolvable residual garnet effect given that mg isotopic compositions of the dabie lma are identical with local mafic basement within the current precision 26mgmelt cpx is estimated to be around 0 02 limited isotope fractionation of the dabie lma could be attributed to the dilution effect of residual clinopyroxene co existing with garnet the estimated 26mgmelt cpx is then applied to predict the possible isotope fractionation that may occur during hydrous and dehydration melting of subducted oceanic crust with a lower melting temperature and more abundant garnet in the residua slab derived melts may have 26mg higher than their sources by up to 0 36 which however can be easily erased by later melt mantle interaction in this regard mg isotopes can serve as a powerful tracer of recycled supracrustal materials

melts_excel a microsoft excel based melts interface for research and teaching of magma properties and evolution the thermodynamic modeling software melts is a powerful tool for investigating crystallization and melting in natural magmatic systems rhyolite melts is a recalibration of melts that better captures the evolution of silicic magmas in the upper crust the current interface of rhyolite melts while flexible can be somewhat cumbersome for the novice we present a new interface that uses web services consumed by a vba backend in microsoft excel the interface is contained within a macro enabled workbook where the user can insert the model input information and initiate computations that are executed on a central server at ofm research results of simple calculations are shown immediately within the interface itself it is also possible to combine a sequence of calculations into an evolutionary path the user can input starting and ending temperatures and pressures temperature and pressure steps and the prevailing oxidation conditions the program shows partial updates at every step of the computations at the conclusion of the calculations a series of data sheets and diagrams are created in a separate workbook which can be saved independently of the interface additionally the user can specify a grid of temperatures and pressures and calculate a phase diagram showing the conditions at which different phases are present the interface can be used to apply the rhyolite melts geobarometer we demonstrate applications of the interface using an example early erupted bishop tuff composition the interface is simple to use and flexible but it requires an internet connection the interface is distributed for free from http melts ofm research org

geochemistry of north cleft segment vent fluids temporal changes in chlorinity and their possible relation to recent volcanism hydrothermal vent fluids from the north cleft segment of the juan de fuca ridge between 44 54 and 45 00 n were sampled in 1988 1990 1991 and 1992 in 1988 chloride depleted relative to seawater diffuse fluids with low dissolved metal concentrations relative to other mid ocean ridge mor fluids were sampled over a distance of 10 km along axis in 1990 and 1991 both high temperature 200 c and diffuse vents were consistently metal and chloride enriched the end member compositions of high temperature vents and nearby diffuse vents were very similar there were small but significant correlated changes in diffuse and high temperature composition from 1990 to 1991 the large scale but temporary venting of low chlorinity fluids over the entire north cleft segment provides a resolution to the apparent mass imbalance implied by observations of continuous venting of chloride enriched fluids it is possible that a volcanic event along the north cleft segment prior to 1987 for which there is firm geologic evidence initially caused a boiling event which resulted in the preferential venting of vapor enriched fluids through 1988 followed by a transition to brine enriched fluids by 1990 high iron low sodium and low sr ca ratios in the high chlorinity fluids suggest that the brine phase has continued to react and approach reequilibration with an alteration mineral assemblage after the phase separation event the absence of chloride depleted fluids from 1990 onward and the systematics of lithium boron and manganese with chloride in the high temperature fluids from north cleft suggest that the evolution toward lower chlorinity at monolith vent from 1990 to 1992 is caused by progressive dilution of a brine with hydrothermal seawater

characteristics of 4 day oscillations trapped by the juan de fuca ridge current meter observations from different studies along the crest of the juan de fuca ridge have been combined to show new features of the relatively large broad spectral peak centered near a period of about 4 days the coherence range of the 4 day oscillations has been extended to the extreme ends of the ridge and the trapped waves appear to propagate northward along the entire length of the ridge at about 1 m s spectral amplitudes are about double on the south end compared to the north end and a recent model for bottom trapped subinertial waves over topography shows this may be due to variations in ridge width allen and thomson 1993 new observations show coherence between temperatures and currents near the south cleft vent field and suggest 4 day oscillations can play a major role in the local advection of hydrothermal plumes these observations combined with earlier results demonstrate that 4 day oscillations are a significant ridge trapped feature and they suggest that other ridges may be important in transmitting energy over much longer distances

iron isotopic compositions of adakitic and non adakitic granitic magmas magma compositional control and subtle residual garnet effect abstract here we present iron fe isotopic compositions of 51 well characterized adakitic and non adakitic igneous rocks from the dabie orogen central china and panama costa rica central america twelve i type non adakitic granitoid samples from the dabie orogen yield 56fe ranging from 0 015 to 0 184 the good correlations between 56fe and indices of magma differentiation e g sio2 feot mg and fe3 fe suggest fe2 rich silicate and oxide minerals dominated fractional crystallization with 56femelt crystal 0 06 may account for the 56fe variation in these samples one a type granite sample from the dabie orogen has 56fe as high as 0 447 likely indicating less magnetite crystallization and an increase in 103ln melt with magma na k ca mg combined with the literature data most high silica sio2 71 wt granitic rocks define a good positive linear correlation between 56fe and na k ca mg 56fe 0 0062 na k ca mg 0 130 r2 0 66 given that fractional crystallization also tends to increase 56fe with na k ca mg this correlation can serve as the maximum estimate of the magma compositional control on fe isotope fractionation low mg adakitic samples lma have 56fe ranging from 0 114 to 0 253 the melt compositional control on lma 56fe could be insignificant due to their limited na k ca mg variation except for one sample that may be affected by late differentiation 14 out of 15 lma have 56fe increasing with dy yb n reflecting a subtle but significant effect of residual garnet proportion this serves as evidence for that source mineralogy may play an important role in fractionating fe isotopes during partial melting dabie and central america high mg adakitic samples have homogeneous fe isotopic compositions with mean 56fe of 0 098 0 038 2sd n 11 and 0 085 0 045 2sd n 11 respectively these samples have undergone melt mantle interaction which may neutralize the possible 56fe variation of their pristine magmas derived by partial melting of eclogitic rocks

origin of heavy fe isotope compositions in high silica igneous rocks a rhyolite perspective abstract the origin of heavy fe isotope compositions in high silica 70 wt sio 2 igneous rocks remains a highly controversial topic considering that fluid exsolution in eruptive rocks is more straight forward to constrain than in plutonic rocks this study addresses the problem of fe isotope fractionation in high silica igneous rocks by measuring fe isotope compositions of representative rhyolitic samples from the neoproterozoic volcanic sedimentary basins in southern china and the triassic tu le basin in northern vietnam the samples show remarkably varied 56 fe irmm014 values ranging from 0 05 0 05 to 0 55 0 05 which is among the highest values reported from felsic rocks the extensional tectonic setting and short melt residence time in magma chambers for the studied rhyolites rule out soret diffusion and thermal migration processes as causes of the high 56 fe values effects of volcanic degassing and fluid exsolution on bulk rock 56 fe values for the rhyolites are also assessed using bulk rock geochemical indicators and rayleigh fractionation models and these processes are found to be insufficient to produce resolvable changes in fe isotope compositions of the residual melt the most probable mechanism accounting for heavy fe isotope compositions in the high silica rhyolites is narrowed down to fractional crystallization processes in the magma before rhyolite eruption removal of isotopically light fe bearing minerals i e ulvospinel rich titanomagnetite ilmenite and biotite is proposed as the main cause of fe isotope variation in silicic melts during magmatic evolution this study implies that crystal fractionation is the dominant mechanism that controls fe isotope fractionation in eruptive rocks and fe isotopes could be used to study magmatic differentiation of high silica magmas

origin of potassic c type adakite magmas experimental and field constraints the postcollisional mesozoic c type or high k adakitic intrusions k adakites of china lack either temporal or spatial association with subduction and have k2o na2o around unity otherwise their geochemistry is very similar to ordinary adakites new experimental data geological observations and theoretical considerations lead us to suggest that these k adakites were high t 1050 and probably 1075 c rather hydrous magmas 6 wt h2o produced by fluid absent partial melting of metatonalites meta andesites or possibly potassic metabasalts at pressures exceeding 2 gpa their peculiar chemistry is a consequence of both the elevated k2o na2o in the protolith and the very high pressure of partial melting the most likely tectonic setting is one of extreme crustal thickening followed by delamination of the eclogitic keel and partial melting of this continental crustal material at mantle depths with high mantle heat flow during orogenic collapse

molybdenum and boron isotopic compositions of porphyry cu mineralization related adakitic rocks in central eastern china new insights into their petrogenesis and crust mantle interaction

fe isotopic fractionation during the magmatic hydrothermal stage of granitic magmatism abstract felsic intrusive magmatic systems are more complex than volcanic systems as they involve prolonged magmatic processes in addition to potentially multiple phases of magmatic hydrothermal activity within a single pluton in particular the competition between exsolved magmatic fluids and crystallizing minerals for elements and isotopes during the magmatic hydrothermal stages of evolution of these systems remains unclear despite fluid exsolution being one of the key steps in ore formation this study focuses on a suite of representative late jurassic w sn mineralization related granites within the nanling range of southern china and investigates the behavior of fe isotopes during magmatic differentiation and magmatic hydrothermal processes recorded within these granites the granites in the study area have variable 56fe values relative to irmm 014 ranging from 0 12 0 04 0 42 0 01 these values do not correlate with bulk rock geochemical indicators of fluid exsolution e g zr hf nb ta k rb and f cl ratios qualitatively indicating that fluid exsolution has an insignificant effect on fe isotopic fractionation we further quantified the relative contributions of fluid exsolution and crystal fractionation on bulk rock fe isotopic variability using a rayleigh fractionation model this modeling indicates that fluid exsolution can only generate a

ridge collision slab window formation and the flux of pacific asthenosphere into the caribbean realm mantle wedge derived arc volcanism ceased in southern costa rica after ca 8 ma because of subduction of the aseismic cocos ridge beneath the central american arc and the subsequent opening of a slab window geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5 8 and 2 ma identify a source derived from the galapagos plume the presence of this source is explained by an influx of pacific upper mantle into the caribbean mantle wedge through a slab window where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted cocos ridge by using geochemical and isotopic signatures we trace this upper mantle flow beneath central america from southern costa rica northward at a rate of 40 mm yr

iron and lithium isotope systematics of the hekla volcano iceland evidence for fe isotope fractionation during magma differentiation abstract in this study potential iron isotope fractionation by magmatic processes in the earth s crust was systematically investigated high precision iron isotope analyses by mc icp ms were performed on a suite of rock samples representative for the volcanic evolution of the hekla volcano iceland the whole series of hekla s rocks results from several processes i basaltic magmas rise and induce partial melting of meta basalts in the lower part of the icelandic crust the resulting dacitic magma evolves to rhyolitic composition through crystal fractionation during this differentiation the 56 54feirmm 014 values increase successively from 0 051 0 021 for the primitive dacites to 0 168 0 021 for the rhyolites this increase can be described by a rayleigh fractionation model using a constant bulk fractionation factor between all mineral phases m and the silicate liquid l of 56 54fem l 0 1 ii the basaltic magma itself differentiates by crystal fractionation to basaltic andesite composition no fe isotope fractionation was found in this series all basalts and basaltic andesites have an average 56 54feirmm 014 value of 0 062 0 042 2sd n 9 identical to mean terrestrial basaltic values reported in previous studies this observation is consistent with the limited removal of iron from the remaining silicate melt through crystal fractionation and small mineral melt fe isotope fractionation factors expected at temperatures in excess of 1050 c iii andesites are produced by mixing of basaltic andesite with dacitic melts the iron isotope composition of the andesites is matching that of the basaltic andesites and the less evolved dacites in agreement with a mixing process in the hekla volcanic suite li concentrations are positively correlated with indicators of magma differentiation all hekla rocks have 7li values typical for the upper mantle and demonstrate the absence of resolvable li isotope fractionation during crystal fractionation as a fluid mobile trace element li concentrations and isotopes are a potential tracer of magma fluid interaction at hekla li concentrations and isotope compositions do not indicate any extensive fluid exsolution hence the heavy fe isotope composition of the dacites and rhyolites can be predominately attributed to fractional crystallisation iron isotope analyses on single samples from other icelandic volcanoes torfajokull vestmannaeyjar confirm heavy fe isotope enrichment in evolving magmas our results suggest that the iron isotope composition of highly evolved crust can be slightly modified by magmatic processes

a forced baroclinic vortex around a hydrothermal plume a steady frictional flow around a hydrothermal plume is described which is forced by the entrainment of water into the buoyant part of the plume as it rises above its source the circulation is anticyclonic clockwise in the northern hemisphere near the equilibrium density level or spreading level of the plume and cyclonic below this level the strength of this flow is adequate to trap anomalous properties from the hydrothermal fluid near the source in the presence of weak background flow some observational support for such a flow is presented

variations in mean currents affecting hydrothermal plumes on the juan de fuca ridge a summary of current meter observations near the juan de fuca ridge shows new features of topographic steering of flow that determine the fate of plumes from hydrothermal vents an anticyclonic circulation exists over the ridge with north and south mean flows along the west and east flanks speeds are up to 2 3 cm s 1 near the ridge and decrease to zero about 25 km from the crest contributions from hydrothermal forcing and tidal rectification may generate the along ridge flows average anomalous heat fluxes associated with the north and south flows were estimated at 1400 and 2600 mw respectively and they are larger than previous estimates based on single point current measurements variability of flow occurs on several time scales and space scales daily average currents over a 4 day period can rotate over the entire ridge with sufficient cross axis speeds to move plumes away from the vents into the stronger north south flows over the flanks currents along the west side of north cleft diverge with near ridge flow 10 km from the ridge turning eastward around the north end of the segment and offshore flow turning westward farther west in winter off axis currents can be several times as large as the long term mean and could advect plume water relatively long distances significant cross axis flow occurs on the north side of axial volcano that may be the result of interaction between the general circulation and topography generating eddy circulations around or near axial or the entire axial cobb massif

fractionation of oxygen and iron isotopes by partial melting processes implications for the interpretation of stable isotope signatures in mafic rocks abstract recycling of oceanic crust into the deep mantle via subduction is a widely accepted mechanism for creating compositional heterogeneity in the upper mantle and for explaining the distinct geochemistry of mantle plumes the oxygen isotope ratios 18o of some ocean island basalts oib span values both above and below that of unmetasomatised upper mantle 5 5 0 4 and provide support for this hypothesis as it is widely assumed that most variations in 18o are produced by near surface low temperature processes here we show a significant linear relationship between 18o and stable iron isotope ratios 57fe in a suite of pristine eclogite xenoliths the 18o values of both bulk samples and garnets range from values within error of normal mantle to significantly lighter values the observed range and correlation between 18o and 57fe is unlikely to be inherited from oceanic crust as 57fe values determined for samples of hydrothermally altered oceanic crust do not differ significantly from the mantle value and show no correlation with 18o it is proposed that the correlated 57fe and 18o variations in this particular eclogite suite are predominantly related to isotopic fractionation by disequilibrium partial melting although modification by melt percolation processes cannot be ruled out fractionation of fe and o isotopes by removal of partial melt enriched in isotopically heavy fe and o is supported by negative correlations between bulk sample 57fe and cr content and bulk sample and garnet 18o and sc contents as cr and sc are elements that become enriched in garnet and pyroxene bearing melt residues melt extraction could take place either during subduction where the eclogites represent the residues of melted oceanic lithosphere or could take place during long term residence within the lithospheric mantle in which case the protoliths of the eclogites could be of either crustal or mantle origin this modification of both 57fe and 18o by melting processes and specifically the production of low 18o signatures in mafic rocks implies that some of the isotopically light 18o values observed in oib and eclogite xenoliths may not necessarily reflect near surface processes or components

iron isotope fractionation during magmatic differentiation in kilauea iki lava lake magmatic differentiation helps produce the chemical and petrographic diversity of terrestrial rocks the extent to which magmatic differentiation fractionates nonradiogenic isotopes is uncertain for some elements we report analyses of iron isotopes in basalts from kilauea iki lava lake hawaii the iron isotopic compositions 56fe 54fe of late stagemeltveins are 0 2 permil per thousand greater than values for olivine cumulates olivine phenocrysts are up to 1 2 per thousand lighter than those of whole rocks these results demonstrate that iron isotopes fractionate during magmatic differentiation at both whole rock and crystal scales this characteristic of iron relative to the characteristics of magnesium and lithium for which no fractionation has been found may be related to its complex redox chemistry in magmatic systems and makes iron a potential tool for studying planetary differentiation

vertical and temporal vorticity observations at juan de fuca ridge hydrothermal signatures moored current measurements made from april 1994 to june 1995 reveal the vertical and temporal structure of the vorticity over the northern segment of the juan de fuca ridge at a depth of 2100m near the thermal and particle maximum associated with the neutrally buoyant hydrothermal plume the relative vorticity is anticyclonic with a mean value of nearly 0 09f with variability of the same magnitude this arises from cross ridge shear in the horizontal currents at this level in the vertical the anticyclonic shear has a peak value at plume level decreasing in magnitude both upward and downward from depths of 2100 2150m as the bottom is approached the transverse shear changes sign with cyclonic values of up to 0 09f below 2250m depth this vorticity signature is suggestive of dynamics of a line source of buoyant fluid entering the plume at its neutral density horizon and not due to individual point sources

characteristics of hydrothermal plumes from two vent fields on the juan de fuca ridge northeast pacific ocean deep ctd transmissometer tows and water bottle sampling were used during 1985 to map the regional distribution of the neutrally buoyant plumes emanating from each of two major vent fields on the southern symmetrical segment sss and endeavour segment es of the juan de fuca ridge at both vent fields emissions from point and diffuse hydrothermal sources coalesced into a single 200 m thick plume elongated in the direction of current flow and with characteristic temperature anomalies of 0 02 0 05 c and light attenuation anomalies of 0 01 0 08 m 1 10 80 g l above background temperature anomalies in the core of each plume were uniform as far downcurrent as the plumes were mapped 10 15 km downcurrent light attenuation trends were non uniform and differed between plumes apparently because different vent fluid chemistries at each field cause significant differences in the settling characteristics of the hydrothermal precipitates vent fluids from the sss are metal dominated and mostly precipitate very fine grained hydrous fe oxides that remain suspended in the plume vent fluids from the es are sulfur dominated and precipitate a high proportion of coarser grained fe sulfides that rapidly settle from the plume the integrated flux of each vent field was estimated from measurements of the advective transport of each plume heat flux was 1700 1100 mw from the es and 580 351 mw from the sss particle flux varied from 546 312 g s to 204 116 g s at the es depending on distance from the vent field and was 92 48 g s from the sss