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

petrological geochemical and geochronological constraints on the origin of the xiadong ural alaskan type complex in nw china and tectonic implication for the evolution of southern central asian orogenic belt abstract the xiadong mafic ultramafic complex is located in the central tianshan terrane the southern part of the central asian orogenic belt it is composed of dunite hornblende clinopyroxenite hornblendite and hornblende gabbro several dykes of hornblendite hornblende gabbro and gabbroic diorite cut the main body these rocks are characterized by adcumulate textures the dunites contain high fo 92 3 96 6 olivines with rare orthopyroxene and plagioclase hornblende chromite and magnetite are common phases in all of the rock types geochemically the rocks of the xiadong complex display high mgo contents and extremely low trace element abundances although the rocks of the main body show higher mgo contents and lower trace element abundance compared to the dykes both are characterized by flat ree patterns and enrichment of lile relative to hsfe these petrological mineralogical and geochemical features indicate that the xiadong complex is a typical ural alaskan type complex the cross cutting and intrusive relationships of the rock units and their distinct geochemical features suggest that the main body represents a different pulse from the intrusive dyke the positive whole rock e nd t values together with variable 87 sr 86 sr i ratios show a depleted mantle source metasomatized by subduction related material zircon u pb dating on one hornblende gabbro from the main body of the complex yields a u pb 206 pb 238 u age of 479 ma and zircons from three samples of the dykes yield ages of 477 379 and 313 ma all the zircons have positive e hf t and slightly higher 18 o than normal mantle and their hf o isotopic values are correlated to their ages the older samples are comparable to those from the permian mafic ultramafic complexes in the beishan terrane which were derived from a mantle source metasomatized by subduction of the south tianshan ocean while the younger samples are similar to those from the paleozoic complexes in the eastern tianshan which record the subduction features of the junggar ocean these results suggest that the central tianshan was a continental arc from at least the ordovician to the carboniferous the lithospheric mantle beneath the central tianshan was depleted in sr nd hf isotopic compositions and was metasomatized by melts and or fluids from the subduction of the south tianshan and junggar oceans in paleozoic the formation of the xiadong ural alaskan type complex was the product of arc magmatism via high degree melting of the lithospheric mantle beneath the southern central asian orogenic belt

iron and magnesium isotopic constraints on the origin of chemical heterogeneity in podiform chromitite from the luobusa ophiolite tibet we present high precision measurements of iron fe and magnesium mg isotopic compositions of olivine orthopyroxene and chromite separates from harzburgites dunites and chromitites in the mantle section of the luobusa ophiolite southern tibet to investigate the origins of podiform chromitite two harzburgites in the zedong ophiolite southern tibet are also reported for comparison the olivine and orthopyroxene in the luobusa and zedong harzburgites have similar fe and mg isotopic compositions with fe 56 values ranging from 0 to 0 083 in olivine from 0 034 to 0 081 in orthopyroxene and mg 26 values ranging from 0 25 parts per thousand to 0 20 parts per thousand in olivine from 0 29 parts per thousand to 0 26 parts per thousand in orthopyroxene respectively the olivines of two dunites from the luobusa display small fe and mg isotopic variations with fe 56 values of 0 014 parts per thousand and 0 116 parts per thousand and mg 26 values of 0 21 parts per thousand and 0 29 parts per thousand all chromites in the luobusa chromitites have lighter fe isotopic compositions than the coexisting olivines with fe 56 values ranging from 0 247 parts per thousand to 0 043 parts per thousand in chromite and from 0 146 parts per thousand to 0 215 parts per thousand in olivine fe 56 chr ol 0 294 to 0 101 parts per thousand the chromite mg 26 values span a significant range from 0 41 parts per thousand to 0 14 parts per thousand large disequilibrium fe and mg isotope fractionation between chromite and olivine as well as positive correlation of chromite fe 56 values with their mgo contents could be attributed to fe mg exchange between chromite and olivine in the disseminated chromitites the higher modal abundances of olivine than chromite would result in a more extensive fe mg exchange whereas chromite in the massive chromitite where olivine is rare could not be affected by this process

crystallographic control on lithium isotope fractionation in archean to cenozoic lithium cesium tantalum pegmatites the age distribution of lct pegmatites largely overlaps with major phases of collisional orogenic events and assembly of super continents some of the largest known lct pegmatite deposits formed in very short intervals 2 7 2 5 and 1 9 1 8 billion years ago ga corresponding to two major pulses of continental crust growth however the exact process of generation and segregation of large volumes of li bearing pegmatite liquids perhaps involving disequilibrium fractional crystallization and leaving residual melts enriched in fluxing elements such as b f h2o li and p remains largely obscure the new data on li contents and isotope compositions in major mineral phases from temporally and geographically separated pegmatite bodies document extreme variations in d7li values among individual large lct pegmatites in particular archean occurrences the observed 10 variations in d7li values for the same mineral phases from different localities i e beryl petalite spodumene lepidolite amblygonite muscovite contrast with globally homogeneous li isotope systematics of major mineral phases from unmodified mantle rocks consistent li isotope offsets between coexisting mineral phases are best explained by li isotope fractionation as a function of the bond length between li and neighboring ions o oh f we suggest that spatially distinct li isotope patterns act as fingerprints for different pegmatites and can be explained by the pre existing li isotope differences of their crustal sources at the time of pegmatite formation owing to differences in crustal age and evolution this would imply secular evolution of the continental crust over earth history toward present day globally broadly uniform crustal 7li 6li ratios d7li 0 the differences among archean occurrences could reflect possible archean paleogeography and perhaps be linked with different thermal regimes of individual cratons as a consequence of variations in crustal thickness

behavior of li and its isotopes during serpentinization of oceanic peridotites analyses of li and li isotopes in serpentinized peridotites have been performed using thermo ionisation mass spectrometry tims and secondary ion mass spectrometry sims techniques on samples collected from the southwest indian ridge swir in the bulk samples li concentrations range from 0 6 to 8 2 ppm while whole rock 6li values range from 2 9 to 14 in situ analyses display a greater range in both li concentration 0 1 19 5 ppm and li isotopic composition 27 to 19 with the serpentinized portions having higher li concentrations than the associated relict phases these variations may reflect changes in li partitioning and isotopic fractionation between serpentine and fluid with temperature and water rock ratio they may also be explained by changes in the composition of the serpentinizing fluid over the course of serpentinization as the serpentine forms by interaction with a circulating fluid it preferentially removes 6li causing the li in the fluid to become isotopically heavier the isotopic composition of the initial hydrothermal fluid is dominated by basalt derived li which easily overwhelms the very low li content originally present in seawater as this fluid circulates through ultramafic rocks it induces the formation of serpentine that incorporates this mantle derived li hence li in serpentine is mainly derived from oceanic crust rather than from seawater and serpentinization involves li recycling within this crust consequently li isotopes are good tracers of the hydrothermal contribution in serpentinizing fluid these results imply that serpentinized peridotites are probably only a minor sink of oceanic li

lithium isotope geochemistry of the hawaiian plume results from the hawaii scientific drilling project and koolau volcano 1 we determined lithium isotopic compositions of mauna loa and mauna kea basalts from the 3 1 km drill hole of the hawaiian scientific drilling project hsdp for comparison li isotopic ratios were also determined for basalts from koolau volcano these two suites of samples define geochemical extremes in the range of hawaiian shield lavas the 400 ka record of mauna kea in the hsdp core shows temporal fluctuations between low 7li 4 relative to the l svec standard and high 7li 5 6 suggesting that the source components in the hawaiian plume are heterogeneous in li isotopic composition based on sio2 content and isotopic ratios of he li nd hf and pb three geochemical groups are identified in mauna kea lavas mauna kea basalts between 1900 and 2500 mbsl have relatively low 7li of about 4 they are low sio2 lavas distinguished by the highest 3he 4he and 208pb 204pb and low 176hf 177hf and 143nd 144nd like basalt from loihi seamount this mauna kea group is considered to originate from the core of the plume above 1900 mbsl high 7li lavas with high sio2 contents appear in both the submarine and subaerial sections they are marked by low 3he 4he and high 176hf 177hf the 7li rich signature of some samples 7li up to 5 7 is indicative of recycled oceanic crust in the plume this magma group defines the kea component the low sio2 lavas in the subaerial section have low 7li 4 3he 4he and 208pb 204pb their 7li values overlap the range of 7li in unaltered mid ocean ridge basalt morb and are consistent with upper mantle material entrained by the plume or contamination of plume derived magmas by the pacific lithosphere the 7li of koolau lavas mostly fall within the range of 4 5 0 3 exceptions are two samples that have 7li of 2 3 the lightest isotopic values may indicate subducted li that was isotopically fractionated during slab dehydration in contrast to other isotopic systems most koolau samples however resemble mauna kea samples in li isotopic composition mauna loa samples have 7li values of 3 5 to 4 9 within the range of the koolau and mauna kea lavas based on these data the loa trend volcanoes and kea trend volcanoes have largely overlapping li isotopic compositions in summary the hawaiian plume is not highly variable in li isotopic composition 7li is typically 4 with perturbations by subducted components to lower and higher ratios 2 5 to 5 7 the overlap of most hawaiian basalt and morb in their range of li isotopic ratios suggests minor influence of recycled oceanic crust in the plume and perhaps similar li isotopic ratios in the upper and lower mantle

lithium isotopic composition of alaskan type intrusion and its implication abstract to determine whether fractionation of li isotopes could occur at high temperatures we examined major element and li isotopic compositions of olivine from the xiadong intrusion an alaskan type complex in the central asian orogenic belt olivine in thirteen dunites displaying characteristic cumulus textures yielded large variations in li concentration 0 10 to 11 18 ppm and isotopic composition 7 li 7 18 to 34 41 these variations are too large to be attributed entirely to diffusive processes the correlations between li elemental or isotopic composition and differentiation indices such as fo and mno contents of olivine and nio content of chromite suggest probable li isotope fractionation during early stage of differentiation we speculate that while li behaves mildly incompatible during differentiation 7 li is preferentially incorporated into olivine relative to 6 li during early stage crystallization from arc magmas relatively high li concentrations and low 7 li in arc magmas might be the result of substantial olivine fractionation prior to eruption

geochemical modeling of sedimentary rocks in the central hokkaido japan episodic deformation and subsequent confined basin formation along the eastern eurasian margin since the cretaceous abstract late cretaceous to paleogene tectonic episodes around the eastern eurasian margin are described utilizing one dimensional 1d basin modeling technique on the basis of organic maturation vitrinite reflectance and t max parameter values of the rock eval pyrolysis fine grained marine sediment of the yezo supergroup in central hokkaido is one of the most extensive cretaceous constituents along the plate margin to evaluate organic maturation within the yezo supergroup a surface section hakkin zawa and a deep borehole miti yubari were selected from which quite different maturation trends were obtained notably low and constant maturation was suggested throughout the thick hakkin zawa section and it was confirmed on the basis of various biomarker analyses it requires anomalously rapid burial probably related to thrust stacking or large scale slumping followed by prompt tilting exhumation event in sharp contrast late cretaceous strata in the miti yubari do not indicate coeval tectonic disturbances in a short period a wider view of the ancient convergent margin suggests that deformation of the forearc propagated westward during the cretaceous high maturation levels in the uppermost thrust sheet around the miti yubari are optimized in 1d geochemical modeling on the assumption that a thick missing paleogene unit has been eroded out as a result of thrust forming contraction scheme emerged during the neogene a similar cenozoic burial pattern is adopted for the once tilted and exhumed hakkin zawa section in order to match the present maturation levels although such an active subsidence of a forearc is generally interpreted as an effect of the subduction erosion heavy mineral composition of the shallow marine to fluvial paleogene implies uplift and exhumation of ultramafic rocks to the east trench side thus the confined basin formation in central hokkaido is to be understood in a different tectonic framework that may reflect transcurrent motions on the plate margin

arrested kinetic li isotope fractionation at the margin of the ilimaussaq complex south greenland evidence for open system processes during final cooling of peralkaline igneous rocks li contents li and isotopic composition 7 li of mafic minerals mainly amphibole and clinopyroxene from the alkaline to peralkaline ilimaussaq plutonic complex south greenland track the behavior of li and its isotopes during magmatic differentiation and final cooling of an alkaline igneous system li in amphibole increase from b10 ppm in caamphiboles of the least differentiated unit to n3000 ppm in na amphiboles of the highly evolved units in contrast li in clinopyroxene are comparatively low b85 ppm and do not vary systematically with differentiation the distribution of li between amphibole and pyroxene is controlled by the major element composition of the minerals ca rich and na rich respectively and changes in oxygen fugacity due to li incorporation via coupled substitution with ferric iron during magmatic differentiation 7 li values of all minerals span a wide range from 17 to 8 with the different intrusive units of the complex having distinct li isotopic systematics amphiboles which dominate the li budget of whole rocks from the inner part of the complex have constant 7 li of 1 8 2 2 2 n 15 this value reflects a homogeneous melt reservoir and is consistent with their mantle derivation in agreement with published o and nd isotopic data clinopyroxenes of these samples are consistently lighter with 7 liamph cpx as large as 8 and are thus not in li isotope equilibrium these low values probably reflect late stage diffusion of li into clinopyroxene during final cooling of the rocks thus enriching the clinopyroxene in 6 li at the margin of the complex 7 li in the syenites increases systematically from 2 to high values of 14 this coupled with the observed li isotope systematics of the granitic country rocks reflects post magmatic open system processes occurring during final cooling of the intrusion although the shape and magnitude of the li isotope and elemental profiles through syenite and country rock are suggestive of diffusion driven isotope fractionation they cannot be modeled by one dimensional diffusive transport and point to circulation of a fluid having a high 7 li value possibly seawater along the chilled contact in all this study

combined sr nd pb and li isotope geochemistry of alkaline lavas from northern james ross island antarctic peninsula and implications for back arc magma formation abstract we present a comprehensive geochemical data set for a suite of back arc alkaline volcanic rocks from james ross island volcanic group jrivg antarctic peninsula the elemental and isotopic sr nd pb and li composition of these cenozoic basalts emplaced east of the antarctic peninsula is different from the compositions of the fore arc alkaline volcanic rocks in southern shetlands and nearby bransfield strait the variability in elemental and isotopic composition is not consistent with the jrivg derivation from a single mantle source but rather it suggests that the magma was mainly derived from a depleted mantle with subordinate oib like enriched mantle component em ii the isotopic data are consistent with mantle melting during extension and possible roll back of the subducted lithosphere of the antarctic plate magma contamination by triassic early tertiary clastic sediments deposited in the back arc basin was only localized and affected li isotopic composition in two of the samples while most of the basalts show very little variation in 7 li values as anticipated for mantle driven li isotopic composition these variations are difficult to resolve with radiogenic isotope systematics but li isotopes may prove sensitive in tracking complex geochemical processes acting through the oceanic crust pile including hydrothermal leaching and seawater equilibration

petrogenesis of the ultramafic zone of the stillwater complex in north america constraints from mineral chemistry and stable isotopes of li and o to investigate the petrogenesis of cyclic units in layered intrusions we examined chromitite dunite poikilitic harzburgite and bronzitite from the ultramafic zone of the stillwater complex and measured stable isotopes of li and o in their major minerals the li isotopes in olivine range from 4 to 26 in 7li with uniform li contents of 1 3 ppm whereas orthopyroxene and clinopyroxene have li contents of 0 5 5 ppm and 4 8 ppm and 7li ranges of 13 to 7 and 14 to 6 respectively the 18o values vary from 4 91 to 5 72 in olivine from 5 11 to 5 87 in orthopyroxene and from 4 64 to 5 86 in clinopyroxene for a given sample olivine displays more variable and higher 7li but lower 18o values than orthopyroxene indicating that olivine experienced more extensive compositional modification after crystallization relative to orthopyroxene the general li and o isotopic compositions are interpreted as the result of re equilibration between interstitial liquids from which pyroxenes crystallized and cumulus minerals the inter mineral and inter sample isotopic variations correlate with mineral assemblages crystal sizes and major and trace element compositions revealing that the interstitial liquids varied compositionally mainly due to mixing between fractionated magma and newly injected primitive magma abrupt mineralogical and geochemical changes from silicate rocks to chromitites imply that hydrous fluids which collected on chromite surfaces and were later released from chromite seams played an additional critical medium of chemical exchange between minerals in the chromitites

temperature dependent isotopic fractionation of lithium between clinopyroxene and high pressure hydrous fluids the fractionation of lithium isotopes between synthetic spodumene as representative of li bearing clinopyroxene and cl and oh bearing aqueous fluids was experimentally determined between 500 and 900 c at 2 0 gpa in all the experiments 7li was preferentially partitioned into the fluid the fractionation is temperature dependent and approximated by the equation 7li clinopyroxene fluid 4 61 1 000 t k 2 48 r 2 0 86 significant li isotopic fractionation of about 1 0 exists even at high temperatures of 900 c using neutral and weakly basic fluids revealed that the amount of fractionation is not different the li isotopic fractionation between altered basalt and hot spring water 350 c in natural samples is in good agreement with our experimentally determined fractionation curve the data confirm earlier speculations drawn from the li isotopic record of dehydrated metamorphic rocks that fluids expelled from a dehydrating slab carry heavier li into the mantle wedge and that a light li component is introduced into the deeper mantle li and li isotopes are redistributed among wedge minerals as fluids travel across the wedge into hotter regions of arc magma production this modifies the li isotopic characteristics of slab derived fluids erasing their source memory and explains the absence of cross arc variations of li isotopes in arc basalts

the absence of lithium isotope fractionation during basalt differentiation new measurements by multicollector sector icp ms we report measurements of the isotopic composition of lithium in basalts using a multicollector magnetic sector plasma source mass spectrometer mc icp ms this is the first application of this analytical technique to li isotope determination external precision of multiple replicate and duplicate measurements for a variety of sample types averages 1 1 2 population the method allows for the rapid 8 min sample analysis of small samples 40 ng li relative to commonly used thermal ionization methods the technique has been applied to a suite of samples from kilauea iki lava lake hawaii the samples range from olivine rich cumulitic lava to sio2 and k2o enriched differentiated liquids and have 7li per mil deviation of sample 7li 6li relative to the l svec standard of 3 0 to 4 8 the data indicate a lack of per mil level li isotope fractionation as a result of crystal liquid fractionation at temperatures greater than 1050 c this conclusion has been tacitly assumed but never demonstrated and is important to the interpretation of li isotope results from such geochemically complex environments as island arcs

diffusion driven extreme lithium isotopic fractionation in country rocks of the tin mountain pegmatite abstract lithium concentrations and isotopic compositions in the country rocks amphibolites and schists of the tin mountain pegmatite show systematic changes with distance to the contact both li and 7 li decrease dramatically along a 10 m traverse from the pegmatite into amphibolite with li concentration decreasing from 471 to 68 ppm and 7 li decreasing from 7 6 to 19 9 rubidium and cs also decrease from the pegmatite contact into the country rock but only within the first 2 m of the contact after which their concentrations remain constant neither mixing between pegmatite fluids and amphibolite nor li isotope fractionation by rayleigh distillation during fluid infiltration is a likely explanation of these observations due to the extremely light isotopic composition required for the amphibolite end member in the mixing model 7 li 20 and the similarly extreme isotopic fractionation required in a rayleigh distillation model rather these variations are likely due to isotopic fractionation accompanying li diffusion from the li rich pegmatite li 450 to 730 ppm into amphibolites li 20 ppm the fact that other alkali element concentrations vary only within 2 m of the contact reflects the orders of magnitude faster diffusion of li relative to heavier elements quartz mica schists in contact with the pegmatite also show large variations in both li and 7 li as a function of distance from contact 1 wt to 70 ppm and 10 8 to 18 6 respectively but over a longer distance of 30 m lithium concentrations of the schist decrease from 1 wt adjacent to the contact to 70 ppm 300 m from the contact the latter is a typical concentration in metapelites the nature of the 7 li variations in the schists is different than in the amphibolites schists within the first 2 m of the contact have nearly identical 7 li of 10 which mimics that of the estimated bulk pegmatite 8 to 11 at a distance of 30 m the 7 li reaches the lowest value in the schists of 18 6 similar to the lowest amphibolite measured at a distance of 300 m the 7 li climbs back to 2 5 which is within the range of 7 li of other schists in the region and metapelites worldwide the behavior of li in the schists can also be modeled by li diffusion with the effective diffusion coefficient in the schist being 10 times greater than that in the amphibolite the effective diffusion coefficients of li in the amphibolite and schist are 2 orders of magnitude greater that those in minerals which implicates the importance of fluid assisted grain boundary diffusion over solid state diffusion in transporting li through these rocks

lithium isotope evidence for light element decoupling in the panama subarc mantle the systematics of fluid mobile trace elements in arc lavas from panama relative to their li isotopic compositions provide unique evidence for the fertilization and subsequent differential extraction of mobile species from the subarc mantle calc alkaline lavas that crystallized between 20 and 5 ma old group that possess 7 li as high as 11 2 have low b be otherwise identical and similarly old calcalkaline lavas with high b be to 23 have mid ocean ridge basalt morb like 7 li 4 7 to 5 6 adakite lavas 3 ma young group possess 7 li from 1 4 to 4 2 and have consistently lower b be than old group lavas consistent with derivation from melting of a devolatilized morb slab if li and b had comparable fluid mobility in the subarc mantle then slab fluids would carry both high b concentrations and elevated 7 li signatures into arc sources and samples with the highest 7 li would also have the highest b be our data suggest that although both li and b are initially derived from the slab older 7 li signatures may be preserved in the mantle beneath arcs as a result regions of the lithospheric mantle will develop li isotope signatures that are heavier than typical morb mantle

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

the effect of chrysotile nanotubes on the serpentine fluid li isotopic fractionation we determined the lithium isotope fractionation between synthetic li bearing serpentine phases lizardite chrysotile antigorite and aqueous fluid in the p t range 0 2 4 0 gpa 200 500 c for experiments in the systems lizardite fluid and antigorite fluid 7li preferentially partitioned into the fluid and 7li values followed the t dependent fractionation of li bearing mica fluid wunder et al 2007 by contrast for chrysotile fluid experiments 7li weakly partitioned into chrysotile this contrasting behavior might be due to different li environments in the three serpentine varieties in lizardite and antigorite lithium is sixfold coordinated whereas in chrysotile lithium is incorporated in two ways octahedrally and as li bearing water cluster filling the nanotube cores low temperature ir spectroscopic measurements of chrysotile showed significant amounts of water whose freezing point was suppressed due to the li contents and the confined geometry of the fluid within the tubes the small inverse li isotopic fractionation for chrysotile fluid results from intra crystalline li isotope fractionation of octahedral li 6 with preference to 6li and lithium within the channels li ch of chrysotile favoring 7li the nanotubes of chrysotile possibly serve as important carrier of li and perhaps also of other fluid mobile elements in serpentinized oceanic crust this might explain higher li abundances for low t chrysotile bearing serpentinites relative to high t serpentinites isotopically heavy li bearing fluids of chrysotile nanotubes could be released at relatively shallow depths during subduction prior to complete chrysotile reactions to form antigorite during further subduction fluids produced during breakdown of serpentine phases will be depleted in 7li this behavior might explain some of the li isotopic heterogeneities observed for serpentinized peridotites

restitic or not insights from trace element content and crystal structure of spinels in african mantle xenoliths the lithospheric architecture of africa consists of several archean cratons and smaller cratonic fragments stitched together and flanked by polycyclic fold belts here we investigate the structure and chemistry of spinels from lithospheric mantle xenoliths from distinct tectonic settings i e from the saharan metacraton in libya waw en namus which could show archaic chemical features cameroon barombi koto and nyos lakes where the sub continental lithospheric mantle was modified during the pan african event and fluxed by asthenospheric melts of the tertiary cameroon volcanic line and morocco tafraoute bou ibalrhatene maars in the middle atlas where different metasomatic events have been recorded from a structural point of view it is to notice that the libyan spinels can be divided into two groups having different oxygen positional parameter u 0 2632 and u 0 2627 respectively while those from cameroon are in between those values as the moroccan ones already studied by other authors the intracrystalline closure temperature tc of the here studied spinels is different among the different samples with one libyan group lb i showing tc in the range 490 640 c and the other 680 950 c lb ii cameroon and morocco spinels show a tc in the range 630 760 c about 150 different spinels have been studied for their trace element content and it can be seen that many of them are related to cr content while zn and co are not and clearly distinguish the occurrences differences in the trace element chemistry in the structural parameters and in the intracrystalline closure temperatures suggest that a different history should be considered for cameroon morocco and lb i and lb ii spinels even if it was not considered for this purpose we tentatively used the fe2 fe3 vs tio2 diagram that discriminate between peridotitic and the so called magmatic spinels i e spinel crystallized from melts lb i and lb ii spinels plot in the peridotitic field while cameroon and morocco spinels fall in the magmatic one consequently the xenoliths sampled from a probably juvenile sclm at the edge of the most important lithospheric roots i e cameroon and morocco apparently have spinels possibly fractionated in situ from percolating melts and do not represent a real spinel peridotite facies on the contrary mantle xenoliths from libya exhibit spinels with peridotitic features compatible with a slow ascent of a mantle diapir plume

evidence for high temperature fractionation of lithium isotopes during differentiation of the moon lithium isotope and abundance data are reported for apollo 15 and 17 mare basalts and the lapaz low ti mare basalt meteorites along with lithium isotope data for carbonaceous ordinary and enstatite chondrites and chondrules from the allende cv3 meteorite apollo 15 low ti mare basalts have lower li contents and lower 7li 3 8 1 2 all uncertainties are 2 standard deviations than apollo 17 high ti mare basalts 7li 5 2 1 2 with evolved lapaz mare basalts having high li contents but similar low 7li 3 7 0 5 to apollo 15 mare basalts in low ti mare basalt 15555 the highest concentrations of li occur in late stage tridymite 20 ppm and plagioclase 11 3 ppm with olivine 6 1 3 8 ppm pyroxene 4 2 1 6 ppm and ilmenite 0 8 0 7 ppm having lower li concentrations values of 7li in low and high ti mare basalt sources broadly correlate negatively with 18o 16o and positively with 56fe 54fe low ti 7li 4 56fe 0 04 18o 5 7 high ti 7li 6 56fe 0 18 18o 5 4 lithium does not appear to have acted as a volatile element during planetary formation with subequal li contents in mare basalts compared with terrestrial martian or vestan basaltic rocks observed li isotopic fractionations in mare basalts can potentially be explained through large degree high temperature igneous differentiation of their source regions progressive magma ocean crystallization led to enrichment in li and 7li in late stage liquids probably as a consequence of preferential retention of 7li and li in the melt relative to crystallizing solids lithium isotopic fractionation has not been observed during extensive differentiation in terrestrial magmatic systems and may only be recognizable during extensive planetary magmatic differentiation under volatile poor conditions as expected for the lunar magma ocean our new analyses of chondrites show that they have 7li ranging between 2 5 and 4 the higher 7li in planetary basalts than in the compilation of chondrites 2 1 1 3 demonstrates that differentiated planetary basalts are on average isotopically heavier than most chondrites

cosmic ray exposure histories and 235u 136xe dating of apollo 11 apollo 12 and apollo 17 mare basalts the history of exposure to cosmic rays was studied for nine basaltic rocks based on the cosmic ray produced noble gas isotopes the results for the five basalts 10003 10047 12004 12051 and 77135 are consistent with a single stage exposure four basalts 10057 10069 12040 and 12053 experienced a multistage exposure using the 235u 136xe dating method the time of their early exposure to cosmic rays on the lunar surface was dated to have begun 3 4 aeons ago immediately after rock formation as defined by their crystallization ages the durations and shielding depths for the proposed two stage exposure models of the four basalts were calculated from the shielding at the early stage we estimate a lower limit of 50 cm for the thickness of the lava flows from which the investigated basalts originate

lithium isotopic composition of central american volcanic arc lavas implications for modification of subarc mantle by slab derived fluids abstract li contents and isotopic compositions were determined for a suite of well characterized basaltic lavas from the central american volcanic arc cava variable li y 0 2 0 5 li sc 0 1 0 4 and 6 li values 2 6 to 7 7 attest to significant compositional heterogeneity in the subarc mantle within specific arc segments these parameters correlate strongly with each other and with a number of other constituents e g k rb ba b la 10 be 9 be 87 sr 86 sr u ce and 230 th 232 th among others these correlations are particularly strong for nicaragua samples coupling of this particular set of constituents is best explained in terms of addition of subduction components to the subarc mantle moreover their selective enrichment with respect to relatively fluid immobile incompatible elements signifies the dominance of fluid vs silicate melt transport of slab components to the subarc mantle several interesting nuances are revealed by the li data first although li and b are strongly correlated in both costa rica and nicaragua there are systematic along strike variations in li b that are consistent with these elements having different fluid release patterns from subducted slab segments for example li b is highest in costa rica where auxiliary evidence indicates higher subduction zone temperatures apparently b is preferentially depleted and li retained in the slab under warmer conditions the same relations are reflected in li 10 be and other subduction tracer systematics all of which point to larger subduction contributions below nicaragua yet even nicaragua lavas vary widely in levels of subduction enrichment high ti basalts from nejapa are the least enriched and have the highest 6 li 1 4 to 2 6 these values are greater than in fresh morb ca 4 and are not easily explained by additions of subducted li because most oceanic crustal rocks and marine sediments have lower 6 li than morb with typical values between 8 and 20 thus it appears the nejapa data may be representative of isotopically light mantle domains relatively light 6 li values in an undepleted spinel lherzolite 11 3 from zabargad is red sea and in primitive backarc basalts 1 6 to 0 5 from lau basin support this conclusion considering representative fluid and mantle endmember compositions the cava data are consistent with limited up to a few percent additions of slab derived fluids to a heterogeneous mantle containing variably depleted and enriched domains to form the respective magma sources in our view the subarc mantle is heterogeneous on a small scale but some arc sectors clearly received greater slab inputs than others