controls of eustasy and diagenesis on the 238u 235u of carbonates and evolution of the seawater 234u 238u during the last 1 4 myr using a leaching protocol designed for the study of u isotopes in recent carbonates we measured the u isotope composition both 238 u 235 u and 234 u 238 u of modern and ancient corals n 6 a limestone and a dolostone as well as 43 shallow water carbonate sediments from the odp leg 166 site 1009 drill core on the slope of the bahamas platform although bulk corals record the seawater 238 u value within 0 02 differences of up to 0 30 in the 238 u of individual leachates suggest a control of the coral structure and a more positive 238 u 235 u ratio in the centers of calcification the drill core 238 u data shows that the 238 u 235 u ratio of shallow water carbonates is controlled mainly by 1 variations in sea level through the mixing of different amounts of platform derived sediments with 238 u 0 50 0 60 heavier than seawater and pelagic sediments with seawater like 238 u values 2 authigenic u enrichment via pore watercirculation and u reduction both on the platform and down to 5 m below the surface mbsf after deposition of the sediment and to a lesser extent by 3 early diagenetic processes i e carbonate dissolution and or recrystallization during sediment burial the global effect of these processes leaves the 238 u values of shallow water carbonates offset relative to that of seawater by carbonates sw 0 24 0 06 95 ci including all samples this shift can be used in seawater paleoredox reconstructions based on carbonates deposited on shallow water platform shelf and slope environments i e most of the carbonate sedimentary record prior to the mesozoic to account for the average effect of carbonate diagenesis assuming that the 238 u 235 u ratio of carbonate platform sediments directly records the seawater 238 u 235 u ratio would underestimate the extent of ocean seafloor anoxia by at least a factor 10 the rapid fluctuations in 238 u values due to sea level changes i is a factor that should be considered before interpreting 238 u variations as reflecting changes in oceanic paleoredox conditions and ii reinforces the need for statistically meaningful data sets the 234 u data suggest that the 234 u 238 u ratio of the seawater has remained within 20 of the modern seawater value during the last 1 1 4 myr furthermore we find that small scale 1 15 variations in seawater 234 u mirror sea level changes during the penultimate glacial interglacial period 140 to 200 ka thus confirming the record of lower 234 u sw during periods of low sea level stand and expanding it to at least the last two glacial interglacial events i e 0 23 ma such fluctuations in 234 u initial values should be taken into account when screening carbonate sediments u th ages on the basis of the initial 234 u 238 u ratios of the samples

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

zn isotopic evidence for fluid assisted ore remobilization at the balmat zinc mine ny abstract shear zone hosted ore bodies at the balmat ny zinc deposit were remobilized at the kilometer scale during amphibolite facies metamorphism ca 1180 ma despite there being little evidence for interaction of hydrous fluids with ores during deformation such translocation distances are considered unlikely without the assistance of fluids measurements of zn isotopic composition of sphalerite from six balmat ore bodies that originated from the same stratigraphic level upper marble unit 6 reveal variation between ore bodies as well as intra orebody trends of isotopic lightening down plunge in general increasing distance of remobilization correlates with decreasing 66zn the syn tectonic isotopic fractionation recorded in balmat sphalerite is interpreted to have resulted from the interaction between the ore and sulfide melts that were fluxed by h2s localized primarily in upper marble unit 7 fetid dolomite lighter isotopes of zn were enriched in the melt leaving the residual ore enriched in heavier zn isotopes these observations support previous petrographic evidence for the presence of anatectic melts at balmat and help to explain the unusual scale of translocation at balmat by means of fluid assisted remobilization during which intergranular melts decreased rock competency

magnesium isotopic systematics of mafic rocks during continental subduction abstract magnesium isotopic compositions of a set of prograde metamorphosed rocks greenschists amphibolites and eclogites from east china were measured in order to understand the behavior of mg isotopes during continental subduction the 26mg values of the greenschists vary from 0 269 0 057 to 0 133 0 042 2sd and correlate negatively with the mgo contents and nb u ratios possibly due to the assimilation of isotopicallly heavy felsic schists 26mg up to 0 099 during the genesis of morb like protoliths the two subgroups of amphibolites have slightly different mg isotopic compositions the group i amphibolites with cumulate origin have 26mg values ranging from 0 243 0 061 to 0 192 0 050 which reflects clinopyroxene accumulation nature of the protolith the group ii amphibolites have relatively light 26mg values varying from 0 358 0 061 to 0 224 0 056 which may result from either source contamination or crustal assimilation of isotopically light components e g carbonates eclogitic garnet and clinopyroxene display large inter mineral mg isotope fractionations 26mgcpx grt 1 097 0 056 to 1 645 0 081 equilibrated at peak metamorphic temperatures the 26mg values of the eclogites vary from 0 348 0 041 to 0 137 0 063 overlapping the ranges of greenschists and amphibolites such mg isotopic variations are not related to unrepresentative sampling nor host eclogite chemical interactions instead they reflect the protolith heterogeneities collectively the similarity of mg isotopic compositions among the greenschists 0 196 0 044 amphibolites 0 271 0 042 and eclogites 0 226 0 044 suggests that mg isotope fractionation during continental subduction if any is limited

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

iron and zinc isotope fractionation during magmatism in the continental crust evidence from bimodal volcanic rocks from hailar basin ne china abstract this study presents fe zn isotope data for a suite of well characterized bimodal volcanic rocks from hailar basin northeast china to understand the mechanism of fe isotope fractionation in highly differentiated igneous rocks the samples range from basaltic trachyandesites to trachytes rhyodacites and rhyolites the 56 fe values increase with increasing sio 2 contents with the rhyolites having the highest 56 fe up to 0 64 0 02 among the previously reported data for igneous rocks at a similar sio 2 the lack of correlation between 56 fe and rb la argues against the effect of fluid exsolution on fe isotopes the 56 fe do not show a clear correlation with 66 zn and radiogenic isotopes suggesting that thermal diffusion or crustal contamination cannot produce the high 56 fe in hailar volcanic rocks fe isotopic variation in hailar volcanic rocks can be explained by two steps of magmatism during the first step partial melting of basaltic trachyandesites with an average 56 fe of 0 09 0 14 produced trachytes rhyodacites with an average 56 fe of 0 24 0 27 modelling using rhyolite melts shows that fe isotopes can be fractionated by preferential partitioning of isotopically different fe 3 and fe 2 between the solid residue and partial melt the second step involves formation of rhyolites with significantly high 56 fe through partial melting or extensive crystallization of crust materials during which isotopically heavy fe preferentially partition into the rhyolitic melt therefore fractionation of fe isotopes between melts and minerals can result in high 56 fe in sio 2 rich igneous rocks and apparent fe isotope heterogeneity within the continental crust

zn he ar and sr nd isotopic compositions of the tongkeng tin polymetallic ore deposit in south china implication for ore genesis abstract the tongkeng tin polymetallic ore deposit in nandan county guangxi zhuang autonomous region southern china is the representative deposit of the largest scale in the famous dachang tin polymetallic ore field it consists of the upper tin polymetallic ores and the lower newly found zinc copper ores with the sn metal reserve of 680 000 tons average grade 1 4 500 000 tons of zn average grade 4 and 80 000 tons of cu average grade 0 2 the orebodies occur as layers and veins within the middle to late devonian siliceous rocks and carbonate rocks in this paper the isotopic compositions of zn he ar and sr nd of the tongkeng deposit have been studied systematically the results show that the isotopic composition of zn is quite homogenous with 66zn 0 22 0 34 in sphalerite samples of various colors from brown to black collected from different orebodies upper tin polymetallic and lower zinc copper ores in the tongkeng deposit it indicates that zn source mainly came from magmatic fluid as the calculated 66zn 0 42 0 54 of ore forming fluid is almost same as that of the longxianggai granite 0 41 0 55 but quite different from that of the marine carbonate average 0 91 he ar isotope study of pyrite shows that the exhalative substances might have been involved in the mineralization with 3he 4he 1 2 ra 2 5 ra in the upper tin polymetallic ores and 3he 4he 0 8 ra 1 9 ra in the lower zinc copper ores in the tongkeng deposit which is different from that of the longxianggai granite 3he 4he 0 7 0 8 ra but close to that of the devonian liujiang formation exhalative sedimentary sequences with 3he 4he 1 5ra the longxianggai granite has isr 0 719 in average end t 7 9 9 6 and nd model age t2dm 1 54 1 68 ga based on the geochemical characteristics of granite and sr nd and o isotopes the source rocks of the longxianggai granite may be mainly derived from the continental crust which is a typical remelted s type granite based on the results of this study and the previous data in the literature it is concluded that the mineralization in the dachang ore field is closely related to the cretaceous longxianggai granite in time and space with the main metal substances of zn most likely derived from the magmatic fluid classified into the type of post magmatic hydrothermal filling metasomatism the admixing of the early devonian spouting sediments in the mineralization was evidenced in the study of source trace and has led to the longtime dispute over the ore genesis

on the development of electron microprobe zircon fission track geochronology abstract the fission track method has been applied for decades to quantify rates and timing of processes in the shallow crust the most widely used approach the external detector method involves counting fission tracks a decay product resulting from the spontaneous fission of 238 u and a paired set of induced tracks parent proxy from the thermal neutron irradiation of 235 u we propose an alternate method of dating zircons that utilizes an electron microprobe to directly measure uranium concentration u and image the number of spontaneous fission tracks or etch figures that intersect the surface of the crystal using an electron backscatter detector the electron microprobe fission track ep ft method is used to date six zircon samples four of which are widely used as standards fish canyon tuff peach springs tuff buluk member of the bakata formation tuff tardee rhyolite mt dromedary complex hypabyssal granite and browns park formation tuff all samples yield ages that overlap within two standard deviations of published reference ages determined using other radiometric techniques i e k ar 40 ar 39 ar and u pb and also by the traditional fission track external detector method to correct for poorly constrained parameters such as the spontaneous fission decay constant weight to volume percent conversions etching efficiency and selection bias we calculate a preliminary z proportionality factor of 4469 661 1 the ep ft technique avoids the hazard and expense of thermal neutron irradiation allows simultaneous chemical compositions to be determined removes the step of counting an external detector manually and will likely allow much higher track densities to be counted than would be normally possible with optical microscopy the technique is ideal for dating moderate to high u zircons 100 ppm u

investigating the molybdenum and uranium redox proxies in a modern shallow anoxic carbonate rich marine sediment setting of the malo jezero mljet lakes adriatic sea abstract the molybdenum mo and uranium u isotope compositions recorded in carbonate rich sediments are emerging as promising paleo redox proxies however the effects of early diagenetic effects within the sediments on these isotope systems are not well constrained we examined the mo and u isotopic systematics in anoxic carbonate rich sediments in a semi enclosed karstic marine lake malo jezero of the island of mljet adriatic sea measurements of water column redox behavior in the lake since the 1950s have shown a transition from anoxic sulfidic conditions in the deeper water column to more oxic conditions and anoxia refined to the sediment and pore waters a 50 cm long sediment core from the deepest part of the lake show a transition from moderate to high authigenic mo and u accumulation with depth consistent with the changing lake redox environment in the past in the deep euxinic part of the core the authigenic mo and u are isotopically lighter and heavier respectively than seawater following similar systematics as observed in other modern euxinic basins with high but non quantitative mo and u uptake into the sediments based on bahamas bank carbonate sediments it has been suggested that the 238u 235u ratio is 0 25 higher compared to seawater from the effects of early carbonate sediment diagenesis and this carbonate vs seawater off set is applicable to carbonate rich sediments across the geological past the shallower part of lake sediment core was deposited under similar redox conditions as the bahamas sediments and these sediments show an average 238u 235u ratio 0 31 0 01 2se higher than seawater although the average 238u 235u ratios for these two carbonate rich settings are similar caution is necessary when inferring seawater 238u 235u compositions from such sediments as they contain u from different sources e g diagenetic uptake and carbonate bound the mo isotope compositions within the same malo jezero sediments are variable but approaches the seawater composition at low pore water h2s concentrations this show the potential of using the mo isotope composition from carbonate rich sediments to infer the seawater composition however further work is required to establish the link between the mo isotope composition and the chemistry of the pore water environment

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

apatite chemical composition determined by electron microprobe and laser ablation inductively coupled plasma mass spectrometry as a probe into granite petrogenesis abstract major minor and trace element abundances in apatites from various i and s type igneous and sedimentary granites of the lachlan fold belt have been determined using electron microprobe and laser ablation inductively coupled plasma mass spectrometer the results show that apatite can accommodate many minor and trace elements whose concentrations and ratios are relatively sensitive to factors controlling many of the fundamental differences between i and s type granites apatites from s type granites generally have higher f but lower cl contents than those from i type granites which is ascribed mainly to the loss of cl during the weathering processes forming the source rocks of s type granites although fractional crystallisation can cause significant enrichment in f as well high mn and fe contents in apatites from s type granites and high s and as abundances in apatites from mafic i type granites result from different oxygen fugacities and degrees of al saturation or aluminosity between metaluminous mafic i type magmas and peraluminous s type and felsic i type magmas there are systematic and distinctive differences in absolute rare earth element ree abundances ree distribution patterns and element ratios e g la y sm nd etc between apatites from different types of granite the strong eu depletion that characterises apatites from s and felsic i type granites is interpreted here to be a result of the uniqueness of crystal chemistry of apatite and high eu2 eu3 ratios in s type and felsic i type magmas which are more reduced and peraluminous than mafic i type magmas strong ree la to eu and th enrichment in apatites from mafic i type granites and marked nd depletion in apatites from most s type and felsic i type granites are caused by the precipitation and fractionation of monazite in the parental magmas of the latter rocks substitution mechanisms are responsible for high na in apatites from s type and felsic i type granites and for high si in apatites from mafic i type granites and may also have important effects on ree partitioning between apatite and melt thus apatite chemistry can be used as an excellent indicator of granite petrogenesis the results have important implications for identifying different types of granite and are potentially significant for determining the provenance of sedimentary rocks

interlaboratory comparison of magnesium isotopic compositions of 12 felsic to ultramafic igneous rock standards analyzed by mc icpms to evaluate the interlaboratory mass bias for high precision stable mg isotopic analysis of natural materials a suite of silicate standards ranging in composition from felsic to ultramafic were analyzed in five laboratories by using three types of multicollector inductively coupled plasma mass spectrometer mc icpms magnesium isotopic compositions from all labs are in agreement for most rocks within quoted uncertainties but are significantly up to 0 3 in 26mg 24mg 4 times of uncertainties different for some mafic samples the interlaboratory mass bias does not correlate with matrix element mg ratios and the mechanism for producing it is uncertain but very likely arises from column chemistry our results suggest that standards with different matrices are needed to calibrate the efficiency of column chemistry and caution should be taken when dealing with samples with complicated matrices well calibrated standards with matrix elements matching samples should be used to reduce the interlaboratory mass bias

mineral composition control on inter mineral iron isotopic fractionation in granitoids abstract this study reports elemental and iron isotopic compositions of feldspar and its coexisting minerals from four dabie i type granitoids to evaluate the factors that control inter mineral fe isotopic fractionation in granitoids the order of heavy iron isotope enrichment is feldspar pyrite magnetite biotite hornblende feldspar has heavier iron isotopic compositions than its co existing magnetite 56 fe plagioclase magnetite 0 376 to 1 084 56 fe alkali feldspar magnetite 0 516 to 0 846 which can be attributed to its high fe 3 fe tot ratio and low coordination number tetrahedrally coordinated of fe 3 56 fe magnetite biotite of coexisting magnetite and biotite ranges from 0 090 to 0 246 based on homogeneous major and iron isotopic compositions of mineral replicates the inter mineral fractionation in this study should reflect equilibrium fractionation the large variations of inter mineral fractionation among feldspar magnetite and biotite cannot be simply explained by temperature variation but strongly depend on mineral compositions the 56 fe plagioclase magnetite and 56 fe alkali feldspar magnetite are positively correlated with albite mode in plagioclase and orthoclase mode in alkali feldspar respectively this could be explained by different fe o bond strength in feldspar due to different fe 3 fe or different crystal parameters the 56 fe magnetite biotite increases with decreasing fe 3 fe biotite and increasing mole na k mg biotite indicating a decrease of factor in low fe 3 fe and high na k mg biotite high silica leucosomes from dabie migmatites with a feldspar accumulation petrogenesis have higher 56 fe values 56 fe 0 42 0 567 than leucosome that represents pristine partial melt 56 fe 0 117 0 016 indicating that accumulation of feldspar could account for high 56 fe values of these rocks high 56 fe values are also predicted for other igneous rocks that are mainly composed of cumulate feldspar crystals e g anorthosites feldspar accumulation however cannot explain high 56 fe values of most high silica granitoids reported in the literature based on their low sr ba contents and negative eu anomalies

characterization and origin of aluminous a type granites from the lachlan fold belt southeastern australia the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are a distinctive group of igneous rocks on the basis of chemical and mineralogical criteria those granites that contain 65 72 sio2 can be distinguished from other types on the basis of higher abundances of fetotal fetotal mg high field strength elements trivalent rare earth elements ga and zn mineralogically they contain fe rich hydrous mafic minerals and primary ilmenite and hence are reduced relative to the ni nio buffer however the extremely felsic a type granites sio2 72 have the same chemical and mineralogical characteristics as felsic fractionated i type granites recent analyses indicate that the lachlan fold belt a type granites have sc f alkali element trace transition element and h2o contents similar to those of other unfractionated i type granites rb sr and nd sm isotopic compositions are highly variable probably reflecting source region heterogeneity the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are distinct from peralkaline rocks in terms of chemical composition petrography and field associations although these rocks have been grouped together as a single type in current classification schemes we propose that the metaluminous to weakly peraluminous a type granites such as those of the lachlan fold belt should be defined as aluminous a type granites and should not be grouped with peralkaline granites the lachlan fold belt aluminous a type granites have relatively high calculated zircon saturation temperatures we suggest that these granites were produced by high temperature partial melting of a felsic infracrustal source

uranium isotope fractionation in saanich inlet a modern analog study of a paleoredox tracer abstract uranium isotope fractionation was studied in the sediment and water of saanich inlet a seasonally anoxic fjord on the east coast of vancouver island the concentration of dissolved u is the same above and below the redoxcline at 120 m depth with an average 238u value of 0 45 0 06 2 s e which is indistinguishable from seawater values reported in other studies this finding is consistent with water renewal times for the inlet that are roughly seasonal in frequency thus hiding the impact of the u losses to the anoxic sediment of the inlet complete digests of the anoxic sediment yielded higher 238u values than seawater 0 21 0 11 2 s e by contrast complete digests of the sediment trap samples yielded lower 238u values than seawater 0 52 0 10 2 s e and marine plankton from the inlet yielded the lowest 238u value of 1 24 0 17 2 cross plots of 238u and 234u vs th u reveal strong correlations indicative of two component mixing when the data from traps and anoxic inlet sediment are combined one mixing end member is fractionated uranium of seawater origin with a 238u value of 0 17 0 14 2 the other is detrital uranium with a 238u value of 0 83 0 12 2 the detrital end member is lower than the range of 238u values reported in the literature for granitoid igneous rocks 0 44 to 0 17 2 telus et al 2012 suggesting that continental weathering fractionates uranium isotopes with preferential release of 238u development and application of u isotopes as a paleoredox proxy has its basis in the nuclear volume fractionation the data from saanich inlet meets this expectation with a positive fractionation factor aq vi sed iv of 0 62 0 17 2 calculated as the difference in 238u between authigenic u in anoxic sediments and sediment traps 0 17 and u dissolved in seawater 0 45 however it is widely believed that u vi reduction in the marine environment occurs on the surfaces of particles and that the negative isotope effect associated with u vi sorption to plankton seawater plankton 0 79 0 17 2 opens up the possibility that the particulate pool of seawater derived u vi is fractionated from the dissolved pool accordingly the reduction of u vi in the marine environment might involve two steps 1 u vi sorption to particles with a negative fractionation and 2 reduction to u iv on particles with a positive fractionation environmental factors that are not yet well understood may influence the relative reaction rates for the two steps thus affecting the magnitude and sign of the overall fractionation this has the potential to explain conflicting results reported for u vi reduction experiments in the literature laboratory and field where both positive and negative fractionations have been observed and in some instances no fractionation at all variation in the fractionation factor has the potential to complicate application of u isotopes as a paleoredox proxy but this study and a study of the black sea romaniello 2012 give the same results within their respective uncertainties suggesting that 0 62 0 17 2 is a robust assessment of the apparent u isotope fractionation factor associated with reductive deposition of seawater u vi in both anoxic and euxinic marine sediments of the present day

equilibrium high temperature fe isotope fractionation between fayalite and magnetite an experimental calibration abstract the iron isotopic fractionation factor between magnetite and fayalite was measured at high temperature in a piston cylinder apparatus for the first time the three isotope method matsuhisa j goldsmith j r and clayton r n 1978 mechanisms of hydrothermal crystallisation of quartz at 250 c and 15 kbar geochim cosmochim acta 42 173 182 has been used to determine equilibrium fractionation between two minerals directly the experimentally determined temperature dependent iron isotope fractionation between fayalite and magnetite is described by 10 3 ln 57 mag fa 57 fe mag fa 0 30 0 024 10 6 t 2 the equation predicts measurable fractionation at magmatic temperatures our work bears on the magnitude of fe isotope fractionation attending differentiation in magmatic systems and provides a new isotope thermometer for co existing fayalite and magnetite

melt segregation in the continental crust before melt will segregate from its restite and source there must be both a driving force and space into which the melt can move the continental crust is anisotropic and in most cases is subject to deviatoric stresses during melting under these conditions melt segregation begins when an interconnected network of melt forms and the source becomes permeable melt is driven by pressure gradients from its restite toward dilatant sites set up in the heterogeneously deforming source the source is continually drained of melt and so need never contain more melt than that required to maintain permeability 26 where the solid matrix breaks down and diapirism can occur

spatially controlled fe and si isotope variations an alternative view on the formation of the torres del paine pluton we present new fe and si isotope ratio data for the torres del paine igneous complex in southern chile the multi composition pluton consists of an approximately 1 km vertical exposure of homogenous granite overlying a contemporaneous 250 m thick mafic gabbro suite this first of its kind spatially dependent fe and si isotope investigation of a convergent margin related pluton aims to understand the nature of granite and silicic igneous rock formation results collected by mc icp ms show a trend of increasing 56fe and 30si with increasing silica content as well as a systematic increase in 56fe away from the mafic base of the pluton the marginal torres del paine granites have heavier fe isotope signatures 56fe 0 25 0 02 2se compared to granites found in the interior pluton 56fe 0 17 0 02 2se cerro toro country rock values are isotopically light in both fe and si isotopic systems 56fe 0 05 0 02 30si 0 38 0 07 the variations in the fe and si isotopic data cannot be accounted for by local assimilation of the wall rocks in situ fractional crystallization late stage fluid exsolution or some combination of these processes instead we conclude that thermal diffusion or source magma variation is the most likely process producing fe isotope ratio variations in the torres del paine pluton

investigation of magnesium isotope fractionation during basalt differentiation implications for a chondritic composition of the terrestrial mantle to investigate whether magnesium isotopes are fractionated during basalt differentiation we have performed high precision mg isotopic analyses by multi collector inductively coupled plasma mass spectrometry mc icp ms on a set of well characterized samples from kilauea iki lava lake hawaii usa samples from the kilauea iki lava lake produced by closed system crystal melt fractionation range from olivine rich cumulates to highly differentiated basalts with mgo content ranging from 2 37 to 26 87 wt our results demonstrate that although these basalts have diverse chemical compositions mineralogies crystallization temperatures and degrees of differentiation their mg isotopic compositions display no measurable variation within the limits of our external precision average 26 mg 0 36 0 10 and 25 mg 0 20 0 07 uncertainties are 2sd this indicates that mg isotopic fractionation during crystal melt fractionation at temperatures of 1055 c is undetectable at the level of precision of the current investigation calculations based on our data suggest that at near magmatic temperatures the maximum fractionation in the 26 mg 24 mg ratio between olivine and melt is 0 07 two additional oceanic basalts two continental basalts bcr 1 and bcr 2 and two primitive carbonaceous chondrites allende and murchison analyzed in this study have mg isotopic compositions similar to the kilauea iki lava lake samples in contrast to a recent report u wiechert a n halliday non chondritic magnesium and the origins of the inner terrestrial planets earth and planetary science letters 256 2007 360 371 the results presented here suggest that the bulk silicate earth has a chondritic mg isotopic composition 2007 published by elsevier b v

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

iron isotopic evolution during fractional crystallization of the uppermost bushveld complex layered mafic intrusion we present 56fe 56fe 54fe relative to standard irmm 014 data from whole rock and magnetite of the upper and upper main zones uumz of the bushveld complex with it we assess the role of fractional crystallization in controlling the fe isotopic evolution of a mafic magma the uumz evolved by fractional crystallization of a dry tholeiitic magma to produce gabbros and diorites with cumulus magnetite and fayalitic olivine despite previous experimental work indicating a potential for magnetite crystallization to drastically change magma 56fe we observe no change in whole rock 56fe above and below magnetite saturation we also observe no systematic change in whole rock 56fe with increasing stratigraphic height and only a small variation in 56fe in magnetite separates above magnetite saturation whole rock 56fe errors twice standard deviation 2 throughout the uumz ranges from 0 01 0 03 to 0 21 0 09 56feaveragewr 0 10 0 09 n 21 isotopically light outlier 56fewr 0 15 and magnetites range from 0 28 0 04 to 0 86 0 07 56feaveragemgt 0 50 0 15 n 20 similar to values previously reported for other layered intrusions we compare our measured 56fewr to a model that incorporates the changing normative mineralogy calculated temperatures and published fractionation factors of fe bearing phases throughout the uumz and produces 56fewr values that evolve only in response to fractional crystallization our results show that the fe isotopic composition of a multiply saturated multiple phases on the liquidus magma is unlikely to change significantly during fractional crystallization of magnetite due to the competing fractionation of other fe bearing cumulus phases this article is protected by copyright all rights reserved

uranium and molybdenum isotope systematics in modern euxinic basins case studies from the central baltic sea and the kyllaren fjord norway abstract recent investigations have revealed significant fractionation of 238u 235u between organic rich sediments of anoxic marginal seas and seawater indicating redox dependent u isotope fractionation this study explores the conditions controlling u isotope fractionation in selected modern anoxic basins baltic sea landsort and gotland deeps and the kyllaren fjord in norway and compares u with mo isotope fractionation therefore the concentrations and isotope compositions of dissolved u and mo from the water column and organic rich sediments from three currently anoxic basins have been analysed the water column samples from the kyllaren fjord display a moderate depletion of u and a strong depletion of mo with increasing depth these variations are associated with a decrease in 238u and an increase in 98mo with depth from 0 35 to 0 70 and from 2 4 to 2 6 respectively from the u isotope composition of the deep euxinic water column of the kyllaren fjord a minimum value of 238ured 0 7 for the u isotope fractionation during reduction is inferred due to the high sedimentation rate in the kyllaren fjord surface sediment samples are only moderately enriched in u and mo 4 g g and 6 g g to 37 g g respectively and display 238u and 98mo averages of 0 26 0 10 and 2 18 0 21 respectively water column samples from the baltic sea display a minor decrease in the u and mo concentrations and only negligible u and mo isotope fractionation compared to open seawater the sediment layers from the baltic sea landsort deep which were deposited under mostly anoxic conditions are moderately enriched in u up to 8 g g and highly enriched in mo up to 222 g g however u and mo isotopic compositions are significantly lighter with 238u of 0 36 0 08 and 98mo of 0 04 0 11 than those of typical organic rich sediments from anoxic basins these findings reveal that compared to mo the u isotopic composition of sediments in restricted and strongly euxinic basins like the kyllaren fjord is significantly more dependent on 1 the extent of u removal from the water column which is lower than that of mo and on 2 the sedimentation rate i e the fraction of authigenic u relative to detrital u in the sediment which is also lower compared to that of mo in more open and only temporarily euxinic basins such as the investigated basins of the baltic sea strong mo is coupled with weak u isotope fractionation between water and sediment these signatures were likely the result of isotope fractionation under weakly sulfidic conditions and the generation of isotopically light mo during frequently occurring flushing events with oxygen rich water from the open sea this implies that mo and u isotope signatures of sediments only record paleo water column redox conditions of restricted basins if the water column was permanently stratified

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

an earth moon silicon isotope model to track silicic magma origins abstract a comparison between terrestrial and lunar igneous rocks reveals that si isotope compositions become slightly though significantly enriched in heavy isotopes from basalts to granites and anorthosites as a function of the rock sio 2 concentration and or tectosilicate content this is interpreted as the result of a global igneous differentiation process that leads to an increased amount of tectosilicates in the rocks this relationship of increasing degree of melt polymerization with increasingly heavy silicon isotope composition is particularly apparent in lunar rocks the terrestrial trend however is more scattered given the sensitivity of si isotopes to water rock interactions it is likely that the more erratic terrestrial trend reveals the involvement of water that does not occur on the moon hence si isotopes appear to reflect the occurrence of low temperature water rock interaction products like clay minerals in the source of peraluminous leucogranites conversely the heavy silicon isotope composition of some andesites possibly trace the input of fluids involved in subduction zones and or interaction of the oceanic crust with isotopically heavy seawater before subduction

iron isotope fractionation during skarn type alteration implications for metal source in the han xing iron skarn deposit abstract the han xing iron mineralization in the central north china craton is a typical fe skarn deposit associated with altered diorites here we report the fe isotopic compositions of whole rocks and mineral separates from this deposit with a view to evaluate the fe isotope fractionation during the formation of fe skarn deposit and to constrain the metal source the fe isotopes show a large variation both in whole rocks and mineral separates altered diorites show a wide range in 56 fe values 0 07 to 0 21 relative to the fe isotope standard irmm 014 which positively correlate with their tfe 2 o 3 tio 2 ratios fe 2 o 3 and feo calculated as tfe 2 o 3 the positive correlation indicates that heavy fe isotopes were preferentially leached from diorites during the skarn type alteration among the metallic minerals pyrite and pyrrhotite are isotopically heavier 0 12 to 0 48 than the magnetite 0 07 to 0 21 fe isotope fractionation between mineral pairs demonstrates that magnetite did not attain fe isotopic equilibrium with pyrite and pyrrhotite whereas pyrite and pyrrhotite might have attained isotopic equilibrium petrological observations and major element data also suggest that iron was leached from the diorites during the skarn type alteration if the leached iron provides the main fe budget of the han xing fe skarn deposit magnetite in ores would be isotopically heavier than the unaltered diorite however our results are in contrast with the magnetite being isotopically lighter than the unaltered diorite this suggests that the major fe source of the han xing fe skarn deposit is not from the leaching of diorites and might be from magmatic fluid which is isotopically lighter than the silicate melt our data demonstrate that fe isotopes can be used as important tracers in deciphering the metal source of fe skarn deposits

magnesium isotopic systematics of the makran arc magmas iran implications for crust mantle mg isotopic balance abstract continental crust loses mg by lower crustal foundering and chemical weathering to become its intermediate silicic composition both processes should preferentially retain heavy mg isotopes in the crust yet the estimated mg isotopic ratio for the bulk continental crust is indistinguishable from that for unmodified mantle this can be elucidated by two notions that are not mutually exclusive i the basaltic parent to the crust has mg isotopes lighter than the mantle and has since become mantle like as a result of mg loss by igneous and weathering fractionation and ii heavy mg isotopes in the continental crust are constantly balanced by a hidden input of light mg isotopes arc magmas are arguably building blocks of the continental crust and their mg isotopic signature could be used to place limits on the above views here we conducted a mg isotopic study of the makran arc a rare continental arc in iran within the alpine himalayan orogenic belt the measured mg isotopic ratios for most mafic and intermediate samples are mantle like with 26mg values ranging from 0 32 to 0 20 after excluding dubious samples affected by alteration the silicic samples span a broad range of isotopic ratios with 26mg values ranging from 0 32 to 0 32 beyond that of unmodified mantle 26mg 0 25 0 04 the isotopic data form a trend from mantle like 26mg at relatively high mgo towards elevated 26mg at low mgo requiring the isotopic variability be related to differentiation in an open system involving assimilation of high 26mg crustal rocks our results are generally consistent with mg isotopic data compiled for global arcs where magmas have mg isotopic ratios ranging from mantle like to 0 7 per mil above the mantle range and compositions substantially below that range are rare thus heavy mg isotopes in the continents predicted by igneous and weathering fractionation might have been compensated by a hidden input of light mg isotopes which is presumably in the form of carbonates leading to a bulk crustal mg isotopic composition that is mantle like

lava effusion a slow fuse for paroxysms at stromboli volcano questo lavoro ha analizzato i risultati del monitoraggio termico delle eruzioni di stromboli del 2002 2003 e 2007 ed ha mostrato come prima di ogni parossisma il volume di lava eruttato nel corso dell attivita effusiva sia stato confrontabile questa osservazione ha fatto ipotizzare l esistenza di una decompressione critica nel sistema di alimentazione superficiale del vulcano raggiungibile anche lentamente che innesca la veloce risalita del magma ricco in gas responsabile dei parossismi durante l attivita effusiva a questa decompressione critica e associabile una soglia di volume di magma emesso che diventa la misura discriminante per valutare la fase critica del vulcano e predire il parossisma

application of proton microprobe data to trace element partitioning in volcanic rocks abstract in situ proton microprobe analyses are used to calculate partition coefficients d for mn fe cu zr ga rb sr y zr ba and pb and more restricted data for ni ge nb and as in pyroxenes olivines biotites hornblendes feldspars and fe ti oxides from 32 volcanic rocks all with glassy matrices these range from leucitite and picritic basalt to high silica and peralkaline rhyolites data for pigeonite aenigmatite allanite and leucite are included where these occur as additional phases coefficients have also been rayleigh fractionation corrected this correction being significant for those elements with high coefficients in specific mineral phases and where pronounced crystallisation has occurred partition coefficients are presented as plots vs nbo t ratios of coexisting glasses melts negative correlations are observed based on log transformed data for mn zn fe and y pyroxenes mn zn and fe olivines mn and zn fe ti oxides nb y sr and zr hornblendes sr plagioclase and ba cu zn and fe feldspars these element partition coefficients are not correlated with bulk mineral compositions zr and ga are not correlated with either glass or mineral compositions except zr in hornblende the correlations support the role of melt polymerisation in controlling element partitioning at least for those elements listed although possible effects of temperature cannot be uniquely isolated in the present data set the coefficients presented here are generally consistent with published values for equivalent rock compositions one difference being that some of the higher published coefficients e g sr in plagioclase are not matched in the present data set from a practical viewpoint the relative regularity of the coefficient data presented does facilitate choice of relevant coefficient values that are required for petrogenetic modelling procedures

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

fe isotope fractionation in magmatic hydrothermal mineral deposits a case study from the renison sn w deposit tasmania abstract we present 50 new iron isotopic analyses of source granite and mineral separates from the renison tin deposit in western tasmania the aim of the study is to characterise the composition of minerals within a tin deposit associated with a reduced s type magma we have analysed bulk samples of granite and separates of pyrrhotite pyrite arsenopyrite magnetite chalcopyrite and siderite by multi collector inductively coupled mass spectrometry the isotopic compositions of mineral separates are consistent with theoretical predictions of equilibrium fractionation based on mossbauer spectroscopy and other parametric calculations mineral mineral pairs yield temperatures of formation that are in agreement with prior detailed fluid inclusion studies but are spatially inconsistent with declining fluid temperatures with distance from the causative intrusion limiting the use of fe isotopes as a potential geothermometer at least in this case comparison of our data with published data from other deposits clearly demonstrates that pyrite magnetite and chalcopyrite from the hottest ore fluids 300 400 c at renison are isotopically heavier than minerals sampled from a deposit formed at similar temperatures but associated with a more oxidised and less differentiated intrusion

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

iron isotope fractionation during sulfide rich felsic partial melting in early planetesimals abstract new fe isotope data of feldspar rich meteorites graves nunataks 06128 and 06129 gra 06128 9 reveal that they are the only known examples of crustal materials with isotopically light fe isotope compositions fe 56 0 08 0 06 56fe is defined as the per mille deviation of a sample s 56fe 54fe ratio from the irmm 014 standard in the solar system in contrast associated brachinites as well as brachinite like achondrites have fe isotope compositions fe 56 0 01 0 02 that are isotopically similar to carbonaceous chondrites and the bulk terrestrial mantle in order to understand the cause of fe isotope variations in the gra 06128 9 and brachinite parent body we also report the fe isotope compositions of metal silicate and sulfide fractions from three ordinary chondrites semarkona kernouve saint severin metals from ordinary chondrites are enriched in the heavier isotopes of fe average fe 56 0 15 sulfide fractions are enriched in the lighter isotopes of fe average fe 56 0 14 and the 56fe values of the silicates are coincident with that of the bulk rock average fe 56 0 03 the enrichment of light isotopes of fe isotopes in gra 06128 9 is consistent with preferential melting of sulfides in precursor chondritic source materials leading to the formation of fe s rich felsic melts conceptual models show that melt generation to form a gra 06128 9 parental melt occurred prior to the onset of higher temperature basaltic melting

crystallization conditions and evolution of magmatic fluids in the harney peak granite and associated pegmatites black hills south dakota evidence from fluid inclusions abstract a microthermometric study of inclusions in granites and pegmatites in the proterozoic harney peak granite system identified four types of inclusions type 1 inclusions are mixtures of co2 and h2o and have low salinities on average 3 5 wt nacleq type 2 inclusions are aqueous solutions of variable salinities from 0 to 40 wt nacleq type 3 inclusions are carbonic dominated by co2 with no detectable water and type 4 inclusions consist of 20 to 100 solids with the remaining volume occupied by a co2 h2o fluid many inclusions have a secondary character however a primary character can be unambiguously established in several occurrences of the type 1 inclusions these inclusions were trapped above the solidus and represent the exsolved magmatic fluid the secondary populations of types 1 2 and 3 probably formed as a result of reequilibration and unmixing of the type 1 fluid that progressively changed composition and density with decreasing temperature and pressure and was finally trapped along healed microfractures under subsolidus conditions type 4 inclusions are primary and are interpreted to be trapped fluid bearing complex silicate melts that subsequently solidified or underwent other posttrapping changes it is demonstrated that primary type 1 fluid inclusions that coexist with crystallized melt inclusions in the complex li bearing tin mountain pegmatite were trapped along the two fluid phase boundary in the system co2 h2o nacleq consequently the temperature and pressure conditions of trapping are identical to the bulk homogenization conditions on average 340 c and 2 7 kbar these conditions indicate that this li cs rb p and b rich pegmatite crystallized at some of the lowest known temperatures for a silicate melt in the crust an internally consistent empirical solvus surface in p t xco2 coordinates was generated for the pseudobinary co2 h2o 4 3 wt nacleq pegmatite fluid system distribution coefficients for the major species co2 h2o nacl and ch4 between the immiscible co2 rich and h2o rich fluid phases as a function of pressure and temperature were extracted from data for the two cogenetic fluid inclusions types

uranium isotopes distinguish two geochemically distinct stages during the later cambrian spice event a b s t r a c t anoxic marine zones were common in early paleozoic oceans 542 400 ma and present a potential link to atmospheric po2 via feedbacks linking global marine phosphorous recycling primary production and organic carbon burial uranium u isotopes in carbonate rocks track the extent of ocean anoxia whereas carbon c and sulfur s isotopes track the burial of organic carbon and pyrite sulfur primary long term sources of atmospheric oxygen in combination these proxies therefore reveal the comparative dynamics of ocean anoxia and oxygen liberation to the atmosphere over million year time scales here we report high precision uranium isotopic data in marine carbonates deposited during the late cambrian spice event at ca 499 ma documenting a well defined 0 18 negative 238 u excursion that occurs at the onset of the spice event s positive 13

on apatite fission track dating and the tertiary evolution of west greenland topography abstract combined with geological information apatite fission track aft data can impose valid thermal and temporal constraints however their uncritical interpretation may also lead to unsupported conclusions because fission tracks in apatite undergo measurable shortening even at surface temperatures model artefacts can potentially be mistaken for geological cooling events this problem can be especially acute when older fanning kinetic models are utilized instead of the newest curvilinear ones in any circumstances aft model results must be interpreted with considerable caution and should rarely be considered to supersede conventional geological wisdom the possibility that young cooling events are simply artefacts ought always to be entertained as an example this study considers aft data and model results that have been cited to suggest that today9s high mountains of west greenland are erosional remnants of a landmass uplifted during the neogene however in the best of circumstances the aft data in question are heavily contaminated and in the worst case they carry virtually no geological meaning although geological constraints require the faulted westward facing escarpment of west greenland9s disko region to have been erected after commencement of sea floor spreading in baffin bay the aft model based hypothesis that it was constructed in purely neogene time remains an unproven speculation

isotopic composition of zinc copper and iron in lunar samples abstract we determined by icp ms the concentrations and isotopic ratios of fe cu and zn in the ti rich lunar basalt 74275 in the lunar orange glass 74220 and in up to 10 lunar soils namely 14163 15231 64501 66041 68841 69941 70011 72501 75081 and 76501 two analyses of zinc in lunar basalt 74275 give 66 zn 0 17 and 0 75 values within the range of those measured in terrestrial basalts copper in lunar basalt 74275 has 65 cu 1 4 which is isotopically heavier than values observed in terrestrial basalts in the orange glass we measured 56 fe 0 24 65 cu 0 42 and 66 zn 3 6 these values of are more negative than those obtained for 74275 and for typical lunar basalts but for cu comparable to those observed in terrestrial sulfides and meteorites in lunar soils we found 0 11 56 fe 0 51 2 6 65 cu 4 5 and 2 2 66 zn 6 4 insofar as we can generalize from a small sample set s fe cu zn and cd show similar trends in isotopic fractionation on the moon lunar basalts have nearly terrestrial isotopic ratios relative to the lunar basalt 74275 the pyroclastic glass 74220 is enriched in the lighter isotopes of fe cu and zn and the soils are enriched in the heavier isotopes of fe cu and zn the patterns in the basalts are likely inherited from the source material the light isotope enrichments seen in the orange glass originated during lava fountaining or less probably during partial condensation of vapor and the heavy isotope enrichments in the lunar soils were likely created by a combination of processes that included micrometeorite vaporization and sputtering in the orange glass the light isotope enrichments relative to lunar basalts of zn are larger than those of cu if these enrichments reflect accurately the isotopic composition of the gas they suggest that cu is more volatile than zn in the liquid from which the gas derived a simple model built on the known flux of micrometeorites to the lunar surface and a published estimate that micrometeorites generate 10 times their own mass of vapor predicts heavy isotope enrichments comparable to those observed in soils but only if the regolith gardening rate is set at about one twentieth of the generally accepted value of 1 cm my this discrepancy may reflect the difference in the time constants for micrometeorite milling and decimeter scale gardening or the importance of sputtering

uranium isotope fractionation suggests oxidative uranium mobilization at 2 50ga abstract geochemical data from late archean sedimentary rocks point to photosynthetic o 2 production and at least intermittent occurrences of locally mild oxidative weathering and surface ocean oxygenation oxygen oases prior to the early paleoproterozoic great oxidation event for example distinctive authigenic enrichments of mo and re in euxinic anoxic and sulfidic black shales are best explained by the oxidative mobilization of these metals from crustal sulfide minerals and their accumulation as oxyanions in seawater in contrast it is not clear if low u enrichments in the same shales reflect negligible oxidation of u from the upper crust or a very small oceanic u inventory that was derived from oxidative u mobilization here we report u isotope data for the 2 50 ga mt mcrae shale hamersley basin western australia which provides a more sensitive test for the presence or absence of authigenic u compared to u concentrations and enrichment factors normalized to average shale or upper crustal compositions that may not be representative of the local detrital composition we find instances where the u isotope composition in the upper mt mcrae shale 238 u 0 2 to 0 0 relative to standard srm950a is isotopically heavier than average upper crust 238 u 0 31 0 14 2sd based on granitoids and basalts the high 238 u values point to u isotope fractionation in the late archean marine environment and hence indicate the presence of a small amount of dissolved u in seawater and authigenic u in the mt mcrae shale volume dependent equilibrium isotope fractionation during the reduction of dissolved u vi to insoluble u iv like that observed in the modern black sea may explain the high 238 u signatures if u removal from bottom waters was not quantitative alternatively quantitative u removal would require that late archean seawater had high 238 u which could have arisen from the preferential sequestration of 235 u to fe oxyhydr oxide minerals elsewhere in the hamersley basin the supracrustal 238 u signatures are associated with some of the highest mo and re enrichments in the mt mcrae shale as well as distinctive mo s and n isotope signatures that are indicative of mild environmental oxygenation hence our findings suggest that small amounts of u were oxidatively mobilized from the upper crust at 2 50 ga unlike mo and re however u oxidation may not have occurred on land instead we hypothesize that oxidative u mobilization occurred by submarine weathering in an oxygen oasis

fe isotope behaviours during sulfide dominated skarn type mineralisation abstract fe isotope compositions of mineral separates and bulk samples from dongguashan and fenghuangshan skarn type cu fe deposits in tongling district middle lower yangtze valley cu fe au mo polymetallic belt have been investigated differences in 57 fe values exist in co precipitated bornite chalcopyrite and pyrite chalcopyrite mineral pairs the bornite is enriched in light iron isotopes relative to coexisting chalcopyrite whereas pyrite samples show a tendency of heavy isotopes enrichment relative to chalcopyrite moreover the 57 fe values of endoskarn and the earliest formed fe mineral phase magnetite of the three deposits investigated in tongling district are consistently lower relative to ore related igneous rocks suggesting fe isotopic fractionation occurs during fluid exsolution and exsolved fluid is enriched in light fe isotopes relative to the stock furthermore spatial and temporal variations in 57 fe values within single mineral phases are observed suggesting iron isotopic fractionation occurs between mineral and fluid phases as a result of precipitation of fe bearing minerals during fluid evolution overall according to fe isotope compositions of three representative skarn deposits formed under the same geological setting in tongling district this study indicates that fe isotopes have the potential to unraveling ore forming processes as well as constraining the metal sources of skarn deposits combining with results of previous studies a protocol for using fe isotopes to trace skarn type metallogeny is proposed

the generation and crystallization conditions of the proterozoic harney peak leucogranite black hills south dakota usa petrologic and geochemical constraints the mineralogy petrology and geochemistry of the proterozoic harney peak granite black hills south dakota were examined in view of experimentally determined phase equilibria applicable to granitic systems in order to place constraints on the progenesis of peraluminous leucogranites and commonly associated rare element pegmatites the granite was emplaced at 3 4 kbar as multiple sills and dikes into quartz mica schists at the culmination of a regional high temperature low pressure metamorphic event principally along the periphery of the main pluton and in satellite intrusions the sills segregated into granite pegmatite couplets the major minerals include quartz k feldspar sodic plagioclase and muscovite biotite mg no molar mgo mgo feo 0 32 0 38 is the predominant ferromagnesian mineral in the granite s core whereas at the periphery of the main pluton and in the satellite intrusions tourmaline mg no 0 18 0 48 is the dominant ferromagnesian phase almandine spessartine garnet is also found in the outer intrusions there is virtually a complete overlap in the wide concentration ranges of sio2 cao mgo feo sr zr w of the biotite and tourmaline bearing granite suites with no discernable differentiation trends on harker diagrams precluding the derivation of one suite from the other by differentiation following emplacement this is consistent with the oxygen isotope compositions which are 11 5 0 6 for the biotite granites and 13 2 0 8 for the tourmaline granites suggesting derivation from different sources the concentrations of tio2 and possibly ba are higher and of mno and b are lower in the biotite granites the normative orthoclase albite ratio is extremely variable ranging from 0 26 to 1 65 in the biotite granites to 0 01 1 75 in the tourmaline granites very few sample compositions fall near the high pressure watersaturated haplogranite minima eutectic trend indicating that the granites for the most part are not minimum melts generated under conditions with a text h text 2 text o 1 instead most biotite granites are more potassic than the water saturated minima and eutectics and in analogy with experimentally produced granitic melts they are best explained by melting at 6 kbar a text h text 2 text o 1 and temperatures 800 c such high temperatures are also indicated by oxygen isotope equilibration among the constituent minerals nabelek et al 1992 several of the tourmaline granite samples contain virtually no k feldspar and have oxygen isotope equilibration temperatures 716 775 c therefore they must represent high temperature accumulations of liquidus minerals crystallized under equilibrium conditions from melts more sodic than the water saturated haplogranite minima or during fractionation of intruded melts into granite pegmatite couplets accompanied by volatile aided differentiation of the alkali elements the indicated high temperatures a text h text 2 text o 1 the relatively high tio2 and ba concentrations and the relatively low partial 18 text o values of the biotite granites suggest that they were generated by high extent biotite dehydration melting of an immature archean metasedimentary source the ascent of the hot melts may have triggered low extent muscovite dehydration melting of schists higher in the crust producing the high b low ti melts comprising the periphery of the main pluton and the satellite intrusions alternatively the different granite types may be the result of melting of a vertical section of the crust in response to the ascent of a thermal pulse with the low partial 18 text o biotite granites generated at a deeper hotter region and the high partial 18 text o tourmaline granites at a higher cooler region of the crust the low ti and high b concentrations in the high partial 18 text o melts resulted in the crystallization of tourmaline rather than biotite which promoted the observed differentiation of the melts into the granitic and pegmatitic layers found along the periphery of the main pluton and the satellite intrusions

magnesium isotopic behaviors between metamorphic rocks and their associated leucogranites and implications for himalayan orogenesis abstract magnesium isotopic compositions along with new sr nd pb isotopic data and elemental analyses are reported for 12 miocene tourmaline bearing leucogranites 15 eocene two mica granites and 40 metamorphic rocks to investigate magnesium isotopic behaviors during metamorphic processes and associated magmatism and constrain the tectonic magmatic metamorphic evolution of the himalayan orogeny the gneisses granulites and amphibolites represent samples of the indian lower crust and display large range in 26mg from 0 44 to 0 09 in mafic granulites 0 44 to 0 10 in amphibolites and 0 70 to 0 03 in granitic gneisses the average mg isotopic compositions of the granitic gneisses 0 19 0 34 mafic granulites 0 22 0 17 and amphibolites 0 25 0 24 are similar indicating the limited mg isotope fractionation during prograde metamorphism from granitic gneisses to mafic granulites and retrograde metamorphism from mafic granulites to amphibolites the eocene two mica granites and miocene leucogranites are characterized by large variations in elemental and sr nd pb isotopic compositions the leucogranites and two mica granites have their corresponding 87sr 86sr i varying from 0 7282 to 0 7860 and 0 7163 to 0 7191 143nd 144nd i from 0 511888 to 0 512040 and 0 511953 to 0 512076 207pb 204pb from 15 7215 to 15 7891 and 15 7031 to 15 7317 208pb 204pb from 38 8521 to 39 5286 and 39 2710 to 39 4035 and 206pb 204pb from 18 4748 to 19 0139 and 18 7834 to 18 9339 however they have similar mg isotopic compositions 0 21 to 0 06 versus 0 24 to 0 09 which did not originate from fractional crystallization nor source heterogeneity based on hornblende biotite muscovite dehydration melting reaction and mg isotopic variations in two mica granites and leucogranites with the proceeding metamorphism along with elemental discrimination diagrams eocene two mica granites and miocene leucogranites could be related to hornblende dehydration melting and muscovite dehydration melting respectively mg isotopic compositions of eocene two mica granites become heavier compared to the source because of residues of isotopically light garnet in the source while those of miocene leucogranites become lighter because of entrainment of isotopically light garnet from the source region thus a new model for crustal anatexis and himalayan orogenesis was proposed based on the mg isotope fractionation in the leucogranites and metamorphic rocks this model emphasizes a successive process from indian continental subduction to rapid exhumation of the higher himalayan crystalline series hhcs the former underwent high temperature ht and high pressure hp granulite facies prograde metamorphism which resulted in the hornblende dehydration melting and the formation of eocene two mica granites while the latter experienced amphibolite facies retrogression and decompression which resulted in the muscovite dehydration melting and the formation of miocene leucogranites

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

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

north atlantic intermediate to deep water circulation and chemical stratification during the past 1 myr benthic foraminiferal carbon isotope records from a suite of drill sites in the north atlantic are used to trace variations in the relative strengths of lower north atlantic deep water lnadw upper north atlantic deep water unadw and southern ocean water sow over the past 1 myr during glacial intervals significant increases in intermediate to deep 13c gradients commonly reaching 1 2 are consistent with changes in deep water circulation and associated chemical stratification bathymetric 13c gradients covary with benthic foraminiferal 18o and covary inversely with vostok co2 in agreement with chemical stratification as a driver of atmospheric co2 changes three deep circulation indices based on 13c show a phasing similar to north atlantic sea surface temperatures consistent with a northern hemisphere control of nadw sow variations however lags in the precession band indicate that factors other than deep water circulation control ice volume variations at least in this band

hydrothermally induced changes in mineralogy and magnetic properties of oxidized a type granites abstract the changes in magnetic mineralogy due to the hydrothermal alteration of a type granitic rocks have been thoroughly investigated in samples from the granite of tana corsica france and compared with other a type granites meruoca ne brazil bushveld south africa mount scott wichita mountains oklahoma usa and the stratoid hypersolvus granites of madagascar the altered red colored samples and their non altered equivalents were magnetically characterized by means of magnetic susceptibility measurements hysteresis loops remanent coercivity spectra and lowrie test it is shown that hydrothermalization in magnetite bearing granites is related to the formation of fine grained magnetite and hematite and to coeval depletion in the content of primary low coercive coarse grained magnetite these mineralogical changes give typical rock magnetic signatures namely lower susceptibility magnitudes and anisotropy degrees prolate ams anisotropy of magnetic susceptibility fabrics and increased coercivities optical microscopy and sem scanning electronic microscopy images suggest that the orientation of the secondary magnetic minerals is related to fluid pathways and micro fractures formed during the hydrothermal event and therefore may be unrelated to magma emplacement and crystallization fabrics changes in magnetic mineralogy and grain size distribution have also to be considered for any paleomagnetic and iron isotope studies in granites

exsolution of immiscible vapor and liquid phases from a crystallizing silicate melt implications for chlorine and metal transport abstract the compositional variation of a cl bearing aqueous fluid exsolving from a crystallizing silicate melt is evaluated for the system haplogranitic melt na k c1 h 2 o with attention to the effect of vaporliquid immiscibility on the aqueous fluid above critical pressures for the aqueous fluid p 1 5 kb at 800 c where the fluid is single phase the cl concentration in the aqueous fluid c cl aq gradually decreases with crystallization of a silicate melt under rayleigh fractionation as demonstrated by previous studies however the variation of ca is quite different below critical pressures where vapor liquid immiscibility occurs in particular at low pressures p cl h 2 o ratio in a melt is small and ca will increase with crystallization a high cl h 2 o ratio at low pressure results in liquid exsolution and decreasing ca with crystallization eventually both vapor and liquid phases will exsolve simultaneously regardless of the initial exsolving phase during this simultaneous exsolution the composition of the exsolving vapor and liquid phases the mass ratio of these phases and the cl concentration of the silicate melt will all remain constant comparison of the model and published experimental data indicates that the simultaneous exsolution of vapor and liquid may occur from a normal rhyolite crystallized at pressures below the critical pressure for the aqueous fluid therefore simultaneous exsolution of the immiscible aqueous phases is likely to be a common phenomenon in magmatic hydrothermal systems at conditions for the vapor liquid immiscibility model calculations at 0 6 and 1 2 kb indicate that major amounts of cl in the system will be distributed to the liquid phase particularly at the lower pressure this suggests that the majority of chloridecomplexed metals exsolving from a magma may be transported in the liquid phase under shallow crustal conditions

the experimental calibration of the iron isotope fractionation factor between pyrrhotite and peralkaline rhyolitic melt abstract a first experimental study was conducted to determine the equilibrium iron isotope fractionation between pyrrhotite and silicate melt at magmatic conditions experiments were performed in an internally heated gas pressure vessel at 500 mpa and temperatures between 840 and 1000 c for 120 168 h three different types of experiments were conducted and after phase separation the iron isotope composition of the run products was measured by mc icp ms i kinetic experiments using 57 fe enriched glass and natural pyrrhotite revealed that a close approach to equilibrium is attained already after 48 h ii isotope exchange experiments using mixtures of hydrous peralkaline rhyolitic glass powder 4 wt h 2 o and natural pyrrhotites fe 1 x s as starting materials and iii crystallisation experiments in which pyrrhotite was formed by reaction between elemental sulphur and rhyolitic melt consistently showed that pyrrhotite preferentially incorporates light iron no temperature dependence of the fractionation factor was found between 840 and 1000 c within experimental and analytical precision an average fractionation factor of 56 fe 54 fe pyrrhotite melt 0 35 0 04 2se n 13 was determined for this temperature range predictions of fe isotope fractionation between fes and ferric iron dominated silicate minerals are consistent with our experimental results indicating that the marked contrast in both ligand and redox state of iron control the isotope fractionation between pyrrhotite and silicate melt consequently the fractionation factor determined in this study is representative for the specific fe 2 fe ratio of our peralkaline rhyolitic melt of 0 38 0 02 at higher fe 2 fe ratios a smaller fractionation factor is expected further investigation on fe isotope fractionation between other mineral phases and silicate melts is needed but the presented experimental results already suggest that even at high temperatures resolvable variations in the fe isotope composition can be generated by equilibrium isotope fractionation in natural magmatic systems

distinct 238 u 235 u ratios and ree patterns in plutonic and volcanic angrites geochronologic implications and evidence for u isotope fractionation during magmatic processes angrites are differentiated meteorites that formed between 4 and 11 myr after solar systemformation when several short lived nuclides e g 26 al 26 mg 53 mn 53 cr 182 hf 182 w were still alive as such angrites are prime anchors to tie the relative chronology inferred from these short lived radionuclides to the absolute pb pb clock the discovery of variable u isotopic composition at the sub permil level calls for a revision of pb pb ages calculated using an assumed constant 238 u 235 u ratio i e pb pb ages published before 2009 2010 in this paper we report high precision u isotope measurement for six angrite samples nwa 4590 nwa 4801 nwa 6291 angra dos reis d orbigny and sahara 99555 using multi collector inductively coupled plasma mass spectrometry and the irmm 3636 u double spike the age corrections range from 0 17 to 1 20 myr depending on the samples after correction concordance between the revised pb pb and hf w and mn cr ages of plutonic and quenched angrites is good and the initial 53 mn 55 mn 0 ratio in the early solar system ess is recalculated as being 7 1 10 6 at the formation of the solar system the error bar incorporates uncertainty in the absolute age of calcium aluminum rich inclusions cais an uncertainty remains as to whether the al mg and pb pb systems agree in large part due to uncertainties in the pb pb age of cais a systematic difference is found in the u isotopic compositions of quenched and plutonic angrites of 0 17 a difference is also found between the rare earth element ree patterns of these two angrite subgroups the 238 u values are consistent with fractionationduring magmatic evolution of the angrite parent melt stable u isotope fractionation due to a change in the coordination environment of u during incorporation into pyroxene could be responsible for such a fractionation in this context pb pb ages derived from pyroxenes fraction should be corrected using the u isotope composition measured in the same pyroxene fraction

chemical exchange in migmatites during cooling abstract the chemistry of the leucosome components in limited size migmatitic associations is generally believed to reflect the composition of the partial melt that could have been extracted to yield an anatectic magma several well documented studies of migmatites indicate that the distributions of some trace elements in leucosome melanosome pairs are inconsistent with equilibrium partial melting but are rather in agreement with chemical reequilibration during cooling in migmatites the liquid fraction does not leave the system this may induce an alteration of the chemical composition of the whole migmatite system by extensive chemical exchange between the liquid and the solid residue during cooling if this is the case the leucosome and melanosome components are not representative of the chemical composition of the liquid and solid residue respectively at the time of partial melting a rough evaluation of the residences time of the melt fractions in cooling in situ migmatitic bodies yields minimum estimates of 1 to 10 ma for such time spans the characteristics distances of diffusion distances of diffusion induced chemical transfer exceed the sizes of individual mineral grains which is consistent with a re equilibration process the release of latent heat of crystallization in cooling migmatitic systems and the presence of an interstitial liquid phase are critical in determining the extent and scale of chemical exchange the mechanism involves 1 reducing the cooling rate and 2 promoting the efficiency of chemical diffusion through the liquid phase thus some doubt is cast on the ability of small scale migmatitic systems to provide relevant information about chemical mass balance during partial crustal melting

high precision iron isotope measurements of terrestrial and lunar materials we present the analytical methods that have been developed for the first high precision fe isotope analyses that clearly identify naturally occurring mass dependent isotope fractionation a double spike approach is used which allows rigorous correction of instrumental mass fractionation based on 21 analyses of an ultra pure fe standard the external precision 1 sd for measuring the isotopic composition of fe is 0 14 mass for demonstrated reproducibility on samples this precision exceeds by at least an order of magnitude that of previous attempts to empirically control instrumentally produced mass fractionation dixon et al 1993 using the double spike method 15 terrestrial igneous rocks that range in composition from peridotite to rhyolite 5 high ti lunar basalts 5 fe mn nodules and a banded iron formation have been analyzed for their iron isotopic composition the terrestrial and lunar igneous rocks have the same isotopic compositions as the ultra pure fe standard providing a reference fe isotope composition for the earth and moon in contrast fe mn nodules and a sample of a banded iron formation have iron isotope compositions that vary over a relatively wide range from 56fe 0 9 to 1 2 this range is 15 times the analytical errors of our technique these natural isotopic fractionations are interpreted to reflect biological vital effects and illustrate the great potential fe isotope studies have for studying modern and ancient biological processes

ree distribution in some layered migmatites constraints on their petrogenesis abstract the ree distributions in mesosomes neosomes leucosomes and melanosomes of four layered migmatites have been investigated in one example arvika migmatites the ree patterns in adjacent paragneisses the presumed parent rock of the migmatites were also determined ree patterns of neosomes and mesosomes of arvika migmatites are similar to the finegrained layers and coarse grained layers respectively observed in the adjacent paragneiss this is in agreement with the layer by layer paragneiss migmatite transformation model the ree patterns of mesosomes and neosomes indicate that these lithologies may have been closed systems for ree during the formation of the migmatites no indication of metasomatic reactions melt segregation or injection could be detected within the neosomes leucosomes are depleted and melanosomes enriched in ree contents this is interpreted to be due to separation and concentration of accessory minerals monazite epidote allanite zircon sphene apatite garnet into the melanosomes the behaviour of accessory minerals during migmatite formation is closely allied to that of biotite which is also concentrated in the melanosomes

zinc isotopic composition of the lower continental crust estimated from lower crustal xenoliths and granulite terrains abstract this study presents high precision zn stable isotope analyses for lower crustal rocks 9 granulites from archean terrains and 30 lower crustal xenoliths from the north margin of the north china craton ncc to understand the behavior of zn isotopes during deep crustal processes and the zn isotopic composition of the lower continental crust lcc the lower crustal xenoliths range in composition from mafic to felsic with mgo contents of between 0 4 to 20 2 wt the 66zn the permil deviation of the 66zn 64zn ratio from the jmc lyon standard values of lower crustal xenoliths range from 0 17 to 0 38 the lack of clear correlation of 66zn with geochemical indicators e g al2o3 cr ba la 87sr 86sr and 143nd 144nd indicates that assimilation of precambrian lower crust fluid metasomatism and accumulation of pyroxene and plagioclase had a limited effect on zn isotopic compositions of these lower crustal xenoliths the 66zn values 0 18 to 0 34 of garnet bearing mafic granulites decrease with increasing feo t and v contents which are likely results of the accumulation of fe ti oxides the average 66zn of the lower continental crust is estimated to be 0 29 0 02 95 se using lower crustal xenoliths from the neogene hannuoba basalts this 66zn value is similar to the estimated value 0 28 0 04 95 se obtained from the granulites from archean terrains suggesting that there is no significant difference in the zn isotopic composition between the archean and present day lower continental crust combining the 66zn data of the lower crustal xenoliths and granulite terrains and different weighting methods the zn isotopic composition of the lower continental crust is estimated to be 0 28 0 04 95 se

geochemical evidence from coesite bearing jadeite quartzites for large scale flow of metamorphic fluids in a continental subduction channel abstract metamorphic fluids produced by dehydration of subducting crust transport mass and energy at the slab mantle interface in subduction channels it is commonly assumed that fluid flow is significant in oceanic subduction channels but insignificant in continental subduction channels this assumption is challenged by a combined study of whole rock geochemistry mg and o isotopes zircon u pb ages and trace elements in coesite bearing jadeite quartzites from the dabie orogen china although the target samples were collected from different outcrops in an area of 50 km2 zircon u pb dating yields similar discordia lines with not only consistent upper intercept ages of 1 9 2 0 ga but also consistent lower intercept ages of 224 235 ma this indicates the same paleoproterozoic protolith and the same triassic metamorphism for these jadeite quartzites the o isotope analysis of mineral separates and whole rock yields variable 18o values from 6 3 to 9 4 indicating involvement of supracrustal components except for one outlier at 0 43 all the other rocks give variable 26mg values from 0 16 to 0 61 much higher than normal mantle values the whole rock mg isotopes show significant positive correlations not only with mgo contents but also with rb la rb gd and rb nb ratios but a negative correlation with na2o contents these observations indicate that the middle paleoproterozoic protolith of jadeite quartzites was weathered to produce a kind of sedimentary rocks in a passive continental margin and then underwent significant metasomatism by metamorphic fluids with high 26mg values during the continental subduction in the triassic the metamorphic fluids were produced by the breakdown of biotite in the metasedimentary rocks during their subduction to subarc depths for ultrahigh pressure metamorphism they would have acquired their geochemical compositions not only from the biotite breakdown but also through leaching reaction with the ttg provenance in view of the spatial occurrences of the target samples the metamorphic fluids would have flowed inside the continental subduction channel on a large scale this is the first report of the large scale fluid flow in the continental subduction zone and therefore demonstrates that fluid flow can be significant in continental subduction channels

variations in the uranium isotopic compositions of uranium ores from different types of uranium deposits abstract variations in 238u 235u and 234u 238u ratios were measured in uranium minerals from a spectrum of uranium deposit types as well as diagenetic phosphates in uranium rich basins and peraluminous rhyolites and associated autunite mineralisation from macusani meseta peru mean 238u values of uranium minerals relative to nbl crm 112 a are 0 02 for metasomatic deposits 0 16 for intrusive 0 18 for calcrete 0 18 for volcanic 0 29 for quartz pebble conglomerate 0 29 for sandstone hosted 0 44 for unconformity type and 0 56 for vein with a total range in 238u values from 0 30 to 1 52 uranium mineralisation associated with igneous systems including low temperature calcretes that are sourced from u rich minerals in igneous systems have low 238u values of ca 0 1 near those of their igneous sources whereas uranium minerals in basin hosted deposits have higher and more variable values high grade unconformity related deposits have 238u values around 0 2 whereas lower grade unconformity type deposits in the athabasca kombolgie and otish basins have higher 238u values the 234u values for most samples are around 0 in secular equilibrium but some samples have 234u values much lower or higher than 0 associated with addition or removal of 234u during the past 2 5 ma these 238u and 234u values suggest that there are at least two different mechanisms responsible for 238u 235u and 234u 238u variations the 234u 238u disequilibria ratios indicate recent fluid interaction with the uranium minerals and preferential migration of 234u fractionation between 235u and 238u is a result of nuclear field effects with enrichment of 238u in the reduced insoluble species mostly uo2 and 235u in oxidised mobile species as uranyl ion uo22 and its complexes therefore isotopic fractionation effects should be reflected in 238u 235u ratios in uranium ore minerals formed either by reduction of uranium to uo2 or chemical precipitation in the form of u6 minerals the 238u values of uranium ore minerals from a variety of deposits are controlled by the isotopic signature of the uranium source the efficiency of uranium reduction in the case of uo2 systems and the degree to which uranium was previously removed from the fluid with less influence from temperature of ore formation and later alteration of the ore uranium isotopes are potentially superb tracers of redox in natural systems

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

coupled molybdenum iron and uranium stable isotopes as oceanic paleoredox proxies during the paleoproterozoic shunga event the paleoproterozoic era was a time of remarkable importance in the redox evolution of earth s atmosphere and oceans here we present a multi proxy study of mo u and fe isotopes together with fe speciation of black shales and siltstones from the upper zaonega formation of the onega basin in karelia we attempt to better understand oceanic redox conditions during the 2 05 ga shunga event as the next step following the great oxidation event goe and the lomagundi carbon isotope excursion event a cautious examination of the fe speciation data shows that the studied section was deposited under dominantly euxinic conditions anoxic and sulfidic and that the lower part of the section experienced metamorphism through which pyrite was altered to pyrrhotite during this episode the system was closed with respect to fe but not sulfur the mo and u isotopic compositions corrected for detrital input were not affected by the metamorphism and loss of s and are fairly uniform throughout the entire section the fe isotope compositions are exceptionally heavy in the lower part of the section up to 56 fe irmm 14 0 83 and become lighter towards the upper intervals which also show significant mo and u enrichments we suggest that this pattern reflects changes in the position of the deposition site relative to the redox structure of the water column the upper part was deposited within a locally euxinic portion of the basin where h 2s availability was highest removal of mo and u was more efficient and precipitated pyrite captured relatively non fractionated dissolved fe in other words quantitative uptake of fe was favored in contrast the lower interval was deposited on the lower margin of a euxinic wedge where h 2s availability was lower and removal of mo and u was less efficient pyrite precipitation in this part of the water column reflected a more fractionated dissolved fe reservoir due to more protracted non quantitative fe uptake because of less efficient pyrite formation under lower sulfide conditions and greater access to the large oceanic pool of fe the u isotopic signal was corrected for detrital contribution giving compositions similar to the riverine input and suggesting that co precipitation into carbonates was the main process of u removal at this time we estimate the mo isotope composition of the contemporaneous ocean to be 98 mo sw 0 85 0 21 this is the lowest value yet reported for the proterozoic ocean suggesting that the oceanic mo cycle was dominated by euxinic and anoxic sinks with negligible mo removal into oxic environments recent studies have proposed a sharp increase in ocean atmosphere oxygen levels during the lomagundi event followed by a dramatic crash our results from black shales of the 2 05 ga shunga event are consistent with a post lomagundi decrease in biospheric oxygen levels

uranium and sm isotope studies of the supergiant olympic dam cu au u ag deposit south australia the olympic dam cu u au ag deposit in the archean proterozoic gawler craton south australia is a type example of the iron oxide copper gold iocg spectrum of deposits and one of the largest cu u au resources known mineralization is hosted in a lithologically and texturally diverse hematite rich breccia complex developed within a granite of the 1 59 ga gawler silicic province emerging evidence indicates that both the breccia complex and its metal content developed over 1000 ma responding to major tectonic events e g at 1300 1100 825 and 500 ma however metal sources and exact mechanism s of ore formation remain poorly known new high precision 238u 235u data for a set of 40 whole rock samples representing all major lithological facies of the breccia complex show a narrow range 238ucrm112a 0 56 to 0 04 at the scale of sampling there is no correlation of 238u with lithology degree of alteration or u mineralogy although ores with u 5 wt have subtly higher 238u values 0 20 to 0 00 than the majority of samples 0 7 wt u 0 56 to 0 23 the new u isotope data are consistent with published data for uraninites from olympic dam and with published results from high temperature u deposits they overlap completely with the range of 238u values in granitoids including the host granite 0 18 to 0 32 and with estimates of the upper continental crust in general this similarity suggests that olympic dam 238u values reflects the crustal sources of u which probably include felsic volcanic rocks and granitoids the isotopic homogeneity suggests depositional mechanisms that involve minimal isotopic fractionation of u alternatively primary fractionation signatures may have been erased during the long history of the u mineralization high grade u ores may record isotopic neutron capture effects related to fissionogenic neutrons high precision sm isotope data for five high u 5 wt u u sm 500 olympic dam ores define a neutron capture line with correlated depletions in 149sm up to 2e units and excesses in 150sm up to 4e units but fission fragment contributions to sm are below detection these observations provide evidence for small scale neutron capture effects with calculated neutron fluences of 1015 to 1016 n cm 2 similar to those observed in several proterozoic and phanerozoic u deposits the apparent lack of fission fragment contributions in olympic dam high grade ores can be explained with an age of u deposition or re deposition that is substantially younger than the initial 1 59 ga age of the oldest iocg style mineralization the results presented here thus i suggest uranium sources in common likely igneous upper crustal lithologies ii support geochronological evidence for gradual addition of u in several stages over 1000 ma at elevated temperatures of mineralization and iii do not show the high 238u signatures expected from low temperature reworking of older low 238u ores

lithium isotopic systematics of granites and pegmatites from the black hills south dakota to study li isotopic fractionation during granite differentiation and late stage pegmatite evolution li isotopic compositions and concentrations have been measured for the s type harney peak granite the spatially associated tin mountain pegmatite and possible metasedimentary source rocks in the black hills south dakota the harney peak granite is isotopically heterogeneous with 7 li varying from 3 1 to 6 6 the 7 li values of proterozoic metasedimentary rocks that are possible sources of the harney peak granite range from 3 1 to 2 5 and overlap with post archean shales and the harney peak granite for the granite suite there is no correlation between 7 li and elements indicative of degrees of granite differentiation sio2 li rb etc the li isotopic composition of the harney peak granite therefore appears to res ect the source composition minerals from the zoned tin mountain pegmatite have extremely high li contents and heavier li isotopic compositions than the granite or surrounding black hills metasedimentary rocks the heavier compositions may res ect li isotopic fractionation resulting from extensive crystal melt fractionation lithium concentrations decrease in the order spodumene 3 7 wt muscovite 0 2 to 2 0 wt plagioclase 100 1100 ppm quartz 30 140 ppm plagioclase muscovite and spodumene in all zones display a relatively narrow range in 7 li of 7 9 to 11 4 in contrast quartz is isotopically heavier and more variable 14 7 to 21 3 with 7 li showing an inverse correlation with li concentration this correlation res ects the mixing of isotopically heavy li in quartz and lighter li in s uid inclusions as documented by s uid inclusion compositions

geochemical and multi isotopic 87sr 86sr 143nd 144nd 238u 235u perspectives of sediment sources depositional conditions and diagenesis of the marcellus shale appalachian basin usa abstract we investigate sediment sources depositional conditions and diagenetic processes affecting the middle devonian marcellus shale in the appalachian basin eastern usa a major target of natural gas exploration multiple proxies including trace metal contents rare earth elements ree the sm nd and rb sr isotope systems and u isotopes were applied to whole rock digestions and sequentially extracted fractions of the marcellus shale and adjacent units from two locations in the appalachian basin the narrow range of end values from 7 8 to 6 4 at 390 ma is consistent with derivation of the clastic sedimentary component of the marcellus shale from a well mixed source of fluvial and eolian material of the grenville orogenic belt and indicate minimal post depositional alteration of the sm nd system while silicate minerals host 80 of the ree in the shale data from sequentially extracted fractions reflect post depositional modifications at the mineralogical scale which is not observed in whole rock ree patterns limestone units thought to have formed under open ocean oxic conditions have 238u values and ree patterns consistent with modern seawater the 238u values in whole rock shale and authigenic phases are greater than those of modern seawater and the upper crust the 238u values of reduced phases the oxidizable fraction consisting of organics and sulfide minerals are 0 6 greater than that of modern seawater bulk shale and carbonate cement extracted from the shale have similar 238u values and are greater than 238u values of adjacent limestone units we suggest these trends are due to the accumulation of chemically and more likely biologically reduced u from anoxic to euxinic bottom water as well as the influence of diagenetic reactions between pore fluids and surrounding sediment and organic matter during diagenesis and catagenesis

zinc and lead isotope signatures of the zhaxikang pbzn deposit south tibet implications for the source of the ore forming metals abstract stable zn isotopes may be applied to trace the source of ore forming metals in various types of pb zn deposits to test this application zn and pb isotope systematics for sulfides and associated basement rocks as well as fe mn carbonates gangue from the zhaxikang pb zn deposit in south tibet have been analyzed the basement in this region includes metamorphosed mafic to felsic rocks dolerite quartz diorite rhyolite porphyry pyroclastics and porphyritic monzogranite these rocks have similar 66 zn values of 0 33 to 0 37 with an average value of 0 36 0 03 2 except for the more evolved porphyritic monzogranite that has a heavier value of 0 49 fe mn carbonates are present as hydrothermal veins and were probably precipitated from magmatic fluids they have an average 66 zn value of 0 27 0 05 which is slightly lighter than the basement rocks possibly representing 66 zn isotopic compositions of the hydrothermal fluids sphalerite and galena have similar zn isotopic compositions with 66 zn ranging from 0 03 to 0 26 and 0 21 to 0 28 respectively considering the zn isotope fractionation factor between sphalerite and fluids of 0 2 at 300 c as reported in literature hydrothermal fluids from which these sulfides precipitated will have 66 zn values of ca 0 39 0 10 which are consistent with the values of basement rocks and the fe mn carbonates this similarity supports a magmatic hydrothermal origin of the zhaxikang pb zn deposit both pb and s isotopes in these sphalerite and galena show large variations and are consistent with being derived from a mixture of basement and sedimentary rocks in various proportions zn isotopic compositions of the sulfides significantly extend the range of regional basement rocks suggesting that sedimentary rocks e g shales are also a significant source of zn however the zn isotopic compositions of sphalerite and galena differ from those of marine carbonates and those of typical sedex type deposits e g kelley et al 2009 confirming a magmatic hydrothermal model combined with regional geological observations and the age constraints of 20 ma zheng et al 2012 2014 the results of our investigation indicate that the zhaxikang pb zn deposit is most likely a magmatic hydrothermal deposit

a reassessment of the iron isotope composition of the moon and its implications for the accretion and differentiation of terrestrial planets abstract the fe isotope composition of planetary bodies may provide constraints on their accretion modes and or differentiation processes but to do so the fe isotope systematics of key planetary reservoirs needs to be determined to investigate this for the moon we measured the fe isotope compositions for a suite of 33 bulk lunar mare basalts and highland rocks combined with published data a compendium of 73 different lunar bulk rocks reveals a statistically significant fe isotope difference between low ti and high ti mare basalts yielding average 57fe 0 127 0 012 2se n 27 and 57fe 0 274 0 020 2 se n 25 respectively relative to the irmm 14 isotopic reference material as lunar basalts are thought to reflect the fe isotope composition of their respective mantle sources the estimated relative proportion of the low ti and high ti source mantle suggests that the lunar upper mantle 57fe value should be close to 0 142 0 026 whilst the composition of highland rocks ferroan anorthosites and mg suite rocks should provide a more global view of the moon the calculation of the mean 57fe value of 15 available highland rock analyses yields 57fe 0 078 0 124 such a value is not defined precisely enough to be of critical use for comparative planetology ferroan anorthosites and mg suite rocks also give unresolvable means it appears that fe isotope heterogeneity among the lunar highland rocks is caused by non representatively too small sample aliquots of coarse grained rocks it can also be the result of mixed lithologies for some when the kinetic effect of olivine tending towards low 57fe and feldspar with predominantly high 57fe is cancelled a more precise 57fe value of 0 094 0 035 is calculated it is indistinguishable from the mean 57fe of impact melts and is also similar to the upper lunar mantle estimate obtained from mare basalts collectively this newly determined fe isotope composition of the bulk moon is indistinguishable from that of the earth and heavier than those reported for other planetary bodies this planetary isotope relationship is only observed for silicon given the currently available mass dependent stable isotope database because both iron and silicon reside in the earth s metallic core in significant quantities this may point to the involvement of metallic cores of the earth and moon in the interplanetary fe and si isotope fractionation rather than via high pressure metal silicate fractionation at the core mantle boundary this would more likely be achieved by partial vaporization of the liquid outer metallic core in the aftermath of a moon forming giant impact

redox conditions across the cambrian ordovician boundary elemental and isotopic signatures retained in the gssp carbonates the final publication is available at elsevier via https doi org 10 1016 j palaeo 2015 09 014 2015 this manuscript version is made available under the cc by nc nd 4 0 license http creativecommons org licenses by nc nd 4 0

comment on spatially controlled fe and si isotope variations an alternative view on the formation of the torres del paine pluton by gajos et al contrib mineral petrol 171 93 2016 the torres del paine pluton is one of the best examples of an incrementally constructed shallow crustal intrusion contrib gajos et al contrib mineral petrol 171 93 2016 show that relatively large differences in 56fe and 30si exist in the mafic to granitic rocks of the torres del paine pluton they propose that the data is spatially distributed from the bottom of the intrusion to its roof they propose that thermally induced diffusion or source magma variations are the most likely processes to explain the data while we do believe that non traditional stable isotopes indeed contribute significantly to the understanding of igneous and hydrothermal processes we suggest that the interpretation of the fe and si isotopic data by gajos et al 2016 is not consistent with field relations the observed field relations and geochronology of the torres del paine are adequately explained by our original interpretation in this comment we will show that a the preferred interpretation of gajos et al 2016 that thermal migration zone refining can explain fe and si isotopic data from the torres del paine is incorrect based on simple field observations b that ignoring the known geological context as published on maps and cross sections will lead to erroneous interpretations of how granitic magmas are assembled and c the correlation of sr si and fe isotopes as shown by gajos et al 2016 can be readily interpreted as a combination of fractionation and eventual source magma variation and or late stage fluid exsolution

trace element constraints on pegmatite genesis tin mountain pegmatite black hills south dakota the tin mountain pegmatite is a small zoned granitic body that is extremely enriched in rb and li but has moderate concentrations of sr and ba these trace elements are modelled using granitic distribution coefficients in order to test the potentials of partial melting of metasedimentary rocks and fractionation of a less evolved granitic melt to have produced the parental liquid to the tin mountain pegmatite batch melting of any reasonable metasedimentary source rock would likely have yielded melts that were either insufficiently enriched in rb and li to be the parental liquid or that had sr and ba concentrations that were much higher than those estimated for the parental liquid the modelling of simple fractional crystallization and equilibrium crystallization of a granitic melt within the compositional range of the spatially associated harney peak granite gives calculated melt compositions with either lower sr and ba concentrations or inadequate rb and li enrichments to be the parent liquid of the pegmatite at least two variants from simple crystal liquid fractionation models can however successfully account for the derivation of the parent liquid 1 generation of a rb li ba and sr rich granitic melt outside of the compositional range of the sampled portions of the harney peak granite complex by low degrees of partial melting of metasedimentary rocks found in the black hills followed by moderate extents of fractional or equilibrium crystallization 2 derivation from harney peak granite via a complex multi stage crystal liquid fractionation process such as progressive equilibrium crystallization

investigation of magnesium isotope fractionation during granite differentiation implication for mg isotopic composition of the continental crust abstract high precision mg isotopic analysis was performed on a suite of well characterized i type granitoids and associated hornblende and biotite minerals from the dabie orogen in central china to address the behavior of mg isotopes during granite differentiation although these granitoids formed through different degrees of partial melting and fractional crystallization with large variations in elemental and mineral compositions their 26 mg values vary from 0 26 to 0 14 and are indistinguishable within our analytical precision 0 07 2 sd coexisting hornblendes and biotites in these granitoids display similar mg isotopic composition with 26 mg ranging from 0 31 to 0 14 in hornblendes and 0 23 to 0 12 in biotites the inter mineral fractionation factors 26 mg hbl bt 26 mg hbl 26 mg bt vary from 0 10 to 0 02 with an average 0 06 0 08 2 sd the limited inter mineral fractionation agrees with the theoretic prediction that mg cations in both hornblende and biotite are octahedrally coordinated with oxygen which restricts the magnitude of equilibrium isotope fractionation overall data from both bulk granitoids and associated mineral separates suggest that mg isotope fractionation during i type granite differentiation is limited collectively granitoids studied here have mg isotopic composition similar to that of terrestrial basalts and peridotites 26 mg 0 21 0 07 vs 0 25 0 07 2 sd confirming that magmatic processes do not significantly fractionate mg isotopes the continental crust in the dabie orogen as sampled by these i type granitoids has a mantle like mg isotopic composition given that significant mg isotope fractionation occurs during chemical weathering processes mg isotopes may potentially be used for tracing granite genesis in particular if sedimentary materials are involved in granite sources

on the iron isotope homogeneity level of the continental crust abstract assessing the level of homogeneity of different planetary reservoirs is an essential step to establish a new isotopic tracer in recent studies we concluded that most igneous rocks from the continental crust were homogeneous to 71 wt sio2 that we concluded were significantly heavier than less silicic rocks from the crust on the basis of fe isotopic analyses of seven bulk rock samples from different geological settings beard and johnson beard b l johnson c m 2006 comment on heavy iron isotope composition of granites determined by high resolution mc icp ms by f poitrasson and r freydier chemical geology volume 222 pages 132 147 chem geol this issue challenge this finding on the basis of previously published data it is shown here that this controversy stems from the confusion between whole rock and mineral iron isotope analyses since this isotopic tracer like many others is more heterogeneous at the mineral scale than the bulk rock scale thus minerals cannot be used in place of bulk rock fe isotope compositions the controversy also arises from different levels of analytical uncertainties if only the best fe isotope probes of planetary reservoirs bulk rocks are considered then the conclusion that igneous rocks with sio2 71 wt are significantly heavier than the mean 57fe of the continental crust poitrasson f freydier r 2005 heavy iron isotope composition of granites determined by high resolution mc icp ms chem geol 222 132 147 remains valid nonetheless given that granites represent only a minor fraction of the continental crust this finding does not change the bulk igneous fe isotope composition estimate previously defined for the earth

fe isotope systematics of coexisting amphibole and pyroxene in the alkaline igneous rock suite of the ilimaussaq complex south greenland abstract the ilimaussaq intrusion south greenland provides an exceptional test case for investigating the changes of stable fe isotope fractionation of solidus phases with changes in the fe 3 fe ratio of an evolving melt the intrusion comprises a sequence of four melt batches that were fed from the same parental alkali basaltic magma differentiation produced cumulate rocks that range from augite syenite phase i over peralkaline granite phase ii to agpaitic syenites phases iiia and iiib fe 3 fe ratios in amphiboles increase substantially from phase i to phase ii and iii rocks and mark a major change in the parental magma composition from augite syenites to peralkaline granites and agpaitic syenites before this transition olivine clinopyroxene and amphibole in augite syenite the most primitive rock type in the ilimaussaq complex have a uniform fe isotope composition that is identical to that of the bulk of igneous crustal rocks and approximated by the average isotopic composition of basalts 56 54 fe irmm 014 0 072 0 046 after the transition amphiboles in the peralkaline granites and agpaitic syenites yield significantly heavier fe isotope compositions with 56 54 fe irmm 014 values ranging from 0 123 to 0 237 contamination of the ilimaussaq magma by ongoing crustal assimilation as cause for this increase can be excluded on the grounds of nd isotope data large scale metasomatic overprint with an external fluid can also be dismissed based on amphibole o and li isotope systematics rather the increase towards heavy fe isotope compositions most likely reflects the change in chemical compositions of amphiboles calcic in augite syenite to sodic in the agpaitic syenites and their fe 3 fe ratios that mirror changes in the chemical composition of the melt and its oxygen fugacity a sensitive adjustment of equilibrium fe isotope fractionation factors to amphibole ferric ferrous ratios is also supported by beta factors calculated from mossbauer spetroscopy data comparison of the measured isotope fractionation between clinopyroxene and amphibole with that predicted from mossbauer data reveal fe isotope systematics close to equilibrium in augite syenites but fe isotopic disequilibrium between these two phases in phase iiia agpaitic syenites these results are in agreement with o and li isotope systematics while amphiboles in all ilimaussaq lithologies crystallized at temperatures between 650 and 850 c textural evidence reveals later clinopyroxene crystallization at temperatures as low as 300 400 c therefore isotopic equilibrium at crystallization conditions between these two phases can not be expected but importantly subsolidus reequilibration can also be dismissed