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

new isotopic evidence for the origin of groundwater from the nubian sandstone aquifer in the negev israel the geochemistry and isotopic composition h o s osulfate c sr of groundwater from the nubian sandstone kurnub group aquifer in the negev israel were investigated in an attempt to reconstruct the origin of the water and solutes evaluate modes of water rock interactions and determine mean residence times of the water the results indicate multiple recharge events into the nubian sandstone aquifer characterized by distinctive isotope signatures and deuterium excess values in the northeastern negev groundwater was identified with deuterium excess values of 16 which suggests local recharge via unconfined areas of the aquifer in the negev anticline systems the d 18 oh2o and d 2 h values 6 5 and 35 4 of this groundwater are higher than those of groundwater in the sinai peninsula and southern arava valley 7 5 and 48 3 that likewise have lower deuterium excess values of 10 based on the geochemical differences between groundwater in the unconfined and confined zones of the aquifer a conceptual geochemical model for the evolution of the groundwater in the nubian sandstone aquifer has been reconstructed the isotopic composition of shallow groundwater from the unconfined zone indicates that during recharge oxidation of pyrite to so4 d 34 sso4 13 d 18 oso4 7 7 and dissolution of caco3 87 sr 86 sr 0 70787 d 13 cdic 3 7 occur in the confined zone of the aquifer bacterial so4 reduction removes a significant part of dissolved so 2

magnesium isotopic composition of the deep continental crust to constrain the behavior of mg isotopes during deep crustal processes and the mg isotopic composition of the middle and lower continental crust 30 composite samples from high grade metamorphic terranes and 18 granulite xenoliths were investigated the composites derive from eight different high grade metamorphic terranes in the two largest archean cratons of china including 13 ttg gneisses 5 amphibolites 4 felsic 4 intermediate and 4 mafic granulites they have variable bulk compositions with sio 2 ranging from 45 7 to 72 5 representative of the middle crust beneath eastern china the 26 mg values of these samples vary from 0 40 to 0 12 reflecting heterogeneity of their protoliths which could involve upper crustal sediments the granulite xenoliths from the cenozoic hannuoba basalts also have a diversity of compositions with mgo ranging from 2 95 to 20 2 these xenoliths equilibrated under high temperatures of 800 950 c corresponding to depths of the lower continental crust 30 km they yield a large 26 mg variation of 0 76 to 0 24 the light mg isotopic compositions likely result from interactions with isotopically light metamorphic fluids probably carbonate fluids together with previously reported data the average 26 mg values of the middle and lower continental crusts are estimated to be 0 21 0 07 and 0 26 0 06 respectively the bulk continental crust is estimated to have an average 26 mg of 0 24 0 07 which is similar to the average of the mantle the large mg isotopic variation in the continental crust reflects the combination of several processes such as continental weathering involvement of supracrustal materials in the deep crust and fluid metasomatism

strontium isotope quantification of siderite brine and acid mine drainage contributions to abandoned gas well discharges in the appalachian plateau unplugged abandoned oil and gas wells in the appalachian region can serve as conduits for the movement of waters impacted by fossil fuel extraction strontium isotope and geochemical analysis indicate that artesian discharges of water with high total dissolved solids tds from a series of gas wells in western pennsylvania result from the infiltration of acidic low fe fe 10 mg l coal mine drainage amd into shallow siderite iron carbonate cemented sandstone aquifers the acidity from the amd promotes dissolution of the carbonate and metal and sulfate contaminated waters rise to the surface through compromised abandoned gas well casings strontium isotope mixing models suggest that neither upward migration of oil and gas brines from devonian reservoirs associated with the wells nor dissolution of abundant nodular siderite present in the mine spoil through which recharge water percolates contribute significantly to the artesian gas well discharges natural sr isotope composition can be a sensitive tool in the characterization of complex groundwater interactions and can be used to distinguish between inputs from deep and shallow contamination sources as well as between groundwater and mineralogically similar but stratigraphically distinct rock units this is of particular relevance to regions such as the appalachian basin where a legacy of coal oil and gas exploration is coupled with ongoing and future natural gas drilling into deep reservoirs

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

massive magnesium depletion and isotope fractionation in weathered basalts abstract magnesium isotopes have been shown to fractionate significantly during continental weathering however the degree and direction of fractionation varies from one study to the next and the main factors that control mg isotope fractionation during weathering have yet to be delineated here we report mg contents and isotopic compositions for two 10 m deep drill cores through bauxites developed on columbia river basalts crbs samples from these two drill cores have very low mgo contents 0 12 0 25 wt and variable but high 26mg values from 0 1 to up to 1 8 the heaviest isotopic composition ever reported for mg relative to the average values of fresh crbs 26mg 0 23 0 07 and mgo 5 9 wt these features reflect the near complete loss of mg in the isotopically heavy regolith the most highly weathered bauxites at the tops of the profiles have the lowest 26mg values reflecting the addition of isotopically light eolian dust excluding these samples 26mg in bauxites displays a weak positive correlation with gibbsite abundance suggesting that gibbsite preferentially retains 26mg in the bauxites the integrated mg isotopic fractionation factors between bauxite and fluid 26mgbauxite fluid inferred from rayleigh fractionation calculations vary from 0 05 to 0 4 which is similar to the range inferred from previous studies of igneous rock weathering profiles though none of these previous studies found the extremely heavy 26mg and massive mg depletion seen here collectively studies of weathering profiles suggest loss of isotopically light mg from the continents which must contribute to the very light 26mg seen in river water and seawater

the influence of weathering processes on riverine magnesium isotopes in a basaltic terrain this study presents major trace element and mg isotope data for the dissolved load and suspended particulates of icelandic rivers draining dominantly basaltic catchments including both glacier fed and direct runoff rivers these samples provide the opportunity to understand the behaviour of mg isotopes during chemical weathering where variations due to lithology are not extant given the significant role of mg in the carbon cycle such variations may provide important information on the regulation of earth s climate hydrothermal waters groundwater precipitation glacial ice basalt glass olivine and representative soils have also been analysed the dissolved load shows a wide range of 26mg compositions compared to the parent basaltic glass 26mg 0 29 ranging from 0 96 to 0 64 while precipitation and hydrothermal waters possess 26mg values of 0 83 and 0 85 respectively with lower mg concentrations than the dissolved load biomass activity in vegetation and organic material in soils and rivers colloids appear to have little effect on the mg isotope compositions rather the data suggest that mg elemental and isotopic variations are largely controlled by the formation and stability of secondary phases in response to differing hydrological conditions in some samples seawater in the form of direct precipitation or glacial runoff also appears to be an important source of mg glacier fed rivers groundwaters and some direct runoff rivers with a high ph have higher 26mg than basalt which is most likely due to the incorporation of light mg isotopes in secondary minerals in contrast those direct runoff rivers which have a relatively low ph have low 26mg relative to basalt consistent with preferential incorporation of heavy mg isotopes into secondary phases although it is not possible to rule out some contribution from precipitation riverine suspended particulates are depleted in mobile elements and have 26mg compositions values both higher and lower than unweathered basalt in the glacier fed and direct runoff rivers where the 26mg of the dissolved phase is heavy due to the formation of secondary phases the suspended load is light because it contains more of those phases the opposite is true for the remainder of the direct runoff rivers which have low ph this could be due to dissolution of secondary minerals enriched in light mg which are unstable at low ph or the formation of new secondary phases

carbonate and carbon isotopic evolution of groundwater contaminated by produced water brine with hydrocarbons abstract the major ionic and dissolved inorganic carbon dic concentrations and the stable carbon isotope composition of dic 13 c dic were measured in a freshwater aquifer contaminated by produced water brine with petroleum hydrocarbons our aim was to determine the effects of produced water brine contamination on the carbonate evolution of groundwater the groundwater was characterized by three distinct anion facies hco 3 rich so 4 2 rich and cl rich the hco 3 rich groundwater is undergoing closed system carbonate evolution from soil co 2 g and weathering of aquifer carbonates the so 4 2 rich groundwater evolves from gypsum induced dedolomitization and pyrite oxidation the cl rich groundwater is contaminated by produced water brine and undergoes common ion induced carbonate precipitation the 13 c dic of the hco 3 rich groundwater was controlled by nearly equal contribution of carbon from soil co 2 g and the aquifer carbonates such that the 13 c of carbon added to the groundwater was 11 6 in the so 4 2 rich groundwater gypsum induced dedolomitization increased the 13 c such that the 13 c of carbon added to the groundwater was 9 4 in the produced water brine contaminated cl rich groundwater common ion induced precipitation of calcite depleted the 13 c such that the 13 c of carbon added to the groundwater was 12 7 the results of this study demonstrate that produced water brine contamination of fresh groundwater in carbonate aquifers alters the carbonate and carbon isotopic evolution

isotopic imaging of surface water groundwater interactions sacramento valley california groundwater isotope data across the sacramento valley california establish two types of groundwater mining 1 overdraft of ancient groundwater with limited recharge by surface waters producing cones of depression 2 ancient groundwater withdrawal followed by rapid recharge of irrigation water reducing groundwater quality the first type occurs in the sacramento metropolitan area where meteoric runoff is unnaturally high and 40 years of pumping have depressed water levels to 25 m below sea level inducing recharge from losing reaches of the sacramento and american rivers lateral migration rates are quantified by the binary mixing between river water 18o 10 8 and natural groundwater 18o 7 0 the second type of mining occurs in agricultural regions to the west where 14c ages indicate that irrigation waters constitute more than 80 of modern recharge this recharge has several characteristics of evaporated irrigation water including 1 high 18o values to 6 0 that define closed contour patterns 2 elevated no3 concentrations to 100 ppm 3 low 14c ages of less than 500 years stable isotope contours augmented by 14c data provide dynamic recharge patterns in this profoundly disturbed giant alluvial aquifer on a large scale 100 km2 the lateral permeabilities of alluvial aquifers are essentially isotropic whereas on a smaller scale 25 km2 anisotropy is evident and isotope values can be geographically complex and seasonally transient groundwater flow patterns implied by the isotope data can differ substantially from steady state models based on head measurements

planar non planar and refolded sheath folds in the phulad shear zone rajasthan india abstract progressive ductile shearing in the phulad shear zone of rajasthan india has produced a complex history of folding with development of planar non planar and refolded sheath folds there are three generations of reclined folds f 1 f 2 and f 3 with a striping lineation l 1 parallel to the hinge lines of f 1 the planar sheath folds of f 1 have long subparallel hinge lines at the flanks joining up in hairpin curves at relatively small apices l 1 swerves harmoniously with the curving of f 1 hinge line there is a strong down dip mineral lineation parallel to the striping lineation in most places but intersecting it at apices of first generation sheath folds both the striping and the mineral lineation are deformed in u patterns over the hinges of reclined f 2 and f 3 folding of axial surfaces and hinge lines of earlier reclined folds by later folds was accompanied by very large stretching and led to the development of non planar sheaths the reclined folds of all the three generations were deformed by a group of subhorizontal folds each generation of fold initially grew with the hinge line at a very low angle with the y axis of bulk non coaxial strain and was subsequently rotated towards the down dip direction of maximum stretching the patterns of deformed lineations indicate that the stretching along the x direction was extremely large much in excess of 6000 percent

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

structural assemblage of high pressure mantle and crustal rocks in a subduction channel cabo ortegal nw spain 1 high pressure metamorphic fold nappes and ductile thrusts occur in the cabo ortegal upper allochthons they involve arc root lithospheric mantle peridotites high pressure volcanic arc granulites normal mid oceanic ridge basalt eclogites and shallow crustal volcanosedimentary high pressure gneiss units confluence of units from such disparate tectonic settings occurred at an eo hercynian convergent plate margin their internal deformation juxtaposition and high pressure synmetamorphic reworking resulted from kinematically simple but intense and polyphase ductile deformations fold nappes exhibit a remarkable sheath like form and contain mineral and stretching lineations defined by high pressure mineral assemblages subparallel to fold axes we argue that these structures outline the internal organization of high pressure sheets at deep tectonic realms of an oblique subduction collision orogenic channel

discrimination of water inrush source and evolution analysis of hydrochemical environment under mining in renlou coal mine anhui province china with the development of mining the hydrochemical environment has changed in the renlou coal mine anhui province china it will be of great significance to coal mine safety production by exploring the temporal and spatial evolution law of hydrochemistry of main water inrush aquifers in renlou coal mine under the effect of mining groundwater samples from three main water inrush aquifers in different periods were collected to analyze their principal components and establish the discrimination model of water inrush source the first and second principal components with eigenvalues greater than 1 were selected in which the first principal component represents sulfate dissolution and pyrite oxidation and the second principal component represents desulfurization and cation alternating adsorption the evolution law of hydrochemical environment was analyzed by combining the distribution characteristics of principal component scores with the spatial location of misjudged groundwater samples the results show that 1 quaternary aquifer is greatly affected by mining and the hydraulic connections with carboniferous limestone aquifer in the central region leads to two misjudged groundwater samples in the discriminant model 2 permian sandstone aquifer is to a certain extent affected by mining which shows a relatively closed hydrogeological environment in most areas of coal mine and an opened environment in some areas with time there are five misjudged samples in the discriminant model due to hydraulic connections with the quaternary aquifer and carboniferous aquifer in some areas 3 carboniferous limestone aquifer is controlled by typical faults and collapse columns and mining has a slight influence on it in the central area there are hydraulic connections with the quaternary aquifer and the permian sandstone aquifer due to the collapse columns which lead to four misjudged groundwater samples in the discriminant model

heterogeneous magnesium isotopic composition of the lower continental crust a xenolith perspective 1 we report 26 high precision whole rock mg isotopic analyses for two suites of well characterized granulite xenoliths from chudleigh and mcbride north queensland australia in order to constrain the behavior of mg isotopes during deep crustal processes and the mg isotopic composition of the lower continental crust previous studies suggest that the chudleigh granulites are a suite of cogenetic cumulates crystallized from mafic magmas that intruded into and assimilated the preexisting lower crust via combined assimilation and fractional crystallization afc the 26mg values of the xenoliths range from 0 31 to 0 21 and correlate with radiogenic isotopes reflecting mixing of mantle derived mafic magma 26mg 0 31 with preexisting isotopically heavy crustal materials 26mg 0 5 through the afc process the mcbride granulites range compositionally from mafic to felsic and originated as cumulates solidified mafic felsic melts and restites that formed during basaltic underplating and reworking of preexisting lower crust their 26mg values vary widely from 0 72 to 0 19 the large mg isotopic variation in the mcbride xenoliths reflects both distinct source compositions and metamorphic enrichment of garnet which is isotopically light based on these results the lower continental crust has a heterogeneous mg isotopic composition with a weighted average 26mg of 0 18 the bulk continental crust based on available data has an average mg isotopic composition of 0 19 and is slightly heavier than the mantle the highly heterogeneous mg isotopic distribution in the crust indicates that chemical weathering not only modifies the upper crust compositions but also significantly influences lower crust compositions through emplacement of upper crustal materials into the deep crust

anatomy of a 10 km scale sheath fold mount hay ridge arunta region central australia the structural record of deep crustal flow 1 mount hay ridge arunta region central australia exposes a 10 km scale antiformal sheath fold formed in the deep crust p 0 6 0 9 gpa in proterozoic time tilting and exhumation during the devonian alice springs orogeny resulted in an oblique cross section such that the once subhorizontal sheath fold now has a moderately dipping gently folded axial plane and overturned southern limb fabric types e g s l l s were mapped in the field and finite strain data were collected from a widely distributed gabbroic granulite unit fabric and finite strain data sets exhibit remarkable correspondence and confirm the common assumption that fabric type is directly related to finite strain type for deep crustal rocks both data sets reveal a distinct pattern of oblate strain on fold limbs grading into prolate strain in the fold hinge there is no field evidence for significant strain localization and measured strain magnitudes are consistent across the fold thus we infer that different structural domains record different though coeval strain paths and that strain compatibility was maintained across domain boundaries the uniformity of foliation lineation and mesoscale fold orientations across mount hay and the consistent magnitude of maximum stretch across structural domains indicate that the sheath fold and subsidiary structures formed during a single progressive deformational event based on its geometry the inferred scale of the enclosing shear zone 8 km in thickness and regional tectonic correlations we infer that the mount hay sheath fold formed in a subhorizontal crustal scale flow zone during the paleoproterozoic strangways orogeny 1740 1690 ma the record of deformation is consistent with fold formation either on one side of a channel or through detachment flow

high sr y magmas reflect arc maturity high magmatic water content and porphyry cu mo au potential examples from the tethyan arcs of central and eastern iran and western pakistan the temporal and geochemical evolution of arc magmatism that culminated in porphyry cu mo au deposit formation has been studied in three separate neo tethyan arc systems in central and eastern iran and western pakistan porphyry cu au deposits in the lut block of eastern iran formed in the middle eocene at the end of a period of extensive paleocene eocene volcanism porphyry cu mo deposits in the kerman belt of central iran formed in the middle miocene at the end of a period of voluminous eocene oligocene volcanism and porphyry cu au deposits in the chagai belt of western pakistan formed in four pulses during the eocene early miocene middle late miocene and late miocene pliocene after a prolonged period of arc magmatism that began in the late cretaceous and is still active in each region the late porphyry related magmas are more geochemically evolved and more hydrous as evidenced by the presence of hornblende phenocrysts than the preceding volcanic rocks we suggest that this reflects maturation of the arc magmatic system over a period of tens of millions of years leading to the generation of more evolved volatile rich magmas at later stages of the arc s history high magmatic water contents are a prerequisite for the shallow crustal emplacement of arc magmas and the subsequent generation of potentially ore forming subvolcanic magmatic hydrothermal systems it is thus suggested that the fertility of arc magmas within a given arc terrane can be assessed by observing the relative timing of plutonic suites later suites are more prospective noting the common presence of hornblende or biotite phenocrysts indicating high magmatic water contents and through lithogeochemical fingerprinting of magmatic fractionation processes relatively high sr y and la yb ratios and eun eu ratios 1 indicating abundant early hornblende fractionation and suppression of plagioclase crystallization in hydrous magmas

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

application of multiple isotopic and geochemical tracers for investigation of recharge salinization and residence time of water in the souss massa aquifer southwest of morocco summary groundwater and surface water in souss massa basin in the west southern part of morocco is characterized by a large variation in salinity up to levels of 37 g l 1 the high salinity coupled with groundwater level decline pose serious problems for current irrigation and domestic water supplies as well as future exploitation a combined hydrogeologic and isotopic investigation using several chemical and isotopic tracers such as br cl 18 o 2 h 3 h 87 sr 86 sr 11 b and 14 c was carried out in order to determine the sources of water recharge to the aquifer the origin of salinity and the residence time of water stable isotope 3 h and 14 c data indicate that the high atlas mountains in the northern margin of the souss massa basin with high rainfall and low 18 o and 2 h values 6 to 8 and 36 to 50 is currently constitute the major source of recharge to the souss massa shallow aquifer particularly along the eastern part of the basin localized stable isotope enrichments offset meteoric isotopic signature and are associated with high nitrate concentrations which infer water recycling via water agricultural return flows the 3 h and 14 c data suggest that the residence time of water in the western part of the basin is in the order of several thousands of years hence old water is mined particularly in the coastal areas the multiple isotope analyses and chemical tracing of groundwater from the basin reveal that seawater intrusion is just one of multiple salinity sources that affect the quality of groundwater in the souss massa aquifer we differentiate between modern seawater intrusion salinization by remnants of seawater entrapped in the middle souss plains recharge of nitrate rich agricultural return flow and dissolution of evaporate rocks gypsum and halite minerals along the outcrops of the high atlas mountains the data generated in this study provide the framework for a comprehensive management plan in which water exploitation should shift toward the eastern part of the basin where current recharge occurs with young and high quality groundwater in contrast we argued that the heavily exploited aquifer along the coastal areas is more vulnerable given the relatively longer residence time of the water and salinization processes in this part of the aquifer

sheath fold formation around slip surfaces terra nova 24 417 421 2012 abstract sheath folds are associated with shear zones and have been used as high strain indicators we present results from a three dimensional analytical study showing that the flow perturbation around a planar weakness acting as a slip surface in simple shear leads to the formation of sheath folds at low shear strain closed traces of marker layers are exhibited in cross sections cut perpendicular to the shear direction in the vicinity of the slip surface tip in cross sections cut parallel to the shear direction and normal to the shear plane flanking folds are observed our model captures the first order observations of natural sheath folds such as diversity of shapes and multiple eye structures natural equivalents of the planar weaknesses that we model include fractures veins and rheologically weak layers flow perturbation around these zones of weaknesses is proposed as an alternative mechanism of sheath fold formation which emphasizes the coeval operation of brittle and ductile processes in shear zones

magnesium isotopic variations in loess origins and implications abstract loess deposits serve as important continental archives for studying quaternary climatic variations and for estimating the average chemical composition of the upper continental crust here we report high precision mg isotopic data for 19 loess samples from china argentina and europe which were previously used to estimate the composition of the upper continental crust the results show that global loess samples have heterogeneous mg isotopic compositions with 26mg per mil deviation of the 26mg 24mg ratio from the dsm3 standard values ranging from 1 64 to 0 25 and a weighted average of 0 89 which is lighter than both crust and mantle silicates mgo content and 26mg of loess positively correlate with cao al2o3 ratio suggesting a two component mixing between carbonates and secondary silicate minerals the large variation in mg isotopic composition of loess results from a combination of factors including source heterogeneity eolian sorting during transport of the loess and chemical weathering during formation of the loess deposit our results suggest that mg isotopic composition of loess has potential to be a proxy indicator to characterize the paleoclimatic change but may not represent the average mg isotopic composition of the upper continental crust due to mixing sorting of isotopically distinct components and isotope fractionation during loess deposit formation

when shearing is a cause of folding abstract numerous publications on shearing as a cause of folding do not refute the idea of buckling being the main process responsible for the formation of fold structures deformations of other types may either produce conditions for layer parallel shortening needed for buckling or modify fold shapes but do not generate new fold structures on any scale the origin of similar folds commonly considered to be a result of inhomogeneous shearing across layering may be explained by buckling of a multilayer where layers have uniform favourable rheological properties tightening and amplification of folds near fault surfaces is not necessarily a result of shearing caused by friction at fault surfaces on the contrary in these cases faulting is a result of strain localization during folding recumbent isoclinal folds so common in metamorphic rocks originate mainly by buckling of subvertical planar structures intrafolial folds are usually very flattened relics of former folds the idea of sheath folds being produced mostly by shearing is not confirmed by the available geological data sheath folds that were not produced by shearing are rather common more likely they are formed by constriction or flattening of folds with curved hinges the latter are a result either of fold interference or of the constriction in the layering plane

the geotectonic history of the makran and adjacent areas of southern iran abstract the makran mountain range in southern iran has been entirely covered by regional mapping on a scale of 1 250 000 but in contrast to the mapping of oman on the other side of the gulf the recent mapping is little known the cenozoic accretionary prism in the south fronts onto a mesozoic melange zone representing earlier subduction and to the north of this is an eastwards tapering microcontinental sliver which is followed by a zone of largely intact mesozoic ophiolite complexes to the north of this lies the jaz murian depression a desert expanse with no bedrock outcrop the sanandaj sirjan bajgan dur kan microcontinental sliver which extends from turkey through the zagros to the makran is one of a number of microcontinents which detached from gondwana no later than the early triassic nw iran yazd tabas lut birk helmand farah during the mesozoic these were enclosed by tracts of ocean now represented by ophiolites and the sectors closed at different times ranging from early cretaceous to end palaeogene an analogy is drawn with the present day mediterranean the sistan ocean in the east was infilled with an immense thickness of abyssal turbidites during the eocene before it closed the field evidence indicates that there was a second cenozoic accretionary prism here but sedimentation ceased after the eocene on its collision with the back of the main makran prism this eastern sistan trough now completely occluded is compared with the still open tract separating the makran from oman at the present time and its northern extension which has been similarly occluded by collision beneath the zagros some residual very shallow inland seas persisted inland from the makran through the neogene the occurrence of blueschists in the inner ophiolite belt of the makran is discussed abyssal turbidite sedimentation continued in the main southern zone of the makran into the early miocene to be succeeded by thick shallow water sequences including reefal limestones intense tectonic dislocation at the end of the miocene was accompanied by a shift of the subduction front to its present position out in the indian ocean and uplift leading to the deposition of spectacular pliocene pleistocene fanglomerates

magnesium isotope fractionation during granite weathering abstract the mg isotope compositions of weathering profiles developed on granite and granodiorite bedrock have been analyzed to investigate mg isotope behavior during the weathering of felsic rocks in a continental environment 26mg generally exhibits a small increase from the bedrock values which range from 0 17 0 06 to 0 07 0 06 until the upper two meters in which a decrease is observed producing near surface values between 0 14 0 06 and 0 09 0 06 the small observed increase in isotopic composition below 2 m is likely due to mg loss to the hydrosphere during biotite weathering and in situ illite production while the decrease at depths

study of multi aquifer groundwater interaction in a coal mining area in china using stable isotopes and major ion chemical data investigations have been undertaken to determine the interactions between the main aquifers in a coal mining area near the taihang mountains in china to determine the sources of water environmental isotopes 18o 2h 3h 34s and water chemistry ions ca2 mg2 na k hco3 so42 cl were used as tracers furthermore batch sampling and testing were conducted on mountain spring water and karst water in the aquifers of the taiyuan group fengfeng group and majiagou group on the seatearth of the coal mining area via field observations and laboratory experimental analysis the 18o and 2h values of the mountain spring water and karst water in the coal mining area showed a common distribution with the local meteoric water line el1 of karst water in the majiagou group but significantly deviated from the surface water evaporation line el2 in the coal mining area when combined with the distribution of karst water level values thus karst water in the coal mining area is mainly supplied by groundwater from mountainous areas furthermore the concentration of sulfate ions increased dramatically in the groundwater flow of karst water from mountain spring water to karst water in the fengfeng group and majiagou group of the coal mining area when equivalent concentrations of ca2 mg2 hco3 and so42 hco3 reached their peak the chemical type of groundwater gradually evolved from ca mg hco3 to ca mg so4 hco3 in addition significant positive correlation was found between the 34s and so42 values of the water samples indicating that gypsum is involved in groundwater lixiviation in contrast the relationships of ca na versus mg na na hco3 versus total dissolved solids tds and na versus cl revealed that tds and salinity accumulate from the mountain spring water and karst water in the taiyuan group of the coal mining area furthermore the chemical type of groundwater gradually evolved from ca mg hco3 to na hco3 as revealed by a piper trilinear diagram

mg o isotopes trace the origin of mg rich fluids in the deeply subducted continental crust of western alps abstract fluids are important for mass transfer at the slab mantle interface in subduction zones however it is usually difficult to trace fluids from specific sources in a subducting slab especially those derived from dehydration of serpentinite coesite bearing whiteschist at dora maira in the western alps is characterized by strong mg enrichment relative to the country rocks which requires infiltration of mg rich fluids into the supracrustal rock in order to constrain the origin of such mg rich fluids we have performed an integrated study of whole rock mg and o isotopes zircon u pb ages and o isotopes for the whiteschist and related rocks zircons in the whiteschist show two groups of u pb ages at 262 ma and 34 ma respectively for relict and newly grown domains the permian u pb ages of relict magmatic domains are consistent with the protolith age of host metagranite suggesting that their common protolith is the permian granite the tertiary u pb ages occur in coesite bearing metamorphic domains consistent with the known age for ultrahigh pressure metamorphism the metamorphic domains have 18o values of 5 8 6 8 whereas the relict magmatic domains have high 18o values of 10 such high 18o values are also characteristic of the metagranite indicating that the whiteschist protolith underwent metasomatism by metamorphic fluids with low 18o value of 2 4 the whiteschist mostly has whole rock 26mg values of 0 07 to 0 72 considerably higher than country rock 26mg values of 0 54 to 0 11 thus the metamorphic fluids are not only rich in mg but also heavy in mg isotopes they were probably derived from the breakdown of mg rich hydrous minerals such as talc and antigorite in serpentinite at the slab mantle interface in the subduction channel therefore the dehydration of mantle wedge serpentinite during the subduction and exhumation of continental crust can provide the mg rich fluids responsible for the metasomatism of crustal rocks at subarc depths

geologic controls on movement of produced water releases at us geological survey research site a skiatook lake osage county oklahoma abstract highly saline produced water was released from multiple sources during oil field operations from 1913 to 1973 at the usgs research site a on skiatook lake in northeastern oklahoma two pits designed to hold produced water and oil were major sources for release of these fluids at the site produced water spills from these and other features moved downslope following topography and downdip by percolating through permeable eolian sand and colluvium underlying permeable sandstone and to a lesser extent through shales and mudstones saline water penetrated progressively deeper units as it moved through the gently dipping bedrock to the north and nw a large eroded salt scar north of the pits coincides with underlying fine grained rocks that have retained substantial concentrations of salt causing slow revegetation where not eroded thick eolian sand or permeable sandstone bedrock is near the surface and vegetation has been little affected or has reestablished itself after the introduced salt was flushed by precipitation the extent of salt contaminated bedrock extends well beyond existing surface salt scars these results indicate that one of the legacies of surface salt spills can be a volume of subsurface salinization larger than the visible surface disturbance

investigating the behaviour of mg isotopes during the formation of clay minerals abstract we present elemental and isotopic data detailing how the mg isotope system behaves in natural and experimentally synthesized clay minerals we show that the bulk mg isotopic composition 26 mg of a set of natural illite montmorillonite and kaolinite spans a 2 range and that their isotopic composition depends strongly on a balance between the relative proportions of structural and exchangeable mg after acid leaching these natural clays become relatively enriched in isotopically heavy mg by between 0 2 and 1 6 results of exchange experiments indicate that the mg that has adsorbed to interlayer spaces and surface charged sites is relatively enriched in isotopically light mg compared to the residual clay the isotopic composition of this exchangeable mg 1 49 to 2 03 is characteristic of the isotopic composition of mg found in many natural waters further experiments with an isotopically characterized mgcl 2 solution shows that the clay minerals adsorb this exchangeable mg with little or no isotopic fractionation although we cannot discount the possibility that the uptake of exchangeable mg does so with a slight preference for 24 mg to characterize the behaviour of mg isotopes during clay mineral formation we synthesized brucite mg oh 2 which we consider to be a good analogue for the incorporation of mg into the octahedral sheet of mg rich clay minerals or into the brucitic layer of clays such as chlorite in our experiment the brucite mineral becomes enriched in the heavy isotopes of mg while the corresponding solution is always relatively enriched in isotopically light mg the system reaches a steady state after 10 days with a final fractionation factor solid solution of 1 0005 at near neutral ph this result is consistent with the general consensus that secondary clay minerals preferentially take up isotopically heavy mg during their formation however our results also show that exchangeable mg is an important component within bulk clay minerals and can have an important influence over the bulk clay 26 mg value modeling shows that in certain soils or sediments where the percentage of exchangeable mg is 30 and the isotopic composition of the exchangeable mg is around 2 the generation of bulk 26 mg values of 26 mg values that are closer in composition to the ucc despite this results of modeling together with experimental observation suggests that the uptake of exchangeable mg into these clay rich sediments could cause a decrease in the bulk 26 mg value by up to 0 3 0 4 this should be accounted for when assessing the 26 mg value of sediments on a crustal scale

the magnesium isotope budget of the modern ocean constraints from riverine magnesium isotope ratios we have measured the magnesium mg isotope ratios in 45 rivers including 16 of the largest rivers in the world covering a range of geologic tectonic and climatic drainage basin environments the range in riverine 26mg is 2 5 half the variation in terrestrial rock although the mg isotopic composition of the source rock is important for small rivers at a global scale lithology is of limited significance for mg isotope ratios in rivers part of the variability at a global scale may be attributed to fractionation in the weathering environment the rivers analysed constitute 30 of the global mg riverine flux to the oceans and a flux weighted mg isotope composition of global runoff has been estimated at 1 09 even taking into account uncertainty this is distinct from seawater at 0 82 this difference arises either from the fractionation of mg isotope ratios in the ocean and or a mg budget which is not at steady state the difference is consistent with fractionation by carbonate precipitation in the simplest steady state scenario where the oceanic mass balance is maintained by riverine input and hydrothermal and dolomite output mg isotope ratios imply a minimum dolomite mg flux of 9 of the total output mg flux the implications of the distinct isotopic composition of the riverine flux from modern seawater is that the mg isotope ratio of seawater has probably varied outside of current analytical uncertainty mg isotope ratios may therefore provide a valuable new tracer in palaeo oceanography

the three dimensional shape and localisation of deformation within multilayer sheath folds abstract sheath folds are widely believed to develop by the passive geometric amplification of folds in which layering has no mechanical influence during non coaxial deformation where layering becomes rheologically significant then active sheath folds may form in which the inner nose of the sheath decouples along detachments and undergoes a translation relative to the outer folded layers that surround and envelope the nose we present a systematic 3 d analysis of multiple folded layer geometries in a serially sectioned natural sheath fold weak layers which are reactivated as detachments define different shaped sheath folds relative to other layers with the sense of cut off along detachments reversing across the axial surface as the inner fold nose has protruded into the surrounding sheath envelope detachment layers are more tightly folded meaning that such active sheath folds are non similar shapes the obliquely oriented bifurcating en echelon nature of the fold hinges developed in adjacent layers suggest that pre cursor folds formed with pronounced 3 d obliquity relative to the subsequent shear plane mineral lineations folded around sheath closures display asymmetric star burst patterns consistent with recrystallisation during active folding and hinge rotation we show that the eye fold shapes exposed in any 2 d y z slice can be used to predict the geometry of marker horizons back along the x axis in the third dimension this self similarity may be of value when tracing stratiform mineralised horizons in large scale sheath folds

sheath folds as discriminators of bulk strain type abstract nested elliptical closures defining eye folds represent classic y z cross sections through highly curvilinear sheath folds generated during intense ductile deformation in metamorphic rocks systematic analysis of 1425 such eye folds based on our own field observations and examples from the published literature reveals distinct and consistent differences in ellipticites measured from the outer r yz to the inner most r y z elliptical rings of individual sheaths the variation in overall aspect ratios from outer to inner rings is defined as r where r r yz r y z and may display a relative increase or decrease in ellipticity to define cats eye r bulls eye r 1 fold patterns respectively layer thicknesses may also be measured along the y axis parallel to the axial surface t y and at 90 to this along the z axis t z to define the ratio of t yz sheath folds generated during broadly simple shear deformation k 1 display mean r yz 4 61 t yz 3 31 and cats eye folds r 0 69 sheath folds generated during general shear also display cats eye folds with identical mean r 0 69 values but greater thickness variations and elliptical ratios t yz 4 35 r yz 5 76 thus within both simple and general shear dominated deformations the overall variation in layer thickness t yz and ellipticity of eye folds r yz increases with increasing deformation and a greater component of pure shear whilst the r value remains constant and reflects original fold patterns sheath folds formed during constrictional k 1 deformation display markedly lower aspect ratios r yz 2 42 and thickness variations t yz 2 94 together with distinctive bulls eye folds r 1 23 these empirical relationships suggest fundamental and universal constraints on curvilinear fold generation across this broad spectrum of deformation types and allow sheath folds to act as both effective 95 consistent and robust discriminators of bulk strain type

evaluation of petroleum hydrocarbon biodegradation in shallow groundwater by hydrogeochemical indicators and c s isotopes biodegradation is one of the main natural attenuation processes in groundwater contaminated with petroleum hydrocarbons in this work preliminary studies have been carried out by analyzing the concentrations of total petroleum hydrocarbons tph dissolved inorganic carbon dic dominant terminal electron accepters or donors as well as 13cdic and 34sso4 to reveal the biodegradation mechanism of petroleum hydrocarbons in a contaminated site the results show that along groundwater flow in the central line of the plume the concentrations of electron acceptors ph and e h increased but tph and dic decreased the 13cdic values of the contaminated groundwater were in the range of 14 02 to 22 28 pdb and 7 71 to 8 36 pdb which reflected a significant depletion and enrichment of 13c respectively the increase of dic is believed to result from the non methanogenic and methanogenic biodegradation of petroleum hydrocarbon in groundwater meanwhile from the contaminated source to the downgradient of the plume the 34s in the contaminated groundwater became more depleted the rayleigh model calculation confirmed the occurrence of bacterial sulfate reduction as a biodegradation pathway of the petroleum hydrocarbon in the contaminated aquifers it was concluded that stable isotope measurements combined with other biogeochemical measurements can be a useful tool to prove the occurrence of the biodegradation process and to identify the dominant terminal electron accepting process in contaminated aquifers

magnesium isotope evidence for a recycled origin of cratonic eclogites the mg isotopic compositions of garnet and clinopyroxene mineral separates and whole rocks from 21 xenolithic eclogites 11 low mgo eclogites and 10 high mgo eclogites from the koidu kimberlite complex erupted within the archean man shield sierra leone west africa provide new evidence bearing on the origin of cratonic eclogites garnet and clinopyroxene in both low mgo and high mgo eclogites generally record equilibrium inter mineral mg isotope partitioning with 26 mg varying from 2 15 to 0 46 in garnets and from 0 49 to 0 35 in clinopyroxenes bulk 26 mg values 1 38 to 0 05 constructed from garnet and clinopyroxene data are similar to results from rock powders 1 60 to 0 17 suggesting that kimberlite infiltration has had negligible influence on the mg isotopic compositions of the xenoliths the 26 mg values of low mgo eclogites 0 80 to 0 05 exceed the range of mantle peridotite xenoliths 0 25 0 04 consistent with the eclogite9s derivation from recycled altered oceanic crust similarly variable 26 mg values in high mgo eclogites 0 95 to 0 13 together with their high mgo and low feo contents suggest that high mgo eclogites were produced by mg fe exchange between partially molten low mgo eclogites and surrounding peridotites our study shows that cratonic xenolithic eclogites preserve a record of mg isotopic compositions produced by low pressure surficial isotope fractionations the recycling of oceanic crust therefore increases the mg isotope heterogeneity of the mantle

adsorption and desorption controlled magnesium isotope fractionation during extreme weathering of basalt in hainan island china magnesium isotopic compositions of a set of clay rich saprolites developed on the neogene tholeiitic basalt from hainan island in southern china have been measured in order to document the behavior of mg isotopes during continental weathering compared with unaltered basalts 26mg 0 36 the overlying saprolites are strongly depleted in mg i e th mg 99 1 to 92 9 and display highly variable 26mg ranging from 0 49 to 0 40 magnesium concentration and 26mg value of the saprolites display a general increasing trend upwards in the lower part of the profile but a decreasing trend towards the surface in the upper part the variations of mg concentration and isotopic composition in this weathering profile can be explained through adsorption and desorption processes 1 adsorption of mg to kaolin minerals kaolinite and halloysite with preferential uptake of heavy mg isotopes onto kaolin minerals and 2 desorption of mg through cation exchange of mg with the relatively lower hydration energy cations in the upper profile evidence for adsorption is supported by the positive correlation between 26mg and the modal abundance of kaolin minerals in saprolite of the lower profile while negative correlations between 26mg and concentrations of lower hydration energy cations e g sr and cs in the upper profile support the desorption process our results highlight that adsorption and desorption of mg on clay minerals play an important role in behavior of mg isotopes during extreme weathering which may help to explain the large variation in mg isotopic composition of river waters

strain and kinematic analysis in general shear zones we present a unifying theory for the development and distribution of strain markers and kinematic indicators in zones of general shear and thus provide a framework in which data previously considered contradictory may be understood rigid and deformable porphyroclast systems including and complex grains are potential indicators of both strain and flow the shapes and distribution of such porphyroclast systems may be used to distinguish among different tectonic regimes general shear is divided into two fields sub simple shear in which the rotational component of the strain is less than that for simple shear and super simple shear in which the rotational component is greater than for simple shear sub simple shear may involve narrowing or broadening of shear zones super simple shear regimes are possible in local regions such as the vicinities of deformable porphyroclasts but must be enclosed by regimes of sub simple shear the polar mohr constructions for finite deformation and flow are useful to analyze general shear in theory the hyperbolic net is employed for practical plotting of real data and derivation of the kinematic vorticity number wn this number represents the relative contributions of pure and simple shear in steady flow in nature deformation is thought to build up and decay by processes that may invalidate the assumption of constant flow regime we therefore introduce the concept of accelerating deformation and analyze the implications of non steady flow for the shearing histories of deformed objects

arabia eurasia collision and the forcing of mid cenozoic global cooling the end of the eocene greenhouse world was the most dramatic phase in the long term cooling trend of the cenozoic era here we show that the arabia eurasia collision and the closure of the tethys ocean gateway began in the late eocene at 35 ma up to 25 million years earlier than in many reconstructions we suggest that global cooling was forced by processes associated with the initial collision that reduced atmospheric co2 these are 1 waning volcanism across southwest asia 2 increased organic carbon storage in paratethyan basins e g black sea and south caspian 3 increased silicate weathering in the collision zone and 4 a shift towards modern patterns of ocean currents associated with increased vigour in circulation and organic productivity

empirical calibration of the clinopyroxene garnet magnesium isotope geothermometer and implications the large equilibrium mg isotope fractionation between clinopyroxene and garnet observed in eclogites makes it a potential high precision geothermometer but calibration of this thermometer by natural samples is still limited here we report mg isotopic compositions of eclogite whole rocks as well as mg and o isotopic compositions of clinopyroxene and garnet separates from 16 eclogites that formed at different temperatures from the dabie orogen china the whole rock 26mg values vary from 1 20 to 0 10 among them 11 samples display limited 26mg variations from 0 36 to 0 17 similar to those of their protoliths the mineral separates exhibit very different 26mg values from 0 39 to 0 39 for clinopyroxenes and from 1 94 to 0 81 for garnets the clinopyroxene garnet mg isotope fractionation 26mgclinopyroxene garnet 26mgclinopyroxene 26mggarnet varies from 1 05 to 2 15 the clinopyroxene garnet o isotope fractionation 18oclinopyroxene garnet 18oclinopyroxene 18ogarnet varies from 1 01 to 0 98 equilibrium mg isotope fractionation between clinopyroxene and garnet in the investigated samples is selected based on both the 26mgclinopyroxene versus 26mggarnet plot and the state of o isotope equilibrium between clinopyroxene and garnet the equilibrium 26mgclinopyroxene garnet and corresponding temperature data obtained in this study together with those available so far in literatures for natural eclogites are used to calibrate the clinopyroxene garnet mg isotope thermometer this yields a function of 26mgclinopyroxene garnet 0 99 0 06 106 t2 where t is temperature in kelvin the refined function not only provides the best empirically calibrated clinopyroxene garnet mg isotope thermometer for precise constraints of temperatures of clinopyroxene and garnet bearing rocks but also has potential applications in high temperature mg isotope geochemistry

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

environmental isotopes and hydrochemical study applied to surface water and groundwater interaction in the awash river basin an environmental isotope and hydrochemical study was carried out to conceptualize the surface water and groundwater interaction and to explore the groundwater flow pattern in relation to the geological setting more emphasis is given to the afar depression where groundwater is a vital source of water supply conventional field hydrogeological study and river discharge records support the isotope and hydrochemical analysis the region is tectonically active comprising rift volcanic terrain bordered by highlands the result revealed that recent meteoric water is the major source of recharge three distinct groundwater zones were identified associated with the highlands transitional escarpment and the rift towards the rift the ionic concentration and isotopic enrichment 2h and 18dego increases following the groundwater flow paths which is strongly controlled by axial rift faults the groundwater flow converges to the seismically active volcano tectonic depressions with internal drainage and to the awash river within the afar depression at least four groundwater regimen are identified 1 fresh and shallow groundwater associated with alluvial deposits ultimately recharged by isotopically depleted recent highland rainfall and the evaporated awash river 2 cold and relatively younger groundwater within localized fractured volcanics showing mixed origin in axial fault zones 3 old groundwater with very high ionic concentration and low isotopic signature localized in deep volcanic aquifers and 4 old and hot saline groundwaters connected to geothermal systems the study demonstrated that dependable groundwater can only be obtained from the first two aquifer types in aerially restricted zones in flat plains following river courses local wadis and volcano tectonic depressions the conventional hydrogeological survey and discharge records indicate substantial channel losses from the awash river which becomes a more dominant source of recharge in central and lower awash valleys copyright 2007 john wiley sons ltd

high temperature inter mineral magnesium isotope fractionation in eclogite from the dabie orogen china abstract to investigate the magnitude and mechanism of inter mineral mg isotope fractionation at high temperatures we report high precision analyses of mg isotopes for 10 whole rocks and 13 mineral separates for a set of eclogites from bixiling in the dabie orogen china magnesium isotopic compositions of whole rocks 26mg of 0 44 to 0 26 are similar to the estimated 26mg values of the mantle suggesting mg isotopic inheritance from a gabbroic protolith with limited mg isotope fractionation during eclogite facies metamorphism by contrast mineral separates are highly heterogeneous with 26mg values ranging from 0 30 to 0 60 in phengite from 0 16 to 0 40 in omphacite and from 0 95 to 0 74 in garnet phengite and omphacite are 1 heavier in 26mg than coexisting garnet indicating large high temperature inter mineral mg isotope fractionations the constant 26mgomphacite garnet 26mgomphacite 26mggarnet value 1 14 0 04 together with homogeneous mineral chemistry and equilibrium oxygen isotopic partitioning between omphacite and garnet suggests an equilibrium mg isotope fractionation controlled by the difference in coordination number of mg between omphacite six and garnet eight the 1 14 fractionation is the largest high temperature equilibrium inter mineral mg isotope fractionation observed so far and makes the omphacite garnet mg isotope fractionation a potential geothermometer by contrast mg isotope fractionations between phengite and garnet and between phengite and omphacite vary from 1 25 to 1 47 and from 0 14 to 0 32 respectively this implies mg isotopic disequilibria between phengite and garnet omphacite which might result from the mg isotopic variation in phengites due to mg isotope exchange between phengites and retrograde fluids in bixiling eclogites