the thermal and mineral springs of aitoloakarnania prefecture function mechanism and origin of groundwater the study area is located in the northwestern part of greece in aitoloakarnania prefecture in this region where no volcanic activity exists thermal springs such as kremasta and kokkino stefani well known for their healing properties occur the objective of this study was the investigation of these springs as well as the study of the chemical composition and origin of water relationships between these springs were also examined the geological setting of the area comprises sedimentary rocks of the pindos gavrovo tripolis and ionian geotectonic zones deformed by orogenic movements followed by neogene extensional tectonism the thermal and mineral springs were classified into three main groups the first group is characterized by ca hco3 water type and low water temperatures it corresponds to the springs that are hosted in the ionian zone and their possible enrichment in so4 is mainly attributed to the evaporites the other two groups consist of alkaline thermal water mainly hosted in the formations of gavrovo tripolis zone in these two groups the very strong reducing conditions that prevail are expressed by high amounts of nh3 and h2s moreover na f li sr and ba display elevated concentrations the second includes mineral waters of ca na hco3 type that are depleted in calcium their residence time is rather long and they originate from deep water circulation through siliceous rocks the third group includes thermal waters of ca mg na hco3 water type of higher water temperatures that reveal characteristics of deep circulation directly associated with the underlain limestones

orthopyroxene oikocrysts in the mg1 chromitite layer of the bushveld complex implications for cumulate formation and recrystallisation two typical mineral textures of the mg 1 chromitite of the bushveld complex south africa were observed one characterised by abundant orthopyroxene oikocrysts and the other by coarse grained granular chromitite with only minor amounts of interstitial material oikocrysts form elongate clusters of several crystals aligned parallel to the layering and typically have subhedral almost chromite free core zones containing remnants of olivine the core zones are surrounded by poikilitic aureoles overgrowing euhedral to subhedral chromite chadacrysts chromite grains show no preferred crystal orientation whereas orthopyroxene grains forming clusters commonly share the same crystallographic orientation oikocryst core zones have lower mg and higher concentrations of incompatible trace elements compared to their poikilitic aureoles core zones are relatively enriched in ree compared to a postulated parental magma b1 and did not crystallise in equilibrium with the surrounding minerals whereas the composition of the poikilitic orthopyroxene is consistent with growth from the b1 magma these observations cannot be explained by the classic cumulus and post cumulus models of oikocryst formation instead we suggest that the oikocryst core zones in the mg1 chromitite layer formed by peritectic replacement of olivine primocrysts by reaction with an upwards percolating melt enriched in incompatible trace elements poikilitic overgrowth on oikocryst core zones occurred in equilibrium with a basaltic melt of b1 composition near the magma crystal mush interface finally adcumulus crystallisation followed by grain growth resulted in the surrounding granular chromitite

compositional variations in the mesoarchean chromites of the nuggihalli schist belt western dharwar craton india potential parental melts and implications for tectonic setting the chromite deposits in the archean nuggihalli schist belt are part of a layered ultramafic mafic sequence within the western dharwar craton of the indian shield the 3 1 ga ultramafic mafic units occur as sill like intrusions within the volcano sedimentary sequences of the nuggihalli greenstone belt that are surrounded by the tonalite trondhjemite granodiorite ttg suite of rocks the entire succession is exposed in the tagdur mining district the succession has been divided into the lower and the upper ultramafic units separated by a middle gabbro unit the ultramafic units comprise of deformed massive chromitite bodies that are hosted within chromite bearing serpentinites the chromitite bodies occur in the form of pods and elongated lenses 60 500 m by 15 m detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate rich chromitite 50 modal chromite and serpentinite 2 modal chromite throughout the entire ultramafic sequence however the primary composition of chromite is preserved in the massive chromitites 60 75 modal chromite from the byrapur and the bhaktarhalli mining district of the nuggihalli schist belt these are characterized by high cr ratios cr cr al 0 78 0 86 and moderate mg ratios mg mg fe2 0 38 0 58 the compositional variability occurs due to sub solidus re equilibration in the accessory chromite in the serpentinite mg ratio 0 01 0 38 cr ratio 0 02 0 99 and in silicate rich chromitite mg ratio 0 06 0 48 cr ratio 0 60 0 99 in the massive chromitites the sub solidus re equilibration for chromite is less or absent however the re equilibration is prominent in the co existing interstitial and included olivine fo96 98 and pyroxene grains mg numbers 97 99 compositional variability on the scale of a single chromite grain occurs in the form of zoning and it is common in the accessory chromite grains in serpentinite and in the altered grains in chromitite in the zoned grains the composition of the core is modified and the rim is ferritchromit in general ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains in this case ferritchromit formation is not very extensive this indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization in some occurrences the ferritchromit rim is very well developed and only a small relict core appears to remain in the chromite grain however complete alteration of the chromite grains to ferritchromit without any remnant core is also present the regular well developed and continuous occurrence of ferritchromit rims around the chromite grain boundaries the complete alteration of the chromite grains and the modification of the core composition indicate the alteration in the nuggihalli schist belt to be intense pervasive and affected by later low grade metamorphism the primary composition of chromite has been used to compute the nature of the parental melt the parental melt calculations indicate derivation from a high mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the western dharwar craton tectonic discrimination diagrams using the primary composition of chromites indicate a supra subduction zone setting ssz for the archean chromitites of nuggihalli and derivation from a boninitic magma the composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back arc rift settings formation of the massive chromitites in nuggihalli may be due to magma mixing process involving hydrous high mg magmas or may be related to intrusions of chromite crystal laden magma however there is little scope to test these models because the host rocks are highly altered serpentinized and deformed the present configurations of the chromitite bodies are related to the multistage deformation processes that are common in archean greenstone belts

origin of the ug2 chromitite layer bushveld complex chromitite layers are common in large mafic layered intrusions a widely accepted hypothesis holds that the chromitites formed as a consequence of injection and mixing of a chemically relatively primitive magma into a chamber occupied by more evolved magma this forces supersaturation of the mixture in chromite which upon crystallization accumulates on the magma chamber floor to form a nearly monomineralic layer to evaluate this and other genetic hypotheses to explain the chromitite layers of the bushveld complex we have conducted a detailed study of the silicate rich layers immediately above and below the ug2 chromitite and another chromitite layer lower in the stratigraphic section at the top of the lower critical zone the ug2 chromitite is well known because it is enriched in the platinum group elements and extends for nearly the entire 400 km strike length of the eastern and western limbs of the bushveld complex where we have studied the sequence in the central sector of the eastern bushveld the ug2 chromitite is embedded in a massive 25 m thick plagioclase pyroxenite consisting of 60 70 vol granular cumulus orthopyroxene with interstitial plagioclase clinopyroxene and accessory phases throughout the entire pyroxenite layer orthopyroxene exhibits no stratigraphic variations in major or minor elements mg number1 4 79 3 81 1 however the 6 m of pyroxenite below the chromitite footwall pyroxenite is petrographically distinct from the 17 m of hanging wall pyroxenite among the differences are 1 phlogopite k feldspar and quartz are ubiquitous and locally abundant in the footwall pyroxenite but generally absent in the hanging wall pyroxenite and 2 plagioclase in the footwall pyroxenite is distinctly more sodic and potassic than that in the hanging wall pyroxenite an45 60 vs an70 75 the lower critical zone chromitite is also hosted by orthopyroxenite but in this case the rocks above and below the chromitite are texturally and compositionally identical for the ug2 we interpret the interstitial assemblage of the footwall pyroxenite to represent either interstitial melt that formed in situ by fractional crystallization or chemically evolved melt that infiltrated from below in either case the melt was trapped in the footwall pyroxenite because the overlying ug2 chromitite was less permeable if this interpretation is correct the footwall and hanging wall pyroxenites were essentially identical when they initially formed however all the models of chromitite formation that call on mixing of magmas of different compositions or on other processes that result in changes in the chemical or physical conditions attendant on the magma predict that the rocks immediately above and below the chromitite layers should be different this leads us to propose that the bushveld chromitites formed by injection of new batches of magma with a composition similar to the resident magma but carrying a suspended load of chromite crystals the model is supported by the common observation of phenocrysts including those of chromite in lavas and hypabyssal rocks and by chromite abundances in lavas and peridotite sills associated with the bushveld complex indicating that geologically reasonable amounts of magma can account for even the massive 70 cm thick ug2 chromitite the model requires some crystallization to have occurred in a deeper chamber for which there is ample geochemical evidence

chemical interactions between a sedimentary diapir and surrounding magma evidence from the phepane dome and bushveld complex south africa the phepane dome is a circular outcrop of metasedimentary rock within the eastern lobe of the bushveld complex hypothesized to have formed as a diapir when underlying wallrock rose into the overlying magma body interactions between the metasedimentary rock of the phepane dome and the magma of the bushveld complex were investigated through measurements of oxygen and lithium isotopic compositions determination of mineral modes and major element mineral compositions cathodoluminescence imaging and dihedral angle analysis evidence from cathodoluminescence imaging and dihedral angle analysis suggest that heat transfer during diapirism caused partial melting and complete recrystallization of the phepane dome metasedimentary rock oxygen isotope analysis of samples from traverses spanning the contact between metasedimentary and igneous rocks demonstrates that relatively minimal exchange of oxygen over distance 4 m occurred across the contact between the phepane dome and the surrounding bushveld magma the lithium concentrations and isotopic compositions of metasedimentary rock are significantly different from the associated igneous rocks lithium isotope analysis of samples from traverses across the contact demonstrates exchange of li over somewhat greater distances 60 m than oxygen consistent with evidence that suggests a higher diffusivity of li than most major elements models of oxygen diffusion through intergranular melt and aqueous fluid are used to place maximum constraints on the duration of diffusive exchange across the contact resulting in estimates ranging from 5 kyr to 5 myr these values are consistent with previous estimates of the duration of crystallization of the bushveld complex and phepane diapir development

re os isotopic constraints on magma mixing in the peridotite zone of the stillwater complex montana usa chromite separates within the peridotite zone of the 2 700 ma old stillwater complex are characterized by low 187re 188os 0 009 to 1 74 and relatively high os concentrations 8 74 to 78 2 ppb their calculated initial os isotopic compositions likely reflect the compositions of the magmas from which they crystallized the chromites show variable initial os isotopic compositions os of 2 0 to 16 4 over the vertical extent of the peridotite zone implicating at least two sources of os both the range of os and values of os decrease upsection these variations in os were caused by mixing of variable proportions of two magmas having different os isotopic compositions one of the magmatic components was a more primitive magma with a nearly chondritic os isotopic composition the other magma had a radiogenic os isotopic composition as a result of assimilation of crust perhaps of sedimentary rocks beneath the stillwater complex the gradual decrease in the initial os values of the chromite layers with increasing stratigraphic height implies a decreasing relative contribution from the contaminated magma throughout the growth of the peridotite zone small variations in os between different chromite occurrences within the h multicyclic unit reflects the petrologic requirement that chromite layers crystallize from slightly different proportions of the magmas compared to chromite from olivine and orthopyroxene rich layers

genesis of silicate exsolution lamellae in chromite of the stillwater complex a challenge to the high pressure crystallization of ophiolitic chromitite abstract models for deep derivation of ophiolitic chromite from the mantle transition zone versus magmatic crystallization at lithospheric level are now highly controversial due to the discovery of ultrahigh pressure uhp and super reduced sur minerals in ophiolitic chromitites and the presence of silicate mineral exsolution lamellae in chromite grains according to experimental studies chromite grains crystallized at low pressures cannot exsolve silicate phases because of their low contents of cao and sio2 however clinopyroxene needles in chromite grains from the stillwater complex which is a typical layered mafic ultramafic intrusion crystallized at crustal level show morphological compositional and crystallographic features very similar to exsolution lamellae in ophiolitic chromite grains we propose that the stillwater clinopyroxene lamellae represent minute melt inclusions that were attached parallel to branches of skeletal chromite where they were trapped during continued growth of the grain this interpretation is supported by the orientations of the lamella like clinopyroxene and the coexistence of larger clinopyroxene inclusions and apatite crystals along with the clinopyroxene needles another possibility is that these lamella like minerals precipitated from melt fluid that filled cracks in the chromite grains trails of tiny inclusions preserved in some healed fractures are consistent with this mechanism but are inconsistent with the regular orientations of most of the needle like inclusions the presence of such features in the stillwater chromitites raises serious questions regarding the origin and significance of silicate lamellae in ophiolitic chromite grains and the use of such features to determine the depth of crystallization

magmatic unconformities and stratigraphic relations in the peridotite zone stillwater complex montana new stratigraphic relations between cumulate layers and units are documented in the peridotite zone of the stillwater complex a late archean mafic to ultramafic layered intrusion exposed along the northern edge of the beartooth uplift montana first low angle unconformities occur between all types of successive stratigraphic units and at all scales within the peridotite zone second new stratigraphic units and divisions based on the unconformities and type of cumulate layering within the peridotite zone are proposed and are correlated with published stratigraphic columns the stratigraphic units identified on the basis of rock type s and contacts with adjacent units from smallest to largest are cyclic units multicyclic units megacyclic groups and subzones the three larger units are usually bounded by low angle magmatic unconformities the three subzones and five of the six megacyclic groups identified in the peridotite zone can be traced over a distance of 17 7 mi 28 3 km the stratigraphic relations in the peridotite zone indicate that low angle crosscutting magmatic unconformities occur at all scales and that lateral variations within and between the identified stratigraphic units are common the relations suggest an extremely dynamic and rapidly changing environment during formation of the peridotite zone overprinted on the stratigraphic relations is a larger evolution of cumulus rock types in the peridotite zone from more primitive stratigraphic units dominated by olivine and olivine bronzite cumulates to more evolved stratigraphic units dominated by bronzite and bronzite olivine cumulates several major magmatic or possibly structural events appear to have affected the peridotite zone during its formation as indicated by thickness variation patterns in megacyclic groups and subzones the thickest massive chromite layers in the peridotite zone are associated with two of the major magmatic events

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

the genesis of archaean chromitites from the nuasahi and sukinda massifs in the singhbhum craton india abstract the chromitite deposits of the nuasahi and sukinda massifs are part of layered ultramafic bodies which occur within archaean low grade metamorphic rocks of the iron ore group iog in the singhbhum craton of the indian shield the chromitite seams are interlayered with dunite and associated with orthopyroxenite detailed electron microprobe study reveals very high mg numbers 0 66 0 82 and cr numbers 0 75 0 87 for chromite in massive chromitite from the seams high fo content fo 92 95 for olivine in dunite and high en content en 89 94 for orthopyroxene in orthopyroxenite our study suggests that the original igneous compositions for these minerals are preserved in the respective monomineralic rocks and can be used to evaluate the primary magmatic petrogenesis of the rocks the primitive compositions of the minerals in monomineralic rocks suggest high degree of partial melting of the source mantle during the mesoarchaean the parental magma from which the massive chromitites crystallized was of siliceous high mg basaltic or boninitic compositions similar to the compositions of spatially associated chromite bearing siliceous high mg basalts of the iron ore group the parental magma may have generated due to the interaction of a depleted mantle with a fluid enriched melt possibly derived in response to the dehydration of a subducting slab monomineralic chromitite layers formed either in response to mixing of magmas or due to the suppression of silicate mineral crystallization in response to elevated h 2 o concentrations in the parent magmas the parental magmas that produced chromitites may have intruded into the volcano sedimentary greenstone belts of a supra subduction zone setting

phosphogenesis carbon isotope stratigraphy and carbonate platform evolution along the lower cretaceous northern tethyan margin we distinguish three different stages in the evolution of the tithonian late jurassic to aptian early cretaceous northern tethyan carbonate platform 1 carbonate production in the coral oolite mode late tithonian to early valanginian early barremian to early aptian 2 carbonate production in the crinoid bryozoan mode early valanginian haute rivian late aptian and 3 platform retrogradation and destruction condensation and phosphogenesis that is platform drowning early valanginian to early hauterivian middle hauterivian late hauterivian to early barremian early to early late aptian and latest aptian to earliest albian transitions from the coral oolite mode to the crinoid bryozoan mode and consequently to platform drowning may have been driven by increases in nutrient levels on the shelf phases of relative sea level rise in times of carbonate production in the coral oolite mode are named constructive because of the observed platform regeneration following phases of widespread emersion during late sea level highstands in contrast phases of relative sea level rise in times of platform drowning are termed destructive the 13 c stratigraphies obtained from valanginian hauterivian and aptian albian hemipelagic successions beyond the platform correlate well with the early cretaceous pelagic 13 c record positive excursions in the pelagic 13 c record correspond in time to episodes of platform drowning this suggests the existence of a coupling mechanism between changes in the global carbon cycle and platform drowning in our view the early cretaceous crises in carbonate platform growth were the consequence of reinforced greenhouse conditions which may have been triggered by episodes of extensive flood basalt volcanism strong greenhouse conditions may have induced the following chain of feedback mechanisms enabling the biosphere to return to normal conditions climate warming sea level rise accelerated water cycle intensified weathering nutrient mobilization platform destruction increased productivity increased phosphogenesis and carbon burial weakened greenhouse conditions

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

the significance of magmatic erosion for bifurcation of ug1 chromitite layers in the bushveld complex abstract one of the most puzzling features of the ug1 chromitite layers in the famous exposures at dwars river eastern bushveld complex is the bifurcation i e convergence and divergence of layers along strike that isolate lenses of anorthosite the bifurcations have been variously interpreted as resulting from 1 the intermittent accumulation of plagioclase on the chamber floor as lenses terminated by crystallization of continuous chromitite layers the depositional model 2 late stage injections of chromite mush or chromite saturated melt along anastomosing fractures that dismembered semi consolidated plagioclase cumulates the intrusive model 3 post depositional deformation of alternating plagioclase and chromite cumulates resulting in local amalgamation of chromitite layers and anorthosite lenses that wedge out laterally the deformational model none of these hypotheses account satisfactorily for the following field observations a wavy and scalloped contacts between anorthosite and chromitite layers b abrupt lateral terminations of thin anorthosite layers within chromitite c in situ anorthosite inclusions with highly irregular contacts and delicate wispy tails within chromitite many of these inclusions are contiguous with footwall and hanging wall cumulates d transported anorthosite fragments enclosed by chromitite e disrupted anorthosite and chromitite layers overlain by planar chromitite f protrusions of chromitite into underlying anorthosite g merging of chromitite layers around anorthosite domes we propose a novel hypothesis that envisages basal flows of new dense and superheated magma that resulted in intense thermo chemical erosion of the temporary floor of the chamber the melting and dissolution of anorthosite was patchy and commonly inhibited by chromitite layers resulting in lens like remnants of anorthosite resting on continuous layers of chromitite on cooling the magma crystallized chromite on the irregular chamber floor draping the remnants of anorthosite and merging with pre existing chromitite layers excavated by erosion with further cooling the magma crystallized chromite bearing anorthosite emplacement of multiple pulses of magma led to repetition of this sequence of events resulting in a complex package of anorthosite lenses and bifurcating chromitite layers this hypothesis is the most satisfactory explanation for most of the features of this enigmatic igneous layering in the bushveld complex

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

the effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions a series of calculations have been carried out to evaluate the effect on cumulus mineral compositions of solidification of trapped intercumulus liquid in orthocumulates the calculation assumes local equilibrium between phases and that the system remains chemically closed during crystallization of the trapped liquid the latter assumption is held to be valid on a scale of tens to hundreds of centimeters it is not necessary to know the composition of the trapped liquid as the calculation only requires an estimate of feo content and trapping temperature the change in composition of a mineral from that of the initially precipitated cumulus crystals to the final composition after complete solidification is termed the trapped liquid shift its magnitude depends on the modal proportions of cumulus phases and the initial porosity and is only weakly dependent on initial phase compositions trapped liquid shifts are significant when compared with mineral composition changes occurring during fractional crystallization crystallization of 30 trapped liquid gives rise to shifts of up to 10 mol percent in mg number of olivine or pyroxene the size of the shift becomes greater when the initial cumulus assemblage has a lower total feo mgo content and vice versa as a result of the relationship between trapped liquid shift and cumulus mode mineral composition variations and trends may be generated in sequences of cumulates which originally had constant compositions of cumulus minerals for example in a cyclic unit grading from a pyroxenitic base to an anorthositic top crystallization of a uniform proportion of trapped liquid will result in an apparent iron enrichment trend from bottom to top of the cycle as has been observed in the upper critical zone of the bushveld complex

lithium isotopic composition of submarine basalts implications for the lithium cycle in the oceans abstract we have measured the li isotope composition of young pristine basalts from active ocean ridge crests of progressively older basalts along a dredging transect and a limited number of hydrothermally altered basalts the data significantly extend our limited knowledge of the isotopic abundance ratio of li in geological material fresh mid ocean ridge basalts have 6li values of between 3 4 and 4 7 relative to isotope standard l svec during low temperature weathering on the seafloor the isotopic composition of the rock becomes increasingly heavier due to addition of seawater li 6li 32 3 the oldest 46 m y and most altered rock studied has an isotopic composition of 14 a linear relationship exists between 6li and the inverse of li concentration suggesting that li in weathered basalts can be regarded as a two component mixture of basaltic li and seawater derived li that has been incorporated in alteration minerals most likely secondary clays such as smectite and phillipsite the inferred li isotopic composition of the alteration endmember indicates an apparent isotopic fractionation factor of 1 019 relative to the seawater source thus li uptake by secondary minerals from the low temperature weathering process and by analogy incorporation in similar authigenic minerals in marine sediments provides a mechanism for preferential removal of the lighter li isotope from ocean water however isotopic fractionation due to authigenic clay formation alone cannot account for the isotopic difference between seawater and its principal sources unless the hydrothermal flux is comparable to the river flux alternatively more important sinks of6li must exist if the steady state isotopic composition of li in ocean water is to be maintained

isotope fractionation by chemical diffusion between molten basalt and rhyolite abstract experimental diffusion couples were used to study chemical diffusion between molten rhyolite and basalt with special emphasis on the associated fractionation of calcium and lithium isotopes diffusion couples were made by juxtaposing firmly packed powders of a natural basalt suny morb and a natural rhyolite lake county obsidian and then annealing them in a piston cylinder apparatus for times ranging from 0 1 to 15 7 h temperatures of 1350 1450 c and pressures of 1 2 1 3 gpa profiles of the major elements and many trace elements were measured on the recovered quenched glasses the diffusivities of all elements except lithium were found to be remarkably similar while the diffusivity of lithium was two to three orders of magnitude larger than that of any of the other elements measured chemical diffusion of calcium from molten basalt into rhyolite was driven by a concentration ratio of 18 and produced a fractionation of 44ca from 40ca of about 6 because of the relatively low concentration of lithium in the natural starting materials a small amount of spodumene lialsi2o6 was added to the basalt in order to increase the concentration difference between basalt and rhyolite which was expected to increase the magnitude of diffusive isotopic fractionation of lithium the concentration ratio between li doped basalt and natural rhyolite was 15 and the resulting diffusion of lithium into the rhyolite fractionated 7li from 6li by about 40 we anticipate that several other major rock forming elements such as magnesium iron and potassium will also exhibit similarly larger isotopic fractionation whenever they diffuse between natural melts with sufficiently large differences in the abundance of these elements

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

the stable cr isotope inventory of solid earth reservoirs determined by double spike mc icp ms abstract we present the first comprehensive set of stable cr isotope data for the major igneous silicate earth reservoirs cr iii rich ores and minerals and hydrothermal chromates these were determined by mc icp ms using a double spike technique mantle xenoliths ultramafic rocks cumulates as well as oceanic and continental basalts share a common cr isotope composition with an average 53 52 cr value of 0 124 0 101 2 sd relative to the isotopically certified chromium standard nist srm 979 an isotopic difference between mantle xenoliths and basalts as was reported for iron was not observed for chromium thus the change in oxidation state that is observed when solid mantle rocks containing only trivalent chromium partially melt to form basaltic melts which predominantly contain bivalent chromium does not cause any measurable cr isotope fractionation chromite separates from major chromitite seams of the bushveld and great dyke layered igneous complexes are invariable in their cr isotope compositions and reproduce within uncertainties the average 53 52 cr srm 979 value of igneous silicate earth reservoirs this is important for environmental stable cr isotope studies because it labels the approximate isotope composition of industrial cr vi pollutants the cr isotope compositions of hydrothermal lead chromates crocoites pbcr vi o 4 from various localities yield 53 52 cr srm 979 values between 0 640 and 1 037 these cr isotope compositions are substantially heavier than those of igneous silicate rocks from which the chromium was leached precipitation experiments revealed an isotope fractionation of 53 52 cr crocoite cr vi aq of only ca 0 10 0 05 at a temperature of 20 c thus the heavy cr isotope signature of crocoites is most likely the result of repeated redox cycling of chromium in hydrothermal processes

a study of the trace sulfide mineral assemblages in the stillwater complex montana usa the sulfide assemblages of the stillwater complex away from the well studied ore zones are composed mainly of variable proportions of pyrrhotite chalcopyrite pentlandite and pyrite excluding vein assemblages and those affected by greenschist and lower temperature alteration the majority can be classified into two broad assemblages defined here as pristine multiphase often globular in shape or volatile bearing multiphase high temperature volatile rich minerals such as biotite hornblende or an unmixed calcite dolomite assemblage the volatile bearing assemblages are mainly found within and below the j m reef where native copper and sphalerite are also locally present pristine sulfides are found throughout the stratigraphy both groups can be affected by apparent s loss in the form of pyrite being converted to magnetite and chalcopyrite to a cu fe oxide delafossite with little to no silicate alteration an upward trend from pentlandite rich to pyrrhotite rich to pyrite rich assemblages is observed in the footwall rocks in upper gn i and the same trend repeats from just below the reef and continues into the overlying n ii and gn ii modeling suggests that the sulfide ni in the peridotite zone is largely controlled by silicate ni when taken together observations are most readily explained by the remobilization of selected elements by a high temperature fluid with the apparent loss of s cu ni this could concentrate ore metals by vapor refining eventually producing a platinum group element enriched sulfide ore zone such as the j m reef

lithium isotope evidence for subduction enriched mantle in the source of mid ocean ridge basalts the process of plate tectonics continually returns material from the earth s surface to its interior a major goal of geochemists has been to see if convective flow within the earth ever allows such compositionally distinctive material to return to the surface a study of lithium isotope variations in the upper mantle now provides firm evidence for such a recycled signature in the chemical compositions of some submarine lavas that sample the convecting interior of the earth yet the recycled signature they record appears to arise not from former crustal plates themselves as was expected but from mantle material influenced by the subducting crust at convergence zones the fate of the plates themselves thus remains elusive recycled crustal materials returned from the earth s surface to the mantle by subduction have long been invoked to explain compositional heterogeneity in the upper mantle1 yet increasingly problems have been noted with this model2 3 the debate can be definitively addressed using stable isotope ratios which should only significantly vary in primitive mantle derived materials as a consequence of recycling here we present data showing a notable range in lithium isotope ratios in basalts from the east pacific rise which correlate with traditional indices of mantle heterogeneity for example 143nd 144nd ratios such co variations of stable and radiogenic isotopes in melts from a normal ridge segment provide critical evidence for the importance of recycled material in generating chemical heterogeneity in the upper mantle contrary to many models however the elevated lithium isotope ratios of the enriched east pacific rise lavas imply that subducted ocean crust is not the agent of enrichment instead we suggest that fluid modified mantle which is enriched during residency in a subduction zone is mixed back into the upper mantle to cause compositional variability