water solubility in pyrope to 100 kbar the solubility and incorporation mechanism of water in natural almost pure pyrope from dora maira western alps was investigated the infrared spectrum of the natural untreated sample 58 ppm water shows several exceptionally sharp bands in the oh stretching region including a single band at 3601 9 cm 1 and a band system with main components at 3640 5 3650 8 and 3660 6 cm 1 high temperature and high pressure infrared spectra suggest that the two absorption features arise from almost free oh groups in sites with different compressibility and thermal expansivity with the site causing the 3601 9 cm 1 band being much stiffer pyrope samples were annealed in a piston cylinder or multi anvil apparatus for several days in the presence of excess water excess sio2 and excess al2sio5 to determine the equilibrium solubility of water in pyrope to 100 kbar total solubility increases with pressure however this is exclusively due to the high frequency band system while the intensity of the low frequency band decreases with pressure at 1000 c and the oxygen fugacity of the ni nio buffer the bulk solubility can be described by the equation c oh af h2o 0 5exp p v rt with a 0 679 ppm bar0 5 and v 5 71 cm3 mol this equation implies the incorporation of water in the crystal as isolated oh groups with increasing temperature solubility appears to decrease with h 14 kj mol at fe feo buffer conditions solubility is 30 to 50 lower than with the ni nio buffer suggesting that the incorporation of oh is not coupled to the reduction of fe3 possibly the 3601 9 cm 1 band is associated with the tetrahedral oh b defect and the high frequency system with the dodecahedral oh li defect based on the experimentally established solubility model it is estimated that garnet in a hot subducted slab will transport 170 ppm of water into the mantle beyond the breakdown limit of amphibole in a cold slab 470 ppm of water can be incorporated into garnet at the breakdown limit of phengite these numbers imply that a significant fraction of the total water in the hydrosphere has been recycled into the mantle since the proterozoic

electrical conductivity of mantle clinopyroxene as a function of water content and its implication on electrical structure of uppermost mantle abstract the electrical conductivity of san carlos clinopyroxene aggregates with various water contents were measured under ni nio buffer at 1 5 gpa and 600 1200 k in a dia type apparatus the conductivity increases with increasing water content in clinopyroxene hidden conduction mechanism was detected because of the much smaller iron content in clinopyroxene which was usually covered by small polaron conduction in other nominally anhydrous minerals the identified activation enthalpies ranged from 0 70 0 75 ev to 1 23 1 37 ev our result reveals that the dominant charge carrying species in electrical conductivity could change with temperature and water content at high temperatures relevant to asthenospheric condition activation enthalpy for the conductivity agrees well with that for the hydrogen self diffusion the dominant charge carrier therefore might be m site vacancy however contrary to previous view that all hydrogens contribute to increasing conductivity equally our result shows that only a limited amount 20 40 of hydrogen acts as effective charge carrier in clinopyroxene on the other hand the activation enthalpy for the conductivity at low temperatures is significantly lower than that for the hydrogen self diffusion similar to what has been observed in olivine and orthopyroxene this type of conduction is probably caused by fast diffusion of specific hydrogen or fast hydrogen grain boundary diffusion at low temperatures the proton conduction of clinopyroxene is nearly one order and two orders of magnitude lower than those of olivine and orthopyroxene respectively and tends to converge at high temperatures using the present data combined with conductivity of olivine and orthopyroxene a laboratory based conductivity depth profile in the uppermost mantle shows that hydrous clinopyroxene cannot account for the high conductive regions observed beneath the ocean floor near eastern pacific rise the presence of partial melt would be unavoidable

hydroxide in pyroxene variations in the natural environment a suite of 51 pyroxenes from a wide range of geological environments was examined with infrared spectroscopy small amounts of oh occur in nearly all the samples suggesting that oh groups are a minor component of most pyroxenes the oh concentrations vary 0 002 0 12 wt oh as a function of geological setting with the greatest amounts occurring in mantle derived samples the intensities of certain oh absorption bands are somewhat correlated with pyroxene composition in particular with the presence of trivalent cations this suggests that crystal chemical or compositional factors also control the oh incorporation the oh incorporated in the pyroxene structure is easily distinguishable from oh owing to amphibole lamellae which usually occur in diopsides and often in orthopyroxenes oh in pyroxene is probably a function of the activity of hydrous components during crystallization but may also depend on postcrystallization changes in the geological environment

iron isotope constraints on fe cycling and mass balance in oxygenated earth oceans the fe isotope composition of proterozoic to modern clastic sedimentary rocks and aerosols defines a range ind 56 fe values that is only slightly more variable than the range of fe isotope com positions measured in terrestrial igneous rocks indicating that chemical weathering sedimentary transport and diagenesis play only a minor role in producing fe isotope variations in environ ments where fe redox conditions have been controlled by current levels of atmospheric oxygen in contrast the fe isotope composi tions of hot fluids 300 8c from mid ocean ridge mor spread ing centers define a narrow range that is shifted to lower d 56 fe values by 0 2 0 5 as compared to igneous rocks these new data allow a conceptual model for the fe isotope composition of the oxic oceans that predicts large ranges in fe isotope composition under conditions of changing aerosol and mor fe fluxes such as during periods of major worldwide glaciation

water content in natural eclogite and implication for water transport into the deep upper mantle abstract infrared spectroscopy and ion micro probe measurements showed that the major constituent minerals of eclogites from the kokchetav massif which have been subducted to 180 km depths contain significant amounts of water up to 870 ppm h 2 o by weight in omphacite 130 ppm h 2 o in garnet and 740 ppm h 2 o in rutile omphacite shows three hydroxyl absorption bands at 3440 3460 3500 3530 and 3600 3625 cm 1 garnet has a single band at 3580 3630 cm 1 and rutile has a single sharp band at 3280 cm 1 the hydroxyl absorbance at these wavenumbers changes with the crystal orientation in polarized infrared radiation indicating that the water is structurally incorporated in these minerals the water contents in omphacite and garnet increase systematically with the metamorphic pressure of the host eclogites the partitioning coefficient of the water content between coexisting garnet and omphacite is similar in different eclogites d grt omp 0 1 0 2 but decreases slightly at high pressure based on the mineral proportions of the eclogites we estimate bulk rock water content in the eclogites ranging from 3070 to 300 ppm h 2 o by weight although hydrous minerals are absent in the diamond grade eclogite 60 kbar and 1000 c trace amounts of water are incorporated in the nominally anhydrous minerals such as omphacite and garnet the presence of significant water in these minerals implies that the subducting oceanic crust can transport considerable amounts of water into the deep upper mantle beyond the stability of hydrous minerals such water may be stored in the deep upper mantle and have an important influence on dynamics in the earth s interior

thermobarometric constraints on burial and exhumation of 2 billion year old eclogites and their metapelitic hosts abstract one of the first appearances of eclogite facies mineral assemblages in the geological record occurs in the c 2000 ma palaeoproterozoic usagaran belt in central tanzania where the extended margin of the tanzanian craton is interpreted to have been subducted mafic rocks are interpreted to have contained the mineral assemblage garnet omphacite rutile quartz amphibole this high pressure assemblage has been overprinted by a secondary mineral assemblage containing clinopyroxene plagioclase hornblende ilmenite orthopyroxene mineral equilibria forward modelling indicates that the eclogite facies assemblages reached minimum peak pressure temperature p t conditions of 17 kbar and 700 c inclusions in garnet document a prograde p t history consistent with burial through upper amphibolite facies conditions and possible partial melting petrological and compositional evidence from garnet suggests that following peak metamorphism the eclogite facies rocks were heated while stalled at approximate peak pressures temperature estimates derived from zr concentrations in interpreted texturally retrograde rutile support a near isothermal post peak p t evolution for the eclogite facies rocks an evolution that terminates at retrograde p t conditions of approximately 7 6 8 2 kbar and 680 790 c the relict eclogite domains form part of a larger assemblage with enclosing migmatitic metapelitic lithologies the isimani suite the metapelitic gneisses contain garnet kyanite biotite staurolite hornblende plagioclase muscovite rutile quartz and preserve minimal evidence of a high pressure history conceivably due to post peak mineralogical recrystallisation p t modelling inclusion assemblages and compositional zonation patterns in porphyroblastic garnet suggests the metapelitic gneisses similarly to the relict eclogites experienced burial to minimum peak pressures of approximately 16 5 17 kbar compositional zoning patterns in eclogitic garnet suggest the isimani system was buried reached peak metamorphic conditions and was subsequently exhumed within a timeframe of up to 20 myr a tectonic regime involving crustal thickening and subduction followed by extensional exhumation of the entire isimani suite is our preferred model for the development of the c 2000 ma usagaran belt

very high pressure orogenic garnet peridotites mantle derived garnet peridotites are a minor component in many very high pressure metamorphic terranes that formed during continental subduction and collision some of these mantle rocks contain trace amounts of zircon and micrometer sized inclusions the constituent minerals exhibit pre and postsubduction microstructures including polymorphic transformation and mineral exsolution experimental mineralogical petrochemical and geochronological characterizations using novel techniques with high spatial temporal and energy resolutions are resulting in unexpected discoveries of new phases providing better constraints on deep mantle processes

experimental deformation of synthetic wet jadeite aggregates 1 predictions on interplate coupling and shear heating in the deeper levels of subduction zones require an understanding of the rheology of eclogite the strength of eclogite is probably limited by the flow strength of its major constituent omphacite representing a solid solution between the clinopyroxene end members diopside and jadeite here we report the results of deformation experiments on the end member jadeite naalsi2o6 carried out on fine grained synthetic aggregates crystallized in the stability field of jadeite from a synthetic glass precursor a standard procedure was established to yield samples with a uniform microstructure and an average grain size of approximately 10 m the presence of micropores and the detection of small amounts of freezable water by fourier transform infrared spectroscopy ftir in both undeformed and deformed samples indicates the presence of a free aqueous fluid hence wet conditions deformation experiments were carried out in a modified griggs type apparatus at a confining pressure of 2 5 gpa temperatures between 800 c and 1100 c and at strain rates of 4 10 4 s 1 to 2 10 6 s 1 a molten eutectic cscl nacl mixture was used as pressure medium the microfabrics of the deformed samples and the mechanical data indicate deformation in the dislocation creep regime the mechanical data are fit by a power law using a global inversion method yielding an activation energy of q 326 27 kj mol 1 a stress exponent n 3 7 0 4 and a preexponential factor of ln a 3 3 2 0 extrapolation of this flow law for synthetic jadeitite to low geological strain rates and comparison with available flow laws for diopside indicates jadeite to be significantly weaker than diopside in the dislocation creep regime

silica precipitates in omphacite from eclogite at alpe arami switzerland evidence of deep subduction observations of oriented sio2 precipitates in omphacite from eclogite with tholeiitic basalt protolith bordering the alpe arami garnet peridotite massif ticino switzerland and petrological studies of the eclogitic mineral assemblages suggest that this rock was subjected to higher pressure metamorphism than previously realized we employed various calibrations of the fe2 mg exchange thermometer and calculations of equilibria with thermodynamic data considering the calcium tschermak s component caal2sio6 of garnet pyroxene pairs from these calculations it is concluded that the eclogitic lenses have recorded at least four stages of mineral growth corresponding to the following stage i prograde c 2 4 gpa 700 c stage iia maximum recorded grade c 7 0 gpa 1100 c stage iib retrograde c 3 7 gpa 900 c stage iii retrograde c 2 1 gpa 750 c because of the preservation of stage i a relatively rapid subduction and exhumation of alpe arami eclogite is suggested the exhumation path of the eclogitic rock is in good agreement with most exhumation paths inferred for the alpe arami garnet lherzolite proposed previously by several authors based upon a variety of different observations although the eclogite and peridotite exhumation paths may diverge at depths greater than 120 km

an experimental investigation of hydroxyl solubility in jadeite and na rich clinopyroxenes the solubility and incorporation mechanisms of water in synthetic water saturated jadeite and na rich clinopyroxenes have been experimentally investigated infrared spectra for water saturated jadeite synthesised from 2 0 to 10 gpa show two prominent sharp peaks at 3 373 and 3 613 cm 1 together with several weaker features in the oh stretching region indicating that there are at least 5 distinct modes of hydrogen incorporation in the structure water solubility in pure jadeite reaches a maximum of about 450 ppm by weight at 2 gpa and slowly decreases with increasing pressure to about 100 ppm at 10 gpa solubility can be described by the function coh a fh2o0 5 exp p vsolid rt where coh is water solubility in ppm h2o by weight a is 7 144 ppm bar0 5 fh2o is water fugacity and vsolid 8 019 cm3 mol is the volume change of the clinopyroxene upon incorporation of oh jadeite provides a good model for understanding hydrogen incorporation mechanisms in more complex omphacite compositions assignment of absorption bands in ir spectra verifies the importance of cation vacancies on the m2 site in providing mechanisms for hydrogen incorporation however results also suggest that substitution of lower valency cations onto the m1 site may also be important solid solution of jadeite with diopside and in particular with ca eskola component leads to a drastic increase of water solubility and the bulk composition has a more important effect on the capacity of omphacite to store water than pressure and temperature omphacite is expected to be the major carrier of water in a subducted eclogite after the breakdown of hydrous minerals

oriented quartz calcic amphibole inclusions in omphacite from the saualpe and pohorje mountain eclogites eastern alps an assessment of possible formation mechanisms based on ir and mineral chemical data and water storage in eastern alpine eclogites abstract the composition of mineral phases and their modal proportions have been determined for three representative eoalpine eclogites from the saualpe type locality eastern austria sample skp31 and the pohorje massif slovenia cm31 03 and cm15 01 using electron microprobe laser icp ms ir spectroscopy and modal analysis to evaluate possible mechanisms for the formation of composite oriented calcic amphibole quartz inclusions cois in omphacite and to assess the relative importance of hydrous and nominally anhydrous phases as h 2 o carriers in these eclogites for omphacites in cm31 03 with a zonal distribution of cois a comparison of water and trace element concentrations of areas containing cois and those free of cois and a comparison with the trace element concentration of calcic amphibole indicate that cois have formed through an open system alteration of clinopyroxene and not through a closed system exsolution process in sample skp31 both textural and mineral chemical evidence suggests that cois did not form by exsolution involving a ca eskola component in clinopyroxene but formed by progressive growth under eclogite facies p t conditions and prior to the onset of retrogressive symplectite formation analogous to the formation of poikiloblastic quartz calcic amphibole grains in the matrix bulk h 2 o contents of the eclogites are between ca 750 and 2150 ppm with 6 25 of the total water contributed by nominally anhydrous minerals nams because of high modal amounts of 37 65 omphacite is the major nominally anhydrous water carrier containing 145 580 ppm h 2 o with significant concentration variations on a thin section scale due to their very low h 2 o concentrations of 2 o content calcic amphibole forming part of cois may be a major carrier of h 2 o as evidenced by cm31 03 containing both cois and texturally primary calcic amphibole in this sample calcic amphibole of the cois contributes 63 whereas primary calcic amphibole only accounts for 13 of the bulk water the relative order of h 2 o concentrations in nams is h 2 o rutile h 2 o omphacite h 2 o garnet for cm31 03 and cm15 01 and h 2 o rutile h 2 o omphacite h 2 o garnet in skp31

pressure and temperature dependence of water solubility in fe free wadsleyite the effects of temperature and pressure on water solubility in wadsleyite in the system mgo sio 2 h 2 o were investigated experiments were carried out using a 1000 ton multi anvil press one series of experiments was performed at a fixed pressure of 15 gpa and at various temperatures and in a second series the temperature was fixed at 1200 c and pressure was varied from 13 to 18 gpa the starting material was a mixture of oxides and hydroxides equivalent to the composition mg 2 sio 4 5 wt h 2 o run products consisted of wadsleyite quenched hydrous melt and minor amounts of clinoenstatite the water content of wadsleyite was quantified by ion probe results show that at 15 gpa the water solubility in wadsleyite decreases significantly with increasing temperature from 2 2 wt h 2 o at 900 c down to 0 9 wt h 2 o at 1400 c the corresponding mg si ratios increase from 1 80 to 1 91 over this temperature range this effect appears to be largely due to changes in the water activity in the coexisting melt the partition coefficient of water between wadsleyite and coexisting melt is nearly independent of temperature with d w a t e r w a d s l e y i t e m e l t 0 08 no significant effect of pressure on water solubility was observed at 1200 c our data suggest that the water storage capacity of wadsleyite in the transition zone is much lower than previously suggested together with previous results on ringwoodite our data imply a strong decrease of the water partition coefficient between wadsleyite and ringwoodite with temperature this decrease could have two important consequences 1 the width of the 520 km discontinuity may vary strongly as a function of temperature 2 during cooling of the earth s mantle since the hadean water may have increasingly partitioned from the lower into the upper part of the transition zone

reply to comment by c miller and j konzett on first evidence for ultrahigh pressure metamorphism of eclogites in pohorje slovenia tracing deep continental subduction in the eastern alps received 27 june 2005 revised 26 september 2005 accepted 19 october 2005 published 9 december 2005 citation jana k m n froitzheim m vrabec and e j kroghravna 2005 reply to comment by c miller and j konzett on first evidence for ultrahigh pressure metamorphism of eclogitesin pohorje slovenia tracing deep continental subductionin the eastern alps tectonics 24 tc6011 doi 10 1029 2005tc001875

crystal structure refinement of na bearing clinopyroxenes from mantle derived eclogite xenoliths nine omphacitic clinopyroxenes with a jadeitic content between 25 and 57 from the udachnaya and zagadochnaya diamond bearing kimberlites were investigated using single crystal x ray diffraction electron microprobe analysis and mossbauer and infrared spectroscopy crystal structure refinements incorporating constraints from chemical analyses and mossbauer data show a significant concentration of vacancies at the m2 site in three of the samples the cell volumes of the non stoichiometric samples plot below the linear trend defined by stoichiometric diopside jadeite compositions where the deviation from the linear trend is positively correlated with the concentration of vacancies charge balance appears to be achieved primarily through substitution of al3 for mg2 on the m1 site which causes a contraction of the m1 site volume compared to stoichiometric clinopyroxenes with the same composition vacancies are not associated with oh incorporation as the sample with the highest concentration of vacancies contains only 89 ppm wt h2o significant hydrogen loss through iron oxidation during ascent is ruled out based on the low fe3 concentrations h content in omphacite samples from kimberlites could provide a sensitive tool for determining f h2o during diamond formation

the high pressure stability of mg sursassite in a model hydrous peridotite a possible mechanism for the deep subduction of significant volumes of h2o the stability of the high pressure phase mg sursassite previously mgmgal pumpellyite in ultramafic compositions has been determined in experiments in the system mgo al2o3 sio2 h2o mash the breakdown of mg sursassite forsterite enstatite to pyrope vapour with increasing temperature was bracketed at 6 0 and 7 0 gpa below 6 0 gpa mg sursassite forsterite vapour reacts to chlorite enstatite this reaction provides a mechanism for transfer of water from chlorite to mg sursassite bearing assemblages at pressures of 7 0 gpa and above the assemblage mg sursassite phase a enstatite was found phase relations involving mg sursassite and phase a are considered for bulk compositions with a low water content the vapour absent reaction mg sursassite forsterite pyrope phase a enstatite determines the upper pressure stability of mg sursassite and provides a mechanism for the complete transfer of water from mg sursassite to phase a bearing assemblages mg sursassite plays an important role in peridotite compositions in the subducting slab because at temperatures below 700 c it can transfer water from hydrous phases such as antigorite and chlorite to high pressure stable phases such as phase a

isotopic composition of disseminated sulfur in precambrian sedimentary rocks abstract isotopic analyses of trace sulfur in precambrian carbonates and clastic sedimentary rocks support the results of previous studies which have shown that 34s measurements of archean sedimentary sulfur cluster mostly around mantle like values and that a significant overall dispersion of 34 s 32 s ratios commenced at about 2 7 ga ago the outstanding feature of the proterozoic record is the predominance of heavy 34ssulfide values mostly in excess of 0 cdm consideration of these data in combination with the sparse 534s record for proterozoic sulfates suggests that 34ssulfide sulfide may have been smaller than was the case during the phanerozoic a possible scenario that is compatible with the above data is based on a proposition that the terrestrial exogenic sulfur cycle evolved in a tripartite manner in the archean it was coupled to and controlled by the magmatic endogenic cycle the proterozoic for the most part represents a transitional stage into a phanerozoic like exogenic cycle the latter essentially decoupled from its endogenic counterpart and its redox balance maintained and mediated by the biosphere the indication that the mean combined 34s of proterozoic sulfate and sulfide may have significantly exceeded the phanerozoic mean value of 2 cdm is enigmatic and poses problems for balance considerations

unusual exsolution phenomena in omphacite and partial replacement of phengite by phlogopite kyanite in an eclogite from the erzgebirge in the erzgebirge crystalline complex eclogites occur in three different high pressure hp units 1 2 and 3 recording contrasting pressure p temperature t conditions eclogites from hp unit 1 experienced peak metamorphic conditions in the coesite stability field at about 33 kbar 850 c commonly these eclogites from hp unit 1 are all very similar with an eclogitic peak assemblage of omphacite garnet coesite k feldspar rarely accompanied by kyanite and omphacites systematically deviating from a stoichiometric composition in contrast an eclogite recently found near blumenau is mineralogically and geochemically different from the typical eclogites of hp unit 1 this unusual eclogite reveals the eclogitic equilibrium assemblage omphacite garnet coesite phengite phlogopite kyanite and yields metamorphic peak conditions of 870 c and 29 kbar there is clear textural evidence of the formation of phlogopite and kyanite under partial consumption of phengite and garnet moreover the omphacite is stoichiometric and contains abundant exsolution lamellae the thickest of which were identified as quartz by the electron microprobe the finer lamellae were studied by transmission electron microscopy tem oligoclase was identified as an exsolution phase other lamellae proved to consist of k white mica also interpreted as exsolution prior to exsolution the omphacite composition must have been cation deficient as that of the other common hp unit 1 eclogites these non stoichiometric compositions are ascribed to partial substitution by the ca eskola pyroxene component which calculates to an average of 8 mol for omphacite in hp unit 1 eclogites according to experiments this substitution becomes significant at p 30 kbar exsolution of k white mica may indicate hydroxyl defects in the original omphacite also favoured by high pressure oligoclase and k white mica exsolution from ca eskola rich clinopyroxene has not previously been reported the omphacite has a disordered c2 c structure and in just one case very small a few tens of nanometres antiphase domains resulting from the c2 c to p2 n transformation are present these features may indicate a brief thermal history and rapid tectonic processes

stable isotope characterization of the thermal profile and subsurface biological activity during oxidation of the great australia deposit cloncurry queensland australia the application of stable isotope thermometry and phase relations was used to examine the thermal evolution of oxidation and secondary mineralization at the great australia cu au co deposit cloncurry queensland australia results show that early secondary mineralization consisted of djurleite crystallization at temperatures 93 c followed by calcite mineralization at 66 to 75 c the secondary mineralization paragenesis continued with azurite that formed at about 51 c followed by malachite crystallization at 41 to 38 c and malachite after azurite at 34 c the final stage of secondary mineralization consists of calcite that formed at 25 to 30 c followed by goethite the general trend observed is for early oxidation and supergene enrichment at apparent temperatures less than 93 c and between 66 and 75 c cooling to ambient surface temperatures late in the period of mineralization a correlation is observed between decreasing isotope thermometry temperatures and increasing 13c values as oxidation and supergene enrichment progressed this is interpreted to result from exothermic sulfide oxidation and associated subsurface thermophilic bacterial oxidation within the oxidation front over the period of oxidation and supergene enrichment temperatures decreased and 13c values increased recording in the secondary minerals the passing of the oxidation front and a diminishing subsurface bacterial carbon source

quartz exsolution in clinopyroxene is not proof of ultrahigh pressures evidence from eclogites from the eastern blue ridge southern appalachians u s a oriented quartz needles in clinopyroxene have become one of the diagnostic indicators of ultrahigh pressure uhp metamorphism the presence of apparently exsolved quartz is taken as evidence of decompression of a non stochiometric ca eskola component ca0 5 0 5alsi2o6 caes that is presumed to be stable only at uhp conditions eclogite from the eastern blue ridge north carolina contains clinopyroxene jd20cats5ac5caesdi65hd5 with oriented needles of quartz and calcic amphibole that appear to have exsolved together the quartz amphibole intergrowths are surrounded by 1 5 m haloes of neoformed pyroxene jd10cats10ac5caesdi70hd5 the modes of quartz amphibole and clinopyroxene haloes were determined using bse images and reintegrated with the host clinopyroxene viewing the quartz and amphibole needles down the c axis of the pyroxene host provides a better estimate of their proportions than in prismatic sections reintegrated pyroxene compositions were nearly identical to the analyzed host pyroxene with no caes component clinopyroxene with caes solid solution has been repeatedly synthesized at uhp conditions however examination of the phase equilibria usually cited as evidence for caes stability at conditions of 25 kbar shows that clinopyroxene with 10 mol caes is stable well within the quartz field and provides a pressure minimum similar to the albite jadeite quartz barometer exsolution of quartz and associated amphibole is commonplace in clinopyroxene from the blue ridge eclogite that lacks coesite or other evidence for uhp metamorphism the presence of a diluted 5 10 caes component in clinopyroxene does not require uhp conditions

water contents of roberts victor xenolithic eclogites primary and metasomatic controls a suite of eclogites from the roberts victor kimberlite has been extensively characterized in terms of petrology and geochemical compositions greau et al in geochim cosmochim acta 75 22 6927 6954 2011 huang et al in lithos 142 143 161 181 2012a in the present study the water contents of eclogitic garnet and omphacite were analyzed by fourier transform infrared spectrometry garnet does not contain measureable oh in any sample the water content of omphacite in the studied eclogites ranges from 211 to 1 496 ppm mantle metasomatism has modified the water content of some of the eclogites while others retain water contents characteristic of their original environment the oh contents of the metasomatized eclogites may be mainly controlled by the h2o fugacity and mineral compositions the oh contents of the non metasomatized samples are interpreted to be more sensitive to their mantle equilibration temperature pressure and the local fugacities of h2o and o2 the calculated water content of the metasomatic medium is similar to that of carbonatitic kimberlitic melts fluids eclogites contain more water than peridotites recorded in the literature 341 161 vs 122 54 ppm and represent an important water reservoir in the lithospheric mantle wherever they occur this is an important parameter to be considered in the interpretation of mantle processes and geophysical data such as seismic wave speeds and electrical conductivity and in geodynamic modeling

direct dating of stromatolitic carbonates from the schmidtsdrif formation transvaal dolomite south africa with implications on the age of the ventersdorp supergroup pb isotopic analyses of stromatolitic limestones from the schmidtsdrif formation transvaal dolomite of south africa give a late archean isochron age of 2 557 plus minus 49 2 sigma ma the new date significantly changes the position of the archean proterozoic boundary in the kaapvaal province and suggests that a possible division of the transvaal supergroup is necessary it also resolves the long standing controversy about the age of the ventersdorp supergroup whose zircon u pb data of ca 2 7 ga must be more acceptable than the rb sr dates of about 2 3 ga the study provides a new example that demonstrates the great potential of employing the u pb systems of carbonate rocks for direct dating of sedimentary sequences

h2o contents and d h ratios of nominally anhydrous minerals from ultrahigh pressure eclogites of the dabie orogen eastern china abstract garnet and omphacite from ultrahigh pressure eclogites from the dabie orogen eastern china were investigated by micro ftir the results show that all garnet and omphacite grains contain structural water occurring as hydroxyl oh with h2o contents varying from 14 to 1915 ppm h2o wt and from 105 to 695 ppm respectively within the same sample the water contents are either homogeneous at the grain scale or vary systematically from higher in the core to lower in the rim low water contents at crystal rims possibly result from hydroxyl exsolution after pressure decrease upon rock exhumation the d values of omphacites are between 108 4 and 114 2 and independent of water contents heterogeneous water contents of garnet occur at the centimeter scale and fluid mobility during uhp metamorphism was very limited the estimated whole rock water content based on mineral h2o contents is between 260 and 750 ppm thereby implying that eclogitic rocks formed during continental subduction have the potential to recycle at least several hundreds ppm water to mantle depths the preserved chemical differences likely indicate that the eclogitic rocks resided at mantle conditions for a limited time span and imply that they were exhumed shortly after subduction the water released during decompression might represent the early stage retrograde fluid

biogeochemical cycling of iron in the archean paleoproterozoic earth constraints from iron isotope variations in sedimentary rocks from the kaapvaal and pilbara cratons abstract iron isotope compositions of low metamorphic grade samples of archean paleoproterozoic sedimentary rocks obtained from fresh drill core from the kaapvaal craton in south africa and from the pilbara craton in australia vary by 3 in 56 fe 54 fe ratios reflecting a variety of weathering and diagenetic processes depositional ages for the 120 samples studied range from 3 3 to 2 2 ga and fe c and s contents define several compositional groups including samples rich in fe organic carbon carbonate and sulfide the 56 fe values for low c org low c carb and low s sedimentary rocks are close to 0 the average of igneous rocks this range is essentially the same as that of c org poor late cenozoic loess aerosol river loads and marine sediments and those of c org poor phanerozoic proterozoic shales that these 56 fe values are the same as those of igneous rocks suggests that fe has behaved conservatively in bulk sediments during sedimentary transport diagenesis and lithification since the archean these observations indicate that if atmospheric o 2 contents rose dramatically between 2 4 and 2 2 ga as proposed by many workers such a rise did not produce a significant change in the bulk fe budget of the terrestrial sedimentary system if the archean atmosphere was anoxic and fe was lost from bedrock during soil formation any isotopic fractionation between aqueous ferrous fe fe aq 2 and fe bearing minerals must have been negligible in contrast if the archean atmosphere was oxic fe would have been retained as fe 3 hydroxides during weathering as it is today which would produce minimal net isotopic fractionation in bulk detrital sediments siderite rich samples have 56 fe values of 0 5 0 5 and experimentally determined fe aq 2 siderite fractionation factors suggest that these rocks formed from fe aq 2 that had similar or slightly higher 56 fe values the 56 fe values calculated for fe aq 2 overlaps those of modern submarine hydrothermal fluids but it is also possible that fe aq 2 had 56 fe values higher than those of modern hydrothermal fluids depending upon the fe aq 2 fe carbonate fractionation factor that is used in contrast c org rich samples and magnetite rich samples have strongly negative 56 fe values generally between 2 3 and 1 0 and available fluid mineral fractionation factors suggest that the fe bearing minerals siderite and magnetite in these rocks formed in the presence of fe aq 2 that had very low 56 fe values between 3 and 1 reduction of fe 3 hydroxide by sulfide precipitation of sulfide minerals or incongruent dissolution of silicate minerals are considered unlikely means to produce significant quantities of low 56 fe fe aq 2 we interpret microbial dissimilatory fe 3 reduction dir as the best explanation for producing such low 56 fe values for fe aq 2 and our results suggest that dir was a significant form of respiration since at least 2 9 ga

metamorphic evolution of diamond bearing and associated rocks from the kokchetav massif northern kazakhstan representative diamond bearing gneisses and dolomitic marble eclogite and ti clinohumite bearing garnet peridotite from unit i at kumdy kol and whiteschist from unit ii at kulet eastern kokchetav massif northern kazakhstan were studied diamond bearing gneisses contain variable assemblages including grt bt qtz pl kfs zo chl tur cal and minor ap rt and zrn abundant inclusions of diamond graphite chlorite or calcite phengite clinopyroxene k feldspar biotite rutile titanite calcite and zircon occur in garnet diamond bearing dolomitic marbles consist of dol di grt phl inclusions of diamond dolomite graphite biotite and clinopyroxene were identified in garnet whiteschists carry the assemblage ky tlc grt rt garnet shows compositional zoning and contains abundant inclusions of talc kyanite and rutile with minor phlogopite chlorite margarite and zoisite inclusions and zoning patterns of garnet delineate the prograde p t path inclusions of quartz pseudomorphs after coesite were identified in garnet from both eclogite and gneiss other ultrahigh pressure uhp indicators include na bearing garnet up to 0 14 wt na2o with omphacitic cpx in eclogite occurrence of high k diopside up to 1 56 wt k2o and phlogopite in diamond bearing dolomitic marble and cr bearing kyanite in whiteschist these uhp rocks exhibit at least three stages of metamorphic recrystallization the fe mg partitioning between clinopyroxene and garnet yields a peak temperature of 800 1000 c at p 40 kbar for diamond bearing rocks and about 740 780 c at 28 35 kbar for eclogite whiteschist and ti bearing garnet peridotite the formation of symplectitic plagioclase amphibole after clinopyroxene and replacement of garnet by biotite amphibole or plagioclase mark retrograde amphibolite facies recrystallization at 650 680 c and pressure less than about 10 kbar the exsolution of calcite from dolomite and development of matrix chlorite and actinolite imply an even lower grade greenschist facies overprint at c 420 c and 2 3 kbar a clockwise p t path suggests that supracrustal sediments together with basaltic and ultramafic lenses apparently were subjected to uhp subduction zone metamorphism within the diamond stability field tectonic mixing may have occurred prior to uhp metamorphism at mantle depths during subsequent exhumation and juxtaposition of many other tectonic units intense deformation chaotically mixed and mylonitized these lithotectonic assemblages

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

mineral oxygen isotope and hydroxyl content changes in ultrahigh pressure eclogite gneiss contacts from chinese continental scientific drilling project cores a combined study of mineral o isotopes and hydroxyl contents was carried out for the contacts between ultrahigh pressure eclogite and gneiss from main hole of the chinese continental scientific drilling project in the sulu orogen while there is a large d 18 o variation from 8 3 to 7 3 for all minerals different styles of mineral pair fractionation occur at the boundaries of different lithologies both equilibrium and disequilibrium o isotope fractionations are observed between quartz and the other minerals with reversed fractionations between omphacite and garnet in some samples of eclogite this suggests that both eclogite and gneiss acquired their negative d 18 o values by meteoric hydrothermal alteration of their protoliths at high temperatures before subduction and that fluid assisted o isotope exchange did take place across the boundary of different lithologies at local scales during amphibolite facies retrogression fourier transform infrared spectroscopy analysis yielded h2o concentrations of 50 to 1144 p p m by weight for garnet and 139 to 751 p p m for omphacite the state of equilibrium or disequilibrium o isotope fractionations between omphacite and garnet are correlated with variations in their water content at local scales indicating that the internally derived fluid plays a critical role in retrograde metamorphism during exhumation the retrograde metamorphism results in mineral reactions and o isotope disequilibria between some of the minerals but the fluid for retrogression was derived from the decompression exsolution of structural hydroxyl and thus internally buffered in the o isotope composition a quantitative estimate suggests that a hand specimen 3 6 9 cm of eclogite composed of 70 garnet and 30 omphacite can release 0 316 g water by the decompression exsolution of structural hydroxyl which can form 14 4 g amphibole during exhumation this provides sufficient amounts of water for the amphibolite facies retrogression

early archean foredeep sedimentation related to crustal shortening a reinterpretation of the barberton sequence southern africa abstract the upper dominantly terrigenous sedimentary successions of the early archean barberton sequence kaapvaal province record an evolution similar to that of phanerozoic foredeeps and may represent the oldest known foredeep sequence rather than a passive continental margin sequence as previously proposed pre foredeep ocean crust the 3 5 ga old onverwacht group consists of bimodal volcanics with intercalated shallow water sediments gravity modeling and boundary shear zones indicate that deformed onverwacht crust was obducted northward onto sialic continental crust immediately before during or after burial by the synorogenic terrigenous clastic fill foredeep basin deepening due to crustal loading from the south is recorded by basinal iron formation chert and mudstone of the basal fig tree group which accumulated paraconformably on the onverwacht overlying sediments of the fig tree group consist of 2 km of submarine fan greywackes and mudstones shoaling of the basin is indicated by the transition to the overlying braided alluvial and shallow marine sediments of the ca 3 3 ga old moodies group 3 km thick changing ree patterns and relative abundances of ni cr sr th and ur in the sediments of the fig tree and moodies groups indicate a significant ultramafic mafic volcanic component diluted upward in the stratigraphic sequence by an increasing granitic contribution a similar trend is reflected in sandstone petrography in which the ratio of extrusive to intrusive rock fragments decreases upwards in the fig tree group whereas the moodies group sediments are enriched in quartz and potassium feldspar a persistent chert component in both units suggests that the tectonic setting of the basin is analogous to that of phanerozoic recycled orogens the distribution of facies in the fig tree and moodies groups and paleocurrent petrographic and geochemical data indicate progressive unroofing of a southerly source terrane consisting of the onverwacht volcanics and intercalated sediments and the 3 5 3 2 ga old ancient gneiss complex agc in addition older components of the fig tree group were recycled into overlying stratigraphie units the agc consists of mantle derived 3 5 ga old bimodal metavolcanics and metasediments a tonalite gneiss batholith metabasite dikes and meta anorthosites its structures record progressive northwestward uplift beginning around 3 4 ga ago intense homogeneous ductile strains were overprinted by northwest verging major folds rotated by large scale northwestward simple shear followed by imbricate faulting and pseudotachylite formation by 3 3 ga ago the fig tree and moodies groups had also shortened with similar kinematics to those of the agc by partly syndepositional folding and volume loss during the formation of transecting cleavage further folding and slide formation followed during subsolidus diapiric rise of the initially tabular kaap valley tonalite by 3 2 ga and emplacement of trondhjemite plutons from 3 3 to 3 0 ga ago

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

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

dehydroxylation of omphacite of eclogite from the dabie sulu dehydroxylation of omphacites in eclogites from uhp dabie sulu terrain has been investigated using a combined method of petrological observations mineral chemistry heat treatment mossbauer and micro ftir spectroscopy progressive heating of omphacite in air up to 1100 c results in complex multi stage processes involving oxidation and dehydrogenation at different temperatures during heating fe2 at the m1 site is oxidized to fe3 accompanied by a decrease in the intensity of the 3460 cm 1 oh band this reaction can be described as fe2 m1 oh fe3 m1 o2 o3 1 2h2 and most likely represents loss of h bonded to o1 adjacent to the m1 site at higher temperatures continued oxidation results in further dehydroxylation due to h loss from oh bands 3530 cm 1 and 3630 cm 1 these bands represent h defects with higher thermal stability slower diffusivity in this case oxidation of fe2 provides a means for charge balancing h loss over the whole crystal structure rather than locally oh bands at 3530 and 3630 cm 1 are consistent with h incorporation onto o2 with vibration of oh dipoles towards adjacent tetrahedral vacancies rapid loss of h during annealing at even low temperatures implies that significant h loss from eclogites occurs during exhumation furthermore changes in relative oh band intensity are expected during exhumation due to different thermal behaviour of h defects in omphacite as such the ratio of oh bands in omphacite spectra could be used to deduce exhumation conditions

water transport by subduction clues from garnet of erzgebirge uhp eclogite a key question concerning the water budget of earth s mantle is how much water is actually recycled into the mantle by the subduction of eclogitized oceanic crust hydrous phases are stable only in quartz eclogite not coesite eclogite so that water transport to greater depths is mainly governed by structural water in omphacite and garnet here we explore if garnet can be used as a proxy to assess the amount of this water available data on the water contents of garnet in coesite eclogite vary over orders of magnitude from a few up to ca 2000 ppm by implication the maximum bulk rock water contents are unrealistically high wt level new data from the erzgebirge indicate moderate amounts of structural h 2 o stored in garnet 43 84 ppm omphacite 400 820 ppm and in the bulk coesite eclogite ca 280 460 ppm higher garnet water contents occur but these are not primary features they are related to molecular water in fluid inclusions that can be attributed to eclogite facies fluid influx postdating the metamorphic peak fluid influx also caused the uptake of additional structural water in garnet domains close to fluid inclusions such secondary h 2 o incorporation is only possible in the case of primary water deficiency indicating that garnet hosted less water than it was able to store this is insofar astonishing as comparably high h 2 o amounts are liberated by the breakdown of prograde eclogite facies hydrous minerals as a result of ultrahigh pressure uhp metamorphism judging from erzgebirge quartz eclogite dehydration of 5 10 hydrous minerals equal portions of zoisite calcic amphibole produces 1500 3000 ppm water we infer that the largest part of the liberated water escaped probably due to kinetic reasons and hydrated exhuming uhp slices in the hanging wall depending on the physical conditions water influx in eclogite during exhumation 1 produces fluid inclusions and simultaneously enhances the structural water content of nominally anhydrous minerals as in the erzgebirge and or 2 it may give rise to retrograde hydrous minerals we conclude that eclogite transports moderate quantities of water several hundred parts per million to mantle depths beyond 100 km an amount equivalent to that in ca 1 calcic amphibole

submicron sized fluid inclusions and distribution of hydrous components in jadeite quartz and symplectite forming minerals from uhp jadeite quartzite in the dabie mountains china tem and ftir investigation abstract fluid inclusions and clusters of water molecules at nanometer to submicron scale in size have been investigated using transmission electron microscopy tem and fourier transform infrared spectroscopy ftir in jadeite quartz and symplectite aegirine augite albite taramite and magnetite corona minerals from ultrahigh pressure uhp jadeite quartzite at shuanghe the dabie mountains china fluid inclusions from 0 003 m to 0 78 m in size occur in jadeite and quartz crystals and a small number of fluid inclusions from 0 001 m to 0 25 m have also been detected in symplectite forming minerals most of the fluid inclusions have round or negative crystal morphology and contain aqueous fluids but some contain co 2 rich fluids they are usually connected to dislocations undetectable at an optical scale the dislocations represent favorable paths for fluid leakage accounting for non decrepitation of most fluid inclusions when external pressure decreased at later stages although there was partial decrepitation of some fluid inclusions unconnected to defect microstructures resulting from internal overpressure non decrepitation and partial decrepitation of fluid inclusions resulted in changes of original composition and or density it is clear that identification of hidden re equilibration features has significant implications for the petrological interpretation of post peak metamorphic processes micro ftir results show that all jadeite and quartz samples contain structural water occurring as hydroxyl ions oh and free water h 2 o in the form of clusters of water molecules the h 2 o transformed from oh during exhumation and could have triggered and enhanced early retrograde metamorphism of the host rocks and facilitated plastic deformation of jadeite and quartz grains by dislocation movement and thus the h 2 o released during decompression might represent early stage retrograde metamorphic fluid the nominally anhydrous mineral nam jadeite is able to transport aqueous fluids in concentrations of at least several hundred ppm water along a subduction zone to mantle depths in the form of clusters of water molecules and hydroxyl ions within crystals

hydroxyl in clinopyroxene from the deep subducted crust evidence for h2o transport into the mantle clinopyroxenes in eclogites from the deep subducted crust in the kokchetav kazakhstan ultrahigh pressure metamorphic terrane were found by infrared spectroscopy to contain up to 3020 ppm oh these rocks were recrystallized at pressures over 60 kbar and temperatures over 1000 c where no hydrous minerals are stable infrared spectra of the clinopyroxenes show three hydroxyl absorption bands in the regions 3440 3460 cm 1 3500 3530 cm 1 and 3600 3625 cm 1 the hydroxyl absorbance increases with recrystallization pressure of the eclogites and with vacancy concentration in the pyroxene structure ca eskola component clinopyroxenes represent about 40 50 vol of eclogites such that the bulk eclogites contain approximately 1300 ppm hydroxyl at depths greater than 150 km thus subducted oceanic crust can transport trace amounts of h2o into the upper mantle even in the absence of nominally hydrous minerals

pressure induced phase transitions in coesite high pressure behavior of coesite was studied on single crystals using diamond anvil cells with neon as the pressure transmitting medium by means of in situ raman spectroscopy up to pressures of 51 gpa the experimental observations were complemented with theoretical computations of the raman spectra under similar pressure conditions we find that coesite undergoes two phase transitions and does not become amorphous at least up to 51 gpa the first phase transition coesite i to coesite ii is reversible and occurs around 23 gpa the second transition coesite ii to coesite iii at about 35 gpa is also reversible but involves a large hysteresis samples recovered from the highest pressure achieved 51 gpa show raman spectra of the initial coesite the ab initio calculations gave insight into the initiation mechanism of the first phase transition implying from the analysis of unstable phonon modes that it is probably a displacive phase transition due to shearing of the four membered rings of sio4 tetrahedra upon compression the transition to the lowest symmetry phase coesite iii is possibly a first order phase transition that leads to a very distinct structure none of the metastable high pressure phases of coesite has been previously studied and it was widely accepted that coesite undergoes pressure induced amorphization at significantly lower pressures 30 gpa the study of the high pressure behavior of coesite is important to better constrain the metastable phase diagram of silica further crystallographic investigations are necessary for characterizing the structures of these metastable coesite forms crystalline or amorphous metastable phases derived from coesite under high pressure conditions are of particular interest because they can be used as potential tracers of peak transient pressures stress reached in processes such as impacts or faulting

thermodynamics of water solubility and partitioning when it was realized that nominally anhydrous minerals could be a major storage site of water in the mantle smyth 1987 bell and rossman 1992 it also became rather obvious that many mantle samples probably have lost most of their water during ascent e g ingrin and skogby 2000 while analyses of natural samples therefore in many circumstances only provide a lower limit of the actual water content in the mantle measurements of water solubility give an upper limit of the amount of water that might be stored in a mineral experimental measurements of water solubility have therefore naturally evolved in the last decades as a major tool for understanding the water storage capacity of the mantle closely related to studies of water solubility are studies of water partitioning among minerals if the water solubility in two minerals is known under given conditions the partition coefficient is just the ratio of the water solubilities under the same conditions provided that the composition of the fluid coexisting with the two minerals is the same since the pioneering work of bai and kohlstedt 1992 and of kohlstedt et al 1996 the water solubility in all major upper mantle minerals has been studied as well as the partitioning of water between the minerals of the lower mantle and the transition zone solubility studies by themselves do not directly give the water content in the mantle they constrain the storage capacity of the mantle i e the maximum amount of water that may be stored if the mantle were water saturated however as most of the mantle is very likely water undersaturated the actual water content is probably far below the storage capacity estimates of the actual abundance and distribution of water in the mantle may be obtained by combining solubility and partitioning data with direct

origin of retrograde fluid in ultrahigh pressure metamorphic rocks constraints from mineral hydrogen isotope and water content changes in eclogite gneiss transitions in the sulu orogen abstract by taking advantage of having depth profiles between contrasting lithologies from core samples of the chinese continental scientific drilling ccsd project a combined study was carried out to examine changes in mineral h isotope total water and hydroxyl contents in garnet and omphacite across the contacts between ultrahigh pressure uhp eclogite and gneiss in the sulu orogen east central china the samples of interest were from two continuous core segments from the ccsd main hole at depths of 734 21 737 16 and 929 67 932 86 m respectively the results show d values of 116 to 64 for garnet and 104 to 82 for omphacite consistent with incorporation of meteoric water into protoliths of uhp metamorphic rocks by high t alteration both equilibrium and disequilibrium h isotope fractionations were observed between garnet and omphacite suggesting fluid assisted h isotope exchange at local scales during amphibolite facies retrogression while bulk water analysis gave total h2o concentrations of 522 1584 ppm for garnet and 1170 20745 ppm for omphacite structural hydroxyl analysis yielded h2o contents of 80 413 ppm for garnet and 228 412 ppm for omphacite it appears that significant amounts of molecular h2o are present in the minerals pointing to enhanced capacity of water storage in the uhp eclogite minerals hydrogen isotope variations in the transition between eclogite and gneiss show correlations with variations in their water contents petrographically the degree of retrograde metamorphism generally increases with decreasing distance from the eclogite gneiss boundary thus retrograde metamorphism results in mineral reactions and h isotope variation because hydroxyl solubility in nominally anhydrous minerals decreases with dropping pressure significant amounts of water are expected to be released from the minerals during decompression exhumation decompression exsolution of structural hydroxyl from 1 m3 volume of eclogite composed of only garnet and omphacite results in release of a quantitative estimate of 3 07 3 44 kg water that can form 140 156 kg amphibole during exhumation therefore it is concluded that fluid for retrogression of the eclogites away from the eclogite gneiss boundary was derived from the decompression exsolution of structural hydroxyl and molecular h2o in nominally anhydrous minerals for the eclogites adjacent to gneiss in contrast the retrograde metamorphism was principally caused by aqueous fluid from the gneiss which is relatively rich in water consequently both the origin and availability of metamorphic fluid during exhumation of deeply subducted continental crust are deciphered by this combined study focusing on the transitions and the retrograde processes between the felsic and mafic uhp rocks

kinetic iron stable isotope fractionation between iron ii and iii complexes in solution abstract iron stable isotope fractionation between tris 2 2 bipyridine iron ii fe ii bipy 3 2 and iron iii chloride complexes has been measured using plasma source mass spectrometry the experimental protocol involves complexing fe ii ion with 2 2 bipyridine in a fe ii fe iii chloride solution and then separating the fe ii and fe iii solution species in 6 m hcl on an anion exchange resin large isotopic variations of 57 fe and 56 fe are experimentally measured in the two separated solution fractions with isotopic fractionations increasing from fe ii fe iii 25 to 174 units for 57 fe 54 fe and 17 to 117 units for 56 fe 54 fe the increase in fractionations correlates with a decrease in the mole fraction of fe ii in the solution fe fe ii fe ii fe iii that results from the dissociation and breakdown of fe ii bipy 3 2 complex in 6 m hcl solution the data variations are mainly ascribed to a kinetic fractionation occurring during this dissociation reaction mass balance calculations assuming that a rayleigh law describes the overall isotopic trends suggest a kinetic fractionation of ca 1 010 100 units the magnitude of this fractionation is attributed to the rupturing of the strong covalent bonds between 2 2 bipyridine and fe ii ion the experimental data confirm that the coordination chemistry of iron exhibits a profound control on its isotopic behaviour and that kinetic fractionations may play an important role in its isotope geochemistry as was also found in the pioneering experimental studies of the sulphur isotopic system in solution

site specific hydrogen diffusion rates during clinopyroxene dehydration the rate of hydrogen diffusion in clinopyroxene is relevant to interpreting hydrogen water concentrations in xenoliths phenocrysts and clinopyroxene hosted melt inclusions to provide insight into the deep earth water cycle and volcanic explosivity here we determine bulk and site specific hydrogen diffusivities in two diopsides and an augite by heating initially homogeneous water bearing samples in a 1 atm co co2 gas mixing furnace at 800 1000 c and oxygen fugacity at the quartz fayalite magnetite buffer and observing h loss profiles the o h stretching range between wavenumbers 3000 and 4000 cm 1 in ftir spectra is resolved into 4 6 peaks each of which is assumed to represent a distinct defect site for the hydrogen to determine peak specific diffusivities using our previously published whole block method for the diopside from the kunlun mts in china arrhenius relations are reported for peaks at 3645 3617 3540 3443 and 3355 cm 1 based on measurements at 816 904 and 1000 c bulk and site specific diffusivities are determined for the same set of peaks at 904 c for the second diopside jaipur the augite pmr 53 was a triangular thin slab and hydrogen diffusivities were determined for bulk hydrogen and peaks at 3620 3550 3460 and 3355 cm 1 in the thickness direction at 800 c bulk hydrogen diffusivity in the jaipur diopside is consistent with previous work and hydrogen diffusivity in augite pmr 53 is slightly lower than the fast direction diffusivities measured 100 and 001 in jaipur diopside both diopsides show 1 2 orders of magnitude differences in the peaks specific diffusivities with the fastest diffusivities at 3450 cm 1 and the slowest at 3645 cm 1 however the hydrogen diffusivities in jaipur diopside are 2 4 orders of magnitude higher than those in kunlun diopside for bulk hydrogen and all peaks thus peak specific differences cannot by themselves adequately explain the 5 orders of magnitude range in hydrogen diffusivities observed experimentally in different diopsides the results are broadly consistent with a previously proposed increase in hydrogen diffusivity in diopside with fe up to 0 6 0 8 a p f u although there may be an opposing relationship with al iv the results of this study and others predict high water diffusivities in fe bearing mantle xenolith clinopyroxene on the order of 10 9 to 10 11 m2 s at 1000 c the common observation of hydrogen zonation in mantle xenolith olivine but not in clinopyroxene implies that hydrogen diffusion is much faster in olivine than in pyroxene which then requires the operation of the fastest diffusion mechanism quantified in olivine and diffusivities in clinopyroxene at the lower end of this 2 orders of magnitude range such high diffusivities strongly suggest that water in mantle xenoliths has at least partially equilibrated with the host magma and that the diffusion profiles observed in mantle xenolith olivine reflect only the final stage of ascent after water begins to exsolve

the stratigraphy of the 3 5 3 2 ga barberton greenstone belt revisited a single zircon ion microprobe study abstract recent field and geochemical studies indicate a need to test the stratigraphy of the ca 3 5 ga barberton greenstone belt as it is presently adopted 1 2 this work uses the ion microprobe shrimp to attempt such a test results show that 1 volcaniclastic sediments of the theespruit formation

the oh content of pyrope at high pressure the oh contents of pyrope garnets synthesised at 1000 c and pressures between 2 and 13 gpa have been measured by infrared spectroscopy we find that under the same conditions of pressure temperature ah2o and asio2 the oh content of pyrope is similar to that of grossular this shows that observed differences in nature between water contents of pyrope and grossular are related to paragenesis rather than reflecting higher intrinsic solubility in grossular in the presence of excess sio2 the h2o content increases with pressure to 5 gpa 1000 ppm then decreases to below the detection limit at pressures above 7 gpa thus garnet becomes more hydrous with pressure to some critical value beyond which dehydration occurs even under h2o saturated conditions this observation is consistent with the measured partial molar volume of water in hydrogarnet which becomes greater than that in fluid water within the pressure range of this study when corrected to upper mantle conditions we find that pyrope should dehydrate with increasing depth below about 250 km pyrope is unlikely therefore to be a major site of water storage in the transition zone and upper mantle