cretaceous tectonic evolution of coastal southern california insights from the catalina schist the relatively high t high p t subduction related basement in the southern california continental borderland preserves important information regarding the late mesozoic tectonic evolution of coastal southern california integrated petrologic geochemical and geochronological study of the subduction related catalina schist exposed on santa catalina island documents the formation of lawsonite blueschist through amphibolite facies rocks during progressive underplating in a progressively cooling newly initiated subduction zone petrologic and 40ar 39ar thermochronologic data demonstrate similar perhaps shared retrograde pt histories for all but the lawsonite albite facies rocks under pumpellyite actinolite facies conditions as early as 95 100 ma the 40ar 39ar data together with the abundance of pumpellyite actinolite facies vein assemblages and lack of higher p t epidote or lawsonite blueschist over printing in high grade units are consistent with the rapid emplacement of the catalina schist into the shallow to mid crust by 95 ma by combined underplating and tectonic erosional removal of overlying rocks in the accretionary complex cf platt 1986 the catalina schist and the adjacent peninsular ranges batholith potentially represent paired metamorphic belts formed along southwest north america at 120 to 90 ma combining the evidence for the evolution of the catalina schist with the previously advanced hypotheses regarding tectonic evolution of the batholith suturing of a fringing island arc to the continent may have triggered an oceanward trench stepout and the formation of a new subduction zone at 120 115 ma in which the catalina schist was accreted and metamorphosed

tectonic and metasomatic mixing in a high t subduction zone melange insights into the geochemical evolution of the slab mantle interface abstract the catalina schist california contains an amphibolite grade 0 8 1 1 gpa 640 750 c melange unit consisting of mafic and ultramafic blocks in high mg schistose melange matrix with varying modal proportions of talc chlorite anthophyllite calcic amphibole enstatite and minor phases including zircon rutile apatite spinel and fe ni sulfides this melange unit is interpreted as a kilometer scale zone of tectonic and metasomatic mixing formed within a juvenile subduction zone the study of which may yield insight into chemical mixing processes at greater depths in subduction zones relationships among the major and trace element compositions of the mafic and ultramafic blocks in the melange the rinds developed at the margins of these blocks and the surrounding melange matrix are compatible with the evolution of the melange matrix through a complex combination of infiltrative and diffusional metasomatism and a process resembling mechanical mixing simple linear mixing models are compatible with the development of the melange matrix primarily through simple mixture of the ultramafic and mafic rocks with cr al ratios serving as indicators of the approximate proportions of the two lithologies this conclusion regarding mafic ultramafic mixing is consistent with the field observations and chemical trends indicating strong resemblance of large parts of the melange matrix with rinds developed at the margins of mafic and ultramafic blocks the overall process involved development of metasomatic assemblages through complex fluid mediated mixing of the blocks and matrix concurrent with deformation of these relatively weak rind materials which are rich in layer silicates and amphibole this deformation was sufficiently intense to transpose fabrics progressively disaggregate more rigid block derived materials in weaker chorite and talc rich melange and in some particularly weak lithologies e g chlorite talc and amphibole rich materials intimately juxtapose adjacent lithologies at the sub cm scale approaching grain scale sampled by the whole rock geochemical analyses chemical systematics of various elements in the melange matrix can be delineated based on the cr al based mixing model simple mixing relationships exhibited by al cr mg ni fe and zr provide a geochemical reference frame for considerations of mass and volume loss and gain within the melange matrix the compositional patterns of many other elements are explained by either redistribution local stripping or enrichment at varying scales within the melange ca na k ba and sr or massive addition from external sources si and h2o the latter probably in infiltrating h2o rich fluids that produced the dramatic o and h isotopic shifts in the melange melange formation resulting in the production of high variance ultramafic assemblages with high volatile contents may aid retention of volatiles in this case h2o to greater depths in subduction zones than in original subducted mafic and sedimentary materials the presence of such assemblages i e containing minerals such as talc chlorite and mg rich amphiboles would impact the rheology of the slab mantle interface and perhaps contribute to the low velocity seismic structure observed at near the slab mantle interface in some subduction zones if operative along the slab mantle interface complex mixing processes such as these involving the interplay between fluid mediated metasomatism and deformation also could impact slab incompatible trace element and isotopic signatures ultimately observed in arc magmas producing fluids with geochemical signatures inherited from interactions with hybridized rock compositions

accessory minerals and subduction zone metasomatism a geochemical comparison of two melanges washington and california u s a abstract the ability of a subducted slab or subducted sediment to contribute many incompatible trace elements to arc source regions may depend on the stabilities of accessory minerals within these rocks which can only be studied indirectly in contrast the role of accessory minerals in lower t and p metasomatic processes within paleo subduction zones can be studied directly in subduction zone metamorphic terranes the gee point iron mountain locality of the shuksan metamorphic suite north cascades washington state is a high t melange of metamafic blocks in a matrix of meta ultramafic rocks this melange is similar in geologic setting and petrology to the upper part of an unnamed amphibolite unit of the catalina schist santa catalina island southern california both are interpreted as shear zones between mantle and slab rocks that formed during the early stages of subduction some garnet amphibolite blocks from the gee point iron mountain locality display trace element enrichments similar to those in counterparts from the catalina schist some catalina blocks are highly enriched in th rare earth elements ree the high field strength elements ti nb ta zr and hf hfse u and sr compared to mid ocean ridge basalt morb and to other garnet amphibolite blocks in the same unit textural and geochemical data indicate that accessory minerals of metamorphic origin control the enrichment of th ree and hfse in blocks from both areas the mg rich rinds around blocks and the meta ultramafic matrix from both melanges are highly enriched in a large number of trace elements compared to harzburgites dunites and serpentinites evidence for recrystallization or formation of accessory minerals in the former rocks suggests that these minerals control some of the trace element enrichments data from the gee point and catalina melanges suggest that the accessory minerals titanite rutile apatite zircon and ree rich epidote play a significant role in the enrichment of trace elements in both mafic and ultramafic rocks during subduction related fluid rock interaction mobilization of incompatible elements and deposition of such elements in the accessory minerals of mafic and ultramafic rocks may be fairly common in fluid rich metamorphic environments in subduction zones

enrichment of trace elements in garnet amphibolites from a paleo subduction zone catalina schist southern california the abundance p t stability solubility and element partitioning behavior of minerals such as rutile garnet sphene apatite zircon zoisite and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones the influence of these minerals on the composition of subduction related magmas has been inferred and disputed from inverse modelling of the geochemistry of island arc basalt or by experiment although direct samples of the dehydration partial melting region of a mature subduction zone have not been reported from subduction complexes garnet amphibolites from melanges of circumpacific and caribbean blueschist terranes reflect high t 600 c conditions in shallower regions such rocks record geochemical processes that affected deep seated high t portions of paleo subduction zones in the catalina schist a subduction zone metamorphic terrane of southern california metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta ultramafic rocks this mafic and ultramafic complex may represent either slab derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab mantle wedge contact or both the trace element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by inaa xrf electron microprobe and sem in order of increasing alteration from a probable metabasalt protolith three common types of garnet amphibolite blocks in the catalina schist are 1 non migmatitic clinopyroxene bearing blocks which are compositionally similar to morb that has lost an albite component 2 garnet amphibolite blocks which have rinds that reflect local interaction between metabasite metaperidotite and fluid and 3 migmatites that are extremely enriched in th hfse lree and other trace elements these trace element enrichments are mineralogically controlled by rutile garnet sphene apatite zircon zoisite and allanite alkali and alkaline earth elements are much less enriched in the solid assemblage and thus appear to be decoupled from the other elements in the inferred metasomatic process es the compositions of migmatitic garnet amphibolite blocks seem to complement that of average island arc tholeiite trace element metasomatism reflects fluid solid rather than melt solid interaction the metasomatic effects indicate that h2o rich fluid perhaps with a significant component of na al silicate and alkalis carried th u sr ree and hfse fractionations of lree in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex exotic lree deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment if the paleo subduction zone represented by the mafic and ultramafic complex of the catalina schist had continued its thermal and fluid evolution a selvage of similarly enriched rocks might have been generated along the slab mantle wedge contact between 30 and 85 km depth rocks affected by subduction zone metasomatism although rarely recognized at the surface could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments

petrology of amphibolite facies mafic and ultramafic rocks from the catalina schist southern california metasomatism and migmatization in a subduction zone metamorphic setting the catalina schist of southern california is a subduction zone metamorphic terrane it consists of three tectonic units of amphibolite high p greenschist and blueschist facies rocks that are structurally juxtaposed across faults forming an apparent inverted metamorphic gradient migmatitic and non migmatitic metabasite blocks surrounded by a meta ultramafic matrix comprise the upper part of the catalina amphibolite unit fluid rock interaction at high p high t conditions caused partial melting of migmatitic blocks metasomatic exchange between metabasite blocks and ultramafic rocks infiltration of silica into ultramafic rocks and loss of an albitic component from nonmigmatitic clinopyroxene bearing metabasite blocks partial melting took place at an estimated p 8 11 kbar and t 640 750 c at high h2o activity the melting reaction probably involved plagioclase quartz trondhjemitic melts were produced and are preserved as leucocratic regions in migmatitic blocks and as pegmatitic dikes that cut ultramafic rocks the metasomatic and melting processes reflected in these rocks could be analogous to those proposed for fluid and melt transfer of components from a subducting slab to the mantle wedge aqueous fluids rather than melts seem to have accomplished the bulk of mass transfer within the mafic and ultramafic complex

distinction between tectonic mixing and mass transfer processes in a ductile shear zone abstract a petrological geochemical and microtextural study of an upper amphibolite facies shear zone developed in interlayered mafic and felsic layers permits the relative roles of tectonic mixing and fluid assisted mass transfer processes to be determined geochemical evidence indicates that the chemical changes in the deformed rocks result from mixing of mafic and felsic layers together with fluid assisted mass transfer within the shear zone during mylonitization most major elements and some trace elements lree rb sr ba cu ni exhibited mobile behavior the hrees ti v sc co and fe on the other hand were immobile based on mass conservation of these elements a two component mixing model using mafic and felsic rocks from outside the shear zone as end members explains the major and trace element data the chemical composition of the felsic mylonite is modeled by mixing 12 5 mafic rock and 88 5 felsic rock whereas the mafic mylonite is modeled by mixing 55 4 mafic rock and 45 4 felsic rock a closed system mixing model yields a good fit for immobile elements such as hrees ti v and sc but significant chemical differences between the calculated data and observed data are explained by fluid assisted mass transfer of mobile elements under open system conditions a 24 volume loss in the felsic mylonite and 13 volume loss in mafic mylonite are derived from the mass balance assuming the hree ti v and sc were immobile

relationship between the sr and o isotope compositions of hydrothermal fluids and the spreading and magma supply rates at oceanic spreading centers we find a first order correlation between the sr and o isotope compositions of hydrothermal fluids and spreading rate at oceanic spreading centers the isotope signatures of hydrothermal fluids at slow spreading ridges are more rock dominated than those at fast spreading ridges implying that they have undergone a greater extent of sr and o exchange with the crust the local spatial and temporal variability in sr and o isotope signatures within individual hydrothermal sites can be attributed to short term variations in the hydrothermal system in response to magmatic or tectonic events phase separation at depth and mixing with seawater in the upwelling zone however these processes cannot explain the systematic global relationship of an increasing rock signature in hydrothermal fluids with decreasing spreading rate a greater extent of sr and o exchange between rocks and the circulating fluids can be achieved by 1 a longer fluid flow pathway through the oceanic lithosphere 2 more fractured rock that exposes a greater reaction area 3 an increased residence time of the fluid due to slower hydrothermal circulation or 4 a combination of all three our preferred interpretation is that fluid flow paths are longer and depths of penetration are likely greater at slow spreading ridges

vent fluid chemistry of the rainbow hydrothermal system 36 n mar phase equilibria and in situ ph controls on subseafloor alteration processes abstract the rainbow hydrothermal field is located at 36 13 8 n 33 54 15 w at 2300 m depth on the western flank of a non volcanic ridge between the south amar and amar segments of the mid atlantic ridge the hydrothermal field consists of 10 15 active chimneys that emit high temperature 365 c fluid in july 2008 vent fluids were sampled during cruise knox18rr providing a rich dataset that extends in time information on subseafloor chemical and physical processes controlling vent fluid chemistry at rainbow data suggest that the mg concentration of the hydrothermal end member is not zero but rather 1 5 2 mmol kg this surprising result may be caused by a combination of factors including moderately low dissolved silica low ph and elevated chloride of the hydrothermal fluid combining end member mg data with analogous data for dissolved fe si al ca and h2 permits calculation of mineral saturation states for minerals thought appropriate for ultramafic hosted hydrothermal systems at temperatures and pressures in keeping with constraints imposed by field observations these data indicate that chlorite solid solution talc and magnetite achieve saturation in rainbow vent fluid at a similar ph t p 400 c 500 bar of approximately 4 95 while higher ph values are indicated for serpentine suggesting that serpentine may not coexist with the former assemblage at depth at rainbow the high fe mg ratio of the rainbow vent fluid notwithstanding the mole fraction of clinochlore and chamosite components of chlorite solid solution at depth are predicted to be 0 78 and 0 22 respectively in situ ph measurements made at rainbow vents are in good agreement with ph t p values estimated from mineral solubility calculations when the in situ ph data are adjusted for temperature and pressure calculations further indicate that ph t p and dissolved h2 are extremely sensitive to changes in dissolved silica owing to constraints imposed by chlorite solid solution fluid equilibria indeed the predicted correlation between dissolved silica and h2 defines a trend that is in good agreement with vent fluid data from rainbow and other high temperature ultramafic hosted hydrothermal systems we speculate that the moderate concentrations of dissolved silica in vent fluids from these systems result from hydrothermal alteration of plagioclase and olivine in the form of subsurface gabbroic intrusions which in turn are variably replaced by chlorite magnetite talc tremolite with important implications for ph lowering dissolved sulfide concentrations and metal mobility

effect of paleoseawater composition on hydrothermal exchange in midocean ridges variations in the mg ca sr and so4 concentrations of paleoseawater can affect the chemical exchange between seawater and oceanic basalt in hydrothermal systems at midocean ridges mor we present a model for evaluating the nature and magnitude of these previously unappreciated effects using available estimates of paleoseawater composition over phanerozoic time as inputs and 87sr 86sr of ophiolite epidosites and epidote quartz veins as constraints the results suggest that modern hydrothermal fluids are not typical due to low ca and sr relative to mg and so4 in modern seawater at other times during the last 500 million years particularly during the cretaceous and ordovician hydrothermal fluids had more seawater derived sr and ca a prediction that is supported by sr isotope data the predicted 87sr 86sr of vent fluids varies cyclically in concert with ocean chemistry with some values much higher than the modern value of 0 7037 the seawater chemistry effects can be expressed in terms of the transfer efficiency of basaltic ca and sr to seawater in hydrothermal systems which varies by a factor of 1 6 over the phanerozoic with minima when seawater mg and so4 are low this effect provides a modest negative feedback on seawater composition and 87sr 86sr changes for the mid cretaceous the low 87sr 86sr of seawater requires either exceptionally large amounts of low temperature exchange with oceanic crust or that the weathering flux of continentally derived sr was especially small the model also has implications for mor hydrothermal systems in the precambrian when low seawater so4 could help explain low seawater 87sr 86sr

evolution of the blueschist and greenschist facies rocks of sifnos cyclades greece the metamorphism on the island of sifnos is characterized by the eocene development of a coherent highpressure blueschist terrane and an early miocene greenschist facies overprint this study documents the metamorphic evolution of the blueschist assemblages still preserved in the northern parts of the island and their subsequent transformation into greenschists in the central and southern parts the oxygen isotope geothermometry is based on calibrations for quartz pyroxenes and magnetite matthews et al 1983a augmented by revised calibrations for the minerals muscovite qz mu 1 55 106 t 2 epidote qz ep 1 56 1 92 ps 106 t 2 and rutile qz ru 4 54 106 t 2 oxygen isotope analyses of minerals from the blueschist unit of northern sifnos give consistent fractionations which are independent of rock type an average temperature of 455 c was obtained although the scatter in temperatures deduced from the various geothermometers suggests that equilibration occurs under slightly changing physicochemical conditions analyses of minerals and whole rocks shows that pervasive equilibration in the presence of a common metamorphic fluid has not occurred the minerals and whole rocks of the greenschists of central sifnos are systematically enriched in 18o relative to the blueschist assemblages chemical data indicate that the greenschist overprint was accompanied by a metasomatic enrichment of ca2 and co2 the petrologic isotopic and chemical evidence favour a metamorphism governed by the infiltration of 18o co2 enriched aqueous solutions it is reasonable to assume that this is connected with the miocene magmatic activity observed throughout the cyclades the marbles separating the blueschist from the greenschist unit probably acted as barriers to fluid infiltration into the blueschists and were responsible for their preservation the pressure of the blueschist metamorphism is estimated at 14 2 kbar corresponding to a depth of ca 50 km the structural style and stratigraphy of sifnos are suggestive of the subduction of a continental margin sequence it is clear that the considerable tectonic depression may be associated with continental collision and underthrusting

the influence of kinetics on the oxygen isotope composition of calcium carbonate paleotemperature reconstructions rely on knowledge of the equilibrium separation of oxygen isotopes between aqueous solution and calcium carbonate although oxygen isotope separation is expected on theoretical grounds the temperature dependence remains uncertain because other factors such as slow exchange of isotopes between dissolved co2 species and water can obscure the temperature signal this is problematic for crystal growth experiments on laboratory timescales and for interpreting the oxygen isotope composition of crystals formed in natural settings we present results from experiments in which inorganic calcite is precipitated in the presence of 0 25 m dissolved bovine carbonic anhydrase ca the presence of dissolved ca accelerates oxygen isotope equilibration between the dissolved carbon species co2 h2co3 hco3 co32 and water thereby eliminating this source of isotopic disequilibrium during calcite growth the experimental results allow us to isolate for the first time kinetic oxygen isotope effects occurring at the calcite water interface we present a framework of ion by ion growth of calcite that reconciles our new measurements with measurements of natural cave calcites that are the best candidate for having precipitated under near equilibrium conditions our findings suggest that isotopic equilibrium between calcite and water is unlikely to have been established in laboratory experiments or in many natural settings the use of ca in carbonate precipitation experiments offers new opportunities to refine oxygen isotope based geothermometers and to interrogate environmental variables other than temperature that influence calcite growth rates

stable calcium isotopic composition of meteorites and rocky planets abstract new measurements of mass dependent calcium isotope effects in meteorites lunar and terrestrial samples show that earth moon mars and differentiated asteroids e g 4 vesta and the angrite and aubrite parent bodies are indistinguishable from primitive ordinary chondritic meteorites at our current analytical resolution 0 07 sd for the 44 ca 40 ca ratio in contrast enstatite chondritic meteorites are slightly enriched in heavier calcium isotopes ca 0 5 and primitive carbonaceous chondritic meteorites are depleted in heavier calcium isotopes ca 0 5 the calcium isotope effects cannot be easily ascribed to evaporation or intraplanetary differentiation processes the isotopic variations probably survive from the earliest stages of nebular condensation and indicate that condensation occurred under non equilibrium undercooled nebular gas conditions some of this early high temperature calcium isotope heterogeneity is recorded by refractory inclusions niederer and papanastassiou 1984 and survived in planetesimals but virtually none of it survived through terrestrial planet accretion the new calcium isotope data suggest that ordinary chondrites are representative of the bulk of the refractory materials that formed the terrestrial planets enstatite and carbonaceous chondrites are not the enrichment of light calcium isotopes in bulk carbonaceous chondrites implies that their compositions are not fully representative of the solar nebula condensable fraction

stability and composition relations of calcic amphiboles in ultramafic rocks calcic amphiboles are observed in ultramafic rocks that have equilibrated under a broad span of geological conditions and might prove to be good indicators of metamorphic grade if their stabilities could be determined as a function of their compositions experiments were performed on the stability of tremolite plus forsterite in the system h2o cao mgo sio2 from 5 to 20 kbar a univariant curve was fitted to the experimental brackets using volume water fugacity and heat capacity data the results indicate that the maximum stability of tremolite in the presence of forsterite is about 825 c at 5 kbar addition of al2o3 to this system increases the stability of tremolitic amphibole by only 20 40 c and induces solubility of 5 7 wt al2o3 in the amphibole as determined from quantitative sem analyses of individual amphibole crystals thus substitution of the tschermakite component ca2 mg3al2 si6al2 o22 oh 2 alone cannot lead to the greatly enhanced al2o3 contents or thermal stability of natural calcic amphiboles

tracing the evolution of hydrothermal fluids in the upper oceanic crust sr isotopic constraints from dsdp odp holes 504b and 896a

influence of particle shape and sorting upon sample size estimates for a coarse grained upland stream abstract present bulk sampling recommendations for the characterisation of fluvial sediments based upon ellipsoids of revolution do not take into account variations in particle shape and sorting disc shaped particles with equivalent b axes to spheres have lower relative masses and require more relaxed sampling criteria compared to existing recommendations whereas heavier cube and rod shaped particles require more stringent criteria empirical data from the river rede northumberland uk an upland gravel bed channel indicate that samples are dominated by discs however particle shape was not constant in every grain size fraction blocky material cubes and rods was more frequent in the 63 mm fraction in comparison to the other finer fractions data based on the weight of the d 99 9 particle indicate that a larger sample weight is required when the coarsest size category of that sample is dominated by blocky material sorting also affects the size of the sample required to characterise a deposit the more poorly sorted the deposit the larger the sample has to be or the finer should be the truncation grain size this is particularly evident in samples with a high matrix concentration two sample reference lines developed from these data using the 0 1 by mass criteria recommend larger minimum sample sizes or finer truncation in comparison to other published standards data from a second site support these results indicating that they may be applicable to other poorly sorted upland gravel bed streams analysis of the size category variance demonstrates that the new guidelines returned the lowest variance levels suggesting that they should be used where particularly accurate sampling procedures are required

reconstructing the oxygen isotope composition of late cambrian and cretaceous hydrothermal vent fluid oxygen isotope analyses 18o of 16 quartz epidote pairs from late cambrian betts cove and mings bight newfoundland ordovician thetford mines quebec canada and cretaceous troodos cyprus ophiolites are used to calculate the 18o of the hydrothermal fluids from which they crystallized we combine these with 3 quartz fluid inclusion measurements and 3 quartz magnetite measurements from the cambrian ophiolites to explore how the range in the 18o of submarine hydrothermal vent fluid has varied between the late cambrian cretaceous and today the range of calculated 18o values of vent fluid 4 to 7 4 is larger than that of modern seafloor hydrothermal vent fluid 0 to 4 we employ two numerical models to ascertain whether this range is most consistent with changes in paleo seawater 18o or with changes in the reactive flow path in ancient hydrothermal systems a static calculation of the vent fluid oxygen isotope composition as a function of the water rock ratio suggests that in an ocean with a lower 18o than today the range of vent fluid 18o should be larger our data however show little evidence that the 18o of the ocean was much lower than the global ice free value of 1 2 a dual porosity model for reactive flow through fractured and porous media is used to model the relative evolution of the 87sr 86sr and 18o of vent fluid in contact with rock our 87sr 86sr and 18o for cretaceous epidotes suggest the strontium concentration of the cretaceous oceans may have been much higher than at present the 87sr 86sr and 18o data from cambrian epidotes are strikingly different from the younger samples and are difficult to model unless fluid rock interaction in the cambrian hydrothermal systems was substantially different it is also possible that some of the quartz epidote veins have been reset by obduction related metamorphism our data suggest that the high calcium to sulfate ratio in early and cretaceous seawater may have affected the degree of strontium isotope exchange causing hydrothermal fluids to have 87sr 86sr closer to that of seawater than in modern systems

plagioclase and epidote buffering of cation ratios in mid ocean ridge hydrothermal fluids experimental results in and near the supercritical region abstract experiments have been performed with na ca k cl fluids of seawater chlorinity and diabase basalt and plagioclase bearing mineral mixtures at 350 425 c and 250 400 bars to help constrain hydrothermal alteration processes at mid ocean ridges dissolved ca na and ph for all experiments responded systematically to differences in dissolved sio2 concentrations and the compositions of plagioclase reactants diabase alteration at low fluid rock mass ratios 0 5 to 1 produces fluids undersaturated with respect to quartz during hydration of primary olivine and orthopyroxene whereas basalt alteration under similar conditions yields fluids slightly supersaturated with respect to quartz during breakdown of glass to smectite and amphibole fluid chemistry in all experiments appears to approach a partial equilibrium state with the albite and anorthite components in plagioclase and approaches a ph consistent with plagioclase alteration to epidote trace element data from vent fluids specifically b and sr together with major element chemistry ca na sio2 ph provides evidence that the reaction zone for black smoker fluids at mid ocean ridges is composed of only slightly altered diabase and is characterized by small amounts of epidote nearly fresh plagioclase and clinopyroxene and partially to completely hydrated olivine and orthopyroxene fluids reacting with this rock may be undersaturated with respect to quartz so pressure estimates based on the quartz geobarometer should be regarded as minimums using equilibrium between plagioclase the dominant reactant and epidote the dominant reaction product in experiments we estimate that temperatures in reaction zones are in excess of 375 c for most vent systems these temperatures are higher than measured vent temperatures suggesting that hotspring fluids commonly loose heat during ascent to the seafloor

an oxygen isotopic profile through the upper kilometer of the oceanic crust dsdp hole 504b dsdp hole 504b is the deepest basement hole in the oceanic crust penetrating through a 571 5 m pillow section a 209 m lithologic transition zone and 295 m into a sheeted dike complex an oxygen isotopic profile through the upper crust at site 504 is similar to that in many ophiolite complexes where the extrusive section is enriched in18o relative to unaltered basalts and the dike section is variably depleted and enriched basalts in the pillow section at site 504 have 18o values generally ranging from 6 1 to 8 5 smow mean 7 0 although minor zeolite rich samples range up to 12 7 rocks depleted in18o appear abruptly at 624 m sub basement in the lithologic transition from 100 pillows to 100 dikes coinciding with the appearance of greenschist facies minerals in the rocks whole rock values range to as low as 3 6 but the mean values for the lithologic transition zone and dike section are 5 8 and 5 4 respectively oxygen and carbon isotopic data for secondary vein minerals combined with the whole rock data provide evidence for the former presence of two distinct circulation systems separated by a relatively sharp boundary at the top of the lithologic transition zone the pillow section reacted with seawater at low temperatures near 0 c up to a maximum of around 150 c and relatively high water rock mass ratios 10 100 water rock ratios were greater and conditions were more oxidizing during submarine weathering of the uppermost 320 m than deeper in the pillow section the transition zone and dikes were altered at much higher temperatures up to about 350 c and generally low water rock mass ratios 1 and hydrothermal fluids probably contained mantle derived co2 mixing of axial hydrothermal fluids upwelling through the dike section with cooler seawater circulating in the overlying pillow section resulted in a steep temperature gradient 2 5 c m across a 70 m interval at the top of the lithologic transition zone progressive reaction during axial hydrothermal metamorphism and later off axis alteration led to the formation of albite and ca zeolite rich alteration halos around fractures this enhanced the effects of cooling and18o enrichment of fluids resulting in local increases in 18o of rocks which had been previously depleted in18o during prior axial metamorphism

field based evidence for devolatilization in subduction zones implications for arc magmatism metamorphic rocks on santa catalina island california afford examination of fluid related processes at depths of 15 to 45 kilometers in an early cretaceous subduction zone a combination of field stable isotope and volatile content data for the catalina schist indicates kilometer scale transport of large amounts of water rich fluid with uniform oxygen and hydrogen isotope compositions the fluids were liberated in devolatilizing relatively low temperature 400 degrees to 600 degrees c parts of the subduction zone primarily by chlorite breakdown reactions an evaluation of pertinent phase equilibria indicates that chlorite in mafic and sedimentary rocks and melange may stabilize a large volatile component to great depths perhaps 100 kilometers depending on the thermal structure of the subduction zone this evidence for deep volatile subduction and large scale flow of slab derived water rich fluids lends credence to models that invoke fluid addition to sites of arc magma genesis

geometry conditions and timing of off axis hydrothermal metamorphism and ore deposition in the solea graben the recent discovery of fossil spreading axes along the northern margin of the troodos ophiolite1 provides a unique opportunity to compare a fossil seawater hydrothermal system with systems of modern oceanic spreading centres2 6 whereas previous accounts of ophiolite hydrothermal systems7 13 including troodos14 16have elucidated many aspects of sea floor hydrothermal metamorphism earlier workers have not placed ophiolite hydrothermal features within any specific spreading centre structural framework most ophiolites exhibit complex histories of post accretion deformation that mask or obliterate early structures related to seafloor spreading the troodos ophiolite is apparently still in the process of being emplaced 17 and most structural elements at troodos appear to have originated beneath a cretaceous sea floor1 we have discovered an extensive area of intense seawater hydrothermal alteration within the sheeted dyke complex of the solea graben fig 1 the metamorphic petrology and oxygen isotope geochemistry of 100 dyke samples from this graben reveal the size and three dimensional geometry of the hydrothermal system and yield estimates of temperature salinity oxygen isotope composition and recharge discharge geometry for fluids that transported the constituents of massive sulfide orebodies geometric discordance between the trends of the hydrothermally altered area and of earlier seafloor spreading structures compels us to consider a post spreading off axis origin for the orebodies of the solea graben

strontium alteration in the troodos ophiolite implications for fluid fluxes and geochemical transport in mid ocean ridge hydrothermal systems new and published strontium isotope analyses from the troodos ophiolite constrain fluid solid exchange processes and the magnitude and circulation paths of the hydrothermal fluids the87sr 86sr profile reflects alteration in the recharge zone of an evolving hydrothermal system fluid rock strontium isotope exchange in the upper 1 5 km of extrusive lavas was kinetically limited and seawater derived fluids emitted from the base of this zone were buffered to87sr 86sr ratios between 0 7047 and 0 7059 in contrast over the next 1 km depth interval of sheeted dykes and the uppermost plutonics 87sr 86sr values cluster about0 7054 7 2 and fluid flow is inferred to have been pervasive with near equilibrium fluid rock exchange quartz chlorite and epidosite zones the probable pathways of the concentrated high temperature upwelling fluids have identical87sr 86sr ratios to adjacent diabase dykes on troodos a time integrated fluid flux in excess of2 9 107 kg m 2 is required to transport the strontium isotope composition of 0 7054 set in the kinetically controlled exchange zone through the 1 km of sheeted dykes and into the zones of concentrated upwelling the uniformity of the87sr 86sr ratios in the diabase sheeted dykes and high temperature epidosite and quartz chlorite rocks indicate that the strontium isotopic alteration took place during the high temperature phase of hydrothermal circulation the inferred minimum time integrated fluid flux of2 9 107 kg m 2 substantially exceeds that of 5 106 kg m 2 inferred from thermal models of high temperature circulation but is comparable with estimates of the hydrothermal flux from oceanic budgets of3he mg and87sr the high flux estimate for troodos is consistent with the ophiolite venting fluids with87sr 86sr elevated significantly above rock values which contrasts with the near morb87sr 86sr ratios of fluids from active high temperature vents at mid ocean ridges and the87sr 86sr profile in dsdp hole 504b this requires that either the modern systems are immature and there is a protracted phase of lower temperature circulation or that circulation in the troodos ophiolite differed from that in normal mid ocean ridges and oceanic strontium budgets are balanced by another source the lack of very elevated87sr 86sr values in the vent fluids precludes significant channelling in the recharge zones of the active systems

high temperature kinetic isotope fractionation of calcium in epidosites from modern and ancient seafloor hydrothermal systems abstract calcium isotope ratios in epidote from epidosites in ophiolites of varying phanerozoic ages have 44ca 40ca ratios that are lower by 0 1 to 0 6 relative to typical mid ocean ridge hydrothermal fluids epidosites are inferred to form in high temperature parts of seafloor hydrothermal systems at temperatures above 300 c and where fluid fluxes are high so the ca isotopic composition of the epidote is likely to reflect fractionation during growth of crystals from aqueous solution available ca isotope data from mor hydrothermal vent fluids and mantle peridotites constrain the 44 ca of likely modern hydrothermal fluids to a narrow range at 44 ca 0 05 0 1 a reactive transport model is used to evaluate whether the 44 ca of hydrothermal fluids might have been higher during the cretaceous and late cambrian the ages of the troodos oman and betts cove ophiolites from which we have data for these calculations we use the epidosite 87sr 86sr as a guide to the extent of ca isotopic exchange that affected the ancient hydrothermal fluids which were derived from seawater with higher ca and sr and lower sulfate concentration than modern seawater the calculations suggest that the ancient hydrothermal fluid 44 ca values were not much different from modern values with the possible exception of the late cambrian example we infer that the epidote fluid ca isotope fractionation averaging 44 ca 0 2 to 0 6 is most likely due to kinetic isotope fractionation during mineral precipitation there is evidence from the literature that hydrothermal epidote may commonly form from oversaturated solutions which makes the kinetic isotope interpretation plausible the equilibrium epidote fluid ca isotope fractionation is estimated to be 44 ca eq 0 based on recently reported dft calculations our results suggest that kinetic calcium isotope fractionation can affect hydrothermal silicate minerals and may be only slightly smaller in magnitude than the effects observed in ca bearing minerals at low temperature kinetic isotope effects during mineral growth could provide new insights into the formation mechanisms of hydrothermal silicate minerals

sulfur isotopic profile through the oceanic crust sulfur mobility and seawater crustal sulfur exchange during hydrothermal alteration measurements of the abundance of anhydrite and pyrite veins and the distribution of alteration types in oceanic basement enable estimates of global mass and isotopic fluxes of sulfur during hydrothermal alteration of oceanic crust fluxes of 2 5 10 12 g yr s 34 s 2 7 from the volcanic section to seawater and 2 1 10 12 g yr of seawater s 21 into the crust result in an insignificant change in the s content of altered ocean crust these are clearly second order fluxes compared to the riverine input of s to the oceans and the output of sedimentary pyrite most of the anhydrite that forms in the high temperature axial convection cell must later be dissolved at lower temperatures during off axis circulation the mean 34 s of altered oceanic basement is 0 9 only slightly changed from the primary value of 0 1 unless s is preferentially mobilized from 34 s rich sulfide deposits or mineralized zones subduction of altered basaltic crust is unlikely to be the source of high 34 s values of arc volcanic rocks 4 but could contribute to isotopic heterogeneities of s in the mantle

physical and chemical properties of the epidote minerals an introduction epidote minerals are known since the 18th century but at that time the greenish to dark colored varieties were termed actinolite or schorl and not distinguished from the minerals to which these names apply today hauy defined the mineral species and introduced the name epidote in 1801 whereas werner in 1805 used the term pistacite quoted from hintze 1897 epidote is derived from greek epidosis to increase because the base of the rhombohedral prism has one side larger than the other and pistacite refers to its green color all references for names after luschen 1979 blackburn and dennen 1997 weinschenk 1896 proposed the name clinozoisite from its monoclinic symmetry and zoisite like composition for those monoclinic members of the epidote family that are fe poor optically positive and have low refractive indices and birefringence zoisite was probably confused with tremolite until the beginning of the 19th century in 1804 siegmund zois baron von edelstein 1747 1819 an austrian sponsor of mineral collections found and described a new mineral in a handspecimen from the saualpe mountains in carinthia that was named zoisite by werner hauy 1822 interpreted zoisite as a variety of epidote and included it in his epidote spezies weiss 1820 presented a theory of the epidote system and also discussed crystal morphological features of the epidote minerals weiss 1828 rammelsberg 1856 studied the relationship between epidote and zoisite and presented a compilation of chemical analyses of zoisite he already noticed that the relative concentrations of di tri and tetravalent cations are identical in zoisite and epidote but that the fe content in zoisite about 2 3 5 wt fe2o3 is generally less than in epidote about 9 12 wt fe2o3 piemontite see bonazzi and menchetti 2004 was probably first described in 1758 by cronstedt

calcium isotope record of phanerozoic oceans implications for chemical evolution of seawater and its causative mechanisms a total of 280 brachiopods of ordovician to cretaceous age complemented by published data from belemnites and planktonic foraminifera are used to reconstruct the evolution of calcium isotope composition of seawater 44 40casw over the phanerozoic the compiled 44 40casw record shows a general increase from 1 3 nist srm 915a at the beginning of the ordovician to 2 at present superimposed on this trend is a major long term positive excursion from the early carboniferous to early permian as well as several short term mostly negative oscillations a numerical model of the global cycles of calcium carbon magnesium and strontium was used to estimate whether the recorded 44 40casw variations can be explained by varying magnitudes of input and output fluxes of calcium to the oceans the model uses the record of marine 87sr 86sr ratios as proxy for seafloor spreading rates a record of oceanic mg ca ratios to estimate rates of dolomite formation and reconstructed atmospheric co2 discharge and erosion rates to estimate continental weathering fluxes the model results indicate that varying magnitudes of the calcium input and output fluxes cannot explain the observed 44 40casw trends suggesting that the isotope signatures of these fluxes must also have changed as a possible mechanism we suggest variable isotope fractionation in the sedimentary output flux controlled by the dominant mineralogy in marine carbonate deposits i e the oscillating calcite aragonite seas the ultimate control of the calcium isotope budget of the phanerozoic oceans appears to have been tectonic processes specifically variable rates of oceanic crust production that modulated the hydrothermal calcium flux and the oceanic mg ca ratio which in turn controlled the dominant mineralogy of marine carbonates hence the 44 40casw as to the causes of the short term oscillations recorded in the secular 44 40casw trend we tentatively propose that these are related to variable rates of dolomite formation and or to changing chemical composition of the riverine flux in particular and ratios induced by variable proportions of silicate vs carbonate weathering rates on the continents

channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust sr isotope evidence from odp hole 1256d abstract odp hole 1256d in the eastern equatorial pacific is the first penetration of a complete section of fast spread ocean crust down to the dike gabbro transition and only the second borehole to sample in situ sheeted dikes after dsdp hole 504b here a high spatial resolution record of whole rock and mineral strontium isotopic compositions from site 1256 is combined with core observations and downhole wireline geophysical measurements to determine the extent of basalt hydrothermal fluid reaction and to identify fluid pathways at different levels in the upper ocean crust the volcanic sequence at site 1256 is dominated by sheet and massive lava flows but the sr isotope profile shows only limited exchange with seawater however the upper margins of two anomalously thick 25 m massive flow sequences are strongly hydrothermally altered with elevated sr isotope ratios and appear to be conduits of lateral low temperature off axis fluid flow elsewhere in the lavas high 87 sr 86 sr are restricted to breccia horizons mineralised hyaloclastic breccias in the lava dike transition are strongly altered to mg saponite silica and pyrite indicating alteration by mixed seawater and cooled hydrothermal fluids in the sheeted dike complex 87 sr 86 sr ratios are pervasively shifted towards hydrothermal fluid values 0 705 dike chilled margins display secondary mineral assemblages formed during both axial recharge and discharge and have higher 87 sr 86 sr than dike cores indicating preferential fluid flow along dike margins localised increases in 87 sr 86 sr in the dike gabbro transition indicates the channelling of fluids along the sub horizontal intrusive boundaries of the 25 to 50 m thick gabbroic intrusions with only minor increases in 87 sr 86 sr within the cores of the gabbro bodies when compared to the pillow lava dominated section from hole 504b the sr isotope measurements from site 1256 suggest that the extent of hydrothermal circulation in the upper ocean crust may be strongly dependent on the eruption style sheet and massive flow dominated lava sequences typical of fast spreading ridges may experience relatively restricted circulation but there may be much more widespread circulation through pillow lava dominated sections in addition the hole 1256d sheeted dikes display a much greater extent of sr isotopic exchange compared to dikes from hole 504b because seawater derived hydrothermal fluids must transit the dikes during their evolution to black smoker type fluids the different sr isotope profiles for holes 504b and 1256d suggest there are significant variations in mid ocean ridge hydrothermal systems at fast and intermediate spreading ridges which may impact geochemical cycles of elements mobilised by fluid rock exchange at different temperatures

a seismological constraint on the depth of basalt eclogite transition in a subducting oceanic crust the oceanic crust primarily has a basaltic composition1 2 which must transform into eclogite on subduction of the oceanic crust beneath continents3 4 there has been a considerable debate about the role of the basalt eclogite transformation in driving lithospheric plate motion5 and in the thermal regieme of subducting plates6 how important this transformation is in tectonics critically depends on the depth of transformation which is affected by factors such as temperature7 8 bulk rock chemical composition2 water content9 and perhaps stress environment becuase it would be practically impossible to evaluate all these factors correctly we attempt here to constrain the transformation depth by a more direct seismological method

metal depleted root zones of the troodos ore forming hydrothermal systems cyprus the cupriferous pyrite deposits of cyprus were precipitated from hydrothermal solutions derived by interaction of contemporaneous seawater with hot mafic rock at the ancient troodos spreading centre here we identify the zones in which this interaction took place the zones occur in the lower part of the sheeted dyke complex and within them 30 50 of the rock is made up of epidosite an epidote quartz rock replacing the dykes as sheets and pipes the epidosites contain abundant fluid inclusions which give trapping temperatures of 350 400 c or even higher and contain water normally near seawater in salinity zones of epidosite are elongate parallel to the strike of the sheeted dykes and are up to 1 km wide the rocks throughout these zones are strongly depleted in cu and zn and the metals removed are sufficient to supply the ore deposits in fact several large ore deposits lie along strike from zones of epidosite all of these features support the identification of the epidosites as the hydrothermal reaction zones the location of the epidosite zones immediately above the gabbros of the plutonic complex supports the hypothesis that the heat to drive the ore forming systems came from the underlying magma as is also likely for modern black smoker springs

molecular simulations of kinetic stable calcium isotope fractionation at the calcite aqueous interface abstract the measurement of calcium stable isotope fractionation during crystal growth can provide insights into the dominant pathways and mechanisms of crystal growth but these interpretations require a detailed understanding of mass dependent reaction rates and free energy states at the molecular scale here we report the results of free energy calculations used to determine the rate limiting step controlling kinetic stable isotope fractionation during calcium ion attachment to active growth sites kink sites at the calcite surface the free energy profile for attachment is combined with ion coordination states to reconstruct the multi step molecular pathway for the ion attachment process dissociative desolvation of a water molecule coordinating the calcium ion approaching the kink is identified as the rate limiting step with a free energy barrier of 18 kj mol which is equal to the energetic cost of water molecule leaving ca2 coordination shell in the aqueous phase molecular dynamics simulations were used to investigate the dependence of this desolvation step on calcium ion mass both brute force md and an analysis of mass dependent water exchange coefficients kwex give identical kinetic stable isotope fractionation factor values f44 40 0 992 which corresponds to 44 40ca of 8 originating from a mass dependent transition frequency for the ca bound solvation water at the kink this value is significantly higher than the fractionation factor associated with water exchange dynamics around ca2 in bulk aqueous solution determined here f44 40 0 999 our results confirm that the growing calcite surface should be depleted in 44ca with respect to the aqueous phase due to kinetically preferred desolvation of the lighter isotope we conclude that the maximum kinetic stable isotope fractionation associated with calcium attachment to surfaces is not constant but can vary due to changes in the interfacial environment that affect local solvent dynamics during desolvation

hydrothermal alteration of basalt by seawater under seawater dominated conditions fresh mid ocean ridge basalt glass and diabase have been reacted with seawater at 150 300 c 500 bar and water rock mass ratios of 50 62 and 125 using experimental apparatus which allowed on line sampling of solution to monitor reaction progress these experiments characterize reaction under what we have called seawater dominated conditions of hydrothermal alteration in an experiment at 300 c basalt glass undergoing alteration removed nearly all mg2 from an amount of seawater 50 times its own mass in the process the glass was converted entirely to mixed layer smectite chlorite anhydrite and minor hematite removal of mg from seawater occurred as a mg oh 2 component incorporated into the secondary clay this produced a precipitous drop in solution ph early in the experiment accompanied by a dramatic increase in the concentrations of fe mn and zn in solution as mg removal neared completion and the glass was hydrolyzed ph rose again and heavy metal concentrations dropped at water rock ratios of 62 and 125 and 150 300 c the mineral assemblage produced was similar to that at a water rock ratio of 50 solution chemistry however contrasted with the earlier experiment in that mg concentrations in solution were greater and ph lower this caused significant leaching of heavy metals at 300 c nearly all of the na ca cu zn and co2 and most of the k ba sr and mn were leached from the silicates h2s al si and possibly co were also significantly mobilized whereas v cr and ni were not little or no seawater sulfate was reduced although submarine hot spring solutions sampled to date along mid ocean ridges clearly come from rock dominated hydrothermal systems evidence from ocean floor metabasalts and from heat flow studies indicates that seawater dominated conditions of alteration prevail at least locally both in axial hightemperature systems and in ridge flank systems at lower temperatures

subseafloor phase equilibria in high temperature hydrothermal fluids of the lucky strike seamount mid atlantic ridge 37 17 n abstract as part of an integrated study conducted at the lucky strike seamount mid atlantic ridge 37 17 n in 2008 gas tight sampling devices were used to collect high temperature 300 c hydrothermal fluids issuing from sulfide structures distributed throughout the vent field located in the summit depression compared with previous observations from 1993 to 1997 the most substantial changes in vent fluid compositions are dramatically increased co2 concentrations 5 up to 133 mmol l and the observation of vent fluids enriched in dissolved chloride relative to seawater combined with an increase in 13 c co 2 values by 4 in 2008 the elevated co2 indicates replenishment of the magmatic heat source and may be indicative of a recent magmatic event the additional supporting fluid chemistry is however similar to that of the previous sampling intervals necessitating a reassessment of the subseafloor controls on vent fluid chemistry at lucky strike in the context of recently obtained geophysical data that provides the depth extent of a steady state magma chamber two phase behavior is indicated by the chloride variability in the vent fluids and comparison with experimental data for the associated chloride dependent partitioning of minor trace elements suggests the possibility of a similar source fluid for all the vent structures while limiting the likelihood of shallow phase separation and subseafloor mixing for the hydrothermal end members a recently calibrated fe mn geothermometer indicates minimum subseafloor equilibration temperatures of 350 385 c however constraints imposed by dissolved si cl in conjunction with geophysical observations are consistent with peak reaction conditions at temperatures of 430 475 c and pressures near the top of the axial magma chamber 410 480 bars where magmatic co2 becomes entrained in the circulating fluids the distance between the magma chamber and the seafloor at lucky strike is substantially greater than at most faster spreading ridges and we propose the resulting increased residence time in the up flow zone leads to the re equilibration of temperature sensitive transition metals at conditions less extreme than those associated with peak reaction agreement between experimental data thermodynamic model calculations and dissolved concentrations of fe cu zn h2 and h2s in the lucky strike fluids reinforce the hypothesis of ph redox equilibria for transition metals at relatively oxidizing conditions and temperatures predicted by the empirical fe mn geothermometer in situ ph measurements of the high temperature fluids exiting the seafloor are also consistent with the model calculations

calcium isotope fractionation in calcite and aragonite abstract calcium isotope fractionation was measured on skeletal aragonite and calcite from different marine biota and on inorganic calcite precipitation temperatures ranged from 0 to 28 c calcium isotope fractionation shows a temperature dependence in accordance with previous observations 1000 ln cc 1 4 0 021 t c for calcite and 1000 ln ar 1 9 0 017 t c for aragonite within uncertainty the temperature slopes are identical for the two polymorphs however at all temperatures calcium isotopes are more fractionated in aragonite than in calcite the offset in 44 40 ca is about 0 6 the underlying mechanism for this offset may be related to the different coordination numbers and bond strengths of the calcium ions in calcite and aragonite crystals or to different ca reaction behavior at the solid liquid interface recently the observed temperature dependence of the ca isotope fractionation was explained quantitatively by the temperature control on precipitation rates of calcium carbonates in an experimental setting lemarchand et al 2004 we show that this mechanism can in principle also be applied to caco 3 precipitation in natural environments in normal marine settings following this model ca isotope fractionation in marine ca carbonates is primarily controlled by precipitation rates on the other hand the larger ca isotope fractionation of aragonite compared to calcite can not be explained by different precipitation rates the rate control model of ca isotope fractionation predicts a strong dependence of the ca isotopic composition of carbonates on ambient co 3 2 concentration while this model is in general accordance with our observations in marine carbonates cultured specimens of the planktic foraminifer orbulina universa show no dependence of ca isotope fractionation on the ambient co 3 2 concentration the latter observation implies that the carbonate chemistry in the calcifying vesicles of the foraminifer is independent from the ambient carbonate ion concentration of the surrounding water

calcium isotope δ 44 40 ca fractionation along hydrothermal pathways logatchev field mid atlantic ridge 14 45 n we investigate the logatchev hydrothermal field at the mid atlantic ridge 14 degrees 45 n to constrain the calcium isotope hydrothermal flux into the ocean during the transformation of seawater to a hydrothermal solution the ca concentration of pristine seawater ca sw increases from about 10 mm to about 32 mm in the hydrothermal fluid endmember ca hydend and thereby adopts a delta ca 44 40 hydend of 0 95 0 07 parts per thousand relative to seawater sw and a sr 87 sr 86 isotope ratio of 0 7034 4 we demonstrate that delta ca 44 40 hydend is higher than that of the bedrock at the logatchev field from mass balance calculations we deduce a delta ca 44 40 of 1 17 0 04 parts per thousand sw for the host rocks in the reaction zone and 1 45 0 057 parts per thousand sw for the isotopic composition of the entire hydrothermal cell of the logatchev field the values are isotopically lighter than the currently assumed delta ca 44 40 for bulk earth of 0 92 0 187 parts per thousand sw skulan j depaolo d j and owens t l 1997 biological control of calcium isotopic abundances in the global calcium cycle geochim cosmochim acta 61 12 2505 2510 and challenge previous assumptions of no ca isotope fractionation between hydrothermal fluid and the oceanic crust zhu p and macdougall j d 1998 calcium isotopes in the marine environment and the oceanic calcium cycle geochim cosmochim acta 62 10 1691 1698 schmitt a d chabeaux f and stille p 2003 the calcium riverine and hydrothermal isotopic fluxes and the oceanic calcium mass balance earth planet sci lett 6731 1 16 here we propose that ca isotope fractionation along the fluid flow pathway of the logatchev field occurs during the precipitation of anhydrite two anhydrite samples from the logatchev hydrothermal field show an average fractionation of about delta 44 4c ca 0 57 parts per thousand relative to their assumed parental solutions ca isotope ratios in aragonites from carbonate veins from odp drill cores indicate aragonite precipitation directly from seawater at low temperatures with an average delta ca 44 40 of 1 54 0 08 parts per thousand sw the relatively large fractionation between the aragonite precipitates and seawater in combination with their frequent abundance in weathered mafic and ultramafic rocks suggest a reconsideration of the marine ca isotope budget in particular with regard to ocean crust alteration c 2008 elsevier ltd all rights reserved

subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab mantle interface seismological studies of the interface between downgoing slabs of subducted lithosphere and the overriding mantle provide constraints against which models of mantle and slab temperature and bulk composition can be tested these studies employing waves reflected and converted at the slab mantle interface suggest that a velocity contrast of 5 10 occurs over a distance of 10 20 km to depths of at least 350 km the authors have investigated whether such sharp contrasts can result form thermal structure compositional differences phase changes or other effects such as preferred mineral orientation using a mantle and slab temperature model a petrologic model of mantle and slab mineralogy and a data base of elastic properties of mantle and slab phrases theoretical seismic velocities were calculated and compared with the observed velocity contrast and boundary thickness constraints at 9 6 gpa and 1000c conditions modeled at the slab mantle interface at approximately 280 km depth cooler temperatures in the slab interior produce at constant composition only approximately a 1 75 v sub p increase into the slab in any of the peridotitic and eclogitic mineralogies investigated variation in v sub p among the different bulk compositions is only approximately 0 5 these contrasts are substantially lower than those obtained from the seismological studies andmore indicate that temperature and bulk compositional differences between mantle and slab are inadequate to cause the observed velocity contrasts they propose that phase transformations in mantle and slab mineralogies notably elevation of the olivine spinel phase change normally occurring at 400 km depth are important factors in these sharp contrasts other contributing effects may include preferential mineral orientation near the slab surface and at shallower depths the presence of partial melt or hydrous fluid evolved from dehydration reactions along the interface less

metasomatism and partial melting in a subduction complex catalina schist southern california partial melting accompanied high pressure amphibolitization of eclogitic blocks and metasomatism of blocks and their peridotite matrix in a unit of the catalina schist a subduction zone metamorphic terrane migmatitic blocks contain leucocratic zones veins and pods with albitic plagioclase quartz muscovite mineralogically similar dikes and veins cut the ultra mafic matrix a few veins can be traced back into migmatitic blocks uniform phase proportions and igneous textures indicate that the dikes and veins crystallized from silicate melts field relations mineral and bulk compositions and comparisons with experimental data suggest that the migmatites and dikes are partial melts of variably metasomatized amphibolites element partitioning phase equilibrium and fluid inclusion data are compatible with partial melting of the amphibolites at p 8 11 kbar and t 640 750 c in the presence of a low salinity aqueous fluid although the catalina metamorphism occurred at much shallower levels than those inferred for subduction related magmatism the process envisioned is one of multistage slab derived metasomatic contamination of the mantle in the hanging wall of a subduction zone

nitrogen isotope tracers of high temperature fluid rock interactions case study of the catalina schist california abstract nitrogen isotope data for metasomatized rocks veins and pegmatites in the catalina schist subduction zone metamorphic complex allow futher characterization of complex high p t metasomatic proceses and evaluation of the scales of isotopic equilibration and fluid transport during subduction zone metamorphism throughout the catalina schist n resides predominantly as nh4 in white mica which occurs in nearly all bulk compositions i e metasedimentary metamafic and to a lesser extent metaultramafic melange at all grades within each metamorphic unit of the catalina schist ranging in grade from lawsonite albite to amphibolite facies 15n values of mica in metasomatized metamafic and metaultramafic rocks are consistent with the metasomatic addition of n from nearby devolatilizing metasedimentary rocks into the initially n poor mafic and ultramafic rocks within each unit uniformity of mica 15n in metasomatized rocks relative to the 15n of metasedimentary rocks in the same unit implies mixing of n from nearby heterogeneous metasedimentary sources perhaps producing fluids with unifrom 15n at up to the kilometer scale however the trend in 15n of metasedimentary sources with increasing metamorphic grade is inconsistent with larger scale up temperature transfer of fluid in this case n2 bearing in the catalina schist paleosubduction zone such flow at scales of up to tens of kilometers has been inferred through previous oxygen isotope study nitrogen isotope compositions are instead believed to have been controlled at a more local scale than the o isotope systematics due to the more rock dominated fluid rock mass balance for n the 15n of muscovite in leucosomes and pegmatites in amphibolite grade metasedimentary exposures matches that of muscovite in metasedimentary hosts implying minimal n isotope fractionation during migmatization processes and possible transfer of metasedimentary n isotope signatures in silicate melts these results illustrate the potential of the n isotope system to yield valuable information regarding fluid rock interactions in the crust and mantle the data for the catalina schist demonstrate the ability of n isotopes to trace the transfer of sediment derived c o h s n fluids and silicate melts and show the expected benefit of the n isotope system in having a differing fluid rock mass balance relative to the more commonly used stable isotope systems that can yield unique constraints in quantitative models of crustal fluid processes

calcium isotopes in the global biogeochemical ca cycle implications for development of a ca isotope proxy abstract at the earth s surface calcium ca is a critical element at a variety of scales it is soluble in natural waters a biological nutrient and a major constituent of the dominant mineral sink for carbon in the ocean caco3 there is a 4 variation in the ca isotopic composition 44ca 40ca expressed as 44ca of various ca reservoirs on earth suggesting ca isotopes as a promising tracer of ca cycling in both the present and the past fifteen years of high precision ca isotope measurements has revealed much about the behavior of ca isotopes in the earth surface environment but there remain fundamental questions concerning how ca isotopes are used to elucidate the marine and terrestrial ca cycles the current work presents a data compilation of over 70 published ca isotope studies totaling over 2600 measurements presented on a common delta scale that includes data on rivers and groundwater dust soils and soil pore fluids vegetation rainwater silicate minerals rocks and authigenic marine minerals carbonates sulfates and phosphates both modern and ancient the data compilation suggests that 1 there is a significant difference between carbonate 0 60 and silicate 44ca 0 94 2 riverine 44ca 0 88 does not simply reflect the compiled carbonate 44ca and 3 terrestrial vegetation exhibits the largest range of ca isotopic compositions 3 5 in the terrestrial setting we discuss these observations in the context of the global ca cycle exploring the extent to which seawater 44ca variability is feasible and how we can achieve accurate reconstructions of seawater 44ca over geologic time scales the current study presents simple mass balance models that quantify the leverage of inputs to change the ca isotopic composition of the ocean as this directly impacts the manner in which ca isotopes are interpreted although ca fractionates isotopically in the modern system during continental cycling the 44ca range of riverine inputs to the ocean is considerably smaller than the variability observed in putative seawater proxies such as nannofossil ooze and marine barite in the terrestrial realm plants exhibit a wide 44ca range and there is evidence that ca fluxes via biomass degradation are significant at the catchment scale we therefore assess the ability of the continental biosphere to influence riverine and consequently seawater 44ca a steady state biosphere has little leverage to alter riverine 44ca except in cases where the 44ca of the recycling flux is isotopically distinct from the 44ca of the uptake flux a non steady state biosphere can substantially impact both soil and riverine 44ca driving exchangeable ca either heavier or lighter depending on the magnitude of the recycling flux relative to the uptake flux based on estimates of the size of the global biosphere 1 5 1015 mol ca we suggest a decaying biosphere has the potential to impact riverine 44ca by tenths of a permil over time scales in the marine realm we evaluate the effect of a variable fractionation factor accompanying global removal of ca from the ocean on seawater 44ca and suggest methods by which such a mechanism can be recognized in the rock record experimental data suggest that there is considerable leverage