silicon isotope fractionation during silica precipitation from hot spring waters evidence from the geysir geothermal field iceland this study aims to explore the extent and controls of silicon isotope fractionation in hot spring systems of the geysir geothermal area iceland a setting where sinter deposits are actively formed the 30si values of dissolved silica measured in the spring water and sampling sites along outflowing streams covering a temperature range between 20 and 100 c were relatively constant around 0 2 whereas the 30si signatures of associated opaline sinters from the streambeds were between 0 1 and 4 0 becoming progressively more negative in the downstream parts of the aprons here the deposited sinters represent some of the most 30si depleted abiotically produced terrestrial materials documented to date compared to the data reported for icelandic basalts considered to be the source of the silicon the 30si values of the fluids and sinter deposits are higher and lower respectively the resulting values for apparent solid water isotope fractionation 30sisolid water decreased with decreasing temperature from ca 0 7 at 80 c to 3 7 at 20 c locally down to 4 4 this temperature relationship was reproducible in each of the investigated hot spring systems and is qualitatively consistent with recent findings in laboratory experiments on kinetic fractionation for a flowing fluid however the apparent fractionation magnitudes observed in the field are ca 2 more negative and thus significantly larger we infer that solid water silicon isotope fractionation during deposition of amorphous silica from a flowing fluid correlates inversely with temperature but is essentially a function of the precipitation rate such that the fractionation factor decreases with increasing rate as an important corollary the effective fractionation behavior during precipitation of silica from saturated solutions is a system dependent feature which should be taken into account when using silicon isotopes for paleo environmental reconstructions

jotunites from the grenville province quebec petrological characteristics and implications for massif anorthosite petrogenesis abstract we report new results on the geology mineralogy and chemical compositions of jotunites mainly pyroxene monzodiorites from the morin st urbain and labrieville anorthosite complexes all in the grenville province of quebec jotunites occur between anorthosite and mangerite as marginal units to anorthosite or mangerite plutons or as fine grained dikes in either rock type some occurrences are probably cumulates but the dikes indicate the existence of jotunitic liquids jotunites consist predominantly of sodic plagioclase an25 an45 clinopyroxene and orthopyroxene having intermediate xmg e g en60 en40 along with significant ilmenite magnetite and apatite most samples contain small amounts of k feldspar locally mesoperthite and quartz it is noteworthy that plagioclase and pyroxene compositions in jotunites lie between those in associated anorthosite and mangerite jotunites from all areas span a similar range in sio2 content 40 55 wt and are characterized by high levels of fe2o3 t 30 wt tio2 6 wt and p2o5 3 25 wt all samples are alkalic with k2o 3 6 wt increasing in the sequence morin st urbain labrieville they contain high sr up to 1400 ppm and ba up to 4600 ppm and concentrations correlate well with those in associated anorthosite in each case jotunites also are high in ga high ga al zr zn y and ree but low in rb high k rb u th ni cr co sc and v high ti v jotunites can be linked spatially mineralogically and chemically with associated anorthosite and mangerite and we interpret them as transitional rocks in a largely comagmatic sequence from anorthosite to mangerite we postulate a model for andesine anorthosite petrogenesis involving early crystallization and accumulation of plagioclase from parental magmas of intermediate sio2 content broadly dioritic in composition resulting in fe ti and p enrichment in the remaining magma leading to jotunite high o2 table causes early crystallization of fe ti oxides which suppresses fe mg enrichment in pyroxene fractionation of fe ti oxides pyroxene and apatite ultimately results in si and k enrichment in residual liquids leading to quartz mangerite significantly liquids follow an initial trend of si depletion fe enrichment followed by a reversal of this trend this view contrasts strongly with current models that regard anorthosite jotunite and mangerite as unrelated products of bimodal magmatism

sm nd evidence for a major 1 5 ga crust forming event in the central grenville province the majority of gray gneisses from an 80 000 km 2 area in the central quebec grenville province define a sm nd isochron that yields an age of 1 53 0 07 ga major element trace element and initial nd isotopic data all point to a subduction related orogenic source for this terrane it is attributed to a major crustal extraction event that formed a mid proterozoic island arc the age of the terrane agrees with published data from the eastern and western grenville province in newfoundland and ontario respectively we suggest that 1 5 ga are fragments were accreted along much of the length of the laurentian foreland within 100 m y of their formation in an event marked by widespread granitoid plutonism

absolute accumulation and isotope fractionation of si and fe during dolomite weathering and terra rossa formation abstract the behavior of silicon si and iron fe and their isotope fractionation during carbonate weathering and terra rossa formation are unclear here we focus on a 4 05 m thick terra rossa profile developed on dolomite in the yunnan guizhou plateau china we measured the mineral and geochemical composition magnetic susceptibility and si and fe isotopic composition of the terra rossa ferromanganese concretions fmcs and the dolomite and its insoluble residues the results indicate that there is absolute accumulation of si and fe during dolomite weathering and terra rossa formation due to the extraneous input of dissolved si and fe this means that the insoluble residues of dolomite are not the only source for terra rossa formation in addition we suggest that the high frequency dependent magnetic susceptibility fd and rubification of the terra rossa are the result of the oxidation of extraneous dissolved fe and the neoformation of finely dispersed fe oxides overall the terra rossa is characterized by the enrichment of light si and fe isotopes compared with insoluble dolomite residues and bulk dolomite the extraneous input of dissolved fe is characterized by a light isotopic signature whereas the si isotopic signature of the extraneous source is unclear due to modification by late desilicification si isotope fractionation in the terra rossa is mainly controlled by chemical weathering and associated mobilization and redistribution of si the 30 si values of terra rossa decrease with increasing weathering intensity while the re precipitation of mobilized si leads to the significant enrichment of isotopically heavy si in the basal terra rossa the 30 si values of terra rossa are not significantly affected by the adsorption of the fe and al oxyhydr oxides by contrast the major process that fractionates fe isotopes in the dolomite terra rossa weathering system is unclear in the terra rossa profile the 56 fe values of the fmcs are significantly lower than those of the surrounding terra rossa matrix these low 56 fe signatures of the fmcs reflect the preferential translocation of the light fe isotope while the fmcs may be able to record the low 56 fe characteristics of pore water

crystallization and emplacement of the lac st jean anorthosite massif quebec canada the lac st jean anorthosite massif underlies an area of over 20 000 km2 and has been emplaced into migmatitic gneisses of the central granulite terrain of the grenville province of the canadian shield field data and petrography in an area straddling the anorthosite gneiss contact close to chicoutimi quebec permits an outline of its tecto magmatic evolution depositional magmatic textures in the massif reveals that it crystallized from a magma in a relatively calm tectonic environment the absence of fusion in pelitic gneisses at the contact proves that the crystallization did not take place at the level presently exposed the parallelism of subvertical foliation in the enveloping gneisses and the anorthosite indicates that both were deformed together it is suggested that the deformation results from a diapiric ascent of the anorthosite massif after its consolidation at depth the depth of consolidation of the anorthosite is estimated at 25 30 km from subsolidus reaction between plagioclase and olivine the diapiric ascent is further substantiated by the fact that three sets of mafic dykes of different ages intrusive into the anorthosite have a mineralogy which indicates successively decreasing p t conditions of emplacement from granulite fades to amphibolite facies an evolution of the basement gneisses and the anorthosite is proposed as a working hypothesis it relies on the fact that metabasite dyke swarms in the basement gneisses represent a period of major crustal extension and could be used as a stratigraphic subdivision of the grenville province

the δ15n of nitrate in the southern ocean consumption of nitrate in surface waters we report nitrogen isotope data for nitrate from transects of hydrocast and surface samples collected in the eastern indian and pacific sectors of the southern ocean focusing here on the data from the upper water column to study the effect of nitrate consumption by phytoplankton the 15n of nitrate increases by 1 2 from deep water into the antarctic summertime surface layer due to kinetic isotopic fractionation during nitrate uptake estimation of the nitrate uptake isotope effect from antarctic depth profiles yields values in the range of 5 6 in east indian sector and 4 5 in the east pacific sector surface transect data from the pacific sector also yield values of 4 5 the major uncertainty in the profile based estimation of the isotope effect involves the 15n of nitrate from the temperature minimum layer below the summertime antarctic surface layer which deviates significantly from the predictions of simple models of isotope fractionation for the subantarctic surface it is possible to distinguish between nitrate supplied laterally from the surface antarctic and nitrate supplied vertically from the subantarctic thermocline because of the distinctive relationships between the 15n and concentration of nitrate in these two potential sources our subantarctic samples collected during the summer and fall indicate that nitrate is supplied to the subantarctic surface largely by northward transport of antarctic surface water isotopic data from the pacific sector of the subantarctic suggest an isotope effect of 4 5 indistinguishable from the antarctic estimates in this sector

silicon and chromium stable isotopic systematics during basalt weathering and lateritisation a comparison of variably weathered basalt profiles in the deccan traps india global biomass production is fundamentally affected by the hydrological cycling of elements at the earth s surface continental weathering processes are the major source for most bio essential elements in marine environments and therefore affect primary productivity in addition critical zone biomass depends on energy and chemical exchange reactions in weathering profiles the latter reservoirs are in turn influenced by different climatic conditions that control weathering and pore water parameters like ph and eh which regulate mineral break down rates and dictate the mobility and mass flux of elements two deccan traps basalt weathering profiles of contrasting age and alteration intensity provide a natural laboratory for investigating the effects of rock alteration on si and cr and their isotopic systematics the quaternary chhindwara profile has progressed to a moderate degree of alteration saprolite while the paleogene bidar example displays an extremely altered laterite the chhindwara saprolite profile shows a near uniform cr and si concentration and isotopic composition whereas the bidar laterite profile is characterised by an intense loss of si a large enrichment of cr within the most altered uppermost levels and a wide range of cr stable isotope ratios 0 85 to 0 36 parts per thousand delta cr 53 52 a co variation between si and cr isotopes as well as their co variation with iron content provides empirical evidence that iron redistribution within the profile has a large effect on cr mobility and si isotopic fractionation therefore it is concluded that iron oxides exert a primary control over the isotopic composition of both cr and si in pore waters of laterites since laterite formation is promoted by tropical climates the results of this study provide new evidence to suggest that the hydrological cr and si fluxes originating from continental weathering have changed in accordance with large scale deep time climate variation and continental plate configuration an increased flux of si and greater magnitude of cr mobility and isotopic fractionation are possibly amplified under co2 rich greenhouse episodes and or when large landmasses were tectonically arranged at near equatorial latitudes

isotopic constraints on the genesis of the rogaland anorthositic suite southwest norway demaiffe d weis d michot j and duchesne j c 1986 isotopic constraints on the genesis of the rogaland anorthositic suite southwest norway in s deutsch and a w hofmann editors isotopes in geology picciotto volume chem geol 57 167 179 isotopic data were obtained on the sveconorwegian 1200 900 ma anorthosites and associated norites and charnockites of the rogaland igneous complex sw norway the isotopic data isr 0 703 0 7077 end 0 4 to 5 5 suf 4pb 1 8 1 8 2 so 5 2 to 6 6 c suggest an upper mantle origin for the parental magma of the anorthosites and related norites and jotunites or an origin in the lower crust by melting of mantle derived basic rocks shortly after their formation in two well documented cases the egersund ogna and the hidra bodies a progressive contamination by u depleted but not rb depleted lower crustal material is apparent during the differentiation process the late stage liquids have higher i up to 0 7086 lower but still slightly positive end less radiogenic pb isotopic composition the intrusive acidic rocks some of them of the charnockitic type display variable isotopic data is 0 7055 0 709 end 0 5 to 0 8 pb isotopic compositions comparable to that of some of the surrounding gneisses suggesting that there are in general no direct cogenetic relations between the anorthosites and the charnockites our data point to an origin of some of these acidic rocks by anatectic melting of the surrounding high grade metamorphic gneisses

mechanisms controlling the silicon isotopic compositions of river waters it has been proposed that silicon si isotopes are fractionated during weathering and biological activity leading to heavy dissolved riverine compositions in this study the first seasonal variations of stable isotope compositions of dissolved riverine si are reported and compared with concomitant changes in water chemistry four different rivers in switzerland were sampled between march 2004 and july 2005 the unique high resolution multi collector icp ms nu1700 has been used to provide simultaneous interference free measurements of 28si 29si and 30si abundances with an average limiting precision of 0 04 on 30si this precision facilitates the clarification of small temporal variations in isotope composition the average of all the data for the 40 samples is 30si 0 84 0 19 1 sd despite significant differences in catchment lithologies biomass climate total dissolved solids and weathering fluxes the averaged isotopic composition of dissolved si in each river is strikingly similar with means of 0 70 0 12 for the birs 0 95 0 22 for the saane 0 93 0 12 for the ticino and 0 79 0 19 for the verzasca however the 30si undergoes seasonal variations of up to 0 6 comparisons between 30si and physico chemical parameters such as the concentration of dissolved si and other cations the discharge of the rivers and the resulting weathering fluxes permits an understanding of the processes that control the si budget and the fate of dissolved si within these rivers the main mechanism controlling the si isotope composition of the mountainous verzasca river appears to be a two component mixing between the seepage of soil ground waters with heavier si produced by clay formation and superficial runoff associated with lighter si during high discharge events a biologically mediated fractionation can be excluded in this particular river system the other rivers display increasing complexity with increases in the proportion of forested and cultivated landscapes as well as carbonate rocks in the catchment in these instances it is impossible to resolve the extent of the isotopic fractionation and contributed flux of si contributed by biological processes as opposed to abiotic weathering the presence of seasonal variations in si isotope composition in mountainous rivers provides evidence that extreme changes in climate affect the overall composition of dissolved si delivered to the oceans the oceanic si isotope composition is very sensitive to even small changes in the riverine si isotope composition and this parameter appears to be more critical than plausible changes in the si flux therefore concurrent changes in weathering style may need to be considered when using the si isotopic compositions of diatoms sponges and radiolaria as paleoproductivity proxies

u pb zircon dates of morin anorthosite suite rocks grenville province quebec u pb zircon ages of samples of anorthosite pyroxene monzodiorite jotunite and pyroxene quartz monzonite quartz mangerite of the morin anorthosite complex grenville province quebec are 1155 plus minus 3 1146 plus minus 4 and 1135 plus minus 3 ma respectively these dates are very similar to available dates for equivalent units of the lac st jean and adirondack anorthosite suite occurrences and slightly predate estimates of 1075 1100 ma for peak metamorphic conditions during the grenville orogeny in this region sup 87 sr sup 86 sr initial ratios of 0 7048 0 7051 for the three morin units sampled permit a comagmatic origin if the different emplacement or cooling times can be reconciled the mangerite sampled cannot have been formed by fusion of the upper crust but may have been derived from a relatively juvenile crust at depth all three units have interacted with continental crust given the likely depleted nature of the mantle in this region

a lower crustal origin for massif type anorthosites the origin of batholith sized masses of plagioclase rich crustal rocks mainly proterozoic in age has mostly been ascribed to aluminous magmas of mantle derivation here we propose that such masses are derived by partial melting of an aluminous lower continental crust which has developed following extraction of a granitic upper crust by intra crustal melting this segregation of upper and lower crusts occurred principally in the late archaean as the lower crust was depleted in heat producing elements during the initial melting to produce granites an external source of heat is required to produce proterozoic massif type anorthosites we speculate that this heat resulted from the continental crust moving over a mantle hotspot or line plume

the geochemistry of the la galissonniere pluton a middle proterozoic late orogenic intrusion from the eastern grenville province quebec the la galissonniere pluton is a late orogenic pluton located in the eastern grenville province of quebec the pluton contains biotite hornblende zircon titanite allanite and apatite along with secondary sericite and epidote the pluton features elevated values of nb y and f the normalized rare earth element patterns have light rare earth element concentrations of 400 500 chondrite moderate slopes 10 20 and noticeable eu anomalies trace element discriminant diagrams suggest that the pluton formed in a late orogenic environment and indicate a possible period of late precambrian crustal attenuation in the eastern grenville province

the continental si cycle and its impact on the ocean si isotope budget abstract the silicon isotope composition of biogenic silica 30sibsi in the ocean is a function of the 30si of the available dissolved si dsi h2sio4 the degree of utilisation of the available dsi and for some organisms the concentration of dsi this makes 30sibsi in sediment archives a promising proxy for past dsi concentrations and utilisation at steady state mean 30sibsi must equal a weighted average of the inputs the majority of which are of continental origin variation in the functioning of the continental si cycle on timescales similar to the residence time of dsi in the ocean 10 ka may therefore contribute to downcore variability in 30sibsi on millennial or longer timescales the direction and magnitude of change in published 30sibsi records over the last few glacial cycles is consistent among ocean basins and between groups of silicifiers they document glacial values that are typically 0 5 to 1 0 lower than interglacial values and together hint at coherent and predictable glacial interglacial variability in whole ocean 30si driven by a change in mean 30si of the inputs in this contribution we review the modern inputs of dsi to the ocean and the controls on their isotopic composition and assess the evidence for their variability on millennial plus timescales today 9 55 1012 mol yr 1 dsi enters the ocean of which roughly 64 and 25 are direct riverine inputs of dsi and dsi from dissolution of aeolian and riverborne sediment respectively the remainder derives from alteration or weathering of the ocean crust each input has a characteristic 30si with our current best estimate for a weighted mean being 0 74 although much work remains to be done to characterise the individual fluxes many aspects of the continental si cycle may have differed during glacial periods that together can cumulatively substantially lower the mean 30si of dsi entering the ocean these changes relate to i a cooler drier glacial climate ii lowered sea level and the exposure of continental shelves iii the presence of large continental ice sheets and iv altered vegetation zonation using a simple box model with a monte carlo approach to parameterisation we find that a transition from a hypothesised glacial continental si cycle to the modern si cycle can drive an increase in whole ocean 30si of comparable rate and magnitude to that recorded in 30sibsi this implies that we may need to revisit our understanding of aspects of the si cycle in the glacial ocean although we focus on the transition from the last glacial our synthesis suggests that the continental si cycle should be seen as a potential contributory factor to any variability observed in ocean 30sibsi on millennial or longer timescales

silicon isotope variations accompanying basalt weathering in iceland abstract this study presents the si isotope compositions of the dissolved phase of twenty rivers from across iceland the high resolution nu plasma 1700 mc icp ms was used to provide interference free 30 si 28 si ratios with an average limiting precision of 0 04 30 si the si isotope composition ranges from 0 08 to 1 46 30 si averaging 0 63 0 38 1 sd this is only slightly lighter than the recently reported mean of 0 84 0 19 for high precision measurements of swiss rivers but is significantly lighter than compositions reported for some other rivers the si isotope composition varies slightly with the amount of glacial cover which provides evidence that the continental si input to the oceans may be climatically controlled and thus likely to vary over glacial interglacial cycles after correction for the dissolution of trace amounts of calcite in one region a correlation is found between 30 si and ca si this is consistent with the measured basaltic composition and indicates isotopic fractionation during si removal from the dissolved phase by calculating the fraction of si that is removed from the dissolved phase into secondary phases it is possible to use the riverine si flux si denudation rate to calculate the initial dissolution rate of si within these catchments from this study we estimate that the total amount of si being dissolved by primary weathering of iceland is 3 million tons per yr however only 50 of this si is released into the rivers and contributes to denudation using the si isotope composition and the si fraction removed it is possible to model the si isotope fractionation as either a rayleigh type fractionation or a steady state system both yield similar fractionation factors of about 1 5 30 si implying only one underlying fractionation process that is controlled by external parameters such as runoff furthermore rivers that follow a steady state fractionation model are associated with higher si dissolution rates and lower 30 si values indicating an inverse relationship between weathering rates and dissolved riverine si isotope composition

columnar concretions in the visakhapatnam red sediments on the east coast of india abstract calcareous and siliceous columnar concretions occur in some of the visakhapatnam red sediments on the east coast of india the carbonate externally derived was rapidly precipitated in the intergranular space of the sediments by displacement of the fine material intense evaporation at the surface in the dry periods and the initial high permeability of the red sediments are considered to have produced the columnar forms the fine grained nature low mg calcite and the negative 13 c and 18 o ratios of the carbonate cement suggest an evaporative precipitation in a fresh water environment late diagenetic changes and compaction of the red sediments caused the modern development of the concretions to cease irregular basement topography largely controlled the localized distribution of the concretions in the red sediments siliceous concretions are confined to a narrow zone along the coast amorphous silica biogenous matter and silica microspheres constitute the cementing material of these concretions the microspheres indicate a precipitation of silica from saturated solutions in a protected environment the siliceous concretions are interpreted to have replaced the carbonate ones during the post pleistocene sea level rise in the visakhapatnam region

silicon isotopes in allophane as a proxy for mineral formation in volcanic soils abstract weathering of basaltic ash in volcanic areas produces andosols rich in allophane and ferrihydrite since the rate of mineral formation is very useful in climate and geochemical modelling this study investigates si isotope compositions of allophane as a proxy for mineral formation allophane formed in contrasting conditions in five icelandic soil profiles displays silicon isotope signatures lighter than the basalt in less weathered soils 0 64 0 15 and heavier in more weathered organic rich soils 0 23 0 10 the fate of the dissolved si in those volcanic soils strongly depends on al availability in organic rich soils most of al is humus complexed and the results support that si precipitates as opaline silica by super saturation leaving an isotopically heavier dissolved si pool to form allophane with uncomplexed al this study highlights that si isotopes can be useful to record successive soil processes involved in mineral formation which is potentially useful in environmental paleo reconstruction

weathering dust and biocycling effects on soil silicon isotope ratios abstract silicon isotope ratios 30si of bulk mineral materials in soil integrate effects from both silicon sources and processing here we report 30si values from a climate gradient of hawaiian soils developed on 170 ka basalt and relate them to patterns of soil chemistry and mineralogy the results demonstrate informative relationships between the mass fraction of soil si depletion and 30si in upper 1 m deep soil horizons generally correlate with the isotope fractionation predicted by a study of dissolved si in basalt watershed rivers and driven by preferential 28si removal from the dissolved phase during precipitation in contrast after correcting for the influence of dust secondary mineral si depletion and 30si values in shallow

micrometer silicon isotope diagnostics of soils by uv femtosecond laser ablation this study presents the first si isotope data of the principle si pools in soils determined by a uv femtosecond laser ablation system coupled to a multicollector inductively coupled plasma mass spectrometer mc icp ms this method reveals accurate and precise si isotope data on bulk materials and at high spatial resolution on the mineral scale the following si pools have been investigated a the si source to soils on all major silicate minerals on thin sections from bedrock fragments in the soil profiles b bulk soils particle size 2 mm after fusion to glass beads with an iridium strip heater or pressed into powder pellets c separated clay fractions as pressed powder pellets and e separated phytoliths as pressed powder pellets multiple analyses of three rock standards bhvo 2 agv 1 and rgm 1 as fused glass beads and as pressed powder pellets reveal 30si values within the expected range of igneous rocks the mpi ding reference glass kl2 g exhibits the same si isotope composition after remelting by an iridium strip heater showing that this technique does not alter the isotope composition of the glass we used this approach to investigated two immature cambisols developed on sandstone and paragneiss in the black forest germany respectively bulk soils show a largely uniform si isotope signature for different horizons and locations which is close to those of primary quartz and feldspar with 30si values around 0 4 soil clay formation is associated with limited si mobility which preserves initial si isotope signatures of parental minerals an exception is the organic horizon of the paragneiss catchment where intense weathering leads to a high mobility of si and significant negative isotope signatures as low as to 1 00 in bulk soils biogenic opal in the form of phytoliths exhibits negative si isotope signatures of about 0 4 these results demonstrate that uv femtosecond laser ablation mc icp ms provides a tool to characterize the si isotope signature of the principle si pools left behind after weathering and si transport have altered soils these results can now serve as a fingerprint of the residual solids that can be used to explain the isotope composition of dissolved si in soil solutions and river water which is mostly enriched in the heavy isotopes

chemical weathering mass loss and dust inputs across a climate by time matrix in the hawaiian islands we determined the total mass loss and rate of chemical weathering from three minimally eroded hawaiian lava flows that are 10 170 and 350 ka old using a backhoe we sampled the entire weathering zone at 28 sites and measured the depletion or enrichment of each major element in each soil horizon relative to parent material we were able to assess the influence of both climate and substrate age on chemical weathering because each flow crosses a precipitation gradient from 600 to 2500 mm yr 1 mass loss rates were highest for the 0 10 ka interval under the wettest climatic conditions 54 t km 2 yr 1 and decreased to near zero in the wet sites during the 10 170 and 170 350 ka intervals not surprisingly weathering rates were lower in drier sites 24 t km 2 yr 1 from 0 10 ka to b 2tk m 2 yr 1 thereafter however the effects of precipitation were non linear there was a precipitation threshold below which mass loss was relatively small and above which mass loss was substantial but insensitive to increased rainfall chemical weathering rates depend on tectonic uplift erosion climate rock type or some combination thereof by working on stable uneroded surfaces of a single rock type across a well constrained precipitation gradient we were able to identify another potential driver the rate of dust deposition although hawai i is one of the least dusty places in the northern hemisphere dust inputs reached 82 of the total mass loss from the weathering zone at some sites and averaged 30 on the 170 ka flow this highlights the potential importance of dust as a component of observed weathering fluxes from catchments worldwide

sr isotopes as a tracer of weathering processes and dust inputs in a tropical granitoid watershed luquillo mountains puerto rico abstract sr isotope data from soils water and atmospheric inputs in a small tropical granitoid watershed in the luquillo mountains of puerto rico constrain soil mineral development weathering fluxes and atmospheric deposition this study provides new information on pedogenic processes and geochemical fluxes that is not apparent in watershed mass balances based on major elements alone 87sr 86sr data reveal that saharan mineral aerosol dust contributes significantly to atmospheric inputs watershed scale sr isotope mass balance calculations indicate that the dust deposition flux for the watershed is 2100 700 mg cm 2 ka 1 nd isotope analyses of soil and saprolite samples provide independent evidence for the presence of saharan dust in the regolith watershed scale sr isotope mass balance calculations are used to calculate the overall short term chemical denudation velocity for the watershed which agrees well with previous denudation rate estimates based on major element chemistry and cosmogenic nuclides the dissolved streamwater sr flux is dominated by weathering of plagioclase and hornblende and partial weathering of biotite in the saprock zone a steep gradient in regolith porewater 87sr 86sr ratio with depth from 0 70635 to as high as 0 71395 reflects the transition from primary mineral derived sr to a combination of residual biotite derived sr and atmospherically derived sr near the surface and allows multiple origins of kaolinite to be identified

natural variations of δ30si ratios during progressive basalt weathering hawaiian islands abstract silicon stable isotopes can be used to trace the biogeochemical pathways of si as it moves from its continental sources to its sink in ocean sediments along the way si is incorporated into clay minerals taken up by plants where it forms plant opal and leached into rivers the major land to ocean conduit compared to igneous rocks the waters that drain continents are enriched in heavy si isotopes but the mechanisms that control fractionation have not been elucidated we studied si isotope fractionation along a 4 million yr basaltic soil chronosequence on the hawaiian islands using the natural context of these samples in combination with laboratory experiments we demonstrate that the isotopic composition of dissolved si in weathering systems is determined by the combined effects of rock disintegration clay mineral neosynthesis and si biocycling weathering preferentially releases 28si into solution whereas secondary mineral formation preferentially removes 28si from solution in humid environments leached soils have lost large amounts of this soluble si thus creating a net loss of 30si from the entire soil system as soils develop and greater fractions of si reside in neoformed clay minerals 30sibulk soil values change progressively toward more negative values basalt 30si values are about 0 5 but older soils have 30si values up to 2 5 the difference between the solid and solution 30si values remains more or less constant with progressive weathering and therefore soil water from older soils has a more negative 30si composition in the upper horizons of the hawaiian soils this weathering driven 30si shift is modified by the addition of unweathered primary minerals via dust carrying 30si values of about 0 5 and by biocycling of si via plants producing negative 30si values in phytoliths and positive 30si values in soil solutions derived from upper horizons due to the high concentrations of dissolved si in these near surface layers rivers have more positive 30si values than predicted based on the weathering status of the lower horizons when combined with published 30si values from large rivers worldwide we find that the results from hawaii point to weathering control of si isotopes delivered to the oceans and thus to an important continent ocean linkage that warrants further investigation

precambrian si isotope mass balance weathering and the significance of the authigenic clay silica sink abstract the geological cycles of carbon and silicon are linked via the silicate weathering feedback but growing understanding of reverse weathering the formation of authigenic clay minerals via back reaction of silica with cations indicates that the nature of the connection between these two elemental cycles may be more complex than previously assumed recent carbon cycle modeling has suggested that enhanced reverse weathering during precambrian time could have played an important role in regulating climate the topology of the precambrian silica cycle including the size of the authigenic clay sink and its influence on the carbon cycle is challenging to reconstruct due to inherent preservation biases in the rock record here we used an alternative approach using si isotope values to invert for the magnitudes of precambrian silica fluxes by assuming that the silica cycle must be in mass balance with the bulk silicate earth over geological time scales with respect to si isotopes to estimate uncertainties associated with these flux reconstructions we used monte carlo simulations based on the natural variation exhibited in si isotope ratio data from the precambrian sedimentary record n 2118 spanning 3 8 ga to 525 ma and modern authigenic clays n 123 the perspective of si isotope mass balance shows that the previously observed increase in chert 30si values across precambrian time is a signal of an evolving silica cycle rather than a secular change in seawater temperature results yielded a secular increase in the relative contribution of the authigenic clay sink across most of precambrian time this view is consistent with previous interpretations of a concomitant decrease in the concentration of dissolved silica in seawater this result is consistent among a variety of model versions incorporating different assumptions about the abundance of iron formations and possible relationships between authigenic clay formation and concepts that invoke slow growth in the absolute amount of the continental crust this approach also highlights key areas where improving si isotope records could further enhance our constraints on the precambrian silica cycle and its relationship with the carbon cycle and climate stability

impact of low denudation rates on soil chemical weathering intensity a multiproxy approach abstract quantifying the influence of denudation i e physical erosion and chemical weathering on soil weathering intensity is an important component for a comprehensive understanding of element biogeochemical cycles the relation between the weathering intensity and the denudation rate is not clear and requires further investigation in a variety of climatic and erosional settings here in the betic cordillera southern spain we assess the soil chemical weathering intensity with a multiproxy approach combining different indicators of chemical weathering of the soil the total reserve in bases trb the content in fe oxides the quartz and clay content the soil cation exchange capacity cec and the silicon si isotope composition of the clay sized fraction our multiproxy approach demonstrates that in this semi arid environment at low denudation rates an increasing denudation rate decreases the soil weathering intensity whereas si mobility remains limited our results converge with previous conclusions based on chemical mass balance methods in the same geological setting mass balance methods and particularly chemical depletion fractions cdf are based on the immobility of a refractory element commonly zirconium zr relative to major cations in soils interestingly our study suggests that a weathering index such as the trb may provide a useful complement to assess soil chemical weathering intensity in eroding landscapes where the application of chemical mass balances may be hampered by potential zr mobility in the soil or by heterogeneity of zr concentrations in the bedrock

aeolian controls of soil geochemistry and weathering fluxes in high elevation ecosystems of the rocky mountains colorado abstract when dust inputs are large or have persisted for long periods of time the signature of dust additions are often apparent in soils the of dust will be greatest where the geochemical composition of dust is distinct from local sources of soil parent material in this study the influence of dust accretion on soil geochemistry is quantified for two different soils from the san juan mountains of southwestern colorado usa at both study sites dust is enriched in several trace elements relative to local rock especially cd cu pb and zn mass balance calculations that do not explicitly account for dust inputs indicate the accumulation of some elements in soil beyond what can be explained by weathering of local rock most observed elemental enrichments are explained by accounting for the long term accretion of dust based on modern isotopic and geochemical estimates one notable exception is pb which based on mass balance calculations and isotopic measurements may have an additional source at one of the study sites these results suggest that dust is a major factor influencing the development of soil in these settings and is also an important control of soil weathering fluxes after accounting for dust inputs in mass balance calculations si weathering fluxes from san juan mountain soils are within the range observed for other temperate systems comparing dust inputs with mass balanced based flux estimates suggests dust could account for as much as 50 80 of total long term chemical weathering fluxes these results support the notion that dust inputs may sustain chemical weathering fluxes even in relatively young continental settings given the widespread input of far traveled dust the weathering of dust is likely and important and underappreciated aspect of the global weathering engine

silicon isotope variations in central siberian rivers during basalt weathering in permafrost dominated larch forests abstract this work is devoted to the characterization of natural mechanisms of silicon isotope fractionation within siberian watersheds and predicting the climate warming effect on si fluxes from the land to the arctic ocean to unravel the different sources of silica generated by basalt weathering in central siberia under permafrost and larch deciduous forest conditions we measured the si isotopic composition of large and small rivers surface flow interstitial soil solutions plant litter and soils the average annual discharge weighted 30 si values of the second largest tributary of the yenissei river nyzhnaya tunguska and its main northern tributary kochechum are equal to 1 08 0 10 and 1 67 0 15 respectively while their average annual si concentrations are very similar 3 46 and 3 50 mg l respectively during summer baseflow the dissolved si isotope composition of both large rivers and a small stream ranges between 1 5 and 2 5 this is much heavier compared to the source basaltic rocks but similar to the fresh litter of larix gmelinii the dominating tree species in this region it could be consistent with litter degradation in the uppermost soil horizons being the dominant source of solutes annually exported by central siberian rivers during spring flood accounting for 60 80 of annual si flux the 30 si of the large rivers dissolved load decreases by 1 1 5 thus approaching the value of the bedrock and the silicate suspended matter of the rivers rsm this may reflect the dissolution of the silicate suspended load at high water mineral ratio the winter 30 si values of the large river dissolved load range between 1 0 and 2 5 during this period contributing to 10 of the annual si chemical flux the interaction between bedrock porous tuffs and deep ground waters occurs at a very high solid solution ratio leading to the precipitation of isotopically light secondary minerals and enrichment of 30 si in the fluids that feed the river through the unfrozen flowpaths results of this study imply that more than a half of the silica transported by siberian rivers may transit through the biogenic pool and that like in other stable basaltic regions bedrock water interactions account for a lesser fraction of the silica flux as a result of projected future climate warming and weathering increases in boreal regions the 30 si isotopic composition of large siberian rivers is likely to shift towards less positive values

long term base cation weathering rates in forested catchments of the canadian shield abstract a number of methods exist for estimating mineral weathering rates yet quantifying this crucial biogeochemical reaction in the field has always been a challenge because mineral weathering takes place in open systems whose initial state conditions are not well known and where material inputs and outputs vary in time and space here using the soil profile mass balance method we estimated long term weathering rates of base cations bcs for a set of 21 watersheds and we investigated the links between environmental variables and soil properties and the spatial variation in bc weathering rates in the study area the watersheds are located in southern quebec and vary with respect to hydro climatic conditions soil properties and forest cover average long term estimates of annual bc weathering rates for the study area were 0 16 0 10 0 09 and 0 06 kmolc ha 1 year 1 for ca mg na and k respectively overall redundancy analysis rda indicated that soil surface area percent deciduous canopy cover elevation as well as soil albite content were the most significant variables explaining together 56 of the total variation in bc weathering rates consistent with previous findings our results showed that beside soil properties climate related environmental variables played key roles in determining the variability of bc weathering rates in the study area in the context of global climate change more insights are needed for a better understanding of the effects of discrete climatic variables on bc weathering rates

another continental pool in the terrestrial silicon cycle silicon is the second most abundant element on earth and is involved in important terrestrial and marine biogeochemical processes however its terrestrial cycle is not well quantified and there is a missing reservoir in the budget of silicon isotopes water in seas and rivers is enriched in silicon 30 compared with igneous rocks suggesting that there is an unknown continental reservoir that is depleted in silicon 30 now with the help of a new generation mass spectrometer basile doelsch et al have obtained in situ measurements of silicon isotopes in quartz from sandstones in southern france the results showing greater silicon 30 depletion than in any previously reported terrestrial samples suggest that silicified rocks formed from quartz precipitates in soils and groundwaters can account for the missing reservoir silicon is the second most abundant element on earth it is an important nutrient for phytoplankton1 and is readily absorbed by terrestrial vegetation2 it also assists the removal of carbon dioxide from the atmosphere through the weathering of silicates3 but the continental cycle of silicon is not well known and only a few studies have attempted to use silicon stable isotopes 28si 29si and 30si 4 5 6 7 8 9 10 11 12 13 to quantify the continental silicon reservoirs dissolved silicon in sea and river waters forms a reservoir of mean isotopic value 1 1 refs 7 10 it is enriched in 30si with respect to the igneous rocks reservoir which has a mean isotopic value of 0 3 refs 4 9 this enrichment can only be produced by a major fractionation during weathering and should result in the formation of a continental 30si depleted reservoir such a reservoir however has not been identified to date here we analyse silicon isotopes of in situ quartz from a sandstone series in france using a new generation secondary ion mass spectrometry apparatus we show that quartz that precipitates as siliceous cements forms a strongly 30si depleted reservoir with isotopic values down to 5 7 a more negative value than any previously published for terrestrial samples our findings suggest that quartz re precipitation plays an important role in the biogeochemical cycle of silicon

distinct silicon and germanium pathways in the soil plant system evidence from banana and horsetail plants strongly impact the continental silicon cycle by taking up si and precipitating opal phytoliths which are recycled into the soil studying ge incorporation a chemical analog of si relative to si may provide a useful tracer of si pathways however ge uptake and transport through plants and the impact on ge si of phytoliths remain poorly understood here we report ge uptake and accumulation and ge si fractionation in all plant parts and solutions from 1 hydroponic banana 2 in situ sampled banana and 3 horsetails we further combine these data with delta si 29 from banana plants our data reconcile opposite conclusions drawn from previous studies on ge uptake and pathways no discrimination of ge occurred at the root solution interface banana and horsetails were shown to accumulate ge in roots a previous study provided evidence of low ge si ratios in root phytoliths which contrasts with high bulk ge si ratios in roots we report here this suggests that ge is organically trapped in roots consequently shoots display lower ge si ratios without fractionation between shoot parts since ge would follow transpiration stream as silicon and is not discriminated between shoot parts this contrasts with the two step discrimination against heavy si isotopes at the root solution interface and then within the shoots the soil composition clays versus fe oxides has a leading role on the ge si signatures of plants which may in turn impact on the si and ge fluxes to the global ocean

sims analyses of silicon and oxygen isotope ratios for quartz from archean and paleoproterozoic banded iron formations banded iron formations bifs are chemical marine sediments dominantly composed of alternating iron rich oxide carbonate sulfide and silicon rich chert jasper layers isotope ratios of iron carbon and sulfur in bif iron bearing minerals are biosignatures that reflect microbial cycling for these elements in bifs while much attention has focused on iron banded iron formations are equally banded silica formations thus silicon isotope ratios for quartz can provide insight on the sources and cycling of silicon in bifs bifs are banded by definition and microlaminae or sub mm banding are characteristic of many bifs in situ microanalysis including secondary ion mass spectrometry is well suited for analyzing such small features in this study we used a cameca ims 1280 ion microprobe to obtain highly accurate 0 3 and spatially resolved 10 lm spot size analyses of silicon and oxygen isotope ratios for quartz from several well known bifs isua southwest greenland 3 8 ga hamersley group western australia 2 5 ga transvaal group south africa 2 5 ga and biwabik iron formation minnesota usa 1 9 ga values of d 18 o range from 7 9 to 27 5 and include the highest reported d 18 o values for bif quartz values of d 30 si have a range of 5 from 3 7 to 1 2 and extend to the lowest d 30 si values for precambrian cherts isua bif samples are homogeneous in d 18 o to 0 3 at mm to cm scale but are heterogeneous in d 30 si up to 3 similar to the range in d 30 si found in bifs that have not experienced high temperature metamorphism up to 300 c values of d 30 si for quartz are homogeneous to 0 3 in individual sub mm laminae but vary by up to 3 between multiple laminae over mm to cm of vertical banding the scale of exchange for si in quartz in bifs is thus limited to the size of microlaminae or less than 1 mm we interpret differences in d 30 si between microlaminae as preserved from primary deposition silicon in bif quartz is mostly of marine hydrothermal origin d 30 si 0 5 but silicon from continental weathering d 30 si 1 was an important source as early as 3 8 ga

δ30si systematics in a granitic saprolite puerto rico granite weathering and clay mineral formation impart distinct and interpretable stable si isotope 30 si signatures to their solid and aqueous products within a saprolite clay minerals have 30 si values 2 0 more negative than their parent mineral and the 30 si signature of the bulk solid is determined by the ratio of primary to secondary minerals mineral specific weathering reactions predominate at different depths driving changes in differing 30 si pore water values at the bedrock saprolite interface dissolution of plagioclase and hornblende creates 30 si pore water signatures more positive than granite by up to 1 2 these reactions are the main contributor of si to stream water and determine its 30 si value throughout the saprolite biotite weathering releases si to pore waters but kaolinite overgrowth formation modulates its contribution to pore water si the influence of biotite on 30 si pore water is greatest near the bedrock where biotite derived si mixes with bulk pore water prior to kaolinite formation higher in the saprolite biotite grains have become more isolated by kaolinite overgrowth which consumes biotite derived si that would otherwise influence 30 si pore water because of this isolation which shifts the dominant source of pore water si from biotite to quartz 30 si pore water values are more negative than granite by up to 1 3 near the top of the saprolite

biogenic and pedogenic controls on si distributions and cycling in grasslands of the santa cruz soil chronosequence california abstract biogenic and pedogenic processes control silica cycling in grasslands growing on a soil chronosequence and dominated by strong seasonal variabilities of a mediterranean climate shallow pore water si in spite of significant annual uptake and release by plant growth and dieback exhibits only moderate seasonal fluctuations reflecting strong buffering from labile biogenic si dominated by phytoliths and by secondary pedogenic silicates long phytolith residence times 340 900 yrs reflect the seasonally dry climate and high solute si concentrations water extractable si is closely associated with al indicating seasonal precipitation and dissolution of a highly labile 1 1 hydroxyaluminosilicate has probably allophane which transforms in deeper soil into fine grained poorly crystalline kaolinite shallow plant roots extract greater proportions of biogenic si and deeper plant roots larger amounts pedogenic si high pore water ge si in late winter and spring reflects the reinforcing effects of plant fractionation and concurrent dissolution of ge enriched has the same processes produce pore waters with depleted 30 si 28 si in the summer and fall ge si declines and 30 si 28 si increases reflecting the cessation of plant uptake continued dissolution of soil phytoliths and re precipitation of less soluble has si inputs from weathering 2 90 mmol m 2 yr 1 and losses from pore water discharge 18 68 mm m 2 yr 1 are comparable for individual soils decline with soil age and are significantly less than amounts of si annual cycled through the vegetation 42 171 mm m 2 yr 1 mobile si is generally balanced in the soils with upward bio pumping by the shallow rooted grasses efficiently competing against downward leaching and pore water discharge small net annual increases in si in the present day soils could not have been maintained over the time scale represented by the chronosequence 65 225 yrs implying past changes in environmental conditions

the protocontinental nature and regional variability of the central metasedimentary belt of the grenville province lead isotope evidence the lead isotopic compositions of sulphide samples mainly galenas from a representative set of mineral deposits within the central metasedimentary belt of the grenville province have been determi

silicon isotope study on rice plants from the zhejiang province china silicon isotope compositions of 16 rice plants from the zhejiang province were studied and potential applications of silicon isotope study to agriculture and biogeochemistry were discussed it is found that the sio2 contents in ignited rice plant samples increased generally from roots through sl stem and leaves to husks large and systematic silicon isotope fractionation was observed among different organs of individual rice plants their 30si values show a generally increasing trend from roots and sl through husks to grains which can be explained by kinetic isotope fractionation in a rayleigh process a 30si value of 6 1 was obtained from two samples of rice grains which is much greater than the highest value 3 4 observed in terrestrial samples before it is inferred that monosilicic acid is the major silicon bearing component taken up by rice roots passive uptake of silicon is important for rice and evapotranspiration may be the major mechanism for the transportation and precipitation of silicon in rice plants it is suggested that silicon isotope study has a potential application to agriculture and biogeochemistry

halloysite as a kinetically controlled end product of arid zone basalt weathering abstract mineralogical and isotope results paired with field observations suggest that halloysite is the favored albeit metastable aluminosilicate end product of arid zone basalt weathering on kohala hawai i and that the formation of smectite has been inhibited by kinetic factors soils sampled on constructional lava flows having ages ranging from 4 to 350 ka provide a chronosequence that has had minimal physical disturbance thus allowing us to interpret chemical and mineralogical changes as the result of a time dependent process the halloysite content of the soil increases with increasing soil age its growth is at the expense of allophane which in turn forms at the expense of primary minerals these mineral relationships suggest that halloysite has been forming continuously throughout the lifetime of the soil smectite the thermodynamically stable phyllosilicate phase predicted by soil solution composition is not found in these arid soils we determined that our soil system is controlled by kinetic factors and that therefore thermodynamic predictions do not reflect reality the main factors favoring kinetic control of halloysite formation are intense but short wet periods followed by prolonged extremely dry seasons and microenvironmental conditions leading to immediate uptake of released al by the halloysite precursor mineral allophane the 18o relationship between present day soil water and halloysite formed over the last 170 ka was used as a tracer of past soil conditions results from a reconstruction of paleo climate and soil conditions combined with 18o data observed mineral relationships along the 350 ka chronosequence and field evidence for long term aridity rule out the possibility that the arid side of kohala was affected by prolonged periods of higher rainfall that could have produced more dilute soil waters and therefore altered mineralogical stabilities we conclude that pedogenic halloysite has been continually forming from the early stage of soil formation and that it has consistently formed under isotopic equilibrium conditions indicating that halloysite 18o compositions imply paleoclimatic conditions over the time of its formation that are similar to today s

age and radiogenic isotopic composition of a late to post tectonic anorthosite in the grenville province the labrieville massif quebec abstract the labrieville anorthosite massif lbv is found in the central granulite terrain of the grenville structural province but it displays no evidence of post emplacement deformation or metamorphism implying intrusion following peak grenvillian metamorphic conditions we report u pb zircon dates of 1008 3 4 ma for border leucogabbro and 1010 5 6 ma for a cogenetic jotunite dike intruding anorthosite we interpret these dates as igneous crystallization ages and regard 1010 ma as a reasonable estimate of the emplacement age for lbv lbv is thus the youngest massif anorthosite yet recognized in north america and its age is consistent with late tectonic emplacement relative to the 1 1 1 0 ga grenville orogeny we also report a u pb date of 1015 1 8 ma for metamorphic zircon in a country rock amphibolite this could reflect the age of grenvillian regional metamorphism or perhaps a later heating episode resulting from the intrusion of numerous late felsic plutons in this area rb sr sm nd and u th pb isotopic compositions for four rock types anorthosite jotunite leucogabbro and a plagioclase megacryst span narrow ranges in each case consistent with comagmatism among these units isr t 1010 ma range from 0 7032 0 7034 and are among the lowest yet reported for anorthosite in the grenville province initial nd values are positive 0 8 to 2 5 like other grenville anorthosites pb isotopic compositions lie near the model mantle evolution curve of zartman and doe 1981 implying no involvement of significantly older crust in the petrogenesis of these rocks collectively these data suggest a source for lbv in the mantle or mafic lower crust lbv is a compositionally extreme anorthosite characterized by alkalic plagioclase an32or12 and high levels of sr 2000 ppm and ba 1000 ppm these properties cannot be attributed to simple crustal contamination of mantle derived basalt we suggest alternatively that lbv s compositional features may be linked with its late tectonic character perhaps reflecting partial melting of mafic lower crust brought about by crustal thickening during the grenville orogeny

the sept iles layered mafic intrusion geophysical expression eighty eight percent of the cambrian sept iles layered mafic intrusion possibly the second largest on earth lies under the estuary of the st lawrence river a marine geophysical cruise in 1986 t

halloysite versus gibbsite silicon cycling as a pedogenetic process in two lowland neotropical rain forest soils of la selva costa rica abstract halloysite and gibbsite although known to require quite different conditions for their formation commonly occur together in the same horizon in oxisols derived from andesitic parent materials in tropical costa rica we selected two soils of similar parent material but of different ages and soil moisture regimes to identify possible clues to the coexistence of these two minerals we employed selective dissolution procedures x ray fluorescence analysis and x ray diffraction on field moist and air dry bulk soil samples to investigate how mineralogy changes with depth we further separated the size fraction we found both soils to be depleted of primary minerals and pedogenesis to have progressed to advanced weathering stages particularly in the subsoils gibbsite xrd signal intensities were linearly and significantly related to weathering indices corroborating the residual nature of gibbsite as an endproduct of weathering processes the si bearing quartz and kaolinite group minerals were enriched in the topsoils indicating i their independence from a primary mineral si source and ii the existence of a mechanism capable of protecting them against the continuous tropical weathering pressure as we found no indications for retrospective additions of soil material through mass movement or aeolian additions we believe a vegetation dependent biological pumping mechanism to be the most plausible explanation for the presence of silica bearing minerals in the la selva topsoils the vertical distribution of 1 0 nm halloysite and its accumulation in the lower reaches of the wetter alluvial soil suggest that this metastable mineral forms as a result of si enrichment where the residence time of the pore water is long enough to allow for si concentrations to exceed the halloysite precipitation threshold taken together our evidence indicates gibbsite in the la selva soils to be the endproduct of intense tropical weathering while the presence of hydrated halloysite seems to have mainly kinetic reasons and is most probably coupled to the contemporary soil moisture regime

evaluation of three methods for estimating the weathering rates of base cations in forested catchments abstract this paper investigates three techniques that are commonly used to generate estimates of base cation bc weathering rates i e the profile mass balance pedon the watershed input output budget watershed and the profile model model these methods were compared for their relative performance in estimating bc weathering rates for 21 watersheds located in southern quebec that vary with respect to hydro climatic conditions soil properties and forest cover average total bc weathering rates for the 21 watersheds were 0 41 0 09 se 1 20 0 17 and 1 71 0 22 kmol c ha 1 yr 1 for pedon watershed and model respectively passing and bablok regression analysis demonstrated good agreement between watershed and model regression formula watershed 0 08 0 74 model with 95 ci for intercept 1 13 0 25 and for slope 0 40 1 43 while poorer agreements were observed between these two methods and pedon contrary to the watershed and the model methods bc weathering rates obtained with pedon were not significantly associated with the spatial variation of the soil calcite content and of the size of the soil exchangeable bc pools we hypothesized that in the calcite containing watersheds the performance of pedon was negatively impacted by environmental conditions that favored the partial dissolution and leaching of the calcite contained in the initial parent material including in the material situated at the base of the profile c horizon

silicon isotope compositions of dissolved silicon and suspended matter in the yangtze river china abstract silicon isotope compositions of main channel samples of the yangtze river were systematically investigated along with their chemical compositions the concentration of suspended matter in the yangtze river tends to decrease from the upper reaches to the lower reaches corresponding to settling of the sediments in the lakes and reservoirs due to reduction of the velocity of water flow the silica contents of suspended matter vary from 52 1 to 56 9 and their 30si values vary from 0 to 0 7 both similar to those of shales from the upper to lower reaches the silica contents of suspended matter tend to increase whilst their 30si values tend to decrease both trends reflect the increase of clay minerals and decrease of carbonates in suspended matter the concentrations of dissolved silicon vary from 97 to 121 mol l and their 30si values vary over a wide range from 0 7 to 3 4 from the upper to lower reaches dissolved silica concentrations tend to decrease and their 30si values tend to increase these trends mainly reflect the change of chemical and isotopic characteristics of the tributaries from the upper to lower reaches the major factors responsible for these changes may be the high meteoric precipitation and significant silicon absorption by grass in wetlands and rice in paddy fields in drainage areas of the middle and lower reaches there is no correlation between 30si of dissolved silicon and that of suspended matter the 30sidiss spm values vary over a wide range of 1 0 3 7 indicating that 1 they are out of isotopic equilibrium 2 dissolved silicon and the associated suspended matter do not belong to one physico chemical system and 3 isotopic exchange rate between them is very slow the 30si value of dissolved silicon output from the yangtze river to the east sea is estimated to be 3 0 much higher than the values reported for the amazon and congo rivers this increases the 30si range of dissolved silicon in the world s rivers from 0 4 1 2 to 0 4 3 4

the silicon isotope composition of the upper continental crust the upper continental crust ucc is the major source of silicon si to the oceans and yet its isotopic composition is not well constrained in an effort to investigate the degree of heterogeneity and provide a robust estimate for the average si isotopic composition of the ucc a representative selection of well characterised continentally derived clastic sediments have been analysed using high precision mc icpms analyses of loess samples define a narrow range of si isotopic compositions 30si 0 28 to 0 15 this is thought to reflect the primary igneous mineralogy and predominance of mechanical weathering in the formation of such samples the average loess 30si is 0 22 0 07 2 s d identical to average granite and felsic igneous compositions therefore minor chemical weathering does not resolvably affect bulk rock 30si and loess is a good proxy for the si isotopic composition of unweathered crystalline continental crust the si isotopic compositions of shales display much more variability 30si 0 82 to 0 00 shale si isotope compositions do not correlate well with canonical proxies for chemical weathering such as cia values but do correlate negatively with insoluble element concentrations and al si ratios this implies that more intensive or prolonged chemical weathering of a sedimentary source with attendant desilicification is required before resolvable negative si isotopic fractionation occurs shale 30si values that are more positive than those of felsic igneous rocks most likely indicate the presence of marine derived silica in such samples using the data gathered in this study combined with already published granite si isotope analyses a weighted average composition of 30si 0 25 0 16 2 s d for the ucc has been calculated

sr and nd isotope geochronology geologic history and origin of the adirondack anorthosite abstract we have analyzed samples from the adirondack marcy massif for rb sr and sm nd isotopes in an attempt to determine directly the primary crystallization age of a proterozoic massif type anorthosite rock suite the oldest age obtained 1288 36ma is from a 4 point sm nd isochron defined by igneous textured whole rock and mineral separate data from a local layered sequence gradational from oxiderich pyroxenite to leuconorite this age is older than silver s 1969 1113 ma zircon age of associated charnockites but is within the window of permissible anorthosite ages based on previous geochronology and field relationships as such 1288 ma may represent the time of crystallization of the massif for the most part however both sm nd and rb sr isotopic systems did not survive granulite facies metamorphism internal isochrons based on whole rocks and minerals yield ages between 995 and 919 ma these isotopic data suggest that the granulite fades metamorphism experienced by the massif was a prograde event that occurred a minimum of 100 ma and as much as 350 ma after crystallization of the massif the relatively large range in rb abundance and in calculated initial 87 sr 86 sr 0 7039 0 7050 and 143 nd 144 nd ratios among anorthosite suite rocks particularly those at or near the contacts of the marcy massif is explicable by variable contamination with crustal materials and or fluids derived from surrounding acidic metaplutonic rocks paragneisses and marbles despite uncertainies caused by crustal contamination and metamorphic resetting of primary ages marcy samples have epsilon nd values between 0 44 and 5 08 implying a source for the massif with long term depletion in light rare earth elements a probable source material would be depleted mantle

strontium isotopic constraints on the origin of proterozoic anorthosites abstract the proterozoic giles complex central australia contains an almost complete range of anorthosite types from minor or major layers in gabbronorite intrusions to large anorthosite troctolite bodies to small orthopyroxene anorthosite massifs each type has a distinctive sr isotopic signature anorthosite dominated masses have a regular relationship between ferromagnesian mineralogy initial 87 sr 86 sr and anorthite contents in plagioclase anorthosite troctolite bodies have significant olivine relatively low initial 87 sr 86 sr 0 7038 0 7043 and an 50 69 orthopyroxene dominant anorthosites have relatively high 87 sr 86 sr 0 7045 0 7063 and an 45 60 the pattern is found worldwide detailed study of one intrusion demonstrates that contamination by wall rock granulite produces the higher 87 sr 86 sr values anti correlation between 87 sr 86 sr and an and determines olivine orthopyroxene proportions olivine bearing anorthosites form from a primary aluminous tholeiite magma with plagioclase dominating the liquidus progressive contamination of this parent magma produces a gradation to orthopyroxene anorthosites

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

geochemical constraints on the genesis of the ekou banded iron formation shanxi province north china the ekou banded iron formation bif in the wutai area is hosted within the late archean baizhiyan formation the mineral assemblage is used to identify oxide and silicate facies the oxide facies is composed of magnetite and quartz and the silicate facies is characterized by the presence of silicate minerals a geochemical analysis shows that the major elemental compositions are dominantly sio2 and fe2o3t with very little al2o3 and tio2 and minor abundances of incompatible elements and transition elements these results indicate that negligible terrigenous materials were involved in the bif deposition the rare earth elements rees normalized by post archean australian shale exhibit the characteristics of light ree lree depletion heavy ree hree enrichment and positive la y and eu anomalies the y ho ratios are superchondritic these results indicate that the material that formed the ekou bif originated from the mixing of seawater and submarine hydrothermal fluids a ce anomaly deficiency and heavy fe isotope enrichment indicate that the ekou bif formed in an anoxic marine environment the 30sinbs 28 values for quartz in the ekou bif are similar to those of other bifs with distribution ranges of modern sinters and black smokers the 18ov smow distribution is similar to that of hydrothermal sedimentary siliceous rocks these results suggest that the formation of the ekou bif was closely connected to submarine volcanic exhalation activity

proterozoic anorogenic magmatism and its orogenic connections there is a major belt of proterozoic so called anorogenic igneous rocks that extends from the baltic shield through southern greenland and eastern canada to southern usa all rocks are situated within a zone of 1 9 1 6 ga arc accretionary orogens that consist of juvenile mantle derived material especially accretionary prism sediments that provided the fertile source for abundant granitic crustal melts there were two main periods of formation of the anorogenic rocks 1 76 1 55 ga rocks are mainly rapakivi granites and rhyolites that formed about 60 200 ma after the last compressional events in the svecofennian ketilidian and penokean orogens the time lag being caused by the slow thermal relaxation of thickened lower crust that had commonly been depleted by the extraction of earlier granitic crustal melts 1 45 1 4 ga anorogenic rocks are mainly rhyolites and peraluminous granites in central southern usa and anorthosites in eastern canada that formed in the continental margin of the grenvillian ocean the subduction of which led to the grenville orogen these anorogenic igneous rocks formed in extensional regimes during the formation and breakup of a 1 5 ga supercontinent

correlated study of initial sr 87 sr 86 and al mg isotopic systematics and petrologic properties in a suite of refractory inclusions from the allende meteorite we have studied a suite of six coarse grained ca al rich inclusions cais from the allende meteorite applying in concert detailed petrographic and chemical characterization ion probe mass spectrometric analyses of the al mg isotopic system to determine the abundance and distribution of 26a1 and thermal emission mass spectrometric analyses of the rb sr system to determine initial 87sr86sr ratios the simplest inclusion is a type bl usnm 3529 z that shows evidence of minor alteration and recrystallization since solidification from a melt droplet e g some of the coarse grained anorthite in the inclusion especially near the inclusion rim apparently formed by recrystallization or melting of fine grained secondary anorthite that had replaced melilite four other type bs contain evidence of more widespread recrystallization in addition to evidence of local replacement of melilite by coarse anorthite e g melilite zoning is complex and not igneous in origin and in one inclusion usnm 3529 21 melilite and pyroxene crystals enclose relicts of an earlier generation of the same phases the sixth inclusion usnm 3898 is a type a whose properties appear to be largely metamorphic in origin all six inclusions show variations in mg isotopic compositions indicating that they incorporated 26a1 in 3529 z the relative abundance of 26a1 26al27al 4 0 0 1 10 5 is close to the canonical value 26al27al 5 10 5 for cais but both anorthite and melilite show minor departures from a strict isochron relationship between mg and al in particular the coarse anorthite near the inclusion rim has initial 26al27al ratios of 1 2 10 5 implying formation at least 1 5 million years after the first generation anorthite the other four type bs exhibit larger disturbances of the al mg system that can be understood in terms of local recrystallization and isotopic exchange primarily between anorthite and melilite only 3898 is consistent with a strict al mg isochron relationship but this cai lacks primary anorthite and the small range of al mg among the phases analyzed may preclude resolution of any small isotopic disturbances the rb sr isotopic system also typically indicates some level of disturbance some of which must have been relatively recent nevertheless phases characterized by very low rb sr permit precise identification of initial 87sr86sr our al mg and sr data are largely consistent with a simple chronological interpretation of both systems i e the 26al results suggest that all the initial 87sr86sr ratios should have the same value and with one exception our initial 87sr86sr ratios are consistent with a single value this value is somewhat higher than the primitive value all reported by gray et al 1973 however also with one exception the very low rb sr cai data reported by gray et al 1973 are consistent with the same value the exception is the same inclusion in both studies our analysis of usnm 3898 yields an initial 87sr86srhigher than that of the other cais while gray et al 1973 obtained an initial 87sr86srlower than that of the other cais for the same inclusion labeled as d7 on which all is based neither result can be readily explained as a chronological aberration since 3898 has nearly canonical 26al27al the present data are the first which establish a firm association between primitive 26al27al and primitive 87sr86sr by analysis of both isotopic systems in the same inclusions the comparison of both isotopic systems interpreted as simple nebular chronometers does not reveal any chronological inconsistencies that demand resolution in terms of a grossly heterogeneous distribution of 26al

refractory element mobility in volcanic soils refractory trace element concentrations in strongly weathered hawaiian soils ranging in age from 20 to 4100 ka are highly elevated over parent rock values due to extensive mass loss of more soluble major elements during pedogenesis nb and ta exhibit virtually no mobility soil nb ta ratios are within the range of fresh bedrock even when soil nb concentrations are residually enriched by a factor of 10 in contrast al zr and hf are depleted relative to nb in surface soil horizons but are enriched at depth clearly indicating mobility of these elements variations in th nb ratios in soil profiles indicate significant th mobility within the soil column however mass balance calculations require that accretion of th enriched asian dust has resulted in a net increase in th in some soils soils developed on a 150 ka rainfall gradient show that the mobility and loss of zr increase with mean annual precipitation

a partisan review of proterozoic anorthosites most anorthosites of the massif type crystallized in the episode 1 7 1 2 gyr with a pronounced peak of the age distribution near 1 4 gyr they were emplaced anorogenically at depths as shallow as 7 km where the ambient temperature of country rocks was probably less than 250 c depths of emplacement may have been as great as 25 km or more in rare cases the greater depths of equilibration estimated from granulite facies metamorphism may be incorrectly interpreted as emplacement depths but in any case they are demonstrably not required or characteristic of anorthosite emplacement penetrative deformation and metamorphism of anorthosites are post emplacement accidents of the local geologic history and are not directly caused by the presence of anorthosites granitic rocks mangerite charnockite suite associated with anorthosite are in general later or contemporaneous products of crustal anatexis with chemical and isotopic signatures distinct from the anorthosites and their residua such granitic rocks should not therefore be summed with the anorthositic rocks to obtain bulk compositions the magmas that produced most anorthosites were dry as shown by high temperature mineralogy and anhydrous mineral assemblages in contact aureoles residua from their crystallization are ferrodiorites to ferrosyenites typical of closed system fractionation these residua were locally and frequently ejected into contemporaneously molten granite where they formed pillows and cooled rapidly the overall chemistry of anorthosites and residua is broadly tholeiitic and consistent with derivation from the mantle olivine bearing magmas locally ranged from leucotroctolite anorthosite to later but coexisting picrite or melatroctolite in the same pluton confirming a wide spectrum of magma types a signal feature of troctolitic and noritic magmas is their low augite content implying high content of spinel component large anorthosite complexes such as nain and harp lake consist of many plutons representing repeated injections of separate magma batches with varying chemistry the abundant true anorthosites richer in plagioclase than magmas cosaturated with a mafic phase must represent plagioclase enrichment by either mechanical or chemical processes or both the role of kinetics in nucleation and solidification of such rocks may be centrally important it is proposed that hyperfeldspathic plagioclase supersaturated liquids were generated by quasi isothermal extraction of mafic minerals from tholeiitic magma enroute to and at the site of emplacement and that such a kinetic process was uniquely permitted in an environment of aborted continental rifting anorthositic rocks may have much to say about the episodic versus continuous geochemical evolution of the earth s mantle

provenance of dust in the pacific ocean eolian dust preserved in deep sea sediment cores provides a valuable indicator of past atmospheric circulation and continental paleoclimate in order to identify the provenance of eolian dust nd and sr isotopic compositions and rb sr and rare earth element ree concentrations have been determined for the silicate fractions of deep sea sediments from the north and central pacific ocean different regions of the pacific ocean are characterized by distinct air borne inputs producing a large range in end 10 to 1 stsr 86sr 0 705 0 721 la yb 5 15 eun eu 0 6 1 0 and sr nd 4 33 the average nd isotopic composition of pacific deep sea sediments eno 6 is more radiogenic than the average from the atlantic ena 8 in contrast the average 1475m 14and ratio for pacific sediments 0 114 is identical to that of atlantic sediments and to that of global average riverine suspended material the values of ena and 147sm 144nd are positively correlated for the pacific samples but negatively correlated for atlantic samples reflecting a fundamental difference between the dominant components in the end members with radiogenic nd island arc components in the pacific and lree enriched intraplate ocean island components in the atlantic samples from the north central pacific have distinctive unradiogenic end values of 10 87sr 86sr 0 715 high la yb 12 and low eun eu 0 6 and sr nd 3 6 these data are virtually identical to the values for loess from asia and endorse the use of these sediments as indicators of asian paleoclimate and paleowind directions island arc contributions appear to dominate in the northwest pacific resulting in higher end 1 to 1 and lower 87sr 86sr 0 705 and la yb 5 sediments from the eastern pacific tend to have intermediate sr and nd isotopic compositions but regionally variable sr nd and ree patterns they appear to be derived from the west margin of the north and south american continents rather than from asia our results confirm that dust provenance can be constrained by isotopic and geochemical analyses which will facilitate reconstructions of past atmospheric circulation and continental paleoclimate

accretion of asian dust to hawaiian soils isotopic elemental and mineral mass balances hawaiian soils contain a mixture of material derived from in situ weathering of parent material plus atmospheric inputs including sea salt aerosols and asian dust we use soil mineralogy and radiogenic isotope geochemistry sr and nd to evaluate the impact of asian dust on a chronosequence of hawaiian soils dust becomes an important constituent of soils 20 ky and older near surface 50 cm depth horizons contain as much as 30 quartz a mineral absent from local parent material basaltic sr and nd isotope signatures in these horizons have been completely overprinted by asian dust signatures with 87sr 86sr ratios as high as 0 723 and end values as low as 7 ree patterns in these soils are indistinguishable from that of average upper continental crust quartz abundance and nd isotopes provide two independent tracers of long term dust additions to the chronosequence soils the two tracers indicate a minimum long term average dust accretion rate of 125 mg cm 2 ky 1 at the 150 ka chronosequence site roughly a factor of three higher than holocene dust accumulation rates estimated from north pacific sediment cores we find that the mass of dust preserved in these soil profiles does not increase with age in soils 150 ka and older requiring a loss mechanism for accreted dust on the basis of the geomorphic stability of these sites observed preferential loss of dust derived mica relative to quartz and estimates of soil si leaching rates we argue that chemical weathering is the dominant loss mechanism for dust from these soils dust has a profound effect on the budgets of elements that are susceptible to leaching losses dust becomes the dominant source of soil nutrients si and p in the oldest most intensely weathered soils we calculate a dust derived p input flux of 0 8 mg cm 2 ky 1 and a dust derived si input flux of 35 mg cm 2 ky 1 si leaching fluxes are high 1400 mg cm 2 ky 1 in the youngest 2 ka soils and drop systematically with soil age to a value that closely balances the dust derived si input flux by 4100 ka extremely refractory elements such as nb which are concentrated in soils by residual enrichment processes are much less readily impacted by dust addition although dust has had a pronounced impact on sr and nd isotopic budgets and on soil mineral composition at the 150 ka site dust cannot have contributed 4 of the total nb contained in this soil profile

the age and origin of grenville province uraniferous granites and pegmatites uranium mineralization in pegmatites and granites occurs widely in the grenville province but is especially important near bancroft ontario and north of mont laurier and johan beetz in quebec most investigators have concluded that these rocks were produced by anatexis of a uranium enriched sedimentary protolith we have used rb sr isotopic methods to test this and other hypotheses and have reexamined the petrologic and structural features that led to these hypotheses rb sr whole rock isochron ages from all three areas range from 937 to 980 ma some 150 ma younger than the age of the peak of grenville metamorphism as inferred from numerous ages from syntectonic intrusive rocks and high grade metasediments the ages of the pegmatites are similar to or just slightly greater than k ar ages from this region the initial 87sr 86sr ratios range from 0 7028 to 0 7054 plus one poorly defined value of 0 709 excluding this last one these values are only slightly greater than mantle ratios and calculations show

silicon in the earth s core the earth and moon have iron isotopic abundances that are slightly heavy compared with mars the asteroid vesta and primitive meteorites the reason for this has been unclear one suggestion is that there was a large scale isotopic equilibration during the giant impact the collision between the early earth and a mars sized body that created the moon but this has seemed unlikely given that most other elements do not show the effect an analysis of 44 meteorite and terrestrial samples now shows that the silicon isotopic compositions of basaltic rocks from the earth and moon are also distinctly heavy the finding is consistent with the isotopic equilibration model and it suggests that silicon was already partitioned into the earth s core as a light element before the moon formed the silicon isotopic compositions of basaltic rocks from the earth and moon are distinctly heavy the similar isotopic composition of the bulk silicate earth and the moon are consistent with large scale isotopic equilibration during the giant impact indicating that si was already a light element in the earth s core before the moon formed small isotopic differences between the silicate minerals in planets may have developed as a result of processes associated with core formation or from evaporative losses during accretion as the planets were built up basalts from the earth and the moon do indeed appear to have iron isotopic compositions that are slightly heavy relative to those from mars vesta and primitive undifferentiated meteorites1 2 3 4 chondrites explanations for these differences have included evaporation during the giant impact that created the moon when a mars sized body collided with the young earth however lithium5 and magnesium6 lighter elements with comparable volatility7 8 9 reveal no such differences rendering evaporation unlikely as an explanation here we show that the silicon isotopic compositions of basaltic rocks from the earth and the moon are also distinctly heavy a likely cause is that silicon is one of the light elements in the earth s core we show that both the direction and magnitude of the silicon isotopic effect are in accord with current theory10 based on the stiffness of bonding in metal and silicate the similar isotopic composition of the bulk silicate earth and the moon is consistent with the recent proposal11 that there was large scale isotopic equilibration during the giant impact we conclude that si was already incorporated as a light element in the earth s core before the moon formed

plant impact on the biogeochemical cycle of silicon and related weathering processes abstract the contribution of plants to the biogeochemical cycle of si and related weathering processes was studied in an equatorial rainforest ecosystem congo where the biologic turnover of si is high 58 to 76 kg ha y litterfall leaves a soil profile and groundwaters were analysed phytoliths and organic matter have a similar distribution with depth in the soil profile the model of a bicompartmental distribution of organic matter is applied to phytolith distribution and shows that about 92 of the biogenic silica input is rapidly recycled while about 8 of the biogenic silica input supplies a stable pool of phytoliths with a lower turnover reprecipitation of silica was observed at the base of the soil profile indicating a local geochemical environment that is oversaturated with respect to amorphous silica a balance in biogeochemical cycle of si requires that the vegetation absorb dissolved silicon released from weathering of minerals which otherwise would be available for mineral neoformation or export from the profile towards regional drainages plant uptake of si increases the chemical weathering rate without increasing the denudation rate this study shows that the uptake storage and release of si by the vegetation have to be taken into account when using dissolved si for tracing chemical weathering dynamics

the dynamics of si cycling during weathering in two small catchments in the black forest germany traced by si isotopes abstract silicon stable isotopes have emerged as a powerful proxy to investigate weathering because si uptake from solution by secondary minerals or by the vegetation causes significant shifts in the isotope composition in this study we determined the si isotope compositions of the principle si pools within two small catchments located on sandstone and paragneiss respectively in the temperate black forest germany at both settings clay formation is dominated by mineral transformation preserving largely the signature of parental minerals with 30si values of around 0 7 bulk soils rich in primary minerals are similar to bulk parental material with 30si values close to 0 4 topsoils are partly different because organic matter degradation has promoted intense weathering leading to 30si values as low as 1 0 water samples expose highly dynamic weathering processes in the soil zone 1 after spring snowmelt increased release of doc and high water fluxes trigger clay mineral dissolution which leads to 30si values down to 0 7 and 2 in course of the summer si uptake by the vegetation and secondary mineral formation drives dissolved si to typical positive 30si values up to 1 1 groundwater with 30si values of around 0 4 records steady processes in bedrock reflecting plagioclase weathering together with kaolinite precipitation an isotope mass balance approach reveals incongruent weathering conditions where denudation of si is largely driven by physical erosion erosion of phytoliths contributes 3 to 21 to the total si export flux which is in the same order as the dissolved si flux these results elucidate the si dynamics during weathering on catchments underlain of sedimentary origin prevailing on the earth surface and provide therefore valuable information to interpret the isotope signature of large river systems

mineralogy of aeolian dust reaching the north pacific ocean 1 sampling and analysis the mineral composition of the 2 m clay and 2 to 20 m silt size fractions of aerosol particles collected near the margins of the north pacific ocean was determined by x ray diffraction analysis although the same mineral phases are present in the two size fractions there are significant differences in the quartz plagioclase and kaolinite concentrations north pacific aerosols are primarily composed of illite in both size fractions in addition to kaolinite in the 2 m size class and quartz and plagioclase in the 2 to 20 m size class quartz and plagioclase are present in significantly higher amounts in the 2 to 20 m size fraction while kaolinite is greater in the 2 size class western north pacific aerosol particles contain significantly higher 2 m quartz than samples collected in the east and eastern north pacific aerosols have significantly higher concentrations of 2 m smectite than those from the west

texture specific si isotope variations in barberton greenstone belt cherts record low temperature fractionations in early archean seawater sedimentary cherts are unusually abundant in early archean pre 3 0 ga sequences suggesting a silica cycle that was profoundly different than the modern system previously applied for the purpose of paleothermometry si isotopes in ancient cherts can offer broader insight into mass fluxes and mechanisms associated with silica concentration precipitation diagenesis and metamorphism early archean cherts contain a rich suite of sedimentological and petrographic textures that document a history of silica deposition cementation silicification and recrystallization to add a new layer of insight into the chemistry of early cherts we have used wavelength dispersive spectroscopy and then secondary ion mass spectrometry sims to produce elemental and si and o isotope ratio data from banded black and white cherts from the onverwacht group of the barberton greenstone belt south africa this geochemical data is then interpreted in the framework of depositional and diagenetic timing of silica precipitation provided by geological observations sims allows the comparison of si and o isotope ratios of distinct silica phases including black carbonaceous chert beds and bands many including well defined sedimentary grains white relatively pure chert bands including primary silica granules early cavity filling cements and later quartz filled veins including all chert types and textures analyzed the 30 si dataset spans a range from 4 78 to 3 74 with overall mean 0 20 median 0 51 and standard deviation 1 30 n 1087 most samples have broadly similar 30 si distributions but systematic texture specific 30 si differences are observed between white chert bands mean 0 60 n 750 which contain textures that represent primary and earliest diagenetic silica phases and later cavity filling cements mean 1 41 n 198 we observed variations at a 100 m scale indicating a lack of si isotope homogenization at this scale during diagenesis and metamorphism although fractionations during diagenetic phase transformations may have affected certain textures we interpret these systematic variations to reflect fractionation during silica precipitation as well as isotopically distinct fluids from which later phases originated sims 18 o values fall in a range from 16 39 to 23 39 n 381 similar to previously published data from bulk gas source mass spectrometry of onverwacht cherts we observed only limited examples of texture related variation in 18 o and did not observe correlation of 18 o with 30 si trends this is consistent with hypotheses that si isotope ratios are more resistant to alteration under conditions of rock buffered diagenesis marin carbonne et al 2011 our results indicate that low temperature processes fractionated silicon isotopes in early archean marine basins a behavior that probably precludes the application of chert 30 si as a robust paleothermometer the values we observe for facies that sedimentological and petrographic observations indicate formed as primary and earliest diagenetic silica precipitates from seawater are more 30 si rich than that expected for bulk silicate earth this is consistent with the hypothesis that the silicon isotope budget is balanced by the coeval deposition of 30 si enriched cherts and 30 si depleted iron formation lithologies precipitation of authigenic clay minerals in both terrestrial and marine settings may have also comprised a large 30 si depleted sink with the corollary of an important non carbonate alkalinity sink consuming cations released by silicate weathering

silicon isotope systematics of acidic weathering of fresh basalts kilauea volcano hawai i silicon stable isotopes are fractionated by a host of low temperature aqueous processes making them potentially useful as a weathering proxy here we characterize the silicon isotope signature of surficial chemical weathering of glassy basaltic lava flows at kilauea volcano hawaii fresh basalt flow surfaces 40 years old frequently feature opaque amorphous silica surface coatings up to 80 m thick these silica coatings and associated silica cements are enriched in the heavier isotopes of si 30 si nbs 28 0 92 to 1 36 relative to their basaltic substrate 30 si nbs 28 0 3 to 0 2 secondary clays and opals are typically depleted in 30 si relative to the dissolved reservoirs from which they precipitated so this sense of isotopic fractionation is unusual mechanisms capable of producing isotopically heavy secondary minerals were explored by conducting batch alteration experiments on fresh basaltic glass batch acidic alteration of basalt glass in hcl h 2so 4 and hf produced silica rich surface layers resembling the hawaiian surface coatings differences in fluid chemical composition affected the direction and magnitude of si isotope fractionation basalt leaching in hcl or h 2so 4 produced 30 si enriched fluids 1000 in precip si aq 0 8 and 30 si depleted secondary silica layers in contrast hf bearing experiments produced highly 30 si depleted fluid compositions 1000 in precip si aq up to 8 larger isotopic fractionations were observed in experiments with lower fluid rock ratios in hawaii where altering fluids contain h 2so 4 and hcl but minimal hf high 30 si values for the silica coatings were likely achieved by rayleigh fractionation aqueous 30 si enriched silica was released during incongruent basalt dissolution then subsequently transported and deposited from an evaporating solution at the flow surface our results indicate that 1 altering fluid chemistry and fluid rock ratio impact the si isotope signature of chemical weathering and 2 30 si of solids produced by low temperature aqueous alteration may diverge sharply from watershed or landscape scale weathering trends

modeling novel stable isotope ratios in the weathering zone when rock is converted to weathering products the involved processes can be fingerprinted using the stable isotope ratios of metals for example li mg ca fe sr and metalloids b si here we construct a framework for interpreting these novel stable isotope ratios quantitatively in the compartments of the weathering zone in a geomorphic context the approach is applicable to any novel stable isotope system and is based on a simple steady state mass balance model that represents the weathering zone from the scale of a soil column to that of entire continents our model is based on the assumption that the two main processes associated with isotope fractionation are formation of secondary precipitates such as clays and uptake of nutrients by plants the model results show that the isotope composition of a given element in the weathering zone compartments depends on 1 the ratio between the release flux to water through primary mineral dissolution and the erosion flux of isotopically fractionated solid material consisting of secondary precipitates and organic matter 2 the isotope fractionation factors associated with secondary mineral precipitation and uptake by plants a relationship is established between isotope ratios isotope fractionation factors and indexes for chemical weathering such as chemical depletion fractions cdf and elemental mass transfer coefficients derived from simple elemental concentration measurements from this relationship isotope fractionation factors can be calibrated from chemical and isotope data measured on field material furthermore we show how the ratio of solid export to dissolved export of a given element from the weathering system can be estimated from the comparison of the isotope composition between bedrock water and sediment this calculation can be applied to samples from soils from rivers and from the sedimentary record and does not require knowing the isotope fractionation factors involved in the reactions finally we apply the model to the oceanic li isotope record reconstructed from marine carbonate sediments in order to discuss changes in global geomorphic regimes through the cenozoic

changing sources of strontium to soils and ecosystems across the hawaiian islands abstract strontium isotope ratios assist ecosystem scientists in constraining the sources of alkaline earth elements but their interpretation can be difficult because of complexities in mineral weathering and in the geographical and environmental controls on elemental additions and losses hawaii is a natural laboratory where a number of important biogeochemical variables have either limited ranges or vary in systematic ways providing a unique opportunity to understand the impact of time climate and atmospheric inputs on the evolution of base cation sources to ecosystems there are three major sources of strontium sr to these ecosystems each with distinct isotopic compositions basalt lava asian dust and rainfall we present sr isotope and concentration data on both bulk soil digests and nh4ac extracts from soil profiles covering a wide range of environments and substrate ages bulk soil material from dry climates and or young substrate ages with 80 g g 1 sr retain basalt like sr isotopic signatures whereas those with sr concentrations 40 by mass at elevated dust levels lava derived sr is low and dust derived sr is the dominant control of 87sr 86sr in bulk soils however 87sr 86sr of nh4ac extractable sr largely reflects atmospheric deposition of marine aerosol in these situations overall whole soil sr isotope values are controlled by complex interactions between sr provided by lava weathering but partially lost by leaching and sr provided by dust but held in more resistant minerals the isotopic composition of nh4ac extractable sr and of the biota is controlled by lava weathering and rainfall contribution of sr with only minor contributions from radiogenic dust sources

the impact of local geochemical variability on quantifying hillslope soil production and chemical weathering soil mantled upland landscapes are widespread across the habitable world support extensive life and are the interface between the atmosphere hydrosphere and lithosphere but typically are not cultivated soil found across such landscapes fits the conceptual framework of a physically mobile layer derived from the underlying parent material along with some locally derived organic content the extent and persistence of these upland soils depend on the long term balance between soil production and erosion here we briefly review methods used to quantify the physical and chemical processes of soil production and erosion and revisit three granitic study areas in southeastern australia and northern california that enabled early quantification of the soil production function and topographic controls on chemical weathering we then present new major and trace element data from 2 m by 2 m pits dug at each field site to quantify local variability of zr concentrations and the chemical index of alteration cia weathering indices used to determine chemical weathering rates and extents in soils and saprolites using both new and previously published data we compare differences between local variability and regional as well as intersite variability of these important indices for each of the 2 m pits we collected 25 samples and found that the simple mean and the 2 standard deviation best describe the local variation in the data we also find that the variability in the 2 m pit data lies within variability observed in the same data from samples collected in individual soil pits across each of the field sites and that the differences between sites are consistent with previously published results these observations highlight the importance of quantifying local scale variability in studies that use similar multifaceted measurements to quantify hillslope soil production and erosion processes

anorogenic magmatism and the grenvillian orogeny the grenvillian orogeny was preceded by extensive anorogenic volcanism and plutonism in the period 1500 1300 ma in the form of rhyolites epizonal granites anorthosites gabbros alkaline complexes and basic dykes an analogue for the mid proterozoic anorogenic complexes is provided by the 2000 km by 200 km belt of anorogenic complexes in the hoggar niger and nigeria which contain anorthosites gabbros and peralkaline granites and were generated in a cambrian to jurassic rift that farther south led to the formation of the south atlantic an analogue for the 1 106 km2 area of 1500 1350 ma rhyolites and associated epizonal granites that underlie the mid continental united states is provided by the 1 7 106 km2 area of jurassic tobifera rhyolites in argentina which were extruded on the stretched continental margin of south america immediately preceding the opening of the south atlantic the mid proterozoic complexes were intruded close to the continental margin of the grenvillian ocean and were c

silicon isotopes and the tracing of desilication in volcanic soil weathering sequences guadeloupe abstract silicon si stable isotopes have the potential to become a useful weathering proxy given that light si isotopes are preferentially incorporated into secondary clay minerals here we investigate how si depletion in soils and associated clay mineralogy influence the si isotope fractionation associated with clay mineral formation we report 30 si compositions in bulk soils and clay fractions relative to their parent andesite in three soil weathering sequences from guadeloupe that were formed under contrasting climatic conditions strongly desilicated soils containing kaolinite that formed in wet areas high precipitation are compared with less desilicated soils containing smectite formed in drier conditions low precipitation clay fractions are isotopically lighter than the parent andesite 30 si 0 23 and increasingly lighter with si depletion in soils which supports the view that the si isotope composition in secondary clay fractions is controlled by the degree of soil desilication it is shown that the si isotope fractionation factor between the parent silicate material and the secondary clay minerals is smaller for si rich secondary clay minerals such as smectite and larger for si poor secondary clay minerals such as kaolinite this study provides new insights to better define si isotopes as a proxy for environmental conditions for clay neoformation

isotopic evidence from the eastern canadian shield for geochemical discontinuity in the proterozoic mantle most workers agree that proterozoic anorthosite massifs represent the crystallization products of mantle derived magmas1 2 although the composition of the parental melts is a major unsolved petrological problem3 as mantle derived rocks the massifs can be used as geochemical probes of their late precambrian upper mantle sources we report here nd and sr isotopic compositions of anorthosites and related rocks from the grenville and nain provinces of the eastern canadian shield here 75 of the earth s known anorthosite is found in a 1 600 km belt from the adirondack mountains of northern new york state to the eastern coast of labrador4 fig 1 the results indicate that the massifs were derived from at least two distinct mantle source regions which were established before 1 650 myr ago and were episodically involved in magmatism over 500 myr one reservoir below the grenville province and probably below much of the eastern superior province was isotopically similar to the depleted modern day mid ocean ridge basalt morb source the other reservoir was chondritic to moderately enriched and is most easily identified in the nain province but may have occurred scattered throughout the superior province

the silicon isotopic composition of fine grained river sediments and its relation to climate and lithology abstract the 30si stable isotopic composition of silicon in soils and fine grained sediments can provide insights into weathering processes on continents with important implications on the si budget of modern and past oceans to further constrain the factors controlling the distribution of si isotopes in sediments we have analysed a large number n 50 of separate size fractions of sediments and suspended particulate materials collected near the mouth of rivers worldwide this includes some of the world s largest rivers e g amazon congo mackenzie mississippi murray darling nile yangtze and rivers from the case study areas of the congo river basin and northern ireland silt size fractions exhibit a mean si isotopic composition 30si 0 21 0 19 2 s d similar to that previously inferred for the upper continental crust in contrast clay size fractions display a much larger range of 30si values from 0 11 to 2 16 which yield a global 30siclay of 0 57 0 60 2 s d representative of the mean composition of the average weathered continental crust overall these new data show that the si isotopic signature transported by river clays is controlled by the degree of chemical weathering as inferred from strong relationships with al si ratios at a global scale the clay bound si isotopic composition of the world s largest river systems demonstrates a link with climate defining a general correlation with mean annual temperature mat in corresponding drainage basins while the distribution of si isotopes in river sediments also appears to be influenced by the tectonic setting lithological effects and sediment recycling from former sedimentary cycles our results pave the way for their use as paleo weathering and paleo climate proxies in the sedimentary record

the impact of climate on the biogeochemical functioning of volcanic soils abstract rainfall and the amount of water available to leach ions from soil are among the most important features determining mineral weathering secondary mineral synthesis and soil chemical properties along an arid to humid climosequence on kohala mountain hawaii we sampled 16 soil profiles and found that weathering and soil properties change in a nonlinear fashion with increased rainfall the lavas are influenced by a strong rain shadow with mean annual precipitation map averaging 160 mm near the coast and rising to 3000 mm near the summit a temperature decline from 24 to 15 c with increasing elevation is matched by lower potential evapotranspiration et a water balance model monthly precipitation minus monthly et defines three broad climate zones along the sampling transect an arid zone with moisture deficit in every month an intermediate zone with moisture deficit during low rainfall summer months and moisture surplus during high rainfall winter months and a humid zone with moisture surplus during every month the annualized water balance can be ratioed with the integrated porosity of the top meter of soil to provide a leaching index the index reaches 1 total filling of the pore space on an annual basis at about 1400 mm map index values 1 imply intense leaching conditions because of pore water replacement in these 170 ka soils leaching losses of soluble base cations and si are nearly complete at index values 1 whereas only 60 of al has been lost at index values 1400 mm soil buffering capacity has been totally exhausted leading to low ph and low ecec the nonlinear decline in ecec is irreversible under natural conditions base cation depleted soils will remain so even if the climate shifts to drier conditions in contrast a climate shift to wetter conditions can drastically modify surface chemical properties existing in the drier soils as weathering depletes primary minerals elements are lost to leaching and surface chemistry is modified there is a time dimension implied in climate gradient studies soils forming in recently rejuvenated landscapes contain more primary minerals and should experience loss of buffering capacity at higher rainfall loss of buffering capacity means that biological acidity will move more deeply into the vadose zone or into the aquatic system the details of this transfer depend on present and past climate and the age and erosional stability of landscapes

dissolved silicon and its isotopes in the water column of the bay of bengal internal cycling versus lateral transport abstract the concentration of dissolved si and its isotope composition are measured in the bay of bengal bob region of the northern indian ocean the isotope data are the first data set from the northern indian ocean the measurements are made in eight depth profiles closely along the 87 e transect gio1 section of the international geotraces program and in a few samples from the northern shelf of the bay dissolved si in the water column varies from 0 6 to 152 5 mol kg whereas the 30si data cover a range 1 2 to 3 6 the depth profiles of dissolved si show generally lower values in the surface increasing with depth whereas the pattern reverses in the case of 30si these vertical distribution patterns of si and 30si are similar to those reported in other oceanic regions and suggestive of the significant role of biological processes in governing si biogeochemistry in the upper layers top 1500 m in contrast dissolved si in near surface waters of the northern shelf and the southernmost station is exceptionally high these results indicate a continental supply of dissolved si from the ganga brahmaputra river system g b and submarine groundwater discharge sgd to the shelf region and an intrusion of high salinity waters from the arabian sea in the southern bay the 30si values of 1 34 0 10 for deep bottom waters of the bob depth 1500 m are similar to those reported for the deep southern ocean and indicate the dominant control of water mass mixing the dissolved si concentrations in the bottom waters of the bob are generally higher than those of the water mass endmembers which suggest the need for an additional source of si in situ particle dissolution and or benthic release in the central bay seem to be the potential candidate the annual si budget in the top 100 m of the bob seems to suggest that meso scale eddies frequently occurring during non monsoon periods can supply at the most 2 6 g si m2 year which is about 33 of the si requirement to support new production in the bay the supply of dissolved si 1 3 0 5 1011 mol year from the g b river system and sgd has been calculated based on the distributions of dissolved si concentration and 30si in the northern shelf waters a comparison of this supply with the reported si flux upstream of the estuarine zone indicates about 40 removal of dissolved si in the g b estuary the mass balance of si isotopes in the deep waters indicates that the dissolution of diatoms is the main cause of excess si in the bay

the age of the lac saint jean anorthosite complex and associated mafic rocks grenville province canada u pb analyses of zircon and baddeleyite from the south central and southeastern parts of the lac saint jean anorthosite complex lsja give an igneous crystallization age of 1157 3 ma parts of the anorthosite were deformed in the solid state and subsequently intruded by a diorite megadyke which also gives a crystallization age of 1157 3 ma indicating that crystallization and deformation of the anorthosite were essentially synchronous the diorite megadyke was intruded into a north northeast trending shear zone and deformed by sinistral strike slip movements emplacement was followed by intrusion of a subparallel leucotroctolite megadyke that again gives the same crystallization age and hence dates movement of the shear zone at 1157 3 ma this short history of crystallization and synchronous deformation rules out slow diapiric rise as the emplacement mechanism for the anorthosite instead anorthosite parental magmas probably rose up offsets in subvertical strike slip shear zones to their present

sr nd and pb isotopes in proterozoic intrusives astride the grenville front in labrador implications for crustal contamination and basement mapping abstract we report sr nd and pb isotopic compositions of mid proterozoic anorthosites and related rocks 1 45 1 65 ga and of younger olivine diabase dikes 1 4 ga from two complexes on either side of the grenville front in labrador anorthositic or diabasic samples from the mealy mountains grenville province and harp lake nain churchill provinces complexes have very similar major minor and trace element compositions but distinctly different isotopic signatures all mealy mountains samples have isr 0 7025 0 7033 end 0 6 to 5 6 and pb isotopic compositions consistent with derivation from a mantle source depleted with respect to nd sm and rb sr pb isotopic compositions for the mealy mountains samples are slightly more radiogenic than model mantle compositions all harp lake samples have isr 0 7032 0 7066 end 0 3 to 4 4 and variable but generally unradiogenic 207pb 204pb and 206pb 204pb compared to model mantle suggesting mixing between a mantle derived component and a u depleted crustal contaminant crustal contaminants are probably a variety of archean high grade quartzofeldspathic gneisses with low u pb ratios and include a component that must be isotopically similar to the early archean 3 6 ga uivak gneisses of labrador or the amitsoq gneisses of west greenland this would imply that the ancient gneiss complex of coastal labrador and greenland is larger than indicated by present surface exposure and may extend in the subsurface as far west as the labrador trough if harp lake and mealy mountains samples were subjected to the same degree of contamination as suggested by their chemical similarities then the mealy contaminants must be much younger probably early or middle proterozoic in age the labrador segment of the grenville front therefore appears to coincide with the southern margin of the archean north atlantic craton and may represent a pre mid proterozoic suture

terrestrial ecosystems and the global biogeochemical silica cycle 1 most research on the global si cycle has focused nearly exclusively on weathering or the oceanic si cycle and has not explored the complexity of the terrestrial biogeochemical cycle the global biogeochemical si cycle is of great interest because of its impact on global co2 concentrations through the combined processes of weathering of silicate minerals and transfer of co2 from the atmosphere to the lithosphere a sizable pool of si is contained as accumulations of amorphous silica or biogenic silica bsi in living tissues of growing plants known as phytoliths and after decomposition of organic material as remains in the soil the annual fixation of phytolith silica ranges from 60 200 tmol yr 1 and rivals that fixed in the oceanic biogeochemical cycle 240 tmol yr 1 internal recycling of the phytolith pool is intense with riverine fluxes of dissolved silicate to the oceans buffered by the terrestrial biogeochemical si cycle challenging the ability of weathering models to predict rates of weathering and consequently changes in global climate consideration must be given to the influence of the terrestrial bsi pool on variations in the global biogeochemical si cycle over geologic time and the influence man has had on modifying both the terrestrial and aquatic biogeochemical cycles

isotopic constraints on emplacement age of anorthositic rocks of the marcy masiff adirondack mts new york the igneous emplacement age of anorthositic rocks comprising the marcy massif have been constrained by fixing minimum and maximum ages for their intrusion minimum ages have been fixed by a u pb dating of cores of air abraded zircons from the anorthosite 1113 ma b u pb dating of baddeleyite within the anorthosite 1087 ma c u pb dating of baddeleyite from an olivine metagabbro crosscutting marcy massif anorthosite 1109 ma d u pb zircon dating of jotunitic rocks crosscutting the marcy anorthosite massif 1143 pm 31 ma maximum emplacement ages are based upon a sheets of anorthositic rock crosscutting mangeritic charnockitic gneiss dated by u pb zircon at 1125 pm 10 ma 1134 pm 4 ma and 1135 pm 43 ma and b anorthosite related pyroxenite dikes crosscutting mangerite that contains xenoliths of identical pyroxenite these results constrain emplacement of the marcy anorthosite to 1113 1138 ma with the most probable age in the interval 1125 1135 ma this time of in