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Alex N Halliday - One of the best experts on this subject based on the ideXlab platform.

  • Silicon Isotopes in granulite xenoliths insights into isotopic fractionation during igneous processes and the composition of the deep continental crust
    Earth and Planetary Science Letters, 2013
    Co-Authors: Helen M. Williams, Paul S Savage, Bastian R Georg, Alex N Halliday
    Abstract:

    The Silicon (Si) cycle is of great current interest but the isotopic composition of the continental crust has not been determined. Magmatic differentiation generates liquids with heavier Si and the lower crust, thought to be dominated by cumulates and restites, is predicted to have a light isotopic composition. This is borne out by the composition of many types of granite, which appear to have relative light Si for their silica content. Here we report the Si isotopic compositions of two granulite facies xenolith suites, from the Chudleigh and McBride volcanic provinces, Australia, providing new constraints on deep crustal processes and the average composition of the deep continental crust. The xenoliths display a range of isotopic compositions (δ30Si=−0.43‰ to −0.15‰) comparable to that measured previously for igneous rocks. The isotopic compositions of the McBride xenoliths reflect assimilation and fractional crystallisation (AFC) and/or partial melting processes. Silicon and O Isotopes are correlated in the McBride suite and can be explained by AFC of various evolved parent melts. In contrast, the Chudleigh xenoliths have Si isotope compositions predominantly controlled by the specific mineralogy of individual cumulates. Using the xenolith data and a number of weighting methods, the Si isotope compositions of the lower and middle crust are calculated to be δ30Si=−0.29±0.04‰ (95% s.e.) and −0.23±0.04‰ (95% s.e.) respectively. These values are almost identical to the composition of the Bulk Silicate Earth, implying minimal isotope fractionation associated with continent formation and no light lower crustal reservoir.

  • Silicon Isotopes and the tracing of desilication in volcanic soil weathering sequences guadeloupe
    Chemical Geology, 2012
    Co-Authors: R B Georg, Sophie Opfergelt, Kevin W. Burton, Bruno Delvaux, Y M Cabidoche, Alex N Halliday
    Abstract:

    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 δ 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 (δ 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.

  • Silicon Isotopes in lunar rocks implications for the moon s formation and the early history of the earth
    Geochimica et Cosmochimica Acta, 2012
    Co-Authors: R. M. G. Armytage, R B Georg, Helen M. Williams, Alex N Halliday
    Abstract:

    Silicon isotopic data from a range of lunar samples are presented to assess the degree of heterogeneity of the lunar mantle and its similarity to bulk silicate Earth (BSE). Multi-collector inductively-coupled-plasma mass spectrometry (MC-ICPMS) was used to analyse 24 samples, including both high and low-Ti basalts, as well as Highland anorthosites and picritic glasses, covering all the Apollo sample return missions. No systematic δ30Si differences are found between any of the bulk sample lithologies (±2σSD) (δ30SiLow-Ti basalt = −0.29 ± 0.06, δ30SiHigh-Ti basalt = −0.32 ± 0.09, δ30Silunar glass = −0.29 ± 0.05 and δ30SiHighland rocks = −0.27 ± 0.10). The average of the lunar samples is δ30Si = −0.29 ± 0.08 (2σSD), which is identical to the composition of BSE, δ30Si = −0.29 ± 0.08 (2σSD), from Savage et al. (2010). The BSE Si isotope composition is thought to be the result of Si partitioning between metal and silicate, and consequent isotopic fractionation during core formation. The Moon-forming impactor would not be expected to share that composition, because it is thought to have been relatively small (∼0.1 Earth masses) like Mars and formed under relatively low temperatures and pressures that are insufficient for Si to partition into the core. Therefore, the identical lunar and BSE Si isotope data show that Si Isotopes, like those of oxygen, must have homogenised in the aftermath of the Moon-forming impact, if smooth particle hydrodynamic simulations of the Giant Impact are correct in showing that most lunar material should have originated from the impactor rather than the Earth. The data presented here are in agreement with other isotope systems and experimental studies that indicate that the majority of core formation happened early and before the Giant Impact. It has been predicted (Pahlevan et al., 2011) that the Moon and the BSE should show a ∼0.14‰ offset in δ30Si for the Moon to have an Fe/(Fe + Mg) ratio twice that of BSE. The current resolution and sample population size of the Si data for the Moon and Earth would allow such an offset to be detected. The fact that is it not observed can put constraints on the element ratios and lunar budgets as modelled by Pahlevan et al. (2011); in particular, it constrains the Fe/(Fe + Mg) ratio of the Moon to be only 1–1.3 BSE.

  • quantifying the impact of freshwater diatom productivity on Silicon Isotopes and Silicon fluxes lake myvatn iceland
    Earth and Planetary Science Letters, 2011
    Co-Authors: Sophie Opfergelt, Kevin W. Burton, Christopher Siebert, Sigurdur R. Gislason, Eydis Salome Eiriksdottir, Arni Einarsson, Alex N Halliday
    Abstract:

    Diatom productivity in the oceans plays a crucial role in the carbon cycle, but is strongly dependent upon the continental Silicon supply. However, the relative influence of weathering and biological processes on continental Si fluxes remains poorly constrained. This study aims to quantify the impact of terrestrial diatom productivity on Si fluxes to the ocean. Lake Myvatn in North Iceland is one of the most productive lakes in the Northern Hemisphere, with nutrient-rich waters almost uniquely sourced by groundwater. The primary production is mainly controlled by diatom growth but also by cyanobacteria, and the lake output is via a single river, thereby providing a relatively simple natural laboratory to quantify the impact of diatom growth on the chemistry and Si budget of lake waters. Silicon stable Isotopes (o^30Si) provide a tracer of this biocycling, and have been measured in groundwater inputs to the lake, and in time-series monitoring of waters at the lake outlet. The o^30Si values at the outlet range from + 0.70 ± 0.08 to + 1.42 ± 0.06‰, which is significantly heavier than the groundwater input (average cold and hot springs: + 0.50 ± 0.17‰, 2SD) and consistent with the preferential uptake of light Si Isotopes by diatoms. The o^30Si value at the outlet increases by up to 0.9‰ in spring and autumn relative to the Si isotope composition of the inflow. These seasonal diatom blooms can be modeled by an open system of Si uptake and affect Si fluxes at the outlet of the lake by up to 79%, or 53% integrated over the year. In the summer a shift to lighter o^30Si values is correlated with a higher pH, which results in dissolution of diatoms releasing light Si Isotopes. From mass balance, this seasonal diatom dissolution affects Si fluxes by up to 33%, but is limited to 3.7% integrated over the year. These results clearly illustrate that biological activity can have a significant impact on both isotope composition and elemental abundance of continental derived Si. They also demonstrate the pH dependency of diatom dissolution and/or preservation, which is likely to affect not only the continental Si fluxes to the ocean but also the Si recycling in the oceans themselves.

  • Silicon Isotopes in antarctic sponges an interlaboratory comparison
    Antarctic Science, 2011
    Co-Authors: Katharine R Hendry, Bastian R Georg, Melanie J Leng, Laura F Robinson, Hilary J Sloane, Jerzy Blusztjan, Rosalind E M Rickaby, Alex N Halliday
    Abstract:

    Cycling of deepwater Silicon (Si) within the Southern Ocean, and its transport into other ocean basins, may be an important player in the uptake of atmospheric carbon, and global climate. Recent work has shown that the Si isotope (denoted by δ29Si or δ30Si) composition of deep sea sponges reflects the availability of dissolved Si during growth, and is a potential proxy for past deep and intermediate water silicic acid concentrations. As with any geochemical tool, it is essential to ensure analytical precision and accuracy, and consistency between methodologies and laboratories. Analytical bias may exist between laboratories, and sponge material may have matrix effects leading to offsets between samples and standards. Here, we report an interlaboratory evaluation of Si Isotopes in Antarctic and sub-Antarctic sponges. We review independent methods for measuring Si Isotopes in sponge spicules. Our results show that separate subsamples of non-homogenized sponges measured by three methods yield isotopic values within analytical error for over 80% of specimens. The relationship between δ29Si and δ30Si in sponges is consistent with kinetic fractionation during biomineralization. Sponge Si isotope analyses show potential as palaeoceaongraphic archives, and we suggest Southern Ocean sponge material would form a useful additional reference standard for future spicule analyses.

Sophie Opfergelt - One of the best experts on this subject based on the ideXlab platform.

  • Silicon Isotopes in arctic and sub arctic glacial meltwaters the role of subglacial weathering in the Silicon cycle
    Proceedings of The Royal Society A: Mathematical Physical and Engineering Sciences, 2019
    Co-Authors: Jade E Hatton, Sophie Opfergelt, Katharine R Hendry, J Hawkings, Jemma L Wadham, Tyler J Kohler, Jacob C Yde, Marek Stibal, Jakub D žarský
    Abstract:

    Glacial environments play an important role in high-latitude marine nutrient cycling, potentially contributing significant fluxes of Silicon (Si) to the polar oceans, either as dissolved Silicon (D...

  • desilication in archean weathering processes traced by Silicon Isotopes and ge si ratios
    Chemical Geology, 2016
    Co-Authors: Sophie Opfergelt, Damien Cardinal, Camille Delvigne, Axel Hofmann, Luc Andre
    Abstract:

    Abstract The influence of continental weathering on oceanic seawater compositions in the Precambrian is poorly understood. Here we combine Si Isotopes and Ge/Si ratios of a Mesoarchean paleosol (~ 2.95 Ga) and overlying shales as proxies for weathering processes and Si mass transfer at the early Earth's surface. Results show that, as with modern soils, neoformation of secondary clay minerals in the paleosol was associated with the fractionation of Si Isotopes and Ge/Si ratios in response to chemical weathering and soil desilication. Furthermore, the loss of Fe-bearing minerals, most likely Fe-bearing smectites, produced additional controls on Si and Ge mobility, through the coupled dissolution and re-precipitation of clay minerals under reductive conditions. Opposite fractionation behaviors are observed: neoformed secondary clay minerals acted as a 28 Si and Ge sink, whereas the leaching of Fe-bearing minerals released 28 Si and Ge into soil solutions. An attempt to estimate δ 30 Si and Ge/Si signatures of released dissolved elements into soil solution provided δ 30 Si and Ge/Si signatures of about + 0.1‰ and 1.4 μmol/mol, respectively. Furthermore, shales deposited shortly after paleosol formation display δ 30 Si and Ge/Si compositions that may be explained as mixtures of the recognized paleosol components. The shale record suggests that weathering-induced desilication and leaching of Fe-bearing minerals, to a lesser extent, might have been widely effective during the Mesoarchean for the transfer of Si from the continents to the hydrosphere. Comparison of Si Isotopes recorded in detrital sediments and chemical precipitates over time might thus provide a useful supplementary tool to decipher the impact of putative increases in continental weathering and desilication, relative to changes in hydrothermal and continent-derived solute inputs to ocean and ocean cooling.

  • Silicon Isotopes and continental weathering processes assessing controls on si transfer to the ocean
    Comptes Rendus Geoscience, 2012
    Co-Authors: Sophie Opfergelt, Pierre Delmelle
    Abstract:

    Knowledge of the weathering processes controlling continental transfer of dissolved Silicon (DSi) to the ocean is fundamental to the estimate of the long-term atmospheric CO2 -budget. Studies at the soil profile and catchment scales demonstrate that the Si isotopic composition of soil and riverine DSi is a marker of abiotic and/or biotic weathering processes. In this paper, we first review Si isotopic fractionation in the natural environment. We then evaluate the application of Si Isotopes to gain new insights into weathering processes on distinct spatial and temporal scales. We suggest that bulk soil δ30Si is inversely correlated to weathering degree, regardless of variability in climate, soil age and lithology. We also posit that the Si isotopic composition of DSi and suspended matter in rivers is indicative of the weathering regime (weathering- vs. transport-limited) prevailing in the area where DSi flux originates. Finally, we propose some future research directions.

  • Silicon Isotopes and the tracing of desilication in volcanic soil weathering sequences guadeloupe
    Chemical Geology, 2012
    Co-Authors: R B Georg, Sophie Opfergelt, Kevin W. Burton, Bruno Delvaux, Y M Cabidoche, Alex N Halliday
    Abstract:

    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 δ 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 (δ 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.

  • quantifying the impact of freshwater diatom productivity on Silicon Isotopes and Silicon fluxes lake myvatn iceland
    Earth and Planetary Science Letters, 2011
    Co-Authors: Sophie Opfergelt, Kevin W. Burton, Christopher Siebert, Sigurdur R. Gislason, Eydis Salome Eiriksdottir, Arni Einarsson, Alex N Halliday
    Abstract:

    Diatom productivity in the oceans plays a crucial role in the carbon cycle, but is strongly dependent upon the continental Silicon supply. However, the relative influence of weathering and biological processes on continental Si fluxes remains poorly constrained. This study aims to quantify the impact of terrestrial diatom productivity on Si fluxes to the ocean. Lake Myvatn in North Iceland is one of the most productive lakes in the Northern Hemisphere, with nutrient-rich waters almost uniquely sourced by groundwater. The primary production is mainly controlled by diatom growth but also by cyanobacteria, and the lake output is via a single river, thereby providing a relatively simple natural laboratory to quantify the impact of diatom growth on the chemistry and Si budget of lake waters. Silicon stable Isotopes (o^30Si) provide a tracer of this biocycling, and have been measured in groundwater inputs to the lake, and in time-series monitoring of waters at the lake outlet. The o^30Si values at the outlet range from + 0.70 ± 0.08 to + 1.42 ± 0.06‰, which is significantly heavier than the groundwater input (average cold and hot springs: + 0.50 ± 0.17‰, 2SD) and consistent with the preferential uptake of light Si Isotopes by diatoms. The o^30Si value at the outlet increases by up to 0.9‰ in spring and autumn relative to the Si isotope composition of the inflow. These seasonal diatom blooms can be modeled by an open system of Si uptake and affect Si fluxes at the outlet of the lake by up to 79%, or 53% integrated over the year. In the summer a shift to lighter o^30Si values is correlated with a higher pH, which results in dissolution of diatoms releasing light Si Isotopes. From mass balance, this seasonal diatom dissolution affects Si fluxes by up to 33%, but is limited to 3.7% integrated over the year. These results clearly illustrate that biological activity can have a significant impact on both isotope composition and elemental abundance of continental derived Si. They also demonstrate the pH dependency of diatom dissolution and/or preservation, which is likely to affect not only the continental Si fluxes to the ocean but also the Si recycling in the oceans themselves.

Frederic Moynier - One of the best experts on this subject based on the ideXlab platform.

  • an oceanic subduction origin for archaean granitoids revealed by Silicon Isotopes
    Nature Geoscience, 2019
    Co-Authors: Zhengbin Deng, Frederic Moynier, Marc Chaussidon, Martin Guitreau, Igor S Puchtel, N Dauphas
    Abstract:

    Modern oceanic crust is constantly produced at oceanic ridges and recycled back into the mantle at subduction zones via plate tectonics. An outstanding question in geology is whether the Earth started in a non-plate tectonic regime, and if it did, when the transition to the modern regime occurred. This is a complicated question to address because Archaean rocks lack modern equivalents to anchor interpretations. Here, we present a Silicon isotopic study of 4.0–2.8-Gyr-old tonalite–trondhjemite–granodiorites, as well as Palaeozoic granites and modern adakites. We show that Archaean granitoids have heavier Silicon isotopic compositions than granites and adakites, regardless of melting pressure. This is best explained if Archaean granitoids were formed by melting of subducted basaltic crust enriched in sedimentary silica through interaction with seawater. Before the appearance of silica-forming organisms 0.5–0.6 billion years ago, the oceans were close to Silicon saturation, which led to extensive precipitation of cherts on the seafloor. This is in contrast to modern oceans, where silica biomineralization maintains dissolved Silicon at low concentration. The unique heavy Silicon isotope signature of cherts has been transferred to Archaean granitoids during an oceanic subduction process, which was probably responsible for the formation of felsic rocks on Archaean emerged lands. Archaean granitic rocks formed by melting of silica-enriched subducted basaltic crust through interaction with seawater, according to heavy Silicon Isotopes measured in Archaean samples.

  • Silicon Isotopes reveal recycled altered oceanic crust in the mantle sources of ocean island basalts
    Geochimica et Cosmochimica Acta, 2016
    Co-Authors: Emily A Pringle, Frederic Moynier, Paul S Savage, Matthew G Jackson, Manuel Moreira
    Abstract:

    Abstract The study of Silicon (Si) Isotopes in Ocean Island Basalts (OIB) has the potential to discern between different models for the origins of geochemical heterogeneities in the mantle. Relatively large (∼several per mil per atomic mass unit) Si isotope fractionation occurs in low-temperature environments during biochemical and geochemical precipitation of dissolved Si, where the precipitate is preferentially enriched in the lighter Isotopes relative to the dissolved Si. In contrast, only a limited range (∼tenths of a per mil) of Si isotope fractionation has been observed from high-temperature igneous processes. Therefore, Si Isotopes may be useful as tracers for the presence of crustal material within OIB mantle source regions that experienced relatively low-temperature surface processes in a manner similar to other stable isotope systems, such as oxygen. Characterizing the isotopic composition of the mantle is also of central importance to the use of the Si isotope system as a basis for comparisons with other planetary bodies (e.g., Moon, Mars, asteroids). Here we present the first comprehensive suite of high-precision Si isotope data obtained by MC-ICP-MS for a diverse suite of OIB. Samples originate from ocean islands in the Pacific, Atlantic, and Indian Ocean basins and include representative end-members for the EM-1, EM-2, and HIMU mantle components. On average, δ30Si values for OIB (−0.32 ± 0.09‰, 2 sd) are in general agreement with previous estimates for the δ30Si value of Bulk Silicate Earth (−0.29 ± 0.07‰, 2 sd; Savage et al., 2014). Nonetheless, some small systematic variations are present; specifically, most HIMU-type (Mangaia; Cape Verde; La Palma, Canary Islands) and Iceland OIB are enriched in the lighter Isotopes of Si (δ30Si values lower than MORB), consistent with recycled altered oceanic crust and lithospheric mantle in their mantle sources.

  • Silicon Isotopes reveal recycled altered oceanic crust in the mantle sources of Ocean Island Basalts
    Geochimica et Cosmochimica Acta, 2016
    Co-Authors: Emily Pringle, Frederic Moynier, Manuel Moreira, Paul Savage, Matthew Jackson, James M.d. Day
    Abstract:

    The study of Silicon (Si) Isotopes in Ocean Island Basalts (OIB) has the potential to discern between different models for the origins of geochemical heterogeneities in the mantle. Relatively large (∼several per mil per atomic mass unit) Si isotope fractionation occurs in low-temperature environments during biochemical and geochemical precipitation of dissolved Si, where the precipitate is preferentially enriched in the lighter Isotopes relative to the dissolved Si. In contrast, only a limited range (∼tenths of a per mil) of Si isotope fractionation has been observed from high-temperature igneous processes. Therefore, Si Isotopes may be useful as tracers for the presence of crustal material within OIB mantle source regions that experienced relatively low-temperature surface processes in a manner similar to other stable isotope systems, such as oxygen. Characterizing the isotopic composition of the mantle is also of central importance to the use of the Si isotope system as a basis for comparisons with other planetary bodies (e.g., Moon, Mars, asteroids). Here we present the first comprehensive suite of high-precision Si isotope data obtained by MC-ICP-MS for a diverse suite of OIB. Samples originate from ocean islands in the Pacific, Atlantic, and Indian Ocean basins and include representative end-members for the EM-1, EM-2, and HIMU mantle components. On average, δ30Si values for OIB (−0.32 ± 0.09‰, 2 sd) are in general agreement with previous estimates for the δ30Si value of Bulk Silicate Earth (−0.29 ± 0.07‰, 2 sd; Savage et al., 2014). Nonetheless, some small systematic variations are present; specifically, most HIMU-type (Mangaia; Cape Verde; La Palma, Canary Islands) and Iceland OIB are enriched in the lighter Isotopes of Si (δ30Si values lower than MORB), consistent with recycled altered oceanic crust and lithospheric mantle in their mantle sources.

  • Silicon Isotopes in angrites and volatile loss in planetesimals
    Proceedings of the National Academy of Sciences of the United States of America, 2014
    Co-Authors: Emily A Pringle, Frederic Moynier, Paul S Savage, J. Badro, Jean-alix Barrat
    Abstract:

    Inner solar system bodies, including the Earth, Moon, and asteroids, are depleted in volatile elements relative to chondrites. Hypotheses for this volatile element depletion include incomplete condensation fromthe solar nebula and volatile loss during energetic impacts. These processes are expected to each produce characteristic stable isotope signatures. However, processes of planetary differentiation may also modify the isotopic composition of geochemical reservoirs. Angrites are rare meteorites that crystallized only a few million years after calcium - aluminum-rich inclusions and exhibit extreme depletions in volatile elements relative to chondrites, making them ideal samples with which to study volatile element depletion in the early solar system. Here we present high-precision Si isotope data that show angrites are enriched in the heavy Isotopes of Si relative to chondritic meteorites by 50-100 ppm/amu. Silicon is sufficiently volatile such that it may be isotopically fractionated during incomplete condensation or evaporative mass loss, but theoretical calculations and experimental results also predict isotope fractionation under specific conditions of metal-silicate differentiation. We show that the Si isotope composition of angrites cannot be explained by any plausible core formation scenario, but rather reflects isotope fractionation during impact-induced evaporation. Our results indicate planetesimals initially formed from volatile-rich material and were subsequently depleted in volatile elements during accretion.

Paul S Savage - One of the best experts on this subject based on the ideXlab platform.

  • Silicon Isotopes reveal recycled altered oceanic crust in the mantle sources of ocean island basalts
    Geochimica et Cosmochimica Acta, 2016
    Co-Authors: Emily A Pringle, Frederic Moynier, Paul S Savage, Matthew G Jackson, Manuel Moreira
    Abstract:

    Abstract The study of Silicon (Si) Isotopes in Ocean Island Basalts (OIB) has the potential to discern between different models for the origins of geochemical heterogeneities in the mantle. Relatively large (∼several per mil per atomic mass unit) Si isotope fractionation occurs in low-temperature environments during biochemical and geochemical precipitation of dissolved Si, where the precipitate is preferentially enriched in the lighter Isotopes relative to the dissolved Si. In contrast, only a limited range (∼tenths of a per mil) of Si isotope fractionation has been observed from high-temperature igneous processes. Therefore, Si Isotopes may be useful as tracers for the presence of crustal material within OIB mantle source regions that experienced relatively low-temperature surface processes in a manner similar to other stable isotope systems, such as oxygen. Characterizing the isotopic composition of the mantle is also of central importance to the use of the Si isotope system as a basis for comparisons with other planetary bodies (e.g., Moon, Mars, asteroids). Here we present the first comprehensive suite of high-precision Si isotope data obtained by MC-ICP-MS for a diverse suite of OIB. Samples originate from ocean islands in the Pacific, Atlantic, and Indian Ocean basins and include representative end-members for the EM-1, EM-2, and HIMU mantle components. On average, δ30Si values for OIB (−0.32 ± 0.09‰, 2 sd) are in general agreement with previous estimates for the δ30Si value of Bulk Silicate Earth (−0.29 ± 0.07‰, 2 sd; Savage et al., 2014). Nonetheless, some small systematic variations are present; specifically, most HIMU-type (Mangaia; Cape Verde; La Palma, Canary Islands) and Iceland OIB are enriched in the lighter Isotopes of Si (δ30Si values lower than MORB), consistent with recycled altered oceanic crust and lithospheric mantle in their mantle sources.

  • Silicon Isotopes in angrites and volatile loss in planetesimals
    Proceedings of the National Academy of Sciences of the United States of America, 2014
    Co-Authors: Emily A Pringle, Frederic Moynier, Paul S Savage, J. Badro, Jean-alix Barrat
    Abstract:

    Inner solar system bodies, including the Earth, Moon, and asteroids, are depleted in volatile elements relative to chondrites. Hypotheses for this volatile element depletion include incomplete condensation fromthe solar nebula and volatile loss during energetic impacts. These processes are expected to each produce characteristic stable isotope signatures. However, processes of planetary differentiation may also modify the isotopic composition of geochemical reservoirs. Angrites are rare meteorites that crystallized only a few million years after calcium - aluminum-rich inclusions and exhibit extreme depletions in volatile elements relative to chondrites, making them ideal samples with which to study volatile element depletion in the early solar system. Here we present high-precision Si isotope data that show angrites are enriched in the heavy Isotopes of Si relative to chondritic meteorites by 50-100 ppm/amu. Silicon is sufficiently volatile such that it may be isotopically fractionated during incomplete condensation or evaporative mass loss, but theoretical calculations and experimental results also predict isotope fractionation under specific conditions of metal-silicate differentiation. We show that the Si isotope composition of angrites cannot be explained by any plausible core formation scenario, but rather reflects isotope fractionation during impact-induced evaporation. Our results indicate planetesimals initially formed from volatile-rich material and were subsequently depleted in volatile elements during accretion.

  • Silicon Isotopes in granulite xenoliths insights into isotopic fractionation during igneous processes and the composition of the deep continental crust
    Earth and Planetary Science Letters, 2013
    Co-Authors: Helen M. Williams, Paul S Savage, Bastian R Georg, Alex N Halliday
    Abstract:

    The Silicon (Si) cycle is of great current interest but the isotopic composition of the continental crust has not been determined. Magmatic differentiation generates liquids with heavier Si and the lower crust, thought to be dominated by cumulates and restites, is predicted to have a light isotopic composition. This is borne out by the composition of many types of granite, which appear to have relative light Si for their silica content. Here we report the Si isotopic compositions of two granulite facies xenolith suites, from the Chudleigh and McBride volcanic provinces, Australia, providing new constraints on deep crustal processes and the average composition of the deep continental crust. The xenoliths display a range of isotopic compositions (δ30Si=−0.43‰ to −0.15‰) comparable to that measured previously for igneous rocks. The isotopic compositions of the McBride xenoliths reflect assimilation and fractional crystallisation (AFC) and/or partial melting processes. Silicon and O Isotopes are correlated in the McBride suite and can be explained by AFC of various evolved parent melts. In contrast, the Chudleigh xenoliths have Si isotope compositions predominantly controlled by the specific mineralogy of individual cumulates. Using the xenolith data and a number of weighting methods, the Si isotope compositions of the lower and middle crust are calculated to be δ30Si=−0.29±0.04‰ (95% s.e.) and −0.23±0.04‰ (95% s.e.) respectively. These values are almost identical to the composition of the Bulk Silicate Earth, implying minimal isotope fractionation associated with continent formation and no light lower crustal reservoir.

  • Silicon Isotopes in meteorites and planetary core formation
    Geochimica et Cosmochimica Acta, 2011
    Co-Authors: R. M. G. Armytage, R B Georg, Paul S Savage, Helen M. Williams, Alex N Halliday
    Abstract:

    The Silicon (Si) isotope compositions of 42 meteorite and terrestrial samples have been determined using MC-ICPMS with the aim of resolving the current debate over their compositions and the implications for core formation. No systematic δ30Si differences are resolved between chondrites (δ30Si = −0.49 ± 0.15‰, 2σSD) and achondrites (δ30Si = −0.47 ± 0.11‰, 2σSD), although enstatite chondrites are consistently lighter (δ30Si = −0.63 ± 0.07‰, 2σSD) in comparison to other meteorite groups. The data reported here for meteorites and terrestrial samples display an average difference Δ30SiBSE−meteorite∗ = 0.15 ± 0.10‰, which is consistent within uncertainty with previous studies. No effect from sample heterogeneity, preparation, chemistry or mass spectrometry can be identified as responsible for the reported differences between current datasets. The heavier composition of the bulk silicate Earth is consistent with previous conclusions that Si partitioned into the metal phase during metal–silicate equilibration at the time of core formation. Fixing the temperature of core formation to the peridotite liquidus and using an appropriate metal silicate fractionation factor (e ∼0.89), the Δ30SiBSE−meteorite∗ value from this study indicates that the Earth core contains at least 2.5 and possibly up to 16.8 wt% Si.

Mark A Brzezinski - One of the best experts on this subject based on the ideXlab platform.

  • coupling of the distribution of Silicon Isotopes to the meridional overturning circulation of the north atlantic ocean
    Deep-sea Research Part Ii-topical Studies in Oceanography, 2015
    Co-Authors: Mark A Brzezinski, Janice L Jones
    Abstract:

    Abstract The distribution of Silicon Isotopes within silicic acid, δ 30 Si(OH) 4 , was examined along a section in the North Atlantic from the Cape Verde Islands off Africa to Cape Cod, Massachusetts in North America. Surface water displayed elevated δ 30 Si(OH) 4 associated with biological fractionation of Si during silica production. Below 300 m variations in δ 30 Si(OH) 4 were closely tied to the distribution of water masses as diagnosed through optimum multiparameter analysis, confirming a tight relationship between δ 30 Si(OH) 4 and the meridional overturning circulation in the Atlantic. A linear relationship between δ 30 Si(OH) 4 and the inverse of silicic acid concentration supported control of Si isotope distribution by conservative mixing of end member water masses of different isotopic composition in the Atlantic. There was a suggestion of a weak local minimum in δ 30 Si(OH) 4 in deep waters above the Trans-Atlantic Geotraverse hydrothermal zone on the mid-Atlantic Ridge consistent with the light δ 30 Si(OH) 4 of hydrothermal waters. The lightest δ 30 Si(OH) 4 values were observed in the deep western and deep eastern basins where Antarctic Bottom Water (AABW) dominated. The heaviest values in subsurface waters occurred in North Atlantic Deep Water due to strong ventilation and the contribution of heavy northern source waters that are influenced by the Arctic Ocean. The concept of a Silicon isotope bipole is introduced to explain how the isotopic differences between the northern and southern end-member water masses arise, and how they influence Si isotope distributions. Northern end-member water masses are heavy due to the influence of the Arctic Ocean. Bottom topography prevents light deep waters from entering the Arctic and the further removal of light Isotopes through local biological productivity results in extremely heavy δ 30 Si(OH) 4 within the Arctic. Light AABW dominates the southern end member. The Southern Ocean silicic acid trap distills heavier Isotopes of Si out of the Southern Ocean as preformed silicic acid within Antarctic mode waters, while retaining light Isotopes that become incorporated into AABW. The influence of the Silicon isotope bipole is predicted to be strongest in the Atlantic as the net flow of waters through the Arctic Ocean is from the Pacific to the Atlantic.

  • using Silicon Isotopes to understand the role of the southern ocean in modern and ancient biogeochemistry and climate
    Quaternary Science Reviews, 2014
    Co-Authors: Katharine R Hendry, Mark A Brzezinski
    Abstract:

    Abstract The growth of siliceous phytoplankton, mainly diatoms, in the Southern Ocean influences the preformed nutrient inventory in the ocean on a global scale. Silicic acid use by diatoms and deep circulation combine to trap dissolved Si in the Southern Ocean resulting in high levels of silica production and expansive diatom oozes in Southern Ocean sediments. The analysis of the Silicon isotope composition of biogenic silica, or opal, and dissolved silicic acid provide insight into the operation of the global marine Silicon cycle and the role played by the Southern Ocean in nutrient supply and carbon drawdown, both in the modern and in the past. Silicon isotope studies of diatoms have provided insight into the history of silica production in surface waters, while the analysis of spicules from deep sea sponges has defined both the spatial and the temporal variability of silicic acid concentrations in the water column; together these – and other – proxies reveal variations in the northward flow of Southern Ocean intermediate and mode waters and how changes in Southern Ocean productivity altered their preformed nutrient content. We present a new hypothesis – the “Silicic Acid Ventilation Hypothesis” (SAVH) – to explain the geographical variation of opal-based proxy records, in particular the contrasting patterns of opal burial change found in the low and high latitudes. By understanding the Silicon isotope systematics of opal and silicic acid in the modern, we will be able to use opal-based proxies to reconstruct past changes in the Southern Ocean and so investigate its role in global carbon cycling and climate.

  • fractionation of Silicon Isotopes during biogenic silica dissolution
    Geochimica et Cosmochimica Acta, 2009
    Co-Authors: Mark S Demarest, Mark A Brzezinski, Charlotte P Beucher
    Abstract:

    Abstract Silicon Isotopes have been investigated for their potential to reveal both past and present patterns of silicic acid utilization, primarily by diatoms, in surface waters of the ocean. Interpretation of this proxy has thus far relied on characteristic trends in the isotope composition of the dissolved and particulate Silicon pools in the upper ocean, as driven by biological fractionation during the production of biogenic silica (bSiO2, or opal) by diatoms. However, other factors which may influence the Silicon isotope composition of diatom opal, particularly post-formational aging and maturation processes, remain largely uninvestigated. Here, we report a consistent fractionation of Silicon Isotopes during the physicochemical dissolution of diatom bSiO2 suspended in seawater under closed conditions. This fractionation acts counter to that occurring during bSiO2 production and at about half its absolute magnitude, with dissolution discriminating against the release of the heavier Isotopes of Silicon at an enrichment factor eDSi–BSi of −0.55‰, corresponding to a fractionation factor α30/28 of 0.99945. The enrichment factor did not vary with source material, indicating the lack of a significant species effect, or with temperature from 3 to 20 °C. Thus, the dissolution of bSiO2 produces dissolved Silicon with a δ30Si value that is 0.55‰ more negative than its parent bSiO2, an effect that must be accounted for when interpreting oceanic δ30Si distributions. The δ30Si values of both the dissolved and particulate Silicon pools increased linearly as dissolution progressed, implying a measurable (±0.1‰) change in the relative δ30Si of opal samples whenever the difference in preservation efficiency between them is >20%. This effect could account for ∼10–30% of the difference in diatom δ30Si values observed between glacial and interglacial conditions. It is unlikely, however, that the inferred maximum possible change in δb30SiO2 of +0.55‰ would be manifested in situ, as a high mean percentage of dissolution would include complete loss of the more soluble members of the diatom assemblage.

  • sources and biological fractionation of Silicon Isotopes in the eastern equatorial pacific
    Geochimica et Cosmochimica Acta, 2008
    Co-Authors: Charlotte P Beucher, Mark A Brzezinski, Janice L Jones
    Abstract:

    Abstract Silicon Isotopes in dissolved silicic acid were measured in the upper four kilometers between 4°N and 3°S latitude at 110°W longitude in the eastern Equatorial Pacific. Silicon Isotopes became progressively heavier with silicic acid depletion of surface water as expected from biological fractionation. The value of e estimated by applying a steady-state isotope fractionation model to data from all stations between 4°N and 3°S was −0.77 ± 0.12‰ (std. err.). When the analysis was restricted to those stations whose temperature and salinity profiles indicated that they were directly influenced by upwelling of the Equatorial Undercurrent (EUC), the resulting value of e was −1.08 ± 0.27‰ (std. err.) similar to the value established in culture studies (−1.1‰). When the non steady state Rayleigh model was applied to the same restricted data set the resulting value of e was significantly more positive, −0.61 ± 0.16‰ (std. err.). To the extent that the equatorial system approximates a steady state these results support a value of −1.1‰ for the fractionation factor for Isotopes of Si in the sea. Without the assumption of steady state the value of e can only be constrained to be between −0.6 and −1.1‰. Silicic acid in Equatorial Pacific Deep Water below 2000 m had a near constant δ 30 Si of +1.32 ± 0.05‰. That value is significantly more positive than obtained for North Pacific Deep Water at similar depths at stations to the northwest of our study area (0.9–1.0‰) and it is slightly less positive than new measures of the δ 30 Si of silicic acid from the silicic acid plume centered over the Cascadia basin in the Northeast Pacific (Si(OH) 4  > 180  μM, δ 30 Si = +1.46 ± 0.12‰ (SD, n  = 4). We show that the data from the equator and Cascadia basin fit a general trend of increasing δ 30 Si(OH) 4 with increasing silicic acid concentration in the deep sea, but that the isotope values from the Northeast Pacific are anomalously light. The observed level of variation in the Silicon isotope composition of deep waters from this single ocean basin is considerably larger than that predicted by current models based on fractionation during opal formation with no isotope effect during dissolution. Confirmation of such high variability in deep water δ 30 Si(OH) 4 within individual ocean basins will require reassessment of the mechanisms controlling the distribution of Isotopes of Silicon in the sea.

  • biological fractionation of Silicon Isotopes in southern ocean surface waters
    Global Biogeochemical Cycles, 2004
    Co-Authors: Carol J Pride, Diana E Varela, Mark A Brzezinski
    Abstract:

    [1] The fractionation of Silicon Isotopes by diatoms during silicification (i.e., opaline cell wall formation) provides a new tool for paleoceanographic studies of the Silicon cycle. Here we examine the natural variations of the 30Si:28Si ratio in silicic acid (Si(OH)4) and biogenic silica (bSiO2) in surface waters of the Antarctic Circumpolar Current (ACC) along 170°W. The results provide direct evidence of biologically mediated fractionation of Silicon Isotopes, with an enrichment factor (ɛ) of between −1.1 and −1.9‰, depending on the model employed. Comparison of the mass flux of bSiO2 captured in sediment traps deployed in the study area with the Silicon isotopic composition of that material establishes a direct linkage between diatom dynamics in surface waters and the isotopic signature of exported particles. We calculated Si(OH)4:NO3− utilization ratios from variations in Silicon and nitrogen isotopic ratios in surface waters and sediment traps that agree well with direct observations of 4:1 in this region. This work supports the use of variations in Silicon Isotopes in sedimentary opal as a proxy for relative Si(OH)4 utilization in surface waters.