Stable W isotope evidence for redistribution of homogeneous 182W anomalies in SW Greenland
Affiliations | Corresponding Author | Cite as | Funding information
Published by the European Association of Geochemistry
under Creative Commons License CC BY-NC-ND 4.0
Keywords: Stable tungsten isotopes, 182W anomaly, Isua, Eoarchean
- Share this article
Article views:447Cumulative count of HTML views and PDF downloads.
- Download Citation
- Rights & Permissions
top
Abstract
Figures
 Figure 1 Plot of stable W isotope compositions and W concentrations for Eoarchean rocks from the Itsaq Gneiss Complex. The observed range in δ186/184W is even larger when compared to samples from modern OIBs, MORBs and arcs, and the variations are probably related to stable W isotope fractionation during local metasomatic processes. |  Figure 2 Plots of measured W isotope compositions (a) versus MgO as an indicator for magmatic differentiation, (b) versus W/Th as an indicator for selective W mobility by metasomatic fluids and (c) versus Nb/Ta as an indicator for the presence of residual rutile during partial melting. We observe no co-variation of δ186/184W values neither with any element concentration nor with any element ratio. |  Figure 3 Despite variable δ186/184W, the excess of 182W is homogeneously distributed in igneous rocks from the IGC (Tusch et al., 2019). We hypothesise that mass dependent stable W isotope fractionation during multi-stage metasomatic redistribution of W caused locally variable δ186/184W without affecting the regionally homogeneous mass independent excesses of 182W. |
| Figure 1 | Figure 2 | Figure 3 |
top
Introduction
The early Earth differed significantly from its present day geological setting. Higher potential mantle temperatures but lower heat fluxes (Korenaga, 2003
Korenaga, J. (2003) Energetics of mantle convection and the fate of fossil heat. Geophysical Research Letters 30.
) affected most relevant geological processes such as partial mantle melting with higher melt proportions, the mode of fractional crystallisation, and global tectonics (e.g., de Wit et al., 1992de Wit, M.J., de Ronde, C.E.J., Tredoux, M., Roering, C., Hart, R.J., Armstrong, R.A., Green, R.W.E., Peberdy, E., Hart, R.A. (1992) Formation of an Archaean continent. Nature 357, 553.
; Van Kranendonk, 2010Van Kranendonk, M.J. (2010) Two types of Archean continental crust. American Journal of Science 310, 1187–1209.
). For example, it remains controversial if modern style plate tectonics operated on a global or local scale or were even absent in Hadean and Eoarchean times (e.g., de Wit et al., 1992de Wit, M.J., de Ronde, C.E.J., Tredoux, M., Roering, C., Hart, R.J., Armstrong, R.A., Green, R.W.E., Peberdy, E., Hart, R.A. (1992) Formation of an Archaean continent. Nature 357, 553.
; Nutman et al., 1996Nutman, A.P., McGregor, V.R., Friend, C.R.L., Bennett, V.C., Kinny, P.D. (1996) The Itsaq gneiss complex of southern West Greenland; the world’s most extensive record of early crustal evolution (3900-3600 Ma). Precambrian Research 78, 1–39.
; Van Kranendonk, 2010Van Kranendonk, M.J. (2010) Two types of Archean continental crust. American Journal of Science 310, 1187–1209.
). Alternative models suggest vertical stagnant lid tectonics, where thickened and dense eclogitic crustal restites sink in sagduction regimes (e.g., Bédard, 2006Bédard, J.H. (2006) A catalytic delamination-driven model for coupled genesis of Archaean crust and sub-continental lithospheric mantle. Geochimica et Cosmochimica Acta 70, 1188–1214.
). In the latter case, crustal recycling would have differed, also affecting the geochemical cycling of elements and thus, the geochemical composition of early crustal nuclei.Stable W isotopes represent an emerging geochemical tool that might help to reconstruct the geologic and tectonic history of ancient rocks as well as the cycling of W through different geochemical reservoirs (Kurzweil et al., 2019
Kurzweil, F., Münker, C., Grupp, M., Braukmüller, N., Fechtner, L., Christian, M., Hohl, S.V., Schoenberg, R. (2019) The stable tungsten isotope composition of modern igneous reservoirs. Geochimica et Cosmochimica Acta 251, 176–191.
; Mazza et al., 2020Mazza, S.E., Stracke, A., Gill, J.B., Kimura, J.-I., Kleine, T. (2020) Tracing dehydration and melting of the subducted slab with tungsten isotopes in arc lavas. Earth and Planetary Science Letters 530, 115942.
). Distinct types of modern igneous rocks show resolvable variation in the stable W isotope compositions. For example, subduction related rocks that carry a significant subducted sediment component show isotopically heavy compositions for W. In contrast, isotopically lighter compositions were observed in back-arc lavas that formed above deeper subduction zone settings, more distant to the trench (Mazza et al., 2020Mazza, S.E., Stracke, A., Gill, J.B., Kimura, J.-I., Kleine, T. (2020) Tracing dehydration and melting of the subducted slab with tungsten isotopes in arc lavas. Earth and Planetary Science Letters 530, 115942.
). Fractional crystallisation of olivine and pyroxene as well as partial melting of mantle peridotite, however, have limited and so far not resolvable effects on the stable W isotope composition of mafic and intermediate melts. Accordingly, plume related rocks from ocean island basalt settings and mid-ocean ridge basalts show limited variation in δ186/184W values (Eq. S-1), calling for a homogeneous modern mantle stable W isotope composition of +0.085 ± 0.019 ‰. Fig. 1; Kurzweil et al., 2019Kurzweil, F., Münker, C., Grupp, M., Braukmüller, N., Fechtner, L., Christian, M., Hohl, S.V., Schoenberg, R. (2019) The stable tungsten isotope composition of modern igneous reservoirs. Geochimica et Cosmochimica Acta 251, 176–191.
; Mazza et al., 2020Mazza, S.E., Stracke, A., Gill, J.B., Kimura, J.-I., Kleine, T. (2020) Tracing dehydration and melting of the subducted slab with tungsten isotopes in arc lavas. Earth and Planetary Science Letters 530, 115942.
).Figure 1 Plot of stable W isotope compositions and W concentrations for Eoarchean rocks from the Itsaq Gneiss Complex. The observed range in δ186/184W is even larger when compared to samples from modern OIBs, MORBs and arcs, and the variations are probably related to stable W isotope fractionation during local metasomatic processes.
In this study, we present stable W isotope data for some of the Earth’s oldest rocks from the Itsaq Gneiss Complex, SW Greenland (IGC, 3620 to >3850 Ma; e.g., Nutman et al., 1996
Nutman, A.P., McGregor, V.R., Friend, C.R.L., Bennett, V.C., Kinny, P.D. (1996) The Itsaq gneiss complex of southern West Greenland; the world’s most extensive record of early crustal evolution (3900-3600 Ma). Precambrian Research 78, 1–39.
) including peridotites, tholeiitic metabasalts, boninite-like metabasalts, non-gneissic tonalite-trondhjemite-granodiorites (TTGs) and metasediments, with the aim to reconstruct the sources and processes that controlled the inventory of W in Eoarchean time. This region is of particular interest, because of widespread metasomatic alteration (Rosing et al., 1996Rosing, M.T., Rose, N.M., Bridgwater, D., Thomsen, H.S. (1996) Earliest part of Earth’s stratigraphic record. Geology 24, 43–46.
) and a previously reported large scale mobility of W (Rizo et al., 2016Rizo, H., Walker, R.J., Carlson, R.W., Touboul, M., Horan, M.F., Puchtel, I.S., Boyet, M., Rosing, M.T. (2016) Early Earth differentiation investigated through 142Nd, 182W, and highly siderophile element abundances in samples from Isua, Greenland. Geochimica et Cosmochimica Acta 175, 319–336.
; Tusch et al., 2019Tusch, J., Sprung, P., van de Löcht, J., Hoffmann, J.E., Boyd, A.J., Rosing, M.T., Münker, C. (2019) Uniform 182W isotope compositions in Eoarchean rocks from the Isua region, SW Greenland. Geochimica et Cosmochimica Acta 257, 284–310.
) that even led to ore grade enrichment of W at a local scale (Appel, 1986Appel, P.U. (1986) Strata bound scheelite in the Archean Malene supracrustal belt, West Greenland. Mineralium Deposita 21, 207–215.
). Moreover, the IGC is one of the first localities on Earth, where significant 182W excesses were reported (e.g., Willbold et al., 2011Willbold, M., Elliott, T., Moorbath, S. (2011) The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment. Nature 477, 195.
; the excess of 182W is described by µ182W, which is defined in the SI), and stable W isotopes may aid to elucidate the significance of these isotope anomalies. We combine the stable W isotope data with previously published major and trace element concentration data as well as with µ182W data obtained for the same set of samples (Tusch et al., 2019Tusch, J., Sprung, P., van de Löcht, J., Hoffmann, J.E., Boyd, A.J., Rosing, M.T., Münker, C. (2019) Uniform 182W isotope compositions in Eoarchean rocks from the Isua region, SW Greenland. Geochimica et Cosmochimica Acta 257, 284–310.
) to place constraints on the geodynamic setting of the magmatic rocks, their relationship among each other and possible hydrothermal and metasomatic overprint.top
Results
A detailed description of the sample material, the geological setting and the analytical procedure can be found in the SI. The peridotite samples show extreme variations in W concentrations between 59 and 4380 ng/g, corresponding to W/Th between 0.02 and 143. Their stable W isotope compositions range from +0.008 to +0.134 ‰ in δ186/184W (Table S-1, Fig. 1). Tholeiitic metabasalts show similarly variable W concentrations between 117 and 1835 ng/g (W/Th from 0.2 to 2.3) and an even larger range in δ186/184W (−0.072 to +0.136 ‰), clearly outside the range predicted for igneous processes (Kurzweil et al., 2019
Kurzweil, F., Münker, C., Grupp, M., Braukmüller, N., Fechtner, L., Christian, M., Hohl, S.V., Schoenberg, R. (2019) The stable tungsten isotope composition of modern igneous reservoirs. Geochimica et Cosmochimica Acta 251, 176–191.
; Mazza et al., 2020Mazza, S.E., Stracke, A., Gill, J.B., Kimura, J.-I., Kleine, T. (2020) Tracing dehydration and melting of the subducted slab with tungsten isotopes in arc lavas. Earth and Planetary Science Letters 530, 115942.
). In contrast, the boninite-like metabasalts are more depleted in W with concentrations between 14 and 100 ng/g. These rocks show elevated δ186/184W values between +0.069 and +0.249 ‰. Non-gneissic TTGs range in W concentrations between 29 and 430 ng/g (W/Th between 0.01 and 0.35). The stable W isotope compositions vary between +0.016 and +0.179 ‰ in δ186/184W, thus showing a similar range as most mafic and ultramafic rocks (Table S-1, Fig. 1). In contrast, metasediments that are relatively rich in W (928 to 1547 ng/g) exhibit a smaller range in δ186/184W between +0.050 and +0.077 ‰.In the complete dataset we observe no clear co-variation of δ186/184W values with any other geochemical parameters (Fig. 2), neither with major or trace element concentrations nor with element concentration ratios. The variability in δ186/184W values is independent of the geographic position, the age, and the rock type of the sample. Similarly, W concentrations show no co-variation with other element concentrations or with element concentration ratios (Fig. S-1).
Figure 2 Plots of measured W isotope compositions (a) versus MgO as an indicator for magmatic differentiation, (b) versus W/Th as an indicator for selective W mobility by metasomatic fluids and (c) versus Nb/Ta as an indicator for the presence of residual rutile during partial melting. We observe no co-variation of δ186/184W values neither with any element concentration nor with any element ratio.
top
Discussion
The major and trace element composition of tholeiitic and boninite-like metabasalts from the IGC is mainly controlled by fractional crystallisation of olivine and pyroxene (Polat and Hofmann, 2003
Polat, A., Hofmann, A.W. (2003) Alteration and geochemical patterns in the 3.7–3.8 Ga Isua greenstone belt, West Greenland. Precambrian Research 126, 197–218.
), whereby the boninite-like melts originate from an already depleted, more harzburgitic mantle source. The peridotites of this study represent relict mantle rocks (van de Löcht et al., 2018van de Löcht, J., Hoffmann, J.E., Li, C., Wang, Z., Becker, H., Rosing, M.T., Kleinschrodt, R., Münker, C. (2018) Earth’s oldest mantle peridotites show entire record of late accretion. Geology 46, 199–202.
), and the TTGs likely formed by partial melting of hydrated mafic crust (e.g., Hoffmann et al., 2014Hoffmann, J.E., Nagel, T.J., Muenker, C., Næraa, T., Rosing, M.T. (2014) Constraining the process of Eoarchean TTG formation in the Itsaq Gneiss Complex, southern West Greenland. Earth and Planetary Science Letters 388, 374–386.
; SI). The close genetic relationship of the rock suites analysed here might explain the observed tight co-variations between various compatible and incompatible elements (Table S-2, Fig. S-1; Tusch et al., 2019Tusch, J., Sprung, P., van de Löcht, J., Hoffmann, J.E., Boyd, A.J., Rosing, M.T., Münker, C. (2019) Uniform 182W isotope compositions in Eoarchean rocks from the Isua region, SW Greenland. Geochimica et Cosmochimica Acta 257, 284–310.
). Most elements that behave incompatibly during partial melting and fractional crystallisation of olivine and pyroxene (Zr, Th, Ba, Pb) show expected enrichments in more differentiated rocks, whereas compatible elements such as Mg, Ni and Co are more depleted (Fig. S-1). These differentiation trends are even preserved for some fluid mobile elements (Ba, Pb), which is noteworthy, considering the complex geologic history of igneous rocks from the IGC that experienced several high grade metamorphic, hydrothermal and metasomatic events (Nutman et al., 1996Nutman, A.P., McGregor, V.R., Friend, C.R.L., Bennett, V.C., Kinny, P.D. (1996) The Itsaq gneiss complex of southern West Greenland; the world’s most extensive record of early crustal evolution (3900-3600 Ma). Precambrian Research 78, 1–39.
; Rosing et al., 1996Rosing, M.T., Rose, N.M., Bridgwater, D., Thomsen, H.S. (1996) Earliest part of Earth’s stratigraphic record. Geology 24, 43–46.
). Partial recrystallisation during amphibolite facies grade metamorphism (e.g., the formation of secondary amphibole, feldspar and biotite) apparently changed the mineralogy without significantly changing the overall bulk geochemical composition for most elements. However, the incompatible and fluid mobile element W shows no co-variation with any other element (Fig. S-1), indicating that its budget was severely affected during metamorphism, metasomatism and hydrothermalism. Extreme W concentrations in peridotites up to 4380 ng/g (more than 100 fold compared to 12 ng/g of the modern mantle; König et al., 2011König, S., Münker, C., Hohl, S., Paulick, H., Barth, A.R., Lagos, M., Pfänder, J., Büchl, A. (2011) The Earth’s tungsten budget during mantle melting and crust formation. Geochimica et Cosmochimica Acta 75, 2119–2136.
) and W/Th (0.02–143) that are, with the exception of sample 10–12A, highly elevated compared to the canonical value defined by modern igneous rocks and clearly indicate secondary enrichment of W (Rizo et al., 2016Rizo, H., Walker, R.J., Carlson, R.W., Touboul, M., Horan, M.F., Puchtel, I.S., Boyet, M., Rosing, M.T. (2016) Early Earth differentiation investigated through 142Nd, 182W, and highly siderophile element abundances in samples from Isua, Greenland. Geochimica et Cosmochimica Acta 175, 319–336.
; Tusch et al., 2019Tusch, J., Sprung, P., van de Löcht, J., Hoffmann, J.E., Boyd, A.J., Rosing, M.T., Münker, C. (2019) Uniform 182W isotope compositions in Eoarchean rocks from the Isua region, SW Greenland. Geochimica et Cosmochimica Acta 257, 284–310.
). Primary hosts for secondary W enrichments by metasomatic fluids are minor mineral phases such as serpentine, Ti-rich mineral phases, phlogopite and sulfide or grain boundary assemblages (Liu et al., 2018Liu, J., Pearson, D.G., Chacko, T., Luo, Y. (2018) A reconnaissance view of tungsten reservoirs in some crustal and mantle rocks: Implications for interpreting W isotopic compositions and crust-mantle W cycling. Geochimica et Cosmochimica Acta 223, 300–318.
). Indeed, fine serpentine veinlets as well as small metasomatic sulfides have been reported for most peridotites investigated here (van de Löcht et al., 2018van de Löcht, J., Hoffmann, J.E., Li, C., Wang, Z., Becker, H., Rosing, M.T., Kleinschrodt, R., Münker, C. (2018) Earth’s oldest mantle peridotites show entire record of late accretion. Geology 46, 199–202.
, 2020van de Löcht, J., Hoffmann, J.E., Rosing, M.T., Sprung, P., Münker, C. (2020) Preservation of Eoarchean mantle processes in ∼3.8 Ga peridotite enclaves in the Itsaq Gneiss Complex, southern West Greenland. Geochimica et Cosmochimica Acta 280,1–25.
). Consistent with a previous study (Rizo et al., 2016Rizo, H., Walker, R.J., Carlson, R.W., Touboul, M., Horan, M.F., Puchtel, I.S., Boyet, M., Rosing, M.T. (2016) Early Earth differentiation investigated through 142Nd, 182W, and highly siderophile element abundances in samples from Isua, Greenland. Geochimica et Cosmochimica Acta 175, 319–336.
), we thus suggest that W was strongly mobilised by metasomatic fluids. This secondary disturbance of W probably increased the variation in δ186/184W beyond the range that is expected for unaltered igneous rocks (Kurzweil et al., 2019Kurzweil, F., Münker, C., Grupp, M., Braukmüller, N., Fechtner, L., Christian, M., Hohl, S.V., Schoenberg, R. (2019) The stable tungsten isotope composition of modern igneous reservoirs. Geochimica et Cosmochimica Acta 251, 176–191.
; Mazza et al., 2020Mazza, S.E., Stracke, A., Gill, J.B., Kimura, J.-I., Kleine, T. (2020) Tracing dehydration and melting of the subducted slab with tungsten isotopes in arc lavas. Earth and Planetary Science Letters 530, 115942.
). Therefore, our data provides no straightforward insight into tectono-magmatic processes active during the formation of the IGC.The chemical composition of metasomatic fluids in the Isua region was shown to be variable on local to regional scales depending on the ambient rock assemblage (Rosing et al., 1996
Rosing, M.T., Rose, N.M., Bridgwater, D., Thomsen, H.S. (1996) Earliest part of Earth’s stratigraphic record. Geology 24, 43–46.
). This local heterogeneity probably also accounts for heterogeneous W concentrations and stable W isotope compositions of the ambient fluids and for the decoupling of W that is typically dissolved as an oxyanion, which can be complexed with cationic ligands such as K+ or H+, from other nominally fluid mobile elements such as Ba or Pb that are transported as metal cations, which are complexed with anionic ligands such as Cl− (Wood and Samson, 2000Wood, S.A., Samson, I.M. (2000) The hydrothermal geochemistry of tungsten in granitoid environments. Economic Geology 95, 143–182.
). The observed variability in δ186/184W values, which is not correlated with W concentrations (Fig. 1), argues against a single, regionally homogeneous metasomatising fluid that overprinted primary stable W isotope compositions of IGC rocks to the same extent. The large variation of δ186/184W within different rock types rather calls for the involvement of different metasomatic fluids with locally (and temporally) variable stable W isotope compositions. To explain this local and temporal variability in δ186/184W, stable W isotope fractionation at various geologic conditions is required.In general, the magnitude of equilibrium isotope fractionation in geological environments depends on differences in bond strengths, whereby heavy isotopes prefer stronger bonding environments such as high oxidation states and low coordination numbers. Stable W isotope fractionation caused by redox changes in terrestrial igneous environments is not expected (Kurzweil et al., 2019
Kurzweil, F., Münker, C., Grupp, M., Braukmüller, N., Fechtner, L., Christian, M., Hohl, S.V., Schoenberg, R. (2019) The stable tungsten isotope composition of modern igneous reservoirs. Geochimica et Cosmochimica Acta 251, 176–191.
), because the oxidised W6+ species is predominant in these settings (Fonseca et al., 2014Fonseca, R.O.C., Mallmann, G., Sprung, P., Sommer, J.E., Heuser, A., Speelmanns, I.M., Blanchard, H. (2014) Redox controls on tungsten and uranium crystal/silicate melt partitioning and implications for the U/W and Th/W ratio of the lunar mantle. Earth and Planetary Science Letters 404, 1–13.
). However, during metasomatic alteration the preferential leaching of isotopically heavy W, which is mainly abundant as a tetrahedrally coordinated anion [WO4]2− in fluids, is expected, similar as in modern fluid-dominated subduction zone settings (Mazza et al., 2020Mazza, S.E., Stracke, A., Gill, J.B., Kimura, J.-I., Kleine, T. (2020) Tracing dehydration and melting of the subducted slab with tungsten isotopes in arc lavas. Earth and Planetary Science Letters 530, 115942.
). Moreover, octahedral coordination is preferred in hexagonally close packed mineral structures (e.g., wolframite), whereas the complex remains in tetrahedral coordination in cubic closed packed minerals (e.g,. scheelite; Kuzmin and Purans, 2001Kuzmin, A., Purans, J. (2001) Local atomic and electronic structure of tungsten ions in AWO4 crystals of scheelite and wolframite types. Radiation measurements 33, 583–586.
). As such W-rich minerals are reported in the IGC (Appel, 1986Appel, P.U. (1986) Strata bound scheelite in the Archean Malene supracrustal belt, West Greenland. Mineralium Deposita 21, 207–215.
), we speculate that the local remobilisation of W by metasomatic and hydrothermal fluids (and possibly seawater) as well as the formation of secondary W-rich mineral phases caused mass dependent stable W isotope fractionation in different directions due to localised changes in coordination.To understand the metasomatic processes in more detail the separate consideration of distinct rock types and their specific petrographic properties is necessary. For example, TTGs with sub-canonical W/Th show low δ186/184W, whereas TTGs with canonical or supra-canonical W/Th show higher δ186/184W (Fig. 2b). A similar relationship is observed for peridotites. Most peridotite samples show strong W enrichment (high W/Th) and relatively high δ186/184W. The only peridotite with sub-canonical W/Th was also hydrated most strongly (10–12A; van de Löcht et al., 2020
van de Löcht, J., Hoffmann, J.E., Rosing, M.T., Sprung, P., Münker, C. (2020) Preservation of Eoarchean mantle processes in ∼3.8 Ga peridotite enclaves in the Itsaq Gneiss Complex, southern West Greenland. Geochimica et Cosmochimica Acta 280,1–25.
) and shows the lowest δ186/184W of all peridotites. These relationships indicate the preferential mobilisation of isotopically heavy W by metasomatic fluids, leaving behind W depleted rocks that are isotopically lighter (Fig. 2b). The apparently higher number of W-rich peridotites with high δ186/184W might reflect sampling bias, as strongly altered samples such as 10–12A are usually excluded. Thus, strong W enrichments in peridotites do not necessarily require external sources of W but could also relate to very local W redistribution within single rock types. Importantly, W-rich peridotites in SW Greenland were metasomatised by chemically different fluids (H2-rich vs. CO2-rich; Rosing et al., 1996Rosing, M.T., Rose, N.M., Bridgwater, D., Thomsen, H.S. (1996) Earliest part of Earth’s stratigraphic record. Geology 24, 43–46.
) as also indicated by crosscutting fine serpentine veins (SOISB1 and NUB peridotites) and carbonate veins (sample 10–34), respectively (van de Löcht et al., 2020van de Löcht, J., Hoffmann, J.E., Rosing, M.T., Sprung, P., Münker, C. (2020) Preservation of Eoarchean mantle processes in ∼3.8 Ga peridotite enclaves in the Itsaq Gneiss Complex, southern West Greenland. Geochimica et Cosmochimica Acta 280,1–25.
). Generally high W/Th in these samples indicate that both fluids were rich in W but it seems that W concentrations were higher and δ186/184W slightly lower in CO2-rich fluids that percolated the carbonate bearing peridotite 10–34 (Fig. 2b). These relationships clearly suggest that the source of W in metasomatic fluids was heterogeneous and of different isotopic polarity.Compared to modern igneous rocks, samples from the IGC show uniform excesses in the radiogenic isotope 182W. This excess was explained by either a missing late veneer component that was depleted in 182W (Willbold et al., 2011
Willbold, M., Elliott, T., Moorbath, S. (2011) The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment. Nature 477, 195.
; Dale et al., 2017Dale, C.W., Kruijer, T.S., Burton K.W. (2017) Highly siderophile element and 182W evidence for a partial late veneer in the source of 3.8 Ga rocks from Isua, Greenland. Earth and Planetary Science Letters 458, 394–404.
), or by early silicate differentiation during the lifetime of short lived 182Hf and the respective formation of 182W-enriched and 182W-depleted silicate reservoirs, respectively (Rizo et al., 2016Rizo, H., Walker, R.J., Carlson, R.W., Touboul, M., Horan, M.F., Puchtel, I.S., Boyet, M., Rosing, M.T. (2016) Early Earth differentiation investigated through 142Nd, 182W, and highly siderophile element abundances in samples from Isua, Greenland. Geochimica et Cosmochimica Acta 175, 319–336.
; Tusch et al., 2019Tusch, J., Sprung, P., van de Löcht, J., Hoffmann, J.E., Boyd, A.J., Rosing, M.T., Münker, C. (2019) Uniform 182W isotope compositions in Eoarchean rocks from the Isua region, SW Greenland. Geochimica et Cosmochimica Acta 257, 284–310.
). In all these studies, mass dependent isotope fractionation that occurred during the chemical separation of W and the measurement of W isotope abundances is corrected assuming the exponential mass fractionation law and a given reference value of the 186W/184W ratio. However, natural mass dependent isotope fractionation during fluid controlled processes might have followed a different mass fractionation law (e.g., Hart and Zindler, 1989Hart, S.R., Zindler, A. (1989) Isotope fractionation laws. International Journal of Mass Spectrometry and Ion Processes 89, 287–301.
; SI). The application of inappropriate mass fractionation laws may therefore theoretically create apparent mass independent isotope effects in µ182W (Hart and Zindler, 1989Hart, S.R., Zindler, A. (1989) Isotope fractionation laws. International Journal of Mass Spectrometry and Ion Processes 89, 287–301.
; SI). Assuming the equilibrium mass fractionation law during natural stable W isotope fractionation, however, the observed range in δ186/184W can maximally account for 182W excesses that are smaller than 2 ppm in µ182W (Fig. S-2, Table S-3) a similar range that was also previously predicted by stable W isotope data of lower precision (Rizo et al., 2016Rizo, H., Walker, R.J., Carlson, R.W., Touboul, M., Horan, M.F., Puchtel, I.S., Boyet, M., Rosing, M.T. (2016) Early Earth differentiation investigated through 142Nd, 182W, and highly siderophile element abundances in samples from Isua, Greenland. Geochimica et Cosmochimica Acta 175, 319–336.
). Moreover, we observe no co-variation of µ182W and δ186/184W values (Fig. 3). Thus, 182W excesses of around +13 ppm in rocks of the IGC (Willbold et al., 2011Willbold, M., Elliott, T., Moorbath, S. (2011) The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment. Nature 477, 195.
; Rizo et al., 2016Rizo, H., Walker, R.J., Carlson, R.W., Touboul, M., Horan, M.F., Puchtel, I.S., Boyet, M., Rosing, M.T. (2016) Early Earth differentiation investigated through 142Nd, 182W, and highly siderophile element abundances in samples from Isua, Greenland. Geochimica et Cosmochimica Acta 175, 319–336.
; Dale et al., 2017Dale, C.W., Kruijer, T.S., Burton K.W. (2017) Highly siderophile element and 182W evidence for a partial late veneer in the source of 3.8 Ga rocks from Isua, Greenland. Earth and Planetary Science Letters 458, 394–404.
; Tusch et al., 2019Tusch, J., Sprung, P., van de Löcht, J., Hoffmann, J.E., Boyd, A.J., Rosing, M.T., Münker, C. (2019) Uniform 182W isotope compositions in Eoarchean rocks from the Isua region, SW Greenland. Geochimica et Cosmochimica Acta 257, 284–310.
) represent no analytical artefacts caused by the application of inappropriate mass fractionation laws.Figure 3 Despite variable δ186/184W, the excess of 182W is homogeneously distributed in igneous rocks from the IGC (Tusch et al., 2019
Tusch, J., Sprung, P., van de Löcht, J., Hoffmann, J.E., Boyd, A.J., Rosing, M.T., Münker, C. (2019) Uniform 182W isotope compositions in Eoarchean rocks from the Isua region, SW Greenland. Geochimica et Cosmochimica Acta 257, 284–310.
). We hypothesise that mass dependent stable W isotope fractionation during multi-stage metasomatic redistribution of W caused locally variable δ186/184W without affecting the regionally homogeneous mass independent excesses of 182W.Despite variable secondary enrichment of W and heterogeneous δ186/184W beyond the range observed in fresh igneous rocks, the excess of 182W is homogeneously distributed in igneous rocks from the IGC (Tusch et al., 2019
Tusch, J., Sprung, P., van de Löcht, J., Hoffmann, J.E., Boyd, A.J., Rosing, M.T., Münker, C. (2019) Uniform 182W isotope compositions in Eoarchean rocks from the Isua region, SW Greenland. Geochimica et Cosmochimica Acta 257, 284–310.
). We therefore suggest that mass dependent stable W isotope fractionation during multi-stage metasomatic redistribution of W increased the variability in δ186/184W on a local scale without affecting the mass independent excesses of 182W. The sources of the metasomatic W likely tap different rock types as indicated by extremely variable W/Th and δ186/184W in all analysed rock types. These observations argue against initially variable excesses in 182W that were fully homogenised but are more consistent with an originally widespread and homogeneously distributed excess of 182W within the Eoarchean assemblage of the IGC prior to the metasomatic redistribution of W. From a W perspective, this would permit combination with other geochemical parameters such as 142Nd isotope or HSE signatures (e.g., Rizo et al., 2016Rizo, H., Walker, R.J., Carlson, R.W., Touboul, M., Horan, M.F., Puchtel, I.S., Boyet, M., Rosing, M.T. (2016) Early Earth differentiation investigated through 142Nd, 182W, and highly siderophile element abundances in samples from Isua, Greenland. Geochimica et Cosmochimica Acta 175, 319–336.
; Dale et al., 2017Dale, C.W., Kruijer, T.S., Burton K.W. (2017) Highly siderophile element and 182W evidence for a partial late veneer in the source of 3.8 Ga rocks from Isua, Greenland. Earth and Planetary Science Letters 458, 394–404.
; Saji et al., 2018Saji, N.S., Larsen, K., Wielandt, D., Schiller, M., Costa, M.M., Whitehouse, M.J., Rosing, M.T., Bizzarro, M. (2018) Hadean geodynamics inferred from time-varying 142Nd/144Nd in the early Earth rock record. Geochemical Perspectives Letters 7, 43–48.
). In the light of coupled 182W-142Nd anomalies (Rizo et al., 2016Rizo, H., Walker, R.J., Carlson, R.W., Touboul, M., Horan, M.F., Puchtel, I.S., Boyet, M., Rosing, M.T. (2016) Early Earth differentiation investigated through 142Nd, 182W, and highly siderophile element abundances in samples from Isua, Greenland. Geochimica et Cosmochimica Acta 175, 319–336.
) and recently reported Ru isotope data (Fischer-Gödde et al., 2020Fischer-Gödde, M., Elfers, B.-M., Münker, C., Szilas, K., Maier, W.D., Messling, N., Morishita, T., Van Kranendonk, M., Smithies, H. (2020) Ruthenium isotope vestige of Earth’s pre-late-veneer mantle preserved in Archaean rocks. Nature 579, 240–244.
), contributions from both missing late veneer and early silicate differentiation to the 182W excesses now appear likely. For future 182W isotope studies, the combination with stable W isotope analyses might represent an additional tool to assess the pristine character of W signatures.top
Acknowledgements
This study is part of the “Infant Earth” project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 669666). We thank Austin Boyd and Minik Rosing for providing sample material. Helen Williams is acknowledged for editorial handling. Constructive comments raised by two anonymous reviewers and Helen Williams helped to improve the quality of the article.
Editor: Helen Williams
top
References
Appel, P.U. (1986) Strata bound scheelite in the Archean Malene supracrustal belt, West Greenland. Mineralium Deposita 21, 207–215.
Show in contextAs such W-rich minerals are reported in the IGC (Appel, 1986), we speculate that the local remobilisation of W by metasomatic and hydrothermal fluids (and possibly seawater) as well as the formation of secondary W-rich mineral phases caused mass dependent stable W isotope fractionation in different directions due to localised changes in coordination.
View in article
This region is of particular interest, because of widespread metasomatic alteration (Rosing et al., 1996) and a previously reported large scale mobility of W (Rizo et al., 2016; Tusch et al., 2019) that even led to ore grade enrichment of W at a local scale (Appel, 1986).
View in article
Bédard, J.H. (2006) A catalytic delamination-driven model for coupled genesis of Archaean crust and sub-continental lithospheric mantle. Geochimica et Cosmochimica Acta 70, 1188–1214.
Show in contextAlternative models suggest vertical stagnant lid tectonics, where thickened and dense eclogitic crustal restites sink in sagduction regimes (e.g., Bédard, 2006).
View in article
Dale, C.W., Kruijer, T.S., Burton K.W. (2017) Highly siderophile element and 182W evidence for a partial late veneer in the source of 3.8 Ga rocks from Isua, Greenland. Earth and Planetary Science Letters 458, 394–404.
Show in contextThis excess was explained by either a missing late veneer component that was depleted in 182W (Willbold et al., 2011; Dale et al., 2017), or by early silicate differentiation during the lifetime of short lived 182Hf and the respective formation of 182W-enriched and 182W-depleted silicate reservoirs, respectively (Rizo et al., 2016; Tusch et al., 2019).
View in article
Thus, 182W excesses of around +13 ppm in rocks of the IGC (Willbold et al., 2011; Rizo et al., 2016; Dale et al., 2017; Tusch et al., 2019) represent no analytical artefacts caused by the application of inappropriate mass fractionation laws.
View in article
From a W perspective, this would permit combination with other geochemical parameters such as 142Nd isotope or HSE signatures (e.g., Rizo et al., 2016; Dale et al., 2017; Saji et al., 2018).
View in article
Fischer-Gödde, M., Elfers, B.-M., Münker, C., Szilas, K., Maier, W.D., Messling, N., Morishita, T., Van Kranendonk, M., Smithies, H. (2020) Ruthenium isotope vestige of Earth’s pre-late-veneer mantle preserved in Archaean rocks. Nature 579, 240–244.
Show in contextIn the light of coupled 182W-142Nd anomalies (Rizo et al., 2016) and recently reported Ru isotope data (Fischer-Gödde et al., 2020), contributions from both missing late veneer and early silicate differentiation to the 182W excesses now appear likely.
View in article
Fonseca, R.O.C., Mallmann, G., Sprung, P., Sommer, J.E., Heuser, A., Speelmanns, I.M., Blanchard, H. (2014) Redox controls on tungsten and uranium crystal/silicate melt partitioning and implications for the U/W and Th/W ratio of the lunar mantle. Earth and Planetary Science Letters 404, 1–13.
Show in contextStable W isotope fractionation caused by redox changes in terrestrial igneous environments is not expected (Kurzweil et al., 2019), because the oxidised W6+ species is predominant in these settings (Fonseca et al., 2014).
View in article
Hart, S.R., Zindler, A. (1989) Isotope fractionation laws. International Journal of Mass Spectrometry and Ion Processes 89, 287–301.
Show in contextThe application of inappropriate mass fractionation laws may therefore theoretically create apparent mass independent isotope effects in µ182W (Hart and Zindler, 1989; SI).
View in article
Hoffmann, J.E., Nagel, T.J., Muenker, C., Næraa, T., Rosing, M.T. (2014) Constraining the process of Eoarchean TTG formation in the Itsaq Gneiss Complex, southern West Greenland. Earth and Planetary Science Letters 388, 374–386.
Show in contextThe peridotites of this study represent relict mantle rocks (van de Löcht et al., 2018), and the TTGs likely formed by partial melting of hydrated mafic crust (e.g., Hoffmann et al., 2014; SI).
View in article
König, S., Münker, C., Hohl, S., Paulick, H., Barth, A.R., Lagos, M., Pfänder, J., Büchl, A. (2011) The Earth’s tungsten budget during mantle melting and crust formation. Geochimica et Cosmochimica Acta 75, 2119–2136.
Show in contextHowever, the incompatible and fluid mobile element W shows no co-variation with any other element (Fig. S-1), indicating that its budget was severely affected during metamorphism, metasomatism and hydrothermalism. Extreme W concentrations in peridotites up to 4380 ng/g (more than 100 fold compared to 12 ng/g of the modern mantle; König et al., 2011) and W/Th (0.02–143) that are, with the exception of sample 10–12A, highly elevated compared to the canonical value defined by modern igneous rocks and clearly indicate secondary enrichment of W (Rizo et al., 2016; Tusch et al., 2019).
View in article
Korenaga, J. (2003) Energetics of mantle convection and the fate of fossil heat. Geophysical Research Letters 30.
Show in contextHigher potential mantle temperatures but lower heat fluxes (Korenaga, 2003) affected most relevant geological processes such as partial mantle melting with higher melt proportions, the mode of fractional crystallisation, and global tectonics (e.g., de Wit et al., 1992; Van Kranendonk, 2010).
View in article
Kurzweil, F., Münker, C., Grupp, M., Braukmüller, N., Fechtner, L., Christian, M., Hohl, S.V., Schoenberg, R. (2019) The stable tungsten isotope composition of modern igneous reservoirs. Geochimica et Cosmochimica Acta 251, 176–191.
Show in contextStable W isotopes represent an emerging geochemical tool that might help to reconstruct the geologic and tectonic history of ancient rocks as well as the cycling of W through different geochemical reservoirs (Kurzweil et al., 2019; Mazza et al., 2020).
View in article
(Fig. 1; Kurzweil et al., 2019; Mazza et al., 2020).
View in article
Tholeiitic metabasalts show similarly variable W concentrations between 117 and 1835 ng/g (W/Th from 0.2 to 2.3) and an even larger range in δ186/184W (−0.072 to +0.136 ‰), clearly outside the range predicted for igneous processes (Kurzweil et al., 2019; Mazza et al., 2020).
View in article
This secondary disturbance of W probably increased the variation in δ186/184W beyond the range that is expected for unaltered igneous rocks (Kurzweil et al., 2019; Mazza et al., 2020).
View in article
Stable W isotope fractionation caused by redox changes in terrestrial igneous environments is not expected (Kurzweil et al., 2019), because the oxidised W6+ species is predominant in these settings (Fonseca et al., 2014).
View in article
Kuzmin, A., Purans, J. (2001) Local atomic and electronic structure of tungsten ions in AWO4 crystals of scheelite and wolframite types. Radiation measurements 33, 583–586.
Show in contextMoreover, octahedral coordination is preferred in hexagonally close packed mineral structures (e.g., wolframite), whereas the complex remains in tetrahedral coordination in cubic closed packed minerals (e.g,. scheelite; Kuzmin and Purans, 2001).
View in article
Liu, J., Pearson, D.G., Chacko, T., Luo, Y. (2018) A reconnaissance view of tungsten reservoirs in some crustal and mantle rocks: Implications for interpreting W isotopic compositions and crust-mantle W cycling. Geochimica et Cosmochimica Acta 223, 300–318.
Show in contextPrimary hosts for secondary W enrichments by metasomatic fluids are minor mineral phases such as serpentine, Ti-rich mineral phases, phlogopite and sulfide or grain boundary assemblages (Liu et al., 2018).
View in article
Mazza, S.E., Stracke, A., Gill, J.B., Kimura, J.-I., Kleine, T. (2020) Tracing dehydration and melting of the subducted slab with tungsten isotopes in arc lavas. Earth and Planetary Science Letters 530, 115942.
Show in contextIn contrast, isotopically lighter compositions were observed in back-arc lavas that formed above deeper subduction zone settings, more distant to the trench (Mazza et al., 2020).
View in article
However, during metasomatic alteration the preferential leaching of isotopically heavy W, which is mainly abundant as a tetrahedrally coordinated anion [WO4]2− in fluids, is expected, similar as in modern fluid-dominated subduction zone settings (Mazza et al., 2020).
View in article
Stable W isotopes represent an emerging geochemical tool that might help to reconstruct the geologic and tectonic history of ancient rocks as well as the cycling of W through different geochemical reservoirs (Kurzweil et al., 2019; Mazza et al., 2020).
View in article
(Fig. 1; Kurzweil et al., 2019; Mazza et al., 2020).
View in article
Tholeiitic metabasalts show similarly variable W concentrations between 117 and 1835 ng/g (W/Th from 0.2 to 2.3) and an even larger range in δ186/184W (−0.072 to +0.136 ‰), clearly outside the range predicted for igneous processes (Kurzweil et al., 2019; Mazza et al., 2020).
View in article
This secondary disturbance of W probably increased the variation in δ186/184W beyond the range that is expected for unaltered igneous rocks (Kurzweil et al., 2019; Mazza et al., 2020).
View in article
Nutman, A.P., McGregor, V.R., Friend, C.R.L., Bennett, V.C., Kinny, P.D. (1996) The Itsaq gneiss complex of southern West Greenland; the world’s most extensive record of early crustal evolution (3900-3600 Ma). Precambrian Research 78, 1–39.
Show in contextIn this study, we present stable W isotope data for some of the Earth’s oldest rocks from the Itsaq Gneiss Complex, SW Greenland (IGC, 3620 to >3850 Ma; e.g., Nutman et al., 1996) including peridotites, tholeiitic metabasalts, boninite-like metabasalts, non-gneissic tonalite-trondhjemite-granodiorites (TTGs) and metasediments, with the aim to reconstruct the sources and processes that controlled the inventory of W in Eoarchean time.
View in article
For example, it remains controversial if modern style plate tectonics operated on a global or local scale or were even absent in Hadean and Eoarchean times (e.g., de Wit et al., 1992; Nutman et al., 1996; Van Kranendonk, 2010).
View in article
These differentiation trends are even preserved for some fluid mobile elements (Ba, Pb), which is noteworthy, considering the complex geologic history of igneous rocks from the IGC that experienced several high grade metamorphic, hydrothermal and metasomatic events (Nutman et al., 1996; Rosing et al., 1996).
View in article
Polat, A., Hofmann, A.W. (2003) Alteration and geochemical patterns in the 3.7–3.8 Ga Isua greenstone belt, West Greenland. Precambrian Research 126, 197–218.
Show in contextThe major and trace element composition of tholeiitic and boninite-like metabasalts from the IGC is mainly controlled by fractional crystallisation of olivine and pyroxene (Polat and Hofmann, 2003), whereby the boninite-like melts originate from an already depleted, more harzburgitic mantle source.
View in article
Rizo, H., Walker, R.J., Carlson, R.W., Touboul, M., Horan, M.F., Puchtel, I.S., Boyet, M., Rosing, M.T. (2016) Early Earth differentiation investigated through 142Nd, 182W, and highly siderophile element abundances in samples from Isua, Greenland. Geochimica et Cosmochimica Acta 175, 319–336.
Show in contextConsistent with a previous study (Rizo et al., 2016), we thus suggest that W was strongly mobilised by metasomatic fluids.
View in article
Assuming the equilibrium mass fractionation law during natural stable W isotope fractionation, however, the observed range in δ186/184W can maximally account for 182W excesses that are smaller than 2 ppm in µ182W (Fig. S-2, Table S-3) a similar range that was also previously predicted by stable W isotope data of lower precision (Rizo et al., 2016).
View in article
This region is of particular interest, because of widespread metasomatic alteration (Rosing et al., 1996) and a previously reported large scale mobility of W (Rizo et al., 2016; Tusch et al., 2019) that even led to ore grade enrichment of W at a local scale (Appel, 1986).
View in article
However, the incompatible and fluid mobile element W shows no co-variation with any other element (Fig. S-1), indicating that its budget was severely affected during metamorphism, metasomatism and hydrothermalism. Extreme W concentrations in peridotites up to 4380 ng/g (more than 100 fold compared to 12 ng/g of the modern mantle; König et al., 2011) and W/Th (0.02–143) that are, with the exception of sample 10–12A, highly elevated compared to the canonical value defined by modern igneous rocks and clearly indicate secondary enrichment of W (Rizo et al., 2016; Tusch et al., 2019).
View in article
This excess was explained by either a missing late veneer component that was depleted in 182W (Willbold et al., 2011; Dale et al., 2017), or by early silicate differentiation during the lifetime of short lived 182Hf and the respective formation of 182W-enriched and 182W-depleted silicate reservoirs, respectively (Rizo et al., 2016; Tusch et al., 2019).
View in article
From a W perspective, this would permit combination with other geochemical parameters such as 142Nd isotope or HSE signatures (e.g., Rizo et al., 2016; Dale et al., 2017; Saji et al., 2018).
View in article
In the light of coupled 182W-142Nd anomalies (Rizo et al., 2016) and recently reported Ru isotope data (Fischer-Gödde et al., 2020), contributions from both missing late veneer and early silicate differentiation to the 182W excesses now appear likely.
View in article
Rosing, M.T., Rose, N.M., Bridgwater, D., Thomsen, H.S. (1996) Earliest part of Earth’s stratigraphic record. Geology 24, 43–46.
Show in contextThe chemical composition of metasomatic fluids in the Isua region was shown to be variable on local to regional scales depending on the ambient rock assemblage (Rosing et al., 1996).
View in article
This region is of particular interest, because of widespread metasomatic alteration (Rosing et al., 1996) and a previously reported large scale mobility of W (Rizo et al., 2016; Tusch et al., 2019) that even led to ore grade enrichment of W at a local scale (Appel, 1986).
View in article
These differentiation trends are even preserved for some fluid mobile elements (Ba, Pb), which is noteworthy, considering the complex geologic history of igneous rocks from the IGC that experienced several high grade metamorphic, hydrothermal and metasomatic events (Nutman et al., 1996; Rosing et al., 1996).
View in article
Importantly, W-rich peridotites in SW Greenland were metasomatised by chemically different fluids (H2-rich vs. CO2-rich; Rosing et al., 1996) as also indicated by crosscutting fine serpentine veins (SOISB1 and NUB peridotites) and carbonate veins (sample 10–34), respectively (van de Löcht et al., 2020).
View in article
Saji, N.S., Larsen, K., Wielandt, D., Schiller, M., Costa, M.M., Whitehouse, M.J., Rosing, M.T., Bizzarro, M. (2018) Hadean geodynamics inferred from time-varying 142Nd/144Nd in the early Earth rock record. Geochemical Perspectives Letters 7, 43–48.
Show in contextFrom a W perspective, this would permit combination with other geochemical parameters such as 142Nd isotope or HSE signatures (e.g., Rizo et al., 2016; Dale et al., 2017; Saji et al., 2018).
View in article
Tusch, J., Sprung, P., van de Löcht, J., Hoffmann, J.E., Boyd, A.J., Rosing, M.T., Münker, C. (2019) Uniform 182W isotope compositions in Eoarchean rocks from the Isua region, SW Greenland. Geochimica et Cosmochimica Acta 257, 284–310.
Show in contextWe combine the stable W isotope data with previously published major and trace element concentration data as well as with µ182W data obtained for the same set of samples (Tusch et al., 2019) to place constraints on the geodynamic setting of the magmatic rocks, their relationship among each other and possible hydrothermal and metasomatic overprint.
View in article
The close genetic relationship of the rock suites analysed here might explain the observed tight co-variations between various compatible and incompatible elements (Table S-2, Fig. S-1; Tusch et al., 2019).
View in article
Despite variable δ186/184W, the excess of 182W is homogeneously distributed in igneous rocks from the IGC (Tusch et al., 2019).
View in article
Despite variable secondary enrichment of W and heterogeneous δ186/184W beyond the range observed in fresh igneous rocks, the excess of 182W is homogeneously distributed in igneous rocks from the IGC (Tusch et al., 2019).
View in article
This region is of particular interest, because of widespread metasomatic alteration (Rosing et al., 1996) and a previously reported large scale mobility of W (Rizo et al., 2016; Tusch et al., 2019) that even led to ore grade enrichment of W at a local scale (Appel, 1986).
View in article
However, the incompatible and fluid mobile element W shows no co-variation with any other element (Fig. S-1), indicating that its budget was severely affected during metamorphism, metasomatism and hydrothermalism. Extreme W concentrations in peridotites up to 4380 ng/g (more than 100 fold compared to 12 ng/g of the modern mantle; König et al., 2011) and W/Th (0.02–143) that are, with the exception of sample 10–12A, highly elevated compared to the canonical value defined by modern igneous rocks and clearly indicate secondary enrichment of W (Rizo et al., 2016; Tusch et al., 2019).
View in article
This excess was explained by either a missing late veneer component that was depleted in 182W (Willbold et al., 2011; Dale et al., 2017), or by early silicate differentiation during the lifetime of short lived 182Hf and the respective formation of 182W-enriched and 182W-depleted silicate reservoirs, respectively (Rizo et al., 2016; Tusch et al., 2019).
View in article
Thus, 182W excesses of around +13 ppm in rocks of the IGC (Willbold et al., 2011; Rizo et al., 2016; Dale et al., 2017; Tusch et al., 2019) represent no analytical artefacts caused by the application of inappropriate mass fractionation laws.
View in article
van de Löcht, J., Hoffmann, J.E., Li, C., Wang, Z., Becker, H., Rosing, M.T., Kleinschrodt, R., Münker, C. (2018) Earth’s oldest mantle peridotites show entire record of late accretion. Geology 46, 199–202.
Show in contextThe peridotites of this study represent relict mantle rocks (van de Löcht et al., 2018), and the TTGs likely formed by partial melting of hydrated mafic crust (e.g., Hoffmann et al., 2014; SI).
View in article
Indeed, fine serpentine veinlets as well as small metasomatic sulfides have been reported for most peridotites investigated here (van de Löcht et al., 2018, 2020).
View in article
van de Löcht, J., Hoffmann, J.E., Rosing, M.T., Sprung, P., Münker, C. (2020) Preservation of Eoarchean mantle processes in ∼3.8 Ga peridotite enclaves in the Itsaq Gneiss Complex, southern West Greenland. Geochimica et Cosmochimica Acta 280,1–25.
Show in contextThe only peridotite with sub-canonical W/Th was also hydrated most strongly (10–12A; van de Löcht et al., 2020) and shows the lowest δ186/184W of all peridotites.
View in article
Indeed, fine serpentine veinlets as well as small metasomatic sulfides have been reported for most peridotites investigated here (van de Löcht et al., 2018, 2020).
View in article
Importantly, W-rich peridotites in SW Greenland were metasomatised by chemically different fluids (H2-rich vs. CO2-rich; Rosing et al., 1996) as also indicated by crosscutting fine serpentine veins (SOISB1 and NUB peridotites) and carbonate veins (sample 10–34), respectively (van de Löcht et al., 2020).
View in article
Van Kranendonk, M.J. (2010) Two types of Archean continental crust. American Journal of Science 310, 1187–1209.
Show in contextHigher potential mantle temperatures but lower heat fluxes (Korenaga, 2003) affected most relevant geological processes such as partial mantle melting with higher melt proportions, the mode of fractional crystallisation, and global tectonics (e.g., de Wit et al., 1992; Van Kranendonk, 2010).
View in article
For example, it remains controversial if modern style plate tectonics operated on a global or local scale or were even absent in Hadean and Eoarchean times (e.g., de Wit et al., 1992; Nutman et al., 1996; Van Kranendonk, 2010).
View in article
Willbold, M., Elliott, T., Moorbath, S. (2011) The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment. Nature 477, 195.
Show in contextMoreover, the IGC is one of the first localities on Earth, where significant 182W excesses were reported (e.g., Willbold et al., 2011; the excess of 182W is described by µ182W, which is defined in the SI), and stable W isotopes may aid to elucidate the significance of these isotope anomalies.
View in article
This excess was explained by either a missing late veneer component that was depleted in 182W (Willbold et al., 2011; Dale et al., 2017), or by early silicate differentiation during the lifetime of short lived 182Hf and the respective formation of 182W-enriched and 182W-depleted silicate reservoirs, respectively (Rizo et al., 2016; Tusch et al., 2019).
View in article
Thus, 182W excesses of around +13 ppm in rocks of the IGC (Willbold et al., 2011; Rizo et al., 2016; Dale et al., 2017; Tusch et al., 2019) represent no analytical artefacts caused by the application of inappropriate mass fractionation laws.
View in article
de Wit, M.J., de Ronde, C.E.J., Tredoux, M., Roering, C., Hart, R.J., Armstrong, R.A., Green, R.W.E., Peberdy, E., Hart, R.A. (1992) Formation of an Archaean continent. Nature 357, 553.
Show in contextHigher potential mantle temperatures but lower heat fluxes (Korenaga, 2003) affected most relevant geological processes such as partial mantle melting with higher melt proportions, the mode of fractional crystallisation, and global tectonics (e.g., de Wit et al., 1992; Van Kranendonk, 2010).
View in article
For example, it remains controversial if modern style plate tectonics operated on a global or local scale or were even absent in Hadean and Eoarchean times (e.g., de Wit et al., 1992; Nutman et al., 1996; Van Kranendonk, 2010).
View in article
Wood, S.A., Samson, I.M. (2000) The hydrothermal geochemistry of tungsten in granitoid environments. Economic Geology 95, 143–182.
Show in contextThis local heterogeneity probably also accounts for heterogeneous W concentrations and stable W isotope compositions of the ambient fluids and for the decoupling of W that is typically dissolved as an oxyanion, which can be complexed with cationic ligands such as K+ or H+, from other nominally fluid mobile elements such as Ba or Pb that are transported as metal cations, which are complexed with anionic ligands such as Cl− (Wood and Samson, 2000).
View in article
top
Supplementary Information
The Supplementary Information includes:
- Geological Setting and Sample Material
- Methods, Analytical Precision and Accuracy
- The Influence of Different Mass Fractionation Laws on μ182W
- Figures S-1 and S-2
- Tables S-1 to S-3
- Supplementary Information References
Download Tables S-1 to S-3 (Excel).
Download the Supplementary Information (PDF).
Figures
