Mantle heterogeneity through Zn systematics in oceanic basalts: Evidence for a deep carbon cycling

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Abstract

Subduction at convergent margins introduces a range of sedimentary and crustal materials into the mantle, providing the most dominant form of heterogeneity in the source of oceanic basalts. Yet, the relationship between geochemical variability and lithologic heterogeneities in the Earth's mantle remains controversial. In this paper, we comprehensively review Zn, δ66Zn and Sr-Nd isotope systematics in near-primary basalts erupted at mid-ocean ridges (MORB) and ocean islands (OIB) to help constrain the nature and proportion of the carbon (C) bearing slab-derived component in their mantle sources. We show that Zn elemental and isotopic composition of oceanic basalts differs according to their tectonic settings, increasing from MORB (Zn = 62 ± 10 to 73 ± 11 ppm; δ66Zn = +0.24 ± 0.01 to +0.31 ± 0.02‰) to OIB (Zn = 74 ± 9 to 124 ± 7 ppm; δ66Zn = +0.21 ± 0.07 to +0.40 ± 0.04‰). Unlike MORB, the high Zn and δ66Zn recorded in OIB cannot be explained by partial melting of a fertile peridotite mantle source only. Importantly, global correlations between Zn content and Sr-Nd isotopes in oceanic basalts suggest that the Zn enrichment in OIB is inherited from a recycled component in their mantle source rather than melting processes. We demonstrate that involvement of neither typical MORB-like oceanic crust nor subducted sediments can achieve the whole range of Zn composition in OIB. Instead, addition of ≤6% C-bearing oceanic crust to a fertile peridotite mantle fully resolves the Zn heterogeneity of OIB, both in terms of magnitude of Zn enrichment and global trends with Sr-Nd isotopes. Such scenario is corroborated by the elevated δ66Zn of OIB relative to MORB and mantle peridotites, reflecting the contribution of isotopically heavy C-bearing phases (δ66Zn = +0.91 ± 0.24‰) to the mantle source (δ66Zn = +0.16 ± 0.06‰). Our study thus emphasizes the use of Zn and δ66Zn systematics to track the nature and origin of mantle carbon, highlighting the role of subduction in the deep carbon cycle. Finally, the positive correlation between Zn content and temperature of magma generation of oceanic basalts suggests that hotter mantle plumes are more likely to carry a higher proportion of dense C-bearing eclogite. Zinc systematics therefore may provide evidence that the presence of heterogeneous domains in the source of OIB is, at least partly, linked to plume thermal buoyancy, bringing new insights into mantle dynamics.

Introduction

Geophysical and geochemical heterogeneities have long been recognized within the Earth's mantle. The latter was initially inferred from trace elements, and long-lived radiogenic isotopes (e.g., Sr, Nd, Hf, Pb) variations in basalts erupted at mid-ocean ridges (MORB) and ocean islands (OIB). Their geochemical signature reflects the compositional heterogeneity of the mantle and its evolution through time (e.g., Hofmann, 1997; Willbold and Stracke, 2006; Zindler and Hart, 1986). It is widely accepted that these heterogeneities involve ancient and deeply subducted sediments, oceanic crust and underlying lithosphere introduced in the mantle at convergent margins. Eclogites (i.e., olivine-free, clinopyroxene- and garnet-bearing rocks) derived from the recycled oceanic crust and sediments are then entrained and stirred by convection, providing one of the most dominant form of heterogeneity in the mantle source of oceanic basalts. However, the specific nature and proportion of the slab-derived eclogitic component in the mantle is still a matter of speculation (see Anderson, 2006 and references therein). Subducted eclogites may derive from a typical MORB oceanic crust (e.g., silica-excess and volatile-free, hereafter referred as MORB-eclogite) that can be carbonatized during seafloor hydrothermal alteration (e.g., silica-deficient and carbon-bearing, hereafter referred as C-bearing eclogites; Nakamura and Kato, 2004; Kitajima et al., 2001). However, little is known about the relationship between geochemical variability and lithologic heterogeneities in the Earth's mantle. While incompatible trace element variations highlight distinct geochemical imprints between MORB and OIB, they do not provide information about the exact nature and extent of mineralogical variability in the source regions of oceanic basalts. Yet, characterization of such lithologic heterogeneities is of prime importance for constraining the physical properties of the mantle (e.g., thermal state, viscosity and density), understanding the dynamics of the Earth (e.g., differentiation and melting processes) and bridging geochemical and geophysical observations. Several studies have tried to tackle the question of mineralogical heterogeneities in the mantle focusing on major oxide (Dasgupta et al., 2010; Jackson and Dasgupta, 2008; Sobolev et al., 2005, Sobolev et al., 2007; Prytulak and Elliott, 2007), moderately incompatible first-row transition element (FRTEs, e.g., Zn/Fe, Fe/Mn, Ni; Le Roux et al., 2010, Le Roux et al., 2011, Le Roux et al., 2015; Humayun, 2004; Sobolev et al., 2007) and non-traditional stable isotope systematics (Williams and Bizimis, 2014; Pringle et al., 2016; Krienitz et al., 2012) in mantle rocks and mantle-related melts. Among the FRTEs, Zn has been subject to a growing interest for tracing eclogite fragments in the mantle through Zn/Fe ratio systematics in oceanic basalts (Le Roux et al., 2010, Le Roux et al., 2011, Le Roux et al., 2015). In parallel, recent studies have also shown the great potential of some metal stable isotopes, including Zn, as sensitive tracers of the Deep Carbon Cycle (DCC) since carbonates and Bulk Silicate Earth display very distinct isotopic compositions (δ66Zncarbonates = +0.91 ± 0.24‰ and δ66ZnBSE = +0.16 ± 0.06‰ using the per mille deviation of 66Zn/64Zn from the JMC-Lyon standard, Liu et al., 2016; Pichat et al., 2003; Sossi et al., 2018; Liu and Li, 2019; Debret et al., 2018a). The concomitant use of Zn and δ66Zn offers some interesting possibilities to distinguish between MORB-eclogite and C-bearing eclogite contribution in the source of oceanic basalts.

In this paper, we present a comprehensive review of Zn, δ66Zn and Sr-Nd isotope systematics in near-primary oceanic basalts worldwide and reassess the use of Zn as a tracer of eclogite-derived melts. We show that Zn abundances correlate with Sr-Nd isotopes on a single ridge and ocean island group basis. This result, together with the extreme Zn enrichment in OIB cannot be readily explained by melting and mixing of fertile peridotite and MORB-eclogite as previously suggested. Instead, our results point towards the involvement of Zn-rich C-bearing eclogites. On the basis of a compilation of Zn isotopic data from previous studies and new unpublished data from the Crozet archipelago, we argue that such scenario is corroborated by the heavy δ66Zn of OIB relative to MORB and mantle peridotites. Thus, Zn systematics may provide a valuable tool to fingerprint recycling of C-bearing subducted materials and deep carbon cycling in the Earth's mantle.

Section snippets

Data selection and filtering

We compiled zinc abundances, major oxide concentrations and radiogenic Sr-Nd isotopic composition of oceanic basalts from the GEOROC (http://georoc.mpch-mainz.gwdg.de/georoc) and PetDB (www.earthcem.org/petdb) databases. Basalts with LOI > 3 wt% and sum of oxides <97 wt% or > 102 wt% calculated on a dry basis were systematically excluded from the filtered database. Major element contents of the remaining samples were then normalized to 100 wt% on a dry-weight basis with all Fe reported as FeOT.

Zinc behavior during igneous differentiation and subduction processes

A primary implicit assumption for the use of Zn and Zn isotopes as tracers of recycled material and deep carbon cycling in the mantle is that they suffer little or no fractionation during igneous and subduction processes. The composition of oceanic basalts, however, may vary considerably during magmatic differentiation and no longer truly reflect the signature of their mantle sources (e.g., Teng et al., 2008; Williams et al., 2009 for Fe; Savage et al., 2011 for Si). Fractionation of

Conclusions

Zinc elemental and isotopic composition of oceanic basalts differs according to their tectonic settings, increasing from ridges to ocean islands. Unlike MORB, the high Zn and δ66Zn recorded in OIB cannot be explained by partial melting of a fertile peridotite mantle source only. Importantly, global correlations between Zn content and Sr-Nd isotopes in oceanic basalts suggest that the Zn enrichment in OIB is inherited from a recycled component in their mantle source rather than melting processes.

Declaration of competing interest

None.

Acknowledgements

This research was supported by the F.N.R.S (Fond National de la Recherche Scientifique, Belgium)1141117F to HB. HB acknowledges his F.R.S.-F.N.R.S. research fellowship (Aspirant). The authors wish to thank J. De Jong (ULB) for technical support during MC-ICP-MS analyses. We are grateful to the thorough and detailed reviews from Veronique Le Roux and Paolo Sossi and from the editor, Arturo Gomez-Tuena. We also thank Cin-Ty Lee for helpful discussion and critical comments on earlier version of

References (113)

  • B. Debret et al.

    Ore component mobility, transport and mineralization at mid-oceanic ridges: a stable isotopes (Zn, Cu and Fe) study of the Rainbow massif (Mid-Atlantic Ridge 36°14′N)

    Earth Planet. Sci. Lett.

    (2018)
  • D.J. DePaolo

    Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization

    Earth Planet. Sci. Lett.

    (1981)
  • L.S. Doucet et al.

    Zn isotopic heterogeneity in the mantle: a melting control?

    Earth Planet. Sci. Lett.

    (2016)
  • M. Ducher et al.

    Equilibrium zinc isotope fractionation in Zn-bearing minerals from first-principles calculations

    Chem. Geol.

    (2016)
  • A. Ewart et al.

    Application of proton-microprobe data to trace-element partitioning in volcanic rocks

    Chem. Geol.

    (1994)
  • T. Fujii et al.

    The origin of Zn isotope fractionation in sulfides

    Geochim. Cosmochim. Acta

    (2011)
  • T. Fujii et al.

    Density functional theory estimation of isotope fractionation of Fe, Ni, Cu, and Zn among species relevant to geochemical and biological environments

    Geochim. Cosmochim. Acta

    (2014)
  • D. Grassi et al.

    Element partitioning during carbonated pelite melting at 8, 13 and 22GPa and the sediment signature in the EM mantle components

    Earth Planet. Sci. Lett.

    (2012)
  • T. Hammouda

    High-pressure melting of carbonated eclogite and experimental constraints on carbon recycling and storage in the mantle

    Earth Planet. Sci. Lett.

    (2003)
  • A.W. Hofmann et al.

    Mantle plumes from ancient oceanic crust

    Earth Planet. Sci. Lett.

    (1982)
  • J. Huang et al.

    Zinc isotopic systematics of Kamchatka-Aleutian arc magmas controlled by mantle melting

    Geochim. Cosmochim. Acta

    (2018)
  • M.G. Jackson et al.

    Compositions of HIMU, EM1, and EM2 from global trends between radiogenic isotopes and major elements in ocean island basalts

    Earth Planet. Sci. Lett.

    (2008)
  • S.D. King et al.

    Hotspot swells revisited

    Phys. Earth Planet. Inter.

    (2014)
  • V. Le Roux et al.

    Zn/Fe systematics in mafic and ultramafic systems: implications for detecting major element heterogeneities in the Earth’s mantle

    Geochim. Cosmochim. Acta

    (2010)
  • V. Le Roux et al.

    Mineralogical heterogeneities in the Earth’s mantle: constraints from Mn, Co, Ni and Zn partitioning during partial melting

    Earth Planet. Sci. Lett.

    (2011)
  • C.-T.A. Lee et al.

    Constraints on the depths and temperatures of basaltic magma generation on Earth and other terrestrial planets using new thermobarometers for mafic magmas

    Earth Planet. Sci. Lett.

    (2009)
  • S.-A. Liu et al.

    Tracing the deep carbon cycle using metal stable isotopes: opportunities and challenges

    Engineering

    (2019)
  • S.-A. Liu et al.

    Zinc isotope evidence for a large-scale carbonated mantle beneath eastern China

    Earth Planet. Sci. Lett.

    (2016)
  • A. Mallik et al.

    Reaction between MORB-eclogite derived melts and fertile peridotite and generation of ocean island basalts

    Earth Planet. Sci. Lett.

    (2012)
  • C.N. Maréchal et al.

    Precise analysis of copper and zinc isotopic compositions by plasma-source mass spectrometry

    Chem. Geol.

    (1999)
  • K. Nakamura et al.

    Carbonatization of oceanic crust by the seafloor hydrothermal activity and its significance as a CO2 sink in the Early Archean

    Geochim. Cosmochim. Acta

    (2004)
  • H.St.C. O’Neill et al.

    The effect of melt composition on trace element partitioning: an experimental investigation of the activity coefficients of FeO, NiO, CoO, MoO2 and MoO3 in silicate melts

    Chem. Geol.

    (2002)
  • H. Palme et al.

    Cosmochemical estimates of mantle composition

  • C. Patten et al.

    Partition coefficients of chalcophile elements between sulfide and silicate melts and the early crystallization history of sulfide liquid: LA-ICP-MS analysis of MORB sulfide droplets

    Chem. Geol.

    (2013)
  • S. Pichat et al.

    Zinc isotope variations in deep-sea carbonates from the eastern equatorial Pacific over the last 175 ka

    Earth Planet. Sci. Lett.

    (2003)
  • T. Plank

    The chemical composition of subducting sediments

  • E.A. Pringle et al.

    Silicon isotopes reveal recycled altered oceanic crust in the mantle sources of Ocean Island Basalts

    Geochim. Cosmochim. Acta

    (2016)
  • J. Prytulak et al.

    TiO2 enrichment in ocean island basalts

    Earth Planet. Sci. Lett.

    (2007)
  • P.S. Savage et al.

    Silicon isotope fractionation during magmatic differentiation

    Geochim. Cosmochim. Acta

    (2011)
  • D.M. Shaw

    Trace element fractionation during anatexis

    Geochim. Cosmochim. Acta

    (1970)
  • K. Shimizu et al.

    Two-component mantle melting-mixing model for the generation of mid-ocean ridge basalts: implications for the volatile content of the Pacific upper mantle

    Geochim. Cosmochim. Acta

    (2016)
  • P.A. Sossi et al.

    Zinc isotope composition of the Earth and its behaviour during planetary accretion

    Chem. Geol.

    (2018)
  • H. Staudigel et al.

    Large scale isotopic Sr, Nd and O isotopic anatomy of altered oceanic crust: DSDP/ODP sites417/418

    Earth Planet. Sci. Lett.

    (1995)
  • R.J. Sweeney et al.

    Selected trace and minor element partitioning between peridotite minerals and carbonatite melts at 18–46 kb pressure

    Geochim. Cosmochim. Acta

    (1995)
  • G.F. Davies

    Mantle Convection for Geologists

    (2011)
  • B. Debret et al.

    Isotopic evidence for iron mobility during subduction

    Geology

    (2016)
  • B. Debret et al.

    Carbonate transfer during the onset of slab devolatilization: new insights from Fe and Zn stable isotopes

    J. Petrol.

    (2018)
  • B.R. Doe

    Zinc, copper, and lead geochemistry of oceanic igneous rocks—ridges, islands, and arcs

    Int. Geol. Rev.

    (1995)
  • M.S. Duncan et al.

    Rise of Earth’s atmospheric oxygen controlled by efficient subduction of organic carbon

    Nat. Geosci.

    (2017)
  • J. Eguchi et al.

    Great oxidation and Lomagundi events linked by deep cycling and enhanced degassing of carbon

    Nat. Geosci.

    (2020)
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