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