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

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, δ 66 Zn 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; δ 66 Zn = +0.24 ± 0.01 to +0.31 ± 0.02‰) to OIB (Zn = 74 ± 9 to 124 ± 7 ppm; δ 66 Zn = +0.21 ± 0.07 to +0.40 ± 0.04‰). Unlike MORB, the high Zn and δ 66 Zn 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 δ 66 Zn of OIB relative to MORB and mantle peridotites, reflecting the contribution of isotopically heavy C-bearing phases (δ 66 Zn = +0.91 ± 0.24‰) to the mantle source (δ 66 Zn = +0.16 ± 0.06‰). Our study thus emphasizes the use of Zn and δ 66 Zn 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.

Mg and Zn Isotope Evidence for Two Types of Mantle Metasomatism and Deep Recycling of Magnesium Carbonates

“…Zinc isotopes may be fractionated during crystal‐melt differentiation involving olivines that are isotopically lighter than the melts (McCoy‐West et al, 2018; Sossi et al, 2015; Yang & Liu, 2019), but detectable fractionation (~0.1‰) only occurs at the very end of the differentiation sequence when MgO falls below 3 wt.% as observed in the Kilauea Iki lavas (Chen et al, 2013). The studied alkali basalts have near‐primary compositions (MgO = 8.7–12.2 wt.%) and have δ 66 Zn values above the trend of Kilauea Iki lavas (Chen et al, 2013) as well as of global OIBs (Beunon et al, 2020; Wang et al, 2017) (Figure 7a). There is also no visible correlation of δ 66 Zn or δ 26 Mg with indices of magmatic differentiation (e.g., Mg # and CaO, Figure 6; SiO 2 , not shown), which is consistent with a negligible influence of magmatic differentiation on Mg and Zn isotopic systematics of the alkali basalts.…”

“…(a) Relationship between δ 66 Zn and Zn concentrations. Data for OIBs are from Chen et al (2013), Wang et al (2017), and Beunon et al (2020). (b) Relationship between δ 26 Mg and δ 66 Zn.…”

“…Thereupon, lighter Mg and heavier Zn isotopic compositions of basaltic melts relative to the mantle values may indicate the incorporation of carbonate into the mantle via subduction (Li et al, 2017; Liu et al, 2016). For example, significant higher‐than‐mantle Zn isotopic compositions of basaltic rocks, such as late Mesozoic to Cenozoic intraplate basalts in Eastern China (Liu et al, 2016; Wang et al, 2018) and global ocean island basalts (Beunon et al, 2020), have been explained as a result of recycled carbonate‐bearing components in their mantle sources. By contrast, melts derived from a mantle source hybridized by recycled siliciclastic sediments should have heavier Mg and possibly lighter Zn isotopic compositions.…”

Zinc Isotopes of the Mariana and Ryukyu Arc‐Related Lavas Reveal Recycling of Forearc Serpentinites Into the Subarc Mantle

“…Marine carbonates have heavier Zn isotope compositions than the normal mantle (Figure 1). Accordingly, the high δ 66 Zn values of the East China basalts and OIB (Figure 4a) are ascribed to the involvement of recycled/subducted isotopically heavy carbonate materials in mantle sources (Beunon et al, 2020;Jin et al, 2020;Liu et al, 2016;. In fact, previous studies, based on DSDP Sites 294, 295, and ODP Site 801 near the Ryukyu and Mariana trenches, have suggested that marine sediments are dominated by abyssal pelagic clays rather than carbonates (Plank, 2014;Plank & Langmuir, 1998).…”

“…In contrast, mantle melting has only a limited effect on the Zn isotope compositions of residual peridotites (Sossi et al, 2018;. Therefore, Zn isotope compositions of basaltic magma are insensitive to melt extraction, as indicated by the MORB-like δ 66 Zn values of Kamchatka-Aleutian arc basalts and could be used to identify the incorporation of recycled crustal components in their mantle sources via subduction only if Δ 66 Zn subduted component-mantle (i.e., δ 66 Zn subduted component -δ 66 Zn mantle ) is >0.08‰ (Beunon et al, 2020). For instance, sedimentary carbonates (0.91 ± 0.24‰; Liu et al, 2017;Lv et al, 2018;Pichat et al, 2003;Sweere et al, 2018;X.…”