Zinc isotopic evidence for recycled carbonate in the deep mantle

Abstract

Carbonate, the major carbon reservoir on Earth’s surface, can enter into the mantle by subduction. However, evidence for recycled surficial carbonates in the deep mantle is still scarce. Ocean island basalts from Cook-Austral islands and St. Helena Island, widely called HIMU basalts because of their high μ = 238U/204Pb sources, are thought to be fed by mantle plumes originating in the lower mantle. Here we report exceptionally high δ66Zn values (δ66Zn = 0.38 ± 0.03‰) of these HIMU lavas relative to most published data for oceanic basalts (δ66Zn = 0.31 ± 0.10‰), which requires a source contributed by isotopically heavy recycled surficial carbonates. During subduction of the oceanic lithosphere, melting of mixed surficial carbonates and basaltic crust in the deep mantle generates carbonatite melts, which metasomatizes the nearby mantle and the resultant carbonated mantle ultimately evolves into a high-δ66Zn HIMU source. High-δ66Zn signatures of HIMU basalts, therefore, demonstrate that carbonates can be transported into Earth’s deep mantle.

Effects of Spin Transition and Cation Substitution on the Optical Properties and Iron Partitioning in Carbonate Minerals

“…Carbonated slabs could have penetrated the boundary between the mantle transition zone and the uppermost lower mantle, even reaching the base of the lower mantle (Zhang et al, 2022). During subduction, iron-bearing carbonate minerals not only undergo a pressure-induced spin-pairing transition, but also exchange cations like Fe 2+ , Mg 2+ , and Ca 2+ with surrounding mantle minerals.…”

“…The above factors might dictate how iron and other metal elements (e.g., Ca, Mg, and Zn) exchange between carbonate and coexisting mantle minerals. Such an exchange might have contributed to iron, zinc, calcium, and magnesium isotopic signatures observed in basalts (Li et al, 2017;Liu et al, 2017;Zhang et al, 2022), and this demands further work.…”

High‐precision zinc isotope measurement of chemically different geostandards by multicollector inductively coupled plasma mass spectrometry

“…For example, given the non‐negligible fractionation during igneous processes and melt/fluid‐peridotite interaction, Zn isotopes (expressed as δ 66 Zn) are an effective index of partial melting and magmatic differentiation as well as mantle metasomatism 1–6 . Zinc isotopes have also been proposed as a tool for tracing deep carbonate recycling, owing to the distinct isotopic difference between sedimentary carbonates and the terrestrial mantle 7–11 . Recent studies found that high‐silica (SiO 2 > 70 wt%) granites commonly have Zn isotope compositions shifted toward heavier values compared to the global basalt average, 6,12–14 making Zn isotopes a valuable tracer for continental crust formation and differentiation.…”

“…For example, global oceanic basalts are isotopically heavier than the terrestrial mantle by ~0.04–0.14‰, 1–3,22,23 which is only marginally beyond the currently available analytical uncertainties. The Zn isotope ratios of some oceanic island basalts (OIB), explained as a result of recycled carbon‐bearing crustal materials in their sources, are higher than those of mid‐ocean ridge basalts (MORB) by only ~0.1‰ 10,11,24 . Recently, the tiny (~0.1‰) variation of δ 66 Zn among MORB samples has been attributed to isotope fractionation during basaltic magma differentiation 25 .…”

“…[1][2][3][4][5][6] Zinc isotopes have also been proposed as a tool for tracing deep carbonate recycling, owing to the distinct isotopic difference between sedimentary carbonates and the terrestrial mantle. [7][8][9][10][11] Recent studies found that high-silica (SiO 2 > 70 wt%) granites commonly have Zn isotope compositions shifted toward heavier values compared to the global basalt average, 6,[12][13][14] making Zn isotopes a valuable tracer for continental crust formation and differentiation. Lunar mare basalts are characterized by heavier Zn isotope composition relative to terrestrial basalts, [15][16][17] suggesting that Zn isotopes are a powerful tool for deciphering volatilization-related events during planet formation and evolution.…”