Molybdenum isotopes in plume-influenced MORBs reveal recycling of ancient anoxic sediments
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Keywords: molybdenum isotopes, subduction zones, MORB, sediments, mantle plume, redox
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Abstract
Figures and Tables
 Figure 1 Covariation diagram of MORBs. (a) δ98/95Mo vs. 87Sr/86Sr; (b) δ98/95Mo vs. 143Nd/144Nd. Mixing of a 1.5 Ga old pelagic sediment end member (best fit parameters from linear regression) with the ambient depleted mantle (see Table S-2 for mixing parameters). Mixing with upper continental crust (UCC) is plotted for comparison. External reproducibility on each isotope value is considered for regression, and the shaded area indicates 95 % CI error envelope. PAR samples are excluded from regression. Error bars indicate 2 s.d. external reproducibility. |  Figure 2 Least squares error (Δ98/95Mo in ‰; up to 6 ‰) between calculated mixing lines and analysed samples. Variables are the δ98/95Mo and [Mo] values derived from (a) the δ98/95Mo vs. 87Sr/86Sr and (b) the δ98/95Mo vs. 143Nd/144Nd relationship (Fig. 1). Literature values for potential (concentration averaged) recycled lithologies, and anoxic sediments sorted by age intervals are shown for comparison. See Table S-2 and Supplementary Information S-5 for references, mixing parameters, and further details. |  Figure 3 Illustration of the subducted sedimentary Mo cycle during the Precambrian (left) and Phanerozoic (right). Deep sea sediments carry variable redox budgets influencing Mo mobility and hence isotope fractionation during subduction over Earth’s history (see text). UCC, upper continental crust; DM, depleted mantle; EM, enriched mantle. |  Table 1 Average Mo isotope composition of MORBs from the S-MAR and PAR together with radiogenic isotope data (Douglass et al., 1999) and Se isotope compositions (Yierpan et al., 2020). Individual measurements are listed in Table S-1.A The external reproducibility (2 s.d.) is 0.05 ‰ (see Supplementary Information S-4).B The external reproducibility (2 s.d.) is 0.08 ‰, except for EW9309 2D-1g and EW9309 9D-3g, for which it is 0.04 ‰ (Yierpan et al., 2020). |
| Figure 1 | Figure 2 | Figure 3 | Table 1 |
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Introduction
The quantification of element and isotope fractionation in subduction zones plays an important role in understanding (re)cycling mechanisms between the Earth’s surface and its interior over geologic time scales. Element and isotope fractionation in this setting can be controlled by the prevailing redox conditions of the subducted material. As aerobic conditions prevail at the Earth’s surface today, deep sea sediments and altered oceanic crust (AOC) are largely oxidised and influence the redox budget of the subducting slab (e.g., Evans, 2012
Evans, K.A. (2012) The redox budget of subduction zones. Earth-Science Reviews 113, 11–32. https://doi.org/10.1016/j.earscirev.2012.03.003
). It is still unclear whether the redox budget of slabs is directly correlated with the oxygenation of Earth’s atmosphere and oceans, and whether the redox state of the Earth’s surface influences the behaviour of redox-sensitive elements in subduction zone settings through Earth’s history.A particularly suitable element to study the relationship between oxidised and reduced geochemical reservoirs is the redox-sensitive element Mo. The mobility of Mo from the subducted material is controlled by the redox state of slab-derived aqueous fluids and hydrous melts (Bali et al., 2012
Bali, E., Keppler, H., Audetat, A. (2012) The mobility of W and Mo in subduction zone fluids and the Mo–W–Th–U systematics of island arc magmas. Earth and Planetary Science Letters 351–352, 195–207. https://doi.org/10.1016/j.epsl.2012.07.032
; Skora et al., 2017Skora, S., Freymuth, H., Blundy, J., Elliott, T., Guillong, M. (2017) An experimental study of the behaviour of cerium/molybdenum ratios during subduction: Implications for tracing the slab component in the Lesser Antilles and Mariana Arc. Geochimica et Cosmochimica Acta 212, 133–155. https://doi.org/10.1016/j.gca.2017.05.025
). Significant Mo mobilisation and isotope fractionation has been observed during subduction of oxidised lithologies, leaving behind a Mo depleted and isotopically light residual slab (e.g., Freymuth et al., 2015Freymuth, H., Vils, F., Willbold, M., Taylor, R.N., Elliott, T. (2015) Molybdenum mobility and isotopic fractionation during subduction at the Mariana arc. Earth and Planetary Science Letters 432, 176–186. https://doi.org/10.1016/j.epsl.2015.10.006
; König et al., 2016König, S., Wille, M., Voegelin, A., Schoenberg, R. (2016) Molybdenum isotope systematics in subduction zones. Earth and Planetary Science Letters 447, 95–102. https://doi.org/10.1016/j.epsl.2016.04.033
; Chen et al., 2019Chen, S., Hin, R.C., John, T., Brooker, R., Bryan, B., Niu, Y., Elliott, T. (2019) Molybdenum systematics of subducted crust record reactive fluid flow from underlying slab serpentine dehydration. Nature Communications 10, 4773. https://doi.org/10.1038/s41467-019-12696-3
; Ahmad et al., 2021Ahmad, Q., Wille, M., König, S., Rosca, C., Hensel, A., Pettke, T., Hermann, J. (2021) The Molybdenum isotope subduction recycling conundrum: A case study from the Tongan subduction zone, Western Alps and Alpine Corsica. Chemical Geology 576, 120231. https://doi.org/10.1016/j.chemgeo.2021.120231
). Under reducing conditions, only limited Mo mobility is expected to occur in subducted lithologies (Bali et al., 2012Bali, E., Keppler, H., Audetat, A. (2012) The mobility of W and Mo in subduction zone fluids and the Mo–W–Th–U systematics of island arc magmas. Earth and Planetary Science Letters 351–352, 195–207. https://doi.org/10.1016/j.epsl.2012.07.032
; Skora et al., 2017Skora, S., Freymuth, H., Blundy, J., Elliott, T., Guillong, M. (2017) An experimental study of the behaviour of cerium/molybdenum ratios during subduction: Implications for tracing the slab component in the Lesser Antilles and Mariana Arc. Geochimica et Cosmochimica Acta 212, 133–155. https://doi.org/10.1016/j.gca.2017.05.025
) thus preserving pre-subducted Mo signatures of the surface. This redox dependent mobilisation of Mo during subduction metamorphism potentially allows reconstructing the redox budget of ancient subduction zones through Mo isotope compositions of mantle-derived material enriched by ancient recycled crustal components.This study investigates Mo isotope systematics in a well characterised basaltic sample suite from the South Mid-Atlantic ridge (S-MAR) that shows evidence for interaction with the enriched Shona and Discovery mantle plumes (Supplementary Information S-1, Fig. S-1). Previous studies on these samples suggested recycling of ancient oceanic crust and sediments in their mantle source with an age between 1 and 2 Ga based on radiogenic isotopes, and the absence of mass independent fractionation of S isotopes (Douglass et al., 1999
Douglass, J., Schilling, J.-G., Fontignie, D. (1999) Plume-ridge interactions of the Discovery and Shona mantle plumes with the southern Mid-Atlantic Ridge (40°-55°S). Journal of Geophysical Research: Solid Earth 104, 2941–2962. https://doi.org/10.1029/98JB02642
; Andres et al., 2002Andres, M., Blichert-Toft, J., Schilling, J.-G. (2002) Hafnium isotopes in basalts from the southern Mid-Atlantic Ridge from 40°S to 55°S: Discovery and Shona plume–ridge interactions and the role of recycled sediments. Geochemistry, Geophysics, Geosystems 3, 1–25. https://doi.org/10.1029/2002GC000324
; Labidi et al., 2013Labidi, J., Cartigny, P., Moreira, M. (2013) Non-chondritic sulphur isotope composition of the terrestrial mantle. Nature 501, 208–211. https://doi.org/10.1038/nature12490
). These basalts additionally feature increasingly heavier S and Se isotope compositions with indicators of mantle source enrichment, and are interpreted to reflect subduction recycling of reduced sediments from a redox stratified Proterozoic ocean (Labidi et al., 2013Labidi, J., Cartigny, P., Moreira, M. (2013) Non-chondritic sulphur isotope composition of the terrestrial mantle. Nature 501, 208–211. https://doi.org/10.1038/nature12490
; Yierpan et al., 2020Yierpan, A., König, S., Labidi, J., Schoenberg, R. (2020) Recycled selenium in hot spot–influenced lavas records ocean-atmosphere oxygenation. Science Advances 6, abb6179. https://doi.org/10.1126/sciadv.abb6179
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Results
The δ98/95Mo signatures of S-MAR basaltic glass samples range between −0.24 ‰ and −0.10 ‰. These δ98/95Mo values show correlations with 87Sr/86Sr as well as 143Nd/144Nd, 176Hf/177Hf, and δ82/76Se (Figs. 1, S-4a,b). The correlations indicate that Mo isotopes trace mantle source enrichment. The range in δ98/95Mo of the samples is similar to that of MORBs (and seamounts) from the East Pacific Rise (EPR), the Pacific-Antarctic ridge (PAR), and the Mohns Knipovich ridge (MKR; Figs. 1, S-2). However, the samples from the S-MAR show a much larger variability in 87Sr/86Sr and 143Nd/144Nd and more systematic source enrichments (Figs. 1, S-2, S-4a,b).
Figure 1 Covariation diagram of MORBs. (a) δ98/95Mo vs. 87Sr/86Sr; (b) δ98/95Mo vs. 143Nd/144Nd. Mixing of a 1.5 Ga old pelagic sediment end member (best fit parameters from linear regression) with the ambient depleted mantle (see Table S-2 for mixing parameters). Mixing with upper continental crust (UCC) is plotted for comparison. External reproducibility on each isotope value is considered for regression, and the shaded area indicates 95 % CI error envelope. PAR samples are excluded from regression. Error bars indicate 2 s.d. external reproducibility.
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Discussion
Origin of enriched mantle component. Combined element and isotope systematics of S-MAR samples strongly suggest that secondary mantle melting processes (such as sulfide melt segregation and fractional crystallisation), or seawater alteration, are unlikely the causes for the observed Mo isotope variations. This is because all these processes would have obliterated any correlation between Mo isotope composition and radiogenic isotopes (see Supplementary Information S-2 for further discussion). Low degree melting of mantle plume material has been suggested to affect geochemical signatures of investigated samples (le Roux et al., 2002a
le Roux, P., le Roex, A., Schilling, J.G. (2002a) MORB melting processes beneath the southern Mid-Atlantic Ridge (40–55°S): a role for mantle plume-derived pyroxenite. Contributions to Mineralogy and Petrology 144, 206–229. https://doi.org/10.1007/s00410-002-0376-3
), and might have affected their δ98/95Mo (e.g., Chen et al., 2022Chen, S., Sun, P., Niu, Y., Guo, P., Elliott, T., Hin, R.C. (2022) Molybdenum isotope systematics of lavas from the East Pacific Rise: Constraints on the source of enriched mid-ocean ridge basalt. Earth and Planetary Science Letters 578, 117283. https://doi.org/10.1016/j.epsl.2021.117283
). We argue that this process did not result in a first order modification of the enriched source signatures of our samples. Their distinct radiogenic isotope compositions, their covariation with stable Se and S signatures, and trace element systematics are independent of partial melting variations, suggesting the plume source was enriched prior to low degree mantle melting (le Roux et al., 2002ale Roux, P., le Roex, A., Schilling, J.G. (2002a) MORB melting processes beneath the southern Mid-Atlantic Ridge (40–55°S): a role for mantle plume-derived pyroxenite. Contributions to Mineralogy and Petrology 144, 206–229. https://doi.org/10.1007/s00410-002-0376-3
; Labidi et al., 2013Labidi, J., Cartigny, P., Moreira, M. (2013) Non-chondritic sulphur isotope composition of the terrestrial mantle. Nature 501, 208–211. https://doi.org/10.1038/nature12490
; Yierpan et al., 2020Yierpan, A., König, S., Labidi, J., Schoenberg, R. (2020) Recycled selenium in hot spot–influenced lavas records ocean-atmosphere oxygenation. Science Advances 6, abb6179. https://doi.org/10.1126/sciadv.abb6179
). Moreover, the absence of a covariation of δ98/95Mo and Pb isotopes (Fig. S-4c,d) does not support the influence of an ancient low degree melt (Chen et al., 2022Chen, S., Sun, P., Niu, Y., Guo, P., Elliott, T., Hin, R.C. (2022) Molybdenum isotope systematics of lavas from the East Pacific Rise: Constraints on the source of enriched mid-ocean ridge basalt. Earth and Planetary Science Letters 578, 117283. https://doi.org/10.1016/j.epsl.2021.117283
). Rather, a recycled sediment component can explain the Pb isotope signature of LOMU-affinity samples (cf. Douglass et al., 1999Douglass, J., Schilling, J.-G., Fontignie, D. (1999) Plume-ridge interactions of the Discovery and Shona mantle plumes with the southern Mid-Atlantic Ridge (40°-55°S). Journal of Geophysical Research: Solid Earth 104, 2941–2962. https://doi.org/10.1029/98JB02642
; Andres et al., 2002Andres, M., Blichert-Toft, J., Schilling, J.-G. (2002) Hafnium isotopes in basalts from the southern Mid-Atlantic Ridge from 40°S to 55°S: Discovery and Shona plume–ridge interactions and the role of recycled sediments. Geochemistry, Geophysics, Geosystems 3, 1–25. https://doi.org/10.1029/2002GC000324
). In addition, mantle melting degrees of 2.5 % (le Roux et al., 2002ale Roux, P., le Roex, A., Schilling, J.G. (2002a) MORB melting processes beneath the southern Mid-Atlantic Ridge (40–55°S): a role for mantle plume-derived pyroxenite. Contributions to Mineralogy and Petrology 144, 206–229. https://doi.org/10.1007/s00410-002-0376-3
) cannot explain the observed variations of δ98/95Mo indicating that heavy Mo is likely a characteristic primary signature of the enriched plume material (see Supplementary Information S-2 for further details). Previous studies attributed this source enrichment to the influence of a recycled component, such as delaminated subcontinental lithospheric mantle, lower continental crust, or subducted sediment (±AOC) (Douglass et al., 1999Douglass, J., Schilling, J.-G., Fontignie, D. (1999) Plume-ridge interactions of the Discovery and Shona mantle plumes with the southern Mid-Atlantic Ridge (40°-55°S). Journal of Geophysical Research: Solid Earth 104, 2941–2962. https://doi.org/10.1029/98JB02642
; Andres et al., 2002Andres, M., Blichert-Toft, J., Schilling, J.-G. (2002) Hafnium isotopes in basalts from the southern Mid-Atlantic Ridge from 40°S to 55°S: Discovery and Shona plume–ridge interactions and the role of recycled sediments. Geochemistry, Geophysics, Geosystems 3, 1–25. https://doi.org/10.1029/2002GC000324
; le Roux et al., 2002ble Roux, P.J., le Roex, A.P., Schilling, J.G., Shimizu, N., Perkins, W.W., Pearce, N.J.G. (2002b) Mantle heterogeneity beneath the southern Mid-Atlantic Ridge: trace element evidence for contamination of ambient asthenospheric mantle. Earth and Planetary Science Letters 203, 479–498. https://doi.org/10.1016/S0012-821X(02)00832-4
). In line with previous investigations showing covariations between other redox-sensitive stable Se-S isotope systematics and radiogenic isotopes (Labidi et al., 2013Labidi, J., Cartigny, P., Moreira, M. (2013) Non-chondritic sulphur isotope composition of the terrestrial mantle. Nature 501, 208–211. https://doi.org/10.1038/nature12490
; Yierpan et al., 2020Yierpan, A., König, S., Labidi, J., Schoenberg, R. (2020) Recycled selenium in hot spot–influenced lavas records ocean-atmosphere oxygenation. Science Advances 6, abb6179. https://doi.org/10.1126/sciadv.abb6179
), we suggest that a sediment contribution to the S-MAR mantle source is the most likely scenario for the observed heavy Mo isotope enrichment in our samples (see Supplementary Information S-3 for further discussion). This sedimentary source was previously inferred to have a mid-Proterozoic age (1 to 2 Ga; Douglass et al., 1999Douglass, J., Schilling, J.-G., Fontignie, D. (1999) Plume-ridge interactions of the Discovery and Shona mantle plumes with the southern Mid-Atlantic Ridge (40°-55°S). Journal of Geophysical Research: Solid Earth 104, 2941–2962. https://doi.org/10.1029/98JB02642
; Andres et al., 2002Andres, M., Blichert-Toft, J., Schilling, J.-G. (2002) Hafnium isotopes in basalts from the southern Mid-Atlantic Ridge from 40°S to 55°S: Discovery and Shona plume–ridge interactions and the role of recycled sediments. Geochemistry, Geophysics, Geosystems 3, 1–25. https://doi.org/10.1029/2002GC000324
; Labidi et al., 2013Labidi, J., Cartigny, P., Moreira, M. (2013) Non-chondritic sulphur isotope composition of the terrestrial mantle. Nature 501, 208–211. https://doi.org/10.1038/nature12490
; Yierpan et al., 2020Yierpan, A., König, S., Labidi, J., Schoenberg, R. (2020) Recycled selenium in hot spot–influenced lavas records ocean-atmosphere oxygenation. Science Advances 6, abb6179. https://doi.org/10.1126/sciadv.abb6179
).Mobility of Mo during subduction: the role of fO2. Recycled Proterozoic sediments that enriched the S-MAR mantle source with isotopically heavy Mo are in stark contrast to inferred, isotopically light Mo in dehydration residues of subducted Phanerozoic metasediments (Ahmad et al., 2021
Ahmad, Q., Wille, M., König, S., Rosca, C., Hensel, A., Pettke, T., Hermann, J. (2021) The Molybdenum isotope subduction recycling conundrum: A case study from the Tongan subduction zone, Western Alps and Alpine Corsica. Chemical Geology 576, 120231. https://doi.org/10.1016/j.chemgeo.2021.120231
). The trend towards higher observed (co-) variations of δ98/95Mo with increasing degrees of mantle source enrichment within the S-MAR suite therefore implies recycling of a sedimentary δ98/95Mo or even a total Mo budget unaffected by dehydration and melting during subduction. This may be reconciled with experimental studies showing immobility of Mo in low fO2-bearing slab fluids and hydrous slab melts (Bali et al., 2012Bali, E., Keppler, H., Audetat, A. (2012) The mobility of W and Mo in subduction zone fluids and the Mo–W–Th–U systematics of island arc magmas. Earth and Planetary Science Letters 351–352, 195–207. https://doi.org/10.1016/j.epsl.2012.07.032
; Skora et al., 2017Skora, S., Freymuth, H., Blundy, J., Elliott, T., Guillong, M. (2017) An experimental study of the behaviour of cerium/molybdenum ratios during subduction: Implications for tracing the slab component in the Lesser Antilles and Mariana Arc. Geochimica et Cosmochimica Acta 212, 133–155. https://doi.org/10.1016/j.gca.2017.05.025
; Chowdhury et al., 2022Chowdhury, P., Dasgupta, R., Phelps, P.R., Costin, G., Lee, C.-T. A. (2022) Oxygen fugacity range of subducting crust inferred from fractionation of trace elements during fluid-present slab melting in the presence of anhydrite versus sulfide. Geochimica et Cosmochimica Acta 325, 214–231. https://doi.org/10.1016/j.gca.2022.02.030
). These experiments were conducted at subduction zone P-T and at reducing conditions, and showed that Mo4+ is immobile in fluids in the presence of rutile (Bali et al., 2012Bali, E., Keppler, H., Audetat, A. (2012) The mobility of W and Mo in subduction zone fluids and the Mo–W–Th–U systematics of island arc magmas. Earth and Planetary Science Letters 351–352, 195–207. https://doi.org/10.1016/j.epsl.2012.07.032
) and mobilisation of Mo is inefficient in melts due to increased partitioning of Mo4+ relative to Mo6+ into the residual phases such as sulfides or rutile (Skora et al., 2017Skora, S., Freymuth, H., Blundy, J., Elliott, T., Guillong, M. (2017) An experimental study of the behaviour of cerium/molybdenum ratios during subduction: Implications for tracing the slab component in the Lesser Antilles and Mariana Arc. Geochimica et Cosmochimica Acta 212, 133–155. https://doi.org/10.1016/j.gca.2017.05.025
; Chowdhury et al., 2022Chowdhury, P., Dasgupta, R., Phelps, P.R., Costin, G., Lee, C.-T. A. (2022) Oxygen fugacity range of subducting crust inferred from fractionation of trace elements during fluid-present slab melting in the presence of anhydrite versus sulfide. Geochimica et Cosmochimica Acta 325, 214–231. https://doi.org/10.1016/j.gca.2022.02.030
). These findings are also consistent with subduction of organic matter (OM)-rich black shales in the Lesser Antilles Arc, where lavas south of Martinique exhibit high δ98/95Mo along with lower Mo/Ce (Freymuth et al., 2016Freymuth, H., Elliott, T., van Soest, M., Skora, S. (2016) Tracing subducted black shales in the Lesser Antilles arc using molybdenum isotope ratios. Geology 44, 987–990. https://doi.org/10.1130/G38344.1
; Gaschnig et al., 2017Gaschnig, R.M., Reinhard, C.T., Planavsky, N.J., Wang, X., Asael, D., Chauvel, C. (2017) The Molybdenum Isotope System as a Tracer of Slab Input in Subduction Zones: An Example From Martinique, Lesser Antilles Arc. Geochemistry, Geophysics, Geosystems 18, 4674–4689. https://doi.org/10.1002/2017GC007085
), suggesting the minute contribution of unfractionated slab-derived Mo to the mantle sources relative to melts originating from more oxidising sediments. Therefore, significant loss of heavy Mo during subduction metamorphism did not occur in the enriched mantle source component. This would have resulted in a preferential loss of more incompatible (isotopically heavy) Mo6+ during melting and dehydration (e.g., Chen et al., 2019Chen, S., Hin, R.C., John, T., Brooker, R., Bryan, B., Niu, Y., Elliott, T. (2019) Molybdenum systematics of subducted crust record reactive fluid flow from underlying slab serpentine dehydration. Nature Communications 10, 4773. https://doi.org/10.1038/s41467-019-12696-3
; McCoy-West et al., 2019McCoy-West, A.J., Chowdhury, P., Burton, K.W., Sossi, P., Nowell, G.M., Fitton, J.G., Kerr, A.C., Cawood, P.A., Williams, H.M. (2019) Extensive crustal extraction in Earth’s early history inferred from molybdenum isotopes. Nature Geoscience 12, 946–951. https://doi.org/10.1038/s41561-019-0451-2
) and would shift δ98/95Mo of the residual subducted material towards lighter values. The interpretation is in line with the fO2-sensitive stable isotope systematics of S and Se (Table 1), which indicate negligible mobilisation and isotope fractionation during subduction (Labidi et al., 2013Labidi, J., Cartigny, P., Moreira, M. (2013) Non-chondritic sulphur isotope composition of the terrestrial mantle. Nature 501, 208–211. https://doi.org/10.1038/nature12490
; Yierpan et al., 2020Yierpan, A., König, S., Labidi, J., Schoenberg, R. (2020) Recycled selenium in hot spot–influenced lavas records ocean-atmosphere oxygenation. Science Advances 6, abb6179. https://doi.org/10.1126/sciadv.abb6179
). It is noteworthy that in some cases, that do not apply here, sediments may also buffer subduction zone fluids towards oxidising conditions, such as Fe- and Mn-rich (meta)sediments that show a high metamorphic fO2 (Ague et al., 2022Ague, J.J., Tassara, S., Holycross, M.E., Li, J.-L., Cottrell, E., Schwarzenbach, E.M., Fassoulas, C., John, T. (2022) Slab-derived devolatilization fluids oxidized by subducted metasedimentary rocks. Nature Geoscience 15, 320–326. https://doi.org/10.1038/s41561-022-00904-7
and references therein). Yet, our S-MAR data can be attributed to an immobile behaviour of Mo with unchanged δ98/95Mo under reducing conditions, which is in sharp contrast with the mobility of Mo under oxidising conditions where prograde subduction metamorphism can cause Mo mobilisation and alter the primary slab Mo isotope signature.Table 1 Average Mo isotope composition of MORBs from the S-MAR and PAR together with radiogenic isotope data (Douglass et al., 1999
Douglass, J., Schilling, J.-G., Fontignie, D. (1999) Plume-ridge interactions of the Discovery and Shona mantle plumes with the southern Mid-Atlantic Ridge (40°-55°S). Journal of Geophysical Research: Solid Earth 104, 2941–2962. https://doi.org/10.1029/98JB02642
) and Se isotope compositions (Yierpan et al., 2020Yierpan, A., König, S., Labidi, J., Schoenberg, R. (2020) Recycled selenium in hot spot–influenced lavas records ocean-atmosphere oxygenation. Science Advances 6, abb6179. https://doi.org/10.1126/sciadv.abb6179
). Individual measurements are listed in Table S-1.| Sample | Type | δ98/95Mo (‰)A | n | δ82/76Se (‰)B | 87Sr/86Sr | 143Nd/144Nd |
| Southern Mid-Atlantic ridge | ||||||
| EW9309 40D-1g | Depleted N-MORB | −0.231 | 3 | −0.18 | 0.702997 | 0.513033 |
| EW9309 33D-1g | Discovery influenced MORB (North) | −0.098 | 3 | −0.03 | 0.704475 | 0.512726 |
| EW9309 28D-1g | Discovery influenced MORB (North) | −0.239 | 2 | −0.14 | 0.703028 | 0.513077 |
| EW9309 2D-1g | Discovery influenced MORB (South) | −0.148 | 2 | −0.08 | 0.704127 | 0.512652 |
| EW9309 4D-3g | Discovery influenced MORB (South) | −0.152 | 3 | −0.04 | 0.703762 | 0.512732 |
| EW9309 9D-3g | LOMU MORB | −0.183 | 2 | −0.03 | 0.704284 | 0.512873 |
| EW9309 15D-1g | Shona influenced MORB | −0.207 | 2 | −0.13 | 0.702741 | 0.513008 |
| EW9309 21D-1g | Shona influenced MORB | −0.188 | 2 | −0.12 | 0.703115 | 0.512818 |
| EW9309 22D-3g | Shona influenced MORB | −0.187 | 2 | −0.08 | 0.703576 | 0.512893 |
| Pacific-Antarctic ridge | ||||||
| PAC2 DR3 3-1 | Depleted N-MORB | −0.245 | 3 | −0.15 | 0.702488 | 0.513082 |
| PAC1 CV-02g | Depleted N-MORB | −0.297 | 3 | −0.23 | 0.702568 | 0.513135 |
A The external reproducibility (2 s.d.) is 0.05 ‰ (see Supplementary Information S-4).
B The external reproducibility (2 s.d.) is 0.08 ‰, except for EW9309 2D-1g and EW9309 9D-3g, for which it is 0.04 ‰ (Yierpan et al., 2020Yierpan, A., König, S., Labidi, J., Schoenberg, R. (2020) Recycled selenium in hot spot–influenced lavas records ocean-atmosphere oxygenation. Science Advances 6, abb6179. https://doi.org/10.1126/sciadv.abb6179).).
Recycled sediments from an anoxic deep ocean. The δ98/95Mo-87Sr/86Sr-143Nd/144Nd covariations (Fig. 1) in the S-MAR data combined with the previously established model of the linear δ34S-δ82Se-87Sr/86Sr(-143Nd/144Nd) relationship (Labidi et al., 2013
Labidi, J., Cartigny, P., Moreira, M. (2013) Non-chondritic sulphur isotope composition of the terrestrial mantle. Nature 501, 208–211. https://doi.org/10.1038/nature12490
; Yierpan et al., 2020Yierpan, A., König, S., Labidi, J., Schoenberg, R. (2020) Recycled selenium in hot spot–influenced lavas records ocean-atmosphere oxygenation. Science Advances 6, abb6179. https://doi.org/10.1126/sciadv.abb6179
) allows extrapolation of δ98/95Mo and [Mo] to constrain the nature of the recycled sediment. Extrapolation of the linear regression to a model composition of 1.5 Ga old sediment (87Sr/86Sr = 0.7203, 143Nd/144Nd = 0.5117; Fig. 1, see Table S-2 for details) yields two vastly different end member Mo signatures with δ98/95Mo of 0.78 ± 0.20 ‰ and 0.12 ± 0.06 ‰, and [Mo] of ∼0.76 μg/g and 2.98 μg/g, respectively (see blue mixing lines in Fig. 1). This indicates that the linear extrapolation might not sufficiently constrain the Mo composition of the recycled sediment source. Considering the present day 87Sr/86Sr and 143Nd/144Nd model composition of 1.5 Ga old subducted sediment, the isotopic variations of the samples represents only a small fraction of the mixing space between the ambient depleted mantle and recycled sediment. Therefore, for given radiogenic Nd and Sr signatures of the sedimentary component, different δ98/95Mo and [Mo] combinations can potentially result in a minimum least squares error between the mixing model and the S-MAR data. As both variables cannot be independently constrained, a misfit function (Supplementary Information S-5) was applied to calculate the sedimentary δ98/95Mo and [Mo] that represent the measured δ98/95Mo-87Sr/86Sr-143Nd/144Nd covariation best (Fig. 2).Figure 2 Least squares error (Δ98/95Mo in ‰; up to 6 ‰) between calculated mixing lines and analysed samples. Variables are the δ98/95Mo and [Mo] values derived from (a) the δ98/95Mo vs. 87Sr/86Sr and (b) the δ98/95Mo vs. 143Nd/144Nd relationship (Fig. 1). Literature values for potential (concentration averaged) recycled lithologies, and anoxic sediments sorted by age intervals are shown for comparison. See Table S-2 and Supplementary Information S-5 for references, mixing parameters, and further details.
Least squares errors for variable δ98/95Mo and [Mo] values for a given 1.5 Ga old sediment with 143Nd/144Nd and 87Sr/86Sr end member values (blue field in Fig. 2) show a similar pattern for both radiogenic isotope systems, which indicates that a mid-Proterozoic pelagic sediment is likely to be a valid end member. Best fits with minimum errors overlap with the lower 1σ of Proterozoic OM-rich sediment data from the literature (Ye et al., 2021
Ye, Y., Zhang, S., Wang, H., Wang, X., Tan, C., Li, M., Wu, C., Canfield, D.E. (2021) Black shale Mo isotope record reveals dynamic ocean redox during the Mesoproterozoic Era. Geochemical Perspectives Letters 18, 16–21. https://doi.org/10.7185/geochemlet.2118
; Table S-2) and can also be achieved with a recycled sediment contribution close to that of the UCC (∼0.05 to 0.15 ‰; Willbold and Elliott, 2017Willbold, M., Elliott, T. (2017) Molybdenum isotope variations in magmatic rocks. Chemical Geology 449, 253–268. https://doi.org/10.1016/j.chemgeo.2016.12.011
and references therein) with slightly higher δ98/95Mo and/or [Mo] values. This points towards a minor authigenic Mo enrichment from seawater or a residual enrichment of Mo (e.g., Kendall et al., 2017Kendall, B., Dahl, T.W., Anbar, A.D. (2017) The stable isotope geochemistry of molybdenum. Reviews in Mineralogy and Geochemistry 82, 683–732. https://doi.org/10.2138/rmg.2017.82.16
) and implies that δ98/95Mo has not been affected by oxic conditions during slab dehydration due to subduction of reducing lithologies (Fig. 2).These findings support the notion that the deep ocean remained anoxic until the beginning of the Phanerozoic (e.g., Poulton and Canfield, 2011
Poulton, S.W., Canfield, D.E. (2011) Ferruginous Conditions: A Dominant Feature of the Ocean through Earth’s History. Elements 7, 107–112. https://doi.org/10.2113/GSELEMENTS.7.2.107
; Stolper and Keller, 2018Stolper, D.A., Keller, C.B. (2018) A record of deep-ocean dissolved O2 from the oxidation state of iron in submarine basalts. Nature 553, 323–327. https://doi.org/10.1038/nature25009
). The extent of the biological pump in the Proterozoic ocean, where primary productivity in the oxygenated surface ocean was dominated by cyanobacteria, was lower compared to modern oceans and higher primary surface productivity was restricted to marine environments close to continents (Laakso and Schrag, 2019Laakso, T.A., Schrag, D.P. (2019) A small marine biosphere in the Proterozoic. Geobiology 17, 161–171. https://doi.org/10.1111/gbi.12323
). This limited the OM flux to the Proterozoic deep ocean and therefore authigenic Mo accumulation from seawater. However, deep ocean anoxic conditions increased OM preservation and burial efficiency (Burdige, 2007Burdige, D.J. (2007) Preservation of Organic Matter in Marine Sediments: Controls, Mechanisms, and an Imbalance in Sediment Organic Carbon Budgets? Chemical Reviews 107, 467–485. https://doi.org/10.1021/cr050347q
). Furthermore, under anoxic conditions with an overall low concentration of dissolved SO42− and MoO42−, neither a significant Mo enrichment from seawater into the sediment nor a significant Mo mobilisation during fluid alteration is expected (Lyons et al., 2014Lyons, T.W., Reinhard, C.T., Planavsky, N.J. (2014) The rise of oxygen in Earth’s early ocean and atmosphere. Nature 506, 307–315. https://doi.org/10.1038/nature13068
). The overall lower OM input to the deep ocean and smaller oceanic Mo reservoir can therefore explain the small authigenic heavy Mo contribution to the recycled mid-Proterozoic deep sea sediment (Fig. 2). With respect to the much shorter ocean residence time of Mo relative to the average lifetime of an oceanic crust, the S-MAR enriched end member is likely representative for overall reducing conditions during subduction and an average subducted sedimentary Mo signature, which provides a context of deep ocean redox conditions.Implications for the sedimentary Mo subduction cycle. Due to the anoxic conditions in the Proterozoic deep ocean, oxidised species of major and minor elements like Fe, S and Mn, were absent in deep sea sediments, thus lowering their redox budget/oxidising capacity compared to present day marine lithologies (e.g., Evans, 2012
Evans, K.A. (2012) The redox budget of subduction zones. Earth-Science Reviews 113, 11–32. https://doi.org/10.1016/j.earscirev.2012.03.003
; Ague et al., 2022Ague, J.J., Tassara, S., Holycross, M.E., Li, J.-L., Cottrell, E., Schwarzenbach, E.M., Fassoulas, C., John, T. (2022) Slab-derived devolatilization fluids oxidized by subducted metasedimentary rocks. Nature Geoscience 15, 320–326. https://doi.org/10.1038/s41561-022-00904-7
). This may also explain the preserved δ98/95Mo of a reduced Proterozoic sediment component recycled into the S-MAR mantle source, in contrast to Neoproterozoic, deep mantle recycling of low δ98/95Mo into mantle plume sources (see also Ma et al., 2022Ma, L., Xu, Y.G., Li, J., Chen, L.-H., Liu, J.-Q., Li, H.-Y., Huang, X.-L., Ma, Q., Hong, L.-B., Wang, Y. (2022) Molybdenum isotopic constraints on the origin of EM1-type continental intraplate basalts. Geochimica et Cosmochimica Acta 317, 255–268. https://doi.org/10.1016/j.gca.2021.11.013
). This is in line with Samoan OIBs, where δ98/95Mo signatures are interpreted as a mixture of isotopically heavy terrigenous sediments (δ98/95Mo ≈ UCC) and isotopically light dehydrated mafic oceanic crust, which reflect the influence of a distinct pool of mid-Proterozoic recycled ocean crust (Gaschnig et al., 2021Gaschnig, R.M., Reinhard, C.T., Planavsky, N.J., Wang, X., Asael, D., Jackson, M.G. (2021) The impact of primary processes and secondary alteration on the stable isotope composition of ocean island basalts. Chemical Geology 581, 120416. https://doi.org/10.1016/j.chemgeo.2021.120416
). Altogether, this indicates that changing Earth surface redox conditions have influenced the fO2 conditions during subduction and the mobility of sedimentary Mo (and by analogy that of other redox sensitive elements) and hence, the Mo isotope budget between different Earth (silicate) reservoirs. This emphasises the time- and condition-related variations in Mo mobility during subduction on our planet (Fig. 3). We therefore conclude that the Mo isotope signature of plume-influenced volcanic rocks can be used to reconcile the redox conditions during ancient surface deposition of deep sea sediments (cf. Gaschnig et al., 2021Gaschnig, R.M., Reinhard, C.T., Planavsky, N.J., Wang, X., Asael, D., Jackson, M.G. (2021) The impact of primary processes and secondary alteration on the stable isotope composition of ocean island basalts. Chemical Geology 581, 120416. https://doi.org/10.1016/j.chemgeo.2021.120416
) as well as during subduction-related prograde metamorphism and the inception of modern subduction.Figure 3 Illustration of the subducted sedimentary Mo cycle during the Precambrian (left) and Phanerozoic (right). Deep sea sediments carry variable redox budgets influencing Mo mobility and hence isotope fractionation during subduction over Earth’s history (see text). UCC, upper continental crust; DM, depleted mantle; EM, enriched mantle.
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Acknowledgements
This work was funded by the Swiss National Science Foundation, Switzerland (Grant number 182508) to MW. The MC-ICP-MS at the Institute of Geological Sciences, University of Bern used in this study was acquired within the framework of the NCCR project PlanetS (Grant nr. 1NF40-141881) funded by the Swiss National Science Foundation. SK and CR acknowledge ERC Starting Grant 636808 (project O2RIGIN). SK also acknowledges Ramón y Cajal contract RYC2020-030014-I. Jörg Hermann and Paolo Sossi are acknowledged for discussions that contributed to this manuscript. We also wish to thank Alex McCoy-West and two anonymous reviewers for constructive reviews, as well as Helen Williams for editorial handling.
Editor: Helen Williams
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References
Ague, J.J., Tassara, S., Holycross, M.E., Li, J.-L., Cottrell, E., Schwarzenbach, E.M., Fassoulas, C., John, T. (2022) Slab-derived devolatilization fluids oxidized by subducted metasedimentary rocks. Nature Geoscience 15, 320–326. https://doi.org/10.1038/s41561-022-00904-7
Show in context It is noteworthy that in some cases, that do not apply here, sediments may also buffer subduction zone fluids towards oxidising conditions, such as Fe- and Mn-rich (meta)sediments that show a high metamorphic fO2 (Ague et al., 2022 and references therein)
View in article
Due to the anoxic conditions in the Proterozoic deep ocean, oxidised species of major and minor elements like Fe, S and Mn, were absent in deep sea sediments, thus lowering their redox budget/oxidising capacity compared to present day marine lithologies (e.g., Evans, 2012; Ague et al., 2022)
View in article
Ahmad, Q., Wille, M., König, S., Rosca, C., Hensel, A., Pettke, T., Hermann, J. (2021) The Molybdenum isotope subduction recycling conundrum: A case study from the Tongan subduction zone, Western Alps and Alpine Corsica. Chemical Geology 576, 120231. https://doi.org/10.1016/j.chemgeo.2021.120231
Show in context Recycled Proterozoic sediments that enriched the S-MAR mantle source with isotopically heavy Mo are in stark contrast to inferred, isotopically light Mo in dehydration residues of subducted Phanerozoic metasediments (Ahmad et al., 2021)
View in article
Significant Mo mobilisation and isotope fractionation has been observed during subduction of oxidised lithologies, leaving behind a Mo depleted and isotopically light residual slab (e.g., Freymuth et al., 2015; König et al., 2016; Chen et al., 2019; Ahmad et al., 2021)
View in article
Andres, M., Blichert-Toft, J., Schilling, J.-G. (2002) Hafnium isotopes in basalts from the southern Mid-Atlantic Ridge from 40°S to 55°S: Discovery and Shona plume–ridge interactions and the role of recycled sediments. Geochemistry, Geophysics, Geosystems 3, 1–25. https://doi.org/10.1029/2002GC000324
Show in context Previous studies on these samples suggested recycling of ancient oceanic crust and sediments in their mantle source with an age between 1 and 2 Ga based on radiogenic isotopes, and the absence of mass independent fractionation of S isotopes (Douglass et al., 1999; Andres et al., 2002; Labidi et al., 2013)
View in article
Rather, a recycled sediment component can explain the Pb isotope signature of LOMU-affinity samples (cf. Douglass et al., 1999; Andres et al., 2002)
View in article
Previous studies attributed this source enrichment to the influence of a recycled component, such as delaminated subcontinental lithospheric mantle, lower continental crust, or subducted sediment (±AOC) (Douglass et al., 1999; Andres et al., 2002; le Roux et al., 2002b)
View in article
This sedimentary source was previously inferred to have a mid-Proterozoic age (1 to 2 Ga; Douglass et al., 1999; Andres et al., 2002; Labidi et al., 2013; Yierpan et al., 2020)
View in article
Bali, E., Keppler, H., Audetat, A. (2012) The mobility of W and Mo in subduction zone fluids and the Mo–W–Th–U systematics of island arc magmas. Earth and Planetary Science Letters 351–352, 195–207. https://doi.org/10.1016/j.epsl.2012.07.032
Show in context The mobility of Mo from the subducted material is controlled by the redox state of slab-derived aqueous fluids and hydrous melts (Bali et al., 2012; Skora et al., 2017)
View in article
Under reducing conditions, only limited Mo mobility is expected to occur in subducted lithologies (Bali et al., 2012; Skora et al., 2017) thus preserving pre-subducted Mo signatures of the surface
View in article
This may be reconciled with experimental studies showing immobility of Mo in low fO2-bearing slab fluids and hydrous slab melts (Bali et al., 2012; Skora et al., 2017; Chowdhury et al., 2022)
View in article
These experiments were conducted at subduction zone P-T and at reducing conditions, and showed that Mo4+ is immobile in fluids in the presence of rutile (Bali et al., 2012) and mobilisation of Mo is inefficient in melts due to increased partitioning of Mo4+ relative to Mo6+ into the residual phases such as sulfides or rutile (Skora et al., 2017; Chowdhury et al., 2022)
View in article
Burdige, D.J. (2007) Preservation of Organic Matter in Marine Sediments: Controls, Mechanisms, and an Imbalance in Sediment Organic Carbon Budgets? Chemical Reviews 107, 467–485. https://doi.org/10.1021/cr050347q
Show in context This limited the OM flux to the Proterozoic deep ocean and therefore authigenic Mo accumulation from seawater. However, deep ocean anoxic conditions increased OM preservation and burial efficiency (Burdige, 2007)
View in article
Chen, S., Hin, R.C., John, T., Brooker, R., Bryan, B., Niu, Y., Elliott, T. (2019) Molybdenum systematics of subducted crust record reactive fluid flow from underlying slab serpentine dehydration. Nature Communications 10, 4773. https://doi.org/10.1038/s41467-019-12696-3
Show in context This would have resulted in a preferential loss of more incompatible (isotopically heavy) Mo6+ during melting and dehydration (e.g., Chen et al., 2019; McCoy-West et al., 2019) and would shift δ98/95Mo of the residual subducted material towards lighter values
View in article
Significant Mo mobilisation and isotope fractionation has been observed during subduction of oxidised lithologies, leaving behind a Mo depleted and isotopically light residual slab (e.g., Freymuth et al., 2015; König et al., 2016; Chen et al., 2019; Ahmad et al., 2021)
View in article
Chen, S., Sun, P., Niu, Y., Guo, P., Elliott, T., Hin, R.C. (2022) Molybdenum isotope systematics of lavas from the East Pacific Rise: Constraints on the source of enriched mid-ocean ridge basalt. Earth and Planetary Science Letters 578, 117283. https://doi.org/10.1016/j.epsl.2021.117283
Show in context Moreover, the absence of a covariation of δ98/95Mo and Pb isotopes (Fig. S-4c,d) does not support the influence of an ancient low degree melt (Chen et al., 2022)
View in article
Low degree melting of mantle plume material has been suggested to affect geochemical signatures of investigated samples (le Roux et al., 2002a), and might have affected their δ98/95Mo (e.g., Chen et al., 2022)
View in article
Chowdhury, P., Dasgupta, R., Phelps, P.R., Costin, G., Lee, C.-T. A. (2022) Oxygen fugacity range of subducting crust inferred from fractionation of trace elements during fluid-present slab melting in the presence of anhydrite versus sulfide. Geochimica et Cosmochimica Acta 325, 214–231. https://doi.org/10.1016/j.gca.2022.02.030
Show in context This may be reconciled with experimental studies showing immobility of Mo in low fO2-bearing slab fluids and hydrous slab melts (Bali et al., 2012; Skora et al., 2017; Chowdhury et al., 2022)
View in article
These experiments were conducted at subduction zone P-T and at reducing conditions, and showed that Mo4+ is immobile in fluids in the presence of rutile (Bali et al., 2012) and mobilisation of Mo is inefficient in melts due to increased partitioning of Mo4+ relative to Mo6+ into the residual phases such as sulfides or rutile (Skora et al., 2017; Chowdhury et al., 2022)
View in article
Douglass, J., Schilling, J.-G., Fontignie, D. (1999) Plume-ridge interactions of the Discovery and Shona mantle plumes with the southern Mid-Atlantic Ridge (40°-55°S). Journal of Geophysical Research: Solid Earth 104, 2941–2962. https://doi.org/10.1029/98JB02642
Show in context Previous studies on these samples suggested recycling of ancient oceanic crust and sediments in their mantle source with an age between 1 and 2 Ga based on radiogenic isotopes, and the absence of mass independent fractionation of S isotopes (Douglass et al., 1999; Andres et al., 2002; Labidi et al., 2013)
View in article
Rather, a recycled sediment component can explain the Pb isotope signature of LOMU-affinity samples (cf. Douglass et al., 1999; Andres et al., 2002)
View in article
Previous studies attributed this source enrichment to the influence of a recycled component, such as delaminated subcontinental lithospheric mantle, lower continental crust, or subducted sediment (±AOC) (Douglass et al., 1999; Andres et al., 2002; le Roux et al., 2002b)
View in article
This sedimentary source was previously inferred to have a mid-Proterozoic age (1 to 2 Ga; Douglass et al., 1999; Andres et al., 2002; Labidi et al., 2013; Yierpan et al., 2020)
View in article
Average Mo isotope composition of MORBs from the S-MAR and PAR together with radiogenic isotope data (Douglass et al., 1999) and Se isotope compositions (Yierpan et al., 2020)
View in article
Evans, K.A. (2012) The redox budget of subduction zones. Earth-Science Reviews 113, 11–32. https://doi.org/10.1016/j.earscirev.2012.03.003
Show in context As aerobic conditions prevail at the Earth’s surface today, deep sea sediments and altered oceanic crust (AOC) are largely oxidised and influence the redox budget of the subducting slab (e.g., Evans, 2012)
View in article
Due to the anoxic conditions in the Proterozoic deep ocean, oxidised species of major and minor elements like Fe, S and Mn, were absent in deep sea sediments, thus lowering their redox budget/oxidising capacity compared to present day marine lithologies (e.g., Evans, 2012; Ague et al., 2022)
View in article
Freymuth, H., Vils, F., Willbold, M., Taylor, R.N., Elliott, T. (2015) Molybdenum mobility and isotopic fractionation during subduction at the Mariana arc. Earth and Planetary Science Letters 432, 176–186. https://doi.org/10.1016/j.epsl.2015.10.006
Show in context Significant Mo mobilisation and isotope fractionation has been observed during subduction of oxidised lithologies, leaving behind a Mo depleted and isotopically light residual slab (e.g., Freymuth et al., 2015; König et al., 2016; Chen et al., 2019; Ahmad et al., 2021)
View in article
Freymuth, H., Elliott, T., van Soest, M., Skora, S. (2016) Tracing subducted black shales in the Lesser Antilles arc using molybdenum isotope ratios. Geology 44, 987–990. https://doi.org/10.1130/G38344.1
Show in context These findings are also consistent with subduction of organic matter (OM)-rich black shales in the Lesser Antilles Arc, where lavas south of Martinique exhibit high δ98/95Mo along with lower Mo/Ce (Freymuth et al., 2016; Gaschnig et al., 2017), suggesting the minute contribution of unfractionated slab-derived Mo to the mantle sources relative to melts originating from more oxidising sediments
View in article
Gaschnig, R.M., Reinhard, C.T., Planavsky, N.J., Wang, X., Asael, D., Chauvel, C. (2017) The Molybdenum Isotope System as a Tracer of Slab Input in Subduction Zones: An Example From Martinique, Lesser Antilles Arc. Geochemistry, Geophysics, Geosystems 18, 4674–4689. https://doi.org/10.1002/2017GC007085
Show in context These findings are also consistent with subduction of organic matter (OM)-rich black shales in the Lesser Antilles Arc, where lavas south of Martinique exhibit high δ98/95Mo along with lower Mo/Ce (Freymuth et al., 2016; Gaschnig et al., 2017), suggesting the minute contribution of unfractionated slab-derived Mo to the mantle sources relative to melts originating from more oxidising sediments
View in article
Gaschnig, R.M., Reinhard, C.T., Planavsky, N.J., Wang, X., Asael, D., Jackson, M.G. (2021) The impact of primary processes and secondary alteration on the stable isotope composition of ocean island basalts. Chemical Geology 581, 120416. https://doi.org/10.1016/j.chemgeo.2021.120416
Show in context This is in line with Samoan OIBs, where δ98/95Mo signatures are interpreted as a mixture of isotopically heavy terrigenous sediments (δ98/95Mo ≈ UCC) and isotopically light dehydrated mafic oceanic crust, which reflect the influence of a distinct pool of mid-Proterozoic recycled ocean crust (Gaschnig et al., 2021)
View in article
We therefore conclude that the Mo isotope signature of plume-influenced volcanic rocks can be used to reconcile the redox conditions during ancient surface deposition of deep sea sediments (cf. Gaschnig et al., 2021) as well as during subduction-related prograde metamorphism and the inception of modern subduction
View in article
Kendall, B., Dahl, T.W., Anbar, A.D. (2017) The stable isotope geochemistry of molybdenum. Reviews in Mineralogy and Geochemistry 82, 683–732. https://doi.org/10.2138/rmg.2017.82.16
Show in context This points towards a minor authigenic Mo enrichment from seawater or a residual enrichment of Mo (e.g., Kendall et al., 2017) and implies that δ98/95Mo has not been affected by oxic conditions during slab dehydration due to subduction of reducing lithologies (Fig. 2)
View in article
König, S., Wille, M., Voegelin, A., Schoenberg, R. (2016) Molybdenum isotope systematics in subduction zones. Earth and Planetary Science Letters 447, 95–102. https://doi.org/10.1016/j.epsl.2016.04.033
Show in context Significant Mo mobilisation and isotope fractionation has been observed during subduction of oxidised lithologies, leaving behind a Mo depleted and isotopically light residual slab (e.g., Freymuth et al., 2015; König et al., 2016; Chen et al., 2019; Ahmad et al., 2021)
View in article
Laakso, T.A., Schrag, D.P. (2019) A small marine biosphere in the Proterozoic. Geobiology 17, 161–171. https://doi.org/10.1111/gbi.12323
Show in context The extent of the biological pump in the Proterozoic ocean, where primary productivity in the oxygenated surface ocean was dominated by cyanobacteria, was lower compared to modern oceans and higher primary surface productivity was restricted to marine environments close to continents (Laakso and Schrag, 2019)
View in article
Labidi, J., Cartigny, P., Moreira, M. (2013) Non-chondritic sulphur isotope composition of the terrestrial mantle. Nature 501, 208–211. https://doi.org/10.1038/nature12490
Show in context These basalts additionally feature increasingly heavier S and Se isotope compositions with indicators of mantle source enrichment, and are interpreted to reflect subduction recycling of reduced sediments from a redox stratified Proterozoic ocean (Labidi et al., 2013; Yierpan et al., 2020)
View in article
Their distinct radiogenic isotope compositions, their covariation with stable Se and S signatures, and trace element systematics are independent of partial melting variations, suggesting the plume source was enriched prior to low degree mantle melting (le Roux et al., 2002a; Labidi et al., 2013; Yierpan et al., 2020)
View in article
In line with previous investigations showing covariations between other redox-sensitive stable Se-S isotope systematics and radiogenic isotopes (Labidi et al., 2013; Yierpan et al., 2020), we suggest that a sediment contribution to the S-MAR mantle source is the most likely scenario for the observed heavy Mo isotope enrichment in our samples (see Supplementary Information S-3 for further discussion)
View in article
Previous studies on these samples suggested recycling of ancient oceanic crust and sediments in their mantle source with an age between 1 and 2 Ga based on radiogenic isotopes, and the absence of mass independent fractionation of S isotopes (Douglass et al., 1999; Andres et al., 2002; Labidi et al., 2013)
View in article
The interpretation is in line with the fO2-sensitive stable isotope systematics of S and Se (Table 1), which indicate negligible mobilisation and isotope fractionation during subduction (Labidi et al., 2013; Yierpan et al., 2020)
View in article
The δ98/95Mo-87Sr/86Sr-143Nd/144Nd covariations (Fig. 1) in the S-MAR data combined with the previously established model of the linear δ34S-δ82Se-87Sr/86Sr(-143Nd/144Nd) relationship (Labidi et al., 2013; Yierpan et al., 2020) allows extrapolation of δ98/95Mo and [Mo] to constrain the nature of the recycled sediment
View in article
This sedimentary source was previously inferred to have a mid-Proterozoic age (1 to 2 Ga; Douglass et al., 1999; Andres et al., 2002; Labidi et al., 2013; Yierpan et al., 2020)
View in article
le Roux, P., le Roex, A., Schilling, J.G. (2002a) MORB melting processes beneath the southern Mid-Atlantic Ridge (40–55°S): a role for mantle plume-derived pyroxenite. Contributions to Mineralogy and Petrology 144, 206–229. https://doi.org/10.1007/s00410-002-0376-3
Show in context Low degree melting of mantle plume material has been suggested to affect geochemical signatures of investigated samples (le Roux et al., 2002a), and might have affected their δ98/95Mo (e.g., Chen et al., 2022)
View in article
In addition, mantle melting degrees of 2.5 % (le Roux et al., 2002a) cannot explain the observed variations of δ98/95Mo indicating that heavy Mo is likely a characteristic primary signature of the enriched plume material (see Supplementary Information S-2 for further details)
View in article
Their distinct radiogenic isotope compositions, their covariation with stable Se and S signatures, and trace element systematics are independent of partial melting variations, suggesting the plume source was enriched prior to low degree mantle melting (le Roux et al., 2002a; Labidi et al., 2013; Yierpan et al., 2020)
View in article
le Roux, P.J., le Roex, A.P., Schilling, J.G., Shimizu, N., Perkins, W.W., Pearce, N.J.G. (2002b) Mantle heterogeneity beneath the southern Mid-Atlantic Ridge: trace element evidence for contamination of ambient asthenospheric mantle. Earth and Planetary Science Letters 203, 479–498. https://doi.org/10.1016/S0012-821X(02)00832-4
Show in context Previous studies attributed this source enrichment to the influence of a recycled component, such as delaminated subcontinental lithospheric mantle, lower continental crust, or subducted sediment (±AOC) (Douglass et al., 1999; Andres et al., 2002; le Roux et al., 2002b)
View in article
Lyons, T.W., Reinhard, C.T., Planavsky, N.J. (2014) The rise of oxygen in Earth’s early ocean and atmosphere. Nature 506, 307–315. https://doi.org/10.1038/nature13068
Show in context Furthermore, under anoxic conditions with an overall low concentration of dissolved SO42− and MoO42−, neither a significant Mo enrichment from seawater into the sediment nor a significant Mo mobilisation during fluid alteration is expected (Lyons et al., 2014)
View in article
Ma, L., Xu, Y.G., Li, J., Chen, L.-H., Liu, J.-Q., Li, H.-Y., Huang, X.-L., Ma, Q., Hong, L.-B., Wang, Y. (2022) Molybdenum isotopic constraints on the origin of EM1-type continental intraplate basalts. Geochimica et Cosmochimica Acta 317, 255–268. https://doi.org/10.1016/j.gca.2021.11.013
Show in context This may also explain the preserved δ98/95Mo of a reduced Proterozoic sediment component recycled into the S-MAR mantle source, in contrast to Neoproterozoic, deep mantle recycling of low δ98/95Mo into mantle plume sources (see also Ma et al., 2022)
View in article
McCoy-West, A.J., Chowdhury, P., Burton, K.W., Sossi, P., Nowell, G.M., Fitton, J.G., Kerr, A.C., Cawood, P.A., Williams, H.M. (2019) Extensive crustal extraction in Earth’s early history inferred from molybdenum isotopes. Nature Geoscience 12, 946–951. https://doi.org/10.1038/s41561-019-0451-2
Show in context This would have resulted in a preferential loss of more incompatible (isotopically heavy) Mo6+ during melting and dehydration (e.g., Chen et al., 2019; McCoy-West et al., 2019) and would shift δ98/95Mo of the residual subducted material towards lighter values
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Poulton, S.W., Canfield, D.E. (2011) Ferruginous Conditions: A Dominant Feature of the Ocean through Earth’s History. Elements 7, 107–112. https://doi.org/10.2113/GSELEMENTS.7.2.107
Show in context These findings support the notion that the deep ocean remained anoxic until the beginning of the Phanerozoic (e.g., Poulton and Canfield, 2011; Stolper and Keller, 2018)
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Skora, S., Freymuth, H., Blundy, J., Elliott, T., Guillong, M. (2017) An experimental study of the behaviour of cerium/molybdenum ratios during subduction: Implications for tracing the slab component in the Lesser Antilles and Mariana Arc. Geochimica et Cosmochimica Acta 212, 133–155. https://doi.org/10.1016/j.gca.2017.05.025
Show in context The mobility of Mo from the subducted material is controlled by the redox state of slab-derived aqueous fluids and hydrous melts (Bali et al., 2012; Skora et al., 2017)
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Under reducing conditions, only limited Mo mobility is expected to occur in subducted lithologies (Bali et al., 2012; Skora et al., 2017) thus preserving pre-subducted Mo signatures of the surface
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This may be reconciled with experimental studies showing immobility of Mo in low fO2-bearing slab fluids and hydrous slab melts (Bali et al., 2012; Skora et al., 2017; Chowdhury et al., 2022)
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These experiments were conducted at subduction zone P-T and at reducing conditions, and showed that Mo4+ is immobile in fluids in the presence of rutile (Bali et al., 2012) and mobilisation of Mo is inefficient in melts due to increased partitioning of Mo4+ relative to Mo6+ into the residual phases such as sulfides or rutile (Skora et al., 2017; Chowdhury et al., 2022)
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Stolper, D.A., Keller, C.B. (2018) A record of deep-ocean dissolved O2 from the oxidation state of iron in submarine basalts. Nature 553, 323–327. https://doi.org/10.1038/nature25009
Show in context These findings support the notion that the deep ocean remained anoxic until the beginning of the Phanerozoic (e.g., Poulton and Canfield, 2011; Stolper and Keller, 2018)
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Willbold, M., Elliott, T. (2017) Molybdenum isotope variations in magmatic rocks. Chemical Geology 449, 253–268. https://doi.org/10.1016/j.chemgeo.2016.12.011
Show in context Best fits with minimum errors overlap with the lower 1σ of Proterozoic OM-rich sediment data from the literature (Ye et al., 2021; Table S-2) and can also be achieved with a recycled sediment contribution close to that of the UCC (∼0.05 to 0.15 ‰; Willbold and Elliott, 2017 and references therein) with slightly higher δ98/95Mo and/or [Mo] values
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Ye, Y., Zhang, S., Wang, H., Wang, X., Tan, C., Li, M., Wu, C., Canfield, D.E. (2021) Black shale Mo isotope record reveals dynamic ocean redox during the Mesoproterozoic Era. Geochemical Perspectives Letters 18, 16–21. https://doi.org/10.7185/geochemlet.2118
Show in context Best fits with minimum errors overlap with the lower 1σ of Proterozoic OM-rich sediment data from the literature (Ye et al., 2021; Table S-2) and can also be achieved with a recycled sediment contribution close to that of the UCC (∼0.05 to 0.15 ‰; Willbold and Elliott, 2017 and references therein) with slightly higher δ98/95Mo and/or [Mo] values
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Yierpan, A., König, S., Labidi, J., Schoenberg, R. (2020) Recycled selenium in hot spot–influenced lavas records ocean-atmosphere oxygenation. Science Advances 6, abb6179. https://doi.org/10.1126/sciadv.abb6179
Show in context These basalts additionally feature increasingly heavier S and Se isotope compositions with indicators of mantle source enrichment, and are interpreted to reflect subduction recycling of reduced sediments from a redox stratified Proterozoic ocean (Labidi et al., 2013; Yierpan et al., 2020)
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In line with previous investigations showing covariations between other redox-sensitive stable Se-S isotope systematics and radiogenic isotopes (Labidi et al., 2013; Yierpan et al., 2020), we suggest that a sediment contribution to the S-MAR mantle source is the most likely scenario for the observed heavy Mo isotope enrichment in our samples (see Supplementary Information S-3 for further discussion)
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Their distinct radiogenic isotope compositions, their covariation with stable Se and S signatures, and trace element systematics are independent of partial melting variations, suggesting the plume source was enriched prior to low degree mantle melting (le Roux et al., 2002a; Labidi et al., 2013; Yierpan et al., 2020)
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The interpretation is in line with the fO2-sensitive stable isotope systematics of S and Se (Table 1), which indicate negligible mobilisation and isotope fractionation during subduction (Labidi et al., 2013; Yierpan et al., 2020)
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Average Mo isotope composition of MORBs from the S-MAR and PAR together with radiogenic isotope data (Douglass et al., 1999) and Se isotope compositions (Yierpan et al., 2020)
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The external reproducibility (2 s.d.) is 0.08 ‰, except for EW9309 2D-1g and EW9309 9D-3g, for which it is 0.04 ‰ (Yierpan et al., 2020)
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The δ98/95Mo-87Sr/86Sr-143Nd/144Nd covariations (Fig. 1) in the S-MAR data combined with the previously established model of the linear δ34S-δ82Se-87Sr/86Sr(-143Nd/144Nd) relationship (Labidi et al., 2013; Yierpan et al., 2020) allows extrapolation of δ98/95Mo and [Mo] to constrain the nature of the recycled sediment
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This sedimentary source was previously inferred to have a mid-Proterozoic age (1 to 2 Ga; Douglass et al., 1999; Andres et al., 2002; Labidi et al., 2013; Yierpan et al., 2020)
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Supplementary Information
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