Research ArticleGEOLOGY

Barium isotope evidence for pervasive sediment recycling in the upper mantle

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Science Advances  11 Jul 2018:
Vol. 4, no. 7, eaas8675
DOI: 10.1126/sciadv.aas8675
  • Fig. 1 Summary of Ba concentrations and isotope compositions for different reservoirs relevant for crustal recycling.

    Sources for the data are cited in the text or listed in table S4. Note the logarithmic scale on the y axis. EMM, enriched MORB mantle; NIST, National Institute of Standards and Technology.

  • Fig. 2 Map of different samples investigated for Ba isotopes in this study.

    Squares are MORB glasses, green circles are AOC, and brown diamonds are sediments. MORB glasses are color-coded according to the ridge system/ocean basin they represent, and the same colors are used in Figs. 3 and 4.

  • Fig. 3 Barium isotope data for MORB and melt mixing at the ridge.

    Barium isotope data for MORB glasses plotted against (A) Rb/Sr ratios, (B) La/Sm ratios, (C) Nd isotopes, and (D) Sr isotopes. Also shown are mixing lines between two different D-MORB compositions [see (3) and most depleted sample from this study] and E-MORB (3) melts (bold and dashed black lines). These mixing lines show that all the isotopic and trace element data can be explained via mixing between two distinct end-member melts at the mid-ocean ridge. Error bars for isotope ratios are 2 SDs, and trace element ratios are ±10%.

  • Fig. 4 Barium isotope data for MORB and sediment addition in a subduction zone.

    Barium isotope data for MORB glasses plotted against (A) Rb/Sr ratios, (B) Ce/Pb ratios, (C) Nd isotopes, (D) Ba/Th ratios, (E) Sr isotopes, and (F) La/Sm ratios. Also shown are mixing lines between the DMM (17, 52) and sediment-rich components derived from subducting slabs. These mixing lines indicate the processes taking place in subduction zones that could be capable of producing the EMM reservoir. The mixing lines for bulk pelagic (pel.) clay, bulk terrigenous (terr.) sediment, bulk global subducted sediment (GLOSS), GLOSS melt, and residues of GLOSS melting are stopped at 0.4, 0.3, 0.1, 0.08, and 0.8% addition to the DMM, respectively. Sediment compositions are average subducted pelagic/brown clays, terrigenous/turbidite sediments, and GLOSS (20). Partition coefficients for Ba and Th in slab fluids released at 4 GPa and 800°C (18) are assumed to represent 10% fluid release from GLOSS. Partition coefficients for Ba, Th, La, Nd, and Sm during GLOSS melting assume 10% batch melting at 3.5 GPa and 900°C (43). Residual sediment calculations assume that 50% of the sediment is removed at 3.5 GPa and 900°C before mixing with the DMM. The degrees of melting and fluid release used to construct the mixing lines are not critical as different melt and fluid fractions produce broadly similar results.

  • Fig. 5 Simplified geodynamic schematic of EMM generation.

    (A) During subduction, a hydrous sediment-rich component is added at depth to the DMM. Addition of this component triggers incipient partial melting of the DMM (arrow from green to red square). These low-degree partial melts that are contaminated by sedimentary Ba freeze back into the mantle and create the EMM reservoir. (B) Convection of the upper mantle for 100 to 500 Ma causes radiogenic ingrowth (downward arrow) in the EMM reservoir that diverges from the DMM. (C) Melting at ridges generates melts from EMM and DMM that mix in variable proportions to produce hyperbolic mixing relationships between Ba, Nd isotopes, and La/Sm element ratios.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/7/eaas8675/DC1

    Table S1. Isotope data for MORB glass samples.

    Table S2. Trace element data for MORB glasses in microgram per gram.

    Table S3. Major element compositions of MORB glasses.

    Table S4. Barium isotope data for AOC and sediment core tops.

    Table S5. Partition coefficients used to calculate sediment melting.

    Table S6. Barium isotope data for international reference materials.

    Fig. S1. Barium isotopes plotted against Sr isotopes for MORB samples.

    Fig. S2. Calculated Ba isotope variation in MORB mantle reservoirs generated by hypothetical isotope fractionation during melt depletion.

    References (5371)

  • Supplementary Materials

    This PDF file includes:

    • Table S1. Isotope data for MORB glass samples.
    • Table S2. Trace element data for MORB glasses in microgram per gram.
    • Table S3. Major element compositions of MORB glasses.
    • Table S4. Barium isotope data for AOC and sediment core tops.
    • Table S5. Partition coefficients used to calculate sediment melting.
    • Table S6. Barium isotope data for international reference materials.
    • Fig. S1. Barium isotopes plotted against Sr isotopes for MORB samples.
    • Fig. S2. Calculated Ba isotope variation in MORB mantle reservoirs generated by hypothetical isotope fractionation during melt depletion.
    • References (5371)

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