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Redox state of the convective mantle from CO2-trace element systematics of oceanic basalts

J. Eguchi1,

1Department of Earth, Environmental and Planetary Sciences, Rice University, 6100 Main Street, MS 126, Houston, TX 77005

R. Dasgupta1

1Department of Earth, Environmental and Planetary Sciences, Rice University, 6100 Main Street, MS 126, Houston, TX 77005

Affiliations  |  Corresponding Author  |  Cite as  |  Funding information

Eguchi, J., Dasgupta, R. (2018) Redox state of the convective mantle from CO2-trace element systematics of oceanic basalts. Geochem. Persp. Let. 8, 17–21.

NSF grants EAR-1255391, OCE-1338842, and the Sloan Foundation through Deep Carbon Observatory.

Geochemical Perspectives Letters v8  |  doi: 10.7185/geochemlet.1823
Received 20 April 2018  |  Accepted 01 August 2018  |  Published 18 September 2018
Copyright © The Authors

Published by the European Association of Geochemistry
under Creative Commons License CC BY-NC-ND 4.0




Figure 1 Pressure-temperature conditions for generation of partial melts from crustal/mafic lithologies in the mantle with colour map for wt. % SiO2 in the melt. Also plotted for reference are mantle adiabats for mantle potential temperatures of 1350 and 1650 °C (dashed lines), solidi of various volatile-free lithologies (1-metapelite, 2-MORB-eclogite, 3-silica-deficient garnet pyroxenite, 4-peridotite, references given in footnote of Table S-3 – the table of compiled experiments), and graphite-diamond transition (dotted line).
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Figure 2 CO2 concentrations calculated at the CCO buffer for each experiment in Figure 1. Vertical blue lines are highest CO2 concentrations measured in natural oceanic basalts from different locations (Table S-2) that are thought to receive contributions from subducted lithologies. Red band is for CO2 contents expected in depleted peridotite-derived partial melts based on undegassed melt inclusions from Siqueiros (Saal et al., 2002) and calculations using an experimental bulk partition coefficient for CO2 and source CO2 estimate of ~75 ppm at F = 10 % (Rosenthal et al., 2015

Rosenthal, A., Hauri, E.H., Hirschmann, M.M. (2015) Experimental determination of C, F, and H partitioning between mantle minerals and carbonated basalt, CO2/Ba and CO2/Nb systematics of partial melting, and the CO2 contents of basaltic source regions. Earth and Planetary Science Letters 412, 77–87.

).
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Figure 3 CO2-Nb and CO2-Ba mixing lines between depleted peridotite melts and graphite-saturated melts of subducted lithologies. Mixing lines are calculated at different logfO2s relative to FMQ up to the CCO buffer as denoted by numbers above mixing line (the average fO2 of eclogites based on CLM xenoliths at this pressure is marked with bold text). Each marker along a mixing line represents 10 wt. % incremental contribution from subducted lithology partial melt. Data points are the CO2-Nb/CO2-Ba concentrations recorded in the least degassed natural basalts at each location. Data point and mixings lines coloured for melt SiO2 (wt. %) contents. Pink shaded region shows CO2-Nb-Ba contents at graphite-saturation from CCO to FMQ-2.5 for all relevant experimental partial melts for a particular lithology from Figure 1. Gray band shows CO2-Nb-Ba calculations for a depleted peridotite from 1-10 % melting degree (see also Fig. S-1). Data for generation of Figure 3 given in Supplementary Information.
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Figure 4 Pressure vs. fO2 computed for continental lithospheric mantle xenoliths versus those that have been estimated here for oceanic mantle sources (gray region). DCDG/D is the graphite/diamond transition in eclogite/pyroxenite (Luth, 1993

Luth, R.W. (1993) Diamonds , eclogites , and the oxidation state of the Earth’s Mantle. Science 261, 66–68.

). Graphite/diamond transition in peridotite is shown for reference (EMOG/D). References for xenolith data are in Table S-4.
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