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Fe-XANES analyses of Reykjanes Ridge basalts: implications for oceanic crust’s role in the solid Earth oxygen cycle

Fe-XANES analyses of Reykjanes Ridge basalts: implications for oceanic crust’s role in the solid Earth oxygen cycle
Fe-XANES analyses of Reykjanes Ridge basalts: implications for oceanic crust’s role in the solid Earth oxygen cycle
The cycling of material from Earth’s surface environment into its interior can couple mantle oxidation state to the evolution of the oceans and atmosphere. A major uncertainty in this exchange is whether altered oceanic crust entering subduction zones can carry the oxidised signal it inherits during alteration at the ridge axis, past the arc front and into the deep mantle for long-term storage. However, recycled oceanic crust may eventually be entrained into mantle upwellings and undergo melting at ocean islands, creating the potential for basalt chemistry to constrain the nature of past solid Earth–hydrosphere redox coupling.

Numerous independent observations suggest that Iceland contains a significant recycled oceanic crustal component, making it an ideal locality to investigate links between redox proxies and geochemical indices of enrichment. We have interrogated the elemental, isotope and redox geochemistry of basalts from the Reykjanes Ridge, which forms a 700 km transect of the Iceland plume. Over this distance, geophysical and geochemical tracers of plume influence increase dramatically, with the basalts recording both longand short-wavelength heterogeneity in the Iceland plume. We present new high-precision Fe-XANES measurements of Fe3+=P Fe on a suite of 64 basalt glasses from the Reykjanes Ridge. These basalts exhibit positive correlations between Fe3+=P Fe and trace element and isotopic signals of enrichment, and become progressively oxidised towards Iceland: fractionation-corrected Fe3+=P Fe increases by ? 0:015 and ?QFM by ? 0:2 log units. We carefully rule out a role for sulfur degassing in creating this trend, and by considering various redox melting processes and metasomatic source enrichment mechanisms, conclude that an intrinsically oxidised component within the Icelandic mantle is required. Given the previous evidence for entrained oceanic crustal material within the Iceland plume, we consider this the most plausible carrier of the oxidised signal.

To determine the ferric iron content of the recycled component ([Fe2O3]source[Fe2O3]source) we project observed liquid compositions to an estimate of Fe2O3 in the pure enriched endmember melt, and then apply simple fractional melting models, considering lherzolitic and pyroxenitic source mineralogies, to estimate [Fe2O3](source)[Fe2O3](source) content. Propagating uncertainty through these steps, we obtain a range of [Fe2O3](source)[Fe2O3](source) for the enriched melts (0.9–1.4 wt%) that is significantly greater than the ferric iron content of typical upper mantle lherzolites. This range of ferric iron contents is consistent with a hybridised lherzolite–basalt (pyroxenite) mantle component. The oxidised signal in enriched Icelandic basalts is therefore potential evidence for seafloor–hydrosphere interaction having oxidised ancient mid-ocean ridge crust, generating a return flux of oxygen into the deep mantle.
Mantle fO2, XANES, oxygen, mantle heterogeneity, pyroxenite
0012-821X
272-285
Shorttle, Oliver
3ecb584f-8ca0-4c22-81ef-842305a894f2
Moussallam, Yves
8d226827-b7cf-45f0-9144-f6f5c33fdd9a
Hartley, Margaret
e19aa84e-d38c-4d20-8c2c-3720e19464e1
Maclennan, John
c7e5c171-3bed-4e71-8ba8-7f1047184e83
Edmonds, Marie
77edb34f-ead5-4ec5-afda-05b7388c471b
Murton, Bramley
9076d07f-a3c1-4f90-a5d5-99b27fe2cb12
Shorttle, Oliver
3ecb584f-8ca0-4c22-81ef-842305a894f2
Moussallam, Yves
8d226827-b7cf-45f0-9144-f6f5c33fdd9a
Hartley, Margaret
e19aa84e-d38c-4d20-8c2c-3720e19464e1
Maclennan, John
c7e5c171-3bed-4e71-8ba8-7f1047184e83
Edmonds, Marie
77edb34f-ead5-4ec5-afda-05b7388c471b
Murton, Bramley
9076d07f-a3c1-4f90-a5d5-99b27fe2cb12

Shorttle, Oliver, Moussallam, Yves, Hartley, Margaret, Maclennan, John, Edmonds, Marie and Murton, Bramley (2015) Fe-XANES analyses of Reykjanes Ridge basalts: implications for oceanic crust’s role in the solid Earth oxygen cycle. Earth and Planetary Science Letters, 427, 272-285. (doi:10.1016/j.epsl.2015.07.017).

Record type: Article

Abstract

The cycling of material from Earth’s surface environment into its interior can couple mantle oxidation state to the evolution of the oceans and atmosphere. A major uncertainty in this exchange is whether altered oceanic crust entering subduction zones can carry the oxidised signal it inherits during alteration at the ridge axis, past the arc front and into the deep mantle for long-term storage. However, recycled oceanic crust may eventually be entrained into mantle upwellings and undergo melting at ocean islands, creating the potential for basalt chemistry to constrain the nature of past solid Earth–hydrosphere redox coupling.

Numerous independent observations suggest that Iceland contains a significant recycled oceanic crustal component, making it an ideal locality to investigate links between redox proxies and geochemical indices of enrichment. We have interrogated the elemental, isotope and redox geochemistry of basalts from the Reykjanes Ridge, which forms a 700 km transect of the Iceland plume. Over this distance, geophysical and geochemical tracers of plume influence increase dramatically, with the basalts recording both longand short-wavelength heterogeneity in the Iceland plume. We present new high-precision Fe-XANES measurements of Fe3+=P Fe on a suite of 64 basalt glasses from the Reykjanes Ridge. These basalts exhibit positive correlations between Fe3+=P Fe and trace element and isotopic signals of enrichment, and become progressively oxidised towards Iceland: fractionation-corrected Fe3+=P Fe increases by ? 0:015 and ?QFM by ? 0:2 log units. We carefully rule out a role for sulfur degassing in creating this trend, and by considering various redox melting processes and metasomatic source enrichment mechanisms, conclude that an intrinsically oxidised component within the Icelandic mantle is required. Given the previous evidence for entrained oceanic crustal material within the Iceland plume, we consider this the most plausible carrier of the oxidised signal.

To determine the ferric iron content of the recycled component ([Fe2O3]source[Fe2O3]source) we project observed liquid compositions to an estimate of Fe2O3 in the pure enriched endmember melt, and then apply simple fractional melting models, considering lherzolitic and pyroxenitic source mineralogies, to estimate [Fe2O3](source)[Fe2O3](source) content. Propagating uncertainty through these steps, we obtain a range of [Fe2O3](source)[Fe2O3](source) for the enriched melts (0.9–1.4 wt%) that is significantly greater than the ferric iron content of typical upper mantle lherzolites. This range of ferric iron contents is consistent with a hybridised lherzolite–basalt (pyroxenite) mantle component. The oxidised signal in enriched Icelandic basalts is therefore potential evidence for seafloor–hydrosphere interaction having oxidised ancient mid-ocean ridge crust, generating a return flux of oxygen into the deep mantle.

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Accepted/In Press date: July 2015
Published date: 1 October 2015
Keywords: Mantle fO2, XANES, oxygen, mantle heterogeneity, pyroxenite
Organisations: Marine Geoscience

Identifiers

Local EPrints ID: 379555
URI: http://eprints.soton.ac.uk/id/eprint/379555
ISSN: 0012-821X
PURE UUID: c17a5641-5af9-44d3-93ca-398f9c53aa6e

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Date deposited: 11 Aug 2015 12:42
Last modified: 14 Mar 2024 20:43

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Contributors

Author: Oliver Shorttle
Author: Yves Moussallam
Author: Margaret Hartley
Author: John Maclennan
Author: Marie Edmonds
Author: Bramley Murton

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