Arc magmas oxidized by water dissociation and hydrogen incorporation in orthopyroxene


Elevated H2O concentrations and oxygen fugacities are two fundamental properties that distinguish magmas formed in subduction zones from new crust generated at mid-ocean ridges. However, the mechanism of magma oxidation and how it relates to the increase in H2O remain unclear. In this study, we used infrared spectroscopy of mantle wedge orthopyroxene to trace the temporal and spatial evolution of oxygen fugacity during the transport of hydrous arc melts towards the crust. A positive correlation between equilibrium oxygen fugacity and orthopyroxene H2O concentrations for the peridotite samples studied allowed the assignment of specific, commonly observed absorption bands to redox-sensitive crystallographic defects. H2O content associated with these redox-sensitive defects increases in concentration across individual crystals, uniquely preserving the time-dependent transition from reduced to oxidized conditions during the migration of hydrous melts through the mantle wedge. A separate but related process of reaction with H2 occurring primarily during the earliest stages of melt–mantle reaction may be fundamental in generating the oxidized nature of hydrous melts. Our study proposes that the oxidized nature of arc magmas may not be a primary feature, but is instead acquired progressively as hydrous primary melts react with the surrounding mantle.

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Fig. 1: Summary of the data and interpretations.
Fig. 2: Compilation of key FTIR data.
Fig. 3: An example of H2O distribution in an orthopyroxene from sample 67–02B(1).

Data availability

All supporting data for the manuscript are provided as a Supplementary Data file, and are also available online at Any additional data may be requested from the authors.


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We thank R. Arculus for providing the samples, and D. Rubatto and M. Jollands for discussions and feedback. This work was supported by the SNF project: 200021_169062.

Author information

P.T. designed and conducted the FTIR measurements, and processed the data. P.T. and J.H. interpreted the data and wrote the manuscript.

Correspondence to Peter Tollan.

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Supplementary information

Supplementary Information

Supplementary information and Figs. 1–7

Supplementary Data

Supplementary data for orthopyroxene analyses.

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