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Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle


Komatiites are volcanic rocks mainly of Archaean age that formed by unusually high degrees of melting of mantle peridotite. Their origin is controversial and has been attributed to either anhydrous melting of anomalously hot mantle1,2,3 or hydrous melting at temperatures only modestly greater than those found today4,5. Here we determine the original Fe3+/ΣFe ratio of 2.7-Gyr-old komatiitic magma from Belingwe, Zimbabwe6, preserved as melt inclusions in olivine, to be 0.10 ± 0.02, using iron K-edge X-ray absorption near-edge structure spectroscopy. This value is consistent with near-anhydrous melting of a source with a similar oxidation state to the source of present-day mid-ocean-ridge basalt. Furthermore, this low Fe3+/ΣFe value, together with a water content of only 0.2–0.3 wt% (ref. 7), excludes the possibility that the trapped melt contained significantly more water that was subsequently lost from the inclusions by reduction to H2 and diffusion. Loss of only 1.5 wt% water by this mechanism would have resulted in complete oxidation of iron (that is, the Fe3+/ΣFe ratio would be 1). There is also no petrographic evidence for the loss of molecular water. Our results support the identification of the Belingwe komatiite as a product of high mantle temperatures (1,700 °C), rather than melting under hydrous conditions (3–5-wt% water), confirming the existence of anomalously hot mantle in the Archaean era.

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Figure 1: Iron K-edge XANES spectra of quenched silicate melts.
Figure 2: Calibration curve for determining Fe 3+ /ΣFe from the XANES pre-edge centroid energy.
Figure 3: Optical image of an olivine-hosted melt inclusion (after heating) from a komatiite of the Belingwe belt, Zimbabwe.


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We thank D. R. Scott for sample preparation, N. Métrich for providing the pantellerite glass standards and G. J. Foran for assistance with XANES experiments at the Australian National Beamline Facility that provided the foundations for the present study. We also thank the Australian Research Council (DP0450252), the Access to Major Research Facilities Programme (funded by the Commonwealth of Australia) and the Natural Environment Research Council for financial support. GeoSoilEnviroCARS is supported by the US National Science Foundation (EAR-0622171) and the US Department of Energy (DE-FG02-94ER14466). Use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. W-31-109-Eng-38.

Author Contributions A.J.B. and H.StC.O’N. prepared the synthetic samples and collected the XANES spectra, with assistance from M.N. and S.R.S. The natural samples were prepared and characterised by L.V.D., who identified the importance of determining Fe3+/ΣFe in komatiite melt inclusions. A.J.B. interpreted the spectra and produced the manuscript with significant contributions from H.StC.O’N. and L.V.D.

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Berry, A., Danyushevsky, L., St C. O’Neill, H. et al. Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle. Nature 455, 960–963 (2008).

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