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Stable vanadium isotope fractionation at high temperatures: a proxy for oxygen fugacity?

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Understanding and quantifying oxygen fugacity (fO2) is critical to our concepts of mineral partitioning, fluid speciation, and the nature and causes of mantle melting. Furthermore, the secular evolution of mantle fO2 may be directly related to the rise of atmospheric oxygen and our planets ability to sustain life.

The established view holds that mantle fO2 is tectonically associated, with subduction zones ‘more oxidizing’ than the ambient upper mantle source of mid-ocean ridge basalts [e.g., 1]. Routine determination of fO2 uses the oxidation state of iron. However, the mantle is not well-buffered with respect to Fe3+ and there has been some question of whether oxidized values are in fact tracking later processes such as degassing and metasomatism, rather than recording mantle source fO2. The argument that the oxidation state of iron reflects later events is largely based on conflicting evidence yielded by the transition metal element vanadium (V) [e.g., 2, 3]. Vanadium is well-suited to investigations of oxidation-reduction processes as it exists in five valence states. Indeed, the partitioning of V is strongly fO2 dependent [e.g., 4]. However, uncertainties arise from models based on vanadium abundances or trace element ratios (e.g., V/Sc). This is due to unknowns such as the source abundance of vanadium, the degree of melting, and the fractionating phases. Taking uncertainties into account using Fe or V, mantle fO2 cannot be confidently determined to better than 2 log units [5]. This is far too gross a scale to be useful for the detailed study of mantle melting, secular evolution, and fluid speciation.

The past decade has seen a major proliferation in the exploration of so-called ‘non-traditional’ stable isotope systems. This is particularly exciting for igneous geochemists in that these new systems offer very different insight to high temperature processes then we have thus far been able to deduce from radiogenic isotopes and trace element ratios. Instead of tracking the amount of sediment in a mantle source, for example, we may now be able to determine physical conditions such as fO2 using isotope geochemistry.

Here I will discuss recent advances in the measurement and use of stable vanadium isotopes [6, 7]. I will present and explore the first (and only) precise and accurate stable V isotope data for peridotites and igneous suites including the Mariana arc and Hekla volcano in Iceland. I shall discuss the exciting potential of this isotope system in the context of the surprisingly large and resolvable isotope variations seen at high temperatures.

[1] Wood, B.J., Bryndzia, L.T., Johnson, K.E. 1990. Science, 248, 337-345. [2] Lee, C-T., Brandon, A.D., Norman, M. 2003. Geochim. Cosmchim. Acta. 67, 3045-3064. [3] Lee, C-T., Leeman, W.P., Canil, D., Li, Z-X. A. 2005. J. Pet. 46, 2313-2336. [4] Canil, D. 1997. Nature, 389, 842-845. [5] Frost, D.J., McCammon, C.A. 2008. Ann. Rev. Earth Sci. 36, 389-420. [6] Nielsen, S.G., Prytulak, J., Halliday, A.N. 2011. Geostand. Geoanaly. Res. 35, 293-306. [7] Prytulak, J., Nielsen, S.G., Halliday, A.N. 2011. Geostand. Geoanaly. Res, 35, 307-318.

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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