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SUMMARY:Stable vanadium isotope fractionation at high temperatures: a prox
 y for oxygen fugacity?  - Julie Prytulak\, Imperial College
DTSTART:20121113T163000Z
DTEND:20121113T173000Z
UID:TALK38987@talks.cam.ac.uk
CONTACT:John Maclennan
DESCRIPTION:Understanding and quantifying oxygen fugacity (fO2) is critica
 l to our concepts of mineral partitioning\, fluid speciation\, and the nat
 ure and causes of mantle melting.  Furthermore\, the secular evolution of 
 mantle fO2 may be directly related to the rise of atmospheric oxygen and o
 ur planets ability to sustain life.\n\nThe established view holds that man
 tle fO2 is tectonically associated\, with subduction zones ‘more oxidizi
 ng’ than the ambient upper mantle source of mid-ocean ridge basalts [e.g
 .\, 1]. Routine determination of fO2 uses the oxidation state of iron.  Ho
 wever\, 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 mantl
 e source fO2.  The argument that the oxidation state of iron reflects late
 r events is largely based on conflicting evidence yielded by the transitio
 n metal element vanadium (V) [e.g.\, 2\, 3].  Vanadium is well-suited to i
 nvestigations of oxidation-reduction processes as it exists in five valenc
 e states.  Indeed\, the partitioning of V is strongly fO2 dependent [e.g.\
 , 4].  However\, uncertainties arise from models based on vanadium abundan
 ces or trace element ratios (e.g.\, V/Sc).  This is due to unknowns such a
 s the source abundance of vanadium\, the degree of melting\, and the fract
 ionating phases. Taking uncertainties into account using Fe or V\, mantle 
 fO2 cannot be confidently determined to better than 2 log units [5].  T
 his is far too gross a scale to be useful for the detailed study of mantle
  melting\, secular evolution\, and fluid speciation.\n\nThe past decade ha
 s seen a major proliferation in the exploration of so-called ‘non-tradit
 ional’ stable isotope systems.  This is particularly exciting for igneou
 s geochemists in that these new systems offer very different insight to hi
 gh temperature processes then we have thus far been able to deduce from ra
 diogenic isotopes and trace element ratios.  Instead of tracking the amoun
 t of sediment in a mantle source\, for example\, we may now be able to det
 ermine physical conditions such as fO2 using isotope geochemistry. \n\nHer
 e I will discuss recent advances in the measurement and use of stable vana
 dium isotopes [6\, 7]. I will present and explore the first (and only) pre
 cise and accurate stable V isotope data for peridotites and igneous suites
  including the Mariana arc and Hekla volcano in Iceland. I shall discuss t
 he exciting potential of this isotope system in the context of the surpris
 ingly large and resolvable isotope variations seen at high temperatures.\n
 \n[1] Wood\, B.J.\, Bryndzia\, L.T.\, Johnson\, K.E. 1990.  Science\, 248\
 , 337-345.\n[2] Lee\, C-T.\, Brandon\, A.D.\, Norman\, M. 2003. Geochim. C
 osmchim. Acta. 67\, 3045-3064.\n[3] Lee\, C-T.\, Leeman\, W.P.\, Canil\, D
 .\, Li\, Z-X. A. 2005. J. Pet. 46\, 2313-2336.\n[4] Canil\, D. 1997. Natur
 e\, 389\, 842-845.\n[5] Frost\, D.J.\, McCammon\, C.A. 2008. Ann. Rev. Ear
 th Sci. 36\, 389-420.\n[6] Nielsen\, S.G.\, Prytulak\, J.\, Halliday\, A.N
 . 2011. Geostand. Geoanaly. Res. 35\, 293-306.\n[7] Prytulak\, J.\, Nielse
 n\, S.G.\, Halliday\, A.N. 2011. Geostand. Geoanaly. Res\, 35\, 307-318.
LOCATION:Harker 1 seminar room\, Department of Earth Sciences
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