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SUMMARY:Constraints on mantle geochemistry and planetary differentiation d
 erived from Fe isotopes - Helen Williams\, University of Oxford
DTSTART:20091013T153000Z
DTEND:20091013T163000Z
UID:TALK19562@talks.cam.ac.uk
CONTACT:John Maclennan
DESCRIPTION:Recent advances in mass spectrometry (specifically multi-colle
 ctor inductively coupled plasma mass spectrometry\; MC-ICPMS) allow for pr
 ecise measurements of the stable isotope compositions of “heavy” eleme
 nts such as the transition metals.  In this talk I will present variations
  in the iron isotope compositions of igneous rocks\, meteorites and high-p
 ressure experiments and discuss the significance of these results with res
 pect to mantle geochemistry and the differentiation of the terrestrial pla
 nets.\n\nPart i) Iron isotopes in mantle peridotites and eclogites\n\nLarg
 e variations exist in the iron isotope compositions (δ57/54Fe\, deviation
  in parts per 1000 with respect to the IRMM-14 pure Fe standard) of mantle
  peridotites from different tectonic settings (0.9‰) although the range 
 in the δ57/54Fe values of MORB and OIB is comparatively small (~0.1‰\; 
 average MORB 0.14 ±0.06 ‰). Mineral separates prepared from the same m
 antle peridotites and pyroxenites show a surprisingly large range in δ57/
 54Fe (olivines 0.6‰\, clinopyroxenes 0.9‰ and orthopyroxenes 0.8‰.)\
 , with spinels showing the greatest total variation of 1.7‰.  There are 
 positive correlations between the δ57/54Fe values of coexisting orthopyro
 xene\, clinopyroxene and olivine\, strongly suggesting that the δ57/54Fe 
 values of these minerals reflect intra-sample mineral-mineral isotopic equ
 ilibrium. Bulk-rock\, clinopyroxene and spinel δ57/54Fe values correlate 
 with chemical indices of both melt extraction and oxidation.  Coupled with
  simple models\, these data suggest that iron isotope fractionation takes 
 place during spinel-facies partial melting\, with the residue becoming iso
 topically light relative to the melt and to the initial source region. \n\
 nThe extent to which Fe isotopes might fractionate during partial melting 
 and melt-rock reaction in the garnet stability field was investigated in a
  follow-up study of fresh eclogite xenoliths originating from South Africa
 .  Surprisingly large isotopic variations were observed.  The δ57/54Fe va
 lues of the eclogite garnets analysed range between –0.41 and 0.61 ‰ w
 hereas those of co-existing pyroxenes range from –0.21 to 0.57 ‰.  Cal
 culated bulk rock δ57/54Fe values range from -0.59 to 0.33 ‰\, and enco
 mpass a total variation of 0.92 ‰. Oxygen isotope measurements were also
  carried out on these samples and positive correlations between both garne
 t and bulk sample δ18O and δ57Fe are present.  These correlations sugges
 t that both Fe and O isotopes are fractionated by the same underlying proc
 ess in this sample suite.  Comparisons of these data with published data f
 or altered and fresh MORB and for samples from the sheeted dike part of th
 e ODP 504B drill core suggest that these co-variations in Fe and O isotope
 s were not inherited from oceanic crust protoliths. However\, we observed 
 that the samples with the isotopically lightest δ18O and δ57Fe values ar
 e also enriched in Sc and Cr\, elements which become concentrated in the r
 esidues of melting. We therefore suggest that Fe and O isotopes are fracti
 onated by either partial melting or by melt-rock reaction processes\, whic
 h could take place either in the lithospheric mantle or in the downgoing s
 lab. \n\nPart ii) Iron isotopes in iron meteorites and high pressure exper
 iments\n\nMagmatic iron meteorites are considered to be remnants of the me
 tallic cores of differentiated asteroids\, and may be used as analogues of
  planetary core formation.  Sulphur is believed to be a significant light 
 element in the cores of iron meteorite parent bodies and the terrestrial p
 lanets\, although its exact abundance remains unknown.  We have found evid
 ence for significant equilibrium Fe isotope fractionation (~0.26 ‰/amu)
  between metal and troilite (FeS) in iron meteorites. Coupled with publish
 ed data for pallasites\, which constrains the magnitude of metal-silicate 
 Fe isotope fractionation\, Fe isotopes may be used to provide information 
 about the S contents of cores of the iron meteorite parent bodies and the 
 terrestrial planets. However\, it is not yet clear if the observed fractio
 nations can be extrapolated to the pressure and temperature conditions of 
 planetary core formation. In order to test this we are currently investiga
 ting Fe isotope fractionation between silicate melt and liquid Fe-S alloys
  and between liquid iron and basaltic melt using high-pressure experiments
 . \n\nWeyer\, S. Anbar\, A.\,G.P. Brey\, G.\, Munker\, C\, Mezger\, K.  an
 d Woodland\, A.B. Iron isotope fractionation during planetary differentiat
 ion\, Earth and Planetary Science Letters 240(2)\, 251-264\, 2005.\n\nWill
 iams\, H.M.\, Nielsen\, S.G.\, Renac\, C\, Griffin\, W.L.\, O’Reilly\, S
 .Y.\, McCammon\, C.\, Pearson\, N.\, Viljoen F\, Alt\, J.C. (2009) Fractio
 nation of oxygen and iron isotopes in the mantle: implications for crustal
  recycling and the source regions of oceanic basalts. Earth and Planetary 
 Science Letters\, 283 (1-4): 156-166.\nWilliams\, H. M.\, Markowski\, A.\,
  Quitté\, G\, Halliday\, A. N.\, Teutch\, N. and Levasseur\, S. (2006): F
 e isotope fractionation in iron meteorites: New insights into metal-sulphi
 de segregation and planetary accretion. Earth andPlanetary Science Letters
 \, 250\, 486–500\nWilliams\, H. M.\, Peslier\, A.\, McCammon\, C.\, Hall
 iday\, A.N. Teutch\, N.\, Levasseur\, S.\, and Burg\, J.-P. (2005):Iron is
 otope fractionation in mantle minerals\, partial melting and mantle oxygen
  fugacity. Earth and PlanetaryScience Letters\, 235\, 435-452\nWilliams\, 
 H. M.\, McCammon\, C.\, Peslier\, A.\, Halliday\, A.N. Teutsch\, N. Levass
 eur\, S.\, and Burg\, J.-P. (2004):Iron Isotope Fractionation and the Oxyg
 en Fugacity of the Mantle. Science\, 304\, 1656-1659.
LOCATION:Tilley Lecture Theater\, Department of Earth Sciences
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