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Udachnaya garnet peridotites xenoliths

Udachnaya garnet peridotites xenoliths. EGU 2012. Application of Fe K-edge XANES determinations of Fe 3+ /∑Fe in garnet to peridotite xenoliths from the Udachnaya Kimberlite.

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Udachnaya garnet peridotites xenoliths

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  1. Udachnaya garnet peridotites xenoliths EGU 2012 Application of Fe K-edge XANES determinations of Fe3+/∑Fe in garnet to peridotite xenoliths from the UdachnayaKimberlite G.M. Yaxley1, A.J. Berry1,2,A.B. Woodland3,V.S. Kamenetsky4, D. Paterson5, M.D. de Jong5 and D.L. Howard5 1Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia (greg.yaxley@anu.edu.au) 2Department of Earth Science and Engineering, Imperial College London, UK 3Institut fürGeowissenschaften, Universität Frankfurt, Frankfurt/M, Germany 4ARC Centre of Excellence in Ore Deposits, University of Tasmania, Hobart TAS 7001, Australia 5Australian Synchrotron, Clayton VIC 3168, Australia

  2. ƒO2 influences many mantle processes EGU 2012 McCammon et al. (2001) Woodland & Koch (2003) EMOD/G: MgSiO3 + MgCO3 = Mg2SiO4 + C + O2 enstatitemagnesite olivine • Understanding the variation in ƒO2 is essential for understanding diamond stability, • metasomatism and the nature of fluids and melts in the cratonic lithosphere and • elsewhere in the mantle • First application of Fe K-edge XANES to determination of Fe3+ in mantle garnets • (Berry et al. 2010)

  3. Udachnaya East kimberlite and xenoliths EGU 2012 • Udachnayakimberlites emplaced 360±7 Ma (U-Pbperovskite – Kinny et al. 1997) • 21 very fresh garnet peridotites presented as small fragments of xenoliths • or grain separates • Redox characteristics of Siberian Craton not currently well known Ashchepkov et al. (2010)

  4. Analytical techniques EGU 2012 Analytical techniques • Major and minor elements of mineral phases by WDS EPMA (University of Tasmania) • Trace elements by LA-ICPMS (ANU) • Fe3+ contents of garnets by Fe K-edge XANES on XFM beamline • (Australian Synchrotron, Melbourne)

  5. Garnet compositions EGU 2012 • Garnets are almost all G9 = lherzolitic • Ca-Cr well correlated G0 = unclassified G1 = low-Cr megacrysts G3 = eclogitic G4 = pyroxenitic G5 = pyroxenitic G9 = lherzolitic G10 = harzburigitic G12 = wehrlitic Classification from Grütter et al. (2004)

  6. Thermobarometry EGU 2012 • 2-pyroxene thermometry - Taylor (1998); • Brey & Köhler (1990) • Al-in-opx barometry – Nickel & Green (1985) • Nimis and Grütter (2010) • P[NG85] = 1.18 – 7.06 GPa • T[TA98] = 723 - 1364 °C Ionov et al. (2010) Mostly sheared textures • Low T samples lie close to 40 mW/m2cratonicgeotherm, with hotter, sheared • xenoliths deriving from P>5 GPa • In broad agreement with earlier studies of Boyd (1984), Boyd et al. (1997) • and Ionov et al. (2010)

  7. Depleted and enriched samples EGU 2012 Griffin & Ryan (1995) • Garnet and clinopyroxene chemistries indicate samples vary from depleted • to strongly metasomatised

  8. Depleted and enriched layers EGU 2012 • Depleted material is present throughout the entire sampled pressure • interval (2.6 – 7.1 GPa) • Enriched material derived nearly exclusively from narrower pressure interval • of ≈4.5 – 6.6 GPa

  9. XANES technique and results Diavik + Kaapvaal standards EGU 2012 Fe K-edge XANES technique for determining Fe3+ in garnet (Berry et al. 2010) 7138.4 eV 7161.7 eV 7138.4 eV 7161.7 eV Intensity Ratio Post-edge Fe K-edge Normalised intensity Fe3+/∑Fe (Mössbauer) Fe3+/∑Fe (Udachnaya garnets) = 0.03 ± 0.01 – 0.14 ± 0.01 Pre-edge Natural garnet standards (MB) From Diavik & Kaapvaal – Fe3+/∑Fe from 0.022to 0.122 7100 7120 7140 7160 7180 Energy (eV)

  10. Redox profile through Siberian Craton EGU 2012 • Overall decrease in ƒO2 with increasing • depth, consistent with molar volume • change of buffering reaction • Most samples from the enriched • pressure interval are oxidised relative • to the depleted samples EMOG Enriched interval Carbonate Graphite Diamond • Oxybarometry • ƒO2 calculated using T[TA98] and P[NG85] and Gudmundsson & Wood (1995) • calibration of reaction 2Fe32+Fe23+Si3O12 = 2FeSiO3 + 4Fe2SiO4 + O2 • skiagite in gaferrosilite in opxfayalite in ol • ΔlogƒO2[FMQ] = -2.5 ± 0.5 to -5.9 ± 0.5 log units EMOD NiPC

  11. Conclusions EGU 2012 Conclusions • ƒO2 in the cratonic mantle under Udachnaya decreases with increasing depth, • as noted in other cratons (e.g. Kaapvaal), consistent with molar volume change • of buffering reaction in garnet peridotite assemblages • Metasomatic enrichment in the 4.5 – 6.6 GPa interval under Udachnaya is associated • with an increase in ƒO2 of 1 – 2 log units, but this is insufficient to destabilise diamond • Metasomatism is associated with the high temperature sheared xenolithsuite(s) • and probably relates to deep low %, local melting fluxed by increased H2O activity • caused by oxidation of CH4-rich fluids influxing from the asthenosphere. • Similar depth-ƒO2-metasomatism profiles have been observed in the Kaapvaal • and Slave Cratons, although the depleted Siberian samples are somewhat lower in • oxygen fugacity, probably because they are more depleted. • Fe K-edge XANES provides a rapid, highly spatially resolved technique for • determining Fe3+ abundances in garnet with precision comparable to Mössbauer • Spectroscopy or Flank Method.

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