Application of Fe K-edge XANES determinations of Fe3+/∑Fe in garnet to peridotite xenoliths from the

Udachnaya KimberliteG.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, UK3Institut für Geowissenschaften, Universität Frankfurt, Frankfurt/M, Germany

4ARC Centre of Excellence in Ore Deposits, University of Tasmania, Hobart TAS 7001, Australia5Australian Synchrotron, Clayton VIC 3168, Australia

Udachnaya garnet peridotites xenoliths EGU 2012

Woodland & Koch (2003)

• 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)

EMOD/G: MgSiO3 + MgCO3 = Mg2SiO4 + C + O2enstatite magnesite olivine

McCammon et al. (2001)

ƒO2 influences many mantle processes EGU 2012

• Udachnaya kimberlites emplaced 360±7 Ma (U-Pb perovskite – 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)

Udachnaya East kimberlite and xenoliths 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)

Analytical techniques EGU 2012

Classification from Grütter et al. (2004)

• Garnets are almost all G9 = lherzolitic

• Ca-Cr well correlated

G0 = unclassified G1 = low-Cr megacrystsG3 = eclogitic G4 = pyroxeniticG5 = pyroxeniticG9 = lherzolitic G10 = harzburigiticG12 = wehrlitic

Garnet compositions 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

Mostly sheared textures

Ionov et al. (2010)

• Low T samples lie close to 40 mW/m2 cratonic geotherm, 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)

Thermobarometry EGU 2012

• Garnet and clinopyroxene chemistries indicate samples vary from depleted to strongly metasomatised

Griffin & Ryan (1995)

Depleted and enriched samples 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

Depleted and enriched layers EGU 2012

Fe K-edge XANES technique for determining Fe3+ in garnet (Berry et al. 2010)

7100 7120 7140 7160 7180Energy (eV)

Post-edge

Pre-edge

Fe K

-edg

e

7138.4 eV 7161.7 eVN

orm

alise

d in

tens

ity

Natural garnet standards (MB)From Diavik & Kaapvaal –

Fe3+/∑Fe from 0.022 to 0.122

Fe3+/∑Fe (Mössbauer)

Inte

nsity

Rati

o

Diavik + Kaapvaal standards

7138.4 eV 7161.7 eV

Fe3+/∑Fe (Udachnaya garnets) = 0.03 ± 0.01 – 0.14 ± 0.01

XANES technique and results EGU 2012

EMOG

Carbonate

EMOD

Graphite

Diamond

NiPC

• Overall decrease in ƒO2 with increasing depth, consistent with molar volume change of buffering reaction

Oxybarometry• ƒO2 calculated using T[TA98] and P[NG85] and Gudmundsson & Wood (1995) calibration of reaction 2Fe3

2+Fe23+Si3O12 = 2FeSiO3 + 4Fe2SiO4 + O2

skiagite in ga ferrosilite in opx fayalite in ol

• ΔlogƒO2[FMQ] = -2.5 ± 0.5 to -5.9 ± 0.5 log units

• Most samples from the enriched pressure interval are oxidised relative to the depleted samples

Enriched interval

Redox profile through Siberian Craton 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 xenolith suite(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.

Conclusions EGU 2012