Major and Trace Elements

Analytical method

Major and trace elements

Major elements of minerals were determined with a CAMECA SX51 electron microprobe (EPMA) at the Institute of Geology and Geophysics (IGG), Chinese Academy of Sciences. Analyses were performed using a 15 kV accelerating voltage and a 20 nA current beam with electron beam size of 2 μm in diameter. Natural and synthetic glasses were used for standard calibration and all matrix corrections were made using the CAMECA PAP correction program (Pouchou and Pichoir 1991). The results are given in Table 1. Trace element abundances of clinopyroxene separates were determined with ICP-MS (VG PQ2) at the IGG. Samples were dissolved in distilled HF+HNO3 in 15 ml Savillex Teflon screw-cap beakers at 120°C, dried and then re-dissovled in 50 mL 0.5M HCl for analysis. Indium was used as an internal standard to correct for matrix effects and instrument drift. National Silicate reference material GSR-3 was analyzed to check the reproducibility and accuracy during the analyses. The precision is generally within 5% for the elements given in Appendix B. The total procedural blank was  50 ng for all trace elements.

Thermobarometry and oxygen fugacity calculation

Equilibration temperature for the Hannuoba peridotites is estimated by different thermometers (Wells 1977 (TWells); Sachtleben and Seck 1981 (TSS); Brey and Köhler 1990 (TBK1), (TBK2)) in order to avoid systematical errors within a single thermometer.

Oxygen fugacity is another important parameter for upper mantle and calculated using two independent calibrations: Ballhaus et al. (1991) and Wood (1990), which is based on the reaction of Fe2SiO4 + O2 = 3Fe2Si2O6 + 2Fe3O4. These two oxygen barometers are identical in handling Cr-A1 correction and temperature and pressure dependence relative to the fayalite-magnetite-quartz (FMQ) buffer (O’Neill and Wall 1987) and give results within 0.3 to 0.4 log units of each other. The compositional parameter that is most sensitive to oxygen fugacity is the Fe2+ and Fe3+ in spinel. Ballhaus et al. (1991) concluded that the difference of calculated (Δlogƒo2)FMQ between the Fe3+ calculated from electron microprobe assuming that spinel obey perfect RO : R2O3 (1:1) and Fe3+ determined by Mössbauer spectroscopy are less than 0.4 log units and the difference are random. The barometer of Ballaus Ballhauset al. (1991) is also insensitive to temperature within analytical error if spinel is equilibrated along ƒo2-T paths parallel to FMQ although it is slightly dependent on pressure. A 3-Kbar pressure difference gives  0.1 unit variation in calculated (Δlogƒo2)FMQ. Therefore, the Fe2+ and Fe3+ in spinel calculated from electron microprobe assuming perfect crystal stoichiometry and the algorithm of Ballaus Ballhaus et al. (1991) and Wood (1990) in combination with the equilibrium temperatures and a pressure of 1.5 GPa have been used in the oxygen fugacity calculation.

Appendix B: Trace element analysis of GSR-3 Standard by ICP-MS

n: number of analyses; Meas.: measured value; Ref.: reference value; R.: relative difference between measured and recommended values.

References：

Ballaus Ballhaus C, Berry RF, Green DH (1991) High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobatometer: implications for the oxidation-state of the upper mantle. Contrib Mineral Petrol 107:27–40

Brey GP, Köhler T (1990) Geothermobarometry in four-phase lherzolites II: new thermobarometers, and practical assessment of existing thermo barometers. J Petrol 31:1353–1378

O’Neill HS, Wall VJ (1987) The olivine orthopyroxene spinel oxygen geobarometer, the nickel precipitation curve, and the oxygen fugacity of the Earth’s upper mantle. J Petrol 28:1169–1191

Pouchou JL, Pichoir F (1991) Quantitative analysis of homogeneous or stratified microvolumes applying the model "PAP". In: Heinrich KFJ, Newbury DE (eds.) Electron Probe Quantification Plenum, New York, pp 31–75

SachtlebenT, Seck HA (1981) Chemical control of Al-solubility in orthopyroxene and its implocations on pyroxenegeothermomete. Contrib Mineral Petrol 78:157–165

Wood BJ (1990) An experimental test of the spinel peridotite oxygen barometer. J Geophys Res 95:15845–15851

Wells PRA (1977) Pyroxene thermometry in simple and complex systems. Contrib Mineral Petrol 62:129–139