Computer Science
Scientific paper
Jul 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988gecoa..52.1781v&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 52, Issue 7, pp.1781-1794
Computer Science
18
Scientific paper
Ilmenite samples from four kimberlite localities were studied using electrochemical, Mössbauer spectroscopic, and microprobe analytical techniques in order to infer the oxidation state of their source regions in the mantle. The values of Fe 3+ / Fe calculated from analyses, using three different electron microanalytical instruments assuming ilmenite stoichiometry, are consistently higher than those derived from the Mössbauer data, by as much as 100%. Furthermore, the range in Fe 3+ / Fe calculated using the analyses from different instruments and/or different correction schemes is nearly as large. Thus Fe 3+ / Fe calculated from microprobe analyses should be taken with caution, even if the precision appears high. An yttria-doped zirconia, double-cell electrochemical apparatus was used to measure the oxygen fugacity ( f O 2 ) of ilmenite as a function of temperature. Samples that contain carbon show irreversible, "auto-oxidation" trends of sharply increasing f O 2 with increasing temperature upon initial heating. The one sample that lacked C did not "autooxidize". 57 Fe Mössbauer spectroscopy on the electrochemical experiment run products demonstrates that Fe 3+ / Fe is significantly lower than it is for the natural C-bearing ilmenites. In contrast, the ilmenite that lacked C did not change Fe 3+ / Fe during the electrochemical experiment. Examination of the reduced samples with SEM established that the natural, single-phase ilmenites exsolved during the electrochemical experiment to form ilmenite ss , + spinel ss . Reduction-exsolution was caused by reaction with C, which shifted the bulk composition from the stability field of ilmenite ss along an oxygen reaction line into the two-phase stability field of ilmenite ss + spinel ss that lies between the ilmenite-hematite and the ulvöspinel-magnetite joins in the TiO 2 -(Fe,Mg)O-(Fe,Al) 2 O 3 system. The initial, reduced trends in the electrochemical experiments for the C-bearing ilmenites are attributed to disequilibrium interactions between the decomposing sample and the evolved gas in the electrochemical cell and do not represent the quenched "mantle memory" nor the intrinsic f o 2 of the sample prior to reduction. Futhermore, the oxidized f o 2 trend is interpreted, for the carbon-bearing samples, as representing the f o 2 of the ilmenite ss + spinel ss assemblage and not the intrinsic f o 2 of the mantle-derived ilmenite ss .
Luth Robert W.
Moats Mark A.
Ulmer Gene C.
Virgo David
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