caa a22 4500 465790771 CHVBK 20180323112023.0 cr unu---uuuuu 170327e19900101xx s 000 0 eng 10.1007/BF00310646 doi (NATIONALLICENCE)springer-10.1007/BF00310646 Ferric iron in mantle-derived garnets [Elektronische Daten] Implications for thermobarometry and for the oxidation state of the mantle [Robert Luth, David Virgo, Francis Boyd, Bernard Wood] The oxidation state of a mantle assemblage may be defined by heterogeneous reactions between oxygen and iron-bearing minerals. In spinel lherzolites, the presence of Fe3+ in spinel allows use of the assemblage olivine-orthopyroxene-spinel to define f O 2 at fixed T and P. As a first step towards establishing an analogous reaction for garnet lherzolites, garnets from mantle-derived xenoliths from South Africa and the USSR have been analyzed with 57Fe Mössbauer spectroscopy at 298 and 77K to determine Fe3+/Fe2+ and the coordination state of iron. Garnets from South African alkremites (pyrope+Mg-spinel) and eclogites, as well as garnet-spinel and low-temperature garnet lherzolites from both South Afica and the USSR, have Fe3+/ΣFe<0.07. In contrast, garnets from high-temperature garnet lherzolites from within the Kaapvaal craton of South Africa have Fe3+/ΣFe>0.10. Ferric iron is octahedrally coordinated, and ferrous iron is present in the dodecahedral site in all samples. The occurrence of significant Fe3+ in these garnets necessitates caution in the use of geothermometers and geobarometers that are applied to mantle samples. For example, the presence of ∼12% of the Fe as Fe3+ in garnets can increase temperatures calculated from existing Fe/Mg geothermometers by>200°C. The concomitant increase in pressures calculated from geobarometers that use the Al content in orthopyroxene coexisting with garnet are ∼10-15 kbar. Results of calculations based on heterogeneous equilibria between garnet, olivine, and pyroxene are consistent with the derivation of the peridotite samples from source regions that are relatively oxidized, between the f O 2 of the FMQ (quartz-fayalite-magnetite) buffer and that of the WM buffer. No samples yield values of f O 2 as reduced as IW (iron-wüstite buffer). Springer-Verlag, 1990 Luth Robert Bayerisches Geoinstitut, Universität Bayreuth, Postfach 101251, D-8580, Bayreuth, Federal Republic of Germany aut Virgo David Geophysical Laboratory, Carnegie Institution of Washington, 2801 Upton Street, NW, 20008, Washington, D.C., USA aut Boyd Francis Geophysical Laboratory, Carnegie Institution of Washington, 2801 Upton Street, NW, 20008, Washington, D.C., USA aut Wood Bernard Department of Geological Sciences, Northwestern University, 60208, Evanston, IL, USA aut Contributions to Mineralogy and Petrology Springer-Verlag 104/1(1990-01-01), 56-72 0010-7999 104:1<56 1990 104 410 https://doi.org/10.1007/BF00310646 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/BF00310646 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Luth Robert Bayerisches Geoinstitut, Universität Bayreuth, Postfach 101251, D-8580, Bayreuth, Federal Republic of Germany aut NATIONALLICENCE 700 1- Virgo David Geophysical Laboratory, Carnegie Institution of Washington, 2801 Upton Street, NW, 20008, Washington, D.C., USA aut NATIONALLICENCE 700 1- Boyd Francis Geophysical Laboratory, Carnegie Institution of Washington, 2801 Upton Street, NW, 20008, Washington, D.C., USA aut NATIONALLICENCE 700 1- Wood Bernard Department of Geological Sciences, Northwestern University, 60208, Evanston, IL, USA aut NATIONALLICENCE 773 0- Contributions to Mineralogy and Petrology Springer-Verlag 104/1(1990-01-01), 56-72 0010-7999 104:1<56 1990 104 410 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer