Eur. J. Mineral. 1996, 8, 153-163 Oxygen thermobarometry of the Semail harzburgite massif, Oman and United Arab Emirates SOBHI NASIR Department of Geology, United Arab Emirates University, P. O. Box 17551, Al Ain, U.A.E. Abstract: Mossbauer spectra for 15 spinels separated from sub-oceanic, upper-mantle tectonic peridotites from the Ibra, Hatta and Dibba areas, Semail massif, were measured in an effort to determine their Fe + contents. The determined Fe 3+ fractions were used to calculate oxygen fugacity. The average Δ fo2 (relative to FMQ) is -1.11 ± 0.4 log-units. This is in agreement with that reported for abyssal peridotites (-0.9 ± 0.7 log units relative to FMQ) and the orogenic massif of Ronda (-1.1 ±0.7 relative to FMQ), and in contrast to the oxidized continental xenolith (+0.24 ± 0.5 relative to FMQ). Key-words: Semail massif, Oman, United Arab Emirates, oxygen fugacity, thermobarometry. Introduction Mantle oxygen fugacity has been the subject of many recent studies. It has been determined either by direct intrinsic measurement on upper mantle spinel peridotites and megacrysts (UΓ mer et al, 1987; Arculus & Delano, 1981; Arculus et al, 1984) or through thermobarometric calcula- tions based on the mineral assemblages olivine- orthopyroxene-spinel (Mattioli & Wood 1986, 1988; O'Neill & Wall, 1987; Mattioli et al, 1989; Bryndzia & Wood, 1990; Wood et al, 1990; Ballhaus et al, 1991; Luth & Canil, 1993), olivine-orthopyroxene-garnet (Luth et al, 1990), olivine-clinopyroxene-orthopyroxene (Luth & Canil, 1993). Results obtained from the intrinsic measurements have suggested that the upper mantle is reduced and has fo2-values near the iron-wiistite buffer {e.g. Ulmer et al, 1987). However, these measurements are now known to be artifacts of the intrinsic fo2 method {e.g. Virgo et al, 1988). The majority of thermobarometric studies indicated that fo2 lies between the fay- alite-magnetite-quartz (FMQ) and the wiistite- magnetite (WM) oxygen buffers (Wood & Virgo, 1989; Canil et al, 1990; Haggerty, 1990; Wood et al, 1990; Daniels & Gurney, 1991; Ionov & Wood, 1992; Nasir et al, 1992, 1993). Several studies have demonstrated significant differences in fo2 as a function of tectonic environment (Mattioli et al, 1989; Bryndzia & Wood, 1990; Wood et al, 1990; Canil et al, 1994). Results from spinel peridotites indicate that the sub- oceanic mantle is somewhat more reduced than the subcontinental mantle (Bryndzia & Wood, 1990; Wood et al, 1990; Woodland et al, 1992). These values are in excellent agreement with fo2 estimates obtained from the quenched rims of mid-ocean ridge basalt (MORB) pillow basalts (Christie et al, 1986). 0935-1221/96/0008-0153 $ 2.75 > 1996 E. Schweizerbart'sche Verlagsbuchhandlung. D-70176 Stuttgart