eclogites. The raised prevalence of diamonds in eclogites compared to peridotites may, therefore, reflect more effective scavenging of carbon by melts in these rocks. The ferric iron contents of monomineralic layers of clinopyroxene and garnet contained in the same experiments were also measured using Mössbauer spectroscopy. A preliminary model was derived for determining the fO 2 of eclogitic rocks from the compositions of garnet and clinopyroxene, including the Fe 3+ /ΣFe ratio of garnet, using the equilibrium,The model, which reproduces the independently determined fO 2 of the experimental data to within 0.5 log units, can be used to estimate the fO 2 of ultrahigh-pressure metamorphic eclogites and cratonic eclogitic xenoliths. Although there are very few analyses of garnet Fe 3+ /ΣFe ratios from eclogite samples, the range in fO 2 recorded by available eclogitic xenoliths is similar to that reported for peridotitic xenoliths and generally within the graphite/diamond stability field. Estimates for the average bulk Fe 3+ / ΣFe ratio of modern basaltic oceanic crust, however, are higher than the values for most of these xenoliths, and upon subduction, crustal carbon is likely to remain in the carbonate stability field to depths of at least 250 km. If eclogite xenoliths originated from subducted oceanic crust, then their generally lower fO 2 most likely reflects either lower initial basaltic Fe 3+ /ΣFe ratios, loss of Fe 2 O 3 through partial melting or the initial presence of organic carbon. AbstractThe oxygen fugacity (fO 2 ) at which carbonatebearing melts are reduced to either graphite or diamond in synthetic eclogite compositions has been measured in multi-anvil experiments performed at pressures between 3 and 7 GPa and temperatures between 800 and 1,300 °C using iron-iridium and iron-platinum alloys as sliding redox sensors. The determined oxygen fugacities buffered by the coexistence of elemental carbon and carbonatebearing melt are approximately 1 log unit below thermodynamic calculations for a similar redox buffering equilibrium involving only solid phases. The measured oxygen fugacities normalized to the fayalite-magnetite-quartz oxygen buffer decrease with temperature from ~−0.8 to ~−1.7 log units at 3 GPa, most likely as a result of increasing dilution of the carbonate liquid with silicate. The normalized fO 2 values also decrease with pressure and show a similar decrease with temperature at 6 GPa from ~−1.5 log units at 1,100 °C to ~−2.4 log units at 1,300 °C. In contrast to previous arguments, the stability field of the carbonatebearing melt extends to lower oxygen fugacity in eclogite rocks than in peridotite rocks, which implies a wider range of conditions over which carbon remains mobile in natural Communicated by Chris Ballhaus. Electronic supplementary materialThe online version of this article (

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The Pabdeh Formation is part of a thick carbonate-siliciclastic succession in the Zagros Basin of SW Iran which includes carbonate reservoirs of Cretaceous and Cenozoic ages. From field observations and petrographic and facies analysis of exposures in the type section of the Pabdeh Formation, four lithofacies were recognized. These are from oldest to youngest: (i) a mottled, bioturbated bioclastic wackestone/mudstone facies; (ii) a wackestone/packstone facies with horizontal burrows on bedding planes; (iii) a thin-bedded bioclastic wackestone/mudstone facies alternating with thin bioclastic-oolitic-intraclastic intervals; and (iv) a bioclastic foraminiferal / algal / peloidal packstone facies. These observations indicate that facies evolved upwards from deep outer-ramp deposits to inner-ramp deposits within a shoal complex, suggesting progradation of the ramp depositional system.Storm events significantly influenced the ramp system. Storm-generated surges transported sediments from nearshore to the deeper outer-ramp environment where they were deposited as shell-lags, composed mostly of bioclastic packstones, rich in pelagic microfauna with sharp, undulatory erosional basal contacts. The packstones rest on outer ramp mudstones deposited below storm base level. Sedimentary structures in the Pabdeh Formation are those typical of storm deposits, such as hummocky cross-stratification, ripple cross-lamination, ripple marks, escape burrows on the tops of the beds, couplets of fine-and coarse-grained laminae and mixed fauna, as well as intraclasts derived from underlying facies. These distinctive sequences are interpreted to have been generated by waning storm-generated currents. The dominance of fine-grained sediments (medium to fine sand); the lack of large-scale hummocky cross-stratification; the minor amounts of intraclasts derived from underlying facies; the paucity of amalgamated tempestite beds; and the finely-laminated (mm to cm scale) couplets of coarse and fine lamina all suggest a distal tempestite facies. Palaeogeographic reconstruction of the Zagros Basin during the Eocene indicates that the study area was situated in tropical, storm-dominated palaeolatitudes.