ORIGINAL PAPER Deep solid-state equilibration and deep melting of plagioclase-free spinel peridotite from the slow-spreading Mid-Atlantic Ridge, ODP Leg 153 Thomas M. Will & Esther Schmädicke & Hartwig E. Frimmel Received: 10 March 2010 / Accepted: 1 September 2010 # Springer-Verlag 2010 Abstract A petrological investigation of abyssal, plagioclase-free spinel peridotite drilled during ODP cruise 153 in the North Atlantic revealed that the peridotite represent refractory, partial residual mantle material that experienced depletion of incompatible trace elements during upper mantle melting. The degree of partial melting as estimated from spinel compositions was c. 12%. Fractionated middle and heavy rare earth elements imply polybaric melting, with c. 1–4% initial melting in the garnet peridotite stability field and subsequent partial melting of ~7–10% in the spinel peridotite stability field. Geothermo- barometric investigations revealed that the solid-state equilibration of the spinel peridotite occurred at some 1,100–1,150°C and c. 20–23 kbar, corresponding to an equilibration depth of c. 70±5 km and an unusually low thermal gradient of some 11–17°C/km. A thermal re- equilibration of the peridotite occurred at ~850–1,000°C at similar depths. Naturally, the initial mantle melting in the garnet-peridotite stability field must have commenced at depths greater than 70±5 km. It is likely that the residual peridotite rose rapidly through the lithospheric cap towards the ridge axis. The exhumation of the abyssal peridotite occurred, at least in parts, via extensional detachment faulting. Given the shallow to moderate dip angles of the fault surfaces, the exhumation of the peridotite from its equilibration depth would imply an overall ridge-normal horizontal displacement of c. 50–160 km if tectonic stretching and detachment faulting were the sole exhuma- tion mechanism. Introduction Serpentinised peridotite directly exposed on the seafloor is known through dredging and drilling from several loca- tions. Examples include sections of the Mid-Atlantic Ridge (e.g. Miyashiro et al. 1969; Bonatti and Honnorez 1976; Tamura et al. 2008), the Gakkel Ridge in the Arctic Ocean (e.g. Michael et al. 2003) and the Central and Southwest Indian Ridge (e.g. Kimball et al. 1985; Dick 1989). In addition to the seafloor exposures some ophiolite com- plexes, for example, in the western Alps and Ligurian province (e.g. Lagabrielle and Cannat 1990; Rampone et al. 1995) or on the island of Zabargad in the Red Sea (e.g. Bonatti et al. 1986) provide further insights into mantle- derived peridotite that was accreted to the seafloor. Typically, these occurrences are associated with oceanic fracture zones in the close vicinity of slow-spreading ridges (with full spreading rates less than c. 5 cm/yr). Abyssal peridotite from or near slow-spreading ridge segments was recovered during several Ocean Drilling Program (ODP) expeditions (e.g. Legs 153, 209, 304/305). In this paper we focus on plagioclase-free spinel-bearing abyssal peridotite that was drilled along the Mid-Atlantic Ridge in the vicinity of the Kane Fracture Zone (‘MARK area’, 23°N; full spreading rate 2.5 cm/yr, Kong et al. 1988) during Leg 153. The primary aim of drilling Leg 153 was to characterise the variation of the mantle composition, melt migration Editorial handling: J. Raith T. M. Will (*) : H. E. Frimmel Geodynamics and Geomaterials Research Division, University of Würzburg, Am Hubland, 97074 Würzburg, Germany e-mail: thomas.will@uni-wuerzburg.de E. Schmädicke GeoZentrum Nordbayern, University of Erlangen-Nürnberg, Schlossgarten 5a, 91054 Erlangen, Germany Miner Petrol DOI 10.1007/s00710-010-0125-7