The Othris peridotite massif, Greece, shows conflicting evidence for a mid-ocean ridge and supra-subduction zone tectonic setting with the presence of plagioclase peridotite that may represent an area of either incomplete melt extraction, or melt impregnation and accumulation. To address these problems we focus on a 3 km continuous section in the Fournos Kaïtsa area, consisting of layers of harzburgite, plagioclase harzburgite and plagioclase lherzolite with accurately known structural and petrographic control. Refractory, Cr-rich spinel compositions and light rare earth element depleted clinopyroxenes in the harzburgites are consistent with ∼15% dry partial melting. Simple batch and fractional melting models are not sufficient to explain the composition of the residual phases and a multistage model with some melting in the garnet stability field is proposed. The pyroxenes from the plagioclase peridotites have higher Ti and rare earth element contents than those from the harzburgites, but similar refractory spinel compositions in both rock types indicate that the plagioclase peridotites may be products of impregnation of harzburgites with a fractionating melt. These observations are in good agreement with previous structural studies and suggest that the moderately depleted Fournos Kaïtsa mantle section most probably originated at a slow-spreading mid-ocean ridge.

We present the results of a structural and petrological study of mantle rocks from the strongly dismembered Othris Ophiolite. Part of the mantle section was impregnated with melt, crystallizing plagioclase and clinopyroxene as cumulate phases and refertilizing previously depleted peridotites. Melt impregnation occurred late in the deformation history of the host peridotites. The deformation took place at stresses of 13–26 MPa and at temperatures around 1000–1200°C, at the base of the thermal lithosphere. The melt therefore impregnated relatively cold mantle rocks, implying that the thermal lithosphere reached into the mantle during magmatic activity. We conclude that the Othris Ophiolite represents a spreading environment with a relatively thick lithosphere, such as that near an axial discontinuity or transform fault of a slow-spreading ridge. The proposed magmatic and deformation history of the peridotites is in agreement with episodic magmatism at slow-spreading ridges. We thus conclude that the heterogeneous character of the mantle section of the Othris Ophiolite results from melt impregnation processes. We suggest that the presence of lherzolitic ophiolite types among harzburgitic ophiolite types in the Hellenic–Dinaric chain reflects variable degrees of melt impregnation and refertilization rather than partial melting and melt extraction.

An unusual late Neoproterozoic ( c . 572 Ma) ophiolite is exposed in the Dariv Range (western Mongolia), which contains intermediate to acidic lavas and sheeted dykes, and an igneous layered complex consisting of gabbro–norites, websterites, orthopyroxenites and dunites underlain by serpentinized mantle harzburgites. Based on the compositions of the crustal units and the crystallization sequences in the mafic and ultramafic cumulates we conclude that the entire oceanic crust, including the cumulates, was made from arc magmas with boninitic characteristics. The Dariv rocks bear a strong resemblance to rocks recovered from the modern Izu–Bonin–Mariana fore-arc, a fragment of proto-arc oceanic basement, and we propose that the Dariv Ophiolite originated in a similar tectonic setting. A metamorphic complex consisting of amphibolite- to granulite-facies metasedimentary and meta-igneous rocks was thrust over the ophiolite. This metamorphic complex probably represents a Cambrian arc. Thrusting started before 514.7 ± 7.6 Ma as constrained by new sensitive high-resolution ion microprobe U–Pb zircon analyses from a syn- to post-tectonic diorite. The Dariv Ophiolite is a type-example of a proto-arc ophiolite, a special class of supra-subduction zone ophiolites.

Macquarie Island (Southern Ocean) is a fragment of Miocene ocean crust and upper mantle formed at a slow-spreading ridge system, uplifted and currently exposed above sea-level. The crustal rocks on the island have unusually enriched compositions and the strong signature of an enriched source requires low overall degrees of melt depletion in the underlying mantle. Peridotites on the island, however, are highly refractory harzburgites that can be modeled as residues of >20–25% of near-fractional melting from which all the free clinopyroxene was melted out. The peridotites have some of the highest spinel Cr-numbers (0·40–0·49) and lowest orthopyroxene-core Al₂O₃ concentrations (2·7–3·0 wt %) reported so far for oceanic peridotites. The peridotites were subsequently modified by melt–rock reactions underneath the Miocene ridge system. The refractory character of the peridotites is inconsistent with the slow-spreading ridge setting as well as with the enriched character of the overlying crust, and must indicate a previous depletion event; the peridotites are not the source residue of the overlying ocean crust on Macquarie Island. Osmium isotopic compositions of peridotite samples are very unradiogenic (¹⁸⁷Os/¹⁸⁸Os = 0·1194–0·1229) compared with normal abyssal peridotites and indicate a long-lived rhenium depletion. Proterozoic rhenium-depletion ages indicate that these rocks have preserved a memory of an old mantle melting event. We argue that the Macquarie Island harzburgites are samples from an anciently depleted refractory mantle reservoir that may be globally important, but that is generally overlooked because of its sterility; that is, its inability to produce basalts. This reservoir may preserve key information about the history of the Earth’s mantle as a whole.