Thematic Articles

Ophiolites are a newly documented host of diamonds on Earth. Abundant diamonds have indeed been separated from peridotites and chromitites of ophiolites in China, Myanmar, and Russia. In addition, diamond grains have recently been discovered in chromite from the Cretaceous Luobusa ophiolite (Tibet) and the early Paleozoic Ray-Iz ophiolite (polar Urals, Russia). These diamonds are accompanied by a wide range of highly reduced minerals, such as Ni–Mn–Co alloys, Fe–Si and Fe–C phases, and moissanite (SiC); these have been found as either mineral separates or inclusions in diamonds and indicate growth under superreducing conditions. The diamond-bearing chromite grains likely formed near the mantle transition zone and were then brought to shallow levels in the upper mantle to form podiform chromitites in oceanic lithosphere. Because these diamond grains occur widely in peridotites and chromitites of many ophiolites, we refer to them as ophiolite-hosted diamonds. It is possible that such diamonds may be common in the upper oceanic mantle.

Volcanic glass from pillow lavas and hyaloclastites displays distinctive alteration textures that suggest the activity of boring microbes. Analogous textures are common in volcanic sections of the seafloor, in ophiolites, and in greenstone belts up to 3.5 Ga in age. While the origin of such trace fossils remains poorly understood, they offer much potential for investigating processes in the present-day, deep-ocean, crustal biosphere and their role in biogeochemical cycles.

Recent geological and geophysical surveys in the Izu-Bonin-Mariana forearc have revealed the occurrence on the seafl oor of oceanic crust generated in the initial stages of subduction and the earliest stage of island arc formation. The earliest magmatism after subduction initiation generated forearc basalts, and subsequently, boninitic and tholeiitic to calc-alkaline lavas were produced. Collectively, these rocks make up the extrusive sequence of the Izu-Bonin-Mariana forearc oceanic crust. This volcanic stratigraphy and its time-progressive development are analogous to those documented from many suprasubduction zone ophiolites. Most suprasubduction zone ophiolites may be on-land fragments of forearc oceanic crust, produced during the initiation of subduction and the early stages of island arc development.

The Oman–UAE ophiolite is the largest piece of oceanic crust exposed on land, yet debate continues about its origin. It has been variously considered as an ideal analogue for a fast-spreading mid-ocean ridge and as a typical suprasubduction zone ophiolite. A resolution to this conundrum comes from the recognition of at least two different phases of magmatism, with the second phase being most voluminous in the northern blocks of the ophiolite. The first phase was formed at an oceanic spreading centre; petrological and geochemical evidence clearly shows that the second phase was formed above a subduction zone.

Much of our understanding of ocean ridges has come from the collection and analysis of glasses recovered from ridge axes. However, applying the resulting methodologies to ophiolite complexes is not straightforward because ophiolites typically experience intense alteration during their passage from ridge to subduction zone to mountain belt. Instead, immobile element proxies for fractionation indices, alkalinity, mantle temperature, mantle flow and subduction addition may be used to classify ophiolite lavas and fingerprint the precise setting of the ridge at which an ophiolite formed. The results can help us recognise and interpret past spreading centres and so make plate tectonic reconstructions.

Ophiolites are suites of temporally and spatially associated ultramafic, mafic, and felsic rocks that are interpreted to be remnants of ancient oceanic crust and upper mantle. Ophiolites show significant variations in their internal structure, geochemical fingerprints, and emplacement mechanisms. These differences are controlled by (1) the proximity, when formed at the magmatic stage, to a plume or trench; (2) the rate, geometry, and nature of ocean-ridge spreading; (3) mantle composition, temperature, and fertility; and (4) the availability of fluids. The oceanic crust preserved in ophiolites may form in any tectonic setting during the evolution of ocean basins, from the rift–drift and seafloor spreading stages to subduction initiation and terminal closure. An ophiolite is emplaced either from downgoing oceanic lithosphere via subduction-accretion or from the upper plate in a subduction zone through trench–continent collision. Subduction zone tectonics is thus the most important factor in the igneous evolution of ophiolites and their emplacement into continental margins.