Ophiolites, and discussions on their origin and significance in Earth's history, have been instrumental in the formulation, testing, and establishment of hypotheses and theories in earth sciences. The definition, tectonic origin, and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century. Here, we present a review of these ideas as well as a new classification of ophiolites, incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological, geochemical, and tectonic processes during formation in different geodynamic settings. We define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments. Their geochemical characteristics, internal structure, and thickness vary with spreading rate, proximity to plumes or trenches, mantle temperature, mantle fertility, and the availability of fluids. Subduction-related ophiolites include suprasubduction-zone and volcanic-arc types, the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle, melting of the subducting sediments, and repeated episodes of partial melting of metasomatized peridotites. Subduction-unrelated ophiolites include continental-margin, mid-ocean-ridge (plume-proximal, plume-distal, and trench-distal), and plume-type (plume-proximal ridge and oceanic plateau) ophiolites that generally have mid-ocean-ridge basalt (MORB) compositions. Subduction-related lithosphere and ophiolites develop during the closure of ocean basins, whereas subduction-unrelated types evolve during rift drift and seafloor spreading. The peak times of ophiolite genesis and emplacement in Earth history coincided with collisional events leading to the construction of supercontinents, continental breakup, and plume-related supermagmatic events. Geochemical and tectonic fingerprinting of Phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in Earth history, and it can be extended into Precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the Archean.

Ophiolite genesis and global tectonics: Geochemical and tectonic fingerprinting of ancient oceanic lithosphere

Here we argue that life emerged on Earth from a redox and pH front at c. 4.2 Ga. This front occurred where hot (c. 150°C), extremely reduced, alkaline, bisulphide-bearing, submarine seepage waters interfaced with the acid, warm (c. 90°C), iron-bearing Hadean ocean. The low pH of the ocean was imparted by the ten bars of CO 2 considered to dominate the Hadean atmosphere/hydrosphere. Disequilibrium between the two solutions was maintained by the spontaneous precipitation of a colloidal FeS membrane. Iron monosulphide bubbles comprising this membrane were inflated by the hydrothermal solution upon sulphide mounds at the seepage sites. Our hypothesis is that the FeS membrane, laced with nickel, acted as a semipermeable catalytic boundary between the two fluids, encouraging synthesis of organic anions by hydrogenation and carboxylation of hydrothermal organic primers. The ocean provided carbonate, phosphate, iron, nickel and protons; the hydrothermal solution was the source of ammonia, acetate, HS − , H 2 and tungsten, as well as minor concentrations of organic sulphides and perhaps cyanide and acetaldehyde. The mean redox potential (ΔEh) across the membrane, with the energy to drive synthesis, would have approximated to 300 millivolts. The generation of organic anions would have led to an increase in osmotic pressure within the FeS bubbles. Thus osmotic pressure could take over from hydraulic pressure as the driving force for distension, budding and reproduction of the bubbles. Condensation of the organic molecules to polymers, particularly organic sulphides, was driven by pyrophosphate hydrolysis. Regeneration of pyrophosphate from the monophosphate in the membrane was facilitated by protons contributed from the Hadean ocean. This was the first use by a metabolizing system of protonmotive force (driven by natural ApH) which also would have amounted to c. 300 millivolts. Protonmotive force is the universal energy transduction mechanism of life. Taken together with the redox potential across the membrane, the total electrochemical and chemical energy available for protometabolism amounted to a continuous supply at more than half a volt. The role of the iron sulphide membrane in keeping the two solutions separated was appropriated by the newly synthesized organic sulphide polymers. This organic take-over of the membrane material led to the miniaturization of the metabolizing system. Information systems to govern replication could have developed penecontemporaneously in this same milieu. But iron, sulphur and phosphate, inorganic components of earliest life, continued to be involved in metabolism.

The emergence of life from iron monosulphide bubbles at a submarine hydrothermal redox and pH front

Boninite-like volcanic rocks in the 3.7–3.8 Ga Isua greenstone belt, West Greenland: geochemical evidence for intra-oceanic subduction zone processes in the early Earth

Alteration and geochemical patterns in the 3.7–3.8 Ga Isua greenstone belt, West Greenland

Turbiditic and pelagic sedimentary rocks from the Isua supracrustal belt in west Greenland [more than 3700 million years ago (Ma)] contain reduced carbon that is likely biogenic. The carbon is present as 2- to 5-micrometer graphite globules and has an isotopic composition of δ13C that is about –19 per mil (Pee Dee belemnite standard). These data and the mode of occurrence indicate that the reduced carbon represents biogenic detritus, which was perhaps derived from planktonic organisms.

https://science.sciencemag.org/content/sci/283/5402/674.full.pdf

13C-Depleted carbon microparticles in >3700-Ma sea-floor sedimentary rocks from west greenland

Investigations of the Isua supracrustal rocks in West Greenland allow us to identify the protoliths and alteration history for most Isua rocks. The protoliths consisted of alternating basalt and banded iron formation. This sequence was invaded by dunitic sills, metamorphosed, and later intruded by felsic gneisses. Pervasive carbonation and K metasomatism produced a sequence of lithologies, mimicking those found in modern platform deposits. However, the protoliths could have originated in a purely oceanic environment with no sialic detrital components. The Isua sequence probably consists of several tectonic panels, some of which are cut by >3810 Ma felsic intrusive rocks.

Nicholas M. Rose

Papers

Earliest part of Earth's stratigraphic record: A reappraisal of the >3.7 Ga Isua (Greenland) supracrustal sequence

The role of ultramafic rocks in regulating the concentrations of volatile and non-volatile components during deep crustal metamorphism