Earth’s biggest ‘bull’s eye’

Since astronauts and satellite imaging devices first made pictures from orbit, top of the list for oddness is the Richat structure of Mauritania. Sitting out in the Sahara is series of perfectly concentric rings that are almost circular. The structure is at least 40 km across, and even today, many geoscientists use images of Richat as a superb example of a meteorite impact. It is not (Matton, G. et al. 2005. Resolving the Richat enigma: Doming and hydrothermal karstification above an alkaline complex. Geology, v. 33, p. 665-668). Spectacular from space, Richat is not easily accessible. Early field work reported a breccia on a kilometric scale at its high-relief core, which unsurprisingly added to its designation as an impact structure. There are other possibilities: a structural dome, perhaps due to interference between open folds of a couple of generation; the result of upward forces from magmatic activity, such as an underlying plutonic diapir.

The rocks involved are Neoproterozoic to Ordovician sediments of various kinds, which dip radially outwards from Richat’s core, so it is some kind of dome, rather than the sort of circular breach expected of an impact. Two large, basaltic ring dykes, whose centre coincides with that of the dome, cut the sediments. Other igneous materials are: carbonatites (formed from unusual carbonate-rich magmas) in dykes and sills; alkaline silicate-rich intrusions and flows occurring close to the central breccia; kimberlites in the form of plugs and sills. The central breccia is in fact a roughly horizontal lens, about 3 km across, that is made mainly of local sedimentary material, mainly once carbonates, set in a silica-rich matrix. The clasts range from highly angular to rounded, but show abundant evidence of some kind of corrosion and silicification. Matton et al. interpret the breccia as a zone of intense dissolution that caused the original sediments at the structure’s core to collapse as volume was reduced as magmatic gases (supercritical fluids) rushed to the surface. So the Richat structure has all the hallmarks of doming above an alkaline igneous pluton, followed by intense hydrothermal activity that was able to dissolve carbonates and produce features akin to those formed by weathering in areas of karst. Rather than being particularly ancient, the igneous activity dates to the Middle Cretaceous. Richat is still unique. Diatremes (vertical breccia tubes) formed by explosive release of fluids from alkaline magmas are quite common, especially in areas dotted with kimberlites, but nowhere else have they produced doming on such a grand scale and with such a spectacular shape.

Detecting the effects of slab to wedge fluid transfer in subduction zones

A fundamental hypothesis concerning the formation of magmas above subduction zones is that partial melting in the over-riding wedge of mantle is induced by upward transfer of water vapour produced by dehydration of the descending lithospheric slab. Many aspects of the chemistry of igneous rocks in supra-subduction zone settings are explained by such dehydration-hydration. However, such fluid transfer is difficult to demonstrate, other than by its ‘second-hand’ geochemical effects on crustal magmas. It should have another, physical effect: in the presence of water vapour, some of the dominant olivine in mantle rocks should break down to form hydrated minerals of the serpentine family. Since olivine is an iron-magnesium silicate, whereas serpentine contains only magnesium, the hydration reaction should release iron to crystallise in the form of iron oxide; specifically Fe3O4 or magnetite. Geophysicists at the US Geological Survey have been able to detect at first hand the effects of this process, thereby allowing zones of hydration in the mantle wedge to be mapped (Blakely, R.J. 2005. Subduction-zone magnetic anomalies and implications for hydrated forearc mantle. Geology, v. 33, p. 445-448). As well as finding substantial magnetic anomalies caused by the release of magnetite by olivine dehydration over the forearc of the Cascadia subduction zone in Oregon, they show gravity anomalies that reflect density variations in the underlying mantle. The other aspect of the olivine-serpentine transformation is a large decrease in density, which should result in a decrease in gravity anomaly should sufficient olivine have been transformed. The coincidence of gravity lows with magnetic highs allowed Blakely et al. to model the location of hydrated mantle wedge in the Cascadia subduction system: probably just above the zone where subducting oceanic crust is transformed to ecologite.

Serpentinite also has a marked effect on the rheology of mantle rocks, because of its ease of ductile deformation. It should allow subduction deformation to proceed in a continuous fashion within the part of the system where it occurs, yet may focus sudden strain in great earthquakes to shallow levels up-dip of its position.

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