Dostal reported in 2008 that Komatiites are ultramafic volcanic rocks that occur mainly in Archean and Paleoproterozoic greenstone belts. These olivine-rich rocks are assumed to have crystallized from magmas that have about 28–30 wt% MgO. They are characterized by spinifex texture - platy or skeletal crystals of olivine set in a glassy matrix. Chemically, komatiites resemble peridotites and have high MgO but low SiO2, TiO2 (<1 wt%), K2O (<0.5 wt%) and incompatible trace element contents. Most of their compositional variations can be accounted for by olivine fractionation.
Komatiites are conventionally considered to be derived from high temperature melts that have eruption temperatures of about 1600°C and are produced
by high degrees of anhydrous melting of mantle plumes. The abundance of Archean komatiites, their decrease through the Proterozoic and extreme rarity in the Phanerozoic have been taken as evidence for secular cooling of the mantle. However, the plume model has recently been challenged.
In marked contrast, it has been proposed that komatiites originate via hydrous melting at a shallow depth in subduction environments at significantly lower melting temperatures than those invoked by the plume hypothesis. This new model thus challenges traditional views of the early evolution of the Earth. Nevertheless, it appears that many komatiites are plume-related. In addition to the information they provide about the tectonics and the thermal
evolution of Archean Earth, komatiites are economically important because they host locally significant magmatic Ni-sulfide (Ni-Cu-PGE) mineralization.
Dotal concluded that komatiites provide a window into the composition and thermal dynamics of the Archean mantle. They are usually
considered to be the product of mantle plumes generated at depth, possibly near the core–mantle boundary (Campbell et al. 1989; Arndt et al.
1998; Sproule et al. 2002).
The chemical compositions of komatiites and related Mg-rich rocks appear to vary with time. For example, with respect to the MgO contents of common ultramafic magmas, komatiites characteristic of the Archean have >18 wt% MgO, komatiitic basalts dominant in the Proterozoic have 12–18 wt% MgO and
picrites in the Phanerozoic have 10–13% wt% MgO. Because the MgO contents of dry magmas are related to their melting temperature, the trend is consistent with secular cooling of the Earth’s interior.
However, recently, some researchers (Parman et al. 1997; Grove and Parman 2004; Wilson et al. 2003) have challenged the plume model and attendant volatile-free mantle melting, and postulated that komatiites may be produced by hydrous melting at shallow mantle depth in a subduction (arc) environment. In this scenario, temperatures would be considerably lower than those required by the plume model.
Since many current models of chemical differentiation and thermal evolution of the Earth are based on mantle plume-generated komatiites, the subduction model, if correct, would significantly change current views of the evolution of Archean Earth. Although recent data imply that many komatiites are derived from a dry mantle source, there are still some rocks that could be subduction-related, such as high-SiO2 komatiites from the ~3.33 Ga Commondale greenstone belt in South Africa (Wilson et al., 2003), and some komatiitic basalts that resemble boninites. More studies of the trace element and isotopic characteristics of komatiites and their minerals as well as experimental studies, particularly under wet melting conditions, are needed to constrain their origin.
Obviously, there is much left to learn about these fascinating rocks.
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