The Earths earliest mantle

Unlike the modern mantle, direct samples of the Archaean mantle are rare and geophysical measurements impossible. This means that our prime evidence comes from the study of Archaean mafic and ultramafic magmas. 

Basaltic rocks are common in Archaean terrains and are particularly abundant in Archaean greenstone belts. However, they are frequently altered and metamorphosed, and only the best preserved samples should be used to obtain information about the composition of the Archaean mantle. Recent research has shown that Archaean greenstone belts formed in a number of different tectonic environments (see Chapter 1, Section 1.3.1). This means that greenstone belt basalts potentially offer an insight into a variety of mantle environments in the early Earth. 

Arndt et al. (1997) compared the compositions of Archaean tholeiites with those of modern basalts from ocean-ridge, ocean-island, and arc environments (Fig. 3.22). They showed that Archaean tholeiites have higher SiOa and FeO, and lower incompatible trace element concentrations (although enriched in Rb and Ba relative to modern MORB), compared to their modern equivalents. Many of these geochemical features of Archaean basalts match those of modern arc basalts, but there are also important differences. This led Arndt et al. (1997) to the important conclusion that the major- and trace-element characteristics of Archaean basalts are matched by no common type of modern basalt. The uniqueness of Archaean tholeiitic basalts requires that they are either from a source different from that of modern tholeiites, or are the product of a different melting process. Arndt et al. (1997) interpreted the low incompatible trace element concentrations and high Si, Fe, Ni, 

FIGURE 3.22 The trace element composition of Archaean tholeiites compared to modern enriched-MORB and normal-MORB, normalized to the primitive mantle (after Arndt et al.( 1997). 

Typically komatiites are ultramafic rocks with volcanic features such as glassy flow tops, flow top breccias, amygdales, and pillows. They form lava flows varying in thickness 

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