Komatiites mantle evolution

Fewer reliable osmium isotopic data exist for Precambrian samples owing to the problems of disturbance of rhenium and osmium during alteration and metamorphism. Key points used to define mantle evolution are indicated in Figure 7. The osmium isotopic constraints on the composition of the Archean mantle are, in many cases, derived from Late Archean komatiite suites. Komatiites are high-MgO lavas that often form differentiated flow sequences, such that Re-Os isochrons can be determined chromite-rich layers are commonly present and provide a robust estimate of the initial osmium isotopic... 

Jochum K. P., Arndt N. T., and Hofmann A. W. (1991) Nb-Th-La in komatiites and basalts constraints on komatiite petrogenesis and mantle evolution. Earth Planet. Set Lett 107, 272-289. 

Brevart, O., Dupre, B. Allegre, C. 1986. Lead-lead age of komatiitic lavas and limitations on the structure and evolution of the Precambrian mantle. Earth and Planetary Science Letters, 77, 293-302. 

Brevart tfr a/. (1986) use Pb isotope initial ratios to demonstrate mantle heterogen ty at 2.7 Ga and have constructed a depleted mantle curve from 2.7 Ga to the present day from the initial ratios of komatiites and basalts on a isotope evolution diagram. 

Figure 6J7 The contrasting evolution of [ 1 in the continental crust and mantle. CHUR is the chondridc uniform reservoir it is equivalent to the bulk Earth value and is used as the reference against which other measurements are made. Hence for CHUR is zero. Five curves are shown for the evolution of the depleted mantle. Curve (1), after Liew and McCulloch (1985), assumes the derivation of die depleted mande from CHUR at 2.7 Ga and linear evolution until the present av value of +10. Curve (2), after DePaolo (198 la,b), may be plotted from the expression Q.25r — 3/ + 8.5, where t is Ae time before the present in Ga. Curve (3) is taken from Nelson and DeF lo (1984 — Figure 2). Curve (4) is f m Goldstein et ai (1984) and assumes the linear evolution of the depleted mande between a value of +10 at the present day and 0 at 4.5 Ga, Curve (5) is based upon data from MORB, ophiolites, komatiites and meteorites and is calculated from the expression given both by Ben Othman et ai (1984) and Allegre and Rousseau (1984) ( Nd/ Nd)o( = + t + C, where A -...

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