Basalt Archaean

To evaluate these questions further, we have looked again at the variations in Pb/ Pb ratios (and thus pi) of Archaean basalts and granitoids worldwide in the age range 3.7-3.8, 3.5, 3.2, 2.9 to 2.7-2.6 Ga. We show that there are provinces with different Pb/ Pb and pj values, and note that some unexpectedly coincide with Mezosoic continental flood basalts and oceanic DUPAL rocks with similar Pb isotope features. 

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,... 

Earth, has been prominent in models of the mantle for several decades (see Section 3.1.2). Such models are based upon a chondritic starting composition for the Earth, modified during core formation and perhaps in a magma ocean, and presuppose that there was a point in time when the mantle was totally homogenized. Whether the remnants of this primitive mantle can be identified today in modern basalts, or even in the early history of the Earth in Archaean basalts, is the subject of some debate. In favor are mantle melts with chondritic Os-isotope ratios (Fig. 3.32) and against are calculations which show that it is possible that the entire mantle has been processed through the subduction system (Section 3.1.6.4) during the history of the Earth. 

However, Archaean basalts do have average compositions which are different from modern basalts (see Section 3.2.1.1) which could imply a difference in mantle source composition and raises the possibility that they sampled primitive mantle. This argument has been developed by Condie (2005) who used immobile incompatible trace element ratios to demonstrate a difference in source composition between modern and Archaean plume basalts. One of his principal findings is that Archaean plume... 

It can be seen from these diagrams that the complement to Archaean SCLM (A-SCLM) is Archaean komatiite (of the undepleted, Munro-type, as exemplified by the Sula Mountains lavas) and not Archaean basalt and not Archaean TTG. These relations show clearly that Archaean TTG crust cannot be the complement to the Archaean SCLM and that Archaean TTG and SCLM are not related as melt and residue. 

Condie, K.C., 2005. High field strength element ratios in Archaean basalts a window into evolving sources of mantle plumes. Lithos, 79, 491-504. 

Smithies, R.H., Van Kranendonk, M.J., and Champion, D.C., 2005. It started with a plume -early Archaean basaltic protocontinental crust. Earth Planet. Sci. Lett., 238, 284-97. 

Campbell I. H., Griffiths R. W., and Hill R. I. (1989) Melting in an Archaean mantle plume heads it s basalts, tails it s komatiites. Nature 339, 697-699. 

Re-Os studies of mantle nodules demonstrate that the craton keel is Archaean in age and that there is no correlation with depth in the keel. The keel, therefore, did not thicken over time. Many eclogite inclusions in diamond are the same age as depleted peridotite or even slightly older. The age data on eclogite inclusions, coupled with evidence from stable isotope measurements that the eclogites formed from seawater-altered basalts, indicate that subduction processes were probably important in Archaean... 

The mineralogical and chemical composition of peridotite from subcontinental lithosphere differs from that of peridotite from other parts of the mantle (Boyd 1989 Berstein et al. 1997). Peridotites from subcontinental lithosphere is depleted , which means it contains only a small amount of clinopyroxene and an aluminous phase, which together make up the so-called basaltic component. The lithosphere beneath the oldest Archaean cratons has a composition markedly different from that of younger subcontinental lithosphere (Boyd Mertzman 1987 Griffin et al. 1999). Old unmetasomatized lithosphere is harzburgitic, a mixture of olivine... 

Pb isotope variations in Archaean time and possible links to the sources of certain Mesozoic-Recent basalts... 

Preservation of UjPb heterogeneities through time are the spatial links between Pi in the Archaean, Mesozoic CFB and DU PAL oceanic basalts just coincidence ... 

Chauvel, C., Dupre, B. Arndt, N. T. 1993. Pb and Nd isotopic correlation in Belingwe komatiites and basalts. In Bickle, M. J. Nisbet, E. G. (eds) The Geology of the Belingwe Greenstone Belt, Zimbabwe - a Study of the Evolution of Archaean Continental Crust. Geological Society of Zimbabwe Special Publication, 2, 167-174. 

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