Komatiites melt depletion

Walter M. J. (1998) Melting of garnet peridotite and the origin of komatiite and depleted lithosphere. J. Petrol. 39, 29-60. 

The counter argument, summarized by Arndt (2003) and Arndt et al. (1998), stresses the inability of low viscosity komatiitic melts to contain dissolved volatiles. Furthermore, most komatiites are chemically depleted implying that they are the melts of a source which was previously chemically fractionated. If this was the case, then any volatiles in the source would be expected to have been removed in the first melt and would not be present to enter a komatiitic melt. 

Inoue, T., Rapp, R.P., Zhang, J., Gasparik, T., Wedner, D.J., and Irifune, T., 2000. Garnet fractionation in a hydrous magma ocean and the origin of Al-depleted komatiites melting experiments of hydrous pyrolite with REEs at high pressure. Earth Planet. Sci. Lett., 177, 81-7. 

The results of the fast decay scheme (Fig. 3.29b) are similar to the original curve by Vervoort and Blichert-Toft (1999) and imply a very early (pre-3.5 Ga) mantle differentiation event, followed by less extreme mantle depletion. The early depletion event was most likely caused by the production of a very early mafic crust. Thus the ultimate source of the parental magmas of the Jack-Hills zircons, the Amitsoq gneisses, and some Barberton komatiites, was mantle that had already been depleted in the early Archaean. Bizzarro et. al. (2003) proposed that this early differentiation of the mantle took place prior to 4.33 Ga. This result is consistent with evidence from 142Nd for a very early mantle melting event, as outlined in the previous section of this chapter. 

It is clear that the Earth s mantle has at least two Os-isotopic reservoirs - a plume-related isotopically enriched reservoir and a chondritic upper mantle reservoir. Both have long histories (Fig. 3.32). The variations in composition within the upper mantle reservoir reflect Re-depletion and enrichment related to melt extraction. The isotopically enriched plume reservoir represents chemically isolated, rhenium-enriched, recycled oceanic lithosphere. There is some evidence to suggest that this enriched reservoir may have been in existence since the early Archaean (Walker Nisbet, 2002) and was the source of some Archaean komatiites and the 3.81 Ga Itsaq Gneiss chromitites. If this is true, then basaltic crust was being created and recycled even before 4.0 Ga. Estimates of the present size of this high Re/Os basaltic reservoir vary from 5% to >10% of the whole mantle (Bennett et al., 2002 Walker et al., 2002). 

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