Komatiitic basalt

Puchtel I. S., Briigmann G. E., Hofmann A. W., KuMkov V. S., and Kulikova V. V. (2001b) Os isotope systematics of komatiitic basalts from the Vetreny belt, Baltic Shield evidence for a chondritic source of the 2.45 Ga plume. Contrib. Mineral. Petrol. 140, 588-599. 

The mafic rocks are komatiites, basalts, gab-bros and dolerites, and the felsic rocks are both tonalite-trondhjemite-granodiorites (TTG) and granodiorite-granite-monzogranites (GGM). Analyses of galena and K-feldspar provide initial Pb isotopic compositions, as they contain little or no uranium. 

Belingwc belt Reliance Formation Komatiitic basalt Z2I.34, 25 36... 

Normalized incompatible trace element diagrams are shown in Figure 4. The basalts of the Belingwe belt are generally characterized by flat incompatible element patterns (1.7-4,6x primitive mantle) with marked enrichment of Th (28.3x) and Sr (8.2x). The komatiitic basalts show comparable patterns to those of the basalts but with spiked large ion lithophile element... 

Nisbet, E. G., Martin, A., Bickle, M. J. Orpen, J. L. 1993. The Ngezi Group komatiites, basalts and stromatolites on continental crust. In Bickle, M. j. Nisbet, E. G. (eds) The Geology of the Belingwe Greenstone Belt, Zimbabwe. Geological Society of Zimbabwe Special Publications, 2, 69-86. 

Sproule, R.A., Lesher, C.M., Ayer, J.A., Thurston, PC., and Herzberg, C.T., 2002. Spatial and temporal variations in the geochemistry of komatiites and komatiitic basalts in the Abitibi greenstone belt. Precambrian Res., 115, 153-86. 

The main importance of this diagram, however, is that it shows komatiites clearly as a separate field from basalts and from calc-alkaline rocks, and so is useful for Archean metavolcanics. The original diagram of Jensen (1976) was slightly modified by Jensen and Pyke (1982), who moved the komatiitic basalt/komatiite field boundary to a lower Mg value. This is the version presented here (Figure 3.11). The plotting parameters of the field boundaries are taken from Rickwood (1989). 

Puchtel I. and Humayun M. (2000) Platinum group elements in Kostomuksha komatiites and basalts implications for oceanic crust recycling and core-mantle interaction. Geochim. Cosmochim. Acta 64, 4227-4242. 

Copper. The abundance of copper in the depleted mantle raises a particular problem. Unlike other moderately compatible elements, there is a difference in the copper abundances of massive peridotites compared to many, but not all, of the xenolith suites from alkali basalts. The copper versus MgO correlations in massive peridotites consistently extrapolate to values of 30 ppm at 36% MgO, whereas those for the xenoliths usually extrapolate to <20 ppm, albeit with much scatter. A value of 30 ppm is a relatively high value when chondrite normalized ((Cu/Mg)N = 0.11), and would imply Cu/Ni and Cu/Co ratios greater than chondritic, difficult to explain, if true. However, the copper abundances in massive peridotites are correlated with sulfur, and may have been affected by the sulfur mobility postulated by Lorand (1991). Copper in xenoliths is not correlated with sulfur, and its abundance in the xenoliths and also inferred from correlations in basalts and komatiites points to a substantially lower abundance of 20 ppm (O Neill, 1991). We have adopted this latter value. 

Figure 49 Bulk analyses of eclogite and alkremite (alkremite-corgaspinite-corganite Table 1) xenoliths from kimberlites compared with massif eclogites and Archean basalts and komatiites (open field). Data sources Kushiro and Aoki (1968), Sobolev (1974), Mazonne and Haggerty (1989), Hills and Haggerty (1989), Taylor and Neal (1989), Ireland et al. (1994), Pyle and Haggerty (1994), Snyder et al. (1993), Jacob et al. (1994), and Jacob and Foley (1999). Outlined field is for Archean basalts and Komatiites taken from Ireland et al. (1994).

Figure 54 Comparison of present-day osmium isotopic compositions of eclogite xenoliths from Udachnaya, Yakutia (Pearson et ah, 1995c) and S. Africa (Pearson et al, 1992 Menzies et al, 1999 Shirey et ah, 2001) with continental crust, oceanic basalts (Shirey and Walker, 1998), and Archean komatiites and basalts (Walker et al, 1989b). Udachnaya peridotite data from Pearson et al (1995a).

Figure 6 Effect of silicate melt composition on metal/silicate partition coefficients for cobalt ( ), gallium (+), tungsten (o), and phosphorus ( ) (Jaeger and Drake, 2000 Pak and Fruehan, 1986). NBO/t is calculated according to Mysen (1991) and corresponds to basalt values of 1, komatiite —1.7, and peridotite —2.8. In general, high-valence elements such as tungsten and phosphorus are affected more strongly than lower valence elements such as cobalt (or nickel).

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