Komatiites Barberton

Parman S. W., Dann J. C., Grove T. L., and Wit M. J. d. (1997) Emplacement conditions of komatiite magmas from the 3.49 Ga komati formation, Barberton Greenstone belt, South Africa. Earth Planet. Sci. Lett. 150, 303-323. 

Some of the oldest preserved oceanic plateau sequences are those found in —3.5 Ga Barberton and Pietersberg belts of the Kaapvaal Shield of southern Africa (De Wit et aL, 1987 Smith and Erlank, 1982). These belts contain pillow basalts and komatiites, with chemical signatures (Lahaye et aL, 1995) suggesting a likely origin as part of an oceanic plateau. The Pilbara craton of Australia appears to possess some of the oldest oceanic plateau material so far identified (Green et aL, 2000) in the —3.5 Ga Coonterunah and Warrawoona Groups. 

Smith H. S. and Erlank A. J. (1982) Geochemistry and petrogenesis of komatiites from the Barberton greenstone belt. In Komatiites (eds. N. T. Arndt and E. G. Nisbet). Allen and Unwin, pp. 347-398. 

Parman, S. W., Grove, T. L. Dann, J. C. 2001. The production of Barberton komatiites in an Archean subduction zone. Geophysical Research Letters, 28(13), 2513-2516. 

A very specific challenge in the Archaean geological record are those rocks which are without modern counterparts. One example is the ultramafic rock komatiite, first discovered as a lava in the Barberton area of South Africa. At the time of their discovery, in the early 1960s, ultramafic lavas were thought to be an impossibility. Over the past 30 years we have learned a great deal about komatiites and they are discussed in detail in Chapter 3 (Section 3.2.1.2). Nevertheless, why they are more common in the early history of the Earth than at the present is an important scientific question. 

Were komatiites weU Researchers working on the komatiites of the Barberton Greenstone belt have recently argued that komatiites are the product of wet mantle melting. Their evidence comes from both field observation and from experiment. Field observations of vesicles in komatiite lavas and pyroclastic komatiites lead to the inference that the lavas were degassing water when they erupted. In addition, experimental... 

FIGURE 3.25 The secular cooling curve for the potential temperature of the Earth s mantle (from Richter, 1988) and calculated upper mantle potential temperatures based on basalt chemistry (Abbott et al., 1994 - shaded field). Also shown are the potential temperatures for komatiites from Barberton Greenstone Belt (Barb) and the Belingwe Greenstone Belt (Bel), estimated komatiite temperatures from Grove and Parman (2004) and the calculated temperature of the subcontinental lithosphere from beneath the Kaapvaal Craton (Richardson et al., 1984). 

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. 

A, zircons from Amitsoq Gneisses, west Greenland B [black squares], komatiites and basalts from Barberton Greenstone Belt, South Africa JH, zircons from the Jack Hills,... 

Parman, S.W., Schimizu, N., Grove. T.L., and Dann, J.C., 2003. Constraints on the pre-metamor-phic element composition of Barberton komatiites from ion probe analyses of preserved clinopyrox-enes. Contrib. Mineral. Petrol., 144, 383-96. 

Yoshihara, A. and Hamano, Y., 2004. Palaeomagnetic constraints on the Archaean geomagnetic field intensity obtained from komatiites of the Barberton and Belingwe greenstone belts, South Africa and Zimbabwe. Precambrian Res., 131, 111—42. 

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