Greenstone belt 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. 

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. 

De Ronde C. E. J., de Wit M. J., and Spooner E. T. C. (1994) Early Archean (>3.2Ga) iron-oxide-rich hydrothermal discharge vents in the Barberton greenstone belt. South Africa Geol. Soc. Am. Bull. 106, 86-104. 

FIGURE 1.6 A geological map of the Barberton greenstone belt in South Africa, showing the main geological components (after De Ronde De Wit, 1994). 

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). 

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,... 

Nahcolite. The mineral nahcolite (imaginatively named after its formula NaHCOs) is a primary evaporite mineral that has been found in 3.4 Ga sediments in the Barberton greenstone belt and in 3.2 Ga silicified sediments at Pilbara in western Australia (Lowe Tice, 2004). Experimental studies show that, although nahcolite is not normally a stable phase in evaporites, it may become so if there is a high enough partial pressure of COa. On the basis of their observations Lowe and Tice (2004) calculated a minimum C02 level of between 1.4 and 10% in the atmosphere, for Archaean surface temperatures of between 55 and 85°C. It should be noted, however, that this value would be much lower if lower surface temperatures were used in the calculation. 

Paleosols. Paleosols formed before 2.2 Ga tend to contain iron silicates, rather than siderite-iron carbonate. Rye et al. (1995) used carbonate-silicate mineral equilibria in the 2.75 Ga Mount Roe paleosol to estimate the partitioning of COa between soil and air in the late Archaean. Their calculations suggest a maximum partial pressure of 10 14 (0.04) atmospheres COa in the late Archaean atmosphere, significantly lower than the estimate of Lowe and Tice (2004) based on nahcolite. A lower limit for atmospheric COa comes from the study of siderite-clay mineral equilibria for weathering rinds on clasts in river gravels from the 3.2 Ga Moodies Group of the Barberton greenstone belt, in south Africa (Hessler et al., 2004). In this study the minimum partial pressure for atmospheric COa at 3.2 Ga was calculated to be 2.5 X 10 3 bars at 25°C. 

De Ronde, C.E.J. and De Wit, M.J., 1994. Tectonic history of the Barberton greenstone belt, South Africa 490 million years of Archaean crustal evolution. Tectonics, 13, 983-1005. 

Westall F, de Ronde CEJ, Southam G, Grassineau N, Golas M, Cockell C, Lam-mer H Implications of a 3.472-3.333 Gyr-old subaerial microbial mat from the Barberton greenstone belt. South Africa for the UV environmental conditions on the early Earth. PhilosTrans RSoc Lond B Biol Sci 2006, 36l(l474) 1857— 1875. 

Volcanics from the Onverwacht Group, Swaziland and South Africa, comprise the basal part of the Early Precambrian Barberton greenstone belt found both in Swaziland the Transvaal. Data from them gave an age of about 3.54 Ga calculated using an initial [ Nd/ Nd]o ratio of 0.50809 00004 (Hamilton et aL 1979). The strontium and neodymium isotopic record of Apollo-12 basalts from the Moon were examined. Isochrons were derived for an achondrite (eucrite) and another meteorite and gave ages of about 4.58 and 4.562 Ga, respectively, with primordial [ Nd/ Nd]p ratios of 0.50684 0.00008 and 0.506664. An initial [ Sr/ Sr]o ratio was termed ADOR. The isotopic evolution of neodymium in the Earth has been described by means of a model termed CHUR (GHondritic Uniform Reservoir). And the present value of the [ Nd/ Nd] ratio is 0.512638 relative to a [ Nd/ Nd] ratio of 0.7219. The present value of the [ Sm/ Nd] ratio of CHUR is 0.1967, which permits calculation of [ Nd/ Nd] ratio in any past time interval. 

Barberton greenstone belt, South Africa. 2014. Geochem Geophys Geosys. 15(4), p. 1054. DOI 10.1002/2014GC005229. 

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