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ETUDE DU MANTEAU ARCHEEN. APPORTS ET LIMITES DES TRACEURS GEOCHIMIQUES (REE, Nd, Hf, Sr et 0)

Abstract : In this study, an attempt is made to constrain further the compositional heterogeneity and evolution of the Archaean mantle (T ranging from 2500 to 3800 Ma) using major, trace element (Rare Barth Elements) and isotopie (Nd, Hf, Sr, and 0) compositions of ancient basic-ultrabasic rocks (komatütes, basalts and gabbros). We also investigate the behaviour of chemical elements and isotopie systems during alteration and metamorphism; the aim is to evaluate the effects of nonmagmatic processes on the preservation of primary chemical and isotopie memories in old terrestrial rocks. The sa~ples come from (1) West Greenland (Isua supracrustal belt; T=3750 Ma); (2) South Africa (Barberton and Schapenburg geenstone belts; T=3500 Ma); (3) Western Australia (Pilbara Craton; T=3500 Ma); and (4) Eastern Finland (Khumo and Tipasjarvi greenstone belts; T=2700 Ma). An intriguing aspect of many komatiites in early Archaean grrenstone belts ('P-:3400 Ma; e.g. komatiites from South Africa and Western Australia) is their Al- and HREE-depleted signature. In fact, many early Archaean komatütes have CaO/Al203 and (Gd!Yb)N higher, but Al203fTi02 lower than in chondrites, whereas most late Archaean komatiites (T=2700 Ma; e.g. komatütes from Canada and Zimbabwe) have roughly chondritic ratios of these elements. A third less common type, also largely restricted to early Archaean komatiites, has relative enrichment of Al and HREEs. Geochemical modelling conducted during the course of this study show that these variations are best explained by the removal or addition of gamet. Lu-Hf isotopie results obtained on komatüte samples from the Barberton greenstone belt in South Africa (EHf(T)=O) indicates that the gamet fractionation processes took place during the melting events that led to the various groups of komatiites. This result, together with the recent finding that majorite gamet is the high-temperature mantle phase when P~15GPa, provides a vehicle for discussing the origin of the various groups of komatütes. Most likely, the Archaean mantle had essentially chondritic ratios of CaO/Ah03, Ah03/Ti02 and (Gd/Yb )N, and the various groups of komatütes arise from differences in the depth of mel ting which controlled whether or not gamet was residual during the melting process. At any rate, the contrasted geochemical signatures of komatütes are not the consequence of a long-lived mineralogical and chemical stratification in the early Archaean mantle, such as might have formed during the solidification of a 4500-4400 Ma-old terrestrial magma ocean. As a general rule, the study of the behaviour of chemical elements and isotopie systems during metamorphism of basic-ultrabasic rocks leads us to question the capacity of these rocks to preserve their primary (magmatic) signatures. Results from the komatiite flows from Eastern Finland (Tipasjarvi and Siivikkovaara areas) show that metamorphic recrystallization may erase all the primary chemical and isotopie memories, including the REE patterns and ENd(T) values. The open system type of behaviour is particularly weil documented by the komatüte flows from Süvikkovaara; although the age of emplacement of the lava flows is 2700 Ma, whole-rock Sm-Nd data yield an age of 1800 Ma, which corresponds to the age of the metamorphic episodes recorded in the secondary mineral phases present in the samples. The fact that the Nd-isotopie re-equilibration occurred =1Ga after the emplacement of the flows cause strong shifts in the calculated tNd(T) values (-8 to +3) relative to the true magmatic values (=+2.0?). A consideration of petrographie and Oxygen isotopie data indicates that temperature conditions were in the range 400-500°C and that the REE carrier was likely a C02-rich fluid. The same type of behaviour is observed in the rocks from the Isua supracrustal belt of West Greenland. Although sorne authors have interpreted the scatter in calculated ENd(T) values displayed by the rocks from this belt (ENd(3750) ranging from -1.0 to +3.5) as a true initial isotopie variation, new Sm-Nd results obtained during the course of this study provide flrm evidence that the Sm-Nd system did not become a closed system in lsua rocks ca. 3750 Ma ago. A Sm-Nd mineral isochron age of 2849±116 Ma indicates that the earliest recognizable amphibolite facies metamorphism of the lsua belt is ofmid-Archaean age. Very probably, ail the rocks from Isua underwent sorne Nd isotopie reequilibration and/or fractionation of their 147SmJ144Nd ratios during the 2850 Ma-old metamorphic episode. Nevertheless, Nd-rich rocks (> 20 ppm) - such as the Amîtsoq grey gneisses and certain metasediments of the Isua belt - are relatively resistent to secondary fractionation due to the presence of REE-acceptor phases. In view of this behaviour, only ENd(T) values calculated from rocks having Nd contents greater than 20 ppm (ENd(T) = +2.0) appear to be sufflciently reliable for use in characterizing the ENd value of the Earth's mantle at 3800-3700 Ma. In fact, among the four investigated rock sequences, only the komatiites and komatiitic basalts from South Africa (Barberton and Schapenburg greenstone belts) appear to have preserved most of their primary isotopie and chemical memories.
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  • HAL Id : tel-01576414, version 1

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Gérard Gruau. ETUDE DU MANTEAU ARCHEEN. APPORTS ET LIMITES DES TRACEURS GEOCHIMIQUES (REE, Nd, Hf, Sr et 0). Géochimie. Université de Rennes 1 [UR1], 1993. Français. ⟨tel-01576414⟩

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