STUDY OF THE ARCHEEN MANTLE. INPUTS AND LIMITS OF GEOCHEMICAL TRACERS (REE, Nd, Hf, Sr and 0)

• Main Author: Gruau, Gérard
• Other Authors: Centre Armoricain d'Etude Structurale des Socles (CAESS), Université de Rennes (UR), Université de Rennes 1 UR1, Bohr-Ming Jahn
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 1993
• Subjects: geochemistry; REE; archean; Mantle; Geochimie; terres rares; archeen; manteau; [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; Greenland
• Online Access: https://hal-insu.archives-ouvertes.fr/tel-01576414; https://hal-insu.archives-ouvertes.fr/tel-01576414/document; https://hal-insu.archives-ouvertes.fr/tel-01576414/file/Gerard%20GRUAU%20-%201993.pdf

In this study, an attempt is made to constrain further the compositional heterogeneity and evolutionof the Archaean mantle (T ranging from 2500 to 3800 Ma) using major, trace element (Rare BarthElements) 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 andisotopie systems during alteration and metamorphism; the aim is to evaluate the effects of nonmagmaticprocesses on the preservation of primary chemical and isotopie memories in old terrestrialrocks. The sa~ples come from (1) West Greenland (Isua supracrustal belt; T=3750 Ma); (2) SouthAfrica (Barberton and Schapenburg geenstone belts; T=3500 Ma); (3) Western Australia (PilbaraCraton; 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 Al203fTi02lower than in chondrites, whereas most late Archaean komatiites (T=2700 Ma; e.g. komatütes fromCanada 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 bestexplained by the removal or addition of gamet. Lu-Hf isotopie results obtained on komatüte samplesfrom the Barberton greenstone belt in South Africa (EHf(T)=O) indicates that the gamet fractionationprocesses took place during the melting events that led to the various groups of komatiites. Thisresult, together with the recent finding that majorite gamet is the high-temperature mantle phase whenP~15GPa, provides a vehicle for discussing the origin of the various groups of komatütes. Mostlikely, the Archaean mantle had essentially ...