GEOCHEMISTRY OF AMPHIBOLE FROM ARCHEAN AND EARLY PROTEROZOIC ULTRAMAFIC ROCKS: IMPLICATIONS FOR THE SECULAR EVOLUTION OF THE EARTH¿S MANTLE

Since its formation at 4.5 Ga, the Earth underwent a complex evolution that progressively differentiated its original composition into the reservoirs that we presently know. Our knowledge on the composition and differentiation mechanisms active in the Early and Ancient Earth are still fragmentary for the paucity of suitable preserved rock records. The poor knowledge on the Archean mantle composition arises a series of problems spanning from the effective chondritic composition of the Earth to how volatile elements (hydrogen, oxygen, chlorine and fluorine) were added to the Earth. For the unavailability of mantle sectors preserving the Archean geochemical signature, valuable information on the Archean mantle composition can be exclusively extracted from Archean mantle-derived igneous rocks. In the Archean greenstone belts, different products of mantle melting are found as lavas and sills spanning in composition from tholeiites through Fe-picrites to komatiites. All these rocks are generally affected by extensive alteration which prevent the bulk rocks to be fully informative on the primary mantle melt composition and particularly on its volatile element contents. However, in some of these rocks primary igneous mineral phases such as amphibole are preserved that may be useful to constrain the primary melt composition including its volatile budgets. In this thesis amphibole-bearing ultramafic rocks of late Archean and Early Proterozoic age (Stone et al., 2003; Fiorentini et al., 2004; Fiorentini et al., 2008) were selected. For comparison amphibole-bearing ultramafic rocks from different tectonic settings of the Phanerozoic were also considered. The Archean and Early Proterozoic rocks share many petrographic and textural similarities with hornblendites and amphibole-bearing pyroxenites from Phanerozoic orogenic settings. In all studied rocks the crystallisation of amphibole follows that of the early crystallising minerals: olivine + spinel ± orthopyroxene + clinopyroxene. The chemical composition of Archean and Early Proterozoic amphiboles is more similar to that of amphibole from alkaline lavas than that of amphibole in orogenic settings. The geobarometric calculations on Archean and Early Proterozoic rocks yield large uncertainty on the pressure of crystallisation with values between 0 and >3 Kbar, which are not conclusive about the deep or shallow origin of amphibole. In the Archean and Early Proterozoic rocks amphibole is in clear disequilibrium with the early crystallizing clinopyroxene. Modelling of melt differentiation suggests that amphibole crystallized from a melt percolating the cumulate pile. Such melt evolved by crystallization of olivine and pyroxene and subsequently modified its composition in response to olivine assimilation. A major problem in the studied Archean and Early Proterozoic rocks is about the origin of the H2O necessary to stabilize amphibole. The H2O concentrations in the Archean and early Proterozoic amphiboles are comparable to those of either subduction-related or amphibole megacrysts from alkaline lavas, thus suggesting that melts in equilibrium with amphiboles possessed almost the same water contents irrespective of age. According to the composition of amphibole in fluid-mobile elements (e.g., F, Cl, B and Sr) a contribution of seawater-derived fluid in the Archean and Early Proterozoic rocks is unlikely. The range of δD values of the Archean and Paleoproterozoic amphiboles is between -99.5 ‰ and -129.8 ‰, that is slightly lower than the mantle range but still consistent with a magmatic origin for water. The hypothesis of a crustal contribution in the origin of the amphiboles (and in turn a crustal origin of water) contrasts with the oxygen isotope signature of amphibole showing δ18O values lighter than those of the mantle. Because the involvement of recycled crustal materials, able to provide the required seawater-like geochemical anomalies, is unlikely for the genesis of the studied amphiboles, the light δ18O signature is interpreted as a primary feature of the mantle source. In order to monitor possible changes marked by amphibole in the secular evolution of the Earth’s mantle, the trace element composition of the melt in equilibrium with amphibole from Archean and Early Proterozoic rocks was calculated and compared with that of melts produced nowadays at the different geodynamic settings. Equilibrium melts show increasing Nb/Y ratios from komatiites through tholeiites to Fe-picrites that are in agreement with the increased alkalinity of the parental melt as inferred from the literature. All calculated melts share an incompatible trace element pattern paralleling that of present-day OIB. The comparison of the water content in primary melts calculated from Archean-early Proterozoic amphiboles and present-day primary mantle melts reveals that the mantle source of the Archean komatiites had a much higher water content than that characterizing present day OIB. The highly variable water contents in Fe-picrites however suggest a large heterogeneity in the composition of the mantle source. The comparison between the Archean-early Proterozoic amphiboles and those from the Phanerozoic has also revealed heterogeneities in the Nb/Ta ratios of the mantle through the Earth’s history. Some of the calculated melts (since early Proterozoic) show an enriched Nb/Ta signature that is independent from space (geological setting) and time and that was interpreted as a primary feature of the different mantle sources. The observed heterogeneous Nb/Ta signature of the Earth’s mantle was interpreted as related to the addition of extra-terrestrial material after the mantle-core equilibration prior to 4.4 Ga and to an incomplete equilibration of these domains during the Earth’s evolution. In conclusion, the data of this thesis suggest that the Earth’s mantle is much more heterogeneous than commonly assumed. The occurrence in the Archean and Early Proterozoic of mantle domains enriched in volatile elements but unrelated to subduction processes has been documented. An extra-terrestrial signature for some mantle domains was also reported and I do not exclude that the light oxygen isotope signature of the Archean and Early Proterozoic rocks is also a reminiscence of extra-terrestrial inputs possibly related to the meteoritic Late Veneer.