T9.1 : Magma differentiation


The works carried out since the turn of the century have shown the great homogeneity of the iron isotope composition of igneous rocks from the continental crust. An exception has however been found: granites with more than 70% SiO2 (Poitrasson and Freydier, 2005). This discovery at first generated a controversy (Beard and Johnson, 2006; Poitrasson, 2006), but it was subsequently confirmed by the scientific community (Schoenberg and von Blanckenburg, 2006; Dauphas and Rouxel, 2006). The enrichment in heavy isotopes of iron in the most differentiated silicate rocks is now well established (Figure) and a new research avenue opened. What are the mechanisms responsible for this increase in heavy iron isotopes in magmas? Fractional crystallization (Schoenberg and von Blanckenburg, 2006; Schuessler et al., 2009), a late exsolution of magmatic fluids accompanied by a change in oxidation state of iron in the magma (Poitrasson and Freydier, 2005; Heiman and al., 2008) or a more complex process of thermodiffusion (Huang et al., 2009)? Solving this problem will affect the contribution of iron isotopes on our knowledge of some metallic ore deposits and / or silicate melts evolution. This work also helped to refine the continental crust isotopic baseline (Poitrasson, 2006) which is the benchmark for all studies of soils, rivers or oceans.

Granite de Mantelluccio-en

Photo: Anorogenic granite from Mantelluccio, Corsica. Picture: F. Poitrasson.

Afficher le pied de page