2 - Mass conservation and elemental fractionation

Summary

Before discussing the composition of the different systems of geological interest and the exchanges of matter that make those systems evolve relative to each other, it is worth recalling the principles governing the geochemical differentiation of our planet. These seemingly simple principles conceal what are often daunting complexities. They are: the principle of conservation of mass, elementary and isotopic fractionation induced by phase changes, kinetic fractionation, and radioactivity. In the Introduction, we alluded to the contrast between mixing processes and differentiation processes. We will now look at a number of examples.

1. Partial melting of the mantle beneath mid-ocean ridges produces basaltic liquids whose chemical composition is different from the ultramafic chemical composition of the source peridotite. This chemical fractionation of elements between the molten fluid and its parent medium can be described by thermodynamic rules. The former makes up the oceanic crust while the latter forms the refractory base of the lithosphere (located in the oceanic plates beneath the crust). It must be kept in mind that there is no chemical or isotopic fractionation in the system unless at least two phases co-exist (solid/liquid, vapor/liquid, mineral A/mineral B, …) each to host a different share of the initial inventory. Conversely, when the oceanic crust and oceanic lithospheric mantle plunge at subduction zones, they begin a long journey within the mantle, where convection folds and stretches them, in much the same way as a baker kneads dough, progressively eliminating the differences that initially existed between the two constituent parts. Convective mixing, or better, stirring, therefore undoes the effect of magmatic differentiation.

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