Geoscience Reference
In-Depth Information
2
Mass conservation and elemental fractionation
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 recall-
ing 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 differen-
tiation 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 inven-
tory. 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 elimi-
nating the differences that initially existed between the two constituent parts. Convective
mixing, or better, stirring, therefore undoes the effect of magmatic differentiation.
2. The erosion of a granite yields clay minerals and quartz grains, and adds to the dis-
solved load of run-off water; the clay accumulates on the ocean floor, the quartz is
deposited as sandstone on the continental shelf or slope, and the river water mixes with
