Geoscience Reference
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the ocean waters. Through the process of erosion, a granite body of initially homoge-
neous composition is separated into three different products that meet very different
fates. Over the course of time, the sediments laid down are scraped off along subduc-
tion zones, where they are buried under considerable thicknesses of rock. There tectonic
deformation and metamorphic mineralogical transformations at high temperature and
high pressure, combined with fluid action, provide conditions amenable to the partial
melting of crustal rocks (anatexis), and the siliceous fluids formed escape toward the
surface producing granite intrusions. The work of differentiation performed by erosion
is therefore destroyed by metamorphic and anatectic recombination.
3. In the ocean, chemical differentiation occurs at the surface, as biological activity draws
on nutrient elements as well as calcium carbonate and silica. The redissolution of dead
organisms and waste materials, together with bottom-water circulation, remixes the
components that were separated out by biological activity. Once again the opposing
processes of differentiation and mixing are at work.
We turn now to the methods of mass balance analysis, which are the essence of these differ-
entiation and mixing processes. By such methods we can determine the relative abundances
of the constituent parts mixed together in a composite product and evaluate the proportions
of phases that have separated out in a differentiation product. First some essential defini-
tions. Any system, whether natural or artificial, is made up of components and species.
Components are chemical entities (atoms, ions, or, for a rock, metal oxides, such as Na,
Na + ,Na 2 O) from which the chemical composition of the rock can be described fully and
uniquely. Rock compositions are commonly given in weight percent of its constitutive
oxides SiO 2 ,Al 2 O 3 ,Na 2 O, etc., while the composition of solutions is normally given as
moles of ionic component, Na + ,Ca 2 + ,Cl , etc., per kilogram of water. One unit system
is as good as another, provided consistency is maintained. The set of components itself
may not be unique: a rock can be described as proportions of atoms or as proportions of
oxides. Components are not normally found as such in the system; a familiar but often mis-
understood example being that of oxygen whose pressure (fugacity) can always be defined
even when no gas phase is present. Components are preserved during reactions and phase
changes, and their abundances are independent of the physical conditions in which the sys-
tem finds itself. Species are all the forms of chemical associations of components, whether
present or not (expressed or virtual), in the system. A species may be a KAlSi 3 O 8 feldspar
in a rock or an HCO 3 ion in a solution. Species abundances are altered by temperature,
reactions, and phase changes.
2.1 Conservation of mass
Let us begin with a few simple questions of geochemistry.
1. A river receives the input of a tributary. Given the flow rate and chemical composition
of the two streams ahead of their confluence, what is the composition of the river water
downstream of the confluence?
 
 
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