Book contents
- Frontmatter
- Contents
- Foreword to the English edition
- Foreword to the French edition
- Acknowledgments
- Introduction
- 1 The properties of elements
- 2 Mass conservation – elemental and isotopic fractionation
- 3 Geochronology and radiogenic tracers
- 4 Element transport
- 5 Geochemical systems
- 6 Waters present and past
- 7 Mineral reactions
- 8 The solid Earth
- 9 The Earth in the Solar System
- 10 The geochemical behavior of selected elements
- Appendix A Composition of the major geological units
- Appendix B The mixing equation for ratios
- Appendix C A refresher on thermodynamics
- Appendix D The Rayleigh distillation equation
- Appendix E The geological time scale
- Appendix F An overview of analytical methods
- Appendix G Physical and geophysical constants
- Appendix H Some equations relative to residence time
- Further reading
- Index
2 - Mass conservation – elemental and isotopic fractionation
- Frontmatter
- Contents
- Foreword to the English edition
- Foreword to the French edition
- Acknowledgments
- Introduction
- 1 The properties of elements
- 2 Mass conservation – elemental and isotopic fractionation
- 3 Geochronology and radiogenic tracers
- 4 Element transport
- 5 Geochemical systems
- 6 Waters present and past
- 7 Mineral reactions
- 8 The solid Earth
- 9 The Earth in the Solar System
- 10 The geochemical behavior of selected elements
- Appendix A Composition of the major geological units
- Appendix B The mixing equation for ratios
- Appendix C A refresher on thermodynamics
- Appendix D The Rayleigh distillation equation
- Appendix E The geological time scale
- Appendix F An overview of analytical methods
- Appendix G Physical and geophysical constants
- Appendix H Some equations relative to residence time
- Further reading
- Index
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.
Partial fusion 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). 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 therefore undoes the effect of magmatic differentiation.
The erosion of a granite yields clay minerals and quartz grains, and adds to the dissolved 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 the ocean waters. […]
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- Information
- GeochemistryAn Introduction, pp. 23 - 46Publisher: Cambridge University PressPrint publication year: 2003