- © The Mineralogical Society Of America
Hydrogen is the most abundant element (Fig. 1⇓) in the galaxy and our solar system (Lodders 2003). Therefore it is not astonishing that hydrogen is a key player in the geodynamic evolution of planets. Its fate in the early Earth, after condensation of the solar nebula, the accretion of our planet and hydrogen reprocessing through early asteroidal and cometary bombardment (Dauphas et al. 2000) and segregation of a proto-Earth into iron core and silicate mantle is described elsewhere in this volume (Marty and Yokochi 2006).
This chapter concerns itself with the geodynamics of the modern Earth, where nowadays hydrogen occupies only a very small mass fraction (less than 0.1 wt%). Although its abundance is thus drastically diminished, it is still believed to have a profound influence on the geodynamics of the present planet. The reason why hydrogen can have a profound influence on geodynamics, even when it is diluted to ppm-levels, relies on its fundamental influence on both rock mechanics (Kohlstedt 2006) and the melting relationships (Kohn 2006). However, both the micromechanical mechanism and the large scale implications are not yet fully understood. In this review geodynamic processes which may be triggered by a rock mechanical hydrogen threshold value are reviewed in addition to resulting styles of planetary convection controlled by melt and rock mechanics. This review will conclude with a more speculative discussion of the possible implications for planetary evolution from an early Earth to the present day and highlight areas of future work. Following current practice in the literature the term “water” will be used interchangeably for hydrogen.
Recent breakthroughs in computational geodynamics are throwing a new light on the possible evolution of planetary tectonics. Simulations …