Abstract

Magmatic, metamorphic, hydrothermal, and dynamic processes at and near convergent plate boundaries offer unique opportunities to examine the physics and chemistry of materials recycling in the earth.

The upper mantle near convergent plate boundaries typically consists of the following major tectonic units. There is the subducting slab itself, which at the beginning of descent into the underlying upper mantle, consists of pelagic sediments, oceanic crust of basalt, dolerite and gabbro, hydrated and pristine oceanic upper mantle. This slab undergoes series of metamorphic reactions, and may possibly melt, as it descends into the upper mantle and perhaps even togreater depth.

Above the subducting slab is an upper mantle wedge. The material in this zone consists of peridotite that may be altered by ingress of fluids or melts from the subducting slab. This metasomatism might affect the chemical composition of the upper mantle wedge itself, both in regard to major and trace element chemistry. This alteration in bulk composition, in turn, results in the formation of minerals not found in peridotite elsewhere in the Earth's upper mantle. Furthermore, melting processes ultimately responsible for formation of overlying crust as well as near-surface volcanic and hydrothermal activity, most likely originate in the mantle wedge immediately above the subducting slab.

Above the upper mantle wedge, there is crust, often referred to as an island arc although in continent-continent collision environments, the surface manifestation will be uplift and mountain chain formation. The crust in island arcs consists of lower and intermediate portions that include igneous intrusives dominated by intermediate granitoids, but does also include more mafic components such as gabbro, in particular at at greater depth. Active volcanism, often, but not always, of an explosive nature, is a typical feature in island arcs. The explosive nature of this volcanism can be traced to H2O released during dehydration of the subducting slab during its descent into the mantle.

Metamorphic reactions that occur in the descending slab and in the overlying mantle wedge during plate descent require identification and characterization through field and experimental studies. To this end, experimental data on element partitioning behavior and phase relations among relevant minerals during hydration and dehydration and during partial melting have been the focus of much research.

Study of the relevant mineral assemblages provides the key for establishing the mechanisms by which the bulk compositions, temperature and pressure control the of both the subducting slab and the overlying mantle wedge. Viable physical and chemical models for upper mantle near convergent plate boundaries can be tested only with experimentally-determined partitioning of trace and major elements between minerals, melts and fluids, by detailed laboratory studies of phase equilibria, volatile solubility in melts and silicate solubility in fluids, and by thermochemical properties of the phases and phase assemblages involved.

In view of the chemical complexity of natural materials, information such as that illustrated above can realistically be obtained by integrated research efforts that include field observations, laboratory experiments, and theoretical modeling. Significant experimental progress has been made in this area. In this review, some of the relevant natural observations will be combined with published results that offer experimental constraints on the mineralogy and physicochemical properties of the upper mantle.

In this chapter, we will review the most important natural, experimental, and theoretical information relevant to the mineralogy of the upper mantle near convergent plate boundaries. Information from natural observations will be summarized first. This summary will be followed by a discussion of experimental data on stability of minerals and mineral assemblages. Some aspects of materials transport from the subducting slab to the overlying mantle, and the consequences of this transport for the chemical composition and phase relations in the overlying mantle will also be examined.