RICERCA ITALIANA     

LITHOSPHERE EVOLUTION INDUCED BY MIGRATION OF MANTLE-DERIVED MELTS AT DIFFERENT GEODYNAMIC SETTINGS

Università degli Studi di Genova

Abstract

The compositional heterogeneity of the lithospheric mantle is well established from detailed petrologic, geochemical and geophysical studies. However, the distribution, the scale and the magnitude of lithospheric mantle heterogeneity are still controversial, such as the origin and the consequences of this heterogeneity. Reliable informations on these debated issues are provided by mantle xenoliths in alkaline lavas and mantle peridotite massifs, which clearly document that reactive melt migration and melt-rock interaction strongly modify mineralogical-chemical composition and rheology of the lithospheric mantle. The large number of complex factors and parameters that are involved prevents the development of reliable models that accurately describe the effects of the melt-peridotite interaction, as well as the origin and characteristics of distinct geochemical reservoirs in the lithosphere.
The present research program is aimed at describing and investigating the effects of melt-mantle reactions at different geodynamic settings. The objective is twofold: i) to define the petrologic and geochemical constraints which must be used in developing realistic and reliable models of melt reactive percolation through sectors of the lithospheric upper mantle; ii) to reconstruct the temporal and compositional evolution of the lithosphere depending on the geodynamic setting. The physical-chemical features of the reactive migration and their space-time changes along the percolated >>>

Principal Investigator

Giovanni Battista PICCARDO Università degli Studi di GENOVA

Research Objectives

The heterogeneity of the lithospheric mantle is well established from detailed petrologic, geochemical and geophysical studies. However the distribution, scale, and magnitude of the heterogeneity of lithospheric mantle are still controversial, as still controversial remain the origin and the consequences of this heterogeneity. A critical question in erupted magma studies is the extent to which lithospheric mantle domains are capable of producing and releasing melts with peculiar geochemical signatures upon partial melting rather than pervasively reacting with the surrounding mantle column. On the other hand, solid Earth's researchers need to constrain the extent to which texture, matrix mode and geochemistry of lithospheric mantle sectors have been modified by peridotite-melt reactions. Many studies have demonstrated the theoretical potential of reactive melt transport for modifying the geochemical signatures in magmas (McKenzie, 1984; Kelemen, 1986; Iwamori, 1993a, 1993b; Kelemen et al., 1995; Spiegelman, 1996). Similarly, mantle xenoliths and peridotite massifs world-wide show evidence that migration of melt strongly modifies the composition of solid mantle (Bodinier et al., 1990; Vasseur et al., 1991; Godard et al., 1995; Van der Wall & Bodinier,1996).
Unfortunately, the development of complete models of geochemical exchange between migrating melts and lithospheric mantle has been hampered by the high number of complex factors and parameters involved, such as >>>

Timescale

24 months

National and international background

The bulk of petrographic-structural and petrologic-geochemical data collected in the last years on the lithospheric mantle clearly shows that this latter is highly heterogeneous and that the great variability of its structural features and modal and chemical composition is induced by percolation of melts and by melt-rock interaction processes. These processes have a fundamental role in the evolution of the lithosphere and must be interpreted within the more general frame of the thermochemical interaction between asthenosphere and lithosphere at the different geodynamic settings.
Major factors controlling the migration of melts and their interaction with the lithosphere:
- the ionic exchange mechanisms, such as dissolution and precipitation of mineral in the solid, the element diffusion, the volatiles transfer between solid and melt, the melt propagation in veins;
- the characteristics of the melt-solid system, such as the type of melt flow ("porous flow" or hydrofracturing), the melt velocity and distribution, the solid compaction;
- the physico-chemical parameters, such as the primary melt composition, closely related to that one of their mantle sources, the composition of mantle lithologies, the volatile elements content, the thermal gradient in the lithosphere and between melts and lithospheric rocks, the mass ratios of melt and peridotite;
- the kinetic aspects, such as the diffusion velocity of elements in solid and melt, the >>>