RICERCA ITALIANA     

MANTLE HETEROGENEITY AND GEODYNAMIC EVOLUTION OF THE LITHOSPHERE-ASTHENOSPHERE SYSTEM

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, scale, and magnitude of these heterogeneities are still controversial, as still debated remain their origin. A critical question is the extent to which lithospheric mantle domains are capable of producing and releasing melts with peculiar geochemical signatures upon partial melting or pervasive reaction with upwelling asthenospheric melts.
Magma sources in the asthenosphere-lithosphere system bring records od distinct metasomatic components in the different geodynamic settings, as indicated by the peculiar geochemical imprintings of the magmatic associations from both anorogenic and supra-subduction settings, either continental or oceanic, and intra-plate and plume-related settings. In all these settings, the lithopsheric peridotites record evident effects of processes of reactive percolation of metasomatic components and asthenosheric melts, which significantly modify their structural and compositional characteristics.

This project aims at investigating the profound compositional heterogeneities of the mantle lithosphere at the different geodynamic settings (divergent and convergent plate margins and intraplate settings) to unravel processes which govern evolution and modification of the mantle lithosphere in space and time, evidencing the relevance of asthenosphere interaction >>>

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 these heterogeneities are still controversial, as still debated remain their origin and consequences. A critical question is the extent to which lithospheric mantle domains are capable of producing and releasing melts with peculiar geochemical signatures upon partial melting or pervasive reaction with upwelling asthenospheric melts. To correctly address this point a better understanding is needed of the interaction processes between lithosphere and asthenosphere, and particularly of how texture, matrix mode and geochemistry of lithospheric mantle sectors can be modified by interaction with percolating melts.
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 pervasive 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 metasomatic exchange between migrating melts and lithospheric mantle has been hampered by the high number of the involved complex factors and >>>

First Results

The bulk of petrographic-structural and petrologic-geochemical data collected in the last years on the lithospheric mantle clearly shows that, at the different geodynamic settings, its structural and compositional characteristics are highly heterogeneous in response to the processes active at the different settings.

This research project aims at investigating characteristics and causes of these heterogeneities at the different settings, to unravel the involved agents and processes, to contribute to the understanding of the geodynamic evolution of the lithosphere-asthenosphere system in relation with the chemical-physical parameters, the kinetic factors, the characteristics of the solid-liquid system and of the ionic exchange mechanisms active at the different geodynamic settings.

The multidisciplinary approach of this project, which involves petrographic-structural, petrologic, geochemical and isotopic studies on the mantle material and the associated melts at the different geodynamic settings, will drive to the understanding of: (i) the characteristics and processes which induce the compositional heterogeneity of the lithospheric mantle, either partial melting or melt interaction with percolating melts, (ii) the relationships between chemical-physical characteristics of the process responsible of these modifications, its evolution in space and time and the tectonic characteristic of the particular geotectonic setting, (iii) the formation >>>

Timescale

24 months

National and international background

1) INTRODUCTION
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 and compositional features is induced, besides partial melting processes, 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 are: 1) the ionic exchange and element diffusion mechanisms, 2) the characteristics of the melt-solid system, 3) the physico-chemical parameters, and 4) the kinetic aspects.
All the above parameters are significantly variable in function of the peculiar geodynamic setting. Including all of these parameters in a single model is extremely difficult (Navon and Stolper, 1987; Vasseur et al., 1991; Iwamori, 1993a, 1993b; Godard et al., 1995; Bedini et al., 1997; Vernières et al., 1997). However, detailed investigation of some carefully selected study cases can furnish important information for the development of simplified, but reliable models of melt-lithosphere interaction.
Of particular relevance for the comprehension of characteristics and processes which >>>