Heat is the primary driving energy for all geological processes: volcanism is the surface expression of thermal heterogeneities within the Earth. Mantle-derived volcanism provides a tool to unravel internal Earth processes: the nature of mantle convection, fluid transport, rheological properties and heat transfer mechanisms and the chemical and physical nature of the asthenosphere/lithosphere/crust system. The volcanic rocks yield geochemical averages of the melt/solid interfaces below the lithosphere but may be contaminated by overlying lithosphere during their ascent and may therefore reflect the geochemical signature of such lithosphere. Deep-seated xenoliths in volcanic rocks sample different levels and can be used to define empirical paleogeotherms characterizing the ambient thermal regime at the time of eruption. The characterization of the thermal structure of the Earth using xenoliths entrained in mantle-derived volcanics is a basic and very powerful tool for investigating the nature of the lithosphere in space and time. Xenoliths and high-pressure mineral debris in volcanic rocks yield the paleostratigraphies and thermal profiles of different types of lithosphere and measure the depths to the crust-mantle boundary and the lithosphere-asthenosphere boundary. Such xenoliths provide the raw materials to link geological, geochemical and petrophysical parameters with present-day remotely-sensed geophysical information. Repeated volcanic episodes that have occurred in the same crustal region can be used trace thermal, physical and chemical modifications of lithosphere through time. Ancient lithosphere rejuvenated by fluid infiltration (metasomatism) may source younger volcanism in response to a new thermal pulse as old cratonic keels erode and transform. Xenoliths in basaltic rocks, kimberlites and lamproites are the key to realistic interpretation of geophysical data in terms of lithosphere stratigraphy: geophysical data can then be used to extend the xenolith-derived stratigraphic profiles laterally. Xenoliths therefore provide the means to map the evolution of Earth's lithosphere in space and time (4-D Lithosphere Mapping).
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|Published - 30 Sept 1996