Superplumes – A Geochemical and Petrological view of Mantle Plume

Australia: The Land Where Time Began

Superplumes – A Geochemical and Petrological view of Mantle Plume

It has been considered that basaltic lavas that are erupted at hotspot volcanoes or oceanic plateaus are a manifestation of buoyant plumes that ascend through the mantle. Key insights into the activity of mantle plumes, including ordinary ones and superplumes, and styles of convection in the mantle, may therefore be provided by the chemical composition of such basalts that are plume-related. Recycling of subducted crustal materials are strongly suggested to play an important role in the ocean island basalts (OIB) by isotopic and major element compositions. Isotopic diversity of OIB, however, appears to be too homogeneous to have been derived from subducted materials which are expected to have compositions that are extremely heterogeneous because of the variability of their compositions, history of modification during subduction modification, and residence times in the mantle. According to Kogiso¹ it requires detailed knowledge of the dynamic and rheological behaviour of mantle and crustal materials under the conditions in the mantle, as well as their chemical characteristics, to gain an understanding of the origin of the enriched components (EM1, EM2, and HIMU), which characterise the isotopic diversity of OIB. It is suggested by the compositions of major elements of OIB that OIB are generated by peridotite-pyroxenite heterogeneous mixtures at temperatures above the normal mantle geotherm. Major element compositions of the basalts in oceanic islands, in contrast, are similar to those of basalts from mid-ocean ridges, MORB, which does not require excess temperature or significant amounts of extra components such as pyroxenite in their sources. It is implied by these observations that OIB are generated from small hot plumes during normal periods of the history of the Earth, whereas basalts from oceanic plateaus are generated from plumes that of high flux but not hot plumes that have been forced by the flushing of subducted slabs down to the lower mantle, to ascend through the upper mantle. This in turn brings into question the conception that superplumes are generated from the bottom of the mantle with abnormally high temperatures.

In plume-related volcanics, such as those at hotspot volcanoes, oceanic plateaus, and continental flood basalt provinces, the major product is basalt. The chemical composition of basaltic lavas that are plume-related are generally a reflection of the plume material (e.g. Hoffmann, 1997), the physical conditions of plume magmatism, including the temperature and the rate of upwelling of the source mantle (Watson & McKenzie, 1991; Sims et al., 1999). In this contribution Kogiso¹ reviews the geochemical and petrological features of oceanic basalts that are plume-related in order to reveal the chemical properties of mantle plumes, which may provide some constraints on the physical conditions of plume magmatism and the dynamics of plumes magmatism, as well as the dynamics of mantle plumes, including the ordinary type and the so-called superplumes.

Isotopic ratios and the compositions of major elements in oceanic basalts are powerful tools used in the determination of the chemical properties of mantle plumes. Important chemical characteristics of basalt sources are revealed by isotopic ratios of some key elements, such as Sr, Nd, Pb and Os (Zindler & Hart, 1986), as the characteristics of source materials are reflected by the isotopic ratios of these elements that do not change during the partial melting process. Major element compositions of basalts, on the other hand, are controlled by the chemical composition of their sources and the pressures and temperatures that prevail during the generation of magma, allowing them to be used for constraining the physical setting of partial melting of basalt sources (McKenzie & Bickle, 1988). Isotopic ratios and major-element compositions of basalts therefore provide complementary information on the chemical and physical manifestations of plumes. While considering the isotopic and major-element characteristics of plume-related basalts, Kogiso¹ considers how the chemical compositions of plume-related basalts can or cannot be linked to the chemical features of plume source materials and to the physical expression of mantle plumes.

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