The Chemical Structures of the Earth and their Evolution: Arguments and Debate

Claude Jean Allègre Laboratoire de Géochimie et Cosmochimie, Université Paris 7 et IPGP,

4 Place Jussieu, 75252 Paris Cedex 05, France

The first question that has been debated over the years is the alternative between whole mantle versus two layer convection. An approach to this problem can be made by the use of :

* Lithophile isotopic systems 87Rb/87Sr, 147Sm/143Nd, 176Lu/176Hf, or 232Th/208Pb, 235U/207Pb, 238U/206Pb (including or excluding the continental lithosphere).

* The rare gas isotopic systems 4He/3He, 20Ne/22Ne, 21Ne/22Ne, 40Ar/36Ar, 129Xe/130Xe, 136Xe/130Xe.

The result of quantitative modelling of present data suggests :

* Two layer convection with limited exchange between the two reservoirs.

* The establishment of this structure occurred relatively early in Earth history.

The second question is the origin of OIB. Since the proposition by Hofmann and White about the affinity of OIB (ocean island basalts) with recycling oceanic crust the origin of OIB has been a matter of debate. The alternatives for the generating place are the boundary layers either at the core-mantle interface or at 670 km. The use of the ISOTER (ISOtope-like Trace Element Ratios) Ce/Pb, Nb/U etc... suggest a common source reservoir for MORB and OIB. The rare gas data may be interpreted as evidence that the same OIB come directly from the lower mantle. Courtillot's hypothesis that OIB should be divided into two sets, one from the lower mantle, one from the upper mantle, is supported by 3He characteristics; Hart's Fozology should also be discussed in this context.

The origin of the different end members in the OIB elemental and isotopic ratio multispace is still a matter of debate. Are the end members and components similar? Should we consider rare gases in our characterisation? Linked to these questions, the origin of CFB (continental flood basalts) also offers much room for debate. Are the CFB linked to OIB at their point of origin, the lower mantle? A systematic survey that considers all isotopic tracers is complicated because of the interaction with continental lithosphere and crust.

Since the discovery of the Indian Ocean isotopic singularity, the question of regional domains in the mantle is posed. How many provinces exist? What do the boundaries look like? Can we map the mantle geochemically? Should we split the mantle into domains before doing any mass balance? Should we consider the continental lithosphere as an important reservoir?

The dynamics of the mantle is also a matter of debate for geochemists. One problem concerns the steady-state model for the upper mantle as proposed by Galer and O'Nions. Subsequent to their original proposal, the mass transfer between the lower mantle and the upper mantle has been estimated to be quite small. The steady-state then exists with respect to the continental crust and the recycling of the surface material. What is the role of the ocean and the sedimentary pile in this context? The extension of the steady-state model to the rare gases appears to be quite successful at least for the light ones. Can we define general steady-state parameters, like for the ocean, for the different elements. On the other hand, since the first suggestion by Tatsumoto and Hart of "mantle isochrons" the significance of those patterns have been under debate. In a modern view, the question is whether we can extract significant information about the stirring time of the mantle by the use of geochemical data. What kind of time scale will we obtain and how will such a time scale fit in general model, considering e.g. the time scale(s) of convective revervoir(s) or the time scale of generation of OIB?