EARTH/Mantle 399 Figure Temperature (degrees C) variation curve for the interior of the Earth Lines are isotherms (reproduced with permission from Mason B (1966) Principles of Geochemistry New York, London, Sydney: John Wiley and Sons, Inc.) carbonaceous chondrites fall to Earth extremely rarely compared with common chondrites can be explained both by their fragility and by the fact that common chondrites, according to theory, are derived from them by metamorphic processes However, doubts must remain about whether the material of chondrites, which accreted as asteroids in the interval between Mars and Jupiter at about 4500 Ma, is really representative of the material from which the Earth’s mantle accreted at about the same time There is an assumption of a homogeneous dust cloud from the Sun outwards The metal forming the core is supposed to have melted and gravitated to the centre of the planet (Figure 4) Whether accretion of the mantle from the dust cloud was homogeneous or heterogeneous (layered) remains undecided – both possibilities are shown in Figure Mantle Composition Concepts concerning the composition of the mantle have inevitably changed since the acceptance of the plate-tectonic paradigm It is relevant here to go back to the earlier statements Those of Arthur Holmes and Brian Mason are concise summaries of what was then believed Holmes, in a 1965 reissue of his famous book Physical Geology, wrote It has long been thought that meteorites, stony and iron, might be direct clues to the nature of the Earth’s mantle and core Stony meteorites are like our terrestrial peri dotites in many ways and, moreover a few varieties have compositions not unlike some of our basalts For these and other reasons a similar range of composition for the mantle, with ultrabasic materials predominating, has seemed to be a plausible guess The sudden change of seismic velocities at the Moho indicates either a change in composition (e.g from basic rocks above to ultrabasic below) or a change in state (from, say, gabbro or amphibolite to a high pressure modification, which, for convenience, we may refer to as eclogite), without much change in composition Olivine rich peridotites and garnet bearing varieties, at the appropriate tempera tures and pressures, give seismic velocities that are about right for most parts of the upper mantle Eclogite would equally well, and various associations of the rocks would match seismic and density requirements That eclogite does occur in the mantle is proved by the fact that it occurs in diamond pipes as inclusions More over, basaltic magma ascends from the mantle and