Peridotite-chondrite mantle model

Peridotite-chondrite models Rather than assuming a composition for the mantle, its composition may be estimated by directly measuring the composition of mantle samples. The biggest difficulty here is... 

FIGURE 3.9 The geochemical/ cosmochemical fractionation diagram for the Earth showing the various estimates of primitive mantle composition discussed in the text. The elliptical shaded area is the field for Cl chondrites and the analyses in Table 3.1 are plotted as weight ratios, for the chondritic model (C), the pyrolite model (P) and the peridotite-chondrite model (PC). 

The melting temperature of the Earth s proto-mantle is governed by its initial composition. Most experimental models assume a peridotitic composition for the Earth s mantle with liquidus temperatures of about 2500 °C at 25 GPa. However, if the Earth was undifferentiated and chondritic then the melting temperature might be several hundred degrees lower (Ohtani et al., 1986 Agee, 1997). Another possibility is that the proto-mantle... 

Pyrolite models Pyrolite models for the mantle are based upon the assumption that the composition of the mantle can be calculated by "reconstituting" the mantle from depleted mantle residues plus basaltic melt. The method was first proposed by Ringwood (1962), who showed that his model agreed well with heat flow data, and with the composition of carbonaceous chondrites minus a core component (Ringwood, 1966). More recently McDonough and Sun (1995) have used a similar approach. These authors examined a suite of fertile mantle peridotites,... 

The range of values measured on mantle peridotites overlaps with other estimates for the composition of the convecting upper mantle such as PUM and ophiolitic chromites. Nevertheless, the mean yQs value of ca. —2.0 differs from the estimate based on ophiolitic chromitites by 3.0 yQs units and from PUM by about 4.0 yQs units (Fig. 3.32). This result does not sit easily with a simple chondritic model for the upper mantle. 

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