Melting primitive mantle

The primitive mantle-normalized trace-element spider diagram of felsic rocks shows negative Sr and Eu anomalies that are indicative of either plagioclase restite or plagioclase fractionation resulting from a combination of the partial melting and fractional crystallization processes (Fig. 5), and later changed by hydrothermal alteration. 

The least fractionated rocks of the Earth are those that have only suffered core formation but have not been affected by the extraction of partial melts during crust formation. These rocks should have the composition of the PM, i.e., the mantle before the onset of crust formation. Such rocks are typically high in MgO and low in AI2O3, CaO, Ti02, and other elements incompatible with mantle minerals. Fortunately, it is possible to collect samples on the surface of the Earth with compositions that closely resemble the composition of the primitive mantle. Such samples are not known from the surfaces of Moon, Mars, and the asteroid Vesta. It is, therefore, much more difficult to reconstruct the bulk composition of Moon, Mars, and Vesta based on the analyses of samples available from these bodies. 

The late veneer hypothesis has gained additional support from the analyses of the osmium isotopic composition of mantle rocks. Meisel et al. (1996) determined the Os/ Os ratios of a suite of mantle xenoliths. Since rhenium is more incompatible during mantle partial melting than osmium, the Re/Os ratio in the mantle residue is lower and in the melt higher than the PM ratio. By extrapolating observed trends of Os/ Os versus AI2O3 and lutetium, two proxies for rhenium, Meisel et al. (1996) determined a Os/ Os ratio of 0.1296 0.0008 for the primitive mantle. This ratio is 2.7% above that of carbonaceous... 

Figure 19 Left (Pd/Ir) , where n refers to values normalized to primitive mantle values (McDonough and Sun, 1995) versus bulk rock AI2O3. Right (Os/Ir) versus (Pt/Ir) plot of cratonic and olf-craton whole-rock peridotites. Curves on left-hand plot are trends expected for progressive melting of mantle peridotite. On the right-hand plot, large circle with error bars = mean and 1 SD for cratonic peridotites. Large square with error bars = mean and 1 SD for off-craton peridotites. Cratonic peridotite data are isotope dilution data from Pearson et al. (2002), Irvine (2002), Pearson et al. (2004), and Irvine et al. (2003). Off-craton xenolith data from Handler and Bennett (1999) and...

Figure 6 Major-element oxides (wt.%) versus FeO as a function of pressure (GPa) and degree of batch melt extraction (sources the 1 GPa and 2 GPa trends are based on the Kinzler and Grove (1992a, 1993) model for melting of primitive mantle of McDonough and Sun (1995) (composition 1, Table 1), and the trends at higher pressures are based on the data of Walter (1998) for melting of fertile peridotite KR4003).

Figure 10 shows major-element oxides versus Mg for off-craton and oceanic mantle, as well as some estimated compositions for primitive mantle (Table 1). As expected from the normative plots, the two sets of mantle compositions have distinct trends for all oxides. Previous models for primitive upper mantle have a range in Mg from 89 to 90, and Figures 9 and 10 show that the oceanic and off-craton trends also converge within this range. Assuming that the off-craton and abyssal mantle trends are due primarily to melt extraction from a common protolith, then the intersection of the trends should provide a good estimate for the composition of fertile upper mantle for major elements. 

Melt extraction at higher pressures as applied to cratonic mantle is modeled using the experimental data of Walter (1998) for a composition that is very similar to the primitive mantle of McDonough and Sun (1995). 

Asahara Y. and Ohtani E. (2001) Melting relations of the hydrous primitive mantle in the CMAS-H2O system at high pressures and temperatures, and imphcations for generation of komatiites. Pkys. Earth Planet. Inter. 125, 31-44. 

The formation of basalts by partial melting of the upper mantle at mid-oceanic ridges and hot spots provides the opportunity to determine mantle composition. Early studies of radiogenic isotopes in oceanic basalts (e.g., Eaure and Hurley, 1963 Hart et al, 1973 Schilling, 1973) showed fundamental chemical differences between OIBs and MORBs (see Chapter 2.03). This led to the development of the layered mantle model, which consists essentially of three different reservoirs the lower mantle, upper mantle, and continental cmst. The lower mantle is assumed primitive and identical to the bulk silicate earth (BSE), which is the bulk earth composition minus the core (see also Chapters 2.01 and 2.03). The continental cmst is formed by extraction of melt from the primitive upper mantle, which leaves the depleted upper mantle as third reservoir. In this model, MORB is derived from the depleted upper mantle, whereas OIB is formed from reservoirs derived by mixing of the MORB source with primitive mantle (e.g., DePaolo and Wasserburg, 1976 O Nions et al., 1979 Allegre et al., 1979). 

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