Mantle melting residue

Figure 6 Correlation of Sm versus Sn in partial melts and residues of mantle melting. The constant Sm/Sn ratio suggests that this ratio is representative of the PM. Sn is depleted relative to Cl-chondrites by a factor of 35. The knowledge of the abundance of the refractory element Sm in PM allows the calculation of the Sn PM abundance. Many of the PM abundances of siderophile and chalcophile elements are calculated from similar correlations (after Jochum et al, 1993).

Improved analytical capabilities have led to the analysis of several hundred xenoliths for osmium isotopic composition. The compatible nature of osmium during mantle melting means that, unlike incompatible-element-based isotope systems, peridotite residues have much higher osmium contents than mantle melts and thus the system is less readily disturbed by later metasomatism (see Section 2.05.2.5.3). This is clearly shown by rhenium and osmium abundances (Figure 21). The vast majority of rhenium contents of both cratonic and noncratonic peridotite xenoliths are below the PUM value proposed by Morgan et al (1981) and many are P-PGE depleted. This contrasts with almost universal TREE enrichment of whole-rock peridotites. That the Re-Os system is not immune from the effects of metasomatism is illustrated by the consideration of extended PGE patterns (Figure 20 Section 2.05.2.5.3 Pearson et al., 2002, 2004). Dismption of both rhenium and osmium in some mantle environments may have occurred (Chesley et al, 1999), especially where sulhde metasomatism is involved (Alard et al, 2000). However, Pearson et al. (2002, 2004) and Irvine et al (2003) have shown that coupled PGE and Re-Os isotope analyses can effectively assess the level of osmium isotope disturbance in peridotite suites. 

Walter M. J. (1999) Melting residues of fertile peridotite and the origin of cratonic lithosphere. In Mantle Petrology Field Observations and High Pressure Experimentation (eds. Y. Fei, C. Bertka, and B. O. Mysen). The Geochemical Society, Houston, vol. 6, pp. 225-240. 

In this chapter the mineral-melt phase equilibria that control the compositions of partial melts are examined on the basis of experimental and thermodynamic databases, and this information is used to predict the effects of partial melt extraction from fertile upper mantle on residual mineralogy and major-element chemistry. 

Full fluid-mechanically consistent melt transport models with reactive channeling extend the results of the two-porosity models and produce distributions of compositions for both stable and radiogenic tracers in melts and residues. These models suggest that much information on the structure and rates of magmatic process might be contained in the observed variability of mantle melts but they need to be explored more rigorously. 

Figure 1 Systematics of Nd- and Hf-isotopic evolution in the bulk Earth, continental crust, and mantle. Daughter elements Nd and Hf are more incompatible during mantle melting (more likely to go into a partial melt of mantle rock) than Sm and Lu, respectively. As a result, the continental crust has a lower Sm/Nd and Lu/Hf ratio than the mantle, and lower Nd- and Hf-isotope ratios. Young continental crust has isotope ratios similar to the mantle, and the older the continental terrain, the lower the Nd- and Hf-isotope ratios. Rb-Sr behaves in the opposite sense, such that the parent element Rb is more incompatible than the daughter element Sr. (a) Schematic example of the evolution of Nd- and Hf-isotope ratios of a melt and the melt residue from a melting event around the middle of Earth history from a source with the composition of the bulk Earth, (b) The same scenario as in (a), but with the isotope ratios plotted as e d and snf. The bulk Earth value throughout geological time is defined as e d and SHf = 0> and e-value of a sample is the parts per 10 deviation from the bulk Earth value.

The Re-Os isotope system differs from isotope systems based on lithophile elements (Sr-Nd-Hf-Pb) because Os is a compatible element during mantle melting and thus remains in the residue following melt extraction. Re is moder-... 

Normal mantle melting does not produce a residue with the mineralogical composition of old subcontinental lithosphere... 

Normal mantle melting, such as that which leads to the formation of oceanic crust and oceanic islands, produces magma of basaltic to picritic composition and leaves a residue consisting of olivine, orthopyroxene, chnopyroxene and an aluminous phase. The compositions of minerals in this residue, and their relative proportions, are unlike those in old subcontinental hthosphere Mg-Fe ratios of the ferromagnesian minerals are too low, and the amount of chnopyroxene and spinel or garnet is too high. 

Fig. 2. Calculated proportions of refractory residue formed during mantle melting, (a) Radial profiles for cylindrical symmetry and the radial distribution of residue, of source and of geometric volume (b) the McKenzie Bickle (1988) relationship between temperature and local degree of melting (c) and (d) the distribution as function of temperature and degree of melting. It should be noted that because the two horizontal scales are not linearly related, the distribution maximum around 35-40% melting corresponds to the fiattest part of the McKenzie Bickle curve.

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