Xenoliths composition

The major element composition of the subcontinental lithosphere The principal chemical feature of the subcontinental lithosphere is one of strong chemical depletion. This is evident when xenolith compositions are plotted relative to both the Bulk Silicate Earth and the oceanic lithosphere (Fig. 3.12). There are also compositional differences between Phanerozoic and Archaean subcontinental lithosphere as is illustrated by the... 

Xenoliths from Siberian continental lithosphere, with Archean model ages, had b Li as low as +0.5 (Eouman et al. 2000). If these values accurately represent the Archean mantle, they suggest the potential for Li isotopic evolution in the Earth, from lighter compositions in the ancient mantle to what is seen in present-day MORE. In spite of the analytical challenges presented by ultramafic rocks, more data from these materials are crucial to an understanding of Li in the mantle, and in resolving questions about the appropriateness of the accepted MORE mantle range. 

Metasomatized xenoliths studied by Nishio et al. (2004) show a diversity of Li isotopic compositions (ST i = -17.1 to +6.8), some of these correlated with variations in radiogenic... 

Elliott et al. (2003). The data set for unmetasomatized peridotite xenoliths is too sparse to aid much in this controversy at present. As measurement reproducibility betters the 0.5%o barrier, it is just a matter of analyzing the most appropriate samples before such questions can be answered and an accurate estimate can be made of the Li isotopic composition of the mantle, and hence of the bulk Earth. 

Bouman C, Elliott TR (1999) Li isotope compositions of Mariana arc lavas Implications for crust-mantle recycling. Ninth Goldschmidt Conf Abst, LPI Contribution 971, Lunar Planetary Institute, 35 Bouman C, Elliott TR, Vroon PZ, Pearson DG (2000) Li isotope evolution of the mantle from analyses of mantle xenoliths. J Conf Abst 5 239... 

Moecher DP, Valley JW, Essene EJ, (1994) Exhaction and carbon isotope analysis of COj from scapolite in deep crustal granulites and xenoliths. Geochim Cosmochim Acta 58 959-967 Mojzsis SJ, Harrison TM, Pidgeon RT (2001) Oxygen-isotope evidence from ancient zircons for liquid water at the Earth s surface 4,300 Myr ago. Nature 409 178-181 Muehlenbachs K, Clayton RN (1976) Oxygen isotope composition of the oceanic crust and its bearing on seawater. J Geophys Res 81 4365-4369... 

Figure 14. Inter-mineral Fe isotope fractionations among olivine and clinopyroxene from spinel peridotite mantle xenoliths. Data are from Zhu et al. (2002) ( ) and Beard and Johnson (2004) ( ). In the study by Beard and Johnson (2004), the difference in the Fe isotope composition between clinopyroxene and olivine is larger as a function of their 5 Fe values, suggesting disequilibrium fractionation.

Piccardo G. B. and Ottonello G. (1978). Partial melting effects on coexisting mineral compositions in upper mantle xenoliths from Assab (Ethiopia). Rend. S.I.M.P, 34 499-526. Pitzer K. S. (1973). Thermodynamics of electrolytes. I Theoretical basis and general equations. J. Phys. Chem., 77 268-277. 

Galy et al. (2001) suggested that the mantle should have a homogeneous Mg isotope composition. Pearson et al. (2006), however, demonstrated that olivines from mantle xenoliths have a heterogeneous compositions with a 5 Mg range of about 4%c. These authors suggested that the differences are due to diffusion-related meta-somatic processes. 

Sources of information about the isotopic composition of the upper portion of the lithospheric mantle come from the direct analysis of unaltered ultramafic xenoliths brought rapidly to the surface in explosive volcanic vents. Due to rapid transport, these peridotite nodules are in many cases chemically fresh and considered by most... 

Interesting differences in sulfur isotope compositions are observed when comparing high-S peridotitic tectonites with low-S peridotite xenoliths (Fig. 3.7). Tec-tonites from the Pyrenees predominantly have negative 5 " S-values of around —5%c, whereas low-S xenoliths from Mongoha have largely positive 8 S-values of up to - -l%o. Ionov et al. (1992) determined sulfur contents and isotopic compositions in... 

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