Xenoliths, isotopic composition

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.

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... 

To interpret xenolith data, however, it is important to consider the general question of how well they can be expected to preserve the record of their mantle environment independently of the record in their host magmas (cf. Hofmann Hart, 1978, for solid elements Trull Kurz, 1993, for noble gases and Kellog, 1992, for a general discussion). Studies of Sr, Pb, and Nd isotopic compositions in various xenoliths indicate that these elements generally have not equilibrated with their host lavas. Experimental studies of diffusion coefficients of Sr, Mg, Al, and other cations indicate that this is reasonable, and that isotopic disequilibrium on a centimeter scale could persist for periods at least of the order of 10s to 109 year in dry crystalline... 

Figure 6.13 Kr isotopic ratios in MORB, OIB, diamond, and mantle xenolith are plotted as percentage deviation from the air Kr isotopic composition. MORB Hiyagon et al. (1992). OIB Hiyagon et al. (1992). Diamond Ozima and Zashu, 1991. Xenolith Poreda and Farley (1992). N number of data analyzed.

Meisel T., Walker R. J., Irving A. J., and Lorand J.-P. (2001) Osmium isotopic compositions of mantle xenoliths a global perspective. Geochim. Cosmochim. Acta 6S, 1311-1323. 

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 31 Whole-rock versus mineral separate Nd-Sr isotopic compositions for kimberlite derived xenoliths from Bultfontein (Richardson et al, 1985) and one sample from Jagersfontein (Walker et al., 1989a). Lines connect coexisting phases/whole-rocks. Initial isotopic composition of the Bultfontein kimberlite is also plotted. Jagersfontein...

A consequence of this varied elemental partitioning and hence parent-daughter isotope fractionation, is that the extent to which isotopic equihbria is achieved by xenohths is also very variable, despite the high equilibration temperatures of mantle rocks. This indicates that many of the processes that affect the incompatible element isotope systematics in mantle xenoliths are of a relatively recent nature compared to the age of the rock. Hence, it is difficult to simply use the measured parent/daughter element ratios of xenohths and their minerals to predict what their measured isotopic compositions should be. 

Figure 37 Frequency distribution plots for Os, Nd, and Sr isotope compositions of cratonic and noncratonic peridotite xenoliths. Upper right plots give the range for ocean island basalts (OIB) and arrows show the direction of isotopic evolution for melt depletion and enrichment events. Data compiled from sources cited in Menzies (1990b), Pearson (1999a,b), and those given in Figure 21 (after Pearson and Nowell, 2002).

If continental peridotite xenoliths are divided into cratonic and noncratonic compositions, the vast majority of highly enriched neodymium isotope compositions originate in cratonic mantle and very few are evident in noncratonic mantle. Enriched neodymium isotope compositions can also be found in mantle sampled by orogenic peridotites (Reisberg and Zindler, 1986 Pearson et ai, 1993 Chapter 2.04) but not the extreme values evident in cratonic CLM. This is expected, given the great antiquity of cratonic CLM. Further subdivision of cratonic samples (Figure 37) shows... 

Four cratonic, ultradeep xenoliths from S. Africa and Sierra Leone have been analyzed for their strontium and neodymium isotopic compositions (Macdougall and Haggerty, 1999). The neodymium isotopic compositions of minerals from these xenoliths suggest that they were emplaced into the African lithosphere at times ranging from approximately the time of kimberlite emption to hundreds of millions of years earlier. The samples show a complex history of melt... 

Figure 39 Cross-section of typical cratonic and noncratonic lithospheres as sampled hy xenoliths showing the variation in ohvine Mg, density (p). Os and Nd isotope compositions with depth.

Compared with neodymium and strontium, there are relatively few studies of the lead isotopic compositions of mantle xenoliths and the systematics are probably biased towards samples that show some degree of patent metasomatism in the form of introduction of amphibole and/or mica. Much of the data come from noncratonic metasomatized peridotites (e.g., Stolz and Davies, 1988) and cratonic MARID xenoliths. Some type I xenoliths that do not have patent metasomatism, from cratonic and noncratonic settings (Kramers, 1977 Galer and O Nions, 1989 Walker et al., 1989 Lee et al., 1996) together with various... 

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. 

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