Strontium peridotites

The distribution of lithophile trace elements (REE + mbidium, caesium, strontium, barium, yttrium, zirconium, hafnium, niobium, tantalum, thorium, and uranium) normalized to primitive mantle (PM) values are illustrated in Figure 16 for a range of peridotite lithologies from the Ronda orogenic Iherzolite massif, and in Figure 17 for ophiolitic and abyssal refractory peridotites. 

The abyssal peridotites from the Izu-Bonin-Mariana Forearc resemble the ophiolitic peridotites. They are, nevertheless, distinguished by more prominent enrichments in alkaline and alkaline-earth elements (rubidium, caesium, strontium, and barium), possibly reflecting a supra-subduction imprint (Parkinson and Pearce, 1998). Except for one sample, they lack the niobium enrichment relative to FREE that is observed in the Ronda and... 

In addition, a detailed study of a metasomatized wall rock adjacent to an amphi-bole-pyroxenite dike has revealed that the contamination in " Nd/ " " Nd and Sr/ Sr is spatially limited to a distance of <25 cm from the dike (Bodinier et al., 2003). This distance is interpreted as the chromatographic front of neodymium and strontium since the infiltration of small volume melts probably occurred on a greater distance in the host peridotite, as attested by the enrichment of the highly incompatible elements observed at 25-80 cm from the dike (e.g., lanthanum and cerium, see Section... 

The silicate minerals account for aU the HREE budget and 50-90% of the LREE, strontium, and Zr-Hf in apatite-free peridotites. 

Clinopyroxene shows a range of REE patterns from extremely enriched to very depleted TREE signatures (Figure 22). Noncratonic peridotites are subdivided on the basis of clinopyroxene REE patterns into LREE-depleted (type lA) and LREE-enriched (type IB Menzies, 1983 Figure 17). LREE-enriched type IB pyroxenes are the norm in most suites. LREE-depleted varieties are relatively scarce. Very few clinopyroxenes show simple LREE-depleted REE patterns that can be interpreted solely in terms of melt depletion, i.e., LREE depletion, fiat, unfractionated MREE-HREE patterns (e.g., UM-6 or 2905 Eigure 22). For peridotites that do have LREE-depleted clinopyroxenes, a correlation of HREE with other incompatible trace elements (e.g., yttrium, strontium, zirconium) in xenoliths suites worldwide requires fractional melting to be the principal means of depletion in the mantle (Norman, 2001). 

Ionov et ai, 1997). Both strontium and barium are higher in amphibole from xenohths than those from massif peridotites (e.g., Vannucci et ai, 1995). Numerous ion-probe and laser/solution ICP-MS studies have found wide variation in amphibole REE patterns depending on textural context and tectonic setting (Witt- Eickschen and Harte, 1994 Vannucci et al, 1995 Ionov and Hofmann, 1995 Johnson et al, 1996 Chazot etai, 1996 Vasellieta/., 1995 Ionov eta/., 1997 Pearson and Nowell, 2002 Gregoire et ai, 2002). 

Most apatites are chlorine-rich (2-4.3%) with low F/Cl (0.1 -0.3). High fluorine ( 5%) and low chlorine (—0.25%) have been reported for apatite in spinel Iherzolites from Pacific OIB (Hauri et al., 1993). Extremely high strontium contents, commonly >2X10 ppm and up to 7 wt.% (Ionov et al., 1997 Table 9) are common in mantle apatites meaning that this phase is a major repository for strontium when present in peridotites at abundances of 0.1% or above. Rb/Sr is very low. Apatites have high levels of REE and are LREE-enriched (Table 9). Lanthanum and cerium concentrations can reach >1 wt.% and neodymium concentrations can be above 1,000 ppm. Sm/Nd is below PUM. HFSE are low and so the presence of this phase does not affect bulk rock HFSE chemistry. 

One of the first studies to show this was performed on Kilboume Hole spinel Uierzolites (Jagoutz et al, 1980). Equihbrated neodymium isotopes in orthopyroxene and diopside defined essentially zero age isochrons, consistent with the very recent eruption age of the host volcanic rocks, while strontium isotopes were un-equilibrated. Stolz and Davies (1988) found varying degrees of equihbration between amphibole, clinopyroxene and apatite in peridotite xenoliths from S.E. Australia. Several samples contained coexisting amphibole and clinopyroxene and had almost reached isotopic equilibrium for strontium but displayed disequilibrium relations for lead and neodymium isotopes. This was taken to indicate more rapid diffusion of strontium than lead and neodymium. Some peridotite and eclogite... 

Few systematic Sr-Nd isotope studies have been performed on ocean island xenolith suites. Ducea et al. (2002) analyzed clinopyroxenes from plagioclase-spinel and spinel peridotites from Pali, (Oahu, Hawaii) and found relatively depleted strontium and neodymium isotope systematics that they interpret as representing their evolution as residues from the extraction of Pacific Ocean crust. Consistent with this is a 61 20Ma errorchron defined by the pyroxene separates that is within error of the 80-85 Ma age of Pacific lithosphere beneath Hawaii. 

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... 

Figure 52 Initial ENd ssr isotope plot of calculated whole-rock eclogites from Africa (Koidu—Hills and Haggerty, 1989 Orapa—Smith et al., 1989 Viljoen et al, 1996 Roberts Victor—Jagoutz et al, 1984 Smith et al., 1989 Jacob and Jagoutz, 1994 Bellsbank—Neal et al., 1990), Yakutia (Snyder et al, 1993 Jacob et al, 1994 Pearson et al., 1995a Snyder et al, 1997), and eclogitic inclusions in diamonds (Richardson, 1986 Richardson et al, 1990 1999 Smith et al, 1991). Eclogite data are initial calculated whole-rock neodymium isotope compositions. Strontium isotope compositions are from clinopyroxene only. Cratonic peridotite minerals and diamond inclusions are plotted for comparison. Arrow points to a sample from Udachnaya that has a calculated initial SNd of 484. dashed lines are...

Roe G. D. (1964) Rubidium-strontium analyses of ultramahe rocks and the origin of peridotites. DPhil. Thesis, Massachusetts Institute of Technology. 

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