Peridotites isotopic ratios

O, H, C, S, and N isotope compositions of mantle-derived rocks are substantially more variable than expected from the small fractionations at high temperatures. The most plausible process that may result in variable isotope ratios in the mantle is the input of subducted oceanic crust, and less frequent of continental crust, into some portions of the mantle. Because different parts of subducted slabs have different isotopic compositions, the released fluids may also differ in the O, H, C, and S isotope composition. In this context, the process of mantle metasomatism is of special significance. Metasomatic fluids rich in Fe +, Ti, K, TREE, P, and other large ion lithophile (LIE) elements tend to react with peridotite mantle and form secondary micas, amphiboles and other accessory minerals. The origin of metasomatic fluids is likely to be either (1) exsolved fluids from an ascending magma or (2) fluids or melts derived from subducted, hydrothermally altered crust and its overlying sediments. 

Osmium isotopes currently provide the strongest case for mineral-to-mineral disequilibrium, and for mineral-melt disequilibrium available from observations on natural rocks. Thus, both osmium alloys and sulfides from ophiolites and mantle xenoliths have yielded strongly heterogeneous osmium isotope ratios (Alard et al., 2002 Meibom et al., 2002). The most remarkable aspect of these results is that these ophiolites were emplaced in Phanerozoic times, yet they contain osmiumbearing phases that have retained model ages in excess of 2 Ga in some cases. The melts that were extracted from these ophiolitic peridotites contained almost certainly much more radiogenic osmium and could, in any case, not have been in osmium-isotopic equilibrium with all of these isotopically diverse residual phases. 

Figure 9 Osmium isotope ratios in MORE and abyssal peridotites. This diagram shows that osmium is generally compatible in peridotites during MORE melting. The systematic differences in Os/ Os ratios between MORE and peridotites suggest that the melts may not be in isotopic equilibrium with their residual peridotite (sources Martin, 1991 Roy-Earman and Allegre, 1994 Snow and Reisberg, 1995 Schiano et al., 1997 Erandon et al., 2000).

Major-element compositions of most lithospheric peridotites reflect an origin as melt-residues. However, as with parent-daughter isotope ratios, compilation of their strontium and neodymium mineral isotopic compositions reveals that very few samples show the characteristics of ancient melt residues (Figure 37). Osmium isotopes are the exception and dominantly reflect ancient extraction of high Re/Os melts, leaving rhenium-depleted residues to develop time inte-... 

Radiogenic isotopes have proved a most powerful tool for understanding mantle processes. By studying mantle peridotites and mantle-derived melts from a variety of different geological time periods it is possible to define evolutionary curves for the different isotopic systems within the mantle. These curves, when plotted on isotope ratio versus time diagrams, can be used to characterize the chemical evolution of the mantle over time. Deviations from the chondritic trend are used to identify chemical fractionation events in the mantle during Earth history. Of particular... 

A few reports are available where TIMS has been used for cosmochemical analysis. The recent report of Yamakawa et al. (2009) deals with sequential chemical separation technique for Cr, Fe, Ni, Zn, and Cu in terrestrial and extraterrestrial silicate rocks for precise and accurate determination of elemental concentration. The chemical technique uses a combination of cation-anion exchange chromatography and Eichrom nickel specific resin. The developed method was tested to a variety of matrixes bulk meteorite (Allende), terrestrial peridotite (JP-1), and basalt (JB-lb). Concentrations of each element were determined by TIMS. The analytical procedure was further extended to obtain high-precision isotope data for Cr. The method is capable to determine the isotopic ratios of Cr/ Cr and Cr/ Cr with precision of 5 X 10 and 1 x 10, respectively. The method can be equally applicable in cosmochemical studies, like Mn-Cr chronology and investigation of nucleosynthetic isotopic anomalies in meteorites. The elemental concentrations measured by TIMS in Allende are, for example, Cr 3,600 0.007 ppm, Fe 233,400 0.009 ppm, Ni 14,020 0.006 ppm, Cu 107 0.013 ppm, and Zn 117.8 0.01 ppm. The detection of Al (0.7 Ma) in the Allende meteorite proves that nucleosynthesis is still active in the Milky Way as the half-life of A1 is much less than the timescale of galactic evolution (10 years). 

Ultrabasic Rocks. Although a number of determinations of lead isotope ratios have been reported for ultrabasic rocks, only those for the kimberlite from South Africa (Lovering and Tatsumoto, 1968) and peridotitic portions of a layered mafic intrusion in Scotland (Moorbath and Welke, 1968b) can be considered reliable whole-rock values. In view of the great importance of data on these kinds of rocks and the difficult analytical problems, only those data considered reliable whole-rock or mineral values are included in Table 21. 

Figure 5. Histogram Th/U for clinopyroxenes in peridotites and pyroxenites from the Ronda peridotite massif Concentrations were measured by isotope dilution mass spectrometry in acid-leached clinopyroxenes. This histogram shows that pyroxenites do not have larger Th/U ratios than peridotites. Thus, the correlation found between ( °Th/ U) and Th/U cannot be explained by mixing of peridotite and pyroxenite melts as advocated in Sigmarsson et al. (1998). Data from Hauri et al. (1994) and Bourdon and Zindler (unpublished). It can be shown with a simple Student t-test that the two populations are indistinguishable.

The Se/ Se ratios of CDT and 3 other iron meteorite samples were determined by Rouxel et al. (2002). CDT had the greatest ratio, and the other meteorites ranged from -0.2%o to -0.6%o relative to CDT. Four basaltic reference materials, two glassy MORB s, and one peridotite also analyzed by Rouxel et al. (2002) were within 0.2%o of CDT. These results suggest that the earth s mantle is close in Se isotope composition to CDT, and that CDT is, tentatively, a reasonable proxy for the bulk composition of the earth. 

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