Xenoliths

Lowenstem et al. (2000) used the same approach to date several zircons found in granitoid xenoliths ejected by Medicine Lake volcano (California) during two eruptions, 1065 and 2000 years ago. The ages obtained at 29 and 90 ka are thought to represent previous intrusive episodes in this magmatic system. 

Olafsson M, Eggler DH (1983) Phase relations of amphibole, amphibole-carbonate, and phlogopite-carbonate peridohte petrologic constraints on the asthenosphere. Earth Planet Sci Lett 64 305-315 Olson P, Schubert G, Anderson C, Goldman P (1988) Plume formahon and lithosphere erosion a comparison of laboratory and numerical experiments. J Geophys Res 93 15065-15084 Pearson DG, Shirey SB, Carlson RW, Boyd FR, Nixon PH (1995) Stabilisahon of Archean lithospheric manhe A Re-Os isotope isotope study of peridohte xenoliths. Earth Planet Sci Lett 134 341-357... 

Blatter DL, Carmichael ISE (1998) Hornblende peridotite xenoliths from central Mexico reveal the highly oxidized nature of subarc upper mantle. Geology 26 1035-1038 Blundy J, Wood B (2003) Mineral-melt partitioning of uranium, thorium and their daughters. Rev Mineral Geochem 52 59-123... 

Maury RE, Defant MJ, Joron J-L (1992) Metasomatism of the sub-arc mantle iirferred from trace elements in Philippine xenoliths. Nature 360 661-663... 

Olsen EJ, Davis AM, Hutcheon ID, Clayton RN, Mayeda TK, Grossman L (1988) Murchison xenoliths. Geochim Cosmochim Acta 52 1615-1626... 

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... 

Figure 9. Plots of Li and radiogenic isotopes for mantle rocks, (a) 5 Li vs. Sr/ Sr (b) 5 Li vs. Nd/ Nd (c) "Sr/ Sr vs. Pb/ Pb (d) 5"Li vs. Pb/ Pb (Nishio et al. 2003, 2004). Symbols + = south Pacific island basalts (six islands) O = Iherzolite xenolith, Bullenmerri, Australia = Iherzolite xenolith, Sikhote-Alin, Russia (three localities) A = dunite-peridotite-pyroxenite xenolith, Kyushu, Japan (two localities) V = Iherzolite xenolith, Ichinomegata, Japan. The ocean island data are from bulk rocks, the xenolith data are clinopyroxene separates. For explanations of the derivation of radiogenic isotope fields (DM, EMI, EM2, HIMU), see Zindler and Hart (1986). The estimate for Li isotopes in DM is based on MORE. The Li isotopic ranges for the other mantle reservoirs are based on Nishio et al. (2004) and Nishio et al. (2003), but these will require further examination (hence the use of question marks).

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.

Irving A. J. (1974). Geochemical and high-pressure experimental studies of garnet pyroxenite and granulite xenoliths from the Delegate basaltic pipes, Australia. J. Petrol, 15 1-40. 

Ottonello G. (1980). Rare earth abundance and distribution in some spinel peridotite xenoliths from Assab (Ethiopia). Geochim. Cosmochim. Acta, 44 1885-1901. 

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. 

The youngest eruptive event is of Paleocene age ( 60 Ma) whereas the youngest preserved host rocks capping the Buffalo Head Hills are of Campanian age ( 78 Ma). In this instance, the only record of now eroded latest Campanian through Paleocene host rocks is provided by sedimentary xenoliths preserved in the truncated intra-crater facies ultramafic bodies (e.g., K1 body). 

Skelton, D., Clements, B., McCandless, T.E., Hood, C. Aulbach, S., Daview, R,. Boyer, L.P. 2003. The Buffalo Head Hills kimberlite province, Alberta. In Kjarsgaard, B.A. (ed.). Slave Province and Northern Alberta Field Trip Guidebook. Geological Survey of Canada, Miscellaneous Publication G-293. Skinner, E.M.W. Marsh, J.S. 2004. Distinct kimberlite pipe classes with contrasting eruption processes. Lithos, 76, 183-200. Sweet, A.R., Boyce, K,. Eccles, D.R. In prep. Palynological constraints on kimberlite emplacement models chronostratigraphy of host rock and clastic xenoliths, Buffalo Head Hills, Alberta. 

The layered quartzite occurs as xenoliths in the granite. The contact between the two units is sharp and discordant, and is locally undulatory. The xenoliths have rounded edges and range in size xenoliths are very rare within the Main Double S Zone and do not contain U mineralization (e.g., 250-300 c/s). 

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... 

Water in the mantle is fonnd in different states as a fluid especially near sub-duction zones, as a hydrous phase and as a hydroxyl point defect in nominally anhydrous minerals. 8D-values between -90 and -110%c have been obtained by Bell and Ihinger (2000) analyzing nominally anhydrous mantle minerals (garnet, pyroxene) containing trace quantities of OH. Nominally anhydrous minerals from mantle xenoliths are the most D-depleted of all mantle materials with 5D-values 50%c lower than MORE (O Leary et al. 2005). This difference may either imply that these minerals represent an isotopically distinct mantle reservoir or that the samples analyzed have exchanged hydrogen dnring or after their ascent from the mantle (meteoric/water interaction ). 

---