Clinopyroxenes mantle

Mineral-Melt Partitioning ofU-Series Nuclides Clinopyroxene along mantle solidus... 

Landwehr et al. (2001) extended the model of Wood and Blundy (1997) to include and Th". They measured experimentally Aj and Z)xh in a wide variety of synthetic clinopyroxene compositions in order to evaluate the crystal compositional dependence of U-Th fractionation. Their observations confirm the predictions of Wood et al. (1999), namely that as the M2 site becomes smaller, so D-m becomes smaller than Du (Figs. 1 and 11). The M2 site becomes smaller as the enstatite component of the clinopyroxene increases and Ca on M2 is replaced by Mg. Enstatite solubility in clinopyroxene increases with increasing temperature, consequently clinopyroxene coexisting with orthopyroxene will show higher Du/Dih at higher temperature. For this reason, DuIDjb increases with increasing pressure along the mantle solidus, as discussed above. 

Lanthanides are more incompatible in orthopyroxene than clinopyroxene, typically by a factor of 4-8 on the mantle solidus (Blundy and Wood 2003 Salters et al. 2002 McDade et al. 2003a,b). Not surprisingly there are fewer lanthanide partitioning data for... 

There is only one determination of Z)pb in orthopyroxene, that of Salters et al. (2002) at the mantle solidus at 2.8 GPa. This value (0.009 0.006) is within error of that calculated from the Dsr value of McDade et al. (2003a) under similar conditions, using the lattice strain model, i.e., 0.0024 0.0012. However, the uncertainties on both measurements should not be taken as strong support for the potential of Sr as a proxy for Pb. Still, there is no evidence for the anomalously low Z)pb values observed in clinopyroxene. 

Blundy JD, Dalton JA (2000) Experimental comparison of trace element partitioning between clinopyroxene and melt in carbonate and silicate systems and implications for mantle metasomatism. Contrib Mineral Petrol 139 356-371... 

Wood BJ, Blundy JD, Robinson JAC (1999) The role of clinopyroxene in generating U-series disequilibrium during mantle melting. Geochim Cosmochim Acta 63 1613-1620 Zou H, Zindler A, Niu Y (2002) Constraints on melt movement beneath the East Pacific Rise from °Th- U disequilibrium. Science 295 107-110... 

At shallow pressure (less than 1 GPa), Th becomes slightly more compatible than U in clinopyroxene (Landwehr et al. 2001) and garnet is not stable even in pyroxenite or eclogite. On the other hand, it is believed (mostly based on observations) that Pa remains more incompatible than U through the entire length of the melt column. Thus, melting at intermediate pressure should yield small °Th or U excesses, but significant Pa excess. This has been observed in the lithospheric mantle in the Colorado plateau by... 

Beattie (1993) argued rather convincingly, that only zircon could have a significant effect, but zirconium is also present in garnets and clinopyroxenes such that zircon is extremely unlikely in mantle assemblages. 

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).

Chondrules from Chainpur LL3 meteorite have somewhat lower 5 Mg values than Chainpur LL3 matrix. Cl, and terrestrial mantle clinopyroxene. Chondrules from the Bjurbole L/LL4 ordinary chondrite also have slightly lower 8 Mg values than Cl and LL3 matrix. Chondrules, matrix, and whole rock samples from the Allende CV3 meteorite span a larger range in 5 Mg that overlaps the Cl and ordinary chondrite data. 

The values of A Mg for a given datum are different on the SRM 980 and DSM3 scales. The SRM 980 O standard has a A Mg value of +0.0315%o on the DSM3 scale (Galy et al. 2003). Two analyses of Cl carbonaceous chondrite and the clinopyroxene from a terrestrial mantle nodule have A Mg values that are indistinguishable from 0.00 on the DSM3 scale (Table 2). 

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.

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