Orthopyroxenes mantle

Mantle rocks may be classified into those which are relatively enriched in the elements Ca, Al, Ti, and Na and those which are not. Lherzolites are "enriched" peridotites and are thought to be mantle rocks which have not melted and are known as "fertile" mantle -mantle from which a basaltic melt has not been extracted. Typically they contain a few percent of clinopyroxene and an aluminum-rich mineral (plagioclase, spinel, or garnet) in addition to the statutory olivine and orthopyroxene. Mantle which has experienced melt extraction is known as "depleted" mantle and has lower concentrations of the elements Ca, Al, Ti, and Na. This type of mantle is represented by the rock types harzburgite (peridotite without clinopyroxene and an aluminous mineral) and dunite. The relationship between a fertile mantle Iherzolite, depleted mantle harzburgite/ dunite and a basaltic melt is... 

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... 

Where Af° and Ca denote atoms per 6-oxygen formula unit. These values of combined with of 360 GPa allow the ratios D olDui and DbJDi to be calculated. Typical values for mantle solidus orthopyroxenes are in the range 0.001-0.009 and 1.1-1.2, respectively. 

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. 

In detail, however, the picture is not so simple. All mantle peridotites (whether massive peridotites or xenoliths) are metamorphic rocks that have had a complex subsolidus history after melt extraction ceased. As well as subsolidus recrystallization, peridotites have undergone enormous amounts of strain during their emplacement in the lithosphere. Massive peridotites show modal heterogeneity on the scale of centimeters to meters, caused by segregation of the chromium-diopside suite of dikes, which are then folded back into the peridotite as deformation continues. The net result is more or less diffuse layers or bands in the peridotite, which may be either enriched or depleted in the material of the chromium-diopside suite, i.e., in climopyroxene and orthopyroxene in various proportions, minor spinel, and sulfide. This process should cause approximately linear correlations of elements versus MgO, broadly similar to, but not identical with, those caused by melt extraction. Indeed, there is... 

Figure 1 Depth-varying phase proportions in a pyrolite model mantle after the manner of Ringwood (1989), Ita and Stixmde (1992), and Bina (1998h). Phases are (a) ohvine, (fi) wadsleyite, (y) ringwoodite, (opx) orthopyroxene, (cpx) clinopyroxene, (gt-mj) garnet-majorite, (mw) magnesiowiistite, ((Mg,Fe)-pv) ferromagnesian sihcate perovskite, and (Ca-pv) calcium silicate perovskite. Patterned region at base denotes likely heterogeneity near core-mantle boundary.

In large part, this ability of anhydrous basaltic eclogites to seismically blend into an ultramafic mantle arises from the behavior of elastic moduli in pyroxenes. In the shallowest upper mantle, orthopyroxene is —6% slow in Vp relative to olivine and about equal in Vj. However, the bulk... 

Ozawa K. (1997) P-T history of an ascending mantle peridotite constrained by A1 zoning in orthopyroxene a case study in the Horoman peridotite complex, Hokkaidi, northern Japan. Mem. Geol. Soc. Japan 47, 107—122. 

Figure 6 P-T arrays compiled for garnet peridotite xenoliths from several suites using two-pyroxene thermometry and Al-in-orthopyroxene barometry (Tbkn and Rbkn methods, Table 5). Data sources given in Rudnick and Nyblade with additional data here for Vitim (Ionov et al., 1993a) and Canada (MacKenzie and Canil, 1999 Schmidberger and Francis, 1999). The best-fit line for the Kaapvaal data is plotted in each figure for reference. Intersection of P-T array with mantle adiabats (shaded field) represents an estimate of the thickness of lithosphere at the time of sampling.

Ballhaus C., Berry R. F., and Green D. H. (1991) High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen barometer implications for the oxidation state of the upper mantle. Contrib. Mineral. Petrol. 107, 27-40. 

Smith D., Riter J. C. A., andMertzman S. A. (1999) Water rock interactions, orthopyroxene growth and Si-enrichment in the mantle evidence in xenohths from the Colorado Plateau, southwestern United States. Earth Planet. Sci. Lett. 167, 347-356. 

Orthopyroxene. Orthopyroxene has been studied less than olivine. There were no mantle-derived orthopyroxenes in the study of Skogby and Rossman (1989) that of Skogby et al. (1990) included an enstatite (Mg/(Mg -p Fe) = 0.90) from the Premier kimberlite. South Africa, which contained 500 ppm OH, or 265 ppm H2. O. According to Ingrin and Skogby (2000), these values should be reduced by a factor of 0.4 to correspond to the new calibration for orthopyroxene of BeU et al. (1995). Bell and Rossman (1992a) report a range from 40-460 ppm H2O in mantle-derived orthopyroxene. Peslier et al. 

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