Orthopyroxene

Figure 1.74. Zonal sequence of the propylitic alteration in E-W section of the Seigoshi-Toi mine area (Yug = yugawaralite Heu = heulandite Stil = stilbite Opx = orthopyroxene Mont = montmorillonite Mor = mordenite Lm = laumontite Wr = wairakite Chi = chlorite pr = prehnite ep = epidote Py = pyrite Kf = K-feldspar Cpx = clinopyroxene) (Shikazono, 1985a).

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. 

Orthopyroxene has a Vl-fold Ml site and a Vl-fold M2 site. Both are predominantly filled by Mg and Fe. The smaller Ml site shares many characteristics with the clinopyroxene Ml site. It is therefore reasonable to assume that no U-series cations, including Pa (see above) enter that site. We will confine our discussion to the octahedral M2, which is smaller than the equivalent (Vni-fold) clinopyroxene site, even after allowing for the different coordination number. Consequently most of the U-series elements have very low orthopyroxene-melt partition coefficients. 

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. 

By analogy with clinopyroxene it is likely that Pa enters the orthopyroxene M2 site. In light of the fact that Du and Dxh in orthopyroxene are approximately ten times lower than in clinopyroxene, it is likely that Z)pa is also lower in orthopyroxene. However, this effect is offset to some extent by the smaller M2 site in orthopyroxene, which will tend to be more favourable to Pa than the M2 site in clinopyroxene. We have used the electrostatic model, applied to the two orthopyroxene-melt partitioning experiments of McDade et al. (2003a,b) to derive (Fig. 13). Both datasets, at 1.5 and 3 GPa, are... 

Figure 13. Electrostatic model fitted to partition coefficients for cations entering the M2-site in orthopyroxene, based on the experiments of McDade et al. (2003a,b). The curves are fits to Equation (7) and can be used to estimate and Do(m2) > from which D-p ui) can be calculated via the lattice strain model. The fit parameters are given in the legend.

A (cf orthopyroxene). We have used the same Vl-fold ionic radii for and th" as for orthopyroxene (Table 2), which gives Du/Dxh of 6.3, in broad agreement with the two experimental values. Dpo/Du calculated from the same lattice strain parameters is 0.03-0.10. 

Argon partition coefficients for olivine have recently been determined by Brooker et al. (1998). Their measured Z)at values are around 10 , which is higher than measured for clinopyroxene, but similar to the estimated values for orthopyroxene. We propose a value... 

Jones JH (1995) Experimental trace element partitioning. In Ahrens TJ (ed) Rock physics and phase relations A handbook of physical constants Am Geophys Union Reference Shelf 3 73-104 Kennedy AK, Lofgren GE, Wasserburg GJ (1993) An experimental study of trace element partitioning between olivine orthopyroxene and melt in chondrales equilibrium values and kinetic effects. Earth Planet Sci Lett 115 177-195... 

Meteorite ALH84001 was found in the Allan Hills in Antarctica in 1984 and was the subject of an extraordinary NASA press announcement in 1996. It weighed 1.93 kg when it was collected (Figure 6.10) and is probably the most extensively studied lump of rock of all time. It is 95 per cent orthopyroxene, the volcanic rock mineral that accumulated in a molten-lava-reducing environment on Mars some... 

Figure 13. Plot of lO lnaA-B values versus lOVT (T in K) for inter-mineral fractionation between magnetite-olivine, orthopyroxene-olivine, and clinopyroxene-olivine, and Fe metal-olivine as calculated from spectroscopic data by Polyakov and Mineev (2000), and as measured from natural samples by Zhu et al. (2002), Beard and Johnson (2004). Also shown is the Fe isotope fractionation factor between magnetite and Fe-silicates measured by Berger and von Blanckenburg (2001).

Sprenkel-Segel and Hanna (28) during their Mossbauer analysis of meteorites also determined the shifts (relative to Fe) and splittings of olivine (13 to 26% Fe) and an orthopyroxene (15-26% Fe). Their values are summarized in Table VII. 

Recently, Evans and co-workers (11) studied the spectra of three orthopyroxenes with Fe/(Mg + Fe) ratios of 0.758, 0.532, and 0.280. They obtained two somewhat asymmetric lines, and their results are summarized in Table VIIIA. One of these authors had previously established by x-ray analysis (12, 13) that the sample with the 0.532 ratio exhibits Mg-Fe ordering, with iron preferring the larger, more distorted M2 site. Consequently it was concluded that the two-line Mossbauer spectra must consist of a superposition of two quadrupole-split doublets nearly coinciding. Mossbauer parameters for iron in these... 

A. Mossbauer parameters for two-line orthopyroxene spectra of varying iron content as determined by Evans, Ghose, and Hafner (11/... 

XD Zou, EA Ferrow, S Hovmoller. Correcting for crystal tilt in HREM images of minerals the case of orthopyroxene. Physics and Chemistry of Minerals 22 517-523, 1995. 

Pyroxenes are poly silicates ( inosilicates in the mineralogical classification), crystallizing, respectively, in the monoclinic clinopyroxenes spatial groups C2lc, Pljlc, P2ln) and orthorhombic systems orthopyroxenes spatial groups... 

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