Olivine mg-number

Fig. 10. Histograms of Trd ages for on- and off-craton peridotites from southern Africa co-plotted with histograms of olivine mg-numbers. Kaapvaal low-L peridotites xenohths, n = 96. Data from Nixon et al. (1983), Walker et al. (1989), Pearson et al. (1995a), Carlson et al. (1999), Menzies et al. (1999) and Irvine et al. (2002). Lrd ages for peridotite xenoliths from kimberlites from East Griqualand, 170 km from the Lesotho on-craton kimberlites (Pearson et al. 1998) and for peridotites from the Farm Louwrencia kimberlite. Southern Namibia (Pearson et al. 1994 Pearson 1999a). It should be noted that no Archaean Trd ages are observed for off-craton peridotites. Olivine compositional data from Boyd Nixon (1979), Nixon (1987) and Boyd (unpubl. data).

Chondrules display a wide variety of compositions and textural types (see Fig. 8.5). The volume fraction occupied by chondrules in chondritic meteorites ranges from 85% (ordinary chondrites) down to 0% (Cl chondrites). Without question the most abundant type of chondrule is dominated by ferromagnesian (Fe, Mg-rich) silicates (Figs. 8.4 and 8.5 Lauretta et al. 2006). Ferromagnesian chondrules are primarily composed of olivine, pyroxene (minor Ca-rich pyroxene), glass, spinels, Fe, Ni-rich metal, FeS, and other minor phases (< 1 vol%). These are subdivided into FeO-poor and FeO-rich, or Type-I and Type-II chondrules (Brearley Jones 1998 Jones et al. 2000, and references therein Lauretta et al. 2006, and references therein). The division is delineated by the Mg-number [or Mg defined as 100 x Mg/(Mg + Fe)] of the olivine and low-Ca pyroxene > 90 is FeO-poor, and < 90 is FeO-rich, which roughly translates in a bulk difference of 10 wt% FeO < 10 wt% = FeO-poor and >10 wt% = FeO-rich. 

To illustrate the effects of metasomatism on the silicate mineralogy of peridotite xenoliths it is instructive to compare the modal abundances of garnet and clinopyroxene, expected to be the first minerals to be exhausted during partial melting, with indices of melt extraction such as the mg-number of olivine (Fig. 1). The observed abundances of garnet plus clinopyroxene with olivine... 

This form of the partition coefficient, analogous to that used for Fe-Mg fractionation between olivine and melt (see Chapter 1), is necessary only for the rare cases where trace substitution affects Cj and Cp substantially. A number of reviews (O Nions and Powell, 1977 Michard, 1989) describe the various sorts of partition coefficients expressed either in mass-fractions, atom fractions, or normalized to a major element and their respective merits. If the discussion is restricted to a narrow range of chemical compositions (e.g., basaltic systems, Irving, 1978, Irving and Frey, 1984), enough experimental information exists on trace-element partitioning to resort to the wonderfully simple equation (9.1.1). 

Figure 3.7 Mg isotope ratio data for terrestrial mantle-rocks and minerals, whole-rock chondrites, pallasite olivines, Martian meteorites, and lunar basalts and breccia. Error bars represent the external reproducibility in 2SE (= 2a/v/n, where n is the number of measurements per sample). Not considering the dunite DTSl and the harzburgite PCC-1 (with 32% serpentine [15]), which are both likely affected by processes occurring after their emplacement within the Earth s crust, the average (5 Mg of the BSE is —0.54 + 0.04 (2SE), which overlaps with that of average chondrites (—0.52 + 0.04), pallasites (—0.54 + 0.04), Mars (—0.57 + 0.02), and the Moon (—0.51 0.03), showing that the stable Mg isotopic composition of the inner solar system (dotted lines, <5 Mg = —0.54) is homogeneous to within 0.05%o.

Iron and manganese occur in a number of soil minerals. Sodium and chlorine (as chloride) occur naturally in soil and are transported as atmospheric particulate matter from marine sprays (see Chapter 10). Some of the other micronutrients and trace elements are found in primary (unweathered) minerals that occur in soil. Boron is substituted isomorphically for Si in some micas and is present in tourmaline, a mineral with the formula NaMg3AlgB3Sig027(0H,F)4. Copper is isomorphically substituted for other elements in feldspars, amphiboles, olivines, p5Toxenes, and micas it also occurs as trace levels of copper sulfides in silicate minerals. Molybdenum occurs as molybdenite (M0S2). Vanadium is isomorphically substituted for Fe or A1 in oxides, pyroxenes, amphiboles, and micas. Zinc is present as the result of isomorphic substitution for Mg, Fe, and Mn in oxides, amphiboles, olivines, and pyroxenes and as trace zinc sulfide in silicates. Other trace elements that occur as specific minerals, sulfide inclusions, or by isomorphic substitution for other elements in minerals are chromium, cobalt, arsenic, selenium, nickel, lead, and cadmium. 

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