Iron peridotites

The Dumas Formation is uncomformably overlying the Archean Superior Province. It hosts metasediments and intercalated iron formations, as well as peridotite and gabbro bodies, which were identified and separated with the help of both Band ratio and Principal Component Analysis. 

The Se/ Se ratios of CDT and 3 other iron meteorite samples were determined by Rouxel et al. (2002). CDT had the greatest ratio, and the other meteorites ranged from -0.2%o to -0.6%o relative to CDT. Four basaltic reference materials, two glassy MORB s, and one peridotite also analyzed by Rouxel et al. (2002) were within 0.2%o of CDT. These results suggest that the earth s mantle is close in Se isotope composition to CDT, and that CDT is, tentatively, a reasonable proxy for the bulk composition of the earth. 

Williams H, Lee D-C, Levasseur S, Teutsch N, Poitrasson R, Halliday AN (2002) Iron isotope composition of mid-ocean ridge basalts and mantle peridotites. Geochim Cosmochim Acta 66 A838... 

Samples 6 and 7 in table 5.32 are from the Zabargad peridotite (Red Sea) and are representative of the chemistry of upper mantle pyroxenes (Bonatti et al., 1986). The absence of Fe203 in these samples is due to the fact that microprobe analyses do not discriminate the oxidation state of iron, which is thus always expressed as FeO. It must be noted here that the observed stoichiometry (based on four oxygen ions) is quite consistent with the theoretical formula and that no Fe is required to balance the negative charges of oxygen. 

Table 5.32 Compositions (in weight %) of natural pyroxenes (samples 1-5 from Deer et al., 1983 samples 6 and 7 from Bonatti et al., 1986) (1) enstatite from a pyroxenite (2) ferrosilite from a thermometamorphic iron band (3) hedembergite (4) chromian augite from a gabbroic rock of the Bushveld complex (5) aegirine from a riebeckite-albite granitoid (6) diopside from a mantle peridotite (7) enstatite from a mantle peridotite. ...

Serpentinite A metamorphic rock derived from the hydration of iron- and magnesium-rich minerals, such as olivine and pyroxenes, in peridotites and other ultramafic igneous rocks. 

Sudo, M., Ohtaka, O., Matsuda, J. (1994) Noble gas partitioning between metal and sihcate under high pressures The case of iron and peridotite. In Noble Gas Geochemistry and Cosmochemistry, J. Matsuda, Ed., pp. 217-27. Tokyo Terra Scientific Publ. Co. 

O Hara, M.J., 1963. Distribution of iron between coexisting olivines and calcium-poor pyroxenes in peridotites, gabbros, and other magnesian environments. Am. J. Sci., 261 32-46. 

Densities and bulk sound velocities in the lower mantle are also consistent with the high-pressure mineralogy of a bulk composition approximating an upper-mantle peridotite, such as pyrolite. Seismic constraints provide no support for iron enrichment of the lower mantle relative to such an upper mantle. Silica enrichment of the lower... 

Konzett and Ulmer (1999) bracketed the phlogopite-out reaction in a natural peridotite at 6-7 GPa at 1,150 °C and in a subalkaline iron-free bulk composition at 8-9 GPa at 1,150 °C and at <8 GPa at 1,200 °C (Figure 3). Phlogopite becomes unstable with increasing pressure relative to a potassium-rich amphibole the stability of this amphibole has been studied by a number of workers (Figure 3). 

Lateritic weathering, promoted by warm-humid environments and low rates of erosion, can enhance some geochemical anomalies to the point where they may be mined at a profit. For example, lateritic weathering of dunites and peridotites or their serpentinized equivalents can produce ores of nickel with 1 -2% nickel. Nickel is commonly hosted by either iron oxides or silicates (garnierite). The deposits may be cmdely vertically zoned with pisolitic iron-oxide and nickel-bearing zones above richer saprolitic silicate ore. 

---