Peridotites elemental abundances

Garuti G., Gorgoni C., and Sighinolli G. P. (1984) Sulfide mineralogy and chalcophile and siderophile element abundances in the Ivrea-Verbano zone mantle peridotites (Western Italian Alps). Earth Planet. Sci. Lett. 70, 69-87. 

Figure 24 Chondrite-normalized abundances of REEs in a wall-rock harzburgite from Lherz (dotted lines— whole-rock analyses), compared with numerical experiments of ID porous melt flow, after Bodinier et al. (1990). The harzburgite samples were collected at 25-65 cm from an amphibole-pyroxenite dike. In contrast with the 0-25 cm wall-rock adjacent to the dike, they are devoid of amphibole but contain minute amounts of apatite (Woodland et al., 1996). The strong REE fractionation observed in these samples is explained by chromatographic fractionation due to diffusional exchange of the elements between peridotite minerals and advective interstitial melt (Navon and Stolper, 1987 Vasseur et al, 1991). The results are shown in (a) for variable t t ratio, where t is the duration of the infiltration process and t the time it takes for the melt to percolate throughout the percolation column (Navon and Stolper, 1987). This parameter is proportional to the average melt/rock ratio in the percolation column. In (b), the results are shown for constant f/fc but variable proportion of clinopyroxene at the scale of the studied peridotite slices (<5 cm). All model parameters may be found in Bodinier et al. (1990). As discussed in the text, this model was criticized by Nielson and Wilshire (1993). An improved version taking into account the gradual solidiflcation of melt down the wall-rock thermal gradient and the isotopic variations was recently proposed by Bodinier et al. (2003).

Copper. The abundance of copper in the depleted mantle raises a particular problem. Unlike other moderately compatible elements, there is a difference in the copper abundances of massive peridotites compared to many, but not all, of the xenolith suites from alkali basalts. The copper versus MgO correlations in massive peridotites consistently extrapolate to values of 30 ppm at 36% MgO, whereas those for the xenoliths usually extrapolate to <20 ppm, albeit with much scatter. A value of 30 ppm is a relatively high value when chondrite normalized ((Cu/Mg)N = 0.11), and would imply Cu/Ni and Cu/Co ratios greater than chondritic, difficult to explain, if true. However, the copper abundances in massive peridotites are correlated with sulfur, and may have been affected by the sulfur mobility postulated by Lorand (1991). Copper in xenoliths is not correlated with sulfur, and its abundance in the xenoliths and also inferred from correlations in basalts and komatiites points to a substantially lower abundance of 20 ppm (O Neill, 1991). We have adopted this latter value. 

Figure 16 Abundances of lithophile trace elements normalized to PM values in orogenic peridotites from the Ronda massif (whole-rock analyses). Data from Remaidi (1993). Normalizing values after Sun and McDonough (1989).

Figure 17 Abundances of lithophile trace elements normalized to PM values in ophiolitic peridotites from Oman and Cuba, and in abyssal peridotites from the East Pacific Rise and the Izu-Bonin-Mariana Forearc (whole-rock analyses). Ophiolitic peridotites data from Godard et al. (2000) (reproduced by permission of Elsevier from Earth Planet. Set Lett. 2000,180, 133-148) for Oman, and from Proenza et al. (1999) for Cuba abyssal peridotites data from Niu and Hekinian (1997a) and Godard (unpublished) for the East Pacific Rise, and from Parkinson and Pearce (1998) for the Izu-Bonin-Mariana Forearc. Normalizing values after Sun and McDonough (1989).

Mildly incompatible elements. The abundance of V in several different mantle residues was recently reviewed by Canil (2(X)2). The trends for abyssal and massif peridotites are compared with on- and off-craton xenolith suites in Figure 14. Abyssal peridotites contain the most V at a given degree of depletion. Massif peridotites are shifted from the abyssal samples and parallel the off-craton samples, but the latter... 

There are few measurements of the Fe (Fe203) abundance in mantle xenoliths (O Neill et al, 1993 Canil et al., 1994 Canil and O Neill, 1996). Wet chemical determinations for FeaOs in peridotites are fraught with errors, but better and more precise results are obtainable with Moss-bauer spectroscopy of individual minerals (O Neill et al., 1993). Fe " " behaves as a mildly incompatible element during melting with a bulk D of —0.1, similar to that of scandium or V (Figure 12(e)). 

In contrast, peridotites metasomatized by small melt fractions show enrichment in platinum and palladium and elevated (Pd/Ir) . Bulk mineral separate PGE-Re analyses of two fertile xenoliths from southeastern Australia indicate less than 6% of the whole-rock PGE budget resides in either silicate or oxide phases and further implicates sulfides and alloys as the main controls of PGE-Re abundance. Comparison of sulfide versus whole-rock budgets by Lorand and Alard (2001) demonstrates the dominance of sulfide as the main PGE host in relatively fertile peridotites. This confirms the results of earlier studies of xenolith PGE mass balance (Hart and Ravizza, 1996 Mitchell and Keays, 1981) plus xenolith-derived and diamond inclusion sulfide studies (Jagoutz et al, 1979 Pearson et al, 1998b). As with cratonic xenoliths, sulfur-PGE and major-element-PGE correlations in more depleted noncratonic peridotites indicate that I-PGEs are probably not hosted entirely by sulfide (Lee, 2002). 

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