Beni Bousera

In Morocco a 50-km-long slab called Beni Bousera contains chunks of graphite that were probably once diamonds formed in the deposit when it was buried 150 km underground. As this slab slowly rose to the surface over millions of years, the very slow reaction changing diamond to graphite had time to occur. That is, on this time scale thermodynamic control could exert itself. In the diamond-rich kimberlite deposits in South Africa, which rise to the surface much faster, kinetic control exists. 

When Beni Bousera and Ronda are excluded, the tme orogenic garnet peridotites have not been much studied from the geochemical point of view. As they witness deep mantle processes that are hardly accessible to the observation otherwise, these rocks represent an exciting field of investigation for the future. 

The chemical diversity of mantle pyroxenites is also patent from REE variations. Figure 26 shows the chondrite-normahzed REE patterns of representative pyroxenites from orogenic peridotite massifs in Eastern Pyrenees (Lherz and Freychi-nede) and the Betico-Rifean Belt (Ronda and Beni Bousera). These rocks show variable REE distributions, but some relationships are observed between the structural and/or mineralogical types of pyroxenites and the REE patterns ... 

Corandum-bearing garnet pyroxenites (Beni Bousera)... 

Figure 26 Chondrite-nomialized abundances of REEs in representative mafic rocks from the Beni Bousera, Ronda, and Pyrenees orogenic peridotite massifs. Data from Bodinier et al. (1987a, 1987b, 1990), Bodinier (1989), Komprobst et al. (1990), Pearson et al. (1993), Remaidi (1993), McPherson (1994), Garrido (1995), McPherson et al. (1996), and Garrido and Bodinier (1999). Normalizing values after Sun and McDonough (1989).

At the first sight, this model is supported by the isotopic characterization (Nd-Sr-Pb-O-S) of the corundum and diamond pseudomorph-bearing garnet pyroxenites from Beni Bousera by Pearson et al. (1991a,b, 1993), which indicates a derivation from hydrothermafly altered oceanic crust... 

Blichert-Toft J., Albarede F., and Kornprobst J. (1999) Lu-Hf isotope systematics of garnet pyroxenites from Beni Bousera, Morocco implications for basalt origin. Science 283, 1303-1306. 

Komprobst J., Piboule M., Roden M., and Tabit A. (1990) Corundum-bearing garnet clinopyroxenites at Beni Bousera (Morocco) original plagioclase-rich gabbros recrystallized at depth within the mantle J. Petrol 31, 717-745. 

Kumar N., Reisberg L., and Zindler A. (1996) A major and trace element and strontium, neodymium, and osmium isotopic smdy of a thick pyroxenite layer from the Beni Bousera ultramafic complex of northern Morocco. Geochim. Cosmochim. Acta 60, 1429-1444. 

Pearson D. G., Davies G. R., Nixon P. H., Greenwood P. B., and Mattey D. P. (1991a) Oxygen isotope evidence for the origin of pyroxenites in the Beni Bousera peridotite massif, north Morocco derivation from subducted oceanic hhtho-sphere. Earth Planet. Set Lett. 102, 289—301. 

The robustness of the Lu-Hf isotope system in some mantle environments is demonstrated by the precise Lu-Hf isochron of 1,413 67 Myr dehned by clinopyroxene separates from the Beni Bousera peridotite massif (Pearson and Nowell, 2003). This age probably dates the time of melt extraction from these rocks and is considerably more precise than the Sm-Nd isochron or the scattered Re-Os isotope systematics of these rocks. This indicates the potential power of this system in dating mantle rocks. The initial results from the Lu-Hf isotope system indicate that of the incompatible element isotope systems, it is the more robust to metasomatic effects, with signatures frequently recording the time-integrated response to melt depletion. 

Figure 43 CHf versus cnj isotope diagrams for lithospheric mantle peridotite minerals. Kaapvaal peridotite data are ah garnets and clinopyroxenes (Simon et al, 2002). Slave peridotite data are garnets and whole rocks from Schmidberger et al (2002). Salt Lake Crater peridotites (Hawaii), Kilboume Hole and Abyssal peridotites, are from Salters and Zindler (1995). Siberian and Mongohan peridotite Held are chnopyroxene data from cratonic and off-craton peridotites (field taken from Ionov and Weis, 2002). Fields for MORE (N-MORB) and OIB are from Nowell et al (1998). Field for Beni Bousera peridotites from Pearson and Noweh (2003).

Pearson D. G. and Nixon P. H. (1995) Diamonds in young orogenic belts graphitised diamonds from Beni Bousera N. Morocco, a comparison with kimberlite-derived diamond occurrences and implications for diamond genesis and exploration. Africa Geosci. Rev. 3, 295-316. 

Pearson D. G. and Nowell G. M. (2003) Dating mantle differentiation a comparison of the Lu-Hf, Re-Os and Sm-Nd isotope systems in the Beni Bousera peridotite massif and constraints on the Nd-Hf composition of the lithospheric mantle. Geopkys. Res. Abstr. 5, 05430. 

Montel J.-M., Kornprobst J., and Vielzeuf D. (2000) Preservation of old U-Th-Pb ages in shielded monazite example from the Beni Bousera Hercynian kinzigites (Morocco). J. Metamorph. Geol. 18, 335-342. 

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