Mantle fertile

Ref.) Chondrite values Primirive mantle Fertile mande (6) Archaean mande (7)... 

Falloon TJ, Green DH (1989) The solidus of carbonated, fertile peridotite. Earth Planet Sci Lett 94 364-370 Galer SJG, O Nions RK (1986) Magmagenesis and the mapping of chemical and isotopic variations in the mantle. Chem Geol 56 45-61... 

Figure 4 The FeO contents of samples from the same suites as in Figures 2 and 3, samples from Zabargad island and samples from two xenoliths suites from Vitim (Baikal region) and Hessian Depression (Germany). The FeO contents are, similar to Cr in Figure 3, independent of the fertility of the mantle rocks as reflected in their MgO...

The constancy of refractory element ratios in the Earth s mantle, discussed before, is documented in the most primitive samples from the Earth s mantle. Figure 8 plots (modified from Jochum et ai, 1989) the PM-normalized abundances of 21 refractory elements from four fertile spinel Iherzolites. These four samples closely approach, in their bulk chemical composition, the primitive upper mantle as defined in the previous section. The patterns of most of the REEs (up to praseodymium) and of titanium, zirconium, and yttrium are essentially flat. The three... 

This group includes peridotite massifs dominated by fertile Iherzolites that were exhumed in the early stages of continental rifting and exposed as denuded mantle on the seafloor, along passive continental margins. 

In spite of their variable provenance (subcontinental lithosphere, supra-subduction mantle wedge, or oceanic mantle), most of the tectonically emplaced and abyssal peridotites show coherent covariation trends for major elements (Eigure 5). These variations reflect their variable modal compositions between a fertile end-member— comparable to proposed estimates for pristine... 

Fertilization may alternatively result from the fractional solidification of partial melts incompletely drained from residual mantle, upon cooling of melting domains due to mantle convection or tectonic upwelling. In this situation, the refertilized peridotites (sometimes associated with replacive pyroxenites—see Section 2.04.4.2.2) may dehne a layering interpreted in terms of high-porosity compaction waves or porous-flow channels (Obata and Nagahara, 1987 Van der Wal and Bodinier, 1996 Garrido and Bodinier, 1999). 

Peridotite fertilization may also result from the fractional solidihcation of exotic (deep-seated) melts inhltrated in wall rocks of translithospheric magma conduits. This process was hrst described in composite mantle xenoliths (Wilshire and Shervais, 1975 Gurney and Harte, 1980 Irving, 1980 Wilshire et al., 1980 Boivin, 1982 Harte, 1983 Harte et al, 1993 Menzies et ah, 1987), where it is referred to as modal metasomatism when new (generally hydrous) minerals are precipitated (Dawson, 1984 Kempton, 1987), or Fe-Ti metasomatism (Menzies et al., 1987) when the attention is focused on chemical enrichment. In contrast with ultramafic xenoliths, the tectonically emplaced and oceanic peridotites contain only sparse rock types attributable to mantle metasomatism by deep-seated melts. Examples of wall-rock, modal, and Fe-Ti metasomatism were nevertheless described in IP orogenic Iherzolites, notably in the Pyrenees (Fabrics et al., 1989 Bodinier et al., 1988, 1990, 2003 McPherson et al., 1996 Woodland et al., 1996). 

An alternative to this scenario is to envisage that some fertile orogenic Iherzolites have acquired their geochemical signature as a result of melting and melt-rock interaction processes associated with the thermomechanical erosion of lithospheric mantle by upweUing asthenosphere (e.g., Lenoir et al., 2001). In this scheme, refertilization of lithospheric peridotites by (and reequilibration with) MORB melts is an alternative to the small degrees of melt extraction to account for LREE depletion in otherwise fertile Iherzolites (e.g., Piccardo and Rampone, 2001). 

Most of the orogenic spinel Lherzolites also contain small amounts of amphibole ( 1 %) textu-rally equilibrated with the peridotite minerals. These amphiboles have been ascribed to the infiltration of melts/fluids in mantle conditions (Fabries et al., 1991), but their presence is not associated with noticeable chemical enrichments (Vannucci et al., 1995). A possible explanation is that the liquid/rock ratio was so low that the fluid composition was buffered by the LREE-depleted composition of the peridotites. Alternatively, these amphiboles might represent the products of a late crystallization stage during fertilization of the peridotites by LREE-depleted basaltic melts. 

However, the information conveyed by mantle xenoliths indicates that stable subcontinental lithosphere is dominated by refractory peridotites which are enriched in HIE and LREE and have often acquired an enriched isotopic signature as a result of time integration of their chemical enrichment (see Chapter 2.05). Therefore, an alternative to the porous-flow model is to consider that the harzburgite layers represent strips of lithospheric peridotites embedded into more fertile material derived from the asthenospheric mantle (e.g., the Lherz massif, Eigure 30). In this scheme, the and Sr/ Sr versus... 

Figure 30 Simplified geological map of the Lherz orogenic peridotite (Eastern Pyrenees), after Conquere (1978) and Monchoux (unpublished data), showing the hazburgite layering concordant with peridotite foliation. Based on their enriched Nd-Sr isotopic composition (e.g., Downes et al, 1991), the harzburgite layers are interpreted as residual strips of aged, isotopically enriched, but refractory, mantle lithosphere, embedded into younger, depleted but fertile, asthenospheric mantle (spinel Iherzolites). This marble cake stmcture is tentatively ascribed to thermomechanical erosion of lithospheric mantle by upwelling asthenosphere.

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