Subcontinental lithosphere

Archaean subcontinental lithosphere is normally underlain by enriched lithosphere (EMI of Zindler and Hart (1986) — low Rb/Sr, low Sm/Nd), but depleted examples are also known. The lithosphere beneath Proterozoic mobile belts, however, more closely resembles the depleted mantle found beneath older ocean basins (Menzies, 1989). Froterozic to Phanerozoic subcondnental lithosphere is characterized by enrichment in Rb and the Ught REE resulting in radiogenic Sr and non-radiogenic Nd isotopes. This is similar to enriched mantle EMII described above and in Table 6.5. 

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Williams RW, Collerson KD, Gill JB, Deniel C (1992) High Th/U ratios in subcontinental lithospheric mantle mass spectrometric measurement of Th isotopes in Gaussberg lamproites. Eartli Planet Sci Lett 111 257-268... 

McDonough W. F. et al. (1985) Isotopic and geochemical systematics in Tertiary-Recent basalts from southeastern Australia and implications for the evolution of the subcontinental lithosphere. Geochim. Cosmochim. Acta 49, 2051-2067. 

These basalts represent the oceanic subclass of so-called intraplate basalts, which also include continental varieties of flood and rift basalts. They will be collectively referred to as OIE, even though many of them are not found on actual oceanic islands either because they never rose above sea level or because they were formed on islands that have sunk below sea level. Continental and island arc basalts will not be discussed here, because at least some of them have clearly been contaminated by continental crust. Others may or may not originate in, or have been contaminated by, the subcontinental lithosphere. For this reason, they are not considered in the present chapter, which is concerned primarily with the chemistry of the sublitho-spheric mantle. 

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... 

The formation of replacive pyroxenites can be explained by melt-consuming reactions at pressure and temperature conditions close to the peridotite solidus. Ronda represents a situation where the reaction was associated with relaxation of thinned and thermally eroded subcontinental lithosphere (Garrido and Bodinier, 1999). Upon cooling of the melting domain developed during the thermal erosion (Lenoir et al., 2001), the interstitial melt became saturated in pyroxene ( aluminous phases) and reacted with olivine to produce secondary cpx, opx, and spinel... 

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... 

Garrido C. and Bodinier J.-L. (1999) Diversity of mafic rocks in the Ronda peridotite evidence for pervasive melt/rock reaction during heating of subcontinental lithosphere by upwelling asthenosphere. J. Petrol. 40, 729-754. 

Garrido C. J., Bodinier J.-L., and Alard O. (2000) Distribution of LILE, REE and HFSE in anhydrous spinel peridotite and websterite minerals from the Ronda massif insights into the nature of trace element reservoirs in the subcontinental lithospheric mantle. Earth Planet. Sci. Lett. 181, 341-358. 

Lenoir X., Garrido C. J., Bodinier J.-L., Dautria J.-L., and Gervilla F. (2001) The recrystallization front of the Ronda peridotite evidence for melting and thermal erosion of subcontinental lithospheric mantle beneath the Alboran basin. J. Petrol 42, 141-158. 

Piccardo G. B., Rampone E., Vannucci R., Shimizu N., Ottohni L., and Bottazzi P. (1993) Mantle processes in the subcontinental lithosphere the case study of the rifted sp-Uierzohtes from Zabargad (Red Sea). Euro. J. Mineral. 5, 1039-1056. 

Gautheron C. and Moreira M. (2002) Helium isotopic signature of the subcontinental lithospheric mantle. Earth Planet. Sci. Lett. 199, 39-47. 

Carlson R. W. and Irving A. J. (1994) Depletion and enrichment history of subcontinental lithospheric mantle— an Os, Sr, Nd, and Pb isotopic study of ultramafic xenoliths from the northwestern Wyoming craton. Earth Planet. Sci. Lett. 126, 457-472. 

The abundances of these potassic hydrous phases, and hence the amount of water they can host, are limited by the potassium content of the mantle, but they are of considerable interest because of their high thermal stability. Depending on the precise mantle geotherm (adiabat) assumed, these hydrous phases may be stable in convecting mantle as well as in the cooler portions of the mantle such as the subcontinental lithosphere. This stability to high temperatures provides a mechanism not only to transport water into the mantle by subduction, but also to retain it. In this respect, the potassic hydrous phases contrast with the hydrous magnesium-rich phases, as outlined below. 

It is worth emphasizing that the temperatures over which many of these hydrous phases in the MASH system are stable are sufficiently low that their presence is only possible in cold subducting slabs or subcontinental lithosphere. They will not be present in normal mantle, given what we believe to be the case concerning mantle geothermal gradients. 

Farmer G. L., Perry F. V., Semken S., Crowe B., Curtis D., and DePaolo D. J. (1989) Isotopic evidence on the structure and origin of subcontinental lithospheric mantle in southern Nevada. J. Geophys. Res. 94, 7885-7898. 

Furman T. (1995) Melting of metasomatized subcontinental lithosphere undersaturated mafic lavas from Rungwe, Tanzania. Contrib. Mineral. Petrol. 122, 97-115. 

Hassler D. R. and Shimizu N. (1998) Osmium isotopic evidence for ancient subcontinental lithospheric manUe beneath the Kerguelen Islands, southern Indian Ocean. Science 280, 418-421. 

R. Prendergast, M. D. A. 1997. Growth of subcontinental lithospheric mantle beneath Zimbabwe started at, or before 3.8 Ga Re-Os study on chromites. Geology, 25, 983-986. 

Richardson, S. H., Erlank, A. J. Hart, S. R. 1985. Kimberlite-bome garnet peridotite xenoliths from old enriched subcontinental lithosphere. Earth and Planetary Science Letters, 75, 116-128. 

The mineralogical and chemical composition of peridotite from subcontinental lithosphere differs from that of peridotite from other parts of the mantle (Boyd 1989 Berstein et al. 1997). Peridotites from subcontinental lithosphere is depleted , which means it contains only a small amount of clinopyroxene and an aluminous phase, which together make up the so-called basaltic component. The lithosphere beneath the oldest Archaean cratons has a composition markedly different from that of younger subcontinental lithosphere (Boyd Mertzman 1987 Griffin et al. 1999). Old unmetasomatized lithosphere is harzburgitic, a mixture of olivine... 

Current models for the formation and evolution of subcontinental lithosphere... 

Normal mantle melting does not produce a residue with the mineralogical composition of old subcontinental lithosphere... 

To extract the refractory constituents to form a thick, uniform layer of refractory components like the subcontinental lithospheric mantle requires an efficient sorting process. To emphasize the problem we need only to consider that if high-temperature residue constitutes 20% of the total residue of melting, to accumulate a 200 km thick layer of Kaapvaal-type lithosphere requires the elimination of five times the volume (the equivalent of a 1000 km thick layer) of less depleted material. 

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