Xenoliths, mantle-derived

Rocholl, A., Heusser, E., Kirsten, T., Oehm, J., Richter, H. (1996) A noble gas profile across a Hawaiian mantle xenolith Mantle-derived cognate and accidental noble gas components and evidence for anomalous krypton isotopes. Geochim. Cosmochim. Acta, 60, 4773-83. 

Nishio Y, Nakai S, Yamamoto J, Sumino H, Matsumoto T, Prikhod ko VS, Arai S (2004) Lithium isotope systematics of the mantle derived ultramafic xenoliths imphcations for EMI origin. 

Small (normally up to about 10 cm) xenoliths are found in some Quaternary basanitic and nephelinitic lavas (e.g. Sapienza and Scribano 2000 and references therein). These include both mantle-derived and crustal lithologies. 

To obtain information on the noble gas state in the mantle, it is necessary to analyze mantle-derived materials that have trapped mantle noble gases. Accessible samples include volcanic rocks, volcanic gases, mantle xenoliths, and diamonds. Among various mantle-derived materials, submarine volcanic rocks are particularly useful because of their wide occurrence and their relatively large (for mantle samples) amounts of trapped noble gases. So far, information has been obtained mainly from... 

Canil D. (1990) Experimental study bearing on the absence of carbonate in mantle-derived xenoliths. Geology 18, 1011-1013. 

Ionov D. (1998) Trace element composition of mantle-derived carbonates and coexisting phases in peridotite xenoliths from alkali basalts. J. Petrol. 39, 1931 — 1941. 

Kramers J. D. (1977) Lead and strontium isotopes in Cretaceous kimberlites and mantle-derived xenoliths from Southern Africa. Earth Planet. Sci. Lett. 34, 419-431. 

In this section, we review the evidence for volatiles in the mantle obtained from the study of mantle-derived samples. The primary focus is on xenoliths brought to the surface in alkali basalts and kimberlite host magmas, but some relevant results obtained from study of alpine peridotites are also discussed. 

The presence of diamond and graphite in mantle-derived samples such as kimberlites and the xenoliths they host is prima facie evidence that neutral carbon is stable in the Earth s mantle. Outstanding questions remain, however, concerning the stability of neutral carbon in areas other than those beneath continental cratons, and concerning the mechanism by which diamond forms. To a large extent, the latter question revolves around the unresolved problem of the oxidation state of the mantle, and how—and if— the oxidation state is controlled. 

Carbonate is rarely found in mantle-derived xenoliths. It has been found in mantle-derived garnets (McGetchin and Besangon, 1975 Smith, 1987), clinopyroxenes (Hervig and Smith, 1981) and in rare xenoliths (e.g., Ionov et al., 1993a, 1996 Lee et al., 2000 Laurora et al., 2001). 

Sulfur is almost always present in mantle-derived magmas and mantle samples as sulfide, which has been documented from mantle xenolith suites, abyssal peridotites, peridotite massifs, and diamonds (Meyer and Brookins, 1971 Desborough and Czamanske, 1973 Frick, 1973 Vakhrushev and Sobolev, 1973 Bishop et al., 1975 De Waal and Calk, 1975 Meyer and... 

Based on the work of Philippot et al. (1998), one might expect to observe a certain proportion of chlorine-rich fluid inclusions in mantle-derived xenoliths, but inclusions in these xenoliths are overwhelmingly C02-rich, and chlorine-rich inclusions have not been reported (cf. reviews by Roedder, 1984 Pasteris, 1987 Andersen and Neumann, 2001), with the intriguing exception of the brines reported as inclusions in some diamonds (Johnson et al., 2000 Izraeli et al., 2001). The lack of direct observation of chlorine-rich fluid inclusions in mantle-derived xenoliths may be a result of the lack of examination of appropriate samples that record a previous history as subducted oceanic crust, an absence of these fluids in deeper samples because of participation of these fluids in other petrological processes, such as melt production, or because such fluids do not survive subduction below the slab dehydration limit. Conversely, the presence of chlorine in fluid inclusions in diamonds argues for the existence of chlorine-rich fluids at least in some circumstances in the mantle in the pressure range of diamond stability. 

Xenoliths from these localities are probably derived from mid-crustal levels based on either equilibration pressures, lack of mantle derived xenoliths in the same hosts and/or chemically evolved character of the host. 

Kramers, J. D. 1979. Lead, uranium, strontium, potassium and rubidium in inclusion-bearing diamonds and mantle derived xenoliths from southern Africa. Earth and Planetary Science Letters, 42, 58-70. 

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