Xenoliths noble gases

Some of the main questions that might be sought in the xenolith noble gas record are... 

Distinctive lithospheric mantle He. As discussed in the Possible xenolith noble gas components section, radiogenic production within the lithosphere can lower the He/ He ratio of He derived from the convecting mantle. Therefore, the He represents a distinctive lithospheric component. This may be created in the xenolith source region, or may involve He that is remobilized in the lithosphere. As discussed below, regional studies often contend with distinguishing between distinctive asthenospheric and lithospheric sources for such He. 

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... 

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. 

A number of noble gas studies have focused on xenoliths from the Pliocene to Recent alkali basalts of the Newer Volcanics in SE Australia... 

Mafic minerals contained in volcanic rocks (phenocrysts) or in xenoliths are widely exploited in noble gas studies as they frequently contain fluid and/or melt inclusions that trap noble gases. Olivines and pyroxenes are the most commonly utilized minerals. Whole-rock samples are first crushed to 0.5 to 2 mm or larger, depending on the size of the crystals. The olivines and pyroxenes are then separated using a Frantz-Isodynamic magnet separator, followed by hand-picking under a binocular microscope to remove any... 

Shirane volcano, Japan. J Volcanol Geotherm Res 97 329-346 Ohmoto H. (1986) Stable isotope geochemistry of ore deposits. Rev Mineral 16 491-559 Ozima M, Podosek FA (1983) Noble Gas Geochemistry. Cambridge University Press Patterson DB, Honda M, McDongall 1 (1994) Noble gases in mafic phenocrysts and xenoliths from New Zealand. Geochim Cosmochim Acta 58 4411-4427... 

In general, in order to characterize the noble gas state of the continental lithosphere, it is necessary to measure or clearly deduce the pre-entrainment noble gas isotope inventories of xenolith samples. This first requires subtracting the effects of post-eruption... 

Many noble gas studies have concentrated on ultramafic xenoliths and phenocrysts that are found in alkali basalts. Relatively recent eruptions from a wide range of locations provide suites of samples that are relatively unweathered, and so have retained mantle noble gases. Most of the available data is for He isotopes, and other data are discussed separately in the section The heavier noble gases. 

Decoupling between He and Ne is also required, since only MORE He isotope ratios are observed in the apatites. Also, Matsumoto et al. (2000) subsequently reported similar Ne characteristics in a garnet pyroxenite with MORE He/ He ratios. Since all the Ne data do overlap with MORE Ne when two sigma errors are considered (rather than the one sigma errors routinely reported for noble gas analyses), the possibility that there is hotspot Ne in the xenoliths idea requires confirmation. 

There is no unequivocal evidence for He/ He ratios in ultramafic xenoliths that are higher than those of MORE, suggesting that high He/ He mantle sources, such as found in Iceland and Hawaii, are not important for the noble gas signature of the continental lithospheric mantle. 

Porcelli D, O Nions RK, Galer SJG, Cohen AS, Mattey DP (1992) Isotopic relationships of volatile and lithophile trace elements in continental ultramafic xenoliths. Contrib Mineral Petrol 110 528-538 Porcelli D, Ballentine CJ, Wieler R (2002) An introduction to noble gas geochemistry and cosmochemistiy. Rev Mineral Geochem 47 1-18... 

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