Mantle krypton

He found that both the melting curves (Figure 2.1a) and the densities (Figure 2.1b) for xenon and krypton are well above the estimated temperature and density of the lower mantle. One might then suppose that Xe and Kr are in solid form in the lower mantle. If this is indeed the case, implications on noble gas degassing from the mantle and hence on the evolution of the atmosphere would be far-reaching (cf. Section 6.9). However, the formation of solid Xe or Kr seems to be unlikely because of its... 

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. 

Noble gases and nitrogen in martian meteorites reveal several interior components having isotopic compositions different from those of the atmosphere. Xenon, krypton, and probably argon in the mantle components have solar isotopic compositions, rather than those measured in chondrites. However, ratios of these noble gas abundances are strongly fractionated relative to solar abundances. This decoupling of elemental and isotopic fractionation is not understood. The interior ratio in martian meteorites is similar to chondrites. 

In this section we present an overview of the principal means available to sample mantle-derived noble gases, followed by a summary of their main isotope and relative abundance characteristics in the mantle. Mostly, the mantle shows a wide range in noble gas isotope variations serving to impart information on a variety of topics. The only exception is krypton whose isotopic composition is steadfastly air-like in mantle materials. Consequently, we do not consider krypton in this review. 

There are various terrestrial reservoirs that have distinct volatile characteristics. Data from midocean ridge basalts (MORBs) characterize the underlying convecting upper mantle, and are described here without any assumptions about the depth of this reservoir. Other mantle reservoirs are sampled by ocean island basalts (OIBs) and may represent a significant fraction of the mantle (Chapter 2.06). Note that significant krypton isotopic variations due to radiogenic additions are neither expected nor observed, and there are no isotopic fractionation observed between any terrestrial noble gas reservoirs. Therefore, no constraints on mantle degassing can be obtained from krypton, and so krypton is not discussed further. Comparison between terrestrial and solar system krypton is discussed in Chapter 4.12. 

The noble gas geochemistry of natural waters, including formation waters in sedimentary basins, has been used to determine paleotemperatures in the recharge areas, to evaluate water washing of hydrocarbons, and to identify mantle-derived volatiles (Pinti and Marty, 2000). The dissolved noble gases, helium, neon, argon, krypton, and xenon in sedimentary waters, have four principal sources the atmosphere, in situ radiogenic production, the deep crust, and the mantle. These sources have characteristic chemical and isotopic compositions (Ozima and Podosek, 1983 Kennedy et al., 1997). 

Krypton isotopes in oceanic basalts have not been very diagnostic of mantle processes. The Kr isotopic composition of oceanic basalts is typically the same as modem... 

Lewis RS (1975) Rare gases in separated whitlockite from the St. Severin chondrite xenon and krypton from fission of extinct Geochim Cosmochim Acta 39 417-432 Leya I, Wider R (1999) Nucleogenic production of Ne isotopes in the Earth s cmst and upper mantle induced by alpha particles from the decay of U and Th. J Geophys Res 104 15439-15450 Lin WJ, Manuel OK (1987) Xenon decay products of extinct radionuclides in the Navajo, New Mexico well gas. Geochem J 21 197-207... 

Finally, we note that deviations from atmospheric-like krypton isotope ratios have not been found for any back-arc basin however, there are two reports of anomalous xenon isotope variations for the Mariana Trough. Ikeda et al. (1998) found coupled " Xe/ °Xe and Xe/ °Xe deviations from air—similar to those found in MORE, whereas Sano et al. (1998a) reported only Xe excesses (relative to Xe). These signatures reinforce the idea that the mantle wedge is the principal source of volatiles in the Mariana Trough particularly where circumstances limit the amount of atmosphere-derived contributions. At all other localities, only atmospheric-like xenon isotope ratios have been found. 

Krammenacher D, Merrihue CM, Pepin RO, Reynolds JH (1962) Meteoritic krypton and barium versus the general isotopic anomalies in meteoritic xenon. Geochim Cosmochim Acta 26 231-249 Kunz J (1999) Is there solar argon in the Earth s mantle Nature 399 649-650... 

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