Helium, neon and argon isotope systematics in Kerguelen ultramafic xenoliths: implications for mantle source signatures

Noble Gases in the Earth's Mantle

“…The ratios rise approaching Iceland (Poreda et al 1986), and ratios on Iceland itself are even higher (to 28 R A ; Condomines et al 1983, Kurz et al 1985). As demonstrated by 3 He/ 4 He of ∼11 R A in Easter microplate basalts, offaxis plumes can also influence MORB He (Poreda et al 1993). Similarly, interactions with nearby hotspots carrying low 3 He/ 4 He ratios (see below) can lower MORB ratios below the strong 8-R A peak, at least slightly (Graham et al 1992b).…”

“…Midocean ridge basalt (MORB) He isotope ratios range from 6 to 16 R A , with the distribution strongly peaked at 8 R A (Figure 1). The general interpretation of this distribution is that the MORB mantle carries rather uniform 3 He/ 4 He ratios in the range 7-9 R A , but the MORB source may be modified to apparently "anomalous" values as a result of input from other sources. Most commonly, these anomalous ratios are attributed to involvement of a component derived from a nearby hotspot.…”

“…The discovery that the nonradiogenic isotope 3 He emanates from mid-ocean ridges provided the first demonstration that terrestrial degassing is both incomplete and ongoing (Clarke et al 1969; see also Mamyrin et al 1969). Because the mantle is not completely devoid of volatiles, degassing and atmosphere formation processes are likely recorded by the composition of mantle volatiles.…”

“…In addition, the isotopic variability in mantle samples is enormous. Relative to the atmospheric 3 He/ 4 He ratio of 1.39 × 10 −6 (commonly denoted as R A ), mantle samples range from about 3 to 30 times higher (3 to 30 × R A , or simply 3 to 30 R A ). (Helium isotope ratios are commonly reported as the ratio of nonradiogenic 3 He to radiogenic 4 He, unlike other radiogenic isotope ratios in which the nonradiogenic isotope is the denominator.…”

“…A great deal of data on the He isotopic composition of volcanic materials has been published over the years, providing what is probably a fairly complete representation of the variability in mantle 3 He/ 4 He, at least to the extent that such variability is expressed at the Earth's surface. For interpreting the evolution of the mantle-atmosphere system, the most important volcanic settings are midocean ridges, which are believed to sample the uppermost mantle, and ocean islands, derived from mantle plumes that sample other and possibly deeper portions of the mantle.…”