He ppm

100-

140-

100-

Plume (383)

TOM Iceland Loihi Shona Samoa Kola other

12 14 1e 4He/3He, x104

MORB (206) i i Indian i i Pacific I I Atlantic

14 1e

□ Mariana Trough

He/3Hex104

Arc and continental volcanics (153)

12 14 1e

Fig. 27.6 Histogram of He-isotope compositions in plume, MORB, arc and continental volcanics. Note the narrow range of 4He/3He ratios in the MORB samples, in contrast with their highly variable concentrations, 10-13 < [3He] < 10-9 cm3 STP g-1. Back-arc rocks generally show higher ratios, indicating a contribution of crustal radiogenic He. The 4He/3He ratios in plume-related rocks vary within a wide range, but a number of plumes show low ratios that cannot be reproduced by mixing MORB-source mantle He and crustal He. After Tolstikhin and Hofmann (2005); see the data sources in that paper, © Elsevier Science 2005, reproduced by permission.

are well below the MORB end-member value, 0.06 (Moreira et al., 2001; Tolstikhin et al., 2002).

Thus, in contrast with the isotope systematics discussed in Section 27.3, the plume-related noble-gas isotope signatures cannot have been produced by the mixing of MORB-source mantle and any subducted matter.

Decoupling of rare gases from other isotopic families: the need for a reservoir unrelated to subduction

The above important conclusion is further strengthened by the observation that the 4He/3He (and 21Ne/22Ne) ratios in plume materials are decoupled from other isotopic systematics. The mantle sources of the Hawaii and Iceland plumes, which of all plume sources are the most similar to the MORB source in their Sr-, Nd-and Pb-isotope compositions, are the most different from the MORB source in He isotopes (Figs. 27.6 and 24.9). In contrast, some of the plumes with anomalous Sr, Nd and Pb isotopes, such as Samoa, which has a strong EM II signature, have 4He/3He ratios closer to MORB, overlapping with it.

considering MORB, and using the mean Atlantic isotopic ratios as a reference, in Iceland quite low 4He/3He ratios in rocks and fluids (Figs. 27.5 and 27.6) accompany enhanced 206Pb/2°4Pb and 87Sr/86Sr ratios. Along the Atlantic ridge to the south, in most Azores and Sierra Leone rocks all three ratios exceed the average MORB values. Still further to the south, at Shone, the relationships between the trio are again similar to those seen in Iceland rocks (Kurz et al., 1982b; Graham, 2002).

Within plumes, interrelationships between isotopic signatures of He, Ne and other daughter nuclides have occasionally been observed. The U-Th-Pb system-atics appear to be the most promising for finding recurring patterns; the parent isotopes are the same and Pb is more depleted in the DMM than Sr and Nd. Indeed, Eiler et al. (1998) reported a co-variation of 3He and the enhanced 208Pb*/204Pb ratios on Hawaii, which could be modelled as the three-component mixing of distinct end-member compositions. They considered that a contribution of primitive mantle material caused an enhancement of both 3He and 208Pb*/204Pb. However, this type of relationship has not been observed in other plumes yet; therefore it is not clear whether excess 208Pb* is a real feature of 3He-rich material or whether the observed co-variation results from occasional mixing between 208Pb* -rich and 3He-rich "carriers" belonging to different mantle domains. For example, Moreira et al. (1999) showed that the highest abundances of 3He in lavas of the Azores archipelago coincided with enhanced 206Pb*/204Pb ratios due to the mixing of subducted crust (a source of 206Pb*) and enriched deep-mantle material (3He).

As discussed in Section 27.3, in OIBs, Sr-, Nd- and Pb-isotope deviations from the DMM signatures are generally considered as resulting from subducted or delam-inated matter stored in the mantle (up to — 2 Gyr ago). For 3He, this presents problems. The recycled materials are either almost He-free or contain radiogenic He, so that their 4He/3He ratios are expected to exceed both the MORB and OIB values. The subduction of continuously accreting 3He-rich cosmic dust together with oceanic sediments has been proposed by e.g. Anderson (1993) as a mechanism for introducing He with low 4He/3He ratios into the mantle. This mechanism is unlikely, however, as He is easily released from the tiny dust grains, so that little He could survive the devolatilization and partial melting of subducting slabs (Hiyagon, 1994). Also, plume-like low 4He/3He ratios are not at all typical for arc magmatic rocks and fluids (Hilton et al., 2002). Thus it would appear that no 3He-enriched matter can be introduced into the mantle by subduction or delamination. This is in accord with the observed decoupling of high-3He material from Sr, Nd and Pb carriers and highlights the need for a specific noble-gas-bearing reservoir (Graham, 2002). The entrainment of noble-gas-rich material or a diffusion flux from such a reservoir could maintain the noble-gas isotope abundances in the MORB source mantle (Chapter 28).

Characteristics of the 3He-bearing reservoir and solar-type light rare gases

The low initial 4He/3He ratio (< 15 000) required for a 3He-bearing reservoir must be due either to a low time-integrated (U + Th)/3He ratio or to a young age.

The latter alternative is in contradiction with analyses on ancient rocks such as 370 Myr ultramafic-carbonatitic intrusive complexes (Tolstikhin et al., 2002), Archaean komatiites (Matsumoto et al., 2002) and other examples. Therefore the required deep-seated ancient reservoir must have a low time-integrated (U + Th)/3He ratio.

As such a reservoir was formed early in Earth history, the time-integrated (U and Th)/3He ratios in it must always have been at least a factor ~ 100 lower than the DMM values (Table 28.3). As was shown in Section 24.3, no large reservoir with [U] ^ [U]DMM ~ 5-7 ppb exists in the silicate Earth. Also, as discussed above, the relations between U-Th-He and other isotopic systematics do not point to a reservoir that is much more depleted in incompatible elements than the DMM (e.g. Graham, 2002).

It is now generally accepted that the mantle was almost completely degassed after the Moon-forming giant impact (Section 16.2), and thus material with such a high 3He concentration can only be of post-giant-impact extraterrestrial origin. Its identification from He isotopes alone is hardly possible, because He has only two stable isotopes. In this respect a major advance has come from Ne-isotope studies (Honda et al., 1993): Ne has three isotopes and, after correction for atmospheric contamination, Ne-isotope compositions in most MORB and plume-related samples are similar to those observed in implanted solar gases, plus a nucleogenic 21Ne component (Table 11.3). After correction for elemental fractionation, a 3He/22Ne elemental ratio ~ 7 ± 3 is derived for MORBs and OIBs (Graham, 2002). This is only slightly above the solar value, 3.6 (Table 11.3). The difference could result from the better retention of He, which is more soluble in melts than Ne, in the course of mantle degassing (Azbel and Tolstikhin, 1990). Overall, an important inference from these observations is that implanted solar gases appear to fit the primordial light-noble-gas component in the mantle best of all.

0 0

Post a comment