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The (U-Th)/He dating method applied to U-rich phases such as zircon and apatite has sufficient sensitivity and precision to be of potential use for dating relatively recent geologic events such as volcanic eruptions. However, in phases with crystallization ages less than ∼ 1 Ma, chemical fractionation within the238U decay series may modify the He ingrowth rate, causing He ages computed from the secular equilibrium age equation to be incorrect. The resulting systematic error depends on the [230Th/238U] activity ratio of the dated phase when it is erupted, and on the eruption age. Zircons, which exclude Th relative to U, will likely have secular equilibrium He 'ages' that underestimate the eruption age by up to a few tens of %, decreasing with increasing eruption age. Apatites tend to accommodate U and Th with little fractionation, so apatite secular equilibrium He ages will be nearly concordant with eruption age. If minerals are erupted immediately after crystallization, the disequilibrium effect can be reasonably accounted for based on Th/U systematics. However, crystals are likely to reside for unknown but potentially long periods in a magma chamber, such that the degree of secular disequilibrium will be reduced prior to the onset of He accumulation. (U-Th)/He analyses of co-genetic phases that fractionate the U/Th ratio differently, like apatite and zircon, can be used to better constrain eruption age, as well as to provide insights into magma chamber residence time. We illustrate this approach with (U-Th)/He analyses of zircons and apatites of the Pleistocene-age Rangitawa Tephra, New Zealand.

Noble gas isotopic ratios in mantle-derived samples require variability in the time-integrated ratio of volatile to lithophile elements in the Earth. Documentation of mantle ³He/⁴He variability is becoming increasingly complete, but for the heavier noble gases, the picture is still partly clouded by the effects of atmospheric contamination of mantle samples. Nevertheless, clear variations in mantle Ne, Ar, and Xe isotopic ratios exist, are apparently correlated with ³He/⁴He, and may be the product of varying degrees of mantle degassing. However, uncertainties in noble gas geochemical behavior and several conflicting observations leave open other possibilities. Recent Ne isotopic data are particularly important because they require that the atmosphere has not been closed to exchange with space. Derivation of much of the atmosphere from a source other than degassing of the mantle is a strong possibility that complicates efforts to model the geochemical evolution of the Earth.

In order to investigate the behavior of slab-derived volatiles in the subduction environment, helium isotope ratios have been measured in geothermal gases from the Tabar-Lihir-TangaFeni (TLTF) chain in the Bismarck Archipelago of Papua New Guinea. As recorded by several geochemical tracers, these volcanos carry an exceptionally large slab-derived component, and therefore may provide new insights to the old question of volatiles in subduction zones. Geothermal gases from Lihir Island have homogeneous ^3He/^4He ratios of 7.18±0.07 times the atmospheric ratio (R_A), while those from Ambitle Island (Feni Group) have lower ratios of 6.61±0.13 R_A. These ^3He/^4He ratios are within the range defined by more-typical arc volcanos, but lie at the low end of the spectrum observed in arc volcanos erupted through purely oceanic crust. Although a small slab-derived signature (^3He/^4He ratio lower than depleted mantle) exists in the TLTF gases, these data demonstrate that even in volcanos with a comparatively large slab component, He is overwhelmingly derived from the depleted mantle wedge. This observation further confirms the relative insensitivity of He isotopes to the presence of slab fluids. He isotope ratios of 6.25 R_A were measured in geothermal gases from the Rabaul Caldera on New Britain Island. Coincidentally, these samples were taken six months prior to the major 1994 eruption at Rabaul. In conjunction with samples taken from the same locality 8 years earlier, these data allow us to test whether increasing He isotope ratios associated with fresh ascending magmas precede volcanic eruptions. Although some of the 1986 samples had much lower ^3He/^4He ratios (5 R_A than observed in 1994, one did not. We thus find no strong evidence for a systematic rise in the He isotope ratio of the Rabaul fluids between 1986 and 1994. If a ^3He/^4He increase did precede the Rabaul eruption, then it occurred either prior to 1986 or sometime between our 1994 sampling and the eruption.

Helium isotope analyses of ground waters from the Mirror Lake drainage basin in central New Hampshire (USA) show helium in excess of air-saturated water by up to 200x. The freon ages of these waters are younger than 50 years, consistent with the local hydrology. This excess helium has an isotope ratio of ^3He/^4He = 1.65 ± 0.10 x 10^(-6). It is shown that this component cannot be the result of cosmogenic production or mixing with young water containing ^3He from the decay of (bomb) tritium. Measurements of the helium isotope ratio of local rocks indicate that they cannot be the source of this excess component. This strongly suggests that the excess helium component is the result of the addition of some external source of mantle helium. The generally accepted view suggests that mantle helium in continental environments is the result of active volcanism and extensional tectonics. The latest episodes of volcanism in this region of New England are related to the New England hot spot track (95-190 Myr) and the closure of the Iapetus (> 300 Myr). Thus, either the timescale for helium transport through the crust is of the order of 100's of Myr or the signature of mantle helium can be preserved in (e.g.) fluid inclusions for significant periods of time.

New noble gas measurements have been made on Samoan ultramafic xenoliths in order to characterize the composition and nature of entrapment of a postulated mantle metasomatic agent. The new measurements were performed on gases extracted from severely tectonized harzburgites and dunites by both bulk crushing and laser microprobe. The tectonized specimens have the highest noble gas concentrations yet reported from mantle materials and attest to deformation in a highly gas-charged environment. The noble gas isotopic systematics are similar to those observed in undeformed specimens from the same locality, and are consistent with mixing between a mantle component (e.g. ^3He/^4He = 12 R_A, ^(40)Ar/^(36) Ar > 10,000) and an atmospheric contaminant. Within the xenoliths, the mantle component is spatially associated with features previously attributed to metasomatism (e.g. HAURI et al., 1993). Although this metasomatic component has many characteristics suggesting derivation from material returned to the mantle by subduction, its relatively high ^3He/^4He ratio is enigmatic. Whatever its source, this fluid appears to have existed within the Samoan mantle over fairy large temporal and spatial scales, and plays an important role in the geochemistry of Samoan basalts. Just as with the mantle component, the deformed xenoliths are also enriched in the atmospheric contaminant. This enrichment suggests pervasive penetration of air into the ubiquitous micro fractures and decrepitated fluid inclusions of the deformed specimens. In addition to source and contamination effects, the noble gases within these xenoliths record variable degrees of elemental fractionation. While the gas-rich (deformed) xenoliths have ^4He-^(21)Ne*-^(40)Ar* systematics close to long-term closed-system behavior, the comparatively gas-poor samples have lost up to 90% of their helium without concomitant loss of neon and argon. This likely represents diffusive loss of helium after fluid inclusion entrapment.