Ichiro Kaneoka

The uniform and low 3He/4He ratios of HIMU basalts as evidence for their origin as recycled materials

Several hypotheses have been proposed for the origin of the group of lavas having the isotopic signature known as ‘high μ’ (HIMU, where μ=  238U/204Pb); these explanations have invoked processes involving recycled oceanic crust and sediment, metasomatically enriched subcontinental lithosphere, or intra-mantle metasomatism. Here we present helium isotope analyses of HIMU basalts, with ages of 10–18 Myr, from three islands of the Cook–Austral Archipelago in the southern Pacific Ocean. We find that the HIMU samples have a relatively uniform and low 3He/4He ratio of 6.8 ± 0.9 RA compared with mid-ocean-ridge basalt, whereas samples of other enriched-mantle lavas from this region have more variable and higher signatures. The consistency of our HIMU results with those obtained from previous analyses of HIMU lavas at St Helena in the Atlantic Ocean lead us to conclude that a relatively low and uniform 3He/4He ratio represents a general characteristic of the mantle source region for HIMU lavas. Also, the uniform 3He/4He ratio (in both space and time) suggests that recycled oceanic crust and/or sediments are present in the source region for HIMU lavas, as it seems less likely that the other candidate processes, invoking metasomatism, would produce such consistent values.

Rare Gas Isotopes in Hawaiian Ultramafic Nodules and Volcanic Rocks: Constraint on Genetic Relationships

Differences in the rare gas isotopic ratios, especially the ratios of helium-3 to helium-4 and of argon-40 to argon-36, in Hawaiian ultramafic nodules and phenocrysts in volcanic rocks indicate that the nodules and phenocrysts were derived from different sources. The isotopic ratios in ultramafic nodules are similar to those in oceanic tholeiites. The phenocrysts seem to have formed in equilibrium with source materials richer in primordial components than those of the oceanic tholeiites. Mixing between the sources is quite likely.

KAr ages of submarine basalts dredged from seamounts in the western pacific area and discussion of oceanic crust

Abstract KAr ages of 16 submarine basalts dredged from 5 seamounts in the Western Pacific area are presented. The KAr ages range from 30 my to 80 my. Because of large amounts of radiogenic argon in these samples (more than 10 −10 moles/g), excess argon would not affect the KAr ages significantly and the discordant KAr ages can be best explained by diffusion loss of radiogenic argon. We conclude that the KAr ages obtained here can give at least a good estimate of minimum age of the seamounts. In the Western Pacific area, the KAr ages of the seamounts lie between the Cretaceous and the Early Tertiary in marked contrast to the relatively quiet volcanic activity in the Western Pacific Island arcs. Likelihood of the long time span for the growth of seamounts (more than 10 my) sets a minimum depth of 50 km to the supposed moving oceanic layer.

Noble gas systematics for coexisting glass and olivine crystals in basalts and dunite xenoliths from Loihi Seamount

Noble gas isotopes including 3He/4He, 40Ar/36Ar and Xe isotope ratios were determined for coexisting glass and olivine crystals in tholeiitic and alkalic basalts and dunite xenoliths from Loihi Seamount. Glass and coexisting olivine crystals have similar 3He/4He ratios (2.8–3.4) × 10−5, 20 to 24 times the atmospheric ratio (RA), but different 40Ar/36Ar ratios (400–1000). Based on the results of noble gas isotope ratios and microscopic observation, some olivine crystals are xenocrysts. We conclude that He is equilibrated between glass and olivine xenocrysts, but Ar is not. The apparent high 3He/4He ratio (3 × 10−5; = 21 RA) coupled with a relatively high 40Ar/36Ar ratio (4200) for dunite xenoliths (KK 17-5) may be explained by equilibration of He between MORB-type cumulates and the host magma. Except for the dunite xenoliths, noble gas data for these Loihi samples are compatible with a model in which samples from hot spot areas may be explained by mixing between P (plume)-type and M (MORB)-type components with the addition of A (atmosphere)-type component. Excess 129Xe has not been observed due to apparent large mass fractionation among Xe isotopes.

Rare gas isotopes and mass fractionation: An indicator of gas transport into or from a magma

A simple model of mass fractionation may explain the isotopic ratios of rare gases in volcanic materials. Single-stage mass fractionation of atmospheric rare gases predicts an upper limit for20Ne/22Ne of 10.3 and a lower limit for40Ar/36Ar of 280. The rare gas data in volcanic materials seem to support this interpretation. Relatively low40Ar/36Ar ratios, as low as 282, have been observed in recent Japanese volcanic rocks. Such a low40Ar/36Ar ratio may be explained by mass fractionation of the atmospheric value if the rare gases represent those which were transported into the magma chamber with other volatile elements. Both the amounts and the fractionated rare gas abundance pattern of lighter elements which are observed in pumices from the recent eruption of Mt. Usu, Southern Hokkaido, Japan, suggest the possibility of air injection into its magma chamber. Thus, the fractionation of rare gases in volcanic materials may be a common occurrence, and it must be considered in models for the origin of isotopic differences between rare gases in volcanic materials and the atmosphere.

Excess 129Xe and high 3He/4He ratios in olivine phenocrysts of Kapuho lava and xenolithic dunites from Hawaii

Abstract The occurrence of excess 129 Xe and a higher 3 He/ 4 He ratio than that of the atmosphere has been confirmed in olivine phenocrysts of Kapuho lava, Kilauea, and Hualalai xenolithic dunites with CO 2 inclusions from Hawaii. The 3 He/ 4 He ratio observed in dunites is similar to those of submarine pillow basalts, whereas that of olivine phenocrysts is higher than the former. This may be related to the different characteristics of their sources with respect to rare gas isotopes. Normalized to 36 Ar, Ne is depleted in olivine phenocrysts and enriched in dunites, whereas Xe is more enriched in both samples than in the atmosphere.

Constraints on the evolution of the Japan Sea based on 40Ar- 39Ar ages and Sr isotopic ratios for volcanic rocks of the Yamato Seamount chain in the Japan Sea

40 Ar- 39 Ar and Sr isotope analyses were performed on basalts and andesites dredged from the Yamato Seamount chain in the Japan Sea. The 40 Ar- 39 Ar plateau ages range from about 11 to 17 Ma, though most samples show ages between 10 and 14 Ma. The seamounts seem to have formed within a period of a few million years, although some of them might have formed earlier. Based on the present results together with previously reported radiometric age data, it is thought that the Yamato Basin formed during some period prior to 17 Ma and probably later than around 25 Ma. Taking into account the radiometric age data on rocks from the Japan Basin, it is conjectured that the opening of the Japan Sea might have started almost at this time or a little earlier. The observed 87 Sr 86 Sr ratios range from 0.70357 to 0.70388, suggesting incorporation of some time-integrated components enriched in incompatible elements such as continental crustal materials. This may indicate that in the Japan Sea area, at least the Yamato Basin had not developed enough to show the characteristics of typical N-type MORB source materials without being affected by pre-existing continental crustal materials.

Noble-gas state in the Earth's interior ― some constraints on the present state

The noble-gas state in the present Earths interior is discussed on the basis of selected data on recent samples. For this purpose, careful sample selection, experimental procedures and the usage of suitable isotope systems are discussed in detail. Based on the 3He/4He-40Ar/36Ar diagram, we can infer at least four typical sources for noble-gas isotopes: MORB type (M-type), plume type (P-type), atmosphere type (A-type) and crust type (C-type). Among them, only M- and P-type sources are directly related to the mantle materials. The M-type source is assigned to the depleted mantle, whereas the P-type source to the fertile mantle which is supposed to be located beneath the former. The P-type source materials are characterized by higher 3He4He and lower 40Ar36Ar ratios compared to the MORBs values. All observed data for terrestrial samples seem to be explained by the mixing of these components including those derived from the arc areas and from the hot-spot areas. The apparent lesser heterogeneity of noble-gas isotopes in the Earths deep interior probably reflect the easier mobility of noble gases at high temperatures compared with those of solid elements. Based on higher 3He4He and lower 40Ar36Ar ratios in the P-type source materials compared with MORBs values, we infer that noble gases (and probably volatiles) are still retained in the Earths deep interior with significant amounts, which might play an important role to form mantle plumes and some other processes. The 40Ar36Ar ratio for the total Earth is estimated to be ∼350 or less at present. Compared with the values for the other planets, it is conjectured that the K36Ar ratio for each planet might have been already determined during the accretion process.

Noble gas study of the Reunion hotspot: Evidence for distinct

We present an extensive He, Ne and Ar isotope data set from the Reunion hotspot that demonstrates the presence of a homogeneous plume source that has unique isotopic characteristics. 3He/4He ratios of the two volcanoes on Reunion (Piton de la Fournaise, <0.53 Ma; Piton des Neiges, 2–0.44 Ma) are uniform 12–13.5 Ra regardless of 4He concentration and sample age. The shield-building Older Series of Mauritius (5–8 Ma) has a constant 3He/4He ratio around 11.5 Ra. The similarity of 3He/4He and Sr–Nd isotope ratios among them demonstrate that the volcanoes have had a common homogeneous source related to the mantle plume activity over a period of 8 Myr. 20Ne/22Ne and 21Ne/22Ne of these volcanoes define a linear trend on a Ne three-isotope diagram with a slope between the Loihi and MORB correlation lines. There is a clear correlation between 20Ne/22Ne and 40Ar/36Ar. In contrast, Intermediate (2–3 Ma) and Younger Series (<1 Ma) of Mauritius and Rodrigues (1.5 Ma) have 3He/4He ratios similar to MORB and Sr and Nd isotope ratios closer to MORB than lavas from Reunion and Older Series of Mauritius. These Intermediate and Younger Series lavas therefore record a late stage thermal rejuvenation beneath Mauritius derived from a source that is unrelated to the mantle plume. The isotopic characteristics of the source of the Reunion magmatism are relatively low 3He/4He (13 Ra), an intermediate slope in a Ne three-isotope diagram and relatively radiogenic Sr isotope ratios. These source characteristics cannot be explained by either crustal contamination or MORB source mixing with Loihi-type primitive mantle. Thus He–Ne–Ar–Sr–Nd isotopes demonstrate that this plume source is distinct from the source of other large plumes (Loihi and Iceland), clearly showing that the mantle contains several relatively less-degassed reservoirs and not a single primitive source. Two possible models can account for the different isotopic signature of Reunion and Hawaii hotspots; (1) the Reunion source contains more recycled material than Loihi source and (2) the Reunion source experienced stronger degassing/differentiation than the Loihi source in the early stage of mantle evolution. In both cases a convection mode in the mantle is required that isolates and preserves several less-degassed reservoirs in the convectively stirred lower mantle.

Fossil pressures of fluid inclusions in mantle xenoliths exhibiting rheology of mantle minerals: implications for the geobarometry of mantle minerals using micro-Raman spectroscopy

Abstract Micro-Raman spectroscopic analysis allows us to estimate the internal pressure of small fluid inclusions. We applied this method to CO2-dominated fluid inclusions in mantle-derived xenoliths. The pressures estimated from the equilibration temperature and density of the fluid range from 0.96 to 1.04 GPa corresponding to depths of up to 30 km, which confirms that these rocks and fluids are of uppermost mantle origin. Furthermore, the inclusions show pressures specific to the individual host minerals (spinel≥orthopyroxene≈clinopyroxene≫olivine). In particular, the densities of CO2 in pyroxenes are 10% higher than in olivine. Such an enormous difference cannot be explained by elastic deformation of the minerals during ascent of the xenoliths, although the process may explain the slightly higher density of CO2 in spinel. During the ascent, the strain rate of orthopyroxene calculated using the ‘constitutive equation’ is several orders of magnitude lower than that of olivine. The difference in densities of CO2 among the host mineral species is therefore attributable to the rheological properties of the minerals. Present internal pressures of fluid inclusions can be a sensitive strength marker of mantle minerals. Conversely, the density of CO2 inclusions in pyroxene (and spinel) may be a useful geobarometer.