Shigeo Zashu

Noble gases in submarine pillow volcanic glasses

Fifteen submarine glasses from the East Pacific Rise (CYAMEX), the Kyushu-Palau Ridge (DSDP Leg 59) and the Nauru Basin (DSDP Leg 61) were analysed for noble gas contents and isotopic ratios. Both the East Pacific Rise and Kyushu-Palau Ridge samples showed Ne excess relative to Ar and a monotonic decrease from Xe to Ar when compared with air noble gas abundance. This characteristic noble gas abundance pattern (type 2, classified by Ozima and Alexander) is interpreted to be due to a two-stage degassing from a noble gas reservoir with originally atmospheric abundance. In the Kyushu-Palau Ridge sample, noble gases are nearly ten times more abundant than in the East Pacific Rise samples. This may be attributed to an oceanic crust contamination in the former mantle source. There is no correlation between the He content and that of the other noble gas in the CYAMEX samples. This suggests that He was derived from a larger region, independent from the other noble gases. Except where radiogenic isotopes are involved, all other noble gas isotopic ratios were indistinguishable from air noble gas isotopic ratios. The3He/4He in the East Pacific Rise shows a remarkably uniform ratio of (1.21±0.07)×10−5, while the40Ar/36Ar ranges from 700 to 5600.

Noble gas state of the ancient mantle as deduced from noble gases in coated diamonds

Cores and coats of five coated diamonds, one from Botswana and four from Zaire, were separately analyzed for their noble gases. Noble gases in the diamonds are essentially of a trapped origin, including radio- and nucleogenic components such as4He, 40Ar, 21Neexcess and excesses in Xe isotopes (129, 131–136). The fairly precise elemental and isotopic abundances allow us to infer the noble gas state in the ancient mantle. 20Ne/22Ne ratios are fairly constant (11.8 ± 0.4), and very close to that of SEP (solar energetic particle)-Ne, but distinctly different from the atmospheric ratio. 21Ne/22Ne ratios range from 0.028 to 0.06, which is attributed to nucleogenic 21Ne from 18O(α, n)21Ne and 24Mg(n, α)21Ne reactions. The difference in 20Ne/22Ne between atmosphere and mantle can be attributed to the hydrodynamic escape of hydrogen from the primitive atmosphere during the very early stage in the Earths history. 38Ar/36Ar and Kr isotopic ratios are identical to the atmospheric values within 1%. After correction for 238U- or 244Pu-fission Xe, the 131–136Xe abundance ratios are indistinguishable from atmospheric ratios. Lighter Xe isotopes (124–128Xe) are also likely to be atmospheric, but a final conclusion must wait until better data are obtained. In a 136Xe/130Xe−129Xe/130Xe diagram, diamond data lie on the same line as defined for MORB. The observed identical correlation for both diamonds and MORBs appears to suggest that the progenitor of the excess131–136Xe is 244Pu, but not238U, though the direct Xe isotopic measurements was not precies enough to decide unanimously the progenitor.

K/Rb and (87Sr/86Sr)0 ratios of dredged submarine basalts

Abstract K/Rb and ( 87 Sr/ 86 Sr) 0 ratios were measured for 14 submarine basalts dredged from seamounts in the Pacific Ocean. The K/Rb ranges from 200 to 700, which is significantly lower than that of oceanic ridge tholeiites. Petrographic examination and the low value of K/Rb indicate that seamount basalts are alkaline. Submarine basalts from the east side of the Japan trench have significantly lower ( 87 Sr/ 86 Sr) 0 than basalts from the Izu-Mariana islands located on the west (continental) side of the trench. This may be explained on the basis of difference in the depth of their magma source region. Samples from Shatsky Rise have comparatively high ( 87 Sr/ 86 Sr) 0 , which may be in accordance with a recent suggestion that Shatsky Rise was a fossil boundary between rigid oceanic plates.

Primitive Helium in Diamonds

Thirteen diamond stones from various unspecified mines in South Africa were analyzed for the isotopic ratio of helium-3 to helium-4. Values of the ratio ranged from less than 10–7 to (3.2 � 0.25) x 10–4. The latter value is higher than the primordial helium-3/helium-4 ratio in meteorites and close to the ratio for solar-type helium. Such extremely high values may represent primitive helium that evolved very little (that is, showed very little increase in radiogenic helium-4) since the formation of the earth.

Sr isotopic studies of volcanic rocks from island arcs in the western pacific

Abstract Systematic measurements were made on initial ( 87 Sr 86 Sr ) ratios and Rb, Sr and K contents of the rocks from Izu, Ogasawara (Bonin), Iwojima, Mariana, Yap and Palau Islands. The initial ( 87 Sr 86 Sr ) ratios for volcanic rocks from Izu, Iwojima and Mariana Islands fall in a range from 0.7030 to 0.7050, which is similar to previously reported values for island-arc volcanics. However, a high initial ( 87 Sr 86 Sr ) ratio (~ 0.7052) was found for rocks from Ogasawara Islands, suggesting that they are genetically different from other islands. This result may be related to the particular topographical feature of the trench discontinuity in the eastern side of the Ogasawara Islands. On the contrary, rocks from the Palau Islands show low initial ( 87 Sr 86 Sr ) ratios (~ 0.7028), which are similar to those of oceanic ridge basalts, and Rb and Sr contents similar to those of normal island-arc volcanics. A low initial ( 87 Sr 86 Sr ) ratio was found for the rock from the Yap Islands, supporting the oceanic origin of them as suggested by Shiraki. It is likely that Palau, Yap and Mariana Island arcs represent the three different stages of development of island arcs. These results suggest that except for the Ogasawara Islands, the island arcs in the western Pacific have not been formed by the splitting or drifting of continental margin, but have grown from oceanic structure.

Additional evidence of existence of ancient rocks in the mid-Atlantic ridge and the age of the opening of the Atlantic

Abstract A metabasalt dredged at a junction of the median valley with the Atlantis fracture zone (30°01/tN, 42°04/tW) in the Mid-Atlantic Ridge shows complete recrystallization under a metasomatic condition, though the original igneous texture of a coarse-grained basalt is still recognizable. There is strong circumstantial evidence suggesting that this rock is not an ice-rafted erratic, but an authentic Mid-Atlantic Ridge rock. The 40 Ar- 39 Ar age of this sample is 169 m.y. (Jurassic) which should represent the time of recrystallization. The initial value ( 87Sr/ 86 Sr ) O is 0.720, far above the values previously observed in oceanfloor basalts, including both tholeiitic and alkalic rocks (0.701–0.704). Sr with such a high isotopic ratio is considered to have been introduced by metasomatism during metamorphism by a solution coming from a continental mass or masses which were then located very close to the Mid-Atlantic Ridge. The 40 Ar− 39 Ar age of sample AM50 may approximate the time of the commencement of the opening of the Atlantic. All these data support the possible existence of ancient rock masses in the Mid-Atlantic Ridge, as was formerly claimed by Bonatti, Melson and others.

Ar and Sr isotopes of mantle-derived rocks from the Japanese Islands

Abstract40Ar-30Ar and Sr isotope analyses have been made for ultramatic nodules in basaltic rocks from Itinome-gata and the Oki-Dōgo Island, northeast and southwest of Japan respectively, in order to examine the state of the upper mantle under the Japanese Islands. 40Ar-30Ar analyses have revealed that ultramafic nodules from Oki-Dōgo Island were involved in events which caused Ar redistribution in these rocks and probably were related to their formation not older than several tens of millions of years ago.These nodules are relatively depleted in both Rb and Sr contents together with some other incompatible elements compared to similar nodules from the other regions in the world. This may be typical for ultramafic nodules from the Japanese Islands.Ultramafic nodules show mostly a little higher87Sr/86Sr ratios (0.704–0.706) than those of host basaltic rocks (0.703–0.705) indicating the accidental origin for these nodules. Present results also suggest both horizontal and vertical inhomogeneities of the uppermost part of the mantle with respect to Sr isotopes on a regional scale. Hence, the ultramafic nodules in volcanic rocks may not always directly represent the upper mantle materials from which the magma is produced at present. Such ultramafic nodules might be relatively old material from the uppermost part of the mantle under the Japanese Islands.

Nitrogen isotopic composition of a gas-rich ordinary chondrite, ALHA-77216

Abstract Nitrogen abundance and isotopic composition in a gas-rich chondrite ALHA-77216 (L3 or H3) was measured in order to obtain an independent estimate of the average nitrogen isotopic composition of the solar system. Nitrogen, Ne and Ar were extracted, by a stepped combustion method, and measured using a static mass spectrometer. The cosmic ray exposure age based on the 21 Ne abundance is about 35.3 ± 1.1 million yr. Isotopically heavy nitrogen is observed at 700–1100°C. The abundance in a bulk sample is 1.38 ppm with 0.61 ppb excess 15 N, of which 0.17 ppb could be due to cosmogenic nitrogen. Heavy nitrogen is enriched in the metal phases compared with silicates. A δ 15 N value as large as 174‰ was observed for a the 800°C step of a magnetic separate. The abundance ratio of nitrogen to solar type rare gases in this meteorite is higher than those in lunar samples. We suggest that the isotopically heavy nitrogen is of solar origin, but other possibilities (presolar and indigenous nitrogen) cannot be completely ruled out.