Minoru Ozima

Partition of noble gases between olivine and basalt melt

Abstract The distribution coefficients (Ki) of noble gases were determined for synthesized olivine-basalt melt pairs. Seven samples (BH-series) were synthesized at 1370 ~ 1300°C under a one atmospheric pressure (~ 105 Pa) noble gas mixture, and four samples (HPP- and HPG-series) at 1360-1050°C at high pressures (0.2–1.5 GPa). The two different experiments gave consistent results with each other. Some BH-series olivine samples were apparently contaminated with gas-rich inclusions or the glass phase, and the effects of such interferences on the calculated distribution coefficients were evaluated. The possible ranges of the distribution coefficients of noble gases thus obtained are: KHe ≲ 0.07, KNe = 0.006−0.08, KAr = 0.05−0.15, KKr ≲ 0.15, and KXe ≲ 0.3. These show remarkable features compared to solid element distribution coefficients: the distribution coefficients of noble gases are rather insensitive to, or even positively correlated with the atomic size of noble gases. The striking contrast between the partition of noble gases and that of solid elements may be attributed to the electrical neutrality of noble gases and the increasing electronic polarizability from He to Xe.

Sedimentary noble gases

Abstract This paper presents experimental data for the trapped noble gas contents in a variety of shales and related samples. These data, along with those previously available, indicate that the normal sedimentary rock pattern is one of progressive enrichment of the heavier gases in comparison with the proportions in air, from which sedimentary rock gases were presumably acquired. Within this normal pattern, however, variations considerably beyond an order of magnitude characterize both abundance and composition, so that multiple processes appear to be necessary to account for trapping of noble gases in sedimentary rocks. Relative to the normal pattern a number of cases of anomalous Ne enhancement occur. This unexplained phenomenon, previously thought to be exotic, must now be considered unexceptional. Noble gases in air occur in approximately the ‘planetary’ proportions characteristic of meteorites, except for more than an order of magnitude deficiency of Xe. It is commonly held that atmospheric noble gases are indeed planetary, the Xe deficiency in air being made up by the inventory in sedimentary rocks. We consider that while this hypothesis cannot be proved false the available data do not support it. At least equal weight should be given to the alternatives: either that (preferentially) Xe has failed ever to become part of the atmosphere or that the total terrestrial noble gas inventory is simply not planetary.

Ar isotopes and Earth-atmosphere evolution models

Earth-atmosphere evolution models are mathematically simulated and the resulting present isotopic ratio (40Ar/36Ar) in the mantle is given for each. Differential outgassing experiments on several recent submarine glasses were made to estimate an isotopic ratio (40Ar/36Ar) in the present mantle. Estimations of (40Ar/36Ar) in the mantle by various methods are also critically reviewed. From the experimental results and these considerations a minimum value of 2000 for (40Ar/36Ar) ratio in the present mantle is inferred. By assuming that (40Ar/36Ar)M is larger than 2000 and that the potassium content in the present mantle is larger than 50 ppm, we can limit considerably a choice among various Earth-atmosphere evolution models, i.e. (1) a continuous degassing process can not explain rare gas evolution in the atmosphere, (2) early sudden degassing is more likely and (3) such sudden degassing must have occurred earlier than 4.35 b.y.

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.

Solar-type Ne in Zaire cubic diamonds

Elemental abundances and isotopic compositions of noble gases have been measured in five cubic diamonds from Zaire. /sup 20/Ne//sup 22/Ne ratios range from 11.5 to 13.5 and /sup 21/Ne//sup 22/Ne from 0.047 to 0.074. The authors interpret these data to indicate that neon in the diamonds is a mixture of atmospheric neon and diamond neon, the latter consisting of solar-type neon trapped from the mantle source and nucleogenic neon produced by /sup 18/O(..cap alpha.., n) /sup 21/Ne and /sup 24/Mg(n, ..cap alpha..) /sup 21/Ne reactions in the diamonds. Measured /sup 3/He//sup 4/He ratios range from 4.2 to 13.0 x 10/sup -6/, while /sup 38/Ar//sup 36/Ar = 0.187 +/- 0.005 and is identical with atmospheric argon. Kr and Xe isotopic compositions are indistinguishable from air, except for slight excesses in /sup 129/Xe and fissiogenic Xe isotopes. The noble gas elemental abundance pattern in the diamonds shows a systematic fractionation trend, with the heavier noble gases being enriched relative to air.

Noble gas distribution in oceanic basalt glasses

The distribution of noble gases has been investigated in six MORB glass samples using stepwise heating, vacuum crushing and the analysis of grain size fractions. These experiments indicate a strong noble gas partitioning into CO2-filled vesicles. An inhomogeneous distribution of argon isotopes within individual glasses is observed in several cases and is believed to result from the combined effects of the deep-seated component partitioning into vesicles and the contamination of the residual component dissolved in the glass by atmospheric noble gases. Using a mixing diagram, we are able to discriminate against atmospheric contamination and estimate the Ar partition coefficients. These coefficients are in qualitative agreement with the Henrys law constant for silicate melts.

Comparative Studies of Solar, Q-Gases and Terrestrial Noble Gases, and Implications on the Evolution of the Solar Nebula

Abstract We selectively review noble gas data for the Earth and meteorites (the Q component) and for the sun (solar wind) and seek to understand possible genetic relationships among these components. The widespread distribution of Q gases suggests that it was established as a distinct component prior to incorporation in planetary solids, e.g., in the gas phase of the solar nebula. We suggest that mass might be the major factor involved in generating both elemental and isotopic fractionations which characterize Q, e.g., by some form of Rayleigh distillation. The empirical relationship between Q and its presumed antecendant solar-composition source reservoir further suggests that He in Q is post-deuterium-burning He, implying active exchange of materials between the early sun and the solar nebula. A comparison of terrestrial noble gases with Q and solar wind suggests that primordial terrestrial gases are unlikely to have been derived from either component alone, even with some superposed fractionation; a mixture of the two components is a more plausible source.

40Ar-39Ar age of rocks, and the development mode of the Philippine Sea

THE origin and development of the Phillipine Sea have been central issues in tectonic studies of a marginal sea: the deep-sea drilling project (DSDP), Leg 31, was primarily intended to resolve the question1. Unfortunately, at only two of the Leg 31 sites (292 and 296) were microfossils indicating the age of the basement recovered, so the age of the ocean basin, had to be deduced by dating the drilled basement rocks. There seems to be some confusion in the literature2 in interpreting the fossil data with regard to the age of the ocean basin. We have dated several DSDP Leg 31 drilled rocks by the 40Ar–39Ar step-heating technique (Table 1 and Fig. 1) and re-evaluated the fossil ages and various hypotheses concerning the development of the western Philippine Sea and the Shikoku Basin. We applied two statistical criteria to judge the reliability of 40Ar–39Ar ages, (1) the experimental data with MSUM values (Table 1) less than 3.50 and (2) the apparent ages should agree with each other, within Iσ, for more than 65% of 40Ar released. Although such criteria do not prove the reliability of this dating technique, they provide a measure of it3. Of the nine drilling sites, six reached the basement—site 290, 291, 292, 293, 294 and 296—their ages were determined by both the 40Ar–39Ar step-heating dating and micropalaeontological methods (Table 1).

40Ar39Ar stepwise degassing experiments on some submarine rocks

40Ar39Ar stepwise degassing experiments were made on six submarine rocks. The40Ar39Ar ages thus obtained were compared with the conventional K-Ar ages. Samples with high K-content generally give a linear correlation on a (40Ar*/36Ar*) - (39Ar*/36Ar*) diagram, showing insignificant excess40Ar. The excess40Ar is shown to reside in higher temperature fractions, though the amount is small (< 10−11 mol/g) and becomes significant only for low K-content samples. Basalt drilled from the Experimental Mohole gives a40Ar39Ar age of 19 my which agrees with magnetic anomaly lineation isochron. The K-Ar age of 42 my reported for this sample by Kulp is shown to be in error owing to excess40Ar. The geophysical implications of the40Ar39Ar ages of other samples are also discussed.

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.