Jun Okano

Geochemical implications from Sr isotopes and K-Ar age determinations for the Cook-Austral islands chain

Abstract Sr isotopes and K-Ar ages were determined for volcanic rocks from three islands, Samoa, Rarotonga and Rurutu in the Austral-Gilbert-Marshall chain. We have established that Rurutu originated from the same “hot spot” as Tubuai and Raivavae in the Cook-Austral chain as indicated by its 11 m.y. K-Ar age and ( 87 Sr 86 Sr ) ratios. The ( 87 Sr 86 Sr ) ratios for Rurutu, Tubuai and Raivavae lie in the restricted range 0.7026–0.7035. K-Ar ages of all volcanic rocks from Samoa and Rarotonga are all less than 4 m.y. and their ( 87 Sr 86 Sr ) ratios are much higher than those in Rurutu, Tubuai and Raivavae. These findings suggest that Samoa and Rarotonga did not originate in a single hot spot for the Cook-Austral islands.

An Ion Microprobe Analyzer

An ion microprobe mass analyzer was constructed for the analysis of solids. A beam of argon ions was used as a probe. The current and the diameter of the probe were 10-7 A and 0.25 mm respectively. A double focusing mass spectrometer was used to analyze the mass of the secondary ions. The mean radii of the ions in the electric field and in the magnetic field were both 10.0 cm. The deflection angles in these fields were 63.6° and 90° respectively. A Cu-Be secondary electron multiplier followed by a d.c. amplifier was used as an ion detector. Some characteristics with the variation of the peak height due to the change of the surface conditions were clarified. The relative sensitivities for Ti, V, Cr, Mn, Fe, Co, Ni in an iron alloy were determined. The detectable limits for boron in silicon and for chromium in iron were estimated to be 5 ppm and 1 ppm respectively. The distributions of nickel and chromium in the iron meteorite, Odessa, were briefly studied.

Isotope Analysis on Iron Meteorites with Ion Probe Mass Spectrometer

In order to investigate the isotopic effect of nickel which might have been brought into by the diffusion of nickel isotopes during the crystallization of an iron meteorite, the isotopic ratio 58Ni/60Ni and the nickel concentration were measured with the ion probe mass spectrometer. Krypton and argon ions were used as a primary ions, whose energy was 5 keV. The diameter and the current of the primary ion beam were 250 µm and 2×10-7 A on the surface of the sample target. Three iron meteorites, Odessa, Canyon Diablo and Hembury were analyzed. Two former samples belong to the class of coarse octahedrite, and the latter one medium octahedrite. The point-to-point analysis was carried out every 0.25 mm or 0.5 mm in the range of about 4 mm on the surface of the sample. The distribution of 58Ni/60Ni was likely to correlate to the nickel concentration in the case of Odessa and Canyon Diablo.

Isotopic Ratio of Lithium in Chondrite Measured by an Ion Probe Mass Spectrometer

Isotopic abundance ratios of lithium, 7Li/6Li, for four stony meteorites and for a terrestrial granite sample, G-1, are measured by an ion probe mass spectrometer. The measured ratios for the meteorites are about 13 percent lower than the ratio for G-1. The results are inconsistent with those obtained by conventional mass spectrometers with surface ionization ion sources at other laboratories. The mass discrimination effects and the systematic errors, which may lower the measured ratios preferentially for the meteorites, are thoroughly investigated in connection with the characteristics of the ion probe mass spectrometer. The origin of the instrumental bias, however, is not found. Although further experimental work with both types of mass spectrometer should be required to reach the final conclusion, the difference is likely existent from the results.