Okio Arai

Isotopic ratios of potassium in magnetic spherules from deep-sea sediments

Abstract Isotopic ratios of potassium in individual magnetic spherules were measured. The magnetic spherules were collected from deep-sea sediments dredged at the middle South Pacific Ocean (35°04′S, 138°39′W). The diameters of the spherules ranged from 170 to 520 μm and their weights from 17 to 350 μg. Each sample was dissolved in distilled hydrochloric acid and then mounted on a filament of a mass spectrometer without any chemical separation. Measurements were carried out with a highly sensitive mass spectrometer operating in a ion counting mode. Anomalous values of isotopic ratios of 40 K/ 41 K are observed in a few magnetic spherules. The values found in them are higher by about 40 ∼ 70% than in reagent potassium. These high values may be caused by cosmic ray irradiation in outer space. We conclude that these spherules are of extra-terrestrial origin or are ablation droplets scattered from the melted surface of a meteorite entering the earths atmosphere.

Isotopic Fractionation of Magnesium Sputtered by 2 keV He^+ Ions

Mass spectrometric measurement is made to see isotopic fractionation of magnesium caused by bombardment with 2 keV He-ions. A thin layer of magnesium of thickness 100~1000 A is prepared and irradiated with a dose of 1017~1018 ions/cm2. Comparison of mass spectrometric data on bombarded and non-bombarded samples shows an increase of 2% for the isotopic ratio 26Mg/24Mg and 1% for 25Mg/24Mg. The mechanism of enrichment is looked for in the sputtering, and the model of cascade atomic collisions for sputtering is found to give a reasonable explanation.

Isotopic compositions of potassium and calcium in magnetic spherules from marine sediments

Abstract Isotopic compositions of potassium and calcium in individual magnetic spherules were determined. No significant anomaly was observed for potassium within twice the statistical error (2σ), although for calcium isotopes enrichments of 46 Ca, 44 Ca and 42 Ca were observed in one spherule. The relative excess of 46 Ca, 44 Ca and 42 Ca in the spherule agrees with the relative yield of spallogenic calcium isotopes observed in iron meteorites. This fact indicates that the enrichment in the calcium isotopes was caused by cosmic ray irradiation of the spherule in outer space.