PRECISE DETERMINATION OF THE ISOTOPIC COMPOSITION OF POTASSIUM: APPLICATION TO TERRESTRIAL ROCKS AND LUNAR SOILS

Abstract

We detail a method for the precise and accurate determination of isotopic variations in the ratio with a precision of ±0.5‰ (2σm). Purified potassium is chemically extracted from rocks, soils, minerals, or solutions by ion exchange chromatography. Complete chemical yields (>99.8%) are achieved in order to avoid laboratory-induced isotopic fractionations. The purified potassium is converted to a glass by melting with barium borate flux, and the resultant bead is mounted for ion probe analysis. The SIMS method utilized by the ion probe produces extremely stable K+ ion beams, with no measurable temporal variability in the isotope ratio. The instrumental fractionation is steady at about −4‰, and is corrected for by measurement of a standard. The measurement of gravimetrically prepared isotopic standards indicates that the method is accurate at the stated level of precision and free of egregious errors. Analysis of terrestrial samples including peridotite, basalts, granites, carbonatite, biotite schists, and seawater, indicate the complete absence of isotopic variations in δ41 1K among terrestrial materials at the 0.5%o level. Application to lunar soils and a regolith breccia confirms previously observed large isotopic fractionation effects (Garner et al., 1975a; Church et al., 1976). Some lunar soils, e.g., 14163, are shown to have large sample heterogeneity (≈7%o), while others, e.g., soils and a regolith breccia at several Apollo 15 sites (Station ), are homogeneous at the level of analytical precision. The presence of potassium isotopic effects in bulk soils (up to +12.7‰ in this study) with magnitudes comparable to the Rayleigh fractionation factor (25‰) indicates that volatility during micrometeorite impact melting played a large role in lunar regolith formation. As much as 15% of the regolith potassium has been lost from the Moon, through the tenuous lunar atmosphere.