Evidence from ¹⁰Be and U series disequilibria on the possible contamination of mid-ocean ridge basalt glasses by sedimentary material

Abstract : It has often been suggested that U series disequilibria measured in mid-ocean ridge basalts (MORB) can be perturbed by contamination with sedimentary or hydrothermal material found near the ridge axis where the basalts are erupted. Here we provide an independent way of estimating the maximum degree of contamination by sediment using constraints from ¹⁰Be. Since ¹⁰Be is mostly a cosmogenic nuclide and has a half life of 1.5 × 10⁶ years, any ¹⁰Be found in a MORB glass must result from contamination by sedimentary material where cosmogenic ¹⁰Be is enriched. Four MORB samples with a wide range in Th concentrations (87-550 ppb) were measured for U decay series and ¹⁰Be. No ¹⁰Be above the detection limit (6 × 10⁴ to 1 × 10⁵ atoms) was measured in the glasses except for one sample that was not picked. Considering that the sedimentary contaminant contains 10⁹ to 10¹⁰ atoms/g, the maximum fraction of contaminant in the picked glasses is 10^-4 to 10^-5. Mass balance calculations between a surficial contaminant and a hypothetical pristine glass with (²³¹Pa/²³⁵U) = 1 and (¹⁰Be/⁹Be) = 0 show that only for the unpicked sample, nearly all the measured excess ²³¹Pa can be accounted for by contamination. For the other samples, <15% to <1% of the excess ²³¹Pa and ²³⁰Th can be explained by incorporation of sedimentary material. This study reemphasizes the need for careful sample preparation for U series measurements in MORB, especially at low U levels (<50 ppb). It also confirms that the large U series excesses in carefully picked and cleaned MORB samples are magmatic in origin.