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

It has often been suggested that U series diseuqilibria 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 Be-10. Since Be-10 is mostly a cosmogenic nuclide and has a half life of 1.5 x 10(6) years, any Be-10 found in a MORB glass must result from contamination by sedimentary material where cosmogenic Be-10 is enriched. Four MORB samples with a wide range in Th concentations (87-500 ppb) were measured for U decay series and Be-10. No Be-10 above the detection limit (6 x 10(4) to 1 x 10(5) atoms) was measured in the glasses except for one sample that was not picked. Considering in the picked glasses is 10(4) to 10 (5). Mass balance g, the maximum fraction of contaminant and a hypothetical pristine glass with (Pa-231/U-235) = 1 and calculatios between a surficial contaminant and a hypothetical pristine glass with Pa-231 and (230) Th can (Be-10/Be-9) = 0 show that only for the unpicked sample, nearly all the measured excess Pa-231 and Th-230 can accountetd for by contamination. For the other samples, <15% to <1% of the excess Pa-231 and Th-230 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.