EarthRef.org Reference Database (ERR) -- Bourdon et al. 2000

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 Be. Since 10Be is mostly a cosmogenic nuclide and has a half life of 1.5 x 106 years, any 10Be found in a MORB glass must result from contamination by sedimentary material where cosmogenic 10 Be is enriched. Four MORB samples with a wide range in Th concentrations (87--550 ppb) were measured for U decay series and 10 Be. No 10 Be 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 that the sedimentary contaminant contains 10 9 to 10 10 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 (10 Be/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.