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

Publisher Website
Google Scholar
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, 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.