SAMPLE CONTAMINATION EXPLAINS THE PB ISOTOPIC COMPOSITION OF SOME RURUTU ISLAND AND SASHA SEAMOUNT BASALTS

Palacz and Saunders [1] reported Pb isotopic compositions for Rurutu island basalts that form a trend oblique to the main oceanic basalt array and interpreted them as representing a mixture of HIMU and DUPAL (or EM II) mantle sources. Detailed leaching experiments on aliquots of the same rock powders as those measured by Palacz and Saunders [1] demonstrate that these powders have been heavily contaminated by a foreign Pb component. Two contaminants with different Pb isotopic compositions are identified. They represent together more than 80% of the total Pb present in the sample. The residues display a single coherent Pb isotopic trend consistent with more recent measurements [2,3] of Rurutu island basalts and with the main oceanic basalt array. Pb isotopic compositions similar to those of Rurutu basalts [1] were reported by Fornari et al. [4,5] for Lamont seamount basalts. They interpreted them as reflecting a Rapa or Rurutu-type (EM II) hotspot influence in their sources. Strong leaching of glass chips from one of the Fornari et al. [4,5] samples demonstrates that these glass chips were contaminated by a foreign Pb component similar to one of the Rurutu contaminants. In both of these cases the contamination of the samples occurred, most likely, prior to the initiation of isotope analyses. These findings highlight the importance of careful sample preparation procedures and we suggest using acid-washed rock chips for even the freshest looking lavas for Pb isotopic analyses.