Anomalous lead isotope ratios and provenance of offshore sediments, Gulf Carpentaria, northern Australia

Anomalous Pb isotope ratios measured by Inductively Coupled Plasma Mass Spectrometry in terrigenous marine sediments (<63 m fraction) from the Gulf of Carpentaria originated from depositional mixing of clay/silt with average modern crustal Pb isotope ratios and detrital monazite with high 208Pb/206Pb and low 207Pb/206Pb. This interpretation is supported by strong correlations between Pb isotope ratio and Th, U and light rare-earth element concentrations in the sediments as well as by monazite compositional data. A likely source of the detrital monazite is the western portion of the Georgetown Inlier of mainly Proterozoic S-type granitic rocks. A clear distinction between Pb isotope ratios in sediments deposited from the Norman and Bynoe Rivers in the southeast Gulf of Carpentaria and the persistence of catchment-specific Pb isotope ratios 45 km offshore suggest that Pb isotope data are useful in tracing the provenance of terrigenous offshore sediments when the source rocks of catchments show sufficient chemical and/or mineralogical variation