Lead and uranium isotopic behavior in diagenetic and epigenetic manganese nodules, Timna Basin, Israel, determined by MC-ICP-MS

Abstract

Isotope ratios of U and Pb were measured in two types of Mn nodules from the Cambrian Timna Formation, Israel. Type A nodules are mainly composed of pyrolusite and hollandite, with Mn, Ba, Pb and U concentrations of 30–60%, 0.2–2.5%, 0.2–1.0% and 500–3500 ppm, respectively, whereas type B nodules were formed by alteration of the former, and contain mainly coronadite, with Mn, Ba, Pb and U concentrations of 7–48%, 0.2–7%, 0.6–5% and 10–160 ppm, respectively. The isotopic composition of U and Pb was measured by MC-ICP-MS on Mn-rich solutions (up to 100 mg/L) without and with chromatographic separation. The values for the 207/206 and 208/206 ratios have been determined with precisions of up to 50 ppm and those of 206/204, 207/204 and 208/204 – up to 200 ppm. The values for the 234/238 ratios have been determined with precisions of 0.4–1%. The results of the separated and unseparated solutions were shown to be equal within the error. Thus there is no significant matrix effect while measuring U and Pb in Mn rich solution using the MC-ICP-MS. The isotopic composition of Pb and U support the distinction between the two types of Mn nodules. Type A nodules have a wide range of 206Pb/ 204Pb ratios (18.278–19.776), and an almost constant ratio of 208Pb/ 204Pb. In contrast, type B nodules have almost constant 206Pb/ 204Pb ratios and a wide range of 208Pb/ 204Pb ratios (37.986–38.079). Type A nodules form a linear array on a 207Pb/ 204Pb vs 206Pb/ 204Pb diagram, while type B nodules form a tight group characterized by lower Pb isotope ratios that slightly deviate from the type A array. The 234U/ 238U ratio differs between the two types of nodules; type A nodules exhibit a uniform and close to equilibrium 234U/ 238U ratio while type B nodules show a wide range of 234U/ 238U ratios above and below the equilibrium value. The isotopic composition of Pb in type A nodules might reflect Pb contributions from plutonic rock weathering, exposed at the time of deposition or later, to the Cambrian sea. These nodules have remained unaffected by processes that occurred since the Cambrian. The higher 208Pb/ 204Pb values of type B indicate that these nodules were formed from a Th-enriched solution probably during epigenetic processes which occurred also during the last 1 Ma. Thus the two isotopic systems of U and Pb can record formation, leaching and redeposition of Mn ores.