Nd isotopic composition of Jurassic Tethys seawater and the genesis of Alpine Mn-deposits: Evidence from Sr-Nd isotope data

Abstract

Jurassic metabasalts, metasediments and Mn ores from the Pennine realm of the Alps were examined in order to establish the Nd isotopic composition of the Jurassic Tethys seawater and to elucidate the genesis of the Mn deposits. The highly positive initial ϵ Nd values (+7 to +9.8) of the metabasalts and their low 87Sr 86Sr ratios (0.7028 to 0.7049) indicate that they originated from a depleted mantle. The initial ϵ Nd values of the cherts range between −5 and −9. The Sm-Nd isotope data indicate that they are primary mixtures of basalt and continental detritus. The smaller than 2 μm fractions of the cherts and of a marble, which probably represent the formerly authigenic material, show initial ϵ nd values ranging between −5.9 and −6.6. The average initial Sr isotopic composition of five Mn ores is 0.70730 ± 22, identical to that of contemporaneous Jurassic seawater. The initial Nd isotopic composition values of the Mn ores are very homogeneous. The average value of five ores, a leachate and residue of one of them, calculated for an age of 170 Ma, is 0.512082± 19. It is suggested that the ores, together with the smaller than 2μm fractions of the sediments, define the Jurassic Tethys seawater isotopic composition. The resulting average 143Nd 144Nd initial value is 0.512089 ± 17 ( ϵ Nd −6.5 ± 0.6).