Trace and rare earth element geochemistry of Jurassic mudstones in the northern Qaidam Basin, northwest China

Trace and rare earth element (REE) of Jurassic mudstones from the northern Qaidam Basin of northwest China were measured by inductively coupled plasma mass spectrometry (ICP-MS) analysis. The analytical results show that the trace element and REE concentrations are higher than in the average of the upper crust. Three aspects evolved: (i) the paleoclimate index, Σ(Fe + Mn + Cr + Ni + V + Co)/Σ(Ca + Mg + Sr + Ba + K + Na), varies between 0.27 and 1.14 indicating semi-arid to semi-moist conditions during the Jurassic; the Middle Jurassic is more arid than the Early Jurassic; (ii) absence of marine transgression during the sedimentation period of the Jurassic mudstones, mainly evidenced by the Sr content ranging from 0.05‰ to 0.33‰ and Sr/Ba < 1.0; and (iii) the redox condition during the sedimentation of the Jurassic mudstones were slightly oxic as evidenced by Ce anomalies of 0.83–1.01.