Rare-earth element (REE) geochemistry of late Paleocene–middle Eocene phosphate nodules in the Subathu Basin of Solan District, Himachal Pradesh, India

The phosphate nodules are solely confined to the Green facies of late Paleocene–middle Eocene Subathu Formation in the Subathu Basin of Solan District, Himachal Pradesh, India. The phosphate nodules are hard and compact, elliptical, rounded to well rounded, usually dull earthy to dark grayish in color and break with sharp and conchoidal surfaces. The present study deals with the rare-earth elements (REE) geochemical characteristics viz. hat-shaped, enrichment of middle REE relative to light REE and heavy REE, negative Eu and weak negative to weak positive Ce anomalies and values of ratios such as Er/Nd (0.07–0.19), Y/Ho (32–48), La/Nd (0.46–0.90), (La/Sm) N (0.18–0.47) and (La/Yb) N (0.26–0.51). It appears that the seawater conditions or growth mechanisms were different at the time of formation of cores and rims of the nodules as indicated by the contrast REE concentrations, respectively. Although in both cases very similar REE spectra are observed, rims are much more likely to show a diagenetic alteration front than cores. The Y/Ho ratio (32–48) suggests that the principal source of REE was the terrigenous sediments followed by seawater in the rims and cores of these nodules. The results of this study suggest a dominant role of diagenetic processes in the REE concentration and distribution as well as possible suboxic to anoxic conditions during the formation of cores and rims of phosphate nodules, as also evident by the presence of pyritic nodules in the Subathu Basin of Solan District.