This dataset contains major element compositions of the KWR4 sample series collected from the high-resolution interval of the Kawaruppu section, Nemuro Group, Hokkaido, Japan. These data were obtained to evaluate whether the osmium isotope ratios and platinum group element concentrations of the same samples were affected by diagenetic alteration. Major element compositions were measured by a RIGAKU ZSX Primus II X-ray fluorescence (XRF) spectrometer at Atmosphere and Ocean Research Institute (AORI), the University of Tokyo (Kashiwa).