The major element, rare earth element, and platinum-group element chemistry of the Margi umbers have been investigated in order to evaluate these sediments as recorders of past variations in the Os isotopic composition of seawater. Because of the compositional similarity of the Margi umbers to metalliferous sediments and hydrothermal-plume particles from the modern ocean, we conclude that these supraophiolite sediments record the Os isotope composition of Cretaceous seawater. This conclusion requires that the Margi umbers have in large part retained their compositional integrity throughout burial diagenesis and subsequent uplift. That the 187Os/188Os ratio (0.52–0.55) of ca. 90 Ma seawater is similar to, or more radiogenic than, seawater throughout the Paleogene provides evidence of decoupling of the marine Sr and Os isotope records. Beyond constraining the 187Os/188Os of Cretaceous seawater, these results are significant in that they provide a set of chemical criteria that can be used to evaluate the potential of other metalliferous sediment sequences as recorders of the 187Os/188Os ratio of ancient seawater. Establishing these criteria represents an important step toward extending the marine Os isotope record into the Mesozoic and Paleozoic.