FIGURE 5 Coupled ¹⁸⁶Os-¹⁸⁷Os isotope variation in direct samples of the mantle (abyssal peridotites and PGE-alloys and chromites from ophiolites) and volcanic rocks believed to originate by melting of plumes rising from the core-mantle boundary. Although crustal rocks (e.g. basalt) and various types of sediment have high Re/Os and Pt/Os ratios, their low Os concentrations coupled with only small ingrowth of ¹⁸⁶Os due to ¹⁹⁰Pt decay lead to nearly horizontal mixing lines with mantle peridotite on this diagram. In contrast, due to the very high concentrations of Os in the core, even a small addition of core material to the mantle will shift the Os isotope composition of the mixture towards the core component. The lines on the diagram show mixing trajectories expected for fertile mantle peridotite mixed with two types of crustal rocks (tick marks give 10% by mass increments of crustal rock in the mixture) and outer core (tick marks show 1% increments in core addition). The mixing lines were constructed using the end member parameters given in Carlson (2005), with data sources summarized in Carlson (2005) and Brandon and Walker (2005).