1-s2.0-S0012821X17300353-gr004.sml

Fig. 4. Cu/Pd ratios in shergottites and in terrestrial mantle-derived magmatic rocks. Because sulfide–silicate melt partition coefficients of Cu (∼103) and Pd (∼105–106) are very different (Mungall and Brenan, 2014), sulfide saturation and segregation during fractional crystallization leads to highly variable Cu/Pd in evolving melts and cumulates, e.g., up to 3 orders of magnitude in lower crustal gabbros (Peucker-Ehrenbrink et al., 2012) and in MORB glasses (e.g., Lissner et al., 2014). In contrast, terrestrial komatiites from Victoria's lava lake (Puchtel et al., 2016) display constant Cu/Pd in rocks that display a range of MgO contents. Note Cu/Pd ratios of these komatiites overlap Cu/Pd of Earth's mantle, indicating that the Cu/Pd ratio of sulfide-undersaturated magma generally reflects its mantle source. As in terrestrial komatiites, Cu/Pd in most shergottites are relatively constant (2650 ± 950, blue band) over a large range of MgO contents, consistent with sulfide-undersaturated magmatic evolution. The mean Cu/Pd of shergottites is slightly higher than that calculated from Cu and Pd contents of the Martian mantle because of the large uncertainty of the estimated Pd value of the Martian mantle (Day et al., 2016, Taylor, 2013). Literature Pd contents in shergottites are from Brandon et al. (2012), and the corresponding Cu contents are from this study and Meyer (2013). Because of sample heterogeneity, literature data on Y-980459 and Dhofar 019 display highly variable PGE contents and thus Cu data obtained on different aliquots yield apparently different Cu/Pd (gray symbols with dashed lines). The Pd value of a replicate of Dhofar 019 is from Jones et al. (2003) whereas a lower Pd content for Y-980459 was measured by Yang et al. (2015). Because Cu and Pd contents are not from the same sample aliquots, this contributes to the scatter of Cu/Pd ratios (a factor of 3–4 variation). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)