Ancient high Pt/Os crustal contaminants can explain radiogenic 186Os in intraplate magmas

The origin of variations in 186Os/188Os ratios amongst mantle-derived basaltic and komatiitic lavas remains controversial, with opposing models arguing for deep core-mantle versus shallow mantle sources. Crustal contamination has generally not been favoured due to the low Os contents of such sources, meaning that variations in 186Os/188Os would require involvement of extremely high proportions of crustal material. Here we re-examine crustal contamination as an effective means for generating significant 186Os/188Os variations in Earth materials. Using chromitites and peridotites from the Stillwater, Muskox and Rum layered intrusions, we show that radiogenic 186Os/188Os ratios are correlated with 187Os/188Os ratios and can only be explained by shallow-level mixing processes and crustal contamination. The samples have 186Os ([{(186Os/188Ossample[t]/186Os/188OsPM(t)) -1} × 1000], where the modern primitive mantle [PM] 186Os/188Os is 0.1198388) values ranging between 0.04 to 0.15 for the ~2.7 Ga Stillwater Igneous Complex, -0.05 to 0.17 for the ~1.27 Ga Muskox Intrusion, and 0.02 to 0.13 for the ~0.06 Ga Rum Layered Suite. The highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) contents of the chromitites and peridotites can be modelled through high sulfide-melt partitioning (typically >8000) and emphasise the role of S-saturation and HSE scavenging. Considering the high sulfide-melt partitioning and accounting for high silicate melt to sulfide melt ratios (R-factor), it is possible to explain the variations in 186Os-187Os in layered intrusions using calculated Os isotope crustal evolution growth models. These calculations indicate that <4% of ancient high Pt/Os crustal contributions can explain the composition of the chromitites and peridotites that were examined. Our observations are consistent with published models for chromitite genesis that invoke either crustal melt-primitive melt mixing, or cumulate assimilation. A crustal origin for radiogenic 186Os is a possible cause for 186Os/188Os ratio variations observed in some komatiites. It is more difficult to explain radiogenic 186Os/188Os measured in Hawaiian lavas by crustal contamination processes. Instead, ancient high Pt/Os oceanic crust, shallow mantle sources such as metasomatic sulfide, or metal-rich large low-shear wave velocity provinces at the core-mantle boundary, all remain valid explanations.