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

摘要

The origin of variations in Os-186/Os-188 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 favored due to the low Os contents of such sources, meaning that variations in Os-186/Os-188 would require involvement of extremely high proportions of crustal material. Here we re-examine crustal contamination as an effective means for generating significant Os-186/Os-188 variations in Earth materials. Using chromitites and peridotites from the Stillwater, Muskox and Rum layered intrusions, we show that radiogenic Os-186/Os-188 ratios are correlated with Os-187/Os-188 ratios and can only be explained by shallow-level mixing processes and crustal contamination. The samples have delta Os-186 ([{(Os-186/Os-188(sample[t])/Os-186/Os-188(PM(t))) - 1) x 1000], where the modern primitive mantle [PM] Os-186/Os-188 is 0.1198388) values ranging between 0.04 to 0.15 for the similar to 2.7 Ga Stillwater Igneous Complex, -0.05 to 0.17 for the similar to 1.27 Ga Muskox Intrusion, and 0.02 to 0.13 for the similar to 0.06 Ga Rum Layered Suite. The highly siderophile element (HSE: Os, IL Ru, Pt, Pd, Re) contents of the chromitites and peridotites can be modeled through high sulfide-melt partitioning (typically >8000) and emphasize 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-factors), it is possible to explain the variations in Os-186/Os-188 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 as