Atmospheric outgassing and native-iron formation during carbonaceous sediment–basalt melt interactions

• Published in: Earth and Planetary Science Letters
• Main Authors: Pernet-Fisher, John F., Day, James M D, Howarth, Geoffrey H., Ryabov, Victor V., Taylor, Lawrence A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2017
• Subjects: crustal contamination; greenhouse gases; highly siderophile elements; mass extinction; native iron; osmium isotopes; Permo–Triassic; Dzhaltul; Greenland; Siberia
• Online Access: https://research.manchester.ac.uk/en/publications/6c5054ee-217a-4246-b82b-51be571c2fed; https://doi.org/10.1016/j.epsl.2016.12.022; http://www.scopus.com/inward/record.url?scp=85007518359&partnerID=8YFLogxK

Organic carbon-rich sediment assimilation by basaltic magmas leads to enhanced emission of greenhouse gases during continental flood basalt eruptions. A collateral effect of these interactions is the generation of low oxygen fugacities (fO 2 ) (below the iron-wüstite [IW] buffer curve) during magmatic crystallization, resulting in the precipitation of native-iron. The occurrence of native-iron bearing terrestrial basaltic rocks are rare, having been identified at three locations: Siberia, West Greenland, and Central Germany. We report the first combined study of Re–Os isotopes, highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), and trace-element abundances for these three occurrences, in addition to host sediments at West Greenland. To quantify the amount of crustal assimilation experienced by the magmas, we present combined crystallization and assimilation models, together with fractional crystallization models, to assess how relative abundances of the HSE have been modified during crystallization. The radiogenic osmium isotopic compositions (γOs initial +15 to +193) of mafic igneous samples are consistent with assimilation of old high Re/Os crustal contaminants with radiogenic 187 Os/ 188 Os, whereas the HSE inter-element fractionations (Pd/Os 2 to >10,000) suggest that some Siberian samples underwent an early stage of sulfide removal. Metalliferous samples from the Siberian intrusions of Khungtukun and Dzhaltul (associated with the Siberian flood basalts) yield internal 187 Re– 187 Os ages of 266±83Ma and 249±50Ma, respectively, reflecting late-Permian emplacement ages. These results imply that crustal assimilation took place prior to crystallization of native-Fe. In contrast, metalliferous samples from Disko Island and Bühl (associated with the West Greenland flood basalts, and the Central European Volcanic Province, respectively) have trends in 187 Re/ 188 Os– 187 Os/ 188 Os space corresponding to apparent ages older than their reported crystallization ages. These anomalous ages probably reflect ...