Journal article

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

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Publication Details

Author list: Pernet-Fisher JF, Day JMD, Howarth GH, Ryabov VV, Taylor LA

Publisher: Elsevier

Publication year: 2017

Journal: Earth and Planetary Science Letters


Volume number: 460

Start page: 201

End page: 212

Total number of pages: 12

ISSN: 0012-821X

eISSN: 1385-013X


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-wustite [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 (gamma Os-initial +15 to +193) of mafic igneous samples are consistent with assimilation of old high Re/Os crustal contaminants with radiogenic (OS)-O-187/Os-188, 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 Re-187-Os-187 ages of 266 +/- 83 Ma and 249 +/- 50 Ma, 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 Buhl (associated with the West Greenland flood basalts, and the Central European Volcanic Province, respectively) have trends in Re-187/Os-188-Os-187/Os-188 space corresponding to apparent ages older than their reported crystallization ages. These anomalous ages probably reflect concurrent assimilation of high Re/Os, radiogenic Os-187 crust during crystallization of native-Fe, consistent with the character of local West Greenland sediments. In all three locations, calculations of combined assimilation of crustal sediments and fractional crystallization indicate between 1-7% assimilation can account for the Os-isotope systematics. In the case of Siberian samples, incompatible trace-element abundances indicate that lower crustal assimilation may have also occurred, consistent with the suggestion that crustal assimilation took place prior to native-Fe precipitation. The extent of local crustal contamination at Siberia, West Greenland, and Buhl necessitates that significant quantities of CH4, CO, CO2, SO2 and H2O were released during assimilation of carbonaceous sediments. Consequently, carbonaceous sediment basalt melt interactions have collateral effects on total gas output from flood basalt volcanism into the atmosphere. However, the amount of carbonaceous sediment contamination experienced by melts forming the Khungtukun and Dzhaltul intrusions alone, cannot explain the major C-isotope excursions at the Permo-Triassic mass-extinction event. Instead, further unsampled intrusions that also experienced significant carbonaceous sediment-melt interactions would be required. Enhanced greenhouse gas emission during the Permo-Triassic mass extinction may have been facilitated by a combination of mantle melting and carbonaceous sediment-melt interactions, together with other proposed mechanisms, including wildfires, or by microbial metabolic exhalation. (C) 2016 Elsevier B.V. All rights reserved.


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