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

Abstract

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 ±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 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 concurrent assimilation of high Re/Os, radiogenic 187 Os 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 to 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 Disko Island and Bühl necessitates that significant quantities of CH 4 , CO, CO 2 , SO 2 and H 2 O were released during assimilation of carbonaceous Pernet-Fisher et al. The origin of native-iron 3 sediments. Consequently, carbonaceous sediment-basalt melt interactions have collateral effects on total gas output from flood basalt volcanism into the atmosphere. However, the amount ...

A Low O/Si Ratio on the Surface of Mercury: Evidence for Silicon Smelting?

“…Smelting processes are not limited to anthropogenic influence. In fact, there are several examples of naturally occurring smelting processes that have occurred on Earth (Disko Island, Greenland; Northern Siberia, Russia; and Kassel, Germany; Bird & Weathers, 1977;Iacono-Marziano et al, 2012;Melson & Switzer, 1966;Pernet-Fisher et al, 2017;Ulffmoller, 1990), and smelting has also been invoked to explain the petrologic history of the parent body of the unusual ureilite meteorites (Goodrich et al, 2007;Singletary & Grove, 2003;Walker & Grove, 1993).…”

Precious metal enrichment at low-redox in terrestrial native Fe-bearing basalts investigated using laser-ablation ICP-MS

“…The IPGE are less soluble than the PPGE in a silicate melt, indicating that the PPGE enrichment is linked to an early-stage fractionation of olivine, chromite and metallic IPGE phases that preferentially incorporate IPGE over PPGE (e.g., Barnes and Fiorentini, 2008;Song et al, 2009;Mungall and Brenan, 2014). The tholeiitic magmas of the Siberian and Disko Island flood basalt sequences are typically interpreted to result from assimilation-fractionation of picritic magmas within staging chambers in the lower or upper crust (e.g., Lightfoot et al, 1993;Keays and Lightfoot, 2007;Pernet-Fisher et al, 2016). Picritic magmas in flood basalt sequences are generally believed to represent primary magmas unaffected by assimilation-fractionation process, and thus to best represent the mantle source.…”

“…In Figure 11, we show model R-factor pathways for Re versus Os and Pt versus Pd for LA-ICP-MS data and whole-rock data from Pernet-Fisher et al (2016). The R-factor, described by Naldrett and Campbell (1979), is typically used in the study of sulfide ore bodies and represents the mass of silicate magma that a segregated sulfide liquid (ore body) has equilibrated with in order to account depth within the staging chamber, implying that parental melts were already relatively enriched in these elements, as noted above.…”

“…CI-chondrite after Horan et al (2003) and Noril'sk sulfide normalizing factor after. The field for whole-rock analyses is from the same suite of samples and is presented inPernet-Fisher et al (2016).…”

“…a) HSE profiles for average native Fe metal (FeNi metal) analyses; b) HSE profiles normalized to the average HSE contents of all data collected for Siberian native Fe ores, allowing direct comparison with the HSE enrichments between the two localities; c) HSE profiles for average cohenite and sulfide analyses. The field for whole-rock analyses is from the same suite of samples and is presented inPernet-Fisher et al (2016).…”