Iron isotope and trace metal variations during mantle metasomatism: In situ study on sulfide minerals from peridotite xenoliths from Nógrád-Gömör Volcanic Field (Northern Pannonian Basin)

Abstract

Sulfides from lherzolite and wehrlite xenoliths from the Nógrád-Gömör Volcanic Field (NGVF), located in the Northern Pannonian Basin, were studied to understand the behavior of chalcophile and siderophile elements during mafic melt – peridotite interaction. We applied in situ methods to analyze the major and trace elements, as well as Fe isotope compositions of sulfide minerals. Sulfides are more abundant in wehrlites (~0.03 vol%) and are often enclosed in silicates, whereas in lherzolites, they are scarcer (~0.01 vol%) and predominantly interstitial. Monosulfide solid solution and pentlandite are the most common sulfide phases in the lherzolite xenoliths, whereas in wehrlite xenoliths it is pyrrhotite and chalcopyrite. Consequently, wehrlitic sulfides show higher bulk Fe and Cu but lower bulk Ni and Co contents compared to the lherzolitic sulfides. Trace elements with both chalcophile and siderophile character (Ge, Se, Te, and Re) show lower, whereas highly chalcophile elements (Zn, Cd, Sb, and Tl) show higher concentrations in wehrlitic sulfides compared to lherzolitic ones. Highly siderophile elements show no systematic difference between the sulfides of the two xenolith series, which suggests moderate enrichment in these elements in wehrlite bulk rocks due to their higher sulfide content. Sulfide δ56Fe signature indicates variable isotopic composition both in lherzolites (δ56Fe: −0.13 to +0.56‰) and wehrlites (δ56Fe: −0.20 to +0.84‰) relative to the terrestrial mantle (δ56Fe: +0.025 ± 0.025‰; Craddock et al., 2013). However, irrespectively of the xenolith lithology, there is a significant difference between the δ56Fe of sulfides from the two sampling localities: NTB /North/: vary from −0.20 to +0.04‰ and NME /South/: vary from +0.56 to +0.84‰. This suggests that the Fe isotopic ratios of sulfides are not modified by the wehrlitization process. Difference in sulfide δ56Fe between the two xenolith localities is probably because of the higher, isotopically heavier (δ56Fe: from +1.28 to +1.60‰; Ciążela et al., 2019) chalcopyrite content in sulfides from the NME xenoliths compared to those from the NTB xenoliths irrespectively to their lithology. Our results also indicate sulfide and chalcophile element enrichment resulting from metasomatism in the subcontinental lithospheric mantle. We suggest that this process affected the regional metal distribution and has implications for global metal mass balance within the subcontinental lithosphere.