Chemistry and origin of mantle sulfides in spinel peridotite xenoliths from alkaline basaltic lavas, Nógraád-Go¨mo¨r Volcanic Field, northern Hungary and southern Slovakia

Abstract

Neogene to Quaternary alkaline basalts from the Nógrád-Go¨mo¨r Volcanic Field (NGVF) in the Carpathian-Pannonian Region of northern Hungary and southern Slovakia contain a large number and great variety of sulfide-bearing mantle xenoliths. A previous detailed textural and chemical study of spinel peridotite (Cr-diopside) xenoliths from the NGVF identified multiple mantle processes (i.e., partial melting (s), metasomatism, and entrainment of the peridotite xenoliths into the host lavas ), which might be genetically associated with sulfides and which could account for their variability in texture and chemistry. Eleven spinel peridotite nodules (three protogranular to porphyroclastic, three equigranular, four secondary recrystallized, and one strongly metasomatized xenolith) were selected to characterize the sulfide assemblages. Two types of primary sulfide grains and two types of secondary sulfide grains were identified based on occurrence and distribution. Type-i primary sulfide assemblages are interstitial to mantle silicates, whereas Type-e primary sulfide assemblages are enclosed in mantle silicates. Secondary sulfides are connected either to healed fractures in mantle silicates (Type-f) or to borders of mantle silicates (Type-b). Type-i and Type-e primary sulfide assemblages consist mostly of the phases pentlandite (Pn), chalcopyrite (Cp), monosulfide solid solution (MSS), and violarite (Vi). Type-f and Type-b secondary sulfide grains are indistinguishable mineralogically and chemically from the Type-i and Type-e sulfide assemblages, but can be distinguished based on texture and occurrence. MSS sulfide blebs in the metasomatized xenolith developed their compositions and textures as a result of mantle metasomatism. Sulfides in the protogranular/porphyroclastic, equigranular, and recrystallized xenoliths represent immiscible melts trapped during partial melting event (s) in the mantle. In the less deformed (protogranular/porphyroclastic) xenoliths the Cp + Pn assemblage (either Type-e or Type-i) crystallized from a Cu-Ni-bearing sulfide liquid which was in equilibrium with MSS at high temperature. In addition, an MSS + Cp + Pn assemblage (either Type-e or Type-i) was produced as a result of exsolution of Cp and Pn from the high-temperature MSS during cooling. In the more deformed equigranular and recrystallized xenoliths Pn + MSS or Pn ± Cp ± Po are present. The abundance of sulfide assemblages in individual samples can be correlated to the texture of the xenoliths. The protogranular/porphyroclastic and metasomatized xenoliths show sulfide concentrations up to 0.5 vol%, whereas the other xenoliths have less than 0.02 vol% sulfides. This correlation between sulfide abundance and textural type of host xenolith indicates that the heterogeneous distribution of sulfides within the upper mantle is related, at least in part, to deformation and recrystallization processes.