Ulv�spinel in native iron-bearing assemblages and the origin of these assemblages in basalts from Ovifak, Greenland, and B�hl, Federal Republic of Germany

Abstract

Euhedral crystals of ulvospinel are found in many of the native-iron-bearing xenoliths from the basalt of Buhl near Kassel (West Germany) and Ovifak on Disko Island (West Greenland). The typical assemblage of these xenoliths at both localities is: native Fe, troilite, cohenite, ulvospinel, ilmenite, olivine, and plagioclase, as well as silicate glass containing droplets of former Fe and troilite melt. The ulvospinel subsolidus textures and intergrowths also indicate identical cooling histories for the xenoliths in both cases. Ulvospinel crystallized after the formation of iron, but still above the Fe-FeS eutectic at 988° C. A subsequent strong drop in oxygen fugacity revealed partial breakdown of ulvospinel according to the reaction $${\text{Fe}}_{\text{2}} {\text{TiO}}_{\text{4}} {\text{ = FeTiO}}_{\text{3}} {\text{ + Fe + }}\tfrac{{\text{1}}}{{\text{2}}}{\text{O}}_{\text{2}} $$ . Microprobe analyses of a Buhl xenolith indicate that ulvospinel contains up to 4.7 w.t.% MnO, while olivine compositions correspond to Fa64–74Fo12–24Te12–15. The entire xenolith contains 1.9 w.t.% MnO. This fact, together with the geological evidence and the occurrence of corroded quartz relicts within some of the xenoliths provides clear evidence for reduction under near-surface conditions in a blast-furnace-like process. The reducing agent was coal from the Tertiary seams cut by the erupting basalt, while the xenolith source material most probably was spherosiderite, which is very common in the coals and would explain the high MnO content. Consequently, the presence of cohenite is not necessarily an indicator of high pressure. The analogies between the Buhl and Ovifak localities and their xenoliths strongly suggest a similar formation through near surface reduction and not derivation from the mantle.