Mantle Origin of Olivine-rich Troctolites in a Rift Environment

Abstract:

Olivine-rich troctolites from Krivaja-Konjuh massif in the Dinarides (Bosnia and Herzegovina) represent a unique natural laboratory to study melt segregation and retention in originally fertile rift-related mantle rocks. Petrographic and chemical analyses of coexisting minerals (olivine and interstitial clinopyroxene, spinel and plagioclase) show that olivine and spinel are of mantle origin, and interstitial clinopyroxene and plagioclase are products of crystallization of trapped melt. Geochemical and microstructural observations (dissolution of pyroxene) indicate that this trapped melt started off at larger depth and that it was silica undersaturated when it reached the plagioclase stability field. The original mantle olivine remained and texturally equilibrated with the melt, which is evident from the high Ni content and euhedral crystal habit in thin sections. This suggests that the melt that crystallized the interstitial phases, clinopyroxene and plagioclase, was saturated in olivine and probably similar to MORB-type melt. The interstitial phases show no sign of deformation. A mantle origin of olivine is corroborated by EBSD data that show a progressive misorientation across individual olivine grains due to previous episodes of deformation within the mantle. By contrast, grains originating from cumulate crystallization should show little internal deformation. Therefore we suggest that olivine-rich troctolites are not crystallization products, but rather products of reaction of melt with the lherzolitic mantle. The evidence of trapped melt in those rocks provides a new insight into the crust-mantle transition at slow spreading ridges and continental rifts where olivine-rich troctolites have been observed.