Generations of Melt Extraction, Melt–Rock Interaction and High-Temperature Metasomatism Preserved in Peridotites of the ∼497 Ma Leka Ophiolite Complex, Norway

Abstract

Ophiolites allow spatial and temporal assessment of the causes and length-scales of upper mantle compositional heterogeneity because they permit field-based observations to be coupled with geochemical investigations of upper mantle lithologies. The � 497 Ma Leka Ophiolite Complex (Norway) comprises a section of early Palaeozoic (Iapetus) oceanic lithosphere with well-exposed mantle and lower crustal sections and generally low degrees of serpentinization. The Leka upper mantle section is heterogeneous at the centimetre to metre scale, manifested by abundant dunite lenses and sheets in harzburgitic host-rock, especially within � 500 m of the palaeo Moho. Abundant chromitite (� 60 vol. % Cr-spinel) and pyroxenite lenses and layers also occur in the uppermost 200‐300 m of the mantle section. These diverse mantle lithologies probably developed in a suprasubduction-zone (SSZ) setting, as a result of fluid-assisted melt extraction, offering an opportunity to interrogate the nature of chemical heterogeneities developed in such rocks. At � 497 Ma, the Os isotopic compositions of Leka harzburgites averaged � 2% more radiogenic than the projected average for abyssal peridotites at that time, yet they exhibit nearly chondritic relative abundances of the highly siderophile elements (HSE). Several of the harzburgites are characterized by low initial 187 Os/ 188 Os (<0� 121), reflecting Proterozoic melt depletion. Preservation of Os isotopic compositions consistent with ancient (<0� 5 to 2 Ga) melt depletion episodes is a common characteristic of melt-depleted oceanic peridotites. There is no clear evidence that SSZ melt extraction had a discernible impact on the bulk Os isotopic composition of the Iapetus oceanic mantle, as represented by the Leka harzburgites. By contrast, non-harzburgitic lithologies are generally characterized by more radiogenic initial 187 Os/ 188 Os and more variable HSE abundances. The dunites, chromitites and pyroxenites of the LOC can be separated into two groups on the basis of their trace element geochemistry and the Re-Os isotope errorchrons that they define, yielding ages of 485 6 32 Ma and 589 6 15 Ma, respectively. The former age corresponds, within error, to the accepted age of the ophiolite (497 6 2 Ma). The meaning of the latter age is uncertain, but possibly corresponds to the early stages of Iapetus opening. The Leka ophiolite reveals the importance of oceanic lithosphere formation processes for mantle heterogeneity at metre to kilometre scales, but also emphasizes the robustness of Os isotopes in recording older melt-depletion events.