Ophiolite Perspectives on Oceanic Mantle Heterogeneity

Abstract:

The mantle sections of ophiolites offer a useful approach to studying compositional heterogeneities in the oceanic mantle. A potential caveat is that the tectonic provenance of ophiolites is often not easy to decipher, although many have undergone at least supra-subduction zone (SSZ) processing. Significant outstanding questions include the degree to which ophiolite peridotites preserve evidence of pre-SSZ events and the way that SSZ melt extraction modifies the character of these peridotites. A suite of Caledonian ophiolites associated with the closure of the Iapetus Ocean offers an opportunity to shed light on these issues, in particular to assess the degree to which long (regional) wavelength compositional heterogeneities survive SSZ melting. Observations on the combined highly-siderophile element (HSE) and ¹⁸⁷Os/¹⁸⁸Os systematics of the broadly coetaneous (490-500 Ma) Shetland Ophiolite Complex (Scotland) and Leka Ophiolite Complex (LOC; Norway) are presented here.

Generally, the lithological composition of each locality is harzburgitic, and hosts lenses and layers of dunite, chromitite and pyroxenite that are interpreted as representing SSZ-related (channelised) melt migration and melt-rock interaction. Although the bulk of the harzburgitic rocks have approximately chondritic initial ¹⁸⁷Os/¹⁸⁸Os and HSE abundances, ancient (Proterozoic) melt depletion (T_RD =1.4 to 1 Ga) is recorded in ~10% of samples from each locality. This is also commonly observed in abyssal peridotites. One important implication of the data is that SSZ melt generation/migration has had no discernible impact on the bulk Os isotopic composition of the Iapetus oceanic mantle. By contrast, non-harzburgitic lithologies consistently exhibit more radiogenic initial ¹⁸⁷Os/¹⁸⁸Os and more variable HSE abundances. The dunites, chromitites and pyroxenites of the LOC can be separated into two groups on the basis of isochrons that they define; yielding ages of 481±22 Ma and 589±15 Ma, respectively. The former age corresponds, within error, to the accepted age of the ophiolite (497 ± 2 Ma). The meaning of the latter age is uncertain, but may correspond to the early stages of Iapetus opening. The data imply that the oceanic mantle represented by both ophiolites resembles a ‘patchwork’ of peridotites of different ages and compositions.