New paper by Brian O’Driscoll and colleagues discusses terrestrial mantle melting processes.
Earth’s mantle is the major terrestrial reservoir by volume (>80%), and its mechanical properties and chemical composition are markedly different from those of the crust and core, reflecting over 4.5 billion years (Ga) of thermally-driven differentiation. Our understanding of the upper part of the mantle has developed significantly over the past few decades, from being relatively homogeneous and convectively well-mixed, to a mantle that is chemically and lithologically heterogeneous from the grain scale to the scale of entire ocean basins. Unfortunately, the mantle is not easily accessible, and we are restricted to specific geological environments such as abyssal peridotites (dredged or drilled from modern ocean floors) and ophiolites, in order to obtain mantle samples.Ophiolites represent slivers of oceanic lithosphere thrust (obducted) onto continental crust at destructive plate margins, and commonly preserve a portion of mantle in their lowermost regions. Our study combines measurements of major and trace elements with analysis of the 187Re/188Os systematics and highly siderophile element (HSE; Os, Ir, Ru, Pt, Pd and Re) abundances of a suite of mantle peridotites from the ~497 Ma Leka Ophiolite (Norway). Our datasets show that much of the character of these peridotites is dominated by the final melt extraction event that they experienced, above a subduction zone during closure of the Iapetus Ocean. However, some of the Leka peridotites preserve compositional and chronological information pointing towards a different melting event, at ~590 million years (Ma). The meaning of this age is still uncertain, but it may correspond to the early stages of Iapetus opening. Finally, some Leka rocks yield model ages that are even older, at 1 billon years old. The new data imply that the oceanic mantle represented by the Leka Ophiolite resembles a ‘patchwork’ of peridotites of different ages and compositions. The Leka peridotites comprised the root to the Iapetus Ocean basin, and they have retained a memory of the full cycle of ocean opening and closing, as well as evidence of an enigmatic ancient melting event long before the existence of Iapetus.
Full citation is: Brian O’Driscoll, Richard J. Walker, James M. D. Day, Richard D. Ash, and J. Stephen Daly (2015) Generations of Melt Extraction, Melt–Rock Interaction and High-Temperature Metasomatism Preserved in Peridotites of the ∼497 Ma Leka Ophiolite Complex, Norway. Journal of Petrology 2015 56: 1797-1828. doi: 10.1093/petrology/egv055
Publisher’s website: http://petrology.oxfordjournals.org/content/suppl/2015/10/05/egv055.DC1
University of Manchester holdings: https://www.escholar.manchester.ac.uk/uk-ac-man-scw:274469