Element and Sr–O isotope redistribution across a plate boundary-scale crustal serpentinite mélange shear zone, and implications for the slab-mantle interface

Abstract

Fluid flow along crustal plate boundary-scale serpentinite shear zones may provide insights into element transfer at one of Earth's most important but least accessible lithological boundaries: the slab-mantle wedge plate interface. For ∼55 km in the Southern Alps of New Zealand, a 10 to 100s m wide block-in-matrix serpentinite mélange shear zone – the Livingstone Fault – separates the harzburgitic base of the Dun Mountain Ophiolite Belt from arc-derived metasedimentary and minor metabasaltic rocks. Transects across the shear zone reveal systematic chemical and isotopic changes over several hundred meters, from harzburgite (<1 wt% H2O, 87Sr/86Sr = 0.7031–0.7045, δ18O=∼+5.3‰) to massive and then foliated mélange serpentinite (up to 13.7 wt% H2O, 87Sr/86Sr = 0.7084, δ18O=+7.8‰). These changes require hydration of peridotite by fluids derived from, or that interacted with, the metasediment-dominated wallrock (87Sr/86Sr = 0.7073–0.7093, δ18O≥+9.6‰). Tremolite-veined serpentinite and metasomatized greyschist blocks (87Sr/86Sr = 0.7063–0.7071; δ18O=+8.7) within the serpentinite mélange also require ultramafic- and metasediment–derived element mixing. Halogens (Br and I) were enriched in serpentinized ultramafic rocks. By analogy with the crustal Livingstone Fault, the slab-mantle plate interface may have a strongly anisotropic basal serpentinite mélange shear zone that grades into a thicker zone of massive and variably serpentinized peridotite with a progressively decreasing “slab” element and isotope signature. Permeability in subduction-related shear zones is predicted to be mainly parallel to the slab but channelized fluid mobility across the Livingstone Fault was aided by transient fracturing. Oxidation of harzburgite during serpentinization appears to have been accompanied by mobilization of Si, Ca, Mg, Sr, Br, I, Ga, Mo, Ta, W, V and Y. As many of these elements occur in the sources of arc magmas, serpentinization may play an important role in mobilizing them. The Livingstone Fault lithologies and their element and isotope transfer records means this shear zone may represent a field-accessible analogue for the shallow slab-mantle plate interface.