Vertical depletion of ophiolitic mantle decodes melt focusing and interaction in the asthenospheric column under oceanic spreading centers

Abstract

Adiabatic decompressional melting of asthenosphere under spreading centers has been accepted to produce vertical compositional variations of oceanic lithospheric mantle. However, theoretical estimates of the compositional gradients are much smaller than those observed from ophiolites, clearly requiring additional processes. Here we conduct systematic high-density sampling and whole-rock and mineral compositional analyses of harzburgites in a Tibetan ophiolitic mantle section (~2 km thick), which shows a primary upward depletion (~12% difference over ~2 km) and local depleted anomalies. Thermodynamic modeling demonstrates that these features cannot be produced by decompressional melting or proportional compression of residual mantle. Instead, they can be explained by reaction between silica-undersaturated melts and peridotite with lateral melt/rock variations in the topmost asthenospheric upwelling column, showing stronger depletion in its melt-focusing center and local zones. This column will split from the center into two parts, which rotate in the mantle flow to become horizontal, thus forming the oceanic uppermost lithospheric mantle characterized by vertical depletion and local anomalies within a sub-spreading-center regime.