Fig. 6: Schematic cartoons (a-c) illustrate melt focusing and melt-peridotite interaction in the upwelling asthenospheric column under a typical oceanic spreading center.
Panel a shows a vertical cut plane (100 km deep and 600 km wide) of a sub-spreading-center regime far from transform faults, modified from Ligi et al.7. Lithosphere spreading and asthenosphere upwelling produce the color-coded decompressional melting regions with melting degrees (F) of 1–20% marked (a). Red dashed curves with temperatures from 1300 °C to 200 °C show the thermal structure of the lithosphere and asthenosphere7, 9, 10, 42, and the 1100 °C curve marks the lithosphere-asthenosphere boundary (LAB) as proposed by Niu and Green43. Two black dashed lines show the boundaries between spinel- and garnet-facies mantle and the garnet-spinel transitional zone (85–60 km) in between. The white dashed curve encloses the upwelling asthenospheric mantle where anhydrous melting occurs. The red short arrows show that the melts converge and focus into the sub-axis narrow zone under the spreading center. The black thick curves with black arrows display the flow patterns of upwelling asthenosphere. The residual mantle columns represented by the color-coded zones have the compositional gradients too small to be consistent with the Kangjinla situation (Figs. 2–4). A model of melt focusing and melt-rock reaction (pyroxene consumption and olivine addition) in the top region of the upwelling asthenospheric column (b) can well explain the vertical depletion and local depleted anomalies displayed in (c). The variations of melt/rock (M/R) ratios in the column result in the lithological and compositional variations, which form the laterally symmetrical mantle column (b). After it splits into two parts which each rotates ~90° to become the oceanic uppermost lithospheric mantle, the primarily more-depleted axial region forms the top of the lithospheric mantle section while the more-fertile bilateral region becomes the bottom (c), as observed in the Kangjinla and other ophiolites. Sp spinel, Gt garnet, Py pyroxene, Ol olivine, NDZ northern dunite zone, NHZ northern harzburgite zone, CHZ central harzburgite zone, SLZ southern lherzolite zone.
