Application of a Premelting Model to the Lithosphere‐Asthenosphere Boundary

Abstract A seismic discontinuity known as the G‐discontinuity, which has been prevailingly detected in the oceanic mantle, is considered to provide evidence of a sharp lithosphere‐asthenosphere boundary (LAB). We investigate whether this sharp LAB is explained by grain boundary premelting, which causes a solid‐state weakening of rock from just below the solidus temperature. We use an experimentally derived mechanical model describing the effect of premelting and calculate vertical profiles of VS and QS−1 for various sets of oceanic geotherm and solidus values. Based on the realistic volatile distribution and solidus profile obtained from a model of fractional decompression melting, we show that even when the oceanic upper mantle is melt free, geotherm sufficiently approaches the solidus at various plate ages, resulting in a steep reduction in VS due to the anelastic effect of premelting. A small amount of volatile adsorbed to grain boundary plays an important role in this result. The predicted seismic discontinuity sharpness (mostly ∼15 km) is consistent with seismological observations (≤15 km), predicted discontinuity depth (along ∼1300°C isotherm) is deeper than observations (along 1100°C isotherm), and predicted velocity reduction amplitude (∼1%–2%) is considerably smaller than observations (≥5%). Because the predicted velocity reduction amplitude is subject to uncertainty resulted from the limited experimental data, further study is needed for the mechanical effect of premelting. We further discuss a discrepancy of seismological results from receiver function and SS precursor studies and from surface wave studies, making it difficult to explain both data consistently.