Toward a Comprehensive Understanding of Transition Zone Seismic Discontinuities: Part II. Inferences on the State of the Transition Zone with a Stagnant Slab

Abstract:

Plate tectonic processes operating over much of the Earth’s history induce long-term mantle mixing of chemical heterogeneities, recycling of volatiles and regulate basalt geochemistry. Through broadband analysis of P-wave receiver function conversions and topside reflections, we characterize the 410 and 660 seismic discontinuities beneath China (see Shen et al., DI013, this meeting) and simultaneously determine the shear velocity and density jumps, the sharpness of the boundary as well as the gradient immediately above/below the boundary. Within the thermodynamic framework, HeFESTo, we explore these observations with respect to seismic properties predicted in a volatile/melt free mantle, where harzburgite and basalt rock assemblages are either fully equilibrated or mechanically mixed.

We find that observed shear velocity (5-6%) and density jumps (1.5-2%) across the 410 are both too low to be consistent with the olivine-wadsleyite phase transition in a pyrolitic mantle; the 410 is sharp (<< 5 km) and it requires Mg-rich harzburgite (Mg# >~ 0.93) in the mantle; the gradient near the 410 is very low, inconsistent with a dry pyrolitic mantle. The observed 660 not only consists of a sharp boundary (<< 5 km) with relatively small shear velocity (3.5-4%) and density jumps (4-4.5%), but it also involves a substantial velocity/density gradient near the boundary (> 0.003 km/s/km; > 0.003 g/cm³/km). These observations coincide very well with the post-spinel and post-garnet phase transitions in a mantle with about 75% harzburgite fully equilibrated with 25% basalt.

Taking into account of previous mantle triplication analyses and electric conductivity profiles in the region, these results lead us to examine the following hypotheses: The harzburgite assemblage in the mantle transition zone beneath China is probably at least as depleted as Archean cratonic peridotites. Near the 410, the mantle is either relatively dry (e.g., the water content is well below the solubility of olivine) and basalt-enriched (~40%), or extremely wet (e.g., the water content is near or beyond wadsleyite saturation limit) and pyrolitic. While the 660 is consistent with a dry mantle in full equilibrium, we can’t rule out the possibility of a hydrated lower transition zone and uppermost lower mantle that is mechanically mixed.