Tracking deep mantle reservoirs with ultra-low velocity zones

摘要

Some regions of the Earth's lowermost mantle exhibit anomalous seismic properties within a thin zone, less than tens of kilometers in thickness, that directly overlies the core-mantle boundary (CMB). These regions have been dubbed Ultra-Low Velocity Zones (ULVZs) due to their greater than 10% drop in seismic velocities. High resolution seismic array studies have found small, localized ULVZs (e.g., 10 km thick and 50-100 km wide) with a large increase in ULVZ density (~ 10%) relative to the background mantle. Many studies note that ULVZ material may be chemically distinct, though P-to-S-wave velocity reductions are sometimes consistent with partial melting. The apparent absence of ULVZs in many regions of the CMB is consistent with having a distinct chemical signature, regardless of melt content. However, it is unknown how a small volume of very dense ULVZ material can be locally elevated, particularly in the presence of large-scale compositional reservoirs predicted by seismology, geochemistry, and geodynamics. We perform ultra-high resolution, kilometer-scale, thermochemical convection calculations for an entire mantle system containing three distinct compositional components in order to investigate how a ULVZ interacts with large-scale lower mantle compositional reservoirs. We demonstrate that convection can dynamically support small scale accumulations of dense ULVZ material, consistent with the size and density inferred from seismology. Furthermore, we show that ULVZs preferentially reside at the boundaries of large compositional reservoirs, which periodically break apart and merge together in response to changes in downwelling patterns. As they do, ULVZ material migrates and recollects in a systematic fashion. ULVZ material can become entrained in mantle plumes forming from reservoir boundaries, contributing to isotopic anomalies found in hotspot volcanism. Thus ULVZ detection helps to constrain large-scale mantle convection patterns, the locations of compositional reservoir boundaries, and the evolution of geochemical reservoirs.