2003 Goldschmidt Conference Abstract

Bina, C. R., Seismic signatures of heterogeneity from subducted basalts, Abstracts of the 2003 Goldschmidt Conference, Kurashiki, Japan, A40, 2003.


Products of chemical differentiation near the surface, such as oceanic crustal basalts, may be transported downward by subduction to give rise to chemical heterogeneity at depth. In the upper mantle, anhydrous metabasalts (e.g., eclogites) should be only slightly fast relative to ambient peridotite mantle, with a signature arising primarily from the thermal anomaly rather than composition. Hydrous metabasalts (e.g., lawsonite blueschists, lawsonite eclogites), on the other hand, should be significantly slow in the 100-250 km depth range, becoming faster (e.g., stishovite eclogites) below 250 km.

In the lower mantle, anhydrous metabasalts (e.g., perovskitites) should be fast, growing progressively more so with increasing depth. This signature arises from the compositional contrast, as the thermal signal is small and falls with increasing depth (as does the negative buoyancy anomaly). If dense hydrous phases persist to such depths, they may decrease this predicted velocity anomaly. It is important to note that these fast anomalies arise largely from the presence of free silica in metabasaltic mineralogies. Thus, onset of a CaCl2-structured post-stishovite phase of silica may induce a drop in shear moduli resulting in slow shear velocity anomalies, and any subsequent stabilization of a PbO2-structured post-stishovite phase of silica may yield larger fast anomalies.

These anomalies largely depend upon the survival of free silica phases, which are unstable in contact with the surrounding peridotite. In the lower mantle, for example, silica reacts with ferropericlase to form perovskite. Thus, these metabasalt mineralogies (free silica + perovskite) can persist only to the extent that they are preserved as armored relics from contact with surrounding metaperidotite (perovskite + ferropericlase). Therefore, efficient mixing in the deep mantle would cause decay in such velocity anomalies due not only to volumetric averaging but also to chemical reaction.

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