Bina, C. R., Seismic velocity and density anomalies from subducted basalts, Abstracts of the 8th European Workshop on Numerical Modeling of Mantle Convection and Lithospheric Dynamics, Zámek Hrubá Skála, Czech Republic, 13-15, 2003.
Products of chemical differentiation near the surface, such as oceanic crustal basalts and gabbros, may be transported downward by subduction to give rise to chemical heterogeneity at depth. In the shallow upper mantle, anhydrous metabasalts (e.g., eclogites) should be 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. The properties of observed seismic reflectors along the upper surfaces of subducting slabs are consistent with phase relations in hydrothermally altered basalts [Helffrich et al., 1989; Helffrich & Stein, 1993; Peacock, 1993; Hacker, 1996; Helffrich, 1996; Connolly & Kerrick, 2002].
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 temperature-dependence of seismic velocities falls with increasing depth in the lower mantle, and the magnitude of temperature anomalies from subducted material should also fall with increasing depth due to thermal assimilation [Bina & Wood, 2000; Mattern et al., 2002; Bina, 2003]. Thus, subducted metabasalts can explain many apparently fast seismic scatterers in the lower mantle, but how can some apparently slow scatterers (e.g., at ~1115 km) be explained [Kawakatsu & Niu, 1994; Niu & Kawakatsu, 1997; Hedlin et al., 1997; Kaneshima & Helffrich, 1998, 1999; Vinnik et al., 1998, 2001; Castle & Creager, 1999; Kruger et al., 2001; Niu et al., 2003]? Possible causes of such low velocity features include dense hydrous silicate phases and post-stishovite phases of free silica.
If dense hydrous phases persist to such depths, they may decrease the predicted velocity anomaly (as in the case of hydrous metabasalts in the upper mantle). However, dense hydrous silicates are unstable below ~1200-1300 km depth [Shieh et al., 1998; Ohtani et al., 2001]. Furthermore, the elasticity of these phases appears to depend primarily upon density rather than upon water content, so that any associated slow velocity anomalies should be small [Angel et al., 2001]. Thus, significantly slow seismic anomalies in the lower mantle may be difficult to generate from dense hydrous silicate phases in subducted material.
An unusual shear-mediated phase transition in silica, from rutile-structured stishovite to a CaCl2-structured phase, occurs at high pressures. As a result, the shear modulus of SiO2 should drop by ~20% in the pressure interval ~40-47 GPa before rising again subsequently [Karki et al., 1997; Shieh et al., 2002]. This transition thus has the potential to generate slow VS anomalies in the ~1200-1500 km depth range which are accompanied by positive density anomalies but small VP anomalies, in accord with recent seismic observations below 1100 km [Niu et al., 2003]. At greater depths, such high-pressure silica phases may be expected to yield fast anomalies. It has recently been suggested, however, that mantle temperatures may possibly depress this transition in silica to greater depths, perhaps ~1900 km, rendering it unsuitable to explain velocity anomalies at shallower levels [Ono et al., 2002]. Thus, the temperature-dependence of this transition pressure in silica remains an important topic for further study. In summary, slow shear velocity anomalies at ~1200-1500 km may arise from a post-stishovite phase change in silica (as may large fast anomalies at greater depths) unless dP/dT for the transition proves to be large.
It is important to note that both fast and slow anomalies in material subducted into the lower mantle arise largely from the presence of free silica in metabasaltic mineralogies. Hence, such anomalies 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 (with perovskite rinds) from contact with surrounding metaperidotite (perovskite + ferropericlase). Therefore, high temperatures and efficient mixing in the deep mantle may induce decay in such velocity anomalies, due not only to volumetric averaging but also to chemical reaction.
Subducted metagabbro/basalt should remain negatively buoyant (relative to peridotite) throughout the lower mantle [Bina & Wood, 2000; Mattern et al., 2002; Bina, 2003; Mattern, pers. comm. 2003; Ohtani et al., 2003]. While it has been suggested that an abundance of low-density (Ca-ferrite structured) Na-Al phase in sodic compositions may allow for density crossover (to neutral buoyancy) near ~1500-2000 km [Ono et al., 2001], high-pressure alkali phase behavior remains a complex puzzle for further study [Miyajima et al., 2001].