Insights into the nature of plume–asthenosphere interaction from central Pacific geophysical anomalies

Abstract

The central Pacific is one of the type localities where the influence of a plume on the properties of the asthenosphere can be investigated based on the geophysical observations. Recent seismological studies showed enigmatic structures of the upper mantle in the central Pacific. The asthenosphere (~ 100–200 km depth) in this region has unusually strong V_SH/V_SV > 1 anisotropy associated with relatively weak azimuthal anisotropy. Although less well-constrained, there are suggestions of higher than normal viscosity and lower than normal electrical conductivity in this region. Previous models for the plume–asthenosphere interaction do not easily explain these observations. I propose that these observations are the consequence of a single process: depletion of water by deep partial melting in the plume column. The plume-fed materials in the asthenosphere will be dominated by the olivine A-type fabric whereas in the surrounding normal asthenosphere olivine E-type fabric dominates. This contrast in olivine fabrics provides an explanation for the observed anomalous seismic anisotropy in the central Pacific. Water depletion will also cause high viscosity and low electrical conductivity. The present model implies that plumes supply depleted materials (rather than enriched materials) to the asthenosphere.

Introduction

The OIB (ocean island basalt) and MORB (mid-oceanic ridge basalt) are the two major volcanic rocks on Earth that are believed to be derived from two different regions or components in Earth's mantle. It is generally considered that MORB is derived from “depleted” materials and OIB is from “undepleted” or “enriched” materials (e.g., Zindler and Hart, 1986, Hofmann, 1997, Hofmann, 2004). The way in which these materials (or “reservoirs”) interact or mix is one of the main questions in global geodynamics (e.g., Kellogg et al., 1999, Phipps Morgan and Morgan, 1999, Albarède and van der Hilst, 2002, Tackley and Xie, 2002, van Keken et al., 2002, Bercovici and Karato, 2003). In almost all of geodynamic models, materials that produce OIB (“undepleted” or “enriched” regions) are considered to ascend through the upper mantle, and to be added to the asthenosphere that is believed to be the source region of MORB. Consequently, it is important to understand how the injection of plume materials modifies the properties of the asthenosphere. In many models, the plume flux is considered to be so small that the influence of plume materials is negligible (e.g., Davies, 1988, Sleep, 1990). However, there are some hints as to a more important role of plumes affecting the materials' circulation in Earth's mantle (e.g., Phipps Morgan et al., 1995b, Nolet et al., 2006).

The central Pacific is the best place to investigate the nature of plume–asthenosphere interaction because in this area, the Hawaii plume, one of the largest plumes (Sleep, 1990, Montelli et al., 2006), interacts with the Pacific upper mantle that moves with high velocity (relative to the plume). Consequently, a large region of the Pacific upper mantle may show some influence of plume–asthenosphere interaction (e.g., Ribe and Christensen, 1994). In this paper, I will discuss that the reported anomalous properties of the central Pacific asthenosphere including anomalous seismic anisotropy suggest an extensive interaction between plume and ambient asthenospheric materials that causes the asthenosphere in these regions be anomalously depleted with water (hydrogen). I will first review studies reporting various anomalous properties in the asthenosphere of the central Pacific, and then will present a model to explain them based on recent mineral physics and petrological observations. Particularly important in this analysis are the recent finding of the influence of water content on lattice-preferred orientation in olivine (and hence seismic anisotropy) (Jung and Karato, 2001, Karato et al., 2008), the influence of water on viscosity and electrical conductivity of upper mantle minerals (e.g., Mei and Kohlstedt, 2000, Karato, 2006) and the new knowledge on the melting behavior of upper mantle materials (e.g., Hirschmann, 2006).