Bina, C. R., Lower mantle mineralogy and the geophysical perspective, Reviews in Mineralogy, 37, 205-239, 1998.
Introduction. A variety of observations (Jeanloz 1995, Helffrich and Wood 1996, Irifune and Isshiki 1998, Agee this volume) suggest that the upper mantle may be largely peridotitic in bulk composition, perhaps approaching the composition of the model pyrolite (Ringwood 1989). What can be said about the composition and mineralogy of the lower mantle? Certainly we expect the mineralogy of the lower mantle to differ from that of shallower regions, if only due to high-pressure phase transformations, but why might the bulk composition of the lower mantle differ from that of the overlying material? Early partial melting of the mantle (Herzberg and O'Hara 1985) may have resulted in large-scale differentiation between upper and lower mantle, although elemental partitioning data appear not to support such a model (Kato et al. 1988). Diffusive (Garlick 1969, Bina and Kumazawa 1993) or convective (Weinstein 1992) processes acting across regions of phase transition may have generated chemical separation between upper and lower mantle, but such processes would occur over inordinately long time scales in the absence of fluid phases (Mao 1988). Perhaps more fundamentally, if chondritic meteorites (Anders and Grevesse 1989) are taken as representative of the cosmochemistry of the solar nebula from which the planet condensed, then a bulk earth whose whole mantle is of pyrolite composition (Ringwood 1989) is deficient in silicon. Unless the excess silicon was taken up in the core or volatilized during planetary accretion (Ringwood 1975), or unless the chondritic model is not an appropriate model for the bulk earth (McDonough and Sun 1995), then the lower mantle should be enriched in silica relative to a pyrolitic upper mantle. Early models of solar cosmochemistry indicated an iron deficit, suggesting that the lower mantle might be enriched in iron relative to the upper mantle (Anderson 1989a), but subsequent calibration of photospheric spectra (Holweger et al. 1990, Biémont et al. 1991) brought solar abundances into agreement with those of chondrites, thus removing the cosmochemical argument for iron enrichment.
Aside from a few diamond inclusions that appear to represent low-pressure back-transformation products of lower mantle mineral assemblages (Kesson and Fitz Gerald 1991, Harte and Harris 1994, Harte et al. 1994), the lower mantle is not amenable to direct sampling. Thus, the mineralogy and composition of this region must be inferred via geophysical remote sensing, from (for example) observations of seismic waves and electric fields at the surface. The processing of raw geophysical data lies beyond the scope of this review, but an outline of how geophysical models are constructed (and exposure to their associated terminology) can give valuable insight into potential uncertainties. Furthermore, the models (e.g., three-dimensional seismic velocity structures) which result from inversion of these data can be used to construct models of lower mantle composition and mineralogy. Ideally, the resulting lower mantle mineralogical models could then be tested by using them to predict the primary geophysical observations, in the sort of forward modeling approach already applied in the transition zone (Helffrich and Bina 1994). Many uncertainties remain in our understanding of the geophysical properties of minerals under lower mantle conditions, so that conclusions tentatively drawn herein may be subject to change in the wake of future measurements. However, while we may not yet be ``in possession of the talismans which are to open to thee the mineral kingdoms and the centre of the earth itself'' (Beckford 1786), the topics reviewed herein should provide a framework for addressing unanswered questions as more data become available.Copyright © 1998 Mineralogical Society of America