Mineralogy of the lower mantle: A review of ‘super-deep’ mineral inclusions in diamond

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Abstract

Starting from the late 1980s, several groups of lower-mantle mineral inclusions in diamond have been found. Three associations were established among them: juvenile ultramafic, analogous to eclogitic, and carbonatitic. The juvenile ultramafic association strongly predominates, and it is composed of ferropericlase, MgSi-, CaSi- and CaTi-perovskites, stishovite, tetragonal almandine-pyrope phase (TAPP), and some others. The association analogous to the upper-mantle eclogitic association, formed from subducting lithosphere, comprises: majorite, CaSi-perovskite bearing compositional Eu anomalies, phase ‘Egg’ with a tetragonal structure, and stishovite. The carbonatitic association is represented by various carbonates, halides, and associated minerals. Some mineral associations (wüstite + periclase and native iron + iron carbides) are, possibly, related to the D″ layer at the core/mantle boundary. The mineralogical composition of the lower mantle is now understood to be more complex than had been suggested in theoretic and experimental works. The proportion of ferropericlase in the lower mantle is higher than it was suggested before, and its composition is more iron-rich (mg = 0.36–0.90) as compared to experimental and theoretical data. Free silica (stishovite) is always present in lower-mantle associations, and a separate aluminous phase (TAPP) has been identified in several areas. These discrepancies suggest that the composition of the lower mantle differs to that of the upper-mantle, and experiments based solely on ‘pyrolitic’ compositions are not, therefore, applicable to the lower mantle. These data indicate a probability of an alternative to the CI-chondrite model of the Earth's formation, for example, an enstatite-chondrite model.

Introduction

It has long been assumed that our view of the structure and composition of the lower mantle (over a half of the Earth's total volume) relied solely upon indirect methods, such as geophysical and/or experimental data, direct observation being out of the question. However, in the last two decades, a series of lower mantle minerals has been discovered, encapsulated and preserved in diamond crystals formed in the lower mantle and delivered to near-surface regions by kimberlite eruptions. While these most frequently appear as single minerals they sometimes form paragenese with other phases. Some of these were formed before their being entrapped by diamonds while the remainder were probably encapsulated by the diamonds formed in lower-mantle provenance melts and fluids. Irrespective of these considerations, they probably represent material from the deepest levels of the Earth available for direct investigation. Compositionally, such lower-mantle minerals resemble those either predicted theoretically or synthesized experimentally under ultra-high pressure conditions (20–25 GPa and higher), corresponding to depths of 500–600 km and greater.

However, the observed diamond-bearing phases are not identical to the experimentally-produced phases, and are characterized by specific chemical features and, sometimes, distinct crystallographic structures.

At the present time being, the number of lower-mantle material ‘samples’ available for study is very limited, less than, for example, the number of Lunar samples retrieved by the Apollo and Luna missions. This does not diminish the scientific value of lower-mantle mineral samples, however, as they allow for the preliminary reconstruction of real compositions and conditions of formation in the deepest zones of the Earth providing critical information relating to early stages of the Earth's formation. Accordingly, it is appropriate to summarize the existing relevant data as a basis for formulating models and drawing preliminary conclusions. With these goals in view, the author has made use of material obtained primarily from Brazil and Venezuela, in conjunction with the existing experimental data and published materials from other regions, where similar material has been found.

Section snippets

General

Experiments performed in the 1980s demonstrated the presence a series of phase transitions in the Earth's interior within the pressure interval 20–25 GPa. The most important of these was: (Mg,Fe)2SiO4  (Mg,Fe)O + (Mg,Fe)SiO3 (wadsleyite, ringwoodite  ferropericlase + ‘MgSi-perovskite’) which occurs at c. 24 GPa and marks a transition from the upper to the lower mantle corresponding to a depth of 650–670 km (Ringwood and Irifune, 1988). According to this reaction, the lower-mantle indicator association

General

Geological data have shown that the mineral composition of the lower mantle is more complex than previously thought, as shown in Table 1, listing lower-mantle minerals in diamond from Brazil, Guinea, and Canada identified to date. Only minerals confirmed as lower-mantle phases are included in the table: ferropericlase and MgSi-perovskite (as lower-mantle indicator mineral phases) and minerals associated with these in a single diamond grain, frequently as intergrowths (‘touching association’) (

Minerals suggested from deeper levels of the Earth

Two peculiar mineral associations were found in a diamond containing the carbonatitic association: wüstite + periclase and iron carbides + native iron.

Composition of the lower mantle

In Table 11 and Fig. 19 we present frequencies of lower-mantle minerals included in diamond in three major regions, Brazil, Canada, and Guinea, which have more or less representative numbers (tens and low hundreds) of lower-mantle mineral inclusions. The proportions of minerals vary in different areas, which may be explained either as regional differences or not full representativity of the sample sets (particularly for Canadian kimberlitic pipes, where only twenty grains of lower-mantle

Discussion and conclusions

Among lower-mantle minerals identified as inclusions in ‘super-deep’ diamond, can be distinguished three mineral associations: juvenile ultramafic, ‘eclogitic’-type, and carbonatitic. The juvenile (ultramafic) obviously predominates among them. The ‘eclogitic’-type association is related only to subducted slabs which likely comprise an insignificant volume of the deep Earth, and a carbonatitic association, most likely, is associated with lower-mantle, carbon-rich partial melts caused by

Acknowledgements

Many people have contributed to the subject of this paper, and I would particularly like to thank: P. Andreazza, E. Belousova, W. Griffin, S. O'Reilly, R. Wirth, and O. Zakharchenko for past collaborations. G. Bulanova, B. Harte, and T. Stachel helped considerably by providing spreadsheets for analytical data. L. Kogarko and Yu. Litvin made useful comments to the text. This paper is based upon the presentation of the 2010 Vernadsky Reading, and I thank the Vernadsky Institute of Geochemistry

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