Mantle minerals

Paul UH (2001) Melt retention and segregation beneath mid-ocean ridges. Nature 410 920-923 Feineman MD, DePaolo DJ, Ryerson FJ (2002) Steady-state Ra/ °Th disequilibrium in hydrous mantle minerals. Geochim Cosmochim Acta 66 A345 (abstr)... 

Over the past 10 years a great deal of effort has gone into constraining the values of the partition coefficients for U and Th between mantle minerals and basalt. Chapter 3 (Elundy and Wood 2003) reviews this work in more detail however, a basic review relevant to observations in MORE and modeling is given here. 

Water in the mantle is fonnd in different states as a fluid especially near sub-duction zones, as a hydrous phase and as a hydroxyl point defect in nominally anhydrous minerals. 8D-values between -90 and -110%c have been obtained by Bell and Ihinger (2000) analyzing nominally anhydrous mantle minerals (garnet, pyroxene) containing trace quantities of OH. Nominally anhydrous minerals from mantle xenoliths are the most D-depleted of all mantle materials with 5D-values 50%c lower than MORE (O Leary et al. 2005). This difference may either imply that these minerals represent an isotopically distinct mantle reservoir or that the samples analyzed have exchanged hydrogen dnring or after their ascent from the mantle (meteoric/water interaction ). 

Since boron concentrations in mantle minerals are exceedingly low, boron isotope analysis of mantle minerals are very restricted. On the basis of a boron budget between mantle and crust, Chaussidon and Marty (1995) conclnded that the primitive mantle had a 5 B-value of-10 2%c.ForMORB Spivack and Edmond (1987) and Chaussidon and Marty (1995) reported a 5 B-value of aronnd -4%c. Higher and lower 5 B-values observed in some ocean island basalts shonld be due to crustal assimilation (Tanaka and Nakamura 2005). 

Considerable interest centres on the Mantle constituting, as it does, more than half of the Earth by volume and by weight. Attention has been focussed on several problems, including the chemical composition, mineralogy, phase transitions and element partitioning in the Mantle, and the geophysical properties of seismicity, heat transfer by radiation, electrical conductivity and magnetism in the Earth. Many of these properties of the Earth s interior are influenced by the electronic structures of transition metal ions in Mantle minerals at elevated temperatures and pressures. Such effects are amenable to interpretation by crystal field theory based on optical spectral data for minerals measured at elevated temperatures and pressures. 

Increased pressure, on the other hand, results in compression of a crystal structure. The compressibilities of oxides and silicates show that the molar volumes of these phases decrease with rising pressure, indicating that interatomic distances within coordination sites become shorter. This is demonstrated by the data for Mantle minerals at elevated pressures summarized in table 9.1. Equation (2.20)... 

Crystal field spectra of low-spin Fe2 in Mantle minerals... 

Various estimates have been made of the CFSE of Fe2+ in dense oxide structures modelled as potential Mantle mineral phases (Gaffney, 1972 Bums, 1976a). All estimates indicate that octahedrally coordinated Fe2+ (in periclase, for example) has a considerably higher CFSE than Fe2+ ions in the eight- to twelve-coordination sites in the perovskite structure. Thus, the CFSE of Fe2+ in... 

Gaffney, E. S. (1972) Crystal field effects in mantle minerals. Phys. Earth Planet. Interiors, 6, 385-90. 

Bertka and Fei (1997) experimentally determined mantle mineral stabilities using the Wanke and Dreibus (1988) model composition. The mineral stability fields and resulting mantle density profile, as well as core densities and positions of the core-mantle boundary for a range of model core compositions, are illustrated in Figure 9. The moment of inertia calculated from these experimental data (0.354) is consistent with the Mars Pathfinder measurement (Bertka and Fei, 1998). However, this model requires an unrealistically thick crust. 

The least fractionated rocks of the Earth are those that have only suffered core formation but have not been affected by the extraction of partial melts during crust formation. These rocks should have the composition of the PM, i.e., the mantle before the onset of crust formation. Such rocks are typically high in MgO and low in AI2O3, CaO, Ti02, and other elements incompatible with mantle minerals. Fortunately, it is possible to collect samples on the surface of the Earth with compositions that closely resemble the composition of the primitive mantle. Such samples are not known from the surfaces of Moon, Mars, and the asteroid Vesta. It is, therefore, much more difficult to reconstruct the bulk composition of Moon, Mars, and Vesta based on the analyses of samples available from these bodies. 

Figure 3 Na, Cr, and Ni for the same suite of rocks as in Figure 2. Na is incompatible with mantle minerals, Ni is compatible, and Cr partitions equally between melt and residue.

Three kinds of evidence have been put forward in support of a lower mantle with a different composition from the upper mantle. The first was the apparent lack of a match between the seismic and other geophysical properties observed for the lower mantle, and the laboratory-measured properties of lower mantle minerals (MgSi03-rich perovskite and magnesiowiistite) in an assemblage with the upper mantle composition (meaning, effectively, with the upper mantle s Mg/Si and Mg/Fe ratios). Jackson and Rigden (1998) reinvestigated these issues and conclude that there is no such mismatch (see Chapter 2.02). 

Similar results have been reported by Mattern et al. (2002), using more recent equations of state for lower-mantle minerals and incorporating the solubility of alumina in silicate perovskite. They also used a three-layered slab model (midocean ridge basalt (MORE) over harzburgite over pyrolite), but with a MORE composition (Si/(Mg - - Fe) = 2.29) intermediate between our extreme end-members of the Helffrich et al. (1989) eclogite (1.65) and the Helffrich and Stein (1993) gabbro (2.58). 

Burton K. W., Schiano P., Birck J.-L., Allegre C. J., and Rehkamper M. (2000) The distribution and behavior of Re and Os amongst mantle minerals and the consequences of metasomatism and melting on mantle lithologies. Earth Planet. Set Utt. 183, 93-106. 

Schiano P. and Clocchiatti R. (1994) Worldwide occurrence of silica-rich melts in sub-continental and sub-oceanic mantle minerals. Nature (London) 368, 621-624. 

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