Olivine mantle

Different minerals contain different metal cations to balance the -4 charge on the orthosilicate ion. Examples Include calcium silicate (Ca2 Si04), an important ingredient in cement, and zircon (ZrSi04), which is often sold as artificial diamond. One of the most prevalent minerals in the Earth s mantle is olivine, Af2(Si04), in which M is one or two of the abundant metal cations, Fe -, Mg -, and Mn +. 

Olivine is the principal mineral (in terms of mass) of the earth s upper mantle where it is present as a combination of the two main components forsterite (85-95 mol.%) and fayalite (5-15 mol.%). In the earth mantle the increase in seismic velocity with depth is believed to be due to the presence of denser modifications of common minerals. 

At temperatures germane to melting and erystallizrng mantle magmas, Li isotopes do not show permil-level mass fractionation (Fig. 4 Tomascak et al. 1999b). This has since been corroborated by examination of bulk rocks and olivine separates from basaltic lavas, which yield consonant isotopic values (Chan and Frey 2003). Also, whole rocks and omphacite mineral separates from alpine eclogite with metamorphic peak temperatures approximately 650°C (Zack et al. 2003) show no consistent Li isotopic difference. 

Support for this conclusion comes from laser ablation analyses of mantle olivines recently reported by Norman et al. (2004). The loess and continental basalt samples suggest that evolved crustal materials may be on average approximately 0.4-0.6%o lower in 5 Mg than the primitive Cl/mantle reservoir (Fig. 1). 

Figure 14. Inter-mineral Fe isotope fractionations among olivine and clinopyroxene from spinel peridotite mantle xenoliths. Data are from Zhu et al. (2002) ( ) and Beard and Johnson (2004) ( ). In the study by Beard and Johnson (2004), the difference in the Fe isotope composition between clinopyroxene and olivine is larger as a function of their 5 Fe values, suggesting disequilibrium fractionation.

Davis L. L. and Smith D. (1993). Ni-rich olivine in minettes from Two Buttes, Colorado A connection between potassic melts from the mantle and low Ni partition coefficients. Geochim. Cosmochim. Acta, 57 123-129. 

Kirby S. H. and Wegner M. W. (1978). Dislocation substructure of mantle-derived olivine as revealed by selective chemical etching and transmission electron microscopy. Phys. Chem. Minerals, 3 309-330. 

Because Li isotopes may be used as a tracer to identify the existence of recycled material in the mantle, systematic studies of arc lavas have been undertaken (Morignti and Nakamura 1998 Tomascak et al. 2000 Leeman et al. 2004 and others). However, most arc lavas have 5 Li values that are indistinguishable from those of MORB. Thus Li seems to be decoupled from other fluid mobile elements, becanse Li can partition into Mg-silicates (pyroxene, olivine) in the mantle (Tomascak et al. 2002). 

Galy et al. (2001) suggested that the mantle should have a homogeneous Mg isotope composition. Pearson et al. (2006), however, demonstrated that olivines from mantle xenoliths have a heterogeneous compositions with a 5 Mg range of about 4%c. These authors suggested that the differences are due to diffusion-related meta-somatic processes. 

Mackwell S.J. (1992) Oxidation kinetics of fayalite (Fe2Si04). Phys. Chem. Miner. 19, 220-228. Mackwell S.J. and Kohlstedt D.J. (1990) Diffusion of hydrogen in olivine implications for water in the mantle. /. Geophys. Res. 95, 5079-5088. 

Young T.E., Green H.W, Hofmeister A.M., and Walker D. (1993) Infrared spectroscopic investigation of hydroxyl in beta-(Mg, Pe)2Si04 and coexisting olivine implications for mantle evolution and dynamics. Phys. Chem. Minerals 19, 409-422. 

Figure 4-19 Plagioclase phase diagram and plagioclase melting Figure 4-20 Free falling velocity of a mantle xenolith in a basalt Figure 4-21 Sketch of boundary layer, and boundary layer thickness Figure 4-22 MgO diffusion profile in olivine and in melt during olivine growth...

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