Peridotites mantle

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.

O, H, C, S, and N isotope compositions of mantle-derived rocks are substantially more variable than expected from the small fractionations at high temperatures. The most plausible process that may result in variable isotope ratios in the mantle is the input of subducted oceanic crust, and less frequent of continental crust, into some portions of the mantle. Because different parts of subducted slabs have different isotopic compositions, the released fluids may also differ in the O, H, C, and S isotope composition. In this context, the process of mantle metasomatism is of special significance. Metasomatic fluids rich in Fe +, Ti, K, TREE, P, and other large ion lithophile (LIE) elements tend to react with peridotite mantle and form secondary micas, amphiboles and other accessory minerals. The origin of metasomatic fluids is likely to be either (1) exsolved fluids from an ascending magma or (2) fluids or melts derived from subducted, hydrothermally altered crust and its overlying sediments. 

Figure 11 Bulk sound velocity anomalies relative to peridotite mantle versus depth in the lower mantle (blue) for candidate bulk compositions for the basaltic layer (red) and the underlying harzburgite layer (green). Note that the basaltic layer is fast and grows faster with increasing depth.

Yaxley G. M. and Green D. H. (1998) Reactions between eclogite and peridotite mantle refertilisation by subduction of oceanic crust. Schweiz. Mineral. Petrogr. Mitt. 78, 243 -255. 

Canil D. (2002) Vanadium in peridotites, mantle redox and tectonic environments Archean to present. Earth Planet. Sci. Lett. 195, 75-90. 

In natural samples, clinohumite is invariably titanium-rich and often fluorine-bearing (in those cases where fluorine analyzed). There is no apparent systematic composition difference (e.g., Mg/Fe, Ti content) between clinohumites found in massifs and those found in kimberlites and xenoliths. Ulmer and Trommsdorff (1999) argue that titanium-saturated clinohumite has the maximum thermal stability, with titanium-undersatu-rated clinohumite breaking down at lower temperatures in a divariant reaction to form a titanium-saturated clinohumite + olivine + ilmenite + H2O, based on experiments by Weiss (1997) (Figure 9). Because of the dependence of the stability of clinohumite on Ti02, Ulmer and Trommsdorff (1999) argue that the amount of titanoclinohumite in the peridotitic mantle would be controlled by the low Ti02 content... 

The andesitic continental cmst composition is drflicult to explain if the cmst is generated by single-stage melting of peridotitic mantle, and additional processes must therefore be involved in its generation. All of these processes entail return of mafic or ultramafic cmstal material (which is complementary to the present continental cmst) to... 

A key feature is that all the nuclides of interest are highly incompatible in common mantle mineral phases (Table 2). Clinopyroxene and garnet (present in normal peridotitic mantle at depths greater than —80 km) are the principal host minerals for uranium and thorium in the solid phase, although even in these phases partition coefficients do not exceed 0.05. An important consideration is the sense of uranium and thorium fractionation imparted by the presence of different minerals. This can be conveniently expressed in terms of Du/ >Th- Minerals with Du/ >Th > 1 retain uranium over thorium and contribute to 23 Th-excesses in a coexisting melt, whereas those with Du/ >Th < 1 help to create 23 3xh-deficits. The effects of different minerals are then weighted by their absolute partition coefficients and modal abundance to control the bulk partition coefficient of the mantle, and thus determine the sense of fractionation of thorium from uranium in a coexisting melt. 

The transition zone in the oceanic crust is represented by serpentine, the product of the Hess reaction of olivine with water. The serpentine forms in the lower oceanic crust, because of water percolation from the ocean through brittle -fissured basalts, and exhibits true plastic behavior, which prevents further migration of oceanic water into the peridotite mantle. Since the true plastic, that is, impermeable state of the serpentine is reached at pressures of 0.2-0.4 GPa and temperatures below 550°C, the thickness of the oceanic crust is about 11 km, if we account for the ocean water weight, Nikolaevskiy (1979), Lobkovsky (1988). 

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