Olivine oceanic mantle

The off-craton mantle subset is shown relative to isobaric batch melt extraction curves on plots of normative olivine and major-element oxides versus Mg in Figure 17. Generally speaking, off-craton mantle compositions are consistent with effectively 0-30% melt extraction from fertile upper mantle in the range of 1-5 GPa. The chemical signature recorded in off-craton mantle mimics closely the maximum degree of melt extraction recorded in oceanic mantle, but in contrast there are many samples that show little or... 

First, volatiles exert an important control on the physical properties of the mantle. For example, the presence of water reduces the strength of olivine aggregates and seriously alters the viscosity of the mantle. Experimental studies show that at 300 MPa, in the presence of water, the viscosity of olivine aggregates deformed in the dislocation creep regime is reduced by up to a factor of 140. Thus a wet mantle is a low viscosity mantle. Conversely a mantle that is dried out by partial melting will be stiffer and more refractory, as is the case for the lithospheric "lid" to modern oceanic mantle. Thus, if it is possible to estimate the volatile content of the mantle both now and in the Archaean, it will be possible to set some physical constraints on models of mantle evolution over time. 

Scarsi P (2000) Fractional extraction of helium by crashing of olivine and chnopyroxene phenocrysts effects on the He/ He measured ratio. Geochim Cosmochim Acta 64 3751-3762 Scarsi P, Craig H (1996) Helium isotope ratios in Ethiopian Rift basalts. Earth Planet SciLett 144 505-516 Schiano P, Clocchiatti R (1994) Worldwide occurrence of silica-rich melts trapped in sub-continental and sub-oceanic mantle minerals. Nature 368 621-624... 

The formation of the Moon s crust, composed primarily of feldspar (the rock is called anorthosite) illustrates how physical fractionation can occur during differentiation. Early in its history, a significant portion of the Moon was melted to form a magma ocean. The first minerals to crystallize, olivine and pyroxene, sank because of their high densities and formed an ultramafic mantle. Once feldspar began to crystallize, it floated and accumulated near the surface to produce the crust. 

In the oceanic setting, spinel Iherzolite xenoliths from Pali (Hawaii) have olivine 5 0 values of 5.09-5.12 per mil, typical of olivines from other oceanic and continental mantle rocks (Ducea et al., 2002). In contrast, olivines from plagioclase peridotites are enriched by 0.5 per mil. This is interpreted to be due to the formation of plagioclase by reaction with or crystallization from melts intruding the Pacific lithospheric mantle. 

Flotation of olivine in the mid-mantle during an early magma ocean would perhaps satisfy the superchondritic Mg/Si ratio (Agee and Walker, 1988). However, McFarlane and Drake (1990) have shown that the Ni/Co ratio of such a layer would be 25% higher than that in the observed upper mantle. In addition, the incompatible nature of platinum, palladium, gold, and rhenium in olivine compared to iridium, ruthenium, and rhodium would undoubtedly lead to nonchondritic HSE ratios such as Pd/Ir, Au/Ru, etc. in the upper mantle, and this is not observed (e.g., Righter et al., 2000). As a result, early olivine flotation seems unlikely. 

Whereas a number of reactions at intermediate to elevated temperatures are important for hydration of peridotite in the overlying mantle wedge directly adjacent to the top of the oceanic cmst (Figures 6 and 7), almost any subduction P-T-path will keep the slab in the serpentine stability field to at least >2 GPa (Ulmer and Trommsdorff, 1995). As a consequence, hydrated peridotite in the downgoing lithosphere will remain as serpentine and chlorite (4-olivine - -clinopyroxene) while a multitude of reactions is... 

Normal mantle melting, such as that which leads to the formation of oceanic crust and oceanic islands, produces magma of basaltic to picritic composition and leaves a residue consisting of olivine, orthopyroxene, chnopyroxene and an aluminous phase. The compositions of minerals in this residue, and their relative proportions, are unlike those in old subcontinental hthosphere Mg-Fe ratios of the ferromagnesian minerals are too low, and the amount of chnopyroxene and spinel or garnet is too high. 

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