Melt extraction composition

At each step, a fraction of fluid f is added to the mantle wedge from the slab. The bulk partition coefficients used for fluid dehydration can be derived from published mineral/fluid partition coefficients (see Tables Al and A2). The composition of the residual slab is estimated as follows after At which is the time step between two melt extractions (similar equation for Th and Pa) ... 

We assume that a constant mass fraction fr remains in the mantle wedge after melt extraction. As in Section A2 of the Appendix, the ratio of slab mass to wedge mass is assumed to be equal to 1 but more complex models are also possible. The bulk composition of the mantle wedge after melt extraction is calculated with the following equation after each extraction increment ... 

While all spinel-lherzolite facies suites show remarkably similar compositional trends as a function of depletion, some garnet peridotite xenoliths in kimberlites and lamproites from ancient cratonic lithospheric keels show signih-cantly different trends (e.g., see Boyd, 1989 Chapters 2.05 and 2.08). Most of these xenoliths are extremely depleted extrapolation of the trends back to the PM MgO of 36.7% gives similar concentrations of Si02, EeO AI2O3, and CaO to the spinel Iherzolites (O Neill and Palme, 1998) the difference in their chemistry is due to a different style of melt extraction, and not a difference in original mantle composition. 

Figure 23 Chondrite-normalized abundances of REEs in representative harzburgites from the Oman ophiolite (symbols—whole-rock analyses), compared with numerical experiments of partial melting performed with the Plate Model of Vemieres et al. (1997), after Godard et al. (2000) (reproduced by permission of Elsevier from Earth Planet. Set Lett. 2000, 180, 133-148). Top melting without (a) and with (b) melt infiltration. Model (a) simulates continuous melting (Langmuir et al., 1977 Johnson and Dick, 1992), whereas in model (b) the molten peridotites are percolated by a melt of fixed, N-MORB composition. Model (b) is, therefore, comparable to the open-system melting model of Ozawa and Shimizu (1995). The numbers indicate olivine proportions (in percent) in residual peridotites. Bolder lines indicate the REE patterns of the less refractory peridotites. In model (a), the most refractory peridotite (76% olivine) is produced after 21.1% melt extraction. In model (b), the ratio of infiltrated melt to peridotite increases with melting degree, from 0.02 to 0.19. Bottom modification of the calculated REE patterns residual peridotites due to the presence of equilibrium, trapped melt. Models (c) and (d) show the effect of trapped melt on the most refractory peridotites of models (a) and (b), respectively. Bolder lines indicate the composition of residual peridotites without trapped melt. Numbers indicate the proportion of trapped melt (in percent). Model parameters...

Hartmann G. and Wedepohl K. H. (1993) The composition of peridotite tectonites from the Ivrea complex northern Italy residues from melt extraction. Geochim. Cosmochim. Acta 57, 1761-1782. 

Yoshikawa M. and Nakamura E. (2000) Geochemical evolution of the Horoman Peridotite Complex implications for melt extraction, metasomatism and compositional layering in the mantle. J. Geophys. Res. 105, 2879-2901. 

Melt Extraction and Compositional Variability in Mantle Lithosphere... 

In this chapter the mineral-melt phase equilibria that control the compositions of partial melts are examined on the basis of experimental and thermodynamic databases, and this information is used to predict the effects of partial melt extraction from fertile upper mantle on residual mineralogy and major-element chemistry. 

Direct evidence for the compositional effects of partial melt extraction is preserved in samples of upper-mantle lithosphere with a range of ages, including Archean cratonic mantle, Proterozoic subcontinental mantle, and modern oceanic mantle. Samples of upper mantle are collected as xenoliths, peridotites dredged from oceanic fracture zones, and slices of upper mantle tectonically exposed at the surface, and extensive samples exist from both oceanic and continental settings (see Chapters 2.04 and 2.05). Here, data sets are assembled for oceanic and subcontinental mantle lithosphere, and compositional trends are compared to those predicted for partial melt extraction from fertile peridotite in order to deduce the role that melt extraction has played in producing compositional variability in upper-mantle lithosphere, and to place constraints on the thermal evolution of the mantle. 

In this section a general review is presented of the melting phase relations for fertile upper-mantle peridotite, concentrating on how variations in the depth and degree of partial melt extraction impart compositional variability to the residual source rock. 

Phase Equilibrium and Melt Extraction Table 1 Model fertile upper-mantle compositions. 

Figure 4 Normative spinel Iherzolite mineral abundances (wt.%) in batch partial melt extraction residues (0-25%) from fertile peridotite (composition 8, Table 1) as a function of Mg (molar Mg/(Fe + Mg)) at 0.5 GPa, 1 GPa, and 2 GPa, based on the melting model of Kinzler and Grove (1992a, 1993). Normative mineral compositions are calculated using the spinel Iherzolite normative algorithm of Kelemen et ai, (1992).

A complimentary perspective of compositional variation in melting residues can be made from major-element oxide variation diagrams. Figure 6 shows the variation in major-element oxides versus FeO content as a function of pressure (1-7 GPa) and degree of batch melt extraction. FeO was chosen as the abscissa because of its large relative variation in partial melts and residues as a function of pressure. At 1 GPa the FeO content in the residue shows a mild increase with degree of melt extraction, but by 2 GPa FeO shows a mild decrease. This reflects the fact that low-pressure partial melts have FeO contents less than that of the bulk rock, but as pressure increases so does the FeO... 

Figure 6 Major-element oxides (wt.%) versus FeO as a function of pressure (GPa) and degree of batch melt extraction (sources the 1 GPa and 2 GPa trends are based on the Kinzler and Grove (1992a, 1993) model for melting of primitive mantle of McDonough and Sun (1995) (composition 1, Table 1), and the trends at higher pressures are based on the data of Walter (1998) for melting of fertile peridotite KR4003).

Before presenting a detailed account of the compositional variability of mantle lithosphere and the role that melt extraction has played in producing that variation, it is important to acknowledge the likelihood of intrinsic heterogeneity in the mantle that is not a direct consequence of primary melt extraction, and to assess whether such... 

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