Melt extraction cratonic mantle

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. 

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 order to focus more closely on the melt extraction component in olf-craton mantle, a subset of data has been selected. A single criterion... 

Figure 10 shows major-element oxides versus Mg for off-craton and oceanic mantle, as well as some estimated compositions for primitive mantle (Table 1). As expected from the normative plots, the two sets of mantle compositions have distinct trends for all oxides. Previous models for primitive upper mantle have a range in Mg from 89 to 90, and Figures 9 and 10 show that the oceanic and off-craton trends also converge within this range. Assuming that the off-craton and abyssal mantle trends are due primarily to melt extraction from a common protolith, then the intersection of the trends should provide a good estimate for the composition of fertile upper mantle for major elements. 

Here, the fertile upper-mantle composition derived in the previous section is assumed for oceanic and olf-craton mantle, and the model of Kinzler and Grove (1992a, 1993) is used to model melt extraction at pressures of < 2.5 GPa. 

Melt extraction at higher pressures as applied to cratonic mantle is modeled using the experimental data of Walter (1998) for a composition that is very similar to the primitive mantle of McDonough and Sun (1995). 

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... 

Figure 18 Normative olivine and opx (garnet Uierzo-Ute norm) versus rock Mg showing low-T cratonic mantle relative to batch melt extraction trends from 3 GPa to 7 GPa (see Figure 6). Open circles are compositions from the Kaapvaal, Siberian, and Slave cratons, and the filled circles are compositions from the Tanzanian, Canadian, and Greenland cratons.

Experimental studies show that volatile components such as H2O and CO2 can have profound effects on melting temperature and melt composition (e.g., Kushiro, 1972 Hirose and Kawamoto, 1995 Kawamoto and Holloway, 1997 Dalton and Presnall, 1998 Gaetani and Grove, 1998 Hirschmann et al., 1999a Lee et al., 2000 Asahara and Ohtani, 2001). It has been implicitly assumed in the assessment above that melt extraction occurred in nominally anhydrous mantle. This assumption is most robust in the case of MORE mantle, which has been shown to have a low volatile content (e.g., Michael, 1988 Saal et al., 2002 Chapter 2.07). Inasmuch as off-craton mantle has isotopic characteristics that indicate similar long-term incompatible element depletion to the MORE source, and considering that volatiles are very incompatible elements, a low volatile content at the time of melt extraction from off-craton mantle is implied. Indeed, off-craton mantle may be genetically related to modern MORE mantle. 

Assuming an average pressure of melt extraction of —2 GPa for ofif-craton mantle, with a maximum of —30% and an average of 12% melt extraction, an average temperature of 1,440 50 °C is estimated. 

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