Melt extraction oceanic mantle

Niu Y. (1997) Mantle melting and melt extraction processes beneath ocean ridges evidence from abyssal peridotites. J. Petrol. 38, 1047-1074. 

Hirth G. and Kohlstedt D. L. (1996) Water in the oceanic upper mantle implications for rheology, melt extraction and the evolution of the lithosphere. Earth Planet. Set Lett. 144, 93-108. 

In the modern tectonic environment, melt extraction from the uppermost mantle occurs in numerous settings that include oceanic and continental spreading centers, hot-spots, and sub-duction zones, with the vast majority of melt... 

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. 

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. 

Figure 13 Normative spinel Iherzolite mineral abundances (wt.%) versus rock Mg for oceanic mantle (as in Figure 7) relative to trends for 0-25% batch melt extraction at 0.5-2 GPa. The starting composition is fertile upper mantle as determined in this study (Table 1, 8), and residues are calculated using the melting model of Kinzler and Grove (1992a, 1993).

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... 

As an initial premise, it is assumed in the following that in general water played a relatively minor role in the melt extraction events that led to the primary depletion characteristics of upper-mantle lithosphere. In this case, compositional distinctions among oceanic and continental lithosphere of variable age can be related directly to average depth and temperature of melt... 

Relatively young (<250 Ma) oceanic upper mantle is formed at the lowest average pressures and temperatures by melt extraction at mid-ocean ridges. Melt extraction is expected to be a polybaric process, and likely involves a combination of near-fractional and batch melt extraction. Assuming an average pressure of melt extraction of 1 GPa and 15-20% melt extraction at the ridge axis, an average temperature of 1,315 50 °C is estimated. 

Another very active area of recent research is the identification of several different source components in primary arc basalts, including (i) fluids derived by dehydration of subducting metabasalt, (ii) fluids derived by dehydration of subducting metasediment, (iii) partial melts of subducting basalt, (iv) partial melts of subducting sediment, (v) fertile mantle peridotite similar to the MORE source, (vi) mantle peridotite depleted by melt extraction beneath a mid-ocean ridge and/or a back-arc basin, and (vii) enriched mantle similar to... 

---