Mantle melt extraction

Pearson D. G., Irvine G. J., Ionov D. A., Boyd F. R., and Dreibus G. E. (2004) Re-Os isotope systematics and platinum group element fractionation during mantle melt extraction a study of massif and xenolith peridotite suites. Chem. Geol. (special volume) Highley Siderophile Elements (in press). 

Ribe NM Smooke MD (1987) A stagnation point flow model for melt extraction from a mantle plume. J Geophys Res 92 6437-6443... 

Woodhead J, Eggins S, Gamble J (1993) High field strength and transition element systematics in island arc and back-arc basin basalts evidence for multi-phase melt extraction and a depleted mantle wedge. Earth Planet Sci Lett 114 491-504... 

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 ... 

Nicolas, A. (1986). A melt extraction model based on structural studies in mantle peridotites. J. Petrol., 27, 999-1022. 

In detail, however, the picture is not so simple. All mantle peridotites (whether massive peridotites or xenoliths) are metamorphic rocks that have had a complex subsolidus history after melt extraction ceased. As well as subsolidus recrystallization, peridotites have undergone enormous amounts of strain during their emplacement in the lithosphere. Massive peridotites show modal heterogeneity on the scale of centimeters to meters, caused by segregation of the chromium-diopside suite of dikes, which are then folded back into the peridotite as deformation continues. The net result is more or less diffuse layers or bands in the peridotite, which may be either enriched or depleted in the material of the chromium-diopside suite, i.e., in climopyroxene and orthopyroxene in various proportions, minor spinel, and sulfide. This process should cause approximately linear correlations of elements versus MgO, broadly similar to, but not identical with, those caused by melt extraction. Indeed, there is... 

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. 

Osmium is of great interest to mantle geochemists because, in contrast with the geochemical properties of strontium, neodymium, hafnium, and lead, all of which are incompatible elements, osmium is a compatible element in most mantle melting processes, so that it generally remains in the mantle, whereas the much more incompatible rhenium is extracted and enriched in the melt and ultimately in the crust. This system therefore provides information that is different from, and complementary to, what we can learn from... 

An alternative to this scenario is to envisage that some fertile orogenic Iherzolites have acquired their geochemical signature as a result of melting and melt-rock interaction processes associated with the thermomechanical erosion of lithospheric mantle by upweUing asthenosphere (e.g., Lenoir et al., 2001). In this scheme, refertilization of lithospheric peridotites by (and reequilibration with) MORB melts is an alternative to the small degrees of melt extraction to account for LREE depletion in otherwise fertile Iherzolites (e.g., Piccardo and Rampone, 2001). 

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

Sparks D. W. and Parmentier E. M. (1991) Melt extraction from the mantle beneath spreading centers. Earth. Planet. Set Lett. 105. 

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. 

The robustness of the Lu-Hf isotope system in some mantle environments is demonstrated by the precise Lu-Hf isochron of 1,413 67 Myr dehned by clinopyroxene separates from the Beni Bousera peridotite massif (Pearson and Nowell, 2003). This age probably dates the time of melt extraction from these rocks and is considerably more precise than the Sm-Nd isochron or the scattered Re-Os isotope systematics of these rocks. This indicates the potential power of this system in dating mantle rocks. The initial results from the Lu-Hf isotope system indicate that of the incompatible element isotope systems, it is the more robust to metasomatic effects, with signatures frequently recording the time-integrated response to melt depletion. 

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