Peridotites melt extraction

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... 

In addition, many peridotites bear the obvious signatures of metasomatism, which re-enriches the rock in incompatible components subsequent to depletion by melt extraction. Where this is obvious (e.g., in reaction zones adjacent to later dikes) it may be avoided easily but often the metasomatism is cryptic, in that it has enriched the peridotite in incompatible trace elements without significantly affecting major-element chemistry (Frey and Green, 1974). Peridotites thus have very variable contents of highly incompatible trace... 

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. 

Partly based on the ophiolite record (see Section 2.04.2.2), the refractory degree of abyssal peridotites was tentatively associated with spreading rate (e.g., Boudier and Nicolas, 1985 Nicolas and Boudier, in press). Mildly refractory peridotites after moderate melt extraction degrees (Iherzolites and cpx-harzburgites) would be... 

Ytterbium versus AI2O3 covariation trend and fertile orogenic Iherzolites. Similar to the correlations observed between major (and minor transition) elements, the good correlation between ytterbium and AI2O3 in tectonically emplaced and abyssal peridotites (Figure 10) is classically ascribed to variable degrees of melt extraction. However, any of the alternatives envisioned for the major elements (melt-rock reactions, melt... 

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). 

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. 

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

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. 

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). 

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. 

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).

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