Melting metasomatism

McPherson E. (1994) Geochemistry of silicate melt metasomatism in Alpine peridotite massifs. University of London, London. 

Positive slopes at moderate angles may be related to silicate melt-metasomatism, whereas subvertical trends of increasing AI2O3 and (Pd/Ir) are indicative of sulhde addition (Rehkamper et al, 1999). Hence, the bulk rock PGE systematic s of cratonic peridotites are indicative of their major-element depleted characters, but with metasomatic effects superimposed on this signature to varying degrees. 

Figure 24 Covariation of ) ttrium and zirconium in mantle garnets showing fields ascribed to different mantle protoUths (undepleted, depleted) and processes (high-T melt metasomatism, low-T phlogopite metasomatism) and zonation patterns from cores to rims of garnets (after Griffin et al., 1999c).

Harry D. L. and Leeman W. P. (1995) Partial melting of melt metasomatized subcontinental mantle and the magma source potential of the lower lithosphere. J. Geophys. Res. 100, 10255-10269. 

Bourdon E., Eissen J.-P., Monzier M., Robin C., Martin H., Gotten J., and Hall M. L. (2002) Adakite-Uke lavas from Antisana Volcano (Ecuador) evidence from slab melt metasomatism beneath the Andean northern volcanic zone. J. Petrol. 192, 561-570. 

Fresh, Cenozoic oceanic basalts typically span a narrow range in 5 O ( 5 to 7 %o for whole rocks), restricting the abundance of subducted crust in their mantle sources and crustal contaminants added to them during differentiation to amounts less than 10 wt %. Furthermore, these data suggest that isotopic fractionations during melting, metasomatism, and metamorphism in the upper mantle are small... 

Blundy JD, Dalton JA (2000) Experimental comparison of trace element partitioning between clinopyroxene and melt in carbonate and silicate systems and implications for mantle metasomatism. Contrib Mineral Petrol 139 356-371... 

Migration of fluids in a porous matrix with solid-liquid fractionation results in a process much similar to the chromatographic separation of elements (DeVault, 1943 Korzhinskii, 1970, Hofmann, 1972). This mechanism has recently been revived in the context of mantle metasomatism by Navon and Stolper (1987), Bodinier et al. (1990), Vasseur et al. (1991), in the context of hydrothermal systems by Lichtner (1985) and, for stable isotopes, by Baumgartner and Rumble (1988). Only a simplified account of this model will be given here. Let

solid matrix and melt, respectively, and vHq the fluid velocity relative... 

Regionally, three magmatic processes are envisioned to have ultimately controlled mineralization at Morila 1) re-melting of metasomatized lithospheric mantle during crustal-scale regional transpression 2)... 

Considerable geochemical and isotopic evidence has accumulated supporting the concept that many parts of the mantle have experienced a complex history of partial melting, melt emplacement, crystallization, recrystallization, deformation, and metasomatism. A result of this complex history is that the mantle is chemically and isotopically heterogeneous. 

O, H, C, S, and N isotope compositions of mantle-derived rocks are substantially more variable than expected from the small fractionations at high temperatures. The most plausible process that may result in variable isotope ratios in the mantle is the input of subducted oceanic crust, and less frequent of continental crust, into some portions of the mantle. Because different parts of subducted slabs have different isotopic compositions, the released fluids may also differ in the O, H, C, and S isotope composition. In this context, the process of mantle metasomatism is of special significance. Metasomatic fluids rich in Fe +, Ti, K, TREE, P, and other large ion lithophile (LIE) elements tend to react with peridotite mantle and form secondary micas, amphiboles and other accessory minerals. The origin of metasomatic fluids is likely to be either (1) exsolved fluids from an ascending magma or (2) fluids or melts derived from subducted, hydrothermally altered crust and its overlying sediments. 

The one-time addition of fluids can be regarded as a one-time fluid metasomatism of the solid rocks just as it starts melting. This is a potential important model for mantle metasomatism. 

The amount of one-time fluid metasomatism is related to the mass of the melting zone source rocks by... 

Note that for one-time metasomatism, is not related to the degree of melting. 

Hf-isotope ratios also fall close or within the field of crustal rocks (Fig. 2.4 Table 2.2). This has been interpreted as evidence that metasomatic modification of lamproitic mantle sources in Tuscany was provided by addition of crustal material (e.g. metapelites). Interestingly, the mantle-normalised incompatible element patterns of Tuscany lamproites are similar to those of gneisses and schists in almost eveiy detail. This has been interpreted as evidence for addition of bulk upper crustal material to the mantle, with little element fractionation during metasomatism and the subsequent partial melting (Peccerillo 2002). This makes the Tuscany Province a zone where the upper mantle magmas look very much like the upper crust in terms of trace element and radiogenic isotope compositions. 

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