Metasomatism, mantle

Petrological and geochemical variations along the arc are considered to result from variable relative amounts of slab-released fluids and melts added to the mantle wedge, as well as from higher amounts of subducted sediments added to the mantle beneath eastern islands. Variation of pre-metasomatic mantle composition, from MORB-type in the central arc to OIB-type in the external islands, is also considered as an important factor for compositional diversity along the arc. 

As it provides easy access to a variety of metasomatized mantle rocks, the Lherz massif has been recently the focus of detailed geochemical studies—as well as the source of debates—concerning melt infiltration and melt-rock interaction processes in wall rocks of... 

Grieco G., Ferrario A., von Quadt A., Koeppel V., and Mathez E. A. (2001) The zircon-bearing chromitites of the phlogopite peridotite of Finero (Ivrea Zone, Southern Alps) evidence and geochronology of a metasomatized mantle slab. J. Petrol. 42, 89-101. 

Mclnnes B. I. A., Gregoire M., Binns R. A., Herzig P. M., and Hannington M. D. (2001) Hydrous metasomatism of oceanic sub-arc mantle, Lihir, Papua New Guinea petrology and geochemistry of fluid-metasomatized mantle wedge xeno-liths. Earth Planet. Sci. Lett. 188, 169-183. 

The apatite in mantle rocks studied by O Reilly and Griffin (2000) include an unusual carbonate-bearing (0.7-1.7 wt %) hydroxyl-chlorapatite (1.5-2.5 wt % Cl, 6-40 ppm Br, 1 wt % F) associated with C02-rich fluid inclusion-bearing veins in mantle Iherzolites (from both Australia and Alaska), and also a hydroxyl-fluorapatite variant that is igneous in origin. The former occurs in metasomatized mantle wall-rock peridotites. 

As was discussed previously, Th in lavas from Vesuvius, Stromboli, Etna and Mt. St. Flelens contains a component (recorded in mineral phases) with a higher Th isotopic composition (lower A") than that of the host whole rocks. Because most Th in the w hole rocks probably was fed to the magma from an interstitial, metasomatic mantle component, the lower K com-... 

Metasomatism seems to generally produce an increase in n and a decrease in K of the metasomatized mantle region. 

The major source of U, Th, Ra and Pb in a magma probably is the metasomatic mantle component. Instead, the major source of Sr and Nd in a magma is the non-metasomatic, more refractory mantle component. If this is so, the lead paradox is easily explained. 

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