This chapter discusses the general characteristics of rare earth element (REE) distribution in the most common varieties of ultramafic rock (alpine-type, or “metamorphic’’ peridotites): dunites, harzburgites, and lherzolites). These rocks form numerousmaficultramafic massifs within folded belts on the continents and mid-oceanic ridges. These formations are usually considered to be mantle restites that have undergone some degree of REE depletion in the process of partial melting of the mantle. We define these rocks as orthomagmatic ultramafites to underline their primarily magmatogenic nature. By contrast, complex mafic-ultramafic massifs contain paramagmatic ultramafic rocks, including wehrlites, clinopyroxenites, websterites, and orthopyroxenites, all of which formation is connected with the magmatic and metasomatic interactions of mantle mafic melts with orthomagmatic ultramafic restites and their serpentinous varieties (Lesnov, 1986). A common feature of ultramafites that differs frommost other types of magmatogenic rocks is that ultramafites contain relatively low total concentrations of all REE, especially light ones. For a long time this characteristic was the main obstacle to geochemical investigations of the REE composition of ultramafites. Another feature of ultramafic rocks is that their REE are weakly fractionated. At the same time, monotypic petrographical varieties of ultramafites from different massifs can differ from one another both in total REE content and in extent of fractionation. Not infrequently, however, the latter is due not so much to the primary processes involved in ultramafite restite formation, as to the later redistribution of light REE under the influence of metamorphic and hydrothermal processes. As a result, variable amounts of REE, especially light ones, were added to the initial rocks by epigenetic fluids circulating through microcracks, thus creating a false effect of REE fractionation. These REE are not part of the mineral structure and, thus, represent a non-structural admixture sometimes referred to as a “contaminant’’ (Kovalenko et al., 1989). Note that, as will be shown later (see Chapter 2), the presence of such a “contaminant’’ is very characteristic of ultramafic restites brought up to the Earth’s surface by basaltic or kimberlitic melts in the form of deep xenoliths. Relatively little analytical data have been obtained on the geochemistry of REE in ultramafic restites from mafic-ultramafic massifs that are widespread both within the folded regions of the continents and in the mid-oceanic ridges. Particularly scarce are data on paramagmatic ultramafites such as
e in t ramaf i c and maf ic and i r
plagioclase-bearing peridotites, plagioclase-bearing wehrlites and plagioclase-bearing pyroxenites. (Frey, 1984; Laz’ko, 1988b; Lesnov, 2003c,d).