Processes of mantle metasomatism; Constraints from observations of composite peridotite xenoliths

Available major, trace element, and isotopic data from peridotite xenoliths show that the chemical signature of “cryptic” metasomatism (characterized by enrichment of light rare-earth elements, LREE, relative to heavy rare-earth elements, HREE, in bulk rock and clinopyroxenes) also is seen in rocks affected by “patent” metasomatism (characterized by the presence of secondary hydrous minerals). Detailed major element, REE, and isotopic analyses of minerals in two similar composite xenoliths from Dish Hill, California, demonstrate that patent and cryptic metasomatism, as currently defined, can be caused by the intrusion of a mafic melt. Clinopyroxenes may become enriched in LREE at the same time that hydrous minerals are deposited and minerals of the peridotite wallrock are enriched in Fe, Ti, and Al by reaction with volatile-rich fluid from a dike (Fe-Ti metasomatism). Microscopic solid and fluid inclusions have been identified as sources of incompatible elements in peridotite xenoliths. Distributions of such inclusions in the composite xenoliths show that they can be added by mantle fluids. Limited data suggest that bulk rock enrichment of LREE/HREE in clinopyroxene-poor, low-CaO metasomatized anhydrous peridotite xenoliths can be controlled by olivine rather than by pyroxene. This control may be due to LREE-enriched inclusions in olivine and other minerals that do not accommodate REE in structural sites. If the chemical signature of cryptic metasomatism is caused by microscopic additions, the distinction between “patent” and “cryptic” categories becomes arbitrary. The evidence suggests that mantle metasomatism results from reaction between fluid differentiates of mafic melts and peridotite wallrocks. Variations of initial melt compositions, degrees of melt differentiation, and ratios of peridotite to melt all influence the LREE and isotopic compositions of metasomatized mantle rock.