Serpentinized mantle peridotites from Site 897 are highly variable in composition and mineralogy, ranging from lherzolites to harzburgites and, more infrequently, to dunites. Lherzolites are enriched in Al₂O₃, TiO₂, CaO, Cr₂O₃ and rare-earth elements relative to harzburgites and dunites, which are enriched in MgO and NiO. Texturally some of the lherzolites are characterized by mylonitic bands of fine-grained plagioclase, clinopyroxene, orthopyroxene, and olivine between larger crystals of olivine, orthopyroxene, clinopyroxene, plagioclase, and chrome spinel. These textural characteristics suggest the lherzolites have been enriched by intrusion of basaltic magma. Moreover, some of the lherzolites have flat rare-earth-element patterns near chondritic abundance, and regarded as typical of primitive mantle, whereas others have rare-earth-element patterns similar to normal mid-ocean ridge basalt, suggesting metasomatic enrichment by the addition of a normal mid-ocean ridge basaltic magma. The vertical and lateral heterogeneity of mantle peridotites at Site 897 shows that the mantle can be locally heterogeneous and that modeling of the mantle as a few layers of relatively homogeneous material represents a vast oversimplification that has no significance at the local level.

Date of initial receipt: 1 December 1994
Date of acceptance: 27 April 1995

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