Rare earth geochemistry of fused ophiolitic and alpine lherzolites |
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| Authors: | Martin Menzies Douglas Blanchard Joyce Brannon Randy Korotev |
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| Institution: | (1) NASA Johnson Space Center, 77058 Houston, Texas, USA;(2) Lockheed Electronics, 77058 Houston, Texas, USA;(3) University of Wisconsin, 53700 Madison, Wisconsin, USA;(4) Present address: Dept. Geology Geophysics, University of Minnesota, 108 Pillsbury Hall, 55455 Minneapolis, Minnesota, USA |
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| Abstract: | Partial fusion hypotheses have been proposed for the origin of lherzolite-harzburgite alpine peridotite associations. Analyzed lherzolites from Othris, Ronda, Lanzo and Beni Bouchera, have light REE depleted to chondritic REE abundances, and clinopyroxenes contain most of the REE relative to depleted olivine and orthopyroxene. Variation in the level of REE enrichment within these lherzolites indicates mantle heterogeneity probably caused by partial melting processes. The Beni Bouchera spinel lherzolite and the Othris plagioclase lherzolite are the best candidates for relatively  undepleted mantle based on REE studies. Fractional fusion calculations (15–25%) reveal that partial melts have REE characteristics somewhat similar to oceanic tholeiites. Conversely, computed source peridotites from oceanic tholeiites (Schilling, 1975) are similar to the alpine lherzolites reported here. Alpine lherzolites are, however, depleted in trace elements (K, Rb, Sr and Ba, Menzies and Murthy 1976). Since the lherzolites have an undepleted major, minor and REE chemistry close to that of pyrolite, the lost trace element-rich fraction must represent a small degree of melting. It is proposed that alpine lherzolites are residue left after the loss of a nephelinitic/alkalic fraction, (Ce/Yb]N=2.0–4.01) representing a small degree of partial fusion. This labile fraction may have existed as an intergranular phase or hydrous mineral prior to melting. |
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