THE COMPOSITION OF PERIDOTITE TECTONITES FROM THE IVREA COMPLEX, NORTHERN ITALY - RESIDUES FROM MELT EXTRACTION

A total of forty-three samples from the Balmuccia, Baldissero, and Fin ero peridotite tectonites have been analyzed for major elements, forty minor elements, mineral composition, and a subset of samples have bee n investigated for deltaD, deltaO-18, deltaS-34, and Sr-87/Sr-86 ratio s. The spinel lherzolites from Balmuccia and Baldissero contain on ave rage 56% olivine, 28% orthopyroxene, 14% clinopyroxene, and 1.5% spine l. They have a neodymium and strontium isotopic signature similar to M ORB. These peridotites were moderately depleted in Al and Ca and highl y depleted in incompatible elements by the separation of 4.5% P-MORB. The MORB in excess to the formation and subduction of the oceanic crus t fractionated into a tonalitic continental crust and a pyroxenitic re sidue which was recycled into the mantle. The crustal mass fraction of the upper mantle-crust system is 2.8%. A primitive mantle composition can be calculated from 97.2% Balmuccia model peridotite plus 2.8% bul k crust. The new set of primitive mantle concentrations is in accordan ce, within about 10%, with the data from fertile peridotite xenoliths and primitive meteorites for the elements Li, Mg, Si, Ca, Sc, V, Mn, F e, Co, Ni, Zn, Ga, Y, Zr, La, Ce, Nd, Eu, Th, Dy, Yb, Lu, and Hf. Larg er differences occur for the elements F, Na, P, Ti, Cr, and Cu. The bu lk crust concentrations of the highly incompatible and mobile elements K, Rb, Ba, Th, and U exceed those being supplied from a primitive upp er mantle reservoir of 660 km depth. Ah additional lower mantle source is required for about half of the crustal accumulation of these eleme nts. Moderately depleted peridotite bodies with a cross section of 3 k m can be sufficiently well homogenized in compatible and moderately in compatible elements to represent MORB producing upper mantle. Concentr ations of Na, Sc, Ti, V, Ga, Y, Zr, and Yb are well correlated with th e Al abundance so that primitive mantle values can also be estimated u sing the cosmically derived primordial Al concentration. The spinel ha rzburgites from Finero contain on average 74% olivine, 16% orthopyroxe ne, 3% clinopyroxene, 5% amphibole, 1% phlogopite, and 1% spinel. Thes e peridotites were depleted by a separation of about 18% MORB. Subsequ ently they have undergone metasomatic alteration probably from dehydra tion of a subducted lithospheric slab. They received two pulses of wat er-rich fluids at temperatures of about 800-degrees-C to form amphibol e low in incompatible elements or amphibole plus phlogopite high in in compatible elements without complete reequilibration of the preexistin g rocks. The accumulation of elements is correlated to their incompati bility with the exception of a depletion of U, Th, Nb, and Ti. Isotopi c ratios of H, O, S, Sr, Nd, and Pb, chlorine in phlogopite and a rela tive deficit of Nb and Ti are consistent with a crustal imprint in the metasomatic fluids.