GEOCHEMICAL PROCESSES IN THE EARTH'S MANTLE AND THE NATURE OF CRUST-MANTLE INTERACTIONS: EVIDENCE FROM STUDIES OF Nd AND Sr ISOTOPE RATIOS IN MANTLE-DERIVED IGNEOUS ROCKS AND LHERZOLITE NODULES

Investigations have been undertaken to provide constraints on the nature of geochemically active processes in the mantle and their effects on mantle evolution and magma generation. Three types of approaches have been implemented: (1) To study isotopic systematics and trace element abundances in young, mantle-derived volcanic rocks, which, because of their tectonic setting, had little or no chance for interaction with crustal material prior to their eruption. (2) To compare and contrast the Precambrian mantle with the present-day mantle via geochronological studies of ultramafic and mafic igneous rocks from Archean and Proterozoic greenstone belts. (3) To examine isotopic systematics and trace element distributions in mantle-derived lherzolite nodules, in an effort to reconcile first order observations of mantle materials with models of magma production in the mantle. Picritic basalts to quartz-normative tholeiites from the western Reykjanes Peninsula have been analyzed for Nd and Sr isotopic ratios and rare earth element (REE) abundances. The results of this study show that the mantle beneath the Reykjanes Peninsula is both vertically and laterally zoned with respect to LIL elements, and that mixing of liquids produced in different mantle segments must be called upon to explain the chemical variation in the basalts. Alkalic volcanics from Central Europe have also been investigated and are found to lie within the range defined by young oceanic volcanic rocks. The mantle source region for these lavas has changed in character from the Miocene to the Quaternary, as reflected by changes in the Nd and Sr isotopic compositions of these rocks to lower and higher values respectively. Sm-Nd systematics for the Abitibi belt and the Cape Smith belt yield ages 2622 and 1871 m.y. respectively. HoweverRb-Sr systematics in both areas have been affected by alteration processes subsequent to crystallization and no longer carry information about the primary crystallization ages. Relative to a model for the bulk earth which predicts chondritic relative abundances for REE's, the Abitibi magmas appear to be derived from an unfractionated, "chondritic", reservoir, while the Cape Smith magmas are derived from a reservoir with a history of LREE depletion. Sr isotopic systematics in inclusion-free clinopyroxenes provide evidence for both old oceanic crust and island arc material in the Cape Smith belt. Nd and Sr isotopic data show that the magma which crystallized as gabbro of the Shobogamo Intrusive Suite was contaminated by crustal component prior to crystallization. The nature of the process whereby a mantle-derived magma is contaminated, is constrained by Nd and Sr isotope ratios and concentrations. The process described is considered applicable to modern volcanic provinces which have been interpreted as being derived from "enriched" mantle source regions. The constituent phases of some ultramafic nodules from San Carlos, Arizona, ranging from dunite to "fertile" spinel lherzolite have been analyzed for Nd and Sr isotopes and trace element concentrations. Adjacent phases with different parent-daughter ratios were found to be in isotopic equilibrium suggesting recent equilibration events in the mantle, a result which is contrary to that expected on the basis of previous work. Alkali element budgets of the nodules are not accounted for by the alkalies in silicate phases, and liquid inclusions and sulfides are thought to play an important role. Sr, Nd, and Sm are shown to reside in clean silicate phases in both the LREE-depleted, "fertile" nodules, and the LREE-enriched harzburgitic nodules. The processes which have affected these nodules are complex and multi-facetted, and involve the migrationwithin the mantleof liquids or fluids enriched in alkali and LIL elements. It is proposed that these metasomatic components can play an important role in determining the chemistry of magmas generated in the mantle. Stanley R. Hart, Professor of Geology and Geochemistry Thesis supervisor: