Evaluating mantle and crustal processes using isotope geochemistry
Geochemical studies are fundamental for understanding how the dynamic Earth works and evolves. These studies place constraints on the composition, formation, age, distribution, evolution and scales of geochemically distinct reservoirs such as the Earth's crust, mantle and core. In this dissertation the strategy has been to work on a broad range of topics to evaluate crustal and mantle processes. This study presents Re-Os systematics to constrain the composition, formation and age of the lower continental crust and the mantle lithosphere, examines melt inclusion from oceanic island basalts to evaluate the scale of the mantle heterogeneities, and uses U-series isotope to constrain geodynamic parameters, such as the upwelling velocities and porosities of mantle plumes. The lower continental crust plays a pivotal role in understanding the composition and evolution of the continental crust and the petrogenesis of continental basalts. This chapter presents Re/Os isotope measurements which allow us to further our understanding of these problems. Two well-characterized suites of lower crustal xenoliths from Northern Queensland, Australia, which have average major and trace element compositions similar to bulk lower crust, were analyzed for Re/Os isotope systematics. From this data, we infer that the lower crust has 1 to 2 times as much as, about half of the Re and is less radiogenic in 187OS/88OS than the upper continental crust. Our data show that assimilation and fractional crystallization (AFC) are important processes in the formation of the lower crust and lead to dramatic changes in the Os isotopic composition of basalts that pond andfractionate there. Because of this, the Re-Os system cannot be relied upon to yield accurate mantle extraction ages for continental rocks. Chapter 2 examines the Re-Os isotopic composition of the Horoman massif, Japan. These data indicate that the Os isotope composition is controlled by the Re content, through radiogenic ingrowth, while the Re content is governed by the extent of depletion in "basaltic component" of the ultramafic rocks. Re-Os systematics suggest that depletion model ages of ≈ 1.8 Ga represent the age of the melting event. The colinearity between mafic and ultramafic rocks in the Re-Os isochron diagram defines an apparent age of ≈ 1Ga.. The similar "ages" determined by Re-Os and Sm-Nd isotopes and the high Re/Os ratios in the most fertile peridotites plotting to the right of the geochron, indicate that the mafic layers and the ultramafic rocks are genetically related by a refertilization process which took place ≈ 1 Ga ago. The Re-Os systematics for' other ophiolitic massifs indicate that refertilization of the lithospheric mantle seems to be a more Widespread process than previously thought. Previous studies have suggested that melting processes are responsible for the trace element variability observed in olivine-hosted basaltic melt inclusions. Melt inclusions from four individual lava samples representing three mantle end-members HIMU, EMl and EMIl (two from Mangaia, Cook Islands, one from Pitcairn, Gambier chain, and one from Tahaa, Society chain), have heterogeneous Pb isotopic compositions, even though the erupted lavas are isotopically homogeneous. The range of Pb isotopic compositions from individual melt inclusions in a single lava flow spans 50% of the world-wide range observed for ocean island basalts (OlB). The melt inclusion data can be explained by two-component mixing for each island. Our data imply that magmas with different isotopic compositions existed in the volcanic plumbing system prior to or during melt aggregation. Evaluation of U-series disequilibrium, trace element composition and He, Sr, Nd and Pb isotopes of Galapagos lavas indicates that magma mixing between plume and asthenospheric melts has been the main process responsible for the geochemical variation observed in the archipelago. Correlations between He isotopes and TilTi*, K/Rb and Nb/La ratios suggest that the mantle plume has positive anomalies of Nb and Ti and negative anomalies of K. 230Th excesses measured in the lavas indicate that the basalts from Galapagos originated completely or partially in the garnet stability field. Mantle upwelling velocity for the Galapagos plume (Fernandina) ranges from ≈ 1 to 3 cm/y with a maximum porosity of 0.3%, indicating that Galapagos is a mildly buoyant plume. Very slow mantle upwelling rates and very low porosity for Pinta (0.5 to 1 crnIy and 0.1%) and Floreana (0.1 em/year and <0.1%) islands, support the hypothesis that the movement of the plume across the 91°50' transform fault into a younger and thinner lithosphere produced slow upwelling and small extents of melting.
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Massachusetts Institute of Technology and Woods Hole Oceanographic Institution