Detail Processes and mantle sources generating basaltic volcanism: Insights from thorium isotopes Bibliografi Author: Ramos, Frank Calles ; Reid, Mary R. (Advisor) Topik: GEOCHEMISTRY Bahasa: (EN )    ISBN: 0-599-78736-8     Penerbit: University of California     Tahun Terbit: 2000     Jenis: Theses - Dissertation Fulltext: 9973154.pdf (0.0B; 0 download) Abstract Thorium isotopes have been instrumental in evaluating the processes and time scales involved in the generation and differentiation of magmas. In addition, Th isotopes may constrain the geochemical and isotopic character of mantle sources involved in magmagenesis and the processes and environments in which these mantle sources melt. Such constraints are, however, critically dependent on the absence of magmatic modifications imposed by the assimilation of crust. This dissertation focuses on evaluating the effects of crustal assimilation and, where these effects can be discounted, constrains the mantle sources that are involved in magmatism. Modifications imposed by mafic crustal assimilation in basalts erupted at Pisgah Crater in the Mojave Desert between 28 to 18 ka has resulted in variable signatures in trace element, 87Sr/86Sr, and 143Nd/144Nd signatures. Variations in 87Sr/86Sr are seen in both whole rock and individual minerals including olivine, plagioclase, and clinopyroxene/amphibole. 87Sr/86Sr variations are also exhibited within individual whole rock matrices of single hand samples, suggesting recent modification prior to eruption. In contrast, such extensive variations in 87Sr/ 87Sr are accompanied by only limited changes in (230Th)/( 232Th) and 3He/4He, suggesting that mafic crust has only limited influence in modifying Th and He isotopes. In contrast to results from Pisgah Crater, andesites and dacites erupted at San Pedro Volcano in the SVZ of Chile have geochemical characteristics such as variable trace element ratios and elevated δ18O which are consistent with assimilation of silicic crust. These evolved lavas generally have near secular equilibrium (230Th)/(232 Th) that cluster in a limited range that is consistent with contamination. Basalts, present in more evolved lavas as quenched mafic inclusions, reflect more variable Th isotope signatures if allowed to regain to secular equilibrium) but such signatures are not observed in andesites and dacites. As a result, the basalts, are interpreted to reflect only limited contamination and are presumed to retain the isotopic signatures and trace element characteristics of their sources. Insights gained from Pisgah Crater and San Pedro Volcano are used evaluate the possible effects of crustal assimilation on the geochemical and isotopic signatures, including Th isotopes, on continental basalts, erupted in the western U.S. Specifically, these results offer the constraints necessary to dismiss the modifying effects resulting from crustal contamination in recently erupted basalts in the Colorado Transition Zones, with the notable exception of basalts from the Zuni Bandera Volcanic Field. The remaining basalts have not undergone extensive crustal contamination and exhibit characteristics inherited from lithospheric mantle sources. Such characteristics include enriched isotopic signatures that are dissimilar to those of ocean island basalts (OIB) and therefore to those expected from a mantle plume. In addition, select basalts exhibit asthenospheric mantle signatures similar to those of basalts erupted in the Basin and Range Province. The presence of basalts with large degrees of disequilibrium in regions characterized by thick lithosphere in the CP Transition Zones suggest that modes of Th enrichment other than those resulting from ingrowth of upwelling asthenosphere may be able to produce large amounts of enrichments. More specifically, small degrees (<2%) of partial melting may be responsible for both Th enrichments and Sm/Nd fractionations in CP Transition Zone basalts. The presence of lithospheric mantle signatures in these basalts are also inconsistent with their derivation from asthenospheric mantle reservoirs as which have been proposed for basalts erupted in the Basin and Range Province. Opini Anda Klik untuk menuliskan opini Anda tentang koleksi ini! Lihat Sejarah Pengadaan  Konversi Metadata   Kembali

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