In order to evaluate the electron microprobe (EMP) method for chemical dating of monazite, we chemically analyzed selected suites of monazite grains that were previously dated by standard U/Pb isotopic methods at three laboratories each equipped with a sensitive high resolution ion microprobe (SHRIMP). Representing diverse igneous and metamorphic lithologies, these grains yielded conventional isotopic ages ranging in age from Neoarchean to Devonian. Chemical dating was performed at Virginia Tech using a Cameca SX-50 EMP in which the analytical routines and settings were specifically optimized for monazite geochronology, including correction of analytical peaks for all major spectral interferences and correction of peak intensities for local background emission. Placement of cross-grain analytical traverses was based on backscattered electron (BSE) images together with wavelength-dispersive (WD) generated X-ray maps for Y, Th, U, and Ca, which revealed the internal compositional complexity of each grain. Shorter EMP traverses were selected adjacent to each SHRIMP pit in order to provide the best possible comparison of ages obtained by the two dating methods. Synthesis protocol for key elemental measurements (Y, Th, U, and Pb) was developed utilizing the 1Ï elemental errors associated with individual analyses, providing an objective approach for data synthesis. Analytical dates were either accepted or excluded based on analytical and spatial justifications. Isotopic dating techniques utilize three independent age calculations, provided the sample is old enough to have accumulated sufficient 207Pb (i.e., ≥~1000 Ma). Similarly, the chemical dating method can utilize two independent age calculations (i.e., Th/Pb and U/Pb) and a U-Th-Pbtotal centroid age in Th/Pb vs. U/Pb space, verified independently against the calculated Th* or U* CHIME ages. Across the entire 2,200 m.y. age range represented by the sample set, the chemical ages calculated from the EMP data chemical ages are internally consistent (within 2Ï error) with the previously measured SHRIMP isotopic ages, except in one case where bulk mixing of discrete age domains within an ablation pit led to an isotopically discordant apparent age. Overall, this study illustrates that EMP chemical dating (1) represents both an accurate and precise primary method for dating monazite from igneous and polymetamorphic terrains; (2) provides superior spatial resolution for obtaining meaningful ages from small and/or irregular domains of discrete age that may be irresolvable or misinterpreted by other dating techniques that sample larger volumes; and (3) illuminates the geological meaning of isotopically discordant monazite ages obtained using conventional methods with lower spatial resolution (e.g., SHRIMP). / Ph. D.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18803号 / 理博第4061号 / 新制||理||1584(附属図書館) / 31754 / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 平田 岳史, 教授 田上 高広, 教授 土`山 明 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

La tsavorite, grossulaire vert à V-Cr-Mn, est contenue dans des gneiss et roches calco-silicatées graphiteux, souvent associés à des marbres dolomitiques, et appartenant à la ceinture métamorphique néoprotérozoïque mozambicaine. La tsavorite se trouve soit dans des nodules ou des veines de quartz (gisements primaires), soit dans des placers (gisements secondaires). L'étude minéralogique des tsavorites propose un nouveau protocole de certification de leur origine géographique, à partir du rapport V/Cr, de la teneur en Mn et du [delta]18 O. L'étude des gisements de Lemshuku et Namalulu en Tanzanie montre que le métamorphisme des protolithes sédimentaires riches en matière organique et évaporites s'est effectué à P = 7,0 ± 0,4 kbar et T = 677 ± 14°C, à 634 ± 22 Ma (datation U-Th-Pb sur monazite). Le bâti métamorphique s'est refroidi vers 500 Ma (datation 40Ar-39Ar sur muscovite). Deux stades de métasomatose sont reliés à la formation de la tsavorite : (i) une métasomatose de diffusion formant les nodules à P = 5,0-7,4 kbar et T = 580-691°C; (ii) une métasomatose calcique d'infiltration contemporaine de la formation des veines de quartz à P = 3,6-4,9 kbar et T = 505-587°C. Ces dernières sont datées in situ par la méthode Sm-Nd à 606 ± 36 Ma. Les évaporites continentales, déposées dans une sabkha de côte marine avec des sédiments silico-calcaires, sont transformées en tsavorite dans le cas des nodules, alors que les sels fondus sont associés à la formation des veines de quartz. Les minéralisations sont contrôlées par la lithostratigraphie et la tectonique / Tsavorite, a (V, Cr, Mn)-bearing green grossular, is hosted by graphitic gneisses or calc-silicates, often asssociated with dolomitic marbles, and belonging to the Metamorphic Neoproterozoic Mozambique Belt. Tsavorite is found either as nodules or in quartz veins (primary deposits), or in placers (secondary deposits). The mineralogical study of tsavorites suggests a new protocol to certificate their geographical origin, based on the V/Cr ratio, Mn content and delta18O. The study of the Lemshuku and Namalulu deposits in Tanzania has shown that the metamorphism of organic matter-rich and evaporites-rich sedimentary protoliths occurred at P = 7.0 ± 0.4 kbar and T = 677 ± 14°C, at 634 ± 22 Ma (U-Th-Pb dating on monazite). The metamorphic series cooled down at around 500 Ma (40Ar-39Ar dating on muscovite). Two metasomatic stages are linked to the formation of tsavorite : (i) diffusion metasomatism forming nodules at P = 5.0-7.4 kbar and T = 580-691°C; (ii) calcitic infiltration metasomatism forming quartz veins at P = 3.6-4.9 kbar and T = 505-587°C. These last have been dated in situ with Sm-Nd dating at 606 ± 36 Ma. Continental evaporites, deposited in a coastal marine sabkha with (Si, Ca)-bearing sediments, transformed into tsavorite in the case of the nodules, while the molten salts are associated with the formation of the quartz veins. The mineralisations are controlled by lithostratigraphy and structure