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Crystal chemistry, melt composition and redox controls on the behavior of trace elements during lunar magmatism

dc.contributor.advisorFonseca, Raúl O. C.
dc.contributor.authorLeitzke, Felipe Padilha
dc.description.abstractThe behavior of trace elements in magmatic systems is controlled by physicochemical conditions prevalent during mantle partial melting and magmatic differentiation. Among these controlling factors are temperature, depth, type and degree of melting, source mineralogical composition and fO2. In order to generate a coherent, systematic dataset of crystal/silicate melt partition coefficients for trace elements as a function of crystal and silicate melt composition, and fO2, this thesis provides new partition coefficients for the REE, Y, HFSE, Sc, Ba, Sr, Cr, U, Th, Mo, W, Sn, In, Ga, Pt, and Rh between crystalline mantle phases (Fe-Ti oxides, pyroxene, plagioclase, spinel, and olivine) and basaltic melts, in particular under conditions relevant to lunar magmatism. Experiments were conducted in vertical tube gas-mixing furnaces (1 bar) at temperatures between 1100 C - 1300 C, and fO2 from 5.5 log units below the fayalite-magnetite-quartz (FMQ) redox buffer to air, thereby covering the range of temperatures and oxygen fugacities during planetary differentiation processes. At first, two element species, namely Al2O3 and Na2O, were chosen to be investigated since they are known to affect the structure of silicate melts and especially clinopyroxene crystal chemistry. The amount of [4]Al in clinopyroxene will result in an increase of clinopyroxene/melt partition coefficients even after applying a correction factor to account for the effect of melt polymerization. Moreover, the positive correlation between [4]Al and cpx/melt partition coefficients is not restricted to the REE, but also applies for Sn, Ga, In, and Ba. The addition of up to 2.6 wt% Na2O to the silicate melt universally increases the cpx/melt partition coefficients without any concomitant change in crystal chemistry or a significant effect in melt polymerization. This compositional effect is likely due to the ability of Na to break REE-Al-O-complexes in the melt. Our results emphasize the importance of considering all variables that affect the behaviour of trace elements in magmatic systems before applying the lattice strain model and derive meaningful results for changes in the parameters of crystallographic sites. A specific feature of some basaltic lunar rocks is that their TiO2 contents can reach concentrations as high as 16 wt. %. The High-field strength elements (HFSE) group, which includes Ti, may provide valuable information on the processes that occurred in the lunar mantle to generate high-Ti mare basalts. With the exception of Nb, all cpx/silicate melt HFSE partition coefficients show a strong negative correlation with the TiO2 content of the silicate melt. Olivine/silicate melt partition coefficients for Zr, Hf, Nb, Ta and Th decrease slightly from 0 to ca. 5 wt. % TiO2, above which they remain constant up to ca. 20 wt. % TiO2 in the silicate glass. In addition, redox sensitive elements, i.e. U, Mo, and W show clearly distinct crystal/silicate melt partition coefficients at different fO2, implying that these elements are relatively more compatible at reduced (ca. IW 1.8) than in oxidized (FMQ and air) environments. Molybdenum is shown to be volatile at oxygen fugacities above FMQ and its compatibility in pyroxene and olivine increases three orders of magnitude towards the more reducing conditions covered in this study. The partitioning results show that Mo is dominantly tetravalent at redox conditions below FMQ-4 and dominantly hexavalent at redox conditions above FMQ. Given the differences in oxidation states of the terrestrial (oxidized) and lunar (reduced) mantles, molybdenum will behave significantly differently during basalt genesis in the Earth (i.e. highly incompatible; average peridotite/melt Mo partition coefficient ca. 0.008) and Moon (i.e. moderately incompatible/compatible; peridotite/melt Mo partition coefficient ca. 0.6). Thus, it is expected that Mo will strongly fractionate from W during partial melting in the lunar mantle, given that W is broadly incompatible at FMQ-5. Finally, the new dataset of crystal/silicate melt partition coefficients was used to perform simple melting models of the lunar mantle cumulates. Results indicate that to reproduce the fractionation of W from the HFSE, as well as U and Th observed in lunar mare basalts, metal saturation and the presence of Fe-Ti oxides in the mantle sources is required. Moreover, the depletion of Mo and the Mo/W range in lunar samples can be reproduced by simply assuming a primitive Earth-like Mo/W for the bulk silicate Moon. Such a lunar composition is in striking agreement with the Moon being derived from the primitive terrestrial mantle after core formation on Earth.
dc.rightsIn Copyright
dc.subject.ddc550 Geowissenschaften
dc.titleCrystal chemistry, melt composition and redox controls on the behavior of trace elements during lunar magmatism
dc.typeDissertation oder Habilitation
dc.publisher.nameUniversitäts- und Landesbibliothek Bonn
ulbbnediss.affiliation.nameRheinische Friedrich-Wilhelms-Universität Bonn
ulbbnediss.instituteMathematisch-Naturwissenschaftliche Fakultät : Fachgruppe Erdwissenschaften / Steinmann-Institut für Geologie, Mineralogie und Paläontologie
ulbbnediss.fakultaetMathematisch-Naturwissenschaftliche Fakultät
dc.contributor.coRefereeBallhaus, Christian

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