Geochemical measurements of OIB (ocean island basalt) samples have demonstrated that the Earth's mantle is compositionally heterogeneous, but the cause of this heterogeneity is a point of debate within the scientific community. One such OIB location is Mangaia, Cook Islands (Polynesia). Its lead isotopic composition defines the HIMU (high μ = high ^238U/^204Pb) mantle endmember, which many attribute to recycled oceanic crust being present in the mantle source. If true, this endmember represents an important vehicle for returning surface material to the mantle and an opportunity to study volatile element cycling through the mantle. Sulfur isotopic measurements were made on sulfides hosted in melt inclusions from Mangaia. Prior to 2.45 Ga, the Earth's atmosphere was not oxygenated, allowing photochemical cycles to fractionate sulfur isotopes. This form of fractionation results in a mass independently fractionated (MIF) sulfur isotopic signature in surface materials containing sulfur. We have found such a signal in sulfide inclusions from Mangaia, indicating that the material erupted at this young (~19 Ma) ocean island was once at the surface over 2.45 Ga. This finding confirms the recycled origin hypothesis for the generation of the HIMU mantle endmember. Lead isotopes and major elements were measured in olivine hosted melt inclusions from the island. Previous studies by Saal et al. (1998) and Yurimoto et al. (2004) have revealed large lead isotopic variability, spanning half of the global range for OIBs. A more recent study by Paul et al. (2011) has shown much reduced lead isotopic variability using a different analysis technique. We find the lead isotopic variability in glassy melt inclusions to be less than previously found and attribute much of the earlier observed variability to contaminant lead. Volatile and trace elements were measured in the same olivine hosted melt inclusions, providing the first ever coupled lead isotopes, major, trace, and volatile elements in glassy melt inclusions from the island. We observe some of the highest water and carbon dioxide contents found in OIBs globally. This allows us to constrain volatile abundances in the HIMU mantle source as well as volatile cycling in the mantle, from subduction zones to hotspots.

abstract: With the InSight mission deploying a seismometer , Martian bulk chemical compositional models are more important than ever. Three largely consistent models for the Martian mantle have been suggested over the past two decades. Of these three, two are fairly similar and one is dramatically different. Of these three, the EH70 (Sanloup et al., 1999) models have the systematically lower divalent cation to silicon ratios as compared to the other model, the DW85 (Dreibus and Wanke, 1985) model. However, impact of such a low (Mg+Fe+Ca)/Si ratio on mineralogy has not been experimentally investigated. Measurements have been made of the mineralogy of the EH70 bulk mantle composition (Sanloup et al., 1999)) through in-situ laser-heated diamond anvil cell (LHDAC) and large volume press (LVP). Majorite-garnet (Mj) dominated mineralogy has been observed up to 25 GPa. Bridgmanite (Bm) begins to appear from 25.2 GPa and continues in a mixed phase with Mj up to 27 GPa at which point only Bm and calcium perovskite (CaPv) remain. Akimotoite (Ak) is stable up to 1873 K, higher by ≈300 K compared to numerical calculations (Connolly, 2009). This may result in an Ak layer in the Martian mantle, something missing in Earth’s mantle. The overall ratio of pyroxene to olivine polymorphs by volume is high, approaching pure pyroxene. This agrees with numerical calculations. Additionally, ferropericlase (Fp) is stable at lower temperatures, suggesting a higher dependence on temperature for its stability, something that is different from Perple_X calculations which show a strong dependence on pressure. Furthermore, Mj, which make up a majority of the volume of EH70 mantles, was measured to increase in Fe content as pressure increases. The more oxidizing conditions coupled with the silicon-rich composition resulted in three times higher Fe3+ content in Mj as opposed to a pyrolite model. This increased Fe3+ meant our Mj composition approached that of skiagite (Ski,Fe2+ 3 Fe3+ 2 Si3O12) and this caused Mj to have a very low compressibility of only 152.8 GPa, lower than any other Mj compositions in literature. This result suggests that a mantle with EH70 bulk composition would have lower than predicted seismic wave velocities , lower than Perple_X predicts. The Al content of Mj was also found to suppress the first derivative of compressibility to 4.45, lower than that of Ski100 at 6.7. Such differences compared with pyrolitic composition are important to estimate the velocity profiles and to model the dynamics of the Martian mantle. This dataset of mineralogy and composition can also model terrestrial exoplanetary mantles. Current measurements of stellar abundances show a wide range of compositions, and especially compositions with (Mg+Fe+Ca)/Si ratios approaching 1 (Brewer and Fischer, 2016). This experimental study of EH70 composition can fill-in this gap. / Dissertation/Thesis / Masters Thesis Natural Science 2019

Through geodynamic modeling, I investigate how Mars could have produced the extensive volcanism required to form the Tharsis rise early in its history, as well as continue to produce small amounts of melt up to present-day, in order to account for the evidence of limited geologically recent volcanism. InSight is the first interplanetary mission dedicated primarily to the study of a planet's deep interior, and has provided useful constraints for the present structure and interior temperature of Mars. I use the results from InSight and other spacecraft missions to more accurately model Mars, and evaluate the results of my geodynamic models, so as to constrain the properties that are necessary for or consistent with both the InSight results and the volcanic history reflected on the surface. This modeling has required extensive modification to the CitcomS geodynamic code I use, the bulk of that effort being in implementing and testing the melting calculations. One of the useful constraints that would have been provided by InSight would have been ground truthing the heat flow from the interior at the landing site, and this required determining, among other quantities, the thermal conductivity of the regolith into which the heat flow probe (mole) was placed. While the mole could not penetrate to its designed depth, thus disallowing the complete heat flow measurement, the team were able to obtain the necessary data determine the thermal conductivity, and how it varies seasonally. My rapid analytical method of estimating thermal conductivity produces results that agree surprisingly well with those of the team's complex numerical model, despite the mole not meeting the assumption of a sufficiently high length to width ratio. / Doctor of Philosophy / I investigate how Mars could have produced the extensive volcanism required to form the Tharsis rise early in its history, as well as continue to produce small amounts of melt up to present-day, in order to account for the evidence of limited geologically recent volcanism. I use 3D computer models of the mantle--the solid, but slowly flowing layer that makes up the bulk of rocky planets like Earth and Mars. InSight is the first interplanetary mission dedicated to the study of a planet's deep interior, and has provided useful constraints for the present structure and interior temperature of Mars. I use the results from InSight and other spacecraft missions to more accurately model Mars, and evaluate the results of my models, so as to constrain the properties that are necessary for or consistent with both the InSight results and the volcanic history reflected on the surface. This modeling has required extensive modification to the modeling code I use, the bulk of that effort being in implementing and testing the melting calculations. One of the useful constraints that would have been provided by InSight would have been ground truthing the heat flow from the interior at the landing site, and this required determining, among other quantities, the thermal conductivity of the soil into which the heat flow probe (mole) was placed. While the mole could not penetrate to its designed depth, thus disallowing the complete heat flow measurement, the team were able to obtain the necessary data determine the thermal conductivity, and how it varies seasonally. My rapid analytical method of estimating thermal conductivity produces results that agree surprisingly well with those of the team's complex numerical model, despite the mole not meeting the assumption of a sufficiently high length to width ratio.

Results from two-station surface-wave inversions across the Archean Kaapvaal craton of southern Africa are compared with seismic velocities estimated from approximately 100 mantle xenoliths brought to the surface in kimberlite pipes. As the xenoliths represent a snapshot of the mantle at the time of their eruption, comparison with recently recorded seismic data provides an opportunity to compare and contrast the independently gained results. These cratonic xenoliths from the southern Kaapvaal, all less than 100Ma in age, have been analyzed geothermobarometrically to obtain the equilibrium P-T conditions of the cratonic mantle to about 180km depth [James et al 2004]. Seismic velocity-depth and density-depth profiles calculated on the basis of these P-T data and the mineral modes of the xenoliths are used to produce theoretical surface-wave dispersion curves and to generate roughly the upper 200km of a starting/reference model. A regionally-developed crustal structure [Niu and James 2002] was used for the crust and 300km of mantle values taken from PREM filled in down to 500km depth. This composite model was used as the starting/reference model for a Neighbourhood Algorithm surface-wave inversion using fundamental-mode Rayleigh-wave phase velocities for 16 paths within the Kaapvaal Craton from five events. The velocity structures found by that inversion are consistent with those derived from the xenolith data. Hence the velocity structure (i.e. thermal structure) of the mantle to a depth of 180km beneath the Kaapvaal craton is basically the same today as it was 80-90Ma. Further, synthetics runs show that for this surface-wave dataset, there is no strong low-velocity zone at depths shallower than at least 200km. / Master of Science

The oceanic lithosphere forms as a residue of partial melting of the mantle beneath the mid-ocean ridge axis. Subduction of this residual layer has a profound impact on the Earth’s thermal and geochemical cycles as the recycling of this layer facilitates heat loss from the Earth’s interior and induces geochemical heterogeneities in the mantle. The goal of this study is to understand the thermal and geochemical evolution of the oceanic lithospheric mantle from a petrological perspective. An empirical geobarometer is calibrated for ocean island xenoliths in order to understand the thermal structure of the oceanic lithospheric mantle. The results of 0.1 MPa experiments from this study and high-pressure experiments from previous studies are used in the calibration. The uncertainties on pressures derived using the above geobarometer are high and hence could not be tested against thermal models for the oceanic lithosphere. The geochemical evolution of the oceanic lithospheric mantle involves post-melting geochemical modifications such as metasomatism. The geochemical evolution of the uppermost oceanic lithospheric mantle is studied using harzburgites from Hess Deep ODP Site 895, which are depleted in moderately incompatible elements relative to the global suite of abyssal peridotites. A comparison between Yb-abundances in Hess Deep harzburgites iii iv and those of a model depleted MORB mantle (DMM) residue reveals that the harzburgites have undergone up to 25% melting, assuming 0.5% melt porosity. Higher light and middle rare earth elements in the Hess Deep harzburgites than the model DMM melting residue are interpreted as the result of plagioclase crystallisation from melts being extracted by diffuse porous flow through the upper mantle. The effect of plagioclase crystallisation does not affect the chemistry of residual mineral phases as evidenced from the depleted light rare earth element abundances in clinopyroxene relative to the bulk rock. Ocean island xenoliths are studied to understand when and where metasomatism occurs in the deeper portion of the oceanic lithosphere. The median values of measured and reconstructed bulk concentration of Al2O3 in most ocean island xenoliths is lower than in abyssal peridotites, which generally would be interpreted as indicating a higher extent of melting in the former. However, a comparison between Yb- abundances in ocean island xenoliths and abyssal peridotites with a model DMM melting residue suggests that the extents of melting in the suites of rocks are broadly similar. Although fewer in number than ocean island xenoliths, abyssal peridotites from several locations have low concentrations of moderately incompatible elements. Metasomatism is observed in both, ocean island xenoliths and abyssal peridotites in the form of higher bulk rock Ce and Nd concentration than the model DMM melting residue but the extent of metasomatism is higher in ocean island xenoliths. There is no correlation between the concentrations of bulk rock Ce, Nd, Sm and Eu of ocean island xenoliths and age of the oceanic lithosphere from which the xenoliths originate. It is interpreted that metasomatism in the lower oceanic lithospheric mantle occurs near the ridge axis above the wings of the melting column. / Graduate / 0996 / 0372

Brandt, Martin B.C. 2011. Imaging the African Superplume – Upper mantle, Tomography and Moment tensor. Ph.D. thesis, Faculty of Science, University of the Witwatersrand, Johannesburg, South Africa. The African Superplume, African Superswell and East African Rift System are amongst the most prominent geophysical features on Earth, but the structure, evolution and interaction between these features is controversial. In my thesis I conducted a range of investigations in an effort to better understand these issues. The thesis presents the investigations into the structure and expressions of these features. These include: (I) A study of the upper mantle shear velocity structure beneath southern Africa to investigate the source of the buoyancy that has powered the Superswell; (II) Statistical hypothesis testing of middle-mantle shear velocity tomographic models to evaluate evidence for links between the Superplume and low velocity features in/near the transition zone; and (III) Computation of three new regional moment tensors for South Africa to assess crustal stress in the Kalahari craton, and its link with mantle structure and dynamics. Waveform data were obtained for the study on the upper mantle shear velocity structure and the moment tensor inversions from the Southern African Seismic Experiment Kaapvaal craton array. For the statistical hypothesis testing on global tomography images, new travel-time data from both global and AfricaArray stations were added to Grand’s global shear velocity data set. The principal findings of this study are summarized below. I. The upper mantle shear velocity structure beneath the Kalahari craton is similar to that of other shields, except for slightly slower velocities from 110–220 km depth. The difference may be due to higher temperatures or a decrease in magnesium number (Mg#). If the slower velocities in the deep lithosphere are due solely to a temperature anomaly, then slightly less than half of the unusually high elevation of the Kalahari craton can be explained by shallow buoyancy from a depleted hot lithosphere. Decreasing the Mg# of the lower lithosphere would increase density and counteract higher temperatures. If an excess temperature of 90 K over a 110 km depth range and a corresponding decrease in Mg# of -2 between the Kalahari and the other cratons are assumed, this would match the seismic velocity difference but would result in essentially no buoyancy difference. We conclude that the high elevation of the Kalahari craton can only be partially supported by shallow mantle buoyancy and must have a deeper source. We determined a thickness of 250±30 km for the mantle transition zone below eastern southern Africa, which is similar to the global average, but the corresponding velocity gradient is less steep than in standard global models (PREM and IASP91). Velocity jumps of 0.16±0.1 km/s (eastern) and 0.21±0.1 km/s (central) across the 410 km discontinuity were found. Our results indicate a thermal or chemical anomaly in the mantle transition zone, but this cannot be quantified due to uncertainty. II. Statistical hypothesis testing on our global tomography images indicated that the African Superplume rises from the core-mantle boundary to at least 1150 km depth, and the upper mantle slow-velocity anomaly extends from the base of the lithosphere to below the mantle transition zone. The model that links the African Superplume with the slow-velocity anomaly in the upper mantle under eastern Africa has an equal probability to an alternative hypothesis with a thin slow-velocity “obstruction zone” at 850 to 1000 km depth. III. Finally, we calculated three regional moment tensors for South Africa and made progress towards resolving the discrepancy between the local and moment magnitudes we observe for the region. Moment tensors/focal mechanisms in southern Africa change from normal faulting (extension) in the northeast near the East African Rift to strike-slip faulting in the southwest. This confirms previous studies stating that not only eastern Africa, but also southern Africa is being actively uplifted by lithospheric modification at its base and/or the African Superplume.

No description available.

Thesis (doctoral)--Johann Wolfgang Goethe Universität, 2004. / Includes bibliographical references (p. [270]-299).

A região sul da placa Sul-Americana, hoje pertencente à região da Patagônia Argentina e Chilena, formou-se por consequência de acreções continentais desde o Proterozóico. Atualmente, a região é caracterizada por um complexo sistema de placas tectônicas, no qual as placas oceânicas de Nazca, Antártica e Scotia interagem diretamente com a placa continental Sul-Americana através dos processos de subducção e transcorrência. Entre as placas de Nazca e Antártica, ocorre a dorsal do Chile, e a subducção desta dorsal sob a placa Sul-Americana forma a Junção Tríplice do Chile, ocorrendo o soerguimento da astenosfera na região. O magmatismo Cenozóico de composição alcalina que ocorre na região da Patagônia Argentina e Chilena hospeda xenólitos mantélicos ultramáficos de classificação espinélio- e granada-peridotitos. Estes xenólitos são de extrema importância para a caracterização e identificação dos processos atuantes no manto superior abaixo dessa complexa região que hoje é a Patagônia. Estudos do sistema isotópico Re-Os nos xenólitos de Prahuaniyeu (41°20’09.4”S, 67°54’08.1”W), e Chenque (43°38’39.3”S, 68°56’22”W), na região norte da Patagônia Argentina, sugerem que a litosfera abaixo de Prahuaniyeu (TRD ~ 1.69 Ga) é mais antiga que Chenque (TRD ~ 0.71 Ga). Dados de Rb-Sr mostram que a litosfera da região norte da Patagônia possui altas razões 87Sr/86Sr (Prahuaniyeu: 0,7037 a 0,7041; Chenque: 0.7037 a 0.7086), devido fluidos relacionados a desidratação de uma placa de subducção. Através destes dados e dos dados geoquímicos, o manto litosférico subcontinental da região norte da Patagônia sofreu metassomatismo relacionado a slabs derivados de antigas placas de subducção e que proporcionou características de metassomatismo por líquidos/fluidos do tipo-OIB, e atualmente sofreu metassomatismo relacionado aos fluidos derivados da desidratação da placa de subducção atual (Nazca), caracterizados pelo enriquecimento em calcófilos. Todos os peridotitos de Laguna Timone (52°01’39” S, 70°12’53” W), no Campo Vulcânico de Pali Aike, região sul da Patagônia Chilena, também apresentam expressivo enriquecimento nos elementos calcófilos sugerindo que o manto litosférico subcontinental da região sul da Patagônia também foi metasomatisado pelos fluidos derivados da desidratação da placa de subducção atual (Antártica). Em Laguna Timone também há a ocorrência de um glimerito entre os xenólitos e a presença de flogopita e pargasita nos peridotitos classificados como gr-sp lherzolitos, sp-lherzolitos e gr-sp harzburgitos. A presença de um glimerito, de peridotitos com minerais hidratados (flogopita e pargasita) e as similaridades com peridotitos metassomatisados por líquidos astenosféricos (peridotitos do distrito de Manzaz, Argélia e do campo vulcânico Vitim, no lago de Baikal, Sibéria) com baixas razões Ba/Nb, Ba/La e U/Nb, indicam que a litosfera da região sul da Patagônia sofreu metassomatismo por fluidos astenosféricos, ocasionado devido o soerguimento da astensofera durante a passagem da Junção Tríplice do Chile pela região de Pali Aike. / The southern of the South-American plate, today is the Chile and Argentina Patagonia region, was formed as a result of continental accretions since the Proterozoic.Currently, this region is characterized for a complex tectonic plates system, in which Nazca, Antartica and Scotia oceanic plates interact directly to the South-American continental plate by subduction and transcorrent process. Between Nazca and Antartica plate occurs the Chile Ridge, and the Chile Ridge subduction under the South-American plate creates the Chile Triple Junction and the upwelling of underlying asthenospheric mantle in this region. The Cenozoic alkali magamtism that occurs in Patagonia Argentina and Chilena hosts ultramafic mantle xenoliths (spinel- and garnet-peridotites). These xenoliths are extremely important to characterization and identification of the processes that occurred in the upper mantle underneath the Patagonia region. The Re-Os isotopic studies in Prahuaniyeu (41°20’09.4”S, 67°54’08.1”W), and Chenque (43°38’39.3”S, 68°56’22”W) xenoliths, in north Patagonia Argentina, suggests the Prahuaniyeu lithosphere (TRD ~ 1.69 Ga) were formed previously to Chenque (TRD ~ 0.71 Ga). Rb-Sr data show high 87Sr/86Sr ratio (Prahuaniyeu: 0.7037 to 0.7041; Chenque: 0.7037 to 0.7086), suggesting interactions with subduction plate dehydration related fluids. Trough this data, and geochemistry data, the sucontinental lithospheric mantle underneath the north Patagonia region suffered two metasomatic events: one related to the OIB-like melt/fluids from slabs derived by ancient subductions; and another related to the fluids derived from the current subducted plate (Nazca) dehydration, characterized by the chalcophiles enrichment. Peridotites from Laguna Timone (52°01’39” S, 70°12’53” W), in the Pali Aike Volcanic Field, southern Patagonia Chilena region, also shows expressive enrichment in chalcophile elements suggesting metasomatism by fluids from currently subduction (Antartica plate). Another kind of metasomatism occurs in subcontinental lithospheric mantle underneath Pali Aike due the glimmerite occurrence, hydrated minerals (phlogopite and pargasite) in peridotites and similarities with peridotites that suffered metasomatism by asthenospheric melts (Manzaz, Argelia peridotites and Vitim Volcanic Field, Baikal, Siberia peridotites), with low Ba/Nb, Ba/La and U/Nb. All these carachteristics suggest that lithosphere suffered interactions between asthenosphere-lithosphere due upwelling of underlying asthenospheric mantle when the Chile Triple Junction was on the same latitude of Pali Aike.

xv, 151 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / To explain the origin of several distinct aspects of the Galápagos volcanic hotspot, such as the broad geographical extent of recent volcanism and the unusual pattern of geochemical anomalies, we conducted seismic tomography studies of the upper mantle and crust beneath the Galápagos Archipelago. The studies combine measurements of group and phase velocities of surface waves and delay times of body waves. We find that upper mantle seismic velocities are lower than those beneath other regions of comparable age in the Pacific and consistent with an excess temperature of 30 to 150°C and ∼0.5% melt. We attribute the excess temperature and presence of melt to an upwelling thermal mantle plume. Crustal seismic velocity is up to 25% lower than that of very young crust at the East Pacific Rise (EPR) and is comparable to that of Hawaii, which we attribute to heating by increased intrusive activity above the Galápagos plume and the construction of a highly porous volcanic platform. In addition, we find that the Galápagos hotspot is underlain by a high-velocity region whose thickness varies from 40 to 100 km. The tomographic images reveal that the upwelling mantle plume tilts northward (towards the nearby Galápagos Spreading Center) as it rises and then spreads laterally when it reaches the bottom the lid. The lid, which we attribute to residuum from melting, is thickest where it is farthest from the spreading center, suggesting that ridge processes may affect the generation and amount of thinning of the residuum layer. In addition, the thickness of the lid correlates well with the geographical pattern of geochemical anomalies of erupted lavas, suggesting that the lid may control the final depth of decompression melting. We conclude that many of the distinct characteristics of the Galápagos can be attributed to the interaction of the upwelling plume with the lid and the nearby ridge. We further suggest that the ridge affects the geometry of plume upwelling in the upper mantle and also the pattern of lateral spreading of the plume due to its effect on the thickness of the residuum layer. This dissertation includes previously published co-authored material. / Committee in charge: Dr. Douglas R. Toomey, Chairperson; Dr. Eugene Humphreys, Member; Dr. Emilie Hooft Toomey, Member; Dr. Paul Wallace, Member; Dr. John Conery, Outside Member