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Group 2 kimberlites from South Africa, and lamproites from Western Australia and North America are relatively unfractionated mantle-derived igneous rocks, situated on or close to ancient cratonic areas. They are characterised by high trace element contents, while the range in N d and Sr isotopes encompasses much of that reported for various upper and lower crustal rocks. It is argued that these features are not due to crustal contamination during magma ascent, rather they are source and extraction phenomena. The mantle source regions of these rocks were ancient (~ 1.0 to 2.5 Ga) and variably trace element enriched. Preservation of such regions within the mantle is most probable in the relatively 'cold' and 'rigid' subcontinental mantle lithosphere, which is believed to have been isolated from the convecting asthenosphere for a long time. The source regions of the kimberlites and lamproites were situated at various depths within the subcontinental mantle lithosphere, from within the amphibole stability field « 100 km) to within the diamond stability field (> 150 km). Low degrees of partial melting « 1 %), together with volatile composition and depth of melting, have significantly influenced the composition of the resultant kimberlite and lamproite magmas. Those magmas that originated from within the diamond stability field contain abundant entrained and dis aggregated mantle peridotite. This feature is related to melt migration and rapid ascent to the surface, from these mantle depths. The Sr, Nd and Pb isotope data record evidence of variable, but related trace element enrichment styles. The origin of these trace elements is either from recycled continental crust (e.g. pelagic sediment), or from intra-mantle processes (e.g. the migration and crystallisation of small volume silicate melts with variable volatile compositions). The available data are insufficient to determine between the models and further work in this area is required.

Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Insets of large anhedral minerals in kimberlites are proposed to mainly have their origin from the disaggregation of mantle-derived xenoliths through mechanical abrasion by the ascending kimberlite magma. Garnet, despite being an important constituent of both mantle-derived peridotite and eclogite, forms only a minor constituent of kimberlites. This suggests that a large proportion of garnet liberated into the kimberlite through the disaggregation of such xenoliths, is consumed before the emplacement of kimberlite. This study records the breakdown mechanism of garnet by the ascending kimberlite magma by conducting experiments between pressures of 2 and 4 GPa and temperatures between 1100 and 1300°C. The starting material used in the experiments was prepared from a natural hypabyssal kimberlite that closely resembles the composition of a proposed primary Group I kimberlite magma. To the kimberlite material 5 wt % garnet, sourced from a natural peridotite, was added. It was found that garnet is not a stable equilibrium phase within the kimberlite magma under the investigated temperature and pressure conditions. Based on large volumes of phlogopite in the experiments it is concluded that garnet melts incongruently in the kimberlite magma to form phlogopite as a peritectic product and melts transitional in composition between silicate and carbonate melts. This is in contrast to more SiO2 -rich melt compositions produced as a consequence of the incongruent breakdown of both orthopyroxene and omphacite within kimberlite magmas. The consequence of the melt compositions produced in the experiments is increased solubility of CO2 in the form of carbonate (CO32-). This finding has implications for both kimberlite ascent mechanisms and the solubility of diamond transported within kimberlite magmas. / AFRIKAANSE OPSOMMING: Dit word voorgestel dat groot oneenvormige mineraalinsluitsels in kimberliet hoofsaaklik afkomstig is van xenolitiese mantelmateriaal wat verbrokkel weens die meganiese werking van die stygende kimberliet magma. Ten spyte daarvan dat granaat ‘n belangrike komponent is van peridotiet en eklogiet xenoliete afkomstig uit die mantel, vorm granaat slegs ‘n geringe deel van die kimberliet mineraalsamestelling. Dit dui daarop dat ‘n beduidende gedeelte van die granaat wat vrygstel word in die kimberliet magma deur die verbrokkeling van xenolitiese materiaal, afgebreek word deur die stygende kimberliet magma voordat dit stol. Hierdie studie ondersoek die afbreekmeganisme van granaat afkomstig van die mantel in die stygende kimberliet magma deur eksperimente uit te voer by drukke tussen 2 en 4GPa en temperature tussen 1100 en 1300°C. Die materiaal met die aanvang van die eksperimente was voorberei uit ‘n natuurlike hipabissale kimberliet wat soortgelyk is in samestelling as ‘n voorgestelde primêre Groep I kimberliet magma se samesteling. Vyf gewigspresent (5 wt %) granaat, verkry van n natuurlike peridotiet, is bygevoeg tot die kimberlietmateriaal. Daar is gevind dat granaat nie ‘n stabiele ewewigsfase is in die kimberliet magma binne die temperatuur-en drukstoestande ondersoek nie. Groot volumes flogopiet teenwoordig in die eksperimente dui daarop dat granaat inkongruent smelt in die kimberliet magma om flogopiet as ‘n peritektiese produk te vorm te same met ‘n smelt wat neig na ‘n karbonatitese smeltsamestelling. Die bevinding is in teenstelling met meer SiO2-ryke smeltsamestellings as gevolg van die inkongruente afbreek van beide ortopirokseen en omfasiet in die kimberliet magma. Die gevolg van die meer karbonatitese smeltsamestellings, is ‘n toename in die oplosbaarheid van CO2 in die smelt in die vorm van karbonaat (CO32-). Die bevinding het nagevolge vir beide kimberliet stygingsmeganismes asook die oplosbaarheid van diamant tydens die vervoer van diamant deur kimberliet magmas.

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This dissertation consists of four chapters, each of which is either published in a peer-reviewed journal, or in submission. These chapters developed from the testing of the hypothesis that the lithospheric mantle contains significant magnetic regions that contribute to the magnetism observed/measured, either at or close to the Earth’s surface, or from satellite data. Chapter 1 “Eight good reasons why the mantle could be magnetic” (2014) published in Tectonophysics by Ferré, Friedman, Martín-Hernández, Feinberg, Till, Ionov and Conder, addresses the motivation for this project and establishes the probability of upper mantle contribution to magnetic anomalies. My role with this manuscript was to produce figures using my previously collected data (Figures 2, 4, and 6), compile known data on the magnetic properties of minerals in mantle peridotites (Table 1), provide discussion for and against each argument made, and edited the manuscript. Chapter 2 “Remanent magnetization in fresh xenoliths derived from combined demagnetization experiments: Magnetic mineralogy, origin and implications for mantle sources of magnetic anomalies” (2014) published in Tectonophysics by Martín-Hernández, Ferré, and Friedman, investigates the natural remanent magnetization of mantle xenoliths. Notably, it establishes that the natural remanent magnetization of these xenoliths is derived from a thermoremanent magnetization (primary) and not from chemical remanent magnetization (secondary) origin. My primary role in this study was to provide preliminary magnetic and petrologic data and analysis of the samples. Secondary responsibilities were to prepare the samples, edit the manuscript and provide discussion on the results. Chapter 3 “Craton vs. rift uppermost mantle contributions to magnetic anomalies in the United States interior” (2014) published in Tectonophysics by Friedman, Feinberg, Ferré, Demory, Martín-Hernández, Conder, and Rochette begins to compare magnetic properties across different tectonic settings. The metasomatized cratonic upper mantle of the United States interior contains ferromagnetic phases that exist at temperatures lower than the Curie temperature. This upper mantle would likely contribute to magnetic anomalies. Alternatively, the high geotherm and sulfide-rich mantle near the Rio Grande Rift precludes this area from mantle contribution to magnetic anomalies. As first author I prepared samples, ran experiments, processed data, produced figures, wrote the manuscript and applied for funding. Chapter 4 “What is magnetic in the mantle wedge?” (2015) submitted to Geology, examines the mantle wedge beneath multiple island arcs. Magnetic anomalies in island arc settings have been attributed to a serpentinized mantle wedge. While this material is not available to test, metasomatized mantle, common to the mantle wedge, is available. Metasomatized mantle is mostly paramagnetic, and thus supports that stepwise dehydration of a subducting slab may produce positive and negative anomalies in the mantle wedge. As first author I prepared samples, ran experiments, processed data, produced figures, wrote the manuscript and applied for funding.

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

Mantle pyroxenites are important components of mantle rocks, because they provide important insights into bulk composition and heterogeneity of the Earth's upper mantle and therefore, direct evidence of mantle evolution throughout the Earth history. Studied pyroxenites from localities Bečváry (the Kutná Hora Complex), Níhov, Nové Dvory, Mohelno, Horní Kounice, Karlstteten and Meidling (the Gföhl Unit of the Moldanubian Zone) occur as dykes and/or layers within spinel and garnet peridotites from the Bohemian massif. Whole-rock concentrations of rare earth (REE) and other trace elements in studied pyroxenites yield extreme variability, which most likely reflect: 1) variable garnet/clinopyroxene ratios in bulk rocks, 2) different degree of fractionation of parental melts and 3) different concentrations trace elements in the source host material. Pyroxenites from locality Mohelno (LREE-depleted) may be derived from depleted or only slightly enriched suboceanic mantle. In contrast, pyroxenites from Karlstetten, Meidling, Horní Kounice and Nové Dvory (LREE-enriched) crystallized from the melts derived from enriched mantle source with possibly significant contribution of recycled crust. Studied pyroxenites are characterized by extremely variable 187 Os/188 Os ratios. While the pyroxenites from Mohelno and...

Thesis advisor: John E. Ebel / Thesis advisor: John C. Hepburn / Using the seismic refraction technique with a least squares inversion methodology, arrival time data from 1985 to the present are analyzed to delineate, with improved spatial resolution, the upper mantle P-velocity structure throughout northeastern North America (NENA). A total of one hundred and sixty-eight earthquakes are analyzed utilizing over one hundred seismic stations throughout NENA. Seismic data analyzed between 200 - 400 km, 400 - 600 km, and 600+ km throughout NENA are used to study the increase in velocity with depth in the upper mantle. A jackknife analysis was carried out to put constraints on the uncertainties of the velocity measurements. The P-wave velocity of the upper mantle through the New England Appalachians is found to be uniformly 7.94 - 8.07 km/s at depths down to 75 km. Upper mantle Pn velocities throughout the southeastern Grenville Province show velocities ranging from 8.15 km/s to 8.54 km/s as epicentral distances increase. Uncertainties of P velocities range from 0.01- 0.12 km/s. Based on laboratory measurements of simulated upper mantle conditions and the orogenic history of the Grenville Province and northern Appalachians, upper mantle mineral compositions of eclogite (Grenville Province) and pyroxenite (northern Appalachians) are proposed to be the factor controlling seismic velocity variation in the upper mantle. Variations in upper mantle temperatures between the Grenville Province and northern Appalachians are ruled out as affecting the difference in upper mantle velocities between southeastern Canada and New England. / Thesis (MS) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Geology and Geophysics.

The České středohoří Mts. forming the most important and active part of Ohře/Eger rift are formed by volcanic rocks of Tertiary age containing upper mantle xenoliths which can provide us information about internal structure of upper mantle. This diploma thesis is focused on the study of mantle xenoliths from this part of Bohemian massif close to the city Litoměřice (4 locations: Dobkovičky, Prackovice, Kuzov, Medvědí hill) and for comparison another 4 locations of mantle xenoliths from the northern Bohemia locaties Brtníky in Šluknovský ledge, Kraslice and Zámeček at Fláje in Krušné Mts. and Venuše volcano in Nízký Jeseník at Bruntálská Highlands. Almost all studied xenoliths are spinel harzburgites or lherzolites with mineral association olivine + orthopyroxene + clinopyroxene + spinel (the most often Cr-spinel). They have usually protogranular texture followed by porfyroclastic texture (porfyroclasts are represented by olivine, orthopyroxene and in some cases also by clinopyroxene, and fine-grained matrix of all these minerals with olivine showing undulose extinguishes). An equigranular texture is the least common. Host rock of the xenoliths is always basanite. The most abundant mineral in peridotite xenoliths is olivine with #Mg value 89,4-91,5; followed by orthopyroxene with #Mg value 90,8-92,1...