Éditeur(s) : HAL CCSD European Geosciences Union
Résumé : International audience
We calculated the seismic properties of 47 mantle xenoliths from 9 kimberlitic pipes in the Kaapvaal craton based on their modal composition, the crystal-preferred orientations (CPO) of olivine, ortho- and clinopyroxene, and garnet, the Fe content of olivine, and the pressures and temperatures at which the rocks were equilibrated. These data allow constraining the variation of seismic anisotropy and velocities within the cratonic mantle. The fastest P and S2 wave propagation directions and the polarization of fast split shear waves (S1) are always subparallel to olivine [100] axes of maximum concentration, which marks the lineation (fossil flow direction). Seismic anisotropy is higher for high olivine contents and stronger CPO. Maximum P wave azimuthal anisotropy (AVp) ranges between 2.5 and 10.2% and the maximum S wave polarization anisotropy (AVs), between 2.7 and 8%. Changes in olivine CPO symmetry result in minor variations in the seismic anisotropy patterns, mainly in the apparent isotropy directions for shear wave splitting. Seismic properties averaged over 20 km-thick depth sections are, therefore, very homogeneous. Based on these data, we predict the anisotropy that would be measured by SKS, Rayleigh (SV) and Love (SH) waves for five endmember orientations of the foliation and lineation. Comparison to seismic anisotropy data from the Kaapvaal shows that the coherent fast directions, but low delay times imaged by SKS studies, and the low azimuthal anisotropy with with the horizontally polarized S waves (SH) faster than the vertically polarized S wave (SV) measured using surface waves are best explained by homogeneously dipping (45°) foliations and lineations in the cratonic mantle lithosphere. Laterally or vertically varying foliation and lineation orientations with a dominantly NW-SE trend might also explain the low measured anisotropies, but this model should also result in backazimuthal variability of the SKS splitting data, not reported in the seismological data. The strong compositional heterogeneity of the Kaapvaal peridotite xenoliths results in up to 3% variation in density and in up to 2.3% variation of Vp, Vs, and Vp / Vs ratio. Fe depletion by melt extraction increases Vp and Vs, but decreases the Vp / Vs ratio and density. Orthopyroxene enrichment due to metasomatism decreases the density and Vp, strongly reducing the Vp / Vs ratio. Garnet enrichment, which was also attributed to metasomatism, increases the density, and in a lesser extent Vp and the Vp / Vs ratio. Comparison of density and seismic velocity profiles calculated using the xenoliths' compositions and equilibration conditions to seismological data in the Kaapvaal highlights that (i) the thickness of the craton is underestimated in some seismic studies and reaches at least 180 km, (ii) the deep sheared peridotites represent very local modifications caused and oversampled by kimberlites, and (iii) seismological models probably underestimate the compositional heterogeneity in the Kaapvaal mantle root, which occurs at a scale much smaller than the one that may be sampled seismologically.
ISSN: 1869-9510

hal-01053516
https://hal.archives-ouvertes.fr/hal-01053516
DOI : 10.5194/se-5-45-2014

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We have analyzed the microstructures and crystal preferred orientations (CPO), and calculated the seismic properties of 53 mantle xenoliths from four localities within the North Tanzanian Divergence of the East African rift: two within the rift axis and two in the transverse volcanic belt. Olivine OH concentrations were measured in 15 xenoliths.Most samples have harzburgitic to dunitic compositions and high olivine Mg#. Microstructures and olivine CPO patterns vary strongly depending on the location. In-axis peridotites display mylonitic to porphyroclastic microstructures, which record recent deformation by dislocation creep. Highly stretched orthopyroxenes in mylonites indicate that the deformation was initiated under high stress and probably low temperature. Orthopyroxene replacement by olivine in mylonitic and porphyroclastic peridotites suggest syn-kinematic melt–rock reactions and further deformation under near-solidus conditions. Exsolutions in orthopyroxene imply significant cooling between melt-assisted deformation and xenolith extraction. Late metasomatism is evidenced by the occurrence of veins crosscutting the microstructure and interstitial clinopyroxene and phlogopite. Axial-[100] olivine CPOs predominate, suggesting activation of the high temperature, low pressure [100] {0kl} slip systems and, probably, transtensional deformation. In the volcanic belt, Lashaine peridotites display very coarse-granular textures, indicating deformation by dislocation creep under low deviatoric stress conditions followed by annealing. Axial-[010] olivine CPOs are consistent with transpressional deformation or simultaneous activation of the [100](010) and [001](010) slip systems. Intermediate microstructures and CPOs in Olmani suggests heterogeneous deformation within the volcanic belt. Olivine OH concentrations range between 2 and 12 ppm wt. H2O. No systematic variations are observed between in- and off-axis samples. Maximum P wave azimuthal anisotropy (AVp) ranges between 3.3 and 18.4%, and the maximum S wave polarization anisotropy (AVs) between 2.3 and 13.2%. Comparison between seismic properties of in-axis peridotites and SKS splitting data suggests transtensional deformation in the lithospheric mantle beneath the rift.
ISSN: 0040-1951

hal-01121552
https://hal.archives-ouvertes.fr/hal-01121552
DOI : 10.1016/j.tecto.2014.11.011

Éditeur(s) : HAL CCSD
Résumé : This study provides additional constraints on the relations between deformation, hydration and percolation of fluids and melts in the subcontinental lithospheric mantle beneath a craton and a rift, as well as their implication on its geodynamical behaviour. I have analysed the microstructures, the CPOs, and the hydrogen content of mantle xenoliths from the Kaapvaal craton, and two sets of xenoliths from different localities along the East African Rift (North Tanzanian Divergence and SE Ethiopia).The coarse-granular microstructures and the well-defined CPOs in Kaapvaal peridotites suggest a deformation followed by a long quiescence time. Orthorhombic olivine CPOs predominates, but axial-[100] and axial-[010] are also measured. Cratonic peridotites record multiple metasomatic episodes, leading to a significant compositional heterogeneity, which cannot be imaged by seismic studies. Olivine hydrogen contents are variable, but tend to increase until 150 km depth, reaching up to 50 ppm wt. H2O. The deeper samples are almost dry. Piston-cylinder experiments on hydrogen diffusion between a volatile-rich kimberlitic melt and forsterite suggest that the presence of CO2 in the system could significantly decrease water fugacity and thus forsterite hydration. These experimental results indicate that the hydrogen contents measured in olivine were acquired during a metasomatic event rather than during xenolith extraction by kimberlites. However, this metasomatism was not followed by remobilization of the cratonic root.In the North Tanzanian Divergence, localities within the rift axis and the volcanic transverse belt (Lashaine and Olmani) show significant differences in microstructures and olivine CPO patterns. In Lashaine, coarse-granular microstructures and orthorhombic to axial-[100] CPO patterns in olivine can be explained by transpressional deformation during the formation of the Mozambique belt, or by the occurrence of a remnant of a cratonic domain embedded within the Mozambique belt. Within the rift axis, porphyroclastic to mylonitic microstructures suggest a recent rift-related deformation accompanied by syn-kinematic melt-rock reactions, and followed by variable annealing. The strong heterogeneity in microstructures and olivine CPO suggests that this deformation was acquired during multiple tectonic events probably linked to episodic magma percolation, separated by quiescence episodes. The axial-[100] patterns in olivine and the oblique fast directions reported by SKS studies are coherent with transtensional deformation within the lithospheric mantle beneath the rift.The peridotites from SE Ethiopia are less recrystallized than the rift-axis Tanzanian peridotites, displaying coarse-porphyroclastic microstructures. Microstructures and orthorhombic CPOs in olivine suggest syn- to post-metasomatic deformation. S-waves polarization anisotropies calculated for these samples cannot explain alone the delay times reported by SKS studies in this part of the East-African Rift.
Les travaux réalisés durant cette thèse apportent de nouvelles contraintes sur les relations entre déformation, hydratation et percolation de fluides et/ou de magmas dans le manteau subcontinental sous un craton et sous un rift, et leurs implications sur son comportement rhéologique. Il repose sur l'analyse des microstructures, des OPRs et des teneurs en hydrogène de xénolites mantelliques du craton du Kaapvaal, et sur deux séries de xénolites provenant de différentes localités le long du rift Est-Africain (Divergence Nord Tanzanienne et SE de l'Ethiopie). Les microstructures granulaires à gros grains et les OPRs bien définies des péridotites du craton du Kaapvaal sont cohérentes avec un épisode de déformation suivi d'une longue période de quiescence. Les OPRs de l'olivine sont majoritairement à symétrie orthorhombique, mais des symétries axiale-[100] et axiale-[010] sont aussi mesurées. Les péridotites cratoniques enregistrent de multiples épisodes métasomatiques, ayant entraîné une hétérogénéité de compositions à petite échelle ne pouvant être détectée par les études sismiques. Les teneurs en hydrogène mesurées dans l'olivine sont variables, mais ont tendance à augmenter jusqu'à 150 km de profondeur, atteignant alors jusqu'à 50 ppm wt. H2O. En dessous de cette profondeur, les échantillons montrent des teneurs en hydrogène très faibles. Les expériences réalisées en piston-cylindre sur la diffusion de l'hydrogène issue d'un liquide kimberlitique vers de la forstérite suggèrent que la fugacité en eau pourrait fortement être diminuée par la présence de CO2, empêchant l'hydratation de l'olivine durant extraction des xénolites par les kimberlites. Ces résultats expérimentaux suggèrent que les teneurs en hydrogène dans l'olivine des péridotites du craton du Kaapvaal ont été acquises durant un épisode métasomatique en profondeur et non pendant leur extraction par les kimberlites. Ces teneurs n'ont toutefois pas à ce jour entraîné de remobilisation de la racine cratonique. Enfin, le calcul des propriétés sismiques des péridotites cratoniques révèle que les anisotropies générées par les OPRs de ces échantillons sont suffisantes pour expliquer les anisotropies mesurées par les ondes SKS et les ondes de surface.Les xénolites de la Divergence Nord-Tanzanienne, montrent des variations significatives de microstructures et d'OPR de l'olivine entre les péridotites des localités dans l'axe du rift et celles de la chaîne volcanique transverse (Lashaine et Olmani). A Lashaine, les microstructures granulaires à gros grains et les OPRs de type orthorhombique et axial-[010] peuvent être expliquée par une déformation en transpression liée à la formation de la chaîne Mozambique ou par la présence d'une relique d'un domaine cratonique à l'intérieur de la chaîne Mozambique. Dans l'axe du rift, les microstructures porphyroclastiques à mylonitiques suggèrent une déformation plus récente, accompagnée de réactions magma-roche sous des conditions proches du solidus, suivie d'un recuit variable. L'hétérogénéité des microstructures enregistrées par les échantillons du rift suggère de multiples épisodes de déformation localisée, probablement liés à l'injection percolation épisodique de magmas, espacés de périodes d'accalmie. Les OPRs de l'olivine de type axial-[100] et l'orientation des directions de polarisation des ondes SKS suggère que le rift s'est formé en régime de transtension Les péridotites du Sud-Est de l’Éthiopie présentent des microstructures porphyroclastiques à gros grains moins recristallisées qu'en Tanzanie. Les microstructures et les OPRs principalement de type orthorhombique suggèrent une déformation syn- à post-métasomatisme. Les anisotropies de polarisation des ondes S calculées pour ces échantillons sont insuffisantes pour expliquer à elles seules les déphasages des ondes SKS dans cette partie du rift.
https://tel.archives-ouvertes.fr/tel-01684759

Droits : info:eu-repo/semantics/OpenAccess
NNT : 2014MON20139
tel-01684759
https://tel.archives-ouvertes.fr/tel-01684759
https://tel.archives-ouvertes.fr/tel-01684759/document
https://tel.archives-ouvertes.fr/tel-01684759/file/43443_BAPTISTE_2014_archivage_cor.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We explore the relations between deformation, annealing, and melt percolation during rifting and the effect of these processes on seismic anisotropy by analyzing the microstructures and crystal preferred orientations (CPO) in a suite of mantle xenoliths from Mega, in the southern end of the Ethiopian rift. Previous geochemical studies on these xenoliths showed evidence for interactions with variable melt types and volumes during the rifting process. The peridotites have dominantly coarse-porphyroclastic microstructures, but coarse granular or partially recrystallized microstructures also occur. The olivine CPO, characterized by orthorhombic to fiber-[100] patterns and moderate intensities, the common occurrence of (100) tilt walls, and the predominance of <0vw> rotation axes accommodating low angle misorientations in olivine support deformation by dislocation creep with dominant activation of the [100](010) system. Annealing (static recrystallization) of variable intensity followed this deformation. Modal enrichment in pyroxenes in > 60% of the studied peridotites corroborates extensive, but spatially heterogeneous reactive melt percolation leading to refertilization of the lithospheric mantle beneath the southern Ethiopian rift. The common interstitial shapes of the pyroxenes and lack of correlation between the pyroxenes and the olivine CPOs in many samples suggest that part of the refertilization is post-kinematic. However, there is no simple relation between reactive melt percolation and annealing of the olivine deformation microstructure. Comparison with data from other xenolith localities points to changes in the metasomatic imprint in the lithospheric mantle along the East African rift system correlated with the evolution in the rift maturity. Seismic properties averaged over all samples show typical lithospheric mantle patterns with fast propagation of P- and polarization of the fast S-waves parallel to the lineation. The anisotropy is moderate (< 6% for P-waves and < 4% for S-waves), implying that aligned melt must contribute to the strong seismic anisotropy measured in the Main Ethiopian rift, north of Mega.
ISSN: 0040-1951

hal-01355918
https://hal.archives-ouvertes.fr/hal-01355918
DOI : 10.1016/j.tecto.2016.05.027

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
To constrain the relations between deformation and metasomatism in the subcratonic lithospheric mantle, we have analyzed the microstructures and crystal preferred orientations in 50 mantle xenoliths from the Kaapvaal craton. Water contents in olivine and pyroxenes were measured in 14 samples equilibrated at different depths. Coarse-granular microstructures recording deformation by dislocation creep followed by annealing predominate. Mylonitic (sheared) peridotites with partially or totally recrystallized microstructures are however common below 140 km. Refractory compositions predominate, but multiple metasomatic events resulted in orthopyroxene enrichment or secondary crystallization of clinopyroxene and phlogopite. Coherent orthopyroxene and olivine CPO in most coarse-grained peridotites implies in pre- to syn-kinematic orthopyroxene enrichment or epitaxial growth on primary orthopyroxene. Undeformed, interstitial orthopyroxene, clinopyroxene, and phlogopite with random orientations in coarse-grained peridotites record post-kinematic modal metasomatic events. Deformation of these phases in the sheared peridotites implies that mylonitization results from a later event, which affected locally the deep cratonic lithosphere. Olivine CPO recording dominant [100] glide predominate at all depths. Only two samples, equilibrated at ~ 3.3 GPa show olivine [001] and orthopyroxene [001] axes subparallel, suggesting dominant [001] glide. Water contents in olivine are maximum (150 wt. ppm H2O) in peridotites equilibrated at ~ 160 km depth. Peridotites equilibrated below 180 km depth are, in contrast, almost dry. Lack of correlation between olivine mg# and water content indicates that the high water contents in olivine record re-hydration after the extensive partial melting, which produced the cratonic root. The vertical variation in water contents in olivine observed in the Kaapvaal peridotites may result from hydrogen addition or loss during extraction by the kimberlites. Comparison with magnetotelluric electrical conductivity data suggests, however, that the observed vertical variation of water contents in olivine may be representative of the present-day state of the Kaapvaal mantle, implying that extensive metasomatism resulted in hydration of the cratonic mantle at intermediate depths. The annealed microstructures of Kaapvaal peridotites indicate however that this metasomatism was not followed by remobilization of the cratonic root.
ISSN: 0024-4937

hal-00750328
https://hal.archives-ouvertes.fr/hal-00750328
DOI : 10.1016/j.lithos.2012.05.001

Éditeur(s) : HAL CCSD Mineralogical Society of America
Résumé : International audience
To test if hydrogen incorporation by ionic diffusion can occur between a volatile-rich kimberlitic liquid and forsterite, results of high-pressure and high-temperature experiments using a piston-cylinder apparatus at 1200–1300 °C and 1 GPa for durations of 1 min, 5 h, and 23 h, are reported here. Kimberlitic liquid in the system CaO-MgO-Al2O3-SiO2-CO2-H2O and synthetic forsterite single crystals were chosen as a first simplification of the complex natural kimberlite composition. Unpolarized Fourier transform infrared spectroscopy was used to quantify the concentrations of OH in the crystallographically oriented forsterite. Scanning electron microscopy, electron backscattered diffraction, electron microprobe analyses, and transmission electron microscopy were performed to identify the run products. After 5 and 23 h, a forsterite overgrowth crystallized with the same orientation as the initial forsterite single crystal. The kimberlitic liquid has crystallized as micrometer-scale euhedral forsterite neocrystals with random crystallographic orientations, as well as a nanoscale aluminous phase and a calcic phase. Despite theoretical water-saturation of the system and long duration, none of the initial forsterite single crystals display signs of hydration such as hydrogen diffusion profile from the border toward the center of the crystal. Most likely, the presence of CO2 in the system has lowered the H2O fugacity to such an extent that there is no significant hydration of the starting forsterite single crystal or its overgrowth. Also, the presence of CO2 enhances rapid forsterite crystal growth. Forsterite growth rate is around 2 × 108 μm3/h at 1250 °C. These experimental results suggest a deep mantle origin of the high OH content found in natural mantle-derived xenoliths transported in kimberlites, as reported from the Kaapvaal craton. In agreement with previous studies, it also points out to the fact that significant hydration must take place in a CO2-poor environment.
ISSN: 0003-004X

hal-01217157
https://hal.archives-ouvertes.fr/hal-01217157
DOI : 10.2138/am-2015-5200