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In subduction zones, sediments and hydrothermally altered oceanic crust, which together form part of the subducting slab, contribute to the chemical composition of lavas erupted at the surface to form volcanic arcs. Transport of this material from the slab to the overlying mantle wedge is thought to involve discreet melts and fluids that are released from various portions of the slab. We use a meta-analysis of geochemical data from eight globally representative arcs to show that melts and fluids from individual slab components cannot be responsible for the formation of arc lavas. Instead, the data are compatible with models that first invoke physical mixing of slab components and the mantle wedge, widely referred to as high-pressure mélange, before arc magmas are generated.
This chapter reviews the boron isotopic composition of the ocean floor, including pristine igneous oceanic crust such as mid-ocean ridge basalts and ocean island basalts and their implications for the B isotopic composition of the mantle. The chapter further discusses the B isotopic effects of assimilation of altered crustal materials in mantle-derived magmas. The systematics of seawater alteration on oceanic rocks are discussed, including sediments, igneous crust and serpentinization of ultramafic rocks and the respective marine hydrothermal vent fluids. The chapter concludes with a discussion of the secular evolution of the B isotopic composition of seawater.
We present a study characterizing aerosol particles resulting from a skyscraper blasting. High mass concentrations with a maximum of 844.9 μg m-3 were present for a short time period of approximately 15 minutes. They result in a day mean of 32.6 μg m-3 compared to a 27.6 μg m-3 background not exceeding the 50 μg m-3 EU maximum permissive value. The increase in particle number concentration was less pronounced with a maximum concentration of 6.9 ⋅ 104 cm-3 compared to the local background value of 1.8 ⋅ 104 cm-3. The size-resolved number concentration shows a single mode of ultrafine particles at approximately 93 nm. The spatial distribution of deposited dust was investigated with Bergerhoff glass collection vessels, showing a decrease with distance. In the deposited dust samples the concentrations of twelve metals was determined, non of them exceeded the regional background concentrations significantly. The chemical composition of individual particles emitted by the demolition was studied by Scanning Electron Microscopy. They were mainly concrete and steel particles, with 60% calcium carbonates, 19% calcium sulfates, 19% silicates and 2% steel. In energy-dispersive X-Ray Spectroscopy, no fibers like asbestos were observed. Using a broad spectrum of instruments and methods, we obtain comprehensive characterization of the particles emitted by the demolition.
The African continent is regularly portrayed as an indolent space with a well-known reputation as a chaotic continent. Viewed as lacking vision, means and capacities, Africa is perceived at best as a place that is marked by a permanent status quo, stagnation, or in worst case scenarios, as a declining continent. Various references to the continent are synonymous with famine, poverty, war, etc. Such portrayals are all the more intriguing given that the continent is known for its abundant natural resources, such as timber, oil, natural gas, minerals, etc., whose reserves are, moreover, not well known both by the African people and their leaders. As a result, there is still much progress to be made in tapping into the resources in order to improve the daily lives of African citizens.
In such a context dominated by infantile carelessness throughout the continent, the interventions of actors from outside the continent are the only hopes of bringing some vitality to this continent which is cloaked in "la grande nuit – the great darkness" (Mbembé 2013). Thus during the main sequences of recent history, representing different forms of Western penetration and activity on the African continent (slavery, imperialism, colonization), all the Western world’s contributions have obviously not sufficed to boost Africa and take it out of its never ending childhood. It has remained just as passive and apathetic today as it was yesterday.
The attraction of Asian actors to the continent is even more recent. And consistent with its abovementioned indolence, Africa is seen as an easy and defenceless prey for the Korean, Japanese, Indian, Malaysian, or Chinese conquerors. In the latter case, the insatiable appetite for natural resources whose reserves are being rapidly depleted is the cornerstone of their foreign aid policy. This led China to colonize the continent, showing a preference for Pariah Regimes which held no appeal for the West, by sending an army of workers to extract those resources (Lum et al. 2009), in defiance of all national and international regulations and based on completely opaque contracts.
Although the concept of African Agency was rapidly developed in several African countries, the aim of this study was more specific to Cameroon’s mining sector in which different entrepreneurs from abroad got involved over time. The thesis investigates whether indigenous citizens took part in any way in the development of mining projects in the country. Thus, the work assesses and analyses actions and reactions initiated and undertaken by local people in the context of China’s presence within Cameroon’s mining sector to promote and advance their interests over those of foreign investors. In addition, the author has no knowledge of any other study investigating African Agency in the mining sector as a whole in Cameroon.
In conducting this study, a multi-method research framework was developed including a series of methods used to collect data and analyse concepts of African Agency associated Political Ecology as they developed within Cameroon’s mining sector. Specifically, those methods comprised quantitative research when it came to collecting data using a positivist and empirical approach constructed by deducing evidence from statistical data collected by means of the 167 questionnaire surveys administered to local inhabitants and workers randomly selected on mining sites and in riparian communities. The questionnaires helped to capture Cameroonians' perceptions of the recent phenomenon of the gradual but significant influx of international actors and precisely Chinese players in the mining sector on the one hand, and on the other hand, observational data was collected across the GVC as developed in the Betare-Oya region. As a complement to the former technique, qualitative methods helped to study and deepen understanding of human behaviour and the social world in a holistic perspective through individual interviews, focus groups, and direct observations on the ground. In addition, the spatial analysis method based on the land use classification technique served to detect changes to land use/land cover that have been brought on by mechanised mining activities undertaken in this region. The sequencing of data collected and their processing from a ground theory perspective led to the formulation and specification of Cameroon’s Ecological Agency theory.
One of the earliest steps of this work consisted in a literature review and in placing the African Agency concept in a broader context. It then led to the state of the art, specifications about research content of the work and the main theories undergirding this thesis. Before examining developments that emerged during the last decade, a historical perspective was provided to the topic in order to show how African societies started mining operations and how they dealt with foreign partners interested in their mining resources. The aim was to show that while Western imperialism presented a challenge for the sector, it did not erase local participation, even despite the constraints associated with such involvement.
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A recent CLOUD (Cosmics Leaving OUtdoor Droplets) chamber study showed that sulfuric acid and dimethylamine produce new aerosols very efficiently, and yield particle formation rates that are compatible with boundary layer observations. These previously published new particle formation (NPF) rates are re-analyzed in the present study with an advanced method. The results show that the NPF rates at 1.7 nm are more than a factor of 10 faster than previously published due to earlier approximations in correcting particle measurements made at larger detection threshold. The revised NPF rates agree almost perfectly with calculated rates from a kinetic aerosol model at different sizes (1.7 nm and 4.3 nm mobility diameter). In addition, modeled and measured size distributions show good agreement over a wide range (up to ca. 30 nm). Furthermore, the aerosol model is modified such that evaporation rates for some clusters can be taken into account; these evaporation rates were previously published from a flow tube study. Using this model, the findings from the present study and the flow tube experiment can be brought into good agreement. This confirms that nucleation proceeds at rates that are compatible with collision-controlled (a.k.a. kinetically-controlled) new particle formation for the conditions during the CLOUD7 experiment (278 K, 38% RH, sulfuric acid concentration between 1×106 and 3×107 cm-3 and dimethylamine mixing ratio of ~40 pptv). Finally, the simulation of atmospheric new particle formation reveals that even tiny mixing ratios of dimethylamine (0.1 pptv) yield NPF rates that could explain significant boundary layer particle formation. This highlights the need for improved speciation and quantification techniques for atmospheric gas-phase amine measurements.
Chlorine and bromine atoms can lead to catalytic destruction of ozone in the stratosphere. Therefore the use and production of ozone depleting substances (ODS) containing chlorine and bromine is regulated by the Montreal Protocol to protect the ozone layer. Equivalent Effective Stratospheric Chlorine (EESC) has been adapted as an appropriate metric to describe the combined effects of chlorine and bromine released from halocarbons on stratospheric ozone. Here we revisit the concept of calculating EESC. We derive a new formulation of EESC based on an advanced concept of ODS propagation into the stratosphere and reactive halogen release. A new transit time distribution is introduced in which the age spectrum for an inert tracer is weighted with the release function for inorganic halogen from the source gases. This distribution is termed the “release time distribution”. The improved formulation shows that EESC levels in the year 1980 for the mid latitude lower stratosphere were significantly lower than previously calculated. 1980 marks the year commonly defined as the onset of anthropogenic ozone depletion in the stratosphere. Assuming that the EESC value must return to the same level in order for ozone to fully recover, we show that it will take more than 10 years longer than currently assumed in this region of the stratosphere. Based on the improved formulation, EESC level at mid-latitudes will reach this landmark only in 2060. We also present a range of sensitivity studies to investigate the effect of changes and uncertainties in the fractional release factors and in the assumptions on the shape of the release time distributions. We conclude that, under the assumptions that all other atmospheric parameters like stratospheric dynamics and chemistry are unchanged, the recovery of mid latitude stratospheric ozone would be expected to be delayed by about a 10 years, in a similar way as EESC.
The East African Rift System (EARS) was initiated in the Eocene epoch between 50 and 21 Ma probably due to the influence of mantle plumes that caused volcanism, flood basalts and rifting extensions in Ethiopa and the Afar region. As a result of magmatic intrusions and adiabatic decompression melting within the lithosphere caused by the impact of the Kenya plume, there was a southward propagation of the EARS of about 30 – 15 Ma from Ethiopia to Kenya, which coincide with the occurrence of volcanism. The EARS developed towards the south along the margins of the Tanzania Craton between 15 and 8 Ma. Previous findings of low-velocity anomalies within the upper mantle and the mantle transition zone indicate an upwelling of hot mantle material in the vicinity of the Afar region and the East African Rift. This study includes the analysis of P- and S-receiver functions in order to determine further impacts on the lithosphere from below. The aim was to determine the topographic undulations of further boundary layers and to identify their variability owing to the rifting processes and the formation of the EARS. The study area included the Tanzania Craton and the surrounding rift branches of the East African Rift System.
The region of the Rwenzori Mountains can be analysed in detail because of the large dataset of the RiftLink project. The use of the P-receiver function technique and the H-K stacking method enabled to determine different vP /vS ratios depending on the tectonic setting in the Rwenzori region: Rift shoulders (vP /vS =1.74), Albert Rift segment (vP /vS =1.80), Edward Rift segment (vP /vS =1.87) and Rwenzori Mountains (vP /vS =1.86). To determine the topography of the Moho, it is necessary to take into account the thickness of the sedimentary layer, the surface topography, the azimuthal variations in crustal thickness and the impact of local anomalies. After correcting these effects on the Moho depths, significant variations in Moho topography could be determined. The Moho depths range from 29 to 39 km beneath the rift shoulders of the Albertine Rift. Within the rift valley, the crustal thickness varies between 25 – 31 km in the Edward Rift segment and 22 – 30 km in the Albert Rift segment. An averaged crustal thickness of about 26 km within the rift valley indicates the lack of the crustal root beneath the Rwenzoris. Similar variations in crustal thickness were determined by using an automatic procedure for analysing S-receiver functions that was developed in this study.
The S-receiver functions are created by applying a rotation criterion in order to rotate the Z, N and E components into the L, Q and T components. It is necessary to perform trial rotations using different incident and azimuth angles to determine the correct rotation angles. The latter are identified by the use of the rotation criterion, including the amplitude ratio of the converted Moho signal to the direct S/SKS-wave signal. The L component is rotated correctly in the direction of the incident shear wave in the case of the maximum amplitude ratio. After analysing the frequency content of the receiver functions in order to sort out harmonic and long-periodic traces, the individual Moho signals are checked for consistency in order to remove atypic signals. To increase the signal-to-noise ratios on the traces, the S-receiver functions are stacked. For this purpose, the signals of the direct shear waves must originate from similar epicenters. On the basis of similar ray paths, the receiver functions show comparable waveforms and converted signals. To perform the stacking procedure, it is necessary to merge the datasets of the adjacent stations in order to obtain a sufficient number of receiver functions. This analysis is based on the assumption that the incident seismic waves arriving at the adjacent stations penetrate to some extent the same underground structures in the case of similar wave propagation paths. This approach accounts for the fact that the converted signals do not result exclusively from the piercing points at the boundary layers. Further signals originate from the conversions at the boundary layer within the Fresnel Zone. The piercing points are derived from the significant signals in the receiver functions. Depending on the order of arrival of the converted phases on the traces, the signals are attributed to the theoretical discontinuities DIS1, DIS2, DIS3 and DIS4. However, partly due to the low signal-to-noise ratios on the traces, it is difficult to identify the real conversions on the traces and to ensure that the converted signals are attributed to the correct boundary layers. For this reason, it is necessary to check the consistency of the conversion depths among each other. In the case of inconsistent conversion depths, the corresponding signals are either adjusted to another seismic boundary layer or removed from the dataset. To verify the functionality of the automatic procedure and to determine the resolvability with respect to two boundary layers, several models are tested including horizontal and dipping discontinuities. To resolve distinct discontinuities, their depths must differ by at least 60 km, otherwise, due to similar depth ranges of the different boundary layers, the converted signals cannot be separated from each other. As a consequence, the converted signals that originate from different discontinuities are attributed to a single one. Further tests including break-off edges of seismic discontinuities are performed to check the attributions of the converted signals to the discontinuities. Owing to the varying number of boundary layers, the converted signals cannot be attributed to the discontinuities according to the order of their arrivals on the traces. It is necessary to correct their attributions to the seismic discontinuities in order to resolve the boundary layers.
The crust-mantle boundary and further discontinuities within the lithospheric mantle are investigated by applying this automatic procedure. Depending on the tectonic setting, the conversion depths of the Moho range from about 30 – 45 km beneath the western rift shoulder to 20 – 35 km within the rift valley up to 30 – 40 km beneath the eastern rift shoulder. The long wavelengths of the shear waves hamper the correct identification of the converted phases in the S-receiver functions. With respect to the relative differences in conversion depth, the topographic undulations of the crust-mantle boundary are consistent with the Moho depths derived from P-receiver functions. In contrast to the Rwenzori region, it is difficult to resolve completely the trend of the Moho in the remaining area of the East African Rift due to the small dataset provided by IRIS. The results exibit an increase in crustal thickness to up to 45 km in the region of the Cenozoic volcanics such as Virunga, Kivu, Rungwe and Kenya. The greatest Moho depths of more than 50 km are located near Mount Kilimanjaro. In addition to the Moho, the analysis of the S-receiver functions revealed two further boundary layers at depths of 60 – 140 km and 110 – 260 km, which are associated with a mid-lithospheric discontinuity and the lithosphere-asthenosphere boundary, respectively. The shallowest conversion depths of the LAB are focussed to small-scale regions within the rift branches, namely the northern Albertine Rift, the Chyulu Hills and the Mozambique Belt, which are located around the Tanzania Craton. The larger thickness of the lithosphere beneath the cratonic terrain indicates that the Tanzania Craton is not significantly eroded. However, there are indications that the lithosphere beneath the craton and the rift branches is penetrated by ascending asthenospheric melts to depths of up to 140 and 60 km, respectively. The top of the ascending melts is associated with the occurrence of the mid-lithospheric discontinuity. The shallowest conversion depths of this boundary layer (60 – 90 km) are related to the rifted areas of the EARS and the Cenozoic volcanic provinces, which are located along the Albertine Rift, the Kenya Rift and the Rukwa-Malawi rift zones. The deepest conversion depths of up to 140 km are related to the Rwenzori Belt, the Ugandan Basement Complex and the interior of the Tanzania Craton.
Over the last several decades, spinel-structured minerals with the chemical formula AB2O4 (where A and B stand for divalent and trivalent cations, respectively) have attracted more and more attention, particularly with regards to their breakdown at high pressures and temperatures and the nature of the so-called "post-spinel" phases. Spinel-structured phases with different endmember compositions, like magnetite (Fe3O4), hercynite (FeAl2O4) or spinel (MgAl2O4), are known to breakdown differently at high pressure-temperature conditions (e.g., Akaogi et al. 1999; Schollenbruch et al. 2010; Woodland et al. 2012). Such phases are of particular interest when they incorporate ferric (Fe3+) and ferrous (Fe2+) cations as this makes their stability sensitive to redox conditions. Since magnetite and magnesioferrite (MgFe3+ 2O4) have been found as inclusions in diamond (e.g., Stachel et al. 1998; Harte et al. 1999; Wirth et al. 2014; Palot et al. 2016; Jacob et al. 2016), understanding their phase relations is important for setting constraints on the conditions of their formation.
This study aimed to experimentally investigate the phase relations of Fe-Mg spinel-structured phases at conditions of the deep upper mantle and transition zone. Exploring the stability of new post-spinel phases and their characterization were also major goals of this study. Approaching a pyrolitic mantle composition by adding amounts of SiO2 in the system allowed constraints on the relevance of Fe-Mg post-spinel phases coexisting with mantle silicates to be made. ...