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Diamonds, Xenoliths and Kimberlites: A Small Window into the Earth’s Interior

How are diamonds found? On a macroscale, one unique way is by a Zeppelin Airship to identify kimberlites on the Earth’s surface. Kimberlite is the diamond bearing rock much younger than the diamonds itself and, therefore, only carrying them to the Earth’s surface. On a microscale the latest method, and one of the most promising way to distinguish diamond-bearing from non-diamond bearing kimberlites, is the use of synchrotron infrared radiation, as demonstrated recently at ANKA, Karlsruhe.Figures 1 & 2
The principal aim of our study is to develop a model describing the formation of diamonds. Hence, our project incorporates the study of the Earth’s interior. The formation of diamonds deep inside the Earth can be understood better by investigating the petrology and geodynamic setting of diamond bearing rock. Especially, new insights can be gained by studying hydrogen concentration in nominally OH free minerals, such as olivine, pyroxene and garnet, to calculate the ascent rates of diamond bearing xenoliths. The reason is thus kimberlites with high ascent rates are rich in diamonds, whereas kimberlites with low ascent rates are poor in diamonds.
Two decades ago, water in nominally anhydrous minerals was studied by only very few scientists. Now it is part of the main stream of research in mineralogy, geochemistry and geophysics. Nominally anhydrous minerals1 (NAMS) constitute the main reservoir of water in the Earth’s mantle. Very likely, this reservoir is comparable in size to the bulk of all the oceans combined. The exchange of water between the mantle and the surface of the Earth may be responsible for slow variations in sea level. Even traces of water in minerals, such as olivine, drastically reduce mechanical strength 2-5, with major consequences for mantle convection and the formation of minerals, such as diamonds. Without traces of water in olivine, there would be no plate tectonics on Earth. Partitioning of water between partial melts and NAMS controls the water content and therefore, the mobility of mantle melts. Some models suggest that the chemical evolution of the Earth’s mantle is largely controlled by water partitioning between NAMS. Water partitioning probably controls the widths of some seismic discontinuities in the Earth’s mantle. Moreover, water has a major impact on the bulk and shear moduli of minerals and, therefore, on seismic velocities. For more information about the Earth’s mantle and the formation of diamonds, we are studying xenoliths, kimberlites and diamonds, because they open up a window into the Earth’s mantle (Fig. 1).
One questions still remaining open is the distribution of water is in NAMS. To address this problem, we are exploring the utility of Fourier-Transform Infrared (FTIR) micro-spectroscopy in imaging the distribution of H2O around inclusions and cracks in mantle minerals, such as olivine. The required high spatial resolution is provided by the brilliant synchrotron IR light of ANKA. Large numbers of overlapping spots are analyzed with a step size of 2 mm and an aperture of 6 mm f in a grid pattern accessed by an automated stage. The resulting maps serve as a basis for quantitative water analysis (Fig. 2). For example, Fig. 2 shows water-free Cr-spinel inclusion embedded in a olivine matrix. The water content significantly increases towards the Cr-spinel inclusion up to values of 800 ppm.

Afterwards, we compare these results with other multi-scale laboratory analyses (SEM, TEM, EBSD, Nano-SIMS, PIXE, FIB, EPM, LA-IC-PMS) to derive the passage of aqueous fluids through the lithosphere, thus obtaining more detailed information about the ascent rates of kimberlitic melts and their potential for diamond deposits.

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Adsorption mechanism of AS in rice paddy soils and on mineral surfaces for water remediation

Bangladesh and the neighboring Indian state of West Bengal (the Bengal Basin) are the sites of what has been called the largest mass poisoning in history: millions of people drink water heavily contaminated with arsenic. People in the Bengal Basin drink a lot of water because of the hot climate, the demands of physical labour, and the lack of alternative beverages. The region has constantly struggled in its attempt to administer safe and adequate water resources, to make it easier for people in the Bengal Basin to have access to safe drinking water.
By the late 1980s, surface water was the main source of drinking water in the region. This water was severely polluted and people suffered from diarrhea and various water-borne diseases. Many people, mostly children, died. In order to help people in the Basin, the Bangladesh government, supported by UNICEF and various other aid groups, began to install approximately 4 million tube wells to provide drinking water from the ground. Unfortunately, the ground water was heavily polluted with arsenic. People did not realize that there was arsenic in the drinking water since it is colorless, tasteless and odorless. Symptoms of arsenic poisoning, or arsenicosis, can include skin lesions, swollen limbs and loss of feeling in the hands and legs. Long-term exposure to arsenic can also lead to cancer, possibly affecting the lungs, bladder and kidneys. There are an estimated 40,000 cases of arsenicosis in Bangladesh, and public health experts believe there will be more than 2.5 million cases in the next 50 years. As large portions of these ground waters are used for irrigation in many areas, figures could become far worse if food is also taking up the toxic element. However, not only potable water is a way for arsenic to enter the human body. People can also be poisoned by arsenic via their daily dietary. In the Bengal Delta, groundwater is used for irrigation of cereals, especially rice, as well as for cooking. The WHO currently suggests a provisional maximum tolerable daily intake of 2 µg/kg bodyweight per day. Thus, not only the pathway of potable water but also the total dietary has to be considered.

Figure 2: Arsenic distribution in a rice grain from contaminated soils. Note the As- enrichment in the germ and in the husk.

Figure 2

The Institute of Mineralogy and Geochemistry is investigating arsenic flow from the sediments into the groundwater, which becomes drinking water, and via irrigation towards plants and soils. Especially in cases of high arsenic concentrations of several 100 µg/L in irrigation water, arsenic is enriched in the top soil horizons. In soils, arsenic is mainly co-precipitated with Fe-oxides in the soil matrix but also in the coatings of roots, especially rice. From the root coatings, small amounts of arsenic are taken up by the roots and are transported into other parts of the plant and into the rice grains. Analyses at ANKA made such enrichment visible as shown on the following figures. Further comprehensive studies are necessary to assess the fate of As arsenic in agricultural systems with respect to different Asarsenic concentrations in irrigation water. Only if the total pathway of As arsenic from sediments to human beings is fully understood, the development of sustainable mitigation measures embedded in a comprehensive management of the affected areas will be possible. Meanwhile, we extended our investigations to Vietnam and China where similar problems with arsenic- contaminated groundwater exist.

 

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