Monitor for Crystallisation Conditions

During the past two decades it has repeatedly been shown that most nominally anhydrous mantle minerals contain significant amounts of hydrous defects, and that the knowledge of their OH-defect incorporation and distribution is important for many areas within the geosciences such as petrology, geochemistry and mineral physics. In the Earth`s upper mantle the pyroxenes may be regarded as major hosts for water. The nature, occurrence and concentration of specific hydrous defects in pyroxenes depend on many petrologically interesting parameters such as pressure, temperature, silica activity and oxygen fugacity, and, hence, the defect characteristics may be used as sensor for the physical and chemical conditions. In this project we intend to investigate the distribution of OH-defects in pyroxenes from high-pressure experiments. The defect distribution can be analysed on the m-scale by the application of a novel technique: samples are analysed by FT-IR microscopy equipped with a focal plane array (FPA) detector. The FPA detector consists of 64 x 64 MCT detectors, where the locus of the measured point is determined by the position of the respective detector element and no optical aperture is necessary. In this way, we are able to IR-image an area of 170 x 170 m with a spatial resolution of 2.65 m (i.e. as good as or better than the physical resolution limited by the wavelength of the IR-radiation) within less than a minute. In this way are able to detect spatial variations of OH-defects even within small crystals and can use the variations as monitor for the prevailing crystallisation conditions during crystal growth. Samples will also be investigated by transmission electron microscopy (TEM), in order to check whether the formation of planar defects (a very efficient way to incorporate OH) has to be considered. Furthermore, samples will be characterised by electron microprobe, secondary ion mass spectrometry, Raman and Mößbauer spectroscopy. The new insights gained from the experiments will be applied to natural samples from the Earth`s mantle. Results of this project are expected to give deeper insight into the influence of silica activity and oxygen fugacity on the defect chemistry of pyroxenes and the evolution of crystallisation conditions in high-pressure experiments in general. Besides the contribution in the field of experimental petrology, the proposed project may provide a significant for understanding of the global water cycle, which affects plate tectonics and especially rates of volcanic activity that - in turn - is accepted as a major influence of climatic change.

Defects in crystals contain important information concerning the formation history of crystals. In this context, very interesting defects are those in which protons are incorporated into the crystal, because the resulting OH-defects have significant influence on the physical properties of crystals and are easily detected by infrared (IR) spectroscopy. In this project the influence of pressure and chemical composition of the starting material on OH defects in pyroxenes, important rock-forming minerals of the earth's crust and upper mantle, has been investigated. The most important result is the formulation of a barometer that enables the estimation of the formation pressure. In addition to the study of natural samples, in a first step a chemically highly simplified model system was investigated, which has the advantage that the formation of defects is significantly easier to understand. In this way, in the model system consisting of the chemical components MgO-SiO2-H2O two types of defects were observed by IR spectroscopy, which abundance showed a significant and opposite pressure dependence. A comparison with a data set from well-characterized natural mantle samples of global provenance showed (especially in terms of depth of origin) that the formation of defects in nature is much more complicated due to the higher chemical complexity and that the amount of defect in the simple model system positively correlates with pressure, whereby in natural samples the opposite trend is observed. In more nature-like systems, where other important defect forming chemical elements such as chromium, aluminum, calcium and sodium have been considered, the complexity of the defect chemistry and the pressure trend was in between the trends of the simple model system and the natural samples. The results therefore indicate that based on crystal chemistry considerations an application as proxy for pressure for natural mantle samples is justified. In addition to pressure, the influence of accompanying minerals is important for the formation of OH-defects (and thus indirectly of "water"). The trace water content incorporated in this way can thus vary by more than a factor of 2. In this project, the water content of the (nominally anhydrous) pyroxene was 0.05 to 0.25 wt%.