Light Element Incorporation in Nominally Anhydrous Minerals

California Institute Of Technology, Pasadena CA

Investigators

Abstract

Water is essential not only to life, but also to geological processes such as volcanic eruption, magma melting and rising, and rock deformation. In the geological world, a major reservoir of water is tied up in solid rock deep in the Earth where it is chemically bound to the minerals. In these minerals, water is present both as the H2O molecule, and also as its precursor, the hydrogen atom bound to oxygen atoms in the form of hydroxide groups. Although the concentrations are relatively low, usually less than a few hundred parts per million, the volume of this rock is great. A large proportion of this water is incorporated into minerals we normally think of as anhydrous. These nominally anhydrous minerals include feldspars, quartz, olivine, garnets, and pyroxenes. For a couple decades, the Caltech lab has developed the first generation of analytical standards for determining the 'water' content of nominally anhydrous minerals using a variety of analytical tools. These standards have seen wide distribution across the United States and abroad. The standards were originally developed for use with infrared spectroscopy, but now, another analytical technique known as secondary ion mass spectrometry offers comparable or better sensitivity on smaller areas. The new method is not self-calibrating and has relied, in part, on the standards previously developed for infrared spectroscopy. In some ways, the first generation of standards is proving less than optimal in view of the improved spatial sensitivity offered by the new methods. The proposed study will re-evaluate existing standards and develop new, second generation ones to optimize the new-found analytical capabilities. Part of this work will involve an extensive characterization of feldspars, one of the few major rock-forming minerals that still need to be investigated using the mass spectrometry methods. This effort is particularly topical due to interest from the community in measuring H in feldspars from both terrestrial and extraterrestrial samples. It will also study fluorine ion incorporation in minerals with the goal of improving calibration protocols while systematically studying samples derived from the Earth's mantle were indications of a coupling between hydrogen and fluorine have been previously suggested. These studies are critical to the ultimate fundamental question of where the important volatile components such as water reside in the earth and how they influence the properties of rocks and minerals. They address the questions of which phases contain trace 'water' and at what concentrations. They also contribute to understanding the spatial distribution of these volatiles throughout our planet. The analytical standards are also used to address questions about the existence of critical volatiles elsewhere in the solar system such as the moon. The presence or absence of small amounts of 'water' in nominally anhydrous synthetic minerals and related synthetic solids of technological importance plays an important role in operational success of devices such as supports for high power electronic circuits, electro-optic crystals used in data communication and fiber optics, and timing circuits used in computers, watches and telecommunication devices. This research will broaden the communities understanding of the important role that trace amounts of hydrogen and fluorine play in such minerals and materials, and will benefit the international community through the development of new standards and assessment of calibration protocols for these elements. This work also provides opportunities for direct participation in the research process by students training to be professional scientists and engineers.