This EaGER will support the preliminary experiments designed to test if temperature cycling has a marked effect on the crystal size distribution and crystal alignment in igneous rocks. The motivation for this study comes from the observation that crystal size relationships in plutonic rocks commonly do not match the predictions of phase equilibrium experiments nor of crystal-size distribution theory. For example, K-feldspar commonly occurs in huge crystals (>10 cm in length) even though it co-crystallizes last in granites along with quartz. Crystal size relationships suggest that these large crystals grow by cannibalism of smaller crystals. Experiments in various materials science fields (e.g., food processing) show that large crystals can grow rapidly at the expense of smaller ones if the temperature field oscillates. The preliminary study will use a three-fold approach to studying the effects of temperature cycling on crystal size relationships: (1) experiments in the ammonium thiocyanate-cobalt chloride magma analog system; (2) temperature cycling experiments in natural mafic magmas in a one-atmosphere gas-mixing furnace; and (3) temperature cycling experiments in the granite-water system using cold-seal pressure vessels. Experiments in the magma analog system will be continued in order to develop a quantitative dataset on crystal size development, and experiments at 1-atm and in gas-mixing furnaces will examine the role of varying temperature of crystal growth. An important facet of these experiments is the possibility that oscillating temperature will dramatically increase crystal growth rates, as is seen in other materials; if so, kinetic problems that plague experiments in high-silica systems may be partially avoided in future studies.
The intellectual merit of this work lies mainly in developing an understanding of the processes that govern the crystal size and crystal interrelationship characteristics (i.e., texture) of plutonic rocks. Texture is one of the few things one can easily observe in the field and is often the basis for drawing geologic maps in plutonic terranes, but much recent work casts doubt on the standard textbook interpretations of what plutonic texture means. Oscillating temperature in a melt-present system profoundly affects the texture of many materials, and the study will test the hypothesis that crystallization in a varying temperature field exerts a strong control on the texture of igneous rocks as well. In particular, crystal size and crystal alignment, both of which are critical to igneous and structural interpretation of magmatic rocks, may be strongly affected by oscillating temperature.
The broader impacts of this work are many-fold. This work will bear on the fields of petrology, structural geology, tectonics, and volcanology. If crystal alignment in plutonic rocks can result from crystallization in a thermal gradient, then features used to understand magma emplacement (e.g., crystal alignment) must be reevaluated. The project may provide a partial solution to the vexing kinetic problems that plague experiments in high-silica systems, because analog experiments demonstrate that crystal growth is greatly accelerated by oscillating temperature. The project will facilitate the development of new techniques of microanalysis using the JEOL Hyperprobe that was recently been installed at Fayetteville State University (FSU). It is expected that the project will provide a pathway for students from FSU to work and study at the main campus of the University of North Carolina. Movies of crystallization processes will be posted online for use in classroom instruction. The project has a strong educational component, with several undergraduate theses as well as one Ph.D. thesis planned.
Understanding how crystals grow is critical to many fields of materials science. In geology, the distribution of crystal sizes in an igneous rock such as granite should contain a great deal of information about the history of the magma that produced it, including how long the magma took to cool and what its time-temperature history was. However, this information has proven to be difficult to extract. In particular, many igneous rocks contain surprisingly large crystals (up to several cm in length) whose origin cannot be explained by existing models of crystal growth. In this project we examined the effects of cyclical variations in temperature (T) on crystal growth. This work was inspired by studies of ice crystal growth in various foods; such studies convincingly show that T cycling has a profound effect on crystal growth, moving mass from smaller crystals (which commonly disappear) to larger ones, strongly skewing the crystal size distribution. T cycling also hastens conversion of small snow crystals to larger ice crystals in nature. We performed T cycling on a magma analog system (ammonium thiocyanate-cobalt chloride) on a microscope stage and produced movies showing this crystal coarsening process in action in experiments lasting several days. An unexpected result of this work was production of a strong fabric in the resulting large crystals; crystals were consistently elongated parallel to the temperature gradient in the microscope field. To test whether crystal coarsening happens during T cycling in natural magmas, we performed experiments on a basalt (low-silica) lava at 1 atmosphere pressure, and on a rhyolite (high-silica) lava at 0.1 GPa pressure, water-saturated. In both sets of experiments, T cycling dramatically coarsened crystals (olivine and plagioclase in the basalt, and feldspar and hornblende in the rhyolite). Linear growth rates increased by a factor of around 10 for the conditions explored. Thus, T cycling can produce much larger crystals than the simple monotonic T decrease that is commonly assumed for cooling magmas. In the rhyolite, T cycling produced crystals large enough (10’s of micrometers in length) for routine chemical analysis on an electron microprobe. This process of crystal synthesis offers a way around the crippling effects of slow kinetics in studying crystal growth in silica-rich systems. These results have important implications for studies of volcanic hazards, ore deposits, and other geologic systems where crystal growth is important.
