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Hydrothermal fluid systems drive the transfer of both material and heat within the Earth’s crust.  Equally important, hydrothermal systems govern the mobility of dissolved ore species and organic compounds related to oil and natural gas formation.  As a result, the character and evolution of crustal fluid systems are the subject of extensive study.  Paramount to understanding these systems is a quantitative description of the timing and evolution of individual hydrothermal channels.  Unfortunately, all that is preserved of the fluids themselves is contained in the solid residue that precipitated from them while they were cooling down.  This residue typically consists of veins composed predominately of individual quartz crystals.  Trapped within every hydrothermal quartz crystal are tiny fluid inclusions and chemical impurities that document the changing conditions of the host fluid system.  We have recently shown the potential of micro-FTIR analyses for revealing the growth evolution of individual quartz crystals and for offering insights into the chemical evolution of natural hydrothermal systems (Ihinger and Zink, 2000).  Below, we document recent advances made in our laboratory to further our understanding of the evolution of hydrothermal fluid systems.  These studies have resulted in two poster presentations at the 2008 Goldschmidt Conference in Vancouver, BC (Ihinger, 2008; Henke, Ihinger and Thomas, 2008), and two additional poster presentations at the 2009 Goldschmidt Conference in Davos, Switzerland (Conde, Ihinger, and Frahm, 2009; Ihinger, Kawatski, and Steltz, 2009).

Every hydrothermal quartz crystal contains small amounts of impurities that were trapped on the growing crystal face at the time of mineral growth.  These elements really don’t belong in the host crystal lattice, and are considered ‘defects’ by mineralogists.  Crystalline defects allow geologists to understand the character and evolution of the hydrous systems that were responsible for their growth.  The development of the high-resolution micro-FTIR instrument and its application to detailed studies of single crystals of quartz can potentially revolutionize our understanding of hydrothermal systems.  Recently, I and my students have pioneered a new infrared (IR) technique for imaging the internal structure of crystals by mapping variations in the abundances of impurities contained within individual crystals (Ihinger and Zink, 2000; Ihinger et al., 2009).  We have shown that variable impurity concentrations record variable growth rates, and that with a calibration of our chemical ‘speedometer’, the actual time it took for the crystal to grow in the natural environment can be determined (Ihinger and Zink, 2000).

During the 2008-2009 fiscal year, in collaboration with UWEC undergraduate students Daniel Steltz, Jae Erickson, and Giselle Conde, we have characterized the morphologic evolution of four quartz crystals grown from a single vein from the LeChang hydrothermal system in China.  We have used micro-FTIR analyses on polished wafers extracted from various heights within individual crystals and mapped out the distribution of impurity concentrations within each specimen.  Impurity abundances vary by orders of magnitude within each crystal and delineate clearly two types of sector zones: those that result from growth on the terminal rhombohedral r and z faces, versus those that result from growth on the six m prism faces.  Each crystal extracted from the vug shows a thick mantling on the prism faces of material with a chemical fingerprint (low AlOH and high KOH) distinguishable from that observed in their cores (high AlOH with no KOH).  Optical analyses reveal striking Dauphine and concentric Brazil twinning, with the Brazil twins confined to the prism sector zones.  The axial-symmetric distribution of impurities in the prism sector zones measured via micro-FTIR, the constant mantle thickness up the length of each crystal, and the uniformity in relative thickness of the prism sector zones across all analyzed specimens suggest that the second ‘coating’ event occurred only after the completion of growth in the core, which was confined entirely to growth on the terminal r and z faces.

Our micro-FTIR analyses on polished wafers extracted from various heights within individual crystals can further be used to document the thermal evolution of the hydrothermal system.  As discussed, the core of each LeChang crystal grew as successive layers were added to the terminal r and z faces that preceded Stage 1 growth from the bottom to the top of the crystal.  Our measurements of impurities across the cores of each crystal document time-dependent diffusion profiles within each successive level.  After having been trapped by succeeding growth layers, the impurities migrated toward the edge of the crystal and back into the fluid.  We observe more pronounced diffusion profiles at the bottom of each crystal compared to the profiles observed in layers near the terminus of the crystal.  Each profile can be used to measure the time that elapsed between growth of that level and the quenching that occurred when the hydrothermal system shut down.  The time elapsed between growth at the bottom and growth at the terminus can be used to constrain the time of growth of the crystal.  This represents a novel new approach to discerning crystal growth kinetics in natural hydrothermal systems, which we aim to continue to investigate in the coming year.