46990-B2
Evolution of Petroleum-Bearing Fluid Systems: Insights from Growth Dynamics of Quartz Crystals
Hydrothermal quartz veins are the fossillized remnants of fluid-filled fractures that facilitate the majority of material and heat transfer within the Earth’s crust. The behavior of hydrothermal systems governs the mobility of dissolved ore species and organic compounds related to oil and natural gas formation. Sadly, the timing and evolution of natural hydrothermal channels in the crust are poorly known. We have recently shown the potential of micro-FTIR analyses for revealing the growth evolution of individual quartz crystals within veins of variable hydrothermal grade, and thus for offering insights into the thermal and chemical evolution of natural hydrothermal systems (Ihinger and Zink, 2000).
Over the past year, my students and I have begun to characterize the morphologic evolution of a series of quartz crystals grown from a single hydrothermal vein. 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 of material with a distinguishable chemical fingerprint (low AlOH and high KOH) 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.
The distribution of impurities in the crystals from LeChang, China differs markedly from the distribution of impurities measured in the only other hydrothermal quartz crystal well-characterized by micro-FTIR, that of Minas Geras, Brazil (Ihinger and Zink, 2000). In the Brazilian specimen, the sector zones are well-defined triangles (or hexagons, depending on the relative size of the neighboring terminal faces) with distinct boundaries that extend to the edge of the crystal at the prism face. In the LeChang crystals, the six sector zones associated with the rhombohedral faces are clearly distinguishable in plane-polarized light, but the variations in impurity concentrations measured by FTIR are not sufficient to distinguish them from one another. However, the boundaries between the rhombohedral sectors and the prism sectors in the LeChang crystals are easily distinguished both in plane-polarized light and with micro-FTIR analyses.
The infrared absorbances offer a quantifiable means of monitoring changing fluid composition, crystal growth rate, and post-crystallization diffusion profiles. It is still unclear as to whether the dramatic difference in chemical fingerprint between sector zones observed in the core and in the mantle of the LeChang crystals resulted from differences related to variable exposure of the crystal lattice (involving impurity uptake on the {01-11} and {10-11} lattice planes for the r and z faces versus uptake on {1010} for the m faces) or whether they result from subsequent influx of a chemically distinct fluid (with higher K+ and lower Al3+). We aim to address this question in future spectroscopic studies of quartz crystals sampled from other hydrothermal environments. In these studies, we also hope to explore further the notion that diffusion profiles observed near crystal edges document thermal events that crystals experienced after their growth, and thereby constrain the life times of elevated thermal events in natural hydrothermal systems. We presented the preliminary results of our faculty-undergraduate student research at last year’s Goldschmidt Conference in Vancouver, BC (Henke et al., 2008) and we will be presenting our latest results at this year’s Goldschmidt Conference in Davos, Switzerland (Ihinger et al., 2009).
