Hydrothermal Quartz Oxygen Isotope Ratios in Altered Post-Collapse Rhyolite at Sevenmile Hole, Grand Canyon of the Yellowstone River, Yellowstone National Park, WY
Abstract
The Grand Canyon of the Yellowstone River, Yellowstone National Park, Wyoming, displays regions of pervasively hydrothermally altered rock formed in the shallow, epithermal portions of a hydrothermal system. Hydrothermal fluid circulation causing the alteration is driven by magmatism related to the Yellowstone Caldera thermal anomaly. The protolith, the Tuff of Sulfur Creek, is a 480 ka high silica, low δ18O rhyolitic tuff that erupted after the Yellowstone caldera collapse at 640 ka. Incision of the canyon has exposed 350 vertical meters in the Sevenmile Hole vicinity. Hydrothermal mineralogy determined by standard XRD powder techniques and PIMA on over 90 samples shows both vertical and lateral variation. A vertical transition occurs from kaolinite at depths less than about 100 meters below the present day canyon rim, to illite in deeper exposures. This transition may correspond to a temperature of 150°C, based on a similar transition in the active Yellowstone hydrothermal system. A lateral variation of mineral assemblages in the altered tuff suggests temperatures that may range up to 330°C. Alteration was most likely caused by a liquid due to the presence of pyrite throughout. Local zones of suspected hydrothermal fluid upwelling correspond to the most intense silicification and highest temperature mineral assemblages. This alteration includes quartz + illite ± hyalophane, slawsonite, and buddingtonite. At similar depths outside inferred fluid upwelling zones, lower temperature assemblages are quartz + illite/smectite ± alunite and buddingtonite. At shallow depths, the lowest temperatures are suggested by the presence of quartz + kaolinite ± alunite and opal. Dickite, a kaolinite polymorph, may indicate locally higher temperatures in the shallow kaolinite zones. Oxygen isotope ratios of silica phases were measured for approximately 50 samples using laser fluorination techniques with an error of ±0.2‰. Hydrothermal quartz displays δ18O signatures more negative (ranging from -5.1 to -0.7 ‰) than quartz phenocrysts in the unaltered tuff (typically 1.7 ‰). Relict quartz phenocrysts in altered tuff have an average value of 0.5 ‰ (n=23, σ=0.7), suggesting minimal oxygen exchange with the hydrothermal fluid. Paragenetically later prismatic vug-filling quartz yielded the lowest average values of -4.5 ‰ (n=5, σ=0.4) while earlier massive quartz in the same vugs range up to -1.0 ‰. The variation shows that the fluid ratio and/or temperature evolved as the vug crystallized inward. Matrix silicification produced quartz δ18O values averaging -4.1 ‰ (n=11, σ =0.8). The low hydrothermal silica signatures suggest that they formed in equilibrium with a fluid that was dominantly meteoric water. There is obvious disequilibrium between open space filling hydrothermal quartz and matrix silicification that may be due to the timing of interaction with circulating waters, progressing water-rock ratios, and/or variable temperatures of alteration. Higher temperature mineral assemblages appear to correspond laterally with the more negative quartz δ18O values. Water-rock interaction was likely greater in these areas and they may mark the location of intense hydrothermal fluid upflow and/or higher temperatures.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V11A2017P
- Keywords:
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- 3616 Hydrothermal systems (0450;
- 1034;
- 3017;
- 4832;
- 8135;
- 8424)