K-Ar Dates Of Hydrothermal Clays From Core Hole VC-2B, Valles Caldera, New Mexico And Their Relation To Alteration In A Large Hydrothermal System

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Journal Article: K-Ar Dates Of Hydrothermal Clays From Core Hole VC-2B, Valles Caldera, New Mexico And Their Relation To Alteration In A Large Hydrothermal System

Abstract
Seventeen K/Ar dates were obtained on illitic clays within Valles caldera (1.13 Ma) to investigate the impact of hydrothermal alteration on Quaternary to Precambrian intracaldera and pre-caldera rocks in a large, long-lived hydrothermal system (~ 1.0 Ma to present). Clay samples came from scientific core hole VC-2B (295°C at 1762 m) which was spudded in the Sulphur Springs thermal area and drilled into the boundary between the central resurgent dome and the western ring-fracture zone. Six illitic clays within Quaternary caldera-fill debris flow, tuffaceous sediment, and ash-flow tuff (48 to 587 m depth) yield ages from 0.35 to 1.09 Ma. Illite from Miocene pre-caldera sandstone (765 m) gives an age of 6.74 Ma. Two dates on illite from sandstones in Permian red beds (1008 and 1187 m) are 4.33 and 4.07 Ma, respectively. Surprisingly, three dates on illites from altered andesite pebbles within the red beds (1010-1014 m) are 0.95 to 1.06 Ma. Four illite dates on variably altered Precambrian quartz monzonite (1615-1762 m) range from 2.90 to 276 Ma. Post-Valles age illite is not correlated with alteration style (argillic to propylitic). Rather, post-Valles ages are uniformly obtained from illites in highly fractured, intensely altered, caldera-fill rocks and the Permian volcanic clasts. Generally, finer clay fractions from identical samples yield younger ages. Plots of 40Ar/36Ar versus 40K/36Ar and 40Ar* versus 40K for the illites in caldera-fill rocks lie close to a 1-Ma isochron. Most illite dates older than Valles caldera are difficult to interpret because they correspond to the ages of pre-Valles volcanic and hydrothermal episodes in the Jemez volcanic field (≤ 13 Ma). In addition, older dates may be caused by co-mingling of different illites during sample preparation, or by inherited argon or lost argon in illites from rocks with potentially complex hydrothermal histories. However, the range of ages obtained from illites in Permian sands and pebbles and from Precambrian crystalline rocks indicates that Valles hydrothermal activity is overwhelming illite produced by earlier geologic events.

Authors 
Giday WoldeGabriel and Fraser E. Goff








Published Journal 
Journal of Volcanology and Geothermal Research, 1992





DOI 
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Online 
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Citation

Giday WoldeGabriel,Fraser E. Goff. 1992. K-Ar Dates Of Hydrothermal Clays From Core Hole VC-2B, Valles Caldera, New Mexico And Their Relation To Alteration In A Large Hydrothermal System. Journal of Volcanology and Geothermal Research. 50(3):207-230.