Ar-40/Ar-39 and U-Pb dating of the Fish Canyon magmatic system, San Juan Volcanic field, Colorado: Evidence for an extended crystallization history

摘要

The similar to 5000 km(3) Fish Canyon Tuff (FCT) is an important unit for the geochronological community because its sanidine, zircon and apatite are widely used as standards for the Ar-40/Ar-39 and fission track dating techniques. The recognition, more than 10 years ago [Oberli, F., Fischer, H. and Meier, M., 1990. High-resolution U-238-Pb-206 zircon dating of Tertiary bentonites and Fish Canyon Tuff, a test for age "concordance" by single-crystal analysis. Seventh International Conference on Geochronology, Cosmochronology and Isotope Geology. Geological Society of Australia Special Publication Canberra, 27:74], of a >= 0.4 Ma age difference between the U-Pb zircon ages and Ar-40/Ar-39 sanidine ages has, therefore, motivated efforts to resolve the origin of this discrepancy. To address this controversial issue, we initially performed 37 U-Pb analyses on mainly air-abraded zircons at ETH Zurich and nearly 200 Ar-40/Ar-39 measurements on hornblende, biotite, plagioclase and sanidine obtained at the University of Geneva, using samples keyed to a refined eruptive stratigraphy of the FCT magmatic system. Disequilibrium-corrected Pb-206/U-238 ages obtained for 29 single-crystal and three multi-grain analyses span an interval of similar to 28.67-28.03 Ma and yield a weighted mean age of 28.37 +/- 0.05 Ma (95% confidence level), with MSWD=8.4. The individual dates resolve a range of ages in excess of analytical precision, covering similar to 600 ka. In order to independently confirm the observed spread in zircon ages, 12 additional analyses were carried out at the Berkeley Geochronology Center (BGC) on individual zircons from a single lithological unit, part of them pre-treated by the "chemical abrasion" (CA) technique [Mattinson, J.M., 2005. Zircon U-Pb chemical abrasion ("CA-TIMS") method: Combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages. Chemical Geology, 220(1-2): 47-66]. Whereas the bulk of the BGC results displays a spread overlapping that obtained at ETH, the group of CA treated zircons yield a considerably narrower range with a mean age of 28.61 +/- 0.08 Ma (MSWD = 1.0). Both mean zircon ages determined at ETH and BGC are older than the similar to 28.0 Ma Ar-40/Ar-39 eruption age of FCT-even when considering the possibility that the latter may be low by as much as similar to 1% due to a miscalibration of the K-40 decay constants-and is thus indicative of a substantial time gap between magma crystallization and extrusion. The CA technique further reveals that younger FCT zircon ages are likely to be associated with chemically unstable U-enriched domains, which may be linked to crystallization during extended magma residence or may have been affected by pre-eruptive and/or post-eruptive secondary loss of radiogenic lead. Due to their complex crystallization history and/or age bias due to Pb loss, the FCT zircon ages are deemed unsuitable for an accurate age calibration of FCT sandine as a fluence monitor for the Ar-40/Ar-39 method. Even though data statistics preclude unambiguous conclusions, Ar-40/Ar-39 dating of sanidine, plagioclase, biotite, and hornblende from the same sample of vitrophyric Fish Canyon Tuff supports the idea of a protracted crystallization history. Sanidine, thought to be the mineral with the lowest closure temperature, yielded the youngest age (28.04 +/- 0.18 Ma at 95% c.l., using Taylor Creek Rhyolite [Renne, P.R. et al., 1998. Intercalibration of standards, absolute ages and uncertainties in Ar-40/Ar-39 dating. Chemical Geology, 145: 117-152.] as the fluence monitor), whereas more retentive biotite, hornblende and plagioclase gave slightly older nominal ages (by 0.2-0.3 Ma). In addition, a laser step-heating experiment on a 2-cm diameter feldspar megacryst produced a "staircase" argon release spectrum (older ages at higher laser power), suggestive of traces of inherited argon in the system. Thermal and water budgets for the Fish Canyon mag