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Title:
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Dating young basalt eruptions by (U-Th)/He on xenolithic zircons
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Reference Number:
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7568
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ISSN:
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0091-7613
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Publication Year:
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2007
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Publisher:
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Geological Society of America (GSA), Boulder, CO, United States (USA)
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GeoREF Number:
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2007-011011 (View Original Data File)
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Authors:
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Blondes, Madalyn S; Reiners, Peter W; Edwards, Benjamin R; Biscontini,Adrian
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Descriptors:
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(U-Th)/He; absolute age; Alaska; Big Pine volcanic field; BritishColumbia; California; Canada; Canadian Cordillera; Cenozoic; dates;diffusion; Fish Springs Vent; helium; inclusions; Little BearMountains; nesosilicates; noble gases; North America; North AmericanCordillera; Oak Creek Vent; orthosilicates; Pleistocene; Prindle;Quaternary; silicates; Southern Alaska; techniques; thermochronology;United States; vents; volcanic fields; volcanoes; Western Canada;xenoliths; zircon; zircon group
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Source:
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Geology (Boulder), vol.35, no.1, pp.17-20, Jan 2007
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Abstract:
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Accurate ages for young (e.g., Pleistocene) volcanic eruptions areimportant for geomorphic, tectonic, climatic, and hazard studies.Existing techniques can be time-consuming and expensive when many agesare needed, and in the case of K/Ar and (super 40) Ar/ (super 39) Ardating, extraneous Ar often can limit precision, especially forcontinental basalts erupted through old lithosphere. We present a newtechnique for dating young basaltic eruptions by (U-Th)/He dating ofzircons (ZHe) from crustal xenoliths. Single-crystal ZHe datesgenerally have lower precision than typical (super 40) Ar/ (super 39)Ar dates, but can be determined relatively easily on multiplereplicate grain aliquots. We dated zircons from xenoliths from fourvolcanic centers in western North America: Little Bear Mountain,British Columbia (157+ or -3.5 [2.2%] ka weighted 95% confidenceinterval [CI], mean square of weighted deviates [MSWD] = 1.7) andPrindle Volcano, Alaska (176+ or -16 [8.9%] ka, MSWD = 13), in thenorthern Cordilleran volcanic province, and Fish Springs (273+ or -23[8.6%], MSWD = 43) and Oak Creek (179+ or -8.1 [4.5%] ka, MSWD = 12),in the Big Pine Volcanic Field, California. All ZHe ages are eitherequivalent to or younger than previously determined K/Ar or (super 40)Ar/ (super 39) Ar ages, indicating the possibility of inherited (super40) Ar in some of the previous measurements. Zircons from uppercrustal xenoliths in the Oak Creek and Fish Springs vents show poorerreproducibility and multiple apparent age distribution peaks,consistent with either intracrystalline U-Th zonation or <99.99% Hedegassing (assuming ca. 100 Ma pre-entrainment ZHe ages) of somezircons during magmatic entrainment. Removal of clear outliers in theolder age-distribution peaks of the upper crustal xenoliths, most ofwhich have extremely high U compared to other zircons of the samexenolith, improve the reproducibilities of Fish Springs to 4.7% (95%CI, MSWD = 4.8) and Oak Creek to 3.4% (95% CI, MSWD = 6.2). Coupledthermal and He diffusion modeling using appropriate xenolith sizes andmagma temperatures and assuming published diffusion kinetics forzircon indicate that incomplete He degassing would require entrainmenttimes <1 h. However, the observation of extremely high U in mostzircons with older ages raises the possibility that zircons with highradiation dosages may have more retentive He diffusioncharacteristics.
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Notes:
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With GSA Data Repository Item 2007013
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Copyright:
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GeoRef, Copyright 2007, American Geological Institute. Referenceincludes data from GeoScienceWorld, Alexandria, VA, United States |Reference includes data supplied by the Geological Society of America,Boulder, CO, USA, United States
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