Date File Created	Name of Creator
3/20/2009	Sheryl Singerling
GeoRef Data	2007-011011
DN	GeoRef
TI	Dating young basalt eruptions by (U-Th)/He on xenolithic zircons
AU	"Blondes, Madalyn S; Reiners, Peter W; Edwards, Benjamin R; Biscontini,Adrian"
AF	"Yale University, Department of Geology and Geophysics, New Haven, CT,United States (USA)"
AF	"Dickinson College, United States (USA)"
SO	"Geology (Boulder), vol.35, no.1, pp.17-20, Jan 2007"
IS	0091-7613
CD	GLGYBA
PB	"Geological Society of America (GSA), Boulder, CO, United States (USA)"
DE	(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
AB	"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."
LA	English
NT	With GSA Data Repository Item 2007013
FE	References: 37; illus. incl. 1 table													
PY	2007													
PT	Serial; Analytic													
CP	United States (USA)													
CL	24 Quaternary geology; 03 Geochronology													
CY	"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"													
UD	200704													
DO	10.1130/G22956A.1													
AN	2007-011011													
JV	35													
JI	1													
JP	17-20													
Sample Data														
Item	Age	Max_age	Min_age	LatDD	LongDD	Loc_prec	Volcanic_Field	Unit	Rock_Class	Rock_Type	Sample_Description	Collected_by	Material	State
Units	Ma	Ma	Ma	DD	DD									
Method	1	1	1	2	2									
Sample Name														
Oak Creek	0.176	0.1821	0.1699	36.8474	-118.2906	0.0001	Big Pine	Oak Creek Flow	granodiorite	plutonic	16 zircons from xenolith	Blondes	Mineral	California
Fish Springs	0.243	0.254	0.232	37.0767	-118.2612	0.0001	Big Pine	Fish Springs Cone	granodiorite	plutonic	15 zircons from xenolith	Blondes	Mineral	California
Method Data														
1	U-TH	352												
2	TABLE	2007-011011