Date File Created	Name of Creator
11/7/07	Taylor Brown
GeoRef Data	2007-058887
DN	GeoRef
TI	"(super 40) Ar/ (super 39) Ar and U/Pb dating of the Fish Canyonmagmatic system, San Juan volcanic field, Colorado; evidence for anextended crystallization history"
AU	"Bachmann, O; Oberli, F; Dungan, M A; Meier, M; Mundil, R; Fischer, H"
AF	"Universite de Geneve, Section des Sciences de la Terre, Geneva,Switzerland (CHE)"
AF	"ETH Zuerich, Switzerland (CHE); Berkeley Geochronological Center,United States (USA)"
SO	"Chemical Geology, vol.236, no.1-2, pp.134-166, 15 Jan 2007"
RL	[U
RL	http://www.sciencedirect.com/science/journal/00092541]
IS	0009-2541
CD	CHGEAD
DE	absolute age; Ar/Ar; calderas; Cenozoic; Colorado; crystallization;dates; eruptions; Fish Canyon Tuff; geochemistry; igneous rocks; LaGarita Caldera; magma chambers; magmas; magmatism; mass spectra;mineral assemblages; mineral composition; Nutras Creek Dacite;Oligocene; Pagosa Peak Dacite; Paleogene; petrography; processes;pyroclastics; rhyolites; San Juan volcanic field; spectra; TaylorCreek Rhyolite; Tertiary; thermal history; thermal ionization massspectra; tuff; U/Pb; United States; volcanic features; volcanicfields; volcanic rocks; volcanism; X-ray diffraction data
AB	"The approximately 5000 km (super 3) Fish Canyon Tuff (FCT) is animportant unit for the geochronological community because itssanidine, zircon and apatite are widely used as standards for the(super 40) Ar/ (super 39) Ar and fission track dating techniques. Therecognition, more than 10 years ago [Oberli, F., Fischer, H. andMeier, M., 1990. High-resolution (super 238) U- (super 206) Pb zircondating of Tertiary bentonites and Fish Canyon Tuff; a test for age""concordance"" by single-crystal analysis. SeventhInternational Conference on Geochronology, Cosmochronology and IsotopeGeology. Geological Society of Australia Special Publication Canberra,27:74], of a > or =0.4 Ma age difference between the U-Pb zirconages and (super 40) Ar/ (super 39) Ar sanidine ages has, therefore,motivated efforts to resolve the origin of this discrepancy. Toaddress this controversial issue, we initially performed 37 U-Pbanalyses on mainly air-abraded zircons at ETH Zurich and nearly 200(super 40) !"
" Ar/ (super 39) Ar measurements on hornblende, biotite,plagioclase and sanidine obtained at the University of Geneva, usingsamples keyed to a refined eruptive stratigraphy of the FCT magmaticsystem. Disequilibrium-corrected (super 206) Pb/ (super 238) U agesobtained for 29 single-crystal and three multi-grain analyses span aninterval of approximately 28.67-28.03 Ma and yield a weighted mean ageof 28.37+ or -0.05 Ma (95% confidence level), with MSWD = 8.4. Theindividual dates resolve a range of ages in excess of analyticalprecision, covering approximately 600 ka. In order to independentlyconfirm the observed spread in zircon ages, 12 additional analyseswere carried out at the Berkeley Geochronology Center (BGC) onindividual zircons from a single lithological unit, part of thempre-treated by the """"chemical abrasion"""" (CA) technique[Mattinson, J. M., 2005. Zircon U-Pb chemical abrasion(""""CA-TIMS"""") method: Combined annealing and multi-steppartial dissolution analysis for improv!"	
" ed precision and accuracy ofzircon ages. Chemical Geology, 220(1-2): 4"	
"7-66]. Whereas the bulk ofthe BGC results displays a spread overlapping that obtained at ETH,the group of CA treated zircons yield a considerably narrower rangewith a mean age of 28.61+ or -0.08 Ma (MSWD = 1.0). Both mean zirconages determined at ETH and BGC are older than the approximately 28.0Ma (super 40) Ar/ (super 39) Ar eruption age of FCT--even whenconsidering the possibility that the latter may be low by as much asapproximately 1% due to a miscalibration of the (super 40) K decayconstants--and is thus indicative of a substantial time gap betweenmagma crystallization and extrusion. The CA technique further revealsthat younger FCT zircon ages are likely to be associated withchemically unstable U-enriched domains, which may be linked tocrystallization during extended magma residence or may have beenaffected by pre-eruptive and/or post-eruptive secondary loss ofradiogenic lead. Due to their complex crystallization history and/orage bias due to Pb loss, the FCT zircon age!"	
" s are deemed unsuitable foran accurate age calibration of FCT sandine as a fluence monitor forthe (super 40) Ar/ (super 39) Ar method. Even though data statisticspreclude unambiguous conclusions, (super 40) Ar/ (super 39) Ar datingof sanidine, plagioclase, biotite, and hornblende from the same sampleof vitrophyric Fish Canyon Tuff supports the idea of a protractedcrystallization history. Sanidine, thought to be the mineral with thelowest closure temperature, yielded the youngest age (28.04+ or -0.18Ma at 95% c.l., using Taylor Creek Rhyolite [Renne, P. R. et al.,1998. Intercalibration of standards, absolute ages and uncertaintiesin (super 40) Ar/ (super 39) Ar dating. Chemical Geology, 145:117-152.] as the fluence monitor), whereas more retentive biotite,hornblende and plagioclase gave slightly older nominal ages (by0.2-0.3 Ma). In addition, a laser step-heating experiment on a 2-cmdiameter feldspar megacryst produced a """"staircase"""" argonrelease spectrum (older ages at hig!"	
" her laser power), suggestive oftraces of inherited argon in the system"	
". Thermal and water budgets forthe Fish Canyon magma indicate that the body remained above itssolidus ( approximately 700 degrees C) for an extended period of time(>10 (super 5) years). At these temperatures, argon volumediffusion is thought to be fast enough to prevent accumulation ofradiogenic Ar. If this statement were true, an existing isotopicrecord should have been completely reset within a few hundred years,regardless of the phase and initial age of the phenocryst. As theseminerals are unlikely to be xenocrysts that were incorporated withinsuch a short time span prior to eruption, we suggest that a fractionof radiogenic Ar can be retained >10 (super 5) years, even at Tapproximately 700 degrees C."""	
LA	English
NT	Includes appendices
FE	"References: 79; illus. incl. 7 tables, geol. sketch map"
PY	2007
PT	Serial; Analytic
CP	Netherlands (NLD)
LL	"Latitude:N370000,N380000 Longitude:W1060000,W1080000"
CL	05A Igneous and metamorphic petrology; 03 Geochronology
CY	"GeoRef, Copyright 2007, American Geological Institute. Referenceincludes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam,Netherlands"
UD	200715
DO	10.1016/j.chemgeo.2006.09.005
AN	2007-058887																																				
JV	236																																				
JI	2-Jan																																				
JP	134-166																																				
Sample Data																																					
Item	LatDD	LongDD	Loc_prec	Age	Max_age	Min_age	Age	Max_age	Min_age	Age	Max_age	Min_age	Age	Max_age	Min_age	Age	Max_age	Min_age	Age	Max_age	Min_age	Age	Max_age	Min_age	Age	Max_age	Min_age	Volcanic_Field	Sample_Description	Location_Name	Location_Quad	Collected_by	Rock_Type	Sample Comment	Rock_Class	Material	State
Units	DD	DD		Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma	Ma										
Method	1	1		2	2	2	3	3	3	4	4	4	5	5	5	6	6	6	7	7	7	8	8	8	9	9	9										
Sample Name																																					
PPDcc	37.5708	-106.7583	0.0001	27.94	28.03	27.85										28.42	28.52	28.32	28.33	28.48	28.18	28.61	28.69	28.53	28.9	30.4	27.4	San Juan	Pagosa Peak Dacite (pre-FCT pyroclastic unit)	Columbine Creek	Mt Hope	Bachmann	Volcanic		Dacite	Mineral	Colorado
PPDlc	37.4881	-106.87	0.0001				28.22	28.33	28.11																			San Juan	Pagosa Peak Dacite	Lake Creek	Wolf Creek Pass	Bachmann	Volcanic		Dacite	Mineral	Colorado
FCTar	37.7111	-106.5561	0.0001	28.04	28.13	27.95	28.19	28.28	28.1	28.26	28.36	28.16	27.49	27.6	27.38	28.51	28.64	28.38	28.49	28.67	28.31							San Juan	"Fish Canyon Tuff (outflow facies, Agua Ramon location"	Agua Ramon	South Fork East	Bachmann	Volcanic		Tuff	Mineral	Colorado
FCTfv	37.6133	-106.7022	0.0001													28.44	28.66	28.22	28.41	28.51	28.31							San Juan	"Fish Canyon Tuff (outflow facies, Fun Valley  location)"	Nutras Creek	Beaver Creek Reservoir	Bachmann	Volcanic		Tuff	Mineral	Colorado
NCD	38.0269	-106.8342	0.0001	28.065	28.155	27.975										28.31	28.55	28.07	28.31	28.61	28.01							San Juan	Nutras Creek Dacite (post-FCT lava flow)	Willow Creek	Elk Park	Bachmann	Volcanic		Dacite	Mineral	Colorado
MegaX	37.9275	-106.8961	0.0001	28.23	28.32	28.14																						San Juan	Fish Canyon Tuff intracaldera	Willow Creek	San Luis Peak	Bachmann	Volcanic		Tuff	Mineral	Colorado
TonX	37.9275	-106.8961	0.0001													31.39	31.55	31.23	31.31	31.36	31.26							San Juan	tonalitic xenolith in intracaldera Fish Canyon tuff	Machin Lake area	San Luis Peak	Bachmann	Xenolith	Tonalite	Xenolith	Xenolith	Colorado
GRDX1	37.9372	-106.8589	0.0001													28.51	28.77	28.25	28.31	28.62	28							San Juan	granodioritic xenolith in intracaldera Fish Canyon Tuff	Machin Lake area	Halfmoon Pass	Bachmann	Xenolith	Granodiorite	Xenolith	Xenolith	Colorado
GRDX2	37.9372	-106.8589	0.0001													28.12	28.97	27.27	28.4	28.55	28.25							San Juan	granodioritic xenolith in intracaldera Fish Canyon Tuff	Machin Lake area	Halfmoon Pass	Bachmann	Xenolith	Granodiorite	Xenolith	Xenolith	Colorado
GRNX	37.9264	-106.85	0.0001													28.36	28.62	28.07	28.37	28.81	27.93							San Juan	Porphyritic granitic xenolith in intracaldera Fish Canyon Tuff	Machin Lake area	Halfmoon Pass	Bachmann	Xenolith	Granite	Xenolith	Xenolith	Colorado
Method Data																																					
1	TABLE	2007-058887																																			
2	AR-AR:SAN	53																																			
3	AR-AR:BIO	53																																			
4	AR-AR:PLAG	53																																			
5	AR-AR:HB	53																																			
6	U-PB7:ZIRC	224																																			
7	U-PB6:ZIRC	224																																			
8	U-PB6:ZIRC	451																																			
9	U-PB7:ZIRC	451