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Division of Geological & Geophysical Surveys
RAW-DATA FILE 2014-16
Zircon U-Pb Age Data, Alaska Highway Corridor,
Tanacross and Nabesna quadrangles, Alaska
by
Diana N. Solie, Paul O’Sullivan, Melanie B. Werdon, Larry K. Freeman,
Rainer J. Newberry, David J. Szumigala, and Trent D. Hubbard
$3.00
July 2014
THIS REPORT HAS NOT BEEN REVIEWED FOR
TECHNICAL CONTENT OR FOR CONFORMITY TO
THE EDITORIAL STANDARDS OF DGGS
Released by
STATE OF ALASKA
DEPARTMENT OF NATURAL RESOURCES
Division of Geological & Geophysical Surveys
3354 College Road
Fairbanks, Alaska 99709-3707
TABLE OF CONTENTS
Abstract ......................................................................................................................................................................... 1
Introduction ................................................................................................................................................................... 1
Methodology ................................................................................................................................................................. 1
Field Methods ............................................................................................................................................................ 1
Analytical Methods .................................................................................................................................................... 2
Analytical Results ........................................................................................................................................................... 2
Igneous Rocks ............................................................................................................................................................ 2
Metamorphic Rocks ................................................................................................................................................... 2
Detrital Zircons .......................................................................................................................................................... 2
Acknowledgments ......................................................................................................................................................... 2
References Cited ............................................................................................................................................................ 3
Appendix A: Data plots for individual samples .............................................................................................................. 5
09MBW247A:......................................................................................................................................................... 5
09LF454A: .............................................................................................................................................................. 6
09TDH75A: ............................................................................................................................................................. 7
09LF279A: .............................................................................................................................................................. 8
09Z213A: ................................................................................................................................................................ 9
09LF441A: ............................................................................................................................................................ 10
09TDH74A: ........................................................................................................................................................... 11
09MBW103A:....................................................................................................................................................... 12
09LF419A: ............................................................................................................................................................ 13
09Z172B: .............................................................................................................................................................. 14
09LF254A: ............................................................................................................................................................ 15
09LF433A: ............................................................................................................................................................ 16
09LF647A: ............................................................................................................................................................ 17
09LF637A: ............................................................................................................................................................ 18
09LF644A: ............................................................................................................................................................ 19
09MBW338A:....................................................................................................................................................... 20
09LF206A: ............................................................................................................................................................ 21
09MBW400A:....................................................................................................................................................... 22
09MBW243A:....................................................................................................................................................... 23
09LF233A: ............................................................................................................................................................ 24
09LF234A: ............................................................................................................................................................ 25
09RN242A: ........................................................................................................................................................... 26
Appendix B: Zircon U-Pb Dating Methodology ............................................................................................................ 27
Sample Preparation ................................................................................................................................................. 27
LA-ICP-MS Session Details ....................................................................................................................................... 27
U-Pb Data Analysis ................................................................................................................................................... 27
Moving-Median Smoothing ..................................................................................................................................... 28
Appendix C: Zircon age standards ............................................................................................................................... 29
RDF 2014- P a g e | 1
ZIRCON U-PB AGE DATA, ALASKA HIGHWAY CORRIDOR,
TANACROSS AND NABESNA QUADRANGLES, ALASKA
by
Diana N. Solie1, Paul O’Sullivan2, Melanie B. Werdon1, Larry K. Freeman1,
Rainer J. Newberry3, David J. Szumigala1, and Trent D. Hubbard1
ABSTRACT
Twenty-two igneous and metamorphic rock samples were collected in a 20-km-long swath centered on the Alaska
Highway between Tetlin Junction, Alaska, and the Canada border. The 15 felsic plutonic rock samples and one gabbro
sample yield Cretaceous zircon U-Pb weighted-mean ages ranging from 95.7 Ma to 106.2 Ma. Zircons from a
metadiorite yield a Triassic weighted-mean age of about 241.9 Ma. Metamorphic zircons from two samples with
felsic igneous protoliths yield Mississippian weighted-mean ages of 351.7 Ma and 354.6 Ma. Detrital zircons from
two metaconglomerates yield individual grain ages from 100 Ma to 1,811 Ma, but are predominantly between 300
Ma and 345 Ma. Detrital zircons from a quartzite yield peaks at about 2,648 Ma, 1,946 Ma, and 1,807 Ma.
INTRODUCTION
The Alaska Division of Geological & Geophysical Surveys (DGGS) conducted a multi-year project (from 2006 through
2010) studying the geology and geologic hazards along the Alaska Highway corridor (Solie and Burns, 2007). As part
of the bedrock geologic mapping, samples were collected for geochronologic analyses using 40Ar/39Ar, fission-track,
and U-Pb dating techniques. Summaries of analytical results appear on bedrock-geologic maps that are being
published in three segments. This report supplies the supporting data for the U-Pb age results from the easternmost
segment of the project, in the Tanacross and Nabesna quadrangles.
DGGS mapped the easternmost segment of the Alaska Highway corridor, from Tetlin Junction to the Canada
border, in 2009. Twenty-two igneous and metamorphic rock samples were submitted to the Apatite to Zircon, Inc.
laboratory (A2Z), where U-Pb age analysis was performed by Paul O’Sullivan in 2011. The resulting data, excerpted
from the laboratory report, are presented in this publication. All data are available from the DGGS website
(http://dggs.alaska.gov/pubs) at no charge.
METHODOLOGY
Field Methods
DGGS field geologists collected rock samples from the surface or within 0.5 m of the surface. Care was taken to
collect fresh, unweathered samples representative of rock types in the map area. Locations were recorded using
hand-held GPS units with an estimated horizontal accuracy of approximately 10 m. Location coordinates for all
dated samples are provided in an accompanying .csv file, in North American Datum of 1927 decimal degrees. This
file also contains brief descriptions of each sample based on field observations.
1 Alaska Division of Geological & Geophysical Surveys, 3354 College Road, Fairbanks AK 99709-3707 2 Apatite to Zircon, Inc., 1075 Matson Road, Viola, ID 83872-9709 3 University of Alaska Fairbanks, Department of Geology & Geophysics, P.O. Box 755780, Fairbanks AK 99775-5780
RDF 2014- P a g e | 2
ANALYTICAL METHODS
DGGS submitted selected rock samples to Apatite to Zircon, Inc. in 2011. Paul O’Sullivan directed the processing
and analyses of the samples in the A2Z laboratory. Laser ablation–inductively coupled plasma–mass spectrometry
(LA–ICP–MS) analyses were performed at the Geoanalytical Laboratory, Washington State University, Pullman,
Washington, U.S.A. Detailed discussion of laboratory techniques is provided in appendices to this report.
ANALYTICAL RESULTS
Twenty-two rock samples were analyzed. Data plots for each sample are provided in Appendix A, from youngest to
oldest.
IGNEOUS ROCKS
Sixteen of the analyzed samples were igneous rocks. Fifteen were felsic plutonic rocks, either granite, granodiorite,
or quartz monzonite; the other was gabbro. The weighted-mean age of all measurements from zircons in each
igneous sample are recorded in the accompanying .csv table. All 16 ages fall within a span of about 10.5 Ma,
between 95.7 ± 2.5 Ma (09MBW247A) and 106.2 ± 2.8 Ma (09MBW338A). There is no apparent pattern to the age
distribution across the map area, or correlation of rock type with age. The mafic igneous rock, a gabbro
(09LF647A), yields a U-Pb weighted-mean age of 103.8 ± 2.7 Ma.
METAMORPHIC ROCKS
A mafic sample from the southern map area, described in the field as diorite or possibly amphibolite with no
foliation (09LF206A), yields an age distribution between 1,372 Ma and 114 Ma. We interpret these results to
include lead loss yielding the younger measurements, and the weighted-mean age (based on 20 of 42
measurements) yields a Triassic age of 241.9 ± 13.5 Ma.
Two metamorphosed felsic igneous rocks from the northern half of the map area were dated as lower
Mississippian and upper Devonian. One, a metarhyolite (09MBW400A), yielded a weighted-mean age of 351.7 ±
9.3 Ma based on 49 tightly grouped zircon measurements. The other, a granitic orthogneiss (09MBW243A), yielded
a weighted-mean age of 354.6 ± 9.3 Ma based on 45 zircon measurements.
DETRITAL ZIRCONS
Detrital zircons from two metaconglomerate samples collected in the southern map area yield ages that are mostly
younger than 350 Ma. Sample 09LF234A yielded two measurements around 1,800 Ma, and another around 225
Ma; the other 17 measurements are Carboniferous, between ~300 Ma and 345 Ma. Sample 09LF233A yielded two
zircon measurements of ~100 Ma, and 321 Ma.
Detrital zircons from a quartzite collected about 3 km from the Canada border (09RN242A) yield a significantly
older age distribution, with measurements ranging from ~3,000 Ma to 1,030 Ma. The largest peak in the
distribution (see plots in Appendix A) is at 1,807 Ma, with two smaller but prominent peaks at 1,946 Ma and 2,648
Ma.
ACKNOWLEDGMENTS
This project was supported by Alaska State Capital Improvement Project funds. Samples used in this report were
collected by Larry Freeman, Melanie Werdon, David Szumigala, and Trent Hubbard of DGGS; and Rainer Newberry
RDF 2014- P a g e | 3
of the University of Alaska Fairbanks Department of Geology and Geophysics. Margaret Donelick and Jim McMillan
(A2Z) provided technical assistance with sample preparation; Charles Knaack (Washington State University) provided
technical assistance with LA-ICP-MS data collection; Paul O’Sullivan and Ray Donelick (A2Z) provided valuable
assistance with LA-ICP-MS data interpretation.
REFERENCES CITED
Black, L.P., Kamo, S.L., Allen, C.M., Davis, D.W., Aleinikoff, J.N., Valley, J.W., Mundil, Roland, Campbell, I.H., Korsch,
R.J., Williams, I.S., and Foudoulis, Chris, 2004, Improved 206Pb/238U microprobe geochronology by the monitoring
of trace-element-related matrix effect—SHRIMP, ID-TIMS, ELA-ICP-MS, and oxygen isotope documentation for
a series of zircon standards: Chemical Geology, v. 205, p. 15–140.
Chang, Zhaoshan, Vervoort, J.D., McClelland, W.C., and Knaack, Charles, 2006, U-Pb dating of zircon by LA-ICP-MS:
Geochemistry, Geophysics, Geosystems, American Geophysical Union, v. 7, no. 5, 14 p.,
doi:10.1029/2005GC001100.
Chew, D.M., and Donelick, R.A., 2012, Combined apatite fission track and U-Pb dating by LA-ICP-MS and its
application in apatite provenance analysis: Mineralogical Association of Canada Short Course, v. 42, p. 219–247.
Donelick, R.A, O’Sullivan, P.B., and Ketcham, R.A., 2005, Apatite fission-track analysis: Reviews in Mineralogy and
Geochemistry, Mineralogical Society of America, v. 58, p. 49–94.
Gehrels, G.E., Valencia, V.A., and Ruiz, J., 2008, Enhanced precision, accuracy, efficiency, and spatial resolution of U-
Pb ages by laser ablation–multicollector–inductively coupled plasma–mass spectrometry: Geochemistry
Geophysics Geosystems, American Geophysical Union, v. 9, 13 p.
Kuiper, K.F., Deino, A., Hilgen, P.J., Krijgsman, W., Renne, P.R., and Wijbrans, J.R., 2008, Synchronizing rock clocks of
Earth history: Science, v. 320, p. 500–504.
Lanphere, M.A., and Baadsraard, H., 2001, Precise K-Ar, 40Ar/39Ar, Rb-Sr and U-Pb mineral ages from the 27.5 Ma
Fish Canyon Tuff reference standard: Chemical Geology, v. 175, p. 653–671.
Paces, J.B., and Miller, J.D., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern
Minnesota—Geochronological insights to physical, petrogenic, paleomagnetic, and tectonomagmatic processes
associated with the 1.1 Ga Midcontinent Rift System: Journal of Geophysical Research, v. 98, no. B8, p. 13,997–
14,013.
Paton, C., Woodhead, J.D., Hellstrom, J.C., Hergt, J.M., Greig, A., and Maas, R., 2010, Improved laser ablation U-Pb
zircon geochronology through robust downhole fractionation correction: Geochemistry, Geophysics,
Geosystems, v. 11, no. Q0AA06, doi:10.1029/2009GC002618.
Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., and DePaolo, D.J., 1998, Intercalibration of
standards, absolute ages and uncertainties in 40Ar/39Ar dating: Chemical Geology, v. 45, p. 117–152.
Solie, D.N., and Burns, L.E., 2007, Alaska Highway corridor geology and geophysics: Alaska GeoSurvey News, v. 10,
no. 1, p. 1–4.
Steiger, R.H., and Jäger, E., 1977, Subcommission on geochronology—Convention on the use of decay constants in
geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, p. 369–371.
Zhang, M., Ewing, R.C., Boatner, L.A., Salje, E.K.H., Weber, W.J., Daniel, P., Zhang, Y., and Farnan, I., 2009, Pb*
irradiation of synthetic zircon (ZrSiO4); Infrared spectroscopic study—Reply: American Mineralogist, v. 94, p.
856–858.
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RDF 2014- P a g e | 5
APPENDIX A: DATA PLOTS FOR INDIVIDUAL SAMPLES
09MBW247A:
0
100
200
300
400
500
600
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09MBW247Ahistogram
0
100
200
300
400
500
600
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09MBW247A gauss pdf
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09MBW247A peak
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0
100
200
300
400
500
600
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09MBW247Apdf
U/Th
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09MBW247A U/Th
0
1000
2000
3000
4000
5000
6000
7000
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09MBW247A U ppm
0
200
400
600
800
1000
1200
1400
1600
1800
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09MBW247A Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09MBW247A
RDF 2014- P a g e | 6
09LF454A:
0
50
100
150
200
250
300
350
400
450
500
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF454Ahistogram
0
50
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150
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300
350
400
450
500
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF454A gauss pdf
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09LF454A peak
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0
50
100
150
200
250
300
350
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450
500
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09LF454Apdf
U/Th
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09LF454A U/Th
0
2000
4000
6000
8000
10000
12000
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09LF454A U ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09LF454A Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09LF454A
RDF 2014- P a g e | 7
09TDH75A:
0
50
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450
0 500 1000 1500 2000 2500 3000
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U-Pb Age (Ma)
09TDH075histogram
0
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450
0 500 1000 1500 2000 2500 3000
Re
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req
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U-Pb Age (Ma)
09TDH075 gauss pdf
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09TDH075 peak
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0
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450
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
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ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09TDH075pdf
U/Th
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09TDH075 U/Th
0
100
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900
1000
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09TDH075 U ppm
0
100
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500
600
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09TDH075 Th ppm
0
500
1000
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2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09TDH075
RDF 2014- P a g e | 8
09LF279A:
0
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U-Pb Age (Ma)
09LF279Ahistogram
0
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0 500 1000 1500 2000 2500 3000
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U-Pb Age (Ma)
09LF279A gauss pdf
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09LF279A peak
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0
50
100
150
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250
300
350
400
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09LF279Apdf
U/Th
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09LF279A U/Th
0
1000
2000
3000
4000
5000
6000
7000
8000
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09LF279A U ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09LF279A Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09LF279A
RDF 2014- P a g e | 9
09Z213A:
0
50
100
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500
0 500 1000 1500 2000 2500 3000
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req
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U-Pb Age (Ma)
09Z213Ahistogram
0
50
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0 500 1000 1500 2000 2500 3000
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U-Pb Age (Ma)
09Z213A gauss pdf
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09Z213A peak
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
0
50
100
150
200
250
300
350
400
450
500
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09Z213Apdf
U/Th
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09Z213A U/Th
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09Z213A U ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09Z213A Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09Z213A
RDF 2014- P a g e | 10
09LF441A:
0
50
100
150
200
250
300
350
400
450
500
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF441Ahistogram
0
50
100
150
200
250
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450
500
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF441A gauss pdf
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09LF441A peak
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0
50
100
150
200
250
300
350
400
450
500
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09LF441Apdf
U/Th
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09LF441A U/Th
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09LF441A U ppm
0
500
1000
1500
2000
2500
3000
3500
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09LF441A Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09LF441A
RDF 2014- P a g e | 11
09TDH74A:
0
50
100
150
200
250
300
350
400
450
500
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09TDH074histogram
0
50
100
150
200
250
300
350
400
450
500
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09TDH074 gauss pdf
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09TDH074 peak
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0
50
100
150
200
250
300
350
400
450
500
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09TDH074pdf
U/Th
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09TDH074 U/Th
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09TDH074 U ppm
0
200
400
600
800
1000
1200
1400
1600
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09TDH074 Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09TDH074
RDF 2014- P a g e | 12
09MBW103A:
0
50
100
150
200
250
300
350
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09MBW103Ahistogram
0
50
100
150
200
250
300
350
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09MBW103A gauss pdf
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09MBW103A peak
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0
50
100
150
200
250
300
350
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09MBW103Apdf
U/Th
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09MBW103A U/Th
0
200
400
600
800
1000
1200
1400
1600
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09MBW103A U ppm
0
200
400
600
800
1000
1200
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09MBW103A Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09MBW103A
RDF 2014- P a g e | 13
09LF419A:
0
50
100
150
200
250
300
350
400
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF419Ahistogram
0
50
100
150
200
250
300
350
400
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF419A gauss pdf
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09LF419A peak
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0
50
100
150
200
250
300
350
400
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09LF419Apdf
U/Th
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09LF419A U/Th
0
100
200
300
400
500
600
700
800
900
1000
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09LF419A U ppm
0
100
200
300
400
500
600
700
800
900
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09LF419A Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09LF419A
RDF 2014- P a g e | 14
09Z172B:
0
10
20
30
40
50
60
70
80
90
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09Z172Bhistogram
0
10
20
30
40
50
60
70
80
90
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09Z172B gauss pdf
0.00
0.05
0.10
0.15
0.20
0.25
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09Z172B peak
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
0
10
20
30
40
50
60
70
80
90
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09Z172Bpdf
U/Th
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09Z172B U/Th
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09Z172B U ppm
0
100
200
300
400
500
600
700
800
900
1000
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09Z172B Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09Z172B
RDF 2014- P a g e | 15
09LF254A:
0
50
100
150
200
250
300
350
400
450
500
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF254Ahistogram
0
50
100
150
200
250
300
350
400
450
500
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF254A gauss pdf
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09LF254A peak
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0
50
100
150
200
250
300
350
400
450
500
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09LF254Apdf
U/Th
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09LF254A U/Th
0
200
400
600
800
1000
1200
1400
1600
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09LF254A U ppm
0
100
200
300
400
500
600
700
800
900
1000
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09LF254A Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09LF254A
RDF 2014- P a g e | 16
09LF433A:
0
50
100
150
200
250
300
350
400
450
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF433Ahistogram
0
50
100
150
200
250
300
350
400
450
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF433A gauss pdf
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09LF433A peak
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0
50
100
150
200
250
300
350
400
450
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09LF433Apdf
U/Th
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09LF433A U/Th
0
200
400
600
800
1000
1200
1400
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09LF433A U ppm
0
100
200
300
400
500
600
700
800
900
1000
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09LF433A Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09LF433A
RDF 2014- P a g e | 17
09LF647A:
0
50
100
150
200
250
300
350
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF647Ahistogram
0
50
100
150
200
250
300
350
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF647A gauss pdf
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09LF647A peak
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
0
50
100
150
200
250
300
350
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09LF647Apdf
U/Th
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09LF647A U/Th
0
2000
4000
6000
8000
10000
12000
14000
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09LF647A U ppm
0
500
1000
1500
2000
2500
3000
3500
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09LF647A Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09LF647A
RDF 2014- P a g e | 18
09LF637A:
0
50
100
150
200
250
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF637Ahistogram
0
50
100
150
200
250
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF637A gauss pdf
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09LF637A peak
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
0
50
100
150
200
250
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09LF637Apdf
U/Th
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09LF637A U/Th
0
500
1000
1500
2000
2500
3000
3500
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09LF637A U ppm
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09LF637A Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09LF637A
RDF 2014- P a g e | 19
09LF644A:
0
50
100
150
200
250
300
350
400
450
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF644Ahistogram
0
50
100
150
200
250
300
350
400
450
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF644A gauss pdf
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09LF644A peak
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0
50
100
150
200
250
300
350
400
450
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09LF644Apdf
U/Th
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09LF644A U/Th
0
200
400
600
800
1000
1200
1400
1600
1800
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09LF644A U ppm
0
500
1000
1500
2000
2500
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09LF644A Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09LF644A
RDF 2014- P a g e | 20
09MBW338A:
0
50
100
150
200
250
300
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09MBW338Ahistogram
0
50
100
150
200
250
300
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09MBW338A gauss pdf
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09MBW338A peak
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0
50
100
150
200
250
300
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09MBW338Apdf
U/Th
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0 500 1000 1500 2000 2500 3000
U/T
h
U-Pb Age (Ma)
09MBW338A U/Th
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 500 1000 1500 2000 2500 3000
U p
pm
U-Pb Age (Ma)
09MBW338A U ppm
0
500
1000
1500
2000
2500
0 500 1000 1500 2000 2500 3000
Th p
pm
U-Pb Age (Ma)
09MBW338A Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09MBW338A
RDF 2014- P a g e | 21
09LF206A:
0
1
2
3
4
5
6
7
8
9
10
-100 100 300 500 700 900 1100 1300 1500
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF206Ahistogram
0
1
2
3
4
5
6
7
8
9
10
-100 100 300 500 700 900 1100 1300 1500
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09LF206A gauss pdf
0.00
0.05
0.10
0.15
0.20
0.25
-100 100 300 500 700 900 1100 1300 1500
Fre
qu
en
cy
U-Pb Age (Ma)
09LF206A peak
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
0
1
2
3
4
5
6
7
8
9
10
-100 100 300 500 700 900 1100 1300 1500
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09LF206Apdf
U/Th
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
-100 100 300 500 700 900 1100 1300 1500
U/T
h
U-Pb Age (Ma)
09LF206A U/Th
0
200
400
600
800
1000
1200
1400
1600
1800
2000
-100 100 300 500 700 900 1100 1300 1500
U p
pm
U-Pb Age (Ma)
09LF206A U ppm
0
100
200
300
400
500
600
-100 100 300 500 700 900 1100 1300 1500
Th p
pm
U-Pb Age (Ma)
09LF206A Th ppm
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Low
er-
Age
Inte
rcep
t (M
a)
Upper-Age Intercept (Ma)
09LF206A
RDF 2014- P a g e | 22
09MBW400A:
0
20
40
60
80
100
120
140
160
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09MBW400Ahistogram
0
20
40
60
80
100
120
140
160
0 500 1000 1500 2000 2500 3000
Re
lati
ve A
ge F
req
uen
cy
U-Pb Age (Ma)
09MBW400A gauss pdf
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0 500 1000 1500 2000 2500 3000
Fre
qu
en
cy
U-Pb Age (Ma)
09MBW400A peak
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0
20
40
60
80
100
120
140
160
0 500 1000 1500 2000 2500 3000
U/T
h (
blu
e p
oin
ts)
Re
lati
ve A
ge F
req
uen
cy (r
ed
cu
rve)
U-Pb Age (Ma)
09MBW400Apdf
U/Th
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0 500 1000 1500 2000 2500 3000
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09MBW243A:
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09MBW243Apdf
U/Th
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09MBW243A U/Th
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09MBW243A U ppm
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09MBW243A Th ppm
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09LF233A:
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09LF233Ahistogram
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09LF233A gauss pdf
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09LF233A Th ppm
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09LF233A
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09LF234A:
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09LF234Ahistogram
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09LF234Apdf
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09LF234A U ppm
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RDF 2014- P a g e | 26
09RN242A:
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09RN242A Th ppm
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09RN242A
RDF 2014- P a g e | 27
APPENDIX B: ZIRCON U-PB DATING METHODOLOGY
Sample Preparation
Zircon grains were isolated and prepared for Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-
MS) analysis using standard procedures combined with specific customized procedures described by Donelick et al.
(2005). Whole rock samples were crushed using a jaw crusher with the minimum jaw separation set to 2-3 mm,
sieved through 300 µm nylon mesh, and the <300 µm size fraction washed with tap water and allowed to dry at
room temperature. Zircon was separated from other mineral species using a combination of lithium metatungstate
(density ~2.9 g/cm3), Frantz magnetic separator, diiodomethane (density ~3.3 g/cm3), and hand-panning separation
procedures. Epoxy wafers (~1 cm x 1 cm) containing zircon grains for LA-ICP-MS were polished manually using 3.0
µm and 0.3 µm Al2O3 slurries to expose internal zircon grain surfaces. The polished zircon grain surfaces were washed
in 5.5 M HNO3 for 20 s at 21⁰ C prior to introduction into the laser system sample cell.
LA-ICP-MS Session Details
LA-ICP-MS data collection was performed at the Geoanalytical Laboratory, Washington State University, Pullman,
Washington, U.S.A. Individual zircon grains were targeted for data collection using a New Wave YP213 213 nm solid
state laser ablation system using a 20 µm diameter laser spot size, 5 Hz laser firing rate, and ultra-high purity He as
the carrier gas. Isotopic analyses of the ablated zircon material were performed using a ThermoScientific Element2
magnetic sector mass spectrometer using high purity Ar as the plasma gas. The following masses (in amu) were
monitored for 0.005 s each in pulse detection mode: 202, 204, 206, 207, 208, 232, 235, and 238. At time = 0.0 s, the
mass spectrometer began monitoring signal intensities; at time = 6.0 s, the laser began ablating zircon material; at
time = 30.0 s, the laser was turned off and the mass spectrometer stopped monitoring signal intensities. A total of
250 data scans were collected for each zircon spot analyzed comprising: approximately 55 background scans;
approximately 20 transitions scans between background and background+signal, approximately 175
background+signal scans. A scheme was developed to check whether mass 238 experienced a switch from pulse to
analog mode during data collection and a correction procedure was employed to ensure the use of good quality
intensity data for masses 235 and 238 when such a switch was observed.
U-Pb Data Analysis
Previous LA-ICP-MS studies of U-Pb zircon dating used the so-called intercept method which assumes that isotopic
ratio varies linearly with scan number due solely to linearly varying isotopic fractionation (Chang et al., 2006; Gehrels
et al., 2008). The data modeling approach favored here was the modeling of background-corrected signal intensities
for each isotope at each scan. Background intensity for each isotope was calculated using a fitted line (for decreasing
background intensity) or using the arithmetic mean (for non-decreasing background intensity) at the global
minimum of selected isotopes (206Pb, 232Th, and 238U) for the spot. Background+signal intensity for each isotope at
each scan was calculated using the median of fitted (2nd-order polynomial) intensity values for a moving window (7
scans wide here) that includes the scan. The precision of each background-corrected signal intensity value was
calculated from the precision of background intensity value and the precision of the background+signal intensity
value.
Zircon U-Pb age standards used during analysis are summarized in Appendix C, including the 1099±0.6 Ma FC zircon
(FC-1 of Paces and Miller, 1993) used here as the primary age standard. Isotopic data for FC were used to calculate
Pb/U fractionation factors and their absolute errors for each FC data scan at each FC spot; these fractionation factors
were smoothed session-wide for each data scan using the median of fitted (1st-order polynomial) fractionation factor
values for a moving window (11 FC spots wide here) that includes the current FC spot and scan. Under the operating
RDF 2014- P a g e | 28
conditions of the LA-ICP-MS sessions in this study, fractionation factors were found to vary strongly with scan
number, decreasing with increasing scan number (presumably due to increasing ablation pit depth and the effect
this had on fractionation, (e.g., Paton et al., 2010). The zircon crystal lattice is widely known to accumulate α-
radiation damage (e.g., Zhang et al., 2009 and references therein). It was assumed here that increased α-damage in
a zircon leads to a decrease in the hardness of the zircon; this in turn leads to a faster rate of laser penetration into
the zircon during ablation leading to dependence of isotopic fractionation on the degree of zircon lattice radiation
damage. Ages calculated for all zircon age standards, when those standards were treated as unknowns, were used
to construct a fractionation factor correction curve (exponential form) in terms of accumulated radiation damage.
The notion of matrix-matched zircon standard and zircon unknown has been proposed largely on the basis of trace
element chemistry (e.g., Black et al., 2004). In this study, time and lattice damage, parameters invisible to
instruments used to characterize trace element chemistry, were introduced and applied based on measured U and
Th chemistries to effectively matrix-match standard and unknown zircons.
Uranium decay constants and the 238U/235U isotopic ratio reported in Steiger and Jäger (1977) were used in this
study. Errors for the isotopic ratios 207Pb/235Uc (235Uc = 137.88238U), 206Pb/238U, and 207Pb/206Pb at each scan included
errors from the background-corrected signal values for each isotope, the fractionation factor error, and an additional
relative error term required to force 95% of the FC ages to be concordant. Ages for the ratios 207Pb/235Uc, 206Pb/238U,
and 207Pb/206Pb were calculated for each data scan and checked for concordance; concordance here was defined as
overlap of all three ages at the 1 level (the use of 2 level was found to skew the results to include scans with any
significant common Pb). If the number of concordant data scans for a spot was greater than zero, the more precise
age from the concordant-scan-weighted ratio 207Pb/235Uc, 206Pb/238U or 207Pb/206Pb was chosen as the preferred age.
Asymmetrical negative-direction and positive-direction age errors were calculated by subtracting and adding,
respectively, the isotopic ratio errors in the appropriate age equation (Chew and Donelick, 2012).
Moving-Median Smoothing
Moving-median smoothing (MMS) is applied here to a subset (window) of N data points x,y of width m values of x
to which a polynomial of order n is fitted. For each value of x at each position of the data window, a value of y is
calculated for the fitted polynomial. The window is positioned with the right-hand boundary at the left-hand x value
and then shifted N-1 times until the left-hand boundary of the window is positioned at the right-hand x value. At
each x position, m fitted values of y are calculated and the median of these fitted values is taken.
RDF 2014- P a g e | 29
APPENDIX C: ZIRCON AGE STANDARDS
Standard Standard U-Pb age (2) Reference
FC Duluth complex
1099.0 0.6 Ma Paces and Miller, 1993
F5 Duluth complex
1099.0 0.6 Ma (assumed equal to FC-1)
Paces and Miller, 1993
IF Fish Canyon Tuff
28.201 0.012 Ma Lanphere et al., 2001; Kuiper et al., 2008
MD Mount Dromedary
99.12 0.14 Ma Renne et al., 1998
PX Peixe 563.5 1.6 Ma Gehrels et al., 2008
R3 Braintree complex
418.9 0.4 Ma Black et al., 2004
T2 Temora 2, Middledale gabbroic diorite
416.78 0.33 Ma Black et al., 2004
TR Tardree Rhyolite
61.23 0.11 Ma Dave Chew, pers. commun.
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