Layer, P.W.
Solie, D.N.
200811
40Ar/39Ar ages from the Selawik A-2, A-3 and A-4 and Candle B-5 quadrangles, Alaska
tabular digital data
Raw Data File
RDF 2008-5
Fairbanks, Alaska - USA
State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys
14 p.
http://dx.doi.org/10.14509/17861
40Ar/39Ar data from seven plutonic samples and one metamorphic sample collected during mineral assessment studies in 1991 show that various rock units in the Selawik Hills plutonic complex were emplaced in the order syenite/monzonite (nonfoliated, 106.6 ± 0.8 Ma; foliated, 101.7 ± 0.7 Ma; both on hornblende), followed by nepheline syenite (103.7 ± 0.8 Ma on hornblende) and then quartz monzonite (100.4 ± 3.7 Ma on biotite). 40Ar/39Ar data from Granite Mountain hornblendes yielded ages slightly older, but overlapping, those from Selawik Hills. In this suite, the silica-undersaturated phase appears to be younger (105.7 ±0.8 Ma) than the silica-saturated phase(s) (106.8 ± 0.8 Ma and 107.1 ± 0.8 Ma).
In 1991, the Alaska Division of Geological & Geophysical Surveys (DGGS) conducted helicopter-supported field work in west-central Alaska as part a state-wide project to review selected areas of federal land that were eligible at the time for selection by Alaska to become state land. DGGS's role in the project was to evaluate mineral resource potential in the eligible units of land by acquiring data to outline permissive mineral terranes and to define mineral deposit types that could occur in each land selection unit (DGGS Staff, 1992). The data in this report resulted from investigations in the Selawik Hills and Granite Mountain areas as part of the land selection project. Geochemical and major oxide data (Solie and others, 1993a) and electron microprobe data (Solie and others, 1993b) were previously published.A brief discussion of our 40Ar/39Ar results from the Selawik Hills was presented at a Geological Society of America meeting (Solie and Layer, 1993). The purpose of this DGGS Raw Data File (RDF) is to present all our 40Ar/39Ar results from the Selawik Hills and Granite Mountain and make them available for use.
Attribute information for the followingtables (entities) is included in this metadata file under the "Entity_and_Attribution_Information" section:
>Locations_Results_AnalyticalData coordinates of sample locations, 40Ar/39Ar age summary, analytical data (Tables 1, 2, 3)
1991
observed
None planned
-161.5744
-159.8142
66.17045
65.40369
None
40Ar/39Ar
Age Dates
Analytical Results
Ar-Ar
Geochronology
Granite
Hornblende
Igneous Rocks
Plutonic Rocks
None
Granite Mountain
Candle Quadrangle
Selawik Quadrangle
None
Cretaceous
Albian
Early Cretaceous
Mesozoic
This report and/or dataset is available directly from the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (see contact information below).
Any hard copies or published datasets utilizing these datasets shall clearly indicate their source. If the user has modified the data in any way, the user is obligated to describe the types of modifications the user has made. User specifically agrees not to misrepresent these datasets, nor to imply that changes made by the user were approved by the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys.
State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys
GIS Data Manager/Cartographer
mailing and physical
3354 College Road
Fairbanks
AK
99709-3707
(907) 451-5029
(907) 451-5020
(907) 451-5050
dggspubs@alaska.gov
You may view our web site at http://www.dggs.alaska.gov for the latest information on available data.Please e-mail your questions and data requests when possible since our web site and e-mail address will remain current even if our phone number and mailing address change.
This research was supported by State funding to the Alaska Division of Geological & Geophysical Surveys. Additional Acknowledgments: members of the 1991 DGGS field teams: Thomas K. Bundtzen, Rainer J. Newberry, Greg M. Laird, Ellen E. Harris, Milt A. Wiltse, Jeffrey T. Kline, Thomas E. Smith, Shirley A. Liss and Karen H. Clautice.Review of draft manuscript by D.J. Szumigala.
All field locations were visually determined by the geologists in the field and recorded on a 1:63,360 topographic map. Coordinates were digitized visually from the field maps. DGGS submitted eight rock samples to the UAF Geochronology Laboratory for 40Ar/39Ar dating; three were collected from Granite Mountain in the Candle B-5 quadrangle and four were collected from the Selawik Hills in the Selawik quadrangle.Major oxide and selected trace element analyses for some of the samples are included in Solie and others (1993a). Separates for 40Ar/39Ar dating were prepared to >99.5 percent purity (visual inspection) using standard heavy liquid and magnetic separation techniques followed by hand-picking under a binocular microscope.Thin section examination of the samples prior to crushing indicated that the chosen minerals were free from alteration and sufficiently coarse-grained for mechanical separation.For all minerals, grains in the size range of 250 - 500 microns were used.For each sample, ~50-80 mg of biotite or 250-350 mg of hornblende or ‘whole rock’ (phenocryst-free groundmass) was packaged in an aluminum foil tube and irradiated in position 5C at the McMaster University nuclear reactor, in Hamilton, Ontario.Approximately 20 samples were irradiated at a time.Six packages containing ~20 mg of the standard mineral mmhb-1 (Samson and Alexander, 1987) with an age of 513.9 Ma (Lanphere and others, 1990) were also irradiated with the samples to determine the irradiation parameter (J) and the flux gradient in the reactor.Samples and standards were analyzed 45 to 90 days after irradiation. The irradiated samples were step-heated on-line in a Modifications Ltd. low-blank furnace.Temperature control was better than 5 degrees and a maximum temperature in excess of 1600o C was achievable to ensure complete sample fusion.The extracted argon was purified in a two-stage process using a liquid nitrogen cold finger and two SAES Zr-Al getters.Prior to measurement of the Ar, the gas was collected on an activated charcoal finger.The purified Ar gas was measured using a Nuclide 6-60-SGA 15 cm mass spectrometer.The sensitivity of the spectrometer is 6.5 x 10-15 mol/mV and system noise is generally around 0.02 mV.System blanks are generally better than 1 X 10-14 mol for 40Ar.Argon isotopic measurements for both samples and standards were corrected for the system blanks, for decay of37Ar and 39Ar, and for reactor-induced isotopic interferences.Ages were calculated using the equations and corrections from McDougall and Harrison (1999) and the constants from Steiger and Jaeger (1977).Age, Ca/K and Cl/K spectra plots are shown in figure 3, andanalytical age data are shown in Tables 2 and 3.All errors on analyses are reported at the 1-sigma level. For each mass spectrometer analysis, five Ar isotope abundances are measured.36Ar is used to determine the amount of atmospheric or initial Ar in the sample, 37Ar provides an estimation of the Ca content in the mineral, 38Ar provides an estimation of the Cl content, 39Ar reflects the K content and 40Ar is a mixture of initial and radiogenic Ar.The age of the sample is proportional to the ratio of the amount of radiogenic 40Ar to the amount of 39Ar produced by neutron bombardment from 40K.All samples were dated by the 40Ar/39Ar step-heating method.In this method, a sample is heated to progressively higher temperatures in a double-vacuum resistance-heated furnace, and the argon isotopes are measured and age determined for each step (fraction).The integrated age is the age given by the total gas measured in all fractions and is equivalent to a potassium-argon (K-Ar) age. The fractions are commonly plotted on an age spectrum plot for analysis.A spectrum provides a plateau age if three or more consecutive gas fractions represent at least 50% of the total gas release and are within two standard deviations of each other (Mean Square Weighted Deviation less than ~2.5).If the fractions do not meet the criteria for a plateau due to intrafraction scatter in ages, then a weighted mean age of ‘plateau-like’ fractions, with each fraction weighted by the volume of 39Ar released, is calculated.
No topologic relationships are present in the data.
This dataset includes all of the samples submitted for 40Ar/39Ar age date analyses from the Selawik and Candle quadrangle which were collected by DGGS during the summer of 1991.Hand samples and thin sections are no longer available for the samples dated. The brief sample descriptions in text are compiled from field notes and some preliminary petrographic notes from 1991.
Sample locations were determined visually by geologist in the field, using a brunton compass and altimeter and topographic map. Locations were then digitized visually from the field maps using ArcGIS v9.2.
100
The sample locations were found in the field using a compass and altimeter. Considering the errors inherent in the USGS topographic maps, locating oneself in the field,and the methods of digitizing stations, samples are located to an accuracy of between 10 and 100 meters.
Lanphere, M.A.
Dalrymple, G.B.
Fleck, R.J.
Pringle, M.S.
1990
Intercalibration of mineral standards for K/Ar and 40Ar/39Ar age measurements
journal article
EOS Transactions of the American Geophysical Union
v. 71
(abstract), P. 1658
paper
1990
publication date
Lanphere and others, 1990
Describes intercalibration standards
McDougall, Ian
Harrison, T.M.
1999
Geochronology and Thermochronology by the 40Ar/39Ar method
2
book chapter
Oxford
Oxford University Press
269 pp
paper
1999
publication date
McDougall and others, 1999
Source reference for equations and corrections used in age calculation.
Samson, S.D.
Alexander, E.C.
1987
Calibration of the interlaboratory 40Ar/39Ar dating standard, MMhb-1
journal article
Chemical Geology
v. 66
p.27-34
paper
1987
publication date
Samson and Alexander, 1987
Interlaboratory calibration data for age determination
Steiger, R.H.
Jager, E
1977
Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology
journal article
Earth and Planetary Science Letters
v.36
p. 359-363
paper
1977
publication date
Steiger and Jager, 1977
discussion of use of decay constants in age determination
Solie, D.N.
Harris, E.E.
Bundtzen, T.K.
Wiltse, M.A.
Newberry, R.J.
Kline, J.T.
Smith, T.E.
199303
Land Selection Unit 16 (Selawik, Candle, Norton Bay, Unalakleet, Kateel River, and Nulate quadrangles): references, DGGS sample locations, geochemical and major oxide data
document
Public Data File
PDF 93-16a
Fairbanks AK
Alaska Division of Geological & Geophysical Surveys
54 p.
paper
1993
observed
Solie and others, 1993a
geochemical and major oxide data from study area
Solie, D.N.
Severin, K.P.
Lear, K.G.
199305
Electron microprobe data from Selawik Hills and Granite Mountain plutonic rocks, western Alaska
document
Public Data File
PDF 93-16b
Fairbanks AK
Alaska Division of Geological & Geophysical Surveys
11 p.
paper
1993
observed
Solie and others, 1993b
electron microprobe data from rocks in study area
Solie, D.N.
Layer, P.W.
1993
Evidence of synmagmatic foliation in the Selawik Hills, NW Alaska, based on 40Ar/39Ar age determinations
document
Geological Society of America Abstracts with Programs
v.25, no.5
abstract, p.149.
paper
1993
observed
Solie and Layer, 1993
preliminary presentation of age results
Fieldwork - Alaska Division of Geological & Geophysical Surveys (DGGS) conducted helicopter-supported field work in west-central Alaska as part a state-wide project to review selected areas of federal land that were eligible at the time for selection by Alaska to become state land.Rock samples were collected in the Selawik Hills and Granite Mountain areas during summer 1991. Members of the field crew were Diana N. Solie, Thomas K. Bundtzen, Rainer J. Newberry, Greg M. Laird, Ellen E. Harris, Milt A. Wiltse, Jeffrey T. Kline, Thomas E. Smith, Shirley A. Liss and Karen H. Clautice.Geochemical and major oxide data from samples collected during the Selawik Hills project were published (Solie and others, 1993a). Selected electron microprobe data from rocks collected from Selawik Hills and Granite Mountain were published in Solie and others, 1993b. A brief discussion of the 40Ar/39Ar age results was given in an abstract by Solie and Layer (1993).
Solie and others, 1993a
Solie and others, 1993b
Solie and Layer, 1993
199107
Labwork - Separates for 40Ar/39Ar dating were prepared to >99.5 percent purity (visual inspection) using standard heavy liquid and magnetic separation techniques followed by hand-picking under a binocular microscope.Thin section examination of the samples prior to crushing indicated that the chosen minerals were free from alteration and sufficiently coarse-grained for mechanical separation.For all minerals, grains in the size range of 250 - 500 microns were used.For each sample, ~50-80 mg of biotite or 250-350 mg of hornblende or ‘whole rock’ (phenocryst-free groundmass) was packaged in an aluminum foil tube and irradiated in position 5C at the McMaster University nuclear reactor, in Hamilton, Ontario.Approximately 20 samples were irradiated at a time.Six packages containing ~20 mg of the standard mineral mmhb-1 (Samson and Alexander, 1987) with an age of 513.9 Ma (Lanphere and others, 1990) were also irradiated with the samples to determine the irradiation parameter (J) and the flux gradient in the reactor.Samples and standards were analyzed 45 to 90 days after irradiation.
The irradiated samples were step-heated on-line in a Modifications Ltd. low-blank furnace.Temperature control was better than 5 degrees and a maximum temperature in excess of 1600o C was achievable to ensure complete sample fusion.The extracted argon was purified in a two-stage process using a liquid nitrogen cold finger and two SAES Zr-Al getters.Prior to measurement of the Ar, the gas was collected on an activated charcoal finger.The purified Ar gas was measured using a Nuclide 6-60-SGA 15 cm mass spectrometer.The sensitivity of the spectrometer is 6.5 x 10-15 mol/mV and system noise is generally around 0.02 mV.System blanks are generally better than 1 X 10-14 mol for 40Ar.Argon isotopic measurements for both samples and standards were corrected for the system blanks, for decay of37Ar and 39Ar, and for reactor-induced isotopic interferences.Ages were calculated using the equations and corrections from McDougall and Harrison (1999) and the constants from Steiger and Jaeger (1977). All errors on analyses are reported at the 1-sigma level.
For each mass spectrometer analysis, five Ar isotope abundances are measured.36Ar is used to determine the amount of atmospheric or initial Ar in the sample, 37Ar provides an estimation of the Ca content in the mineral, 38Ar provides an estimation of the Cl content, 39Ar reflects the K content and 40Ar is a mixture of initial and radiogenic Ar.The age of the sample is proportional to the ratio of the amount of radiogenic 40Ar to the amount of 39Ar produced by neutron bombardment from 40K.
Samson and Alexander, 1987
Lanphere and others, 1990
McDougall and Harrison, 1999
Steiger and Jaeger, 1977
1991
Point
Universal Transverse Mercator
4
9.9996
-153.000
0
500000
0
coordinate pair
1
1
meters
Locations_Results_AnalyticalData
North American Datum of 1927
Clarke 1866
6378206.4
294.978698
Locations_Results_AnalyticalData
Table1_coordinates.csv
Coordinates for 40Ar/39Ar samples
Alaska Division of Geological & Geophysical Surveys
SampleNo
Items in the Sample Number field
Alaska Division of Geological & Geophysical Surveys
Sample numbers are unique identifiers derived from the last two digits of the year the sample was collected, initials of the geologist's name who collected it, and the consecutively numbered station location. If more than one sample was collected from the station location, each is assigned a letter (a, b, c, etc) designator as well.
Easting
UTM easting coordinate, Zone 4, NAD 27
Alaska Division of Geological & Geophysical Surveys
394311
463220
m
Northing
UTM northing coordinate, Zone 4, NAD 27
Alaska Division of Geological & Geophysical Surveys
7255163
7339612
m
Latitude
Latitude coordinate, NAD 27
Alaska Division of Geological & Geophysical Surveys
65.40369
66.17045
decimal degree
Longitude
Longitude coordinate, NAD27
Alaska Division of Geological & Geophysical Surveys
159.81421
161.57441
decimal degree
Locations_Results_AnalyticalData
Table2_agesummary.csv
Summary table for 40Ar/39Ar data
Alaska Division of Geological & Geohysical Surveys
Min.
Material used for age date
Alaska Division of Geological & Geophysical Surveys
Material used for dating. Includes Bio (biotite) or Hbd (hornblende).
Integrated Age (Ma)
Integrated age in million years,
Alaska Division of Geological & Geophysical Surveys
98.8+-0.7
107.4+-0.8
Ma
Plateau Age
Summarizes the Plateau (P) Age or the Weighted Mean (W) Age in Million years for each sample
Alaska Division of Geological & Geophysical Surveys
100.4+-3.7
107.1+-0.8
Ma
Plateau Information
Summarizes number of fractions, percent 39Ar released and MSWD for each sample
Alaska Division of Geological & Geophysical Surveys
# of fractions, % 39Ar released, MSWD for each sample
SampleName
Items in the Sample Name field
Alaska Division of Geological & Geophysical Surveys
Sample numbers are unique identifiers derived from the last two digits of the year the sample was collected, initials of the geologist's name who collected it, and the consecutively numbered station location. If more than one sample was collected from the station location, each is assigned a letter (a, b, c, etc) designator as well.
Rock type
name of rock type of sample
Alaska Division of Geological & Geophysical Surveys
rock type, either a type of plutonic rock or a generalized metamorphic rock
Mass
mass of sample that was dated, in grams
Alaska Division of Geological & Geophysical Surveys
0.0605
0.3588
grams
#
number of fractions in the step heat
Alaska Division of Geological & Geophysical Surveys
9
12
each
K2O
Weight % K2O as estimated from 39Ar release
Alaska Division of Geological & Geophysical Surveys
0.72
7.15
weight %
CaO
weight % CaO estimated from 37Ar release
Alaska Division of Geological & Geophysical Surveys
0.64
15.37
weight %
Isochron age
Isochron age and intial 40Ar/36Ar, both calculated from regression of isotopic compositions of individual step-heated fractions. Initial is the intercept of regression line.
Alaska Division of Geological & Geophysical Surveys
101.6+-0.8
107.1+-0.8
Ma
Locations_Results_AnalyticalData
Table3_analytical data.csv
For each sample, the 40Ar/39Ar analytical data associated with each temperature recorded is presented.
Alaska Division of Geological & Geophysical Surveys
Temp
Temperature in Degrees C of step for which data were recorded
Alaska Division of Geological & Geophysical Surveys
400
1600
Degrees C
Cum. 39Ar
Cumulative 39Argon
Alaska Division of Geological & Geophysical Surveys
0.0000
1.0000
40Ar/39Ar measured
Measured 40Ar/39Ar ratio at each heating step, corrected for blank and 39Ar decay
Alaska Division of Geological & Geophysical Surveys
0.0001
634.0000
+/-
Defines the size of the error bars for each ratio measurement to one standard deviation
Alaska Division of Geological & Geophysical Surveys
0.0000
509.0000
37Ar/39Ar measured
The ratio of 37Ar to 39Ar measured at each temperature interval for each sample corrected for blank and 37Ar and 39Ar decay
Alaska Division of Geological & Geophysical Surveys
0.00000
23.00000
36Ar/39Ar
Ratio of amounts of 36Ar to 39Ar measured at each temperature interval for each sample corrected for blank and 39Ar decay
Alaska Division of Geological & Geophysical Surveys
0.0000
3.2000
% Atm. 40Ar
Percent of 40Ar which is atmospheric in composition
Alaska Division of Geological & Geophysical Surveys
0.0
150.0
Ca/K
ratio of Calcium to Potassium as determined from 37Ar/39Ar
Alaska Division of Geological & Geophysical Surveys
0.0000
43.0000
Cl/K
ration of Chloring to Potassium as determined from 38Ar/39Ar
Alaska Division of Geological & Geophysical Surveys
0.00000
0.15000
40Ar*/39ArK
ratio of radiogenic 40Ar to 39Ar derived from potassium
Alaska Division of Geological & Geophysical Surveys
0.0000
12.0000
Age (Ma)
calculated age for each temperature step for each sample
Alaska Division of Geological & Geophysical Surveys
0.0
185.0
Ma
+-(Ma)
Defines the size of the error bars for each measurement to one standard deviation
Alaska Division of Geological & Geophysical Surveys
0.2
3167.0
Ma
State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys
Natural Resource Technician
mailing and physical
3354 College Road
Fairbanks
AK
99709-3707
USA
907-451-5020
None available.
907-451-5050
dggspubs@alaska.gov
8 am to 4:30 pm, Monday through Friday, except State holidays
Please view our Web site (http://www.dggs.alaska.gov) for the latest information on available data. Please contact us using the e-mail address provided above when possible.
Raw-Data File
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20081031
State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys
Metadata Manager
mailing and physical address
3354 College Road
Fairbanks
AK
99709-3707
USA
907-451-5039
907-451-5050
dggspubs@alaska.gov
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Please contact us through the e-mail address above whenever possible.
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If the user has modified the data in any way they are obligated to describe the types of modifications they have performed in the supporting metadata file. User specifically agrees not to imply that changes they made were approved by the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys.