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National Geochronological Data Base

Table of Contents

• Background
• Extent of Coverage
• Data Characteristics
• Data Organization
• Applications and Related Data Sets
• References
• Appendix

Background

The National Geochronological Data Base (NGDB) was established by the United States Geological Survey (USGS) to collect and organize published isotopic (also known as radiometric) ages of rocks in the United States. The NGDB was started in 1974 by a committee that was appointed by the Director of the USGS. Their mission was to investigate the feasibility of compiling the published radiometric ages for the United States into a computerized data bank for ready access by the user community. A successful pilot program, which was conducted in 1975 and 1976 for the State of Wyoming, led to a decision to proceed with the compilation of the entire United States.

For each dated rock sample reported in published literature, a record containing information on sample location, rock description, analytical data, age, interpretation, and literature citation is constructed and included in the NGDB. The NGDB and a program to search the data files are published and distributed on Compact Disc-Read Only Memory in standard ISO 9660 format as USGS Digital Data Series DDS-14. As of May 1994, the NGDB consists of more than 18,000 records containing over 30,000 individual ages, which is believed to represent approximately one-half the number of ages published for the United States through 1991. Building the isotopic age data base is an ongoing process as geochronological information becomes available.

Extent of Coverage

The NGDB provides coverage for the United States. See the Appendix for a table containing number of records and percent of coverage for each State.

Data Characteristics

The NGDB is contained in a Helix Express relational data base program on a Macintosh computer. The data base is organized into records containing a number of relationships. A separate relationship is maintained for each analytical technique, which is designed to contain all the relevant analytical information available regarding each record's analysis.

Each record provides a Location and Geology Form containing general data about the sample and a Methods Form containing the results of analysis for each analytical method used. See the Appendix for more information about each analytical method.

Analytical Methods

• Fission Track
  
• Lead-Alpha (Pb-Alpha)
  
• Potassium-Argon (K-Ar and including ⁴⁰Ar/³⁹Ar)
  
• Rubidium-Strontium (Rb-Sr)
  
• Samarium-Neodymium (Sm-Nd)
  
• Uranium-Thorium-Lead (U-Th-Pb)
  

More information about the data contained in the forms may be found at Universal Resource Locator http://ncgmp.cr.usgs.gov/ncgmp/ngdb/ngdb.htm.

Data Organization

The occurrence of isotopic ages in scientific literature is identified primarily by performing keyword searches on the GeoRef data base (e.g., geochronology, isotope ages). Some ages have been located by cross references when geochronology constituted a minor part of an article in a publication identified in the GeoRef data base. When an age is reported more than once in the literature, its first appearance accompanied with supporting analytical data is used as the primary reference. There are currently more than 1,300 references entered in the NGDB.

For each dated sample reported, a record containing information on sample location, rock description, analytical data, age, interpretation, and literature citation is constructed. The data and metadata are stored in fixed field ASCII files.

Calculated ages in the original data have been recomputed using currently accepted decay constants, although the original constants have been retained. Throughout the data base, the same numerical precision was maintained as that contained in the original source publication. However, in preparation of the constant format ASCII flat files for the CD-ROM, all values within a field have been given the same number of digits. The user should be aware that the values may not be as precise as indicated. Some fields in the relational data base were left blank if no value was given in the source publication. These may have been converted to zeroes on the CD-ROM.

Applications and Related Data Sets

The data from the NGDB may be read directly into other data bases or commercial spreadsheet programs for customized searches of the records. This data set may be useful to geologists, other Earth scientists, mining companies, and Federal, State, and local agencies in the application of renewable and nonrenewable resource management, land use planning, and environmental quality studies.

In addition to the NGDB, the CD-ROM contains the complementary Natural Radioelement Data Base (NRDB). The NRDB contains quantitative analyses of over 8,500 rock and ore samples for three natural radioelements (uranium, thorium, and potassium).

References

Zartman, R.E., Bush, C.A., and Abston, Carl, 1995, National Geochronological and Natural Radioelement Data Bases, [data base] A, National Geochronological Data Base [and data base] B, Natural Radioelement Data Base, U.S. Geological Survey Digital Data Series DDS-14. [Published on CD-ROM.]

Bates, R.L., and Jackson, J.A., eds., 1980, Glossary of geology (2d ed.): Falls Church, Va., American Geological Institute, 749 p.

Disclaimer: Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Appendix

• State Compilation Table
• Analytical Methods of Isotopes

State Compilation Table

The following table gives a State-by-State breakdown of the number of records and estimated percent of coverage through 1991. Percent of coverage is based on a ratio of compiled to total identified literature. This is a highly imprecise estimate because the number of dated samples varies considerably among published articles.

State		Number of Records 
	Estimated Percent of Coverage

Alabama			17			60
Alaska                 511                      20
Arizona                936                      40
Arkansas                10                      10
California           2,748                      70
Colorado               720                      50
Connecticut            402                      70
Delaware                15                      20
D.C.                     0                       0
Florida                 29                      60
Georgia                245                      50
Hawaii                   0                       0
Idaho                  748                      70
Illinois                26                      50
Indiana                  8                      30
Iowa                    10                      30
Kansas                 118                      70
Kentucky                 3                      50
Louisiana                0                       0
Maine                  542                      70
Maryland                19                      40
Massachusetts          346                      70
Michigan               137                      60
Minnesota              570                      60
Mississippi              6                      50
Missouri               183                      60
Montana                990                      50
Nebraska                41                      60
Nevada               2,160                      70
New Hampshire          280                      60
New Jersey              21                      20
New Mexico             928                      40
New York               194                      40
North Carolina         685                      70
North Dakota            14                      50
Ohio                    17                      50
Oklahoma                45                      50
Oregon               1,016                      60
Pennsylvania            41                      40
Rhode Island            88                      60
South Carolina         217                      70
South Dakota            62                      50
Tennessee               78                      50
Texas                  190                      40
Utah                   770                      60
Vermont                 76                      60
Virginia               184                      60
Washington             828                      70
West Virginia           16                      50
Wisconsin              186                      70
Wyoming                779                      70

Analytical Methods of Isotopes

Fission Track

A method of calculating an age in years by determining the ratio of the spontaneous fission-track density to the amount of uranium present. The method, which has been used for ages from 20 years to 1.4 X 10⁹ years, works best for micas, tektites, and meteorites, and is also useful for determining the amount and distribution of the uranium in the sample. (Bates and Jackson, 1980)

Lead-Alpha

A method of calculating an age in years by spectrographically determining the total lead content and the alpha-particle activity representing the uranium-thorium content. This age method is less precise than the potassium-argon or rubidium-strontium age methods, and is best used for rocks younger than Precambrian. (Bates and Jackson, 1980) This method is no longer used.

Potassium-Argon

Determination of the age of a mineral or rock in years, based on measurement of the ratio of radiogenic argon-40 to potassium-40 and the known radioactive decay rate of potassium-40 to argon-40. (Bates and Jackson, 1980) A few ages are included using the ⁴⁰Ar/³⁹Ar method which can produce more precise ages from smaller samples.

Rubidium-Strontium

Determination of an age for a mineral or rock in years based on the ratio of radiogenic strontium-87 to rubidium-87 and the known radioactive decay rate of rubidium-87. If ratios are measured for more than one phase of a single rock, or for a number of related rocks that differ in rubidium content, an isochron may be drawn. (Bates and Jackson, 1980)

Samarium-Neodymium

A method of age determination based on the alpha decay of samarium-147 to neodymium-143 ( = 6.54 X 10^_12yr^_1). The ratios ¹⁴⁷Sm / ¹⁴⁴Nd and ¹⁴³Nd / ¹⁴⁴Nd are measured and plotted on an isochron diagram. Lunar rocks 4.40 X 10⁹ years old have been dated by this method. (Bates and Jackson, 1980)

Uranium-Thorium Lead

Calculation of an age in years for geologic material, often zircon, based on the known radioactive decay rate of uranium-238 to lead-206, uranium-235 to lead-207, and thorium-232 to lead-208, whose ratios give three independent ages for the same sample. The determined lead-207 / lead-206 ratio can be used to compute a fourth age (lead-lead age). The method is most applicable to minerals that are Precambrian in age. Whether all four possible dates are concordant or discordant is useful for evaluating the results of this method, used alone or in comparison with other methods, and in determining whether the initially closed system has been disturbed. (Bates and Jackson, 1980)

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