Potassium-
Argon
| Rubidium-Strontium
| Sediment
Chronology
| Shipping
Information
Potassium-Argon
- Age Determination
- Radiogenic Ar-40 Analysis
Sample Size:
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Clean Mineral:
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5 - 10 grams
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Mica:
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2 - 5 grams
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Whole Rock:
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3 or 4 hand specimens or 200-300 grams
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Precision, Accuracy, and Sensitivity
The determination of an isotopic age is a
function of both geological and analytical factors. We
try to evaluate the suitability of a given rock for age
determination, but we cannot be responsible for the
effect of geological factors such as argon loss, excess
argon, high percentage of atmospheric argon, or sample
inhomogeneity on the precision and accuracy of the
calculated age. We do accept responsibility for high
analytical standards including:
- Calibration of both potassium and argon
measurements with widely accepted reference
materials
- Intercalibration with recognized experts in the
field
Because the absolute age of available reference
materials is not known to better than 2.5% (at the one
sigma level) due to sample inhomogeneity and
interlaboratory differences, we assign a minimum error of
2.5% on all calculated ages even though the precision of
the measurements is often better than 2.5%.
All measurements are performed in duplicate so
that the precision of each measurement can be assessed
for the actual sample being analyzed. If the uncertainty
of the measurement exceeds 2.5%, the observed value is
reported.
Our
nominal limit of analytical sensitivity is 10E-08 scc
radiogenic Ar-40/g and 0.01% K. Because of the
variability of geological and analytical parameters, a
single value for minimum detectable age cannot be
cited. For a biotite with 7.5% K and 10E-08 scc
radiogenic Ar-40 /g, the age would be 0.03 Ma. For a
basalt with 1.0% K and 10E-08 radiogenic Ar-40 /g, the
age would be 0.26 Ma. These values can serve as
nominal limits of sensitivity for assessing the
applicability of our present measurement technology to
your problems.
Rubidium-Strontium
- Rb-Sr analysis.
- 3 cogenetic rocks for Rb-Sr isochron.
- Additional samples on same isochron
- Sr-87/Sr-86 Isotopic ratio.
Sample Requirements:
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Whole-rock isochron:
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3 - 5 cogenetic rock samples
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Mineral isochron:
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Rock with 2 to 3 appropriate minerals
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Mineral date:
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Mineral with high Rb-Sr ratio
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Methods
All methods assume:
- Homogeneity of initial strontium isotope
ratios
- No re-equilibration since formation
- Existence of closed system since formation
For the whole-rock method, the assumptions apply
to the entire geologic formation or terrain. For the
mineral isochron and mineral date, they apply only to the
specimen analyzed.
In the whole-rock and mineral isochron methods,
the value of the initial strontium isotope ratio is
calculated. For the mineral date, the initial ratio is
estimated.
Sr-87/Sr-86 isotopic ratios can be used to
characterize rocks with regard to petrogenesis or for
making distinctions between differing geologic
provinces.
Mineral association and genetic relationships
can be established for diverse geologic problems ranging
from the study of ore deposits to determining the
provenance of deep sea sediments.
Our measurements are standardized using NIST
Standard Reference Materials or other internationally
recognized reference materials.
Sediment
Chronology
- Pb-210 analysis
- Cs-137 / Be-7 analysis
- Sediment Chronology Interpretation
Sample Requirements
Sediment core 5 cm or greater in diameter,
generally one meter or longer. Shipped in plastic core
liner or segmented by agreement with the laboratory.
Description
Analyses of Pb-210, Cs-137 and Be-7 in sediment
cores are used to interpret sedimentation rates in
rivers, lakes and estuaries. Results lead to the
calculation of calendar dates at various depths in the
sediment over the last 100 years, which are useful for
characterizing buried artifacts or contaminants.
Methods
At least 6 transverse segments of a core, often
2 cm thick, are required to establish a data trend which
can be interpreted. Radiochemical analyses of
Pb-210 and gamma spectral analyses of Cs-137 and Be-7 are
standardized with NIST traceable reference materials.
Methodology and interpretation are described
in:
Jeter, H. W. (2000), "Determining the Ages of
Recent Sediments Using Measurements of Trace
Radioactivity", Terra Et Aqua, 78, pp. 21-28
and
www.iadc-dredging.com/terra-et-aqua/2000/78-4.htm
Shipping
Information
Mineral Separations
If required, we will do mineral separations on
age determination samples. Please be sure to supply a
sample that, once separated, will provide the minimum
sample size that is specified for the age determination
procedure involved.
Delivery
Please allow 4 to 6 weeks for delivery of
results for K/Ar determinations and 6 to 8 weeks for
Rb/Sr determinations
Priority Service - 2 weeks - 50% premium on
total
Shipping Instructions
Samples that must pass United States Customs and
should be labeled:
Scientific Samples - No Commercial
Value
Please wrap and label all powdered samples
carefully.
Send Samples To:
Mass Spec Services
103 South Greenbush Road
Orangeburg, NY 10962 USA
Mass Spec
Services
Reading the isotopic
clocks
Tel:
845-359-1151
Fax:
845-359-1102
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