NTRS - NASA Technical Reports Server

Back to Results
Rubidium-Strontium Formation Age of Allan Hills 84001 CarbonatesOur preferred age for the formation of carbonates in the martian meteorite ALH 84001 is 3.90 plus or minus 0.04 Ga for Lambda (Rubidium 87) equals 0.01402 Ga (exp -1), or 3.85 plus or minus 0.04 Ga for Lambda (Rubidium 87) = 0.0142 Ga(exponent -1). This age is determined by a three-point Rb-Sr isochron defined by leachates of high-graded carbonate-rich material. Major cation and especially phosphorous analyses of the leachates permit contributions from igneous whitlockite to be recognized for low-acidity leachates, and the corresponding data are omitted from the isochron. Data for the two highest acidity leachates plot close to the preferred isochron, but are omitted because we believe they contain contributions leached from the pyroxene substrate on which most of the carbonates are found. Nevertheless, the isochron age for all five highest-acidity leachates is 3.94 plus or minus 0.04 Ga, and is within error of the age obtained for the more restricted data set. All leachates used to define the isochron have major cation compositions that are singular to those obtained by microprobe analyses of the carbonate rosettes and are consistent with progressive digestion of the carbonates according to their composition. The age thus obtained for the carbonates is about 600 m.y. younger than the crystalization age of ALH 84001 determined by Sm-Nd analyses but is within error limits of the age of impact metamorphism inferred from the Rb-Sr and Ar-Ar systematics of silicates. which yield ages of 3.85 plus or minus 0.05 Ga and 4.05- 3.80 Ga to 4.3-3.8 Ga, respectively. Similarities between the carbonate crystallization age and the age of impact metamorphism as determined by Ar-Ar and Rb-Sr suggest that the carbonate formation is impact-related. Nevertheless, both high and low- temperature scenarios for the origin of the carbonates are possible.
Document ID
Acquisition Source
Johnson Space Center
Document Type
Borg, L. E.
(NASA Johnson Space Center Houston, TX United States)
Nyquist, L. E.
(NASA Johnson Space Center Houston, TX United States)
Shih, C.-Y.
(Lockheed Martin Corp. Houston, TX United States)
Weismann, H.
(Lockheed Martin Corp. Houston, TX United States)
Reese, Y.
(Lockheed Martin Corp. Houston, TX United States)
Connelly, J. N.
(Texas Univ. Austin, TX United States)
Date Acquired
August 19, 2013
Publication Date
January 1, 1998
Publication Information
Publication: Workshop on the Issue Martian Meteorites: Where do we Stand and Where are we Going?
Subject Category
Distribution Limits
Work of the US Gov. Public Use Permitted.

Available Downloads

There are no available downloads for this record.
No Preview Available