Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.376j&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 376
Other
Acfer Meteorites, Cosmogenic Nuclides, Nullarbor Meteorites, Terrestrial Ages
Scientific paper
The terrestrial age or residence time on the Earth's surface is important in determining the history of a meteorite. Carbon-14 has been used for a terrestrial-age indicator since 1962 [1,2]. Since 1984, small samples of meteorites of 0.1 to 0.5 g have been dated using accelerator mass spectrometry [3-5]. The precision of terrestrial age estimates is limited by the accuracy to which the saturated activity of ^14C in the meteorite is known. Jull et al. [4,5] used Bruderheim and some other chondrites to establish a saturated activity reference level. It is important to be aware that ^14C can vary with the depth and size of the object, and ^14C as a function of accurate depth has so far been measured only for one object, Knyahinya [7]. Carbon-14 is of particular interest in warmer climatic regions, where the storage time before a meteorite weathers away is expected to be much less than other locations, for example, Antarctica. This view was originally based on the work of Boeckl [7], who determined a "weathering half life" of some 3500 yr for chondrites from the southwestern U.S. This work was reinvestigated [5] and it was determined that the ^14C age distribution of the meteorites was longer than the earlier report. We have studied ^14C ages of meteorites from Roosevelt County, New Mexico [8], and from the western Libyan desert [9]. In both these areas meteorites of ages as old as 35,000 yr are observed, and the mean survival time at both locations is well over 10,000 yr. We have studied the ^14C age distribution of a large number of meteorites from Acfer, Algeria, and the Nullarbor Plain, Australia. Figure 1 presents the ^14C age distribution of Acfer samples compared to some other locations where a substantial number of ^14C ages have been obtained. The Algerian site shows a simple exponential dependence of terrestrial age vs. time, and no meteorites of >25 K.y. age. This is in contrast to the results from the southwestern U.S. [7] and from Roosevelt County [8]. One might expect that meteorites would be more well preserved in a very arid, hot climate, and some meteorites of longer age would be present, but this appears not to be the case. This interpretation is strengthened by the results from Nullarbor Plain, although the Australian collection does show some older samples. However, these two regions do show the expected exponential drop-off in number of meteorites of a given terrestrial age with time, which indicates the collections have been undisturbed over at least the last 20,000 yr. This is not seen in the U.S. meteorites. The less arid and colder high plains of Texas and New Mexico may be more conducive to storage of meteorites for long periods of time than these areas, but we believe some selection processes must be at work here and there is a deficit of "young" meteorites. References: [1] Suess H. and Wanke H. (1962) GCA, 26, 475. [2] Fireman E. L. (1978) Proc. LPSC 9th, 1647. [3] Beukens R. P. et al. (1988) Proc. NIPR Symp. Antarc. Met., 1, 224. [4] Jull A. J. T. et al. (1989) GCA, 53, 2095. [5] Jull et al. (1993) Meteoritics, in press. [6] Reedy R. C. (1993) LPS XXIV. [7] Boeckl R. P. (1972) Nature, 236, 25. [8] Jull A. J. T. et al. (1991) LPS XXII, 665. [9] Jull A. J. T. et al. (1990) GCA, 54, 2895. Fig. 1, which appears here in the hard copy, shows terrestrial ^14C ages from desert regions.
Bevan Alex W. R.
Brown Shaun T.
Donahue Douglas J.
Timothy Jull A. J.
Wlotzka Frank
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