Two-Stage Exposure of the Fayetteville Meteorite Based on 129I

Details Two-Stage Exposure of the Fayetteville Meteorite Based on 129I Two-Stage Exposure of the Fayetteville Meteorite Based on 129I

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.412n&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 412

Other

Abee, Allende, Cosmogenic Nuclides, Dhajala, Fayetteville

Scientific paper

Cosmogenic nuclides and cosmic ray track densities in the solar noble gas-rich brecciated H-chondrite Fayetteville have been reported [1] . The authors found a discrepancy between exposure ages based on noble gases (27-32 m.y.) and on cosmic ray tracks (4-7 m.y.). We obtained nearby samples from Dr. P. Pellas and measured ^129I (half-life = 1.57 x 10^7 year) in two locations, "S" and "UV" of Fayetteville. The steep track density gradient indicates that sample "S" was a few centimeters from the preatmospheric surface [1]. The distance between "S" and "UV" is 3-4 cm. The ^129I AMS measurements were made at the University of Rochester and the results are shown in Table 1. The blank correction was (49 +/- 27) x 10^-15 ^129I/^127I. Measurements of ^129I in the two samples are in good agreement. Since the main target element for production of cosmogenic ^129I in meteorites is Te, the ^129I concentrations were normalized to Te content and are shown in Table 1. We adopted 0.50 ppm as the Te concentration in Fayetteville [2]. The measured ^129I concentration is higher than the saturation values of Abee or Dhajala and similar to that of a sample of Allende (USNM 3529) that was located 25-30 cm from the surface of a preatmospheric body with a 55-65-cm radius [3]. Iodine-129 in Fayetteville was produced by ^128Te(n,gamma) reactions in addition to ^130Te(n,2n) reactions. Based on comparison with other (n,gamma) products such as ^60Co, ^41Ca, and ^36Cl in Allende and in the Apollo 15 core, we conclude that the ^129I in Fayetteville was produced at a depth below 20 cm within a 60-150-cm-radius object. The observed ^129I activity cannot be explained by either a shallow depth within an object of the recovered size or by 2-pi bombardment in a large parent body. Our ^129I results indicate that Fayetteville experienced a two- stage irradiation with 4-pi production dominating in both bombardments. In this scenario most of the ^129I and cosmogenic noble gases were produced at or below a depth of 20 cm in a large body during a 25-40-m.y. first-stage bombardment. The meteoroid (or parent body) was then broken up and sample "S" was exposed in a near-surface location. The cosmic ray tracks and short half- life nuclides were produced during the 4-7 m.y. of this second-stage bombardment. Measurement of ^36Cl in the stony phase and measurement of ^53Mn will constrain the duration of the second-stage exposure and the size of the object during this stage. The slightly high ^22Ne/^21Ne ratio observed in the meteorite [1] may suggest a complex irradiation prior to the first stage described here. The measurement of ^129I extends the study of exposure histories of extraterrestrial materials to tens of million years. References: [1] Wieler R. et al. (1989) GCA, 53, 1449-1459. [2] Xiao X. and Lipschutz M. E. (1991) GCA, 55, 3407-3415. [3] Nishiizumi K. et al. (1983) Nature, 305, 611-612. Table 1, which appears here in the hard copy, shows ^129I results in the Fayetteville meteorite.

Affiliated with

Also associated with

No associations

Rating

Two-Stage Exposure of the Fayetteville Meteorite Based on 129I does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.

If you have personal experience with Two-Stage Exposure of the Fayetteville Meteorite Based on 129I, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Two-Stage Exposure of the Fayetteville Meteorite Based on 129I will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFWR-SCP-O-1073080