Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30s.518h&link_type=abstract
Meteoritics, vol. 30, no. 5, page 518
Other
Scientific paper
Dalgety Downs (Western Australia) is a large (>200 kg), heavily weathered find that was recovered in many pieces. Herzog and Cressy [1] reported the 26Al activities and noble gas contents in a set of eight samples. The average 22Ne/21Ne ratios are all less than 1.08 and the 21Ne contents correlate weakly but positively with 22Ne/21Ne ratios. Both observations indicate heavy shielding. Surprisingly, while the 21Ne concentrations vary more than 50%, presumably in response to intersample shielding variations, the 26Al activities in a subset of those samples increase by only 25% (Figure 1). The difference is noteworthy because 26Al and 21Ne production rates normally depend on shielding in a similar way [2], as in Keyes or Knyahinya for example [3]. Herzog and Cressy interpreted the difference in shielding behavior in terms of weathering losses. Since then, studies of several other large meteorites have yielded similar if more pronounced patterns of cosmogenic nuclide concentrations, e.g., Jilin [4], Tsarev [5], and Bur Gheluai [6]. Such patterns are now thought to reflect complex exposure histories so a reexamination of Dalgety Downs seemed in order. Figure 1 shows 26Al and 10Be activities obtained by accelerator mass spectrometry for the same specimens analyzed by [1]. The new analyses refer to samples from which magnetic removal of metal lowered Fe contents to 16-18%, compared to 22% for bulk L chondrites. With allowance for the compositional change, the new 26Al results agree well with the old and confirm a 25% variation in 26Al activities. To date, 10Be activities have been measured in eight samples, namely, silicate- and metal-rich portions of 4190D, E, F, and G. The 10Be activities of the silicate-rich samples differ by 35%, perhaps slightly more than do the 26Al activities (Figure 1). Two observations point to a complex exposure for Dalgety Downs. First, as noted above, the 26Al/21Ne ratio varies more than expected. Second, the measured 26Al/10Be ratios of 4.0 and 3.5 exceed the value of 3.0 characteristic of silicate-rich material at saturation [2] even though the average 21Ne content of 6.610^-8 cm3 STP/g and 22Ne/21Ne ratio of 1.06 correspond to a nominal single-stage exposure age (15 My [7]) that is more than long enough to insure that 26Al and 10Be activities did reach saturation. 26Al/10Be ratios greater than 3.0 imply that the most recent stage of irradiation lasted only a few million years. If 26Al reached saturation during the second stage, then model calculations [6] indicate a preatmospheric radius of 100 cm or so for Dalgety Downs in the recent past. References: [1] Herzog G. F. and Cressy P. J. (1976) Meteoritics, 11, 59-68. [2] Vogt et al. (1990) Rev. Geophys., 28, 253-275. [3] Cressy P. J. (1975) JGR, 80, 1551-1554; Reedy R. C. et al. (1993) LPS XXIV, 1195-1196. [4] Honda et al. (1982) EPSL, 57, 101-109; Heusser et al. (1985) EPSL, 72, 263-272. [5] Nagai et al. (1993) GCA, 57, 3705-3723. [6] Vogt et al. (1993) Meteoritics, 28, 71-85. [6] Graf et al. (1990) GCA, 54, 2521-2534; Michel et al. (1991) Meteoritics, 26, 221-242. [7] Eugster O. (1988) GCA, 52, 1649-1659.
Cloudt S.
Herzog Gregory F.
Klein Jeff
Middleton Richard
Timothy Jull A. J.
No associations
LandOfFree
Cosmogenic Nuclides in the Dalgety Downs (L4) Chondrite: Evidence for Complex Exposure 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 Cosmogenic Nuclides in the Dalgety Downs (L4) Chondrite: Evidence for Complex Exposure, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Cosmogenic Nuclides in the Dalgety Downs (L4) Chondrite: Evidence for Complex Exposure will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-831031