EXPOSURE HISTORIES OF ALH 84001 AND ALHA 77005.  K. Nishiizumi1, M. W. Caffee2, and R. C. Finkel2, 1Space Sciences Laboratory, University of California, Berkeley CA 94720, USA, 2CAMS, Lawrence Livermore National Laboratory, Livermore CA 94551, USA.

Published in Meteoritics, 29, p. 511.

From cosmogenic nuclide studies of SNC meteorites exposure histories, ejection conditions from the hypothesized martian parent body, and genetic relationships between SNC meteorites have been determined. Previous studies show ablation to have been very low in at least three shergottites, ALHA 77005, Shergotty, and EETA 79001 [1,2]. This suggests that the atmospheric entry velocity and/or entry angle of shergottites must have been much lower than of ordinary chondrites. We report here the results of cosmogenic radionuclides in the newly classified SNC meteorite ALH 84001 and additional studies of ALHA 77005. Table 1 shows 10Be (half-life = 1.5 m.y.) and 36Cl (0.30 m.y.) results for these two meteorites along with previous measurements of the shergottite LEW 88516. Aluminum-26 (0.71 m.y.) measurements are in progress.

TABLE 1.  10Be and 36Cl concentrations in ALH 84001 and ALHA 77005 meteorites.

MeteoriteDepth10Be (dpm/kg)36Cl (dpm/kg)
ALH 84001,970.5-3.5 mm from
fusion crust
20.03 ± 0.705.93 ± 0.10
ALH 84001,977-9 mm from
fusion crust
19.81 ± 0.376.16 ± 0.12
ALH 84001,13910-12 mm from
fusion crust
20.25 ± 0.296.01 ± 0.11
ALHA 77005,125below fusion crust14.04 ± 0.237.63 ± 0.16
ALHA 77005,126bottom of sample13.94 ± 0.395.52 ± 0.13
ALHA 77005,12715-20 mm from
the nearest surface
14.02 ± 0.235.87 ± 0.10
ALHA 77005,7843 mm from pre-
atmospheric surface
16.23 ± 0.82*4.82 ± 0.19
LEW 88516,13homogenized bulk16.70 ± 0.278.35 ± 0.12
                            *  From Nishiizumi et al. [2].
                            †   From Nishiizumi et al. [8].

ALH 84001:  We received two chips on opposite sides of ALH 84001. Two subsamples, at depths of 0.5-3.5 mm and 7-9 mm, from fusion crust were separated from ALH 84001,97. The 14C terrestrial age is 6.5 ± 1.3 k.y. [3]. The noble gas exposure age is reported to be 14 ± 2 m.y., which is slightly longer than Chassigny [4]. The 10Be and 36Cl concentrations in three subsamples are nearly constant. A reasonable interpretation is that there are no SCR (solar cosmic ray) effects at these sample depths, implying at least 3 cm ablation. The 10Be production rate is estimated to be 21-24 atom/min·kg based on recovered size (17 × 9.5 × 6.5 cm) and over 3 cm of ablation depth, although the chemical composition of our sample is not yet measured. The 10Be concentration indicates that ALH 84001 was exposed to cosmic rays 4-7 m.y. in a 4-pi geometry. The 10Be exposure age is significantly shorter than noble gas exposure age. If ALH 84001 was ejected from a shallow depth of the parent body 4-7 m.y. ago this would require nearly half of the cosmogenic noble gases to be produced in the parent body. However, since the 10Be concentration is near the saturation value and the chemical composition and depth of samples are not well known, this model is not unequivocal. Manganese-53 (3.7 m.y.) measurements are required to resolve the particular conditions under which this exposure occurred. The measured activities in ALH 84001 is similar to that of Chassigny, 20.5 ± 3.1 dpm10Be/kg [5] and 6.60 ± 0.16 dpm 36Cl/kg (unpublished).

ALHA 77005:  Three of the subsamples of ALHA 77005 measured for cosmogenic radionuclides are aliquots from the noble gas study [6]. The size of this recovered meteorite is 9.5 × 7.5 × 5.25 cm. The preatmospheric size is estimated to be 5-6 cm in radius based on 1.5-2.5 cm of ablation [2]. Based on 36Cl in deep sample (,78) and 81Kr [7], the terrestrial age of the meteorite is 210 ± 70 k.y. The high 36Cl concentrations in ALHA 77005,125 relative to the concentration at a greater depth indicate 36Cl production by SCR. Chemical analysis and 26Al measurements for these three subsamples are in progress.

References:  [1] Nishiizumi K. et al. (1986) GCA, 50, 1017-1021. [2] Nishiizumi K. et al. (1986) Meteoritics, 21, 472-473. [3] Jull A. J. T. et al. (1994) LPS XXV, 647-648. [4] Miura Y. N. et al. (1994) LPS XXV, 919-920. [5] Pal D. K. et al. (1986) GCA, 50, 2405-2409. [6] Garrison D. H. et al. (1994) LPS XXV, 403-404. [7] Schultz L. and Freundel M. (1984) Meteoritics, 19, 310. [8] Nishiizumi K. et al. (1992) Meteoritics, 27, 270.