Terrestrial ages of antarctic meteorites
Elmore, D., F.A. Rickey, P.C. Simms, M.E. Lipschutz, K.A. Mueller, and T.E. Miller. In preparation. PRIME Lab: A dedicated AMS facility at Purdue University. In A. Long (Ed.), Proceedings of the Fourteenth International Radiocarbon Conference, 20-24 May 1991, Tucson, Arizona, Center for the Study of Environmental Radioisotopes, University of Arizona. Koeberl, C., and W.A. Cassidy. 1991. Differences between Antarctic and non-Antarctic meteorites: An assessment. Geochimica et Cosmochitnica Acta, 55, 3-18. Lindstrom, MM., D.W. Mittlefehldt, R.R. Martinez, M.E. Lipschutz, and M.-S. Wang. 1991a. Geochemistry of Yamato 82192, 86032 and 793274 Lunar Meteorites. Proceedings of the NIPR Symposium on Antarctic Meteorites, 4, 12-32. Lindstrom. M.M., S.J. Wentworth, R.R. Martinez, D.W. Mittlefehldt, D.S. McKay, M.-S. Wang, and M.E. Lipschutz. 1991b. Geochemistry and Petrology of the MacAlpine Hills Lunar Meteorites. Geochimica et Cosmochimica Acta, 55, 3089-3103. Lipschutz, M.E. 1991a. Meteorite studies: Terrestrial and extraterrestrial applications, 1990. Antarctic Journal of the U.S., 25(5), 49-50.
Lipschutz, M.E. 1991b. The impact of Antarctic meteorites on the conventional view of the inner solar system. Observatory, 111, 7-8. Lipschutz, M.E., and S.M. Samuels. 1991. Ordinary chondrites: Multivariate statistical analysis of trace element contents. Geochmnmica et Cosmochimica Acta, 55, 19-47 Paul, R.L., and M.E. Lipschutz. 1990a. Chemical studies of differentiated meteorites—I. Labile trace elements in Antarctic and nonAntarctic eucrites. Geochimica et Cosmnochimica Acta, 54, 3185-3195. Paul, R.L., and M.E. Lipschutz. 1990b. Consortium study of labile trace elements in some Antarctic carbonaceous chondrites: Antarctic and non-Antarctic meteorite comparisons. In K. Yana (Ed.), Proceedings of the NIPR Symposium on Antarctic Meteorites, 3, 80-95, 6-8 June 1989, Tokyo, Japan, National Institute of Polar Research. Sack, R.O., W.J. Azeredo, and M.E. Lipschutz. 1991. Olivine diogenites: The mantle of the eucrite parent body. Geochimica et Cosmochimica Acta, 55, 1111-1120.
Terrestrial ages of antarctic meteorites
antarctic mountains and moraines to understand duration of exposure above glaciers and erosion rates for the antarctic continent (Nishiizumi et al. 1986; Nishiizumi et al. 1991). We have measured terrestrial ages of meteorites from 29 locations in Antarctica although only one or two meteorites were measured at many sites. We found stony meteorites with ages greater than 200,000 years at seven locations (Allan Hills, Dominion Range, Elephant Moraine, Lewis Cliff, MacAlpine Hills, Pecora Escarpment, and Thiel Mountain). Four iron meteorites (DRP7800I-9, 1LD83500, Lazarev, and Mount Wegener), which were found on bedrock or in moraines, are all old (0.26-5 million years). The figure shows a histogram of terrestrial ages of Yamato, Allan Hills, and other antarctic meteorites. Pairs of meteorites are shown as one object plotted at the average age. Although the total amount of data is nearly double, the general trend remains the same as in the previous publication on this subject (Nishiizumi, Elmore, and Kubik 1989). There are four interesting points. • Many of the Lewis Cliff meteorites are as old as the Allan Hills (Main Icefield**) meteorites. Although we have measured only nine Lewis Cliff meteorites, five out of nine of them have terrestrial ages greater than 200,000 years. No clear correlation has been found between the terrestrial ages and the locations of Lewis Cliff meteorites. Very old and young meteorites were found on both the lower and upper ice tongues of Lewis Cliff. Fireman (1988) measured the age of one dust-containing ice sample from lower Lewis Cliff ice tongue using a uranium-thorium dating method. The terrestrial ages of Lewis Cliff meteorites, which we have measured, are significantly longer than the age of this Lewis Cliff ice, 25,000 years. If we accept the young age for the Lewis Cliff ice, then we must conclude either that ice from Law Glacier must have flowed continuously into the Lewis Cliff ice tongue
K. NisHlizuMi
Department of Chemistry University of California, San Diego La Jolla, California 92093-0317 P. SHARMA
and P.W. KuBIK*
Nuclear Structure Research Laboratory University of Rochester Rochester New York 14627 We continue to study cosmogenic nuclides in antarctic meteorites. Through our measurement program on cosmogenic nuclides in antarctic ice, Greenland ice, antarctic rocks, and antarctic meteorites, we hope to understand meteorite accumulation mechanisms as well as the history of both polar ice sheets and climatic change. In addition, we are studying the history of antarctic meteorites and cosmic rays. Our probes are mainly the long-lived, cosmic-ray-produced radionuclides berylium-lO, aluminum-26, clorine-36, calcium-41, manganese-53, and iodine-129. The major objective is to measure terrestrial ages of meteorites based on chlorine-36 concentrations. Chlorine-36 was determined using the University of Rochester tandem accelerator. We, at the University of California, San Diego, have developed a new dating method; we measure in situ produced berylium-10 and aluminum-26 in terrestrial quartz (Lal and Arnold 1985). We have applied this technique to several
*Present address: Inst it ut für Mittelenergiephysik,Eidgenossische Technische
Hochschule, Hon ggerherg, Zürich,Switzerland. 1991 REVIEW
**The designation "Main Icefield" is not an official name, but the feature is a distinct geographic unit. 57
60 50 I-
40
0
30
I.-
Yamato (42)
1
20 E = z 10 .0
0 60 50 I-
40
0
30
.0
20
I.-
E = z
10 0 60 50
I-
40
0
30
I-
E = z
20 10 lU0 2U0 420 560 700 840 980 Terrestrial Age (10 years)
Histogram of terrestrial ages of Vamato, Allan Hills, and other antarctic meteorites. Pairs of meteorites are shown as one object plotted at the average age.
for more than a few hundred thousand years or that the young and old ice sheets were mixed together at the ice tongue. • The histogram of terrestrial ages of Elephant Moraine meteorites shows a smooth exponential decrease with age. Thus far, old meteorites have been found on the southwest side of the moraine, and younger meteorites have been found in the eastern and northern regions. According to Faure and Taylor (1985), Elephant Moraine consists of pieces of bedrock that were fairly recently transferred to the surface by upward ice movement. We measured berylium-10—aluminum-26 surface exposure ages of four moraine rocks using the University of Pennsylvania accelerator. They are calculated to be about 100, 400, 10,000, and 62,000 years. Although all samples were collected from the same area, their exposure ages are very different. This is evidence for continuous production of Elephant Moraine to this day. We conclude that the age of the moraine is greater than 60,000 years. • Only four meteorites (ALH84243, 85037, 85048, and 85123) were collected on soil or on bedrock at Allan Hills. Three of them have terrestrial ages of less than 100,000 years, but one (ALH 85048) has a terrestrial age of 920,000 years. The rocky 58
outcrops located southeast of the Allan Hills Main Icefield and the west side of Allan Hills were essentially ice free for at least 1.4 to 2.5 million years based on in situ produced cosmogenic nuclides, berylium-10, aluminum-26, and neon21 in quartz (Graf et al. 1991; Nishiizumi et al. 1991). We do not yet understand the relationship between the terrestrial ages and the history of the outcrops on which the meteorites were found. S ALH85118 was collected from a steeply sloping ice surface. If the meteorite was recently exposed from inside the ice, the age of the ice should be same as the terrestrial age of the meteorite, 650,000 years. The meteorite age, however, is much older than the measured age of ice of the Allan Hills Main Icefield (approximately 100,000 years older) based on uranium-thorium (Fireman 1988) and krypton-81 (Craig et al. 1990) dating, and the age of ice at Allan Hills Cul de Sac*** (approximately 300,000 years) (Fireman 1988). Although some trends were found for the Allan Hills Main Icefield, both old and young meteorites were found there. The oldest terrestrial ages of these meteorites are far older than the measured age of the ice. If these meteorites fell on the snow accumulation area and were transferred to the present location by the currently accepted mechanism (Yanai 1978; Whillans and Cassidy 1983), we cannot explain this discrepancy. One possible explanation for this and other similar discrepancies is that meteorites fell on the accumulation area but stayed on the ice or snow for a long time due to a steadystate balance between the snow accumulation and ablation rates. Then climate changed, the snow accumulation rate increased, and young and old meteorites were trapped in the ice and moved to the area where they were currently found. We will perform uranium-thorium dating of ice according to the idea of Fireman and will also make further measurements of terrestrial ages of meteorites and in situ surface exposure age dating of rocks to investigate the history of meteorites and ice. This work involves a collaboration between the University of California, San Diego (K. Nishiizumi and J.R. Arnold) and the University of Rochester (P. Sharma and P.W. Kubik). The berylium-lO and aluminum-26 measurements in terrestrial quartz were performed in collaboration with the University of Pennsylvania, Eidgenossische Technische Hochschule (Zurich), and Lawrence Livermore National Laboratory accelerator groups. This research was supported in part by National Science Foundation grants DPP 89-16036, DPP 89-16236, and National Aeronautics and Space Administration grant NAG 9-33. References Craig, H., T.E. Cerling, R.D. Willis, W.A. Davis, C. Joyner, and N. Thonnard. 1990. Krypton 81 in Antarctic ice: First measurement of a krypton age on ancient ice. Transactions of the American Geophysical Union, 71, 1825. Faure, C., and K.S. Taylor. 1985. The geology and origin of the Elephant Moraine on the east antarctic ice sheet. Antarctic Journal of the U.S., 20(5), 11-12. Fireman, E.L. 1988. Ice chronology at meteorite standing sites, Antarctica. Antarctic Journal of the U.S., 23(5), 49-50.
***The designation "Cul de Sac" is not an official name, but the feature is a distinct geographic unit. ANTARCTIC JOURNAL
Graf, T., CF. Kohl, K. Marti, and K. Nishiizumi. 1991. Cosmic-ray produced Ne in Antarctic rocks. Geophysical Research Letters, 18, 203206. La!, D., and J. Arnold 1985. Tracing quartz through the environment. Proceedings of the Indian Academy of Sciences (Earth and Planetary Science), 94, 1-5. Nishiizumi, K., D. Elmore, and P.W. Kubik. 1989. Update on terrestrial ages of Antarctic meteorites. Earth and Planetary Science Letters, 93, 299-313. Nishiizumi, K., CF. Kohl, J.R. Arnold, J . Klein, D. Fink, and R. Middleton. 1991. Cosmic ray produced `Be and "Al in Antarctic rocks:
1991 REVIEW
Exposure and erosion history. Earth and Planetary Science Letters, 104, 440-454. Nishiizumi, K., D. La!, J . Klein, R. Middleton, and J . R. Arnold. 1986. Production of Be and "Al by cosmic rays in terrestrial quartz in situ and implications for erosion rates. Nature, 319, 134-136. Whillans, I.M., and W.A. Cassidy. 1983. Catch a falling star: Meteorites and old ice. Science, 222, 55-57 Yanai, K. 1978. Yamato-74 meteorites collection, Antarctica from No-
vember to December 1974. Memoirs of the National Institute for Polar Research, special issue 8, 1-37
59
Lipschutz, M.E. 1991b. The impact of Antarctic meteorites on the conventional view of the inner solar system. Observatory, 111, 7-8. Lipschutz, M.E., and S.M. Samuels. 1991. Ordinary chondrites: Multivariate statistical analysis of trace element contents. Geochmnmica et Cosmochimica Acta, 55, 19-47 Paul, R.L., and M.E. Lipschutz. 1990a. Chemical studies of differentiated meteorites—I. Labile trace elements in Antarctic and nonAntarctic eucrites. Geochimica et Cosmnochimica Acta, 54, 3185-3195. Paul, R.L., and M.E. Lipschutz. 1990b. Consortium study of labile trace elements in some Antarctic carbonaceous chondrites: Antarctic and non-Antarctic meteorite comparisons. In K. Yana (Ed.), Proceedings of the NIPR Symposium on Antarctic Meteorites, 3, 80-95, 6-8 June 1989, Tokyo, Japan, National Institute of Polar Research. Sack, R.O., W.J. Azeredo, and M.E. Lipschutz. 1991. Olivine diogenites: The mantle of the eucrite parent body. Geochimica et Cosmochimica Acta, 55, 1111-1120.
Terrestrial ages of antarctic meteorites
antarctic mountains and moraines to understand duration of exposure above glaciers and erosion rates for the antarctic continent (Nishiizumi et al. 1986; Nishiizumi et al. 1991). We have measured terrestrial ages of meteorites from 29 locations in Antarctica although only one or two meteorites were measured at many sites. We found stony meteorites with ages greater than 200,000 years at seven locations (Allan Hills, Dominion Range, Elephant Moraine, Lewis Cliff, MacAlpine Hills, Pecora Escarpment, and Thiel Mountain). Four iron meteorites (DRP7800I-9, 1LD83500, Lazarev, and Mount Wegener), which were found on bedrock or in moraines, are all old (0.26-5 million years). The figure shows a histogram of terrestrial ages of Yamato, Allan Hills, and other antarctic meteorites. Pairs of meteorites are shown as one object plotted at the average age. Although the total amount of data is nearly double, the general trend remains the same as in the previous publication on this subject (Nishiizumi, Elmore, and Kubik 1989). There are four interesting points. • Many of the Lewis Cliff meteorites are as old as the Allan Hills (Main Icefield**) meteorites. Although we have measured only nine Lewis Cliff meteorites, five out of nine of them have terrestrial ages greater than 200,000 years. No clear correlation has been found between the terrestrial ages and the locations of Lewis Cliff meteorites. Very old and young meteorites were found on both the lower and upper ice tongues of Lewis Cliff. Fireman (1988) measured the age of one dust-containing ice sample from lower Lewis Cliff ice tongue using a uranium-thorium dating method. The terrestrial ages of Lewis Cliff meteorites, which we have measured, are significantly longer than the age of this Lewis Cliff ice, 25,000 years. If we accept the young age for the Lewis Cliff ice, then we must conclude either that ice from Law Glacier must have flowed continuously into the Lewis Cliff ice tongue
K. NisHlizuMi
Department of Chemistry University of California, San Diego La Jolla, California 92093-0317 P. SHARMA
and P.W. KuBIK*
Nuclear Structure Research Laboratory University of Rochester Rochester New York 14627 We continue to study cosmogenic nuclides in antarctic meteorites. Through our measurement program on cosmogenic nuclides in antarctic ice, Greenland ice, antarctic rocks, and antarctic meteorites, we hope to understand meteorite accumulation mechanisms as well as the history of both polar ice sheets and climatic change. In addition, we are studying the history of antarctic meteorites and cosmic rays. Our probes are mainly the long-lived, cosmic-ray-produced radionuclides berylium-lO, aluminum-26, clorine-36, calcium-41, manganese-53, and iodine-129. The major objective is to measure terrestrial ages of meteorites based on chlorine-36 concentrations. Chlorine-36 was determined using the University of Rochester tandem accelerator. We, at the University of California, San Diego, have developed a new dating method; we measure in situ produced berylium-10 and aluminum-26 in terrestrial quartz (Lal and Arnold 1985). We have applied this technique to several
*Present address: Inst it ut für Mittelenergiephysik,Eidgenossische Technische
Hochschule, Hon ggerherg, Zürich,Switzerland. 1991 REVIEW
**The designation "Main Icefield" is not an official name, but the feature is a distinct geographic unit. 57
60 50 I-
40
0
30
I.-
Yamato (42)
1
20 E = z 10 .0
0 60 50 I-
40
0
30
.0
20
I.-
E = z
10 0 60 50
I-
40
0
30
I-
E = z
20 10 lU0 2U0 420 560 700 840 980 Terrestrial Age (10 years)
Histogram of terrestrial ages of Vamato, Allan Hills, and other antarctic meteorites. Pairs of meteorites are shown as one object plotted at the average age.
for more than a few hundred thousand years or that the young and old ice sheets were mixed together at the ice tongue. • The histogram of terrestrial ages of Elephant Moraine meteorites shows a smooth exponential decrease with age. Thus far, old meteorites have been found on the southwest side of the moraine, and younger meteorites have been found in the eastern and northern regions. According to Faure and Taylor (1985), Elephant Moraine consists of pieces of bedrock that were fairly recently transferred to the surface by upward ice movement. We measured berylium-10—aluminum-26 surface exposure ages of four moraine rocks using the University of Pennsylvania accelerator. They are calculated to be about 100, 400, 10,000, and 62,000 years. Although all samples were collected from the same area, their exposure ages are very different. This is evidence for continuous production of Elephant Moraine to this day. We conclude that the age of the moraine is greater than 60,000 years. • Only four meteorites (ALH84243, 85037, 85048, and 85123) were collected on soil or on bedrock at Allan Hills. Three of them have terrestrial ages of less than 100,000 years, but one (ALH 85048) has a terrestrial age of 920,000 years. The rocky 58
outcrops located southeast of the Allan Hills Main Icefield and the west side of Allan Hills were essentially ice free for at least 1.4 to 2.5 million years based on in situ produced cosmogenic nuclides, berylium-10, aluminum-26, and neon21 in quartz (Graf et al. 1991; Nishiizumi et al. 1991). We do not yet understand the relationship between the terrestrial ages and the history of the outcrops on which the meteorites were found. S ALH85118 was collected from a steeply sloping ice surface. If the meteorite was recently exposed from inside the ice, the age of the ice should be same as the terrestrial age of the meteorite, 650,000 years. The meteorite age, however, is much older than the measured age of ice of the Allan Hills Main Icefield (approximately 100,000 years older) based on uranium-thorium (Fireman 1988) and krypton-81 (Craig et al. 1990) dating, and the age of ice at Allan Hills Cul de Sac*** (approximately 300,000 years) (Fireman 1988). Although some trends were found for the Allan Hills Main Icefield, both old and young meteorites were found there. The oldest terrestrial ages of these meteorites are far older than the measured age of the ice. If these meteorites fell on the snow accumulation area and were transferred to the present location by the currently accepted mechanism (Yanai 1978; Whillans and Cassidy 1983), we cannot explain this discrepancy. One possible explanation for this and other similar discrepancies is that meteorites fell on the accumulation area but stayed on the ice or snow for a long time due to a steadystate balance between the snow accumulation and ablation rates. Then climate changed, the snow accumulation rate increased, and young and old meteorites were trapped in the ice and moved to the area where they were currently found. We will perform uranium-thorium dating of ice according to the idea of Fireman and will also make further measurements of terrestrial ages of meteorites and in situ surface exposure age dating of rocks to investigate the history of meteorites and ice. This work involves a collaboration between the University of California, San Diego (K. Nishiizumi and J.R. Arnold) and the University of Rochester (P. Sharma and P.W. Kubik). The berylium-lO and aluminum-26 measurements in terrestrial quartz were performed in collaboration with the University of Pennsylvania, Eidgenossische Technische Hochschule (Zurich), and Lawrence Livermore National Laboratory accelerator groups. This research was supported in part by National Science Foundation grants DPP 89-16036, DPP 89-16236, and National Aeronautics and Space Administration grant NAG 9-33. References Craig, H., T.E. Cerling, R.D. Willis, W.A. Davis, C. Joyner, and N. Thonnard. 1990. Krypton 81 in Antarctic ice: First measurement of a krypton age on ancient ice. Transactions of the American Geophysical Union, 71, 1825. Faure, C., and K.S. Taylor. 1985. The geology and origin of the Elephant Moraine on the east antarctic ice sheet. Antarctic Journal of the U.S., 20(5), 11-12. Fireman, E.L. 1988. Ice chronology at meteorite standing sites, Antarctica. Antarctic Journal of the U.S., 23(5), 49-50.
***The designation "Cul de Sac" is not an official name, but the feature is a distinct geographic unit. ANTARCTIC JOURNAL
Graf, T., CF. Kohl, K. Marti, and K. Nishiizumi. 1991. Cosmic-ray produced Ne in Antarctic rocks. Geophysical Research Letters, 18, 203206. La!, D., and J. Arnold 1985. Tracing quartz through the environment. Proceedings of the Indian Academy of Sciences (Earth and Planetary Science), 94, 1-5. Nishiizumi, K., D. Elmore, and P.W. Kubik. 1989. Update on terrestrial ages of Antarctic meteorites. Earth and Planetary Science Letters, 93, 299-313. Nishiizumi, K., CF. Kohl, J.R. Arnold, J . Klein, D. Fink, and R. Middleton. 1991. Cosmic ray produced `Be and "Al in Antarctic rocks:
1991 REVIEW
Exposure and erosion history. Earth and Planetary Science Letters, 104, 440-454. Nishiizumi, K., D. La!, J . Klein, R. Middleton, and J . R. Arnold. 1986. Production of Be and "Al by cosmic rays in terrestrial quartz in situ and implications for erosion rates. Nature, 319, 134-136. Whillans, I.M., and W.A. Cassidy. 1983. Catch a falling star: Meteorites and old ice. Science, 222, 55-57 Yanai, K. 1978. Yamato-74 meteorites collection, Antarctica from No-
vember to December 1974. Memoirs of the National Institute for Polar Research, special issue 8, 1-37
59
