35'E

YAMATOMETEORITEICE FIELD

36'Er '%..MASSIFI

Y751

N

ICEFLOW

KURAKAKE NUNATAK

'I

g.Y86692

Y86032

Y82192'182193

tICE

K26-_SNLOW

Y86771V86770

72'S

TEPHRABAND

• TEPHRASAMPLE SITE

METEORITESAMPLE SITE

72.5'S

010 km

The comparisonof the uranium-series age

of Yamato K-26 icewith those of

Allan Hills and LewisCliff ice

E.L. FIREMAN

Smithsonian Astrophysical ObservatoryCambridge, Massachusetts 02138

The ice-ablation zone near the Yamato Mountains was thefirst meteorite-collecting area discovered in Antarctica (Yanai1981). During the past year, we received a 20-kilogram blockof ice with a tephra band labeled K-26 from F. Nishio of theNational Institute for Polar Research, Tokyo, Japan. The tephrain this ice had been studied (Katsushima and Nishio 1985;Fukuoka, Arai, and Nishio 1987) and the terrestrial ages ofseveral of the many meteorites in the neighborhood of this icesample had been measured. The figure is a map of a sectionof the Yamato icefield, which shows the direction of the iceflow and the locations of the K-26 tephra band and severalnearby tephra bands (the wiggly lines in the map) and thelocations of the meteorites: Y75102, Y86032, Y82192, Y86692,Y82193, Y86770, and Y86771. The Y75102 meteorite was datedby the carbon-14 method to be 4.3 ± 1.0 thousand years old(Fireman 1983) and to be 3.0 ± 1.2 thousand years old (lull et

al. 1984). The Y82192 and Y82193 meteorites were dated 83 ±35 thousand and 75 ± 30 thousand years old, respectively, bythe krypton-81 method (Eugster and Niedermann 1987).

The ages of tephra-laden polar ice samples were measuredby the uranium-series method (Fireman 1986). W.A. Cassidysupplied us with a number of tephra-banded ice samples fromthe Allan Hills (76°15'S 159°E) and Lewis Cliff (84°20'S 162°25'E)meteorite collecting sites, which are the second and third rich-est in meteorite specimens. The uranium-series ages of icesamples from the Allan Hills Main Icefield are very old, rangingfrom 100,000 to 300,000 years; a comparatively young age,25,000 years, was determined for the Lewis Cliff 86-3 ice sam-ple (Fireman 1986, 1987, 1988).

The physical properties and the chemical compositions ofthe particulates in the Lewis Cliff and Allan Hills tephra havealso been studied (Koeberl et al. 1987; Nishio, Katsushima,and Ohmae 1985). The tephra, in the samples for which wehave ages, consists mainly of glass shards. The Yamato glassshards resemble magmas associated with volcanos in the SouthSandwich Islands. The Lewis Cliff and Allan Hills shards re-semble magmas associated with volcanos in the McMurdo group.

The table compares the results of our measurements in Ya-mato K-26 ice with those in Allan Hills 85-1 and Lewis Cliffice.

The uranium abundance in Yamato tephra is 0.02 parts permillion, which is 250 times lower than in the Allan Hills 85-1tephra; nevertheless, the ice is datable by the uranium-seriesmethod. The radium-226/thorium-230 age of K-26 ice is ap-proximately 38,000 years old.

This work is supported by National Science grant DPP 87-16835.

References

Yamato meteorite field. (km denotes kilometer.)

Eugster, 0., and S. Niedermann. 1987. Cosmogenic and trapped noblegas isotopes, exposure age, and terrestrial age of lunar meteoritesY82192 and 82193. (Abstract.) 12th Symposium on Antarctic Meteorites.Tokyo: National Institute of Polar Research.

Fireman, E.L. 1983. Carbon-14 terrestrial ages of Antarctic meteorites.8th Symposium on Antarctic Meteorites. Tokyo: National Institute ofPolar Research.

Fireman, E.L. 1986. Uranium-series dating of Allan Hills ice. Journalof Geophysical Research, 91(134), D539-D544.

Fireman, E.L. 1987. Uranium-series dated ice, 100,000 to 350,000 yearsold, arranged sequentially at Allan Hills. Antarctic Journal of the U.S.,22(5), 75-77.

Fireman, E.L. 1988. Ice chronology at meteorite stranding sites, Ant-arctica. Antarctic Journal of the U.S., 23(5), 49-50.

Fukuoka, T., F. Arai, and F. Nishio. 1987. Correlation of tephra layersin antarctic ice by trace element abundances and refractive indicesof glass shards. Bulletin of the Volcanological Society of Japan, 2, 103-118.

Jull, A.J.T., D.J. Donahue, T.A. Zabel, and E.L. Fireman. 1984. Car-bon-14 ages of antarctic meteorites with accelerator and small-vol-ume counting techniques. Journal of Geophysical Research, 89, C329-C335.

Katsushima, T., and F. Nishio. 1985. Volcanic ash in dirt layers fromthe Allan Hills bare ice area in Victoria Land, Antarctica. Memoirsof the National Institute for Polar Research Special issue, 39, 193-208.

Koeberl, C., K. Yanai, W.A. Cassidy, and J.W. Schutt. 1988. Inves-tigations of dust bands from blue ice fields in the Lewis Cliff (Beards-more) area, Antarctica: A progress report. Proceedings of the NationalInstitute of Polar Research Symposium Antarctic Meteorites, 1, 291-309.

Nishio, F., T. Katsushima, and H. Ohmae. 1985. Volcanic ash layers

471989 REVIEW

in bare ice areas near the Yamato Mountains, Dronning Maud Landand Allan Hills, Victoria Land, Antarctica. Annual Glaciology, 7, 34-41.

Yanai, K. 1981. Collection of Yamato meteorites in the 1979-1980 fieldseason, Antarctica. Memoirs of the National Inst it ute for Polar Research,20, 1-8.

Tephra weight and uranium-series nuclide activities in ice.

Sample Yamato K-26 Allan Hills 85-1 Lewis Cliff 86-3

Dusty ice (wgt.)Dust (g)/ice (kg )a226 R (filtered H20)b230Th (filtered H20)"234 U (filtered H20)d238 U (filtered H2 0)e226Ra/230ThAge of ice (years)

Clear ice (wgt.)Dust (g)/ice (kg )a226 R (filtered H20)b230Th (filtered H20)c234 U (filtered H2 0)d238 U (filtered H2 0)e

1.50 kga0.1680.0317 ± 0.00100.0182 ± 0.00080.0157 ± 0.00080.0137 ± 0.00084.0± 0.7

(38±7)x103

1.78 kga0.0020.004 ± 0.0020.005 ± 0.0020.004 ± 0.0020.004 ± 0.002

1.63 kg 1.80 kg0.210 0.0380.144 ± 0.004 0.169 ± 0.0040.080 ± 0.001 0.064 ± 0.0010.049 ± 0.001 0.048 ± 0.0010.031 ± 0.001 0.039 ± 0.0012.3± 0.1 5.2± 0.3

(100± 10) x 10 (25± 5) x 101.4 kg 2.71 kg0.002 0.0020.014 ± 0.0020.013 ± 0.002 0.004 ± 0.0020.014 ± 0.002 0.005 ± 0.0020.015 ± 0.002 0.005 ± 0.002

ag denotes gram; kg denotes kilogram.b226 Ra denotes radium-226; H 20 denotes water; activity is measured in decays per minute per kilogram.c23oTh denotes thorium-230; H 20 denotes water; activity is measured in decays per minute per kilogram.d234 U denotes uranium-234; H 20 denotes water; activity is measured in decays per minute per kilogram.

238 U denotes uranium-238; H 20 denotes water, activity is measured in decays per minute per kilogram.1226 R denotes radium-226; entries are activity minus uranium-238 activity; activity is measured in decays per minute per kilogram.

Gravity measurementsacross and between

the meteorite-bearingicefields west-southwest

of the Allan Hills

ROBERT F. FUDALI

Department of Mineral SciencesSmithsonian Institution

Washington, D.C. 20560

Several years ago I reported the results of a gravity survey,which used a previously established triangulaton network, acrossthe Main Icefield* immediately adjacent to the Allan Hills,southern Victoria Land (Fudali 1982). One direct ice thicknessdetermination by radio-echo sounding was used to calibrate

* The designations "Far Western Icefield," Mid Western Icefield," NearWestern Icefield," and "Main Icefield" are not official names but thefeatures are distinct geographic units.

the gravity data, and relative gravity differences were con-verted to differences in ice thickness, assuming the unknownregional gravity gradient was modest and significant local bed-rock density differences were not present. Calculated ice thick-nesses and survey-determined surface elevations (both relativeto a bedrock base station) were then combined to generate sub-ice, bedrock profiles across the triangulation network. This waspart of an ongoing effort to understand the ice dynamics inthis blue-ice area containing high surface concentrations ofmeteorites. Our 1981-1982 antarctic search for meteorites party,however, also found meteorite concentrations approximately20 kilometers west-southwest of the westernmost triangulationstations and the following year, meteorite concentrations werefound at the Far Western Icefield, approximately 60 kilometerswest-southwest of the westernmost triangulation station. Itthus became clear that any attempt to understand the ice dy-namics had to await additional information on the ice/bedrockinterface all the way out to the Far Western Icefield.

Our 1983-1984 antarctic search for meteorites team was putinto the field by LC-130 flights to a landing site just north ofGriffin Nunatak from which we journeyed overland to Ele-phant Moraine to collect meteorites on the blue ice there. Sub-sequently, we moved overland to the Far Western Icefield, andfinally we moved east-northeast to the Allan Hills (figure).Twenty-four topographic and gravity stations were establishedalong this latter traverse and tied into the triangulation andgravity stations on the Main Icefield. This extends our coverage

48 ANTARCTIC kLkN\I