The CR carbonaceous chondrite group comprises sixindependent meteorites, four of which are from Antarctica(Weisberg et al. 1993; Kallemeyn, Rubin, and Wasson 1994).These chondrites have refractory lithophile element abun-dances of approximately 1.OxCI, that is, they are approxi-mately equal to those in CI carbonaceous chondrites (thegroup most similar in composition to the Sun). The abun-dance of metallic iron-nickel (Fe-Ni) is 100-160 milligramsper gram, unusually high for carbonaceous chondrites. Chon-drules are large (approximately 900 micrometers) and some-what irregular in shape; magnetite framboids are common.Previous studies of metallic Fe-Ni grains in CR chondrites(Lee, Rubin, and Wasson 1992) have shown that these mete-orites experienced some reduction during weak thermalmetamorphism. The magnetite framboids formed fromhydrothermal alteration of metal-rich precursors on the par-ent asteroid.

Several years ago, we described a new grouplet of chon-drites that were not members of the ordinary, carbonaceous orenstatite chondrite classes; we called these meteorites CarlisleLakes chondrites after the largest member (Rubin and Kale-meyn 1989). Since that time, several new specimens have beendescribed, most from Antarctica (e.g., Rubin and Kallemeyn1994), but one observed 1934 fall from Rumuruti, Kenya,recently turned up in the Berlin collection, and the group hasbeen renamed R chondrites after this specimen (Schulze et al.1994). The R chondrites constitute the 12th major chondritegroup: they are characterized by a high degree of iron oxida-tion, a low chondrule/groundmass modal abundance ratio,and a high abundance of the oxygen isotope 170 (e.g., Weis-berg et al. 1991; Bischoff et al. 1994). Refractory lithophileabundances are about 0.9xCI, indicating a moderate depletionin these elements relative to CI chondrites. All of the R chon-drites exhibit moderate metamorphic re crystallization and

most exhibit significant brecciation and/or moderate impactmelting. Several of the samples are regolith breccias that onceresided at the surface of their parent asteroid: they consist oflight-colored, highly metamorphosed clasts surrounded byless-metamorphosed, dark-colored material containing solar-wind-implanted noble gases.

This work was supported in part by National ScienceFoundation grant EAR 91-19065.

References

Bischoff, A., T. Geiger, H. Palme, B. Spettel, L. Schultz, P. Scherer, T.Loeken, P. Bland, R.N. Clayton, T.K. Mayeda, U. Herpers, B. Melt-zow, R. Michel, and B. Dittrich-Hannen. 1994. Acfer 217—A newmember of the Rumuruti chondrite group (R). Meteoritics, 29(2),264-274.

Kallemeyn, G.W., A.E. Rubin, and J.T. Wasson. 1994. The composi-tional classification of chondrites: VI. The CR carbonaceous chon-drite group. Geochimica et CosmochimicaActa, 58(13), 2873-2888.

Lee, M.S., A.E. Rubin, and J.T. Wasson. 1992. Origin of metallic Fe-Niin Renazzo and related chondrites. Geochimica et CosmochimicaActa, 56(6), 2521-2533.

Rubin, A.E, and G.W. Kallemeyn. 1989. Carlisle Lakes and Allan Hills85151: Members of a new chondrite grouplet. Geochimica et Cos-mochimicaActa, 53(11),3035-3044.

Rubin, A.E., and G.W. Kallemeyn. 1994. Pecora Escarpment 91002: Amember of the new Rumuruti (R) chondrite group. Meteoritics,29(2), 255-264.

Schulze, H., A. Bischoff, H. Palme, B. Spettel, G. Dreibus, and J. Otto.1994. Mineralogy and chemistry of Rumuruti: The first meteoritefall of the new R chondrite group. Meteoritics, 29(2), 275-286.

Weisberg, M.K., M. Prinz, H. Kojima, K. Yanai, R.N. Clayton, and T.K.Mayeda. 1991. The Carlisle Lakes-type chondrites: A new groupletwith high A 170 and evidence for nebular oxidation. Geochimica etCosmochimica Acta, 55(9), 2657-2669.

Weisberg, M.K., M. Prinz, R.N. Clayton, and T.K. Mayeda. 1993. TheCR (Renazzo-type) carbonaceous chondrite group and its implica-tions. Geochimica et CosmochimicaActa, 57(7), 1567-1586.

Stability of antarctic blue icefields over the last 300,000 years:Meteorites as surface exposure markers

PAUL H. BENOIT, JOYCE ROTH, HAZEL SEARS, and DEREK W.G. SEARS, Department of Chemistry and Biochemistry, University ofArkansas, Fayetteville, Arkansas 72701

Bare "blue" icefields are found in the inland portions of theantarctic ice sheet and are especially common in places

where ice flow is disturbed by interaction of the ice withmountain chains, nunataks, and sub-ice topography (Delisleand Sievers 1991; Takahashi et al. 1992, pp. 128-139). Theablation of deep ice on these icefields sometimes results in theaccumulation of entrained rocks on the ice surface, and thecomposition of these "moraines" can be indicative of regionaliceflow dynamics (Faure et al. 1987). Among the rocks in themoraines are sometimes found meteorites; at some icefields,meteorites are the most common type of morainal specimen,with hundreds of meteorite fragments collected on the ice sur-

face (Whillans and Cassidy 1983; Cassidy et al. 1992). Using theactivity of cosmogenic radionuclides, one can determine howlong each individual meteorite has been on Earth, that is, itsterrestrial age. In this article, we discuss the use of thermolu-minescence of antarctic meteorites to determine how longindividual meteorites have been exposed on the ice surface asopposed to being buried within the ice, that is, a surface expo-sure age. These data, coupled with terrestrial age data, permitstudy of the stability of the iceflelds on which the meteoriteswere found over the last 500,000 years or so.

The interaction of cosmic rays with meteorites in spaceresults in the production of radionuclides, including carbon-

ANTARCTIC JOURNAL - REVIEW 199451

14 ('C), aluminum-26 (26jJ), and chlorine-36 (36C1). Underthe much lower dose rates at the Earth's surface, the activityof these radionuclides decreases to much lower levels and canthus be used to determine how long a meteorite has been onEarth (Nishiizumi, Elmore, and Kubik 1989). It is apparentthat most antarctic meteorites have terrestrial ages rangingbetween a few thousand years to about 400,000 years,although a few meteorites have terrestrial ages up to approxi-mately 1 million years. The presence of meteorites with veryold terrestrial ages has been used as evidence for the long-term stability of the icefields on which they were found(Drewry 1980).

Natural thermoluminescence (TL) is also produced as aresult of the interaction of cosmic rays with a meteorite inspace. Unlike the decay of radionuclides, however, the rate ofTL decay is determined by ambient temperature. In temper-ate climates, the meteorites' average ambient temperature isfairly close to that of the air, and thus one can use the averagetemperature of the discovery site as a guide to the decay rateof TL. We find a correspondence between natural TL levelsand terrestrial age for meteorite finds from the Sahara andfrom the deserts of the western United States which is inaccord with calculated TL decay rates based on the averagetemperatures at these sites (Benoit et al. 1993). Antarcticmeteorites can reside in two very distinct thermal regimesduring their terrestrial history, however; namely, they can beheld at very low temperatures during burial in the ice or atmuch warmer temperatures (by solar heating) on the ice sur-face. In figure 1, we show calculated TL levels of meteorites ineach regime; it is apparent that the rate of TL decay for mete-orites buried in the ice is negligible when compared to thatfor meteorites exposed on the ice surface. A meteorite with atwo (or more) step history involving burial and exposure willhave a more complex TL response, such as that shown bycurve C of figure 1, but TL decay is still dominated by periodsof surface exposure and the TL level thus reflects the durationof surface exposure of the meteorite.

In figure 2, we show a comparison of surface exposureages, derived from natural TL levels and terrestrial ages(derived largely from 14C and 36Cl activities) for a group ofantarctic meteorites, most of which are from the large [75-square-kilometer (km2)] "Main Icefield* near the Allan Hills(76045'S 159020'E) in Victoria Land (Benoit, Sears, and Sears1993). We find that meteorites with terrestrial ages less thanor equal to 300,000 years exhibit a broad range of surfaceexposure ages, ranging between almost no surface exposureto the full duration of their terrestrial history. Meteorites withterrestrial ages greater than 300,000 years, however, generallyexhibit a more restricted range of surface exposure ages, gen-erally between 200,000 and 300,000 years.

One interpretation of these data is that the icefield wasformed between about 300,000 to 350,000 years ago, perhapsafter a pulse of unusually high iceflow related to the begin-ning of an interglacial period (Cassidy et al. 1992). With the

*The designation "Main Icefield" is not an official name, but the feature is adistinct geographic unit.

100 A.

-o 50U)()ccD0co0)

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0)-c1-10

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5 B.0200400600800

Terrestrial Age (ka)Figure 1. Calculated TL histories for antarctic meteorites. Curve A isfor a meteorite buried within the ice (average temperature-30 0C), andcurve B is for a meteorite on the ice surface (-10°C). Curve C showsthe TL level for a meteorite that is buried for the first 200,000 years ofits terrestrial history and then brought to the surface. (krad denotesthousand rad; ka denotes thousand years.)

restabiization of iceflow and the icefield following the pulse,"old" meteorites (with terrestrial ages greater than 300,000years) were carried to the field within the ice. Since that time,newly fallen meteorites have been added to these "old" mete-orites, either by direct infall onto the field or by local trans-port within the ice, thus producing the large range ofobserved surface exposure ages in meteorites with terrestrialages less than 300,000 years. It appears that glacial-inter-glacial cycles following the event at about 300,000 years agowere not intense enough to remove large numbers of mete-orites from the field.

We anticipate more detailed analysis of the surface expo-sure histories of meteorites from other icefields in Antarcticain the future. Although it appears that the meteorites frommost other iceflelds have younger average terrestrial ages thanthose at Allan Hills and thus may not document the event atabout 300,000 years ago, it is likely that some of these fieldswere more sensitive to ice pulses in more recent glacial-inter-glacial transitions, and this may be recorded in the natural TLof meteorites found on their surfaces.

We thank W.A. Cassidy and S.W.S. McKeever for discus-sions on natural TL and antarctic meteorites. This researchwas supported by National Science Foundation grant OPP 91-15521 and National Aeronautics and Space Administrationgrant NAG 9-81.

ANTARCTIC JOURNAL - REVIEW 1994

52

A 1'

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Figure 2. Surface exposure ages for antarctic meteorites, derived from natural TL levels, vs. ter-restrial age, derived from 36Cl and 14C activities. Some meteorites plot above the 1:1 line andwere heated prior to Earth impact (that is, "reheated"). Terrestrial ages are from Nishiizumi et al.(1989). Meteorites from the Allan Hills region are shown as circles and those from other antarcticsites are shown as squares and triangles. (ka denotes thousand years.)

References

Benoit, P.H., A.J.T. Jull, S.W.S. McKeever, andD.W.G. Sears. 1993. The natural thermo-luminescence of meteorites VI: Carbon-14, thermoluminescence and terrestrialages of meteorites. Meteoritics, 28(2),196-203.

Benoit, P.H., H. Sears, and D.W.G. Sears.1993. The natural thermoluminescence ofmeteorites 5. Ordinary chondrites at theAllan Hills ice fields. Journal of Geophys-ical Research, 98(132), 1875-1888.

Cassidy, W., R. Harvey, J. Schutt, G. Delisle,and K. Yanai. 1992. The meteorite collec-tion sites of Antarctica. Meteoritics, 27(5),490-525.

Delisle, G., and J. Sievers. 1991. Sub-ice topog-raphy and meteorite finds near the AllanHills and the Near Western Icefield, Victo-ria Land, Antarctica. Journal of GeophysicalResearch, 96(E1), 15577-15587.

Drewry, D.J. 1980. Pleistocene bimodalresponse of antarctic ice. Nature,287(5779), 214-216.

Faure, G., M.L. Strobel, E.H. Hagen, and D.Buchanan. 1987. Glacial geology of theReckling Moraine on the east antarctic icesheet. Antarctic Journal of the U.S., 22(5),61-63.

Nishiizumi, K., D. Elmore, and P.W. Kubik.1989. Update on the terrestrial ages ofantarctic meteorites. Earth and PlanetaryScience Letters, 93(3/4), 299-313.

Takahashi, S., T. Endoh, N. Azuma, and S.Meshida. 1992. Bare ice fields developedin the inland part of Antarctica. Pro-ceedings of the NIPR Symposium on PolarMeteorology and Glaciology (No. 5). Tokyo:National Institute of Polar Research.

Whillans, I.M., and W.A. Cassidy. 1983. Catcha falling star: Meteorites and old ice. Sci-ence, 222(4619), 55-57.

800AntarcticOrdinary

700 Chondrites

Measurements of ice thickness and related studies on theLewis Cliff Ice Tongue, 1993-1994

KEITH ECHELMEYER, Geophysical Institute, University ofAlaska, Fairbanks, Alaska 99775WILLIAM CASSIDY and JOHN SCHurr, ANSMETProject, Department of Geology and Planetary Science, University of Pittsburgh,

Pittsburgh, Pennsylvania 15260

The Lewis Cliff Ice Tongue (84 0 16'S 161 020'E) is a 2x10-kilo-meter (km) patch of exposed ice that terminates in a large

moraine below Mount Achernar (84°12'S 160056'E). We haverecovered approximately 2,000 meteorite specimens on thisand adjacent ice surfaces (Cassidy et al. 1992), with the highestconcentration located on the ice tongue itself; therefore, we areinterested in characterizing this ice tongue with a view to

learning its origin and history. Specifically, during the earlypart of the 1993-1994 field season, we were interested in thetransverse and longitudinal variations in ice thickness and howthese patterns relate to the flow of the ice tongue and, ulti-mately, to the distribution of meteorites on the surface. Addi-tional work was also carried out to determine if an ice-strati-graphic sequence is exposed on the surface of the ice tongue.

ANTARCTIC JOURNAL - REVIEW 199453