the worlds beyond: meteorite studies

9 Irons Fe-Ni

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Zhondrites

Achondrites

Hexahedrites Coarse cctahedriies

Medium cctahedriies

=“e octahedriies Ataxies

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Pallasites 1 MeSOSiderites

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Aubriies

Dibgenites Howardtes

Eucrites Shergonites

Nakhlites I

Angrite CaO

Flgure 1. Chemical classification of meteorites Metmite classes (IeW are based on proportions of metallic FbNi (red). FeS (orange), and silicate (white). Each class is divided imo a number of groups (right) by major chemical ConstiluBnls. i.e.. chondritic groups are delineated by amounts 01 metallic Fe-Ni and iron in ferromagnesian silicates

eteorite studies constitute the major, but not the sole, M part of cosmochemistry, the

discipline initiated by H. C. Urey in the mid-1950s to determine chemical and physical processes important in the Solar System’s formation and evolution. Meteorites are of essential interest because they contain the oldest Solar System materials available for research and sample a wide range of parent bodies-exteriors and interi- on-some primitive, some highly evolved. Meteorites carry decipher- able records of certain solar and galac- tic effects and yield otherwise unob- tainable data about the genesis, evolution, and composition of the Earth and other major planets, satellites, asteroids, and the Sun. Some contain iu- clusions tracing events from before the Solar System formed; others contain organic matter derived from giant mo- lecular clouds in the interstellar medium. Meteorites also provide an important body of “ground truth,” in a chemical and physical sense, which is critical to interpreting planetary data obtained by remote sensing. It is especially advantageous that meteorites occur on the Earth’s surface, where the full spectrum of laboratory analytical techniques can he applied, ranging from the simplest~to the most sophis-

ticated. As the recently released report of the U S . National Commission on Space put it: If one picture is worth 10,000 words, then one sample is worth 10,ooO pictures ( I ) .

Because of the interdisciplinary nature of meteorite studies--overlap- ping chemistry, physics, geology, and astronomy-no brief article can sum- marize the full scope of current research. (Several recent general books of varying complexity [ 2 4 ] cover specific aspects and are highly recom- mended; many other books and references are cited in them.) After intro- ducing some basic cosmochemical facts and approaches, this REPORT will illustrate the nature of questions that cosmochemists ask and how they go about answering them. For reasons to be described, we will focus on certain trace elements-especially Ag, Au, Bi, Cd, Co, Cs, In, Rh, Se, Te, TI, and Zn-that are particularly respon- sive to relatively low temperature processes and that yield important genetic information.

The genetic framewak

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orbital plane is the ecliptic plane and whose mean distance to the Sun, 1.5 X 108 km, is 1 astronomical unit (AU). There are, in addition, two belts of smaller objects: the putative Oort cloud a t 50,000 AU-the comet source-and the asteroid belt, mainly located a t 2.2-3.6 AU, i.e., between the orbits of Mars and Jupiter. The asteroid belt contains more than 3000 num- bered minor planets detected from Earth The short-lived (May 1983 to March 1984) infrared astronomy satel- lite, IRAS, ohserved 15,000. About 30

ble 1. NowAntarctic meteor- I

A major goal of meteorite studies is to establish the origin and early evolutionary history of the Solar System, which consists of the Sun and nine major planets including Earth, whose

968A ANALYTICAL CHEMISTRY. VOL. 58, NO. 9, AUGUST 1986 0003-2700/8610358-968A$01.50/0 @ 1986 American Chemical Society

objects, the Apollo asteroids, have orbits that cross the Earth‘s and are, therefore, inherently unstable.

To attain Earth-crossing orbits, an object or its fragments must he acted on by an external force, usually a mas- sive collision. Fragments from a large object, i.e., the Moon or Mars, must reach escape velocity-2.4 and 5.0 kmls, respectively. Ring asteroids must acquire an impulse of 1-3 km/s to attain a Mars-crossing orbit, following which Mars’s gravitational field can perturb objects into more elliptical, Earth-crossing (Apollo-like) orbits. Mars may he the largest object from which we can expect meteorites because shock velocity and shock heating are related. Velocities greatly ex- ceeding 5 kmls imply shocks large enough to vaporize planetary materials. Whether special conditions can be found permitting such high velocities without substantial shock loading is currently being investigated.

The asteroid-meteorite connection is well established ( 2 , 3 , 5 ) . Three ordinary meteorite falls recovered in Czechoslovakia (19591, Oklahoma (1970), and Canada (1977) were each photographed simultaneously from two or more points during atmospheric passage so that their orbits could he calculated. These orbits resembled

those of Apollo asteroids, having peri- helia (closest solar approach) of 51 AU and aphelia of 2-4 AU. Plots of re- flectance (or albedo) vs. wavelength from 0.4 to 2.2 fim demonstrate matches between specific meteorite types and asteroid surfaces, suggesting a linkage. For example, many asteroids, including the largest-1 Ceres with a diameter of 1025 km-are of the C type; that is, they have spectral reflectances like those of carbonaceous chondrites, a primitive meteorite type that can contain up to 5% organic matter of considerable complexity (6) .

Meteorites are classified as stones (Chondrites and achondrites), stony irons, and irons based on their relative proportions of Fe-Ni, silicate, and FeS (Figure 1). Chondrites contain spheri- cal millimeter- to centimeter-sized chondrules or their fragments, silicates that were rapidly melted and cooled in a few minutes early in the Solar System’s history. Such rapid heating and cooling are easy to per- form on the laboratory scale but diffi- cult to achieve on the Solar System scale (7). Yet, large volumes of chondrules must have been present in the Solar System because the number of chondrites is large (Table I). Chon- drites date hack to the Solar System’s formation-indeed provide chronome-

ters for it-and represent accumulat- ed primary nebular condensate and accretionary products. A portion of this condensate formed from the hot nebula as millimeter-sized Ca- and Al- rich inclusions (CAI) that are aggre- gates of minerals predicted as vapor deposition products by thermodyuam- ic calculations. These CAI, found mainly in carbonaceous chondrites, exhibit many isotopic anomalies and may contain presolar material (2,6,8). Other condensates formed at much lower temperatures.

Heating by a variety of sources (short- or long-lived radioactivities, gravitational energy release, etc.), mainly in asteroidal-sized parent bodies early in the Solar System’s history, caused partial or complete melting (Figure 2) that transformed some chondrites into differentiated meteorites (igneous achondrites, irons, and stony irons). These transformations involved chemical and physical fractionations that are being studied and that are not yet well understood. Simi- lar events must also have occurred early in the history of the terrestrial planets and Moon. Thus, establishing the genetic linkage between specific chondritic and differentiated meteoritic types should provide a model system for understanding the Earth’s

ANALYTICAL CHEMISTRY, VOL. 58, NO. 9. AUGUST 1986 969A

Present L

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bodii Thermal m.1.morphi.m,

panial melting,

Evolved parent

Source l k a k m.1.morphism, + objects (abnoopkerk ablmim),

Flgure 2. Stepwise formation of meteorites (bottom) from the nebula (top) Spscific processes in parentheses (at right) do not Chemically hamionate meteaitic material; the omera do. b is not clear Whether primary wbuW podUctD were small planetesimals u asteroidBI-sIzed prlmhive bodies. The &IcBI line is not lo sale

Figure 5. A meteoritic impact breccia The whhe hwt is an achOndrite (aubre): sterite chondrite parent body (phdo munesy of Smimsanian Institution)

970A ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986

blaCk iwlusim are collision debrb tom the primnlve fa-

early evolution. The idea that the Earth's core consists of metallic Fe-Ni was originally suggested by the exis- tence of iron meteorites; subsequent data support this.

Using compositional information, chondrites and differentiated meteorites can he further classified into vari- ous chemical groups (Figure 1) deriv- ing from different starting materials or parts of the chemically inhomogen- eous nebula (Figure 2). Each meteoritic group has a characteristic stable ox- ygen isotopic composition ('6OPO/ lSO) trend that seems characteristic of the batch of nebular material from which it condensed (9). (Isotopic anomalies in meteorites yield many sorts of genetic information [SI.) Be- cause isotopic homogenization is much easier than chemical homogenization, nebular chemical heterogeneity is as- sumed.

Chondrites are classified into six or more groups, based on proportions of iron as metal and silicate and on total iron (as Fe, FeO, and FeS) content, that is, high, low, or very low (H, L, or LL, respectively) Fe concentration. Some parts of primitive chondritic parent bodies were heated less than others, and in these parts, solid-state processes occurred that can he recog- nized petrolpgically (e.g., in making chondrules less distinct) or chemically (e.g., in homogenizing Fez+ contents of olivine (Fe,Mg),SiO,, pyroxene (Fe,Mg)SiOs), or ferromagnesian silicates. Ten such criteria permit each chondritic chemical group to he classified into one of seven petrologic types (2,3), the higher numbers indicating the greater degree of secondary metamorphism. Some chemical-petrologic types are unknown (e.g., H, L, LL or E l or 2); others are especially abundant (e.g., H5 or L6). Which, if any, group (e.g., E3-7, LL3-7) was meta- morphosed under open-system conditions, so that mobile elements and compounds could have been lost, is being debated. Metamorphism occurred shortly after the Solar System formed 4.6 X l o 9 yr ago and involved temperatures of about 400 "C for type 3 to almost 1100 "C for type 7.

Because no internal energy source known is sufficient to partly or totally disrupt an asteroid, collisions must have produced meteoroids (which he- came meteorites upon landing on Earth), each of which, in principle, records shock loading. In practice, different meteorites experienced different shock intensities: Very low pressures will not register, whereas high shock pressures induce marked changes observable petrographically or by thermoluminescence. Effects in chondrites permit estimation of shock pressures or facies ranging from a (<5 GPa or <50 kbar) through f

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0 5 7 GPa). Immediate postshock temperatures could have been considerable, 21200 “C for facies f, and persis- tently high for years, if heavily shocked material was covered with a thick, insulating ejecta blanket. Hence, although classification of a meteorite as L6f generally tells much about it, specific information must he obtained by detailed study. Some- times, collisions mix meteorites of different types or grades. Such breccias (Figure 3) are the only known source (IO) of forsterite chondrites (Figure l), for example. Other sorts of breccias, which sample the very surface of asteroids, contain implanted solar wind, that is, ions that stream from the Sun’s surface and give physical, chemical, and isotopic information about it (2).

Present-day meteorites record relatively recent catastrophic collisions. From their contents of radiogenic 4oAr (produced by radioactive decay of 1.28 X lo9 yr ‘OK), it is apparent that L chondrite falls were strongly heated, presumably during disruption of their parent body 500-650 X 106 yr ago (11). Catastrophic collisions were probably much more frequent in the early Solar System, but the ejecta has long since been swept up. The Eartb, therefore, receives a sample of extraterrestrial material biased in space and time: There are many asteroids, for example, with spectral properties unlike those of known meteorites (5).

Collisional ejecta usually is decime- ter to kilometer sized When later collisions reduce chondritic meteoroids to meter sizes, energetic (GeV) cosmic rays induce nuclear spallation reactions, producing stable and radioac-

tive nuclides that permit determination of exposure ages. Typical chondritic cosmic ray exposure ages are on the scale of 107 yr or less, and iron meteorite ages are 108-109 yr. Age distributions vary with chemical group (2, 11). When meteorites land on Earth, the kg/cm2-thick atmosphere shields them from further cosmic ray hom- bardment and cosmogenic radionu- clides decay. Measurement of radionu. clides with different half-lives then permits estimation of a meteorite’s terrestrial age, if it is not an observed fall.

During a meteoroid’s brief passage through the atmosphere, frictional heating and ablation occur a t essentially equal rates so that heat-affected surfaces in meteorites are very thin (a few millimeters) even in thermally conductive irons. Hence, a meteorite’s interior is unaffected by atmospheric transit and is representative of its state in space unless it is subsequently affected by terrestrial weathering (Figure 2). Recovered meteorites vary widely in size. The smallest weighed 1 g and landed on a frozen lake without penetrating the ice. The largest weigh many tons and have kinetic energies equivalent to those of hydrogen bombs, creating explosion craters. Im- pact of exceptionally large meteoroids or comets was the cause of the Tungu- ska explosion of June 30,1908, and may have been the means by which di- nosaurs (and other phyla) were extin- guished a t the Cretaceous-Tertiary boundary 70 X 106 yr ago (12).

Meteorites can and do land any- where: About 2600 have been found or seen to fall outside of Antarctica during the period in which records have

been kept-essentially the past 200 years (Table I). During the past 15 years, more than 7500 meteorite fragments, representing 12003800 different impacts, have been recovered from Antarctica (Figure 4). mainly by Japa- nese and U S . annual field expeditions (13,14). Meteorites are abundant in Antarctica because the ice sheet seems to collect, preserve, transport, and concentrate them in old ice regions near barriers (Figure 5). Meteorites of types rare or nonexistent outside Ant- arctica are frequently found in Ant- arctica. Even ordinary chondrite types (H, L, and LL) distribute differently, with non-Antarctic H/L ratios being 1 and Antarctic ones being 3 (Table I).

somerecentimestigasons Meteorites can he quite complex

mineral mixtures: Their petrology (or metallography), ages, and composition are of major interest to cosmochemists. Petrologic investigations can be qualitative (using optical or electron microscope techniques to establish textures) or quantitative, as are chron- ologic and compositional studies. Quantitative petrology mainly uses electron probe microanalysis. Deter- mining meteorite ages involves the use of mass spectrometers-either gas or solid source. And the laser probe mass spectrometer and ion prohe microana- lyzer permit determination of ages of individual grains or parts of them. A recent development is the use of accel- erator mass spectrometry (AMS) to increase measurement sensitivities of cosmic-ray-induced radioactivities that quantify cosmic ray exposure and terrestrial ages. The compositions of meteorites or of grains in them are de-

972,. * ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986

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termined using classical or bulk chemical techniques for major elements, gas chromatographylmass spectrometry for organics, and neutron activation analysis, with and without radiochem- ical separations, for trace elements.

With an especially complex or important sample, an international consortium is often formed in which each participant studies selected samples with known relation to the others to investigate a major problem. For example, in 1983 a consortium composed of 22 groups studied the complete petrology, chemistry, and chronology of samples from a peculiar 31.4-g Antarc- tic meteorite found by a US. team to determine whether it derived from the Moon. A total of 1 g of material was consumed, and the consortium mem- bers unanimously concluded that i t was, in fact, the first lunar sample ever found on Earth (25). Subsequently, two additional Antarctic samples were identified by Japanese teams, and the National Institute of Polar Research in Tokyo organized consortia to study them. Because the three meteorites show different chracteristics, came from different depths below the lunar surface, and were separated by unknown horizontal distance on the Moon, the consensus is that each was

ejected Earthward by a different mas- sive impact.

A few years ago, circumstantial evidence indicated the possibility of a Martian origin for six non-Antarctic, igneous SNC meteorites (two shergot- tites, three nakhlites, and a chassig- nite). Discovery of three lunar meteor. ites produced in separate events-two of which appear relatively unshocked despite ejection a t lunar escape veloci- ' ty-strengthens this possibility. Fw- thermore, an additional shergottite- the fvst known meteorite containing an igneous contact-and a unique related meteorite were found in Victoria Land, Antarctica, the former containing noble gases and nitrogen with isotopic compositions that indicate mix- ture of Martian (data from Viking lander results) and terrestrial atmospheric components (26). Although there is no consensus yet, some or all SNC meteorites may well be Martian samples, providing chemical information about that planet in advance of a sample return mission (I 7).

Genetic processes summarized in Figure 2 can fractionate trace elements (parts-per-million to parts-per- trillion levels), especially the 12 listed earlier. In fact, trace elements are good markers of these processes be-

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cause of their initially low concentra- tions: A small absolute loss (or gain) caused by an external force (e.g., shock) is magnified into a large relative change. Typically, one establishes fractionations and their causes by ex- amining the trace element pattern in a given meteorite or group or by statistically comparing compositions (element by element) in sample populations. Trace element distributions can vary widely in a sample population; therefore, lognormal distributions, rather than normal ones, are compared for some elements. In such statistical comparisons, the larger the number of significant differences, the greater the reason to doubt that the two sample populations compared derive from the same parent population.

A trace element's physical properties may not be those of macroscopic quantities of the element or its compounds. Trace elements are not usually sited in a single phase, and nearest neighbors are unknown; bond lengths and strengths are also unknown and probably vary. Hence, trace element thermodynamic properties are unknown, and kinetic response to heating must be established experimental- ly. For example, the three stable Ar isotopes respond differently during week-long heating of Aliende, a primitive carbonaceous chondrite, under temperature and ambient atmospheric ( 10-5 atm H2) conditions reasonable for early Solar System objects. Trapped 36Ar and 3sAr are incipiently lost at 900 "C, whereas radiogenic 40Ar loss is measurable at 500 "C: Apparent activation energies are 190 kJ/mol for "Ar and 38Ar and 34 kJ/mol for 40Ar (18). The differences apparently reflect 40Ar association with radiation damage in K sites: "Ar and 38Ar are sited elsewhere.

Many other elements were deter- mined in Allende samples: Bi and T1, for example, are incipiently lost at 400 "C and In at 600 "C, with apparent activation energies of 10-12 kJ/ mol for Bi and T1 and 88 kJ/mol for In (18). Hence, Bi and T1 are more mobile (probably because they diffuse by fast-ion conduction) than *OAr, whereas In is less mobile: All are initially present at parts-per-billion levels in meteorites. It would be expected, therefore, that if mobile trace elements are present in a meteorite that is sufficiently shock-heated to lose 40Ar, Bi and T1 at least would also be lost. In fact, 11 of 12 trace elements are significantly lost from L chondrite falls shocked above 22 GPa compared with those shocked to lower pressures (19). The pattern in heavily shocked L chondrite falls suggests vaporization of some elements and loss of others that behave geochemically like metallic Fe and FeS (Le., siderophile and

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in H chondrites Nelther (a) Amarctic sample (symbols m i a l e ( (symbols associated wlth twDlener d e s [241) dlsblbutiOn6 wree with theoretical predictions (2s) for cmd+wation from a nebvla of solar mmposition at pressures 01 5 X lo-' atm to 5 X I O V aim. Sbongly shocked or -ped samples are indicated by filled 8ymbols. Asterisks indlcate cases where Shmk his- tules are not yet kmwn. Data arrays lw mn-Antarctic and Antarctic samples clearly differ (wves act as fiducial marks). polntlng to differem prstenestrial thermal hlstwies

. n u m b [.?SI) IXX (b) m-Amarctlo I

978A ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUOUST 1986

chalcophile elements, respectively) in FeS-Fe eutectic melt at 2988 "C (19). Similar fractionations apparently oc- ourred in the shock-melted LL7 chondrite, Yamato 74160 (ZO), and during secondary thermal alteration of primitive enstatite meteorite material form- ing E4-7 chondrites, auhrites, and associated irons (21).

The remainder of this REPORT will deal with a recent, rather controversial question: Has the nature of the meteoroid flux on Earth changed over the past 106years? Dennison et al. (22) pointed out that the large numbers of rare or unique Antarctic meteorites and different relative proportions of ordinary chondrites in Antarctic meteorites and non-Antarctic falls (Table I) hint at such a change. To examine this possibility, 13 trace elements-including the ones most easily mobilized by evolutionary processes-were de- termined by neutron activation analysis in more than 30 H5 chondrites in two sample populations-non-Antarc- tic falls and recoveries from Victoria Land, Antarctica (22). More recent data for H44 chondrites represent analysis of 25 Victoria Land samples and 43 non-Antarctic falls (23). When the data are compared statistically, 8 of the 13 elements differ significantly in concentration, giving strong reason to doubt that the two sample populations derive from the same parent population (22,23). Differences are more apparent when data are treated in other ways, that is, when relation- ships are considered between two mobile elements with substantially different apparent activation energies (Figure 6).

These differences could, in principle, reflect one or more of the following causes: an artifact of the data treatment, alteration of Antarctic samples by weathering during their terrestrial residence of 0.3-1 X lo6 years, or extraterrestrial source varia- tions, probably with time. The first possible cause was eliminated by treating the data in a variety of ways; essentially the same differences appear (22,23). Antarctic weathering can be eliminated by several lines of evidence. First, it was shown that Ant- arctic weathering in meteorites involves leaching loss from sample inte- riors to the environment (23). Con- tents of six of the eight elements differing significantly are higher in the Antarctic sample population than in the non-Antarctic one, precluding leaching. (The latter population should not he affected by weathering because all samples were collected shortly after fall and were suhse- quently preserved in museums.) Sec- ond, shock effects (established petrographically) and cosmogenic 53Mn contents differ significantly in the two

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9 8 0 A ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986

sample populations: Neither can be affected by terrestrial weathering. Fi- nally, shock effect differences have been confirmed by thermoluminescence studies.

This leaves an extraterrestrial cause, probably a time-dependent one, as the source of the difference, but this hypothesis is highly controversial. Monte Carlo (statistical random walk) calculations of asteroid ring ejecta indicate a flux constancy on Earth on a scale of a t least lo7 yr (26). Hence, meteoroids arriving at Earth from the asteroid ring today and 105-106 years ago should be sampling the same sources. However, this need not be the case for Apollo asteroids, which include the only three S-type objects known, i.e., those with spectral properties like those of ordinary chondrites. In 1983, Greenberg and Chap- man (27) showed that Apollo asteroids contribute only slightly to the meteoroid flux on Earth over a 200-yr span, such as that represented by non-Ant- arctic falls. However, calculations prompted by the report of Dennison et al. (22) indicate that on the 105-106 year scale of Antarctic meteorites, Apollo asteroidal material should be effectively captured on Earth. Hence, Apollo debris may predominate in the Antarctic population, and asteroid ring debris may be represented by the steady background of contemporary falls.

Clearly, many additional tests are needed if this hypothesized meteoroid flux variation can be regarded as prov- en. Whether it proves correct or not, the hypothesis has sparked reconsid- eration of the dynamic processes by which meteorites arrive on Earth. It may turn out that Antarctica is a source of extraterrestrial material from objects not previously sampled, indeed that may no longer exist to be sampled.

concbion

objects of interdisciplinary interest. Results of one study-say, chemical analysis-often can be applied to other areas, orbital dynamics in the last case considered. Meteorites from Ant- arctica are used not only to study extraterrestrial processes but also as probes of ice sheet history and dynamics. Early experience gained from meteorite studies provided guidance for proper handling, preservation, and analysis of Apollo lunar samples. Studies of these samples, in turn, led to the development of extremely sen- sitive techniques that are now being used to analyze meteorites and micro- gram-sized interplanetary dust parti- cles of probable cometary origin collected by high-altitude aircraft and from sea sediments. The past 30 years

Meteorites are, by their very nature,

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ANALYTICAL CHEMISTRY, VOL. 58. NO. 9. AUGUST 1986 981 A

of meteorite studies has provided an enormous amount of knowledge about the Solar System, and the re is no indi- cation that the scientific growth curve in this area is beginning to level off.

Acknowletlgment Suppor t of this research b y the Na-

tional Aeronautics and Space Admin- istration (grant NAG 9-48) and b y the National Science Foundation (grant DPP 8415061) is gratefully acknowl- edged.

References (1) "Pioneerine the Soace Frontier," Re-

bort of the UYS. Naiional Commission on Space; Bantam Books: New York, 1986.

(2) Dodd, R.,T. Meteorites, A Petrologi- cal-Chemreol Svnthests: Cambridee University Press: Cambridge, E n g h d , 1983.

(3) Dodd, R. T. Thunderstones and Shooting Stars; Harvard University Press: Cambridge, Mass., 1986.

(4) Adler, 1. The Analysis of Extraterres- triol Materials; Wiley-Interscience: New York, 1986.

(5 ) Asteroids; Gehrels, T., Ed.; University of Arizona: Tucson, 1982.

(6) Nagy, B. Carbonaceous Meteorites; El- sevier: New York, 1975.

(7) Chondrules and Their Origins; King, E. A., Ed.; Lunar and Planetary Insti- tute: Houston. 1983.

(8) Begemann, F. Repts. Prop. Phys. 1980, 43,1309-56.

(9) Clayton, R. N.; Onuma, N.; Mayeda, T. K. Earth Planet. Scr. Lett. 1976,30, 10-18.

(10) Verkouteren, R. M.; Lipschutz, M. E. Geoehim. Cosmoehim. Acto 1983.47, 1625-33.

(11) Anders, E. Space Sci. Reus. 1964,3, 583-714.

(12) Alvarez, L. W.; Alvarez, W.; Asaro, F.; Michel, H. V. Science 1980.208,1095- i ina

(24) Lingner. D. W.; Huston, T. J.; Hutson, M.; Lipsehutz, M. E., submitted for publication in Geoehim. Cosmochim. Acta.

(25) Larimer, J. W. Geochim. Cosmochim. Acta. 1973,37,1603-23.

(26) Wetherill, G. W. Nnture 1986,319. 357-58.

(27) Greenherg, R.: Chapman, C. R. Icarus 1983.55,455-81.

(13) Workshop on Antarctic Glaciology and Meteorites; Bull, C . B. B.; Lips- chutz, M. E., Eds.; Lunar and Planetary Institute: Houston, 1982.

(141 Inremotmnal Workehop on Antorcric Mereonrev. Annexstad. J 0: Schulri. L : Wanke. H . Eds . Lunar and Planetarv

rnorhtrn.Arlo 1986.50, in press.

chin, Cormorhim &la 19X0,44,701-39. (181 Ngo.H.T. ; I.ipwhutz, M E Geo-

1191 Hurton.T J : Liouchutr. .M E Ceo. chim. Coshoehih. Acto 1984,48,1319- 99

(20)'Takeda. H.; Huston, T. J.; Lipschutz, M. E. Earth Planet. Sci. Lett. 1984.71, 290 ?O Y * I - Y I .

(21) Biswas, S.; Walsh? T.; Bart, G.; Lip- sehutz, M. E. Geoehrm. Cosmoehim. Acta 1980,44,2097-2110.

(22) Dennison, J. E.; Lingner, D. W.; Lip- schutz, M. E. Nnture 1986,329,390-93.

(23) Dennison, J. E.; Lipschutz, M. E:, submitted for publication in Geoehm. Cosmoehim. Acto.

Michael E . Lipschutz earned a B.S. degree i n chemistry from Pennsylva- nia State University and M.S. and Ph.D. degrees in physical chemistry from the University of Chicago. His research interests include trace element analysis, geo- and cosmochemistry , radiochemistry, solid-state chemistry, high-pressure and high-temperature chemistry, and Antarctic studies. He is currently professor o f chemistrv a t Purdue Uniuersitv.

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