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Solar System Research, Vol. 36, No. 2, 2002, pp. 125–135. Translated from Astronomicheskii Vestnik, Vol. 36, No. 2, 2002, pp. 140–150.Original Russian Text Copyright © 2002 by Shukolyukov, Nazarov, Schultz.

INTRODUCTION

Since 1815, new additions to meteorite collectionshave occasionally included achondrites, which are sim-ilar in composition to terrestrial basic and ultrabasicrocks. The number of these odd extraterrestrial objectsincreased appreciably with the discovery of meteorite-rich regions in Antarctica and in the rocky deserts ofAfrica, Australia, and America. A specific group amongthe achondrites are the so-called SNC meteorites, aterm based on the initial letters of the names of threeunusual meteorites: Shergotty (1865), Nakhla (1911),and Chassigny (1815). Today there are 16 meteorites ofthis special class in the collections of the world. Theyare now subdivided into groups according to theirpetrology.

A more than bold idea was voiced (Walker

et al.

,1979, McSween

et al.

, 1979a, 1979b; Wasson andWetherill, 1979) in 1979: that the SNCs are fragmentsof Martian igneous rocks ejected by the impact ofimpactor meteorites of considerable size on the surfaceof Mars. The assumptions of the researchers were based

on the small crystallization age (<1.3 Gyr) of the SNCs,which is possible only on very large parent bodies, andon the similarity of the chemical composition of theSNCs and of Martian rocks studied by remote-sensingtechniques. This idea was developed in a paper byWood and Ashwal (1981) and received its first reliabledirect experimental confirmation when it was estab-lished in 1983 that the isotopic composition of thetrapped argon and nitrogen in the EETA 79001 shergot-tite matched the isotopic composition of the same gasesin the Martian atmosphere, as determined by the

Viking

spacecraft (Bogard and Johnson, 1983). Theoreticalestimates (Melosh, 1985) showed that Martian rockfragments ejected as a result of an impact event canacquire a Martian escape velocity of over 5 km/s. Thetotal mass of the ejecta was estimated to be

10

–2

–10

–3

ofthe mass of the impactor meteorite. The laws of solar-system dynamics permit the transfer of material fromone terrestrial planet to another: the trajectories of rockfragments ejected from Mars may intersect the orbits ofother planets and fall on their surface (Gladman

et al.

,1996). Approximately 7% of the total number of frag-

A New Martian Meteorite: the Dhofar 019 Shergottite with an Exposure Age of 20 Million Years

Yu. A. Shukolyukov

1, 3

, M. A. Nazarov

1

, and L. Schultz

2

1

Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 117975 Russia

2

Max Planck Institute of Chemistry, Mainz, Germany

3

Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, nab. Makarova 2, St. Petersburg, 199034 Russia

Received May 10, 2001; in final form, August 27, 2001

Abstract

—The isotopic composition of the noble gases of the new Martian meteorite, the Dhofar 019 shergot-tite, found in the desert in the territory of the Sultanate of Oman on January 24, 2001, was investigated. Stepwisethermal annealing with isotopic analysis of each of the noble-gas temperature fractions was employed to deter-mine the component composition. The concentration of the trapped noble gases in the new Martian meteoriteDhofar 019 is relatively high, although it lies within the range of concentrations in known SNC meteorites. Acharacteristic feature of all the trapped noble gases is the presence of two main components: a low-temperature,probably terrestrial atmospheric, component, trapped during the weathering of the meteorite on Earth, and ahigh-temperature trapped Martian component. Owing to the different ratios of the quantities of the two compo-nents, the trapped neon, argon, krypton, and xenon differ markedly in the kinetics of their release. The isotopiccomposition of the noble gases varies accordingly. The trapped xenon was found to contain two Martian com-ponents. One of them, with typical ratios of

129

Xe/

132

Xe and

132

Xe/

84

Kr, is representative of xenon and kryptonof the Martian atmosphere; the other, of gases of the Martian mantle. Variations of the isotopic compositions ofhelium, neon, and argon (and also, to a lesser extent, of krypton and xenon) during the thermal annealing of theDhofar 019 meteorite clearly point to a large proportion of cosmogenic as well as trapped components. The con-centration of cosmogenic neon and argon in the meteorite is unusually high. This corresponds to a maximumexposure age among other SNC meteorites: 20 million years. Estimates of the potassium–argon age (gas-reten-tion age) yielded the figure of 560 million years, which is within the range of values obtained for SNC meteor-ites by other authors, who used the rubidium–strontium and the potassium–argon technique.

126

SOLAR SYSTEM RESEARCH

Vol. 36

No. 2

2002

SHUKOLYUKOV

et al

.

ments ejected from the surface of Mars into interplane-tary space may reach Earth in the form of meteorites.There is a probability, albeit small, of finding rocksfrom Mercury among the meteorites (Gladman

et al.

,1996).

The idea of the possible natural transfer of Martianmaterial to Earth received fresh support when lunarmeteorites were recovered on Earth (Marvin, 1983;Yanai and Kojima, 1984, 1985; Eugster, 1987; Nishii-zumi

et al.

, 1991). The meteorites exhibit mineralogi-cal–petrographic, geochemical, and isotopic character-istics similar to those of the lunar rocks returned toEarth by

Luna

unmanned probes and

Apollo

missions.

One of the latest Martian meteorite finds occurred inthe desert in the territory of the Sultanate of Oman onJanuary 24, 2000. The new meteorite, a shergottite witha mass of 1.056 kg, was named Dhofar 019 after thelocation where it was found, a point with the coordi-nates

18°18.97

′

N,

54°08.87

′

E (Grossman, 2000).

The main aims of our study of the meteorite Dhofar019 were:

1. To determine the absolute and relative abun-dances of the trapped noble gases, their isotopic com-position and origin;

2. To determine the concentration of radiogenic

40

Ar

and assess the K–Ar age of the meteorite; and

3. To determine the abundances of the cosmogenicisotopes of the noble gases and assess the exposure age(radiation age) of the meteorite.

In our work, we used the methodology of determin-ing the isotopic composition and concentration of noblegases in meteorites developed at the Max Planck Insti-tute of Chemistry in Mainz, Germany.

The main purpose of the work was to perform an ini-tial investigation of the regularities governing the iso-tope and element abundances of the noble gases in thenew meteorite Dhofar 019. Its petrology and chemicalcomposition will be dealt with in other papers. The ini-tial results obtained in classifying the meteorite and adescription of its mineralogical–petrographic featuresand chemical composition are reported in a paper byTaylor

et al.

(2000).

10 100

0.1 1.0 10.0 100.0

DaG 476ALH 77005

QUE 94201ALH 84001

NakhlaChassigny

Y 793605

Dhofar 019

Dhofar 019

ShergottyEETA 79001

Trapped

36

Ar, cm

3

/g

×

10

–10

ALH 77005DaG 476

ChassignyEET 79001

ALH 84001Y 793605

NakhlaShergotty

Trapped

132

Xe, cm

3

/g

×

10

–12

2E–8

1E–8

5E–9

0E+0

6E–9

4E–9

2E–9

0E+0

2.0E–11

1.5E–11

1.0E–11

5.0E–12

0E+0

6E–12

4E–12

2E–12

0E+0500 1000 1500

20

Ne

36

Ar

84

Kr

132

Xe

T

,°C

Fig. 1.

A comparison of

36

Ar

and

132

Xe concentrations inDhofar 019 and other Martian meteorites.

Fig. 2.

Kinetics of the release of noble gases from the Dho-far 019 meteorite.

SOLAR SYSTEM RESEARCH

Vol. 36

No. 2

2002

A NEW MARTIAN METEORITE 127

MINERALOGICAL–PETROGRAPHIC FEATURES OF THE DHOFAR 019 METEORITE

The meteorite is of a fine-grained (grain size 0.2–0.5 mm) doleritic rock, which consists of pigeonite[

En

40–70

, Wo

9–15

, (Fe/Mn)

‡t

= 20–40], augite (

En

40–55

,Wo

30–40

), olivine [

Fo

25–60

, (Fe/Mn)

‡t

= 50–60)], andfeldspar (

An

36–68

) converted to diaplectic glass (maske-lynite). Just as in nakhlites, in olivine the Fe/Mg ratio ishigher than in the coexisting pyroxenes. The share ofpyroxene

≈

65

, that of maskelynite

≈

25

, and that of oli-vine

≈

10 vol %. The accessory minerals identified inthe meteorite are silica, potassium-rich feldspar, whit-lockite, chlorapatite, chromite, ilmenite, magnetite, andpyrrhotine. The secondary mineral phases formed byterrestrial weathering are calcite, gypsum, smectite,celestine, and iron hydroxides. The shock-effect signa-tures are increased fracturing, mosaic pyroxene extinc-tion, the formation of planar elements, and the presenceof maskelynite and of pockets of shock-melted mate-rial. In mineralogy, bulk chemical composition, and theabundance of rare elements, the Dhofar 019 meteoritehas been classified as an olivine-containing basalticshergottite.

TRAPPED NOBLE GASES IN THE DHOFAR 019 METEORITE

The concentration of trapped

36

Ar

and

132

Xe (andalso

84

Kr

) in the new shergottite Dhofar 019 lies withinthe range of concentrations in known SNCs (Fig. 1)(Becker and Pepin, 1984; Ott and Begemann, 1985;Swindle

et al.

, 1986; Wiens

et al.

, 1986; Becker andPepin, 1986; Ott, 1988; Swindle

et al.

, 1995; Miura

et al.

, 1995; Marti

et al.

, 1995; Goswami

et al.

, 1997;

Murty and Mohapatra, 1997; Eugster

et al.

, 1997;Zipfel

et al.

, 1998; Terribilini

et al.

, 1998; Mathew

et al.

, 1998; Garrison and Bogard, 1998; Eugster, 1988;Eugster

et al.

, 1993; Eugster and Michel, 1995).

1.2

1.1

1.0

8E–12

6E–12

4E–12

2E–12

0E+0500 1000 1500

T

, °C

129

Xe/

132

Xe

132

Xe, cc/g

×

10

–12

Fig. 3.

A comparison of variations of the

129

Xe/

132

Xe isoto-pic ratio and kinetics of xenon release from the Dhofar 019meteorite.

Table 1.

Concentration and isotopic composition of xenon in the Dhofar 019 meteorite

T

,

°

C

132

Xe,cm

3

/g

×

10

–12

136

Xe

134

Xe

132

Xe

131

Xe

130

Xe

129

Xe

128

Xe

126

Xe

124

Xe

Stepwise annealing, 0.159 g700 7.3

±

0.7 0.3341

±

0.0041

0.4009

±

0.0079

1.000 0.8289

±

0.0138

0.1578

±

0.0038

0.9976

±

0.0137

0.0726

±

0.0030

0.0039 ± 0.0007

0.0043 ± 0.0009

1000 6.6 ± 0.7 0.337 ± 0.0198

0.4241 ± 0.0245

1.000 0.8077 ± 0.0068

0.1528 ± 0.0150

0.9967 ± 0.0148

0.0821 ± 0.0076

0.0064 ± 0.0035

0.0034 ± 0.00033

1200 3.4 ± 0.3 0.3211 ± 0.0121

0.3726 ± 0.0146

1.000 0.8418 ± 0.0288

0.1630 ± 0.0086

1.199 ± 0.0272

0.1277 ± 0.0060

0.035 ± 0.0040

0.0161 ± 0.0025

1400 1.5 ± 0.2 0.3303 ± 0.083

0.3757 ± 0.0218

1.000 0.8558 ± 0.0133

0.1596 ± 0.0109

1.212 ± 0.0158

0.0943 ± 0.0086

0.0157 ± 0.0028

0.0066 ± 0.0036

1600 0.64 ± 0.06 0.3529 ± 0.0056

0.3909 ± 0.0083

1.000 0.8823 ± 0.0207

0.1382 ± 0.0094

0.9925 ± 0.0158

0.0706 ± 0.0176

0.0031 ± 0.0021

0.0090 ± 0.0055

Total 19.4 ± 1.9 0.3359 ± 0.0164

0.4015 ± 0.0170

1.000 0.8260 ± 0.0194

0.1509 ± 0.0107

1.0624 ± 0.0174

0.0876 ± 0.0067

0.0131 ± 0.0027

0.0070 ± 0.0025

Bulk sample, 0.202 g1650 42.2 ± 4.2 0.3248 ±

0.00150.3897 ± 0.0037

1.000 0.7924 ± 0.040

0.1522 ± 0.018

1.0060 ± 0.0025

0.0747 ± 0.0005

0.0067 ± 0.0003

0.0058 ± 0.0002

128

SOLAR SYSTEM RESEARCH Vol. 36 No. 2 2002

SHUKOLYUKOV et al.

The kinetics of the release of 20Ne, 36Ar, 84Kr, and132Xe differs markedly during the thermal annealing ofDhofar 019 (Fig. 2). The shape of the lines suggests theexistence of two unresolved peaks of the release rates:a low-temperature (500–700°C) and a high-tempera-ture (1000–1200°C) peak. The ratio of the componentscorresponding to these peaks changes from 20Ne to13Xe. The low-temperature component probably com-prises gases of the terrestrial atmosphere trapped dur-ing the presence of the meteorite on Earth. Such aneffect of “irreversible adsorption,” which increasesfrom the lighter Kr to the heavier Xe, was observed ear-lier during the prolonged presence of lunar rocks in ter-restrial conditions (Niedermann and Eugster, 1992).Consequently, it is necessary to distinguish betweentrapped indigenous and terrestrial atmospheric gases.

This is corroborated when we examine isotopic vari-ations during the thermal annealing of the meteorite(Table 1). For example, the 129Xe/132Xe isotopic ratios inthe 1200 and 1400°C temperature fractions are higherand differ substantially from the ratios in the 700, 1000,and 1600°C fractions (Fig. 3). This also indicates thatthe Dhofar 019 shergottite contains at least two compo-nents of trapped xenon, whose isotopic compositiondiffers and whose atoms occupy different structuralpositions. The possible presence of still other compo-nents is suggested by the distinctly different ratios ofother xenon isotopes that are released at 1200–1400 and700–1000, 1600°C (Fig. 4): it is not only excess 129Xe

that is formed at 1200–1400°C, but also xenon highlyenriched in the cosmogenic isotopes 124Xe, 126Xe, and128Xe.

One of the components of the xenon trapped in theDhofar 019 shergottite, which is released mainly at700–1000°C, may be interpreted as a mixture of Mar-

2.5

2.0

1.5

1.0

0.51 10

84Kr/132Xe

Mars Earth

Mars129Xe/132Xe

ALHA 84001

Chassigny

EET 79001

Shergotty

Nakhla

Y 793605

ALHA 77005

QUE 94201

Dhofar 019

100 1000 1000040Ar/36Ar

Literary dataOur result

20 30

4000

3000

2000

1000

0

–1000124 126 128 130 132 134 136

1200–1400°C

700–1000, 1600°C

‰

å

XeM

Xe132⁄( )

XeM

Xe132⁄( )Buik sample

------------------------------------------------------ 1– 103×T°C

Gr. sam

Fig. 4. Differences in the isotopic composition of xenonreleased at various temperatures in annealing the Dhofar019 meteorite.

Fig. 5. Ternary composition of xenon released in the ther-mal annealing of the Dhofar 019 meteorite.

Fig. 6. The unusually low 40Ar/36Ar isotopic ratio in theDhofar 019 shergottite compared with all the other SNCs.

(atmosphere)

(mantle)



tian mantle xenon with terrestrial atmospheric xenon,which was trapped during the weathering of the mete-orite on the Earth’s surface (points on the correspond-ing line of mixing in Fig. 5).

The second component (points near the line of themixing of atmospheric and Martian mantle xenon),which was released at 1200–1400°C, was formed dur-ing rock melting in Martian conditions. Judging fromthe isotopic composition of the xenon and the132Xe/84Kr ratio, which coincide with the isotopic char-acteristics of the Martian atmosphere, this process wasaccompanied by the trapping of a certain amount of

xenon by the planet’s atmosphere, which constitutes astrong argument in favor of the Martian origin of theDhofar 019 meteorite.

POTASSIUM–ARGON (GAS-RETENTION)AGE OF THE DHOFAR 019 SHERGOTTITE

The 40Ar/36Ar ratio in the Dhofar 019 shergottite isunusually low compared with other SNCs (Fig. 6). The40Ar/38Ar ratio is also very low. This is due mainly tothe increased concentration of the cosmogenic compo-nent, the somewhat reduced abundance of 40Ar, and,

900

800

700

600

500

400

300

200

100

0500 1000 1500

300

200

100

0

40Ar/38Ar

40Ar/36Ar

40Ar/36Ar40Ar/38Ar

T, °ë

9.0E+8

8.0E+8

7.0E+8

6.0E+8

5.0E+8

4.0E+8

3.0E+8

2.0E+8

1.0E+8

0E+00 100 200 300 400 500 600 700

(40Ar/36Ar)trap

( 36Ar/ 38A

r)trap = 3.50

( 36Ar/ 38A

r)trap = 5.50

Terr

estr

ial a

tmos

pher

ic A

r

Apparent age, years

Fig. 7. Variations of the 40Ar/36Ar and 38Ar/36Ar isotopicratios during the thermal annealing of the Dhofar 019 mete-orite.

Fig. 8. Estimates of the K–Ar age of the Dhofar 019 mete-orite for various assumptions concerning the value of the40Ar/36Ar and 36Ar/38Ar isotopic ratios in trapped argon.

Table 2. Concentration and isotopic composition of argon in the Dhofar 019 shergottite (K2O = 0.053 wt %)

T, °C 40Ar, cm3/g × 10–8 40Ar/36Ar 36Ar/38Ar 40Ar/38Ar

Stepwise annealing, 0.159 g

700 23.2 ± 0.3 206.9 ± 4.4 4.200 ± 0.029 869.0 ± 1.5

1000 67.9 ± 0.9 318.0 ± 5.0 1.070 ± 0.022 340.1 ± 5.0

1200 89.1 ± 1.2 135.0 ± 1.0 0.7205 ± 0.0050 97.2 ± 1.1

1400 23.9 ± 0.3 63.9 ± 0.6 0.6908 ± 0.0044 44.1 ± 0.3

1600 1.45 ± 0.02 117.2 ± 2.6 3.734 ± 0.025 437.6 ± 10.3

Total of fractions 205.7 ± 2.7 149.8 ± 2.7 0.8136 ± 0.0134 121.8 ± 2.4

Bulk sample, 0.202 g

1650 235.0 ± 3.1 160.9 ± 0.9 0.8043 ± 0.0056 129.4 ± 0.2

130


SHUKOLYUKOV et al.

probably, a very small amount of trapped argon ofwhich the 40Ar/36Ar and 36Ar/ 38Ar ratios are a prioriunknown.

This is confirmed by observations of the isotopiccomposition of argon in the course of thermal labora-tory annealing. The 40Ar/36Ar and 40Ar/38Ar isotopicratios undergo very pronounced changes, attaining val-ues of 63.9 and 44.1, respectively, at a temperature of1400°C (Fig. 7). The 40Ar/36Ar and 40Ar/38Ar ratioschange differently, which apparently reflects the mix-ing of three components—radiogenic, cosmogenic, andtrapped—in different proportions. Such features of theisotopic composition of argon make it difficult to deter-mine the K–Ar age of the Dhofar 019 shergottite. Nev-ertheless, some estimates of that age can be made. Ini-

tial data are listed in Table 2. Calculations were per-formed for different possible values of 38Ar/36Ar intrapped argon in the range 3.50–5.50 and on theassumption that in cosmogenic argon 36Ar/38Ar = 0.65(Fig. 8). The simplest assumption is that the trappedargon is terrestrial atmospheric argon, which becamepart of the meteorite during its weathering on Earth. Inthat case, the apparent age of Dhofar 019 is 540 ± 20 Myr.

If, however, we assume that all the 40Ar is radiogenic(in the trapped argon the 40Ar/36Ar ratio is very low),the apparent age turns out to be equal to 855 Myr. Thesequantities fall into the range of relatively low K–Arages that are usually obtained for SNCs. For example,the age of the Zagami Martian meteorite is 180 Myr(Marti et al., 1995); the Shergotty, 196 Myr;

1.7

1.6

1.5

1.4

1.3

1.2

1.1

1.00.5 1.0

20Necumul

Cosmogenic neon

700°ë

1000°ë

1200°ë 1400°ë

1600°ë

22Ne/21Ne

Bulk

(c) 5.55.04.54.03.53.02.52.01.51.00.5

0 0.5 1.036Arcumul

Bulk

700°ë

1000°ë1200°ë 1400°ë

1600°ë

Cosmogenic neon

Terrestrial atmospheric Ar

36Ar/38Ar (d)

8

7

6

5

4

3

2

1

0 0.5 1.0

20Ne/21Ne

Bulk700°ë 1000°ë 1200°ë

1400°ë

20Necumul

Cosmogenic helium

1600°ë

(b)

0.20

0.15

0.10

0.05

0 0.5 1.0

Cosmogenic helium

700°ë1000°ë

1200°ë

1400°ë1600°ë

Bulk

3He/4He

4Hecumul

(‡)

0

Fig. 9. Variations of He, Ne, and Ar isotopic composition due to the contribution of cosmogenic components during the annealingof the Dhofar 019 meteorite.

sample

sample

sample

sample



QUE 94201, 328 Myr (Terribilini et al., 1998);DaG 400, 900 Myr (Zipfel et al., 1998); ALH 77005,180−1330 Myr (Miura et al., 1995); LEW 88516,180−1300 Myr; Y 793605, 180–1300 Myr (Terribiliniet al., 1998); Nakhla and Chassigny, 1300 Myr (Ter-ribilini et al., 1998; Ott, 1988; Ganapathy and Anders,1969). As we see from Fig. 8, the maximum possible40Ar/36Ar ratios in the trapped argon of Dhofar 019 can-not exceed 660–690 (the intersection points of the cal-culated curves with the x-coordinate).

Close values of the age were obtained recently forDhofar 019: 575 ± 7 Myr by the Sm/Nd method and525 ± 56 Myr by the Rb/Sr method (Borg et al., 2001).These values are consistent with the value of 540 ± 20 Myr,which we found.

COSMOGENIC ISOTOPES AND THE EXPOSURE AGE

OF THE DHOFAR 019 SHERGOTTITE

Thermal annealing experiments revealed a high con-centration of cosmogenic isotopes in the Dhofar 019shergottite (Fig. 9). Thus, the high 3He/4He isotopicratio in the 700–1000°C temperature fractions indicatesthat the cosmogenic component makes up more than90% of the 3He in this shergottite (Table 3).

The 20Ne/21Ne and 22Ne/21Ne isotopic ratios in Dho-far 019 are typical of cosmogenic neon (see Fig. 9,Table 3). The (22Ne/21Ne)cosm average value in twoinvestigated aliquots of the meteorite is 1.195 ± 0.008.Such a quantity points to a low shielding coefficient ofthe meteorite substance.

Table 3. Concentration and isotopic composition of helium and neon in the Dhofar 019 shergottite

T, °C4He, cm3/g

×10–83He/4He 20He, cm3/g × 10–9 20He/21He 20He/22He 21He/22He


700 155 ± 8 0.1239 ± 0.0012 0.637 ± 0.032 1.0026 ± 0.0028 0.7554 ± 0.0040 0.7534 ± 0.0041

1000 25.9 ± 13 0.1254 ± 0.0025 1.36 ± 0.06 0.9796 ± 0.0056 0.8290 ± 0.0025 0.8462 ± 0.0047

1200 4.24 ± 0.42 0.03392 ± 0.00025 1.15 ± 0.05 0.9505 ± 0.0045 0.8232 ± 0.0082 0.8641 ± 0.0083

1400 3.05 ± 0.15 0.005585 ± 0.000042 0.603 ± 0.3 0.9576 ± 0.0004 0.8446 ± 0.0056 0.8811 ± 0.0020

1600 3.16 ± 0.15 0.000179 ± 0.000008 0.0042 ± 0.0004 6.56 ± 0.53 4.15 ± 0.13 0.616 ± 0.020

Total of fractions

191 ± 9 0.1182 ± 0.0015 3.75 ± 0.20 0.9774 ± 0.0051 0.8210 ± 0.0051 0.8413 ± 0.0053


1650 189 ± 9 0.1043 ± 0.0001 3.15 ± 0.15 0.9584 ± 0.0049 0.7954 ± 0.0043 0.8312 ± 0.0035

Table 4. Concentration and isotopic composition of krypton in the Dhofar 019 meteorite

T, °C84Kr, cm3/g

×10–1286Kr 84Kr 83Kr 82Kr 80Kr 78Kr


700 21.6 ± 2.2 0.3052 ± 0.0074

1.000 0.2020 ± 0.0082

0.1999 ± 0.0047

0.0433 ± 0.0024

0.00614 ± 0.00062

1000 21.6 ± 2.2 0.3035 ± 0.0063

1.000 0.2197 ± 0.0059

0.2183 ± 0.0058

0.0627 ± 0.0013

0.00959 ± 0.00065

1200 10.6 ± 1.1 0.2907 ± 0.0176

1.000 0.3543 ± 0.0156

0.3271 ± 0.0119

0.1684 ± 0.0090

0.0334 ± 0.0024

1400 5.9 ± 0.6 0.2684 ± 0.0203

1.000 0.2883 ± 0.0191

0.2781 ± 0.0091

0.0837 ± 0.0100

0.0338 ± 0.0036

1600 1.8 ± 0.2 0.2305 ± 0.0127

1.000 0.1936 ± 0.0277

0.2288 ± 0.0181

0.0346 ± 0.0081

0.00325 ± 0.00080

Total 61.5 ± 6.2 0.2963 ± 0.0100

1.000 0.2426 ± 0.0100

0.2367 ± 0.0072

0.0754 ± 0.0041

0.0146 ± 0.0012


1650 137 ± 14 0.2963 ± 0.025

1.000 0.2362 ± 0.0019

0.2317 ± 0.0015

0.0729 ± 0.0011

0.01354 ± 0.00016

132


SHUKOLYUKOV et al.

A substantial part of the argon in the Dhofar 019shergottite is cosmogenic argon. The 36Ar/38Ar ratio inthe 1200–1400°C temperature fractions (more than90% of the argon) almost coincides with the ratio ofcosmogenic isotopes, which is equal to 0.65 (see Fig. 9and Table 2).

A cosmogenic component is also present in thecomposition of the krypton in the Dhofar 019 meteorite

(Table 4). This is evident from the correlation betweenthe krypton isotopic ratios 83Kr/84Kr, 82Kr/84Kr, and80Kr/84Kr (Fig. 10). The excess of 80Kr and 82Kr iso-topes is probably due to the trapping of thermal or epi-thermal neutrons by bromine isotopes. Cosmogenicisotopes of krypton could not be used in calculating theexposure age of Dhofar 019 because it had become con-taminated in terrestrial conditions by the target ele-

0.35

0.30

0.25

0.20

0 0.10 0.2080Kr/84Kr

82Kr/84KrCosmogenic Kr

Epitherm

al neutro

ns

Thermal neutrons

Br + n

Trapped Kr

0.04

0.03

0.02

0.01

00.05 0.10 0.15

128Xe/132Xe

126Xe/132Xe

0.4

0.3

0.2

0.20 0.25 0.30 0.3582Kr/84Kr

83Kr/84Kr

0.04

0.03

0.02

0.01

0 0.005 0.010 0.015 0.020124Xe/132Xe

126Xe/132Xe

(‡)

(b)

(c) (d)

Fig. 10. Krypton and xenon isotopic correlations due to the contribution of cosmogenic components during the annealing of theDhofar 019 meteorite.

Table 5. Concentration of cosmogenic isotopes and exposure age of the Dhofar 019 meteorite

Sample3Hecosm,

cm3/g ×10–8

21Necosm,cm3/g ×10–8

38Arcosm,cm3/g ×10–8 (22Ne/21Ne)

Exposure age, Myr

T3 T21 T38 Tav

Dhofar 019, bulksample, 0.202 g

21.1 3.277 1.76 1.202 (13.1) 17.0 22.6 19.8

Dhofar 019, sum of fractions in annealing, 0.159 g

22.7 3.826 1.63 1.187 (14.3) 18.7 20.1 19.7

Average value 21.9 ± 1.9 3.55 ± 0.27 1.70 ± 0.05 1.195 ± 0.008 (13.7 ± 0.6) 17.8 ± 1.1 21.4 ± 1.3 19.6 ± 0.4



ments Sr, Rb, and others. For the same reason, theexposure age was not calculated according to the cos-mogenic isotopes of xenon either, although the isotopiccorrelations of xenon (see Fig. 10) are also due to thepresence of a certain amount of 124Xe, 126Xe, 128Xe,129Xe, 130Xe, and 131Xe cosmogenic isotopes.

The position of the Dhofar 019 experimentalpoint on the graph in the coordinate system Mg/(Si +Al)(21Ne/22Ne)cosm shows (Fig. 11) that this meteoriteappears to have been irradiated by Galactic cosmic raysonly, and the contribution of solar cosmic radiation maybe neglected (Eugster et al., 1997).

The concentrations of the cosmogenic isotopes 3He,21Ne, and 38Ar in two aliquots of Dhofar 019 and theirgeneration rates derived from chemical compositiondata (Taylor et al., 2000) were used to calculate itsexposure age according to the method and parameterspresented by Eugster (1988), Eugster et al., (1993), andEugster and Michel (1995). The value obtained accord-ing to the concentration of 3He is clearly lower thanthose derived from the concentrations of 21Ne and 38Ar(Table 5). This is a result of the loss of part of the cos-mogenic 3He owing to solar heating of the meteorite.

Among the SNCs, the maximum exposure age so far(15 Myr) has been obtained for the meteorite ALH84001 (Fig. 12). The exposure age of the Dhofar 019shergottite—approximately 20 million years—showsthat the delivery time of meteorites from Mars to Earthcould be 5 million years longer than we previouslyassumed. This does not contradict estimates of theorbital history of SNCs by Gladman et al. (1996), whoassume that a small share of the ejecta from Mars mayreach the Earth after a journey of more than 20 millionyears through space. The data obtained also tend to sup-port notions about the multiplicity of the discrete eventsthat eject rock fragments from Mars to Earth.

CONCLUSIONS

The isotopic composition of the trapped xenon andthe low K–Ar isotopic age that we have determined for

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

00.75 0.80 0.85 0.90

Mg/(Si + Al)

21Ne/22Ne

Chassigny

ALHA 77005

LEV 88516ALHA 84001

Dhofar 019

GCR only

EET 79001

Lafayette

Nakhla

GovernadorValadores

Zagami

Shergotty

QUE 94201

SCR

Increase

5

4

3

2

1

01.0 100.1Exposure age, Myr

Number of meteorites

Basaltic

Chassignites

Lercolithic

Nakhlites

ALH 84001

Dhofar 019

Fig. 11. Determination of the nature of the cosmic radia-tion—Galactic or solar—that affected the Dhofar 019 mete-orite from the isotopic composition of neon (Eugster et al.,1997).

Fig. 12. A comparison of the exposure ages of Dhofar 019 and all the other known SNCs.

shergottites

shergottites

134


SHUKOLYUKOV et al.

the new meteorite Dhofar 019 indicate unambiguouslythat it belongs to the group of shergottites—Martianrocks of basaltic composition that trapped volatilesfrom the atmosphere of Mars. A specific feature of thenew Dhofar 019 shergottite is the very high relative andabsolute abundance of cosmogenic isotopes of all thenoble gases. The exposure age derived from themexceeds the values obtained earlier for all other SNCs.

ACKNOWLEDGMENTS

We thank the Russian Foundation for BasicResearch for supporting this study (project no. 01-05-64247) and the reviewers for helpful remarks.

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