Ejection times of Martian meteorites

Geochimicaet CosmochimicaActa, Vol. 61, No. 13. pp. 2749-2757, 1997 Copyright 0 1997Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037197$17.00 + .oo

Pergamon

PI1 SOO16-7037( 97)00115-4

Ejection

times of Martian

meteorites

0. EUGSTER, A. WEIGEL, and E. POLNAU Physikalisches Institut, University of Bern, Sidlerstrasse

5. 3012 Bern, Switzerland

Abstract-Noble

gas isotopic analyses were made for four meteorites whose origin is Mars: basaltic shergottites Queen Alexandra Range 94201, Zagami, the lherzolite LEW885 16, and orthopyroxenite Allan Hills 84001. The latter one was separated into orthopyroxene, chromite, and maskelynite fractions. Ejection times were calculated for all Martian meteorites using cosmic-ray exposure ages based on ‘He, “Ne, and ‘“Ar concentrations and literature data on the terrestrial ages. We discuss the arguments for a scenario in which they were ejected from Mars by asteroidal or cometary impact as small meteoroids and were delivered to Earth within and up to 15 million years. The basaltic shergottites QUE94201, Shergotty, and Zagami show an ejection age cluster at 2.76 Ma, the fourth basaltic shergottite EET79001 being younger (0.82 Ma). Lherzolites LEW88516 and ALH77005 were ejected by a later event 3.84 Ma ago. All nakhlites, Nakhla, Governador Valadares, and Lafayette, originate from an event 1I .O Ma ago. Although Chassigny is chemically distinct from the nakhlites, its ejection age is I I .6 Ma. Whether this is a separate event cannot be decided from the available data. Finally, orthopyroxenite ALH84001 was launched from Mars 14.4 Ma ago. The distribution of the delivery times for Martian meteorites to Earth reflect individual impact events onto five or six different source terrains. This result is significant in the context of a comprehensive knowledge of the Martian crust composition. Finally, we derived the K-““Ar gas retention age of ALH84001 mineral fractions adopting two different models: Assuming a component of Martian atmospheric Ar with 40Ar/“hAr = 1790 trapped by this meteorite we obtained an age of 3390 Ma. On the other hand, assuming no contribution of Martian atmospheric Ar, the resulting K-‘“‘Ar age is about 4100 Ma. that is. roughlv 500 Ma younger than the reported Sm-Nd crystallization age. Co&right 0 1997 El&u Sc&&L~d 1. INTRODUCTION

and LEW885 16 (LEW) are Iherzolites. Yamato-793605 (Yamato) has recently been recognized to be another Iherzolites (field number Y25-95, Mayeda et al., 1995), but no data on its chronology have been reported until now. The three nakhlites, Nakhla, Lafayette, and Governador Valadares. are pyroxenites, whereas Chassigny is a dunite. Finally, Allan Hills 84001 (ALH84) is a coarse grained orthopyroxenite. For a more detailed description of the petrology and chemistry of the Martian meteorites see McSween (1994). The study of the Martian meteorites allows us to improve our knowledge of the composition of the Martian crust. The question from how many sites on Mars the twelve meteorites originate can be answered if we know the number of impact events that were responsible for their ejection. The ejection times, 7;). are the sum of the duration that the meteoritic material spent in free space (the cosmic-ray exposure age, 7’,) and the duration of its terrestrial residence after fall on Earth (the terrestrial age, T,,,,) Usually, r, is obtained from the concentration of noble gas nuclei that were produced by the interaction of energetic cosmic rays with the meteoritic material, whereas T,,,, can be derived from the activity of cosmic-ray produced radionuclides, such as “C. 3hC1, ‘hAl. and “Kr (cf. Nishiizumi. 1987). It is not a priori clear that T,,. calculated as indicated above, represents the time when the meteoritic material was ejected from Mars. Bogard et al. ( 1984) discussed three scenarios for the delivery of the Martian meteorites from their parent planet. In scenario I the 180 Ma Rb-Sr age of the four shergottites (Shih et al., 1982 ) represents the time

Most meteorites originate from asteroids. However, during the past two decades twelve objects have been recognized to be Martian rocks in the form of SNC (Shergottite-NakhliteChassigny) meteorites and ALH84001, the latter being distinct from the SNC classification (Mittlefehldt, 1994). These meteorites are now generally acknowledged to be material that was ejected from the Martian crust by asteroidal or cometary impact: their late crystallization ages (Shih et al., 1982) suggest that they are derived from a large planet and the isotopic composition of the gases trapped within some of them (Bogard and Johnson, 1983; Marti et al., 1995) perfectly matches that of the Martian atmosphere as determined by the Viking landers (Owen and Biemann, 1976). The case for a Martian origin was further strengthened when it became apparent that thirteen specimens representing ten independent meteorite falls are rocks launched from the Moon (cf. Warren, 1994). A lunar origin for these meteorites was generally accepted because they could be compared with the returned Apollo and Luna samples. The fact that rocks could survive being ejected from the moon suggested that a Martian origin for meteorites is also possible. The Martian meteorites fall into several groups with little internal variations in terms of mineralogy and chemical composition. The basaltic shergottites are represented by Shergotty, Zagami, and the antarctic find Queen Alexandra Range 94201 (hereafter QUE). Elephant Moraine 79001 (EET) is also classified as a basaltic shergottite, but it contains two basaltic lithologies termed A and B that differ in grain size with lithology B being coarser. Allan Hills 77005 (ALH77) 2749

2750

0. Eugster,

A. Weigel,

of a strong shock event which ejected the Martian meteorites from their parent body in one or several large blocks ( >5 m diameter). The recovered meteorites. however, yield much shorter cosmic-ray exposure ages. We shall show below that the cosmic-ray exposure lasted - 15 Ma for orthopyroxenite ALH8400 1, - 11 Ma for the nakhlites and Chassigny, and -4 Ma or less for the shergottites and lherzolites. Consequently, these meteorites must have been completely shielded from cosmic rays during most of the 180 Ma during which these large blocks were stored in space. Multiple fragmentation of these blocks during the past 15 Ma initiated the cosmic-ray exposure for the eventually recovered meteorites. This scenario was also discussed in detail by Vickery and Melosh ( 1987). Scenario II assumes that the cosmic-ray exposure age for each Martian meteorite represents the time when they were ejected directly from the Martian surface. In scenario III the differences in exposure age for the Martian meteorites results from a pre-exposure on Mars prior to ejection for all but the meteorite with the shortest cosmic-ray exposure. Numerous arguments can be put forward for scenario II and against the other ones: ( 1) The ratios of cosmic-ray produced “Ne/2’Ne and “‘Xe/‘2hXe are sensitive depth indicators. They depend on the target element concentrations, in the case of 22Ne/2’Ne mainly on Mg, Al, and Si. The lherzolites, Chassigny, and ALH84 are quite close in chemical composition to chondrites for these elements: e.g., LEW88 (see Table 3) 15.4% Mg, 1.68% Al, 205% Si, Lchondrites (Wasson and Kallemeyn, 1988) 14.9% Mg. 1.22% Al, 185% Si. The cosmogenic ratio “Ne/“Ne as a function of depth in the St. Severin chondrite was studied by Schultz and Signer ( 1976). Using their data we conclude that the “Ne/“Ne ratios in the previously mentioned Martian meteorites are consistent with pre-atmospheric radii of less than 0.5 m. Ott ( 1988) also concluded based on the ( ‘i’Xe/“hXe), ratio that Shergotty and Nakhla were shielded before exposure to cosmic rays as small objects. This argues against a pre-exposure on the Martian surface (scenario III) or in the outer layer of a large object in space (scenario I). If the recovered meteorites originate from large blocks in space, it is very unlikely that all were from the center and none from surface layers where they would show a pre-exposure (Wetherill, 1984). (2) This author also argues that if shergottites and nakhlites are from a secondary break-up of a large block in space, it would be a coincidence that more than one meteorite of the same class is recovered from the same break-up event. (3) Theoretical conclusions show that it is much easier to accelerate small objects (< 1 m) to escape velocity than large (>7 m) ones (Wetherill, 1984). (4) McSween (1985) argued that the cosmic-ray exposure ages are similar to the calculated transit times from Mars to Earth supporting scenario II. (5) Bhandari et al. ( 1986) measured nuclear tracks, ‘hAl, and other cosmic-ray effects in Shergotty, ALH77, and EET79 and found them to be consistent with a single stage exposure in space as small objects. (6) The most complete study of the ejection scenarios for Martian meteorites was performed by Gladman et al. ( 1996). These authors found that the exposure age distribution is consistent with a model in which all fragments were launched at speeds modestly

and E. Polnau

above the escape velocity as small objects and were delivered independently to Earth. An argument against scenario II may be based on the chronological data for the shergottites and lherzolites. Shih et al. ( 1982) performed a detailed study of the chronology of Shergotty, Zagami, and ALH77. They interprete the RbSt-, Sm-Nd, and ‘9Ar-J”Ar systematics for these three meteorites as follows: ( 1) Shock metamorphism age - 180 Ma; (2) A best estimate for the crystallization from magma of 900-1300 Ma. The same history was found by Wooden et al. ( 1982) for basaltic shergottite EET79 based on Rb-Sr and Sm-Nd dating. We shall show in the following discussion, adopting scenario II, that three separate impacts must have occurred to eject the shergottites and Iherzolites. Hence, this scenario requires that three impacts occurred into a terrain with these age characteristics. Considering all arguments discussed above we conclude that the most probable scenario is the direct ejection of the Martian meteorites as small objects. The ejection age as defined above represents a crater forming event on Mars. The number of such events and consequently the number of different sites on Mars sampled by the Martian meteorites is given by the number of different ejection times. The aim of this work is the calculation of the cosmic-ray exposure ages for all Martian meteorites using a uniform method for deriving the production rates that take into account the target element abundances and the shielding conditions for the production of cosmogenic noble gases. The database consists of results obtained by us for QUE, LEW, Zagami, and ALH84, as well as of the literature data presently available for all Martian meteorites. Preliminary data on LEW (Eugster and Weigel, 1992), ALH84 (Eugster, 1994)) and QUE (Eugster et al., 1996) have been published in abstracts. 2. EXPERIMENTAL

PROCEDURE

AND RESULTS

The antarctic meteorites QUE, LEW, and ALH84 were obtained from the NASA/NSF Meteorite Working Group (MWG). QUE was allocated in the form of several chips (0.198 g), all of them interior samples, at least 3 mm from the fusion crust, We crushed these samples to a grain size of <750 pm and analyzed two bulk samples (Table I ). The analyzed Zagami sample, crushed to <750 pm, represents a chip with no visible fusion crust. A powder sample ( <250 p,rn) of 0.196 g from the interior part of LEW was obtained in the framework of a consortium study organized by the NSF/ NASA Meteorite Working Group. We analyzed two bulk samples. Finally, two adjacent chips (2.13 g) from the interior of ALH84 were crushed by us to a grain size of <340 pm and mineral separates were prepared by handpicking. The meteorite samples were analyzed using our standard procedure and mass spectrometer system B. For details of the analytical procedure, background, and blank corrections see Eugster et al. ( 1993). The analytical results are given in Table 1. All errors correspond to a 95% confidence level (2~ errors). The reproducibility for multiple analyses is generally within error limits except for &‘Ar and “‘Ar. These variations are typically due to inhomogeneities of the K and trapped gas concentrations, respectively. 3. COSMIC-RAY

PRODUCED NOBLE GASES AND PRODUCTION RATES

For the calculation of the cosmic-ray produced (cosmogenie) noble gas abundances in the meteorites analyzed in this work we made the assumptions as given by Eugster et

Ejection

times of Martian

Table 1. Results of helium, neon, and argon analyses

meteorites

of QUE94201,

275

Zagami,

LEW885 16, and ALH8400

1

1,

IO-’ cm’ STP/g “‘Ne

‘He Basaltic shergottites QUE9420 I ,23 Bulk 0.02 16 g

“‘Ar

“Ne,“Ne

‘He/‘He

‘“Ar/“Ar

1.50

499 I6

16.6 ? 0.7

0.540 + 0.025

414 -e 20

4.84 r 0.12

0.967 t 0.010

I.313 2 0.030

Iir 0.07

16.9 2 0.6

0.547 2 0.025

304 rt I5

5.00 + 0.12

0.964 5 0.010

I.323 +- 0.035

I.33 2 0.10

16.8 -t 0.6

0.544 + 0.025

359 2 I5

4.92 +- 0.12

0.966 ? 0.010

1.318 i 0.030

I .32 t 0.07

210 i4

0.630 i 0.020

364 t 25

46. I + 0.8

0.837 -t 0.045

1.244 2 0.025

I .07 2 0.01

34 2 I

I .24 ? 0.02

804 I 55

4.67 + 0.07

I.014 +- 0.020

1.260 2 0.060

2.24 r 0.03

f

807 I2

34 ? I

I .34 -+ 0.07

758 -+ 55

4.81 -c 0.12

0.980 -t 0.060

I.255 i 0.045

I .9x -t 0.03

f

847 IO

34 -t I

1.25 -t 0.02

781 t 40

4.7 I ? 0.06

I.010 -c 0.020

I.257 % 0.035

2.1 I 2 0.02

487 i- 35

4.63 % 0.35

1319 + 55

19.14 t 0.20

0.973 + 0.01 I

I .205 -c 0.010

1.24 ? 0.08

435 ? 30

4.12 -c 0.30

1439 ? 55

17.01 * 0.17

0.878 -t 0.009

1.199 + 0.010

1.06 t- 0.03

2419 % 90

458 -+ I7

4.35 -t 0.16

1378 -c 40

17.96 + 0.13

0.916 t 0.010

I .202 t 0.010

I .08 % 0.03

2333 i- 80

Orthopyroxene 0.0448 g

248 &6

4.23 i 0.14

III3 i- 55

9.79 2 0.10

0.876 2 0.012

1.205 -+ 0.013

I .09 t 0.04

1882 + 95

Orthopyroxene 0.0329 g

256 t7

4.26 2 0.17

902 2 45

10.36 2 0.11

0.878 % 0.019

I .204 t 0.024

0.97 + 0.04

1753 + 110

Orthopyroxene weighted average’

251 ?5

4.24 + 0.11

987 5 35

10.05 + 0.07

0.877 +- 0.010

I.205 -f 0.01 I

I .03 + 0.03

I828 -t 73

g

735 ? 25

I.17 -c 0.10

1324 2 60

29.74 2 0.35

I.062 5 0.080

I.172 -c 0.095

1.79 t 0.07

430 +- I2

Maskelynite o.oonss g

354 % 25

1.34 t- 0.13

38140 ? 2500

124.4 2 6.0

2.30 t 0.45

I .54 -c 0.45

2.84 * 0.20

3350 t 650

Bulk 0.0301

g

Bulk average

Zagami BE-532 Bulk 0.0430 g

Lherzolite LEW8851613 Bulk 0.0216

g

Bulk 0.0105

Bulk average

Orthopyroxenite ALH8400 I .99 Bulk 0.0 I95 .g

Bulk 0.0432

g

Bulk weighted average’

Chromite 0.00193

The experimental

al. ( 1993 ). The

errors are 20 mean.

results

do

not

’Weighted

critically

depend

calculated

on these

of “Ne are cosmogenic. More than 60% of “Ar is cosmogenic except for the maskelynite of ALH84; but these two mineral fractions will not be used for exposure age calculations. Table 2 gives the cosmogenic (c) and trapped (tr) components of the noble gases in the meteorites studied in this work. The duration of cosmic-ray exposure in space, T,. can be calculated from T, = CJP,, where C, is the concentration of a stable cosmogenic noble gas and P, its production rate. assumptions

as all ‘He and >99.9%

average

according

to the square of the experimental

t

521 2 0.20

t

510 I5

765 t8

X31 28

i

2092 150

error

P, is, among other parameters, a function of the target element abundances of the particular meteorite and of a suitable shielding indicator, such as (‘2Ne/“Ne)c. In this work we use the method for the production rate calculation derived by Eugster and Michel ( 1995) for achondrites. This method is applicable for meteoritic material that was exposed to galactic cosmic rays only, that is, the investigated material must have been far enough away from the pre-atmospheric surface to exclude the contribution of solar cosmic-ray (SCR) produced noble gases. For most meteoritic samples

2152

0. Eugster, A. Weigel, and E. Polnau Table 2. Cosmogenic and trapped components of Martian meteorites (concentrations in IO-’ cm’ STP/g). Trapped

Cosmogenic

Basaltic shergottites QUE9420 1,23 Zagami BE-532 Lherzolite LEW88516,13 Orthopyroxenite ALHEOOI,99 Bulk Orthopyroxene Chromite Maskelynite

‘He

“Ne

‘*Ar

3.40 -+ 0.20 4.54 ? 0.1 I

0.43 t 0.03 0.60 t 0.04

0.41 -+ 0.03 0.40 i- 0.03

7.23 -c 0.12

t 0.04

25.5 t 0.9 25.0 i 0.5 24.7 ? 0.9 2.84 ? 0.25

3.93 + 0.14 4.01 t 0.12 0.94 + 0.13 0.37 i- 0.13

I .oo

this is the case because ablation that occurred during passage through the atmosphere is high enough to cause the loss of the outer layer containing SCR-produced noble gases. However, Garrison et al. ( 1995) observed solar proton produced neon in some shergottite samples probably as a result of different orbital parameters compared to other meteorites, which caused slower atmospheric entry velocities and lesser surface ablation. Using a plot of Mg/( Si + Al) vs. (“‘Ne/“Ne), Garrison et al. ( 1995) proposed a method for detecting SCR-produced Ne in meteorites of various classes. Fig. 1 shows the data for the meteorites discussed in our work as given in Table 3 for the chemical composition and Table 4 for the cosmogenic noble gases. The data points falling to the left of the shaded zone might contain SCR-produced noble gases. This is the case for samples of the lherzolite ALH77 analyzed by Garrson et al. (1995) and Miura et al. (1995). respectively. in contrast to the sample measured by Bogard et al. (1984). The only other Martian meteorite indicating a contribution of SCR-produced Ne is QUE. The Swindle et al. (1996) datum point plots slightly to the left of our value. For the calculation of the exposure ages of ALH77 and QUE we use the cosmogenic abundances of the samples with no indication for a SCR Ne contribution. For deriving the production rates the following chemical abundances were used: QUE, Zagami, LEW, and ALH84, as given in Table 3; all other Martian meteorites, compilation by McSween (1985) and Treiman et al. (1986). The shielding sensitive (“Ne/“Ne), ratios are given in Table 4. For all Martian meteorites we use the production rate calculation method proposed by Eugster and Michel ( 1995) derived from achondrites. These authors demonstrated that valid production rates can be obtained for achondrites of strongly varying chemical composition such as the eucrites and the diogenites. Equations 2 and 10 derived by these authors were adopted for the calculation of the ‘He produc-

“lNe

“Ne,“Ne

‘“Ar

1.282 + 0.025 1.238 -t 0.025

t 0.05 0.03 i- 0.04

0.43 + 0.04 0.21 + 0.01

-t 0.01

I .227 i 0.035

0.29 i- 0.02

0.13 ? 0.04

0.49 t 0.03 0.49 -t 0.03 1.31 -+ 0.11 1.64 + 0.35

1.183 -t 0.012 1.198 ? 0.012 1.133 -t 0.090 1.245 t 0.350

0.57 +- 0.04 0.39 -+ 0.05 0.32 -c 0.08 0.96 i- 0.14

0.28 t 0.02 0.23 i 0.01 2.23 t 0.13 7.70 t 1.23

0.31

0. IO

tion rates, P3, and the ‘8Ar production rates, Pax, respectively. For these two production rates the shielding corrections do not depend on the chemical composition; however, the

j : 8' : 3' 3' 8'

.

CHABIW

:'

: : -

-&

ALHA 17005

isffim

Eh

!’

:

0.75

AlHA @doOl

:

:

,’

: : : :

t' : : : : : : &-+M$rn : ,' : , :

.

.

EETA 75X-I(Al

??LAFAYETTE

?? pVV1 Ylnni_n

.

1

MV.VAL.

,$zA%rir!M . SIfKiOTTY : : 9' :

:

,

I

0.80

0.85

PNeP2Ne) c

0.9

_

Fig. I, Mg/(Si + Al) elemental concentration ratios vs. the ratio of cosmic-ray produced “Ne/“Ne for Martian meteorites, following Begemann and Schultz (1988) and Garrison et al. (1995). The shaded zone represents the boundary between essentially pure galactic cosmic-ray (GCR) Ne on the right and increasing solar cosmicray (SCR) Ne on the left. Two samples of ALH77005 and one sample of QUE94201 contain a contribution of SCR Ne.

Ejection

Table 3. Concentrations Na % Basaltic shergottites QUE9420 1 Zagami Lherzolitic shergottite LEW88516 Orthopyroxenite ALH84001 bulk opx chromite mask.

1.12 0.90

times of Martian

meteorites

of maior and minor elements

Mg %

Al %

Si %

3.72 6.4

5.41 3.2

21.5 23.1

415 114

2753

relevant

for this work.

co

Ni

ppm

ppm

Ref.

15.4 14.2

25.9

67

1 2 3

Ti %

Cr %

Mn %

Fe 410

7.6 7.6

1.08 0.48

0.10 0.23

0.388 0.41

K

Ca

ppm

%

0.34

15.4

1.68

20.5

216

3.24

0.19

0.47

0.38

16.0

64

260

0.066 0.022

16.0 14.1 3.2 3.6

0.5 0.35 3.2 15.4

24.0 25.2

129 148

0.085 0.09 1.34

0.36 0.38 0.3 0.009

13.3 13.7 39.5 3.1

20

6260

0.70 0.27 32.6 0.0009

44

28.9

1.23 1.19 0.014 4.5

5.2

4 5, 6 5, 6 5, 6

References: (1) Mittlefehldt, pers. comm. (1996). (2) Av. of data from Treiman et al. (1986) and McCoy et al. (1992). (3) Av. of data from Delaney (1992), Dreibus et al. (1992) and Treiman et al. (1994). (4) Av. of data from Mittlefehldt (1994), Dreibus et al. (1994). and Sack et al. (1991). (5) Krlhenbtihl, pers. comm. (1996). (6) Mittlefehldt (1994).

shielding term as given in Table 6 of Eugster and Michel ( 1995) for the Ne production rate, Pzl, depends on the ratio of Mg:Si:Al. The basaltic shergottites QUE, Shergotty, and Zagami are similar to eucrites, the lherzolites and Chassigny are similar to diogenites, and for the nakhlites an average for the shielding term of eucrites and howardites was adopted. EET and ALH84 are similar to howardites in this respect. The minor mineral constituents of ALH84 analyzed in

Table 4. Cosmic-ray-produced

noble gases, production

lo-* cm3 STP/g

Basaltic shergottites EET79001 A QtE9420

I

Shergotty Zagami Lherzolites ALH77005 LEW885 16 Chassigny Nakhlites Nakhla Gov. Valadares Lafayette Orthopyroxenite ALH84001 bulk

oPx bulk

this work are chromite and maskelynite. It is, in principle, possible to derive production rates for cosmogenic noble gases for these minerals. However, because the target element abundances strongly differ from those in the bulk material for achondrites the resulting values are not reliable. Furthermore, the shielding dependency of the production rates is not known. Using the chemical abundances given in Table 3 and calculating production rates according to the equations in Table 4 of Eugster and Michel (1995) we obtain for

rates, and cosmic-ray

exposure

ages of Martian

10e8 cm3 STP/g, Ma

‘He,

*lNe,

“Arc

(**Ne/*‘Ne),

P3

Pz,

0.98 0.83 3.40 3.57 3.0 4.20 4.54 4.65

0.134 0.12 0.43 0.54 0.92 0.53 0.60 0.55

0.043 0.16 0.41 0.41 0.44 0.35 0.40 0.35

1.21

1.316 1.245

1.61 1.61 1.57 1.62 1.57 1.60

0.207 0.171 0.138 0.208 * 0.169

0.110 0.157 0.160 0.164 0.160 0.147

1.238

1.60

0.181

0.156

6.17 7.23 6.92 22.4

0.75 1.00 0.87 3.94

0.18 0.31 0.23 0.26

1.19

1.61

0.233

0.062

1.227

1.60

0.204

0.079

1.156

1.60

0.30

20.3 19.8 19.2

2.56 1.63 2.40

2.10 1.76 2.02

1.146 1.173 1.195

1.63 1.62 1.61

25.5 25.0 24.1

3.93 4.01 3.90

0.49 0.49 0.52

1.183 1.198 1.20

1.63 1.62 1.63

1.282 -

meteorites.

Ma T3

T21

T,*

0.61 0.52 2.17 2.2 1.91 2.62 2.84 2.91

0.65 0.70 3.12 2.6 * 3.14 3.31 3.02

0.39 1.02 2.56 2.5 2.75 2.38 2.56 2.24

0.55 0.75 2.62 2.43 2.33 2.71 2.90 2.72

0.034

3.83 4.52 4.32 14.0

3.22 4.90 4.26 13.1

2.90 3.92 2.91 7.65

0.205 0.183 0.184

0.214 0.191 0.214

12.5 12.2 11.9

12.5 8.91 13.0

9.81 9.21 9.44

0.295 0.261 0.256

0.0400 0.0382 0.0356

15.6 15.4 14.8

13.3 15.4 15.2

P1X

12.2 12.9 14.6

T,b (220)

-c t -c 2 t ? i 5

Ref.

0.16 0.28 0.55 0.23 0.84 0.45 0.46 0.50

1, 2. 11 2 this work 3 4 5-9 this work 2, 6, 10

3.32 4.45 3.83 11.6

-+ 0.55 t 0.58 5 0.90 24.0

2 this work 13-16 9, 17, 18

11.6 10.1 Il.4

+ 1.8 t 2.2 * 2.1

9, 19 20 IO, 19

13.7 14.6 14.9

2 2.0 2 1.7 t 0.4

this work this work 12,21

* No production rate calculated due to possible contribution of solar cosmic-ray Ne. References: (1) Becker and Pepin (1984). (2) Bogard et al. (1984). (3) Dreibus et al. (1996). (4) Swindle et al. (1996). (5) Eberhardt and Hess (1960). (6) Heymann et al. (1968). (7) Miiller and Zlhringer (1969). (8) Becker and Pepin (1986). (9) Ott (1988). (10) Ott (1989). (11) Sarafin et al. (1985). (12) Miura et al. (1995). (13) Becker and Pepin (1993). (14) Bogard and Garrison (1993). (15) Ott and LGhr (1992). (16) Treiman et al. (1994). (17) Lancet and Lancet (1971). (18) Schultz et al. (1996). (19) Ganapathy and Anders (1969). (20) Swindle et al. (1989). (21) Swindle et al. (1995).

0. Eugster, A. Weigel, and E. Polnau

2754

the chromite an exposure age of 12.3 2 6.0 Ma. For the maskelynite T3 and T2, are about 1.5 Ma, whereas TX8is 15.9 ? 3.7 Ma. It appears that the glassy character of maskelynite caused diffusion losses of 3He and “Ne whereas 38Ar is completely retained and yields an exposure age in agreement with those for the bulk and the orthopyroxene fraction.

15 Ma for ALH84 (Eugster, 1994; Miura et al., 1995; Swindle et al., 1995). The exposure ages and the terrestrial ages are the basis for deriving the ejection times from Mars. The results are given in Table 5. For nonantarctic meteorites the terrestrial age was assumed to be negligibly brief. The ejection ages of the basaltic shergottites QUE, Shergotty, and Zagami fall into a narrow range from 2.7 to 2.9 Ma, whereas the lherzolites were ejected about 1 Ma earlier. This difference of 1 Ma is also recognized for the “He ages alone (Table 4): basaltic shergottites: 1.9-2.9 Ma, lherzolites: 3.8-4.5 Ma. The production rates for T3 are quite insensitive to the chemical composition except for Fe, which is almost the same for basaltic shergottites ( 14.6%) and lherzolites ( 15.9%). Thus, the difference in exposure age between basaltic shergottites and lherzolites cannot be due to an error in the choice of the respective production rates. Except for Fe the basaltic shergottites and the lherzolites strongly differ in mineralogical and chemical composition: the basaltic shergottites mainly consist of the clinopyroxenes pigeonite and augite, whereas the lherzolites consist of olivine, chromite, and orthopyroxene. These differences are reflected by large differences in chemical composition: average for the three basatlic shergottites: 5.3% Mg, 4.0% Al, 7.4% Ca, and 0.70% Ti; average for the two lherzolites: 15.8% Mg, 1.8% Al, 2.8% Ca, and 0.23% Ti. Obviously the impacts

4. COSMIC-RAY EXPOSURE AND EJECTION AGES From the resulting production rates and the cosmogenic isotope abundances the exposure ages were calculated (Table 4). The exposure ages derived from the analyses of other workers, in most cases using the same method as for our data are also given. The only exception is QUE of Dreibus et al. (1996) where we adopted the age published by these authors, because they did not give the (**Ne/“Ne), ratio. All references for the various noble gas analyses are indicated as notes to Table 4. Where several authors reported noble .gas isotopic abundances, we calculated average T3, T2,, and T3* values and then a grand average for these three ages. The errors are 20 mean. Agreement between several analyses of a meteorite is generally within error limits. In order to avoid the possibility of a contribution of solar cosmic rays to the ALH77 sample, the sample measured by Bogard et al. ( 1984) was adopted for the calculation of the exposure age. Inspection of the resulting exposure ages shows remarkable groupings of the Martian meteorites. The shergottites have long been known to yield exposure ages around 3 Ma with the exception of EET ( < 1 Ma) and Chassigny and the nakhlites at about 11 Ma (cf. Bogard et al., 1984). We now realize that the three basaltic shergottites, QUE, Shergotty, and Zagami, are distinct in age from the lherzolites, the latter ones showing a -1 Ma higher exposure age. Furthermore, we confirm the earlier results for the exposure age of about

Table 5. Ejection

Meteorite EET7900 1 QUE94201 Shergotty Zagami Av. for QUE, Shergotty, and Zagami ALH77005 LEW88516 Av. for ALH77005 and LEW Chassigny Nakhla Gov. Valadares Lafayette Av. for nakhlites ALH8400 1

for the ejection

of these

ages of Martian

meteorites.

T,,,.. r/1 (Ma)

Terrestrial age, T,,, (Ma)

0.65 2.46 2.71 2.81

two

classes

of Martian

meteorites

did not occur into the same terrain. Shergottite EET differs from the other basaltic shergottites and the lherzolites not only in ejection time but also in its petrologic characteristics as it consists of several lithologies in contrast to the other more homogeneous shergottites. The initial strontium, neodymium, and lead isotopic composition of Shergotty and Zagami are sufficiently distinct from EET to require that

-t + ? i

0.20 0.17 0.45 0.18

3.32 ? 0.55 4.14 +- 0.62 11.6 11.6 10.1 11.4

? 2 2 t

4.0 1.8 2.2 2.1

14.4

2 0.7

0.17? 0.29’

o.24 0.0215’ -

Ejection age Tc, = Tav,(cr , + T,,, (Ma) 0.82 2.75 2.71 2.81 2.76 3.52 4.16 3.84 11.6 11.6 10.1 11.4

11.0 0.00655

14.4

+ 0.20 2 0.17 2 0.45 i 0.18 I! 0.06 5 0.55 t 0.62 ? 0.64 2 4.0 + 1.8 t 2.2 +2.1 t 0.9 % 0.7

Errors are 2u mean. ’Average cosmic-ray exposure age calculated from the results of this work and those of other workers as given in Table 4. ’ Average for data from Sarafin et al. (1985) and Jull and Donahue (1988). 3 Nishiizumi and Caffee (1996). 4 Schultz and Freundel (1984) and Nishiizumi et al. (1992). ’ Jull et al. (1994).

Ejection

I

I

I

I

I

times of Martian

I

I

I

I

I

I

I

I

I

I

I

_ SHERGOTTITES

a

s

-

&

_

kl

-

S$%TY

LHE!?ZOLlTES NAKHLITES

633 ?zl_ is2

2155

meteorites

EET79

l-

I

0

0

ALM4

i%G Y-19 I

2

4

G-IASSIW I

I

I

6

I

8

I

II

IO

I

12

Ill

I

14

16

-

MARS EJECTIONAGE (MILLION YEARS) Fig. 2. Histogram

of the Mars ejection

ages for the Martian meteorites dated until now. For data and experimental

errors see Table 5.

typical Rb-Sr, Nd-Sm, and U/Th-Pb mineral isochron ages of about 180 Ma (Jones, 1986; McSween, 1994). Obviously a large fraction is trapped Martian atmospheric Ar. Bogard and Johnson ( 1983) and Becker and Pepin ( 1984) found Martian atmospheric Ar in lithology C of EET whereas Marti et al. (1995) discovered Ar originating from the Martian atmosphere in shock-melted glass of Zagami. These two shergottites show clear signatures based on isotopic ratios for entrapped Martian atmospheric gases and all investigators of LEW come to the conclusion that most Ar is trapped (Ott and LGhr, 1992; Becker and Pepin, 1993; Bogard and Garrison, 1993; Treiman et al., 1994). Orthopyroxenite ALH84 crystallized much earlier than all other Martian meteorites studied until now. Jagoutz et al. (1994) obtained a Sm-Nd age of 4560 Ma that was confirmed by Nyquist et al. ( 1995) who reported a Sm-Nd age of 4570 Ma. Ash et al. (1996) reported a 4000 Ma shock age based on 39Ar- 40Ar dating. Table 6 gives the K- 40Ar ages for the two phases for which K and Ar concentrations were determined. The resulting age critically depends on the assumption for the quantity of trapped 40Ar. For the maskelynite and orthopyroxene we can tentatively derive the ratio 40Ar/36Ar of the trapped component from the ordinate intercept in a 40Ar/36Ar vs. K/“6Ar plot. We obtain (40Ar/“6Ar),, = 1790, corrected for radiogenic “OAr. This ratio indicates

these formed from different magmas (Jones, 1989). The ejection ages of the nakhlites and Chassigny are identical within error limits and form the well known cluster at 11 Ma. Finally, EET and ALH84 represent two independent events on Mars 0.82 and 14.4 Ma ago, respectively. The orthopyroxenite ALH84 is unique among the Martian meteorites in its mineralogy as well as in the time when it left its parent planet. We conclude that at least five different impact events on Mars are responsible for ejecting meteorites that were collected on Earth until now (Fig. 2). The ejection time for Chassigny cannot be resolved from that of the nakhlites with the present data. If the Chassigny event is not related to the nakhlite event the number of required impacts rises to six. We propose that these ejection age clusters represent individual impact processes into distinct source areas. 5. K-4UAr AGE

OF ALH84001

From our data of QUE, Zagami, and LEW, we cannot derive which fraction of 40Ar is of trapped Martian atmospheric origin or in situ produced radiogenic Ar. The nominal K-40Ar ages that correspond to the 40Ar and K content (Tables 1 and 2, respectively), assuming no trapped Ar are in the range of 1600-3400 Ma. This is much higher than the

Table 6. K-Ar ages of maskelynite

and orthopyroxene

from ALH84001.

Assuming (40Ar/36Ar),, = 1790 K (ppm)” Maskelynite

Assuming (““A#‘Ar),, = 295.5

40Ar,

36Artr

40Ar,

TAO (Ma)

“OAr,

6260 F 300

38140 t 2500

7.10 -c 1.23

24360 2 3300

3390 % 200

35865 2 2525

148 28

987 * 35

0.23 + 0.01

575 2 40

3390 100

919 5 35

Orthopyroxene

Noble gas concentrations

in lo-’ cm3 STP/g.

I) See Table 3.

t

T4” (Ma)

t

4000 150 4130 2 80

2156

0. Eugster,

A. Weigel, and E. Polnau

the presence of Martian atmospheric gas. The present-day ratio 40Ar/36Ar is 2000 (Owen et al., 1977; Bogard and Johnson, 1983), and that trapped by ALH84 4000 Ma ago may have been lower. It is also close to the value of about 1600 for Zagami glass (Marti et al., 1995). However, the obtained (40Ar/36Ar),, ratio is very uncertain because there are only two data points relatively far from the origin. Table 6 gives the K-40Ar gas retention ages ( Tbo) obtained with different assumptions for the trapped argon isotopic composition. Using (40Ar/36Ar)tr = 1790, T40 is 3390 Ma. On the other hand, if we adopt 40Ar/3hAr to be terrestrial atmospheric contamination with ( 40Ar/36Ar)tr = 295.5 we obtain T4(, = 4000 Ma and 4 130 Ma for the maskelynite and orthopyroxene sample, respectively. These ages are considerably younger than the Sm-Nd crystallization ages mentioned above and indicate that this material experienced a thermal event later in its history. The question whether Martian atmospheric Ar was trapped by ALH84 remains unsolved. 6. CONCLUSIONS Evidence from the isotopic composition of cosmogenic nuclei supports the numerical simulations by Gladman et al. ( 1996) showing that the most probable scenario for the ejection of Martian meteorites, and for their transfer to Earth, is the production as small meteoroids by asteroidal or cometary impact. The temporal distribution of the ejection times from Mars indicates that five (or perhaps six) impact events on Mars produced all Martian meteorites recovered until now. The three basaltic shergottites (QUE94201, Shergotty, and Zagami) were launched from the Martian surface 2.76 t 0.06 Ma ago. EET79001, also classified as a basaltic shergottite but differing from the other three in some petrographic characteristics and strontium, neodymium, and lead isotopic composition, was ejected 0.82 t 0.20 Ma ago. Both lherzolites (ALH77005 and LEW88516) left Mars 3.84 +0.64 Ma ago. Preliminary (unpubl.) data obtained by us for the cosmic-ray exposure age of the third known lherzolite, Yamato-793605 (Mikouchi and Miyamato, 1996) agree with this age. The nakhlites Nakhla, Govemador Valadares, and Lafayette yield the previously known ejection ages of 11 .O 2 0.9 Ma. Chassigny, although being mineralogically distinct from the nakhlites, also falls onto this age cluster and may, thus, originate from the same event. Finally, the orthopyroxenite ALH84001 was ejected 14.4 ? 0.7 Ma ago. It is remarkable that all Martian meteorites from a certain class are characterized by the same ejection time. This implies that these age clusters represent five to six individual impacts of an asteroid or comet into six distinct source areas, improving our knowledge of the Martian crust composition. All Martian meteorites except for ALH84 yield a crystallization age of 1300 Ma or less, depending on the interpretation of the data from Rb-Sr, Sm-Nd, U-Pb, and 39Ar-40Ar dating (McSween, 1994). Consequently, all impacts except the one for ALH84 occurred into a relatively young terrain. It appears that the oversampling of the young geologic units results from three or more impact events. If the Martian meteorites provide representative sampling of the Martian crust, magmatism extending until 1300 Ma ago must have been more widespread than currently believed.

thank U. Krtienbtihl for the unpublished data of ALH84001. The authors are indebted to the NSF/NASA

Acknowledgments-We

Meteorite Working Group for providing the antarctic meteorites studied in this work. We thank M. Zuber for preparing the samples, P. Guggisberg and A. Schaller for their help in mass spectrometry, and B. Balsiger and R. Btitzer for preparing the manuscript. We are grateful to Donald Bogard, Brett Gladman, and an anonymous reviewer for constructive reviews. This work was supported by the Swiss National Science Foundation,

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