Thermal history of the nakhlites by the40Ar-39Ar method

EARTH AND PLANETARY SCIENCE LETTERS 19 (1973) 135-144. NORTH-HOLLAND PUBLISHING COMPANY

[]

THERMAL HISTORY OF THE NAKHLITES

BY T H E 4 0 A r - 3 9 A r M E T H O D F.A. PODOSEK Division of Geological and Planetary Sciences*, California Institute of Technology, Pasadena, California 91109, USA Received 10 November 1972 Revised version received 18 March 1973 "The discovery of the significance of these glimpses and of their relation to the ancient histories revealed the Third A g e . . . " [23]. This paper reports the results of thermal-release argon analyses of neutron-irradiated samples of the two nakhlite meteorites, Lafayette and Nakhla. The initiation of retention of radiogenic 4°At in Lafayette appears to have been a reasonably well-defined event which occurred (1.33 ± 0.03) X 109 yr ago, as determined by the '*OAr- 39Ar method. Nakhla also appear~ to have been retaining argon no longer than 1.3 X 109 yr, but its gas-retention age cannot be considered well-defined because its apparently most-retentive sites have nominal gas-retention ages shorter than those of the less-retentive sites which contain most of its potassium.

1. Introduction The nakhlites are a class of calcium-rich achondrites comprised of only two members: Nakhla and Lafayette. Nakhla fell in Egypt in 1911. It has an igneous texture and consists mainly of diopside with some iron-rich olivine and accessory plagioclase and augite [ 1,2]. Lafayette was discovered in a geological collection in 1931; it is described and classified as a nakhlite by Nininger [3]. Its prior history is thus uncertain and it is possible, and has been argued [4], that it is one of the Nakhla stones. The nakhlites (or nakhlite) are particularly interesting because of recent evidence suggesting that their thermal history is significantly different from that of other meteorites. Pinson et al. [5] found that the ( R b - S r ) chemical differentiation age of Nakhla was no different from that o f other meteorites. All other isotope chronology data for the nakhlites are based on noble gas measurements, and thus refer to their thermal history. The U - H e and (isotopic dilution) K - A r ages of both meteorites are quite low [6]. Low ages of this * Contribution number 2239.

type are, however, neither unusual nor customarily interpreted as indicating formation (in any sense of the word) at any time significantly different from the 4.5 AE (1 AE = 10 9 yr) primary meteorite age. Since low ages are presumably due to partial diffusive loss of 4°Ar they cannot, except in a statistical sense [7], be interpreted as dating any specific event in the history of a meteorite. Observation o f the decay products of extinct radionuclides in the xenon in low-age meteorites also indicates that in spite o f low U - H e and K - A r ages at least some phases have been retaining noble gases for 4.5 AE. More detailed examination of thermal history by the 39Ar-4°Ar method [8] has also shown that even in low K - A r age chondrites some phases have been retaining 4°Ar for 4.5 AE or at least substantially longer than the bulk K - A r age [9,101. Rowe et al. [4] measured xenon in both Nakhla and Lafayette. These authors noted the absence of any detectable decay products of 244pu but also observed definitive excesses of 129Xe, presumably due to decay of 129 I. This observation seems to indicate that the generalizations in the preceding paragraph are applicable to nakhlites, and that they have been retaining noble gases for the customary 4.5 AE.

F.A. Podosek, Thermal history of the nakhlites

136

TABLE 1 Argon compositions in neutron-irradiated (LAV2) nakhlites Release temp °C*

(4°At)** ccSTP/g X 10 --'s

36Ar/4°Ar

37Ar/4°Ar

38Ar/4°Ar

39Ar/4°Ar

Apparent aget × 109 yr

0.0307 0.0031 0.0138 0.0003 0.0099 0.0001 0.0086 0.0002 0.0205 0.0002 0;0736 0.0002 0.1503 0.0006 0.1861 0.0002 0.263 0.0O3 0.0175 0.0024 0.0076 0.0010 0.0407 0.0007

0.0230 0.0040 0.0372 0.0004 0.0390 0.0001 0.0409 0.0001 0.0408 0.0001 0.0406 0.0001 0.0422 0.0001 0.0646 0.0003 0.1377 0.0026 0.0138 0.0009 0.0099 0.0004 0.0404 0.0001

0.0'* 0.2 1.2'* 0.1 1.35'* 0.05 1.331 0.002 1.330 0.004 1.337 0.003 1.300 0.007 1.333 0.016 1.5 0.6 1.5 2.2 0.6 2.7

0.0289 0.0007 0.0289 0.0001 0.0065 0.0001 0.0103 0.0001 0.0253 0.0005 0.1261 0.0005 0.3467 0.0013 0.3018 0.0016 0.425 0.006 0.0331 0.0019 0.0082 0.0008 0.0441 0.0008

0.0238 0.0002 0.0409 0.0001 0.0391 0.0001 0.0398 0.0001 0.0402 0.0002 0.0492 0.0002 0,0678 0,0005 0,1190 0,0004 0,229 0.015 0.0231 0.0014 0.0082 0.0005 0.0408 0.0002

1.2'* 0.1 1.25'* 0.02

La~yettett 550

21

650

81

700

193

750

129

800

106

900

139

1000

39

1150

39

1300

2.5

1500

5.1

1600

11

Total

765

550

110

650

359

700

119

0.0035 ±0.0007 0.0010 ±0.0004 0.0002 ±0.0001 0.0001 ±0.0002 0.0001 ±0.0002 0.0003 ±0.0002 0.0009 ±0.0006 0.0333 ±0.0005 0.154 ±0.008 0.010 ±0.005 0,001 ±0.002 0.0026 ±0.0001

0.034 0.002 0.031 0.005 0.033 0.002 0.040 0,002 0.055 0.003 0.114 0.002 0.605 0.005 31.41 0.09 155.2 4.2 9.3 0.2 0.418 0.008 2.25 0.09

0.0027 ±0.0003 0.0003 ±0.0001 0.0003 ±0.0003 0.0000 ±0,0003 0.0002 ±0.0006 0.0001 ±0.0007 0.0021 ±0.0013 0.043 ±0.004 0.242 ±0.008 0,019 ±0.005 0.0036 ±0.0016 0.0028 ±0.0002

0.058 0.001 0.037 0.001 0.034 0.002 0.071 0.001 0.093 0.004 0,326 0.006 2.089 0.014 34.32 0.09 193.8 2.4 14.8 0.2 0.606 0.015 1.91 0.08

Nakhla*

750

102

800

50

900

44

1000

25

1150

15

1300

4.7

1500

6.1

1600

19

Total

854

1.29'* 0.04 1.329 0.005 1.315 0.008 1.20 0.02 0.89 0.01 0.66 0.02 0.5 0.2 0.4 2.6 1.7 1.5

F.A. Podosek, Thermal history o f the nakhlites

137

Notes to table 1 * Temperature-RF power calibration determined by optical pyrometer assuming spectral emissivity of 0.5; temperatures accurate

only within 50°C. ** Absolute gas amounts uncertain by 10%; relative amounts uncertain by 5 %.

i" Computation of apparent gas retention age from raw data requires several stages of correction, made as described in text and references. Procedural blanks ('*OAr,ccSTP X 104 ) for both samples are 0.05 up to 1150°C and 0.12, 0.40, 1.0, at 1300°C, 1500°C, 1600°C, respectively. l'i" Sample weight 0.097 g; neutron fluence (relative to LAV2 normalization) 1.010 ± 0.003. 4: Sample weight 0.071 g; neutron fluence (relative to LAV2 normalization) 0.986 :t 0.007. *~ Includes correction for atmospheric 4°Ar indicated by presence of 36Ar. Hohenberg [11] measured xenon in a thermalrelease experiment on neutron-irradiated Lafayette. Later analysis of the data also indicated the absence of 244pu fission products [12]; since the neutron irradiation also allows simultaneous measurement of U abundance, this interpretation indicated a chronology inconsistent with the apparent presence of radiogenic 129Xe [13] in the same sample. Subsequent reappraisal of the same data, however, indicated that radiogenic 129 Xe was not present and that the absence of 224pu fission products limited the initiation of noble gas retention in Lafayette to at least 350 my after the corresponding event for chondrites [14]. More recently, Podosek and Huneke [ 15] reported a 4°Ar-39Ar analysis of Lafayette, concluding that its 4°Ar-39Ar age was also quite low (essentially the same as its bulk K - A r age) and, within the limitations of the experiment, well-def'med, in that its pattern of apparent age variation was constant and not similar to the steeply rising patterns characteristic of low K - A r age chondrites [9, 10]. By contemporary standards, however, this experiment was inferior in that 37At measurements were not possible, and the age evaluation accordingly hedged by the inapplicability of many current interpretational techniques. This paper reports the results of a more thorough 4°Ar-a9Ar experiment on both Lafayette and Nakhla.

2. Procedures and results The samples analyzed were obtained from the Nininger Collection through the courtesy of C.B. Moore; specimen numbers are No. 167 ax (Lafayette) and No. 84 a (Nakhla). The samples were irradiated in the LAV2 irradiation and analyzed in eleven stages of thermal release by induction heating in an extraction manifold connected

on-line to the Henearkrxe mass spectrometer. The results are presented in table 1. The tabulated data have been corrected for the presence of gases other than argon (C1 and HC1) in the spectrometer and for radioactive decay of 3TAr and agAr but are otherwise "raw"; they include no corrections for irradiation interferences, blanks, etc. All techniques for irradiation, gas analysis, data reduction, corrections, and age computations are documented elsewhere [15, 16]. Subsequent discussion is based on data with all appropriate corrections made. Table 1 and the references above contain sufficient information to duplicate all calculations implicit in this paper. In this paper the conventional asterisk (*) notation will be used to designate argon data corrected for all contributions other than kaliogenic (potassium-derived): 4°Ar* from natural decay of 4°K and 39Ar* from 39K(n, p).

3. Potassium, calcium, and chlorine

Analysis of argon in neutron-irradiated samples permits computation of the abundances of K, Ca, and C1 through measurement of the daughters of the S9K(n, p)39Ar*, 4°Ca(n, a) s7 Ar, and 37Cl(n, 7~3)3aAr reactions. Calibrations for the K and Ca measurements are given elsewhere [ 16, 17 ]. Concentrations, listed in table 2, are uncertain by the 10% uncertainty in absolute spectrometer sensitivity. The average K/Ca ratio in sites contributing to a given release fraction may be inferred from the 39Ar*/aTAr ratio in that fraction. For both samples this ratio exhibits the normal pattern of relative enrichment of Ca at higher temperatures, but in both cases the magnitude of the skewing is extreme. Three regimes may be distinguished in the thermal release.

138

F.A..Podosek, Thermal history of the nakhlites

TABLE 2 Summary and comparison of results for nakhlites Lafayette This work K (%)* Ca (%)* CI (ppm)** Cosmic-ray age (million years)* K-At age (AE) (bulk) 4°Ar-39Ar age (AE) (high-temperature

0.094 10.1 65 6.5 1.36 ± 0.03 1.33 ± 0.03

Nakhla This work

Other 0.10 10 1.1 ± 0.3 1.4 - 1.7

(15) (6) (6) (15)

0.109 9.8 80 8.0 1.30 ± 0.03 < 1.3

Other 0.11 10.8

'[1] [1]

11

[6]

1.4 ± 0.3

[61

* Uncertainty for values reported in this work is 10%. ** Uncertainty for values reported in this work is 25%. The K/Ca ratio is relatively high and constant (0.3 - 0 . 6 ) up to 800°C or 900°C, accounting for most of the K. The K/Ca ratio then drops through more than three orders of magnitude by 1300°C. Most of the Ca is accounted for by the 1150°C and 1300°C fractions. In both samples the K/Ca ratio increases to about 0.0004 in the 1500°C fraction, presumably reflecting some highly-retentive accessory phase (0.1-0.2% of the K). The "diopside" (in Nakhla) is (Cao.39 Mg0.37 Feo.24) SiO 3 [1, 2] and accounts for most of the Ca. Major degassing of the diopside thus seems to occur in the high-temperature regime, particularly in the 1150°C and 1300°C fractions. The plagioclase probably accounts for most of the K and is thus tentatively identified with the lower temperature regime. It is unclear whether the 39Ar in the diopside regime represents small amounts of K actually in the pyroxene, the tail end of plagioclase degassing, or another phase altogether. The LAV2 irradiation included a sample of opal glass (NBS SRM 91), which serves as a chlorine monitor. The glass was found to have 38Ar/C1 = • = 4.4 × 10 -3 ccSTP/g; the accuracy of this calibration is limited by the uncertainty in the C1 content of the glass, about 25%. In the general case, 38Ar attributable to C1 cannot be resolved rigorously, since three components (trapped, spallation, and chlorine) may contribute to the two-isotope system 36Ar-aaAr. For both Lafayette and Nakhla, however, 3aAr is much more abundant than 36Ar, indicating that at most only a small fraction (less than 10%) of the 38Ar can be attributed to

either trapped or spallogenic Ar; the error inherent in this resolution is thus small compared to the calibration error. The C1 abundances are given in table 2. The C1 release pattern does not correlate particularly well with that of either K or Ca. Fractional release is high (relative to K and Ca) in the first extractions (perhaps indicating superficial contamination), declines with increasing temperature, but then reaches a strong peak around 1000°C. It may thus be inferred that C1 probably does not reside in the same phase(s) as K, and certainly not in the same phase as most of the Ca.

4. Cosmic-ray exposure ages The technique of determining the ratio of spallogenic 3aAr to its principal target element (Ca), and thus determining the cosmic-ray exposure age, by means of comparison with 37Ar in neutron-irradiated samples was first used by Turner et al. [17]. In the present case 3BAr is useless for this purpose because it is dominated by production from C1, but 36Ar serves as well. 36Ar is also produced by neutron irradiation of 3s C1, but because of the long half-life of 36C1 chlorine-derived 36Ar is only 10 -3 times chlorinederived 3BAr (approximately 150 days between irradiation and analysis) and this interference is inconsequential. In both samples 36Ar is present in (relatively) large amounts in the first two extractions, but it is unlikely that it can be spallogenic; atmospheric contamination or trapped gas is more reasonable. At higher tempera-

F.A. Podosek, Thermal history of the nakhlites

tion. A monotonic increase of apparent age with increasing extraction temperature may be understood in terms of partial diffusive loss of 4°Ar*. Coincidence of apparent age in several successive release fractions (a plateau) may be taken to represent a well-defined event in the thermal history of the sample; if this occurs throughout the highest temperatures the event is the initiation of argon retention and defines the gas retention age. The normal correction procedure for calcium-derived 39Aris inadequate for Lafayette and Nakhla. The distributions of K and Ca are highly skewed and the corrections are critical in the high-temperature release fractions; the extreme case is Lafayette 1300°C, where only 12% of the 39Ar is 39Ar*. In such a case we may profitably employ a more general approach

tures, 36Ar is essentially undetectable until it rises with 3TAr at 1150°C. For 36Ar/37Ar = 0.00076-+ 0.00003 (Lafayette 1150°C and 0.00092 + 0.00007 (Nakhla 1300°C), a production rate of 1.75 × 10 -s ccSTP (36Ar)/g(Ca)/my [17], and a small correction for spallation from Fe [1, 18], we obtain the exposure ages given in table 2. The other high-temperature ratios are compatible with the ones used above, but have larger statistical errors.

5. G a s r e t e n t i o n

ages

In the conventional evaluation o f a 4 ° A r - 3 9 A r experiment, a nominal or apparent gas r e t e n t i o n age m a y be c o m p u t e d from 4°Ar* '39Ar* in each release fray-

650

'

139

~

'

700

'

o 700

24 ~ 7 5 0 - 9 0 0 25

i 900

24 1000 { ~-

16

1150

,

i lO

,

tel

8

LAFAYETTE t

~

~ ~_.1~00

(LAV2) 4

I \ I 200

I 400

I 600

I 800

I 1000

-

I~ 1200

3%/3% Fig. 1. Three-isotope correlation diagram for Lafayette. 4°Ar has been corrected for blanks, spallation, and production from neutron irradiation of K; 39At and 3TAr have been corrected for radioactive decay and normalized to unit neutron fluence for the LAV2 irradiation. Points are labeled by release temperature in °C. The line shown is the least-squares (20) fit to the high-temperature data (solid circles); its intercept on the Y-axis corresponds to a K - A t age of 1.33 AE. Lower temperature points (open circles, not included in fit) probably include contamination with terrestrial atmosphere as indicated by presence of 36Ar. Correc• . . . . • tlon of 4 0 Ar on this basis produces normal sequence of increasing 4 0 Ar * / 3 9 Ar * with temperature.

140

F.A. Podosek, Thermal history of the nakhlites

first used by Turner [19]. Fig. 1 is a three isotope correlation diagram for Lafayette; 4°Ar in this diagram has been corrected for neutron production (from K), blanks, and spallation, but no corrections (other than decay and neutron fluence normalization) have been applied to either 39Ar or 3TAr. On such a diagram the simple case of a well-defined gas retention age with no diffusion loss would be represented by a linear array: a two-component mixture of kaliogenic and calcigenic argon. If this situation obtains, the 4°Ar*/39Ar* ratio, and thus the age, are determined by the intercept of the correlation line with the Y-axis. Independent knowledge of the calcigenic composition is unnecessary; this composition is, in fact, determined by the intercept of the correlation line with the X-axis. As seen in fig. 1, the 750°C, 800°C, and 900°C fractions, which account for 51% of the K, are essentially identical, thus constituting a plateau. The three previous fractions, accounting for 37% of the K, have higher 4°Ar/39Ar ratios, opposite to the sequence expected if diffusive losses were significant. A source of the "excess" 4°Ar is suggested by the presence of 36Ar in these fractions in amounts greater than may be attributed to spallation (sect. 4). While the possibilities of indigenous trapped Ar, an internal atmosphere, etc., cannot be excluded, the less exotic possibility of superficial contamination with terrestrial atmosphere is equally satisfactory, and the corresponding adjustments to 4°At make the release pattern quite orthodox. Corrections to 4°Ar on this basis are (102 +--20)%, ( 2 8 -+ 12)%, and (3.3+3.5)% at 550°C, 650°C, and 700°C, respectively. The corresponding sequence of corrected 4°Ar/39Ar is the normal one expected for diffusive loss and the 700°C point is within error limits the same as the three succeeding points. The degree of 4°At* loss inferred in the first two fractions is small: 3.6% of total 4°Ar*. Because of the large Ca contributions, apparent ages for the 1150°C and 1300°C points can be computed only with relatively low accuracy. To the extent that they are consistent with linear relation to the low temperature points they may be considered to have the same apparent age. The calcigenic composition determined by a least-squares [20] fit is 39Ar/3TAr = = 7.80 X 10-4; an independent calibration for the same reactor position is (7.32 + 0.15) × 10 -4 [17]. The difference is not great, and could be caused, for

example, by an error of 0.7 day in the 35-day halflife of 3TAr used in the decay corrections. The apparent ages of the individual release fractions, incorporating the correction for calcigenic 39Ar by 39Ar/3TAr = 0.000780, are included in table 1. They are also included in fig. 3, a conventional agerelease plot which more clearly exhibits each fraction's contribution to the total potassium content. The 1000°C point is clearly not on the presumed correlation line (fig. 1), and is the only hindrance to the simple interpretation that argon retention began at a well-defined time for all phases of the rock with only minor diffusive loss from the least-retentive sites subsequent to that time. In cases such as this, outright experimental error must always be considered a possibility. If the effect is real, no simple explanation is readily forthcoming. It may be significant, however, that the 1000°C release marks the transition between the low-temperature, high K/Ca and the high-temperature, low K/Ca regimes of the release pattern. We will return to this point later. The 1000°C datum notwithstanding, it seems reasonably clear that at least the major potassium-bearing phase or phases of Lafayette have been retaining radiogenic 4°Ar for a fairly well-defined (at least within about 20 my) time interval. This interval, as determined by the least-squares correlation [20] intercept on the Y-axis in fig. 1, is 1.33 -+ 0.03 AE; the uncertainty is due chiefly to the irradiation calibration [16]. There is also no evidence that even the most highlyretentive phases have been retaining argon for any longer than that. Apparent ages for the individual release fractions of Nakhla are also listed in table 1, and displayed in fig. 3. Since Lafayette and Nakhla have similar K and Ca abundances and release patterns, and were analyzed within a few days of each other, the Ca corrections for Nakhla were made on the basis of the calcigenic composition derived for Lafayette. A three-isotope diagram analogous to fig. 1 is presented as fig. 2, and the same remarks apply. It is immediately apparent that a simple interpretation such as that made for Lafayette cannot be extended to Nakhla. The first two fractions, and probably the third, contain presumably non-spallogenic 36Ar. Again interpreting this as atmospheric contamination and correcting 4°Ar accordingly, the sequence of the first three points is that expected for partial

F.A. Podosek, Thermal history 01"the nakhlites

700 24 o 7 5 0

Z5

650, 800

141

' 700 } 750 T

9OO

800

24 '~

20

650

NAKHLA (LAV2) i

0•16

I

10 1000

12

8

0 1150

4

1300

@

Co --

L 2OO

1 400

I 600

I 800

I 1000

12oo

3ZAr/39A~ Fig. 2. Three-isotopecorrelation diagram for Nakhla. Cf. fig. 1. Low temperature points also contain atmospheric 4°At. Absence of linear correlation in the high-temperaturedata precludes assignment of a well-definedK-Ar age to all phases of the meteorite. diffusive loss (6.5%) of 4°Ar* from the least retentive sites. The higher-temperature data, however, are wholly inconsistent with the model of two-component mixing. The standard rules for three-isotope mixing diagrams apply: as seen in fig, 2, at least three components contribute to the 4°Ar-aTAr-agAr system. Since only the calcigenic component includes 3TAr, the others must lie on the Y-axis. Although more cannot be excluded, only two Y-axis components are required, and discussion will be continued on the basis of two. The location of these components on the X-axis must be such that the triangle between them and the Ca component includes all data points, i.e. such that the apparent ages corresponding to the 4°Ar/39Ar ratios span the apparent ages in table 1. Any source of 4°Ar other than in situ decay of 4°K, e.g. a (very high 4°Ar/36Ar) trapped component, will lie at infinity on the Y-axis, satisfying the requirement for one of the components, as will any

mineral phase older than 1.3 AE. The clustering of points around 4°Ar/39Ar = 24.5, followed by the rapid drop-off in 4°Ar/39Ar beginning at 900°C, is, however, rather suggestive that the "high" component has this value. The suggestiveness is strengthened by the observation that this corresponds to the same apparent age as found for Lafayette; it seems unlikely that such a coincidence is accidental. These relatively low temperature points (up to 800°C) constitute the high K/Ca regime for Nakhla and account for 83% of the total K. Beginning with the 900°C fraction both K/Ca and 4°Ar/39Ar decrease sharply, presumably indicating the prominence of release from another phase with a different 4°Ar/39Ar component. This "low" component could have 4°Ar/39Ar anywhere between zero and 10, the limit set by the 1150°C point, corresponding to apparent ages between zero (e.g. complete loss of 4°Ar*) and 0.7 AE. The data are only weakly suggestive that 0.6 - 0.7 AE is anything other

F.A. Podosek, Thermal history o f the nakhlites

142

2.0

i

(LAV2)

LAFAYETTE bJ

(.9

1.6

I-Z LU ,"r

,oo

~ 1.2 Q. 65O

I

I

I

I

I

I

I

I

i

i

i

i

i

i

i

i

1.2

I

[S~ 900 550

1000

0.8 N~KHLA

(LAV 2)

I-Z uJ w"

1150

0.4

13OO

13..

t

0

i

0.2

i

i

0.4

i

L

0.6

I

I

I

0.8

FRACTIONAL ~9Art RELEASE Fig. 3.4°Ar-39Ar age-release plot for Lafayette and Nakhla. • O Boxes are labeled by release temperature in C. Apparent ages plotted on the Y-axis are taken from table 1, and incorl~orate a correction for calcigenic argon with the composition 39 Ar/ 37 Ar = 0.000780. The X-axis represents fractional release of 39Ar*: the length of each box is the fraction of 39Ar* released at that temperature. The dashed line in the lower part (Nakhla) is the 1.33 AE age inferred for Lafayette by the high-temperature correlation shown in fig. 1.

than a limit. This sequence, a plateau followed by lower apparent ages at the highest temperatures, is quantitatively more extreme but qualitatively similar to patterns observed in some lunar rocks [17]. The phenomenon is not understood but the high-temperature drop-off in apparent age has been shown to be associated with pyroxenes [21], as also appears to be the case for Nakhla. One way in which such a pattern might be generated is by mechanical admixture of two (or more) phases with separate gas retention histories. Nakhla is not brecciated, however [2], and this mechanism appears unlikely on textural grounds. An alternative possibility suggested in this context [ 17] is that the high-temperature phase (in the laboratory experiment) may have a lower activation energy for diffusion than the (laboratory) low-temperature phase. It is thus possible that a

thermal event involving longer times and lower temperatures than the laboratory experiment could selectively degas the phase which, in the laboratory, appears to be the more retentive. Lacking supportive evidence, however, this possibility remains only conjectural. Other models based on more complex igneous and/ or metamorphic histories may be proposed, but also only conjecturally. Until such patterns are understood, interpretation of results such as those for Nakhla must remain incomplete. It seems unlikely that gas retention in any potassium-bearing phase extends beyond 1.3 AE but even this cannot be stated with certainty. It is noteworthy, however, that the low-temperature plateau in lunar rocks showing this type of pattern are attributable to plagioclase and give the same ages as Rb-Sr isochrons for these rocks [21]. In view of the requirement that at least two components are present in Nakhla (fig. 2), the 1000°C point for Lafayette (fig. 1) is suggestive of a similar effect, albeit on a lesser scale. That is, this point may signal a high-temperature drop in apparent age similar to that of Nakhla, but a drop of only about 0.03 AE rather than at least 0.6 AE. Because of larger statistical uncertainty and the large calcigenic contribution to 39Ar, the succeeding data cannot resolve alternative apparent ages differing by only 0.03 AE. The arguments regarding release patterns and chronological interpretations may be summarized as follows: (i) The degassing patterns of both meteorites may be divided into two major regimes identified with the degassing of plagioclase and diopside. The mineralogical identifications are only tentative and are best regarded as shorthand for "low-temperature, high K/Ca" and "high-temperature, low K/Ca", respectively. The plagioclase accounts for most of the K, the diopside for most of the Ca. (ii) There appears to have been only minor loss of radiogenic 4°Ar* from the plagioclase regimes: 3.6% for Lafayette, 6.5% for Nakhla. (iii) The plagioclase in Lafayette has apparently been retaining radiogenic 4°Ar* for a rather well-defined time, 1.33 + 0.03 AE. With somewhat less precision and confidence, this statement and age also apply to Nakhla. (iv) The apparent gas-retention age of the diopside in Lafayette is the same as the plagioclase, or perhaps somewhat lower (about 0.03 AE). To this extent, at least, the gas-retention age of Lafayette, as a rock, is well-defined.

F.A. Podosek, Thermal history o f the nakhlites

(v) The gas-retention age of Nakhla is not well-def'med. Apparent ages in the diopside are significantly lower than in the plagioclase regime. This pattern cannot be unambiguously attributed to thermal diffusive loss, either continuous or episodic. (vi) Within the limitations of the data there is no evidence that any potassium-bearing phase in either meteorite has been retaining radiogenic 4°Ar* any longer than 1.3 AE.

6. Discussion The most straightforward interpretation of the results of the experiment reported here is that both Lafayette and Nakhla were thoroughly degassed 1.3 AE ago, and that since that approximate time Lafayette has experienced only minor gas loss from its least-retentive sites but Nakhla has experienced some more complicated history not easily explainable as simple heating with consequent loss of noble gas from its least retentive sites. This interpretation for Lafayette is consistent with the results of previous analyses of argon [15] and xenon [11, 14] in neutronirradiated specimens. All of these are inconsistent with the excesses of 129Xe observed in (unirradiated) specimens of both meteorites [4], whether attributed to in situ decay of 129I or to the maintenance of an internal atmosphere including this decay product. Unless one or both sets of data reflect gross experimental error, or different parts of the same meteorite may have essentially unrelated thermal histories, this inconsistency challenges our interpretation of retention of noble gas radioactive decay products on an equally gross scale. Irrespective of such difficulties, however, it seems reasonably clear that a major event in the histories of both meteorites, resulting in complete or nearly complete degassing of at least argon, occurred 1.3 AE ago. Beyond this statement, the nature of the event cannot be inferred from noble gas data alone. The event could have been, for example, a collision, such as that possibly responsible for partial degassing of a large number of hypersthene chondrites [7, 9]; if so, it was apparently substantially more intense than the hypersthene chondrite event. At another extreme, this event evidently did not re-

143

suit in chemical fractionation of Rb relative to Sr on a macroscopic scale (large compared to the sizes of samples studied) [5]. In between these extremes lies the important question of whether or not these meteorites "formed" at 1.3 AE, in the sense of solidification from a melt. An unambiguous answer cannot be obtained from the noble gas data. In all previous instances in which it has been possible to identify both a well-def'med gas-retention age by the 4°Ar-39Ar technique and a solidification age by a Rb-Sr internal isochron, the ages have agreed. In the limited number of cases (all lunar rocks) where a definable a°Ar-39Ar plateau has been followed by lower apparent ages, as observed in Nakhla, and perhaps in Lafayette, the plateau age has also agreed with Rb-Sr isochrons [ 17, 21 ]. Historical extrapolation thus suggests that the nakhlites were molten 1.3 AE ago. This extrapolation has no basis other than precedent, however; noble gas mobilization does not necessarily require melting, and the nakhlites could be the first exceptions. The xenon results of Rowe et al. [4] argue that they are. Even if the nakhlites were molten at 1.3 AE, such melting could be caused by basically accidental processes such as collisions as well as by natural igneous activity in their parent body. The former possibility is interesting; the latter is relevant to more fundamental considerations of solar system evolution. This topic has been discussed in the literature and will not be extended here. Resolution of these alternatives is impossible with currently available data. It may turn out that 4°Ar-a9Ar patterns such as that observed for Nakhla are associated with metamorphism, but this is highly speculative. Accumulation of evidence in recent years has strengthened the generalization that chondrites formed, in all senses of the word, within a remarkably narrow time span. Evidence has also accumulated that other classes of meteorites are distinct from chondrites in their chronology as well as in their chemistry and petrology. To date, clear evidence for chemical fractionation at any time demonstrably different from the chondrite age remains limited to a single case, the iron meteorite Kodaikanal [22]. Recent work also indicates that at least some calcium-rich achondrites genuinely differ from chondrites in their thermal histories. In this respect the nakhlites are extreme and at least on this basis are attractive candidates for further investigation.

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Acknowledgements I am grateful for the c o o p e r a t i o n o f C.B. Moore and the tolerance o f stupid questions s h o w n b y A. Albee and A. Gancarz. My largest d e b t is to J.C. Huneke, b o t h for l a b o r a t o r y operations and m a n y and fruitful discussions. This w o r k was supp o r t e d by the National Science F o u n d a t i o n u n d e r grant GP-28027.

References [1] G.T. Prior, The meteoritic stones of El Nakhla El Baharia (Egypt), Mineral. Mag. 16 (1912) 274. [2] J.A. Wood, Physics and chemistry of meteorites, in The Moon, Meteorites, and Comets, B. Middlehurst and G. Kuiper, eds. (University of Chicago Press, 1963) 337. [3] H.H. Nininger, The Lafayette meteorite, Popular Astronomy 43 (1935) 404. [4] M.W. Rowe, D.D. Bogard and P.K. Kuroda, Mass yield spectrum of cosmic-ray-produced xenon, J. Geophys. Res. 71 (1966)4679. [5] W.H. Pinson, Jr., C.C. Schnetzler, E. Beiser, H.W. Fairbairn and P.M. Hurley, R b - S r age of stony meteorites, Geochim. Cosmochim. Acta 29 (1965) 445. [6] R. Ganapathy and E. Anders, Ages of calcium-rich achondrites, II. Howardites, nakhlites and the Angra dos Reis angrite, Geochim. Cosmochim. Acta 33 (1969) 775. [7] D. Heymann, On the origin ofhypersthene chondrites: ages and shock effects of black chondrites, Icarus 6 (1967) 189. [8] C.M. Merrihue and G. Turner, Potassium-argon dating by activation with fast neutrons, J. Geophys. Res. 71 (1966) 2852. [9] G. Turner, J.A. Miller and R.L. Grasty, The thermal history of the Bruderheim meteorite, Earth Planet. Sci. Letters 1 (1966) 155.

[ 10] G. Turner, Thermal histories of meteorites by the 39Ar-4°Ar method, in Meteorite Research, P.M. Millman, ed. (Reidel, 1969) 407. [ 11 ] C.M. Hohenberg, Ph.D. Thesis, University of California, Berkeley (1968). [12] F.A. Podosek, The abundance of 244puin the early solar system, Earth Planet. Sci. Letters 8 (1970) 183. [13] F.A. Podosek, Dating of meteorites b~¢ the high-temperature release of iodine-correlated Xe12", Geochim. Cosmochim. Acta 34 (1970) 341. [14] F.A. Podosek, Gas retention chronology of Petersburg and other meteorites, Geochim. Cosmochim. Acta 36 (1972) 755. [15] F.A. Podosek and J.C. Huneke, Argon 4 0 - a r g o n 39 chronology of four calcium-rich achondrites, Geochim. Cosmochim. Acta 37 (1973) 667. [16] F.A. Podosek and J.C. Huneke, Absolute and relative ages of the chondrites St. Severin and Guarena (in preparation). [17] G. Turner, J.C. Huneke, F.A. Podosek and G.J. Wasserburg, 40A r - 39Ar ages and cosmic-ray exposure ages of Apollo 14 samples, Earth Planet. Sci. Letters 12 (1971) 19. [18] J.C. Huneke, F.A. Podosek, D.S. Burnett and G.J. Wasserburg, Rare gas studies of the galactic cosmic ray irradiation histories of lunar rocks, Geochim. Cosmochirn. Acta 36 (1972) 269. [ 19 ] G. Turner, Argon-40/argon-39 dating of lunar rock samples, Proc. Apollo 11 Lunar Science Conference, Geochim. Cosmochim. Acta, Suppl. 1 (1970) 1165. [20] D. York, Least-squares fitting of a straight line, Can. J. Phys. 44 (1966) 1079. [21] G. Turner, J.C. Huneke, F.A. Podosek and G.J. Wasser• m • rocks and separated burg, 40A t - 39Ar systematlcs minerals from Apollo 14, Proc. Third Lunar Science Conf., Geochim. Cosmochim. Acta, Suppl. 3 (1972) 1589. [22] D.S. Burnett and G.J. Wasserburg, Evidence for the formation of an iron meteorite at 3.8 × 109 years, Earth Planet. Sei. Letters 2 (1967) 137. [23] J.R.R. Tolkien, The Fellowship of the Ring (Ballantine Books, New York, 1965).