Fission-track analysis of meteorites: Dating of the Marjalahti pallasite

Radiation Measurements 40 (2005) 522 – 527 www.elsevier.com/locate/radmeas

Fission-track analysis of meteorites: Dating of the Marjalahti pallasite Yu.V. Bondara,∗ , V.P. Perelyginb,
a Institute of Environmental Geochemistry, 34a Palladin ave., Kiev 03142, Ukraine b Joint Institute for Nuclear Research, Dubna 141980, Russia

Received 27 August 2004; received in revised form 4 May 2005; accepted 2 June 2005

Abstract The results of the Marjalahti pallasite fission-track age determination are presented. Thorough examination of fossil tracks in the phosphate (whitlockite) crystals coupled with U-content determination in whitlockites can make it possible to estimate the contributions of all possible track sources to the total track density and to calculate a model fission-track age. It is found that whitlockite crystals of the Marjalahti pallasite contain fossil tracks due to galactic cosmic rays (VH, VVH nuclei); fission of U and Th induced by cosmic rays; spontaneous fission of 238 U; and spontaneous fission of extinct, shortlived 244 Pu present in significant quantities in the early solar system. A great track density attributed to the extinct 244 Pu testifies to the high fission-track age. The model fission-track ages of (4.31 ± 0.02) × 109 yr for the Marjalahti pallasite are calculated. Petrographic studies allow us to interpret the fission-track age as the time of the last shock/thermal event in the cosmic history of the pallasite. © 2005 Elsevier Ltd. All rights reserved. Keywords: Fission-track age; Marjalahti pallasite; Phosphate grains; 244 Pu spontaneous fission

1. Introduction Following detailed studies on mineralogy, petrology and geochemistry of some pallasites by Buseck (1977) and Scott (1977), minor studies have been addressed to this group of stony-iron meteorites. However, some questions on shockmetamorphic structures and measurement of chronological data which are the basis for unraveling the cosmic history of pallasite parent bodies still remain. The pallasite mineralogy is remarkably simple: kamacite (
-FeNi), taenite (-FeNi) and olivine are the major


Deceased.

∗ Corresponding author. Fax: +380 44 424 00 60.

E-mail address: [email protected] (Yu.V. Bondar). 1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2005.06.013

minerals (89–97 vol%); troilite (FeS), schreibersite ([FeNi]3 P) and chromite are less abundant, but nevertheless generally present; and low-Ca pyroxene and phosphates are the accessory minerals (Buseck, 1977). Chronometry is a weak point in the investigation of pallasites since minerals that can be used for traditional radiometric dating methods (K–Ar, U–Pb, Rb–Sr, Sm–Nd) are practically absent. The fission-track method is very attractive in this respect. Only the uranium-rich minerals, as phosphates, can be good candidates for this method to be applied. As was noted by Buseck and Holdsworth in 1977, the phosphates (whitlockite, stanfieldite and farringtonite) are widespread accessory minerals of pallasites; however, Crozaz et al. (1982) studied U-containing minerals in some pallasites and pointed out the fact that the small size of the pallasite phosphates and their low abundance can make the track analysis very difficult.

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523

Only two attempts to date pallasites by fission-track method have been reported (Pellas et al., 1983; Bondar and Perelygin, 2001). The fossil tracks in extraterrestrial samples in contrast to terrestrial samples result from the number of sources additional to 238 U spontaneous fission (Fleischer et al., 1967). However, only tracks of 244 Pu and 238 U isotopes that undergo spontaneous fission can be used as a chronometer for fission-track dating. According to Durrani (1981), the ratio of the track densities, Pu /U due to 244 Pu and 238 U nuclei spontaneous fission is related to the fission-track age as shown below

an attempt to determine the pallasite fission-track age. They found a great excess of fission tracks (17 × 106 cm−2 ) and attributed them to 244 Pu. According to their estimate the fission-track age of ∼ 4.3 × 109 yr dates the onset of track retention in the Marjalahti phosphates. However, this value can be considered only as a qualitative one, owing to some methodical oversights. The aim of this work is to carry out a detailed fissiontrack analysis, to calculate the fission-track age, to interpret obtained data.


 fPu DU Pu fPu = fU DPu fU U 0 exp(−DPu
t) − exp(−DPu t0 ) , × exp(−DU
t) − exp(−DU t0 )

2. Meteorite samples and experimental procedure

(1)

where fPu = 1.06 × 10−11 yr−1 (Jaffey et al., 1971), DPu = 8.47 × 10−9 yr−1 (Fields et al., 1966), fU = 8.46 × 10−17 yr−1 (Galliker et al., 1970; Guedes et al., 2000), DU =1.551×10−10 yr−1 (Jaffey et al., 1971) are the spontaneous fission and total decay constants of 244 Pu and 238 U, respectively; t0 is the “zero” time (for practical considerations, t0 is often taken as the time of meteorite formation);
t = t0 − t is the time interval between meteorite formation and cooling below the track retention temperature, or the interval before the onset of track retention; and (Pu/U)0 is the ratio of 244 Pu to 238 U at time t0 . Thus if (Pu/U)0 and t0 are known, the meteorite fission-track age t (t = t0 −
t) can be calculated from Eq. (1). This age is referred to as the model fission-track age, because it depends on the choice of a zero time t0 and the ratio 244 Pu/238 U at that moment. Eq. (1) can be solved for
t (Fleischer et al., 1975). According to Green et al. (1978) 
t = 1.198 × 108 ln

 5371 (Pu/U)0 . fPu /fU

(2)

At present there is no consensus on the value of the primary ratio of 244 Pu to 238 U isotopes, (Pu/U)0 . A majority of experimental data are grouped around the intervals 0.015–0.017 and 0.004–0.006. In the analysis of all known experimental determinations of (244 Pu/238 U)0 ratio for the early Solar System, Burnett et al. (1982) recommended use of a value of (244 Pu/238 U)0 = 0.015. This value is in satisfactory agreement with the theoretical computations of Schramm (1982) for the ratios of actinide isotope contents at the moment of the Solar System formation. This gives the following value for the (244 Pu/238 U)0 ratio: −2 2.5+0.7 −1.0 × 10 . Marjalahti pallasite belongs to the Main Group of pallasites (Scott, 1977). Megrue (1968) and Kolesnikov et al. (1977) reported its cosmic exposure age to be ∼ 178 Ma and K–Ar age to be 4.3 × 109 yr. Pellas et al. (1983) made

Phosphate grains of up to 0.2–0.5 mm with whitlockite composition (CaO—44.33; MgO–3.83; P2 O5 —51.61; Sum—99.77) have been hand-picked from the silicate residues obtained from the Ukrainian Meteoritic Committee and Joint Institute for Nuclear Research. Whitlockites are visually identified according to the criteria described by Buseck and Holdsworth (1977) and Pellas et al. (1983). One problem to be solved before fission-track dating of the Marjalahti pallasite is to differentiate between all possible track sources. Potentially pallasite phosphates can register fossil tracks of (1) iron group nuclei of galactic cosmic rays (GCR), (2) induced fission of heavy nuclei (U, Th and others) by cosmic ray (CR) bombardment, (3) spontaneous fission of 238 U and (4) spontaneous fission of 244 Pu (Fleischer et al., 1967). By correcting the total fossil track density for sources (1)–(3), the track density due to spontaneous fission of 244 Pu can be obtained. The maximum track density due to spontaneous fission of 238 U is calculated from the actual uranium content and a maximum estimation of the sample age; then the fission-track age is calculated from the track ratio Pu /U . The procedure for fission-track analysis is shown in Fig. 1. All selected grains are divided into two parts: the first part is used to calculate the uranium content, while the second part is used to study fossil tracks and to estimate the contribution of each possible track source to the total fossil track density. At least 10 crystals are utilized for each examination step and about 80 phosphates are used in general. Uranium content: The phosphate grains annealed at 500 ◦ C for 1 h to remove all fossil tracks were irradiated with thermal neutron fluences of 1016 –1018 n/cm2 at the JINR reactor. The thermal neutron fluence was measured by a reactor operator according to gold foil activation; the fluence determination error was ±10% error. Then the irradiated grains were mounted in epoxy resin, polished and etched. The uranium content was calculated according to Fleischer et al. (1975). The U content uncertainty was determined from the following equation: 
(U ) =

 
2
(F ) 2 (R) 2 1  + + , F R Ni

(3)

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U content determination

Annealing 500°C, 1h

Mounting, polishing, etching

Irradiation by thermal neutrons

Induced tracks counting

Phosphate grains

Fossil track analysis

Fossil tracks counting

Estimation of track densities from:

Fission-track age calculation

GCR Induced fission by cosmic ray bombardment 238

U spontaneous fission 244

Pu spontaneous fission

Fig. 1. The procedure for fission-track analysis in the Marjalahti pallasite phosphate grains.

 where Ni is the total number of 235 U-induced fission tracks; F is a value of neutron fluence; and R is the length of the etchable track of 235 U induced fission. Fe-group nuclei of GCR: There are two approaches to evaluate the contribution of this track source - to calculate track density in olivine crystals chosen from phosphateneighboring localizations, in which the spontaneous fission track density is three orders of magnitude lower, or to apply the differential annealing method. Since the location of the selected whitlockite grains was unknown we used the latter approach. Tracks caused by Fe-group nuclei of GCR are thermally less stable than spontaneous fission tracks (Carver and Anders, 1976a,b; James and Durrani, 1988). GCR tracks can be removed by partial annealing of the samples at low temperatures, while virtually all fission tracks are preserved. CR-induced fission tracks were estimated using the experimental curve of the total rate of fission caused by cosmic particles as a function of the depth below the lunar surface given by Damm et al. (1978). The number of induced-fission tracks per unit area of a crystal surface was obtained from the equation (Reedy, 1981) cri = 21 Nv RT cos m ,

(4)

where R is the mean length of the etchable track of 235 U fission, Tcos m is the cosmic exposure age, and Nv is the total cosmic-ray-induced fission rate. 238 U spontaneous fission tracks: Taking the actual uranium content of the sample and assuming a maximum age of the sample of 4.6 × 109 yr the maximum value of 238 U spontaneous fission tracks over this period can be calculated from the equation (Kothari and Rajan, 1982) U =

CU N0 R fU d[exp(DU t0 ) − 1] A 2 DU

where N0 is the Avagadro number, A is the atomic wt (g), CU is the U content (g), R is the mean length of the etchable track of 235 U fission (cm), d is the density of a studied sample (3.2 g/cm3 for whitlockite grain), DU is the total decay constant of 238 U, and fU is the spontaneous fission decay constant of 238 U. 244 Pu spontaneous fission tracks: The total fossil track density corrected for the track contributions discussed above was ascribed to the spontaneous fission of 244 Pu. Finally, the experimentally obtained Pu /U ratio allowed us to calculate a fission-track age by Eq. (2), or more accurately by means of Eq. (1). Track revelation: For the revelation of both fossil and induced tracks the phosphate grains were mounted in epoxy resin, polished and etched in 0. 2% HNO3 solution for 30 s at 20 ◦ C. Tracks were observed by a transmission optical microscope at magnifications of 600–900×. Track lengths were measured by the TINT method; then the mean track length was used to estimate the contribution of CR-induced fission tracks (Eq. (4)), to calculate the uranium content and the upper limit of 238 U spontaneous fission tracks (Eq. (5)).

(5)

3. Results The experimental results are shown in Table 1. Uranium content was calculated from the track density of 235 U-induced fission in completely annealed samples irradiated with thermal neutron fluence. Table 2 reports the track densities and calculated U concentrations obtained with three different fluences of F1 = 1.5 × 1016 cm−2 ; F2 = 1.34 × 1017 cm−2 ; and F3 = 1.8 × 1018 cm−2 . The result obtained from F2 is neglected in the calculation of the average uranium content because of the small crystal area

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Table 1 Components of fossil track density, uranium content and fission-track age of the Marjalahti pallasite Uranium

Track density (106 cm−2 )

content, (ppb)

Totala

After partial annealing GCRa

CR-induced fission

238 U + 244 Pu

238 U (over 4.60 × 109 yr)

244 Pu

fission-track age, (yr); (Pu/U)0 = 0.015

50 ± 7

1.4 ± 0.17− 3.8 ± 0.27

1.39 ± 0.04

< 0.03

1.39 ± 0.04a

0.17 ± 0.02a

1.22 ± 0.04a

(4.31 ± 0.02) × 109

Model

a Calculation uncertainty is 1.

Table 2 U content in whitlockites of the Marjalhti pallasite, determined by induced fission-track analysis Flux of thermal

Area,

Number

Track density,a

Content, ppbb

neutrons, F, cm−2

10−6 cm2

of tracks

cm−2

235 U

U (total)

1.50 × 1016 1.34 × 1017 1.80 × 1018

1141 56 543

21 63 1184

(1.82 ± 0.40) × 104 (1.13 ± 0.14) × 105 (2.18 ± 0.06) × 106

0.36 ± 0.09 0.25 ± 0.05 0.36 ± 0.05

50.0 ± 12.0 34.7 ± 6.4 49.9 ± 6.9

a Calculation uncertainty is 1. bAverage calculation uncertainty.

under study for searching induced tracks; the average uranium content obtained from F1 and F3 reaches 50 ppb. The mean length of 235 U-induced fission tracks, 14.4 ± 1.3 m, is rather close to the length of 15 m, evaluated by Bhandari et al. (1971). The total fossil track density was measured in 10 individual crystals. The dispersion between [(1.40 ± 0.17) × 106 cm−2 ] and [(3.80±0.27)×106 cm−2 ] reflects the range within minimum and maximum values of the total fossil track density obtained by us. The density of iron group nuclei of GCR estimated by the partial annealing method is shown in column 3. Whitlockites with fossil tracks were annealed at a temperature of 320 ◦ C for 1–15 h with a 1 h interval and the resultant track density was also measured. As the ultimate track density after annealing for 10 h calculated in 12 crystals remained invariable with the average value of (1.39 ± 0.04) × 106 cm−2 (Table 3), we reached the conclusion that annealing at 320 ◦ C for 10 h makes it possible to eliminate the GCRtracks contribution from the total density of fossil tracks. Samples irradiated with 235 U-induced fission tracks were annealed along with the untreated samples. The decrease of the induced track density by no more than 10% after annealing for 10 h confirms the relative stability of fossil fission tracks compared to Fe-group nuclei-induced tracks. The track density due to CR-induced fission of U and Th by cosmic ray bombardment is calculated from Eq. (4). At a maximum value of the induced fission rate Nv = 2.2 × 10−7 s−1 cm−3 (Damm et al., 1978; Reedy, 1981) and a cosmic exposure age of 178 × 106 yr (Megrue, 1968), the density of CR-induced fission tracks attains ∼ 6.2 × 105 cm−2 . However, considering that the uranium content in the stud-

Table 3 Fossil track density in whitlockite crystals of the Marjalahti pallasite after annealing (320 ◦ C, 10 h) Sample

Area, 10−7 cm2

Number of tracks

Track density, 106 cm−2

5

1279 551 710 2420 710 350 2020 315 372 672 567 1734

173 81 99 316 105 49 272 39 46 101 96 219  1596

1.35 1.47 1.39 1.31 1.47 1.40 1.35 1.24 1.24 1.50 1.69 1.26 1.39 ± 0.04a

9 8

10

11

1 2 3 4 1 1 2 3 1.1 2.3 2.4 1

a Calculation uncertainty is 1.

ied phosphates (50 ppb) is one-twentieth of that in the lunar soil (1000 ppb), for which the curve was plotted, this value must be 20 times reduced. In addition, it is reasonable that the pallasite should have a lower induced fission rate due to the capture of epithermal neutrons because of its large iron content (∼ 50%) (Durrani and Bull, 1987). Therefore, the contribution of induced fission is negligible. Consequently, the value of (1.39 ± 0.04) × 106 cm−2 comprises spontaneous fission of 238 U and 244 Pu nuclei. An upper limit of the contribution from the spontaneous fission of 238 U shown in column 6 is calculated from

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Eq. (5), using the measured uranium content and the age of 4.60 × 109 yr. A uranium concentration as low as 50 ppb cannot produce more than (0.17 ± 0.02) × 106 spontaneous fission tracks per cm2 over this period. The track excess attributed to the spontaneous fission of 244 Pu is defined as the difference between the total density of fossil tracks and the sum of all the above-mentioned sources. The track fraction assigned to this source is a minimum value because of the upper limit estimation of 238 U spontaneous fission tracks. The obtained value of 1.22 × 106 cm−2 is ten times smaller than that reported by Pellas et al. (1983). The fission track age value is calculated from Eq. (2) on the assumption that the (244 Pu/238 U)0 ratio in the pallasite phosphates 4.60×109 yr ago was identical to the initial solar system ratio of 0.015 (Burnett et al., 1982). The uncertainty of fission track age value was determined from the following equation: 

  (fU ) 2 (fPu ) 2 8 (
t) = 1.198 × 10 + (6) fPu fU From Eqs. (2) and (6) we obtain a fission-track age of (4.31± 0.02) × 109 yr for the whitlockite grains of the Marjalahti pallasite.

4. Discussion Among the few mineral constituents of the pallasites, only the phosphates (whitlockite, stanfieldite and farringtonite), which have an enhanced uranium concentration, are suitable for fission-track dating. Our investigations of a crumbled silicate fraction enabled us to find and to identify the whitlockite grains. Extraterrestrial phosphate grains in general may contain fossil tracks due to four sources, with a total track density which depends on the fission track age (as an age of fission tracks retention), location of a crystal relative to the surface of the host meteorite in preatmospheric time and its cosmic exposure age. After the correction of the total fossil track density for other possible track sources, the tracks revealed were unambiguously identified as those due to the spontaneous fission of 244 Pu and 238 U. The largest part of them was attributed to the spontaneous fission of the extinct isotope 244 Pu, indicating that the Marjalahti pallasite began to retain tracks at a comparatively early stage of the cosmic history. The calculated model fission-track age is (4.31 ± 0.02) × 109 yr. In order to understand the real significance of a fission track age one should consider those processes which occur within the time interval
t prior to the onset of track retention. Two cases can be considered. If cooling down of a meteorite parent body following its formation to the track retention temperature takes place during this interval, the fission-track age reflects the date of the cooling through the track retention temperature of the parent body. If a later thermal (shock/thermal) event (or a few events) causes signif-

icant heating and erasing of the fission tracks accumulated prior to this event, a fission-track age will reflect the date of such an event or the date of the last thermal event. In any case, to interpret the obtained data, the results of fission track dating should be combined with the other meteorite investigations. Fossil track analysis and comprehensive petrographic investigation have been carried out for the purpose of unraveling the cosmic history of the Marjalahti pallasite. It allowed us to find out numerous evidences of shock/thermal metamorphism (Bondar, 1994), similar to those found earlier in some severely reheated chondrites (Smith and Goldstein, 1977). They are the following: polycrystalline kamacite with regular monocrystals (0.5–2 mm); both polycrystalline troilite grains and eutectic intergrowths of troilite-metal in metal–troilite veins and grains; spherical inclusions of a few micrometers in diameter in the olivine crystals, consisting of troilite or troilite, metal and chromite; and sheared plessite fields. These features indicate that the Marjalahti pallasite underwent a moderate shock/thermal metamorphism after complete solidification. Our observations agreed with the suggestion of Scott et al. (1991) on two types of reheating found for shocked chondrites: local heating, which results from the shock PT peak (it forms irregular structures of shock melting), and residual heating of the bulk of a host meteorite caused by equilibrium shock pressure. As it follows from the presence of the shock-induced viens of troilite (metal–troilite eutectic intergrowths) and spherical inclusions of troilite in the olivine crystals of the Marjalahti pallasite the shock temperature reached 1000 ◦ C; however, the presence of cloudy taenite in the taenite grains points out that the residual reheating temperature did not rise above 450–500 ◦ C (Scott, 1973). The regular monocrystals of polycrystalline kamacite point to a relatively low (∼ 10 ◦ C/Ma) cooling rate through 400–450 ◦ C (Wood, 1967; Wasson and Choi, 2003). Shock pressure, indicated by wavy and mosaic extinction in olivine grains, has not exceeded 20–25 GPa. As mentioned above, the petrographic data testify to the shock/thermal event(s) in the cosmic history of the Marjalahti pallasite, leading to elevated temperatures of 400–450 ◦ C and in local sites up to 1000 ◦ C. Taking into account the fact that all fossil tracks have been removed completely at 450–500 ◦ C for 1h when carrying out our experiments, we can conclude that the shock/thermal event erased all fission tracks accumulated prior to this event. Thus, the fission-track age of the Marjalahti pallasite reflects the time of the onset of track retention in the phosphates after this event took place.

5. Conclusions • Examination of fossil tracks in the whitlockite crystals of the Marjalahti pallasite coupled with U content determination allowed us to estimate the contributions of all

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possible track sources to the total track density. A high track density is attributed to the extinct 244 Pu and pointing obviously to the high value of the fission-track age. • Model fission-track age of (4.31 ± 0.02) × 109 yr for the Marjalahti pallasite was calculated. • The comparison of represented data with petrographic analysis allowed us to interpret the value of fission-track age as the moment of the last intensive shock/thermal event in the cosmic history of the pallasite.

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