Anal. Chem. 1996, 68, 4130-4134
Determination of 18 Siderophile Elements Including All Platinum Group Elements in Chondritic Metals and Iron Meteorites by Instrumental Neutron Activation Ping Kong, Mitsuru Ebihara,* and Hiromichi Nakahara
Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan
An instrumental neutron activation method is developed to analyze chondritic metals and iron meteorites. By changing irradiation and decay times, and selecting suitable γ-ray and X-ray photopeaks, not only all platinum group elements (Ru, Rh, Pd, Os, Ir, Pt) but also other siderophilic elements (Fe, Co, Ni, Cu, Ga, Ge, As, Mo, Sb, W, Re, Au) can be nondestructively determined in the meteoritic metal samples. To obtain analytical data as accurate as possible, interfering reactions and neutron flux gradients during irradiation are considered. Siderophile elemental abundances measured for the Odessa iron meteorite are highly consistent with the literature values. Rh abundances for bulk H, L, LL, EH, and EL chondrites, which had been scarcely reported in the literature, are derived from Rh/Ni abundance ratios in the metal separates of the corresponding chondritic groups.
life of the neutron-activated radionuclide 104(m)Rh is too short to allow its radiochemical separation. As Rh is a strongly siderophilic element,8,9 it is concentrated in the metal phase, and its content in chondritic metals becomes measurable by INAA.10 Considering the similarity in chemical affinities and condensation temperatures of Rh and Ni,11 it may be reasonable to infer Rh abundances in bulk chondrites from Rh/Ni abundance ratios for the chondritic metals. In this paper, the INAA procedure for the determination of elemental abundances in meteoritic metal samples is described. By using this procedure, all platinum group elements can be nondestructively determined in chondritic metal separates. In addition, interfering reactions and neutron flux gradients among multiple samples are discussed. Rh abundances for bulk ordinary and enstatite chondrites are derived from Rh/Ni abundance ratios for their metal fractions.
One of the reasons chondrites are believed to be the most primitive materials in the solar system is that they are similar to the sun in chemical composition.1-3 The chemical composition of chondrites and the compositional variations among chondritic clans may give hints in considering the formation and evolution of planetary bodies whose interiors have never be reached.3-5 Representative chemical compositions of chondrites, however, are not easily obtained. This is especially true for metal-bearing chondrites where metal grains are inhomogeneously distributed, hence, representative sampling is not always guaranteed.6 Jarosewich6,7 proposed a method for obtaining representative elemental compositions of chondrites. In his procedure, a powdered chondritic sample is first separated into magnetic and nonmagnetic portions, and the bulk composition is calculated from precise elemental abundances of the two complementary fractions. Neutron activation analysis (NAA) is often applied for the determination of trace element abundances in meteoritic samples. Rh, a monoisotopic element, is one of the elements that are difficult to determine by this technique. Rh abundances in chondrites are not well established because they are too low to be determined by instrumental NAA (INAA); furthermore, the half-
EXPERIMENTAL SECTION Sample Preparation. One or two lumps (g1 g) of each chondritic specimen were ground in an agate mortar. The powdered chondrite was roughly separated into magnetic and nonmagnetic fractions using a hand magnet. The two fractions thus isolated were again carefully ground and subjected to further separation to eliminate intercontamination. The magnetic fraction was leached by concentrated HF in an 80 °C water bath for 2 min and was ultrasonicated in deionized H2O for 30 min. The metal fraction was finally purified under an optical microscope. A quantity of 10-20 mg of each metal fraction was doubly sealed in polyethylene bags for INAA. Our previous studies showed that the separated metal fractions are representative of the bulk metals of chondrites in chemical composition within 10%.12,13 Known amounts of high-purity Fe, Si, and Al metals were also sealed in clean polyethylene bags for correction of interfering reactions. All the inner polyethylene bags used were prepared to the same size (1 cm × 1 cm). Chemical Standards. Chemical standards were prepared from high-purity chemical reagents for elements of interest, including several nonsiderophilic elements. A reasonable amount of each reagent was dissolved in an appropriate solvent, which
(1) Anders, E.; Ebihara, M. Geochim. Cosmochim. Acta 1982, 46, 2363-2380. (2) Anders, E.; Grevesse, N. Geochim. Cosmochim. Acta 1989, 53, 197-214. (3) Wasson, J. T.; Kallemeyn, G. W. Phil. Trans. R. Soc. London 1988, 325A, 535-544. (4) O’Neill, H. St. C.; Dingwell, D. B.; Borisov, A.; Spettel, B.; Palme, H. Chem. Geol. 1995, 120, 255-273. (5) McDonough, W. F.; Sun, S. S. Chem. Geol. 1995, 120, 223-253. (6) Jarosewich, E. Meteoritics 1990, 25, 323-337. (7) Jarosewich, E. Geochim. Cosmochim. Acta 1966, 30, 1261-1265.
4130 Analytical Chemistry, Vol. 68, No. 23, December 1, 1996
(8) Fleet, M. E.; Stone, W. E. Geochim. Cosmochim. Acta 1991, 55, 245-253. (9) Campbell, I. H.; Barnes, S. J. Can. Mineral. 1984, 22, 151-160. (10) Kong, P.; Ebihara, M. Geochim. Cosmochim. Acta 1996, 60, 2667-2680. (11) Fegley, B., Jr.; Palme, H. Earth Planet. Sci. Lett. 1985, 72, 311-326. (12) Kong, P.; Ebihara, M.; Endo, K.; Nakahara, H. Proc. NIPR Symp. Antarct. Meteorites 1995, 8, 237-249. (13) Kong, P.; Ebihara, M.; Nakahara, H.; Endo, K. Earth Planet. Sci. Lett. 1995, 136, 407-419. S0003-2700(96)00477-5 CCC: $12.00
© 1996 American Chemical Society
was made up as a stock solution. Running solutions were further prepared from the stock solutions. Some 30-60 µL of running solutions were pipetted on six sheets of filter papers (0.8 cm × 0.8 cm), which were square-shaped and were similar in size to the meteorite samples. These filter papers were sealed in polyethylene bags (1 cm × 1 cm). To minimize the number of samples and the geometric difference in irradiation, the following seven mixed elemental standards were prepared: I, Ge-Rh; II, AlCa-V-Cu-Pd; III, Mg-Mn-Ga-Mo-Re; IV, Cr-Fe-Co-Ni-Zn-Se; V, GaMo-Sb-W-Au; VI, Ru-Ag-Re-Os-Ir-Pt; and VII, Ge-Pd. Irradiation and Measurement. Activation was carried out in a TRIGA Mark II reactor at the Institute for Atomic Energy, St. Paul’s University (Yokosuka, Japan). All samples were first irradiated for 100 s at a thermal neutron flux of 1.5 × 1012 cm-2 s-1 for the determination of Mg, Al, V, Mn, Cu, Ge, and Rh using short-lived radionuclides. Metal samples were irradiated individually using a pneumatic transfer system. After the outer polyethylene bags were changed, the metal samples were counted in contact with a high-efficiency Ge detector (relative efficiency, 35%). Cu and Cd plates (1 mm thick each) were used to absorb lowenergy activities, mainly the 58.6 keV γ-ray emitted by 60mCo (halflife, 10.48 min), to reduce the dead-time. The counting time for a metal sample was 100 s, and the measurement started generally within 60 s after irradiation. After the short-term irradiation analyses, the samples were reirradiated for 6 h at the same neutron flux for the determination of Na, Sc, Cr, Fe, Co, Ni, Ga, As, Mo, Ru, Pd, Sb, W, Re, Os, Ir, Pt, and Au using long-lived radionuclides. The radioactivity measurement was performed at the RI Research Center of Tokyo Metropolitan University using Ge detectors. Three successive sets of measurements were performed after each 6 h irradiation. The first measurement started 1 day after irradiation, and each sample was counted for 5000 s for the determination of Na, Ga, As, Pd, W, and Re. In the second counting, each sample was measured for 3-4 h soon after the first measurement for the determination of Sc, Cr, Fe, Co, Ni, Mo, Sb, Ir, Pt, and Au. The third counting was done for 12 h for each sample, after 20 days of cooling, for the determination of Ru and Os. The nuclides and γ-rays used in this work are summarized in Table 1. The abundances of Na, Mg, Al, and Sc, which are typical lithophilic elements, were used to assess the purity of chondritic metal separates, and the abundances of V, Cr, and Mn are significant in understanding the formation of chondritic metals.10 RESULTS AND DISCUSSION Interfering Nuclides and Reactions. In Table 1, interfering nuclides and reactions are listed. In the case that a γ-ray peak was interfered with, either another γ-ray peak was used, decay analysis was used, or hand-fitting of a spectrum was applied. The interfering reactions are discussed separately below. Rh. Rh abundances in the metal specimens were calculated from the 555.8 keV γ-ray photopeak emitted by 43 s 104Rh. 104mRh, produced also via (n,γ) reaction with 103Rh, decays to 4.4 min 104Rh. The contribution from 104mRh was corrected according to the change in radioactivity ratio of 104Rh to 104mRh after irradiation. The initial radioactivity ratio of 104Rh to 104mRh was obtained by measuring the Rh standard. Counting started immediately after the irradiation of a Rh standard, and the decay was followed for (14) Friedlander, G.; Kennedy, J. W.; Macias, E. S.; Miller, J. M. Nuclear and Radiochemistry; John Wiley and Sons: New York, 1981.
Figure 1. Decay line of Rh. By neutron irradiation of 103Rh, radioactive 104Rh and 104mRh formed. 104mRh transits to 4.39 min 104Rh, which in turn decays to 43.7 s 104Pd, emitting a 556 keV γ-ray. The decay lines were obtained on the basis of a series of measurements of radioactivity of the 556 keV γ-ray in the Rh standard sample immediately after irradiation. Counting time is 30 s for each point. Table 1. Nuclear Data Related to the INAA Used in This Studya element isotope Mg Al V Mn Cu Ge Rh
27Mg
Na Sc Cr Fe Co Ni Ga As Mo Ru Pd Sb W Re Os Ir Pt Au
24Na
28Al 52V 56Mn 66Cu 75Ge 104Rh
46Sc 51Cr 59Fe 60Co 58Co 72Ga 76As 99Mo 103Ru 109Pd 122Sb 187W 188Re 191Os 192Ir 199Au 198Au
half-life
energy (keV)
interfering nuclideb and reaction
Short Irradiation (100 s) 27Al(n,p)27Mg 9.46 min 1015 28Si(n,p)28Al 2.24 min 1778.9 3.8 min 1434.4 56Fe(n,p)56Mn 2.58 h 1811 5.1 min 1039 48 s 139.8 104mRh(IT)104Rh 43 s 555.8 15.0 h 83.3 d 27.7 d 44.6 d 5.27 y 71.3 d 14.1 h 26.5 h 2.75 d 39.4 d 13.4 h 2.74 d 23.9 h 17.0 h 15.4 d 74.2 d 3.15 d 2.70 d
Long Irradiation (6 h) 24Mg(n,p)24Na 1368 889 54Fe(n,R)51Cr 320.1 1291, 1099 1173, 1332 810.8 630 559.1 59Fe (142 keV) 140.5 497.9 88.0, 22.2 76As (559 keV) 563 685.5 155 129.4 51Cr (320 keV) 316, 468 197Au(n,γ)198Au(n,γ)199Au 158 411.8
a From Friedlander et al.14 in parentheses.
b
Energies of interfering γ-rays are shown
10 min by counting the sample for 30 s at each measurement. The decay line of Rh is shown in Figure 1. The activity ratio of 104Rh/104mRh is expressed as 33.9e-0.79t, where t is the decay time in min. The radioactivity ratio of 104Rh to 104mRh at t ) 0 is 33.9, which is roughly consistent with theoretical calculations of the radioactivity ratios of 104Rh/104mRh after a 100 s irradiation for both thermal neutron (a ratio of 42) and epithermal neutron (a ratio of 45). Al and Mg. Al and Mg abundances were calculated from the radioactivities of 28Al and 27Mg. 28Si(n,p)28Al and 27Al(n, p)27Mg Analytical Chemistry, Vol. 68, No. 23, December 1, 1996
4131
reactions interfere with the determination of Al and Mg, respectively. The contributions from 28Si and 27Al were monitored by irradiating high-purity Si and Al metals in each run. It was observed that, after the 100 s irradiation at a total neutron flux (in cm-2 s -1) of 2.05 × 1012 (1.5 × 1012 for thermal neutron and 5.5 × 1011 for fast neutron), 1 g of Si contributed the equivalent of 0.0112 g of Al, and 1 g of Al contributed the equivalent of 0.463 g of Mg. Mn and Cr. Abundances of Mn and Cr were calculated from the radioactivities of 56Mn and 51Cr, respectively. During irradiation, 56Mn and 51Cr were also produced via 56Fe(n,p)56Mn and 54Fe(n,R)51Cr, respectively. The Mn and Cr abundances of meteoritic metal samples were therefore corrected for the contributions from these reactions with Fe. The correction was made by measuring the radioactivities of 56Mn and 51Cr in a high-purity Fe metal. It was found that 1 g of Fe contributed the equivalent of 55 µg of Mn after the 100 s irradiation and 33.3 µg of Cr after the 6 h irradiation at thermal and fast neutron fluxes of 1.5 × 1012 and 5.5 × 1011 cm-2 s-1, respectively. Pt. Data for Pt were obtained using the 158 keV γ-ray emitted by 199Au. 199Au could also be produced from 197Au by a double neutron capture reaction, and this contribution was examined by irradiating a pure Au monitor. Since an activity of 199Au in the Au monitor sample was below the detection limit, no correction was made in calculating Pt abundances. Pd. The 88.0 keV γ-ray emitted by 13.4 h 109Pd is a suitable γ-ray for the determination of Pd. Because of its relatively short half-life and low energy, this γ-ray is not always detectable in chondritic metals. In place of this γ-ray, the 20.2 keV Rh KR X-ray emitted by 17 d 103Pd is often used in the literature.15 In measuring the Pd standard, a peak at 22.2 keV was also detected in addition to the 20.2 keV X-ray. The intensity of the 22.2 keV peak was about 100 times greater than that of the 20.2 keV peak shortly after a 6 h irradiation. The 22.2 keV peak corresponds to the Ag KR X-ray, which is emitted by 109mAg, produced through the following processes: 108
β-
Pd(n,γ)109Pd (T1/2 13.4 h) 98 IT
Ag (T1/2 39.8 s) 98 109Ag
109m
To confirm this, decay of the 22.2 keV X-ray was followed and compared with that for the 88.0 keV γ-ray emitted by 109Pd (Figure 2). As expected, the 22.2 keV X-ray and the 88.0 keV γ-ray decayed with the same half-life. To verify the applicability of the 22.2 keV Ag X-ray for the determination of Pd, known amounts of Mo, Ru, Rh, and Agswhose atomic weights are similar to that of Pdswere irradiated and measured by an intrinsic Ge detector designated for low-energy photon spectrometry (LEPS). Although the 22.2 keV X-ray was found to be emitted by Ag, its radioactivity was so low that it is negligible for meteorite samples. No other elements were confirmed to interfere with the 22.2 keV peak. Thus, the 22.2 keV X-ray can be used for the determination of Pd in meteoritic metal specimens. The intensity of the 22.2 keV X-ray was 7 times greater than that of the 88.0 keV γ-ray for a LEPS spectrum of the Pd standard sample. When the 88.0 keV γ-ray was not detectable in a metal sample, its Pd abundance was (15) Anders, E.; Wolf, R.; Morgan, J. W.; Ebihara, M.; Woodrow, A. B.; Janssens, M.-J.; Hertogen, J. Report NAS-NS-3117, U.S. National Technical Information Service: Arlington, VA, 1988.
4132
Analytical Chemistry, Vol. 68, No. 23, December 1, 1996
Figure 2. Decay line of 109Pd. Two lines are compared: one is for the 88.0 keV γ-ray of 109Pd, and the other is for the 22.2 keV X-ray. The same slope for these two lines indicates that the 22.2 keV X-ray comes from 13.5 h 109Pd via 109Pd f (β-) 109mAg f IT 109Ag. The radioactivity of the 22.2 keV X-ray is about 7 times greater than that of the 88.0 keV γ-ray for the Pd chemical standard sample. I, intensity of radioactivity (counts/s).
Figure 3. Self-absorption factors for chondritic metal samples. The factors are calculated by comparing Pd abundances calculated from the 88.0 keV γ-ray with those from the 22.2 keV X-ray (using the same chemical standard) for chondritic metal samples of different weights. The points tend to scatter because of variable size distribution and nonreproducible geometry. When the correction of selfabsorption is necessary, the average line (solid line shown) is typically used.
determined by using the 22.2 keV photopeak. In this case, the radioactivity of the 22.2 keV X-ray was corrected for self-absorption of the sample. The degree of self-absorption was obtained by comparing Pd abundances calculated from the 88.0 keV γ-ray with those from the 22.2 keV X-ray for chondritic metal fractions of different weights. A relationship between these two abundance ratios and the sample thickness is shown in Figure 3. All points are not well located on a straight line, partly because the size distribution of metal grains is not uniform for different samples. There also is another reason for the scatter: since metal exists as grains, the weight of a sample is not proportional to the thickness when the sample is e10 mg cm-2. This may explain why the line does not intersect with the ordinate at 1. When needed, self-absorption was corrected for by using the average line as shown in Figure 3. This line does not fit the theoretical line but can be regarded as a practical line.
Table 2. Decay-Corrected Radioactivities (counts/min) for Replicate Samples of Chemical Standards Irradiated at Different Geometric Positionsa standard group element set 1 IV
V
VI
Cr Fe Co Ni Zn Se Ga Mo Sb W Au Ru Ag Re Os Ir Pt
948 352 1260 265 1400 12100 88800 4580 4000 22300 6490 672 760 23300 1250 2350 2550
vertical horizontal set 2 deviationb set 1 set 2 deviationb 977 353 1250 265 1410 12100 86900 4620 3950 22200 6260 670 760 24200 1240 2360 2580
0.1 -0.3 0.8 0.0 -0.7 0.0 2.2 -0.9 1.3 0.4 3.6 0.3 0.0 -3.8 0.8 -0.4 -1.2
514 480 112 105 721 677 140 132 679 639 8270 7840 34200 31800 1680 1540 1220 1140 6580 6050 2030 1880 331 318 454 440 2250 2130 726 689 374 366 988 950
6.8 5.9 6.3 5.9 6.1 5.3 7.3 8.7 6.8 8.4 7.7 4.0 3.1 5.5 5.2 2.2 3.9
a Two sets of chemical standards with identical elemental abundances (reproducibilities are better than 2%) were irradiated either in the vertical or in the horizontal direction in the TRIGA-II reactor, and activities in the replicate samples are compared. Counting statistical errors are within 2% for all the elements listed. b A deviation (in %) is calculated by 2(I1 - I2)/(I1 + I2), where In is the activity of a specific nuclide in the standard set n.
Gradients of Neutron Flux. It is well acknowledged that there is a neutron flux gradient in the vertical direction of a TRIGAtype reactor. There could also be a neutron flux gradient in the horizontal direction. In INAA of geological and cosmochemical samples, multiple samples are simultaneously irradiated in a capsule. To evaluate the accuracy of the data, both gradients must be known. Therefore, these gradients were examined in this study. In a typical case of our previous studies, about 30 samples (including metals, nonmetal specimens, and chemical standards) were sealed in a single thick polyethylene bag (2 cm × 2 cm × 2 cm). Two sets of chemical standards were included and sandwiched among the samples. The samples were placed in a capsule either in the vertical direction or in the horizontal direction. The radioactivities of duplicate chemical standards placed parallel to the horizontal level were compared in the left half of Table 2. No systematic difference in specific activities of the elements is observed between the duplicate samples. The deviations of specific activities between duplicate chemical standards are
