Geochemical Journal, Vol. 39, pp. 311 to 316, 2005
Mn-Cr systematics of pallasites WEIBIAO HSU 1,2 1
Laboratory for Astrochemistry and Planetary Sciences, Purple Mountain Observatory, 2 West Beijing Road, Nanjing 210008, China 2 Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, U.S.A. (Received August 17, 2004; Accepted January 8, 2005) I report here on a comprehensive ion microprobe study of Mn-Cr systematics of six pallasites. Four pallasites show excesses of radiogenic 53Cr (53Cr*), which correlate well with the Mn/Cr ratios. For these pallasites, the inferred initial (53Mn/ 55Mn) o ratios are ~1 × 10–5, which indicates that pallasites formed very early in the solar system. Eagle Station and Imilac show hints of 53Cr*, but the excesses are not resolved from terrestrial standards. Comparisons of Mn-Cr results with those of Pd-Ag indicate that these two short-lived radionuclide chronometers are self-consistent in pallasites. Inferred (53Mn/55Mn) o ratios in this study are generally higher than those previously observed by TIMS (1–3 × 10 –6). It is not clear what mechanism caused such a discrepancy. Nevertheless, it should be noted that ion probe depends on the zoning profiles to give high Mn/Cr ratios at the edges of the olivine crystals. Thus, it dates the closure time of diffusive fractionation of Mn and Cr in pallasite olivines. Keywords: pallasite, short-lived nuclides, Mn-Cr systematics, ion microprobe, early solar system
Pallasites are of particular interest. They consist of roughly equal amounts by weight of olivine and Fe-Ni metal and have historically been thought to represent samples from the core-mantle boundary of differentiated parent bodies (Mason, 1962, 1963). Excesses of radiogenic 53 Cr (53Cr*), correlated with Mn/Cr ratios, have previously been found in three pallasites, Eagle Station, Springwater, and Omolon (Birck and Allègre, 1988; Hutcheon and Olsen, 1991; Lugmair and Shukolyukov, 1998; Shukolyukov and Lugmair, 2001). In addition, precise Pd-Ag data are available for some pallasites (Chen and Wasserburg, 1996). It is therefore possible to make direct comparison of two short-lived nuclides, 53Mn and 107 Pd, in these meteorites. The purposes of this study were to confirm the previous results, to seek widespread evidence of 53Mn, to correlate possible relationships with other short-lived radiochronometers in pallasites, and thereby to constrain the chronology of pallasites. In this work, I studied Mn-Cr systematics in six pallasites, Albin, Brenham, Eagle Station, Imilac, Glorieta Mountain, and Springwater.
INTRODUCTION It is generally believed that the formation of early solar-system objects and the planetary differentiation processes occurred within the first 10 Ma of solar system history (Podosek and Cassen, 1994). The time scales for these events are not well-defined, but are vital for understanding solar-system history. Due to technical limitations, long-lived radionuclide chronometers have considerable difficulty resolving small time intervals at ~4,560 Ma ago. However, short-lived radionuclides (half life ≤ 100 Ma), now extinct, potentially provide the required high-resolution chronometers. Among them, 53Mn, 60Fe, 107Pd, and 182 Hf have relatively long half lives (several Ma) and should persist through the period of early planetary differentiation. Indeed, evidence for their existence has been found in pallasites, eucrites, and iron meteorites (Birck and Allègre, 1988; Shukolyukov and Lugmair, 1993; Chen and Wasserburg, 1996; Lee and Halliday, 1996; Lugmair and Shukolyukov, 1998, 2001; Sugiura and Hoshino, 2003). The presence of short-lived radionuclides in these differentiated meteorites provides a means of determining their relative formation times and the time scales for silicate-metal differentiation in the parent bodies. In this paper, I report on an ion microprobe study of the 53Mn-53Cr system (half life = 3.7 Ma) in pallasites.
ANALITICAL METHODS A key to finding evidence for 53 Mn is identifying phases with high Mn/Cr ratios. Phosphates in pallasites can have high Mn/Cr ratios (Hutcheon and Olsen, 1991). All pallasite samples are one-inch round thick sections coated with carbon and were first studied with a JEOL JSM 35-CF scanning electron microscope (SEM)
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Table 1.
53
Cr excesses in pallasites δ5 3 Cr* (‰)
55
Mn/5 2 Cr
Albin ( 5 3 Mn/5 5 Mn) o = (1.3 ± 1.0) × 10– 5 Chromite Olivine Olivine Olivine
0.73 ± 1.00 3.26 ± 3.16 3.72 ± 3.98 6.75 ± 5.72
(85 ± 4) × 10– 4 13 ± 1 36 ± 2 55 ± 3
Brenham ( 5 3 Mn/5 5 Mn) o = (1.9 ± 1.3) × 10– 5 Troilite Olivine Olivine Olivine
–0.98 ± 2.74 2.57 ± 4.16 4.00 ± 4.48 5.72 ± 3.93
(56 ± 3) × 10– 3 13 ± 1 24 ± 1 36 ± 2
Eagle Station ( 5 3 Mn/5 5 Mn) o = (5.9 ± 8.1) × 10– 6 Chromite –0.35 ± 1.00 (37 ± 2) × 10– 4
Fig. 1. Cr and Mn zoning profiles in an Albin olivine grain. Cr content decreases and Mn increases toward the edge of olivine grains, suggesting diffusive loss of Cr and gain of Mn during sub-solidus cooling period.
Olivine Olivine Olivine
2.04 ± 4.44 2.12 ± 6.96 5.09 ± 7.66
21 ± 1 57 ± 3 94 ± 5
Glorieta Mountain ( 5 3 Mn/5 5 Mn) o = (1.5 ± 1.0) × 10– 5 Troilite
0.29 ± 1.78
Olivine Olivine Olivine
1.33 ± 2.32 2.98 ± 2.39 5.73 ± 4.00
(127 ± 6) × 10– 3 11 ± 1 26 ± 1 40 ± 2
Imilac ( 5 3 Mn/5 5 Mn) o = (8.0 ± 8.1) × 10– 6 Olivine Olivine Olivine
0.52 ± 2.31 2.21 ± 2.85 3.64 ± 3.68
12 ± 1 34 ± 2 50 ± 3
Springwater ( 5 3 Mn/5 5 Mn) o = (9 ± 6) × 10– 6 Troilite –1.44 ± 1.88 (24 ± 1) × 10– 2 Olivine Olivine Olivine
0.78 ± 2.98 2.07 ± 2.04 8.84 ± 5.17
17 ± 1 42 ± 2 91 ± 5
Errors quoted are 2 σ of means.
Fig. 2. Histogram of Cr isotopic measurements in the standard and meteoritic olivines. (a) The measured δ53Cr (uncorrected) for San Carlos olivine (–5.41 ± 2.00‰), (b) the LEW88774 ureilite olivine (–5.06 ± 1.40‰), and (c) the Brenham olivine (0.30 ± 3.90‰). The 53Cr excess was obtained by comparing the measured δ53Cr in the sample with that in San Carlos olivine (–5.41‰).
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equipped with a Tractor Northern energy dispersive (EDS) X-ray analysis system. Phosphates and Mn-poor, Cr-rich phases, such as chromite and troilite, were located in the pallasites with the SEM. Unfortunately, all phosphates I found have Mn/Cr ratio less than 20. I then focused on pallasite olivines in which Cr decreases and Mn increases toward the edges of the crystals and yield Mn/Cr ratios up to 100 (Fig. 1). The Cr isotopic compositions of olivine, troilite, and chromite in the pallasites were analyzed with a modified Cameca IMS-3f secondary ion mass spectrometer (SIMS) at the California Institute of Technology. An 16O– primary beam of ~1 nA was used and the measurements were carried out at a mass resolving power of ~6000, which eliminated molecular and most isobaric interferences. The beam spot is about 5 µm. It is intended to determine instrumental mass fractionation during each measurement
Fig. 3. Mn-Cr systematics in pallasites and comparison with TIMS results of Birck and Allègre (1988) and Papanastassiou (priv. comm., 1997).
Mn-Cr systematics of pallasites 313
using the 50Cr/52Cr ratio. 49Ti+ and 51V+ were monitored to permit corrections for unresolved isobaric interferences at mass 50 by 50Ti + and 50V+. This procedure was successful for troilite and chromite, which have small interference corrections. But for olivine, large corrections for 50 + Ti on 50Cr+ made the resulting 50Cr/52Cr ratios too variable to use to correct for instrumental mass fractionation. Therefore, for olivine, I compared the 53Cr/52Cr measured in the sample minerals with that measured in standard minerals of similar composition to determine the excesses of 53Cr* (Fig. 2). For olivine, terrestrial San Carlos olivine was the primary standard. The measured 53Cr/52Cr, expressed as δ53Cr, the deviation in part-per-thousand (‰) of the measured 53Cr/52Cr ratio from the standard ratio ([( 53Cr/52Cr)measured/( 53Cr/52Cr)standard – 1] × (1000), was –5.41 ± 2.00‰ (2 σ mean ). A Cr-rich (Cr = 2500 ppm) olivine from the LEW 88774 ureilite yielded the same ratio (δ53Cr = –5.06 ± 1.40‰) (Fig. 2). The mean value for San Carlos olivine ( δ53Cr = –5.41 ± 2.00‰) was then compared with the measured 53Cr/52Cr ratios in pallasite olivines (Fig. 2). The standard ratios for Cr isotopes used in this study are 0.051859 ( 50Cr/52Cr) and 0.11346 (53Cr/ 52 Cr) (Papanastassiou, 1986). Data in Table 1 and in Fig. 3 are corrected for instrumental fractionation and are shown as delta values relative to 53Cr/52Cr = 0.11346. The errors reported reflect both uncertainties of the standard and meteorite samples. The values reported in Table 1 are error-weighted means of ~20 individual measurements of olivine with similar 55Mn/52Cr ratios. 55Mn/52Cr ratios were calculated from 55Mn+/52Cr+ using sensitivity factors determined in the mineral standards. It should be recognized that the effects being sought are small and that the precision achievable by the current SIMS technique is limited. RESULTS Table 1 presents the Mn-Cr isotopic data for six pallasites. In pallasites, the manganese-poor, chromiumrich phases, e.g., chromite and troilite, have normal chromium isotopic compositions relative to terrestrial standards. Olivines from Albin, Brenham, Glorieta Mountain, and Springwater pallasites show variable chromium isotopic compositions, ranging from normal to resolved 53Cr excesses ( δ 53 Cr* up to 8.84 ± 5.17‰ (2 σ mean ) in Springwater olivine). The δ53Cr* values in olivine correlate well with Mn/Cr ratios (Fig. 3), which vary substantially due to the core-to-rim zoning of manganese and chromium (Fig. 1). Error-weighted least-squares fits of the data forced through normal chromium at 55Mn/52Cr = 0 yield slopes of (1.3 ± 1.0) × 10–5, (1.9 ± 1.3) × 10–5, (1.5 ± 1.0) × 10–5, and (9 ± 6) × 10–6 for Albin, Brenham, Glorieta Mountain, and Springwater respectively (Fig. 3), the latter being consistent with the result of Hutcheon ((9
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± 5) × 10 –6, priv. comm.), who measured olivine and silico-phosphates in Springwater (Hutcheon and Olsen, 1991). Eagle Station and Imilac show hints of correlated 53 Cr* excesses (Fig. 3). DISCUSSION Are the measured 53Cr excess from in situ decay of 53Mn? Before discussing the chronologic significance of the observed 53Cr excesses in pallasitic olivine, it must be sure that those excesses are indeed due to the in situ decay of 53Mn. Pallasites, like irons, usually have long exposure ages (Megrue, 1968; Honda, 1985; Honda et al., 1996). I estimated the contributions of cosmogenic 53Mn and 53Cr in the Brenham pallasite, which has an exposure age of 200 Ma (Honda et al., 1996). The main target element in pallasites is iron. The 200 Ma exposure age of Brenham is long compared to the half life of 53Mn (3.7 Ma), so cosmogenic 53Mn will have reached equilibrium between production and decay. Because the terrestrial age of Brenham is short relative to the half life of 53Mn, the production rate of cosmogenic 53Mn can be obtained from the decay rate of 53Mn. The maximum measured decay rate for 53 Mn in pallasites is ~500 dpm/kg of iron (Nishiizumi, 1987). The production rates of cosmogenic 53 Cr in irons were given by Birck and Allègre (1985), Shima and Honda (1966), and Shimamura et al. (1986). Using these data, I estimate the contribution of 53Cr from cosmic-ray interactions in Brenham olivine (Fe ~ 10 wt. %) to be negligible (10 Ma. Relatively precise Pd-Ag data (Chen and Wasserburg, 1996) also indicate that Eagle Station is “younger” than Brenham by >5 Ma (Fig. 4). Both Mn-Cr and Pd-Ag indicate that Brenham is 1–2 Ma “older” than Glorieta Mountain, although the differences between the two meteorites are not resolved (Fig. 4). Comparisons of Mn-Cr results with those of Pd-Ag must recognize that these two systems date different events. The Pd-Ag system dates time of the palladium fractionating from silver, which probably occurred when metallic iron was segregated from silicates and sulfide. In contrast, the zoning profiles that permit us to measure Mn-Cr in pallasitic olivine reflect local redistribution of parent and daughter elements between a variety of phases during cooling and dates the time of the end of diffusive fractionation. Thus, it would not be surprising to find that Pd-Ag was dating a much earlier event than Mn-Cr, even
Comparisons of SIMS results with TIMS Although the picture described above is encouraging, the results cannot be accepted uncritically. There is some indication that the ion probe (SIMS) gives higher (53Mn/ 55 Mn)o than the thermal ionization technique (TIMS). The best value for (53Mn/55Mn)o in Eagle Station is a factor of 2.5 higher than that given by Birck and Allègre (1988), but the uncertainties in the ion probe result are too large that the two numbers cannot be resolved. The TIMS estimate for (53Mn/55Mn)o in Omolon, (~1.3 × 10–6) (Lugmair and Shukolyukov, 1998), is a factor of five to ten lower than any of the ion microprobe results. TIMS data for Salta (v. Imilac) reported by Papanastassiou (priv. comm.) gave (53Mn/55Mn)o = 3 × 10–6 vs. (8.0 ± 8.1) × 10–6 by SIMS (Fig. 3). It should be noted that the two methods handle the samples differently and may date different events. Although the precision achievable with current SIMS techniques is considerably lower than that achievable by TIMS, SIMS can measure zoning profiles and other features that require micron-scale spatial resolution. For the pallasites, the ion probe results depend on the zoning profiles in the olivine crystals, while for TIMS, the olivines are dissolved and the chromium separated for analysis. Thus SIMS dates the closure time of diffusive fractionation of Mn and Cr in olivine whereas TIMS dates the segregation of Mn and Cr into the olivine, an event that may predate the establishment of the zoning profiles. However, in this case the TIMS estimate of (53Mn/55Mn)o should be greater than or equal to the ion probe measurement, not the other way around. Another difference between the two techniques is that SIMS depends on the zoning profiles to give high Mn/Cr ratios at the edges of the crystals (Fig. 1), while during the TIMS sample preparation the outer surfaces of the olivine crystals are abraded off prior to dissolution (Papanastassiou, priv. comm.; Lugmair, priv. comm.). If a mechanism could be identified that concentrates 53Cr* in the surface layers, the apparent discrepancies between the two techniques might be explained. Of course in this case neither technique would be giving the true (53Mn/ 55Mn)o for the meteorite. Additional work on pallasites will be necessary to resolve this issue. Pallasites were previously thought to have formed at the core-mantle boundary of differentiated parent bodies (Mason, 1962, 1963). In addition, cooling rates estimated from kamacite-taenite diffusion profiles in pallasite FeNi metal are in the range of 0.5–2°C/Ma from 700 to 300°C (Buseck and Goldstein, 1969), which indicates a burial depth of ~200 km (Fricker et al., 1970). Studies of minor element zoning in pallasite olivine show that Ca,
Mn-Cr systematics of pallasites 315
Cr, Ti, V, and Ni concentrations decrease from center to rim by factors of up to 10 and that Mn is generally unzoned or increases slightly at the very edge of some olivine grains (Leitch et al., 1979; Miyamoto, 1997; Hsu, 2003). These zoning profiles are thought to have developed through thermal diffusion during sub-solidus cooling (Miyamoto, 1997; Hsu, 2003). These authors argued that pallasites cooled rapidly (~50°C/year) at high temperature ranges (1300 to 600°C). It is therefore expected that pallasite parent bodies cooled down to the closure temperature of Mn-Cr diffusive fractionation within a few hundred years after their formation. This time period is very short compared to the half-life of 53Mn. Therefore, the Mn-Cr isochron recorded in pallasite olivine could provide additional constraints on planetary differentiation in the early solar system. Alternatively, pallasites could have formed by a collision mixing between a body rich in metallic iron and an olivine layer near the surface of a planetary body. It is also possible that the two bodies carried different Cr isotopic compositions. The mixing process and thermal diffusion transported cosmogenic 53 Cr from metal to olivine and resulted in the observed 53 Mn/55Mn in these meteorites. In this case, the 53Mn/ 55 Mn ratios of pallasites have no chronologic meaning. Acknowledgments—The author likes to thank the Smithsonian Institution for the loan of the samples used in this study. G. J. Wasserburg initiated this study and Gary R. Huss helped with the measurements. They both made significant contributions to this work. Financial supports by NASA grant NAGW-3297, by Chinese National Natural Science Foundation for Distinguished Young Scholars (Grant No. 40325009), and by “One-HundredTalent Program” of Chinese Academy of Sciences, are greatly appreciated. The author thanks two anonymous reviewers for their thorough and constructive reviews.
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