Ar–Ar and I–Xe ages and thermal histories of three unusual metal-rich meteorites

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Geochimica et Cosmochimica Acta 73 (2009) 6965–6983 www.elsevier.com/locate/gca

Ar–Ar and I–Xe ages and thermal histories of three unusual metal-rich meteorites Donald D. Bogard a,*, Daniel H. Garrison b a b

ARES, code KR, NASA Johnson Space Center, Houston, TX 77058, USA Barrios Technology, JE23, 2224 Bay Area Blvd., Houston, TX 77058, USA

Received 27 May 2009; accepted in revised form 11 August 2009; available online 18 August 2009

Abstract Portales Valley, Sombrerete, and Northwest Africa (NWA) 176 are three unrelated meteorites, which consist of silicate mixed with substantial amounts of metal and which likely formed at elevated temperatures as a consequence of early impacts on their parent bodies. Measured 39Ar–40Ar ages of these meteorites are 4477 ± 11 Ma and 4458 ± 16 Ma (two samples of Portales Valley), 4541 ± 12 Ma, and 4524 ± 13 Ma, respectively (Ma = million years; all one-sigma errors). The Ar–Ar data for Portales Valley show no evidence of later open system behavior suggested by some other chronometers. Measured 129 129 I– Xe ages of these three meteorites are 4559.9 ± 0.5 Ma, 4561.9 ± 1.0 Ma, and 4544 Ma, respectively (relative to Shallowater = 4562.3 ± 0.4 Ma). From stepwise temperature release data, we determined the diffusion characteristics for Ar and Xe in our samples and calculated approximate closure temperatures for the K–Ar and I–Xe chronometers. Adopting results and interpretations about these meteorites from some previous workers, we evaluated all these data against various thermal cooling models. We conclude that Portales Valley formed 4560 Ma ago, cooled quickly to below the I–Xe closure temperature, then cooled deep within the parent body at a rate of 4 °C/Ma through K–Ar closure. We conclude that Sombrerete formed 4562 Ma ago and cooled relatively quickly. NWA 176 likely formed and cooled quickly 4544 Ma ago, or later than formation times of most meteorite parent bodies. For all three meteorites, the Ar–Ar ages are in better agreement with I–Xe ages and preferred thermal models if we increase these Ar–Ar ages by 20 Ma. Such age corrections would be consistent with probable errors in 40K decay parameters in current use, as suggested by others. The role of impact heating and possible disruption and partial reassembly of meteorite parent bodies to form some meteorites likely was an important process in the early solar system. Published by Elsevier Ltd.

1. INTRODUCTION Most parent bodies of meteorites formed and many partially or wholly differentiated over a period of several Ma about 4.56 Ga ago (Carlson and Lugmair, 2000; Wadhwa et al., 2008; Halliday and Kleine, 2008). Some meteorite types, e.g., chondrites and mesosiderites, indicate lengthy (e.g., 0.1 Ga and longer) periods of internal parent body metamorphism (Trieloff et al., 2003; Bogard and Garrison, 1998; Krot et al., 2008). Impacts and collisional disruption

*

Corresponding author. Tel.: +1 281 483 5146. E-mail address: [email protected] (D.D. Bogard).

0016-7037/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.gca.2009.08.009

also were important processes on many early meteorite parent bodies, and textural and chronological evidence of impact heating has been given for several meteorite types (Bogard, 1995; Mittlefehldt et al., 1998; Bogard et al., 2000; Scott et al., 2001; Scott 2002; Dixon et al., 2004; Scott and Wilson, 2005; Yang et al., 2007; Benedix et al., 2008; Rubin 2009). However, it often is difficult to separate the roles of impact heating and parent body metamorphism in the thermal history of individual meteorites. Nearly all age-dated meteorites give 39Ar–40Ar ages tens to hundreds of Ma younger than the times of parent body formation determined by other isotopic chronometers. Given the thermal histories experience by many meteorites and the relative ease with which the Ar–Ar chronometer

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can be reset by heating, these younger Ar–Ar ages may date the times of parent body cooling or collisional heating. However, the accuracy of the 40K decay coefficients in use have been questioned, and these decay coefficients may give Ar–Ar ages that are too young by a few 10s of Ma for a 4.5 Ga-old meteorite (Renne et al. 1998; Renne, 2000; Trieloff et al., 2001). One-way to test the accuracy of the 40K decay coefficients would be through comparison of Ar–Ar and I–Xe ages obtained for old meteorites that experienced a sudden thermal event followed by either a relatively fast cooling or slow cooling that is well characterized. Such ages, along with Ar and Xe diffusion data to indicate the relative ease of resetting, may indicate whether different Ar–Ar and I–Xe ages were produced during the meteorite thermal history or are due to inaccurate 40K decay parameters. Not much data exist for the early chronology and thermal history of individual meteorites that can elucidate the role of large, early collisions in meteorite formation history. We report here 39Ar–40Ar and 129I–129Xe ages obtained on silicate from three unusual metal-rich meteorites that derived from different parent bodies. Other workers (e.g., Kring et al., 1999; Liu et al., 2001; Ruzicka et al., 2005; Ruzicka et al., 2006) have argued that each of these silicate-metal assemblage may have formed by an impact event that involved considerable heating. When K and I are located in similar silicate phases, the Ar–Ar chronometer is typically more sensitive to resetting by heating than the I– Xe chronometer, although few data exist for Xe diffusion in meteorites (Burkland et al., 1995; Bogard et al., 2000). Thus, comparison of their Ar–Ar and I–Xe ages, along with Ar and Xe diffusion data, may elucidate their thermal histories and may set limits on errors in the 40K decay coefficients. The chronological history of each of these meteorites is interesting. For example, one meteorite studied, Portales Valley, is H-chondrite material disseminated in a metal matrix, and Portales Valley metal uniquely among chondrites shows the Widmansta¨tten structure indicative of slow cooling at depth in the parent body. Different possible origins have been given for this meteorite. However, previous data by the Re–Os, Sm–Nd, Rb–Sr, and U–Pb chronometers for Portales Valley have suggested ages across the wide range of 1.16–4.5 Ga (Chen et al., 2000; Papanastassiou et al., 2001), which are hard to interpret against these possible origins.

900 °C produced fluidization of metal veins (Ruzicka et al., 2005). Further, the metal phase shows a Widmansta¨tten structure, possibly the only chondrite with such a characteristic, and indicates slow cooling at depth in the parent body (Kring et al., 1999; Haack et al., 2000; Sepp et al., 2001; Ruzicka et al., 2005). The Portales Valley sample #1 (PV-1) we analyzed was acquired from Everett Gibson and came from a 950 g, unsawed piece collected by Robert Haag the day after the meteorite fall. This piece, now in the American Museum in New York, has massive metal rather typical of other pieces. The sample #2 (PV-2) we analyzed was acquired from Dimitri Papanastassiou, and is the same piece for which Cal Tech workers reported their original Rb–Sr, Sm–Nd, U–Pb, and Re–Os data (Chen et al., 2000). This sample was acquired by them from Marvin Kilgore and derived from a separate, water cut piece of Portales Valley. The Cal Tech lab also later reported data on a sub-sample from our piece #1. Thus, both the Ar–Ar and I–Xe data reported here and at least part of the other chronology data reported by Cal Tech were acquired on common samples. 2.2. Northwest Africa (NWA) 176 NWA 176 consists of polymineralic silicate inclusions embedded in a metal matrix (Liu et al., 2001), and was described as being very similar to Bocaiuva, described by Malvin et al. (1985). Like Bocauuva, silicate in NWA 176 has approximately chondritic composition, was strongly metamorphosed, does not show relic chondrules, and may have experienced partial melting (Liu et al., 2001). Determined two pyroxene equilibration temperatures for NWA 176 averaged 1100 ± 60 °C, and nickel concentration profiles in taenite grains indicate a cooling rate of 1000 °C/ Ma (Liu et al., 2001). These authors gave three possible origins for NWA 176: strong metamorphism of an asteroid with significant metal; breakup and reaccretion of a partially melted asteroid; and impact melting of a chondrite, followed by quenching and incomplete separation of silicate and metal. We made our analysis on a chip of the silicate, which contained some tiny metal inclusions. This sample was obtained from Ed Scott at the University of Hawaii and derived from the same piece as the one studied by Liu et al. (2001). 2.3. Sombrerete

2. SAMPLES AND METHODS 2.1. Portales Valley Portales Valley, which fell in 1998 and was immediately recovered, is unusual in that it consists of H-chondrite material that occurs as angular clasts in an extensive metal matrix (McHone et al., 1999). This Portales Valley assemblage has been attributed either to impact-induced melting, mixing, and metamorphism (Kring et al., 1999; Rubin et al., 2001) or to internal parent body heating and metamorphism (Pinault et al., 1999). Formation of Portales Valley by a combination of these mechanisms also has been suggested, whereby a large impact when temperatures were

Prinz et al. (1983) described Sombrerete as an iron similar to IIE irons and containing globular, highly fractionated silicate inclusions that resembled minimum melts. Ruzicka et al. (2006) further characterized Sombrerete inclusions as consisting predominately of alkali siliceous glass (69 vol.%) and smaller amounts of orthopyroxene, plagioclase, Cl-apatite, and other minerals, which formed immiscibly in the metal. Based on a variety of evidence, Ruzicka et al. (2006) concluded that Sombrerete inclusions were initially melted and flowed, before cooling rapidly near a parent object surface. The metal was probably substantially liquid, and both silicate and metal may have derived from a chondritic protolith. Ruzicka et al. (2006)

Ar–Ar and I–Xe Ages Metal-Rich Meteorites

suggested that the Sombrerete parent body was partially melted by decay of short-lived nuclides when a large impact further heated the body, produced silicate and metal separation, and temporarily broke the parent body into smaller fragments to facilitate rapid cooling. After this, the fragments reaccreted into another parent body. This scenario is similar to models of parent body breakup proposed for silicate-containing IAB irons (Benedix et al., 2000) and for Portales Valley (see above). Our Sombrerete sample was obtained from Glen MacPherson of the Smithsonian Institution and was mostly metal with “pearl-like” strings of tiny white and grey inclusions. For Ar–Ar, we analyzed a single, 13 mg white inclusion, and for I–Xe we analyzed 8 inclusions, ranging from white to light grey in color and totaling 75 mg. We previously published our Ar–Ar results for Sombrerete (Bogard et al., 2000), but we give a more detailed interpretation here. 2.4. Experimental methods For Ar–Ar analyses, Portales Valley (PV-1 and PV-2), NWA 176, and Sombrerete were neutron irradiated at separate times, and irradiation parameters (J-values) were 0.0315 ± 0.0003, 0.02199 ± 0.00004, 0.02095 ± 0.00008, and 0.0725 ± 0.0005, respectively. Several samples of the NL-25 hornblende age and neutron fluence standard were included in each irradiation. For I–Xe analyses, these three meteorites and two samples of the Shallowater age standard (Brazzle et al., 1999; Gilmour et al., 2006, 2009) were irradiated together in a single irradiation, which also included additional samples for Ar–Ar ages. Meteorite samples were loaded into cylindrical Al foil packages and interspersed with several NL-25 hornblendes (2–4 mg each) into a quartz tube of 6 mm inside diameter. Each irradiation involved two or three quartz tubes, clustered in fixed relative positions in the reactor. This geometric arrangement of hornblende monitors (up to 12 for some irradiations) allows complete determination of the neutron flux gradient along the tubes (typically 61% over 3 cm) and the maximum gradient between adjacent, parallel tubes (typically a few percent). Care is taken to position samples within the center of each quartz tube at a known location along the tube. The irradiation parameter, J, for each sample was determined from the relative positions of the sample and hornblende samples in that tube and adjacent tubes. An approximate one-sigma uncertainty for J was determined from the variation of monitors about the flux gradient determined by all tubes. For the I–Xe experiment, the two Shallowater samples were alternated in line with the other meteorite samples over a length of 2.3 cm. Neither thermal shielding nor sample rotation was used. Noble gases were extracted in a series of increasing temperature steps, each 10–30 min duration, by induction heating in a deep-well, Ta furnace equipped with a thermocouple. This system is optimal for determining Ar and Xe diffusion characteristics. Isotopic measurements were made on a VG-3600 mass spectrometer. Isotopic ratios were extrapolated to gas inlet time, although for the Xe measurements this extrapolation was minimal. Measured compositions were corrected for blanks, mass dis-

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crimination, and reactor-produced interferences for Ar. Uncertainties assigned to Ar–Ar ages presented in temperature release spectra are calculated from uncertainties in ratio measurements and applied corrections, but do not include uncertainties in J. All reported Ar–Ar ages determined from age spectra or from isochron plots do include the uncertainty in J. All uncertainties given are one-sigma. Corrected data for Ar are given in electronic annex EA-1 and for Xe in electronic annex EA-2. Additional experimental and data reduction details, as well as characteristics of the NL-25 hornblende, are given in Bogard et al. (1995, 2000, 2005). 3. RESULTS 3.1.

39

Ar–40Ar ages

3.1.1. Portales Valley The 39Ar–40Ar age spectra for the two analyzed samples of Portales Valley are shown in Fig. 1. For the last few extractions of both samples, the age increases and the K/ Ca decreases substantially, suggesting Ar degassing from pyroxene. Across the first 94% of the 39Ar release, PV-1 shows a relatively constant age and K/Ca ratio. The average age for 0.8–95% 39Ar release of sample PV-1 is 4480 ± 15 Ma, and the average age for 8–88% 39Ar release is 4477 ± 11 Ma. Approximately half of the error in both ages (one-sigma) derives from the error in J. The total PV-1 age across all extractions is 4483 Ma. Across the first 88% of the 39Ar release of PV-2, the K/Ca is relatively constant, whereas the age steadily decreases. The average PV-2 age for 6–90% 39Ar release is 4458 ± 16 Ma. Much of this age uncertainty is produced by the slope in the age spectrum. The total age across all extractions of PV-2 is 4464 Ma. The slightly older ages at highest temperature release of both samples are not easily explained. They may represent radiogenic 40Ar diffusion into pyroxene during slow cooling at significant depths within the parent body, as discussed in Section 4.2. To check whether the plateau ages for PV-1 and PV-2 might be influenced by 39Ar recoil or by trapped or redistributed 40Ar, we examined these data in isochron plots (Fig. 2). Because 36Ar abundances are relatively small and may represent both cosmogenic and trapped components, we normalized isochron plots to both total 36Ar and 37Ar (Bogard and Park, 2008). Argon-37 is a single component produced from Ca in the reactor and resides in the same lattice sites as cosmogenic 36Ar. Table 1 summarizes the ages and intercepts obtained from these isochron plots for different ranges of extraction temperature of both Portales Valley samples. In these plots we do not weight individual ratios with their uncertainties. Use of a common isotope in both plotted ratios (36Ar or 37Ar) will produce some correlation in the uncertainties, especially in the case of normalization to 36Ar, whose uncertainties tend to be larger than those for other isotopes. We do give the linear goodness of fit, r2 (Bevington, 1969), where the maximum permissible r2 value of 1.0 indicates perfect linear correlation between the plotted ratios. All isochrons summarized in Table 1 for all four analyzed samples are strongly linear, i.e.,

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1.000

4.60

Portales Valley #2

0.100

4.55 K / Ca

39Ar-40Ar Age,

Ga

4.65

4.50 4.45

0.010

4.40 [K]= 842 ppm, [Ca]= 1.04% 4.35 4.30

0.001

0.0

0.1

0.2

0.3

0.4 39Ar

0.5

0.6

0.7

0.8

0.9

1.0

Cumulative Fraction

4.80

1.000

Ga

4.70

39Ar-40Ar Age,

4.75

4.65

0.100

Portales Valley # 1

4.55

K / Ca

4.60 [K]= 915 ppm, [Ca]= 0.91%

4.50

0.010

4.45 4.40 4.35

0.001

0.0

0.1

0.2

0.3 39Ar

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Cumulative Fraction

Fig. 1. 39Ar–40Ar ages (rectangles, left scale) and K/Ca ratios (stepped line, right scale) for stepwise temperature extractions of Portales Valley samples #1 (lower) and #2 (upper). Individual age uncertainties are indicated by the width of the rectangles. K and Ca concentrations measured in the analyzed samples are indicated.

r2 values are very close to 1.0. Further, uncertainties of individual plotted ratios are mostly very much smaller than the range of plotted data (e.g., Fig. 2). For both Portales Valley samples, the isochron intercepts are very small compared to the range in plotted ratios and most are within error of zero (Table 1). This characteristic indicates that significant trapped or redistributed 40Ar was not released in these extractions. For PV-2 the isochron ages are slightly higher than, but within uncertainty of, the “plateau” age. Slightly higher ages and slightly negative intercepts for these isochrons suggest some isochron rotation as a consequence of the sloped age plateau (Park et al., 2009). For PV-1, the isochron ages show greater variation both above and below the plateau age, but the average of the four isochron ages (4478 ± 10 Ma) agrees with the preferred plateau age (4477 ± 11 Ma). An isochron normalized to 36Ar and forced through the origin for 8–88% release of PV-1 (which shows the largest value and uncertainty in its intercept) gives an age of 4478 ± 11 Ma, in agreement with the plateau age.

We adopt the Ar–Ar plateau age for PV-1 of 4477 ± 11 Ma. Because of possible isochron rotation for PV-2, we adopt the plateau age of 4458 ± 16 Ma and note that the error on this age overlaps the four isochron ages. These two Portales Valley ages, which overlap within their uncertainties, represent the time of last significant heating of this chondrite. However, below we suggest that the K– Ar closure ages of these two samples are slightly different. We see no suggestion of subsequent thermal events, as might be implied by the much younger apparent ages and open system behavior reported for other radiometric systems (Section 4.2) by Chen et al. (2000) and Papanastassiou et al. (2001). 3.1.2. NWA 176 The Ar–Ar age spectrum for NWA 176 is shown in Fig. 3 (lower). High K/Ca and 36Ar/37Ar ratios and erratic ages for the first few extractions suggest weathering contamination, which is common for hot-desert meteorites. Lower K/Ca ratios and ages for the last few extractions

Ar–Ar and I–Xe Ages Metal-Rich Meteorites

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500

Portales Valley #2

300

40Ar

/

37Ar

400

200

100

0 0.0

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0.6 39Ar

0.8

1.0

/ 37Ar

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15000

40Ar

/

36Ar

20000

10000

5000

0

0

10

20

30 39Ar

40

50

60

70

/ 36Ar

Fig. 2. Isochron plots of 40Ar/36Ar vs. 39Ar/36Ar for 0.8–95% of the 39Ar release for sample PV-1 (lower) and 40Ar/37Ar vs. 39Ar/37Ar for 2– 100% of the 39Ar release from sample PV-2 (upper). Most analytical uncertainties are contained within the symbol sizes.

suggest transfer of recoiled 39Ar into pyroxene. The age and K/Ca are relatively constant over 8–90% of the 39Ar release, and these extractions give an average plateau age of 4524 ± 13 Ma. An isochron plot of extractions releasing 8–90% of the 39Ar and normalized to total 36Ar (Fig. 4) gives an age of 4520 ± 10 Ma, and an isochron for all extractions >8% give an age of 4525 ± 8 Ma. Isochrons normalized to 37Ar give similar ages. All isochron intercepts are much smaller than the values of individual plotted ratios and in three cases pass through the origin within their uncertainty. These characteristics indicate no significant presence of redistributed 40Ar or recoiled 39Ar (Park et al., 2009). The average of these four isochron ages is 4520 ± 10 Ma. Because of uncertainties in the isochron ages, we adopt an Ar–Ar age for NWA 176 of 4524 ± 13 Ma. 3.1.3. Sombrerete The Ar–Ar age spectrum for Sombrerete is shown in Fig. 3 (upper). The first few extractions suggest diffusive loss of 40Ar; extractions releasing 4–11.5% of the 39Ar

and showing higher ages suggest recoil loss of 39Ar; and the last extraction, releasing 99–100% of the 39Ar suggests gain of recoil 39Ar. Ages over 11.5–99% 39Ar release also show a very slight decrease, which may also reflect some 39 Ar recoil redistribution. The K/Ca ratios decrease slightly throughout the extraction. The age summed across 2–100% 39 Ar release, omitting those extractions suggesting 40Ar diffusive loss, is 4542 ± 19 Ma. Thirteen extractions releasing 11.5–99% of the 39Ar give an average age of 4541 ± 12 Ma. Table 1 gives results of isochron plots for both of these 39Ar data ranges and normalized to 36Ar and 37Ar. All isochron ages are significantly older than the plateau ages and give negative intercepts outside of their individual errors. A negative isochron intercept can be produced by recoil loss of 39 Ar from lattice sites with higher 36Ar and 37Ar and plotting nearer the origin, combined with recoil gain of 39Ar by sites lower in 36Ar and 37Ar and plotting further from the origin, and thus producing counter clock-wise isochron rotation and an older age (Park et al., 2009, Appendix A). Thus, we discount the isochron ages and adopt an Ar–Ar age for Sombrerete of 4541 ± 12.

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Table 1 Various Ar–Ar Ages (Ma) and

40

Ar/xxAr Isochron Intercepts for Portales Valley, NWA 176, and Sombrerete.

PV-1

PV-2

NWA 176

Sombrerete

Plateau ages 39 Ar range Plateau age 39 Ar range

4477 ± 11 8–88% 4480 ± 15 0.8–95%

4458 ± 16 6–90%

4524 ± 13 8–90%

4541 ± 12 11.5–99% 4542 ± 19 2–100%

Isochron ages Age to 37Ar 39 Ar range r2 40 Ar/37Ar

4485 ± 11 0.8–95% 0.9998 0.4 ± 0.5

4462 ± 5 6–99.7% 0.9997 0.08 ± 0.8

4529 ± 8 8–100% 0.9995 0.53 ± 0.25

4557 ± 12 11.5–99% 0.9999 0.34 ± 0.04

Age to 37Ar 39 Ar range r2 40 Ar/37Ar

4487 ± 12 8–88% 0.9998 1.4 ± 0.8

4463 ± 6 6–90% 0.9996 0.3 ± 1.0

4506 ± 18 8–90% 0.9966 1.0 ± 1.0

4579 ± 13 2–99% 0.9991 0.77 ± 0.16

Age to 36Ar 39 Ar range r2 40 Ar/36Ar

4475 ± 11 0.8–95% 0.9999 39 ± 15

4464 ± 4 6–99.7% 0.9998 15 ± 30

4520 ± 10 8–90% 0.9994 23 ± 37

4556 ± 12 11.5–99% 0.9999 5.7 ± 0.8

Age to 36Ar 39 Ar range r2 40 Ar/36Ar

4465 ± 11 8–88% 0.9999 133 ± 23

4465 ± 4 6–90% 0.9998 29 ± 38

4525 ± 8 8–100% 0.9999 9±9

4560 ± 13 2–99% 0.9990 5.4 ± 2.7

All uncertainties are one-sigma. Age uncertainties include uncertainties in irradiation parameter, J. The r2 parameter is a measure of the degree by which the plotted ratios are correlated and define a linear isochron, where a value of 1.0 indicates perfect correlation (Bevington, 1969).

3.2.

129

I–129Xe ages

3.2.1. Shallowater To determine the I–Xe age of a sample requires comparison of the 129Xe*/128Xe* ratio in the sample with that in a standard sample of known age, where 129Xe* is the isotopic component arising from the in situ decay of 129I and 128Xe* is the isotopic component produced from 127I during neutron irradiation (Hohenberg and Pravdivtseva, 2008). We utilized the Shallowater aubrite as a standard (Brazzle et al., 1999; Hohenberg and Pravdivtseva, 2008; Gilmour et al., 2009). Xenon isotopic data for the two Shallowater samples irradiated with the new meteorite data presented here generally resemble those for Shallowater samples reported by Bogard et al. (2005). For all Shallowater samples we have analyzed, lower temperature iodine sites have lost part of their radiogenic 129Xe*. For each sample reported here, five extractions at temperatures of >1050 °C released 70% of the total 128Xe and define a constant slope on an isochron plot of measured 129Xe/132Xe versus 128 Xe/132Xe (Fig. 5, lower). For Shallowater #1, the unweighted 129Xe*/128Xe* slope is 0.530 ± 0.003 and the goodness of linear fit is r2 = 0.9999. For sample #2 the unweighted slope is 0.545 ± 0.003 and r2 = 0.9999 (one-sigma errors). As is the case with Ar–Ar isochrons, errors in individual ratio measurement are very small compared to the values of the ratios. If we calculate isochrons by weighting each measured ratio by its uncertainty (Williamson, 1968), the two slopes change very little. The slight difference in slopes for the two samples, although overlapping within

uncertainties, may measure a small gradient in neutron flux along the sample irradiation tube. Because of relatively large uncertainties in measuring 126Xe, we did not apply spallation corrections to the ratios in determining the slopes. Such corrections are very small (Bogard et al., 2005) and for these two Shallowater samples tended to increase scatter in the plotted data. From the isochron slope and intercept we calculate the 129Xe/132Xe ratio of the trapped component by assuming trapped 128Xe/132Xe has the terrestrial atmospheric ratio of 0.0714. These calculated 129 Xe/132Xe ratios are 0.93 ± 0.06 and 0.84 ± 0.03, respectively, and are slightly lower than the atmospheric value of 0.98. Lower trapped 129Xe/132Xe ratios from isochrons have been observed before and are not well understood (Gilmour et al., 2001; Hohenberg et al., 2002). 3.2.2. Sombrerete Based on 128Xe concentrations, most of which was released at temperatures >1000 °C, our Sombrerete sample contained 13 times as much I as did Shallowater. Fig. 5 shows the 129Xe/132Xe versus 128Xe/132Xe isochron plot for measured Sombrerete data. We applied a correction for spallation 132Xe using the 126Xe/132Xe ratios, which generally were measured to a precision of 10% (for details, see Bogard et al., 2005). These corrections move the ratios along the isochron slope (Fig. 5). The two lowest temperature data indicate loss of 129Xe* and/or iodine contamination. The six highest temperature extractions (1200– 1400 °C; some points overlap), releasing 56% of the total 129 Xe and not corrected for spallation, define an un-

Ar–Ar and I–Xe Ages Metal-Rich Meteorites

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4.6 5

0.9

Sombrerete silicate

4.5 5

0.6

K / Ca

39Ar-40Ar Age,

Ga

4.6 0

4.5 0

4.4 5

0.3

[K]= 2850 ppm, [Ca]= 2.3% 4.4 0

4.3 5

0.0 0.1

0.2

0.3 39Ar

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Cumulative Fraction

4.70

0.4

NWA 176 silicate

4.65

39Ar-40Ar

Age, Ga

Series3

Series2

0.3

4.60 4.55

0.2 4.50 4.45

0.1

4.40 4.35 0.0

K / Ca

0.0

[K]= 515 ppm, [Ca]= 1.11% 0.0 0.1

0.2

0.3 39Ar

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Cumulative Fraction

Fig. 3. 39Ar–40Ar ages (rectangles, left scale) and K/Ca ratios (stepped line, right scale) for stepwise temperature extractions of NWA 176 (lower) and Sombrerete (upper). Individual age uncertainties are indicated by the width of the rectangles.

weighted isochron slope (r2 = 0.9959) of 0.521 ± 0.017. The same data corrected for spallation, give an unweighted slope (r2 = 0.9966) of 0.525 ± 0.015. The trapped 129 Xe/132Xe ratios calculated from the isochron slopes and intercepts, assuming trapped 128Xe/132Xe has the atmospheric value of 0.0714, are 0.67 ± 0.44 and 0.46 ± 0.44. Assuming trapped 128Xe/132Xe has a value of 0.082 typical of primitive meteorites only increases the trapped 129 Xe/132Xe ratios by about 0.01. These derived trapped ratios are uncertain because of the small range of plotted ratios relative to the distance the isochron is extrapolated. Five intermediate extractions (800–1100 °C), releasing 25% of the 128Xe, define a linear trend parallel to the high-temperature data (i.e., slope = 0.525 ± 0.023) but offset toward excess 128Xe, such that the calculated trapped 129 Xe/132Xe ratio is negative ( 1.8 ± 0.8). These data defining the offset isochron may reflect the same I–Xe age with admixed iodine contamination not accompanied by 129Xe. We estimated fission 134Xe and 132Xe concentrations for Sombrerete by subtracting the atmospheric composition

from the spallation-corrected 134Xe/130Xe and 132Xe/130Xe ratios. These fission 134Xe and 132Xe values were small, and a few negative values indicate the significant uncertainty in applying fission corrections to these data. Fission corrections made to 132Xe tend to move the data points along the isochron without significantly changing its slope. To calculate an I–Xe age for Sombrerete, we adopt a 129 Xe*/128Xe* isochron slope of 0.525 ± 0.015. From the relative positions of Sombrerete and the two Shallowater samples during irradiation, we adopt a 129Xe*/128Xe* ratio for Shallowater of 0.532 ± 0.015. This value assumes that a small gradient existed in neutron flux along the sample tube, but the assigned uncertainty includes the 129 Xe*/128Xe* slopes for both Shallowater samples. The ratio of 129Xe*/128Xe* isochron slopes for Sombrerete compared to Shallowater is thus 0.986 ± 0.040, where individual errors are propagated statistically. This ratio is set equal to N/N0 in the radioactive decay equation N/N0 = e kt, and the formation age is calculated relative to the I–Xe age for Shallowater,. Adopting a mean life

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Sombrerete 11.5-99% 39Ar

30

40Ar

/

37Ar

40

20

10

0 0.00

0.05

0.10

0.15 39Ar

0.20

/

0.25

0.30

37Ar

12000

NWA 176 silicate

10000

40Ar

/

36Ar

8-90% 39Ar 8000 6000 4000 2000 0 0

2

4

6

8

10 39Ar

12

14

16

18

20

/ 36Ar

Fig. 4. Isochron plots of 40Ar/36Ar vs. 39Ar/36Ar for 8–90% of the 39Ar for NWA 176 (lower) and 40Ar/37Ar vs. 39Ar/37Ar for 11.5–99% of the 39 Ar release of Sombrerete (upper). Most analytical uncertainties are contained within the symbol sizes.

for 129I of 22.66 Ma (Brazzle et al., 1999), we calculate an 129 129 I– Xe age that is 0.32 ± 0.91 Ma younger than the I– Xe age of Shallowater. The absolute I–Xe age of Shallowater is based on comparison of I–Xe ages with other radiometric ages for several meteorites and has been determined in more than one study (Brazzle et al., 1999; Gilmour et al., 2006; Hohenberg and Pravdivtseva, 2008; Gilmour et al., 2009). We adopt the most recent reported age of 4562.3 ± 0.4 Ma (Gilmour et al., 2009), which differs only slightly from earlier values. Using this Shallowater age, the absolute I–Xe age for Sombrerete is 4561.9 ± 1.0 Ma. This age is 3 Ma older than an I–Xe age we reported for the silicate from the Caddo County IIE iron meteorite using the same Shallowater normalization (Bogard et al., 2005; corrected for newer Shallowater age). 3.2.3. NWA 176 Xenon released from NWA 176 is more complex. Eighty percent of the total 128Xe (with a 129Xe/128Xe ratio of 0.14) released in the first, 450 °C extraction, which probably reflects hot-desert weathering. Subsequent extractions released only 12% as much 128Xe as was released by Shallowater. As with Sombrerete, we used the 126 Xe/132Xe ratios to make cosmogenic corrections. These corrections were small and uncertain, and produced only very small changes in the isotopic composition. Correc-

tions for fission Xe, however, are more substantial, especially in the 650–800 °C extractions where the great majority of xenon isotopes 132, 134, and 136 is fissionproduced. Assuming trapped Xe has the atmospheric composition, these extractions show a relative fission Xe composition of 132Xe/134Xe/136Xe = 0.53/1.00/1.41. The most likely sources of this fission Xe are neutron induced fission of 235U produced during reactor irradiation and spontaneous fission of 238U and 232Th over geologic time. The expected Xe mass yield from spontaneous fission is 0.71/1.00/1.20 (Wetherill, 1953), and the Xe mass yield from 235U fission induced by fission spectra neutrons is 0.54/1.00/0.80 (Katcoff, 1958, 1960). The fission 136 Xe/134Xe ratio observed in NWS 176 is larger than the reported values from either spontaneous or induced fission. This is commonly observed in irradiations and occurs because fission 135Xe produced in the reactor has a large cross section for neutron capture, which enhances the 136Xe yield (Hohenberg and Kennedy, 1981). Thus, to correct 132Xe in NWA 176 for fission, we utilize the 132 Xe/134Xe ratio. We did not apply a correction for fission 129Xe, because its fission yield is more than an order of magnitude less than that of 132Xe (Wetherill, 1953; Katcoff, 1960; Rao and Kuroda, 1966). A plot of measured 129Xe/132Xe versus 128Xe/132Xe for the 1050–1400 °C extractions of NWA 176 is shown in

Ar–Ar and I–Xe Ages Metal-Rich Meteorites

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35

Sombrerete

30

Measured Corrected Atmosphere

/ 132Xe

20

129Xe

25

15 10 5 0 0

10

20

30

40

128Xe

50

60

70

/ 132Xe

24 Shallowater #1

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Shallowater #2

129Xe

/ 132Xe

Atmosphere

16 12 8 4 0 0

4

8

12

16

20 128Xe

24

28

32

36

40

/ 132Xe

Fig. 5. Isochron plots of 129Xe/132Xe vs. 128Xe/132Xe for two Shallowater standard samples (lower) and Sombrerete (upper). The terrestrial atmospheric composition is indicated. Circles for Sombrerete represent measured data, whereas diamonds have an applied correction for spallation Xe. Intermediate temperature data for Sombrerete define a similar 129Xe*/128Xe* slope as high-temperature data, but displaced to the right. Analytical uncertainties for measured data are mostly contained within symbol sizes.

Fig. 6. Also shown in Fig. 6 are these data corrected for fission 132Xe using fission ratios of 132Xe/134Xe = 0.54 and =0.71. The type of fission Xe assumed is much less important than making a fission correction. For fission 132 Xe/134Xe = 0.54 and =0.71, the unweighted isochron slopes are 0.235 ± 0.060 (r2 = 0.886) and 0.254 ± 0.024 (r2 = 0.982), respectively. Substituting atmospheric 128 Xe/132Xe = 0.0714 into the equation for the isochron, the trapped 129Xe/132Xe ratios are 0.96 ± 0.13 and 1.01 ± 0.06, respectively. Using a chondritic 128Xe/132Xe ratio of 0.082 does not change these trapped values. We use these isochron slopes to calculate I–Xe formation intervals relative to Shallowater, in a manner analogous to that described for Sombrerete. The slope ratios relative to Shallowater are 0.431 ± 0.11 and 0.466 ± 0.044. These correspond to calculated I–Xe formation intervals of 19.1 ± 5.8 Ma and 17.3 ± 2.1 Ma after Shallowater, or an absolute age of 4544 ± 7 Ma. 3.2.4. Portales Valley Our Portales Valley sample contained 61% as much iodine as Shallowater, and the release of 128Xe* as a function of temperature mostly occurred in two peaks around 900–

1000 °C and 1300 °C. Xenon released at 1375 °C and 1500 °C were near blank levels and are not further considered. Spallation Xe corrections were very small. The 780– 1000 °C extractions released the most fission Xe and gave a fission 132Xe/134Xe ratio of 0.53, which is similar to the expected fission ratio and the measured ratio for NWA 176. We corrected all 132Xe for fission contribution using this ratio, although for the highest temperature extractions, corrections were minor. Measured 129Xe/128Xe and 128 Xe/132Xe ratios and spallation- and fission-corrected ratios are plotted in Fig. 7. The four high-temperature extractions, 1175–1325 °C, are also shown in the Fig. 7 inset. These four extractions released 34% of the total 128Xe and 77% of the total 129Xe, and define an unweighted isochron slope (r2 = 0.9998) of 0.4846 ± 0.0052. The trapped 129 Xe/132Xe ratio, defined by the isochron at a trapped 128 Xe/132Xe of 0.082, is 1.10 ± 0.02, which is slightly higher than trapped Xe in primitive chondrites. The two lowest temperature extractions lie on a line with a slope of 0.021 and passing through the trapped Xe composition and define a lower limit to the I–Xe age. Because Portales Valley is a fall, these lower 129*Xe/128*Xe ratios may not represent weathering loss of 129Xe* or gain of iodine. If

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NWA 176

129Xe

/ 132Xe

2.0

1.5

Measured

1.0

Fission 132/134=0.54 Fission 132/134=0.7 Atmosphere

0.5 0

1

2

3

128Xe

4

5

/ 132Xe

Fig. 6. Isochron plot of 129Xe/132Xe vs. 128Xe/132Xe for NWA 176. Circles give measured data and uncertainties for four extractions (1050– 1400 °C). Diamonds show the same data corrected for spallation and fission Xe, using fission 132Xe/134Xe = 0.54; triangles show same data corrected for spallation and fission Xe, using 132Xe/134Xe = 0.7. The terrestrial atmospheric composition is indicated.

5.0 4.5 4.1

4.5

3.7 3.3 2.9

4.0

2.5 2.1

129Xe / 132Xe

3.5

1.7

1.3 0.9

3.0

0.5 0

1

2

3

4

5

6

7

2.5 2.0 1.5

Portales Valley

1.0 0.5

0

10

20

30

40

50

60

70

80

128Xe / 132Xe

Fig. 7. Isochron plot of 129Xe/132Xe vs. 128Xe/132Xe for Portales Valley. Circles are measured data; whose uncertainties are mostly contained within the symbol sizes; triangles show data corrected for spallation and fission Xe, using 132Xe/134Xe = 0.53. The figure inset expands that portion of the plot near the origin and shows data for four extractions, 1175–1325 °C, releasing 77% of the total 129Xe.

real, these lower temperature extractions may represent a mild Xe degassing episode that did not affect the higher temperature data, as discussed later. We use the Portales isochrons to calculate I–Xe formation intervals relative to Shallowater in a manner analogous to that described for Sombrerete. For the high-temperature isochron, the slope ratio relative to Shallowater is 0.900 ± 0.017 and the calculated I–Xe formation interval is 2.37 ± 0.04 Ma after Shallowater, or an absolute age of 4559.9 ± 0.5 Ma. The calculated formation interval for

the approximate “isochron” defined by the first two extractions is about 88 Ma after Shallowater, or an absolute age of about 4.47 Ga. Intermediate temperature extractions, releasing the most fission Xe, plot between these two isochrons. 3.3. Argon and xenon diffusion In order to compare the ease with which Ar–Ar and I– Xe ages may have been reset in these meteorites by thermal

Ar–Ar and I–Xe Ages Metal-Rich Meteorites

heating, we calculated Arrhenius diffusion plots (Langerwall and Zimen, 1964; Fechtig and Kalbitzer, 1966) from the Ar and Xe data collected as a function of extraction temperature. Both Portales Valley samples released most of their 39Ar from K and a minor part of their 37Ar from Ca in a peak centered around 850 °C, which probably reflects degassing of plagioclase. Thus, we calculated Ar diffusivities for just this phase, as shown in Fig. 8 (lower). Each Portales sample gives a linear trend for which the 39Ar and 37 Ar data are in agreement. All data converge at a D/a2 value of 10 4 s 1 at 900 °C. However, PV-1 and PV-2 give slightly different slopes, which correspond to activation

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energies of 63 and 45 kcal/mol, respectively. This suggests some physical difference in plagioclase (other than grain size) between these two samples. Our analyzed sample of NWA 176 had a similar argon release pattern as Portales Valley, with most of the 39Ar and a fraction of the 37Ar released from plagioclase in a peak centered at 850 °C. For Sombrerete, most of the 39 Ar and 37Ar show a correlated release around 800 °C. The Arrhenius plot for 39Ar in NWA 176 (Fig. 8, upper) is relatively linear and indicates an activation energy of 60 kcal/mol. Part of the 37Ar diffusion data is consistent with this trend, but the lower temperature 37Ar data suggest

-3

NWA 39Ar, 1-93% -4

NWA 37Ar, 0-46% Som 39Ar, 2-92%

Log D / a2 sec-1

-5

Som 37Ar, 1-89% -6 -7 -8

NWA 176 & Sombrerete

-9

-10 -11

0.6

0.7

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0.9

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1.1

1.2

1.3

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Log D / a2 sec-1

-5 -6 -7 -8 #2, 0.5-92% 39Ar

-9

#2, 0-19% 37Ar #1, 0-95% 39Ar

-10

#1, 0-27% 37Ar

-11 0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1000 / T Fig. 8. Arrhenius plots of log D/a2 vs. reciprocal temperature (in Kelvin) for 39Ar and 37Ar diffusion in PV-1 and PV-2 (lower), and in NWA 176, and Sombrerete (upper). The ranges of total release of 39Ar and 37Ar used in constructing these plots are given in percent in the legend.

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diffusivities lying below this trend. Part of the low-temperature 37Ar may have been released from a phase other than plagioclase, such as phosphate. The Arrhenius trends for 39 Ar and 37Ar from Sombrerete (Fig. 8, upper) are similar to each other, but give a slight convex upward pattern. These deviations from linear diffusion trends may indicate mixing of Ar from different diffusion domains releasing at lower and higher temperatures. Activation energies of 45–60 kcal/mol for Ar diffusion from these three meteorites lies at the upper range of determined Ar activations energies for K-bearing phases of various meteorite types (Turner, 1978; Bogard and Hirsch, 1980; Bogard and Garrison, 1998; Bogard, 2009). The Arrhenius plot for 128Xe diffusion in Sombrerete, utilizing all extractions except the first (Fig. 9, lower), is well defined and corresponds to an activation energy of 61 kcal/ mol. The 128Xe Arrhenius plot calculated for those Portales Valley extractions exceeding 1100 °C (Fig. 9, upper) is moderately well defined and gives a calculated activation energy

of 160 kcal/mol. Arrhenius data calculated for Portales Valley extractions <1100 °C (not shown) scatter considerably and suggest a much lower activation energy, consistent with lower 129Xe/128Xe ratios for these extractions (Fig. 7). Arrhenius data for 128Xe extracted from NWA 176 at >650 °C suggest a nearly flat slope (Fig. 9), but this is inconsistent with the relatively high temperature required to degas this Xe. Most 128Xe in NWA 176 was released from iodine contamination at lower temperatures, and contamination probably also contributes to higher temperature data and affects this Arrhenius plot. Note that for Portales and Sombrerete, values for D/a2 at a given temperature are much lower for Xe than for Ar. Little literature data have been reported for Xe diffusion in meteorites, and the data that exist give variable results (Burkland et al., 1995). From I–Xe studies of silicates in IAB meteorites, Bogard et al. (2005) reported activation energies of 82 and 189 kcal/ mol for two meteorites and, from literature data on a third IAB silicate, estimated an activation energy similar to the

-2 -3

Log D / a2 sec-1

-4 -5 -6 -7

Portales Valley >850C

-8

NWA 176 >650C

-9 0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1000 / T -3

Log D / a2 sec-1

-4

Sombrerete, 3-100% 128Xe

-5

-6

-7

-8

-9 0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1000 / T Fig. 9. Arrhenius plots of log D/a2 vs. reciprocal temperature (in Kelvin) for 128Xe diffusion in Sombrerete (lower) and Portales Valley and NWA 176 (upper). The temperature extractions used in constructing the plots for Portales Valley and NWA 176 are indicated in the legend.

Ar–Ar and I–Xe Ages Metal-Rich Meteorites

lower of these two values. The wide variation of reported activation energies for Xe diffusion probably reflects diverse iodine-bearing phases in meteorites. 4. DISCUSSION The Ar–Ar and I–Xe ages of Sombrerete, Portales Valley, and NWA 176 presented above must be interpreted in terms of known early solar system chronology and early processes that affected this chronology. Most parent bodies of meteorites formed and many partially or wholly differentiated over a period of several Ma about 4.56 Ga ago (Carlson and Lugmair, 2000; Wadhwa et al., 2008; Halliday and Kleine, 2008). Data for some meteorite types, e.g., chondrites and likely mesosiderites, indicate lengthy periods of internal parent body metamorphism, possibly P0.2 Ga in length (Bogard and Garrison, 1998; Trieloff et al., 2003; Krot et al., 2008). Impacts and collisional disruption also were important processes on many early meteorite parent bodies, and textural and chronological evidence of impact heating has been given for several meteorite types (Bogard, 1995; Mittlefehldt et al., 1998; Bogard et al., 2000; Scott et al., 2001; Scott 2002; Dixon et al., 2004; Scott and Wilson, 2005; Yang et al., 2007; Benedix et al., 2008; Rubin, 2009). As discussed in Section 2, the three meteorites reported here likely experienced impacts, which produced silicate-metal mixing and resetting of their Ar–Ar ages and possibly of their I–Xe ages. Because of its greater sensitivity to thermal heating, most chronology of early impact events has been determined using the Ar–Ar chronometer. However, for meteorite K–Ar ages that postdate parent body formation by only a few tens of Ma, another relevant issue is whether accurate 40K decay coefficients are being used to calculate these ages (Renne et al. 1998). A common approach to evaluating the best 40K decay coefficients is to compare, for a given sample, the K–Ar (Ar–Ar) age with the U– Th–Pb age, where decay coefficients are best known. As applied to meteorites, this approach would be most appropriate for old meteorites that experienced a sudden thermal event followed by either a relatively fast cooling or slow cooling that is well characterized. The Acapulco meteorite derives from a chondrite-like parent body that experienced metamorphism up to partial melting. The Ar–Ar age of Acapulco feldspar (4507 ± 18 Ma; Renne 2000, and references therein), the U–Th–Pb age of Acapulco phosphate (4557 ± 2 Ma; Go¨pel et al., 1992), and the I–Xe age of phosphate (4562 ± 3 Ma; Nichols et al., 1994; Brazzle et al., 1999) have each been accurately measured, although the effective closure temperature of each of these chronometers may differ. Based on these studies, Renne (2000) suggested that the 40K decay coefficients should be adjusted such that the true Ar–Ar age of Acapulco is 4554 Ma, an increase of 47 Ma. However, as Trieloff et al. (2001) pointed out, such a large upward adjustment of Ar–Ar ages contradicts other evidence for a slow, extended cooling history for Acapulco and the existence of Ar–Ar ages older than 4.51 Ga for some other meteorites. In a separate study reporting 244Pu fission track and Ar–Ar ages (4.53–4.43 Ga) of H-chondrites, Trieloff et al. (2003)

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showed that this parent body accreted fast and then cooled slowly in a layered body, and that those H-chondrites located deeper and more heavily metamorphosed give younger ages. These authors concluded that an upward revision in K–Ar ages was desirable, but suggested that an increase of about 30 Ma was more appropriate for H-chondrites and would be consistent with Acapulco data. In an extensive determination of Ar–Ar ages (4340–4540 Ma) of IAB silicates, another class of meteorites consisting of evolved silicate disseminated inside a metal phase (Mittlefehldt et al., 1998; Bogard et al., 2005), Vogel and Renne (2008) suggested old Ar–Ar ages of meteorites should be increased by 22 Ma, which would make the oldest among these ages consistent with I–Xe ages of IABs and formation of the parent body. 4.1. Sombrerete The I–Xe age for Sombrerete is relatively well defined at 4561.9 ± 1.0 Ma, which is similar to I–Xe ages for silicate in some IAB irons (Bogard et al., 2005) and some chondrites (Brazzle et al., 1999; Gilmour et al., 2000). Because these I–Xe ages are similar to typical formation ages of meteorite parent bodies, the Sombrerete age likely dates formation of the silicate phase and the silicate-metal mixture within the parent object. This age is consistent with the Sombrerete parent body having been strongly heated by short-lived radionuclides, followed by a collision that further heated it and produced the silicate-metal separation, as suggested by Ruzicka et al. (2006). Our preferred Ar–Ar age for Sombrerete is 4541 ± 12 Ma, which ranks among the oldest Ar– Ar ages for meteorites. The Ar–Ar age appears younger than the I–Xe age, although the ages overlap at the two-sigma level. Although a younger Ar–Ar age might be interpreted as slow cooling within the parent body, this would not be consistent with the suggestion of Ruzicka et al. (2006) that Sombrerete cooled quickly after formation. On the other hand, it might have experienced a two stage cooling history, with initial fast cooling followed by slow cooling through closure of the K–Ar chronometer, as might occur upon reassembly of the collisionally disrupted fragments into a still warm parent body (Ruzicka et al., 2006). A second possible explanation for the difference in I–Xe and Ar–Ar ages of Sombrerete might be a later thermal event that reset the Ar–Ar age but not the I–Xe age. Only the first two extractions, which release 20% of the 128Xe, give low 129Xe/128Xe that might be indicative of Xe loss in a later thermal event. In a slowly cooling system, the Ar and Xe diffusion data permit calculation of the temperatures at which Ar and Xe diffusion stop and the K–Ar and I–Xe chronometers close (Dodson, 1973; Bogard et al., 2005). This calculated closure temperature depends on diffusion characteristics of the gas and the cooling rate. We assumed cooling rates of 10 and 1000 °C/Ma and used the diffusion data in Figs. 8 and 9 to calculate closure temperatures for Sombrerete. The calculated Ar and Xe closure temperatures were 600–660 and 675–750 K, respectively. The similarity of Ar and Xe closure temperatures suggests that thermal resetting is not the explanation for 20 Ma difference in Ar–Ar and I–Xe ages. Further,

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Ruzicka et al. (2006) do not describe any evidence for later shock heating events. A third possible explanation for the slightly lower Ar– Ar age for Sombrerete is uncertainty in the 40K decay coefficients, as discussed above. If we assume Ar–Ar ages, as usually calculated, are too low by 20 Ma (Vogel and Renne, 2008), then the corrected Ar–Ar age for Sombrerete would become 4561 Ma, in good agreement with the I–Xe age. This third explanation, which we prefer, would permit Sombrerete to have formed early and cooled quickly, as preferred by Ruzicka et al. (2006). 4.2. Portales Valley Higher temperature extractions of Portales Valley give an I–Xe age of 4559.9 ± 0.5 Ma, which indicates formation of Portales Valley silicate occurred relatively early in solar system history. This age is essentially identical to that of Sombrerete, only slightly lower than the Shallowater standard age, and similar to I–Xe ages of several other meteorite types (Nichols et al., 1994; Brazzle et al., 1999; Gilmour et al., 2000; Bogard et al., 2005; Hohenberg and Pravdivtseva, 2008; Vogel and Renne, 2008). However, the first two temperature extractions of Portales Valley suggest a possible I–Xe age of only 4.47 Ga. This younger I–Xe age, although problematic, is in agreement with the determined Ar–Ar ages of 4.46–4.48 Ga for two Portales Valley samples. We suggest that either slow parent body cooling kept the K–Ar system open, or that a strong thermal event reset the Ar–Ar chronometer completely in Portales, but only reset I–Xe in the lower temperature iodine phase and not the higher temperature iodine phase. It is unlikely that the younger Ar–Ar age is the result of uncertainties in the 40 K decay coefficients. Other Ar–Ar ages of meteorites, including the Sombrerete Ar–Ar age, are older than the Portales Valley Ar–Ar age by several tens of Ma. Increasing the Ar–Ar age for Portales Valley to coincide with its I–Xe age would cause these other Ar–Ar ages to exceed the formation time of most meteorite parent bodies 4.56 Ga ago. The Cal Tech lab reported a variety of isotopic chronology of silicate and metal for Portales Valley (Chen et al., 1999, 2000; Papanastassiou et al., 2001, 2002). In the metal phase, 187Os/188Os and 187Re/188Os data were consistent with early formation, but the data for the silicate indicated disturbance and open system behavior. For the silicate, Sm–Nd data indicated very young model ages of 1.16– 1.57 Ga., and the TBABI Rb–Sr model age was older than the solar system. The silicate U–Pb data indicated an early formation age at 4571 ± 10 Ma, but a disturbance at 94 ± 5 Ma. These results are hard to reconcile with the I– Xe and Ar–Ar ages reported above, which show no evidence of disturbance later than 4.46 Ga. Because at least Ar–Ar, if not I–Xe, should be sensitive to resetting by thermal events, our results suggest that open system behavior exhibited by these other chronometers was not produced by heating. The thermal environment that produced the Portales Valley Ar–Ar age may have been a large impact 4.47 Ga ago that mixed silicate and metal. However, this explanation does not readily account for the I–Xe age and

for older ages indicated by Re–Os data of the metal and U– Pb data of the silicate (Chen et al., 2000). On the other hand, the metal apparently did not have live 107Pd, which suggests that it formed >6 Ma after some other irons (Chen et al., 1999). Alternatively, the Ar–Ar age may date the time during slow cooling of the parent body at which the K–Ar chronometer closed to diffusion loss of 40Ar. This would permit heating and mixing of Portales Valley to have occurred much earlier. The thermal history of Portales Valley may have involved both early impact heating and slow cooling, as suggested by Ruzicka et al. (2005). Because Ar diffusion from PV-1 occurs more slowly than from PV-2 (Fig. 8), in a slowly cooling body, PV-1 might be expected to close to 40Ar loss earlier than PV-2 and thus to show an older age. This is in fact what is observed; the Ar–Ar age for PV-1 appears to be 20 Ma older than the age of PV-2. We can use the Ar and Xe diffusion data for Portales Valley to calculate Ar and Xe closure temperatures (see above) and examine a probable cooling history in more detail. For reasons justified below, we assume a cooling rate at Ar–Ar closure of 4 °C/Ma and at I–Xe closure of 10 °C/ Ma. Using the Ar diffusion data from Fig. 8, we calculate closure temperatures for PV-1 and PV-2 of 600 K and 500 K, respectively. Using the Xe diffusion data from Fig. 9, we calculate a I–Xe closure temperature of 1045 K. These calculated closure temperatures are not very sensitive to assumed cooling rate, and changing the cooling rate by a factor of two changes the calculated closure temperatures for Ar by less than 10 °C and that for Xe by 60 °C. Fig. 10 plots these three closure temperatures against the respective ages and compares these results against two types of possible thermal histories for the formation of Portales Valley. Ruzicka et al. (2005) concluded that some parts of Portales Valley were heated to 940– 1150 °C (1213–1423 K) and partially melted. We assume heating to this temperature range occurred 4560 Ma ago. The upper, dot-dashed line in Fig. 10 gives the cooling curve with these initial conditions and the requirement that the curve pass through the plotted Ar–Ar data for samples PV-1 and PV-2. This cooling curve agrees with the two Ar– Ar data sets and gives a cooling rate of 11 °C/Ma at 1000 K and 4 °C/Ma at 550 K. This cooling curve assumes a surface temperature of 173 K for the parent body (Ghosh and McSween, 1998). However, assuming a surface temperature of 273 K and slightly adjusting the cooling rate would give a similar cooling curve in equal agreement with the Ar–Ar data. (Bogard et al., 2005, give details of such calculations.) In contrast, the I–Xe point for Portales Valley is not in agreement with this cooling model, and the only way it could be made compatible would be to assume an improbably high I–Xe closure temperature. The second cooling model examined is that where the Hchondrite parent body is relatively hot when it was impacted by another body, which produced additional heterogeneous heating and mixing (Ruzicka et al., 2005). If hot metal is mixed with cooler silicate, the mixture will rapidly quench to a lower equilibrium temperature, then cool more slowly. This rapid quenching is represented by the downward pointing arrow in the upper right of Fig. 10. In this second cooling model, we assume the quenched equilibrium

Ar–Ar and I–Xe Ages Metal-Rich Meteorites 1400 1300

1213-1423 K

Portles Valley Cooling History

1200

Temperature, Kelvin

1100

11oC/Ma 8oC/Ma 10oC/Ma

1000

I-Xe Age

900

800 700 600

Ar-Ar Ages

4.4oC/Ma 3.3oC/Ma 4.2oC/Ma

500

400 300

4.42

4.44

4.46

4.48

4.50

4.52

4.54

4.56

4.58

Time, Gyr B.P. Fig. 10. Possible thermal histories for Portales Valley investigated by plotting measured Ar–Ar ages (dark circles) and I–Xe age (diamond) against calculated closure temperatures. The uppermost cooling curve (dot-dash) assumes an initial temperature of 1350 K at 4560 Ma ago, cooling at a rate consistent with the two Ar–Ar ages, and is not consistent with the I–Xe data. Assuming early impact heating of an already hot body (Ruzicka et al., 2005), followed by partial quenching and slow cooling from near the I–Xe closure temperature (solid and dashed curves), is consistent with the I–Xe data and with either the measured Ar–Ar ages, or these ages increased by 20 Ma (light circles) to compensate for likely errors in the 40K decay coefficients. The cooling rates for these three thermal models are given at 1000 K and 550 K, and are equivalent to burial in the center of a parent body 30 km in radius.

temperature is 1050 K, or about the I–Xe closure temperature. To be compatible with the plotted Ar–Ar data, cooling of Portales Valley from this point would have a rate of 8 °C/Ma at 1000 K and 3.3 °C/Ma at 550 K, i.e., the solid line in Fig. 10. This cooling scenario is compatible with the I–Xe age and both Ar–Ar ages of Portales Valley, and is the one we favor. If we assume that uncertainty in the 40K decay coefficients require the Ar–Ar ages to be increased by 20 Ma, then we get the lower, dashed cooling curve in Fig. 10. This curve also is consistent with the corrected Ar–Ar ages and requires a similar cooling rate of 4.2 °C/Ma. Note that cooling curves with rates differing by as much as 25% from these values would be inconsistent with the Ar–Ar ages. Also note that this model is not very sensitive to the I–Xe closure temperature; a lower closure temperature would lower the initial temperature of the second cooling stage and modestly decrease the cooling rate. Other evidence seems consistent with the thermal model given above for Portales Valley. These include formation of phosphate, pyroxene geothermometry, and metal textures, as discussed in Ruzicka et al. (2005). Different workers

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applying different methods to the metal phase determined overlapping cooling rates of a few degrees per Ma for the temperature range of 350–700 °C (Pinault et al., 1999; Ruzicka et al., 1999; Haack et al., 2000; Sepp et al., 2001), in good agreement with the Ar–Ar cooling rates (Fig. 10). All these thermal models indicate relatively slow cooling for the Portales Valley parent body, whether heating was due to internal metamorphism, impact, or both processes. By comparison, determined metallographic cooling rates of ordinary chondrites at 700 K show a wide range of 1–100 °C/Ma (Willis and Goldstein, 1981; Bennett and McSween, 1993; Bennett and McSween, 1996; Trieloff et al., 2003), whereas the cooling rate of mesosiderites was considerably slower (Yang et al., 1997; Bogard and Garrison, 2008). If Portales Valley was located at the center of its parent object immediately after its formation, a cooling rate of 10 °C/Ma would indicate this object was at least 30 km in radius. For a location nearer the surface, the object would have been even larger (Bogard and Garrison, 2008; Bennett and McSween, 1996.). If impact heating was a factor in forming Portales Valley, this must have occurred during an early period of rapid accretion and impact disruption in order to bury the meteorite so deeply. A second impact disruption that did not appreciably heat the meteorite would have been required to liberate Portales Valley from the interior of its parent, just as other higher metamorphic grade chondrites were liberated from their parent interiors. Although rare, evidence of early impact melting of chondrites does exist (e.g., Benedix et al., 2008). 4.3. NWA 176 Our preferred Ar–Ar age for NWA 176 is 4524 ± 13 Ma, and our preferred I–Xe age is 4544 Ma. We used our determined activation energy for Ar diffusion in NWA 176 (Fig. 8) and the cooling rate of 1000 °C/Ma calculated from Ni profiles in taenite (Liu et al., 2001) to calculate a K–Ar closure temperature of 650 K. If we assume a slower cooling rate of 100 °C/Ma, the Ar closure temperature would modestly decrease to 620 K. Closure of Ni diffusion in metal probably would occur at a somewhat higher temperature, or 700–850 K. Xenon data for NWA 176 do not define diffusion characteristics, so we assume the same Xe diffusion characteristics and closure temperature as Portales Valley. Fig. 11 plots the Ar–Ar and I– Xe ages against closure temperature. Also plotted is the Ar– Ar age increased by 20 Ma to correct for a possible error in the 40K decay coefficients (see above). Liu et al. (2001) calculated two pyroxene equilibration temperatures for NWA as 970–1280 °C, with an average of 1100 ± 60 °C. A cooling curve that begins at this temperature (1373 K) at an assumed starting time of 4556 Ma and passes through the I–Xe and Ar–Ar ages (dashed line, Fig. 11), represents a cooling rate of 16 °C/Ma at the Ni closure temperature in metal. This model would imply that NWA 176 formed early and cooled slowly. Such a slow cooling rate, however, contradicts the much faster cooling rate actually measured in the metal (Liu et al., 2001). Any cooling history that begins at the I–Xe age 4544 Ma ago would require both an

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1400

1100 +-60oC

1300

Temperature, Kelvin

1200 1100

NWA 176 Cooling History

16oC/Ma

I-Xe Age

1000 900

Ni 1000oC/Ma

Ar-Ar Age

800 700 600 500 400 300 4.49

23oC/Ma 4.50

4.51

4.52

4.53

4.54

4.55

4.56

4.57

Time, Gyr B.P. Fig. 11. Measured Ar–Ar age (dark circle), Ar–Ar age increased by 20 Ma to correct for probable error in 40K decay coefficients (light circle), and measured I–Xe age (diamond) are plotted against closure temperatures. The open rectangle represents the approximate initial temperature range of 1100 ± 60 °C, and the shaded rectangle represents a cooling rate of 1000 °C/Ma determined from NWA 176 metal (Liu et al., 2001), and neither are associated with a defined age. The dashed cooling curve begins 4556 Ma ago and is consistent with the measured Ar–Ar and I–Xe ages, but not with the measured Ni cooling rate. Cooling that began at the time of the I–Xe age of 4544 Ma (dot-dash curve) also is not consistent with all data. However, increasing the Ar–Ar age by 20 Ma does permit a cooling history consistent with all three data sets (solid curve), and implying impact heating of the parent body at this time. Calculated cooling rates apply to a temperature of 700–800 K.

unrealistic high I–Xe closure temperature and a cooling rate much slower than that indicated by the metal (e.g., dotdashed curve). A third, more likely cooling history (solid line in Fig. 11) began 4544 Ma ago, cooled through the temperature of Ni closure in metal at a rate of 1000 °C/Ma, and agrees with the I–Xe age. This cooling curve does not pass through the measured Ar–Ar age. However, if we again invoke a bias in 40K decay parameters and increase the Ar–Ar age by 20 Ma, the Ar–Ar age becomes completely consistent with this third cooling history for NWA 176. This rapid cooling history for NWA 176 seems the most consistent with all available data, and could be attained in a silicate parent body only a few km in radius. The I–Xe age is slightly younger than determined formation times of most parent asteroids and suggests that NWA 176 formed in a later impact by melting and mixing. Liu et al. (2001) list three possible origins for NWA 176: impact disruption and reassembly of a partly melted parent body; impact melting of a chondritic parent; or strong interior metamorphism. Given the fast metal cooling rate and the fact that even the decay-corrected Ar–Ar age of 4544 Ma is younger than asteroidal parent body formation times, the third option seems unlikely. For either of the other two options, the Ar–Ar and the I–Xe data suggest that the impact event occurred 4544 Ma ago. This interpretation also is fully consistent with the suggestion that 40 K decay coefficients in common use are incorrect, and that K–Ar ages of 4.5 Ga should be increased by 20 Ma.

5. CONCLUSIONS The Ar–Ar age for Sombrerete is 4541 ± 12 Ma, and the absolute I–Xe age is 4561.9 ± 1.0 Ma, relative to an age of 4562.3 ± 0.4 Ma (Gilmour et al., 2009) for the Shallowater standard. Our preferred interpretation is that the slightly younger Ar–Ar age does not represent an extended period of parent body metamorphism nor impact heating, but rather reflects an error in the 40K decay parameters in current use. Although the Ar–Ar age is within two-sigma of the I–Xe age, an upward adjustment of 20 Ma in the Ar–Ar age brings these two ages into complete agreement. Closure of both chronometers 4562 Ma ago is consistent with the model of Ruzicka et al. (2006) that Sombreretre formed early and cooled quickly. The Ar–Ar ages for Portales Valley samples #1 and #2 are 4477 ± 11 Ma and 4458 ± 16 Ma, respectively, whereas the I–Xe age is 4559.9 ± 0.5 Ma, relative to an age for Shallowater of 4562.3 ± 0.4 Ma. The Ar–Ar ages give no evidence of later heating events, as might be inferred from disturbance observed in some other chronometer data (Chen et al., 2000; Papanastassiou et al., 2001). The difference in the Ar–Ar and I–Xe ages are too large to be produced by errors in the 40K decay parameters. We conclude that the I–Xe age dates the approximate time of formation of the silicate–metal assemblage by an impact event into an already hot parent body, as suggested by Ruzicka et al. (2005). After this event, Portales Valley was quickly quenched to near or below the I–Xe closure temper-

Ar–Ar and I–Xe Ages Metal-Rich Meteorites

ature (1045 °C), then cooled much more slowly (at 4 °C/ Ma) through the Ar–Ar closure temperature for PV-1 (600 °C) and PV-2 (500 °C). This cooling history and differences in Ar diffusion data for PV-1 and PV-2 also explain the 20 Ma difference in measured Ar–Ar ages between the two PV samples. Increasing the Ar–Ar ages by 20 Ma to accommodate errors in the 40K decay coefficients changes the cooling history only slightly. This thermal history for Portales Valley is consistent with cooling rates determined from Ni concentration profiles in the metal phase reported by several labs and the conclusion of Ruzicka et al. (2005) that the silicate-metal mixture likely formed during impact heating and mixing, followed by slow cooling. Our preferred Ar–Ar age for NWA 176 is 4524 ± 13 Ma, and our preferred I–Xe age is 4544 Ma. The difference in these ages cannot be explained by any cooling history that is also consistent with the reported Ni cooling rate for the metal of 1000 °C/Ma (Liu et al., 2001). If we increase the Ar–Ar age by 20 Ma, justified by a probable error in the 40K decay coefficients, then the Ar–Ar and I–Xe ages come into agreement. This suggests that mixing of silicate and metal in NWA 176 occurred 4544 Ma ago during impact heating, followed by rapid cooling, as suggested by Liu et al. (2001). For all meteorite data reported here we conclude that an upward adjustment in Ar–Ar ages by 20 Ma results in better agreement with I–Xe ages and thermal histories suggested by other workers. An increase in K–Ar ages of 22 Ma for a 4.5 Ga-old sample based on biases in the 40 K decay coefficients in current use was suggested by Vogel and Renne (2008). A 20 Ma bias in a 4.5 Ga-old sample would imply that the half life of 40K decay to 40Ar is 0.5% longer than the value in current use. However, K40 decays by two modes with different half-lives (to 40Ar and 40Ca), and it is not obvious in which decay mode such an error might occur. In addition to NWA 176 and Portales Valley, some other meteorites consisting of silicate-metal mixtures give evidence for formation by early impact melting of a portion of their parent bodies (Mittlefehldt et al., 1998). A major impact undoubtedly was responsible for various chronometer resetting and mixing 3.7 Ga ago in some IIE iron-silicate meteorites (Bogard et al., 2000 and references therein). Benedix et al. (2000) suggested formation of IAB iron-silicate meteorites involved impacts, and Takeda et al. (2000) described heterogeneous melting and differentiation of the Caddo Co. IAB meteorite. Argon–Ar ages of several IAB meteorites show a wide range from 4.54 Ga to several hundreds of Ma younger, suggesting slow, variable cooling in the parent body (Bogard et al., 2005; Vogel and Renne, 2008). The brecciated nature of mesosiderites, along with their younger Ar–Ar ages and very slow cooling rates indicate early parent body disruption, mixing and reassembly (Bogard and Garrison; 1998; Scott et al., 2001). Benedix et al. (2008) described a meteorite formed by impact melting of L-chondrite material 4.46 Ga ago, Weirich et al. (2008) reported an impact melt age of >4.5 Ga for another L-chondrite, and Dixon et al. (2004) reported early, impact reset Ar–Ar ages for some LL chon-

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drites. These chondrite impacts, however, were likely on a smaller scale compared to early impacts involving parent bodies of metal-rich meteorites. Likely the role of impact heating and possible disruption and partial reassembly of meteorite parent bodies was an important process in the early solar system (Scott 2002), but one about which we currently have only limited chronological information. ACKNOWLEDGEMENTS This research was supported by NASA’s Cosmochemistry Program. We thank those individuals identified in Section 2 for furnishing samples. We appreciate helpful reviews that improved the paper by C. Hohenberg, T. Swindle, J. Gilmour, and T. Ireland.

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