40Ar–39Ar dating of plagioclase grain size separates from silicate inclusions in IAB iron meteorites and implications for the thermochronological evolution of the IAB parent body

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Geochimica et Cosmochimica Acta 72 (2008) 1231–1255 www.elsevier.com/locate/gca

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Ar–39Ar dating of plagioclase grain size separates from silicate inclusions in IAB iron meteorites and implications for the thermochronological evolution of the IAB parent body Nadia Vogel *, Paul R. Renne Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA Received 25 January 2007; accepted in revised form 3 December 2007; available online 28 January 2008

Abstract In order to better constrain the thermochronological evolution of the IAB parent body we performed a 40Ar/39Ar age study on individual silicate inclusions of the IAB irons Caddo County, Campo del Cielo, Landes, and Ocotillo. In contrast to earlier studies, several plagioclase separates of different grain sizes and quality grades were extracted from each inclusion to reduce the complexity of the age spectra and study the influence of these parameters on the Ar–Ar ages. In nearly all inclusions we found significantly different Ar–Ar ages among the separates (Caddo County: 4.472 ± 0.02–4.562 ± 0.02 Ga; Campo del Cielo 2: 4.362 ± 0.04–4.442 ± 0.03 Ga; Landes 2: 4.412 ± 0.05–4.522 ± 0.04 Ga; Ocotillo: 4.382 ± 0.04–4.462 ± 0.03 Ga). These ages were calculated using the new 40K decay constant published by [Mundil R., Renne P. R., Min K. and Ludwig K. R. (2006) Resolvable miscalibration of the 40Ar/39Ar geochronometer. Eos Trans. AGU 87, Fall Meet. Suppl., Abstract V21A-0543]. The ages did not systematically correlate with the respective grain size of the separate as expected, i.e., smaller grains did not necessarily show younger ages due to later closure to Ar diffusion or easier re-opening of the system in the course of a reheating event compared to larger grains. Based on the large range of Ar–Ar ages we suggest that the individual inclusions are composed of silicate grains from different locations within the IAB parent body. While some grains remained in a hot (deep) environment that allowed Ar diffusion over an extended time period—in some cases combined with grain coarsening—, others cooled significantly earlier (near surface) through the K/Ar blocking temperature. These different grains where brought together during an impact followed by mixing and reassembly of the debris as proposed by Benedix et al. [Benedix G. K., McCoy T. J., Keil K. and Love S. G. (2000) A petrologic study of the IAB iron meteorites: constraints on the formation of the IAB-Winonaite parent body. Meteorit. Planet. Sci. 35, 1127–1141]. Due to rapid cooling after the impact some of the age differences among the grains could be preserved. Based mainly on our Caddo County Ar–Ar age information, the IAB parent body was destroyed by impact and reassembled between 4.5 and 4.47 Ga. However, IAB silicate Ar–Ar ages should depend much more on the pre- and post-impact cooling rate and burial depth than on the time of the actual impact. This is supported by a compilation of our and literature IAB and winonaite Ar–Ar ages ranging smoothly from the time of accretion of the chondritic IAB parent body down to the time of its final cooling through the K–Ar blocking temperature after impact and reassembly, instead of showing a peak in Ar–Ar ages at the time of the destructive impact.  2007 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

* Corresponding author. Present address: Institute of Physics, Sidlerstrasse 5, University of Berne, 3012 Berne, Switzerland. E-mail address: [email protected] (N. Vogel).

0016-7037/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2007.12.001

Considerable controversy exists about the origin of IAB irons. In contrast to other major iron meteorite groups, IABs did not form by fractional crystallization of a slowly cooling asteroidal core (see Mittlefehldt et al., 1998, for a comprehensive review; Wasson and Kallemeyn, 2002). For example it is

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difficult to explain the broad ranges of Ni, Ge, and Ga concentrations in IAB metal as a result of fractional crystallization. Also the presence of micron- to cm-sized silicate inclusions with roughly chondritic chemical compositions (e.g., Bild, 1977; Mittlefehldt et al., 1998; Olsen and Schwade, 1998; Takeda et al., 2000) and abundant primordial and radiogenic noble gases (e.g., Jentsch and Schultz, 1987; Niemeyer, 1979a; Palme et al., 1991; Takeda et al., 2000) in IAB silicates contradicts slow cooling of the clasts under core conditions. Various formation models for IAB iron meteorites have been published, of which recent ones were presented by Benedix et al. (2000), Takeda et al. (2000), and Wasson and Kallemeyn (2002). Benedix et al. (2000) favor impact disruption of a partially differentiated chondritic body followed by mixing and gravitational reassembly of the debris. Takeda et al. (2000) propose local incomplete differentiation due to internal heating of a chondritic body. Wasson and Kallemeyn (2002) prefer IAB formation by impacts on one (or more) porous chondritic body (or bodies) forming localized metallic melt pools peppered with silicate debris from the host asteroid. Also, there are a number of studies, in which radiometric dating of IAB silicate inclusions was used to elucidate the thermochronological evolution of the IAB parent body. I–Xe (e.g., Niemeyer, 1979b), Sm–Nd (e.g., Liu et al., 2002; Stewart et al., 1996), and Rb–Sr (e.g., Burnett and Wasserburg, 1967; Liu et al., 2002, 2003) radioisotopic ages of IAB silicates are often identical with or close to the time of the formation of first solids in the solar system 4.567 Ga ago. In contrast, literature K–Ar and Ar–Ar ages of bulk IAB silicate inclusions span a fairly large age range from 4.52 Ga down to 4.3 Ga and lower. These ages often do not reflect the time of primary closure of the K/Ar system in the mineral assemblage analyzed, but are affected by secondary events like parent body metamorphism, impact resetting, or loss of radiogenic 40Ar (40Ar*) due to weathering. Additional complexity of Ar–Ar age spectra is often caused by loss and/or redistribution of Ar isotopes during neutron irradiation of the samples (McDougall and Harrison, 1999) and by the presence of different K-bearing mineral phases with different Ar diffusion coefficients in the mineral assemblage used for dating (e.g., Bogard, 1995). Additionally, if a K-bearing phase has a large range of grain sizes, different behavior with respect to Ar retention can be expected (Bogard, 1995; Dodson, 1973). We present 40Ar/39Ar data from individual silicate inclusions of the IAB iron meteorites Caddo County (CC), Campo del Cielo (CdC1, CdC2), Landes (L1, L2), and Ocotillo (O). In order to reduce the complexity of the Ar–Ar age spectra and thus contribute to a better understanding of the thermochronological evolution of the IAB parent body we analyzed plagioclase separates instead of commonly used bulk silicates. By analysis of different quality grades and grain size separates effects of these parameters on the age spectra were studied. 2. EXPERIMENTAL 2.1. Sample separation Samples were obtained as already separated inclusions (Caddo County, Campo del Cielo; Fig. 1a–c) or as silicate

inclusions embedded in metal groundmass (Landes, Ocotillo; Fig. 1d–f). In the latter case, small parts of the inclusions were removed from the specimens using a dentist drill and stainless steel tools. The inclusions consisting mainly of olivine, pyroxene, plagioclase, sulfides, and metal were carefully crushed to their natural grain size spectra in an agate mortar. Lithological differences within the inclusions, i.e., darker and lighter areas, were observed in the case of Caddo County, Campo del Cielo 2, and Landes 2 (arrows in Fig. 1). However, due to the scarcity of sample material the inclusions were not further subdivided. Plagioclase separates of two, in the case of Caddo County three, different grain sizes (l, large; m, medium; s, small) were obtained by sieving, magnetic and density separation (not Campo del Cielo, see below), and—most importantly— careful hand picking. During this final step, two quality grades were distinguished depending on the relative amounts of impurities (I = smaller amount of impurities than II), i.e., mineral inclusions and adhering material as, e.g., pyroxene and sulfides (Fig. 2). Campo del Cielo separates were obtained by hand picking only and some of them were split into two aliquots (a, b) and analyzed separately. The Caddo County grade II separates were additionally treated with diluted HF to remove surface staining from the grains. Digital photographs (Fig. 2) were taken from all separates to determine grain lengths and widths. Approximate grain dimensions and sample weights are listed in Table 1. 2.2.

40

Ar/39Ar analytical methods

2.2.1. Irradiation For irradiation the separates were loaded in Al disks as described by Renne et al. (1998). As flux monitors, grains of the Hb3gr hornblende (Jourdan and Renne, 2007) were used. To monitor nucleogenic interferences, Fe-doped KAlSiO4 glass and optical grade CaF2 were loaded in each disk. Ratios for interference corrections deduced from these samples are listed in Table 2. The samples were irradiated in three different batches at the CLICIT (Cd-lined in-core irradiation tube) facility of the Oregon State University (OSU) TRIGA Reactor for 200 h (Campo del Cielo, #311NV), without Cd-shielding at the OSU TRIGA Reactor for 300 h (Ocotillo, #314NV), and with Cd-shielding at the McMaster Nuclear Reactor for 200 h (Caddo County, #320NV-A; Landes, #320NV-B). 2.2.2. Ar analysis and data reduction Argon was extracted from the separates by stepwise heating with a continuous wave Synrad CO2 laser using an expanded beam to assure uniform heating of the samples. After cleaning the gas by admission to SAES getters and a cryogenic condensation trap, the argon isotopic composition was measured in static mode on a MAP 215C spectrometer described, e.g., in Nomade et al. (2005) and Renne et al. (1998). The neutron fluence (J) was monitored by analyzing single grains of Hb3gr from four to five locations within each disk. Resulting J values were averaged for each disk (see Appendix A) as no significant variabilities within the disks were detected. After every three sample steps

40

Ar–39Ar dating of IAB iron plagioclase separates

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Fig. 1. Photographs of silicate inclusions of Caddo County (a), Campo del Cielo 1 (b), Campo del Cielo 2 (c), Landes 1 (d), Landes 2 (e), and Ocotillo (f). Macroscopically visible lithological differences (i.e., lighter and darker areas) within inclusions are marked by arrows.

two blanks were measured. Usually a curve fit to the blank data to correct the individual sample steps was used. In some cases sample steps were corrected with the mean value of the bracketing blanks. In those cases the assigned blank uncertainties were determined from the long term variability of the blanks. Typical blank values and their long term variability for 40Ar, 39Ar, 38Ar, 37Ar, and 36Ar in moles were (1.2 ± 0.3) · 10 16, (2.4 ± 1.0) · 10 18, (1.2 ± 0.7) · 10 18, (3.7 ± 1.3) · 10 18, and (4.0 ± 1.9) · 10 18, respectively. Their contributions to an average sample step were typically well below 1% except for 36Ar, for which the blank contribution could reach several %. Several air aliquots were analyzed during each day to constantly monitor mass discrimination. The latter was on average 1%/amu favoring the light isotope and very stable over months. Ar isotopic data corrected for mass discrimination, blanks, and radioactive decay are given in Appendix A. Measurements, data reduction, isochrons, and apparent age spectra were obtained using the program Mass Spec 7.4 developed at the Berkeley Geochronology Center. Isochron and plateau ages given in Table 1 and Figs. 3–6 were calculated using an age of 1.074 ± 0.005 Ga for Hb3gr (Jourdan and Renne,

2007) and the 40K decay constants and 40K/K ratio given in Steiger and Jaeger (1977). ‘‘Revised plateau ages’’ in Table 1 and Figs. 7 and 8 were recalculated using new 40K decay constants deduced from a new total 40K decay constant and a revised age of the Fish Canyon sanidine (FCs) standard of 28.28 Ma given in Mundil et al. (2006), as well as a 40 Ar*/40K ratio for FCs of 1.6407 · 10 3 (Jourdan and Renne, 2007). Unless stated otherwise, age uncertainties are 2r and include analytical uncertainties, all uncertainties associated with neutron fluence, blank, neutron interferences, and mass discrimination. Not included are uncertainties of the monitor age and 40K decay constants, which would, if included, add another 1–2% to the age uncertainty. 3. RESULTS 3.1. Presentation and interpretation of

40

Ar/39Ar age data

Although the predominant part of 40Ar in our samples is radiogenic, a small part can, e.g., be cosmogenic with 40 Ar/36Ar 6 1, and/ or trapped ArQ with (40Ar/36Ar)Q < 0.12 (Ott, 2002, and references therein). Also terrestrial

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Fig. 2. Plagioclase separates of Caddo County, Landes 2, Ocotillo, Campo del Cielo 1, and Campo del Cielo 2. Pl, plagioclase large; Pm, plagioclase medium; Ps, plagioclase small. Roman figures I, II refer to quality grades (I contains less impurities than II). Images are about 2 · 2 mm for Caddo County and Landes, and 1.3 · 1.3 mm for Ocotillo and Campo del Cielo separates. Average grain dimensions are given in Table 1.

atmospheric Ar (40Ar/36Ar = 298.6, Lee et al., 2006) is a common contaminant in meteorite samples. 40Ar from those reservoirs can be corrected for using the isochron approach to determine an initial 40Ar/36Ar ((40Ar/36Ar)ini) ratio and deduce for each extraction step the appropriate 40Ar

amount via the measured 36Ar to obtain an apparent age spectrum. Unless stated otherwise, isochrons are produced using all extraction steps except the ones containing <1% of the total 39ArK. While isochron plots are not shown, resulting (40Ar/36Ar)inis and isochron ages (excluding J

Table 1 Ages, grain dimensions, Ca/K ratios, and approximate sample amounts for Caddo County, Campo del Cielo, Landes, and Ocotillo plagioclase separates Isochron age [Ma]

±

(40Ar/36Ar)ini

±

Plateau age [Ma]

Revised plateau age [Ma]

± (incl. J unc.)

Grain length [lm]

Grain width [lm]

Caddo County CC-Pl-I CC-Pl-II CC-Pm-I CC-Pm-II CC-Ps-I CC-Ps-II

4454 4518 4528 4520 4515 4536

12 11 12 11 11 11

0.08 14.8 2.6 5.1 5.7 9.4

0.02 1.3 0.8 0.4 1 0.8

4450 4520 4530 4530 4510 4540

4472 4542 4552 4552 4532 4562

20 20 20 20 20 20

250 253 226 162 165 151

160 170 147 134 108 100

12 8 60 8 10 6

0.9 9 0.7 18 3.7 35

Campo del Cielo CdC1-Pl-IIa CdC1-Pl-IIb CdC1-Ps-IIa CdC1-Ps-IIb

4425 4430 4430 4423

17 20 30 17

1.4 6.8 11 8.3

0.7 0.7 3 1

4430 4460 4430 4400

4452 4482 4452 4422

30 90 30 30

247 247 149 149

143 143 92 92

22 27 17 21

2.5 1.5 0.9 0.7

CdC2-Pl-I CdC2-Pl-IIa CdC2-Pl-IIb CdC2-Ps-I

4420 4410 4350 4420

19 30 30 60

0.4 0.09 0.4 22

1 0.06 0.18 7

4420 4410 4340 4410

4442 4432 4362 4432

30 30 40 50

250 228 228 158

171 131 131 112

14 14 22 15

0.9 0.7 0.5 0.3

Landes L1-P L2-Pl-I L2-Pl-II L2-Ps-I L2-Ps-II-high L2-Ps-II-low

472 4390 4467 4498 4471 4424

3 30 12 19 11 15

21 2.2 0.12 0.8 2.3 28

0.1 0.6 0.07 0.2 0.2 7

470 4390 4470 4500 4470 4420

474 4412 4492 4522 4492 4442

30 50 20 40 20 20

149 206 277 149 150 150

93 136 162 105 100 100

7 112 55 58 24 24

0.1 0.05 1 0.07

4350 4387

30 12

0.01 0.06

30 17

4360 4440 4370 4360 4420

4382 4462 4392 4382 4442

40 30 30 50 30

163 193 193 87 110

107 124 124 55 71

244 43 43 119 53

0.029

4310 4423

0.06 0.93 0.93 2.8 0.05

Sample

Ar–39Ar dating of IAB iron plagioclase separates

0.5 0.02

Approximate sample weight [mg]

40

Ocotillo O-Pl-I O-Pl-II-high O-Pl-II-low O-Ps-I O-Ps-II

Ca/K

1.4

0.18 0.02 0.13

Age uncertainties are 2r. Plateau age uncertainties include analytical and J uncertainties but exclude the uncertainty associated with the age of the monitor. Revised plateau ages were calculated using new the 40K decay constant of (Mundil et al., 2006). See text for further information. Grain lengths and widths represent median values with uncertainties usually not above 30%.

1235

1.21E 02 1.21E 02 1.24E 02

3.66E 05 5.35E 05 4.85E 05

6.67E 04 5.37E 03 8.71E 04

3.88E 04 5.76E 04 5.01E 04

uncertainties) are shown in the plots of the respective apparent age spectra (Figs. 3–6). A plateau within an age spectrum is assigned when three or more adjacent steps releasing at least 20% of the total 39ArK overlap within 2r excluding the J uncertainty, which is systematic when comparing age steps within one separate. Stepwise gas release patterns are interpreted as functions of their release temperatures. A plateau defines the time of the last major degassing of a sample when the mineral assemblage in question has cooled through its K/Ar blocking temperature. An increase of apparent ages during the first extraction steps as particularly prominent for Campo del Cielo (Fig. 5) is due to a loss of radiogenic 40Ar (40Ar*) from unstable lattice sites in response to weathering or mild metamorphic reheating of the sample. In the case of Campo del Cielo the transition to plateau steps correlates with an increase of the Ca/K ratio (deduced from the measured ratio 37ArCa/39ArK). This generally indicates argon degassing from a different, more retentive phase within the plagioclase. However, most of our separates show higher average Ca/K ratios than those common for IAB plagioclase. This is attributed to contamination of our separates with one (or more) Ca-rich mineral phase(s) like pyroxene or apatite. While this does not have a direct influence on the age of a separate, our Ca/K ratios cannot be used for deciphering different (usually less and more stable) lattice sites within the plagioclase as it is common practice for terrestrial and extraterrestrial rocks. As expected for separates with grain dimensions generally P100 lm, the spectra generally do not show effects of recoil redistribution of reactor produced 39ArK, i.e., there is no correlation of strongly increasing Ca/K ratios and decreasing ages for the highest extraction steps.

6.74E 04 3.34E 05 7.99E 04

3.2. Caddo County (Fig. 2a–f)

1.51E 05 4.39E 04 2.29E 05

6.50E 06 4.72E 06 2.31E 06

2.06E 05 4.60E 07 8.92E 05 2.74E 04 1.73E 04 2.85E 04 # 311NV # 314NV # 320NV

Uncertainties are 1r.

(38Ar/39Ar)K ± (39Ar/37Ar)Ca ± (38Ar/37Ar)Ca ± (36Ar/37Ar)Ca Irradiation

Table 2 Correction factors for interfering isotopes determined from optical grade CaF2 and Fe-doped KAlSiO4 co-irradiated with each sample batch

±

(40Ar/39Ar)K

±

N. Vogel, P.R. Renne / Geochimica et Cosmochimica Acta 72 (2008) 1231–1255

1.70E 05 2.28E 06 3.81E 06

1236

The three grade I Caddo County separates show flat well behaved spectra without significant loss of 40Ar* except in the very first extraction steps usually contributing 61% of the total 39ArK released. The (40Ar/36Ar)ini ratios are low, between 0 and 6. Plateau ages are identical within uncertainties for CC-Pm-I (4.530 ± 0.02 Ga) and CC-Ps-I (4.510 ± 0.02 Ga), while the age for CC-Pl-I is significantly lower (4.450 ± 0.02 Ga). The three grade II HF treated separates show identical plateau ages within uncertainties that range between 4.520 and 4.540 Ga with (40Ar/36Ar)ini values of 5–15. Their gas release spectra look slightly disturbed. This is attributed to the large amounts of sample material (compare Fig. 2d–f and Table 1) that hindered gas release as a function of release temperature only, as grains lying at the bottom of the sample pit were shielded from the laser beam by the uppermost grain layers. The slightly lower (40Ar/36Ar)ini ratios of the grade I separates compared to the grade II ones might be due to a slightly higher degree of weathering of the latter ones. Our average Ca/K ratios (see Table 1) agree with literature Ca/ K ratios for Caddo County plagioclase of 5 (Benedix et al., 2000; Bogard et al., 2005) over large parts of gas extraction. Only the last extraction steps show significantly higher Ca/K ratios indicating the degassing of a Ca-rich

40

Ar–39Ar dating of IAB iron plagioclase separates

a

b

Caddo County Plagioclase large I Isochron age = 4454 ± 12 Ma 40Ar/36Ar Intercept = 0.08 ± 0.02 Plateau age incl. J unc. = 4450 ± 20 Ma

1237

Caddo County Plagioclase large II Isochron age = 4518 ± 11 Ma 40Ar/36Ar Intercept = 14.8 ± 1.3 Plateau age incl. J unc. = 4520 ± 20 Ma

30 20

20 15 10

10 0 4600

5

4700

4454 ± 5 Ma (MSWD = 1.51)

4400

B

D

C

E

G

F

I

H

4523 ± 3 Ma (MSWD = 1.18)

4600

J

AC

K

4200

4500

4000

AJ

LM PQ R V XY Z AA ADAEAFAHAK D E G HI J K NO AB STW AGAM C F AN

4400

A 3800 10

c

20

30

40

50

60

70

80

10

90

d

Caddo County Plagioclase medium I Isochron age = 4528 ± 12 Ma 40Ar/36Ar Intercept = 2.6 ± 0.8 Plateau age incl. J unc. = 4530 ± 20 Ma

20

30

40

50

60

70

80

90

Caddo County Plagioclase medium II Isochron age = 4520 ± 11 Ma 40Ar/36Ar Intercept = 5.1 ± 0.4 Plateau age incl. J unc. = 4530 ± 20 Ma

200

20 15

100

10

0

5

4528 ± 8 Ma (MSWD = 0.88)

4700

4650 4526 ± 2 Ma (MSWD = 1.45)

4600

4550

4500

K O Q RS H F G LM P I 4450 E

C

B

H

E

D

10

e

20

30

40

50

60

70

80

XY

Z

AC AJ AA AEAK AB

KL

I

F

4400

J

G

T UV W

4350

90

10

f

Caddo County Plagioclase small I

60

70

80

90

25 20 15 10

0

4700

50

Caddo County Plagioclase small II

Ca/K

20

40

Isochron age = 4536 ± 11 Ma 40Ar/36Ar Intercept = 9.4 ± 0.8 Plateau ages incl. J unc. = 4540 ± 20 Ma

60 40

30

Ca/K

Isochron age = 4515 ± 11 Ma 40Ar/36Ar Intercept = 5.7 ± 1.0 Plateau age incl. J unc. = 4510 ± 20 Ma

20

5

4700

4500 D

4514 ± 3 Ma (MSWD = 0.86) E

G

H

I

K

J

F

L M

N OPR T S

4400

Age [Ma]

Age [Ma]

4537.9 ± 1.5 Ma (MSWD = 1.16) 4600

4600 C 4500

E

AFAHAK J K L M N PRS T UVWXYZ ABAD AGAJAL AC AE AA AIAX 4535.9 ± 1.6 Ma (MSWD = 0.86)

F GH I

D 4400

10

20

30

40

50

60

70

80

90

10

Cumulative release Ar-39 [%]

20

30

40

50

60

70

80

90

Cumulative release Ar-39 [%]

Fig. 3. Caddo County apparent age spectra with Ca/K ratios. Uncertainties of individual age steps and those given with the plateau ages are 2r excluding uncertainties of the irradiation parameter J. Isochron ages and their 40Ar/36Ar intercepts as well as plateau ages including J uncertainties are given in the text field of each plot.

mineral phase, probably pyroxene. CC-Pm-I is exceptional as most of the extraction steps have Ca/K ratios distinctly higher than the literature value (average Ca/K of CC-PmI  60) and increase to ratios of up to 250 in the last third of gas extraction. We attribute this to degassing of a Ca-rich mineral phase other than pyroxene which does not directly influence the age of the steps, possibly chlorapatite. As the three HF treated grade II separates show slightly lower average Ca/K ratios than the grade I separates, we suggest that the HF treatment might have reduced the amount of adhering pyroxene or other Ca-rich mineral phases. Except for CC-Pl-I, all Caddo separates show identical 40 Ar/39Ar ages within uncertainties despite variable grain

sizes, sample amounts, quality grades, Ca/K, and initial Ar/36Ar ratios. There is no obvious explanation for the younger age of CC-Pl-I compared to the other Caddo samples. We speculate that during hand picking of the largest clearest grains we might have concentrated grains from the light area of the inclusion (Fig. 1a) with possibly different age information stored than in the other parts of the inclusion. 40

3.3. Campo del Cielo 1 (Fig. 4a–d) and 2 (Fig. 4e–h) All Campo del Cielo separates show significant losses of Ar* over large parts of the spectra (P50 % of the total

40

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N. Vogel, P.R. Renne / Geochimica et Cosmochimica Acta 72 (2008) 1231–1255

a

b

Campo del Cielo 1 Plagioclase large IIa Isochron age = 4425 ± 17 Ma 40Ar/36Ar Intercept = 1.4 ± 0.7 excluded steps = A-I Plateau age incl. J unc. = 4430 ± 30 Ma

Campo del Cielo 1 Plagioclase large IIb Isochron age = 4430 ± 20 Ma 40Ar/36Ar Intercept = 6.8 ± 0.7 excluded steps = A-I Plageau age incl. J unc. = 4460 ± 90 Ma

40 30 20

40 20

10 0 4400

H

4000

4600

K L MN

J

I

G

E

4200

4425 ± 8 Ma (MSWD = 0.84)

F

D

3600 3200

3400 3000

A 10

c

F

C

B 2800

G

3800

C

20

30

40

50

60

70

80

D E B A 10

90

Isochron age = 4430 ± 30 Ma 40Ar/36Ar Intercept = 11 ± 3 excluded steps = A-E, M Plateau age incl. J unc. = 4430 ± 30 Ma

20

30

40

50

60

70

80

90

Campo del Cielo 1 Plagioclase small IIb

d

Campo del Cielo 1 Plagioclase small IIa

J K L M N I H 4460 ± 90 Ma (MSWD = 3.33)

Isochron age = 4423 ± 17 Ma 40Ar/36Ar Intercept = 8.3 ± 1.0 excluded steps = A-E Plateau age incl. J unc. = 4400 ± 30 Ma

20 10

80 60 40 20

0 M F 4200 E

D

G H I J K L

4400

M 4434 ± 12 Ma (MSWD = 1.42)

4000

E

C

3800

H

3200 10

20

30

40

50

60

70

80

4400 ± 20 Ma (MSWD = 1.14)

A 10

90

f

Campo del Cielo 2 Plagioclase large I Isochron age = 4420 ± 19 Ma 40Ar/36Ar Intercept = 0.4 ± 1.0 excluded steps = A-H,N,O Plateau age incl. J unc. = 4420 ± 30 Ma

20

30

40

50

60

70

4200

15

20

10

10

5

4200

D C 10

g

20

30

40

50

60

70

80

K

90

H

J

10

20

h

Campo del Cielo 2 Plagioclase large IIb

O 4406 ± 15 Ma (MSWD = 0.85)

Isochron age = 4410 ± 30 Ma 40Ar/36Ar Intercept = 0.09 ± 0.06 excluded steps = A-O Plateau age incl. J unc. = 4410 ± 30 Ma 30

40

50

60

70

80

90

Campo del Cielo 2 Plagioclase small I Isochron age = 4420 ± 60 Ma 40Ar/36Ar Intercept = 22 ± 7 excluded steps = A-F Plateau age incl. J unc. = 4410 ± 50 Ma

40

Ca/K

20

RS

30 20

Ca/K

Isochron age = 4350 ± 30 Ma 40Ar/36Ar Intercept = 0.4 ± 0.2 excluded steps = A-I Plateau age incl. J unc. = 4340 ± 40 Ma

L M N

3400 F E G 3000

O

E

Q

P

I

3800

N

3800

3000

90

30

G H I J K L M 4420 ± 15 Ma (MSWD = 0.03)

F

80

Campo del Cielo 2 Plagioclase large IIa

4600

3400

J KL

B

A

e

I

CD

3600 B 3400

G

F

10

0

F

4000 3600 3200

J

K L

MN

H

4400

E

4000

D C

D

G

H

IJ K M L N

F 4410 ± 40 Ma (MSWD = 0.96)

3600 B

C 10

P O

I 4340 ± 30 Ma (MSWD = 0.89)

E

A B

G

Age [Ma]

Age [Ma]

4400

20

30

40

50

60

70

Cumulative release Ar-39 [%]

80

90

10

20

30

40

50

60

70

80

90

Cumulative release Ar-39 [%]

Fig. 4. Campo del Cielo 1 and 2 apparent age spectra with Ca/K ratios. Uncertainties of individual age steps and those given with the plateau ages are 2r excluding uncertainties of the irradiation parameter J. Isochron ages and their 40Ar/36Ar intercepts as well as plateau ages including J uncertainties are given in the text field of each plot. Also given are steps that were excluded from isochron production due to loss of 40Ar*. See text for further information.

40

Ar–39Ar dating of IAB iron plagioclase separates

a

b

Landes 1 Plagioclase Isochron age = 472 ± 3 Ma 40Ar/36Ar Intercept = 20.7 ± 0.1 Plateau age incl. J unc. = 470 ± 30 Ma

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Landes 2 Plagioclase large I Isochron age = 4390 ± 30 Ma 40Ar/36Ar Intercept = 2.2 ± 0.6 Plateau age incl. J unc. = 4390 ± 50 Ma

60 40

200 150 100 50

20 D

4390 ± 40 Ma (MSWD = 0.46)

2500 2000

4800 E

4600

1500 F

1000

4400

470 ± 30 Ma* (MSWD = 10.18)

G

I

H

500

J

K

L

B

469 ± 3 Ma (MSWD = 0.34) 10

20

c

30

40

50

60

70

80

C

4200

90

K

J

H

L

I

10

20

d

Landes 2 Plagioclase large II Isochron age = 4467 ± 12 Ma 40Ar/36Ar Intercept = 0.12 ± 0.07 Plateau age incl. J unc. = 4470 ± 20 Ma

G

F

E D

30

40

50

60

70

80

90

Landes 2 Plagioclase small I Isochron age = 4498 ± 19 Ma 40Ar/36Ar Intercept = 0.8 ± 0.2 Plateau age incl. J unc. = 4500 ± 40 Ma

60 40

200 100

20 5200 4500 ± 30 Ma (MSWD = 0.86)

4600 4467 ± 5 Ma (MSWD = 0.20)

4550

4800

4500 4450 4400

C F D

G H I

JK

M L

N

4400 B

R

O PQ

D

C

G

E

H

F

I

O

4000

4350 10

e

20

30

40

50

60

70

80

10

f

40

50

60

70

80

90

100 50

Ca/K

4

30

Isochron age = 4471 ± 11 Ma 40Ar/36Ar Intercept = 2.3 ± 0.2 Plateau age incl. J unc. = 4470 ± 20 Ma

Ca/K

Isochron age = 4424 ± 15 Ma 40Ar/36Ar Intercept = 28 ± 7 Plateau age incl. J unc. = 4420 ± 20 Ma

20

Landes 2 Plagioclase small II, high

6

4600

KL

90

Landes 2 Plagioclase small II, low

M

0

2

4500

4400

4424 ± 6 Ma (MSWD = 0.60)

D E

F

G

H

I

J K

M

L

Age [Ma]

Age [Ma]

4600 4473 ± 6 Ma (MSWD = 1.27) 4500 P

Q

4400 NO

U

S

R

T

4300 4300 10

20

30

40

50

60

70

80

90

Cumulative release Ar-39 [%]

10

20

30

40

50

60

70

80

90

Cumulative release Ar-39 [%]

Fig. 5. Landes 1 and 2 apparent age spectra with Ca/K ratios. Uncertainties of individual age steps and those given with the plateau ages are 2r excluding uncertainties of the irradiation parameter J. Isochron ages and their 40Ar/36Ar intercepts as well as plateau ages including J uncertainties are given in the text field of each plot.

39

ArK released). Generally, isochron-like structures with reasonable (40Ar/36Ar)ini ratios could be obtained from few high temperature steps only. Steps excluded from the isochrons are indicated in the text fields of the plots. Resulting plateaux are often small and the ages suffer from relatively large uncertainties. Due to technical problems CdC1-Pl-I, CdC1-Ps-I, and CdC2-Ps-II were not homogeneously degassed, thus, these data are not shown. All Campo del Cielo 1 separates show identical plateau ages within uncertainties (CdC1-Pl-IIa: 4.430 ± 0.03 Ga, CdC1-Pl-IIb: 4.460 ± 0.09 Ga, CdC1-Ps-IIa: 4.430 ± 0.03 Ga, CdC1-PsIIb: 4.400 ± 0.03 Ga).

The small plateaux of CdC2-Pl-I, CdC2-Pl-IIa, and CdC2Ps-I are within uncertainties identical to the ages of CdC1, 4.420 ± 0.03 Ga, 4.410 ± 0.03 Ga, and 4.410 ± 0.05 Ga, respectively. CdC2-Pl-IIb shows a lower plateau age of 4.340 ± 0.04 Ga. The (40Ar/36Ar)ini ratios of the Campo del Cielo separates vary between 0 and 20, indicating variable degrees of atmospheric Ar in the samples. The average Ca/ K ratios of all Campo del Cielo separates range between 9 and 18, somewhat higher than literature (Wlotzka and Jarosewich, 1977) and our own microprobe CdC plagioclase Ca/K ratios of 5. This indicates variable amounts of one (or more) contaminating Ca-rich mineral phase(s) in the separates.

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a

b

Ocotillo Plagioclase large I Isochron age = 4350 ± 30 Ma 40Ar/36Ar Intercept = 0.06 ± 0.01 Plateau age incl. J unc. = 4360 ± 40 Ma

Ocotillo Plagioclase large II 80

1500

60

1000

40

500

20

0

5000 4360 ± 40 Ma (MSWD = 0.59)

4700

O N

4600

4442 ± 12 Ma (MSWD = 2.00) 4369 ± 13 Ma (MSWD = 1.71)

4500 4300

F

D

4200

G

E

4100

J K L H

3800

M

3900

I

C 10

20

c

30

40

50

60

70

80

E C

F

20

d

Ocotillo Plagioclase small I

M

N OP

30

40

50

60

70

80

90

Ocotillo Plagioclase small II

Ca/K

200 100

0 4600 CD

0

M

4422 ± 17 Ma (MSWD = 0.98)

Age [Ma]

Age [Ma]

4600 4400

4400

G

4200

I

H

4200

F E 4000 10

F

E

H

G

J

D

L

4360 ± 50 Ma (MSWD = 0.07) 20

30

40

50

60

J 70

I 4000

K 80

Ca/K

Isochron age = 4423 ± 17 Ma 40Ar/36Ar Intercept = 0.050 ± 0.02 Plateau age incl. J unc. = 4420 ± 30 Ma

200 100

L

J

Isochron age = 4387 ± 12 Ma 40Ar/36Ar Intercept = 0.93 ± 0.06 Plateau ages incl. J unc. = 4370 ± 30 Ma 4440 ± 30 Ma 10

Isochron age = 4310 ± 30 Ma 40Ar/36Ar Intercept = 2.8 ± 0.5 Plateau age incl. J unc. = 4360 ± 50 Ma

H I

D

B

90

K

G

90

K

C 10

20

Cumulative release Ar-39 [%]

30

40

50

60

70

80

90

Cumulative release Ar-39 [%]

Fig. 6. Ocotillo apparent age spectra with Ca/K ratios. Uncertainties of individual age steps and those given with the plateau ages are 2r excluding uncertainties of the irradiation parameter J. Isochron ages and their 40Ar/36Ar intercepts as well as plateau ages including J uncertainties are given in the text field of each plot.

CAI formation 4550

Caddo high

Age [Ma]

4500

Landes high Caddo low

4450

CdC1 high CdC2 high

Impact disruption and gravitational reassembly of Ocotillo high debris

Landes low

4400 Ocotillo low CdC2 low

4350

Diffusive loss of 40 Ar* due to weathering? Discrete thermal events?

Fig. 7. High and low average ages of Caddo County, Campo del Cielo 1 and 2, Landes 2, and Ocotillo (using new 40K decay constants—see text for further information). Also given is the time of formation of the first solids in the solar system as defined by Amelin et al. (2002), and an approximate time of impact disruption of the IAB parent body based on our Caddo ages. It is unclear if the low ages of CdC2 and Ocotillo represent discrete thermal events (e.g., local impacts) or diffusive loss of 40Ar* due to weathering.

3.4. Landes 1 (Fig. 4a) and 2 (Fig. 5b–f) Landes 1: Due to the very small fraction of plagioclase in this inclusion only a single separate could be produced and

this separate shows severe disturbance. An isochron-like structure was found by only using the four high temperature steps I–L, resulting in a (40Ar/36Ar)ini of 20. The resulting age spectrum points to impact heating of this

40

Ar–39Ar dating of IAB iron plagioclase separates

1241

o]

o

el

i lC

4600

[B

] de e] 2 [T ] po II o] o # [B ] ] [B 7 [B Ps -I -II ni* nty ] [N ) [N f o] #5 #2 CC -Pm-Pm tly CouII e [Nilla hed e] [B ty ty n B - n b c H ] CC CC Po ddo -Pl dbi dra (et a* [ I ounoun #E [N o y ed) Ca CC Wo MunPitts non -Ps do C o C I t n i d o] W CC CadCad -Ps- Cou etch [B y nt gh L2 do (un u d ts i Co -h b Ca Pit ll II -II I o] ] da Pl- Ps Pl-I [B Be ] en - K L2 L2 C1- a* [ I [N h elo i g Cd non -Pl iapo -hi l C a a I i e I I W CCCop Pl-I o d l-I s-I N] w P P [ e] O amp C1- C1- es l-I I-lo [B C Cd Cd and 2-P s-I s-II l-IIa -I s* L dC 2-P -P P -Ps orri IIb C L O C2 C2 M st P Cd Cd un C1I o M Cd -Plow L2 I-l l-I -P O Pl-I Ps-I O O-

m

Ca

4550

Age [Ma]

4500

4450

4400

CAI formation

Impact disruption and gravitational reassembly of debris

Ib

l-I

C

4350

o]

-P

2 dC

3

33

83

[B

n

io at

T t EE ei S d U

o] Diffusive loss [B 40 Ar* due to

of

weathering? Discrete thermal events?

Fig. 8. Compilation of literature and our own IAB and winonaite (marked with asterisks) Ar–Ar ages. All ages are calculated using the most recent monitor ages and the new 40K decay constants. See text for further information. Equivalent to Fig. 7, the time of formation of the first solids in the solar system (Amelin et al., 2002), and an approximate time of impact disruption of the IAB parent body based on our Caddo ages are given. [Bo], Bogard et al. (2005); [Be], Benedix et al. (1998); [T], Takeda et al. (2000); [N], Niemeyer (1979a).

sample (compare, e.g., Bogard and Garrison, 2003). The first age steps are comparably high indicating excess 40Ar redistributed by a shock event. A ‘‘plateau’’ of steps I and J indicates an age of 0.470 ± 0.03 Ga, identical to the age resulting from a forced plateau (labeled with an asterisk in Fig. 5a) from steps I to L and to the isochron age. Thus, this age is adopted as an upper limit for the time of the last major degassing of this sample by impact. The average Ca/ K ratio of 5 is in agreement with literature (Ca/K  7, Bunch et al., 1972) and our own microprobe analyses of Landes plagioclase. Landes 2: The age spectra of Landes 2 separates generally display large plateaux except for L2-Ps-II. Due to the small amounts of sample material the ages of the grade I separates are accompanied by rather large uncertainties. The plateau ages are quite variable: 4.390 ± 0.05 Ga (L2Pl-I), 4.470 ± 0.02 Ga (L2-Pl-II), 4.500 ± 0.04 Ga (L2-PsI), and two plateaux for L2-Ps-II of 4.420 ± 0.02 Ga and 4.470 ± 0.02 Ga (low and high temperature parts of the spectrum). To obtain the two latter plateaux two separate isochrons and (40Ar/36Ar)inis were produced from the respective age steps (see Fig. 5e and f). One common isochron and (40Ar/36Ar)ini using all extraction steps would have led to one plateau with an apparent age of 4.450 ± 0.02 Ga for L2-Ps-II. The initial 40Ar/36Ar ratios are generally low (0–3) indicating little atmospheric contamination, except for the (40Ar/36Ar)ini (28) of the low temperature part of L2-Ps-II. Ca/K ratios for Landes 2 separates range from about 16 to 75, significantly higher than the literature value, indicating again variable amounts of contaminating Ca-rich minerals within the separates. The Landes 2 plagioclase separates seem to contain age information about more than one degassing event. Ages do not correlate with grain size, Ca/K ratio, initial 40Ar/36Ar ratio or other easily controllable parameters. The most extreme ages however are connected to the lowest total 40Ar amounts degassed (compare Fig. 2g–j and Table 1), while

the intermediate ages occur in separates with larger 40Ar totals. We speculate that the latter separates are more homogenized in terms of age than the former ones. We emphasize that both Landes inclusions (1 and 2) were located within the same specimen less than 5 cm apart. The contrasting data impressively show how variable the age information can be from inclusions in close proximity to each other and how local the influence of shock resetting obviously is. 3.5. Ocotillo (Fig. 6a–d) Because of the small amount of sample material available the age uncertainties for the Ocotillo separates are relatively large. Also the apparent age spectra seem to be somewhat disturbed. The two grade I separates show plateau ages of 4.360 ± 0.04 Ga (O-Pl-I) and 4.360 ± 0.05 Ga (O-Ps-I). The grade II spectra O-Pl-II and O-Ps-II show higher ages of 4.440 ± 0.03 Ga and 4.420 ± 0.03 Ga, respectively. Additionally, O-Pl-II has a second plateau at low extraction temperatures with an age of 4.370 ± 0.03 Ga repeating the age information of the grade I separates. Initial 40Ar/36Ar ratios are between 0 and 3, thus significant degrees of terrestrial contamination can be excluded. The Ca/K ratios for our Ocotillo separates are much higher than the literature Ocotillo plagioclase ratio of 3 (Olsen and Schwade, 1998). While the grade II separates show average Ca/K ratios of 30, grade I separates are distinctly higher, 80 (O-Ps-I) and 160 (O-PlI). This suggests substantial contamination of the separates with one or more Ca-rich mineral phases, in particular for the grade I separates. Again, ages are not clearly correlated with grain size, Ca/K ratio, or (40Ar/36Ar)ini. It seems however that the two grade I separates with very low 40Ar totals (indicating small sample amounts) have the lower ages, while the larger grade II separates are older. Possibly also here do we see the

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effect of more or less homogenization of plagioclase grains with different age informations. 4. DISCUSSION 4.1. Complex age information in IAB silicate inclusions The studied IAB silicate inclusions display in part significant age ranges among the separates larger than the 2r plateau age uncertainties including the J uncertainties. Thus, these age differences are interpreted as representing different degassing events whose ages are stored within the inclusions. Indeed, heterogeneous ages had been expected as in the course of an impact small grains are supposed to be more readily reset than large ones (e.g., Bogard, 1995). Also, small grains are supposed to close later to Ar diffusion than large ones due to smaller diffusion dimensions (e.g., McDougall and Harrison, 1999), which would have allowed to deduce cooling rates using closure temperatures for grains of different sizes within one inclusion. However, none of the studied inclusions systematically shows this expected age—grain size correlation. In fact, in several cases (Caddo, CdC2, Landes 2) large grains even show particularly low ages. A potential reason for this might be grain coarsening during a metamorphic event. The ‘‘low age grains’’ within an inclusion might then have the largest influence on the age in separates consisting of very few plagioclase grains only, as homogenization of different ages is less probable here than in a separate consisting of very many grains. It is not easy to imagine that the significant 40Ar/39Ar age differences in the studied inclusions can develop within one small silicate inclusion embedded in a consolidated metal groundmass. However, silicate grains with different age information can have developed in different thermal environments within the parent body altogether and be brought together to form one inclusion later on. If not further reset, it is possible that such an inclusion has preserved the different age information until today. The impact disruption model presented by Benedix et al. (2000) does not only deliver a ready mechanism to mix liquid core metal and solid silicates, but also highly metamorphosed or even partially molten silicates from the interior of the parent body with solid silicates from cooler areas. The authors (Benedix et al., 2000, and references therein) report silicate–silicate mixing in several winonaites, which are almost certainly genetically related to IABs. Fig. 8 of the same publication suggests mixing of partial silicate melts with chondritic material for example in Caddo County. This process might also be an alternative explanation for the close spacial contact of coarse-grained gabbroic areas enriched in plagioclase and more chondritic winonaite-like material within Caddo County described by Takeda et al. (2000) and Bogard et al. (2005). Mineralogical variability within single IAB silicate inclusions, which might be explained by silicate–silicate mixing during disruption of the IAB parent body, has also been described by Olsen and Schwade (1998). They suggest that Ocotillo silicate inclusions contain a mixture of grains from different sources based on their unequilibrated olivine chemistry.

Also in our inclusions of, e.g., Caddo and Landes 2 (arrows in Fig. 1), more or less well separated lighter and dark areas were macroscopically visible which might point to silicate–silicate mixing at some point in the history of these inclusions. Unfortunately, as we did not mineralogically/ petrologically examine our inclusions, we cannot further prove or reject this hypothesis. Nonetheless, we speculate that the light and darker lithologies within those inclusions might be due to silicate–silicate mixing from different sources within the IAB parent body and correlate with the different 40Ar/39Ar ages found in the inclusions. We emphasize that analyzing each inclusions’ plagioclase grains together would have resulted in uniform mixed ages, equivalent to the ‘‘whole rock’’ analyses of IAB silicate inclusions frequently done in the past. Such mixed ages, however, do not necessarily reflect cosmochronological age information with the precision suggested by the assigned uncertainties. Such information can only be deduced from the youngest and oldest ‘‘endmembers’’ of which the mixed age is composed. This also has to be kept in mind for our high- and low-age separates: although they are certainly closer to the real highest and lowest endmember ages than a mixed age, some of the real endmembers can be expected to be even more extreme than our actual measured highest and lowest ages. 4.2. Thermochronological evolution of the IAB parent body 4.2.1. Adaptation of 40Ar/39Ar ages to a new 40K decay constant To elucidate the early thermal evolution of the IAB parent body we will compare our 40Ar/39Ar data to literature thermochronological information of IAB iron meteorites obtained from various chronometers. So far, our ages were calculated using the 40K decay constants of Steiger and Jaeger (1977), which are widely used but have repeatedly been called into question (e.g., Begemann et al., 2001; Min et al., 2000; Renne, 2000; Trieloff et al., 2003). Renne (2001) pointed out that the partial decay constant for the electron capture decay branch given by Steiger and Jaeger (1977) is likely to be overestimated by about 1–2%. Using U/Pb and 40 Ar/39Ar data pairs for 10 quickly cooled terrestrial volcanic rocks, and the statistical methods of Kwon et al. (2002), Mundil et al. (2006) have derived values of 5.530 · 10 11/a for the total 40K decay constant, and 28.28 Ma for the age of the Fish Canyon sanidine standard. Using a 40Ar*/40K ratio for FCs of 1.6407 · 10 3 (Jourdan and Renne, 2007) indeed results in a reduction of the ke value of 1%. Besides the 40Ar/39Ar ages calculated with the Steiger and Jaeger (1977) decay constants, Table 1 also lists all plateau ages recalculated with the new ones. The latter ages are 20 Ma higher than the former ones, a shift similar to a value (+30 Ma) also suggested realistic, e.g., by Trieloff et al. (2001, 2003). Whereas this difference is irrelevant while comparing only 40Ar/39Ar ages to each other, it is indispensable when comparing 40Ar/39Ar ages to those obtained from different chronometers. Therefore the following discussion is based on ages calculated with the revised 40K constant published so far in an abstract by Mundil et al. (2006). For easier handling, we averaged (error weighted)

40

Ar–39Ar dating of IAB iron plagioclase separates

for each inclusion all revised plateau ages identical within their 2r uncertainties (including J uncertainties). Doing so, we obtain one high- and one low-age endmember for each inclusion except for CdC1 (Fig. 7). Uncertainties assigned to these average values represent standard deviation, internal, or external errors, depending on which of these was largest. Due to large uncertainties CdC2-Pl-IIb and O-Ps-I do slightly overlap with some members of the respective high age (not with the averaged data, though). Keeping this in mind, we still refer to them as low age. 4.2.2. Thermochronological evolution of the IAB parent deduced from Caddo County Of all IAB irons Caddo County seems to be studied most extensively in terms of radiometric dating. Therefore we will try to decipher the thermochronological evolution of the IAB parent body based on Caddo County data before adding other IABs. The data will be integrated into the model presented by Benedix et al. (2000) as it—in contrast to many other models presented in the literature— delivers a straightforward mechanism not only for mixing liquid metal with solid silicates but also for mixing silicate grains from different sources and different ages to form IAB silicate inclusions. It is generally accepted that the IAB parent body was chondritic (although not represented in our meteorite collections). The body was heated—probably internally—to temperatures that allowed differentiation of metal from silicate at least in parts of its interior. A recent high precision Hf–W age of Caddo metal (4.561 ± 0.002 Ga, Markowski et al., 2006) shows that metal and silicate segregated only few Ma after CAI formation about 4.567 Ga (dashed line in Fig. 7) (Amelin et al., 2002). Thereby, metal peak temperatures might have been as high as 1400 C (Scott, 1982). Metamorphic temperatures of IAB silicates were at least equivalent to those of type 6 ordinary chondrites (Benedix et al., 2000), i.e., between about 750–950 C (Dodd, 1981). These temperatures were not necessarily sufficient to reset the I–Xe chronometer in Caddo (4557.9 ± 0.1 Ma, Bogard et al., 2005), but probably the K–Ar chronometer. The high average age of our Caddo sample (4.548 ± 0.015 Ga, Fig. 7) could therefore date cooling through the K–Ar blocking temperature after internal metamorphic heating of the body, equivalent to the interpretation of the revised H4 ordinary chondrite Ar–Ar ages of Ste Marguerite and Forest Vale (Trieloff et al., 2003), which are identical to our high Caddo age. Taking into account the age uncertainty of 15 Ma, it is even possible that the K–Ar chronometer was not reset at all, i.e., some areas of the parent body were not heated to the temperatures mentioned above, or heating was too short to reset the K–Ar chronometer. In this case the high Caddo age might as well reflect plagioclase formation. In any case, a rather surface near location of the high age Caddo silicates can be inferred. The low age grains of Caddo (4.472 ± 0.020 Ga, Fig. 7) cooled through the K–Ar blocking temperature much later. We assume that these grains originated from deeper (and hotter) within the parent body, where cooling after internal

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heating was distinctly slower than in surface near regions. Impact disruption of the parent body and reassembly of the debris represents a mechanism to mix these younger grains with old (cold) grains and metal. It is possible that at the time of impact disruption the young grains were still above the K–Ar blocking temperature and that final closure of Ar diffusion did not happen until some time after reassembly of the IAB parent body. This is supported by petrologic observations in Caddo pointing to metal injection into still hot (even partially molten) silicates during a shock event (Takeda et al., 2000). Thus, the age of the young Caddo grains gives a lower limit for the time of the destructive impact. Keeping in mind that the separation of young and old grains was probably not quantitative, the ‘‘real’’ low age endmember and thus the lower limit for the time of the impact might even be somewhat lower than 4.472 ± 0.02 Ga. We emphasize that only by delaying the time of the impact towards the age of the young Caddo grains it is possible to mix plagioclase with different ages to form one inclusion, and only by rapid cooling after reassembly of still molten metal, hotter, and cold silicates it is possible to maintain theses differences, i.e., not resetting the K–Ar clock of the high age Caddo plagioclase. Assuming that the temperature of the metal at the time of impact disruption was just above the FeNi–FeS eutectic of 950 C (Kubaschewski, 1982; Kullerud, 1963) and expanding the Ni-cooling rates of IAB metal of 20 to 200 /Ma (Herpfer et al., 1994) down to an average K–Ar blocking temperature of 350 C, the catastrophic impact happened between about 5 and 30 Ma before the low Caddo age was established, i.e., not before 4.50 Ga (shaded area in Fig. 7). Reports about ‘‘quench textures’’ for example in Caddo (Benedix et al., 2000, and references therein) also suggest a very short time interval between the impact and final cooling through the K–Ar blocking temperature 4.47 Ga. If the impact was assumed to have taken place before the high Caddo ages were established, younger and older Caddo ages must have developed within the very same inclusion over time, which is considered improbable. If assembly of the different lithologies happened at much lower silicate temperatures (<<350 C), i.e., much later than the young Caddo age was established, a higher shock stage than S2 (Takeda et al., 2000) (indicating very weak shock only (Sto¨ffler et al., 1991)) due to brittle deformation were to be expected. Also, the metal must have still been liquid at the time of impact disruption, which additionally limits time frame for impact disruption. Takeda et al. (2000) published an Ar–Ar age of 4.520 ± 0.0005 Ga for Caddo County plagioclase-rich material with the highest temperature extractions being as high as 4.54 Ga. Adjustment of these ages to the new decay constants brings them into agreement with our Caddo high age separates, i.e., between 4.54 and 4.56 Ga. The authors interpreted these ages as dating cooling of a gabbroic partial melt after local metal and silicate segregation from a chondritic (winonaite-like) parent. If so, we have to conclude that even in the source areas of the high age Caddo silicates temperatures were sufficient to produce partial

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melts within the first 20 Ma after CAI formation. From the study however it is somewhat unclear if the sample used for Ar–Ar dating indeed consisted exclusively of gabbroic material or if also significant amounts of winonaite-like precursor material were present, as both lithologies occur intimately mixed in the studied sample. In the latter case it is thinkable that a mixture between a reset (i.e., younger) gabbroic material and older metamorphosed chondritic precursor was dated. Three Ar–Ar plateau ages of Caddo County silicates were published by Bogard et al. (2005), which are, adjusted to the new decay constants, 4.528 ± 0.013 Ga (Caddo #5), 4.527 ± 0.01 Ga (Caddo #27), and 4.487 ± 0.023 Ga (Caddo #EH). While the former samples agree within uncertainties with our high Caddo age, the third sample, described as coarse-grained, overlaps with our low Caddo age. With regard to our hypothesis, we assume that also the Caddo silicates dated by Bogard et al. (2005) represent a mix of fine-grained old and coarse-grained younger grains, and the ages range between these endmembers depending on their relative proportions in the sample used for dating. There also exist two Sm–Nd ages of 4.53 ± 0.02 Ga (Stewart et al., 1996) and 4.50 ± 0.04 Ga (Liu et al., 2002) for Caddo County, and Liu et al. (2002) present a Rb–Sr isochron age of 4.57 ± 0.23 Ga and Rb–Sr model ages ranging between 4.55 and 4.59 Ga for the same meteorite. Using only the ages with the best precision for an age comparison with our Ar–Ar data (i.e., Sm–Nd: 4.53 ± 0.02 Ga, Rb–Sr: 4.57 ± 0.02 Ga) shows that they are both identical within uncertainties with each other and our high Caddo age, but not with the low one. According to our hypothesis, also these ages were deduced from surface near material not reset at the time of impact disruption and reassembly but rather represent cooling after internal heating of the accreting chondritic IAB precursor parent body. Lower Sm–Nd and Rb–Sr ages or those with high uncertainties can be interpreted as ages from the hotter interior portion of Caddo silicates and/or mixtures of both portions. 4.2.3. Integrating Landes, Campo del Cielo, and Ocotillo into the model An I–Xe age of 4.560 ± 0.010 Ga for Landes published by Niemeyer (1979b) is within uncertainties identical to that of Caddo, thus, they are interpreted identically, too. Based on the observations on Caddo above we assume that the destructive impact followed by reassembly of the debris happened around 4.47 Ga. Therefore, it is possible that our high average age of Landes (4.498 ± 0.017 Ga, Fig. 7) does represent pre-impact age information while our low Landes age (4.433 ± 0.021 Ga, Fig. 7), identical to the revised Landes Ar–Ar age of Niemeyer (1979a) (4.448 ± 0.030 Ga), postdates this event. The high age of Landes is lower than its respective Caddo counterpart which can be explained by a deeper location of the pre-impact age grains within the parent body resulting in slower cooling and later closure of those grains. The lower post-impact age of Landes compared to Caddo consequently points to a deeper burial of the mixed age sample after reassembly of the debris and thus

slower cooling also after the impact. This is indirectly supported by the metal cooling rate of Landes of 30/Ma (Herpfer et al., 1994), which is at the lower end of the cooling rate range given by Herpfer et al. (1994). Although Caddo County metal was not included in this study, its higher Ni concentration compared to that of Landes (Wasson and Kallemeyn, 2002) might further support faster cooling of the former, as Herpfer et al. (1994) found a correlation between Ni content and cooling rate in IAB metal. Note that the burial depth of Landes after the impact was not deep enough to fully reset its pre-impact age information, although a certain degree of resetting cannot be ruled out. Also we emphasize that in particular for Landes the ‘‘real’’ endmember ages might be more extreme than the actual measured ones, because the separation of high and low age grains obviously was rather bad as shown by the complex age information within separate L2-Ps-II. Based on a Hf–W age of Campo del Cielo metal of 4.559 ± 0.004 Ga published by Kleine et al. (2005) and an I–Xe age of Campo silicates of 4558.6 ± 0.7 Ma by Podosek (1970) we assume that the thermochronological evolution of Campo del Cielo during the first Ma after CAI formation was identical to that of Caddo and Landes. A Rb–Sr age for Campo del Cielo published by Liu et al. (2003), 4.54 ± 0.08 Ga, has too large an uncertainty to assign it as either pre- or post-impact. Both of our Campo del Cielo samples (average ages CdC1 4.446 ± 0.030 Ga, CdC2 4.436 ± 0.020 Ga) clearly post-date an impact 4.47 Ga (Fig. 7). This might indicate that the Campo del Cielo K–Ar clocks in the silicates studied here were fully reset due to deep burial before and/or after impact and reassembly of the IAB parent body. This is in agreement with the finding of Bild (1977) that Campo del Cielo silicates were heated more strongly than Caddo and other IAB silicates. We doubt that the low plateau age of CdC2-Pl-IIb reflects a discrete heating event but prefer diffusive loss of 40 Ar* as an explanation for the low age. Not only do all of our Campo separates show the effects of at least partial diffusive loss of 40Ar*, but also others, e.g., Bogard et al. (2005) found that their Campo spectra were severely disturbed probably due to weathering. Bogard et al. (2005) found in their sample of Campo del Cielo silicates not only an Ar–Ar plateau age of 4.454 ± 0.011 Ga (adjusted to new decay constants) identical within uncertainties to our Campo del Cielo ages, but also an Ar–Ar isochron age for the highest extraction steps, which is—adjusted to the new decay constants—indistinguishable from the canonical age of the solar system. This indicates that resetting until after impact was not complete or that in the sample of Bogard et al. (2005) hot silicates from deep within the parent body were mixed with some cold surface near and thus old silicates during the impact. In this case the burial depth of Campo del Cielo material after reassembly of the asteroid was not deep enough to fully reset the K–Ar systems of cold old grains. Apart from our Ar–Ar ages no other radiometric ages of Ocotillo have been published to our knowledge. Thus, we unfortunately lack information about the high temperature thermal history of Ocotillo.

40

Ar–39Ar dating of IAB iron plagioclase separates

We divided our four Ocotillo Ar–Ar plateau ages into two groups with the higher ages averaging at 4.452 ± 0.021 Ga, the lower ones at 4.386 ± 0.022 Ga (Fig. 7). The older Ocotillo plagioclase thus has closed to Ar diffusion shortly after the assumed time of the impact 4.47 Ga, and its age is within uncertainties identical to the major Campo del Cielo age cluster (CdC1 4.446 ± 0.030 Ga, CdC2 4.436 ± 0.020 Ga) and also to the post-impact age of our Landes sample (4.433 ± 0.019 Ga). This points to a comparable post-impact burial depth and cooling rate for all three meteorites. Potential pre-impact Ar–Ar age information however is fully erased in our Ocotillo sample, in agreement with the granoblastic recrystallized texture of Ocotillo silicates described by Olsen and Schwade (1998). Therefore, the pre-impact burial depth might have been somewhat deeper than for Landes and Campo. However, our dataset is small, and further Ar–Ar analyses on Ocotillo silicates might also reveal higher ages. The potential presence of grains of different age also within Ocotillo is supported by a mineralogical study of Ocotillo silicates by Olsen and Schwade (1998), who explain the inter-grain variability of the Fa content in olivines as being due to a mixture of grains from different sources. For the lower Ocotillo average age of 4.386 ± 0.022 Ga we consider—similar to the low Campo age—diffusive loss of 40Ar* in response to terrestrial weathering as the most probable reason. Ocotillo is described as weathered (Olsen and Schwade, 1998), and also our separates displayed partially severe staining. 4.2.4. Further literature IAB Ar–Ar ages In contrast to other models, e.g., by Choi et al. (1995), Kracher (1983), or Wasson and Kallemeyn (2002), the IAB formation model of Benedix et al. (2000) allows mixing of silicates from different locations from within the parent body (and thus with different Ar–Ar ages) with liquid metal during gravitational reassembly of a previously disrupted partially differentiated parent body. It thus represents a straightforward mechanism to explain large variabilities of Ar–Ar ages even within a single inclusion (composed of silicate grains from different locations), as long as the destructive impact did not occur too early in the parent body’s history—in order to let significant age differences evolve—and cooling after reassembly was fast enough to not fully reset all K–Ar clocks within the composite inclusion. The range of Ar–Ar ages to be expected within single inclusions and among silicate inclusions from different IABs thereby depends much more on the combination of pre- and post-impact burial depth of a given sample than on the actual time of the impact. Therefore a more or less continuous Ar–Ar age spectrum for IAB silicates from the time of accretion of the parent body down to the time of its final post-impact cooling through the K/Ar blocking temperature is to be expected. This is indeed the case as shown in Fig. 8, where Ar–Ar literature ages of IAB silicates in addition to our new data are compiled. All ages are adjusted to the most recent monitor ages and the new decay constants discussed above. Only separation of high and low Ar–Ar (and potentially also Sm–Nd and Rb–Sr) age endmembers, if present, from within a single inclusion allows to set at least a rough time frame for the destructive impact on the IAB parent body.

1245

5. CONCLUSION 1. The separation of plagioclase from individual IAB silicate inclusions for Ar–Ar analyses generally reduced the complexity of the age spectra resulting in generally flat plateaux and hardly any signs of recoil effects. 2. Two inclusions of Landes which were located within cm to each other in the specimen showed highly different ages (L1: 470 Ma, L2: >4.4 Ga). This impressively shows how local the influence of impact resetting obviously is, and that mixing of different inclusions for dating purposes is a questionable venture. 3. Different separates of plagioclase from individual inclusions did show in part significantly different ages indicating different thermal events recorded in some inclusions. If analyzed together (equivalent to ‘‘whole rock’’ analyses) these age differences would have remained undetected and the resulting mixed age would not necessarily have been of thermochronological relevance. 4. However, we did not find the expected systematic correlation of age and grain size due to later closure and/or easier resetting of smaller grains compared to larger ones. In contrary, some large grain separates did show the lowest ages within an inclusion. Based on this observation we assume that some grains remained in a hot (deep) environment that allowed Ar diffusion over an extended time period—in some cases combined with grain coarsening—, while other grains cooled significantly earlier (surface near) through the K/Ar blocking temperature. The existence of different age information within individual inclusions (in particular in Caddo and Landes 2) is explained by mixing high age silicates from surface near regions of the IAB parent body with younger silicates from deeper, hotter regions in the course of a destructive impact followed by gravitational reassembly of the debris. 5. From many IAB formation models presented in the literature, we consider only the one of Benedix et al. (2000) capable of letting different ages evolve over time in different regions of the parent body, combine these different age silicates to individual inclusions due to an impact event, and finally conserve potential age differences within the inclusions depending on their post-impact cooling rate and burial depth within the reassembled IAB parent body. 6. In general, Ar–Ar ages of IAB silicates thus depend much more on their pre- and post-impact burial depths than on the actual time of the impact, thus, a more or less continuous age range for IAB silicates between the time of accretion of the parent asteroid and its final cooling through the K–Ar blocking temperature is to be expected. This is supported by an Ar–Ar age compilation of many IAB silicates showing a continuous age spectrum between the canonical age of the solar system down to 4.4 Ga and lower.

ACKNOWLEDGMENTS We thank the Smithsonian National Museum of Natural History for providing a Caddo County silicate inclusion and the

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Arizona State University Center for Meteorite Studies for a slice of Ocotillo. Samples from Campo del Cielo and Landes were generously provided by B. Mack Kennedy at the Lawrence Berkeley National Laboratory. Detailed reviews of G. Benedix, M. Trieloff, an anonymous reviewer, and the associate editor N. Kita helped to considerably improve the manuscript. Sebastien Nomade

and Tim Becker provided invaluable help with sample handling and measurements. Additionally, N.V. thank Ruth Ziethe for valuable discussions on cooling of asteroidal bodies as well as Anne Brennwald and Urs Lauterburg for frequent computer access. This work was supported by the Ann and Gordon Getty Foundation.

APPENDIX A Full Ar data corrected for decay, blank, and mass discrimination Step

Power [W]

40

Ar

±

39

Ar

±

38

Ar

±

37

Ar

±

36

Ar

±

Caddo County plagioclase large I, irradiation # 320A, J = 0.0458697 (0.0003046) A 0.3 1227 2 6.87 0.11 0.71 B 0.5 4439 3 18.89 0.14 0.68 C 0.7 5974 5 25.64 0.11 0.53 D 0.9 11,698 6 49.68 0.18 1.07 E 1 7121 6 30.29 0.16 0.44 F 1.1 3690 3 15.90 0.14 0.31 G 1.3 9924 7 41.93 0.20 1.51 H 1.5 6249 8 26.40 0.27 1.36 I 1.8 11,921 7 50.66 0.25 7.75 J 3 16,603 9 71.32 0.18 7.42 K 5 2195 3 9.60 0.15 2.46 L 10 585 2 2.31 0.14 0.25 Total 81,626 349 24

0.06 0.07 0.08 0.11 0.10 0.10 0.09 0.09 0.12 0.14 0.09 0.08

12.2 46.5 70.4 145.1 85.4 41.4 130.2 84.8 537.9 732.7 146.2 11.2 2044

1.4 1.6 1.7 2.2 2.1 2.0 2.0 1.6 3.5 3.5 1.9 1.4

1.49 1.07 0.49 1.06 0.49 0.14 3.16 3.56 31.55 26.11 8.60 1.12 79

0.12 0.12 0.12 0.18 0.18 0.17 0.17 0.17 0.20 0.16 0.13 0.12

Caddo County plagioclase medium I, irradiation #320A, J = 0.0458697 (0.0003046) A 0.3 260 2 0.78 0.15 0.41 B 0.6 3157 3 12.85 0.17 0.81 C 0.9 21,331 11 87.02 0.27 2.45 D 1 8731 6 35.52 0.24 0.83 E 1.1 9223 7 37.74 0.19 3.00 F 1.2 1994 3 8.34 0.17 1.26 G 1.3 5817 5 25.73 0.18 26.32 H 1.5 5424 4 23.05 0.18 17.03 I 1.7 1915 3 8.22 0.15 4.52 J 3 4412 3 17.92 0.18 3.15 K 5 2159 3 9.12 0.16 3.85 L 10 1034 2 4.19 0.15 0.39 Total 65,457 270 64

0.11 0.09 0.11 0.10 0.13 0.10 0.21 0.08 0.10 0.12 0.10 0.10

5.0 38.2 303.6 123.5 462.6 136.7 3183.6 2147.3 541.4 435.0 613.2 151.4 8142

1.7 1.8 2.5 1.9 2.7 1.9 13.5 5.0 3.0 2.7 3.2 2.2

1.70 1.43 5.08 1.52 2.14 1.35 5.87 5.04 1.63 4.02 15.76 0.65 46

0.19 0.19 0.20 0.13 0.13 0.13 0.13 0.12 0.12 0.16 0.19 0.16

Caddo County plagioclase small I, irradiation #320A, J = 0.0458697 (0.0003046) A 0.2 3160 4 13.06 0.15 1.20 B 0.3 52 2 0.24 0.09 0.22 C 0.4 797 3 3.36 0.11 0.34 D 0.6 15,180 12 61.99 0.24 1.22 E 0.7 36,322 25 148.59 0.35 2.47 F 0.8 10,330 10 43.03 0.22 0.60 G 0.9 17,637 24 72.45 0.25 1.37 H 1 55,389 42 227.35 0.45 4.28 I 1.1 27,071 30 111.56 0.32 1.97 J 1.2 40,699 50 167.83 0.60 3.03 K 1.3 30,912 32 127.07 0.37 3.12 L 1.4 16,430 7 67.48 0.27 1.37 M 1.5 31,506 13 129.29 0.32 3.58 N 1.6 22,141 15 90.78 0.32 2.67 O 1.7 8285 6 34.00 0.18 1.03 P 1.8 7013 6 28.46 0.19 1.29 Q 1.9 2675 3 11.29 0.14 0.54 R 2.2 5503 4 22.36 0.19 1.52 S 3 4944 4 20.76 0.17 2.97 T 5 8408 7 34.70 0.20 7.28 U 10 2305 3 9.74 0.13 1.60 Total 346,761 1425 44

0.06 0.04 0.04 0.11 0.13 0.10 0.12 0.16 0.14 0.13 0.13 0.11 0.13 0.11 0.10 0.07 0.07 0.08 0.10 0.10 0.07

36.9 2.2 11.6 173.7 432.3 125.5 218.0 727.0 336.2 562.9 452.0 237.7 720.1 461.4 146.2 143.1 66.7 299.5 657.1 1070.8 182.2 7063

1.0 1.0 1.0 2.2 2.4 1.9 2.1 3.7 2.4 3.2 2.7 2.2 3.7 2.4 1.8 2.0 1.7 2.3 3.5 4.2 2.1

2.54 0.28 0.41 1.33 1.27 0.23 1.31 2.55 1.49 2.42 5.13 1.47 6.52 5.20 2.03 3.12 0.79 3.01 8.21 20.57 4.10 74

0.10 0.09 0.09 0.15 0.16 0.15 0.14 0.18 0.15 0.18 0.18 0.17 0.15 0.13 0.12 0.16 0.16 0.16 0.15 0.16 0.14

40

Ar–39Ar dating of IAB iron plagioclase separates

1247

Appendix A (continued) Step

Power [W]

40

Ar

±

39

Ar

±

38

Ar

±

Caddo County plagioclase large II (HF-ed), irradiation #320A, J = 0.0458697 (0.0003046) A 0.2 623 2 2.68 0.11 0.34 0.06 B 0.3 5466 5 23.03 0.19 0.69 0.08 C 0.4 8983 6 37.20 0.19 0.87 0.08 D 0.5 33,833 20 139.34 0.32 2.15 0.14 E 0.5 30,849 16 127.51 0.27 2.04 0.12 F 0.5 10,584 7 44.19 0.22 0.64 0.11 G 0.6 52,517 42 215.71 0.42 3.44 0.13 H 0.6 17,498 13 72.18 0.27 1.24 0.09 I 0.6 23,351 14 96.21 0.30 1.56 0.11 J 0.7 19,091 15 78.71 0.27 1.07 0.10 K 0.7 39,594 19 162.49 0.37 2.44 0.12 L 0.7 43,105 30 175.41 0.40 2.84 0.11 M 0.7 11,959 10 48.48 0.27 0.77 0.10 N 0.8 20,088 20 82.07 0.30 1.24 0.11 O 0.8 51,422 35 211.84 0.47 3.52 0.13 P 0.8 15,914 10 64.90 0.25 0.99 0.10 Q 0.8 18,625 16 76.12 0.27 1.31 0.10 R 0.9 47,037 50 191.80 0.40 3.49 0.13 S 0.9 19,520 25 80.88 0.45 1.27 0.11 T 0.9 13,871 7 57.26 0.25 1.08 0.11 U 0.1 2 2 0.17 0.14 0.06 0.07 V 1 16,363 13 66.73 0.27 1.56 0.09 W 1 9068 7 37.27 0.22 0.72 0.07 X 1.1 25,954 11 106.35 0.32 1.92 0.10 Y 1.1 16,907 11 69.43 0.27 1.24 0.11 Z 1.2 35,689 35 145.68 0.40 2.94 0.14 AA 1.2 20,854 13 84.71 0.30 1.60 0.12 AB 1.3 23,995 14 97.74 0.27 1.85 0.09 AC 1.3 24,797 15 101.06 0.27 2.12 0.11 AD 1.4 29,885 23 122.63 0.37 3.28 0.11 AE 1.5 43,285 42 176.66 0.40 7.28 0.15 AF 1.6 18,555 11 75.73 0.30 2.18 0.10 AG 1.7 13,291 8 53.96 0.30 1.34 0.09 AH 1.8 16,909 14 69.20 0.27 2.01 0.11 AI 1.8 6330 6 26.07 0.18 0.77 0.10 AJ 1.9 14,263 10 58.37 0.24 1.53 0.11 AK 2 11,356 8 46.38 0.20 1.51 0.15 AL 2.3 7509 7 30.91 0.18 1.23 0.15 AM 3 19,547 16 79.74 0.27 5.53 0.17 AN 5 8601 8 34.98 0.20 2.92 0.11 AO 10 524 2 2.34 0.14 0.44 0.09 Total 847,616 3474 77 Caddo County plagioclase medium II (HF-ed), irradiation #320A, A 0.2 53 2 0.22 B 0.3 1640 3 6.55 C 0.3 766 2 3.20 D 0.4 3157 4 13.73 E 0.5 20,824 20 87.66 F 0.6 59,453 30 245.19 G 0.7 88,965 85 366.00 H 0.7 111,074 107 453.01 I 0.7 31,777 30 130.99 J 0.7 16,185 14 66.54 K 0.8 128,108 107 520.80 L 0.8 27,331 22 112.55 M 0.8 46,227 32 191.04 N 0.8 8719 11 36.38 O 0.9 31,859 30 129.54 P 0.9 17,823 30 72.48 Q 1 133,364 187 544.10

J = 0.0458697 (0.0003046) 0.11 0.11 0.08 0.14 0.55 0.08 0.13 0.15 0.08 0.15 0.46 0.07 0.32 1.77 0.08 0.45 3.98 0.12 0.55 5.56 0.15 0.87 6.62 0.18 0.37 1.97 0.13 0.30 1.03 0.10 0.90 7.71 0.18 0.32 1.75 0.12 0.65 2.74 0.23 0.25 0.50 0.11 0.45 2.27 0.15 0.30 1.47 0.09 1.25 8.96 0.18

37

Ar

8.8 67.4 103.2 399.6 360.4 131.2 635.9 209.7 293.2 239.3 489.0 521.2 144.6 238.0 637.0 191.7 232.5 591.2 287.6 169.3 0.8 208.7 112.8 357.8 222.3 515.1 299.9 337.5 347.0 488.9 1827.5 386.9 214.3 428.0 141.6 324.4 334.3 336.3 913.6 404.5 58.1 14,211 5.0 31.5 13.1 42.4 252.6 719.7 1070.4 1361.2 388.0 200.1 1585.3 343.2 574.5 106.3 410.1 226.4 1733.6

± 1.4 1.6 1.4 2.7 3.0 2.0 3.7 2.3 2.7 2.5 2.7 3.2 1.9 2.7 3.7 2.5 2.5 3.7 2.1 2.5 1.8 2.4 2.1 2.7 2.4 3.0 2.7 3.0 2.7 3.7 5.7 3.0 2.7 3.2 1.7 3.0 2.7 2.7 4.2 2.7 2.0

36

Ar

±

0.59 1.03 0.62 0.51 0.88 0.25 0.76 0.13 0.31 0.07 0.46 0.56 0.06 0.12 0.56 0.09 0.09 1.01 0.64 0.39 0.25 0.58 0.45 0.99 0.84 1.87 0.72 0.75 2.26 3.54 4.40 2.26 1.00 2.93 1.42 1.03 2.56 2.33 16.80 9.01 0.95 66

0.11 0.11 0.12 0.14 0.13 0.13 0.16 0.14 0.14 0.12 0.17 0.17 0.11 0.12 0.21 0.12 0.12 0.19 0.21 0.15 0.13 0.13 0.12 0.14 0.16 0.19 0.17 0.15 0.15 0.16 0.17 0.15 0.15 0.15 0.15 0.16 0.18 0.17 0.20 0.15 0.14

1.2 0.50 1.4 1.45 1.2 0.42 1.6 0.49 2.5 1.42 3.2 1.23 3.7 1.09 4.7 0.61 3.0 0.71 2.5 0.19 5.5 0.65 3.2 0.60 3.2 0.95 2.5 0.57 2.7 1.82 2.7 1.23 5.7 1.53 (continued on next

0.08 0.09 0.08 0.13 0.15 0.18 0.20 0.20 0.12 0.11 0.23 0.16 0.27 0.14 0.15 0.15 0.24 page)

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Appendix A (continued) Step

Power [W]

40

R S T U V W X Y Z AA AB AC AD AE AF AG AH AI AJ AK Total

1 1 1.1 1.1 1.1 1.2 1.2 1.2 1.3 1.3 1.3 1.4 1.4 1.5 1.6 1.7 1.8 2 5 10

33,968 84,046 108,706 48,182 63,786 148,988 31,140 22,080 134,306 48,707 19,219 26,417 16,021 21,196 16,401 15,552 4610 8623 76,410 36,384 1,692,068

Ar

± 24 60 90 32 32 142 19 14 67 30 9 13 8 11 9 9 5 5 27 25

39

Ar

±

38

±

37

±

Ar

±

137.84 343.99 442.62 196.38 260.05 607.84 128.03 90.83 547.71 200.62 79.74 108.16 65.45 87.81 67.34 64.09 18.88 35.39 313.67 149.15 6926

0.42 0.65 0.90 0.45 0.55 0.95 0.42 0.35 1.02 0.42 0.30 0.35 0.30 0.35 0.30 0.30 0.23 0.25 0.47 0.32

2.42 6.26 8.17 3.51 4.60 13.99 2.79 2.01 11.27 4.14 1.77 2.47 1.90 2.63 2.34 3.16 0.36 1.20 21.05 5.51 149

0.14 0.16 0.20 0.14 0.17 0.18 0.11 0.09 0.20 0.15 0.12 0.12 0.12 0.13 0.12 0.12 0.10 0.09 0.20 0.14

415.9 1073.9 1409.0 606.3 839.9 2648.9 492.6 355.5 2471.4 781.3 303.1 455.3 361.4 491.8 489.7 540.9 103.4 198.7 3451.9 1108.3 27,663

3.2 3.2 5.0 3.7 3.7 9.2 3.5 3.0 6.7 4.0 2.7 3.2 2.7 3.0 3.0 4.0 2.1 2.5 20.2 4.2

1.02 1.24 2.05 0.76 1.23 3.29 0.95 0.79 9.54 2.12 1.20 1.23 1.25 1.40 1.49 2.22 0.41 0.67 31.90 5.37 86

0.17 0.21 0.20 0.16 0.17 0.23 0.16 0.15 0.21 0.16 0.13 0.17 0.17 0.17 0.15 0.15 0.14 0.15 0.25 0.18

90.1 52.8 574.0 381.1 2357.1 1107.1 1491.9 801.5 3175.5 1584.7 910.1 1272.2 1040.5 2475.9 259.4 708.1 338.6 731.0 882.8 1698.5 1103.3 888.4 656.3 885.6 997.5 639.6 534.9 1090.7 1037.9 1078.0 944.5 881.9 578.0 820.8 560.2 865.8 986.8 498.9 356.2 176.5 294.0

1.9 1.6 3.0 2.5 10.0 4.0 5.0 3.5 14.0 5.2 4.5 4.0 4.2 11.5 2.4 3.2 2.5 4.2 4.0 5.0 4.7 4.2 3.2 3.7 3.7 3.7 3.5 4.2 4.2 4.7 3.7 4.7 3.2 4.0 3.7 4.0 4.5 3.2 2.7 2.1 2.7

3.46 0.69 2.38 0.59 2.53 0.93 7.73 1.10 1.80 6.17 1.15 0.94 0.98 2.70 0.42 1.44 0.37 1.21 2.02 3.30 1.88 1.55 0.93 1.85 2.38 1.89 1.10 3.34 2.82 2.77 2.42 1.80 1.77 2.89 5.25 4.12 4.00 3.70 2.60 1.88 2.83

0.16 0.15 0.19 0.21 0.30 0.25 0.38 0.27 0.37 0.27 0.20 0.22 0.22 0.30 0.20 0.22 0.20 0.23 0.22 0.25 0.23 0.24 0.22 0.22 0.24 0.23 0.20 0.23 0.22 0.27 0.27 0.27 0.22 0.23 0.22 0.22 0.21 0.19 0.18 0.18 0.18

Caddo County plagioclase small II (HF-ed), irradiation #320A, J = .0458697 A 0.3 8397 6 33.87 0.20 B 0.4 4943 4 20.32 0.18 C 0.5 50,814 40 208.52 0.45 D 0.6 33,110 35 137.22 0.38 E 0.7 204,546 98 825.34 1.25 F 0.8 96,206 123 390.21 1.20 G 0.9 127,096 83 514.28 1.00 H 0.9 68,989 47 280.52 0.55 I 1 272,728 195 1105.31 1.40 J 1 135,705 75 548.91 0.95 K 1 75,478 42 306.77 0.47 L 1 107,313 72 435.72 0.70 M 1.1 86,723 40 351.39 0.57 N 1.1 208,805 140 846.50 1.35 O 1.1 21,767 21 87.75 0.45 P 1.2 58,890 35 238.53 0.52 Q 1.2 28,002 21 113.00 0.37 R 1.3 60,257 40 243.31 0.47 S 1.3 71,550 47 289.80 0.55 T 1.4 139,718 85 567.33 1.07 U 1.4 87,885 42 355.26 0.62 V 1.5 71,514 70 290.20 0.57 W 1.5 53,640 40 217.48 0.50 X 1.5 71,508 60 290.27 0.50 Y 1.5 80,979 57 326.32 0.55 Z 1.6 51,059 52 206.08 0.47 AA 1.6 42,073 25 170.46 0.45 AB 1.7 82,311 45 331.89 0.60 AC 1.7 80,820 102 327.11 0.57 AD 1.8 85,322 60 345.20 0.57 AE 1.8 71,449 50 289.42 0.55 AF 1.8 67,690 102 274.46 0.55 AG 1.9 45,169 37 182.72 0.42 AH 1.9 60,869 47 246.77 0.50 AI 1.9 43,621 37 177.23 0.42 AJ 2 63,018 30 254.44 0.52 AK 2.1 69,543 60 282.18 0.52 AL 2.1 32,707 21 132.55 0.35 AM 2.2 22,574 21 92.04 0.32 AN 2.2 11,977 10 48.27 0.27 AO 2.3 18,413 13 74.48 0.32

Ar

(0.0003046) 1.49 0.07 0.62 0.06 4.24 0.08 2.46 0.11 12.48 0.21 6.00 0.16 7.61 0.21 5.04 0.15 16.50 0.27 8.57 0.17 4.86 0.14 6.96 0.19 5.56 0.15 12.98 0.21 1.64 0.11 3.91 0.11 1.91 0.10 4.43 0.13 5.02 0.17 10.23 0.19 6.05 0.19 5.26 0.17 3.91 0.14 5.15 0.17 6.01 0.16 3.87 0.18 3.18 0.14 6.50 0.16 6.33 0.18 6.47 0.15 5.58 0.18 5.18 0.15 3.57 0.15 5.13 0.18 4.39 0.16 5.62 0.14 5.68 0.17 3.51 0.11 2.22 0.12 1.30 0.11 2.01 0.12

Ar

36

40

Ar–39Ar dating of IAB iron plagioclase separates

1249

Appendix A (continued) Step

Power [W]

40

±

38

±

37

AP AQ AR AS AT AU AV AW AX AY AZ Total

2.3 2.4 2.4 2.5 2.5 2.6 2.8 4 6 8 10

10,729 20,178 14,949 18,474 14,628 7210 8790 18,274 38,854 11,709 7189 3,246,163

Ar

±

0.23 0.27 0.27 0.32 0.27 0.21 0.18 0.25 0.32 0.24 0.21

1.00 2.34 1.58 2.98 1.92 1.09 1.20 8.94 13.87 3.34 1.33 259

0.10 0.10 0.11 0.09 0.08 0.06 0.09 0.12 0.14 0.11 0.10

156.3 317.9 245.1 359.1 243.1 130.5 199.5 1053.6 2014.7 376.4 162.7 43,067

2.2 3.0 2.4 3.0 2.4 2.0 2.4 4.5 5.0 3.2 2.4

1.39 3.38 2.47 4.68 1.53 1.56 2.15 24.36 32.97 8.38 3.40 182

0.13 0.14 0.13 0.15 0.14 0.14 0.15 0.19 0.17 0.19 0.18

Campo del Cielo 1 plagioclase large II-a, irradiation #311, J = 0.0528815 (0.0004195) A 2 494 1 6.69 0.09 0.46 B 3 1225 2 13.47 0.10 0.51 C 4 3321 3 30.62 0.12 0.68 D 5 6048 4 44.61 0.15 1.24 E 6 7081 5 43.95 0.18 1.49 F 7.5 9785 6 65.05 0.18 2.56 G 9 8151 6 47.40 0.17 2.45 H 11 9356 8 47.40 0.19 4.21 I 14 7351 8 37.55 0.16 8.76 J 18 5700 4 28.55 0.14 8.40 K 21 6432 5 31.89 0.14 4.21 L 25 5599 4 27.70 0.14 5.64 M 32 4807 3 23.67 0.12 7.52 N 40 3639 4 18.09 0.10 5.63 Total 78,988 467 54

0.02 0.02 0.02 0.03 0.03 0.03 0.03 0.04 0.06 0.05 0.03 0.05 0.04 0.04

7.5 19.7 61.9 102.9 113.4 211.3 200.4 221.9 698.9 511.3 291.1 345.0 396.2 380.1 3562

0.8 0.9 0.9 1.0 1.1 2.2 1.9 1.9 3.3 2.8 2.2 2.1 2.3 2.3

1.62 1.17 1.25 1.72 2.53 4.34 5.99 11.11 24.33 26.37 11.36 18.73 26.97 21.80 159

0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.03 0.07 0.08 0.05 0.06 0.07 0.07

Campo del Cielo 1 plagioclase large II-b, irradiation #311, J = 0.0528815 (0.0004195) A 2 290 1 3.76 0.07 0.24 B 3 1391 2 14.06 0.09 0.44 C 4 2414 3 17.90 0.11 0.70 D 5 1550 2 13.71 0.10 0.60 E 6 2138 3 18.59 0.10 0.76 F 7.5 5217 4 37.76 0.13 2.01 G 9 4282 3 27.79 0.13 2.43 H 11 4700 4 29.51 0.12 2.67 I 14 4570 4 26.69 0.13 4.19 J 18 4583 5 23.52 0.12 8.57 K 21 3369 4 17.29 0.13 4.30 L 25 5344 4 26.96 0.13 8.06 M 32 3142 5 17.09 0.12 7.31 N 40 3149 4 16.94 0.12 3.48 Total 46,139 292 46

0.02 0.02 0.02 0.02 0.03 0.03 0.03 0.03 0.03 0.05 0.04 0.06 0.04 0.04

5.9 26.6 37.1 29.5 45.6 135.3 106.1 174.9 232.9 424.8 293.2 493.6 419.8 248.5 2674

0.9 0.7 1.1 0.7 1.1 1.6 1.6 1.4 1.9 2.5 2.6 2.6 2.8 2.1

0.74 0.81 0.94 0.94 1.14 4.01 7.20 6.32 11.27 27.13 11.86 22.08 23.17 10.61 128

0.01 0.02 0.02 0.02 0.02 0.02 0.04 0.03 0.05 0.08 0.05 0.07 0.07 0.05

Campo del Cielo 1 plagioclase small II-a, irradiation #311, J = 0.0528815 (0.0004195) A 3.5 1010 2 10.15 0.08 0.35 B 4.5 1545 2 12.73 0.09 0.39 C 5.5 2696 3 18.31 0.11 0.51 D 6.5 3993 4 24.16 0.11 1.06 E 9 4560 3 26.56 0.09 2.16 F 11 3339 3 16.62 0.08 2.05 G 14 1866 3 9.66 0.10 1.58 H 18 1589 2 8.19 0.08 1.61 I 22 1324 2 6.77 0.09 1.40 J 26 954 2 4.86 0.07 1.13 K 30 1361 3 6.97 0.08 1.00 L 35 1346 4 7.00 0.08 0.71 M 40 941 2 5.36 0.07 0.47 Total 26,524 157 14

0.02 0.02 0.02 0.03 0.03 0.05 0.03 0.03 0.02 0.03 0.02 0.02 0.02

14.0 30.2 48.7 84.7 126.6 83.8 109.1 91.7 78.3 69.0 55.1 64.6 38.9 895

0.7 0.8 1.1 1.2 1.8 1.3 1.5 1.1 0.8 1.2 0.8 0.8 1.0

Ar

± 7 17 11 12 12 6 7 11 19 10 6

39

Ar

43.50 82.33 61.02 74.61 59.46 29.71 35.51 74.63 157.30 47.36 29.49 13,154

Ar

Ar

36

±

0.52 0.45 0.53 1.62 5.02 6.15 4.04 4.79 3.43 3.25 3.44 2.14 1.72 37 (continued on next

0.02 0.02 0.02 0.03 0.04 0.05 0.04 0.04 0.03 0.03 0.02 0.02 0.02 page)

1250

N. Vogel, P.R. Renne / Geochimica et Cosmochimica Acta 72 (2008) 1231–1255

Appendix A (continued) Step

Power [W]

40

Ar

±

39

Ar

±

38

Ar

±

37

Ar

±

0.9 0.7 0.7 1.2 0.9 1.0 1.2 0.9 1.3 1.4 1.2 1.1 1.2

0.43 0.39 0.63 1.81 3.51 2.89 4.78 2.62 2.67 4.77 2.81 3.31 1.80 32

0.02 0.02 0.02 0.02 0.02 0.02 0.03 0.03 0.03 0.03 0.02 0.03 0.02

0.4 0.8 12.6 18.9 71.5 67.5 42.3 71.1 90.6 131.0 56.5 74.0 74.0 25.6 5.5 742

0.6 0.6 0.7 0.9 0.9 0.8 1.1 1.1 1.3 1.4 1.1 1.2 1.0 0.7 0.6

0.04 0.05 0.51 0.39 0.37 1.76 0.75 5.66 5.32 8.57 8.60 4.50 4.40 1.00 0.84 43

0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.04 0.03 0.05 0.05 0.03 0.04 0.02 0.02

0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.03 0.02 0.04 0.03 0.03 0.03 0.03 0.03 0.03 0.03

0.6 1.1 2.2 2.8 4.3 6.5 13.4 15.6 13.0 18.1 26.4 51.7 35.2 54.5 67.7 67.1 103.6 53.5 98.9 635

0.8 0.6 0.6 0.6 0.6 0.6 0.8 0.7 0.7 0.6 0.8 1.0 0.7 0.7 0.8 1.1 1.3 0.7 0.8

0.04 0.01 0.09 0.18 0.27 0.25 0.46 0.24 1.16 1.12 1.25 12.90 2.32 2.28 3.24 4.27 8.49 2.97 6.95 48

0.03 0.02 0.02 0.02 0.02 0.02 0.03 0.03 0.03 0.02 0.02 0.05 0.02 0.02 0.02 0.04 0.05 0.02 0.05

0.02 0.02 0.02 0.02 0.02 0.03 0.02

0.8 2.9 8.6 11.4 16.9 36.0 14.5

0.7 0.6 0.7 0.7 0.7 1.1 0.7

0.31 0.46 0.67 0.71 0.70 1.88 2.35

0.02 0.03 0.02 0.03 0.03 0.03 0.02

0.02 0.02 0.02 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.02 0.03 0.02

14.1 17.4 26.3 49.7 117.2 135.3 96.3 98.1 92.2 103.8 50.9 47.1 83.6 932

Campo del Cielo 2 plagioclase large I, irradiation #311, J = 0.0528815 (0.0004195) A 2 62 2 0.64 0.06 0.01 B 3 92 2 1.13 0.07 0.04 C 4 235 2 2.79 0.08 0.39 D 5 611 2 6.88 0.07 0.26 E 6 3212 4 28.12 0.12 0.53 F 7.5 4822 4 25.65 0.12 0.87 G 9 3100 3 14.59 0.13 0.41 H 12 2370 3 10.86 0.11 1.58 I 16 2087 3 10.39 0.11 1.65 J 20 1804 4 9.04 0.11 2.20 K 24 1336 4 6.72 0.09 2.01 L 28 3388 3 16.76 0.12 1.29 M 32 2832 4 14.13 0.13 1.35 N 37 827 2 5.31 0.08 0.31 O 40 312 2 2.56 0.06 0.19 Total 27,092 156 13

0.02 0.02 0.02 0.02 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.02

Campo del Cielo 2 plagioclase large II-a, irradiation #311, J = 0.0528815 (0.0004195) A 0.3 1 2 0.00 0.10 0.02 B 1 2 2 0.06 0.08 0.03 C 1.5 11 2 0.17 0.07 0.03 D 2.5 63 2 0.86 0.07 0.03 E 3.5 130 2 1.68 0.07 0.08 F 4.5 178 2 2.17 0.07 0.09 G 5.5 571 2 6.21 0.09 0.15 H 6 982 2 7.67 0.09 0.16 I 6.8 1054 2 5.70 0.10 0.36 J 7.8 917 2 7.08 0.11 0.32 K 9.2 1208 2 8.56 0.09 0.40 L 11.2 1307 2 7.52 0.09 2.98 M 13.2 1076 2 6.14 0.08 0.61 N 15.5 1774 3 10.73 0.11 0.77 O 18 1837 6 11.97 0.11 0.97 P 21.5 2032 6 10.42 0.11 1.18 Q 26 3652 7 18.38 0.13 2.13 R 30 2717 4 13.64 0.13 0.88 S 36 2704 4 13.57 0.13 1.80 Total 22,217 133 13 Campo del Cielo 2 plagioclase large II-b, irradiation #311, J = 0.0528815 (0.0004195) A 2 122 2 1.26 0.07 0.10 B 3 214 2 2.31 0.07 0.18 C 4 648 2 6.63 0.09 0.28 D 5 900 2 7.66 0.08 0.28 E 6 1025 2 7.89 0.10 0.23 F 7.5 2666 3 15.14 0.14 0.69 G 8.5 1247 2 7.01 0.07 0.59

36

Ar

Campo del Cielo 1 plagioclase small II-b, irradiation #311, J = 0.0528815 (0.0004195) A 3.5 685 2 7.15 0.07 0.32 B 4.5 862 2 7.27 0.08 0.30 C 5.5 1424 2 10.57 0.09 0.33 D 6.5 1691 2 12.69 0.09 0.84 E 9 3477 3 21.26 0.12 1.87 F 11 4080 3 20.12 0.12 1.63 G 14 2431 3 11.91 0.09 1.90 H 18 2040 3 10.90 0.10 1.15 I 22 1541 2 8.01 0.09 1.09 J 26 1263 2 6.55 0.08 1.43 K 30 1157 1 6.00 0.08 0.89 L 35 639 1 3.28 0.07 1.10 M 40 347 1 1.85 0.06 0.45 Total 21,638 128 13

±

40

Ar–39Ar dating of IAB iron plagioclase separates

1251

Appendix A (continued) Step

Power [W]

H I J K L M N O P Total

10.5 14 17 21 25 29 33 37 40

40

Ar

39

Ar

±

0.02 0.03 0.04 0.04 0.03 0.03 0.03 0.03 0.04

17.2 80.0 114.2 144.1 83.7 46.0 64.7 40.3 7.8 689

0.7 1.3 1.5 1.5 1.3 1.1 1.3 1.1 0.7

2.07 7.06 6.92 12.48 6.15 8.60 5.61 5.70 12.36 74

0.02 0.05 0.04 0.06 0.04 0.05 0.03 0.04 0.03

Campo del Cielo 2 plagioclase small I, irradiation #311, J = 0.0528815 (0.0004195) A 2 29 2 0.21 0.05 0.00 B 3.5 212 2 2.07 0.06 0.11 C 5.5 1489 3 10.25 0.11 0.41 D 7.5 1539 3 9.25 0.10 0.70 E 9.5 1179 3 5.28 0.09 1.09 F 11.5 371 2 1.89 0.07 0.70 G 13.5 1194 2 5.74 0.09 0.82 H 16.5 587 2 2.65 0.08 0.29 I 19.5 228 2 1.08 0.06 0.10 J 22.5 356 2 1.59 0.07 0.27 K 26.5 436 2 2.08 0.08 0.15 L 30.5 144 2 0.68 0.06 0.08 M 35 292 2 1.34 0.06 0.17 N 40 341 2 1.72 0.06 0.14 Total 8395 46 5

0.02 0.02 0.03 0.02 0.03 0.03 0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02

0.2 3.9 24.9 30.2 42.7 18.9 33.7 24.4 17.3 4.9 15.4 3.1 9.3 10.6 240

0.8 0.7 1.0 1.1 0.8 0.8 1.0 1.0 0.8 0.7 0.7 0.7 0.7 0.7

0.10 0.27 0.98 2.40 4.49 2.73 3.01 1.19 0.35 1.29 0.51 0.18 0.85 0.40 19

0.02 0.02 0.02 0.02 0.04 0.03 0.02 0.02 0.02 0.02 0.03 0.02 0.02 0.02

Landes 1 plagioclase, irradiation #320B, J = 0.0455419 (0.0002992) A 0.2 129 1 1.05 0.08 B 0.3 110 1 0.46 0.17 C 0.5 847 2 7.70 0.15 D 0.7 1067 2 12.83 0.15 E 0.9 3902 4 86.59 0.27 F 1 1703 2 70.09 0.21 G 1.1 1792 2 87.55 0.23 H 1.3 2749 3 258.49 0.35 I 1.5 1372 3 183.81 0.30 J 1.7 1113 2 136.01 0.30 K 2 1016 2 84.84 0.24 L 3 2361 3 47.08 0.18 M 5 160 1 0.43 0.10 N 10 52 1 0.06 0.09 Total 18,371 977

0.15 0.07 0.43 0.55 2.62 1.65 2.01 8.60 5.00 5.53 13.15 62.94 2.76 0.32 106

0.06 0.09 0.08 0.06 0.07 0.06 0.06 0.07 0.06 0.06 0.08 0.27 0.05 0.06

23.1 7.3 24.3 42.2 29.2 20.7 24.8 171.1 42.1 63.0 402.5 1647.7 12.7 3.6 2500

1.3 2.4 2.0 1.6 1.1 1.2 1.0 2.1 1.4 1.5 2.5 5.7 1.1 1.3

0.30 2.15 0.70 0.02 1.80 0.96 1.45 9.16 8.10 10.92 21.00 100.34 8.65 0.51 160

0.21 0.70 0.52 0.37 0.25 0.23 0.22 0.20 0.20 0.20 0.21 0.30 0.23 0.24

Landes 2 plagioclase large I, irradiation #320B, J = 0.0455419 (0.0002992) A 0.3 8 1 0.02 0.07 B 0.5 118 1 0.41 0.08 C 0.7 408 1 1.74 0.08 D 0.8 218 1 0.92 0.09 E 0.9 513 2 2.23 0.09 F 1.1 592 2 2.60 0.09 G 1.3 545 2 2.35 0.09 H 1.5 536 1 2.39 0.09 I 1.7 361 1 1.68 0.09 J 2 640 2 2.82 0.08 K 3 1955 3 8.30 0.18 L 5 241 1 0.93 0.10 M 10 77 1 0.22 0.12 Total 6212 27

0.01 0.19 0.35 0.13 0.23 0.62 0.72 1.03 2.30 3.38 37.29 1.80 1.35 49

0.05 0.05 0.04 0.04 0.05 0.06 0.06 0.08 0.06 0.06 0.13 0.07 0.07

0.6 0.1 5.1 2.9 9.0 8.9 10.0 16.9 75.1 56.4 788.1 16.6 5.5 994

1.1 1.0 1.0 1.0 1.1 1.1 1.1 1.1 1.8 1.4 3.7 1.6 1.7

0.35 0.47 0.85 0.52 0.57 2.08 2.06 2.56 4.00 8.21 80.75 2.77 0.95 106

0.18 0.17 0.17 0.17 0.18 0.18 0.18 0.25 0.24 0.24 0.35 0.40 0.50

Landes 2 plagioclase large II, irradiation #320B, J = 0.0455419 (0.0002992) A 0.2 450 2 2.11 0.11 B 0.3 301 2 1.14 0.09

0.71 0.24

0.08 0.07

2.5 2.3

0.08 0.10 0.09 0.09 0.09 0.07 0.08 0.08 0.08

Ar

±

36

0.61 2.03 2.18 3.30 1.61 2.03 1.31 1.29 2.54 19

3.74 9.13 7.23 5.80 5.63 2.49 3.72 2.30 1.96 90

±

37

±

3 3 5 5 4 3 2 2 2

Ar

38

Ar

635 1378 1362 1117 1061 503 754 414 372 14,418

±

1.0 1.49 0.23 1.2 0.56 0.21 (continued on next page)

1252

N. Vogel, P.R. Renne / Geochimica et Cosmochimica Acta 72 (2008) 1231–1255

Appendix A (continued) Step

Power [W]

C D E F G H I J K L M N O P Q R S Total

0.4 0.5 0.6 0.7 0.8 0.85 0.9 1 1.1 1.3 1.5 1.6 1.7 1.8 2 5 10

40

Ar 4631 1824 849 2906 11,456 3859 2969 4929 1806 3842 8229 5025 6265 2839 5228 63,721 1028 132,155

± 4 2 2 3 12 3 3 3 2 5 6 5 6 3 4 42 2

39

Ar 19.29 7.64 3.47 12.18 47.71 16.10 12.43 20.65 7.49 16.20 34.51 21.26 26.44 11.89 22.09 271.80 4.92 559

±

38

±

37

±

36

±

0.22 0.11 0.11 0.13 0.35 0.14 0.14 0.25 0.11 0.20 0.20 0.18 0.17 0.13 0.16 0.35 0.13

2.43 0.70 0.35 1.54 7.05 2.03 1.90 3.05 0.97 2.28 7.26 3.76 8.39 3.70 10.65 421.71 22.68 501

0.07 0.07 0.07 0.07 0.14 0.06 0.08 0.09 0.08 0.08 0.12 0.10 0.12 0.10 0.08 0.77 0.17

20.5 10.8 3.8 17.2 111.7 48.4 38.8 73.0 22.0 51.5 169.8 101.3 267.5 69.4 332.8 8616.6 476.0 10,436

1.4 1.2 1.1 1.3 1.9 1.5 1.2 1.6 1.4 1.6 2.2 1.7 2.7 1.6 2.5 29.9 3.2

4.42 1.05 0.49 1.95 7.23 1.99 1.84 2.72 0.61 2.46 9.44 5.08 11.16 8.82 18.13 907.27 42.20 1029

0.20 0.19 0.19 0.19 0.21 0.19 0.20 0.22 0.24 0.24 0.25 0.30 0.24 0.22 0.24 0.87 0.25

0.09 0.17 0.69 0.54 0.39 0.38 1.80 1.63 0.73 0.45 3.01 4.29 12.28 1.03 2.60 0.02 30

0.06 0.06 0.07 0.06 0.06 0.06 0.07 0.06 0.07 0.06 0.06 0.06 0.08 0.07 0.05 0.07

0.6 3.3 24.0 17.0 10.1 11.0 49.0 46.6 16.5 6.4 90.4 110.2 273.8 13.5 7.0 0.8 679

1.1 1.2 1.1 1.3 1.0 1.0 1.6 1.4 1.3 1.0 1.9 1.9 2.4 1.0 1.2 1.1

0.00 0.43 1.05 0.95 0.63 0.72 2.42 3.12 1.58 0.90 5.29 6.89 22.94 2.51 5.70 0.02 55

0.20 0.20 0.20 0.19 0.17 0.17 0.23 0.22 0.21 0.20 0.21 0.21 0.23 0.20 0.20 0.24

0.08 0.08 0.07 0.09 0.08 0.09 0.09 0.10 0.09 0.12 0.08 0.08 0.15 0.08 0.09 0.17 0.12 0.11 0.27 0.25 0.32 0.16

1.4 0.6 3.0 24.8 34.4 43.3 46.2 138.4 63.4 146.8 100.6 59.6 332.3 47.8 37.4 474.1 160.8 214.3 1398.5 1071.0 1710.6 264.0 6373

1.3 1.1 1.3 1.3 1.4 1.1 1.6 1.8 1.5 2.3 1.3 1.5 2.7 1.3 1.3 1.9 2.1 2.2 5.2 4.5 5.2 2.3

0.27 0.12 0.17 2.04 2.27 1.67 2.02 4.63 2.36 7.17 4.78 3.31 18.39 2.85 2.49 33.00 12.54 16.00 126.59 141.32 209.92 34.41 628

0.30 0.27 0.27 0.25 0.27 0.27 0.25 0.24 0.22 0.24 0.25 0.21 0.22 0.20 0.21 0.23 0.25 0.27 0.35 0.35 0.37 0.27

Landes 2 plagioclase small I, irradiation #320B, J = 0.0455419 (0.0002992) A 0.3 50 1 0.27 0.08 B 0.5 407 2 1.52 0.10 C 0.7 1325 2 5.42 0.12 D 0.8 915 2 3.62 0.11 E 0.9 547 2 2.26 0.09 F 1 565 2 2.41 0.09 G 1.2 2026 3 8.44 0.12 H 1.3 978 2 4.04 0.10 I 1.4 455 2 1.77 0.10 J 1.6 39 1 0.25 0.08 K 1.8 214 1 1.00 0.09 L 2 198 1 0.95 0.08 M 2.5 473 2 2.04 0.10 N 3 38 1 0.16 0.07 O 5 130 1 0.38 0.10 P 10 15 1 0.04 0.08 Total 8375 35

Ar

Landes 2 plagioclase small II, irradiation #320B, J = 0.0455419 (0.0002992) A 0.2 315 2 2.55 0.12 0.43 B 0.3 339 2 1.79 0.10 0.12 C 0.4 463 2 2.18 0.10 0.23 D 0.5 4756 3 20.34 0.14 1.71 E 0.6 5392 5 22.96 0.15 2.03 F 0.7 5223 6 22.26 0.15 2.26 G 0.8 4544 5 19.20 0.15 2.35 H 0.9 10,974 7 46.22 0.35 5.39 I 1 4674 3 19.75 0.17 2.30 J 1.1 10,792 6 45.34 0.19 6.02 K 1.2 6988 8 29.79 0.27 3.89 L 1.3 4123 3 17.32 0.15 2.38 M 1.4 21,588 9 90.68 0.27 13.50 N 1.4 2865 3 11.96 0.14 2.04 O 1.5 2209 3 9.19 0.14 1.47 P 1.7 25,463 14 105.54 0.25 19.73 Q 1.8 7188 7 30.03 0.18 6.71 R 2 7411 5 31.09 0.20 8.60 S 2.5 38,880 23 162.16 0.40 63.38 T 3 15,804 8 66.45 0.35 59.94 U 5 7761 6 31.19 0.20 93.80 V 10 975 2 4.47 0.11 15.90 Total 188,725 792 314

Ar

Ar

40

Ar–39Ar dating of IAB iron plagioclase separates

1253

Appendix A (continued) 38

±

Ocotillo plagioclase large I, irradiation #314, J = 0.0871983 (0.000579) A 0.3 1 2 0.04 0.20 B 0.6 24 2 0.33 0.18 C 0.9 244 3 2.75 0.14 D 1.1 442 3 3.91 0.13 E 1.3 296 3 2.63 0.12 F 1.5 2336 5 20.32 0.25 G 1.6 574 3 4.86 0.13 H 1.7 90 3 0.75 0.11 I 1.9 124 3 1.25 0.13 J 2.2 192 3 1.63 0.12 K 2.5 240 3 2.13 0.12 L 2.8 229 3 2.15 0.12 M 3.2 158 3 1.62 0.13 N 4 85 3 1.33 0.13 O 6 87 3 1.23 0.21 P 10 23 3 0.40 0.17 Total 5142 47

0.03 0.32 15.28 105.55 48.95 238.51 25.33 3.36 3.70 8.67 22.17 16.92 18.96 25.65 77.20 13.75 624

0.07 0.07 0.10 0.32 0.23 0.42 0.22 0.06 0.07 0.11 0.14 0.11 0.15 0.19 0.32 0.15

6.7 20.4 52.8 343.1 165.3 956.8 98.0 19.5 30.3 43.5 97.9 167.3 155.0 637.3 873.1 216.8 3843

Ocotillo plagioclase large II, irradiation #314, J = 0.0871983 (0.000579) A 0.3 0 0 0.00 0.00 B 0.6 332 3 3.45 0.13 C 0.9 1047 3 9.33 0.18 D 1.1 1749 3 15.46 0.18 E 1.3 2424 5 20.42 0.18 F 1.4 1782 3 14.75 0.16 G 1.5 3268 4 27.23 0.15 H 1.6 1977 3 16.76 0.16 I 1.8 1527 3 13.12 0.15 J 2.1 3284 3 29.05 0.14 K 2.4 2230 4 18.16 0.18 L 2.8 4276 5 34.63 0.18 M 3.2 2722 7 22.12 0.37 N 4 997 3 7.87 0.14 O 6 3543 4 28.31 0.19 P 10 2425 4 19.19 0.17 Total 33,583 280

0.00 1.82 3.87 5.56 12.04 12.84 38.11 32.41 68.90 347.54 71.59 39.29 156.53 19.77 99.13 60.65 970

0.00 0.05 0.09 0.11 0.11 0.13 0.20 0.21 0.25 0.39 0.27 0.20 0.21 0.15 0.30 0.25

Ocotillo plagioclase small I, irradiation #314, J = 0.0871983 (0.000579) A 0.1 1 6 0.18 0.24 B 0.3 3 2 0.00 0.11 C 0.6 56 2 0.48 0.17 D 0.9 111 3 0.89 0.15 E 1.2 240 3 1.99 0.14 F 1.5 266 3 2.22 0.12 G 1.8 795 3 6.66 0.14 H 2.1 417 3 3.50 0.12 I 2.5 497 3 4.12 0.12 J 2.9 189 3 1.65 0.11 K 4 474 3 4.62 0.12 L 6 443 3 3.80 0.14 M 10 112 3 1.00 0.13 Total 3604 31

0.17 0.03 2.20 11.37 52.86 23.54 72.34 9.87 11.83 9.18 16.53 101.27 10.62 321

Ocotillo plagioclase small II, irradiation #314, J = 0.0871983 (0.000579) A 0.1 1 1 0.01 0.35 B 0.3 0 2 0.36 0.67 C 0.6 150 3 1.52 0.37 D 0.9 954 2 7.83 0.37 E 1.2 1697 3 13.65 0.39 F 1.5 2340 4 18.47 0.37 G 1.8 2261 3 18.63 0.15 H 2.3 5834 6 48.15 0.32

0.05 0.02 0.87 1.80 4.62 3.27 14.11 69.90

Step

Power [W]

40

Ar

±

39

Ar

±

Ar

37

Ar

±

36

Ar

±

8.7 7.9 4.7 4.7 4.4 7.1 4.7 4.2 4.4 4.4 4.2 4.4 4.7 5.9 7.1 5.7

0.35 0.47 3.02 14.92 7.10 37.46 3.87 0.32 0.88 1.75 3.92 4.77 4.97 17.63 24.81 6.31 132

0.30 0.27 0.19 0.17 0.18 0.22 0.18 0.18 0.18 0.17 0.17 0.17 0.16 0.17 0.21 0.21

0.0 9.6 15.8 27.4 54.3 53.8 151.2 118.1 235.4 1151.8 261.7 195.5 606.1 109.0 936.1 254.5 4180

0.1 3.7 4.4 4.2 4.2 3.9 3.7 4.7 4.7 4.9 5.2 5.2 5.2 4.4 5.4 4.9

0.00 1.28 1.25 1.70 2.81 2.60 6.98 5.60 13.07 52.20 12.70 10.69 33.23 8.30 59.97 16.93 229

0.00 0.17 0.19 0.19 0.19 0.18 0.17 0.17 0.18 0.19 0.21 0.20 0.21 0.19 0.22 0.19

0.12 0.04 0.07 0.13 0.21 0.14 0.24 0.12 0.07 0.12 0.14 0.32 0.08

0.6 0.9 3.2 43.2 185.9 82.9 262.1 41.2 46.8 39.0 76.7 439.5 52.0 1274

8.4 4.5 8.6 4.2 4.4 3.9 3.9 3.2 3.5 3.7 4.4 4.7 4.2

0.19 0.00 0.02 1.97 7.78 3.43 10.95 1.70 2.21 1.61 3.40 18.28 2.41 54

0.39 0.19 0.32 0.19 0.19 0.17 0.18 0.16 0.16 0.17 0.18 0.19 0.19

0.08 0.16 0.10 0.11 0.13 0.11 0.12 0.30

1.0 6.5 5.9 10.9 29.8 36.8 121.0 1559.3

4.6 0.00 9.1 0.22 4.4 0.62 4.7 1.08 4.9 1.62 4.7 1.74 3.9 6.05 6.9 46.15 (continued on next

– 0.24 0.14 0.14 0.15 0.14 0.18 0.22 page)

1254

N. Vogel, P.R. Renne / Geochimica et Cosmochimica Acta 72 (2008) 1231–1255

Appendix A (continued) Step I J K Total

Power [W] 2.7 3.7 6

40

Ar 347 587 144 14,314

39

± 4 3 3

Argon concentrations are given in moles · 10

Ar 3.18 4.84 1.27 118

16

±

38

±

0.15 0.15 0.14

26.15 23.74 33.22 178

0.17 0.16 0.19

Ar

37

Ar 132.0 146.1 115.2 2162

± 4.4 4.4 3.9

36

Ar

±

9.72 17.73 6.34 91

0.19 0.20 0.18

, uncertainties are 1r.

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