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Earth and Planetary Science Letters

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Rheasilvia provenance of the Kapoeta howardite inferredfrom ∼1 Ga 40Ar/39Ar feldspar ages

Fara N. Lindsay a,∗, Jeremy S. Delaney a, Gregory F. Herzog a, Brent D. Turrin b, Jisun Park a,c, Carl C. Swisher III b

a Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ 08854, USAb Department of Earth and Planetary Science, Rutgers University, Piscataway, NJ 08854, USAc Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 August 2014Received in revised form 22 December 2014Accepted 29 December 2014Available online xxxxEditor: B. Marty

Keywords:Vestahowarditeargon datingRheasilviaKapoeta

We report 40Ar/39Ar ages for several lithological components of the brecciated howardite Kapoeta and compare the ages with results for asteroid 4 Vesta as observed by the Dawn mission. Our Kapoeta sample has an unusual, millimeter wide glass vein that intruded into a complex breccia. The plateau ages of three lithic clasts of basaltic composition that were remote from the glass vein range from 4.2 to 4.5 Ga. Such ages are typical of eucritic material; the oldest reflects early magmatic crystallization (∼4.5 Ga), the younger (4.2–4.5 Ga) reflect magmatism associated with protracted cooling. Samples of the glass vein itself, which include relict grains, give apparent ages between 3.1 and 3.9 Ga as do chips from the matrix. We consider both glass and bulk matrix ages as mixing ages; not marking the time of a single event, but dating regolith activity (<3.1–4.1 Ga).Eight feldspar grains close to the glass vein give markedly younger plateau ages averaging 1.4 Ga. The Ar release spectra for glass vein and breccia subsamples indicate a disturbance in the last 1.4 Ga. Taken together, these younger ages suggest a recent, major thermal event in the history of howardites that has been under-reported – perhaps the impact that formed the Rheasilvia basin on Vesta.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Conventional radiometric ages for multi-milligram samples of polymict breccias and relict containing glass chips may be either ambiguous or misleading because they average the contributions of chronologically distinct components. The howardite Kapoeta, a member of the howardite–eucrite–diogenite clan (HED) of mete-orites, is a polymict breccia. It is a mechanical mixture of mineral and rock clasts, each with a distinct provenance. The specimen studied here is typical of howardites, except that it also includes prominent glassy vein material (Fig. 1). Many workers believe that the howardite parent body is asteroid 4 Vesta (McCord et al., 1970;Consolmagno and Drake, 1977; cf., Wasson, 2013 for a different view). We report 40Ar–39Ar ages of individual feldspar crystals and rock clasts taken from specific lithologies within the meteorite.

* Corresponding author at: 610 Taylor Rd., Piscataway, NJ, USA. Tel.: +1 848 445 3436.

E-mail addresses: flindsay@rci.rutgers.edu (F.N. Lindsay), jsd@rci.rutgers.edu(J.S. Delaney), herzog@rutchem.rutgers.edu (G.F. Herzog), bturrin@rci.rutgers.edu(B.D. Turrin), jp975@rci.rutgers.edu (J. Park), cswish@eps.rutgers.edu (C.C. Swisher).

http://dx.doi.org/10.1016/j.epsl.2014.12.0490012-821X/© 2015 Elsevier B.V. All rights reserved.

Published 40Ar/39Ar ages of Kapoeta and other HED meteorites (Kaneoka et al., 1979; Rajan et al., 1979; Shukolyukov et al., 1984;Caffee et al., 1985; Bogard, 1995; Kunz et al., 1997; Bogard and Garrison, 2003; Korochantseva et al., 2005; Shankar et al., 2008;Bogard and Garrison, 2009; Bogard, 2011; Cohen, 2013; Kennedy et al., 2013) lie between 3.1 and 4.5 Ga and are distributed as shown in Fig. 2. The prominent peak in the conventional data at approx-imately 3.7 Ga and the absence of ages younger than ∼3 Ga are striking. With one notable exception (Kaneoka et al., 1979), young (<1.3 Ga) 40Ar/39Ar have not been reported in bulk HED mete-orites. As noted above, however, the ages for polymict breccias and glasses are likely averages for the many distinct lithologies present.

In contrast, ages from the unbrecciated eucrites fall in the range between 4.3–4.5 Ga and can be reasonably interpreted as the ages of discrete events, perhaps associated with Vestan global differen-tiation.

We report 40Ar/39Ar studies of 25 small (3–1330 μg) samples, chosen to be mono-lithologic in most cases, including 14 felds-pathic grains extracted from documented locations in a slab of Kapoeta. The chronology of 4 Vesta implied by the range of dates from Kapoeta clasts can elucidate a more precise history of that

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Fig. 1. Top (polished) and bottom faces of the Kapoeta AMNH4788A slab. Discrete clasts studied are outlined with solid lines. Regions within the breccia from which feldspar clasts were excavated are outlined with dashed lines.

Fig. 2. Distribution of Ar–Ar apparent plateau ages from samples in this study and in the literature for HEDs (see text for references). Each age is represented as a Gaus-sian with equal area. Literature samples include eucrites, diogenites, clasts from howardites (eucritic, matrix and impact melt), and both mg and μg sized feldspar samples.

body and add detail to events outlined by more conventional dat-ing techniques.

The crust of Vesta may be as much as 60 km thick. Vesta’s northern hemisphere preserves craters that record much of the planetoid’s early history (Marchi et al., 2012). Unaltered mete-oritic material from this region is likely to be old, >4.0 Ga. Ves-ta’s southern hemisphere is dominated by two large, overlapping impact basins, Veneneia and Rheasilvia that are younger (Marchi et al., 2012; Schmedemann et al., 2014). Meteoritic material pre-served from this region is more likely to be young (<3.7 Ga). The depth of regolith mixing by the southern hemisphere basins on Vesta is probably 10–30 km. If the products of the southern basin impact events were mixed more or less uniformly through-out the regolith of Vesta, they would represent only a relatively

small fraction of the total material. Unsurprisingly then, previously recognized, young material in HEDs is rare.

Results of the Dawn mission to 4 Vesta, have provided miner-alogical/compositional data from many parts of the Vestan surface and a range of cratering ages between >4 Ga and ∼200 Ma, for major morphological features on the surface of the asteroid. In par-ticular, the very prominent basins in the south hemisphere have been dated at 1.0 or 3.6 Ga for Rheasilvia, and 2.1 or 3.8 Ga for Veneneia (Marchi et al., 2012; Schmedemann et al., 2014). The di-vergent ages for each basin, derive from differing interpretations of the same data set from the Dawn mission and highlight continu-ing uncertainties in crater counting methodologies. Williams et al.(2014) present the case that Rheasilvia is 1.2 Ga old, but, do not resolve the methodological uncertainties associated with the cra-tering methodologies.

The absolute ages of Rheasilvia and Veneneia are of particu-lar importance as they may be the only viable source for many vestoid asteroids. The vestoids are typically of a size – several kilo-meters – that requires large impacts for intact ejection from Vesta (Binzel and Xu, 1993; Harderson et al., 2014). The vestoids are also thought to be the way-stations from which HED meteorites are launched (at a much later time) and eventually delivered to Earth.

We compare the 40Ar–39Ar ages measured in single crystals of feldspar, which reflect the petrological history of discrete litholo-gies preserved in the Kapoeta howardite, with the inferred ages of the basins on Vesta based on cratering records of specific features. The dated samples show Ar signatures set (or reset) by events ranging from early HED magmatism and planetoidal differentia-tion, through regolith impact events on the parent body (presumed to be 4 Vesta), to impacts that ejected them into Earth-bound tra-jectories.

2. Materials and methods

2.1. Sample

We received a thick slab (∼20 × 10 × 3 mm) of Kapoeta 4788 courtesy of the American Museum of Natural History, NY, NY. The slab was sliced parallel to its largest face; we processed one of the two slices, 4788A. One face was dry polished and the other left alone prior to electron microprobe analyses. After microprobe re-connaissance of the polished surface, the mounting material and carbon coating were taken off with a mixture of ethanol and ace-tone, and the surface was washed in distilled, deionized water.

Areas of interest in the slab were identified optically and mapped (Fig. 1). They fall into three categories.

1. Two discrete, unbrecciated types of clasts: the first are dark, fine grained mixtures of plagioclase and pyroxene with sub-ophitic textures (MC001 is an example); the second, also com-posed of coarser granular plagioclase and pyroxene, appear as distinct orange and white areas (MC102 is an example). We refer to these objects as ‘mafic’ clasts (MC) because of their basaltic mineralogy.

2. Matrix is the extensive portions of Kapoeta composed of rel-atively fine-grained, brecciated material (<50 μm) although it often contains larger clasts (>250 μm). One area of matrix that we sampled has clear petrographic boundaries (e.g., BR002). Most do not, however, and for bookkeeping purposes we have assigned arbitrary boundaries to them (e.g., BR 103).

3. Glass occurs as extended ribbons of a black, clast-laden phase of high reflectance (e.g., G002). Kapoeta AMNH 4788 is un-usual among howardite specimens in that it contains >20% by volume of glassy vein material.

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2.2. Separation of components

Samples from the areas of interest were excavated and sepa-rated manually; grains ranged in size from 75 to 500 μm. We dated bulk samples of the three types of components. As K-rich material is best for dating, we searched the loosened materials for the clear, light-colored, or milky grains, which were thought most likely to be feldspathic. After separation, these grains were characterized by using optical microscopy and Raman and energy dispersive spec-troscopies (Lindsay et al., 2014).

2.3. Neutron irradiation and mass spectrometry

After an 80-hour irradiation (with Cd-shielding) at the USGS TRIGA facility, samples were incrementally step-heated using a New Wave 40-watt CO2 laser. The grains were co-irradiated with the reference mineral Fish Canyon sanidine (FC-2) to calibrate the neutron flux. Values of J ranged from 0.01791 to 0.01807. The age adopted for FC-2 is 28.201 Ma (Kuiper et al., 2008), just within the 2-σ uncertainty of the determination presented by Rivera et al. (2011), namely, 28.172 ± 0.028 Ma. Argon isotopes were an-alyzed using a Mass Analyzer Products (MAP) 215-50 noble gas mass spectrometer with an ion pulse-counting detector upgrade (Turrin et al., 2010).

For all reported ages, uncertainties are expressed as 1σ unless otherwise specified. For data reduction, we use Mass Spec (version 7.816) software from Alan Deino at the Berkeley Geochronology Center and decay constants: 40K λε = 5.81 × 10−11 y−1; 40Kλβ =4.962 × 10−10 y−1; 39Arλ = 7.068 × 10−6 d−1; 37Arλ = 1.975 ×10−2 d−1; 40K/Ktotal = 1.167 × 10−4 (Steiger and Jäger, 1977). We define a plateau as three or more consecutive steps that: 1) are indistinguishable from adjacent ones at the 95% confidence level; and 2) comprise at least 50% of the total 39ArK released from the sample (see Dalrymple and Lanphere, 1969, 1974). The plateau ages and their errors are calculated using the variance-weighted mean of the plateau steps (see Taylor, 1997). Reactor constants, blanks, and other procedural specifications are discussed in Lindsay et al. (2014).

3. Results

3.1. Mineralogy

Texturally, two of the clasts (MC001 and MC003) are fine-grained, subophitic, and have radiating pyroxene (En40–50Wo2–40), plagioclase (An86–94Or0.15–0.6), quartz and tridymite, troilite, and intergrown chromite and ilmenite. The other clast (MC102) is coarser grained and granular. Its pyroxene is richer in Mg (En56–62) than that of the other mafic clasts, the plagioclase slightly less anorthitic (An85–90Or0.2–0.4) and no silica phases were observed. Overall, the compositions of the three dated mafic clasts span the range of composition for all reported howardites (Delaney et al., 1981).

The meteorite matrix is a fine grained breccia. Clasts of min-erals, microbreccia chips, diogenitic and mafic clasts of various sizes sit within this matrix. Feldspars within the matrix show a range of compositions (An67–90Or0.3–2.1) as do the pyroxenes (En28–41Wo3–33). The pyroxenes compositions are modally lower in En % than those previously reported for Kapoeta (Prinz et al., 1980)and probably do not represent the pyroxene modality of the entire meteorite that includes the coarser diogenitic clast component.

The glass vein that runs through Kapoeta AMNH 4788 con-tains relict grains of pyroxene, feldspar and quartz. Although not stoichiometric, the composition of the relict-free glass can be un-derstood as a mixture of pyroxene and feldspar and is somewhat Fe-rich compared to the other components.

Table 1Plateau ages of Kapoeta AMNH4788A. Refer to Fig. 1 for clast locations. Asterisks denote samples whose release spectra show evidence of a disturbance at low tem-perature (see text and Appendix A; Fig. A1). Error = 1σ .

Sample ID Mass (μg)

Plateau Age (Ga)

Sample type

BR002_bulk1∗ 163 3.28 ± 0.10 brecciaBR002_bulk2∗ 226 3.37 ± 0.12

Fsp1_BR002 10 1.25 ± 0.70 feldspar from brecciaFsp2_BR002 3 0.63 ± 0.20Fsp7_BR002 5 0.76 ± 0.40Fsp13_BR002 3.5 1.03 ± 0.30Fsp15_BR002 8 0.84 ± 0.30

Fsp12_BR004 38 4.60 ± 0.23 feldspar from brecciaFsp13_BR004 49 1.76 ± 0.84Fsp18_BR004 8 0.82 ± 0.20

FspC_BR103 16.5 4.48 ± 0.27 feldspar from brecciaFspF_BR103 15.5 4.49 ± 0.26

Fsp19_BR105 7 2.52 ± 0.39 feldspar from brecciaFsp17_BR105 3 1.17 ± 0.67

FspE1_MC102 26 4.29 ± 0.12 feldspar from mafic clastFspE2_MC102 7 4.24 ± 0.27

G002_A∗ 216 3.99 ± 0.05 glass chipsG002_D∗ 17 3.06 ± 0.20G002_G∗ 40 3.91 ± 0.16

MC001_bulk1 109 4.46 ± 0.16 mafic clastMC001_bulk2 71 4.50 ± 0.07MC001_bulk3 53 4.45 ± 0.09

MC003_bulkA 795 4.31 ± 0.01 mafic clastMC003_bulkB 448.5 4.36 ± 0.03MC003_bulkC 1330 4.39 ± 0.02

3.2. Dating

The amounts and total concentrations of 36–40Ar measured in step heating experiments are given in Appendix A (Tables A2 and A3) for each of 25 samples. Plots of age vs. cumulative frac-tion of 39Ar released form plateaus for 24 of these samples; the plateau ages are listed in Table 1. Integrated ages are presented in Table A1. Only the results for the largest samples were pre-cise enough to define good isochrons. In these cases the intercepts were indistinguishable from zero, suggesting that any corrections for trapped 40Ar are small in Kapoeta and that plateau ages are reliable.

3.2.1. Mafic clastsBulk subsamples of two mafic clasts (MC001 and MC003)

(Fig. 1) gave weighted average ages, 4.47 ± 0.05 Ga and 4.34 ±0.01 Ga, respectively (Table 1). We did not date a ‘bulk’ sample of MC102. However, plateau ages of two feldspars separated from MC102, 4.24 ±0.27 and 4.29 ±0.12 are within error of the plateau age for MC003.

3.2.2. Feldspar grains from brecciated regionsFourteen feldspar grains extracted from breccia gave plateau

ages ranging from 0.63 ± 0.20 to 4.6 ± 0.23 Ga (Figs. 1 and 2; Ta-ble 1). Seven of the grains closest to the glass vein (closer than 400 μm) yield remarkably young ages of ≤1.25 Ga (Fig. A1); one feldspar, in moderate proximity to the glass vein (∼600 μm), gave an age of 1.76 ± 0.84 Ga. The unweighted mean for the group is 1.03 ± 0.36 Ga (σmean = 0.14 Ga) and the weighted mean is 0.84 ± 0.12 Ga. The individual isochrons of the young grains have large errors due to small amounts of gas and are not meaningful. A combined isochron gives an age of 1.11 ± 0.23 Ga, an intercept (the 40Ar/36Ar ratio of a trapped component) of 4.6 ± 4.1, and an MSWD of 0.17 (see Appendix A; Fig. A2).

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One feldspar, relatively remote from the glass vein, gave an age of 2.52 ± 0.39. Feldspar from brecciated regions most remote from the glass vein gave the oldest ages (4.60 ±0.23; 4.48 ±0.27; 4.49 ±0.26 Ga).

3.2.3. Brecciated regionsBreccia region BR002 (Fig. 1) abuts the glass vein. The appar-

ent ages of bulk BR002 samples, 3.3–3.4 Ga, are older than the plateau ages of some of the feldspars quarried from it (<1.2 Ga). The ages of individual aliquots do not differ significantly from their unweighted average of 3.33 ± 0.02 Ga.

3.2.4. GlassTwo chips of the glass vein region G002 (Fig. 1) yield high-

temperature plateau ages of 3.99 ± 0.05 and 3.91 ± 0.16 Ga; a third, smaller chip of the glass gave a plateau age of 3.06 ±0.20 Ga. 40Ar/39Ar ages reported for howarditic impact clasts have a comparable range, 3.3–3.8 Ga (Cohen, 2013).

3.2.5. Cosmic ray exposureUsing argon isotope measurements, standard methods for de-

convolving cosmogenic 38Ar after the necessary reactor corrections (Turner et al., 1997), and the production rate equations of Eugster and Michel (1995), we calculate a cosmic-ray exposure age of 4.1 ±1.0 Ma for Kapoeta (see Appendix A; Table A1), in agreement with other reported CRE ages for Kapoeta of ∼3 Ma (Rajan et al., 1979;Pun et al., 1998; Caffee and Nishiizumi, 2001).

4. Discussion

4.1. Mafic clasts

These plateau ages of the Kapoeta-4788 mafic clasts are con-sistent with ages of monomict and cumulate eucrites reported by Bogard (1995), Korochantseva et al. (2005), and Kennedy et al.(2013). Although petrologically similar (Section 3.1), the ages of MC001 and MC003 differ at the α-95% confidence level. The pri-mary igneous textures and lack of metal within these two clasts suggest that they represent distinct magmatic events.

4.2. Glass and bulk samples: argon mixing ages

The glass in Kapoeta contains relict grains of differing sizes and compositions (see Appendix A; Fig. A2). We therefore infer that the apparent ages of the glass chips reflect the mixing of Ar re-leased from the younger melt with Ar released from the older relict grains. Our smallest sample from region G002 is also the youngest, perhaps because it contains the smallest mass fraction of relict grains. It seems likely that the vein-forming event occurred more recently than 3.1 ± 0.2 Ga ago. This event probably predates the creation of the Kapoeta meteoroid and the onset of significant exposure to cosmic rays.

Similarly, the apparent age of bulk BR002 likely represents a mixture in which the young feldspars are only one component. A margin of material in BR002 that fringes the glass is texturally more sintered than the rest of the clast (see Appendix A; Fig. A3). Such a texture is compatible with the greater recrystallization ex-pected in a thermal metamorphic aureole.

The low temperature 40Ar/39Ar release data give ages of 1 Ga or less, compatible with later disturbances, and suggest the occur-rence of some younger feldspar within the bulk sample. The fact that we see evidence of more resent isotopic resetting in our bulk samples suggests that some percent volume of the bulk samples is young feldspar grains. Our work shows, however, that these grains have varying ages and occur in varying proportions depending on location and lithology.

4.3. Glass as a source of energy for Ar resetting

The youngest feldspars are found in closest proximity to the glass vein. Additionally, the plateau ages of three feldspar grains (Fsp12,13,18_BR004) increase with distance from impact melt vein region G007 (which is part of the vein network) toward clast G003, as would be expected with increasing distance from a source of contact metamorphism. Likewise, Fsp19_BR105, quarried from the surface of the unpolished side of the slab, is older than Fsp17_BR105, which was extracted from the interior of the slab (toward the polished face), closer to the glass vein. The variation of age with distance from the glass vein may be consistent with a metamorphic overprint.

4.4. Low temperature disturbances at ∼1 Ga

The 40Ar/39Ar release spectra for the glass vein and bulk breccia samples show disturbances at low temperatures (see Appendix A; Fig. A1). Specifically, the first 10–30% of the 39Ar released from 6 starred samples noted by an asterisk in Table 1 show evidence for thermal disturbance within the last 1.40 ± 0.30 Ga.

Similar indications of recent disturbance in the 40Ar/39Ar sys-tematics of other HED meteorites have been reported previously. The 40Ar/39Ar age for granoblastic diogenite Y-74097 is perhaps the best example. This recrystallized meteorite has concordant plateau and isochron ages of 1.10 ± 0.06 Ga (Kaneoka et al., 1979). Moreover, ages of less than 2.2 Ga appear in the first 10–30% of the 39Ar release spectra of several eucrites (Bogard, 1995;Bogard and Garrison, 2003; Korochantseva et al., 2005; Bogard and Garrison, 2009). Additional evidence for a 1 Ga event is discussed separately (Appendix A).

The 0.6–1.7 Ga ages of some of our Kapoeta samples, and the comparable 40Ar/39Ar age disturbances of both our bulk samples and other HEDs record the effects of an event of sufficient magni-tude to have modified Vesta’s regolith, the near surface eucritic flows, the diogenitic lower crust/upper mantle, and everything in between. In particular, the 1.1 Ga ages in diogenitic samples (Kaneoka et al., 1979; the Yamato Type-A suite), which have no observed intrusive glassy veins (Takeda and Mori, 1981; Takeda, 1991), imply a pervasive shock/thermal metamorphic event at that time (Mori and Takeda, 1981).

The various HEDs, containing young radiometric ages are con-sistent with the observed mineralogy of the Rheasilvia basin on Vesta from the Dawn mission. The Rheasilvia basin contains abun-dant diogenitic material that underlay a basaltic crust (De Sanctis et al., 2013).

4.5. The role of impacts

We suggest that an impact, possibly the Rheasilvia event, cre-ated a network of fractures in the adjacent crustal regolith and upper mantle of Vesta, a network through which shock-generated melt intruded (Fig. 3). Prior to this event, proto-Kapoeta was in a location that was penetrated by such a fracture and was partially metamorphosed by the intruding melt vein. Either si-multaneous or subsequent ejection from the Vestan surface as a Vestoid could have delivered the meteorite to Earth. Under fa-vorable circumstances, relict-free glassy impact melt in Kapoeta and other HED meteorites may be a good medium for recording ages of events such as the Rheasilvia formation. Even when im-pact glass appears to be free of clasts, however, caution may be needed. Consistent with the simplest hypothesis that glass should be no older than the matrix it cross-cuts, glass from the howardite Bununu is younger (Rajan et al., 1975) than nearby fine-grained host material. In the Malvern howardite, glass and host have simi-lar ages (Kirsten and Horn, 1974; Rajan et al., 1975). On the other

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Fig. 3. Cartoon of the Rheasilvia cratering event. The impactor, shown here as inci-dent at ∼60◦ from vertical axis, initially generates an asymmetrical thermal plume (cf. Davison et al., 2014) that propagates a molten zone from the locus of the im-pact. Some molten material penetrates fractures in less heated regions of the Vestan regolith and mantle and cools to produce the glass veins seen in meteorites such as Cachari, Kapoeta, and other howardites. Depending on grain size, shock state and localized temperature-time history of the surrounding material, the injection of the molten glass will produce partial to complete thermal resetting of 40Ar–39Ar ages. An event of the magnitude depicted would eject much debris from Vesta and ac-count for the vestoids seen in the vicinity of Vesta (Harderson et al., 2014). Star symbols represent plausible sites of Kapoeta (K), Cachari (C), and Yamato-A (Y) on Vesta prior to their ejection as kilometer-scale Vestoids. The heavy dashed line labeled “Final surface” approximates the final shape, including slump material, of what is now the Vestan southern hemisphere.

hand, in the eucrite Cachari (Bogard et al., 1985) as well as in a few L chondrites (McConville et al., 1988; Bogard et al., 1995;Beard et al., 2014; Lindsay, pers. comm.) glass appears to be olderthan the “host”. Fortunately, this complication does not affect Kapoeta, in which the range of glass ages is comparable to or less than the corresponding range of matrix ages. A more focused study of relict-free melts from Kapoeta would be desirable.

Re-setting of the argon clock is generally thought to occur in locations blanketed by hot impact ejecta (e.g., Bogard, 2011). As noted by Marchi et al. (2013), however, no widespread kilogram-scale re-setting of radiochronometers in HED meteorites took place 1 Ga ago (except in Y-74097). This observation is consistent with the low impact velocities of collisions, the generally low temper-atures expected for asteroidal impact debris, and the difficulty of maintaining slow cooling rates (Keil et al., 1997) on a small ob-ject such as a Vestoid. If most HED meteoroids came from vestoids and vestoid launch requires an unusually large event, then the age of the last large event on Vesta, Rheasilvia, probably sets a lower limit on the Ar/Ar likely to be observed in HEDs. Material re-set by later, smaller events is probably still on Vesta.

Given the difficulty of keeping asteroidal material hot enough for long enough to re-set the argon clock, one may doubt the like-lihood that the glass in Kapoeta generated enough heat to re-set the ages of the young plagioclase grains. To first order, the dis-tance scales for Kapoeta seem plausible in the sense that our re-set grains lie within about one vein thickness, ∼1 mm, of the glass. Detailed thermal modeling would be desirable.

Also desirable, would be improved precision of the 40Ar/39Ar ages of the plagioclase grains that date the young event. At present, the uncertainties of the measured ages reflect the small sample size. The analysis of a larger mass of plagioclase grains har-vested from the region surrounding Kapoeta glass almost certainly would yield better statistics by increasing counting rates for minor

argon isotopes and decreasing the relative size of the blank correc-tions. Unfortunately, the hunt for such particles is labor-intensive and the amount of material available limited. More generally, while the pooling of samples for analysis makes good sense for a miner-alogically homogeneous population of grains, in our view the ho-mogeneity of plagioclase grains – and in particular how uniformly they respond to a thermal pulse – is not yet well established.

5. Conclusion

The Kapoeta howardite contains individual feldspar clasts with 40Ar/39Ar ages as young as 0.6 Ga, embedded in a breccia with a variety of feldspar ages as old as 4.5 Ga. Disturbed 40Ar/39Ar release patterns from bulk samples of Kapoeta and other HED me-teorites are consistent with a recent thermal event at ∼1 Ga.

The thermal pulse responsible for the resetting of the feldspar grains in Kapoeta was provided by shock-generated intrusive melt veins, which quenched to glass but contain older relict grains that compromise their ages, yielding only an upper limit to the event.

Comparison of the ages of the youngest grains from Kapoeta with Vestan crater count ages from the Dawn mission suggests an association between these young crystals and the formation of the large young Rheasilvia basin.

Acknowledgements

We thank the American Natural Museum of History in NYC, NY for allotting us a part of the sample Kapoeta 4788 and Joseph Boesenberg for preparing the initial slabs. We thank Bill Bottke and two anonymous reviewers for constructive and thoughtful reviews and Bernard Marty for considerate handling. This study was made possible by NASA’s Cosmochemistry Program grants NNX09AG37G and NNX12AL40G, and the National Science Foun-dation grant EAR1057650.

Appendix A. Supplementary material

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2014.12.049.

References

Beard, S.P., Kring, D.A., Isachsen, C.E., Lapen, T.J., Zolensky, M.E., Swindle, T.D., 2014. Ar–Ar analysis of Chelyabinsk: evidence for a recent impact. Lunar Planet. Sci. XLV, 1807. (abstr.).

Binzel, R.P., Xu, S., 1993. Chips off of asteroid 4 Vesta: evidence for the parent body of basaltic achondrite meteorites. Science 260, 186–191.

Bogard, D.D., 1995. Impact ages of meteorites: a synthesis. Meteoritics 30, 244–268.Bogard, D.D., 2011. K–Ar ages of meteorites: clues to parent-body thermal histories.

Chem. Erde 71, 207–226.Bogard, D.D., Garrison, D.H., 2003. 39Ar–40Ar ages of eucrites and thermal history of

asteroid 4 Vesta. Meteorit. Planet. Sci. 38, 669–710.Bogard, D.D., Garrison, D.H., 2009. Ar–Ar impact heating ages of eucrites and timing

of the LHB. Lunar Planet. Sci. XL, 1131 (abstr.).Bogard, D.D., Taylor, G.J., Keil, K., Smith, M.R., Schmitt, R.A., 1985. Impact melting of

the Cachari eucrite 3.0 Gy ago. Geochim. Cosmochim. Acta 49, 941–946.Bogard, D.D., Garrison, D.H., Norman, M., Scott, E.R.D., Keil, K., 1995. 39Ar–40Ar age

and petrology of Chico: large-scale impact melting on the L chondrite parent body. Geochim. Cosmochim. Acta 59, 1383–1399.

Caffee, M.W., Nishiizumi, K., 2001. Exposure history of separated phases from the Kapoeta meteorite. Meteorit. Planet. Sci. 36, 429–437.

Caffee, M.W., Hohenberg, C.M., Swindle, T.D., 1985. Pre-compaction irradiation of Kapoeta: 40Ar–39Ar ages of individual grains. Meteoritics 20, 620.

Cohen, B.A., 2013. The Vestan cataclysm: impact-melt clasts in howardites and the bombardment history of 4 Vesta. Meteorit. Planet. Sci. 48, 771–785.

Consolmagno, G.J., Drake, M.J., 1977. Composition and evolution of the eucrite par-ent body – evidence from rare earth elements. Geochim. Cosmochim. Acta 41, 1271–1282.

Dalrymple, G.B., Lanphere, M.A., 1969. Potassium–Argon Dating, Principles Tech-niques and Applications to Geochronology. W.H. Freeman and Company, San Francisco, 258 pp.

JID:EPSL AID:13035 /SCO [m5G; v1.145; Prn:19/01/2015; 12:33] P.6 (1-6)

6 F.N. Lindsay et al. / Earth and Planetary Science Letters ••• (••••) •••–•••

Dalrymple, G.B., Lanphere, M.A., 1974. 40Ar/39Ar age spectra of some undisturbed terrestrial samples. Geochim. Cosmochim. Acta 38, 715–738.

Davison, T.M., Ciesla, F.J., Collins, G.S., Elbeshausen, D., 2014. The effect of impact obliquity on shock heating in planetesimal collisions. Meteorit. Planet. Sci. 49, 2252–2265.

Delaney, J.S., Prinz, M., Nehru, C.E., Harlow, G.E., 1981. A new basalt froup from Howardites: mineral chemistry and relationships with basaltic achondrites. Lu-nar Planet. Sci. XII, 211–213 (abstr).

De Sanctis, M.C., Ammannito, E., Capria, M.T., Capaccioni, F., Combe, J.-P., Frigeri, A., Longobardo, A., Magni, G., Marchi, S., McCord, T.B., Palomba, E., Tosi, F., Zambon, F., Carraro, F., Fonte, S., Li, Y.J., McFadden, L.A., Mittlefehldt, D.W., Pieters, C.M., Jaumann, R., Stephan, K., Raymond, C.A., Russell, C.T., 2013. Vesta’s mineralogi-cal composition as revealed by the visible and infrared spectrometer on Dawn. Meteorit. Planet. Sci. 48, 2166–2184.

Eugster, O., Michel, Th., 1995. Common asteroid break-up events of eucrites, dio-genites, and howardites and cosmic-ray production rates for noble gases in achondrites. Geochim. Cosmochim. Acta 59, 177–199.

Harderson, P.S., Reddy, V., Roberts, R., Mainzer, A., 2014. More chips off of Asteroid (4) Vesta: characterization of eight Vestoids and their HED meteorite analogs. Icarus 242, 269–282.

Kaneoka, I., Ozima, M., Yanigisawa, M., 1979. 40Ar–39Ar age studies of four Yamato-74 meteorites. Mem. Natl. Inst. Polar Res., Spec. Issue 12, 186–206.

Keil, K., Stöffler, D., Love, S.G., Scott, E.R.D., 1997. Constraints on the role of impact heating and melting in asteroids. Meteorit. Planet. Sci. 32, 349–363.

Kennedy, T., Jourdan, F., Bevan, A.W.R., Gee, M.A.M., Frew, A., 2013. Impact history of the HED parent body(ies) clarified by new 40Ar/39Ar analyses of four HED mete-orites and one anomalous basaltic achondrite. Geochim. Cosmochim. Acta 115, 162–182.

Kirsten, T., Horn, P., 1974. 39Ar–40Ar dating of basalts and rock breccias from “Apollo-17” and the Malvern achondrite. Moon 11, 411–414.

Korochantseva, E.V., Trieloff, M., Buikin, A.I., Hopp, J., Meyer, H.-P., 2005. 40Ar/39Ar dating and cosmic-ray exposure time of desert meteorites: Dhofar 300 and Dho-far 007 eucrites and anomalous achondrite NWA 011. Meteorit. Planet. Sci. 40, 1433–1454.

Kuiper, K.F., Deino, A., Hilgen, F.J., Krijgsman, W., Renne, P.R., Wijbrans, J.R., 2008. Synchronizing rock clocks of Earth history. Science 320, 500–504.

Kunz, J., Falter, M., Jessberger, E.K., 1997. Shocked meteorites: argon-40–argon-39 evidence for multiple impacts. Meteorit. Planet. Sci. 32, 647–670.

Lindsay, F.N., Herzog, G.F., Park, J., Delaney, J.S., Turrin, B.D., Swisher III, C.C., 2014. 40Ar/39Ar dating of microgram feldspar grains from the paired feldspathic achondrites GRA 06138 and 06129. Geochim. Cosmochim. Acta 129, 96–110.

Marchi, S., McSween, H.Y., O’Brien, D.P., Shenk, P., De Sanctis, M.C., Gaskell, R., Jau-mann, R., Mottola, F., Raymond, C.A., Roatsch, T., Russell, C.T., 2012. The violent collisional history of asteroid 4 Vesta. Science 336, 690–694.

Marchi, S., Bottke, W.F., Cohen, B.A., Wünnemann, K., Kring, D.A., McSween, H.Y., de Sanctis, M.C., O’Brien, D.P., Schenk, P., Raymond, C.A., Russell, C.T., 2013. High-velocity collisions from the lunar cataclysm recorded in asteroidal meteorites. Nat. Geosci. 6, 303–307.

McConville, P., Kelley, S., Turner, G., 1988. Laser probe 40Ar–39Ar studies on the Peace River shocked L6 chondrite. Geochim. Cosmochim. Acta 52, 2487–2499.

McCord, T.B., Adams, J.B., Johnson, T.V., 1970. Asteroid Vesta: spectral reflectivity and compositional implications. Science 168, 1445–1447.

Mori, H., Takeda, H., 1981. Thermal and deformational histories of diogenites as inferred from their microtextures of orthopyroxene. Earth Planet. Sci. Lett. 53, 266–274.

Prinz, M., Nehru, C.E., Delaney, J.S., Harlow, G.E., Bedell, R.L., 1980. Modal studies of mesosiderites and related achondrites, including the new mesosiderite ALHA 77219. In: Proc. Lunar Planet. Sci. 11th Conf., pp. 1055–1071.

Pun, A., Keil, K., Taylor, G.J., Wieler, R., 1998. The Kapoeta howardite: implications for the regolith evolution of the howardite–eucrite–diogenite parent body. Meteorit. Planet. Sci. 33, 835–851.

Rajan, R.S., Huneke, J.C., Smith, S.P., Wasserburg, G.J., 1975. 40Ar–39Ar chronology of isolated phases from Bununu and Malvern howardites. Earth Planet. Sci. Lett. 27, 181–190.

Rajan, R.S., Huneke, J.C., Smith, S.P., Wasserburg, G.J., 1979. Argon 40 – argon 39 chronology of lithic clasts from the Kapoeta howardite. Geochim. Cosmochim. Acta 43, 957–971.

Rivera, T.A., Storey, M., Zeeden, C., Hilgen, F.J., Kuiper, K., 2011. A refined astro-nomically calibrated 40Ar/39Ar age for Fish Canyon sanidine. Earth Planet. Sci. Lett. 311, 420–426.

Schmedemann, N., Kneissl, T., Ivanov, B.A., Michael, G.G., Wagner, R.J., Neukum, G., Ruesch, O., Hiesinger, H., Krohn, K., Roatsch, T., Preusker, F., Sierks, H., Jaumann, R., Reddy, V., Nathues, A., Walter, S.H.G., Neesemann, A., Raymond, C.A., Russell, C.T., 2014. The cratering record, chronology and surface ages of (4) Vesta in comparison to smaller asteroids and the ages of HED meteorites. Planet. Space Sci. http://dx.doi.org/10.1016/j.pss.2014.04.004.

Shankar, N., Swisher III, C.C., Turrin, B., Herzog, G.F., 2008. 40Ar/39Ar – CO2 laser incremental heating release spectra for the Pasamonte eucrite and martian me-teorites ALHA77005, Shergotty, and Y000749. Lunar Planet. Sci. XXXIX, 1924 (abstr.).

Shukolyukov, I.A., Fugzan, M.M., Dang, V.M., Kolesov, G.M., Skripnik, A.I., 1984. AR-40–AR-39 dating of the meteorite Vetluga. Geokhim., 291–301.

Steiger, R.H., Jäger, E., 1977. Subcommission on geochronology: convention on the use of decaly constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359–362.

Takeda, H., 1991. Comparisons of Antarctic and non-Antarctic achondrites and pos-sible origin of the differences. Geochim. Cosmochim. Acta 55, 35–47.

Takeda, H., Mori, H., 1981. Mineralogy of the Yamato diogenites as possible pieces of a single fall. Mem. Natl. Inst. Polar Res., Spec. Issue 20, 81–99.

Taylor, J.R., 1997. An Introduction to Error Analysis, second ed. University Science Books, Sausalito, CA. p. 327.

Turner, G., Knott, S.F., Ash, R.D., Gilmour, J.D., 1997. Ar–Ar chronology of the Mar-tian meteorite ALH84001: evidence for the timing of the early bombardment of Mars. Geochim. Cosmochim. Acta 61, 3835–3850.

Turrin, B.D., Swisher III, C.C., Deino, A.L., 2010. Mass discrimination monitor-ing and inter-calibration of dual collectors in noble-gas mass spectrometer systems. Geochem. Geophys. Geosyst. 11, Q0AA09. http://dx.doi.org/10.1029/2009GC003013.

Wasson, J.T., 2013. Vesta and extensively melted asteroids: why HED meteorites are probably not from Vesta. Earth Planet. Sci. Lett. 381, 138–146.

Williams, D.A., Jaumann, R., McSween, H.Y., Marchi, S., Schmedemann, N., Raymond, C.A., Russell, C.T., 2014. The chronostratigraphy of protoplanet Vesta. Icarus 244, 158–165.