Argon Isotopic Analysis of Breccia Veins from the Roter Kamm Crater, Namibia, and Implications for Their Thermal History

Meteoritics & Planetary Science 40, Nr 6, 841–854 (2005) Abstract available online at http://meteoritics.org

Argon isotopic analysis of breccia veins from the Roter Kamm crater, Namibia, and implications for their thermal history

David RAJMON*, Peter COPELAND, and Arch M. REID

Department of Geosciences, University of Houston, Houston, Texas 77204, USA *Corresponding author. E-mail: [email protected] (Received 02 April 2002; revision accepted 23 March 2005)

Abstract–The rocks exposed in the rim of the 2.5-km-wide and 3.7-Ma-old Roter Kamm crater in southwest Namibia are cut by breccia veins that macroscopically resemble, and were originally described as, pseudotachylytes. The veins were later shown to be cataclasites with no evidence for melting. 40Ar/39Ar data for vein and host rock samples indicate a low-grade metamorphic event at around 300 Ma, but provide no evidence for an impact age. The samples have suffered 5–7% Ar loss, which we associate with the impact event. All the samples record similar ranges of possible time- temperature conditions and there are no resolvable differences between the results for the vein and the host rock samples, as would be expected if frictional heating played an important role in breccia formation. Modeling the 40Ar/39Ar data, assuming instantaneous impact heating followed by extended cooling, and coupling these results to published data on fluid inclusions, quartz precipitation, shock effects, and crater degradation, suggest that the veins reached maximum temperatures of 230–290 °C during impact and never approached melting temperatures of the precursor rocks.

INTRODUCTION large impact craters (Reimold 1995, 1998; Spray 1998) and on major tectonic faults (e.g., Camacho et al. 1995). The chronology of impact events on Earth is important in Pseudotachylytes have been generated in shock experiments addressing various geologic problems, such as the role of (e.g., Fiske et al. 1995; Kenkmann et al. 2000) and in impacts in geologic evolution of the Earth (e.g., Dressler et al. experiments involving localized cataclasis and frictional 1994), the relationship between impacts and biologic and melting (Spray 1987). The term pseudotachylyte has been environmental evolution (e. g., Ryder et al. 1996; Vonhof often used in a much wider sense for similar-looking rocks, et al. 2000), reconstruction of the terrestrial meteorite flux including various types of tectonic and impact breccias, (e.g., Farley et al. 1998), and stratigraphic and reflecting limitations in our understanding of the genesis and paleogeographic reconstructions (e.g., Courtillot et al. 2000). the difficulty in proper recognition of these various rock types Isotopic ages of impact craters have been obtained both from (Reimold 1995, 1998). However, a misinterpretation of impact melt rocks and from pseudotachylytes (e.g., Deutsch unmelted breccias as pseudotachylyte can lead to erroneous and Sch‰rer 1994; Spray et al. 1995). Smaller impacts isotopic dating results. produce little if any melt but, in some cases, generate breccias Breccia veins occurring in the Roter Kamm crater of SW similar to pseudotachylyte (Reimold and Miller 1989; Namibia were originally described as pseudotachylytes Degenhardt et al. 1994). (Reimold and Miller 1989). Degenhardt et al. (1994) Pseudotachylyte is a very fine-grained, glass-like rock concluded that the veins did not actually contain melted that occurs as veins and irregular breccia matrices within host material but retained the term pseudotachylyte. We report rocks. The veins may appear intrusive into the host rock and here results of 40Ar/39Ar analysis of samples from the contain abundant clasts of the host rock. True Degenhardt study and interpret these data in terms of the pseudotachylytes show evidence of the former presence of a thermal history of the Roter Kamm breccia veins. The results melt (such as glass, microlites, vesicles, chilled margins; e.g. bear on general understanding of thermal effects of impacts Reimold 1998 and references therein) of frictional origin. on target rocks. While the thermal effects of large impact Pseudotachylytes have been found associated with several structures have been studied extensively (e.g., Staudacher

841 © The Meteoritical Society, 2005. Printed in USA. 842 D. Rajmon et. al.

to about 35 GPa (Huffman and Reimold 1996; French 1998), were reported by Reimold and Miller (1989) and Degenhardt et al. (1994). Multiple sets of PDFs in quartz within suevite were also reported by Reimold et al. (1997). Shock features, reported from a single impact melt rock sample (Reimold and Miller 1989), include diaplectic quartz (corresponding to 35– 45 GPa according to compilation in French 1998, or > 27 GPa according to Huffman and Reimold 1996) and vesicular glass (∼45–50 GPa, French 1998). Besides the dated impact melt rock, melt was reported only in the form of several fragments in suevite (Reimold et al. 1997). Degenhardt et al. (1994) found that the pseutotachylyte- like breccias contained irregular clasts of the host rock displaying a wide range of grain sizes from several millimeters to about 10 µm. The finest grains displayed granoblastic texture suggestive of recrystallization. The dark color of the breccias was attributed to the presence of fine opaque minerals in quartz grains. No evidence for the presence of melt was found. According to these authors, the overall textural and geochemical characteristics of the veins suggested minimal transport of the vein material relative to the immediate host rock and minimal introduction of exotic Fig. 1. The location of the Roter Kamm crater. material. They concluded that the breccias were formed by local comminution followed by recrystallization of mainly et al. 1982; Boer et al. 1996; Moser 1997; Gibson et al. 1998; fine-grained material and that the heat generated during this Ivanov and Deutsch 1999), data on the thermal effects of process was insufficient to melt the rock. small impacts are rare (Koeberl et al. 1989). ANALYTICAL TECHNIQUES BACKGROUND Selected samples of vein and host rock material (Table 1, Roter Kamm is a simple crater, 2.5 km in diameter, Fig. 2) were crushed and processed by density and magnetic located in southwestern Namibia (27°46′S, 16°18′E, Fig. 1). separation in order to concentrate K-feldspar for 40Ar/39Ar Its impact origin has been established by a number of analyses. Composition of each concentrate was roughly geological, petrological (Reimold and Miller 1989; Reimold estimated based on XRD data and visual checking under a et al. 1997), and geophysical (Fudali 1973; Brandt et al. 1998) binocular microscope. The small grain sizes (∼1–200 µm in studies. Erosion has lowered the crater rim by 40–130 m and the vein matrix) made recognition and manual separation of most of the ejecta, apart from a few patchy occurrences, have different feldspars and other minerals difficult. Therefore, the been stripped away (Grant et al. 1997). 40Ar/39Ar analysis of K-feldspar concentrates contain some quartz and two of them an impactite sample provided an age for the impact event of also contain some albite. 3.7 ± 0.3 (1σ) Ma (Koeberl et al. 1993). Analytical and data reduction procedures for Ar isotopic The rocks exposed at the crater rim are gneisses and analyses follow those used by Spell et al. (1996) at the granites belonging to the Namaqualand Metamorphic University of Houston, except for a few differences. The K- Complex with a metamorphic age of 900–1200 Ma (Reimold feldspar concentrates were irradiated at a reactor at the and Miller 1989). At the time of impact, the Namaqualand University of Michigan together with fluence monitors gneisses were probably covered by a thin layer of Late (Australian National University 92-176, Fish Canyon Tuff Proterozoic Gariep metasedimentary rocks, which are sanidine, assumed age = 27.9 Ma, Steven et al. 1967; Cebula ∼700 Myr old. et al. 1986). Correction factors for reactions on Ca were (39Ar/ The breccias occur as numerous fine-grained dark- 37Ar) = 6.689 (±0.071) × 10−4 and (36Ar/37Ar) = 2.678 colored veins and dykes of submillimeter to meter widths. (±0.036) × 10−4, and for reactions on K (38Ar/39Ar) = 1.077 × The larger veins appear to extend radially from the crater, 10−2 and (40Ar/39Ar) = 4.950 (±0.090) × 10−2; discrimination whereas the fine veinlets may form networks without factor was 9.960 (±0.018) × 10−1 (all errors are 1σ). preferred orientation. The rocks show rare evidence for shock Irradiation factors with their respective errors are presented metamorphism. Planar deformation features (ω and π in together with the data in Table 2. The samples were heated by quartz), together indicating maximum shock pressures of 20 steps in a double-vacuum resistance furnace. Argon isotopic analysis of breccia veins from the Roter Kamm crater 843

Fig. 2. Hand specimen RK-SR and RK-1. RK-SR is granitic gneiss with a dark-to-greenish gray breccia vein. The host rock is dominated by pink microcline with smaller amount of albite and quartz, and is strongly fractured with many narrow breccia veinlets (indicated by arrow). The main vein displays a flow-like texture. The large fragments within the breccia match composition of the host gneiss. The network of the fine light fractures (indicated by arrows) was generated during sample manipulation. RK-1 is light gray granite cut by networks of narrow breccia veinlets. Locations, from which the analyzed K-feldspar concentrates were separated, are indicated. See text for further explanations.

Table 1. List of samples. Hand specimen K-feldspar concentrate RK-SR Compact vein of dark breccia, 4–5 cm thick, in pink granitic gneiss. Host rock → RK-SR-VR Microcline > plagioclase, quartz, mica, a few percent of From fragment of the brecciated host rock adjacent to the main oxides, and minor secondary carbonate. Multiple narrow vein, fractures filled with narrow breccia veinlets. The apophyses of the breccia material intrude the gneiss. All concentrate contained microcline (70–80 vol%) plus albite and minerals highly fractured, grain sizes 0.5–5 mm. quartz in the grain size range of 125–177 µm. Vein → RK-SR-P Multiple rock fragments (matching composition and texture of Vein core material avoiding oxide accumulations and feldspar the host) in a fine-grained cataclastic matrix. Crude flow clasts >0.2 mm in diameter. The concentrate contained texture developed. Matrix in the vein core contains microcline, microcline (∼60 vol%) and quartz (∼40 vol%). The fragments in quartz, and some oxides, margin of the vein also has albite and this concentrate ranged from 177–564 µm in diameter but more oxides. Grain sizes in the matrix down to ∼1 µm. consisted of grains smaller than 200 µm. RK-1 → RK-1-VR Gray granite cut by breccia veinlets. Contained material from both host rock and veins: microcline Contains subequal amounts of quartz, microcline and (∼70 vol%), albite (∼15 vol%), and quartz (∼15 vol%). plagioclase, plus minor mica and chlorite. Incipient alteration Fragments in the concentrates were 125–177 µm in diameter but of feldspars. The rocks are cut by irregular networks of dark sizes of individual grains ranged down to ∼20 µm. breccia veinlets up to 3 mm wide. The breccia has the same composition as the host granite.

40 39 In order to be able to obtain precise results on the early Ar*/ ArK, we estimated the age from the adjoining (and presumably youngest) fractions of gas released, we measured steps, taking into consideration the relative chose large sample sizes (150–200 mg), which had the importance of those steps in terms of the fraction of released advantage of providing an easily measurable amount of gas Ar and the general trend within this particular part of released in the low-temperature steps but the disadvantage of spectrum. We chose the uncertainty in these estimated steps an inconveniently large amount of gas in some high- so as to encompass the ages of the surrounding steps, or in the temperature steps. The setup of the mass spectrometer would case of steps on the plateau, to encompass uncertainties in all cause the gas to saturate the electrometer above a certain measured steps on the plateau. We calculated plateau ages and amount of signal and this threshold was exceeded with 40Ar in associated uncertainties as error-and-gas-weighted averages several steps; this leads to an underestimate of the apparent of the selected analytical steps (e.g., Mahon 1996). Total-gas age of these steps. In these steps without a reliable value for ages are averages of ages on all the steps in respective spectra 844 D. Rajmon et. al. σ 1 ± 0.7 0.7 0.6 0.7 0.7 0.7 0.8 0.8 0.8 0.7 1.0 0.9 0.9 0.9 0.9 3.1 2.7 8.3 1.0 0.5 0.5 0.5 0.4 0.8 2.1 0.6 0.5 0.5 0.5 0.6 2.0 5.1 0.7 0.9 12.1 2.8 11.2 3.4 2.7 p 0.6 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Ar 39 % cumul. Age (Ma) Ar 39 % Ar* 40 % Ar ) 39 2 − Ar*/ Err 40 (×10 Ar ) 39 2 − Ar*/ 40 (×10 Ar ) 2 39 − Ar/ 40 (×10 Ar ) 5 39 − Ar/ 38 (×10 Ar ) 5 39 − Ar/ Ar step heating analyses. 37 (×10 39 Ar/ 40 Ar ) 5 39 − Ar/ 36 (×10 550550560560570 83570 32580 48580 37590 52 0590 40 0600 35 0600 65 0610 36 153 0610 94 146 0620 41 142 0620 49 147 0 32 146 0 15,213 55 144 0 15,587 42 147 0 149 16,015 25 150 125 16,203 14,962 162 170 16,667 15,487 156 16,955 15,869 150 0 148 412 17,406 16,089 140 17,688 16,509 48 153 18,120 16,833 46 18,618 17,298 46 156 141 18,392 17,492 19,162 48 18,011 19,316 98.4 47 18,336 19,833 99.4 47 18,266 99.1 55 19,012 20,409 20,796 99.3 57 19,217 99.1 0.34 58 19,666 99.3 50 0.56 20,280 99.4 76 0.90 20,717 63 98.9 0.74 64 11.2 99.4 0.89 69 11.8 98.5 0.77 12.7 150 216.0 99.3 66 0.82 99.2 13.4 223.1 0.58 99.5 14.3 228.3 0.53 99.2 15.1 231.2 0.71 15.9 99.6 236.9 99.4 0.85 0.75 16.5 241.3 0.79 17.0 247.5 0.52 17.7 250.1 18.6 19.3 0.62 257.0 0.47 20.1 261.3 20.6 260.4 270.3 21.7 273.0 21.1 278.9 292.6 286.9 354397441435 6662460 8946461 3653481 3067481 0 939501 0 703500 0 397520 0 287520 1521 187530 0 1910 131530 0 848 108540 0 742540 0 83 24,946 312 0 64 32,115 278 0 66 210 17,523 0 60 190 17,362 41 5255 0 167 5674 12,214 0 207 11,990 0 6723 151 11,984 0 8293 208 148 12,039 0 185 9434 149 12,019 9907 573 146 12,125 10,805 147 12,905 11,186 70 21.1 148 11,463 13,088 17.7 34 11,733 13,571 38 38.4 12,581 32 13,928 47.8 30 14,447 0.35 12,837 57 14,729 0.15 13,376 77.3 149 13,729 82.7 1.19 90.2 39 14,264 0.3 93 0.25 14,604 35 0.5 95.4 38 0.65 96.8 1.7 38 0.47 78.8 97.5 0.72 41 1.9 85.0 98.1 46 0.64 100.3 1.04 2.6 98.6 0.05 3.1 122.9 98.6 3.8 1.16 98.8 139.2 4.4 99.2 0.92 5.5 145.9 5.5 158.5 0.99 0.84 6.7 163.9 167.7 0.84 171.5 7.6 0.60 8.6 183.3 9.4 10.3 186.8 10.9 194.3 199.1 206.5 211.1 526633710750 324790830 7890 30950 1171 82 0 34 0 232 0 892 152 35 0 404 12 0 537 135 0 22,181 57 22,319 22,081 21,220 220 0 170 23,916 22,226 22,056 25,874 392 23,671 23,277 24,899 24,878 25,916 66 55 949 95.7 23,167 24,965 24,879 219 99.6 99.9 99 213 883 0.33 266 100.2 2.15 22.0 100.3 3.31 100 99.5 1.90 299.2 3.71 27.5 25.4 0.30 29.4 4.83 3.06 312.2 33.1 310.0 330.7 33.4 41.3 36.5 359.2 347.2 324.3 346.1 RK-SR-VR Temp. Temp. (°C) Table 2.Table Results of Argon isotopic analysis of breccia veins from the Roter Kamm crater 845 σ 1 ± 1.8 0.4 12 0.9 6* * 1.4 0.7 1.2 1.5 1.3 1.6 1.4 1.8 2.1 1.7 1.2 p 1.4 p 1.4 p 1.7 p 10 p 2.6 p 2.3 p* 1.8 p 2.3 p p p 0.2 0.4 0.3 4.3 2.3 2.7 p 3.3 p 39.1 p 3.0 p p 2.8 2.8 p 4.8 p 4.9 p 0.4 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Ar 39 % cumul. Age (Ma) Ar 39 % Ar* . 40 % % 1.1 ± 6.9 Ar ) 39 = 2 − Ar*/ Err 40 (×10 Ar ) 39 2.9 Ma, Ar lost 2 ± − Ar*/ 40 (×10 323.3 = Ar ) 2 39 − Ar/ 40 (×10 2.2 Ma, Plateau age ± Ar ) 5 39 − 300.9 Ar/ = 38 (×10 Ar step heating analyses. 39 Ar/ Ar ) 5 39 − 40 , Total-gas age , Total-gas 4 Ar/ − 37 (×10 Ar ) Results of 5 39 − 0.006) 10 ± Ar/ 36 (×10 8.500 ( Continued. = 418439242510 193530 312570 214152650 0690 0 23730 0 17770 0 66810 32850 0 45 19 26 390890 189 43 332930 188 0 642970 0 833 137 16 44 914 74 28 9653 214 9584 n.d. 179 19 172633 19 31 159 161 n.d. 154 10,168 417 9079 8657 11,486 208 148 11,722 n.d. 179 15,294 67 161 163 10,199 16,812 130 11,431 188 n.d. 18,702 11,524 26 166 15,195 1108 19,609 n.d. 170 16,753 68 20,368 20,490 18,572 100 94.1 n.d. 20,145 47 90.4 19,613 89 19,952 306 114 100.3 n.d. 20,362 19,622 20,439 101 99.6 20,059 0.01 n.d. 98.4 19,893 122 0.34 99.4 19,560 n.d. 99.7 106 143 0.04 0.60 99.3 164 1.2 0.73 131 100 1.5 4.45 96 2.17 4.25 100 2.2 130.3 99.8 6.93 n.d. 2.1 124.5 99.6 4.81 5.1 99.7 9.5 13.8 145.7 4.3 99.7 4.89 135.0 20.7 162.6 3.48 3.86 n.d. 25.5 163.8 213.1 3.88 151.0 4.62 30.4 233.6 6.00 257.2 37.8 34.3 41.6 n.d. 270.6 46.2 52.3 281.1 280.1 276.3 274.1 269.9 18.14 94.3 273.0 398387389 841 621 485 0 0 0 320 249 226 6844 8378 9245 4354 6537 7808 26 24 27 63.7 78.1 84.5 0.26 0.32 0.15 0.7 1.0 1.2 63.7 94.8 112.6 340 734 0 296 4635 2461 17 53.1 0.45 0.4 36.3 1010105010901170 41210 121250 181290 148 0 219 34 257 105 130 169 171 0 644 0 169 1138 19,593 166 19,705 184 187 198 19,852 19,575 19,666 19,794 19,255 20,118 20,315 21,020 105 108 19,410 19,803 128 20,208 20,635 99.9 99.8 206 139 99.7 176 178 7.44 100 8.09 98.5 99.5 8.43 98.2 59.7 67.8 76.2 0.62 1.65 1.23 1.33 270.1 271.3 272.9 95.0 97.8 96.2 99.2 268.0 273.0 278.2 283.6 RK-SR-P 10701110113511601185 771210 85 12112351245 0 4011255 34 175 215J-factor 89 198 5948 0 228 755 0 0 231 867 156 38227 0 23,771 408 98 0 23,346 436 23,972 0 23,540 16,742 23,091 22,861 23,611 62 23,828 24,203 177 17,309 30,985 22,223 208 23,722 23,616 261 30,861 2926 84,589 99 220 98.9 237 371 98.5 30,984 53,383 103.4 11.57 97.2 6.35 99.6 97.6 406 7.20 272.3 0.03 53.2 17.59 59.5 13.90 100.4 1.24 66.7 329.1 0.00 66.7 323.3 85.6 330.0 99.5 68.0 247.6 0.52 99.5 312.2 330.0 331.4 977.4 100.0 421.8 Temp. Temp. (°C) 1070 28 0 97 27,851 27,763 344 99.7 0.30 41.6 382.2 Table 2. Table 846 D. Rajmon et. al. σ 1 ± 8.1 13.1 18.9 3.4 1.8 1.1 0.9 17.5 0.7 0.9 10.5 5.9 5.7 1.5 4.8 2.3 2.6 2.8 3.0 3.4 3.6 3.3 3.2 3.1 p 3.2 p 3.0 p 3.0 p 20.0 p 4.0 p p* 4.1 6.6 p p 3.1 2.7 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Ar 39 % cumul. Age (Ma) . σ Ar 39 % ateau. All errors are 1 All errors are ateau. Ar* . . 40 % % % 3.4 1.8 ± ± 4.9 Ar 6.1 = ) = 39 step included in pl 2 = − Ar*/ Err 40 (×10 Ar ) 39 4.0 Ma, Ar lost 1.8 Ma, Ar lost 2 age estimated; p ± − ± = Ar*/ 40 (×10 300.8 271.8 = = Ar ) 2 39 − Ar/ 40 (×10 9.6 Ma, Plateau age 4.7 Ma, Plateau age ± ± Ar ) 5 39 − Ar yielding saturation of the detector; * 286.1 255.2 40 Ar/ = = 38 (×10 Ar step heating analyses. 39 Ar/ Ar ) 5 39 − 40 , Total gas age , Total age , Total-gas 4 4 Ar/ − − 37 (×10 Ar ) Results of 5 39 − 0.006) 10 0.006) 10 ± ± Ar/ 36 (×10 6.650 ( 8.250 ( Continued. = = not determined because of too high amounts = 373399441462 28455481 17066489 8114502 4191510 0 2107 507520 1241530 0 658570 179 495 5689620 3485 63 349660 0 196700 1703 117 329 945740 130 148 148,450780 104,591 549820 0 52 395 74 130900 1374 280 55,972940 64,361 54,158 11 216 40,322 507980 5 31,863 453 200 12 879 173 609 31,990 22,297 24 1556 27,933 2143 19,904 36 214 0 16,328 25,633 65 207 0 286 18,626 15,940 346 0 206 17,955 16,103 178 14,403 43.4 51.8 408 14,860 168 101 19,692 539 14,904 163 19,273 1633 15,519 86 13,429 167 57.2 216 23,022 69.3 0.04 0.09 277 19,250 63 80.5 296 23,852 19,116 84 22,637 966 90.2 539 24,430 83.6 0.11 25,716 0.1 27,161 0.17 0.2 531 91.0 23,814 27,191 0.21 143 27,078 24,410 463 642.7 93.5 554.9 96.4 25,675 93.3 0.45 0.3 27,085 0.10 0.5 27,082 97.8 219 0.20 0.7 26,882 348.0 99.2 255 307.4 98.3 270 0.12 1.3 299 0.96 0.8 0.19 284.0 334 3.18 1.5 99.9 354 203.5 210.7 3.67 99.9 1.6 5.59 2.6 99.9 2.7 99.7 170.0 99.6 5.9 6.43 99.3 170.5 177.2 154.3 9.6 15.2 5.80 4.35 217.3 3.41 3.05 21.6 215.9 253.0 2.71 27.4 31.8 35.2 265.2 38.2 271.4 40.9 284.4 298.8 298.7 296.7 338 15136 338 3285 51,060 6327 702 12.4 0.09 0.1 74.4 10201060110011401180 74 1051220 1121260 1001300 617 1181340 459 1681582 288J-factor 0 120 313 238 287 0 213 530 1401 0 1016 167 157 1124 3092 159 27,264 27,603 408 199 361 28,250 28,151 232 27,041 884 27,289 28,184 n.d. 26,257 27,914 27,850 29,832 320 29,642 310 27,829 25,755 8950 305 28,905 n.d. 315 28,793 99.2 298 98.9 7383 289 392 98.8 n.d. 98.9 403 2.84 98.8 3.34 575 98.1 96.9 4.05 4.09 n.d. 97.1 43.8 47.1 5.13 82.5 9.81 51.2 55.2 298.3 8.50 300.8 20.15 0.88 60.4 307.2 306.5 70.2 0.29 98.8 90.3 306.3 99.7 285.2 100.0 317.2 305.0 316.1 86.5 RK-1-VR 1550J-factor 163 2663 343 21,670 21,190 213 97.8 0.39 100.0 290.7 Temp. Temp. (°C) 1330 128 589 264 22,061 21,678 243 98.3 0.44 99.6n.d. 296.8 Table 2. Table Argon isotopic analysis of breccia veins from the Roter Kamm crater 847

Fig. 3. Breccia vein texture: a, b) RK-SR, contact of the vein (left) with the host rock (right). The vein matrix is fine-grained, loaded with angular fragments of the host rock. The host rock is fractured with many narrow breccia veinlets; a) polarized light, b) crossed polarizers, c) RK-1, crossed polarizers, ∼3-mm-thick vein cutting through host rock, displaying much smaller grain-size than the host rock, with which it is in sharp contact, d) RK-SR, SEM image of the breccia matrix documents its extremely fine-grained nature (photograph courtesy of John Degenhardt). weighted by fractions of released gas at each step. Calculation consists of microcline > plagioclase, quartz, mica, a few of both plateau and the total-gas ages included the estimated percent of oxides, and minor secondary carbonate. The vein ages. The uncertainties in the ages include uncertainties in Ar contains multiple rock fragments in a fine-grained cataclastic isotopic ratios, J-factor, blank, correction factors, and matrix (Fig. 3). We found on SEM images that many small discrimination, but they do not include the uncertainties in the “grains” are actually aggregates of even smaller grains age of the standard or in the decay constant (see McDougall ranging in size down to ∼1 µm. According to Degenhardt et and Harrison 1999). The uncertainties in ages were al. (1994) the mineral composition of the vein is the same as propagated in calculated Ar loss and diffusivities for each that of the host rock, except that mica is not present. Our XRD heating step used for Arrhenius diagrams. The uncertainties in data indicate that the vein core contains mostly microcline Ar loss were propagated in time-temperature plot and in and quartz and almost no albite. The host rock is fragmented thermal history models. All calculations of Ar diffusion were and penetrated by fine veinlets of the breccia. The sample done for infinite sheet geometry. appears fresh without any significant alteration. We separated and analyzed two K-feldspar concentrates. The first (RK-SR- SAMPLES VR) was from a fragment of the brecciated host rock adjacent to the main vein. It contained microcline (70–80 vol%) plus For this study, we selected the two breccia samples RK- albite and quartz in the grain size range of 125–177 µm. The SR and RK-1 (Fig. 2, Table 1) petrologically characterized by second concentrate (RK-SR-P) was vein material with Degenhardt et al. (1994). We analyzed mineral separates. K- feldspar clasts <0.2 mm in diameter. The concentrate feldspar is the most abundant potassium-bearing phase in contained microcline (∼60 vol%) and quartz (∼40 vol%). The these specimens. fragments in this concentrate ranged from 177–564 µm in Sample RK-SR represents a compact vein of dark diameter but consisted of grains smaller than 200 µm. breccia, 4–5 cm wide, in granitic gneiss. The gneiss host rock Sample RK-1 is a granite with subequal amounts of 848 D. Rajmon et. al.

in diameter but sizes of individual grains ranged down to ∼20 µm. Degenhardt et al. (1994) did not observe shock features in any of the samples selected for our study, but they described planar deformation features in other samples of the vein breccia indicative of shock pressure in the range of 20– 35 GPa.

Ar DATA

40Ar/39Ar analyses of the three K-feldspar concentrates yielded three similar age spectra (Fig. 4, Table 2). For each concentrate, the first heating step at ∼340–350 °C yielded the lowest apparent age. The age of the succeeding steps then gradually increased with increasing heating temperature until ∼900 °C. This part of the spectrum represents ∼40% of total released Ar. The remaining heating steps formed a plateau representing ∼60% of total released Ar. The heating schedule for the sample RK-SR-VR also involved double heating steps to check for possible fluid inclusion signature (Harrison et al. 1994); no such signature was observed. The plateau, lowest apparent. and total gas ages for the individual concentrates are: 323.9 ± 2.9, 78.8 ± 3.1 and 300.9 ± 2.2 Ma for RK-SR-VR, 271.8 ± 1.8, 36.3 ± 0.2 and 255.2 ± 4.6 Ma for RK-SR-P, and 300.8 ± 4.0, 74.4 ± 8.1 and 286.1 ± 9.6 Ma for RK-1-VR.

COMPARISON TO PREVIOUS DATING

Koeberl et al. (1993) investigated several isotopic systems of rocks exposed at the Roter Kamm crater. Rb-Sr analyses of granitic samples yielded several ages. Whole-rock data yielded 1292 ± 49.5 (1σ) Ma interpreted to be associated with the Namaqualand metamorphism. Ages of 671 Ma (no uncertainty given) and 466 ± 15.5 (1σ) Ma were derived from whole rock and mineral separates from two samples. The authors associated these ages with two different periods of Damaran igneous and metamorphic activity. The same authors reported K-Ar ages of 464 ± 21 (1σ) and 479 ± 22 (1σ) Ma for two “schistose melt breccias” (according to Degenhardt et al. 1996 “autochthonous impact breccias”) and 251 ± 12 (1σ) and 255 ± 12 (1σ) Ma for a “pseudotachylyte” and a “quarzitic melt breccia” (i.e., “vein Fig. 4. 40Ar/39Ar apparent age spectra for the vein (RK-SR-P), its host rock (RK-SR-VR), and the mixed sample (RK-1-VR). Height of each breccia” in this study). Koeberl et al. (1993) interpreted this box indicates 1σ error. age as being dominated by incompletely degassed mineral fragments with 470 Ma Damaran metamorphic ages. The quartz, microcline, and plagioclase plus minor mica and 255 Ma K-Ar age is identical with the total gas age we chlorite. Feldspars show incipient alteration. The rocks are obtained for the pure vein sample RK-SR-P. cut by irregular networks of dark breccia veinlets up to 3 mm Fission-track ages from apatite of about 20–28 Ma were wide (Fig. 3). The breccia has the same composition as the related by Koeberl et al. (1993) to either Burdigalian host granite. We analyzed the K-feldspar concentrate RK-1- peneplanation at 25 Ma or to partial impact resetting. VR, contained material from both host rock and veins: The ∼300 Ma plateau ages obtained in this study (Fig. 4) microcline (∼70 vol%), albite (∼15 vol%), and quartz are similar to 342 ± 4 (1σ) Ma 40Ar/39Ar plateau age reported (∼15 vol%). Fragments in the concentrates were 125–177 µm by Onstott et al. (1986) for plagioclase from ∼200 km Argon isotopic analysis of breccia veins from the Roter Kamm crater 849

Table 3. Summary of ages, Ar loss, and diffusion parameters. Plateau age Min. age Tot. gas age 2 −1 Sample (Ma) (Ma) (Ma) Ar loss % E (kJ/mol) E (kcal/mol) D0/a (s ) RK-SR-VR 323.0 ± 2.9 78.8 ± 3.1 300.9 ± 2.2 6.9 ± 1.1 165.4 39.5 1.36 × 105 RK-SR-P 271.8 ± 1.8 36.3 ± 0.2 255.2 ± 4.7 6.1 ± 1.8 167.5 40 4.90 × 104 RK-1-VR 300.8 ± 4.0 74.4 ± 8.1 286.1 ± 9.6 4.9 ± 3.4 164.6 39.3 9.01 × 103 = 2 = σ E activation energy D0/a frequency factor. All errors are 1 . southeast of Roter Kamm. Onstott et al. (1986) related this excursion will be brief. Various studies on granitic rocks have age to Karoo igneous activity. indicated peak temperatures during pseudotachylyte formation at ∼700–1500 °C (e.g., Kelley et al. 1994; Lin THERMAL MODELING 1994; Camacho et al. 1995). Tectonic pseudotachylyte- forming events are estimated to last only a few seconds The observed patterns of the age spectra can be explained (Spray 1987; Kelley et al. 1994; Spray et al. 1995) and are by two hypotheses: 1) slow cooling since the Carboniferous followed by rapid cooling. Data of Kelley et al. (1994) or 2) relatively fast cooling in the Carboniferous followed by indicate that the K-Ar system of the studied host paragneiss a impact-related heating event in the Pliocene. We did not from Outer Hebrides was disturbed within 800 µm from the analyze samples from outside of the crater for a regional contact with the pseudotachylyte and remained essentially thermal signature, therefore, we cannot rule out the first unaffected at 840 µm and farther from the contact. scenario. As the geological context indicates that vein Based on our laboratory data, diffusion parameters were formation is related to the impact, we further consider only calculated for each sample and the results plotted in Arrhenius the second scenario. From this it follows that the ratio of the diagrams (Fig. 5). Linear fits through low-temperature points total-gas age over the plateau age leads to the fraction of in the plots define activation energies, E, and frequency 2 argon lost from our samples during reheating processes factors, D0/a , for the diffusion modeling. The data for RK-1- associated with the impact event: ∼7% for host rock (RK-SR- VR up to 700 °C provide good linear fit. RK-SR-VR data VR), ∼6% for the vein (RK-SR-P) and ∼5% for host rock with could be fitted only up to ∼500 °C because the higher- veinlets (RK-1-VR). We assert that the temperature and temperature data points deviate from a linear trend. This is a duration of the impact-related thermal event were sufficient common pattern observed in data on feldspars and has been only to induce Ar loss from the margins of the relevant interpreted as a result of analyzing a multidomain sample with diffusion domains. variation in sample domain sizes (Lovera et al. 1989). RK- By itself, shock pressure is known to have little effect on SR-P data are quite scattered. We attribute this scatter to the Ar loss. Unless a sample was pervasively shocked at brecciated nature of the sample resulting in large variation of >>30 GPa involving considerable melting, all Ar loss occurs grain-size and multidomain behavior during the step-heating during the post-impact cooling period (Davis 1977; experiment (Lovera et al. 1989). Attempted fitting of the RK- Jessberger and Ostertag 1982; Bogard et al. 1987; Deutsch SR-P data yielded activation energy values between 22 and and Sch‰rer 1994). As evidence for shock is only slight in the 31 kcal/mol, which were too low compared to common Roter Kamm veins, we assume that the K-Ar system has not values for K-feldspar 46 ± 6 (1σ) kcal/mol (Lovera et al. been affected by shock-related effects. 1997). In the absence of a better solution, we used the value of The impact, however, produces a significant amount of 40 kcal/mol, which is similar to the values derived for the heat in the target rocks, which dissipates over a prolonged other two samples and corresponds to a slope of a line in period of time. For example, Ar data from Ries fallout suevite Arrhenius diagram consistent with the distribution of the RK- indicate cooling from the peak temperature of 450 °C to SR-P data points. Given the activation energy we then 100 °C over some 103 years, under dry conditions (Staudacher calculated the corresponding frequency factor (Table 3). et al. 1982). Newsom et al. (1986) included a possible effect With the diffusion parameters thus obtained, we can from the presence of water in their cooling scenario and calculate the duration of square-pulse heating events at suggested rapid cooling of the suevite from >550 °C to 100 °C various temperatures that would induce loss of argon in years to days due to steam convection in a hydrothermal determined for each sample. Figure 6 shows that the ranges of system. Their estimate of the peak temperature is consistent possible time-temperature combinations are very similar for with the study of Pohl (1977), which showed that magnetite all three samples. We attribute differences among the curves present in the suevite reached the Curie point at 580 °C. After to uncertainties in the diffusion parameters and in the the water stopped boiling, cooling slowed down drastically estimates of the Ar loss. For these reasons, we cannot resolve and could have been even significantly extended by any difference in thermal histories between RK-SR-VR (host exothermal reactions (Newsom et al. 1986). rock) and RK-SR-P (vein). Frictional heating apparently did Frictional heating can raise temperature rapidly above the not play an important role in the vein formation and, thus, we melting point of granite, although the duration of this only consider the overall impact heating. 850 D. Rajmon et. al.

Fig. 6. Possible time-temperature combinations for square-pulse heating events that would produce loss of Ar estimated for each sample. The shaded zone indicates the total range of uncertainty introduced by 1σ uncertainties in estimated Ar loss for all three samples. Parameters used for these calculations are activation energy (kcal/mol), frequency factor (s−1), and estimated Ar loss.

In order to limit probable duration and peak temperature of the impact heating event, we modeled various cooling histories for our samples. We assumed that a sample located at a chosen depth was instantaneously heated by the impact and then cooled to ambient surface temperature. We further assumed one-dimensional diffusive heat loss (Crank 1975) and thermal diffusivity of 1 mm2/s (Garland 1979; Turcotte and Schubert 1982). Samples located close to the surface cool faster than the samples located deeper and, therefore, require higher peak temperature to lose a given amount of argon (Fig. 7). To achieve ∼5% Ar loss from our most retentive sample (RK-1-VR), the cooling had to start after an instantaneous heating to a temperature of 365 °C, if the sample resided at 5 m depth, 340 °C at 10 m, 310 °C at 20 m, 290 ºC at 40 m, 270 °C at 80 m, or 255 °C at 130 m. To satisfy ∼6% Ar loss from RK-SR-P the cooling had to start at 350, 325, 300, 280, 260, or 250 °C, for the same depths, respectively. To satisfy ∼7% Ar loss from the least retentive sample (RK-SR-VR) the cooling had to start at 330, 305, 280, 260, 245, or 230 °C, respectively. The modeled peak temperatures for all our samples and various depths are summarized in Table 4. The samples presently located on the crater rim were previously covered by tens of meters of rock. Considering the depth of erosion at Roter Kamm (Grant et al. 1997), the modeled peak temperatures for the depths of 40, 80, and 130 m are particularly relevant. The most reasonable estimate of the Fig. 5. Arrhenius diagrams for the 40Ar/39Ar data. Open circle data peak temperature is then between 230 °C (RK-SR-VR, points were used for linear regressions. One sigma error bars mostly depth = 130 m) and 290 °C (RK-1-VR, depth = 40 m). do not show up in the plots because they are smaller then the data points. The diagrams for RK-SR-VR and RK-1-VR also display It is reasonable to assume that the impact heated the regression equations and correlation coefficients. The diagram for target rocks to temperatures similar to those we estimated, RK-SR-P shows a line with a slope corresponding to the activation and some material of the crater floor and ejecta to even higher energy of 40 kcal/mol assumed to be valid for this sample. temperatures. Heat dissipating from this hot material would Argon isotopic analysis of breccia veins from the Roter Kamm crater 851

Table 4. Summary of modeled peak temperatures for various depths. Sample Modeled peak temperature (°C ) 5 m 10 m 20 m 40 m 80 m 130 m RK-SR-VR 330 305 280 260 245 230 RK-SR-P 350 325 300 280 260 250 RK-1-VR 365 340 310 290 270 255 Note: The model is calculated in steps by 10 °C. Therefore each temperature in the table is rounded to the nearest 5 °C, which generates an uncertainty in the temperatures of about ±3 °C. Uncertainties introduced by the model parameters are discussed in the text.

slow down the cooling of rocks at the crater rim, and the peak temperature required for the observed Ar loss could, thus, be lower. On the other hand, hydrothermal activity would accelerate the cooling. As the studied samples do not show much alteration, and the only evidence for impact-related hydrothermal activity is derived from fluid inclusions (Koeberl et al. 1989) and interpretation of recrystallization of the finest breccia particles, the extent of such activity was probably small. The range of our peak temperature estimates is in good agreement with shock pressures of 20–35 GPa estimated for the breccia veins (Degenhardt et al. 1994), which corresponds to a post-shock temperature in the range of 170 to 300 °C (see compilation in French 1998). The lower limit of the peak temperatures can also be derived from fluid inclusions in quartz pebbles found at the crater rim. The inclusions have been interpreted as a product of a hydrothermal system associated with the impact (Koeberl et al. 1989). They indicated a range of equilibration temperatures between 165–250 °C, with most of the data between 200 and 230 °C (Koeberl et al. 1989). Thus, any cooling scenario should start at or above 200 °C. The 200–230 °C recorded by these inclusions can be used to estimate how much time a sample spent at this temperature range. Our calculations of quartz precipitation rates (following Rimstidt and Barnes 1980; Dove and Crerar 1990) in the temperature range of 200–230 °C from a slightly quartz-supersaturated saline solution, which is in equilibrium with quartz at ∼220–260 °C, indicate that at least several months and probably several years are necessary to precipitate a 200 µm layer of quartz. Although we have no control over the time that our samples spent at 200–230 °C, the time indicated by most of our models is sufficient to precipitate enough quartz to contain the reported inclusions. If the solution was in equilibrium with quartz at still higher temperature (i.e., more supersaturated at 200–230 °C) the precipitation would be even faster and still be consistent with our models.

Fig. 7. Model cooling histories for the three analyzed samples. The DISCUSSION model assumes one-dimensional heat diffusion (heat diffusivity D = 1 mm2/s) and is based on Ar loss and Ar diffusion parameters The presented interpretation is dependent on several determined for each sample. Each curve describes a cooling history assumptions, which require further discussion and of a sample located at a particular depth (x) below the surface, starting at a particular peak temperature and approaching surface justification. A problem could be that we analyzed mineral temperature. Each cooling model has been calculated in 10 °C steps. separates rather than individual grains. Attempts to date The last two steps of each model have not been plotted because the Vredefort pseudotachylytes with the 40Ar/39Ar whole-rock corresponding times approach infinity. sample method (Reimold et al. 1990) yielded complex 852 D. Rajmon et. al. apparent age spectra difficult to interpret. Later studies of Vredefort pseudotachylytes, which utilized step-heating analyses of mineral separates and laser probe analyses, either resulted in well-defined plateau ages or related the complex age spectra to hydrothermal alteration (see discussion in Reimold and Colliston 1994). Trieloff et al. (1994) obtained consistent plateau ages even for whole-rock pseudotachylyte samples from Witwatersrand basin. Our vein sample contained only microcline plus some quartz. Unlike the Reimold et al. (1990) age spectra, the apparent age spectra of our samples resemble simple theoretical spectra calculated for corresponding Ar loss via volume diffusion (e.g., McDougall and Harrison 1999). Another issue concerns the quality of our Ar data. Spectrum of RK-1-VR contains one step without measured age within the plateau accounting for 20.15% of the released Ar. Spectrum of RK-SR-P contains three problematic steps. Two steps (0.60% and 2.17% released Ar) are in the rejuvenated part of the spectrum, the second one (18.14% released Ar) is within the plateau. Given the regular behavior of most steps in the spectra we are confident that the interpolation of the apparent ages of these steps is reasonable and the assigned uncertainties are rather conservative. The estimated steps were included in total gas ages otherwise these ages, as well as the uncertainties, would be strongly underestimated. Plateau ages were calculated as error- weighted averages and we included the estimated steps in the plateau ages as well. This approach 1) ensured consistency in the way the plateau and the total gas ages were calculated, 2) held the plateau age insensitive to the ages of the estimated steps, and 3) allowed the large uncertainty in ages of the estimated steps to be propagated in uncertainty in the plateau ages. While the problematic analytical steps introduced additional uncertainty to our results, we feel that the results, taken as a whole, are still quite robust and useful. The next step in data interpretation involved derivation of diffusion parameters. Although we were not able to obtain 40Ar data for several heating steps, we did measure 39Ar. These heating steps therefore provided valid data points in the Arrhenius plots (Fig. 5). Errors calculated for individual data Fig. 8. Model cooling histories for sample RK-SR-P for activation points were typically smaller than the symbols. However, the energies of 30, 40, and 50 kcal/mol and corresponding frequency factors, respectively. All other parameters are same as in Fig. 7. The data scatter introduced an error in the determination of the plot illustrates the range of variation of the calculated cooling diffusion parameters. This error contributes to the spatial histories due to uncertainty in Ar diffusion parameters. separation among curves for each sample in the time- temperature plot (Fig. 6) and also to the variation in modeled 15 to 25 °C for 5 to 130 m depths, respectively. Such results peak temperatures among the samples (Figs. 7 and 8). would be consistent with the low temperature origin of the If we lower the activation energy for RK-SR-P to vein and with the other geological constraints. The range of 30 kcal/mol and adjust the frequency factor accordingly, the ±1 sigma uncertainty in the estimated argon loss would result modeled peak temperatures will drop by 55 to 65 °C for 5 to in an about ±5 °C range of the modeled peak temperature for 130 m depths, respectively (Fig. 8). Such results would be RK-SR-VR, less then ±10 °C for RK-SR-P, and about ±15 °C consistent with the low temperature origin of the vein but for RK-1-VR. If we assume ten times faster heat diffusion in inconsistent with the minimum peak temperature indicated by our model, the duration of the cooling decreases by a factor of fluid inclusions. If we increase the activation energy for RK- ten and the peak temperature rises by 30–40 °C for RK-SR- SR-P to 50 kcal/mol and adjust the frequency factor VR, 30–45 °C for RK-SR-P, and 35–50 ºC for RK-1-VR. accordingly, the modeled peak temperatures will increase by The assumption that the Ar loss is related to impact and not Argon isotopic analysis of breccia veins from the Roter Kamm crater 853 to previous thermal history is difficult to substantiate as data for Wendorff L. 1998. Geophysical profile of the Roter Kamm samples from outside the crater are not available. However, if impact crater, Namibia. Meteoritics & Planetary Science 33: this assumption is wrong then our estimate of the peak 447–453. Camacho A., Vernon R. H., and Fitzgerald J. D. 1995. 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