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O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System Rb-Sr, Sm-Nd from MONTAGNAIS & POPIGAI Stecher, 0 1048 LPSC XX Rb-Sr AND Sm-Nd SIGNATURES FROM THE MONTAGNAIS AND POPIGAI LMPACT CRATERS 0. Stecher,l H. H. Ngo,l D. A. Papanastassi~u,~G. J. WasserburgY1 and L. F. ~ansa~.lThe Lunatic Asylum, Div. Geol. & Planet. Sci., Caltech 170-25, Pasadena, CA 91125. 2~eol. Survey of Canada, Dartmouth B2Y 4A2, Canada. Tektites from each of the established strewnfields display narrow ranges of 87~r/86~r(cSr) and 143~d/14~Nd(E~~) that are distinctive for each tektite strewnfield, potentially allowing tektites to be assigned to appropriate tek- tite groups based on isotopic characteristics [1,2]. The isotopic compositions also permit characterization of the provenance of the target material. Tektite fragments and microtektites from DSDP-612, off the New Jersey coast, show a wide range in EN^ and especially ES~[3] and are not compatible with North American Tektites (NAT). The EN^ and csr characteristic of NAT and 612-tek- tites indicate that the sources of these tektites can be found on the conti- nental shelf off the eastern part of N. America. An underwater impact crater, Montagnais [4], on the continental shelf, south of Nova Scotia, has been sug- gested as the source for 612-tektites [5]. We report an isotopic study of rocks from the Montagnais crater. Melt rocks from Montagnais were recently shown to have a 40~r-39~rage of 51 Ma [6]. This age is not consistent with either the Montagnais impact event being the source of NAT or 612-tektites (35Ma) [7] . 40~r-39~rages of melt rocks from the Popigai impact crater, W. Siberia, are identical to those of NAT [8]. A melt rock from Popigai, gener- ously supplied by R. A. F. Grieve, is included in this study. The Montagnais samples come from an oil exploration drill hole through the central part of the impact crater. To check potential contamination of the samples during drilling, we subjected several samples to cleaning and leaching experiments. The surfaces of several samples were also abraded and the coarse particles leached. The typical results of these experiments, on one of the samples are shown in Table 1. The data show an easily leachable component, presumably associated with alteration by seawater. The residues have distinct compositions. No evidence for contamination from drill mud was identified. The suevite sample from the upper part of the impact crater, the lower melt horizon, and the metagraywacke basement have csr values (202 to 262) in- termediate between those of NAT and 612-tektites (Fig. 1) and cNd values (-11.5 to -12.5) that are clearly lower than those of the tektites. The schist from the basement has a large eSr=888 and has a cNd=-13.5 that is only margin- ally different from other Montagnais samples. The post-impact marine sediment (MON-2080) has Esr=119 that is comparable to the range of values found for NAT (tzSr=l10 to 130), but the low cNd (cNd=-14.9) is very different from the val- ues of NAT and 612-tektites (Fig.1) and indicates an older mean crustal age. The geochemical and isotopic values of the melt rocks indicate that the melts found within the crater were derived mainly from material from the base- ment. The relationship between TEand l/f(Rb/Sr) for Montagnais (Fig. 2) show an array similar to that of 612-tektites and indicate an Rb enrichment=400 Ma ago. However, ti^^ model ages of the Montagnais samples indicate that the Montagnais material was derived from continental crust of older mean age than the source material for NAT and 612-tektites. Rb-Sr and Sm-Nd isotopic determinations of samples from Montagnais clearly indicate that the Monta nais impact could not be the source of either of these groups of tektites. TCHURN% for NAT and 612-tektites suggest that their source crater is located on the eastern North American continent or its shelf, but further away from the Canadian craton than the Montagnais crater. The old TEmodel age of the impact glass from Popigai is clearly too old to be associated with NAT or 612-tektites. Nd measurements are in progress. Based on these attempts to identify source materials, tektites, including O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System Rb-Sr, Sm-Nd FROM MONTAGNAIS & POPIGAI Stecher, 0. et al. LPSC XX 1049 NAT, may reflect a surficial layer at the target, of relatively uniform compo- sition, ejected to great distances. Glassy material found closer to the impact crater, or fallen back in the crater may reflect a greater local diversity. Therefore, the source areas of different tektite groups may remain elusive, except when an impact structure is well-preserved. (#658) [I] Shaw 6 Wasserburg, EPSL 60 (1982) 155. 121 Ngo et al., GCA 2 (1985) 1479. [3] Stecher et al., Meteoritics (1988'1 submitted. [41 Jansa 6 ~e-~ipr,Nature 327 (1987) 612. [5] Glass, LPSC XIX (1988) 391. 161 Bottomley 6 York, GRL 2 (1988) 1409. [7] Keller et al., Meteoritics 22 (1987) 25. [8] Bottomley 6 York, EOS 2 (1988) 1290. Table 1. LEACHING EXPERIME* ABRASION EXPERIMENT ABRASION + LEACHING EXP.' MON-3660 Sample weight: 315 mg Sample weight: 336 mg Sample weight: 144 mg Lower melt Leaching loss: 24 mg Amount abraded: 33 mg Amount Abraded: 29 mg horizon Leaching loss: 3 mg Cold Warm Leach of Leach H20 HN03 HN03 Leaching Fine Coarse Calc. coarse coarse leach leach leach residue TOTAL partic. partic. total partic. partic. ~b~ 0.17 0.89 7.4 91.5 105.8 - - - - - Sr 0.43 11.0 10.3 78.3 171.3 200 165 168 15 146 Nd 0.31a 3.9 14.3 81.3 25.6 - - - - - Sm 0,4Za 6.3 16.7 79.2 4.8 - - - - - 'Sr 106 80 201 216 199 189 204 202 9 1 219 f2o 1.6 0.6 0.9 1.2 1.0 0.4 0.3 0.4 4 4 f(Rb/Sr) 7.40 0.74 15.56 24.27 20.62 - - - - - - - - - - T~~ 862 6460 827 535 580 - - 'Nd - -10.5 -11.6 -11.5 -11.4 - - - f2o - 0.8 0.5 0.5 0.5 - - - - - f(Sm/Nd) - -0.205 -0.386 -0.445 -0.426 - - - - - bllR - 2038 1196 1027 1069 - - - - - a Determinations from unseparated aliquots. Entries for Leaching Experiment, in percent rela- tive to the total amount (in ppm) for each element given in column labeled TOTAL. Data for coarse (interior) particles; entries for both columns in ppm relative to net sample weight of 115mg. Table 2. Sample ~b~ Sr Nd Sm cSr f(Rb/Sr) T;(M~) cNd f(Sm/~d)~~~,,~(tia) Cover Sediment MON-2080 WR 68 172.9 17.63 3.18 119.4M.7 12.94 554 -14.9t0.3 -0.446 1330 Suevite MON-2140 Res 72 86.5 18.37 3.48 262.4+0.3 28.02 562 -12.5M.4 -0.418 1190 Lower Melt Horizon HON-3660 Res 97 134.2 20.75 3.75 216 *1 24.27 535 -11.5M.5 -0.445 1027 Meta-graywacke MON-5282 Res 29 68.4 4.9a9b lapb 202 *I08 14.3a 847a Schist MON-5284 WR 261 74.8 48.23 9.30 887.7M.4 121.9 437 -13.5M.4 -0.407 1319 Popigsi Glass POP-3P WR 95 271.3 47a 8.5a 345 *2 11.5 1816 a Aliquot determinations. Only -20% of REE remained after leaching. DSDP612 WINS r' r' I &S r Fig. 2. l/f (Rb/Sr) vs. T% ~ig.1. &sr and EN^ relationship of model ages for Montagnais rocks from the Montagnais impact crater samples (open circles), 612- and different groups of tektites. NAT= tektites (solid dots) and NAT North American tektites, Irg = Irghiz- (open triangles). The outlined ites, Mol = Moldavites and OW sands, trend indicates an Rb enrich- Aus = Australasian tektites. ment age of -400 Ma. O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .
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