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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Analytical Procedure
  6. Dating Results and Interpretation
  7. Discussion
  8. Conclusions
  9. References
  10. Supporting Information

Abstract–40Ar/39Ar dating of recrystallized K-feldspar melt particles separated from partially molten biotite granite in impact melt rocks from the approximately 24 km Nördlinger Ries crater (southern Germany) yielded a plateau age of 14.37 ± 0.30 (0.32) Ma (2σ). This new age for the Nördlinger Ries is the first age obtained from (1) monomineralic melt (2) separated from an impact-metamorphosed target rock clast within (3) Ries melt rocks and therewith extends the extensive isotopic age data set for this long time studied impact structure. The new age goes very well with the 40Ar/39Ar step-heating and laser probe dating results achieved from mixed-glass samples (suevite glass and tektites) and is slightly younger than the previously obtained fission track and K/Ar and ages of about 15 Ma, as well as the K/Ar and 40Ar/39Ar age data obtained in the early 1990s. Taking all the 40Ar/39Ar age data obtained from Ries impact melt lithologies into account (data from the literature and this study), we suggest an age of 14.59 ± 0.20 Ma (2σ) as best value for the Ries impact event.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Analytical Procedure
  6. Dating Results and Interpretation
  7. Discussion
  8. Conclusions
  9. References
  10. Supporting Information

Among the approximately 180 impact structures currently known on Earth, the 24 km Nördlinger Ries crater in southern Germany is unique in terms of the state of preservation of the crater shape and its impact ejecta blanket (e.g., Hüttner and Schmidt-Kaler 1999; Buchner and Schmieder 2009) and offers a considerably wide age data set achieved by various types of material for isotopic dating and analytical methods (Table 1). A first serial of age determinations was carried out in the 1960s by the use of the K/Ar and fission track dating technique resulting in an age of about 15 Ma for the Ries impact event (Gentner et al. 1963, 1967, 1969; Kaufhold and Herr 1967) (Table 1). All age data obtained in this time (before 1977) and reported in this study were recalculated to the decay constants of Steiger and Jäger (1977) resulting in an age increase of 0.40 Ma and of 0.30 Ma for the results of McDougall and Lovering (1969), respectively (compare Table 1). In the aftermath, further dating campaigns (by the 40Ar/39Ar step-heating method; Jessberger et al. 1978; Staudacher et al. 1982) (Table 1) yielded slightly younger ages of about 14.8 Ma and mean ages at about 14.9 Ma were compiled by Storzer et al. (1995), Hüttner and Schmidt-Kaler (1999), and Heizmann and Reiff (2002) (Table 1). With the exception of dating results presented by Abdul Aziz et al. (2008) (14.89 ± 0.10 Ma) and Di Vincenzo and Skála (2009) (14.68 ± 0.11 Ma), all of the recent age determinations performed by the 40Ar/39Ar step-heating and 40Ar/39Ar laser probe method suggested ages of about 14.3 to 14.5 Ma for the Nördlinger Ries impact (Lange et al. 1995; Schwarz and Lippolt 2002; Buchner et al. 2003; Laurenzi et al. 2003) (Table 1). The Ries suevite, a melt-bearing polymictic lithic impact breccia thought to originate from the collapsing cloud of rock fragments in all stages of shock metamorphism, molten rocks, and rock vapor (e.g., von Engelhardt and Graup 1984; von Engelhardt 1997; see also Osinski et al. 2004 for discussion), occurs in the form of a coherent impact breccia lens within the crater (“crater suevite”) and a patchy suevite blanket within a distance of some tens of kilometers outside the Ries impact structure (“fallout suevite”). Melt particles in the suevite (some are aerodynamically deformed and therefore locally known as “flädle”) have often been used for age determinations; however, these mixed-glass particles are generally rich in crystalline basement-derived rock and mineral fragments not completely molten upon postimpact heating (schlieren). Furthermore, tektites of the Central European strewn field (moldavites and Lusatian tektites, respectively; e.g., Bouška 1998; Skála et al. 2009) that consist of essentially pure glass produced by the vaporization of the uppermost Ries target rock layer (Miocene sandy deposits) were used as dating material. Thus, so far, all isotopic ages for the Nördlinger Ries crater were obtained by the use of mixed-glass samples (suevite glass and tektites). In this study, we analyzed, for the first time, monomineralic recrystallized K-feldspar melt particles extracted from an impact-metamorphosed crystalline target rock clast in Ries melt rocks from the abandoned Polsingen quarry (isolated patches of coherent Ries melt rocks at Polsingen, Amerbach, and Enkingen; e.g., Osinski 2004; Schmieder and Buchner 2008; Pohl et al. 2008) by the 40Ar/39Ar step-heating method.

Table 1.   Compiled isotopic ages (40Ar-39Ar step-heating and total fusion ages) for the Nördlinger Ries impact crater; mean ages (bold) are recalculated to updated age monitor values (see Discussion).
Samples datedMethodUsed standard1Age (Ma)Mean age (Ma)Total mean age (Ma)References
1 recrystallized monomineralic K-feldspar meltAr-Ar step-heating (plateau)BMus/2 328.5 ± 1.1 Ma14.37 ± 0.30 (0.32)14.37 ± 0.30 (0.32)This study
1 moldaviteAr-Ar step-heating (plateau)Bern4/M 18.5 ± 0.4 Ma15.21 ± 0.30 (0.50)15.19 ± 0.58 (0.71)Staudacher et al. (1982)
1 suevite glassAr-Ar step-heating (plateau)14.98 ± 0.98 (1.04)
1 moldavite (Lusatia)Ar-Ar step-heating (plateau)HD-B1 24.18 ± 0.18 Ma14.51 ± 0.08 (0.12)14.51 ± 0.08 (0.12)Lange et al. (1995)
6 moldavites (Bohemia)Ar-Ar step-heating (plateau)HD-B1 24.18 ± 0.18 Ma14.40 ± 0.14 (0.16) to 14.68 ± 0.20 (0.22)14.48 ± 0.16 (0.18)14.47 ± 0.18 (0.20)Schwarz and Lippolt (2002)
1 moldavite (Lusatia)14.36 ± 0.26 (0.28)14.36 ± 0.26 (0.28)
6 moldavites (Bohemia)Ar-Ar total fusion/ageFCTb 28.26 ± 0.18 Ma14.42 ± 0.14 (0.16) to 14.59 ± 0.14 (0.16)14.51 ± 0.14 (0.16)14.50 ± 0.17 (0.19)Laurenzi et al. (2003)
1 moldavite (Moravia)14.46 ± 0.14 (0.16)14.46 ± 0.14 (0.16)
3 suevite glassesAr-Ar total fusionTCs 28.34 ± 0.28 Ma14.47 ± 0.18 (0.24) to 14.58 ± 0.12 (0.22)14.53 ± 0.08 (0.20)Buchner et al. (2003)
15 suevite glassesAr-Ar total fusionFCs 28.02 ± 0.16 Ma14.79 ± 0.10 (0.14) to 17.44 ± 0.26 (0.28)14.88 ± 0.11 (0.15)Abdul Aziz et al. (2008)
5 moldavites (Cheb area)Ar-Ar step-heating (2) (plateau)FCs (FCTb) 28.02 ± 0.16 Ma (28.26 ± 0.18 Ma)14.69 ± 0.10 (0.14)14.68 ± 0.09 (0.13)14.68 ± 0.11 (0.15)Di Vincenzo and Skála (2009)
Ar-Ar total fusion (5)14.66 ± 0.08 (0.12)
1 moldavite (Bohemia)Ar-Ar step-heating (plateau)14.68 ± 0.10 (0.12)14.67 ± 0.09 (0.13)
Ar-Ar total fusion14.66 ± 0.08 (0.12)
1 moldavite (Moravia)Ar-Ar step-heating (plateau)14.64 ± 0.10 (0.14)14.66 ± 0.11 (0.15)
Ar-Ar total fusion14.68 ± 0.11 (0.15)

Samples

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Analytical Procedure
  6. Dating Results and Interpretation
  7. Discussion
  8. Conclusions
  9. References
  10. Supporting Information

The rock sample investigated in this study was found as a clast in an outcropping impact melt rock in the abandoned Polsingen quarry near the eastern rim of the Nördlinger Ries crater (N48°55′03″/E10°42′21″; see Fig. 1 for location). Macroscopically, the rock is composed of yellowish-white domains of K-feldspar of foamy appearance, sometimes intergrown with minor plagioclase, white aggregates of quartz of lowered transparency, and brownish semiopaque to opaque mineral phases (mainly Fe-Ti-oxides), as well as accessory zircon. K-feldspar domains exhibit the original shape of idiomorphic to hypidimorphic K-feldspar crystals up to approximately 2 cm in size (Fig. 2) and, occasionally, of Karlsbad twins. The K-feldspar domains are locally rich in vesicles (see Figs. 2, 3, and 4A and 4B); the lack of secondary minerals within the vesicles indicates a largely unaltered state of the feldspar. Plagioclase occurs as small spherulitic aggregates. Silica domains (microcrystalline α-quartz) exhibit the characteristic ballen texture (Fig. 4C; e.g., Ferrière et al. 2008; Schmieder et al. 2009a; see also Fig. 3B), as well as traces of planar fractures, as an expression of shock-metamorphism (e.g., French 1998). Microcrystalline aggregates of Fe-Ti-oxides occasionally trace the shape of biotite flakes (Fig. 4D).

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Figure 1.  Digital elevation model (DEM) of the Nördlinger Ries area with the position of the sample location of the abandoned Polsingen quarry.

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Figure 2.  Polished section of the partially molten biotite granite (component in melt rock) recovered from the Polsingen quarry, Nördlinger Ries. The pristine porphyric-granitic texture is extraordinary well preserved; although transformed into vesicular recrystallized feldspar glass (compare Figs. 3A and 3B), the K-feldspar domains largely preserved their original protolith crystal shape and position (compare Schmieder and Buchner 2008).

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Figure 3.  Scanning electron microscope images of the vesicle-rich recrystallized K-feldspar in the partially molten biotite granite clast from Polsingen impact melt rock, Nördlinger Ries. A) Secondary electron image of a separated K-feldspar particle. B) Backscattered electron image of the biotite granite (thin section); qtz, quartz; Kfs, K-feldspar; ves, vesicles.

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Figure 4.  Thin section photomicrographs of the partially molten biotite granite from Polsingen, Nördlinger Ries (crossed polarizers; compare Schmieder and Buchner 2008). A) The vesicular recrystallized potassium feldspar melt (bottom left) is surrounded by ballen quartz and aggregates of decomposed biotite. B) Detail of (A), the recrystallized potassium feldspar melt shows numerous vesicles. C) Ballen quartz with microcrystalline ballen. D) An aggregate of former biotite (in relict shape) transformed into opaque iron oxides is surrounded by recrystallized vesicular potassium feldspar melt; qtz, quartz; Kfs, K-feldspar; bt, decomposed biotite.

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Microprobe measurements (Table 2) revealed a composition of the K-feldspar domains of Or74–76Ab21–23An2–3 (K2O 10.05–12.26 wt%; Na2O 2.73–4.18 wt%; CaO 0.26–0.75 wt%; Fig. 5), with a minor celsian component (BaO 0.05–0.47 wt%; compare Table 2). In accordance with the electron microprobe data (Table 2; Fig. 5) and the complementary XRD analyses, the K-feldspar variety corresponds to sanidine (see supplementary electronic Fig. S1). The fresh and homogeneous state, comparatively large size, and high potassium content of the vesicular K-feldspar domains suggests that this material is preferentially suitable for 40Ar/39Ar dating.

Table 2.   Geochemical composition of 9 K-feldspar domains from the partially molten biotite granite (values are given in wt%).
Polsingen granite K-feldsparMeasurement numberAverage
123456789
  1. The K-feldspar variety corresponds to an impure (Na-rich) sanidine (Or74–76Ab21–23An2–3). Electron microprobe measurements (CAMECA SX 100) were carried out on 1 μm-polished thin section at acceleration voltage of 15 kV with a beam current of 10 nA and a beam size: 8 μm. Microprobe silicate measurement standards: Na: albite; K: orthoclase; Mg: periclase; Al: corundum; Ba: barite; Ti: rutile; Mn: rhodonite; Ca and Si: wollastonite; Fe: hematite.

SiO265.465.3665.4765.5165.9664.6965.1365.3264.8865.30
TiO20.010.010.020.01n.d.0.14n.d.n.d.n.d.0.02
Al2O318.8818.5818.8518.7718.6818.9818.5318.6718.8818.76
Fe2O30.120.010.210.100.080.090.070.100.220.11
MgO0.020.01n.d.0.01n.d.0.01n.d.n.d.n.d.0.01
CaO0.340.260.380.40.410.750.520.550.540.46
MnOn.d.n.d.n.d.0.02n.d.0.01n.d.0.020.010.01
BaO0.050.470.140.180.220.130.150.160.210.19
Na2O3.523.383.473.613.574.182.733.223.353.45
K2O11.5511.5711.4611.4111.2110.0512.2611.4911.211.36
Total99.8999.65100.00100.02100.1399.0399.3999.5399.2999.66
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Figure 5.  Ternary feldspar diagram showing the composition of potassium feldspar separates from impact-metamorphosed granite, Polsingen, Ries crater (squares). Geochemical data obtained using a CAMECA SX 100 microprobe.

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Analytical Procedure

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Analytical Procedure
  6. Dating Results and Interpretation
  7. Discussion
  8. Conclusions
  9. References
  10. Supporting Information

Petrography and Geochemistry

The partially molten biotite granite (Schmieder and Buchner 2008) from the Polsingen quarry was analyzed by optical microscopy, scanning electron microscopy (SEM), electron microprobe measurements, and X-ray diffractometry (XRD). High resolution SEM imaging was performed by using a CamScan™ SC44 SEM coupled with an EDAX™ PV 9723/10 Energy Dispersive X-Ray system (Institut für Geologie und Paläontologie, Universität Stuttgart, Stuttgart, Germany). The geochemical composition of K-feldspar was determined by a CAMECA™ SX 100 electron microprobe equipped with five wavelength dispersive spectrometers (Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Stuttgart, Germany; the electron-beam diameter was 5 μm at a current of 8 nA and an acceleration voltage of 15 kV; microprobe silicate compound standards: Na: albite; K: orthoclase; Mg: periclase; Al: corundum; Ba: barite; Ti: rutile; Mn: rhodonite; Ca and Si: wollastonite; Fe: hematite). Complementary XRD measurements were carried out using a Bruker D8 Advance affiliation (2 theta range from 11°–75°; 0.02° step length; step time 3 s; measured using CoKα radiation; Institut für Mineralogie und Kristallchemie, Universität Stuttgart).

40Ar/39Ar Dating

Optically fresh and pure aggregates of K-feldspar were crushed to particles 0.25–2 mm in size and were rinsed in an ultrasonic cleaner (in demineralized H2O and ethanol). 40Ar/39Ar dating of handpicked K-feldspar separate (105 mg) was done at the Institut für Geowissenschaften, University of Heidelberg (see Trieloff et al. 2005; Schwarz and Trieloff 2007a for technical details), using the decay constants recommended by Steiger and Jäger (1977). The samples were wrapped in Al-foil, packed in quartz ampoules (12 × 55 mm), evacuated and sealed, Cd-shielded (to reduce isotopic interferences), and irradiated for 120 h at the GKSS Geesthacht nuclear reactor, Germany. After irradiation, the samples were heated in an inductively heated Mo-furnace up to approximately 1500 °C in several temperature steps. The gas was cleaned with Zr-, Ti-, Cu/CuO-, and Al-getters. Argon isotopes were measured using a Finnigan MAT GD150 mass spectrometer, with a modified digital instrumentation following the Faraday cup. The argon results were corrected for mass discrimination (40Ar/36Ar ratio for pipetted air approximately 280), argon decay, isotopic interferences (all information listed in Table 3), and blanks from the extraction line and furnace (40Ar blank between 1.5 × 10−9 cm3 at 600 °C and 1.4 × 10−7 cm3 at approximately 1450 °C and no measurable 37Ar and 39Ar, with no detectable mass spectrometer background). The argon isotopic composition of blanks was always atmospheric within uncertainty. Ages were calculated using the decay constants determined by Steiger and Jäger (1977). The age standard used was the BMus/2 (Bärhalde muscovite, Black Forest, Germany) with an age of 328.5 ± 1.1 Ma (1σ), which was calibrated against five further international Ar-Ar dating standards (see Schwarz and Trieloff 2007a for details on the standard intercalibration and further information about the analytical procedure). All errors in this study are ±2σ, unless otherwise noted; errors given in parentheses include the fluence monitor uncertainty.

Table 3.   Isotopic argon dataset and ages far recrystallized feldspar separated from biotite granite, Polsingen (Nördlinger Ries).
Step no.Temp (°C)36Ar (fA)±1σ37Arca (fA)±1σ38Ar (fA)±1σ39Ark (fA)±1σ40Ar (fA)±1σ40Ar* (fA)±1σ36Ar/40Ar (×10-4)±1σ39Ar/40Ar (×10-2)±1σ39Ar (%)Age (Ma)±1σ
  1. Bold and italic notations indicate data included in the plateau calculation.

  2. The values given in parenthesis are represented as ±1σ.

 16002.8990.0800.890.111.130.1830.940.28918.52.6622431.550.873.3690.0320.7810.03.8 (3.8)
 26901.410.139.950.253.360.12166.780.41921.13.15053915.31.418.1050.0754.2015.11.2 (1.2)
37601.620.1524.220.4813.540.19718.91.22536.17.62058466.380.6128.3470.09718.1214.310.32 (0.33)
47800.8790.09533.870.6720.660.141144.91.93560.66.53301282.470.2732.1550.08028.8514.410.13 (0.14)
57900.510.1511.40.2810.520.19578.460.441822.85.21672452.790.8331.7340.09314.5814.450.38 (0.39)
68200.7570.0570.3960.0847.4210.084399.090.171368.95.51145I65.530.4129.150.1210.0614.340.20 (0.21)
78950.850.123.690.167.510.14405.800.441406.08.11154336.060.8228.860.1710.2314.220.41 (0.41)
810000.190.1232.160.644.080.11212.390.5566515609342.91.932.030.715.3714.290.81 (0.81)
9115000.1286.161.84.530.12246.610.78715327151701.734.51.66.2214.480.34 (0.34)
1014500.200.5413.620.281.170.2463.380.152411601831982326171.6014.41.5 (1.5)
Weighted plateau: 14.37±0.15 (0.16) Ma (1σ), n = 7 steps, MSWD = 0.05, P = 0.99J = 0.0027814 ± 0.0000015Std: BMus/2; t = 328.5 ±1.1 Ma100.0014.370.16 (0.17)

Dating Results and Interpretation

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Analytical Procedure
  6. Dating Results and Interpretation
  7. Discussion
  8. Conclusions
  9. References
  10. Supporting Information

40Ar/39Ar step-heating analysis yielded a plateau age of 14.37 ± 0.30 (0.32) Ma (MSWD = 0.05, P = 0.99) for about 4.2–100% of the 39Ar released during eight of 10 high temperature steps (steps 3–10) with ages of individual extractions that overlap within the 1σ error limit (Fig. 6). The age spectrum exhibits younger apparent ages in the lowest temperature steps (steps 1 and 2 with 4.2% of 39Ar released). Steps 3–10 (95.8% of 39Ar released) show a very restricted range of individual ages between 14.22 and 14.48 Ma. An indistinguishable plateau age (or integrated total age) of 14.37 ± 0.32 (0.34) Ma (MSWD = 0.67) results, if all steps (n = 10) are considered. The inverse isochron plot yields an age of 14.43 ± 0.34 (0.36) Ma, coeval with the plateau age within uncertainties, and a 36Ar/40Ar intercept ratio of approximately 0.0035 (Fig. 7) indistinguishable within error from the atmospheric value. The atmospheric intercept value suggests no notable portion of excess argon (e.g., Kelley and Spray 1997; Kelley 2002). Due to the largely Ca-free nature of the recrystallized K-feldspar, the Ca/K ratio (calculated from the 37Ar/39Ar value) is very low, increasing at the high temperature steps, indicating a small contamination (e.g., of Ca-rich plagioclase), but obviously not disturbing the age spectrum. With respect to the widely accepted criteria for defining plateaux in 40Ar/39Ar geochronology (e.g., McDougall and Harrison 1999), the new age can be considered as a well-defined and statistically robust plateau age for the Nördlinger Ries impact.

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Figure 6. 40Ar/39Ar age spectrum for recrystallized potassium feldspar melt separate from partially molten biotite granite (Polsingen, Nördlinger Ries); uncertainties are at the 1σ level; parentheses include the error on the standard.

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Figure 7.  Inverse isochron plot; note that all extractions (black circles) are included in the particle plateau fraction; cross symbols give errors; uncertainties are at the 1σ level; parentheses include the error on the standard.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Analytical Procedure
  6. Dating Results and Interpretation
  7. Discussion
  8. Conclusions
  9. References
  10. Supporting Information

Petrogenesis

The presence of ballen quartz in the Polsingen biotite granite, relict planar fractures in quartz, and foamy domains of K-feldspar suggest that the rock was partially molten by intensive shock-metamorphism, accompanied by the selective melting of feldspars (Figs. 4A and 4B; compare Bischoff and Stöffler 1984). The formation of ballen quartz from either diaplectic quartz glass or lechatelierite (e.g., Ferrière et al. 2008; Schmieder et al. 2009a; Figs. 4C and also 3B) indicates high postshock temperatures of ≥1200 °C, in turn suggestive of peak-shock pressures of ≥35 GPa (Schmieder and Buchner 2008); however, the presence of relict planar fractures in quartz and a nonfluidal texture suggest that silica was not molten but rather transformed into diaplectic glass upon shock and subsequently recrystallized to α-quartz via α-cristobalite (e.g., French 1998; Ferrière et al. 2008; Schmieder et al. 2009a). Numerous vesicles in the recrystallized K-feldspar (Fig. 3B) indicate intensive degassing processes in the local monomineralic feldspar melt (Figs. 3A, 3B, and 4A, 4B). Microcrystalline Fe-Ti–oxides obviously represent shock-thermal breakdown products from former biotite (Fig. 4D; e.g., Schneider 1974). The overall fabric of the rock points to a porphyritic and coarse-grained biotite granite derived from the Moldanubian (Variscan) basement as preimpact protolith (Schmieder and Buchner 2008). Despite the high degree of shock metamorphism (stage III after Stöffler 1971, 1984), the impact rock from the Polsingen quarry, as a whole, shows an exceptionally distinct original porphyritic texture of the granitic protolith (Figs. 2, 3A, 3B, and 4A–D).

40Ar/39Ar Dating

K-feldspar is known to be preferentially suitable for 40Ar/39Ar dating (e.g., McIntosh et al. 1990; McDougall and Harrison 1999). The 28.03 ± 0.08 Ma (Jourdan and Renne 2007) Fish Canyon sanidine, for example, is widely used as standard in 40Ar/39Ar geochronology. In impact geochronological studies, K-feldspar (sanidine) has been used as dating material for the approximately 74 Ma Manson impact structure (Iowa, USA; Izett et al. 1998), and the ≥142 Ma Gosses Bluff impact structure (Australia; Milton and Sutter 1987), and recently for the 201 ± 2 Ma Rochechouart impact structure (France; Schmieder et al. 2009b); for a recent discussion and recommendation of the Manson and Gosses Bluff impact ages see Jourdan et al. (2009). The characteristics of the 40Ar/39Ar age plateau for recrystallized K-feldspar in the impact-metamorphosed Ries granite (Fig. 6), together with a representative number of heating steps with concordant step ages (8/10; with an 39Ar release of more than 95%) and a high probability of 99% (Fig. 6; Table 3), provide a robust internal data set. In terms of the plateau shape, the age spectrum is notably similar to the well-defined age plateaux obtained for, e.g., the Pilot (Canada; Bottomley et al. 1990), Rochechouart (France; Schmieder et al. 2009b), or Keurusselkä (Finland; Schmieder et al. 2009c) impact structures. Furthermore, age plateaux showing similar characteristics were obtained, for instance, from Chicxulub melt rocks and microtektites associated with the well-established approximately 65 Ma Cretaceous/Paleogene boundary event (Swisher et al. 1992), as well as the Ries tektites (e.g., Schwarz and Lippolt 2002).

The virtually fresh state of the impact-metamorphosed biotite granite (i.e., there is no indication of stronger alteration of K-feldspar domains in thin section), comparatively large domains of recrystallized K-feldspar (up to 2 cm in protolith crystal length), and the lack of visible fluid inclusions suggest that the internal K-Ar isotopic system is not disturbed by argon loss, recoil effects, or excess argon (e.g., McDougall and Harrison 1999; Kelley 2002; Jourdan et al. 2007a), which is obviously supported by a good fit of the data in inverse isochron plots (Fig. 7). The stable position of the Ries crater on the Swabian-Franconian Alb plateau (S German terrane) suggests that no postimpact thermometamorphism occurred that could have affected the Ries impactites and disturbed the K-feldspar age (or that of suevite glasses). This is underlined by the dating results acquired in the last 50 yrs from mixed and monomineralic melt lithologies from the Nördlinger Ries (see Table 1), as well as for the crystalline rocks of the underlying Paleozoic Moldanubian basement (predominantly about 320 Ma granites, gneisses, amphibolites, and pegmatites; e.g., Jessberger et al. 1978; Staudacher et al. 1982; von Engelhardt and Graup 1984), respectively, ranging in a comparatively narrow frame.

In the course of the last 50 yrs, a large number of impactites from the Ries crater (Suevite glasses) and from several locations across the Central European strewn field (moldavites and microtektites) were analyzed by the use of different dating methods. The results obtained by fission track methods range from 14.4 ± 0.6 to 15.1 ± 0.6 Ma (Gentner et al. 1967, 1969; Bigazzi and De Michele 1996; Balestrieri et al. 1997; Bouška et al. 2000). K-Ar measurements on Ries glasses and moldavites give a range between 14.7 ± 0.4 and 15.2 ± 0.7 Ma (Gentner et al. 1963, 1967, 1969; Zähringer 1963; McDougall and Lovering 1969; for fission track and K-Ar age data see supplementary electronic Table S1). In Table 1, we exclusively summarize 40Ar/39Ar data, recalculated to updated age monitor values that are presented and discussed in the following section.

The first 40Ar/39Ar measurement on a single Ries glass sample and a moldavite was performed by Staudacher et al. (1982) yielding ages of 14.98 ± 0.49 (1.04) Ma and 15.21 ± 0.30 (0.50) Ma, respectively, with a mean age of 15.19 ± 0.59 (0.71) Ma. The 40Ar/39Ar ages were calibrated against the fluence monitor Bern4/M with an age of 18.5 ± 0.4 Ma (Jäger 1970); uncertainties of the age monitors are included in external errors, displayed in parentheses in the text. The first Lusatian moldavite age was presented by Lange et al. (1995) with a plateau age value of 14.51 ± 0.08 (0.12) Ma, measured against the HD-B1 monitor. This value is recalculated for the most recent HD-B1 age of 24.18 ± 0.18 Ma (Schwarz and Trieloff 2007a); for recalculation procedure, see Schwarz and Lippolt (2002). A comparison between the age of the Bohemian and a Lusatian moldavite was reported by Schwarz and Lippolt (2002) yielding a weighted mean age of 14.48 ± 0.16 (0.18) Ma for the Bohemian moldavites and 14.36 ± 0.26 (0.28) Ma for the Lusatian moldavite, with a weighted mean of 14.47 ± 0.18 (0.20) Ma for all seven samples (all data recalculated against the new HD-B1 monitor age of 24.18 ± 0.18 Ma). Laurenzi et al. (2003) carried out 4–14 individual measurements on seven individual tektites (six from Bohemia and one from Moravia) that resulted in total fusion ages of 14.51 ± 0.14 (0.16) Ma and 14.46 ± 0.14 (0.16) Ma, respectively; the weighted mean age of all 77 single measurements totals to 14.50 ± 0.17 (0.19) Ma. Sample ages were recalculated by Di Vincenzo and Skála (2009) on the basis of the new age value for the FCTb mineral standard of 28.26 ± 0.18 Ma. Buchner et al. (2003) measured one suevite glass from proximal Ries ejecta (Seelbronn) and two from Graupensand sediments (reworked Ries ejecta in North Alpine Foreland Basin deposits) with a weighted mean age for three individual samples (10 measurements each) of 14.53 ± 0.08 (0.20) Ma, recalculated against the TCs-1 mineral standard (monitor age: 28.34 ± 0.28 Ma, Renne et al. 1998). Abdul Aziz calculated a mean age of 14.88 ± 0.11 (0.15) Ma for 15 suevite glass samples from Otting (replicate measurements) against the FCs monitor of 28.02 ± 0.16 Ma (e.g., Jourdan and Renne 2007). The most recent work on moldavites (Di Vincenzo and Skála 2009) was carried out by both 40Ar/39Ar step-heating (four samples, two from Cheb area and one from Moravia and Bohemia each), and total fusion analyses on five Cheb area moldavites and one from Moravia and Bohemia each; the results of the four step-heating analyses were included in total fusion measurements. In that study, Di Vincenzo and Skála (2009) obtained mean ages of 14.68 ± 0.09 (0.13) Ma for the Cheb area moldavites and of 14.67 ± 0.09 (0.13) and 14.66 ± 0.11 (0.15) Ma for the Bohemia and Moravia moldavites, respectively, with a total weighted mean age of 14.68 ± 0.11 (0.15) Ma, compiled from all seven samples (against FCTb with an age of 28.26 ± 0.18 Ma, calibrated in the study by Di Vincenzo and Skála [2009] against FCs with an age of 28.02 ± 0.16 Ma, see, e.g., Jourdan and Renne 2007). All 40Ar/39Ar dating results are compiled in Table 1.

The results in the present study as well as the recalculated mean ages obtained by Staudacher et al. (1982) (suevite age), Lange et al. (1995), Schwarz and Lippolt (2002), Laurenzi et al. (2003) and Buchner et al. (2003) agree very well even within 1σ error limits, while the mean age presented by Di Vincenzo and Skála (2009) is still coeval within the 2σ error limit (see Figs. 8 and 9), as already demonstrated by these authors who recalculated all 40Ar/39Ar data available against the FCs fluence monitor. The moldavite age data reported by Staudacher et al. (1982) is consistent with the other 40Ar/39Ar ages within 3σ error limits; furthermore, the ages presented by Abdul Aziz et al. (2008) are only consistent with the age data of Di Vincenzo and Skála (2009) (see Fig. 8).

image

Figure 8.  Weighted mean ages of 40Ar/39Ar dating obtained from mixed-glass samples (suevite), Ries tektites (moldavites), and monomineralic melts (K-feldspar) published since 1995; error bars are 2σ external errors (including monitor uncertainty); see Table 1.

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image

Figure 9.  Weighted mean ages of all step-heating plateau ages and total fusion samples obtained by 40Ar/39Ar dating of Ries impact glasses and moldavites, listed in the more recent publications since 1995 (except data of Abdul Aziz et al. 2008; see Discussion); error bars are 2σ external errors (including monitor uncertainty); see Table 1.

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For their age calculation, Staudacher et al. (1982) used the monitor Bern4/M that is not directly calibrated against one of the standards used in other 40Ar/39Ar suevite/moldavite age determinations. All other monitors were directly (FCTb and TCs-1 against FCs) or indirectly (HD-B1, BMus/2, and FCs against NL-25, GA1550, Hb3gr) intercalibrated, see Renne et al. (1998), Schwarz and Trieloff (2007a), Jourdan and Renne (2007), and Di Vincenzo and Skála (2009). Due to possible miscalibration of Bern4/M, the results by Staudacher et al. (1982) have not been included in further calculations.

In the study by Abdul Aziz et al. (2008), the 15 total fusion ages of the suevite glass samples exhibit nearly indistinguishable ages with one exception of 17.44 ± 0.26 (0.28) Ma indicating excess argon in this sample, which is commonly observed in impact melt glasses (e.g., von Engelhardt et al. 1969; their fig. 22; Buchner et al. 2003). Likewise, Ries tektites may contain a small portion of extraneous (inherited or excess) argon (see, e.g., Schwarz and Lippolt 2002 and Di Vincenzo and Skála 2009). The presence of excess argon in dating samples tends to increase the apparent age of the sample by up to 2%. Schwarz and Lippolt (2002), for instance, measured a 14.68 Ma plateau age while the (total) age of the same sample is 15.01 Ma. This slightly older apparent age is in the range of the age data presented by Abdul Aziz et al. (2008). The interpretation of the calculated isochron age and the 40Ar/36Ar intercept of total fusion ages used to generate an isochron (as done by Abdul Aziz et al. 2008) is difficult and its validity might be problematic, especially as excess argon in Ries glasses cannot be ruled out (see also the discussion by Di Vincenzo and Skála 2009). Furthermore, when only a single step in a step-heating experiment is contaminated with excess argon (due to sample impurity or absorbed argon) and plotted in an isochron diagram, excess argon as the cause of older apparent ages cannot be clearly identified. Accordingly, the age data of Abdul Aziz et al. (2008) are also not included in the further age calculations (see Figs. 8 and 9).

To compare fission track, K-Ar, and 40Ar/39Ar dating results, the miscalibration between the U and K decay serie(s) has to be taken into account. The K-Ar or 40Ar/39Ar ages have to be considered to be 1% higher compared with the fission track ages (see, e.g., Begemann et al. 2001; Mundil et al. 2006; Schwarz and Trieloff 2007b). Due to relatively large errors of the fission track and conventional K-Ar ages, the data for all measurements agree within uncertainties with the 40Ar/39Ar ages obtained in the last decade. In the case of K-Ar ages, inherited radiogenic 40Ar from microscopic inclusions of older (Paleozoic) basement-derived phases could be considered as a potential source of error (e.g., Staudacher et al. 1982; Jourdan et al. 2007b). Incompletely degassed basement-derived “schlieren” that may have been present in larger glass separates (several 100 mg to g) used for K/Ar analyses of suevite glasses (Buchner et al. 2003) also account for older apparent ages. Therefore, in order to obtain a best value for the Nördlinger Ries impact age, fission track and K-Ar ages were not considered, accordingly.

As discussed above, the (partially recalculated) 40Ar/39Ar age data of this study, by Lange et al. (1995), Schwarz and Lippolt (2002), Laurenzi et al. (2003), Buchner et al. (2003) and Di Vincenzo and Skála (2009) (see Figs. 8 and 9) agree within uncertainties. Thus, we propose a new mean age for the Ries impact event from the largely concordant individual Ries ages published. In a first step, we calculated a mean age for all 13 40Ar/39Ar step-heating plateau ages available, resulting in an age of 14.56 ± 0.22 Ma. Secondly, we calculated a mean age for all 17 total fusion measurements resulting in an age of 14.60 ± 0.18 Ma (see Figs. 8 and 9). Although the numbers of replicate measurements per sample ranged from 4 to 23, each sample was weighted equally. As both mean values (step-heating and total fusion) are in perfect agreement within uncertainties, we calculated a weighted mean value of 14.59 ± 0.20 Ma for the Ries impact (see Figs. 8 and 9) from a total of 30 samples (13 plateau and 17 total fusion ages). The 14.37 ± 0.30 Ma age obtained from the Polsingen granite is about 0.2 Ma younger than this weighted mean age but still within uncertainties at the 2σ level; this effect can be explained by the 14.68 ± 0.11 Ma Ries tektite ages reported by Di Vincenzo and Skála (2009) that slightly increase the mean age for the Nördlinger Ries impact (for summary and references see Table 1, Fig. 8).

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Analytical Procedure
  6. Dating Results and Interpretation
  7. Discussion
  8. Conclusions
  9. References
  10. Supporting Information

1. We carried out the first 40Ar/39Ar dating of the Nördlinger Ries impact structure using (1) monomineralic (recrystallized K-feldspar) melt (2) separated from a shocked crystalline target rock clast (granite) in (3) impact melt rocks and therewith add a new age to the extensive isotopic age data set for this impact structure.

2. 40Ar/39Ar step-heating analysis yielded a 14.37 ± 0.30 (0.32) Ma (2σ) age for the Nördlinger Ries supported by the age plateau characteristics, as well as the robust data set statistics and a concordant total fusion age.

3. The 14.37 ± 0.30 (0.32) Ma Nördlinger Ries age is indistinguishable within error from the majority of the Nördlinger Ries ages obtained in recent years from suevite glasses and Ries tektites.

4. We suggest an age of 14.59 ± 0.20 Ma (2σ) as best value for the Ries impact event, based on current decay constant values for K/Ar and 40Ar/39Ar dating and 30 individual Ries ages so far published in the literature and obtained in this study.

Acknowledgments— This manuscript strongly benefited from critical and thorough reviews by Fred Jourdan (Curtin University of Technology, Perth, Australia), an anonymous reviewer, and the associate editor Christian Koeberl (University of Vienna, Austria) who are kindly acknowledged. We also kindly thank Jens Hopp (University of Heidelberg, Germany) for sample preparation before neutron irradiation and Thomas Theye (University of Stuttgart, Germany) for his technical support (electron microprobe and XRD analyses). Furthermore, we are grateful to the Klaus Tschira Stiftung gGmbH for support to perform isotopic dating reported in this study. E. B. and M. S. kindly acknowledge a grant by the Stifterverband für die deutsche Wissenschaft and the Landesgraduiertenförderung Baden-Württemberg, respectively.

Editorial Handling— Dr. Christian Koeberl

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  6. Dating Results and Interpretation
  7. Discussion
  8. Conclusions
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  10. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples
  5. Analytical Procedure
  6. Dating Results and Interpretation
  7. Discussion
  8. Conclusions
  9. References
  10. Supporting Information

Fig. S1. X-ray diffractogram of potassium feldspar separates from impact-metamorphosed granite, Polsingen, Ries crater.

Table S1. Compilation of isotopic ages for the Nördlinger Ries impact crater.

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