On the significance of crater ages: New ages for Dellen (Sweden) and Araguainha (Brazil)

Tectonophysics, Elsevier

205

216 (1992) 205-218

Science

Publishers

B.V., Amsterdam

On the significance of crater ages: new ages for Dellen (Sweden) and Araguainha (Brazil) * Alexander Deutsch ‘, Dieter Buhl b and Falko Langenhorst a u Institut fiir Planetologie, Unicersitiit Miinster, Wilhelm-Klemm-Strasse IO, D-4400 Miinster, Germany h lnstitut fib Geologic, Ruhr-Unir~ersitiitBochum, Postfach 102 148, D-4630 Bochum, Germany (Received

November

29, 1990; revised version

accepted

September

30, 1991)

ABSTRACT Deutsch, A., Buhl, D. and Langenhorst, Araguainha (Brazil). In: L.J. Pesonen with a Special Focus on Fennoscandia.

F., 1992. On the significance of crater ages: new ages for Dellen (Sweden) and and H. Henkel (Editors), Terrestrial Impact Craters and Craterform Structures Tectonophysics, 216: 205-218.

Samples of a few grams in weight from the impact melt at the Dellen crater (Sweden) and the Araguainha dome (Brazil) were analyzed by Rb-Sr and Sm-Nd techniques. For Dellen, pure labradorite, orthopyroxene, and glass separates from a clast-free impact melt rock with holohyaline ground mass yield an internal Rb-Sr isochron with 89.0 k 2.7 Ma t I,, = 0.83571 f 0.00007; 20-1, compared to Ar-Ar plateau ages of 102 + 3.2 Ma and 109.6 f 2.0 Ma published for total melt rocks. Various whole-rock values not plotting on this Rb-Sr isochron indicate primary variations in s’Sr/s’Sr over the melt sheet. The age of the Araguainha dome (Brazil) is bracketed by the Rb-Sr model age of 243 k 19 Ma (I,, = 0.7226 + 0.0005), for altered cordierite in the impact melt, and by the internal Rb-Sr isochron age of 449 f 9 Ma (I,, = 0.7140 + 0.0007), for an uplifted granite (shock stage lb). In melt rocks from both impact structures ““Nd/‘44Nd was not homogenized during total rock melting: Sm-Nd data either scatter around the primary isochron (Araguainha) or point to a partial resetting (Dellen). This data array may be caused by tiny refractory REE-rich inclusions in the main mineral phases which dominate the Sm-Nd system of the impact melt rocks. The significant difference between the new isochron age for Dellen and Ar-Ar “ages” show that the data base for analysis of periodicity in cratering record most probably lack precision and accuracy.

Introduction Among the approximately 140 impact structures identified on Earth about one third have been dated absolutely with radiometric methods (Grieve, 1987; Henkel, this issue). Crater ages are usually determined on whole rock samples from the impact melt sheet and are studied either by the 40Ar-39Ar (e.g., Mak et al., 1976; Jessberger and Reimold, 1980; Bottomley and York, 1988; Bottomley et al., 1990; Mtiller et al., 1990) or the Rb-Sr technique (e.g., Reimold et al., 1981; 1990).

Correspondence

to: A. Deutsch,

versitat

Miinster,

Minister,

Germany.

* Dedicated

Institut

fir Planetologie,

Wilhelm-Klemm-Strasse

to Prof. B. Grauert

10,

on the occasion

UniD-4400

of his 60th

birthday.

0040-1951/92/$05.00

0 1992 - Elsevier

Science

Publishers

Crater ages have been used to calculate the crater production rates (e.g., Grieve and Dence, 1979) and, more recently, for proposing a periodicity in impact events (e.g., Alvarez and Miiller, 1984; Shoemaker and Wolfe, 1986; Grieve et al., 1988). In addition to crater ages, isotopic studies on shock-metamorphosed rocks from terrestrial impact structures could yield a better understanding of the so-called shock ages for extraterrestrial material (e.g., Bogard and Hirsch, 1980; Bogard et al., 1990). Based on Rb-Sr and Sm-Nd data for carefully selected samples from the impact melts at Araguainha and Dellen (Fig. 11, we will show in this paper that sometimes even precisely defined “impact ages” lack accuracy. This work is part of an extended program for systematic studies on isotope systematics in naturally and experimen-

B.V. All rights reserved

A. DEUTSCH

206

El‘ AL.

lochthonous polymict breccia form an annular ring, which is surrounded by a pronounced topographical high built up from sediments of the Furnas Formation (Fig. la). Impact melt rocks are only found as relics in pockets on top of breccia and granite. The latter shows local, in situ, melting due to thermo-metamorphic overprint by the impact melt (Swietlik, 1989). The age of the Araguainha dome has been confined by stratigraphic observations: one limit is given by fossil-bearing marine sediments of the Permian Passa Dois Formation which outcrop in the annular graben system at a distance of 14-20 km from the assumed crater centre (Fig. la). Undeformed basaltic dykes transecting the structure post-date the impact event: they are probably of pre-Jurassic age (Theilen-Willige, 1982; Crosta, 1983). K-Ar dating of felsic minerals from the molten granite and a poorly defined 40Ar-‘yAr whole-rock age for the impact melt,

tally shocked rocks and minerals (Deutsch, 1987, 1990; Deutsch and Schgrer, 1990; Schlrer and Deutsch, 1990; Stephan and Jessberger, 1992). Geological outlines and samples Araguainha (16”46’S, 52”59’W) is a circular structure with a diameter of about 40 km. It is situated on the border between the states of Matto Gross0 and Goias in Brazil (Fig. la). First proposed by Dietz and French (1973) as an astrobleme, its exogenic origin is well established now by morphological (Theilen-Willige, 19821, structural and petrographic criteria (Crosta et al., 1981; Crosta, 1982; Projeto Radambrasil, 1983; Von Engelhardt et al., 1985; Pohl and Von Engelhardt, 1986; Swietlik, 1989). In the middle of this structure, uplifted rocks are exposed, with a highly brecciated alkali granite as the central morphological depression. Remnants of the al-

6 ‘-

o km

500

0.H.”

[Ifl

PATCHES

B

ALLOCHTHONOUS

OF IMPACT

a

PASSA

0

AQUIDUANA

B

PONTA

fXl

FURNAS

BRECCIA

DOIS FORMATION FORMATION

GROSSA

m

UPLIFTED

m

SAMPLE

Fig. 1. Geological

a

MELT

POLYMICT

FORMATION

FORMATION ALKALI

O&km

GRANITE

LOCATIONS sketch maps of (a) Araguainha

(modified

A = Araguainha;

after

Swietlik,

1989) and (b) Dellen

stars = sample locations.

(b) (modified

after

Delin,

1990).

ON ‘THE SIGNIFICANCE

OF CRATER

AGES:

NEW

AGES

FOR

DELLEN

both result in an age of 250 + 25 Ma (la>, confirming a Late Permian/Early Triassic age for the Araguainha event (Crosta, 1983; HammerSchmidt, unpublished, cited in Pohl and Von Engelhardt, 1986). We have analyzed two samples from Araguainha. Sample A 235f is a medium- to coarse-grained granite from the central uplift, which was shock metamorphosed at a pressure of 26 GPa (shock stage Ib according to Stbffler, 1984). This pressure was determined by Martinez (1990) by measuring refractive indices of diaplectic quartz and albite crystals with a spindle stage. The impact melt rock sample A 300 consists of a fine-grained crystalline matrix with fan-shaped intergrowths of quartz, albite and minor K-feldspar, some tiny biotite flakes and euhedral titano-magnetite, ilmenite, hematite and barite crystals. Felt-like aggregates, a few millimetres in size, of very finegrained sericite and Al-Mg-chlorite (talc-chlorite with Fe/Fe + Mg = 0.114) are prominent features in this rock. Due to their characteristic finning (trilling to sixling) and the SiO,-rich bulk rock chemistry (Table 1) these pseudomorphs are interpreted as altered high magnesium cordierite (Fig. 2). Mineral analyses of A 300 are given in Table 2. DelEen (61”55’N, 16”32’E; Fig. lb), a highly eroded complex structure with a topographic diameter of about 20 km and forming a 350 m depression in Precambrian bedrocks of Karelian to Svecofennian age (1.7-2.6 Ga; Lundeg%rdh, 1967; Svensson, 1968) was originally interpreted as remnant of a Tertiary volcano (Svenonius,

AND

207

ARAGUAINHA

TABLE 1 Chemical com~sition of impact melt rocks at Ara~uainha (sample A 300) and Dellen (samples De 3, 3/4, 4 and 5) Component (wt%‘o)

Sample

SiO, TiO, AI,0 Fe0 MnO

P,O, L.O.I.

75.1 0.5 13.2 2.25 < 0.02 0.83 0.67 4.58 1.79 0.17 1.36

69.1 0.56 14.1 4.12 0.06 0.97 2.22 3.01 3.68 0.12 2.46

68.3 0.55 13.63 3.93 0.06 0.96 2.10 2.82 4.24 0.13 2.87

71.1 0.55 14.5 3.34 0.05 0.47 2.13 2.56 4.91 0.15 0.62

70.7 0.55 14.1 3.92 0.05 0.83 2.09 2.49 4.83 0.14 0.64

Total

100.47

100.40

99.59

100.38

100.34

MgO CaO Na,O K,O

A300

De3

De 3/4

De

De5

X-ray fluorescence analyses using natural standards. L.O.1. = loss of ignition. Total Fe expressed as FeO.

1888). In 1910, Hogborn suggested an impact origin for Dellen, which was later confirmed by the discovery of planar elements in shocked quartz crystals (Svensson, 3968). The impact melt (“Dellenite”) is forming a sheet about 9 km in diameter as seen on low altitude aeromagnetic maps (Henkel, this issue; compare also Delin, 1990; Fig. lb). Previous attempts to date Dellen yielded ambiguous results for the age of the impact melt: Paiaeomagnetic results point to a Tertiary age (Bylund, 19741, whereas two groups of mirly well defined 40Ar-3YAr degassing plateaux cluster at

Fig. 2. Photomicrographs of impact melt A 300 from Araguainha. (a) Transmitted light. (b) Polarized light. Abundant crystals with cyclic twinning are supposed to represent cordierite altered to sericite and Al-Mg-chlorite.

A. DEUTSCH

208

ET AL.

TABLE 2 Chemical composition of minerals and glass from impact melt rocks at Araguainha (A 300) and at Dellen (De 3/4) De 3/4

Component wt%

A 300 Albite

K-feldspar

Sericite ’

Chlorite ‘,*

SiO,

K2O

68.9 19.8 0.1 n.d. 0.1 11.4 0.3

68.6 17.3 n.d. 0.2 0.4 1.6 12.6

48.9 30.0 2.9 3.5 0.2 0.3 9.0

37.3 19.9 5.1 22.1 0.5 0.2 0.2

Total

100.5

100.7

94.8

85.3

A1203

Fe0 MgG CaO Na,O

.

Hypersthene

Glass

53.2 1.2 23.6 20.1 0.9 0.4 n.d.

74.4 12.3 2.9 0.2 0.9 3.2 5.0

100.0

98.9

EDS analyses on a JEOL-840A electron microscope using polished thin sections and natural standards. n.d. = not detected. ’ Alteration products of cordierite. * Talc-chlorite with Fe/Fe + Mg = 0.114; ions per formula unit on the basis of 20 oxygens: KrrWONa,,,,,

Mg,,,,,

Fe,,,,,s Al,,,,,

Ala.4s0 Si7.5211.

Fig. 3. Photomicrographs of impact melts from Dellen. (a) De 3/4; transmitted light. The glassy matrix is full of titanomagnetite (black) and shows numerous perlitic cracks. (b) Polarized light. Rosettes are hypersthene crystallites. (c) De 5; transmitted light. The groundmass is cryptocrystalline with dentritic hypersthene crystals. (d) De 3/4. H-shaped labradorite An,, with glass inclusions and whiskers at the edges of the laths; transmitted light.

ON THE

SlGNlFlCANCt

OF CRATER

AGES:

NEW

AGES

FOR

DELLEN

AND

The SiO,-rich impact melt at Dellen (Table 1) displays a very peculiar texture with unusual, H-shaped plagioclase embracing geometrically confined inclusions of silica-rich, magnesium-poor glass (Table 2; Fig. 3dI. This feature can most probably be explained by the extremely quick cooling of the impact melt sheet which caused growth of whiskers at the edges of the plagioclase laths below 1350°C and at still lower T” entails perlitic cracks in the solidified glass matrix.

240 Ma and at 100-110 Ma (Bottomley et al., 1977, 1990; Miiller et al., 1990). For age determination, we selected four samples of nearly fragment-free, fine-grained impact melt, which has been described previously by Maerz (1979). Sample De 3/4 consists of homogeneous high-labradorite An,, as indicated by the optic axial angle (21/, = 75X), hypersthene (optic axial angle, 21/, = 60.0”; Table 2), and titano-magnetite in a completely glassy matrix (Fig. 3a,b). In samples De 3, De 4 and De 5, the main constituents are identically to De 3/4. However, the holohyaline matrix of De 3 and De 4 is partially replaced by chlorite and De 5 shows a cryptocrystalline matrix with plagioclase needles and dendritic orthopyroxene (Fig. 3~).

TABLE Rb-Sr

209

ARAGUAINHA

Sample preparation

and analytical

methods

For age dating, samples up to 10 g in weight were sawn out of a hand specimen and crushed in an agate mortar. A binocular microscope was

3 analytical

Sample

results

Comments

Araguainha

Weight

Rb

Sr

XhSr

“Rb/s?Sr

(mg)

(ppm)

(ppm)

(~mol/g)

’

X7Sr,XhSr

2.3

+2a,

impact melt

A 300

Whole rock

83.9

64.21

A 300

Feldspar,

h.p.

39.61

43.05

A 300

Magnetic

enriched

feldspar

22.16

36.07

A 300

Magnetic

enriched

biotite

17.07

57.96

A300

“Cordierite”,

Araguainha

+ quartz > 80%

green aggregates

35.44

259

98.68

0.1108

1.887

0.72938 k 2

0.1394

1.0057

0.72585 k 7

76.41

0.08584

1.368

0.72736 i 6

69.38

0.07793

2.422

124.1

50.76

14.89

0.72873 k 3 0.77309 f 4

central uplift, granite shocked at 26 GPa

A 235f

Whole rock

A 235f

Albite + quartz,

A 235f

Magnetic

enriched

91.7

166

150.8

0.1693

3.197

0.73631 f 4

h.p. > 99%

33.4

158

189.9

0.2132

2.424

0.72949 + 3

biotite

19.1

747

18.25

0.01901

128.1

1.53369 f 7

Dellen impact melt

De 3

Whole rock

75.48

220

122.4

0.1360

5.267

0.83586 f 4

De 4

Whole rock

79.29

208

120.0

0.1333

5.077

0.84119 of- 1

De 5

Whole rock

82.73

221

117.3

0.1304

5.509

0.84088 f 2

De 3/4

Whole rock

96.5

213.8

117.2

0.1302

5.345

0.84254 f 1

De 3/4

Labradorite,

564.3

0.6273

0.1652

0.83594 f 1

De 3/4

Glass h.p.

0.08173

9.497

0.84802 k 2

De 3/4

Hypersthene,

0.83696 + 2

De 3/4

Opx leach

De 3/4

Magnetic

’ Analytical

error

’ Normalized ’ 2a errors

1.42

x

lo-”

18.93 h.p., leached

46.06

31.83 238 20.50 0.027

enriched

opx + glass

30.12

203

73.61 58.13 0.03 105.8

0.06462

1.033

3.4 x lo-’

2.546

0.75561 f 4

0.1175

5.621

0.84266 + 1

< 1%.

refer to the last digits. according

analyses

2u,,,,). During

19.6

to ?Sr /XhSr = 8.37521.

Sr was loaded replicate

h.p.

to Birk (1986) on outgassed

of NBS SRM 987 SrCO,

this work total blanks a-’

h.p. = handpicked,

(Steiger

and JIger,

opx = hypersthene.

ranged 1977).

Ta single filaments

with loo-125

ng Sr yielded

and measured

in the static mode. Using this procedure

a mean value of 0.710234 k 0.000017

(unweighted

7

mean

from 0.04 to 0.1 ng for Rb and from 0.04 to 0.3 ng for Sr. The s7Rb decay constant

is

A. DEUTSCH

210

ET AL.

(Institut fiir Geologie, RU Bochum) equipped with five variable collectors. For further details see Tables 3 and 4.

then used to inspect for rock and mineral clasts. Mineral fractions were obtained by magnetic separation and selection by hand only, however the hand-picked orthopyroxene from De 3/4 was pre-enriched by centrifuging for 30 s in bromoform. Mineral fractions as well as whole rock samples were rinsed with ethanol and mixed spikes (“7Rb-84Sr and 14’Sm- 146Nd) were added prior to sample digestion in HF-HNO,. For Rb-Sr separation we followed Deutsch and Stiiffler (1987) and Sm and Nd purification was performed with modified Kel-F columns (Cerrai and Testa, 1963; Scharer et al., 1990). Isotope analyses were measured in the static mode on a Finnigan MAT 262 solid source mass spectrometer

Dating results

In sample A 235f a granite shocked to stage Ib from the central uplift, albite + quartz, biotite, and the corresponding whole rock define an Rb-Sr isochron with an age of 449 &-9 Ma for the kinked biotite from this granite published by Crosta (1983). Our data for A 235f Araguainha:

TABLE 4 Sm-Nd analytical results Sample

Comments

t‘%7Sm,“t‘tNd 1 14+Jd,‘dhNd2.3

Weight

Sm

Nd

‘44Nd

hg)

bpd

kvd

(pmol/g)

83.9 39.61 17.07

5.342 11.49 4.955

27.31 23.45 23.51

0.04506 0.03869 0.03880

0.11824 0.29611 0.12737

0.511954 * 9 0.511950 k 6 0.511984 k 17

35.44

2.026

10.41

0.01717

0.11769

0.511956 f 16

0.01455

0.12280

0.512019 + 5

0.0695 1 0.06446 0.06759 0.06674 0.02285 0.07884 0.009306 6 x lo-’ 0.06419

0.10561 0.11987 0.13693 0.11955 0.093044 0.11566 0.18146 0.114 0.11981

0.511611 f 0.511567 + 0.511595 * 0.511613 f 0.511628 + 0.511732 + 0.511684 + 0.51146 f 0.511635 +

(*2L&,)

Araguainha impact melt A300

A300 A300 A300

Whole rock Feldspar, h.p. Magnetic enriched biotite > 80% <4Opm “Cordierite”, green aggregates h.p.

Araguainha central uplift, granite shocked at 26 GPa A235f

Albite + quartz, h.p. > 99%

33.4

1.792

75.48 79.29 82.73 96.5 19.6 18.93 46.06

7.361 7.748 9.280 8.002 2.132 9.143 1.6933 6.8 x 10-3 7.711

8.821

Dellen impact melt

De 3 De 4 De5 De 3/4 De 3/4 De 3/4 De 3/4 De 3/4 De 3/4

Whole rock Whole rock Whole rock Whole rock Labradorite, h.p. Glass, h.p. Hypersthene, h.p., leached Opx leach Magnetic enriched opx + glass

30.12

42.13 39.07 40.96 40.45 13.85 47.78 5.640 3.6 x lo-’ 38.90

17 5 7 20 25 30 25 9 21

’ Analytical error d 0.3%. * Normalized to 145Nd/‘44Nd = 0.348442 (corresponding to 146Nd= 0.7219). 3 2v,,, errors refer to the last given digits. Nd was loaded with 0.25 N H,PO, on outgassed Re double filaments and measured in the static mode as metal ion. Using this procedure 14 replicate analyses of the La Jolla Nd standard with 250-350 ng Nd yield 0.511848 f 0.000007 (unweighted mean f 217~) for 143Nd/144Nd. Blanks were in the order of 10 pg for Sm and 80 pg for Nd. The r4’Sm decay constant is 6.54 x lo-‘*a-’ (Lugmair and Marti, 1978); Nd model ages TNd DM were calculated relative to a LREE depleted mantle source after DePaolo (1981). h.p. = handpicked;

opx = hypersthene.

ON THE

SIGNIFICANCE

OF CRATER

AGES:

AGES

NEW

FOR

DELLEN

AND

ARAGUAINHA

I

1

0.76

& 8

2

6

ARAGUAINHA A235 central upliftgranlte 26 GPa ISr = 0.7140 f 0.0007

1.6

“-

bfotlte

0.76

/,’

0.74

/

$

3

1.3

,’ ,’

**?

,’ ,’

tP

,’ ,’ ,’

@/”

lP/’

2,s ,’ ,’

1 .a

/

,’

,’

(a) 0.7

I 100

1

Impact melt ISr = 0.7226 f 0.0005

0.72

(b) 67Rb166Sr

67Rbla6Sr 150

ARAGUAINHA A300

0.70

I 5

1

1

10

15

feldspar l

ARAGUAINHA A300 Impact melt lNd=O.5115g?O.M)M)8

Fig. 4. Isochron diagrams for rocks from the Araguainha structure. (a) Rb-Sr for granite A 235f of shock stage Ib from the central uplift. (b) and (c) impact melt A 300 (b) Rb-Sr Cc)Sm-Nd. The whole-rock(WR) data point is not defined by the mineral fractions, at least one additional component with low Sm/Nd and a non-radiogenic 143Nd/ 144Nd ratio similar to the hypersthene leach must exist.

confirm observations from other craters that the impact-related Rb-Sr fractionation in crystalline rocks is very limited (Horn et al., 1985; Scharer and Deutsch, 1990); whereas even at low shock pressures severe Ar loss occurs (Stephan and Jessberger, 1992). Rb-Sr dating on the impact melt sample A 300 yields a complicated pattern. Data points for two feldspar separates, the whole rock and

“cordierite”, scatter along a regression line with a corresponding age of 243 + 19 Ma and an I,, of 0.7226 + 0.0005 (Fig. 4b). Since the slope is dominated by the “cordierite” fraction, which consists predominantly of secondary sericite, this isochron age dates the alteration of the impact melt and sets only an upper age limit for the Araguainha impact event. The magnetic fraction from A 300, a mixture of 80% biotite and ore minerals, plots

A. DEUTSCH ET AL.

212

60

DELLEN impact melt

(4

0

200 6r

DELLEN DE314

400

40.

600

@ml

0

2

4

6

6

10

Sm &pm1

Fig. 5. Concentration plots for the Dellen impact melt. (a) Rb-Sr. For De 3/4 the following modal composition can be calculated using Rb and Sr data from Table 3 and estimated densities, p, of 2.2 g cm-* for glass, 2.65 g cm-’ for plag, 3.53 g cm-’ for opx, and 2.33 for the whole rock (WR): 82.6 ~01% glass, 10.1% plag, and 6.8% opx. This result of mass balance computation is in excellent agreement with data given by Maerz (1979) from point counting (81.2 ~01% glass, 11.6% plag, 6.7% opx, 0.5% titanomagnetite). It shows that all principle Rb and Sr carrying phases have been analyzed. (b) Sm-Nd. The whole rock value for De 3/4 and De 4 plot near the opx-glass tie line, but De 3 and De 5 clearly lie outside the triangle defined by mineral separates from De 3/4.

on the primary isochron of A 235f. This fact clearly demonstrates that impact-induced total rock melting does not necessarily result in resetting of radiometric clocks for all components of the precursor rocks. Sm-Nd data for A 300 underline this result: within the large error limits, the age of 476 f 110 Ma for a regression line through whole rock, “cordierite” and “magnetic-enriched biotite” data points is identical to the age of the moderately shocked granite (449 + 9 Ma) but significantly higher compared to the Rb-Sr “isochron age” of 243 k 19 Ma for the Araguainha impact melt (Fig. 4~). The feldspar fraction, with a high Sm/Nd, ratio plots far off this regression and has a large negative Nd model age CT:&), whereas the whole rock, A 300, and a feldspar concentrate from the uplifted granite, A 235f, both have geologically reasonable T&f, ages of 1.7 Ga. We interpret the Sm-Nd systematics in melt rock A 300 from inclusions of REE phases in “cordierite” and “biotite” which have not been reset; whereas data from the A 300 feldspar are an indication of Sm/Nd fractionation not accompanied by a homogenization of the Nd isotope composition.

Dellen: All whole rock samples of the impact

melt show quite similar Rb and Sr concentrations. In a Rb-Sr correlation diagram all wholerock data lie within a triangle defined by the values for mineral separates from the glassy impact melt sample De 3/4 (Fig. 5a). The internal Rb-Sr isochron for this sample is defined by very pure plagioclase, orthopyroxene (opx) and glass fractions, an opx-glass mixture and the corresponding whole-rock and give a precise age of 89.0 k 2.7 Ma (I,, = 0.83571 t- 0.00007) for the cratering event (Fig. 6a). This Rb-Sr isochron age for De 3/4 is significantly younger than the current estimates for the age of the impact event; namely 102 k 1.6 Ma (40Ar-39Ar “plateau age” + la; Bottomley et al. (1990)) or 109.6 + 1.0 Ma (40Ar-39Ar “plateau age” k la; Miiller et al. (1990)). The other whole rock samples, De 3, De 4 and De 5, plot aside the internal Rb-Sr isochron for De 3/4 (Fig. 6a). In the case of De 3 and De 4, which both contain up to 3 wt% H,O (Table l), this scatter could be due to a reopening of the Rb/Sr system in the glass during the alteration stage. However, for the unaltered sample De 5, which displays a (87Sr/86Sr)T=89 Ma of 0.83390, in

213

ON THE SIGNIFICANCE OF CRATER AGES: NEW AGES FOR DELLEN AND ARAGUAINHA

contrast to 0.83577 for De 3/4, ence in Sr isotope ratios has to An attempt to date De 3/4 Sm-Nd isochron failed. The

labradorite and hypersthene corresponds to an age of 97 Ma (&-89 Ma, 2a), but glass and the whole rock data point plot far off this line (Fig. 6b). Moreover, in the Sm-Nd isochron diagram

a primary differbe assumed. with an internal tie line between

0.85

I

I

1

I

1

DELLEN Impactmelt

DE314

j

OdMmnt

whole rocks

1

0.83 l 0

/ 5

I *'w*%r

1 to

O,SW

O&m6

DELLEN impact melt

qs114

VP-

i

Opxkach

0.12

0.16

Fig. 6. Isochron diagrams for the DeHen impact melt. (a) Rb-Sr. (b) Sm-Nd.

020

A. DEUTSCH

214

BOTTOMLEY

BoTToMLEY et al. (1990)

et al. (1990)

TPLATEAU - 240 Ma

120

300

i *

TPLATEAu’ 102 +-1.6 Ma vmIpht.d rn..”Of4 .p.c11.s DELLEN D3 396

DELLEN D6 323 250:_ s 3 w 200. s

ET AL.

_

90: Rb-Sr ISOCHRON AGE

150. 60.

!I % D loo3

70. Rb-Sr-ISOCHRON

AGE

“$,, , , , , .‘“‘i

(b)

1

0.5 FRACT10NAL3S~r

60.

FRACTlONAl. %f

RELEASE

RELEASE

MULLER et al. (1990) TpLATEAU = 109.6 _*1.0 Ma 1201 DELLEN

pJ

z l”

1oa

9 Y

90

s

-

i Rb-Sr ISOCHRON AGE

5 2 70 60

(c) 3

60 I_-5o0

0,5 FRACTIONAL %f

Fig. 7. Dellen

impact melt: “‘Ar-“Ar

in this paper. (a) and (b) Modified Fleck et al. (1977).

RELEASE

degassing spectra in comparison after Bottomley

et al. (1990).

to the internal

Cc) Modified

spectra (b) and Cc) do not display age plateaux.

Rb-Sr

after Miiller

isochron age for sample De 3/4 et al. (1990).

Applying

It is assumed that similar over-interpretation

given

the criteria

of

of Ar data for

impact melt rocks is very common.

the De mineral plots in triangle

3/4 whole rock is not defined by its fractions, despite the fact that De 3/4 a Sm/Nd correlation diagram inside the feldspar-opx-glass (Fig. 5b). Again, the

escape of REE carriers during the total melting of precursor rocks and chemical homogenization in the melt sheet must be proposed. Differences of more than 2eN,, units at T = 89 Ma (Fig. 6b) calculated for the whole rocks analyzed, point to an impact melt sheet which still bore a resemblance to the variations in Sm/Nd systematics of

the precursor lithologies. accordance with Rb-Sr samples.

This interpretation is in data for the identical

Discussion

Results presented here give a new, precise age of 89 + 2.7 Ma for the Dellen event and set a narrow limit of 243 + 19 Ma for the age of the Araguainha crater. Moreover, these data are of

ON THE

SIGNIFICANCE

OF CRATER

AGES:

NEW

AGES

FOR

DELLEN

general importance: they demonstrate that, for age determination on impact melt rocks, the internal Rb-Sr isochron method is superior to RbSr or Ar-Ar dating techniques using whole rock samples. Concerning the widely applied isochron approach using whole-rock sampIes for determining cratering ages, the results for Dellen prove that mixing and total melting of precursor rocks do not necessarily result in an isotopically homogenized impact melt. In the case of the Dellen melt, the ~~~~~~~~~ eve”t parameter varies from - 18.95 to -20.01, and Tgi is 2.0-2.9. The cs7Sr/ X6sr)T=X9.2 Ma r atio, even for identical samples, ranges from 0.8292 to 0.8358. Some of these variations in Sr and Nd systematics may be due to alteration processes; however, in certain samples it is demonstrably a primary feature. The generally accepted assumption of isotopically homogenized impact melt sheets, which provides the base for total rock isochron dating, is therefore not valid. Close inspection of published data arrays (e.g. Reimold et al., 1981; 1990) reveal that a scatter in trT’impai-t eVentis a common feature for ST impact melt sheets of terrestrial craters. Looking at 4”Ar-“‘)Ar dating on whole rock samples from impact melt sheets, Dellen, again, could be used as an example. For the Dellen samples, the discrepancy between the new internal Rb-Sr isochron age and Ar-Ar “ages” far exceeds given errors. In consequence, even fairly defined or well defined degassing plateaux of unaltered meh rocks, which consist of several gas fractions with more than 90% of the 39Art,t,,, do not necessarily have a strict chronological significance (Fig. 7). In the Dellen case, the extremely fast cooling of the impact melt most probably prevented a tota loss of the 40Arrad accumulated in precursor lithologies of the melt sheet. Stepwise degassing experiments cannot resolve this inherited Ar component as it is apparently distributed uniformly over sites of different retentivity. Uptake of 40Ar released from shocked rocks in the crater floor during annealing by the impact melt sheet is another, but more unlikely, explanation for the high 4oAr-39Ar “plateau ages”. Similar mismatches between internal isochron ages and argon data are reported for Manicoua-

AND

ARAGUAINHA

215

gan (Wolfe, 1971; Jahn et al., 1978; Hodych and Dunning, 1992) and for subophitic lunar impact melts: Ar-Ar dating on a whole rock chip of sample 68416 yields a plateau “age” (Kirsten et al., 1973) about 150 Ma in excess of the internal Rb-Sr isochron age (Papanastassiou and Wasserburg, 1975); a plagiociase separate from sample 68415 shows a well defined degassing plateau (Huneke et al., 1973) roughly 200 Ma older than the precise Rb-Sr isochron age (Papanastassiou and Wasserburg, 1972). Complex degassing spectra appear to be a common feature of impact melt rocks rather than the exception (e.g. Mak et al., 1976; 3ottomley and York, 1988; Bottomley et aI., 1990; Miiller et ai., 19901, and prerequisites for a validity of K-Ar ages, such as total loss of Ar accumulated in precursor lithologies and closed system behaviour, are often not fulfilled. Major impIi~ations for planetary questions of more general interest arise from the difference between Ar-Ar results and self-consistent internal Rb-Sr isochron ages. The proposed periodicity in the terrestrial cratering record seems doubtful as Ar-Ar ages provide the predominant part of the data base (e.g. Grieve et al., 1985). Grieve et al. (1988) have already pointed out that some of the Ar-Ar data lack sufficient precision for analysis in periodi~i~. The new internal RbSr isochron age for De 3/4 shows now that even well defined “precise ” 40Ar- “9Ar plateau ages for impact melt rocks may be wrong; that is, they have no time significance. Thus, the verification of the periodicity hypothesis, as we11 as calculations of crater production rates, need some reconsideration. The questioning of Ar-Ar ages for cratering events also complicates the search for appropriate impact structures linked to geochemically anomalous Iayers (e.g., DePaolo et al., 1983) and to proposed mass extinctions (e.g., Alvarez et al., 1980). Based on the new results for Dellen it is suggested that published Ar-Ar ages for impact events should be confirmed by repeated measurements on different samples and mineral separates, or by using other decay schemes, including fission track dating on moderately shocked rocks (e.g., Hartung and Anderson, 1988) and the Rb-Sr small slab technique applied to annealed breccias (Deutsch et al., 1989).

A. DEUTSCH

216

Conclusions For dating hypervelocity collisional events small samples of clast-free impact melts have shown to yield the most reliable results. This is demonstrated by an example from Dellen: extremely pure mineral separates from one impact melt rock with a glassy matrix define a internal Rb-Sr isochron with an age of 89.0 f 2.7 Ma (2a), which is more than 10 Ma lower than previously published Ar-Ar ages. The current best age estimate for the Araguainha event is 243 & 19 Ma (2~1 based on a Rb-Sr model age for alteration products of cordierite in the impact melt. In addition, data for Dellen demonstrate that the internal Rb-Sr isochron technique on carefully selected melt rock samples is superior to any other dating method, especially to the widely used Ar-Ar analysis on whole-rock samples of impact melts. In such rocks, uniformly distributed, inherited 4oAr may veil the true age of the impact. Due to the presence of unmolten tiny refractory REE-rich inclusions in the main mineral phases which dominate the Sm-Nd system, the 143Nd/ 144Nd ratio does not seem to become homogenized in melt sheets from impact structures smaller than about 40 km in size, an estimate based on our results for Araguainha. In impact melt sheets of large craters such as Sudbury (Grieve et al., 19911, however, the Sm-Nd system is totally re-equilibrated (Faggart et al., 1985). Crater ages determined by Rb-Sr analyses on total rock samples have to be critically reviewed: fragment-free small volumes of the melt sheet display primary differences in “Sr/@Sr, violating the basic assumption for isochron dating. Applying strict criteria, the precision and accuracy of most published crater ages are not good enough for statistical treatment in the search for periodicity in impact events. Acknowledgements We would like to thank P. Kunz for participating in painstaking mineral separation, M. Flucks for help with analytical work and F. Bartschat, G. McCormack and T. Verhoeven for technical assistance during preparation of the manuscript.

ET AL.

Financial support of analytical work (DFG De 401/2-2) and of field work at Araguainha (DFG Bi 176/4-4) by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. References Alvarez, L.W. and Muller, R.A., 1984. Evidence from crater ages for periodic impacts on the Earth. Nature, 308: 718720. Alvarez, L.W., Alvarez, W., Asaro, F. and Michel, H.V., 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science, 208: 1095-1108. Birk, J.L., 1986. Precision K-Rb-Sr isotopic analysis: application to Rb-Sr chronology. Chem. Geol., 56: 73-83. Bogard, D.D. and Hirsch, W.C., 1980. 40Ar/39Ar dating, Ar diffusion properties, and cooling rate determinations of severely shocked chondrites. Geochim. Cosmochim. Acta, 44: 1667-1682. Bogard, D.D., Garrison, D.H., Jordan, J.L. and Mittlefehldt, D., 1990. 39Ar-4”Ar dating of mesosiderites: Evidence for major parent body disruption < 4 Ga ago. Geochim. Cosmochim. Acta, 54: 2549-2564. Bottomley, R. and York, D., 1988. Age measurement of the submarine Montagnais Impact crater. Geophys. Res. Lett., 15: 1409-1412. Bottomley, R.J., York, D. and Grieve, R.A.F., 1977. 40Ar-39Ar dating of Scandinavian impact craters. Meteoritics, 12: 182-183. Bottomley, R.J., York, D. and Grieve, R.A.F., 1990. 40Argon39Argon dating of impact craters. Proc. Lunar Planet. Sci. Conf., 20: 421-431. Bylund, G., 1974. Palaeomagnetism of a probable meteorite impact, the Dellen structure. Geol. Foren. Stockholm F&h., 96: 275-278. Cerrai, E. and Testa, C., 1963. Separation of rare earth by means of small columns of Kel-F supporting di(2-ethylhexyllortho-phosphoric acid. J. Inorganic Nucl. Chem., 25: 1045-1050. Crosta, A.P., 1982. Estruturas de impact0 no Brasil; uma sintese do conhecimento atual. In: 32nd Congr. Bras. Geol. An. Salvador, Sot. Bras. Geol., 4: 1372-1377. Crosta, A.P., 1983. Mapeamento geologico do Domo de Araguainha utilizando tecnicas de sensoriamento remoto. Diploma Thesis, Univ. Sao Paulo, 109 pp. Crosta, A.P., Gaspar, J.C. and Candia, M.A.F., 1981. Feicoes de metamorfismo de impact0 no Domo de Araguainha. Rev. Bras. Geoci., 11 (3): 139-146. Delin, H., 1990. Berggrundskartan 16G Ljusdal So. Geol. Surv. Seden, Ser. Ai, No. 36. DePaolo, D.J., 1981. A neodymium and strontium isotopic study of the mesozoic talc-alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, Calif. J. Geophys. Res., 86: 10470-10488. DePaolo, D.J., Kyte, F.T., Marshall, B.D., O’Neil, J.R. and

ON THE SIGNIFICANCE OF CRATER AGES: NEW AGES FOR DELLEN AND ARAGUAINHA

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