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Ar age and large ion lithophile trace element abundances in rocks and glasses from the Wanapitei Lake impact structure
Geochimica et Cormochlmica
Acta.1976,
Vol. 40, pp. 51 to 57.
PergamonPress.Printedin
Great
Britain
Rb. Sr and strontium isotopic composition. K/Ar age and large ion lithophile trace element abundances in rocks and glasses from the Wanapitei Lake impact structure STEPHEN R. WINZER Martin Marietta Laboratories, 1450 South Rolling Road. Baltimore, Maryland 21227, U.S.A. R. K. L. LUM and SHUFORD SCHUHMANN Astrochemistry
Branch, Code 691.2, Goddard Space Flight Center, Greenbelt, Maryland 20770, U.S.A.
(Rrcriwd IO Morclr 1975:trcccpfrd in revised ,fbrm13 May 1975)
Abstract-Shock metamorphosed rocks and shock-produced melt glasses from the Wanapitei Lake impact structure have been examined petrographically and by electron microprobe. Eleven clasts exhibiting varying degrees of shock metamorphism and eight impact-produced glasses have been analyzed for Rb, Sr and Sr isotopic composition. Five clasts and one glass have also been analyzed for large ion lithophile (LIL) trace element abundances including Li, Rb, Sr, and Ba and the REE’s. The impact event forming the Wanapitei Lake structure occurred 37 m.y. ago based on K/Ar dating of glass and glassy whole-rock samples. Rb/Sr isotopic dating failed to provide a meaningful whole-rock or internal isochron. The isotopic composition of the glasses can be explained by impact-produced mixing and melting of metasediments. Large ion lithophile trace element abundance patterns confirm the origin of the glasses by total shock melting of metasediments. INTRODC’CTION
THE STUDY of chemical and
isotopic relationships between target rocks and impact produced melts derived from them are of interest because of their relevance to lunar and planetary studies. Impact produced glasses and shocked rocks from the Wanapitei Lake impact structure are well suited for chemical and isotopic studies because of their lack of alteration, and because their origin by impact has been well established. The structure, at least 8.6 km in diameter, is large enough to simulate smaller lunar structures. The target rocks are varied enough in composition to allow for such processes as selective melting controlled by rock composition and partial melting caused by longer residence times at higher temperatures. The rocks studied are not examples of melt puddles, such as those found in the Brent or Manicouagan structures (DENCE, 1965, 1971). To understand such large structures and the origins of their accompanying melt pools requires detailed structural analyses. Such a detailed analysis is not possible at Wanapitei Lake, except by extensive drilling. The purpose of this study was to: (1) obtain an age for the structure which could be interpreted with reasonable certainty as being the time of impact. (2) To look at the Rb/Sr system in both the relatively anshocked rocks and the glasses to determine whether meaningful ages for impact could be obtained and to gain insight into the behavior of the Rb/Sr system in highly shocked rocks. (3) To add further data on the source of the glasses through use of the large-ion-lithophile trace element suite as an indicator. The origin of impact produced melt glasses is a problem which has been discussed either by means 51
of major element compositions of bulk samples (DENCE, 1971; FUDALI. 1974) or by electron microprobe analyses of glasses for major elements (e.g. VON ENGELHARDT, 1972). Only a few studies (e.g. SCHNETZLERet a[., 1967) have dealt with the relationship between impact glasses and country rocks from data on their trace element abundances. Because the large-ion-lithophile trace elements are sensitive to such processes as partial melting and fractional crystallization, they are useful for studies relating the parent materials to impact-produced glasses. GEOLOGIC SETTING Wanapitei Lake lies on the Canadian Shield just northeast of the Sudbury Basin and about 40 km northeast of the city of Sudbury. Gravity studies by POPELAR(1972) indicated a nearly circular depression of about 15 mgal centered over the circular portion of the lake. Later reconnaissance along the southern shore of Wanapitei Lake turned up pebbles and boulders in glacial drift with features indicative of shock metamorphism (DENCEand POPELAR, 1972). Subsequent identification of shock-melted glasses, maskelynite, thetomorphic minerals and coesite confirmed an impact structure (DENCEand POPELAR,1972; DENCEet al., 1974). Target rocks are likely to be the Huronian Gowganda and Mississagi Formations, the Nipissing Diabase and younger diabase dikes (DENCE and POPELAR, 1972). The most probable occupants of the center of the lake are rocks from the Mississagi Formation and the Nipissing Diabase. Other possible target rocks are Archean gneisses and rocks of the Superior province, based on outcrops in the vicinity of the lake (DENCEand POPELAR,1972). Based on studies of boulders recovered from glacial drift, most rock fragments appear to be derived from the Mississagi Formation and the Nipissing Diabase (DENCE and POPELAR, 1972). Studies of suevite breccias by Dence and Popelar, and by Winzer (this study), indicate that the following rock types are involved in the event: quart&e, arkosic quartzite, siltstone, and diabase, in descending order of importance.
S.
52
R.
WINZER,
R. K. L. LUM and S. SCHUHMANN
Table 1. Modal analyses of rocks from the Wanapitei Lake impact structure* Sample No.
22122
22123
22124
ou?.rtz
16. 2
37. 3
13. 1
2. 4
Feldspar
7s. 1
1. 5
39. 5
26. 6
7. 9
0. 2
15. 5
1. 3
Muscovite
3. 3
Biotite
2. 4
3. 2
Amphibole
22125A
22125B
22126
22121
22128
1. 6 25. 7
6. 8
35.82
25. 1
2
44.
6
34.72
25. 0
4. 6
8. 4
3. 4
0. 1
7. 7
52. 2
1. 4 51. 9
Pyroxene Clear
221210 12. 3
32. 9
9. 8
6. 1
22129
Glass
34. 7
Brown Glass
4. 2
23. 4
Lithic Fragments
17.7’
25. 9 16. 8
27. 1
31. 8
0. 5
12. 5
20. 8
Opaque Calcite
43. 4 18. 0
12. 8
49. 6
9. 3
1. 9
WZ-b
W3-a
W3-b
Sample No.
Wl-a
WI-C
Quartz
13.73
13. 63
25. l3
15. o3
22.53
25. 33
9. 7
7. 3
6. 3
24. 3
21. 7
16. 8
23. 84
46. 64
Feldspar
WZ-a
4. 6
2. 0
3. 9
w-4
W5-a
W5-b
10.03
1O.23
0. 2
8. 5
10. 0
41.54
30. 4
43. 4
46. 7
32. 7
9.83
Muscovite Biotite
0. 1
Amphibole 2. 7
Pyroxene Clear
Glass
Brown Glaes
40. 5
74. 7
35. 0
4. 4
28. 2
43. 0
Lithic Fragments Opaque
14. 0 0. 9
27. 9
27. 6
27. 3
24. 5
4. 1
1. 3
4. 1
Calcite *
23. 8
Based on 2000 points; expressed in ‘%.
1Includes maskelvnite and thetomornhic quartz. ’ Mostly maskelynite. 3 Includes thetomorphic quartz. 4 Includes devitrified material. ‘Granitic’ rocks, greenstones and amphibolite were found, but are rare. Several boulders of suevite breccia were collected by the senior author during the course of the 1972 International Geological Congress field trip to the Sudbury area. These boulders were collected from glacial drift near the Skead mining area on the south shore of Lake Wanapitei. The boulders range from polymict breccias containing shocked and unshocked lithic fragments in a matrix of finely comminuted rock and mineral fragments (with a minimum of glass) to glassy grey or greyish white boulders composed mainly of fresh or slightly devitrified glass with some thetomorphic minerals remaining. This latter type of boulder was found to contain coesite by DENCE et al. (1974). PETROGRAPHY
AND PETROLOGY
Both suevite breccias and glassy breccias were used in this study. Two large boulders of suevite, containing clasts of unshocked to mildly shocked arkosic quartzite, siltstone and diabase were recovered from the drift. Thin sections of the clasts reveal textures indicative of low to moderate shock pressures, and encompass comminution of rock fragments, planar elements in quartz and feldspar, kinking in biotite, maskelynite and a small amount of glass. Little actual melting was observed, and few thetomorphic minerals were present [a thetomorphic mineral is defined as a glass or glassy phase, commonly of quartz or feldspar, produced by solid state alteration of an original crystalline mineral by action of shock waves and retaining the form and original textures of the pre-existing mineral or grain (Glossary of Geology,p. 736)]. Larger lithic fragments included in the cornminuted matrix also show textures indicative of low to moderate shock pressures. In general, thetomorphic minerals are minor, and flowed glass is
absent. Examples of the lithic types were extracted from the breccia matrix and used for subsequent chemical study. Petrographic descriptions of these lithic fragments can be found in the Appendix, modal analyses in Table 1. Three of the boulders collected, ranging in size from 15 to 50cm in diameter, were of the glassy type. These glassy boulders are pale grey, with a pitted surface suggesting vesicularity. A few rounded, vesiculated clasts, resembling mineral or lithic fragments, were found, but the most striking feature is the clear grey to brown glass veins which lace the boulders. The vein glass is fresh, no signs of alteration appear in hand specimen. In thin section, the glass is either clear or brown, with flow texture evident in both cases. The surrounding partly devitrified grey glass is filled with small crystallites of feldspar and possibly zeolite, giving the mass a felted texture. Thetomorphic quartz grains are scattered throughout the grey glass. but thetomorphic feldspar glass is minor or absent, and where present, is free of the crystallites present in the glass matrix. Electron microprobe analyses are presented in Table 2. The normative calculations, from the electron microprobe analyses, indicate that the glasses have formed from the melting of a rock rich in feldspar and quartz. The mafic component, indicated by the ‘diopside’ content, is variable and could indicate either the diabase component or an inhombgeneously distributed phase such as biotite which, through local melting, contributes varying amounts of Fe and Mg to the glass. There is no systematic variation in chemistry between clear glass and brown glass, suggesting that the color may be due either to a dispersed phase or minor elements. The glass compositions support the hypothesis of DENCE and POPELAR (1972) that the Mississagi Formation and the Nipissing Diabase are the major groups involved. The major element data suggest an arkosic quartzite as the main contributor, with smaller. variable
53
Rb, Sr and strontium isotopic composition Table 2. Electron microprobe analyses of glasses from Lake Wanapitei** Vf1-b
Si02 TiO2
1
63. 87
76. 67
0. 40
0. 04
lb.
bl
10. 26
FeO’
5. 30
2. 15
MllO
0. 02
0.03
MgO
3. 52
1. 24
cao
1. 04
1. 02
i-h20
1. 98
K2O
*‘Z”3
TOTAL. Norm
WI-a 2
0
Sample
77.48
WI-c
74.
0. 12 10.84
26
WZ-a
1
74. 36
76. 86
0. 04 12.05
2
75.80
12. 95
11.33
11.38
13. 26
w4-3
74.
w4-4
60
0. 17 13.40
77. 57 0. 05 12.78
0. 70
0. 97
0. 94
0. 34
0. 78
1. 25
1. 04
2. 48
2. 08
1. 54
1. 59
2. 99
3. 52
5. 22
6. 28
2. 58
97. 47
98. 12
22. 0
32. 5
0. 86
1. 50
0. 03
0. 05
0. 08
0. 82
1. 37
0. 23
0. 83
1. 14
1. 04
1. 14
0. 73
0. 88
1. 19
2. 22
2. 53
2. 04
1. 81
2. 35
5. 07
4. 03
3. 62
3. 98
6. 47
3. 43
97. 81
97. bb
97. 78
96. 95
97. 46
97. 2b
97.17
2,.
5
-
2
73.40
0.11
-
2. 04
1. 33
w4-
1. 95
1. 76 0. 10
100.
2
38. 5
IS.
5
(Nigglil 0. 2
0. 2
0.b
-
0. 2
0. 1
Or
31. 0
25. 0
22. 5
25. 0
40.
Ab
18. 5
21. 0
23. 5
19. 5
17. 0
22. 0
19. 5
14. 5
15. 0
27. 0
An
5. 5
5. 5
5. 5
6. 0
4. 0
4. 5
6. 5
6. 5
5. 5
12. 5
Corundum
6. 5
0. 3
1.0
2.5
1. 8
3. 5
2. 2
3. 2
2. 3
0. 6
17. 8
7. 0
4. 4
7. 3
2. 0
4. 8
6. 4
5. b
2. 0
3. 8
43. 0
39. 7
35. 2
44. 6
43. 3
37. 8
36. 6
Ilrn
Di
(En
+ Fs,
20. 1
atz
’
Clear glass;
41.
2
’ Brown glass;
* Total Fe as FeO;
amounts of diabase contributing the mafic component. This is supported further by the petrography of low to moderately shocked clasts included in suevite breccias. These clasts are usually either a feldspathic quartzite, siltstone, diabase or amphibolite. The ‘siltstone’ and feldspathic quartzites are low in ferromagnesian minerals. The surprising fact about the glass is the low calcium content. Both rock types examined have either carbonate or calcic plagioclase in significant amounts (up to several per cent), yet the normative calculations indicate smaller amounts of calcium than expected. No immediate explanation presents itself for this apparent difference. K/Ar AGES Rb, Sr, strontium isotopic composition, K, Ar and argon isotopic composition were obtained on selected glasses, clasts and whole-rocks from the Wanapitei Lake structure. K/Ar dates were obtained on one glass vein and one whole-rock sample obtained from the glassy boulders. The analyses were done by Geochron Laboratories. Clean, clear and unaltered glass from vein material cutting one of the larger glassy boulders has a date of 37.8 & 1.6 m.y. Grey, glassy whole-rock material from a separate boulder gave a date of 36.0 + 1.6m.y. (Data on these analyses are available on request from the senior author.) These two dates overlap within the quoted error and may be considered as the same date. In the absence of subsequent heating or alteration, the date quoted should be the age of formation of the glass, and thus a reasonable approximation of the time of impact. As the vein glass shows no sign of recrystallization or alteration. this age is the best approximation of the time of impact. The whole-rock date includes thetomorphic clear glass and devitrified glass. This devitrification may be the reason for the younger date of the whole-rock. Rb/Sr AND Sr ISOTOPIC Procedure
COMPOSITION
Small (8 cm3) blocks of sample were cut from boulders and clasts selected for examination and analysis. One side of the block was slabbed and a
0
40.
5
** Expressed as wt o/W
polished thin section made. This section was used for modal analyses and microprobe study. The remainder of the block was crushed to powder in a boron carbide mortar. Two samples were chosen for further separation. These were less finely ground, and glass and minerals were separated using heavy liquids and handpicking. One bulk glass sample was used for K/ Ar age work, and one sample was separated into four fractions (by density) in an attempt to obtain an internal Rb/Sr isochron. Rb and Sr were determined by isotope dilution mass spectrometry. An aliquant of powder was taken and spiked with 84Sr with a purity of 99.892% (SRM988). The use of this spike allowed 87Sr/8hSr to be determined at the same time as Sr abundance. The 84Sr spike and powder were treated successively with HF + HClO,, HCl and H,O. Sr was separated by ion-exchange chromatography. Rb was treated in the same manner as Sr, using “Rb spike on another sample aliquant. K was analyzed along with Rb, using K4r spike. Sr samples were mass analyzed using a Shields-type 12 in. radius, 60” sector mass spectrometer utilizing triple filament surface ionization and expanded scale chart recording. Rb and K were done on a Shieldstype 6 in. radius, 60” sector mass spectrometer using triple filament surface ionization but without expanded scale chart recorder. Precision and accuracy are within the limits quoted for trace element analyses by this laboratory (SCHNETZLERet al., 1969). Blanks are generally 7 nng for Sr and @6nng for Rb. Precision of s’Sr/*%r ratios (2a, normalized to ‘%/*%r = 01194) varies from sample to sample and is quoted in the table (Table 3). SRM-987 and the Eimer and Amend SrC03
54
S. R. WINZER,R. K. L. LUM and S. SCHUHMANN
t=104 IR= x
be established for the glass points. This ‘age’ is neither that of the impact nor that of the country rock, although it does come close to the Grenville ages found to the south and east of the Wanapitei Lake structure. The isotopic compositions of the clasts comprise 2 groups with 4 samples scattering generally between the two groups. The clasts do not define an isochron, but do contribute to the understanding of the glass compositions. First, the glasses are all enriched in radiogenic strontium. The ‘initial ratio’ for the glasses is also enriched relative to the initial ratio (0.7063) of the Gowganda Formation studied by FAIRBAIRN rt al. (1969). The clasts, with two exceptions. can be said to approximate a line with an age of 180& 2100 m.y. with an initial ratio close to that found by FAIRBAIRN et al. (1969) for the Gowganda and Bruce formations in the Sudbury area. No data are available for the Mississagi Formation. The clasts, in any case, are indicative of Huronian metasediments of age between 1800 and 2100m.y. If the metasediments can be considered coeval, then the glasses derived from them can be interpreted in terms of either loss of Rb or gain of radiogenic Sr, or both, or lack of equilibration or homogenization during impact. The most enriched samples studied are feldspathic ‘quartzites’ with lithic fragments of ‘siltstone’ and carbonate. The group with the lowest s7Sr/86Sr and Rb/ Sr ratios consists of amphibolites or diabase with the exception of 22125A, which has a large amount of opaque material and glass. The glass compositions, being far more enriched in radiogenic Sr as well as Rb than the amphibolites and diabase, suggest that they are derived from feldspathic quartzite.
x 10%
7508
= 1.39 x IO”yr
22:25(A) 73w-
2y29
72W-
l22126
22727
7lco/ 0
1
2
3 “Rb /=
5
4
Sr
Fig. 1. Rubidium-strontium evolution diagram for glasses and clasts from the Wanapitei Lake impact structure. 0 Glass. A Metasediment. n Diabase or amphibolite. I Error bar (glass only).
were run throughout the procedure. “Sr/ *%r ratios were: SRM-987 07104 f 00016 and Eimer and Amend 07083 f 00003. Analytical data are presented in Table 3 and Fig. 1. The. data indicate that no good isochron relationship exists for either glasses or clasts (Fig. 1). The Rb/Sr ratio for glasses does not vary greatly, but the precision of the analysis is sufficient to allow an isochron. A least-squares regression line corresponding to an age of 1043 m.y. and an initial ratio of 0.7508 can standards
Table 3. Rb, Sr and Sr isotopic composition of Wanapitei Lake impactites Sample 22121
Rb(PPM) 3. 22
87Rb(PPM) 0. 90
22l22
57. 6
16. 0
22123
37. 5
10. 4
22124 22125(A) 22125(B) 22126 22127 22128 22129 22lZlO
143.
9
37. 62 133.2 42. 46 38. 24 149.4 16. 7 190.4
SrN(PPM)
86
Sr(PPM)
87
87
Sri%
Rb/%
162.
2
15.70
106.
5
10. 29
7561
t
0006
1. 56
4. 96
. 7652
t
0015
2. 10
9. 91
8096
3
0009
4. 04
7309
+
0005
0. 62
9. 62
8146
f
0003
3. 85
14. 64
. 7272
+
0002
0. 81
7227
2
0008
0. 62
51. 42
40. 05
102.7
10.47
176.
37. 07
100.1
11.82
151
10. 64
175.7
41. 58
123.
4. 64
183.7
52. 99
103.4
0
17
17. 1 6
.7451
*
11.68
.8159
17.85
.7253~.0012
i
9. 87
. 8438
+
.0003
.0005
0. 057
3. 56 0. 26
0007
5. 37
Glass WI-a
91. 99
25. 6
97. 58
9. 40
. 7897
k
0008
2. 72
w-c
90. 16
25. 09
94.63
9. 12
7932
+
0001
2. 75
WZ-A
84. 67
23. 56
92. 76
8. 98
7896
k
0003
2. 62
WZ-B
82.93
23. OS
94. 57
9. 11
7907
+
0003
2. 53
W4
71. 04
19.77
9. 93
1198
3
0001
1. 99
103
Internal lsochron WI-b,
109.
W-lb3
100.4
W-lb4
6
96. 59
30. 5
104.5
10. 06
. 7881
+
0017
3. 03
30.17
109.2
10. 52
.7850
*
0004
2. 87
3
0007
3. 38
26. 88
82. 59
7. 96
7898
55
Rb, Sr and strontium isotopic composition Table 4. Large ion lithophile trace element abundances 22123
22122 Li
3. 10
Rb
104
Ba
570
Ce
22129
6. 63
6. 59
57. 6
Sr
22124
152
50. 6
102
168
809
147
385
16. 5
W2-b
221210
11. 8
37. 5
in Wanapitei Lake samples*
5. 43
BCR-1
4. 27
178
12. 9 46. 7
85. 1
95. 8
91. 9
1230
328
426
681
28. 4
40. 0
9. 42
12. 5
17. 3
5. 77
4. 12
1. 82
2. 86
1. 42
0. 796
1. 83
b, 61
0.448
0.751
0. 548
0. 231
0. 432
1. 92
0. 611
1. 13
1. 89
0.555
0. 969
6. 74
0.406
0. 290
0. 563
1. 03
0.435
0. 644
4. 68
Yb
0. 376
0. 261
0. 558
0. 916
0.490
0. 622
3. 25
L”
0. 064
0. 065
0. 098
0.159
0.188
0.106
0.533
16. 3
Nd
7. 46
Sm
1. 21
EU
0. 325
DY
0. 732
Er
zr
22. 2
Hf
15. 5
0. 634
43. 5
29. 7
10. 4
46. 0
1. 91
25. 9
50. 7
11. 0
27. 6
43. 4
1. 52
1. 64
* Expressed as ppm.
If all clearly igneous or amphibolitic material is left out, no age relationship can be inferred for the
clasts. All metasedimentary clasts scatter, but are more enriched in radiogenic strontium than the amphibolites or diabase. 22121 comes close to the ‘initial ratio’ of the glasses. With the exceptions of 22122 and 22123, the glasses appear as a mix between metasediments of the mineralogical compositions of 22121, 22124, 22125B, 22128 and 221210. In order to evaluate further the contribution of various clast components, several clasts and one glass were analyzed for large-ion-lithophile trace elements (Li, K, Rb, Sr, Ba and the REE’s).
11
, L,
K
Rb
Sr
Ba
Ce
Nd
TRACE ELEMENT
ABUNDANCES
Procedure
Aliquants of material prepared for Rb and Sr isotopic analysis were taken and analyzed for Li, K, Rb, Sr, Ba and the rare-earth elements (REE’s) by stable isotope dilution mass spectrometry. The method used is that reported by SCHNETZLERet al. (1967b). BCR-1 was analyzed as a monitor. The analytical blank is generally less than 1% of the amount present (PHILPINTS et al., 1972). Data are presented in Table 4 and in graphical form in Fig. 2. It can be noted, by comparing Rb and Sr values
Sm Eu Gd
DY
Er
Yb Lu
2r
Hf
Fig. 2. Large ion lithophile trace element abundances in clasts and glass from the Wanapitei Lake impact structure. Solid line = metasediment. Dashed line = glass. Broken line = amphibolite or diabase.
56
S. R. WINZER,R. K. L.
between Tables 3 and 4, that repeat analyses by separate dissolutions and using different spikes are generally good. Sr for 22129 and Rb for 221210 are outside the range expected from the comparison run for BCR1. This may be due to sample inhomogeneity. The trace element abundance patterns show a variation in absolute abundance of about 5. with the exception of 22129. The patterns are strongly enriched in light REE’s and depleted in heavy REE’s, with the general exception of Zr and Hf. Enrichment in Zr and Hf may be expected for metasediments as zircon, the principle mineral phase containing these elements, survives erosion and redeposition and may be concentrated in sedimentary deposits. The pattern for 22129 is quite different from the other clast patterns in that the light REE’s are not nearly as enriched over the heavy REE’s. This is a function of lithology, as 22129 is a diabasic rock and the others are feldspathic quartzites or arkoses. The abundance pattern for 22129 is similar to those found by PHILPOTTS and SCHNETZLEK(1968) for other diabases, but the absolute abundance is lower. It is immediately obvious that the compositional trend of the glass parallels that of the metasediments. The heavy REE abundances are somewhat higher than most of the metasediments; this suggested some contribution from rocks of the 22129 type, but the light pattern, Rb. Sr and Li values virtually rule out such a contribution. The trace element patterns strongly suggest thdt the glasses formed by shock melting of metasedimentary material, and that contribution from diabase or amphibolite was negligible. The similarity of the patterns also shows that little or no volatile/involatile ratio changes have taken place, suggesting that these elements do not fractionate during the impact melting process. The similarity of glass and clast patterns confirms the relationship of crater glass to country rock shown by SCHNETZLERet a/. (1967a). CONCLUSIONS
Glass and glassy whole-rock samples from the Wanapitei Lake impact structure were found to be 37 m.y. old. This age is considered to approximate the time of impact, thus confirming the expected large time interval between the formation of the Sudbury and Wanapitei Lake structures (DENCE and POPELAR. 1972). Study of Rb/Sr and Sr isotopic composition failed to yield a meaningful isochron for glasses and glassy whole-rock samples. An attempt to obtain an internal isochron also failed. This is likely due to incomplete re-equilibration of Rb and Sr isotopes, or to selective loss or gain of Rb or “Sr. The latter seems unlikely in the light of study of the large ion lithophile trace element abundance patterns, which do not indicate that fractionation has taken place. Large-ion-lithophile trace element abundance patterns indicate that the Wanapitei Lake glasses were formed by total melting of metasedimentary material,
JIM
and S.
SCHUHMANN
most likely of the arkosic quartzite type. This conclusion is supported by the Rb/Sr isotopic work which indicates little contribution from diabase of low s’Sr/ ‘%r ratio. The conclusion indicated by the isotopic and trace element abundance work is somewhat different than would be drawn from major element analyses of the glasses alone. No quantitative comparison can be made on this set of rocks because major element analyses of the clasts are lacking. Aclinowledyemrtlts-This work was done while the senior author was under a National Academy of Sciences Resident Research Associateship, for which thanks are given. I (Winzer) would also like to thank J. A. PHILPO~~ for discussions and review of this work. REFERENCES D~~NCE M. R. (1965) The extraterrestrial origin of Canadian craters. Ann. N.Y. Acad. Sci. 123. 941-969. DENCE M. R. (1971) Impact melts. J. Geophys. Res. 76. 5552-5565. DENCE M. R. and POPELARJ. (1972) Evidence for an impact origin for Lake Wanapitei, Ontario. Geol. Ass. Can. Spec. Paper 10. I17-l24. DENCEM. R.. ROL&RTKINP. B. and WIRTHLINR. L. (1974) Coesite from the Lake Wanapitei Crater, Ontario. Earth Planet. Sci. Lett. 22. 118-l 22. FAIRBAIRNH. W., HURLEYP. M., CARDK. D. and KNIGHT C. J. (1969) Correlation of radiometric ages of Nipissing Diabase and Huronian metasediments with Proterozoic erogenic events in Ontario. Can. J. Earth Sci. 6.489-496. FLIUALIR. F. (1974) Genesis of melt rocks at Tenoumer Crater, Mauritania. J. Geophys. Res. 79. 21152121. PHILPOTTSJ. A. and SCHNETZLER C. C. (1968) Genesis of continental diabases and oceanic tholeiites considered in light of rare-earth and barium abundances and partition coefficients. In Origin and Distribution of the Elements, (editor L. H. Ahrens), Vol. 30, 939-947. Pergamon Press. PHILPOTTSJ. A., SCHUHMANNS.,BICKELA. and LUM R. K. L. (1972) Luna 20 and Apollo 16 core fines: large ion lithophile trace element abundances. Earth Planet. Sci. Lett. 17. 13-18. POPULARJ. (1972) Gravity interpretation of the Sudbury area. Grol. Ass. Can. Spec. Paper 10. 103-115. SCHNETZLER C. C., PHILPOTT~J. A. and PINSON W. H. (I 969) Rubidium-strontium correlation study of moldavites and Ries Crater material. Geochim. Cosmochim. Acta 33. 1015.-1021.
SCHNETZLER C. C., PHILPOTTSJ. A. and THOMASH. H. (1967a) Rare-earth and barium abundances in Ivory Coast tektites and rocks from the Bosumtwi Crater area, Ghana. Grochim Cosmochim. Acta 31. 1987-1993. SCHNETZLER C. C., THOMASH. H. and PHILPO~ J. A. (1967b) Determination of rare-earth elements in rocks and minerals by mass spectrometric stable isotope dilution technique. Anal. Chem. 39. 1888-1890. VON EXELHARDT W. (1972) Shock produced glasses from the Ries Crater. Corltrih. Mineral. Petrol. 36. 265292. APPENDIX Petrography 22121
No thin section. 22122 Weakly shocked arkose. Feldspar grains occasionally show shock lamellae while quartz grains usually show two or more sets of planar features. Some biotite oxidation.
Rb, Sr and strontium isotopic composition 22 123 Hyalocrystalline, strongly shocked ‘arkose’. The rock is vesicular and contains about 60% glass. The remaining quartz and feldspar is strained but shock lamellae are absent. Two varieties of glass occur. Clear glass shows flow texture by slight differences in refractive index while brown gfass contains dark inclusions which are aligned. also indicating gowage. 22124 Weakly shocked ‘arkose’. The rock consists of quartz, feldspar. biotite. muscovite, amphibole and grey. amorphous or microcrystalline fragments (siltstone?). Quartz and feldspar grains are cloudy, and many show planar features. Some grains have rims of clear quartz which has not been shocked or strained. suggesting some later annealing. 22125A H~locrystalline-allotriomorphi~ ‘breccia’ consisting of feldspar, quartz, hthic fragments and glass. The lithic fragments are similar to the ‘siltstone’ fragments in 22124. Most of the glass is nearly opaque with inclusions, but a small amount of clear glass does occur. 221258 Moderately shocked lithic arkose. This sample differs from 22125A in that it contains less glass. Quartz shows several sets of planar features, while a few feldspar grains are now maskelynite. The maskelynite is included with glass in the modal analysis. 22126 Moderately shocked amphibolite. In this section most of the feldspar is maskelynite, but no glass or flowed glass is present. Little or no comminution has occurred. Grains are idiomorphic, with sharp boundaries which do not show signs of incipient melting. Some alteration of plagioclase to sericite or epidote has occurred. 22127 Moderately shocked amphibolite. Texture is very similar to 22126. but some of the maskelynite shows more alteration to scricite. Biotite has been oxidized. and amphibole is brecciatcd but not comminuted. No glass is present in the slide. 22128 Weakly shocked greywacke. This rock consists of feldspar, quartz, opaque minerals (mainly hematite) and microcrystalline rounded lithic fragments (‘siltstone’). A few feldspar and quartz grains show deformation lamellae or a few planar features. 22129 Moderately shocked diabase. Holocrystalline, hypidiomorphic inequigranular poikilitic rock consisting of ortho- and clinopyroxene, plagioclase and minor biotite, amphibole and carbonate. The orthopyroxene is dominant: it shows coarse schiller structure and poikiliti-
57
tally encloses smaller grains of clinopyroxene. The pyroxenes have been fractured so that one grain has many domains within it. These domains are characterized by irregular boundaries. All plagioclase is maskelynite. and some grains are partly altered to sericite. Carbonate occurs in veins cutting the maskelynite. 221210 Weakly shocked greywacke. Texture and shock indicators are the same as 22128. WI -a Hyalocrystalline breccia consisting of clear glass, brown glass. thetomorphic quartz and feldspar and fragments of quartz and feldspar. Clear glass shows flow texture and contains small euhedral feldspar(?) crystals. Brown glass includes bubbles and opaque inclusions. Mineral grains remaining have planar features in multiple sets. WI-b No thin section. WI-c Hyatocrystahine breccia, with texture similar to Wl-a, but containing more glass and less quartz and feldspar. W2-a Hyalocrystalline breccia. This sample differs from WI-a and c in that the glass is partly recrystallized. This gives the slide a ‘fehed’ appearance. The crystal phase [feldspar~?). zeolitef?)] is acicular, euhedral and has a refractive index greater than the glass. Mineral fragments of quartz and feldspar show multiple sets of planar features. W2-b Hyalocrystalline breccia, similar to WZ-a but with a higher proportion of mineral fragments. W3-a Polymict breecia. consisting of mineral and lithic fragments in a matrix of glass and finely ~omminuted rock powder. Glass has a Row texture. Fragments of quartz and feldspar contain planar features. Lithic fragments are generally mildly shocked quartz& or arkose. but a few are strongly shocked and glassy. W3-b Polymict breccia, almost identical to W3-a. W-4 Hyalocrystalline breccia, consisting of glass, thetomorphic quartz, mineral and lithic fragments. The glass in this sample differs from that in the others in that it is not Rowed. W5-a Pofymict breccia consisting of glass, lithic fragments of diabase, amphibolite, quartzite and arkose and mineral fragments of quartz, feldspar and pyroxene. This rock is a typical seuvite breccia, fragments range from unshocked to moderately shocked. with a few giassy ciasts included. W5-b Polymict breccia very similar to W5-a.
Acta.1976,
Vol. 40, pp. 51 to 57.
PergamonPress.Printedin
Great
Britain
Rb. Sr and strontium isotopic composition. K/Ar age and large ion lithophile trace element abundances in rocks and glasses from the Wanapitei Lake impact structure STEPHEN R. WINZER Martin Marietta Laboratories, 1450 South Rolling Road. Baltimore, Maryland 21227, U.S.A. R. K. L. LUM and SHUFORD SCHUHMANN Astrochemistry
Branch, Code 691.2, Goddard Space Flight Center, Greenbelt, Maryland 20770, U.S.A.
(Rrcriwd IO Morclr 1975:trcccpfrd in revised ,fbrm13 May 1975)
Abstract-Shock metamorphosed rocks and shock-produced melt glasses from the Wanapitei Lake impact structure have been examined petrographically and by electron microprobe. Eleven clasts exhibiting varying degrees of shock metamorphism and eight impact-produced glasses have been analyzed for Rb, Sr and Sr isotopic composition. Five clasts and one glass have also been analyzed for large ion lithophile (LIL) trace element abundances including Li, Rb, Sr, and Ba and the REE’s. The impact event forming the Wanapitei Lake structure occurred 37 m.y. ago based on K/Ar dating of glass and glassy whole-rock samples. Rb/Sr isotopic dating failed to provide a meaningful whole-rock or internal isochron. The isotopic composition of the glasses can be explained by impact-produced mixing and melting of metasediments. Large ion lithophile trace element abundance patterns confirm the origin of the glasses by total shock melting of metasediments. INTRODC’CTION
THE STUDY of chemical and
isotopic relationships between target rocks and impact produced melts derived from them are of interest because of their relevance to lunar and planetary studies. Impact produced glasses and shocked rocks from the Wanapitei Lake impact structure are well suited for chemical and isotopic studies because of their lack of alteration, and because their origin by impact has been well established. The structure, at least 8.6 km in diameter, is large enough to simulate smaller lunar structures. The target rocks are varied enough in composition to allow for such processes as selective melting controlled by rock composition and partial melting caused by longer residence times at higher temperatures. The rocks studied are not examples of melt puddles, such as those found in the Brent or Manicouagan structures (DENCE, 1965, 1971). To understand such large structures and the origins of their accompanying melt pools requires detailed structural analyses. Such a detailed analysis is not possible at Wanapitei Lake, except by extensive drilling. The purpose of this study was to: (1) obtain an age for the structure which could be interpreted with reasonable certainty as being the time of impact. (2) To look at the Rb/Sr system in both the relatively anshocked rocks and the glasses to determine whether meaningful ages for impact could be obtained and to gain insight into the behavior of the Rb/Sr system in highly shocked rocks. (3) To add further data on the source of the glasses through use of the large-ion-lithophile trace element suite as an indicator. The origin of impact produced melt glasses is a problem which has been discussed either by means 51
of major element compositions of bulk samples (DENCE, 1971; FUDALI. 1974) or by electron microprobe analyses of glasses for major elements (e.g. VON ENGELHARDT, 1972). Only a few studies (e.g. SCHNETZLERet a[., 1967) have dealt with the relationship between impact glasses and country rocks from data on their trace element abundances. Because the large-ion-lithophile trace elements are sensitive to such processes as partial melting and fractional crystallization, they are useful for studies relating the parent materials to impact-produced glasses. GEOLOGIC SETTING Wanapitei Lake lies on the Canadian Shield just northeast of the Sudbury Basin and about 40 km northeast of the city of Sudbury. Gravity studies by POPELAR(1972) indicated a nearly circular depression of about 15 mgal centered over the circular portion of the lake. Later reconnaissance along the southern shore of Wanapitei Lake turned up pebbles and boulders in glacial drift with features indicative of shock metamorphism (DENCEand POPELAR, 1972). Subsequent identification of shock-melted glasses, maskelynite, thetomorphic minerals and coesite confirmed an impact structure (DENCEand POPELAR,1972; DENCEet al., 1974). Target rocks are likely to be the Huronian Gowganda and Mississagi Formations, the Nipissing Diabase and younger diabase dikes (DENCE and POPELAR, 1972). The most probable occupants of the center of the lake are rocks from the Mississagi Formation and the Nipissing Diabase. Other possible target rocks are Archean gneisses and rocks of the Superior province, based on outcrops in the vicinity of the lake (DENCEand POPELAR,1972). Based on studies of boulders recovered from glacial drift, most rock fragments appear to be derived from the Mississagi Formation and the Nipissing Diabase (DENCE and POPELAR, 1972). Studies of suevite breccias by Dence and Popelar, and by Winzer (this study), indicate that the following rock types are involved in the event: quart&e, arkosic quartzite, siltstone, and diabase, in descending order of importance.
S.
52
R.
WINZER,
R. K. L. LUM and S. SCHUHMANN
Table 1. Modal analyses of rocks from the Wanapitei Lake impact structure* Sample No.
22122
22123
22124
ou?.rtz
16. 2
37. 3
13. 1
2. 4
Feldspar
7s. 1
1. 5
39. 5
26. 6
7. 9
0. 2
15. 5
1. 3
Muscovite
3. 3
Biotite
2. 4
3. 2
Amphibole
22125A
22125B
22126
22121
22128
1. 6 25. 7
6. 8
35.82
25. 1
2
44.
6
34.72
25. 0
4. 6
8. 4
3. 4
0. 1
7. 7
52. 2
1. 4 51. 9
Pyroxene Clear
221210 12. 3
32. 9
9. 8
6. 1
22129
Glass
34. 7
Brown Glass
4. 2
23. 4
Lithic Fragments
17.7’
25. 9 16. 8
27. 1
31. 8
0. 5
12. 5
20. 8
Opaque Calcite
43. 4 18. 0
12. 8
49. 6
9. 3
1. 9
WZ-b
W3-a
W3-b
Sample No.
Wl-a
WI-C
Quartz
13.73
13. 63
25. l3
15. o3
22.53
25. 33
9. 7
7. 3
6. 3
24. 3
21. 7
16. 8
23. 84
46. 64
Feldspar
WZ-a
4. 6
2. 0
3. 9
w-4
W5-a
W5-b
10.03
1O.23
0. 2
8. 5
10. 0
41.54
30. 4
43. 4
46. 7
32. 7
9.83
Muscovite Biotite
0. 1
Amphibole 2. 7
Pyroxene Clear
Glass
Brown Glaes
40. 5
74. 7
35. 0
4. 4
28. 2
43. 0
Lithic Fragments Opaque
14. 0 0. 9
27. 9
27. 6
27. 3
24. 5
4. 1
1. 3
4. 1
Calcite *
23. 8
Based on 2000 points; expressed in ‘%.
1Includes maskelvnite and thetomornhic quartz. ’ Mostly maskelynite. 3 Includes thetomorphic quartz. 4 Includes devitrified material. ‘Granitic’ rocks, greenstones and amphibolite were found, but are rare. Several boulders of suevite breccia were collected by the senior author during the course of the 1972 International Geological Congress field trip to the Sudbury area. These boulders were collected from glacial drift near the Skead mining area on the south shore of Lake Wanapitei. The boulders range from polymict breccias containing shocked and unshocked lithic fragments in a matrix of finely comminuted rock and mineral fragments (with a minimum of glass) to glassy grey or greyish white boulders composed mainly of fresh or slightly devitrified glass with some thetomorphic minerals remaining. This latter type of boulder was found to contain coesite by DENCE et al. (1974). PETROGRAPHY
AND PETROLOGY
Both suevite breccias and glassy breccias were used in this study. Two large boulders of suevite, containing clasts of unshocked to mildly shocked arkosic quartzite, siltstone and diabase were recovered from the drift. Thin sections of the clasts reveal textures indicative of low to moderate shock pressures, and encompass comminution of rock fragments, planar elements in quartz and feldspar, kinking in biotite, maskelynite and a small amount of glass. Little actual melting was observed, and few thetomorphic minerals were present [a thetomorphic mineral is defined as a glass or glassy phase, commonly of quartz or feldspar, produced by solid state alteration of an original crystalline mineral by action of shock waves and retaining the form and original textures of the pre-existing mineral or grain (Glossary of Geology,p. 736)]. Larger lithic fragments included in the cornminuted matrix also show textures indicative of low to moderate shock pressures. In general, thetomorphic minerals are minor, and flowed glass is
absent. Examples of the lithic types were extracted from the breccia matrix and used for subsequent chemical study. Petrographic descriptions of these lithic fragments can be found in the Appendix, modal analyses in Table 1. Three of the boulders collected, ranging in size from 15 to 50cm in diameter, were of the glassy type. These glassy boulders are pale grey, with a pitted surface suggesting vesicularity. A few rounded, vesiculated clasts, resembling mineral or lithic fragments, were found, but the most striking feature is the clear grey to brown glass veins which lace the boulders. The vein glass is fresh, no signs of alteration appear in hand specimen. In thin section, the glass is either clear or brown, with flow texture evident in both cases. The surrounding partly devitrified grey glass is filled with small crystallites of feldspar and possibly zeolite, giving the mass a felted texture. Thetomorphic quartz grains are scattered throughout the grey glass. but thetomorphic feldspar glass is minor or absent, and where present, is free of the crystallites present in the glass matrix. Electron microprobe analyses are presented in Table 2. The normative calculations, from the electron microprobe analyses, indicate that the glasses have formed from the melting of a rock rich in feldspar and quartz. The mafic component, indicated by the ‘diopside’ content, is variable and could indicate either the diabase component or an inhombgeneously distributed phase such as biotite which, through local melting, contributes varying amounts of Fe and Mg to the glass. There is no systematic variation in chemistry between clear glass and brown glass, suggesting that the color may be due either to a dispersed phase or minor elements. The glass compositions support the hypothesis of DENCE and POPELAR (1972) that the Mississagi Formation and the Nipissing Diabase are the major groups involved. The major element data suggest an arkosic quartzite as the main contributor, with smaller. variable
53
Rb, Sr and strontium isotopic composition Table 2. Electron microprobe analyses of glasses from Lake Wanapitei** Vf1-b
Si02 TiO2
1
63. 87
76. 67
0. 40
0. 04
lb.
bl
10. 26
FeO’
5. 30
2. 15
MllO
0. 02
0.03
MgO
3. 52
1. 24
cao
1. 04
1. 02
i-h20
1. 98
K2O
*‘Z”3
TOTAL. Norm
WI-a 2
0
Sample
77.48
WI-c
74.
0. 12 10.84
26
WZ-a
1
74. 36
76. 86
0. 04 12.05
2
75.80
12. 95
11.33
11.38
13. 26
w4-3
74.
w4-4
60
0. 17 13.40
77. 57 0. 05 12.78
0. 70
0. 97
0. 94
0. 34
0. 78
1. 25
1. 04
2. 48
2. 08
1. 54
1. 59
2. 99
3. 52
5. 22
6. 28
2. 58
97. 47
98. 12
22. 0
32. 5
0. 86
1. 50
0. 03
0. 05
0. 08
0. 82
1. 37
0. 23
0. 83
1. 14
1. 04
1. 14
0. 73
0. 88
1. 19
2. 22
2. 53
2. 04
1. 81
2. 35
5. 07
4. 03
3. 62
3. 98
6. 47
3. 43
97. 81
97. bb
97. 78
96. 95
97. 46
97. 2b
97.17
2,.
5
-
2
73.40
0.11
-
2. 04
1. 33
w4-
1. 95
1. 76 0. 10
100.
2
38. 5
IS.
5
(Nigglil 0. 2
0. 2
0.b
-
0. 2
0. 1
Or
31. 0
25. 0
22. 5
25. 0
40.
Ab
18. 5
21. 0
23. 5
19. 5
17. 0
22. 0
19. 5
14. 5
15. 0
27. 0
An
5. 5
5. 5
5. 5
6. 0
4. 0
4. 5
6. 5
6. 5
5. 5
12. 5
Corundum
6. 5
0. 3
1.0
2.5
1. 8
3. 5
2. 2
3. 2
2. 3
0. 6
17. 8
7. 0
4. 4
7. 3
2. 0
4. 8
6. 4
5. b
2. 0
3. 8
43. 0
39. 7
35. 2
44. 6
43. 3
37. 8
36. 6
Ilrn
Di
(En
+ Fs,
20. 1
atz
’
Clear glass;
41.
2
’ Brown glass;
* Total Fe as FeO;
amounts of diabase contributing the mafic component. This is supported further by the petrography of low to moderately shocked clasts included in suevite breccias. These clasts are usually either a feldspathic quartzite, siltstone, diabase or amphibolite. The ‘siltstone’ and feldspathic quartzites are low in ferromagnesian minerals. The surprising fact about the glass is the low calcium content. Both rock types examined have either carbonate or calcic plagioclase in significant amounts (up to several per cent), yet the normative calculations indicate smaller amounts of calcium than expected. No immediate explanation presents itself for this apparent difference. K/Ar AGES Rb, Sr, strontium isotopic composition, K, Ar and argon isotopic composition were obtained on selected glasses, clasts and whole-rocks from the Wanapitei Lake structure. K/Ar dates were obtained on one glass vein and one whole-rock sample obtained from the glassy boulders. The analyses were done by Geochron Laboratories. Clean, clear and unaltered glass from vein material cutting one of the larger glassy boulders has a date of 37.8 & 1.6 m.y. Grey, glassy whole-rock material from a separate boulder gave a date of 36.0 + 1.6m.y. (Data on these analyses are available on request from the senior author.) These two dates overlap within the quoted error and may be considered as the same date. In the absence of subsequent heating or alteration, the date quoted should be the age of formation of the glass, and thus a reasonable approximation of the time of impact. As the vein glass shows no sign of recrystallization or alteration. this age is the best approximation of the time of impact. The whole-rock date includes thetomorphic clear glass and devitrified glass. This devitrification may be the reason for the younger date of the whole-rock. Rb/Sr AND Sr ISOTOPIC Procedure
COMPOSITION
Small (8 cm3) blocks of sample were cut from boulders and clasts selected for examination and analysis. One side of the block was slabbed and a
0
40.
5
** Expressed as wt o/W
polished thin section made. This section was used for modal analyses and microprobe study. The remainder of the block was crushed to powder in a boron carbide mortar. Two samples were chosen for further separation. These were less finely ground, and glass and minerals were separated using heavy liquids and handpicking. One bulk glass sample was used for K/ Ar age work, and one sample was separated into four fractions (by density) in an attempt to obtain an internal Rb/Sr isochron. Rb and Sr were determined by isotope dilution mass spectrometry. An aliquant of powder was taken and spiked with 84Sr with a purity of 99.892% (SRM988). The use of this spike allowed 87Sr/8hSr to be determined at the same time as Sr abundance. The 84Sr spike and powder were treated successively with HF + HClO,, HCl and H,O. Sr was separated by ion-exchange chromatography. Rb was treated in the same manner as Sr, using “Rb spike on another sample aliquant. K was analyzed along with Rb, using K4r spike. Sr samples were mass analyzed using a Shields-type 12 in. radius, 60” sector mass spectrometer utilizing triple filament surface ionization and expanded scale chart recording. Rb and K were done on a Shieldstype 6 in. radius, 60” sector mass spectrometer using triple filament surface ionization but without expanded scale chart recorder. Precision and accuracy are within the limits quoted for trace element analyses by this laboratory (SCHNETZLERet al., 1969). Blanks are generally 7 nng for Sr and @6nng for Rb. Precision of s’Sr/*%r ratios (2a, normalized to ‘%/*%r = 01194) varies from sample to sample and is quoted in the table (Table 3). SRM-987 and the Eimer and Amend SrC03
54
S. R. WINZER,R. K. L. LUM and S. SCHUHMANN
t=104 IR= x
be established for the glass points. This ‘age’ is neither that of the impact nor that of the country rock, although it does come close to the Grenville ages found to the south and east of the Wanapitei Lake structure. The isotopic compositions of the clasts comprise 2 groups with 4 samples scattering generally between the two groups. The clasts do not define an isochron, but do contribute to the understanding of the glass compositions. First, the glasses are all enriched in radiogenic strontium. The ‘initial ratio’ for the glasses is also enriched relative to the initial ratio (0.7063) of the Gowganda Formation studied by FAIRBAIRN rt al. (1969). The clasts, with two exceptions. can be said to approximate a line with an age of 180& 2100 m.y. with an initial ratio close to that found by FAIRBAIRN et al. (1969) for the Gowganda and Bruce formations in the Sudbury area. No data are available for the Mississagi Formation. The clasts, in any case, are indicative of Huronian metasediments of age between 1800 and 2100m.y. If the metasediments can be considered coeval, then the glasses derived from them can be interpreted in terms of either loss of Rb or gain of radiogenic Sr, or both, or lack of equilibration or homogenization during impact. The most enriched samples studied are feldspathic ‘quartzites’ with lithic fragments of ‘siltstone’ and carbonate. The group with the lowest s7Sr/86Sr and Rb/ Sr ratios consists of amphibolites or diabase with the exception of 22125A, which has a large amount of opaque material and glass. The glass compositions, being far more enriched in radiogenic Sr as well as Rb than the amphibolites and diabase, suggest that they are derived from feldspathic quartzite.
x 10%
7508
= 1.39 x IO”yr
22:25(A) 73w-
2y29
72W-
l22126
22727
7lco/ 0
1
2
3 “Rb /=
5
4
Sr
Fig. 1. Rubidium-strontium evolution diagram for glasses and clasts from the Wanapitei Lake impact structure. 0 Glass. A Metasediment. n Diabase or amphibolite. I Error bar (glass only).
were run throughout the procedure. “Sr/ *%r ratios were: SRM-987 07104 f 00016 and Eimer and Amend 07083 f 00003. Analytical data are presented in Table 3 and Fig. 1. The. data indicate that no good isochron relationship exists for either glasses or clasts (Fig. 1). The Rb/Sr ratio for glasses does not vary greatly, but the precision of the analysis is sufficient to allow an isochron. A least-squares regression line corresponding to an age of 1043 m.y. and an initial ratio of 0.7508 can standards
Table 3. Rb, Sr and Sr isotopic composition of Wanapitei Lake impactites Sample 22121
Rb(PPM) 3. 22
87Rb(PPM) 0. 90
22l22
57. 6
16. 0
22123
37. 5
10. 4
22124 22125(A) 22125(B) 22126 22127 22128 22129 22lZlO
143.
9
37. 62 133.2 42. 46 38. 24 149.4 16. 7 190.4
SrN(PPM)
86
Sr(PPM)
87
87
Sri%
Rb/%
162.
2
15.70
106.
5
10. 29
7561
t
0006
1. 56
4. 96
. 7652
t
0015
2. 10
9. 91
8096
3
0009
4. 04
7309
+
0005
0. 62
9. 62
8146
f
0003
3. 85
14. 64
. 7272
+
0002
0. 81
7227
2
0008
0. 62
51. 42
40. 05
102.7
10.47
176.
37. 07
100.1
11.82
151
10. 64
175.7
41. 58
123.
4. 64
183.7
52. 99
103.4
0
17
17. 1 6
.7451
*
11.68
.8159
17.85
.7253~.0012
i
9. 87
. 8438
+
.0003
.0005
0. 057
3. 56 0. 26
0007
5. 37
Glass WI-a
91. 99
25. 6
97. 58
9. 40
. 7897
k
0008
2. 72
w-c
90. 16
25. 09
94.63
9. 12
7932
+
0001
2. 75
WZ-A
84. 67
23. 56
92. 76
8. 98
7896
k
0003
2. 62
WZ-B
82.93
23. OS
94. 57
9. 11
7907
+
0003
2. 53
W4
71. 04
19.77
9. 93
1198
3
0001
1. 99
103
Internal lsochron WI-b,
109.
W-lb3
100.4
W-lb4
6
96. 59
30. 5
104.5
10. 06
. 7881
+
0017
3. 03
30.17
109.2
10. 52
.7850
*
0004
2. 87
3
0007
3. 38
26. 88
82. 59
7. 96
7898
55
Rb, Sr and strontium isotopic composition Table 4. Large ion lithophile trace element abundances 22123
22122 Li
3. 10
Rb
104
Ba
570
Ce
22129
6. 63
6. 59
57. 6
Sr
22124
152
50. 6
102
168
809
147
385
16. 5
W2-b
221210
11. 8
37. 5
in Wanapitei Lake samples*
5. 43
BCR-1
4. 27
178
12. 9 46. 7
85. 1
95. 8
91. 9
1230
328
426
681
28. 4
40. 0
9. 42
12. 5
17. 3
5. 77
4. 12
1. 82
2. 86
1. 42
0. 796
1. 83
b, 61
0.448
0.751
0. 548
0. 231
0. 432
1. 92
0. 611
1. 13
1. 89
0.555
0. 969
6. 74
0.406
0. 290
0. 563
1. 03
0.435
0. 644
4. 68
Yb
0. 376
0. 261
0. 558
0. 916
0.490
0. 622
3. 25
L”
0. 064
0. 065
0. 098
0.159
0.188
0.106
0.533
16. 3
Nd
7. 46
Sm
1. 21
EU
0. 325
DY
0. 732
Er
zr
22. 2
Hf
15. 5
0. 634
43. 5
29. 7
10. 4
46. 0
1. 91
25. 9
50. 7
11. 0
27. 6
43. 4
1. 52
1. 64
* Expressed as ppm.
If all clearly igneous or amphibolitic material is left out, no age relationship can be inferred for the
clasts. All metasedimentary clasts scatter, but are more enriched in radiogenic strontium than the amphibolites or diabase. 22121 comes close to the ‘initial ratio’ of the glasses. With the exceptions of 22122 and 22123, the glasses appear as a mix between metasediments of the mineralogical compositions of 22121, 22124, 22125B, 22128 and 221210. In order to evaluate further the contribution of various clast components, several clasts and one glass were analyzed for large-ion-lithophile trace elements (Li, K, Rb, Sr, Ba and the REE’s).
11
, L,
K
Rb
Sr
Ba
Ce
Nd
TRACE ELEMENT
ABUNDANCES
Procedure
Aliquants of material prepared for Rb and Sr isotopic analysis were taken and analyzed for Li, K, Rb, Sr, Ba and the rare-earth elements (REE’s) by stable isotope dilution mass spectrometry. The method used is that reported by SCHNETZLERet al. (1967b). BCR-1 was analyzed as a monitor. The analytical blank is generally less than 1% of the amount present (PHILPINTS et al., 1972). Data are presented in Table 4 and in graphical form in Fig. 2. It can be noted, by comparing Rb and Sr values
Sm Eu Gd
DY
Er
Yb Lu
2r
Hf
Fig. 2. Large ion lithophile trace element abundances in clasts and glass from the Wanapitei Lake impact structure. Solid line = metasediment. Dashed line = glass. Broken line = amphibolite or diabase.
56
S. R. WINZER,R. K. L.
between Tables 3 and 4, that repeat analyses by separate dissolutions and using different spikes are generally good. Sr for 22129 and Rb for 221210 are outside the range expected from the comparison run for BCR1. This may be due to sample inhomogeneity. The trace element abundance patterns show a variation in absolute abundance of about 5. with the exception of 22129. The patterns are strongly enriched in light REE’s and depleted in heavy REE’s, with the general exception of Zr and Hf. Enrichment in Zr and Hf may be expected for metasediments as zircon, the principle mineral phase containing these elements, survives erosion and redeposition and may be concentrated in sedimentary deposits. The pattern for 22129 is quite different from the other clast patterns in that the light REE’s are not nearly as enriched over the heavy REE’s. This is a function of lithology, as 22129 is a diabasic rock and the others are feldspathic quartzites or arkoses. The abundance pattern for 22129 is similar to those found by PHILPOTTS and SCHNETZLEK(1968) for other diabases, but the absolute abundance is lower. It is immediately obvious that the compositional trend of the glass parallels that of the metasediments. The heavy REE abundances are somewhat higher than most of the metasediments; this suggested some contribution from rocks of the 22129 type, but the light pattern, Rb. Sr and Li values virtually rule out such a contribution. The trace element patterns strongly suggest thdt the glasses formed by shock melting of metasedimentary material, and that contribution from diabase or amphibolite was negligible. The similarity of the patterns also shows that little or no volatile/involatile ratio changes have taken place, suggesting that these elements do not fractionate during the impact melting process. The similarity of glass and clast patterns confirms the relationship of crater glass to country rock shown by SCHNETZLERet a/. (1967a). CONCLUSIONS
Glass and glassy whole-rock samples from the Wanapitei Lake impact structure were found to be 37 m.y. old. This age is considered to approximate the time of impact, thus confirming the expected large time interval between the formation of the Sudbury and Wanapitei Lake structures (DENCE and POPELAR. 1972). Study of Rb/Sr and Sr isotopic composition failed to yield a meaningful isochron for glasses and glassy whole-rock samples. An attempt to obtain an internal isochron also failed. This is likely due to incomplete re-equilibration of Rb and Sr isotopes, or to selective loss or gain of Rb or “Sr. The latter seems unlikely in the light of study of the large ion lithophile trace element abundance patterns, which do not indicate that fractionation has taken place. Large-ion-lithophile trace element abundance patterns indicate that the Wanapitei Lake glasses were formed by total melting of metasedimentary material,
JIM
and S.
SCHUHMANN
most likely of the arkosic quartzite type. This conclusion is supported by the Rb/Sr isotopic work which indicates little contribution from diabase of low s’Sr/ ‘%r ratio. The conclusion indicated by the isotopic and trace element abundance work is somewhat different than would be drawn from major element analyses of the glasses alone. No quantitative comparison can be made on this set of rocks because major element analyses of the clasts are lacking. Aclinowledyemrtlts-This work was done while the senior author was under a National Academy of Sciences Resident Research Associateship, for which thanks are given. I (Winzer) would also like to thank J. A. PHILPO~~ for discussions and review of this work. REFERENCES D~~NCE M. R. (1965) The extraterrestrial origin of Canadian craters. Ann. N.Y. Acad. Sci. 123. 941-969. DENCE M. R. (1971) Impact melts. J. Geophys. Res. 76. 5552-5565. DENCE M. R. and POPELARJ. (1972) Evidence for an impact origin for Lake Wanapitei, Ontario. Geol. Ass. Can. Spec. Paper 10. I17-l24. DENCEM. R.. ROL&RTKINP. B. and WIRTHLINR. L. (1974) Coesite from the Lake Wanapitei Crater, Ontario. Earth Planet. Sci. Lett. 22. 118-l 22. FAIRBAIRNH. W., HURLEYP. M., CARDK. D. and KNIGHT C. J. (1969) Correlation of radiometric ages of Nipissing Diabase and Huronian metasediments with Proterozoic erogenic events in Ontario. Can. J. Earth Sci. 6.489-496. FLIUALIR. F. (1974) Genesis of melt rocks at Tenoumer Crater, Mauritania. J. Geophys. Res. 79. 21152121. PHILPOTTSJ. A. and SCHNETZLER C. C. (1968) Genesis of continental diabases and oceanic tholeiites considered in light of rare-earth and barium abundances and partition coefficients. In Origin and Distribution of the Elements, (editor L. H. Ahrens), Vol. 30, 939-947. Pergamon Press. PHILPOTTSJ. A., SCHUHMANNS.,BICKELA. and LUM R. K. L. (1972) Luna 20 and Apollo 16 core fines: large ion lithophile trace element abundances. Earth Planet. Sci. Lett. 17. 13-18. POPULARJ. (1972) Gravity interpretation of the Sudbury area. Grol. Ass. Can. Spec. Paper 10. 103-115. SCHNETZLER C. C., PHILPOTT~J. A. and PINSON W. H. (I 969) Rubidium-strontium correlation study of moldavites and Ries Crater material. Geochim. Cosmochim. Acta 33. 1015.-1021.
SCHNETZLER C. C., PHILPOTTSJ. A. and THOMASH. H. (1967a) Rare-earth and barium abundances in Ivory Coast tektites and rocks from the Bosumtwi Crater area, Ghana. Grochim Cosmochim. Acta 31. 1987-1993. SCHNETZLER C. C., THOMASH. H. and PHILPO~ J. A. (1967b) Determination of rare-earth elements in rocks and minerals by mass spectrometric stable isotope dilution technique. Anal. Chem. 39. 1888-1890. VON EXELHARDT W. (1972) Shock produced glasses from the Ries Crater. Corltrih. Mineral. Petrol. 36. 265292. APPENDIX Petrography 22121
No thin section. 22122 Weakly shocked arkose. Feldspar grains occasionally show shock lamellae while quartz grains usually show two or more sets of planar features. Some biotite oxidation.
Rb, Sr and strontium isotopic composition 22 123 Hyalocrystalline, strongly shocked ‘arkose’. The rock is vesicular and contains about 60% glass. The remaining quartz and feldspar is strained but shock lamellae are absent. Two varieties of glass occur. Clear glass shows flow texture by slight differences in refractive index while brown gfass contains dark inclusions which are aligned. also indicating gowage. 22124 Weakly shocked ‘arkose’. The rock consists of quartz, feldspar. biotite. muscovite, amphibole and grey. amorphous or microcrystalline fragments (siltstone?). Quartz and feldspar grains are cloudy, and many show planar features. Some grains have rims of clear quartz which has not been shocked or strained. suggesting some later annealing. 22125A H~locrystalline-allotriomorphi~ ‘breccia’ consisting of feldspar, quartz, hthic fragments and glass. The lithic fragments are similar to the ‘siltstone’ fragments in 22124. Most of the glass is nearly opaque with inclusions, but a small amount of clear glass does occur. 221258 Moderately shocked lithic arkose. This sample differs from 22125A in that it contains less glass. Quartz shows several sets of planar features, while a few feldspar grains are now maskelynite. The maskelynite is included with glass in the modal analysis. 22126 Moderately shocked amphibolite. In this section most of the feldspar is maskelynite, but no glass or flowed glass is present. Little or no comminution has occurred. Grains are idiomorphic, with sharp boundaries which do not show signs of incipient melting. Some alteration of plagioclase to sericite or epidote has occurred. 22127 Moderately shocked amphibolite. Texture is very similar to 22126. but some of the maskelynite shows more alteration to scricite. Biotite has been oxidized. and amphibole is brecciatcd but not comminuted. No glass is present in the slide. 22128 Weakly shocked greywacke. This rock consists of feldspar, quartz, opaque minerals (mainly hematite) and microcrystalline rounded lithic fragments (‘siltstone’). A few feldspar and quartz grains show deformation lamellae or a few planar features. 22129 Moderately shocked diabase. Holocrystalline, hypidiomorphic inequigranular poikilitic rock consisting of ortho- and clinopyroxene, plagioclase and minor biotite, amphibole and carbonate. The orthopyroxene is dominant: it shows coarse schiller structure and poikiliti-
57
tally encloses smaller grains of clinopyroxene. The pyroxenes have been fractured so that one grain has many domains within it. These domains are characterized by irregular boundaries. All plagioclase is maskelynite. and some grains are partly altered to sericite. Carbonate occurs in veins cutting the maskelynite. 221210 Weakly shocked greywacke. Texture and shock indicators are the same as 22128. WI -a Hyalocrystalline breccia consisting of clear glass, brown glass. thetomorphic quartz and feldspar and fragments of quartz and feldspar. Clear glass shows flow texture and contains small euhedral feldspar(?) crystals. Brown glass includes bubbles and opaque inclusions. Mineral grains remaining have planar features in multiple sets. WI-b No thin section. WI-c Hyatocrystahine breccia, with texture similar to Wl-a, but containing more glass and less quartz and feldspar. W2-a Hyalocrystalline breccia. This sample differs from WI-a and c in that the glass is partly recrystallized. This gives the slide a ‘fehed’ appearance. The crystal phase [feldspar~?). zeolitef?)] is acicular, euhedral and has a refractive index greater than the glass. Mineral fragments of quartz and feldspar show multiple sets of planar features. W2-b Hyalocrystalline breccia, similar to WZ-a but with a higher proportion of mineral fragments. W3-a Polymict breecia. consisting of mineral and lithic fragments in a matrix of glass and finely ~omminuted rock powder. Glass has a Row texture. Fragments of quartz and feldspar contain planar features. Lithic fragments are generally mildly shocked quartz& or arkose. but a few are strongly shocked and glassy. W3-b Polymict breccia, almost identical to W3-a. W-4 Hyalocrystalline breccia, consisting of glass, thetomorphic quartz, mineral and lithic fragments. The glass in this sample differs from that in the others in that it is not Rowed. W5-a Pofymict breccia consisting of glass, lithic fragments of diabase, amphibolite, quartzite and arkose and mineral fragments of quartz, feldspar and pyroxene. This rock is a typical seuvite breccia, fragments range from unshocked to moderately shocked. with a few giassy ciasts included. W5-b Polymict breccia very similar to W5-a.