Composition of meteorite impact glass across the Henbury strewnfield

Composition of meteorite impact glass across the Henbury strewnfield

Gcochimica et Cosmochimica Acta,1967,Vol. 31,pp. 961to 968. Perplamon PressLttl. Printedin Northern Irelan~l Composition of meteorite impact glass ac...

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Gcochimica et Cosmochimica Acta,1967,Vol. 31,pp. 961to 968. Perplamon PressLttl. Printedin Northern Irelan~l

Composition of meteorite impact glass across the Henbury strewnfield S. R. TAYLOR* Dept. of Geophysics and Geochemistry, Australian Sational University, Canberra (Received

14 November

1966; accepted in revised form

9

Januay

1967)

Abstract-Glass formed by meteorite impact occurs over an area of at least 1500 m2 at, Henhllry. Northern Territory, Australia (24”35’S. Lat.; 133”09’E. Long.). Extensive collecting across t,his strewnfield was undertaken because of variable compositions of potential parent materials. Eight samples of meteorite impact glass from widely separated areas have been analysed. 1Yhrn the iron due to meteorite contamination is removed, the average composition (wt, “/,) for the major elements is: SiO,, 74.5; Al,O,, 13.1; FeO, 4.0; MgO, 2.3; CaO, 0.6; Na,O, 1.0; Ti,O, 3.4; TiO,, 1.0. The glass is uniform in composition and the variation in the eight samples is \vit,hinthca limit,s of analytical error. Fourteen analyses of country rocks arc given. Those exposed in thtx crater walls range from 76 to 90% SiO,. The glass composition matches most closely tllai of t,hrownout green subgreywacke, which appears to have been derived from near the centfsr*of the northwest crater of the double main crater.

AN EXTENSIVE strewnfield

of glass fragments has been formed by meteorit’e impact, on subgreywacke type sedimentary rocks at Henbury, Northern Territory, Sust’rnlia (24“ 36’S Lat.; 133” 09’ E Long.). Geochemical studies of a restricted number of glass samples indicated a close correspondence in composition for fifty elements, with that of subgreywacke exposed at the north wall of the main crater (TAYLOR, 1966; TAYLOR and KOLBE, 1964, 1965). ALDERalAN (1932) suggested that the oval shape of the main crater (Figs. 1 and 2) was due to the overlap of two craters, and MILTON (1965) has confirmed this interpretration on the basis of detailed mapping The northwest crater is excavated mainly in subgreywacke, but more silica-rich sediments are exposed in the south and east walls of t,he southeast crater. Because of these variations in composition of potential parent materials, the strewnfield was sampled to see whether the composition of the glass varied or was uniform and whether glass was derived from one or both craters of the double main crater. GLASS DISTRIBUTION

Individual pieces of glass were found within a sector extending from 270” through north to 120” and from the rim of the main crater up to distances of 300 m. The glass appears to be concentrated along radial directions from the crater and to be sparse or absent in intervening areas. Collecting along a north bearing, and on Localised concentrations bearings at 55, 75 and 90”, was particularly fruitful. occurred along the radial directions. Redistribution by wind and water may have altered the initial distribution. The glass occurs on a gently sloping stony surface, on top of the thrownout sediment. It is fragile and would not survive much transport by water. Although the climate is arid (average rainfall is 8 in.), surface flooding by occasional heavy rain occurs. Areas to west and southeast have been * On leave at: California 92038.

Department

of Chemistry, University 961

of California, San Diego, La .Jolla,

S. R. TAYLOR

962

slightly disected by stream erosion. Immediately to the south are the \Vatc:~ Crater and No. 8 Crater. These craters, identified in Fig. 1, presumably formed simultaneously with the pair forming the main crater since throwout from later craters would have otherwise modified earlier formed ones. Collecting has also disturbed the original strewnfield. The formerly abundant iron meteorite fragments (ALDERMAN, 1932) have now been nearly completely removed. The glass which is not so conspicuous nor elegant has been less thoroughly collected, except near t’he rim.

‘L

MAIN CRATER

’ 142;;

\&y

;_,, 34

0

130

_.__A

200 ___

METERS

:

.I

s'_' .. x

Lx. 120"

ANALYSED GLASS LOCALITIES ADDITIONAL AREAS OF GLASS COLLECTION ANALYSED SEDIMENTARY ROCKS

Fig. 2. Sketch map showing distributionof glass around main crater at Henbury.

The areas from which glass was collected are shown in Fig. 2. Those area’s with diagonal shading yielded enough glass for chemical analysis of the major element+. Localities are given in Appendix I. SAMPLE PREPARATION AND ANALYSIS The slaggy frothy friable glass samples were cleaned using an ultrasonic cleaner and gently fragmented in a new agate mortar. The highly magnetic glass fraction was separated by use of a hand magnet, and cleaned in the ultrasonic cleaner. -1 Franz separator was used to separate the less magnetic glass, and the nonmagnetic glass was separated from adhering soil and clay by use of bromoform-acetone solutions. After final cleaning by handpicking under a binocular microscope, the various clean glass fractions (about 80 mesh) were combined, and ground to pass 120 mesh silk bolting cloth. The sedimentary rock samples were prepared by reducing the samples to 1 cm size fragments in a jaw crusher, followed by reduction to pass 20 mesh silk cloth, by an agate cone grinder, followed by grinding in a mechanical agate mortar to pass 120 mesh silk. Sample localities are given in Appendix II. Analytical data are given in Table 1 for the glasses. The glasses contain varying amounts of iron and nickel derived from the impacting meteorite. Although some

Fig. 1. Aerial view of Henbury craters, looking northwest. Photo taken above qnartxite ridge. The main crater is the pronlin~nt, feature to the right and consists of two coalescing craters. 1mmediatt:ly to tho left of t)hc main crate is So. 8 crater. Behind this is the w&er crater, with rim breached by stream. (‘ma&w 3 is scdirrwnt is visibl(l as t lw Arisible at 11 o’clock from t’he mater crater. Thrownout dark pat~ch to the right of the main crater (crater numbers are from i%LfmRXAN, 1932).

Photo

by W. D. Ehmann.

Composition of meteorite impact glass across the Hr?nbury strewmfield

963

of this is in the form of spherules, much is finely dispersed through the glass, and it wa,s not possible to make an effective separation. TAYLOR and KOLBE (1965) removed the meteoritic iron contribution from the analyses of the glasses by assuming that the parental iron content was given by t’hnt of the sediment. closest, in composition. The variation in nickel content and the apparent) change in Fe/Ni ratios from those observed in the iron meteorite makes the nickel cont,ent an The procedure unreliable index of the amount of meteoritic contamination. adopted here is to reduce the glasses t,o a uniform iron content, of 4% FeO. This is Table 1. Analyses of Wenbury impact glasses, expressed as wt. 0A oxides, reduced to uniform iron content Sample HB

I.

2

SiO, AJO, Fe0 * MgO

75.1 12.3 4.0 2.39

cao Na,O K,O TiO 1 2

11

28

is.6

742

754

12.3 4.0 2.33

13.3 4.0 2.25

O-74

0.59

0.92 3.19

O-87 3-27

;;::

g;:y

29

31

1213

1413

a,-.

12.9 4.0 2.14

74.1 13.5 4.0 2.29

74.1 13.3 4.0 2.30

73.7 13.2 4.0 2.26

71.3 13.6 4.0 2‘28

74.5 13.1 4.0 2.28

o-57

0.47

0.38

0.59

0.51

0.53

0.53

0.96 3.61

0.94 3-45

O-81 3-62

1.01 3.48

1.10 3.62

0.91 3.13

0.94 3.42

g;:r

9;:;2

g;:;g

Original iron content(s of glasses, as analysed (wt. %) Fe,03 3.91 2.39 2.15 3.45 4.70 Fe0 5.81 11.43 4.15 3.35 3.95

y;:;6 4.15 4.30

$“,2 0.32 Il.5

ii:;”

9;:;5

3.60 3.90

* All iron expressed as FeO.

close to the amount in the subgreywaekes, and enables a comparison in composition to be made. The original iron contents of the glasses are given in Table 1. ‘Table 2 lists the compositions of the sedimentary rocks. These are expressed on a water and CO, free basis to enable comparison with the glass compositions.

When the effects of varying iron content due t’o meteoritic material are removed, the glasses exhibit a uniform composition (Ta’ble 1). The significance of the small fluctuations observed is difficult to evaluate. The differences are much less than the variations in composition of the sedimentary rocks, and are probably not outside the limits of analyt.ical error of silicate analysis by wet chemical methods. The conclusion adopted here is that the impact glass composition is uniform over the strewnfield. GEOLOGICAL RELATIONS OF SEDIMENTARY ROCKS Figure 3 shows a north-south cross section through the Henbury main crater, No. 8 crater (Fig. 1) (crater numbers from ALDER~~A~, 1932), and the quartzite ridge to the south. The location of the analysed samples is shown, and should be compared with the sketch map (Fig. 2). Samples 18 and 20 are from small blocks The other samples are from large masses of rather friable which may be throwout. sediment which appear to form part of the crater walls.

~____-----27

75.5 12.1 445

SiO, A& FeO*

*

0‘85

TiO,

99-6

0.88

0.62 3.19

0.08

2.16

76.8 11.3 4.52

21

AII iron expressed aa FeO.

99.8

o*ln 3.81

*@ GO

I:

2.32

011

W3O

C&O

No.

Sample

T&la

99.3

0.82

I.00 294

084

2.02

57.2 10.8 3.71

A

2.14

99,6

0.72

0.82 2.77

0.48

1.81

99.7

0.90

0.80 3.35

0.59

99.5

0.98

0.69 2.83

0.66

1.84

18.9 9.41 4.22

sodimentctry

18.7 [email protected] 3.58

of Henbury

77.4 11.0 4.28

2. Anelyses

99.6

0.80

0.75 2.94

0.48

i*i”

79.8 9.58 3.53

99.9

0.92

1.23 2.67

0.08

1.67

79.8 lot? 3.34

90.9

0.86

08% 3.06

0.41

l-96

78.1 106 4.04

99.6

0.60

0.73 1.84

0.28

1.04

884 6.84 2.96

100.0

0.50

0.43 1.52

0.08

0.80

84.2 4.65 2.59

rocks, expresscvi ~8 wt.n/O ouldc9,with watrr, (‘0,. ch.,

ijB.6

0.45

0.30 1.20

0.73

0.48

89.8 5.18 1.48

rrmoxwl

09.8

0.36

0.30 1.28

0.29

P41

92.0 3.42 1.72

Q9.8

0.36

0.32 0.90

0.20

0.36

94.3 3-2s 0.08

100-O

0.68

0.23 2.07

0+36

0.94

82.6 8.85 3.96

8

i$

2

?j

i”

Composition of meteorite impact glass across the Henbury strewnfield

96.5

Although there has been great disturbance of the strata in the crater walls due to t,he impact, the interpretation adopted here is that the blocks exposed in the crater walls are, with minor exceptions, not far from their original stratigraphical position. This inclusion has also been confirmed by MILTON (1965). The inferred subsurface structure based on an average dip southwards of about 35’ is shown in Fig. 3. There is no evidence of faulting or folding between the ridge and the craters, but drilling is required to substantiate the present interpretation. The silica percentage of the analysed samples is given on Fig. 3. The ridge is due to the hard quartzites containing”92-94°/o SiO,. Towards the north, interbedded units of quarizite and subgreywacke outcrop in the walls of No. 8 and the Water Crat’er and in the QUARTZITE

NO 8

RIDGE SAMPLENO. F,G SI 02%.

MAIN CRATER

CRATER D,20

92,94

90,80

C 85

E1,21 !S 77

80 ,

1

SO;TH

,A’ 0 '50 IS, METERS

,

/ 100

,/’

A,l6,17

27

77,79

15

(THROWOUT

,. NORTH

Fig. 3. Cross section through quart&e ridge and main crater at Henbury. Inferred pre-impact structure is shown. Locations and SiO, percentages of analysed samples are shown. southeastern walls of the main crater. The harder bands thin out and softer subgreywackes predominate northwards. Thus the northwest crater is excavated in less siliceous sediments than the southeast crater of the double main crater. MILTON (1965) has provided a detailed map of the Main Crater, No. 8 Crater and the Water Crater. This was not available until after this paper was submitted for publication. A tentative correlation of his mapped units with the analysed samples is given in Appendix II. At the south end of the crater, hard quartzite blocks (MILTON’S H-l unit) contain 90% SiO, (TAYLOR samples D, 19). A small block at this location (TAYLOR sample 20) contains 80% SiO, and is probably MILTON’S E or F unit. Farther north, blocks on the east wall (TAYLOR sample C, MILTON Unit E4) contain 85% SiO, and are intermediate in composition between subgreywacke and quartzite. TAYLOR samples B and 21 (MILTON Unit D) are from prominent projections on the east and west crater walls, and contain 77% SiO,. A small block at the northeast corner (TAYLOR sample 18, MILTON Unit C?) contains 80% SiO,. Three samples (TAYLOR A, 16,17, MILTON Unit B) from the north wall contain 77-79% SiO,. Finally, a sample (27) exposed in the side of a small gully, has the lowest silica content (75%) of any of the analysed samples. This sample is from an oval tongue of thrownout sediment, about 150 m long, with a maximum width of about 30 m. At the sample locality the tongue is about 0.5 m thick. The colour contrast between the green sedimentary rock and the underlying red-brown soil is striking. An analysis of the soil is given in Table 2. The soil is derived by weathering of similar sedimentary rock, together with some down slope wash from the harder quartzites. Enrichment in SiOz and some loss of Na, K and Mg are the major changes.

S. R. TAYLOR

966

PARENT MATERIAL OF HENBURY IMPACT GLASS The question of change of composition during melting and formation of glass meteorite impact at Henbury has been studied by TAYLOR and KOLBE (1964,196;3) and TAYLOR (1966). No effective change in composition has occurred for fift) elements. The evidence relating to this is given in the papers cited and the subsequent discussion will be concerned with identification of the parent material of the glass by comparison of compositions. Firstly, the soil (Table 2, Sample 27A) and the quartzites (Table 2, Samples (_‘: 19, and D) are clearly excluded, being at least 10% higher in SiO, content? and having many other differences in composition well outside analytical error. Table 3. Comparison of composition of Henbury impact glass with subgreywacke closest in composition (27), and with average subgreywacke Henbury glass

SO, *‘& FeO* MgO

CaO Na,O KZO

TiO, 1

Henbury subgreywacke

averago

(27)

av.

74.6 13.1

75.5 12.1 4.45 2.32 0.11 0.67 3.81 0.85 99.8

77.8 10.6 4.14 2.00 0.46 0.76 3.12 0.85 99.7

4.0 2.28 0.55 0.94 3.42 0.95 99.7

* All iron expressed

as FeO.

The glass compositions are rathel Table 3 lists some relevant comparisons. similar to those of the various subgreywackes exposed in the Northwest Crater. The closest match is with sample 27 from the thrown-out sediment. The subgreJ-wacke blocks from within the crater walls are slightly higher in silica’ and the average of 77.8% is significantly higher than the 74.5% SiOz average in the glasses. The match with sample 27 is very close, except that CaO is low in the sediments. An examination of the data in Table 2 shows that CaO is a variable constituent iu the sediments, due to the presence of variable amounts of calcite cement. (The CO, content of the Henbury subgreywackes varies from 0.15 to 0.800/6.) Thus little significance can be attached to small random fluctuations in the CaO content. Similar variations are observed in the CaO content of tektites (TAYLOR, 1962, 1966; TAYLOR and SAWS, 1964). The identification of sample 27 as the closest in composition to that of the impact glass places restrictions on the point of origin of the glass. Sample 27 is tentatively correlated with MILTON’S Unit D, which occurred as a stratum passing through the center of the northwest crater of the double main crater according to the reconstruction of the pre-impact geology. This indicates that the glass was derived from the northwest crater.

Composition of meteorite impact glass across the Henbury strewnfield

967

CoNCLUsIoNs

The following conclusions emerge from this study of the composition of the glass across the Henbury strewnfield: (1) The glass composition is uniform over a wide area, when the variable iron content due to meteoritic contaminat.ion is removed. This uniformity argues for derivation from a localised source because of the variability of bedrock compositions around the crater walls. (2) The glass is close in composit’ion to t’he sedimentary rocks exposed in the northwest crater of the double main crater. The closest match is to a specimen of thrownout green subgreywacke, which appears to be derived from a stratum (MILTON’S Unit U) which would have passed through the northwest crat’er, in the pre-impact geological sett,ing. dcli~~ouleclgements-The author is grateful to Dr. P. KOLBE for assistance in the field, to Mrs. A. HALSEY for preparing the glass samples, to Dr. W. D. EHMANNand to the University of Kentucky for use of Fig. 1, and to Dr. D. J. MILTONfor permission to quote unpublished information on the geology of the Henbury craters and for helpful comments on this manuscript. The preparation of this paper has been supported by XASA Grants NsG-321 and X-sG-322,

R El?EREXCES ALDER&IANA. R. (1932) The meteorite craters at Henbury, Central Australia. Mineral. &!ug. 23, 19-32. MILTON D. C. (1965) Structural geology of the larger Henbury craters. U.S. Geol. Snrr. Astrogeologic Studies Annual Progress Report 1965, 26-49. TAYLOR S. R. (1962) The chemical composition of aust,ralites. Geochim. Cosmochim. dctu 26, 685-722. TAYLOILS. R. (1966) Australites, Henbury impact glass and subgreywacke: a comparison of the abundances of 51 elements. Geochim. Cosmochim. Acta 30, 1121-1136. TAYLOR S. R. and KOLBE P. (1964) Henbury impact glass: Parent material and behaviour of volatile elements during melting. Nature 203, 390-39 1. TAYLOR S. R. and KOLBE P. (1965) The geochemistry of Henbury impact glass. &ochim. Cosmochim. Acta 29, 741-754. TAYLOR S. R. and &CHS M. (1964) Gcochemical evidence for the origin of australites. Geochim. Cosmochim.

Acta 28, 236-264.

APPENDIX Location

1

of analysed impact glass samples

Distance and bearing from main crat,er HB-1 2

Approximate area (m2)

R’orth rim

100

100 m North

300

11

Same as No. 1

300

28 29 31

20-50 m, 55” T 100 m, 55OT 300 m, 75” T

400

1213 1415

100-150 m North Northeast rim 45-55” T

300 300

1000 400

968

5. R. TAYLOR APPENDTX

2

_tioc#th ofimsa.lysed SedirnWta~ [email protected] --

--

.I

...-.

MILTON (1965)

unit

TAYLOR NO.

D

27

B I3 B C? D D E4 E or F H,l H,l -

A 16 17 18 B 21 C 20 19 D F G

_____. ,_.. __~___,-- ~.____- _..--_. _I-- .._.-.-.

.~ ..~

Green subgreywacke, outthrown flap of sediment, creek bank l00 1~ +II 310” from orater rim. Green subgreywaoke block in north wall, main crater. Grey subgreywaeke? north wali main crater, same locality (t8 A. Grey sub~~a~ke blouk in north wall, main crater 3 m XW of 16. Gray su~e~~ke, northeast wall main crater. Grey subgreywaoke, east wall, main crater. Grey subgreywacke, west wall, main crater 270” from B, on line of strike. Grey subgreywacke or quartzits, east wall main crater, south of B. Grey subgreywaoke, loose block, south waI1 main crater, 2 m from 19. Quart&e, south wall main crater. Quartzite, south wall main mater. Quart&e, top of ridge, 260 m south of main crater. Quartzite, top of ridge, 650 m west of F.