Iridium anomalies and shocked quartz in a Late Archean spherule
layer from the Pilbara craton: New evidence for a major asteroid
impact at 2.63 Ga
Birger Rasmussen* School of Earth and Geographical Sciences, University of Western Australia, Crawley, WA 6009, Australia
Christian Koeberl* Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
ABSTRACT
A thin (1–5 mm) spherule layer in ca. 2.63 Ga shale from the Jeerinah Formation
(Pilbara craton, northwestern Australia) has been identified at two new localities. The
layers have Ir concentrations as high as 15.5 ppb, significantly higher than the surrounding carbonaceous shale (,1.5 ppb). The sand-sized spherules display quench and devitrification textures and are interpreted as former silicate melt droplets that were replaced
by K-feldspar, carbonate, and sulfide during diagenesis. In one spherule-layer sample, an
angular quartz grain (;100 mm in size) with planar deformation features was found,
which represents the oldest known shocked grain in distal ejecta by .2000 m.y. The
survival of shocked quartz in ca. 2.63 Ga rocks, which have undergone multiple metamorphic events, suggests that their absence in other impact ejecta layers may not only be
a question of preservation. The presence of shocked quartz and anomalously high Ir
contents in a layer containing melt spherules provides compelling evidence for an extraterrestrial impact with a target area that was at least partly silicic, favoring a continental
impact site. Estimates based on geochemical data suggest that the spherule layer comprises
as much as 2–3 wt% of a chondritic meteorite component. If proposed correlations with
the Carawine (eastern Pilbara craton) and Monteville (South Africa) layers are correct,
then the combined ejecta blanket represents fallout from a single major impact with an
areal distribution of .32,000 km2, among the largest yet documented in the Precambrian
rock record.
Keywords: Archean impact layers, shocked quartz, Ir anomaly, impact ejecta.
INTRODUCTION
Impact events have played a crucial role in
the geologic and biologic history of Earth.
Many of the larger impacts have left traces in
the form of craters or as geochemical and mineralogical signatures in the stratigraphic
record (see Koeberl, 2001, for a review). Although impact structures have not been found
in rocks older than ca. 2 Ga (Grieve et al.,
1995), possible impact debris layers have been
documented in South African and Australian
Archean successions (see review by Simonson
and Glass, 2004). Shocked quartz, commonly
regarded as unambiguous evidence for a hypervelocity impact event (e.g., Grieve et al.,
1996), has not yet been found in distal ejecta
horizons older than ca. 600 Ma (Gostin et al.,
1986). Only a single Precambrian ejecta layer
(i.e., from the ca. 590 Ma Acraman impact
structure, Australia) is known to contain
spherules, geochemical anomalies, and
shocked grains.
A series of spherule layers was discovered
in sedimentary successions of Late Archean
age from the Pilbara craton of northwestern
Australia (Fig. 1A): three of the layers occur
in the Hamersley province and a single layer
*E-mails: [email protected]; christian.
[email protected].
was found in the Oakover River area (Simonson, 1992) (Figs. 1A, 1B). The beds contain
sand-sized spherules interpreted as former
droplets of silicate melt produced during impact (Simonson, 1992). All have Ir anomalies
(see review by Simonson and Glass, 2004),
and two have chromium isotope signatures indicating an extraterrestrial component (Shukolyukov et al., 2002). Possible equivalents
have been found in the Monteville and Reivilo
Formations in South Africa (Simonson et al.,
2000b; Simonson and Sumner, 2004). The
oldest of the three spherule beds from the
Hamersley province occurs in the uppermost
Roy Hill Shale Member of the Jeerinah Formation and has been documented in drill hole
FVG-1 (Simonson et al., 2000a) and two outcrop localities, the Hesta railway siding and
the Tarra Tarra turnoff (Simonson et al., 2002)
(Fig. 1A). In the drill hole, the spherule layer
is #3 mm thick, whereas the layer at the outcrop sites is significantly thicker: ;1.5 m at
the Hesta locality and ;10 m at the Tarra Tarra turnoff site (Simonson et al., 2002). We report here a spherule layer in the same stratigraphic position from two new localities (Fig.
1A); this layer provides additional mineralogical and geochemical evidence, including
the oldest known shocked quartz, strongly
supporting an impact origin for the horizon.
SAMPLES AND METHODS
Samples were collected in 1999 and 2000
from diamond core recovered from two drill
holes ;50 km apart, WRL-1 and DDH 186,
located in the Pilbara craton of northwestern
Australia (Fig. 1). The material is from the top
of the Roy Hill Shale Member of the Jeerinah
Formation (Fortescue Group), in a region that
has undergone negligible strain and prehnitepumpellyite–facies metamorphism (Smith et
al., 1982) and a low-temperature thermotectonic event dated by U-Pb analysis of monazite as 2192 6 5 Ma (Rasmussen et al., 2001).
The shale was deposited in a marine shelf or
an upper-slope environment (Blake and Barley, 1992; Simonson et al., 2000a) and conformably passes into the overlying Marra
Mamba Iron Formation. The age of the Jeerinah Formation is between 2684 6 6 Ma
(Arndt et al., 1991) and 2629 6 5 Ma from
U-Pb zircon analyses of tuffaceous rocks
(Nelson et al., 1999). A tuff in the overlying
Marra Mamba Iron Formation has an age of
2597 6 5 Ma (Trendall et al., 1998).
Polished thin sections of shale from the upper Jeerinah Formation, 15 from WRL-1 and
36 from DDH 186, and 9 polished thin sections from the spherule beds (7 from WRL-1
and 2 from DDH 186) were examined by optical microscope. Key sections were subsequently studied by scanning electron microscope (SEM) using backscattered-electron and
SEM-cathodoluminescence techniques. Minerals were identified by optical microscopy,
SEM–energy-dispersive X-ray spectrometry
(SEM-EDX), and X-ray diffraction analysis.
The host rock in which the spherule beds
occur is a carbonaceous shale with thinly interbedded siltstone and sandstone. The shale
contains quartz, K-feldspar, muscovite, and
chlorite, interspersed organic matter (up to 9.0
wt% total organic carbon; Brocks et al., 1999)
and trace amounts of heavy minerals. Diagenetic and metamorphic minerals include Kfeldspar, quartz, chlorite, sericite, calcite, ankerite, sulfides (pyrite, chalcopyrite, sphalerite),
phosphates (apatite, xenotime, monazite), titanium oxide, and thorite. Diagenetic pyrite is
common as nodules and thin, bedding-parallel
bands, typically surrounded by fibrous quartz
and sericite pressure shadows.
q 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; December 2004; v. 32; no. 12; p. 1029–1032; doi: 10.1130/G20825.1; 2 figures; 1 table.
1029
The spherules, which comprise ,1%–50%
of the layer (Figs. 2A, 2B), are now composed
of K-feldspar, quartz, calcite, chlorite, pyrite,
chalcopyrite, and sphalerite that probably replaced original constituents after deposition.
The spherules are between 0.1 and 0.9 mm
(typically ;0.5 mm) in diameter, spherical to
elliptical in shape, and range from intact
grains (Fig. 2C) to broken fragments. Some
spherules and fragments display outlines of
fan-shaped, bladed, and lath-shaped crystals
that radiate inward from the margins, suggestive of nucleation around the outer margin of
former melt droplets, possibly along a
quenched rim. A few spherules contain rounded to irregular cores typically composed of
quartz, carbonate, chlorite, or sulfide, probably
representing vesicles or areas formerly filled
by glass that were replaced by diagenetic cement. The spherules from the Jeerinah Formation closely resemble spherules from other
impact layers (John and Glass, 1974; Bohor
and Glass, 1995), and display quench and devitrification textures (cf. Lofgren, 1971, 1977)
interpreted to be former droplets of silicate
melt generated during a large impact (cf. Simonson and Glass, 2004).
Figure 1. A: Map showing geology of Pilbara region and locations of drill holes (open
circles) and outcrop localities (x) in Hamersley province and Oakover River area containing Jeerinah and Carawine spherule layers, respectively. B: Stratigraphic column of
upper Fortescue Group and Hamersley Group from Hamersley province and Oakover
River area. Geochronology is from Arndt et al. (1991), Trendall et al. (1998), Woodhead
et al. (1998), and Nelson et al. (1999).
SPHERULE BEDS
In drill hole WRL-1, a spherule layer between 1 and 2 mm thick was identified at a
depth of 684.1 m, ;5.2 m below the stratigraphic contact between the Jeerinah Formation and the overlying Marra Mamba Iron Formation (Fig. 2). In drill hole DDH 186, the
spherule bed ranges from 1 to 5 mm in thickness and is located at a depth of 147.5 m,
;5.5 m below the contact (Fig. 1B). The base
of the spherule bed consists of fine sandstone
;0.5–1.0 mm thick and passes into organicrich shale laminae (Figs. 2A, 2B). The spherules first appear ;0.5–1.0 mm above the basal
contact, immediately above the organic-rich
shale laminae, and grade from coarse (up to
0.9 mm), intact spherules to smaller spherules
and broken fragments toward the top of the
bed.
1030
The spherule beds contain angular, silt- to
fine-sand–sized grains of nonluminescent Kfeldspar, with lesser amounts of chlorite and
quartz. Chlorite aggregates are more abundant
in the top of the spherule bed, whereas most
quartz grains are scattered in the lower section. Quartz grains are complexly zoned (seen
by SEM-CL) and comprise composites of two
or more crystals, which share common crystal
boundaries. A few quartz grains display complex intergrowths, possibly with igneous feldspar, that have been replaced by nonluminescent diagenetic K-feldspar. Most quartz grains
are surrounded by irregular nonluminescent
overgrowths that probably precipitated after
deposition. The spherule bed also contains
trace amounts of sulfide (pyrite, chalcopyrite,
sphalerite, and galena), apatite, zircon, titanium oxide, and sericite.
SHOCKED QUARTZ
Because of the limited amount of drill core,
only nine polished thin sections were made
across the two spherule beds. In a polished
thin section from WRL-1, an angular grain of
quartz (;100 mm) with two intersecting sets
of planar deformation features (PDFs) was
found adjacent to a small (;0.2 mm in diameter) spherule (Fig. 2D). The nature of the
grain was confirmed by SEM-EDX analysis
and universal-stage studies. Universal stage
work indicates the possible presence of a third
set of PDFs. The lamellae in the quartz are
sharp, parallel, and closely spaced (3–10 mm),
and extend across the entire grain; they are
identical to those of shocked quartz from impact craters and ejecta horizons (cf. Koeberl,
2001). Shocked quartz is scarce in the stratigraphic record and is restricted to impact craters (ca. 2 Ga or younger) and a few distal
ejecta horizons (younger than 600 Ma).
Shocked minerals are considered the strongest
single evidence for extraterrestrial impacts
(e.g., Stöffler and Langenhorst, 1994; Grieve
et al., 1996), and their absence in early Precambrian spherule beds has, in part, been attributed to the effects of postdepositional alteration (Simonson et al., 1998). However, the
discovery of shocked quartz in 2630 Ma
shales shows that (1) in favorable circumstances, such grains can survive diagenesis
and low-grade metamorphism and (2) the absence of such grains in other distal ejecta horizons cannot be dismissed as a problem of
preservation, but may reflect the composition
GEOLOGY, December 2004
Figure 2. A and B: Photomicrographs of spherule beds in Jeerinah Formation;
scale bars represent 1 mm. A: Drill hole DDH 186 (147.5 m depth). B: Drill
hole WRL-1 (684.1 m depth). C: Coarse spherule (~0.9 mm in diameter) composed of K-feldspar with core displaying planar faces; drill hole WRL-1 (684.1
m depth). D: Angular quartz grain with two sets of planar deformation features; scale bar represents 20 mm; drill hole WRL-1 (684.1 m depth).
of the target. The presence of shocked quartz
in the Jeerinah spherule bed indicates that the
impact site contained quartz, favoring a continental target rather than an oceanic site.
GEOCHEMISTRY
Eight samples (three from DDH 186 and
five from WRL-1) were analyzed for selected
major and trace element contents by instrumental neutron activation analysis. For the
two spherule-layer samples, the cores were cut
and then broken, and chips (;200 mg) were
used for analysis. Above and below the spherule layer, ;5 g samples were collected and
were ground to powders in an agate mill; from
these, 200 mg subsamples were analyzed. One
sulfide sample chip from WRL-1, ;5 cm below the spherule layer, was also analyzed to
see whether the sulfides had concentrated any
siderophile trace elements. Details of the analytical method were given by Koeberl
(1993); the only deviation from the method
described there is that samples were counted
for up to 100 h each instead of 24 h, for better
detection limits.
The results are given in Table 1. In terms
of major element contents, the samples are
very poor in Na, high in K, and have variable
Fe contents, depending on pyrite concentrations. The spherule-layer samples are in core
GEOLOGY, December 2004
DDH 186 at 147.50 m depth and in core
WRL-1 at 684.10 m depth. Both samples have
significantly elevated Ir contents, at 15.5 and
12.1 ppb, respectively. One other sample has
an Ir content of 0.5 ppb, which is at the detection limit. This sample is slightly above the
spherule layer and may represent a tailing in
the abundance. The spherule layers also have
high abundances of Cr, Co, Ni, and Au, and
the Ir/Au ratios are close to chondritic ratios.
Owing to minor sulfide mineralization, some
of the other samples have elevated contents of
Au and a few other elements, but without Ir.
The high contents of Ir and other siderophile
elements clearly indicate an extraterrestrial
component in the spherule layers, which can
be estimated to represent 2–3 wt% of a chondritic meteorite. Compositionally the Jeerinah
spherule layers are very similar to the probably coeval spherule layer in the Monteville
Formation, South Africa (Simonson et al.,
2000b); in samples of this spherule layer, from
the Pering Mine, Ir contents as high as 6.4 ppb
were found, and other trace element contents
are very similar to the Jeerinah samples analyzed here.
CORRELATION OF SPHERULE
LAYERS
The spherules in drill holes DDH 186 and
WRL-1 are similar in size, shape, composi-
tion, and texture, and occur in the same stratigraphic position, suggesting that the beds
formed during the same event. Uranium-lead
analysis of zircon from an andesitic ignimbrite
from the uppermost Jeerinah Formation yielded an age of 2629 6 5 Ma (Nelson et al.,
1999), which is considered a close approximation of the depositional age of the spherule
layer. The horizon from the newly discovered
localities is equivalent to spherule beds from
the uppermost Jeerinah Formation in drill hole
FVG-1 (Simonson et al., 2000a) and nearby
outcrop localities (Simonson et al., 2002), extending the Jeerinah spherule layer by thousands of square kilometers. Based on petrographic similarities and stratigraphic position,
the Jeerinah spherule layer was recently correlated with the Carawine horizon in the Oakover River area of the Pilbara craton (Fig. 1)
(Simonson et al., 2000a, 2002) and the Monteville spherule layer (2.65–2.60 Ga) in the
Griqualand West Basin, South Africa (Simonson et al., 1999; Simonson and Glass, 2004).
In the Pilbara craton, the layer covers an area
of ;15,000 km2, whereas in South Africa it
covers ;17,000 km2 (Simonson et al., 2000b),
for a total minimum areal distribution of
;32,000 km2. Further geochronology is required to test these correlations, but if correct,
then the Jeerinah-Carawine-Monteville layer
represents one of the most extensive fields of
Precambrian impact ejecta yet documented.
CONCLUSIONS
Our mineralogical, petrographical, and geochemical study of the Jeerinah spherule layer
provides compelling evidence for a major asteroid impact ca. 2.63 Ga. Geochemical estimates suggest that the layer comprises as
much as 2%–3% projectile material. For the
first time in these Archean impact layers,
shocked quartz with planar deformation features was found, indicating that the impact site
contained quartz, and favoring a continental
site rather than oceanic crust. Such an interpretation is supported by an increase in the
maximum spherule size (up to 2 mm) and layer thickness toward the inferred basin margins
in the north and east (Simonson et al., 2002).
If proposed correlations are valid, then the
Jeerinah, Carawine, and Monteville layers
were produced from the same impact event,
demonstrating the likely global extent of the
ejecta fallout.
ACKNOWLEDGMENTS
We thank S. Brown, M. Doyle, B. Krapez, S.
Sheppard, and B.M. Simonson for discussion; G.
Broadbent and Rio Tinto Exploration for access to
WRL-1; the Geological Survey of Western Australia for access to DDH 186; and the staff of the Centre for Microscopy and Microanalysis, University of
Western Australia, for technical help. We are grateful to John Spray and an anonymous reviewer for
constructive comments. Laboratory work by Koeberl was supported by the Austrian Science Foun1031
TABLE 1. CHEMICAL DATA FOR SAMPLES FROM THE JEERINAH FORMATION IN DRILL HOLES WRL-1
AND DDH 186
DDH 186 samples
Drill depth:
Na
K (wt%)
Sc
Cr
Fe (wt%)
Co
Ni
Zn
As
Se
Br
Rb
Sr
Zr
Sb
Cs
Ba
La
Ce
Nd
Sm
Eu
Gd
Tb
Tm
Yb
Lu
Hf
Ta
W
Ir (ppb)
Au (ppb)
Th
U
WRL-1 samples
145.1 m
shale
147.50 m
spherule
bed
148 m
shale
678.9 m
shale
684.10 m
spherule
bed
684.15 m
shale
688.02 m
shale
693.17 m
shale
413
8.72
19.1
120
3.15
10.3
30
720
4.51
1.8
0.4
290
4
320
0.63
3.6
250
19.0
44.5
24.2
6.35
1.8
5.25
1.0
0.59
4.21
0.66
7.11
0.81
2.7
0.5
0.8
7.11
3.63
365
6.18
12.5
157
13.8
42.5
190
25100
1.33
5.1
0.2
241
5
90
0.56
2.79
220
14.1
31.5
18.1
5.62
1.88
4.44
0.74
0.31
1.92
0.29
2.45
0.35
2.5
15.5
5.1
3.21
1.59
375
8.10
21.3
140
6.89
38.7
40
2210
27.1
4.6
0.2
266
5
145
4.98
4.11
220
28.8
52.9
25.8
4.07
1.17
3.38
0.63
0.37
2.48
0.38
3.86
0.80
2.8
,0.7
2.4
8.68
2.32
100
0.035
12.2
38.2
5.86
18.2
60
82
22.4
1.7
0.4
1.6
55
50
2.11
0.32
15
10.3
18.4
12.2
3.84
1.82
4.05
0.66
0.30
1.79
0.27
0.86
0.19
0.6
,0.5
1.2
2.40
0.70
725
6.15
52.9
208
4.27
15.9
145
2050
28.1
2.8
0.6
136
53
225
0.70
4.21
200
5.25
10.9
15.2
6.70
2.83
7.75
1.67
0.77
5.10
0.77
4.61
0.99
1.8
12.1
4.5
9.32
2.71
475
1.98
8.56
19.3
30.3
230
520
280
161
5.7
0.2
48.7
10
40
28.9
1.76
25
0.67
0.82
1.5
0.56
0.18
0.55
0.095
0.055
0.38
0.058
0.89
0.27
0.7
,1.5
24.5
0.64
0.23
135
0.017
1.25
8.21
18.5
1.72
13
14
0.32
0.5
0.3
2
24
15
0.15
1.98
20
2.35
4.35
2.76
0.67
0.94
0.8
0.17
0.11
0.77
0.13
0.13
0.02
0.08
,0.5
0.3
0.30
0.062
942
7.66
18.7
172
1.88
15.7
30
950
7.15
3.1
2.5
240
5
230
1.17
15.9
280
15.6
25.1
12.7
2.47
0.85
3.87
0.75
0.46
3.09
0.46
4.80
1.05
0.05
,1
0.9
10.1
4.13
Note: Data are in ppm, except as noted.
dation. This work was supported by an Australian
Research Council fellowship and grant to
Rasmussen.
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Manuscript received 11 May 2004
Revised manuscript received 24 August 2004
Manuscript accepted 25 August 2004
Printed in USA
GEOLOGY, December 2004