Download Saltpan impact crater, South Africa: Geochemistry of target rocks

Geochimica et Cosmochimica Acta, Vol. 58. No. 13, pp. 2893-29 IO, 1994
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Saltpan impact crater, South Africa: Geochemistry of target rocks, breccias, and impact
glasses, and osmium isotope systematics
CHRISTIAN KoEBERL,‘,“~ WOLF UWE REIMOLD,’ and STEVEN B. SHIREY~
‘Institute of Geochemistry, University of Vienna, Dr.-Karl-Lueger-Ring 1, A-1010 Vienna, Austria
*Economic Geology Research Unit at the Department of Geology, University of the Witwatersrand, Johannesburg 2050, South Africa
3Department of Terrestrial Magnetism, Carnegie Institution, Washington, DC 200 i 5, USA
~~eceised October 28, 1993; accepted in revisedfbrrn Feb~i~ffr~~
10, 1994)
Abstract-The
Pretoria Saltpan crater is a well-preserved
220,000 year-old, 1.13 km-diameter,
simple
impact crater. The crater was formed in Nebo granites of the Bushveld Complex. Some minor intrusions
thought to be younger than the Nebo granite are present at the crater and have earlier been believed to
support a volcanic origin of the structure, but recent geological studies showed them to be part of the
regional geology and of Proterozoic age. We studied the petrology and geochemistry
of fourteen target
granite samples, three suevitic breccias, nine intrusive rocks, as well as melt agglutinates, handpicked
impact glass fragments and sulfide spherules from the Saltpan impact crater. Unconsolidated
suevitic
breccias recovered from different depths in the crater were found to contain abundant evidence of shock
metamorphism.
The target rock granites show only limited compositional
variability. The major and trace element
composition
of the bulk breccia is very similar to that of average basement granite. Impact glass fragments
recovered from the unconsolidated
suevitic breccia have a CIPW normative composition
similar to that
of the basement granites. No evidence for admixture from any of the minor intrusions was found. The
similarity of trace element abundances and ratios, and REE patterns between impact glasses and granites
favors derivation of the glasses from the granites.
The impact glass fragments show considerable en~chments
of Mg, Cr, Fe, Co, Ni, and Ir, compared
to the basement granites. The abundances of these elements in the glasses (after correction for indigenous
concentrations)
can be explained by admixture of about ~10% o f a chondritic component.
High Ir
concentrations
(= 100 ppb) have been found in sulfide spherule samples, which may complement
the
(lower) lr abundances in the glasses and could indicate some fractionation during impact. Re-0s isotopic
studies were applied to further investigate the presence of a meteoritic component in the suevitic breccia.
The target granite shows very low osmium abundances
of about 7 ppt and high ‘s70s/‘880s ratios of
about 0.72 that would be expected for old continental crust. In contrast, the breccia samples were found
to have much higher osmium abundances (-80 ppt) and lower ix70s/18xOs ratios of about 0.205. These
values can be explained by mixing of target rocks with a chondritic component.
(BRAND-F and REIMOLD, 1993) include
lNTRODUCI’lON AND SUMMARY
OF CRATER GEOLOGY
lamprophyre
dikes
and possibly sills, trachyte, and minor carbonatite.
About 3 km south-southeast
of the Saltpan crater a perfectly circular depression with a diameter of about 400 m
was recently discovered
(BRANDT et al., 1993a,b, 1994).
Complete lack of exposure and only limited, still inconclusive
geophysical evidence (BRANDT et al., 1993a,b, 1994) have so
far precluded positive identification
of the origin and nature
of this small structure. However, no magnetic anomaly was
found at this satellite structure in a first magnetic survey,
which clearly rules out the possibility that it could be a volcanic structure with a central magnetic body (such as a kimberlite pipe). The conspicuous shape, its vicinity to the Saltpan
crater, and regional uniqueness suggest that this feature could
represent a twin or satellite crater to the Pretoria Saltpan
structure.
In the past the origin of the Saltpan crater has been controversial. The first visitors to the crater in the mid- 1800s felt
strongly that the unique presence of this crater indicated a
volcanic origin (for a historical account of reports on the
Pretoria Saltpan see, e.g., LEVIN, 1991). The first detailed
geological study of the structure was reported by WAGNER
THE PRETORIA SALTPAN (also called “Zoutpan,”
or
“Tswaing”-“The
Place of Salt”) structure has a diameter of
I. 13 km (Fig. 1) and is located at 25”24’30” S and 28”04’59”
E, about 40 km Noah-no~hwest
of Pretoria (Transvaal Province, South Africa, Fig. 2). The interior of the structure consists of a flat crater floor that is partiafly covered by a highly
saline lake, the extent of which depends largely on annual
rainfall patterns. The maximum elevation of the crater rim
over the present crater floor is 119 m, and the rim rises up
to 60 m above the surrounding plains.
The Saltpan crater was formed about 220 ka ago (STORZER
et al., 1993; KOEBERLet al., 1994b) in the crystalline basement
of the 2.05 Ga old {WAI_RAVEN et al., 1990) Nebo granite of
the Bushveld Complex. Granite exposures are confined to
the crestal areas of the crater rim and to isolated plateau
outcrops of restricted extent in the crater environs. Other
lithologies that can be sampled in limited outcrop of often
not more than 50 cm exposure along the inner rim wall and
in a few exposures in the region surrounding
the structure
2893
2894
c‘ Koeheri. W. U. Relmold. and S. R. Shire!
FIG. 1. Low altitude aerial view of the Saltpan crater. View from the northeast (courtes! I). Brandit.
( 1920. 1922) who discussed findings of a dolomitic hreccia
thought to be of volcanic origin. F~~IcHT~~N~~R (1973) reported additional
findings of mafic and alkaline volcanic
rocks, which led to wide acceptance of the volcanic hypothesis
for the origin of this crater. A gravity survey by FIJDALI et
al. (1973) yielded results that were interpreted by these authors
to be in support of a volcanic origin as well: they proposed
the possible existence of a carbonatite or kimberlite vent below the central part of the crater.
In contrast, ROHLEDER (1933) and LEONAKD ( I YJ6) suggested a meteorite impact origin for the Pretoria Saltpan crater, largely based on morphological
observations.
MILTOK
and NAESER {1971)conducted some first structural studies
on the crater rim and compared the rim deformations
observed with those at Meteor Crater in Arizona. These workers
also determined
fission track ages for zircon and apatite separates from carbonatite and interpreted the ages of I .9 + 0.4
.,
/
BOTSWANA
.’
\
,’
.‘IMRARWL
,
-
JOHAN~ESEUR~=
DOUt
SWAZILAND
/
!_--
--_,..__
.-_A
____
_~ _~____
.-..
_ .-__
-._.. -_..-
FIG. 2. Location of the Pretoria Saltpan and other impact structures
in southern Africa.
(?a and 0.6 I 0.09 Ga as indicative ofa long timespan between
the emplacement
of such intrusive rocks and the formation
of the crater. Both &JDALl et al. (1973) and MII ~‘0% nnd
NAESER (197 1) commented,
however. on the compfctc lack
of unambiguous genetic information at that time and stressed
the need for a borehole for delinite resolution ofthis dilemma
In 1988 such a borehole was drilled to obtain the neccssar!
information
for the establishment
of the true origin of the.
crater. It also provides a complete and undisturbed
record
of paleoenvironmental
changes since formation of the crater
through the study of the accumulated
crater sedirnemr
(SCOX I‘, 1988: PARTRIDCE et al.. 1993). When the drillcorc
became available for detailed analysis in 1989. PAN KIIX~I
et al. ( 1990) at first faiied to identify evidence 111favor of an
impact origin. However, they reported a ncM’ K-Ar age 01
1.36 Ga for biotite from a Saitpan lamprophyrr.
which ix
incompatible
with the fresh appearance of the cratei’.
The drillcore revealed an internal crater stratigraphy (Rk,i
NXD et al.. 1992; PARTRIDGE et al.. 1993) comprising ~XJUI
90 m of crater sediments
that are underlain b! 53 m unconsolidated granitic breccia (Fig. 3). This “sandy ._ breccia was
found to contain numerous shock metamorphosed
quarti..
feldspar, and biotite grains. besides glass and melt breccirt
fragments. The impact origin of the Saltpan crater was. thub.
established without doubt (R~IM~)I_u Ed al.. I W I, i YX!. I‘ll<
hreccia layer was classified as unconsolidated
sucvitic impact
breccia (REIMOL.Det al., 19%). This layer is. in &urn. underlain
by strongly fractured and locally brecciated rmonomict
c‘~Itaclastic breccia) granite, which is occasionally intercalated
with narrow layers of sandy breccia to a depth of 161 m. ‘I he
amount of solid granite increases gradually with depth, until
drilling was suspended in solid and completely undeformed
granitic crater floor at a depth of 200 m. Together with a
present-day apparent depth of the crater from the crater rim
of about 100 m. this results in a true crater depth of about
300 m. Such an estimate is slightly shallower compared to
determinations
and models for some other small simplr cra-
2895
The Saltpan crater of South Africa
and KOEBERL,1992; REIMOLDet al., 1992). In this paper we
present a geochemical characterization of the target rocks
and the granitic breccia, as well as of impact glasses and melt
breccia fragments. The principal objectives for this study are
to (1) investigate the respective contributions of granite and
volcanic rocks to the impact breccias and glasses, and (2) to
attempt identification of a meteoritic projectile. In addition,
Re-0s isotopic data were collected for selected granite and
breccia samples, with the result that the impact origin of the
Sahpan is further documented and traces of a meteoritic projectile were confirmed in the breccia samples.
Depth(m)
PETROGRAPHIC OBSERVATIONS
SAMPLE CHARACTERISTICS
m
Granitic “sand”
E
l
l
m
Pebbles/fragments
<3cm
Pebbles/fragments
>3cm
Fractured granite
m
Solid granite
15-,7&m
long coherent grafnits.
wthln sand-partial
recovery
00 •I Mylonite
*
FIG. 3. Stratigraphic column for the Saltpan borehole (after REIet al., 1992). Sampled intervals are indicated in the right margins.
MOLn
ters (e.g., GRIEVE et al., 1989). However, as discussed by REIMOLDet al. (1992), this might be because the Saltpan crater
is a small crater which formed in crystalline rock, in contrast
to some other crater of this size which formed in sedimentary
targets.
Since then BRANDTand REIMOLD(1993) and BRANDTet
al. (1993a,b, 1994) presented more detailed geological and
geophysical results. It was shown that the occurrence of volcanic rocks is not restricted to the crater area and is part of
a regional volcanic event that took place at about 1.2- 1.4
Ga ago. Apparently the well-preserved crater and the Late
Proterozoic regional voicanism are unrelated. This was further
corroborated by fission track dating of impact glass separates
from the suevitic breccia layer, giving an age of 220 -t 52 ka
(STORZERet al., 1993; KOERERLet al., 199413).This age for
the cratering event is in excellent agreement with an estimate
of 180 ka for the period of deposition of the crater sediments
by PARTRIDGE et al. (1993) based on extrapolation of “‘C
dating of the upper sediment units.
Only preliminary geochemical results on the various Saltpan crater litholo~es have been reported to date (REIMOLD
AND
Exposures of country rocks and breccias in the Saltpan
crater are limited to the crater rim crest and occasional outcrop on the inner part of the upper rim slope (Fig. 4). In
addition to samples of autochthonous granite, several float
samples of brecciated granite were cohected on the flat crater
floor in the vicinity of the crater lake. Figure 3 shows the
drillhole stratigraphy with sampling depths, and Fig 4 shows
the locations of surface samples. Characteristic microphotographs of target rocks and breccias are given in Figs. 5a-f
and 6a-h.
The main country rock is Nebo granite of the Bushveld
complex. It is found at places along the rim in contact with
overlying Karoo grits (sandstone composed of rounded grantic rocks up to 1 cm in diameter and mineral grains cemented
by iron oxides and silica), or with fragmental breccia of extremely variable fragment size (from < 1 cm to > I m). Ail
fragments studied were granite derived. The typical basement
granite consists of coarse-grained (up to 1 cm crystals) quartz
and feldspar (partially exsolved, often piagiociase-bearing
perthitic alkali feldspar and microciine), and minor primary
plagioclase, biotite, and hornblende. From sample to sample
these minerals display a highly variable degree of alteration,
manifested in partial sericitization of feldspar, chioritization,
or uralitization of hornblende, and rare oxidation or chloritization of biotite. The same granite occurs in the drillcore,
in form of clasts in the crater sediments (probably part of
debris flows off the crater rim), as pebbles and meter-sized
boulders in the breccia of intermediate depths, and as fractured or locally brecciated granite just above the solid granite
of the crater floor.
In some first examinations to assess the presence of characteristic shock metamorphic features, solid granites from
the rim and the drillcore were studied. Only weakly deformed
granite samples were described from the crater rim and the
drillcore (PARTRIXE et al., 1990; REIMOLD et al., 1991).
Isolated shear fractures (Fig. 5b), microfaults, and deformation bands (Fig. 5d) of typically tectonic origin were found.
it can, however, not be excluded that such deformation phenomena have been introduced during the cratering event. A
recent reexamination of ail available thin sections of rim and
drillcore granite samples revealed a single pebble, collected
from 94.69 m depth, that contains two small (~250 pm)
quartz crystals with one set of characteristic planar deformation features (PDFs) each.
Breccia specimens collected for this project comprise severa1 samples of cataclastic (figments)
breccia that either
396
(
Koeherl, IQ’. II. R&mold. and S. R. Shire!,
FIG. 4. Exposure map of the crater area (after II. Bran&. III prep.) with sampling sites for thl\ stud!. I’hc tliamonti
symbol indicates the position of the 198X/I989 borehole.
occur along contacts between granite and carbonatitc or trachyte intrusions, or as small isolated and irregularly shaped
‘pockets’ of breccia in solid granite. in the first case (Fig.
5e,f), brecciation was accompanied
by considerable thermal
annealing leading to widespread recrystallization
of breccia
fragments-but
not of the adjacent intrusion. It is therefore
thought that this annealing and brecciation event took place
during emplacement
of the intrusion and is not impact related. The other breccia type shows no such thermal effect.
and as D. Brandt (pers. commun..
1993) recently observed
a few sets of PDFs in quartz from such breccia, these monomict fragmental breccias are believed to be of impact origin.
In addition, a single, about I5 cm-sized float sample was
collected near the shore of the crater lake. This sample macroscopically
resembles a network of fine pseudotachylite
veinlets (Fig. 5a). However, the vein fill consists of strongly
angular clasts, cemented by later Fe,Mn-oxides.
Due to this
alteration the primary nature ofthis sample is uncertain. The
lack of significant annealing of the clasts and the angular
shapes of most clasts suggest an origin as fragmental breccia.
No shocked particles were identified in this sample. FELJCWWANGER (1973) indicated the presence of pseudotachylite-
like breccia, but did not provide further detail :\t this stage
it has to be concluded that no bona fide psc-udotachylitic
breccia has yet been found in the Saltpan crater.
Mafic and alkaline intrusions belonging to :I magmaul
phase of about 1300 Ma ago are frequently cncountcrcd
it1
the crater rim, mainly along the inner slopes of the northern
and northeastern sectors. BRI\NII’I and REIW)L I) C19Y3) haie
shown that these intrusions are analogues to the intrusion\
of regional provenance. Trachytes and carbonates. occurring
in form of thin (<30 cm) veins or sheets are general11 yultc
strongly altered. Lamprophyre
dikes (or sheets) occur mainly
in form of discontinuous,
blocky exposures WI the eastern
rim and occasionally are of fresher appearance (Fig. 5~). Nones
of these samples displays any severe microdeformation
01
any shock metamorphic
effects (BRANDI and REIMOI I).
1993). Only a few pyroxene grains (possible iraccs of a t (Acanic target rock component)
were observed in the central
drillcore (REIMOLD et al., 1992). No significant carbonate
component
was detected.
Below the crater sediments, a package of about 60 m 01
“granitic sand” was transected, mainly consisting of unconsolidated granite-derived
mineral fragments. The most con-
The Saltpan crater of South Africa
2897
(b)
le)
RG. 5. Microphotographs of basement rocks. (a) ~‘Pseudotachylite-like” breccia-float sample collected on the shore
of the crater lake (courtesy D. Brandt). See text for description. (b) Two parallel micro-shear fractures (horizontal)
interconnected by extension fractures in a granite “pebble” from suevitic breccia at 99.8 m depth; crossed nicols. ca.
2 mm wide. (c) Typical microtexture of a lamprophyre sample, consisting of acicular clinopyroxene and some large
plagioclase crystals (right half) in a fine-grained groundmass with additional titanomagnetite and-often fresh-biotite.
Crossed nicols, 3.4 mm wide. (d) “Planar features” in a quartz grain from granite at 94.7 m depth; these features are
believed to represent a series of deformation bands that are not typical of shock metamorphism; crossed nicols, 1 mm
wide. fe) and (f) microphotographs of cataclastic granite breccia (ZP-5) from a contact between fragmental granite
breccia and a vein of altered trachyte; as the trachyte is not affected by annealing (note annealed clasts in Fig. 59, this
brecciation event is related to the intrusion of the trachyte, and not to the cratering event; planar (e) and crossed (f)
nicols, 1 mm wide.
(C!
2899
The Saltpan crater of South Africa
(a)
(b)
FIG. 7. Secondary electron images of impact glass fragments in suevitic breccia 116-l 18 m. (a) A smooth globular
particle with adherent glass spherule. (b) A second composite particle with a small, scalloped melt fragment and a
protuberance of surhcially pitted, probably vesicular glass. (c) A glass particle with schlieren, with spherical to ovoid
vesicles. (d) An irregularly shaped, partially vesicular glass containing a quartz clast. Scale bar lengths in pm.
spicuous com~nen~
of this incoherent breccia are abundant
glass particles, many of which occur in the form of perfect
spheruies (Fig. 6a) or droplets. Other forms include irregular
‘shards’ or composite grains, such as the ones shown in Fig,
7. Glass may be completely homogeneous
or be composed
of schlieren of different color and chemical composition (see
below). They appear either translucent,
or are of brownish.
yellowish or greenish color. Some particles (e.g., Fig. 6a) are
attached to mineral fragments that occasionally display shock
deformation
in form of PDFs or diapiectic glass. In most
glass fragments and often in spherules tiny sulfide spherules
or droplets are visible. Due to the presence of melt in this
breccia, REIMOLD et al. ( 1992) concluded that these “granitic
sands” represented an unconsolidated
equivalent of suevite.
Numerous
shocked mineral grains, mostly quartz and
feldspar, were found in samples from the breccia layer hetween 90 and 146 m depth (REIMOLD et al., 1992; this work:
Fig. 6b-d). Quartz grains with up to three sets of PDFs were
FIG. 6. Microphotographs of constituents of suevitic breccia from the Saltpan crater: (a) Silica-rich impact glass
spherule with tiny mineral clasts, connected to a quartz fragment with planar deformation features (PDFs): parallel
nicols, 230 pm wide; depth: 102-103.5 m; (b) Two sets of PDFs in quartz clast. Parallel nicols. 355 pm wide: depth:
113-l 16 m; (c) Another quartz fragment with PDFs: the crystal is twinned; PDFs are perfectly visible because they
have been enhanced by etching in the natural environment: parallel nicols, 355 pm wide: depth: 102-103.5 m: (d)
Two sets of PDFs in quartz; parallel nicols, 230 pm wide; depth: 105.1-107.15 m; (e) Small clasts of cataclastic granite
(monomict fragmental breccia) in suevitic breccia from 134.55- 136.25 m depth: crossed nicols. 900 pm wide: (t) Melt
breccia (so-called “agglutinate”) fragment from 1 I8 m depth. G = diaplectic glass fragment. Q = quartz clast, M
= mesostasis; parallel nicols, 1.1 mm wide; (g) “Ballen” structure in diaplectic quartz glass fragment; small particles
of high relief that are partially surrounded by radial fractures were analyzed by SEM-EDAX and found to consist of
pure silica; they are thought to be coesite; parallel nicols, 355 pm wide: depth: 113-l 16 m: fh) Diatom&e fragment
from 1! 3-l 16 m sample; parallel nicols, 355 grn wide.
2900
(‘ Koeherl.
W. II. Retmold.
frequently observed, and alkali feldspar, microcline. and plagioclase grains with up to two sets of PDFs occur. With regard
to length, width, spacing between individual features. as well
as their overall appearance, the Saltpan microdeformations
rn quartz and feldspar fulfil the criteria for bona fide PDFs
as. for example, discussed by ALEX~POULOS et al. (1988).
However. the statistics for crystallographic
orientations
of
the Saltpan PDFs are different from those measured for some
other impact craters (e.g., ROBERTSON et al.. 1968). Orientations that are characteristic
for shock. such as w 110I3 1 or
r’, 1012 i. were measured for PDFs in Saltpan quartz. but.
contrary to the normal case, these orientations are not dominant in the frequency diagram oforientations
(to be discussed
further by W. U. Reimold and D. Brandt. unpubl. data).
Besides impact glass, the suevitic breccia also contains a
few rare fragments of a elastic breccia (Fig. 6e). At certain
depths, namely between 113 and 118 m, and at about I 39. IO
m, several up to 20-cm-wide layers were detected that appear
oxidized due to the presence of abundant goethite and contain
numerous aggregates of melt and mineral fragments. We have
termed these aggregates “agglutinates”
(Fig. 61). These particles consist of a mesostasis of yellowish (probably due to
the presence of much Fe3+) glass that surrounds mineral and
less abundant lithic clasts and abundant diaplectic quartz
and feldspar particles. PDFs in these fragments are vzry rare.
In addition, these layers contain numerous. mostly spherical
sulfide grains that are generally less than 350 pm in si,e, but
occasionally as large as 1 mm. It is possible that alteration
of these sulfides caused the formation of goethite and other
ferrous oxides and hydroxides that are enriched in these Lanes.
Other ore minerals are rare throughout the drillcore, but some
angular fragments of pyrite and some oxides (magnettte.
chromite). as well as some euhedral pyrite of probable target
rock association. have been observed. Unfortunately
in this
depth interval the first drill-bit used was partially lost and in
fact disintegrated.
Accordingly it was necessary to perform
detailed analyses of some recovered remnants of this tlrillbit in order to assess whether the breccia in this /one had
been contaminated.
A single quartz grain (Fig. 6g) was found that may contain
a high-pressure polymorph, coesite. The grain contains small.
highly refractive particles of pure silica (as determined
by
secondary electron microscopy-energy
dispersive spectrometry), which are often the nuclei for radial fractures typically
observed around high-pressure
SiOZ polymorphs.
In the absence of more such material. but on the basis of textural
appearance and chemical composition,
we suggest that this
grain of ballen-structured
diaplectic quartz glass might contain
small nuclei of coesite. In addition. a fraction of a percent
of Karoo-derived
fragments or of diatomite particles (Fig.
6h) were found in the breccia.
Specimens of all rock types described above were analyeed
for this study. These samples are listed and described in more
detail in Table I.
ANALYTICAL
METHODS
Major element analyses were done on powdered samples by standard X-ray fluorescence (XRF) procedures (see REIM~III et al., 1994.
for details on procedures. precision. and accuracy). The concentrations
of V, Cu. Y. and Nb were also determined
by XRF analysis. All
and S. B. Shire)
other trace elements were analyzed by instrumental neutron actrvatron
analysis (INAA) following procedures described by KOEREKLet al
(1987) and KOEBERL(1993). For most samples. Sr and Zr concerttrations were determined by both XRF and INAA. and for some 111
the low abundance samples, Ni data were also obtained by XRF. F<)i
reasons of similarity in major element composition
one trachy te and
some granite samples were not analyzed by INAA; for those samples.
XRF data are also given for Cr. Co, Rb. and Ba. Impact glass fragment\
(see Fig. 7) and sulfide spherules were separated from the hull\ brccct;r
by standard magnetic separation techniques followed by handpicking.
Composite
samples of impact glasses and spherules. respective&.
weighing a few milligrams, were first analyzed by INAA. and afterwards mounted for electron microprobe analysis (EPMA). Individual
glass shards and spherules were then analyzed with a f’ameca
Camebax electron microprobe,
following standard procedure\
I #1
avoid Na volatilization.
20 X 20 pm areas of glass were rastered hi
the electron beam.
For rhenium and osmium isotope studies, two target rock sample\
(unshocked granite clasts from 136 m and I54 m depth), representing
the basement granite, and two breccia samples (granitic sands knoart
to contain impact glass and shocked minerals. from I 13 m and 1! fs
m depth), were selected. About 6-10 g of sample were used for thL
measurement
of abundances
and isotopic compositions
folloulng
procedures described by KOEBERI and SHIREY(1993). Total andlytical blanks for this study were (on average) 2 pg for 0s and 12pg
for Re. The precision of abundance
and isotopic compositions
lix
Re and OS is usually ~0.3%: total errors including error propagation
from spike calibration are on the order of 3%
GEOCHEMISTRY
OF TARGET ROCKS, BRECCJAS. ANI)
IMPACT GLASSES
The target rocks at the Saltpan crater comprrse I~IIII~
granitic rocks, and rare intrusive volcanic rocks. Fourteen
granite samples (two surface and twelve drillcore sumplcs
from various depths. see Fig. 3 and fable 1i lrcrc analyzed
to determine
the average composition
and compositionai
Lariation of the target rocks. The major and trace &~netri
compositions
of these samples are listed in iablc 2 Also
given are the chemical data for three bulk samples of uncon.
solidated suevitic breccia (from the depth intervals 1I 1-i i it
m. 116-t IX m. and 139.1 m).
The granite shows a limited but noticeable <omposrtmnai
variation between the different samples from various depths
Most of the minor differences do not seem signiticant or systematic. The only prominent systematic variation exists for
the rare earth elements (REEs) and Th between granite samples from the surface and the upper part of the drill core and
granites from the lower part of the core. REF. and Th abundances are significantly higher in the upper granite samples.
The cause for this difference is not known. but could bc due
to diverse trace mineral abundances or primary compositional
differences in the granites. The major and trace element
compositions
of the intrusiva rocks (Table / i arc grven in
fable 3. ‘There is a considerable
variation I?: Lomposrtron
between the different types of intrusions. and even between
samples of the same rock type. However, the granites and
the volcanic intrusions are distinctly different in composition.
4 comparison
between impact hreccias and target rocks
shows that their composition
is virtually identrcal to that ot
the granites. and shows no similarity to that ot‘ the volcanrc
rocks. The large difference in composition
between granites
and the intrusive volcanic rocks rules out a contribution from
the volcanic rocks to the breccias. This is in agreement with
geological observations,
which show that the volcanic rocks
290 I
The Saltpan crater of South Africa
TABLE 1. SAhWLFS
DISCUSSED AND ANALYZED
IN THlS STUDY.
ZP-1
Autoehthonous
ZP-4
Cataclastic granite breccia, collected at contact to &wed trachyte vein.
94.69 m
Drillcore sample: a 3 cm granite pebble in suevitic breccia, with strongly altered feldspar and hornblende; contains
2 small quartz grains with 1 set of PDFs each (Fig. 5d).
128.00 m
Fractured granite from boulder-size
hornblende.
129.15 m
Fractured granite fragment in suevitic breccia; minor feldspar sericitization.
129.90 m
Fractured granite fragment in suevitic breccia; some feldspar sericitization.
crater rim granite (fresh hut slightly microfractured).
fragment in suevitic breccia; locally chloritization
or uralitization
of
131.9+lm
Fractured, reasonably fresh granite pebble (ca. S-cm-long) in suevitic breccia.
136.75 m
A ca. S-cm-long piece of fractured granite in “granitic sand”; hornblende
(chloritization/sericitization,
respectively).
146.00 m
Hardly fractured, fresh, solid granite (probably a m-size clast in fragmental breccia).
151.26 m
Fractured granite (underlain by some 25 cm of “granitic sand” again) from large clast: some feldspar and minor
biotite alteration.
154.10 m
Fractured/locally
u~lit~tion.
160.90 m
Slightly fractured, rather fresh (only some biotite alteration)
brecciated crater floor.
and some microcline are altered
brecciated granite of ca. IO-cm-long pebble; some feldspar sericitization
and hornblende
granite, clearly belonging to fractured/iocaily
177.95 m
Fresh granite pebble from zone of extensive core-loss; hardly any fractures or microfractures.
196-198 m
Composite sample of several S-cm-wide specimens of solid, fresh basement granite.
SP 113-116 m
Bulk suevitic breccia; composite of several scoops of “granitic sand” from this depth interval.
SP 116-118 m
Bulk suevitic bnccia - composite sample.
SP 139.10 m
Bulk suetitic breccia from only this depth, collected for its abundance of ‘a~utinates’.
SP-95’
Altered carbonatite.
SP-107’
Altered carbonatite.
ZP-12
Slightly altered lamprophyre from ca. 15 cm-wide, rim-parallel, discontinuous dikelet or sheet.
SP-104’
Lamprophyre, similar to ZP-12.
SP-109*
Lamprophyre, similar to ZP-12.
SP-22’
Altered traehyte; mainly kaolinitic matrix, but some fresh biotite phenocwts.
SP-76*
Altered trachyte; strongly Fe-stained. some secondary carbonate.
ZP-5
Altered trachyte from apparently radial vein (ca. IS-cm-wide); similar to SP-76.
ZP-14
Slightly altered tmchyte (float sample); some Fe-staining.
Glass 113-116 m
Impact glass separate, handpicked from bulk suevitic breccia SP113-116”.
Glass 116-118 m
Impact glass separate, handpicked from bulk suevitic hreccia SPt16-118”.
Melt agglutinate
Composite of up to 0.8 cm-large melt fm~en~
STAR agglutinate
A particularly fresh and glass-rich melt breccia fragment, ca. 1 cm long, from breccia sample 139.10 m.
from breccia sample SP 139.10 m.
Spherules 113-116 m
Composite sample of handpicked sulfide sphcrules < 500 pm, from breccia sample SP113-116”.
Spherules 116-118 m
As previous sample, but from breccia
Samples marked with
l
sample
SPllC-118".
are courtesy D. Brandt.
are a feature of the local geology (BRANDT and REIMOLD,
1993), and with drillcore observations that show an insignificant contribution of volcanic rocks to the breccia. As stated
above, a causal relationship between the breccia-forming
event and the intrusives is also unlikely because of the age
difference between the intrusives and the cratering event
(STORZER et al., 1993; KOEBERL et al., 1994b). The major
and most trace elements show no significant differences be-
tween ind~vidu~ breccia samples, or between the breccias
and the granite samples (Table 2, last 2 columns). The REE
abundances in the breccia samples are similar to those in the
granite samples from the upper part of the core, while the
Th abundance is intermediate. The only significant differences
are much higher W abundances in the breccia samples which
are caused by contamination from the drill-bit (see section
on petrographic observations, and below).
80s
126
37
6J
348
192
351
211
298
1.9
105
gs
w
16.8
0.72
2.2
Cl
,,O
211
24
95
8
CO.4
0.29
0.64
194
795
1%
71.5
11.8
I56
8.8
1.45
9.1
0.77
5.04
0.72
17.7
in
03
-1
52
*:O.!
32.4
5.54
10110
99.44
lb
w
8
11.5
0.1
49
0.18
11.69
2.7,
0.01
0.15
on
237
Sdl
0.02
111
76.87
m.21
0.26
8.1,
4.18
O.OZ
CL30
0.27
1.22
3.67
0.04
1.20
825
:x7
M
46
303
,i
i,
!.26
i4
i
"4
58
Jso
4*,4i
8.oy
4.45
1.08
2.40
,w
II
7
8
5
0.i
a
14.9
1.97
I.2
1.1
1.55
61
68
354
22
CO.4
0.27
1.63
75.5
15.8
40.2
26.4
6s7
139
6.8
1.15
72
0.65
4.45
0.61
752
0.92
0.)
.i
4.6
~ o.,
355
3.3
97.95
98.13
12
!1
8
n
I
50
98.29
IO.54
3.72
0.0,
0.0,
I.44
2.45
4sl
0.03
0.99
0.29
7180
0.27
,153
35,
0.03
0.01
,.I0
2.92
4.99
II.03
2.10
i3.90
7352
0.18
11.7,
2.70
0.02
0.0,
1.00
3.14
4.97
0.03
0.85
98.47
97.M
4x3
0.04
2.04
99.73
v)9*
693
675
723
22
5s
55
17
2.11
.e1
OAR
140
82
48
330
16
c,
0.27
2.44
,a5
88.2
168
79.6
11.9
2.47
7.95
1.34
8.7
0.88
521
ml
t03
0.43
06.9
*2
8.h
:I
165
2.7
95
!2”
17
6,
126
zz
4
39.6
135
6s
58
xu
Ih
3a.8
0
2.5
II
1.93
w.95
150
0.04
‘La3
3.22
,.,I
0.19
I6
2.95
I.43
II
W.7h
037
0.0,
453
2.50
1.18
3.10
494
0.05
0.95
0.6,
0.22
0.04
2.96
0.19
0.01
II.23
0.06
0.04
3.77
329
0.24
7441
I,36
0.25
73.28
0.01
11.69
0.29
73.70
1.26
Ion
0.08
,*5
Ni
70
328
18
11.72
2.2,
0.w
0.04
0.66
2.25
5.62
0.03
0.46
0.16
7538
25
O!
JB
7198
0.33
11.8,
3.95
0.05
0.02
141
2.82
5.09
0.03
0.3,
12
2
r.5
.I
0,
58
v
2
IO.47
2.99
0.06
0.05
1.00
1.85
444
0.w
1.41
0.27
75.89
0.26
,,sl
353
0.06
0.16
1.10
294
4.97
0.03
1.74
7157
1.93
27
6.7
12.2
120
9
59
15
1.4!
1.45
OAIr
14,
74
50
297
,a
0.w
026
,.LB
749
46.38
kiwa
4558
8.74
1.62
7.35
1.26
7.m
0.67
4A,
0.6,
107
OS9
Ol.7
0110
55
0.00
j8.2
3.57
1.84
13
34.1
31.9
73
478
458
2.3
532
I.2
0.05
t42
68
46
332
16
332
038
2.49
7.51
965
186
s3.7
116
2.12
10.7
1.77
103
0.92
53,
0.69
10.7
IJ
732
~^
12.28
7.11
98.81
98.96
15
07
18.7
,.Y?
74.63
0.21
11.19
3 14
0.03
0.07
0.92
262
4.94
0.03
0.96
7339
0.23
11.48
458
0.05
0.05
LO2
205
4.80
OR3
12.3
12.39
1.m
15
36.2
45.6
39.3
197
197
30
356
0.70
033
147
80
48
336
163
11.07
034
2.46
770
96.90
182.67
82.10
12.a
226
9.27
155
9.27
0.a
5a
0.70
Ill.6
1.49
'I
0.m
3.8
023
!8.O
2.69
c9955
73.74
0.24
11.36
3.61
0.04
O.,S
1.19
2.74
4.&4
0.w
1.6,
c
2903
The Saltpan crater of South Africa
hb.fOR
TABLE 3.
AND TRACE ELEMENT COMPOSITION OF SAIXPAN INTRUSIVE ROCKS.
ALTERED CARBONAWES
SP-95
AtJEW
LMPROPHYREE
TP.ACH~
SP-107
ZP-12
SP-104
SP-109
SP-22
SP-76
ZP-s
ZP-14
52.04
SiO,
11.85
23.92
39.86
39.18
39.58
57.67
50.02
50.80
TiO,
1.22
0.34
5.78
0.79
5.91
1.35
1.24
1.01
1.17
ALO,
1.34
5.15
5.64
13.43
5.87
17.04
14.00
15.05
14.24
%O,
MnO
9.63
11.48
18.09
25.74
19.37
8.85
10.89
11.28
13.01
0.78
0.75
0.36
0.41
0.32
0.03
0.17
0.17
0.11
MS0
ChO
4.16
7.75
8.20
3.90
8.23
0.97
2.34
7.40
1.62
36.94
19.16
14.17
52s
13.73
0.09
6.28
10.40
2.15
0.37
k0
0.01
0.19
0.65
0.92
0.34
0.01
4.51
1.75
%O
0.03
3.24
1.73
1.63
1.46
9.85
2.94
0.60
8.86
PZO,
L.0.I
1.16
1.32
0.50
0.24
0.55
0.03
1.14
0.15
1.63
32.34
25.85
5.83
9.19
4.96
4.05
4.87
0.15
4.04
Total
99.46
99.75
1043.81
100.68
105.32
99.94
98.71
99.84
SC
10.1
V
9.02
73
146
45.7
18.5
417
87
19
398
12
354
376
286
29.3
22.1
369
5
75.4
CO
30.1
12.8
91.6
12.3
Ni
32
2.5
24
72
Zn
0.6
157
$8
Ga
6.2
As
2.28
13.4
3.14
41.5
237
67.7
7.7
3.31
89
Cr
cu
4.55
34.9
99.w
14
5.8
201
4.8
11.8
76.9
9.78
15.2
53.2
161
17.9
14.8
195
267
11.6
11.3
255
409
126
100
11s
30
11
2.5
32
2.39
1.44
2.64
122
0.4
1.3
7.3
0.9
1.2
0.5
0.41
0.6
0.28
0.42
0.21
0.28
0.23
0.52
0.19
0.07
124
130
379
478
179
Y
36
759
zr
123
511
5.1
Nb
30.4
A&
Sb
0.08
Cs
84.3
96.5
414
874
89
87.7
0.11
0.12
0.25
0.67
0.89
0.58
0.098
0.26
1.52
2.12
27.5
141
1375
La
26.4
203
110
ce
56.2
375
215
Nd
28.3
198
sm
6.36
EU
1.41
Gd
7.6
Tb
1.32
DY
Tm
7.9
Yb
4.11
LU
0.57
42.7
6.82
38
6.98
46
0.65
Hf
2.05
Ta
2.34
4.37
31.3
4.14
14.1
1.36
Au @pb)
5.8
Hg
Th
0.22
7.13
u
5.64
61.6
110
98.4
56.6
16.8
12.6
5.34
16.6
2.72
0.55
1.72
0.25
10.9
2.27
2.13
35.8
1.6
1.45
1.8
2.71
439
31
232
313
128
189
582
99.7
54
10
0.29
0.17
0.36
13.1
642
0.07
0.18
0.41
0.35
0.09
6.61
0.89
0.18
344
777
747
37.8
104
45.3
207
89.4
77.6
20.3
102
51.2
49.1
il.7
12.8
12.2
15.4
4.42
15
1.73
16.2
2.98
2.53
12
19
1.81
0.61
2.21
12.9
0.27
1.91
3.32
2.01
1.15
6.7
4.32
7.03
0.82
6.3
4.9
0.56
0.39
3.45
2.47
0.45
0.38
9.64
13.9
11.7
2.47
8.46
IS.9
1.54
14.1
0.27
0.93
<l
1.2
CO.24
< 0.5
29.8
11.2
36.1
3.35
11.1
1.&i
<2
1.87
0.9
10.4
1.7
0.06
14.8
0.08
1.45
3.25
0.36
44
60.58
36.24
37.98
60.28
42.95
55.32
49.74
93.93
HfjTa
0.88
2.50
1.20
9.35
1.14
0.83
9.15
bF
3.70
95.31
9.02
2.07
9.29
0.87
1.25
2.55
6.65
wu
1.26
0.06
4.50
2.57
3.34
3.25
4.55
4.03
La,/Yb,
EU/EU’
4.34
4.38
43.22
4.12
31.80
2.37
7.40
2.64
0.620
0.517
0.367
0.679
0.928
All major elements
Blank spafes
indicate
in wt.%,
1165
0.977
trace
0.428
3613
7499
7356
0.889
elements
in ppm,exceptas noted.
For analytical
705
9.64
K/U
5292
40
769
Zr/Hf
750
151
414
< 0.5
11.6
21.4
210
16
<2
0.8
3.3
12.2
53.6
31.1
1.55
1.11
79.9
515
133
10.1
<2
0.08
29.7
1.51
14.5
1.48
0.8
2.73
17.2
9.11
< 0.4
1.99
16
14.5
124
1010
0.08
160
5.65
<1.5
<2
11 @pb)
52.4
42.5
Ba
W
113
0.5
31
0.21
Se
Rb
3
3.8
1.49
Br
SC
6
13817
details see text.
that this sample has not been analyzed by INAA.
Figure 8a,b shows the major element composition of granites, intrusive+ breccias, and impact glasses in terms of (CaO
+ Na20 + &O)-Fe,O,-MgO
contents (Fig. 8a) and in terms
of CIPW normative mineralogy (Fig. 8b). In both diagrams
the granites show a limited compositional variability. The
breccias occupy the same areas in the diagrams as the granites,
while the intrusive rocks show quite a different composition,
which is especially obvious from Fig. 8b. The difference in
the distribution of points between the two diagrams is due
the choice of parameters. For example, Fig. 8a clearly shows
the enrichment of Fe and Mg in the impact glasses (see below).
Table 4 gives the major and trace element composition of
impact glass fragments (Fig. 7), agglutinates, and sulfide
spherules separated from bulk breccia samples. The major
2904
(a)
C. Koeberl. W. U. Reimold, and S. B. Shirey
CoO+KzO+No20
(b)
iP1
4
l
Orth.
FIG. 8. Major element composition of granites, intrusive rocks,
and impact glasses from the Saltpan crater: (a) in terms of (CaO
+ NazO + K20)-Fe*O,-MgO contents: (b) in terms of CIPW normative mineralogy; the choice in coordinates is responsible for the
different appearance of the two diagrams; Fig. 8a shows more clearly
the enrichment of Fe and Mg in the impact glasses.
element data given here are averages of about 30-50 microprobe analyses of ten to fifteen different glass fragments or
spherules. Examples of sulfide spherules were shown by REIMOLD and KOEBERL ( 1992; their Fig. 1b-d). Two main types
of sulfide spherules were observed: firstly, solid sulfide particles (Fig. lc in REIMOLD and KOEBERL, 1992), and, secondly, sulfide shells enclosing silicate cores thought to represent glass spherules (Fig. 7a, this work, and Fig. Id in REIMOLD and KOEBERL, 1992). Typically, these particles are
< 150 pm in size, but individual sulfide spherules as large as
I mm in diameter were noticed. In a typical bulk breccia the
sulfide spherule component
does not exceed 0.5 vol%, but,
because of their generally small grain size, a grain mount
may contain as many as 150 such particles. The trace element
data were obtained by INAA of a composite sample prior to
EPMA.
The two glass samples that were isolated from different
breccia samples show virtually identical elemental abun-
dances. Figure 9 shows the chondrite-normalized
REE abundance patterns for the granitic target rocks and breccias and
the impact glasses (Fig. 9a) in comparison with those for the
intrusive rocks (Fig. 9b). White the patterns for the breccias
and glasses are almost identical to that of granite from the
upper part of the drillcore. significant differences exist reiativc
to those of the intrusive rocks (e.g.. differences in slope, Ia&
luN ratio, or Eu anomaly). The differences m REE patterns.
together with other significant diflerences in major and trace
element contents, provide further arguments against a cootribution from the intrusions to the impact glasses.
The two agglutinate samples (both from IV. IO mj wert’
initially thought to be fresh melt rock samples but turned
out to be altered and strongly contaminated
by the drill bit
which disintegrated at that depth. Different parts ofthe drilibit (e.g., crown. crown barrel) were analyzed by EPMA and
1NAA and were found to be composed of either mainly W
with minor Cu and Zn (crown). or of mainly 1-c with minor
Mn (crown barrel). Table 4 shows that the agglutinates arc
contaminated
mainly with W, Zn. and Fe. However. because
of the contamination
we decided not to use the agglutinates
for the following discussion. impact glasses and spherules an
not compromised
because they were recovered from a much
higher location in the drill core and because they represent
handpicked
separates devoid of any contaminating
metai
particles. Spheruies with a metallic luster were zommottl:~
found in the unconsolidated
suevitic breccra and. using
EPMA, identified as pyrite spheruics (may& after troilitc,
with some minor silicate inclusions (some first results were
given by REIMOLD et al.. 1992. and REIMOL.I) and KO~:HFKI ,
1992). The data given in Table 4 are averages of about sixty
analyses on twenty spheruies from each depth fraction. Similar spheruies are absent from the basement rocks mJ arc.
thus. assumed to be related to the impact event and the postimpact alteration stage. The composite spheruie sample 11h-I I8 m used for INAA possibly contained a few zircon crystals
as shown by the high Zr, Hf, and heavy REE abundances in
this fraction.
A comparison
between average breccia and glasses. and
average granitic target rocks is shown for major elements in
Fig. IOa and for trace elements in Fig. lob. ‘I’hc close sinti
iarity between the two glass fractions is obvious. Breccia and
glasses show mostly similar enrichment
or depletion
trends.
Among the major elements, an enrichment
in Mg is most
prominent,
followed by Mn and Fe. For trace elements. cnrichments in Cr. Co, Ni, and Ir are most striking in the glasses
and somewhat more subdued in the bulk breccia. The significant W anomaly mainly in the bulk breccia is due to the
disintegrated
drill-bit (with the highest W value in th<
139.10 m breccia; see above). ‘The relatively low W value II!
the glasses ( 1.8 ppm) indicates that the glasses arc not ~WItaminated by the drill bit. Figure I I shows the correlation
between Fe and Mg from the individual microprobe analyses
of glasses in the 1 l3- I 16 m fraction. The positive correlation
indicates a common source for these two elements.
REIMOLD and KOEBERL (1992) suggested that the impact
glasses are either derived from bulk granites, or from mixtures
of the mineral constituents
of the granite, mainly quartz.
feldspar, and mafic minerals fbiotite, hornblende).
Such
mixing relationships
are also suggested by Fig. 8a. and Fig.
2905
The Saltpan crater of South Africa
TABLET. M~~ORANDTRACEELEME~COMPOSITION
OFHANDPICKED IMPACT
GLASSFRAGMEhTS,AGGLUTINATEAND
MELTFRAGMENQAND
HANDPICKEDSULFIDE
SPHERULES FROM BRECCIAS AT 113-116 AND 116-118 M DEPTH.
Glass
Glass
Melt
STAR
Spherules
Sphetules
113-116 m
116-118 m
Agglutinate
Agglutinate
113-116 m
116-118 m
SiO,
71.06
73.15
0.30
0.85
TiO,
0.27
0.27
0.01
0.06
AI@,
Fe0
13.54
11.92
0.09
0.26
5.86
5.64
47.52
47.53
MnO
0.13
0.12
0.05
0.05
M&t
CaO
2.62
2.28
0.01
0.05
0.97
0.67
0.01
0.03
Na,O
1.48
2.30
2.03
0.79
0.05
0.13
%O
3.80
3.37
2.89
0.95
0.05
0.13
P,O,
S
0.08
0.03
0.005
0.001
Total
99.82
SC
290
CO
4.89
279
55.4
8.84
99.75
4.75
Cr
20.9
59.7
3.23
304
0.01
52.01
50.86
100.11
99.96
1.18
143
68.8
3.88
12.7
205
230
101
138
Ni
1140
1200
180
220
500
250
Zll
8
6
7870
2220
300
330
Ga
14
12
83
10
<80
< 150
14.4
286
4.2
12
AS
2.49
1.24
Se
0.9
1.4
Br
0.37
Rb
168
Sr
c 140
ZI
405
Ag
Sb
<1
Cs
Ba
625
0.01
0.09
12
0.27
201
0.4
114
570
0.4
0.1
22
84.5
105
120
< 2200
30
30
<3Oca
1660
140
700
<loo
128
46.5
<35
0.36
0.48
2.92
3.57
8.51
1.61
1.71
0.85
0.56
3.5
520
500
310
189
<lO
330
<loo0
11.4
1.85%
<20
8.9
2.1
4Oa
La
116
126
149
42.1
10.1
33.5
Ce
208
252
307
88.6
15
70
Nd
89
107
151
40.8
Sm
17.5
EU
19.8
2.21
Gd
13.8
Tb
2.21
Yb
LU
1.77
36.1
2.32
6.21
1.45
1.31
0.28
7.3
<5
1.8
6.1
9
n.d.
<0.7
1.2
3.1
0.45
9.03
17.2
2.63
5.18
4.76
0.39
0.68
2.5
0.5
1.91
1.03
Ta
1.31
1.29
W
1.8
1.7
1.02
<8
1.23
1.05
12.8
21.9
6.85
7.39
11.8
79
35.9
5.85
14.8
0.98
Hf
2.43
14.2
13.5
DY
Tm
20.5
<lO
14.1
1.73%
<4
3.7
12.8
518
1.7
605
445
100
<50
fr @pb)
3
5.5
Au @pb)
2
4
2.5
1.7
< 0.9
<0.7
8.5
0.5
26.8
30.5
28.4
6.76
2.1
49.1
0.68
< 1.5
56.9
Hg
Tb
u
3.88
R/U
ZrfHf
8119
<5
1.65
2760
n.d.
103
4.18
6684
4.97
4821
11582
180
<lO
185
<20
19
34.32
44.53
664.00
280.00
366.49
35.71
Hf/Ta
9.01
9.92
0.18
0.30
0.52
304.71
Lw-flJ
4.33
4.13
5.25
6.23
4.81
0.68
n/u
6.91
7.30
5.71
9.94
0.86
10.61
9.43
5.85
10.82
0.453
0.204
k/Yb,
EU/EU’
0.445
All major elements
in wt.%;
0.609
1.32
0.22
0.222
0.123
trace elements in ppm, exe:ept as noted. For analytical details see text
8b, in which the CIPW analyses for the glasses fall on a line
between granite and the quartz component.
Enrichments
of
Mg and Fe in impact melts or glasses compared to target
rocks are known from a few other craters (see, e.g., the reviews
by STOFFLER,~~~~,~~~
BouS~A,1993,
pp, 165-173). The
Fe and Mg enrichment
could be due to preferential melting
of ferromagnesian
minerals. However, the enrichment in Mg
is considerably
higher than that in Fe. Furthermore,
such
minerals do not explain the trace element enrichments
mentioned above. We therefore considered the possibility of a
substantial meteoritic contribution
to the impact glass composition.
The concentrations
of the elements Mg, Cr, Fe, Co, Ni,
and Ir in the impact glasses were corrected for indigenous
concentrations
(average granite; Table 2) and then used to
compare with a possible meteoritic component.
The results
79Oh
(-. KoehurI. W. U. Relmold, and S. R. Shire!
FK,. 9. Chondrite-normalized
REE patterns (nommahratlon
~alucs
from TAYLOR and MCLENNAN. 1985) for: (a) two granites. avcrage
breccia. and two composite samples of handpicked glass fragments:
(h) typical intrusive rocks at the Saltpan crater. ‘fhc granites and
impact glasses (a) show REE patterns that are distinctly different in.
c.g.. their slope. LaN/YbN ratios. and Eu anomalies.
front those 01
the intrusive rocks.
Flcr. IO. C‘ompositlons of aberagc brccc13 ( 1~ihlc:J ami rmo3( t
glasses (Table 4). normalized to average Saltpall gr;rrnrc U;lhlc 2%
(a) ma.jor elements: lb) trace clcmcnt\.
biased toward\ the sciccuo~:
glasses. First. a magnetic preconc~ntratiorl
\\a\
performed. and second, brow>nish and greetmh glasses *t~crr
preferred during handpicking because of cas~cr recopnitlon
from
the bulk hreccia was doubl!
of metal-rich
are shown
in Fig. 12. A relatively good fit (i.c.. normalized
values near unity) for the abundances of Mg. Cr. 1-c. Co, and
Ni is obtained for about 105%Cl-chondritic
or Xci cnstatitechondritic contributions.
The elevated abundances of these
elements in the impact glasses can thus be rather clegantl)
explained by a chondritic contribution.
An iron meteorite
seems an unlikely source because it would not explain the
enrichments
in Mg and Cr, and produce a different pattern
for the other elements as well. On the other hand. the abundance of Ir from such a chondritic
component
would bc
higher than actually observed in the impact glasses. However.
the Ir may very well have been fractionated during the impact
(see, e.g., ATTREP el al., 199 I ; MI ITLEFEHLI~I~et al.. 1992a.b).
This assumption is supported by the high Ir abundances (on
the order of 100 ppb!) found in the spherule fraction.
The meteoritic contribution
to the impact glasses is higher
than in many other cases where a meteoritic component
has
been documented
in impact glass or impact melt rock (see.
e.g., PALME et al., 1978; MORGAN. 1978: PALMF. 1982). except, for example, impact melt from the West Clearwater
crater with up to about 10% meteoritic contribution
(PALME
et al.. 1979). However, the separation
of the impact glasses
1
1.5
?
2.5
7
.7’
j
.I
.:
MgO (wt.%)
FIG. I I. Correlation between Fe0 and MgO in 1ndlvldual Saltpan
Impact glass fragments from the breccla at I 1% I I6 m depth. intlteating that the enrichment
of these elements in the glasses relative
IO the target granites is due to a common source. I’he five high-t c
analyses show no other significant (major element) deviatmn from
an average glass composition.
2907
The Saltpan crater of South Africa
Corrected for Indigenous Concentrations
p
10
B
$
B
2
H
2 0.1’
9
Mg
Cr
Fe
-m- 10% Cl-Chondrite
Co
-A-
Ni
Ir
8% E-Chondrite
FIG. 12. Meteoritic contribution to impact glass composition. The
abundances derived from 10% of an average C I chondrite (ANDERS
and GREVESSE,
1989)and 8% of an average enstatite chondrite (MASON, I97 1: lr from BAEDECKER
and WASSO~*~,
I973 are compare
to the average composition of Saltpan impact glass, corrected for the
indigenous concentrations derived from the basement granite.
We thus conclude that a chondritic contamination is the most
likely explanation for the selective enrichment of the elements
Mg, Cr, Fe, Co, Ni, and Ir in the impact glasses (and some
spherules).
RHENIUM-OSMIUM
ISOTOPE SYSTEMATIC
To further establish the presence of a meteoritic component
in the Saltpan breccias, we performed a Re-0s isotopic study.
KOEBERLand SHIREY( 1993) and REIMOLDet al. ( 1993) have
shown that Re-0s isotopes can be used as a powerful tool to
identify the presence of a meteoritic component in impact
breccias, melts, or glasses. The usage of the Re-0s isotope
system for impact studies is based on the behavior of these
two elements during crust formation and the contrast with
rhenium and osmium abundances and isotopic compositions
in meteorites. During partial melting of mantle rocks, rhenium is moderately incompatible, while osmium is highly
compatible and retained in the residue. Low to moderate
degree of melting of the mantle yields melts with high rhenium
TABLE
5. &OS IS~~IC DATAFORTWOBRECCIAS
Sample
‘=os
Re (PW
(IP
motes&)
TotalOS
but low osmium concentrations, resulting in high Re/Os ratios. As a result of the high rhenium concentrations, crustal
rocks accumulate significant abundances of ls70s from the
decay of ‘*‘Re (half-life: 42.3 Ga). In contrast, meteorites
have high osmium abundances (e.g., ANDERSand GREVESSE,
1989), and isotopic ratios that are distinct from those of old
continental crust containing substantially less ‘*‘OS. Meteorites (and mantle rocks) have low ‘x70s/‘RROsratios of about
0.11 to 0.18 (is’Os/‘860s = 0.95-1.5) (e.g., WALKER and
MORGAN, 1989; MORGANet al.. 1992; HORA~Jet al., 1999).
Typical values of ‘*‘OS/‘*~~Sratios for the average continental crust are about 0.67 to 1.6 I (‘870s/18hOs = 5.6- 13.4:
ESSER,199 I f, with ‘~‘~/‘*~Os ratios of about 1.2- I .3 thought
to be an average for presently eroding continental crust (ESSER
and TUREKIAN, 1993). Meteorites have much higher (i.e.,
about 5 orders of magnitude higher) absolute abundances of
osmium, as well as ‘*‘Re/‘**Os and ‘870s/18RO~ratios that
are signi~cant~y different compared to those in old crustal
target rocks. Therefore, small amounts of a meteoritic component can easily be detected in impact breccias or melts
derived from continental crustal rocks. The first application
of the osmium isotopic system to impact craters was attempted by FEHTVet al. (1986) who found evidence for a
meteoritic component in melt rocks from the East Clearwater
crater. Recent analytical improvements (e.g., negative thermal
ionization mass spectrometry) allow the determination of
abundances and isotopic ratios of osmium and rhenium at
low abundance levels found in impact-derived materials and
associated target rocks using the limited sample quantities
available (see KOEBERL.and SHIREY. 1993, and KOEBERI.et
al., 1994a).
The results of the Re-0s isotopic analyses of two basement
granites and two suevitic breccias from the Saltpan crater are
given in Table 5 and shown in Fig. 13. The granitic target
rocks have osmium abundances of 4.3 and 7.0 ppt, which
are relatively typical values for granite, and high ‘x70s/‘“80s
ratios of 0.7 13 and 0.736, which are within the range of old
crustal rocks. The breccias have more than ten times higher
osmium abundances at 81.4 and 75.5 ppt, and much lower
‘s70s/‘880s ratios of 0.205 and 0.206. Figure 13 shows that
the breccias have isotopic ratios consistent with derivation
by mixing of a meteoritic component with basement granites.
AND TWO
‘Q (%)
BASEMENT
GRANITJZS
FROM
THESALTPAN
CRATER.
‘“RCPOs
‘“RePGs
I%Qs,/18Qs
‘%POs
@pb)
Brezcia113-116m
Breccia 116.118m
0.171
0.110
56.3
523
0.081
0.076
2.69
2.71
10.8
7.05
84.1
58.6
0.2OSi4
0.206i8
1.70
1.71
Granite 136m
Granite l.%m
0.054
0.0089
4.06
4.50
0.@%3
0.0070
8.73
9.00
44.9
6.68
373‘3
55.6
0.713*19
0.736i13
5.92
6.12
ForReaad 0s aaalyses, about 6-10 8 of sample powder were spiked with enriched “‘0s and ““Re. Re and 0s wete measured as ReO; and 0~0,‘. respectively,
using negative thermal ionizatian mass speetrometry using a 15” mass specuometer. For details on the acid digestion - distillation - anion exchange procedure, as well
as fat for mom details on correction of mass spectrometq measuremems, see Koeheri and Shirey (1993). Total 0s (column 3) includes mdiogoaically derived lmOs,
dte percemage of which is @en in column 4. We adopt tbe convention of~~~zing
to ‘%s, because ‘“OS, aad not ‘%Is, is the no-~iogeaic
0s isotope directly
mea%medby aii workers. ‘%s notmaIized data (columns 6 and 8) are presented here to allow comparison with earlier literatzue data. Errors include uncertaiaty in
spike calibration.
290x
C’. Moehcri. W. L:. Keimold. and S. R. Shin3
Saltpan Crater,
0.80,
,
,
,
South
,
Africa
/ ‘-.-.-7
which arc higher In samples ij_onl
the upper part of the drill core. The intruslvt: rocks S~I!M
a wide variety in composition
and arc‘ dictlnrtl~ Jitfcrc*nr
from the granites.
suevitlc hreccias hu~c !XY*IIt~co\t’r~~t
i) llnconsolidated
from different depths. Three ofthcse samples were studied
and were found to contain abundant e~xlcncc trt‘ shuck
metamorphism.
e.g., shocked minerals and mtpact glasstx
(see also RPIMOLD et al.. 1W2 i. Chc qwt‘lr grain ii-~;;
the suevitic breccia is th~~u~ht tct contain cocsitc. a highand Th concentrations,
pressure p~~l~rno~h ofquanr.
diagram for two target rock
FKi. 13. 1”‘os/~~80s
vs. ‘X’Re/‘XWOs
samples (unshocked basement granite clasts from the drillcore at I 36
and IS4 m depth) and two breccia samples (from I I3 and I I6 m
depth). compared with data for carbonaceous chondrites and iron
meteorites. The two breccias have higher OS abundances and lower
‘X70s/‘RROs ratios than the basement granites. indicating the presence
of a meteoritic component.
(‘orrection of the average osmium abundance in the hreccias
for the indigenous osmium content ofthe granites yields an
excess of about 0.072 ppb OS. Assuming a C’l-chondritic
abundance of 486 ppb (ANDFRSand GREVWL 1989) yields
an estimate of about 0.0 15% of a chondritic component
in
the bulk breccia. Considering that impact glasses make up
5 1 vol% of the bulk breccia, and that the handpicked glasses
discussed above are a specifically selected subset of the impact
glasses. this value agrees roughly with the number inferred
above from the analyses of the glass fragments. Handpicked
impact glasses could not be measured because too much material (several hundred miiligrams to a few grams) is needed
for the Re-0s isotope analyses.
SliMMARY
AND CONCLIISIONS
We have studied the petrology and major and tract ciement
geochemistry of fourteen target granites. three sucvitic breccias, nine intrusive rocks. as well as melt agglutinates, handpicked impact glass fragments and sulfide spherules from the
Saltyan impact crater. In addition. WCperformed Kc-OS isotopic analyses on two target granites and two sucvitic breccias.
Our study comprises the most thorough petrologlcal and
geochemical examination
of the Sattpan crater to date. Some
major observations
and conclusions
resulting from our investigations are listed below.
The target lithology of the Saltpan crater comprtscs predominantly Nebo granites of the Bushveld Complex. Some
minor occurrences of volcanic rocks intruding the granites
at the crater have earlier been interpreted
to support a
volcanic origin of the structure. However, recent geological
studies (e.g.. BRANDT and REIMOLD,1993) showed them
to be part of the regional geology. We have not found an>
evidence for incorporation
of any intrusive rocks into the
suevitic breccias (see alsoREIMOLD etal..
i992).
2) The compositions
of the target granites recovered as clasts
from various depths of the drillcore show a limited variability. The only signi~cant difference i!: evident in REE
c)i the bulk
St) The major and trace element compasmon
breccia is very similar to that ofaverage hascment granite
Enrichments in W (and, to some degree, in %n). cspcciail!
in the 139. IO m breccia sample and in ;lgglutinates IX
covered from the same depth, arc due to <~,ntaminativ~~!
from a first drill-bit which disintegrated
.H thal depth
Analyses ofremains of the drill bit have shown th;it othct
elements are not compromi5cd.
‘1 impact glass fragments werr recovered from Eht’ unconsolidated sucvitic breccia h!- magnetic scparalion. folIo~ed
by iiandpi~king.
Their ncrmrati\c
ctxnp~:~ition P. it:;_:
similar to that of the granites from whi~*h the;, ha\-c h~c’ri
derived. and distinct from that of the intrusive rocks. I-hi
similarity to the granites is firrthcr supportctf hy trace eic.
ment abundances and ratios and REE pa~tcrn~
il) Compared
to the average target granite composition.
Ihc
impact glass fragments she\{ remarkable cnrxhments
!I?
Mg. C‘r. Fe, Co, Ni. and Ir. sspecialll a> Ihq habc ,;thcrwise granitic compositions.
Although przfcrential melt.
ing of mafic minerals in the granites ma> 1~ a fhctor. w
prefer to explain these enrichments
h? admixture of‘ 4
significant
meteoritic
component.
Ahout ‘. 10’5 ti ;t
chondritic
component
provide an cxecllen~ tit lix- thy
abundances of these elements in the glasses (after C’OWKtion for indigenous concentrations).
The Ir ~:ontcnt r&~h(:
glasses is lower than predicted by such ;m admixturt..
tfowevcr. very high Ir concentrations
( :- IO0 ppb) hark
been found in sulfide spherule samples whxh complcmt~ni
the Ir abundances
in the glasses and indicate some fractionation during impact. Although a mett<rrlriz contrabution of about &IO% is rather high. it should be noted
that the glass sample is biased towards nirial-rich CW~positions
because of magnetic
prccotti‘t:rllr;ltioi?
,mri
handpicking of brownish or greenish fragments.
7) The presence of a meteor-i(i~ i‘omponent
in the ~uf\lit~
breccia was ~~~n~rrned hy &-OS isotoprc srudics. 3~~
abundances and isotopic ~omp~~sitjons ofthc target granir~.~
arc typical for those of old continental crust t I.P..LCQ it,t+
osmium concentrations
oi’ at~ut 7 ppt. anti high “‘OS;
“‘0s ratios ofabout 0.72). In contrast, the hrcccia sample5
contained much higher osmium abundances at abour 80
ppt, and lower ‘x70s/‘xxOs ratios of about i)._‘.OS,which i\
close to meteoritic values The fact that the meteori&.
component was measured in bulk brcccia. z.mJ not in rhc
handpicked impact glasses (ofwhich there was noi enough
material for Re-0s isotope studies), shows the extreme
sensitivity of Re-0s isotope measurements
a:, a t~i tii
study extraterrestrial
contaminations
in ~nl~~i~t-deri~~~~
l-o&S.
2909
The Saltpan crater of South Africa
In conclusion, the Saltpan crater is a recent addition to
growing list of mostly small and young impact craters in
Southern Africa (e.g., KOEBERL et al., 1993; REIMOLD et al.,
1993). Its excellent preservation state and the availability of
a drillcore make the Saltpan crater a prime object for the
study of morphological, geophysical, geochemical, and petrological characteristics of a simple impact crater.
the
ilcknoH,~ed~mt~~ts-We
hatefully appreciate the support of the Chief
Director of the Geological Survey of South Africa, C. F&k. for making
the samples available for study. We are grateful to D. Brandt, Univ.
of the Witwatersrand, for samples of volcanic intrusive rocks. Mr.
V. Govender of the Department of Geology, University of the Witwatersrand, carried out the XRF analyses. CK is grateful to G. McKay,
NASA Johnson Space Center, Houston, for allowing the use of the
Cameca microprobe, and to L. Le for help with the microprobe analyses. This study was supported by the Austrian Fonds zur FSrderung
der wissenschaftlichen Forschung, Project P9026-GE0 (to CK) and
by the National Science Foundation, Project EAR-92 I8847 (to SBS).
C.K. wants to thank colleagues at the DTM-Carnegie Institution of
Washington for hospitality and the opportunity to do Re-0s isotopic
analyses. the University of the Witwatersrand, Johannesburg, for a
visiting research fellowship during which this paper was written, and
C. R. Anhaeusser and T. S. McCarthy for the invitation to work at
EGRU and the Depa~ment of Geology, University of the Witwatersrand. We are grateful to R. Anderson and B. French for perceptive
reviews.
Editorial
hundling: S. R. Taylor
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