saltpan impact crater, south africa: geochemistry …...which clearly rules out the possibility that...

Geochimica et Cosmochimica Acta, Vol. 58. No. 13, pp. 2893-29 IO, 1994

Pergamon Copyright 0 1994 Elsevier Science Ltd Printed in the USA. All rights reserved

0016-7037194 $6.00 + .OO

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.

lNTRODUCI’lON AND SUMMARY (BRAND-F and REIMOLD, 1993) include lamprophyre dikes OF CRATER GEOLOGY and possibly sills, trachyte, and minor carbonatite.

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 Prov-

ince, 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; KOEBERL et 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

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 kim-

berlite 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 con- troversial. 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

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 be-

low the central part of the crater. In contrast, ROHLEDER ( 1933) and LEONAKD ( I YJ6) sug-

gested 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

/ ., \ .‘IMRARWL

BOTSWANA , - ,’

.’ 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 111 favor 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‘~I- taclastic breccia) granite, which is occasionally intercalated with narrow layers of sandy breccia to a depth of 16 1 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-

The Saltpan crater of South Africa 2895

Depth(m)

m Granitic “sand”

E Pebbles/fragments <3cm

l l Pebbles/fragments >3cm

m Fractured granite

m Solid granite

* 15-,7&m long coherent graf wthln sand-partial recovery

00 •I Mylonite

nits.

FIG. 3. Stratigraphic column for the Saltpan borehole (after REI- MOLn et al., 1992). Sampled intervals are indicated in the right mar- gins.

ters (e.g., GRIEVE et al., 1989). However, as discussed by REI- MOLD et 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.

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 gran- tic rocks up to 1 cm in diameter and mineral grains cemented by iron oxides and silica), or with fragmental breccia of ex- tremely 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.

Since then BRANDT and REIMOLD (1993) and BRANDT et In some first examinations to assess the presence of char- al. (1993a,b, 1994) presented more detailed geological and acteristic shock metamorphic features, solid granites from geophysical results. It was shown that the occurrence of vol- the rim and the drillcore were studied. Only weakly deformed canic rocks is not restricted to the crater area and is part of granite samples were described from the crater rim and the a regional volcanic event that took place at about 1.2- 1.4 drillcore (PARTRIXE et al., 1990; REIMOLD et al., 1991). Ga ago. Apparently the well-preserved crater and the Late Isolated shear fractures (Fig. 5b), microfaults, and defor- Proterozoic regional voicanism are unrelated. This was further mation bands (Fig. 5d) of typically tectonic origin were found. corroborated by fission track dating of impact glass separates it can, however, not be excluded that such deformation phe- from the suevitic breccia layer, giving an age of 220 -t 52 ka nomena have been introduced during the cratering event. A (STORZER et al., 1993; KOERERL et al., 199413). This age for recent reexamination of ail available thin sections of rim and the cratering event is in excellent agreement with an estimate drillcore granite samples revealed a single pebble, collected of 180 ka for the period of deposition of the crater sediments from 94.69 m depth, that contains two small (~250 pm) by PARTRIDGE et al. (1993) based on extrapolation of “‘C quartz crystals with one set of characteristic planar defor- dating of the upper sediment units. mation features (PDFs) each.

Only preliminary geochemical results on the various Salt- pan crater litholo~es have been reported to date (REIMOLD

Breccia specimens collected for this project comprise sev- era1 samples of cataclastic (figments) breccia that either

and KOEBERL, 1992; REIMOLD et 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.

PETROGRAPHIC OBSERVATIONS AND SAMPLE CHARACTERISTICS

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.

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 mac- roscopically 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. FELJCW- WANGER (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) C 19Y3) 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 (A- canic 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-


(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.


(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 he- tween 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. and S. B. Shire)

frequently observed, and alkali feldspar, microcline. and pla-

gioclase 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). Orien-

tations that are characteristic for shock. such as w 1 10 I3 1 or

r’, 1012 i. were measured for PDFs in Saltpan quartz. but.

contrary to the normal case, these orientations are not dom-

inant 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 tlrill-

bit 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 ab- sence 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~I II 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

other trace elements were analyzed by instrumental neutron actrvatron analysis (INAA) following procedures described by KOEREKL et al (1987) and KOEBERL (1993). For most samples. Sr and Zr concert- trations 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 after- wards 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 and- lytical blanks for this study were (on average) 2 pg for 0s and 12 pg 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 1 i 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 1 I 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

The Saltpan crater of South Africa

TABLE 1. SAhWLFS DISCUSSED AND ANALYZED IN THlS STUDY.

290 I

ZP-1

ZP-4

94.69 m

128.00 m

129.15 m

129.90 m

131.9+l m

136.75 m

146.00 m

151.26 m

154.10 m

160.90 m

177.95 m

196-198 m

SP 113-116 m

SP 116-118 m

SP 139.10 m

SP-95’

SP-107’

ZP-12

SP-104’

SP-109*

SP-22’

SP-76*

ZP-5

ZP-14

Glass 113-116 m

Glass 116-118 m

Melt agglutinate

STAR agglutinate

Spherules 113-116 m

Spherules 116-118 m

Autoehthonous crater rim granite (fresh hut slightly microfractured).

Cataclastic granite breccia, collected at contact to &wed trachyte vein.

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).

Fractured granite from boulder-size fragment in suevitic breccia; locally chloritization or uralitization of hornblende.

Fractured granite fragment in suevitic breccia; minor feldspar sericitization.

Fractured granite fragment in suevitic breccia; some feldspar sericitization.

Fractured, reasonably fresh granite pebble (ca. S-cm-long) in suevitic breccia.

A ca. S-cm-long piece of fractured granite in “granitic sand”; hornblende and some microcline are altered (chloritization/sericitization, respectively).

Hardly fractured, fresh, solid granite (probably a m-size clast in fragmental breccia).

Fractured granite (underlain by some 25 cm of “granitic sand” again) from large clast: some feldspar and minor biotite alteration.

Fractured/locally brecciated granite of ca. IO-cm-long pebble; some feldspar sericitization and hornblende u~lit~tion.

Slightly fractured, rather fresh (only some biotite alteration) granite, clearly belonging to fractured/iocaily brecciated crater floor.

Fresh granite pebble from zone of extensive core-loss; hardly any fractures or microfractures.

Composite sample of several S-cm-wide specimens of solid, fresh basement granite.

Bulk suevitic breccia; composite of several scoops of “granitic sand” from this depth interval.

Bulk suevitic bnccia - composite sample.

Bulk suetitic breccia from only this depth, collected for its abundance of ‘a~utinates’.

Altered carbonatite.

Altered carbonatite.

Slightly altered lamprophyre from ca. 15 cm-wide, rim-parallel, discontinuous dikelet or sheet.

Lamprophyre, similar to ZP-12.

Lamprophyre, similar to ZP-12.

Altered traehyte; mainly kaolinitic matrix, but some fresh biotite phenocwts.

Altered trachyte; strongly Fe-stained. some secondary carbonate.

Altered trachyte from apparently radial vein (ca. IS-cm-wide); similar to SP-76.

Slightly altered tmchyte (float sample); some Fe-staining.

Impact glass separate, handpicked from bulk suevitic breccia SP113-116”.

Impact glass separate, handpicked from bulk suevitic hreccia SPt16-118”.

Composite of up to 0.8 cm-large melt fm~en~ from breccia sample SP 139.10 m.

A particularly fresh and glass-rich melt breccia fragment, ca. 1 cm long, from breccia sample 139.10 m.

Composite sample of handpicked sulfide sphcrules < 500 pm, from breccia sample SP113-116”.

As previous sample, but from breccia sample SPllC-118".

Samples marked with l are courtesy D. Brandt.

are a feature of the local geology (BRANDT and REIMOLD, 1993), and with drillcore observations that show an insignif- icant 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).

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The Saltpan crater of South Africa

TABLE 3. hb.fOR AND TRACE ELEMENT COMPOSITION OF SAIXPAN INTRUSIVE ROCKS.

ALTERED CARBONAWES LMPROPHYREE AtJEW TP.ACH~

SP-95 SP-107 ZP-12 SP-104 SP-109 SP-22 SP-76 ZP-s ZP-14

SiO, 11.85 23.92 39.86 39.18 39.58 57.67 50.02 50.80 52.04

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, 9.63 11.48 18.09 25.74 19.37 8.85 10.89 11.28 13.01

MnO 0.78 0.75 0.36 0.41 0.32 0.03 0.17 0.17 0.11

MS0 4.16 7.75 8.20 3.90 8.23 0.97 2.34 7.40 1.62

ChO 36.94 19.16 14.17 52s 13.73 0.09 6.28 10.40 2.15

k0 0.01 0.19 0.65 0.92 0.34 0.01 4.51 1.75 0.37

%O 0.03 3.24 1.73 1.63 1.46 9.85 2.94 0.60 8.86

PZO, 1.16 1.32 0.50 0.24 0.55 0.03 1.14 0.15 1.63

L.0.I 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 99.w 98.71 99.84

SC 10.1 9.02 45.7 18.5 34.9 4.55 3.31 41.5 V 146 73 89 237 417 87 19 398

Cr 67.7 75.4 354 376 286 29.3 22.1 369 CO 30.1 12.8 91.6 12.3 76.9 9.78 15.2 53.2

Ni 32 2.5 24 72 161 17.9 14.8 195 cu 7.7 0.6 14 5.8 267 11.6 11.3 255 Zn $8 157 201 409 126 100 11s 122 Ga 6.2 13.4 4.8 30 11 2.5 32 3.8

As 2.28 3.14 11.8 2.39 2.64 1.44 1.49 0.21 Se 0.4 1.3 7.3 0.9 1.2 0.5 0.41 0.6

Br 0.28 0.42 0.21 0.28 0.23 0.52 0.19 0.07 Rb 5.1 124 130 113 124 515 79.9 21.4 SC 379 478 179 52.4 1010 53.6 210 439 Y 36 759 42.5 96.5 29.7 133 31.1 16 zr 123 511 414 874 414 769 582 232 Nb 30.4 84.3 89 87.7 128 189 99.7 54

A& 0.08 0.11 0.12 0.08 0.29 0.17 0.07 0.18 Sb 0.25 0.67 0.89 0.58 0.36 0.41 0.35 0.09 Cs 0.098 0.26 1.52 2.12 13.1 6.61 0.89 0.18 Ba 27.5 141 1375 160 642 777 344 747

La 26.4 203 110 61.6 104 45.3 37.8 9.64 ce 56.2 375 215 110 207 89.4 77.6 20.3 Nd 28.3 198 98.4 56.6 102 51.2 49.1 il.7 sm 6.36 42.7 16.8 12.6 15.4 12.8 12.2 3.32

EU 1.41 6.82 5.34 1.99 4.42 1.73 2.01 1.15 Gd 7.6 38 16.6 16 15 16.2 6.7 4.32

Tb 1.32 6.98 2.72 2.73 2.53 2.98 7.03 0.82

DY 7.9 46 14.5 17.2 12 19 6.3 4.9 Tm 0.65 4.37 0.55 1.51 0.61 1.81 0.56 0.39 Yb 4.11 31.3 1.72 10.1 2.21 12.9 3.45 2.47 LU 0.57 4.14 0.25 1.45 0.27 1.91 0.45 0.38 Hf 2.05 14.1 10.9 14.5 9.64 13.9 11.7 2.47

Ta 2.34 5.65 9.11 1.55 8.46 IS.9 14.1 0.27 W 1.36 1.48 2.27 1.11 0.93 1.54 1.&i 1.87

11 @pb) <2 <1.5 1.6 <2 <l <2 <2 0.9

Au @pb) 5.8 0.8 1.8 3.3 1.2 0.8 10.4 1.7

Hg 0.22 < 0.4 0.08 < 0.5 CO.24 < 0.5 0.06 0.08 Th 7.13 2.13 12.2 29.8 11.2 36.1 14.8 1.45

u 5.64 35.8 2.71 11.6 3.35 11.1 3.25 0.36

12

5

6

3

0.5

31

151

40

31

313

10

705

K/U

Zr/Hf

HfjTa

bF

wu

La,/Yb, EU/EU’

44 750

60.58 36.24

0.88 2.50 3.70 95.31

1.26 0.06

4.34 4.38

5292

37.98

1.20

9.02

4.50

43.22

1165 3613 7356 7499 13817

60.28 42.95 55.32 49.74 93.93

9.35 1.14 0.87 0.83 9.15

2.07 9.29 1.25 2.55 6.65

2.57 3.34 3.25 4.55 4.03

4.12 31.80 2.37 7.40 2.64 0.620 0.517 0.977 0.428 0.889 0.367 0.679 0.928

All major elements in wt.%, trace elements in ppm, except as noted. For analytical details see text.

Blank spafes indicate that this sample has not been analyzed by INAA.

2903

Figure 8a,b shows the major element composition of gran- which is especially obvious from Fig. 8b. The difference in ites, intrusive+ breccias, and impact glasses in terms of (CaO the distribution of points between the two diagrams is due + Na20 + &O)-Fe,O,-MgO contents (Fig. 8a) and in terms the choice of parameters. For example, Fig. 8a clearly shows of CIPW normative mineralogy (Fig. 8b). In both diagrams the enrichment of Fe and Mg in the impact glasses (see below). the granites show a limited compositional variability. The Table 4 gives the major and trace element composition of breccias occupy the same areas in the diagrams as the granites, impact glass fragments (Fig. 7), agglutinates, and sulfide while the intrusive rocks show quite a different composition, spherules separated from bulk breccia samples. The major

C. Koeberl. W. U. Reimold, and S. B. Shirey 2904

(a) 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 REI- MOLD and KOEBERL ( 1992; their Fig. 1 b-d). Two main types of sulfide spherules were observed: firstly, solid sulfide particles (Fig. lc in REIMOLD and KOEBERL, 1992), and, sec- ondly, sulfide shells enclosing silicate cores thought to represent glass spherules (Fig. 7a, this work, and Fig. Id in REI- MOLD 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 abun-

dance 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 coo-

tribution 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 drili-

bit (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. Sim-

ilar spheruies are absent from the basement rocks mJ arc. thus. assumed to be related to the impact event and the post- impact alteration stage. The composite spheruie sample 1 1 h-- 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. cn- richments 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 ~WI-

taminated 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.


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

TiO, 0.27 0.27

AI@, 13.54 11.92

Fe0 5.86 5.64

MnO 0.13 0.12

M&t 2.62 2.28

CaO 0.97 0.67

Na,O 1.48 2.30

%O 3.80 3.37

P,O, 0.08 0.03

S 0.005 0.001

20.9 8.84

2.03 0.79

2.89 0.95

0.30 0.85

0.01 0.06

0.09 0.26

47.52 47.53

0.05 0.05

0.01 0.05

0.01 0.03

0.05 0.13

0.05 0.13

0.01 0.01

52.01 50.86

Total 99.82 99.75 100.11 99.96

SC 4.75 4.89 3.23 1.18 3.88 12.7

Cr 290 279 304 143 205 230 CO 55.4 59.7 625 68.8 101 138

Ni 1140 1200 180 220 500 250

Zll 8 6 7870 2220 300 330

Ga 14 12 83 10 <80 < 150

AS 2.49 1.24 0.09 14.4 286 189

Se 0.9 1.4 12 4.2 12 <lO

Br 0.37 0.27 0.4 0.1 22 11.4

Rb 168 201 114 84.5 105 120

Sr c 140 <loo 30 30 <3Oca < 2200

ZI 405 570 1660 140 700 1.85%

Ag <1 0.4 128 46.5 <35 <20

Sb 0.36 0.48 2.92 3.57 8.51 8.9

Cs 1.61 1.71 0.85 0.56 3.5 2.1

Ba 520 500 310 330 <loo0 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 <lO 79

Sm 17.5 19.8 20.5 6.21 1.45 21.9

EU 2.21 2.43 1.77 1.31 0.28 1.02

Gd 13.8 14.2 36.1 7.3 <5 <8

Tb 2.21 2.32 5.85 1.23 1.8 6.1

DY 13.5 14.8 35.9 6.85 9 n.d.

Tm 1.05 1.2 3.1 0.45 <0.7 n.d.

Yb 7.39 9.03 17.2 2.63 5.18 103

LU 0.98 1.03 4.76 0.39 0.68 12.8

Hf 11.8 12.8 2.5 0.5 1.91 518

Ta 1.31 1.29 14.1 1.65 3.7 1.7

W 1.8 1.7 1.73% 2760 605 445

fr @pb) 3 5.5 <5 <4 100 <50

Au @pb) 2 4 2.5 1.7 180 185

Hg < 0.9 <0.7 8.5 0.5 <lO <20

Tb 26.8 30.5 28.4 6.76 2.1 49.1

u 3.88 4.18 4.97 0.68 < 1.5 56.9

R/U 8119 6684 4821 11582 19

ZrfHf 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

k/Yb, 10.61 9.43 5.85 10.82 1.32 0.22

EU/EU’ 0.445 0.453 0.204 0.609 0.222 0.123

All major elements in wt.%; 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 men- tioned 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.

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 cnstatite- chondritic 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). ex-

cept, 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

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\.

from the bulk hreccia was doubl! biased toward\ the sciccuo~: of metal-rich 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

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. intlt- eating 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.


p 10 Corrected for Indigenous Concentrations

B

$ B 2

H 2 0.1’ 9

Mg Cr Fe Co Ni Ir

-m- 10% Cl-Chondrite -A- 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 (MA- SON, 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. KOEBERL and SHIREY ( 1993) and REIMOLD et 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

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., ANDERS and GREVESSE, 1989), and isotopic ratios that are distinct from those of old continental crust containing substantially less ‘*‘OS. Mete- orites (and mantle rocks) have low ‘x70s/‘RROs ratios of about 0.11 to 0.18 (is’Os/‘860s = 0.95-1.5) (e.g., WALKER and MORGAN, 1989; MORGAN et al.. 1992; HORA~J et al., 1999).

Typical values of ‘*‘OS/‘*~~S ratios 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 at- tempted by FEHTV et 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.

TABLE 5. &OS IS~~IC DATA FOR TWO BRECCIAS AND TWO BASEMENT GRANITJZS FROM THE SALTPAN CRATER.

Sample

Brezcia 113-116m Breccia 116.118m

Granite 136m Granite l.%m

Re (PW

0.171 0.110

0.054 0.0089

‘=os Total OS (IP motes&) @pb)

56.3 0.081 523 0.076

4.06 0.@%3 4.50 0.0070

‘Q (%) ‘“RCPOs ‘“RePGs I%Qs,/18Qs ‘%POs

2.69 10.8 84.1 0.2OSi4 1.70 2.71 7.05 58.6 0.206i8 1.71

8.73 44.9 373‘3 0.713*19 5.92 9.00 6.68 55.6 0.736i13 6.12

For Re aad 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%med by 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, South Africa 0.80, , , , , / ‘-.-.-7

FKi. 13. 1”‘os/~~80s vs. ‘X’Re/‘XWOs diagram for two target rock

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 (ANDFRS and 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. WC performed 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 in- vestigations are listed below.

The target lithology of the Saltpan crater comprtscs pre- dominantly 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 also REIMOLD et al.. 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

i)

St)

‘1

il)

7)

and Th concentrations, 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. llnconsolidated suevitlc hreccias hu~c !XY*II t~co\t’r~~t

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.. 1 W2 i. Chc qwt‘lr grain ii-~;; the suevitic breccia is th~~u~ht tct contain cocsitc. a high-

pressure p~~l~rno~h ofquanr.

The major and trace element compasmon c)i the bulk

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. impact glass fragments werr recovered from Eht’ uncon-

solidated 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~ 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 ,;th- crwise 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’OWK- tion 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 contra- bution 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. 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.


In conclusion, the Saltpan crater is a recent addition to

the 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 pet- rological characteristics of a simple impact crater.

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 Wit- watersrand, 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 Witwa- tersrand. We are grateful to R. Anderson and B. French for perceptive reviews.

Editorial hundling: S. R. Taylor

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