Iridium and other platinum-group elements as geochemical markers in sedimentary environments

Palaeogeography, Palaeoclimatology,Palaeoecology, 104 (1993): 253-270

253

Elsevier Science Publishers B.V., Amsterdam

Iridium and other platinum-group elements as geochemical markers in sedimentary environments Z. S a w l o w i c z

Institute of Geological Sciences, Jagellonian University, ul. Oleandry 2A, 30-063 Krakow, Poland (Received January 17, 1992; revised and accepted October 10, 1992)

ABSTRACT Sawlowicz, Z., 1993. Iridium and other platinum-group elements as geochemical markers in sedimentary environments. Palaeogeogr., Palaeoclimatol., Palaeoecol., 104: 253-270. Iridium anomalies are commonly used as indicators of extraterrestrial processes which caused global extinctions. However, platinum-group element (PGE) anomalies may result from several different processes, both syn- and postdepositional, acting separately or jointly. The sources and processes of enrichment involved are: extraterrestrial (asteroid, comet, cosmic dust, impact ejecta); volcanogenic(PGE-rich condensates); precipitation from seawater (low sedimentation rate, anoxic conditions); microbial (concentration, dissolution, reprecipitation); exhalative-hydrothermal; leaching, transport and precipitation at a redox boundary. Geochemical characteristics of PGE and accompanying elements in sediments can be used to identify the contributions of some specific sources and processes: (1) extraterrestrial: chondritic Ru/Ir and Os/Ir ratios, low lS~Os/tSrOS ratio, high Ir compared to Pt, Pd and Au; (2) volcanogenic: high iridium and Se, As, Sb and In contents; (3) exhalative-hydrothermal:high Au, Pd and Pt (relatively low Ir) contents; (4) precipitation from seawater: high Pt and low Pd contents, also high Cd and Ag contents under anoxic conditions; and (5) reduction from intermediate and low-temperature solutions: high PGE and Cu (or Mo, U, Ni) contents.

Introduction Occurrences and behaviour of the platinumgroup elements (PGE), especially iridium, have been gaining increasing attention since the discovery of the Ir-rich anomaly at the Cretaceous/ Tertiary boundary (KTB) at Gubbio (Italy) by Alvarez et al. (1980). Because Ir is highly depleted in the Earth's crust, the authors explained the anomaly by the impact of a large asteroid, which also initiated a global catastrophic extinction. Since then, discussion on the origin of this anomaly has been dominated by extraterrestrial (see Alvarez, 1986) vs. volcanic hypotheses (see Officer and Drake, 1985; Hallam, 1987), although alternative mechanisms (precipitation from seawater, microbial activity, metal-rich pore solutions rising in the sedimentary sequences) have also been sug0031-0182/93/$06.00

gested (e.g. Strong et al., 1987; Schmitz et al., 1988; Dyer et al,, 1989). During the last few years, Ir and other P G E anomalies have been discovered not only at K / T boundary sites but also in m a n y other horizons, which may or may not be related to several global extinction events. The P G E represent a generally coherent group of siderophile elements, but on the basis of association the P G E may be divided into two groups: the Ir g r o u p - - c o n s i s t i n g of Ir, Ru and Os; and the Pt g r o u p - - c o n s i s t i n g of Pt, Rh, Pd (and Au) (Barnes et al., 1985). There are no comprehensive data on all the P G E in sedimentary rocks, however, sparse data suggest that their ratios vary significantly. The main objective of this study is to discuss the complex nature of P G E enrichments, to present alternative concentration processes (both syndepositional and postdepositional and to review the

© 1993 - - Elsevier Science Publishers B.V. All rights reserved.

254 occurrence and behaviour of PGE in sedimentary environments and the problems in the use of PGE as geochemical markers. Distribution of PGE concentrations in time

PGE anomalies (especially Ir) have been studied extensively in specific stratigraphic horizons, like the KTB, and were identified at many other locations. An iridium anomaly (up to 2 ppb) was found in the Acraman impact ejecta horizon of late Precambrian age in Australia (Gostin et al., 1989). Upper Proterozoic black shales in the Bohemian Massif, which are closely associated with submarine volcanic activity (Pasava, 1991), are enriched in PGE (max. values: Pt, 9 ppb; Pd, 102 ppb; Rh, 1.7 ppb; Au, 130 ppb). High concentrations of Ir and Os in Precambrian-Cambrian (P/C) sediments of China have been attributed either to impact (Hs/i et al., 1985) or nonmeteoritic (Zhou and Kyte, 1987) sources. Zhang et al. (1985) have found Ir enrichments (up to 4 p p b - - 2 0 times the background value) in a P/C boundary clay at four localities in China. Thin, apparently syngenetic sulphide (Ni-Mo) beds directly above or close to the P/C boundary have enrichments of Au (0.7 ppm), Pt (0.3 ppm), Pd (0.4 ppm), Ir (30 ppb), as well as other PGE (Coveney and Nansheng, 1991). Small enrichments in Ir (max. 230 ppt, 10 times the background), probably indicative of low sedimentation rates, have been found at the Ordovician/Silurian boundary in China (Wang et al., 1992). Evans et al. (1991) have found a peak Ir value of 5.6 ppb, and Ru/Ir ratios about 1.0 at the Frasnian/Famennian (F/F) boundary, suggesting an extraterrestrial source. Small Ir anomalies (max. 230 ppt) are also present at the F/F boundary in South China, caused probably by reducing conditions or an oceanic impact (Wang et al., 1991). Anomalous contents of Ir (about 20 times the local background) and Pt are associated with beds formed by the cyanobacterium Frutexites (Playford et al., 1984). This occurrence in Western Australia is in the erepida zone, i.e. later than the F/F boundary, and is not associated with a major

z. SAWLOWlCZ extinction event. Similar PGE-rich (max. values: 1.1 ppb Ir; 14 ppb Pt; 1.2 ppb Ru) ferruginous stromatolitic beds occur in the Early Cambrian and in the Oligocene of Australia. The PGE have probably been derived from seawater (Wallace et al., 1991). Tredoux et al. (1987) found an enrichment in PGE relative to average shales in Tournasian shales from Ireland. Ni-Zn mineralization in thick black shale sequences of Devonian-Permian age from the Selwyn Basin (Western Canada) is characterized by very high concentrations of PGE (max. values: Pt, 510 ppb; Pd, 210 ppb; Re, 60 ppb; Au, 86 ppb) (Coveney and Nansheng, 1991). Orth et al. (1988) described four PGE anomalies from the Lower Mississippian of Oklahoma. Concentrations reach 0.56, 150, 0.51 and 18 ppb of Ir, Pt, Os and Au, respectively. Different origins are proposed for the individual anomalies, ranging from adsorption by sulfide- and organic-rich shale particles, through precipitation from seawater by bacteria, to accumulation of detrital material from erosion of ultramafic source rocks. All strata accumulated at very slow rates of deposition. Anomalous concentrations of PGE (Ir--0.4 ppb, O s - - 4 ppb, P t - - 6 ppb), together with Cr and U, have also been described from the Mississippian/Pennsylvanian boundary in Oklahoma and Texas. These anomalies are associated with an increase in the phosphate contents of the rocks and may be related to increased upwelling, stagnation, or nearby submarine volcanism (Orth et al., 1986). The Zechstein copper-bearing shale (Kupferschiefer) in Poland is characterized by high concentrations of PGE. Locally present are millimeter-thick precious metal-bearing shales containing up to 1000ppm Pd, 340ppm Pt and 3000 ppm Au (Kucha, 1990) (Table 1). The source of metals could be oxidizing brines leaching the molasse lithologies underlying the Kupferschiefer. Very variable, generally minor enrichments (max. values: Ir, 1.0 ppb; Os, 5.2 ppb; Au, 3.2 ppb) at the Permian/Triassic (P/T) boundary are reported from China, Pakistan, India and the Alps (Xu and Yan, 1991; Bhandari and Shukla, 1991). Anomalous Ir concentrations were registered in Lower/Middle Jurassic (Rocchia et al., 1986) and

255

lr AND OTHER Pt-GROUP ELEMENTS AS GEOCHEMICAL MARKERS

TABLE I Abundances of PGE from selected sources Ru Seawater CI chondrites 712 Iron meteorites 7.3 Mantle nodules Ultramafic rocks 6.1 Manganese nodules -3.8 Pelagic sediments Volcanic dust bands, Antarctica Kilauea particles K/T Boundary -43 Kupferschiefer, North Sea Kupferschiefer, mineralized, Poland - 52 Black shales, China 16 Sulphide beds within black shales, China 23 Coals -43 Deformed coals - 130

Rh

Pd

Re

Os

Ir

134

560 4.5 4.5 6.7 5.3 3.7

36 0.4 0.3

486 5.4 4.0 5.l -2.4 0.14

- 12

- 110

- 59

-49

481 4.0 4.1 3.8 -8.5 - 2.0 -7.5 630 -650 0.6

25 -2 -6l

- 1000 37 13 184 -70 -6 -6100

3.0

3 150 -17 -24

0.08 1.7 -0.8 -22

Pt

Au

0.9 990 9.4 6.7 9.1 -940 - 22

- 250 140 1.3 0.8

Re~.

7 ! 6 2 17 34 4,7,19,20 2.7 4,5,7 -28 13 16 - 131 -43 3,11,12,14,15,21 130 57 15 - 340 - 3000 8 15 48 18 295 334 18 -210 -38 9,10 > 10,000 - 140 9

Values for iron meteorites and Kupferschiefer in ppm, seawater in pg/l, the other values in ppb, -xx=maximum values, xx= average values. References: I: Anders and Grevesse (1989), 2: Sun (1982), 3: Ganapathy (1980), 4: Esser and Turekian (1988), 5: Crocker et al. (1973), 6: Crocker (1972), 7: Hodge et al. (1986), 8: Kucha (1990; pers. commun., 1991), 9: Van der Flier-Keller (1991), 10: Chyi (1982), ll: Martinez Ruiz et al. (1992), 12: Schmitz (1988), 13: Koeberl (1989), 14: Elliot et al. (1989), 15: Kyte et al. (1985), 16: Officer and Drake (1985) (calculated from Zoller et al. 1983), 17: Crocket (1981) (ophiolitic and Alpine peridotites tectonite), 18: Coveney et al. (1992), 19: Palmer and Turekian (1986), 20: Bekov et al. (1984), Alvarez et al. (1992).

M i d d l e / U p p e r Jurassic s t r a t a in Spain a n d P o l a n d ( B r o c h w i c z - L e w i n s k i et al., 1985, 1986). A L a t e Jurassic a n o m a l y in P o l a n d is a s s o c i a t e d with green clay a n d f e r r u g i n o u s s t r o m a t o l i t e s (up to 7 p p b Ir) o f b a c t e r i a l origin. A c o m p l e x source for this a n o m a l y was suggested: c o s m i c dust, s u b a q u e ous e x h a l a t i o n s , h y d r o t h e r m a l vein m i n e r a l i z a t i o n , a n d volcanic ash. A n o m a l o u s c o n c e n t r a t i o n s o f P G E (up to 19.2 p p b Ir) were f o u n d in a 5 c m thick layer o f p h o s p h a t i c limestone at the J u r a s s i c / C r e t a c e o u s b o u n d a r y o f N o r t h Siberia. A s h a r p r e d u c t i o n in s e d i m e n t a t i o n rate was the possible cause o f the P G E - r i c h c o s m i c d u s t enrichment. Wezel et al. (1981) have f o u n d an Ir a n o m a l y at the C e n o m a n i a n / T u r o n i a n (C/T) b o u n d a r y at Bonarelli (Italy). T h e a u t h o r ' s p r e l i m i n a r y d a t a on samples f r o m C / T b o u n d a r y shales at C i s m o n (Italy) s h o w t h a t the shales are enriched in P G E . Small Ir e n r i c h m e n t s (only 0.1 p p b , b u t 10 times b a c k g r o u n d ) were r e p o r t e d b y O r t h et al. (1987) j u s t b e l o w the C / T b o u n d a r y in C o l o r a d o . O v e r 75 C r e t a c e o u s / T e r t i a r y b o u n d a r y sites with

a n o m a l o u s l y high Ir ( a n d o t h e r P G E ) concent r a t i o n s are k n o w n o n a g l o b a l scale (Alvarez, 1986). T h e origin o f this a n o m a l y is the subject o f a h o t dispute. T h e i n t e r p r e t a t i o n s v a r y as m u c h as the m a g n i t u d e o f the a n o m a l y itself (from b e l o w 1 p p b up to 580 p p b Ir, Table 1). T h e s u p p o s e d d u r a t i o n o f a b n o r m a l Ir a c c u m u l a t i o n ranges f r o m less t h a n one y e a r ( A l v a r e z et al., 1980) u p to 250 k y r ( R o c c h i a et al., 1984). T h e p r o p o s e d P G E sources range f r o m a s t e r o i d s (e.g., A l v a r e z et al., 1980), t h r o u g h v o l c a n o e s (e.g., Officer a n d D r a k e , 1985), p o s t d e p o s i t i o n a l s o l u t i o n s rising into the s e d i m e n t (e.g. Schmitz, 1985), m i c r o b i a l activity (e.g. Schmitz et al., 1988), to seawater (e.g. S t r o n g et al., 1987). A n a n o m a l o u s (relative to the b a c k g r o u n d ) value o f 0.41 p p b Ir, resulting f r o m a m a j o r b o l i d e i m p a c t , was o b s e r v e d at the E o c e n e / O l i g o c e n e b o u n d a r y in C a r r i b e a n sediments ( A l v a r e z et al., 1982). N o b l e m e t a l a n o m a l i e s (up to 20 p p b Ir), t o g e t h e r with m e t e o r i t i c particles suggesting meteorite i m p a c t o c c u r in U p p e r Piiocene deep-sea sediments f r o m the Pacific ( K y t e et al., 1988).

256

z. SAWLOWICZ

•. .". .~/~

.

• asteroid,

."...

_L comet,cbsl~ic

dust •

votca n i s m

.7.

fS's zation

t o w k intermed, temperature

exhcllotiVehydroterrnaI

leaching

Fig. I. Platinum-group elements in sedimentary environments-occurrences, sources and concentration processes.

Hosts and associations of PGE occurrences

The carriers or phases in which high PGE concentrations occur in sedimentary rocks are various and often unclear. They seem to depend on the source material, environment of deposition and postdepositional alteration processes. High concentrations of PGE occur in extraterrestrial spherules of submillimeter size which represent micrometeorites and ablation spherules. Koeberl and Hagen (1989) found that spherules contain 10->1000 ppb Ir, <200-1000 ppb a s and 19-225 ppb Au. In meteoritic debris from Upper Pliocene sediments, Kyte and Brownlee (1985) found 3150 ppb Ir and 3880 ppb Au in metal particles, and 140-209 ppb Ir and 20-970 ppb Au in vesicular particles. According to Crocket (1972), all PGE in iron-rich meteorites show a positive correlation, except for Pd which correlates well with other non-platinum metals like Ni, As, Au, Cu and Sb. Koeberl and Hagen (1989) suggest that the observed variations of siderophile ratios in spherules probably result from composition variations in the original micrometeorites or fractionation into sulphides, which were subsequently lost as volatile compounds during melting and quenching in the atmosphere. Bonte et al. (1987) report that PGE in cosmic iron spherules occur as

discrete metal phases ("nuggets") of two types, Irrich (Ir, as, Ru, Pt, Rh) and Pt-rich (Pt, Ru, Rh). Resistant Pt-metal nuggets were also observed in stony spherules (chondrules) (Brownlee et al., 1984). Native gold and platinum, as well as palladium alloys and Pd-arsenides, have been observed in the Zechstein Kupferschiefer (Kucha, 1981, 1982). Association of PGE with organic matter is very common. In coals, where levels of PGE are extremely variable (see Table 1), PGE occur as discrete particles of unknown nature and are organically associated (especially Pt) (Chyi, 1982; Van der Flier-Keller, 1991). Strong organometallic complexes of Ir and other PGE have been supposed to occur in the Kupferschiefer by Kucha (1981, 1982) and at the K/T boundary (KTB) shales by Schmitz et al. (1988) who found that as much as 50% of Ir may be present in kerogen (1100-1500 ppb). High concentrations of Pt and Pd (270 and 310ppm, respectively) were found in U-bearing organic matter (thucholite) from the Kupferschiefer (Kucha, 1981). There is a high degree of correlation between carbon black (soot) and Ir from the KTB (Wolbach et al., 1985). The carbon black from the KTB in Poland contains 16 ppb Ir and 244ppm Au (Hansen et al., 1989). Soot may originate from

257

lr AND OTHER Pt-GROUP ELEMENTS AS GEOCHEMICAL MARKERS

forest fires (Wolbach et al., 1985) or volcanic eruptions during which Ir-fluorides could be adsorbed onto the soot (Hansen et al., 1987). Wolbach et al. (1990) warn that soot may be confused with kerogen. PGE are commonly present in sulphides, arsenosulphides and arsenides. The PGE are dispersed in the crystal structure or occur as inclusions of < 200 ,~ in size (Chryssoulis et al., 1989). Different sulphides concentrate different platinum group elements and the absolute abundances of PGE in sulphides vary over a range of five orders of magnitude (Chyi and Crocket, 1976; Naldrett and Duke, 1980). Knowledge about PGE in sulphides and arsenosulphides from sediments is limited to mineralized black shales. In N i - M o ores from Cambrian black shales, PGE occur in sulphides and sulphosalts (vaesite, millerite, polydimite and jordisite) (Fan, 1983; Pasava, 1991), whereas in the Cu-bearing shales (Kupferschiefer) PGE occur as an admixture in Ni-diarsenides (Kucha, 1981, 1982). In a number of sediments, high PGE concentrations are associated with silicates and oxides. Ir occurs in the smallest grain size fractions of volcanic ash from blue ice fields in Antarctica (Koeberl, 1989), in the KTB Fish Clay (Elliot et al., 1989) and is closely associated with clay minerals in the KTB at Gubbio (Rocchia et al., 1990). Elliot et al. (1989) propose that Ir may be incorporated into smectite as it formed from vitric ash. Most of the Ir in the Acraman impact ejecta horizon (Australia) is carried either by the ejected clasts and/or by clasts of devitrified glass (Gostin et al., 1989). In some KTB locations, the iridium peak correlates with chromium and individual grains of chromite of probably ultramafic origin (Hansen, 1990). Chromite may have high contents of It, Os and Ru relative to Pt, Pd and Au (Wilson et al., 1991). PGE occur in inclusions within chromite (McElduff and Stumpfl, 1989), and Ir and Os may substitute for Cr in the chromite lattice (Agiorgitis and Wolf, 1984). At the KTB at Gossau (Austria) the high iridium content is associated with titanium minerals and with magnetite (Preisinger et al., 1986). Magnetite may be enriched in Ir relative to other noble metals (Chyi and Crocket, 1976). Magnesioferrite and Ni-rich spi-

nels, common at the KTB (Bohor et al., 1987; Robin et al., 1991), may also be carriers of PGE. The association of PGE with microbiologicallymediated processes (Dyer et al., 1989) and with magnetite, sulphides and organic matter suggests that a potential carrier of PGE could be bacterial magnetosomes, including magnetite and iron sulphides, which have been formed by magnetotactic bacteria (see Mann et al., 1990; Bazylinski et al., 1990). Rocchia et al. (1990) and Robin et al. (1991) proposed that Ir in KTB sections may not be attached to a particulate mineralogic phase but is present in a very dispersed fraction, i.e., on an atomic scale. During diagenesis the carrier of PGE may change, e.g. PGE are concentrated in clay minerals as a result of alteration of the original microtektite host (Wilson et al., 1991), and PGE are found in goethite and hematite spherules (26-72 ppb Ir) which are weathering products of diagenetic pyrite concretions (Brooks et al., 1985). The decomposition of organic matter and bacteria may release organically bound PGE which are then incorporated into diagenetic iron sulphides. Extraterrestrial sources

The rarity of some PGE, e.g. Ir and Os, in the Earth's crust (about 0.03 ppb Ir as compared with 500 ppb in carbonaceous chondrites, Table 1), make extraterrestrial materials an attractive source of PGE. Some PGE have been used as tracers of meteoritic input (Luck and Turekian, 1983; Kyte and Wasson, 1986). For example, Esser and Turekian (1988) calculated that 62-70% of Os in pelagic clay sediment is extraterrestrial. Kyte and Wasson (1986) estimate that the extraterrestrial Ir influx is 9 _+3 ng cm- 2 Myr- 1. An extraterrestrial source of Ir and other PGE anomalies at the KTB and other boundaries has been proposed by many authors. Among extraterrestrial matter reaching the Earth are asteroids, comets and sub-millimeter dust particles. The latter originate probably from the ablation of both asteroids and comets. The continual rain of micrometeorites results in a relatively constant background accumulation rate of

258 Ir (and probably other PGE) in marine, especially deep-water, sediments (Kyte, 1988). This background was periodically modified by large asteroids and comets (Alvarez et al., 1980; Hut et al., 1987) which perhaps caused world-wide PGE anomalies. Kyte and Brownlee (1985) suggest that impacts of small asteroids ( < 1 km) should be very common in Earth's history, and the sedimentary record must contain numerous localized horizons with Ir and other PGE enrichments, like the one in Late Pliocene Pacific sediments. Extraterrestrial materials may supply PGE to seawater not only in detrital form, but also in dissolved form as a result of vaporization of dust in the atmosphere and shock vaporization of impacting large asteroids (Kyte, 1988). PGE-rich material may originate both from an asteroid itself and from its impact ejecta. An oceanic asteroid impact results in relatively "pure" extraterrestrial material, whereas during a continental impact some target material is ejected from the center of the developing crater. Typically PGEenrichments in sedimentary rocks are a mixture of these two sources (e.g. Acraman impact ejecta horizon; Gostin et al., 1989). In such a case, the abundances of PGE and their ratios depend on asteroid composition (chondritic, achondritic, iron, Table 1), on the type of target rocks (PGErich or PGE-poor), and on the relative masses of asteroid and ejecta materials. Volcanic and hydrothermal sources

Volcanism has been suggested to be a major process of Ir enrichment at the K/T boundary (Officer et al., 1987; Hallam, 1987 and references herein). The enormous amounts of Deccan Trap basalts, erupted during the magnetic zone embracing the KTB (Courtillot et al., 1988), could have supplied sufficient amounts of iridium (Olmez et al., 1986), but supporting evidence is lacking. The content of Ir in Deccan basalts is low (below 20 ppt) (Bhandari and Shukla, 1991). These authors have found only low Ir contents (below 275 ppt) at all but one KTB section in India and assumed that the Ir contribution by the Deccan volcanism was small. However, unusual iridium enhancements have been found in aerosols emitted

z. SAWLOWlCZ by the Kilauea volcano (6.4 ppb Ir) (Zoller et al., 1983; Olmez et al., 1986), and by the hot-spot volcano on the island of Reunion (up to 7.5 ppb Ir in sublimates, Toutain and Meyer, 1989). Volcanic ashes containing up to 7.5 ppb Ir have been described from the blue ice fields of Antarctica (Koeberl, 1989). Felitsyn and Vaganov (1988) found up to 4 ppb Ir in ashes from acidic volcanics in Kamchatka. Ir concentrations both in aerosols and ashes correlate well with Se, Au, TI, and are also associated with high contents of As and Sb. This relationship is common in most KTB sediments. Toutain and Meyer (1989) suggest that Ir can be released from hot-spot volcanoes as gaseous fluoride compounds. The enrichments of Ir in sulphate and fluorhydroxide-rich condensates (some of which are water soluble) from Kilauea were up to 75 times higher than in Kilauea basalts (Crocket and Kabir, 1987). PGE may be transferred to sedimentary environments also as PGE carbonyls within volcanic aerosols (Tredoux et al., 1991; McDonald et al., 1992). Such iridium-rich volcanic condensates, derived probably from a deep magma source, could be adsorbed onto inorganic and organic particles. Deposition of such particles, postdepositional release of Ir, or dissolution in seawater could be responsible for some elevated concentrations of PGE, especially of iridium. Hydrothermal enrichments of PGE have been suggested for different anomalous concentrations in sediments. It should be stressed here that although PGE may be transported by hydrothermal fluids, the behaviour of individual elements is very different, depending on temperature and fluid composition. Pd and Pt are more soluble at low and intermediate temperatures, whereas Ru, Rh, Os and Ir are among the least soluble PGE (McCallum et al., 1976; Paktunc and Gandhi, 1989). Mountain and Wood (1988) suggest that Pt and Pd may be transported by some hydrothermal ore-fluids as chloride complexes, whereas in other ore-fluids they may be present as bisulphide and/or hydroxide complexes. Submarine volcanic exhalations may contribute significantly to the Pd and Au contents in deep-sea sediments (Crocket et al., 1973). Salton Sea geothermal brines carry up to about 1 ppb each of Au, Pt, Pd, and Rh at

lr AND OTHER Pt-GROUP ELEMENTS AS GEOCHEMICAL MARKERS

temperatures near 300°C and near-neutral pH (McKibben et al., 1990). A strong PGE-Au fractionation has been observed from mid-ocean ridge settings. For example, the Au/Ir ratio in basalts is 20, whereas it is above 1000 in black smoker precipitates (Crocket, 1989). The Ir, Pd and Au contents in the latter are typically 0.2, 2 and 100 200 ppb, respectively. Thus, anomalous Ir concentrations, if not matched by high Pd (and Pt) contents, should not be related to exhalativehydrothermal activity. Continental sources

The main sources of PGE are mafic and ultramafic rocks (Table 1), and associated ore deposits, like Sudbury, Noril'sk, or Merensky Reef. Erosion and run-off may supply PGE directly, both in detrital or hydrogenous form, to the oceans, or to molasse lithologies from which PGE can be leached during postdepositional processes. The role of weathering in supplying PGE to sediments is difficult to evaluate. A number of different processes, like oxidation, hydration, effect of sulphuric acid generated by weathering of sulphides, effect of saline brines, presence of lower oxidation state oxyanions of sulphur, and attack by organic acids, influence weathering and some of these processes may participate in the mobilisation of PGE (Fuchs and Rose, 1974; Bowles, 1986; Plimer and Williams, 1987; Jaireth, 1992). It is interesting to note that, on the basis of a strontium isotope study, Martin and Macdougall (1991) suggest enhanced weathering by global, highly acidic rain at the Ir-rich K/T boundary. Data on the behaviour of different Pt-group metals during weathering are often contradictory, although inertness of Ir (and to a lesser extent Ru) is commonly reported during these processes (McCallum et al., 1976). Weathering does not lead to significant enrichments. For example, Crocket et al. (1973) describe only small enrichments of Pd and Au, and even less for Ir, in weathering products of basalts. Low-temperature depositional processes

The extent to which PGE can be mobilized and transported at low temperatures is currently being

259

investigated. Preliminary data suggest that oxidizing, acidic to neutral, saline solutions containing chloride and organic complexes are the most plausible transporting agents for PGE (Bowles, 1986; Goldberg, 1987; Jaireth, 1992). Changes in the above conditions, especially rapid lowering of Eh, lead to PGE precipitation and deposition. Sources of PGE in seawater may be both extraterrestrial and terrestrial, and in both cases PGE may occur in dissolved and detrital form. Contributions from different sources have been estimated on the basis of Os/Ir ratios or the content of cosmic spherules (Esser and Turekian, 1988; Halbach et al., 1989). The best example of PGErich (except Pd) oceanic deposits are ferromanganese crusts and nodules. Pt, Pd, Ir, Ru and Os concentrations in ferromanganese nodules show similar relationships in pelagic sediments (Koide et al., 1991) (Table 1). Typically Fe-Mn minerals are especially enriched in Pt. Precipitation from seawater involves several processes; coprecipitation with manganese oxides, surface adsorption of anionic Pt-tetrachloro-complexes onto positively charged amorphous iron hydroxides (Halbach et al., 1989), and uptake of reducible elements by organic films sorbed on surfaces of Fe-Mn minerals (due to bio- or abiological reduction) (Koide et al., 1991). The seawater Pt/Ir ratio is found only in nearly pure hydrogenous Fe-Mn deposits (Goldberg et al., 1986). Oceanic residence times are 0.04 and 1 Myr for Ir and Pt, respectively, which suggests that Ir is more susceptible to scavenging (Hodge et al., 1986; Goldberg et al., 1986). The contribution of PGE-rich spherules of cosmogenic origin to the PGE budget is often significant (McMurtry et al., 1989; Halbach et al., 1989). Ferruginous microstromatolites of Cambrian, Devonian and Oligocene age in Australia are similar to deep-sea ferromanganese crusts. They are also enriched in Pt and Ir, probably derived from seawater as the result of inorganic or biological processes (Playford et al., 1984; Hurley and Van der Voo, 1990; Wallace et al., 1991). However, seawater could also temporarily have an abnormal composition, being enriched in some metals as the result of impact- or volcanism-related processes and their degradation products. The depositional rate plays a significant role in

260 PGE concentration. Values of 0.1-0.5 ppb Ir or Os in deep-sea sediments are common and due to low sedimentation rates (Barker and Anders, 1968). Ir-rich sediments at the K/T boundary in deep-sea sections are interpreted to represent extremely condensed intervals or hiatuses (MacLeod and Keller, 1991). Wallace et al. (1991) suggest that Ir-rich (up to 3 ppb Ir) Cambrian and Oligocene stromatolitic beds in Australia represent periods of condensed marine sedimentation. At low sedimentation rates normal cosmic influx alone can account for some Ir anomalies. High concentrations of cosmic spherules, which can be the carriers of PGE accumulated on hard ground surfaces at the Middle/Upper Jurassic boundary during times of low sedimentation rates (Czajkowski et al., 1982; Zolensky and Murali, 1985). Wang et al. (1992) predict 0.34 ppb Ir at the O/S boundary in China assuming a sedimentation rate of 0.35 mm/ka whereas the maximum observed value was 0.23 ppb Ir. Precipitation of Ir from seawater at the Danish KTB has been suggested, on the basis of very high REE contents, by Schmitz et al. (1988). Multiple Ir-anomalies at the KTB in the Bavarian Alps may have been preserved due to high sedimentation rates and due to the absence of carbonate dissolution (Graup and Spettel, 1989). However, the source of Ir in such sediments had to be different from that in normal deep-sea sediments. Anoxic conditions in sea- and pore-water during sedimentation and early diagenesis can favour accumulation of PGE. Koide et al. (1991) have described PGE enrichments in phosphorites and anoxic coastal deposits formed under reducing conditions. In Danish KTB strata iridium concentrations range from 0.003 ppb at Kjolby Gaard (no clear indication of anoxic eonditions) to about 500 ppb Ir in the Fish Clay at Stevns Klint, which was deposited in shallow anoxic parts of the Danish Maastrichtian sea (Ekdale and Bromley, 1984). Extensive reducing conditions may lead to a reduction in the oceanic residence time and enhance deposition of some siderophiles (Kyte et al., 1980), mainly by their incorporation in iron sulphides, which in turn can limit Pd and Pt mobility (Wood et al., 1989). Rhenium, unlike other noble metals, seems to occur in seawater in

z. SAWLOWlCZ comparably high concentrations as unreactive perrhenate ion, which under strongly reducing conditions is removed and precipitated as an insoluble tetravalent sulphide or oxide (Goldberg, 1987). PGE enrichment may result from a combination of the factors mentioned above, as well as from an increase in organic matter content, due to upwelling and higher productivity. The Mississippian/Pennsylvanian boundary in North America may serve as an example. Here the increase in the concentrations of Ir, Pt and Au could have been caused by low sedimentation rates, upwelling and reducing conditions facilitating precipitation of these siderophiles as metals or insoluble sulphides (Orth et al., 1986). Immobilisation of PGE in organic-rich reducing sediments is common. For example, Dissanayake and Kritsotakis (1984) describe enrichments of Pt (up to 5 ppm) at a sudden change from oxic to anoxic conditions in polluted river sediments. Plimer and Williams (1989) describe precipitation of oxidized chloro and/thio complexes of PGE at an organic redox barrier in salt lakes. In specific cases, for example in salt lakes, a significant increase of PGE content may also. result from evaporation (Plimer and Williams, 1989).

Biological processes The important role of biological processes in concentrating PGE from seawater has been suggested by a number of authors (e.g., Playford et al., 1984; Schmitz, 1985; Wallace et al., 1991). The microbial role in the enhancement, erasure, dispersal and creation of certain sedimentary iridium anomalies has been documented in experiments by Dyer et al. (1989). Recently, Watterson (1992) described the process of gold biomineralization by Pedomicrobium-like bacteria. Some bacteria and fungi have the ability to dissolve iridium both from igneous rocks and from meteoritic materials (Dyer et al., 1989). The mobilized PGE (both by biological and inorganic action) are transported into a sedimentary environment, where they may be concentrated by living or dead cyanobacterial mats, and some other bacteria and fungi (Dissanayake and Kritsotakis, 1984; Dyer et al., 1989). Microorganisms may also

lr AND OTHER Pt-GROUP ELEMENTS AS GEOCHEMICAL MARKERS

dissolve Ir during diagenesis and redeposit it in adjacent sediments (Dyer et al., 1989). Magnetotactic bacteria, producing biogenic magnetite and iron sulphides (Mann et al., 1990; Bazylinski et al., 1990) and Fe(III)-reducing bacteria (Lovley et al., 1990) may also contribute to the PGE cycle.

Postdepositional processes Extensive postdepositional PGE mobilization in sediments can be generated by low-temperature aqueous solutions, hydrothermal fluids, and oxidation fronts. The mechanism of PGE behaviour and transport in low-temperature solutions is still poorly known, but chemical fractionation among the PGE certainly occurs. Diagenetic remobilization may produce different dispersal patterns around primary PGE anomalies. Commonly, PGE form postdepositional halos or are partly reconcentrated in neighbouring beds (typically clays) by low-temperature fluids from PGE-rich horizons (e.g., impact ejecta horizon, Wallace et al., 1990; Crocket et al., 1988; Graup and Spettel, 1989). Redistribution seems to be more prominent for Pt, Pd, Au than for Ir. Several Pt anomalies at the Stevns Klint KTB were not associated with other PGE or siderophile anomalies. On the basis of higher Pd mobility, relative to Pt, Tredoux et al. (1987) suggest that these anomalies may be primary signatures. Micro-scale redistribution may result from diagenetic degradation of organometallic compounds or the presence of radioactive minerals. For example, Kucha (1981, 1982) attributes reconcentration of PGE in thucholite from the Zechstein Kupferschiefer to catalysed oxidation of organic matter by v-radiation and Pt-group metals. Remobilization appears to be absent in the continental KTB facies of North America. Samples taken from above and below the Ir-rich KTB coal bed, including the bases of similar coal beds, showed no evidence of Ir enrichment (Gilmore et al., 1984). Progressive oxidation fronts migrating downward into reducing sediments can lead to appreciable metal enrichments (Wilson et al., 1986; Pruysers et al., 1991). The experiment by De Lange et al. (1991) has shown that a sharp Ir anomaly may be

261

explained by post-depositional mobilization and redox-controlled precipitation at distinct depth levels within the sediment. Differential stratification of many elements may occur over the thin vertical extent of the oxidation front (Wallace et al., 1988). The often observed different positions of Ir, Pt, Pd and other PGE anomalies across vertical profiles of the KTB may result from the mechanism described above. Colodner et al. (1992) report redistribution of Pt, Ir and Re due to changes in sedimentary redox conditions during early diagenesis. PGE enrichments typically occur in shaly and organic-rich beds, associated with thick complexes of sandstones or carbonates. Organic-rich sediments serve as low-permeability redox boundaries. The movement of Ir towards such a redox boundary can be both upward and downward (Gilmore et al., 1984). Reduction of PGE by sedimentary organic matter has been described by Baranager et al. (1991). Higher concentrations of PGE in coal splits from Canada were caused by leaching of overlying volcanic units. The enrichments may be related to faults and fissures acting as channels for postdepositional PGE-rich hydrothermal fluids or ground waters. Van der Flier-Keller (1991) has found in deformed samples of coals (related to faults) high concentrations Ir and Pd, in contrast to other PGE and Au (Table 1). She suggests that ground waters infiltrating the Pt-rich Tulameen Ultramafic Complex may carry PGE which precipitate on contact with the reducing environment of coals. Reducing diagenetic conditions may also influence earlier PGE-rich minerals. Colodner et al. (1992) found that significant amounts of Ir and Pt in Fe-Mn oxyhydroxides may be remobilized if oxides are reduced. Regional enrichments may be caused by basinal fluids. Such brines, responsible for Mississippi Valley-type Pb-Zn mineralization, are capable of mobilizing PGE (Macdonald, 1987; Lechler and Hsu, 1989; Coveney and Nansheng, 1991). McKibben et al. (1990) report that saline, relatively oxidized hydrothermal fluids can transport much greater amounts of PGE than predicted by theoretical models. Extremely favourable conditions for PGE concentrations seem to be black shales, underlain by red beds. Sediment-hosted, strata-

262

z. SAWLOWlCZ

bound Cu-Ag deposits in Namibia and Poland contain high concentrations of PGE and Au (Tables 1 and 2), leached from underlying red beds and precipitated at the redox-boundary in overlying black shales (Borg et al., 1987; Kucha, 1982). Au, Pt and Pd are also present in high concentrations in base metal deposits of the African Copper Belt, in association with organic carbon and uranium (Unrug, 1985). In the Zechstein copper-bearing shale of Poland, PGE (mainly Pt and Pd, also Au) are especially concentrated in a millimeter-thick lowermost layer of black shales (typically rich in U and Ag), at the contact with underlying white sandstones (Kucha, 1982). Leaching of red beds and transport of base metals from these deposits probably depends on the presence of oxidizing, chloride-rich solutions (Lur'ye, 1977). Mobilization of gold in such solutions and precipitation at a reduction barrier was described by Schade (1987). Recently, Jaireth (1992) presented a model of transport for PGE, Au and U as chloro-complexes, by oxygen-saturated saline fluids and their precipitation at an oxidationreduction interface. Anthony and Williams (1989) found that thiosulphates are very effective in the dissolution of PGE. Thiosulphates and sulphites were suggested by Kucha and Piestrzynski (1991) as possible compounds capable of accumulating metals and sulphur in the Kupferschiefer. The stability fields for thiosulphate and chloride complexes are complementary: the former is stable at a low Eh and high pH and the latter is stable at a high Eh and low pH (Anthony and Williams, 1989). Wallace et al. (1990) observed that where an ejecta horizon is enriched in Cu, elements like

Au, Pt and Pd are greatly enriched relative to Ir and Ru. This observation suggests that copper may be used as a tracer of some PGE mobilization processes. The geochemical distribution of PGE is influenced by pH, Eh, chloride concentration, organic acid content and mode of occurrence in the primary source (Ling Ong and Swanson, 1969; Fuchs and Rose, 1974; Jaireth, 1992). A possible model of enrichment involves: diagenetic leaching and release of PGE metals (together with Cu and Ag, and possibly Zn and Pb) from magmatic rocks and detrital minerals of a molasse succession; retention and transport in chloride-rich basinal brines; precipitation at the contact with black shales as the result of reduction by organic matter, formation of organometallic compounds and coprecipitation with sulphides. PGE concentrations are observed in oxidized zones of reducing sediments and in oxidation products of sulphides. Oxidation may cause redistribution of platinum metals, for example degradation of PGE-rich organic compounds and precipitation of native metals and their alloys (Kucha, 1981, 1982). PGE enrichment may be the result of removing part of the primary components, or the result of supplying mineralizing solutions to a redox boundary, as for example iron oxidestained laminae and wisps in the KTB black Fish Clay (Elliot et al., 1989) and iron oxide spherules that probably resulted from oxidation of primary iron sulphides, at the KTB in New Zealand (Brooks et al., 1985). These materials are highly enriched in Ir and noble metals, relative to nonoxidized counterparts. The secondary oxidized

TABLE 2 Abundances of some PGE in the Permian Kupferschiefer(bulk organic-rich shale) in Poland (weighted average) Facies

TOC (%)

S (%)

Cu (%)

Re Au Pt Pd (ppm) (ppb) (ppb) (ppb)

n

Cu-rich 13.8 Rote Faule primaryoxidized 0.5 Rote Faule secondaryoxidized* 4.1

4.5 0.03 0.21

18.3 0.1 0.5

8 0.2 2

10 2 6

25 <5 540

<5 <5 280

6 2 60

Analyses:Au, Pt and Pd = PbS-fireassayand ICP-MSor INAA, Re= ICP-MS, TOC (total organiccarbon) and S (total sulphur)= coulometric,Cu = XRF, n = number of analyses,* = primarilyprobably Cu-rich.

lr AND OTHER PI-GROUP ELEMENTS AS GEOCHEMICAL MARKERS

zones of the Zechstein black shales, adjacent to Cu-rich shales, are up to 50 times richer in Pt, Pd and Au than the primary Cu-rich zones (Table 2). These zones are significantly depleted in Re (Table 2), which agrees with the observation of Kyte et al. (1985) that Os and Re tend to be leached during oxidation. A long-term circulation of oxygen-rich mineralizing solutions (enriched in PGE) across these zones could be responsible for the observed concentrations. The other possibility is that oxidizing solutions migrating across extensive sulphide-rich zones leached PGE from sulphides. Oxidation of sulphides (and organic matter) favours the reduction of the PGE to elements (Westland, 1981). These elements could be transported with organic ligands (Plimer and Williams, 1989) and reprecipitated at the contact of an oxidizing front with reducing black shales. The balance between oxidizing and reducing conditions in black shales seems to be very sensitive and the mechanism of PGE mobilization and precipitation under such conditions is still poorly known. Discussion of PGE concentrations in sedimentary environment The foregoing chapters illustrate that PGE anomalies may not represent a single event but many processes. Furthermore, these processes may be closely related to each other. The problem is best illustrated by the K/T boundary where the following events or processes could be responsible for the observed PGE concentrations: impacts of asteroids, comets, extensive volcanism, tsunamis, acid rain enhancing erosion, different sedimentation rates, anoxic conditions, abnormal seawater composition, microbial activity and postdepositional processes of enrichment or dispersion of syndepositional concentrations. Contributions from different PGE sources may be proposed using ratios between individual PGE and associated elements. However, it is very difficult to estimate the relative contributions from these sources, except for some "pure" cases. Os/Ir and Ru/Ir ratios, suggested as indicators of extraterrestrial materials (Kyte et al., 1985; Evans et al., 1991; Wang and Chai, 1991), seem to reflect relatively well the primary source. Wang

263

et al. (1992) studied several stratigraphic boundaries and found that the Os/Ir ratio at K/T boundaries is similar to this ratio in the solar system (close to 1.0) while the Os/Ir ratios from the PC/C, D/C and P/T boundaries vary from 10 to 30. However, several pitfalls in the use of inter-element ratios of PGE should be considered. Os/Ir ratios vary for some KTB sections (Smit and Hertogen, 1980; Bhandari and Shukla, 1991) and the Os/Ir ratios in terrestrial rocks and meteorites are similar (Agiorgitis and Wolf, 1984; Esser and Turekian, 1988). Although Ir, Os and Ru are the least mobile elements, there are probable differences in their solubility and behaviour under different Eh conditions. While a strong influence of microorganisms on Ir behaviour has been found (Dyer et al., 1989), little is known about their influence on Os, Ru and Rh. The 1 8 7 0 s / 1 8 6 O s ratio has the capability of distinguishing between continental (ratio of 10) and meteoritic or mantle sources (ratio of I) (Luck and Turekian, 1983). Unusual concentrations of Re (lSTRe, the radioactive source of 18~Os), present in anoxic environments or at diagenetic redox boundaries, may disturb the 187Os/iS6Os ratio significantly. However, the expected values would be shifted even more away from meteoritic or mantle values. Neither Os/Ir nor iSVOs/lS6Os ratios are able to distinguish between extraterrestrial and mantle sources of PGE, but the Ru/Ir ratio may be useful in that case, because mantle values seem to be higher than chondritic ones (Evans et al., 1991). Typically the primary chondritic ratios can be substantially modified by terrestrial processes. Tredoux et al. (1989) found that none of the K/T sites studied had chondritic PGE patterns and suggested modification of the original pattern by secondary terrestrial processes. However, good agreement of chondrite-normalized PGE abundances in sediments and meteorites can be a good indicator of extraterrestrial origin. Nonchondritic ratios of siderophile elements may result from a nonchondritic projectile composition, vapour fractionation during impact, contribution of ejecta material, weathering and postdepositional remobilisation of individual elements. The unusually high concentrations of Ir, compared to the other metals, at the KTB is

264 unique among all other stratigraphic boundary strata. The anomaly appears to be the result of complex terrestrial processes triggered by an extraterrestrial impact. The latter seems to be well illustrated by a PGE anomaly in the KTB profile from South Pacific pelagic clays (Zhou et al., 1991). At this site, high concentrations of Ir (max. 14 ppb), occurring together with relict highpressure and high-temperature mineral phases, are not matched by high concentrations of other siderophiles and chalcophiles (e.g., As and Co), like at other KTB sites. In summary, Ir in sedimentary rocks is probably still the most sensitive and convenient (easily detected by neutron activation) geochemical tracer of extraterrestrial matter, but a high Ir content alone cannot be used as a unique marker. Variations in the concentration of PGE in sedimentary environments may have been caused by differences in source, mobility, reactivity dependent on Eh and pH, and by microbiological processes. Concentrations of Pt, Pd and Au very often do not coincide with those of Ir (Crocker et al., 1988; Gostin et al., 1989). In some cases the Ir peak is sharper and offset relative to other PGE anomalies due to remobilisation and/or different sources. High concentrations of Pd, Au and Pt suggest exhalative-hydrothermal activity, whereas high Pt and low Pd concentrations suggest precipitation from seawater (Koide et al., 1991). Enrichment of PGE together with anomalously high concentrations of Cd and Ag may suggest precipitation from seawater under anoxic and low depositional conditions. Koide et al. (1986) suggest that Re can be helpful in identifying reducing depositional environments, but leaching of large volumes of sediments and precipitation at the redox boundary could also lead to high concentrations of Re. High PGE contents, coupled with increased phosphate content may suggest high organic productivity under upwelling conditions (Orth et al., 1986). Contributions from volcanic processes may be invoked by a high Ir content coupled with high Se, As, Sb and In contents. Such a positive correlation has been reported from KTB sediments by Graup and Spettel (1989). At the KTB, As/Ir and Sb/Ir ratios are three orders of magnitude greater

z. SAWLOWICZ than chondritic values, but are in accord with a mantle origin (Officer and Drake, 1985). High concentrations of Pd, Pt and Au, together with a high content of Cu, and perhaps Re, may suggest a contribution from basinal intermediate and low temperature solutions which leached large volumes of molasse-like lithologies and precipitated PGE, together with base metals, at a redox boundary. The association of PGE with Au and U (Hulbert et al., 1989; Jaireth, 1992) may suggest similar sources and mechanisms of enrichment to those of unconformity-related uranium deposits. According to some authors, the shape (sharp versus diffuse) of an Ir anomaly in a vertical sequence may suggest different processes of enrichment. However, the matter is much more complicated. A single sharp peak taken as evidence of an impact event (Alvarez et al., 1980), may in fact be an artifact of a temporally incomplete (or extremely condensed) deep-sea record (MacLeod and Keller, 1991), or may represent diagenetic enrichment at a sharp redox boundary (De Lange et al., 1991). A multiple or diffused peak may result from episodic or long-duration events, both extraterrestrial (Hut et al., 1987) or volcanic (Crocket et al., 1988), in a depositional setting characterized by high sedimentation rates, absence or presence of carbonate dissolution (Graup and Spettel, 1989; Rocchia et al., 1990); lateral and biological reworking (Alvarez et al., 1990; Zhou et al., 1991); remobilization (Wallace et al.,, 1990) or even Milankovitch cyclicity (Alvarez et al., 1990). The distribution pattern of PGE between different carriers can indicate their source and time of formation. Thus, the separation and identification of PGE carriers must be attempted. Extraterrestrial spherules, for example, are an important carrier of PGE, and identify both the source and its contribution. A high content of Ti and its common association with Ir are a useful marker to identify the KTB in the Western Interior of North America (Gilmore et al., 1984). The occurrence and amount of PGE-bearing detrital chromite grains may indicate contributions from ultramafic rocks. Sampling is crucial in detecting a PGE anomaly. PGE are often enriched in a thin layer within

lr AND OTHER Pt-GROUPELEMENTSAS GEOCHEMICALMARKERS

thicker strata and their concentrations may significantly vary laterally across the same outcrop. Tredoux et al. (1989) report that PGE patterns vary not only between K/T sites but also within layers at individual sites. If possible, sampling should be made in very small vertical intervals, even on a millimeter-scale, correlatable across the outcrop. Locally, remobilisation processes can cause an integrated profile to have siderophile abundances more closely approaching a chondritic pattern than do individual layers (Kyte et al., 1985). The complex occurrence and behaviour of PGE in sedimentary environments means that geochemical investigations cannot be limited to this group, but must incorporate other indicative elements and be coupled with detailed mineralogical analyses. Sedimentological and paleontological observations are necessary for a correct interpretation of PGE anomalies. The role of Ir and other PGE can be properly evaluated after more information has been obtained from other sedimentary environments, not just those related to catastrophic events. The most obvious targets for PGE anomalies are sharp lithological changes, especially black shales (redox boundary) associated with thick permeable oxic sequences; extremely condensed anoxic sediments, ferromanganese nodules and stromatolites; sediments associated with submarine volcanism. The extraordinary Cretaceous/Tertiary boundary is the best example to illustrate how difficult it is to identify the source and the processes that caused the observed anomalies. Very intense studies of the KTB over more than 10 years have produced an enormous amount of data and several interesting hypotheses but there is still no agreement on the source and the processes. The very high PGE, especially Ir, anomalies at some KTB sites could be caused by a meteoritic impact alone whereas at other KTB sites the anomalies could also be related to intense volcanism. The presence of smaller anomalies at many other stratigraphic levels could be linked to any of the processes and sources described in this paper.

Conclusions PGE anomalies may result from several different sources and processes, both syn- and postdeposi-

265

tional, acting separately or jointly. The sources and processes involved are: extraterrestrial (asteroid, comet, cosmic dust, impact ejecta); volcanogenic (PGE-rich condensates); precipitation from seawater (low sedimentation rate, anoxic conditions); microbial (concentration, dissolution, reprecipitation); exhalative-hydrothermal; leaching, transport and precipitation at a redox boundary (molasse-type lithologies; ultramafics). Anomalous concentrations of different PGE and associated elements can be used to identify contributions of specific processes: (1) extraterrestrial: chrondritic Ru/Ir and Os/Ir ratios, low 187Os/186Os ratio, high Ir content compared to Pt, Pd and Au; (2) volcanogenic: high iridium and Se, As, Sb and In contents; (3) exhalative-hydrothermal: high Au, Pd and Pt (relatively low Ir) contents; (4) precipitation from seawater: high Pt and low Pd contents, also high Cd and Ag contents under anoxic conditions; (5) reduction from intermediate and lowtemperature solutions: high PGE and Cu (or Mo, U, Ni) contents. PGE analyses should be accompanied by other elemental analyses, and by mineralogical, sedimentological and paleontological studies. More extensive investigations of all PGE in sedimentary environments, not only at stratigraphic boundaries, are needed to better understand the real value of PGE as geochemical markers.

Acknowledgements Linguistic help of A. Gize and A. Wodzicki, logistic help of w. The Macallan, and comments of J.H. Crocker and H. Kucha on the manuscript are appreciated.

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