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The Canadian Mineralogist
Vol. 41, pp. 331-352 (2003)

LAURITE AND RUARSITE FROM PODIFORM CHROMITITES AT KRAUBATH
AND HOCHGRÖSSEN, AUSTRIA: NEW INSIGHTS FROM OSMIUM ISOTOPESKRESHIMIR N. MALITCH§
Institute of Geological Sciences, University of Leoben, Peter Tunner Strasse 5, A-8700 Leoben, Austria and NATI Research
JSC, Otechestvennaya 3-3a, St. Petersburg 195030, Russia

STEPHAN A. JUNK
Institute of Archaeometry, Technical University of Mining and Metallurgy, Gustav-Zeuner Strasse 5,
D-09596 Freiberg, Germany

OSKAR A.R. THALHAMMER
Institute of Geological Sciences, University of Leoben, Peter Tunner Strasse 5, A-8700 Leoben, Austria

FRANK MELCHER
Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, D-30655 Hannover, Germany

VLADIMIR V. KNAUF
NATI Research JSC, Otechestvennaya 3-3a, St.Petersburg 195030, Russia

ERNST PERNICKA
Institute of Archaeometry, Technical University of Mining and Metallurgy, Gustav-Zeuner Strasse 5,
D-09596 Freiberg, Germany

EUGEN F. STUMPFL
Institute of Geological Sciences, University of Leoben, Peter Tunner Strasse 5, A-8700 Leoben, Austria

ABSTRACTThe chemical and osmium-isotope composition of platinum-group minerals (PGM) [e.g., laurite–erlichmanite (RuS2–OsS2),
ruarsite–osarsite (RuAsS–OsAsS) series and Os–Ir alloy (Os,Ir)] from variably altered podiform chromitites of the Kraubath and
Hochgrössen dunite–harzburgite massifs are reported for the first time. These massifs, the largest dismembered mantle relics inthe Eastern Alps of Austria, were interpreted as a strongly metamorphosed ophiolite sequence, which forms part of the Speik
Complex. Unaltered podiform chromitites from both localities display negatively sloped chondrite-normalized platinum-groupelement (PGE) patterns. The highly altered podiform chromitite at Kraubath is dominated by less refractory PGE (PPGE: Pd, Pt,
Rh) over refractory PGE (IPGE: Os, Ir and Ru). The chemical composition of chromite varies from a Cr# [100*Cr/(Cr + Al)] of
74 to 87 and a Mg# [100*Mg/(Mg + Fe2+)] of 44 to 61, values typical of podiform chromitites from the mantle section of an
ophiolite. The PGM assemblage in the unaltered podiform ores is dominated by laurite (43% and 75% of all PGM at Kraubath and
Hochgrössen, respectively). Sperrylite, PtAs2, is the most abundant PGM (61%) in the altered chromitite, whereas minerals of
laurite–erlichnmanite series are subordinate (4%). At Kraubath, Os-bearing PGM (laurite, erlichmanite, ruarsite and Os–Ir alloy)
occur as (a) single grains and (b) complex polyphase assemblages. At Hochgrössen, laurite and Os–Ir alloy are present as solitarygrains only. In situ osmium-isotope measurements of 16 PGM grains from bedrock (e.g., laurite and ruarsite) by laser-ablation
multiple-collector inductively coupled plasma – mass spectrometry (LA–MC–ICP–MS) revealed low 187Os/188Os and Ost=0
values, indicative of a subchondritic source of the PGE in the mantle. Combined with less radiogenic Os isotopic values measured
by negative thermal ionization mass spectrometry (N–TIMS), 187Os/188Os ranges from 0.11580 to 0.12437, and Ost=0 values,§

E-mail address: malitch@unileoben.ac.at

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from –9.97 to –3.31, respectively. The Os isotopic composition of PGM from altered chromitite at Kraubath is similar or even lessradiogenic than that from typical podiform chromitite. The Re–Os system within PGM thus has not been disturbed during later
metamorphic events. Furthermore, the wide range of 187Os/188Os values is similar to that in detrital Os-rich alloys derived fromother dunite–harzburgite complexes worldwide (i.e., Ost=0 ranges from –14.95 to –2.12). On the basis of the osmium isotopic
composition of laurite and ruarsite, ultramafic protoliths in the Speik ophiolite complex yield model ages in the range of 1758 to
584 Ma. The mineral-isotopic data are consistent with a prolonged melting history of the parent ultramafic rocks from the mantle
section of an ophiolite and provide further evidence for the closed-system behavior of the Re–Os isotopic system within PGM.
Keywords: laurite, ruarsite, LA–MC–ICP–MS study, osmium isotopes, chromitite, ophiolite, upper mantle, Kraubath,
Hochgrössen, Austria.

SOMMAIRE
Nous avons déterminé pour la première fois la composition chimique et les rapports isotopiques d’osmium des minéraux dugroupe du platine [e.g., des séries laurite–erlichmanite (RuS2–OsS2) et ruarsite–osarsite (RuAsS–OsAsS) et de l’alliage Os–Ir(Os,Ir)] provenant de chromitites podiformes variablement altérées des massifs à dunite–harzburgite de Kraubath et Hochgrössen.
Ces massifs, les plus volumineux représentants du manteau démembré des Alpes orientales en Autriche, étaient attribués à uneséquence ophiolitique fortement métamorphisée faisant partie du complexe de Speik. Les chromitites podiformes non altérées desdeux endroits montrent une courbe à pente négative dans un diagramme illustrant les teneurs en éléments du groupe du platine(normalisées par rapport aux concentrations chondritiques). Les échantillons de chromitite fortement altérés à Kraubath montrent
une dominance des éléments moins réfractaires (Pd, Pt, Rh) par rapport aux éléments réfractaires (Os, Ir, Ru). La composition duspinelle chromifère varie en Cr# [100*Cr/(Cr + Al)] de 74 à 87 et en Mg# [100*Mg/(Mg + Fe2+)] de 44 à 61, valeurs typiques pourles chromitites podiformes de la section mantellique d’une ophiolite. Le minerai podiforme non altéré contient surtout la laurite(43% et 75% de tous les grains de minéraux du groupe du platine à Kraubath et Hochgrössen, respectivement). La sperrylite,
PtAs2, est davantage abondante (61%) dans la chromitite altérée, tandis que les membres de la série laurite–erlichmanite sontsubordonnés (4%). A Kraubath, les minéraux porteurs d’osmium (laurite, erlichmanite, ruarsite at alliage Os–Ir) se présentent (a)en grains individuels et (b) en assemblages complexes polyphasés. A Hochgrössen, la laurite et l’alliage Os–Ir sont présents en
grains isolés seulement. Nos analyses isotopiques in situ de seize grains de ces minéraux (e.g., laurite et ruarsite) par analyse sur
plasma avec couplage inductif, spectrométrie de masse, ablation au laser et avec multicollecteurs révèle de faibles valeurs durapport 187Os/188Os et de Ost=0, indications d’une source subchondritique des éléments du groupe du platine dans le manteau.
Considérés à la lumière des valeurs isotopiques moins radiogéniques obtenues par spectrométrie de masse par ionisation thermiquenégative (N–TIMS), 187Os/188Os varie de 0.11580 à 0.12437, et Ost=0 varie de –9.97 à –3.31, respectivement. La composition
isotopique de l’osmium des minéraux du groupe du platine provenant de la chromitite altérée à Kraubath est semblable, voiremême moins radiogénique, que ce qui est typique d’une chromitite podiforme typique. Le système Re–Os n’a donc pas subi demodification au cours des événements ultérieurs. De plus, l’intervalle étendu de valeurs de 187Os/188Os ressemble à celle d’alliages
riches en osmium d’origine détritique, dérivés d’autres complexes à dunite–harzburgite à l’échelle mondiale (i.e., Ost=0 dansl’intervalle de –14.95 à –2.12). Selon la composition isotopique de l’osmium dans la laurite et la ruarsite, les protolithes
ultramafiques du complexe ophiolitique de Speik ont un âge modèle dans l’intervalle de 1758 à 584 Ma. Les données isotopiquesdéterminées étayent l’hypothèse d’un épisode prolongé de fusion partielle d’un socle ultramafique dans la section mantellique del’ophiolite et du comportement en système isotopique fermé de Re–Os dans ces minéraux.
(Traduit par la Rédaction)
Mots-clés: laurite, ruarsite, étude ICP–MS avec ablation au laser et multicollrcteurs, isotopes d’osmium, chromitite, ophiolite,
manteau supérieur, Kraubath, Hochgrössen, Autriche.

INTRODUCTION
Recent Os-isotope studies have clearly shown that
the Os isotopic system of minerals like chromite, olivine, base-metal (BM) sulfides, and Os-rich alloy, may
contribute to a better understanding and more accurateinterpretation of the processes leading to fractionationof Os isotopes in different environments in the mantle(Burton et al. 1999, 2000, Standish et al. 2001, Alard etal. 2002, Malitch et al. 2002a, Meibom & Frey 2002,
Pearson et al. 2002, Spetsius et al. 2002, among others). Taking into consideration that Ru–Os sulfides encapsulated in chromian spinel retain primordial
Os-isotope signatures, these platinum-group minerals

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(PGM) are very promising targets to avoid ambiguity inthe interpretation of whole-rock Os isotopic results.
To our knowledge, few Os isotopic analyses of bedrock Ru–Os sulfides have been done from an ophiolitic
environment. Results of three analyses of laurite measured with the MIT–Harvard–Brown Cameca 3f ion
microprobe have been reported by Hattori et al. (1992).
Two other sets of Os isotopic results on laurite have beenpresented, by Walker et al. (1996) and Ohnenstetter etal. (1999). To fill this gap and to demonstrate the highpotential of Os isotopes measured in bedrock PGM, we
conducted a detailed laser-ablation (LA) multiple collector – inductively coupled plasma – mass spectrometry (MC–ICP–MS) study of single and polyphase

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Os-rich PGM grains, which comprise laurite, (Ru,Os)S2,
and ruarsite, (Ru,Os)AsS. These PGM were liberated
from variably altered podiform chromitites of the
Kraubath (Malitch et al. 2001b) and Hochgrössen (thisstudy) dunite–harzburgite massifs, the largest dismembered mantle relics in the Eastern Alps, Austria (Fig. 1).

Our aim in this paper is to discuss for the first time
results of LA–MC–ICP–MS isotopic analyses of Ru–
Os sulfides and sulfarsenides from variably alteredpodiform chromitites at Kraubath and Hochgrössen inorder to: (1) test the closed-system behavior of the Re–
Os isotopic system within PGM, (2) determine the

FIG. 1. Tectonic scheme of the Eastern Alps (a) and schematic geological map of Middle Austroalpine
units east of the Tauern Window, showing the location of the Hochgrössen, Kraubath and Pernegg
ultramafic massifs (b), after Frank (1987) and Neubauer (1988), respectively.

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source of the PGE (e.g., chondritic, subchondritic,suprachondritic mantle), (3) establish if the PGM frompodiform chromitites are isotopically similar at all
localities, and (4) provide age constraints on the formation of ultramafic protolith hosting the PGE mineralization.
In the past, some Os isotope data have been presented without reporting the mineral chemistry of Osrich PGM (Hart & Kinloch 1989, Hirata et al. 1998,
Meibom & Frey 2002, among others). To ultimately
characterize the PGM from the podiform chromitites forwhich LA–MC–ICP–MS analyses have been performed, the composition of Os-bearing PGM (e.g.,laurite–erlichmanite and ruarsite–osarsite series) also ispresented, along with bulk platinum-group elements(PGE) concentrations in mantle-derived chromitites.

BACKGROUND INFORMATION
Re–Os is potentially the best isotopic system withwhich to determine mantle-depletion ages for ultramafic rocks. Owing to considerable progress in the development of analytical techniques in recent years, it has
been widely applied in evaluating distinct sources anddating melting events in the mantle in different geological settings (Reisberg & Lorand 1995, Snow & Reisberg
1995, Roy-Barman et al. 1996, Walker et al. 1996, Handler et al. 1997, Nägler et al. 1997, Hirata et al. 1998,
Parkinson et al. 1998, Shirey & Walker 1998, Burton etal. 1999, Blusztajn et al. 2000, Bowles et al. 2000, Brandon et al. 2000, Malitch et al. 2000, Alard et al. 2002,
Malitch 2002, Malitch & Thalhammer 2002, among
many others). Recent Os isotopic results for bulk rocks
and ultramafic xenoliths led to the conclusion that the
Re–Os system may have been disturbed by interaction
with melt or fluid (Brandon et al. 1996, 1998, Peslier et
al. 2000, Becker et al. 2001), thus questioning theclosed-system behavior of the Re–Os system. In addition, ambiguity in interpretation of whole-rock Os isotopic data also arose from mineral Os isotopic studies,
which proved Os heterogeneity in mantle minerals evenin one sample (Burton et al. 1999, 2000, Alard et al.
2001a, b, 2002, among others). Alard et al. (2001b,2002) documented two generations of isotopically distinct BM sulfides, which may occur even in one peridotite xenolith. Subordinate BM sulfides enclosed inolivine had unradiogenic Os isotope composition,
whereas dominant pyroxene-hosted or interstitial BM
sulfide were much more radiogenic (187Os/188Os up to
0.159), contributing in different proportions to the radiogenic whole-rock Os isotope composition. Thus, theprimordial signature of BM sulfide enclosed in silicate
is masked by highly radiogenic secondary BM sulfide,
which cannot be estimated from the whole-rock Os isotope systematics alone. Furthermore, in situ LA–MC–
ICP–MS analyses of BM sulfides from abyssal andophiolitic peridotites indicate a wide range of 187Os/
188
Os values, from 0.1113 ± 0.0008 to 0.1382 ± 0.0014

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(Alard et al. 2001a), which are in obvious disagreementwith the average estimates for both (1) depleted midocean ridge (MOR) mantle (DMM; 0.1246 ± 0.0014)
deduced from whole-rock Os isotopic composition ofabyssal peridotites (Snow & Reisberg 1995) and (2)primitive upper mantle (PUM; 0.1296 ± 0.0008) definedon the basis of whole-rock Os isotopic data for mantle
xenoliths (Meisel et al. 2001).
Chromite, as the most resistant mantle mineral, iswell protected from high-temperature serpentinizationand low-temperature ocean-floor weathering in an
ophiolite environment, and, thus, has been proposed torepresent initial 187Os/188Os values (Walker et al. 2002).
However, it has been shown that the Os budget of the
mantle is mainly controlled by sulfide and alloy (Martin et al. 1993, Hart & Ravizza 1996, Burton et al. 1999).
Therefore, inclusions of Os-rich platinum-group minerals (PGM) in chromite, commonly the earliest precipitates in ultramafic systems, are the best mineral phases
to constrain initial 187Os/188Os values and to avoid ambiguity in the interpretation of Os isotopic results inoceanic mantle.
The advantage of the Re–Os system applied to Osrich minerals is that PGM contain osmium as a main ortrace element in their crystal structures, and are essentially devoid of Re. This feature allows one to obtainaccurate initial Os isotope values, assuming that the Os
isotopic composition of PGM has not changed after theirformation and, thus, reflects that of the source. Furthermore, since osmium is one of the six PGE, Os isotopes
are particularly useful (1) to distinguish between crustal
and mantle sources for PGE due to the large fractionation of Re over Os between crust and mantle, and (2)to date Ru–Os sulfides and Os–Ru–Ir alloys of mantle
origin. Both approaches have been applied in detail to
PGM from placer deposits derived mainly from dunite–
harzburgite (e.g., of Alpine or ophiolite type) andclinopyroxenite–dunite (e.g., of zoned, Uralian, Alaskan or Aldan type) massifs (Allègre & Luck 1980,
Hattori & Hart 1991, Hattori et al. 1992, Hattori & Cabri
1992, Borg & Hattori, 1997, Lyon et al. 1997, Hirata
et al. 1998, Malitch & Kostoyanov 1998, 1999, Bird
et al. 1999, Malitch 1999, Ohnenstetter et al. 1999,
Rudashevsky et al. 1999, Malitch et al. 2001c, 2002a,
b, Malitch & Badanina 2002, Malitch & Thalhammer
2002, Meibom & Frey 2002). Therefore, Os-rich PGM,
in most cases the earliest precipitates in ultramafic systems, are potentially the best tracers of melting events
that might be unequivocally recognized during the formation of the ultramafic protolith in the mantle.
In the past, application of Os isotope studies of PGM
from bedrock have been hampered for several reasons.
First, PGM are relatively small (usually less than 10–20
m; grains more than 50–100 m are rare). It is difficult, therefore, to liberate them from bedrock in significant amounts. A reliable technique to quantitatively
separate the PGM was lacking. However, liberation isneeded to avoid uncertainties from excitation effects

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from host minerals (chromite, olivine, etc.) during Os
isotope analysis. Developments in concentration techniques, including hydroseparation (Knauf 1996, Malitch
et al. 2001b, 2002c, Malitch & Knauf 2002), and in Os
isotope measurements of Os-rich alloys by N–TIMS(Creaser et al. 1991, Kostoyanov et al. 2000) and by insitu LA–MC–ICP–MS (Hirata et al. 1998, Junk 2001)
now make it possible to obtain quantitative data on PGM
grains.

GEOLOGICAL SETTING AND SAMPLE LOCATIONThe Kraubath and Hochgrössen ultramafic massifs,
situated within the Austrian province of Styria, havebeen interpreted as parts of a strongly deformed, metamorphosed and dismembered ophiolite complex presumably of Late Proterozoic to Early Paleozoic age (ElAgeed et al. 1980, Stumpfl & El Ageed 1981, Neubauer
1988, Neubauer et al. 1989), that originated in a suprasubduction zone setting (Melcher et al. 1999b, 2002,
Puhl 2000). Ultramafic and mafic rocks of the
Hochgrössen massif are generally correlated with similar rocks in the Speik Complex located to the east,
namely at Kraubath and Pernegg (Fig. 1B). However,
Neubauer et al. (1989) have suggested that the ultramafic body at Hochgrössen might represent a separate unit
not linked to the Speik Complex.
The Kraubath and Hochgrössen massifs are composed of metamorphosed, foliated harzburgite and dunite. Layers, lenses and stocks of coarse-grained
orthopyroxenite (“bronzitite”) are present at Kraubath
only. A pre-Variscan (>400 Ma) high-pressure eclogitefacies metamorphic event has been recognized in the
Hochgrössen massif only (Faryad et al. 2002); thatmassif subsequently underwent Variscan amphiboliteand Alpine (~100 Ma) greenschist-facies conditions(Puhl 2000, Faryad & Hoinkes 2001). The pre-Variscan
event has not been recognized at Kraubath yet, owing
to a stronger Alpine amphibolite-facies imprint.
Numerous bodies of chromitite of generally small
size (less than 0.5 m thick) are exposed in the northernpart of the Kraubath and in the northwestern part of the
Hochgrössen massifs. The chromitites are hosted byrefractory dunite and harzburgite having preserved magmatic olivine (Mg# between 90 and 92, NiO in the range
0.3–0.5 wt.%) and orthopyroxene (Mg# between 91 and93, <1 wt.% Al2O3), metamorphic talc, tremolite, serpentine, and chlorite. The host rocks are highly depleted
in incompatible elements (Melcher et al. 1999b, 2002,
Puhl 2000), displaying U-shaped REE patterns typicalof highly depleted restitic mantle formed by multistage
melting processes above supra-subduction zones(Parkinson & Pearce 1998, Monnier et al. 1999).
The predominant type of chromitite at Kraubath and
Hochgrössen is schlieren, deformed stringers andstreaks of massive chromite not more than a few centimeters thick. These are considered to be typical of
podiform chromitite. The chemical composition of

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chromite at Kraubath varies in Cr# [100*Cr/(Cr + Al)]
from 80 to 86 and in Mg# [100*Mg/(Mg + Fe2+)] from44 to 50, whereas that at Hochgrössen varies in the Cr#
range 74–84 and Mg# range 53–61. Since the first investigation of PGM in ophiolitic rocks of the Eastern
Alps (Thalhammer & Stumpfl 1988), an increasing
amount of information has been gathered on PGM in
association with chromite together with whole-rock concentrations of the PGE (Thalhammer et al. 1990,
Melcher & Mali 1998, Melcher et al. 1999b, Melcher
2000, Malitch et al. 2001b, d). These podiform occurrences of chromitite are distinct from banded chromitite,which has been recognized closely above the typicalmantle section at Kraubath (Malitch 2001, Thalhammer
et al. 2001, Malitch & Knauf 2002, Malitch et al.
2002c). The Os isotopic composition of PGM from thebanded type of chromitite will be presented elsewhere(Malitch et al., in prep.).
The PGM in three samples of podiform chromitite(sample HG 71 from Hochgrössen and samples K 137
and K 142 from Kraubath) were studied by LA–MC–
ICP–MS. Sample HG 71 is a massive sample ofpodiform chromitite from an extensive outcrop in thenorthwestern part of the Hochgrössen massif. It consistsof coarse-grained chromite that contains abundant inclusions of chlorite, Ni arsenide and Ni sulfide. A rim
of magnetite around a core of chromite is common. Irregular thin veinlets and fractures in chromite filled withserpentine and chlorite result in a network-like secondary texture. Sample K 142 consists of unaltered massive chromitite (type 1) with a schlieren texture; it was
taken from a small outcrop in the old chromite mine area
south of the Sommergraben (Fig. 1 in Malitch et al.
2001b). Sample K 137 consists of massive chromitite;
it was taken from the mine dumps of the same area. This
chromitite is highly altered, with zones completely decomposed to talc and tremolite (type-2 chromitite).

ANALYTICAL TECHNIQUES
Bulk analyses of the chromitites for the PGE and Au
were made in the Laboratory of Analytical Research andMonitoring of Mekhanobr-Analyt (St. Petersburg, Russia) using fire assay combined with chemical spectroscopy (for Pt, Pd, Rh, Ru, Ir, Au) and the kinetic method
for Os (Alekseeva et al. 1978, Malitch 1990, Ushinskaya
et al. 1999).
To constrain textural relationships of the Os-rich
PGM with the associated minerals, all three samples ofchromitite were first investigated optically. Ru–Os sulfides have been observed invariably as euhedral inclusions in chromite (Figs. 2a, b), thus revealing theirprimary signature. The chromitite samples were subsequently disintegrated by gradual milling followed byremoval and sieving of the fine fraction (<90 m). To
avoid the possibility of artificial contamination from themill, chromitite samples weighing 2–3 kg were first disintegrated to a grain-size fraction of <3 mm, and the

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FIG. 2. Back-scattered electron images of euhedral single and polyphase PGM grains frompodiform chromitite at Hochgrössen (a, c, d) and Kraubath (b, e, f). LR: laurite, (Os, Ir):
iridian osmium; Pt–Fe: Pt–Fe alloy close to Pt3Fe; K: keithconnite, Cu–Ir–T:
cuproiridsite; CHR: chromite; numbers 1–6 denote areas of electron-microprobe analyses, and correspond to the same number in Table 1.

fraction <0.5 mm was removed from further concentration. The remaining 0.5–3 mm fraction was further disintegrated, and two fractions (i.e., <37 m and 37–90m) were separated. The heavy minerals (includingPGM) of these two fractions were concentrated by ahydroseparation technique (Knauf 1996) at NATI Research JSC, St. Petersburg, Russia. Concentration factors between 4  104 to 1  105 times were attained.

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Finally, each concentrate with PGM was mounted in
epoxy blocks and polished in separate sections for further detailed mineralogical and Os-isotope studies.
Chemical analyses were carried out with an ARL–
SEMQ electron microprobe equipped with four wavelength-dispersion spectrometers (WDS) and with a
LINK energy-dispersion analyzer (EDS) at the Instituteof Geological Sciences (Department of Mineralogy and5/2/03, 15:21

LAURITE AND RUARSITE FROM PODIFORM CHROMITITES, AUSTRIAPetrology), University of Leoben (Austria); analyticaldetails are described in Malitch et al. (2001b).
After the microprobe analyses, the 16 grains of PGM
from the Kraubath and Hochgrössen massifs were investigated by LA–MC–ICP–MS using a Microprobe II
LA device (Thermo Elemental, Nd:YAG laser, 266 nm
wavelength, up to 4 mJ per shot, 3 ns pulse width) andan AXIOM MC–ICP–MS (Thermo Elemental). Allanalyses were performed at the Technical University of
Mining and Metallurgy (Institute of Archaeometry),
Freiberg, Germany.
The ICP–MS was tuned using a desolvating nebulizer (MCN 6000, CETAC), a solution of 33 g/L Re,
330 g/L Os, and 330 g/L Ir in 2% nitric acid, a nebulizer flow of 0.8 L/min Ar, and a radio frequency (RF)forward power of 1330 W. Helium was used as an ablation chamber gas, with a flow of 85 mL/min in a chamber 2.5 cm in diameter that has minimized dead volume.
The air capacitor of the ICP–MS was optimized to obtain a RF reflected power of 12 to 18 W with this He
addition to the plasma gas. Usually, it was necessary tostop the He flow to ignite the plasma of the AXIOM
MC–ICP–MS. The remaining ICP parameters were optimized with the tune solution for signal stability andmaximum signals during laser ablation of an Ir foil. The
MC set-up, and corrections for Re and W contents, werechecked with combined laser-ablation analyses of members of the ferberite–hübnerite series and the aerosol
generated by the desolvating nebulizer, as described by
Junk (2001).
Laser-ablation spots of 5 to 10 m were used with ascan field that was adapted to the size of each samplingarea, a laser-shot frequency of 20 Hz, and an energy
output of up to 0.5 mJ. The energy output yielded power
densities in the range of 109 to 1015 W/m2. This corresponds to vaporization as the main process, using themodel described by Steen (1991). Vaporization or phase
explosion seemed to be able to generate smaller particles
in contrast to ablation by a heating or melting process.
Helium used as the ablation-chamber gas yielded a finer
aerosol, and the redeposition of the ablated material near
the ablation spot was minimized owing to the better thermal conductivity and, to a lesser extent, the lower density of He compared to Ar. Thus, the use of He as theablation-chamber gas proved to be beneficial.
The aerosols generated by laser ablation were transported by a gas stream to the MC–ICP–MS (ThermoElemental Axiom, multicollector version with nine
Faraday cup detectors, resolution 400). The signals were
measured at m/z 183 (W), 184 (W + Os), 185 (Re), 186(W + Os), 187 (Re + Os), 188 (Os), 189 (Os), 191 (Ir)and 193 (Ir) using the multichannel collector of the ICP–
MS. The mass bias was corrected using an exponential
fractionation law and the 188Os/189Os value. Isobaric
interferences were very rare and were thus correctedusing the natural abundances of Re or W. Molecularinterferences and problems connected with the abundance sensitivity were checked for by using the 191Ir/

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193
Ir value as a second possibility to corrected the massbias, as described by Junk (2001). For the reported values, no significant contribution of these possible sourcesof error was detected. The abundances used for the calculations were published as best experimental values byRosman & Taylor (1997).
Values of the isotope ratios are reported with experimental uncertainties that take into account the contributions of the Faraday cup efficiencies, the normalization
values for mass-bias corrections using 188Os/189Os(Rosman & Taylor 1997), interference corrections, thesignal noise, and the within-run standard deviations. The
reported uncertainties are expanded uncertainties using
a coverage factor of 2 which gives a level of confidenceof approximately 95% (Ellison et al. 2000).
The use of within-run standard deviations or errors(i.e., Pearson et al. 2002), which confirm that the
IUPAC 188Os/189Os and 191Ir/193Ir values are within theexperimental errors of the experiments, should be
avoided. Such an approach does not include all sourcesof uncertainty, it only allows limited comparison among
methods, and it lacks the traceability to SI units as required by DIN EN ISO/IEC 17025 (1999). The expanded uncertainty used in our study simplifies thecomparison of isotope values determined by different
methods or instruments.

PGE GEOCHEMISTRY AND PGE MINERALOGY
The Hochgrössen massif
Bulk concentrations of the PGE in podiform
chromitite at Hochgrössen are low (in ppb): Os 77, Ir
10, Ru 88, Rh <5, Pt <6 and Pd <20. The rock thus has
a negatively sloped chondrite-normalized PGE pattern,similar to those characteristic of ophiolite-type podiform
chromitites (e.g., Page & Talkington 1984, Barnes etal. 1985, Volchenko & Koroteev 1990).
Six solitary grains of laurite and two of Os–Ir alloy
belong to PGM of the refractory IPGE-group (Figs. 2c,d). The PGM conform to the bulk-distribution patternof the PGE expected and the predominance of Ru–Os
sulfides and Os-rich alloys over other PGM reported
from the mantle section of an ophiolite (Talkington etal. 1984, Legendre & Augé 1986, Augé & Johan 1988,
Palandzhian et al. 1994, Melcher et al. 1997, Garuti etal. 1999).
The Kraubath massif
Bulk concentrations of the PGE and the PGM present
in two samples of chromitite (K 142 and K 137) fromthe Sommergraben area of the Kraubath massif havebeen studied in detail by Malitch et al. (2001b) and are,thus, only briefly summarized here. Distribution patternsof PGE in these chromitites show considerable differences in the behavior of the less refractory PGE (PPGE
group: Rh, Pt, Pd) compared to the refractory PGE

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THE CANADIAN MINERALOGIST(IPGE group: Os, Ir, Ru). Platinum and Pd are moreenriched in chromitite showing features of pronounced
alteration (type 2, sample K 137). The unaltered
chromitite (type 1, sample K 142) displays a negatively
sloped chondrite-normalized PGE pattern typical of
ophiolitic–podiform chromitite.
About 26 different PGM and two gold-rich minerals(e.g., 21 PGM in type-1 chromitite and 13 PGM and
two Au-rich minerals in type-2 chromitite) have been
documented from a data base of 1117 PGM grains(Malitch et al. 2001b). Type-1 chromitite is dominatedby laurite (43%), which occurs in complex polyphaseassemblages with PGE alloys (Ir–Os, Os–Ir, Pt–Fe),
PGE sulfides (kashinite, bowieite, cuproiridsite,
cuprorhodsite, unnamed Ir-rich variety of ferrorhodsite,
braggite, unnamed Ni–Fe–Cu–Rh and Ni–Fe–Cu–Ir
sulfides) and Pd telluride (keithconnite). A variety of
PGE sulfarsenides (33%) have been identified, including irarsite, hollingworthite, platarsite, ruarsite and anumber of intermediate species, whereas sperrylite andstibiopalladinite are subordinate (2%). In contrast, type2 chromitite is dominated by sperrylite (61% of all
PGM), followed by PGE sulfarsenides (26%), Pd-richminerals [e.g., potarite (4.7%) and stibiopalladinite

331 vol 41#2 avril 03 - 07338

(3.5%)], Ru–Os sulfides (4%) and other PGM, which
are present in minor amounts. The occurrence of such awide variety of PGM is highly unusual for an ophiolitic
environment. Three PGM assemblages have been recognized and attributed to processes ranging from magmatic to hydrothermal and weathering-related (Malitch
et al. 2001b).

CHEMICAL COMPOSITION OF OS-BEARING MINERALS
To avoid confusion with osmium and iridium as elements, we refer to the Os-rich alloy grains as iridian
osmium (Os, Ir) and osmian iridium (Ir, Os). Membersof the Ru–Os sulfide solid-solution series show a complete compositional spectrum, from laurite (Ru,Os)S2
to erlichmanite (Os,Ru)S2, whereas Ru–Os sulfarsenide
is represented by ruarsite (Ru,Os)AsS. The compositionof the Ru–Os sulfides, Os-rich alloys and PGE
sulfarsenides, along with their morphology and detailsof their internal texture, are presented in Figures 2–4.
The chemical composition of single PGM and polyphase PGM assemblages, which were measured by LA–
MC–ICP–MS and N–TIMS, is given in Tables 1 and 2.

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339

Columns 17-26 refer to analysis points on Figure 3. Column 15: sample from Hochgrössen;columns 16-26: samples from Kraubath. Symbols of PGM: (Os,Ir): iridian osmium, (Ir,Os):osmian iridium, Rua–Plt and Plt–Rua: uncommon ruarsite–platarsite solid-solution series,
RuNiAsS: unnamed Ru-Ni sulfarsenide, Irs: irarsite. **Composition 16 refers to osmian iridium shown in Malitch et al. (2001b).

Laurite–erlichmanite series

Os–Ir alloy

At Kraubath, the Ru–Os sulfides, in which Ru# [100Ru/(Ru + Os)] varies from 95 to 26, are dominated bylaurite, documented both in single and polyphase grains,
whereas Os-rich sulfide (erlichmanite) is less common(Table 1, Figs. 2b, e, f, 3a–h, 4a, b). Laurite frompodiform chromitite at Hochgrössen has values of
Ru #, ranging from 84 to 63 (Table 1, Figs. 2a, c, d, 4a).
The composition of the laurite–erlichmanite series, plotted on the Os–Ir–Ru diagram (Figs. 4a, b), shows common Ru substitution for Os. Laurite usually carries
moderate concentrations of both Ir and Rh, whereas
erlichmanite contains a notable abundance of Ir only
(Table 1). The wide compositional trend recorded at
Kraubath is similar to that of Ru–Os sulfide inclusions
from chromitites at Kempirsai and Rai-Iz, Urals(Melcher et al. 1997, Garuti et al. 1999), whereas themore restricted compositional range of laurite at
Hochgrössen is in good agreement to that of laurite inclusions reported by Thalhammer & Stumpfl (1988) and
Thalhammer et al. (1990).

The Os–Ir alloy occurs in a polyphase assemblage
with laurite, Pt–Fe alloy and Ir–Os alloy at Kraubath,and as single euhedral grains at Hochgrössen. The Ir–
Os alloy is typical of unaltered podiform chromitite at
Kraubath only (Malitch et al. 2001b, 2002c). Compositions of Os–Ir and Ir–Os alloys cluster at both sides ofthe miscibility gap defined by Harris & Cabri (1991) in
the Os–Ir–Ru diagram. The Os–Ir alloy contains low
concentrations of Fe (2.10–2.38 at.%) and Ru (2.30–
2.49 at.%), whereas the Ir–Os alloy carries significant
concentrations of Rh (up to 5.51 at.%) and Pt (up to 2.86at.%) (Table 2, anal. 15 and 16, respectively).

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The PGE sulfarsenides
The PGE sulfarsenides at Kraubath are commonly
associated with Ru–Os sulfides and show complex, andin some cases unconventional substitutions (Malitch etal. 2002c, this study, Table 2). Such phenomena are
exemplified by several grains (Figs. 3a, d, g, 4c–f, Table

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THE CANADIAN MINERALOGIST

1, anal. 7, 11, 12; Table 2, anal. 17, 18, 20–22, 24, 25);
grain K 137–121 shows the association of at least four
minerals [laurite (Ru# 70), ruarsite (Ru# 73), unnamed
Ru–Ni sulfarsenide (possibly a solid-solution series ofruarsite–gersdorffite) and Ru-rich oxide], whereas
grains K 142–5 and K 142–9 reveal associations of threedifferent PGE sulfarsenides [ruarsite (Ru# 90), irarsite

and Pt–Ru–Rh sulfarsenide] and two distinct compositions of laurite (Ru# 83 and 77, respectively) associatedwith two PGE sulfarsenides (irarsite and Pt–Ru
sulfarsenide), respectively.
The PGE sulfarsenides are dominated by irarsite(Figs. 3d, e, g, h, 4e) and Ru–Pt sulfarsenide (Figs. 3d,g, 4f) followed by ruarsite and hollingworthite (Figs.

FIG. 3. Back-scattered electron images of PGM assemblages from the Kraubath massif before (a, c–h) and after (b) laser ablation MC–ICP–MS. LR, LR 1 and LR 2: laurite, RURS: ruarsite, IRS: irarsite, R–P and P–R: members of the uncommon
ruarsite–platarsite series, Cu–Ir–T: cuproiridsite, Ru–ox: Ru oxide(?), Ru–Ni–As–S: unnamed Ru–Ni sulfarsenide, PNTL:
pentlandite. Numbers 7–13 and 17–26 denote areas of electron-microprobe analyses corresponding to the same numbers in
Tables 1 and 2, respectively. Number 4 in black hole (Fig. 3b) indicate the area of the laser-ablation MC–ICP–MS analysis,
which corresponds to the same number in Table 3. Scale bar is 10 m

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341

Ir

FIG. 4. Composition of Ru–Os sulfides (a, b and d) and ruarsite (c) in the triangular diagram Ru–Os–Ir (at.%) and various PGE
sulfarsenides in the diagrams Rh–Ir–Pt (e) and Ru + Os–Pt–Rh + Ir (f) from podiform chromitites at Kraubath (samples K 137
and K 142) and Hochgrössen (sample HG 71).

3a, c, d, 4c, e). Ruarsite (Table 2, anal. 17, 19, 20) shows
common Ru-for-Os substitution (up to 19.33 wt.% Os),and Ru # varies from 90 to 73. Minor elements detectedin ruarsite include Ir, Ni and Fe (up to 1.97, 0.40 and
0.26 wt.%, respectively).

OSMIUM-ISOTOPE DATA
The 187Os/188Os values of PGM from podiform
chromitites at Kraubath and Hochgrössen are listed in

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341

Table 3 and shown in Figure 5. Since the concentrationof Re in all samples appears to be less than 0.05 wt.%,
the isotopic effect caused by in situ radioactive decayof 187Re is negligible. Consequently, the value of 187Os/
188Os in the PGM corresponds to that in the source atthe time of PGM formation. The 187Os/188Os value inlaurite from Kraubath and Hochgrössen varies between
0.11940 ± 0.00059 and 0.12437 ± 0.00050; the 187Os/
188Os value in ruarsite at Kraubath ranges from 0.12105± 0.00158 to 0.12299 ± 0.00027 (Table 3, Fig. 5; the5/2/03, 15:21

342

THE CANADIAN MINERALOGIST

expanded uncertainties correspond to the 95% confidence interval). The dispersion of 187Os/188Os values invarious PGM from samples K 142 and HG 71 exceeds
the analytical uncertainty (Fig. 5). Therefore, isotopic
fractionation among the majority of PGM is significant
and similarly expressed in both massifs. For instance,
the range of the Os isotopic composition of PGM at
Kraubath measured by LA–MC–ICP–MS is from
0.12026 ± 0.00088 to 0.12437 ± 0.00050 (Table 3,
Fig. 5, samples K 142 and K 137), closely matching thatat Hochgrössen (Table 3, Fig. 5, sample HG 71). However, two Ru–Os sulfides from sample K 142 (grains K
142–11 and K 142–10, Figs. 2e, f) measured by N–
TIMS (Malitch 2001) do not show significant variationand provide the most unradiogenic 187Os/188Os values,
0.1158 ± 0.0007 to 0.1162 ± 0.0004 (Fig. 5). An evennarrower range of 187Os/188Os values (from 0.12105 ±0.00158 to 0.12117 ± 0.00014) characterizes PGM
grains from the highly altered chromitite (Table 3, anal.
3 and 4, Figs. 3a–c, 5, sample K 137). These PGM grains
are less radiogenic than the majority of the PGM fromtypical podiform chromitite (Fig. 5, samples K 142 and
HG 71). Examples of primary and secondary PGM as-sociations, which show unradiogenic Os isotope compositions, are represented by the grains K 137–121(Figs. 3a, b) and K 137–83 (Fig. 3c) from highly altered
chromitite. Apart from primary laurite, the first one contains secondary PGM [unnamed Ru–Ni sulfarsenide andRu-rich oxide], whereas in the second one, ruarsite isrimmed and cross-cut by Ru-rich oxide. Unradiogenic
187
Os/188Os values of PGM grains from highly altered
chromitite (sample K 137) indicate that the Re–Os system in Os-bearing minerals (laurite and ruarsite) fromthese assemblages has remained unchanged, despite asecondary metamorphic overprint. The stability of the
Os isotope system at the single-crystal scale has also
been demonstrated for detrital 3.0–4.0 Ga old Os-richalloys and gold from the Evander and Klerksdorp goldfields at Witwatersrand, South Africa (Malitch et al.
2000, 2001a, Kirk et al. 2001).
Since the 187Os/188Os values in all analyzed PGM
do not exceed the value of the contemporary undifferentiated mantle material (CHUR) (0.12863 ± 0.00046,
Chen et al. 1998), a model Re–Os age can be calculated
according to the method of Allègre & Luck (1980). In
order to calculate model (mantle-derived) ages of PGM,

FIG. 5. Normal probability plot of 187Os/188Os and model age values of PGM from podiform chromitites at Kraubath and
Hochgrössen. N–TIMS data (PGM grains K 142–11 and K 142–10) from Malitch (2001) also are included. Sum probability
is defined as (i–1/n)/n for the ith sample out of n; quantile Q is calculated using normal distribution and sum probability. The
expanded uncertainty for each PGM is also indicated.

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the 187Os/188Os evolution curve of the mantle has to be
known and constrained by initial and present-day 187Os/
188Os values. In this study, we are using the primary andcontemporary 187Os/188Os values (0.12863 ± 0.00046
and 0.0953 ± 0.0013, respectively) for an undifferentiated mantle reservoir (CHUR) as estimated by Chen etal. (1998). Alternative 187Os/ 188Os values for thepresent-day mantle, in widespread use for calculationof model ages, are 0.12736 (Yin et al. 1996), 0.1270(Shirey & Walker 1998) and 0.1296 ± 0.0008 (Meisel
et al. 2001). Calculations using the latter would resultin model ages that are approximately 0.2 Ga older. However, as is clear from recent studies on BM sulfides(Alard et al. 2001a, b, 2002), the latter estimate cannot
be considered a valid proxy material for mantle sulfides.
Since most ultramafic rocks from oceanic upper mantle
follow the chondritic evolution curve (Snow & Reisberg
1995), we favor the initial and present-day 187Os/188Os
estimates for chondrites (Chen et al. 1998), which areconsidered to be more reliable for the dating of primary
PGM from the mantle section of an ophiolite.
The wide range of 187Os/188Os values of different Osbearing PGM (e.g., laurite and ruarsite) from the
Kraubath massif results in Re-depletion ages in therange of 1147 to 584 Ma (Table 3, LA–MC–ICP–MS
study, n = 16), if the equation for the calculation of
187Os/ 188Os model ages is used: (0.12863 – 187Os/
188
Ossample)/0.0073 (Ga). The Os isotopic compositionof two grains of laurite at Hochgrössen yielded model
ages of 1264 Ma and 645 Ma, which is in accordance343

with the total variation defined by those at Kraubath(Table 3, Fig. 5, samples HG 71 and K 142, respectively). In contrast, model 187Os/188Os ages for PGM
from the highly altered chromitite vary between 1038
and 1022 Ma only (Table 3, Fig. 5, sample K 137). The
Os isotope system thus has not been disturbed duringlater thermal events, which affected both massifs in preVariscan, Variscan and Alpine times (Puhl 2000, Faryad& Hoinkes 2001, Faryad et al. 2002).

DISCUSSION
Podiform chromitites within residual mantle from an
ophiolite complex are generally well characterizedbased on geological, geochemical and mineralogicalgrounds (e.g., Lago et al. 1982, Dick & Bullen 1984,
Talkington et al. 1984, Legendre & Augé 1986, Prichard
et al. 1986, Augé 1987, Augé & Johan 1988, Prichard& Tarkian 1988, McElduff & Stumpfl, 1990, Nilsson
1990, Volchenko & Koroteev 1990, Lord 1991,
Palandzhian et al. 1994, Melcher et al. 1997, Garuti etal. 1999, Ohnenstetter et al. 1999, Bai et al. 2000). Most
podiform chromitites show negatively sloped chondritenormalized PGE patterns, which are mineralogically
expressed by the preponderance of a rather limited variety of PGM of the IPGE group (i.e., dominated by laurite
or Ru–Os–Ir alloy). Rarely documented positive chondrite-normalized PGE patterns in podiform chromitites
are usually ascribed to secondary processes leading toremobilization and reconcentration of the PPGE (e.g.,
Thalhammer et al. 1990, Malitch et al. 2001b).
Extensive Os isotopic data on Proterozoic ophiolite
occurrences based on chromite concentrates andchromitites have been reported by Walker et al. (1996)
and Tsuru et al. (2000). Os isotopic analyses for Phanerozoic ophiolites reported by Meisel et al. (1997),
Melcher et al. (1999a) and Walker et al. (2002) havebeen restricted to a limited number of chromite-richsamples from the mantle portion or lower crustal sequence (or both) within a particular ophiolite locality
studied. The present study provides new constraints on
the osmium isotopic composition of the oceanic upper
mantle based on extensive dataset of Ru–Os sulfides andsulfarsenides from variably altered podiform chromititesof the Speik Complex within the Eastern Alps.
Main PGM assemblages in mantle environments
The dominance of Ru–Os sulfides or Os-rich alloys
over other PGM is not unusual in mantle sections ofdunite–harzburgite and lherzolite ultramafic complexes.
In fact, it is considered a typical feature of PGE occurrences in dunite–harzburgite and lherzolite complexes
worldwide (e.g., Stumpfl 1974, Cabri & Harris 1975,
Legendre & Augé 1986, Augé & Johan 1988, Dmitrenko
et al. 1990, Palandzhian et al. 1994, Cabri et al. 1996,
Torres-Ruiz et al. 1996, Nakagawa & Franco 1997,
Garuti et al. 1999, Melcher 2000). However, the331 vol 41#2 avril 03 - 07343

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THE CANADIAN MINERALOGIST

Kraubath massif is characterized by a high diversity of
PGM assemblages (Malitch 2001, Malitch et al. 2001b,
2002c, Thalhammer et al. 2001, Malitch & Knauf 2002).
The occurrence of about 35 different PGM and Au-richminerals documented from only three bedrock samplesof chromitite is uncommon for an ophiolitic environment. Such unusual diversity is due to the applicationof a new technique of concentration applied for variously altered and distinctly different chromitites of the
Kraubath massif. The relative abundance of PGM in
podiform chromitite at Hochgrössen corresponds wellto that in mantle-hosted chromitites as reported elsewhere (e.g., Tamvatnei, Russia: Dmitrenko et al. 1990;
Krasnogorsky, Russia: Palandzhian et al. 1994; Ojen,
Spain: Torrez-Ruiz et al. 1996; Kempirsai, Kazakhstan:
Melcher et al. 1997; Rai–Iz, Russia: Garuti et al. 1999;
Ust’–Bel’sky, Russia: Rudashevsky et al. 1999).
The composition of Ru–Os sulfides from podiform
chromitite (Figs. 4a, b) is close to the wide compositional range for the laurite–erlichmanite series ofmantle-hosted chromitites (Garuti et al. 1999, and references cited therein). A broad spectrum of element
substitutions is also characteristic of PGE sulfarsenides
at Kraubath and Hochgrössen (Thalhammer & Stumpfl
1988, Thalhammer et al. 1990, Melcher 2000, Malitch
et al. 2001b, 2002c, this study). These PGM are represented by (1) irarsite, platarsite and hollingworthite (Fig.
4e), all of which have cubic symmetry (space group:Pa3) and commonly form extensive solid-solution
among end members, (2) Ru–Os sulfarsenide, namely
ruarsite (Fig. 4c), characterized by monoclinic symmetry (space group: P21/c), and (3) Pt–Ru sulfarsenide,
which shows an uncommon solid-solution between cubic PGE sulfarsenide (e.g., platarsite, hollingworthiteand irarsite) and monoclinic ruarsite (Fig. 4f). Similarcomplex PGE sulfarsenides have been reported from anumber of dunite–harzburgite and lherzolite complexes
worldwide (e.g., Shetland, United Kingdom: Prichard
et al. 1986, Prichard & Tarkian 1988; Osthammeren,
Norway: Nilsson 1990; Ojen, Spain: Torres-Ruiz et al.
1996). Such PGE sulfarsenides commonly exhibit acomplex intergrowth (Figs. 3d, g), and also are associated with laurite or sperrylite (Figs. 3a, e, g, h).
Mineralogical and Os isotopic constraints
for the source of the PGE mineralization
We assume that the primary PGM, generally exhibiting equilibrium assemblages, formed early as part ofthe chromite precipitation event, similar to what has
been suggested by Talkington et al. (1984), Augé &Johan (1988), Melcher et al. (1997), Garuti et al. (1999)
among others. Mineralogical evidence from podiform
chromitite at Kraubath and Hochgrössen suggests that
the PGM were trapped partly as solids (i.e., single grainsof laurite or Os–Ir alloy, and polyphase grains containing PGE alloys + laurite) as well as in the liquid state(other polyphase PGM).

331 vol 41#2 avril 03 - 07

344

The Os isotope compositions of PGM from Kraubathand Hochgrössen correspond to those estimated for the
mantle and mantle peridotites, which have relatively low
187
Os/188Os values as a result of evolution in a low Re/
Os environment (e.g., Hattori & Hart 1991, Luck &Allègre 1991, Martin 1991, Snow & Reisberg 1995,
Shirey & Walker 1998). However, the Os isotopic compositions are somewhat different from that of laurite
measured by N–TIMS (Fig. 5) and from the whole-rock
Os isotopic composition of chromite concentrates fromchromitites at Kraubath and Hochgrössen (Meisel et al.
1997, Melcher et al. 1999b). The fact that the Ru–Os
sulfides from sample K 142 measured by N–TIMS do
not show significant variation may be due to the limited
number of PGM grains studied (n = 2), a statement thatis also valid for the two PGM grains from highly altered
chromitite (sample K 137) measured by LA–MC–ICP–MS. It is noteworthy that these grains (K 137–121 andK 137–83) contain secondary PGM that have been attributed to late overprinting events at Kraubath (Malitch
et al. 2001b). If, therefore, the Re–Os system was perturbed, then the PGM would be expected to yield themost radiogenic values. However, the PGM grains fromhighly altered chromitite (Table 3, Fig. 5, sample K 137)
are less radiogenic than many other samples of laurite
from typical podiform chromitite (Table 3, Fig. 5,
samples K 142 and Hg 71). In contrast, two whole-rock187
Os/ 188 Os values determined on chromite frompodiform chromitite at Kraubath and Hochgrössen
(Meisel et al. 1997) and ten 187Os/188Os values determined on chromite concentrates at Kraubath (Melcher
et al. 1999b) closely match either present-day depleted
MOR mantle (DMM) or an enriched mantle source (e.g.,
0.1239–0.1271 and 0.1329–0.2028, respectively). Thus,
Os isotopic data obtained on chromite concentrates of
the Kraubath and Hochgrössen massifs tend toward
more radiogenic values compared to results for PGM
obtained by N–TIMS (Malitch 2001) and LA–MC–
ICP–MS studies. The high degree of Os isotopic heterogeneity obtained implies that comprehensive sets of
Os isotope data (e.g., on PGM, chromite, chromitite,etc.) will be needed to provide better understanding of
Os isotopic behavior in the ophiolitic mantle. Therefore,a restricted number of Os isotope analyses, especiallyobtained on whole rocks from a particular ophiolite occurrence (Walker et al. 2002), should be treated withcaution since these can lead to ambiguous interpretations.
The early formation of Os-rich minerals at high temperatures implies that the Os isotopic composition oflaurite and ruarsite reflects that of the source region in
the mantle. Therefore, the low 187Os/188Os values, corresponding to the Os isotopic composition of the mantle,
indicate a common subchondritic mantle source for the
PGE. Furthermore, the unradiogenic 187Os/188Os valuesof laurite and ruarsite are similar to those of detrital Osrich alloys derived from other dunite–harzburgite andlherzolite complexes worldwide (Hattori & Hart 1991,

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LAURITE AND RUARSITE FROM PODIFORM CHROMITITES, AUSTRIA

Hattori et al. 1992, Hattori & Cabri 1992, Lyon et al.
1997, Hirata et al. 1998, Bird et al. 1999, Ohnenstetter
et al. 1999, Rudashevsky et al. 1999, Malitch et al.
2002b, Meibom & Frey 2002, among others). For instance, the bedrock laurite and ruarsite from the Eastern
Alps, Austria and the detrital Os-rich alloys derived
from several other dunite–harzburgite and lherzolitecomplexes (e.g., the Kunar Complex, Taimyr Peninsula,and the Ust’–Bel’sky Complex, Far East, both in Russia, and the Klamath Mountains, California and Oregon,
U.S.A.) have a wide range of subchondritic 187Os/188Os
values (0.1094–0.1259, n = 67, Fig. 6) and Ost=0 values (e.g., from –14.95 to –2.12, n = 67). On the contrary, the range of 187Os/188Os and the Os values in
PGM derived from clinopyroxenite–dunite complexes(also known as zoned, Uralian, Alaskan or Aldan type)are distinctly different from those observed in dunite–
harzburgite complexes, as pointed out by Malitch et al.
(2002a). Indeed, PGM derived from the Guli, Kondyor
and Inagli clinopyroxenite–dunite complexes (Siberian
Craton, Russia) show a narrow range of subchondritic
187
Os/188Os values (e.g., 0.12432–0.12520, n = 30), consistent with a single-stage process of PGE concentra-

345

tion in these complexes and with a significant
metallogenic potential of their parent ultramafic
protoliths (Malitch 1999, Malitch et al. 2001c, 2002a,
Malitch & Badanina 2002, Malitch & Thalhammer
2002).
A wide scatter of osmium isotope compositions oflaurite and ruarsite is in obvious disagreement withpresent-day 187Os/188Os estimates for various mantle
reservoirs [i.e., chondritic mantle (CHUR) 0.12736 (Yinet al. 1996) or 0.1270 (Shirey & Walker 1998), depleted
upper MOR mantle (DMM) 0.1246 (Snow & Reisberg
1995), convecting upper mantle 0.12809 (Walker et al.
2002), and primitive upper mantle (PUM) 0.1296(Meisel et al. 2001)]. Our results suggests that the 187Os/
188Os value for the ophiolitic upper mantle, deduced
from bedrock PGM, lies within the range of 0.1158–
0.12437 (n = 18).
Os isotopic age constraints andgeodynamic implications
With rare exceptions (Kusky et al. 2001), mostophiolites worldwide are considered to be younger than

FIG. 6. Os isotopic composition of PGM from ophiolite-type complexes. LA–MC–ICP–MS data: this study. N–TIMS data aretaken from Malitch (2001) for PGM from Speik, from Malitch et al. (2002b) for PGM from Kunar, from Rudashevsky et al.
(1999) for PGM from Ust‘–Bel‘sky, and from Meibom & Frei (2002) for PGM from the Klamath Mountains. Error bars areshown in cases where expanded uncertainty exceeds the size of the diamond symbol.

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1 Ga. Their ages of formation show distinct peaks in the
Late Proterozoic (Late Riphean), Cambro-Ordovician
and Jurassic-Cretaceous (Abbate et al. 1985, Nicolas
1989, Ishiwatari 1994). The similarity of the Kraubathand Hochgrössen massifs to an ophiolite has been recognized by El Ageed et al. (1980) and Stumpfl & El
Ageed (1981). Consequently, these massifs have beeninterpreted as strongly metamorphosed Early Paleozoic
or Precambrian dismembered ophiolites, emplaced during Variscan nappe tectonics (Neubauer 1988, Neubauer
et al. 1989). The Speik Complex rests tectonically on
the Core Complex (Fig. 1b), a Late Proterozoic to Early
Paleozoic magmatic arc assemblage, which underwent
a complex polyphase metamorphic evolution. Metamorphism was polyphase, including (1) pre-Variscan (older
than 400 Ma) eclogite facies (700°C, >1.5 GPa) documented in mafic rocks at Hochgrössen (Faryad et al.
2002), (2) Variscan amphibolite facies, and (3) early
Alpine (~100 Ma) greenschist facies (Puhl 2000, Faryad& Hoinkes 2001). Metamorphism was accompanied bypervasive flow of fluid along faults and fissures.
The age of the ultramafic protoliths in the Speik
Complex is still poorly constrained. Recent attempts
using whole-rock analyses (Sm–Nd and Re–Os systematics) of ultramafic rocks of the Speik Complex gaveerrorchron “ages” ranging from about ~780 Ma for the
harzburgites to ~550 Ma for intrusive orthopyroxenites,
respectively (Fig. 12 in Melcher et al. 2002, Melcher &Meisel, submitted). Suprachondritic 187Os/188Os values(187Os/188Os(I) = 0.178 ± 0.003) characteristic of orthopyroxenites are different from subchondritic 187Os/188Os
values of the harzburgites hosting them (0.12270–
0.12578). On the basis of these results, a two-stage scenario has been proposed (Melcher et al. 2002): (1)partial melting of undepleted mantle during the Late
Proterozoic resulted in formation of residual harzburgite
in a mid-ocean ridge system or evolved back-arc basin;(2) second-stage melting in a supra-subduction-zone
setting during the Early Cambrian led to the formationof a highly depleted residual mantle (harzburgites anddunites) and mantle-derived melts, from which theorthopyroxenites formed.
Our age estimates on the formation of the ultramafic
protoliths in the Eastern Alps, based on osmium isotopic composition of PGM (N–TIMS and LA–MC–ICP–MS studies), yielded model ages in the range of 1758 to
584 Ma, assuming a chondritic mantle reservoir(CHUR) (Figs. 5, 6). Furthermore, U–Pb(Th) model
ages of highly radiogenic uraninite (U,Pb,Th)O2 frombanded chromitite, which occurs above the mantle section at Kraubath, vary from 1400 to 1200 Ma (Malitch
et al., in prep.). These Re–Os and U–Pb model ages areindicative of the existence of a Precambrian parent ultramafic protolith, which most likely formed during theLate Proterozoic.
The unradiogenic 187Os/188Os values of laurite andruarsite from podiform chromitites at Kraubath and
Hochgrössen (0.11580–0.12437, n = 18, N–TIMS and

331 vol 41#2 avril 03 - 07

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LA–MC–ICP–MS studies) provide isotopic evidence
for locally restricted but temporally extended periodsof PGM formation. The Os isotopic composition ofthese PGM indicates that they record much older melting events than would be expected from single-stage
melting of undepleted mantle. As one of the possible
explanations of this phenomenon, the PGM remained
isolated from the convecting upper mantle after theirformation. In this case, peridotites of the Speik complex are not simple residues after partial melting at a
mid-ocean ridge system or evolved back-arc in the Late
Proterozoic. Instead, they may represent a mixture of(1) refractory isolated blocks that retain much older agesand (2) ultramafic rocks formed during a partial melting episode (~780 Ma). A similar scenario has been
advocated by Parkinson et al. (1998) and Snow &Schmidt (1999) for the Izu–Bonina–Mariana andZabargad peridotites, respectively. Furthermore, Os isotope model ages of PGM at Kraubath are older than thetime of formation of orthopyroxenite (~550 Ma), implying that subduction zones indeed may represent
graveyards for ancient oceanic lithosphere, as proposed
by Parkinson et al. (1998).
On the other hand, another option to interpret theresults obtained would be to consider the mantle source,from which the PGM were derived, as much less radiogenic than presently assumed. Within this scenario, Os
isotope heterogeneity might also reflect the effect ofvariable degrees of partial melting of this mantle source.
Finally, the origin of this heterogeneity may be also attributed to the presence of subcontinental lithosphericmantle (SCLM), characterized by unradiogenic 187Os/
188Os values [i.e., <0.1220 (Handler et al. 1997) or<0.1160 (Nägler et al. 1997)], which has been lately
incorporated into asthenospheric mantle with more radiogenic 187Os/188Os values [i.e., 0.1220–0.1230 (Snow& Reisberg 1995, Shirey & Walker 1998)].
In our opinion, the wide range of subchondritic
187Os/188Os values of PGM (e.g., laurite, ruarsite andOs-rich alloys) derived from the dunite–harzburgite
complexes worldwide (Fig. 6) is consistent with a modelin which a prolonged history of melting events of parent ultramafic source-rocks took place in the mantle.
The Os isotopic system of PGM thus unequivocallyrecords multiple events in the chemical differentiation
history of the Eastern Alpine mantle relics, represented
by the mantle sections at Kraubath and Hochgrössen.
We further propose that substantial heterogeneity among
the unradiogenic 187Os/188Os values is a feature typicalof PGM from the mantle section of an ophiolite. Consequently, suprachondritic 187Os/ 188Os values (i.e.,>0.12863), which also have been detected in detrital
PGM grains (e.g., Hattori & Hart 1991, Rudashevsky etal. 1999, Meibom & Frey 2002) could indicate derivation from a distinct source other than dunite–harzburgite
sequences from the mantle section of an ophiolite. Most
conceivable sources for such PGM would be the transition zone or crustal sequence of an ophiolite. Once ex-

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LAURITE AND RUARSITE FROM PODIFORM CHROMITITES, AUSTRIA

tensive Os isotopic data on bedrock PGM from
ophiolites of different age (i.e., Proterozoic, Paleozoic,
Mesozoic, Cenozoic) become available, a more plausible explanation could be developed.
Finally, Os isotope systematics suggest that meltdepletion event(s) recorded by unradiogenic 187Os/188Os
values at Kraubath and Hochgrössen and certain otherperidotite occurrences worldwide (e.g., Hattori & Hart
1991, Parkinson et al. 1998, Snow & Schmidt 1999,
Malitch & Kostoyanov 1999, Brandon et al. 2000,
Malitch et al. 2002a; Fig. 6) are older than the time of
their emplacement in the crust. This observation is similar to some extent to the phenomenon recorded by Re–
Os isotopes in sulfide inclusions in diamonds (Spetsius
et al. 2002).

CONCLUSIONS
1. A combination of the unique dataset on PGM,
obtained by a combination of various techniques ofseparation and concentration (including the hydroseparation method), and osmium isotope study (LA–MC–
ICP–MS and NTI–MS), has allowed us for the first time
to measure relatively small PGM grains from bedrockand unequivocally to determine the Os isotopic sourceof PGE-mineralization in podiform chromitites of Eastern Alpine mantle sections. Podiform chromitites at
Kraubath and Hochgrössen are characterized byunradiogenic 187Os/188Os and Ost=0 values, indicativeof a subchondritic mantle source of the PGE. These
values yield a very wide range of 187Os/188Os (0.11580
to 0.12437) and Os (–9.97 to –3.31) values, which isalmost identical to the Os isotope composition of detrital Os-rich alloys derived from other ophiolite-type ultramafic massifs. The unradiogenic 187Os/188Os values,
combined with the high degree of Os isotopic heterogeneity documented, are considered a typical feature of
the mantle section of an ophiolite.
2. Calculated 187Os/188Os model ages of laurite andruarsite, assuming a chondritic mantle reservoir(CHUR), vary from 1758 to 584 Ma. Therefore, they
likely indicate the existence of a Precambrian ultramafic protolith. The range of model ages for PGM at
Kraubath is in basic accordance with that for PGM at
Hochgrössen. The Os isotopic evidence supports theassumption of a genetic link between the ultramafic
protoliths for the Kraubath and Hochgrössen dunite–
harzburgite bodies as parts of the Speik Complex.
3. The Os isotope results show that the Re–Os system in PGM remained unchanged from the time of formation of the PGM until now, despite later thermal
events affecting both massifs. The evidence of longlived osmium isotope heterogeneity obtained implies
that comprehensive sets of Os isotope data can only
provide a valid understanding of Os isotopic behavior
in the ophiolitic upper mantle. Therefore, a restricted
number of Os isotope analyses should be treated withcaution. Finally, we infer that the Os isotopic composi-

331 vol 41#2 avril 03 - 07

347

347

tion of PGM can be employed to test the validity ofvarious petrological models.

ACKNOWLEDGEMENTS
Financial support by (1) the Austrian Science Fund
(Project 601–CHE to K.N. Malitch), (2) the BMBF,
Forschungszentrum Jülich (Project BEO21–04040323–
03PE9FRE/7 to E. Pernicka) and (3) the Austrian Academy of Science (Project “Secondary remobilisation of
PGE in the Kraubath ultramafic complex” to O.A.R.
Thalhammer) is gratefully acknowledged. The seniorauthor (K.N.M.) greatly appreciates the productive atmosphere in the Department of Mineralogy and Petrology, Institute of Geological Sciences, Mining Universityof Leoben and the financial support of the Austrian
Academy of Sciences, which helped to speed up thecompletion of this study. We express our sincere thanks
to Yury G. Shvyrkov, Alla M. Tikhonova, Lyudmila A.
Ushinskaya (Mekhanobr-Analyt, St. Petersburg, Russia)
and Helmut Mühlhans (University of Leoben, Austria)
for analytical help. This manuscript has benefitted
greatly from stimulating discussions with Anatoly I.
Kostoyanov (All-Russia Research Geological Institute,
St. Petersburg, Russia), and thorough reviews by
Giorgio Garuti and Antonio Simonetti. Finally, we express our gratitude to Robert F. Martin for his very careful handling of the manuscript and helpful comments.

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