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Geochimica et Cosmochimica Acta 100 (2013) 158–175

Highly siderophile element geochemistry of peridotitesand pyroxenites from Hornı Bory, Bohemian Massif: Implications

for HSE behaviour in subduction-related upper mantle

Lukas Ackerman a,b,⇑, Lynnette Pitcher c, Ladislav Strnad d, Igor S. Puchtel c,Emil Jelınek d, Richard J. Walker c, Jan Rohovec a

a Institute of Geology v.v.i., Academy of Sciences of the Czech Republic, Rozvojova 269, 165 00 Praha 6, Czech Republicb Czech Geological Survey, Geologicka 6, 165 00 Praha 5, Czech Republic

c Isotope Geochemistry Laboratory, Department of Geology, University of Maryland, College Park, MD 20742, USAd Faculty of Science, Charles University, Albertov 6, 128 43 Praha 2, Czech Republic

Received 13 May 2012; accepted in revised form 26 September 2012; Available online 12 October 2012

Abstract

Highly siderophile element (HSE – Re, Os, Ir, Ru, Rh, Pd, Pt, and Au) and S abundances, sulphide petrography and187Os/188Os compositions of two distinct suites – Mg-lherzolite and Fe-dunite/wehrlite accompanied by pyroxenites fromHornı Bory, Bohemian Massif, Czech Republic are reported. The Mg-lherzolite suite is enriched in IPGE (Os, Ir, andRu), moderately depleted in Pd, Pt and S, and has osmium isotopic compositions similar to that of the Phanerozoic convect-ing mantle. Such a composition can be best explained by reaction between previously depleted peridotite and sulphur-under-saturated melt at low melt-rock ratios. This reaction was associated with precipitation of IPGE alloys and Pt–As. The Fe-dunite/wehrlite suite is depleted in IPGE, enriched in Pd, Pt, and S and has radiogenic osmium isotopic compositions.The origin of this suite reflects melt-rock reaction between peridotite and subduction-related melt at high melt/rock ratiosresulting in breakdown of primary sulphides and their replacement by HSE-poor but 187Os-enriched sulphides. Mass-balancecalculations and relations between HSE and S contents show that, while most of the Re is trapped in base metal sulphides(BMS), only up to 28% of the rest of the HSE (Os, Ir, Ru, Pd, Pt) are accommodated in BMS. Several Pt–As phases discov-ered in Mg-lherzolite and Fe-dunite/wehrlite and Os–Ru phase found in Mg-lherzolite suggesting complex behaviour of HSEduring melt-rock reactions depending on melt composition and melt/rock ratios. The Re–Os data for the Fe-wehrlite/duniteand pyroxenite rocks yield an errorchon age of 334 ± 19 Ma, which is similar to the age of other high temperature/pressurerocks from Gfohl Nappe (Moldanubian Zone, Bohemian Massif); this age likely corresponds to the time of melt-rock reac-tions and subduction.� 2012 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

Highly siderophile elements (HSE, including Os, Ir, Ru,Pt, Rh, Pd, Re, and Au), together with the 187Re–187Os iso-

0016-7037/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.gca.2012.09.050

⇑ Corresponding author at: Institute of Geology v.v.i., Academyof Sciences of the Czech Republic, Rozvojova 269, 165 00 Praha 6,Czech Republic. Tel.: +420 233087240; fax: +420 220922670.

E-mail address: ackerman@gli.cas.cz (L. Ackerman).

topic system, can provide important insights to mantle ref-ertilization processes that occur via interactions betweenperidotites and migrating basaltic melts of various compo-sitions (e.g., Becker et al., 2001; Saal et al., 2001; Buchlet al., 2002, 2004; Lorand et al., 2003; Harvey et al.,2010; van Acken et al., 2010; Liu et al., 2011). Osmium,Ir and Ru typically behave as compatible elements duringmantle melting (basaltic melt formation), while Pt, Rh,Pd, Au, and Re are usually incompatible, depending onthe degree of partial melting (e.g., Lorand et al., 2004; Pear-

L. Ackerman et al. / Geochimica et Cosmochimica Acta 100 (2013) 158–175 159

son et al., 2004; Puchtel et al., 2004). Therefore, the reactionof basaltic melts with previously melt-depleted peridotitescan result in complex modifications to the absolute and rel-ative abundances of the HSE, as well as Os isotopic ratios.Prior studies have documented both addition of some HSEto refertilized rocks (e.g., Becker et al., 2001; Luguet et al.,2003, 2004; Pearson et al., 2004; Ionov et al., 2006; vanAcken et al., 2008, 2010), as well as depletion (Lorandet al., 2000; Becker et al., 2001; Buchl et al., 2002; Acker-man et al., 2009a). Such processes can also lead to increasesor decreases in 187Os/188Os ratios in the hybridized rocks(e.g., Brandon et al., 1996, 1999; Becker et al., 2004). Thevariable behaviour reflects the fact that HSE distributionsin mantle rocks are mostly controlled by sulphides, whichcan be removed and/or added during the melt-rock reac-tions (e.g., Buchl et al., 2002; Lorand et al., 2004; Luguetet al., 2004; Sen et al., 2011).

Osmium isotopes can also potentially be affected byaqueous transport, so while some studies have suggested187Os enrichment of mantle peridotites by aqueous fluids(Brandon et al., 1996; McInnes et al., 1999), other studieshave pointed to enrichment by melt-rock reactions (Beckeret al., 2004; van Acken et al., 2010). The mechanisms ofsuch isotopic enrichments are still unclear and likely includediverse processes. For e.g., mantle pyroxenites, formed ascumulates from subduction-related basaltic melts, typicallyhave supra-chondritic 187Os/188Os compositions (Beckeret al., 2004; Pearson and Nowell, 2004; van Acken et al.,2010). Consequently, the interactions of such melts withmantle peridotite would likely lead to an increase in187Os/188Os. On the other hand, these melts cannot be di-rectly derived from a subducting slab, because slab meltingexperiments have shown that during melting, aqueous fluidsor melts of tonalitic, not basaltic compositions originateand penetrate the mantle wedge (e.g., Sen and Dunn,1994; Rapp and Watson, 1995; Rapp et al., 2003).

Here, we present HSE data and 187Os/188Os isotopiccompositions of two distinct, well-characterized peridotitesuites from the Hornı Bory locale, of the Bohemian Massif,Czech Republic. One suite, consisting of Mg-lherzolites, hasgenerally similar compositions to other worldwide occur-rences of peridotites that experienced variable melt extrac-tion. The second suite consists primarily of unusually Fe-rich dunite/wehrlite, accompanied by pyroxenites. Previousstudies by Medaris et al. (2005) and Ackerman et al.(2009b) have concluded that some Mg-lherzolites weretransformed to Fe-dunite/wehrlite by interaction withSiO2-undersaturated, subduction-related basaltic melts.Thus, study of these two suites may allow documentationof the effects these processes have on HSE fractionationsand 187Os/188Os exchange during subduction-related melt-rock reactions.

2. GEOLOGICAL SETTING

The Bohemian Massif represents the largest Variscanorogenic belt in Europe, which formed during collision ofGondwana, Laurussia and some microcontinents duringthe Late Paleozoic (e.g., Matte, 2001). Four major tectonicunits can be distinguished within the Bohemian Massif: the

Moldanubian, the Tepla-Barrandien, the Saxothuringianand the Moravo-Silesian (see Electronic Supplementary filefor Tectonostratigraphic map). Within the Moldanubian,the uppermost lithostratigraphic unit – the Gfohl Nappe,is characterized by high-pressure granulite, orthogneissand migmatites. This unit contains abundant disruptedbodies of spinel/garnet peridotites, pyroxenites andeclogites.

At Hornı Bory, the ultramafic rocks form boudins rang-ing from decimetres to tens of metres across in high-temper-ature, high-pressure Bory granulites are exposed in a quarrynear the village of Hornı Bory. Based on mineralogy andgeochemistry, three types of ultramafic boudins have beendistinguished (Medaris et al., 2005; Ackerman et al.,2009b): (1) Mg-lherzolite with #Mg (100 � [Mg/Mg+Fe])from 91 to 89, (2) Fe-rich dunite/wehrlite with #Mg from88 to 81, and (3) pyroxenite with #Mg from 85 to 77.The Mg-lherzolite is usually a strongly serpentinized, five-phase peridotite, consisting of variable proportions of oliv-ine, orthopyroxene, clinopyroxene, Cr-spinel and garnet.

The Fe-rich dunite/wehrlite consists of olivine (83–88%),orthopyroxene (<5%), clinopyroxene (4–11%) and spi-nel + garnet (2–7%). This type of fresh rock representsthe predominant type of ultramafic boudins in the HornıBory quarry. It has a slightly foliated, equigranular textureand, locally, a layered aspect due to thin layers of clinopy-roxene. Some Fe-dunite/wehrlite boudins have a compositestructure with interlayering of Fe-dunite/wehrlite andpyroxenite on a scale of 1–6 cm (e.g., sample 06HB2 – seeFig. 2A in Ackerman et al., 2009b).

The pyroxenite compositions range from clinopyroxe-nite to websterite, all containing clinopyroxene, orthopy-roxene, garnet, and minor or accessory amounts ofolivine, ilmenite, rutile, phlogopite, and secondary amphi-bole. Compared to pyroxenites from ultramafic massifs(e.g., Ronda, Beni Bousera: Bodinier and Godard, 2003and references therein), all Hornı Bory pyroxenites haveconvex-upward REE patterns, coupled with slight LREEenrichment (CeN/YbN = 1.6–1.9; Ackerman et al., 2009b).modeling of Mg–Fe exchange between Mg-lherzolite andFe-rich melt reveals that the modal and chemical composi-tions of the Fe-dunite/wehrlite suite can be produced bymelt-rock reaction between Mg-lherzolite and SiO2-under-saturated melts of basaltic composition, with melt/rock ra-tios ranging from 0.3 to 2 (Ackerman et al., 2009b).

The Fe-rich dunite/wehrlite suite is significantly enrichedin large-ion lithophile elements (LILE), depleted in high-field strength elements (HFSE), and has radiogenic calcu-lated initial 87Sr/86Sr ratios ranging from 0.7077 to0.7087, all of which point to a significant component of sub-ducted crustal material (Ackerman et al., 2009b). Thepyroxenites could represent crystal cumulates (± trapped li-quid) from melts migrating along conduits, and reactingwith, peridotite in a mantle wedge above a Variscan sub-duction zone (Medaris et al., 1995; Ackerman et al., 2009b).

3. SAMPLES AND ANALYTICAL METHODS

Five samples of Mg-lherzolite, five Fe-rich dunite/wehrliteand three pyroxenites were selected for this study (Table 1).

Table 1Petrography and modal abundances of sulphide assemblages.

Sample Rock type Sulphide modal abundance (wt.%) Pn Hz Gd Po Mss Cp Mau Cub Mt

HB2 Mg-lherzolite 0.02 ++ +++97CZ3C Mg-lherzolite 0.05 ++ ++ + (+) (+) + +++06HB4 Mg-lherzolite 0.06 ++ ++ +++99BY1B Mg-lherzolite 0.04 ++ + +++

HB13 Fe-wehrlite 0.29 ++ ++ ++ (+) + +85GM8B Wehrlite-clinopyroxenite 0.08 ++ ++ (+) + + (+)HB7 Fe-wehrlite 0.26 ++ ++ + ++ + (+)HB14 Fe-wehrlite 0.09 ++ + + + + +99BY3A Fe-dunite 0.26 ++ ++ + + + +

HB3 Clinopyroxenite 0.17 + + ++HB4 Websterite 0.26 + + ++HB5 Websterite 0.09

06HB2B Fe-wehrlite 0.76 ++ ++ + +++06HB2C Clinopyroxenite 0.72 + +++ (+) (+)06HB2D Fe-wehrlite 0.34 ++ ++ + +++06HB2E Olivine clinopyroxenite 0.35 + + ++ (+)

Pn, pentlandite; Hz, heazlewoodite; Gd, godlevskite; Po, pyrrhotite; Mss, mono sulphide solid solution; Cp, chalcopyrite; Mau, maucherite.Cub, cubanite; Mt, magnetite. Sulphide modal abundance: +++, very common; ++, abundant; +, common; (+), rare.Sulphide modal abundance calculated from bulk rock sulphur contents and composition of sulphide assemblages.

Fig. 2. Osmium concentration variations with other highly siderophile elements (Ir, Ru, Pt, Pd). Note positive correlations with Ir and Ru(r = 0.92 and 0.95, respectively).

160 L. Ackerman et al. / Geochimica et Cosmochimica Acta 100 (2013) 158–175

L. Ackerman et al. / Geochimica et Cosmochimica Acta 100 (2013) 158–175 161

Also, one composite sample showing interlayering of Fe-richdunite/wehrlite and pyroxenite was chosen to assess HSEbehaviour resulting from possible interactions betweenperidotite and a percolating melt. Petrography, major andtrace element concentrations, as well as Sr–Nd isotopic geo-chemistry of these samples, have been previously reported(Ackerman et al., 2009b). The GCDkit geochemical softwareprogram (Janousek et al., 2006) was used for whole-rockdata processing and plotting.

Total sulphur concentrations in whole-rocks were deter-mined by Inductively Coupled Plasma-Optical EmissionSpectrometry (ICP-OES) at the Institute of Geologyv.v.i., Academy of Sciences of the Czech Republic usingthe procedures outlined in Ackerman et al. (2012).

Highly siderophile element concentrations and Os isoto-pic data were obtained at the Isotope Geochemistry Labora-

tory, University of Maryland (UMd) using the techniquesoutlined in Puchtel et al. (2009a,b) and Ackerman et al.(2009a). In brief, 1–2 g of whole-rock sample powder wassealed with appropriate amounts of 185Re–190Os and191Ir–99Ru–105Pd–194Pt spikes and digested in pre-cleanedCarius tubes using 4 mL concentrated HCl and 6 mL con-centrated HNO3 at 280 �C for 48–72 h (Shirey and Walker,1995). Osmium was extracted from the aqua regia solutioninto CCl4 and then back-extracted into HBr (Cohen andWaters, 1996). The final Os-fraction was purified by micro-distillation (Birck et al., 1997). Iridium, Ru, Pt, Pd and Rewere separated by anion exchange chromatography usingAG 1 � 8 resin (BioRad). Osmium concentrations and iso-topic ratios were obtained using N-TIMS (Creaser et al.,1991; Volkening et al., 1991). Samples were analyzed usinga VG Sector 54 thermal ionization mass spectrometer onFaraday cups in static mode, or using an electron multiplierin a peak hopping mode. Blanks were analyzed using anNBS 12-inch mass spectrometer using an electron multiplierin a peak hopping mode. In-run precision for 187Os/188Osfor both instruments was always better than ±0.2%(2rm). The measured Os isotopic ratios were correctedfor mass fractionation using 192Os/188Os = 3.08271 (Shireyand Walker, 1998). Iridium, Ru, Pt, Pd, and Re were ana-lyzed using a Nu Plasma multi-collector ICP-MS with twoor three electron multipliers in a static mode. The total pro-cedural blanks were 2–6 pg for Os, 1–7 pg for Re, 1.0 pg forIr, 10–11 pg for Ru, 12–41 pg for Pd and 42–56 pg for Pt.Blank contributions to samples were <1% except for extre-mely HSE-poor sample 06HB2E with blank contributionup to 9% for Pt and 5% for Os. In situ analyses of Os, Ir,Ru, Rh, Pd, Pt, Re, and Au in sulphides were performedin polished thin sections using a quadrupole-based ICP-MS (X-Series II, Thermo Bremen, Germany) coupled to aNewWave UP 213 laser microprobe (New Wave Research,USA) installed at Charles University in Prague. Before theanalyses, all sulphide phases were inspected by optical andelectron microscopy to select only fresh phases devoid ofany visible inclusions on surface. Analyzed sulphide grainsranged in size from 100 lm to 2 mm. The data were ac-quired in the time resolved (TRA) and peak jumping modesand processed offline to correct for possible heterogeneityof the analyzed samples. External calibration was per-formed using SRM Po727 (Memorial University of New-

foundland, Canada) and correction factors were cal-culated from signals of the 34S isotope. The analyticalprotocol closely followed a strategy described by Hincheyet al. (2003) and Strnad et al. (2012). The laser was firedwith the energy of 0.25 mJ, repetition rate of 6 Hz andbeam (spot) size of 80 lm. Rhenium concentrations werecalculated only semi-quantitatively using PlasmaLab soft-ware, as this element is not present in SRM Po727. The sig-nal of Re was measured using a peak jumping acquisition inthe same run as other elements present in SRM Po727 (Os,Ir, Ru, Pt, Pd and Au) and Re senstivity was integratedagainst a response curve generated from the sensitivitiesof Os, Ir, Ru, Pt, Pd and Au.The analytical precision ofindividual laser ablation ICP-MS analyses (one relativestandard deviation, RSD) strongly depend on heterogeneityof the samples and analyzed concentration. On average, thestandard deviations ranged from 5 to 80% for samples andfrom 3 to 7% for the SRM Po727 standard. Determinationof HSE by the laser ablation ICP-MS was facilitated by thelow background, relatively good sensitivity and a minimumlevel of contamination. Problems may occur in the analysisof samples with very high transition metals content, espe-cially in the case of sulphides, leading to the formation ofspectral interferences on Ni and Cu (e.g., 40Ar60Ni+ M

100-

Ru or 40Ar65Cu+ M105Pd; van Acken et al., 2010; Guillong

et al., 2011). In this study, we tested the effect of the61Ni40Ar+ and 63Cu40Ar+ species on the signals of101M(101Ru) and 103M(103Rh), respectively. The formationof argides (MAr+/M+) was monitored using both Ni andCu pure directly via ablation. The measured ratios variedstrictly below a 0.000015 for nickel argides (61Ni40Ar+/61-

Ni+) and around 0.000025 for copper argides(63Cu40Ar+/63Cu + and/or 65Cu40Ar+/65Cu+). No signifi-cant effect of nickel argides on the mass 101M(101Ru) wasdetected, and corrections were not required. However, theformation of the 63Cu argide significantly influences mass103M(103Rh) and, therefore, Rh in Cu-rich minerals (e.g.chalcopyrite). Hence, Ru is not reported for chalcopyrite.

Sulphide modal abundances for all samples were calcu-lated from total sulphur concentrations and compositionof sulphide assemblage within individual samples assumingthat no sulphur is hosted by silicate fraction.

We searched for platinum-group minerals (PGM) usinga scanning electron microscope TESCAN Vega 3SEM withX-Flash detector 5010 (ED) and accelerating voltage of20 kV at the Institute of Geology v.v.i., Academy of Sci-ences of the Czech Republic.

4. RESULTS

4.1. Sulphide petrography

All studied peridotites and pyroxenites from Hornı Borycontain abundant sulphides. Modal abundances of sulp-hides in the rocks and their petrography are summarizedin Table 1. In general, three textural types can be identified:(1) isolated, usually rounded, inclusions in silicates, (2)interstitial anhedral sulphide grains commonly found at tri-ple junction boundaries, and (3) small veinlets ofchalcopyrite.

162 L. Ackerman et al. / Geochimica et Cosmochimica Acta 100 (2013) 158–175

The Mg-lherzolites contain variable amounts of basemetal sulphides (BMS). Total sulphide modal abundancesrange from 0.02 to 0.06 wt.%. This level of variation islikely mostly due to the heterogeneous distribution of sulp-hides within the studied thin sections. The sulphides presentpredominantly consist of Ni-rich phases, mostly pentlandite[(FeNi)9S8], with only trace amounts of pyrrhotite [Fe(1�x)-

S]. These phases are present as mostly irregular or triangu-lar-shaped grains of variable size (from 10 to 50 lm) ontriple junction boundaries, or as rounded blebs within the

Fig. 1. Petrography of sulphides in Hornı Bory mantle suite (A:plane-polarized reflected light, B–H: back scattered electron (BSE)images). (A) Rounded pentlandite inclusion in olivine altered tomagnenite (Mg-lherzolite 97CZ3C). (B) Irregular grain of pent-landite (Pn) replaced by heazlewoodite (Hz-Ni3S2) and magnetite(Mt) from Mg-lherzolite (sample 97CZ3C). (C) Pt–As inclusion(likely sperrylite) in magnetite (Mt) rim replacing pentlandite (Pn)grain located on triple-junction boundary between silicate grains(Mg-lherzolite 97CZ3C). (D) Pentlandite (Pn) – chalcopyrite (Cp)grains within post garnet sympletite (Fe-wehrlite 85GM8B. (E)Large pentlandite (Pn) – pyrrhotite (Po) grain with exsolution ofmonosulphide solid solution (MSS) located within pentlandite (Fe-wehrlite HB7). (F) Intergranular sulphide grain composed ofpentlandite (Pn), pyrrhotite (Po) and exsolution lamellaes ofbornite (Cb) (Fe-wehrlite HB13). (G) Maucherite (Mau) locatedwithin pentlandite (Pn) rounded inclusion enclosed in olivine (Fe-wehrlite HB13). (H) Chalcopyrite (Cp) grain and veinlets frompyroxenite sample HB3.

silicates (Fig. 1A). Interstitial grains in contact with serpen-tine are strongly altered to magnetite. Locally, pentlanditeis replaced by NiS sulphides, mainly heazlewoodite(Ni3S2; Fig. 1B), but also rare godlevskite [(NiFe)9S8]. Allof these secondary phases are commonly associated withNi–Fe alloys, suggesting low-temperature alteration atstrongly reducing conditions (e.g., Lorand, 1985). Chalco-pyrite (CuFeS2) was found only in one thin section of sam-ple 97CZ3C, where it occurs as very rare, small (<5 lm)irregular grains located in spinel-pyroxene symplectitesreplacing garnets. Euhedral grains of maucherite (Ni11As8)are locally found within pentlandite grains of this sample.Scanning electron microscope (SEM) analysis revealed thepresence of two Pt–As phases (� 0.5 lm in diameter) thatwere detected in magnetite rims surrounding pentlanditegrains that occur on silicate triple junction boundaries(e.g., Fig. 1C).

In comparison to Mg-lherzolite, rocks from the Fe-du-nite/wehrlite suite contain Ni–Fe–Cu base metal sulphidesof variable shapes (usually euhedral, triangular) and sizes(20–700 lm) located on grain boundaries, or as roundedinclusions within the silicates. Except for sample HB14,these rocks also tend to be richer in sulphides comparedto the Mg-lherzolite, as reflected by the much higher andheterogeneous modal sulphide abundances (0.08 to0.29 wt.%). Pentlandite (>50%) and pyrrhotite (20–50%)predominate in sulphide mineral paragenesis, but chalcopy-rite is widespread with variable contents (Table 1). In somesamples, chalcopyrite is common (e.g., HB13), while in oth-ers it is rare (e.g., 85GM8B). Chalcopyrite usually occurs assmall individual grains, or accompanying pentlandite andpyrrhotite in complex Ni–Fe–Cu grains (Fig. 1D). Mono-sulphide solid solution (MSS) was found in only two sam-ples (HB7 and HB14; Table 1). In sample HB7, it formscommon exsolution lamellae within pentlandite in largepyrrhotite–pentlandite grains (Fig. 1E). Conversely, MSSwas found in small pentlandite–chalcopyrite rounded inclu-sions enclosed in olivine in sample HB14. Rare cubanite(CuFe2S3) was found as an exsolution phase within pent-landite–pyrrhotite grains (Fig. 1F) in samples 85GM8Band HB13 (Table 1). Maucherite is common in these rocks,and its presence points to the As-rich nature of the Fe-du-nite/wehrlite suite. It was found in all such samples as smalleuhedral, or needle-like grains within or adjacent to sulp-hides (Fig. 1G). In sample HB7, one euhedral Pt–As grain(likely sperrylite) was found in chalcopyrite located on sili-cate grain boundaries.

The pyroxenites show a wide range of sulphide concen-trations in studied samples (from 0.09 to 0.26 wt.%), withgrains up to 1 mm across usually enclosed within silicates.Compared to the peridotites, the sulphides consist of pent-landite, pyrrhotite, and chalcopyrite. Large grains are com-monly composed of pyrrhotite + pentlandite, in variableproportions, and in some cases accompanied by chalcopy-rite. Smaller euhedral grains, in comparison, are very richin chalcopyrite and may or may not contain pyrrhotiteand pentlandite. Chalcopyrite also occurs in hydrother-mally formed assemblages in veinlets, preferentially fillingthe cracks and commonly cross-cutting several silicatephases (Fig. 1H).

L. Ackerman et al. / Geochimica et Cosmochimica Acta 100 (2013) 158–175 163

Composite sample 06HB2, consisting of Fe-dunite/wehrlite and pyroxenite layers, shows the same sulphidepetrography and mineralogy as the individual rocks de-scribed above (Table 1). However, these samples have high-er calculated sulphide concentrations ranging from 0.34 to0.76 wt.%; Table 1).

4.2. Sulfur concentrations

Total sulphur concentrations of studied samples are gi-ven in Table 2. The Mg-lherzolites have S concentrationsranging from 71 to 199 ppm, reflecting low sulphide con-tents (Table 1). In contrast, Fe-dunite/wehrlite has variable,but much higher S contents ranging from 288 to 1020 ppm.The pyroxenites have similar S concentrations to the Fe-du-nite/wehrlite ranging from 306 to 923 ppm. Two layers ofthe composite Fe-wehrlite and clinopyroxenite sample06HB2 (06HB2B, 06HB2C) have similar, high S contentsof 2662 and 2503 ppm, respectively Two other layers withsimilar bulk composition (06HB2D, 06HB2E) have lowerS concentrations of 1180 and 1230 ppm, respectively, thatare similar to individual Fe-dunite/wehrlite and pyroxenitesamples.

4.3. Whole rock HSE concentrations

Highly siderophile element concentrations as well asMg# (100 � [Mg/Mg+Fe2+]) and Al2O3 contents are listedin Table 2. Significant differences are present among the dif-ferent peridotite lithologies (Mg-lherzolite and Fe-dunite/wehrlite), as well as pyroxenite. Two samples of Mg-lherz-olite (HB2 and 99BY1B) have similar HSE contents to esti-mates for Earth’s primitive mantle (PM – Becker et al.,2006). Also, their HSE composition is similar to other oro-genic peridotites and mantle xenoliths reported from world-wide locations (e.g., Lorand et al., 2000; Lorand and Alard,2001; Pearson et al., 2004; van Acken et al., 2010), as well asmantle peridotites from nearby Sklene locality (Medariset al., 2009) which is located in the same Bory granulitemassif. Two samples (97CZ3C and 06HB4), however, aremarkedly enriched in IPGE (Iridium-group PGE), withOs contents up to 10.7 ppb.

In contrast, except for one sample (HB14) with a veryhigh Os content of 9.3 ppb, the Fe-dunite/wehrlite all showdepletion in IPGE and selective enrichment in the Pd-groupPGE (PPGE: Pt, Pd) and Re (up to 19.4 ppb of Pd,15.2 ppb of Pt and 1.85 ppb of Re), relative to the Mg-lherzolites and PM. Sample HB14 has similar IPGE andRe contents to the Mg-lherzolites, but is enriched in PPGE.Composite Fe-wehrlite-clinopyroxenite sample 85GM8Bhas HSE concentrations that are similar to those presentin the pyroxenites. The pyroxenites have HSE signaturesthat are typical of basalts, with strong depletions in IPGE(Os below 0.44 ppb) and moderate enrichments in Re (upto 2.22 ppb). Rhenium and Os concentrations of pyroxe-nites from Hornı Bory are in the range of pyroxenites re-ported from Beni Bousera, Lower Austria and the Totalpmassif (Becker et al., 2004; Pearson and Nowell, 2004;van Acken et al., 2010). As far as we are aware, Ir, Ru,

Pd and Pt data have been published only for pyroxenitesfrom the Totalp Massif (van Acken et al., 2010). Comparedto clinopyroxenites from the Totalp Massif, the Hornı Borypyroxenites have similar (very low) HSE concentrations.The Fe-wehrlite and pyroxenite from composite sample06HB2 have HSE compositions similar to individual Fe-du-nite/wehrlite and pyroxenite boudins (Table 2).

For most of the samples studied, Os abundances corre-late positively with Ir and Ru (Fig. 2; r = 0.92 and 0.95

for Ir and Ru, respectively). When only Fe-dunite/wehrliteand pyroxenite samples are considered (except for sampleHB14 that is highly enriched in Os), Os also correlates pos-itively with Pt and Pd (Fig. 2; r = 0.88 and 0.85, respec-tively). Similarly, when only Mg-lherzolites areconsidered, Os contents are negatively correlated with Pd(Fig. 2; r = 1.00). Rhenium does not correlate with Yb con-tents or garnet modal abundances (not shown) which argueagainst possible control of Re by garnet (e.g., Righter andHauri, 1998), as suggested for garnet pyroxenite by Roy-Barman et al. (1996). On the other hand, Re show excellentpositive correlation with S contents (r = 0.97; Fig. 3). TheFe-dunite/wehrlite suite (except for sample HB14) showsa positive correlation between IPGE and S (Fig. 3), butnot for Pt and Pd. The Hornı Bory suite does not show cor-relations between HSE concentrations and Al2O3, exceptfor Pd, which shows a broad negative correlation withAl2O3 for Fe-dunite/wehrlite (Fig. 4).

Primitive mantle-normalized patterns for the Mg-lherzo-lites show small and variable enrichments and/or depletionsin Pd and Re (Fig. 5). On the other hand, the normalizedpatterns for the Fe-dunite/wehrlite suite are characterizedby U-shaped HSE patterns (OsN/IrN = 1.3–2.3, PtN/IrN = 1.9–7.5). Sample D from composite rock 06HB2has a high Pt/Pd ratio, resulting in a positive Pt anomalyin the normalized plot (Fig. 5). All pyroxenite samples showlow IPGE concentrations (OsN = 0.01–0.17) and strongenrichments in PPGE and Re relative to IPGE (PtN/IrN = 4.3–9.2 and ReN/IrN = 65–861).

4.4. Whole rock 187Os/188Os compositions and Re–Os

systematics

Present-day 187Os/188Os ratios show large variationsamong the different rock types, ranging from 0.1190 to1.2281. With respect to the individual lithologies, Os isoto-pic ratios in Mg-lherzolites range from 0.1190 to 0.1245. Incontrast, samples of the Fe-dunite/wehrlite suite have moreradiogenic 187Os/188Os ratios ranging from 0.1717 to0.1960, except for anomalously Os-rich sample HB14,which has a ratio of 0.1391. The pyroxenites exhibit veryheterogeneous and highly radiogenic Os isotopic composi-tions ranging from 0.2781 to 1.228. Duplicate analyses ofpyroxenite HB4 yield similar Os contents but different187Os/188Os of 0.3313 and 0.2971, suggesting a heteroge-neous osmium isotopic composition of a principal Os-bear-ing phase within this sample.

Initial 187Os/188Os ratios, calculated for an age of330 Ma (Sm–Nd geochronology of garnet pyroxenite/eclogite – Medaris et al., 2006; Ackerman et al., 2009b),

Table 2Whole-rock HSE and S concentration, Re–Os isotopic data, Al2O3 contents and #Mg* for Hornı Bory peridotites and pyroxenites.

Sample Rock # Mg Al2O3 S Re Os Ir Ru Pt Pd 187Re/188Os 187Os/188Os c Os T MA T RD

wt.% (ppm) (ppb) (ppb) (ppb) (ppb) (ppb) (ppb) 330 Ma (Ga) (Ga)

HB2 Mg-lherzolite 89 1.82 71 0.131 4.14 3.31 8.30 5.46 5.72 0.120 0.1242 �1.0 0.6 0.597CZ3C Mg-lherzolite 91 1.91 180 0.512 7.5 3.28 10.9 5.71 4.41 0.328 0.1220 �3.7 4.0 1.006HB4 Mg-lherzolite 91 2.23 199 0.082 10.7 6.79 17.7 5.44 2.72 0.037 0.1190 �4.8 1.3 1.299BY1B Mg-lherzolite 90 1.35 145 0.361 4.46 4.66 7.87 8.42 5.84 0.391 0.1245 �2.0 0.7

HB13 Fe-wehrlite 86 1.46 1020 1.49 1.77 1.00 2.46 4.66 8.20 4.08 0.1717 19.6 0.785GM8B Wehrlite-clinopyroxenite 80 5.41 288 0.95 0.53 0.31 0.72 1.96 4.35 8.66 0.1960 18.8 0.5HB7 Fe-wehrlite 86 1.15 911 1.43 1.78 1.20 2.53 9.52 13.6 3.85 0.1775 25.2 0.9HB14 Fe-wehrlite 88 1.54 320 0.304 9.3 3.62 8.6 15.2 19.4 0.16 0.1391 10.899BY3A Fe-dunite 86 1.74 924 1.85 1.19 0.89 1.73 4.85 12.7 7.47 0.1881 17.8 0.5

HB3 Clinopyroxenite 82 5.73 572 1.27 0.08 0.06 0.08 b.d. 0.22 81.4 0.6812 86.3 0.4HB4 Websterite 77 4.58 923 2.23 0.44 0.34 0.62 b.d. 3.13 24.9 0.2781 12.9 0.4HB4-dupl 1.93 0.38 0.28 0.49 2.71 2.38 24.5 0.3313 57.1 0.5HB5 Websterite 4.84 306 0.58 0.13 0.06 0.17 0.64 0.74 21.3 0.2971 43.9 0.5

06HB2B Fe-wehrlite 83 3.59 2662 3.97 3.08 2.05 4.55 18.2 23.3 6.20 0.1787 15.8 0.506HB2C Clinopyroxenite 84 5.43 2503 3.87 0.65 0.45 0.83 4.16 11.7 28.5 0.3160 27.1 0.406HB2D Fe-wehrlite 87 2.46 1180 1.82 1.77 1.17 2.18 19.1 6.99 4.94 0.1817 23.8 0.706HB2E Olivine clinopyroxenite 85 7.05 1230 1.72 0.04 0.02 0.10 0.40 1.02 197.6 1.2281 11.1 0.3

*Al2O3 contents and #Mg from Ackerman et al. (2009b). c Os, TMA and TRD values were calculated using chondritic composition of Shirey and Walker (1998).

164L

.A

ckerm

anet

al./G

eoch

imica

etC

osm

och

imica

Acta

100(2013)

158–175

Fig. 3. Variations of total S concentrations with HSE.

L. Ackerman et al. / Geochimica et Cosmochimica Acta 100 (2013) 158–175 165

expressed using the cOs notation (Walker et al., 1989; Ta-ble 2), show distinct variations from moderately negative(�1.0 to �4.8) for the Mg-lherzolite, through strongly var-iable and radiogenic values from +10.8 to +25.2 for Fe-du-nite/wehrlite, and +11.1 to +86.3 for the pyroxenite(Table 2). While these values are in the range of values re-ported for other pyroxenites worldwide (Reisberg et al.,1991; Pearson and Nowell, 2004; van Acken et al., 2010),these studies showed much more variable and radiogenic

compositions with cOs values as high as +4000 comparedto the Hornı Bory pyroxenites.

The Mg-lherzolites show a large variation in Os TRD

model ages (Re depletion age – Walker et al., 1989) from0.5 to 1.2 Ga that is typical of Phanerozoic lithosphericmantle (e.g., Carlson and Irving, 1994; Pearson et al.,1999; Peslier et al., 2000; Gao et al., 2002; Harvey et al.,2006; Lee and Walker, 2006; Liu et al., 2008). In contrastto similar suites from Lower Austria (dunite/wehrlite-

Fig. 4. HSE concentrations vs. Al2O3 contents.

166 L. Ackerman et al. / Geochimica et Cosmochimica Acta 100 (2013) 158–175

pyroxenite) examined by Becker et al. (2001), a strong po-sitive correlation exists between 187Re/188Os and187Os/188Os ratios among Fe-dunite/wehrlite and pyroxe-nite samples (Fig 4). These data are regressed using ISO-PLOT (Ludwig, 2003) to give an errorchron age of334 ± 19 Ma with an initial 187Os/188Os of 0.155 ± 0.020(MSWD = 1318).

4.5. Concentrations of HSE in sulphides

The only Mg-lherzolite sample in which sulphides wereanalyzed is 97CZ3C. The sulphides are dominated by pent-landite that is strongly altered to magnetite. In general, ana-lyzed HSE concentrations in the Hornı Bory pentlanditesare much lower than those reported for pentlandites from

abyssal peridotites (Luguet et al., 2001), xenolithic perido-tites (Lorand and Alard, 2001) and ophiolite peridotites(Luguet et al., 2004). Four analyses of different sulphidegrains show very heterogeneous and fractionated distribu-tions of HSE (e.g., Os contents ranging from <0.02 to0.63 ppm, Os/Ir ratios from 0.6 to 14.7), with one grain(grain 4, Table 3) being exceptionally rich in Pt and Pd(83 and 77 ppm, respectively). In spite of the fact that Reconcentrations were only semi-quantitatively determined,three analyzed pentlandite grains have much higher Re con-tents (1.5–3.2 ppm; Re/Os = 5–41) compared to the low Recontents in the respective whole rocks samples.

Fe-wehrlite sample HB13 hosts very abundant yet differ-ent sulphide phases (pyrrhotite, pentlandite, and chalcopy-rite) compared to those present in sample 97CZ3C. Except

Fig. 5. Primitive upper mantle (PM – Becker et al., 2006)normalized HSE patterns. Symbols are the same as in Fig. 2. Thegrey field represents composition of basalts and other mafic rocks(e.g., Rehkamper et al., 1999; Bezos et al., 2005; Dale et al., 2008,2009; Schulte et al., 2009; Day et al., 2010).

L. Ackerman et al. / Geochimica et Cosmochimica Acta 100 (2013) 158–175 167

for Re, all analyzed sulphide phases of different composi-tions have generally similar, very low HSE concentrations,usually below the detection limits (Table 3). In contrast tothe high concentration of Re in the whole-rock(1.49 ppb), Re concentrations in analyzed sulphide grainsrange only from 0.2 to 1.8 ppm. Similarly, most of the ana-lyzed grains have low concentrations of Pd and Pt. How-ever, chalcopyrite + pentlandite grains have significantlyhigher Pt contents (0.99 ppm) compared to other sulphides.

On the other hand, Rh contents (not analyzed in whole-rock) varied in a similar manner as in the Mg-lherzolite.

Sulphides from websterite sample HB4 have generallysimilar, very low HSE concentrations, compared to sulp-hides from the Fe-wehrlite sample HB13. These concentra-tions are significantly lower than those in sulphides fromthe Totalp massif pyroxenites (van Acken et al., 2010). Incontrast to all other analyzed sulphides, some of the grainscontain higher Au contents than detection limits (0.06–0.13 ppm).

5. DISCUSSION

5.1. HSE carriers and role of sulphides in HSE fractionation

Prior studies have shown that the BMS present in man-tle rocks can accommodate HSE, with concentrations ofsome elements as high as hundreds of ppm (e.g., Alardet al., 2000; Luguet et al., 2001, 2004). On the other hand,recent studies have also revealed the presence of PGM with-in orogenic lherzolites and harzburgites (Luguet et al., 2007;Lorand et al., 2008, 2010). For the Hornı Bory suite, HSEconcentrations measured in sulphides are very low (Table 3)and, except for Re, there is no positive correlation betweenHSE and total S contents. This indicates complex control ofHSE distribution within the Hornı Bory rocks. In situ anal-yses combined with calculated sulphide modal abundances,allows an assessment of the role of BMS in the accommo-dation and fractionation of HSE (e.g., Lorand and Alard,2001; Luguet et al., 2004; Lorand et al., 2008). In Mg-lherz-olite, pentlandite is the dominant sulphide (>90 vol.%). Forsample 97CZ3C, the modal abundance of pentlandite deter-mined for three thin sections, varied around 0.04 wt.%while the total sulphide modal abundance calculated frommeasured S contents in the whole-rock is 0.05 wt.% (Ta-ble 1). The total HSE concentrations of four grains ana-lyzed by LA-ICP-MS are highly variable (from 2.4 to167 ppm; Table 3). Nevertheless, from mass balance calcu-lations (Table 4), it is clear that pentlandite cannot accountfor the total IPGE budget. Only �2–4% of the total IPGEbudget can be concentrated in this mineral. Therefore IPGEare most likely hosted by PGM, as reported for global per-idotites examined by other studies (e.g., Luguet et al., 2007;Lorand et al., 2010). This conclusion is also consistent withour discovery of an unidentified Os–Ru phase. If one pent-landite (grain 4; Table 3) that is strongly enriched in Pt andPd (83 and 77 ppm, respectively) is excluded, the same(�1%) is true for Pd. On the other hand, the behaviour ofPt is more complex. Mass balance calculations show thatpentlandite can host up to 28% of total Pt budget. How-ever, pentlandite grain 4 has high Pt and Pd contents, yetconcentrations of IPGE that are similar to other analyzedgrains. This suggests that Pt and Pd are hosted by some dif-ferent phase. Platinum and Pd partitioning coefficients forNiS phases (heazlewoodite, godlevskite) commonly presentin sample 97CZ3C are not known, but it is unlikely thatthey can accommodate high concentrations of Pt and Pd.Therefore, the best explanation is that Pd and Pt are hostedby PGM present within the pentlandite. This conclusion isin agreement with the discovery of Pt–Te/Pt–Ir–(Os) and

Table 3HSE (Os, Ir, Ru, Rh, Pd, Pt, Re and Au) concentration of selected sulphide grains from Hornı Bory peridotites and pyroxenites.

Sample Rock type Mineral Grain Re Os Ir Ru Rh Pt Pd Au(ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

97CZ3C Mg-lherzolite Pentlandite 1 3.2 0.56 0.04 0.91 0.19 2.88 0.04 <0.05Pentlandite 2 1.5 0.04 <0.01 0.20 <0.02 0.41 0.25 <0.05Pentlandite 3 <0.03 <0.02 <0.01 <0.09 0.07 6.19 0.05 <0.05Pentlandite 4 3.0 0.63 0.99 0.99 0.61 83 77 <0.05

HB13 Fe-wehrlite Pyrrhotite 1 0.4 0.03 <0.01 <0.06 <0.005 <0.03 <0.03 <0.04Pyrrhotite 1 0.3 0.02 <0.01 <0.06 <0.005 <0.03 <0.03 <0.04Pentlandite 1 0.6 0.03 <0.01 0.08 0.03 0.04 <0.03 <0.04Pentlandite 1 0.3 0.02 <0.01 <0.06 0.03 0.03 <0.03 <0.04Pentl + pyrr 2 0.2 <0.01 <0.005 0.15 0.07 0.04 <0.05 <0.04Pentl + pyrr + chalc 2 1.8 0.13 <0.01 0.12 0.20 0.20 <0.05 <0.04Pyrrhotite 3 0.7 0.03 <0.005 0.11 0.10 <0.05 <0.04Chalcopyrite + Pentlandite 4 0.0 <0.01 <0.005 0.47 0.99 <0.05 <0.04Pentlandite 5 0.2 <0.01 <0.005 0.09 0.16 <0.05 <0.04

HB4 Websterite Pentlandite 1 <0.02 0.05 <0.01 0.15 0.14 0.11 <0.03 0.09Pentlandite 1 0.0 0.02 <0.01 0.06 0.09 0.07 <0.03 <0.04Pyrrhotite 2 0.0 <0.01 <0.01 <0.06 <0.005 <0.03 <0.03 0.06Pyrrhotite 2 0.0 <0.01 <0.01 <0.06 <0.005 <0.03 <0.03 <0.04Pentlandite 2 0.9 0.08 0.03 0.15 0.01 0.06 <0.03 0.08Pyrrhotite 3 0.0 <0.01 <0.01 <0.06 <0.005 <0.03 <0.03 <0.04Pentlandite 3 0.6 0.40 0.16 0.78 0.13 0.72 <0.03 0.13Chalcopyrite 4 0.8 0.01 <0.01 <0.06 – 0.04 <0.03 0.11

168 L. Ackerman et al. / Geochimica et Cosmochimica Acta 100 (2013) 158–175

Pd-Te microphases in peridotites (Lorand et al., 2008,2010). Our discovery of two Pt–As grains (probably sperry-lite) located in the magnetite rim replacing pentlandite(Fig. 1C), however, suggests additional complexity in thedistribution of Pt in the presence of the high As contentof this sample expressed in the common occurrence of mau-cherite (NiAs). Taking into account errors rising from cal-culation of modal abundance and highly variable Reconcentrations detected in pentlandite, calculated wholerock concentrations are in general agreement with its modalabundance and whole-rock concentrations. This suggeststhat the abundance of Re was controlled by BMS, whichis in agreement with excellent positive correlation betweenRe and S (Fig. 3) and experimental studies showing highRe solubility in sulphides and high sulphide/silicate parti-tioning coefficients (Sattari et al., 2002; Fonseca et al.,2007; Brenan, 2008).

Except for sample HB14, with similar sulphide and HSEconcentrations as samples comprising the Mg-lherzolitesuite, Fe-dunite/wehrlite rocks are rich in sulphides (0.08–0.29 wt.%). For e.g., sample HB13 (analyzed in situ forHSE concentrations in sulphide) contains 0.29 wt.% sul-phide. However, all analyzed sulphides (pentlandite, pyr-rhotite and chalcopyrite) are very poor in HSE (Table 3)without any measurable difference between the differentsulphides (pentlandite, pyrrhotite, and chalcopyrite). Ex-cept for Re, most of the analyzed sulphides have HSE con-centrations below detection limits (<60 ppb). Mass balancecalculations (assuming calculated average HSE concentra-tion for all analyzed sulphides, and detection limits as max-imum HSE contents) indicate that they likely host less than14% of the total Os, Ir, Ru, Pt, and Pd budgets (Table 4).Nevertheless, positive correlation between IPGE and S con-

tents (Fig. 3) suggests a close association between IPGEand sulphides. The Pt–As phase (sperrylite) found in chal-copyrite in sample HB7 supports the idea that at least thePt budget is controlled by PGM, not base metal sulphides.Rhenium concentrations are highly heterogeneous (from0.04 to 1.83 ppm) without any systematic variations amongdifferent sulphide phases. Using the same modal composi-tion for mass balance calculations as for other HSE, weestimate that �99% of the total Re budget can be accountedfor by BMS. Therefore, it is concluded that the Re budgetin this sample is completely controlled by BMS.

Sulphide modal abundances in the pyroxenites are veryheterogeneous (0.09–0.26 wt.%). The heterogeneity is man-ifested in the apparent nugget effect observed for duplicatewhole-rock analyses of the HSE in pyroxenite HB4 (Ta-ble 2). Sample HB4 with a total sulphide content of0.26 wt.%, has similarly low (usually below detection limits)and heterogeneous HSE concentrations in the analyzedsulphides compared to the Fe-dunite/wehrlite (Table 3).Mass balance calculations with the same parameters usedfor Fe-wehrlite sample HB13 (Table 4) suggest that thesulphides can potentially host as much as 67% of the IPGEand 37% of the Re. In contrast, they accommodate only asmuch as 13 and 2% of the Pt and Pd, respectively.

5.2. Nature of Mg-lherzolite: constraints from HSE

systematics

The HSE patterns of the Mg-lherzolites are character-ized by generally unfractionated IPGE distributions cou-pled with modest Pd depletions in all, and Pt depletionsin most of the studied rocks. The Os isotopic compositions,with calculated initial cOs values ranging between �1.0 and

Tab

le4

Mas

sb

alan

ceca

lcu

lati

on

of

bas

em

etal

sulp

hid

e(B

MS

)co

ntr

ibu

tio

nto

wh

ole

-ro

ckH

SE

bu

dge

tso

fse

lect

edH

orn

ıB

ory

rock

s.

Sam

ple

Ro

ckty

pe

Su

lph

ide

mo

dal

abu

nd

ance

(wt.

%)

Su

lph

ide

par

agen

esis

Wh

ole

-ro

ckco

mp

osi

tio

nR

eO

sIr

Ru

Pt

Pd

(pp

b)

(pp

b)

(pp

b)

(pp

b)

(pp

b)

(pp

b)

97C

Z3C

Mg-

lher

zoli

te0.

05>

90vo

l.%

pn

Cal

cula

ted

0.97

10.

160.

130.

271.

580.

06M

easu

red

0.51

27.

503.

2810

.90

5.71

4.41

Per

cen

tage

ho

sted

inB

MS

190

24

228

1

HB

13F

e-w

ehrl

ite

0.29

50vo

l.%

pn

,0–

50vo

l.%

po

+cp

Cal

cula

ted

1.47

0.09

0.01

0.33

952

0.51

0.06

Mea

sure

d1.

491.

771.

002.

464.

668.

20P

erce

nta

geh

ost

edin

BM

S99

51

1411

1

HB

4W

ebst

erit

e0.

2620

–50

vol.

%p

n,

50–1

00vo

l.%

po

Cal

cula

ted

0.77

0.18

0.06

0.37

0.35

0.05

5–10

0vo

l.%

cpM

easu

red

2.08

0.41

0.31

0.55

2.71

2.76

Per

cen

tage

ho

sted

inB

MS

3745

2167

132

Pn

,p

entl

and

ite;

po

,p

yrrh

oti

te;

cp,

chal

cop

yrit

e.

L. Ackerman et al. / Geochimica et Cosmochimica Acta 100 (2013) 158–175 169

�4.8, are typical of Phanerozoic convecting upper mantle,based on the data for ophiolites and abyssal peridotites(e.g., Buchl et al., 2002; Harvey et al., 2006; Liu et al.,2008, 2009; Schulte et al., 2009). The minor variationscan largely be ascribed to low and variable extents of meltextraction (e.g., Lorand et al., 1999; Pearson et al., 2004;Becker et al., 2006), as is consistent with an absence of pro-nounced Pt–Pd depletions, and the presence of commonsulphides.

On the other hand, sulphide contents in the studied Mg-lherzolites do not reflect the degree of alteration as there isan absence of positive correlation between total sulphidecontents and indicators of alteration such as Fe2O3/FeO.The PPGE that are variably depleted in the Mg-lherzolite,have lower MSS/sulphide melt partitioning coefficients(e.g., Li et al., 1996; Alard et al., 2000; Ballhaus et al.,2001) and therefore, can be fractionated from IPGE bythe variable degrees of partial melting that most strongly af-fect low temperature (Cu-rich) sulphides the most. How-ever, no MSS was observed in the Mg-lherzolites, mostlikely because it broke down to lower-temperature sulp-hides (e.g., pentlandite) during the exhumation of perido-tites and the surrounding granulites. Despite the relativelyflat HSE patterns for these rocks, some aspects of theHSE systematics for these rocks point to at least minorinteraction between depleted peridotite and melts. Mostnotably, all of the Mg-lherzolites are significantly enrichedin IPGE (Os up to 10.7 ppb) compared to PM (e.g.,Os = 3.9 ppb; Becker et al., 2006). This likely suggests thatIPGE were added to the Mg-lherzolites at some stage oftheir evolution. As noted above, mass-balance calculationsfor sample 97CZ3C, with 7.5 ppb of Os, and an absence ofMSS suggest that the IPGE must be hosted by phases otherthan BMS. Laurite (Os–Ir–Ru sulphide) or an IPGE alloyare the most likely candidates, due their high stability andpotential for formation in sulfur-bearing, but sulphur-undersaturated systems (Brenan and Andrews, 2001; An-drews and Brenan, 2002; Fonseca et al., 2012). In this sam-ple, a very small phase (�0.2 lm across) with highbrightness in BSE was discovered in a rounded inclusionof pentlandite during SEM investigation. In spite of its verysmall size, the presence of Os and Ru, but absence of Ir wasdetermined from EDS spectra. This observation is in agree-ment with whole-rock data showing suprachondritic Os/Irand Ru/Ir ratios and suggests that this phase is likely Os-rich laurite which has a lower solubility of Ir with respectto Os and Ru (Luguet et al., 2007).

Moderate depletions in Pd–Pt, low sulphide contents(Table 1), suprachondritic Os/Ir and Ru/Ir ratios, and thepresence of a discrete Os–Ru phase point to melt-rock reac-tions between previously depleted peridotite and sulphur-undersaturated melts. The chondritic to slightly subchon-dritic 187Os/188Os ratios (0.1190–0.1242) however, suggestthat this was probably not associated with radiogenic187Os input, as has been documented for other peridotitesworldwide (e.g., Becker et al., 2001; Buchl et al., 2002; Ack-erman et al., 2009a). Alternatively, high IPGE contents de-tected in two samples may reflect heterogeneous IPGEdistribution controlled by IPGE alloys and/or high-temper-ature Os–Ir–Ru sulphides. The IPGE alloys may represent

170 L. Ackerman et al. / Geochimica et Cosmochimica Acta 100 (2013) 158–175

relicts of complex partial melting histories of Mg-lherzoliteprotoliths. The experimental study of Fonseca et al. (2012)showed that IPGE alloys can persist in the terrestrial man-tle for billions years.

5.3. Melt-rock reactions by subduction-related melts and

origin of Fe-dunite/wehrlite

In contrast to the Mg-lherzolite suite, the Fe-dunite/wehrlite suite (except for sample HB14) has very differentHSE compositions. The rocks are significantly depleted inIPGE, and enriched in Re compared to the Mg-lherzolites,as well as orogenic peridotites and mantle xenoliths world-wide (e.g., Lassiter et al., 2000; Meisel et al., 2001; Pearsonet al., 2004; Becker et al., 2006; Bizimis et al., 2007; vanAcken et al., 2010; Fischer-Godde et al., 2011; Liu et al.,2011). Furthermore, some of these samples show enrich-ment in Pd, and all have suprachondritic calculated initialcOs values ranging from +10.8 to +25.2 (Table 2). TheHSE patterns of the Fe-dunite/wehrlite suite (Fig. 5) exhibitsignificant IPGE enrichments in comparison to basalts andother mafic rocks (grey field in Fig. 5, based on data ofRehkamper et al., 1999; Bezos et al., 2005; Dale et al.,2008, 2009; Schulte et al., 2009; Day et al., 2010). Thisclearly points to different evolution compared to the Mg-lherzolites. Becker et al. (2001) described similar rock asso-ciations (lherzolite–harzburgite and dunite–pyroxenite)from Lower Austria and discussed Re–Os behaviour duringmelt-rock reactions between peridotites and subduction-re-lated melts. Based on a change in cOs, they concluded thatthe dunites formed during melt percolation at high melt/rock ratios (10–400), while the lherzolite–harzburgites expe-rienced much lower melt/rock ratios of �1. A similar rocksuite (dunite–harzburgite) to Hornı Bory/Lower Austriafrom the Troodos ophiolite and the effects of melt percola-tion on HSE concentrations and Os isotopes were studiedby Buchl et al. (2002) and Buchl et al. (2004). They con-cluded that the dunites were formed during progressive meltpercolation by subduction-related melts leading to primarysulphide breakdown and its replacement by metasomaticsulphides bearing a radiogenic Os signature. This replace-ment process can lead to strong fractionation of the HSE(Buchl et al., 2002). However, some differences exist be-tween the Lower Austria/Troodos peridotite and HornıBory rocks. Most importantly, the Fe-dunite/wehrlite suitefrom Hornı Bory is strongly enriched in Re (up to1.85 ppb), and these rocks have much more radiogenic187Os/188Os compositions, yet show similar Os concentra-tions. Further comparisons are not possible, as neitherHSE data nor other information regarding sulphides havebeen published for the Lower Austria and Troodos rocks.

At Hornı Bory, the HSE systematics of the Fe-dunite/wehrlite suite may be explained by interaction with basalticmelts that usually have PM-normalized HSE patterns thatincrease from Os through Pd to Re. Nevertheless, absenceof correlation between degrees of melt percolation (e.g.,Al2O3 and #Mg) and HSE abundances or their ratios(e.g., Pd/Ir, Pt/Ir) argue against a simple depletion/enrich-ment of HSE by infiltration of basaltic melts, as was sug-gested for the Totalp massif (van Acken et al., 2008). On

the other hand, a good positive correlation exists between187Re/188Os and 187Os/188Os (Fig. 6), as well as between187Os/188Os and decreasing #Mg (Fig. 7). There is, how-ever, a lack of correlation between Mg# and cOs. Theseobservations point to a close relationship between degreeof melt-rock reactions (melt/rock ratios) and the generationof radiogenic 187Os, consistent with a relation between Re/Os and #Mg.

To assess the role of melt/rock ratios, we performedmodel mixing calculations involving initial cOs and eNd val-ues calculated for 330 Ma (eNd data from Ackerman et al.,2009b). The most depleted Mg-lherzolite sample (06HB4)was chosen as a starting end member composition, was cho-sen. The composition of the infiltrating melt is difficult toconstrain. Strong decoupling between cOs and eNd (Fig. 8)indicates that the pyroxenites do not represent original meltcompositions, but most likely formed as a result of perido-tite-melt interaction (Ackerman et al., 2009b). The eNd

(330 Ma) values for Bohemian pyroxenites and eclogitesare highly variable, ranging from +6.1 to �6.0 (Brueckneret al., 1991; Medaris et al., 1995; Becker, 1996). Neverthe-less, the negative correlation among Hornı Bory peridotitesand low eNd values of the Fe-dunite/wehrlites point to thecontribution of significant amounts of a crustal component(negative eNd) to the percolating melts. In terms of Os iso-topic composition, the composition of Hornı Bory perido-tites can be best explained by mixing with melt having acOs value of �+400 at 330 Ma. Unfortunately, no Os dataexist for Bohemian pyroxenites, except for those from Low-er Austria (Becker et al., 2004). Using the most Os radio-genic pyroxenite sample PW221A from this suite, whichalso has the lowest eNd value (Becker, 1996), the Mg-lherz-olite compositions can be explained by interaction with lessthan 0.4% of the percolated melt, while the Fe-dunite/wehr-lites can be accounted for by 9–13% of a melt contribution(Fig. 8).

The Fe-dunite/wehrlite suite contains very abundantFe–Ni–Cu sulphides that are preferentially concentratedalong grain boundaries, suggesting that they likely precipi-tated from percolating melts. However, in contrast to otherin situ HSE data for worldwide magmatic sulphides (e.g.,Lorand and Alard, 2001; Luguet et al., 2004), our HSE con-centrations in sulphides are very low (especially IPGE), butare similar to hydrothermal sulphides from ophiolites (Lug-uet et al., 2004) and oceanic crust (Dale et al., 2009). Thisstrongly suggests that the source of HSE in infiltrating meltwas most likely subducting oceanic crust. The presence ofsulphide inclusions similar in composition to those on grainboundaries, their low HSE contents and radiogenic initialwhole-rock Os isotopic compositions suggest that melt/rockratios were high enough to completely dissolve primarysulphides and precipitate metasomatic radiogenic Fe–Ni–Cu sulphides. Except for one layered sample of wehrlite(06HB2D), which is very strongly enriched in Pt, the HornıBory peridotites show a generally positive correlation be-tween PdN/PtN and decreasing #Mg (Fig. 9). This suggeststhat Pd was imported during melt infiltration, yet our LA-ICP-MS data and absence of positive correlation betweenwhole rock Pd and S contents, show that this Pd is nothosted in BMS. We speculate that Pd, as well as most of

Fig. 7. The 187Os/188Os variation with whole rock #Mg(100 � [Mg/Mg+Fe]) for Hornı Bory suite.

Fig. 8. Diagram of cOs vs. eNd at 330 Ma showing mixing linebetween the most depleted Mg-lherzolite sample (06HB4) andpercolating melt (pyroxenite sample PW221A from Becker, 1996;Becker et al., 2004). For more details see the text.

L. Ackerman et al. / Geochimica et Cosmochimica Acta 100 (2013) 158–175 171

the Pt, are hosted by micro/nano phases, most likely tellu-rides (e.g., Lorand et al., 2008) or arsenides (e.g., PtAsphase discovered in both types of peridotite).

5.4. Age constraints

The Hornı Bory Fe-wehrlite/dunite and pyroxeniterocks show a positive correlation between 187Re/188Osand 187Os/188Os. If interpreted to have age significance,the slope gives an errorchron age of 334 ± 19 Ma(MSWD = 1540; Ludwig, 2003). This age is very similarto the Sm–Nd age obtained for Hornı Bory pyroxenite sam-ple HB3 (337 ± 3 Ma; Ackerman et al., 2009b) and otherpyroxenites and eclogites elsewhere in the Bohemian Massif(e.g., Medaris et al., 2006), which have usually been inter-preted as metamorphic mineral closure ages. Such apparentRe–Os isotopic correlations in mantle rocks are rare, andthey are usually interpreted to reflect melt depletion ages(e.g., Reisberg et al., 1991; Reisberg and Lorand, 1995;Gao et al., 2002). However, the nature of the Hornı BoryFe-dunite/wehrlite-pyroxenite (e.g., replacement of primarysulphides by metasomatic sulphides characterized by radio-genic Os) suite suggests that the apparent Re–Os age of334 ± 19 Ma most likely represents the age of melt/rockreaction and may, therefore, reflect the age of subduction.In contrast, variable Os model ages (TMA) ranging only be-tween 0.3 and 0.7 Ga (Table 2) likely reflect different melt/rock ratios (variable contribution of percolating melt).

The compositions of the Mg-lherzolites suggest that theyrepresent melt depletion residues, however, affected by asubsequent melt percolation event involving a melt thatwas similar to that which affected the Fe-dunite/dunitesuite. For these rocks, only a limited correlation exists be-tween 187Re/188Os and 187Os/188Os (r = 0.59; not shown),but their Os model ages (TMA) are highly variable, rangingfrom 0.6 to 4 Ga, likely because small melt/rock ratios stilldisturbed the Re–Os systematics. Nevertheless, it has beenshown before that model Re-depletion ages (TRD; Walkeret al., 1989; Shirey and Walker, 1998) could provide at least

Fig. 6. Re–Os errorchron diagram for Hornı Bory suite. The age334 ± 19 Ma is obtained by regression (Model 3 regression with±95% confidence level – Ludwig, 2003) of Fe-dunite/wehrlite andpyroxenites suites.

minimum estimates of the age of melt extraction even forsamples affected by melt percolation (e.g., Reisberg et al.,2004). In spite of the limited number of samples examined,TRD ages of the Mg-lherzolites vary between 0.5 and1.2 Ga. However, except for sample 06HB4, strong incon-sistencies between TMA and TRD model ages argue for dis-turbance (Re addition) of the Re–Os systematics. Incontrast, sample 06HB4 contains a very low Re contentof 0.082 ppb and also is characterized by greatest meltdepletion of the suite, as reflected by #Mg, incompatibletrace elements and Sr–Nd isotopic ratios. Thus, the TRD

model age of 1.2 Ga for this sample may represent a mini-mum age for the formation of a major lithospheric domainwithin the Bohemian Massif. This age is similar to 1.3 GaTRD model age of one xenolith sampling subcontinentallithospheric mantle beneath the Bohemian Massif (Acker-man et al., 2009a) and in the range of Nd TDM model agesfor the majority of the Variscan crustal rocks (1.25–1.85 Ga; e.g., Gebauer et al., 1989; Janousek et al., 1995).

Fig. 9. PdN/PtN (N = Primitive mantle – Becker et al., 2006) vs.whole rock #Mg (100 � [Mg/Mg+Fe]) for Hornı Bory suite. Notegenerally positive correlation between PdN/PtN and decreasing#Mg.

172 L. Ackerman et al. / Geochimica et Cosmochimica Acta 100 (2013) 158–175

6. CONCLUSIONS

At Hornı Bory (Bohemian Massif, Czech Republic), twodistinct suites – Mg-lherzolite and Fe-dunite/wehrlite,accompanied by pyroxenites, occur. The HSE concentra-tions, Os isotopic ratios, and sulphide petrography of thestudied Mg-lherzolite and Fe-dunite/wehrlite differ signifi-cantly. The Mg-lherzolite suite exhibits enrichment inIPGE, moderate depletions in Pd, Pt, contains sulphidesof predominantly pentlandite composition and has Os iso-topic compositions similar to those of the Phanerozoic con-vecting mantle. These compositions can be best explainedby reaction of previously depleted peridotite with a S-undersaturated melt at low melt-rock ratios. This reactionwas associated with precipitation of IPGE alloys and Pt–As. In contrast, Fe-dunite/wehrlite is depleted in IPGE,but enriched in Pd–Pt, has radiogenic osmium isotopiccompositions and complex sulphide petrography (pentland-ite + pyrrhotite + chalcopyrite). These compositions re-flects melt-rock reaction with subduction-related melt athigh melt/rock ratios. During this process, primary sulp-hides were likely decomposed and replaced by HSE-poorand 187Os-enriched metasomatic sulphides. Mass balancecalculations involving sulphide abundances and their HSEcompositions have shown, that, while most of the Re is en-trapped in base metal sulphides (BMS), only up to 17% ofthe other HSE (Os, Ir, Ru, Pd, Pt) are hosted in BMS.Search for platinum-group minerals (PGM) revealed pres-ence of several Pt–As phases (Mg-lherzolite and Fe-du-nite/wehrlite) and unidentified Os–Ru phase (Mg-lherzolite). This suggests complex behaviour of HSE duringmelt-rock reactions depending on melt composition andmelt/rock ratios. The presence of As-rich minerals in onesample of Mg-lherzolite (maucherite and Pt–As phase) sug-gests similar source of melt interacted with both types ofperidotites. The Hornı Bory Fe-wehrlite/dunite and pyrox-enite rocks show a good positive correlation between187Re/188Os and 187Os/188Os, consistent with a

334 ± 19 Ma 187Re–187Os errorchron age, which is similarto other high temperature/pressure rocks from Gfohl Nap-pe, Moldanubian Zone, Bohemian Massif. This age likelycorresponds to the age of melt-rock reactions and therefore,possibly represents the age of the subduction. One sampleof Mg-lherzolite, most depleted in composition, yieldsTRD depletion age of 1.2 Ga, which may represents the min-imum age of formation of a lithospheric domain within theBohemian Massif.

‘

ACKNOWLEDGMENTS

We thank Gordon Medaris for the samples and Vlasta Boh-mova for the help with the search for PGM. This research was sup-ported by the Grant Agency of the Academy of Sciences, ProjectNo. KJB300130902 (to L. Ackerman), Grant-in-aid internal pro-gram of international cooperation projects Academy of Sciencesof the Czech Republic, Project No. M100131203 (to M. Svojtka)and the Scientific Programmes CEZ: AV0Z30130516 andRVO67985831 of the Institute of Geology v.v.i., Acad. Sci. CR.Financial support from the Ministry of Education (MSM0021620855) to Charles University is gratefully acknowledged.

APPENDIX A. SUPPLEMENTARY DATA

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.gca.2012.09.050.

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Associate editor: Shichun Huang