The effect of silica contents on Pd, Pt and Rh solubilities in silicate melts:

an experimental study

ALEXANDER BORISOV1,* and LEONID DANYUSHEVSKY2

1 Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences,Staromonetnyi per. 35, Moscow, 119017 Russia

*Corresponding author, e-mail: aborisov@igem.ru2 CODES CoE and School of Earth Sciences, University of Tasmania, Private Bag 79, Hobart, TAS 7001, Australia

Abstract: The effect of silica contents on Pt, Pd and Rh solubilities in CaO–MgO–Al2O3–SiO2 melts was investigated at air condition.In a pseudobinary system diopside-anorthite eutectic (DA)–silica at 1450 �C, the maximum solubility of Pd (391 ppm) was observedin melts with 55 wt% SiO2. In more basic and more silicic melts Pd solubility is lower, being 338 ppm at 50 wt% SiO2 and 316 ppm at70 wt% SiO2. In contrast, Pt and Rh solubilities in these melts systematically decrease with increasing silica, from 8.9 and 35.2 ppm,respectively, in the DA composition down to 4.0 and 21.7 ppm in melts with 70 wt% SiO2. The results on Pt solubility provide a newsupport to the role that PtFe alloys precipitation may play during melt evolution from basaltic to silicic compositions.

In silica-free CaO-Al2O3 melts the solubility of Pt and Rh at 1550 �C was found to be extremely high (230 and 319 ppm,respectively). Adding silica up to 50 wt% results in dramatic decrease in Pt and Rh solubilities (to 5.5 and 29 ppm, respectively).

Our results also demonstrate that the contents of trace level impurities in experimental charges (such as W, Mo and alkalis)originating from furnace contamination from earlier experimental runs decrease by two orders of magnitude with melt SiO2 contentsincreasing from 0 to 70 wt%. The ratio of a contaminant concentration in two different melts from a single run is approximatelyinversely proportional to the ratio of the contaminant activity coefficients in these melts.

Key-words: platinum, palladium, rhodium, tungsten, molybdenum, solubility, silicate melt, silica content, contamination,experiment.

1. Introduction

The solubilities of Pd, Pt and Rh in simple haplobasalticmelts (diopside-anorthite eutectic composition, DA) werethoroughly investigated at 1 atm total pressure and at awide range of oxygen fugacity and temperature (Borisovet al., 1994; Borisov & Palme, 1997; Ertel et al., 1999;Fortenfant et al., 2003). It was demonstrated that all solu-bilities decrease with decreasing fO2, implying that all Pd,Pt and Rh are dissolved in silicate melts as formal oxidesspecies and in this sense are similar to more abundantmetals, like Ni, Co or Mo (e.g., Holzheid et al., 1994;O’Neill & Eggins, 2002). At the same time some anom-alous properties were revealed. First, the formal valencesof platinum group elements (PGE) species dissolved insilicate melt are lower than those in the most stable solidoxides (Pd1þ in melt vs. Pd2þ in PdO, Pt2þ vs. Pt4þ in PtO2

or Rh2þ vs. Rh3þ in Rh2O3), with possible additionalspecies coexisting at the most oxidizing and most reducingconditions (Borisov et al., 1994; Ertel et al., 1999).Second, PGE solubilities at a given fO2 increase withincreasing temperature (Borisov et al., 1994; Borisov &

Palme, 1997; Fortenfant et al., 2003), opposite to thetemperature dependence of the solubilities of other metals(e.g., Wang et al., 1973; Holzheid et al., 1994).

The choice of alkali- and iron-free DA composition as abase melt for those early experiments was dictated partly byusing neutron activation as the analytical method (Borisovet al., 1994) and also by the desire to avoid possible ironalloying with the noble metal material of the loop or crucible(see details on the affinity of iron for the noble metals inBorisov & Palme, 2000). Thus, the investigation of theeffect of melt composition on PGE solubility was restricted(e.g., Borisov & Palme, 1997; Nakamura & Sano, 1997;Dable et al., 2001; Borisov et al., 2004, 2006).

Natural magmas, from ultrabasic to silicic, have a widerange of SiO2 content and the influence of silica on thechemical properties of the components of silicate melts istherefore of special interest. In recent studies (Borisov,2006, 2007) the solubility of Ni, Co and Fe in silicatemelts of a pseudobinary DA–SiO2 system was experimen-tally determined. It was shown that, independent of tem-perature and oxygen fugacity, the solubility of these metalshas a maximum in melts of intermediate SiO2

This paper was presented at the EMPG XIIIsymposium in Toulouse, France (April 2010)

0935-1221/11/0023-2107 $ 5.85DOI: 10.1127/0935-1221/2011/0023-2107 # 2011 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

Eur. J. Mineral.

2011, 23, 355–367

Published online March 2011

compositions. The position of this maximum (SiO2¼ 57.6� 2 wt%) is essentially independent of the element, melttemperature, and metal concentration (from a few ppm to13 wt%). In the present study we investigate the effect ofsilica on Pd, Pt and Rh solubilities.

This study was additionally motivated by a desire tosolve a long-standing controversy. As mentioned above,platinum speciation in silicate melts is Pt2þ. In contrast, thestructural and oxidation state information derived from X-ray absorption fine structure (XAFS) spectroscopy study ofPt-bearing glasses in the CaO–Al2O3–SiO2 ternary (CAS)produced in air suggests the dominance of Pt4þO6 polyhe-dra, with all other platinum species amounting to ,10 at%(Farges et al., 1999). The Pt content in silica-poor CASglasses (SiO2 below 15 wt%) was extremely high (up to200 ppm) in contrast to much lower solubility in the DAcomposition (Borisov & Palme, 1997; Ertel et al., 1999;Fortenfant et al., 2003). In this study we determine thesolubility of Pt (and Rh) in CAS melts that range in com-position from silica-free through silica-poor to silica-rich(up to 50 wt% SiO2).

This study also investigates the effect of melt composi-tion on the extent of contamination of experimentalcharges by alkalies and other impurities found in experi-mental furnaces. Contamination between experimentalcharges in heavily used 1 atm furnaces is well-known.For example, Borisov & Palme (1997) were not able toanalyze some Pt-bearing glasses by instrumental neutronactivation analysis (INAA), because the glasses were con-taminated with W, significantly contributing to the back-ground of INAA spectra. Tungsten contamination was theresult of earlier evaporation experiments on complex W-containing alloys, performed in the same furnace (Palmeet al., 1998). Although in this work we found no effect ofthe impurities on measurements of Pt and Rh contents inexperimental glasses, understanding sample contaminationby ‘‘dirty’’ furnaces is important.

2. Experimental details

The starting mixtures in the DA–SiO2 system were pre-pared from a glass of DA composition (diopside-anorthiteeutectic) modified by adding approximately 6, 12, 25, 30,50, and 70 wt% SiO2 powder (compositions DAS6,DAS12, DAS25, DAS30, DAS50, and DAS70, respec-tively). The compositions were thoroughly mixed in anagate mortar under acetone but not pre-fused. Glass ofCA composition was prepared by mixing appropriateamounts of CaCO3 and Al2O3 followed by melting in analumina crucible at 1500 �C for 30 min in a muffle furnace.CAS mixtures were made by adding �10, 40 and 50 wt%SiO2 powder to the base CA composition and also were notpre-fused (compositions CAS10, CAS40 and CAS50,respectively). In both systems, 99.999 % pure silica (AlfaPuratronic�) was used. According to the certificate ofanalysis the concentrations of K, Na, Li and Mo in thesilica were less than 1 ppm for alkalis and 0.1 ppm for Mo.

Experiments were conducted in a vertical tube furnacein the Institut fur Geologie und Mineralogie, Universitat zuKoln, Koln, Germany (Pd experiments) and in theBayerisches Geoinstitut, Universitat Bayreuth, Bayreuth,Germany (Pt and Rh experiments). Hereafter the first andsecond furnaces will be referred to as ‘‘Cologne furnace’’and ‘‘Bayreuth furnace’’, respectively. Temperature in theworking zone of the furnace was determined using a Pt-Rhthermocouple calibrated against the melting points of Au(1064 �C) and Pd (1552 �C). Experimental temperaturewas estimated to be accurate within �2 �C.

The experiments were conducted using a loop techni-que, in which a melt droplet was suspended from a loopprepared from a narrow strip of Pd foil (0.1 mm thick and99.95 % pure, ChemPure�) or flattened Pt70Rh30 wire 0.35mm in diameter.

The DA composition was earlier used in a number ofstudies to investigate metal solubilities at high tempera-tures (1350–1450 �C) with a loop technique (Borisov et al.,1994; Holzheid et al., 1994; Borisov & Palme, 1997;O’Neill & Eggins, 2002; O’Neill et al., 2008).Attainment of equilibrium in these studies was assessedby either comparing direct and reverse experiments per-formed in a single run, or performing experiments of vari-able durations. These studies have demonstrated that forPd, Pt, Ni, Co, W and Mo equilibrium is achieved within4–24 h, suggesting that equilibration time for DA compo-sition is short and does not change for different metals.However, it was shown that time of equilibration for Niincreases for felsic DAS melts compared to the basic DAmelt (Borisov, 2006). The reason is that melt convection inthe loop plays a role in metal/melt equilibration (Borisov,2001) and silica-rich DAS melts are more viscous than thebasic DA melt. This was taken into account in the presentstudy, and the duration of DAS runs was at least 70 h.

A time series experiments aimed at establishing rundurations required for achieving equilibrium were con-ducted for the unusual silica-free CA composition.

In previous years the Cologne furnace was intensivelyused for studying melt saturation with Na, K and Rb(Borisov et al., 2006, 2008; Borisov, 2008, 2009), andthus some contamination of the furnace with alkalis wasinevitable. We therefore analyzed our Pd experimentalglasses for alkali and other potential impurities by LAICPMS (see below). The full list of analyzed elements isgiven below. Only the elements present at concentrationssignificantly above their detection limits are reported in thepaper. We found a significant contamination of theseglasses in W, whereas the Pt- and Rh samples, producedin Bayreuth, were found to be contaminated with Mo.

The major element composition of quenched glasseswas determined with a JEOL Superprobe electron microp-robe at the Institut fur Geologie und Mineralogie,Universitat zu Koln, Koln, Germany (Pd glasses) and atthe Bayerisches Geoinstitut, Universitat Bayreuth,Bayreuth, Germany (Pt and Rh glasses). Natural corun-dum, clinopyroxene and synthetic diopside glass were usedas standards. The operating conditions were as follows: anaccelerating voltage of 15 kV, a beam current of 15 nA, a

356 A. Borisov, L. Danyushevsky

counting time of 20 s on the peak and �10 s on the back-ground. From 10 to 20 points were analyzed in each sam-ple. A summary of glass compositions and experimentalconditions is presented in Table 1.

Trace element concentrations in the experimentalglasses were determined by laser ablation ICPMS.Analyses were performed in the LA-ICPMS laboratory atthe Centre of Ore Deposit Research (CODES), Universityof Tasmania. The set-up involved a NewWave UP193sslaser microprobe coupled to an Agilent 7500cs quadrupolemass-spectrometer. The laser is a solid state Nd-YAG laserwith the base wavelength of 1064 nm and the outputwavelength of 193 nm. Ablation was conducted in anatmosphere of pure He with a laser beam size of 100 mmat a laser frequency of 10 Hz using laser energy of �2 J/cm2. Helium carrying the ablated aerosol at 0.7 L/min wasmixed with argon carrier gas (0.9 L/min) immediately afterexiting the ablation cell. The following masses were mon-itored on the mass spectrometer: Li7, B11, Na23, Mg24,Al27, Si29, P31, K39, Ca43, Sc45, Ti49, V51, Cr53, Mn55,Fe57, Co59, Ni60, Cu65, Zn66, Rb85, Sr88, Y89, Zr90, Nb93,Mo95, Rh103, Pd105, Pd106, Ag107, Pd108, Cd111, Cs133,Ba137, La139, Ce140, Nd146, Sm147, Eu153, Gd157, Dy163,Er166, Yb172, Lu175, Hf178, Ta181, W182, Pt195, Hg202,Pb208, Th232, U238. Analyses on the mass-spectrometerwere performed in the time resolved mode which involvessequential peak hoping through the mass spectrum, withcounting time of 20 ms on each mass and the total sweeptime of �1.15 s. Each analysis lasted 90 s with 30 s ofbackground measurement at the start (laser off) followedby 60 s of ablation. Quantification was performed using thestandard methods (Longerich et al., 1996) with Ca as theinternal standard. NIST612 silicate glass was used as theprimary standard for all elements but PGEs, which were

quantified using an in-house NiS3 standard containing�25 ppm of each PGE (Gilbert et al., 2010). NIST612was analysed twice every 60 min to account for instrumentdrift. An international standard reference material BCR-2gwas analysed as a secondary standard throughout the ses-sion. The extent Cu and Zn argide interference on Rh andPd was monitored by ablating spec. pure Cu and Zn metalsat the beginning and end of the analytical session.However, the concentrations of Cu and Zn in all experi-mental glasses were negligible, and thus no correction forargide interferences were introduced. Data for three differ-ent Pd isotopes agree within error and an average of thethree was accepted as Pd concentration.

Each sample was analysed five times. Standard devia-tions of the average for all elements were similar to orsmaller then the analytical error of individual spot analysesindicating sample homogeneity within analytical uncer-tainty. The data are presented in Table 2.

Each experimental glass was also analyzed 10 times witha 20 mm laser beam, to increase spatial resolution in order toassess the potential presence of micronuggets. At this scale,no non-homogeneity was found for any of PGEs studied.

In order to further investigate the micronuggets issue, Pdcontents in glasses from Pd-bearing experiments wereanalyzed by EMP, as the excitation volume during anEMP analysis is significantly smaller (�3 mm). The EMPanalyses were performed by focused probe at 15 kV, 500nA and 600 s peak counting time. An experimental glassMP5b with 428 ppm Pd (INAA, Borisov et al., 1994) wasused as the Pd standard. Two electron microprobe sessionswere conducted in order to better constrain the analyticalerrors. As the EMP results (Table 1) are generally consis-tent with the LA-ICPMS data, we concluded that no micro-nuggets are present in our experimental glasses.

Table 1. Experimental conditions and microprobe glass composition (wt%)

1st session 2nd session

Sample Furnace T (�C) Duration (h) SiO2 Al2O3 MgO CaO Total Pd (ppm) s.d. Pd (ppm) s.d.

DAPd-40 Cologne 1450 72 50.3 15.89 10.3 23.65 100.14 314 8 293 16DAS12Pd-40 ‘‘ ‘‘ ‘‘ 54.54 14.48 9.39 21.61 100.02 370 9 352 20DAS25Pd-40 ‘‘ ‘‘ ‘‘ 58.13 12.94 8.43 19.55 99.05 368 8 343 19DAS35Pd-40 ‘‘ ‘‘ ‘‘ 60.19 12.42 8.13 18.82 99.56 340 6 321 17DAS50Pd-40 ‘‘ ‘‘ ‘‘ 66.59 10.42 6.7 15.58 99.28 303 19 291 53DAS70Pd-40sil ‘‘ ‘‘ ‘‘ 69.75 9.12 5.97 13.84 98.69 231 23 - -DAPtRh-44 Bayreuth 1450 70 50.86 15.77 10.28 23.49 100.39 – – – –DAS06PtRh-44 ‘‘ ‘‘ ‘‘ 52.82 15.08 9.81 22.4 100.12 – – – –DAS12PtRh-44 ‘‘ ‘‘ ‘‘ 54.71 14.47 9.49 21.55 100.22 – – – –DAS25PtRh-44 ‘‘ ‘‘ ‘‘ 58.94 13.08 8.55 19.48 100.05 – – – –DAS35PtRh-44 ‘‘ ‘‘ ‘‘ 60.48 12.57 8.2 18.71 99.96 – – – –DAS50PtRh-44 ‘‘ ‘‘ ‘‘ 67.2 10.42 6.76 15.48 99.86 – – – –DAS70PtRh-44sil ‘‘ ‘‘ ‘‘ 70.13 9.35 6.11 13.92 99.51 – – – –CAPtRh-46 Bayreuth 1550 9 0 53.87 0 45.65 99.52 – – – –CAPtRh-45 Bayreuth 1550 15 0 53.78 0 45.64 99.42 – – – –CAPtRh-47 Bayreuth 1550 46 0 53.75 0 45.45 99.2 – – – –CAS10PtRh-47 ‘‘ ‘‘ ‘‘ 10.45 48.29 0 40.94 99.68 – – – –CAS40PtRh-47 ‘‘ ‘‘ ‘‘ 38.62 33.23 0 28.21 100.06 – – – –CAS50PtRh-47 ‘‘ ‘‘ ‘‘ 50.25 26.96 0 22.78 99.99 – – – –DAPtRh-47 ‘‘ ‘‘ ‘‘ 50.85 15.76 10.25 23.43 100.28 – – – –

Note: s.d., standard deviations; ‘‘, the charge was melted in the same experimental run as the sample above; sil, silica-saturated samples.

The effect of silica on Pd, Pt and Rh solubilities 357

Tab

le2

.L

AIC

PM

Sd

ata

on

exp

erim

enta

lg

lass

es(p

pm

).S

ample

seq

uil

ibra

ted

wit

hpure

Pd

met

al

Sam

ple

Pd

s.d.

Rh

s.d.

Pt

s.d.

Mo

s.d.

Ws.

d.

Li

s.d.

Na

s.d.

Ks.

d.

Rb

s.d.

Cs

s.d.

Ps.

d.

Sr

s.d.

Ba

s.d.

DA

Pd-4

0338

16

8.7

90.2

41.6

77

0.0

66

––

––

22.5

0.4

252

31.5

20.0

30.1

90

0.0

01

0.0

29

0.0

001

178

20.0

20

0.0

01

45.3

1.3

79.6

0.5

44.6

0.7

DA

S12P

d-4

0391

67.4

00.2

41.0

19

0.0

42

––

––

17.3

0.4

191

31.9

80.0

40.2

61

0.0

01

0.0

60

0.0

001

412

40.0

51

0.0

03

30.5

1.1

73.7

0.5

40.5

0.6

DA

S25P

d-4

0383

12

7.6

20.3

41.0

47

0.0

09

––

––

12.9

0.5

136

42.4

20.0

20.3

60

0.0

01

0.1

04

0.0

003

855

70.1

26

0.0

01

18.4

0.5

67.5

0.5

37.5

0.5

DA

S35P

d-4

0372

27

7.7

60.2

41.3

03

0.0

76

––

––

12.7

0.4

140

22.5

80.0

40.3

89

0.0

01

0.1

13

0.0

004

975

80.1

75

0.0

09

18.1

1.3

63.8

0.2

36.3

0.2

DA

S50P

d-4

0336

96.2

20.1

21.0

06

0.2

25

––

––

9.9

0.2

114

32.9

80.0

50.4

64

0.0

03

0.1

59

0.0

008

1627

20

0.5

33

0.0

12

14.7

0.9

55.5

0.6

28.4

0.2

DA

S70P

d-4

0316

55.5

40.0

51.0

56

0.0

48

––

––

9.2

0.2

107

53.0

90.0

70.4

98

0.0

04

0.1

78

0.0

012

1891

90.7

31

0.0

39

14.0

2.3

46.7

0.3

26.5

0.1

Sam

ple

seq

uil

ibra

ted

wit

hP

t 70R

h3

0(w

t)al

loy

Sam

ple

Pd

s.d.

Rh

s.d.

Pt

s.d.

Rh

(pure

)þs.

d.

Pt

(pure

)þs.

d.

Mo

s.d.

Ws.

d.

Li

s.d.

Na

s.d.

Ks.

d.

Rb

s.d.

Cs

s.d.

Ps.

d.

Sr

s.d.

Ba

s.d.

DA

PtR

h-4

40.0

51

0.0

13

14.4

0.7

4.6

30.0

835.2

1.6

8.9

30.1

6113

17.4

30.1

63.4

20.2

60.0

24

0.0

004

0.0

14

0.0

001

0.1

70.0

20.0

13

0.0

04

152

786.6

0.5

45.2

0.5

DA

S06P

tRh-4

40.0

45

0.0

13

14.4

0.5

4.4

90.1

935.4

1.2

8.6

50.3

799.1

1.5

7.0

20.2

93.9

60.1

80.0

31

0.0

003

0.0

22

0.0

002

0.2

90.0

20.0

21

0.0

04

175

482.7

0.6

45.9

0.7

DA

S12P

tRh-4

40.0

47

0.0

19

13.5

0.5

3.9

90.0

433.0

1.2

7.6

90.0

882.8

1.4

5.6

60.2

24.3

10.2

90.0

38

0.0

004

0.0

30

0.0

002

0.3

90.0

20.0

28

0.0

02

132

877.6

1.3

44.0

0.6

DA

S25P

tRh-4

40.0

39

0.0

08

12.6

0.3

3.4

00.1

430.8

0.8

6.5

50.2

664.1

1.2

4.3

60.1

05.4

10.2

60.0

51

0.0

004

0.0

52

0.0

003

0.9

80.0

50.0

74

0.0

08

99.0

5.1

70.9

1.1

38.8

0.7

DA

S35P

tRh-4

40.0

45

0.0

13

11.8

0.2

3.2

80.0

328.9

0.5

6.3

20.0

658.4

0.4

4.0

90.1

05.5

40.3

60.0

55

0.0

004

0.0

60

0.0

004

1.2

30.0

40.1

02

0.0

09

95.8

10.5

66.8

0.7

38.2

0.4

DA

S50P

tRh-4

40.0

35

0.0

07

9.8

10.4

02.4

60.1

124.0

1.0

4.7

40.2

041.7

0.8

3.0

20.0

96.7

00.1

70.0

65

0.0

003

0.0

89

0.0

005

2.8

80.0

70.3

57

0.0

14

69.5

5.3

58.4

0.5

29.7

0.6

DA

S70P

tRh-4

40.0

32

0.0

12

8.8

60.5

72.0

90.0

921.7

1.4

4.0

40.1

737.2

0.5

2.4

90.0

77.0

00.4

80.0

88

0.0

006

0.1

25

0.0

009

3.6

10.1

00.5

02

0.0

11

60.0

10.2

50.4

0.5

29.5

0.2

CA

PtR

h-4

60.0

61

0.0

16

133

2125

2323

5239

4831

36.7

20.1

7b.d

.l.

–b.d

.l.

–b.d

.l.

–b.d

.l.

–b.d

.l.

–154

539.0

0.2

3.4

70.0

9

CA

PtR

h-4

50.0

77

0.0

20

132

4125

2319

11

239

31262

10

9.8

90.1

0b.d

.l.

–b.d

.l.

–b.d

.l.

–b.d

.l.

–b.d

.l.

–215

439.2

0.2

3.4

30.1

5

CA

PtR

h-4

70.0

59

0.0

15

132

3120

2319

7230

41722

19

24.1

0.4

b.d

.l.

–b.d

.l.

–b.d

.l.

–b.d

.l.

–b.d

.l.

–367

638.6

0.5

3.0

00.1

3

CA

S10P

tRh-4

70.0

40

0.0

10

62.6

2.4

46.7

1.4

151

689.4

2.6

900

521.6

0.3

0.3

20.0

6b.d

.l.

–b.d

.l.

–b.d

.l.

–b.d

.l.

–247

735.2

0.4

3.3

20.1

9

CA

S40P

tRh-4

70.0

43

0.0

16

15.3

0.1

5.4

70.1

437.0

0.3

10.5

0.3

57.6

0.4

9.0

70.0

92.0

00.1

20.0

13

0.0

002

0.0

09

0.0

001

0.0

30.0

1b.d

.l.

–208

924.1

0.2

2.7

70

.19

CA

S50P

tRh-4

70.0

37

0.0

09

12.0

0.4

2.8

90.1

328.9

1.0

5.5

30.2

526.0

0.4

4.0

70.0

74.0

60.2

00.0

40

0.0

001

0.0

45

0.0

002

0.2

20.0

20.0

11

0.0

02

98.7

2.9

19.2

0.3

2.2

60.0

7

DA

PtR

h-4

70.0

56

0.0

13

16.0

0.6

5.0

40.1

238.6

1.4

9.6

30.2

355.9

1.1

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358 A. Borisov, L. Danyushevsky

3. Results

3.1. Effects of silica on Pd, Pt and Rh solubility: DASmelts

For the base DA composition, both ICP and EMP data onPd solubility are in excellent agreement with the solubilityof 330 ppm, calculated for these T-fO2 conditions from thedata of Borisov et al. (1994), obtained with INAA.

The dependence of Pd solubility on the silica content inthe DAS system at 1450 �C (40th series) is shown on Fig. 1.The data demonstrate a sharp maximum of Pd solubility forDAS12Pd-40 sample. As discussed in the Introduction,Borisov (2006, 2007) demonstrated that Ni, Co and Fesolubilities in the same DAS system all exhibit a maximumof solubility at silica content of 55.4–59.6 wt%. Twoexamples of the Ni and Co data for similar temperaturesare also shown on Fig. 1. The most evident differencebetween the three curves is the asymmetry of the Pd solu-bility curve in comparison with the symmetric parabolicshapes for Ni and Co solubilities. From 70 % (DAS70composition) and down to 54 % silica (DAS12 composi-tion), the log solubility of Pd lineally increases and thensharply decreases for the DA composition with 50 wt%silica. The Ni and Co solubility data presented on Fig. 1

may be fitted by quadratic parabolas with R2 equal 1.00and 0.87, respectively. Similar high R2 (0.92 in average)may be found for Ni, Co or Fe solubility data at any fixedT-fO2 parameters (Borisov, 2006, 2007). It is also evidentfrom Fig. 1 that the maximum of Pd solubility lies at a morebasic composition (54 wt% silica) than the maxima for Niand Co solubilities (between 57 and 59 % SiO2).

The reasons for a maximum in metal solubilities are notwell understood, although some explanations for Ni arereviewed in Borisov (2006). In a recent study, Borisov &McCammon (2010) reanalyzed three reduced glasses fromthe experiments of Borisov (2007) using Mossbauer spec-troscopy. The three glasses are from a single series, pro-duced at 1400 �C and oxygen fugacity of 10�12.3 atm.Similar values of the hyperfine parameters, centre shiftand quadrupole splitting, were observed for all threeglasses (see Fig. 3 in Borisov & McCammon, 2010) sug-gesting that the change of melt structure which resulted in amaximum of iron solubility in the intermediate melts doesnot affect iron speciation, at least within the precision ofthe Mossbauer spectroscopy.

The present data on Pt and Rh concentrations in meltsequilibrated with the Pt70Rh30 alloy at 1450 �C (44thseries) were recalculated into pure Pt and Rh solubilitiesusing thermodynamic data from Jacob et al. (1998) for thePt-Rh binary. For the base DA composition, data on Ptsolubility (8.9 � 0.2 ppm) are in a good agreement withpreviously published data either measured or recalculatedto the same T-fO2 conditions (Borisov & Palme, 1997;Ertel et al., 1999; Fortenfant et al., 2003). Present data onRh solubility (35.2� 1.6 ppm) are slightly lower than 43.6� 2.2 ppm found by Ertel et al. (1999) in the same melt inair at 1300 �C.

The results on Pt and Rh solubilities in DAS melts as afunction of melt silica content are shown on Fig. 2. Incontrast to Pd solubility, no maximum is evident. The Ptand Rh contents systematically decrease with increasing

Fig. 1. Dependence of Pd solubilities on SiO2 contents in silicatemelts for the DAS system at 1450 �C in air. The data on Ni and Cosolubility in the same system (Borisov, 2006, 2007) are given forcomparison (see text for details).

Fig. 2. Dependence of Pt and Rh solubilities on SiO2 contents insilicate melts of DAS system at 1450 �C. Primary data were recal-culated to pure metal solubilities (see text for details).

The effect of silica on Pd, Pt and Rh solubilities 359

silica. Thus these two noble metals differ from the ‘‘com-mon metals’’ (Ni, Co and Fe) more markedly than Pd.

Note, that Pt and Rh solubilities (ppm) vs. SiO2 contents(wt%) in the DAS melts may be well approximated by alinear relation (R2 ¼ 0.99 in both cases) with differentslopes of �0.26 � 0.01 and �0.73 � 0.03, respectively.Thus, the relative solubility of Rh compared to Pt is muchhigher in the silicic melts (Rh/Pt¼ 10 at 80 % SiO2) than inthe basic melts (Rh/Pt ¼ 4 at 50 % SiO2).

3.2. Effects of silica on Pt and Rh solubilities: CASmelts

The results of a time series experiments with a silica-freeCA composition are shown on Fig. 3. The Pt and Rhconcentrations in samples equilibrated for 9, 15 and 46 hruns are the same within 2s error limits demonstrating fastachievement of equilibrium between melt and loop mate-rial. This serves as an additional confirmation for theabsence of Pt- and Rh-rich micronuggets, which areknown to be present in samples produced at reducingconditions (e.g., Borisov & Palme, 1997; Ertel et al.,1999).

An average Pt content recalculated to pure metal for thethree CA samples at 1550 �C (236 ppm) is slightly higherthan 195 ppm Pt found by Farges et al. (1999) at 1627 �Cfor a similar composition (Al2O3/CaO ¼ 1.1), but aresignificantly lower than 357 ppm Pt found by Nakamura& Sano (1997) at 1600 �C, also for similar melt (Al2O3/CaO ¼ 1.2). Possibly, this discrepancy reflects analyticaluncertainties between different labs. Note also that the twoprevious studies analyzed bulk glass contents by solutionICPMS, whereas this study uses an in-situ spatially-resolved analytical method.

Although all three CA compositions attained equili-brium in respect of Pt and Rh contents, it is more reliableto consider the melt composition effect for samples

obtained in a single melting run, i.e. at absolutely identicalparameters. Thus, in the following discussion, we willconsider the results of the longest CAS experiments (47thseries).

The Pt and Rh contents of CAS melts, recalculated intopure Pt and Rh solubilities, are shown on Fig. 4 as afunction of their silica contents. Again, both solubilitiessystematically decrease with increasing silica. But theeffect of silica in the CAS melts is more complicatedthan in the DAS system. The log solubilities (in ppm) areapproximated as a quadratic function of silica content (inwt%):

log CPtðppmÞ ¼ �0:042 � CSiO2þ 0:00020 � C2

SiO2

þ 2:37ðR2 ¼ 1:000Þ; (1)

log CRhðppmÞ ¼ �0:035 � CSiO2 þ 0:00028 � C2SiO2

þ 2:51ðR2 ¼ 1:000Þ: (2)

Thus the effects of silica on Pt and Rh solubilities may bequantitatively estimated. The @logCPt/@CSiO2 value is�0.042 for the CA composition, and only �0.022 forCAS with 50 % silica. This implies that adding silica tothe silica-free CA melt is twice more effective in decreas-ing Pt solubility than adding silica to a silica-rich CASmelt.

The @logCRh=@CSiO2 value is �0.035 for CA, and only�0.007 for CAS with 50 % silica. Thus adding silica to thesilica-free CA melt is five times more effective in decreas-ing Rh solubility than adding silica to a silica-rich CASmelt.

In the discussion above, we addressed the maximum ofmetal solubility (Pd, Ni, Co, Fe), as evidenced by experi-mental results. In the case of Rh, the predicted minimum insolubility of 26 ppm is at 62 wt% SiO2 (Fig. 4), with afurther increase in silica leading to an increase in Rh

Fig. 3. Dependence of Pt and Rh contents in CA melts on experi-mental durations.

Fig. 4. Dependence of Pt and Rh solubilities on SiO2 contents insilicate melts of CAS system at 1550 �C. The data in DA composi-tion melted simultaneously with CAS samples are marked. Primarydata were recalculated to pure metal solubilities (see text for details).

360 A. Borisov, L. Danyushevsky

solubility in CAS melts. As the predicted minimum liesbeyond our experimental range, its existence can bedebated. However, minima of metal solubilities have alsobeen described in the literature. For example, Nakamuraet al. (1998) found minima of Pt solubility in the meltswithin the K2O-SiO2 system at 1600 �C, and in the meltswithin the Na2O-SiO2 and Na2O-P2O5 systems at 1100 �C.

The DAPtRh-47 sample was melted in the same run withthe CAS samples and is comparable in its silica content toCAS50PtRh-47. However, this composition had somewhathigher Pt and Rh solubilities (9.6 vs. 5.3 ppm and 39 vs. 29ppm, respectively, see Fig. 4). The DA composition is alsosimilar to CAS50 in respect to its CaO content (see Table 1).Thus, the difference in the Pt and Rh solubility between theDA and CAS50 melts is most likely related to the differ-ence in their Al2O3 and MgO contents. The CAS50 com-position may be ‘‘converted’’ in DA by exchangingroughly 10 wt% Al2O3 to MgO, implying that a decreaseof alumina results in an increase in Pt and Rh solubilities.Similar effects of alumina on Pt solubility were earlierfound in binary melts CaO–Al2O3 (see Fig. 4 inNakamura & Sano, 1997).

In contrast to the DAS melts at 1450 �C, in the CASsystem at 1550 �C the solubility ratio (Rh/Pt) increaseswith silica increasing, from 1.4 in the CA composition to5.2 in the CAS50.

3.3. Contamination of experimental charges:W and Mo

Experimental series 40 and 44 with DAS samples wereperformed using the same initial compositions, at identicaltemperature of 1450 �C and with similar run durations (seeTable 1). Although we did not analyze our loop material(Pd and Pt70Rh30 alloy), we consider it unlikely that W andMo found in the run products are derived from that source.We believe that high W and Mo contents found in the DASglasses are derived from deposits on the inner surface ofthe furnace tube. At high temperatures such deposits arelikely to produce some W and Mo volatile species withinthe furnace atmosphere, which may be adsorbed by theexperimental melts. Thus under identical conditions, themore contaminated furnace would produce more cont-aminated samples. The DAS samples melted in theCologne furnace contain 30–40 times more W and 4–5times less Mo than the samples produced in Bayreuthfurnace (Fig. 5).

Additionally, within a single series, there is a clear effectof the melt composition on W and Mo contents in theexperimental runs. The concentrations of W and Modecrease �2.4–3.0 times with increasing silica contentfrom DA to DAS70 composition in both 40th and 44thseries of experiments (Fig. 5).

The contamination of CAS glasses with Mo in Bayreuthfurnace even more severe, reaching 1722 ppm in silica-freeCAPtRh-47 sample (Table 2, Fig. 5). Both W and Mo in theCAS glasses also display a roughly log-linear decrease oftheir concentrations with increasing silica (Fig. 5). As we

did not analyzed the initial CA composition used to pro-duce the CAS glasses, we cannot exclude that it was con-taminated during melting in a muffle furnace before ourexperiments. Nevertheless, we are confident that W andMo dependences on SiO2 shown on Fig. 5 reflect theeffects of melt composition on furnace contamination.For example, if the initial CA composition was contami-nated, then its one-to-one dilution by ‘‘pure’’ silica toproduce CAS50 would decrease Mo content from 1722 to861 ppm, whereas we find just 26 ppm Mo in sampleCAS50PtRh-47.

Significant contamination of the furnaces with W andMo in comparison to other metals is not surprising. Palmeet al. (1998) performed experiments on metal evaporationby heating small grains of a homogeneous multi-compo-nent refractory alloy at 1250 �C and variable oxygen fuga-city. After the experiment the samples were analyzed byINAA. The following sequence of losses was found: W .Mo . Re . Os . Ru . Ir with Pt, Fe and Ni beingpractically unaffected by heating. It was also found thatmetal evaporation is much more intense under oxidizingconditions (see Fig. 1 in Palme et al., 1998). Thus, experi-ments performed in air (like in this study) are most likely tobe contaminated by the ‘‘dirty’’ furnaces.

Note that some Mo contamination of modern tube fur-naces is possibly due to the use of MoSi2 heating elements. Itis then likely that muffle furnaces with MoSi2 heating ele-ments located inside the working chamber are prone to evenstronger contamination, especially for prolonged high tem-perature experiments with silica-free or silica-poor melts.

3.4. Contamination of experimental charges: alkalies

As discussed in the Introduction, the Cologne furnace wasintensively used over the last few years for a study of Na, Kand Rb solubility in melts. These experiments utilized thevapor phase formed above simple Na- and K-rich or

Fig. 5. Samples contamination with W and Mo. CF and BF denoteCologne furnace and Bayreuth furnace, correspondently.

The effect of silica on Pd, Pt and Rh solubilities 361

complex Na-K- and Na-K-Rb-rich source melts (seeBorisov, 2008, 2009 for details). Although Borisov(2008, 2009) did not analyze the source melts for Li andCs, some Li and Cs impurities in commercial Na2CO3,K2CO3 and Rb2CO3 chemicals used for preparation ofthese source melts cannot be excluded. As a result, con-tamination of the Cologne furnace with all alkalis can beexpected. Thus, DAS samples produced in the presentstudy were analyzed for all alkali impurities. The absolutealkali contents vs. silica concentrations are presented onFig. 6a. The alkali contents increase with increasing silica.The enrichment of DAS samples relative to the most basicDA sample is shown on Fig. 6b. The effect of silica on thisrelative enrichment increases systematically in the orderLi-Na-K-Rb-Cs. For example, the DAS70/DA ratio for Liis 2 whereas it reaches as high as 37 for Cs.

4. Discussion

4.1. PtFe alloy precipitation during crystalfractionation of melts

The magmatic geochemistry of Pt (and other PGE) is verycomplex and controversial. The abundances of PGE in CI-

chondrite meteorites, bulk material of the terrestrial pla-nets, lie in the range from 140 ppb for Rh to 982 ppb for Pt(Palme & O’Neill, 2003). Pd is mostly dissolved in the Ni-Fe alloy grains whereas all other PGEs are concentrated incomplex alloys found in the Ca-Al-rich inclusions (e.g.,Palme, 2008).

After the accretion of the Earth, all PGEs, being highlysiderophile, should be almost completely removed fromthe mantle and concentrated in the core. However, rela-tively high PGE contents in the upper mantle rocks (from0.93 ppb for Rh to 6.6 ppb for Pt, Palme & O’Neill, 2003)together with nearly chondritic ratios of individual PGEs,demands an addition of the chondritic material after theformation of the core was mostly completed. The hypoth-eses of a late chondritic veneer was first suggested byKimura et al. (1974) and is now accepted by mostcosmochemists.

It is now recognized that more than 90 % of the PGEbudget of the mantle lherzolite resides in base-metal sul-fides and tiny nuggets of platinum-group minerals, such aslaurite, PtIrOs alloys and PtPdTeBi phases (Lorand et al.,2008 and references therein). It is possible that somerefractory alloys have a cosmic origin, and survived sincethe accretion times (Bird & Bassett, 1980).

Very high degrees of partial melting of the upper man-tle rocks would consume all PGE phases. Thus PGEcontents of the Archean komatiitic magmas are usuallysimilar or even higher than in upper mantle rocks (e.g.,Puchtel et al., 2004). In contrast, lower degrees of partialmelting would extract only the low-temperature Cu-Nisulfides containing Pt and Pd, whereas the refractoryIrOsRu alloys would remain in the residue (Lorandet al., 2008). Indeed, Borisov & Palme (2000) usedexperimental data on PGE solubilities in silicate meltsto estimate that�20 % of partial melting would produce amelt saturated in complex IrRuOsFe alloys. The samemelt may be either saturated or undersaturated in a PtFealloy, depending on the T-fO2 conditions, but wouldnever be saturated in a PdFe phase.

At the conditions of mantle melting, Pt is incompatiblein olivine and pyroxenes (e.g., Puchtel et al., 2004). At thesame time, PtFe alloys may play a role. Borisov & Palme(2000) have estimated Pt contents in iron-bearing silicatemelts in equilibrium with a PtFe alloy using availableexperimental data on Pt solubility in the haplobasalticDA composition (Fig. 4 in Borisov & Palme, 2000).They demonstrated that during fractional crystallizationat fO2 conditions corresponding to the QFM buffer, amantle derived melt would precipitate small amount ofPtFe alloys (,2 mm3 from 1 m3 of basalt). However,calculations of Borisov & Palme (2000) did not takeinto account the effect of the change in melt compositionsduring fractional crystallization. A common liquid line ofdescent of a fractionating basaltic melt leads to its evolu-tion towards a more silicic composition. Thus, presentresults, demonstrating a strong decrease of Pt solubilitywith increasing silica content of the melt, provide anadditional argument in support of PtFe alloysprecipitation.

Fig. 6. Absolute (a) and relative (b) alkalis contamination of the DASsamples.

362 A. Borisov, L. Danyushevsky

4.2. Effects of melt composition on Pt and Rhsolubilities

Use of the DA composition both in the 44th and 47th seriesallows for a comparison of samples obtained at two differenttemperatures (1450 and 1550 �C). Consistent with previousresults (Borisov & Palme, 1997; Fortenfant et al., 2003),both Pt and Rh solubilities in the DA melt increase slightlywith increasing temperature. Assuming that the effect oftemperature (log solubility vs. 1/T, K) is the same for allsamples of the DAS system, we recalculated the solubilitiesin the DAS melts to 1550 �C. Thus, for this temperature weobtained the Pt and Rh solubilities in 11 different composi-tions (DA, DAS06, DAS12, DAS25, DAS35, DAS50,DAS70, CA, CAS10, CAS40 and CAS50).

Log solubility of metals may be approximated as a linearfunction of the melt composition (�diXi, where di and Xi

are the empirical coefficients and mole fractions of indivi-dual oxides in the melt, respectively). Earlier, Sack et al.(1980) and Ariskin et al. (1997) used this approach todescribe the effect of melt composition on the ferric/fer-rous ratio in silicate melts and on iron solubility, respec-tively. However, we also include an X2

SiO2term in the

approximation describing metal solubilities CAS (seeFig. 4). The following equations describing Pt and Rhsolubilities (in ppm, recalculated to pure metals) at 1550�C were obtained:

log CPtðppmÞ ¼ �4:59 �XSiO2 þ 2:09 � X2SiO2

þ 1:35 �XA12O3 þ 2:92 ðR2 ¼ 0:998Þ;(3)

log CRhðppmÞ ¼ �3:23 � XSiO2 þ 1:65 � X2SiO2

� 1:03 � XAl2O3

þ 2:91ðR2 ¼ 0:998Þ: (4)

The results are graphically displayed on Fig. 7, where Ptand Rh solubilities are calculated for melts with differentðXMgO þ XCaOÞ=XAl2O3

values (r). This allows for a com-parison of experimental results obtained both with DAS (r¼ 4.36) and CAS (r ¼ 1.54) melts. An agreement ofcalculated and experimental data is exceptional.

Equations (3) and (4) and Fig. 7 clearly demonstrate thataddition of both silica and alumina will decrease Pt and Rhsolubilities. The @logCMe=@XSiO2 values at XSiO2 ¼ 0 are�4.6 for Pt and �3.2 for Rh. A comparison with dAl2O3

values ( � @logCMe=@XAl2O3) from Equations (3) and (4)implies that adding silica is approximately three timesmore effective than adding alumina in decreasing Pt andRh solubilities in silica-poor melts. However, alumina andsilica are ‘‘equally’’ effective (@logCMe=@XSiO2 � dAl2O3)in melts with XSiO2 ¼ 0:78 (in case of Pt) and withXSiO2 ¼ 0:67 (in case of Rh). In more silicic melts aluminavariations will change Pt and Rh solubilities more effec-tively than silica variations.

The free terms in Equations (3) and (4) indicate thatsolubilities in hypothetical network-former-free melt(XSiO2 and XAl2O3 equal zero) should be extremely high,

above 800 ppm both for Pt and Rh. In contrast, calculatedsolubilities in network-modifier-free melt (e.g., in puresilica) should be very low, 3 ppm Pt and 23 ppm Rh,respectively.

Although solubility data may not provide direct infor-mation on metal speciation in silicate melts, our results arein agreement with bond-valence modeling performed byFarges et al. (1999). These authors suggested that in silica-pure CaO–Al2O3 melts platinum-oxide polyhedra arebonded mostly to Ca2þ, which ‘‘may explain the relativelyhigh solubility of Pt in these relatively depolymerizedmelts’’. Note that earlier, Nakamura & Sano (1997, Fig. 3and 4) demonstrated that at 1600 �C addition of basicoxides increases Pt solubility in simple binary melts ofSiO2–Na2O, SiO2–CaO, SiO2–BaO, Al2O3–CaO andAl2O3–BaO systems.

4.3. Platinum valence in silicate melts

As was discussed in the Introduction, the study of Ptsolubility in CAS melts was partly motivated by our desireto verify whether the extremely high Pt contents in silica-free and silica-poor CAS melts found by Farges et al.(1999) are real solubilities, not affected by nugget con-tamination. Using in situ analytical techniques, we havedetermined that at 1550 �C, in air, Pt solubility in the silica-

Fig. 7. Calculated solubilities of Pt and Rh at 1550 �C for the meltswith different (CaOþMgO)/Al2O3 (mol.) ratios, as indicated bydashed lines, compared to experimental results in DAS and CASsystems (see text for details).

The effect of silica on Pd, Pt and Rh solubilities 363

free CA melt is about 24 times higher than in the haploba-saltic DA melt (230 vs. 9.6 ppm, data recalculated to puremetal solubility, see Table 2) confirming the results ofFarges et al. (1999). Thus a question remains, why doXAFS data show Pt4þ as the main species in silicatemelts (Farges et al., 1999), whereas the slope of Pt solubi-lity vs. log fO2 suggests Pt2þ as the main species (Borisov& Palme, 1997; Ertel et al., 1999; Fortenfant et al., 2003)?

One possibility is that the valence of Pt species in silicatemelts is composition dependent. Indeed, those proposingPt2þ as the main species in the silicate melts (Borisov &Palme, 1997; Ertel et al., 1999; Fortenfant et al., 2003)worked mostly with the DA composition, whereas Fargeset al. (1999) studied silica-free and silica-poor CAS melts.However, Nakamura & Sano (1997) performed experi-ments with a silica-free 65.5 mol% CaO–Al2O3 composi-tion (which is similar to our CA composition and theCa0.39 composition from Farges et al., 1999) at constanttemperature of 1600 �C in the fO2 range between air andpure oxygen. The slope of log solubility vs. log fO2 wasfound to be equal to 0.41, which is close to 1/2, implyingthat in these melts Pt2þmust be the main species accordingto the reactions:

PtðmetalÞ þ 1

2O2 ¼ Pt2þOðmeltÞ; (5)

K5 ¼aPtO

aPt � fO1=22

� � ¼ XPtO � gPtOfO1=2

2

; (6)

log CPt ¼1

2logfO2 þ log A � K5

gPtO

� �; (7)

where K5 is the equilibrium constant of reaction (5), A is aconversion factor of weight concentration (Cpt) into molefraction (XPtO), gPtO is the activity coefficient of PtO in thesilicate melt and Pt activity in metal aPt¼ 1 in case of pureplatinum solubility. If Pt4þ were the main species, theslope of log solubility vs. log fO2 should be equal to one,corresponding to a different reaction:

PtðmetalÞ þ O2 ¼ Pt4þO2ðmeltÞ: (8)

Additionally, Pt2þ is also consistent with experimentalresults with a 3SiO2�Al2O3�MgO melt (Borisov & Palme,1997), with silicate laser glass ED-2 (mostly Li2Si2O5,Hornyak & Abendroth, 1970) and even with phosphatemelts (Campbell, 1995; Campbell et al., 1995). The onlyexceptions are the data of Dable et al. (2001), who studiedPt and Rh solubilities in CaO–Al2O3–SiO2 melts at 1700K. The authors declared Pt6þ as the main species dissolvedin most melts at oxidizing conditions, although in equili-brium with solid PtO2, not pure metal. This oxide, theyspeculate, passivates the surface of the Pt or PtRh boatloaded with experimental melts. In summary, the vastmajority of experimental studies come to a conclusionthat Pt2þ is the main species in silicate melts.

On the other hand, Pt4þ by XAFS was found not only byFarges et al. (1999) in silica-free and silica-poor CAS

glasses, but also by Karabulut et al. (2002) in phosphateglasses. A possible issue with these results is that in bothstudies the spectra of Pt-bearing silicate glasses were com-pared with different compounds of Pt4þ. At the same time,no compound of Pt2þ was measured, but the authorsinsisted that there was no Pt2þ in experimental glasses.Nevertheless, in addition to XAFS, the optical spectro-scopy studies of sulphate, borate and phosphate glasses(Duffy & MacDonald, 1971; Paul & Tiwari, 1973; Clicket al., 2003) also show that Pt4þ exists in glasses in adistorted octahedral environment.

We support an explanation for the discrepancy betweensolubility and spectroscopic data given by Click et al.(2003): ‘‘Pt dissolves into the melt as Pt2þ but upon cool-ing to room temperature converts to Pt4þ’’. The mainquestion is to identify a reducing reaction in the melt thatwould allow Pt oxidation from Pt2þ to Pt4þ. In our experi-ments this may be reduction of W or Mo, which are bothpresent in the glasses at concentrations comparable withthose for Pt (Table 2). Both W and Mo can exist in silicatemelts it two valent forms, 4þ and 6þ (O’Neill & Eggins,2002; O’Neill et al., 2008), potentially allowing for thefollowing redox reactions:

Pt2þ þW6þ ¼ Pt4þ þW4þ; (9)

Pt2þ þMo6þ ¼ Pt4þ þMo4þ: (10)

Mutual interaction of redox pairs is well known and widelyused in glass industry (for example, FeO oxidation in glass-forming melts by adding As or Ce oxides). The theory ofthe redox interaction can be found in Bruckner (1986). Inbrief, no mutual interaction between different redox pairsexists at experimental temperature if equilibrium withatmosphere is assumed. However, such interaction occursduring cooling, with a cooling rate playing an importantrole. The redox pair with the larger reaction enthalpywould reduce the pair with the smaller reaction enthalpy.We are not aware of any data on the temperature depen-dence for either Pt4þ/Pt2þ or W6þ/W4þ and Mo6þ/Mo4þ

equilibria, and thus cannot evaluate the likelyhood ofEquations (9) and (10) occuring during quenching.

In summary, the issue of Pt valency in melts and glassesneeds further investigations. A spectroscopy study of Pt-bearing glasses, containing a number of different impuri-ties and different concentrations and quenched at severaldifferent rates, is required. Alternatively, high temperatureXANES spectroscopy of a Pt-bearing melt would be anexcellent solution, similar to the study of Cr3þ/Cr2þ equi-librium by Berry et al. (2003).

4.4. Compositionally dependent contamination ofexperimental charges

Systematically different contamination of samples from asingle experimental series with a component i (impurity)should reflect the difference of its activity coefficients gi

between these samples. Indeed, two samples (A and B)

364 A. Borisov, L. Danyushevsky

placed simultaneously into a contaminated furnace shouldboth have activity of i to be proportional to af

i:

aAi ¼ aBi ¼ k � afi : (11)

where aif denotes ‘‘a furnace activity’’ (e.g., activity of i in

a gas phase in equilibrium with the dirty furnace tube) andk is a proportionality coefficient. If Equation (11) is valid,than Ci

A/CiB ¼ gi

B/giA.

As an example, we may compare some of the present Modata with those of O’Neill & Eggins (2002). These authorsstudied Mo solubility in a variety of compositions at 1400�C and recalculated their data into MoO2 and MoO3 activ-ity coefficients (see Table 7 in O’Neill & Eggins, 2002).Two of their compositions were in the DAS system, withtheir ‘‘AD’’ sample corresponding to our DA melt andtheir ‘‘AD þ Qz’’ corresponding to our DAS50. The ratio

gAdþQzMoO3

.gADMoO3

¼ 2:6. Assuming that MoO3 is the main

molybdenum species in our very oxidized melts, the coin-cidence with Mo impurity ratio is excellent: CMo

DA/CMoDAS50

¼ 2.3 for 40th series (Cologne furnace) and 2.7 for 44thseries (Bayreuth furnace), respectively.

Equation (11) does not always have to hold, however.The samples may be at a different distance from the

impurity source, or the furnace may not be homogenousin respect to the contaminant. Thus, we may only expectthat Ci

A/CiB � gi

B/giA. However we would expect that the

extent of sample enrichment by the contaminant i is con-trolled by gi, values in these samples. On Fig. 8 we com-pare relative values of alkali activity coefficients vs. XSiO2

in silicate melts of the DAS system based on our results at1450 �C (Fig. 8a) and on experiments of Borisov (2009) at1470 �C (Fig. 8b). Although the match is not perfect indetail, the results are remarkably similar: in both studiesrelative gLi2O>gNa2O>gK2O>gRb2O>gCs2O for every meltcomposition, and gi (basic) . gi (silicic) for all alkalis.

5. Conclusions

The maximum solubility of Pd was observed in melts with�55 wt% SiO2 in the DAS system at 1450 �C in air. Incontrast, Pt and Rh solubilities in these melts systematicallydecrease with increasing silica contents. The effect of meltsilica content is even more pronounced in the CAS system,with Pt and Rh decreasing from 230 to 319 ppm, respec-tively, in the silica-free CaO-Al2O3 melts down to 5.5 and29 ppm in the CAS melts with 50 % silica. These resultssupport a suggestion that magmatic fractionation frombasaltic to silicic compositions under sulfur under-saturatedconditions may result precipitation of PtFe alloys.

Strong effects of melt composition on W, Mo and alkalicontents at trace levels in experimental samples contami-nated by ‘‘dirty’’ furnaces was revealed. It was demon-strated that the ratio of a contaminant concentration in twodifferent melts from a single run is approximately inver-sely proportional to the ratio of the contaminant activitycoefficients in these melts.

Acknowledgements: This study was supported by theRussian Foundation for Basic Research, Programme forFundamental Research of the Earth Science Division,Russian Academy of Sciences, the Federal Programmefor the Support of Leading Scientific Schools, and theAustralian Research Council though funding to theCentre of Excellence in Ore Deposits (CODES) at theUniversity of Tasmania. We are grateful to ShigekoNakamura for providing primary data on Pt contents inthe CAS melts and Detlef Krauße for assistance with theelectron microprobe analyses of the Bayreuth samples.Thorough reviews by Philip Kegler and James Brenanhave significantly improved the original version.

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Received 15 June 2010

Modified version received 14 February 2011

Accepted 3 March 2011

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