flux-free fusion of silicate rock preceding acid digestion for icp-ms bulk analysis

Flux-Free Fusion of Silicate Rock Preceding Acid Digestionfor ICP-MS Bulk Analysis

Kenji Shimizu (1, 2)*, Qing Chang (1) and Kentaro Nakamura (2)

(1) Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka237-0061, Japan

(2) Precambrian Ecosystem Laboratory, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka237-0061, Japan

* Corresponding author. e-mail: [email protected]

We present a new method for the decomposition ofsilicate rocks by flux-free fusion in preparation forwhole-rock trace element determination (Sc, Rb, Sr,Y, Zr, Nb, Cs, Ba, rare earth elements and Hf) that isespecially applicable to zircon-bearing felsic rocks.The method was verified by analyses of RMs of mafic(JB-1a, JB-2, JGb-1) and felsic rocks (JG-3, JR-3,JSd-1, GSP-2, G-2). Pellets of powdered sample (upto 500 mg) without flux were weighed and placed ina clean platinum crucible. The samples were thenfused in a Siliconit� tube furnace and quenched toroom temperature. The optimum condition for thefusion of granitic rock was determined to be heatingfor 2–3 min at 1600 �C. The fused glass in theplatinum crucible after heating was decomposedusing HF and HClO4 in a Teflon� beaker.Decomposed and diluted sample solutions wereanalysed using a quadrupole inductively coupledplasma-mass spectrometer. Replicate analyses(n = 4 or 5) of the RMs revealed that analyticaluncertainties were generally < 3% for all elementsexcept Zr and Hf (� 6%) in JG-3. These higheruncertainties may be attributed to sampleheterogeneity. Our analytical results for the RMsagreed well with recommended concentrations andrecently published concentrations, indicatingcomplete decomposition of our rock samples duringfusion.

Keywords: ICP-MS, flux-free fusion, trace elementdetermination, granite, geological reference materials.

Received 21 Jul 09 – Accepted 13 Jan 10

Nous présentons une nouvelle méthode pour ladécomposition des roches silicatées par fusion sansfondant en vue de la détermination des élémentstraces sur roche totale (Sc, Rb, Sr, Y, Zr, Nb, Cs, Ba,terres rares et Hf) qui est surtout applicable pour lesroches felsiques contenant du zircon. La méthode aété vérifiée par des analyses de matériaux deréférence de roches mafiques (JB-1a, JB-2, JGB-1) etfelsiques (JG-3, JR-3, JSD-1, le SGP-2, G-2). Despellets de l’échantillon en poudre (jusqu’à 500 mg)sans fondant ajouté ont été pesés et placés dans uncreuset de platine propre. Les échantillons ont ensuiteété fondus dans un four tubulaire Siliconit� et trempéà température ambiante. Les conditions optimalespour la fusion d’une roche granitique ont étédéterminées et correspondent à un chauffagependant 2–3 min à 1600 �C. Après la phase dechauffage, le verre en fusion contenu dans le creusetde platine a été décomposé en utilisant HF et HClO4

dans un bécher de Teflon�. Les solutionsdécomposées et diluées ont été analysées en utilisantun spectromètre de masse quadripolaire couplée àun plasma inductif. Des analyses répétées (n = 4 ou5) des matériaux de référence ont révélé que lesincertitudes analytiques étaient généralement demoins de 3% pour tous les éléments sauf pour Zr etHf (� 6%) dans JG-3. Ces incertitudes plus fortespeuvent être attribuées au caractère hétérogéne del’échantillon. Nos résultats analytiques pour lesmatériaux de référence sont en bon accord avec lesconcentrations recommandées et celles récemmentpubliées, ce qui indique une décomposition complètede nos échantillons de roche lors de la fusion.

Mots-clés : ICP-MS, fusion sans flux, détermination des élé-ments traces, granite, matériaux géologiques de référence.

Vol. 35 – N� 10311 p . 4 5 – 5 5

doi: 10.1111/j.1751-908X.2010.00059.xª 2010 The Authors. Geostandards and Geoanalytical Research ª 2010 International Association of Geoanalysts 4 5

The concentration of trace elements in silicate rocks isessential information in many environmental, geologicaland material science studies. Concentrations are commonlydetermined by hot acid (HF-HClO4) digestion at � 100-200 �C followed by ICP-MS (e.g., Yokoyama et al. 1999).Silicate rock containing acid-resistant minerals such as zir-con and tourmaline is usually decomposed by bombdigestion (> 200 �C) using HF (e.g., Krogh 1973). However,insoluble aluminium fluoride forms during HF digestion offelsic rocks in a PFA bomb, incorporating large proportionsof trace elements (e.g., Takei et al. 2001). Moreover, it isnot certain that high-pressure digestion is efficient for thecomplete dissolution of refractory minerals in granite (Yuet al. 2001).

Alkali fusion is an efficient method for the decomposi-tion of zircon-bearing granitic rocks. Trace element compo-sitions of fused alkali glasses have been directlydetermined by LA-ICP-MS (Eggins 2003, Orihashi andHirata 2003). In an alternative method, fused alkali glasseshave been dissolved in acid and introduced into an ICP-MS as sample solutions (Awaji et al. 2006). However,fused samples require digestion using large amounts ofacid and must be diluted greatly to reduce matrix effectsfrom the alkali flux (Awaji et al. 2006). Furthermore, criticalcontamination of trace elements can arise from the flux, sothat alkali flux reagents must be checked for target traceelement concentrations.

A flux-free fusion technique was recently developed forLA-ICP-MS analysis of silicate rock samples (Nehring et al.2008, Stoll et al. 2008). The rock samples were melted tohomogeneous glasses at 1300-1800 �C using an iridium-strip heater. Although this method seems to be simple, it isdifficult to obtain homogeneous glasses from felsic samples,which require special preparation techniques such as theaddition of MgO to samples and melting at very hightemperature (1800 �C).

In this study, we present a flux-free fusion method fortreating silicate rock by using a Siliconit� furnace (SiliconitCo. Ltd, Saitama, Japan) followed by HF-HClO4 digestion.Our method does not require fused glasses to be homog-enised as long as refractory minerals are completely dis-solved into amorphous glasses. Decomposed dilutedsample solutions were analysed using a quadrupole ICP-MS (Q-ICP-MS) for determination of trace element [Sc, Rb,Sr, Y, Zr, Nb, Cs, Ba, Hf and rare earth elements (REE)]compositions in silicate rocks, especially for zircon-bearinggranitic samples.

Experimental procedure

Instrumentation, calibration and sampledescription

The Q-ICP-MS apparatus, calibration and reagentsused in this study were the same as those described byNakamura and Chang (2007). Trace element concentra-tions were determined using a Q-ICP-MS (Agilent 7500ce;Agilent Technologies, Japan) at the Institute for Research onEarth Evolution (IFREE), Japan Agency for Marine-Earth Sci-ence and Technology (JAMSTEC). Calibration curves foreach element were obtained from five diluted standardsolutions, including a reagent blank solution. To correct forinstrumental drift and matrix effects, In and Bi were addedto an aliquot of digested sample solution for use as aninternal standard. All element concentrations measured inthis study were corrected based on a linear extrapolationor interpolation of the 115In and 209Bi correction factors asa function of mass.

The geological RMs used were powdered samples ofbasalt (JB-1a, JB-2), gabbro (JGb-1), rhyolite (JR-3), grano-diorite (JG-1a and JG-3) and stream sediment (JSd-1) fromthe GSJ; and powdered granodiorite (GSP-2) and granite(G-2) from the USGS.

Siliconit electronic furnace

Sample fusion was performed using a SiC electric fur-nace at IFREE, JAMSTEC (Figure 1). The maximum tempera-ture of this furnace was � 1650 �C. The vertical furnacetube was 1100 mm long and 42 mm in diameter andwas made of a pure alumina ceramic. The fusion tempera-ture was measured with Pt94Rh6-Pt70Rh30 thermocouples.The hot spot of the furnace was located in the central,50 mm long, section of the tube, within which the variationat a nominal temperature of 1600 �C was � 5 �C. Thetop and bottom of the alumina tube remained close toroom temperature during heating, so the fused samplecould be quenched rapidly by dropping it to the bottom ofthe tube.

Oxygen fugacity in the furnace was controlled by aCO2 and H2 gas mixture. Gas mixing ratios were regu-lated by a mass flow controller unit and calibrated at thehot spot of the furnace against Ni-NiO, FeO-Fe3O4 andFe-FeO equilibria in a temperature range of 1000-1500 �C. Total gas flow rates were 500 cm3 min-1. Thistype of furnace is commonly used for melting experiments

4 6 ª 2010 The Authors. Geostandards and Geoanalytical Research ª 2010 International Association of Geoanalysts

involving silicate rocks and to determine phase equilibriafor magma and crystalline structure at atmospheric pres-sure (e.g., Sugawara 2001).

Flux-free fusion followed by acid digestion

A pressed pellet of powdered sample (30-500 mg)was weighed and placed in a clean platinum crucible(either 5 mm height · 5 mm diameter or 10 mmheight · 10 mm diameter, depending on the amount ofsample). Samples were fused in a Siliconit furnace at 1500or 1600 �C, with regulated oxygen fugacity, and thenquenched to room temperature. Weight losses or gains ofsamples during fusion were less than 0.5% for all of thesamples in this study.

The procedure used to digest fused glass in HF-HClO4

was based on that of Yokoyama et al. (1999), which waseffective in suppressing the formation of insoluble fluorideduring decomposition. A platinum crucible (5 or 10 mmacross) containing fused glass was placed in a 23 ml PFAscrew-cap beaker and HF (2 or 4 ml) and HClO4 (1 or

2 ml) were added to the crucible so that its contents werecompletely soaked with acid. The beaker was tightlycapped and left overnight on a hot plate at 150 �C. Thefollowing morning, the beaker was cooled and openedand the sample was then dried by step heating (100 �Cfor 10 hr, 120 �C for 8 hr and 160 �C until completelydry). HClO4 (1 ml) was added to the residue and the sam-ple was again dried using the same step-heating regimeto completely digest the fluoride (Yokoyama et al. 1999).The residue was converted from the HClO4 form to theHNO3 form by the addition of 6 mol l-1 HNO3 (2 ml),and then dried at 100 �C. The sample residue was thenplaced in a 30 ml solution of 2% v ⁄ v HNO3 with a traceamount of HF (2% v ⁄ v HNO3 + tr HF solution). An aliquotof the resulting solution was diluted with 2% v ⁄ vHNO3 + tr HF solution by a factor of 20000–60000, andIn and Bi were added to provide final concentrations of1 ng g-1 for use as an internal standard for ICP-MSmeasurements.

Results and discussion

Total procedural blanks

Total procedural blanks were determined following theexperimental procedure described above but using emptyplatinum crucibles. One or two procedural blanks wereincluded with each batch of ten to twelve samples analy-sed and their maximum values were recorded (Table 1).Concentrations of trace elements for the procedural blankswere relatively high compared with those expected fromnormal acid digestion (Nakamura and Chang 2007), pre-sumably because the crucibles and furnace were not per-fectly clean. However, the procedural blanks sufficed foruse with samples with high concentrations of trace ele-ments. For example, 20 ng of Zr blank was 0.4% of thetotal Zr in a 50 mg sample containing 100 lg g-1 Zr.

Melting conditions

To find the most appropriate heating temperature andduration for felsic rocks, we used the granodiorite RM JG-3because it contains zircon, which contains largeproportions of the whole-rock Zr and Hf content. Indeed,concentrations of Zr and Hf in JG-3, analysed by generalHF-HClO4 digestion followed by ICP-MS, were nearly 50%lower than the reference values, indicating incompletedigestion of zircon (Dulski 2001). JG-3 was fusedat 1500 �C for 3, 5, 10, 20, 30, 45 and 64 min, and at1600 �C for 2 and 3 min. In addition, the fusionsat 1500 �C were conducted under various oxygenfugacity conditions [atmosphere, argon gas, quartz fayalite

50 mm

Siliconitheater

letCO2-H2 gas inlet

Alumina tube (1100 mm long)

Insulator

Pt-crucible

Tc (B: Pt-Rh)

Figure 1. Cross-section of the Siliconit furnace used

for flux-free fusion in this study.

ª 2010 The Authors. Geostandards and Geoanalytical Research ª 2010 International Association of Geoanalysts 4 7

Tab

le1

.Tr

ace

elem

entc

onc

entr

atio

ns(l

gg)

1),

pre

cisi

on

(%RS

D)

of

refe

renc

em

ate

ria

lsfu

sed

at1

60

0�C

and

2–

3m

inut

esa

ndth

eir

pub

lishe

dva

lues

Na

me

Typ

ee

lem

ent

Tota

lp

roce

du

ral

bla

nk

[ng

]

JG-3

(n=

5)

gra

nd

iori

teJG

-1a

(n=

2)

gra

nd

iori

teG

SP-2

(n=

4)

gra

nd

iori

teG

-2(n

=3

)g

ran

ite

%R

SDre

fere

nce

%R

SDre

fere

nce

%R

SDre

fere

nce

%R

SDre

fere

nce

Sc<0

.38.

33.

18.

76a

5.8

1.3

6.21

a5.

9e6.

33.

96.

3b9.

8f6.

2g3.

22.

23.

5b3.

8g

Rb<0

.466

1.1

67.3

a68

c17

01.

717

8a17

8.0c

247

0.4

245b

199f

274g

165

2.3

170b

201g

172h

Sr<1

362

1.6

379a

367d

180

0.7

187a

186.

0c23

80.

524

0b21

4f25

9g46

92.

747

8b53

8g47

7h

Y<0

.415

1.4

17.3

a17

.1d

280.

732

.1a

29.2

c25

0.6

28b

26.9

f25

g8.

92.

011

b9.

2g9.

24h

Zr<2

015

05.

914

4a14

5d10

11.

611

8a97

.0c

562

2.9

550b

560f

642g

296

5.0

309b

365g

315h

Nb

<0.1

5.5

1.4

5.88

a5.

57d

100.

311

.4a

12.3

e25

0.9

27b

26.7

f23

g11

1.5

12b

11.6

g11

.2h

Cs

<0.0

31.

91.

51.

78a

1.9c

110.

110

.6a

11.0

c1.

20.

51.

2b0.

99f

1.23

g1.

32.

21.

34b

1.36

g1.

32h

Ba<4

472

3.9

466a

479d

508

0.4

470a

468.

0c14

850.

413

40b

1149

f14

61g

1962

5.1

1880

b20

95g

1917

h

La<3

213.

920

.6a

20d

200.

421

.3a

20.0

c18

60.

918

0b17

8f19

7g85

2.5

89b

93g

88h

Ce

<0.8

433.

540

.3a

41.3

d42

0.9

45a

43.0

c44

51.

041

0b40

0f49

8g15

82.

716

0b16

5g17

7h

Pr<0

.02

4.8

2.8

4.7a

4.64

d4.

80.

75.

63a

4.9c

561.

051

b50

.4f

60g

162.

318

b16

.5g

17h

Nd

<0.0

518

1.8

17.2

a16

.9d

181.

120

.4a

17.8

c20

91.

220

0b19

8f22

4g52

2.4

55b

54g

54h

Sm<0

.33.

32.

03.

39a

3.31

d4.

32.

44.

53a

4.1c

271.

327

b25

.4f

27g

7.0

1.6

7.2b

7.3g

7.13

h

Eu0.

843.

50.

9a0.

85d

0.67

0.7

0.7a

0.7c

2.3

1.1

2.3b

2.07

f2.

4g1.

43.

41.

4b1.

4g1.

34h

Gd

<0.0

53.

03.

22.

92a

3.06

d4.

31.

54.

08a

4.4c

12.2

1.4

12b

12.7

f12

.2g

3.8

3.5

4.3b

3.8g

4.08

h

Tb<0

.004

0.44

1.6

0.46

a0.

46d

0.76

0.3

0.81

a0.

7c1.

290.

61.

4g0.

443.

10.

48b

0.46

g0.

512h

Dy

<0.0

12.

71.

32.

59a

2.87

d4.

82.

04.

44a

4.8c

5.8

1.1

6.1b

5.95

f6g

2.0

2.6

2.4b

2.2g

2.24

h

Ho

<0.0

020.

551.

70.

38a

0.57

d0.

990.

90.

82a

1.0c

0.96

0.5

1.0b

1.0g

0.34

2.6

0.4b

0.37

g0.

356h

Er<0

.005

1.6

1.7

1.52

a1.

61d

3.0

1.2

2.57

a3.

0c2.

31.

12.

2b2.

37f

2.5g

0.84

2.0

0.92

b0.

93g

0.90

4h

Tm<0

.007

0.24

2.3

0.24

a0.

25d

0.43

2.5

0.38

a0.

5c0.

261.

50.

29b

0.31

g0.

103.

40.

18b

0.12

5h

Yb<0

.01

1.7

1.0

1.77

a1.

83d

3.0

1.0

2.7a

3.0c

1.6

0.6

1.6b

1.7f

1.77

g0.

652.

10.

8b0.

76g

0.71

2h

Lu<0

.06

0.26

1.3

0.26

a0.

28d

0.44

0.2

0.44

a0.

4c0.

221.

20.

23b

0.24

f0.

25g

0.08

64.

00.

11b

0.11

g0.

099h

Hf

<0.1

54.

16.

24.

29a

3.78

d3.

41.

83.

59a

3.2c

142.

714

b14

.8f

16g

7.2

4.9

7.9b

8.5g

7.18

h


Tab

le1

(co

ntin

ued

).Tr

ace

elem

entc

once

ntra

tions

(lg

g)

1),

pre

cisi

on

(%RS

D)

of

refe

renc

em

ate

ria

lsfu

sed

at1

60

0�C

and

2–

3m

inut

esa

ndth

eir

pub

lishe

dva

lues

Na

me

Typ

ee

lem

en

t

JSd

-1(n

=3

)st

eam

sed

imen

tJR

-3(n

=3

)rh

yoli

teJB

-1a

(n=

2)*

ba

salt

JB-2

(n=

4)*

*b

asa

ltJG

b-1

(n=

2)*

ga

bb

ro

%R

SDre

fere

nce

%R

SDre

fere

nce

%R

SDre

fere

nce

%R

SDre

fere

nce

%R

SDre

fere

nce

Sc10

.81.

510

.9a

10.9

e1.

56.

80.

5a26

.90.

127

.9a

28.9

i52

1.6

53.5

a53

.3j

340.

735

.8a

35.5

i

Rb67

0.8

67.4

a62

.9e

456

0.2

453a

453c

380.

539

.2a

36.3

i6.

21.

27.

37a

6.12

j5.

70.

46.

87a

5.7c

Sr34

20.

434

0a30

2e10

2.2

10.4

a10

c44

50.

444

2a42

8i17

60.

817

8a17

7j32

60.

632

7a32

5c

Y14

1.3

14.8

a15

.7b

157

0.6

166a

157c

210.

524

a24

i22

0.6

24.9

a20

.6j

9.0

0.6

10.4

a9c

Zr13

13.

913

2a13

4b16

780.

614

94a

1674

c13

70.

614

4a15

4i47

0.7

51.2

a46

f28

1.4

32.8

a30

c

Nb

111.

411

.1a

12e

535

1.2

510a

260.

526

.9a

27.4

i0.

485.

91.

58a

0.43

f2.

20.

73.

34a

2.31

i

Cs

2.0

0.8

1.89

a2.

05e

1.0

0.7

1.0a

0.96

4c1.

20.

11.

31a

1.14

i0.

781.

40.

85a

0.76

8j0.

210.

70.

26a

0.2c

Ba53

00.

352

0a50

2e60

.81.

665

.8a

62c

502

0.8

504a

476i

221

0.8

222a

215j

620.

164

.3a

61c

La16

0.9

18.1

a16

.3e

172

0.3

179a

170c

370.

537

.6a

35.8

i2.

34.

62.

35a

2.14

j3.

50.

43.

6a3.

4c

Ce

320.

634

.4a

32e

320

1.2

327a

319c

660.

465

.9a

65.9

i6.

60.

46.

76a

6.39

j8.

20.

28.

17a

8c

Pr4.

20.

64.

05a

4.11

e31

.50.

633

.1a

32.6

c7.

00.

37.

3a6.

7i1.

23.

31.

01a

1.1j

1.1

0.6

1.13

a1.

13c

Nd

170.

617

.6a

16.7

e10

40.

710

7a10

2c26

0.1

26a

25i

6.4

2.0

6.63

a6.

32j

5.3

0.3

5.47

a5c

Sm3.

51.

43.

48a

3.54

e21

.00.

621

.3a

20.5

c5.

10.

25.

07a

4.78

i2.

31.

02.

31a

2.19

j1.

40.

01.

49a

1.33

c

Eu1.

00.

90.

925a

0.94

e0.

431.

40.

53a

0.44

5c1.

50.

61.

46a

1.4i

0.85

0.9

0.86

a0.

818j

0.62

0.5

0.62

a0.

61c

Gd

3.4

1.4

2.71

a3.

26e

23.5

1.8

19.7

a21

.7c

4.9

0.7

4.67

a4.

12i

3.3

0.3

3.28

a3.

2j1.

70.

71.

61a

1.67

c

Tb0.

480.

50.

431a

0.46

e4.

20.

94.

29a

4.12

c0.

710.

50.

69a

0.69

i0.

590.

60.

6a0.

579j

0.28

0.5

0.29

a0.

27c

Ho

0.51

0.3

0.31

8a0.

48e

6.1

1.0

4.7a

6c0.

820.

40.

71a

0.74

i0.

890.

90.

75a

0.86

8j0.

360.

20.

33a

0.34

c

Dy

2.7

0.7

2.23

a2.

56e

28.2

0.4

21.5

a27

.5c

4.2

0.0

3.99

a3.

84i

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magnetite (QFM) buffer and iron-wüstite buffer]. Concentra-tions of trace elements for each of these fugacity conditionsdiffered generally by 3% or less. Although regulation ofoxygen fugacity is not important for flux-free fusion, it wasregulated by QFM-buffer during fusion in our study.

Concentrations of highly volatile elements such as Tland Pb decreased with time, and the normalised valueswere lower than those recommended values by Imai et al.(1995) for all conditions (Figure 2), indicating that ourmethod is not valid for analysing Tl and Pb. Stoll et al.(2008), using an iridium-strip heater, found that concentra-tions of other volatile elements determined by flux-freefusion, such as Rb, Cs and Ba, mostly decreased withincreasing temperature and heating duration. However,our data did not vary significantly from the recommendedvalues of JG-3 (Figure 2). Therefore, our method is consid-ered valid for Rb, Cs and Ba. The heated iridium strip usedby Stoll et al. (2008) was 0.18 mm thick and 33 mm long,and � 40 mg of sample powder was placed on it forfusion. We suggest that the temperature gradient within thesample during heating was the main difference betweentheir fusion experiment (temperature differences > 1000 �C)and ours (temperature differences < 5 �C). Therefore, webelieve that evaporation of volatile elements such as Rb,Ce and Ba may be suppressed by ensuring homogeneousheating of the sample powder during fusion.

Concentrations of Zr and Hf in samples heated at1500 �C increased with heating time and approachedthe reference values after 64 min (Figure 2). This result indi-cated that host minerals for Zr and Hf, such as zircon, grad-ually dissolved with time and that � 60 min was required

to achieve complete digestion of zircon at 1500 �C. Con-centrations of other elements, such as Sc, Sr, Y, Nb andREE, were constant for all heating conditions and agreedwell with recommended values of Imai et al. (1995) (nor-malised values in the range 0.9–1.1), except for Y (� 0.85)and Ho (� 1.4). As reported recently (e.g., Dulski 2001),the measured concentrations of Y and Ho in GSJ RMs areusually higher and lower, respectively, than the ‘true’ values.Our measured Y and Ho concentrations were consistentwith recently published data (Figure 3 and Table 1).

Our results indicate that longer durations (more than� 60 min) of heating at 1500 �C could achieve completefusion for granitic samples. Our results for JG-3 fused at1600 �C for 2–3 min were almost identical to those at1500 �C and 64 min, except for Tl and Pb (Figure 2). Ashort duration of heating suppressed evaporation of vola-tile elements from glass and lowered the risk of contamina-tion from the environment, including the crucible andfurnace. We suggest, therefore, that high temperature andminimum duration of heating (i.e., 1600 �C for 2–3 min)are appropriate conditions for fusion of silicate rocksamples.

Precision and accuracy

Taking into account the above experimental results, wefused RMs at 1600 �C for 2–3 min, except for some maficRMs (JB-1a, JGb-1 and JB-2), which we fused at 1500 �Cfor < 5 min (Table 1 and Figures 3 and 4).

To evaluate the precision and accuracy of our method,we performed replicate analyses for five digestions of geo-

ScRbSr

ZrNb

CsBaLa

NdLuHf

TlPb

Y

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

40 601600 °C 2-3 min

average

TlTlTl

PbPbPb

HfHfHf

ZrZrZr

CsCsCsLuLuLu

Y

Heating duration at 1500 °C (min)

FF

F/r

efer

ence

val

ue

200

Figure 2. Compositional profiles of geo-

logical RM JG-3 for heating durations of

up to 64 min at 1500 �C (left side of dia-

gram) and for 2–3 min at 1600 �C (right

side of diagram). Measured data were

normalised by recommended values for

JG-3 from Imai et al. (1995). The grey

area shows where ratios of measured to

recommended values are in the range

0.9–1.1, indicating that measured and

recommended values are effectively

equal.


1

10

100

1000

10 100

10

100

1000

0.811.2

0.1

1

10

100

1000

1

10

100

1000

0.20.61

10

100

200

0.811.2

10

100

200

015

Rb [µg g-1]

Ba [µg g-1]

Sc [µg g-1]

Y [µg g-1]

Nb [µg g-1]

Published value (PV) FFF/PV

Flu

x fr

ee f

usi

on

(F

FF

)F

lux

free

fu

sio

n (

FF

F)

Flu

x fr

ee f

usi

on

(F

FF

)

Flu

x fr

ee f

usi

on

(F

FF

)

Flu

x fr

ee f

usi

on

(F

FF

)

FFF/PVPublished value (PV)

Published value (PV) FFF/PVFFF/PVPublished value (PV)

Published value (PV) FFF/PVFFF/PVPublished value (PV)

Flu

x fr

ee f

usi

on

(F

FF

)

10

100

1000Zr [µg g-1]

10

100

1000

2000

11.2

JR-3

GSP-2

G-2

JG-3JB-1aJSd-1JG-1a

JGb-1

JB-2

JR-3 (PV = 0.5)

GSP-2

G-2

JG-3

JB-1a

JSd-1

JG-1a

JGb-1

JB-2

JR-3

GSP-2G-2

JG-3JB-1aJSd-1 JG-1a

JGb-1

JB-2

JR-3

GSP-2

G-2

JG-3JB-1a

JSd-1

JG-1a

JGb-1

JB-2

JR-3

GSP-2G-2

JG-3

JB-1a

JSd-1JG-1a

JGb-1

JB-2

JR-3

GSP-2G-2

JG-3

JB-1a

JSd-1

JG-1a

JGb-1JB-2

Recommended value (ref. a & b)Published value (ref. c - k)Pretorius et al., 2006

100

1000

10000

100

1000

10000

0.81

1

10

100

0.811.231

10

100

1 100

010 10 100 1000 0.8

10 100 1000 1.2

10

10

1

10

100

1000

10 100

10

100

1000

0.811.2

0.1

1

10

100

0.1

1

10

100

0.811.2

1

10

100

0.811.2

1

10

100

10.5 10

La [µg g-1]

Yb [µg g-1]

Ho [µg g-1]

FFF/PVPublished value (PV) FFF/PVPublished value (PV)

Published value (PV) FFF/PV

Flu

x fr

ee f

usi

on

(F

FF

)

FFF/PVPublished value (PV)

Flu

x fr

ee f

usi

on

(F

FF

)

Flu

x fr

ee f

usi

on

(F

FF

)F

lux

free

fu

sio

n (

FF

F)

Hf [µg g-1]JR-3

JR-3

GSP-2

GSP-2

G-2

G-2

JG-3JG-3 JB-1aJB-1a

JSd-1JSd-1 JG-1a

JG-1a

JGb-1

JGb-1

JB-2

JB-2

JR-3

GSP-2

G-2

JG-3JB-1a

JSd-1

JG-1a

JGb-1

JB-2

JR-3GSP-2

G-2

JG-3

JB-1a

JSd-1JG-1a

JGb-1JB-2

Compiled data (ref. a & b)Published data (ref. c - k)Pretorius et al. (2006)

0.1

1

10

10.1

1

10

11.41.8

1 10

Figure 3. Mean measured values (this study) of trace element concentrations compared with published values.

For each element, the left side of the diagram shows the mean measured values plotted against published val-

ues, and the right side shows mean measured values plotted against mean measured values normalised by pub-

lished values. Grey bands indicate the ratio of mean to published values in the range 0.9–1.1, indicating that

the mean and published values are effectively equal. References for published values used are listed in Table 1.


1

10

100

1000

1

10

100

1000

Pretorius et al. (2006)

1

10

100

This study (n = 5)Imai et al. (1995)Awaji et al. (2006)

This study (n = 2)Imai et al. (1995)Dulski (2001)

This study (n = 4)Imai et al. (1995)Makishima and Nakamura (2006)

This study (n = 3)Imai et al. (1996)Garbe-Schonberg (1993)

This study (n = 3)Govindaraju (1994)Willbold and Jochum (2005)

Pretorius et al. (2006)

This study (n = 4)Wilson (1998)Nehring et al. (2008)

10

100

La Ce Pr Nd SmEu Gd Tb Dy Ho Er TmYb LuLa Ce Pr Nd SmEu Gd Tb Dy Ho Er TmYb Lu

1

10

100

Ho ErTmYb Lu1

10

100

La Ce Pr Nd Sm Eu Gd Tb Dy Ho ErTmYb LuLa Ce Pr Nd Sm Eu Gd Tb Dy

Ho ErTmYb LuLa Ce Pr Nd Sm Eu Gd Tb DyHo ErTmYb LuLa Ce Pr Nd Sm Eu Gd Tb Dy

JG-3Granodiorite

G-2Granite

JG-1aGranodiorite

GSP-2Granodiorite

JB-2Basalt

JSd-1Stream sediment

Ch

on

dri

te-n

orm

alis

edC

ho

nd

rite

-no

rmal

ised

Ch

on

dri

te-n

orm

alis

ed

Ch

on

dri

te-n

orm

alis

edC

ho

nd

rite

-no

rmal

ised

Ch

on

dri

te-n

orm

alis

ed

Figure 4. Chondrite-normalised (McDonough and Sun 1995) rare earth element abundances of RMs analysed in

this study compared with reference values and published values. Thin grey lines indicate other published values

from the GeoReM database (Jochum et al. 2005). Analytical uncertainties for REEs of RMs were generally better

than 3%.


logical RM JG-3 (granodiorite), and four digestions of JB-2(basalt) and GSP-2 (granodiorite). Reproducibility (% RSD)estimated from individual digestions and from measure-ments of geological RMs was generally better than 3% forJB-2 and GSP-2, and better than 5% for JG-3 (Table 1).Because internal precision obtained by repeat analyses ofthe same JG-3 solution was generally better than 3%,which corresponds to the reproducibility of the other twogeological RMs, the larger RSDs (especially 5.9% for Zrand 6.2% for Hf) may reflect heterogeneous distributions ofminor minerals within the samples. The possible contribu-tion of Zr from a single 100 lm crystal of zircon could be� 4% of the total Zr in a rock sample of 500 mg with Zrcontent of 150 lg g-1. Therefore, great care must be takento avoid heterogeneity of samples analysed.

Six other geological RMs, BIR-1 (basalt, USGS), JB-2(basalt, GSJ), JG-1a (granodiorite, GSJ), JG-3 (granodiorite,GSJ), JGb-1 (gabbro, GSJ) and JGb-2 (gabbro, GSJ) wereanalysed to further test our method (Table 1). Reproducibili-ties for each element of the RMs were generally better than3%, except for G-2, for which the RSD was similar to thatof JG-3.

For almost all of the elements measured, the dataobtained by our flux-free fusion method were well withinthe range of previously published data for all of the RMswe used (Figure 3 and Table 1). In addition, the REE pat-terns for all the RMs (Figure 4) were generally smooth andcoherent, except for Eu, which is anomalously enriched ingabbroic rocks and depleted in granitic rocks owing toaccumulation and removal of plagioclase. These resultsclearly confirm the good accuracy of the data from ournew method.

Prominent differences between the mean concentra-tions from this study and published values were for Zr andHf in GSP-2 and G-2 (Figure 3). Pretorius et al. (2006)report values � 20% higher than those previously reported(and ours), indicating complete decomposition by bombdigestion of acid-resistant minerals (e.g., zircon) in the sam-ples and better resolution of interference in high-resolutionICP-MS determinations. The results we achieved with ourmethod also indicate complete digestion of zircon andhigh recoveries of trace elements, as confirmed by thedigestion of JG-3 (Figure 2). Thus, these differences mayreflect sample heterogeneity caused by different elementconcentrations for different bottles of RM (Cotta et al.2007). Our results for volatile elements such as Rb, Cs andBa in GSP-2 were almost identical to other published val-ues, but considerably higher than those of Nehring et al.(2008), which were obtained by LA-ICP-MS following flux-

free fusion with an iridium-strip heater (Table 1 and Fig-ure 3). These differences indicate that Rb, Cs and Ba inrock powders did not evaporate during heating at1600 �C in our procedure and show that our method isvalid for the determination of Rb, Cs and Ba. Determina-tions for other elements obtained in our study agree well(within analytical uncertainty) with recently published results(Figures 3 and 4).

Complete acid digestion of a pure rock powder with-out flux by our method is an excellent way to prepare sam-ples for trace element determination. The method is alsopotentially usable in sample digestion for isotopic determi-nation of Hf, Sr, Nd and other elements, because ourmethod provided high recovery (� 100%) for these ele-ments in samples and low concentrations of proceduralblanks.

Conclusions

A new method using a Siliconit furnace was developedto accomplish flux-free fusion of silicate rock precedingHF-HClO4 digestion, in preparation for bulk analysis byICP-MS. The method is simple, straightforward and, mostimportantly, achieved complete digestion of silicate rocksamples. It is thus especially effective for felsic samples con-taining refractory minerals such as zircon and tourmaline.Although the method is not valid for the determination ofhighly volatile elements such as Pb and Tl, it is valid for Rb,Cs and Ba.

Acknowledgements

We thank Drs K. Suzuki and A. Ishikawa for helpful dis-cussions. We also acknowledge Dr R. Senda for analyticalsupport. This work was partly supported by grants to K.Shimizu from the Japan Society for the Promotion of Sci-ence (No. 20740311 and No. 19GS0211).

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flux-free fusion of silicate rock preceding acid digestion for icp-ms bulk analysis

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