Calcium Isotopes (δ44/40Ca) in MPI-DING Reference Glasses,USGS Rock Powders and Various Rocks: Evidence for CaIsotope Fractionation in Terrestrial Silicates

Vol. 33 — N° 2 p . 2 3 1 - 2 4 7

We report δ44/40Ca(SRM 915a) values for eight fusedMPI-DING glasses and the respective original powders, six USGS igneous rock reference materials,the U-Th disequilibria reference material TML, IAEA-CO1 (Carrara marble) and several igneousrocks (komatiites and carbonatites). Sample selectionwas guided by three considerations: (1) to addressthe need for information values on reference materials that are widely available in support ofinterlaboratory comparison studies; (2) support the development of in situ laser ablation and ionmicroprobe techniques, which require isotopicallyhomogenous reference samples for ablation; and(3) provide Ca isotope values on a wider range of igneous and metamorphic rock types than is currently available in the scientific literature.Calcium isotope ratios were measured by thermalionisation mass spectrometry in two laboratories(IFM-GEOMAR and Saskatchewan IsotopeLaboratory) using 43Ca/48Ca- and 42Ca/43Ca-doublespike techniques and reported relative to the calcium carbonate reference material NIST SRM915a. The measurement uncertainty in both laboratories was better than 0.2‰ at the 95%confidence level. The impact of different preparationmethods on the δ44/40Ca(SRM 915a) values was foundto be negligible. Except for ML3-B, the originalpowders and the respective MPI-DING glasses showed identical δ44/40Ca(SRM 915a) values; therefore,possible variations in the Ca isotope compositionsresulting from the fusion process are excluded.Individual analyses of different glass fragmentsindicated that the glasses are well homogenised on

Nous présentons ici les valeurs de δ44/40Ca(SRM 915a)

obtenues sur huit verres MPI-DING obtenus parfusion et sur les poudres originales correspondantes,sur six roches ignées de référence de l'USGS, sur lesmatériaux de référence pour le déséquilibre U-Th:TML, IAEA-CO1 (Marbre de Carrar) et sur plusieursroches ignées (komatites et carbonatites). La sélection des échantillons a été guidée par troisconsidérations : (1) répondre au besoin d'avoir desvaleurs pour des matériaux de référence largementdiffusés, en complément d'études comparativesentre laboratoires; (2) aider au développement de l'ablation laser in situ et des techniques demicrosonde, qui nécessitent des matériaux de référence isotopiquement homogènes pour l'ablation; et (3) fournir des valeurs de rapports isotopiques de Ca sur un éventail de roches magmatiques et métamorphiques plus large que ce qu'on trouve actuellement dans la littératurescientifique. Les rapports isotopiques du calcium ont été mesurés par spectrométrie de masses à thermo-ionisation dans deux laboratoires (IFM-GEOMAR et Saskatchewan Isotope Laboratory)en utilisant les techniques de double spike43Ca/48Ca et 42Ca/43Ca, et ils sont rapportés parrapport au carbonate de calcium de référence NISTSRM 915a. L'incertitude de mesure dans les deuxlaboratoires était meilleure que 0.2% pour un intervalle de confiance de 95%. L'impact des différentes méthodes de préparation sur la mesuredes valeurs de δ44/40Ca(SRM 915a) s'est révélé négligeable. Excepté pour ML3-B, les poudres originales et les verres de fusion MPI-DING correspondant ont les mêmes valeurs de

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0609

Marghaleray Amini (1, 2)*, Anton Eisenhauer (1), Florian Böhm (1), Chris Holmden (2), Katharina Kreissig (3), Folkmar Hauff (1) and Klaus Peter Jochum (4)

(1) Leibniz-Institut für Meereswissenschaften, IFM-GEOMAR, Wischhofstrasse 1-3, 24148 Kiel, Germany(2) Saskatchewan Isotope Laboratory, Department of Geological Sciences, University of Saskatchewan, 114 Science Place,

Saskatoon, SK, S7N 5E2, Canada(3) Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV, Amsterdam, Netherlands(4) Max-Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany* Corresponding author. email: marg.amini@usask.ca

© 2009 The Authors. Journal compilation © 2009 International Association of Geoanalysts

GEOSTANDARDS and

RESEARCHGEOANALYTICAL

In recent years calcium has been increasingly used ingeochemical tracer and proxy studies. As the fifth mostabundant element of the Earth’s crust, Ca is one of themajor components for which isotope variations can bedemonstrated. Naturally occurring variations of Ca iso-topes during geological and biological processesresult from either 40Ca generation as a result of 40Kdecay, or from thermodynamically driven mass-depen-dent fractionation (cf. DePaolo 2004).

Stable isotope fractionation of calcium is well docu-mented in low-temperature systems (e.g. , Zhu andMacdougall 1998, Naegler et al. 2000). It occursduring inorganic and biotic Ca-mineral precipitation(e.g., Gussone et al. 2003, Schmitt et al. 2003a, Marriottet al. 2004, Lemarchand et al. 2004), ion exchange(Heumann and Lieser 1972, Russell and Papanastassiou1978) and plant uptake (Schmitt and Sti l le 2005,Wiegand et al. 2005, Holmden and Belanger 2006).However, in high-temperature systems, the behaviour ofCa isotopes is less well studied. Recently, it has beenshown that large isotopic fractionation of Li and Fe maybe driven by diffusion in magmatic systems, related tothe processes of partial melting and melt transport (e.g.,Williams et al. 2005, Lundstrom et al. 2005). Significantfractionation of up to 7‰ for 44Ca/40Ca isotope ratiosbetween rhyolitic and basaltic melts was demonstrated

in experimental studies (Richter et al. 2003). Existing Caisotope data, which comprise only a few contrastingigneous rocks, show very limited natural variations ornone at all (Russell et al. 1978, Skulan et al . 1997,DePaolo 2004). Whether large Ca isotope fractionationexists in natural igneous systems has yet to be verifiedby a systematic investigation of various rock types. Sucha study could reveal undetected isotope variations,contributing to the question of whether the bulk Earthδ44/40Ca(SRM 915a) value of about 1‰ is the most appro-priate reference for Ca isotope studies (cf. DePaolo 2004).

The aim of this study is two-fold: (1) to establishδ44/40Ca values for a suite of rock reference materials,with a particular emphasis on laser and ion microprobeablation calibrators (Vigier et al. 2006, Rollion-Bard etal. 2007); and (2) complement and extend the existingknowledge of δ44/40Ca values in igneous rocks (Skulanet al. 1997, Zhu and Macdougall 1998, Richter et al.2003, Schmitt et al. 2003b, DePaolo 2004).

Materials

MPI-DING glasses and powders

The MPI-DING (Max Planck Institut-Dingwell) refe-rence suite consists of eight igneous rock samples that

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GEOSTANDARDS and

RESEARCHGEOANALYTICAL

© 2009 The Authors. Journal compilation © 2009 International Association of Geoanalysts

the mm scale with respect to Ca. The range ofδ44/40Ca(SRM 915a) values in the igneous rocks studiedwas larger than previously observed, mostly owingto the inclusion of ultramafic rocks from ophiolitesections. In particular, the dunite DTS-1 (1.49 ±0.06‰) and the peridotite PCC-1 (1.14 ± 0.07‰)are enriched in 44Ca relative to volcanic rocks (0.8 ± 0.1‰). The Carrara marble (1.32 ± 0.06‰) was also found to be enriched in 44Ca relative to the values of assumed precursor carbonates (< 0.8‰). These findings suggest that the isotopes of Ca are susceptible to fractionation at high temperatures by, as yet, unidentified igneous andmetamorphic processes.

Keywords: Ca isotope compositions, geological reference materials, MPI-DING glasses,sample preparation.

δ44/40Ca(SRM 915a), par conséquent toute variation dela composition isotopique de Ca liée à la fusion està exclure. Des analyses ponctuelles de différentsfragments de verre montrent que les verres sont trèsbien homogénéisés à l'échelle du mm, pour ce quiconcerne le calcium. La dispersion des résultats deδ44/40Ca(SRM 915a) dans les roches ignées est plusimportante qu'observée précédemment, ceci est dûprincipalement au fait qu'ont été inclus ici desroches ultrabasiques appartenant à des sectionsophiolitiques. La dunite DTS-1 (1.49 ± 0.06‰) et lapéridotite PCC-1 (1.14 ± 0.07‰) en particulier, sonttrès enrichies en 44Ca par rapport aux roches volcaniques (0.8 ± 0.1‰). Le marbre de Carrare(1.32 ± 0.06‰) est aussi enrichi en 44Ca par rapportaux carbonates (< 0.8‰) dont il est censé provenir.Ces découvertes suggèrent que les isotopes de Casont susceptibles de fractionner à haute températurepar le biais de processus ignés et métamorphiquesqui restent encore à élucider.

Mots-clés : compositions isotopiques du calcium, matériaux géologiques de référence, verres MPI-DING,préparation des échantillons.Received 19 Mar 08 — Accepted 15 Nov 08

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© 2009 The Authors. Journal compilation © 2009 International Association of Geoanalysts

Tab

le 1

.C

alc

ium

iso

top

e va

lues

for

the

roc

k sa

mp

les

ana

lyse

d i

n th

e tw

o la

bor

ato

ries

IFM

-GEO

MA

R a

nd S

IL a

nd t

heir

mea

n va

lues

. The

res

ults

wer

e no

rma

lised

to

NIS

T SR

M 9

15a

as

refe

renc

e a

nd c

an

be

conv

erte

d t

o th

e se

aw

ate

r sc

ale

by

sub

tra

ctin

g 1

.82

(IF

M-G

EOM

AR)

and

1.8

9‰

(SI

L),

resp

ectiv

ely.

The

Ca

O a

nd M

gO

con

tent

s a

nd K

/Ca

ra

tios

wer

e ta

ken

from

the

Geo

ReM

da

tab

ase

(ht

tp:/

/geo

rem

.mp

ch-m

ain

z.g

wd

g.d

e)

Sam

ple

Ma

teri

al

δ44

/40C

a(S

RM

915

a)±

2se

aδ4

4/4

0C

a(S

RM

915

a)

bδ4

4/4

0C

a(S

RM

915

a)±

2se

cε4

0/4

4C

a(S

RM

915

a)±

2s

dC

aO

Mg

OK

/Ca

(IFM

-GEO

MA

R)

(SIL

)(m

ean)

[%m

/m]

[%m

/m]

[m/m

]

Com

mon

ly u

sed

RM

sN

IST

SRM

915

aC

alci

um c

arbo

nate

0 ±

0.0

3 (n

= 1

43

)0

±0

.1 (2

s)0

±0

.05

0 ±

0.8

--

<10

-6IA

PSO

Seaw

ater

1.8

2 ±

0.0

3 (n

= 1

35

)1.

89

1.8

6 ±

0.0

50

.5 ±

1.1

--

0.9

68

MPI

-DIN

G g

lass

esAT

HO

-GRh

yolit

e0

.87

±0

.08

(n =

13

)-

-0

.61.

70

.10

30

.901

StH

s6/8

0-G

And

esiti

c as

h0

.75

±0

.05

(n =

16

)-

-0

.35

.28

1.9

70

.14

2T1

-GQ

uartz

dio

rite

0.8

0 ±

0.0

6 (n

= 1

4)

--

0.3

7.1

3.7

50

.16

KL2

-GTh

olei

ite -

Kila

uea

0.7

0 ±

0.0

7 (n

= 1

5)

--

-10

.97.

34

0.0

26

ML3

B-G

Thol

eiite

- M

auna

Loa

0.7

2 ±

0.0

5 (n

= 1

5)

--

-10

.56

.59

0.0

21G

OR1

28

-GKo

mat

iite

(Cre

tace

ous)

0.7

0 ±

0.0

5 (n

= 1

3)

0.8

30

.77

±0

.06

1.8

±2

.06

.24

26

0.0

03

GO

R13

2-G

Kom

atiit

e (C

reta

ceou

s)0

.57

±0

.05

(n =

11

)0

.64

0.6

1 ±

0.0

61.

9 ±

2.3

8.4

52

2.4

0.0

02

BM9

0/2

1-G

Perid

otite

1.01

±0

.07

(n =

16

)-

-0

.7 ±

1.7

2.1

34

.30

.001

Ori

gin

al r

ock

pow

der

sSt

Hs6

/80

And

esiti

c as

h0

.77

±0

.11

(n =

6)

--

--

1.9

70

.14

2T1

Qua

rtz d

iorit

e0

.72

±0

.10

(n =

6)

--

--

3.7

50

.16

KL2

Thol

eiite

- K

ilaue

a0

.76

±0

.07

(n =

6)

--

--

7.3

40

.02

6M

L3B

Thol

eiite

- M

auna

Loa

0.6

2 ±

0.0

6 (n

= 6

)-

--

-6

.59

0.0

21BM

90

/21

Perid

otite

1.01

±0

.09

(n =

19

)-

--

-3

4.3

0.0

01

USG

S re

fere

nce

ma

teri

als

BHVO

-2Th

olei

itic

basa

lt0

.75

±0

.05

(n =

20

)0

.90

.83

±0

.07

1.1

±1.

611

.47.

23

0.0

26

BIR-

1Th

olei

itic

basa

lt0

.77

±0

.09

(n =

14

)-

-0

.713

.49

.70

.001

BCR-

2Th

olei

itic

basa

lt0

.81

±0

.07

(n =

6)

0.9

30

.87

±0

.06

-7.

123

.59

0.1

46

W-2

Dia

base

0.9

4 ±

0.1

0 (n

= 6

)-

--

10.9

6.3

70

.03

3DT

S-1

Dun

ite1.

44

±0

.07

(n =

15

)1.

53

1.4

9 ±

0.0

6-

0.1

74

9.6

0.0

03

PCC

-1Pe

ridot

ite1.

15 ±

0.0

9 (n

= 1

3)

1.13

1.14

±0

.07

-0

.54

42

.90

.011

Oth

er r

efer

ence

ma

teri

als

TML

Tabl

e M

ount

ain

Latit

e0

.73

±0

.10

(n =

6)

--

--

-N

AIA

EA-C

O1

Car

rara

mar

ble

1.2

7 ±

0.0

6 (n

= 1

4)

1.3

71.

32

±0

.06

--

-N

A

Rock

sa

mp

les

OW

501

9Ko

mat

iite

(~3

.5 G

a)0

.97

±0

.03

(n =

6)

1.0

81.

03

±0

.05

0.8

2.3

43

50

.00

5O

W 5

031

Kom

atiit

e (~

3.5

Ga)

0.9

8 ±

0.0

4 (n

= 6

)-

-1.

03

.37

32

0.0

09

82

LM6

6A

(res

)*M

agne

sioc

arbo

natit

e0

.87

±0

.02

(n =

6)

--

-3

8.7

9.5

50

82

LM6

6A

(lea

ch)†

Car

bona

te p

hase

0.6

7 ±

0.0

5 (n

= 6

)-

--

--

-8

3H

V26

(res

)C

alci

ocar

bona

tite

0.6

9 ±

0.0

4 (n

= 6

)-

--

48

.63

.56

0.0

06

83

HV2

6 (l

each

)C

arbo

nate

pha

se0

.74

±0

.04

(n =

6)

--

--

--

aA

vera

ges

of

all

mea

sure

men

ts c

arr

ied

out

at

IFM

-GEO

MA

R, K

iel,

Ger

ma

ny.

b

Mea

sure

d a

t SI

L, S

ask

ato

on/C

ana

da

. No

erro

rs a

re i

ndic

ate

d a

s m

easu

rem

ents

wer

e ca

rrie

d o

ut o

nce.

cM

ean

valu

e d

eter

min

ed f

rom

the

ave

rag

ed v

alu

es o

f IF

M-G

EOM

AR

and

SIL

. For

the

SIL

va

lues

an

exte

rna

l p

reci

sion

(2

s) o

f 0

.1‰

wa

s a

ssum

ed.

dε

= (

40C

a/4

4C

asa

mp

le/4

0C

a/4

4C

aRM

– 1

) x

104

det

erm

ined

at

IFM

-GEO

MA

R.

*

res:

res

idue

aft

er l

each

ing

of

sam

ple

ma

teri

al,

corr

esp

ond

ing

to

the

silic

ate

pha

se.

†Le

ach

: lea

cha

te (

carb

ona

te p

hase

) a

fter

lea

chin

g t

he w

hole

roc

k p

owd

er w

ith d

ilute

HC

l.

cover a wide compositional range (Table 1): ATHO-G(rhyolite), StHs6/80-G (andesite), T1-G (quartz diorite),KL2-G (Kilauea tholeiite), ML3B-G (Mauna Loa tholeii-te), GOR128-G and GOR132-G (Gorgona komatiites)and BM90/21-G (peridotite). The rock powders werefused to glass beads without flux agents (Dingwell etal. 1993). The preparation and overall investigation ofthe glass fragments are described in detail by Jochumet al. (2000, 2006), including some preliminary resultsfor Ca isotopes. In addition, the δ44/40Ca(SRM 915a)

va lues o f se lec ted MPI -DING re ference g lasses(StHs6/80-G, T1-G, KL2-G, ML3B-G and BM90/21-G)were compared to those of their corresponding rockpowders.

USGS reference materials

Six USGS igneous rock reference materials wereanalysed for δ44/40Ca(SRM 915a) (Table 1). The samplescomprised the Columbia River Basalt BCR-2 (Wilson1997a), the Hawaiian tholeiitic basalt BHVO-2 (Wilson1997b), the Icelandic tholeiite BIR-1 and diabase W-2(Flanagan 1984), the peridotite PCC-1 and the duniteDTS-1 (Flanagan 1976).

Other reference materials

We provide the δ44/40Ca(SRM 915a) value for theTable Mountain Latite (TML) often referred to in U-Thdisequilibria studies (e.g., Williams et al. 1992, Turneret al. 2004), and of the δ13C- and δ18O-referencematerial IAEA-CO1 (Carrara marble) distributed by theInternational Atomic Energy Agency.

Other geological materials

In order to extend the spectrum of magmatic rockswith respect to Ca isotope data, we included two car-bonatites from Fuerteventura (Canary Islands) and CapVerde (Hoernle et al. 2002). The calcic carbonatite83HV26 and the magnesian carbonatite 82LM66Aare representative of the two groups of oceanic carbo-natites, namely the high-Ca/low-Mg (calcitic) and thelow-Ca/high-Mg (dolomitic). The silicate and carbona-te phases of these samples were analysed separatelyfor their δ44/40Ca(SRM 915a) values (Table 1).

Since the komatiites GOR128-G and GOR132-Gof the MPI-DING sample suite are young (Cretaceous),and of atypical composition in comparison with otherkomatiites (e.g., Echeverria 1980), we added two typi-cal Archaean komatiites from the Onverwacht Group

in South Africa to our rock compilation (OW5019,OW5031; Nesbitt et al. 1979, Jochum et al. 1991).

Experimental procedure

Sample digestion

Samples were prepared and analysed in two labo-ratories: the Leibniz-Institute of Marine Sciences IFM-GEOMAR, Kiel , Germany, and the SaskatchewanIsotope Laboratory (SIL), University of Saskatchewan,Saskatoon, Canada. In both laboratories the proce-dures were carried out under clean conditions using18.2 MΩ cm-1 water and doubly-distilled ultra-pureacids throughout. The quality of the reagents wascontinuously monitored by procedural blank determi-nations carried out with each set of samples processedthrough the chemistry. Similarly, an aliquot of BHVO-2was processed and measured each time.

Depending on the laboratory and the Ca concen-trations of the samples, between 0.1 and 200 mg ofsample material was weighed into pre-cleaned screw-top Savillex PFA vials. Aliquots of IAEA-CO1 were dis-solved in dilute HCl. All of the other samples weresubjected to two different digestion techniques descri-bed below. Spiking at IFM-GEOMAR was performedafter the dissolutions on aliquots of clear solution; atSIL the sample powders were spiked prior to digestion.In both laboratories a common HF-HNO3 acid attackwas applied and about 2 ml HF and 1 ml 15 mol l-1

HNO3 were added to the samples and heated at 120°C in closed beakers for 48 hours. Thereafter, the dis-solved samples were dried and treated three timeswith 15 mol l-1 HNO3 to dissolve potentially formedCaF2. The final sample cake was finally dissolved in 6mol l-1 HCl. In addition, the seawater salinity referencematerial IAPSO was subjected to the same dissolutionprocedure (IAPSOdiss) and compared to the respectiveuntreated aliquot (Table 2).

At IFM-GEOMAR, a second digestion method wasapplied to the ul tramafic samples (DTS-1, PCC-1,BM90/21, BM90/21-G, GOR128-G and GOR132-G).Samples were leached in 8.8 mol l-1 HBr at ~ 160 °Cfor 72 hours in closed vessels prior to the HF-HNO3

dissolution (Nägler and Kamber 1996). During the lea-ching step the samples were ultrasonically treatedseveral times to promote the release of cations fromthe crystal cage. The brownish solutions were centrifu-ged, decanted and saved. The residues consistingmainly of the silicate framework were then treated with

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GEOSTANDARDS and

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© 2009 The Authors. Journal compilation © 2009 International Association of Geoanalysts

HF-HNO3 as described above. The preserved super-natants were later merged with the final HCl disso-lutions of the HF-HNO3 step. This technique largelyprevented the critical exposure of Ca and Mg to HF,and thus reduced the risk of fractionation effectsfrom the formation of insoluble fluorides. Due to theaddit ional handling, the blank for this digest ionprocedure was, at 16 ng, more than three t imeshigher than the blank for the direct HF-HNO3 disso-lution (< 5 ng) and, as such, the data were blank-corrected.

In the following, we will use an asterisk (*) to identi-fy samples spiked prior to the acid digestion step, forexample HF* as carried out at SIL; and HF and HBrwill denote the two digestion procedures and spikingprotocol applied at IFM-GEOMAR.

Chemical purification

To prevent elemental (40K+, 48Ti+, 84Sr2+, 86Sr2+, 88Sr2+)and molecular (e.g., 24Mg16O+, 27Al16O+) isobars interfe-ring with the Ca isotopes during mass spectrometry, andto ensure efficient ionisation, we applied commonly usedchromatographic clean-ups for seawater and silicates,based on cation exchange and HCl elution. At IFM-GEO-MAR, the ultramafic samples (komatiites, peridotites anddunite) were processed through 1.2 ml columns (BioRad)that were filled with MCI Gel (75-100 mesh). Around 4μg Ca was eluted by 8 ml of 1.5 mol l-1 HCl. For allother samples a column size of 0.6 ml (BioRad) and 5ml HCl was used to yield the same amount of Ca.

At SIL, about 50 μg Ca in 2.45 mol l-1 HCl solutionwas purified from matrix ions using Teflon columns

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Table 2.Comparison of the δ44/40Ca(SRM 915a) values in selected samples subject to different digestion procedures applied at IFM-GEOMAR (HBr-Diss, HF-Diss) and SIL (HF*-Diss). See text for the details

Sample ID Procedure δ44/40Ca(SRM 915a) 2se a n

DTS-1 HF*-Diss 1.53 0.03 1HF-Diss 1.39 0.10 8

HBr-Diss b 1.48 0.09 7

PCC-1 HF*-Diss 1.13 0.08 1HF-Diss 1.14 0.14 7

HBr-Diss b 1.15 0.10 6

BM90/21 HF-Diss 1.04 0.11 11HBr-Diss b 0.98 0.13 8

BM90/21-G HF-Diss 1.07 0.08 12HBr-Diss b 0.94 0.12 4

BHVO-2 HF*-Diss 0.90 0.03 1HF-Diss 0.74 0.04 14

HBr-Diss b 0.76 0.09 6

BCR-2 HF*-Diss 0.93 0.05 1HF-Diss 0.81 0.07 6

GOR128-G HF*-Diss 0.83 0.04 1HF-Diss 0.70 0.05 13

GOR132-G HF*-Diss 0.64 0.06 1HF-Diss 0.57 0.05 11

OW5019 HF*-Diss 1.08 0.10 1HF-Diss 0.97 0.03 6

IAEA-CO1 c SIL 1.37 0.04 1IFM-GEOMAR 1.27 0.06 14

IAPSOdiss HF-Diss 1.78 0.08 4

a For HF*-Diss the internal 2s is indicated as sample was only measured once.b Blank corrected. c Dissolved in diluted HCl.

packed with 3 ml of AG MP50 resin (100-200 mesh)(Holmden 2005). At both laboratories, the doublespike was added before the column chemistry. Thisensured that any isotope fractionation of Ca that mighthave occurred during the chromatographic purificationof Ca ions from matrix ions was corrected for at thesame time as the instrumental mass bias correction(e.g., Eugster et al. 1969, Heumann and Lieser 1972,Russell and Papanastassiou 1978).

Ca isotope homogeneityof MPI-DING glasses

The fusion process of the MPI-DING glasses waschecked for contamination and a potential formationof heterogeneous Ca isotope reservoirs (Hart andZindler 1989) by comparing the results of five MPI-DING reference glasses with those of their originalrock powders. The glassy products were denoted by“-G” as originally proposed by Jochum et al. (2000).For BM90/21, parts of the original hand-specimenwere ground, homogenised and dissolved. Thereby,the purity of the MPI-DING glass BM90/21-G couldbe cross-checked for a potential Ca contamination,since high purity SiO2 (99.95%) was added to therock powder to enhance the melting process (Jochumet al. 2000).

In addition, two different glass shards of the MPI-DING reference glasses were dissolved separately andanalysed in order to detect a possibly heterogeneousCa isotope distribution. The reference materials BHVO-2and BIR-1, assumed to be homogeneous with respectto major and trace elements, were also processedtwice as an internal control on the sample treatment.The different splits are indicated by the addition of(#1) and (#2) to the sample label.

Carbonatite leaching experiments

The two carbona t i te samples (83HV26 and82LM66a) were analysed for their carbonate and theirresidual bulk silicate contents, in order to reveal anydifference in Ca isotope composition between them.Therefore, the sample powders were treated first byadding 0.5 mol l -1 HCl to dissolve the carbonatephase. After decanting the supernatant, the residuumwas washed and then digested by the “HF-Diss” proce-dure. While the carbonate solution was spiked anddirectly analysed, the dissolved residue was spikedand chromatographically cleaned prior to the measu-rements.

Mass spectrometry

Calcium isotope determinations were performed bythermal ionisation mass spectrometry (TIMS) usingThermoFinnigan TRITON instruments at IFM-GEOMARand at SIL, using 43Ca-48Ca and 42Ca-43Ca doublespike techniques, respectively. The analyses at bothlaboratories were performed in dynamic mode. Theexponential law (e.g., Russell et al. 1978) was used tocorrect for instrumental mass fractionation.

The measurements at IFM-GEOMAR were carriedout using a routine method close to that described byHeuser et al. (2002). About 300 ng of Ca was loadedonto out-gassed single Re filaments in 2.2 mol l-1 HClwith a Ta activator. Calcium isotopes were measuredwhen a signal of 4 V on mass 40Ca was achievedusing a 4 s integration time. A typical measurementconsisted of 154 ratios. The results from IFM-GEOMARare averages of replicate analyses from at least threechromatographic column separations. Overall analyt-ical uncertainties were less than 0.2‰ (2s). Analyticalblanks , inc luding sample diges t ion and columnchemistry, were found to be less than 5 ng using theHF-Diss technique and, thus, negligible for any blankcorrections. The “HBr-Diss” sample digestion procedureexhibited a Ca blank of 16 ng with an isotope compo-sition of δ44/40Ca(SRM 915a) = 0.73‰. The results of thisexperiment were therefore blank-corrected.

The measurements at SIL were carried out using amethod that was slightly modified from one describedby Holmden (2005). About 5-8 μg of Ca was loadedonto out-gassed single Ta filaments in 0.2 mol l-1 HNO3

with 1 μl of ultra-pure 10% H3PO4. Measurements wereperformed with 40Ca signals between 10 and 20 V. Thesignal integration time for each scan was 2 s for 40Caand 42Ca, and 16 s for 43Ca and 44Ca. A typical mea-surement consisted of 130 to 180 ratios. The blank was< 1% of the total Ca processed through the chemistry,with a δ44/40Ca(SRM 915a) value of 0.62 ± 0.2‰ (2s).The blank contribution to the sample was negligibleand no corrections were applied.

Calcium isotope measurements are reported in theconventional delta (δ) notation where δ44/40Ca(RM) =(44Ca/40Casample/44Ca/40CaRM) - 1) x 103 (Eisenhaueret al. 2004). The reference material (RM) was theCaCO3 powder NIST SRM 915a, which was measuredfor each group of ten samples and cross-checkedagainst the seawater salinity reference material IAPSO(OSIL). During the course of this study, the average

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long-term value for IAPSO analysed at IFM-GEOMARcomprised 135 single measurements and was 1.82 ±0.15‰ (2s) (40Ca/44Ca = 47.127) relative to NIST SRM915a. Note that the double spikes used at IFM-GEOMARand SIL were not calibrated to determine absolute isoto-pe abundances and the measured 40Ca/44Ca ratioscan only be used for delta value normalisation. At SIL,the performance of the instrument was monitored usingthe CaF2 “isotope normal” that was originally used tocalibrate the double spike, along with samples of deepPacific, Caribbean, North Atlantic and IAPSO seawater.During the course of this work, the same batch of IAPSOused at IFM-GEOMAR was measured in order to confirmthat there were no measurable isotopic differences bet-ween the seawater samples employed by the two labo-ratories. Five measurements of two seawater samples atSIL yielded an average 40Ca/44Ca = 47.093 ± 0.1‰(2s), which is identical within the uncertainties to a singlemeasurement of the IFM-GEOMAR IAPSO seawater(40Ca/44Ca = 47.091). Two measurements of NIST SRM915a yielded -1.88 and -1.89‰. Both values are inagreement with the proposed values for NIST SRM 915ameasured relative to seawater by Hippler et al. (2003).

We checked selected samples (Table 1) for an enrich-ment of radiogenic 40Ca in order to ensure that no radio-genic 40Ca from the decay of 40K superimposed the Caisotope compositions (particularly in the Archaean koma-tiites, and the high-K samples ATHO-G and StHs6/80).For that reason, 40Ca/44Ca ratios of unspiked aliquotswere measured at IFM-GEOMAR at a signal of 10 V forat least fourteen blocks of twenty scans each and an inte-gration time of 4 s. Measured 40Ca/44Ca ratios were nor-malised to a 42Ca/44Ca value of 0.31221 (Russell et al.1978). The radiogenic 40Ca enrichment effect is reportedin ε notation (40Ca/44Casample/40Ca/44CaRM - 1) x 104)relative to the NIST SRM 915a. All data lie within the ana-lytical uncertainty of ± 2 ε units (2s) and no radiogenic40Ca enrichment of the samples compared to NIST SRM915a could be detected.

Results and discussion

Chemical treatment of the samples

At IFM-GEOMAR the spike was added after diges-tion (HF-Diss and HBr-Diss), thus the total recovery of

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O1

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HF-Diss

Figure 1. Comparison of the three digestion experiments applied in this study. HF-Diss is a commonly used

HF-HNO3 attack used for the decomposition of silicates. While in HF*-Diss the spike was added prior to

dissolution, the spike was added afterwards in the HF-Diss and HBr-Diss experiments. In the HBr-Diss

experiment the cations were leached by HBr prior to the HF-HNO3 treatment. The results for all three procedures

overlapped within error limits and show that Ca isotope ratios were unaffected by the sample dissolution and

spiking method. Error bars indicate the external precision (2se) for HF-Diss and HBr-Diss and the internal

precision (2s) for HF*-Diss, as samples treated by the latter were measured only once.

δ44

/40Ca

(SRM

915

a)

Ca during decomposition of silicates was essential toavoid isotope fractionation effects during the dissolu-tion step. We applied both dissolution procedures, HF-Diss and HBr-Diss, in order to monitor the release ofCa from silicate rocks and the effect of chemical pre-t reatment on the Ca isotope composi t ions . Bothmethods were also cross-checked by the digestionmethod HF*-Diss applied at SIL, where the spike wasadded prior to sample dissolution. The results arecontrasted in Table 2 and Figure 1. The δ44/40Ca(SRM

915a) values for all digestions were the same, fallingwithin the range of the quoted uncertainty (Figure 1).This shows that if sample dissolutions are carefully per-formed, isotopic artefacts may be avoided even if spik-

ing is performed after the dissolution step. The sameconclusion applies to samples of IAPSO seawater pro-cessed through the HF-HNO3 treatment (Table 2).Relatively insoluble minerals (e.g., spinels) in the ultra-mafic samples (e.g., BM90/21) may not have beendecomposed entirely. The consistency between the Caisotope ratios in BM90/21 and its entirely dissolvedglassy counterpart BM90/21-G (Table 2, Figure 1)shows that accessory refractory minerals such as spi-nels are unlikely to have contributed significantly to theCa content and isotope composition of the sample.

Spiking the samples prior to chemical separationaccounts for the previously reported effect of isotope

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Table 3.Comparison of δ44/40Ca(SRM 915a) of the MPI-DING reference glasses and their original powders. No variations caused by the fusion process could be detected within analytical uncertainty except for ML3B, for which analysis of variance showed a significant variation at the 98% level

MPI-DING glass Original powder ANOVA

Sample ID δ44/40Ca(SRM 915a) 2se n Variance δ44/40Ca(SRM 915a) 2se n Variance p-value

StHs6/80 0.75 0.05 16 0.02 0.77 0.11 6 0.01 0.68T1 0.81 0.06 14 0.01 0.72 0.10 6 0.01 0.18KL2 0.69 0.07 15 0.01 0.76 0.07 6 0.02 0.31ML3B 0.73 0.05 15 0.01 0.62 0.06 6 0.01 0.02BM90/21 1.02 0.13 16 0.01 1.01 0.07 19 0.04 0.09

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StHs6/80 T1 KL2 ML3B BM 90/21

MPI-DING glass

rock powders

Figure 2. MPI-DING reference glasses are compared to their original rock powders. All of the samples were

identical within error limits (2se) in their Ca isotope compositions and any contamination or formation of isotope

heterogeneities during the fusion process can be excluded. However, for ML3B a significant difference between

the rock powder and its glassy equivalent could be detected by analysis of variance (Table 3).

δ44

/40Ca

(SRM

915

a)

fractionation during ion exchange chromatography(e .g . , Heumann and L ie se r 1972 , Rus se l l andPapanastassiou 1978). The purpose of the columnpurification scheme is to separate Ca from its isobars,namely 40K and 48Ti, doubly-charged isotopes of Sr,and Mg, which may form molecular isobars of MgO+

in the TIMS ion source. We generously cut the Ca peaktails on both sides of the chromatograms and collecteda fairly pure Ca eluate, in particular for the ultramaficsamples that are high in Mg. Consequently, the Cayield was diminished to about 80%.

MPI-DING glasses versusoriginal rock powders

Table 3 lists the δ44/40Ca(SRM 915a) of five MPI-DINGreference glasses and their original powders. Except forML3B, no significant differences could be observedbetween the δ44/40Ca(SRM 915a) values of the samplepowders and their quenched products (Figure 2). This isto be expected as no flux agents were used in the fusionprocess, except for pure SiO2 added to BM90/21.

The δ44/40Ca(SRM 915a) value of sample ML3B diffe-red significantly from that of its glassy counterpart.Although both values agreed within their analyticaluncertainties, analysis of variance (ANOVA) revealed asignificant difference at the 98% confidence level bet-ween the δ44/40Ca(SRM 915a) value of the ML3B powderand its glassy equivalent (Table 3). ANOVA comparesthe variance of the repl icate measurements of asample with the differences between the mean valuesof several samples and calculates the probability (p)of whether the first is equal to or greater than the latter(Davis 2002). Notably, heterogeneities in ML3B-G

have been also reported for Cr abundances (due tothe formation of Cr-rich “islands” during quenching;Jochum et al . 2000), and for L i and Pb isotopes(Jochum et al. 2006).

Homogeneity of the MPI-DING reference glasses

Homogeneity is an important requirement for geo-logical reference materials. In particular, this is true forglass and pressed-powder disc ablation referencematerials for in situ laser and ion microprobe studies.The suite of MPI-DING reference glasses has beensubjected to several detailed studies, all of whichdemonstrate that they are homogeneous at a μm-scale with respect to most elements and isotope sys-tems (e.g., Jochum et al. 2000, 2006).

We checked the homogeneous distribution of theCa isotopes in two different mm-sized shards of eachMPI-DING reference glass sample. No differenceswere detected (Table 4). In addition, we processed ali-quots of the well-established Hawaiian basalt BHVO-2and the BIR-1 powders, as they are proven to behomogeneous in their element and most isotope sys-tems (Flanagan 1984, Wilson 1997b) and are assu-med to be homogeneous for Ca isotopes. The resultingδ44/40Ca(SRM 915a) values are reported in Table 4 andillustrated in Figure 3. The values for each individualsplit were identical within uncertainties. In addition, wecarried out analyses of variances in order to test forthe homogeneity of the samples. GOR132-G showedthe highest variability between the glass splits (Table4). However, differences between sample splits werenot significant at the 95% significance level (Table 4);

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Table 4.Two glass splits were compared for their Ca isotope compositions in order to test the homogeneity of the MPI-DING reference glasses. Two aliquots of BHVO-2 and BIR-1 were processed along with each split as quality controls

Split #1 Split #2 Mean ANOVA

Sample ID δ44/40Ca(SRM 915a) 2se n Variance δ44/40Ca(SRM 915a) 2se n Variance δ44/40Ca(SRM 915a) 2se p-value

ATHO-G 0.89 0.13 7 0.03 0.84 0.06 6 0.01 0.87 0.07 0.50StHs6/80-G 0.74 0.06 9 0.01 0.75 0.08 7 0.01 0.75 0.05 0.78T1-G 0.84 0.10 5 0.01 0.77 0.07 8 0.01 0.81 0.06 0.29KL2-G 0.73 0.11 9 0.03 0.65 0.07 6 0.01 0.69 0.07 0.28ML3B-G 0.70 0.04 11 0.01 0.75 0.10 6 0.02 0.73 0.05 0.28GOR128-G 0.67 0.04 6 0.00 0.73 0.10 7 0.02 0.70 0.05 0.36GOR132-G 0.52 0.06 6 0.01 0.61 0.08 5 0.01 0.57 0.05 0.08BM90/21-G 1.04 0.14 7 0.03 0.99 0.21 5 0.05 1.02 0.13 0.71BHVO-2 0.72 0.07 8 0.01 0.75 0.05 6 0.02 0.74 0.04 0.35BIR-1 0.77 0.08 8 0.01 0.76 0.18 6 0.05 0.77 0.10 0.87

hence, regarding their δ44/40Ca(SRM 915a) values, theMPI-DING glasses do not indicate mm-scale heteroge-neity at the level of the given analytical precision.

Analytical uncertainty

Many systematic error sources were cancelled outby relating the results to a reference material (hereNIST SRM 915a). Additionally, this calibration materialwas cross-checked by a second reference material(IAPSO) that was treated similarly to the samples andanalysed during the course of the analyses, followingthe suggestions of Goldstein et al. (2003). In this res-pect, contributions of the blank, sample-spike ratiosand instrumental bias to the overall analytical uncer-tainty can be neglected. For this reason we neglectextensive error propagation and highlight only someparticular error sources.

A known source of uncertainty is the use of non-representative amounts of sample material. We investi-gated different aliquots of up to 200 mg of samplemass independently as analytical test portions, andcompared the results. There was substantive agree-ment. With regard to in situ microanalytical work, this

study indicates homogeneity on the mm-scale, but maynot resolve micro-heterogeneity on the μm-scale (resul-ting from quench-crystallised mineral phases).

The ultramafic samples provide challenges to ana-lysts because of their par t icular matr ix (high Mgcontent) and the presence of barely soluble minerals,such as spinels, that are not well decomposed by themethods used here. However, thorough adjustments ofthe column chromatographic procedure, checked byindividual elution schemes, and the good agreementbe tween BM90/21 and i t s g las s y coun te rpar tBM90/21-G, excluded matrix effects and refractoryminerals as possible sources of error.

Variations due to sample digestion and CaF2 for-mation were excluded by the comparison of the diffe-rent digestion techniques, in particular by the verygood agreement between HF-Diss and HF*-Diss.

Generally, variations between the analytical runswere small with respect to the overall precision of theanalytical method. External reproducibility was betterthan 0.2‰ (2s). Uncertainty due to sample handlingand during the analytical procedure could be excluded.

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128-

G

OG

R13

2-G

B

01-

M9

/2G

BHVO-2

B

1IR

-

split #1

split #2

Figure 3. Test for the homogeneity in the MPI-DING reference glasses. Variances between δ44/40Ca values

of two different glass splits (split #1 and split #2) are smaller than the analytical uncertainty of 0.2‰ (2s).

δ44

/40Ca

(SRM

915

a)

Variations in the Ca isotope compositions could be ascri-bed to the intrinsic properties of the sample materials.

δ44/40Ca(SRM 915a) values inigneous and metamorphic rocks

δ44/40Ca(SRM 915a) values for twenty-two igneousrock samples are given in Table. 1 and plotted inFigure 4. The reported values are means of all theδ44/40Ca(SRM 915a) values obtained from the differentdissolution procedures. The uncertainty is the standarderror of the mean (2se). Published data on volcanicrocks point to a mean δ44/40Ca(SRM 915a) value of 0.9 ±0.1‰ (2se) relative to NIST SRM 915a (cf. DePaolo2004). The volcanic rocks analysed in our study liewith a δ44/40Ca(SRM 915a) value of 0.8 ± 0.1‰ (2se)within this range. The MPI-DING glasses KL2-G andML3B-G made f rom rock powder s exh ib i tedδ44/40Ca(SRM 915a) values of 0.70 ± 0.07‰ and 0.72 ±0.05‰. They agree with the ocean island basalts

USGS BHVO-2 and BIR-1. Confirming previous results(cf. DePaolo 2004), the δ44/40Ca(SRM 915a) values of thefelsic samples, ATHO-G, StHs6/80 and StHs6/80-Gand TML were indistinguishable from those of themafic samples (basalts, W-2, T1 and T1-G), from thecarbonate and the silicate phases of carbonatites,82LM66A and 83HV26, and also f rom the twoCretaceous komatiites from Gorgona Island, GOR128-G and GOR132-G (Table 1, Figure 4). In contrast, bothArchaean komatiites of the Onverwacht group were0.3‰ heavier than their younger counterparts. TheArchaean komatiites matched the values obtained forthe peridotite BM90/21 and its glassy equivalent,whi le the peridot i te PCC-1 had a sl ight ly higherδ44/40Ca(SRM 915a) value of 1.14 ± 0.06‰. The duniteDTS-1 was the heaviest igneous rock sample yet mea-sured with a δ44/40Ca(SRM 915a) value of 1.49 ± 0.06‰.A l so , t he Car ra ra marb le showed a h ighe rδ44/40Ca(SRM 915a) of 1.32 ± 0.06‰. This value agreedreasonab ly we l l w i th the p rev ious l y repor ted

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FELSIC MAFIC ULTRAMAFIC

Rh

yo

lite

An

de

site

La

tite

Dio

rite

Dia

ba

se

Du

nite

Ma

rble

Figure 4. Overview of the Ca isotope compositions of various rock samples analysed in this study. Most of the rock types

averaged around a value for δ44/40Ca(SRM 915a) of 0.8‰, similar to the currently assumed bulk Earth value of ~ 0.9‰. In

contrast, the Carrara marble and the ultramafic rocks differed significantly from this value, in showing a heavier isotope

signature. The results provide the first compilation of our investigated reference materials and samples. Analytical

uncertainties are given as 2se. Where available, the mean values of both laboratories (Table 1) are illustrated.

δ44

/40Ca

(SRM

915

a)

δ44/40Ca(SRM 915a) of 1.5 ± 0.1‰ determined by MC-ICP-MS (Halicz et al. 1999).

As can be seen from the K/Ca ratios and the mea-sured εCa values (Table 1), none of the samples investi-gated in this study appear to have been affected byradiogenic 40Ca addition from 40K decay since atleast the t ime of rock crys tal l isat ion. Nei ther theArchaean komat i i t e s o f the Onve rwach t g roup(OW5019 and OW5031), nor sample ATHO-G with ahigh K/Ca ratio, showed a detectable radiogenic40Ca excess (Table 1). It is concluded, therefore, thatdifferences in δ44/40Ca(SRM 915a) between different rocktypes reflect mass-dependent fractionation. The fractio-nation mechanism and an interpretation of the isotopicdifferences between the rock types are briefly discus-sed below.

δ44/40Ca in leached carbonatites

Carbonatites are carbonate-rich igneous rocks thatare believed to be sourced directly from the mantle.Their radiogenic Sr, Nd and Pb isotope compositionsresemble that of ocean island basalts, suggesting asimilar isotopic evolution of the source for both rocktypes (Hoernle and Tilton 1991). Carbonatites providethe means to invest igate potent ial di f ferences inδ44/40Ca between carbonates and silicates with thesame evolutionary history. The first perspective is givenby our leaching experiments where the δ44/40Ca(SRM 915a)

values of the leachates in the calcitic and the dolomiticcarbonatites, 83HV26 and 82LM66A (Hoernle et al.2002), were analysed separately from their silicatefraction (Table 1).

In general, the Ca isotope compositions of the car-bonate and si l icate fract ions of both carbonati tesamples ranged between 0.67 and 0.87‰, consistentwith previously reported bulk analyses of carbonatites(0.8 ± 0.4‰; Russell et al. 1978) and the δ44/40Cavalues of basalt ic samples (Table 1, Figure 4; cf .DePaolo 2004). Notably, carbonatites seem also toresemble basalts (MORB, OIBs) in their Li isotope com-positions (Halama et al. 2007).

While the δ44/40Ca(SRM 915a) values of the carbona-te and silicate phases from the calcitic carbonatite83HV26 were identical within analytical error, thedolomitic carbonatite 82LM66A showed a significantdifference of 0.2‰ between its silicate and carbonatefractions. Thereby, the leachate agreed with the calciticcarbonatite; the Ca isotope composition of the silicate

residue was, however, ~ 0.2‰ heavier. Previous studiespropose that the dolomitic carbonatites are formedthrough melting of carbonate-bearing peridotites atgreater depth (e.g., Wallace and Green 1988). Theδ44/40Ca (SRM 915a) o f t he pe r ido t i t e s BM90/21,BM90/21-G and PCC-1 analysed in our study are onthe average 0.2‰ heavier than the basaltic samples(Table 1, Figure 4). The derivation of the dolomitic car-bonatite from such an isotopically heavy source mayexplain the heavier Ca isotope composition of the sili-cate phase in the dolomitic carbonatite.

In contrast, similarities in Sr-Nd-Pb isotope compo-sitions in the calcitic carbonatite 83HV26 and associa-ted si l icate rocks, indicate a genetic relationshipbetween its silicate and carbonate phases (Gerlach etal. 1988, Hoernle and Tilton 1991, Kokfelt 1998). Theconsistency between the Ca isotope compositions ofthe silicate and carbonate phase of the calcitic carbo-natite analysed here is in substantive agreement withthis assumption.

δ44/40Ca(SRM 915a) variability in igneous and metamorphic rocks

The data presented here show that signif icantvariability is present in the Ca isotope compositions ofigneous and metamorphic rocks (Table 1, Figure 4),confirming experimental findings that diffusion at hightemperatures can fractionate the Ca isotopes (Richteret al. 2003). Stable isotope fractionation at ambientmantle temperatures has also been detected for Li andMg, with the latter considered as a chemical analoguefor Ca (e.g., Lundstrom et al. 2005, Pearson et al.2006). We report for the first time δ44/40Ca(SRM 915a)

values in mantle rocks, such as peridotites that are44Ca enriched relative to basalts (Table 1, Figure 4).

For the ultramafic rocks, Ca is negatively, while Mgis positively correlated with δ44/40Ca(SRM 915a) (Figure 5).On the other hand, the δ44/40Ca(SRM 915a) values of thefelsic and mafic rocks are not correlated either withtheir Ca (Figure 5a) or with their Mg contents (Figure5b), despite the wide range of Ca concentrationscovered by these rocks. Variations of Ca and Mgconcentrations in mafic melts are indicative of eitherthe breakdown of calcium- and magnesium-rich mine-ral phases during partial melting, or their removalduring fractional crystallisation. The present data setdoes not allow the observed δ44/40Ca(SRM 915a) variabi-lity in ultramafic rocks to be attributed to either process,partial melting or fractional crystallisation.

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(b)0

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Figure 5. (a) Ca isotope ratios in different rock types as a function of their CaO contents. In contrast to

felsic and mafic rocks, the δ44/40Ca values of ultramafic rocks are correlated with their CaO concentrations.

See text for further explanation. (b) Similarly, the Ca isotope ratios in ultramafic rocks are correlated with

the respective MgO content, while no correlation can be observed for the mafic and felsic rocks.

CaO (% m/m)

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/40Ca

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1.2

1.4

1.6

1.8

0 5 10 15

Interestingly, no significant Li isotope variability wasobserved during basaltic differentiation (Tomascak etal. 1999), but large variations in Li isotope ratios bet-ween dunites and peridotites within an ophiolite com-plex have been described (Lundstrom et al. 2005).Lundstrom et al. (2005) suggest that melt extraction isthe most important mechanism that controls diffusiveisotope fractionation of Li between dunites and harz-burgites. It has also been suggested that melt extrac-tion processes are involved in Mg isotope fractionation(Pearson et al. 2006). Notably, Pearson et al. (2006)reported a positive correlation between δ26Mg andCa concentration in olivines. Although covariant rela-tionships between δ44/40Ca and δ26Mg with both Caand Mg concentrations have been observed previously(Russell et al. 1978, Lee et al. 1978), a satisfactoryexplanation for this behaviour is still lacking.

With respect to Ca isotope fractionation duringpartial melting, there are two processes that maycause variations in δ44/40Ca. First, Ca isotopes couldbe fractionated by different degrees of partial melting.Second, chromatographic isotope exchange effectsmight occur during melt segregation. Inter-mineral frac-tionation may also play a role. Systematic differencesbetween the Li isotope compositions of olivine, ortho-pyroxene and clinopyroxene were recently observed(Seitz et al. 2004) and may also be the case for Ca.Hence, changes in modal mineral assemblages maycause isotope variability. On the basis of the presentdata set, we are unable to identify the mechanismcausing the observed δ44/40Ca(SRM 915a) variations inultramafic rocks and the correlation with Mg concen-tration. A closer consideration of the petrogenesis ofthe different rock types and their comparison, may pro-vide further insight into the underlying mechanism, butare beyond the scope of this work.

The high δ44/40Ca(SRM 915a) value of around 1.3‰ forthe Carrara marble (Table 1, Figure 4) indicates that frac-tionation of Ca isotopes may also occur during metamor-phic processes. The Carrara marble formed duringTertiary greenschist facies metamorphism of a formerLiassic carbonate platform. The δ44/40Ca(SRM 915a) value ofJurassic chalk was determined to be around 0.8‰(Skulan et al. 1997). Farkas et al . (2007) measuredδ44/40Ca(SRM 915a) values of 0.1 to 0.7‰ for Jurassicbelemnites and brachiopods. In addition, a δ44/40Ca(SRM

915a) value of 0.7‰ was determined for a marine fibrouscalcite cement of Pliensbachian age in AmmoniticoRosso limestones from Mt. Hierlatz, Hallstatt, Austria(Böhm et al. 1998). On this basis, it would appear that

the δ44/40Ca value of the precursor carbonate wasshifted towards higher values during metamorphism.

Summary and conclusions

We report δ44/40Ca(SRM 915a) values for variousigneous and metamorphic rocks, including several inter-national reference materials. Different experimentalmethods have been applied in two different laborato-ries to guarantee that no analytical artefacts were obs-curing the results. Our study showed that conventionaldissolution and purification procedures do not seem tofractionate Ca isotopes within the statistical uncertain-ties, if care is taken to avoid fluorides during dissolutionand spike is added before the column separation. Thepresent study does not indicate isotopic heterogeneityat a mm-scale resolution in the MPI-DING glasses. Inaddition, except for ML3B no deviations in δ44/40Ca(SRM

915a) values could be detected between the MPI-DINGglasses and their original rock powders. We thereforeconclude that the MPI-DING glasses may provide asuitable set of reference materials for Ca isotope mea-surements by laser ablation ICP-MS or ion microprobe.

The mean δ44/40Ca(SRM 915a) value of 0.8 ± 0.1‰obtained for the volcanic rocks analysed in this study issimilar to the previously reported values averaging to0.9 ± 0.1‰ (cf. DePaolo 2004), which is the currentlyaccepted δ44/40Ca(SRM 915a) value for the bulk Earth(Skulan et al. 1997, but see discussion in Fantle andDePao lo 2005 and Amin i e t a l . 2008) . Theδ44/40Ca(SRM 915a) values of ultramafic rocks, however,represent a significant departure from this value. Asdunites are supposed to be residues of extensive meltextraction of peridotites, and basalts are formed fromthe partial melts, the question arises as to why the per-idotite and the dunite are, respectively, 0.4‰ and0.7‰ higher in δ44/40Ca(SRM 915a) than basalt. A positi-ve correlation between δ44/40Ca(SRM 915a) and Ca/Mgratios indicates that Ca isotope fractionation in ultra-mafic rocks occurs at ambient mantle temperatures.Further constraints on the fractionation mechanism willrequire a systematic study of a coherent rock assem-blage, including the analyses of mineral separates.Calcium isotopes may also vary during metamorphicprocesses as shown by the high δ44/40Ca(SRM 915a)

value for the Carrara marble.

Acknowledgements

This is a contribution to “SPP1144” and “CASIO-PEIA” which both are funded by the DFG (Ei272/15-

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1/-2 und Ei272/21-1/-2). K. Hoernle is thanked forproviding the carbonatite samples. Two anonymousreviewers and M. Polvé are thanked for constructivereviews and helpful suggestions.

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