Experimentally determined trace and minor element partitioning between clinopyroxene and carbonatite melt under upper mantle conditions

EPSL ELSEVIER

Earth and Planetary Science Letters 133 (1995) 439-448

Experimentally determined trace and minor element partitioning between clinopyroxene and carbonatite melt under upper mantle conditions S. Klemme a~*, S.R. van der Laan b, SF. Foley a, D. Giinther ’ a Minerulogisch-Petrologisches-Institut,

Universitiit Giittingen, Goldschmidtstrasse I, 37077 Gsttikgen, Germany b Faculty of Earth Sciences, Utrecht University, 3508 TA Utrecht, The Netherlands ’ Department of Earth Sciences, Memorial Universiry of Newfoundland, St. John’s, Nfbi. AlB 3X5, Canada

Received 28 October 1994; accepted 16 May 1995 after revision

Abstract Laser ablation microprobe analyses of clinopyroxenes equilibrated with carbonatite melt at 20-22 kbar and 1050-1100°C are used to calculate partition coefficients for an extensive suite of trace elements. Our experiments were performed on a mixture of peridotite minerals and two types of carbonates with differing trace element contents (natural carbonatite and mixtures of sedimentary carbonates). Although trace element concentrations vary by an order of magnitude between the carbonate mixtures, the partition coefficients are similar. Some, however, differ substantially from previously published values. Most of the measured elements have broadly similar partition coefficients of Dcpx/c’q= 0.1-0.4, including Pr, Nd, Eu, Dy, Er and Hf, which are reported here for the first time. Considerably higher partition values are found for Ti and V which are compatible in clinopyroxene with D w-/‘q = 1.4 f 0.6 for Ti and 2.9 f 0.9 for V (la). The clinopyroxene partitioning data for carbonatite (D~x/c'q) are remarkably similar to published values for silicate melts (Dqx/s’q), with significant exceptions for the behaviour of Ti, Ba, Nb and perhaps Ta, whereas values for other silicate minerals differ more strongly between carbonatite and silicate melts. From our results, the most sensitive indicator of mantle metasomatism by carbonatite melt as opposed to silicate melt infiltration should be low Ti/Eu in metasomatised peridotites assuming closed-system metasomatism. In contrast to suggestions from some studies of metasomatised mantle xenoliths Zr/Hf ratios appear to remain unchanged during carbonatite metasomatism. These chemical effects only partly agree with those described from natural mantle rocks interpreted to result from carbonatite metasomatism.

1. Introduction Some peridotite xenoliths derived from the Earth’s mantle show strong enrichments of incompatible trace

Corresponding author: Research School of Earth Sciences, Australia National University, GPO Box 4, Canberra, ACT 2601, Australia l

Elsevier Science B.V. SSDf 0012-821X(95)00098-4

[l-5] relative to estimates of primitive mantle abundances. These enrichments are believed to be generated by metasomatism by either silicate [l-2] or carbonatite [3-51 melts. Carbonatite melts are presumed to be effective agents for metasomatic reactions in the upper mantle 161, because of their low viscosity [7,8] and presumed high contents of trace elements, based on observations of natural elements

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and Planetary Science Letters 133 (I 995) 439-448

carbonatite rocks and theoretical partitioning patterns for very low degree melts [7]. In order to properly quantify the chemical effects of carbonatitic melts interacting with mantle peridotite it is important to obtain more information on the partitioning behaviour of a wide range of elements between mantle minerals and carbonatite melt. Currently, only a limited set of partition coefficients for clinopyroxene, one of the main incompatible-element-bearing phases in the upper mantle, and carbonatite melt is available from the studies of Green et al. [9], Adam et al. [lo] and Sweeney et al. [ll]. These studies show moderate to strong incompatibility for all measured trace elements with D~x’c’q (concentration ratio between clinopyroxene and carbonatite melt) ranging from 0.006 for Ba to 0.7 for Ti. In this study we investigate the partitioning of a wider range of minor and trace elements by laser ablation microprobe-inductively coupled plasmamass spectrometry (subsequently referred to as LAM) on experiments that were performed at upper mantle conditions of 20-22 kbar and 1050-1100°C (thought to be realistic for carbonatite melts) [12-141. The results are used to define more exact partitioning patterns for application to the assessment of metasomatic effects in mantle rocks.

2. Experiments 2.1. Experimental strategy Starting mixtures with 30 wt% silicate phases and 70 wt% natural carbonates were prepared from natural mantle minerals from Kilborne Hole, New Mexico [15], natural carbonatite from Kaiserstuhl, Germany [16], and sedimentary carbonates (calcite, dolomite and siderite from the Mineralogisches Museum, Universitiit Giittingen). Analyses of these minerals and the Kaiserstuhl carbonatite are given in Table 1 and Table 2. All starting materials were ground under acetone or ethanol to a grain size of < 20 pm and sieved to ensure a small and uniform grain size. Two different starting mixtures for the carbonate portion were used, one consisting of natural Kaiserstuhl carbonatite (highly enriched in trace and minor elements), the other consisting of natural

sedimentary CaCO,, FeCO, and MgCO, with relatively low contents of minor and trace elements (Table 2). The two mixtures were used to compare D~x’c’q partitioning for different compositions as a test of the attainment of equilibrium. The choice of 30:70 silicate/carbonate phases was made to facilitate accurate analysis of the quenched liquid, and should not affect the partitioning behaviour between phases. Although suitable for partitioning studies, this experimental approach may not accurately yield the melt composition relevant to mantle conditions. Therefore, appropriate Ca, Mg and Fe contents of the carbonate starting mixtures were chosen based on the equilibrium compositions indicated by Dalton and Wood [13] for the pressure-temperature conditions of our experiments. These are given in Table 3. Experiments were conducted in the pressure-temperature region to the low-pressure side of the extension of the reaction 4En + Do1 = 2Fo + Di + 2C0, into the liquid field in a peridotite-CO, bulk system. Thus orthopyroxene can never be stable in these experiments although included in the starting materials (Table 3). 2.2. Experimental technique Our experiments were performed at the Mineralogisch-Petrologisches-Institut in Giittingen with a single-stage 22 mm piston-cylinder apparatus [17], using CaF, assemblies and a graphite heater. This Table 1 Electron microprobe analysis of minerals used in experimental starting mixtures

opx

cpx

cakite

doloinik

41 1 0.07 50:1

558 33:2 0.8

521 20.1 15:6

~001 4:so 50.2

401 28.8 Is.6

401 0.25 i.6

32 50.7 1:5

0.01 0.03 co.01 9.3 0.2 0.02 CO.01 0.4 co.o1 101.20

0.1 5.1 0.12 5.92 0.15 0.13 CO.01 0.08 0.00 101.45

0.41 6.7 0.5 2.9 0.07 1.55 0.01 0.03 0.01 100.52

.%I.01 0.07 CO.01 0.01 0.02 0.06 co.01 co.01 CO.01 54.88

CO.01 0.02 CO.01 6.89 0.66 0.02 .a01 0.01 0.01 54.97


0.13 0.45 0.04 1.86 0.67 0.28 0.03

divine

so2 z lio2 Al203 cl203 Fe0 hlno NazO KZO NiO F-205 tctal

Kaiserstold carbonatite fkom the Kaisershhl Germany

si&liti

Kakauhl

1.87 60.13

carbonatite complex,

S. Klemme et al. /Earth

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and Planetary Science Letters 133 (1995) 439-448

assembly requires no pressure correction and pressures are believed to be accurate to within 0.5 kbar. Temperatures were measured and controlled with a Eurotherm 812 controller, using Pt/Pt,-Rh,, thermocouples, and are accurate to within 15°C. Graphite-lined Pt capsules (4 mm diameter) were used to maintain the oxygen fugacity at the CC0 buffer and to prevent Fe loss to the Pt capsule. The runs on which we report results were conducted at 20 kbar, 1150-1100°C and 22 kbar, 1100°C (Fig. 1). The experiments were run for 3-5 days, as summarised in Table 3. After the experiments the run products were mounted in epoxy and polished dry to avoid loss of soluble phases during the polishing procedure. 2.3. Analytical techniques Run products were analysed for major elements using a Cameca SX 50 wavelength-dispersive threespectrometer electron microprobe (EPMA) at Memo-

rial University, St. John’s, Newfoundland. An accelerating potential of 15 kV and a beam current of 20 nA was used. Run products were also analysed for minor and trace elements by LAM at the Memorial University. Ablation is achieved by a Nd:YAG laser which was frequency-quadrupled to a wavelength of 266 nm for analyses in this study. This was necessary in order to analyse small crystals in the experimental charges [18]: using the 266 nm beam we managed to reduce the ablation pits to 8-10 pm in ‘diameter in clinopyroxenes (see Fig. 21, enabling analysis of the small clinopyroxene crystals in our experiments (maximum 40 pm). Analyses were made with a repetition rate of 10 Hz; further optimization was achieved by power control through the polariser. The ablation time for clinopyroxenes in the experimental run products is 5-20 s. Detection limits of about 0.3 ppm were achieved. This is considerably higher than in other LAM analyses [19], due to the smaller amount of ablated material [18]. Starting materials

Table 2 Trace and

minor element composition of starting materials (ppm) uaceelements V co Ni Rb Sr Y

zr

Nb MO CS Ba La Ce F&

Kaiserstuhl(2) 64 13 1.4 2735 :; 530 1.0

dOlOmitC(2)eakite(2) 1.3

0.1 78 10 0.3

2.6

101 5.7

0.1

0.1

0.1 0.2

892 234 322 98

0.9 0.03 0.1 1.1

.%I %

:.35

2.0 1.0

4.1 A::

z

6.1 8.8

2.3 3.3

0.2 3.0 1.5

3

0.9

3

1.6

Er Tm Yb LU Hf Ta E U Analyses were performed with

0.4 f: 5.7 2.3

olivine(6) 3.3

8:: 0.2 2.7

8:; 0.9 0.2

0.02 0.1 0.1 0.1 0.01 0.03 0.1 0.02 0.1

0.1 0.01

opx(2)

cpx(6) 233

1020 <0.5 4 1.1 2.1 0.2 <3.9 0.2 0.3 0.03 0.14 0.1 co.02 0.01 <0.06 0. I 0.03 0.1 0.03 c?4 0.1
0.6 2.2 0.4 2.8 1.2 0.5 0.4 2.5 2.1 0.5 1.4 0.3 1.4 0.2

LAM-ICP-MS (Kaiserstuhl analyses with solution ICP-MS); numbers in parentheses represent number of analysed points; elements not determined are indicated by blanks. Starting material analyses were performed with a defocussed laser beam (diameter ca. 30 pm). Longer ablation time in these starting meterial analyses results in lower detection limits compared to experimental run products.

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S. Klemme et al. /Earth and Planetary Science Letters I33 (1995) 439-448

Table 3 Run conditionas and starting mixtures

%(wt.) RI

30

96 (v/t) 10

K2 K3 K4 KS

30 30 30

65

%

(wt., 5%&t., %(wt.) 10

10

30

5

ii;

%“(W.)

70 70 70 70 70

(wt.) 84 Kaiwstaute 19 K.¶iemAe 84

70

Kaim5hlhlcabollatite 79 17

96

9%(W

12

3

17

5

12

:

&I 20 20

I”cl

duration

pharcs

3d 3d

20

1100 1100 1100 1100 1050

5d

CDX.01. mlt &lx, 01. melt cpx, 01. t&t cpx, 01. mlt cpx, 01. men

20

1050

5d

cpx, 01. melt

Individualstartingmaterialmixtures, run conditions and stable phases in every experimentalrun product. nrelr= Quenched carbonatitemelt; Kaiserstuhlcarbonatitefrom the Kaiserstuhlcarbonatitecomplex, Germany.

laser beam to average out inhomogeneities formed during the quench. The ablated material was swept by a continuous argon flow into a Fisons Plasmaquad 2 + S inductively coupled-plasma mass

were analysed with a defocussed laser beam and longer ablation time, thus resulting in considerably lower detection limits (Table 2). Melt pools were also analysed with a defocussed

carbonatite melt

,dolomite Ilherxolite

0

I--

/

lherzolite+ vapour

i--

1000

1100

1200

1300

Temperature [“Cl Fig. I. phase diagramfor peridotitewith CO, [14]. The circles mark the conditions of our experiments.The solidus bends sharply back at a pressureof about 18 kbar where dolomite becomes a solidus phase. Reaction 1 from [24]: Dolomite (mg’(a&) + orthopyroxene (4MgSi0,) = clinopyroxene (CaMgSi,O,) + olivine (2Mg,SiO,) + 5uid (2C0,).

S. Klemme et al. /Earth and Planetary Science Letters 133 (I 995) 439-448

Fig. 2. SEM photograph

of K4 run product. The ablation hole in the clinopyroxene

443

crystal has a diameter of 10 pm.

10

1

b-m 0

A

0.1

4

I

0.01 : Ba

I I

I I

I I

I I

I I

I I

I I

I I

I r

I I

1 I

1 I

I I

I

1 I

I r

Nb

Ta

La

Ce

Pr

Sr

Nd

Sm

Zr

Hf

Eu

Ti

Gd

Dy

Y

Er

V

Fig. 3. D cWc’q values for three experiments, two with natural Kaiserstuhl carbonatite (K2 and K4), the other with natural sedimentary carbonates (Kl). Results show similar partition coefficients, despite differing trace element contents.

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and Planetary Science Letters 133 (1995) 439-448

spectrometer. The accuracy of this analytical technique has been tested against several standard materials (e.g., NIST 612, AGV-1 and BCR-1 [30]) and against other analytical methods [18-201. For our analyses we calibrated against the AGV-1 standard. Drilling through a crystal into the quenched melt or into another crystal can be checked by monitoring an element of different abundance in the analysed phases (e.g., Si or Sr) [18]. The high temporal resolution of the LAM technique allows recognition of boundaries between phases which are crossed during drilling of the ablation holes [19], thus yielding a great advantage over other techniques such as PIXE. In order to show that there was no laser beam overlap onto melt or other crystals during LAM analysis, Fig. 2 shows an SEM photograph of a laser ablation crater in a clinopyroxene.

(i) Results from both carbonate starting materials (sedimentary carbonates and Kaiserstuhl carbonatite), which have about an order of magnitude difference in trace element contents, gave closely similar partition coefficients (Fig. 3). (ii> Clinopyroxenes throughout the capsules in the individual runs show uniform trace and minor element compositions that are considerably changed in comparison to those of the starting materials (compare Table 2 and Table 4). For almost all elements, this involved the enrichment of trace elements in clinopyroxene relative to abundances in the starting clinopyroxene. No zonation of crystals in the run products could be detected with EMP analysis and was not detected in back-scattered electron images. Several clinopyroxenes and melts were analysed in each run. Average values of clinopyroxene analyses in the individual run products (Table 4) were used in the calculation of partition coefficients. These partition coefficients for clinopyroxene and carbonatite melt, including the first experimental values for nine elements (Pr, Nd, Hf, Eu, Dy, Er and

3. Results and discussion Attainment of equilibrium during the experiments is demonstrated by the following lines of evidence: Table 4 Trace and major element contents of run product clinopyroxenes Kl meit (3) std. dcv

cpx (8)

std. &I

and carbonatite

melt

K2 melt(5) std. dev cpx (9) std. de

;ab Nb Tfi

6.1 6

1.6 0.2

1.6 0.8

0.1 0.1

1630 57 236 6.2

01 4h 21 0.2

34 18 r

23 3 0.3

k Pr

2.8 2.7

0.4 1.1

0.2 0.3

0.1

2 26 2% 87 11 34

::

270 5 0.3 3

:: 3 190 10

: 1.3 70 2

& SIU Ef

260 3.1 :

o’.; 0.5 2.9

:.:

0:2

1.6

1.3

$5,

0%

0.02

0.05 0.4

0.01 0.2

1.8

0.3

0.5

0.2

? 1.7 Er t% V 25 mmelemmts

0.4 O?l 5

0.5 0:3 42

0.1 3.3 0 22

Gd

Kl

cho TiCQ Fe0 ‘Q2G3 Na20 K2G z?

I

w. 22.27 0.46 1.64 7.21 0.09 0.75 0.04 1.1 100.1

1

0.i

28

0.64 --

3.2 0.14 8.1 5.2

2333 42

0.3 0.01 0.4 0.2

0.9 0.29 2.8 1.7

0.2 0.09 0.2 0.8

O?l 3

:.: 146

0!2 2a

std. dev

-K

1.6 0.08 0.19 0.75 0.08 0.02 0.01 0.15

5

-

I

K4

23.1 0.23 2.22 4.89 0.09 0.4 co.02 0.76 99.9

K4 elt (5) 6 980 260 7.7

std. dev 104 l.40 25 1.4

cpx (7)

std. dev

94 32 1.1

30 15 0.4

z 4 140

: 0.8 -;.?3 0.5

200 300 26 2080 91 12.3 181 4.3 3.3 0.16 8.5 5.5 z.:

:.‘7

:: 2.9 170 8.8 1.7

0’: 0.5 0.01 1.4 0.8 074

0:‘7 0.5 0.13 1.5 1.4 0.9 7

d.! 0.03 0.06 0.2 0.4 0.5 2

50

4

122

17

-“cmlt

0.46 0.96 0.11 0.16 0.63 0.06 0.01

Kl 45 5:9 36.5 0.06 1.96 1.24 0.10 0.35 0.02

71 ii.1 32.1 0.07 0.64 0.54 0.11 0.22 0.06

K4 a7 219 37.2 0.16 0.43 2.85 0.16 0.26 0.01

50.63

51.94

52.67

0.09

Trace element (ppm) analyses and TiO, (%) analysis were performed with LAM-ICP-MS; the remaining major element (%) analyses were performed with electron microprobe. Trace and major element values are averages, numbers in parentheses represent number of analysed points std.deu. = Calculated standard deviation. Elements not determined are given as blanks.

S. Klemme et al. /Earth

the compatibility changes smoothly from 0.07 f 0.02 for La (LREE) to 0.41 + 0.06 for Er (HREE). The elements Ba, Sr, Nb and Ta are similar in compatibility to the MREE. Relatively high D values are observed for V (W”“q = 2.93 f 0.9) and Ti (Dcpx/‘q = 1.4 f 0.6) and for Zr (W’x/‘q = 0.140.81). Our partition coefficients are compared to those of Adam et al. [lo], Green et al. [9] and Sweeney-et al. [ll] in Fig. 4. The results of Green et al. [9] and Adam et al. [lo] show considerably lower values for elements more incompatible than the MREE Sm and Nd. Sweeney et al. [ll] find higher values for nearly all analysed trace elements, especially for the LILE (Fig. 4). The high values determined for D, and D,, by Adam et al. [lo] may reproduce the partition coefficients indicated by our results if their value for D, can be taken to be representative for the MREE to HREE. In contrast to the behaviour of Ti and Zr, our partition coefficients show that Hf, Nb and Ta are not significantly more compatible in clinopyroxenes crystallised from carbonatite melt than are Ba and Sr. These data do not support the opinion, based on the compatibility of Ti in amphiboles crystallising in

Table 5 Partition coefficients for clinopyroxene and carbonatite melt D this study Rb Ba Nb Ta Ia cc PI Sr Nd Sm zi Hf Eu Ti Gd

0.033-0.27 0.07-0.13 0.15-0.16 0.06-O. 11 0.08-0.09 0.11-0.12 0.08-0.11 0.97-0.19 0.13 0.14-0.81 0.16 0.16-0.28 0.81-2.02 0.17-0.35

? Er V

0.26-0.32 0.22-0.37 0.35-0.48 1.68-3.46

avexa~c D-values D this study Adam ct al. 002 0.07 (0.12) 0:os 0.10 (0.03) 0.01 0.15 (0.005) 0.03 0.07 (0.02) 0.02 0.09 (0.007) 0.11 (0.04) 0.08 (0.015) 0.02 0.11 (0.052) 0.13 (0.04) 0.08 0.48 (0.37) 0.29 0.16 (0.05) 0.22 (0.06) 1.42 (0.61) 0.7 0.26 (0.09) (0.08) 0.29 (0.03) 0.30 0.41 (0.06) 2.93 (0.89)

0.21

D Gmx et al. 0004 Oh06 0.01 0.03 0.02

D Sweeney et a~. 0.28 0.24 0.24 0.23

0.03

0.28

0.08 0.29

0.32

0.17

0.22

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and Planetary Science Letters 133 (1995) 439-448

0.28

Numbers in parentheses show calculated standard deviation. Average D values were calculated from three different runs (Kl, K2 and K4). Other partition coefftents from [9,10,11].

V), are listed in Table 5 and depicted in Fig. 3, in which the elements on the x-axis are ordered following Sun and McDonough 1211. Of the eighteen elements for which partition coefficients are determined, only Ti and V behave compatibly. For REE 10

1

Q .

0.1

I% CI

0.01

l

Adam et al. (1993)

l

Green et al. (1992)

n

Sweeney et al. (1995)

n

thisstudy

0.001 Rb

Ba

Ta

Ce

Nd

Sm

Hf

Ti

Gd

Er

Fig. 4. Partition coefficients between clinopyroxene and carbonatite melt (this study) compared with experimental data from [9] (lo] [II].

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and Planetary Science Letters I33 (1995) 439-448

equilibrium with carbonatites, that carbonatite melt should lead to a general decoupling of HFSE from LILE [6]. Such a decoupling appears to be limited to Ti and maybe to Zr in our results. A pressure effect on clinopyroxene/melt partitioning has been discussed for silicate melts [23], but cannot be the cause of the variation seen in Fig. 4, because our experiments were conducted at a lower pressure than in the other studies, and our partition values are intermediate. However, there may be an influence of the analytical technique used: our data are produced by LAM, whereas all the other three studies analysed for trace elements using PIXE. The depth of penetration of the proton beam during PIXE analysis is several tens of micrometres, but opinions differ as to the effective depth of analysis (15 pm [ll] to 30 pm [25,31]), so that mixed analyses with underlying minerals or matrix are difficult to discount. Unfortunately, specific information on the grain sizes of different minerals is lacking in most experimental reports [9,10], so that the possibility of mixed analyses cannot be properly judged. Higher D~x/c’q for Ba, Nb, Ta, Ce and Sr [ll] may be an indication of this effect, although it would not ex-

plain lower Dcpx’c’q from other studies [9,10]. A possible reason for a strong variation in partition coefficients could be a different bulk composition in the works discussed above, although no significant differences are seen between partitioning data derived from two strongly differing trace element compositions in this study (Fig. 3 and Table 4). Fig. 5 compares our D‘r”“‘q values to a compilation of values for clinopyroxene/silicate melt (Dcpx/s’q) partitioning. These values are exclusively from high-pressure experiments on basaltic compositions for which partition coefficients were determined using the in-situ microbeam methods SIMS or PIXE [10,26,27], as data from these techniques are much more dependable than data from mineral separates [28]. D~x/clq and Dcrx/s’q are in general remarkably similar, with a few significant exceptions: Ba, Nb, Ta and Ti. This partitioning behaviour of clinopyroxene contrasts with that of trace elements in garnet or amphiboles equilibrated with carbonatite or silicate melt [9-11,181 [22] 127,291. This study presents the first partition coefficients for Zr, Hf, Ti and Eu measured on the same experiments. Rudnick et al. [4] suggested from studies of

10

Ba

Nh

Ta

La

Ce

Pr

Sr

Nd

Sm

Zr

Hf

ELI

Ti

GdDyY

Er

Fig. 5. Partition coefficients between clinopyroxene and carbonatite melt (this study), compared with partition coefficients clinopyroxene and basaltic melt [18,26,27]. Bars show range of determinations.

between

S. Klemme et al. /Earth and Planetary Science Letters 133 (1995) 439-448

natural metasomatised rocks that a low Ti/Eu ratio could be used as an indicator of metasomatism by carbonatite as opposed to silicate melts, which is in agreement with the results produced here: assuming that the carbonatite melt was in equilibrium with residual clinopyroxene, the high D, should cause low Ti/Eu in the coexisting melt. If such a carbonatite melt is simply added to a mantle peridotite (closed-system metasomatism) the imprint of trace element contents will result in low Ti/Eu in the peridotite, assuming the trace element content of mantle carbonatites is orders of magnitude higher than that of the peridotite. The high standard deviation of D, (Fig. 4 and Table 5) is due to the low Zr and Hf contents derived from K2 (Table 4), implying the stability of zircon or another Zr-and Hf-rich accessory phase that was not analysed during laser ablation, although such a phase could not be detected optically or during EMP analyses. The presence of 0.1% zircon would be enough to explain the low Zr values, but that cannot explain the Ti anomaly. Because of low I-If contents in the clinopyroxenes D,, is only available from K4. If K4 is taken to be representative for a realistic D, there is no obvious difference between D, and D,,. The for clinopyroxene (Fig. 41, nearly equal D,/DHf however, contrasts with some previous suggestions for the behaviour of these elements from the geochemistry of metasomatised natural rocks: Rudnick et al. [4] and Yaxley et al. [51 predicted high Zr/Hf in a carbonatite melt, whereas Hauri et al. [3] found little fractionation of Zr and Hf. On the basis of our results, it is too early to make definitive statements about Zr/Hf fractionation, but there may be a hint that a carbonatite melt equilibrated with clinopyroxenes should show no significant Zr/Hf fractionation, assuming that clinopyroxene is controlling the trace element budget of the carbonatite melt. From our results, the best criteria for distinguishing between metasomatism by carbonatite as opposed to silicate melts should be the Ti/Eu ratio, and the large difference in DBa, D,, and D,, (Fig. 5). However, precise modelling of enrichment or depletion of trace elements caused by carbonatite metasomatism depends on the style of metasomatism (closed or open system) and will therefore remain uncertain until the trace and minor element composition of carbonatite melt under upper mantle condi-

447

tions has been experimentally determined. The assumption that the trace element composition of carbonatites in the mantle is similar to that of carbonatites at the Earth’s surface, as applied in current models [3-51 in the absence of alternative data, may be unrealistic.

Acknowledgements This is part of SK’s diploma work. S.K. thanks I. Horn, E. Zinngrebe, S.C.H. Melzer, T. Zack and other members of the Mineralogisch-Petrologisches-Institut, Universitiit Giittingen for long discussions and helpful assistance. Financial support was provided by the Deutsche Forschungsgemeinschaft (Fo 181,/2-l, 2-2, 3-l) and by the Natural Sciences and Engineering Research Council of Canada. Use of the LAM facility was coordinated by G.A. Jenner and S.E. Jackson. Thanks to R. Rudnick, R. Sweeney and an anonymous reviewer who helped to improve the manuscript substantially. Thanks also to L. Schnadhorst for catering. [UC]

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