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Earth and Planetary Science Le

The dependence of Nb and Ta rutile–melt partitioning on melt

composition and Nb/Ta fractionation during subduction processes

M.W. Schmidta,*, A. Dardonb, G. Chazotb, R. Vannuccic

aInstitute of Mineralogy and Petrology, ETH, 8092 Zurich, SwitzerlandbUniversite Blaise Pascal–UMR 6524–5, rue Kessler, 63038 Clermont-Ferrand, France

cDipartimento di Scienze della Terra, Universita di Pavia, and CNR-Istituto di Geoscienze e Georisorse (IGG),

Via Ferrata 1, 27100 Pavia, Italy

Received 4 April 2004; received in revised form 14 June 2004; accepted 9 August 2004

Available online 13 September 2004

Editor: B. Wood

Abstract

Partition coefficients between rutile and silicate melts were determined experimentally for Nb and Ta with melt

compositions varying from rhyolite to basalt. Experimental conditions were 1.7–2.5 GPa, 950–1300 8C, at oxygen fugacities

between QFM�2 and QFM+3.5. Both rt/meltDNb andrt/meltDTa increase by almost one order of magnitude with SiO2 content

and polymerization, but decrease with TiO2 content in the melt. The ratio rt/meltDNb/DTa is 0.45–0.55 for basaltic melt

compositions, around 0.6 for andesitic melts and 0.8–1.0 for more silicic melts, remaining V1 for all examined silicate melts.

The fact that rt/meltDNb/DTa is smaller than unity can be explained by a slightly smaller ionic radius of Ta5+ than Nb5+ and

thus a preferred incorporation of Ta into rutile. The variation of rt/meltDNb,rt/meltDTa, and

rt/meltDNb/DTa strongly depends on

melt composition without any significant correlation with rutile composition. The strong positive correlation of rt/meltDNb andrt/meltDTa with rt/meltDTi and SiO2 contents is explained with the decreasing solubility of high charge cations in an

increasingly polymerized melt where the concentration of non-bridging oxygens decreases. The positive correlation ofrt/meltDNb/DTa with

rt/meltDTi and SiO2 contents is more difficult to understand and might be related to the higher polarizibility

of Nb5+ compared to Ta5+.

Magmas resulting from slab melting with residual rutile are slightly Nb-enriched relative to Ta and do not explain the

subchondritic Nb/Ta ratio of continental crust. Rutile in the residue during partial melting or dehydration of subducting crust

is not capable of significantly enriching Nb over Ta in the residue. Excluding residual rutile as a reason for high Nb/Ta

reservoirs outsources this problem to either partial melting of low-Mg amphibolite or metasomatic Nb-enrichment of rutile-

bearing eclogite-lenses in the source region of kimberlites. However, both of these processes cannot produce Nb-enriched

0012-821X/$ - s

doi:10.1016/j.ep

* Correspon

E-mail addr

tters 226 (2004) 415–432

ee front matter D 2004 Elsevier B.V. All rigts reserved.

sl.2004.08.010

ding author. Tel.: +41 1 6327988; fax: +41 1 6321088.

ess: max.schmidt@erdw.ethz.ch (M.W. Schmidt).

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432416

reservoirs sufficiently large to balance the silicate Earth’s Nb/Ta ratio to chondritic, thus, this study supports previous

suggestions that the bmissingQ Nb is stored in the core.

D 2004 Elsevier B.V. All rigts reserved.

Keywords: rutile; partition coefficients; niobium; tantalum; slab melting

1. Introduction

Nb/Ta fractionation during mantle–crustal evolu-

tion has long been regarded as an analytical effect

and not as a natural process [1]. However, recent

analytical developments have demonstrated that such

fractionation is significant, in particular in the arc

setting [2–4]. Reasons for this fractionation are in

discussion (see [2,5–7]), but always include one or

more residual Ti-rich phases (Ti-amphiboles, rutile,

sphene). Some recent experimental studies have

shown that residual titaniferous or Ti-rich minerals

could fractionate Nb over Ta in silicate melt or fluid

[1,2,8–10]. Rutile, which is often invoked to explain

part of the geochemical signature of arc lavas, in

particular the strong negative anomalies of Ti, Nb,

and Ta (e.g. [11,12]), could also fractionate Nb from

Ta in silicate melts [13–15] or aqueous fluids [8,16]

but in different directions, as rt/meltDNb/DTab1

whereas rt/fluidDNb/DTaz1. The few published exper-

imentally determined rt/meltDNb/DTa ratios between

rutile and natural silicate melts of tonalitic, ande-

sitic, and trachytic composition are 0.53 [14], 0.67,

and 0.63 [13], respectively. These results suggest

that rt/meltDNb/DTa varies little in this compositional

range. Nevertheless, partition coefficients for Nb

and Ta measured in a synthetic K2O–Al2O3–TiO2–

SiO2 system between rutile and peralkaline to

peraluminous haplogranitic melts [15] suggest thatrt/meltDNb/DTa could vary from 0.32 to 0.57.

At present, most major geochemical reservoirs, i.e.,

fertile and depleted mantle [17,18], MORB [19], and

continental crust [20] have chondritic [21] or subchon-

dritic Nb/Ta ratios. As both elements are refractory, the

bulk earth is supposed to have a chondritic Nb/Ta ratio.

Depleted mantle and continental crust are assumed to

represent geochemical complementary reservoir by

extraction of the crust from the upper mantle. However,

they both have subchondritic Nb/Ta ratio requiring the

existence of another reservoir with superchondritic Nb/

Ta. It has been suggested to compensate this deficit by

Nb stored in the core [22] or by high Nb/Ta-rutile in

eclogites mixed into the mantle [5].

In this study, we present an experimental determi-

nation of partition coefficients of Nb and Ta between

rutile and melt of basaltic to rhyolitic composition

from experiments realized within a broad range of P–

T–fO2conditions. Our scope is to clarify the role of

rutile in the interplay between different Nb–Ta

reservoirs and in its potential to host the missing Nb

in the Earth.

2. Experimental and analytical techniques

Three types of experiments are presented. Starting

materials of series 1 and 3 (Table 1) are mixtures

between synthetic rutile, Al(OH)3 and a synthetic

basaltic glass. Glasses are fused from specpure oxides

and carbonates around 1400 8C under controlled

atmosphere and then ground in an agate mortar.

Fine-grained rutile (b5 Am) or glass powders were

then mixed with a solution of trace elements with

individual concentrations between 400 and 500 ppm.

After evaporation, powders are homogenized and

either fused (glasses) or heated (rutile) to 1400 8Cfor several hours under controlled oxygen fugacity.

Glasses not doped with trace elements were also fused

a second time and ground in order to homogenize

before final mixing with trace element-doped rutile.

Starting materials of series 1 and 3 contain between 3

and 4 wt.% H2O. For series 2 experiments, three

different starting materials have been synthesized and

enriched in the same way as above and Al-deficient

basaltic, andesitic, and dacitic glasses were mixed

with Al(OH)3, such that each contained 1.7 wt.%

H2O. These three starting materials (Table 1) are

TiO2-saturated at the experimental conditions (1300

8C, 2 GPa) so that only silicate melt and rutile are

present at constant P–T–fO2conditions.

Table 1

Starting material and melt compositions

Run Starting materials Experimental melt compositionsa

TMBb TMBT Gill4c Dacc Kd01 Kd44 Kd57 Kd25 T-1 G-1 D-1

SiO2 50.04 41.64 48.29 56.96 71.51 68.74 60.22 56.32 47.71 53.72 61.74

TiO2 4.21 21.74 16.58 12.52 0.63 1.08 1.92 3.90 12.30 8.70 5.89

Al2O3 16.56 13.36 15.22 15.12 17.06 19.65 18.29 18.85 14.49 16.78 16.28

FeOtot 9.45 7.81 6.65 4.18 1.27 0.98 5.95 2.20 8.05 6.25 3.99

MgO 6.92 4.95 2.90 1.64 0.55 1.21 2.25 4.58 5.65 3.21 1.80

CaO 9.62 7.85 6.50 4.26 2.39 4.32 6.55 9.55 8.91 7.12 4.60

Na2O 2.93 2.45 2.78 3.75 5.17 3.03 4.10 4.18 2.66 2.96 4.02

K2O 0.27 0.20 1.08 1.56 1.42 1.00 0.71 0.42 0.23 1.27 1.69

a Calculated to 100% total; melts contain between 11 and 4 wt.% H2O; compare to Table 2.b Corresponds to average MORB.c Correspond to average andesite and dacite compositions after Gill [55] except for elevated TiO2 contents.

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432 417

All experiments have been conducted in an end-

loaded 1/2-in. piston cylinder apparatus using an outer

Pt or Au capsule with an inner graphite capsule (type

1) or with an inner Au80Pd20 capsule and an external

buffer (NNO, MnO–Mn3O4) for types 2 and 3. The

assemblage was composed of outer NaCl and pyrex

sleeves, a graphite furnace and crushable magnesia

parts inside the furnace. Temperature was measured

by a W74Re26–W95Re5 thermocouple and is accurate

to within F5 8C. Pressure was regulated automatically

during the experiment; its accuracy is better than

F3%. Experiments of type 1 were run between 950

and 1170 8C and 1.7 and 2.5 GPa to evaluate the

consequences of different degrees of partial melting

on the partition coefficients, those of type 2 at 1300

8C, 2.0 GPa, to determine the influence of the melt

composition, and finally those of type 3 at 1050 8C,2.5 GPa to determine the influence of oxygen fugacity

on partitioning coefficients.

Major elements of melt, Ti-minerals and other

phases (clinopyroxene, garnet) have been analyzed by

a Cameca SX100 electron microprobe at University

Blaise Pascal, Clermont-Ferrand, using standard

operating conditions of 15 kV and 8–15 nA. Trace

elements of melt and rutile have been analyzed by

laser ablation ICP-MS at CNR in Pavia using a

Nd:YAG laser source bBrilliantQ (k=266 nm ; repeti-

tion rate of 5 Hz) coupled to a bElementQ, FinniganMAT ICP-MS. 44Ca and 49Ti were used as internal

standards, respectively, for melt and rutile analyses.

NIST610 glass was used as the external standard.

Reproducibility and accuracy of trace element con-

centrations were assessed on the control sample BCR-

2g. An Ar or a mixed Ar–He gas flow to the ICP torch

was used. Ablation spots, were often as small as 6 Amfor rutile, the energy of the laser was reduced to a

minimum by a manually controlled half-wave plate.

The tiny size of experimental rutiles (often less

than 10 Am) made trace element analyses difficult

(possible contamination by glass during ablation,

limited depth of ablation and thus short durations of

analyses) and for several samples with sufficiently

high trace element concentrations, we re-measured Nb

and Ta in rutile by electron microprobe when possible.

For trace elements, operating conditions of 15 to 20

kV and 150.0 nA were chosen, counting times were

between 50 and 400 s for different elements, adapted

to their concentration levels.

3. Results

Partition coefficients between rutile and rhyolitic to

andesitic to basaltic melts have been measured for Nb,

Ta, and Ti. Melt compositions and experimental

conditions are reported in Table 2, element concen-

trations, partition coefficients, and partition coefficient

ratios in Table 3.

3.1. Phase relations—melt compositions

The experiments of series 2 were designed to

investigate rutile–melt pairs at identical pressure,

temperature, and oxygen fugacity conditions. These

experiments crystallized rutile from the initially glassy

starting material at high temperatures (1300 8C). Such

Table 2

Run table

Run T (8C) P (GPa) t (h) Doped Bulk Buffer Melt log( fO2) (D-QFM) Phases

Kd30 960 2.0 68 glass TMB G rhyolite b�1.5 cpx, gar, plag, rt, melt

Kd01 950 1.7 187 rutile TMB G rhyolite b�1.5 cpx, gar, plag, rt, melt

Kd23 1050 2.5 119 rutile TMB G dacite b�1.5 cpx, gar, rt, amph, melt

Kd44 1050 2.5 137 glass TMB G dacite b�1.5 cpx, gar, rt, melt

Kd39 1130 2.5 72 glass TMB G andesite b�1.6 cpx, gar, rt, melt

Kd25 1170 2.5 86 rutile TMB G bas. andesite b�1.7 cpx, gar, rt, melt

Kd48 1160 2.0 79 glass TMB G bas. andesite b�1.8 cpx, gar, rt, melt

Kd47c 1050 2.5 144 glass TMB G dacite b�1.5 cpx, gar, rt, melt

D-1 1300 2.0 2.3 glass Dac MaHa dacite (3.5) rutile + melt

G-1 1300 2.0 2.3 glass Gill4 MaHa andesite (3.5) rutile + melt

T-1 1300 2.0 2.3 glass TMBT MaHa basalt (3.5) rutile + melt

Kd57 1050 2.5 72 glass TMB NNO andesite �0.9 gar, Fe-rich rt, h-ilm, melt

Kd58 1050 2.5 72 glass TMB NNO* andesite b�0.9 cpx, gar, rt, melt

Kd52 1050 2.5 72 glass TMB HM andesite 3.4 h-ilm, Fe-rich rt, cpx, melt

bcQ in Kd47c indicates crystallization experiment, i.e., the liquidus temperature has been exceeded for 2 h (at 1215 8C) before going to the

equilibration temperature. All other experiments did approach equilibration temperature directly. G=inner graphite capsule; Double capsule

buffers: NNO=Ni–NiO; MaHa: MnO–Mn3O4; HM=hematite–magnetite. Amph: amphibole, cpx: clinopyroxene, gar: garnet, h-ilm: hemato-

ilmenite, plag: plagioclase, rt: rutile. NNO*: NiO exhausted in buffer.

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432418

an approach invariably led to a plethora of small

idiomorphic rutiles (Fig. 1a–c) not sufficiently large for

good-quality laser ablation analyses (see Table 3).

Saturation in rutile at 1300 8C requires fairly high TiO2

contents (Table 3) and such melts are not directly

comparable to natural melt compositions and the other

melts of this study. In particular, this is the case because

of the varying influence of TiO2 on melt polymer-

isation: TiO2 acts as a network modifying cation at low

concentrations but is mostly tetrahedrally coordinated

at high concentrations [23,24]. Nevertheless, in this

series, melt composition is the only variable and an

increase of rutile/meltDNb andrutile/meltDTa with increas-

ing SiO2 content or melt polymerisation can be clearly

observed (Fig. 2).

The experiments of series 1 and 3 on a MORB type

bulk composition contain an eclogitic residue com-

posed of garnet+clinopyroxene+rutile (Fig. 1d–f) and

additional plagioclase at 950–960 8C (Table 2). The

experiments included in this study equilibrated in the

P–T region where rutile crystallized as the accessory

Ti-phase; at lower pressures (i.e., V1.5 GPa), ilmenite

was present.1 In all of the experiments, textures are

equilibrated, i.e., idiomorphic crystals are hosted in a

melt matrix (Fig. 1d–f). In general, silicates are

1 Phase relations and silicate phase compositions will be

detailed in a companion paper, see also [44].

homogeneous, but in a few experiments, garnets that

grew to N100 Am show zonations. Generally, garnets

are large (up to 250 Am, e.g. Fig. 1d). In contrast,

clinopyroxene and plagioclase mostly form small

crystals (typically 10–20 Am), distributed over most

of the melt matrix. Nevertheless, in all successful

experiments (i.e., those of Table 2), melt formed

pockets N40 Am in size where Nb and Ta concen-

trations were easily measured by laser ablation.

In the experiments of series 1 and 3, melt

compositions (Table 3) and degree of melting vary

systematically with temperature. At the H2O contents

(3–5 wt.%) and pressures (1.7–2.5 GPa) of this study,

melts at 950 8C are granitic in composition, become

dacites at 1050 8C, and finally basaltic andesites at

1160 8C (Table 3). TiO2 concentrations necessary for

rutile saturation in our experimental melts were

compared to values calculated with the TiO2 solution

model of Ryerson and Watson [11], and agree within

0.4 wt.% except for the experiments bTMBQ and

bGillQ at 1300 8C in which TiO2 concentrations in the

melt reach 8.4 to 11.8 wt.% and calculated deviations

are �1.7 and �1.3 wt.%, respectively.

3.2. Achievement of trace element equilibration

Experiments starting with doped rutile and trace

element free glass and experiments starting with

Table 3

Melt and rutile compositions, Nb, Ta concentrations, and ratios and partition coefficients

Run Melt Rutile

SiO2

(wt.%)

TiO2

(wt.%)

FM nbo/t Nb

(ppm)

Ta

(ppm)

Al2O3

(wt.%)

FeO

(wt.%)

CaO

(wt.%)

Nb Ta Nb Ta

(ppm)

LA-ICP-MS

(ppm)

EMP

Kd30 72.2 0.53 1.21 �0.041 3.15 2.30 0.43 0.60 0.17 471 463 490 480

kd01 71.5 0.63 1.56 �0.023 2.52 2.62 0.77 0.90 0.30 281 304 – –

Kd23 70.1 0.92 1.53 �0.005 3.85 3.45 0.61 0.94 0.31 – – 515 490

Kd44 68.7 1.08 1.46 �0.019 143.0 142.3 0.71 0.73 0.37 – – 12263 12190

kd58 60.3 2.10 3.08 0.169 223.8 147.7 8680 9411 – –

Kd57 60.2 1.92 3.28 0.188 143.9 93.3 0.56 4.28 0.17 – – 10600 11154

Kd39 58.1 3.66 3.35 0.207 344.7 262.7 0.74 0.67 0.41 9593 11234 9024 11550

Kd48 56.9 3.88 4.08 0.275 228.3 192.8 0.65 0.58 0.23 7984 11483 – –

Kd25 56.3 3.90 4.04 0.286 13.5 11.1 0.76 0.21 0.17 480 721 462 655

D-1 61.7 5.89 2.86 0.130 128.2 101.2 0.48 0.98 0.16 – – 3617 4010

G-1 53.7 8.70 4.21 0.254 131.2 91.2 0.55 0.76 0.20 2677 6767 2854 3573

T-1 47.7 12.3 7.02 0.463 137.4 85.8 0.43 0.89 0.23 1957 3663 2013 2571

Kd47c 64.2 1.54 2.53 0.112 150.1 88.9 0.84 0.75 0.13 10419 11211 10543 15137

Run Melt Rutile Partition coefficients rutile/melt

by LA-ICP-MS by EMPNb/Ta S.D.

Nb/Ta S.D. Nb/Ta S.D.

DTi DNb S.D. DTa S.D. DNb/DTa S.D.

Kd30 1.37 0.23 1.02 0.15 1.02 0.116 183.6 156 37 209 37 0.75 0.15

kd01 0.95 0.041 0.925 0.024 – – 155 112 34 116 33 0.97 0.049

Kd23 1.12 0.36 – – 1.05 0.234 105.8 134 20 142 56 0.94 0.37

Kd44 1.00 0.010 – – 1.01 0.041 89.7 85.8 6.9 85.7 3.5 1.00 0.042

kd58 1.52 0.056 0.922 0.030 – – 44.1 38.78 1.7 63.7 2.8 0.61 0.030

Kd57 1.55 0.10 – – 0.95 0.039 48.0 73.7 2.2 119.6 12 0.61 0.048

Kd39 1.32 0.032 0.82 0.11 0.78 0.103 26.4 26.2 5.2 44.0 9.2 0.59 0.079

Kd48 1.19 0.041 0.70 0.020 – – 25.1 35.0 5.0 59.6 9.2 0.59 0.001

Kd25 1.22 0.026 0.71 0.073 0.71 0.111 25.1 34.3 0.8 59.2 11 0.58 0.092

D-1 1.27 0.020 – – 0.90 0.044 16.5 28.2 1.0 39.6 1.4 0.71 0.037

G-1 1.44 0.020 0.61 0.24 0.80 0.039 11.2 21.7 0.7 39.2 1.4 0.56 0.028

T-1 1.60 0.020 0.65 0.35 0.78 0.044 7.9 14.7 0.4 30.0 1.5 0.49 0.028

Kd47c 1.69 0.013 0.97 0.19 0.70 0.050 62.7 70.2 1.2 170 5.5 0.41 0.030

FM=[Na+K+2(Ca+Fe+Mg)]/Al*1/Si [11].

Italic numbers indicate that only two or three very inhomogeneous spots could be measured (typically for 1–3 s), these analyses are disregarded

and replaced by microprobe analysis.

LA-ICP-MS: laser ablation ICP-MS, EMP: electron microprobe.

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432 419

doped glass but pure TiO2 yielded, within error,

identical results. Although considerable scatter is

observed in the determined trace element partitioning,rt/meltDNb and rt/meltDTa at both the basaltic–andesitic

and the rhyolitic end of the compositional array were

successfully reversed (Fig. 3). The larger scatter

towards rhyolitic compositions is probably the result

from lower temperatures and thus much lower

diffusivities and dissolution–reprecipitation rates com-

pared to the higher temperatures in the experiments on

the SiO2-poor melts. In general, melts are homoge-

neous, as large areas can be ablated for long times

(typically 20 s) without changes in the element to Ca

ratios. However, growing large rutiles proved to be a

tedious to impossible task and some of the experi-

ments yielded rutile sizes at the lower limit of ablation

techniques. Thus, in some experiments, Nb and Ta

concentrations in rutiles were also measured by

electron microprobe, yielding identical results within

error.

Fig. 1. BSE images of experimental run products. (a–c): Rutile crystallized from basaltic, andesitic, and dacitic melt, respectively. 2 GPa, 1300

8C, fO2 buffered to MnO–Mn3O4 (experiments T-1, G-1, D-1). (d–f) Partial melting experiments on a MORB composition, graphite capsules. (d)

Plagioclase–garnet–cpx–rutile residue in equilibrium with a granitic melt at 2.0 GPa, 960 8C (Kd30). (e) Eclogitic garnet–cpx–rutile residue in

equilibrium with dacitic melt at 2.5 GPa, 1050 8C, graphite capsule at left lower corner (Kd44). (f) Garnet–cpx–rutile residue equilibrated with

basaltic andesite melt at 2.5 GPa, 1170 8C (Kd25). In this latter experiment, temperature was oscillated with an amplitude ofF15 8C resulting in

much larger garnets and clinopyroxenes at the rim of the melt pool. Graphite capsule visible in the upper left and lower right corners. Mineral

abbreviations as in Table 2, bAQ indicates ablation pit.

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432420

Fig. 2. Dependence of rt/meltDNb andrt/meltDTa on melt composition.

The three experiments have been realised at identical pressure (2.0

GPa), temperature (1300 8C) and oxygen fugacity (MnO–Mn3O4)

and contain melt and rutile only.

2 Who employ Nb2O5 and Ta2O5 concentrations in rutile up to

26 wt.%.

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432 421

One experiment (Kd47c) was conducted as a

crystallization experiment. First the liquidus was

overstepped (1215 8C) and the charge completely

molten, then the sample was rapidly cooled to the

desired temperature (1050 8C). This experiment

yielded spectacular rutile needles 200�20 Am in size

and a single strongly zoned garnet that occupies about

1/3 of the capsule. Major and trace element concen-

trations in this experiment are quite distinct from all

other experiments. In our interpretation, rapid crys-

tallization is probably responsible for only local

equilibrium of minerals with the melt, we thus

disregard this experiment.

3.3. Element distribution

The experimental series 2 on a basaltic, andesitic,

and dacitic bulk composition, conducted at constant

pressure and temperature (2 GPa, 1300 8C),demonstrate that rt/meltDNb and rt/meltDTa vary with

melt composition (Fig. 2). In these three experi-

ments, rt/meltDNb and rt/meltDTa increase with silica

content in the melt. Furthermore, they decrease with

TiO2 contents in the melt and thus increase with

rt/meltDTi. This correlation is equivalent to an

increase of rt/meltDNb,Ta with polymerization of the

melt. Rt/meltDNb/DTa ratios in this series increase

from 0.49 for basaltic melt to 0.71 for dacitic melt.

For the other experiments of this study, realized at

varying experimental conditions (P, T, and fO2),

rt/meltDNb/DTa ratios vary from 0.49 to 1.00 with a

general increase with SiO2 content of the melt (Fig.

3a). Melts with intermediate compositions exhibitrt/meltDNb/DTa ratios around 0.6 (Table 3). For the

more SiO2-rich dacitic and rhyolitic melts of this study,rt/meltDNb/DTa ratios vary between 0.7 and 1. The

positive correlation of rt/meltDNb and rt/meltDTa with

SiO2 contents (Fig. 3a) and the negative correlation

with TiO2 contents (Fig. 3b) are also in agreement with

other experimental results on natural bulk composi-

tions so far available (two data points on an andesite

and on a trachyte from Green and Pearson [13] and one

measurement from Jenner et al. [14]). Further Nb/Ta

partitioning data from 1-atm experiments on K2O–

Al2O3–SiO2–TiO2–melts from Horng and Hess [15]

show that rt/meltDNb/DTa varies between 0.31 and 0.57,

confirming the tendency of an increase of rt/meltDNb,Ta

and rt/meltDNb/DTa with SiO2 contents in the melt.

Whereas there is a clear correlation of partition

coefficients for Nb and Ta with melt composition, a

correlation of these partition coefficients with rutile

composition is almost absent in the experiments of

this study. The Nb/Ta ratio of rutile increases with its

Al contents in the experiments of Horng and Hess

[15]2 and a slight correlation of (Al+Fe) contents is

recognizable for the experiments of type 2 at constant

P, T, and fO2. However, the bulk of the experiments

yields an insignificant correlation of Nb and Ta

contents, partition coefficients, and ratios with any

compositional parameter of rutile.

The evolution of rt/meltDNb andrt/meltDTa (Fig. 3) in

the total of the experiments at varying P, T, and fO2

conditions result in an increase of rt/meltDNb/DTa with

the degree of polymerization in the melt (Fig. 4). A

simple correlation between P–T–fO2conditions and

variations of the ratio rt/meltDNb/DTa at similar melt

compositions is not observed, except when comparing

experiments with basaltic to dacitic melt compositions

Fig. 3. Variations of rt/meltDNb and rt/meltDTa with (a) SiO2 content in the melt; (b) rt/meltDTi, and (c) melt-polymerization expressed as

FM=[Na+K+2(Ca+Fe+Mg)]/Al*1/Si [11]. Experiments of this study. Errors are given for each rt/meltDNb and rt/meltDTa and are smaller than

symbol size where not visible. The errors given in all figures and tables encompass the measured compositional variability. The FM index

employed here correlates linearly with nbo/t calculated after Mysen [52] and is intended as relative measure for polymerization in the melt. The

reason to use FM instead of nbo/t is to avoid Ti in the polymerization index. Arrows at the top and bottom indicate from which side equilibrium

was approached (experiments with trace element doped rutile start at infinity, experiments with trace element doped melt at zero). The square

symbols represent the experiments of Fig. 2 at 1300 8C, other symbols are partial melting experiments on a MORB composition; triangles

indicate the crystallization experiment judged to be unequilibrated.

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432422

from series 1 (1050–1170 8C, graphite capsule) with

experiments from series 2 conducted at 1300 8C. Theselatter experiments yield systematically lower rt/meltDNb

and rt/meltDTa partition coefficients than the other

experiments at comparable SiO2 contents, but have

TiO2 concentrations twice those of the melts from the

experiments at 1050–1170 8C. At this point, it shouldbe emphasized that temperature, and TiO2 contents in

the melt are not independent variables when rutile/

melt partition coefficients are measured, as melts

obviously have to be saturated in TiO2 in order to

crystallize rutile (e.g. contrary to garnet–melt equi-

libria where temperature and TiO2 content of the melt

can be varied independently). Secondly, SiO2 contents

and temperature are also interdependent, as melt

fraction (and thus composition) correlates with tem-

perature in our partial melting series on a MORB

composition (series 1 and 3).

The influence of oxygen fugacity on the partition-

ing of Nb, Ta, and Ti was evaluated through a series

of experiments at fO2VCCO, fO2

=NNO, and fO2=HM.

However, at fO2=HM, hemato-ilmenite was stable

instead of rutile (Kd52, Table 2). Experiments Kd44,

with a graphite-Pt double capsule, and Kd57, with a

double capsule buffered at NNO, were conducted at

identical conditions (1050 8C, 2.5 GPa). However,

Kd57 resulted in a much higher degree of melting and

thus SiO2-poorer melt than Kd44. This can be

explained by hydrogen loss in the graphite-Pt capsule,

hydrogen-gain in the double capsule (the outer

Fig. 4. Variation of rt/meltDNb/DTa relative to (a) SiO2 content in the melt; (b) rt/meltDTi, and (c) polymerization expressed as FM (see Fig. 3).

Experiments of this study. Error bars given for rt/meltDNb/DTa are calculated from errors of Nb/Ta ratios of individual measurements. Note the

systematic variation of rt/meltDNb/DTa with melt polymerization; symbols as in Fig. 3.

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432 423

capsule contained Ni, NiO and H2O), and the

presence of small amounts of CO2 in the graphite

capsule. In addition, rutile in the experiments buffered

to NNO is relatively Fe-rich, making a direct

comparison difficult. Nevertheless, the Nb/Ta ratios

in the experiment buffered to NNO follow the same

trend as all other experiments (Figs. 3 and 4) and, not

surprisingly, it can be concluded that oxygen fugac-

ities varying between VCCO and NNO do not cause

major differences for Nb, Ta, and Ti partitioning.

4. Discussion

In the experiments of this study and from the

literature [13–15], rt/meltDTa is always larger thanrt/meltDNb and rt/meltDNb/DTa increases with SiO2

content in the melt and with rt/meltDTi. This finding

is principally supported by a series of abstracts (see

Fig. 5) that do not specify exact melt compositions.

A relation between rt/meltDNb/DTa and melt compo-

sition is prominent from the data of Wendlandt [25]

where rt/meltDNb/DTa is 0.42 in bhaplobasalticQ melt

and 0.79 in bhaplograniticQ melt. The above range

of rt/meltDNb/DTa values is also consistent with the

partition coefficients reported for a range of melt

types (rhyolite, dacite, andesite, and basalt) by

Green [26].

4.1. Reasons for mineral/meltDTaNmineral/meltDNb in Ti-

rich minerals

Horng and Hess [15], starting from the common

assumption that Nb and Ta have identical ionic radii

(0.064 nm [27]), explained the greater rt/meltDTa relative

to DNb by a difference in the molecular electronic

polarizibility between the twin HFSE5+ (i.e., aG 0.0243

vs. 0.0262 nm3, respectively). In this interpretation, the

higher polarizibility of Nb results in a stronger covalent

bond with oxygen than Ta and in a more severe

distortion of the rutile structure than that operated by

Ta–O, thus causing a smaller quantity of Nb (compared

to Ta) to partition into the rutile structure.

In contrast, Tiepolo et al. [9] observed that the ratio

DNb/DTa correlates positively with M1–O bond

lengths in amphiboles, thus providing strong evidence

in favour of slightly different ionic radii (Drc0.001–

0.002 nm) for Ta and Nb (rNb5+NrTa

5+). If we assume

that the radius of Ta is 0.064 nm [27], the exper-

imental DNb/DTa values for amphiboles obtained by

Tiepolo et al. [9] are consistent with a constant M1-

site Young’s modulus ( M1E5+) of 3500 GPa and a

Nb5+ ionic radius of 0.066 nm [28].

The inferred different ionic radius of Nb and Ta

implies that their relative fractionation differs in

mineral structures with and without Ti-dominated sites

[29] and that preferential incorporation of Ta and

Fig. 5. Rt/meltDNb/DTa (=rutile(Nb/Ta)�melt(Ta/Nb)) as a function of melt composition (SiO2) for all available experiments on natural silicate

melts (this study; [13,14,25,26]; for carbonatite [26]). Several abstracts do not report exact melt compositions but generic melt descriptions (e.g.

bandesiteQ); for these cases, appropriate average SiO2 values are chosen and horizontal bars span the equivalent compositional range. The dashed

line represents an eyeball fit to all data except of Jenner et al. [14], as used for the mass balance calculations.

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432424

rt/meltDNb/DTa values lower than unity are expected in

mineral structures with octahedral sites mostly occu-

pied by Ti (rTi4+=0.0605 nm), such as rutile. In the

case of Ti-pargasite and kaersutite (mineral structures

without Ti-dominant sites), the relative fractionation

of Nb and Ta through amph/meltDNb/DTa varies between

0.71 and 1.63 [9]. This variation is largely controlled

by the dimension of the M1 site, which in turn

depends mostly on both amphibole Mg# and Ti

content. In the case of sphene, sph/meltDNb/DTa is

confined to low values (0.29 to 0.55) due to the

incorporation of HFSE5+ in the Ti-dominated octahe-

dral site [10]. Recent data by Prowatke and Klemme

[30] extend sph/meltDNb/DTa to lower values (down to

0.07). Interestingly, sph/meltDNb/DTa is negatively

correlated with Al3+, since the presence of the latter

(rAl3+=0.0535 nm) in the octahedral site decreases its

mean bond length, thus favouring the incorporation of

the smaller cation, i.e., Ta [10].Spinel/meltDNb/DTa values are close to 0.6 in

ulvospinel (Tiepolo unpublished) and 0.67–1.0 in

magnetites and chromites with small Ti contents

(1.1–2.8 wt.% [31]) thus supporting the view that in

the absence of a Ti-dominated site, the presence of

significant amounts of Fe2+ and, possibly, Mg clearly

result in enlarged octahedral sites and, therefore, in

higher spn/meltDNb/DTa.

Linnen and Keppler [32] modelled partition coef-

ficients for Nb and Ta between rutile and granitic

melts on the basis of experimentally determined

columbite and tantalite solubility data. Although their

general trend of increasing rt/meltDNb/DTa with melt

polymerization is confirmed in this study, their

predicted rt/meltDNb/DTa values of up to 2.1 for granitic

melts (900 8C, 0.2 GPa, H2O-saturated conditions)

overestimate the fractionation of Nb from Ta in fully

polymerized melts.

4.2. The significance of rt/meltDNb and rt/meltDTa

increase with SiO2 concentration, polymerization in

the melt, and with rt/meltDTi

The observed rt/meltDTazrt/meltDNb in the experi-

ments of this study is consistent with the above

argumentation based uniquely on the crystal-chem-

ical behaviour of the solid phase, and particularly on

site-dimensions and the similarity in size of the

major and the trace elements. Nevertheless, the

present experiments clearly show a strong variation

of rt/meltDNb,rt/meltDTa and rt/meltDNb/DTa with melt

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432 425

composition, but not with a compositional parameter

in rutile. Rt/meltDNb,Ta increase with melt composition

(expressed as SiO2 content) and melt-polymerization

(expressed as FM=[Na+K+2(Ca+Fe+Mg)]/Al*1/Si;

[11]) and are negatively correlated with DTi (Figs.

3b and 4b) as the concentration level of TiO2

required by the melt to be saturated in rutile

decreases with the increasing silica content (see also

[11,33]). A relative enrichment of highly charged

cations in ferrobasaltic compared to granitic melts was

predicted by Ryerson and Hess [34], who measured

partition coefficients of REE, Ti, and P between

coexisting immiscible ferrobasaltic and granitic melts.

Their interpretation of the observed Ti partitioning

underlines the relatively low degree of polymerization

in the ferrobasaltic melt and the possibility of highly

charged cations to build their own coordination

polyhedron in such a melt, which has a high

concentration of non-bridging oxygens. Ryerson and

Hess [34] and Ryerson and Watson [11] observed that

cations with an increasing charge density (Z/r)

partition more strongly into a depolymerised melt

when compared to a strongly polymerised granitic

melt. Indeed, Nb5+ and Ta5+ have, after P5+, among

the highest charge densities (77–78 nm�1) of all

commonly used trace elements, and the observed

increase of their rt/meltD values with the increasing

melt polymerization confirms the predictions of [34].

Ryerson and Hess [34] also pointed out that in most

solid solutions (e.g. clinopyroxene, amphibole) the

effect of melt composition on trace element partition

coefficients is obscured by the changes in the major

element composition (and thus in the site properties)

of the solid solution as a function of melt composition.

Therefore, it is not a surprise to only observe a clear

correlation of partitioning coefficients with melt

composition when a unary phase such as rutile is

investigated.

Linnen and Keppler [32] investigated the behav-

iour of Nb and Ta in a simplified model granite system

saturated in columbite or tantalite (MnO–Nb2O5–

Ta2O5–Na2O–K2O–Al2O3–SiO2). The solubility of

columbite (MnNb2O6) and tantalite (MnTa2O6) is

such that the fractionation of Nb over Ta is strongly

correlated with the availability of non-bridging oxy-

gens (NBO). In fact, for most of Linnen and Keppler’s

experiments, the molar concentration of M5+ in the

melt correlates with the available NBOs with a slope

of 5. There are considerable compositional differences

between the columbite/tantalite solubility experiments

in synthetic compositions and our rutile–melt experi-

ments in complex natural compositions; nevertheless,

both studies show the same qualitative tendencies.

4.3. The significance of rt/meltDNb/rt/meltDTa increase

with SiO2 concentration and polymerization in the

melt and with rt/meltDTi

The bulk of all available experiments demonstrate

a strong correlation of rt/meltDNb/DTa with SiO2

concentration and polymerization in the melt (Fig.

5). The influence of melt composition on partition

coefficients can be qualitatively described by two

contrasting effects. As observed above, Nb5+ and Ta5+

have high charge densities and their rt/meltD values

increase with the increasing melt polymerization.

However, if rNb5+NrTa

5+ (see above), then it follows

that the charge density Z/rNb5+ is smaller than Z/rTa

5+

(i.e., ca. 77.5 and 78.7 nm�1, respectively). Following

the arguments of [34], this implies that relative to Nb,

Ta is preferentially incorporated in less polymerized

melts compared to more polymerised melts. As a

consequence, the first melt composition effect one

could expect is that rt/meltDNb increases at a slightly

lower rate than rt/meltDTa with polymerization of the

melt. As the contrary is observed, other effects must

dominate over the effect caused by the small charge

density difference between Nb and Ta.

A second effect deals with the higher polarizibility

of Nb5+ relative to Ta5+, probably resulting in a

competing advantage for non-bridging oxygens. Nb

may thus be relatively favoured in depolymerised

melts. Linnen and Keppler [32] argued that in the

more aluminous melts, free NBOs are not available

and that interaction must occur between high-field

strength cations and bridging oxygen atoms.

Unfortunately, structural data on Nb and Ta in

glasses and melts of geological interest are still

scarce. Some considerations can be extrapolated

from studies of coordination chemistry of Ti and

other geochemically relevant elements, such as REE.

Ponader and Brown [35] showed that the regularity

of the REE sites in the melts decreases as melt

polymerisation and the element size increase, due to

the decreasing number of non-bridging oxygens and

the ability of elements with higher charge density to

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432426

compete for NBO, respectively. These effects lead to

increasing mineral/meltD values, possibly more pro-

nounced for Nb than for Ta in light of its lower

field strength.

Calculations of trace element solution energies in

garnet–melt systems have been carried out by Van

Westrenen et al. [36] to assess the possible influence of

melt. Results indicate that the trace element environ-

ment in the melt affects to some extent both absolute

values of the unstrained ionic radius, and apparent

Young’s modulus of the site incorporating a series of

isovalent cations. Available XANES studies show that[5]Ti and [6]Ti prevail in less polymerised melts,

whereas [4]Ti becomes important in the most poly-

merised melts [37]. Moreover, Nb has been interpreted

to be in tetrahedral coordination in glasses of basaltic

and granitic composition, whereas it changes from[4]Nb to [7]Nb in peralkaline melts [38]. A characteristic

change in slope of columbite/meltDNb and tantalite/meltDTa

versus nbo/t was observed by [32] when melt compo-

sitions changed from fully polymerized model granites

to peralkaline granites. This change in partitioning

behaviour may be caused in part by a coordination

change of Nb (and probably also for Ta) in the melt.

The available data imply that HFSE may have

polyhedral geometries in silicate melts that vary

systematically with melt polymerisation, and that

melt strain energy is not negligible. In fact, both

[32] and [15] suggest that the dissolution mechanism

of Nb5+ and Ta5+ changes when melt compositions

become fully polymerized and peraluminous. The

mechanism might be a change in coordination and

bsiteQ of HFSE in the melt when the concentration of

NBO decreases with increasing polymerization. This

effect is important in the context of granite and

pegmatite crystallisation, but should not affect partial

melting processes of subducted oceanic crust which

produces less siliceous partial melts even at small

melt fractions.

It might be argued that with decreasing concen-

trations of non-bridging oxygens, Nb and Ta have

equal characteristics and thus fractionation between

these elements decreases. Experimental evidence

about mineral/meltDNb/DTa variation with SiO2 con-

centration and polymerization in the melt for

minerals with Ti-dominated sites provides contrast-

ing results. Horng and Hess [15] (their Table 4)

found that in a granitic model system (K2O–Al2O3–

SiO2–TiO2–Nb2O5–Ta2O5) at constant P–T condi-

tions, rt/meltDNb/DTa in haplogranitic melts increases

(from 0.31 to 0.57) with an increase in the

polymerization of these melts (expressed as

K*=K2O/(K2O+Al2O3), which decreases with poly-

merization). Although in accordance with the find-

ings of this study, this observation needs to be

carefully evaluated, as most experiments by Horng

and Hess have Nb2O5 and/or Ta2O5 concentrations

above several wt.% and do not necessarily obey the

Henry’s law behaviour. In contrast, Prowatke and

Klemme [30], in a study of trace element partition-

ing between sphene (a mineral with octahedral Ti-

dominated site) and a range of different melts,

observed that Sph/meltDNb/DTa values decreases from

0.16 to 0.27 in low SiO2 (b60 wt.%) to 0.07–0.12

high SiO2 (up to 67 wt.%) melts.

4.4. Composition of rutile and rt/meltDNb,rt/meltDTa ,

and rt/meltDNb/rt/meltDTa

Natural rutiles from magmatic rocks and also our

experimental rutiles contain Al3+, Fe3+, Fe2+, and Ca2+

inmuch larger concentrations than necessary for charge

balance of Nb5+ and Ta5+. This is also valid for our

experimental rutiles (Table 3). The incorporation of

low-valence cations is thus not a limiting factor for Nb

and Ta and apparently, the relatively low concentra-

tions of Al, Fe, and Ca in the experimental rutiles are

not sufficient to modify the Ti site substantially, such

that an effect on Nb and Ta partitioning would be

recognizable. In this context, it is necessary to point out

that the linear correlation of rt/meltDNb and rt/meltDTa

with Al contents [15] is observed for a much higher

concentration range of Nb and Ta (to max. 26 mol% in

rutile) and are probably caused by charge balance

requirements in a simple K2O–Al2O3–SiO2–Nb2O5/

Ta2O5–TiO2 system.

Rutiles of experiments K*203 and K*204 of Horng

and Hess [15], in equilibrium with peralkaline granitic

melt are Al-poor (0.62 and 0.28 wt.%, respectively)

and are similar to natural rutiles and those of this

study (see Table 3). In contrast, rutile of experiments

K*201 and K*202, in equilibrium with a peralumi-

nous granitic melt, have a high level of Al3+ (N6 wt.%

of Al2O3) and Nb and Ta due to the coupled

substitutions Al3++(Nb,Ta)5+=2Ti4+. This difference

in rutile composition in peraluminous and peralkaline

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432 427

granitic melt is, according to Horng and Hess [15],

due to the high activity of (AlNbO4) and (AlTaO4)

components in the peraluminous granitic melts. An

additional coupled substitution involving vacancies,

i.e.: 5Ti4+=4Nb5++5, for peralkaline melts is pro-

posed by these authors in order to account for

deviations from a linear correlation of Al3+ with

M5+ cations in rutile.

The presence of Fe in this study is one of the major

differences between our experiments in granitic sys-

tems and those by [15,32]. The effect of incorporation

of Fe on rt/meltDNb/DTa cannot be precisely evaluated as

both Fe2+ and Fe3+ may be present in rutile (e.g. [39]).

The ionic radius of (Fe2+)VI is close to (Ti4+)VI (0.061

and 0.0605 nm, respectively). In contrast, the incorpo-

ration of Fe3+ is probably similar to that of Al3+, as Fe3+

and Al3+ have similar ionic radii in six-fold coordina-

tion (0.054 and 0.055 nm, respectively) and have

identical charges. Fe3+ enables the incorporation of Nb

and Ta according to the same type of substitution as

Al3+ (see [15]). Even high levels of Fe in rutile (see

rutile composition in Kd57, and rutile in experiment

1056 T/HM of Green and Pearson [13]) appear not to

modify the ratio rt/meltDNb/DTa.

5. Rutile and Nb/Ta fractionation during

subduction processes and Nb/Ta ratios of

geochemical reservoirs

Depleted mantle and continental crust have sub-

chondritic Nb/Ta ratios of 15.5F1 [5,17] and 12–13

[20], respectively. This (and other ratios such as La/

Nb) suggests that these two reservoirs are not always

complementary and that another reservoir with a

superchondritic Nb/Ta ratio is necessary to balance

the Earth’s Nb/Ta ratio to a supposedly chondritic

bulk value (Nb/Ta=19.9F0.6; [21]). Rudnick et al. [5]

suggested that such a reservoir should amount to 1–6

wt.%, have Nb/Ta of 19–35, and Nb concentrations of

z2 ppm and could be represented by the subducted

oceanic crust hidden in the mantle (refractory rutile

bearing eclogite reservoir of McDonough [40] and

Kamber and Collerson [41]).

Trondhjemitic, tonalitic and granodioritic (TTG)

suites, which have largely contributed to early

crustal genesis [20,42,43] and modern adakitic

magmas are characterized by negative Nb, Ta, and

Ti anomalies relative to other elements with similar

field strength. Adakites are often considered as

modern analogues of Archaean granitoids [42,44];

however, it is commonly assumed that Archean slab

melts resulted from melting at somewhat lower

pressures than most modern adakites due to more

elevated geotherms in Archaean times. Negative

anomalies of the above HFSE are mostly ascribed

to residual rutile during partial melting of oceanic

crust at pressures exceeding 1.4 GPa (see above).

The results of this study show that rt/meltDNb/DTa

varies according to melt composition, which in turn

depends on the degree of melting in the source

MORB and in the following, we will discuss

consequences of rutile being residual to melting

and dehydration processes.

5.1. Nb/Ta ratios of rutile bearing eclogites residual

to partial melting of MORB

Melt extraction from oceanic crust during subduc-

tion is one of the processes able to modify Nb/Ta ratios

of subducting MORB, which has average Nb/Ta ratios

of 15.5F1.0 [19,21]. From the partition coefficients

and rt/meltDNb/DTa ratios obtained in this study, it results

that melting of rutile-bearing residual eclogites will

lead to lower Nb/Ta ratios than their source. Extraction

of low melt fractions (5–10 wt.%) with relatively

siliceous (rhyo-dacitic) composition will lower the Nb/

Ta ratio of a typicalMORB from 15.5 to 15.3 in a rutile-

bearing eclogite residue. Larger melt fractions, which

result in dacitic to andesitic melt compositions,

decrease the Nb/Ta ratio more significantly, at most

down to 11.4 (Fig. 6). These batch melting calculations

employ rt/meltDNb andrt/meltDTa of this study, a degree of

melting F=0.05 to 0.4, a clinopyroxene+garnet+rutile

residue with initial rutile contents of 0.5 and 1.9 wt.%

(corresponding to 1.6 and 3.0 wt.% bulk TiO2 in

MORB), with rt/meltDNb/DTa varying with F between

0.7 and 1 according to Fig. 5 (an eyeball fit was used forrt/meltDNb,

rt/meltDTa andrt/meltDNb/Ta, silicate partition

coefficients and modes from [46]). Any fractional

melting process would tend to decrease Nb/Ta to a

lesser extent, as subsequent melt extraction steps would

produce relatively SiO2-enriched melts and thus

involve rt/meltDNb/DTa values closer to unity. A second

effect of fractional melting is that rutile would persist to

higher accumulated melt fractions (and temperatures)

Fig. 6. Nb/Ta vs. Nb evolution in subducting MORB during dehydration and melting. (a) Dehydration and flushing of MORB modelled afte

[8], see Discussion. Dehydration of MORB at N700 8C is unrealistic [48] but flushing of MORB with fluids provided by serpentine breakdown

in the peridotite situated below the oceanic crust might occur to 800 8C. The Nb and Ta variation in the residue resulting from batch o

incremental dehydration and flushing is barely visible in this diagram (trace to the right of the full dot representing average MORB, stars

represent fluid compositions) for any realistic temperature and amount of fluid. (b) Partial batch melting of the oceanic crust with bulk TiO2

contents of 1.6 and 3 wt.% (as indicated by numbers in parentheses). Other numbers give melt fractions. Rt/meltDNb/DTa varies according to F as

do proportions of clinopyroxene, garnet, and rutile. With a bulk TiO2 of 1.6 wt.%, rutile melts out at F=0.35. Calculated melts and residues fo

melting of garnet-free amphibolite are also shown (amp/meltDNb/DTa=1.47 [9]), modal proportions in the residue from [53] for fluid-saturated

(crosses) and from [54] for fluid-absent melting (triangles). Melting of rutile bearing eclogite lowers the Nb/Ta ratio in the residue, melting o

garnet- and rutile-free amphibolite at low melt fractions may increase the Nb/Ta ratio in the residue by up to 4 units. Horizontal line: average

MORB (at 3.5 ppm Nb [19]), dashed line: chondritic ratio=19.9F0.6 [21], DM=depleted mantle [5,17], star: average continental crust from

[20].

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432428

as more silicic melts have relatively low TiO2

solubilities (Table 3 and [11]). Any affect of rutile on

melt and residue compositions during eclogite melting

is obviously based on its presence and residual amount.

It is thus necessary to bear in mind, that in average

MORB (with 1.6wt.%TiO2), rutile will disappear from

the residue3 at a melt fraction of 30–35%.

Rutile constitutes the residual titaniferous mineral

during high-pressure melting of eclogites. Other

possible titaniferous accessory minerals such as ilmen-

ite and sphene, which may be present in the residuum at

3 Garnets in the relevant experimental range have 1.5–2.2 wt.%

TiO2, clinopyroxenes 0.9–1.3 wt.% TiO2.

r

f

r

f

lower pressures, would not significantly alter the above

conclusion: solid/meltDNb/DTa remain below unity for

ilmenite [13,46] and sphene (sph/liqDNb/DTa=0.3–0.4,

[10,13]).

The arguments reported above reinforce the view

that low Nb/Ta ratios in the tonalite–trondhjemite–

granodiorite gneisses of Archaean terrains (ranging

down to Nb/Ta=3), or more generally, the continental

crust itself, cannot be the partial melting product of

rutile-bearing eclogite when starting from chondritic

or slightly subchondritic initial Nb/Ta ratios [6]. This

contradicts the suggestion of Rapp et al. [7] that the

low Nb/Ta values in early continental crust could

result from melting of eclogites. This latter conclusion

is essentially based on their experimental observation

that their basalt starting material with a bulk Nb/Ta

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432 429

ratio of 6.3F0.24 resulted in a melt with Nb/

Ta=7.7F2.4 coexisting with a garnet+clinopyroxe-

ne+rutile residue. This observed increase in Nb/Ta of

1.4 units is consistent with our results, however,

MOR, ocean island, and continental basalts have Nb/

Ta of 12–19 [21].

Prouteau et al. [4] have measured Nb/Ta ratios of

adakites from the Philippines, and suggested that

these ratios were inherited from the source region.

Measured ratios vary between 12.2 and 17.4 and

correlate negatively with SiO2 content. This finding

would be consistent with an increase of rt/meltDNb/DTa

with SiO2 content in the melt and with melt-fractions

decreasing with increasing SiO2 content from a source

with a subchondritic Nb/Ta ratio V14. However, thiscorrelation needs to be considered with some caution

as SiO2 contents in the melts could be influenced by

AFC processes and thus may have been modified

since the melting process in the source took place.

5.2. Nb/Ta ratios of rutile bearing eclogites residual

to dehydration and flushing of MORB

Nb/Ta ratios between 19 and 37 for the high Nb/Ta

reservoir [5] were estimated on the basis of measured

Nb/Ta ratios (with an average Nb/Ta of 24 [5]) in

rutiles sampled in xenoliths from cratonic kimberlites.

However, these values deviate strongly from Nb/Ta

ratios measured in rutiles from eclogites representing

subducted oceanic crust: Zack et al. [45] report Nb/Ta

ratios of 13.6–17.4 from Trescolmen, Central Alps,

and Dardon [46] obtained an average of Nb/

Ta=17.7F1.6 from four eclogite localities (Norway,

Dabie-Shan, Dora Maira, France).

A significant variation in Nb/Ta ratios of eclogites

will not be achieved by dehydration in presence of

rutile as a result of high rt/fluidDNb and rt/fluidDTa

partition coefficients which range from 400 to N104.Rt/fluidDNb/DTa has been experimentally determined by

[8,16,47]. Brenan et al. [8] found rt/fluidDNb/DTaN1,

increasing with decreasing temperature, but Stalder et

al. [16] argued that within uncertainty, a significant

fractionation of Nb from Ta cannot be ascertained

at 900 8C. In the presence of chlorine, both [8] and

4 This low value of the starting material of Rapp et al. [7] is not

representative of MORB but appears to have resulted from

contamination produced during crushing in WC mills [7].

most experiments from Green and Adam [47] result inrt/fluidDNb/DTa equal to unity within error, only three

experiments from [47] yield an average rt/fluidDNb/DTa

of 0.45F0.27 (1r) indicating a possible Nb-enrich-

ment in the fluid. In the mass balance of Fig. 6, we

used on purpose the values of [8] most favourable for

fractionation and Nb-enrichment in the dehydrated

eclogite. Thus, the effect of fluids on the Nb/Ta ratio

in the residue is maximized, nevertheless, without

obtaining any visible effect on the residue (Fig. 6).

The employed values of rt/fluidDNb/DTa range from 2.4

at 600 8C to 1.3 at 900 8C [8,16], the most extreme

values for rt/fluidDNb and rt/fluidDTa being 14.2�103

and 6.0�103 (at 600 8C) [8]. Absolute concentrationsof Nb and Ta are low (4–300 ppb Nb and 0.1 to 30

ppb Ta) in fluids in the relevant temperature range of

600–800 8C (Fig. 6, at z800 8C, fluid saturation

would lead to partial melting of the basalts). Calcu-

lations maximizing the effect of fluid result in an

increase of Nb/Ta of max. 0.15 units (at 600–800 8C),both for batch or incremental (down to 0.1 wt.% fluid)

dehydration and flushing. For this negligible increase,

extreme amounts of fluids are necessary, i.e., dehy-

drating fluids of the metabasalts of up to 6 wt.% [48]

and 10 wt.% additional fluid provided by dehydrating

serpentinites underlying the oceanic crust. If this latter

water reservoir is fully dehydrated, and if the fluid

passing upwards through the overlying MORB is

equilibrated with rutile therein, an variation in Nb/Ta

of the dehydrated eclogite due to flushing amounts to

at most 0.1 units. Thus, whatever the details of a

dehydration/flushing scenario, the immobility of Nb

and Ta in a fluid in combination with realistic fluid–

rock ratios cannot achieve a significant Nb/Ta

fractionation in the residue as would be necessary to

satisfy the model of Rudnick et al. [5]. Secondly, such

fluids have Nb and Ta concentrations far too low to

produce any reservoir of metasomatized mantle with

Nb and Ta concentrations significant for the global

Nb–Ta mass balance.

5.3. Alternatives to rutile-bearing residues

The rutile-bearing eclogite reservoir is certainly

heterogeneous with respect to Nb/Ta ratios because it

corresponds not only to partially or fully dehydrated

oceanic crust but also to the melting residues of

oceanic crust. Nb/Ta ratios of dehydrated MORB will

5 Rudnick et al. [5] estimated that oceanic crust subducted

during the last 2.4 Ga amounts to 3.3% of the silicate earth’s mass, an

amount that should be increased for possible subduction back to 3.8

Ga. Less than 1/3 of the oceanic crust becomes hydrated and may

possibly melt, melt fractions are typically 30%. Furthermore, only a

small portion of modern subduction zones produce slab melts (which

might have been different in the Archean), leaving at most a few per

mil (of the silicate earth) of oceanic crust residual to partial melting.

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432430

remain subchondritic. Oceanic crust residual to partial

melting in eclogite facies would have a lowered Nb/Ta

ratio. The high Nb/Ta values of 19 to 37 measured by

Rudnick et al. [5] in rutiles from xenoliths in

kimberlites do not directly result from subduction

related dehydration or partial melting processes

involving rutile. Two alternative hypothesis for the

atypical high Nb/Ta ratios in these rutiles are:

(1) Foley et al. [6] suggested that the subchondritic

bulk Nb/Ta ratios of early continental crust

reflect melts originating from partial melting of

amphibolite. Amphiboles with intermediate Mg

numbers, as typical for amphibolite melting,

have amph/meltDNb/DTa=1.3–1.6 [6] and would

leave a amphibole+clinopyroxeneFplagioclase

residue with Nb/Ta values of 16–20.5 at low

pressures. However, residual garnet (and/or

ilmenite) somewhat counterbalances the effect

of amphibole (gar/meltDNb/DTa=0.4–0.7 [46]), 5–

20 wt.% residual garnet limit the increase of Nb/

Ta in the residue to about 2.5 units. While it is

possible to form a high Nb/Ta residue through

melting of amphibolite, this process is effective

only in the absence of garnet at pressures V1.0–1.2 GPa [49] and at low melt fractions leaving a

large amount of residual amphibole (Fig. 6). The

contrasting Nb/Ta partition coefficients of

amphiboles (which may be residual to MORB-

melting at pressures V2.0 GPa) and rutiles

(which may be residual at z1.6 GPa) adds

further evidence to a relatively low-pressure

melting-regime for Archean TTG suites.

(2) It is generally accepted that kimberlites originate

from metasomatized mantle. Metasomatized

rutile-bearing mantle peridotites have whole-

rock Nb/Ta ranging from 14 to 30 [5,18] with

an average of 23F3 [18], which is higher than

any other peridotite value (see also [50]). The

hitherto unidentified metasomatic process that

has caused such an increase in Nb/Ta in the

peridotite may also have altered rutiles in

eclogite lenses contained in the source region

of kimberlites and thus be responsible for the

extreme values measured by Rudnick et al. [5].

A reservoir with an increased Nb/Ta ratio may

have formed during the growth of early continental

crust via partial melting of rutile- and garnet-free

amphibolite, followed by solid-state transformation of

residual amphibolite to rutile–eclogite on further

subduction [6]. However, the Nb/Ta ratio (16–20.5)

of such a reservoir is barely chondritic (Nb/Ta=19.9),

the highest Nb/Ta ratios resulting from relatively low

melt fractions at H2O-saturated conditions (Fig. 5).

Any realistic volume,5 Nb concentration, and Nb over

Ta excess of such a residue seems unable to balance

the silicate earth’s Nb/Ta ratio to a chondritic bulk

value. Further arguments were provided by Mqnker etal. [21]: ocean island basalts which are thought to tap

the eclogite reservoir in the mantle [51] have an

average Nb/Ta ratio of 15.6F0.7 [21]. As any melt

from a source with residual clinopyroxene+gar-

netFrutile will have an increased Nb/Ta ratio, the

Nb/Ta ratios of ocean island basalts provide a strong

argument for the absence of a significant eclogitic

high Nb/Ta reservoir in the mantle.

Finally, as any significant high Nb/Ta reservoir in

the crust or mantle cannot be identified, our measured

Nb/Ta ratios and distribution coefficients support a

subchondritic Nb/Ta ratio of the silicate Earth [21]. If

the Earth’s Nb/Ta has to be balanced to a chondritic

value, the hypothesis of Wade and Wood [22] who

placed the lacking Nb in the core, appears at present

the only valuable solution to the imbalance problem.

6. Concluding remarks

This study establishes an increase of rt/meltDNb andrt/meltDTa by almost one order of magnitude, and ofrt/meltDNb/DTa by a factor of two with melt polymer-

ization. The preferential incorporation of Ta5+ (com-

pared to Nb5+) into rutile, which may be caused by a

slightly smaller ionic radius of Ta5+ compared to Nb5+,

is counterbalanced with increasing melt polymeriza-

tion. Coordination changes of Nb and Ta with melt

composition, availability of NBO sites in the melt, and

M.W. Schmidt et al. / Earth and Planetary Science Letters 226 (2004) 415–432 431

different polarizibilities of Nb and Ta contribute to the

systematic variation in the partition coefficients with

melt composition. The added effects progressively

decrease the fractionation between Nb and Ta,

although the reasons for increasing rt/meltDNb/DTa in

SiO2-rich and highly polymerised melts are at present

not completely understood. Further experiments and

computer simulation studies of cation exchange

reactions between rutile and melts as a function of

the local melt environment of HFSE are desirable and

may isolate the effect of melt composition on

partitioning coefficients thanks to the simplicity of

the investigated mineral phase (i.e., rutile).

Values of rt/meltDNb/DTa for andesitic or more

silicic systems which are relevant for slab melting in

subduction zones are in the range 0.65–1.0, thus

invalidating claims that superchondritic Nb/Ta ratios

in refractory rutile-bearing eclogites form due to

partial melting of subducted oceanic crust.

Acknowledgements

We thank P. Bottazzi for performing the LA-ICP-

MS analyses and M. Vechambre for assistance in

microprobe analysis. G. Prouteau is acknowledged for

making analyses available. M. Tiepolo provided help-

ful suggestions for improving this manuscript, and the

manuscript profited from constructive reviews of J.

Brenan and R. Rudnick.

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