An experimental determination of rare earth partition coefficients between a chloride containing vapor phase and silicate melts

0016-7037/78/0615-0685$02.00/O Geochimica et Cosmochimica Acta Vol. 42, PP. 685-701 @Pergamon Press Ltd. 1978. Printed in Great Britain

AN EXPERIMENTAL DETERMINATION OF RARE EARTH PARTITION COEFFICIENTS BETWEEN A CHLORIDE CONTAINING VAPOR PHASE AND SILICATE MELTS. Ronald T. Flynn* and C. Wayne Burnham The Pennsylvania State University Department of Geosciences, University Park, PA

16801

ABSTRACT The partitioning behavior of cerium, europium, gadolinium and ytterbium between an aqueous "vapor" phase and water saturated silicate melt have been experimentally examined using a new experimental approach employing radioactive tracers and a double-capsule technique. Equilibrium was established by reversing the partition coefficient' and by betatrack autoradiography. Aqueous solution compositions were varied by adding different amounts of chloride, and in some cases fluoride or carbon dioxide. The H20 contents of the Spruce Pine pegmatite melts were varied by conducting experiments at 4.0 kb, 800°C and at 1.25 kb, BOO'C. A jadeite-nepheline composition (75 wt% jadeite) also was employed at 4.0 kb, BOO'C. The chloride experiments (Spruce Pine 4 kb, 8OO'C) show a linear relationship between the cube of the chloride molality and the partition coefficients of the trivalent rare earths. Europium, under the experimental f02 conditions (quartz-fayalite-magnetitebuffer), varied linearly as the fifth power of the chloride molality. At the chloride molalitiesRfxamined (cl.1 mCl), all the rare earths partitioned preferentially into the melt phase (K SP(4.0 kb) > Jd-Ne(i.0 kbr, where SP = Spruce Pine, Jd-Ne = jadeiteKpmodel of Burn nephel P ne. Using the 'kam (1975), it is suggested that the trivalent rare earth partitioning is related to the cube of the melt octahedral site concentration; a property which in hydrous melts is dependent on melt composition and hydroxyl molality. Excellent agreement was found for the Spruce Pine melt, whereas the jadeite-nephelinemelt gave apparent hydroxyl molalfties which were too high for the measured partition coefficient. Additional octahedral sites are proposed for this unusual composition perhaps due to some aluminum in C-fold coordination. The apparent compositional variation of europfum partitioning at a constant oxygen fugacity is believed to be related to both the octahedral melt site concentration for trivalent europium and an 8-coordinated site concentration for divalent europium. Any parameter which affects the numbers of these sites (pH20, melt composition) will affect the rare earth partitioning. The observed dependency of the partition coefficient on the structural state of the melt could be as significant as its dependency on crystalline structural constraints. Furthermore, since H 0 can drastically effect the melt structural state, its effects could be reflected in mePtfcrystal partition coefficients. *Present address: Owens-Corning Fiberglas, Technical Center, P. 0. Box 415, Granville, Ohio 43023 'The partition coefficient ( RE) is defined as; ppm rare earth in "vapor" phase/ppm rare Kp earth in glass.

685

686

R. T. Flynn

and C. W. Burnham

INTRODUCTION The objectives of this study are to attempt to answer the following questions: 1.

Is complexing of the rare earth's a feasible phenomenon at magmatic conditions?

2.

What ligand(s) is(are) important under these conditions?

3.

What effects do temperature, composition and pressure have on the partitioning of the rare earths between silicate melt and vapor phase?

4.

Can complexing

fractionate

This study represents

the rare earths?

the first major ex erimental

attempt

to document

the importance

rare earth complex formation during partPtioning between melt and vapor phase. soviet

Scientists

have been more interested

in the behavior

of rare earths

of

in hydra-

thermal systems, i.e. systems where the volatile phase is important, than their Western counterparts. Soviet literature supports either a carbonate complex (e.g. KOSTERIN, 1959; BEUS, 1958; GANEYEV, 1962) or a fluoride complex for rare earths in hydrothermal fluids (e.g. BANDURKIN, 1969; MINEYEV, 1963). However, in addition to these ligands, sulphate, phosphate and chloride complexes are all mentioned as less likely possibilities. Excepting Mineyev (1966) the arguments are based on the type of minerals in which the rare earths occur e.g. carbonates, fluorides, phosphates, etc. The obvious problem with such an approach is that it assumes that the present mineral complex of the particular rare earth is the one which was stable at high temperatures and pressures. This assumption, without experimental or thermochemical support is tenuous. MINEYEV (1966) attempted to demonstrate the feasibility of fluoride as a complexing agent under hydrothermal conditions in the system mineral/vapor. He succeeded in complexing several rare earths, but the experiment is of limited applicability since he only considered fluoride under concentrated conditions. The Western literature rarely mentions rare earth complex'n rimaril bet se f et al. (1973) determined silicate me\tg/&por ra& ear\\ par$ytion data exist. CULLERS coefficients at magma=cpressures and temperatures but only as a prerequisite to obtain melt/crystal partition coefficients. Their experiments were conducted with "pure" water as the vapor phase and did not consider complexing in the volatile phase. ZLELINSKI and FREY (1974) working in the system diopside/vapor did some preliminary experiments employing both CO and chloride. Effects were noted on the mineral/vapor partition coefficients and an exB lanation involving cationic species in solution was offered. GRAF (1975), in an attempt to model rare earth behavior during the formation of the New Brunswick ore deposits, suggested that rare earth chloride complexing was an important factor in determining the rare earth distribution patterns. He used the one atmosphere and 25'C data of PEPPARD (1962) on mono-chloride rare earth complexes. It is unlikely that such a highly charged, ionized species as a rare earth mono-chloride would be present at magmatic temperatures and pressures. Nonetheless, Graf's effort is significant since complexing was considered to play a major role in the generation of the rare earth patterns and that the complexing ligand was chlorine. Several other workers have considered the potential effects of complexing but lacking experimental verification it is difficult to estimate the magnitude of the effect (see HASKIN et al., 1966; GOLES, 1968; SCHILLING, 1972, etc.). Understanding the geochemical behavior of the rare earths is critical if they are to be used to model petrologic processes (e.g. GAST, 1968; ZIELINSKI and FREY,1970, etc

Most trace element models of magmatic petrogenesis employ phenocryst/matrixpartition toef' ficients obtained from co-existing natural phases. Application of measured partition coefficients to a particular problem is complicated by the pressure, temperature and compositional dependence of the coefficients (e.g. SCHNETZLER and PHILPOTTS, 1970; ONUMA et. al., 1968; DUDAS, 1971; EBY, 1975, etc.). In addition, ALBAREDE and BOTTINGA (1972) questioned the establishment of equilibrium in natural systems and pro osed several theoretical models for kinetic disequilibrium. HART and BROOKS (1974P gave support to Albarede and Bottinga's theoretical arguments by demonstrating disequilibrium in a clinopyroxene/hostlava pair with respect to several large lithophile trace elements. Considering the above, It is not surprlsing that measured phenocryst/matrixpartitioning of a particular trace element can vary over an order of magnitude. In order to sort out the relative influence of pressure, temperature, composition and disequilibrium effects, one must turn to experiments. EXPERIMENTAL METHOD The starting materials for the glass phase were Spruce Pine pegmatite (desiqnated JB-5) and a gel with the composition 75 wt% jadeite - 25 wt% nepheline. The weight percent oxide analyses of both materials are presented in Table 1. The rare earth "stock" solutions of europium, gadolinium and ytterbium were prepared as nitrate solutions (
Determination

Table 1.

of rare earth

partition

687

coefficients

Weight percent oxide analysis of Spruce Pine pegmatite and jadeite-nepheline gel.

-Rock Name

sio2

A1203

Spruce Pine*

73.87

15.26

** Jd-Ne Gel

55.2

27.8

Fe0

CaO

Na20

K20

.3

.15

1.03

4.87

4.12

-

-

-

17.0

-

Fe2o3

*Analyst, C. 0. Ingamells, Mineral Constitution Laboratory **Calculated from gel composition

compositional changes due to the presence of the chloride solution (KILINC, 1969), it was made by combining NaCl, HCl and KC? in the molar proportions 2:2:1. The solution for the fluoride experiment was HF plus distilled water. Carbon dioxide was added as silver oxalate. Glasses doped with a known a~unt of radioactive rare earths were repared in the following manner. Equal a~unts (approxf~tely 160 mg each) of soE; id and radfoactfve fluid' were added to annealed Pt capsules (5 mn x 2.5 cm). The capsules were then tricrimped cfosed, weighed, and welded shut. The capsules were reweighed after welding and any loss of more than 10 mg caused the capsule to be discarded. All runs were conducted in argonpressurized, internally heated vessels (HOLLOWAY, 1971). Three chromel-alumel thermocouples monitored the temperature over the three inch long, one inch diameter sample holder. The thermocouples were calibrated against standardized NBS chrome1 and alumel wire. The temperatures are accurate to +3 C. Furnace temperature was regulated by a solid-state, proportional-band temperature controller (Thermo-Electricmodel #400) and the maximum temperature fluctuation was *5OC of the setpoint.2 The temperature gradient across the capsule holder was less than 3'C. Pressure was monitored by a digital-readout pressure measuring device operating off a manganin cell. Pressures reported are accurate to k50 bars of the stated value. Hydrogen fugacities were maintained via a "Shaw membrane" (SHAW, 1967). The hydrogen pressures are believed to be accurate to +20 psi hydrogen. After loading the capsules into the pressure vessel, it was filled with argon and raised to a pressure less than the desired pressure (PI), the hydrogen pressure was applied (after evacuation of the hydrogen line) and the system heated. The pressure was carefully monitored so as not to exceed P during heating. Isobaric heating was necessary since a pressure rise, above P fncrea4ed the solution of the melt phase and caused precipitation when the pressure was &adjusted to P This effect was most undesirable in the final run, where the mass of the glass was impor&' ant (see below). The temperature was normally held subsolidus for approximately 24 hours while at P to insure that the solution had permeated all parts of the starting material. The temperature was then raised isobarically to the desired temperature (T ). The capsules were held at PI and TI for 3-4 days. The experiment was quenched isobarica1ly. The recovered capsules were weighed and any weight loss amounting to more than 1-2 mg was cause to discard the capsule. The glass slugs were removed and placed in a hardened steel mortar. The fragments were put into an agate mortar and recrushed with an agate pestai under alcohoJ to < 350 mesh. The resulting powder was dried overnight and loaded into 5 mm (dia.) Pt capsule~with approxi~tely 2-3 wt% of distilled H 0 to insure vapor saturated melting during the subsequent refusion. The necessfty for a s$cond fusion was indicated by autoradiography which clearly showed inh~ogeneous distribution of the radioactive tracer after a single fusion (FLYNN, 1977). The samples were subjected to the same pressurization and heating procedure as explained previously. The recovered glass was checked for tracer homoaeneitv and acceotable samples recrushed and sized to splits of 40-60 mesh and less thaii60 mesh. Portions of the ~60 mesh fraction were removed, chemically treated (see below) and analyzed by gamma-ray spectroscopy (the radiochemical procedure taken from FLYNN, 1977). IThe radioactive fluid was obtained by neutron activation of the rare earth "stock" solutions menttoned previously. 2This error includes short and long term drift.

688

Radioactive

R.

glass

T.

F_ynn

anE

C.

ii. ?urn'lam

(40-60 mesh) was loaded into preweigheci, perforated,

2 mm diameter

Pt

inner capsules and the glass weight recorded. Crimped and perforated inner capsules were employed to facilitate complete mechanical separation of the glass and solution after the experiment. The radloactive solution used for the glass doping was alfquots of the same solution used in the glass synthesis. The composite solution was then added to a preweighed 5 nnndiameter Pt capsule containing the inner capsule. The solution weight and total weights were recorded and the total rechecked after welding the outer capsule. The solution/glass weight ratio normally used in the partitioning experiments was approximately 5:l (200 mg solution to 40 mg glass). A large solution/glass ratio minimized the weight fraction of the fluid phase lost to inner capsule. Contamination of the solid by inefficiently separated solution was not significant since the rare earth elements strongly partition into the melt (glass). The capsules containing radioactive solid plus vapor phase were held at P and T for 4-5 days. After an isobaric auench. the caosufes were removed and checkeldfor w&isht loss.

Leaktight capsules were frozen in liquid nitrogen, clipped open and imsersed in a i;eflon evaporating dish under 15 ml of a 2500 ppm lanthanum carrier solution. The lanthanum carrier solution was added to reduce adsorption of the radioactive rare earths on the capsule walls (CU~_LER~ et al., 1970). After 20 minutes, the outer capsule was removed and the inner capsule returned to the same carrier solution for an additional 15 to 20 minutes. The rinse solution containing the experimental vapor phase and carrier was set aside. After drying,the inner capsules and glass were weighed and transferred to teflon evaporating dishes. HF and HC103 (in the volume ratio of 5:l) were added and the glass allowed to digest at 50-60°C. After digestion was judged complete, 15 ml of the lanthanum carrier solution was added and the temperature was increased to 70"80°C for an hour. The platinum inner capsules were removed from the evaporating dishes, dried and weighed. Any apparent weight loss was used to adjust the weight of the recovered glass. Standards were prepared from the same radioactive solutions that were used to prepare the glasses and initial solutions. Ten ml of HF and 2 Ml of HC103 were thgn added to the solutions containing the aqueous phases and they were also heated to 70-80 C. All standards and unkn~ns were evaporated to dryness on a hot plate overnight. The residue was dissolved at 70-80 C in 5 ml of 6N HCl. Next, 15 ml of distilled water and 20 ml of a 4 wt% NaF solution were added to precipitate the rare earths as fluorides.' After 20 minutes, each solution was acid neutralized. Neutralizationminimized degradation of the glass fiber filter paper (Reeve Angel "Ultrafilter"and Whatman GF/F) used to collect the precipitate during suction filtration. Residual radioactivity of the filtrate was ~1% of the radloactivity present before precipitation and filtration. The precipitates were monitored in the gamma-ray spectrometer employing a Ge(Li) detector and the integrated peak areas compared to standards after background corrections (FLYNN 1977). Mass balance calculations showed that normally greater than 95% rare earth recovery was achieved compared to initial masses (this 95% value considers both the glass and vapor phases). The weights of the recovered phases were corrected to account for the soiubiljties the respective phases in each other. Since the starting glasses were

of

run te~erature and pressure (PI and T ), it was assumed that they did not dissolve any additional H 0 during the final run. however, with the large solution to glass weight ratio, disso?ution of the glass was a major factor and was monitored by closely recording the weight changes of the inner capsules. Conversely, addition of a small amount of solute to the large amount of fluid did not make an appreciable difference in the final weight of the fluid phase. After phase weights were corrected, ppm values for REE were calculated and the resultant partition coefficient was compared with the ppm ratios in the starting material. In this manner, transport direction was established and the equilibrium partition coefficient bracketed. Figure 1 graphically displays the application of the experimental procedure. RESULTS A71 the experimental data are tabulated in FLYNN (1977). Table 2 summarizes all the

partition coefficient data. Figures 2 through 5 show the results of the partitioning of Ce, Eu, Gd and Yb using the Spruce Pine pegmatfte and various chloride solutions (4.0 kb, 8OO'C for 4 to 5 days). The artition coefficient is a linear function of the cube of the chloride mo?ality (Ce, Gd, YbP or the fifth power of the chloride molality (Euf. The chlorfde molality is shown on the top abscissa for reference. Each plotted partition coefficient represents at least four independent determinations and in some cases considerably more (see Figure 1). All values plotted and reported in Table 2 represent experimental 'The rare earths were precipitated as fluorides rather than oxalates (see CULLERS et al. 1973, or ZIELINSKI and FREY, 1974) because of the nature of the precipitates. The fluoride precipitate occurs as a gel-like material and bonded to the filter paper while the oxalate tended to form granular crystals which did not bond at all to the filter paper.

689

Determination of rare earth partition coefficients

Summary of Experimentally Derived Partition Coefficients* in the Presence of Chloride

Table 2. Melt

CotlIp**

mcl

P(kb)

T('C)

Eu

Ce

__

Gd .0084+.0021

Yb

__

SP

.449t.023

4.0

800

--

SP

.646?.023

4.0

800

.0286%.0071 .0613+.0153 .0204+.0051 .0170?.0042

SP

.7162.036

4.0

800

.0333+.0083 .0690+.0172 .0244?.0061 .0217*.0054

SP

.753?.038

4.0

800

.0417+.0104 .0980+.0245 .0370+.0093 .0233+.0058

SP

.860~.043

4.0

800

.0666*.0166 ,1587t .0397

.0455+.0114 .0303:.0076

SP

.896?.045

4.0

800

--

.0500*.0125

SP

.914*.046

4.0

800

.0893+.0223 .2778*.0694 .0546+.0137 .0417~.0104

SP

.914t.O46

4.0

800

--

SP

.914k.O46

1.25

800

.1887+.0472 .1191+.0298

.1818+.0455 .0833+.0208

JN

.914+.046

4.0

800

.0143+.0036 .0172+.0043

.0137?.0034 .0149?.0037

.1389*.0347

.0588+.0147

__

__

*The values presented are ppm vapor/ppm melt. The "crossover" values are used i.e. arithmetic average of the "zone of convergence" shown in Figure I. **SP = Spruce Pine; JN-Jadeite-Nepheline ***Experiment conducted with an open membrane.

"reversals". Also, all experiments containing europfum were conducted on the QFM (quartzfayalite-magnetite)buffer, except the one noted in Figure 4 which was conducted with an open membrane and is more oxidizing than the rest. Figure 6 sunxnarizesthe partition coefficients determined in chloride solutions, using the Spruce Pine pegmatite as the melt phase at 4.0 kb, 8OO'C. The partition coefficients for the other rare earths are interpolated f the experimental values for Ce, Gd and Yb. The p8m possibility of significant amounts of Ce + is considered unlikely under the experimental conditions. Over the range of chloride molalities examined (0.45 mC1 to 0.91 mC1) the partition coefficients increase with increasing chloride concentration. The partition coefficient of gadolinfum for example, changes by a factor of seven (Fig. 4). Large positive europium anomalies are observed in the chloride runs made under QFM buffer conditions (Fig. 6). The absolute size of the anomaly is generally independent of the chloride molality. The trivalent rare earths show a consistent relative fractionation, with preference for the vapor phase decreasing in the order, Ce> Gd> Yb. Figure 6 also shows the results of the fluoride ex eriment (.97 mF). A significant europium anomaly or relative fractionation among t e trivalent rare earths was not found. An attempt was made to ascertain the effect of carbon dioxide (mole fraction = .14. mC0 = 6.5) but kinetic problems prevented an approach to equilibrium and no reversal was obta?ned. Thus the results are not plotted, but the data indicate that carbon dioxide is not a complexing agent under the experimental conditions in support of the findings of ZIELINSKI and FREY (1974).

R

Figure 7 shows the results of three runs designed to test the effect of pressure and cMnposftlon. The most striking pressure effect is the marked increase in the partition coefficients of the trivalent rare earths with decreasing pressure. The partition coefficient for qadolfnium increased bv a factor of 3 in resoonse to the Pressure decrease from 4.0 to 1.25-kb. In contrast, Eu partitioning has an opposite dependence on pressure which causes the anomaly to change sense as a function of pressure. The fractionation of the trivalent rare earths is only reflected by ytterbium. The values obtained on the jadeite-nephelinecomposition are much lower than those derived from the Spruce Pine experiments. bv a factor of sliahtlv more than 3. The europium anomaly in the jadeite&epheline <s is distinctly di?ferent from that observed in the Spruce Pine pegmatite melts under similar conditions. In the jadeite-nephelfne composition, the partitioning of europium is only slightly anomalous with respect to the

690

Fig. I.

R. T.

Flynn

and

C, W. Burnham

Illustration of transport direction and convergence of experimental results on the equilibrium partition coefficient for gadoliniu~ at 4.0 kb, 800°C, and ,896 mC1. The shaded area represents the zone of convergence i.e., the equilibrium partition coefficient for gadolini~ under the experimental emions.

trivalent rare earths. Melt composition is obviously very important in ~ete~ining the europium distribution,

DISCUSSIONOF ERRORS The error in the determination of the stated partition coefficients is estimated to be +25% of the reported value, while that of the stated chloride molality is estimated to ~5%. Numerous reversed determinations of partition coefficients under a particular set of conditions gave a two sigma error of t25% (Fig. 1). The largest single source of error in the experimental technique Is incomplete recovery of the aqueous phase. Additional error is introduced by counting statistics, chemical procedures, glass recovery, phase contamination and weighing. The error due to incomplete recovery of the vapor phase was estimated by sealing a known a~unt of radioactive solution into several 5 mm diameter Pt capsules each containing a perforated inner capsule identical to the.actual experiments. The inner capsules contained small pieces of metal rod to simulate the glass slug. These "dumny" runs were pressurized to run pressures, the capsules then opened and the aqueous phase recovered (they were not heated). After the experiment, as much as 10% of the solution was lost, though some capsules showed no loss. This recovery problem Is likely to be more severe when silicate glass is used as the solid ph&se, because it has a tendency to clog some of the holes in the inner capsule. Three standards were analyzed with each group of vapor samples and three with each group of glass samples. The agreement between the standards gives an estimate of the errors involved after the recovery step; those due to the chemical and radiological

Determination of rare earth partition coefficients

I

fig.

2.

~~~~S. *

ma

ubs9 I

024

Q84 I

691.

(x03 f

~x~ar~~~ta~ results for the ~artit~u~~~g of cerium between aqueous chloride solutfon and Spruce Pfne pegmatfte melt at 4-O kb and BOf&. Length of error bars indfcate an estimate of total errors (+5X in determinfng the mC1).

The agreement between

replicated standards was always within 56%

of the true

The final chloride modality was assumed to be within 5X of the initl'alchloride molality. Because chloride solution was mixed with radioactive tracer solution and distilled water, most of the error is attributed to weighing of the respective soluttons. Solution loss by vo~at~~izat~on during welding was minimized by carefully weighjng the capsule before

R. T. Flynn

692

and C. IV. Burnham

0.:

2u

0.2

Y

0.1

4

Fig. 3.

Experimental results for the partitioning of europium between aqueous chloride solution and Spruce Pine pegmatite melt at 4.0 kb, 800°C, and under QFM oxidation conditions. Errors are the same as Fig. 2. The black square data point is the partition coefficient for the open membrane experiment.

and after welding. DISCUSSION runs made at 4 kb and 800°C on the Spruce Pine pegmatite (QFM buffer) exhibit a surprisingly simple relationship. The partition coefficients of cerfum, gadolinfum and ytterbium are all linearly related to the cube of the chloride molality; while europium is apparently related to a fifth power of the chloride molality (Ffgs. 3-6). Under the f0 conditions of these experiments, cerfum, gadolinfum and ytterbium are trivalent, while europ? urnis presumably a mixture of divalent and trivalent ions. Thus the general relationship for the trivalent rare earths is expressed mathematically as:

The results of the chloride

IX d$ dmClx where

=C

I=

cation e.g., Ce3+, Gd3+, Yb3+

x=

ionic charge

C=

slope or constant

flf

Determination of rare earth partition coefficients

I

a00

0.0

Ffg. 4.

I a2

I

I

“,a3a6

08

I

I

I

693

Experimental results for the partftfonfng of gadolfnfum between aqueous chloride solutfon and Spruce Pine pegmatfte melt at 4.0 kb and 8OO'C. Errors are the same as Fig. 2.

The ~latfonshfp for europfum is co~lfcated by the presence of Eu2' ion as well as Eu3+ ion. HOLLAND (1972) found a similar relationship of chloride molality to cationiccharge for several mono and divalent cations when he examined melt/vapor partitioning between chlorfderich fluids and a granftfc melt at pressures from 1.4-2.4 kb and 770-88O*C. Because the linearity persisted up to chloride concentrations of 6 molal, the ratio of activity coefficients apparently remained constant. Holland's findings imply that the rare earth data could be extrapolated considerably beyond the chloride range examined in this study. In each of the Spruce Pin experiments (Fig. 6, Table 2), the partition coefficients decrease in he observed trend is in the same order as the radii of the order Eu > K KpEu2+ Ee the catfons p> ' K';e$+K$ 'BdJ+ > vbS+ The fractionation effect may be due to the relative stabilities of aqueous complexes and/or size restrictions affecting melt substitution. At chloride molalftfes of this study, it is likely that the melt effect should be dominant, as the partition coefficients of all the rare earths strongly favor the melt phase. Furthermore, the relative stabilities of rare earth chloride complexes should leave more heavy REE in the vapor phase. This is in the opposite sense to observations so aqueous complexfng is not as i~ortant as melt sfte availability. By analogy with the crystal chemical arguments of NA~SAWA and SCHNETZLER (1971), for mineral melt partitioning, the size of the site which the rare earth element can occupy is seen as the crftlcal differentiation factor. The data shows that the melt is preferentially enriched in the smaller rare earths. Thus, the melt site into which the trivalent rare earths are partitioned is believed to be octahedral with a preferred cationic radius less than that of ytterbium (.95 Angstrom). The melt substitution site is probably octahedral because rare earths are not known to occur In 4-fold coordination in sflicate minerals, and spectroscopic work by ROBINSON (1974) on the coordination of ErS+ in silicate glass demonstrated an octahedral site preference. CULLERS et al. (1973) found no fractionation of the rare earths in their melt/water experiments. Although the glass composition used by Cullers et al. was very similar to

694

T.

R.

Flynn

and

C. W. Burnham

mCI a59

I

474 1

I

CM4 I

0.93

sot

0.04

g$ a02

0.00

I

I

0.2

I

I

I

0.4

mCI Fig. 5.

I

0.6

I

I

0.8

3

Experimental results for the partitioning of ytterbium between aqueous chloride solution and Spruce Pine pegmatite melt at 4.0 kb and 8OOOC. Errors are the same as Fig. 2.

Spruce Pine composition the experiments were conducted in the presence of virtually pure water. The extremely high preference of rare earths for the melt relative to pure water greatly increases the analytical error of measured partition coefficients and limits the detection of fractionation trends. The large positive europium anomalies in the 4.0 kb Spruce Pine experiments are significant beyond any experimental error. Relative to the exclusively trivalent rare earths europium is excluded from the melt. It is proposed that the Eu2+ cation (approximately20% larger than the largest trivalent rare earth) is in 8-fold coordination in the melt and is excluded from the melt because of a paucity of suitable sites. The evidence for this is two-fold: first, if the trivalent rare earths are in 6-fold coordination in the melt, and this 6-fold melt site prefers the smallest rare e rth (as seen in the relative fractionation trend), it seems unlikely that the much larger Eu2+ could occupy the same site. Se ondly, in silicate minerals the largest europfum anomaly is found in plagfoclases where EuEt partitions into the 8-fold calcium sites. MORRIS (1975) has shown that Eu2+ will partition into octahedral sites in minerals but prefers 8-fold coordination if it is available. 2+

If we assume an 8-fold coordination for the Eu

melt-fluid interaction would be:

ion, then a possible reaction for the

EuClj+EuC12V+30HV1m+20HV111~EuV1mOH3+EuV111mOH2+5C1V rewriting in terms of the equilibrium constant, K=

" a~U"IoH3aW*OH2(acl)

(2)

5 (3)

a~~C13a~uC12(a~H)5 where superscript "v" denotes vapor phase and superscript "m" denotes melt phase and the Roman numerals refer to the cation coordination number. In the 4.0 kb Spruce Pine experiments

Determination of rare earth partition coefficients

s E

695

.'

:

a E k \ z 2 *

.os

----__ .Ol .mCI =.449

ii

a E DOS E

Co

Wd

Sm

Eu

Dd

Dy

Er

Yb

Rare Earth Element Fig. 6.

A sumnary of the experimental results on the Spruce Pine pegmatite composition conducted at 4.0 kb and 8OO'C. The .8g6 mC1 gadolfnium ciatapoint has been omftted for clarity.

all the factors in equation (3) are constant except the chloride activity. Thus, if equation (3) is valid, the partition coefficient for europium should vary linearly as the fifth power of the chloride molalfty. Figure 3 shows that such a relationship is possible within the experimental error. Both carbon dioxide and fluorine were employed as possible complexing a ents in separate ion coefficients experiments. however, experimental "reversals" on the respective part19; were not obtained. Kinetic problems preclude an accurate estimation of the equilibrium partition coefficients for the CO -H O/melt experiments, but the data (unplotted) indicate that at 6.5 mC0 the equflibrium Bar?ition coefficient is greater than eighty to one in favor of the Spfuce Pine melt at 8OO'C and 4.0 kb; apparently carbonate is not an effective complexing agent under the conditions of the experiment. The fluorine data, while not plagued by kinetic problems, is difficult to interpret since melt conposition was severely affected by the HF solution. Since hydrofluorlc acid was used as the fluorine carrier, the final melt composition was much poorer in silica and alumina, and a direct comparison with the chloride data, at a similar molallty, is not possible. However, the data do show that fluoride is effective as a complexer under magmatic conditions and possibly as effective as chloride. According to MUWHAM (1967) fluorine partitions largely into the melt at magmatfc conditions whereas chlorine partitions largely into the vapor. Thus, while fluorine may rival chlorine as a complexfng

696

R. T. Flynn

K

1egmd

.05

E R

_

/I -

Ce Fig. 7.

and C. W. Burnham

Spruce Pine 4kb,8W*C Spruce Pine 1.25 kb, 80ooC Jd-We mix 4kb.800eC

8d

Sm Eu 6d

Dy

Rare

Earth Element

Er

Vb

A sumnary of the experimental results demonstrating the effects of total pressure and composition of sfllcate melt. All the experlments shown were conducted In the presence of a .914 mC1 solution under QFM buffer conditions.

agent, chloride is more important by virtue of its greater abundance in hydrothermal fluids. A jadeite-nephelinegel (75 wt.% jadeite) was prepared in order to examine the dependence of partition coefficients on melt composition. In particular, 1t was thought that variatlon in aluminum content mlght play a significant role in the substitution of the trivalent rare earths into the melt phase. Also, the jadeite-nephelinecomposition was employed since it is completely liquid at the same pressure and tmerature conditions as the experiments on the Spruce Pine pegmatite. The water saturated jadeite-nephelinemelt is considerably less polymerized than the water saturated Spruce Pine melt since it contains more water, on a molal basis, and less tetrahedrally coordinated cations, principally slllca. Because of the higher water content there should be more octahedral sites available for substttution by the rare earths in the jadeite-nephelinenelt (BURNHAM, 1975). Depolymerlzationreduces the relative fracttonation effect since the melt structure is "open" enough to easily acconmudate all the trivalent rare earths in octahedral coordinatfon. The europlum anomaly has apparently decreased In the jadeite-nephellneexperiment despite equilibration of both this experiment and the 4.0 kb Spruce Ptne experiment at equal oxygen fugacities (log oxygen fugacity = -14.65). In the jadeite-nephelinemelt, there are more 8-fold sites than the 4.0 kb Spruce Pine melt due to differences in composition and there are more B-fold sites due to the increased water content.

Determination of rare earth partition coefficients

697

The importance of dissolved water as a structural modifier of the melt was investigated by varylng the water pressure. In order to ensure melting of the Sprucg Pine composltlon, the largest pressure decrease posslble at a constant temperature of 800 C was a lowerlng from 4.0 kb to 1.25 kb. It was reasoned that a pressure decrease would reduce the water fugaclty in the melt, increase polymerization, and by the prevlous arguments, reduce the number of favorable substitutional sites for rare earth elements. The data (Fig. 7, Table 2) lend qualitative support to thls argument as the melt becomes a less favorable host at lower pressure. In addltion, the fractlonatlon scheme consistently seen at 4.0 kb has been altered at 1.25 kb. At 1.25 kb water pressure, the partltfon coefficients of cerium and gadolfnlum are equal, whereas that of ytterbium Is more strongly in favor of the melt. Apparently, the 1.25 kb melt has fewer octahedral sites and reduced capablllty to accept catlons of different site. Perhaps the change in selectivity is due to a change In the geometry of the melt octahedral site. This might explain why the cerfun and gadollnfum partition coefflcfents are slmllar at 1.25 kb but different at 4.0 kb. Such behavior in the observed partition coefficients of phenocryst/matrlxpairs have been similarly ascrlbed to the existence of rlgid substitutional mineral sltes (ONUMA et al., 1968; JENSEN,1973). Another possibllfty is that the partitlon coefficient for gadolinfum Is in error and that the "true" value Is lower. However, the partitlon coefficients for cerlum and gadolinium were consistently the same in all the runs at thls pressure and such an error seems unlikely. The europium anomaly in this low pressure experiment is not resolvable. On a mole fraction basis, one would expect the 1.25 kb Spruce Plne melt to have more 8-fold sites than the 4.0 kb melt of he same conposition due to the decreased water content of the 1.25 kb melt. Thus, the Euh+ should partltlon into the melt phase more readily than trivalent rare earths in the low pressure melt. If dissolved water is one of the geochemical controls in the rare earth substitution In the melt phase and chlorine one of the geochemlcal controls on rare earth substitution in the vapor phase, the followlng reactlon seems feasible for the partltfonlng of a rare earth element between melt and solutlon: GdCl; + 30Hm : Gd(OH); + 3Clv

(4)

where superscrlpts v = vapor phase m = melt phase the equilibrium constant for reactlon (4) Is: K=

amGd(OH) (avCl)3

(5)

aVGdC13 (amOH) rewritlng In terms of molalitles, K'(mm0H)3

mGd(OH3) mVGdC13

=

(mvC1)3

K' = partltion coefflclent if activity coefficients are constant It is understood that Gd could be any trivalent rare earth ion. According to reaction (4). an Increase in the chloride molallty should Increase the partitioning of the rare earths into the vapor phase, with the increase proportional to the cube of the chloride molallty (ratios of the actlvlty coefflclents assumed constant). Conversely, If the OH molallty of the melt is increased, the partltloning of the rare earths into the melt Is Increased in proportion to the cube of the OH molallty. In order to test thls model, It Is necessary to know the OH molallty of the melts examlned. Uslng the thermodynamic model of BURNHAM and DAVIS (1974) one can calculate the OH molallty of any malt of known composition and mole fraction H 0 as a function of pressure and temperature. The mole fraction of hydroxyl in the melt can be obtalned by equatlon (7) below:

(7)

R. T. Flynn and C. W. Burnham

698

To obtain the mass of melt, t

= (X;) (18.02)

MSil.

(8)

= Me (1 - X;)

Mm = f

(9)

+ MSi,

(10)

where,

I( = mass of water in the melt MSil.

= mass of silicate in the melt

Mm = mass of melt Me = mass of rock melt equivalent to one GFW of NaA1Si308 calculated according to (1974). and to obtain the OH molality in the melt, m!r!

= (1iOO) (xtH)

(11)

m

Table 3 shows these calculated values of melt OH molalities and the corresoonding experimental partition coefficients using gadolinium as an example. The dependence of the partition coefficient on the cube of the OH molality, as required by equation (5), is followed by the Spruce Pine pegmatite at both 4.0 kb and 1.25 kb. However, partitioning into the jadeite-nephelinemelt is too large for the calculated OH molality, suggesting that the OH molality is not the only control. According to the model of BURNHAM (1975) the reaction of water with a silicate melt is to dissolve as 2 OH molecules with the melt supplying the oxygen for the second OH molecule. Supply of a "bridging" oxygen depolymerizes the melt creating octahedral sites. In the Spruce Pine pegmatite melt, this is the only reaction creating octahedral sites, but in the jadeite-nephelinemelt, a second reaction is believed to take place which creates additional octahedral sites. These sites are created by the existence of a portion of the aluminum in 6-fold coordination in the jadeite-nepheline melt which breaks down the silicate framework in a way similar to the water interaction. Perhaps a more appropriate general relationship for silicate melts would be a dependence of the partition coefficients on the molality of octahedral melt sites cubed. The implications of this study can be conveniently separated into those oertaining to the solution and those pertaining to the melt. Dissolution of melt in the vapor phase should not alter the melt rare earth content to any significant extent because melts do not contain large amounts of water on a weight percent basis and at reasonable chloride molalities, the rare earths are partitioned into the melt phase. KILINC and BURNHAM (1972) have shown that for a granitic composition at 4.0 kb and 800°C, the KC1 is approximately 40 to 1 in favor of the vapor. Thus for an original melt with 1000 ppm pC1 and 4 wt.% water, the final chloride concentration in the separated vapor phase will be 975 ppm or .7 mC1 if the system is closed. If the system is open, groundwater addition is most likely to dilute Table 3.

Comparison of calculated hydroxyl molalities and experimentally derived partition coefficients.

Melt camp.

S*

P s* P

JN** *S

P

mC1

mOH

KGd P

p(kb)

T('C)

.914

1.25

800

4.42

.1818+.0455

.914

4.00

800

6.36

.0546+.0137

.914

4.00

800

8.60

.0137+.0034

= Spruce Pine

**JN = jadeite-nepheline

Determination of rare earth partition coefficients

699

the concentration of chloride in the vapor phase and thereby decrease the partitioning into the vapor. The simple re'stionship found between the partition coefficient and chloride molality (equation (1) ) in this study, confirms that found by HOLLAND (1972) for several other cations and implies that a similar simple relationship probably exists for any cation of a single valence able to form a stable chloride complex under magmatfc conditions. Theoretically, one determination of the partition coefficient at one chloride molality and knowledge of the cationfc charge would permit calculation of the partition coefficient at other chloride molalitfes. In addition, the relationship predicts that cations of high charge should be extremely sensitive to chloride molality. Further work is needed to substantiate such predictions. Compared to the vapor, the dissolved water in the melt has a significant effect on the melt rare earth abundances. The present work and low temperature spectroscopy by ROBINSON (1974) support an octahedral site preference for the trivalent rare earths in silicate melts and the present work indicates that this preference is related to a cube of the available octahedral melt sites. The findings should pertain to the partitioning of trivalent rare earths between an_yphase and a melt. This may explain why crystal/basalt matrix oartition coefficients are consistentlv smaller than crvstal/andesitematrix partition coefficients (NAGASAWA and SCHNETZLER, 1971). According to the model of BURNHAM (1975) basalt should have more octahedral melt sites than andesite at comparable water pressures. This model may partially explain why the partition coefficients for some phenocryst/matrixpairs increase when in equilibrium with silicic melts (ARTH and BARKER, If one assumes that dissolved water is one of the chief mechanisms for creating 1976). octahedral sites in silicic melts, then some of the variability in measured phenocryst/matrix pairs could occur due to varying melt water pressures. This variability could occur without any chemical evidence in the final product. It further implies that partition coefficients measured on phenocyrst/host lava pairs cannot be applied to plutonic rocks if these plutonic rocks contained significant amounts of water. From other experimental work on crystal/melt oartitionino, e.q., DRAKE and WEILL119751, this "volatile" effect on the oartition coefficient-come more significant than-the temperature effect, since the rare earth partition coefficients between some comnon rock-forming minerals and melt are rather insensitive to temperature. Furthermore, andesitic "volcanic" partition coefficients should not be applied to volcanic alkaline rocks since these alkaline rocks should contain a higher proportion of octahedral sites. Of course, this objection assumes that the crystals Rrecipftated after the lava was extruded and in the-presence of very little water. Therefore, in order to use the published partition coefficients of phenocryst/host lava pairs it may be necessary-to ascertain the sequence of crystallization. the data indicates that the melt phase must be treated as if it were a phase of definite structure with the ability to fractionate the rare earths. However, unlike the mineral phases, the melt's structure changes during differentiation and therefore the partitioning behavior of the rare earths changes. This phenomenon must be considered in petrogenetic models which employ partitioning of the rare earths between melt and co-exfstfng phases.

In summary,

CONCLUSIONS The vapor/melt experiments suggest the following: 1.

The rare earths are complexed by chlorine at magmatfc conditions. This is supported by the increase in the vapor/melt partition coefficient as the vapor chloride molality increases. The partitioning of the trivalent rare earths between melt of constant composition and an aqueous vapor is a cubic function of the chloride concentration of the aqueous phase. The partitioning of the trivalent rare earths between an aqueous vapor of constant composition and a silicate melt is a cubic function of the molality of OH in the melt; a property which increases with increased dissolved water. Application of published phenocrystlmelt rare earth partition coefficients to a particular phenocrystlmelt system is complicated by the unknown volatile content of the co-existing melt. In addition to dissolved volatiles, the bulk composition of the melt determines the number of available octahedral sites. If octahedral site populations can be calculated for v ri us melt compositions, one may have a quantitative model for predicting phenocrystTme?t partitioning.

R. T. Flynn and C. W. Burnham

700

Sincere appreciation is extended to the staff of Breazeale Nuclear Reactor of The Pennsylvania State University, in particular Dr. Jack Smotzer and Dr. William Jester, for their donation of This paper benefited from the careful reviews of irradiation time and counting facilities. This research was supported by the National Science FounRobert A. Zielinski and Nick Davis. dation grants DES-73-00247 and ERA 73-00247.

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