element partitioning between olivine and silicate melt under high pressure

Phys Chem Minerals (1995) 22:411-418 PMICS CHIMIITRY MINIRALS �9 Springer-Verlag 1995

Element Partitioning between Olivine and Silicate under High Pressure Toshihiro Suzuki, Masaki Akaogi

Department of Chemistry, Gakushuin University, Mejiro, Toshima-ku, Tokyo 171, Japan

Received November 7, 1994/Revised, accepted May 8, 1995

Melt

Abstract. Element partitioning between olivine and silicate melt has been investigated at pressures 1-14 GPa, by using a 6-8 type multi-anvil high pressure apparatus. In order to observe systematics in the partitioning of trivalent ions, Li was added to the starting materials in order to increase the concentration of trivalent ions in olivine. With increasing pressure, it was found that partition coefficients of most of the elements gradually decreased. Trivalent ions generally showed parabolic pattern on partition coefficient - ionic radius diagram. When pyrolite-like material was used as the starting material, partition coefficient of A1, DAb gradually increased with increase in pressure while the partition coefficients of the other elements decreased, and the DA1 deviated from the parabolic pattern of other trivalent ions. The deviation of DAI from the D pattern of the other trivalent ions was also found when olivine was employed as main component of the starting material. This result may be ascribed to the compositional change of coexisting silicate melt with increase in pressure.

Introduction

Because of its importance for the geochemical approach to the evolution of the Earth, much effort has been de- voted to the study of element partitioning between minerals and silicate melts. Development of analytical meth- ods has made it possible to expand the information on partitioning behavior to the trace elements, and many data have been accumulated at around normal pressure. Since the genesis of magma is not restricted to the near surface region of the Earth, the pressure effect on the partition behavior is very important for the investigation of the magma genesis and evolution of the Earth. Al- though high-pressure experimental studies on element partitioning have already been made, it was very recent

Correspondence to: T. Suzuki

that element partitioning between melt and high-pressure magnesian silicates, such as majorite and silicate- perovskite, was investigated (e.g., Kato etal. 1988; Drake et al. 1993), and pressure effect on partition coefficients has not yet been systematically investigated.

From the observation of partitioning relation between phenocryst and groundmass in the lava, the relation between partition coefficients and ionic radii was clearly shown by Onuma et al. (1968). Their results indicate that when partition coefficients are plotted in partition coefficient-ionic radius (PC-IR) diagram, peaks which correspond to the most suitable size of cation in the site of the crystal structure appear in the diagram. In the case of olivine, the maximum of the partition coefficient is located near 70 pm, which corresponds approximately to the radius of Mg 2+ ion (Matsui et al. 1977). However, sizes of the most of divalent ions are larger than Mg 2+, and the peak position of partition coefficient can not be determined from the data of the divalent ions. Therefore, the peak position should be determined from the partitioning data for trivalent ions, whose radii vary more widely than those of divalent ions.

In this paper, we intended to observe element partitioning between olivine and silicate melt in the pressure range of 1-14 GPa and observed the change of partition coefficients with increase in pressure. We also examined the effect of the composition of the silicate melt and of olivine on the partition coefficients. It is well known that trivalent ions are scarcely dissolved into olivine, making it difficult to observe the partition behavior of trivalent ions. Therefore, we added to the starting material a small amount of Li § whose ionic radius is close to Mg 2+. Li + is expected to occupy the MI or M2 site of olivine structure to replace divalent ions, and hence, trivalent ions must be incorporated in the olivine structure to maintain the electrostatic neutrality. Finally, we confirmed that the effect of addition of Li + was small enough to examine the element partitioning behavior.

412

Experimental Method

The present experiments were performed by using a split-cylinder type 6-8 multi-anvil high pressure apparatus. Pressure values at room temperature were calibrated against load (oil pressure) of the press, based on the pressure fixed points: Bi I - I I transition at 2.55 GPa, Ba I - I I transition at 5.5 GPa, Bi I I I -V transition at 7.7 GPa, Ba I I - I I I transition at 12.0 GPa and semiconductor- metal transition of ZnS at 15.5 GPa. Pressure calibration at 1000~ C was also attained by using quartz-coesite transition at 2.9 GPa (Bohlen and Boettcher 1982), olivine-spinel transition of F%SiO4 at 5.2 GPa (Yagi et al. 1987), rutile-c~PbOz transition of TiO2 at 7.2 GPa (Akaogi et al. 1992), coesite-stishovite transition at 9.1 GPa (Akaogi and Navrotsky 1984) and olivine-/? spinel transition of MgzSiO4 at 14.0 GPa (Akaogi et al. 1989). The tungsten carbide anvils used in the present experiments were truncated in triangular faces with either 8 or 5 mm edge length. The 8 mm anvils were used for the experiments up to 8 GPa, while the experiments with pressure greater than 9 GPa were performed with 5 mm anvils. The sample assemblies used in the present experiments are illustrated in Fig. 1. The hot junct ion of a P t - P t / 13%Rh thermocouple was placed either at the center of the sample container, or in contact with the outer surface of the Re-heating element. Since the thermocouple could not be used above approximately 1700 ~ C, temperature was estimated from the applied eletric power, by extrapolating the temperature-electric power relation which was obtained below 1700 ~ C.

We prepared four different types of starting materials, as listed in Table 1. Ratios of SiO2, MgO, FeO, CaO, and A1203 of Materi- al-PL were based on pyrolite model (Ringwood 1975), with 1- 4 wt% added NiO, MnO, V205, CrzOa, SczO3, In203, Tm203, LazO3, Na4SiO4 and Li2SiO3. In order to expand the stability field of olivine as the liquidus phase under high pressure, 10 wt% of San Carlos olivine composition was added. The M g / ( M g + F e ) mol ratio of the resulting Material-PL was 0.90. In order to increase the concentrat ion of trivalent ions in olivine, Li20 component was added to the starting material. We also prepared Material-PY which did not contain LizO component, but concentrations of the other components were the same to those of Material-PL. The influence of Li component on parti t ion coefficients for the other elements was tested by using this starting material, al though the accuracy of the analysis of the trivatent ions was significantly re- duced because of their low concentration in olivine. In order to examine the effect of Mg/Fe ratios on parti t ioning behavior, Mg/ ( M g + F e ) ratio of Material-PF and -PG were decreased to 0.36 and 0.06, respectively, by replacing some amount of MgO with equimolar FeO, while the other elemental ratios were not changed. San Carlos natural olivine was used as the main component of Material-FO, and the other minor components were added in the same way as in Material-PL.

In order to prepare the starting materials described above, the following substances were mixed in the desired proport ions: San Carlos olivine, synthetic fayalite, wollastonite and Li2SiO3, and reagents of A1203, NiO, Mn304, VeOs, Cr203, Sc203, In203, Tm203, LazOs, and NagSiO,,. Composit ion of San Carlos olivine used in the present experiments is listed in Table 1. Fayalite was synthesized from a mixture of FezO3 and silicic acid by heating at 1180 ~ C for 30 hours under H2 : CO2 = 1 : 1 gas mixture environment. A mixture of CaCO3 and silicic acid was melted at 1500 ~ C for 2 hours and quenched into water to produce the glass with CaSiO3 composition. This glass was heated at 1250~ for 100 hours to crystallize wollastonite. Li2SiOa was produced by heating a mixture of LizCO3 and silicic acid at 1100~ for 30 hours. Other components were high purity reagents with guar- anteed quality.

These starting materials were held at 1-14 GPa and 150(~ 2450 ~ C for 1-120 minutes, and quenched isobarically. Recovered specimens were polished to sections to observe their texture and chemical composition. Chemical analysis was carried out with an automatic 4-channel EPMA system at the Ocean Research Insti- tute, the University of Tokyo. The accelerating voltage was 25 kV

a Mo electrode LaCrO3

Graphite Heater

nple

3%Rh ocouple

5mm

b Re Heater LaCrO3

MgO, Graphite

Sample

't-13%Rh rmocouple

I I 5mm

Fig . 1. Sample assemblies used in the present experiments with 8 mm anvil type (a), and 5 mm anvil type (b). Semi-sintered magne- sia was used as the pressure transmitting medium

T a b l e 1. Chemical compositions of the starting materials (wt%)

PL PY PF PG FO San Carlos Olivine

SiO2 40.18 39.56 33.89 32.15 33.51 40.51 MgO 35.71 35.73 12.12 1.98 38.32 50.32 CaO 2.30 2.23 1.87 1.84 1.41 0.08 AlzO3 2.44 2.37 1.99 1.96 1.00 0.01 FeO 7.15 7.20 38.71 52.90 6.65 8.72 MnO 0.71 0.90 1.17 0.99 1.28 0.06 NiO 2.48 2.04 0.90 0.73 2.67 0.32 V2Os 1.58 2.08 1.08 0.77 2.76 -- Sc203 0.86 0.85 0.85 0.67 1.50 - In203 1.62 2.33 3.01 1.79 2.84 - Cr203 0.96 0.65 0.95 0.71 1.55 - Tm203 2.53 2.31 1.77 2.06 4.43 - La203 0.84 0.92 0.90 0.71 1.38 - NazO 0.43 0.90 0.52 0.47 -- -- Li20 0.25 0.0 0.19 0.22 0.57 --

Mg/ (Fe+ Mg) a 0.90 0.90 0.36 0.06 0.91 0.91

mol ratio

413

and the beam current on the Faraday Cup was 20 nA. The standard for Si and Ca was wollastonite. Albite and hematite were used for the standard material for A1 and Fe, respectively. Standards for other elements were their pure oxides. Chemical composition was calculated by using ZAF correction method.

Since dendritic texture appeared in the quenched liquid, the composition of the liquid was obtained from the average of five analysis points of this region by using defocused electron beam of EPMA with 10-30 gm in diameter. The olivine which was in contact with this liquid was also analyzed 3 times using focused electron beam to ~ 1 gm, and the averaged composition was compared with the result for the liquid region, and one set of partition data was thus acquired. 5-10 sets of partition data were obtained in each run product, and their averages were used as partition data of the sample.

Since the sample was enclosed in graphite capsule in all of the runs, the oxygen fugacity was fixed as carbon coexisting condition. In order to check the valence of Cr and V in the reducing environment in the capsule, mixtures of Cr203 + MgO and VzOs + MgO were held at 5 GPa and 2100 ~ C for 60 minutes in the present sample assembly. Recovered specimens were examined by powder X-ray diffraction analysis, and MgCr204 and MgVzO4 with spinel structure were found as single phase material in the run products. These results strongly suggested that Cr and V existed as trivalent ions in the present experimental conditions. In contrast, the low oxygen fugacity associated with carbon coexistence resulted in the reduction of Ni 2§ to metallic Ni, which affected to the partition behavior of Ni significantly, as described in the next section.

Resu l t s

Experimental conditions are listed in Table 2. Examples of the observed chemical composition of the run products are listed in Table 3. Although a few wt% of Li20 component must be present in the recovered specimen, Li could not be detected by EPMA. However, Li § ion can be regarded as " t ransparent" to X-ray fluorescence of other elements, and we can regard that a small amount of Li in the sample does not make any serious disturbance in the determination of concentration of other components in the EPMA measurement. Hence, the observed compositions are directly used for the calculation of partition coefficients. In this paper, the partition coefficient, D, is defined as the mol ratio of concentration of elements in olivine and silicate melt.

D = C ~ melt - - x / - - X

where C ~ and Cx melt represent the concentration of an element x in olivine and silicate melt, respectively. As will be discussed in the next section, the lack of information of Li20 content in olivine and silicate melt make no serious error on the calculation of D values.

Figure 2 shows a typical example of the polished section of the run product. The center of the specimen is shown in the upper most part of this figure. Since the temperature gradually decreased from the center to the end of the sample chamber, the upper part of Fig. 2 was totally molten, whereas olivine existed as the liquidus phase in the region imaged in the lower part of this figure. The partition relations between olivine and contacting silicate melt were examined not only near the liquidus, but also under sub-liquidus conditions where silicate melt occupies interstitital regions between the olivine grains. The temperature variation of the re-

Table 2. Experimental conditions

Run No. Pressure Temperature Time (GPa) (~ (rain)

PL-1 1 1900 30 PL-2 5 2100 1 PL-3 5 2100 3 PL-4 5 2100 I0 PL-5 5 2100 30 PL-6 5 2100 120 PL-7 8 2200 22 PL-8 14 2450 30

PY-I 5 2100 60

PF-I 1 1800 25

PG-1 1 1500 60

FO-1 1 1800 30 FO-2 5 2100 60 FO-3 9 2200 30 FO-4 14 2400 30

gion where partition coefficients were measured is estimated to be ~ 60 ~ C in maximum, based on the measured temperature difference between center and edge of the sample chamber, which is approximately 10% of the temperature at the center. However, we can not find any difference in observed partition coefficients between the liquidus and subliquidus regions. The observed variation of D in each run product is mainly caused by the uncertainty of EPMA measurements. The effect of temperature variation within the sample on D values is small enough to be ignored in the present experiments, as will be discussed in the next section.

The change of partition coefficient with experimental time was examined at 5 GPa and 2100 ~ C, and the results are shown in Fig. 3. Although the D values changed during the first few minutes of the experiments, further changes were not observed in most of the elements in run duration time longer than 10 minutes. Hence, we employed the data from experiments with run duration time longer than 20 minutes. DNi still changed with run duration time at 120 rain, and standard deviation of observed DNi increased with time. This phenomenon may be ascribed to the gradual reduction of Ni 2 § to metallic Ni. Ni 2+ content of silicate melt decreased with time, and equilibrium between olivine and silicate melt might be difficult to achieve in the present experiments.

The observed D values of the elements are plotted in PC-IR diagram in which we use the ionic radii for six coordination under normal conditions (Shannon 1976). Figures 4a and 4b show the change of the observed D pattern with increase in pressure for divalent and trivalent ions, respectively, in Material-PL, and the results on Material-FO is shown in Figs. 5a and 5b. The DNI values are not shown in these figures because Ni 2 + may not be in equilibrium because of reduction of Ni 2 +, as mentioned above. La content of olivine was too small to determine D value and the DLa values are not also shown in these diagrams.

The D values of divalent ions decrease with increase in ionic radius. Although the D values of divalent ions

414

Table 3. Examples of composition of recovered specimens

PL-1 PL-6 PL-7 PL-8 PY-1

Olivine Liquid Olivine Liquid Olivine Liquid Olivine Liquid Olivine Liquid

SiO2 39.89 43.43 39.58 40.77 40.16 41.18 39.87 35.93 40.56 40.73 MgO 49.60 26.35 49.79 26.89 51.65 33.67 50.89 29.45 51.88 30.61 CaO 0.15 3.91 0.10 3.63 0.07 2.11 0.13 4.96 0.08 2.95 A1203 0.12 3.69 0.19 3.98 0.21 2.97 0.24 2.84 0.09 3.17 Na20 0.01 0.76 0.02 0.69 0.01 0.49 0.04 0.57 0.03 0.90 FeO 5.62 8.37 5.44 8.76 3.90 7.76 4.39 9.47 5.05 8.60 MnO 0.47 0.79 0.44 0.91 0.32 0.75 0.39 1.00 0.47 0.99 NiO 2.02 0.68 2.17 0.85 2.14 1.55 2.07 1.52 0.32 0.16 V203 0.30 1.59 0.29 1.67 0.18 1.20 0.19 1.80 0.23 2.31 Sc203 0.25 1.08 0.19 1.15 0.13 0.92 0.13 1.12 0.09 0.90 In203 0.25 2.13 0.25 2.33 0.15 1.58 0.21 2.60 0.14 2.52 Cr203 0.53 0.91 0.47 1.09 0.41 1.15 0.27 0.98 0.16 0.57 Tm203 0.15 3.45 0.09 3.58 0.07 2.56 0.06 3 .56 0.03> 3.29 LazO3 0.01 > 1.70 0.01 > 1.25 0.01 > 0.97 0.01 > 1.93 0.01 > 1.32 Total 99.38 98.81 99.03 97.55 99.45 98.85 98.91 97.76 99.90 99.76 (M2+ +M3+)/Si" 2.02 1.41 2.03 1.54 2.03 1.64 2.03 1.88 2.05 1.64

PF-I PG-1 FO-I FO-2 FO-3 FO-4

Olivine Liquid Olivine Liquid Olivine Liquid Olivine Liquid Olivine Liquid Olivine Liquid

S i O 2 34.98 33.81 30.12 33.98 39.89 34.91 40.14 30.81 40.23 31.79 40.83 35.13 MgO 28.48 8.93 5.02 1.26 51.40 33.31 51.14 31.08 50.91 31.01 52.57 36.15 CaO 0.17 2.48 0.19 2.71 0.09 2.31 0.09 2.61 0.11 3.32 0.07 2.89 AlzO3 0.08 4.53 0.05 3.51 0.08 1.51 0.14 1.49 0.14 1.39 0.11 1.34 Na20 0.02 0.68 0.03 0.77 . . . . . . . . FeO 32.09 37.36 57.49 45.28 4.35 8.23 4.12 8.65 4.20 9.13 3.05 7.06 MnO 0.99 1.16 1.10 0.84 01'81 1.71 0.80 1.79 0.75 1.75 0.54 1.44 NiO 0.37 0.12 1.85 0.30 0.75 0.50 0.83 0.19 1.13 0.88 1.64 1.53 V203 0.17 0.70 0.11 0.46 0.30 2.13 0.31 2.85 0.27 2.77 0.19 1.87 Sc203 0.30 0.91 0.37 0.76 0,55 1.83 0.41 2.16 0.34 1.98 0.17 1.43 In203 0.34 3.05 0.69 2.15 0,11 0.79 0.44 3.93 0.46 4.21 0.27 2.91 Cr203 0.49 0.55 0.17 0.40 0,55 1.18 0.58 1.96 0.44 1.55 0.34 1.22 Tm203 0.09 2.11 0.12 2.73 0.29 5.57 0.19 7.14 0.18 6.51 0.08 4.09 La203 0.01 > 1.63 0.01 > 1.09 0,01 > 1.91 0.01 > 2.11 0.01 > 2.09 0.01 > 1.32 Total 98.64 98.04 98.37 96.19 99.21 95.97 99.21 96.79 99.18 98.40 99.91 98.53 (M 2 + + M 3 +)/Si a 2.06 1.73 1.94 1.51 2.07 1.98 2.04 2.27 2.03 2.21 2.02 2.06

mol ratio. M 2 + and M 3 § represent divalent and trivalent ions, respectively

decrease slightly with increase in pressure, this t endency is no t very significant, as shown in Figs. 4a a nd 5a. A m o n g t r ivalent ions, Dcr shows ex t raord inary high values. A n o m a l o u s pa r t i t ion behavior of Cr 3 § had already been po in ted ou t by Matsu i et al. (1977), and it was ascribed to the s t rong crystal field effect s tabi l izat ion of Cr 3 + in olivine. The D values of the other t r ivalent ions general ly show a pa rabo la - shaped pa t t e rn on the P C - I R diagram, similar to the result o f Matsu i et al. (1977). However, the DA1 shows an u n u s u a l behavior : the DA1 value gradual ly increased with increase in pressure while the D values of the other t r ivalent ions decreased. Hence, the curves shown in Figs. 4 -6 were ob- ta ined by least-squares fi t t ing of the values of Dr , Ds~, D~n and Dxm. DA1 and Dcr were excluded f rom the calcu- la t ion of pa rabo la because of their unusua l behavior .

The effect of compos i t ion of olivine is shown in Figs. 6a and 6b, by compar ing the results of Mater ia l -PL,

PF and PG, whose M g / ( M g + F e ) values are 0.94, 0.60 and 0.13, respectively. The par t i t ion coefficients for mos t elements increase with decrease in M g / ( M g + Fe), except for the Dcr and DA1.

Discussion

Li in Olivine and Its Effect on the D Value

As me n t i one d previously, Li + is mos t suitable m o n o v a - lent ion for the M I and M2 sites of olivine structure. Hence, Li § ion exists in M1 or M2 site of the olivine structure, and a considerable a m o u n t of t r ivalent ions should be incorpora ted in olivine a long with the Li § ion to m a i n t a i n electrostatic neutral i ty . A l though above a rguments m e a n that D of the t r ivalent ions will be changed by the add i t ion of Li to the system, the shape

415

Fig. 2. Back-scattered electron image of polished section of run PL-7. The dark grains are olivine. Dendritic texture is found in the quenched silicate melt

10- i i i i i |

oMg

0.1

oFe

. _ + _ ~ i

0 . 0 1 i I I t 0 60 80 100 120 140

Time/rain

q/A1 |Ca

- - - ~ T n l

20 40

Fig. 3. Change of D values with run duration time. Error bars show _+ l~r standard deviation of the observed mean values. The experiment was performed at 5 GPa and 2100 ~ C

of the D pattern which appears in the PC-IR diagram (i.e. the relative values of the partit ion coefficients) will not be seriously changed by the addition of Li. The position and the shape of the parabola are determined dom- inantly by the structure of the crystal (Onuma et al. ]968; Matsui et al. 1977), and the crystal structure of LiScSiO~ olivine had been investigated and its similarity to the crystal structure of forsterite has already been pointed out (Steele et al. 1978). Therefore, the addition of Li to the system will result only in change of the "peak height" of the parabola on the PC-IR diagram. Such a phenomenon was actually found when we corn-

10

0.1

0.01

1

0.1

0.0140

t b i

A PL-1 (1GPa) -_ [] PL-6 (5GPa) - O PL-7 (8GPa) - �9 PL-8 (14GPa) -

F e ~ M n

! Ca

I I I

I I I

2~

Cr I I [I

60 80 Ionic Radius/pro

I I I

I I I

A PL-1 (1GPa) [] PL-6 (5GPa) O PL-7 (8GPa) �9 PL-8 (14GPa)

t 100 120

Fig. 4. PC-IR diagram of divalent ions (a) and trivalent ions (b) in Material-PL. Error bars show _+ Io standard deviation of the observed mean values

pare the results for the two compositions, PL and PY, which differ in Li20 content (Fig. 7). The concentration of trivalent ions in the olivine of PL-6 is 2 ~ 3 times larger than that of PY-1, and the D(IR) curve is dis-. placed accordingly. DTm cannot be observed in PY-1 because of low concentration of Tm in olivine.

In Fig. 7, results of recent partitioning experiments performed by Taura et al. (1994) are also shown. They measured partition relations between olivine and coexisting silicate melt under high pressures by using a second- ary ion mass spectrometer. A spinel lherzolite, KLB-1, was used as their starting material, and partition relations were examined at around its liquidus temperature. Their results are in good agreement with the present experiments, and Fig. 7 clearly shows that the shape of the D pattern appears in the PC-IR diagram is dom-

416

a I 0

] Z

0.1

0.01

b 1

I I I I I

/x FO-1 (1GPa) [] FO-2 (5GPa) O FO-3 (9GPa) �9 FO-4 (14GPa)

Ca

I r I

I I I

I I I.

I I I

FO-1 (1GPa) [] FO-2 (5GPa) O FO-3 (9GPa) �9 FO-4 (14GPa) Z

Cr

i =

0.1 ~~

0"0~0~4 I I I 60 80 I00 t20

Ionic Radius/pm

Fig. 5. PC-IR diagram of divalent ions (a) and trivalent ions (b) in Material-FO. Error bars show _ 1 a standard deviation of the observed mean values

inantly controlled by the crystal structure, and the effect of the different Li concentrat ion on the shape of D pattern is very small. It is interesting that the results of Taura et al. (1994) correspond most closely with those of PL-6, which contains Li20 component , rather than PY-1. This observation suggests that the parti t ioning of trivalent ions is controlled by the relative abundance of monovalent and trivalent ions. I f we assume the following exchange process

2M2 + ~ M 3+ + ( N a + , Li +)

in the M I and M2 sites of olivine, where M 2+ and M a+ represent divalent and trivalent ions, respectively, then the concentrat ion of Li + + N a + in olivine is equal to the total concentrat ion of trivalent ions. In the case of KLB-1, the amount of incorporated Na in olivine, al-

10

0.1!

0.01

I I I I I I

�9 PL-1 (1GPa) A PF-1 (1GPa)

( 1 GP a) M g ~ �9 PG-1

I I [ I I [

b 1~ I), l i J t I E Y �9 PL-1 (1GPa) ]_ ~ /x PF-1 (1GPa)

Cr ~ �9 PG-1 (1GPa)

I~ 0.1

0"0140 60 80 100 120 Ionic Radius/pro

Fig. 6. PC-IR diagram of divalent ions (a) and trivalent ions (b) in Material-PF and PG, in addition to PL-1. Mg/(Mg+Fe) mol ratios of olivine in runs PL-1, PF-1 and PG-1 are 0.94, 0.60 and 0.13, respectively. Error bars show _+ la standard deviation of the observed mean va lue s

though small, may be sufficient to explain the observed D values in KLB-1 (Fig. 7).

It is impossible to measure the concentration of Li + in olivine or silicate melt by using EPMA. Hence we try to estimate Li + content in the olivine of the Li doped run products f rom the abundance of trivalent ions by assuming again the above exchange process. Li20 content of the recovered olivine thus estimated is 0 .3~ 0.7 wt%. I f we also assume that DLi is 0.2-0.4 for the olivine silicate melt parti t ion (Taura et al. 1994), Li20 content of liquid phase is also estimated to be 1-3 wt%. As mentioned previously, the D values used in the present experiments were calculated f rom chemical analyses, which do not include the information of Li20 content

1

01

0.014q

I I I I I I 1

O PL-6 (5GPa) A PY-I (5GPa) �9 Taura et al. (3GPa)

Cr ~ zs

0 0

Y

I I I I I I I 60 80 100 120

Ionic Radius/pm

Fig. 7. Comparison of partitioning behavior of the trivalent ions in the systems with different Li20 content. Results of partitioning experiments on KLB-1 (Taura et al. 1994) are also shown. Dxm of PY-I cannot be determined because of low concentration of Tm in olivine

3.0

2.0

r -~ 1.0

0 . 0 -

- 1 . 0 -

1.2

I I I I I

- �9

0 �9

O

I I I I I I

1.4 1.6 1.8 2.0 2.2 2.4 2.6

(M 2+ + M 3+) / Si

Fig. 8. Change of deviation of DA~ from the "calculated" value, in obs c~1~ (M2 + (DA1 /DA1 ), with +M3+)/Si tool ratio of silicate melt. Dc~ 1~ was obtained by extrapolating the D parabola, which was acquired by the least square fitting of Dv, Dsc, Din and Dvm, to the ionic radius of A13 +. Closed circles are the results of Material- PL, and open circles are Material-FO

of the olivine and silicate melt. Hence, a small correction should be required to evaluate the partition coefficients used in this paper. D values should become 2-5% larger than the values used in the present experiments, if we adopt the above estimation of Li20 content.

In the above estimation, however, the possibility of another exchange reaction, replacement of Si 4§ and M 2+ by two trivalent ions, was not considered. There- fore, the above estimated of content of Li + in the olivine

417

may be regarded as the maximum value. Direct measurement of concentration of Li in olivine and silicate melt is required for detailed discussion, and measurement of Li is planned.

Effect of Temperature and Composition on the Partition Coefficients

The results of the present experiments show that the partition coefficients generally decrease with increase in pressure. When we observe the partition behavior of elements under high pressure, however, the observed change in the D values includes not only the direct effect of pressure, but also the effects of temperature and composition. Liquidus temperature and liquid composition coexisting with olivine vary with increase in pressure, and we have to consider these effects on the partit ion coefficients.

The effect of temperature on D is estimated by ther- modynamic considerations (e.g., Murthy 1992). By assuming Nernst partitioning and neglecting non-ideality of the system, the equilibrium condition is expressed as

xL~ + RTlnCx = #So + RTtnCS

where R is gas constant and T is temperature, and #x L~ and #so represent standard chemical potientials of component x of the liquid phase and solid phase, respectively. Then the partition coefficient is expressed as

lnD L0 so = (#x - #x )/RT.

This equation predicts that every partit ion coefficient approaches 1 with increase in temperature. It is therefore also expected that the relative difference in D of the elements will decrease with increase in temperature, and consequently, the partit ion coefficient curve on the PC- IR diagram should broader under higher temperature conditions. The "sharpness" of the parabolas shown in Figs. 4~6 is only slightly changed: the D parabola broadens with increase in Mg O / (Mg O +F eO ) ratio of the system, or with increase in pressure. Since the observed broadening of the peak corresponds to the increase in liquidus temperature, the change in the shape of the D parabola shown in these figures might be attrib- utable to the difference in liquidus temperature.

As mentioned previously, temperature variation of the region where partit ion coefficients were measured in each run product is estimated to be ~ 60 ~ C in maximum. At around 2000 ~ C, the deviation in InD values with temperature variation of 60 degrees is estimated to be approximately 3%, which causes a very small error on the PC-IR diagram.

Since the liquidus temperature increases with increase in pressure, all D values should be approaching to 1 under high pressure if we consider the temperature effect only. The observed changes of D pattern o f divalent and trivalent ions, however, do not show such a simple change with increase in pressure. The liquidus temperature also increases with increase in MgO/ (MgO + FeO) ratio, but the observed D values gradually decrease with increase in Mg O / (Mg O +F eO) ratio,

418

Therefore, the observed change in part i t ion behavior with increase in pressure cannot be .explained by the effect o f temperature.

In the case of Material-PL, as 'shown in Fig. 4b, the DM values gradually increase wi~h increase in pressure, while the parti t ion coefficients of the other elements decrease. As the result o f that, DA~ value of PL-8 deviates remarkably f rom the D parabola which was constrained by the other trivalent ions. Since SiO2 content o f the silicate melt in Material-PL decreases with increase i.n pressure and becomes more basic .composition (Table 3), the deviation of DAI may be affected by the SiO2 content o f the silicate melt.

In Fig. 8, the deviation of D ~ f rom the D parabola observed in Material-PL and -PY is plotted against the ( M 2 + + M 3 +)/Si mol ratio of the s~ticate liquid. In this figure, the deviation of DA1 is defined as the vertical separation on the PC- IR diagram between the observed DM value and the "ca lcu la ted" DA1 value, which was obtained by extrapolating the D parabola to the ionic radius of A13 +. Figure 8 shows the correlation between the deviation of DM and ( M 2 + + M 3 +)/Si tool ratio.

The results of Onuma et al. (1968) indicate that crystal structure of the solid phase plays a dominant role in determining the shape of the curves in the PC- IR diagram. In the case of the present experiments, the solid phase is always olivine, and it is expected that the D pat tern on PC- IR diagram does not change greatly. This is generally correct, except for DA~. Although this study has emphasized substitution on the octahedral sites, olivine structure also has tetrahedral sites and there should be an additional peak of D pat tern near the ionic radius of Si 4+. From this point of view, ionic radius of the A13 + locates at around the midpoint o f the peaks of tetrahedral site and octahedral sites. Hence, the observed increase in DA1 value may be regarded as the result of increased substitution on the tetrahedral sites. The relative height o f the peaks of tetrahedral site and M1 + M2 sites may vary with the composi t ion of coexisting liquid. This assumption is in agreement with the observed Dsi values, which increases with increase in (M 1+ + M 3 +)/Si mol ratio of silicate melt. Above arguments mean that some amount of AI 3 § should exist in the tetrahedral site of olivine structure. Change in anionic structure of the silicate melt with composi t ion may relate to the partitioning of A1, when we consider substitution of Si 4+ by A13 +.

If SiO2 content o f the silicate melt affects the D values, we also have to consider the effect o f other components on the D values. In the present experiments, the starting materials contain extraordinary large amount of trivalent ions compared with that in the natural rocks, and the D values might be affected by these trivalent cations. As mentioned previously, however, the present experiments are generally in good agreement with the results of the recent element parti t ioning experiments on natural rocks (Taura et al. 1994). Although the D values of trivalent ions changed with concentra-

tion of trivalent ions and Li20, the shape of D patterns appear in the PC- IR diagram are similar in all experiments.

Acknowledgements. We are grateful to T. Ishii for his helpful sug- gestions for EPMA analysis, and to H. Taura and H. Yurimoto for the partitioning data prior to publication. Discussions and comments by P. Beattie are also acknowledged. Critical comments by I. Jackson ,and anonymous reviewers were helpful to improve the manuscript. This work was supported in part by the cooperative program provided by the Ocean Research Institute, the University of Tokyo, and by Grants-in-Aid from Ministry of Education, Sci- ence and Culture, Japan.

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element partitioning between olivine and silicate melt under high pressure

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