Enstatite (En(80)Fs(19)Wo(01)) was synthesized from a hypersthene normative basaltic melt doped at the same time with La, Ce, Nd, Sm, Eu, Dy, Er, Yb and Lu. The rare earth element concentrations were measured in both the basaltic glass and the enstatite. Rare earth element concentrations in the glass were determined by electron microprobe analysis with uncertainties less than two percent relative. Rare earth element concentrations in enstatite were determined by secondary ion mass spectrometry with uncertainties less than five percent relative. The resulting rare earth element partition signature for enstatite is similar to previous calculated and composite low-Ca pigeonite signatures, but is better defined and differs in several details. The partition coefficients are consistent with crystal structural constraints

McKay, Gordon A.

Schwandt,  Craig S.

English

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NAS_TM- 112 964
Rare Earth Element Partition Coefficients from Enstatite/Melt Synthesis Experiments
Craig S. Schwandt t and Gordon A. McKay 2
1Lockheed Martin, Science, Engineering, Analysis and Test, 2400 Nasa Rd 1, C23, Houston, TX
77058
2NASA, Johnson Space Center, SN4, Houston, Texas 77058
October 1997
Manuscript submitted as a Letter to Geochimica et Cosmochimica Acta
10/7/97
https://ntrs.nasa.gov/search.jsp?R=19980002855 2020-06-16T00:37:34+00:00Z
ABSTRACT
Enstatite (EnsoFs_gWool) was synthesized from a hypersthene normative basaltic melt
doped at the same time with La, Ce, Nd, Sin, Eu, Dy, Er, Yb and Lu. The rare earth element
concentrations were measured in both the basaltic glass and the enstatite. Rare earth element
concentrations in the glass were determined by electron microprobe analysis with uncertainties
less than two percent relative. Rare earth element concentrations in enstatite were determined by
secondary ion mass spectrometry with uncertainties less than five percent relative. The resulting
rare earth element partition signature for enstatite is similar to previous calculated and composite
low-Ca pigeonite signatures, but is better defined and differs in several details. The partition
coefficients are consistent with crystal structural constraints.
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INTRODUCTION
As part of on-going research into the petrogenesis of basaltic achondrite meteorites, we
have recently determined mineral/melt partition coefficients for nine rare earth elements (REE) in
enstatite using the percent-level doping technique. Hypersthene normative basaltic glass was
concurrently doped with La, Ce, Sm, Nd, Eu, Dy, Er, Yb, and Lu. Isothermal growth
experiments were conducted at 1210°C under oxygen fugacity conditions equivalent to the iron-
wiistite buffer. These conditions produce enstatite crystals (EnsoFslgWo0_) with dimensions of
approximately 75 x 150 x > 1500 _m. The REE abundances were measured in the pyroxene with
a Cameca 4f ion microprobe. REE abundances in the basaltic glass were measured by electron
microprobe analysis using the Cameca Camebax and Cameca SX-100 microprobes at the Johnson
Space Center. These results provide a REE partition coefficient signature for enstatite. These
partition coefficients may be more appropriate for natural enstatite crystals than composite
partition coefficients derived for low-Ca pigeonite (e.g., McKay et al., 1986, 1991), because in
the present experiments (1) the pyroxene is enstatit¢ rather than pigeonite, and (2) all nine REE
were present simultaneously in each charge rather than as individual REE in different charges as in
the earlier experiments, and were therefore subject to identical experimental and analytical
conditions. This approach provides more assurance of a self-consistent REE partition coefficient
signature than calculated or composite signatures derived from several singly doped experiments,
because experimental and analytical conditions are sometimes difficult to reproduce exactly, and
in singly doped experiments the dopant elements interact in a more simple chemical system.
Therefore we present enstatite partition coefficients for nine of the fifteen rare earth elements.
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EXPERIMENTAL PROCEDURE
Five pure oxides; SiO2, A1203, MgO, Fe203, and CaO were mixed in appropriate
proportions to produce 10 g of a basaltic glass with a composition similar to the basaltic
achonddte meteorites, eucrites, though somewhat more hypersthene normative. The oxide
mixture was ground for half an hour under acetone in a motorized mortar and pestle, then melted
at 1350 °C in a large platinum crucible in air for 24 hours. The melt was quenched in de-ionized
water and then crushed and ground in the mortar and pestle under acetone for about fu'teen
minutes to produce a fine glass powder. Cr203, TiO2, La, Ce, Nd, Sm, Eu, Dy, Er, Yb oxides and
Lu(NO3)3 were weighed out and added to the appropriate amounts of glass powder and then
thoroughly mixed under acetone with a mortar and pestle. Total minor and trace element
concentrations were held to less than 4 wt%.
Mineral growth experiments were initiated by suspending 125 mg pressed glass-powder
pellets on 3 mm diameter platinum wire loops in a flowing CO/CO2 gas-mixing furnace at 1300
°C and iron-wfistite redox conditions for 48 hours to equilibrate the samples with the fugacity of
the experiments and to equilibrate iron exchange with the platinum loop. Approximately 5 wt%
additional Fe203 was added to the original mixture to compensate for iron loss to Pt in all stages
of the experiments. Because of the much higher diffusion rates at 1300°C than at the
crystallization temperature of 1210°C, most iron loss occurred during this high temperature
equilibration. After equilibration, the samples were air quenched by rapid removal from the
furnace. Specific experiments were then initiated by placing a sample back into the furnace at
iron-wiistite fugacity conditions, superheating it to 1260°C (-50 degrees above the liquidus),
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holding it there for --4 minutes, and then dropping the furnace temperature rapidly to 1210°C (2 ±
1 degrees below the liquidus temperature). The samples were then held at constant temperature
for 120 hours, and air-quenched by pulling the sample support assembly out of the furnace. This
procedure produces samples having less than five percent total crystallization and minimizes
nucleation, whereby the few crystals that grow are large enough to analyze without difficulty (75
x 150 by >1000 _tm). Nucleation appears to be internal to the sample charge, as the crystals
predominantly float freely in the basaltic glass droplet and do not appear to nucleate on the inside
surface of the glass sphere or on the wire loop (Lofgren, 1980).
Experiment temperatures are monitored with a thermocouple mounted in the furnace
muffle tube approximately three millimeters from the sample. The Pt and Pt-Rh thermocouple is
calibrated against the melting point of gold (1064 °C). Temperatures are believed to be accurate
within ± 5 degrees, with relative precision of about ± 1 degree. The oxygen fugacity is monitored
using a solid-state oxygen sensor in a separate reference furnace (Jurewicz et al., 1993) calibrated
using a commercially mixed 1:1 CO:CO2 reference gas. The accuracy for the oxygen fugacity
measurements is better than -i- 0.1 log units, and the experimental oxygen fugacity was equivalent
to iron-wiistite minus 0.2 log units ± 0.1 log units (logfO2 = -11.8 at 1210 °C).
ANALYTICAL PROCEDURES
Major and minor element compositions of the pyroxene and glass were determined with a
five spectrometer Cameca SX-100 electron microprobe. Quantitative analyses utilized natural
mineral standards, 15 kV, 40 nA, and the Cameca PAP correction routine (Pouchou and Pichior,
1991). One-sigma counting uncertainties for the major elements are less than one percent of the
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amountpresent.
Rare earth element concentrations of the enstatite crystals were determined using the
University of New Mexico-Sandia National Laboratories, Advanced Materials Laboratory,
Cameca 4fion microprobe. The 20 nA primary O minus beam was focused to a 20-25 _tm beam
diameter. The dopant concentrations in these synthetic pyroxenes yield secondary ion count rates
high enough to provide theoretical standard deviations of less than one percent. However, due to
uncertainties in the standard compositions, the analytical uncertainties are conservatively
estimated at less than 5 percent. The nine REE used in these experiments were chosen to avoid
both x-ray spectra interferences and isobar interferences during secondary ion mass spectrometry.
Ion probe analyses utilized the following isotopes: 13_La+, 14°Ce+, _lqd ÷, 14_Sm÷, mEu÷, mEu÷,
163Dy+, 167Er÷, 174Vh+ and _Ts- +
__, L,U. Enstatite REE analyses were calibrated against two natural
pyroxene standards. The basaltic glass compositions were determined by electron microprobe
analysis using a Cameca Camebax and a Cameca SX-100 microprobe, and synthetic glasses
containing rare earth elements as standards (Drake and Weill, 1972). The counting uncertainties
for the glass analyses are less than two percent.
RESULTS AND DISCUSSION
The synthesized pyroxene crystals have homogeneous major element compositions
consistent with enstatite (Table 1). Longhi and Boudreau (1980) and Longhi (1987) showed that
orthoenstatite was stable to "at least as low as 1211 °C and presumably much lower..."
Interestingly, the enstatite crystals of this study are sector zoned with respect to AI, Cr, and to a
lesser extent the REE. Abundances of heavy REE show positive correlations with AI abundance
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from onesectorto another(Fig. 1,2),thoughthe light REE show very little correlation with AI,
as indicated by the low value of R" value for La as compared with Lu (Fig. 2). The heavy REE
have more mild negative correlations with Mg and Fe. Detailed discussion regarding the
development of minor and trace element sector zoning is in a manuscript in preparation by
Schwandt and McKay.
We define the partition coefficients (D) presented here according to the terminology of
Beattie et ai. (1993), where mi_r'l/mcltDM = mi'_lcMff_ltCM. Mrepresents the cation of interest and
C represents the weight fraction of the cation in each respective phase. The D's determined in
each sector for the measured REEs are reported in Table 2. Figure 3 represents a plot ofthe
partition coefficients against the REE's in order of increasing atomic number.
The resulting REE partition coefficient signature (Fig. 3) is similar to the theoretical
signature as calculated from pyroxene wollastonite content with the equation ofMcKay et al.
(1986) and to the composite low-Ca pigeonite signature (McKay et al. 1991). However, because
more REE were measured, the new partition coefficient pattern is better defined than the older
ones, especially for the HREE. The new pattern also differs in important details from the older
ones. Compared with the composite pattern from McKay et al. (1991), the new pattern has a
steeper slope for the LREE, and a higher Ds, JDNj ratio. This may be a result of the difficulties of
measuring very low partition coefficient values using the electron microprobe (McKay, 1986).
Compared with the pattern calculated using the equation ofMcKay et al. (1986), the new pattern
is steeper overall, and much smoother. This result is not surprising because the calculated pattern
in Fig. 3 represents an extrapolation to significantly lower Wo content than experimental
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pyroxenes used by McKay et al. (1986) to parameterize r_'tDazE as a function of Wo.
The new partition coefficient pattern also shows a significant negative Eu anomaly,
indicating that the partition coefficient for Eu ÷_is less than that for Eu +3. Of course the size of this
Eu anomaly will vary with oxygen fugacity, with the current experiments having been run at
fugacities close to IW. Thus, at lowfth, orthopyroxene will have a negative Eu anomaly even if it
crystallizes from a melt with chondritic Eu/Sm.
The observed REE partition coefficient signatures are easily rationalized based on crystal
structural constraints. The size of the pyroxene M2 site is more flexible than the M1 site, though
cations larger than Ca are not easily incorporated into the structure (Cameron and Papike 1981).
Therefore, as the light REE are significantly larger in ionic size than the heavy REE it is logical
that the light REE are depleted in enstatite relative to the heavy REE. The presence of a negative
Eu anomaly indicates that the larger ionic radius of Eu ÷2(1.25A in eight-fold coordination, vs.
1.07 A for Eu ÷3, Shannon, 1976) must make substitution more difficult than for Eu +3even though
the latter requires a charge-balancing coupled substitution.
Based on pyroxene compositions from lunar and meteoritic basaltic rocks, Papike (1980)
identified a few primary substitution mechanisms for minor elements in pyroxene: 1) XaAl3++
WAl3+ for XaMg2+ + WSi4". 2) XaCr3+ + rVAl3+ for VlMg2+ + rVsi4+. 3) VXTi4÷+ 2NAI 3+ for VlMg2+ +
WSi4÷. Similarly, relative to the sector zoning in our synthetic pyroxenes, Mg and Si show strong
inverse correlations and Cr a strong normal correlation with AI concentration. This suggests the
same coupled substitution mechanisms are important. Incorporation of the REE into the enstatite
structure must also be occurring via a similar coupled substitution (V_REE3+ + WAl 3÷ for VIM22+ +
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IVsi4+)because REE concentrations in the enstatite correlate positively with AI concentration (at
least for the HREE, Fig. 2) and correlate negatively with the typical M2 cations. The REE
correlations are strongest with AI and only moderate relative to the M2 cations individually.
Therefore this REE partitioning behavior suggests that the incorporation of impurity cations, such
as the REE, in the enstatite crystal structure is at least partly controlled by the substitution orAl
for Si in the tetrahedral sites. Moreover, these results suggest there is an additional charge
compensation involving the larger impurity cations in the octahedral M2 site. The mechanism
behind this observation and investigation of the mechanisms leading to the development of sector
zoning of minor and trace elements is explored in depth in a manuscript in preparation by
Schwandt and McKay.
CONCLUSION
We have determined nine REE partition coefficients and the resulting gEE partitioning
signature for enstatite synthesized from a hypersthene normative basaltic melt. We believe these
partition coefficients provide the most complete and reliable experimentally determined partition
coefficient pattern currently available for orthopyroxene. These partition coefficients are useful
for research on the petrogenesis of basaltic achondrite meteorites and terrestrial mantle materials.
In addition, the results also provide clues about mechanisms resulting in the incorporation of
impurity cations into the pyroxene crystal structure.
ACKNOWLEDGMENTS
This research was carried out while the first author held a National Research Council-
Johnson Space Center Research Associateship. The work was also partially supported by NASA
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RTOP 152-12-40-21. We appreciate access to the University of New Mexico-Sandia National
Laboratories Ion Microprobe Facility. We thank Grant Fowler and S. Vincent Yang for analytical
assistance with the UNM-Sandia Ion Microprobe and the JSC Electron Microprobe respectively.
REFERENCES
Beattie P., Drake M., Jones J., Leeman W., Longhi J., McKay G., Nielsen R., Palme H.,
Shaw D., Takahashi E., and Watson B. (1993) Terminology for trace-element partitioning.
Geochim. Cosmochim. Acta 57, 1605-1606.
Cameron M. and Papike J.J. (1981) Structural and chemical variations in pyroxenes. Amer.
Mineral. 66, 1-50.
Drake M.J. and Weill D.F. (1972) New rare earth element standards for electron microprobe
analysis. Chem. Geol. 10, 179-181.
Jurewicz A.J.G., Williams R.J., Le. L., WagstaffJ., Lofgren G., Lanier A., Carter W. and Roshko
A. (1993) Technical update: JSC system using a solid electrolytic cell in a remote location
to measure oxygen fugacities in C0/CO2 controlled-atmosphere furnaces. NASA
Technical Memorandum 104774.
Lofgren G.E. (1980) Experimental studies on the dynamic crystallization of silicate melts. In
Physics of Magmatic Processes (ed. R.B. Hargraves), Chap. 11, pp. 487-552. Princeton
University Press.
Longhi J. (1987) Liquidus equilibria and solid solution in the system CaAI2Si2Os-Mg2SiO4-
CaSiO3-SiO2 at low pressure. Amer. J. Sci. 287, 265-331.
Longhi J. and Boudreau A.E. (1980) The orthoenstatite liquidus field in the system forsterite-
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diopside-silicaat oneatmosphere.Amer. Mineral. 65, 563-573.
McKay G.A. (1986) Crystal/liquid partitioning of KEE in basaltic systems: Extreme
fractionation of KEE in olivine. Geochim. Cosmochim. Acta 50, 69-79.
McKay G., WagstaffJ., and Yang S.-R. (1986) Clinopyroxene KEE distribution coefficients
for shergottites: The KEE content of the Shergotty melt. Geochim. Cosmochim. Acta 50,
927-937.
McKay G., L¢ L., and WagstaffJ. (1991) Constraints on the origin of the mare basalt europium
anomaly: KEE partition coefficients for pigeonite. Lunar Planet. Sci. Conf. XX[I, 883-
884 (abstr.).
Papike J.J. (1980) Pyroxene mineralogy of the moon and meteorites. In Pyroxenes (ed. C.T.
Prewitt) Rev. Mineral., 7, 495-525. Mineralogical Society of America.
Pouchou J. and Pichior F. (1991) Quantitative analysis of homogeneous or stratified
microvolumes applying the model "PAP". In Electron Probe Quantitation (ed. K.F.J.
Heinrich and D.E. Newbury), pp. 31-75. Plenum.
Shannon R. D. (1976) Revised values of effective ionic radii and systematic studies of interatomic
distances in halides and calcogenides. Acta Crsyt. A32, 751-767.
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Table 1. Major element compositions determined by electron
microprobe analysis. Trace element values in glass determined
by electron microprobe analysis. Trace element values in
enstatite obtained by SIMS. Column numbers match SIMS
analysis spots shown in Figure 1.
Glass Enstatite
wt % 1 2 3 4 5
SiO2 53.31 52.96 53.26 53.79 53.33 53.44
A1203 15.07 3.29 3.42 2.52 3.30 3.01
MgO 8.06 28.37 28.49 28.66 28.53 28.35
FeO 12.50 12.64 12.84 12.99 12.77 12.90
CaO 6.93 0.63 0.64 0.63 0.64 0.64
TiO2 0.22 0.05 0.05 0.05 0.06 0.06
Cr203 0.13 0.92 0.98 0.78 0.86 0.84
ppm:
La 5012 1.99 1.62 1.43 2.82 2.65
Ce 6118 6.72 5.74 4.74 7.5 8.49
Nd 4322 18.5 16.18 12.56 17.68 21.39
Sm 4831 59.6 54.0 40.0 57.5 65.6
Eu 4738 21.9 21.3 15.8 21.7 19.8
Dy 3236 171 175 121 173 178
Er 3297 305 320 225 323 317
Yb 3664 381 412 294 419 394
Lu 4641 478 533 384 532 503
Mole Fraction:
re' 0.46 0.19 0.20 0.20 0.20 0.20
Wo 0.012 0.012 0.012 0.012 0.013
En 0.79 0.79 0.79 0.79 0.80
fe '= FeO/(FeO+MgO); Wo = CaO/(CaO+FeO+MgO);
En = MgO/(CaO+FeO+MgO); all quantities molar.
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Table 2. Rare earth element partition coeffi-
cients. Column numbers match SIMS analy-
sis spots shown in Figure 1. Uncertainties
less than 5%.
1 2 3 4 5
La 0.0004 0.0003 0.0003 0.0006 0.0005
Ce 0.0011 0.0009 0.0008 0.0012 0.0014
Nd 0.0043 0.0037 0.0029 0.0041 0.0049
Sm 0.0123 0.0112 0.0083 0.0119 0.0136
Eu 0.0046 0.0045 0.0033 0.0046 0.0042
Dy 0.052 0.054 0.037 0.053 0.054
Er 0.092 0.097 0.069 0.098 0.096
Yb 0.104 0.112 0.080 0.114 0.107
Lu 0.103 0.114 0.082 0.114 0.108
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FigureCaptions:
1. Digital map of A1 concentration.
.
.
Bright indicates high AI concentration and dark indicates
low AI concentration. Enstatite crystal with sector zoning of A1 surrounded by basaltic glass.
Circles outline ion microprobe analysis locations. Analyses 1 and 5 are in { 100 } sectors, 2
and 4 are in {010} sectors and 3 is in a portion of {001} sector overlapped by a low AI zone
that is the result of growth twinning.
Plot ofA1203 wt % versus the REE concentration (ppm) showing the correlation of the REE
with AI. Correlation Coefficients (g z) are given for La and Lu, and indicate a declining
correlation with increasing cationic size, i.e. lighter REE.
Partition coefficient versus atomic number plot reveals the rare earth element partitioning
signature for enstatite. The field represents the variation of the partition coefficients as a
function of the sector zoning. The experimental signature is compared with a pattern
calculated from the variation of partition coefficients with the wollastonite content of synthetic
shergottite pyroxenes 0VlcKay et al., 1986) and a composite low-Ca pigeonite pattern from
several experiments doped with single REE (McKay et al., 1991).
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50 ur,, H| 151},,_lO,i,r,_l
Figure 1
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1000
E
ILl
ILl
100
10
2.4
Sm
Nd
• Ce
La
i I I I
2.8 3.2 3.6
VVt % AI203
Figure 2
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¢-
(9
0
0(0
c-
.o
Q.
1.0000
0.1000
0.0100
0.0010
Range resulting from variation due to sector zoning.
Calculated.
0.0001
La Ce Nd Sm Eu Gd Dy Er Yb Lu
Figure 3
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Rare Earth Element Partition Coefficients from Enstatite/Melt Synthesis Experiments

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