Salt Lake Crater ultramafic nodules that equilibrated at the same T and P, is strongly dependent on Fe ..... gradient exists below the Hawaiian Island chain. Sec-.
Earth and Planetary Science Letters, 39 (1978) 173-178 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
173
[61
COBALT AND SCANDIUM PARTITIONING VERSUS IRON CONTENT FOR CRYSTALLINE PHASES IN ULTRAMAFIC NODULES W.E. GLASSLEY Department o f Geology, Middlebury College, Middlebury, VT 05753 (U.S.A.)
and D.Z. PIPER Pacific-Arctic Branch of Marine Geology, U.S. Geological Survey, Menlo Park, CA 94025 (U.S.A.)
Received March 14, 1977 Revised version received November 22, 1977
Fractionation of Co and Sc between garnets, olivines, and clino- and orthopyroxenes, separated from a suite of Salt Lake Crater ultramafic nodules that equilibrated at the same T and P, is strongly dependent on Fe contents. This observation suggests that petrogenetic equilibrium models of partial melting and crystal fractionation must take into account effects of magma composition, if they are to describe quantitatively geochemical evolutionary trends.
1. Introduction Models of elemental fractionation during partial melting or fractional crystallization is critically dependent upon partition coefficients (KD). Methods used to estimate K D values include the use of observed elemental concentrations in coexisting crystals and matrix of volcanic rocks [ 1,2]. This method requires that chemical equilibrium among coexisting phases be assumed, and that mineral zoning, bulk phase composition, and temperature and pressure variations have not seriously effected element partitioning. Previous work has demonstrated that composition [14] and temperature [17] effects significantly modify KD values for trace alkaline and rare earth elements. These results are consistent with theoretical considerations [3]. However,to our knowledge it has not previously been possible to examine the effects of mineral composition variations on KD in Publication authorized by Director, U.S. Geological Survey.
natural rocks. This results from the difficulty of removing temperature effects on K D in natural systems. During geochemical examination of a suite of ultramafic xenoliths from Salt Lake Crater, Hawaii, we found that clinopyroxene-orthopyroxene equilibration temperatures for the samples were identical within the error limits of the method [61. We have examined the partitioning of Co and Sc for coexisting garnet, clinopyroxene, orthopyroxene, and olivine in this isothermal suite, and have found a strong dependence of K n on mineral composition. The suite of rocks that we investigated consists of ultramafic nodules, recovered from a nephelenite tuff at Salt Lake Crater, on the island of Oahu, Hawaii [4,5]. These xenoliths were collected from the silicaundersaturated, alkali-rich Honolulu volcanic series, which is well known for its abundance of lherzolite, dunite, pyroxenite, and websterite xenoliths [4,5]. The upper stability limit of these rocks is not well defined, but would appear to be on the order of 2 0 -
174 30 kbar, and 1300-1550°C
tive o f e x s o l u t i o n , d e f o r m a t i o n are a b u n d a n t
2. Experimental technique
[4,5]. Textures indicaand recrystallization
F r o m a s u i t e o f several h u n d r e d u l t r a m a f i c n o d u l e s ,
in x e n o l i t h s o f t h i s a n d o t h e r s i m i l a r
nine nodules were selected for neutron activation
alkalic, volcanic terranes [4,5].
analysis (NAA). Their bulk major element composiTABLE 1 Elemental composition o f pure mineral phases separated from ultramafic nodules. Elements reported as oxides (in wt.%) were determined on thc electron microprobe. Co and Sc (in ppm), and Fe (in wt.%), were determined by NAA Clinopyroxencs 8
13
Garnet 14
43
69
71
88
109
1
8
14
71
88
SiO2 TiO2 AI203 Cr203 l:cO * MnO MgO CaO Na20 K20
48.60 1.24 7.92 0.25 6.59 0.12 13.94 17.90 2.06 0.02
-
50.14 0.76 6.20 0.27 5.63 0.11 15.16 19.70 1.50 0.02
52.49 0.62 5.62 0.94 3.90 0.09 15.23 18.80 2.50 0.01
49.16 1.15 7.43 0.21 6.27 0.11 14.15 18.98 1.83 0.01
49.97 0.91 7.47 0.35 7.24 0.13 13.53 16.81 2.70 0.02
47.98 1.47 8.44 0.09 8.51 0.11 12.62 16.86 2.52 0.02
51.82 0.19 4.56 1.09 2.58 0.08 16.31 21.62 1.27 0,02
40.79 0.28 22.64 0.20 12.67 0.44 18.03 4.79 0.04 . .
40.56 0.30 22.47 0.40 12.98 0.41 17.88 4.76 0.04 .
40,94 0,15 22.66 0.32 11.66 0.43 18.46 5.06 0.03 . .
40.18 0.23 22.56 0.14 14.56 0.45 17.01 4.46 0.05
40.17 0.35 22.26 0.11 15.57 0.39 16.02 4.97 0.05
Total
98.64
-
99.49
100.20
99.30
99.13
98.62
99,54
99.88
99.80
99.71
99.64
99.89
('o Sc I:e
28.0 30.0 4.5
48.5 9.7 6.9
30.3 38.4 4.2
20.4 71.1 2.6
31.0 39.0 4.7
30.3 31.3 5.3
38.3 19.8 6.3
16.1 80.5 1.7
41.7 88.9 9.1
46.9 73.9 9.6
45.1 79.0 10.0
55.0 53.0 11.3
Olivines
I SiO 2 TiO 2 AI20 3 Cr203 I:eO * MnO MgO CaO Na20 K~O
. -. .
Total
-
Co Sc I:c
163.0 2.2 13.7
Orthopyroxencs
14
.
. .
69
39.94 40.51 . . 0.11 0.11 0.03 0.03 15.74 18.01 0.10 0.13 44.30 42.58 0.11 0.11 . . . . 100.33
101.33
166.0 2.4 12.6
182.0 2.1 14.4
Mica
109
8
14
43
69
109
41.12
52.67 0.37 4.37 0.14 11.63 0.17 29.67 0.86 0.16 0.01
55.64 0.24 4.54 0.14 10.26 0.16 30.89 0.80 0.13 0.02
55.12 0.17 2.98 0.38 7.99 0.15 32.58 0.72 0.16 0.01
53.74 0.32 3.95 0.11 11.56 0.16 30.00 0.77 0.13 0.01
54.03 0.07 3.56 0.57 5.60 0.12 34.32 0.58 0.08 0.01
100.05
102.82
100.26
100.75
98.94
68.0 12.8 8.8
74.0 14.2 8.6
59.6 14.0 6.1
73.0 13.3 8.7
45.1 21.2 4.17
0.04 0.03 8.59 0.09 50.00 0.06
99.93 112.0 2.5 7.1
44.8 102.0 8.8
Spinel
1
13
1
109
46.7 4.3 4.3
225.0 2.0 15.9
209.0 1.6 10.3
w
h
m
69.5 5.2 6.4
Petrologic classification: 1 = garnet wcbsterite; 8 = garnet pyroxene lherzolite; 13 = mica harzburgite; 14 = garnet pyroxene lherzolite; 43 = orthopyroxenite; 69 = garnet p y r o x e n e Iherzolite; 71 = garnet websterite; 88 = garnet clinopyroxenite; 109 = olivine lher. zolite. Petrologic classification is based on the terminology of Jackson [ 13] but modified to indicate the d o m i n a n t mineral phase in the lherzolite (pyroxene or olivine), and to indicate the presence o f trace garnet or mica in samples 8, 13, 14, 69.
175 tion and mineralogy (Table 1) extends over tire range for the entire group. Each nodule was fragmented by hand with a stainless-steel mortar and pestle. Pure mineral splits of clinopyroxene, orthopyroxene, olivine, garnet, spinel, and mica were hand-picked from the crushed material. No contamination from the mortar and pestle or from coexisting phases was observed under the microscope. The samples were irradiated in a TRIGA reactor and then counted with a Ge(Li) detector and Nuclear Data 2200 analyzer. Elemental abundances were calculated using a single flux monitor, U.S.G.S. standard W-1. The values used in the calculations are given in Table 1. Precision is 6% Co, 5% Sc, and 7% Fe (variability observed for triplicate analysis of a single sample). Electron microprobe analyses of the phases were also made on the same mineral splits, using an ARLEMX SM five-channel instrument. The same mineral species that was being analyzed was used as a standard; corrections for machine deadtime and drift, and fluorescence, absorption, and mean atomic number effects were applied to all of the results. The Fe values obtained by microprobe are systematically lower than the N A A results by an average of 6% except for olivine, for which they are higher by 4%. Analyses are reported only for those samples which contained unzoned minerals.
3. Results The results of the NAA and microprobe analyses are presented in Table 1 and Fig. 1. Parameters of the exponential curves fitted to the data and the correlation coefficients are listed in Table 2. It is clear that a very strong correlation exists between Co and Fe contents, and Sc and Fe contents in the separate phases. As Fe content of each phase increases, Co content increases and Sc content decreases. For each phase the trend is well defined and unique; curve slopes on a semilog plot are not parallel. The rate of change of Co and Sc contents, as a function of Fe content, is greatest for clinopyroxene, is somewhat less pronounced for garnet and orthopyroxene, and is least for olivine. Only two mica and spinel samples were separated from bulk samples (Fig. 1). The analyses are included (Table 1) and plotted, but the trends shown for these minerals (Fig. 1) are not statistically significant.
103
IO2 -
I0
I
I
~ " ~ I010 O°xene 975
I
"'.
V
I
.,
""'
""'. V ~ O t
Mico ~
I
S
thopyroxene "" "'....' "'" ".....
o° ~03
A
,oz Co ppm LOI
,*
,,
0
,
80
,
12 0
,
,
16.0
20.0
Fe %
Fig. 1. Semi-log plot of Co and Sc vs. Fe contents of the individual phases, as determined from NAA. Lines through the data points are exponential curve fits, except for mica and spinel which are visual fits. Broken lines connect coexisting phases. The numbers associated with clinopyroxene data points are equilibration temperatures calculated from coexisting clinopyroxene-orthopyroxene pairs, in degrees celsius.
TABLE 2 Coefficients for exponential curve fits ( Y = a e bx) and correlation coefficients (r 2) for Co and Sc vs. Fe Clinopyroxene
Orthopyroxene
Olivine
Garnet
Scandium
a b r2
178.96 -0.37 0.91
27.79 -0.09 0.77
2.93 -0.02 0.73
833.02 -0.24 0.93
12.11 0.19 0.96
31.33
71.60
18.62
0.10
0.06
0.09
0.94
0.97
0.79
Cobalt
a b r2
176 4. Discussion From the results it is evident that the composition of the phases correlates with the Co and Sc concentrations. In Fig. 2 are plotted the distribution coefficients for coexisting phases, as a function of Fe content of the clinopyroxene. A clear KD vs. Fe correlation exists which could be attributed to differences in T and P, or to effects of mineral composition on mineral structure. To evaluate the role of T, five clinopyroxene-orthopyroxene pairs were used to calculate equilibration temperatures using a version of the Wood and Banno geothermometer [6]. This method assumes that the pyroxene composition is defined by the temperature-sensitive diopside-enstatire solvus, and can be interpreted in terms of temperature once correction is made for solid solution effects. Since our major element data are from microprobe analyses, the reported Fe values are for total Fe. Our calculations assume all of the Fe is divalent although small ( K~Po~t o K~)pXpx < K~Po~, at Fecpx ~ 9%. If the trace metal content of these phases is a function of their Fe content and if the mineral-mineral distribution coefficients for the trace metals are a function of Fe contents, then the mineral-liquid distribution coefficients will also be a function of Fe contents of the minerals. The equations for the slopes of the curves in Fig. 2 are then the equations for the isothermal change in the ratios K~px/KiDL, where K~L is the distribution coefficient for mineral i and liquid L. However, because we do not know the magma composition that would have been in equilibrium with the phases, or the KiDL,it is not possible to predict quantitatively the trend of Sc and Co contents of magmas from which clinopyroxene, orthopyrox-
r
40
a
I
I
I
%
%~ %. % %' %% %% \ %% \ % %%%% ~%'%
SC ppm
in mill
I
~0.
20.
o
".0
~
%
I __ i ~1.0 8.0 ~0.0 % Fe CLINOPYROXENE
I I~.O
Fig. 3. S c a n d i u m c o n t e n t in r e s i d u a l m e l t d u r i n g o r t h o p y r o x e n e a n d c l i n o p y r o x e n e f r a c t i o n a t i o n . Solid circles: r e s i d u a l melt composition for distribution coefficients which change
as a function of mineral composition; open squares: residual melt composition assuming " KDL ~ i's constant, Data points on the curves are for 20%, 40%, 60% and 80% of the original volume removed by fractionation of equal proportion qf clinopyroxene and orthopyroxene. For the constant hnDL u'cpx = 5 , ~Dpx = i .04. For the changing case we assumed "DL K/DL case, we assumed: (1) weight percent Fe in clinopyroxene = 0.15 S, where S = percent of initial liquid crystallized;
cpx • • " cpx • (2) K r ~ f = 5 at 2% F e m c l m o p y r o x e n e , a n d K n r v a n e s as cpx~ . . --. KDL = 5 . 8 0 - 0 . 4 4 ( 0 . 1 5 S); (3) t h e m l h a l m a g m a c o n t a i n e d op_x . opx cpx cpx 1 0 0 p p m Sc. KDL w a s o b t a i n e d f r o m KDL = KDLfK D opx,
where K~opx was determined from fig. 2 at the appropriate weight percent Fe in clinopyroxene. ene, and olivine are fractionating. However, it can be predicted that the trace metal content of the magma from which three or more phases are fractionating will exhibit inflection points when plotted against Fe content of a coexisting phase. Fractionation involving only two solid phases will result in a smooth compositional trend, but the trend will deviate substantially from theoretical trends deduced from nonvarying distribution coefficients. An example of the kind of trend expected is shown in Fig. 3. We emphasize that these trends are the result o f isothermal compositional variation. In natural sys, terns at equilibrium, KD will also vary with t e m perature and pressure and these variations will be superimposed on the compositional variation. In a T-KD-XFeplot, a surface will define the variation in K D with temperature and composition. Thus, for any fractionation or partial melting scheme to be
178
quantitatively appropriate, it is necessary to define how the phase compositions vary during fractionation or partial melting, and what effect this will have on the KD values being used. If equilibrium is not achieved, as has been suggested [ 12] for some rock suites, the equation that defines variations in fractionation of elements between coexisting phases will also have a kinetic term. As the KD's in this suite of rocks appear to be well defined by variations in Fe concentration alone, additional terms apparently are relatively small.
References
5. Conclusions The distribution coefficients for Co and Sc in clinopyroxene, orthopyroxene, olivine, and garnet are demonstrably a function of composition of the phases, at essentially constant temperature and pressure. The strong correlation of K D for both trace metals with Fe content of the phases requires that Kr)L also vary with Fe content. We have observed indications that similar behavior for Cr can be expected. These results suggest that any trace element that either substitutes for iron, or occupies a crystallographic site affected by iron substitution, will be susceptible to the effects we have described, although it must be recognized that correlation with Fe content may be fortuitous. Any attempt to model the behavior of such elements in fractionating systems must be considered only as approximations, if the KD's are defined only as functions of temperature and pressure.
I0 1l 12
13
14
Acknowledgements We would like to thank J. Tatman for generously providing the samples and P. Henshaw for assistance with the chemical analyses. The microprobe analyses were conducted at the University of Washington. We are indebted to D. Clague, J. Bischoff, G. Goles, and T. Vallier for comments and suggestions on the manuscript.
15
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