The behavior of some trace elements during solidification of the Skaergaard layered series

Geochimica et Cosmochimice Acta, 1974, Vol. 38, pp. 1549 to 1577. Pergamon Press.

Printed inNorthern Ireland

The behavior of some trace elements during so~~catio~ of the Skaergaard layered series THEODORE P. PASTER,* DONALD S. SCHAUWECRER,~ and LARRY A. HASKIN: Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, U.S.A. (Received

11 Jtily 1973;

accepted ira revisedform 29 March 1974)

&&&--Six rocks and twelve mineral separates from the layered series of the Skaergaard intrusion were analyzed by neutron activation for REE, Ag, AS, Ba, CO,Cr, CU, Ge, Mn, Ni, Sb,

So and Zn. The logarithmic mode1for trace element partitioning was further developed for use in describing the behavior of the elements during solidification of the layered series. iterations were mede in order to optimize within limits of uncertainty of measured values, suoh parameters as bulk composition, zone size, relative proportions of cumulus plus adcumulus miner&, zone mineralogy, and distribution coefficients, Values for distribution coefficients for the elements entering the cumulus minerals were obtained and have uncertainties of -3.50 per cent. The relative size of the hidden zone is limited, ecoording to the estimates from the model, to between 20 and 50 per oent of the maSs of the layered series. INTRODUCTION THE BEHAVIOR of trace elements during solidification of a silicate liquid has been

the subject of many geochemical studies. A trace element is considered here to be an element that is not essential to the structure of any mineral crysfallizing from its parent liquid, but which distributes itself in some manner among the crystallizing minerals and their parent liquid. Although many insights to the behavior of trace elements have now been achieved, their app~cations to ~derstan~ng the evolution of natural bodies of silicate rock have been mainly qualitative. In most cases, even models that, depend exclusively on empirically determined parameters for trace element behavior have yielded onIy limited success in providing quantitative agreement with natural systems (e.g. MCINTIRE, 1963). Recently, the logarithmic partitioning model has been used with improved success to provide quantitative information about the fractionation of basic magma to produce successively less basic rocks, e.g. at Gough Islands (ZIELINSKI and FREY, 1970), to show genetic relationships among successive basaltic lavas of S&ens ~~ountain (HEL~E and HASKIN, 1973), to describe trace element fractionation during basalt formation (GAST, 1968), and to place limits on compositions and mineralogy of parent materials for lunar rocks (e.g. HASKIN et al., 1970; HUBBARD and GAST, 1971). In this work we apply the logarithmic model to a number of trace elements in the layered series of the Skaergaard intrusion, the classic example of a layered silicate intrusion (e.g. WAGNER and BROWN, 1967). The layered series consists of an inaccessible bidden zone (HZ) at its base, then successively higher zones (lower zone, LZ; middle zone, MZ ; and upper zone, UZ) defined by changes in the cumulate mineralogy with height. In order to provide a minimally satisfctctory body of data

of known accuracy for trace elements in particular representative samples from each * Molybdenum Corporation of America, Louviers, Colorado 80131, U.S.A. t University of Indiana Medical School, Indianapolis, Indiana 46202, U.S.A. $ Code TN, NASA Johnson Space Center, Houston, Texas 77068, U.S.A. 1549

1550

THEODORE P. PASTER, DONALD S, SGHAUIVECEERand LARRY A. HASEIN

accessible zone. we have analyzed 6 whole rocks and 12 mineral separates by neutron activation for the elements La, Ce, Nd, Sm, Eu, Gd, Tb, Ho, Yb, SC, Cr, Mn, Co, Ni, Cu, Zn, Ga, Ba, As, Sb, and Ag. By a more sensible application of the model than that previously used (HU,KIN and X&SKIN, 1968), we have succeeded in obtaining a satisfactory (i.e. within experimental uncertainty) quantitative description for the concentrations of the REE, Ba, So, Zn, Mn, and Ga in rocks of the layered series. Additional factors not accounted for by the model appear to be required for the complete description of Co and Cu. Data for Cr, Ni, As, Sb, Br, and Ag were insufficiently accurate to provide more than qualitative desc~ptions of their behavior. Values of distribution coefficients for most of the trace elements between the silicate liquid and the cumulus minerals were obtained from the study. Possible uses of trace element data to set limits on sizes of zones are examined, and it is concluded that the hidden zone does not comprise more than 60 per cent nor less than 20 per cent of the mass of the entire intrusion, as opposed to earlier estimates of about 70 per cent (WAUER, 1960). EXPERIMENTAL All samples of Skaergaard materials used in this work were kindly provided from the Wager collection through the courtesy of Drs. G. Malcom Brown and C. Kent Brooks. Whole rocks EG4330, EG4427, EG4507, EG5086, and EG5181 were initially received as powdered samples, which were previously analyzed for REE (HASKIN and HASKIN, 1968). Sample of whole rocks EG4272 and EC4312 and an additional sample of EG.5086 were received as single fragments weighing a few grams which were crushed in a Plattner mortar and sized through brass sieves. Mineral samples EG4312 plagioclase, pyroxene and apatite; EG4427 plagioclase and pyroxene; EG5086 plagioclase and pyroxene; and EG5181 plagiolase, pyroxene and olivine were received as already prepared separates. Mineral separates EG4312 magnetite and ilmenite were prepared from the crushed whole-rock samples, using a combination of magnetic separation and hand picking. The purities of the mineral separates were estimated by examination of portions of each under oils of appropriate indices of refraction (for transparent minerals) or under reflected light (for magnetite and ilmenite). The results of the grain counts are given in Table 1. The purities of the separates were such that corrections for contamination proved significant only for REE and SC in plagioclase from EG4272 (apatite contamination), SC in ilmenite from EG4312 (pyroxene contamination), and Eu in olivine from EC5181 (plagioclase contamination). The REE data were taken from earlier work @ASKIN and HASKIN, 1968) for whole-rock samples EG4330, EG4427, EG4507, EG5086 and EG5181. Those for the rest of the rocks and minerals were determined by neutron activation according to the procedures of DENECHAUD Table 1. Modal analyses of mineral separates ( % of grains) EG4312 Ap AP CPX II-mt 01 Gpx PI No. of grains

100 -

tr _ _--

11

Mt

-

-

4 95 -

299 _

-I--

101

-

Pl -6.5

Px -

0.6 I_ -

98.2 0.6 -

98.9

1.2

175

169

EG4427

EG6086

Px

Pl

Px

PI

-

-

96.8 6.3 1.4

I_ --

3.6 96

97.9 0.7 _ -

100

0.4

I.4

94 _ _ 3_._ 3

100

99

128

-

1.6 369

-

EG5181 01 -

I.55 252

Px -.

144

Pl _ 100 155

Some trace elements during solidification of the Skaergaard layered series

1551

et al. (1970). Analyses for the other elements in all samples were done using a modification of procedures of ALLEN et al. (1970). Details are given by SCHAUWE~KER(1972). RESULTS Results of analyses for REE in rocks EG4312 and EG4272, both from UZb, areincluded in Table 2. The results for these rocks show, as anticipated (HASKIN and HASXIN, 1968), REE concentrations intermediate between those of EG5161 (UZa) and EG4330 (Sandwich Horizon, on top of UZc). Rock EG4312 is from a location right at the boundary between UZa and UZb, and rock EG4272 is from higher within the zone UZb. The rocks are essentially ~dist~guishable in REE concentrations except perhaps for those of Eu. Also included in Table 2 are the concentrations of REE in mineral separates from the Skaergaard rocks. The value for Eu in olivine from EC5181 has been corrected from 0.038 to 0.026 ppm according to the observed level of contamination by plagioclase (Table 1). Values for REE in plagioclase from EG4312 have not been corrected for contamination by apatite. A single grain of apatite was found in 175 grams from that separate. The general shape of the eompa~son diagram (not shown) for that separate versus the chilled gabbro (EG4507) and the apparent distribution coefficients for that separate (Fig. 1) show probable effects of contamination by apatite in the amount 04-0.5 per cent (wt). Our further use of the data for EG4312 plagioclase is hampered by the impossibility of making an accurate correction for apatite contamination. Values are included in Table 2 for completeness. No correction was made for the presence of 3 per cent olivine in the pyroxene separate for EG4427. The modal analysis for that rock shows a ratio of chnopyroxene to orthopyroxene of 33:2, or essentially 6 per cent orthopyroxene. The mode for the mineral separate contains 3 per cent orthopyroxene and 3 per cent olivine (Table 1). By making the approximation that 3 per cent of olivine contributes the same to the trace element concentrations as 3 per cent of orthop~oxene, the mineral separate as analyzed is representative of the mixture of ortho- and clinopyroxene in the whole rock. The accuracy of the analytical results in Table 2 varies somewhat with the concentration of each element, but uncertainties may be taken as less than &5 per cent for La, Ce, Sm, Eu, Tb and Yb, less than -f 10 per cent for Nd and Ho, and less than f 15 per cent for Gd. Table 3 contains data for other trace elements in Skaergaard whole rocks and minerals. A correction was made for SC because of the pyroxene in the ilmenite separate from EG4312 (Table I), reducing the concentration for that element slightly from 64 to 62 ppm. Again, no correction for olivine contamination of the pyroxene from EG4427 has been made. The analytical uncertainties for the data in Table 3 vary with concentration for each element, but are approximately as follows: < &5 per cent for Co, Cu, Mn and SC; < 110 per oent for Ba, Cr, Ga, Ni and Zn; and as much as 150 per cent for Ag, As and Sb. Values were obtained for the element Br (SCHAXJWECKER, 1972) but are not reported here Table 2. REE concentrations (ppm) in a Skaergaard whole-rock sample (EG4312) and mineral separates EC4272 EC4312 EG4312 EG4312 EC4312 EG5181 EC5088 EG4427 EC5181 EG4312 EG5086 304427 Mt. W.R. 01 PI PI Pi W.R. Ap II PI* Px Px UZB UZB Zone UZB UZB UZB UZA uza LZB MZ UZB LZB MZ LS Ce Nd Sm Eu Od Tb ::

13.1 37 10-l 3.4 12 1.65 G

13.4 41 33 9.9 2.9 12 1.65 2.4 1.63

320 990 870 280 64 320 48 43 62

3.7 9.6 8.4 2.9 0.69 2.9 0.45 0.44 1.09

0.54 1.43 1.60 0.45 0.17 O-38 0.059 0.115 0.066

0.28 0.76 0.44 0.10 0.026 0.14 0.03 0.14 1.40

4.3 9.2 3.8 0.71 1.45 0.72 0.12 Cl.088 0.11

I.75 2.90 0.99 O-172 1.55 0.14 0.020 0.175 0.026

2.31 4.49 1.62 0.30 3-o 0.25 0.036 0.046 0.032

3.0 14.1 7.4 1.76 4.3 1.94 0.27 0.27 0.24

6.8 26 25 7.6 I.24 9.0 1.55 1.9 3.6

5.5 19 19 6.4 1.21 6.2 1.06 1.26 2.5

EG5181 EG4312 Px Px UZA UZB 2.7 11*6 13.6 5.1 1.02 6.1 1.12 1.42 3.0

2.3 23 18 0.1 I.83 8.6 1.32 I.44 3.1

* Uncorrected for slight contamination by epatite. Subtmctionof REE contained in about 0.45 oA spatite, within the 1111~ certainty of the determined mode for the EC4312 mineral separtate (Table I), givesvaluesthat yield a fairiy smooth comparison diagram and very reasonable distribution coefficients.

1552

THEODORE

P.

PASTEE,

DONALD

S. SGHAUIECEER

and Laarrv A.

HASKIN

Table 3. Concentrations of other trace elements (ppm) in Skattrgaard rocks and minerals EG6086 EG4427 EG5181 EG4312 304330 EG4507 EG4312 EG4312 EC4312 EC4312 EG5181 EG5181 Mt 01 Pi W.R. W.R. W.R. W.R. W.R. W.R. Px Ap Et

Ag

AS BE CO Cr cu GE MD Ni Sb SC zn

0.5 0.2 0.30 41 64 97 98 49 <2 320 140 14 18 2200 1970 es0 100 0.2
0.4 o-3 0.14
0.4 0.7 470 0.5 <2 41 27 1360 <2 <0.05 34 370

O-16


0.3 0.023 92 1.7 <2 14 41 36 <2
because they were found to be erratic, and much higher for most minerals than for the rocks. Presumably, this is & consequence of using dense, Br-containing liquids for the mineral separations, and of handling the sample in laboratories where such liquids are usad.

The basic equations for the logarithmic model have appeared many times in the literature (for example, see the summary by i%ihTIRE, 1963). They are usually derived on the basis of thermodynamic considerations of a trace ion entering into a structural site that would normally be filled by a major ion in the mineral crystal. A diRerent derivation of the model, form~ated earlier in conjunction with some work by LINDSTROM (1963), is presented briefly below, because we believe that it offers a more direct (if less rigorous) introduction to the model than the more general thermodynamic approach. A very similar derivation has been given by SHAW(1970). Let the concentration of the trace element be low compared with the concentrations of the major ions being replaced and with those of ions involved in charge balance so that their activities are not affected in the liquid or the crystal by the presence of the trace element. Then, the behavior of the trace element oan be adequately described by a simple Nernst distribution of the trace element between the parent liquid and the crys~lizing solid (CAPERS etat.,1970). The ratio of the concentration of a trace element in a solid (C$,S) to that of a liquid (C,,.) with which the solid is in equilibrium is given by a constant (Da,s) called the distribution coefficient, as in equation (1). D

E.S =

ckL$&.L*

(11

The assumption leading to logarithmic description of the behavior of a trace element is that each new layer of solid deposited on the surface of a crystal isolates the interior of the crystal from further reaction with the liquid. Thus, equation (1) is taken to apply only to the increment of solid present at the crystal surface and the quantity &‘f.s can be written as the incremental mass dM,*, of the trace element entering the incremental mass of solid diW,. Similarly, the concentration of the elementin the parent liquid is equal to the maw of the trace element (ME,J divided by the mass of the parent liquid (ML). Substitutinginto equation (1)gives the following expression.

d%c,sld%= &MO~Z,I;I-ML).

(2)

Some trace elements during solidification of the Skaergaard layered series From

1653

& mass balance in which the total mass of the trace element in the system

is called _M,,,

and the total mass of the system MT, equation ~,,,/d%

= D,,s(M,,r

which integrates to the following M

E,S

=

-

%,s)/(JG

-

(2) becomes M.s),

form (because when MS = 0, ME,,

Jfz.0

-

[1

-

Ws/JWID”~“>~

(3)

= 0) (4)

Masses can be replaced by concentrations if both sides of equation (4) are divided by Ms, ME,T is multiplied and divided by MT, and MS/MT is called X, the fraction of the system solidified. Equation (5) results. C E.S.X = CE.TP

-

(1 -

XPW.

(5)

Equation (5) gives the average concentration C,,s,x of the element E in solid S at the fraction of solidification of the system given by X. From a mass balance, it follows that the concentration of the trace element in the liquid, CEmL,at any fraction of solidification of the system is given by equation (6). C E,L = CE,T(l

-

x)DE.“l.

(6)

Equations (5) and (6) can also be used to describe nonequilibrium partial melting (HASKIN et al., 1970).] The simplest application of equations (5) and (6) to the Skaergaard layered series would arise if a single value of D,,, for each trace element could be used to describe the partitioning of the elements between all whole rocks (as the solid phases) and the residual liquids that formed each zone. That this naive model fails badly has been shown previously (e.g. HASKIN and HASKIN, 1968). Effects of mineralogy The value of D,,s for the solid that is forming depends on the values of the distribution coefficients for the individual minerals that make up the solid. For a solid that is a solid rock composed of cumulus minerals, all crystallizing in equilibrium with the same parent liquid, D E,R

=

TfiDE.i*

(7)

In equation (7), D,,, is the solid-liquid distribution coefficient for element E in mineral i, andf, is the fraction (by weight) of the rock that is composed of mineral The symbol D,,# for a solid i (e.g. NEUMANN et al., 1954; HELMKE et al., 1972). has been replaced by D,,, for a rock. By analyzing individual minerals from Skaergaard rocks and dividing their trace-element concentrations by those of their parent liquids, it would appear possible to obtain values for D,,, through the use of equation (1). This approach requires that we estimate the concentrations of the trace elements in the parent liquids of the minerals analyzed. The method used here is essentially that proposed by WAGER and DEER (1939) and consists of summing the masses of the elements in each zone above and including the zone from which the minerals were taken, i.e.

1564

'J~EODOBE

P.

PASTER,

DONALD

S. SCIXAUWBXXER and LARBYA, I~~SICDJ

In equation (8) CIF,,,z is the concentration of E in the parent liquid for zone j, CI,R,, is the whole-rock concentration of E in zonej, and Mf is the mass of zonej. Values for subscript j begin with the latest (uppermost) zone to solidify. For the Skaergaard intrusion, j = 1 corresponds to UZc, j = 2 to UZb, and so on. The parent liquid for a particular rock within a zone does not necessarily have the s&me trace element concentrations as the liquid present when the zone first began to crystallize. The composition of the remaining liquid changes continuously as a zone crystallizes. In this particular work, however, we are using the rocks that we analyzed as representing the average for the zones they came from. No more subtle refinement was possible, and the uncertainty resulting from this approach is considered in a later section. The relative zone sizes used in our initial calculations, adapted from WAGER (1960), are as follows : UZc, 1%; UZb, 2%; UZa, 3%; MZ, 12%; LZ, 12%. These zone sizes were based on a combination of field mapping and chemical evidence. WAUER (1960) also gave a value of 70 per cent for the hidden zone, HZ. This value does not enter into the calculation for residual liquid composition, and is discussed in a later section of the paper. The estimated concentrations of trace elements in the parent liquids, obtained by use of the data in Table 2 and in HASHINand HASKIN (1968), the zone sizes given above, and equation (8), are given in Table 4. The concentrations for UZc (the uppermost zone) are taken to be those of rock EG4330. The three si~~cant figures given in Table 4 are not intended to imply that accuracy in our knowledge of the concentrations in the liquid. Rather, they are a necessary artifact of the mass balances, so that the final liquid used in calculations will not have ridiculously high or low concentrations because initial concentrations were rounded off. The proper uncertainties in these values can be estimated from the uncertainties in concentrations for the solidified zones and in zone sizes ; those uncertainties are considered later. Apparent values of QE,i for plagioclase and pyroxene for the REE, obtained through the use of equation (1) and the data from Tables 2 and 4, are shown in Fig. 1. The shapes of the curves for the apparent values of the distribution coefficients reflect clearly the relative preferences of plagioclase and pyroxene for the various REE and are similar to those found by SCHNETZLER and PHILPOTTS (1970). The values for pyroxene for LZb and MZ fall near the top of the range observed by Schnetzler and Philpotts, those for UZa and UZb near the bottom. The values for plagioclase from LZ and UZb also fall near the top of the range reported by Schnetzler and Philpotts, and those for MZ and UZa are in the lower half of that range. The values shown for UZb are uncorrected for contamination by apatite of the sample analyzed and are, therefore, erroneously high by a considerable, but indeterminate, amount. For that reason, those values will be disregarded in the rest of the discussion. From the rest of the results, it would appear that the values for both pyroxene and plagioclase decrease with increasing fraction of solidification of the layered series. This turns out not to be the case. The values for .D,*< may be influenced by ~mperat~e, pressure, mineral and liquid compositions, and trace element con~ntrations (i.e. possible nonlinear correspondence with activities). Effects of pressure are expected to be quite small for a body the size of the Skaergaard intrusion (e.g. MCINTZRE,1963). Effects of

Some trace elements during solidification of the Skaergaard layered series

La Ce Pf

Nd Pm Sm Eu Gd Tb REE

atomic

1556

Dy HO Er Tm Yb Lu No.

Fig. 1. Apparent values of the distribution coefficients for pyroxene and plagioclase, obtained by dividing the concentrations of the REE in separated minerals by the concentrations estimated for the parent liquid at the time of precipitation of each zone, are plotted against REE atomic number.

Table 4. Estimated concentrations

La Ce Nd Sm Eu Gd Tb Ho Yb Ba co cu Ga Mn SC Zn

(ppm) for trace elements in parent liquids for zones of the Skaergaard layered series

LZb

MZ

UZa

UZb

788 17.4 11.8 3.40 1.48 3.77 0.586 0.687 1.53

8.07 196 13.9 4.27 1.85 4.84 0.770 0.852 1.84

18.6 45.8 32.8 10.1 4.10 11.1 1.71 1.78 3.56

34.4 84.3 59.7 18.3 660 19.8 3.02 3.15 6.17

62.5 91.0 613 17.6 2140 36.6 156

76.8 86.4 808 19.9 2250 43.8 170

122 65.2 2143 24-O 2215 34.1 178

184 63.6 1342 21.7 2235 35.1 220

uzc 77 171 113 35 14.0 36 6.8 6.2 13.7 48 0.45 220 19.4 1360 34 69

1556

THEODORE P. PASTER,DONALD S. SCHAUWECKER and LAXUZY A.

HASKIN

trace element concentrations are also probably small (CULLERSet al., 1970, 1973). The effects of mineral and liquid compositions have not yet been carefully studied but appear, to a first approximation, to be small for the range of compositions encountered here (CULLERS et al., 1973; NA~ASAWAand SCHNETZLER, 1971). Effects of temperature can apparently be quite significant (CULLERS et al., 1973). However, the range of temperature decrease between crystallization of the cumulus minerals in the lower and upper zones of the Skaergaard intrusion (TAYLORand EPSTEIN, 1963) is too small to account for the large effect shown in IFig. 1. More concern could be expended on the above effects on values for distribution coefficients. However, those effects appear to be secondary, and most of the apparent changesin the distribution coefficients are artifacts of closed-system crystallization of trapped liquid. The pore spaces left by the random stacking of the cumulus crystals on the floor of the magma chamber are filled with liquid. As long as parent liquid can circulate freely through the pores, the crystals may continue to grow [adcumulus growth, WAGNERand BROWN(1967)]. When the liquid becomes trapped in the remaining pore spaces it must soli~fy more or less in s&% (ortho~umulus growth). Thus, the entire quantity of every element in the liquid at the time that liquid is trapped must become a part of the finally solidified rock. The original (cumulus or cumulus plus adcumulus) crystals will continue to grow as the trapped liquid freezes, and they become zoned as the composition of the liquid changes through fractional crystallization. Eventua~y, new minerals appear. Trace elements with values (1 for DE,< become increasingly enriched in the residual trapped liquid as that liquid solidifies. The cumulus minerals, despite their low values for DE,{ for such elements, will acquire higher and higher concentrations of those elements as the trapped liquid solidifies, assuming that logarithmic partitioning is still obeyed. The equations describing the behavior of a trace element during crystallization in a closed system have been given by HELMKEet al. (1972). These equations, based on the logarithmic model, apply to the crystallization of the trapped liquid. In that (simplest) treatment, the trapped liquid is presumed to solidify to a particular set of minerals (i), each in a constant mass fraction fi’ of the solid that is forming, and each with its own, constant value of IJEei. The fraction of the trapped liquid solidified is given by z. The average concentration of E in the entire ~t~~~~~~us portion of mineral m is given by equation (9) [HELMKEet al., 1972, equation (6)]. Cg,t is the concentration of E in the initially trapped liquid and D,,, is the equivalent of D E,R [equation (7)] for the trapped mineral suite. c

E:.m.z =

icmi.l

-

(1

-

~)~~,~~/~~~[~~‘

+

( 2

~~,j~~)~D~,~l.

p‘#778

(9)

When the trapped liquid has completely crystallized (i.e. when z = 1) c

Earn.1 =

C~.t%mlZ:

i

Qzifi’.

(10)

The fraction of each mineral in the orthooumulus part has been designa~d f$’ to distinguish it from the fraction in the entire rock, given by fi. In the case of the Skaergaard layered series, part of each rock (the cumulus or

Some trace elements during solidification of the Skrtergaardlayered series

1557

cumulus plus adcumulus materials) corresponds to crystallization in equilibrium with large amounts of parent liquid, so that the relationship between Cx,( and C,,, corresponds simply to equation (1) with Cx,< set equal to C&J. No significant change in CE.L occurs during the solidification of the cumulus and adcumulus portions of the minerals in a given rock. The values for C,,, do change significantly during solidification of an entire zone. The rest of the rock is produced by crystallization of the trapped liquid, and the behavior of the trace elements is approximated by equation (9). [The use of equation (9) can be made as exact as is desired by stopping the calculation at every value of x for which there is a significant change in mineralogy, DE,i, or fi’, putting in the appropriate new values, and beginning again, setting x once more equal to zero.] If the cumulus plus adcumulus fraction (by weight) of the mineral is called Y, (and, therefore, the orthocumulus fraction is 1 - Y,) then, from equations (1) (modified) and (lo), the average concentration for E in the mineral m is given by equation (11)

c ~.m

=

CE,LDE,~[Y~ + (1 - KJ/T DE,tfi(l - Yi)/Cfi(l -

Yi)l*

(11)

E

The use of a single parameter (C,,,) for th e l’q 1 ui‘d in equation (11) implies that the parent liquid for the cumulus and adcumulus materials is taken to be the same as the trapped liquid. The quantity fi’ has been replaced by the equivalent expression, fm’ =fWX(l- y7JJ/~fi(1- yi)*

i

The value of the distribution coefficient D EvR for a whole rock, to be used with equations (5) or (6), is given by equation (12). D E'.R = ~DE,J~Y~

+

F.fdl

-

(12)

Yi)*

In equation (12), the second term is merely the fraction of the rock that is composed of orthocumulus minerals. The effective value of the distribution coefficient for the entire orthocumulus portion of the rock is one. The concentration C,, R of E, then, in any whole rock is given by equation (13), which is simply a sum over all of the constituent minerals.

c E.R

-

CE,L

~fiDE.i[Yi

+

(1 -

J'i)/T DE.ifi(l

-

Yi)/T.f
-

Yt)I*

(13)

Distribution coeficients for trace elements in the Skaergaard layered series

Equations (5) and (6) describe the solidification of a body of silicate liquid according to the logarithmic model. In order to match that model to a real system like the Skaergaard layered series it is necessary to evaluate for that system the parameters C,,, (the average concentration for the entire series, which is also the concentration of the initial liquid from which the series crystallized) and DE,R, the distribution coefficient for the element in the forming solid. The direct application of equations (5) and (6) is complicated because values for D,,, are a function of X, the fraction of the original material that has solidified. This change in D,,, with X results from changes in pressure, temperature, composition of the liquid, percentage of trapped liquid, and modal proportions of the crystallizing minerals. Thus, equations (11) and (13) describe the concentration of an element in a mineral 5

1568

THEODORE P. PASTER,DONALD5. SCEAUWECKER and L-Y

A. HMKIN

or a rock, and equation (12) gives an expression for DE,,. To obtain the quantity as described in equation (12) requires a detailed petrographic and chemical D an$Fsis of a rock. Values for C,., oan be obtained from a chilled gabbro for an intrusion (if a proper one exists) or by a mass balance based on field data for zone sizes plus chemical analysis of average rocks for each zone. Values for DE,*, required for use in equation (1.2), can come from measurement of distribution coefficients in the laboratory or from natural systems. However, owing to the wide range in values reported in the literature and to our present lack of information on the magnitudes of the effects of P, T, composition, and kinetics on values for distribution coefficients, we choose to extract values from data on the Skaergaard rocks and minerals themselves. Values for DE,, cannot be extracted from equation (11) even if C,,, is measured for every major mineral because concentrations of E in late-stage orthocumulus portions and in cumulus plus adcumulus portions of minerals cannot be measured separately. Nor can values of Y, be accurately obtained by petrographic studies. Thus, we make the following simplifying approximation, namely, that we can represent Y, by the mode for mineral m. We then obtain equation (14) from (8) and (11). D E.m = QE*, I$ Jf*KF + (1 j

F)/c

f

DE.&l/~

CE,R.lHP

04

j

The approximation made in obtaining equation (14) is tantamount to assuming that the cumulus plus adcumulus and the orthocumulua portions of the rock crystallized with the same minerals and in the same modal proportions. In general, this would be a very poor approximation, It proves to be a satisfactory approximation, however, for rocks that consist almost entirely of cumulus plus adcumulus material. In that case, the quantity (1 - P) in equation (14) is very small, so that the orthocumulus additions of material have a relatively small effect on the average compositions and modal proportions of the cumulus plus adcumulus minerals in the rock. That is, equation (14) can only be sensibly applied to rocks with very low proportions of frozen trapped liquid. Two of the rooks studied, EG5181 (UZa) and EG4312 (UZb), consist mainly of cumulus plus adcumulus material. Thus, 1p approaches unity (092 for UZa and 0.9’7 for UZb), (1 - P) is relatively small, and an approxima~ knowl~ge of values ofl) xef and fi is adequate to generate a value of D,,, whose uncertainty is controlled by factors other than the assumption leading to equation (14). Values for P were initially estimated from the P,O, concentrations of the rocks, as discussed by WAUERand BROWN(1967). The initial estimate of EG4312 (UZb), which contains cumulus apatite, was based mainly on petrographic evidence. The small amount of zoning in that rock indicates extensive adcumulus growth. Refinement of these values is discussed in a later section. The values finally used, however, are those quoted above. To obtain values for the distribution coefficients, Dsam, the estimates of WAGER (1960) (listed earlier) for the relative sizes of the Skaergaard exposed zones (.M& are used, measured values for C,,, are used, and F is set equal to unity for UZa and UZb. The approximate values of D,,m so obtained for the various minerals are then put into equation (14) as D,., values, F is set equal to its proper value, and a

1669

Some trwe elements during eolidifioationof the Skaergwrd layered series

Table 6. Vafues of distribution coefficient8 for minerals from the Skeergaaxd layered series

La 03 Nd Sm EU Cd Tb Ho Yb 338 co ckl

Ga Mn SC Zn

Pl EG6181 uza

EC6181 UZa

0.069 0.062 0.028 0.017 0.08 o-014 0.013 0.011 0.009 0% 0.026 0,004 1.70 0.016 0*008 o-13

0.0084 0.010 0.008 0.006 0~008 0.008 0~010 0.08 0.28 <0~06 3.1 0.023 <0*26 2.6 0.33 1.8

01

Mtl Eg4312 UZb

II Eg4312 UZb

8.6 11.2 14.0 14.6 9.6 15-8 15.4 13.3 8.1 -Co.05

0.016 0.016 0.026 0.024 0.026 0.018 0.019 0.017 0.018 -

0.098 0.11 0.14 0.16 0.10 0.14 0.14 0.13 0.17 -

-Co.03 0.28 <0*26 0.13 0.22
3.4 0.42 2.0 1.4 o-73 2.6

2.2 1.46 0.14 1.9 1.8 O-38

Px EC4312 UZb

AP EG4312 UZb

0.001 0.26 0*29 0.32 0.27 0.41 0.42 0.44 0.44 <0.05 I.2 0.071 CO.25 1.6 3.3 0.49

better value for D,,, is obtained. The procedure is iterated until a self-consistent set of values for DE*, is obtained. Values for DE:,,, for the Skaergaard rocks, obtained essentially by the above procedure and using data from Tables 2 and 3 for pyroxene, apatite, magnetite, and ilmenite from EG4312 (UZb) and for plagioclase and olivine from EG518 1 (UZa) are given in Table 6. To a tit app~ximation {for pyroxene, about a factor of 2, for plagioclase about a factor of 3), values of II,,, obtained from data for plagioclase and pyroxene from the other Skaergaard rocks agree with those in Table 5. Values of D,,,, obtained from data for minerals from LZb and MZ are inherently less precise because of the much higher quantities of trapped liquid (62 per cent and 15 per cent) so that the bulk of most trace elements in those rocks is a part of the orthocumulus material. The values given in Table 6 for REE in plagioclase and pyroxene fall near the bottoms of the ranges for those minerals as determined by Schnetzler and Philpotts, whose values range over about a factor of 10. That range may arisefrom variation in temperatures of crystahization or in compositions, but may also involve effects of closed-system crystallization and further growth of phenocrysts during quenching of the magmas that were used in their determinations. The pyroxenes analyzed from the Skaergaard intrusion are predominantly olinopyroxenes. Modes are given in Table 1. Values for the distribution coefficients for REE entering apatites have been determined by NAGASAWA (X970), and are considerably higher (factors of 16-&2) than those of this work. The reasons for the differences are upon, but may be ~rn~ratu~ and composition. Values determined by NAGASAWA (1970)and by us for Dz, in apatite are considerably lower than that expected from interpolation between values for neighboring Sm and Gd. This can probably be attributed to the concentration ratio of Eue+ to Eua+ in the parent liquid (PHILPOTTS,1970; NAUASAWA, 1970). Values obtained for DRIE in olivine are similar to those reported by SCHNETZLER

1560

THEODORE P. PASTER,DONALDS. SCHATJ~ECKER and LARRYA. H~xm

and PHILPOTTS(1970).Apparently, no values have been reported previously for magnetite or ilmenite. We regard those given in Table 5 for REE in those minerals as upper limits. High concentrations of REE have been observed for lunar ilmenites (e.g. PHILPOTTS and SCHNETZLER, 1970; GOLESet al., 1970)but have been attributed to presence of late-stage trapped liquid. Values for DBa in olivine and pyroxene are within the rather broad ranges reported by PHILPOTTS and SCHNETZLER (1970).That for DBain plagioclase exceeds their largest value of 0.59. The value of DM, in plagioclase is only a third as large as that reported by HIQUCHIand NA~ASAWA (1969), but values for Co, Mn, and SC in pyroxene agree well with those obtained by ONUMAet al. (1968) for augite.

Estimation of uncertainties in distribution coeficients The uncertainties in values for some of the parameters used in equation (14) are known. Those for other parameters must be guessed. Known sources of error are discussed below. The purities of the mineral separates have been measured by grain counts, as described previously. The analytical uncertainties have also already been discussed. The overall analytical uncertainties in C,,,, for the elements used in the modeling are taken to be f 10 per cent. The masses of zones UZb and UZc are not known exactly because much of each zone is buried. Based on WACER’S(1960) estimate of the exposed areas of zones UZb and UZc and various assumptions about the taper in the walls of the intrusion, CHAYES(1970) gives the following extreme estimates for the relative sizes of those zones: UZc:UZb -0.7:6.8 to -6-O: 11.4. How much uncertainty this introduces into values of DE,~ depends on the relative concentrations of the element in the two zones. The extremes of uncertainties caused by Chayes’ limits range from f 1 per cent for SC to & 85 per cent for La. As will be shown later, the extremes given by Chayes fall outside the constraints on zone sizes obtained from the trace element data. Thus, the uncertainty in values of DE,, owing to uncertain knowledge of zone sizes does not appear to exceed &I30 per cent. The sampling uncertainty is considerably more difficult to assess. The rocks we analyzed were carefully selected by Dr. G. M. Brown as being ‘average’ rocks for each zone, by which is meant “ . . . a rock with cumulus crystals corresponding in proportions to those of the total phenocryst assemblage precipitating from the liquid at that particular time...” (WAOERand BROWN,1967). However, an average rock for our purposes needs also to have an average ratio of cumulus plus adcumulus to orthocumulus material for the zone it represents. Wager and Brown note, “... variations in the amounts of (trapped liquid) will affect the compositions of the average rocks as just defined but generally less strongly than variations in the proportions of the cumulus crystals”. That may be true for major elements, but certainly is not for most of the trace elements in this study. We estimate from published P analyses (WAGER and BROUTN, 1967) that the uncertainty in average character of the rocks as required for our purposes may be &25 per cent for trapped liquid content and 110 per cent in the mode for the cumulus minerals. Finally, we combine these uncertainties as if they were random uncertainties and conclude that the values for D ~,i in Table 5 are known to approximately 150 per cent. That uncertainty is indeed large, but far smaller than the ranges for values obtained by other methods. Thus, the values of D E,C in Table 5 are much more restricted than merely being within the range found for other systems and by other methods. The number of significant figures given in Table 5 does not reflect the uncertainty in accuracy for the values given. It does reflect properly the relative values for the various elements as a self-consistent set for the optimized model used in obtaining them. By similar arguments, we estimate the uncertainties in value of DE,R [equation (12)] as &50 per cent, and with the uncertainties in the relative volume of UZc of -&30 per cent and ranging down to - 410 per cent for the lower zones, the uncertainties in values of CE,,,L [equation (S)] are about *30 per cent. Uncertainties for calculated values of CE,~ [equation (13)] are about &30 per cent. We emphasize that, although known uncertainties could be

Some trace elements

during solidification

of the Skaergaard

layered

series

1561

properly propagated through the equations, many of the uncertainties have had to be estimated. Thus, the uncertainties for calculated values, estimated above, are approximate results. Even with these rather cumbersome uncertainties, significant tests of the logarithmic model as applied to the Skaergaard layered series can be made.

Variations in values for distribution coefficients Now that the uncertainties in estimated values for DE,~

have been examined, true variations in values, owing to changes in composition, temperature, etc., can be sought. Figure 2 shows the values for D,,, for plagioclase and pyroxene from zones LZb, MZ and UZa, compared with those values given in Table 5. Note that, in comparison with the spread in values shown in Fig. 1, apparent disparities in values for DE,,, from different zones have been greatly reduced. Orthocumulus portions contain as much as 80 per cent of all the REE in some of the minerals analyzed. The uncertainties of -&50 per cent still apply to each set of values for DREx,m. Much lower uncertainties apply, however, to the relative values within each set. Thus, we regard as meaningful deviations from a straight, horizontal line the values for D,, for pyroxene, for D,, for both for plagioclase from LZ. The values for DL8 plagioclase and pyroxene, and the slope for D,, for pyroxenes suggest that the value in Table 5 is a factor of 2 too low. The reason is not known; no error in the analytical result for La in the pyroxene from UZb could be found. The high values of DE,, for the heavy REE in plagioclase may indicate that anorthite has a greater affinity for heavy REE than albite. Such a possibility is supported somewhat by data obtained by SCHNETZLER and PHILPOTTS (1970) and NAGASAWA and SCHNETZLER (1971). The ‘anomalously’ high value for Eu for pyroxene in LZ appears too large to be accounted for by E lower concentration ratio of Eu 2+ to Eu3+ in the liquid from which the pyroxene in LZ crystallized, as compared to that for UZb (MORRIS and HASKIN, 1974). The estimated concentrations of the REE in that parent liquid (Table 4) do not indicate a significant relative loss of Eu to cumulus plagioclase during crystallization up to that zone. The anomalous Eu value probably results from two factors. First, the difference in relative REE abundances between EG5086 whole rock and those of the chilled gabbro (HASKIN and HASKIN, 1968) suggests that the particular sample of LZ rock analyzed did not quite behave as a closed system. The specimen may have been too small to exceed the range of migration for trapped liquid during crystallization, as found for rocks of the Stillwater intrusion (KOSIEWICZ and HASKIN, to be published). Second, no variability m DE, for plagioclase was considered when equation (14) was used to obtain the value for DEu,pyroxenealthough more than 50 per cent of that rock developed by orthocumulus growth. The values obtained for the other REE and for all REE in plagioclase, however, appear to be reasonable within their limits of uncertainty. The systematic increase in DE, with increasing fraction of solidification of the layered series was anticipated from the work of SCRNETZLER and PHILPOTTS (1970), who showed the dependence of the distribution coefficient for that element on the anorthite content of the feldspar. Apart from the deviations from straight lines, the values of DE,~ shown in Fig. 2 cover a range of about a factor of 2 for pyroxene and a factor of 3 for plagioclase. Such variations leave plenty of latitude for effects of T and composition, but they are so nearly in line with experimental uncertainties in extracting D values from rocks with large fractions of orthocumulus material that they cannot be strongly defended as being real. Thus, in the following sections of this paper we take the approach that the values for D obtained from EG5181 (UZa) and EG4312 (UZb) are valid for all zones of the layered series. We then construct models for the layered series to see whether that assumption requires modification. Paucity of data for other elements in minerals from the lower zones precludes a straightforward analysis of variations for other elements. SCHAUWECKER (1972) estimated the distribution coefficients (corrected for trapped liquid) for Ba, Co, Cu, Ga, Mn and SC by using trace element data from WAGER and MITCHELL (1951) along with the estimates for liquids in Table 4. These values are not included in the tables in this paper. Probable uncertainties for the values obtained range as high as &70 per cent. Within that range, all but four values agreed with those given in Table 5. Values based on the data of Wager and Mitchell for Cu in plagioclase and pyroxene, for Mn in olivine, and for SC in pyroxene fell outside the range of &70 per cent., Questions of purity of mineral separates analyzed by Wager and Mitchell cannot be answered,

lb62

THEODORE

P.

DONALD S. SGEA~~E~~E~

PASTEB,

Nd Pm Sm Eu Gd fb

La Ce Pr

and

Dy. Ho Ef

LABBY

A. ~KIX

Tm yb Lid

REEkwile to&$ Fig. 2. The e&mated values of the distribution ooeffcients for pyroxene fioxn zones LZ, MZ and tTZa have been divided by those for zone UZb, and the estimated vduea of the distributioncoefficientsfor plagioclasehave been divided by those for zone UZa, and the resultingratios have been plotted against BEE atomic number. but as little as O-1 per cent included born&e, a commou copper-rich sulfide in the Skaergaard rocks (WA~ICRa,ndBBO~N, 1967)could account for the variations in vahresfor I)cn. The value for WM,, in olivine may reflect the increased ease of substitution for that element into more fayalite-rich oliviues, as is favored by the greateriouiaradiusof Few than of Mgs’. Qualitatively the apparent increase in W,, for the higher zones follows the imweasingnormative ratio of chuopyroxene to orthopyroxene in the oumulates (Table 6). This is diBioult to reconcile, however, with the modes; the change in the eon~nt~tion ratio for those minerals is small. The details of behavior of SC remain obscure (e.g. NORand BASKIN, 1968).

A~~~~~~~ of the ~u~t~~~c

fez

to the ~0s~

zxmes of tile ~~e~~ series

In the previous section, making use of equation (14), we obtained a set of values for J&Kin for the various trace elements in the various minerals that accumulated to produce the Skaergaard layered series. The values regarded as the most reliable (Table

6) are those obtained from analyses on two rocks high in the series, EG5181

(Uza)

and EM312

(UZb).

Values

calculated

for rooks lower in the intrusion are

less reliable because of the large effects of o~ho~urn~us element concentrations.

cont~butions

Thus, effects of T, P, and composition

to their trace

on values of DE,,,

‘Fable 6. Average modsl mineral Gompositionsfor zones,* predicted modee for cumulusmiuerals for zones,t and modes for cumulus minerals in eamples ax&y-z&it LZ AV. Pi PX

01

&i&n Tr.Liq.

Mod Eo6086 Cum Cum Av.

66 60 26 19 17.3 29 0 tr

*

_t”

* WAOHI end 7 ThiSI work,

MZ

6:

UZS

Mod 3614427 Cum Cum Av.

64f 19 60 2S& 4 36 1842 0 0 tr 2

LB 38 0

6Of 6 36f 4 0

ao+

1:

4 23&3

6

BBOWN(1967).

”

0

0

UZb

EaS181 Cum Cum Au.

Mod

62 20 II 0.2

ii 12 0

6.2 -

7 8

62rf: B 2Of 3 II&3 0 7 6f3

46 28 17 2.6 “”

Mod Cum 46 31 16 2.1 5 3

EU4312 cum 46f 6 28f 3 17f 4 2-5 C8 C6

Some trace

elementsduringsolidificationof the &aegrtard

layered series

1563

were mostly lost within the uncertainties of the estimates. We now construct a model for the exposed zones of the Skaergaard layered series to determine whether the values for L),., in Table 5 can be used to describe the solidification of the entire series or whether values of B,,, must change as soli~fication progresses. We also use the model to determine the effects of changes in parameters such as zone size and mineralogy on the values for D obtained through the use of equation (14). The logarithmic model was used to predict the trace element concentrations in the rooks of the exposed zones of the Skaergaard layered series as follows: (1) The composition of the liquid that crystallized to produce the five exposed zones was obtained through the use of equation (8). Analytical values for ‘average’ rocks from those zones (Tables 2 and 3) were used as the values for C,,,. The relative sizes of the zones (LZ, 12 ; MZ, 12 ; UZa, 3 ; UZb, 2; UZc, 1) were those given by WAGER(1960). [Equation (8) was used only once, with j = 5.1 (2) Distribution coefficients D,,, were calculated for the lower zone by use of equation (12). Values for ft were taken from the modes for average rocks (Table 6) as given by WAGERand BROWN(1967) and those for Y, were based on the concentrations of P in average rocks (Table 6, under heading ‘Cum’). Values used for D,,, are those in Table 5. (3) Values for Cg,g,e were calculated for the lower zone by use of equation (5), letting X = O-40 (the fractional mass of the lower zone), using the values obtained in step (1) for Cx*r, and the values (DEeR) obtained in step (2) for D,,,. The results of this ~~l~~ation can then be compared with the observed concentrations for the average rock from the lower zone (in this case, EG5086). (4) The composition of the residual liquid following the precipitation of the lower zone is calculated through the use of equation (6). The values for the various parameters are the same as those used in step 3. (5) Using the values for the residual liquid in place of those for the entire exposed zone, and using the fraction of the total remaining mass that corresponds to the middle zone, the process is repeated, using parameter values appropriate for that zone, and setting X = 0.667 in equations (5) and (6). (6) Sequential precipitation of the zones is continue until UZb has been produced, then the residual liquid is quenched to produce UZc. The initial attempts to model the trace element concentrations in the exposed zones were done by the above procedure. A computer was used to facilitate the COmpUtatiOIlS

(SCHAUWECKER,

1972).

The anaI~ica1 data for the trace elements (Tables 2 and 3) in average rocks, like the estimates of zone size, estimates of mineralogy for average rocks, etc., are part of the set of observational data on the layered series. Since all of these parameters have considerable latitude of uncertainty, we attempted to optimize the model to see what changes in other parameters would give the best fit to the trace element data. Changes in zone size, mineralogy, etc., produce corresponding ohanges in values for distribution coefficients, etc. Thus, in this exercise, only the analytical data for trace elements were left as inflexible parameters. There proved to be too many variables for the computer to optimize on its own in a reasonable length of time, so the final fit to all parameters was done as a combination of our intuition and computer checks. Few significant improvements in the fit could be

1664

THEODORE P. P_~STER, DONALD S. SCHAUWECKER and LARRYA. I&SKIN

made by reasonable adjustments to the parameters initially chosen. Unreasonable adjustments (e.g. severe changes in miner&logy) gave poorer results. In the model giving the best fit, the values for the relative zone sizes are altered very slightly as follows: LZ, 40; MZ, 40; UZa, 105; UZb, 7; UZc, 3.5. The values given in Table 5 for the distribution coefficients reflect the final mineralogy used [which has a very small effect, because it appears only in the term for orthocumulus material in equation (14), and that term is small for the rocks used in obtaining the values for the distribution coefficients]. The final mineralogy shows the greatest effect on optimization. The optimized cumulus mineralogy is shown in the second oolumn for each zone in Table 6. In the third column for each zone in Table 6 are given the modal analyses for the rocks used in this study, based on our examination of two thin sections of rocks EG6086, EG4427. and EG4312, and on modes supplied by Dr. C. K. Brooks (personal communication) for EG5086 and EG5181. The modes for the cumulus portions of the rock estimated from the thin section analyses (column 3 in Table 6) agree quite well, within their rather substantial uncertainties, with the modes for the average rock, which include the orthocumulus portion. This supports somewhat the use of the appro~mation made in the derivation of equation (14) for application to the Skaergaard rocks. The only significant disagreements between the optimized values for the mineralogy and the estimated cumulus modes are for the relative amounts of olivine and pyroxene in the lower zone. The higher concentration of olivine and lower concentration of pyroxene required by the model could equally well be satisfied by a higher concentration of orthopyroxene and a lower concentration of clinopyroxene. The values for distribution coefficients are much higher for clinopyroxene for most trace elements than are those for olivine or orthop~oxene. The average rock used to describe the entire lower zone came from LZb. Ca-rich clinopyroxene may not be a cumulus phase in LZa (WAGERand BROWN,1967) so the average concentration of that mineral for the entire zone may be significantly overestimated by use of the mode for LZb. Olivine is a cumulus mineral throughout the entire zone. Other significant differences are in the amounts of trapped liquid (orthocumulus component) in the various zones. The quantities of trapped liquid used in the initial calculations were based on an estimated mass balance for P (WAQERand BROWN, 1967). It is assumed that P does not readily enter into any cumulus phase. Accurate chemical analysis for small amounts of P in rocks is quite difficult, however, and can easily lead to errors. The values for P concentrations in rocks from several zones show considerable variation (which may very well be real) so that a properly representative value is hard to select. Actually, P is merely one of a large series of minor and trace elements whose values for DE,< are low for all cumulus minerals (except apatite, in the case of P). Thus, the predictions of the model may be regarded as better in~catio~ of the proportions of o~hocumulus material in the various zones than the prior estimates based on P analyses. Although it is not reflected in the tables, the fit could be slightly improved by inclusion of a small amount of olivine in MZ. Since that mineral is not observed in that zone, the small improvement was rejected. The extent of agreement of the predictions of the optimized model with the observed values for trace elements in the average rocks is analyzed as follows.

1565

Some trace elements during solid~cation of the Skaergaard layered series

Skaergaard intrusion Exposed zones

I.50 Calculated

concentrutions

/experimental

A

z Zn 1.75

worse

concentrations

Fig. 3. Ratios of predicted concentrations for trace elements in the Skaergaard zones to observed values for ‘average’ rocks from those zones are shown as an illustration of the extent of agreement between the model and the samples for the exposed zones of the Skaergaard layered series. Identification of squares is a follows: horizontal lines, LZ; vertical lines, MZ; lines with positive slope, UZa; lines with negative slope, UZb; dashed horizontal lines. UZc.

The predicted value for each trace element is divided by the observed value, and the results are shown as a histogram (Fig. 3). We use, as the criterion for acceptable agreement, a range from 0.7 to l-3 for the ratio, reflecting the unavoidable N -&30 per oent uncertainty estimated earlier for values of C,, x and based on the uncertainties in the parameters in equation (13). Better agreement is welcome, but cannot be expected because of uncertainties in zone sizes, selection of average samples, etc. Figure 3 indicates satisfactory agreement for all zones for REE, Ba, Ga, Mn, Se, and Zn, with the following exceptions: LZ, La; MZ, Zn; UZa, Ce, La, Zn; UZb, Ba, Yb, Zn; UZe, Zn. Values have been omitted from the model for Ag, As, Cr, Hf, Ni, and Sb because their con~ntrations were so low in some of the rocks and minerals analyzed that an attempt to include them in quantitative modelfing was not considered worthwhile. For most, the qualitative behavior expected on the basis of model is quite consistent with the analytical results. Values for Cu, Co, and Eu have been omitted because of known factors not included in the model that seriously affect their behavior. These influences are described in a later section. Thus, from the optimized results for the exposed zones of the layered series, we have provided a set of values (Tables 4-6) for the variable parameters of the model that are set-~nsis~nt and reasonable in view of the ~ce~inties discussed above with respect to the nature of the model itself and the observational data. We caution that, just because these optimized values provide a self-consistent set, they are still subject to considerable uncertainties in terms of describing accurately the structure and composition of the Skaregaard layered series. This is because of the limited number of and the uncertain nature of the observational data (e.g. selection of average rocks) used as inputs in the calculations. We regard it as significant to have shown that the log~thmic model, as applied, can provide a reasonable quantitative description of the exposed zones, consistent with observational data within their uncertainties. The uncertainties of the parameters used are large enough to

1666

TEE~D~RE

P. PASTER,DON&DS. Scrr.~uw~cxxnand L-Y

A. PX~sxm

obscure, at least for most elements, real and significant expected dependence of values for distribution coefficients on temperature and composition. Only a single set of distribution coefficients was needed to provide adequate agreement between the predictions of the model and the values observed for the average rocks.

In the previous section, the #ncentratio~s of the elements for liquid that crystallized to produce the exposed zones were obtained by summing over zone sizes and analyzed rock compositions. The Skaergaard intrusion does have, however, a chilled marginal gabbro whose composition presumably reflects the starting composition for the entire layered series. It is possible to incorporate that composition into the model. In order to do so effectively, it would be necessary to know the size and mineralogy of the hidden zone (HZ). The size of the HZ has been estima~d by WAGER (1960) as 70 per cent. That estimate, although necessarily consistent with petrologic information, is based primarily on mass balances for P and S. Both are minor elements whose values for distribution coefficients in cumulus minerals are quite low and whose analysis in igneous rock is difficult. Therefore, we regard the value of 70 per cent for the size of the HZ with reservation. We chose as fixed values for modeling the composition of the chilled gabbro EC4507 as representative of the starting composition for the entire layered series plus the optimized values given in Tables 5 and 6 and the optimized values given in the last section for the distribution coefficients, the cumulus mineralogy, and the relative sizes of the exposed zones. The proportion of trapped liquid and the mineralogy of the HZ were varied, as was the size of the HZ relative to those of the exposed zones. The earliest rocks in the layered series are inferred to have been cumulates of plagioclase, olivine, and pyroxene, with some evidence favoring their abundances to decrease in that order (WAGERand BROWK,1967). The fraction of trapped liquid was estimated at about 35 per cent (WAGER,1960). Satisfactory combinations of the variable parameters were found that provided satisfactory estimates for most of the elements in most of the zones over a range of ~20-~80 per cent for the size of the HZ. The principal parameter whose value had to be varied sympathetically with zone size in order to provide a good fit was the percentage of trapped liquid (orthocumulus material). Figure 4 shows the results of the fit for assumed values of 20,50, and 80 per cent for the size of the HZ. Table 7 gives the co~spon~ng values for mineralogy. Figure 5 shows the amount of trapped liquid required for a good fit as a function of size for the HZ. A probable upper limit of 50 per cent to the amount of trapped liquid in the II2 has been obtained by experiments using aluminum plates (WAGER and BROWN, 1967). This value agrees well with that of 52 per cent inferred earlier for the LZ. From Fig. 5 it is seen that a limit of 50 per cent for the trapped liquid limits the size of the HZ to not more than half that of the layered series. A value of 70 per cent for the HZ corresponds to the ~ea~nably high value of 62 per cent trapped liquid. Reduction of the trapped liquid concentration to zero still requires that approximately 20 per cent of the mass of the layered series be present in the HZ. Without

Some trace elements during solid&&ion

of the Skaergaanl. layered series

HZ= 20%

IO

5

0

vtuse 0.50

0.75 Colculo+ed

wsse

150

i-25 concentrationqlexper~~ol

concentrO+iinS

Fig. 4. Ratios of predicted concentrations to observed concentrations are shown for the exposed zones in the Skaergaaxd layered series when the composition of the chilled marginal gabbro is used aa the average for the layered series and the relative size of the hidden zone is taken to be 20 per cent (top), 60 per cent (middle), or 80 per oent (bottom).

Table 7. Estimated oumulus mineral and trapped liquid oontents for the hidden Eone according to the size chosen for that zone

Pl CPX opx or 01 T.L.

20%

50%

80%

66 IO 36 0

30 5 15 50

16 2 8 76

1667

1568

THEODORE P. PASTER, DONALD 5. SCEA~~ECKER and L-Y

A. HASKIN

*A Trapped liquid in Hidden Zone

Fig. 5. The fraction of the hidden zone that must consist of frozen trapped liquid is shown as a function of the relative size assumed for the hidden zone in the layered series.

that quantity of cumulus minerals extracted from a liquid of the composition of the chilled gabbro, concentrations for most of the trace elements in the LZ and higher zones would be too low. Thus, the relative size of the HZ appears to be constricted to a value between 20 and 50 per cent by the trace element data. It is interesting to note that, in their original paper on the Skaergaard intrusion, WAUERand DEER (1939) predicted a value of only about 50 per cent for the relative size of the HZ, based on an extrapolation of intrusion boundaries. Recent gravity studies indicate a much smaller size for the HZ than has earlier been supposed (BLANH: and GETTIN(IS,1972). The mineralogy for the HZ that gave the best fit is based on cumulus plagioclase and pyroxene and is not entirely in line with predictions based on fteld data for other Skaergaard rocks (WAGERand BROWN,1967) . Olivine is believed to be an impo~ant constituent. Pyroxene has the advantage over olivine of lower values of its distribution coefficients for Co, Zn, and Mn, which are too rapidly depleted from the liquid when olivine is precipitated. On the other hand, the crystallization of pyroxene too rapidly depletes the liquid in SC. This we have arbitrarily rationalized by suggesting that most of the early pyroxene was orthopyroxene (Table 7). The lower value for the distribution coefficient of SC for orthopyroxene than of clinopyroxene makes the mass balance for that element work out. What may be happening is the following. The olivine expected to precipitate should have the composition Fo,, (BARER and BROWN, 1967). Such a magnesium-rich olivine might be expected to have lower distribution coefficients for Co, Mn, and Zn than that analyzed from UZa, whose composition is approximately Fo,,. This possibility is anticipated because the ionic radii for Goa+, Mn2+, and Zna+ are close to those of Fez+ and, therefore, significantly larger than those of Mg2+. Values of distribution coefficients for

Sometraceelementsduringsolidjficcttion of the Skaergaardlayeredseries

1669

magnesian olivine for Zn of O-95 (GUNN, 1971) and for Mn of 14X-1*8 (GUNN, 1971; HENDERNN and DALE, 1969~1970) as compared with the values of 18 and 2.6 for the highly ferroan olivines in this work are consistent with that suggestion. The values for Co for ma~esian olivine (14-45, average ~3.0; GUNN, 1971; HENDERSONand DALE, 1969/1970) do not support the suggestions. Early precipitation of orthopyroxene in the layered series is expected (WACER, 1960; WACIIRand BROWN, 1967). That the predom~ant early cumulate magnesian silicate was orthopyroxene rather than olivine is perhaps supported by the disappearance of cumulus olivine in MZ. However, the case for orthopyroxene instead of olivine in HZ is far from strong and, in any event, the use of olivine in place of some or all of the orthopyroxens used to obtain the best fit would not significantly affect the predictions of the model for other trace or major elements. The variations of the distribution coeffcients with temperature and liquid and crystal compositions for elements such as Co, Mn, SC, and Zn that readiIy enter mafic minerals are simply not well enough known to set more accurate limits on the cumulus mineralogy of the HZ. That problem does not, however, affect the arguments for the relative amounts of trapped liquid and the size of the HZ.

A successful model for trace elements must, of course, be entirely consistent with observations of and massbalances for major elements. The concentrations of the major elements have been obtained from the model by breaking down the minerals given in Table 7 for the HZ and in Table 6 (predicted cumulus plus trapped liquid) for the other zones into their normative components, then summing the values for the oxides. The results of these calculations are given in Table 8. The columns marked ‘20’ are for an HZ that constitutes 20 per cent of the layered series, those marked ‘50’ refer to a 50 per cent HZ, and those marked ‘80’ refer to an 80 per cent HZ. The mineral compositions used to obtain the estimates of major element analysis are given by SCHAUWE~~ER (1972) and are essentially those given by WAGERand BROWN (1967) as the averages for the minerals in each zone. The HZ was presumed to contain orthopyroxene rather than olivine in these calculations. The fraction of the HZ that is made up of trapped liquid for each case can be found in Fig. 5 or Table 7. The first column in Table 8 gives the concentrations of the major elements in the sample of chilled marginal gabbro regarded by WAGER and BROWH (1967) as the most nearly representative sample for the intrusion. Those values marked ‘obs’ are analytical values for representative rocks from each zone (WAGER and Baow~, 1967) except for the HZ, for which the values estimated by WAGER and BROWN(1967) are given. (The major element Gon~ent~tions for the upper zones vary slightly despite constant zone sizes, cumulus mineral proportions, and cumulus mineral compositions because the calculation for HZ affects the composition of the liquid at the time LZ starts to crystallize. The compositional difference is taken up in the model by the trapped liquid in the upper zones.) The agreement between observed values and those estimated in accordance with the model used to describe the trace element behavior is generally sati~ac~ry, within the limitations of sampling, the remaining uncertainties in zone sizes, etc. The agreement for the highest zones UZb and UZc (not given) is poorer than that for

1570

P. PUTEB, DONALD S. Scn~uwxcxxn and L-Y

TEEODORE

A. EASKXN

Table 8. Major element concentrationsin zones of the Skaergaard layered series* LZ

HZ ~~507 SiO, ~,CS Fe0 MgG CaO NaaO Es0 PsGs TiO,

48.1 17.2 963 862 II-4 237 0.25 0.10 1.17

20

50

80

OBS*

20

50

80

52.5 17.7 3.3 115 13.2 168 0 0 0

49.5 17.9 7.4 9.2 12.0 2.2 0.17 o-07 0.78

48.3 17.3 9.18 8.19 11.5 2-32 0.23 0.09 1.08

48.7 18.0 7.4 9-6 12.0 2.20 O-23 0.03 0.71

48.2 16.1 10.8 Q-8 11.2 2.4 o-21 0.08 0.98

48.0 15.7 11.2 IO.0 11-l 2.5 o-22 0.09 1.05

48.2 16.0 10.9 9.6 11.3 2.4 o-22 0.09 1.03

46.4 168 11.8 9.6 11-3 25 0.20 O-06 0.79

UZb

UZa

MZ

OBS

20

50

80

OBS

20

50

80

OBS

20

60

49-5 16.3 10.3 6.2 11-Q 2-Q 0.12 0.05 l-8

49.4 160 106 6.2 118 3-O o-13 0.05 13

49.5 16.2 10.4 6.0 12.1 3.0 0.12 0.05 l-8

48.2 18.0 11.8 53 10.2 35 0.14 0.05 2-6

47-Q 16.5 14.1 62 8.8 3.6 o-13 0.05 2.2

47.7 16.4 14.5 6.3 8.7 3*6 0.14 O-06 2.2

48-2 16.5 14.3 6-O 8-8 3.6 0.17 0.06 2.2

45.7 14.7 18.4 4.2 8.4 3.7 0.27 0.09 2.5

1 13.0 15.4 4.7 IO.6 1 0.09 0.85 l-07

44.5 12.9 18.7 4.8 IO.8 1 O-10 0.85 l-11

80

OBS

47.0 44.1 13.0 11.7 186 24.5 4.6 1.71 9.6 8.7 1 2.95 0.13 0.35 O-86 1.85 1.11 2.43

* EC4507 is the chilled marginal gabbro. Columns marked 20, 50 and 80 contain predicted values from the logarithmic model with HZ relatively 20, 50 and 80 0/eby weight of the layered series. Values in columns marked OBS are from analyses of average rocks (WAGERand BRO-RN, 1967) except for HZ, for which estimated values (W~oxn and BROYVX,1967) are given.

the lower zones. This is to be expected, since the highest two zones account only for ~2--~8 per oent of the entire layered series. The major elements are fractionated as the cumulus minerals crystallize. With significant uncertainties in relative sizes for the larger zones, it is simply not possible to provide a satisfactory mass balance for the final few per cent of theliquid. Thus, there remains, according to the estimates, insufficient Na,O for UZb and UZc. This observation serves to emphasize the greater sensitivity of the trace than of the major elements for certain kinds of estimates and their compIemen~ty to major element data. For example, the size of the UZc relative to those of the other zones is tied within fairly narrow limits. Because UZc contains 20-40 per cent of the total mass of some of the elements in the intrusion (e.g. REE), any major change in the relative size of that zone causes a considerable change in the ooncentrations of those elements in all zones. The mineralogy of the lower zones must then be adjusted to allow for those changes. If the change in the size of UZc is large enough, the required changes in mineralogy destroy the agreement between the estimated and observed values for the major elements and the mineralogy.

Some trace elements during solidification of the Skaergaard layered series

Somecharacteristics

of trace

1671

element behavior

some of the characteristics of the various trace elements in the Skaergasrd layered series. The model described above have been developed and tested by use of trace element data produced in our laboratory because we know the accuracy of those data and because those data for all elements were obtained from the exact same samples. We consider in addition some of the data for elements not analyzed here. Below

are summarized

REE

Because of the coherent nature of that group, we have relied heavily (but by no means exclusively) on the REE as indicators of trapped liquid concentration and to look for effects of cumulus and adcumulus crystallization on trace element distributions in Skaergaard rocks. As shown previously (HASKINand HASKIN,1968), the relative REE abundances in the rocks from the layered series differ little from that of the chilled gabbro. This occurs because, for most of the rocks, the bulk of the REE is present in the trapped liquid, whose relative REE abundances change but slightly through effects of fractional crystallization. The REE, although selectively accepted by the various cumulus minerals, are mainly excluded by those minerals (except apatite) so that even their selective loss from the liquid has only a small effect on the relative REE abundances in the residual liquid. Deviations of REE distributions in the rocks from that of the chilled gabbro can mostly be attributed to the effects of selective acceptance by the cumulus minerals (and their adcumulus extensions). This has been shown qualitatively in an earlier work (HASKINet al., 1971) and is now quantitatively understood (within the limits of uncertainty imposed by sampling, zone sizes, etc.). Deviations in REE distributions in the rocks from that of the chilled gabbro also tend to be minimized because of the simultaneous precipitation of minerals that prefer the heavier REE (especially calcic clinopyroxene) with those that prefer the lighter REE (plagioclase). The deviation of the REE relative abundances in rock EG5086 (LZb) from that of the chilled gabbro (HASKIN et al., 1971) requires further consideration. The estimated 52 per cent trapped liquid content of that rock is so high that essentially all of the REE contained therein must have come from the trapped liquid. Yet, the REE distribution in EG5086 is somewhat enriched in the lightest REE, Eu, and the heaviest REE as compared with that of the chilled gabbro. Similar, but much more extensive, variations are also seen in rocks from the Stillwater intrusion (KOSIE~ICZ and HASKIN, to be published) and reflect partial solidification of the trapped liquid in situ followed by migration of the final dregs over short distances and out of the sample analyzed. Quantitative accounting of the cumulus, adcumulus, and trapped liquid components of the REE distribution has been achieved for a number of those samples. It is not within the scope of this work to attempt to do so for EG5086, but qualitatively the effects on the REE in that rock appear to be those of precipitation of orthocumulus plagioclase and olivine (or, possibly, pyroxene), with selective loss of the very last portion of the trapped liquid away from the particular sample analyzed. The estimates for the concentrations of Eu, based strictly on the values of the distribution coefficient for that element as determined from the UZa mineral separate

1572

TXEODOREP. PASTER, DONALD S. SCEAUWECICER and

LARRY A. IXASEIN

(Table 6), are not satisfactory. The discrepancies disappear, however, when the values of the distribution coefficient for Eu are adjusted according to the composition of the plagioclase that is crystallizing~ as suggested by SCH~ET~E~ and PHILPOTTS (1970). The uncertainties are. too large for us to evaluate the size of the change in that distribution coefficient independent of the estimates of Schnetzler and Philpotts, but the dependence on plagioclase composition is verified. Note that the values for the distribution coefficients for Eu in pyroxene and apatite (Table 5) are somewhat lower than those of neighboring Sm and Gd, because part of the EU in the liquid is present as Euzf, which is apparently selectively excluded from those minerals (see also PHILPOTTS, 1970). The considerable accumulation of plagioclase in the rocks of the intrusion would be expected to cause the successive residual liquids (Table 4) to be increasingly depleted in Eu relative to the other REE (although the concentration of Eu in the liquids continues to increase). A small effect of this nature is seen (Table 4) except in the liquid parent of UZc. It is absent there because the parent of UZc and rock EG4330 are equivalent and there is no relative Eu deficiency in EG4330, whose composition was taken to represent the final liquid. There is a significant Eu deficiency, however, in the granophyre EG4489 (HASKIN and HASKIN, 1968) whose REE apparently came from the final liquid of the layered series. Thus, the anticipated deficiency of Eu in the last liquid may be present, and an unexplained excess of that element may have found its way into EG4330. If so, the estimates of the Eu concentrations in all of the parent liquids would be lowered slightly, owing to the decreased concentration estimated for that element in UZc. WAKER and MITCEIELL(1951) provided some data for Y and La. Within the stated uncertainties for their data and taking into account possible impurities for some of their mineral separa~s, the results of their analyses are consistent with the model as described in this paper. Ba

Even more than the REE, Ba is excluded from the Skaergaard cumulus minerals. It is accepted better by plagioclase than by the other minerals, but with a value of only O-7 for its ~st~bution coe~cient. Thus, the model must predict a continuing increase in Ba concentrations for the successive residual liquids, and does. The model estimates the concentrations of Ba in all of the zones correctly except UZb, for which the predicted result is considerably too high. We have no explanation for that discrepancy. WAGER and MITCHELL (1951) show that. the concentration of Ba in liquids filter-pressed from the top of the layered series are quite rich in Ba.

The distribution coefficient given in Table 5 for So into pyroxene is mainly for Se entering clinopyroxene. The probable variation of the distribution coefficient for pyroxene in general with the relative amounts of ortho- and clinopyroxene has been discussed earlier. The high value (3.3) of the ~stribution coefficient for SC into caloio clinopyroxene indicates that SC can be efficiently removed into a clinopyroxene cumulate or, at least, the concentration of So in successive residual liquids should not increase if calcic clinopyroxene constitutes at least a third of the cumulus

Sometrace elementsduringsolidificationof the Skmrgaardlayeredseries

1573

plus adcumulus phases in the rocks. In fact, the SC concentration remains nearly constant in the Skaergaard liquid as it crystallizes, increasing somewhat in the HZ where, presumably, little clinopyroxene accumulates. WAGERand MITCHELL(1951) show evidence for the ready acceptance of SC into apatite. Our analyses do not verify that conclusion, and a value of only O-2 for the distribution coefficient of SC entering apatite was obtained. When allowance

is made for the high ratio of ortho- to clinopyroxene in the HZ (or of olivine to clinopyroxene, as discussed previously), the agreement between the estimated and observed concentrations of SC is acceptable. Co, Zn, Mn Values for the distribution coefficients for Co, Zn, and Mn entering olivine are greater than unity, and those for Co and Mn entering clinopyroxene also exceed unity. The values are similar to those reported by GUNN (1971) and HENDERSON and DALE (1969/1970). Thus Co and Mn are expected to become depleted in the successive residual liquids, and the concentrations of Zn should not rise rapidly. These predictions are in qualitative agreement with the estimated concentrations of Table 4. Note, however, the precipitous decrease for the concentrations of Co in the liquid at the time of deposition of UZb. A similar, but less drastic, decrease occurs for the concentration of Mn. (Both elements have very low concentrations in the parent liquid for UZc.) The sudden disappearance of Co from the last liquid of the intrusion we presume to be a result of selective partitioning of that element into immiscible sulfides, as described by WACJERand MITCHELL(1951). The decrease in concentration for Mn in the last liquid, although less drastic, is equally curious. These elements enter ilmenite and magnetite with values of distribution coefficients that exceed unity, but the abundances of those minerals among the cumulus phases are too small to affect significantly the concentrations in the liquids. When the values of the distribution coefficients for Co, Mn, and Zn from Table 5 are used, agreement between the predicted and observed concentrations for those elements is poor. Despite the apparent high values for the distribution coefficients for Co and Mn entering olivine, and the anticipated importance of that mineral in the cumulus portions of the HZ, the concentrations of those elements in the liquid at the time of crystallization of the LZ are higher than in the chilled gabbro (factor of l-7). Thus, either olivine is not an important cumulus phase in the HZ, or the Co and Mn are not entering it as readily as expected. The possible dependence of the distribution coefficients on olivine composition was discussed previously. Ga According to the information in Table 5, Ga readily enters only plagioclase and magnetite, with values for the distribution coefficients of 1.7 and 26. Since the combination of plagioclase and magnetite in the average rocks does not reach 50 per cent of the cumulus material (except possibly in the HZ), the model predicts a continual increase in concentration for Ga in the residual liquid as solidification progresses. The values for the successive liquids (Table 4) show just such an increase,

but with one exception, the liquid parent of UZb. The predictions for Ga (Figs. 3 6

1574

THEODOREP.

PATTER, DONALD S. SCEAUWJZCKER and LARRY A. HASKIN

and 4) agree acceptably with the observed values (to within the uncertainties stated earlier). However, the apparent trend of decrease in the ratio of the predicted to observed values is at first somewhat disturbing and suggests that the behavior of that element may not be as straightforward as the predictions would indicate. Unlike most of the elements studied, however, Ga is not strongly excluded from the rocks that are forming. Because the LZ is much larger than the UZ, the predicted higher value for Ga in the LZ simply depletes the liquid in that element and results in the lower predicted values for the upper zones. cu The only mineral that appears to accept Cu readily is ilmenite, for which the value of the distribution coefficient was found to be l-5. The prediction of the model, therefore, is one of continual increase of Cu concentration in the residual liquid as solidification of the layered series continues. This strong increase is apparent in Table 4 up through UZa, at which point a severe and unexpected decrease occurs, so that the Cu concentration in the parent of UZc is only 40 ppm, well below the 220 ppm of the chilled gabbro and the maximum of 2140 ppm in the parent of UZa. The explanation is apparently that given by WAUERand MITCHELL(1961) of the separation of immiscible sulfides rich in Cu. Other trace elements

From the data obtained, it can only be stated that Ni and Cr are successfully removed from the liquid by the early cumulus minerals. Concentrations of these elements in the rocks and most minerals were too low for accurate analysis under the conditions used for this work. The high concentration of Ni (20 ppm) in magnetite from UZb in conjunction with the very low concentration in the parent liquid ( ~2 ppm) at that stage shows the importance of that mineral even in small modal quantities on the behavior of that element during crystallization of the intrusion. WAGER and ~TCHELL (1961) show concentrations of Ni as high as 2000 ppm in olivine from rocks of the Skaergaard early border group. Thus, the value for the distribution coefficient of Ni into olivine appears to be quite high. This is consistent with the range of 5-20 reported by HENDERSON and DALE (1969/1970) and HXKLI and WRIQHT(1967). The olivine from EG6181was the only mineral analyzed that gave a measurable concentration for Cr. Thus, it can be inferred that the absence of Cr from rocks in the MZ and above is a result of removal of that element from the parent liquid into the early olivine or in spinels (e.g. GUNN, 1971). Such effective early removal of Co and Ni argues well for good stirring of the melt that produced the layered series. Analytical data for Ag, As, and Sb provided by this work have large uncertainties and are incomplete. We consider them briefly along with data obtained using neutron activation analysis by WAGER et al. (1958) for In, by VINCENT and BILEFIELD (1960) for Cd, by VINCENTand CROCKET(1960) for Au, and by ESSONet al.(1966) for AS and Sb. Estimated values for the distribution coefficients for these elements are given in Table 9, and the extent of agreement between predicted values of the model and the analytical values is shown in Fig. 6. The predictions of the model, based on the mineralogy and values for distribution

Some trace elernente during salidifwation of the Skaergaard layered series Tsble 9. Estimated

As AU Cd In Sb

vslue5

1575

for distribution coefficients for minerals from the Skaergaard layered series

PI

01

Px

AP

&It

n

o-4

045

0.2

-

1.0 0.5 O-03 3-4

0.3 1.2 0.6 1.2

0.7 l-5 1.5 0.6

0.6 -

0.5 1.3 1.1 l-4 0.5

0.3 1.0 0.5 2.5 1.4

Exposedtunes

0

WOI Calculated

edncenttations

/experimental

concentrations

Fig. 6. Ratios of predicted to observed concentrations are given for several trace elements for the exposed zones only {top) and an assumed hidden zone of 50 per cent (bottom).

coefficients given in the tables, may be regarded as satisfactory for In and perhaps Sb, but not for the rest uf the elements shown in Fig. 6. The meaning of the values of the distribution coe&ients in Table 9 is somewhat unclear since, in some situations, and BILEFIELD the elements in question show an affinity for sulfide phases, VINCENT (1960) found a factor of 2 difference in Cd concentration between two ‘average’ rocks of the lower zone, even though the rocks came from the stage of differentiation and had only smaIl mineralogist differences. They suggested that the ~~erences in concentration might resuM from different amounts of trapped liquid. This is doubtful because a change in trapped liquid content of a factor of 2 would be required if the cumulus and adcumulus portions of the rock contained no Cd, and a factor very much larger than 2 is required when the va’lue of -O*9 for the cumulus minerals is cunsidered (Table 9). The co~~ntrat~ons uf Ag and Au in the rocks remain approximately constant during so~~~~tio~ of the intrusion.

1576

THEODOREP. PASTER,DONALD S. SCIXAU~ECKER and LARRY A. HASKIN

Acknozuledgements-We are very grateful to Dr. G. MALCOMBROW and Dr. C. KENT BROOKSfor providing the samples and encouragement to carry out the work. We thank the crew of the University of Wisconsin nuclear reactor for irradiating the samples. Thanks to RUTH STURVE and DONNASANDERSfor assistancein preparingthis manuscript. We are especially indebted to FRED FREY for his many helpful suggestionsfor improvement of the original manuscript. We appreciate the partial support of this work by the Research Committee of the Graduate School of the University of Wisconsin and by the National ScienceFoundation through grants GA-1665 and GA-2526. REFERENCES ALLENR. O., HASKINL. A., ANDERSONM. R. and MILLER 0. (1970) Neutron activation for 39 elements in small or precious geologic samples. J. Radioanal. Chem. 6, 115. BLANKH. R., JR. and GETTINGS M. E. (1972) Geophysical studies of the Skaergaardintrusion, East Greenland: initial results. Trans. Amer. Geophys. Union 53, 533. CHAYESF. (1970) On estimating the magnitude of the hidden zone and the composition of the residual liquids of the Skaergaardlayered series. J. Petrol. 11,1-14. CULLERSR. L., MED~LRIS L. G. and HASKIN L. A. (1970) Gadolinium: distribution between aqueous and silicate phases. Science169, 580-583. CULLERS R. L., MEDARISL. G. and H&KIN L. A. (1973) Experimental studies of the distribution of rare earths as trace elements among silicate minerals and liquids and water. Geochim. Coemochim. Acta 37, 1499-1512. DENECHAUD E. B., HELMKEP. A. and HASKINL. A. (1970) Analysis for the rare earth elements by neutron activation and Ge(Li) spectrometry. J. Radioanal. Chem. 6, 97-113. ESSONJ., STEVENSR. H. and VINCENTE. A. (1965) Aspects of the geochemistry of arsenic and antimony, exemplified by the Skaergaardintrusion. Mineral. Mag. 35, 88-107. GASTP. W. (1968) Trace element fractionation and the origin of tholeiitic and alkaline magma types. Geochim. Cosmochim. Acta 52, 1057-1086. GOLESG. G., RANDLE K., OSAWA M., LINDSTROMD. J., JEROE~E D. Y., STEINBORNT. L., BEYER R. L., MARTINM. R. and MCKAY S. M. (1970) Interpretations and speculations on elementalabundancesin lunar samples. Proc. Apollo 11 L-unar Sci. Conf ., Geochim. Cosmochim. Acta SuppZ. 1, pp. 1177-1194. Pergamon Press. GUNN B. M. (1971) Trace element partition during olivine fractionation of Hawaiian basalts. Chem. Geol. 8, 1-13. H&I T. and WRIGHTT. L. (1967) The fractionation of nickel between olivine and augite as a geothermometer. Geochim. Cosmochim. Acta 31, 877-884. HASKIN L. A. and HASKIN M. A. (1968) Rare-earth elements in the Skaergaard intrusion. Geochim. Cosmochim. Acta 32, 433-447. HASKINL. A., ALLENR. O., HELMI~E P. A., PASTERT. P., ANDERSON M. R., KOROTEVR. L. and ZWEIFELK. A. (1970) Rare earths and other trace elements in Apollo 11 lunar samples. Proc. Apollo 11 LunarScLConf., Geochim. Cosmochim. ActaSuppl. 1, pp. 1213-1231. Pergamon Press. HASKINL. A., HELMKEP. A., PASTERT. P. and ALLENR. 0. (1971) Rare earths in meteoritic, terrestrial and lunar material. Proc. NATO Conf. Activation Analysis in Geochemistry. Oslo. HELMKEP. A. and HASKINL. A. (1973) Rare-earth elements, Co, SC, Hf in the Steens Mountain basalts. Geochim. Cosmochim. Acta 37, 1513-1530. HELMKEP. A., HASKIN L. A., KOROT~VR. Land ZIEGE K. E. (1972) Rare earths and other trace elements in Apollo 14 samples. Proc. 3rd Lunar Sci. Conf., Geochim. Cosmochim. Acta Suppl. 3, pp. 1275-1292. M.I.T. Press. HENDERSON P. and DALE I. M. (1969/1970) The partitioning of selected transition element ions between olivine and groundmass of oceanic basalts. Chem. Geol. 5, 267-274. HI~UCHIH. and NAGASAWAH. (1969) Partition of trace elementsbetween rock-formingminerals and the host volcanic rocks. Earth Planet. Sci. Lett. 7, 281-287. HUBBARDN. J. and GAST P. W. (1971) Chemical composition and origin of nonmare lunar basalts. Proc. 2nd LunarSci. Conf., Geochim. Cosmochim. ActaSuppl. 2, pp. 999-1020. M.I.T. Press.

Some trsce elements during solidification of the Sksergaerd layered series

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