LITHOS 0

ELSEVIER Lithos36(1995)141-153

Nd, Pb and Sr isotopic data from the Mount Elgon volcano, eastern Uganda-western Kenya: Implications for the origin and

evolution of nephelinite lavas

A. Simonetti ‘*, K. Bell Ottawa-Carleton Geoscience Centre, Department of Earth Sciences Carleton University, 1125 Colonel By Drive, Ottawa,

Ottawa, Ont. KIS 5B6, Canada

Received 1 November 1994; accepted 22 June 1995

Abstract

Nd, Pb and Sr isotope ratios for nephelinites from the Tertiary Mount Elgon alkaline volcanic centre, eastern Uganda-western Kenya, are highly variable and indicate open system behaviour. The variation in ‘43Ndl’“Nd (0.51219-0.51286) and87Sr/86Sr (0.703 14-0.70604) ratios span almost the entire range documented for carbonatites from several East African alkaline complexes. The whole rock chemical data, mineralogy, composition of diopside phenocrysts, and variation in isotopic ratios from the Mount Elgon nephelinites are similar to those from the nephelinite lavas from the Tertiary Napak volcano, Uganda (Simonetti and Bell, 1994a). The diopside phenocrysts from Mount Elgon nephelinite lavas reveal large core-to-rim compositional variations (which include normal, oscillatory and reverse zoning), and their Nd, Pb and Sr isotopic ratios are not in isotopic equilibrium with their host lavas. Microprobe data along with textural evidence from the Mount Elgon diopside phenocrysts support a model that involves crystallization in an open magma system that was undergoing continuous chemical and isotopic change. The large variation in Pb isotopic ratios (whole rocks -206Pb/Z@‘Pb: 18.45-21.51; 207Pb/Z04Pb: 15.61-15.88; 208Pb/zWPb: 38.6241.02), from the Mount Elgon lavas, best fit a model involving mixing between EM I and HIMU mantle components, and correlations in Pb-Sr and Pb-Nd isotopic plots partly support this interpretation.

The isotopic data from Mount Elgon and Napak nephelinites suggest complex evolutionary histories involving magma mixing, and support the presence of a heterogeneous sub-continental source beneath eastern Uganda, similar to that documented for various types of peralkaline nephelinite lavas from the only active carbonatite-nephelinite volcano, Oldoinyo Lengai, Tanzania (Bell and Dawson, 1995) and other East African volcanoes (e.g. Vollmer and Norry, 1983). The chemical data and large variation in isotopic ratios for the Mount Elgon nephelinites suggests that each lava flow may represent a discrete partial melt derived from an isotopically inhomogeneous upper mantle. In the cases of the Mount Elgon and Napak lavas, magma mixing may have occurred between newly formed, ascending nephelinitic liquids and slightly older nephelinitic melts that ponded at shallower levels.

1. Introduction The origin and evolution of continental volcanics,

such as alkalic basal@ and nephelinites, is controversial because of their extreme enrichment in large-ion-lith-

* Corresponding author. ’ Max-Planck-Institut Wr Chemie, Abteilung Geochemie Post-

fach 3060.55020 Mainz, Germany

ophile elements. The chemical and petrological fea-

tures from such rocks are commonly cited as evidence

for an incompatible element-enriched mantle (e.g.

0024-4937/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIOO24-4937(95)00011-9

142 A. Simonetti, K. Bell/Lirhos 36 (1995) 141-153

Lloyd and Bailey, 1975; Frey et al., 1978). Experi- ments conducted up to 36 kbar pressure have shown that nephelinitic magmas cannot be in equilibrium with a lherzolitic mantle source (Bultitude and Green, 1968,

1971; Allen et al., 1975; Merrill and Wyllie, 1975), but are probably the products of small ( <5%) degrees partial melting of a carbonated (high C02/H20 ratio)

peridotite or pyrolite at high pressures (Brey and Green, 1977; Brey, 1978; Olafsson and Eggler, 1983; Wallace and Green, 1988). Experimental results are consistent with derivation of a primary nephelinitic liq- uid from an amphibole peridotite at pressures of 20-25

kbar (Olafsson and Eggler, 1983; Eggler, 1989).

Fractionation of nephelinitic magmas involves major phenocryst phases, such as olivine, clinopyroxene,

nepheline, and Fe-Ti oxides (e.g. Peterson, 1989a, b) . Of these, clinopyroxene is probably the most useful in monitoring the magmatic evolution of nephelinites

because it remains on the liquidus over a wide range of temperatures (e.g. 1 IOO-16OO”C, Bultitude and Green, 197 1; Thomson, 1974)) it incorporates many minor and

trace elements into its structure, and is relatively slow tore-equilibrate (e.g. Shimizu, 1990). In addition, iso- topic data from clinopyroxenes have proven effective

in monitoring shifts that have occurred during magma evolution (Simonetti and Bell, 1993).

Highly zoned and partially resorbed clinopyroxene

phenocrysts in many nephelinites (e.g. Le Bas, 1987; Donaldson et al., 1987; Peterson, 1989a; Simonetti and Bell, 1993, 1994a) suggest a complex crystallization history, that has been attributed to: ( 1) crustal contam- ination; (2) kinetic (disequilibrium) effects (Leung, 1974; Grove and Bence, 1979; Gamble and Taylor, 1980; Larsen, 1981; Kouchi et al., 1983); (3) changes in oxygen fugacity (Brooks and Rucklidge, 1973; Segalstad, 1979) ; (4) changes in crystallization assem-

blage (e.g. Dobosi, 1986) ; (5) polybaric differentia- tion (e.g. Duda and Schmincke, 1985); and (5) magma mixing (e.g. Brooks and Printzlau, 1978; Dobosi and Fodor, 1992; Simonetti et al., in press).

The eastern branch of the East African Rift Valley System is associated with numerous, young ( < 40 Ma old) nephelinite-carbonatite volcanic centres (Fig. 1) , that are ideal for monitoring the chemical evolution of the sub-continental upper mantle. These volcanic cen- tres are particularly well-suited for isotopic investiga- tion because of their age, and wide compositional range of silica-undersaturated alkaline rocks (e.g. melilite

Fig. 1. Regional geology of East Africa showing the Tanzanian Shield and the East African Rift Valley System (modified after Bell and Blenkinsop, 1987). Also shown is the distribution of some Ter- tiary and Recent carbonatite-nephelinite centres including Mount Elgon and Napak.

nephelinite, olivine nephelinite, melanephelinite, pho- nolite). One such centre, Mount Elgon, is one of the largest alkaline centres in East Africa (Fig. 1) .

This paper reports Nd, Pb and Sr isotopic data from

clinopyroxene phenocrysts and whole rock nephelin- ites from Mount Elgon in an attempt to: ( 1) define the nature of the sub-continental upper mantle below part of East Africa and (2) assess the isotopic evolution of

the nephelinite lavas, and compare the results to those from other nephelinite centres.

2. Geology

The volcanic succession at Mount Elgon consists predominantly of agglomerates and tuffs, and a small volume ( < 1%) of silica-undersaturated lavas (Davies, 1952). The lavas are classified according to their dominant mineralogy (see Table 1) based on a classification proposed by Le, Bas ( 1989). Basanites

Table 1

A. Simonetti, K. BeN/Lithos 36 (1995) 141-153 143

Classification of Mount Elgon laws

Mineralogy ( phenocryst and groundmass)

feldspar pyroxene

feldspar nepheline, pyroxene

nepheline, pyroxene

Samples:

Name: trachyte basanite

D2967 El109 El108 El103 El104 nephelinite

nepheline, melitite, pyroxene,

El101

nepheline, pyroxene olivine

nepheline melitite, pyroxene, olivine El107 D2836 El100

melilite nephelinite mela nephelinite melilitite

and trachytes were the earliest flows to be extruded, and these were followed by the nephelinites, melilite nephelinites and melilitites (Davies, 1952). Potas- sium-Ar whole rock dates of 15 Ma from basanites (Baker et al., 1971) and 22 Ma from nephelinites (Bishop et al., 1969) indicate an early Miocene age for the volcanic activity at Mount Elgon. The spread in dates may be the result of either excess argon, or of argon loss (Bishop et al., 1969). The Mount Elgon volcanic succession is underlain by Precambrian gran- itoid batholiths, and older hornblende gneisses, schists and granulites of the Samia Series (Davies, 1952, 1956).

3. Analytical methods

Analytical procedures for Nd and Sr whole rock analyses are similar to those described in Bell and Blen- kinsop (1987). Whole rock samples ( -0.1-0.2 g) were dissolved in an HF-HNO, mixture under pressure for at least 48 h. Lead was separated in HBr and HCl using a two column anion ion-exchange technique. Details are given in Simonetti et al. ( 1995). Procedures for Nd, Pb, and Sr isotopic analyses of diopside mineral separates (0.14-0.90 g) are similar to those described in Simonetti and Bell (1993). The Nd and Sr blanks were about 0.5 ng; those for Pb range from 0.2-l .5 ng. Neodymium and Sr were analysed using a double Re- filament technique, and all isotope ratios were meas- ured on a Finnigan-MAT 261 multicollector mass spectrometer, operated in the static mode. The Pb sam-

ples were run on a single Re filament using silica gel and phosphoric acid.

4. Results

4.1. Petrographic and chemical data

The nephelinites consist predominantly of clinopy- roxene, iron oxides, and/or olivine phenocrysts set in a groundmass of melilite, nepheline, iron oxides and/ or clinopyroxene. Table 2 lists major element analyses of eight samples from Mount Elgon. Of these, two samples were used for isotopic work in this study (El 100 and El 107). The chemical compositions of sam- ples El 100 and El 107 are similar to the melilite ankar- atrites of Davies ( 1952), and similar to those from olivine-bearing nephelinites from Napak (Simonetti and Bell, 1994a). Table 2 shows that the Mg#‘s (0.76- 0.77) for samples El 100 and El 107 are characteristic of primary mantle melts (Roeder and Emslie, 1970).

Average core, middle, and rim compositions based on 88 microprobe analyses from 27 clinopyroxene phe- nocrysts, the main phenocryst phase of the Mount Elgon nephelinites, are shown in Table 3. All are diop- sides (Fig. 2, Morimoto, 1989). Most of the diopside phenocrysts are highly zoned (normal, oscillatory or reverse, Figs. 2,3 and 4) and in general, the normally- zoned diopside phenocrysts show an increase in Al, Ti and Na contents from core-to-rim, consistent with mag- matic differentiation (Le Bas, 1987).

Zoning is a common feature in diopside phenocrysts from alkalic basal& (Wass, 1979)) foidites and bas-

144 A. Simonetti, K. Bell / Lithos 36 (1995) 141-153

Table 2 Whole rock major element data from Mount Elgon lavas

Sample Wt.% El100 El107 1 2 3 4 5 6 7

SiOz

Al@, TiOz

Fe& Fe0

MgO MnO CaO Na,O K,O P@, Hz0 + Hz0 -

COP Total

Mg# Rb Sr Ba Y Zr V Nb Cr Ni Zn

40.03 39.42 9.01 7.44 3.44 2.74 8.11 8.44 6.12 6.37

11.68 11.33 0.18 0.21

14.87 16.78 2.90 2.22 2.83 1.96 0.41 0.49

99.90 97.77 0.77 0.76

106 50 554 1161 947 715

16 20 114 145 189 124 63 76

190 289 85 150 84 92

42.20 44.12 42.84 12.04 13.89 11.47 2.48 2.61 2.77

13.97 14.11 15.46

6.22 5.03 7.16 0.27 0.25 0.29

15.17 11.42 15.33 3.93 5.52 2.76 2.54 2.78 1.27 0.88 0.95 0.99

_

99.68 0.65’.’

45 910

1211 25

193 211 111

_

100.68 0.60*

58 684

1128 28

214 218 123

100.35 0.66*

60 1946 1857

36 271 286 163

29 30 29 114 121 117

41.44 7.01 0.12 3.16 7.89

26.48 0.18 8.23 1.83 0.94 0.26 1.57 0.35

100.06 0.86

40.38 4.84 3.30 8.93 6.71

10.89 0.36

17.99 1.69 1.54

_ 100.67

0.62

64.30 46.71 19.48 15.03 0.48 2.41 0.19 5.35 0.21 5.41 0.13 4.13 0.02 0.44 0.32 7.83 6.40 4.86 7.72 4.24 0.18 0.62 0.41 2.94 0.35 1.39 0.09 0.00

100.31 101.55 0.47 0.58

Sample nos. l-3 are from Le Bas ( 1987). 1 = melanephelenite lava (EK 6); 2 = nephelinite lava (ElK 7); 3 = melanephelinite lava (ElK 8) .Sample nos. 4-7 are from Davies (1952). 4 = ankaratrite; 5 = melilitite; 6 = trachyte; 7 = basanite.The relative precision of the major element oxide analyses (obtained by XRF) for samples El100 and El107 (this study), with the exceptions of Na,O, Pz05, and MnO, is equal to or less than 3% of the determined value; the precision of Na,O, PzOs and MnO is f lO%.Relative precision of the trace elements (ppm) for samples El100 and El107 is f 10% for all the elements except Y ( f 50%). (-) = not analyzed. n.r. = not reported. A:Mg # for sample nos. 1.2, and 3 calculated assuming a FeO/F%,,, similar to those for samples El100 and E1107.

anites (Duda and Schmincke, 1985)) and nephelinites from other East African carbonatite complexes (e.g. Oldoinyo Lengai: Donaldson et al., 1987; Peterson,

1989a; Napak: Simonetti and Bell, 1993). A micro-

probe traverse of a diopside phenocryst from Mount

Elgon (El 107) shows a negative correlation between

MgO and Al,Os, Fe0 and TiO*. Fig. 4 shows that the

MgO and Fe0 abundances vary by at least 2.0 wt.%, and there are at least two-fold variations in A1203 and TiO,.

The presence of two distinctly coloured (light and dark green) diopside populations from nephelinite sample, El 109, with significantly different chemical compositions (Table 3), cannot be attributed to simul- taneous crystallization from the same melt. The light

green diopside population contains much higher Mg and Si contents and lower Al and Fe contents than the

dark green population.

Composition of the diopside phenocrysts from

Napak (Simonetti et al., in press) are similar to those

from Mount Elgon but contain higher Cr and Na con-

tents (Table 3). The average Cr*O, wt.% values for titanium-bearing aluminian diopsides from several

Napak nephelinite lavas range from 0.19-0.77 (Simo- netti et al., in press), whereas most of those from Mount Elgon are Cr-free (Table 3). The Mount Elgon diop- side phenocrysts contain, in general, higher Mg#‘s (0.83-0.91) than the titanium-bearing aluminian diopsides from Napak (0.79-0.83), and indicate that the Mount Elgon lavas are relatively primitive. In addi-

Tab

le 3

A

vera

ge c

ompo

siti

ons

of d

iops

ide

phen

ocry

sts

from

Mou

nt

Elg

on

Sam

ple

Typ

e #

anal

SiO

Z

Al&

T

iOz

Cr&

F

e0

MgD

M

nO

C

aO

Na,

O

Tot

al

Si

‘“A

l “‘

Al

Ti

Cr

Fe’

+

Fe’

+

Mg

Mn

C

a N

a S

um

Mg#

wo

Fs

En

El

100

El

101

El

108

El

109

Lig

ht

El

109

Dar

k

Cor

e 5

Mid

dle

9 R

im 8

C

ore

5 M

iddl

e 6

Rim

9

D 1

C

ore

5 M

iddl

e 5

Rim

10

Cor

e 4

Mid

dle

6 R

im 6

C

ore

3 M

iddl

e 3

Rim

3

50.4

3 48

.92

48.0

1 51

.84

51.0

5 50

.42

49.3

7 49

.41

49.5

1 49

.83

53.6

5 53

.65

52.7

4 51

.66

51.5

0 52

.58

3.09

3.

74

4.30

2.

51

2.87

3.

13

3.66

4.

39

4.47

4.

17

1.33

1.

33

1.76

2.

71

3.03

1.

95

1.29

1.

88

2.70

0.

95

1.16

1.

23

! .3n

! .

23

I.28

!.

!6

!I35

0.

32

(3.5

9 0.

90

0.92

0.

74

0.00

0.

00

0.00

0.

00

0.00

0.

00

0.00

0.

00

0.00

0.

00

0.00

0.

00

0.13

0.

00

0.00

0.

00

5.68

5.

84

5.75

5.

07

5.34

5.

43

5.80

7.

93

8.01

7.

02

3.55

3.

72

5.29

6.

67

7.61

5.

12

14.8

9 14

.39

14.0

2 15

.29

14.9

8 14

.80

14.1

5 13

.09

13.0

7 13

.85

16.8

2 16

.54

15.5

5 14

.56

14.2

1 15

.77

0.00

0.

00

0.01

0.

00

0.00

0.

00

0.00

0.

02

0.02

0.

00

0.00

0.

00

0.01

0.

00

0.00

0.

01

23.5

1 23

.70

23.8

1 24

.21

24.0

6 23

.93

23.6

4 22

.27

22.4

9 22

.58

24.0

1 24

.27

23.7

0 23

.93

23.2

8 24

.28

0.27

0.

19

0.15

0.

14

0.13

0.

09

0.14

0.

77

0.77

0.

68

0.18

0.

20

0.25

0.

23

0.46

0.

26

99.1

6 98

.66

98.7

5 10

0.01

99

.59

99.0

3 98

.14

99.1

1 99

.62

99.2

9 99

.89

100.

03

100.

02

100.

66

101.

01

100.

71

For

mu

la p

ropo

rtio

ns*

bas

ed o

n 4

cat

ion

s an

d 6

oxyg

en a

tom

s

1.87

3 1.

831

1.80

1 1.

906

1.88

9 1.

877

1.85

8 1.

847

1.84

2 1.

852

1.95

9 1.

957

1.93

8 1.

898

1.88

7 1.

917

0.11

6 0.

154

0.18

5 0.

087

0.10

4 0.

116

0.13

3 0.

137

0.14

1 0.

132

0.03

8 0.

038

0.05

7 0.

093

0.10

0 0.

073

0.01

9 0.

011

0.00

5 0.

022

0.02

1 0.

022

0.03

0 0.

057

0.05

5 0.

051

0.01

9 0.

019

0.01

9 0.

024

0.03

1 0.

010

0.03

6 0.

053

0.07

6 0.

026

0.03

2 0.

034

0.03

9 0.

035

0.03

6 0.

032

0.00

9 0.

009

0.01

6 0.

025

0.02

6 0.

020

0.00

0 0.

000

0.00

0 0.

000

0.00

0 0.

000

0.00

0 0.

000

0.00

0 0.

000

0.00

0 0.

000

0.00

4 0.

000

0.00

0 0.

000

0.06

7 0.

081

0.06

6 0.

037

0.04

2 0.

047

0.05

2 0.

100

0.10

5 0.

098

0.01

8 0.

025

0.02

9 0.

053

0.07

7 0.

060

0.11

0 0.

102

0.11

4 0.

119

0.12

3 0.

122

0.13

1 0.

148

0.14

4 0.

121

0.09

1 0.

088

0.13

4 0.

152

0.15

6 0.

096

0.82

4 0.

803

0.78

4 0.

838

0.82

6 0.

821

0.79

4 0.

729

0.72

5 0.

767

0.91

5 0.

899

0.85

1 0.

796

0.77

6 0.

857

0.00

0 0.

000

0.00

0 0.

000

0.00

0 0.

000

0.00

0 0.

001

0.00

1 0.

000

0.00

0 0.

000

0.00

03

0.00

0 0.

000

0.00

0 0.

936

0.95

1

0.95

7 0.

954

0.95

4 0.

954

0.95

4 0.

892

0.89

6 0.

899

0.93

9 0.

949

0.93

3 0.

942

0.91

4 0.

948

0.01

9 0.

014

0.01

0 0.

010

0.00

9 0.

007

0.01

0 0.

056

0.05

6 0.

049

0.01

2 0.

015

0.01

8 0.

017

0.03

3 0.

018

4.00

0 4.

000

4.00

0 4.

000

4.00

0 4.

000

4.00

0 4.

000

4.00

0 4.

000

4.00

0 4.

000

4.00

0 4.

000

4.00

0 4.

000

0.88

0.

89

0.87

0.

88

0.87

0.

87

0.86

0.

83

0.83

0.

86

0.91

0.

91

0.86

0.

84

0.83

0.

90

Nor

mal

ized

en

d-m

embe

r m

olar

pro

port

ion

s 0.

48

0.49

0.

50

0.49

0.

49

0.49

0.

49

0.48

0.

48

0.48

0.

48

0.48

0.

48

0.48

0.

48

0.48

0.

09

0.09

0.

09

0.08

0.

09

0.09

0.

09

0.13

0.

13

0.13

0.

46

0.06

0.

08

0.12

0.

12

0.08

0.

43

0.42

0.

4 1

0.43

0.

43

0.42

0.

42

0.39

0.

40

0.39

0.

46

0.46

0.

44

0.41

0.

40

0.44

*Fe’

+

con

ten

t was

cal

cula

ted

stoi

chio

met

rica

lly

(Dro

op,

1987

). M

icro

prob

e an

alys

es w

ere

dete

rmin

ed u

sin

g a

Cam

brid

ge M

icro

scan

5 E

DS

sys

tem

. Ope

rati

ng

con

diti

ons

wer

e 20

k

V a

ccel

erat

ing

pote

nti

al a

nd

a be

am c

urr

ent (

abso

rbed

) of

3.0

nA

mea

sure

d on

an

am

phib

ole

stan

dard

. Spe

cim

en,

nat

ura

l an

d sy

nth

etic

sta

nda

rd ra

w X

-ray

spe

ctra

wer

e re

du

ced

by

th

e E

DD

I pr

ogra

m (

Pri

ngl

e,

1989

).

Un

cert

ain

ties

: maj

or o

xide

s, 2

%

of t

he

quot

ed v

alu

e; m

inor

oxi

des,

<

5 w

t.%

(5

%).

<

1 w

t.%

( 1

0%).

C

ompo

siti

ons

are

expr

esse

d as

m

olec

ula

r en

d-m

embe

rs W

o (C

a ca

tion

pro

port

ion

s),

Fs

(Fe’

+ +

Fe’

+

+ M

n c

atio

n p

ropo

rtio

ns)

, an

d E

n (

Mg

cati

on p

ropo

rtio

ns)

. D

= f

iner

-gra

ined

, den

drit

ic d

iops

ide

phen

ocry

st;

Mg#

=M

g/(M

g+F

e’+

).

146 A. Simonetti, K. Bell/Lithos 36 (1995) 141-153

p40h-=-i Fig. 2. Main diagram shows average core (O), middle (O), and rim

(0) compositions for clinopyroxene phenocrysts from five Mount

Elgon nephelinite lavas using the ternary Wo-En-Hd classification

of Morimoto ( 1989). Inset shows the ternary plot with shaded box

representing area of the main diagram. Tie-lines join core-middle-

rim compositions of phenocrysts from the same lava. (*) = average

composition of finer-grained, dendritic crystal from El 101.

8 -A

6-

6 C

1

0’ a 51 52 53 54

Ca/(Ca+Mg)‘lOO

55

Fig. 3. Average major element core-to-rim variations plotted against

Ca/(Ca+Mg) * 100 for Mount Elgon diopside phenocrysts. Sym-

bols as in Fig. 2. (a) Ti* 100 p.f.u. (b) Al * 100 p.f.u. (c) Na* 100

p.f.u.

In view of the high Mg# of both the whole rock

nephelinites and diopside phenocrysts from Mount Elgon, the lavas can be considered as primary mantle melts. The diopside phenocryst populations from Mount Elgon also show large core-to-rim variations (Fig. 3) and fine-scale oscillatory zoning within indi-

vidual diopside phenocrysts (Fig. 4)) both suggestive of a complex crystallization history. On the basis of these observations, it is unlikely that the nephelinite

MgO ~1% Fe0 W/o

tion, the diopside phenocrysts from the Mount Elgon lavas are characterized by low V’Al/rVAl ratios (O- - 0.5)) a feature that is similar to the diopside pheno-

trysts from Napak (Simonetti et al., in press) and typ- ical of clinopyroxene of low-pressure origin (e.g. Wass, 1979; Meyer and Mitchell, 1988). The “‘Al/ rVAl ratio for clinopyroxene formed in melts of differ- ent composition, including olivine nephelinite, at pres- sures of 20-25 kbar (the approximate depth of nephelinitic melt generation), should range from 1 .O- 1.5 (Bultitude and Green, 197 1; Thomson, 1974). The low V’Al/‘VAl ratios for the diopside phenocrysts from Mount Elgon are typical of low pressure ( < 10 kbar)

igneous clinopyroxenes ( Aoki and Kushiro, 1968; Aoki and Shiba, 1973).

lie* WI%

4.0 22

1.8

30

14

2.0 10

- core Rm Core Rim

Fig. 4. Major element variations from electron microprobe core-to-

rim traverse from a diopside phenocryst from sample El 107. Trav-

erse was obtained using a CAMECA electron microprobe. Run

conditions were: 2 mm spot size, 15 kV accelerating potential and

20 nA current standardized on a variety of silicates and oxides.

Uncertainties for A1203 and TiOZ are f 2% of the quoted values. (a)

MgO wt.% (b) Fe0 wt.% (c) A1,03 wt.% (d) TiOZ wt.%.

A. Simonetti, K. Bell / Lithos 36 (1995) 141-153 147

05122i , ‘\+_:I , r’ ; 1 070300 0704co 0.70500 0.70600

*7sr/66sr

Fig. 5. EACL from Bell and Blenkinsop (1987). Symbols:

dots = diopside phenocrysts from Mount Elgon nephelinites; cir-

cles = whole rock Mount E.lgon nephelinites. Tie-lines join diopside

phenocryst with corresponding whole rock analysis. Isotopic data

from Napak whole rock nephelinites and diopside phenocrysts shown

in area enclosed by dashed line.

lavas from Mount E,lgon are the result of different

degrees of differentiation from a single parental melt.

The isotope ratios from both the Mount Elgon diopside phenocrysts and their host lavas will allow a more pre- cise evaluation of the volcano’s magmatic history.

4.2. Radiogenic isotope data

The Nd, Pb, and Sr isotopic results are presented in Tables 4 and 5. In all cases, the isotopic ratios are considered to be similar to those that existed at the time

of formation of the melt, because of the relatively young age of the complex. In addition, the Rb/Sr ratios

are low, ranging from 0.03-0.19 (this work; Le Bas,

1987)) a feature that is characteristic of most nephelin- ites (Simonetti and Bell, 1994a, b). Assuming an age of 20 Ma for Mount Elgon, leads to a correction of < 0.0001 to measured *‘Sr/%jr ratios. Corrections to the ‘43Nd/‘44Nd ratios are < 0.00001 based on Sm/Nd ratios from nephelinites given in Peterson ( 1989a) and Simonetti and Bell ( 1994b).

The Nd and Sr isotopic ratios from Mount Elgon span more than the entire range of values observed from several East African carbonatite complexes (Bell and Blenkinsop, 1987). Two of these samples, El 104 and El 107, both nephelinites, lie in the depleted quadrant of Fig. 5, and are similar to values obtained from car-

bonatites from other Ugandan alkaline centres located within the Eastern Rift (Bell and Blenkinsop, 1987), and values from those carbonatites that define one extreme of the East African Carbonatite Line (EACL, Bell and Blenkinsop, 1987). The EACL is a Nd-Sr

isotopic array defined mainly by young ( < 30 Ma) carbonatites from Kenya, Tanzania and Uganda, and is similar in slope to the “Lo-Nd” array of Hart et al.

( 1986). The EACL may therefore represent carbona- tites derived from a mixture of HIMU and EM I mantle components (Bell and Blenkinsop, 1987)) the two end-

members of the “Lo-Nd” array (Hart et al., 1986). With the exception of the Nd and Sr isotopic ratios from diopside separate El 108, the slope of the regres- sion line ( - 0.22 f 0.06) through the isotopic data from all the remaining Mount Elgon samples (both pyroxene separates and whole rocks) shows similarity to that of the most recent estimate for the EACL of - 0.21 f 0.07 (Bell, unpubl. data).

Also shown in Fig. 5 are Nd and Sr isotopic ratios

from five diopside separates. Most of these have Nd and Sr isotopic ratios significantly different from those of their corresponding whole rocks. The diopsides con-

sistently plot at lower Sr isotopic ratios whereas their Nd isotopic ratios can be either higher or lower than their corresponding whole rock values (Fig. 5). The two different diopside populations from sample El 109

have Nd and Sr isotopic ratios that are distinct from one another and quite different from the ratios from

their host rock. These observations suggest that the diopsides did not precipitate from the same melt, nor

from the melt that transported them to the surface. The

fact that no correlation exists between major element composition (e.g. Mg#) and isotopic ratios for the

Mount Elgon diopside separates supports the involve-

ment of more than one parental magma. The present-day Pb isotopic ratios from all the Mount

Elgon whole rocks and diopside phenocryst separates are listed in Table 5 and shown in Fig. 6. Using Pb and U concentrations determined by isotope dilution for three whole rock samples (Table 5)) calculated initial

Pb isotopic ratios (assuming an age of 20 Ma) are within analytical uncertainty of present-day ratios, and as such we consider all of the whole rock Pb isotopic ratios to approximate initial values. All of the Pb iso- topic ratios plot to the right of the Stacey-Kramers (1975) Pb evolution curve, above the regression line (NHRL) defined by oceanic basalts from the Northern

148 A. Simonetti, K. Bell/Lithos 36 (1995) 141-153

42.00

36.00

15.30

B

A

17.00 18.00 19.00 20.00 21.00 22.00

206Pb/204Pb

Fig. 6. A. 207Pb/2@‘Pb vs. 2”Pb/204Pb and B. *osPb/ZMPb vs. 206Pb/ *04Pb. HIMU and EMI mantle components are from Hart ( 1988). S/K= Stacey-Kramcrs growth curve (Stacey and Kramer% 1975). Open squares along S/K curve are at 200 Ma intervals. NHRL= Northern Hemisphere Reference Line (Hart, 1984). Sym- bols as in Fig. 5. Tie-lines join diopside phenocryst with correspond- ing whole rock analysis. Isotopic data from Napak whole rock nephelinites and diopside phenocrysts shown in shaded areaenclosed by dashed line.

Hemisphere (Fig. 6, Hart, 1984), and between the

HIMU and EM1 mantle components (Hart, 1988)) a distribution that is similar to that defined by oceanic

basalts from the Southern Hemisphere (Hart, 1988).

There is a similarity between the variation in Pb

isotopic ratios from the Mount Elgon diopside sepa- rates and whole rocks to other data collected from East

African carbonatites (AlBgre et al., 1979; Gtinen-

felder et al., 1986)) and the data overlap with the range found in whole-rock nephelinites and diopside sepa-

rates from Napak (Fig. 6, Simonetti and Bell, 1993; Simonetti and Bell, 1994a). Most of the Mount Elgon diopside separates have lower Pb isotopic ratios than

their whole rocks. The two diopside populations from sample El 109 contain Pb isotopic ratios that are sig- nificantly different from one another and support crys- tallization in an open magma system.

The slope of the linear array shown in Fig. 6a is

-0.081 f.O.01 (2~), and a similar slope of -0.073 f 0.02 (2q) is generated by the five diopside

phenocryst seperates. These slopes cannot be second- ary isochrons, since their age of about 1000 Ma ago is well in excess of the age of magmatic activity at Mount Elgon. The slope of the regression lines from the Mount Elgon data in Fig. 6a are similar to those from Napak ( -0.10, Simonetti and Bell, 1994a), the oceanic regression line (0.102, Tatsumoto, 1978) and the NHRL (0.1084, Hart, 1984). A negative correlation

between Pb isotopic ratios and *‘Sr/*%r values, for

both whole rocks and diopsides, and a positive corre- lation between Pb and ‘43Nd/ l”Nd isotopic ratios

(Table 4 and Table 5)) are consistent with open system behaviour involving binary mixing. The data approxi-

mate a mixing model between HIMU and EM I, with no indication of the DMM (Depleted Morb Mantle; Hart, 1988) component (Fig. 7). Mount Elgon neph-

Table 4 Nd and Sr whole rock and diopside phenocryst isotopic data from Mt. Elgon

Sample “Sr/%r measured” ‘43Nd/‘“Nd measured”

Whole rocks El 100 0.70394 0.5 1284 El 101 0.70413 0.5 1272 El 103 0.70500 0.51235 El 104 0.70314 0.5 1280 El 107 0.70328 0.51286 El 108 0.70401 0.51244 El 109 0.70604 0.51236 D 2836 0.70398 0.51252 D 2967 0.70608 0.51219

Diopside phenocrysts El 101 0.70366 0.51278 El 107 0.70324 0.51285 El 108 0.70376 0.51237 El 109 Light 0.70473 0.51241 El 109 Dark 0.70535 0.51229

“Measured Nd and Sr isotope ratios are considered initial. Isotope results normalized to ‘46Nd/‘44Nd=0.7219 ands%r/‘?Sr=0.1194. NBS 987 standard=0.71025 *0.00002; La Jolla Nd standard= 0.51186~0.00002 and BCR-1 ‘“Nd/“=Nd=0.51266f0.00002. Uncertainties are given at the 20 level.

A. Sitnonetti, K. Bell/Lithos 36 (1995) 141-153 149

Table 5 Pb whole rock and diospide phenocryst isotopic data from Mt. Elgon

Sample U Pb ‘06Pb /‘@‘Pb 207Pb/=‘Pb 2osPbl

(ppm) (ppm) *04Pb

Whole rocks El100 - - El 101 1.7 6.4 El 103 - - El104 - - El 107 - - El 108 1.1 7.7 El 109 - _

D2836 - - D 2967 0.2 12.4

Diopside phenocrysts El 101 - 0.2 El 107 - 0.4 El108 - - El 109 - - Light El 109 - - Dark

20.94 15.83 40.84 21.01 (20.96) 15.85 (15.85) 40.66 18.45 15.61 38.62 21.46 15.85 40.66 21.51 15.88 40.92 19.52 (19.49) 15.68 (15.68) 39.11 20.43 15.84 41.04 20.28 15.75 40.50 19.15 (19.15) 15.70 (15.70) 39.83

21.08 15.84 40.63 20.93 15.79 40.28 19.09 15.66 38.74 20.04 15.77 40.29

19.26 15.71 39.53

Reproducibility of Pb ratios is 0.1% at the 2a level. An average fractionation factor of 0.1% per mass unit was applied to all measured ratios based on the analyses of NBS 982. U and Pb concentrations were determined by isotope dilution using a mixed zoaPb-z35U-*33U spike. (-) = not determined. For samples El 101, El 108 and D 2967, values in brackets are initial ratios assuming an age of 20 Ma. On the basis of documented p ( 238U/2arPb) and 232Th/Z04Pb values for clinopyroxenes (e.g. Cohen et al., 1984; Meijer et al., 1990; Davies and Lloyd, 1991), correction to present-day Pb isotopic ratios for the Mount Elgon clinopyroxenes are about the same as our estimated analytical uncertainties for the ratio measurements.

elinites with low 206Pb/204Pb isotopic ratios, however,

contain variable 87Sr/86Sr and 143Nd/‘44Nd values (Fig. 7)) which may suggest the possible involvement of the EM II mantle component (Enriched Mantle II; Hart, 1988).

5. Discussion

The Nd and Sr isotopic ratios from Mount Elgon, for both diopsides and whole rocks, are highly variable and plot in three of the four quadrants of the anticorrelation plot (see Fig. 5)) a finding that is consistent with open system behaviour. A large range in Nd and Sr isotope ratios was also documented for a suite of nephelinite lavas from the Napak volcano (Fig. 5, Simonetti and

Bell, 1994a). The Nd, Pb and Sr isotopic variations from the Napak complex (Fig. 5, Simonetti and Bell, 1994a) span almost the entire range of isotopic data from all the East African nephelinite-carbonatite com- plexes studied to date (Allegre et al., 1979; Grtlnen- felder et al., 1986; Bell and Blenkinsop, 1987). In addition, six diopside phenocryst populations from oli- vine-bearing nephelinites from Napak have isotopic ratios that, in most cases, are significantly different from those of their enclosing lavas (Simonetti and Bell, 1994a). The isotopic disequilibrium demonstrated between the clinopyroxene phenocrysts and their host nephelinite lavas, and large variation in isotopic ratios for the Napak complex as a whole are consistent with clinopyroxene and/or olivine fractionation in a magma open system.

Few examples of isotopic disequilibrium among mineral phases from pfimitiue, mafic oolcunic rocks are cited in the literature. Disequilibrium crystalliza- tion, however, has been documented for clinopyroxene phenocrysts from phonolites from the Laacher See vol- canics, East Eifel (Womer et al., 1984), and this has

0 I I

0.5120 -

I I I B 0.7100 m -

EM II

0.7080 1 I

*O+b/204Pb

Fig. 7. A. ‘43Nd/‘“Nd vs. 206Pb/204Pb. B. a’Sr/?Sr vs. unrPblmPb. HIMU, EM1 and EM II mantle components are from Hart ( 1988). Symbols as in Fig. 5.

150 A. Simonetti. K. Bell / Lithos 36 (1995) 141-153

been attributed to either crustal assimilation, or late- stage fluid exchange between melt and wall rock. Simo- netti and Bell ( 1994a) attempted to explain the isotopic variability of the Napak isotopic data, and the isotopic disequilibrium between diopside phenocrysts and cor-

responding whole rocks in terms of the following three models: (1) crust-magma binary mixing, (2) com- bined assimilation-fractional crystallization (DePaolo, 1981) and (3) mixing of magmas derived from an isotopically heterogeneous mantle source region, all

involving open system behaviour. Any model involv- ing the interaction between melt and continental crust is difficult to reconcile with the Napak isotopic data

because: ( 1) even the most “contaminated” Napak lavas (i.e. samples with the lowest Nd and highest Sr isotopic ratios) still retain a character typical of neph- elinites; (2) of the inherent problem of the thermal budget required for the bulk assimilation of crust and

(3) the lack of agreement between calculated and measured abundances of Nd and Sr using the AFC model (DePaolo, 1981). Due to the similar nature of

the nephelinite lavas from Mount Elgon and Napak, these same problems also apply to the Mount Elgon

isotopic data. Large core-to-rim compositional variations from the

Mount Elgon diopside phenocrysts (Figs. 3 and 4) show that the melt was undergoing a change in chem- ical composition. In addition, the isotopic data show that not all of the diopsides isotopically equilibrated with their enclosing melt. The distinct Nd, Pb and Sr isotopic ratios of the dark and light green diopside

phenocryst populations from sample El 109 rules out crystallization from the same parent melt, or from the melt that hosts them. Such bimodal assemblages of clinopyroxene phenocryst populations, each with their

own distinct chemical compositions, are a common feature of alkaline volcanic flows from several centres (e.g. Wass, 1979; Barton and van Bergen, 1981; Duda and Schmincke, 1985; Dobosi and Fodor, 1992), as well as nephelinite lavas from Napak (Simonetti and Bell, 1993). The data in Fig. 3 indicate that the rim compositions for both diopside populations from sam- ple El 109 approach a similar value, suggesting chem- ical equilibration with their host melt of uniform composition during the late stages of crystallization. On the other hand, nephelinite lavas from Napak, that also contain two populations of diopside phenocrysts, show large compositional variations in the phenocryst

rims (Simonetti et al., in press), suggesting a more complex crystallization history than that indicated by Mount Elgon.

Most of the Nd and Sr data from Mount Elgon plot either in the depleted or the enriched quadrant of the Nd-Sr diagram (Fig. 5) and all (other than sample El 108) define a linear array with a similar slope to the EACL. Similar findings for the silicate lavas from

Oldoinyo Lengai were thought by Bell and Dawson ( 1995) to indicate small volume, discrete mantle melts that involved the mixing of the HIMU and EM I mantle components. The variation in the isotopic data for neph-

elinite lavas from three quite distinct East African vol- canic centres (Mount Elgon, Napak, Oldoinyo Lengai) suggest that the nephelinitic melts from each probably formed in a similar manner, and that the nephelinitic magmas effectively recorded upper mantle isotopic heterogeneities. The large variation in isotope ratios for

the nephelinite lavas could be attributed to the inter- action between metasomatized and depleted sub-con- tinental mantle, since metasomatic activity is a

necessary precursor to nephelinite melt generation. The range in isotopic ratios for the nephelinite lavas from

Mount Elgon, Napak and Oldoinyo Lengai volcanoes can be attributed to the same two mantle end-members, a HIMU-like and EM I-like components. Although it is difficult, at present, to place constraints on the nature and location of these two mantle components, the data from Mount Elgon clearly show that HIMU and EM I are contiguous. In addition, Nd, Pb and Sr isotopic data from young ( <200 Ma) carbonatites world-wide, mostly including those from East Africa, have signa-

tures which lie between HIMU and EM I mantle com- ponents (Tilton and Bell, 1994), and support this interpretation.

The primitive nature (high Mg#) and large isotopic variations exhibited by the Napak and Mount Elgon nephelinite lavas strongly suggest that each lava flow represents a discrete, mantle-derived partial melt, con-

sistent with the findings from Oldoinyo Lengai (Bell and Dawson, 1995). In addition, the occurrence of bimodal assemblages of isotopically-and chemically- distinct diopside phenocrysts found in certain lavas from both Napak and Mount Elgon is consistent with a model that involves magma mixing. It is possible to explain the presence of phenocrysts of quite different chemical and isotope composition by turbulent injec- tions of hot, dense, primitive liquids in a magma cham-

A. Simonetti, K. Bell / Lithos 36 (1995) 141-153 151

ber with compositional and thermal gradients (e.g. Turner and Campbell,. 1986). Such magma mixing may have occurred between newly-formed, ascending nephelinitic liquids and slightly older nephelinitic melts at lower crustal levels, a region favourable to the ponding of mantle-derived melts due to density con- trasts (e.g. Duda and Schmincke, 1985). Stirring or mixing of magmas could also be aided by exsolution of CO2 and HZ0 from fresh influxes of volatile-rich, primitive magma. This latter mechanism may have been important at Mount Elgon and Napak because of the predominantly pyroclastic nature of the volcanics and the abundance of volatiles (particularly COZ) in nephelinitic magmas (Brey and Green, 1977; Brey, 1978; Olafsson and Eggler, 1983; Wallace and Green, 1988).

6. Conclusions References

The Nd, Pb and Sr isotopic ratios from the Mount Elgon nephelinites and their diopside phenocryst sep- arates show large variations that can be attributed to the mixing of melts derived from an isotopically het- erogeneous mantle source beneath eastern Uganda- western Kenya. The similar isotopic variations (between HIMU and EM I) from other East African volcanoes (Napak, Oldoinyo Lengai), suggest that nephelinite-dominated volcanism need not be con- trolled by a single parental melt. The isotopic data from Mount Elgon, Napak and Oldoinyo Lengai show that the sub-continental mantle beneath eastern Uganda probably consists of two end-members, a HIMU-like and the other an EMI-like, mantle components. This finding supports the conclusions made by Bell and Simonetti ( 1994) and Tilton and Bell ( 1994) that oce- anic mantle components, such as HIMU and EM I, may also characterize the sub-continental mantle. The large variation in isotopic data for nephelinites from these East African volcanoes may be attributed to the partial melting of inhomogeneous, metasomatized sub-conti- nental upper mantle.

One of the most important findings to emerge from this study is that silicate whole rock analyses on their own cannot be used to accurately define the isotopic signature of sub-continental mantle. A study involving isotopic ratios from both whole rocks and mineral

phases is essential for understanding the complex crys- tallization history that have effected eruptive centres.

Acknowledgements

One of us (K.B.) would like to thank the Geological Survey of Uganda for donating samples from their ref- erence collection, and providing logistical support many years ago. NSERC provided means to KB that partly supported this project. Mark Shore is thanked for the electron microprobe traverse of diopside phe- nocryst from sample El 107, and Peter Jones for help with electron microprobe analyses. A thoughtful and constructive review by G.R. Tilton improved the qual- ity of the manuscript.

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