Contrib Mineral Petrol (1993) 114:276-287 Contributions to Mineralogyand Petrology �9 Springer-Verlag 1993

Liquid immiscibility between trachyte and carbonate in ash flow tufts from Kenya R. Macdonald 1, B.A. Kjarsgaard 2, 4, I.P. Skilling 1 *, G.R. Davies 3, D.L. Hamilton 2, and S. Black 1

1 Environmental Science Division, Lancaster University, Lancaster LA1 4YQ, UK 2 Department of Geology, The University, Manchester M13 9PL, UK 3 Vrije Universiteit Amsterdam, Faculteit der Aardwetenschappen, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands 4 The Geological Survey of Canada, 588 Booth Street, Ottawa K1A-0E4, Canada

Received August 13, 1992 / Accepted December 10, 1992

Abstract. Three thin, syn-caldera ash flow tufts of the Suswa volcano, Kenya, contain pumiceous clasts and globules of trachytic glass, and clasts rich in carbonate globules, in a carbonate ash matrix. Petrographic and textural evidence indicates that the carbonate was mag- matic. The trachyte is metaluminous to mildly peralka- line and varies from nepheline- to quartz-normative. The carbonate is calcium-rich, with high REE and F con- tents. The silicate and carbonate fractions have similar 143Nd/144Nd values, suggesting a common parental magma. Chondrite-normalized REE patterns are consis- tent with a carbonate liquid being exsolved from a sili- cate liquid after alkali feldspar fractionation. Sr isotopic and REE data show that the carbonate matrix of even the freshest tuffs interacted to some degree with hydro- thermal and/or meteoric water. A liquid immiscibility relationship between the trachyte and carbonate is indi- cated by the presence of sharp, curved menisci between them, the presence of carbonate globules in silicate glass and of fiamme rich in carbonate globules separated by silicate glass, and by the fact that similar phenocryst phases occur in both melts. It is inferred that the carbon- ate liquid separated from a carbonated trachyte magma prior to, or during, caldera collapse. Viscosity differ- ences segregated the magma into a fraction comprising silicate magma with scattered carbonate globules, and a fraction comprising carbonate globules in a silicate magmatic host.

Explosive disruption of the magma generated silicate- and carbonate-rich clasts in a carbonate matrix. The silicate liquid was disaggregated by explosive disruption and texturally appears to have been budding-off into the carbonate matrix. After emplacement, the basal parts of the flows welded slightly and flattened. The Suswa rocks represent a rare and clear example of a liquid immiscibility relationship between trachyte and carbon- ate melts.

* Pre sen t address: British Antarctic Survey, Geology Division, High Cross, Madingley Road, Cambridge CB3 0ET, UK

Introduction

A variety of mechanisms has been suggested for the for- mation of carbonatites, i.e. : (1) Fractionation within the crust of a carbonated, usual- ly nephelinitic, parental magma derived from the mantle. (2) Polybaric immiscible separation of carbonate and silicate liquids from mantle-derived melts, such as nephe- linites and phonolites. (3) Direct melting of carbonate-bearing, metasomatized mantle, to generate separate carbonatite and silicate liq- uids.

It is unlikely that any one mechanism is unique for carbonatites. Each specific occurrence has to be evalu- ated for critical evidence of origin.

The association of carbonatites with strongly silica- undersaturated silicate rocks is a well-known feature of the East African Rift Valleys (Baker 1987). An important role for COa in the petrogenesis of the salic rocks of Kenya has been repeatedly advocated by Bailey (eg. 1978, 1980, 1982, 1987), and Scott (1982) has presented petrographic evidence that a CO2-rich vapour phase of low solubility coexisted with peralkaline trachyte at the Longonot volcano. In this paper, textural, mineralogical and geochemical data on trachytic ash flow tuffs from the Suswa volcano, Kenya, are interpreted as providing exceptionally clear evidence of a liquid immiscibility re- lationship between trachyte and s6vite.

Geological summary

Suswa is a Quaternary, dominantly trachytic-phonolitic volcano, located within the inner trough of the central Kenya Rift Valley (Fig. 1). The volcano has two calderas (Johnson 1969). The first collapsed both incrementally and asymmetrically, at some time be- tween 0.24 Ma and 0.1 Ma. The initial stages were very complex, associated with the eruption of trachybasaltic ashes, carbonate- trachyte ash flow tufts, trachytic pumice lapilli airfalls and tra- chyte-trachyphonolite globule ignimbrites (Fig. 1). During later stages of the collapse of the first caldera, trachytic agglutinate flows were erupted from a zone of ring fractures. Eruption of

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277

magma from a western cupola led to asymmetrical collapse of the caldera to the NW.

The carbonate-bearing ash flow tufts form three units, each 0.2 m to 2.25 m thick, covering large areas of the N, NE and SE flanks of Suswa (Fig. 1). They are pale-yellow to yellow-brown, unwelded, partly consolidated by carbonate cementation, common- ly solution-gulteyed and often honeycomb-weathered from the weathering out of pumice clasts. Locally, the top surfaces are flu- vially reworked, with areas of parallel laminae and infilled, poly- gonal shrinkage cracks. Similar shrinkage cracks are a characteris- tic feature of the carbonatitic Laetolil Beds in Tanzania (Hay 1978). They are possibly "salt polygons" formed by the repeated solution and recrystallization of salts dissolved from the carbonate ash im- mediately post-emplacement. Previously interpreted as lahars (Johnson 1969), the Suswa deposits show strong evidence for an ash flow origin, including a high proportion of juvenile material, the presence of very localized, basal, concentration zones of coarse pumice ( > 3 cm) and megacrysts, and basal flattening and .welding. The highly fluidized nature of the flow is indicated by the low aspect ratio of the tufts and by the presence of basal, blebby con- tacts with underlying vitric ashes. The vents from which the ash flows were erupted have not been identified but were possibly locat- ed on the ring fracture zone. The three units collectively formed a liquid volume estimated to exceed 0.8 km 3 (Skilling 1988).

A n a l y t i c a l a n d e x p e r i m e n t a l t e c h n i q u e s

Microprobe

Major element, and specific trace element analyses were obtained by electron microprobe analysis using standard techniques (Ta- bles 1, 2)_ Metals, oxides, and natural and synthetic minerals were used as calibration standards. Mineral analyses were performed using a beam diameter of 1 gin, carbonate and glass analyses uti- lized a defocussed beam (25 pm diameter). Silicate glass, carbonate and apatite were analysed in Manchester at 15 kV and 14.5 nA by combined EDS/WDS techniques using a Cameca Camebax mi- croprobe with SPECTA software for data reduction. Analyses of feldspar, olivine and clinopyroxene were performed at 15 kV and 3.0 nA by EDS techniques on the same microprobe using ZAF software. Additional pyroxene analyses, plus amphibole, aenigma-

T a b l e 1. Representative analyses of phenocryst phases

Alkali feldspar Olivine

1 2 3

<--Clinopyroxene~

4 5 6

Amphi- Aenigma- ~ Spinel~ Apatite bole tite 7 8 9 10 11

SiO2 66.07 65.67 35.23 51.30 51.27 51.71 TiO2 0.24 0.00 0.05 0.44 0.77 1.13 A1203 19.13 18.51 0.00 1.16 0.67 0.51 FeOt 0.00 0.00 36.10 12.50 16.83 26.66 MnO 0.00 0.00 1.41 0.93 0.95 0.61 MgO 0.00 0.00 26.91 11.92 8.40 0.70 CaO 0.48 0.29 0.11 21.02 19.62 9.23 Na20 7.23 6.76 - 0.89 1.50 8.54 K20 6.21 6.55 - - 0.03 0.00 P 2 0 5 . . . . . . t 7 . . . . .

- O = - F . . . .

Total 99.37 97.78 100.05 100.17 100.04 99.24

43.02 44.97 0.17 2.03 0.34 5.94 2.75 27.45 0.07 0.08 1.00 6.41 0.53 2.39 0.06

38.97 20.44 66.56 73.78 0.89 1.34 0.59 1.47 8.84 0.07 0.88 8.23 0.82 4.23 0.10 0.46 9.63 - - 54.14 7.50 4.24 - - 0.15 0.03 1.34 - - 0.13 . . . . 41.08 - 1.58 - 2.82 - 1 . 1 4 - - 1.19

99.14 99.04 97.10" 91.42 b 98.70 ~

Explanation: 1, 2 - phenocrysts in pumice clast (IS 153); 3 - pheno- cryst contacting silicate glass and carbonate (IS 135); 4 - core of phenocryst in silicate glass (IS 135); 5 - phenocryst in silicate glass (IS 95); 6 - microphenocryst in carbonate matrix (IS 95); 7 - bro- ken phenocryst in carbonate matrix (IS 135); 8 - phenocryst in

carbonate matrix, contacting two silicate globules (IS 95); 9, 10 - phenocrysts in carbonate matrix (IS 95); 11 - microphenocryst in silicate glass (IS 135) Totals include: "ZnO=0 .10 ; b ZnO=0.08; c SRO=0.03 FeOt = total iron as FeO

278

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tite and spinel analyses were performed in Ottawa at 15 kV and 10 or 30 nA (spinels 20 kV and 30 hA) by WDS techniques using a Cameca Camebax microprobe with PAP software.

Major and trace elements, REE and isotopes

Data given in Table 3 for rocks IS 95 and IS 135 were determined as follows�9 Bulk rocks, and the silicate residue in IS 95 after dissolu- tion of the carbonate in dilute acetic acid, were analyzed using a Philips PW 1400 XRF spectrometer at Lancaster, employing fu- sion discs for major elements and powders for trace elements. Sepa- rate preparations were made at Leeds of the silicate (residue) and carbonate (leachate) fractions after dissolution in dilute HC1, fol- lowed by ultrasonic agitation. REE were then determined in the Lancaster and Leeds preparations by isotope dilution, following the method of Thirlwall (1982). Nd and Sr isotopes and Rb and Sr abundances (Table 3) were determined using methods reported in Davies and Macdonald (1987). Data obtained for standards at Leeds during the period of study are: 87Sr/86Sr in NBS 987= 0�9 and 143Nd/14~Nd in BCR 1 =0.5126.

The acid dissolution technique probably did not remove all the carbonate due to the mutually inclusive nature of silicate and carbonate, and may have leached minor amount of trace elements fi-om the silicate glass. However, the close comparability of the compositions of the residue in IS 95 after both dissolutions (Ta- ble 3) suggests that the leacheate is close to the true carbonate composition.

U and Th activity ratios (Table 3) were determined by isotope dilution alpha spectrometry at Lancaster University, using a 232U-22aTh yield monitor. The carbonate was leached from the bulk rock using 4M HNOa. The analytical techniques are similar to those in Williams et al. (1986).

Petrography

The majority of tuff specimens are heavily altered�9 Mafic phenocrysts are limonitized, and silicate glass is either secondarily hydrated or devitrified. There has been con- siderable remobilization and alteration of primary car- bonate. Rare, fresh samples towards the base of the units preserve the original textures and compositions. These fresh samples form the basis of the following discussion.

The ash flow tuffs contain dark grey, pumiceous sili- cate clasts, whitish carbonate-rich clasts, lithic clasts and phenocrysts in an ash grade matrix. There is little varia- tion in the juvenile (magmatic): lithics: phenocrysts ra- tios, about 15:2:2 volume %, but the proportion of these components to matrix is very variable. The size ranges are silicate clasts < 1-3 cm, lithic clasts < 1-7 cm and phenocrysts < 1 3 cm. Exceptions are pmrdce clasts in the concentration zones mentioned above, which may be up to 10 cm. The proportions of silicate to carbonate clasts are highly variable, with some specimens carrying only one type.

A centimetre-scale banding is defined by alternations of layers comprising almost 100% ash grade carbonate and layers rich in silicate globules and phenocrysts (Fig. 2 a). The boundaries between the layers are usually transitional over < 1 mm.

The phenocryst assemblage in the ash flow tufts is alkali feldspar + olivine + clinopyroxene + titanomagne- tite + magnetite + apatite _+ aenigmatite __ amphibole. Re- presentative mineral analyses are given in Table 1. Alkali

Table 3. Partial major, trace and isotopic element data for IS 95 and IS 135

279

IS 95

Bulk Silicate Silicate Carbonate rock (acetic) (HC1) (HC1)

IS 135

Bulk Silicate Carbonate rock (HC1) (HC1)

SiOz 47.59 60.93 - - TiO2 0.60 0.86 - - A1203 11.85 14.34 - - FeO~ 5.64 7.49 - - MnO 0.31 0.28 - - MgO 1.40 0.83 - - CaO 12.90 0.97 - - Na20 4.61 5.79 - - K20 3.80 4.45 - - P 2 0 5 0 . 0 8 0 . 1 0 - -

(Total) (88.78) (96.04) - -

Ba 149 121 - -

Ce 244 257 Nb 235 346 - - Pb 18 26 - - Y 107 112 - - Zn 209 296 Zr 649 943 - -

La 154.5 165 171 88.7 Ce 234 275.3 284 165.7 Nd 80.3 89.2 91.2 59.9 Sm 14.3 15.7 15.9 11.0 Eu 2.40 2.51 2.57 1.72 Gd 13.5 13.6 13.6 13.0 Dy 16.9 16.2 16.2 17.4 Er 11.0 10.5 10.5 12.5 Yb 10.9 10.3 10.4 13.3

Rb 137 143 160 49 Sr 579 18 17 829 STSr/86Sr 0.706142-+ 10 0.704291 _+ 12 0.704276_+ 12 143Nd/144Nd 0.5t269_+ 1 0.51271 + 1 U - - 6.50-+0.10 Th - - 18.8-+0.84 ( 2 3 8 U / 2 3 2 T h ) - - 1.064_+0.033 ( 2 3 ~ - - 0.742_+0.033 ( 2 3 8 U / 2 3 4 U ) - - 1.025 +0.034

0.706337+ 12 0.5t268_+1 2.80 _+ 0.07

20.6_+1.14 0.417+_0.017 0.695 -+ 0.036 1.011 •

28.00 - - 0.40 - - 7.28 - - 2.58 0. t l - - 2.57 - -

29.80 - - 2.89 - - 2.35 - - 0.08 - -

(76.06) - -

597 - - 89 - - 76 - -

8 - - - -

125 - - 49 - -

235 - -

38 117.6 11.47 82.8 214.6 38.8 56.6 77.33 50.5 12.3 14.0 11.7 2.65 3.I5 1.92

13.6 12.1 14.6 16.0 11.4 17.5 10.5 8.8 11.1 10.4 8.3 10.9

42 126 13 598 75 772

0.706060+ 10 0.703866--+ 10 0.706144_+ 12 0.51279-+1 0.51280_+1 0.51263_+1

FeO, = total iron as FeO

fe ldspa r ( < 2 % by v o l u m e ) s t radd les the a n o r t h o c l a s e / s a n i d i n e b o u n d a r y , a n d shows s l ight z o n i n g to m o r e po- tass ic r ims. Ba c o n t e n t s a re m o d e s t a n d Sr low. The re a re two k i n d s o f c l i n o p y r o x e n e ( < 0 . 5 % ) . O n e occur s as e u h e d r a l p h e n o c r y s t s in b o t h si l icate a n d c a r b o n a t e , a n d r anges f r o m co lour less or pa le g reen augi t ic cores (Ca42MgasFezo) to greenish , f e r roaug i t i c r ims (Ca46Mg24Feao) , w i th l i t t le N a - e n r i c h m e n t (Table 1, ana lyses 4, 5). Aeg i r i n e - au g i t e is f o u n d as e u h e d r a l phe- n o c r y s t s i n the c a r b o n a t e m a t r i x (Tab le 1, ana lys i s 6). T i t a n o m a g n e t i t e ( < 1 % ) fo rms e u h e d r a l to s u b h e d r a l p h e n o c r y s t s , u p to 1.5 m m across , s h o w i n g n o z o n i n g or e x s o l u t i o n o f a T i - r i ch phase , even b y b a c k - s c a t t e r e d i m a g i n g . T h e y c o n t a i n f r o m 71 to 78 w t % f i lvosp ine l c o m p o n e n t a n d are t hus typ ica l o f spinels f r o m t r achy tes a n d t r a c h y b a s a l t s b u t qu i t e u n l i k e a n y k n o w n spine l f r o m a c a r b o n a t i t e (Table 1, ana lys i s 9). M a g n e t i t e f o r ms large ( > 1 ram) , s u b h e d r a l to a n h e d r a l p h e n o -

crysts in the c a r b o n a t e ma t r ix . C o m p o s i t i o n a l l y , they are typ ica l o f c a r b o n a t i t i c m agne t i t e s , t h o u g h s l ight ly h ighe r in M n O (Table 1, ana lys i s 10), b u t e m b a y m e n t tex tures sugges t t h a t they m a y h a v e b e e n r eac t ing wi th the c a r b o n a t e in the S uswa rocks .

T h e o l iv ine ( < 0 . 5 % ) in the tuf ts (Fo5 s - s 8) is cons id - e r ab ly m o r e m a g n e s i a n t h a n the o l iv ines in S uswa lavas a n a l y z e d b y N a s h et al. (1969), i.e. Fo < 30. U s i n g , h o w - ever, R o e d e r a n d Ems l i e ' s (1970) va lue o f 0.3 for the p a r t i t i o n coef f ic ien t K ~ it c a n be s h o w n t h a t D / F e - M g ~

ol iv ine o f this c o m p o s i t i o n w o u l d have been in equ i l ib r i - u m wi th m e l t c o n t a i n i n g 1 .2% M g O , s imi la r to the rela- t ively m af i c t r a chy t e glass f o u n d as i n c l u s i o n s in the o l iv ine p h e n o c r y s t s (Table 2). B lue-grey a r fvedson i t i c a m p h i b o l e wi th m o d e r a t e l y h igh F c o n t e n t (Table 1, ana lys i s 7) f o r m s b r o k e n p h e n o c r y s t s in the c a r b o n a t e ma t r ix . T h e a n a l y s e d a e n i g m a t i t e (Table 1, ana lys i s 8) is a e u h e d r a l to s u b h e d r a l , I m m p h e n o c r y s t in the car-

280

Fig. 2. a. Layering defined by an upper band rich in dark, rounded, silicate globules, with alkali feldspar microphenocrysts (see crystal faces) in a carbonate matrix, a lower layer (bottom right), consist- ing of ash grade carbonate only, and a middle, transitional layer (specimen IS 135, PPL, width of field 4 ram). b Globules of trachyte glass in 1S 95. Note the serial range of sizes, rounded vesicles and budding features (e.g. larger globule just right of centre). Globule at bottom left contains carbonate globules. (PPL, field of view 4 ram). e Margin of silicate globule has budded-off into carbonate matrix and has produced a drip-like end. Note slightly variable grain size of matrix. (IS 95; PPL; field of view 0.125 mm). d Partly flattened clast in IS 135 shows carbonate globules (grey) separated by delicately preserved silicate glass (pale) which flowed round the deforming globules. Dark grey material to right and at bottom is matrix carbonate (secondarily stained brown). (PPL, field of

view 1.5 ram). e Strongly flattened clast or fiamma, in which car- bonate globules (grey) have length:breadth ratios exceeding 5:/ . Silicate glass not easily resolvable but forms extremely attenuated films between globules. Some rounded, carbonate-filled structures may represent post-emplacement vesiculation and infilling by lower temperature carbonate. Carbonate matrix to left and right contains phenocrysts of olivine (right), alkali feldspar (top left) and globules and shards of silicate glass (IS 85, PPL, field of view 1.5 ram). f Phenocrysts of titanomagnetite and microphenoeryst of apatite (left centre) at margin of carbonate-rich clast in IS 135. Carbonate globules are rounded and grey, silicate glass is pale coloured (e.g. as wedge between oxides). Note the sharp, curved menisci between silicate and carbonate at centre bottom and bottom right, coales- cence between carbonate globules, and sharp contacts between phe- nocrysts and both silicate and carbonate. PPL, field of view 1.5 mm

bonate matrix. It is close to theoretical end-member composition, with only minor substitution of A1, Mn, Mg and Ca.

The main juvenile components in the tuffs are as fol- lows: (i) Variably vesiculated, pumiceous silicate fragments, range from < 1 cm to 10 cm. The majority of fi'agments are subrounded and uncollapsed, with frayed ends. To- wards the base of the units, the fi'agments are compacted into fiamme-like forms, with flattening ratios up to 10: 1. (ii) Glass globules are the commonest silicate component in the ash fraction, ranging serially in size from 0.1 mm to 2 mm (Fig. 2b). All the smaller globules are essentially undeformed, although larger examples show some de- gree of flattening in the basal zones of the flow units. When the globules are in contact with carbonate, the margins are characteristically budded (Figs. 2b, c). Drip- like ends to buds (Fig. 2c) suggest that the smaller, subs- pherical globules were generated by budding-off from larger globules or clasts. All but the smallest globules carry rounded empty vesicles which probably contained a fluid phase. The silicate clasts and globules commonly contain carbonate globules, which become progressively more flattened as the clasts become compacted. Occa- sionally, carbonate globules are found in glass inclusions within phenocrysts, especially feldspar. Carbonate in the globules is somewhat variable in grain size but is dom- inantly microsparite. There is no consistent internal tex- ture, such as concentric zoning of grain sizes or radial growth of carbonate crystals, but the range of grain sizes in individual globules might indicate that the carbonate grew in more than one stage. At the resolution of the polarizing microscope, contacts between carbonate and silicate are sharp and form curved menisci. (iii) There is a complete transition from carbonate-bear- ing silicate glass to fragments where the proportion of carbonate exceeds 95%. The forms of the carbonate-rich fragments are similar to those of the silicate fragments. Individual carbonate globules may vary from rounded through ovoid to flattened, with flattening ratios > 10 : 1 (Figs. 2d, e). Coalescence of globules is common (Fig. 2 f). In compacted fragments, pristine silicate glass wraps delicately around the globules and intricate tex- tures are perfectly preserved (Fig. 2d). It is possible that round blobs of carbonate in some fiamme (Fig. 2e) rep- resent post-compaction vesicles. (iv) Volumetrically, the most important occurrence of carbonate is as matrix material, enclosing silicate glob- ules, phenocrysts, carbonate-rich clasts and lithic clasts (Figs. 2a-e). Where relatively fresh it is grey in colour but is more commonly brownish or dark grey as a result of alteration. The carbonate is invariably micritic, though there is some variation in grain size (Fig. 2c). There are patches and veinlets of slightly coarser materi- al, while the carbonate in contact with silicate globules is often very fine-grained. There is a sharp junction be- tween carbonate in globules in clasts and the matrix carbonate (Figs. 2d, e). The main phenocryst phases, apart from magnetite and aegirine-augite, are found in both silicate - and carbonate - rich clasts and have sharp, euhedral contacts against both phases (Fig. 2f).

281

In several cases, the same crystal is in contact with both silicate glass and carbonate. It appears that the pheno- cryst phases were in equilibrium with both inferred melts, an important test of an immiscible relationship.

Primary nature of carbonate

We consider that the following lines of textural, mineral- ogical and geological evidence point to an originally pri- mary origin for the carbonate in the Suswa tuffs, al- though we emphasize that in the majority of specimens carbonate has been remobilized during surface weather- ing. (1) The stratigraphical restriction of carbonate to the three ash flow tuffs makes it impossible that it was origi- nally precipitated from influxes of hydrothermal and/or ground water, although isotopic data, discussed below, indicate some hydrothermal/groundwater involvement. (2) The budding textures at silicate-carbonate contacts are best explained as an emulsion texture formed by mechanical disaggregation of one liquid into another. (3) The preservation of pristine silicate glass showing extremely delicate textures in carbonate-rich globules argues against large-scale replacement processes, be- cause alkali silicate glass tends to alter in the presence of CO2-rich, migrating pore water. (4) The presence of coalescing carbonate spheres indi- cates that the system was in a liquid state before quench- ing. (5) The extensive flattening of carbonate globules in- cluded in fiamme would only have been possible if they were liquid or a crystal-liquid suspension. The carbonate is unlikely to be later infillings of flattened vesicles, which would have probably not survived as coherent structures during compaction of the fragments. (6) The absence of carbonate veining or alteration in the lithic clasts is not compatible with a replacement origin for the carbonate. (7) The internal textures of the carbonate globules are not consistent with formation from fluids which migrat- ed into vesicles after eruption or from vesicles filled with a supercritical vapour phase. Precipitation under such conditions tends to produce concentrically zoned or geo- petal type mineralization, rather than the interlocking mosaic of minerals seen in the Suswa rocks and at Shorn- bole (Kjarsgaard and Peterson 1991). (8) The Presence of carbonate globules in fresh silicate glass inclusions within phenocrysts effectively precludes a secondary origin for the carbonate. (9) The banding into silicate-rich and carbonate-rich layers is almost certainly primary and is quite similar to features attributed to mixing of silicate and carbonate liquids in the Kruidfontein complex (Clarke and LeBas 1990).

We suggest that this combination of field and petro- graphic features provides unambiguous evidence that the carbonate in the ash flows was primary and magmatic.

282

Geochemistry

Silicate glass

Microprobe analyses have been made of silicate glass in all its modes of occurrence, viz. inclusions in pheno- crysts, pumice fragments varying from undeformed to flattened, and globules ranging in size from large to small. Apart from smaller globules, the glasses show a modest range in composition, from a more mafic tra- chyte (higher CaO, TiO2, P205, MgO; representative analysis -Table 2, No 1) to a less mafic trachyte (higher Na2 O, SiO2, F; Table 2, No 2).

There is an overall relationship between composition and size in the silicate component. Pumice clasts, larger globules and glass inclusions in phenocrysts most com- monly are mildly peralkaline (molecular (Na20 + KzO)/ A1203 1.0-1.3) and have normative olivine_ nepheline (e.g. Table 2, analyses 1, 2). Cores of medium-sized glob- ules are barely peralkaline to metaluminous and are quartz-normative (Table 2, analysis 3; molecular (Na20 + K20)/A1203 = 1.0, quartz = 6.0), but the rims of these globules, and the smallest sized globules, are corundum-normative and have more normative quartz (Table 2, analysis 4; corundum= 0.3; quartz = 12.8). The relationship is consistent with the observation that smaller globules form by marginal budding- off from larger globules. Overall, therefore, silicate fragments show a trend of decreasing peralkalinity and increasing silica-oversaturation as they become smaller. This trend is very largely a result of lower NazO values at globule rims.

Carbonate

Analytical data for carbonate globules in silicate clasts, globules in variably flattened carbonate-rich pumice fragments, and matrix material are given in Table 2. In all cases, the carbonate is calcium-rich, with moderate MgO levels (_< 5.8 wt%). There are low abundances of SiO 2 (<0.7%), total iron oxides, FeOt (_<0.8%), Na20 (_<1%), BaO (_<0.16%) and SrO (<0.18%). Ce levels are moderate (Ce203_<0.18%) and F high (0.55- 1.38%). No element lies outside the range for calciocar- bonatites given by Woolley and Kempe (1989). We sug- gest that all the compositional data are compatible with, and the F and Ce values strongly suggestive of, a mag- matic origin for the carbonate. The presence of Ca-rich, Na-poor, carbonate as globules within glass inclusions in phenocrysts further suggests that the Na-poor nature is magmatic, and not a result of post-emplacement groundwater leaching, as has been suggested by Hay (1978) and Hay and O'Neil (1983) for the carbonatitic Laetolil Beds of Tanzania. Primary s6vite has been re- ported from many localities, including the Kaiserstuhl (Keller 1981, 1989; Katz and Keller 1981), the lavas of the Fort Portal area, Uganda (Barker and Nixon 1989), Kerimasi, Tanzania (Mariano and Roeder 1983) and the Kruidfontein complex, Transvaal (Clarke and Le Bas 1990).

Isotopic and REE data

Sr and Nd isotopes and the REE have been determined in two specimens (Table 3). In IS 95, the freshest rock in our collection, silicate clasts and globules predominate over carbonate-rich clasts, while in IS 135, the situation is reversed. These relative proportions are reflected in bulk rock compositions (Table 3). The matrix carbonate in IS 135 has been patchily, but extensively, stained by brown, ferruginous material, which also affects the mar- gins of some clasts. The rock has clearly been modified after emplacement by hydrothermal and/or meteoric waters. If the carbonate in the Suswa ash flows is second- ary, it should show differences in isotopic and REE sig- natures from the silicate component, which should be seen in compositional differences between IS 95 and IS 135.

In both specimens, the carbonate (leachate) has more radiogenic Sr isotope ratios than the silicate (residue), the difference being more marked in the altered specimen IS 135. This implies that Sr has been added to the car- bonate during low-temperature alteration, probably by ground-waters. Despite the alteration, the ratios are within the range of basaltic lavas from central Kenya (Davies and Macdonald 1987). Due to the large volcanic input to the Rift Valley, the ground-waters will, however, have relatively low 87Sr/86Sr ratios.

~43Nd/144Nd in both components of IS 95 are simi- lar, suggesting isotopic equilibrium between them. In contrast, the carbonate fraction of IS 135 has significant- ly more radiogenic Nd than the silicate. These data are also consistent with influx of thermal and/or meteoric water to the rock, raising S7Sr/S6Sr, but affecting ~4aNd/ l~4Nd only in the case of the more altered IS 135.

Th and U isotopic data for IS 95 are presented in Table 3; brackets denote activity ratios. A notable fea- ture is the low (238U/232Th), indicating a substantial relative depletion of U. This cannot be related to low- temperature leaching, because (238U/234U), which would have been lowered from unity by this process (Ivanovich and Harmon 1982), are within analytical er- ror of 1. The low activity ratios are, however, consistent with the observations that calciocarbonatites have low U/Th ratios and that U is preferentially partitioned into the silicate phase relative to Th (Woolley and Kempe 1989).

Chondrite-normalized REE patterns are presented in Fig. 3. The patterns for the carbonate and silicate frac- tions of IS 95 are closely similar, both showing negative Eu anomalies, for example. The main difference between the two fractions is that the carbonate has lower LREE/ HREE ratios. For example, (Ce/Yb)N in the carbonate is 3.18, in the silicate 6.94. The data suggest that both fractions were derived from a single parental magma and that the LREE and HREE were partitioned differ- ently into the two melts. Experimental determinations of REE partitioning between silicate and carbonate melts have given conflicting results. Wendlandt and Harrison (1979) presented data for compositions on the join sanidine - potassium carbonate at 5 and 20 kbar. They found that HREE partitioned preferentially into

283

10:100

500 ,,, �9149 . . . . . .

100 ,% ock

---5\

IS 135

10 J ~ I I yI b La Oe Nld Sm Elu GId Dy Er

Fig. 3. Chondrite-normalized REE patterns for two specimens of traehyte-carbonate ash flow tuff. Normalizing factors from Naka- mura (1974)

the carbonate melt and LREE into the silicate melt (Fig. 4). Pressure apparently had little effect on the shape of the patterns. The opposite situation was found by Hamilton et al. (1989) for coexisting phonolite and na- trocarbonatite from the O1 Doinyo Lengai volcano, Tan- zania, the LREE preferentially entering the carbonate. Wendlandt and Harrison (1979) have suggested that the chondrite-normalized pattern is related to partitioning of the LREE into a coexisting, CO2-rich vapour phase. However, this explanation is unlikely; note the similarity of the patterns for vapour-present and vapour-absent runs at 20 kbar and 1300 ~ C. Furthermore, recent experi- mental work by Thibault and Holloway (1992) illustrates that LREE have extremely low solubilities in CO2-rich fluids.

The differences between the two sets of results may alternatively be related to the bulk compositions of the starting materials, sodic in the case of O1 Doinyo Lengai, potassic in the Wendlandt and Harrison experiments. The important point for the Suswa rocks, however, is that REE have partitioned between silicate and carbon- ate fractions in a manner comparable to that in experi- mentally determined coexisting melts. Along with the Sr and Nd isotopic data, this provides strong evidence that the carbonate is primary.

The silicate component of IS 135 is similar to that in IS 95, except that the Eu anomaly is smaller (Fig. 3).

1.0

~W o.~ a

1 3 0 0 0 ( . 0 f/o/~,,...., - . ~ .~.-~ ,~ "

~ " ~ . ~ - 5kb 1300vc ~d~OoOoC c

0.1 I ~ I = I I GId ; b I I I ! yI b I La Ce Pr Nd Sm Eu Dy Ho Er Tm Lu

Fig. 4. Experimental data for the partitioning of REE between silicate and carbonate melts compared to the Suswa case. 5-20 kbar data from Wendtland and Harrison (1979); O1 Doinyo Lengai from Hamilton et al. (1989); and Suswa - this paper, sample IS 95

The carbonate fraction shows a MREE and HREE pat- tern almost identical to the carbonate fraction in IS 135, but has strong depletion of La and Ce. (Ce/Yb)N in the silicate is 6.55, but in the carbonate 0.91, about one- third of the value in the carbonate of IS 95. This suggests that there has been secondary mobilization of LREE from this rock. The removal of LREE is similar to that documented from rocks of the Fen complex by Andersen (1984, 1986, 1987), who considers that they were mobi- lized as LREE - F complexes.

We interpret the isotopic and REE data, and earlier observations of compositional zonation in certain glob- ules, as follows. Globule rims tend to be depleted in Na and Ce, and enriched in F, relative to globule cores. The lower NazO is unlikely to be due to secondary hy- dration of the rims; Na loss relative to the cores is ac- companied by an increase in F, an element also prone to removal from silicate glass during hydration (Noble 1970; Noble et al. 1967). Alternatively, the Na loss may be related to alkali exchange between silicate and car- bonate. It is possible, at Suswa, that Na has been trans- ferred to the carbonate matrix from the silicate globules. However, probe data show n o detectable Na enrichment of the matrix carbonate adjacent to silicate globules. A third possibility is that Na has been lost from the globule rims, and presumably also from the carbonate matrix, by complexing with F in a high-temperature, magmatically-related, fluid phase which streamed through the rocks either when they were liquid or during the post-emplacement welding stage. Although uncer- tainty exists about the composition of fluids in equilibri- um with carbonatite magma, there is much evidence that it is COz- and F-rich and capable of dissolving signifi- cant amounts of alkalies (Gittins et al. 1990). Decreases in Ce abundances in globule rims relative to cores (Ta- ble 2) may indicate that LREE also complexed with F. We suggest that a CO/- and F-enriched, high-tempera- ture fluid flushed through the ash flow tufts, removing

284

(a)

Na~O

Si02+AI203 CaO

Fig. 5. a. General form of the experimentally determined two-liquid field, projected from COz, at 2 kbar and 1250 ~ C (Kjarsgaard and Hamilton 1989, Fig. 15.2) and at 5 kbar and 1250 ~ C (Kjarsgaard and Hamilton 1989, Fig. 15.3). Dots indicate composition of quenched silicate and carbonate liquids; conjugate pairs joined by tie lines. Bulk starting compositions shown as crosses, b Plot o f Suswa, and various experimental, data in the expanded ternary system. The heavy dashed curves are for the 5 kbar solvus in the five component synthetic system (Kjarsgaard and Hamilton, in

Na20 + K 2 0

SiO2 + ]302 50 CaO + MgO

+ AI203 + MnO + FeOt wt%

prep.). The heavy" continuous curves, with two tie-lines, are the sol- vus for Shombole. A and B are tie-lines for high silica, metalumi- nous to peralkaline compositions at 5 kbar and 900-950 ~ C. The stippled field is for Suswa carbonate analyses; solid circles are Sus- wa silicate compositions. Arrow 1 is the compogition vector related to loss of alkalies in a CO2-F-rich phase. Arrow 2 is the vector towards aegirine-augite. The double arrow is the resultant of vec- tors 1 and 2 (see text for explanation)

Na and LREE from the carbonate and silicate globules. Subsequent to emplacement, influx of meteoric water further changed rock compositions. 878r/a6Sr in the car- bonte was slightly elevated compared to the silicate com- ponent. In more altered specimens, LREE were removed and ~43Nd/144Nd in the carbonate became slightly more radiogenic.

Petrogenesis

The textural, mineralogical and geochemical data sup- port a magmatic origin for the carbonate in the tufts. This raises the question of the genetic relationship be- tween the coexisting silicate and carbonate phases (Git- tins 1989 a). These features support a liquid immiscibility origin: the presence of sharp, curved menisci between the two phases, and the presence of globules of carbon- ate in silicate glass.

A condition of an equilibrium liquid immiscibility re- lationship is that both liquids coexist stably with the same phenocryst assemblage. There are complications regarding this condition in the Suswa rocks, viz. the large number and compositional range of the phenocryst phases, the fact that alkali feldspar and titanomagnetite have not been recorded as precipitating from carbona- tite, and the presence of Ti-poor magnetite and aegirine- augite in the tufts despite their absence as phenocrysts in Suswa trachyte lavas (Nash et al, 1969; Skilling 1988).

The silicate component of the tufts varies from a rela- tively mafic trachyte ( M g O > 1 % ) to a more highly evolved trachyte with MgO <0.3%. The phenocryst as- semblage likely to coexist with the more mafic liquid

is alkali feldspar + olivine + Na-poor clinopyroxene + ti- tanomagnetite +apat i te + / - calcic amphibole. This would be replaced in more evolved rocks by alkali feld- spar + ferroaugite + / - alkali amphibole + / - aenigma- tite assemblages (Nash et al. 1969). It seems likely, there- fore, that the silicate component of the tufts came from a compositionally and mineralogically zoned reservoir.

The probability that titanomagnetite and alkali feld- spar did not crystallize from the carbonate melt appar- ently precludes the possibility that the crystals grew after separation of the carbonate and silicate liquids. We sug- gest, therefore, that the carbonate liquid exsolved and separated after growth of the phenocrysts, probably at a relatively low level within the magma reservoir. This is consistent with the presence of negative Eu anomalies in both phases, indicating that there was extensive feld- spar fractionation before the two-liquid field was inter- sected and a carbonate liquid exsolved. Magnetite and aegirine-augite subsequently precipitated from the car- bonate.

We now consider the liquid immiscibility hypothesis in terms of experimental data.

Comparison with the experimentally determined two-liquid f ie ld

The experimentally determined two-liquid field at 2 kbar and 5 kbar and 1250~ C in the five component system S iO2- -AlzO3- -Na20- - CaO-- CO2 (Kjarsgaard and Hamilton (1988, 1989) is shown in Fig. 5a. In Fig. 5b, the Suswa data are shown along with a revised solvus for the five component system at 5 kbar determined by

285

Kjarsgaard and Hamilton (in prep.) and with the 5 kbar polythermal (1025-900 ~ C) solvus determined for lavas from Shombole (Kjarsgaard 1990; Kjarsgaard and Pe- terson 1991). The number of components at each apex in the ternary system (Fig. 5a) has been increased to facilitate plotting data from natural compositions. Com- parison of the Suswa and experimental data at 2 or 5 kbar shows that any tie lines connecting silicate-car- bonate pairs from the tufts have rather different orienta- tions from those found experimentally and that both the Suswa silicate and carbonate compositions lie well outside the solvus. A major problem in comparing the Suswa and experimental data sets is that the size and position of the two-liquid solvus, as well as tie-line orien- tation is affected by several variables (P-T-X), most of which are not well understood. Widening of the two- liquid field at lower temperature and constant pressure (Freestone and Hamilton 1980) is evident in comparing the Shombole and five component synthetic two-liquid fields. In this respect, the Shombole solvus is a much better approximation to the Suswa data as the eruption temperature of the trachytes was probably near 900 ~ C. Furthermore, it has been shown that the peralkalinity of silicate liquids affects the composition of the exsolved carbonate liquid (Kjarsgaard and Peterson 1991). This can be seen in Fig. 5b; silicate liquids from Shombole (molecular (NazO +K20)/A1203 = 1.24-1.35) have con- jugate carbonate liquids more calcic than those in the five component synthetic system (molecular N a 2 0 + K20)/AI203 > 1.70).

The effect of silicate liquid peralkatinity on carbonate liquid composition is especially relevant to interpretation of the Suswa tufts, which are trachytic glasses of metalu- ruinous to weakly peralkaline affinity ( N a 2 0 + K 2 0 / A1203=0.89-1.16). Experiments at 5kbar utilising starting materials which are high silica, metaluminous to weakly peralkaline in composition (Kjarsgaard and Hamilton 1990; Kjarsgaard and Pearce, in prep.) have produced a wide immiscibility field (tie-lines A and B in Fig. 5 b), with the carbonate liquids being very CaO- rich. The Suswa trachytic glasses lie near the silicate limb of this solvus, which to date are the only data available at reasonably similar P-T-X conditions. We note, however, that 2 kbar (and not 5 kbar) is probably a more realistic pressure for the upper part of the magma chamber beneath Suswa (Skilling 1988).

Finally, one needs to compare the compositions of the Suswa carbonates to immiscible carbonate liquids from experiments. The Suswa carbonatite analyses shown in Figs. 5b are not the composition of the ex- solved carbonate liquid. As we have previously dis- cussed, the Suswa carbonate melts have probably under- gone minor alkali loss to a CO2-F-rich fluid. Further- more, we have inferred that the aegirine-augite and mag- netite phenocrysts precipitated from the carbonatite melt after exsolution. Since these minerals have not been in- cluded in the carbonatite analysis, nor are the original alkalies retained, the composition of the exsolved car- bonate melt can only be approximated. This has been indicated on Fig. 5 b by arrows.

Discussion

We have argued on a variety of grounds (petrographic, geochemical and experimental) that the Suswa ash flow tufts represent a case of liquid immiscibility between tra- chyte and carbonate. Proponents of immiscibility (Le Bas 1989; Kjarsgaard and Hamilton 1989; Kjarsgaard and Peterson 1991) envisage a spectrum of silicate-car- bonate relationships; nephelinite evolves to phonolite by migration down the silicate limb of the solvus (Fig. 5) and can continuously evolve a carbonate phase.

Gittins (1989b) points out that the demonstration that carbonate and silicate liquids are immiscible does not prove that they formed by exsolving from a common parental magma. In certain cases, the melts may have been generated by separate mantle fusion events, have come together at high crustal levels and were unable to mix. This situation is not likely at Suswa; the tra- chytes are strongly differentiated rocks (tow Mg, Ba and Sr; high HFSE) and part of this geochemical character has been imparted to the carbonate, such as the Ba and Sr values which are relatively low for carbonatites. The similar REE patterns are also highly suggestive of a com- mon parent; as we noted earlier, the negative Eu anoma- lies, for example, indicate a common history of feldspar fractionation.

It is not possible to estimate accurately the composi- tion of the magma or magmas which were parental to the ash flow tufts. There is great variability in the pro- portions of carbonate and silicate, even at outcrop level. .Furthermore, the proportions of silicate to carbonate in the ash flow tufts may well not represent their propor- tions in the magma reservoir prior to eruption as a result of segregation due to differing viscosities (Treiman and Schedl 1983; Treiman 1989; Sykes et al. 1992).

We suggest that the upper portions of the Suswa mag- ma chamber were occupied, prior to eruption of the ash flow tufts, by a compositionally zoned, carbonated trachyte. After a phase of phenocryst crystallization, a carbonatitic melt exsolved and separated as globules from the silicate. There was some variable segregation into zones rich in silicate melt with isolated carbonate globules and carbonate-rich zones in which the globules commonly coalesced. Aegirine-augite and magnetite pre- cipitated from the carbonate melt. Pressure release relat- ed to caldera collapse resulted in uPward movement of the trachytic melt with entrained carbonatite globules as well as previously coalesced carbonate-rich zones. At this time, segregation of the carbonate and silicate melts was enhanced by the lower viscosity of the carbonatite. After disruption of the magma, in the volcanic conduit, into pumice clasts, ash and a fluid phase, the silicate clasts grew progressively smaller by disaggregating into the carbonate melt by budding. Ejection of well-segre- gated silicate and carbonate liquids, remixed silicate/car- bonatite melts, and trachytes with immiscible carbona- tite resulted in the formation of centimetre-scale banded, silicate-carbonatite tufts with juvenile pumice fragments having variable proportions of silicate glass to carbonate matrix.

286

After emplacement, there was some welding and flat- tening of the basal parts of the units, accompanied by vesiculation of the silicate, and perhaps also the carbon- ate, melts. Later flow of meteoric water through the deposits resulted in alteration and recrystallization of the ash grade carbonate matrix.

The conclusions of this report raise an important issue in the petrogenesis of Kenyan salie alkaline rocks. Are the Suswa tufts a unique case, or are they a uniquely preserved example of a widespread relationship? It may well be that liquid immiscibility between carbonate and trachyte is as common in Kenyan rift magmatism as evolution of a CO2 - rich fluid, but evidence of it is normally lost because carbonate-rich pyroclasts are easi- ly eroded away and/or overlooked, as has been the case at Suswa.

There are many implications of the separation of a carbonatitic liquid from a trachytic parent, e.g. the parti- tioning of trace elements between crystals, vapour and tw o liquids complicates petrogenetic modelling; eruption dynamics of earbonatite-bearing systems are different from those of silicate-only systems; and the mineraliza- tion potential of peralkaline rocks may be affected.

Acknowledgements. We thank the Natural Environment Research Council (UK) for financial support through research grants (RM) and a research studentship (IPS).

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Editorial responsibility: I. Parsons