Journal of Petrology | Volume 39 | Number 11-12 | Pages 2077-2094 | 1998
© Oxford University Press 1998
Peralkaline Nephelinite–Natrocarbonatite Relationships at Oldoinyo Lengai, Tanzania
Department of Geology and Geophysics, University of Edinburgh West Mains Road, Edinburgh EH9 3JW, UK
Received September 30, 1997; Revised typescript accepted May 21, 1998
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ABSTRACT |
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 TOP ABSTRACT IntroductionField Occurrence and PetrographyMineral ChemistryWhole-Rock ChemistryDiscussionConclusionsReferences |
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The younger (<2000 a) nephelinites at Oldoinyo Lengai are peralkaline, with alkalinity indices ([Na + K]/Al) ranging from 1.43 in wollastonite nephelinite to 2.15 in combeite nephelinite. They have a common set of phenocrysts (nepheline, clinopyroxene, Ti-andradite, combeite and apatite) but some phenocrysts and groundmass phases are different (e.g. wollastonite phenocrysts and groundmass melilite in wollastonite nephelinite, and phenocrystal sodalite and groundmass lamprophyllite, ‘delhayelite’ and aegirine-rich pyroxene in combeite nephelinite). Examination of silicate-carbonate spherules from the 1993 eruption showed that natrocarbonatite unmixed from (formerly) carbonate-saturated wollastonite nephelinite, with strong partitioning of alkalis, Ba, Sr and halogens into the carbonate melt. Compared with wollastonite nephelinite, combeite nephelinite is enriched in these elements, and the model proposed here i that their enhancement in combeite nephelinite is due to CO2 loss from the parental melt, precluding the exsolution of natrocarbonatite; the alkalis, Ba, Sr and halogens perforce remain in the silicate melt, enhancing its peralkalinity and leading to precipitation of alkali-, Ba-, Sr- and halogen-rich phases.
KEY WORDS: peralkaline nephelinite; natrocarbonatite; Oldoinyo Lengai; Tanzania
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Introduction |
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TOPABSTRACTIntroductionField Occurrence and PetrographyMineral ChemistryWhole-Rock ChemistryDiscussionConclusionsReferences |
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The silicate lavas extruded from the active carbonatite volcano Oldoinyo Lengai, Tanzania (2°45'S, 35°54'E), comprise phonolites and olivine-free nephelinites (Dawson, 1962a
). The nephelinites associated with the earlies palagonitized pyroclastics that form the bulk of the volcano, and correlated with the Naisiusiu Beds of the Olduvai Gorge succession dated at 22–15ka BP (Hay, 1989), consist of nepheline, clinopyroxene, garnet, apatite, magnetite and glass (Donaldson et al., 1987). In contrast, the nephelinites forming the thin veneer of later black pyroclastics, correlated with the Namorod Ash of the Olduvai succession dated at 2000–1250a bp (Hay, 1989), and later small flows and scoria cones [the melanephelinite unit of Dawson (1962a)], are more highly evolved, with higher (Na + K)/Al values; additionally, they contain wollastonite and/or combeite (Na2Ca2Si3O9), and the most peralkaline contain even more unusual phases such as baryto-lamprophyllite, Na–Sr-rich phosphate and a phase akin to delhayelite (K4Na–Ca2[Al2Si6O19]F,Cl,OH) (Dawson & Hill, 1998). Donaldson et al., (1987) gave modal and chemical data on both older and younger nephelinites.
An intimate temporal and genetic association between the younger nephelinites and natrocarbonatite is indicated by evidence ranging from the production of mixed natrocarbonatite–silicate ashes during the 1966 ash eruption (Dawson et al., 1992), to the presence of wollastonite nephelinite globules in the natrocarbonatite ashes and lavas extruded in June 1993 (Dawson et al., 1994, 1996; Church & Jones, 1995). The purpose of the present paper is to discuss the mineralogical and chemical variations in these later nephelinites and to examine their relationships with the natrocarbonatites.
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Field Occurrence and Petrography |
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TOPABSTRACTIntroductionField Occurrence and PetrographyMineral ChemistryWhole-Rock ChemistryDiscussionConclusionsReferences |
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On the basis of their chemistry and resulting mineralogy, the later nephelinites for a spectrum ranging from wollastonite nephelinites (least peralkaline), through wollastonite–combeite nephelinites to combeite nephelinites (most peralkaline). Wollastonite nephelinites and wollastonite–combeite nephelinites form small flows or scoria cones, whereas both wollastonite–combeite nephelinites and combeite nephelinites occur as lapilli or bombs. Attention is focused on three particular specimens as being representative of th spectrum of younger nephelinite types. Specimen BD4421 (wollastonite nephelinite) is from the most northerly of a chain of small scoria cones on the lower northern slopes of the volcano; this lava is mineralogically very similar to the silicate fraction of the mixed silicate–carbonatite spheroids in the natrocarbonatite lavas and ashes of the June 1993 eruption (Dawson et al., 1994
, 1996). Wollastonite-combeite nephelinite is represented by lava BD119 from a small outward-dipping lava flo on the western rim of the northern crater. Combeite nephelinite is represented by BD70, an ejected block in black agglomerate on the lower NE slopes of the volcano; a detailed study of its mineral chemistry has been made by Dawson & Hill (1998). For the sake of brevity hereafter, in the case of these three samples acronyms for the rock-types will be used and the author's BD collection prefix will be dropped. Thus wollastonite nephelinite BD4421 is referred to as WN4421, wollastonite-combeite nephelinite BD119 as WCN119, and combeite nephelinite BD70 as CN70.
Petrographic examination was made by a combination of light microscopy and back-scattered electron (BSE) imaging, and the mineralogy of the nephelinites is summarized in Table 1. The rocks are porphyritic and common to all the nephelinite types ar abundant euhedral phenocrysts (>1 mm) of nepheline and clinopyroxene (up to 5 mm), and rarer euhedral combeite, titanite, Ti-andradite and rounded grains of apatite. Wollastonite forms phenocrysts in wollastonite nephelinites; these phenocrysts are surrounded by thin rinds of apophyllite. In wollastonite-combeite nephelinites, wollastonite phenocrysts are corroded and surrounded by reaction rims of combeite. In these two rock types, combeite phenocrysts are rare, and BSE images show patchy reflectance which, together with the phase chemistry given below, indicate that the combeite is unstable and variably altered (Fig. 1). In contrast, the combeite phenocrysts in combeite nephelinite are abundant and unaltered (Fig. 2). Thus there is petrographic evidence in the lavas that wollastonite and combeite are mutually incompatible, reinforcing earlier observations (Dawson et al., 1989; Peterson, 1989). Sodalite is an additional phenocryst phase in combeite nephelinite. Nepheline, pyroxene and garnet contain mutual inclusions, pointing to co-precipitation. Garnet also forms aggregates with phenocrystal titanite.
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Most phenocryst species are zoned, as evidenced by variations in inclusion density in nepheline and combeite (Figs 1 and 2), colour (variations from dark to light green in pyroxene and different shades of brown in Ti–andradite), and birefringence in sodalite. Zoning in pyroxene and nepheline is particularly marked, with rounded, resorbed cores surrounded by zoned overgrowths. BSE imaging combined with electron probe micro-analysis (EPMA) shows that the internal variations in nepheline are due mainly to differing Fe contents. EPMA has proved optically undetectable zoning in titanite. BSE imaging of apatite grains in CN70 indicates two stages of replacement of original apatite (Dawson & Hill, 1998
), with initial replacement of apatite cores by a high-Sr variety of apatite, and subsequent replacement by a birefringent (second-order green maximum) Na-rich phosphate that, in some cases, completely replaces the original grain. The groundmass consists of microphenocrysts (<1 mm) and glass. Common to all nephelinites are microphenocrysts of equant euhedral nepheline and Ti andradite, acicular grains of clinopyroxene and tiny (<20 µm) grains of pyrrhotite and magnetite; magnetite is abundant in wollastonite nephelinite and wollastonite-combeite nephelinite but rare in combeite nephelinite. Like the phenocrysts, the nepheline microphenocryts are strongly zoned and the clinopyroxene grains are zoned from light green cores to dark gree rims, though lacking the corroded cores of the phenocrysts. Melilite is an abundant microphenocryst in WN4421. An acicular, colourless phase, chemically resembling delhayelite is found in both WCN119 and CN70. Perovskite is a minor phase, occurring as tiny 10–20 µm equant grains in WN4421; the perovskite in CN70 is a high–Ce–Nb–Sr variety that displays good sector zoning in BSE images. Peculiar to the groundmass of CN70 are rounded microphenocrysts of combeite and sodalite, and also Balamprophyllite, which occurs both as distinct bladed grains or as oikocrysts surrounding other groundmass phases; globular patches in the groundmass in this specimen consist of concentrations of combeite, perovskite and glass and, occasionally, ‘delhayelite’ and oikocrystic lamprophyllite.
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Mineral Chemistry |
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TOPABSTRACTIntroductionField Occurrence and PetrographyMineral ChemistryWhole-Rock ChemistryDiscussionConclusionsReferences |
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Mineral compositions were analysed by wavelength-dispersive techniques on a Camebax Microbeam electron microprobe at the University of Edinburgh. Na and K were analysed early in the routine to minimize migration effects. Counting times were 30 s on peaks, except for the rare earth elements (REE), Nb, U and Th, when counting on peaks was for 60 s; background counts were made for half the peak times. Most minerals were analysed with a beam diameter of
1 µm at 20 kV and a probe current of 20 nA. The combeite, sodalite, ‘delhayelite’ and glasses, however, proved to be unstable under these conditions and were analysed at 10 nA with the beam rastered over a 12.5 µm square. Data were reduced using the PAP routine.
The data given in Tables 2–12 cover the spectrum from wollastonite nephelinite, through wollastonite–combeite nephelinite to combeite nephelinite. The data for CN70 are from Dawson & Hill (1998). Sources for mineral data for other younger nephelinites are: Donaldson et al. (1987) for nepheline, pyroxene, garnet, vishnevite and glass; Peterson (1989) for nepheline, pyroxene, combeite, garnet and sodalite; Keller & Krafft (1990) for combeite; Dawson et al. (1996) for nepheline, pyroxene, wollastonite, melilite, garnet, titanite, perovskite, magnetite, ilmenite, tilleyite, mica, sulphides and glass.
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Phenocrysts
Nepheline
Phenocryst cores contain around 20–22 mol % kalsilite (Ks) and generally contain excess Si. Although some grain cores are homogeneous, others are complex, with zones and patches containing differing K and Fe concentrations, and enrichment in K and Fe is a general feature of the rims (Table 2). Overall the nephelines in CN70 are more potassic (around 25–27% Ks molecule) than in the other nephelinites (20–25% Ks).
Combeite
The combeites from Oldoinyo Lengai have excess Na over the simplified Na2Ca2Si3O9 endmember composition and also contain Fe and Mn as substantial minor elements (Table 3). The petrography indicates that there are two types of combeite: (1) that precipitating from magma to form phenocrysts and, in CN70, microphenocrysts; (2) that forming rims on corroded wollastonite grains in WCN119. In the latter, wollastonite must have reacted with a high-Na magma; in the case of rims on wollastonite in lapilli from the 1966 eruption, Dawson et al. (1992) suggested that magma was natrocarbonatite. The combeite in CN70 is unaltere and gives the most reliable composition for magmatic combeite (Table 3, analyses 6 and 7); the Na/(Na + Ca) ratio is 0.65, FeO is >2.7 w % and MnO >0.90 wt % and, despite evidence from micro-inclusion for several stages of concentric growth (Fig. 2), rim compositions are essentially the same as cores.
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The combeite phenocrysts in WN4421 and WCN119 show evidence of alteration (Fig 1). The least altered parts of the phenocrysts in WCN119 (Table 3, analysis 4) are similar to those in CN70 with Na/(Na + Ca) of 0.62, but with lower Fe and Mn, which may reflect true differences between the phase compositions in the two different rock-types. However, in the more altered parts of these phenocrysts as inferred from reduced brightness in BSE images, even lower concentrations of Fe and Mn accompany the relative Ca enhancement that results in an Na/(Na + Ca) value of 0.58. In WN4421, the least altered parts of the phenocrysts show even lower Fe and Mn; this is accompanied by even greater Ca concentrations relative to Na [Na/(N + Ca) is 0.57], an increase in Si, and a low total (Table 3, analysis 1). The most altered part of the same grain is enriched in Si (SiO2 78.4 wt %), depleted in CaO and Na2O (4.9 and 0.40 wt %, respectively), and the analysis has a very low total; only the Fe and Mn concentrations are undepleted relative to the less altered parts of the grain, and they are in fact higher. These data indicate that during alteration, Na and Ca are selectively removed from the lattice (Na more so than Ca), with a concomitant rise in Si, whereas F and Mn are relatively immobile; whether the low total is due to the presence of some unanalysed element such as O, H or C, or whether it is due to site vacancies, is open to question. In the increase in silica, decrease in large-size cations, and retention of the crystal shape, the alteration of these combeites bears some resemblance to the process referred to as ‘skeletonization’, during which original crystals have undergone selective leaching of Ca, K and F, and replacement by opal, without destroying the silica framework of the crystals (Bailey, 1941
).
Compared with the magmatic combeite, the combeite forming reaction rims on wollastonite in WCN119 contains lower concentrations of Na relative to Ca [Na/(Na + Ca) is 0.59], and lower Fe and Mn (see Table 3, analyses 5 and 6). Dawson et al. (1989) found similar compositional differences between magmatic combeite and combeite formed by replacement of wollastonite and clinopyroxene as the result of postulated interaction with natrocarbonatite during the 1966 ash eruption. Three analyses of combeite given by Keller & Krafft (1990) in their specimen OL7, which is from the same flow as WCN119, are similar to the reaction-rim combeite reported here.
Clinopyroxene
The larger clinopyroxene phenocrysts have rounded, resorbed, broadly zoned cores overgrown by pyroxene characterized by a series of narrower, planar-faced zones. The range in composition of phenocrysts from CN70 (Dawson & Hill, 1998) is shown in Fig. 3 with new data for pyroxenes from WN4421 and WCN119. The phenocrysts listed in Table 4 are mainly inclusions in nepheline and garnet phenocrysts, and lack the corroded cores typical of the larger phenocrysts.
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The main compositional variations are in the molecular proportions of aegirine, diopside and hedenbergite, which together form >95% of the total; with few exceptions, the pyroxenes contain low alumina (<1.50 wt %) and TiO2 (<1.00 wt %). The phenocrysts from all three rock types show a very similar range in composition (Fig. 3). In the zoned crystals, no trends are discernible, and in several cases zoning is towards more Mg-rich rims; others showing oscillatory zoning contain relatively Mg-rich intermediate zones. Both the corroded core-zoned overgrowth relationship seen in the pyroxene from CN70 and the reversion of some zones to more magnesian compositions are features common to the pyroxene phenocrysts in mos other lavas, and in plutonic ijolites and pyroxenite xenoliths from the volcano (Donaldson et al., 1987
; Dawson et al., 1995b). In the xenoliths, this has been attributed to a combination of magma chamber convection and new injections of magma.
Ti-andradite
The garnets (Table 5) fall within the compositional range of garnets reported in other lavas and ijolites from Oldoinyo Lengai, which range from melanite to schorlomite (Huggins et al., 1977; Donaldson et al., 1987; Dawson et al., 1995b), and there are no significant differences in the compositions of the garnets from the different nephelinite types. The main compositional variations are in the amounts of Ti and total Fe, which vary inversely. The Fe tends to be less oxidized in the higher-Ti grains and zones, which are also the most magnesian. Of the minor elements, Na2O (0.52 wt %), MnO (0.45%) and ZrO2 (0.64%) are present in appreciable amounts (values are maximum concentrations). In the analysed grains, there is no consistent sense of zoning, with rims higher in Ti than cores in some grains, whereas the reverse is true in other grains.
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Titanite
There are no significant differences in the titanites from different nephelinite types in terms of the three main elements (Ca, Ti and Si), and they also all contain appreciable FeO (1.86%), and Na2O (0.52%) (maximum concentrations, Table 5). In CN70, the only specimen for which more detailed analyses were obtained, other minor oxide concentrations were Nb2O5 (0.83 wt %), ZrO2 (0.62%), SrO (0.61%) and Ce2O3 (0.17 wt %) (Dawson & Hill, 1998
). Compared with titanites in ijolite and nepheline syenite blocks from Oldoinyo Lengai (Dawson et al., 1995b), those in CN70 contain less Nb, Zr and REE, but higher Sr and Na.
Apatite
The cores of the apatite phenocrysts contain variable amounts of elements other than Ca, P and F (Table 6). SrO (up to 4.26 wt %), La2O3 (0.78 wt %) and Ce2O3 (0.88 wt %) are relatively high in apatites in WC119, compared with those in other nephelinites. In CN70, the core apatite is partly replaced by high-Sr apatite (Table 6, analysis 6). In addition to 10.1 wt % SrO (with commensurate low CaO), it contains 0.4 wt % of both Ce2O3 and La2O3, 3.62 wt % F (all 1.5 times the concentrations in the apatite core), and 0.14 wt % Nd2O3 and 0.23 wt % BaO, both of which are below analysable levels in the core apatite. It is apparent that the original apatite reacted with magma containing enhanced concentrations of Sr, Ba, the light REE (LREE) and F.
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The apatite–Sr apatite grains in CN70 are themselves surrounded by rims of, and sometimes completely replaced by, a CaNa phosphate. Of the known CaNa phosphates, it is compositionally most similar to nefedovite Khomyakov, 1995
), but in addition t CaO and Na2O (the latter making up 21.8 wt %), it also contain appreciable BaO (3.40 %) and K2O (2.09%) (Table 6, analysis 7). Its low analytical total may be due to unanalysed oxides, i this paragenesis the most likely being CO2.
Sodalite
Sodalite phenocrysts occur only in CN70. They are both chemically and optically zoned, with the rims containing higher Fe2O3 but lower K2O than the cores (Table 7, analyses 1 and 2). Compared with most other examples reported in the literature (Deer et al., 1971, table 36), the sodalite is distinctive in containing unusually high Fe2O3 [2.68–3.52 wt % vs a maximum of 0.85 wt % reported by Deer et al., (1971)] and substantial K2O (3.28–4.13 wt %, compared with concentrations of not more than 0.46 wt % in most other reported examples); there is a commensurate reduction in the Na2O concentrations.
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Wollastonite
In containing up to 1.11 wt % FeO and 0.49 wt % MnO (Table 8), the nephelinite wollastonites are very similar to those in plutonic xenoliths from the volcano (Dawson et al., 1995b
). There are no significant differences between the wollastonites in wollastonite nephelinite and wollastonite/combeite nephelinite. The wollastonite in WN4421 is surrounded by a 10–15 µm rim of apophyllite (Table 8, analysis 6), indicating interaction with fluids containing K, F and OH.
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Groundmass phases
Nepheline
The cores of nepheline microphenocrysts are broadly similar to phenocryst rims in their Fe contents, but they are more potassic (Table 2). The rims of groundmass grains are relatively Fe rich and more potassic than the cores. Overall, the nephelin microphenocrysts in CN70 are the most iron rich with >8 wt % Fe2O3.
Combeite and sodalite in CN70
The combeite microphenocrysts in CN70 are slightly more iron rich than the phenocrysts but, with that exception, the combeite and sodalite microphenocrysts in the groundmas are compositionally very similar to the phenocrysts (Tables 3 and 8).
Pyroxene
The acicular groundmass pyroxene grains are all strongly zoned. Their compositions, however, vary considerably between the different nephelinite types (Table 9). The microphenocrysts in WN4421 are all diopsidic, and compositionally similar to the phenocrysts in this rock, with the main variations reflected in Na2O and Fe2O3, and the diopside:hedenbergite ratios. In WCN119, the microphenocrysts are distinctly more sodic than those in WN4421, and in the zoned microphenocrysts (Table 9, analyses 5–7) the sense of zoning is the same as in WN4421, i.e. towards higher Na, Fe3+ (the rim containing nearly 60% aegirine molecule) and Ti in the rim. The microphenocrysts in CN70 have cores consistently more sodic than the phenocrystic pyroxene, with between 37 and 65 mol % aegirine (Table 9; Fig. 3). The rims of the microphenocrysts are even more sodic, with up to 76% aegirine molecule.
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Overall, in the range between wollastonite nephelinite and combeite nephelinite, there is only a small amount of compositional variation in the clinopyroxene phenocrysts. Hwever, the range in microphenocryst compositions is considerable, varying from high diopside in WN4421 to high aegirine in CN70.
Melilite
Previously, melilite has been reported at Oldoinyo Lengai as microphenocrysts in intergranular glass in an alkali pyroxenite cumulate block (Donaldson & Dawson, 1978); in lapilli and discrete grains in ashes of the 1966 eruption (Dawson et al., 1989); in pelletal lapilli ‘from recent explosive eruptions’ (Keller & Krafft, 1990); and in wollastonite nephelinite spheroids from the 1993 eruption (Dawson et al., 1996).
In the younger nephelinites, melilite has been found only as microphenocrysts in WN4421. They are iron and soda rich (Fe2O3 8 to 10 wt %; Na2O 5–6 wt %) (Table 8). They can be most closely matche by melilites in wollastonite–combeite nephelinite spheroids from the 1993 eruption (Dawson et al., 1996), and also by some melilites in ashes from the 1966 eruption, though there are fairly large compositional variations between individual grains in the ashes (Dawson et al., 1989, 1992). Melilites in small olivine melilitite lava flows on the floor of the rift valley to the east of Oldoinyo Lengai (Dawson et al., 1985) are considerably more magnesian and calcic, and contain less Fe and Na (Table 8).
Delhayelite
A colourless acicular phase in the groundmass of WCN119 and CN70 is a halogen-rich KCaNa alumino-silicate (Table 7, analyses 4 and 5). The very few minerals having these compositional characteristics include carletonite and delhayelite. Of the tw it most closely resembles a phase stated to be delhayelite from the Khibin alkaline massif, Kola peninsula (Table 7, analysis 6; Stoppa et al., 1997) but which, like the Oldoinyo Lengai mineral, also shows some differences from the type delhayelite from Mt Shaheru, Zaire (Sahama & Hytönen, 1959). The composition of type delhayelite, which occurs in combeite-bearing melilite nephelinite, is broadly similar to that in the Oldoinyo Lengai nephelinites, although differing in detail in its silica, potash, sulphur and water contents. However, Sahama & Hytönen (1957) acknowledged that, because of admixture with nepheline and alteration products, their analysis was probably only an approximation to the true composition.
Perovskite
Concentrically zoned perovskite in WN4421 contains measurable concentrations of Nb, the LREE, Sr, Ba and Na (Table 10, analyses 1 and 2); the rim contains higher concentrations of the REE, Fe and Na, but lower Sr and Ba than the core. Perovskite in CN70 is sector zoned and contains substantial concentrations of the LREE oxides (between 12 and 15 wt % total REE oxides), SrO (>9 wt %), Nb2O5 (>8 wt %) and Na2O (5–6 wt %), which are not sufficiently high to warrant the phase being classified as loparite, lueshite or tausonite (Dawson & Hill, 1998). Other important minor oxides are ThO2, FeO, BaO and K2O in concentrations of 0.1–0.7 wt %. Sectors appearing brighter in BSE images contain more REE, Th, Sr, Ba and Na, but lower Nb and Ca, than darker zones (Table 10, analyses 4 and 5).
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Lamprophyllite
Lamprophyllite, a mica-like, halogen-bearing Na–K– Ba–Sr titano-silicate, occurs as microphenocrysts and oikocrystic patches in the groundmass of CN70. A representative analysis is: SiO2 29.1, TiO2 28.8, Al2O3 0.17, FeO 3.19, MnO 0.53, MgO 0.41 CaO 1.90, SrO 6.54, BaO 15.6, Na2O 9.20, K2O 2.30, F 1.64, minus 0.69 O
F, total 98.69 wt % (Dawson & Hill, 1998). In general, the most variable components are Sr and BaO, which vary inversely. The CN90 lamprophyllite is a high-Ba variety, most comparable to barytolamprophyllite from the Gardiner complex, Greenland (Johnsen et al., 1994).
Magnetite
Magnetite is an abundant groundmass phase in WN4421 and WCN119, but scarce in CN70. It is titaniferous (Table 11) and contains >1 wt % MnO. The magnetite in CN70 contains no detectable Al2O3 or MgO, which are presen in minor amounts in magnetites from the other nephelinites.
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Glass
Analyses of glasses included in phenocrysts and occurring interstitially to other phases are given in Table 12. Although there are differences in detail betwee inclusion and interstitial glasses in individual rocks, and between glasses in differen rock types, there are none the less some common characteristics. Overall, the glasses are silica undersaturated, high in Fe, alkalis (Na2O + K2O usually 20 wt % or greater) and analysed volatiles (F, Cl, S), and low in Mg. Most analyses have low totals, which would be improved if the iron was expressed as Fe2O3, though there is also the possibility of the presence of unanalysed volatiles in addition to F, Cl and S.
In addition to these general chemical characteristics, glasses from the three nephelinite types have certain individual chemical characteristics. Compared with inclusions i pyroxene phenocrysts, interstitial glass in WN4421 contains much more Al2O3 (22 wt % vs 8–19 wt %), resulting in lower (Na + K)/Al values (1.05–1.39 vs 2.72–3.67), and they are more potassic; however, they are less calcic and contain lower concentrations of halogens. Glass in another wollastonite nephelinite from Oldoinyo Lengai (specimen BD38; Donaldson et al., 1987) contains up to 16 wt % Na2O + K2O, and is more siliceous (up to 53.9 wt % SiO2) and aluminous (up to 18 wt % Al2O3). Relative to the WN4421 glasses, the interstitial glasses in WCN119 are much less aluminous (Al2O3 2.5–3.1 wt %) with resulting high (Na + K)/Al values of 8-12, and they are the most potassic of all the nephelinite glasses with Na/(Na + K) values of 0.63–0.69, compared with values of >0.71 in other glasses. They are also higher in Fe (15.8–17.4 wt % FeO) and MnO but contain the lowest CaO (<1 wt %). Compared with the interstitial glasses in WN4421, they contain higher halogens and SO3, the latter up to 3.35 wt %. In CN70, the analysis of the glass apparently included in nepheline is so similar to that of the interstitial glasses as to suggest that it may be a crack infill. The glasses in CN70 are distinct from those in the other nephelinites in containing the highest concentrations of TiO2 (3.63), FeO (17.5), Na2O (16.7), P2O5 (0.95), F (0.89) and SO3 (3.91) (maximum wt % concentrations in parentheses); they contain the least MgO (<0.4 wt %) and Al2O3 (<3.0 wt %), which, together with Na2O + K2O 24 wt %, result in exceptionally high (Na + K)/Al values varying from 11.9 to 15.7. They are more sodic than those in WCN119, with Na/(Na + K) of 0.76 vs 0.63–0.69. Glass in other combeite nephelinites from Oldoinyo Lengai (Peterson, 1989) although containing up to 24 w % Na2O + K2O, is more aluminous, resulting in maximum (Na + K)/Al values of 7.27; it also contains less FeO (10.9–16.5 wt %).
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Whole-Rock Chemistry |
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TOPABSTRACTIntroductionField Occurrence and PetrographyMineral ChemistryWhole-Rock ChemistryDiscussionConclusionsReferences |
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Compared with other members of the Oldoinyo Lengai silicate lava suite (Donaldson et al., 1987
), the late nephelinites have the lowest SiO2 and Al2O3 but the highest total iron, Na2O, Cl and F concentrations, and amongst the highest concentrations of K2O, Zn, Rb, Zr, Nb, S and Ba. Analyses of the three nephelinite types are given in Table 13. The low MgO (<2 wt %), Ni (<15 ppm) and Cr (<12 ppm) concentrations indicat that these are evolved lavas. They are all silica undersaturated and, although apparently the most siliceous, CN70 contains the least volatiles and would, if all were calculated on a volatile-free basis, be no more siliceous than the others. It also has Al2O3 concentrations similar to the other late nephelinites and, despite th presence of Fe-rich rims on nepheline, sodalite and pyroxene phenocrysts, and the presence of Fe-rich pyroxenes and glass in the groundmass, the total Fe concentration is also very similar to that for the other nephelinites; it is, however, one of the least oxidized of the Oldoinyo Lengai lava suite, with an FeO/(FeO + Fe2O3) value of 0.41 compared with the suite range of 0.44-0.11 (Donaldson et al., 1987), and the inferred lower f(O2) is reflected in the relatively low amount of magnetite in CN70. Other oxides show a range of concentrations, with highest CaO, MgO and P2O5 in WN4421 and the least in CN70, whereas for Na2O and K2O the reverse is true. The concentrations of the alkalis are exceptionally high in CN70 (19 wt % Na2O + K2O), which, together with the rather closely comparable Al2O3 concentrations in the three nephelinite types, results in very high (Na + K)/Al (2.15) in CN70 relative to the others; further, in CN70 there is an increase in Na2O relative to K2O compared with the other nephelinites. For most major and minor oxides, their concentrations and derived ratios in WCN119 are intermediate between combeite nephelinite and wollastonite nephelinite; SO3 is lower than in CN70. For the trace elements, CN70 is characterized by considerably higher concentrations of Ba and especially Cl and F compared with the other nephelinites. Its relatively high U content results in a Th/U value of 1.1, which is exceptionally low for an igneous rock, and compares with a value of 2.0 for WCN119.
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Discussion |
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TOPABSTRACTIntroductionField Occurrence and PetrographyMineral ChemistryWhole-Rock ChemistryDiscussionConclusionsReferences |
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From the similarity in composition of the phenocryst phases in all the nephelinite types (pointing to a common early precipitation history of what must have been volumetrically small magma batches), and their close temporal association, it is apparent that the late nephelinites must be intimately related. Moreover, Bell & Dawson (1995)
have shown that, within the unusually large range in strontium and neodymium isotope values found in the lavas from Oldoinyo Lengai, the younger nephelinites form a ver tight group, together with the natrocarbonatites. The observation from the 1993 eruptio that globules of natrocarbonatite in the groundmass of silicate-dominated spheroids hav unmixed from (formerly) carbonate-saturated wollastonite nephelinite (Dawson e al., 1994, 1996) has demonstrated the intimate relationship between these two particular rock types, so the main concern here is the reason for the range in composition from wollastonite nephelinite to combeite nephelinite.
CN70 is one end of a compositional spectrum, ranging from wollastonite nephelinite through wollastonite-combeite nephelinite to combeite nephelinite. It is relatively enriched in Na, K, F, Cl and Ba, and the presence of sodalite phenocrysts suggests that, even at the onset of crystallization, the magma had high halogen concentrations. As noted earlier by Donaldson et al. (1987), there are major difficulties in linking the members of the Oldoinyo Lengai silicate lava suite by fractional crystallization of the phenocryst phases. For example, when trying to link wollastonite nephelinite to combeite nephelinite, although the trend towards enhancement in alkalis, incompatible elements and halogens in CN70 is what might be expected during normal fractionation (an is observed in many magma series), there is no concomitant build-up of Si and Al. This paradox is particularly highlighted in the groundmass glasses in which the SiO2 concentrations are lower than in the whole-rock analysis, and Al2O3 is particularly low, contrasting with Na, K, Fe and S, which are in high concentrations. This is also in contrast to crystallization trends in the plutonic blocks from the volcano, in which residual glasses in the ijolites are silica enriched (Dawson et al., 1995b). The crucial point in the case of CN70 is the enhancement of alkalis relative to Si and Al, particularly the latter. This could not have been achieved by fractionation of nepheline or sodalite (the known high-Al phases at the volcano) from a precursor melt because, although this might produce the desired low-Al concentrations, it would also lead to alkali depletion and silica enhancement.
Donaldson et al. (1987) argued for the presence of some additional factor and, because the intimate temporal association of natrocarbonatite with the more recent nephelinites has now been established, it is appropriate to consider whether natrocarbonatite itself might have been a complicating factor in the nephelinite compositional variations. The groundmass mineralogy of CN70 particularly serves to highlight the collateral concentrations of Na, K, Sr, Ba and the halogens, i.e. exactly the elements that, apart from CO2, are so strongly concentrated in natrocarbonatite. A further natrocarbonatitic characteristic of CN70 is its high concentrations of Cl relative to F, and other chemical features of CN70 that are intermediate between the other nephelinites and natrocarbonatite are (1) its high Ba/Sr ratio of 1, (2) its high Na2O/K2O ratio of 2.9 (compared with <2 for other nephelinite and 4 for natrocarbonatite) and (3) its low Th/U ratio of 1.1 (compared with2 for WN4421 and WCN119 and 0.38–0.43 for natrocarbonatite (data from Dawson, 1962b; Dawson & Gale, 1970; Dawson et al., 1995a; Keller & Spettel, 1995; Simonetti et al., 1997).
The absence of carbonate globules in CN70 (and the absence of CO2 in its analysis) provides clear evidence against a simple admixture of natrocarbonatite and nephelinite chemically less extreme than CN70, as was the case in the mixed silicate–carbonate spheroids of the 1993 eruption. It is here suggested that the unusual chemistry (and hence, mineralogy) of CN70 is due to loss of CO2 from an evolved magma that under a higher-pressure regime, might have given rise to the wollastonite nephelinite–natrocarbonatite mixture seen in the 1993 eruption spheroids (Dawson et al., 1994, 1996). Put simply, the suggested relationship is
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The left-hand side of this equation is regarded as proven by the products of the 19 eruption; the right-hand side is inferred, but could be tested by experiments on combeit nepheline under CO2-saturated conditions.
A simplified model relating the later nephelinites and natrocarbonatite is given in Fig. 4. The parental magma is suggested as being a mantle-derived carbonate olivine nephelinite. The lava extruded at Oldoinyo Loolmurwak 8 km to the southeast of Oldoinyo Lengai, with an mg-number of 71 and 350 ppm Ni, is the most likely candidate for a primitive, mantle-derived parental melt in the vicinity. Larnite-normative olivine melilitites from other nearby localities have mg-numbers of 60 and relatively low Ni (92–117 ppm), and by these criteria are not as primitive as the Oldoinyo Loolmurwak lava; they may, however, be linked to the primitive Oldoinyo Loolmurwak magma by olivine-spinel fractionation (Dawson et al., 1985). Olivine, phlogopite and aluminous pyroxene, found as megacrysts in some plutonic ijolite and melteigite xenoliths from Oldoinyo Lengai (Dawson et al., 1995b), represent an early stage of fractionation along pathway A. Further cooling causes precipitation of the common set of phases that occur both in the plutonic ijolites and as phenocrysts in most of the Oldoinyo Lengai lavas. If the magma batch remains carbonate saturated, further cooling leads along pathway B, as both wollastonite and combeite precipitate; with falling temperature combeite becomes unstable and, after following pathway C, with falling temperature the carbonate exsolves, stripping alkalis and halogens from the melt to form natrocarbonatite. This final stage is encapsulated in miniature in the 1993 spheroids. The closest experimental approximation to this fractionation is on the system albite–larnite–forsterite (Yoder, 1979) although the experiments were made on a chemically relatively simple system (e.g. CO2-free, and no K, Ba, Sr or halogens).
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In the case of combeite nephelinite, after the initial cooling along pathway A gives the common phenocryst suite plus combeite, CO2 loss, possibly caused by eruptive evacuation of the magma chamber, leads to crystallization along pathway E. The high amounts of alkalis, halogens, Ba and Sr that, under carbonate-saturated conditions, would eventually have been stripped from the magma to form natrocarbonatite, in the magma, causing a rise in the bulk alkalinity and final precipitation of combeite, sodalite, aegirine, halogen-rich glass, ‘delhayelite’, Ba-lamprophyllite, high–Na–Sr perovskite and titanite, and also interact with ‘common’ apatite to form Sr- and Na-rich phosphate.
In the case of wollastonite-combeite nephelinites, the model allows for cooling in the carbonate-saturated field as far as carbonated wollastonite nephelinite, but then CO2 loss leads into the field of carbonate undersaturation along pathway D. Earlier precipitated wollastonite becomes unstable in the alkali-enriched magma, and it is corroded and replaced by corona combeite.
The above is a simplified model that attempts to reconcile the petrographic and chemical data for the later nephelinites, and is compatible with their isotopic and temporal overlap with the natrocarbonatites. It is in some ways similar to the model of Kjaarsgaard et al. (1995), but those workers differed in preferrin an olivine melilitite parent magma, and derived combeite nephelinite from wollastonite nephelinite by low-pressure fractionation. Importantly, however, both models recognize the importance of carbonate undersaturation for combeite stabilization.
Donaldson et al. (1987) suggested that discrete batches of magma had occurred throughout the history of the volcano, and that similar, but not identical fractionation and, possibly, contamination had affected the evolution of these individual magma batches. It is apparent that the later nephelinites, in their extreme alkalinity and constancy of isotopic compositions, represent one such magma batch. The model illustrated in Fig. 4, although aimed at explaining the relationships within this magma batch, does not claim to explain the differences between the later nephelinites and those that occur earlier in the history of the volcano. The earlier nephelinites are of less extreme composition and less exotic mineralogy, and have a wider spread in their strontium and neodymium isotopic compositions. This is a subject requiring further investigation.
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Conclusions |
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TOPABSTRACTIntroductionField Occurrence and PetrographyMineral ChemistryWhole-Rock ChemistryDiscussionConclusionsReferences |
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- The younger nephelinites at Oldoinyo Lengai form a mineralogical and chemical series ranging from wollastonite nephelinite through wollastonite-combeite nephelinite to combeite nephelinite.
- With the exception of wollastonite in the wollastonite-bearing nephelinites and sodalite in the combeite nephelinites, the three nephelinite varieties have a common phenocryst suite, pointing to similar early crystallization histories. The extreme peralkalinity of the combeite nephelinites and enhancement in the alkalis, halogens and incompatible elements in combeite nephelinite relative to the other nephelinites is mainly manifest in the phenocryst rims, and in exotic minerals and peralkaline glass in the groundmass.
- The later nephelinites are temporally closely associated with natrocarbonatite, and have very similar Sr and Nd isotopic characteristics. Mixed silicate-carbonate spheroids in the lavas and ashes of the 1993 eruption testify to the unmixing of natrocarbonatite from (formerly) carbonated wollastonite nephelinite. In contrast, combeite nephelinite lacks carbonate, and it is the silicate phases that are enhanced in alkalis, halogens and incompatible elements; in the wollastonite nephelinite-natrocarbonatite combination all of these elements strongly partitioned into the natrocarbonatite.
- A model is proposed that links the various nephelinite types to each other and to natrocarbonatite bycarbonate under- or over-saturation. The wollastonite nephelinite + natrocarbonatite combination results from fractionation of a parental carbonated olivine nephelinite magma under carbonate-saturated conditions. Conversely, after initia fractionation, CO2 loss, possibly manifest by explosive eruptions, may lead to carbonate undersaturation, when the alkalis, halogens and Ba, that would otherwise have been strongly partitioned into natrocarbonatite, remain in the silicate magma to produce the extreme chemistry of the combeite nephelinites.
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Acknowledgements |
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I am grateful to Simon Burgess, D. Baty and Y. Cooper, for assistance with the microprobe analyses and BSE imaging, diagram drafting and photography, respectively. Dodie James and Godfrey Fitton are thanked for the analysis of WN4421. James Brenan, James Luhr and John Wolff provided helpful reviews of the original manuscript. The specimens described in this paper were collected when I was a geologist with the (then) Tanganyika Geological Survey and during a later visit funded by the Royal Society.
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