Carbonate–silicate immiscibility and extremely peralkaline silicate glasses from Nasira cone and recent eruptions at Oldoinyo Lengai Volcano, Tanzania

Lithos 152 (2012) 40–46

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Carbonate–silicate immiscibility and extremely peralkaline silicate glasses from Nasira cone and recent eruptions at Oldoinyo Lengai Volcano, Tanzania Roger H. Mitchell a,⁎, J. Barry Dawson b a b

Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada P7B 5E1 Grant Institute of Geosciences, University of Edinburgh, Edinburgh, EH9 3JW, UK

a r t i c l e

i n f o

Article history: Received 13 October 2011 Accepted 10 January 2012 Available online 20 January 2012 Keywords: Nephelinite Natrocarbonatite Liquid immiscibility Peralkaline Oldoinyo Lengai Tanzania

a b s t r a c t Phenocrysts of garnet, pyroxene and nepheline in peralkaline nephelinite from the Nasira parasitic cones at Oldoinyo Lengai contain quenched immiscible silicate (peralkalinity= 2–13) and Na–Ca-carbonate melts. Their bulk compositions further define the limits of liquid immiscibility for peralkaline carbonated nephelinite magmas and confirm this process was operative at Oldoinyo Lengai during older stages of activity. Groundmass glasses in Nasira nephelinites are peralkaline (peralkalinity= 5.5–9.5) but less evolved than melt inclusion glasses (peralkalinity= 8–13) in nepheline phenocrysts, implying that these magmas are hybrids formed by magma mixing. Groundmass glass in diverse peralkaline combeite nephelinite ash clasts with and without melilite and/or wollastonite formed in the January–June 2008 eruptions of Oldoinyo Lengai are also exceptionally peralkaline. Two trends in their compositions are evident: (1) increasing peralkalinity from 6 to 10 with SiO2 decreasing from 42 to 33 wt.%; (2) increasing peralkalinity from 6 to 16 with SiO2 decreasing from 45 to 40 wt.%. All recent glasses are considered to be more evolved than groundmass glass in Nasira combeite nephelinite. These data indicate that several varieties of nephelinite exist at Oldoinyo Lengai. Their parental magmas are considered to have been initially enriched in alkalis during partial melting of their metasomatized asthenospheric sources and further by subsequent assimilation, or re-solution, of previously exsolved natrocarbonatite melt in the magma chamber(s) underlying Oldoinyo Lengai. On this basis, none of the bulk compositions of peralkaline stage II lavas at Oldoinyo Lengai, including Nasira, are considered to represent those of liquids as their compositions are determined by rheological factors (phenocryst accumulation; cumulate disruption) and assimilation processes. The formation of combeite is considered to be a consequence of natrocarbonatite melt assimilation. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The silicate lavas extruded from the active natrocarbonatite volcano, Oldoinyo Lengai, Tanzania (2°45′S, 35°54′E) consist principally of nephelinites and phonolites. Although, lavas are rare and the bulk of the volcano is composed of pyroclastic material, they occur as interbedded flows, cognate blocks (Dawson and Hill, 1998) and small latestage extrusions (Dawson, 1962). Compositional data for these lavas has been provided by Dawson (1998), Donaldson et al. (1987) and Klaudius and Keller (2006). The lavas are all phenocryst-rich (10–40 vol.%: nepheline, pyroxene, Ti-andradite) and contain significant amounts of similar “xenocrystal” material derived from genetically-related disaggregated ijolites (Donaldson et al., 1987). Hence, it is important to note that the bulk composition of the lavas does not actually represent that of liquids. Regardless, petrological evolution is considered to be from early weakly peralkaline phonolites

⁎ Corresponding author. Tel.:+1 807 343 8287. E-mail address: [email protected] (R.H. Mitchell). 0024-4937/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2012.01.006

to late strongly peralkaline combeite nephelinites (Dawson, 1998; Kjarsgaard et al., 1995, Klaudius and Keller, 2006, Peterson, 1989a, 1989b). Knowledge of the composition of the magma present at the time of change in the character of the activity at Oldoinyo Lengai from natrocarbonatite lava during much of the 1980s, 1990s and the first 10 years of the 21st century to highly explosive silicate pyroclastics in September 2007 (Mitchell and Dawson, 2007) is important. Dawson et al. (1992, 1995, 1996), Mitchell (2009) and Mitchell and Dawson (2007) have suggested that the pyroclastic activity in 1966–67 and 2007–08 is driven by decarbonation reactions between natrocarbonatite (either solid or liquid) and new batches of hot silicate (? nephelinitic) magma. Because the natrocarbonatites decompose rapidly on weathering, it is as yet unclear whether or not natrocarbonatites have been erupted through the life of the volcano, and the age of partially-decomposed natrocarbonatite lava interbedded with nephelinite tuffs in the summit area (Dawson et al., 1987) is not known. However, melt inclusions in silicate phases are unaffected by such processes and can provide data on the potential presence of natrocarbonates in older eruptions. In addition estimates of the compositions of

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silicate liquids can be made by the analysis of glass inclusions in primary liquidus phases and of glasses forming the matrix of ash particles. In this work data are provided on the composition of glass and carbonate inclusions in phenocrysts from the nephelinitic south parasitic Nasira scoria cone and associated lava flow located on north side of the main Oldoinyo Lengai volcano, together with groundmass glass occurring in ash clasts formed during the January to July 2008 pyroclastic eruptions at the northern summit of Oldoinyo Lengai. These data are relevant to both the proposed formation of the natrocarbonatites by liquid immiscibility (Guzmics et al., 2011; Kjarsgaard et al., 1995; Mitchell, 2009) and to evolution and differentiation of the Oldoinyo Lengai magmatic system (Keller et al., 2006). 2. Analytical methods All silicate glasses and quenched carbonate compositions were determined by quantitative energy dispersive X-ray spectrometry using a JEOL-JSM5900 scanning electron microscope equipped with a LINK ISIS 300 analytical system incorporating a Super ATW light element detector (133 eV FwHm MnK). Raw EDS spectra of rastered areas were acquired for 120 s (live time) with an accelerating voltage of 20 kV and 1 μm diameter beam with a current of 0.475 nA on a Ni standard. The spectra were processed with the LINK ISIS SEMQUANT quantitative software package with full ZAF corrections applied. The following well-characterized mineral and synthetic standards were used: jadeite BM 1913–451 (Na, Al); wollastonite (Ca, Si); orthoclase (K); ilmenite (Fe, Ti); periclase (Mg); Mn-hortonolite (Mn); apatite BM 1926–665 (P); barite (Ba and S); SrTiO3 (Sr); and KCl (Cl). Ba and Sr contents were below the limits of detection (b0.3 wt.%). Fluorine was not analyzed because of significant spectral interferences on the F analytical line. The relatively large size of the inclusions (10–20 μm) coupled with the low beam currents employed for analysis is considered to preclude contributions to their bulk compositions from host crystals. Following the conventional interpretation of the significance of glass inclusions in primary liquidus phases (Frezzotti, 2001; Schiano, 2003) they are considered to reflect the approximate composition of the magma from which they formed. The absence of crystalline phases in the silicate glass inclusions and lack of re-equilibration with host crystals support this contention. The carbonate segregations in the silicate glass inclusions do not quench to a glass and consist primarily of very fine grained aggregates of Na–Ca-carbonates. Analysis of this material using a rastered beam is considered to provide a reliable estimate of their bulk composition (Mitchell, 2009). The compositions of the silicate glasses forming the groundmass of the nephelinites were determined by rastered analysis of large areas (up to 104 μm 2) free from groundmass minerals, and are considered not to have experienced any volatilization of Na during analysis.

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wollastonite nephelinite to melilitite-bearing varieties. The Nasira nephelinites investigated in this work consist of macrocrysts and/or phenocrysts of anhedral-to-subhedral clinopyroxene, subhedral-toeuhedral titanian andradite, macrocrystal wollastonite (commonly resorbed), phenocrystal euhedral-to-subhedral nepheline, two generations of phenocrystal combeite (coexisting altered and fresh) set in a matrix of second generation euhedral nepheline, combeite, Timagnetite, apatite, and glass (Fig. 1a, b). The petrography suggests that the rocks consist of a mixture of disaggregated cumulate material, several distinct batches of phenocrysts, and primary liquidus phases. None of the bulk compositions as determined by Keller et al. (2006) can represent liquids. However, the groundmass silicate glass does represent the liquid carrying this assorted assemblage of crystals but it is possibly only in equilibrium with the late-forming groundmass phases. Melt inclusions consisting of silicate glass or silicate glass with quenched carbonate globules are found in phenocrystal nepheline (but not groundmass nepheline), garnet and clinopyroxene. There is no evidence for the presence of a former gas bubble. Groundmass glass also contains very small (b1 μm) Na–Ca carbonate globules. Dawson (1998) has given limited compositional data for silicate interstitial glass in Nasira nepheliites, but this has very low Na2O (10–14 wt.%) and very high Al2O3 (c. 22 wt.%) contents compared with all groundmass glass analyzed in this work; this may result from different analytical methods with low Na contents possibly reflecting Na loss during analysis even with a rastered 10 nA beam current and high Al from excitation of occult nepheline.

3. Nephelinite, Nasira cones The Nasira cones (Dawson, 1998; Keller et al., 2006) consist of three small north–south orientated Strombolian vents located on the lower northern slope of Oldoinyo Lengai. A short lava flow was emitted from the middle cone. Keller et al. (2006) consider that the activity belongs to the combeite wollastonite nephelinite group of Lengai stage II. The age of the eruptions is not known but is younger than 10 ka; the time of major sector collapse of the main cone. The activity certainly predates significantly the 20th–21st century natrocarbonatite eruptions. Thus, our recognition of preserved immiscible “natrocarbonatite” melt inclusions in the Nasira nephelinites is considered to provide evidence for an earlier period of natrocarbonatite activity at Oldoinyo Lengai. The Nasira cones consist principally of combeite nephelinites with, in some examples, accessory melilitite and/or wollastonite. Keller et al. (2006) consider that the north Nasira cone evolved from combeite

Fig. 1. Representative back-scattered electron images of: (A) hyalo-combeite nephelinite from Nasira with nepheline (Ne) and two generations of combeite (C) set in uniform groundmass glass; (B) Groundmass of combeite nephelinite illustrating late-stage nepheline (Ne), combeite (C), Ti-magnetite (Mt), apatite (A) and abundant glass (G).

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4. Melt inclusions in phenocrysts, Nasira Quenched silicate glass inclusions (5–30 μm) were found in clinopyroxene, garnet, and nepheline phenocrysts in combeite nephelinite from scoria and lava at Nasira. Inclusions are common in nepheline and rarer in garnet and clinopyroxene. Many inclusions in clinopyroxene are composite and contain significant amounts of daughter minerals and thus unsuitable for estimating the composition of the original trapped liquids. The silicate glasses are homogeneous in back-scattered electron images (Figs. 2 and 3). Replicate analysis of different areas of the larger inclusions does not show any significant compositional variation within a given inclusion. Some of the silicate glasses contain globules (b10 μm) of quenched carbonates. These consist predominantly of very fine-grained (≪1 μm) Na–Ca-carbonates with minor amounts of unidentifiable sulfates and chlorides. Bulk compositions of these inclusions are considered to represent those of the liquids prior to quenching (Guzmics et al., 2011; Mitchell, 2009; Nielson et al., 1997; Veksler et al., 1998). Table 1 gives compositional data for silicate glasses in clinopyroxene, garnet and nepheline. All glasses are undersaturated with respect to silica and characterized by high Na, K and Fe contents coupled with low Al contents. Analytical totals are close to 100 wt.%; lower totals are considered to indicate the presence of CO2 only, reflecting the anhydrous character of these magmas. The glasses have very high total alkali contents (20–25 wt.%) and peralkalinity indices [molar (Na + K)/Al] ranging from 1.9 to 11.8. Standard CIPW normative calculations have little relevance to these peralkaline liquids and are thus not provided. However, typical compositions are strongly Or (c. 14 wt.%), Di (c. 16 wt.%) Na2SiO3 (c. 24 wt.%) and K2SiO3 (c. 10 wt.%) normative. The glasses show a well-defined trend of compositional evolution (Fig. 4) with those in the later crystallizing nepheline being more peralkaline and richer in FeOT and poorer in SiO2 and Al2O3 than those in earlierforming clinopyroxene and garnet. Table 2 gives compositions of quenched carbonate liquids and silicate liquids from various host minerals. Compositions are dominated by Na and Ca suggesting that the globules represent quenched “natrocarbonatite” liquids. The carbonate globules are enriched in Na, P, S, and Cl relative to associated silicate glass. Carbonate inclusions in garnet are particularly enriched in sulfur. This might indicate a significant occult gregoryite content in the quench material. Fig. 5 is a

Fig. 3. Melt inclusion of peralkaline silicate glass (white) and globular quenched immiscible carbonate hosted by a nepheline phenocryst. Back-scattered electron image.

Hamilton projection (Kjarsgaard et al., 1995) of the bulk compositions of carbonate globules and coexisting silicate glasses. This projection shows that compositions plot close to the limbs of the immiscible carbonate–silicate field as determined from Lengai 2007 ash (Mitchell, 2009). Compositions of the silicate glasses inclusions evolve to decreasing Si and Al in the sequence pyroxene, garnet, nepheline; and cut across the silicate limbs of both the natural (Mitchell, 2009) and experimentally-determined (Kjarsgaard, 1998; Kjarsgaard et al., 1995) immiscibility fields. Fig. 5 also shows that glasses in garnet and nepheline are similar to those of glass inclusions found in nepheline from the 24 September 2007 ash (Mitchell, 2009). Quenched carbonates are much richer in total alkalis than those found in nephelines from the 2007 Oldoinyo Lengai ash (Mitchell, 2009) and extend the carbonate limb of the immiscibility field towards the total alkali apex of the Hamilton projection. Fig. 5 shows that the carbonate liquids evolve to decreasing alkalis in the sequence from pyroxene, through garnet to nepheline and converge upon the compositions of the carbonate inclusions in nepheline from the 2007 ash. These data confirm that the size of the liquid immiscibility field of carbonated nephelinitic magmas is a function of their peralkalinity and that this field is slightly smaller for the Nasira liquids than for the 2007 ash (Fig. 5).

5. Groundmass silicate glass, Nasira

Fig. 2. Melt inclusion of peralkaline silicate glass (white) and globular quenched immiscible carbonate hosted by a nepheline phenocryst. Back-scattered electron image.

All nephelinites at Nasira are characterized by peralkaline groundmass glass (Fig. 1b). Representative compositions are given in Table 3 which shows that these are undersaturated, Fe-rich and Al-poor glasses with high Na and K contents. All are characterized by high S contents (2.7–4.6 wt.% SO3) with peralkalinity indices ranging from 5.6 to 9.5. Fig. 4 compares their compositions with silicate glass inclusions in Nasira phenocrysts and shows that although they have similar SiO2 contents they are less peralkaline (PI b 10) than the inclusion glasses (PI = 8–13). Thus, the silicate liquids trapped in the nephelines were more evolved than the host groundmass. These data suggest that the phenocrysts and the magma from which they formed was not that which is their current host and these phenocrysts are not in equilibrium with the groundmass-forming liquids. This conclusion is in accord with petrographic observations (see above) which indicate that these combeite nephelinites are mixtures of several generations of cumulate and phenocrystal phases.

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Table 1 Composition (wt.%) of glass inclusions in Nasira nepheline (NEPH), clinopyroxene (CPX) and garnet (GNT) phenocrysts.

NEPH NEPH NEPH NEPH NEPH NEPH NEPH NEPH NEPH NEPH NEPH NEPH NEPH NEPH NEPH NEPH NEPH NEPH NEPH NEPH CPX CPX CPX CPX CPX CPX GNT GNT GNT GNT GNT GNT GNT

SiO2

TiO2

Al2O3

FeOT

MnO

MgO

CaO

Na2O

K2O

P2O5

SO3

Cl

Total

PI

ALK

37.75 43.91 43.78 42.72 42.66 42.30 45.78 42.40 42.03 43.95 43.90 43.58 43.36 45.83 45.94 45.30 42.61 43.87 43.46 44.91 51.09 52.31 45.27 46.32 46.78 46.52 49.07 51.42 50.70 50.68 51.60 47.57 50.86

1.80 1.32 1.80 2.19 1.73 1.94 1.21 2.45 2.19 1.91 2.00 2.11 2.27 1.64 1.74 1.67 2.01 1.49 1.67 1.53 0.85 0.98 0.99 0.90 1.17 0.82 0.50 0.60 0.26 0.49 0.66 0.84 0.30

2.03 3.32 3.19 3.15 3.13 3.46 2.81 2.83 2.93 2.95 3.73 2.90 3.33 3.86 3.71 3.98 3.23 3.19 3.14 3.40 16.17 16.46 10.37 10.10 6.35 7.63 9.74 9.77 16.27 12.23 13.06 11.80 11.66

18.13 13.45 13.11 18.55 14.87 17.28 13.92 18.50 18.09 15.88 16.46 16.70 15.60 12.63 12.50 11.92 16.15 14.91 14.45 13.31 4.53 5.15 11.01 9.70 10.19 9.87 13.01 11.82 6.45 8.73 9.16 10.02 10.90

0.78 0.51 0.55 0.69 0.77 0.73 0.43 0.97 0.77 0.85 0.70 0.69 0.79 0.56 0.63 0.65 0.66 0.87 0.68 0.61 0.24 0.22 0.43 0.59 0.73 0.61 0.50 0.49 0.33 0.28 0.53 0.42 0.32

1.50 0.67 0.89 1.33 1.27 1.10 0.98 1.51 1.48 1.37 1.55 1.30 1.63 0.73 0.81 0.94 1.54 1.17 1.22 0.81 N.D. 0.20 0.69 0.52 0.72 0.57 0.98 1.25 0.65 0.79 0.99 1.12 0.93

4.55 6.30 6.55 3.31 5.10 4.75 6.93 4.09 4.43 5.36 3.30 4.22 4.30 6.35 6.56 5.79 4.42 6.03 6.06 6.42 1.55 1.42 4.56 5.55 7.28 6.57 1.31 1.03 0.48 0.60 0.55 2.85 0.73

17.74 14.31 14.60 14.66 15.86 15.99 14.02 15.57 15.45 15.24 14.87 14.92 15.15 13.03 14.95 13.96 15.31 15.86 16.17 14.65 12.36 11.43 15.81 16.32 17.15 17.02 11.60 11.51 12.45 11.33 11.81 15.27 11.69

7.72 8.44 8.59 8.51 8.48 8.37 9.31 8.63 8.52 7.65 8.21 7.88 7.88 9.31 9.44 9.28 8.26 7.55 7.67 9.51 9.07 9.03 8.40 8.84 8.07 8.53 8.72 9.11 7.28 8.55 8.52 8.90 8.95

0.68 0.50 0.39 0.29 0.60 N.D. 0.23 0.39 0.53 0.68 0.41 0.57 0.46 0.26 0.42 0.14 0.65 0.59 0.28 0.43 N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.31 N.D. 0.44 N.D. N.D. 0.56

2.98 1.63 1.96 2.74 2.35 2.47 1.62 2.87 2.64 1.95 3.50 2.45 2.56 1.80 1.70 1.54 2.17 1.34 1.31 1.87 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D.

0.30 0.35 0.32 0.51 0.47 0.47 0.39 0.37 9.52 0.42 0.63 0.70 0.58 0.36 0.34 0.33 0.42 0.33 0.44 0.41 N.D. N.D. N.D. N.D. N.D. N.D N.D. N.D. N.D. N.D. N.D. N.D. N.D.

93.96 94.71 95.73 97.29 97.29 98.86 97.63 98.58 99.58 98.21 99.26 98.02 97.91 96.36 98.74 95.50 97.43 97.18 96.95 97.86 95.86 97.20 97.53 98.84 98.44 98.14 95.43 97.56 94.87 94.12 96.88 98.79 96.90

13.02 9.84 10.44 10.58 11.27 10.22 11.79 12.35 11.82 11.91 8.94 11.40 10.05 8.16 9.38 8.29 10.57 10.74 11.43 10.12 1.86 1.74 3.38 3.61 5.82 4.88 2.93 2.95 1.74 2.28 2.19 2.95 2.48

23.46 22.75 23.19 23.17 24.34 24.36 23.33 24.20 23.97 22.89 23.08 22.80 23.03 22.34 24.29 23.24 23.57 23.41 24.44 24.16 21.43 20.46 24.21 25.16 25.22 25.55 20.32 20.62 19.73 19.88 20.33 24.17 20.64

FeOT = Total Fe as FeO; N.D. = not detected (b 0.3 wt.%); PI = peralkalinity index; ALK = total alkalis.

6. Groundmass silicate glass in ash clasts from the January–June 2008 eruptions of Oldoinyo Lengai Recent volcanic activity at Oldoinyo Lengai from September 2007 to July 2008 has involved Plinian-to-sub-Plinian pyroclastic eruptions of silicate ash. Initial activity produced an unusual hybrid clinopyroxenefree melilititic magma (Mitchell and Dawson, 2007) which was followed by a diverse group of nephelinitic magmas (Keller et al., 2010; this work). Clasts of the latter built up the ash cone which currently occupies the north crater of Oldoinyo Lengai (Keller et al., 2010). Samples from the ash cone investigated in this paper include combeite nephelinite, wollastonite combeite nephelinite, and melilite combeite nepheline. In terms of their petrography all are typical of Lengai stage

Fig. 4. Compositional variation of silicate glass inclusions in Nasira phenocrysts and groundmass glass.

II wollastonite combeite nephelinites as described by Keller et al. (2006, 2010). The lavas are represented by small vesicular ash clasts or occur as thin vesicular mantles on ijolitic clasts. Their mineralogy typically consists of macrocrysts and/or phenocrysts of wollastonite (commonly resorbed), Ti-andradite, melilite, clinopyroxene, and nepheline set in a pale green glassy groundmass from which crystallized Ti-garnet, apatite, Ti-magnetite nepheline and combeite (Fig. 6a, b). Some of the glasses contain very small (b2 μm) globules of quenched Na–Cacarbonates that are too small to analyze without excitation of the matrix silicate. Keller et al. (2010) have discussed the bulk compositions of these 2008 lavas and suggested that they represent a “new magma” intermediate in composition between natrocarbonatite and nephelinite that is close to the immiscible segregation of natrocarbonate from a carbonated nephelinite. However, it is clear from the petrography of these lavas (Fig. 6a, b) that none of the bulk compositions can represent those of liquids. Keller et al. (2010) did not present any compositional data for groundmass glass. The objective of our study is to investigate only the composition of the residual glasses and to compare these with those present in the older Nasira nephelinites. A subsequent study (Mitchell and Dawson, in prep.) will present a detailed description of the mineralogy of the recently-erupted north crater lavas and plutonic ijolitic clasts. Table 4 gives representative compositions of residual silicate glasses in diverse nephelinites from ash-sized clasts erupted in the north crater of Oldoinyo Lengai between January and July 2008. Data for all 116 compositions determined are available from the authors. All residual glasses are undersaturated, extremely peralkaline (PI = 6–16), enriched in FeOT (13–22 wt.%), Na2O (9–20 wt.%), K2O (5–10 wt.%), and SO3 (2–7 wt.%). Some glasses are potassic (Table 4, compositions 6–7) and enriched in SO3 (5–7 wt.%). Compositional

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Table 2 Compositions (wt.%) of coexisting quenched silicate and carbonate liquids, Nasira. Phase

P2O5 SiO2 TiO2 Al2O3 FeOT MgO CaO SrO Na2O K2O SO3 Cl Total

Garnet

Garnet

Garnet

CPX

NEPH

NEPH

NEPH

NEPH

SL

CL

SL

CL

SL

CL

SL

CL

SL

CL

SL

CL

SL

CL

SL

CL

0.39 45.02 1.75 11.17 10.44 0.73 4.34 N.D. 18.30 7.63 N.D. N.D. 99.77

1.13 1.74 N.D. 0.53 0.47 0.45 3.27 ND. 37.54 2.48 13.74 0.73 62.08

N.D. 49.07 0.50 9.74 13.01 0.98 1.31 N.D. 11.60 8.72 N.D. N.D. 94.93

3.12 1.92 N.D. 0.42 0.52 N.D. 7.60 1.54 38.86 1.37 20.47 N.D. 75.82

N.D. 51.87 0.63 9.58 11.33 1.19 0.88 N.D. 11.08 9.10 N.D N.D 95.66

2.80 0.79 N.D. 0.36 0.27 N.D. 5.52 N.D 39.81 0.88 18.61 N.D. 69.04

N.D. 46.52 0.82 7.63 9.87 0.57 6.57 N.D. 17.02 8.53 N.D. N.D. 97.53

1.87 2.70 N.D. N.D. 0.94 N.D. 4.28 N.D. 42.12 1.50 15.91 0.40 69.32

0.43 44.90 1.53 3.40 13.31 0.81 6.42 N.D. 14.65 9.51 1.87 0.40 97.23

2.47 3.96 N.D. N.D. 1.78 N.D 23.38 N.D. 24.04 3.07 1.06 0.61 60.37

0.60 42.66 1.73 3.63 14.87 1.27 5.10 N.D 15.86 8.48 2.35 0.47 97.02

2.96 1.27 N.D. N.D. 0.79 N.D. 12.66 1.49 31.89 4.30 8.24 2.51 66.11

0.69 43.95 1.91 2.95 15.88 1.37 5.36 N.D. 15.24 7.65 1.95 0.42 97.37

2.63 2.10 N.D. 0.39 0.21 N.D. 8.11 N.D. 25.92 4.84 12.34 1.42 57.96

0.46 43.46 2.27 3.33 15.60 1.63 4.30 N.D. 15.15 7.88 2.56 0.58 97.22

2.82 1.28 N.D. 0.57 0.70 N.D. 5.18 N.D 34.74 8.74 17.23 5.51 76.77

FeOT = total Fe expressed as Fe; N.D. = not detected (b 0.3 wt.%). CPX = clinopyroxene; NEPH = nepheline; SL = silicate liquid; CL = carbonate liquid.

data for nine similar peralkaline groundmass glasses have been given by Dawson and Hill (1998) and (Peterson, 1989a). Fig. 7 illustrates the compositions of recent Lengai ash groundmass glasses in which two trends are evident: (1) increasing peralkalinity (PI = 6–10) with SiO2 decreasing from 42 to 33 wt.%; (2) increasing peralkalinity (PI = 6–16) with SiO2 decreasing from 45 to 40 wt.%.. Trend 1 commences with glasses occurring in combeite nephelinites and terminates with those in highly undersaturated melilite combeite nephelinites. Trend 2 is formed by heterogeneous groundmass K-rich glasses (Table 3) found in a single sample of highly vesicular wollastonite combeite nephelinite lava (Fig. 6a). Fig. 7 also shows that Nasira glasses have compositions which are Si-rich relative to the groundmass glasses in the north crater clasts. Their compositions define the initial part of trend 2, and can be considered to be less evolved than all recent glasses. These data indicate that diverse peralkaline nephelinites are present in the Oldoinyo Lengai magmatic system and that these are evolving to more peralkaline compositions. 7. Discussion The residual glasses in both the Nasira cones and recent lavas from the Oldoinyo Lengai north crater ash cone represent the most

Fig. 5. Hamilton projection of the bulk compositions of co-existing natural (this work, Mitchell, 2009) and 0.1 GPa experimental (Kjarsgaard, 1998; Kjarsgaard et al., 1995) silicate and carbonate melts. Curves labeled 1–4 are the postulated limits of the silicate– carbonate liquid immiscibility fields as a function of increasing peralkalinity.

peralkaline undersaturated liquids known. Only pantellerites (PI = 14) have similar high peralkalinity (Di Carlo et al., 2010). However, these rocks are Si-rich and hydrous and mechanisms for their genesis are not applicable to Oldoinyo Lengai nephelinites. Many mechanisms have been advanced for the development of weakly-tomoderately peralkaline (PI b 3) undersaturated rocks Many of the earlier hypotheses (see MacDonald, 1974 for a review) are based upon only fractionation crystallization processes involving for example alkali feldspar (Bailey and Schairer, 1966) or aluminous clinopyroxene (Peterson, 1989a, 1989b). However, such simple fractional crystallization schemes have been found inadequate to explain the compositional variation of the nephelinite–phonolite suite found at Oldoinyo Lengai (Dawson and Hill, 1998; Donaldson et al., 1987). Recently, more complex genetic processes have been suggested for peralkaline rocks that involve successive melting of compositionally heterogeneous mantle sources coupled with magma hybridization and/or mixing (Marks et al., 2008). In such models alkali enrichment is not caused by fractional crystallization of alkali-poor parental magma. Instead alkali-enrichment is inherited from the source regions of the magmas. Following such reasoning, Dawson (2008) has suggested a multistage modal for the Younger Extrusives of the Gregory Rift Valley that involves their formation from low volume alkali enriched asthenospheric melts that have interacted with metasomatized mantle. Regardless of exactly how the bulk of Oldoinyo Lengai nephelinite– phonolite lavas have been formed, the parental magma to the Recent Lengai stage II combeite nephelinites has not been identified. Following Dawson (2008) we conclude that this must have been initially alkali enriched but the extent of enrichment and how this magma subsequently evolved to produce the characteristic low SiO2, high FeOT residual glasses remains enigmatic. Although it is clear that the recent peralkaline nephelinites have crystallized phenocrystal clinopyroxene, wollastonite, garnet and nepheline together with groundmass Timagnetite and nepheline, any fractional crystallization process involving these phases is not applicable (Dawson and Hill, 1998), as this leads to Fe- and Na-depletion. Fractionation of aluminous clinopyroxene as advocated by Peterson (1989b) is not possible as all of the pyroxenes occurring in the lavas are Al-poor augite and aegirine augite (Mitchell and Dawson in prep.). Alkali feldspar is not a liquidus phase in any of the nephelinites. All stage II Oldoinyo Lengai nephelinites are combeite-bearing. Only two other occurrences of this mineral, both in volcanic rocks, are known, i.e. Mt. Shaheru, a parasitic cone at Mt. Nyiragongo (Sahama and Hytönen, 1957) and Bellerberg volcano, Eiffel Mts. (Fischer and Tillmanns, 1987). Given that nephelinites are relatively common magma types, the rarity of combeite in such lavas might attest to uncommon processes being involved in its formation. Thus, we suggest that as fractional crystallization cannot account for the

R.H. Mitchell, J.B. Dawson / Lithos 152 (2012) 40–46

45

Table 3 Composition (wt.%) of groundmass glass, Nasira.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

SiO2

TiO2

Al2O3

FeOT

MnO

MgO

CaO

Na2O

K2O

P2O5

SO3

Cl

Total

PI

ALK

44.73 43.94 43.76 43.51 44.81 42.74 44.94 45.72 44.93 43.50 45.08 43.86 44.72 45.64 42.23

2.11 2.13 2.48 2.64 2.74 2.50 1.87 2.32 2.22 2.26 2.25 2.21 2.35 2.50 2.51

4.15 4.09 3.37 3.61 3.89 3.95 5.58 4.21 4.47 3.90 3.96 5.65 4.45 5.02 3.70

17.14 17.15 18.65 18.60 17.62 16.93 15.79 16.83 16.63 17.36 16.89 16.07 16.91 16.56 16.67

0.65 0.86 0.64 0.46 0.68 0.50 0.65 0.52 0.69 0.82 0.89 0.78 0.67 0.79 0.53

1.53 1.42 1.79 1.27 1.46 1.62 1.26 1.35 1.57 1.48 1.38 1.45 1.53 0.57 3.70

2.30 2.53 2.78 2.20 2.23 3.81 2.33 2.58 2.74 2.69 2.29 2.52 2.34 2.34 2.48

13.79 12.72 12.20 14.08 13.89 13.69 13.10 11.90 12.71 15.03 13.54 13.78 13.87 12.61 15.85

9.11 9.15 9.89 9.32 9.48 8.87 9.02 9.52 8.87 8.52 9.20 8.78 9.14 9.24 8.53

0.42 0.28 0.45 0.43 0.26 0.51 0.23 N.D. 0.28 0.39 0.26 0.35 0.42 0.77 0.95

4.13 3.88 4.23 3.93 3.96 4.37 2.68 4.21 3.90 4.19 4.26 3.99 4.30 3.61 2.95

0.76 0.97 0.90 0.64 0.88 0.70 0.73 0.76 0.79 0.74 0.74 0.69 0.76 0.76 0.69

100.83 99.12 101.14 100.69 101.81 100.19 98.18 100.02 99.80 100.88 100.74 100.13 101.46 100.41 100.79

7.88 7.50 9.13 9.21 8.71 8.13 5.61 7.09 6.83 7.70 8.64 5.69 7.35 6.12 9.54

22.90 21.97 23.09 23.40 23.37 22.56 22.12 21.41 21.58 23.55 22.74 22.56 23.01 21.85 24.38

FeOT = total Fe expressed as FeO; N.D. = not detected. PI = peralkalinity index; ALK = alkalinity (Na2O + K2O).

composition of the highly peralkaline residual glasses, that their elevated Na and K contents are related to assimilation of natrocarbonatite melt (solute) in a nephelinitic magma (solvent) that was already enriched in alkalis. In physical terms such a process would represent the re-solution of previously exsolved immiscible natrocarbonatite melt that is retained in the Oldoinyo Lengai magma chamber. Such as process would increase K, Na, Ca and S in the hybrid

magma but decrease Al and Si. Further evolution by fractional crystallization of liquidus phases such as wollastonite, apatite, garnet and nepheline would lead to increasing undersaturation and depletion in Ca due to the formation of abundant combeite. Extensive combeite fractionation could lead to the K-enrichment of some batches of magma. In proposing this assimilation/re-solution process for the formation of combeite we do not imply a similar genesis for combeite occurring at Nyiragongo or Bellerberg. The Fe-enrichment remains problematic. In both oversaturated and undersaturated peralkaline plutonic rocks differentiation of hydrous peralkaline melts leads, under conditions of low (bFMQ) and decreasing oxygen fugacity, to the formation of the poikilitic Fe-rich groundmass minerals which are the hallmark of agpaitic magmatism (Markl et al., 2001; Mitchell and Platt, 1978). In a water-free magma such as combeite nephelinite this process might, under plutonic conditions, lead to the formation of aegirine, aegnigmatite and complex Na– Zr-titanosilicates. In this context, Dawson and Hill (1998) have described the formation of groundmass aegirine and Ba-lamprophyllite in an essentially anhydrous combeite nephelinite with a bulk rock peralkalinity index of 2.16. Note that this rock contains clinopyroxenes ranging in composition from diopside (phenocrysts) to aegirine (groundmass) that are all Al2O3-poor (b1.5 wt.%). Hence, given the usual caveat regarding the bulk compositions of phenocryst-rich nephelinites, fractionation of Al–clinopyroxene as a means of increasing the

Table 4 Compositions (wt.%) of groundmass glass in diverse ash clasts from the January to July 2008 eruptions of Oldoinyo Lengai.

Fig. 6. Peralkaline nephelinite ash clasts from the recent eruptions of Oldoinyo Lengai: (a) combeite wollastonite nephelinte; (b) melilite combeite nephelinite. Backscattered electron images. Ne = nepheline; Wo = wollastonite; Gnt = Ti-andradite; Cpx = clinopyroxene; mel = melilite; c = combeite.

Wt.%

1

2

3

4

5

6

7

8

9

P2O5 SiO2 TiO2 Al2O3 FeOT MgO MnO CaO Na2O K2O SO3 Cl Total PI

0.30 41.11 1.87 3.45 16.34 0.55 1.12 4.59 17.34 8.32 4.42 0.30 99.71 10.88

0.80 40.58 1.79 4.93 13.45 0.93 0.70 6.74 18.31 4.91 2.19 0.41 95.74 7.19

0.71 43.39 2.40 3.51 16.83 1.38 0.67 3.17 16.83 6.25 3.67 0.64 99.45 9.82

0.96 40.41 1.77 4.68 14.89 1.09 0.64 7.01 17.30 5.28 2.30 0.43 96.76 7.30

0.69 38.69 1.87 5.54 14.83 1.34 0.64 8.89 18.19 5.42 2.93 0.41 99.44 7.88

0.46 42.54 4.07 2.36 18.92 2.56 0.39 1.64 9.76 10.15 5.11 0.67 98.63 11.46

N.D. 40.51 4.06 1.75 22.20 2.26 0.31 1.61 9.91 9.73 6.68 0.84 99.86 15.77

0.82 0.71 37.50 38.99 1.70 1.90 4.57 4.77 17.27 15.50 1.26 1.64 0.80 0.62 5.64 7.73 17.68 19.69 7.56 5.68 3.46 2.53 0.47 0.42 98.73 100.18 8.34 8.08

10 0.41 39.09 1.59 4.37 13.04 1.00 0.52 8.71 18.94 5.58 2.56 0.41 96.22 8.14

FeOT = total Fe expressed as Fe; N.D. = not detected (b 0.3 wt.%). PI = Peralkalinity index. 1 combeite nephelinite; 2 wollastonite nephelinite; 3 combeite wollastonite nephelinite; 4 wollastonite nephelinite; 5 wollastonite nephelinite; 6–7 K–Ti-rich combeite wollastonite nephelinite; 8 melilite nephelinite; 9 combeite wollastonite nephelinite; 10 combeite wollastonite nephelinite.

46

R.H. Mitchell, J.B. Dawson / Lithos 152 (2012) 40–46

Fig. 7. Compositions of groundmass glass in peralkaline nephelinite (cwn) ash clasts from recent eruptions of Oldoinyo Lengai compared to those of groundmass glass in Nasira combeite nephelinite.

peralkalinity (Peterson, 1989b) to that of the groundmass glass (PI — 12–13) can be discounted. 8. Conclusions Phenocrysts in garnet, pyroxene and nepheline in peralkaline nephelinites from the Nasira parasitic cones at Oldoinyo Lengai contain quenched immiscible silicate and Na–Ca-carbonate melts. Their bulk compositions define the limits of liquid immiscibility for peralkaline carbonated nephelinite magmas and confirm this process was operative at Oldoinyo Lengai during older stages of activity. Groundmass glass in Nasira combeite nephelinites is exceptionally peralkaline but less evolved than glass in nepheline phenocrysts, implying that these magmas are hybrids formed by magma mixing. Groundmass glass in diverse peralkaline nephelinite ash clasts formed in the January–June 2008 eruptions of Oldoinyo Lengai are also exceptionally peralkaline. Their parental magmas are considered to have been initially enriched in alkalis during partial melting of their sources and further by subsequent assimilation, or re-solution, of previously exsolved natrocarbonatite melt in the magma chamber(s) underlying Oldoinyo Lengai. On this basis, none of the bulk compositions of peralkaline stage II lavas at Oldoinyo Lengai, including Nasira, are considered to represent those of liquids as their compositions reflect rheological (phenocryst accumulation; cumulate disruption) and assimilation processes. The formation of combeite is considered to result from natrocarbonatite assimilation. Note that our conclusions are considered to be valid only for Lengai stage II magmas and do not necessarily apply to the older stage I nephelinites and phonolitic nephelinites. Acknowledgments Roger Mitchell's work on alkaline rocks is supported by the Natural Sciences and Engineering Research Council of Canada, Almaz Petrology, and Lakehead University. Barry Dawson's work is supported by the Natural Environment Research Council of the UK, the Carnegie Trust for the Universities of Scotland. Dorobo Safaris of Arusha, Tanzania, are thanked for sample collection and logistical assistance at Oldoinyo Lengai. References Bailey, D.K., Schairer, J.F., 1966. The system Na2O–Al2O3–Fe2O3–SiO2 at 1 atmosphere and the petrogenesis of alkaline rocks. Journal of Petrology 7, 114–170. Dawson, J.B., 1962. The geology of Oldoinyo Lengai. Bulletin of Volcanology 24, 349–387.

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