Immiscible melt inclusions in nepheline phenocrysts from nephelinite lavas, Oldoinyo Lengai: heating experiments and compositional constraints
Sharygin V.V.*, Kamenetsky V.S.**, Zaitsev A.N.***, Kamenetsky M.B.**
* V.S.Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia;** School of Earth Sciences, University of Tasmania, Hobart, Australia; *** Saint Petersburg State University, Saint Petersburg, Russia
Silicate melt inclusions with immiscibility have been found in nepheline phenocrysts of peralkaline nephelinite lavas at the Oldoinyo Lengai volcano, Tanzania. The host rocks are porphyritic and contain abundant euhedral phenocrysts (>1-5 mm) of nepheline and clinopyroxene, and rarer euhedral combeite, titanite, Ti-andradite and apatite. The groundmass consists of microphenocrysts (<1 mm) of the above mentioned minerals and green to brown glass. Delhayelite, perovskite, magnetite, wollastonite with combeite corona, pyrrhotite, K-feldspar, Sr-bearing barite, lamprophyllite, sodalite and calcite are minor or accessory in the groundmass. In general, all studied nephelinite species are similar to those described by Dawson (1998) and Dawson and Hill (1998).
Silicate melt inclusions outline the growth zones in host nepheline phenocrysts. Other inclusion types (fluid, carbonate, sulfide and chloride) are scarce. Clinopyroxene is abundant among crystallites in nepheline, whereas other crystallites (apatite, titanite, Ti-andradite, magnetite and perovskite) occur occasionally.
Silicate melt inclusions (10-100 µm) in nepheline are very specific in phase composition: green glass + gas-carbonate globules + daughter crystals + trapped crystals + fluorite +/- other salts (Fig. 1). The most typical inclusions contain glass and one gas-carbonate globule. In some inclusions gas and carbonate components are separated into individual phases, and gas bubble always contains some amounts of salt crystals on the walls. According to optical observations, SEM and EMPA, trapped crystals are clinopyroxene, apatite, Ti-andradite and titanite, whereas daughter phases are Na-Fe-rich clinopyroxene, fluorite, delhayelite, K-feldspar and unidentified K-rich (20.3 wt.% K2O) alumosilicate. Fluorite occurs both in carbonate globule and in silicate glass. Carbonate globule is commonly fine-devitrified and some individual phases (calcite, nyerereite, Na-carbonate, fluorite) are sometimes fixed in it, like in gregoreyite-hosted inclusions from the Oldoinyo natrocarbonatite (Mitchell, Belton, 2004; Mitchell, 2006). Same immiscible nepheline-hosted inclusions were previously described in nephelinites of the Mosonik carbonatite-related volcano, which is neighboring with the Oldoinyo Lengai volcano (Bazarova et al., 1975).
Fig. 1. Immiscible melt inclusions in nepheline phenocrysts from different samples of nephelinites, Oldoinyo Lengai.
Notes: Gl - silicate glass; Cc - natrocarbonatite globule; g - gas bubble; Lm - leucocratic mineral; Sph - titanite. Scale bar - 10 µm.
Preliminary heating experiments with the Oldoinyo nepheline-hosted inclusions (silicate glass + gas-carbonate globule) were provided for the 20-900oC range. First changes within inclusions are observed in a gas-carbonate globule at 470-510оС and assigned to recrystallization of phases. The carbonate component of gas-carbonate globules melts instantaneously at 550-570оС. Silicate glass melts at 600-670oC. Further increasing of temperature (670-810оС) led to the appearance and then disappearance of blebs of salt liquid (fluoride, chloride or sulfate composition ?) both in silicate and in carbonate melts. Salt liquid blebs occurring in the silicate melt disappear by coalescence with the carbonate melt. In the same temperature range individual carbonate melt globules amalgamate into one large globule, which then gradually decreases in size and gas bubble in it also decreased in size at 800-900oC. The gas-carbonate melt globule homogenizes at 900oC. At this temperature phase composition of the melt inclusions is silicate melt + carbonate melt. Quenching of inclusions showed silicate melt-glass transition at 600oC, appearance of gas bubble in a carbonate globule at 530-540oC and crystallization of phases within carbonate globule at T<500oC (Fig. 2).
Fig. 2. Heating experiment with a nepheline-hosted inclusion from nephelinite, sample Ol7-2000, 1917 eruption, Oldoinyo Lengai.
Note. Si glass - silicate glass; Si melt - silicate melt; Cc - natrocarbonatite solid; Cc melt - natrocarbonatite melt; g - gas bubble. Size of inclusion is 40 µm. Initial phase composition is similar to that on photo at 440oC.
Silicate peralkaline glass of nepheline-hosted inclusions strongly vary (in wt.%): SiO2 - 43.6-53.0; TiO2 - 1.0-2.0; Al2O3 - 2.7-7.1; FeOt - 7.5-18.6; MnO - 0.5-1.2; MgO - 0.2-1.8; CaO - 1.0-7.2; BaO - 0.1-1.1; Na2O - 4.4-10.6; K2O - 9.0-19.3; P2O5 - 0.1-0.3; SO3 - 0.6-1.8; F - 0.4-2.0; Cl - 0.0-0.5. The most Si-undersaturated compositions in partly crystallized inclusions approach the groundmass glasses (author’s data; Peterson, 1989; Dawson, 1998; Dawson, Hill, 1998). Unfortunately, we were unable to fully quantify composition of carbonate globules due to their rapid decomposition after exposing at the surface. After opening they gradually reacted with air to form Na-water-bearing carbonate (trona ?), aphthitalite and Na-K-chlorides. The qualitative analyses by EDS and EMPA show that the carbonate globules are natrocarbonatite in composition and Na, Ca, K, C, Cl, F, S and P are prevailed components, which are common for natrocarbonatite lavas of Oldoinyo Lengai.
Thus, study of nepheline-hosted inclusions in nephelinite lavas advocates very complicated history during cooling of initial silicate melt as suggested by Dawson (1998). The silicate-natrocarbonatite immiscibility took place at temperature above 900oC. In the 900-600oC range the gas phase and salt liquids of different compositions may be separated from natrocarbonatite melt. The carbonate-carbonate immiscibility cannot be excluded (Mitchell, 1997). The enrichment of peralkaline silicate melt in Fe and alkalis was favorable to crystallization of minerals rich in these components and poor in Al. Unlike inclusions (closed system), during crystallization of nephelinite lavas (open system) the silicate-natrocarbonatite immiscibility phenomenon is commonly hidden, owing to degassing of silicate melt, separation of carbonate melt from silicate melt and/or reaction of carbonate melt with primary silicate minerals (combeite coronas around wollastonite, etc.). Evidences of immiscibility were observed only in the Oldoinyo hybrid rocks intermediate between silicate rocks and natrocarbonatites (Church, Jones, 1995; Dawson et al., 1996).
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