Unusual leucite and mica from melt inclusions in nepheline phenocrysts from nephelinite lavas, Oldoinyo Lengai, Tanzania

Sharygin V.V.*, Zaitsev A.N.***

* V.S.Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia;** Saint Petersburg State University, Saint Petersburg, Russia; burbankite@gmail.com

 

Abundant silicate melt inclusions with silicate-carbonate immiscibility have been found in nepheline phenocrysts of peralkaline nephelinite lava of the 1917 eruption at the Oldoinyo Lengai volcano, Tanzania. The host rock is porphyritic and contains 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 and calcite are minor or accessory minerals in the groundmass. In general, the studied rock is wollastonite-combeite nephelinite previously described by Dawson (1998) and Dawson and Hill (1998).

Silicate melt inclusions (10-100 µm) in nepheline phenocrysts contain green glass + gas-carbonate globules + daughter silicate crystals + trapped crystals + daughter fluorite (Sharygin, 2009; Sharygin et al., 2009). Daughter phases in most crystallized inclusions are represented by Na-Fe-rich clinopyroxene, fluorite, delhayelite, K-feldspar, wollastonite and two silicates with atypical composition (Fe-rich leucite and Na-Mg-Ti-rich brown mica). The relations within inclusions have shown that leucite is an earlier daughter phase; whereas mica and delhayelite crystallized later (Fig. 1). Here we present data on chemical composition of unusual leucite and mica from nepheline-hosted inclusions.

 

Fig. 1. Crystallized melt inclusions in nepheline phenocrysts from the 1917 eruption nephelinite lava, Oldoinyo Lengai, BSE images. Notes: Gl - Fe-rich silicate glass; g - gas-carbonate globule; Lc - Fe-rich leucite; Fl - fluorite; Delh - Fe-rich delhayelite; Tfa - Na-Mg-Ti-rich tetraferriannite.

 

Heating experiments with nepheline-hosted inclusions containing mica and leucite have shown that these phases disappeared in silicate melt before homogenization in gas-carbonate melt globule (<900^oC). Delhayelite and mica and then leucite melted in the temperature range of 650-750^oC.

Colorless leucite from inclusions was previously labeled as unidentified K-Al-silicate (Sharygin, 2009; Sharygin et al., 2009) despite the fact that this phase has a leucitic composition. Petrographic observations of inclusions indicate prismatic habit of crystals (Fig. 1). It is known that high-temperature (>1000^oC) leucite is cubic phase. With decreasing of temperature (560-715^oC) it transforms into tetragonal polymorphs. Prismatic crystals, heating experiments and leucitic composition strongly suggest that this leucite crystallized directly from silicate melt as tetragonal phase. This leucite contains high Fe₂O₃ (up to 9 wt.%), low Al₂O₃ (15.3-17.5 wt.%) and appreciable Na₂O (0.1-0. wt.%), indicating high abundance of the KFeSi₂O₆ end-member (up to 26 mole %) (Table 1).

Brown mica is an interstitial anhedral phase in the nepheline-hosted inclusions (Fig. 1). This phase is rich in SiO₂, FeO_t, Na₂O, TiO₂, F and poor in Al₂O₃ and MgO (Table 1). Calculations of formula based on 11 oxygens were shown that this mica may contains water (up to 1.9 wt.%). This complex mica is characterized by equal abundance of different end-members, including annite, tetraferriannite, shirokshinite and hypothetical Ti-rich compositions. However, the predominance (> 50 mole %) of the end-members with Fe²⁺₃[Fe³⁺Si₃O₁₀] gave us possibility to classify this mica as Na-Mg-Ti-rich tetraferriannite. The broad isomorphism between tainiolite KMg₂Li[Si₄O₁₀]F₂ and shirokshinite KMg₂Na[Si₄O₁₀]F₂ in peralkaline systems (Pekov et al., 2003; Armbruster et al., 2007) suggests the possible presence of Li in the Oldoinyo specific mica. Rb is not also excluded as possible minor component. These assumptions are supported by SIMS data for residual peralkaline glasses of partly crystallized inclusions in nepheline phenocrysts. These silicate glasses are peralkaline, strongly vary in SiO₂ (43.6-53.0), FeO_t (7.5-18.6), CaO (1.0-7.2), alkalis (16.5-24.5) and SO₃+F+Cl (1.0-4.3 wt.%) (Sharygin et al., 2009) and contain high Li (240-390 ppm) and Rb (250-460).

 

Table 1. Representative analyses (wt.%) of Fe-rich leucite and Na-Mg-Ti-rich tetraferriannite from nepheline-hosted inclusions, 1917 eruption nephelinite, sample Ol-7-2000, Oldoinyo Lengai, Tanzania

 

Notes: n - number of analyses. SrO, Nb₂O₅, ZrO₂, P₂O₅ are below detection limits. 1-8 - leucite: 1-6 - Fe-leucite from inclusions; 7-8 - ideal compositions K_0.95Na_0.05Fe_0.3Al_0.7Si₂O₆ and K_0.95Na_0.05Fe_0.25Al_0.75Si₂O₆; 9-11 - tetraferriannite: 9-10 - mica from inclusions; 11 - ideal composition K(Fe²⁺_1.8Mg_0.6Ti_0.3Na_0.3)[Al_0.2Fe³⁺_0.5Si_3.3O₁₀](OH)_0.8F_0.6O_0.6. # - leucite and mica from one inclusion. * - Fe₂O₃, FeO and H₂O are calculated from charge balance for formula based on 11 oxygens, Fe₂O₃ is calculated only as tetrahedral Fe³⁺. Na* - shirokshinite K(Mg,Fe)₂Na[Si₄O₁₀](F,OH)₂, Ti* - hypothetical micas KFe₂Ti[Fe³⁺Si₃O₁₀]O₂ and KFe_2.5Ti_0.5[Si₄O₁₀]O₂, Al* - annite K(Fe,Mg)₃[AlSi₃O₁₀](OH)₂, Fe* - tetraferriannite KFe₃[Fe³⁺Si₃O₁₀](OH)₂. Value in brackets is estimated as Na=2F like in ideal shirokshinite KMg₂Na[Si₄O₁₀]F₂.

 

Thus, the increasing of peralkalinity during evolution of the Oldoinyo Lengai nephelinite melt was responsible for appearance of such Al-undersaturated silicates as Fe-rich leucite, Na-Mg-Ti-rich tetraferriannite and Fe-rich delhayelite.

 

References:

Armbruster T., Richards R.P., Gnos E, Pettke T., Herwegh M. Unusual fibrous sodian tainiolite on phlogopite from marble xenoliths of Mont Saint-Hilaire, Quebec, Canada // Canadian Mineralogist. 2007. Vol. 45. P. 541-549.

Dawson J.B. Peralkaline nephelinite-natrocarbonatite relationships at Oldoinyo Lengai, Tanzania // Journal of Petrology. 1998. Vol. 39. P. 2077–2094

Dawson J.B., Hill P.G. Mineral chemistry of a peralkaline combeite lamprophyllite nephelinite from Oldoinyo Lengai, Tanzania // Mineralogical Magazine. 1998. Vol. 62. P. 179–196.

Pekov I.V., Chukanov N.V., Ferraris G., Ivaldi G., Pushcharovsky D.Yu., Zadov A.E. Shirokshinite, K(NaMg₂)Si₄O₁₀F₂, a new mica with octahedral Na from Khibiny massif, Kola Peninsula: descriptive data and structural disorder // European Journal of Mineralogy. 2003. Vol. 15. P. 447-454.

Sharygin V.V. Fe-rich delhayelite from nephelinite lavas, Oldoinyo Lengai, Tanzania // In: Abstracts of XXVI International conference, School “Geochemistry of alkaline rocks”. Moscow, ONTI GEOKHI RAS. 2009. P. 134-136.

Sharygin V.V., Kamenetsky V.S., Zaitsev A.N., Kamenetsky M.B. Immiscible melt inclusions in nepheline phenocrysts from nephelinite lava, Oldoinyo Lengai: heating experiments and compositional constraints // In: Abstracts of XXVI International conference, School “Geochemistry of alkaline rocks”. Moscow, ONTI GEOKHI RAS. 2009. P. 136-138.