Occurrence and compositional variation of high-Na-Al melilites at Oldoinyo Lengai, Tanzania
Wiedenmann, D., 1,2*, Keller, J. 3, Zaitsev, A.N. 4
1 Department of Geoscience, University of Fribourg, PÊrolles, 1700 Fribourg, Switzerland
2 Swiss Federal Laboratories for Material Testing and Research, Hydrogen and Energy,
øberlandstrasse 129, 8600 DØbendorf, Switzerland
3 Institute of Geoscience, University of Freiburg, Albertstrasse 23b, 79104 Freiburg, Germany
4 Department of Mineralogy, St. Petersburg State University, University Emb. 7/9, St. Petersburg 199034, Russia
* Corresponding author Tel.: +41 44 823 48 62; fax: +41 44 823 40 22; Email address: daniel.wiedenmann@gmail.com
Oldoinyo Lengai is the only active carbonatite volcano in the world and is located in the Gregory Rift Valley, the northern Tanzanian sector of the East African Rift System. The volcano is famous for its unique low-temperature and low-viscosity natrocarbonatites, however silicate lavas and pyroclastics dominantly build up the 3000 m high cone (phonolites and nephelinites). Detailed fieldwork at Oldoinyo Lengai and the volcanic centres around showed melilite as a widespread mineral in silicate lavas of the province. Extraordinary Na-rich melilites (> 6 wt.% Na2O; > 9 wt.% Al2O3) from younger explosive eruptions range among the most Na-rich Åkermanites ever reported. Their unusual mineral chemistry leads to unusual optical properties with vivid birefringence colours of the 2nd and 3rd order under crossed nicols. In this context, a new mineral named AlumoÅkermanite (Ca,Na)2(Al,Mg,Fe2+)(Si2O7), was described and accepted by IMA in December 2008. The mineral (space group P`421m) has straight extinction, positive elongation, and is nonpleochroic. AlumoÅkermanite occurs as tabular phenocrysts (up to 1.5 mm) and microphenocrysts in evolved olivine free melilite-nephelinitic ashes and lapilli-tuffs. AlumoÅkermanite has a Mohs' hardness from 4.5 to 6 and is uniaxial (–) with ω = 1.635(1) and ε = 1.624-1.626(1). The variation in mineral chemistry ranging from sodium enriched Åkermanites to high sodium-melilites (alumoÅkermanites) reflects the peralkaline evolution trend from primitive olivine melilitites to highly evolved, melilite-bearing combeite-wollastonite nephelinites. Melilite compositions extend the magmatic field in the Ca2FeSi2O7 - Ca2MgSi2O7 - CaNa(Al,Fe3+)Si2O7 endmember triangle (figure 1).
Fig. 1: Classification of Oldoinyo Lengai melilites in terms of Åkermanite Ca2MgSi2O7, sodium-aluminium melilite CaNa(AlFe3+)Si2O7, and ferro-Åkermanite Ca2FeSi2O7. Filled circles: melilite data of this study. Squares: publishes Lengai data.
This work was supported by Deutsche Forschungsgemeinschaft (grant Ke 136/40) and Alexander von Humboldt Stiftung.
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