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, P╩rolles, 1700 Fribourg, Switzerland
2 Swiss Federal Laboratories for Material Testing and Research, Hydrogen and Energy,
°berlandstrasse 129, 8600 Děbendorf, 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: email@example.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 ┼kermanites 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 Alumo┼kermanite (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. Alumo┼kermanite occurs as tabular phenocrysts (up to 1.5 mm) and microphenocrysts in evolved olivine free melilite-nephelinitic ashes and lapilli-tuffs. Alumo┼kermanite has a Mohs' hardness from 4.5 to 6 and is uniaxial (ľ) with ω = 1.635(1) and ε = 1.624-1.626(1). The variation in mineral chemistry ranging from sodium enriched ┼kermanites to high sodium-melilites (alumo┼kermanites) 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 ┼kermanite Ca2MgSi2O7, sodium-aluminium melilite CaNa(AlFe3+)Si2O7, and ferro-┼kermanite 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.
Dawson J.B. Peralkaline nephelinite-natrocarbonatite relationships at Oldoinyo Lengai, Tanzania // Journal of Petrology. 1998. Vol. 39. P. 2077-2094.
Dawson J.B., Pyle D.M., Pinkerton H. Evolution of natrocarbonatite from a wollastonite nephelinite parent: evidence from the June 1993 Eruption of Oldoinyo Lengai, Tanzania // Journal of Geology. 1996. Vol. 104 P. 41-54.
Dawson, J.B., Smith, J.V. and Jones, A.P. A comparative study of bulk rock and mineral chemistry of olivine melilitites and associated rocks from East and South Africa // Neues Jahrbuch Mineralogischer. Abhandlungen. 1985. Vol. 152. P. 143-175.
Donaldson, C.H. and Dawson, J.B. Skeletal crystallization and residual glass compositions in a cellular alkalic pyroxenite nodule from Oldoinyo Lengai // Contribution to Mineralogy and Petrology. 1987. Vol. 67. P. 139-149.
Donaldson, C.H., Dawson, J.B., Kanaris-Sotiriou, R., Batchelor, R.A. and Walsh J.N. The silicate lavas of Oldoinyo Lengai, Tanzania // Neues Jahrbuch fuer Mineralogie. Abhandlungen 1987. Vol. 156. P. 246-279.
Keller, J. and Krafft, M. Effusive natrocarbonatite activity of Oldoinyo Lengai, June 1988 // Bulletin of Volcanology. 1990. Vol. 52. P. 629-645.
Keller, J., Zaitsev, A.N. and Wiedenmann, D. Primary magmas at Oldoinyo Lengai: The role of olivine melilitites // Lithos. 2006. Vol. 91. P. 150ľ172.
Klaudius, J. and Keller, J. Peralkaline silicate lavas at Oldoinyo Lengai, Tanzania // Lithos. 2006. Vol. 91, 173ľ190.
Wiedenmann, D. Vulcanological position and petrological interpretation of biotite-pyroxene-olivine-tuffs from Oldoinyo Lengai, Tanzania // Diploma thesis (in German language), University of Freiburg im Breisgau, 2004, 89 pp.
Wiedenmann, D., Keller, J., Zaitsev, A.N. and Vogt, U. (2009) Occurrence and compositional variation of high-Na-Al melilite group minerals at Oldoinyo Lengai, Tanzania. 2009. (in preparation).
Wiedenmann, D., Zaitsev A.N., Britvin S.N., Krivovichev, S.V., Keller, J. Alumo┼kermanite, (Ca,Na)2(Al,Mg,Fe2+)(Si2O7), a new mineral from the active carbonatite-nephelinite-phonolite volcano Oldoinyo Lengai, northern Tanzania // Mineralogical Magazine, 2009 (submitted).