Natrocarbonatite melt inclusions in apatite from olivine-mica ijolite, Oldoinyo Lengai volcano, Tanzania

Sharygin V.V.¹, Sekisova V.S.^1,2, Zaitsev A.N.³

¹ V.S. Sobolev Institute of Geology and Mineralogy SD RAS, Novosibirsk, Russia

² Novosibirsk State University, Novosibirsk, Russia

³ St. Petersburg State University, St. Petersburg, Russia

 

Xenoliths of plutonic rocks (ijolite, jacupirangite, nepheline syenite, etc.) in the Oldoinyo Lengai pyroclastics and lavas are considered to be cumulates forming in an intermediate chamber from a parental olivine nephelinite magma. We studied in detail melt inclusion in minerals of olivine-mica-containing ijolite (nepheline, Ti-magnetite, forsterite, diopside, fluorapatite). Here we present data for melt inclusions in fluorapatite, which have natrocarbonatite composition. They essentially differ from nepheline-hosted melt inclusions showing silicate-carbonate immiscibility.

These melt inclusions (size 5-20 µm) are commonly confined to the central zones of the host apatite. Their phase composition is fine-grained aggregate + gas bubble ± daughter crystals. Heating experiments with inclusions indicated the following events: (1) fine-grained aggregate melted at 560^oС; (2) formation of solid phases or liquids near 600^oC; (3) homogenization  at near 850, 930-960 and 1050-1080^oC. The very broad interval for homogenization suggests that discrete inclusions have variable phase and chemical composition. According to scanning microscopy (EDS, elemental maps) Na-Ca-carbonates (nyerereite, rarely gregoryite) are dominant phases in the apatite-hosted inclusions. Other phases are minor and represented by chlorides (halite, sylvite), fluorides (fluorite, villiaumite, neighborite), sulphates (mainly thenardite, baryte), sulphides (pyrrhotite, alabandite), Fe-rich spinel-group minerals, Mg-Fe-carbonates and witherite. Silicate phases are scarce and seem to be haüyne-sodalite, melilite (alumoakermanite ?), Ba-Ti-mica and cuspidine-group minerals. All above mentioned minerals are common of diverse types of natrocarbonatites at Oldoinyo Lengai. The EDS analisis (wide area, 10x10 µm) of one fine-grained inclusion in apatite gave following result  (in wt%): Na₂O – 21.4; K₂O - 8.2; CaO - 20.5; MgO – 1.7; FeO – 3.2; MnO – 0.4; BaO – 1.3; SrO – 1.4; SiO₂ – 2.2; SO₃ – 1.3; P₂O₅ – 0.4; F – 3.9; Cl – 0.9; Total – 66.8; that is very close to average composition of the Lengai natrocarbonatite (Keller, Zaitsev, 2012).

This work is supported by RFBR (grant 11-05-00875).