Natrocarbonatite

The natrocarbonatite lavas of the Oldoinyo Lengai volcano, East African Rift, Tanzania, are composed principally of two water-soluble carbonate minerals, nyerereite (Na,K)2Ca(CO3)2 and gregoryite (Na,Ca,K)2CO3, in a matrix of symplectically intergrown fluorite and (halite-sylvite). Apatite, magnetite, monticellite, cuspidine-niocalite, sellaite, alabandite, sphalerite and galena occur as trace minerals. Subsolidus mineralogical re-equilibration of the lava starts immediately after solidification and cooling, transforming them, under atmospheric influences (air and meteoric water), in a very short time, from dark gray (Fig.4-5) into white secondary products (Fig.6). Immediately after solidification, the natrocarbonatite lavas are cut by a network of thin cracks with hot gases escaping from them. The edges of the cracks become covered by white, brownish and yellowish encrustations up to several centimeters in thickness which are known as "efflorescences" or "salt fringes and tubes" (Fig.7-8-9).

Chemical analysis of the whitish crystalline deposits formed around cracks over fresh flows revealed a composition including approx. 70% thermonatrite (Na2CO3*H2O), 10% aphthitalite (Na,K)2SO4, halite (NaCl) and sylvite (KCl). The deposits are not randomly mixed, individual components crystallize in the order (i) thermonatrite (Na2CO3*H2O) (ii) aphthitalite (Na,K)2SO4, (iii) halite masses, and (iv) halite-sylvite masses. The sequence is attributed to progressive leaching of different components from the deposited lava (causing alteration of the lava in particular adjacent to the cracks), and in particular to the gradually dropping temperature of the gases as the flow cools which influences which materials are leached out and how effectively they crystallize at the surface.

The origin of natrocarbonatite at Oldoinyo Lengai
Natrocarbonatite flows have emerged periodically over the last 4 decades in the summit crater of the Oldoinyo Lengai, accompanied by H2O-CO2 vapor. The currently favored explanation for the genesis of these carbonatites by liquid immiscibility between a silicate and a carbonatite melt is questioned based on the extremely low eruption temperatures of 544-593°C and compositional and mineralogical characteristics not in agreement with experimental constraints. Experimental investigations of the relationship between Oldoinyo Lengai natrocarbonatite and related silicate rock compositions do indicate that alkali-bearing peralkaline carbonatite with liquidus calcite can form by liquid immiscibility. At the same time, these experiments result in evidence which speaks against a liquid immiscibility origin for the highly alkaline and peralkaline Oldoinyo Lengai natrocarbonatite. No natural silicate magma is known to produce natrocarbonatite compositions by liquid immiscibility. The best interpretation of the Oldoinyo Lengai natrocarbonatite involves expulsion of a cognate, mobile, alkaline, and CO2-rich fluid condensate.

The activity at Oldoinyo Lengai can be divided into three main types: (1) dormant stage; (2) carbonatite stage, and (3) Plinian stage. Dormant stages are periods with no activity or only minor degassing, carbonatite stages encompass periods of carbonatite extrusion and enhanced degassin, while Plinian stages refer to silicate dominated eruptions during which the silicate cone of the volcano is constructed.

The dormant stage
During dormant periods (Fig.1) fluid is suggested to separate at depth from cooling low-alkali carbonatite, or carbonated peralkaline silicate melt in the subvolcanic magma chamber of the volcano.
The fluid will react with previously crystallized phases (autometasomatism) and precipitate solids and/or leave droplets of carbonatite liquid behind in the fenitized zone above the magma chamber. This would be due to a decrease in solubility of the "natrocarbonatite component" with falling P and T, and equilibration with the mineral assemblages of the host silicate rocks (evolved nephelinite and phonolite). Volatiles dominated by CO2 and H2O will penetrate and react with the host rocks and be lost. lengai2017(5).jpg

Fig.1: Dormant stage. After Nielsen, T. F (2002).



Carbonate stage
During moderately active silicate magmatism e.g., replenished of the magma chamber, previously deposited natrocarbonatite liquid and solid components could be reactivated to form small melts pools. The natrocarbonatite melt may contain variable amounts of silicate phase as supported by the presence of silicate melt droplets in some natrocarbonatite lavas. The natrocarbonatite liquid accumulates in small pockets, less than 10 m in radius, in the upper part of the volcanic edifice (Fig.2); natrocarbonatite and accompanying vapor are extruded from these little chamber and continued loss of CO2 and H2O vapor transform the fluid into natrocarbonatite melt. The resulting natrocarbonatite liquid will be dry and have strongly enhanced alkali content. lengai2017(3).jpg

Fig.2: Carbonate stage. After Nielsen, T. F (2002).



Plinian stage
The magma chamber is replenished from the feeder system and the residing magma is expelled during violent Plinian eruption (Fig.3). Carbonatite melt in the small pockets or in the conduit may be entrained. After a Plinian eruption the volcano would return to a carbonatite or dormant stage. lengai2017(8).jpg

Fig.3: Plinian stage. After Nielsen, T. F (2002).



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Fig.4: Dark new natrocarbonatite lava flow.



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Fig.5: Dark new natrocarbonatite lava flow on old white-brown lavas.



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Fig.6: Old, altered natrocarbonatite lava flow.



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Fig.7: Salt fringes on a cooling natrocarbonatite lava flow.



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Fig.8: Salt fringes on a cooling natrocarbonatite lava flow.



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Fig.9: Salt fringes on a cooling natrocarbonatite lava flow





Bibliography



• A. Simonetti; K.Bell; C. Shrady (1997): Trace and rare earth element geochemistry of the June 1993 natrocarbonatite la vas, Oldoinyo Lengai (Tanzania): Implications for the origin of carbonatite magmas. Joumal of Volcanology and Geothermal Research 75 (1997) 89-106
• Nielsen, T. F., & Veksler, I. V. (2002). Is natrocarbonatite a cognate fluid condensate?. Contributions to Mineralogy and Petrology, 142(4), 425-435.
• J. Klaudius; J. Keller (2006): Peralkaline silicate lavas at Oldoinyo Lengai, Tanzania. Lithos 91 (2006) 173–190
• A. N. Zaitsev; J. Keller (2006): Mineralogical and chemical transformation of Oldoinyo Lengai natrocarbonatites, Tanzania. Lithos 91 (2006) 191–207
• Roger H. Mitchell (2009): Peralkaline nephelinite–natrocarbonatite immiscibility and carbonatite assimilation at Oldoinyo Lengai, Tanzania. Contrib Mineral Petrol (2009) 158:589–598.

Photo
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 10x (Field of view = 2mm)
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Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 10x (Field of view = 2mm)
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Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 10x (Field of view = 2mm)
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Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite crystals and Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite crystals and Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite crystals and Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite crystals and Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite crystals and Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 2x (Field of view = 7mm)
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Tabular Nyerereite crystals and Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
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Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
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Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)