Natrocarbonatite

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Natrocarbonatite is an extremely rare igneous rock that is counted among the carbonatites .

etymology

The Ol Doinyo Lengai during the 1966 eruption

The rock name natrocarbonatite, also known as lengaiite , is made up of the prefix natro- and carbonatite . Sodium, derived from soda , is used to specify the predominance of sodium in the rock. Carbonatite in turn goes back to carbonate , which comes from the Latin root carbo meaning coal . Lengaiit refers to Ngai , the god of the Maasai .

Discovery and first description

Natrocarbonatite was discovered by Dawson in 1960 and first described in 1962. The rock name was coined in 1963 by Du Bois and colleagues.

Occurrence

Hornito with natrocarbonatite lava flow in the crater of Ol Doinyo Lengai

So far, natrocarbonatite is only known as volcanic rock - as lava from the type locality of the Ol Doinyo Lengai volcano in Tanzania and from a tephra layer on the Kerimasi volcano , also in Tanzania.

Rock specimen with sodalite-burbankite galena from Cerro Sapo in Bolivia

Transitions of type sodalite - Ankerit - barite in Cerro Sapo complex in the alkaline rocks province of Ayopaya in Bolivia are now seen by Schulz and colleagues (2004) as intrusive equivalents of Natrokarbonatitlaven of Ol Doinyo Lengai.

Physical education

Natrocarbonatite laves emerge with a very low viscosity at temperatures between 540 and 593 ° C - compared to basalts , which have an outlet temperature of over 1100 ° C, these are extremely low temperatures. As a result, the emitted lavas, which are reminiscent of mud flows, do not glow during the day and only show a dull reddish glow even at night.

mineralogy

Like all carbonatites, natrocarbonatite also consists of more than 50 percent by weight of carbonates . In the case of natrocarbonatite, these are the carbonates of the elements sodium , potassium and calcium , in particular the minerals Nyerereit (Na, K) 2 Ca [CO 3 ] 2 and Gregoryite (Na 2 , K 2 , Ca) [CO 3 ].

Chemical composition

The following analyzes are intended to clarify the chemical composition of the natrocarbonatite in a study by Le Bas (1981) and on the basis of two different eruptions on Ol Doinyo Lengai:

Oxide
wt.%		 Le Bas (1981) Eruption in 1988 1993 eruption
SiO 2 0.58 0.19 3.07
TiO 2 0.10 0.01 0.10
Al 2 O 3 0.10 0.10 0.85
FeO 0.29 0.32 1.19
MnO 0.14 0.42 0.32
MgO 1.17 0.28 0.38
CaO 15.54 13.81 14.80
Na 2 O 29.56 32.18 27.90
K 2 O 7.14 8.11 5.81
P 2 O 5 0.95 0.86 1.05
CO 2 31.72 34.70 30.40
SO 3 2.48 3.17 2.55
F. 2.26 2.20 1.64
Cl 2.90 2.50 2.54
BaO 1.04 1.39 0.85
SrO 2.09 1.69 1.15

Up to two thirds of the rock is taken up by Na 2 O and CO 2 alone ; the proportions of CaO and K 2 O are also important . Compared to 1988, the 1993 eruption was richer in silicates .

Petrography

Thin section of a natrocarbonatite with rounded gregoryite porphyroclasts

The structure of the natrocarbonatite lava at Ol Doinyo Lengai is porphyry . The Phenocrysts from squat-prismatic Gregoryit and tabular Nyerereite are small (less than 1 millimeter) and are floating in a fine crystalline matrix of the same minerals plus fluorite (CaF 2 ), nahcolite (NaHCO 3 ) and pyrrhotite . The base mass was initially solidified like glass .

Emergence

The petrogenesis of natrocarbonatitic magmas is still highly controversial as two different models collide:

  • Immiscibility of silicate and carbonate melts. In this model, that natrokarbonatische magmas is assumed from stresses peralkaline Nephelinitmagmen ( combeite have -Nepheliniten) as they also occur on Ol Doinyo Lengai unmixed. In 1989, Hamilton and colleagues showed that natrocarbonatites can even coexist with silicate melts at the pressures and igneous temperatures prevailing in the earth's crust .
  • Squeezing out a mobile, alkaline, CO 2 -rich liquid condensate.

A possible crystal fractionation from Sövit (calcite carbonatite) is not considered to be feasible.

Individual evidence

  1. ^ JB Dawson: Sodium carbonate lavas from Oldoinyo Lengai, Tanganyika . In: Nature . tape 195 , 1962, pp. 1075-1076 .
  2. ↑ CGB Du Bois u. a .: Fresh natro carbonatite from Oldoinyo L'engai . In: Nature . tape 197 . London 1963, p. 445-446 .
  3. ↑ RL Hay: Natrocarbonatite tephra of Kerimasi volcano, Tanzania . In: Geology . 1983.
  4. ↑ Schulz, F. u. a .: Carbonatite diversity in the Central Andes: the Ayopaya alkaline province, Bolivia . In: Contributions to Mineralogy and Petrology . tape 148 , 2004, p. 391-408 .
  5. ^ MJ Le Bas: Carbonatite Magmas . In: Mineralogical Magazine . tape 44 , 1981, pp. 133-140 .
  6. ↑ JB Dawson et al. a .: Carbonatite volcanism. Oldoinyo Lengai and petrogenesis of natrocarbonatites. Petrology and geochemistry of Oldoinyo Lengai Lava extruded in November 1988: Magma source, ascent and crystallization . Ed .: K. Bell, J. Keller. Springer, Berlin, Heidelberg, New York 1995, pp. 47-69 .
  7. ↑ CM Petitbon u. a .: Phase relationships of a silicate-bearing natrocarbonatite from Oldoinyo Lengai at 20 and 100 MPa . In: Journal of Petrology . tape 39 , 1998, pp. 2137-2151 .
  8. ^ W. Wimmenauer: Petrography of igneous and metamorphic rocks . Ferdinand Enke Verlag, Stuttgart 1985, ISBN 3-432-94671-6 .
  9. ↑ TD Peterson: Petrology and Genesis of natrocarbonatite . In: Contributions to Mineralogy and Petrology . tape 105 , 1990, pp. 143-155 .
  10. ↑ DL Hamilton et al. a .: The behavior of trace elements in the evolution of carbonatites . Ed .: Bell, K., Carbonatites: Genesis and Evolution. Unwin Hyman, London 1989, pp. 405-427 .
  11. ↑ TFD Nielsen, IV Vekser: Is natrocarbonatite a cognate fluid? In: Contributions to Mineralogy and Petrology . tape 142 , 2002, p. 425-435 .