Abstract Natrocarbonatite magma, erupted as lava flows in the Tanzanian volcano Oldoinyo Lengai in June and November of 1988, has evolved chemically since its formation. The June and November flows of 1988 display increasing Cl, F, Ba, K, Mg and Mn, concomitantly with Na, Ca and P depletion. Furthermore, the June magma, at the time of eruption, had higher Cl, F, Ba and K contents and lower Ca than the November magma and evolved to higher levels of Cl, F, Ba and K content and lower Ca, Na and P. The mineralogy of the lavas reflects these trends. Crystallization of fluorite and halite–sylvite solid solution, usually as a symplectic intergrowth, occurs when Cl and F concentrations reach the critical value necessary to stabilize both minerals and explains why neither occurs as a phenocryst phase. Natrocarbonatite magma has undergone considerable and rapid magmatic evolution, probably in small and separate magma chambers. Two minerals, nyerereite and gregoryite, have dominated the crystallization history of natrocarbonatite magma, and many lavas are phenocryst-rich. However, because most of the lavas are composed principally of these two minerals, crystal accumulation has not greatly changed their composition and, consequently, we suggest that the bulk composition of the lavas closely approximates that of the parental magma.
K. Bell, J. Keller (eds) 1995. Carbonatite Volcanism. Oldoinyo Lengai and the Petrogenesis of Natrocarbonatites. IAVCEI Proceedings in Volcanology Series Volume 4. xi + 210 pp. Berlin, Heidelberg, New York, Barcelona, Budapest, Hong Kong, London, Milan, Paris, Tokyo: Springer-Verlag. Price DM 178.00, Ös 1338.40, SFr 168.00 (hard covers). ISBN 3 540 58299 1. - Volume 133 Issue 5
The Prinsen of Wales Bjerge (PWB), part of the Tertiary volcanic province of East Greenland, consists of tholeiitic basalts overlain by alkalic basalts that were erupted 100–150 km west of the original axis of continental rifting and active ocean-floor development during the creation of the North Atlantic Ocean. They have many features of continental flood basalts but are somewhat enriched in Fe and in Ti relative to Fe and have slightly lower Al 2 O 3 . They have slight enrichments in the light rare-earth elements (La/Yb = 3–4). A nunatak within the PWB displays four cycles of tholeiitic basalt, each about 50 m thick, which are defined by trace-element variations (Ni, Cr, Sr, Zr, and Zr/Y). In three of the four cycles the lowermost flows are the most highly differentiated, and successive flows are increasingly primitive. These changes are thought to be the result of frequent injection of primitive, mantle-derived tholeiitic magma into small crustal magma chambers that contain evolved tholeiitic magma. The resultant mixing and expulsion of hybrid magma produce flows of small volume (0.01–0.03 km 3 ) that display increasingly primitive character upward within each cycle (increasing Mg# and decreasing content of incompatible elements). This process is expected to be more efficient in small reservoirs than in the very large magma chambers that have been invoked by previous exponents of the differentiation–replenishment hypothesis. We suggest that cyclical volcanism in areas well back from the line of active rifting may be more common than is realized and is controlled by the fractionation–magma-replenishment process operating in numerous small reservoirs in an extensively fractured continental crust.
Abstract Calcite-rich carbonatites are commonly attributed to calcitization of alkalic carbonatite of Oldoinyo Lengai type. The interpretation arises from the presumption that magmatic crystallization of calcite at atmospheric pressure is not possible. We show that only a small percentage of fluorine, a common element in carbonatite magmas, permits such crystallization, and we argue that most of the calcite in extrusive carbonatites is magmatic. The presence of any more than minor apatite precludes an alkalic carbonatite parentage. While not denying that calcification of alkalic carbonatite can occur, we suggest that it is not generally responsible for the formation of extrusive calcific carbonatites.
Abstract Several K-Fe sulphide minerals have been described from natrocarbonatite lavas erupted from the Tanzanian volcano Oldoinyo Lengai but uncertainty remains about their exact identity. They do not appear to be any of the established K-Fe sulphides according to Dawson et al. (1995) and Mitchell (1997). Here, we describe yet another variant which is a Mn- and F-bearing variety of rasvumite. It appears to have formed by reaction of Fe alabandite grains with natrocarbonatite magma and occurs as minute rims on the alabandite. Its formula is: (K 0.94 Na 0.06 )(Fe 1.91 Mn 0.13 Ca 0.01 Sr 0.01 )(S 2.06 F 0.24 Cl 0.04 ).
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='rotate(72 0 0)'/%3e%3cuse xlink:href='%23a' transform='rotate(-72 0 0)'/%3e%3cuse xlink:href='%23a' transform='scale(-1 1) rotate(72)'/%3e%3c/g%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h1200v600H0z'/%3e%3cpath stroke='%23FFF' stroke-width='60' d='m0 0 600 300M0 300 600 0' clip-path='url(%23b)'/%3e%3cpath stroke='%23C8102E' stroke-width='40' d='m0 0 600 300M0 300 600 0' clip-path='url(%23c)'/%3e%3cpath stroke='%23FFF' stroke-width='100' d='M300 0v300M0 150h600' clip-path='url(%23b)'/%3e%3cpath stroke='%23C8102E' stroke-width='60' d='M300 0v300M0 150h600' clip-path='url(%23b)'/%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='matrix(45.4 0 0 45.4 900 120)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='matrix(30 0 0 30 900 120)'/%3e%3cg transform='rotate(82 900 240)'%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='rotate(-82 519.022 -457.666) scale(40.4)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='rotate(-82 519.022 -457.666) scale(25)'/%3e%3c/g%3e%3cg transform='rotate(82 900 240)'%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='rotate(-82 668.57 -327.666) scale(45.4)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='rotate(-82 668.57 -327.666) scale(30)'/%3e%3c/g%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='matrix(50.4 0 0 50.4 900 480)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='matrix(35 0 0 35 900 480)'/%3e%3c/svg%3e)