TRACE-ELEMENT AND RARE-EARTH-ELEMENT GEOCHEMISTRY OF THE JUNE 1993 NATROCARBONATITE LAVAS, OLDOINYO-LENGAI (TANZANIA) - IMPLICATIONS FOR THE ORIGIN OF CARBONATITE MAGMAS

Major-, trace- and rare-earth-element data from ten natrocarbonatite l avas collected during the June 1993 extrusive activity define two dist inct groups. Although both groups are characterized by low Nb and Zr c ontents, and low Th/U (similar to 1.0); Ba/Sr > 1.0; (La/Sm)(N) > 40; high Ba, Mo, Sr, W contents; and LREE contents similar to 1000 to 2000 x chondrite, one group has much higher Al2O3, Fe2O3, Nb, Pb, SiO2, Zr and total REEs contents. These differences are attributed to the pres ence of silicate spheroids in natrocarbonatites that form within the l atter group. Similarity in trace- and rare-earth-element-normalized pa tterns for both groups of natrocarbonatite lavas suggest either a comm on source or generation from a common parental melt. Models proposed f or the origin of natrocarbonatites include immiscible separation from a peralkaline, silicate magma, or late-stage fractionation from a pare nt olivine sovite magma. Although natrocarbonatite melt formation may be controlled by either of these differentiation processes, certain tr ace-element ratios for the 1993 lavas, such as Ce/Pb (similar to 9), a nd Th/Nb (similar to 0.1) are similar to those estimated for primitive mantle, and their Sm/Nd ratios (similar to 0.07) are quite different to the average value of 0.15 for most carbonatites world-wide. The sim ilarity in element ratios in many of the older silicate lavas at Oldoi nyo Lengai (e.g., Zr/Nb, La/Nb, Ba/Nb, Rb/Nb, and Ba/La) to those esti mated for HIMU and EM I suggest that source characteristics can be ref lected in such melts. Even if the natrocarbonatites are formed by liqu id immiscibility, recent experiments have shown that partition coeffic ients for trace elements (e.g., Ba, Ce, Mo, Nb, Pb, Th, U) between con jugate carbonate and silicate melts approach unity with increasing tem perature. Alternatively, the similarity in trace-element ratios betwee n those for the silicate lavas from Oldoinyo Lengai and mantle compone nts are simply fortuitous.