CARBONATITE METASOMATISM IN THE NORTHERN TANZANIAN MANTLE - PETROGRAPHIC AND GEOCHEMICAL CHARACTERISTICS

Peridotite xenoliths from the Olmani cinder cone, northern Tanzania, possess distinctive mineralogical and chemical features interpreted to result from interaction of ultra-refractory peridotite residues with carbonatite melts. Chief among these are as follows: (1) The presence of unusually low Al2O3 clinopyroxene and the lack enstatite in refractory dunites (with olivines up to Fo94). (2) The presence of monazite and F-rich apatite in refractory harzburgite and wehrlite xenoliths, respectively. (3) LREE enrichment and strong Ti depletion relative to Eu in all but one peridotite. Ca/Al ratios of the clinopyroxene-bearing dunites are some of the highest ever measured for peridotites (up to 10.8, relative to chondritic Ca/Al of 1.1). In addition, Zr/Hf and Ca/Sc ratios correlate positively, increasing with greater influence of carbonatite on the whole rock. (4) Clinopyroxene-bearing samples have a restricted range of isotopic compositions (epsilon(Nd) = +3.1 to +3.9, Sr-87/Sr-86 = 0.7034 to 0.7035), whereas the monazite-bearing harzburgite has lower epsilon(Nd) (+0.8) at similar Sr-87/Sr-86. The isotopic compositions of the former are similar to young, isotopically primitive east African carbonatites and basalts, suggesting the metasomatism occurred recently and that the carbonatites responsible for the metasomatism were ultimately derived from the asthenosphere. Inferred trace element signatures of carbonatite melts responsible for modal metasomatism of these and other peridotites include: high La/Yb, Nb/La and Ca/Al, very high Zr/Hf and very low Ti/Eu; features similar to those of erupted carbonatites and consistent with partition coefficients for silicates in equilibrium with carbonatite melts. These trace element systematics are used to illustrate that many LREE-enriched spinel peridotite xenoliths may have been affected by addition of very small amounts (less-than-or-equal-to 2%) of carbonatite. Such metasomatism will have small effects on major element compositions of peridotites (e.g., Mg#, Ca/Al) relative to their trace element compositions. Whereas most LREE-enriched peridotites from the literature can be modeled as mixtures between carbonatite and refractory peridotite, the modally metasomatized Olmani peridotites and SE Australian wehrlites require an open-system style of metasomatism, perhaps due to higher proportions of carbonatite melt.