Lithophile trace element data bearing on the origin of peridotite xenoliths, ankaramite and carbonatite from Lashaine volcano, N. Tanzania

Abstract

Rare-earth elements (REE) Rb, Sr and Ba have been determined in three garnet peridotites and a spinel peridotite that occur as xenoliths in ankaramite at Lashaine volcano. These element data have also been determined for the ankaramite and a closely associated carbonatite. Three peridotites have light REE-fractionated patterns, variable Rb, Sr and Sr 87 Sr 86 values which are consistent with mixing of peridotite with ankaramite. However, bulk contamination by either ankaramite or carbonatite is inconsistent with other major and trace element data. Remaining alternatives are selective elemental transfer between xenolitha and host or contamination within the mantle prior to incorporation within the ankaramite/carbonatite magma. The ankaramite has a highly fractionated REE pattern but is less enriched in lithophile trace elements than many alkalic rocks. The carbonatite has a fractionated pattern, but is significantly depleted in large ion lithophile elements compared to most other igneous carbonatites. Single-stage melting of mantle material is unlikely to have produced the carbonatite, and derivation of the carbonatite from a carbonated ankaramite magma requires substantial fractionation of some lithophile trace elements into the carbonatite and others into the ankaramite.