Meimechites are highly magnesian alkaline lavas from the Meimecha-Kotuj region of northern Siberia. They contain abundant large phenocrysts of olivine and smaller grains of chromite in a matrix of smaller olivine grains, titanian clinopyroxene, ilmenite, altered glass, and in most but not all cases, biotite. A small proportion of the larger olivine grains have a pronounced parallel parting or show irregular and patchy extinction or kink bands: these are interpreted as xenocrysts. Abundances of MgO range from 8 to 40 wt.%, SiO2 is low (40-43 wt.%) and FeO (to 16 wt.%) and K2O are high (1-3 wt.%). Trace elements concentrations are generally high and strongly fractionated, ranging from primitive mantle values for the heavy rare earth elements to 100 times greater for incompatible elements such as Rb, Nh and La. Isotopic compositions indicate a depleted source: Nd-143/Nd-144 ranges from 0.51262 to 0.51282 (initial epsilon(Nd) values +2.8 to +5.9): inital Sr-87/Sr-86 values are between 0.70299 and 0.70338. Compositions of olivine phenocrysts and xenocrysts vary from Fo(84) to Fog(93) and compositions of smaller matrix grains from Fo(81) to Fo(92). These values indicate that these rocks formed from highly magnesian liquids: maximum MgO contents of these liquids are calculated to have been at least 25 wt.%, and perhaps as high as 29 wt.%. The high levels of incompatible trace elements and the strongly fractionated patterns are explained by very low degree-melting (similar to 1%) of a source with primitive mantle abundances, or low-degree melting (similar to 7%) of an enriched source. The very high MgO contents in such low-degree melts indicates that the site of melting was very deep, as much as 200 km, and either in the lowermost continental lithosphere or in the underlying asthenosphere. The melting probably was linked with the arrival of the mantle plume that was the source of Siberian basaltic flood volcanism. The olivine xenocrysts, however, were most likely picked up by the magma during its passage through the lithosphere. The composition of the meimechites is therefore a consequence of melting under unusual conditions, with contributions from several mantle sources.
