Meimechites: highly magnesian lithosphere-contaminated alkaline magmas from deep subcontinental mantle

Abstract

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.%, SiO 2 is low (40–43 wt.%) and FeO (to 16 wt.%) and K 2O 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, Nb and La. Isotopic compositions indicate a depleted source: 143Nd/ 144Nd ranges from 0.51262 to 0.51282 (initial ϵ Nd values + 2.8 to + 5.9): initial 87Sr/ 86Sr values are between 0.70299 and 0.70338. Compositions of olivine phenocrysts and xenocrysts vary from Fo 84 to Fo 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 (∼ 1%) of a source with primitive mantle abundances, or low-degree melting (∼ 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.