Alkalis in carbonatite magmas: New evidence from melt inclusions

Concentrations of Na and K are analyzed in carbonatite magmas that give ris e to the carbonatites of deep-seated alkaline-ultramafic complexes and are usually low in alkalis, Based on evidence offered by experimental petrology , distinctive mineralogical features of fenites around carbonatite stocks, and the results obtained by studying fluid inclusions, a hypothesis is prop osed of originally high Na2O and K2O concentrations of the parental carbona tite magmas of various geneses (partial mantle melts and the derivatives of crystallization or liquid fractionation). This concept is confirmed by mic roprobe analyses of alkalis in homogenized melt inclusions in apatite, fors terite, and dolomite in carbonatites of various stages of the Kovdor, Gull, and Magan complexes. The sum of alkalis is demonstrated to increase late d uring the carbonatite process, with the K2O concentration simultaneously in creasing with respect to Na2O.