Chlorine in tertiary basalts from the Hessian Depression in NW Germany

The chlorine concentration has been determined by a chemical method in 7 quartz tholeiites, 19 alkali olivine basalts, 9 basanitic alkali olivine basalts and 11 olivine nephelinites to be on average 80, 280, 720, and 400 ppm Cl respectively. If these basalts are products of decreasing degrees of partial melting of mantle rocks a regular increase of chlorine is to be expected in this sequence. The actual chlorine abundances are a function of partial losses of gases during rock consolidation and optimum stabilities of sodalite group minerals as major chlorine traps in alkalic basalts. The occurrence of sodalite and sodalite nosean solid solutions has been detected by microprobe in 7 out of 10 alkalic basalt species in grains smaller than 70 m. Quantitative analyses of 4 sodalite group minerals from the olivine nephelinites are listed. One contains the sodalite and the nosean molecule in a proportion one to one and must be formed above 1,050 °C according to the experimental results of Tomisaka and Eugster (1968) in the respective system. In the majority of the samples apatite contains less than 20% of the total chlorine of the basalts. Biotite as chlorine containing phase (about 900 ppm Cl) is rare. The proportion of chlorine which could be extracted from rock powders by boiling water is small. No general correlation between the element pairs Cl/S and Cl/K could be observed. Excluding tholeiites a tendency of a reversed correlation between chlorine and potential primary water (as indicated by the Fe₂O₃/FeO ratio) and between chlorine and silica can be derived.