On the behaviour of fluorine and chlorine during magmatic differentiation

The behaviour of fluorine and chlorine during differentiation is considered in the light of halogen data for granites and separated biotites from the Dartmoor granite pluton of S.W. England, hornblende-bearing ultrabasic and basic rocks from a layered intrusion at Rhiw, N. Wales, and recent literature data on halogens in apatites from the Shonkin Sag laccolith and the Skaergaard intrusion. The data indicate that whereas fluorine concentrations are highest in the late stage differentiates, chlorine concentration is highest in the earlier differentiates. It is suggested that this behaviour of the two halogens is due to their relative affinities for silicate melts. Chlorine having a low solubility in silicate melts and a strong affinity for associated aqueous phases will tend to be concentrated in early formed hydrous minerals, leaving fluorine with its strong affinity for the melt to be enriched in the late-formed hydrous minerals.