Cl and F contents of apatite in the Matsumae plutonic rocks, southwestern Hokkaido, Japan. A useful indicator of vapor saturation.

Chlorine and fluorine contents of apatite in the Matsumae plutonic rocks were analyzed with electron microprobe. The Matsumae plutonic rocks are divided into Am and PI series in terms of crystallization sequence and mineralogy. The petrological evidence suggests that the Am series rocks have crystallized under the conditions richer in water than the PI series. Chlorine content of apatite in the Am series is generally low, whereas that in the PI series is rather high in less differentiated rocks and abruptly decreases at about D. I.=45. On the other hand, fluorine content of apatite in both series slightly increases as differentiation proceeds. Such behavior of Cl and F in apatite can be explained in terms of the degassing process; when a magma is saturated with vapor and exsolves free vapor phase, Cl is more strongly partitioned into the vapor relative to F and removed from the magma. Therefore, judging from low Cl content of apatite, the Am series magma may have been saturated with vapor in earlier stage than the PI series one. This is consistent with the previous suggestion that the Am series magma have crystallized under the conditions richer in water than the PI series. Such abrupt decrease in Cl without decrease in F may be a useful indicator of vapor exsolution or vapor saturation of magma. Recognition of vapor saturation gives valuable information on water content of magma.