Crystallization of apatite in natural magmas under high pressure,hydrous conditions,with particular reference to ‘Orogenic’ rock series

Two separate series of hydrous experiments involving (1) imposing apatite saturation on a series of igneous rock compositions from basanite to rhyolite, and (2) crystallizing similar natural rock compositions progressively until apatite appears, demonstrate a close dependence between apatite saturation and silica content of the magma, and determine P₂O₅ levels at a given silica value and temperature at which that composition may be expected to crystallize apatite. The effect of pressure on apatite solubility is not great, and is most significant for silicic compositions.P₂O₅ vs SiO₂ relationships of the low-K island arc suite, calcalkaline suite and high-K calc-alkaline suite, appear regular and characteristic for each suite, and when linked with the experimental work on apatite solubility, indicate the following: (1) the low-K and calc-alkaline series have low P₂O₅ contents (0.1–0.2 wt.%) and relatively flat P₂O₅-SiO₂ patterns; they do not show evidence of reaching apatite saturation until rhyodacite-rhyolite compositions are obtained for the low-K suite, and andesite-dacite compositions for the calc-alkaline suite; (2) the high-K calc-alkaline series has higher P₂O₅ contents (0.4–0.6 wt.%) in mafic compositions, and achieves apatite saturation over a wide compositional range for the series; (3) the calc-alkaline and high-K calc-alkaline series are probably lower temperature, and more hydrous than the low-K series; (4) anomalous P₂O₅-SiO₂ distributions may indicate non-equilibrium crystallization of apatite, magma-mixing and crystal accumulation processes active in generation of the orogenic volcanic series.