Phenocryst formation is decoupled from this process, and as a result, fractionation of amphibole in arc magmas may be cryptic. Methods. Whole rock chemistry.

Figure 1: Photomicrograph of a plagioclase phenocryst with a melt inclusion. This backscattered electron image shows the microtexture of a Mount St Helens lava-dome sample erupted in September 1981.