Toward an understanding of disequilibrium dihedral angles in mafic rocks

The median dihedral angle at clinopyroxene‐plagioclase‐plagioclase junctions in mafic rocks, Θ cpp , is generally lower than equilibrium (109° ± 2°). Observation of a wide range of mafic bodies demonstrates that previous work on systematic variations of Θ cpp is incorrect in several important respects. First, the spatial distribution of plagioclase compositional zoning demonstrates that the final geometry of three‐grain junctions, and hence Θ cpp , is formed during solidification (the igneous process): sub‐solidus textural modification in most dolerites and gabbros, previously thought to be the dominant control on Θ cpp , is insignificant. Θ cpp is governed by mass transport constraints, the inhibiting effects of small pore size on crystallization, and variation in relative growth rates of pyroxene and plagioclase. During rapid cooling, pyroxene preferentially fills wider pores while the narrower pores remain melt‐filled, resulting in an initial value of Θ cpp of 78°, rather than 60° which would be expected if all melt‐filled pores were filled with pyroxene. Lower cooling rates create a higher initial Θ cpp due to changes in relative growth rates of the two minerals at the nascent three‐grain junction. Low Θ cpp (associated with cuspate clinopyroxene grains at triple junctions) can also be diagnostic of infiltration of previously melt‐free rocks by late‐stage evolved liquids (the metasomatic process). Modification of Θ cpp by sub‐solidus textural equilibration (the metamorphic process) is only important for fine‐grained mafic rocks such as chilled margins and intraplutonic chill zones. In coarse‐grained gabbros from shallow crustal intrusions the metamorphic process occurs only in the centers of oikocrysts, associated with rounding of chadacrysts.