Quantifying the building blocks of igneous rocks : are clustered crystal frameworks the foundation ?

Abstract

Most phenocryst populations in volcanic rocks, and those preserved in shallow-level igneous intrusions, are clustered (variously referred to as clots, clumps or glomerocrysts). These clusters of crystals are the building blocks that accumulate to form the high-porosity, touching crystal frameworks from which igneous cumulates form. Examination of touching crystal frameworks in olivine- (komatiite cumulates and experimental charges) and plagioclase-dominant crystal populations (Holyoke flood basalt, Connecticut, USA) reveal complex, high-porosity, clustered crystal arrangements. Olivine touching frameworks in komatiite flows are interpreted to form in hundreds of days. Plagioclase frameworks are calculated to have formed in less than 17 years for a crystal growth rate of 1 x 10-10 mm/s to less than 3 years for a growth rate of 5 x 10-10 mm/s based on crystal size distributions. The origin of crystal clusters is likely to involve either (or a combination of) heterogeneous nucleation, remobilization of cumulate mushes or crystals sticking together during settling and/or flow. The spatial distribution pattern of clustered crystal frameworks from both natural and experimental examples constrains fields on spatial packing diagrams that allow the identification of touching and non-touching crystal populations, and further improve our understanding of crystal packing arrangements and cluster size distributions.