Quartz crystal size distribution of the Ordovician Millbrig K-bentonite

Crystal size distribution (CSD) analysis has been applied to quartz crystals of the Ordovician Millbrig K-bentonite, which represents one of the largest known fallout ash deposits in the Phanerozoic Era, to establish crystal growth histories and conditions in the magma chamber prior to eruption. Specific CSDs were examined for crystal growth conditions. All Millbrig samples exhibit lognormal concave-down shapes in agreement with previously reported CSDs on large silicic systems but in contrast to more mafic systems characterized by linear CSDs. Concave-down CSDs are possibly an intrinsic property of phenocryst growth in large volume silicic magmas as documented in the literature [Bindeman, I.N., 2003. Crystal sizes in evolving silicic magma chambers. Geology, 31: 367–370.]. Quartz crystals of the Millbrig must have been produced by surface-controlled, size-dependent growth with a minor Ostwald ripening component, which could have easily changed to supply-controlled growth due to a limited supply of nutrients, as the crystals grew larger. CSDs were also used to examine the possible origin of multiple ash beds in the Millbrig. These ash beds appear to be a product of a series of separate eruptions that represent separate magma layers or batches, each with slightly different crystal growth conditions.