Crystal growth of clinopyroxene in mafic alkaline magmas

Recent experimental and natural studies have shown that igneous crystals can start to crystallize through skeletal or dendritic growth, and then backfill progressively to form polyhedral crystal morphologies, in contrast to the traditional tree-ring model of crystal growth. Identifying the growth mechanism of crystal is fundamental to understand crystal growth and associated magmatic processes. Clinopyroxene is an ubiquitous phase in magmatic systems. However, crystallographically controlled zoning patterns (e.g., sector zoning) observed in different studies are diverse or even show opposite trends, which make it challenging to understand the crystal growth mechanisms of clinopyroxene. This study presents a new finding of the contemporary occurrence of both clinopyroxenes with Al-rich and -poor {hk0} sectors in the Sailipu alkaline volcanic field in southern Tibet. Based on detailed textural and compositional analyses of a large number of clinopyroxene grains, we suggest a new crystal growth mechanism for clinopyroxene in mafic alkaline magmas. At low degrees of undercooling, all crystal faces grow concurrently, producing equilibrium Si-Mg-rich {-111} sectors and disequilibrium Al-Ti-rich {hk0} sectors; at high degrees of undercooling, the equilibrium sectors {hk0} initially form, which is followed by backfill of disequilibrium Al-rich {-111} sectors. Al2O3 is a useful indicator of equilibrium, and Al2O3-poor sectors, rather than specific crystal faces, are closer to equilibrium. The behavior of other cations is complex and the distribution of their oxide components is variable. Textural observations and the modeling results for P transfer across the crystal-melt diffusive boundary layer show that apatite inclusions in clinopyroxene can indicate rapid crystal growth. These results may contribute to a better understanding of magma storage, ascent and eruption on the basis of clinopyroxene. (C) 2021 Elsevier B.V. All rights reserved.