Dendritic Growth Controls Forsterite–Phosphorus Coupling and Decoupling in Euhedral Olivine: A Straightforward Indicator for Growth-Dominated Zoning

Abstract Olivine compositional zoning is a widely used proxy for deciphering magmatic processes, but the preservation of primary dendritic growth signatures in fast-diffusing Fe–Mg components and their genetic relationship with slow-diffusing phosphorus remain poorly constrained, especially in naturally occurring euhedral olivines. This study presents integrated electron probe microanalysis and quantitative X-ray mapping of a well-preserved euhedral olivine phenocryst from the Cenozoic Datong alkaline basalts in the North China Craton, which exhibits distinct dendritic zoning patterns in both Fo (Mg/(Mg+Fe)) and phosphorus. Our results demonstrate that fast-diffusing Fe–Mg compositions can retain primary growth records under sufficiently short magma residence times, and the olivine exhibits a hopper-shaped high-Fo core (77–78 mol%) that is spatially coincident with P-enriched domains, along with recognizable M-shaped Fo zoning (low-Fo core, high-Fo intermediate zone, and outer low-Fo rim) along rim-core-rim profiles. Three dominant Fo–P covariation patterns are identified: (1) positive coupling generated by early rapid growth along trans-crystallographic secondary branches with synchronous Fo and P enrichment; (2) antithetic decoupling on {110} facets linked to P-rich secondary dendritic growth; (3) uncorrelated signatures marked by oscillatory P zoning and progressive core-to-rim Fo depletion. These patterns are governed by the interplay of growth rate gradients, crystallization sequence, and dynamic evolution of compositional boundary layers during the transition from dendritic to euhedral growth. Diffusion timescale modeling using the DIPRA software yields a maximum magma residence time of only 2–5 days, which effectively prevented diffusive homogenization and preserved fine-scale growth-related zoning, while long-duration simulation indicates that complex primary Fo zoning and the coupling relationship between Fo and slow-diffusing phosphorus would be eliminated after 60–230 days of magmatic residence, leaving only conventional concentric zoning. This study confirms that M-shaped Fo zoning can form as a direct product of crystallization kinetics without invoking magma recharge, and the Fo–P coupling/decoupling system established here provides a straightforward and robust indicator to distinguish growth-dominated zoning from diffusion-modified zoning in natural olivines, reconciling apparent discrepancies between natural observations and laboratory experiments and highlighting that the co-preservation of Fe–Mg and P dendritic zoning records extremely short magma residence times and captures unmodified snapshots of crystal growth kinetics in basaltic magmatic systems.