Complex Zoning Patterns of Skeletal Olivine in Yangyuan Alkaline Basalt: Implications for Diffusion Chronometry and Crystallization Dynamics

Olivine is a pivotal mineral for decoding magmatic processes; however, its compositional and textural complexity—particularly in disequilibrium settings—complicates the interpretation of magma histories. A critical unresolved challenge is distinguishing the respective contributions of rapid crystallization and intracrystalline diffusion to olivine zoning. To address this, this study focuses on a representative skeletal olivine phenocryst from the Yangyuan alkaline basalt in North China, selected for its favorable orientation and size. We integrate whole-rock geochemical analyses, high-resolution mineral chemistry techniques (including electron probe microanalysis and X-ray element mapping), and quantitative intracrystalline diffusion simulations to unravel the mechanistic drivers underlying its zoning patterns. Our results demonstrate that the compositional zoning of the skeletal olivine arises predominantly from disequilibrium growth initiated from an equilibrium core, rather than post-formation diffusional modification or dissolution-reprecipitation, supported by key lines of evidence: a phosphorus-enriched compositional boundary layer (CBL) surrounding the phenocrysts, with elevated P2O5 in glass adjacent to embayed edges reflecting kinetic partitioning during rapid growth; morphological transitions from concentric zoning to skeletal textures, mediated by phosphorus-rich bands that preserve primary growth architectures; normal zoning in forsterite (Fo) and NiO, where narrower bands at concave edges align with skeletal morphology rather than diffusion anisotropy, while diffusion simulations reveal inconsistent timescales across elements (Fe-Mg, Ni, Mn, Ca) and crystallographic directions, ruling out diffusion as a dominant process; oscillatory zoning in Al2O3 and P2O5; and distinct anisotropic relationships where high Fo and NiO contents correspond to depleted P2O5 and Al2O3, diagnostic of growth-driven elemental partitioning under fluctuating kinetic conditions; equilibrium tests utilizing olivine-melt Mn-Mg-Fe partitioning confirm that the most Mg-rich cores (Fo = 79–81) of both skeletal and euhedral olivines are in thermodynamic equilibrium with the host basalt, indicating a single population of phenocrysts that crystallized directly from the parental magma. Thermobarometric and hygrometric constraints indicate initial olivine crystallization occurred under hydrous conditions (2.7 ± 0.5 to 3.4 ± 1.4 wt% H₂O) at ~1168°C and 740 ± 180 MPa. A four-stage growth model—encompassing hourglass initiation, hourglass filling, dendritic growth, and late-stage filling—effectively captures dynamic undercooling and cooling rate variations in this small-volume hydrous magma system, explaining the evolution of skeletal textures and zoning. With an estimated crystallization timeframe of 10–30 days, our results show that short-lived magmatic systems can preserve anisotropic Fo-NiO covariations, underscoring the need for high-resolution compositional profiling to distinguish growth- versus diffusion-induced zoning. These findings refine our understanding of olivine as a robust recorder of magma crystallization histories, particularly in disequilibrium-dominated magmatic environments.