Prolonged crustal storage for mantle xenolith-bearing basanite lava

Abstract

It is widely accepted that magmas must ascend rapidly to transport dense mantle xenoliths to the surface, with minimal residence or evolution during ascent. Here, we investigate the prolonged storage of xenolith-bearing magma within the crust beneath the Cenozoic alkali basalts at Hannuoba, North China. Petrographic observations identify three distinct olivine types. Olivine xenocrysts are relatively large (>500 μm), with pronounced core-to-rim zoning [Fo₉₀₋₉₁ to Fo₆₄₋₆₅; Fo = molar 100 × Mg/ (Mg + Fe)] and distinctively low CaO contents in the cores (<0.1 wt%), which clearly distinguish them from other olivine types. Trace element and Fe-Mg isotope analyses of these xenocrysts reveal strong compositional zoning, with highly variable δ⁵⁶Fe (−2.70 ‰ to 0.40 ‰) and δ²⁶Mg (−0.69 ‰ to 0.68 ‰), strongly indicating Fe-Mg exchange diffusion with the melt. Phenocrysts are smaller (∼100 μm) than xenocrysts, exhibit compositions similar to the rims of the latter, and lack compositional zoning. Olivine microlites in the fine-grained sample matrix, with an abundance of ∼10 %, show a slightly different NiO–Fo systematic, indicative of fractional crystallization.

Zoning in Fe-Mg concentrations and Fe-Mg isotopes from the core to the rim of the xenocrysts was used for diffusion modeling to obtain their residence timescales in the magma prior to eruption. Fo zoning, modeled using DIPRA for six olivine grains, yields timescales of 520–3914 days, while dynamic diffusion modeling of Fe-Mg (Fo) provides longer timescales of 4026–18,139 days (i.e., up to 50 years). Fe-Mg isotopic diffusion modeling yields timescales of 542–3333 days. These values represent minimum estimates for the magma residence time, as xenocrystal olivine may have been mechanically detached from entrained xenoliths during magma ascent. These findings contrast with previous interpretations that xenolith- or xenocryst-bearing magmas ascended to the surface within hours to days and instead suggest that the xenolith-bearing alkali basaltic lavas were stored for an extended period in the crustal plumbing system. Such prolonged storage may have facilitated interaction between xenocrysts and the host magma, potentially modifying the magma's bulk composition. Caution is advised when using these magmas to trace their mantle source, as their bulk composition may have been altered by crustal assimilation or interaction with xenoliths/xenocrysts during extended crustal storage.