Abstract

Melt inclusions (MIs) hosted in euhedral olivine have been proposed to represent droplets of primary melt, protected from processes occurring near Earth’s surface during eruption. The complex zoning of phosphorus (P) in some olivines and the presence of a P-depleted zone around MIs indicate a complex history for the host-MI system. We analyzed P in olivine and MIs from two mid-oceanic ridge basalt (MORB) samples from the Mid-Atlantic Ridge (MAR) by electron probe microanalyzer, secondary ion mass spectrometry (SIMS), and NanoSIMS. Phosphorus dendrites in olivine suggest an initial fast olivine growth followed by a stage of slower growth. Dissolution textures around some MIs were identified and were probably caused by adiabatic decompression melting. Based on diffusion modeling of P in olivine, we infer that olivine beneath the MAR remains in the system (1) for days to weeks after crystallization of P-rich lamellae, and (2) for a few hours after recrystallization of dissolved olivine. Dissolution and reprecipitation of olivine containing boundary layers suggests that most MIs might be affected by late post-entrapment processes.

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