Deciphering magma histories through phosphorus zoning in olivine

Abstract:

Since olivine is usually the first major phase to crystallize from basaltic magma, its primary chemistry is a sensitive tracer of the early evolution of volcanic systems. However, fast diffusion and homogenization under magmatic conditions frequently modifies the original composition of olivine, which hampers the reconstruction of cooling histories and magma evolution from the chemistry and zoning patterns of phenocrysts in erupted products. Phosphorous is a notable exception due to its sluggish diffusion in olivine crystals and silicate melts, as igneous olivines almost always display complex zoning patterns. Phosphorus zoning in olivine has been linked either to crystallization rate variations and diffusion controlled growth or to strong compositional controls on melt-mineral partitioning. We illuminate the versatility of P-in-olivine with a comprehensive EPMA and LA-ICPMS dataset on olivines from Italian potassium rich mafic lavas and the primitive melt inclusions (MI) that they host. The olivines are characterized by P concentrations from limit of quantification (22 ppm) to 435 ppm P with MIs containing up to 2.2 wt.% P₂O₅. High resolution (1-2 µm per pixel) element maps show both fine oscillatory and large scale sector zoning in P, which is uncorrelated with zoning in any other element. The MIs are virtually always surrounded by P-depleted zones that are also depleted in Cr and enriched in Al and Ti, which we attribute to a combination of supply-limited slow growth and melt compositional controls on partitioning behavior imposed by the boundary layer. We demonstrate that P zoning carries valuable information on the nature and timing of magmatic events such as mingling/mixing, wall-rock assimilation and subsequent re-equilibration processes. P-in-olivine is most promising to distinguish multiple generations of MIs, as a guide to study their mode of entrapment and to disclose the origin of primary heterogeneities.