Out-of-sequence skeletal growth causing oscillatory zoning in arc olivines

Pablo Salas,Laura Hernández,Osvaldo Rabbia,Philipp Ruprecht

doi:10.1038/s41467-021-24275-6

Pablo Salas, Laura Hernández + Show 2 more

Open Access

https://doi.org/10.1038/s41467-021-24275-6

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Abstract

Primitive olivines from the monogenetic cones Los Hornitos, Central-South Andes, preserve dendritic, skeletal, and polyhedral growth textures. Consecutive stages of textural maturation occur along compositional gradients where high Fo–Ni cores of polyhedral olivines (Fo92.5, Ni ~3500 ppm) contrast with the composition of dendritic olivines (Fo < 91.5, Ni < 3000 ppm), indicating sequential nucleation. Here we present a new growth model for oscillatory Fo–Ni olivine zoning that contrasts with the standard interpretation of continuous, sequential core-to-rim growth. Olivine grows rapidly via concentric addition of open-structured crystal frames, leaving behind compositional boundary layers that subsequently fill-in with Fo–Ni-depleted olivine, causing reversals. Elemental diffusion modeling reveals growth of individual crystal frames and eruption at the surface occurred over 3.5–40 days. Those timescales constrain magma ascent rates of 40–500 m/h (0.011 to 0.14 m/s) from the deep crust. Compared to ocean island basalts, where dendritic and skeletal olivines have been often described, magmas erupted at arc settings, experiencing storage and degassing, may lack such textures due to fundamentally different ascent histories.

Highlights

Primitive olivines from the monogenetic cones Los Hornitos, Central-South Andes, preserve dendritic, skeletal, and polyhedral growth textures
These findings have been tested experimentally on a Hawaiian basalt composition that shows that phenocryst-size (>100 μm) olivines may form within such twostage growth for undercooling of 25–40 °C, with the crystal experiencing textural maturation as undercooling diminishes[15]
Unlike other volcanoes in the Andean Southern Volcanic Zone that sit on thickened crust (>45 km, ref. 19) where erupted magmas reveal the dominance of storage and differentiation in the crust, Los Hornitos erupted primitive magma with olivine phenocrysts in equilibrium with mantle peridotite

Summary

Introduction

Primitive olivines from the monogenetic cones Los Hornitos, Central-South Andes, preserve dendritic, skeletal, and polyhedral growth textures. The presence of distinct dendritic zoning patterns in slow diffusing elements (e.g., P, Al), as well as the rare preservation of their morphologies in some olivine crystals from ocean island basalts, argue that an early dendritic olivine growth may be followed by an episode of maturation that fills the dendritic network and potentially explains the common occurrence of olivine-hosted melt inclusions. While the evidence for complex olivine growth in hotspot settings has increasingly mounted, similar observations for arc settings are lacking and arc olivines continue to be interpreted exclusively through a paradigm of concentrically grown crystals This view is driven by the polyhedral habits that dominate arc olivines where evidence for dendritic or skeletal growth representing remnants of an earlier growth stage is limited to the occasional presence of oscillatory bands of elevated P and Al zoning In the case of Los Hornitos, structural controls may facilitate their uninterrupted ascent to the surface with limited crustal residence despite the thick Andean crust[19,20]

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