Journal Article

Repacking Accelerates High‐Silica Melts Extraction: Insights From Microstructural Record and Numerical Modeling

Zhao, Kai, Wu, Fu‐Yuan, Ji, Wei‐Qiang, Liu, Boda, Chen, Hou‐Bin, Zhang, Shao‐Hua

2025

Overview

Repacking enhances crystal mush permeability, accelerating melt extraction. However, identifying microstructural records of repacking is challenging, creating a gap in quantifying its effect on magmatic reservoirs. We identified extracted melt (rhyolite) and silicic residue (quartz monzonite) through textures and geochemical characteristics in the Pangduo Basin (Southern Tibet; ∼50 Ma old). By calculating interstitial mineral proportions and modeling incompatible element concentrations in quartz monzonite, we estimate a moderate trapped melt fraction (∼50 vol. %), providing microtextural evidence of repacking at intermediate crystallinities. We interpret that the horizontal preferred orientation of frame‐forming feldspars produces micro‐scale melt channels that accelerate melt extraction. Modeling the intensity of this orientation, we estimated compressive strain to be 20%–30%, likely accelerating melt extraction by at least 15 times. This millennium timescale allows for the growth of a large magma chamber, preventing the melt from freezing or causing multiple small eruptions due to excessive flow‐induced stress. Plain Language Summary Explosive rhyolite eruptions significantly affect climate, the environment, and human life. These devastating events, involving crystal‐melt separation in large upper crustal magma reservoirs, may be accelerated by grain reorganization. Yet, quantifying this acceleration is complex. Our study utilizes samples from the Pangduo Basin in southern Tibet to examine the pertinent microstructure and calculate the acceleration rate of crystal‐melt separation. We found that grain reorganization can reduce the separation duration by a factor of 15, highlighting its critical role in crystal‐melt separation. Key Points The rhyolite and quartz monzonite of the Pangduo Basin represent extracted melt and corresponding residual cumulates, respectively Interstitial minerals fraction and mass‐balance calculations yield a moderate trapped melt fraction of ∼50 vol. % The horizontal preferred orientation of large‐grained feldspars accelerates melt extraction by at least 15 times

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Compressive properties / crystal mush / Crystals / Deformation / Eruptions / Explosive compacting / Feldspars / Freezing / high‐silica rhyolite / Horizontal orientation / Lava / Magma / Magma chambers / melt extraction / Melts (crystal growth) / Microstructure / Modelling / Numerical models / Permeability / Preferred orientation / Quartz / repacking / Reservoirs / Rhyolite / Rhyolites / Separation / Silica / Temperature / Viscosity