Radial Melt Segregation During Extrusion of Partially Molten Rocks

Abstract

We performed a series of extrusion experiments on partially molten samples of forsterite plus 10 vol% of an anorthite‐rich melt to investigate melt segregation in a pipe‐extrusion geometry and test the predictions of two‐phase flow theory with viscous anisotropy. The employed flow geometry has not been experimentally investigated for partially molten rocks; however, numerical solutions for a similar, pipe‐Poiseuille geometry are available. Samples were extruded from a 6‐mm diameter reservoir into a 2‐mm diameter channel under a fixed normal stress at 1350°C and 0.1 MPa. The melt distribution in the channel was subsequently mapped with optical and backscattered electron microscopy and analyzed via quantitative image analysis. Melt segregated from the center toward the outer radius of the channel. The melt fraction at the wall increased with increasing extrusion duration and with increasing shear stress. The melt fraction profiles are parabolic with the melt fraction at the wall reaching 0.17–0.66, values 2 to 16 times higher than at the channel center. Segregation of melt toward the wall of the channel is consistent with base‐state melt segregation as predicted by two‐phase flow theory with viscous anisotropy. However, melt‐rich sheets inclined at a low angle to the wall, which are anticipated from two‐phase flow theory, were not observed, indicating that the compaction length is larger than the channel diameter. The results of our experiments are a test of two‐phase flow theory that includes viscous anisotropy, an essential theoretical frame work needed for modeling large‐scale melt migration and segregation in the upper mantle.

Melt network reorientation and crystallographic preferred orientation development in sheared partially molten rocks

“…The emergence of these "bands" may be the result of viscous anisotropy induced by grain-scale alignment of melt parallel to σ 1 (45˚ to the shear plane) and decreasing compaction length as grains recrystallize to a smaller grain size, documented in Figure 7c. These observations again agree with predictions from the viscous anisotropy theory framework Quintanilla-Terminel et al, 2019;Takei & Holtzman, 2009c;Taylor-West & Katz, 2015), and explain the discrepancy between experimentally obtained value of 25˚ for melt orientation and computationally predicted sub-parallel orientation with respect to σ 1 ; both are correct, just at different observation scales.…”

Melt Network Reorientation and Crystallographic Preferred Orientation Development in Sheared Partially Molten Rocks

“…The emergence of these bands may then be the result of viscous anisotropy induced by grain-scale alignment of melt and decreasing compaction length as grains recrystallize to a smaller grain size, documented in Figure 7c. These observations again agree with predictions from the viscous anisotropy theory framework and the experiments designed to test this theory Qi et al, 2015;Quintanilla-Terminel et al, 2019;Takei & Holtzman, 2009c).…”

Crystal Shape Control on the Repacking and Jamming of Crystal‐Rich Mushes

“…The chemical and physical evolution of crustal magma reservoirs are governed by the mechanical processes that control the separation between melt and crystals. This separation can be driven by gravity (density contrasts) and by external (e.g., regional tectonics) stresses (Bachmann & Bergantz, 2004; Bachmann & Huber, 2016; Davis et al., 2007; Koenders & Petford, 2000; Petford et al., 2020; Quintanilla‐Terminel et al., 2019; Solano et al., 2012). Dufek and Bachmann (2010) argued that melt‐crystal separation is most effective at intermediate crystallinity (between 0.4 and 0.7) when the crystalline framework resists deformation and forms what is often referred to as a crystal mush.…”