Dissertation

Petrogenesis of Hornblendite and Clinopyroxenite Cognate Xenoliths from the West Eifel Volcanic Field, Germany : a Textural Insight into Crystal and Melt Dynamics in an Intraplate Volcanic Setting

Matthews, Zoe

2022

Overview

Glass-bearing clinopyroxenite and hornblendite cognate xenoliths entrained in alkali basalts from the West Eifel Volcanic Field provide a snapshot of a semisolidified mush that was rapidly quenched upon entrainment, preserving the melt distribution as interstitial glass. As such, they provide a unique opportunity to investigate the physical relationship between crystals and melt within a mush. At present, this relationship is poorly constrained and petrologists are largely reliant on using fully solidified plutonic rocks to interpret melt distribution and geometries in two-dimensions. Understanding where melt is distributed, and how magmatic processes act to influence this is integral in improving our understanding of subvolcanic systems, with wider applicability to constraining the mechanisms that govern how melt is extracted from the crystal mush and erupted at the surface. Quantitative textural analysis including crystal size distribution (CSD), spatial distribution pattern (SDP) and shape preferred orientation (SPO), as well as qualitative textural observations, are used in conjunction with supplementary geochemistry to constrain the petrogenetic history of four cognate xenoliths derived from the lower-crust to upper-mantle realm. Together, this data provides evidence for open system magmatic processes driving mineral dissolution and reactive crystallisation. Three-dimensional X-ray Computed Tomography is also utilised to visualise and quantify the geometry of the melt, now glass, which represents the porosity of the mush at the point of entrainment. This provides a valuable insight into how melt is stored within crystal mushes, without textural overprinting by secondary recrystallisation processes that are often pervasive in solidified plutons. Combining the two-dimensional textural data with the threedimensional data identifies significant melt heterogeneity at a micro-scale, related to textural changes within the crystalline fraction acting to enhance melt migration. This not only provides an insight into melt distribution, but permits analysis of how the crystals and melt interact relative to one another in an active mush system. This work will contribute towards improving our understanding of crystal mushes and their significance in controlling volcanic eruptions.

• Publisher: ProQuest Dissertations & Theses
• Subject: Petrogenesis