Insights into the Compositional Evolution of Crustal Magmatic Systems from Coupled Petrological-Geodynamical Models

Abstract

The evolution of crustal magmatic systems is incompletely understood, as most studies are limited either by their temporal or spatial resolution. Exposed plutonic rocks represent the final stage of a long-term evolution punctuated by several magmatic events with different chemistry and generated under different mechanical conditions. Although the final state can be easily described, the nature of each magmatic pulse is more difficult to retrieve. This study presents a new method to investigate the compositional evolution of plutonic systems while considering thermal and mechanical processes. A thermomechanical code (MVEP2) extended by a semi-analytical dike/sill formation algorithm, is combined with a thermodynamic modelling approach (Perple_X) to investigate the feedback between petrology and mechanics. Melt is extracted to form dikes while depleting the source region. The evolving rock compositions are tracked on markers using a different phase diagram for each discrete bulk-rock composition. The rock compositional evolution is thus tracked with a high precision by means of a database with more than 58,000 phase diagrams. This database describes how density, melt fraction, chemical composition of melt and solid fractions and mineralogical assemblages change over crustal to uppermost mantle P-T conditions for a large range of rock compositions. Each bulk rock composition is composed of the 10 major oxides (SiO2-TiO2-Al2O3-Cr2O3-MgO-FeO-CaO-Na2O-K2O-H2O) including an oxygen buffer. The combined modelling approach is applied to study the chemical evolution of the crust during arc magmatism and related melt extraction and magma mixing processes. Basaltic sills are periodically injected into the crust to model heat/magma influx from the mantle. We find that accumulated sills turn into long-lived mush chambers when using a lower rock cohesion or assuming a higher intrusion depth. Associated partial melting of crustal host rocks occurs around densely distributed dikes and sills. High silica rocks (e.g. granites) are generated by partial melting of the host rocks, melt segregation within dikes, and from fractional crystallization of basalts. Although the volume of these rocks is relatively small in our models compared to rocks with a mafic to intermediate composition, they provide important information about the processes of magma differentiation within arc continental crust.

The Mechanical Response of a Magma Chamber With Poroviscoelastic Crystal Mush

“…Petrological studies and thermodynamic models have long indicated that crustal magmatic reservoirs (i.e., magma chambers) contain an abundance of crystal mush, where “mush” refers to a system with melt contained in a framework of crystals (Cashman et al., 2017; Marsh, 1989, 2013; Pritchard et al., 2018; Wieser et al., 2020). Crystals have thermal and geochemical importance, as they can alter the chemistry and thermal state of magma, and provide constrains on the thermal state and timescales of magma storage, ascent, and eruption (Antonelli et al., 2019; Bachmann & Huber, 2016; Cooper, 2019; Costa et al., 2020; Rummel et al., 2020; Singer et al., 2018; Sparks & Cashman, 2017). In recent decades, many research efforts have been devoted to understanding how crystal mush evolves and interacts with magma, using quantitative models and principles in thermodynamics, geochemistry, and geophysics.…”

Magma Chamber Formation by Dike Accretion and Crustal Melting: 2D Thermo‐Compositional Model With Emphasis on Eruptions and Implication for Zircon Records

“…Caricchi et al (2016) explored the magma flux range of 10 −4 -10 −1 km 3 /a, Gelman et al (2013) studied magma injection into a cylindrical domain with the radius of 10 km and magma fluxes of 0.005-0.03 km 3 /a. Other authors (Karakas et al, 2017;Rummel et al, 2020;Tierney et al, 2016) suggest magma fluxes of 0.1-6 ×10 −3 km 3 /a. A global compilation of 170 time-averaged volumetric volcanic output rates was collected in the study White et al (2006).…”

Does Underthrusting Crust Feed Magmatic Flare‐Ups in Continental Arcs?

“…The intruding basalts will thus in turn play an important role in increasing the magma production rate. Magma advection via diking are difficult phenomena to implement even in advanced 2‐D or 3‐D models because the processes depends on many complex, dynamic processes, and parameters such as rock rheology, melt transport, phase transition, and dike formation mechanisms (e.g., Cao et al., 2016a; Liang & Parmentier, 2010; Rummel et al., 2020). While others have quantitatively assessed the process of advective magma transport and the thermal impact of mantle melt on the lithosphere using sophisticated, two‐phase flow dynamics (Keller et al., 2013; Rees Jones et al., 2018).…”

“…(2) We assume all melts are added to the arc crust as soon as they are generated. There is no lag time associated with melt transport, and the physical processes of magma ascent and emplacement (e.g., Cao et al., 2016a; Rummel et al., 2020) are not considered. (3) Magma advection as dikes or diapirs are not included.…”