Rejuvenation of crustal magma mush: A tale of multiply nested processes and timescales

Abstract

Some relatively crystal rich silicic volcanic deposits, including large volume ignimbrites, preserve evidence of a history where a rheologically locked high crystallinity magma was rejuvenated (unlocked) through enthalpy (± mass) exchange with newly injected recharge magma of higher specific enthalpy. This is one event in a possible complex history. That is, the volcanic product of an eruption reflects an array of sequential and nested processes including melt formation and segregation, ascent, cooling, crystallization, crustal assimilation, magma recharge, unlocking, shallow ascent, fluid exsolution, and eruption. Deconvolution of these these nested processes and concomitant timescales is complicated and relies on a multidisciplinary approach; studies that do not clearly associate process and correlated timescale have the potential to provide misleading timescale information. We report the results of thermodynamic and heat transfer calculations that document mass, energy, and phase equilibria constraints for the unlocking of near-solidus rhyolite mush via magma mingling (heat exchange only) with basaltic recharge magma of higher specific enthalpy. To achieve unlocking, defined as the transition from near-solidus to ∼50 percent melt of the host silicic magma, phase equilibria computations provide (1) the enthalpy required to unlock mush, (2) the mass ratio of recharge magma to mush (M_R/M_M) when the two magmas achieve thermal equilibrium, and (3) the changes in melt, mineral, and fluid phase masses, compositions, and temperatures during the approach to unlocking. The behavior of trace elements is computed with knowledge of mineral, fluid, and melt proportions and solid-fluid and solid-melt partition coefficients. Evaluation of unlocking for relatively ‘dry’ (0.5 wt. % H₂O) and ‘wet’ (3.9 wt. % H₂O) rhyolitic mushy (locked) magma by basaltic recharge at upper crustal pressures indicates minimum values of M_R/M_M can be significantly less than 1, assuming the mingling process is isenthalpic with no ‘waste’ heat. For active volcanic systems estimates of M_R may be tested using geodetic data. Wet mush has lower energy requirements for unlocking and thus requires lower M_R/M_M than dry mush. Wet rejuvenated magmas therefore may be more abundant in the volcanic rock record, and unlocked dry mushes may be restricted to extensional tectonic settings with high recharge flux. Temperature changes in dry mush as it unlocks are pronounced (greater than 150 °C) compared to those in wet mush, which are smaller than the resolution of classical geothermometry (∼15 °C). Phase equilibria calculations show that, as required, the net volume of crystals decreases during unlocking. Interestingly, calculations also indicate reactive crystal growth by chemical re-equilibration at the crystal-size scale during unlocking may also take place. In either dissolution by unidirectional resorption or reactive dissolution/ new growth, the chemical signatures of unlocking, potentially preserved in crystals or parts of crystals (for example, rims), are predictable and hence testable. Independent of unlocking thermodynamics, the phase equilibria and elemental consequences of isentropic magma ascent, a transport event that follows unlocking, can also be predicted; detailed examination of several canonical cases reveals a marked contrast with isenthalpic unlocking, thereby providing a means of process deconvolution. Unlocking timescales are estimated by two methods, one that calculates the time to reach thermal equilibrium for recharge magma dispersed in mush as ‘clumps’ of fixed size, and the second where the required volume of recharge magma is initially a single clump and evolves to smaller size through clump stretching and folding. For a range of magma volumes from 0.1 to 5000 km³, unlocking times range from 10⁻² to 10⁶ years. The shorter timescales for any magma volume requires a large number of relatively small clumps (n >10⁶), which implies that large volumes of mush purported to unlock over short timescales (10²–10³ years and less) should preserve and exhibit evidence of intimate magma mingling. The key result of our analysis is that multiple timescales are operative during the potentially long and complex history of silicic mush formation, rejuvenation, and ascent. To correctly ascribe timescales to unlocking requires a holistic understanding of the myriad processes that affect the magma before, during, and after enthalpy exchange. In the absence of this context, unlocking timescales may be incorrectly constrained which, in turn, may hinder eruption forecasting and associated hazard mitigation.