Catherine Annen

Effects of repetitive emplacement of basaltic intrusions on thermal evolution and melt generation in the crust

A one-dimensional thermal conduction model simulates the repetitive intrusion of basalt sills into the deeper parts of the crust. The model assumes geothermal gradients of 10–30°C km−1, and intrusion depths at 20 and 30 km. A range of intrusion rates from 50 m of intruded basalt every 1000, 10 000 and 100 000 years cover a range of geodynamic situations. There is an initial incubation period in which the basalt intrusions solidify. Generation of silicic melts initiates when the solidus temperatures of either the basalt magma or surrounding crust is reached. At an intrusion rate of 50 m per 10 000 years incubation periods in the range 105–106 years are estimated, consistent with geochronological and stratigraphic data on many volcanic systems where there is commonly an evolution from mafic to silicic volcanism. Melt generation involves simultaneous cooling and crystallization of intruding basalt and partial melting of both new basaltic crust and pre-existing old crust. The proportion of these components depends on the fertility of the crust, in particular the abundance of hydrous minerals, and the temperature and water content of the basalt magma. For a wet (2% H2O) and cool (1100°C) basalt, melt generation can be dominated by residual liquids from basalt crystallization. For a dry (0.3% H2O) and hot (1300°C) basalt emplaced into fertile crustal rocks, such as pelite, melt generation can be dominated by partial melting of old crust. Melt proportions and temperature vary greatly across such a deep crustal intrusion zone, resulting in geochemical diversity in magmas. Segregated melts, if mixed together during ascent or in a high-level magma chamber, will be geochemical hybrids with mantle and crustal components. Intrusion rates of 50 m per 100 000 years or less are too low for large-scale melt generation in the crust. Periods of magmatic intrusion create reverse geothermal gradients and thermal anomalies in the crust which will take several million years to decay. Such anomalous zones are predisposed to remelt if a subsequent magmatic episode initiates.

Magma chamber properties from integrated seismic tomography and thermal modeling at Montserrat

It is widely believed that andesitic magmas erupted at arc-volcanoes are stored in shallow reservoirs prior to eruption, but high-resolution images of focused regions of magma in the shallow crust are rare. We integrate seismic tomography with numerical models of magma chamber growth to constrain the magma chamber beneath Soufriere Hills Volcano, Montserrat. Our approach reveals the characteristics and dynamics of the magmatic system with a level of detail that no single method has yet achieved. The integrated analysis suggests that a magma chamber of 13 km3 with over 30% melt fraction formed between 5.5 and at least 7.5 km depth, a significantly higher melt fraction than inferred from the seismic data alone. The magma chamber may have formed by incremental sill intrusion over a few thousand years and is likely to be a transient, geologically short-lived feature. These volume and geometry estimates are critical parameters to model eruption dynamics, which in turn are key to hazard assessment and eruption forecasting.

Rates of magma transfer in the crust: Insights into magma reservoir recharge and pluton growth

Plutons have long been viewed as crystallized remnants of large magma reservoirs, a concept now challenged by high-precision geochronological data coupled with thermal models. Similarly, the classical view of silicic eruptions fed by long-lived magma reservoirs that slowly differentiate between mafic recharges is being questioned by petrological and geophysical studies. In both cases, a key and yet unresolved issue is the rate of magma transfer in the crust. Here, we use thermal analysis of magma transport to calculate the minimum rate of magma transfer through dikes. We find that unless the crust is exceptionally hot, the recharge of magma reservoirs requires a magma supply rate of at least ∼0.01 km 3 /yr, much higher than the long-term growth rate of plutons, which demonstrates unequivocally that igneous bodies must grow incrementally. This analysis argues also that magma reservoirs are short lived and erupt rapidly after being recharged by already-differentiated magma. These findings have significant implications for the monitoring of dormant volcanic systems and our ability to interpret geodetic surface signals related to incipient eruptions.

Factors Affecting the Thickness of Thermal Aureoles

Intrusions of magma induce thermal aureoles in the country rock. Analytical solutions predict that the thickness of an aureole is proportional to the thickness of the intrusion. However, in the field, thermal aureoles are often significantly thinner or wider than predicted by simple thermal models. Numerical models show that thermal aureoles are wider if the heat transfer in the magma is faster than in the country rock due to contrasts in thermal diffusivities or to the effect of magma convection. Large thermal aureoles can also be caused by repeated injection close to the contact. Aureoles are thin when heat transfer in the country rock is faster than heat transfer within the magma or in case of incrementally, slowly emplaced magma. Absorption of latent heat due to metamorphic reactions or water volatilization also affects thermal aureoles but to a lesser extent. The way these parameters affect the thickness of a thermal aureole depends on the isotherm under consideration, hence on which metamorphic phase is used to draw the limit of the aureole.