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Magma intrusive rates and the growth of melt reservoirs and magma chambers

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The concept that granitoids plutons in the upper crust are solidified large magma chambers has been challenged by high precision geochronological data. A strong dichotomy is found between the rate of magma transfer through the crust and the average long-term pluton emplacement rate inferred from geochronology. In addition, field studies show that many plutons are formed by the addition of discrete intrusive sheets. These observations strongly support a model where igneous bodies grow incrementally during pulses of magma injection that are separated by long periods of quiescence. Numerical simulation shows that at low emplacement rate, successive pulses solidify during repose period and the size of any magma chamber is limited to the size of one pulse. A consequence is that large magma chambers in the upper crust must be rare and short-lived and most differentiation from a mafic parent is likely to happen in the lower crust where higher ambient temperatures and higher magma fluxes are more favorable to the formation of long-lived melt reservoirs. Integration of petrological experiments and numerical simulation show that a diversity of melt compositions can be produced in deep crustal hot zones. Episodic ascent of melt from depth can be explained by the dynamics of melt segregation and extraction.

Ref:

Annen, C. (2009), From plutons to magma chambers: Thermal constraints on the accumulation of eruptible silicic magma in the upper crust, Earth and Planetary Science Letters, 284(3-4), 409-416.

Melekhova, E., C. Annen, and J. Blundy (2013), Compositional gaps in igneous rock suites controlled by magma system heat and water content, Nature Geoscience, 6(5), 385-390.

Glazner, A. F., J. M. Bartley, D. S. Coleman, W. Gray, and Z. T. Taylor (2004), Are plutons assembled over millions of years by amalgamation from small magma chambers?, GSA Today, 14(4/5), 4-11.

*Annen, C., J. D. Blundy, and R. S. J. Sparks (2006), The genesis of intermediate and silicic magmas in deep crustal hot zones, J. Petrol., 47(3), 505-539

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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