The dynamics of the magma formATION and intrusion under

mantle-crust conditions

Zhatnuev N.S.

Geological Institute SB RAS, Ulan-Ude, Russia, zhat@gin.bscnet.ru

It is supposed that the mantle and crust magma sources form due to melting the substrate by plumes or at the expense of decompression of the warmed-up rocks. It is also presumed that the magma strains, becomes mobile (Kadik et al., 1986), and intrudes up into the high-placed horizons frequently overcoming sizeable distances after the specified concentration of liquid in the melting substratum is reached. However, it is not clear, what concentration of liquid in the substratum is needed for magma to get enough mobile to rise up to the surface, and does magma strain from pore space of the melting substratum?

The mechanisms of magma rise were repeatedly considered, but there is no sufficiently definite opinion concerning this problem (Fedotov, 2006). The hydrostatics is acknowledged as the main impulsive force. According to other models, the magma moves to the surface owing to zone melting or, owing to increase of pressure during boiling up the gases, tectonic pressure and overpressure, that appears during melting the rocks of mantle and crust, as it follows from review of N. Rast (1972). It is implied that environment represents very viscous liquid, in which «the drop» of less viscous magma floats in models, where buoyancy (hydrostatics) is considered as the main impulsive force. In case of the surrounding matter is solid, but plastic body, it is difficulty to imagine such mechanism of quick moving of magma to the surface. The time of magma rise from mantle and low crust depths, according to some data, is evaluated as days and even hours (Perchuk, http://students.web.ru/db/msg.html?mid=1153763&uri=index.html).

Thus, to present day there is no clearity in understanding the mechanisms of the magma melt migration and moment when they get mobile during melting.

The mechanism of magma melt intrusion by substrate hydraulic fracturing is suggested here. The degree of the substrate melting, configuration of temperature field, strength of host medium, and volume of melt are the determining factors.

The hydraulic fracturing of the host rock takes place in case of exceeding the magma overpressure over the rock strength. The magma overpressure depends on the rock and melt density difference, and height of substrate melted part. An increase in height of magma chamber occurs, and the overpressure correspondingly increases during the hydraulic fracturing formation. The process is self-accelerating. The rate of the substrate melting is the limitative stage.

References

Fedotov S.A. (2006) Magmatic feeding systems and mechanism of volcano eruption // M. Nauka. 456p. (In Russian)

Kadik A.A., Lukanin O.A., Lapin I.V. (1989) Physical and chemical conditions of basalt magma evolution in pre-surface hearths // M., Nauka, 346p. (In Russian)

Perchuk L.L. What can tell us xenoliths of kimberlites and traps? //

http://students.web.ru/db/msg.html?mid=1153763&uri=index.htm

Rast N. (1972) Origin, rise and development of magmas // Mechanism of magma intrusions. M. Mir. P.284-310. (In Russian)