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Basic magma generation and aggregation

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Abstract

Basic magma generation in the mantle at the present stage of earth history probably begins most commonly in metamorphosed garnet peridotite at those points where the four major phases meet; the solidus defines the spatial limits of the region of melting at the site of origin. On the basis of the forsterite-diopside-pyrope system and the melting relations of natural garnet peridotite at high pressures, the melting is invariant-like up to about 30% liquid. If the melt is fractionally removed, melting temporarily ceases after this limit is reached, terminating the production of liquid of invariant-like composition. Because one phase is eventually consumed at the invariant-like point, melting might be resumed at a higher temperature, generating a different liquid of more basic composition at the invariant-like point governing the assemblage of remaining phases. The garnet peridotite becomes permeable to melt almost immediately after the melting process begins, as has been demonstrated by the large increase in measured electrical conductivity. A large volume of relatively homogeneous liquid can, therefore, be extracted as it is produced.

Continuous heating of the parental material by conduction from a hot source below results in a series of liquid compositions, determined predominantly by the thermal gradient, with the greater degree of melting at the bottom. Conversely, heating of the parental material by adiabatic rise also results in a series of liquid compositions, determined predominantly by the pressure gradient, with the greater degree of melting at the top. Tapping from the top of parental materials partially melted in these two different ways results in successions of lavas having opposite sequences of magma composition.

Segregation of the melt occurs under small stress differences, and the melt penetrates the plastic envelope around the magma chamber by means of ephemeral, slowly propagating, ductile faults. The envelope may extend out to a physical boundary at about 0.8 of the temperature of the beginning of melting. The boundary of the zone characterized by plastic behavior may be either sharp or gradual depending on the strain rate. In the brittle region outside the plastic region, magmafracting (similar to hydrofracting) takes place and the cracks are propagated episodically, producing earthquakes.

If a liquid is extracted fractionally under small stress differences from host rocks with temperature or pressure gradients, convective mixing at the site of origin or in an auxiliary chamber may be necessary to account for the limited variation of trace elements in some large-volume extrusions. Isotopic variations between local magma may result from small-scale heterogeneity in the mantle, but is not likely to be due to disequilibrium melting.

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  1. Geophysical Laboratory, Carnegie Institution of Washington, 20008, Washington, D. C.

    H. S. Yoder Jr.

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Yoder, H.S. Basic magma generation and aggregation. Bull Volcanol 41, 301–316 (1978). https://doi.org/10.1007/BF02597365

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