Journal of Petrology | Volume 30 | Number 4 | Pages 885-923 | 1989
© Oxford University Press 1989
research-article |
Fountains in Magma Chambers
Research School of Earth Sciences, Australian National University P.O. Box 4, Canberra, A.C.T. 2601, Australia
Received April 26, 1988; Revised October 19, 1988; ABSTRACT
Cyclic layering is a common feature of the ultramafic zone of layered intrusions and is usually attributed to the entry of new pulses of dense magma into the chamber. Since the crystallization of olivine and bronzite lowers the density of the magma, a new pulse of the parent magma will be denser than the fractionated magma in the chamber. If the new pulse enters with excess momentum it will initially rise up into the host magma to form a fountain, then fall back around the feeder when negative buoyancy forces overcome the initial momentum of the pulse. Laboratory experiments using aqueous solutions with both point and line sources have been conducted to obtain a quantitative understanding of the fluid-dynamical processes that are important in fountains. It is observed that convection within the fountain is highly turbulent, resulting in appreciable entrainment of the host magma. A gravity-stratified hybrid layer develops at the floor and this breaks up into a series of double-diffusive convecting layers if the new pulse is hotter than the host magma. The number of layers that form depends on a number of factors, especially R
, the ratio of the contributions of composition and heat to the total density difference between the host magma and the new pulse. Raising the value of R, results in the formation of more, thinner layers.
The thickness of the hybrid layer at any time t is given by H = h0+(V0/A)t where V0 is the volume flux through the feeder and A is the horizontal area of the chamber. h0 is related to the initial steady-state height of the fountain and, for a line source, is given by h0=CU04/3 d–1(g
/)–2/3 where U0 is the volume flux per unit length, g is the acceleration due to gravity, d is the width of the feeder, is the density of the host magma, is the density difference between the magmas and C is a constant.
Calculations based on these results and the consideration of the flow in the feeder dykes below the chamber indicate that a fountain will rise at least 350 m in a continental magma chamber if the feeder width is greater than 10 m. This will lead to extensive mixing between the new pulse and the fractionated magma in the chamber, producing a zoned hybrid layer at the floor that is commonly over 1000 m thick. If the chamber receives many pulses of dense magma, the resulting zoning may persist throughout much of the life of the chamber, especially if the first pulse to enter becomes contaminated by light magma released by melting at the margins. The highest Mg/Fe ratio for olivine and pyroxenes from cyclic units from the ultramafic zones of layered intrusions is often well below the value expected for minerals crystallizing from a melt derived directly from the mantle, supporting the hypothesis that new pulses of dense magma can mix extensively with the fractionated magma in the chamber.
The feeder dykes to some oceanic magma chambers, such as the Bay of Islands Ophiolite, are believed to be narrower, so that fountains do not rise more than a few metres above the floor of the chamber. This restricts mixing between the input magma and the host magma and can result in the formation of a hybrid zone that is only a few metres thick.
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