Melting in the mantle

Lecture by Timothy L. Grove

Joint MIT, Harvard and WHOI seminar "Mantle Convection"
Spring 1998

Notes Prepared by Clint Conrad, Mary Agner and Thorsten Becker (becker@eps.harvard.edu)

Introduction

Melting in the mantle is a complex process which produces variable amounts of melted material of different mineral composition depending on the conditions during the formation. Important points are:

Melting is an inverse problem

One assumption involved in the interpretation of melt processes is that the melt samples the conditions in which it was created directly prior to segregation.
Petrologists determine the temperature when a rock is totally liquid for a given pressure in order to create a liquidus curve for that material. Basalts are liquids resulting from such melting processes. The temperature at which basalts coexist with many crystals on the liquidus curve equals the temperature of melt extraction. (This concept is more complex then noted by Herzberg and Zhang, 1996). Also, petrologists melt peridotites to determine the first melt composition.
Melts of the mantle have different composition than the parent body. Peridotites are found in stable cratons, extensional environments, and mid ocean ridges.
The problems in determining what exactly happens in the mantle when material melts are:

Why does the mantle melt?

Because there are different pressure-temperature slopes for adiabatically ascending bodies and melting curves. We assume that the convection taking place in the mantle implies adiabatic rise. The change in temperature with depth for silicate is around 0.3 degrees Celsius per kilometer. The slope for the melting curve, however, ranges from 1.3 degrees Celsius per kilometer to 5 degrees Celsius per kilometer. A decompression of 1 GPa will give 90 degrees Celsius of superheat. This implies an output of work about 30 calories per gram. Using a change in enthalpy of fusion of 150 calories per gram, this implies about 20% melt. Another assumption is that the time-scales in mid ocean ridges are approximately equal to spending rates.

Melting Models

The two end-member cases are: Based on melt connectivity and permeability, and observations by Johnson et. al., 1990, fractional melting seems more likely to occur at ridges than batch melting. Most experimental observations, however, assume batch melting, so it can be unclear how to relate them to the earth.

Equilibrium in Fe and Mg bearing systems

Melt formation kinetics (and the final concentration of FeO and MgO in melts) can be described by equilibriums constants as a function of activity parameters. They depend on the temperature but to first order not on the pressure. Hence, deep melts from regions with high temperatures are high in MgO and FeO as a temperature effect.

Effects of Water

References to papers