Melting in the mantle
Lecture by Timothy L. Grove
Joint MIT, Harvard and WHOI seminar "Mantle Convection"
Notes Prepared by Clint Conrad, Mary Agner and Thorsten Becker (firstname.lastname@example.org)
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:
- The melting process itself. Three melting styles are usually considered:
Batch, fractional, and assimilative melting. Since melts react with solids as they
ascend, the time scale of melting becomes important.
- The temperature needed to melt mantle rock increases with pressure
and hence depth.
- Volatiles (such as the best studied one, water) decrease the
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
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:
- melts are not "batches" from mantle resulting from one temperature and depth
- melt freezes as it rises and will crystallize out other minerals,
altering the composition of the melt
- melt assimilates the mantle around it: It is not in equilibrium
with the neighboring rock, since the melt temperature is above the solidus
- melt composition does not equal basalt composition
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.
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 Batch Melting: Melt remains in contact with residual crystals at
all times, so the bulk composition remains constant.
- Fractional Melting: Melt leaves the system as soon as it is formed, so the
bulk composition of the residual solid changes continuously. Fractional
melting requires that the melt pockets get completely interconnected as soon
as they are formed. In addition, the density of most melts is lighter
than the residual only at pressures less than about 8 GPa (250 km), with
a large degree of possible variation in this number. Thus, if melt
occurs below this depth in a high permeability rock, the melt
will sink and not rise to the surface. Melt connectivity is a
function of the dihedral angle at edges of melt regions.
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
Effects of Water
- Additional water lowers the melting temperature - adding only a little bit
of water can lower the melting temperature significantly
- Water also allows melt to occur over a wider temperature range.
The effects of water depend critically on how much water you add
and how you do it.
- Adding water to the system almost completely erases the effect of pressure
- Adding water decreases the amount of FeO and MgO which goes into the melt.
- Computing the composition of the residual and the melt for fractional
melting with water begins to get very complicated. Back-computing
the composition of an original rock from a residuum and a melt is
even more complicated.
References to papers
- Inoue, T. (1994): Effect of water on melting phase relations and melt
composition in the system Mg_2SiO_3-MgSiO_3-H_2) up to 15GPa.
PEPI, 85, 237-263
- Herzberg, C. and Zhang, J. (1996): Melting experiments on anhydrous
peridotite KLB-1: Compositions of magmas in the upper mantle
and transition zone. JGR, 101, B4, 8271-8295
- Johnson, K. T. M., Dick, H. J. B. and Shimizu, N.(1990): Melting in the
oceanic upper mantle: an ion microprobe study of diopsides in
abyssal peridotites. JGR, 95, B3, 2661-2678