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What causes melting to occur?

Pressure and temperature increase as the depth below the earth's surface increases (heat from the core, pressure from overlying rocks, etc.). But, because pressure increases along with temperature, the rocks in the mantle remain solid. For example, a rock's melting temperature on the surface might be 1000 ºC, but 200 km below the surface under much higher pressure, the melting temperature of the rock might be 1300 ºC. So, one process which encourages melting is a decrease in pressure.

Water and volatile content promote melting by lowering the melting temperature of rocks. Thus, a dry rock would have a higher melting point than a rock with water percolating through it or bound up in the minerals. Example includes subduction zones and heating up sedimentary rocks.

Increased temperature-increasing the temperature of a rock will cause melting-the above two cases may be more important for the mantle but there crustal rocks are often thought to be melted by the passage of basaltic magmas (and heat from them) through the crust.

How do magmas form?

Melting rocks located deep in the earth's crust or mantle produces magmas. Melting experiments in the laboratory setting show that materials that are mixtures, like rocks, do not melt at one set temperature - rather they melt over a range in temperature with some minerals melting more easily than others. To completely melt a rock requires much more of a temperature increase than melting only partially. Thus, the most common form of melting is PARTIAL MELTING (usually from <1 % to 30?% melt + the rest mineral residue). Since different minerals (quartz, micas, olivine, etc.) have different compositions, bond strengths, and structures, they will have different melting points.

Where do magmas form?

Divergent plate boundaries - mostly basaltic lava, partial melts of the upper mantle. Thus oceanic crust is dominantly basaltic in composition.

Subduction zones/convergent plate boundaries - mafic, intermediate, and felsic intrusive and extrusive rocks. Mafic rocks are partial melts of the upper mantle (mantle wedge) caused by influx of water from the subducting plate. Intermediate rocks are evolved or mixed in composition (see next topic - differentiation). Felsic rocks are either evolved or, commonly, melted crust (sediments, other felsic rocks) due to emplaced mafic rocks.

Mantle plumes/hot spots - located away from plate boundaries, magma punches through the crust (originating in the lower mantle?) and is generally basaltic and erupted in large quantities.

CRYSTALLIZATION - once in place (intrusive, extrusive) - how does magma become rock? Crystallization - formation of solid minerals from magma.

Magmatic differentiation - a single uniform parent magma may crystallize to form a wide variety of rock compositions based upon the crystallization processes involved.

As with partial melting, the last minerals to melt, due to their high melting temperatures, will be the first minerals to crystallize (and the last to crystallize were the first to melt).
As different minerals start to crystallize, they remove specific elements from the magma and change its composition.
The mineral plagioclase varies considerably in its composition because of solid solution behavior, so it crystallizes over a large, continuous range of magma compositions: it makes up the Continuous Reaction Series.
Mafic minerals only crystallize within specific compositional ranges, and then new minerals take over which are very different in composition. Olivine, Pyroxenes and other make up the Discontinuous Reaction Series.
Continuous reaction series - Feldspars - room for Ca and Na in the Plagioclase Feldspar structure (remember from lab- striations, white to dark gray, 2 directions of cleavage @ 90º).

Ca-rich first to form, highest melting temperature. Magma is depleted in Ca, so next will take in some Na, then more, then all Na near the end of crystallization. Thus, Plagioclase can form over a large span in magma composition. There is a continuous reaction between mineral phase and melt/magma.

Discontinuous reaction series - Magnesium- and Iron-rich minerals (all solid solution, but not a wide crystallization range, changing and increasingly complex Silicate structures):

Olivine: Single tetrahedra, highest melting temperature (1800 ºC)

Pyroxene: Single chains, (~1557 ºC), earlier olivine may be converted to pyroxene.

Amphibole: Double chains, pyroxene may react to form amphibole

Biotite: Sheet silicates, lowest temperature Fe-Mg mineral in series.

Because of the different structures in these minerals, higher temperature forms may not be compatible with the newer phases and will react with the melt and change into the new phase IF NOT SEPARATED from the changing magma.

How do minerals survive intact? FRACTIONAL CRYSTALLIZATION - removal of crystals from the melt. Crystal settling, floating, coating the sides of the magma chamber, removal of the remaining magma elsewhere (eruption, continued upward movement). Supported in field exposures (layered mafic intrusions) and lab experiments (piston cylinders) - pioneer was N.L. Bowen, proposed fractional crystallization and crystallization sequences in the 1920's.

BOWEN'S REACTION SERIES, 1928: Cool a high temperature mafic magma, describes when certain minerals will start to form, and which rocks correspond to which combinations of minerals, also importance of fractional crystallization to preserve the earlier phases. See an example of this series in your textbook.

False interpretation that all rocks could be derived from a common mafic magma - did not fit with the enormous amount of granite present (requires even more [10 times as much] mafic rocks to crystallize before granite could form, and this is not observed in nature!).

Importance of partial melting and the degree of partial melting to provide parent magmas of highly variable composition. Do not need to start with a uniform mafic parent magma, can melt mantle, oceanic crust, continental crust, sediments, and the role of water, MIXING and UNMIXING of different compositions, different temperatures during cooling and crystallization.

Source: University of Illinois at Urbana