Pete Loader 2011

GL4: Q1 June 2010

Partial melting can be achieved when the temperature of the mantle (indicated by the local geotherm) exceeds the melting point of peridotite.

This can occur in a number of ways and in different geological environments as shown by the following :1.By adding water at a subduction zone (convergent boundary) – decreasing the mantle melting point.2.By rapid decompression at a divergent boundary.3.By locally increasing the geotherm (hot spot)

Geotherm is below melting point curve for mantle peridotite (solidus)

Credit: Dougal Jerram

Subduction zone

By adding water at a subduction zone (convergent boundary) – decreasing the melting point of mantle peridotite (wet solidus)

Credit: U S Geological Survey

Credit: Dougal Jerram

Student copyS Student copy

Credit: http://geosun.sjsu.edu/~jmiller/Geo1_Lec5_IgRocks.html

Mid-Ocean Ridge Divergent plate

boundaryA rising convection current at a divergent boundary crosses the solidus and causing rapid decompression melting.

Credit: Dougal Jerram

Credit: U S Geological Survey

Student copy

Credit: http://geosun.sjsu.edu/~jmiller/Geo1_Lec5_IgRocks.html

Partial melting at plate margins summary

GL4: Q1 June 2003

GL4: Q4 June 2005

Basalt Andesite

Credit: US Geological Survey

Magmatic model at

subduction zones

1. 1. Partial melting of Partial melting of

lithospherelithospheresubducted mafic subducted mafic crust,crust,peridotite mantle peridotite mantle wedge,wedge,continental crustcontinental crust

2. Gravity settling -2. Gravity settling -Fractional Fractional crystallisation crystallisation (cumulates)(cumulates)

3. Magma mixing3. Magma mixing

4. Contamination - 4. Contamination - XenolithsXenoliths

Based on Loader et al .(in prep)

In fact the processes of magma formation and evolution can

generate igneous rocks with a variety of compositions regardless

of the plate environment