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

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Mid-Ocean Ridge Divergent plate boundary A rising convection current at a divergent boundary crosses the solidus and causing rapid decompression melting. Credit: Dougal Jerram Credit: U S Geological Survey

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Partial melting at plate margins summary

GL4: Q1 June 2003GL4: Q4 June 2005

Basalt Andesite

Credit: US Geological Survey

1. Partial melting of lithosphere subducted mafic crust, peridotite mantle wedge, continental crust 2. Gravity settling - Fractional crystallisation (cumulates) 3. Magma mixing 4. Contamination - Xenoliths 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