petrology midterm

where are igneous rocks formed

spreading centers (mid-ocean ridge), converging centers (subduction zone)

how does mantle melting mainly occur

decompression melting (when rising mantle material leave the nominal mantle geotherm, rising without much further loss of heat and entering the field in the phase diagram that consists of both solid and melt)

causes of mantle melting

change in pressure (decrease), change in temperature (increase), change in composition (addition of water)

controls on melt composition

degree of partial melting, depth of melting (phases present), composition of mantle that’s melting

why does a change in pressure cause melting

decrease in pressure means less volatiles

why does adding water lead to melting

volatile phases (like water) can rearrange chemical constituents within a system

why don’t granites make it to the surface

usually have water, so they reach exsolution phase and crystallize

three dominant reasons for mantle melting

mantle melting can be caused by temperature changes in places like hot spots where it melts the surrounding material. The dominant cause, from a change in pressure (without a change of heat), can occur from adiabatic upwelling. The change of composition, such as adding water (occurring at subduction zones), can also cause melting