"Me first" elements

Think of elements as having different personalities. Some will take any opportunity to get out of mineral structures and into liquid magma. Others fit nicely into mineral structures and would just as soon stay in the solid peridotite. The behavior depends the mineral structure and on the properties of each element.

As the peridotite continues its upward journey beneath a spreading center, at some depth (~100 km), the pressure will be low enough that melting begins. Minerals with the lowest melting temperatures melt first. Pyroxene melts before olivine, contributing magnesium, iron, and calcium, as well as silicon and oxygen to the melt. This process is called partial melting. The new magma has a different composition, basalt, than its parent rock, peridotite.

A similar situation occurs with a frozen (solid) fruit juice mixture. As the juice melts, the melt contains a higher percentage of fruit flavor than the original juice. The part remaining solid is mostly ice. If the early-melting fruit liquid is removed from the container (a magma chamber), the new liquid will be depleted in fruit flavor and enriched in water when the rest of the solid (ice) melts.