Here's what the magnesium number can tell us about the source of volcanic melts:
1. Degree of Partial Melting:
* High Mg# (above 60): Indicates a melt derived from a mantle source that has undergone minimal partial melting. This means the melt represents a relatively "pristine" sample of the mantle.
* Low Mg# (below 60): Suggests a melt that has experienced significant partial melting or has interacted with other rocks (crustal contamination) during its ascent. This indicates a more evolved melt, potentially influenced by other processes.
2. Source Rock Composition:
* Peridotite Source: Peridotite is a common mantle rock with high Mg#. Volcanic melts with high Mg# are likely derived from peridotite.
* Basalt Source: Basalt is a volcanic rock that can be derived from peridotite, but it generally has a lower Mg# due to partial melting and interaction with other rocks.
* Crustal Contamination: Volcanic melts that have interacted with crustal rocks during their ascent will have a lower Mg# due to the incorporation of iron and other elements from the crust.
3. Evolution of the Melt:
* Crystallization: As a melt cools and crystallizes, the magnesium content in the remaining melt decreases. This results in a lower Mg# in the final volcanic rock.
* Fractional Crystallization: During fractional crystallization, different minerals crystallize out of the melt at different temperatures. This process can also lead to a decrease in Mg#.
4. Tectonic Setting:
* Mid-Ocean Ridges: Magmas erupted at mid-ocean ridges are generally characterized by high Mg#. This is because the mantle source is relatively uncontaminated and undergoes minimal partial melting.
* Subduction Zones: Magmas erupted at subduction zones can have a wide range of Mg#. The Mg# can be influenced by the amount of crustal contamination and the degree of partial melting.
Limitations:
* Mg# is only one parameter that can be used to understand the origin of volcanic melts.
* Other factors such as trace element concentrations, isotopic ratios, and mineral compositions need to be considered for a comprehensive understanding.
In summary, the Mg# provides valuable insights into the source and evolution of volcanic melts. It helps us understand the degree of partial melting, the source rock composition, the processes that influence melt evolution, and the tectonic setting where the melt was generated.
