Model Answer
0 min readIntroduction
Magmatic systems are dynamic environments where the composition of magma evolves through various processes, with fractional crystallization being a dominant one. Magnesium-rich magmas, commonly associated with mantle-derived melts like basalt and komatiite, are particularly susceptible to compositional changes due to the early crystallization of olivine. Olivine, a magnesium iron silicate ((Mg,Fe)2SiO4), preferentially crystallizes at high temperatures. Progressive removal of this mineral significantly alters the remaining melt’s chemical makeup, driving it towards more silica-rich and less magnesium-rich compositions. Understanding these changes is crucial for deciphering the petrogenesis of various igneous rocks.
Initial Magma Composition
Magnesium-rich magmas typically originate from partial melting of the Earth’s mantle. These magmas are characterized by high concentrations of MgO (8-21 wt%), relatively low SiO2 (45-52 wt%), and significant amounts of FeO, CaO, and Al2O3. They are generally ultramafic to mafic in composition, representing the initial melt produced from mantle peridotite. The exact composition varies depending on the degree of partial melting and the source mantle’s heterogeneity.
Progressive Olivine Removal and Compositional Changes
Stage 1: Early Crystallization & Initial Changes
As the magma cools, olivine begins to crystallize. This early-formed olivine is typically Mg-rich (forsterite endmember). Its removal through settling or flow processes depletes the melt in MgO and FeO, while simultaneously increasing the relative proportions of other oxides. The initial effect is a slight increase in SiO2 and CaO content in the residual melt.
Stage 2: Intermediate Olivine Removal & Continued Evolution
Continued removal of olivine leads to more pronounced compositional shifts. The residual melt becomes progressively enriched in SiO2, Al2O3, and CaO. The FeO/MgO ratio increases as the remaining olivine crystals become more iron-rich (fayalite endmember). This stage marks a transition towards more mafic compositions, potentially leading to the formation of high-Mg basalt. The liquidus temperature of the remaining melt also decreases.
Stage 3: Late-Stage Olivine Removal & Advanced Changes
As olivine crystallization nears completion, the remaining melt experiences significant changes. The SiO2 content continues to rise, and the magma may begin to saturate in other minerals like pyroxene. The depletion in MgO and CaO becomes substantial, and the magma’s viscosity increases. The composition can evolve towards intermediate compositions like andesite or dacite, depending on the extent of further fractional crystallization and potential assimilation of crustal material.
Bowen’s Reaction Series & Olivine’s Role
The changes observed during olivine removal are well-explained by Bowen’s Reaction Series. Olivine is one of the first minerals to crystallize from a mafic magma. As the magma cools, olivine reacts with the remaining melt, forming other minerals like pyroxene and plagioclase. The removal of olivine disrupts this equilibrium, driving the magma towards a different evolutionary path.
Quantitative Changes (Illustrative)
| Oxide | Initial Magma (wt%) | After 20% Olivine Removal (wt%) | After 50% Olivine Removal (wt%) |
|---|---|---|---|
| SiO2 | 48 | 50 | 53 |
| MgO | 18 | 14 | 9 |
| FeO | 8 | 9 | 10 |
| CaO | 10 | 11 | 12 |
Note: These values are illustrative and will vary depending on the initial magma composition and the specific conditions of crystallization.
Conclusion
Progressive removal of olivine from magnesium-rich magmas induces a systematic shift in composition, leading to increased silica content, decreased magnesium content, and a corresponding evolution towards more felsic magma types. This process, governed by principles like Bowen’s Reaction Series, is fundamental to understanding the diversity of igneous rocks found on Earth. Further research into magma dynamics and fractional crystallization continues to refine our understanding of these complex geological processes.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.