Questions: Bowen's Reaction Series and Fractional Crystallization

Question 1

Two magma chambers start with identical basaltic compositions. In chamber A, crystals continuously react with the melt as temperature falls. In chamber B, dense crystals sink and are physically removed from contact with the melt. How will the final residual liquid compositions differ?

• A. Both chambers produce identical residual liquids, since they started with the same composition
• B. Chamber A produces a more silicic residual liquid because reactions consume more elements
• C. Chamber B produces a more silicic residual liquid enriched in Na and K, because early Mg- and Ca-rich minerals were removed
• D. Chamber B produces a more mafic residual liquid because it lost the silica-bearing early crystals

Answer: (C)

Fractional crystallization depends on physical separation of early-forming crystals. In chamber B, olivine and Ca-plagioclase sink and are removed, stripping Mg, Fe, and Ca from the melt. The residual liquid becomes progressively enriched in silica, sodium, and potassium — the components that form late in Bowen's series. Chamber A re-equilibrates continuously through crystal-melt reactions, limiting differentiation. The physical separation in chamber B is what drives magmatic differentiation.

Question 2

According to Bowen's reaction series, which mineral pair forms at the highest temperatures and crystallizes first from a cooling basaltic magma?

• A. Quartz and potassium feldspar
• B. Amphibole and sodium-rich plagioclase
• C. Olivine and calcium-rich plagioclase (anorthite)
• D. Muscovite and biotite

Answer: (C)

Bowen's series places olivine (discontinuous branch) and calcium-rich plagioclase/anorthite (continuous branch) at the highest crystallization temperatures. These mafic minerals form first from cooling basaltic magma. Quartz and K-feldspar form last from the most silica-rich residual melts. Amphibole and biotite form at intermediate temperatures. This sequence explains why mafic minerals dominate early-crystallized igneous rocks like gabbro.

Question 3

Early-crystallizing minerals in Bowen's series are enriched in magnesium, iron, and calcium relative to the residual melt.

• T. True
• F. False

Answer: True

High-temperature minerals like olivine (Mg, Fe silicate) and calcium-rich plagioclase incorporate Mg, Fe, and Ca preferentially. Their crystallization removes these elements from the melt, leaving the residual liquid depleted in Mg, Fe, and Ca and enriched in Si, Na, and K. This is precisely why fractional crystallization of basaltic magma can ultimately produce granite — not through mixing of different magmas, but by sequential removal of mafic minerals.

Question 4

Bowen's reaction series describes the crystallization sequence for most rock types, including sedimentary and metamorphic rocks.

• T. True
• F. False

Answer: False

Bowen's series was derived experimentally from cooling basaltic magma and applies specifically to igneous (magmatic) crystallization. Sedimentary rocks form through weathering, transport, deposition, and diagenesis — processes governed by stability at Earth's surface. Metamorphic rocks form through solid-state mineral transformations under heat and pressure without melting. The series has no direct application to non-igneous rock types.

Question 5

Why does fractional crystallization require physical separation of crystals from the melt, and what happens if crystals remain in contact throughout cooling?

Model answer: Physical separation prevents early-formed crystals from reacting back with the melt as temperature falls. If crystals remain in contact, they re-equilibrate with the liquid through reaction (as in the discontinuous branch), consuming the compositional difference that crystallization created and producing a more uniform final rock. When crystals are removed (by sinking, filtering, or wall accumulation), the melt cannot re-equilibrate, and each removal event progressively enriches the residual liquid in components not incorporated by the departed crystals.

This is why 'fractional' crystallization is distinct from 'equilibrium' crystallization. In equilibrium crystallization, all crystals stay in contact with the melt and the system reaches a uniform final composition. Fractional crystallization removes each batch of crystals as it forms — like repeatedly removing partial batches of sugar crystals from a solution, leaving behind an increasingly concentrated remainder. The cumulate rocks that settle out and the evolved liquid that remains preserve a record of this step-by-step compositional evolution.