Bowen's Reaction Series

Bowen's reaction series is the foundational thermodynamic framework of igneous petrology, formulated in 1922–1928 by Canadian experimental petrologist Norman Levi Bowen at the Geophysical Laboratory of the Carnegie Institution of Washington, that predicts the order in which common rock-forming silicate minerals crystallize from a cooling silicate melt (magma) and react with the remaining liquid as temperature declines from ~1,200 °C to ~600 °C. By melting natural basalts and synthetic oxide mixtures in high-temperature platinum-wound furnaces and quenching them at controlled temperature intervals, Bowen demonstrated that a single parent mafic magma—such as basalt or gabbro—can evolve through magmatic differentiation and fractional crystallization into an entire spectrum of intermediate (andesite, diorite) and felsic (rhyolite, granite) igneous rocks and minerals.

Bowen arranged the crystallization sequence of silicate minerals into a downward-converging Y-shaped diagram aligned with decreasing crystallization temperature and increasing polymerization of [SiO₄]⁴⁻ silicate tetrahedra:

1. The Discontinuous Ferromagnesian Branch (Left Arm)

Along the discontinuous branch, iron- and magnesium-rich (ferromagnesian / mafic) silicate minerals crystallize sequentially. At each step, the mineral that crystallized at a higher temperature reacts peritectically with the silica-enriched residual melt at a discrete lower temperature to transform into a new mineral species with a fundamentally different crystal structure and higher Si:O polymerization ratio:

1. Olivine ((Mg,Fe)₂SiO₄ — Isolated Nesosilicate, ~1,200–1,400 °C): Magnesium-rich olivine (forsterite, Mg₂SiO₄) is the first ferromagnesian phase to nucleate from a cooling ultramafic or basaltic magma. Its structure consists of isolated [SiO₄]⁴⁻ tetrahedra (0 shared corner oxygens, Si:O = 1:4) linked by octahedrally coordinated Mg²⁺ and Fe²⁺ cations.
2. Pyroxene ((Mg,Fe,Ca)SiO₃ — Single-Chain Inosilicate, ~1,050–1,150 °C): As the magma cools and silica activity (a_SiO2) rises in the residual melt, early-formed olivine crystals become unstable and react with dissolved silica at a peritectic reaction boundary to form orthopyroxene (enstatite, MgSiO₃) and clinopyroxene (augite):

In pyroxenes, each [SiO₄] tetrahedron shares 2 of its 4 corner oxygen atoms with neighboring tetrahedra (Si:O = 1:3), forming infinite single chains that intersect at 87° and 93° cleavage angles. If cooling is slow under equilibrium conditions, rims of pyroxene(reaction rims / coronas) envelop and replace olivine cores.

1. Amphibole / Hornblende (Ca₂(Mg,Fe,Al)₅(Al,Si)₈O₂₂(OH)₂ — Double-Chain Inosilicate, ~850–950 °C): As temperature drops further and dissolved water (H₂O) concentrates in the residual hydrous melt, single-chain pyroxene reacts with the melt to form calcic amphibole (hornblende). In amphiboles, two parallel single chains cross-link by sharing alternating 2 and 3 corner oxygens (Si:O = 4:11, double-chain structure), producing diagnostic 56° and 124° prismatic cleavage angles and incorporating hydroxyl (OH⁻) groups.
2. Biotite Mica (K(Mg,Fe)₃(AlSi₃O₁₀)(OH)₂ — Sheet Phyllosilicate, ~700–800 °C): At lower temperatures in intermediate-to-felsic melts, double-chain amphibole reacts with potassium- and water-enriched liquid to crystallize biotite mica, in which each tetrahedron shares 3 of its 4 corner oxygens (Si:O = 2:5) in infinite two-dimensional hexagonal sheets held together by interlayer K⁺ ions.

2. The Continuous Plagioclase Solid-Solution Branch (Right Arm)

Simultaneously along the continuous branch, calcium- and sodium-bearing plagioclase feldspars (3D framework tectosilicates in which all 4 corner oxygens of every SiO₄ and AlO₄ tetrahedron are shared) crystallize without changing their fundamental crystal structure. Instead, plagioclase undergoes a continuous coupled cation substitution—trading Ca²⁺ + Al³⁺ (total charge +5) in the high-temperature endmember anorthite (CaAl₂Si₂O₈) for Na⁺ + Si⁴⁺ (total charge +5) in the low-temperature endmember albite (NaAlSi₃O₈):

• Anorthite (An_90–100, calcic) → Bytownite (An_70–90) → Labradorite (An_50–70, typical of gabbro, basalt, and anorthosite) → Andesine (An_30–50, typical of andesite and diorite) → Oligoclase (An_10–30) → Albite (An_0–10, sodic, typical of granite and rhyolite).

Because solid-state diffusion of coupled Ca²⁺ + Al³⁺ ⇌ Na⁺ + Si⁴⁺ inside an already crystallized plagioclase lattice is sluggish, rapid cooling prevents complete re-equilibration with the melt, producing concentric oscillatory or normal compositional zoning (a calcic anorthite-rich core surrounded by progressively more sodic albite-rich outer rims).

3. The Residual Felsic Stem (Bottom Convergence, ~600–750 °C)

Once the discontinuous ferromagnesian branch (at biotite) and the continuous plagioclase branch (at sodic oligoclase/albite) converge, the remaining silica-, potassium-, and aluminum-rich melt crystallizes the three minerals of the felsic eutectic stem:

• Potassium Feldspar (Orthoclase / Microcline, KAlSi₃O₈ — Framework Tectosilicate)
• Muscovite Mica (KAl₂(AlSi₃O₁₀)(OH)₂ — Sheet Phyllosilicate)
• Quartz (SiO₂ — Pure 3D Framework Tectosilicate, Si:O = 1:2)

If all crystals remained suspended in constant chemical contact with the cooling melt (equilibrium crystallization), a basaltic magma would react completely down the branches until it solidified into an ordinary gabbro or basalt of identical bulk composition to the starting melt. However, Bowen recognized that if early-formed dense crystals (olivine, ρ ≈ 3.3–3.4 g·cm⁻³; pyroxene, ρ ≈ 3.2–3.5 g·cm⁻³; and calcic plagioclase) are physically separated from the buoyant silicate liquid (ρ ≈ 2.6–2.8 g·cm⁻³)—by gravitational crystal settling to the floor of a magma chamber to form layered ultramafic cumulates (such as the Bushveld Complex of South Africa or the Palisades Sill of New Jersey), or by filter pressing of interstitial melt—they can no longer react peritectically with the liquid (fractional crystallization).

Because early olivine (38–42 wt% SiO₂) and calcic anorthite (43 wt% SiO₂) contain a lower weight percentage of silica than the parent basaltic melt (~50 wt% SiO₂), removing them selectively strips MgO, FeO, and CaO from the liquid while progressively enriching the residual magma in SiO₂ (up to 70–77 wt%), Na₂O, K₂O, H₂O, and incompatible trace elements (U, Th, Li, Be, Rb). Consequently, fractional crystallization drives the residual melt step-by-step down Bowen's reaction series from basaltic → andesitic → dacitic → granitic composition.

In 1938, American geologist Samuel S. Goldich analyzed the chemical weathering profiles of igneous and metamorphic outcrops across North America and discovered that the relative resistance of silicate minerals to chemical silicate weathering (hydrolysis, oxidation, and carbonic acid dissolution at Earth's surface, ~15–25 °C, 1 atm, oxygenated rainwater) is the exact mirror image of Bowen's reaction series (Goldich's dissolution series):

• Least Stable at Earth's Surface (Weather Rapidly): Olivine and calcic plagioclase (anorthite)—the minerals that crystallize first at the highest temperatures (1,150–1,400 °C) and anhydrous mantle/crustal pressures—are furthest out of thermodynamic equilibrium with cool, wet, oxygenated surface conditions. Furthermore, olivine's isolated [SiO₄]⁴⁻ tetrahedra are held together solely by ionic Mg–O and Fe–O bonds; when carbonic acid (H₂CO₃) leaches Mg²⁺ and oxidizes Fe²⁺ to Fe³⁺ (rust/goethite), the crystal lattice disintegrates rapidly, explaining why green olivine beaches (such as Papakōlea in Hawaiʻi) are geologically ephemeral.
• Most Stable at Earth's Surface (Resist Weathering): Muscovite, orthoclase (which slowly hydrolyzes to kaolinite clay), and above all quartz—the mineral that crystallizes last at the lowest magmatic temperatures (~600–650 °C)—consist of a fully cross-linked 3D covalent Si–O–Si framework (bond enthalpy ~452 kJ·mol⁻¹, Mohs hardness 7, zero cleavage, and no oxidizable Fe²⁺ or leachable Ca²⁺/Mg²⁺ cations). Consequently, quartz survives thousands of kilometers of fluvial and coastal transport to concentrate as mature quartz arenite sandstone.

Why are olivine and quartz almost never found together in the same equilibrium igneous rock?

In Bowen's discontinuous reaction series, magnesium-rich olivine (Mg₂SiO₄) is undersaturated in silica. If free dissolved silica (SiO₂) remains in the cooling magma after olivine crystallizes, olivine reacts with that silica at the ~1,100 °C peritectic boundary to form orthopyroxene (Mg₂SiO₄ + SiO₂ → 2 MgSiO₃). Free quartz (SiO₂) can only crystallize at the bottom of Bowen's series (~650 °C) if there is an excess of SiO₂ left over after all MgO and FeO have been converted into pyroxene, amphibole, or biotite; conversely, olivine only survives to room temperature if the magma ran out of excess SiO₂ before all the olivine could be converted to pyroxene. Therefore, magnesium-rich olivine and quartz are mutually incompatible at thermodynamic equilibrium.

How does water pressure (P_H2O) affect Bowen's reaction series above subduction zones?

Dissolved volatiles—especially water (H₂O) released by dehydration of subducting blueschist and serpentinite slabs—depress the liquidus and solidus temperatures of silicate melts by depolymerizing Si–O–Si bridges into Si–OH groups (flux melting). More importantly, high P_H2O is required to stabilize the hydrous minerals of the lower discontinuous branch—amphibole (hornblende) and biotite/muscovite mica, which contain structural OH⁻ groups—and suppresses early plagioclase crystallization relative to ferromagnesian phases (driving calc-alkaline differentiation typical of continental volcanic arcs such as the Andes and Cascades, rather than iron-enrichment tholeiitic differentiation typical of dry mid-ocean ridge basalts).

What are reaction rims (coronas) in thin section, and how do they prove Bowen's discontinuous series?

When a petrographic thin section of a gabbro or andesite is examined under a polarizing microscope, geologists frequently observe rounded, partially resorbed cores of olivine completely jacketed by a concentric shell (corona or reaction rim) of orthopyroxene, or cores of pyroxene jacketed by rims of hornblende. These rims form when cooling proceeds too rapidly for solid-state diffusion to convert the entire interior of a large olivine crystal into pyroxene at the peritectic boundary: once the outer pyroxene rim armors the olivine core and physically isolates it from the silica-rich melt, the incomplete reaction is frozen in place as direct textural evidence of Bowen's discontinuous reaction sequence.

1. Bowen, N. L. (1922). "The Reaction Principle in Petrogenesis." The Journal of Geology, 30(3), 177–198. https://doi.org/10.1086/622871
2. Bowen, N. L. (1928). The Evolution of the Igneous Rocks. Princeton University Press. ISBN 978-0486603117
3. Goldich, S. S. (1938). "A study in rock-weathering." The Journal of Geology, 46(1), 17–58. https://doi.org/10.1086/624619
4. Winter, J. D. (2014). Principles of Igneous and Metamorphic Petrology (2nd ed.). Pearson Education. ISBN 978-0321592576