Bowen's Reaction Series

Bowen's reaction series is the foundational petrogenetic framework of igneous petrology that describes the predictable sequence in which common rock-forming silicate minerals nucleate, crystallize, and chemically react with residual melt as a mafic (basaltic) magma cools from ~1,200–1,300 °C down to ~600–650 °C. Formulated between 1912 and 1928 by Canadian petrologist Norman Levi Bowen at the Geophysical Laboratory of the Carnegie Institution of Washington through high-temperature quenching experiments on synthetic silicate melts, the reaction series solved the central problem of nineteenth-century geology: how a single parent mafic magma (rich in magnesium and iron, ~45%–52% SiO₂) can evolve by fractional crystallization into intermediate (andesitic/dioritic, ~52%–63% SiO₂) and felsic (granitic/rhyolitic, >63%–75% SiO₂) igneous rocks.

Crucially, Bowen replaced the older misconception of simple "eutectic precipitation" (in which minerals merely drop out of solution and remain inert) with the principle of peritectic and solid-solution reaction: unless early-formed crystals are physically separated from the liquid by gravitational settling or filter pressing, minerals that crystallize at high temperatures continually react with the cooling residual melt to transform into minerals stable at lower temperatures.

Bowen arranged the crystallization sequence of a cooling subalkaline magma into an inverted "Y" diagram consisting of two simultaneous, independent upper branches—the Discontinuous Ferromagnesian Branch on the left and the Continuous Plagioclase Solid-Solution Branch on the right—which converge at lower temperatures (~750–600 °C) into a single felsic residual stem:

1. The Discontinuous Branch: Peritectic Resorption and Structural Polymerization

In the discontinuous reaction series, each successive ferromagnesian (Fe–Mg) mineral possesses a fundamentally different crystal structure with progressively higher polymerization of its [SiO₄]⁴⁻ silica tetrahedra (isolated → single chain → double chain → sheet).

The classic thermodynamic archetype discovered by Bowen and Olaf Andersen (1914) is the peritectic (incongruent melting) reaction of Forsterite Olivine (Mg₂SiO₄) with dissolved silica (SiO₂) in the binary MgO–SiO₂ system at 1,557 °C (1 atm) (occurring at ~1,100–1,150 °C in natural hydrous multicomponent basalts):

As the melt cools to the peritectic reaction boundary, early-formed olivine crystals become thermodynamically unstable in the presence of the silica-enriched liquid; the melt resorbs (corrodes) the outer margins of the olivine grains to form a reaction rim (corona) of orthopyroxene or augite. With further cooling and water (H₂O) enrichment, pyroxene reacts with the melt at a second peritectic boundary to form hydrous hornblende amphibole, which in turn reacts to form biotite mica. Simultaneously, within each ferromagnesian mineral phase, the Mg²⁺ / (Mg²⁺ + Fe²⁺) ratio (Mg-number, *Mg*#) decreases continuously with falling temperature (for example, from high-temperature forsterite [Mg₂SiO₄, T_m = 1,890 °C] toward lower-temperature fayalite [Fe₂SiO₄, T_m = 1,205 °C]).

2. The Continuous Branch: Coupled Ca²⁺ + Al³⁺ ⇌ Na⁺ + Si⁴⁺ Solid Solution

Unlike the discontinuous branch, the continuous reaction series occurs entirely within a single triclinic framework-silicate crystal structure—plagioclase feldspar—via coupled heterovalent cation substitution across the continuous solid-solution binary between Anorthite (CaAl₂Si₂O₈, T_m = 1,553 °C) and Albite (NaAlSi₃O₈, T_m = 1,118 °C):

Because the liquidus and solidus curves of the Ab–An phase diagram form a smooth binary solid-solution loop, the first plagioclase crystals to precipitate from a hot basaltic melt are strongly enriched in calcium (An_70–90) relative to the coexisting liquid. Under equilibrium crystallization (slow cooling with full re-equilibration), Na⁺ and Si⁴⁺ from the cooling melt must continuously diffuse into the interior of the growing plagioclase crystal while Ca²⁺ and Al³⁺ diffuse out, shifting the homogeneous crystal toward sodium-rich oligoclase/albite (An_10–30). However, because solid-state coupled diffusion of NaSi ⇌ CaAl inside the rigid aluminosilicate framework is slow (D < 10⁻²⁰ m² s⁻¹), natural cooling magmas frequently fail to re-equilibrate crystal interiors—producing concentric normal compositional zoning (Ca-rich anorthite/labradorite cores surrounded by progressively more Na-rich andesine/oligoclase rims, readily visible under cross-polarized petrographic microscopy).

If early-crystallizing minerals remained suspended in the magma and reacted to equilibrium down to the solidus, a basaltic parent melt would solidify into a mafic gabbro or basalt (olivine + pyroxene + Ca-plagioclase). Magmatic differentiation occurs when fractional crystallization physically prevents early crystals from reacting with the residual melt via three mechanisms:

1. Gravitational Crystal Settling (or Floatation) and Cumulate Layering: Because early olivine (ρ ≈ 3.3–3.6 g cm⁻³), chromite, and pyroxene (ρ ≈ 3.2–3.5 g cm⁻³) are denser than basaltic liquid (ρ ≈ 2.7–2.8 g cm⁻³), they sink by Stokes flow to the floor of the magma chamber to form ultramafic cumulate layers (dunite, peridotite, pyroxenite—exemplified by the Skaergaard Intrusion in Greenland, the Palisades Sill in New Jersey/New York, and the Bushveld Complex in South Africa).
2. Reaction-Rim Armoring (Zoned Crystals): When a rim of pyroxene armors an olivine core, or a rim of sodic oligoclase armors a calcic labradorite core, the interior of the crystal is chemically isolated from the liquid.
3. Compaction and Filter Pressing of Crystal Mushes: Tectonic compression or buoyancy squeezes low-density, silica- and water-rich interstitial melt out of a rigid crystal mush (>50% crystallinity), allowing the evolved dacitic or rhyolitic/granitic melt (~65%–75% SiO₂) to ascend and erupt or crystallize as a felsic pluton.

What is the Goldich Dissolution Series, and why is it Bowen's Reaction Series in reverse?

In 1938, geochemist Samuel S. Goldich analyzed the chemical weathering rates of igneous rocks exposed at Earth's surface (T ≈ 15–25 °C, P = 1 atm, in the presence of oxygenated rainwater, carbonic acid H₂CO₃, and plant organic acids) and discovered that the order of mineral chemical weathering stability at Earth's surface is the exact inverse of Bowen's Reaction Series:

• Least Stable at Surface (Weather Fastest): Olivine, Pyroxene, and Anorthite (Ca-Plagioclase). Because these minerals crystallized at 1,100–1,300 °C in an anhydrous, reduced mantle/lower-crustal melt, they are furthest from thermodynamic equilibrium at cool, wet, oxidizing surface conditions. Furthermore, olivine consists of isolated [SiO₄]⁴⁻ tetrahedra held together entirely by weak ionic Mg²⁺–O and Fe²⁺–O bonds (Fe²⁺ is rapidly oxidized to Fe³⁺ rust), and Ca-plagioclase has half of its strong Si⁴⁺–O tetrahedral sites replaced by weaker Al³⁺–O bonds.
• Most Stable at Surface (Weather Slowest): Muscovite and Quartz (SiO₂). Quartz crystallizes last (~600–650 °C, closest to surface conditions) and consists of a fully polymerized 3D tectosilicate framework in which every oxygen atom is shared between two silicon atoms via strong covalent Si–O–Si bonds with zero soluble cations (Mg²⁺, Fe²⁺, Ca²⁺, Na⁺) to leach—explaining why mature continental beach sands and sandstones consist overwhelmingly of quartz.

Why are Quartz and Magnesium-Rich Olivine almost never found together in the same igneous rock?

Under Bowen's discontinuous reaction series, forsteritic olivine (Mg₂SiO₄) is a silica-undersaturated mineral that reacts directly with free dissolved silica (SiO₂) in the magma to form orthopyroxene (2 MgSiO₃) at ~1,100–1,557 °C. Consequently:

• If a magma contains an excess of SiO₂ (a silica-oversaturated magma), all early-formed Mg-olivine is consumed to make orthopyroxene long before the residual melt cools to ~650 °C to precipitate free quartz (SiO₂).
• Conversely, if a magma has excess MgO relative to SiO₂ (a silica-undersaturated mafic or ultramafic magma), all free silica is tied up in olivine and pyroxene, leaving zero excess SiO₂ to crystallize quartz. (The rare exception occurs only in iron-endmember fayalite [Fe₂SiO₄] granites, or in disequilibrium volcanic rocks where a basaltic magma containing olivine xenocrysts rapidly mixes with a quartz-bearing rhyolitic magma immediately before eruption.)

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2. Bowen, N. L. (1928). The Evolution of the Igneous Rocks. Princeton University Press. https://doi.org/10.1515/9781400868506
3. Bowen, N. L., & Andersen, O. (1914). "The binary system MgO–SiO₂." American Journal of Science, s4-37(222), 487–500. https://doi.org/10.2475/ajs.s4-37.222.487
4. Goldich, S. S. (1938). "A study in rock-weathering." The Journal of Geology, 46(1), 17–58. https://doi.org/10.1086/624619