Igneous petrology of meteorites
Melting and crystallisation on asteroids, the Moon and Mars produced igneous rocks that geologists read with the same tools used on Earth. This lesson applies partial melting, fractional crystallisation and mineral chemistry.
- Explain partial melting and fractional crystallisation using Bowen's reaction series.
- Calculate the fayalite content of olivine and use it to classify an ordinary chondrite.
- Relate texture to cooling history in achondrites.
Links to: A-level geology: igneous processes, partial melting, fractional crystallisation, Bowen's reaction series, mineral solid solutions and textures; metamorphism.
Melting and crystallising
When a rock begins to melt, the first liquid is enriched in components with low melting points, such as iron, calcium, aluminium and sodium, leaving a residue richer in magnesium-rich olivine and pyroxene. This is partial melting. As a magma cools, minerals crystallise in a broad sequence described by Bowen's reaction series: olivine and calcium-rich plagioclase first, then pyroxene, with later minerals richer in iron, sodium and silica. Removing early crystals, fractional crystallisation, changes the composition of the remaining liquid.
On Vesta these processes produced diogenites, orthopyroxene-rich cumulates from deeper levels, and eucrites, basalts made of pigeonite and plagioclase. Howardites are breccias that mix the two at the surface.
Solid solutions
Olivine is a solid solution between forsterite, Mg₂SiO₄, and fayalite, Fe₂SiO₄; magnesium and iron substitute for each other in the same sites. Its composition is given as the mole percentage of fayalite, Fa = 100 × Fe/(Fe + Mg).
In equilibrated ordinary chondrites, the olivine composition distinguishes the three groups: H chondrites have about Fa16–20, L about Fa23–26 and LL about Fa27–32. The more oxidised groups have less metal and more iron in their silicates.
Texture records cooling
Textures reveal cooling rates. Coarse, interlocking crystals in diogenites indicate slow cooling at depth. Fine-grained eucrites with elongated plagioclase laths cooled quickly in lava flows or shallow intrusions. Many basaltic eucrites also show exsolution lamellae in pyroxene and evidence of later reheating. In chondrites, heating without melting produces the petrologic types 3 to 6: grain growth, homogenisation of olivine compositions and blurring of chondrule boundaries, all forms of thermal metamorphism.
Classifying a chondrite from its olivine
Electron microprobe analysis of olivine in an equilibrated ordinary chondrite gives the formula (Mg₁.₅₂Fe₀.₄₈)SiO₄. Calculate the fayalite content and classify the meteorite.
- Fa = 100 × Fe/(Fe + Mg).
- Fe + Mg = 0.48 + 1.52 = 2.00.
- Fa = 100 × 0.48/2.00 = 24 mol%.
Answer: Fa24 falls in the L range (Fa23–26), so the meteorite is an L chondrite. The petrologic type would then be assigned from its texture.
Compare thin-section photographs of a chondrite, a eucrite and a diogenite from the types and glossary pages. For each, describe grain size, the main minerals and any sign of chondrules, and interpret its formation in terms of melting, crystallisation, cooling rate and metamorphism.
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
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Question 1Compared with the solid residue, what is the first liquid produced by partial melting enriched in?
Partial melts are enriched in low-melting components, leaving a refractory, Mg-rich residue.
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Question 2Which meteorites are basalts from the crust of Vesta?
Eucrites are pigeonite–plagioclase basalts; diogenites are orthopyroxene cumulates from deeper levels.
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Question 3Olivine with formula (Mg₁.₆₄Fe₀.₃₆)SiO₄ has what fayalite content?
100 × 0.36/2.00 = 18 mol%, in the H chondrite range.
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Question 4What does a coarse, interlocking texture in an achondrite suggest?
Slow cooling allows large crystals to grow.
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Question 5What distinguishes petrologic type 6 from type 3 in ordinary chondrites?
Petrologic types 3 to 6 record increasing thermal metamorphism.