ERB222 Chap.2 Basalt Petrogenesis and Mantle Melting
Basalt Petrogenesis and Mantle Melting
How a solid mantle makes basalt
The Week 2 lecture starts from a puzzle: the mantle is solid, yet basalt erupts in every tectonic setting. The answer is decompression melting. Convection carries hot peridotite upward with almost no heat loss, the melting temperature falls with pressure, and the rising rock crosses its solidus.
Only part of it melts, usually less than 30%, because lherzolite is a mixture of olivine, two pyroxenes and an aluminous phase that each melt differently.
Phase diagrams as the explanation
Binary diagrams show why. A solid solution such as plagioclase or olivine melts across a temperature interval, with liquid and crystal differing in composition.
A eutectic system always produces the same first melt, whatever the starting proportions, and incongruent melting of enstatite yields a liquid richer in silica than the solid.
These ideas carry straight into the basalt tetrahedron and its two critical planes.
Pressure and degree of melting
The chapter ends with the comparison examiners return to: alkali basalts from deeper, smaller melt fractions and tholeiites from shallower, larger ones, together with the criteria that separate primary from primitive magmas.
What this chapter covers
- 01
Decompression melting and the mantle solidus
- 02
Partial melting of lherzolite
- 03
Solid solution loops for plagioclase and olivine
- 04
Eutectic, peritectic and incongruent melting
- 05
Basalt tetrahedron and critical planes
- 06
Alkali basalt versus MORB tholeiite
- 07
Primary and primitive magma criteria
Worked example · free
Melting a frozen brine from the eutectic upward
- 1The first liquid appears at the eutectic, -21.1°C, with 23.3 wt% NaCl, whatever the proportions of ice and salt.
- 1Temperature holds at -21.1°C while ice and salt melt together, until the salt is exhausted.
- 1Lever rule just above the eutectic: liquid = 5/23.3 = 21.5%, leaving 78.5% ice.
- 1With further heating the liquid moves along the ice liquidus toward lower NaCl, and the last ice melts where that curve reaches 5 wt% NaCl, between -21.1°C and 0°C.
Key terms
- Decompression Melting
- Melting caused when hot mantle rises adiabatically and its falling pressure lowers the solidus below the rock's temperature.
- Solid Solution
- A mineral series, such as plagioclase or olivine, whose composition varies continuously between two end-members.
- Peritectic Point
- A point on a phase diagram where liquid reacts with an earlier crystal to form a new solid at constant temperature.
- Incongruent Melting
- Melting in which a solid produces a liquid of different composition, as enstatite yields forsterite plus a silica-rich melt.
- Basalt Tetrahedron
- A four-component normative model with olivine, nepheline, clinopyroxene and quartz at its corners, used to classify basalts.
- Thermal Divide
- A boundary that cooling liquids cannot cross, formed by the critical plane of silica undersaturation at low pressure.
- Mg Number
- The molar ratio Mg/(Mg + Fe) in a rock or mineral, used as a measure of how little a basalt has evolved.
Basalt Petrogenesis and Mantle Melting FAQ
Why does the mantle melt at mid-ocean ridges if nothing heats it?
Plates pulling apart let deep peridotite rise. The parcel loses almost no heat on the way up, but the pressure drop lowers its melting temperature, so the rock crosses its solidus. Raising temperature, as in plumes, and adding water, as in subduction zones, are the other two routes the unit covers.
What melts first in a mantle lherzolite?
Clinopyroxene and the aluminous phase, spinel at lower pressure or garnet at higher pressure, melt first, followed by orthopyroxene. Olivine is the last phase to disappear. Removing clinopyroxene turns the residue into harzburgite, which is why depleted mantle is olivine and orthopyroxene rich.
How can the same mantle make both alkali basalt and tholeiite?
Pressure and melt fraction decide it. Small degrees of melting at high pressure give silica-undersaturated, nepheline-normative liquids, whereas larger degrees of melting at lower pressure give hypersthene-normative tholeiites. The petrogenetic grid shows 15% melting at about 1.6 GPa giving alkaline basalt and 30% at the same depth giving tholeiite.
What is the difference between a primary and a primitive magma?
A primary magma is unchanged since leaving its source and should show the highest Mg number, more than 1000 ppm Cr and more than 400 to 500 ppm Ni. A primitive magma only needs to be little evolved, with MgO above 6.0 wt% and few phenocrysts, so it is much more common.
Exam move
Redraw the plagioclase loop from memory using An60, An20 at 1340°C and An87 at 1475°C, then label solidus, liquidus and both tie-lines. Sketch the H2O and NaCl diagram from its three control points and practise the lever rule on two brines. Finish by writing the alkali basalt versus tholeiite comparison as a four-row table and testing two invented basalts against the primary criteria.
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