B2 · Publication Volume 7

Rock Melting and Magma Sources

partial melting, source composition and volatile controls

Pressure, temperature, source and volatile controls on partial melting
Pressure, temperature, source and volatile controls on partial melting

Learning objectives

After this lesson, you should be able to distinguish partial from complete melting, explain solidus and liquidus conceptually, compare decompression, volatile-flux and heat-transfer melting, calculate a simple batch-melting enrichment, and state why an erupted composition rarely identifies a unique source.

Start with a field problem

Two basaltic units have similar major-element compositions. One contains mantle xenoliths and relatively depleted incompatible-element patterns; the other contains hydrous phenocrysts and enriched incompatible elements. Are they products of different mantle sources, different melt fractions, different depths, crustal contamination, crystal accumulation or some combination?

A rock name does not answer the question. The erupted material has passed through source melting, segregation, transport, storage and crystallisation. Major elements can be buffered by crystallising minerals; incompatible trace elements respond strongly to melt fraction and partitioning; volatile minerals may grow or break down after the melt leaves its source. A defensible source model must therefore declare which parts of the signal are expected to survive each filter.

Core process model

A multicomponent rock begins to melt at its solidus and becomes fully molten only above its liquidus for the stated pressure, composition and volatile condition. Between them, crystals and melt coexist. Natural sources commonly produce partial melts because heat and mass are limited and because the first melt may segregate before the source approaches complete melting.

Three end-member triggers organise the problem. Decompression melting occurs when hot rock rises and pressure falls faster than it cools, allowing its path to cross a solidus. Fluid-flux melting occurs when components such as water lower the effective solidus and participate in reactions. Heat-transfer melting occurs when a hotter magma or anomalous heat source raises the temperature of surrounding material. Natural systems can combine all three.

Source composition controls which phases are present and therefore which melt is produced. A peridotitic mantle source, hydrated mafic crust, metapelite and carbonate-bearing source do not share one solidus or one melt composition. Pressure changes mineral stability and the components retained in the residue. Volatiles affect melting relations but are not unlimited catalysts: their amount, speciation and availability must be constrained.

Melt segregation is a separate step. Melt may remain connected along grain edges, collect in channels, react with its residue or freeze before leaving. The extracted magma is therefore not necessarily identical to an equilibrium melt calculated at one point. Source heterogeneity and reactive transport can make pooled melt look more uniform or more complex than any individual source domain.

Evidence and measurement

Build the source argument from several scales. Field relations and xenoliths can constrain accessible lithologies. Mineral compositions and inclusions may record pressure, temperature and volatile conditions. Whole-rock major and trace elements, radiogenic isotopes and stable isotopes constrain possible sources and contamination, provided alteration and crystal accumulation have been evaluated. Experimental or thermodynamic phase relations supply testable boundaries rather than decorative background curves.

Trace-element modelling requires partition coefficients appropriate to the mineral, melt composition, pressure and temperature. Define the bulk distribution coefficient


D = \sum_j X_j K_{d,j},

where X_j is the mass fraction of residual mineral j and K_{d,j} is its mineral–melt partition coefficient. A value of D much less than one describes an element that prefers melt in the stated assemblage; it is not an intrinsic label valid for every mineralogy.

Sampling must represent the material being modelled. A porphyritic lava with abundant accumulated crystals is not automatically a liquid composition. An altered rock may have gained or lost alkalis. A dyke margin may be chilled yet contaminated, and a xenolith may have reacted with its host. Record petrography before treating a table as magma.

Worked example

For simple equilibrium batch melting, mass balance gives


C_0 = F C_l + (1-F)C_s,

where C_0 is the initial concentration, C_l the melt concentration, C_s the mean residue concentration and F the melt fraction. With D=C_s/C_l,


\frac{C_l}{C_0}=\frac{1}{D+F(1-D)}.

Suppose an incompatible element has D=0.05. At F=0.10, the melt is enriched by


\frac{1}{0.05+0.10(0.95)}=6.90

relative to the starting source. At F=0.30, the factor falls to 2.99. A more enriched melt can therefore reflect a smaller melt fraction rather than a more enriched source.

This calculation is a sensitivity test, not a source solution. If residual mineral proportions change during melting, D changes. Fractional extraction, melt–rock reaction, pooling, later crystallisation and contamination alter the result. Report the chosen mineral modes and coefficients, and explore ranges rather than presenting one curve as unique.

Misinterpretations and uncertainty

Common errors include drawing a present-day geotherm and a generic solidus without matching composition or volatile state, treating water as proof of one tectonic setting, and equating an incompatible-element ratio with source composition while ignoring melt fraction. Similar basalts can arise by different combinations of source, melting and differentiation; different basalts can arise from one heterogeneous source.

Pressure estimates from residual mineral signatures are model dependent. Isotopic heterogeneity may reflect source, assimilant or inherited crystals. A xenolith proves transport from depth but may not be representative of the dominant melt-producing region. Absence of a phase in an erupted rock does not prove its absence in the source residue.

Practical investigation

Create three source models using the same starting concentration: batch melting at 5, 10 and 25 percent with D values of 0.01, 0.1 and 1.0. Plot enrichment against melt fraction. Then change the residual mineral mode so that D doubles halfway through melting. Explain which elements are most sensitive to melt fraction and why a multi-element pattern is more informative than one concentration.

For each model, write one observation that would support decompression, fluid flux or heat transfer and one alternative explanation. Keep the source lithology, melting trigger and melt-extraction mechanism in separate fields.

Mastery check

  1. Why does crossing a solidus not imply complete melting?
  2. How can rising mantle melt without receiving additional heat?
  3. Why can a volatile-bearing magma fail to prove fluid-flux melting at its source?
  4. In the batch-melting equation, what happens to an incompatible element as F increases?
  5. Which observations would help distinguish an enriched source from low-degree melting?

Sources and further reading