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Incoming links: Primary magma
The geological record is a collection of thermometers with different sampling biases, not a direct time series of the whole mantle.
Primary melt compositions constrain melting temperatures. Reconstruct primary magma compositions from dated, suitably unaltered basalts by correcting for fractional crystallization and checking equilibrium with olivine. Experimental phase equilibrium constraints and mineral–melt thermometers then constrain the pressure and temperature of melt generation. In laboratory calibrations, a candidate melt and mineral assemblage are equilibrated at controlled pressure and temperature, quenched, and their crystals and glass analyzed. Repetition across adjacent conditions brackets the relevant phase equilibrium boundaries. Correcting for decompression and latent heat then allows comparison as mantle potential temperature. Age-controlled, non-arc basalt suites are especially valuable for separating changes in ambient mantle from exceptional mantle plumes. The 2010 reconstruction by Claude Herzberg and colleagues combines this approach with the temporal distribution of high-Mg parental melts.[ref]
Thermal history of the Earth and its petrological expression by Claude Herzberg and others. Sections on primary magma reconstruction and comparison of non-arc basalts with komatiites
Komatiites provide evidence for unusually hot melting, with qualifications. Many ancient high-Mg lavas, their olivine compositions and spinifex textures indicate large degrees of melting and high parental temperatures. Their abundance in Archean terrains and rarity in the recent record are consistent with a hotter past. However, a hot mantle plume, dissolved water, unusual source composition or crystal accumulation can change the interpretation. A komatiite temperature is not automatically the ambient mantle temperature, and spinifex texture alone is not a calibrated thermometer.
Melting residues and crustal products provide independent checks. Depleted peridotite mantle xenoliths, preserved mineral chemistry and compatible-element depletion constrain former melt fractions. More extensive melting can produce thicker mafic crust and more refractory residual mantle; these are predictions to compare with preserved rock packages and seismic or petrological constraints on ancient lithosphere. Metamorphic facies and pressure–temperature paths provide contextual evidence about geothermal regimes, but tectonic burial, decompression and preservation can mimic temporal changes. They do not directly measure .
The most persuasive inference is therefore agreement between dated melt thermometry, melting residues and their predicted crustal consequences, while separating source fertility, volatile content, mantle plume anomalies, alteration and selective preservation. Those controls explain why hotter ancient melting is better established than one exact global cooling curve.