= Solution
\b[Hotter <Archean> <mantle> would generally produce thicker, more magnesium-rich <oceanic crust>, together with a more strongly depleted residual <mantle>.] Greater melt production, not merely thermal expansion of an unchanged modern column, is the principal reason for the increased crustal thickness.
At a <spreading ridge>, upwelling <peridotite> undergoes <adiabatic decompression melting>. A higher mantle <potential temperature> makes the ascending <mantle> intersect the <solidus> at greater <pressure> and depth. It therefore melts over a longer decompression interval and generally reaches a larger integrated <melt fraction>. With comparable <melt focusing>, more melt is supplied per unit area of newly formed <oceanic crust>.
Modern normal <oceanic crust> is about 6–7 km thick and contains <basaltic> lavas, a <sheeted dike complex>, and <gabbro> with <mafic> cumulates at depth. An illustrative reconstruction for hot ambient <Archean> <mantle> gives about 25–35 km of <oceanic crust>, rather than a universal fixed thickness. Its primary liquids are more magnesium-rich than typical modern primary ridge liquids; their crystallization produces more <olivine>- and <pyroxene>-rich <cumulates>. The extracted liquid and its crystal products must be distinguished: the entire final <oceanic crust> need not be a homogeneous <komatiite> layer.<Thermal history of the Earth and its petrological expression by Claude Herzberg and others>{ref}
The residual <peridotite> loses more melt components, becoming more refractory and commonly <harzburgite>-rich. Thick buoyant <basaltic crust>, its later <eclogitization>, and the depleted underlying <mantle> can change the mechanics of <subduction>. A hotter <mantle> does not by itself establish how fast plates moved or whether a globally connected modern <plate tectonics> regime operated.
<Komatiites>, with very magnesium-rich primary liquids and high inferred eruption <temperatures>, provide evidence for hot melting, but may sample especially hot <mantle plumes>. Estimates of ambient <mantle> conditions also use less exceptional non-arc <basalts>. Variable source composition, <water> content, cooling, melt extraction and later alteration all limit the precision of a single inferred <Archean> thickness. The robust prediction is \b[a larger melting column and melt yield, thicker igneous <oceanic crust>, and a more depleted residue].
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