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The lunar crustal dichotomy combines several observables: the far side of the Moon has generally thicker, higher, feldspathic lunar highlands, whereas most visible lunar maria and the Procellarum KREEP Terrane occupy the near side of the Moon. These need not all have been produced by a single event. The old anorthosite crust formed mainly before the younger basaltic mare infill. Gravity-based crustal models combined with sample and seismic constraints demonstrate that crustal thickness and porosity must be considered alongside surface color.^[ref]

An early thermal and chemical asymmetry is one physically motivated option. In a lunar magma ocean, crystallizing, relatively buoyant plagioclase can accumulate to form feldspathic crust. On a young Moon that rapidly attained synchronous rotation, radiation from a still-hot nearby Earth would preferentially warm the nearside. The primordial Earthshine hypothesis proposes that this imposed a thermal gradient in the atmosphere and accreting material: crust-forming refractory constituents condensed preferentially toward the cooler farside, favoring thicker farside crust. This is more specific than simply saying that the farside cooled faster; it requires a persistent hemispheric chemical contrast.^[ref]

Subsequent concentration of incompatible elements in KREEP-rich late liquids can help preserve a thermal asymmetry: potassium, uranium and thorium provide radiogenic heating, supporting later melting in the nearside source region. Lower surface elevation and a thinner crust also facilitate some magma transport and pooling, although crustal thickness alone cannot specify eruption rates. The origin of the crustal contrast and the later production and emplacement of mare basalt are separate parts of the explanation.

The thermal option is consistent with a thick feldspathic farside crust, a hemispheric chemical contrast and the existence of an early hot Earth. It is not uniquely demonstrated by those observations. Vigorous magma-ocean mixing could erase the proposed gradient; the required irradiation must persist during the relevant condensation and crust-building interval, not merely occur at some early time. A failure to find the predicted bulk compositional contrast in demonstrably ancient deep crust, or a chronology placing the asymmetry after complete crystallization, would challenge this particular mechanism. The existence of farside mare deposits and large thin-crust basins also warns against treating thickness as an on/off volcanic switch.

Accretion of a smaller companion is a contrasting option. A low-speed collision of a smaller co-orbiting moon with the lunar farside could add a broad layer of relatively cold silicate material rather than excavate an ordinary deep crater. Numerical impact models demonstrate that such a depositional event can build thick highlands and displace underlying residual liquid toward the opposite hemisphere.^[ref] Broad crustal excess and asymmetric evolved material are compatible with that mechanism, but are not unique evidence for it. The necessary companion, collision timing and impact geometry must be dynamically feasible. A predicted buried depositional interface, age discontinuity or lateral fabric should survive somewhere; their systematic absence in well-resolved, unmodified ancient crust would count against the model. Closely related bodies could have similar isotopes, so failure to find a large isotopic difference alone would not disprove accretion.

Both scenarios allow later basin impacts, partial melting of the lunar mantle and mare emplacement to modify the original contrast. A useful explanation must reproduce crustal mass, chemistry, chronology and volcanic distribution together, and state which observations would distinguish its initial cause.

Today the usual first melt is basaltic melt from the mantle wedge; in a hotter Archean subduction zone, melting of hydrated basaltic crust itself could generate sodic felsic melt of tonalite-trondhjemite-granodiorite composition. Distinguish a primary melt from the evolved melt ultimately added to the upper continental crust.

On the modern Phanerozoic Earth:

This adds juvenile material derived from the mantle; simply remelting old continental crust is reworking, not net juvenile addition. The amount and depth of slab water release depend on its thermal history, as quantified in Subduction factory: 4. Depth-dependent flux of H2O from subducting slabs worldwide by Peter E. van Keken and others.

For the hotter Archean endmember posited in the question, a hotter mantle and a warm descending plate make it easier for hydrated basaltic crust to reach its solidus. Its partial melting can directly make silica-rich, sodium-rich tonalite-trondhjemite-granodiorite liquids, leaving a more mafic residue. Residual garnet and amphibole, and the presence or absence of residual plagioclase, help determine the rare-earth element and strontium signatures. These are not ultramafic liquids merely because the surrounding mantle was hotter.

The contrast is therefore commonly fluid-fluxed mantle melting followed by differentiation today, versus a greater potential contribution from direct melting of hydrated mafic crust in the hotter case. Tonalite-trondhjemite-granodiorite is not unique evidence for subduction: melting of thickened mafic crust can produce similar liquids. Nor are all modern arcs devoid of slab melting. These qualifications are central to The composite Archaean grey gneisses: Petrological significance, and evidence for a non-unique tectonic setting for Archaean crustal growth by Jean-François Moyen.

At a convergent margin, the incoming oceanic lithosphere is a chemically and mechanically layered package: sediments, oceanic crust and lithospheric mantle. Subduction does not deliver this whole package unchanged to one destination. Solids can be accreted, underplated, exhumed or recycled deeply, while released fluids and melts carry selected components to the overriding plate. The outcome depends on incoming composition and hydration, slab age, convergence rate, thermal structure, interface coupling and subsequent tectonic history.

The incoming material already contains a history. Sediments include terrigenous debris, biogenic carbonates, siliceous deposits, clay minerals, pore water and organic matter. Oceanic crust consists mainly of basalt, sheeted dikes and gabbro; hydrothermal alteration can add water, carbonates and mobile elements before subduction. Beneath it, residual peridotite forms the lithospheric mantle. Faulting at a spreading environment, a transform boundary or an outer-rise bending region can let seawater enter and cause serpentinization. The lithospheric mantle is not uniformly hydrated to its full thickness. Consequently, sediment thickness alone does not measure the incoming volatile inventory.

Sediments: separation starts near the trench. Compaction and increasing effective pressure expel pore fluids, which can escape through the forearc, transport dissolved constituents and influence pore pressure on the plate interface. Some sediments are scraped off into an accretionary prism, shortened by thrusts and retained in the upper plate. Others pass beneath the leading edge, then detach and undergo tectonic underplating beneath the forearc. Deeper underplated material may return to the surface during tectonic exhumation, preserving a record of burial and deformation. At a sediment-starved erosive margin, little prism may form, and tectonic erosion can entrain material from the overriding plate as well. Thus net continental gain and net continental loss are both possible; they can even alternate at one margin.

Sediments that remain attached to the slab undergo metamorphism, dehydration, and potentially decarbonation or carbonate dissolution. The released fluids can transport boron, barium, strontium, lead and other soluble constituents. Carbon can leave as dissolved species, remain in carbonate, or occur as reduced carbon depending on redox conditions and reaction history. Along sufficiently hot paths, sediment-derived melts or mixtures of sediment and altered rock can transfer material into the mantle wedge; buoyant mixed material may rise as a mélange diapir. Cold paths can instead retain more solid material and some volatile-bearing phases to greater depths. These are alternative reaction and transport pathways, not a single compulsory sequence. A carbon-budget study explicitly includes fluid dissolution, sediment diapirism and storage in the upper plate, showing why volcanic carbon dioxide alone cannot close the whole subduction carbon budget.^[ref]

Oceanic crust: metamorphic transformation and chemical filtration. Altered basalt and gabbro are progressively buried. Depending on their pressure-temperature paths and reaction kinetics, they can pass through blueschist facies or warmer assemblages and eventually form eclogite, dominated by garnet and omphacite. Eclogitization increases density and can strengthen negative slab buoyancy, but a kinetic delay can leave metastable rock. It is incorrect to assign every slab the same sequence or the same fixed depths for these changes.

During burial, breakdown of hydrous phases releases water-rich fluids; different mineral assemblages retain different quantities of water. The fluids migrate through or along the interface and into the mantle wedge, where they cause metasomatism and lower the solidus of peridotite. Flux melting of the mantle wedge generates much of the primary magma feeding a volcanic arc. This is different from saying that the entire descending oceanic crust melts beneath the arc. Most of its refractory mass can remain solid. Partial melting of oceanic crust is more plausible for unusually hot or young slabs, or other favorable thermal conditions, and produces a distinct crustal contribution.

The depth of dehydration is controlled by thermal structure and composition. Global thermal-petrological calculations, for example, predict more efficient deep water transport in old, fast, cold slabs than in hot ones, with substantial differences between volcanic crust, gabbro and hydrated peridotite. Such a calculation supports conditional deep retention; its modeled proportions should not be imposed on every margin.^[ref]

Mechanical detachment adds more branches. Upper-crustal slices, seamounts or oceanic plateaus may be accreted or underplated instead of continuing down the slab. A complete or partial ophiolite can be emplaced onto another plate by obduction in suitable convergence histories. Most intact oceanic crust is not saved this way. High-pressure fragments may be entrained in an interface channel or mélange and exhumed; their small preserved volume should not be mistaken for the dominant global mass flux. Magma generated after slab-derived transfer may also pond and crystallize beneath the upper crust by magmatic underplating, which is a different process from mechanical tectonic underplating of solid slices.

Lithospheric mantle: residual rock and a variable water carrier. Most incoming lithospheric mantle is depleted peridotite left after extraction of ridge melts. Its cold, dense, mechanically coherent portion contributes to slab pull and generally sinks with the plate. Hydrated portions contain serpentine and possibly other hydrous minerals. Breakdown of these phases releases water at conditions that depend on the thermal path, feeding further metasomatism of the mantle wedge and sometimes dehydration embrittlement. The latter is a proposed contributor to intermediate-depth earthquakes, not an exclusive explanation for all such events. Very cold paths may transfer some water onward in high-pressure hydrous phases or as hydrogen incorporated in nominally anhydrous minerals.

Do not confuse the incoming hydrated lithospheric mantle with newly serpentinized forearc mantle in the overriding plate. The latter receives slab-derived fluids and can temporarily store their water. Hydrated, relatively buoyant interface material can facilitate return flow and tectonic exhumation, while the colder deeper bulk of the lithospheric mantle continues down. Some peridotite may also be accreted or obducted with an ophiolite. These exchanges mean that even an exposed serpentinite requires structural and chemical context before it can be assigned to its original plate.

Deep recycling and eventual return. The residual eclogite, remaining sediments and lithospheric mantle can enter the deeper mantle. A slab can temporarily stagnate around the mantle transition zone or penetrate into the lower mantle, depending on buoyancy, phase transformations, geometry and mantle viscosity. Crustal material can remain as compositionally distinct domains, mix, or react with surrounding peridotite; it need not retain the near-surface eclogite assemblage indefinitely as pressure rises. Subsequent mantle convection can return recycled components toward spreading regions or mantle plumes, where partial melting incorporates them into new basalt. A primary study of Hawaiian basalt illustrates a specific model in which melts derived from recycled crust react with peridotite to form a pyroxenite source. It is an interpretation of combined chemical evidence, not a unique label attached to every enriched basalt.^[ref]

How the pathways can be observed. Seismic reflection profiles, drilling, wedge geometry and balanced structural sections constrain accretion, fluid escape and tectonic erosion. High-pressure metamorphism, mineral zoning, radiometric dating and pressure-temperature-time paths in exhumed rocks reveal burial, reaction and return. Their reconstructable histories are especially useful when deformation and fluid inclusions are linked to the dated mineral growth. Earthquake distributions, seismic tomography, seismic attenuation and electrical conductivity constrain the geometry and physical state of the slab and mantle wedge, although fluids, melts and temperature can produce nonunique signals.

Arc magmas carry complementary chemical evidence: relatively mobile barium, boron, lead or strontium can trace fluid-mediated transfer, whereas sediment-sensitive thorium and isotope ratios can help identify other contributions. Element mobility changes with fluid composition and pressure-temperature conditions, so no one element is an infallible tracer. Boron isotope ratios, radiogenic strontium isotope ratios and lead isotope ratios should be interpreted with measured incoming sediments and altered oceanic crust. Arc gases and forearc fluid discharge constrain volatile return; ocean-island basalts, mantle xenoliths and diamond inclusions provide possible evidence of deeper recycling. Each observation samples only part of the system.

For any element $X$, a useful control-volume statement isHere permanent accretion is counted outside the chosen reactive control volume, return includes fluid, melt and exhumed-solid exports, and transient underplating or reservoir growth contributes to storage until exported. For a different boundary choice, the terms must be regrouped consistently. The three incoming layers can have very different transfer efficiencies for bulk rock, water, carbon and a particular trace element. A small mass of escaping fluid can dominate an element's return while most of the solid slab continues into the deep mantle.

Partial melting of material within a descending slab, distinct from flux melting of the mantle wedge by fluids released from the slab.