= Solution
\b[Use chemical signatures to reconstruct processes, then test tectonic context.] <Geochemistry> can establish melting conditions, surface alteration and recycling, but <plate tectonics> is a kinematic system. I would sample dated suites across a mapped geological succession and compare competing origins, rather than declare an isolated “arc-like” analysis proof of a global regime. <Radiometric dating> must identify whether the measured age records crystallization, later <metamorphism>, alteration or inheritance.
\b[Hydrous magmatism and selective element transfer.] Analyze fresh <basalt>, associated intrusive rocks and differentiated volcanic suites for <major elements>, <trace elements> and <isotope ratios>. Relative enrichment of fluid-mobile elements such as <barium>, <strontium> and <lead>, together with relative depletion of <niobium> and <tantalum>, can support addition of slab-derived components to a depleted <mantle wedge>. Test coherent variations in <barium>/<niobium>, <thorium>/<niobium> and <rare-earth element> patterns against modeled fluid addition, sediment melting, source depletion and <fractional crystallization>. Do not treat these ratios as fixed universal cutoffs. <Crustal contamination>, remelting of previously arc-modified <crust>, residual <rutile> or other source effects can create some similar patterns. Altered rocks can also gain mobile elements long after their formation.
The case becomes stronger if the chemical evolution occurs in a plausible succession of oceanic, early forearc and mature <arc magmas>, with supporting structure and <geochronology>. The key evidence would be correlated process changes in an independently reconstructed margin, not resemblance to one modern <basalt> plot. A <geochemical discrimination diagram> is a comparison tool; its modern training associations are not a proof that only those settings could exist on a hotter <Earth>.
\b[High-pressure melting and the fate of mafic crust.] <Tonalite-trondhjemite-granodiorite> suites with high <strontium>/<yttrium> or <lanthanum>/<ytterbium> and low heavy <rare-earth element> abundances can indicate melting with residual <garnet> and little <plagioclase>. Model the combined <major element> composition, residual phases and melt fraction to estimate the <pressure>, hydration and source of melting. This could support melting of buried hydrous <oceanic crust>. However, basal melting of thickened <crust>, <delamination> and nonplate vertical burial can produce related conditions. High <strontium>/<yttrium> alone neither proves a descending <slab> nor dates the first <subduction>. Conversely, hotter ancient <subduction> might not produce exactly the same magmatic products as modern cold <slabs>.<The composite Archaean grey gneisses: Petrological significance, and evidence for a non-unique tectonic setting for Archaean crustal growth by Jean-François Moyen>[2011, modeled source mineralogies and distinct melting-pressure groups]{ref}
\b[Radiogenic isotopes distinguish new extraction from reworking.] Couple <zircon> <uranium-lead dating> with <hafnium isotope ratios>, and whole-rock <samarium-neodymium isotope ratios> with <strontium isotope ratios> or <lead isotope ratios>. These systems can constrain contributions from depleted <mantle>, older <continental crust> and altered surface reservoirs. For a closed parent-daughter system,
$$
D^*(t)=P(t)\bigl(e^{\lambda t}-1\bigr),
$$
where $P(t)$ is the present parent abundance, $D^*$ the radiogenic daughter accumulated since closure, and $\lambda$ the <radioactive decay constant>. Initial daughter composition, closure and mixing must be constrained before interpreting an age. A crustal extraction model age depends on a reference <mantle> evolution and subsequent history; it is not automatically a rock's crystallization age or the date of a tectonic transition.
Juvenile signatures can indicate addition from the <mantle>, while evolved signatures can indicate reworking of older <crust>; neither is exclusive to a particular tectonic regime. Spatially organized juvenile additions at a reconstructed margin, followed by collision and reworking, are more diagnostic than an undifferentiated global age histogram. Work combining <zircon> <hafnium isotope ratios>, <oxygen isotope ratios> and age distributions illustrates how crustal generation, reworking and preservation can be separated conceptually, while remaining dependent on sampling and model assumptions.<Rates of generation and growth of the continental crust by Christopher Hawkesworth and others>[2019, interpretation of age distributions and zircon isotope constraints]{ref}
\b[Stable isotopes can identify a visit to the surface.] Zircon <oxygen isotope ratios> substantially different from an appropriate mantle reference may record melting of material that previously underwent low-<temperature> <water-rock interaction>. This would support surface alteration followed by burial or remelting. It would not, by itself, distinguish <subduction> from burial under a thick volcanic pile or another form of recycling. Alteration, metamorphic exchange, inherited cores and analytical domains need checking. Multiple <stable isotope> systems, including <boron isotope ratios> in appropriate rocks, can strengthen a fluid-transfer interpretation if their fractionation and later resetting are modeled.
\b[Deep samples provide an independent archive.] If dated <diamond inclusions> or <mantle xenoliths> preserve <eclogite> compositions together with a surface-alteration chemical signature, they could demonstrate transfer of crustal material into deep <lithospheric mantle>. Dating the relevant mineral assemblage and distinguishing inclusion growth from later host transport are essential. A primary diamond-inclusion study used coupled chemical and isotopic observations to infer changes in deep recycling; the general lesson is to test a transport history, not to make the first preserved sample an infallible onset clock.<Start of the Wilson cycle at 3 Ga shown by diamonds from subcontinental mantle by Steven B. Shirey and Stephen H. Richardson>[2011, diamond-inclusion data and proposed recycling interpretation]{ref} <Delamination> or other sinking processes remain alternatives unless structure and regional history establish plate convergence.
For each candidate signal, I would seek an older comparison population, quantify alteration and analytical uncertainty, reproduce the pattern in several independent regions, and combine chemistry with the tests in the other parts. A securely dated demonstration of <subduction> shows that it operated by that time in that region; it does not demonstrate that it began then everywhere. A secular shift toward more frequent, spatially coherent signatures can support expansion of the regime, but only after correcting for the survival of its archive.
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