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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.

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.

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.^[ref]

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,where $P(t)$ is the present parent abundance, $D^{\ast }$ 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.^[ref]

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.

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.^[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.

A zirconium silicate mineral widely used in uranium-lead dating and studies of hafnium isotope ratios and oxygen isotope ratios.