Rock recycling within the forearcs of subduction zones involves subduction of sediments and hydrated lithosphere into the upper mantle, exhumation of rocks to the surface, and erosion to form new sediment. The compositions of, and inclusions within detrital minerals revealed by electron microprobe analysis and Raman spectroscopy preserve petrogenetic clues that can be related to transit through the rock cycle. We report the discovery of the ultrahigh-pressure (UHP) indicator mineral coesite as inclusions in detrital garnet from a modern placer deposit in the actively exhuming Late Miocene–Recent high- and ultrahigh-pressure ((U)HP) metamorphic terrane of eastern Papua New Guinea. Garnet compositions indicate the coesite-bearing detrital garnets are sourced from felsic protoliths. Carbonate, graphite, and CO₂ inclusions also provide observational constraints for geochemical cycling of carbon and volatiles during subduction. Additional discoveries include polyphase inclusions of metastable polymorphs of SiO₂ (cristobalite) and K-feldspar (kokchetavite) that we interpret as rapidly cooled former melt inclusions. Application of elastic thermobarometry on coexisting quartz and zircon inclusions in six detrital garnets indicates elastic equilibration during exhumation at granulite and amphibolite facies conditions. The garnet placer deposit preserves a record of the complete rock cycle, operative on <10-My geologic timescales, including subduction of sedimentary protoliths to UHP conditions, rapid exhumation, surface uplift, and erosion. Detrital garnet geochemistry and inclusion suites from both modern sediments and stratigraphic sections can be used to decipher the petrologic evolution of plate boundary zones and reveal recycling processes throughout Earth’s history. Significance On Earth, igneous, metamorphic, and sedimentary rocks are recycled from the surface to upper mantle depths and subsequently returned to the surface as a result of tectonic and sedimentary processes. Within active plate boundaries, the rock cycle involves lithospheric deformation to exhume rocks to the surface. We show that trapped inclusions in garnet sand from the actively exhuming high- and ultrahigh-pressure metamorphic terrane of eastern Papua New Guinea preserve a record of crustal subduction and rapid exhumation linking upper mantle and surface processes operative on short geologic timescales (<10 My). Detrital garnet geochemistry and their inclusion suites from both modern sediments and stratigraphic sections can be used to reveal rock recycling processes throughout Earth’s history.

Current models for elastic geobarometry have been developed with the assumption that the host and/or inclusion minerals are elastically isotropic. This assumption has limited applications of elastic thermobarometry to mineral inclusions contained in cubic quasi‐isotropic host minerals (e.g., garnet). Here, we report a new elastic model that takes into account the anisotropic elastic properties and relative crystallographic orientation (RCO) of a host‐inclusion system where both minerals are noncubic. This anisotropic elastic model can be used for host‐inclusion elastic thermobarometric calculations provided that the RCO and elastic properties of both the host and inclusion are known. We then used this anisotropic elastic model to numerically evaluate the effects of elastic anisotropy and RCO on the strains and stresses developed in a quartz inclusion entrapped in a zircon host after exhumation from known entrapment P‐T conditions to room P‐T conditions. We conclude that the anisotropic quartz‐in‐zircon elastic model is suitable for elastic thermobarometry and may be widely applicable to crustal rocks. Our results demonstrate that isotropic elastic models cannot be used to determine the entire strain state of an elastically anisotropic inclusion contained in an elastically anisotropic host mineral, and therefore may lead to errors on estimated remnant inclusion pressures.

The first billion years of Earth history witnessed the emergence of continental magmatism, oceans and life. Yet, the details of how continents formed remain unknown because of the absence of preserved rocks ^1–8 . Two conflicting Hadean models predominate: early onset of subduction and plate tectonics ^2–4 , compared with early stagnant-lid and plume processes with delayed (post-Hadean) plate tectonics ^5–7 . Here we report trace-element ratios (including Nb–Sc–U–Yb) correlated with age and hafnium and oxygen isotope ratios for Hadean detrital zircons from the Jack Hills (JH), Western Australia, which record unprecedented insights into the timing and setting of early magmatism. More than 70% of Hadean JH detrital zircons have Sc/Yb > 0.1, and 47% have U/Nb > 20, fingerprints for continental-arc and subduction settings. The remainder are ocean-island-like with little evidence for ocean-ridge settings. Hadean JH zircons probably originated from distinct terranes with separate tectonic histories. Subduction-related magmatism in the Hadean, as documented by JH zircons, alternated with periods of magmatic quiescence. This contrasts with dominantly stagnant-lid-like signatures for most Barberton Hadean zircons. The diverse settings for Jack Hills and Barberton detrital zircons imply contemporaneous operation of different tectonic styles during the Hadean, as well as a broader diversity of early crustal origins than previously known.

Quartz crystals with zircon inclusions were synthesized using a piston-cylinder apparatus to experimentally evaluate the use of inclusions in “soft” host minerals for elastic thermobarometry. Synthesized zircon inclusion strains and, therefore, pressures (P_inc) were measured using Raman spectroscopy and then compared with the expected inclusion strains and pressures calculated from elastic models. Measured inclusion strains and inclusion pressures are systematically more tensile than the expected values and, thus, re-calculated entrapment pressures are overestimated. These discrepancies are not caused by analytical biases or assumptions in the elastic models and strain calculations. Analysis shows that inclusion strain discrepancies progressively decrease with decreasing experimental temperature in the α-quartz field. This behavior is consistent with inelastic deformation of the host–inclusion pairs induced by the development of large differential stresses during experimental cooling. Therefore, inclusion strains are more reliable for inclusions trapped at lower temperature conditions in the α-quartz field where there is less inelastic deformation of the host–inclusion systems. On the other hand, entrapment isomekes of zircon inclusions entrapped in the β-quartz stability field plot along the α–β quartz phase boundary, suggesting that the inclusion strains were mechanically reset at the phase boundary during experimental cooling and decompression. Therefore, inclusions contained in soft host minerals can be used for elastic thermobarometry and inclusions contained in β-quartz may provide constraints on the P–T at which the host–inclusion system crossed the phase boundary during exhumation.