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Subduction zones regulate Earth's carbon distribution, yet the mechanism of carbon transfer from continental crust to mantle remains elusive. We examined an eclogite‐garnet peridotite interface from the Chinese Continental Scientific Drilling Program in the Sulu orogen, representing the slab–mantle wedge boundary formed during continental subduction. Whole‐rock magnesium (Mg) isotopic and major‐trace element data, together with in situ mineral analyses, identify the presence of carbonic fluids characterized by notably light Mg isotopic compositions (−0.54 to −0.36‰) and elevated Ca, Mg, Sr, and rare earth elements contents. These fluids, generated by slab decarbonation during prograde metamorphism, mobilized carbon from the subducted crust and enriched the mantle wedge. Modeling indicates that continental subduction rivals oceanic systems in transporting carbon to mantle. However, the paucity of mantle‐derived magmatism limits carbon return, promoting long‐term retention of continental carbon in mantle and establishing continental subduction as a major sink in the global carbon budget.

Fil: Guerreiro, Tomás. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales. Instituto de Ciencias de la Tierra, Biodiversidad y Ambiente; Argentina

The Wadi El-Hima Neoproterozoic I- and A-type granites in the Southern Eastern Desert of Egypt are rich in garnets (up to 30 vol%) and are cut by NW–SE strike-slip faults, as confirmed from structure lineament extraction maps. These mineralized granites and garnet mineralization zones can be successfully discriminated using remote sensing techniques. Spectral angle mapper and matched filtering techniques are highly effective for mapping garnet-rich zones and show that the highest garnet concentrations occur along the intrusive contact zone of NW–SE striking faults. El-Hima granites have high SiO2 (73.5–75.1 wt%), Al2O3 (13.4–15.3 wt%) and total alkali (6.7–8.7 wt%) contents, suggesting that they were sourced from peraluminous (A/CNK > 1) parental magmas. Garnet-bearing trondhjemites are metasomatic in origin and formed after I-type tonalite-granodiorites, which originated in a volcanic arc tectonic setting. Garnet-rich syenogranites and alkali-feldspar granites are both post-collisional A-type granites: the syenogranites formed from peraluminous magmas generated by partial melting of lower crustal tonalite and metasedimentary protoliths during lithospheric delamination, and the alkali-feldspar granites crystallized from highly fractionated, felsic and alkali-rich peraluminous magmas in the upper crust. Garnets in El-Hima mineralized granites occur in three forms: (1) subhedral disseminated crystals, (2) vein-type crystals, and (3) aggregated subhedral crystals, reflecting different mechanisms of accumulation. All are dominantly almandine in composition (Alm76Sps10 Prp7Grs6Adr1) and have high average concentrations of heavy rare earth elements (HREE) (ΣHREE = 1636 ppm), Y = (3394 ppm), Zn (325 ppm), Li (39.17 ppm) and Ga (34.94 ppm). Garnet REE patterns show strong negative Eu anomalies with HREE enriched relative to LREE, indicating a magmatic origin. These magmatic garnets are late-stage crystallization products of Al-rich hydrous magmas, and formed at low temperature (680–730 °C) and pressure (2.1–2.93 kbar) conditions in the upper continental crust. Peculiar garnet concentrations in syenogranites near and along contact zones with alkali feldspar granites are related to peraluminous parent hydrous magma compositions. These garnets formed by in situ crystallization from A-type granite melts, alongside accumulation of residual garnets left behind after partial melting of the host garnet-rich granites along the intrusive contact. Magmatic-fluid flow along the NW–SE striking fault of Najd system enhanced garnet accumulation in melts, which formed clots and veins of garnet.

Garnet’s stability in arc magmas and its influences on their differentiation were explored experimentally in a typical basalt, andesite, and dacite at conditions of 0.9–1.67 GPa, 800–1300 °C, with 2–9 wt.% added H 2 O, and with oxygen fugacity buffered near Re + O 2  = ReO 2 (~ Ni-NiO + 1.7 log 10 bars). Garnet did not grow at 0.9 GPa in any of the compositions, even with garnet seeds added to facilitate nucleation. At 1.0–1.2 GPa, garnet grew as thin rims (< 5 µm) on introduced garnet seeds coexisting with dacitic to rhyodacitic liquids at temperatures ≤ 1000 °C. At 1.3 GPa, garnet grew readily with no seeds from 900 to 1100 °C coexisting with liquids ranging from peraluminous basaltic andesite to rhyodacite, and at 1.46 GPa, garnet was stable as hot as 1150 °C in metaluminous basaltic liquid. Garnet grew as a liquidus phase only in the dacite, a composition similar to the average upper continental crust. Inverse experiments on the dacite determined a liquidus multiple-saturation point with garnet, plagioclase, orthopyroxene, calcic clinopyroxene, and amphibole at 975 °C, 1.46 GPa, with 7 wt.% dissolved H 2 O. Such dacitic and more evolved melts can be products of peritectic reactions that with decreasing temperature consume garnet, calcic clinopyroxene, and melt components, producing amphibole and less abundant but more evolved melts. For this reason, experiments on product melts need not produce reactant minerals, accounting for some disparities in published experimental results on the apparent stability of garnet in intermediate-to-evolved arc magmas. Results on more mafic compositions are more reliable guides and show that liquids of arc dacitic composition, and more evolved compositions, would coexist stably with garnet only in the deepest portions of continental-margin arc crust with average thickness and density (~ 43 km, ~ 1.2 GPa) or in the underlying shallow mantle. Metaluminous arc basaltic, basaltic andesitic, and many andesitic liquids would not coexist stably with garnet at pressures ranging from the crust to at least the midpoint of the mantle wedge, but results in the literature allow that some andesitic liquids with higher Fe/Mg than common in arcs may also saturate with garnet in the deeper portions of average-thickness continental arc crust. A persistent issue, however, is that, at pressures of the lower continental crust or shallow mantle (0.7–1.67 GPa), arc basalts may crystallize or differentiate within a regime that includes a clinopyroxene-dominated high-T interval (1250–1150 °C) with lesser orthopyroxene. This crystallizing assemblage drives coexisting liquids to become peraluminous at 53–60 wt.% SiO 2 (normalized anhydrous), whereas arc igneous suites mainly attain peraluminous compositions at 65–70 wt.% SiO 2 . Thus, simple, progressive crystallization-differentiation of appropriately hydrous, oxidized basalts near the base of continental arc crust does not generate, or does not act alone to produce, the dominantly metaluminous arc andesites. Scarcity of natural peraluminous andesites and basaltic andesites, and of correlative intrusions, despite their demonstrated production by deep basalt differentiation, may result from mixing accompanying crystallization-differentiation. Cumulates produced by arc basalts at deep crustal and upper mantle pressures have densities equal to or exceeding those of upper mantle peridotite until coexisting liquid compositions reach or exceed that of silicic andesite or dacite, after which cumulates become buoyant relative to the mantle. Deep differentiation may therefore be efficient until that point, with cumulates being lost to the mantle and melts evolving rapidly to silicic andesite through dacite compositions. This process results in both an intermediate overall composition for the buoyant crust, and deep-crustal dacitic through rhyolitic melts that can mix with deep basalts thereby producing metaluminous basaltic andesites and andesites.

The compositional zoning of the major divalent cations in metamorphic garnet is a useful tool in reconstructing the pressure–temperature path. However, trace elements can provide a better-preserved record of petrogenetic evolution due to their strong affinity in garnet and slow diffusion rates. In this study, three high-pressure micaschist samples of varying composition and garnet textures from the Krušné hory Mountains (Saxothuringian zone, Bohemian Massif) were examined. By utilizing electron probe micro-analysis and laser ablation inductively coupled plasma mass spectrometry, three distinct types of compositional zoning in garnet were identified by compositional mapping. The zoning types were classified as continuous core-to-rim change, concentric annular changes, and overprinting of a pre-existing distribution; all three provide information on the original mineral composition and texture before garnet overgrowth. The transition from overprint to annular zoning shows relation to temperature increment. The annular zoning allowed the identification of several coupled substitutions, including alkali (sodium and lithium) + yttrium and the alkali + phosphorus substitution which is typical of high- to ultra-high-pressure conditions. The formation of annuli zoning was interpreted to originate not only from the decomposition of trace element bearing phases, but also to be related to the availability of fluid medium during garnet growth. Two samples contained atoll texture garnets, interpreted to be originated from the dissolution of the garnet central part, chemically distinct from the new garnet growing coevally on the rim or replacing the original central part. This proposed process is evidenced by the mass balance calculation of yttrium and heavy rare earth elements between the dissolved garnet and newly formed parts.

All available volume and elasticity data for the garnet end-members grossular, pyrope, almandine and spessartine have been re-evaluated for both internal consistency and for consistency with experimentally measured heat capacities. The consistent data were then used to determine the parameters of third-order Birch–Murnaghan EoS to describe the isothermal compression at 298 K and a Mie–Grüneisen–Debye thermal-pressure EoS to describe the PVT behaviour. In a full Mie–Grüneisen–Debye EoS, the variation of the thermal Grüneisen parameter with volume is defined as γ = γ 0 V V 0 q . For grossular and pyrope garnets, there is sufficient data to refine q which has a value of q  = 0.8(2) for both garnets. For other garnets, the data do not constrain the value of q and we therefore refined a q- compromise version of the Mie–Grüneisen–Debye EoS in which both γ / V and the Debye temperature θ D are held constant at all P and T , leading to ∂ C V ∂ P T = 0 , parallel isochors and constant isothermal bulk modulus along an isochor. Final refined parameters for the q- compromise Mie–Grüneisen–Debye EoS are: Pyrope Almandine Spessartine Grossular V 0 (cm 3 /mol) a 113.13 115.25 117.92 125.35 K 0T (GPa) 169.3 (3) 174.6 (4) 177.57 (6) 167.0 (2) K 0 T ′ 4.55 (5) 5.41 (13) 4.6 (3) 5.07 (8) θ D0 771 (28) 862 (22) 860 (35) 750 (13) γ 0 1.185 (12) 1.16 (fixed) 1.18 (3) 1.156 (6) for pyrope and grossular, the two versions of the Mie–Grüneisen–Debye EoS predict indistinguishable properties over the metamorphic pressure and temperature range, and the same properties as the EoS based on experimental heat capacities. The biggest change from previously published EoS is for almandine for which the new EoS predicts geologically reasonable entrapment conditions for zircon inclusions in almandine-rich garnets.

Andradite-rich garnet is a common U-bearing mineral in a variety of alkalic igneous rocks and skarn deposits, but has been largely neglected as a U–Pb chronometer. In situ laser ablation-inductively coupled plasma mass spectrometry U–Pb dates of andradite-rich garnet from a syenite pluton and two iron skarn deposits in the North China craton demonstrate the suitability and reliability of the mineral in accurately dating magmatic and hydrothermal processes. Two hydrothermal garnets from the iron skarn deposits have homogenous cores and zoned rims (Ad 86 Gr 11 to Ad 98 Gr 1 ) with 22–118 ppm U, whereas one magmatic garnet from the syenite is texturally and compositionally homogenous (Ad 70 Gr 22 to Ad 77 Gr 14 ) and has 0.1–20 ppm U. All three garnets have flat time-resolved signals obtained from depth profile analyses for U, indicating structurally bound U. Uranium is correlated with REE in both magmatic and hydrothermal garnets, indicating that the incorporation of U into the garnet is largely controlled by substitution mechanisms. Two hydrothermal garnets yielded U–Pb dates of 129 ± 2 (2 σ ; MSWD = 0.7) and 130 ± 1 Ma (2 σ ; MSWD = 0.5), indistinguishable from zircon U–Pb dates of 131 ± 1 and 129 ± 1 Ma for their respective ore-related intrusions. The magmatic garnet has a U–Pb age of 389 ± 3 Ma (2 σ ; MSWD = 0.6), consistent with a U–Pb zircon date of 388 ± 2 Ma for the syenite. The consistency between the garnet and zircon U–Pb dates confirms the reliability and accuracy of garnet U–Pb dating. Given the occurrence of andradite-rich garnet in alkaline and ultramafic magmatic rocks and hydrothermal ore deposits, our results highlight the potential utilization of garnet as a powerful U–Pb geochronometer for dating magmatism and skarn-related mineralization.

Elastic geobarometry for host-inclusion systems can provide new constraints to assess the pressure and temperature conditions attained during metamorphism. Current experimental approaches and theory are developed only for crystals immersed in a hydrostatic stress field, whereas inclusions experience deviatoric stress. We have developed a method to determine the strains in quartz inclusions from Raman spectroscopy using the concept of the phonon-mode Gruneisen tensor. We used ab initio Hartree-Fock/Density Functional Theory to calculate the wavenumbers of the Raman-active modes as a function of different strain conditions. Least-squares fits of the phonon-wavenumber shifts against strains have been used to obtain the components of the mode Gruneisen tensor of quartz (γ1m and γ3m) that can be used to calculate the strains in inclusions directly from the measured Raman shifts. The concept is demonstrated with the example of a natural quartz inclusion in eclogitic garnet from Mir kimberlite and has been validated against direct X-ray diffraction measurement of the strains in the same inclusion.

Hydrogen distribution between nominally anhydrous minerals (NAMs) of a garnet-lherzolite under subsolidus conditions has been investigated. Separated NAMs from a garnet-peridotite from Patagonia (Chile) are annealed together (olivine, orthopyroxene, clinopyroxene, and garnet) using a piston-cylinder at 3 GPa and 1100 °C using talc-pyrex cell assembly for 10, 25, and 100 h. The talc-pyrex assembly provides enough hydrogen in the system to re-equilibrate the hydrogen concentrations at high pressure. The three coexisting nominally anhydrous minerals (NAMs, i.e., olivine, orthopyroxene, and clinopyroxene) were successfully analyzed using FTIR. The resulting hydrogen concentrations exceed significantly the initial hydrogen concentration by a factor of 13 for olivine and a factor of 3 for both pyroxenes. Once mineral-specific infrared calibrations are applied, the average concentrations in NAMs are 115 ± 12 ppm wt H2O for olivine, 635 ± 75 ppm wt H2O for orthopyroxene, and 1214 ± 137 ppm wt H2O for clinopyroxene, garnet grains are dry. Since local equilibrium seems achieved over time (for 100 h), the calculated concentration ratios are interpreted as mineral-to-mineral hydrogen partition coefficients (i.e., Nernst's law) for a garnet-peridotite assemblage. It yields, based on mineral-specific infrared calibrations, DOpx/Ol = 5 ± 1, DCpx/Ol = 10 ± 2, and DCpx/Opx = 1.9 ± 0.4. While DCpx/Opx is in agreement (within error) with previous results from experimental studies and concentration ratios observed in mantle-derived peridotites, the DPx/Ol from this study are significantly lower than the values reported from mantle-derived xenoliths and also at odd with several previous experimental studies where melt and/or hydrous minerals co-exists with NAMs. The results confirm the sensitivity of hydrogen incorporation in olivine regarding the amount of water-derived species (H) in the system and/or the amount of water in the coexisting silicate melt. The results are in agreement with an important but incomplete dehydration of mantle-derived olivine occurring at depth, during transport by the host magma or during slow lava flow cooling at the surface. The rapid concentration modification in mantle pyroxenes also points out that pyroxenes might not be a hydrogen recorder as reliable as previously thought.