Lawsonite eclogite has received attention due to its high H₂O retainability, thereby contributing to volatile input into the deep Earth via oceanic plate subduction. Because the pressure–temperature (P–T) stability field of lawsonite is limited to high-P and low-T conditions along a cold thermal gradient (~5–6°C/km), continuous subduction of lawsonite eclogite would require either significant cooling of the oceanic lithosphere before subduction or extensive subduction maturation ^[1]. However, scarce geochronological data of natural lawsonite eclogites that include both the protolith and metamorphic ages inhibit the quantitative validation of this idea.

Here, we report metamorphic and protolith ages of lawsonite eclogite from the South Motagua Mélange, Guatemala. These samples ubiquitously include well-preserved lawsonite grains both as inclusions in garnet and in the rock matrix, demonstrating that garnet-forming reactions occurred within the lawsonite stability field. For the determination of the eclogite-facies metamorphic age, we performed LA-MC-ICPMS U–Pb dating of almandine-rich garnet grains ^[2,3] in four thin sections representing four different rock samples. Forty-four spot analyses of the inclusion-free rims in one of the samples plot on a well-defined regression line with a lower intercept of 137 ± 4 Ma. The garnet U–Pb ages from the other samples overlap this value within uncertainty. As for the protolith age, we performed LA-HR-ICPMS U–Pb dating of magmatic zircon in one of the samples, and obtained a concordia age of 187 ± 2 Ma. Hence, the protolith–metamorphic age difference (Δt) is found to be ~50 Myr.

Previously reported Δt values for lawsonite–epidote eclogites from other localities (North Qilian and New Caledonia) are ~20 Myr, which is smaller than our result by ~30 Myr. Those samples formed along warmer thermal gradients than the Guatemalan lawsonite eclogites, as their metamorphic peak conditions were outside the lawsonite stability field. Assuming the subduction duration of eclogite-forming material is generally <10 Myr, we propose that 40–50 Myr of oceanic crust cooling before subduction is required to keep the thermal gradient cold enough for lawsonite stability. This study applied the novel in-situ U–Pb almandine garnet dating method, demonstrating its viability for extracting robust temporal information on convergent plate margin dynamics.

References

[1] Hernández-Uribe, D., & Tsujimori, T. (2023). Geology, 51, 678–682.

[2] Millonig, L. J. et al. (2020). Earth and Planetary Science Letters, 552, 116589.

[3] Shu, Q. et al. (2024). Contributions to Mineralogy and Petrology, 179, 49.

There are abundant shear crack jogs sealed with albite, chlorite, epidote, and quartz in the low-grade basic schists, manifesting jog formation during metamorphism. These shear crack jogs have decompression concaves, that are classified into two types: one is sharp outline type and the other diffusive one. The former displays sharp boundary between albite and quartz sealing jog and chlorite dominant matrix, but the latter does the gradual change of volume ratio of the very fine-grained albite and chlorite from the matrix to the jog interior. It is important that the boundary shape is fitted by sinusoidal curve of the cross-sectional boundary shape, but not concatenated one, thereby suggesting the decompression mechanism of the dent formation. The periodic structure of the concave structure is possibly interpreted by the depression mechanics of the low fluid pressure of jog volume at the time of shear crack motion (growth and propagation). In this talk, the author proposes the kinetics of the decompression dent formation during the plate boundary metamorphism.

The upper mantle is essential for understanding magmatism, crust formation, and element cycling between Earth's interior, hydrosphere, atmosphere, and biosphere. Mantle composition and evolution are inferred through surface sampling and indirect methods. We recovered a 1268 m section of serpentinized mantle peridotite from the Atlantis Massif, with minor gabbroic intrusions, showing depleted compositions and mineralogical variations controlled by melt flow. Located at 30 degrees North on the Central Atlantic Ridge, the Atlantis Massif is an ocean core complex exposing gabbroic rocks and serpentinized peridotite from the lower oceanic crust to the upper mantle. Four previous IODP expeditions were conducted here. IODP Expedition 399 (April 12 - June 12, 2023) by D/V JOIDES Resolution aimed to elucidate the formation of the oceanic core complex, explore reaction processes between the oceanic crust, upper mantle, and seawater, investigate non-biological water-rock reactions representing ancient systems, and evaluate sub-seafloor life activity.

Hole U1601C at the serpentine site in the southern massif was drilled to 1267.8 mbsf, becoming the fifth deepest hard rock hole in IODP history. Continuous temperature, density, porosity, and seismic velocity measurements were taken, along with fluid samples from multiple depths. Dunite zones in these cores exhibit intermediate dips, contrasting with initially steep mantle fabrics, indicating oblique melt transport. Extensive hydrothermal fluid-rock interaction is recorded across the core's full depth, with oxidation in the upper 200 m. Alteration patterns align with vent fluid composition in the nearby Lost City hydrothermal field.

Future analysis of these core samples and data will elucidate the oceanic lithosphere structure, formation of oceanic core complexes, crust alteration, and the generation of hydrogen and methane, essential for the subsurface biosphere.