The Chang’E-5 (CE5) mission has demonstrated that lunar volcanism was still active until two billion years ago, much younger than the previous isotopically dated lunar basalts. How the small Moon retained enough heat to drive such late volcanism is unknown, particularly as the CE5 mantle source was anhydrous and depleted in heat-producing elements. We conduct fractional crystallization and mantle melting simulations that show that mantle melting point depression by the presence of fusible, easily melted components could trigger young volcanism. Enriched in calcium oxide and titanium dioxide compared to older Apollo magmas, the young CE5 magma was, thus, sourced from the overturn of the late-stage fusible cumulates of the lunar magma ocean. Mantle melting point depression is the first mechanism to account for young volcanism on the Moon that is consistent with the newly returned CE5 basalts.

There is extensive geologic evidence of ancient volcanic activity on the Moon, but it is unclear how long that volcanism persisted. Magma fountains produce volcanic glasses, which have previously been found in samples of the Moon’s surface. We investigated ~3000 glass beads in lunar soil samples collected by the Chang’e-5 mission and identified three as having a volcanic origin on the basis of their textures, chemical compositions, and sulfur isotopes. Uranium-lead dating of the three volcanic glass beads shows that they formed 123 ± 15 million years ago. We measured high abundances of rare earth elements and thorium in these volcanic glass beads, which could indicate that such recent volcanism was related to local enrichment of heat-generating elements in the mantle sources of the magma.

Subduction zone fluids are critical for transporting materials from subducted slabs to the mantle wedge. Jadeitites from Myanmar record fluid compositions and reactions in the forearc subduction channel. Here we present high‐precision Mg isotope data of the Myanmar jadeitites and associated rocks to understand the Mg isotope composition of subduction zone fluids at forearc depths. Two types of jadeitites (white and green) exhibit distinct Mg isotope compositions. The white jadeitites precipitated from Na‐Al‐Si‐rich fluids and have low δ²⁶Mg values, varying from −1.55‰ to −0.92‰, whereas the green jadeitites have higher δ²⁶Mg values (−0.91‰ to −0.74‰) due to metasomatic reactions between fluids and Cr spinel. The amphibole‐rich blackwall in the contact boundaries between jadeitites and serpentinites also exhibits low δ²⁶Mg values (−1.17‰ to −0.72‰). Therefore, the jadeite‐forming fluids have not only high concentrations of Na‐Al‐Si but also low δ²⁶Mg values. The low δ²⁶Mg signature of the fluids is explained by the dissolution of Ca‐rich carbonate in subducted sediments or altered oceanic crust, which is supported by the negative correlation of δ²⁶Mg with CaO/TiO₂, CaO/Al₂O₃, and Sr in the white jadeitites. Given the common occurrence of Ca‐rich carbonates in the subduction channel, the Mg isotope composition of low‐Mg aqueous fluids would be significantly modified by dissolved carbonates. Metasomatism by such fluids along conduits has the potential to generate centimeter‐scale Mg isotope heterogeneity in the forearc mantle wedge. Therefore, Mg isotopes could be a powerful tracer for recycled carbonates not only in the deep mantle but also in the shallow regions of subduction zones.

The replacement of rutile by Fe-Ti oxides is a common phenomenon during the retrogression of eclogites. Here, we report an unusual case regarding the replacement of Fe-Ti oxides by rutile during greenschist-facies metamorphic overprinting of veins in amphibolites (retrograded eclogites) from the Dabie ultrahigh-pressure (UHP) terrane, eastern China. The veins mainly consist of plagioclase, Fe-Ti oxides, and quartz, which crystallized from a Ti-rich amphibolite-facies fluid that formed during exhumation of the eclogites. Two types of textures involving the replacement of Fe-Ti oxides by rutile are recognized in the veins: (1) the first type is characterized by the development of rutile coronas (Rt-C) and other silicates (high-Fe epidote, muscovite, and chlorite) around the external boundaries of the Fe-Ti oxide grains, and (2) the second type is characterized by the formation of symplectitic intergrowths of rutile (Rt-S) and magnetite after exsolved hemo-ilmenite (H-Ilm) lamellae in the Fe-Ti oxides. The micro-textures, mineral assemblages, and Zr-in-rutile thermometry indicate that both replacement reactions involved mineral re-equilibration processes in the presence of an infiltrating fluid phase at ~476–515 °C, taking place by an interface-coupled dissolution-precipitation mechanism. Thermodynamic modeling reveals that both replacement reactions occurred during oxidation processes under relatively high-oxygen fugacity (fO₂) conditions, approximately 2.5–4.5 logfO₂ units higher than the fayalite-magnetite-quartz (FMQ) reference buffer. In situ Sr isotopic analyses of epidote (Ep-C) coexisting with the Rt-C suggest that the infiltrating fluid involved in the greenschist-facies replace- ment reactions was externally derived from the surrounding granitic gneisses (the wall rocks of the amphibolites). Compared with the rutile in the UHP eclogites (Rt-E) and amphibolites (Rt-A), the Rt-C is characterized by distinctly lower contents of Nb (<10 ppm) and Ta (<2 ppm) and Nb/Ta ratios (340 ppm) and V (>1580 ppm). These results provide a geochemical fingerprint for distinguishing the low-pressure (LP) rutile from relic high-grade phases in retrograded HP-UHP rocks. Our results reveal that rutile can form during LP retrograde stage in UHP rocks by high-fO₂ fluid-induced replacement reactions. The unusual replacements of Fe-Ti oxides by rutile-bearing assemblages during retrogression provide important constraints on fluid-mineral reactions and fO₂ variations in exhumed UHP slabs.