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We report the surface exploration by the lunar rover Yutu that landed on the young lava flow in the northeastern part of the Mare Imbrium, which is the largest basin on the nearside of the Moon and is filled with several basalt units estimated to date from 3.5 to 2.0 Ga. The onboard lunar penetrating radar conducted a 114-m-long profile, which measured a thickness of ∼5 m of the lunar regolith layer and detected three underlying basalt units at depths of 195, 215, and 345 m. The radar measurements suggest underestimation of the global lunar regolith thickness by other methods and reveal a vast volume of the last volcano eruption. The in situ spectral reflectance and elemental analysis of the lunar soil at the landing site suggest that the young basalt could be derived from an ilmenite-rich mantle reservoir and then assimilated by 10-20% of the last residual melt of the lunar magma ocean.volcanic history | Imbrium basin | lunar rover Yutu | lunar penetrating radar | Chang'e-3 mission

Most magmatism occurring on Earth is conventionally attributed to passive mantle upwelling at mid-ocean ridges, slab devolatilization at subduction zones, and mantle plumes. However, the widespread Cenozoic intraplate volcanism in northeast China 1-3 and the peculiar petit-spot volcanoes 4-7 offshore the Japan trench cannot be readily associated with any of these mechanisms. In addition, the mantle beneath this type of volcanism is characterized by seismic low velocity zones (LVZs) present above and below the transition zone [8][9][10][11][12] , and a comprehensive interpretation of all these intriguing phenomena is lacking. Here we show that most if not all the intraplate and petit-spot volcanism and LVZs present around the Japanese subduction zone can be explained by the Cenozoic interaction of the subducting Pacific slab with a hydrous transition zone. Numerical modelling results indicate that 0.2-0.3 wt.% H2O dissolved in mantle minerals which are driven out from the transition zone in response to subduction and retreat of a stagnant plate is sufficient to reproduce the observations. This suggests that a critical amount of water may have accumulated in the transition zone around this subduction zone as well as others of the Tethyan tectonic belt 13 characterized by intraplate/petit-spot volcanism and LVZs in the underlying mantle.The Cenozoic intraplate volcanism in northeast China is located more than 1000 km westward of the Japan trench 1 , while the young alkaline basalts (0-6 Ma) known as petit-spots outcrop up to 600 km eastward of the trench 4 (Fig. 1). The formation mechanism of these types of on-and off-shore volcanism is still debated as there is no geological and geophysical correlation with mantle plumes or arc volcanism 6,14 .Seismic tomography models indicate that in this region the Pacific Plate is currently stagnant in the transition zone, extending continuously up to nearly 1000 km to the inland of northeast China 3,8,10 . Thus, it has been proposed that the Cenozoic intraplate magmatism is related to the dehydration of the Pacific slab in the mantle transition zone (MTZ) between ~410 km and ~660 km depth 2,15 .The primary petit-spot magma has been determined volatile-rich with extremely EM 1-like isotopic compositions 6,7 . The lack of hotspot tracks in this region excludes the contribution from a mantle

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Layers of Ca-rich garnet-clinopyroxene rocks enclosed in a serpentinite body at Hujialin, in the Su-Lu terrane of eastern China, preserve igneous textures, relict spinel in garnet, and exsolution lamellae of Ca-rich garnet, ilmenite/magnetite, Fe-rich spinel, and also amphibole in clinopyroxene. In terms of their major and trace element compositions, the studied samples form a trend from arc cumulates towards Fe-Ti gabbros. Reconstructed augite compositions plot on the trend for clinopyroxene in arc cumulates. These data suggest that the rocks crystallized from mantle-derived magmas differentiated to various extents beneath an arc. The Ca-rich garnet þ diopside assemblage is inferred to have formed by compressing Ca-rich augite, whereas the relatively Mg-rich cores of garnet porphyroblasts may have formed at the expense of spinel. The protolith cumulates were subducted from near the crust-mantle boundary (c. 1 GPa) deep into the upper mantle (4Á8 AE 0Á6 GPa and 750 AE 50 C). Negatively sloped P-T paths for the garnetclinopyroxene rocks and the corollary of corner flow induced subduction of mantle wedge peridotite are not supported by the available data. Cooling with, or without, decompression of the cumulates after the igneous stage probably occurred prior to deep subduction.

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