IODP Hole U1502B penetrates >180 m into the crystalline basement generated at a rifting margin of the South China Sea (SCS), which is the first confirmed intermediate-type margin worldwide. The recovered lavas show petrographic characteristics of mid-ocean ridge basalt (MORB), but suffered pervasively from hydrothermal alteration. This sequence represents the oldest SCS oceanic crust ever drilled in-situ, and offers a globally unique window to explore the hydrothermal processes during continental breakup. Here, 50 whole-rock samples representative of Hole U1502B were analyzed for major & trace elements and Sr–Nd isotopes, presenting the first report of hydrothermally altered basalts for the SCS. The protolith appears to be tholeiite, enriched MORB, and little affected by crustal contamination due to the constant mantle-values of εNd. However, the altered rocks are characterized by significant Ca depletion and 87Sr/86Sr modification. Major processes are identified to be tightly involved with plagioclase: chloritization and albitization. Both reactions are responsible for the Ca-loss and Sr-mobility, and for the resultant Mg- and Na-uptakes, respectively. Environments varying from the peripheral to deep parts within a discharge zone are evidenced by the co-existence of hydro-fracturing brecciation, high water/rock ratios (~1–25), and lower greenschist facies alteration (albite–chlorite–epidote + quartz). This variability can be attributed to detachment-related faulting, which provides permeability allowing deeply channeled pathways of fluids. Such tectonic effects also permit a penetration of Hole U1502B into the lava–dike transition. With a migration from on-axis to off-axis alterations discerned, our results together imply a more complex and longer SCS rifting than previously thought.

Dissolved rare earth elements (REE) and neodymium isotopic compositions (εNd) were intensively used to evaluate water mass mixing and lithogenic inputs in oceans. The South China Sea (SCS) is the largest marginal sea and a key region for reconstructing past hydrological changes in the West Pacific; however, its REE and εNd distribution are still not well established. This study investigated dissolved REE concentration and εNd distribution at four water stations in the northern and central SCS to better constrain the εNd distribution and REE cycle in the SCS. The results show relatively high concentrations of REE in surface seawater due to the terrigenous inputs. Seasonal variability in the middle REE enrichment is observed, suggesting a controlling role of the lateral mixing of water masses in the REE fractionation. The decreased REE concentrations in bottom water are mainly attributed to the re-suspended particle scavenging. Surface seawater εNd varies from -2.8±0.3 to -6.7±0.3, implying a significant modification due to riverine inputs. The intermediate water is characterized by a slightly negative εNd compared to the North Pacific Intermediate Water (NPIW) suggesting a vertical mixing between the intermediate and deep water within the SCS. εNd of deep water shows a narrow range from -3.4±0.3 to -4.2±0.3 (mean value of ~-3.8), supporting the presence of Pacific Deep Water (PDW) in the deep SCS basins nowadays. εNd of deep water in the SCS behaves conservatively along its pathway from the West Pacific to the SCS even though particle scavenging occurs in bottom water.