USING CR-SPINEL INCLUSIONS TO TRACK CHANGES IN PLAGIOCLASE CRYSTAL CARGOES IN PLAGIOCLASE ULTRAPHYRIC BASALTS FROM SITES U1500 AND U1502 ON THE SOUTH CHINA SEA RIFTED MARGIN

IODP expeditions 367 / 368 / 368X drilled basalt lavas from three sites on the northern rifted margin of the South China Sea. Whole rock major and trace element compositions show N-MORB compositions. Sites U1500 and U1502 contain Plagioclase Ultraphyric Basalt (PUB) horizons, with 18-45% plagioclase macrocysts, that are typically found at slow-spreading ridges. The basalt sequence at U1502 is extensively overprinted by hydrothermal alteration, especially in the upper part where Cr-spinel is the only primary magmatic phase remaining. Therefore, this study focuses on characterizing Cr-spinels, most found as inclusions in plagioclase macrocrysts, to investigate the origins of these PUBs. In order to analyze a statistically representative crystal population, Cr-spinels were isolated using HF-HNO₃ acid digestion to remove the matrix and silicate minerals. For this study, >500 Cr-spinel crystals were analyzed by electron microprobe. Cr-spinels from slightly plagioclase porphyritic flows at site U1503 have compositions broadly consistent with being formed in equilibrium with the host melt. In contrast, most of the Cr-spinels from PUB units at U1500 and U1502 formed from melts more primitive that the carrier melt, based on maximum observed Mg# values and calculated TiO₂ and Al₂O₃compositions of melts in equilibrium with Cr- spinels. This observation, together with the fact that the plagioclase macrocysts in the PUB units are too Ca-rich (~An83) to be in equilibrium with the host melts, indicate that they represent antecrysts from lower crustal mush zones. In detail, different PUB units at U1502 contain distinct plagioclase macrocryst populations based on compositional differences of Cr-spinel inclusions. This suggests that different ascending melt batches are selectively disaggregating specific mush zone levels rather than assimilating material from throughout the vertical trans-crustal mush zone.