Major element and volatile variations of volcanic glasses from Shatsky Rise, sampled from IODP Expedition 324

Shatsky Rise is a late Jurassic-early Cretaceous oceanic plateau located in the northwestern Pacific. It consists of three major massifs: Tamu, Ori and Shirshov Massifs from southwest to northeast. The Rise is 1000km x 200km in size, similar to Japan or California State. Magnetic anomalies of Shatsky Rise indicate that the massifs get younger towards the northeast and have formed along the trace of a triple junction at a mid oceanic ridge. The IODP Expedition 324 drilled five sites on Shatsky Rise, two on Tamu Massif (Sites U1347 and U1348; east flank and north flank), two on Ori Massif (Sites U1349 and U1350; summit and east flank) and one on Shirshov Massif (Site U1346; summit). Unaltered fresh volcanic glasses were sampled from all sites except Site U1349. Glass is preserved as < 2cm thin rims of flow margins or occur in volcanic breccias. Major element compositions of 115 volcanic glasses (68 from Site U1347, 1 from Sites U1346 and U1348, and 45 from Site U1350) were analyzed by an electron probe microanalysis, and about half of them (57 samples) were selected for volatile (H2O, CO2, F, Cl and S) analyses by secondary ion mass spectrometry at WHOI. Based on the variations of SiO2 and Na2O+K2O, all glasses are sub-alkalic (tholeiitic) basalt. MgO and SiO2 contents range from 5 to 8.5 wt% and from 48.5 to 52 wt%, respectively. Although K2O content of most samples are < 0.33 wt%, glasses with higher K2O content (0.46-0.74 wt%) occur at the top and middle of the drilled interval at Site U1350 (156-173mbsf and 222-230mbsf). High-K2O glasses do not fall on the same liquid line of descent as other glasses which cannot be derived from crystallizations of a single magma. Volatile contents of glasses from Site U1347 (H2O: 0.2-0.7 wt%, Cl: 100-550 ppm and S: 250-700 ppm) can be explained by seawater assimilation, whereas those in Site U1350 (H2O: 0.2-0.6 wt%, Cl: 600-1200 ppm and S: 750-1500 ppm) can be explained by assimilation of 40% NaCl brine. The CO2 contents of all volcanic glasses are low (<40 ppm), indicating that the lavas were erupted in shallow water (< 1000m).