Submarine pillow basalts (34 Ma) recovered from the Northern Kerguelen Plateau at ODP Site 1140 contain abundant unaltered glass, providing the first opportunity to measure the volatile contents of tholeiitic basaltic magmas related to the Kerguelen mantle plume. The glasses have La/Sm and Nb/Zr ratios that vary from values similar to Southeast Indian Ridge (SEIR) MORB (Unit 1), to slightly more enriched (Unit 6), to values transitional between SEIR MORB and basaltic magmas formed by melting of the Kerguelen plume (Units 2 and 3). Volatile contents for glasses in Units 1 and 6 are similar to depleted mid-ocean ridge basalt (MORB) values (0·25–0·27 wt % H2O, 1240–1450 ppm S, 42–54 ppm Cl). In contrast, H2O contents are higher for the enriched glasses (Unit 2, 0·44 wt % H2O; Unit 3, 0·69 wt %), as are S (1500 ppm) and Cl (146–206 ppm). Cl/K ratios for all glasses are relatively low (0·03–0·04), indicating that assimilation of hydrothermally altered material did not occur during shallow-level crystallization. H2O/Ce for the enriched glasses (Units 2 and 3) is significantly lower than Pacific and South Atlantic MORB values, suggesting that low H2O/Ce may be an inherent characteristic of the Kerguelen plume source. Vapor saturation pressures calculated using the H2O and CO2 contents of the glasses indicate that 1700 m of subsidence has occurred on this part of the plateau since eruption of the basalts at 34 Ma. Subsidence estimates for Site 1140 and other ODP drill sites indicate that the various parts of the Kerguelen Plateau subsided at a rate comparable with that for normal Indian Ocean lithosphere.
KEY WORDS: basaltic glass; Kerguelen plume; trace elements; volatiles; water
PAUL J. WALLACE*
OCEAN DRILLING PROGRAM, TEXAS A&M UNIVERSITY, COLLEGE STATION, TX 77845, USA