Volatiles in Submarine Basaltic Glasses from the Northern Kerguelen Plateau (ODP Site 1140): Implications for Source Region Compositions, Magmatic Processes, and Plateau Subsidence

Abstract

Submarine pillow basalts (34 Ma) recovered from the Northern INTRODUCTION Kerguelen Plateau at ODP Site 1140 contain abundant unaltered The Kerguelen Plateau and Broken Ridge formed mostly glass, providing the first opportunity to measure the volatile contents during the Cretaceous as a single, giant oceanic plateau of tholeiitic basaltic magmas related to the Kerguelen mantle plume. that is interpreted to represent voluminous magmatism The glasses have La/Sm and Nb/Zr ratios that vary from values associated with emplacement of a mantle plume head similar to Southeast Indian Ridge (SEIR) MORB (Unit 1), to below young Indian Ocean lithosphere (Duncan & slightly more enriched (Unit 6), to values transitional between Storey, 1992; Weis et al., 1992). During Ocean Drilling SEIR MORB and basaltic magmas formed by melting of the Program (ODP) Leg 183, igneous basement rock and Kerguelen plume (Units 2 and 3). Volatile contents for glasses in sediment cores were obtained from five sites on the Units 1 and 6 are similar to depleted mid-ocean ridge basalt Kerguelen Plateau and two on Broken Ridge (Fig. 1). The (MORB) values (0·25–0·27 wt % H2O, 1240–1450 ppm S, results, combined with subsequent radiometric dating, 42–54 ppm Cl). In contrast, H2O contents are higher for the demonstrate that from south to north the age of the enriched glasses (Unit 2, 0·44 wt % H2O; Unit 3, 0·69 wt %), uppermost crust decreases from 120–110 Ma in the as are S (1500 ppm) and Cl (146–206 ppm). Cl/K ratios Southern Kerguelen Plateau, to >110 to 95 Ma in the for all glasses are relatively low (0·03–0·04), indicating that Central Kerguelen Plateau, Broken Ridge and Elan Bank, assimilation of hydrothermally altered material did not occur during to Ζ35 Ma in the Northern Kerguelen Plateau (Coffin shallow-level crystallization. H2O/Ce for the enriched glasses et al., 2000; Pringle & Duncan, 2000; Duncan, 2002). At (Units 2 and 3) is significantly lower than Pacific and South six of seven basement sites, as well as at four sites drilled Atlantic MORB values, suggesting that low H2O/Ce may be an on earlier ODP cruises, subaerially erupted basalt flows inherent characteristic of the Kerguelen plume source. Vapor saturation were recovered, indicating that the growth rate of the pressures calculated using the H2O and CO2 contents of the glasses Kerguelen large igneous province (LIP) was sufficient to indicate that >1700 m of subsidence has occurred on this part of form a subaerial landmass. However, a sequence of the plateau since eruption of the basalts at 34 Ma. Subsidence submarine-erupted pillow basalt flows of 90 m thickness estimates for Site 1140 and other ODP drill sites indicate that the was drilled at Site 1140 on the Northern Kerguelen various parts of the Kerguelen Plateau subsided at a rate comparable Plateau (Coffin et al., 2000). with that for normal Indian Ocean lithosphere. The Site 1140 submarine glasses provide the first