Zeolite-Facies Metamorphism of Central Kerguelen Plateau Basalts

Abstract

Ocean Drilling Program Leg 120 recovered basement samples that consisted of zeolite-facies metabasalts at Sites 747, 749, and 750 on the Kerguelen Plateau. These basalts were metamorphosed in the low to intermediate zones of the zeolite facies, as indicated by the presence of diagnostic zeolites and the absence of chlorite, epidote, prehnite, pumpellyite, and wairakite. Chabazite, natrolite, thompsonite, mesolite, stilbite, huelandite, and smectites occur as amygduloidal fillings in basalts from Holes 747C and 750B, whereas only stilbite, laumontite, and pure and mixed-layered smectites were identified in amygduloidal basalts from Hole 749C. In the lower sections of Hole 749C, only laumontite and mixed-layered smectites coexist.Based on calculations with published experimental phase equilibria, the absence of wairakite in basalts from Hole 749C and of laumontite in basalts from Holes 747C and 750B suggests that metamorphic temperatures did not exceed approximately 225° and 120°C, respectively. The presence of well-developed zeolite mineral assemblages • and the absence of carbonate and clay mineral assemblages restricts X C Q 2 m tne fluid to approximately 0.0075. Low-to intermediate-zone zeolite-facies mineral assemblages in basalts from the Kerguelen Plateau can be accounted for by metamorphism in an active geothermal area such as present-day Iceland.

Trace element mobility during sub-seafloor alteration of basaltic glass from Ocean Drilling Program site 953 (off Gran Canaria)

“…(Schmincke et al 1995). The secondary mineral assemblage consisting of analcite, apophyllite, gmelinite, natrolite, phillipsite, prehnite, smectite (Lindblom and Gérard 1998), gobbinsite, heulandite, paulingite, mordenite, saponite, and montmorillonite (this study) are all characteristic for a low-grade zeolite facies metamorphism (Sevigny et al 1992;Neuhoff et al 1997). The precipitation of zeolites in inter-particle pore spaces represents a late stage in the alteration history because the prior formation of clay minerals causes a depletion of Mg and an increase of pH, which favors the growth of zeolites (Alt et al 1986).…”

Kerguelen Plateau-Broken Ridge: A Large Igneous Province

“…histories of subaerial volcanism and erosion, followed by subsidence caused by cooling. Zeolite mineralogy of the basaltic basement indicates erosion to deeper levels at Site 749 than at Sites 747 and 750 (Sevigny et al, 1992).…”

Kerguelen Plateau-Broken Ridge: A Large Igneous Province

“…histories of subaerial volcanism and erosion followed by subsidence caused by cooling. Higher temperature metamorphism, based on zeolite mineralogy, of the basaltic basement at Site 749 compared to Sites 747 and 750 may indicate erosion to deeper levels at Site 749 (Sevigny et al, 1992).…”

“…Answers to these questions, to be provisionally provided by dating the oldest sediment above basaltic basement and later more definitively by 40 Ar/ 39 Ar dating of the basalts, are required to understand the generation of voluminous magma, the physical processes of magma intrusion and extrusion, and to assess the impact of Cretaceous volcanism on the surficial environment by estimating fluxes into the ocean-atmosphere system. An aspect of oceanic plateau volcanism that has been explored in only cursory detail (e.g., Sevigny et al, 1992) is the role of hydrothermal alteration in controlling elemental and isotopic fluxes. The extent, nature, and duration of hydrothermal processes on the plateau can be determined by drilling several holes with 150-200 m basement penetration.…”