Hydrothermal alteration of oceanic crust in the West Philippine Sea Basin (Ocean Drilling Program Leg 195, Site 1201): inferences from a mineral chemistry investigation

Abstract

Secondary minerals of a 91 meters-thick sequence of pillow basalts cored during ODP Leg 195 (Site 1201, West Philippine Basin) were investigated to reconstruct the hydrothermal alteration history and regime. The basement was first buried by red clays, and then by a thick turbidite sequence, thereby isolating it from seawater. The basalts are primitive to moderately fractionated, texturally variable from hypocrystalline and spherulitic to intersertal, sub-ophitic and intergranular. Relic primary minerals are plagioclase, clinopyroxene and opaques. Hydrothermal alteration pervasively affected the basalts, generating secondary clay minerals (mostly glauconite, minor Al-saponite and Fe-beidellite), ‘‘iddingsite’’, Ca–Na-zeolites, minor alkali-feldspar and calcite. The secondary mineral paragenesis and mutual relationships suggest that the hydrothermal alteration occurred under zeolite-facies conditions, at temperatures <100–150 °C. The main phase of alteration occurred under oxidizing conditions, with a high seawater/rock ratio, in an open-circulation regime, at temperatures of 30–60 °C, with precipitation of abundant glauconite and ‘‘iddingsite’’. A later stage of alteration occurred at ca. 70 °C, with precipitation of abundant Ca–Na-zeolites and minor calcite, in a more restricted circulation regime as a consequence of basement burial under the sedimentary cover, which supplied an altered, Ca-rich and Mg–K-sulfate-poor water causing precipitation of almost pure calcite

Glauconitic laminated crusts from hydrothermal alteration of Jurassic pillow-lavas (Betic Cordillera, S Spain): a microbial influence case

“…Moreover, we must bear in mind the local record of thin films of saponite directly on pillow-lava surfaces, before the development of the thick green laminated crusts. At any rate, there are significant similarities between the products of low temperature hydrothermal alteration in the pillow-lavas of this study as compared with those from ocean-ridge basalts and oceanic crusts (Alt, 1999;Clayton and Pearce, 2000;D'Antonio and Kristensen, 2005), with the presence of early Fe-rich dioctahedral micas followed by a later formation of saponite and carbonates.…”

“…Given the type of deposits related to the infilling of voids and veins among pillow-lavas, the environmental context for the genesis of the studied glauconite must be related to hydrothermal rather than sedimentary processes. However, these glauconites are exceptional in the literature because previous (scarce) descriptions of glauconite in hydrothermal environments refer to granular/pelletal glauconites and not to laminated crusts and columns (Buatier et al, 1989(Buatier et al, , 1993D'Antonio and Kristensen, 2005;Ionescu et al, 2006;Geptner et al, 2008;Clauer et al, 2011). In addition, celadonite is a typical hydrothermal alteration product of marine basalts in the field veinlets (Andrews, 1980;Delmont, 1985;Staudigel et al, 1986;Tazaki and Fyfe, 1992;Renac et al, 2010;Tóth et al, 2010).…”

“…Secondary clay minerals and their association with carbonates can reveal whether the alteration occurred under oxidizing or reducing conditions, as a direct consequence of the fluid/rock mass ratio (D'Antonio and Kristensen, 2005;Renac et al, 2010). If the main phase of hydrothermal alteration occurred under oxidizing conditions with a high sea-water/rock ratio, in an open-circulation regime, it would precipitate abundant glauconite and celadonite.…”

“…Benz et al, 1998), we cannot exclude the activity of iron-oxidizing microbes. Studies on the hydrothermal alteration of basalts by sea-water at low temperature show an early oxidative phase Alt, 1999;Clayton and Pearce, 2000;D'Antonio and Kristensen, 2005). The early alteration mainly proceeds as circulation and diffusion of oxygenated sea-water along veins and inter-pillow spaces coupled with diffusion into the rock circulate more slowly or are trapped, then the environment is dominated by the reducing property of the pillow-basalt.…”

“…The early alteration mainly proceeds as circulation and diffusion of oxygenated sea-water along veins and inter-pillow spaces coupled with diffusion into the rock circulate more slowly or are trapped, then the environment is dominated by the reducing property of the pillow-basalt. According to D'Antonio and Kristensen (2005), glauconite characterises hydrothermal alteration under oxidizing conditions, whereas saponite and calcite characterise alteration under reducing conditions. In this sense, Clayton and Pearce (2000) established a clay-mineral transition from interestratified glauconite-nontronite, interstratified smectite-glauconite and saponite/beidellite mixture depending on the distance from fluid conduits.…”