Chemical and mineralogical changes during basalt-seawater interaction: Site 223, Leg 23, D.S.D.P., north-west Indian Ocean

The development of lowand hightemperature alteration products in a 23 m section of ocean-floor basalts is described. Analcime, calcite and dioctahedral smectite are ubiquitous. Trioctahedral smectite, smectite-chlorite mixed layers, chabazite and scolecite occur in the deeper sections with Fe 3+ oxides/ hydroxides progressively becoming more abundant in the upper regions. The upper layers of the sequence show marked chemical reduction. High-temperature chemical changes include Na and Mg enrichment accompanied by Ca and Fe 2§ losses. Superimposed low temperature changes include gains in Fe 3§ K, Li, and Rb, and losses in Na, Ca, and Fe 2§ Many trace elements also show consistent behaviour. DURING leg 23 of the Deep Sea Drilling Project in the Arabian Sea, basalts were collected beneath Palaeocene sediments from Site 223 (18 ~ 44.98' N., 60 ~ 07.78' E.) in the vicinity of the Owen Fracture Zone. About 23 m of igneous basement was penetrated; the upper 10 m or so being highly brecciated, the whole being progressively altered to a range of secondary minerals. Six core samples, representing the 23 m penetrated, were available for investigation (Table I). The igneous material, believed to represent a single flow, has been designated 'trachybasalt', being an alkali-rich rock atypical of both mid-ocean ridge and island arc environments (Whitmarsh et al., 1974). More recent work shows the basement material to be a transitional basalt with strong sub-alkaline affinities (Papavassiliou, 1979). Mineralogy and mineral chemistry. Secondary minerals in Site 223 basalts occur chiefly in fractures, vesicles or replacing interstitial groundmass material--mainly glass. Table I gives a semiquantitative estimate of their distribution; analcime, smectite and calcite being the most abundant. Analcime is common to abundant throughout * Present address: Laboratory of Mineralogy and Petrology, University of Athens, Ano Ilissia (Panepistimioupolis), Athens, Greece. t~ Copyright the Mineralogical Society the entire igneous sequence. Study of thin sections show that it occurs mainly in vesicles associated with palagonite, the vesicles being commonly rimmed with smectite (Papavassiliou, op. cit.). This type of occurrence suggests direct precipitation from a fluid phase of high salinity being rich in H4SiO4 and AI(OH)3, and with a high Na: K ratio and a high pH. The existence of such fluids has been demonstrated by Jehl et al. (1977) in their study of ocean crust metamorphism in the N. Atlantic Ocean. They showed fluid salinity values up to five times that of normal seawater. Following the experimental work of Keene et al. (1976) a possible reaction for the formation of analcime might be: Al(OH)4(aq) + 2H4SIO4 + Na § (aq) NaA1Si206H20 + 5H20 The AG (reaction) is negative at 25 ~ (-19.053 kcal/mol), implying that thermodynamically, analcime can form spontaneously at 25 ~ given the necessary reactants. The identification of analcime was confirmed by X-ray diffraction which also indicated the presence of the minor zeolites chabazite and scolecite (Table II) which only occur in small quantities in the lower samples. Analcime represents one-sixth of the total zeolite population in D.S.D.P. material from the Indian Ocean where it is present largely in 'basaltic volcanic sediments' (Iijima, 1978). Smectite was identified by X-ray diffraction; orientated slide-mounts of bulk samples showing the characteristic smectite group expansion to 17 ,~ upon glycolation and contraction to 10 A upon heating to 550 ~ from the untreated (001) reflection between 14 A and 14.6 A. The X-ray results for non-orientated samples showed that throughout the igneous basement, smectites having (060) reflections in the range 1.50-1.52 A are present, i.e. a dioctahedral variety. However, the lowest sample (41-2) also contains a more abundant smectite with 142 C. TH. P A P A V A S S I L I O U A N D M. E. C O S G R O V E T A B L E I. Colour, secondary minerals, and oxidation coefficient of basalt core samples, Site 223 Depth in bore below ocean floor 713-22 m 722-31 m 731-40 m Sample No. 39-2 39-3 40-1 40-2 40-2 41-2 (46 48) (132-134) (60-62) (15-17) (105 107) (12-14) Colour Red-brown, yellow with Red-brown Grey-green with Green-grey some green spots green spots some red-brown spots Hematite * * * * Geothite * ** ** ** * * Chabazite and scolecite Analcime **** *** **** Calcite * ** ** Smectite **** **** *** Smectite-chlorite mixed layers Oxidation coefficient 0.57 0.92 0.87 * * * * * * * * * *