Physics
Scientific paper
Mar 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55..731t&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 3, pp.731-749
Physics
17
Scientific paper
Palagonite of basalt and basaltic andesite parentages from hyaloclastite deposits in Iceland has been investigated. SEM studies indicate a sharp to diffuse alteration front which may propagate along microfractures in the glass, resulting in a progressive partial dissolution yielding palagonite of variable, but generally increasing porosity towards grain surfaces. The palagonite has a granular texture. Incipient alteration is indicated by the development of individual globules (ca. 0.01 m in diameter), whereas at an advanced stage chains of globules, defining a sponge-like texture, characterize the palagonite. In the basaltic andesite, the precursor to brown Ti- and Fe-rich palagonite is a white variety, for which an evolutionary model is presented. EDS line-scans across the fresh glass-palagonite boundary show the existence of a 2-4 m thick zone of glass-like material in which all elements have been depleted, except Si, and in some cases Al, which have been relatively enriched. The white palagonite is characterized by strong depletion of Ti, Fe, Na, Mg, and to lesser extent, Ca, Al (in order of decreasing loss). Regardless of the degree of porosity development (ca. 1-43 vol%), the extent of element depletion relative to SiO 2 is constant. This gives evidence for selective element mobility prior to a variable degree of congruent network dissolution of the Si-rich residue, yielding zoned palagonite with a different porosity. In order for Fe, Ti, and Al to dissolve, a pH < 3 is necessary. In an oxidizing environment and at pH-values > 3, Fe, Ti, and Al will precipitate in the pores of the white palagonite as oxides/hydroxides, thus creating the brown variety, which is characterized by highly variable contents of the above-mentioned elements. The applicability of this model to palagonite derived from basalt parentages at different pH conditions is discussed.
Furnes Harald
Thorseth Ingunn H.
Tumyr Ole
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