Mathematics – Logic
Scientific paper
Oct 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.3723s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 56, Issue 10, pp.3723-3731
Mathematics
Logic
Scientific paper
This study had three goals: 1. (1) to study B distribution in a rhyolitic volcanic sequence already extensively investigated for other elements; 2. (2) to interpret the joint behaviour of B and Li during the interaction of such rocks with subsurface waters; 3. (3) to assess the manner in which water affects the behaviour of incompatible elements such as B and Gd. New B, Gd, and Sm analyses have been made on a suite of Yellowstone rhyolites, including fresh and partially devitrified glassy obsidian from surface exposures of several flows, drill-core of increasing degrees of alteration in the Biscuit Basin Flow, and two drill-cores from other flows. Within the Biscuit Basin Flow, the Sm and Gd concentrations remain rather constant (9-12 and 8-10 ppm, respectively) and behave conservatively, independent of alteration. Boron decreases from about 10 to 3 ppm with progressive alteration, and Li increases from about 40 ppm by a factor of 2-3 in the most altered rocks. Thermal waters reflect leaching from the fresh rocks, but contain much less Li than B ( B / Li 3 compared with 0.8 in fresh obsidian). Retention of Li was favoured by abundant illitic alteration. Obsidians from the Valles and Long Valley calderas show greater Li loss during alteration. All the rhyolitic rocks lose B during aqueous alteration; the waters acquire both B and Li, but proportionately much more B. Natural waters of all kinds, including those from the three calderas, show six orders of magnitude range in aqueous B and Li, with a high degree of linear correlation and an average ratio B / Li essentially constant at 4.0. The linearity mainly expresses processes of dilution and concentration: reactions specific to B or Li engender waters with deviating B / Li . The patterns of Sm-Gd-B-Li variation in rocks exemplify two kinds of behaviour. In petrological processes with no hydrous phase (lunar rocks, mantle, fresh seafloor basalt, the lower continental crust), these elements are incompatible and behave similarly; when an aqueous phase is present B and Li behave independently of Sm and Gd.
Shaw Denis M.
Sturchio Neil C.
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