V. Daux et al., GEOCHEMICAL EVOLUTION OF BASALTIC ROCKS SUBJECTED TO WEATHERING - FATE OF THE MAJOR ELEMENTS, RARE-EARTH ELEMENTS, AND THORIUM, Geochimica et cosmochimica acta, 58(22), 1994, pp. 4941-4954
Eleven Icelandic hyaloclastites altered in freshwater have been studie
d. The weathering of basaltic glass, which is their primary constituen
ts, leads to precipitation of clayey and possibly zeolitic phases. The
dissolution reaction progress (mass of dissolved glass per liter of s
olution) governs the chemistry of the secondary phases, which control
the residence time of the solution through their influence on rock per
meability. They contribute to the regulation of reaction advancement.
The reaction progress, xi, can be calculated according to: xi = [Sr(w)
.(I(w) - I(SP)).(I(G) + 9.375)]/[Sr(G).(I(SP) - I(G)). (I(W) + 9.375)]
, where I(W), I(G), and I(SP) are the isotopic strontium ratios of ini
tial water, of pristine glass, and secondary products, respectively, a
nd Sr(W) and Sr(G) the strontium contents of initial water and glass.
The amount of dissolved glass per liter of solution is estimated to be
in the order of 0.01-0.1 g for samples 90000-100000 years old. A glob
al agreement is found between this result and estimations that can be
made on the basis of thermodynamic modelling. Expressing the geochemic
al budget as a function of the reaction progress allows a look at the
evolving aspect of the rock/water interaction: (1) According to the bu
dget calculated for the major elements, the rock undergoes a global lo
ss of matter which decreases with increasing reaction progress (from -
45 to about 0% losses). For the most evolved samples, the transformati
on of the pristine basaltic glass to the alteration products is a near
ly conservative process. (2) There is increasing evidence that rare ea
rth elements, particularly LREEs, can be mobilized during alteration p
rocesses. Our study corroborates this view as we demonstrate that REEs
can be significantly mobilized during basaltic glass weathering. Neve
rtheless, no fractionation among the suite of REEs was observed. We sh
ow that Th and REE behaviours are similar. Mass balance calculations a
ccount for Th and REE losses up to 40%. These losses are not related t
o the reaction progress but could be due to a colloidal or particulate
transport. The REE contents of the secondary clayey phases is showed
to be linked to their degree of crystallinity. The need to take into a
ccount the latter parameter to modelize the long-term behaviour of the
elements adsorbed onto clay minerals is outlined.