Short time leaching experiments (2, 5, and 22 h) have been carried out on the glassy rim of a pillow basalt (MORB, East Pacific Rise) and its microcrystalline interior. The mildly acidic (self-adjusting to about pH 4-3) leaching solution (50 ml bidist. water) reacted with 1.5 g of powdered rock (< 100 µ) in a PE-bottle. A cation exchange resin was present to collect all cations and to prevent their re-adsorption as well as the precipitation of secondary minerals. After elution of the resin, the obtained solution was analyzed with ICP-MS and ICP-AES.
The results (calculated as leached fraction (%) of the whole-rock) represent the initial availability of elements in dependence on the rock´s mineralogy. In order to compare the leaching curves of 31 analyzed elements and to indicate their sources, the leached fractions are plotted vs. the ratio of the leached fractions at two different time steps, e.g. 2 h and 22 h. The latter is a reduced expression for the leaching kinetic or the steepness of the leaching curve as a whole. In the resulting diagram, easily available elements plot in the upper right corner, less easily available in the lower left.
The leaching curves of the glassy rim are similar to each other and plot in the diagram as a close group, indicating the same availability due to homogeneous distribution within the amorphous glass (Fig. 1a). The deviations of some elements are caused by preferential release of the largest ions from the network structure as well as from subcrystallites detected by X-ray diffraction.
The leaching curves of the microcrystalline basalt show a large spread caused by inhomogeneous distribution of the elements between minerals in the rock. The mineralogy is mirrored in the diagram by the similar leaching behaviour (distinct ratio on the
x-axis) of characteristic elements indicating the leached phases (Fig. 1b). In addition, the order from clinopyroxene to glassy matrix reflects the increasing leachability of the different phases.
The highest availability was detected for the REEs (decreasing from La to Lu) which are in preference leached from the glassy matrix together with the rock´s largest ions. Of special interest is the position of Eu, whose leaching curve is different to that of the neighbouring REEs and indicates the leached Eu-fraction as a mixture of at least two sources: plagioclase with its known high Eu concentration and the glassy matrix.
The simple designed experiment demonstrated a high sensitivity for the individual dissolution behaviour of different phases. No distortion of the relative leaching behaviour due to experimental conditions has been detected, e.g. the grinding of the samples for 20 minutes. Hence, the experiment is able to determine the individual leaching/dissolution behaviour of different minerals within one rock sample; it can give more information about the relative behaviour of main and trace elements in the multi-phase system "rock" during fluid-rock interaction by estimating their initial availability.
Fig. 1: The leaching curves, transformed to points within the diagram (see text), enable a detailed comparison between the elements' leaching behaviour as shown for the glassy rim (Fig. 1a) and the microcrystalline basalt (Fig. 1b). The glass is characterized by a mainly
homogeneous leaching behaviour, the microcrystalline basalt by different groups which can be related to their leached phases.