THE EFFECTS OF WEATHERING ON RARE-EARTH ELEMENT, Y AND BA ABUNDANCES IN TERTIARY BASALTS FROM SOUTHEASTERN AUSTRALIA

Tholeiitic basalts showing anomalously high Ba, Y and rare-earth element (REE) concentrations occur within the Tertiary Newer Volcanic Province of Victoria and these geochemical characteristics are related to incipient alteration accompanying weathering. REE and Y released during weathering of basalts are retained either in secondary phosphate or clay minerals and. in rocks showing very high Ba concentrations, the Ba is present as BaSO4. Leaching experiments suggest that glassy or fine-grained rocks are more likely to release REE, Y and Ba while more extensively altered rocks are likely to hold these elements, probably because they substitute in, or are adsorbed on, clay minerals or because secondary BaSO4 and REE phosphates are resistant to acid leaching. Samples taken sequentially across a weathering profile in a tholeiitic basalt flow, from fresh to intensively altered rock, illustrate that, in the very earliest stages of alteration, extreme variations in the abundances of Ba. Y and REE can occur. This is reflected in the broad patterns of trace-element behaviour manifested by an extensive analysed sample suite from the Victorian Newer Volcanic Province where there is no correlation between anomalous enrichments in Ba, Y, or REE and the level of incipient alteration. Ba anomalies are not necessarily accompanied by "abnormal" Y and REE behaviour and this is probably because the mobility of each trace element is a function of. (a) the ionic radius and charge of the ionic species it forms in solution; (b) local groundwater parameters (such as Eh, pH, groundwater flux, and concentrations in the groundwater of species such as SO42-); and (c) the secondary mineral assemblages developed in a particular situation. Sr isotopic compositions are not strongly affected by incipient weathering and do not show significant changes until alteration has progressed to the point where plagioclase begins to break down. The study illustrates that relatively slight alteration at the earliest stages of weathering can produce drastic changes in the concentrations of REE, Y and Ba and petrographic assessment of the freshness of samples is not precise enough to eliminate, prior to analysis, cases where these effects may be manifested. Where weathering has caused changes in abundance patterns these may be recognised by careful comparisons of appropriate elemental ratios (e.g., Ba/Rb) throughout a particular analysed sample suite.