FLUID/ROCK INTERACTION: A COMPARISON OF THE GEOCHEMICAL BEHAVIOR OF ELEMENTS IN NATURAL GEOLOGICAL SYSTEM (QUARRY) AND IN AN MONUMENT ENVIRONMENT

The geochemical/mineralogical behavior of some groups of elements in monument environment are not well known and require considerable groundwork before applications such as stone discrimination/alteration/source material indexes can be studied. Weighing the lack of knowledge against the benefits for the monument preservation, this paper proposes to look at the geochemical behavior of elements in natural geological system (quarry) and in the monument environment. Geochemical study was undertaken using stones from the cathedral of Évora (Portugal) and rock from surrounding quarries. Bulk analytical techniques, including ICPES, INAA, XRF, were performed as well as Mössbauer spectroscopy.

The stones have consistent linear geochemical variations, well constrained by a limited number of inter-element ratios. Taking into account the REE distribution, the Mg, Fe, Ti contents and the Fe³⁺/Fe²⁺ ratio, two types of stones were distinguished. The type-1 stones show REE normalized patterns with negative Eu-anomaly, and are relatively Al-rich. The type-2 stones without Eu-anomaly, more REE-fractionated, are relatively LREE, TiO₂, MgO and Fe₂O₃-rich. In agreement with the geochemical results, the Mössbauer data have shown that the Fe³⁺/Fe²⁺ ratio was correlated with the magmatic differentiation and further suggested that the presence of Fe (Ti) oxides are related to a higher magmatic evolution. Both stone types have a MgO/TiO₂ characteristic of calco-alcaline granite. The chemical inter-element variation along the same constant ratio, as well as the REE pattern and the Fe oxidation state, seem to indicate a filiation from type-2 to type-1 stones.

The aluminum saturation index (Al/(2Ca+Na+K) shows that the cathedral stones differ from the quarry samples. The stones are metaluminous to slightly peraluminous, whereas the quarry samples are clearly peralumonous. The geochemical indexes (Fe/Mg, Ti/Mg, Al saturation index, REE distribution, and Eu anomaly) show that the fluid/rock interactions in the natural weathering system (quarry) differ from the degradation stone process occurring in monument environment.