Christophe Renac

Opals from Slovakia (“Hungarian” opals) a re-assessment of the conditions of formation

Slovakian opals are found in an andesitic host-rock and believed to have formed by water circulation during a tectonic event. Their physical properties are investigated: X-Ray Diffraction (opal-A), Raman spectra (main Raman peak at 437 cm −1 ) and microstructure (large silica spheres 125 to 270 nm in diameter) surprisingly are properties of opals usually found in sedimentary deposits, and differ from those of opals found in other volcanic deposits. The temperature is proposed to control these physical properties rather than the nature of the host-rock. Some preliminary results of oxygen isotopic composition indicate a high δ 18 O for Slovakian and Australian opals (≈ 31‰) consistent with low temperatures of formation (lower than 45°C); by contrast, Mexican opals-CT show a lower δ 18 O at 13‰ consistent with a formation at a higher temperature, possibly up to 190°C.

Hydrothermal fluid interaction in basaltic lava units, Kerguelen Archipelago (SW Indian Ocean)

Hydrothermally altered basaltic lava-units in the northern Kerguelen Archipelago contain a wide variety of secondary silicate and carbonate minerals, including zeolites, hydrothermal calcite, dolomite and magnesite, as well as celadonite, orthoclase (adularia) and quartz. Petrography, fluid-inclusion microthermometry, trace-elements geochemistry, Sr isotopes and stable-isotope compositions indicate hydrothermal fluid cells derived from meteoric water interacting with basalts and Rb-rich subvolcanic peralkaline rocks at temperatures ranging from 50 to 200 °C associated with the cooling of the lava pile. The calculated δ 18 O values of meteoric-hydrothermal waters in fossil hydrothermal systems are identical to those in present-day hot springs, suggesting that meteoric recharge was continuous throughout the igneous cooling cycles of the 28–23 Ma older host basalts and the younger 15–5 Ma old peralkaline intrusions. The Kerguelen northern coastline hydrothermal system in the basaltic pile demonstrates that much of the silicate mineralogy and almost all carbonate secondary minerals in altered basalts were derived from meteoric-hydrothermal fluids, rather than products of seawater interaction, or even magmatic-hydrothermal fluids associated with peralkaline intrusions.