A multi-isotope study (Fe, Ge, O) of hydrothermal alteration in the Limousin ophiolite (French Massif Central)

Geochemical studies using non-traditional stable isotopes can help tracing processes of hydrothermal alteration (or hydrothermal metamorphism) of ancient oceanic lithosphere. In this study, we have measured non-traditional Ge and Fe isotopes and traditional O isotopes in a series of ultrabasic and basic rocks from the Limousin ophiolite (French Massif Central) to decipher the different signatures of hydrothermal vs. magmatic processes. Serpentinites are strongly oxidised rocks (Fe3+/ΣFe: 0.58–0.71) and display δ18O values (+5.0 to +6.1‰) typical of hydrothermally altered ultrabasic rocks. They display δ56Fe (+0.15 to +0.18‰) and δ74Ge values (+0.48 to +0.93‰) similar to heavier than ultrabasic rocks. The negative correlation between δ18O and δ74Ge and between δ18O and δ56Fe, suggests that Ge and Fe isotopes have fractionated during hydrothermal alteration. The δ74Ge shows a slight positive correlation with the δ56Fe, indicating concomittant Ge and Fe isotope fractionation towards heavier values during hydrothermal alteration. However, δ56Fe values display a larger deviation from ultrabasic rocks than δ74Ge and δ18O values, suggesting that oxidising conditions have enhanced Fe isotope fractionation to a larger extent than Ge isotopes. Amphibolites display Fe3+/ΣFe ratios (0.11–0.14) and δ56Fe values (+0.03 to +0.17‰) typical of mid-ocean ridge basalts (MORB). δ18O values are typical of high-T hydrothermally altered MORB (+6.2 to +6.6‰). δ74Ge values show a small range (+0.72 to +0.77‰) and are heavier than most basalts and gabbros. The δ18O slightly decreases with the increase of the δ56Fe, indicating that Fe isotopes may have fractionated towards lighter values during hydrothermal alteration. However, the lack of correlation between δ74Ge and δ56Fe values indicates that Ge isotope fractionation has prevailed over Fe isotope fractionation during hydrothermal alteration of basic rocks in the absence of oxidising conditions.