Tracing the source of gypsum in New Caledonian soils by REE contents and SSr isotopic compositions

Gypsum minerals forming in pedogenic environments in New Caledonia were examined for their S and Sr isotope compositions and REE distributions. Three possible sources were identified: volcanic, soil-derived and marine. However, differentiation based only on the REE contents and normalized patterns was not possible, but, the best preserved volcanic sample yields a negative EuEu∗ anomaly and the lowest GdNYbN ratio due to the high content of organic matter. Mixing calculations of the δ34S values and 87Sr86Sr ratios confirmed that the marine gypsum has a δ34S value close to + 21‰ and an 87Sr86Sr ratio close to 0.70912, and that the volcanic gypsum has a δ34S value of 0‰ and an 87Sr86Sr ratio of about 0.70331. The soil-derived gypsum samples yield intermediate values, as a result of a mixing between the marine and volcanic end members. Negative δ34S values for the volcanic gypsum are explained by low fO2 of the source rocks, while high 87Sr86Sr ratios for the same gypsum are explained by probable atmospheric input. The data confirm also that biological fractionation of δ34S, if any, is limited.