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

Abstract:

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 normalixed patterns was not possible, but, the best preserved volcanic-sample yields a negative Eu/Eu* anomaly and the lowest GdN/YbN ratio due to the high content of organic matter. Mixing calculations of the δ34S values and 87Sr/86Sr ratios confirmed that the marine gypsum has a δ34S value close to + 21‰ and an 87Sr/86Sr ratio close to 0.70912. and that the volcanic gypsum has a δ34S value of 0‰ and an 87Sr/86Sr ratio of about 0.70131. 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 f02, of the source rocks, while high 87Sr/86Sr ratios for the same gypsum are explained by probable atmospheric input. The data confirm also that biological fractionation of δ34S, if any, is limited. © 1998 Elsevier Science B.V.