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 normalized patterns was not possible, but, the best preserved volcanic sample yields a negative Eu Eu ∗ anomaly and the lowest Gd N Yb N ratio due to the high content of organic matter. Mixing calculations of the δ 34S values and 87Sr 86Sr ratios confirmed that the marine gypsum has a δ 34S value close to + 21‰ and an 87Sr 86Sr ratio close to 0.70912, and that the volcanic gypsum has a δ 34S value of 0‰ and an 87Sr 86Sr 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 δ 34S values for the volcanic gypsum are explained by low f O 2 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 δ 34S, if any, is limited.
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