The effect of hydrothermal alteration on the Sr and Nd isotopic signaturesof the Barra do Itapirapua Carbonatite, Southern Brazil

The Cretaceous Barra do Itapirapua carbonatite in southern Brazil experienc ed extensive postmagmatic hydrothermal alteration. In this article, Sr and Nd isotope ratios of coexisting samples of hydrothermally overprinted and o f preserved, nonoverprinted carbonatite are presented. Hydrothermal alterat ion caused strong REE enrichment, leading to the formation of minerals of t he bastnaesite group. In the overprinted carbonatite, Nd contents reach 400 0 ppm, two orders of magnitude higher than in the fresh carbonatite, but ep silon(Nd) varies only within a range of 3.4 units. In contrast, Sr was leac hed from the carbonatite during the postmagmatic alteration; hence values o f around 10,000 ppm in the fresh carbonatite drop to about 1000 ppm in the overprinted samples. Leaching is accompanied by a variation of Sr isotopic composition toward more radiogenic values, resulting in an increase of 15 u nits in epsilon(Sr). Variation of Sr isotopic composition is related to pos tmagmatic alteration and is decoupled from the variation of Nd isotopic com position, ruling out heterogeneities in the mantle source as the main cause of isotopic variability in the data set. Furthermore, this cannot be expla ined by bulk crustal contamination. A two-step model is proposed in which ( 1) a REE-rich, carbonatite-derived hydrothermal fluid overprinted the prist ine carbonatite, causing REE-enrichment with a relative small change of iso topic composition; and (2) crust-derived hydrothermal fluids percolated the cooling carbonatite, leaching the original Sr from the carbonatite and int roducing a more radiogenic Sr isotopic signature. The amounts of carbonatit e-derived Nd with primitive, carbonatite-like Nd isotope ratios introduced during the first stage of hydrothermal alteration are high enough to buffer the effect of crust-derived Nd on the Nd isotopic composition of the overp rinted carbonatite.