⁹⁸Mo/⁹⁵Mo and ²³⁸U/²³⁵U in lamproites, shoshonites, and high-K calc-alkaline rocks from Western Alps: inferences on their genesis

Martina Casalini
Dipartimento di Scienze della Terra, Università degli Studi di Firenze, via Giorgio La Pira 4, I-50121, Firenze, Italy. Bristol Isotope Group, School of Earth Sciences, University of Bristol, Queens Road, Bristol BS8 1RJ, UK. Corresponding author e-mail: martina.casalini@unifi.it.

Oligocene ultrapotassic and associated potassic rocks from Western Alps derived from the crystallisation of magmas originated from partial melting of mantle sources extremely enriched in recycled crustal components. These rocks are well suited for studying the effects of subduction-related, sediment-derived metasomatism on Molybdenum and Uranium isotopes. This paper reports the first ⁹⁸Mo/⁹⁵Mo and ²³⁸U/²³⁵U data on potassic and ultrapotassic rocks at destructive plate margins. Both ⁹⁸Mo/⁹⁵Mo and ²³⁸U/²³⁵U ratios show large variations in the samples from Western Alps. U isotope compositions are consistent with an increasing role of metasomatising melts from recycled sediments, which explains the variable enrichment in potassium and incompatible trace elements passing from high-K calc-alkaline to lamproitic magmas through shoshonitic ones. The variation of Mo isotope compositions is more complex due to the extreme depletion in Mo observed and since their values exceed the range observed for volcanic arcs. These features were investigated considering several possible processes such as secondary weathering, hosting of Mo related to residual mineral phases during sediment melting or physical removal during subduction. The results were also discussed in the framework of the complex processes responsible for the genesis and geochemical characteristics of the Tethyan Realm Lamproites, particularly in relationship with the exotic SALATHO geochemical component.

Molybdenum isotopes, Uranium isotopes, Western Alps, lamproites, shoshonites.