Iwasawa invariant and Massey product

We compute Iwasawa invariant in terms of Massey products in Galois cohomology with restricted ramification. When applied to imaginary quadratic fields and cyclotomic fields, we obtain a new proof and generalization of results of Gold and McCallum-Sharifi. The main tool is the generalized Bockstein map introduced by Lam-Liu-Sharifi-Wake-Wang.