abstract: We prove the Central Limit Theorem (CLT) and a Mixing Local Central Limit Theorem (MLCLT) for the real part, the imaginary part and the absolute value of the Riemann zeta function on the critical line extending the results of Lifschitz & Weber (2009) and Steuding (2012) on "sampling the Lindelöf hypothesis". We obtain these as corollaries of a general result we prove for Birkhoff sums of a class of unbounded heavily oscillating observables over a family of full-branch piecewise C2 expanding maps of the interval.