Monazite in a granite pegmatite from the Ishikawa district, Fukushima Prefecture, Japan has been examined in terms of U, Th, Pb, and rare earth element (REE) chemistry by electron microprobe analysis. Oscillatory and sector zoning were distinguished on the basis of back-scattered electron images. An averaged empirical formula was (Ce_0.383Nd_0.204La_0.136Y_0.062Sm_0.061Pr_0.050Gd_0.040Dy_0.014Er_0.002Th_0.046U_0.005Ca_0.011)

_∑1.014(P_0.952Si_0.040)_∑0.992O₄. Compositional variations suggest that the (Th + U) contents are controlled mainly by the coupled substitution (Th, U)+ Si = REE + P. Analytical data for clear portions of the monazite crystal were arrayed linearly on the PbO-ThO₂* (ThO₂ plus the equivalent of UO₂) diagram and formed a single isochron for a CHIME age of 103 ± 26 Ma (2σ) that is in accord with previously reported radiometric ages for the granitic rocks in the Ishikawa district.

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