Is Impeachment A Pardonable Offense?

Monday's post concerned California's constitutional and statutory provisions governing impeachment.  These provisions are based on the English parliamentary model developed in the 14th century.  In 1678, the Commons impeached Thomas Osborne, the Earl of Danby, despite a royal pardon.  The Commons' action was eventually enshrined in statute: "no Pardon under the Great Seal of England be pleadable to an Impeachment by the Commons in Parliament".  Act of Settlement (1700), Section III.  The California Constitution includes a similar limitation on the power of the Governor to pardon impeachments:

"Subject to application procedures provided by statute, the Governor, on conditions the Governor deems proper, may grant a reprieve, pardon, and commutation, after sentence, except in case of impeachment."

Cal. Const. Art. V, § 8(a).

Thomas Osborne was confined to the Tower of London but was later released.  On June 30, 1688, he became one of the so-called "Immortal Seven" who invited William of Orange and Mary II to depose James II in the "Glorious revolution".