5.6.7 Authorized Assistant at Catholic Marriage and Delegation
1. Only a priest (sacerdos) may celebrate a marriage validly when it involves even one member of an Eastern Catholic Church or of an Eastern non-Catholic (Orthodox) Church, even though the other party is a Latin Catholic. 
2. To protect the validity of marriage, the pastor is to take care that any priest or deacon who assists at a marriage in his parish has the faculty and the canonical capacity to do so.
3. For reasons of family ties, bonds of friendship, or other pastoral advantage in their estimation, pastors and rectors are authorized to permit a deacon to assist at a wedding even though it is celebrated during Mass.  This permission is to be given by way of exception in particular cases rather than as a customary practice in the parish or church.
 De Concordia Inter Codices, c. 1108 §3. The law does not permit delegation to a deacon, since he cannot confer the priestly blessing required by the Eastern law for validity, and this is not a dispensable law.
 c. 1108 §2; cc. 86, 87 §1 and 89.