Service Desk's activities are (mainly) related ti Incident Management process. Incident Management has many activities in scope, so for all of the activities you can define RACI. Additionally, Service Desk in involved in Request Fulfillment process which is also full of activities that can be defined from RACI point of view.
Article "ITIL / ISO 20000 RACI matrix – How to use it to clarify responsibilities" https://advisera.com/20000academy/blog/2016/01/12/itil-iso-20000-raci-matrix-how-to-use-it-to-clarify-responsibilities/
can help you with RACI. Some other articles can help you to learn about Incident Management and Request Fulfillment activities:
"ITIL Incident Management" https://advisera.com/20000academy/knowledgebase/itil/-incident-management/
"ITIL Request Fulfillment: a quick win for customer satisfaction" https://advisera.com/20000academy/blog/2013/08/28/itil-request-fulfillment-quick-win-customer-satisfaction/