What makes you think it is a memeory leak?

SSAS will start to cache queries in order to improve the performance of the queries (this is good!). There are settings to control the memory & SSAS's ability to flush the cache

http://duncansutcliffe.wordpress.com/2010/06/04/memory-settings-on-sql-server-and-analysis-services/

The other thing I would say is that 8Gb is a modest amount of memory for SSAS if your cubes are big

Mack