Section 11.3.3 of the current UML spec (http://www.omg.org/spec/UML/2.4.1/Infrastructure/PDF/) says:
Semantic Variation Points The precise lifecycle semantics of aggregation is a semantic variation point.
That is, the UML specification does not (as far as I understand it) define the precise meaning of filled and black diamonds that specify aggregation or composition. Because of that, we decided not to include such indicators into the UML diagrams in the specification. In other words, we deliberately left the life cycle management of the objects from the OWL 2 specification to implementations, allowing them to chose the model that seems most appropriate for a given setting.