In my estimation you could safely call this a dunite xenolith. Dunite is defined simply as an ultramafic plutonic rock containing >90% olivine. Xenolith means literally “foreign rock” and implies a rock sample that did not originate from the magma in which it ended up. As this sample is most likely a piece of the mantle or lower crust broken off as the basaltic magma ascended it is entirely proper to call it a xenolith. However, one should be cautious not to use the term xenolith for just any inclusion in a crystallized magma body. Sometimes enclaves (a more generic term for inclusions) can be composed of earlier crystallized portions of the same magma chamber they are later incorporated into. A fair amount of fine grained “mafic” enclaves in granitoids are actually cogenetic with the rock in which they are included. I think the following quote from Wikipedia accurately sums up my understanding of the proper usage: “To be considered a true xenolith, the included rock must be identifiably different from the rock in which it is enveloped; an included rock of similar type is called an autolith or a cognate inclusion.” (http://en.wikipedia.org/wiki/Autolith)
Ron, thanks for explaining this. I was also inclined to interprete it as you did.