Overview and images of dunite as a rock type are here: Dunite
Callan Bentley wrote about a xenobomb which is an aggregate of olivine in a basaltic rock.
I also have similar photos of a similar rock. My sample is from Lanzarote, The Canary Islands. It is a xenolith of a dunite? As a mineral aggregate this green stuff in basalt definitely qualifies as a dunite but I was not sure whether it comes as a xenolith from the mantle or is it an olivine cumulate rock formed in the crust. Hence, my question was: is it necessary for a dunite to form deep below or do we only need the rock to be phaneritic and composed of more than 90% olivine? I tend to think its a xenolith because it seems to be surrounded by basalt from every direction.
I got few responses. The consensus seems to be that the rock must be >90% olivine to qualify. I think its a reasonable way to look at it because we often really don’t know and can’t possibly know how a particular rock sample came to be. The Canary Islands, for example, have bneen extensively studied. It is clear that there are very complex interactions and the source(s) of the ultramafic rocks remain often uncertain. Dunite inclusions may be foreign to the host carrier (xenolith) or there might be a genetic link (inclusion).
Here are the pictures of the same rock from different angles:
Dunite xenolith in basalt from Lanzarote. The width of the hand sample is 9 cm.
Dunite xenolith in basalt from Lanzarote. The width of the hand sample is 11 cm.
I have written about somewhat similar rock before: Olivine basalt from Oahu.
Thanks for your explanations. Now I know what the star of my stone collection (
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link to picture:
here
http://1.bp.blogspot.com/-XQ2ADcTS9SM/T0ANYM1qR7I/AAAAAAAAAO8/DkBX8cyNUS0/s0/XenolithOrDuniteFromCanaryIslands.jpg
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.