17 March 2008

If “xenolith” means “foreign rock”, my dunites could be Irish

Posted by Jessica Ball

Happy St. Patrick’s Day! In the spirit of the holiday (green), I’m posting some photos of my favorite field samples – dunites! (Looks like Ron has a few of these in his Deskcrop Geopuzzle – I was a little too slow to guess this time around!)

I collected these from the Vulcan’s Throne cinder cone on the North Rim of the Grand Canyon on my first “big” (meaning month-long) field trip after my freshman year. I hadn’t been expecting to find anything other than the basaltic cinders, but there were a fair number of these little xenoliths lying around, which made me very excited. I remember racing back to camp after the walk to show my advisor, who turned them into a nice impromptu lesson on xenoliths and the mineralogy of dunites.

For those who aren’t familiar with the term, a dunite is an ultramafic igneous rock with greater than 90% olivine (peridot for the gem-hunters), and minor amounts of pyroxene and chromite. One explanation for their origins is that they are the residual portion of a mantle melt that has fractionated and produced basalt. Dunites are basically little chunks of the mantle, and they show up in all sorts of interesting places – at the base of ophiolite sequences, alpine peridotite massifs, and layered intrusions or the bottom of large basaltic magma chambers. These dunites could have come from the bottom of a magma chamber, or from the mantle; two possible eruption histories are A) that they settled out of a magma and were left behind when it erupted, and were then picked up by a new injection of basaltic melt and erupted as xenoliths, or B) fractionated from a melt in the mantle and picked up by another basaltic melt on its way to the surface. There are no ophiolites or massifs near the Grand Canyon, so it’s pretty safe to say those options are out.

Xenolith is a term that often gets associated with mantle rock, but it can mean any fragment of rock that was enclosed in another rock of a different composition during magma emplacement and eruption. Xenoliths are particularly valuable in that they can tell us what was going on in a magma chamber that experienced multiple injections of melt, or what kind of country rock magma was emplaced in, or what the mantle is composed of. They’re like little green windows into processes that go on far below the Earth’s surface – being erupted at a cinder cone in Arizona is only the very end of their story.

I’ll make my next post a larger picture of the area where I found them – it’s one of my favorite stops on that field course.