18 February 2012

The xenobomb

Posted by Callan Bentley

One of the samples I was fortunate to acquire in Texas was this extraordinary thing:

Rotated 90° to the right:

And then rotated another 90° to the right, so now you’re looking at the opposite side relative to the first shot:

This was not a sample I found personally, but one that was collected by Josh Villalobos of El Paso Community College. He sawed it in half, gave it a coat of shellac, and then gave half to me, and half to Elizabeth Nagy-Shadman on our field trip the week before last. The sample was collected out at Kilbourne Hole, a maar volcanic complex west of El Paso, in southern New Mexico. Maars are big explosive craters that form when hot magma encounters groundwater, and steam explosions ensue. Chunks of rock and lava get flung through the air to land like geologic shrapnel around the hole.

Apparently what happened with this sample is that a xenolith of peridotite* (olivine-rich intrusive igneous rock) was tumbling along in the conduit of basaltic magma when the explosion occurred. It was flung through the air as a solid chunk surrounded by a coating of liquid mafic lava, which congealed and solidified as it traveled through the cold air, like a volcanic bomb. It landed, and eventually Josh found it, sliced it, and shared it.

Wild, right? It’s both a xenolith and a volcanic bomb. I hereby propose a new term for such a thing: I call it a xenobomb.

Similar xenobombs can be found at Kilbourne Hole of many other xenolith rock types, including migmatite.

It reminds me of a chocolate-covered almond.


* Several sites online refer to these inclusions not as xenoliths but “giant crystals” of olivine. Others say they’re xenoliths. It’s clearly not a single coherent crystal – you can see the central portion of the xenobomb includes hundreds of 1mm – 2mm sized olivine grains. But are those actually separate crystals with independent nucleations and growth histories? (i.e., crystallization of an ultramafic magma, which then solidified into peridotite and had a chunk of that solid rock stoped off to be included in a later mafic magma?) Or are they clasts still in situ relative to the larger crystal they used to be part of?  (In other words, does this represent a case of positive dilation and ensuing fracturing of a single original crystal into many fragments upon depressurization?) Is this just a case of people on the internet confusing minerals with rocks? That’s my bet.