In the last edition I mentioned the new Atacamaite impact glass and promised a follow-up story. Here it is (largely drawn from our website).
A couple of years ago word started to leak out of Chile regarding a new “tektite strewn field” in the central Atacama Desert. We have recently had the chance to examine a few thousand unsorted specimens, and they are pretty exciting. Figure 1 illustrates some of the finest you are ever likely to see assembled in one place!
Firstly though, I do not think these will ultimately be classed as tektites, but they are unquestionably an impact glass that sits nicely on the shelf next to Irghizites and Wabar glass. In our collection, this new material takes its rightful place as representative of the tiniest little jets of glass ever squirted from the white-hot furnace of an earth/asteroid collision. There were surely numerous smaller events in geological history, but these produced little more than a hail of mechanical pebbles and a cloud of dust. No ejected glass. Atacamaites may define the (current) lower threshold limit for squirting glass!
Of the sample reported in a recent abstract, specimens ranged up to 35 mm in maximum dimension and had a mean weight of about 500 milligrams (Devouard, et al, 2014). In a world where the bragging rights usually go to the biggest known, these are noteworthy for their small size. Of those that we examined, 2 or 3% show nicely developed splashform morphologies like those shown in figure 1. The balance are fairly non-descript fragments, but fully two-thirds of these little bits show enough skin to infer that they are nearly full diameter segments of splashforms. They never were very big.
Atacamaites apparently began life as little squirting fingers of incandescent (but soon to be black) glass. Figure 2 is my geocaricature of the formative event.
These jetting rivulets of fire were seldom larger than half the diameter of a pencil. Reportedly, they are found over an area of some 20 square kilometers, so they must’ve been truly high-pressure squirts. Picture a viscous spray of orange-yellow-white molten glass streaking from the unthinkable place where the kinetic energy of a massive body moving at tens of kilometers per second met an immovable object (earth) and flashed at an instant into shock waves and heat radiating outward at velocities greatly exceeding the speed of sound!
The closest visual analogs of the Atacamaites are Irghizites, Wabar glass, Darwin glass, and Aouelloul glass. While each is unique in some ways, there is considerable overlap in the character and morphologies of these five impactites. Oddly, no source crater has yet been recognized for the Atacamaites. The other four are found in and immediately around obvious craters (Darwin is scattered over a somewhat larger area than the others). There is no reason to think that the Atacamaites were ejected long distances from their source impact, and the area is one of the driest places on earth—hardly a locality where you would postulate erosion sufficient to destroy the crater.
The exact location of the strewn field has not yet been reported, but is expected to be detailed in a forthcoming technical publication currently in preparation. Dating of the impact has been completed, but no results have been released. The glass is relatively dry at about 130 ppm water. There is significant comingling of impactor material, with iron contents varying from about 5 to about 15 weight-percent. In a nice piece of detective work, Devouard et al (2014) report that there is a positive correlation between Ni, Fe, and Co, with correlation coefficients that remain similar at all levels of Fe concentration. This strongly supports contamination of the glass by varying proportions of an impactor containing fixed proportions of Fe, Ni, and Co.
This multi-kilometer spray of tiny glass streamers begs a mental journey to the Australasian event, where millions of tonnes of glass objects weighing up to a kilogram or so flew a third of the way around the earth, some likely in near-orbit. Atacamaites and Australasians are partners at either end of the known spectrum of preserved glass-splashing impact events.
If you have an array of impact glasses from those magical moments, make room at the “tiny” end of the shelf!
Devouard, B., P. et al, 2014, A new Tektite Strewnfield in Atacama, Chile, 77th Annual Meteoritical Society Meeting (2014), Casa Blanca, abst. 5394. (http://www.hou.usra.edu/meetings/metsoc2014/pdf/5394.pdf)
Also see “Meteorite Picture of the Day” November 7, 2014