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The Caldwell Meteorite: Not just any old L imp-Melt!

 

Although an L-impact melt sounds like a delicious sandwich I’d buy, it’s actually a relatively new meteorite class defined as “An ordinary chondrite from the L group that has experienced impact melting.” Seems simple enough, but of the 31 entries for L-imp melt in the Meteoritical Bulletin Database, there are a total of two outside the hot deserts of Africa and the cold deserts of Antarctica. One of the two is named Muckera 007, found in 1991 in Austrailia as a single 14g fragment, and Caldwell, Kansas USA, a 12.9kg stone found in 1961. Another thing Muckera 007 and Caldwell have in common is they are the only two L-imp melt meteorites to be found prior to the year 2000. Actually, this is not likely correct, since the class of L-imp melt is relatively new in the meteorite literature so there could be plenty of historic finds matching the L-imp melt description but have yet to be formally reclassified.

 

 

Caldwell is the oldest and largest of the classified L-imp melt stones, and not by a little. Regarding size, or TKW, only three other L-imp melts exceed a kilogram (one barely at 1280g), and the largest of those three is a rough third of the TKW of Caldwell. And the second oldest L-imp melt is about in the middle of time between today and back when Caldwell was found in 1961. And six of the 31 L-imp melt specimens, about one-fifth, were discovered in 2010 or after, with the most recent, Dominion Range 18623 Antarctica, in 2018.

By definition, Melt-by-impact is a geologic situation where the collision of two or more bodies collide with sufficient force to melt rock which then flows and pools in or on the bodies finally solidifying into highly collectable and beautiful presentations of the violence of the solar system (I added that last part myself). Or in other words (those of the Meteoritical Bulletin again), “An L chondrite that has experienced impact melting.”

 

 

If there are L-imp melts, then it follows that there should be H-imp melts. And there are. However, that list of only 13 includes 12 from Antarctica and one from Libya, and all but one were discovered in 2000 or more recently. The notable exception, Queen Alexandra Range 99396, was discovered, as its name indicates, in 1999. Hardly a historic entry compared to Caldwell’s 40-year head start.

 

 

While my collecting tastes lean strongly towards historic witnessed falls, locating materials to add to my collection is exceedingly hard these days. Either the offered specimen is exceedingly small, or exceedingly expensive, or exceedingly both. Often exceedingly the latter. But hey, as they say, a chondrule is a chondrule. And Caldwell is packed full of them. As a type-4 ordinary chondrite, in Lunar and Planetary Institute speak: “Designates chondrites that are characterized by abundant chondrules, and have been metamorphosed under conditions sufficient to homogenize olivine compositions and recrystallize fine-grained matrix. Some of the low-Ca pyroxene grains may be monoclinic and exhibit polysynthetic twinning. Primary igneous chondrule glass is absent.” Or in other words, there should be many obvious chondrules and quite round at that in a cut surface. Caldwell delivers!

 

 

Often a dark matrix with flashy features such as melt veins and metal blebs can quickly overshadow beautiful but subtile features that are also important players in this 4.6B year old geologic game. Caldwell is like entering a darkened theater. At first, you see very little, but as your eyesight and attitude adjust, there is suddenly a treasure trove of circular chondrules in size from pinpoints to pin heads to BBs and larger.

 

Caldwell Impact melt

A high contrast close up of Caldwell that reveals the densely packed quality chondrules abundant in this type 4 chondrite. The more you look, the more you will see.

 

A spherical tree in the forest:

The 41g slice of Caldwell presented here came from a recent offering by Blaine Reed. When a new list from Blaine comes out, I quickly scan it for historic witnessed falls. No old joy on this list, but a USA impact melt breccia caught my eye. And the 1961 date caught my other eye. Plus the 41g size was large enough to show the action, not just be a souvenir of the impact. As I studied the image in Blaine’s email listing, I could see much beyond the extensive melt rivers crossing the base of this slice. It reminded me of the Snake River flowing across the southern plain of Idaho making a huge smile connecting Yellowstone National Park with a possible location where a meteorite struck in prehistoric Oregon creating the hotspot and inland volcanoes leading to Yellowstone’s current address. With that river metaphor in mind, I could quickly see more and more tributaries feeding the melt river, and a closer look revealed braided flows, lakes, and creek branching. The comparison to a river watershed is likely backwards with flow into smaller areas, not consolidating into ever larger flows. At that moment, I emailed Blaine with my interest.

Studying the image further, I ignored the melted areas and concentrated on the matrix. It soon became obvious that the many small mottled regions were really chondrules and chondrule features. Suddenly my map of Idaho had dozens of small lakes and round areas that on a satellite picture of Idaho would be irrigated farmland, but here, they were early children of our solar system. In person and under magnification, Caldwell is a stadium crowded with bobbing chondrule heads. The contrast between matrix and chondrules, and even between chondrules themselves makes this a specimen where the high-wow of the melt-flows overshadow what would be an already eye-friendly slice.

And as a metaphor for 2024, I am imagining that a cosmic tragedy that left behind melted rock within stones that would crash to earth yet are now valuable things of beauty. If conflict between small bodies of the solar system can create such joy and curiosity, perhaps conflict already here on earth can eventually lead to something good in the long run. Like gravity between asteroids, the fights over space lead to change, and change can be painful. But we all are here today through our ancestors surviving the conflicts of the past. So as the dust settles on the arrival of 2024, I am going to consider the big picture that got us where we are, and hope the conflicts that lay ahead are ultimately productive in avoiding larger conflicts later. I know that’s a big ask, but then again, Caldwell never asked to be the metaphor of an internet article published 63 years after someone dug it from the dirt long after it crash landed onto one of the few dry spots on a planet mostly covered in saltwater.

Until next time….

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