June 30, 2011

Chondrite Town

Posted by Evelyn Mervine

The road to the chondritic uniform reservoir. Switzerland, June 2010.

Last June I spent ten days in Switzerland and Italy exploring the geology of The Alps as part of Woods Hole Oceanographic Institution’s annual Geodynamics Program. As we were driving one day on the trip, several geochemists in the group became very excited about the above sign. Apparently, the mysterious chondritic uniform reservoir (CHUR) is located in Switzerland! We tried our best to persuade our instructors that we should visit this important Earth reservoir, but we were told that Chur is a charming little Swiss town, not a place where we could find chondrites. So, we continued our drive to the mountains, but not before I snapped a photo as we passed a sign for Chur.

***For those who are not familiar, chondrites are primitive, stony meteorites that have not undergone significant melting or differentiation. Thus, chondrites are considered the original “starting material” for planet Earth, which scientists believe formed through accretion of chondritic bodies. The Earth probably started out as roughly chondritic then underwent chemical differentiation. To put it simply, the heavy stuff sunk to the center of the Earth (the core) and the lighter stuff floated to the top of the Earth (the crust) and the medium stuff stayed in the middle (the mantle).  For long-lived chemical systems (for example, the samarium-neodymium isotope system), geochemists like to think about how these chemical systems may have evolved over the ~4.54 billion year history of the Earth. Often, geochemists like to compare the evolution of certain chemical systems to a baseline. One baseline that is sometimes used is CHUR– the chondritic uniform reservoir. The chemical composition of CHUR is based on the average chemical composition of chondritic meteorites. Where do we obtain these chondrites, if planets differentiated? Well, not all bits and pieces of the original chondrite parent bodies were incorporated into planets, which then differentiated. Some chondrite bits and pieces survived and have been zooming around our solar system. Chondrites often fall to Earth as meteorites, where they are sometimes recognized. Since these meteorites are stony, they look like Earth rocks and are tricky to tell apart. There could be one in your backyard! Most chondrite meteorites are found in places where there are few rocks, such as in deserts and in Antarctica.***