Shocked chromites in fossil L chondrite meteorites

Wed, 05/31/2017 - 16:21 -- Anonymous (not verified)

You’ll recall the big news about the Museum’s acquisition of rare fossil meteorites a few years ago. These rocks from space fell into an ancient sea about 470 million years ago, and after three years at the Museum, continue to yield new information at the hands of Pritzker Associate Curator Philipp Heck and colleagues. A new study in Meteoritics & Planetary Science by Philipp's former postdoc Surya Rout and colleagues analyzed these meteorites in order to find out if they show signs of shock. Previous studies of these meteorites indicate that they all are fragments from a major ancient collision of asteroids in the space between Mars and Jupiter that disrupted the parent asteroid of these meteorites (the L chondrite parent body breakup event), but no one has studied the effects of that dramatic shock on the rocks themselves. In the new paper, Surya and Philipp, with collaborators from Argonne National Lab, Field Museum Research Associate Birger Schmitz, a professor at Lund University, and a collaborator from the University of Bayreuth focused on the mineral chromite, which is well preserved in fossil meteorites, to discern the effects of the major shock event. The team analyzed the chromite using Raman spectroscopy and transmission electron microscopy (TEM), and compared it with chromite from recently fallen meteorites of the same type (L chondrites). The team concluded that the fossil meteorites experienced pressures of about 20 Giga-pascals—comparable to the pressure inside Earth’s mantle at a depth of about 600 km—and a temperature of about 1,000 degrees Celsius—thus, the first hard evidence of what these rocks experienced during the collision 470 million years ago. The chromites analyzed in the study are from the matrix of the fossil meteorites, which means that other parts of the specimens may hold evidence of even more intensive shock effects—which could be identified with future research.

Figure caption: Shock features (arrowed) in a electron-transparent section of a chromite mineral from a fossil L chondrite. Transmission electron microscopy bright-field image.