Analysis of micrometeorites reveals H chondrite needles in L Chondrite haystack

Thu, 03/17/2016 - 16:04 -- pheck

Philipp Heck (Robert A. Pritzker Associate Curator of Meteoritics and Polar Studies), Surya Rout (Postdoctoral Research Scientist), Krysten Villalon (Univ. of Chicago Graduate Student) and Lund University’s Professor Birger Schmitz (also a Field Museum Research Associate) with colleagues from the University of Wisconsin published a new paper on 470-million-year-old fossil micrometeorites recovered from marine sediments in Sweden and Russia. Meteorites that are smaller than 2 mm in diameter are called micrometeorites and, perhaps ironically, are easier to find than meteorites because of their higher abundance. Previous studies showed that sediments of this age are unusually rich in meteorites and micrometeorites of one particular type, the L chondrites—usually considered to be debris from a catastrophic disruption of the L chondrite parent asteroid, which arrived on Earth shortly after that event. Examples of such debris include the famous fossil meteorites that will be on exhibit at The Field Museum later this year. In this new study, published in the March 15 issue of Geochimica et Cosmochimica Acta, Philipp and colleagues examined these same sediments to find micrometeorites that were not fragments from the destroyed asteroid. To determine this, Birger acid-extracted the micrometeorites from sediments in his lab at Lund, with samples then prepared at Lund and at the Field. Philipp and Surya used the ion probe at UW-Madison’s WiscSIMS lab to obtain oxygen isotopic data, and Krysten and Philipp obtained elemental data using the Scanning Electron Microscope and X-ray spectroscopy system here at FMNH. The team found that 99% of the micrometeorites were L chondrites and actual debris from the asteroid breakup, with only 1% or less being H chondrites from a different asteroidal source. The new study suggests that ejecta from the L chondrite parent asteroid breakup hitting other asteroids did not generate enough secondary fragments to result in a significantly higher flux of other types of meteorites to Earth than today. The paper can be viewed at online.
Source: FMNH Natural/S&E News, 03/17/2016

Ion microprobe pits in a chromite from one of the rare H chondritic fossil micrometeorites. The pits are caused by the ion probe analysis, which “sputters” away material that is then analyzed for isotopic composition. (“Sputtering” is the use of ions to eject particles from a target material.)