Dr Andy Beard's recent research into ureilites published in Meteoritics and Planetary Science
Research into an Antarctic ureilite (stony meteor) has implications for understanding the geological processes of our early solar system.
Dr Andy Beard’s recent paper [Petrology of a nonindigenous microgranitic clast in polymict ureilite EET 87720: Evidence for formation of evolved melt on an unknown parent body] was the cover publication for Meteoritics and Planetary Science’s August 2015 edition. He and his colleagues, Professor Hilary Downes and Marc Chaussidon, reported on their investigations and the implications these finding have for understanding our early solar system.
Andy Beard explained what ureilites are and why analysing their composition is key to understanding planetary geology:
Ureilites are the second largest group of achondritic or stony meteorites. They are ultramafic, composed of mostly of olivine and pyroxene with lesser amounts of elemental carbon (including microdiamonds), sulphide and metal. Ureilite EET87720, was discovered and collected from the Elephant Moraine region in Antarctica, and is a polymict breccia containing numerous nonindigenous (not from the ureilite parent body) fragments or clasts.
The presence of a nonindigenous microgranitic clast within an ureilitic meteorite reveals that similar geological processes that produce granites on Earth have also occurred on other planetary bodies within the early solar system. Its presence as a fragment within a polymict ureilitic further indicates that these early planetary bodies were subsequently fragmented and reaccredited onto the ureilite parent body.
Image: Elemental distribution maps of Si, Al, Na, K, Ca within the microgranitic clast in EET 87720,41, showing the granophyre-like intergrowth and the zoned oligoclase sharing a common margin. Colors indicate relative intensity of elemental concentrations (i.e., reds, pinks = high concentrations; greens = medium concentrations; blues = low concentrations).
Dr Beard summarises technical findings of the research
In an interior chip of EET 87720, we discovered a microgranitic clast (fragment). The clast consists of a granophyre-like intergrowth of a pure SiO2 phase (tridymite) and albite, mantling a zoned oligoclase phenocryst. In the intergrowth, the tridymite occurs as thin elongate vermicular blebs within larger albite crystals. The granophyre-like intergrowth and the oligoclase phenocryst share a common margin, suggesting that the clast was originally part of a larger fragment. An estimate of its bulk composition is equivalent to that of granite (77 wt.% SiO2). Patches of high-Si K-bearing glass occur interstitially within the clast; they have high concentrations of SO3 (11-12 wt.%) and contain Cl (0.6 wt.%), suggesting that the clast formed on a volatile-rich parent body perhaps resembling early Mars. The mean oxygen isotope composition of the feldspar and tridymite in the clast is very different from the oxygen isotope compositions of ureilites, and is similar to those of silicate inclusions in IIE and IVA irons. Thus the clast is not indigenous to the ureilite parent body, and provides further evidence for formation of evolved melt by either extreme fractional crystallization, liquid immiscibility or very low degrees of partial melting on terrestrial parent bodies in the early solar system. The parent body was subsequently impacted or fragmented and a small fragment of it accreted onto the ureilite parent body.