Metal on the surface of asteroids: geochemistry, petrogenesis and thermal history of IIE iron meteorites and their relationship to ordinary chondrites

Abstract

The formation, evolution and parent asteroid of the IIE iron meteorites is debated in the scientific community. IIE irons are unique in their chemistry and petrography when compared to most iron meteorites. Petrographically, they are a diverse group with most, but not all, members containing silicate inclusions of varied mineralogy and chemistry. Some IIE irons, such as Netschaevo, contain angular, chondritic clasts with relict chondrules. Others, such as Miles, have more globular silicate inclusions that display a range of chemistries from ultramafic/phosphatic to highly felsic. Chemical and physical features of IIE irons are inconsistent with formation in the core of an asteroid, with instead impact processes favoured. This thesis resolves the details of these impact processes, thermal history and cooling rates, how schreibersite and glassy silicate inclusions formed, and the nature of the parent body. In-situ Pb-Pb dating reveals that ~4.542 billion years ago an impact occurred on the H chondrite parent body. Upon release of the initial shock waves, the porous surface region at the impact site experienced heterogeneous heating. The presence of disordered srilankite and tridymite indicates that in Miles temperatures exceeded 1160C at pressures less than 0.7 GPa. In contrast, Netschaevo did not experience temperatures above ~1080C as indicated by the presence of relict chondrules. In the high temperature regions, melt lenses formed with immiscible silicate and metal liquids which under the low pressures and moderate gravity conditions began to undergo density separation. The heavy metallic liquid percolated down through fractures in the basement of the impact crater entraining angular chondritic clasts. Silicate droplets - formed through partial melting of the chondritic parent body material - floated up through the metal. This physical process allowed for effective accumulation of metallic melt bodies containing silicate inclusions of variable petrography at or near the surface of asteroids. The presence of disordered srilankite in Miles and glassy inclusions in Kodaikanal indicate that cooling rates were initially rapid. However, cooling rates slowed significantly as these melts solidified in the near-surface environment of a parent body that was still warm following accretion and radioactive decay of short-lived isotopes. Cooling rates varied depending on the depth of each melt lens within the warm parent body. This resulted in a range of metallographic textures, from a fine-Widmanstatten pattern in rapidly cooled metallic melts, to cm-sized kamacite grains with interstitial taenite and schreibersite in melts that cooled at a moderate rate, to a coarse Widmanstatten pattern in slowly cooled melts. Schreibersite was found to form through three different mechanisms in non-magmatic iron meteorites: direct precipitation from a P- and Ni-rich eutectic melt; subsolidus growth in P-bearing Fe-Ni-phases; and, dephosphorisation of P-rich silicate inclusions into the surrounding Fe-Ni-metal. This final mechanism forms low-Ni schreibersite, resulting in the subsequent diffusion of Ni into the phosphide phase and the presence of distinct diffusion profiles in the surrounding kamacite. From this finding, a potential new method for estimating cooling rates between 700-400C is proposed, however it requires experimental validation. Results from this method estimate that cooling rates experienced by the IIE irons were ~500-26000C/Myr. Evaluation of pre-existing datasets demonstrate that IIE iron meteorites are statistically indistinguishable from H chondrites on the basis of metal chemistry, olivine and pyroxene chemistry, chondrule size, and O isotopes. These results are consistent with the formation of IIE iron meteorites through impact event/s on a H chondrite parent body. In contrast to previous work, statistical analysis of the ordinary chondrite data does not indicate the presence of a further reduced HH ordinary chondrite group. Show more