Rapid cooling of planetesimal core-mantle reaction zones from Mn-Cr isotopes in pallasites

Abstract

doi: 10.7185/geochemlet.1607Pallasite meteorites, which consist of olivine-metal mixtures and accessory phosphates crystallised from silico-phosphate melts, are thought to represent core-mantle reaction zones of early differentiating planetesimals. Pallasite meteorites can be linked to five distinct planetesimals, indicating that they are default products of differentiation. However, their formation modes (deep, shallow, and impact environments) and age are still elusive. We have investigated the trace element and Mn-Cr isotopic signatures of Main-Group pallasite olivine, finding enhanced Mn, P and 53 Cr/ 52 Cr near crystal rims which indicates early ingrowth of radiogenic 53 Cr* in silico-phosphate melts. Mn-Cr isotopic data corroborate previous Hf-W isotopic data, indicating an early metal-silicate separation event but additionally that rapid cooling generated silico-phosphate eutectic melts with high Mn/Cr within ~2.5 to 4 Myr of Solar System formation. These melts formed before most known samples of planetesimal crusts (eucrite and angrite meteorites) and are among the earliest evolved planetary silicates. Additionally, Mn-rich phosphates in other, non-Main-Group pallasite meteorites suggest that core-mantle reaction zones are generic, datable features of differentiation.

Petrogenesis of main group pallasite meteorites based on relationships among texture, mineralogy, and geochemistry

“…Similarly, rapid cooling of the PMG parent body is indicated by Mn‐Cr isotope systematics, yielding diffusive closure of Mn and Cr in olivine of Omolon and Springwater within ~10 Myr of solar system formation (Lugmair and Shukolyukov ), and possible cooling to near‐solidus conditions and segregation of low‐degree Mn‐rich melts in Brahin and Brenham within ~2.5 to 4 Myr (McKibbin et al. ). For the lithophile‐siderophile Hf‐W decay scheme, W‐deficit dating methods have undergone considerable revision due to correction for exposure in space (e.g., Kruijer et al.…”

“…It is likely that continuous variation in olivine phosphorus concentration is possible, but in PMG, most normal olivines have low phosphorus contents at less than 100–200 ppm (e.g., McKibbin et al. , ). The potentially metastable character of this mineral (Boesenberg and Hewins ; Boesenberg et al.…”

“…) and in line profiles approaching olivine margins (McKibbin et al. ). The higher MnO in rims of olivine from low‐MnO PMG indicates potential late diffusion of Mn into olivine, or late overgrowths.…”

Effect of parent body evolution on equilibrium and kinetic isotope fractionation: a combined Ni and Fe isotope study of iron and stony-iron meteorites

“…As such, the broad range of the olivine-metal equilibration temperatures (340 to 1300 ˚C or 255 to 1100 ˚C depending on the thermometric calibration applied) is difficult to explain in terms of a formation model, in which pallasites form at a quiescent core-mantle boundary following parent body metal-silicate segregation (Boesenberg et al, 2012). On the other hand, the equilibration temperature of 1100-1300 ˚C obtained for Marjalahti (Poitrasson et al 2005) appears realistic, close to the schreibersite and Fe-FeS eutectic temperatures (Boesenberg et al, 2012;McKibbin et al, 2016). As such, the equilibrium isotope fractionation observed here and in previous works could be attributed to a later-stage, presently unidentified (possibly incomplete) re-equilibration event, different from the core-mantle segregation.…”

Nickel isotope fractionation during metal-silicate differentiation of planetesimals: Experimental petrology and ab initio calculations

“…Different formation scenarios are invoked for pallasites: these meteorites either sample the mantle core boundary of a differentiated body and result from magmatic processes (e.g. Wasson and Choi, 2003;McKibbin et al, 2016 and references therein), or they formed by impacts between already differentiated planetesimals (Scott, 2007;Solferino et al, 2015;Solferino and Golabek, 2018). Nickel and Fe isotopes data can be discussed relative to these different processes.…”