A petrological and chemical reexamination of Main Group pallasite formation

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

“…Phosphoran olivine (with several wt. % P 2 O 5 ) and Mg-and Si-phosphate minerals crystallised from an unusual P 2 O 5 -rich silicate melt (Boesenberg et al, 2012) which might ultimately be formed by oxidation of metallic P. High temperature merrillite and peritectic stanfieldite formed at or above ~1175 °C and have generally high rare earth element (REE) concentrations, while eutectic stanfieldite, farringtonite and silico-phosphate crystallised near the ~1120 °C eutectic after in situ fractional crystallisation (with generally lower REE concentrations; Ando, 1958;Davis and Olsen, 1991;Hsu, 2003). A parallel trend is recorded by olivine geochemistry: olivine cores exhibit variable Cr and P (Fig.…”

“…The occurrence of genuinely 'phosphoran' olivine rims in some pallasites, with several wt. % P 2 O 5 (Boesenberg et al, 2012) (Hsu, 2005;Qin et al, 2010), due to cosmic ray exposure effects during transport to Earth. However, our 53 Cr/ 52 Cr in olivine rims are higher and inconsistent with the lower susceptibility of olivine to cosmogenic effects relative to metal (via lower Fe/Cr).…”

“…The accumulated oxygen isotopic evidence for five distinct parent bodies of olivine and metal-rich pallasite meteorites (summarised in Boesenberg et al, 2012) suggests that pallasitic material is the default end-product of planetesimal differentiation. Pallasites may represent samples of quiescent core-mantle boundaries (Boesenberg et al, 2012), or violently formed mixtures of core and mantle materials (Yang et al, 2010;Tarduno et al, 2012) and are often discussed 1.…”

“…Pallasites may represent samples of quiescent core-mantle boundaries (Boesenberg et al, 2012), or violently formed mixtures of core and mantle materials (Yang et al, 2010;Tarduno et al, 2012) and are often discussed 1. Analytical, Environmental and Geo-Chemistry (AMGC), Vrije Universiteit Brussel, Brussels 1050, Belgium 2.…”

“…Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia * Corresponding author (email: seann.mckibbin@vub.ac.be, seann.mckibbin@gmail.com) 3. School of Earth Sciences, University of Queensland, Brisbane, QLD 4072, Australia in terms of trapping of olivine in a liquid Fe-Ni metal groundmass progressively becoming saturated in chromite, troilite, schreibersite, and phosphate minerals during cooling (Boesenberg et al, 2012). Despite this diverse mineralogy, the short-lived isotope chronologies of pallasites remain poorly understood due to depletion in crustal components (plagioclase and pyroxene; Nyquist et al, 2009).…”