Fig 5.jpg

Fig. 5. δ30Si versus major element ratios (wt.%/wt.%) in chondrules from the Allende meteorite (a-c: non-porphyritic chondrules, d-f: porphyritic chondrules). (a, b) δ30Si vs. Mg/Fe; (c, d) δ30Si vs. Mg/Si; (e, f) δ30Si vs. Ca/Na. The δ30Si and major element data for bulk Allende and CI chondrites is from (Jarosewich et al., 1987, Lodders, 2003, Georg et al., 2007, Fitoussi et al., 2009, Armytage et al., 2011, Zambardi et al., 2013). Gray symbols are bulk chondrule data for Allende chondrules from Armytage (2011). Chondrule analyses show no systematic variation in δ30Si vs. Ca/Na, except for the radial pyroxene chondrule (RPC). RPC show an inverse correlation between δ30Si vs. Ca/Na. δ30Si correlates with Mg/Fe (black correlation line). Correlated variations between δ30Si and Mg/Si occur in most porphyritic chondrules (c and d). Non-porphyritic chondrules have higher average δ30Si values than CI chondrites and show significant variations in the Mg/Si ratio. This difference can be explained by the variable relative contributions of kinetic and equilibrium isotope fractionation processes in texturally different chondrules. If the δ30Si and Mg/Si values of the chondrule minerals resulted from the equilibrium fractionation of early formed solids and remaining nebular gas with CI chondrite-like initial composition, they would fall between the two dashed lines in Figs. (c) and (d). This area represents the window of solid–gas equilibrium isotope fractionation and interaction of 70% to 20% of forsterite with 30% to 80% remaining SiOgas. Details of the calculations are provided in the Supplementary Information. The two solid lines represent the window of kinetic isotope fractionation between 50% solid and 50% gas (line towards higher Mg/Si) and 20% solid and 80% gas (towards lower Mg/Si). The variation of δ30Si and Mg/Si in all porphyritic chondrules and in some non-porphyritic chondrules reflects equilibrium isotope fractionation between early-crystallized silicates (mainly forsterite) and nebular gas (SiOgas). Non-systematic variations of δ30Si and Mg/Si in barred olivine chondrules that fall into the pink field reflect kinetic isotope fractionation during the formation of these chondrules.