Primordial compositions of refractory inclusions

Abstract

Bulk chemical and oxygen, magnesium and silicon isotopic compositions were measured for each of 17 Types A and B refractory inclusions from CV3 chondrites. After bulk chemical compositions were corrected for non-representative sampling in the laboratory, the Mg and Si isotopic compositions of each inclusion were used to calculate its original chemical composition assuming that the heavy-isotope enrichments of these elements are due to Rayleigh fractionation that accompanied their evaporation from CMAS liquids. The resulting pre-evaporation chemical compositions are consistent with those predicted by equilibrium thermodynamic calculations for high-temperature nebular condensates but only if different inclusions condensed from nebular regions that ranged in total pressure from 10 -6 to 10 -1 bar, regardless of whether they formed in a system of solar composition or in one enriched in OC dust relative to gas by a factor of ten relative to solar composition. This is similar to the range of total pressures predicted by dynamic models of the solar nebula for regions whose temperatures are in the range of silicate condensation temperatures. Alternatively, if departure from equilibrium condensation and/or non-representative sampling of condensates in the nebula occurred, the inferred range of total pressure could be smaller. Simple kinetic modeling of evaporation successfully reproduces observed chemical compositions of most inclusions from their inferred pre-evaporation compositions, suggesting that closed-system isotopic exchange processes did not have a significant effect on their isotopic compositions. Comparison of pre-evaporation compositions with observed ones indicates that 80% of the enrichment in refractory CaO + Al 2 O 3 relative to more volatile MgO + SiO 2 is due to initial condensation and 20% due to subsequent evaporation for both Type A and Type B inclusions.

Timescales for the evolution of oxygen isotope compositions in the solar nebula

“…In addition, after the 16 O-depleted water has arrived from the outer solar system, chondritic silicates must undergo oxygen isotope exchange due to either the ambient temperature of the nebular gas, or to sporadic heating events from protosolar flares and/or other nebular shocks, whereas the most refractory CAIs must resist exchange. Condensation of CAIs from a high pressure (up to $0.1 atm) solar gas (Grossman et al, 2008) is consistent with formation in the vicinity of the X-point. The X-wind could have transported the CAIs to the meteorite-forming region of the nebula.…”

“…Ca and Al are assumed to be exclusively in CAIs, but CAIs also contain some Mg and Si. The amount of Mg and Si in CAIs was taken from the average of all CAI bulk compositions reported by Grossman et al (2008). The remaining Mg and Si (most of it) and all of the Fe is assumed to be in silicates.…”

Calcium isotopic ratios and rare earth element abundances in refractory inclusions from the Allende CV3 chondrite

“…Given a complicated nebular history of refractory inclusions involving multiple episodes of evaporation and condensation, the decoupling of isotopic and chemical fractionations is very likely and, in fact, expected. However, when preserved, the correlated chemical and isotopic fractionations provide a powerful tool for deciphering condensation and evaporation history of a nebular object (e.g., Richter, 2004;Richter et al, 2002;Grossman et al, 2008). Here we use the observed correlation of d 44/40 Ca with the REE patterns of the inclusions studied to evaluate origin of the Group II REE pattern that cannot be done based on REE abundance pattern alone.…”

“…Many refractory inclusions have igneous textures, with some having been modified by secondary alteration processes (e.g., MacPherson, 2004). Refractory inclusions exhibit both large mass-dependent and mass-independent isotopic effects in many elements (Clayton et al, 1973;McCulloch and Wasserburg, 1978a,b;Lee et al, 1979Lee et al, , 2011Jungck et al, 1984;Niederer and Papanastassiou, 1984;Papanastassiou, 1986;Birck and Lugmair, 1988;Harper, 1993;Yin et al, 2002;Grossman et al, 2008;Krot et al, 2008;MacPherson et al, 2008;Bermingham and Mezger, 2010). Some massindependent isotopic effects in refractory inclusions, such as 48 Ca, 50 Ti, 54 Cr, 95 Mo, 100 Mo, 135 Ba,137 Ba and 144 Sm (McCulloch and Wasserburg, 1978a,b;Jungck et al, 1984;Niederer and Papanastassiou, 1984;Papanastassiou, 0016-7037/$ -see front matter Ó 2011 Elsevier Ltd. All rights reserved.…”

Silicon isotopic fractionation of CAI-like vacuum evaporation residues

“…These CAIs are often enriched in the heavy isotopes of magnesium and silicon by a few permil (Clayton et al, 1988;Grossman et al, 2008) and it seems likely that evaporative loss of magnesium and silicon led to the observed heavy isotope enrichment for these elements.…”