Distribution of lithium, beryllium, and boron in meteoritic chondrules

Abstract

The distribution of Li, Be, and B was studied by ion microprobe mass spectrometry in 36 chondrules from the Semarkona, Bishunpur, Allende, Clovis 1, and Hedjaz meteorites. Within a single chondrule, Li-Be-B concentrations can vary up to one order of magnitude. For example, in a chondrule from Hedjaz, concentrations range from 0.3 to 2.4 ppm for Li, from <0.001 to 0.17 ppm for Be, and from 0.4 to 5.5 ppm for B. Among chondrules from Semarkona and Bishunpur, clear crystal-mesostasis partitioning was observed in nine chondrules for Li, in nine chondrules for Be, and in three chondrules for B. The heterogeneities in the distribution of Li, Be, and B in chondrules from Semarkona and Bishunpur appear to be primary features that were inherited from the chondrules' precursors and not totally obscured during the chondrules' formation. A redistribution of B was nevertheless observed at the whole-rock scale for Allende (B-Al2O3 correlation) and Hedjaz (B-SiO2 correlation). At the scale of bulk chondrules, a robust correlation is observed for all studied meteorites between the B/Be and the B/Li ratios, which indicates that Li and Be are much less heterogeneously distributed in chondrites than B. Mean Li, Be, and B concentrations of chondrules ([Li] ~ 0.83-0.42+0.86 ppm; [Be] ~ 0.043-0.017+0.027 ppm; [B] ~ 0.89-0.72+3.71 ppm) are consistent with those of Orgueil ([Li] ~ 1.49 ppm; [Be] ~ 0.0249 ppm; [B] ~ 0.87 ppm), but the mean Li/Be ratio of chondrules (24.5-9.1+6.5) is a factor of ~4 depleted relative to Orgueil (Li/Be ratio of ~78). Such a depletion is puzzling because no correlation between Li and Na or B has been found as would be expected to result from volatilization processes during chondrule melting and cooling. As a consequence, the exact abundance of solar system Li remains an open question.