Chemistry and oxygen isotopic composition of cluster chondrite clasts and their components in LL3 chondrites

Cluster chondrites are characterized by close‐fit textures of deformed and indented chondrules, taken as evidence for hot chondrule accretion (Metzler 2012 ). We investigated seven cluster chondrite clasts from six brecciated LL3 chondrites and measured their bulk oxygen isotopic and chemical composition, including REE, Zr, and Hf. The same parameters were measured in situ on 93 chondrules and 4 interchondrule matrix areas. The CI‐normalized REE patterns of the clasts are flat, showing LL‐chondritic concentrations. The mean chemical compositions of chondrules in clasts and other LL chondrites are indistinguishable and we conclude that cluster chondrite chondrules are representative of the normal LL chondrule population. Type II chondrules are depleted in MgO, Al₂O₃ and refractory lithophiles (REE, Zr, Hf) by factors between 0.65 and 0.79 compared to type I chondrules. The chondrule REE patterns are basically flat with slight LREE < HREE fractionations. Many chondrules exhibit negative Eu anomalies while matrix shows a complementary pattern. Chondrules scatter along a correlation line with a slope of 0.63 in the oxygen 3‐isotope diagram, interpreted as the result of O‐isotope exchange between chondrule melts and ¹⁸O‐rich nebular components. In one clast, a distinct anticorrelation between chondrule size and δ¹⁸O is found, which may indicate a more intense oxygen isotope exchange by smaller chondrules. In some clasts the δ¹⁸O values of type I chondrules are correlated with concentrations of SiO₂ and MnO and anticorrelated with MgO, possibly due to the admixture of a SiO₂‐ and MnO‐rich component to chondrule melts during oxygen isotope exchange. Two chondrules with negative anomalies in Sm, Eu, and Yb were found and may relate their precursors to refractory material known from group III CAIs. Furthermore, three chondrules with strong LREE > HREE and Zr/Hf fractionations were detected, whose formation history remains to be explained.     

From 2D to 3D chondrule size data: Some empirical ground truths

In order to characterize the relation between apparent chondrule sizes (2D) and true chondrule sizes (3D), three ordinary chondrites of the H, L, and LL group have been analyzed. The diameters of a large number of chondrule cut faces in thin sections (2D; n = 2037) and of separated chondrules from the same meteorites (3D: n = 2061) have been measured. The obtained 2D/3D mean chondrule sizes (μm) for the H, L, and LL chondrite are 450/490, 500/610, and 690/830; the corresponding median values (μm) are 370/420, 450/530, and 580/730. The data show that there is a cutoff for small chondrule sizes in each sample. Possibly characteristic minimum sizes exist for the various groups, increasing in the (3D) sequence H (~90 μm) <L (~180 μm) <LL (~240 μm). No systematics were found for the maximum chondrule sizes. The investigated samples show very similar chondrule volume (mass) distributions relative to the mode (peak) of their size-frequency distributions. About 2.6–2.9% and 97.1–97.4% of the total chondrule volume (mass) is present in chondrule sizes smaller and larger than the mode, respectively. It was found that 2D sectioning consistently results in a shift of the true 3D size-frequency distributions toward smaller sizes. This effect leads to the underestimation of the values for (1) the true mean chondrule size by 8–18%, (2) the true chondrule median value by 12–21%, and (3) the true mode value of the size-frequency distributions by 12–17% (50 μm binning). This is the opposite of what popular 2D/3D correction models predict (e.g., Eisenhour 1996 ).