Olsen Mia B, Wielandt Daniel, Schiller Martin, Van Kooten Elishevah M M E, Bizzarro Martin
Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Denmark.
Geochim Cosmochim Acta. 2016 Oct 15;191:118-138. doi: 10.1016/j.gca.2016.07.011.
We report on the petrology, magnesium isotopes and mass-independent Cr/Cr compositions (μCr) of 42 chondrules from CV (Vigarano and NWA 3118) and CR (NWA 6043, NWA 801 and LAP 02342) chondrites. All sampled chondrules are classified as type IA or type IAB, have low Al/Mg ratios (0.04-0.27) and display little or no evidence for secondary alteration processes. The CV and CR chondrules show variable Mg/Mg and Mg/Mg values corresponding to a range of mass-dependent fractionation of ~500 ppm (parts per million) per atomic mass unit. This mass-dependent Mg isotope fractionation is interpreted as reflecting Mg isotope heterogeneity of the chondrule precursors and not the result of secondary alteration or volatility-controlled processes during chondrule formation. The CV and CR chondrule populations studied here are characterized by systematic deficits in the mass-independent component of Mg (μMg*) relative to the solar value defined by CI chondrites, which we interpret as reflecting formation from precursor material with a reduced initial abundance of Al compared to the canonical Al/Al of ~5 × 10. Model initial Al/Al values of CV and CR chondrules vary from (1.5 ± 4.0) × 10 to (2.2 ± 0.4) × 10. The CV chondrules display significant μCr variability, defining a range of compositions that is comparable to that observed for inner Solar System primitive and differentiated meteorites. In contrast, CR chondrites are characterized by a narrower range of μCr values restricted to compositions typically observed for bulk carbonaceous chondrites. Collectively, these observations suggest that the CV chondrules formed from precursors that originated in various regions of the protoplanetary disk and were then transported to the accretion region of the CV parent asteroid whereas CR chondrule predominantly formed from precursor with carbonaceous chondrite-like μCr signatures. The observed μCr variability in chondrules from CV and CR chondrites suggest that the matrix and chondrules did not necessarily formed from the same reservoir. The coupled μMg* and μCr systematics of CR chondrules establishes that these objects formed from a thermally unprocessed and Al-poor source reservoir distinct from most inner Solar System asteroids and planetary bodies, possibly located beyond the orbits of the gas giants. In contrast, a large fraction of the CV chondrules plot on the inner Solar System correlation line, indicating that these objects predominantly formed from thermally-processed, Al-bearing precursor material akin to that of inner Solar System solids, asteroids and planets.
我们报告了来自CV（维加诺陨石和NWA 3118）和CR（NWA 6043、NWA 801和LAP 02342）球粒陨石的42颗球粒的岩石学、镁同位素和非质量分馏Cr/Cr组成（μCr）。所有采样的球粒均归类为IA型或IAB型，具有低Al/Mg比（0.04 - 0.27），并且几乎没有或没有二次蚀变过程的证据。CV和CR球粒显示出可变的Mg/Mg和Mg/Mg值，对应于每原子质量单位约500 ppm（百万分之一）的一系列质量分馏。这种质量分馏的镁同位素被解释为反映了球粒前体的镁同位素不均匀性，而不是球粒形成过程中二次蚀变或挥发性控制过程的结果。本文研究的CV和CR球粒群体的特征是，相对于CI球粒陨石定义的太阳值，Mg（μMg*）的非质量分馏成分存在系统性亏损，我们将其解释为反映了由初始Al丰度低于约5×10的典型Al/Al的前体物质形成。CV和CR球粒的模型初始Al/Al值在（1.5±4.）×10至（2.2±0.4）×10之间变化。CV球粒显示出显著的μCr变异性，定义了一系列与内太阳系原始和分异陨石中观察到的组成相当的组成范围。相比之下，CR球粒陨石的特征是μCr值范围较窄，限于通常在块状碳质球粒陨石中观察到的组成。总体而言，这些观察结果表明，CV球粒由起源于原行星盘不同区域然后被输送到CV母小行星吸积区域的前体形成，而CR球粒主要由具有类似碳质球粒陨石μCr特征的前体形成。在CV和CR球粒陨石的球粒中观察到的μCr变异性表明，基质和球粒不一定由同一储库形成。CR球粒的耦合μMg*和μCr系统表明，这些物体由与大多数内太阳系小行星和行星体不同的热未加工且贫Al的源储库形成，可能位于气态巨行星轨道之外。相比之下，很大一部分CV球粒落在内太阳系相关线上，表明这些物体主要由类似于内太阳系固体、小行星和行星的热加工含Al前体物质形成。
