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Magnesium and 54Cr isotope compositions of carbonaceous chondrite chondrules – Insights into early disk processes

机译：碳质球粒陨石球粒的镁和54Cr同位素组成–早期盘过程的见解

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We report on the petrology, magnesium isotopes and mass-independent 54Cr/52Cr compositions (μ54Cr) 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 27Al/24Mg ratios (0.04–0.27) and display little or no evidence for secondary alteration processes. The CV and CR chondrules show variable 25Mg/24Mg and 26Mg/24Mg 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 26Mg (μ26Mg*) relative to the solar value defined by CI chondrites, which we interpret as reflecting formation from precursor material with a reduced initial abundance of 26Al compared to the canonical 26Al/27Al of ~5 × 10−5. Model initial 26Al/27Al values of CV and CR chondrules vary from (1.5 ± 4.0) × 10−6 to (2.2 ± 0.4) × 10−5. The CV chondrules display significant μ54Cr 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 μ54Cr 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 μ54Cr signatures. The observed μ54Cr variability in chondrules from CV and CR chondrites suggest that the matrix and chondrules did not necessarily formed from the same reservoir. The coupled μ26Mg* and μ54Cr systematics of CR chondrules establishes that these objects formed from a thermally unprocessed and 26Al-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, 26Al-bearing precursor material akin to that of inner Solar System solids, asteroids and planets.
机译：我们报告了来自CV的42个软骨的岩石学，镁同位素和质量无关的 54 Cr / 52 Cr组成（μ 54 Cr）（ Vigarano和NWA 3118）和CR（NWA 6043，NWA 801和LAP 02342）球粒陨石。所有采样的软骨都归为IA型或IAB型， 27 Al / 24 Mg比低（0.04-0.27），几乎没有或没有证据表明存在二次蚀变过程。 CV和CR软骨显示变量 25 Mg / 24 Mg和 26 Mg / 24 Mg值，对应于每原子质量单位约500 ppm（百万分之一）的质量相关分馏范围。这种与质量有关的Mg同位素分馏被解释为反映了球状前体的Mg同位素异质性，而不是反映了球状规则形成过程中二次变质或挥发性控制过程的结果。此处研究的CV和CR软骨群体的特征是相对于CI定义的太阳值， 26 Mg（μ 26 Mg *）的质量独立成分系统性不足球粒陨石，我们将其解释为反映了前体材料的形成，与 26 Al / 27 Al的典范 〜5×10 −5 。 CV和CR软骨的模型初始 26 Al / 27 Al值从（1.5±4.0）×10 -6 到（2.2±0.4 ）×10 −5 。 CV球状体显示出显着的μ 54 Cr变异性，定义了与内部太阳系原始和分化的陨石所观察到的组成范围相当的组成范围。相比之下，CR球粒陨石的特征在于，μ 54 Cr值的范围较窄，仅限于通常在块状碳质球粒陨石中观察到的成分。总体而言，这些观察结果表明，CV球粒状体由起源于原行星盘各个区域的前体形成，然后被转运至CV母体小行星的积聚区，而CR球粒状体主要由具有碳质球粒状μ的前体形成。 54 Cr签名。在CV和CR球粒陨石的球粒中观察到的μ 54 Cr变异性表明，基质和球粒不一定是在同一储层中形成的。 CR球团的μ 26 Mg *和μ 54 Cr耦合系统确定这些物体是由未经热处理且贫铝的 26 源形成的与大多数太阳系内部的小行星和行星体不同的储层，可能位于天然气巨人的轨道之外。相反，大部分CV球状体图绘制在内部太阳系相关线上，表明这些物体主要由与内部太阳系相似的，经热处理的，含 26 Al的前体材料形成。固体，小行星和行星。

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