The Chromium Isotopic Composition of the Ungrouped Carbonaceous Chondrite Tagish Lake

Abstract

Early solar materials bear a variety of isotopic anomalies that reflect compositional differences deriving from distinct stellar nucleosynthetic processes. As shown in previous studies, the stepwise dissolution with increasing acid strengths of bulk rock carbonaceous chondrites liberates Cr with both excesses and deficits in 53 Cr and 54 Cr relative to the terrestrial standard. The magnitude of the 54 Cr variations within a meteorite decreases in the sequence CI1 > CR2 > CM2 > CV3 > CO3 > CK4 and correlates with the degree of metamorphism of each carbonaceous chondrite class. This study shows that the Tagish Lake meteorite presents the highest excesses in 54 Cr ever measured in a bulk silicate phase. According to this study, the Tagish Lake meteorite is composed of the least re-equilibrated material known at this time. The magnitude of 54 Cr variation decreases now in the following sequence: Tagish Lake (ungrouped CI2) > Orgueil (CI1) > Murchison (CM2) > Allende (CV2). Moreover, this study shows that excesses in 53 Cr relative to Earth can be interpreted as representing the extent of aqueous alteration on meteorite parent bodies. Finally, the high 54 Cr anomalies measured in this meteorite make Tagish Lake one of the major targets to decipher the host of these anomalies.

Correlated accretion ages and ε54Cr of meteorite parent bodies and the evolution of the solar nebula

“…; Petitat et al. ). The highest ε 54 Cr values found in these studies were approximately 100, 210, 209, and 139ε, respectively.…”

“…To show that −1ε is a reasonable choice of the preinjection ε 54 Cr value, the stepped dissolution results for Orgueil reported by Petitat et al. () are used. In their 4th dissolution, 72 ppm of Cr with ε 54 Cr = 79ε was detected.…”

“…together with the abundance of Cr (2897 ppm) in Orgueil (Petitat et al. ), i.e., (1.6 + 1) (ε) * 2897 (ppm)/295000 (ε) = 0.026 (ppm). Therefore, the base line of the time versus ε 54 Cr diagram is considered to be approximately −1ε, and the mixing fraction of Cr of the 54 Cr‐rich presolar grains in Orgueil relative to the total Cr is 2.6/295000 ~ 9 ppm.…”

Chromium isotopic systematics of the Sutter's Mill carbonaceous chondrite: Implications for isotopic heterogeneities of the early solar system

“…; Petitat et al. ). The weakly metamorphosed carbonaceous chondrites, such as CI1 and CM2, preserve extreme 54 Cr excesses in silicate phases, while deficits are found in easily dissolved phases (i.e., carbonates, sulfates, sulfides, metal, and magnetite).…”

“…The largest 54 Cr excesses and deficits are found in Tagish Lake, with values of 139.01ε and −16.14ε, respectively (Petitat et al. ). Interestingly, the magnitude of the 54 Cr isotopic variations is well correlated with the petrologic subtype of each carbonaceous chondrite class, and decreases in the following sequence, C2 (ungrouped Tagish Lake) > CI1 > CR2 > CM2 > CM3 > CO3 > CK4 (e.g., Rotaru et al.…”