Major and trace elements of zircons from basaltic eucrites: Implications for the formation of zircons on the eucrite parent body

- The major and trace elements of zircons from six basaltic eucrites were determined.
- The maximum sizes of zircons in each sample may be related to the metamorphic grade.
- Zircons in highly metamorphosed eucrites could have overgrown during metamorphism.

The major and trace elements in zircons obtained from basaltic eucrites Yamato [Y]-75011, Y-792510, Y-82082, Asuka [A]-881467, Juvinas, and Stannern were determined using an electron microprobe analyzer and a sensitive high-resolution ion microprobe, and the formation of zircons on the parent body was discussed. The maximum sizes of zircon grains in each sample are likely to be related to its metamorphic grade. The least-metamorphosed basaltic eucrite Y-75011 contains zircons with sizes of a few μm. Conversely, the highly metamorphosed basaltic eucrites Y-792510 and A-881467 include zircons larger than 20 μm. The relationship between the maximum grain size and metamorphic grade suggests that the formation process of zircons is related to metamorphic events. The Zr/Hf ratios of the zircons found in this study show a wide variation (31.6-71.7), as compared to the chondritic values (Zr/Hf = 32.8-34.3) and bulk basaltic eucrites (34.0-34.7). The Zr/Hf ratios of the zircons from Y-792510 and A-881467 are relatively constant, whereas those of the zircons from Y-82082, Juvinas, and Stannern show large variations. The rare earth element (REE) content in the zircons from Y-792510 and A-881467 is given by La=0.1×CI and Lu = 1000 × CI. On the other hand, the zircons from Stannern show higher REE content (La=0.1-1×CI and Lu=1000-10000×CI) than those from Y-792510 and A-881467. The most reliable REE data of a large zircon from A-881467 show no Ce anomaly and a distinct negative Eu anomaly. Therefore, it is presumed that the zircons formed under reducing condition in which Ce3+ was stable. The melt compositions coexisting with the studied zircons suggest that the zircons from highly metamorphosed eucrites might react with the REE-enriched melt derived from partial melting of the mesostasis region during metamorphism.