4:00 PM - 4:15 PM
[PPS12-21] In-situ U-Pb dating and hydrogen analyses of apatite in the basaltic eucrites
Keywords:eucrite, water contents & hydrogen isotopic ratios, apatite, U-Pb chronology, NanoSIMS
Samples: Agoult is an unbrecciated granulite, which experienced the strong reheating with partial re-melting . The Pb-Pb age of its zircon and plagioclase are 4554 ± 2.0 Ma  and 4532.4 ± 0.8 Ma, respectively . Camel Donga is a monomict breccia with abundant Fe metal (~2wt%), associated with the later reduction of igneous phases . The Pb-Pb age of its zircon is 4531 ± 10 Ma , younger than those of Agoult and other eucrites. By SEM-EDX observation of the polished sections, we identified several euhedral to subhedral apatite grains (20-100µm) in Agoult and anhedral apatite grains (10-200µm) in Camel Donga.
Analyses: Both U-Pb and hydrogen analyses were conducted using NanoSIMS 50 at AORI, UTokyo. The sample sections were first baked at ~100C in the NanoSIMS air-lock overnight and then kept in the vessel at < 5E-9 torr for 1 week, to remove the adsorbed water. The previously established NanoSIMS analytical methods of U-Pb were applied . For hydrogen analyses, negative secondary ions of H, D, 12C and 18O were collected to calculate water contents and D/H ratios. The background level of hydrogen is ~56 ppm as H2O.
Results & Discussion: The Agoult apatite has concordant 238U-206Pb and 207Pb-206Pb ages at 4476 ± 160 Ma and 4486 ± 61 Ma, respectively. The total Pb/U age is 4523 ± 13 Ma, significantly younger than Agoult zircon but consistent with the plagioclase and Camel Donga zircon. It is suggested that these younger minerals may have recorded the same reheating event at ~4523 Ma or the slow cooling in the Vesta’s crust. The Camel Donga apatite has U-Pb and Pb-Pb ages at 4478 ± 86 Ma and 4474 ± 83 Ma, indicating it may have the same thermal records to that of Agoult apatite. The H2O contents of them are < 100 ppm for Agoult and < 1,000 ppm for Camel Donga. Though their D/H ratios cannot be determined due to the low water contents, such dry apatite is consistent with the reductive conditions, previously estimated from the Agoult zircon  and Camel Donga metal . Our results suggest that Vesta’s crust, at least locally, was depleted in water at ~4523 Ma.
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