[P-PS07] Formation and evolution of planetary materials in the Solar System

[PPS07-12] ⁵³Mn-⁵³Cr age of dolomite in the Tarda ungrouped carbonaceous chondrite

Shingo Sugawara¹, *Wataru Fujiya¹, Noriyuki Kawasaki², Naoya Sakamoto³, Takayuki Ushikubo⁴ (1.Ibaraki University, College of Science, 2.Department of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, 3.Creative Research Institution, Hokkaido University, 4.Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology)

Keywords:Tarda meteorite, dolomite, Mn-Cr age, oxygen isotopes, aqueous alteration

The ⁵³Mn-⁵³Cr dating (half-life: 3.7 Myr) of carbonates has widely been used to constrain the timing of aqueous alteration in chondrite parent bodies. Previous studies have suggested that the Mn-Cr ages of carbonates converge to 4-6 Myr after the birth of the solar system [1]. However, a few carbonate grains in CI and CR chondrites have younger ages [2,3], and a recent study reported the age variation of Ryugu dolomite [4]. Moreover, calcite grains with distinct morphologies in CM chondrites (i.e., T1 calcite which precipitated in pore spaces and T2 calcite which replaced other minerals) have distinct Δ¹⁷O values, suggesting multiple generations of CM calcites. These observations imply either prolonged aqueous alteration in chondrite parent bodies or carbonate formation during later impact events, however, the Mn-Cr dating of T2 calcite has not been successful.
Tarda is a carbonaceous chondrite that fell on Morocco in 2020. It does not belong to any existing chondrite groups but has mineralogical, petrological, and geochemical properties akin to the Tagish Lake ungrouped carbonaceous chondrite [7]. We previously found dolomite grains with distinct morphologies in Tarda (Fig. 1) like T1 and T2 calcites in CM chondrites [8]. In this study, we measured the Mn-Cr ages of T1 and T2 dolomites in Tarda to see possible age variation.
We performed Mn-Cr isotope measurement using Cameca ims 1280-HR at Hokkaido University with an ¹⁶O²⁻ primary ion beam from Hyperion radiofrequency (RF) ion source (4-5 µm in diameter and ~50 pA in current). Secondary ions of ^52,53Cr⁺ and ⁵⁵Mn⁺ produced by the primary ion beam were simultaneously detected with three electron multipliers (EMs). The observed ⁵⁵Mn⁺/⁵²Cr⁺ and ⁵³Cr⁺/⁵²Cr⁺ ratios were calibrated using synthetic dolomite standards [9]. The measurement time was ~3600 seconds after 250 seconds pre-sputtering.
The ⁵⁵Mn/⁵²Cr ratios and ⁵³Cr excesses of T1 and T2 dolomites correlate well and plot on a single regression line (Fig. 2). On the other hand, if we plot the data in a 1/⁵²Cr vs. δ⁵³Cr space, then we get worse correlation. These observations suggest that the observed ⁵³Cr excesses are attributed to in-situ decay of ⁵³Mn. Combining the data of T1 and T2 dolomites, the initial ⁵³Mn/⁵⁵Mn ratio at the dolomite formation is (3.56 ± 0.66) ×10⁻⁶, which corresponds to an absolute age of 4563.9 (+0.9/-1.1) Ma. The fact that the data plot on a single regression line indicates that the dolomite precipitation in pore spaces and replacement of other minerals with dolomite took place almost simultaneously. This formation scenario is consistent with similar oxygen isotopic compositions of T1 and T2 dolomites, which indicates that they formed from similar aqueous fluids [8], but different from the case for T1 and T2 calcites in CM chondrites. That is, we did not find any evidence for prolonged aqueous alteration or later impact events of the Tarda parent body, in contrast the case for the Ryugu parent body [4].

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