[P-PS25_2AM1] 隕石解剖学: 太陽系物質の総合的理解に向けて

[PPS25-02] 玄武岩質ユークライトJuvinasにおける147Sm-143Nd年代と146Sm-142Nd年代

*鏡味 沙耶¹、横山 哲也¹ (1.東京工業大学地球惑星科学専攻)

Eucrites are interpreted to have originated from the asteroid 4-Vesta which differentiated into crust and mantle. The chronology of eucrites is important to understand the formation and differentiation of planets/ planetesimals in the early Solar System. The combination of two chronometers, short-lived ¹⁴⁶Sm-¹⁴²Nd system (T_1/2=6.8×10⁷yr) and long-lived ¹⁴⁷Sm-¹⁴³Nd (T_1/2=1.06×10¹¹yr), is useful to decode the thermal history of the eucrite parent body, because they have the same closure temperature [1]. To obtain precise Sm-Nd ages for eucrites and other achondrites, it is indispensable to develop highly precise Nd isotope analysis.We investigated the ¹⁴⁷Sm-¹⁴³Nd and ¹⁴⁶Sm-¹⁴²Nd chronometers for a brecciated basaltic eucrite, Juvinas. We examined samples of whole rock powder (W.R.), 400 mesh grains, plagioclase (Pl), and pyroxene (Px). Plagioclase and pyroxene grains were obtained by handpicking. Samples were dissolved using a mixture of concentrated pure acid (HClO₄, HF, and HNO₃) and heating at 195℃. About ten percent of the solution was removed and spiked with ¹⁴⁹Sm-¹⁴⁵Nd in order to precisely measure Sm and Nd concentrations by ID-ICP-MS (Thermo X-series II at Tokyo Tech). We separated Nd from the rest of the solution via a four-step column chemistry. Nd isotope ratios in W.R., 400mesh, Pl, and Px were analyzed by TIMS (Thermo TRITON-plus at Tokyo Tech) with the dynamic multicollection method.The ¹⁴⁷Sm-¹⁴³Nd mineral isochron diagram of Juvinas, yielded an age of 4610±410 Ma. In contrast, the ¹⁴⁶Sm-¹⁴²Nd systematic for Juvinas yielded an initial ¹⁴⁶Sm/¹⁴⁴Sm ratio of 0.0157±0.0074. This gives ?4618?_(-63)^(+38) Ma for the age of Juvinas when an initial solar system ratio of ¹⁴⁶Sm/¹⁴⁴Sm=0.0094 at 4568 Ma is assumed [1]. The self-consistency of the ¹⁴⁷Sm-¹⁴⁴Nd and ¹⁴⁶Sm-¹⁴²Nd ages for Juvinas supports early crust-mantle differentiation on the eucrite parent body as was proposed by previous chronological studies on basaltic eucrites (e.g., Pb-Pb, Al-Mg, Mn-Cr, and Hf-W; [2-5]). It has been suggested that cumulate eucrites provide internal isochron ages younger than basaltic eucrites due to a longer history in the deep crust or late thermal disturbance during later meteorite bombardment event(s) [6]. Because of the limited Sm/Nd variation in the meteorite components analyzed, the Sm-Nd ages obtained in this study have uncertainties several times larger than those in previous studies [6]. The ¹⁴⁷Sm-¹⁴³Nd age and the initial ¹⁴³Nd/¹⁴⁴Nd ratio for Juvinas are consistent with those obtained by the mineral isochrons of three cumulate eucrites within analytical error (4546±8 Ma; [6]). However, our ¹⁴⁶Sm-¹⁴²Nd age is older than the proposed ¹⁴⁷Sm-¹⁴³Nd age for cumulate eucrites, indicating that the crystallization of basaltic eucrites predates the timing when the Sm-Nd systematics for cumulate eucrites reached a closure temperature. It should be noted that Juvinas is a brecciated basaltic eucrite which may not record a correct Sm-Nd age. Further investigation is required to obtain more precise Sm-Nd ages utilizing unbrecciated basaltic eucrite to reveal the thermal history on the eucrite parent body.References: [1] N. Kinoshita et al., (2012) Science, 335, 1614 [2] S.J.G. Galer and G.W. Lugmair, (1996) MAPS, 31, A47. [3] M. Wadhwa et al., (2003) LPSC XIV, 2055. [4] G.W. Lugmair and A. Shukolyukov, (1998) GCA 62, 2863. [5] T. Kleine et al., (2004) GCA 68, 2935. [6] M. Boyet et al., (2010) EPSL 291, 172.