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147Sm–143Nd and 146Sm–142Nd Systematics of Basaltic Eucrites
1 Department of Earth and Planetary Sciences, Tokyo Institute of Technology (e-mail: [email protected])S. Kagami 1*, T. Yokoyama 1, T. Usui 1
References[1] Bouvier, A. et al. (2015) Meteoritics & Planet. Sci., 50, 1896–1911. [2] Boyet, M. et al. (2010) EPSL, 291, 172–181.JAAS, 22.616-622. [3] Marks, N. E. et al. (2014) EPSL, 405, 15–24. [4] Trinquier A. et al. (2008) GCA, 72, 5146–5163. [5] Kleine, T. et al. (2004) GCA, 68. 2935–2946.
SamplesAcid digestion Determination of
Nd and Sm (HF, HClO4, HNO3)
ICP-MS
145Nd spike149Sm spike
Dynamic multicollection method
X-series Ⅱ(Thermo Scientific)
TIMSTRITON plus(Thermo Fisher Scientific)
Determination of Nd isotope ratios (142Nd/144Nd, 143Nd/144Nd)
First columnremove major elements
10%
90%
resin: 1:1 mixture of AG50WX8 and AG50WX12 1 mL (column = 5 mm) φ
resin: Ln Resin with HCl 0.5 mL (column = 4 mm) φ
【Ce oxidation】Ce3+→Ce4+
Sm3+→Sm3+
Nd3+→Nd3+
Second columnremove Ce
TRU Resinremove matrix elements
Third columnremove Sm
resin: Ln Resin with KBrO3 0.5 mL (column = 7 mm) φ
resin: TRU Resin 0.5 mL (column = 4 mm) φ
Experiments
Introduction
Sm–Nd chronology(Sm・Nd: lithoplile elements)・Long-lived 147Sm →143Nd + α (T1/2= 1.06×1011 yr)
・ Short-lived 146Sm →142Nd + α (T1/2= 1.03×108 yr) [3]
crustmantle
core
http://response.jp/article/
The eucrite parent body has experienced magma ocean associated with core formation and silicate differentiation within
several years after the formation of the solar system. Sm–Nd chronology is useful for determining the crust crystallization
age. Highly precise Sm–Nd ages are needed to constrain the period of igneous activity and subsequent thermal
metamorphism of Vesta. Bouvier et al. [1] investigated the 147Sm–143Nd whole-rock isochron age of basaltic and cumulate
eucrites (Fig. 1). Boyet et al. [2] obtained the 146Sm–142Nd whole-rock isochron age, using three cumulate eucrites with
variable Sm/Nd ratios (Fig. 2). However, the variation of Sm/Nd ratios for basaltic eucrites were several times smaller
than the entire range of Sm/Nd ratios for all eucrites, making it difficult for obtaining the precise Sm–Nd whole-rock
isochron age for basaltic eucrites alone.
Howardite
Eucrite
Diogenite
Basaltic eucrite
Cumulate eucrite
・Subophitic texture・Quenched near the surface
・Gabbroic texture・Crystallized slowly deep in the crust
HED
Objective
・Determine the 147Sm–143Nd and 146Sm–142Nd ages for bulk rocks of basaltic eucrites.
・ Constrain the period of igneous activity of Vesta by comparing the obtained ages with the age of previous studies using different radiometric dating.
Kagami and Yokoyama, 2016 (in review)
Samples: Basaltic monomict eucrites・ Juvinas, NWA 5229, Nuevo Laredo (brecciated)
・ NWA 7188 (slightly brecciated)
・ Agoult (unbrecciated)
4482 ± 170 MaBasaltic eucriteCumulate eucrite
0.5122
0.5126
0.5130
0.5134
0.5138
0.5142
0.18 0.19 0.20 0.21 0.22 0.23 0.24
147Sm/144Nd
143 N
d/14
4 Nd
Cumulate eucrite
Basaltic eucrite
0.25147Sm/144Nd
142 N
d/14
4 Nd
1.14182
1.14184
1.14186
1.14188
1.14190
1.14192
0.18 0.19 0.20 0.21 0.22 0.23 0.24 0.25
Cumulate eucrite
Basaltic eucrite
4553 MaBasaltic eucriteCumulate eucrite
+ 35- 46
Results and Discussion 147Sm–143Nd and 146Sm–142Nd isochrons for basaltic eucrites
147Sm/144Nd14
3 Nd/
144 N
d
AgoultNWA 5229JuvinasNuevo LaredoNWA 7188
0.5123
0.5124
0.5125
0.5126
0.5127
0.5128
0.187 0.190 0.193 0.196 0.199 0.202 0.205147Sm/144Nd
142 N
d/14
4 Nd
1.141798
1.141802
1.141806
1.141810
1.141814
1.141818
1.141822
1.141826
0.187 0.190 0.193 0.196 0.199 0.202
AgoultNWA 5229JuvinasNuevo LaredoNWA 7188
0.205
The 147Sm–143Nd and 146Sm–142Nd ages for bulk basaltic eucrites obtained in this study are consistent with those obtained
by previous studies using basaltic and cumulate eucrites (4482 ± 170 Ma for 147Sm–143Nd [1], 4553 Ma for 146Sm–142Nd [2]). Although the analytical uncertainties are large, the consistent ages suggest that the parent body
processes associated with the last Sm–Nd isotopic closure were contemporaneous for basaltic and cumulate eucrites.
+ 35- 46
4450450045504600 (Ma)
+ 55- 884575 Ma
- 46+ 354553 Ma
4564.9 ± 1.1 Ma
4563.2 ± 1.4 Ma
146Sm–142Nd age of basaltic eucrites (this study)146Sm–142Nd age of cumulate eucrites [2]53Mn–53Cr age [4]182Hf–182W age [5]
According to the 53Mn–53Cr systematics [4], basaltic magmas have formed in the Vesta’ s mantle at 4564.9 ± 1.1 Ma.
This age is consistent with the 146Sm–142Nd whole-rock age obtained in this study. We speculate that the formations of
the crust of Vesta and basaltic magmas in the mantle are contemporaneous. Moreover, the 146Sm–142Nd age in this study
is consistent with the 182Hf–182W whole-rock isochron age for 7 basaltic and 1 cumulate eucrites within analytical
uncertainties, which most likely represents the formation age of the basaltic eucrite source reservoir via metal-silicate
segregation [5]. This implies that there is no apparent age difference between the formation of source mantle and the
crystallization of the crust represented by basaltic eucrites.
4558 ± 300 Ma + 55- 884575 Ma
MSWD = 0.094 MSWD = 6.8
Fig. 5 Previous chronological studies on eucrites.
Fig. 3 147Sm–143Nd isochron diagram for basaltic eucrites. Error bars are 2SE, of which the y-axis is smaller than the size of symbols.
Fig. 4 146Sm–142Nd isochron diagram for basaltic eucrites. Error bars are 2SE. The age was determined by applying 146Sm/144Sm = 0.00828 at 4567 Ma).
Fig. 1 147Sm–143Nd isochron diagram for eucrites modified from [1].
Fig. 2 146Sm–142Nd isochron diagram for eucrites modified from [2].
Conclusion ・The 147Sm-143Nd and 146Sm-142Nd whole-rock isochron ages for basaltic eucrites are 4558 ± 300 Ma and 4575 Ma, respectively.
・The 147Sm–143Nd whole-rock isochron age obtained is consistent with that of previous studies [1].
・ No apparent age difference between basaltic and cumulate eucrites is observed by the whole-rock Sm-Nd isochrons.
・ The timing of global silicate differentiation is nearly contemporaneous to the timing of metal-silicate segregation in Vesta.
+ 55- 88
