Samarium-Neodymium Chronology of an Allende Type A CAI AINO

46th Lunar and Planetary Science Conference (2015)
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SAMARIUM-NEODYMIUM CHRONOLOGY OF AN ALLENDE TYPE A CAI AlNO 1-16.
N. E. Marks1, L. E. Borg1, I. D. Hutcheon1, B. Jacobsen1, R.N. Clayton2, T. K. Mayeda2, 1Lawrence Livermore National Laboratory, 7000 East Ave L-231, Livermore, CA 94550, USA (marks23@llnl.gov), 2Department of Chemistry, The University of Chicago, 5640 S. Ellis Ave, Chicago, IL, 60637, USA.
Sm-Nd Systematics of AlNO-1-16: Results of 147Sm143
Nd isotopic analyses for AlNO 1-16 are presented in
an isochron plot in Figure 1. A linear regression fitted
through five points yields an apparent age of 4.89 ± 2.5
Ga. This regression excludes the lightest density separate, which appears to be disturbed. The large uncertainty in the age reflects the very limited spread in the
147
Sm/144Nd ratios of the mineral fractions. In fact, the
total measured variation in 147Sm/144Nd is less than 3%.
In comparison, the spread we recently determined in
the Type B CAI Al3S4 was over 25% [6]. This sample
yielded a 147Sm-143Nd age of 4569 ± 36 Ma and an
initial 143Nd/144Nd ratio of 0.506646 ± 48. The Al3S4
isochron is in excellent agreement with Pb-Pb ages
determined on CAIs [7,8]. A combined plot of the
ALNO-1-16 and Al3S4 data demonstrate that the density separates from AlNO 1-16 fall along the Al3S4
isochron suggesting both samples have concordant
crystallization ages. A linear regression fitted to all the
points from both CAIs results in an age of 4562 ± 170
Ma. The greater uncertainty in the combined age most
likely reflects the higher proportion of secondary phases in Type A CAI compared to Type B CAIs.
0.5145%
0.5140%
Pyroxene%
143Nd/144Nd'
Introduction: Calcium–aluminum-rich inclusions
(CAIs) are primitive objects that formed within the
protoplanetary disk surrounding the young Sun. Recent
Pb–Pb chronologic studies have demonstrated that
CAIs are the oldest solar system solids, crystallizing
4567 Ma ago. Two primary types of CAIs have been
identified: Type A inclusions are composed almost
entirely of melilite, with minor amounts of spinel, clinopyroxene, grossular, perovskite, and hibonite, while
Type B inclusions contain subequal amounts of melilite and clinopryoxene with lesser amounts of spinel
and anorthite (e.g. [1]).
The isotope systematics of CAIs provide critical insight into the earliest history of the Solar System. Although the Sm–Nd geochronometers are highly effective tools for investigating cosmochemical evolution in
the early Solar System, Type A CAIs have not historically lent themselves to this type of study. Because
they are essentially monomineralic, mineral separates
with variable 147Sm/144Nd are quite difficult to produce. Here we report new data from density separates
of Allende CAI AlNO-1-16 from both the long-lived
(147Sm–143Nd) and short-lived (146Sm–142Nd) isotopic
systems derived from density separates of the Type A
CAI AlNO-1-16.
Petrography and Mineral Fractions: AlNO 1-16
is a coarse-grained compact Type A CAI from Allende
that was first separated by heavy liquid and magnetic
techniques at the University of Chicago [2]. The inclusion is comprised primarily of melilite that poikilitically encloses spinel; the spinel is characterized by
unusual epitaxial overgrowths of perovskite [3]. Although the textures of the spinel-perovskite intergrowths imply condensation from the gas phase (i.e [4]
Hutcheon, 1977) the mineralogy reflects crystallization
from a liquid [4,5]. Five density separates plus a
whole-rock (fines) fraction were analyzed for Sm-Nd
isotopic composition. The density separates comprise
primarily melilite, with variable amounts of spinel and
grossular.
0.5135%
0.5130%
ρ>3.26%
Fines%
0.5125%
0.5120%
0.5115%
0.15%
Px5Mel%
ρ<3.26%
Melilite%
0.20%
0.25%
147Sm/144Nd'
Figure 1. A 147Sm-143Nd isochron for density separates
from AlNO 1-16 (red diamonds). Most of the AlNO-1-16
fractions fall along the isochron defined by mineral separates from Type B CAI Al3S4 (yellow circles) that yielded
an age of 4560 ± 34 [5]. The melilite fraction from AlNO
1-16 appears to be disturbed.
Results of 147Sm-142Nd isotopic analyses for AlNO 116 are presented in an isochron plot in Figure 2. A
linear regression fitted through five AlNO 1-16 fractions yields an age of 4519 +55/-88 Ma (initial ε142Nd = 2.2 ± 1.7). The MSWD for this isochron is 2.2, indicating a good fit of the data. However, like the 147Sm143
Nd isochron, uncertainty in the age is strongly affected by the limited spread in 147Sm/144Nd ratio. An
age of 4567 +11/-12 Ma is calculated from the data obtained from the mineral fractions from both AlNO 1-16
and Al3S4 using 146Sm/144Sm = 0.00828 and 146Sm
t½= 103 Ma. This isochron has an initial of ε142Nd =
46th Lunar and Planetary Science Conference (2015)
3.0 ± 1.1 that is in good agreement with previous estimated of the Solar Systems initial Nd isotopic composition.
1.14190%
Pyroxene%
142Nd/144Nd'
1.14188%
1.14186%
ρ>3.26%
1.14184%
1.14182%
1.141850%
Fines%
Px5Mel%
1.141845%
1.141840%
1.14180%
1.141835%
ρ<3.26%
1.141830%
1.14178% Melilite%
1.14176%
0.17%
0.18%
1.141825%
0.19%
0.19%
0.2%
0.21%
0.22%
geochron plot, as it eliminates issues associated Sm
fractionation from Nd through alteration and also
through laboratory preparation.
Discussion: Despite the very limited spread in Sm/Nd
for the Type A CAI AlNO 1-16, the density separate
data yield 146Sm-142Nd and Nd-Nd ages that are concordant with previous measurements of mineral separates from the Allende CAI Al3S4 [6], as well as PbPb ages determined on other CAIs [7,8]. This agreement confirms evidence from the 26Al-26Mg and 207Pb206
Pb isotope systems [9] that common types of CAIs
crystallized contemporaneously and demonstrates the
Sm-Nd systems of both CAIs were minimally affected
by alteration on the parent body.
1.14192%
Age(all%points)%=%4567%+11/512%
IniGal%ε142Nd=3.0+1.1%
MSWD%=%1.9%
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0.20%
0.23%
1.14192%
0.24%
1.14190%
147Sm/144Nd'
Allende Type A CAIs are typically characterized by
alteration, and AlNO 1-16 is no exception. The longlived and short-lived Sm-Nd systems are vulnerable to
disturbance because this process can fractionate Sm
from Nd. Calculating ages using the 143Nd-142Nd isotopic reduces the affects of disturbance by eliminating
the uncertainties associated with the Sm/Nd ratio. This
approach is illustrated in Fig. 3 where 142Nd/144Nd versus 143Nd/142Nd of mineral fractions from AlNO 1-16
yield an age of 4581 +44/-88 Ma (MSWD = 0.5) that is
more precise than either the 147Sm-143Nd or the 146Sm142
Nd isochron ages determined from these fractions.
If mineral fractions from the Type B CAI Al3S4 [6]
are included in the calculation, an age of 4570 +7/-8 Ma
with an MSWD of 1.3 is determined. The excellent
agreement between the Nd-Nd age and the Pb-Pb ages
determined on CAIs by Amelin et al. [7] and Connelly
et al. [8] indicate that this system most accurately records the formation age of CAIs. Like the Pb-Pb isotopic system, if alteration occurs when both Sm parent
isotopes are extant, then fractionation of parent and
daughter isotopes associated with the alteration event
minimally effects the age.
The lowest density separate for AlNO 1-16, ρ=2.883.20 g/cm3, was determined by [2] to contain primarily
melilite with trace spinel and garnet. Data for this separate falls off the 147Sm-143Nd and 146Sm/142Nd
isochrons, although it does fall on the Nd-Nd isochron.
This is a good illustration of the utility of the Nd-Nd
142Nd/144Nd'
Figure 2 A 146Sm-142Nd isochron for density separates
from ALNO 1-16 (red diamonds). The AlNO 1-16 fractions lie on the isochron defined by Type B CAI Al3S4
(yellow points). A regression through all of the points
yields an age of 4567 +11/-12 Ma with a MSWD of 1.9.
Pyroxene%
1.14188%
1.14186%
ρ>3.26%
1.14184%
Fines%
1.14182%
Px5Mel%
1.14180%
1.14178%
ρ<3.26% Nd5Nd%Age(all%points)%=%4570%+7/58%
Melilite%
1.14176%
0.5115%
0.5120%
Slope%=0.0568±0.0029%(2σ)%%
MSWD%=%1.3%
0.5125%
0.5130%
0.5135%
0.5140%
143Nd/144Nd'
Figure 3 Nd-Nd geochron with CAI isochron defined by
both Al3S4 and AlNO 1-16 data yielding an age of
4570 +7/-8 Ma.
The isochron defined by data from the mineral separates from Al3S4 and the density separates from AlNO
1-16 provides additional confirmation that the 103 Ma
half-life for 146Sm is the best value. The concordance
of the two CAI sample sets indicates that the initial
146
Sm/144Sm of the solar system is well represented by
0.00828 ± 44 determined by Marks et al. [6].
References: [1] Grossman, L (1980) Ann. Rev. Earth
Planet. Sci., 8 559–608. [2] Clayton et al. (1977) Earth Planet. Sci. Lett., 34 209–224 [3] Hutcheon, I.D. (1976) Metoritics, 11, 304. [4] Hutcheon, I.D. (1977) LPSC VII, 8 487-489.
[5] Blander, M. & Fuchs, L.H (1975) Geochim. Cosmochim.
Acta., 39(12) 1605-1619 [6] Marks et al. (2014) Earth Planet
Sci. Lett., 405 15-24 [7] Amelin et al. (2002) Sci. 297, 1678.
[8] Connelly et al. (2008) Sci. 651. [9] Jacobsen et al. (2008)
Earth Planet Sci. Lett., 272, 353-364. This work was performed
under the auspices of the U.S. DOE by LLNL under contract DEAC52-07NA27344.