multiple generations of fractionated hibonite

46th Lunar and Planetary Science Conference (2015)
2750.pdf
MULTIPLE GENERATIONS OF FRACTIONATED HIBONITE-RICH CAIS SAMPLED THE SOLAR
NEBULA AT DIFFERENT DEGREES OF ISOTOPIC HETEROGENEITY L. Kööp1,2,4, A. M. Davis1,2,3,4, P.
R. Heck2,4, N. T. Kita5, A. N. Krot6, P. Mane7, K. Nagashima6, D. Nakashima5,8, C. Park6,9, T. J. Tenner5, M.
Wadhwa7, 1Dept. of Geophysical Sciences, 2Chicago Center for Cosmochemistry, 3Enrico Fermi Institute, Univ. of
Chicago, Chicago, IL, 4Robert A. Pritzker Center for Meteoritics and Polar Studies, Field Museum of Natural History, Chicago, IL, 5WiscSIMS, Dept. of Geoscience, Univ. of Wisconsin, Madison, WI. 6HIGP/SOEST, Univ. of Hawai‘i at Mānoa, Honolulu, HI, 7School of Earth and Space Exploration, Arizona State Univ., Tempe, AZ, 8Tohoku
Univ., Sendai, Japan, 9Korea Polar Research Institute, Incheon, Korea (E-mail: koeoep@uchicago.edu)
Introduction: Calcium-aluminum-rich inclusions
(CAIs) are the oldest dated objects that formed in the
solar system [1]. A few of these show large massdependent enrichments in the heavy isotopes of some
elements, attesting to a formation by melt distillation of
low-temperature precursors, and are often classified as
fractionated (F) CAIs ([2] and references therein). Some
fractionated CAIs also have large mass-independent
anomalies of nucleosynthetic origin and are classified as
FUN (Fractionated and Unidentified Nuclear effects)
CAIs. Other CAIs with large unidentified nuclear (UN)
anomalies lack fractionation effects. While F and FUN
CAIs are often not petrologically distinct from regular
CAIs, UN effects are most commonly found in hiboniterich CAIs like platy hibonite crystals (PLACs) [3].
We have studied 80 hibonite-rich CAIs separated
from the Murchison meteorite for their UN characteristics [4, 5] and incorporation of 26Al at their time of formation. Here, we report the mass-dependent and massindependent isotope effects in O, Mg, Ca and Ti in 7 of
these hibonite-rich CAIs that have F(UN) characteristics.
We also infer their initial 26Al/27Al ratios [(26Al/27Al)0]
from model and internal isochrons.
Methods: Al-Mg and O-isotope analyses were performed with the WiscSIMS Cameca ims-1280 at UW.
The results for O isotopes have been previously reported
[4]. For Mg isotopes, most hibonites were analyzed with
a ~12 µm primary O– beam and mono-EM detection [6].
Ca and Ti isotopes were analyzed with the Cameca ims1280 at UH [7]; part of the results have been previously
reported [5]. Intrinsic isotopic fractionation is expressed
as FTi and FCa as defined in [3].
The major element compositions were determined by
EPMA (Cameca SX-50). Trace elements (REE, Mg, Ti)
were measured with the Cameca ims-6f at ASU. NIST
glasses were used to obtain relative sensitivity factors
and oxide interferences on the heavy REE were corrected using the ion to oxide ratios published by [8].
Results: Mineralogy and morphology: Grains 2-5-1,
2-6-6, and 2-8-7 are stubby crystals of hibonite (70 to
300 µm in size). All contain refractory metal nuggets
(RMNs); those in 2-5-1 and 2-6-6 were large enough for
analysis and are depleted in Mo and W. 2-2-1 and 2-8-3
are single crystals with platy morphologies. In contrast
to other hibonites, their surfaces have abundant hexago-
nal pits. 1-9-1 is a platy CAI fragment consisting of a
spinel and a hibonite layer. 1-10-3 is a hibonite aggregate with inclusions of corundum (<10 µm).
Chemical and isotopic characteristics: The results
are summarized in Table 1 and Figures 1 and 2. The
main observations are as follows: (1) Based on the inferred (26Al/27Al)0, the hibonite-rich CAIs can be divided
into three groups: 26Al-free, with subcanonical ratio, and
with approximately canonical ratios (Table 1). (2) All
except the canonical CAIs are depleted in Mg and Ti
relative to PLAC and SHIB hibonites [4]. (3) All CAIs
show fractionation effects in at least one element, but
never simultaneously in all four studied elements. (4)
The most Mg- and Ti-rich hibonites (canonical CAIs)
have the highest degrees of Ca fractionation, but lack
corresponding effects in Ti, Mg and O (Fig. 2b). (5) Correlated fractionation effects in Ca and Ti are only observed in four CAIs (Fig. 2b), i.e., 1-10-3, two of the
26
Al-free CAIs (2-6-6, 2-5-1) and the subcanonical CAI
2-8-7. (6) Nucleosynthetic anomalies are only observed
in 26Al-free CAIs and are more pronounced in 48Ca
(δ48Ca of up to 43±5‰) than in 50Ti (Fig. 2a). (7) Most
analyzed grains have Δ17O (=δ17O–0.52×δ18O) values
between –23‰ and –25‰ (Fig. 1), like many FUN and
normal CAIs from unmetamorphosed chondrites [e.g., 9,
10]. Only the CAI with the largest nucleosynthetic effects (26Al-free CAI 2-5-1) has a distinct Δ17O of
–14±1‰. (8) Only CAI 2-6-6 shows significant frac-
Table 1: Characteristics of the studied CAIs. Felement
indicates degree of fractionation. Abbreviations: subc –
subcanonical, aggr. – aggregate, cor – corundum, sp –
spinel, n.a. – not analyzed, amu – atomic mass unit.
Name
Classification
26
Al/ 27Al (×10-5)
Δ 17O (‰)
FO (‰/amu)
FCa (‰/amu)
FTi (‰/amu)
FMg (‰/amu)
MgO (wt%)
TiO2 (wt%)
Morphology
Additional
phases
REE
2-5-1 2-6-6 1-9-1 1-10-3 2-8-7
26
Al-free
subc.
~0
~0
~0
n.a. ~ 0.3
–14 –23
–23
–24
–25
~ 24 ~ 4 ~ 27
~ 3 ~ 26
~ 10 ~ 5
~ 15 < 5
~ 15
~ 10 < 5
<5
<5
> 10
~ 0 ~ 10
~0
~0
n.a.
< 0.5 < 0.5 0.5
0.5 < 0.5
< 0.5 < 0.5 ~ 1.5 ~ 1
0.5
stubby
platy aggr. stubby
cor,
RMN RMN sp
RMN
RMN
Ce depletion n.a.
n.a.
2-8-3 2-2-1
canonical
~5
~5
–25
–24
~7
~5
> 15 > 15
~0
~0
~0
~0
~1
~1
~2
~2
platy
perovskite
low
n.a.
46th Lunar and Planetary Science Conference (2015)
2750.pdf
nounced in 1-9-1) and (2) the steep distribution of δ48Ca
vs. δ50Ti compared to PLACs (Fig. 2a), which could
Mg-depleted hibonites:
indicate dilution of 50Ti anomalies in the Ti-poor frac0 0
Mg-depleted
hibonites:
1-9-1
e
ne
tionated CAIs. Alternatively, isotopically normal Mg
n lliin MM
1-10-32-6-6
A
atiioon
tionnat
CCCA
2-5-1 1-9-1
rcatcio
could have entered the hibonite lattice without Ti, possif
C
l
triafra
-20
2-6-6 2-5-1
reisal
-20
Teesrtr
bly facilitated by an overabundance of Ti4+ (and vacanr
r
2-8-7
2-8-7
Te
Corundum-hibonite
cies) relative to Mg2+ in the hibonite lattice, as hinted at
Others:
-40
Canon. fract. CAIs
by our SIMS analyses. Enhanced Ti may be a result of
-40
Others:
(2-2-1 & 2-8-3)
crystallization in a melt with high Ti/Mg due to preferCanon. fract. CAIs
-40
-20 18
0
20
PLACs [4]
ential evaporation of the more volatile Mg. That the exδ O (‰)
PLACs [4]
-40
-20 18
0
20
cess Ti is present as Ti3+ is unlikely, given that Ce depleδ O (‰)
tions in hibonite (Table 1) and W and Mo depletions in
Figure 1. O isotope diagram, modified
after [4].
PLAC-like
CAIs (Kööp et al.RMNs
2014 c) indicate formation under oxidizing conditions.
a
b
Fit through PLAC data
The mass-independent effects suggest that the sam40
15
ples record at least three different stages of solar nebular
Mg-depleted hibonites:
20
history. The range of resolvable anomalies in 48Ca and
10
2-6-6
50
Ti indicates that the 26Al-free fractionated CAIs
1-9-1
0
2-5-1
formed early in a heterogeneous nebula, that was evolv5
2-8-7
-20
ing towards a uniform Δ17O of ~ –24‰ prior to the arri1-10-3
val of 26Al. The subcanonical CAI may have formed
0
-40
Others:
during admixture of 26Al (although formation after sig26
-40 -20
0
20 40
-5
0Al PLACs
5
10
15
50
δ Ti (‰)
FTi (‰/amu)
nificant 26Al decay cannot be excluded), in a nebula
average
uncertainty
Figure 2. Ca- and Ti-isotope data
for 3σfractionated
characterized by a Δ17O of ~ –24‰, in which nucleosynhibonite-rich CAIs, with PLAC data from [5] for com- thetic anomalies had been homogenized to a level of <
parison. a) Nucleosynthetic anomalies. b) Fractionation ~5‰. Finally, the canonical hibonites formed around the
effects. Symbols as in Fig. 1; gray line in Fig. 2a is a fit same time as normal CAIs. Whether or not the subcathrough PLAC data.
nonical level indicates early or late formation, the apDiscussion: The fractionated CAIs 2-2-1 and 2-8-3 proximately constant Δ17O value indicates that the oxyhave particularly enigmatic isotopic compositions (high gen isotopic evolution of the CAI-forming region apfractionation in Ca, yet not in O, Mg and Ti, ~canonical pears to have stagnated for a significant amount of time.
Conclusions: The lack of Mg and Ti isotopic frac(26Al/27Al)0). Their identical morphological, chemical
and isotopic characteristics suggest that they may have tionation in some hibonites with high Ca fractionation
been liberated from the same CAI, possibly during acid suggests that late-stage reintroduction of Mg and Ti may
have diluted isotopic effects in these elements. As Ca
treatment.
A general feature of the studied CAIs is that frac- abundance is fixed by mineral chemistry, this element
tionation effects can be observed in all analyzed ele- likely provides a more robust record of the original
ments, yet never simultaneously in the same CAI. This mass-dependent and -independent isotope effects in
could be a result of complicated condensation and evap- hibonite. The variable nucleosynthetic anomalies and
oration histories or of reintroduction of isotopically levels of 26Al incorporation suggest that these CAIs represent at least three different isotopic populations and
normal Mg and/or Ti after CAI crystallization.
Most fractionated hibonites studied here lack frac- sample different evolutionary stages of the nebula. This
tionation in Mg. This has been observed in previous further implies that conditions favorable for melt distillastudies and attributed to quantitative Mg evaporation, tion existed over a significant period of nebula history.
References: [1] Amelin Y. et al. (2002) Science,
followed by Mg surface contamination [11]. However,
as Mg and Ti contents are generally coupled in the 297, 1678–1683. [2] MacPherson G. J. (2014) Treatise
nd
hibonites studied here, most prominently in the Mg- and on Geochemistry, 2 Ed. [3] Ireland T. (1990) GCA, 54,
Ti-rich canonical CAIs, we argue against contamination, 3219-3237. [4] Kööp L. et al. (2014) LPS, 45, 2508. [5]
but favor late-stage reintroduction of normal Mg (and Kööp L. et al. (2014) MAPS, 49, #5384. [6] Ushikubo T.
possibly also Ti) to explain the lack of Mg fractionation et al. (2013) GCA, 109, 280–295. [7] Park C. et al.
effects. Indicators that reintroduction of Mg also resulted (2014) LPS, 45, #2656. [8] Fahey A. J. et al. (1987)
in addition of isotopically normal Ti include (1) the lack GCA, 51, 329–350. [9] Krot A. N. et al. (2010) ApJ, 713,
of fractionation effects in Ti in the CAIs that show high 1159–1166. [10] Makide K. et al. (2009) GCA, 73,
5018-5050. [11] Ireland T. et al. (1992) GCA, 56, 2503–
degrees of Ca fractionation (2-8-3, 2-2-1 and less pro2520.
FCa (‰/amu)
48
δ Ca (‰)
17
17
δ O (‰)
δ O (‰)
tionation in Mg isotopes. All others are indistinguishable
from or slightly lighter than measured standard values.